Rational Design and Responsive Transdermal Cargo Delivery of Microneedles in the Treatment of Melanoma

Abstract

Melanoma is a type of tumor that originates in the skin. Traditional treatment methods are unable to achieve effective drug concentrations at the tumor site. Many new drug delivery systems and treatment strategies have been developed to enhance the therapeutic effect of melanoma. Based on transdermal drug delivery technology, microneedles (MNs) have become a new strategy for the treatment of melanoma. MNs can surpass the limitations of traditional drug delivery methods, achieving remarkable safety and effectiveness, precise targeting, and painless delivery. Currently, MNs are increasingly being used as drug carriers to enhance therapeutic effects by increasing local drug concentrations. Alternatively, MNs are fabricated into platforms with different responsiveness to construct intelligent drug delivery systems, thereby maximizing the efficacy of active substances. This review systematically summarizes the application of MNs in melanoma treatment, including their preparation methods, treatment modes, and responsive design. The advantages and disadvantages of different types of MNs and their applications in the treatment of melanoma have been systematically expounded. Finally, the prospects of intelligent MNs in the treatment of melanoma were also discussed. Although MNs have made significant progress in clinical applications, issues such as the consistency of their size, biocompatibility, and cost control for large-scale production remain major challenges that need to be addressed urgently. Therefore, the clinical application of therapies based on MNs is still limited, highlighting the key opportunities for future research.

Introduction

Melanoma is an aggressive form of skin carcinoma, characterized by its tendency to metastasize, extremely low survival rate, and the propensity for recurrence.¹ According to the latest estimates from the International Agency for Research on Cancer (IARC), there were nearly 20 million new cancer cases (including non-melanoma skin carcinoma [NMSC]) in 2022, along with 9.7 million cancer-related deaths (including NMSC).² Although there are multiple treatment options for melanoma, including surgical resection, chemotherapy, radiotherapy, immunotherapy and targeted therapy, the issue of drug resistance in existing therapies and the precision of treatment remain major challenges.³ Each mode of administration, including oral administration, intravenous injection, intramuscular injection, mucosal administration, or transdermal administration, has its own unique challenges.⁴ For instance, oral administration is subject to first-pass hepatic biotransformation. Intravenous delivery can inflict greater damage on normal tissues and cells. And intramuscular injection is hindered by injection pain, needle phobia, and the need for skilled technique.⁵ Mucosal delivery requires improving formulation stability. Simultaneously, conventional transdermal patches struggle to overcome the physical barrier composed of the stratum corneum (SC). These issues may hamper effective clinical treatment of melanoma.³ Therefore, it is of utmost importance to develop a new type of drug delivery system that is more efficient and safer for the treatment of melanoma.

Microneedles (MNs) have developed into a promising transdermal drug delivery system. MNs are usually composed of various tiny needle-like structures attached to a supporting base, and they possess a strong mechanical penetration ability. MNs enable efficient drug administration for both local and systemic therapeutic effects by creating temporary microchannels in the skin. Based on superior advantages such as painless application, non-invasiveness, convenience, high patient compliance, relative safety, and elevated bioavailability, MNs effectively integrate the benefits of traditional transdermal patches and become an extremely potent transdermal drug delivery system (TDDS). Compared with traditional therapies, MNs offer unique advantages. First of all, MNs can bypass the first-pass metabolism process and deliver the drugs directly to the deeper melanoma lesions, thereby enhancing the accuracy and efficacy of targeted therapy. Secondly, MNs facilitate the implementation of combined therapies, enabling the combination of chemotherapy drugs, proteins, and other biological molecules. Moreover, they also support the stimulus-responsive release to achieve optimum therapeutic effect while lowering drug resistance.⁶ Third, therapies based on MNs can maximize the prevention of drug diffusion into healthy tissues and organs. This can reduce side effects and extend patient survival. Fourthly, MNs can concentrate drugs at tumor sites to achieve better efficacy at lower doses.⁷ The traditional intravenous administration strategy of anti-tumor drugs has problems such as systemic toxicity risks and the formation of drug resistance. However, the MNs technology can achieve precise local tissue drug delivery and on-demand release. As carriers, the diameters of MNs typically are less than 500 nm. They are composed of various biodegradable materials including natural or synthetic lipids, polymers, or metals.⁸ So far, MNs have been widely studied as a delivery tool for various chemical substances and biological macromolecules, as they have the potential to become a more patient-friendly alternative to traditional drug administration techniques. There have been numerous reports published on their application in the delivery of conditioned medium and monoclonal antibodies.^9,10

The application of MNs in a clinically relevant context, particularly for the management of melanoma, has been addressed by numerous research studies. This review examines studies conducted over the past few years, aiming to explore the optimal design of MNs for efficient treatment, adaptive to melanoma (Figure 1). It provides a comprehensive analysis of various MNs, focusing on their fabrication techniques, therapeutic application, and responsive drug delivery strategy. The review presents rational protocols for achieving responsive release performance and offers strategies for the on-demand design of MNs. This will assist in the development of synergistic MNs-based therapeutic modalities that may facilitate more effective, targeted, and patient-friendly treatments. This review also points out the issues that need to be further explored in future research. Finally, it discusses the obstacles encountered by MNs during their clinical application and large-scale promotion.

Figure 1.

The rational design and responsive transdermal cargo delivery of microneedles in the treatment of melanoma.

The Preparation of MNs

As a TDDS, the functions of MNs largely depend on the preparation method and the production process¹¹ (Figure 2A). From traditional photolithography and etching (ie, micro-electromechanical systems (MEMS) technology) to the emerging processes such as additive manufacturing (AM), template molding, and centrifugal-assisted casting, various methods have achieved different goals. There are examples of the different preparation methods for MNs, such as photolithography, a combination of micromolding method and laser-based fabrication, and a combination of micromolding method and laser-engineered silicone sheet¹¹ (Figure 2B and C). The advancement of these technologies enables precise control over the size, shape, and material of MNs. The development of MNs fabrication technology has approximately suffered from three stages: starting with MEMS technology, using photolithography to fabricate silicon MNs. The MNs are easy to shape but simultaneously prone to breaking and have high manufacturing costs. Later, metal materials (such as titanium and stainless steel) were adopted. Because of immune response limitations and the generation of waste and biologically hazardous materials,¹² now biocompatible polymers such as PLGA and sodium hyaluronate are primarily used. These polymers have high viscosity and are less prone to breaking. It makes them available for low-cost large-scale production.¹³ At the same time, they combine microforming and laser technology to optimize mold preparation, which overcome traditional limitations and also become the mainstream solution.¹¹ The main fabrication methods of MNs are summarized as follows: matched-mold forming, 3D printing method, and micro-electro-mechanical systems.

Figure 2.

(A) Drug delivery approach via different type of MN: (i) solid, (ii) coated, (iii) hollow, (iv) dissolving and (v) hydrogel-forming MNs.¹¹, Copyright 2022, Springer Nature. (B) Schematic representation of MN fabrication using micromoulding method which combined with laser-engineered silicone sheet. (i) An aqueous blend of polymer is poured onto the laser-engineered silicone sheet inside the green silicone elastomer mould. (ii) The cast blend is then centrifuged. (iii) Cross-sectional view of a silicone mould filled with aqueous polymeric blend during the drying process. (iv) The dry MN is removed from the mould. (v) The sidewalls of the MN are cut off using a heated scalpel blade. (vi) The MN following removal of the sidewalls.¹¹, Copyright 2022, Springer Nature. (C) Schematic illustration describing a combination of micromoulding method and laser-based fabrication prior MN manufacturing. (i) Silicone elastomer is poured into the aluminium holder with a metal block inside it. (ii) The aluminium container is filled with silicone elastomer, then this is centrifuged and cured overnight. (iii) The dry silicone elastomer is demoulded from the aluminium holder. (iv) A laser-engineered silicone sheet is placed and adhered onto the bottom part of the cast silicone elastomer. (v) Aerial and (vi) cross-sectional view of adhered laser-engineered mould.¹¹, Copyright 2022, Springer Nature.

Template Molding

MNs can be constructed by injecting liquid materials into molds (such as polydimethylsiloxane, silicon or metal molds).¹² In the process of preparing MNs using this method, three core procedures directly determine the performance of drug-loaded MNs (DMNs). Firstly, the thorough mixing of the drug with the matrix solution and the filling of the mold are the key steps to ensure the performance of DMNs, and this process lays the foundation for the subsequent operations. Chen et al used a compressed tool made of polylactic acid (PLA) to compact the solution after centrifugal drying, repeatedly centrifuging to eliminate bubbles.¹⁴ Secondly, an appropriate curing process is of vital importance, as the use of an improper curing method may result in the drug being damaged or losing its activity. Common curing methods include UV cross-linking or thermal curing.¹² Thirdly, the stable connection between the base and the needle body cannot be ignored, as its quality has a decisive impact on the overall performance and application effect of the DMN.¹⁵ Common template molding methods mainly include photocuring and centrifugal-assisted casting.

Photocuring

Photocuring is a commonly utilized method in the fabrication of MNs. The principle is to expose the photosensitive material to ultraviolet rays, visible light, or other specific wavelengths of light, which triggers a photochemical reaction, causing them to change from a liquid state to a solid state. Luo et al developed a biocompatible and biodegradable MNs based on gelatin methacryloyl using the photocuring method for TDDS.¹⁶ After loading DOX, the drug-free MNs changed from white to pink. Additionally, the GelMAMNs had anti-tumor effects on the melanoma cell line A375. Jia et al used photocuring to create a 10 × 10 array of living photosynthetic MNs, known as MA/CM@MN. The patch measures 10 × 10 mm² and carries conical MNs with a base diameter of 350 μm and a height of 900 μm.¹⁷ Simultaneously, the MNs contain microalgae (MA) and cuttlefish black pigment (CM). Due to the presence of MA and CM, the tips of the MA/CM@MN are dark gray and the bases are light green. The MNs showed high photothermal stability, providing an effective treatment strategy for postoperative melanoma therapy and skin wound healing (Figure 3A and B). Apart from the standalone use of photocuring, this technology can also be combined with other methods to prepare MNs. Zhou et al used a centrifugation and light curing method to create a biodegradable 11 × 11 MNs array for delivering water-insoluble drugs.¹⁸ This patch was made from a natural derivative polymer conjugate of gelatin methacryloyl and β-cyclodextrin (GelMA-β-CD). Curcumin was successfully loaded into the MNs array under a fluorescence microscope. The loaded curcumin maintained its anti-tumor activity, which showed good stability (Figure 3C). The above methods all relate to the preparation of single-layer MNs. Chen et al developed a double-layer MNs patch using template-assisted double-layer molding.¹⁹ The MNs carried hydroxyapatite (HAP) derived from tumor cells and short-chain peptides on its surface. A PDMS mold consisting of a 14 × 14 array of MNs was fabricated, with each MN having a height of 800 μm and an interval of 700 μm. Then, a solution was added and UV cross-linking was adopted. The resulting HVMN hydrogel exhibited good elasticity and proved excellent organ and blood compatibility. In addition, MNs can simultaneously exert anti-tumor and tissue regeneration-promoting effects, demonstrating significant efficacy in the postoperative treatment of malignant melanoma. The above study shows that photocuring demonstrates strong adaptability and innovation in the area of MNs preparation. Whether it is personalized drug loading design for single-layer MNs, composite preparation combined with other technologies, or functional expansion of double-layer structures, this way can prepare MNs by adjusting photochemical reactions and loading drugs to meet different therapeutic needs.

Figure 3.

