Polydopamine-based Surface Modifications for Tissue Engineering and Biosensing: From Understanding Chemistry to Diverse Applications

Abstract

In 2007, researchers were intrigued with how effectively saltwater mussels were able to attach to seemingly any surface, looked further into how this was possible, and developed polydopamine (PDA). The polymerized form of a common neurotransmitter, dopamine, PDA adhesive protein has become known as a powerful biomaterial with broad applications in tissue engineering, drug delivery, biosensing, and antibacterial technologies. Its robust adhesion, due to catechol and amine groups, allows uniform coating on various types of surfaces and enhances properties such as bioactivity, corrosion resistance, and mechanical strength. In this review paper, we aim to look at the last 17 years of research around PDA and to examine its applications, particularly in the biomedical field. Additionally, we have focused on how 3D printing and incorporation into biosensing devices could allow for an even wider range of manufactured products within the biomedical industry that use PDA as a primary component. PDA-coated 3D-printed scaffolds show great biocompatibility and osteogenic potential, providing innovative solutions for bone, neural, and cardiac tissue engineering. In drug delivery, PDA enables controlled release and photothermal therapies, enhancing cancer treatment precision while decreasing side effects. PDA’s antibacterial efficacy and biosensing applications are also discussed.

INTRODUCTION: POLYDOPAMINE AS A UNIVERSAL COATING

Polydopamine (PDA), a powerful and uniform coating on nearly all kinds of surfaces, is acquired by the spontaneous oxidation of its monomer dopamine (DA), which is one of the brain’s main catechol neurotransmitters.¹ Recent research suggests that DA can be easily polymerized to PDA through oxidative self-polymerization in an alkaline aqueous medium. Since its discovery in 2007 by Prof. Messersmith’s group, PDA has been extensively used in the biomedical, energy, consumer, industrial, defense, and other sectors. Investigation into the mussel adhesive protein has revealed the origin and fundamental characteristics of PDA, which is generally known as dopamine–melanin or dopamine-derived coatings. It is a flexible surface modification technique, which is carried out by submerging a substrate for several hours in an aqueous alkaline DA solution, which results in conformal coatings that are 1–10 nm thick.² Taking its original inspiration from mussels and their ability to attach to an extremely wide variety of surfaces, PDA was hypothesized to be able to coat a similarly wide breadth of substrates. This was proven early on, with PDA coatings being successfully applied to numerous organic and inorganic samples made up of metals, ceramics, and polymers. Even typically adhesive-resistant polymers, such as poly(tetrafluoroethylene) were successfully coated in early experiments. Beyond its versatility, in terms of substrates, PDA has also gained attention due to the particularly simple process used to create the coatings. The primary method, demonstrated in early publications, is dip-coating objects by submerging them in a slightly basic, aqueous solution of DA, resulting in a thin film of consistent thickness even over complicated surface geometries. Modern chemical, biological, and materials science heavily rely on surface modification techniques like self-assembled monolayer (SAM) formation, Langmuir–Blodgett deposition, layer-by-layer assembly, and genetically engineered surface-binding peptides.^3–7 In contrast, PDA coatings are relatively simple and can be easily obtained on almost all types of surfaces. For example, to electropolymerize DA onto conducting electrodes, 3,4-dihydroxy-L phenylalanine (DOPA) and other catechol compounds work well as binding agents for coating inorganic surfaces. On the other hand, organic surface coverings are more intangible. For adherence to a variety of materials, both catechol and amine (lysine) groups are essential, and DA has both properties. The catechol groups, via hydroxyl and ortho-quinone functional groups, enable the structure to form strong covalent bonds, while the amine groups further enhance interactions with surfaces, creating a more stable and robust coating.⁸ Since its publication in 2007, the initial dip-coating bath recipe has been modified in many ways. Different buffers and the addition of oxidizers have allowed for more control over the speed of deposition and the resultant thickness of the film. Additionally, organic solvents have been tested for use with materials that are hydrophobic and/or porous.⁹ Methods for “one-pot” coatings combining PDA with other polymers and biomolecules in a single bath with a single dip have also been developed.¹⁰ The robustness of PDA as a coating on a variety of substrates is attributed to its catechol and amine groups. Several different types of molecular interactions have been observed with catechol, including coordination bonding with metallic surfaces and hydrogen bonding with metal oxides.¹¹ However, when similar amine-free catechol mixes have been tested, they have not shown such diverse bonding ability, implying that the amine also plays a crucial role in PDA’s effectiveness; it is possible that the amines enable catechols to get closer to the surface and form stronger bonds by displacing hydrated salts.¹² Its overall bonding ability means that not only is PDA useful as a coating in isolation but it is also frequently used as an intermediate or primer layer for other coating materials. As both a standalone coating, and when used as a component in a composite coating with amino acids, PDA has been shown to have excellent corrosion resistance.¹³ Beyond protecting against corrosion, it has also shown a resistance to the adhesion of calcium carbonate scale and some bacterial resistance, particularly when combined with compounds like curcumin that inhibit biofouling.¹⁴ This variety of resistances makes it suitable for use in numerous applications, ranging from biomedical to industrial waste treatment. The objective of this review is to provide readers with a summary of PDA coating’s applications in various tissue engineering and drug delivery sectors including recent advancement of 3D printing with PDA. The emphasis is on the understanding of PDA chemistry followed by summarizing various applications in the tissue engineering sector. The paper is divided into 6 sections: (1) polydopamine chemistry, (2) mechanical properties of polydopamine coating, (3) utilization of polydopamine in 3D printing and tissue engineering, (4) polydopamine in drug delivery and cancer treatment, (5) antibacterial properties of polydopamine, and (6) biosensing applications of polydopamine.

