New challenges in scar therapy: the novel scar therapy strategies based on nanotechnology

ABSTRACT

The pathological mechanism of pathological scar is highly complex, encompassing the abnormalities of diverse cytokines, signaling pathways and regulatory factors. To discover more preferable scar treatment options, a variety of distinct approaches have been utilized clinically. Nevertheless, these treatments possess certain side effects and are inclined to relapse. Presently, pathological scar treatment remains a clinical conundrum, and there is an urgent demand for treatment methods that are safe, less traumatic and have lower recurrence rates. New drug delivery systems, novel therapeutic drugs and therapy strategies can enable drugs to permeate the skin effectively, decrease side effects, enhance drug efficacy and even achieve pain-free self-administration. Currently, novel nanotechnologies such as nanomicroneedles, photodynamics mediated by novel photosensitizers, bioelectrical stimulation and 3D printed dressings have been developed for the effective treatment of pathological scars. Additionally, innovative nanoscale fillers, including nano-fat and engineered exosomes, can serve as novel therapeutic agents for the efficient treatment of pathological scars. The intervention of nanomaterials can enhance drug absorption, stabilize and safeguard the active ingredients of drugs, delay or control drug release and enhance bioavailability. This article reviews these new treatment strategies for scar to explore novel approaches for efficient and safe for keloid treatment.

Plain language summary

Article highlights.

• The pathological mechanism of pathological scars is highly complex, encompassing abnormalities in various cytokines, signaling pathways and regulatory factors. The treatment of pathological scars remains a challenging clinical issue and there is an urgent demand for methods that are safe, less traumatic and have a low recurrence rate.

• Nanomicroneedles constitute an emerging therapeutic modality that can be utilized either independently or as a delivery system for directly transporting drugs to the epidermis or dermis. It possesses the merits of being painless, minimally invasive, efficient, safe and convenient.

• The newly developed photosensitizers can ensure the depth of treatment and prompt PDT response. Particularly, the combination of photosensitizer-mediated PDT and nanozymes boosts the efficacy of PDT.

• The integration of the self-powered system and the nanocarriers is capable of generating bioelectrical stimulation and achieving synergistic antibacterial and anti-inflammatory effects, thereby significantly expediting wound healing and preventing the formation of scar tissue.

• By integrating 3D printing technology with nanomaterials, it becomes feasible to acquire a dressing scaffold featuring a complex structure, precise size and the ability to promote wound healing and to achieve personalized wound dressings for facilitating wound healing.

• The treatment involving nanofat and combined platelet components can inhibit scar formation through influencing the signaling pathway, which constitutes a potential therapeutic strategy for scar treatment as well.

• Mesenchymal stem cell-derived exosomes and their derivatives suppress ECM deposition in keloids through influencing the TGF-β/Smad and Notch-1 signaling pathways.

1. Introduction

Scarring is a consequence of the skin's reparative process following trauma. In the majority of cases, as a result of delayed or excessive healing, skin wounds will develop non-functional fibrotic tissue masses, namely scars, after going through three stages: inflammation, cell proliferation, migration and tissue remodeling [1]. Hypertrophic scars and keloids are two types of pathological scar that present extremely severe clinical symptoms. Annually, approximately 100 million individuals in developed countries encounter scar-related issues and globally, over 11 million people are solely affected by keloids. Hyperplastic scars and keloids fall under the category of skin fibroproliferative disorders, mainly characterized by abnormal proliferation of fibroblasts, disordered collagen arrangement and excessive deposition of the extracellular matrix. Hyperplastic scars are primarily caused by surgeries and severe burns, whereas keloids can result from minor skin injuries, such as vaccinations and acne. Morphologically, both exhibit hyperplasia, protrusions and redness [1,2]. Moreover, although keloids are regarded as a benign disease, they demonstrate a tumor-like growth pattern, frequently extending beyond the original wound margin [3].

In the early phase of skin wound healing, a significant number of inflammatory factors are secreted through inflammatory responses, among which transforming growth factor-β (TGF-β) is the most prevalent and extensively researched. These factors stimulate the transformation of fibroblasts surrounding the wound into myofibroblasts and the synthesis of a considerable amount of extracellular matrix (ECM) components, such as collagen-I (Col-I) and collagen-III (Col-III). Simultaneously, tumor necrosis factor-α (TNF-α) and other macrophages trigger M1 macrophages to generate a large quantity of pro-inflammatory factors like interleukin (IL-6) and IL-1β, thereby activating fibroblasts and creating a synergistic effect. In the late stage of wound healing, IL-4 and IL-13 activate M2-type macrophages to produce IL-10 and other anti-inflammatory factors as well as matrix metalloproteinases (MMPs), which are involved in the regulation of inflammation and the degradation of ECM. These processes encompass multiple signaling pathways, and the dysregulation of any one of them can result in scar formation.