Preparation of MNs by photocuring method. (A) Schematic illustration of the fabrication of living photosynthetic MN patches and their application for in situ oxygenation and postsurgical melanoma therapy.¹⁷ Copyright 2024, BMC. (B) Characterization of the living microalgae (MA) and cuttlefish melanin (CM)-loading MNs (MA/CM@MN). (i) Photographs of MA/CM@MN. Scale bars indicate 5 mm. (ii) Optical images of the MA/CM@MN. (iii) Optical images of the MA/CM@MN at higher magnification. (iv) Fluorescent images, and (v, vi) SEM images of the MA/CM@MN at different magnifications. (vii, viii) High-resolution SEM images showing the MA embedded in the MN base. (ix) and CM nanoparticles dispersed in the MN tips (x). Scale bars indicate 1 mm in (ii), 500 μm in (iii, iv), 300 μm in (v), 100 μm in (vi), 5 μm in (vii), and 500 nm in (viii). (ix–x) Real-time infrared thermal images (ix) and corresponding photothermal heating curves (x) of membranes under continuous irradiation of an 808-nm laser at the power density of 0.50 W/cm² for 5 min.¹⁷ Copyright 2024, BMC. (C) Schematic for GelMA-β-CD synthesis and GelMA-β-CD based microneedle fabrication and application. Synthesis route of GelMA-β-CD by conjugating GelMA with (i) CM-β-CD conjugation via EDC/NHS coupling. (ii) Curcumin was loaded within GelMA-β-CD pre-gel solution to form curcumin inclusion complex. Schematic for fabrication of GelMA-β-CD/CUR MNs by (iii) centrifugation and UV crosslinking. GelMA-β-CD/CUR MN arrays were tested on 3D B16 F10 skin cancer model (iv).¹⁸ Copyright 2020, Wiley.

Centrifugal-Assisted Casting

Centrifugal-assisted casting is based on the template molding method and simultaneously utilizes centrifugal technology. This can ensure that the material is evenly distributed in the mold and improve the preparation efficiency. Lo et al used centrifuge-assisted solvent casting methods to synthesize dissolvable MN patches (DMNs) loaded with WSG (MN-WSG) using polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP).²⁰ The resulting MNs demonstrated that PVP and PVA possess hardness and elasticity advantages. Simultaneously, these MNs reinforced the stability and bioavailability of WSG. Due to the heterogeneity of tumors, the effect of monotherapy is not satisfactory. Zhao et al used a multi-step centrifugal casting method to construct a multifunctional nanoparticle-integrated dissolving MNs-based drug delivery system. This system is made of 12 × 12 (144) MNs, each 1200 μm in height. The nanoparticles were evenly distributed at the tips of the MNs. This further verifies that the nanoparticle-integrated DMNs are robust enough. Furthermore, these MNs can simultaneously carry multiple drugs, aiming to overcome the limitations of single therapy and traditional administration methods, and significantly enhancing the convenience of treatment.²¹ In summary, the centrifugal-assisted casting method significantly improves the uniform distribution of materials within the mold through the introduction of centrifugal technology. This has effectively improved the efficiency of MNs fabrication and product quality. However, there are still critical issues that need to be solved, such as the impact of MNs length on tumor treatment. Future research should focus more on these aspects.

Template molding methods demonstrate significant advantages for scalable MNs production, making them particularly suitable for transitioning from laboratory research to industrial manufacturing. The scalability of template molding is evidenced by several key factors including cost-effectiveness, manufacturing throughput, and reproducibility. Template molding is recognized as one of the most cost-effective methods for the fabrication of MNs. According to recent studies, this method enables relatively simple, cost-effective MNs production at ambient temperature, making it particularly suitable for mass production applications.²² A single master template can be used to produce hundreds of PDMS (polydimethylsiloxane) replicas, and each replica can subsequently be used to create at least 50 molds for MNs manufacturing.²³ This high replication capability significantly reduces the per-unit cost of production, especially when manufacturing at scale.²³ Template molding methods offer substantial improvements in manufacturing throughput compared to serial production methods. Recent innovations have demonstrated that template molding can be adapted for continuous production processes. For instance, a novel manufacturing process combining injection molding and roller casting has been developed that enables consistent production of 14 × 14 MNs arrays with clear potential for scale-up.²⁴ The development of automated equipment based on positive-pressure microperfusion techniques has further enhanced the scalability of template molding.²³ Template molding methods demonstrate excellent reproducibility, which is critical for regulatory approval and clinical applications. Studies have shown that microneedles produced via template molding exhibit remarkable reproducibility in dimensions and mechanical properties.²³ Recent technical innovations have further improved the scalability of template molding methods. The development of double-penetration female molds (DPFM) with waterproof breather membranes has addressed issues related to gas resistance and solution viscosity, enabling more efficient mold filling.²³ The combination of template molding with other technologies has also enhanced scalability. For example, integrating micromolding with laser technology to optimize mold preparation has overcome traditional limitations and become a mainstream solution for scalable production.²⁵ This hybrid approach combines the precision of laser machining with the cost-effectiveness and scalability of molding processes. When compared to other fabrication methods, template molding offers distinct advantages for large-scale production. Unlike 3D printing, which is often limited by slow production speeds and high material costs, template molding can achieve much higher throughput rates suitable for commercial applications.²⁶ While MEMS technology offers higher precision, it comes with significantly higher equipment costs and complexity, making template molding more economically viable for most applications.²⁷

3D Printing

3D printing is known as additive manufacturing, which achieves MNs formation by layer-by-layer deposition of materials. It does not require pre-made molds and is suitable for complex structures. Thanks to the versatility, ease of operation, high repeatability, and precision of 3D printing, it has sparked extensive research enthusiasm in the field of MNs manufacturing. As an advanced additive manufacturing technology, it not only enables personalized customization of MNs structures but also provides efficient, practical, reliable, and cost-effective manufacturing solutions with potential for scalable production. This technological breakthrough accelerates the transition of MNs from laboratory research to large-scale clinical applications. 3D printing provides strong technical support and effectively promotes industrial development in related fields.²⁸ The main methods of 3D printing technology are summarized below, which include Stereolithography (SLA), Two-photon polymerization, and hybrid laser printing (HLP).

Stereolithography

Stereolithography (SLA) is a high-throughput manufacturing technology, which generates 3D microstructures through an additive process from bottom to top. Spatially controlled curing of a liquid via photopolymerization constructs a solid three-dimensional structure.²⁹ Yeung et al used an SLA printer and IIa class biocompatible resin to create a microfluidic hollow MNs structure.³⁰ They demonstrated the accuracy, consistency, and reproducibility of the 3D printing method through scanning electron microscopy and various tests. Additionally, they confirmed the mechanical robustness, transdermal drug regulation, and delivery capabilities of the microneedles in practical applications. This method provides new degrees of freedom for transdermal drug delivery.

Two-Photon Polymerization

Two-photon polymerization is a laser-based rapid prototyping technique, which can be used to directly manufacture hollow microneedles of various geometries.³¹ Gittard et al used two-photon polymerization (2PP) micromachining and polydimethylsiloxane (PDMS) micro-molding processes to manufacture polymer MNs for transdermal delivery.³² The main structure of the MNs consists of a 5×5 array made up of 25 identical solid MNs (needle height = 500 μm, needle base diameter = 150 μm, center-to-center distance between needles = 500 μm). These MNs can be used for transdermal administration, transporting proteins and other biological drugs into the body. The MNs prepared by two-photon polymerization exhibit good compressive strength. The combination of micro-molding and 2PP nano-fabrication represents a high-throughput approach, making that creating MNs structures with appropriate structural, mechanical, and biological properties become simpler and more convenient. However, whether the MNs fabricated by this method can represent suitable biological features for protein drug delivery still requires further research.

Hybrid Laser Printing

Hybrid Laser Printing (HLP) can be used to manufacture complex, multi-scale, multi-material 3D structures. It contains Continuous Liquid Interface Production (CLIP), which is a new method for manufacturing MNs.³³ This technology differs from SLA, which allows each patch to complete an array of MNs for most designs within less than 10 minutes. The technology is a one-step mold-free manufacturing process and is a continuous rather than layer-by-layer additive manufacturing method. Johnson et al used this approach to quickly create prototypes of sharp MNs with adjustable geometries (size, shape, aspect ratio, and spacing), such as arrow MNs, “layered” MNs, and “turret” MNs.³⁴ These MNs can effectively penetrate mouse skin and release fluorescent drugs, such as rhodamine. This provides a new design scheme for transdermal drug delivery that needs to utilize MNs.

Inkjet Printing

Inkjet printing can be applied for coating transdermal MNs, which involves distributing various active substances in the form of tiny droplets. These liquid drops can form a uniform layer on the needle surface.³³ At the same time, it was found that polyvinylpyrrolidone can be utilized to produce dissolvable MNs, making it a promising polymer. Lucas conducted compression tests on MAP produced by centrifugation and MAP produced by inkjet printing separately. The results showed that both exhibited mechanical properties. However, compared to centrifugation, inkjet printing is more precise and saves API. Inkjet printing can serve as an accurate manufacturing method for personalized MNs array patches.

Despite these advantages, 3D printing faces significant scalability challenges. The layer-by-layer deposition process is inherently time-consuming, making high-throughput production difficult.³⁵ Material costs for 3D printing resins and polymers are substantially higher than bulk polymers used in molding.³⁶ Additionally, the resolution limitations of current 3D printing technologies restrict the fabrication of hollow MNs with precise lumen dimensions, which are critical for fluid delivery applications.³⁶ The thermal sensitivity of many drugs also poses challenges, as some 3D printing methods involve heat that may degrade therapeutic agents.³⁷

Micro-Electromechanical Systems

Micro-electromechanical systems (MEMS) mainly achieve high-precision manufacturing of MNs arrays through micro-scale machining techniques such as photolithography, etching, and thin-film deposition. Kim used sequential steps such as photolithographic patterning, dry and wet etching, and metal-assisted chemical etching (MACE) to design a biodegradable, miniaturized porous silicon (p-Si) needle (Figure 4A).³⁸ It was then proved to have minimal side effects in localized chemotherapy. Additionally, to improve the delivery efficiency and accuracy of DMN, Lee et al utilized centrifugal lithography and dissolving MNs manufacturing methods.³⁹ The prepared P-DMNs have a height of 500 ± 30 μm and is a patchless, microcolumn-integrated DMN coating. Compared to traditional DMNs, it significantly improves drug delivery efficiency and can be applied to diverse types of microbial molecules and large biomolecules for transdermal delivery. The EME lithography system can also be combined with other technologies to fabricate MNs. Balmert et al combined three-dimensional (3D) laser lithography with micro-molding technology to create an OVA ± Poly MNs.⁴⁰ This DDS used a unique MNs array design that contains a sharp pyramidal tip and an inverted conical stem with rounded corners. The MNs have a height of 750 μm, a top angle of 30°, and a stem width of 150 μm. These needles extend from a square pyramid base measuring 250 × 250 μm down to the backing layer of the MNA, which are connected by a radius of 35 μm rounded corner (Figure 4B). This design more significantly stimulates cellular and humoral immunity compared to conventional muscle injection vaccination, making it applicable for transdermal DDS. The above content shows the extensive application and significant effectiveness of MEMS in the manufacturing of MNs. Through innovative combinations of different machining techniques, it not only achieves precise control and performance optimization of MNs structures but also provides efficient and low-side-effect solutions for transdermal drug delivery, vaccination, and other biomedical fields. In the future, with the deep integration of MEMS technology and other advanced technologies, MNs are expected to achieve breakthroughs in more medical scenarios.

Figure 4.

(A) (i) Schematic illustrations for the construction of the p-Si needles on a water-soluble backing. (ii) Optical images of the p-Si needles integrated with a PVA film. (iii) SEM image of the nanopores formed on the surface of the p-Si needle features.³⁸ Copyright 2020, ACS. (B) Microneedle array manufacturing strategy involves six distinct steps. The numbers 1 to 6 indicate the sequential steps in the manufacturing process of the microneedle array.⁴⁰ Copyright 2020, Elsevier.