Polydopamine Chemistry

Polydopamine (PDA) is a dark brown-black insoluble biopolymer which can generate a robust eumelanin-like coating material on versatile surfaces. Tyrosine, or 3,4-dihydroxyphenylalanine (L-DOPA), is the natural source of DA and eumelanin.¹⁵ PDA’s primary adhesion is achieved by covalent connections with polar polymer surfaces or hydrogen bonds between phenolic hydroxyl groups and acceptors, whereas hydrophobic or π–π interactions are essential for non-polar polymer contacts.¹⁶ The presence of high catechol (3,4-dihydroxybenzene) content due to L-DOPA, and a high primary and secondary amine content because of lysine and histidine residues, are the two primary properties of the Mytilus edulis foot proteins-3 and -5 (Mfp-3 and-5) which encouraged PDA. This unique fusion is linked to its excellent surface adhesion.^17–20 Figure 1 shows recent hypotheses about the structure and synthesis of PDA. Dopamine auto-oxidation produces dopamine–quinone, which then goes through a nucleophilic intramolecular cyclization process to produce 5,6-dihydroxyindole (DHI). Dopamine-quinone and DHI are the essential building blocks for PDA. Quinhydrone or trimer assemblies are formed by noncovalent self-assembly of subunits, while eumelanin-like oligo-indoles or catecholamine/quinone/indole heteropolymer are produced by covalent coupling of subunits. It has been suggested that PDA is a substance that resembles eumelanin and contains oligomeric building blocks. It is thought to arise spontaneously by additional oxidation of DHI and coupling through 2–2′, 4–7′, 2–4′, and/or 2–7′ linkages.^1,9,21,22 Due to the obstinate nature of PDA, its structural characterization is difficult. Based on experimental research employing mass spectrometry and chemical spectroscopy, several theories have been proposed. The most popular theory is that PDA is a supramolecular aggregate comprising monomeric and/or oligomeric species stabilized by weak interactions like charge transfer, hydrogen bonding, π–π stacking, and π–cation assembly.²³ Others have suggested that oxidized and cyclized DA monomers are covalently coupled via aryl-aryl bonds to create PDA, which is polymeric.²² After the introduction of PDA, it has gained widespread recognition as “bio glue” due to its superior adhesive capacity. The structure of PDA is dependent on the pH, time, and concentration. PDA coatings can be applied as a conformal film using a dip-coating procedure on any topography and versatile surfaces.¹⁶ Enzymatic oxidation, solution oxidation, and electro-polymerization methods are the most popular formulation techniques of PDA.²⁴ PDA can be produced in different forms such as nanoparticles (NPs), mesoporous NPs, and nanocapsules. These unique forms of PDA are widely applicable in different types of cancer treatment and regenerative medicine applications.^25,26 PDA’s abundant catechol groups promote anti-oxidation and inflammation dampening, and its production is easy without high-tech instruments or harsh reaction conditions.²⁷ Like DA, the Mytilus edulis foot proteins’ (mefp) amino acid sequence includes the modified amino acids, L-DOPA and L-tyrosine, which give mussels their strong and permanent adhesion to a broad range of objects while they are wet. Additional research on the function of proteins in the oxidation and self-assembly process of PDA has been prompted by the formation of narrowly distributed melanin NPs encircled by stabilizing proteins when alcohols are present.²⁸ Since PDA was found to be a viable self-assembling option for coating substrates, further options for customizing and tailoring the coatings have been explored. One option for this is using DA derivatives, such as DOPA and norepinephrine, as the primary deposition agent. Various derivatives of PDA have generally been shown to coat a variety of samples as effectively and simply as PDA, but with differences which can make them more useful in other applications. For example, poly(norepinephrine) coatings are ultrasmooth with a more consistent thickness than that of PDA, and the additional hydroxyl group present allows for polymerization, in two-step coatings, with lactone monomers.³⁰ One of the most thoroughly studied uses of PDA coatings is its use as a primer for a secondary ad-layer. Exploiting the intrinsic chemical reactivity present at the surface of a PDA coating, it allows for a wide array of molecules to layer on top, typically using the same pH-buffered bath technique. Proteins, peptides, and end-functionalized oligonucleotides are the most amenable to this approach, but work has also been carried out showing that synthetic polymers can be modified with additional functional groups to enable bonding with PDA. Although it has shown promise on a laboratory scale, scaling-up PDA production for larger applications has a few roadblocks, while, although PDA formation is simple, dopamine itself is fairly expensive, especially compared to other readily available plant polyphenols that coat in similar manners. Additionally, the rate of polymerization of PDA is fairly slow, but there have been successful attempts to accelerate the deposition using strong oxidizers.³¹ PDA is generally considered environmentally friendly but does present some of the typical concerns of any new material, especially in the biomedical field in terms of testing and obtaining approval. Depending on the exact method, PDA production can also involve organic solvents and reagents that, at a larger scale, could pose environmental challenges, but green synthesis methods, such as ozone-induced polymerization, have been demonstrated.³²

Fig. 1.

Currently proposed polydopamine structure and underlying theories of polydopamine formation from auto-oxidation of dopamine in multiple steps. Adapted with permission from Refs. 1, 9, 10, and 22, Copyright (2014), The American Chemical Society, Copyright (2018), The American Chemical Society, Copyright (2012) Wiley-VCH, and Copyright (2013), The American Chemical Society.

Mechanical Properties of Polydopamine Coating

While the chemical reactivity and bonding behavior of PDA has been the focus of a bulk of research, less emphasis has been placed on its mechanical properties. One mechanical use of PDA is in surface modification of fibers and particles for composites, for which a PDA coating can enhance wetting and adhesion between the constituents. One example is its use in improving the interface strength in the bioceramic composite made of hydroxyapatite and silane-modified gelatin, HAp-Gemosilamine. The incorporation of PDA, when carried out at − 20 °C, increased the compressive strength to 100 MPa compared to the same biocomposite without PDA, which only had a compressive strength of 80 MPa. This improvement of mechanical properties in composites has also been reported for several other materials.³³ Outside of its use in composite interfaces, there has been less research on the mechanical properties of PDA in applications which require resistance to physical abrasion or delamination or as a standalone adhesive. A computational model of PDA films controlled via in silico covalent cross-linking found a predicted Young’s modulus range of 4.1–4.4 GPa when tested in tension, which was backed up in the same study by nanoindentation testing of PDA films on silicon substrates showing a range of 4.3–10.5 GPa.³⁴ This study, however, did not report the film’s thickness. Additional testing of thin films using buckling instead resulted in a slightly lower but comparable result of 2 GPa for Young’s modulus, and speculated that intergranular forces within the PDA film are weaker and lead to a lower modulus than computationally predicted.³⁵ Delamination of PDA coatings from their substrates has been studied both to determine the coating’s viability in environments that experience extended mechanical agitation, such as in flowing liquids, or inside the human body, and to be able to intentionally delaminate to create free-standing, cohesive nanomembranes. The latter, purposeful delamination, has been carried out using an aqueous solution of NaOH or other basic salt solutions to separate the PDA film from a silicon substrate. While the ease with which this is done is ideal for creating PDA membranes, it also demonstrates issues with these films adhering long term. To investigate PDA film delamination, one study used optical monitoring of light scattering to determine macroscopic delamination over time, using different buffer solutions of varying pH and concentration. DA concentration in the initial coating bath and film thickness were also evaluated. The concentration of unreacted DA in these films was measured using UV-Vis spectroscopy to evaluate the extent of polymerization and the process efficiency. The thickness was evaluated by transmission of polarized light via ellipsometry to better determine the mechanical properties and potential applications of the film.³⁵