Traditional treatments for pathological scars can be divided into non-surgical treatments such as drug therapy, laser therapy, cryotherapy, pressure therapy and radiation therapy. Despite the diverse range of treatment options available for pathological scars, the therapeutic outcome remains suboptimal due to the complexity and high recurrence rate. The high recurrence rate associated with surgical resection and the side effects of radiation therapy lead to reduced patient acceptance. The therapeutic efficacy of laser therapy on scars has been established, with minimal effects on surrounding tissues and mild adverse reactions, but it comes at a high cost. Medium frequency electrotherapy, microplasma radiofrequency technology and extracorporeal shock wave therapy are more appropriate for the management of pathological scars in the remodeling stage. Therefore, the exploration of efficient and safe novel treatment approaches and strategies for pathological scars constitutes the focus of current research.

2. Mechanism of pathological scar formation

Current studies indicate that the pathological features of hypertrophic scars and keloids encompass active proliferation of fibroblasts, reduced apoptosis, significant deposition of collagen fibers, infiltration of inflammatory cells and elevated expression of Col-I and Col-III in scars.

The precise mechanism underlying scar formation remains poorly understood, but it might be triggered by a series of disruptions in the normal wound healing process, which is typically classified into three phases: inflammatory response, cell proliferation and tissue remodeling. Scar formation constitutes a crucial pathophysiological process in wound repair. It is generally believed to result from excessive production and deposition of ECM. The augmented synthesis of ECM is considered to be associated with the overactivation of fibroblasts, which is related to the overexpression of inflammatory mediators, such as the increased expression level of TGF-β, leading to abnormal regulation of the TGF-β/SMAD signaling pathway, thereby causing enhanced activity of fibroblasts and the generation of ECM. Excessive ECM undergoes degradation and remodeling. In the early stage, the principal component of ECM is Col-III, and during the remodeling process, it transforms into mature Col-I and eventually forms scars [4,5]. From a histopathological perspective, Col-III in hypertrophic scars are more orderly, and the arrangement is essentially parallel to the long axis, while keloids contain more Col-I, and the arrangement is typically irregular [4]. Additionally, the abnormal expression of TGF-β, platelet-derived growth factor (PDGF), interferon, alpha-melanocyte estrogen, TNF and other cytokines is also associated with scar formation [6,7].

Therefore, as a preventive and therapeutic agent for pathological scars, its mechanism of action is to impede scar formation by interfering with DNA synthesis of fibroblasts, inhibiting their proliferation, reducing collagen synthesis and the expression activities of TGF-β1 and TGF-β2.

3. Novel therapy strategies for scar

3.1. Nano-microneedles for scar treatment

Microneedle (MN) therapy constitutes a novel transdermal drug delivery technology, which consists of a micron-scale sharp needle array. Once the skin is penetrated, micropores are formed on the skin, facilitating the delivery of therapeutic substances such as small molecules, biomacromolecules and exosomes to the local skin, thereby enhancing the permeability of drug molecules on the skin. As a novel approach for transdermal drug delivery, MNs offer advantages such as minimum invasiveness, ease of use and greater acceptance by patients. In contrast to oral administration, along with traditional injections and wound dressings, microneedles do not have to pass through the gastrointestinal tract directly to reach the affected skin, thereby minimizing the risk of systemic side effects and improving drug utilization. Based on these therapeutic benefits, the innovative technique of MNs therapy has been extensively utilized in clinical medicine. The mechanism of microneedles in scar treatment pertains to disrupting the original abnormal collagen fiber bundle using MNs, activating the self-repair mechanism and promoting extracellular matrix regeneration and tissue remodeling. Studies have indicated that MNs technology can offer a promising therapeutic strategy for the treatment of scars.