MEMS technology offers unparalleled precision in MN fabrication, achieving aspect ratios up to 30:1 with nanometer-scale accuracy.²² However, the high equipment costs and complex multi-step processes limit scalability.²⁷ The parallel manufacturing approach of MEMS enables simultaneous production of multiple arrays, ensuring reproducibility and reducing per-unit costs at scale.²²

Others

Apart from the most commonly used preparation methods mentioned above, there are also other techniques. Droplet-born air blowing (DAB) provides gentle (4–25 °C) and fast (≤10 min) MN fabrication conditions without drug loss.⁴¹ Atomized spraying to fill molds can eliminate the need for a centrifugation or vacuuming step from a micromolding process and significantly improve the transition to continuous manufacturing, which is readily scalable.⁴² Laser micromachining (cutting, ablation) is one of the most conventionally used methods for rapid prototyping and cost-effective microfabrication.⁴³ Pulling pipettes are only suitable for the preparation of hollow glass MNs.³³ More and more often, multiple methods are combined, such as the combination of laser technology and microforming technology, the combination of centrifugal separation and photopolymerization technology, and so on. The combination of various methods will make the preparation of MNs more precise and simpler. The MNs obtained by different preparation methods are listed in Table 1.

Table 1.

Different Preparation Methods of MNs

MNs	Material	Preparation Method	Needle Height	The Bottom of Needle	Matrix	Cargo	Drug Loading	Application	Ref.
Biodegradable gelatin methacryloyl microneedles	Gelatin methacryloyl (GelMA)	SLA	600 μm	300 μm	11 × 11	DOX	200 μg per patch	Melanoma	[16]
MNs patch	Methacrylated hyaluronic acid (HAMA) and ethylene glycol dimethacrylate (PEGDA)	Photocuring	900 μm	Diameter of 350 μm	10 × 10	/	/	Melanoma and wound healing	[17]
Biodegradable β-cyclodextrin conjugated gelatin methacryloyl microneedles	Gelatin methacryloyl and β-cyclodextrin (GelMA-β-CD)	Centrifugal molding combined with photocuring	/	Length of 600 μm and widths of 300 μm	11 × 11	Curcumin	/	Melanoma	[18]
Double-layer MNP	Methacrylated gelatin (GelMA)	Template-assisted double-layer casting method	800 μm	700 μm	14 × 14	Hydroxyapatite and biomimetic autophagosomes-based nanovaccine	1 mg/100 mg	Malignant melanoma	[19]
MNPs-dMNs	Hyaluronic acid (HA)	Multi-step centrifugal casting method	/	/	/	Polypharmacy CPT	74 μg per patch	Melanoma	[21]
Biodegradable, miniaturized porous silicon (p-Si) needles	Si wafer (p-type; 525 μm-thick)	MEMS	/	Diameter of 2–4 μm, length of 10–70 μm	/	DOX	/	Melanoma	[38]
P-DMN	Hyaluronic acid	MEMS	300 μm	Diameter of 300 and 500 μm	3 × 3	Rhodamine	/	Transdermal therapy	[39]
Novel dissolution-based undercut MNA	Carboxymethylcellulose (CMC) and trehalose	3D laser engraving and micro injection molding	750 μm	250 μm × 250 μm	5 × 5	OVA and Poly	10 μg per patch	Transdermal drug delivery	[40]
D-MAP	Povidone (PVP), Hypromellose (HPMC)	Inkjet printing	900 μm	/	/	Lisinopril	33 μg per patch	Hypertension and CHD	[44]
MN-WSG	Polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP)	Centrifugal-assisted solvent casting method	/	/		WSG	12.7–51.3 μg per patch	Melanoma	[20]

The Application of MNs in the Treatment of Melanoma

In recent years, researchers have been continuously exploring new methods for treating melanoma in order to overcome the shortcomings such as the non-specific tissue distribution of drugs. In traditional treatments (surgery, chemotherapy, and immunotherapy), oral and transdermal administration often encounter problems such as insufficient local drug accumulation and non-specific toxicity. Melanoma mainly occurs in superficial skin tissues. The skin is composed of the epidermis, dermis, and subcutaneous tissue. The epidermal thickness ranges from 100 to 150 μm, and consists of stratified squamous epithelium, stratum corneum, lucidum layer, and granular layer.⁴⁵ The stratum corneum is composed of 15 to 20 layers of keratinocytes, and its main function is to serve as the first line of defense for protecting the skin from external substances. However, this robust skin barrier significantly hinders the penetration process of drugs through the skin.⁴⁶

The MNs are composed of a series of micrometer-sized needles, with their lengths typically ranging from 400 to 1000 μm. They can penetrate the stratum corneum of the skin, thus overcoming the limitations of traditional transdermal drug delivery in terms of biological molecules and nanoformulations.^47,48 MNs can directly penetrate the dermis of the skin, thus having excellent drug permeability, which greatly facilitates the realization of local targeted therapy. As an innovative drug delivery system, MNs can enhance the transdermal absorption efficiency of drugs, which improves local pharmacological effects while reducing systemic toxicity, providing a new direction that balances efficacy and safety for melanoma treatment.⁷ At the same time, MNs cause minimal damage to the skin, and allow for minimally invasive treatment of melanoma³³. When treating superficial skin tumors (such as melanoma), the combination of photodynamic therapy (PDT), photothermal therapy (PTT), chemotherapy, immunotherapy, and many other treatment methods can significantly enhance the therapeutic effect (Figure 5). Sun et al used the self-assembled nanomicelle-dissolving MNs (DMN) for the chemotherapeutic PTT synergistic approach.⁴⁹ The MNs were used to deliver paclitaxel as the primary chemotherapeutic agent and PSIR780, which successfully eliminated tumors with an 88% cure rate without damaging healthy tissues.

Figure 5.

Schematic diagram of MNs insertion, microchannel formation, drug diffusion gradient, and the synergistic effect with exogenous stimulation.

The insertion of MNs fundamentally alters the pharmacokinetics of drug delivery by creating direct microchannels through the stratum corneum, bypassing the rate-limiting barrier of transdermal absorption.^8,47 This process enables rapid drug diffusion into the epidermis and dermis, achieving therapeutic concentrations at the tumor site within minutes rather than hours.⁵⁰ The microchannel formation also facilitates enhanced penetration of nanoparticles and macromolecules that would otherwise be excluded by the intact skin barrier.^6,48 The insertion of MNs induces localized mechanical stress and microtrauma, triggering inflammatory responses that can enhance immune cell infiltration and improve drug distribution within the tumor mass.⁵¹ The combination of MNs with external stimuli such as light, heat, or pH-responsive materials enables spatiotemporal control over drug release, maximizing therapeutic efficacy while minimizing systemic exposure.⁵¹

In summary, different types of MNs display distinct strengths in tumor penetration, drug retention, and immune activation for cancer therapy (Table 2). Hollow MNs excel in deep-tumor penetration and effective delivery of macromolecules or nanoparticles to avascular tumor regions.⁵² Hydrogel-forming MNs achieve the highest drug retention via swelling-induced physical entrapment, enabling sustained release of therapeutics in the TME.⁵³ Dissolving MNs represent the optimal balanced option, with moderate tumor penetration, long-term drug retention, and superior immune activation—mature dendritic cells, enhance antigen-specific T cell responses, and synergize with immunomodulators to convert cold tumors to hot ones.^53,54 Solid MNs only serve as cost-effective penetration enhancers with limited drug retention and immune activation capacity.⁵⁵ Collectively, the rational selection of MNs based on tumor depth, therapeutic agent properties, and immunotherapy goals is critical to maximizing the efficacy of MNs-mediated tumor treatment.⁵²

Table 2.

Comparative Analysis of Chemical, Photothermal, Immune, and Gene Delivery MN Systems

Delivery System	Advantages	Limitations	Ref.
Chemotherapy	1. High drug loading capacity, 2. Controlled release kinetics, 3. Multiple drug co-delivery, 4.Bypass first-pass metabolism, 5. Reduced systemic toxicity, 6. Enhanced local concentration	1. Potential drug degradation, 2. Limited penetration depth, 3. Burst release issues, 4. pH sensitivity, 5. Stability concerns, 6. Manufacturing complexity	[14,20,21,56,57]
PTT	1. Precise spatial control, 2. Minimal invasiveness, 3. High tumor specificity, 4. Synergistic with chemotherapy, 5.Real-time monitoring, 6. Deep tissue penetration (NIR)	1. Heat shock protein resistance, 2. Normal tissue thermal damage, 3. Inconsistent light distribution, 4. Limited to superficial tumors, 5. Equipment dependency, 6. Thermoresistance issues	[21,57–59]
Immunotherapy	1.Systemic immune activation, 2. Long-term protection, 3. Minimal drug resistance, 4. Checkpoint inhibitor delivery, 5. Tumor microenvironment modulation, 6. Combination therapy potential	1. Low clinical response rates, 2. Immune-related adverse events, 3. Complex preparation, 4. Cold chain requirements, 5. High production costs, 6. Individual variability	[57,60–64]
Gene Delivery	1.Precise gene targeting, 2. High specificity, 3. Reduced off-target effects, 4. Localized action, 5. Minimal systemic exposure, 6. Permanent/lasting effects	1.Complex formulation, 2. Stability challenges, 3. Low transfection efficiency, 4. Degradation risks, 5. High manufacturing costs, 6. Regulatory hurdles	[19,57,65–67]

Critical knowledge gaps remain in understanding the precise mechanisms by which the insertion of MNs affects drug pharmacokinetics and TME modulation. Long-term studies are needed to evaluate the chronic effects of MNs on skin integrity, immune responses, and drug accumulation in healthy tissues.⁶⁸ Furthermore, the relationship between MNs (length, diameter, tip shape) and drug penetration depth in human skin of varying thickness and age remains poorly characterized.⁶⁹

Delivery of Chemotherapy Drugs

Chemotherapy is one of the effective means for tumor treatment currently. Considering the non-differential tissue damage, rapid in vivo metabolism, poor accessibility to the tumor site, adverse side effects, and tumor resistance of chemotherapeutic agents, the delivery of chemotherapy drugs based on MNs is a very promising therapeutic approach.^70,71 In recent years, the combined use of various non-crossing anti-tumor drugs to combat tumors has been widely adopted. Anti-tumor drugs with complementary molecular mechanisms reduce tumor resistance, minimize side effects, accelerate tumor apoptosis, delay the adaptation process of tumors, and comprehensively enhance the anti-tumor effect by intervening different targets. For example, the combination of hydrophilic carboplatin (CBP) and hydrophobic vorinostat (SAHA) is considered a potential solution. However, the difficulties in co-loading arise from their different physical and chemical properties. Li et al developed a kind of porous “sponge-coated” MNs (PF-MNP).⁵⁶ This involves constructing a ten-layer multilayer film of polyacrylic acid/polyethyleneimine on MNP, followed by acid treatment and freeze-drying to form a sponge-like coating. Capillary effects are then used to achieve stepwise adsorption of SAHA and CBP. Finally, the drugs are evenly distributed on the surface of the MNs while maintaining the MNP morphology. The drug loading capacity can be flexibly controlled by adjusting the concentration of the adsorption solution. PF-MNP can rapidly release most of the drugs within 30 min, which demonstrated the best therapeutic effect on multi-drug-resistant melanoma. This “sponge-coated” MNP has the potential to become a safe and efficient platform for transdermal delivery of multiple drugs. Additionally, plasma-activated water (PAW) rich in reactive oxygen and nitrogen species (RONS) has anti-tumor potential. However, conventional injection methods suffer from rapid RONS decay and inefficient PAW delivery. Zhang et al investigated a new approach.⁷² This involves using custom plasma equipment and processes, which are followed by rapid low-temperature micro-molding, and integrating PAW into plasma-activated cryogenic microneedle (PA-CMN) patches. PA-CMNs can be inserted into the skin, releasing RONS and slowing their decay to extend biological activity. The rich mixture of RONS can induce tumor cell death and apoptosis, selectively killing tumor cells. The PA-CMNs significantly inhibited subcutaneous A375 melanoma growth. Due to its lack of systemic toxicity, it is promising to open new pathways for the treatment of more diseases (Figure 6A). Kim designed biodegradable, miniaturized porous silicon (p-Si) needles to handle the issue of difficulty in sustained drug release. The drug loading capacity of these needles is comparable to traditional polymer MNs. The needle tip uses a water-soluble film as a temporary flexible scaffold that can closely conform to irregular tissue surfaces. It completely dissolves within one minute in saline solution, which allows the p-Si needles to remain in the tissue for gradual degradation and sustained drug release. The p-Si needles demonstrated effective localized chemotherapy with minimal side effects and good concealment.³⁸ Above all, MNs combined with chemotherapy can avoid first-pass effect and improve the solubility of chemotherapeutic drugs. At the same time, it can reduce systemic drug exposure and adverse side effects. Currently, the anti-melanoma effects of MNs are mainly studied in mice or other small animals. There are significant differences between mouse skin and human skin. Therefore, more research is needed to determine the optimal application methods.