Polydopamine in 3D Printing and Tissue Engineering

Applications for three-dimensional (3D) printing have grown exponentially in the last two decades because of the technology’s unparalleled progress and the possibilities of experimenting with novel materials. It has encouraged the development of intricate patterns and complex shapes, modernizing a variety of consumer products and industries. In 1983, Chuck Hull, a co-founder of 3D Systems, created the first 3D printer. Since then, this technology has advanced significantly.³⁶ Nowadays, there is a great need for 3D printing (3DP) technology to manufacture complicated structures with extreme accuracy and precision. This versatile technology offers the benefits of customization, rapid prototyping, multiple production methods, and complex designs at a cheap cost in a short timeframe. However, even with these developments, there are various issues with this approach, such as expensive prices, slow printing rates for mass production, small part sizes, and inadequate mechanical properties.³⁷ 3DP technology has transformed mass customization, enabling customers to select products based on their preferences. It enables more flexible production methods, on-demand manufacturing, and improved quality assurance, increasing client accessibility even from home. Furthermore, 3DP has transformed several medical specialties, like surgery, orthopedics, and artificial organ development.³⁸ Polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) are two common thermoplastics used in 3DP. However, because ABS is made from oil, it is not environmentally friendly, which makes it less appropriate for 3DP. In contrast, PLA is made from renewable resources like sugarcane and starch, and is a safer substitute for ABS. Although recycling PLA filaments has advantages for environmental sustainability, it also weakens the filaments’ mechanical qualities. PDA can be utilized as an adhesion promoter to enhance the mechanical qualities of 3D-printed items composed of recycled PLA. Even though PDA coatings are great for their adhesive properties, it has to be ensured that its efficacy and stability are not compromised. It is crucial to consider certain environmental conditions when working with PDA. Studies have shown that exposure to strong alkaline cleaning agents, like DIVOS 130, makes a significant degradation of the PDA layer, and interaction of PDA coatings with different solvents such as DMSO and DMF, can affect their stability. In a study, PDA-coated gold substrates faced up to 56% detachment after immersion in DMSO and up to 31% detachment in DMF. The rate of detachment varies and depends on the solvent’s polarity and its ability to interact with the PDA matrix.^39,40 Tensile testing revealed that applying bio-inspired PDA to the surface of recycled PLA can improve bonding strength, stop PLA from degrading, and increase PLA’s tensile strength.⁴¹ Due to illness and accidents, millions of people suffer from tissue or organ failure each year, which has a significant negative impact on their quality of life.^42–44 The goal of tissue engineering (TE) is to innovate new solutions in terms of technology and materials to replace the affected tissue, organ, or body part which can restore the function of the affected part. PDA is being utilized in various branches of TE, such as bone TE, neural TE, and cardiac TE. Biodegradable and biocompatible polymers with desirable mechanical and machining properties such as poly (L-lactide) (PLLA) and polycaprolactone (PCL) are also valued for making scaffolds. PDA coating of PCL scaffolds greatly improves their biological connections, which in turn promotes improved bone marrow mesenchymal stem cell adhesion and proliferation, increasing the scaffolds’ capacity for bone regeneration. This combination produces efficient scaffolds for orthopedic applications by utilizing the bioactivity provided by PDA and the mechanical advantages of PCL.⁴⁵ Integrating 3DP with PDA-based materials allows for new opportunities for improved customization in many biomedical applications. The advancement of biofabrication techniques allows for more precise deposition of PDA-functionalized biomaterials into scaffolds, of an exact geometry to match with specific patient needs, and with controlled porosity and architecture, allowing for optimization of their mechanical properties. Additionally, coatings can be tailored by incorporating specialized molecules or growth factors into printed designs, improving their performance in TE and regenerative medicine applications.