Based on the scar treatment mechanism, a superior application strategy for MNs treatment mandates that MN possesses outstanding biocompatibility, biodegradability and mechanical strength and can synergize with drugs to attain more favorable therapeutic outcomes. To enhance the utilization rate of drugs, when MNs is employed for transdermal drug delivery, it is essential to select an appropriate carrier for drug encapsulation and subsequently blend the drug-carrying particles with MN patches. According to the distinct modes of action, microneedles can be classified into solid microneedles, hollow MNs, coated MNs, soluble MNs and hydrogel MNs.

In recent years, with the swift advancement of nanotechnology, the utilization of nanomaterials to enhance the performance of MNs has drawn extensive attention. Compared with conventional materials, nanoparticles possess distinctive physical and chemical properties owing to their minute size, large specific surface area, high specific surface energy, quantum size effect and macroscopic quantum tunneling effect. Based on the application requirements of diverse microneedles, nanoparticles with different functionalities can be chosen for combined application with MNs. The integration of nanoparticles and microneedles boosts the performance of transdermal MNs (Table 1).

Table 1.

Summarizing of different strategies in the treatment of scar.

	Therapeutic effect	Advantage	Disadvantage	Therapeutic principle	Type	Ref.
Nano-microneedles for scar treatment
5-FU-CMC-MN	The inhibition of keloid fibroblasts reached 16%; inhibit TGF-β1 expression	Strong tissue penetration; loaded drug capability; promote the dissolution and diffusion of drugs to improve the therapeutic effect; improve the utilization of drugs;	Low drug loading	affects DNA synthesis and collagen expression associated with scar formation	keloids	[8]
siSPARC/Gtn-Tyr	The SPARC gene expression in human dermal fibroblasts was significantly reduced and excessive collagen deposition was inhibited	Strong tissue penetration; protect the carried RNA components from immune clearance and enzymatic degradation	Low drug loading	Silence stromal cell SPARC protein to effectively reduce collagen production, thus effectively inhibiting scar formation	prevent scar formation during the wound-healing process	[9]
GA-QAGN-MN	inhibit the expression of Col-I, Col-III and TGF-β1 in fibroblasts	Strong tissue penetration; the dual-loaded drug system can realize the time-controlled release of different drugs	Low drug loading	Gallic acid and quercetin were released at different time and ECM deposition was reduced by the combined action of two drugs	keloids	[10]

Novel photosensitizer mediated photodynamic therapy for scar
ICG-PDT	The expression of TGF-β, Col-I and III was significantly reduced	Strong tissue penetration	Poor solution stability and concentration quenching characteristics	Application of ICG-mediated photodynamic therapy	keloids	[16,17]
IR-808-ES	Induce HSF apoptosis through intrinsic mitochondrial pathway	Strong tissue penetration;	Not mentioned	The ethosomal (ES) vesicles can disrupt the barrier function of the stratum corneum and squeeze through narrow spaces to reach the dermis; exhibit aggregation induced emission enhancement (AIEE) phenomenon.	keloids	[18]
Ph1-PDT	Increase the apoptosis and the expression of caspase-3; decrease the expression of TGF-β1 and Col- I	Strong tissue penetration	Not mentioned	Application of Ph1-mediated photodynamic therapy	keloids	[13]
A/A-ES	promoting HSF apoptosis; increased MMP-3 expression; Col- I and III reduced	self-generate O2 to enhance the efficacy of HS-PDT; improve the anoxic microenvironment	Not mentioned	possess nanozyme activity; combines photosensitizer mediated PDT with nanozyme	hypertrophic scar	[19]

Bioelectrical stimulation inhibits scar formation
ZGH-MN	inflammatory factors reduced and anti-inflammatory increased; effectively reduce the local blood glucose concentration through the anode oxidation of BFCs to accelerate wound healing; decrease TGF-β to inhibit the formation of scars	Strong tissue penetration; possess the activities of glucose oxidase (GOx) and horseradish peroxidase (HRP); generating stable microcurrents	Not mentioned	Self-powered and enzymatic cascade combination; possess the synergistic effect of antibacterial, anti-inflammatory and bioelectric stimulation	prevent scar formation during the wound-healing process	[25]
ZPFSA	decrease α-SMA and promote collagen remodeling to accelerate the wound healing and effectively prevent the scar tissue formation within 2 weeks	good biocompatibility, antimicrobial properties and piezoelectric healing properties	Not mentioned	dual piezoelectric response models to accelerate wound healing process, prevent scar formation	prevent scar formation during the wound-healing process	[26]