Figure 6.

Application of MNs in melanoma treatment. (A) Conceptual design of PA-CMNs application for melanoma treatment. The arrows indicate a promoting effect.⁷² Copyright 2024, Elsevier. (B) The in vivo antitumor therapeutic efficacy of SNP-Fe@MNs. (i) Conceptual design of postoperative B16 tumor cell residues with defect model. (ii) In vivo heating curves and infrared thermal images of mice under the irradiation of 1.50 W·cm−² NIR. (iii) Tumor volume in groups of control, SNP-Fe@MNs-NIR(−)-NO(+), SNP-Fe@MNs-NIR(+)-NO(−), and SNP-Fe@MNs-NIR(+)-NO(+), respectively. (iv) Photographs of the whole therapeutic course.⁵⁸ Copyright 2024, BMC. (C) Photobleaching of PPIX in the skin of Balb/C mice.⁷³ Copyright 2016, Elsevier.

The Application of Photothermal Therapy

Photothermal therapy (PTT) is a novel tumor treatment modality with significant potential. It utilizes materials with high photothermal conversion efficiency. Upon exposure to near-infrared light, the materials convert light energy into thermal energy, achieving the ablation and killing of cancer cells.⁷⁴ PTT has the characteristics of low invasiveness, high tumor specificity enrichment, and few complications.³³ PTT has become a supplementary therapy for tumors. However, it is incompetent in tissue regeneration and repair. Dong et al developed a novel multifunctional nitroprusside and Fe²⁺ ion-loaded MNs (SNP-Fe@MNs) platform for the ablation of melanoma.⁵⁸ Under ultraviolet light irradiation, the MNs release nitric oxide (NO) explosively while simultaneously exhibiting NIR-responsive photothermal effects. Besides, the MNs steadily releases NO, which promotes tissue regeneration by upregulating HIF-1α/VEGF, thereby ensuring angiogenesis. This method provides a new remedial approach for treating oral and facial melanoma (Figure 6B). Zhao et al developed a multifunctional nanoparticle-integrated dissolving MNs to enhance the limitations of monotherapy and traditional systemic management.²¹ In this system, photothermal agents (CuS) were merged into the zeolitic imidazolate framework-8. At the same time, it was functionalized with hyaluronic acid. This system can load multimodal drugs, enhancing specific uptake and distribution in target tumors, which finally enables low-dose, high-efficiency drug delivery. Additionally, PTT in tumor treatment faces issues such as normal tissue thermal damage, tumor thermoresistance mediated by heat shock proteins (HSPs), and limitations of monotherapy efficacy. Yu’s research team developed a “partitioned MNs” (PMN-SNAP/CuS).⁵⁹ These MNs combined mild photothermal therapy (mPTT) with gas therapy (GT) to reinforce melanoma treatment outcomes. The MNs separate “catalyst” bovine serum albumin-modified copper sulfide nanoparticles (CuS@BSANPs) from “reactants” S-nitroso-N-acetylcysteine (SNAP) in different regions. It also possesses photothermal conversion, Fenton-like catalysis, and nitric oxide (NO) generation capabilities. Under 808 nm laser irradiation, CuS@BSANPs initiate mPTT, gently ablating tumor cells. Simultaneously, NO released from SNAP and reactive oxygen species (ROS) generated by CuS can synergistically block HSP synthesis, thereby overcoming tumor thermo-resistance. The PMN-SNAP/CuS significantly inhibits melanoma growth under laser irradiation. This strategy overcomes the limitations of traditional PTT through partitioned controlled release and multi-mechanism synergy, providing a minimally invasive and highly effective new approach for melanoma treatment. When MNs are applied to PTT treatment, only a single administration is required to achieve good efficacy. Compared with the therapy without using micro-needle technology, it reduces unnecessary inconveniences. At the same time, it can also retain some characteristics of PTT, such as small operational trauma, precise positioning and high safety. The limitations of PTT can also be addressed through multifunctional design, such as promoting tissue regeneration, achieving efficient low-dose drug delivery, etc.²¹ MNs have been widely proved effective in cellular and animal experiments. Due to the obvious differences in weight and melanoma growth between humans and animals, we may be faced with clinical translation issues. In other words, the same dose that is effective for melanoma mice may not be sufficient to achieve the expected efficacy in humans.

Photodynamic Therapy

The principle of photodynamic therapy (PDT) is to locally activate the photosensitizer within the tumor, thereby causing chemical damage and ultimately leading to the death of tumor cells.⁷⁵ The current photosensitizers have problems such as poor stability, unpredictable toxicity, ineffective therapeutic effects, and difficulty in achieving tumor-targeted localization when administered systemically. A large number of nanoparticle delivery systems have been developed to address these issues, including liposomes, solid lipid nanoparticles, and inorganic nanocarriers.⁷⁶ Nevertheless, the off-target toxicity caused by systemic administration remains a significant concern. To overcome this obstacle, local drug delivery has emerged as a highly promising alternative approach. MNs serve as a minimally invasive TDDS, featuring self-repairing, injectability, and stimulus-responsive properties, and are suitable for precise treatment.⁷⁷ Jain’s team evaluated the potential application of coated MNs in optimizing the transdermal delivery of 5-aminolevulinic acid (5-ALA).⁷³ The coated MNs had twice the amount of PPIX photobleaching compared to the cream group. These coated MNs delivery strategy provides a new approach for PDT of skin tumors (Figure 6C). MNs-assisted PDT is an effective approach to reinforce the transdermal delivery rate of photosensitizers, with good safety profiles. The most common side effect of MNs combined with PDT is pain, especially during the pretreatment phase. However, there have been no reports of life-threatening adverse events so far. Nevertheless, extensive high-quality clinical trials are needed to study the efficacy of MNs in PDT and to explore its deeper mechanisms.⁷⁸

The Application of Targeted Therapy

Tumor-targeted therapy is one of the emerging and highly promising tumor treatment methods in recent years. Precisely directing anti-tumor drugs to tumor tissues can increase the local drug concentration, enhance the therapeutic effect, and significantly reduce side effects.⁷⁹ MNs can effectively achieve transdermal targeted delivery of drugs, such as delivering to the lesion site or intradermally. MNs can also be used for delivering targeted drugs, such as inhibitors which are specific to particular gene mutations or signaling pathways. These inhibitors can directly act on tumor cells via MNs to reinforce efficacy. Because of the close relationship between the occurrence of melanoma and the Braf gene, the CRISPR/Cas9-sgRNA ribonucleoprotein complex (RNP) is preferred over plasmids and mRNA due to its superior safety. This makes it a more suitable choice for gene therapy. Additionally, Pan et al constructed a novel biocompatible dissolving MNs system combined with polyethyleneimine (PEI, 25 kDa).⁶⁵ This system is capable of locally delivering STAT3 siRNA to the skin. MNs obviously penetrated the skin and dissolved rapidly. The PEI/siRNA complexes loaded onto the MNs improve the uptake efficiency of siRNA by B16F10 cells through electrostatic interactions. This significantly eventually improved the silencing effect on the STAT3 gene and inhibited tumor cell proliferation. This dissolving MNs-based delivery system provided a safe and effective new strategy for targeted gene therapy of cutaneous melanoma. Chen et al incorporated a small amount of graphene oxide (GO) into biocompatible polymers, thereby obtaining MNs with a variety of new properties.⁶⁶ The mechanical strength of the MNs has been significantly enhanced (10–17 times at 500 mg/mL GO). In addition, the MNs also possessed the properties of moisture resistance, self-bactericidal, anti-bacterial, and anti-inflammatory, which achieved near-infrared light-activated drug release. This system has successfully delivered the chemotherapeutic agent HA15 to melanoma. These characteristics improved the efficiency and ease of transdermal drug delivery, enhanced the control over drug release, and expanded the range of polymers that can be used for the preparation of MNs. Furthermore, the use of a single targeted drug delivery method often leads to drug resistance in melanoma. Tra reverses the multidrug resistance (MDR) mediated by P-glycoprotein, thereby effectively blocking the P-glycoprotein-mediated excretion process of doxorubicin (DOX). Huang et al developed a type of MNs that can simultaneously deliver Tra and DOX, thereby synergistically enhancing the anti-tumor efficacy.⁸⁰ The DexMA hydrogel MNs developed in this study enhance the transdermal delivery efficiency of small-molecule drugs while decreasing systemic toxicity and side effects (Figure 7A). The use of MNs for targeted drug delivery can increase the concentration of drugs at the tumor site, improve the bioavailability of the drugs, and enhance the efficacy of melanoma-targeted therapy. Although significant progress has been made in the design and manufacture of MNs, there are still technical challenges in large-scale production. Precise control over the length, shape, material selection, and structural design of MNs is necessary, which is vital for making sure effective drug delivery, clinical safety, cost reduction, and commercialization.⁶⁰

Figure 7.

Application of MNs in melanoma treatment. (A) Schematic diagram of DexMA hydrogel MNs containing DOX and Tra for continuous transdermal drug delivery. The two asterisks in the DexMA structure represent the repeat unit boundaries of the polymer chain. The arrows represent drug delivery processes.⁸⁰ Copyright 2020, Elsevier. (B) Schematic illustrating the construction of the GOx-loaded PDA nano-shell for long-lasting catalysis, and transdermal delivery via dissolving MNs for percutaneous treatment of melanoma.⁸¹ Copyright 2021, Theranostics. (C) Schematic illustration of the preparation of Cur NDs/IR820/HA MN and Cur NDs/IR820/HA MN for tumor chemo-photothermal therapy and skin tissue regeneration.⁸² Copyright 2022, BMC. (i) Preparation of the Cur NDs/IR820/HA MN. (ii) In-vivo application of the Cur NDs/IR820/HA MN.