BONE TISSUE ENGINEERING WITH POLYDOPAMINE

There is a growing need for bone TE research because musculoskeletal problems affect more than 1.7 billion individuals globally.⁴⁶ The 3DP technology is popular in the scientific community, in particular due to its versatile applications in the bone TE domain. As a result of the global health risk posed by large bone deformities, 3DP technology has been developed to create defect-specific and patient-specific multifunctional scaffolds for bone regeneration.⁴⁷ Allografts and autologous bone grafts are used to treat bone abnormalities, but their uses are limited by contaminated bone, size mismatch, immunological rejection, and donor scarcity.⁴⁸ The great precision and individualized customization of 3DP porous materials have drawn interest in TE.⁴⁹ As TE has advanced, porous scaffolds for the healing of bone defects have been prepared using a variety of 3DP processes, including stereolithography, selective laser sintering, and fused deposition modeling.^50–52 As 3DP ensures regular tissue cell assignment, nutrition transport, and metabolic waste outflow, it can closely duplicate the porous structure of bone.⁵³ PDA is frequently utilized to control tissue and cellular responses to materials because of its remarkable biocompatibility and distinct chemical structure.^54,55 PDA, which has a high cellular affinity and can lower cytotoxicity, inflammation, and immunological response, has been used in scaffolds by researchers. This coating improves cell adhesion. According to recent research, PDA can significantly enhance the adherence and development of bone-related cells on the surface of tissue-engineered scaffolds, as well as aid in the production of calcium phosphate mineralization.^52,56 One of PDA coating’s many beneficial qualities is that it improves the interfacial characteristics of implants, which is linked to bone regeneration.⁵⁷ Other surface modification of materials, such as silane-based coatings and SAMs, are used to modify surface properties and facilitate specific cellular interactions, although they may not provide the same level of versatility and bioactivity as PDA.⁵⁸ Given its nearly universal ability as an intermediate layer, many different modifications have been suggested to make PDA even more effective when used in scaffolds for bone regeneration. PDA has been shown to effectively trap and stabilize different forms of calcium ions to provide oriented nucleation points for new bone growth.^59,60 Other studies used PDA to anchor vascular endothelial growth factor to improve cell growth and osteogenic activity at the sight of an implant.⁶¹ An investigation was conducted to assess the effect of PDA coating on early osteogenesis in porous Ti-6Al-4V scaffolds that were 3D printed. Additionally, it demonstrated that PDA coating enhanced the osseointegration capacity of porous Ti-6Al-4V scaffolds made by solid laser melting, which qualifies them for early titanium implant osteogenesis. The study also found that, during the early stages of bone healing, bone growth occurred within the porous Ti-6Al-4V scaffolds coated with PDA.⁶² In a study, PDA-coated Ti-6A-l4V scaffolds with a pore size of 400 lm and 44.66% porosity were fabricated using additive manufacturing. In vitro studies showed that 40-min immersion in DA significantly improved cell adhesion. Finite element analysis has demonstrated that the porous structure could avoid stress shielding effects, and in vivo experiments have shown early-stage bone ingrowth within PDA-coated scaffolds.⁶² PDA coatings enhanced implant hydrophilicity, improving the adhesion, proliferation, and osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BMSCs), according to research on 3D-printed Ti-6Al-4V implant surfaces employing in situ polymerization. This indicates that PDA coating is a practical and beneficial method for improving the surfaces of 3D-printed Ti-6Al-4V implants. By using oxidative polymerization to create a PDA coating from dopamine hydrochloride, 3DP can alter PLA-based bone plates. Strong resistance against failure is provided by this coating, which improves the bonding between neighboring layers in the porous polymer structure. The coating strengthens the link between neighboring layers by covalently interacting with PLA when it is put to distal ulna bone plates (DUBPs). All DUBPs that were manufactured and coated under various conditions had an increase in tensile and flexural strength as a result of the coating.⁶³

In TE, biomaterial scaffolds are essential for promoting and controlling cell division to produce new tissue. These scaffolds’ mechanical and biological qualities have been improved through the use of NPs. In comparison to planar PDA coatings, 3D PDA-NPs provide a greater surface area for interaction with more cells and bio-functional chemicals. Beyond their adhesiveness and hydrophilicity, PDA-NPs have special qualities such as photothermal conversion capability and redox activities. They can be employed in tumor treatment, imaging, and biosensing.⁶⁴ PDA-assisted surface modification can greatly enhance fibroblast adhesion and cell proliferation by improving the surface properties of polymers utilized in bone TE.⁵⁵ PDA coating may help in modifying stem cell activity and transport stem cells. PDA-modified scaffolds absorb neural stem cells well, proving their healing potential.⁶⁵ Numerous socioeconomic problems as well as substantial morbidity can arise from the loss or dys-function of skeletal tissue that might accompany trauma, injury, illness, or aging. As bone TE has advanced, researchers have started to focus on using synthetic materials to cure bone abnormalities.^66,67 Key factors in the construction of the scaffold for bone TE are pore size and porosity, which can influence both bone growth and the scaffold’s mechanical stability and rate of deterioration.^68,69 In addition to its good biocompatibility and biodegradability, PLLA is a biodegradable polymer with specific mechanical and machinability qualities which have been used in the field of 3D-printed bone TE.⁷⁰ Although 3D-printed PLLA scaffolds have the ability to regenerate bone, their precise use is hampered by their unsatisfactory hydrophilicity, mechanical qualities, cytocompatibility, and osteogenesis. Adding a PDA layer to the surface can improve these characteristics. Furthermore, the addition of active medications through the PDA coating layer can enhance the biological activity of the final products.^71–73 One ingredient in ancient Eastern medicine, quercetin (C₁₅H₁₀O₇, Qu), has anti-inflammatory and anti-oxidant qualities and stops bone loss. By binding to estrogen receptors, it reduces osteoclastic bone resorption and, in a dose-dependent fashion, promotes cell proliferation and osteogenic differentiation.^74–76 Utilizing Qu to surface modified PLLA scaffolds with enhanced cytocompatibility and osteogenic activity via a PDA intermediate layer has not been extensively studied. It was accomplished through a novel study that will serve as a foundation for future use of the resulting Qu-loaded PLLA scaffolds in the field of bone TE (Fig. 2).⁷⁷ The process of oxidative self-polymerization of DOPA and the scaffold preparation process are depicted in Fig. 2a. As seen in Fig. 2b, c, and d, the PLLA scaffold was printed onto a square-pored mesh, and the structures and fiber surface morphologies were examined using a stereomicroscope and a field-emission scanning electron microscope (FESEM). Compared to the pristine PLLA scaffold, the PDA-PLLA scaffold showed a rougher surface. An uneven surface with observably convex rod-like particles was produced upon immobilization with Qu, and the pore size and thread diameter were examined using stereomicroscope images. Pore size is crucial for optimal cellular response. For bone TE, pore size ranges from 300 to 500 μm are ideal for cell infiltration, vascularization, and nutrient exchange. Pore sizes smaller than 300 μm may lead to hypoxic conditions, and potentially adversely affect bone formation. The thread’s diameter also contributes to the scaffold’s mechanical stability. In a study with polydopamine-coated alginate dialdehyde-gelatin scaffolds, the average strand thickness was approximately 1.31 mm, which provided a robust framework that was capable of supporting tissue generation while also maintaining structural stability.^78,79 According to the results, the pore sizes were 8% smaller than expected and the thread diameter was 16.7% larger than anticipated. The PLLA scaffolds’ 56.9% porosity suggested that their porous structure was strongly linked. Less than 10% of the variances were found, suggesting that the 3DP technique is generally reproducible (Fig. 2e). By efficiently and sustainably releasing Qu, the 3D-printed Qu-loaded PLLA scaffolds stimulate MC3T3-E1 cell adhesion, proliferation, alkaline phosphatase activity, calcium nodules, osteogenic-related genes, and protein expression.⁷⁷

Fig. 2.