3D printing technology for the preparation of wound dressings
DECM-2MSNs/Rg3	Inhibit the production of excessive collagen, reduce the expression of vascularization related factors (CD31, VEGF) and fibrosis related factors (TGF-β) and thus inhibit scar formation	The dressing stent with complex structure, precise size and promoting wound healing can be obtained. Easy to load medicine	Lack of the self-adhesive capability	Silica nanoparticles were loaded with Rg3, mixed with DECM hydrogel and combined with 3D printing technology to print the hydrogel scaffold	prevent scar formation during the wound-healing process	[32]

Strategies for treating scar with nanofat
Nanofat	Inhibition of α-SMA, TGF-β1, Col-I expression, reduce collagen accumulation, inhibit scar formation	Autologous transplantation; nanofat combined with other ingredients	Susceptible to infection	The fibrosis of hypertrophic scar was reduced by miR-192-5p/IL-17RA/Smad axis. Promotes skin wound healing through Jagged1/Notch signaling pathway	Pitting scar	[45,46,52,55]

Application of exosomes and derivatives derived from mesenchymal stem cells in the treatment of scar
HBMSCs-Exos	The expression of α-SMA, Col-I and III in keloid fibroblasts was inhibited and the formation of keloid was inhibited	High stability, easy preservation, low immunogenicity, easy quantitative use	Separation acquisition is relatively difficult, the operation cost is relatively high	reduce inflammatory response and collagen deposition	Hyperplastic scar; keloid	[59,60]
ADSC-Exos	The expressions of α-SMA, TGF-β1, Col-I and Smad3 were decreased	The separation is relatively easy to obtain and the operation cost is relatively low	Not mentioned	Inhibition of TGF-β2/Smad3 and Notch-1 signaling pathway, inhibition of ECM deposition in keloid	Hyperplastic scar; keloid	[61–63]
Exosome derived non-coding RNA	The expression of α-SMA, TGF-β1, Col-I and III was inhibited and the formation of keloid was inhibited	Multiple miRNAs affect multiple targets and affect multiple functions	Not mentioned	Influence miR-7846-3p/NRP2/Hedgehog; miR-192-5p/IL-17RA/Smad axis; miR-21/TGF-β-Smad2/3; miR-26b-5p/PTEN-PI3K/AKT; hsa_circ_0020792/ miR-193a-5p/TGF-β1/Smad2/3 signal pathway	Hyperplastic scar; keloid	[40,64–66,69]
Engineering exosomes	The expression of α-SMA, TGF-β1, Col-I and III was inhibited and the formation of keloid was inhibited	Exosomes can be modified to improve their targeting and stability	Not mentioned	EXo-miR-29a/miR-138-5p/miR-138-5p/miR-141-3p are delivered functionally to keloid fibroblasts by exosomal carriers	Hyperplastic scar; keloid	[70–73]

For instance, the combination of nanoparticles and matrix materials can enhance the puncturing performance and drug transdermal performance of MNs. The functional modification of nanoparticles can achieve effects such as regulating release, responsive release and targeted intelligent drug delivery. It represents the tendency of multi-functional MN design in the future. Park et al. fabricated carboxymethyl chitosan (CMC) nanoparticles loaded with 5-fluorouracil (5-FU) and subsequently coated them onto stainless steel solid MNs. The 5-FU-loaded CMC nanoparticles were delivered to the skin through MNs to enhance the treatment of keloids. The outcomes revealed that the inhibitory impact of 5-FU and CMC nanoparticles on human keloid fibroblasts reached 16%, and the expression of TGF-β1 was strikingly suppressed. Nevertheless, microneedles can be employed to locally administer 5-FU and CMC nanoparticles, enabling the drugs to be dissolved and diffused at the administration site and improving the drug efficacy. The findings demonstrate that the MN-mediated drug delivery system not only inhibits human keloid fibroblasts by effectively delivering drugs to keloids but also holds the feasibility of painless self-administration. This technology offers a more efficient and convenient treatment for keloid patients [8]. Chun et al. fabricated a soluble and biocompatible hyaluronic acid (HA) MN patch and a tyramine-modified gelatin for functionalizing a nanoresin loaded with secreted protein acidic and cysteine-rich (SPARC) siRNA to generate MNs capable of preventing scarring by inhibiting excessive collagen deposition. The outcomes of in vitro experiments demonstrated that the MN patch loaded with siSPARC/Gtn-Tyr could notably reduce the expression of the SPARC gene in human dermal fibroblasts, and did not induce obvious cytotoxic effects. In a mouse wound model, siSPARC/Gtn-Tyr nanomicrone was shown to effectively diminish collagen production by silencing the SPARC protein of stromal cells during wound healing, thereby efficiently inhibiting scar formation. The results affirmed the potential of the siSPARC/Gtn-Tyr-loaded HA nanomicroneedle patch for local prevention of pathological scarring [9]. Chen et al. developed a heterogeneous gelatin-structured compound MN for the transdermal dual drug release of gallic acid (GA) and quercetin (Qu) to achieve the combined treatment of keloids. The composite nanoparticle patch, capable of self-pressing into the stratum corneum, enables the early release of GA to retard the proliferation of fibroblasts, followed by the subsequent release of quercetin as a powerful antioxidant to eliminate the production of ROS. Additionally, the amphiphilic gelatin nanocapsules (QAGN) enhanced the transdermal absorption efficiency of GA and quercetin. Hence, in addition to delaying the proliferation of fibroblasts, the nano-microneedle system also suppressed the expression of Col-I, Col-III and TGF-β1 in fibroblasts. The compound two-drug nanomicronal system controlled the release of GA and Qu at distinct times and reduced the excessive deposition of ECM through the combined action of the two drugs, effectively achieving the effect of inhibiting keloid formation (Figure 1) [10].