The Application of Immunotherapy

Tumor immunotherapy can stimulate the immune system to enhance immunity, thereby triggering an immune response against the tumor. However, there are still some challenges with immunotherapy. Systemic administration of the drugs may cause adverse reactions, and due to the obstruction of TME, the drug concentration within the tumor lesions often cannot reach the therapeutic level.⁸³ After surgical resection, infections that occur during the wound healing process often trigger non-tumor-specific inflammatory responses. If this inflammation is automatically suppressed by the body, then tumor-specific cytotoxic T lymphocytes will also be adversely affected, even if immune checkpoint inhibitors have already rescued its activity to some extent. Thus, a reasonable strategy is to increase the drug concentration at the lesion site while avoiding single-drug resistance. The prerequisite is to effectively control infection and inflammation to enhance the therapeutic effect of melanoma.⁸⁰ To convert these design principles into a localized and patient-friendly platform, an MNs-based system incorporating biodegradable hydrogels (such as methacrylated gelatin, GelMA) provides a minimally invasive and continuously releasing interface for transdermal chemical immunotherapy.⁸⁴ Yin et al designed photothermal-immunonanoparticles decorated with M1 macrophage membranes (BD@LM) based on the inflammatory environment after surgical resection.⁶¹ These nanoparticles were then combined with dissolvable MNs to create a new strategy for enhancing tumor immunotherapy. The nanoparticles loaded with photosensitizer black phosphorus quantum dots (BPQD) and DOX can trigger immunogenic cell death in tumor cells through chemo-phototherapy, promoting dendritic cell maturation, and initiating an immune cascade. Additionally, the MNs help overcome the skin barrier, delivering nanoparticles directly to solid tumor sites, significantly improving the efficacy of anti-PD-L1 (aPD-L1) immune checkpoint blockade therapy. By combining photothermal-immunonanoparticles with MNs, this research provided an innovative pathway for efficient and long-lasting treatment of postoperative tumor metastasis and recurrence. Additionally, immune checkpoint blockade (ICB) therapy has low clinical response rates and systemic administration off-target side effects. Han et al focused on local targeted delivery strategies to optimize therapeutic outcomes. They developed transdermal MNs loaded with aPD1 and anti-CTLA-4 antibodies. These MNs continuously release drugs at the lesion site, or trigger drug release by utilizing factors such as the low pH value in TME and abnormal enzyme expression. Hollow MNs loaded with PD-L1 were designed and fabricated, which combined the cold atmospheric plasma (CAP) therapy. The MNs channels can enhance the transdermal transmission effect of CAP and synergistically induce immunogenic cell death. In addition to it, injectable/sprayable hydrogels formed in situ can deliver immunomodulatory antibodies to the surgical bed to inhibit postoperative recurrence. Inspired by platelets targeting inflammatory sites, they constructed aPD-L1-modified natural platelets, platelet-hematopoietic stem cell (HSC) conjugates, and other cellular delivery platforms.⁶² Subsequently, the bone marrow homing ability of HSC was utilized to precisely deliver the aPD1 antibody in the leukemia model. Gene-engineered platelets overexpressing PD1 receptors or HEK293T-derived vesicles (encapsulating indoleamine 2,3-dioxygenase inhibitors) were used to disrupt the PD-1/PD-L1 pathway. These in-situ precise treatment strategies provide innovative approaches for enhancing the anti-tumor efficacy of immunotherapy and reducing side effects. Adoptive cell therapy (ACT) has shown poor efficacy in the treatment of solid tumors. Zhou et al developed a trench MNs to enhance ACT efficacy.⁸⁵ MNs with groove structures were fabricated using the ice template method to ensure the loading capacity of T cells. The surfaces of the MNs were also modified with the chemokine CCL22. The patch can precisely deliver tumor-specific T cells into solid tumors via physical penetration and utilizes a cytokine concentration gradient to specifically migrate Treg cells from the tumor microenvironment to the surface of MNs, which increased the ratio of effector T (Teff) cells to Treg cells in a mouse melanoma model. The groove-shaped nucleus cells treated with CCL22 served as the local microenvironment, and were combined with the strategy of locally delivering T cell receptors and chimeric antigen receptor T cells. This significantly enhanced the anti-tumor effect while reducing the off-target toxicity of ACT. The MNs have great potential in clinical applications, especially in improving the accuracy and efficacy of tumor immunotherapy. It can directly and precisely release nanomedicines continuously to tumor sites. Although MNs have demonstrated excellent performance in tumor immunotherapy, the clinical application is faced with many challenges and limitations. One challenge is the long-term preservation of drugs during delivery, especially for biologically active drugs such as tumor vaccines or oncolytic viruses. NDMNs (MN-based nanodrugs) tend to lose antigenicity and biological activity, thereby often requiring cold-chain storage. This obliges high demands on drug preservation and clinical application.⁶⁰ Increasing investment in equipment and technology, seeking sterile, durable, safe, and higher-quality manufacturing methods, and improving the production volume of MNs through strict quality control are conducive to achieving true cancer prevention and treatment.⁸⁶

Starvation Therapy

The traditional treatment methods for melanoma have some drawbacks, such as severe side effects and low drug concentration. Targeted therapies and immunotherapies have shown higher efficacy compared to conventional treatments. However, a considerable number of patients have developed drug resistance, which leads to unsatisfactory clinical outcomes. To improve treatment effectiveness, researchers have developed more suitable and updated therapies, such as starvation therapy.⁷ Starvation therapy is a novel tumor treatment method, which can block the energy supply to tumors.⁸⁷ Starvation therapy is currently a widely popular treatment method, which mainly achieves its effect by inhibiting tumor angiogenesis, blocking tumor blood vessels, and cutting off the nutrition and energy supply to the tumor.⁸⁸ During the abnormal metabolic process of tumor cells, their growth and proliferation are influenced by nutrients and energy (such as glucose supply). For instance, glucose oxidase (GOx) can consume glucose and convert it into hydrogen peroxide and gluconic acid, thereby cutting off the glucose supply to tumors, which is often used for starvation therapy.⁸⁹ Zeng et al designed a long-lasting nano-catalyst integrated into soluble MNs as a local delivery carrier, effectively consuming glucose in melanoma tissues.⁸¹ The MNs were loaded with polydopamine nanocapsules, enclosing GOx. The nanocapsules could selectively allow glucose to pass through to maintain the catalytic reaction, while preventing larger molecules such as GOx and proteases from penetrating, thereby protecting the activity of GOx. Compared to systemic administration, starvation therapy significantly inhibits tumor growth with minimal side effects. This study provides a new platform for the delivery of GOx in vivo, which can also be extended to other enzyme-based therapeutic fields (Figure 7B).

Tissue Regeneration

Stem cells possess the ability of self-renewal and multi-directional differentiation, making them highly promising for applications in regenerative medicine.⁹⁰ The emergence of induced pluripotent stem cells (iPSCs) has further advanced stem cell research. Traditional cell transplantation methods, such as direct injection or surgical implantation, have issues such as significant trauma and short cell retention time. Recently, MNs have been applied in cell delivery for stem cell therapy, offering a new approach by minimally invasively penetrating tissues to achieve targeted cell transport. This reinforces retention and avoids systemic immune responses.⁹¹ As platforms for PDT, PTT, or starvation therapy, MNs can achieve excellent therapeutic effects with a single administration, avoiding the problem of poor patient adaptability caused by repeated administration.¹⁹ Although the minimally invasive nature of MNs makes them relatively safe, their safety still needs to be carefully evaluated, including possible allergic reactions and infections. Additionally, the frequency of using microneedles (MNs) should be taken into consideration to avoid damaging the normal skin function.

Multi-Modal Therapy

Due to the complexity and heterogeneity of tumors, the current treatment methods do not show significant efficacy.⁹² To enhance the efficacy of a single treatment, the combination of two or more drugs has become an extremely attractive anti-tumor treatment strategy.⁹³ The combination of different drugs overcomes issues such as drug resistance and toxicity, and achieves efficient, multi-pathway, and multi-target anti-tumor treatment through synergistic effects. For example, the combination of photothermal agents and doxorubicin increases the sensitivity of tumor cells to chemotherapy and achieves a synergistic effect.⁹⁴ MNs provide a favorable platform for combined therapy, allowing drugs of different molecular sizes and physicochemical properties to be incorporated into a single system, thereby enhancing the convenience of administration.⁹⁵ Li et al designed DTIC/ICG-Fe₃O₄@TpBDBSP/HA MNs, which integrated PTT with chemotherapy to achieve synergistic anti-tumor effects.⁹⁶ The MNs consist of three parts: magnetic nanoparticles (DTIC/ICG-Fe₃O₄@TpBD) for precise targeting, a soluble matrix composed of Bletilla striata polysaccharide (BSP) and hyaluronic acid (HA), which ensures the penetration and subsequent dissolution and release of the MNs, a polyvinyl alcohol backing layer, which provides structural support. Additionally, tumor killing and wound healing need to be advanced simultaneously in melanoma treatment. Shan et al developed a two-layer MNs, which were prepared through a two-step process and consists of a layer embedded with curcumin nanodrugs/novel indocyanine green/HA (CurNDs/IR820/HA) and a supporting back layer made of sodium alginate/gelatin/HA (SA/Ge/HA).⁸² It can kill tumors synergistically through the photothermal effect of IR820 and the chemotherapeutic effect of curcumin nanodrugs under near-infrared light stimulation. After the MNs dissolve and release drugs, they will cover the wound with the supporting back layer to promote the proliferation of endothelial cells and fibroblasts, accelerating skin regeneration. It finally achieves dual functional effects of chemotherapy-photothermal combined anti-tumor and skin repair. These results provide innovative strategies for melanoma treatment that combine therapeutic efficacy with tissue repair capabilities (Figure 7C). Yu et al developed subcutaneously rapidly dissolving MNs, which utilized multimodal synergistic effects of PTT, chemodynamic therapy, and chemotherapy to enhance anti-tumor efficacy. The MNs used PVP as the substrate and biocompatible Notoginsenoside polysaccharide (PNPS) as the tip. It also encapsulates PVP-stabilized CuO₂ nanoparticles and disulfiram-containing F127 micelles. The water solubility of PNPS enables rapid dissolution under biological conditions, accurately delivering therapeutic components to tumor sites. CuO₂ nanoparticles release Cu²⁺ and H₂O₂ in the acidic tumor environment, achieving self-supply of hydrogen peroxide for chemodynamic therapy. Additionally, the near-infrared photothermal characteristics of CuO₂ induce local hyperthermia. This system effectively inhibits tumor growth in vivo through multi-therapy synergy with minimal side effects that provides a safe and efficient transdermal treatment option for melanoma.⁹⁷

Responsive Transdermal Cargo Delivery of Microneedles

MNs as a novel technology have significant advantages such as painless drug delivery, good biocompatibility, and concise self-administration. This has shown broad application prospects in disease treatment fields (Table 3). Presently, it has been effectively applied in the treatment of various diseases, including diabetes, wound healing, and tumor therapy. In TDDS, precisely controlling the release behavior of loaded drugs in polymer MNs is a core factor affecting therapeutic outcomes and one of the key research directions in this field.⁹⁸ Researchers typically refer to MNs with personalized drug delivery features such as adaptability, controllability, environmental responsiveness, and real-time feedback as “smart MNs”. Traditional anti-tumor drug intravenous administration strategies carry risks of systemic toxicity and resistance. However, MNs technology can achieve accurate local tissue drug delivery and on-demand release. Traditional soluble MNs can only directly release drugs into the skin according to predetermined design schemes, which may fail to meet complex physiological requirements.⁹⁸ By further combination with stimulus-responsive materials, the construction of intelligent stimulus-responsive MNs can be practiced. The MNs can respond to specific physical or chemical stimuli from either internal or external environments, such as pH value, ROS, enzymes, light, temperature, or mechanical force.⁵¹ This approach can further reinforce the precision of tumor treatment while reducing toxicity to surrounding tissues or cells. These systems achieve spatial and temporal control through mechanisms such as separation, degradation, dissociation, and swelling when exposed to specific stimuli, thereby minimizing side effects.⁹⁹ Stimulus-responsive MNs possess characteristics such as spatial and temporal controllability, high drug delivery accuracy, and minimal potential side effects. Depending on different application contexts, they can be designed for rapid release or sustained-release patches, which can improve patient compliance and reduce dosing frequency. This has broad application prospects in areas such as chronic disease drug delivery, wound treatment, and tumor therapy.¹⁰⁰ The stimulus conditions for responsive MNs can be categorized into internal and external stimuli based on the location where the stimulus occurs.

Table 3.