(a) The experimental process of 3D-printed PLLA scaffold fabrication followed by PDA coating and quercetin loading, and the mechanism of DOPA’s oxidative self-polymerization, (b) different 3D printed scaffolds with PLLA, PDA-coated PLLA, and quercetin-loaded PDA-coated PLLA, respectively, (c) stereomicroscopic images of the fabricated scaffolds, (d) FESEM images of the PLLA, PD-PLLA, and different amounts of quercetin loaded and PDA-coated PLLA scaffolds such as 100Qu/PD PLLA, 200Qu/PD-PLLA, and 400Qu/PD-PLLA, (e) The thread diameter, pore size, and porosity were analyzed from the stereomicroscope pictures to assess the accuracy and reproducibility of 3D printing. Reprinted with permission from Ref. 77. Copyright (2019), The American Chemical Society.

One of the most common chronic oral infections that cause damaging inflammation is periodontal disease. A collection of specialized tissues that support teeth, the periodontium, may be destroyed as a result of the more severe form, periodontitis.^80,81 If left untreated, periodontitis causes early tooth loss, progressive loss of periodontal attachment, and loss of surrounding bone.⁸² For periodontal tissue regeneration, several nanofibrous membranes composed of PCL have been created. Biomimetic PDA, which coats these membranes, encourages the adherence of therapeutic proteins and cells, speeding up the osteogenic development of stem cells obtained from teeth.⁸³ The 3DP technology has the advantage of having significantly higher strength and porosity ratios than conventional scaffold production technologies, which allows it to restore critical-sized bone lesions for dental TE.⁸⁴ Because synthetic polymers lack biological functional groups and have hydrophobic surfaces, they have a lower molecular affinity for osteo-related cell adhesion and differentiation.⁸⁵ In order to accelerate cellular differentiation and encourage the development of new bone tissue on substrates, researchers are creating functionalized scaffolds for bone TE using chemical treatments.^86,87 As a natural and environmentally benign material, PDA can be added to synthetic substrates without the use of hazardous chemicals or solvents, and can take part in reduction processes with ions of noble metals.^2,88 The potential of gold nanoparticles (GNPs) in bone TE has been thoroughly investigated because of their capacity to stimulate osteogenic differentiation. Studies have been done on cultivating GNPs on 3D-printed PCLs coated with PDA to produce a hybrid scaffold-free of hazardous substances.^89–91 In vitro and in vivo bone development can be accelerated by a PDA coating on 3D scaffolds, which can function as a reducing agent and enable gold ions to produce GNPs on the scaffold. 3DP systems use computer-aided design (CAD) without any need for intermediate molding and can produce patient-specific artificial bone tissues. With 3DP systems, patient-specific artificial bone tissues can be produced using CAD without the need for intermediary molding. Figure 3a shows a schematic of the scaffold fabrication procedure. PDA was used to treat the PCL surfaces of 3D scaffolds after they were manufactured. When PDA was uniformly applied to the scaffolds, the scaffolds’ color changed from white to brown (Fig. 3b). Due to PDA’s potential as a reductant, the PDA-coated PCL (PCLD) scaffolds may allow GNPs to grow on it’s surface under basic conditions. To create ideal GNP growth conditions, PCLDs were then treated with HAuCl₄ at different doses (0.1, 0.5, 1, and 2 mM mL⁻¹) [Fig. 3c]). Good osteogenic activity resulted from the PDA coating’s considerable improvement of GNP growth in PCL scaffolds. New bone creation activity was demonstrated by in vivo experiments, which bodes well for future bone tissue regenerative therapy in orthopedic and dental situations as well as for improving human health.⁹² In a similar study, PDA-functionalized PCL scaffolds were developed to help with the PCL’s inherent hydrophobicity, which hinders cell adhesion and proliferation. Submerging the PCL scaffolds in a DA solution for 6 h resulted in significant immunomodulatory properties, while also promoting BMSC proliferation, adhesion, and osteogenic differentiation. In vivo experiments have shown that these PDA-PCL scaffolds promoted and enhanced bone regeneration and histocompatibility compared to uncoated scaffolds over periods of 1, 2, and 3 months post-operation.⁴⁵ Additionally, PDA is used to create conductive and dispersing reduced graphene (rGO) nanosheets. Cardiomyocyte maturation is facilitated by gelatin methacrylate hydrogels containing PDA-rGO nanocomposites, which makes them perfect for cardiac TE applications.⁶⁴

Fig. 3.

(a) A schematic of the process used to create hybrid osteoinductive 3D porous PCL scaffolds for the regeneration of bone tissue, (b) PCL scaffolds that are bare (white), PDA-coated (brown), and GNP-grown PDA-coated (deep purple), (c) SEM images of PCLD scaffolds after different HAuCl₄ treatment concentrations (0.1, 0.5, 1, and 2 mM). Reprinted with permission from Ref. 92. Copyright (2018), The Royal Society of Chemistry.