Figure 1.

The preparation of the quercetin (Qu)-loaded amphiphilic gelatin nanocarrier and process of drug release. The prevention of keloid scars by controlling transdermal dual-drug release of Qu and GA via modulating heterogeneous gelatin-structured composite nano-microneedles. Reprinted from [10], published under a CC-BY license.

In summary, nanomicroneedles represent an emerging therapeutic approach that can either be employed independently or function as a delivery system for directly delivering drugs to the epidermis or dermis. They possess advantages such as being painless, minimally invasive, efficient, safe and convenient, thereby demonstrating significant potential in the treatment and prevention of pathological scars.

3.2. A novel photosensitizer mediated photodynamic therapy for scar

Photodynamic therapy (PDT), a non-invasive combination of drugs and devices, has been extensively employed in the treatment of skin tumors and various skin disorders. In recent years, there have emerged clinical reports suggesting that PDT can effectively manage keloids. The three fundamental elements of PDT are photosensitizer, excitation light source (specific wavelength of light) and oxygen. The treatment mechanism lies in that once the photosensitizer is exposed to light irradiation of a certain wavelength, reactive oxygen species (ROS), encompassing singlet oxygen, oxygen free radicals, hydroxyl free radicals, etc., are generated in the presence of molecular oxygen, which act upon specific cellular biomacromolecules, exert cytotoxicity and induce apoptosis or necrosis of cells. Photosensitizers can selectively accumulate in highly active keloid fibroblasts. Consequently, PDT can selectively eliminate target cells, with less damage to normal cells. PDT can not only induce the apoptosis of fibroblasts but also decrease ECM deposition, lower blood vessel density and suppress inflammatory responses, ultimately inhibiting the growth of keloids. Studies have demonstrated that PDT induces the apoptosis of keloid fibroblasts by augmenting the expression of ROS, caspase-3 and caspase-8, and lowering the ratio of Bcl-2/Bax [11]. PDT is capable of reducing the expression of type I and type III collagen, enhancing the expression of matrix metalloproteinase-3 and elastin and minimizing the deposition of ECM, thereby attaining the effect of inhibiting scar [12]. PDT significantly repressed the expression of vascular markers (CD31 and CD34) in keloid grafts. These findings illustrate that PDT can curb keloid hyperplasia by diminishing blood vessel formation [13]. PDT can prominently decrease the expression levels of inflammatory factors IL-8 and IL-1β in keloid grafts and decelerate the progression of keloid. Since the therapeutic efficacy of PDT is closely associated with the photosensitizer, the enhancement of photosensitizer properties will significantly enhance the curative effect of PDT.