Anti-Tumor Applications of Different Responsive Microneedles

Design	Stimulus	Drug Loading	Application	Ref.
Chitosan–2 wt% MGQD–LH MN	Magnetism	Fluorescein sodium (FL)	Transdermal drug delivery	[68]
5-Fu-ICG-MPEG-PCL loaded HA MN	Photothermal	5-fluorouracil and indocyanine green	Epidermoid carcinoma and melanoma	[101]
ZnONW-MGP	Ultrasound	PTX	Tumor	[102]
Nanobubble-modified MNs patch	Ultrasound	Cy5	Transdermal drug delivery	[103]
CuO2 NPs	ROS and light	CuO2 NPs	Melanoma	[104]
a mesoporous nanovehicle with dual loading of photosensitizer	NIR	Dabrafenib and Trametinib	Melanoma	[105]
DOX/AuNC-loaded MNs	NIR	Gold nanocages and DOX	Tumor	[106]
Stimuli-responsive hydrogels form microneedle (MN) arrays	Illumination	Ibuprofen	Achieving “on-demand” delivery of drugs	[107]
EMH-MNA	Electricity	Indomethacin	Transdermal drug delivery	[108]
Multi-layer drug-loaded MNs	Magnetism	Lissamine (LGB) and Rhodamine 6G	Gastrointestinal lesions	[109]
HRMAM system	Hydrothermal response	Docetaxel loaded into polycaprolactone	Melanoma and breast cancer	[110]
MR-EP-MNP	Microwave	PMP and nanomedicine	Localized microwave hyperthermia chemotherapy for tumors	[111]
IRMNs-SWS	Mechanical force	Fluorescein sodium salt	Transdermal drug delivery	[112]
MN-GOx	PH	aPD1	Melanoma	[63]
LCC-NPs MNs	PH	Cisplatin	Anti-tumor	[113]
B’HA-NPs MNs	Hyaluronidase	Cell death protein 1 and immune suppressive enzyme indoleamine 2,3-dioxygenase	Melanoma	[64]
tr-MNP	/	p53 expression plasmid/polyethyleneimine	Subcutaneous tumor	[67]
PDCM@PTX	GSH	Betulinic acid and paclitaxel	Ovarian cancer (OC)	[114]

Exogenous Stimulus-Responsive MNs

Researchers are increasingly interested in exogenous stimulus-responsive MNs because they can achieve remote-controlled drug release. The MNs can enhance therapeutic effects and reduce toxicity to the body. According to actual clinical needs, such MNs can precisely control the immediate on-demand release of drugs through external stimulus signals (such as ultrasound, light, electricity, force, magnetic fields, temperature changes, etc). These MNs represent a more controllable and safer drug delivery method. Yang et al utilized electro-responsive properties to design a rolling MNs electrode array (RoMEA) for large-area local nucleic acid tumor immunotherapy.¹¹⁵ Hao et al used photo-responsive properties to design near-infrared light-responsive 5-fluorouracil (5-Fu) and indocyanine green (ICG)-loaded monomethoxy-polyethylene glycol-polycaprolactone (MPEG-PCL) nanoparticles (5-Fu-ICG-MPEG-PCL), which can be combined with dissolvable MNs (HAMN) for skin carcinoma treatment, including human epidermoid carcinoma and melanoma.¹⁰¹ Below is an introduction to the applications of ultrasound, light, electricity, magnetism, temperature, microwave, and mechanical force-responsive MNs.

Ultrasonic-Responsive MNs

Ultrasonic waves are mechanical waves with frequencies higher than 20 kHz, which can propagate through certain media. Ultrasonics have been used as effective non-invasive stimuli in drug delivery systems. Mechanistically, the cavitation effect and thermal effect of ultrasound can achieve spatiotemporal targeted drug release. A small portion of ultrasonic energy is absorbed by human tissues, causing local heating and promoting drug release.¹¹⁶ Zandi et al developed a microbubble generator probe based on zinc oxide nanowires (ZnONW-MGP).¹⁰² This probe integrates electrochemical stimulators onto micromachined silicon needles covered with zinc oxide nanowires (ZnONW). This generates dense local microbubbles (MB) distributed in the tumor environment. The MBs are then driven to explode by external ultrasound, inducing microcavitation in tumor cells. Shao et al developed a nanobubble-modified MNs for ultrasound-assisted drug delivery.¹⁰³ The patch was used for drug administration with the help of ultrasound (frequency of 850 kHz, amplitude of 10 V, power of 1 W/cm²). The surface of the MNs patch was coated with poly-L-lysine (PLL) to produce a positively charged layer. This layer then attracted and bonded with a negatively charged layer of nanobubbles containing fluorescent dyes via electrostatic contact. Cy5-containing nanobubbles assembled using layer-by-layer (LBL) assembly oscillated, expanded, and collapsed on the MNs surface, beginning to release Cy5 with the application of ultrasound. Cavitation-induced microflows increased the permeation and diffusion of drug molecules, thereby enhancing the effectiveness of transdermal drug delivery. The accompanying microflows caused by cavitation effects promoted the permeation and diffusion of drug molecules in gelatin gel models and ex vivo pig skin models. This drug delivery strategy enhances the permeation and diffusion of released drugs and has potential for rapid transdermal drug delivery.

Light-Responsive MNs

The optical response system has numerous advantages, such as the ability to achieve remote and precise control of dose distribution in both time and space, and its ease of application.¹¹⁷ Light-responsive MNs are prepared using specific photosensitive drugs that respond to particular wavelengths of light, such as ultraviolet (UV), visible, or near-infrared (NIR) light.¹¹⁸ UV light is able to disrupt biological molecules, including nucleic acids, proteins, and lipids.¹¹⁹ However, its high absorption and scattering in tissues limit its clinical applications. Visible light is primarily used for the superficial layers of skin and mucous membranes.¹²⁰ NIR light has gained significant attention because of its low scattering, strong penetration, and minimal tissue damage.¹²¹ Chen et al developed an acid and NIR-light-responsive MNs for melanoma treatment by coupling enhanced chemo-dynamic therapy (CDT) and NIR-mediated PTT. These MNs were based on biocompatible polymers, specifically PVP, and simultaneously loaded with PVP-stabilized CuO₂ NPs. They exhibit good skin insertion capability and dissolution properties, effectively delivering therapeutic agents to melanoma sites. The released CuO₂ NPs generate Cu²⁺ and H₂O₂ in response to acidic environments, catalyzing the production of hydroxyl radicals for self-sufficient CDT. Additionally, they can clear glutathione to reinforce CDT. The system also demonstrates excellent NIR photothermal properties due to the loading of CuO₂ NPs, which can induce localized hyperthermia for PTT. These MNs achieve effective tumor suppression through combined therapy with minimal side effects in vivo, providing a safe and effective option for melanoma treatment.¹⁰⁴ Additionally, MNs-based PDT has been used for localized treatment of various types of skin carcinoma. It is obviously limited by the inability of existing photosensitizers to penetrate deep into the dermal tissue. To overcome these issues, Tham et al developed a light-responsive mesoporous nanocarrier for combined therapy.¹⁰⁵ At the same time, MNs technology is utilized to promote their penetration into deep dermal tissues. NIR-irradiated drug-loaded nanocarriers exhibit synergistic cytotoxic effects on skin carcinoma cells primarily through ROS and caspase-induced apoptosis. The MNs can promote the penetration of nanocarriers through the epidermis layer, reaching deeper melanoma sites, finally demonstrating excellent therapeutic efficacy. For prevalent superficial skin tumors (SST), surgery and systemic treatments are relatively invasive, each potentially causing severe side effects. Dong et al designed dissolvable MNs loaded with gold nanocages (AuNCs) and DOX.¹⁰⁶ The loaded AuNCs not only enhance the mechanical strength of MNs but also serve as effective agents for PTT. This approach achieves efficient transdermal treatment of SST. The resulting MNs can effectively penetrate the skin and dissolve within it. The photothermal effect triggered by NIR laser irradiation of AuNCs synergizes with the chemotherapeutic effect of DOX to destroy tumors. The effective anti-tumor activity of DOX/AuNC-loaded MNs after four doses to mice bearing SST, with no significant side effects. Hardy et al designed a photo-stimuli-responsive hydrogel MNs array using methacrylic acid 2-hydroxyethyl ester (HEMA) and ethylene glycol dimethacrylate (EGDMA) via micro-molding.¹⁰⁷ The system was loaded with up to 5% (w/w) ibuprofen contained in the photo-responsive 3,5-dimethoxy cinnamate conjugate. Raman spectroscopy confirmed the presence of the conjugate in the polymer MNs matrix. The system could deliver up to three doses of 50 mg ibuprofen over an extended period (up to 160 hours) after optical triggering. This made the system attractive as a controlled-release device over a long period. This technology holds promise for “on-demand” delivery of various drugs.

Electro-Responsive MNs

The electro-responsive MNs-based drug delivery system utilizes an electric field to facilitate or regulate the release of drugs.¹²² The research on these intelligent MNs mainly focuses on the design and fabrication of electro-responsive MNs as well as the development of energy supply systems.¹²³ At present, the commonly used electro-responsive materials include conductive polymers, conductive hydrogels, carbon-based materials, metals, and semiconductor materials.¹²² To enhance the efficiency of drug penetration through the stratum corneum, Indermun et al investigated a novel transdermal electro-modulated hydrogel MNs array (EMH-MNA) device.¹⁰⁸ This device consists of nanoporous, embeddable ceramic MNs and optimized EMH. It was used for the electro-responsive delivery of indomethacin through the skin. Ex vivo evaluation of the EMH-MNA device on pig skin showed that there was a significant reduction in drug release without electrostimulation compared to electrostimulation.

Magneto-Responsive MNs

Magnetic induction technology possesses attributes such as long-distance guidance, resilience, and adaptability under complex conditions.^124,125 It has significant potential in fields such as surgical navigation, drug delivery, and blood testing.¹²⁶ Magnetic-responsive materials used in existing research include atomized iron powder, NdFeB particles, mesoporous iron oxide (MIO), etc.¹²⁷ These magnetic materials can be wirelessly driven by a magnetic field. By applying an external magnetic field, it causes material deformation or directional movement towards specific magnetic orientations.¹²⁸ Justin et al prepared biodegradable chitosan-magnetic graphene quantum (MGQD) nanocomposites and investigated the release effects of small and large molecular weight therapeutic agents from both detachable and non-detachable biodegradable MNs (Figure 8).⁶⁸ The presence of MGQDs in chitosan increased its electrical conductivity and biodegradation rate while maintaining its mechanical properties. Detachable MNs were created by incorporating a water-soluble PEG ring at the base of the MNs, allowing rapid separation of the MNs shafts from the base. The chitosan nanocomposite is an excellent material for developing multifunctional MNs for targeted and tracked transdermal drug delivery. Lee et al proposed a capsule driven by an external magnetic field for actively delivering multi-layer drug-loaded MNs.¹⁰⁹ This multi-layer drug-loaded MNs system consists of three drug-loaded MNs units and a polydimethylsiloxane layer. Each drug-loaded unit was prepared from a 10% gelatin solution mixed with drugs and connected to a permanent magnet inside the magnetic-driven capsule. Under the influence of an external magnetic field generated by an electromagnetic driving system, the capsule carrying the multi-layer MNs can reach target lesions, where each unit can deliver drugs to the lesion site for therapeutic effects. The active delivery capability of the magnetic-driven capsule for the multi-layer MNs. This study proposes a magnetic-driven capsule system for actively delivering multi-layer drug-loaded MN patches, which may become a new efficient drug delivery method for different gastrointestinal lesions.

Figure 8.

Experimental procedure to create conventional and detachable chitosan–MGQD microneedle arrays.⁶⁸ Copyright 2018, Royal Society of Chemistry. (a) Conventional intact microneedle array. (1) Viscous nanocomposite solution was used to fill the moulds. (2) Excess solution was cleaned from the surface and the moulds were left to air-dry. (3) Viscous nanocomposite solution was used to fill the moulds. (4) The moulds were left to air-dry. (5) Viscous nanocomposite solution was used to fill the moulds. (6) The moulds were left to air-dry. (7) The base was filled with a viscous pristine chitosan solution. (8) The moulds were left to dry. (b) Detachable chitosan–MGQD microneedle array. (1) Viscous nanocomposite solution was used to fill the moulds. (2) Excess solution was cleaned from the surface and the moulds were left to air-dry. (3) Viscous nanocomposite solution was used to fill the moulds. (4) The moulds were left to air-dry. (5) Viscous nanocomposite solution was used to fill the moulds. (6) The moulds were left to air-dry. (7) A highly viscous water solution of PEG was applied to the mould and excess solution scraped off. (8) The moulds were left to dry and filled with a viscous pristine chitosan solution. (9) The moulds were then dried in a vacuum oven.