Polydopamine in Drug Delivery and Cancer Treatment

According to medical terminology, cancer is defined as an aberrant cell development that proliferates uncontrollably and occasionally spreads (metastasizes).⁹³ Today, approximately 100 cancer kinds have been identified globally.⁹⁴ A number of things, including radiation, chemicals (carcinogens), viruses, and damaged ancestral’ DNA, can cause cancer.⁹⁵ Affected cells’ DNA experiences damage to the section that controls cell division; if this damage is not fixed, cells will proliferate and divide uncontrollably into additional damaged cells or cancer cells. Cancer cells spread to other organs, like the liver, lungs, and bones, where they create new tumors, through the blood arteries, a process known as metastasis.^96,97 With 9.6 million fatalities from the disease in 2018, 70% of which occurred in middle- and low-income nations, cancer is the second most common cause of death worldwide. Given that the yearly cost of cancer treatment was predicted to be US$1.16 trillion in 2010, the economic impact is substantial.⁹⁸ According to recent studies, individual therapies like chemo-, photothermal, and targeted therapy are less effective in treating cancer than combinational therapies like chemo-thermal, chemo-photothermal, and photodynamic–photothermal therapy.^99–101 Drug delivery systems for cancer treatment can be classified into two categories: (1) novel drug delivery systems, and (2) conventional drug delivery systems.¹⁰² Targeted drug release, prolonged and stable blood levels within the therapeutic window, and a lower frequency of dosing are some of the benefits of the novel drug delivery systems over traditional ones.⁹⁸ Since they have a greater specific surface area, a higher carrying capacity, a higher concentration of the drug at the desired location, a steady delivery rate, and a longer half-life, nanocarriers are an essential part of novel drug delivery systems.¹⁰³ The cytotoxicity, pH sensitivity, stimulus response, good biocompatibility, and multi-drug carrier capability of PDA make it a popular polymeric carrier which is inexpensive and simple to manufacture. As seen in Fig. 4, it is widely used in the biomedical field. Since drug-loaded PDA-based nanocarriers delivered the drug at an acidic pH rather than physiological pH, researchers have found that PDA-based nanocarriers are pH-responsive and can target cancer cells without damaging healthy cells.⁹⁸ PDA provides several features that are pertinent to the study and management of cancer. It has the ability to transport antitumor medications, regulating their release, and delivering them precisely to tumors, increasing medicinal effectiveness while lowering side effects and avoiding drug resistance. Additionally, PDA has the ability to deliver imaging agents precisely to tumors, enabling accurate imaging-based cancer detection and characterization.¹⁰⁴ Through hydrogen bonding or π–π conjugation, drugs containing anthraquinone structures can be attached to the surface of PDA-NPs, transforming PDA into a drug carrier. As a result, PDA-modified NPs are frequently employed as drug carriers. Furthermore, PDA offers significant potential for photothermal therapy because it absorbs substantially in the near-infrared spectrum.^105,106 As seen in Fig. 5a, b, c, and d, PDA’s unique set of properties makes it a very attractive platform for drug delivery. In a study combining chemotherapy and photothermal therapy, sorafenib was encapsulated in PDA NPs. The temperature of the NP solution increased by about 10 C when exposed to near-infrared (NIR). This led to significant and localized drug release, showing that NIR light irradiation can trigger the drug release from PDA-NPs.¹⁰⁷ In PDA-based systems, factors including size, charge, and morphology affect how they behave. One study revealed the release of gentamicin from PDA-NPs at different pH levels. They found the amount of release of gentamicin increased up to 40–55% over some period, whereas, at pH 7.4, it was approximately 11.4% over 7 days. Another study highlighted that the cumulative release at pH 7.4 was approximately 25.5% over 24 h in temozolomide-loaded PDA-NPs. At pH 5.0, the release was about 52.4%, and, at pH 4.0, it reached approximately 57%. These studies show that acidic conditions significantly improve the drug release from PDA-NPs.^108,109 Because cellular uptake and drug release relies on the surface-to-volume ratio of the material, variations in size and morphology have an impact on these processes, as shown in Fig. 5e. Blood circulation, drug release mechanisms, and cellular uptake are all influenced by surface charge.¹¹⁰ Biomedical research on diabetes, inflammatory diseases, and Parkinson’s disease has focused on PDA-based multifunctional materials. Depending on the blood glucose level, these NPs can release antidiabetic medications into the blood-stream. PDA materials may help many patients by promoting hemostasis and reducing inflammation in diabetic foot wounds.^111,112 To improve entrapment efficiency, loading therapeutic medicines, and combatting multidrug resistance, PDA-NPs have been used as drug delivery carriers in a number of studies. They can readily load several drugs, reducing negative effects and achieving higher therapeutic results at lower dosages.¹¹³ Additionally, a PDA primary coating can be used as a platform for tethering and releasing small molecule drugs and therapeutic RNAs via weaker interactions, and allowing for use in targeted delivery applications. The studies of PDA in its use as a drug carrier have largely focused on demonstrating its effectiveness without providing a direct juxtaposition to other systems. In a 2023 paper, the authors studied PDA-NPs in antibacterial and antiviral applications and achieved a 37.6% drug-loading rate, noting that its high capacity and controlled release make it an especially useful drug carrier.¹¹⁴ However, this paper, like others, focuses on the standalone performance of PDA.

Fig. 4.

Various polydopamine material types and their uses in biomedicine are shown schematically. Polydopamine (PDA), photothermal treatment (PTT), nanoparticles (NP), and reactive oxygen species (ROS). Reprinted with permission from Ref. 98. Copyright (2019), The Royal Society of Chemistry.

Fig. 5.

The impact of PDA characteristics on drug delivery requirements; an orange triangle stands in for the drug that will be released. (a) The primary need for drug delivery applications is biocompatibility; (b) drug loading and cellular uptake are favored by the simplicity of functionalization, (c) NIR (near-infrared) light irradiation produces photothermal activity, which initiates drug release, (d) the tumor microenvironment (TME) must have an acidic pH in order for pH-dependent medication release to be controlled, and (e) size, shape, and surface charge of PDA-NPs affect processes relevant to drug delivery. Arrows show the effects of each property on blood circulation, drug loading, drug release, and cellular uptake. Reprinted from Ref. 110, under Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Antibacterial Properties of Polydopamine

Bacterial infections and antibiotic resistance are two of the most important problems in global healthcare, resulting in major illness, death, and economic costs. Antibiotic resistance alone is expected to kill 700,000 people each year, with predictions indicating that this number may increase to 10 million by 2050 unless considerable action is taken.¹¹⁵ Bacterial resistance to commonly available antibiotics lowers their effectiveness, leading to longer hospital stays, higher healthcare expenditures, and worse clinical outcomes.¹¹⁶ Infections caused by biofilm growth on medical implants and equipment, such as catheters and joint replacements, present unique challenges.¹¹⁷ Biofilms may protect bacteria from drugs and immunological responses, resulting in implant failure and revision surgeries.¹¹⁸ These circumstances highlight the critical need for alternate techniques for fighting resistant diseases. PDA is a novel option for functionalizing surfaces with antimicrobial agents and has inherent antibacterial properties due to the formation of reactive oxygen species (ROS).⁵⁷ In addition, PDA coatings are stable, strong, and adaptable, making them suitable for a wide range of applications. They can improve medical device performance, minimize biomaterials’ in vivo toxicity, and inhibit biofilm formation, which are some significant limitations in existing antibacterial techniques. Advances in nanotechnology have increased the applications of PDA, which allows more innovation and developments to PDA-based nanomaterials with controllable thickness and multifunctional characteristics. An overview of the mechanisms and applications of PDA as an antibacterial material is shown in Fig. 6.¹¹⁷

Fig. 6.