Indocyanine green (ICG) is a type of amphiphilic carbocyanine dye featuring high photothermal conversion efficacy. Its spectral absorption peak is approximately 800 nm and the emission peak is approximately 832 nm, enabling light to penetrate more deeply into biological tissues and having been approved by the FDA [14]. It has been extensively utilized in the domains of PDT, photodynamic therapy and real-time fluorescence imaging. ICG possesses a higher spectral absorption, enabling a greater depth of light penetration within the tissue. Studies have demonstrated that the topical application of ICG can be effectively absorbed by sebaceous glands. In contrast to conventional photosensitivities such as 5-aminolaevulinic acid (5-ALA) and methyl aminoketovalic acid (MAL), which require time for absorption and conversion into protoporphyrin IX, ICG can induce PDT within a relatively short period [14,15]. Recently, the application of ICG-mediated PDT has made significant advancements in the treatment of keloid (Table 1). For instance, Fakhraei et al. fabricated liposomes encapsulating ICG to investigate the efficacy of its combination with PDT in keloids. To enhance the drug permeability, CO₂ laser pre-treatment was conducted on keloids initially, followed by covering the lesions with liposomes containing ICG and irradiating them with 730 nm light within the near-infrared range once a week. After 6 consecutive weeks, the patient and observer scar assessment scale (POSAS) score was significantly reduced by 23.69% after the treatment. No adverse reactions were witnessed during the treatment and within 3 months of the follow-up. Nevertheless, the POSAS score was significantly increased by 13.77% after the withdrawal. The aforementioned studies demonstrate that CO₂ laser pre-treatment in combination with PDT is a safe and effective approach for the treatment of keloids. Given the high recurrence rate after treatment cessation, the feasibility of its clinical application requires further verification [16]. Shao et al. utilized ICG-mediated PDT in the treatment of postoperative keloid. Through the effect of ICG-PDT, the migration of human keloid fibroblasts can be suppressed and the expression of TGF-β and Col-I and Col-III in the cells can be significantly decreased. A patient with intractable keloids was treated with ICG-PDT for 2 months. It was discovered that the size of keloids reduced and flattened significantly, the VSS score and UNC score decreased prominently and the patient did not report itchiness or discomfort related to surgical wounds during a subsequent follow-up. ICG-mediated PDT therapy has displayed considerable therapeutic potential in the clinical treatment of keloids [17]. Yu et al. fabricated nano-liposomes loaded with IGG IR-808 (IR-808-ES) as a novel nano-photosensitizing agent, which is capable of enhancing the synergistic therapeutic effect of PDT/PTT on hyperplastic scars. The liposome structure of IR-808-ES facilitates the accumulation of IR-808 in hypertrophic scar fibroblasts and induces the apoptosis of hypertrophic scar fibroblasts via the mitochondrial internal pathway. Simultaneously, the tissue penetration of IR-808 under near-infrared irradiation is strengthened. The systematic assessment of the rabbit ear hyperplastic scar model demonstrated that IR-808-ES in PDT/PTT could notably improve the appearance of hyperplastic scars, boost the apoptosis of fibroblasts and the remodeling of collagen fibers, and exert a significant therapeutic effect. IR-808-ES, as a novel nano-photosensitizer, has displayed great potential in the clinical field for the treatment of hypertrophic scars [18].

Phenalen1-one (Ph1), a photosensitizer extracted from Scutellaria baicalensis, is a type of highly efficient type II photosensitizer. Zheng et al. discovered that PDT treatment mediated by PH1 could strikingly inhibit the proliferation, migration and invasion of keloid fibroblasts, as well as raise the apoptosis rate and the expression of caspase-3. In nude mouse models, Ph1-PDT decreased graft volume and vascular density by suppressing the expression of blood vessel density biomarkers (CD31 and CD34) in keloid grafts. Ph1-PDT also notably reduced inflammatory mediators in keloid grafts. Additionally, Ph1-PDT significantly alleviates keloid development by inhibiting TGF-β1 and Col-I proteins in keloid fibroblasts and grafts [13].