Temperature-Responsive MNs

Temperature response is a key characteristic of intelligent biomaterials.¹²⁹ Temperature-responsive MNs can control the delivery behavior of drugs by adjusting the stimulation temperature, application site, and duration.¹³⁰ Temperature-responsive materials play a key role in temperature-responsive MNs, undergoing phase changes based on temperature variations, thereby controlling drug release.¹³⁰ Currently, the temperature-responsive materials include poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), poly(N-isopropylacrylamide) (PNIPAM), and polymethyl methacrylate (PMMA).⁹⁸ Localized treatment of subcutaneous tumors is gaining attention due to advantages such as minimal invasiveness and low systemic toxicity. However, further breakthroughs are needed for deep penetration and spatiotemporal control. Xu et al proposed a hydrothermal-responsive multi-turn movable MNs (HRMAM).¹¹⁰ The docetaxel was loaded into polycaprolactone. The optimized hydrothermal-responsive formulation enabled water-based self-heating responsive drug release and synergistic hyperthermia. The design of the needle tip combined with the groove structure enables deep transportation and enhances the transfer efficiency. The HRMAM system achieved tumor growth inhibition rates of 75.11% and 72.29%. The data were significantly higher than traditional treatments without apparent side effects. This approach holds promise for providing an efficient, low-toxicity, controllable, and user-friendly solution for localized treatment of deep subcutaneous tumors. Besides, some temperature-responsive MNs utilize photothermal catalysts to melt polymers with lower critical temperatures, thereby enabling drug delivery. Hao et al designed a soluble MN system for skin carcinoma treatment, initially creating near-infrared light-responsive 5-fluorouracil (5-Fu) and ICG-loaded methoxy-polyethylene glycol-polycaprolactone (MPEG-PCL) NPs (5-Fu-ICG-MPEG-PCL).¹⁰¹ Then 5-Fu-ICG-MPEG-PCL was integrated with a hyaluronic acid to obtain HAMN loaded with 5-Fu-ICG-MPEG-PCL. This system was used to treat skin carcinomas that include human epidermoid carcinoma and melanoma. After the MNs were inserted, the dissolution of hyaluronic acid led to the delivery of NPs into the body. Upon 5 min of irradiation with 1.5 W/cm² near-infrared light (808 nm), the tumor site temperature rose to 65°C and then caused MPEG-PCL NPs to dissolve and subsequently release 5-fluorouracil. This design improved the cure rate of skin carcinoma and offered new perspectives for clinical treatment.

Microwave-Responsive MNs

The controllable and precise delivery of drugs faces challenges such as tumor targeting and deep penetration. Liu et al developed a novel engineering approach using microwave-responsive magnetic bio-metal-organic frameworks.¹¹¹ The MNs deliver magnetic bio-metal-organic framework nanoparticles, demonstrating excellent microwave heating effects. Engineered platelets release PMP and nanodrugs upon activation in the TME. Furthermore, microwave hyperthermia can also enhance the absorption of drugs by cells and the penetration of drugs into deeper tissues. The separable MNs allow the tips to remain in the target tissue, supporting repeated local hyperthermia treatments. These MNs integrated engineered platelets to enhance transdermal deep drug delivery and achieved localized microwave thermoschemotherapy. This work represents the first try to incorporate microwave-responsive inorganic nanodrugs as cell-based drug delivery systems onto platelets. It provides a new strategy for accurate drug delivery that is activated by microwave thermotherapy.

Mechanical Force-Responsive MNs

When simple movements of body parts or external forces serve as stimulus signals, the mechanically triggered MNs can be constructed to release drugs.¹³¹ The process of drug release via mechanical force can enhance the penetration efficiency of drugs. This allows for the transdermal delivery of large-molecule drugs. The most common form of mechanically responsive MNs involves loading drugs onto hydrogel materials. Simultaneously, gel-based drugs are combined with MNs.¹²⁶ Drugs are released by mechanically compressing the gel-based drugs.¹⁰⁰ Di et al designed a wearable stretch-strain triggered drug delivery device.⁶⁹ The device consists of stretchable elastomers and a microgel reservoir containing drug-loaded nanoparticles. They applied tensile strain to the elastomer film. The expansion of the diffusion surface area and the compression of the micro-reservoir by Poisson’s ratio facilitated the release of drugs from the micro-reservoir. This system has certain issues regarding accurate dosing. This is because the quantitative relationship between force and responsive release is difficult to control.⁵¹ Hyesun et al developed an Inserted Responsive Microneedle (IRMN) system for rapid drug delivery through the skin without the need for skin patches.¹¹² The system comprises square-cone hyaluronic acid (HA) MN tips and poly ε-caprolactone (PCL) base arrays. The HA tips rapidly separate from the matrix under the mechanical force of puncture after insertion into the skin. The MNs remain in the skin and continuously release drugs. The IRMN system holds promise for rapid and accurate delivery of various molecules through the skin. It also reinforces user convenience by eliminating the need to affix MNs to the skin.

Endogenous Stimulus-Responsive MNs

Changes in physiological parameters are major indicators for various diseases such as tumor and endocrine disorders. Based on this, endogenous stimulus-responsive MNs have been developed.¹³² These MNs can autonomously regulate drug release through dynamic changes in biochemical signals such as pH, glucose, and enzymes.¹³³ This differs from exogenous stimuli-responsive MNs, which rely on passive drug delivery.¹³⁰ The MNs can achieve personalized treatment without additional devices, and they also offer high sensitivity and therapeutic adaptability.¹³⁴ Wang et al utilized pH-responsive mechanisms to design a self-degradable MNs patch that continuously releases aPD1 in response to pH changes.⁶³ It induced a strong immune response in B16F10 mouse melanoma models. Chen et al used ROS and NIR light-responsive mechanisms to create MNs that disrupt the redox homeostasis of tumor cells and induce apoptosis, which could be applied for clinical melanoma treatment.¹⁰⁴

pH-Responsive MNs

pH-responsive MNs are primarily made of polymers.⁵¹ Changes in environmental pH values can lead to polymer degradation, swelling, rupture, or collapse.⁵¹ Most tumor microenvironments have a lower pH value than normal tissues.¹³⁵ Therefore, this reactive approach has significant practical value in tumor therapy. Cisplatin, as a first-line chemotherapeutic drug, has systemic toxicity and side effects that limit its clinical application. Lan et al used MNs to mediate pH-responsive liposome-coated cisplatin nanoparticles (LCC-NPs) for transdermal delivery.¹¹³ LCC-NPs are composed of tumor-targeting pH-responsive lipid nanoparticles embedded with cisplatin, with a cisplatin loading rate of 80%, which enhances the solubility of cisplatin and boosts the in vitro anti-tumor efficiency. The MNs loaded with cisplatin nanoparticles significantly enhanced the cytotoxicity and apoptosis rate of tumor cells, with an apoptosis index reaching 58.6%. This indicated a significant reduction in tumor volume and weight. No platinum element was detected in the serum or lungs, liver, and kidneys, which demonstrated the good biocompatibility of MNs. This type of MNs provides new opportunities for tumor treatment, capable of enhancing anti-tumor activity, reducing systemic toxicity, and minimizing side effects. Wang et al reported innovative self-degradable MNs that continuously release aPD1 in TME.⁶³ The MNs are composed of biocompatible hyaluronic acid, pH-sensitive dextran NPs encapsulating aPD1, and GOx. GOx converts blood glucose into gluconic acid, generating an acidic environment that promotes the self-dissociation of NPs, subsequently leading to massive release of aPD1. The MNs induced a strong immune response in B16F10 mouse melanoma models. Additionally, this delivery strategy can be integrated with other immunomodulators (such as anti-CTLA-4) to achieve combined therapy for enhanced antitumor efficacy. Li et al utilized the acidic skin environment to layer-by-layer assemble pH-responsive polyelectrolyte multilayers (PEM) on the surface of PCL MNs for rapid gene release.⁶⁷ PEM consists of two parts: a transition layer (PLL-DMA/polyethyleneimine) and a gene-loading layer (p53 expression plasmid/polyethyleneimine). The PEM-modified microneedle patch was labeled as tr-MNP. In a model DNA tracking modification experiment using Hoechst33258 staining, the fluorescence intensity of tr-MNP was obviously enhanced with increasing bilayer number while maintaining morphology. Under changing environmental pH, PLL-DMA could reverse charge in the acidic skin environment, which caused the transition layer to collapse and promote gene release. Overall, pH-responsive gene-loaded MNP can effectively treat subcutaneous tumors and have potential as a platform for loading many biomacromolecules.

Redox-Responsive MNs

Oxidation reactions are very common in living organisms, and they are even more prevalent in pathological conditions. The most notable oxidizing substances are known as ROS. Mitochondria are considered a primary intracellular site of ROS generation. Generally, tumor cells with mitochondrial genetic abnormalities (copy number change and mutations) have escalated ROS levels compared to normal cells.¹³⁵ Therefore, oxidative molecules such as superoxide radicals and hydroxyl groups can cause the manganese-nitrogen materials to undergo oxidation or structural changes. This will trigger their responsive behaviors, such as drug release or other functions.¹²⁶ However, chemotherapy drugs have poor solubility and numerous adverse side effects, and there may even be cases of multiple drug resistance. The drug delivery system based on MNs is expected to overcome these difficulties and achieve redox-responsive drug release, which is of great significance. Chen et al developed active oxygen and NIR light-responsive MNs for melanoma treatment based on PVP.¹⁰⁴ MNs were used to deliver CuO₂ nanoparticles (namely, MN@CuO₂). These nanoparticles catalyzed the conversion of hydrogen peroxide (H₂O₂) into toxic hydroxyl radicals (·OH), disrupting the redox homeostasis of tumor cells and inducing apoptosis. On the other hand, these CuO₂ NPs were used as near-infrared photothermal agents, which generated significant heat in the tumor area under NIR irradiation, followed by using photothermal therapy to kill melanoma cells. The easy-to-prepare, simple-component, multifunctional MNs have promising clinical applications for efficient and safe melanoma treatment.

Enzyme-Responsive MNs

Enzymes play a crucial role in regulating cells, making them important targets for drug development and treatment. Changes in enzyme activity or dysregulation of expression are the basis for the occurrence of many diseases. In therapeutics, abnormal regulation of enzymes is an important biological trigger factor.¹²⁶ Ye et al developed a synergistic immunotherapy strategy, which employed a transdermal delivery method based on MNs.⁶⁴ Then, it locally targets immune checkpoint receptor PD1 and immune-suppressive enzyme indoleamine 2,3-dioxygenase (IDO) to treat melanoma. Immune therapy nanocapsules loaded with aPD1 were assembled using hyaluronic acid modified with IDO inhibitor 1-methyl-dl-tryptophan (1-MT). Subsequently, these NPs were integrated into the MNs. In the tumor microenvironment, overexpressed hyaluronidase (HAase) digested HA within the MNs, which promoted drug release. The formulation approach based on a combination strategy of “drug A in the carrier formed by the incorporation of drug B” helps enhance the drug loading capacity. Additionally, the resulting delivery device can induce sustained release of checkpoint inhibitors in TME and enhance the retention.

With the continuous development of responsive biomaterials and emerging technologies, stimulus-responsive MNs-based therapy has emerged, providing new possibilities for transdermal on-demand drug delivery. Researchers typically define MNs as “smart MNs”, which possess features such as adaptability, controllability, environmental awareness, and real-time feedback for personalized drug delivery. These intelligent MNs possess excellent spatial, temporal, and dosage control capabilities. The stimulus-responsive MNs therapy system can achieve controlled drug release based on pathological characteristics, external physical, or chemical signals, and is thus used for tumor treatment.¹²⁶ Various responsive MNs are also discussed and examined. These MNs include types such as light-responsive, pH-responsive, temperature-responsive, ultrasound-responsive, etc.