Overview image of mechanisms and applications of PDA as antibacterial material. Reprinted with permission from Ref. 117. Copyright (2022), Elsevier.

PHOTOTHERMAL EFFECTS OF POLYDOPAMINE RESULTING ANTIBACTERIAL EFFICACY

PDA is being used as an effective drug in photothermal treatment (PTT), due to its capacity to transform NIR light into heat for antibacterial applications. PDA’s broad-spectrum NIR absorbance is due to its unique electron donor–acceptor structures, such as 5,6-dihydroxyindole and indole 5,6-quinone, which improve light absorption by decreasing energy bandgaps and increasing electron delocalization. When tested with NIR light (700–1400 nm), PDA materials undergo fast photothermal conversion, resulting in localized hyperthermia that can kill bacteria through thermal ablation processes. Temperatures over 55 °C denature bacterial proteins and enzymes, disrupt metabolism, and damage the cell membrane, resulting in internal leakage and bacterial mortality.^24,119–121 In a study, researchers used mesoporous PDA-NPs and analyzed their antibacterial activity against S. aureus. With exposure to NIR light at 808 nm, the photons become absorbed by PDA, exciting its π-conjugated electron system, which leads to non-radiative relaxation, where the absorbed light energy is converted to heat, therefore increasing the temperature above 50–70 °C and effectively killing the bacteria in both in vitro and in vivo.¹²² PDA’s photothermal characteristics can be improved by structural changes and functional integration, such as donor–acceptor microstructures or metal-ion doping with Fe³⁺ or Cu²⁺. Both of these cations narrow bandgaps and increase photothermal efficiency, making PDA more effective in disrupting biofilms and killing bacteria. In addition to directly killing bacteria, increased temperatures produced by PDA have been found to inhibit biofilm formation, decrease bacterial adhesion, and counter antibiotic resistance, making PTT an effective approach to fighting drug-resistant diseases.^{121,123–125} The antibacterial mechanism of PDA goes beyond biofilm inhibition to impair bacterial cell integrity. The heat produced by PDA-based materials ruptures bacterial membranes and causes permanent damage to proteins and enzymes, limiting physiological functions.¹²⁶ Figure 7shows the antibacterial efficacy of PDA-ferrocene coated TiO₂ nanorods. Photothermal antibacterial effects can also be combined with other therapeutic techniques, such as fluorescence imaging and targeted medication delivery, to improve their accuracy and efficacy in treating bacterial infections.^{120,127–129} PDA-NPs are mixed into hydrogels to provide wound dressings with antibacterial and tissue-regenerative characteristics. These hydrogels take advantage of PDA’s ROS or photothermal properties to eliminate bacteria while encouraging angiogenesis and wound healing. Applications include treating wounds infected with drug-resistant bacteria and lowering the requirement for traditional antibiotics.¹³⁰

Fig. 7.

The photodynamic antibacterial activity of TiO₂ nanorods coated with PDA-ferrocene. Reprinted with permission from Ref. 126. Copyright (2020), The American Chemical Society.

REACTIVE OXYGEN SPECIES (ROS) GENERATION RESULTING ANTIBACTERIAL EFFICACY

The production of ROS is a key component of PDA’s antibacterial properties. PDA is an artificial melanin which promotes electron transport and generates ROS, including hydroxyl radicals (•OH), hydrogen peroxide (H₂O₂), and superoxide anions $\left({\text{O}}_2\cdot -\right)$.²² Bacterial membrane lipids, proteins, and DNA are the targets of these ROS. Protein oxidation affects metabolic processes, lipid peroxidation damages membrane integrity, and DNA damage stops transcription and replication, all of which cause bacterial mortality.^131,132 Surface roughness and hydroxyl groups increase the activity of PDA coatings, which produce ROS through catechol oxidation and are especially efficient against planktonic bacteria.^133,134 On the other hand, PDA nanocomposites use metal ions (such as Ag⁺, Fe³⁺, and Cu²⁺) or photothermal agents (such as TiO₂) to increase ROS generation, penetrate biofilms, and reach over 99% antibacterial activity against S. aureus and E. coli.^{123,131,135,136} Some of the strategies that increase ROS efficiency are metal ion incorporation, NIR irradiation which accelerates electron transfer, and surface modifications like roughening or arginine doping.^128,137 The ability of PDA to maintain endogenous ROS production in mild conditions contrasts with standard agents, minimizes collateral damage, and ensures long-lasting antibacterial actions.^138,139 By breaking down the extracellular matrix of biofilms and putting bacteria under oxidative stress, PDA’s ROS-mediated activity effectively combats biofilm. Because of its versatility, biocompatibility, and ability to target both planktonic and biofilm-embedded bacteria, PDA is a potential material for advanced antibacterial applications.^118,121,135 PDA-based hydrogels use ROS-mediated antibacterial activity to promote tissue regeneration and to sterilize infected wounds. To effectively kill bacteria and reduce infection, PDA-tetracycline composites are integrated into hydrogels and release ROS when needed. These multifunctional hydrogels are ideal for treating persistent or drug-resistant wound infections due to their ability to promote angiogenesis and accelerate wound healing.³⁰ With the healing of wounds in TE, PDA- NPs are becoming more and more popular in antibacterial applications.⁶⁴ When PDA-NPs are treated with ascorbic acid, reduced PDA-NPs are produced, which increases their antibacterial and anti-oxidant properties. PDA-related NPs hold promise for antimicrobial biomaterials, infection therapy, clinical control, and dressing development.