In addition to conventional chemical reagents, novel photosensitizers fabricated through nanotechnology and new materials are also capable of responding to external photothermal stimuli. Some nanoparticles can serve not only as photosensitizers or sources of active oxygen but also as functional carriers to transport photosensitizers. The nanostructured molecular counterpart possesses higher chemical stability and can also act as a quencher for the emission of active photosensitizers to enhance the production efficiency of ROS. The integration of nanoparticles with new materials presents a broader range of excitation possibilities for PDT (Table 1). Chen et al. constructed A/A-ES by loading the photosensitizer 5-aminolevulinic acid (ALA) into the nano-liposomes and immobilizing the Au nanoclusters (ANCs) nanozyme on the surface of the nano-liposomes through electrostatic interaction. The distinct structure of A/A-ES enables it to co-deliver ALA and ANCs to HS tissues. Simultaneously, A/A-ES can also exert the function of catalase, effectively decomposing endogenous hydrogen peroxide in HS to generate O₂, thereby enhancing the efficiency of HS-PDT. In vitro and in vivo experiments indicated that A/A-ES effectively ameliorated the morphology of HS fibroblasts and significantly stimulated the apoptosis and collagen rearrangement of HS fibroblasts. These studies suggest an efficacious HS treatment protocol that combines photosensitizer mediated PDT with and nanozymes, thereby allowing them to exert their beneficial effects, and highlight the potential of enhancing the efficacy of HS-PDT through autogenic O₂, demonstrating its clinical therapeutic potential (Figure 2) [19].

Figure 2.

Nanoliposome loaded photosensitizer combined with nanozyme Au nanoclusters (ANCs), can enhance HS-PDT via self-generating O₂. Reprinted with permission from [19], copyright 2021. American Chemical Society.

Although PDT can provide adjuvant therapy to prevent scars after wound healing. However, due to the limited permeability of conventional photosensitizers. Nevertheless, novel developed photosensitizers can guarantee the depth of treatment and prompt PDT response. In particular, the combination of photosensitizer-mediated PDT and nanozymes boosts the efficacy of PDT, indicating its potential in clinical treatment.

3.3. Bioelectrical stimulation inhibits scar formation

Endogenous electric fields (also referred to as physical electric fields/naturally occurring electric fields), Certain events (such as development, tissue damage, abnormal cell proliferation, etc.) take place in the space and time of cells, tissues and organs, leading to alterations in the original potential difference. This causes ions to re-distribute and generate ion flow, thereby generating an extracellular direct current electric field. Studies have demonstrated that the endogenous electric field is a crucial condition for wound healing. The simulation of the endogenous electric field to facilitate wound healing has been applied in clinical practice, and preliminary outcomes have been achieved. Studies have indicated that the electric field can accelerate neovascularization by guiding endothelial cells and endothelial progenitor cells to migrate directionally, enhancing the migration rate and thus promoting neovascularization [20,21]. Additionally, the electric field can enhance wound healing by stimulating fibroblast proliferation, guiding its directional migration and facilitating fibroblast collagen synthesis [22]. In the study of the scar formation mechanism, it was discovered that the excessive generation of fibroblasts and the loss of bioelectricity around the wound might be the principal causes of scar formation [23]. Particularly, bioelectricity gradually weakens or even vanishes in the sluggish process of wound repair, leading to the disorder of the regulation of wound repair genes, the cascading down-regulation of wound healing, the disorder of collagen fiber deposition, the abnormal remodeling of the extracellular matrix (ECM) and ultimately scar formation [24]. Hence, in the treatment of scars, facilitating the bioelectricity of the wound through nanomaterials is an effective approach to prevent the formation of scars during the process of wound healing (Table 1).

For instance, Zhang et al. innovatively devised and fabricated a self-powered enzyme-linked MN patch. The patch comprises an anode and cathode MN array, respectively coated with glucose oxidase (GO_X) and horseradish peroxidase (HRP) encapsulated within ZIF-8 nanoparticles. Through the enzymatic cascade reaction within the MN patch, the local hyperglycemia of diabetic wounds can be effectively mitigated and a stable micro-current can be generated to facilitate the rapid healing of diabetic wounds. The results indicated that rapid, complete and scar-free healing was achieved within three weeks after the MN patch was utilized to treat diabetic wounds (with a wound area of 1 cm²), attributed to the synergistic effect of the MN patch's hypoglycemic, antibacterial, anti-inflammatory and bioelectrical stimulation. Hence, this study presented an effective approach to promptly promote diabetic wound healing and prevent scar formation, which is anticipated to transform the current predicament of difficult diabetic wound healing and holds broad application prospects in clinical wound repair and scar treatment [25]. Liang et al. designed and 3D printed a piezoelectric gel dressing (ZPFSA scaffold) of PVDF/sodium alginate (SA) modified with zinc oxide nanoparticles (ZnO). The fabricated ZPFSA scaffold possesses a dual piezoelectric response model of vertical swelling and horizontal friction, which can simulate and amplify endogenous bioelectricity via electrical stimulation. Thus, rapid and controllable wound healing is accomplished to prevent scarring, and a novel solution is provided to accelerate the wound healing process and prevent scar formation. Among them, ZPFSA 0.5 (containing 0.5% ZnO nanoparticles) exhibits good biocompatibility, excellent antibacterial performance and a stable piezoelectric response, regulates wound healing through cell migration, vascularization, collagen remodeling and the expression of related growth factors, significantly expedites wound healing and prevents scar tissue formation [26].