Clinical Translation Barriers and Regulatory Considerations

In specific circumstances, the clinical application of MNs in the treatment of melanoma is particularly relevant: (1) For early superficial melanoma, MNs are used as an adjunct to surgical excision to eliminate residual tumor cells and reduce the risk of recurrence.³ (2) For in situ melanoma or lentigo maligna, MNs are employed when achieving surgical margins is difficult.¹³⁶ (3) MNs can be utilized in patients with comorbidities or tumors in locations that pose a high surgical risk.³ (4) MNs can also be combined with systemic therapy for local drug delivery, enhancing drug concentration within the tumor while minimizing systemic toxicity.^33,136

The current standard of care for early-stage melanoma (stage 0-II) is surgical excision with adequate margins, which offers a 5-year survival rate exceeding 90% when properly performed.^33,137 Surgical excision remains the gold standard because it provides complete histological assessment of the tumor, accurate staging, and definitive removal of the primary lesion. For advanced melanoma (stage III–IV), systemic therapies including immune checkpoint inhibitors (ICIs) and targeted therapies are required because the disease has spread beyond the primary site, necessitating whole-body treatment approaches.^138,139 Unlike local delivery methods, systemic therapies can target micrometastatic disease and provide durable responses through immune system activation or targeted inhibition of oncogenic pathways.¹⁴⁰ Despite significant advances in MNs for melanoma treatment, substantial barriers hinder clinical translation and commercialization. Understanding these challenges is crucial for bridging the gap between preclinical research and clinical application.

Manufacturing and Scalability Challenges

The transition from laboratory-scale production to industrial manufacturing faces several obstacles. 1. Good Manufacturing Practice (GMP) Compliance: MN-based combination products must adhere to stringent GMP standards addressing both device and drug components.¹⁴¹ The current manufacturing processes typically lack the standardized quality control systems necessary for regulatory approval. Critical parameters including needle geometry, drug loading uniformity, and mechanical properties require precise control within narrow specifications.⁶⁰ In addition, issues related to the preparation process, such as the scaling up of complex drug delivery systems and the uniform dispersion of photocatalysts, also need to be addressed to achieve broader applications.⁵⁷ 2. Sterility Assurance: Maintaining sterility throughout the manufacturing, packaging, and storage of MNs presents unique challenges. Unlike conventional injections, MNs cannot be terminally sterilized using standard methods without compromising drug stability or needle integrity.⁵ Aseptic manufacturing is required but increases production complexity and costs significantly.¹⁴² 3. Mechanical Failure and Quality Control: To ensure that each array’s thousands of needles have stable mechanical properties while maintaining sharp tips for reliable skin puncture, a complex quality control system is required.¹⁴² Current batch-to-batch variability in mechanical strength and drug loading remains a significant concern for regulatory agencies.¹⁵

Regulatory Approval Pathways

At present, products based on MNs still face complex regulatory classification challenges. The US Food and Drug Administration (FDA) typically classifies drug-loaded MNs as combination products, requiring evaluation of both device and drug components.^142,143 The European Medicines Agency (EMA) has established specific guidelines for MNs-based medicinal products,¹⁴⁴ emphasizing the need for comprehensive safety data, including long-term bio-compatibility studies and clinical trials demonstrating non-inferiority to standard treatments.¹⁴⁴ Phase I–III clinical trials for MNs-based products must demonstrate not only efficacy and safety but also user acceptability, ease of application, and consistent drug delivery across diverse patient populations.¹³⁶

Human Skin Penetration Variability

A critical challenge for the clinical translation of MNs is the substantial variability in human skin properties. Human skin thickness varies significantly based on anatomical location, age, sex, and ethnicity.^25,45 The epidermal thickness ranges from 50–150 μm across different body sites, while dermal thickness can vary from 0.5–3 mm.¹⁴⁵ This variability affects the penetration depth and drug delivery efficiency of MNs. Aging reduces skin elasticity and thickness, potentially affecting the insertion mechanics and drug diffusion rates of MNs.¹⁴⁶ Elderly patients, who represent a significant portion of the melanoma population, may experience different penetration profiles compared to younger individuals. Melanin content and skin structure differences among ethnic groups may influence performance and drug absorption kinetics of MNs.¹⁴⁷ Comprehensive clinical studies across diverse populations are required but often underrepresented in current research. Melanoma and its treatments can alter skin structure, elasticity, and barrier function, potentially affecting the insertion and drug delivery of MNs.¹⁴⁸ Pre-existing conditions such as psoriasis or atopic dermatitis may further complicate the application of MNs.

Knowledge Gaps and Research Priorities

So far, there are several critical knowledge gaps that must be filled. Data on the long-term effects of repeated use of MNs, including the possibility of causing skin scarring, allergic reactions or immune sensitization, are limited.¹⁴¹ Long-term studies (≥2 years) are essential for regulatory approval but are rarely conducted in preclinical research.¹⁴⁹ Current understanding of MNs-mediated drug delivery pharmacokinetics relies heavily on animal models that may not accurately predict human responses.⁸ Advanced computational models incorporating human skin properties are needed. The optimal sequencing and dosing of MNs-delivered agents in combination with systemic therapies remain poorly understood.⁶⁰ Drug-drug interactions at the application site and systemic level require comprehensive evaluation. Real-world usability studies that assess patient acceptance, correct usage methods, and long-term compliance are limited, but they are crucial for clinical success.¹⁴⁹ Addressing these barriers requires collaborative efforts among researchers, manufacturers, and regulatory agencies to establish standardized testing protocols, streamline approval processes, and generate the comprehensive data necessary for clinical translation.

Conclusion

MNs technology, as an innovative transdermal drug delivery platform, has shown significant advantages in melanoma treatment, providing effective solutions to overcome the limitations of traditional therapies. In terms of preparation, advancements in template molding, 3D printing, and MEMS technology have promoted the precision and scalable production of MNs, while the application of biocompatible polymers has addressed the balance between material safety and mechanical performance, laying the foundation for clinical applications. In therapeutic applications, MNs achieve localized efficient drug delivery through chemotherapy, photothermal therapy, targeted therapy, immunotherapy, and multimodal combined strategies, reducing systemic toxicity and lowering tumor resistance through synergistic effects. The development of responsive MNs further enables spatiotemporal controlled drug release, with intelligent design integrating tumor microenvironment factors (such as pH, ROS) or external stimuli (such as light, temperature), significantly enhancing the precision and efficacy of treatment. However, the clinical transformation of MNs in melanoma treatment still faces challenges.

One challenge is that differences in physiological characteristics between animal models and human skin may affect efficacy evaluation, necessitating more preclinical studies for validation. Most current research on MNs in melanoma treatment relies on mouse or other animal models, but there are significant differences in physiological structure and tumor microenvironment between animal skin and human skin, which may lead to efficacy deviations during clinical conversion. For example, the stratum corneum of mouse skin is thinner, and the skin thickness is shallower, whereas the stratum corneum barrier function of human skin is stronger, and the tumor growth patterns and resistance mechanisms are more complex. This makes drug dosage or release rate effective in animal experiments potentially fail to achieve expected efficacy in human patients, even increasing the risk of side effects. Additionally, the metastatic and multidrug-resistant behavior of melanoma in humans differs from that in small animal models. Optimizing MNs design based on human skin characteristics and tumor pathological features (such as needle height, drug loading, responsive trigger conditions) becomes a key obstacle to promoting their clinical application.

The stability of biologically active drugs (such as vaccines and gene-editing tools) and their storage conditions limit their widespread application. MNs face challenges in maintaining drug stability when delivering biologically active drugs such as tumor vaccines, oncolytic viruses, and protein drugs. These drugs are usually sensitive to environmental factors such as temperature and humidity. They may lose activity due to curing during the preparation, fluctuations in storage, and transportation conditions. For example, biological agents commonly used in immunotherapy, such as aPD-1 antibodies and cytokines. It may suffer structural damage because of chemical environment changes during MNs photopolymerization or thermal curing. Oncolytic viruses tend to become inactive at room temperature and often require cold chain storage. This not only increases transportation and storage costs but also limits the application of MNs in resource-limited areas. How to improve the stability of biologically active drugs through material modification (such as using lyoprotectants) or process optimization (such as low-temperature molding techniques) is a critical bottleneck for the practical application of MNs.

Uniformity in the size of MNs, biocompatibility of materials, and cost control in large-scale production still need breakthroughs. The mass production of MNs faces multiple technical challenges including precision control, material consistency, and cost control. For instance, while template molding is suitable for large-scale production, it cannot ensure complete uniformity in needle height, base diameter, and drug loading for each needle, which may lead to unstable skin penetration efficiency or inconsistent drug delivery. It affects treatment outcomes. Although 3D printing can achieve personalized structural design, its slow printing speed and high material costs make it difficult to meet clinical large-scale needs. Additionally, the biocompatibility, mechanical strength, and degradation rate of MNs must be strictly controlled. Sterilization during the production process adds complexity and cost. These factors make it difficult to achieve low-cost, high-quality standardized output in the commercial production of MNs, thereby limiting their widespread clinical application.

Future research could focus on three aspects: First, develop multifunctional smart MNs that integrate diagnostic and therapeutic functions to realize “diagnose-and-treat” integration. Second, promote interdisciplinary technology integration, such as using artificial intelligence to optimize the structure design and leveraging biomaterial engineering to enhance responsiveness. Third, strengthen clinical translational research to establish standardized evaluation systems and promote the transition of MNs from laboratory to clinical settings. With continuous technological advancements, MNs are expected to become an important means for precise melanoma treatment, offering new hope for improving patient outcomes.

Funding Statement

This research was supported by Scientific and Technological Innovation Project of China Academy of Chinese Medical Sciences (CI2021A0O108) and the Fundamental Research Funds for the Central Public Welfare Research Institutes (ZZ18-YQ-046, YZX-202419).

Abbreviations

5-ALA, 5-aminolevulinic acid; 5-Fu, 5-fluorouracil; aPD-1, anti-programmed cell death protein 1; aPD-L1, anti-programmed cell death-ligand 1; CBP, carboplatin; CMC, carboxymethylcellulose; CuO₂, copper peroxide; DDS, drug delivery system; DexMA, dextran methacrylate; DMN, dissolving microneedle; DOX, doxorubicin; EMH, electro-modulated hydrogel; EMA, European Medicines Agency; Gox, glucose oxidase; GelMA, Gelatin methacryloyl; GelMA-β-CD, Gelatin methacryloyl and β-cyclodextrin; HA, hyaluronic acid; HAase, hyaluronidase; HAT, hyaluronic acid-tyramine; HAMA, Methacrylated hyaluronic acid; HPMC, Hypromellose; ICB, immune checkpoint blockade; ICIs, immune checkpoint inhibitors; ICG, indocyanine green; IDO, indoleamine 2,3-dioxygenase; LCC-NPs, liposome-coated cisplatin nanoparticles; MAT, matrine; MEMS, micro-electromechanical systems; mPTT, mild photothermal therapy; mRNA, messenger RNA; NIR, near-infrared; NMSC, non-melanoma skin carcinoma; OVA, ovalbumin; PDT, photodynamic therapy; PDMAEMA, poly(N,N-dimethylaminoethyl methacrylate); PEG, poly(ethylene glycol); PEGDA, ethylene glycol dimethacrylate; PEI, polyethyleneimine; p-Si, porous silicon; PLA, polylactic acid; PLGA, poly(lactic-co-glycolic acid); PMMA, polymethyl methacrylate; PNIPAM, poly(N-isopropylacrylamide); PVA, polyvinyl alcohol; PVP, polyvinylpyrrolidone; RNP, ribonucleoprotein; ROS, reactive oxygen species; SA, sodium alginate; SAHA, vorinostat; SC, stratum corneum; SLA, stereolithography; SNP, S-nitrosothiol; TDDS, transdermal drug delivery system; TPP, two-photon polymerization; VC, vitamin C; TME, tumor microenvironment.

Data Sharing Statement

Data will be made available on request to the corresponding authors.

Ethics Approval and Consent to Participate

Our submission is a review article, which does not include a clinical study and not involve experimentation on animals and human subjects.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.