Polydopamine in Biosensing

An electronic transducer and amplifier are combined with biological sensing elements such as enzymes, nucleic acids, antibodies, and cells to create biosensors, which are analytical instruments. According to the International Union of Pure and Applied Chemistry, “a biosensor is an independent integrated device that uses a biological component to provide certain quantitative or semi-quantitative analytical information.” Biosensors have become a major area of research and innovation due to their wide array of potential applications, from disease identification and prevention to detecting specific bacteria and pathogens. These devices are employed in several fields, including agriculture, food product processing, bioprocess tracking, environmental monitoring, and medical diagnostics.¹⁴⁰ Depending on the type of transducer used to gather and process signals, biosensors are categorized as optical, thermal, piezoelectric, and electrochemical.^141,142 PDA is a potential material for biosensor creation because of its biocompatible nature and versatile ability to form coatings on different substrates. PDA can immobilize different biomolecules on the surface of the transducer, thus, as a promising transducer material for biosensors, PDA is appropriate for biosensing assays due to its excellent stability in neutral or nearly neutral circumstances and ambient environmental conditions.¹⁴³ Other conductive polymers such as polyaniline and polypyr-role are also used for applications that require electrical conductivity. These materials can provide electrical signal transmission but may lack the adhesion and functionalization properties of PDA.¹⁴⁴ Biosensor assemblies can benefit greatly from PDA’s high reactivity and ability to cover nearly infinite materials. By covalently connecting with amino, imidazole, and thiol residues, PDA is a versatile material that may easily encapsulate biomolecules while maintaining their biological function.¹⁴⁵ As a highly biocompatible polymer, PDA can be applied as a flexible coating in sensor and biosensor designs. The properties of the developed sensors are enhanced by the frequent use of PDA as a binding agent.¹⁴⁶ PDA-functionalized self-supported nanoporous gold film electrodes provide advantages in terms of surface area for H₂O₂ and DA detection sensors.¹⁴⁷ PDA has also been studied as a biomimetic analog of eumelanin, specifically in its ability to absorb proteins in a similar manner. In one previous research, electrostatic forces were observed between PDA and a number of model proteins including lysozyme, myoglobin, and α-lactalbumin.¹⁴⁸ Additionally, covalent bonding between PDA and several model polypeptides has been observed.¹⁴⁹ The variation, not just in what molecules are able to adhere to PDA coatings but also the mechanism by which this adherence occurs, again lends to a range of different biosensor types. Beyond its ability to act as an intermediate between biomolecules and a substrate, PDA also has a strong fluorescence-quenching ability, and initial investigation of PDA nanospheres showed comparable results to known super-quenchers, with up to 97% efficiency.²⁹ This capability could be used in biosensing applications for monitoring the adsorption and binding of fluorescently labeled proteins.¹⁵⁰ Another benefit of using PDA in biosensors is in the sub-type of electrochemical biosensors. These devices use an electrical transducer and are especially useful due to their extremely low detection limit and high sensitivity, as well as the potential for further automation. Electron transfer is crucial to these sensors’ functionality, and PDA has been shown to effectively allow this transfer when used as a coating on electrodes. This use was demonstrated in 2014 using PDA to modify a carbon electron with glucose oxidase and create a very high-sensitivity glucose biosensor.¹⁵¹ Later, another group used a similar electrode with PDA and an optimized enzymatic mixture to detect low levels of hydrogen peroxide for applications in food safety and biological research.¹⁵² The major benefit to PDA is its simplicity and cost-effectiveness. Recent research looked at silicon-based photonic biosensors, widely used in rapid testing and diagnostic applications, compared some established surface functionalization methods, and compared them to a new protocol using PDA. The newly proposed PDA protocol required only one step and improved the overall antibody immobilization density, while still allowing for a large variety of different antibodies.¹⁵² Another important biosensing method that has been studied the most is optical bioimaging, which is used in food and environmental monitoring as well as clinical diagnostics. Bioimaging also makes use of PDA’s ability to function as an efficient fluorescence quencher. PDA is primarily being developed for biosensing applications in fluorescence and photoacoustic imaging. Even if there may be benefits and drawbacks to using PDA, it is flexible enough to work with both approaches. PDA combines the following characteristics of an optical biosensor: (1) the ability to generate signals; (2) ease of functionalization; and (3) good biocompatibility. These features have focused researchers’ attention on numerous PDA applications in optical biosensing.¹⁵³ Despite PDA being heavily researched since its discovery, actual clinical trials have been limited, and thus large-scale implementation has not occurred. The process of getting US Food and Drug Administration approval is fundamentally lengthy with a specific focus on safety and standardization. While extensive pre-clinical research has been carried out, particularly in using PDA in the treatment of cancer, and has shown some efficacy, there is a lack of data on the possible toxicity or fouling of PDA on a longer timescale.¹⁵⁴ PDA’s biocompatibility and functional versatility have been used to make electrochemical aptasensors for detecting biomarkers such as glycated albumin (GA), which are crucial to monitor glycemic control in diabetic patients. One study has shown that PDA-functionalized aptasensors could detect GA across a clinically relevant range of 1–10,000 μg/mL, with a low limit of 0.40 μg/mL. These sensors also exhibited appreciable reproducibility (less than 10% variability) and practicality, with recovery rates between 90 and 104%.¹⁵⁵ PDA has also been used to make surface-enhanced Raman scattering biosensors, which enhance sensitivity and specificity for various investigations. However, in-depth research into PDA’s properties is growing to develop multi-mode sensing and integrate diagnostic and therapeutic applications.¹⁵⁶ Consistent fabrication processes and scalability of PDA coatings on various substrates are crucial for clinical applications. Ongoing research aims to address the challenges by optimizing PDA synthesis techniques. While PDA has the benefit of versatility, there are many materials already approved and in use for many of the proposed applications for which it could be used. Both biopolymers, such as chitosan and cellulose, and synthetic polymers, like polyethylene glycol and PLA, are widely used in TE, while metal and ceramic-based coatings for use with implants also exist.¹⁵⁷ Furthermore, lipid-based and protein-based NPs have both been successfully used as drug carriers, though many are highly specific to a single drug.¹⁵⁴ With many of these materials already being fully approved for clinical use, PDA implementation may be slowed, as it has not yet been shown to be definitively more effective than the current market competitors.

Conclusions and Future Directions

In the 17 years since its discovery, polydopamine (PDA) has become a highly researched material due to its versatility and accessibility, and has shown transformative potential across countless industries, especially within the biomedical field. Its straightforward synthesis, universal compatibility, and diverse range of useful properties have driven extensive research and innovation, from tissue engineering to advanced drug delivery systems and antibacterial applications. Despite its simple inspiration, PDA has demonstrated applicability far beyond initial expectations, which has transferred into a seemingly limitless number of sectors. As highlighted in this review, combining advanced and exciting materials like PDA with other emerging technologies such as 3D printing and biosensing devices has paved the way for groundbreaking advancements in the biomedical industry. Moving forward, continued exploration of PDA’s unique properties and the specific mechanisms behind them, along with other innovative applications, will undoubtedly expand its role as a cornerstone material for future technological and medical solutions.