The endogenous electric field at the skin wound assumes a significant role in wound healing and tissue regeneration. Hence, the integration of the self-powered system and nanocarriers can engender bioelectrical stimulation, and collaborate with antibacterial and anti-inflammatory effects, significantly expedite wound healing, prevent the formation of scar tissue and exert a potential impact in the clinical treatment of wound healing for scar formation inhibition.

3.4. 3D printing technology for the preparation of wound dressings

Three-dimensional (3D) printing technology is a manufacturing approach characterized by layer-printing and layer-by-layer accumulation. It takes 3D digital model files as input and employs adhesives and curable materials as raw substances to construct objects. 3D printing technology is an emerging and alluring material processing method in recent years and has been extensively applied in tissue engineering and regenerative medicine [27]. In recent times, 3D printing technology has also been utilized to fabricate wound dressings containing diverse active pharmaceutical ingredients, being particularly suitable for wound healing. The ideal wound dressing ought to be biocompatible, safeguard the wound, maintain the wound in a moist state, and offer physical protection against microorganisms. It is also necessary to stimulate the migration of keratinocytes to achieve the goal of rapid wound healing. Therefore, in the construction of diversified wound dressings, 3D printing can be adopted as a form of wound dressing production to address the issue of the singularity of wound dressing types, such as the ability to adjust the dimensional attributes of wound dressings (like area, thickness or aperture), the simplicity of drug loading, the utilization of multiple materials, and the adjustable oxygen penetration capacity based on pore design. Therefore, the combination of 3D printing technology and nanomaterials can acquire a dressing scaffold with a complex structure, precise size and the capacity to promote wound healing and realize personalized wound dressings to accelerate wound healing [28,29].

For instance, Wang et al. employed 3D printing technology to fabricate a bilayer membrane (BLM) scaffold composed of a poly (lactate-glycolic acid) (PLGA) layer and an alginate hydrogel layer to imitate epidermal and dermal structures. The porous alginate brine gel of the BLM scaffold facilitates cell adhesion and proliferation, and the PLGA membrane prevents bacterial invasion and retains water within the hydrogel, ultimately expediting wound healing [30]. Kim et al. constructed a 3D printed pre-vascularized skin model based on extracellular matrix bioink, which was demonstrated to significantly accelerate wound healing in vivo [31]. Wang et al. fabricated a hydrogel scaffold DECM-2MSNS/Rg3 with uniform pores through blending ginsenoside Rg3 coated with mesoporous silica nanoparticles with extracellular matrix (DECM) hydrogel scaffolds. The hydrogel scaffolds possess excellent biocompatibility and solid-like rheology to guarantee its printing success. DECM-2MSNs/Rg3 hydrogel exhibits good biocompatibility and the capacity to inhibit fibroblast proliferation, along with favorable water absorption, degradability and printability. It was verified that the DECM-2MSNs/Rg3 group could significantly facilitate wound healing in the rat full-layer skin defect model, and the wound healing rate reached 99.04% at 14 days, nearly achieving complete healing. Additionally, DECM-2MSNs/Rg3 can mitigate inflammatory response, restrain excessive collagen production and decrease the expression of vasculization-related factors (CD31, VEGF) and fibrosis-related factors (TGF-β), thereby inhibiting scar formation (Table 1) [32]. Additionally, regarding the wound healing of burn patients, the implementation of individualized wound dressing constitutes the optimal treatment plan. For instance, Teoh et al., employing chitosan methacrylate as a material, via 3D printing, diverse combinations of chitosan methacrylate and drugs can be continuously utilized to fabricate wound dressings of various designs, each with distinct drug dosages, achieving personalized customization while facilitating wound healing. Simultaneously, the addition of antibacterial agents can markedly enhance its antibacterial capacity. Hence, based on nano-loaded drug technology and 3D printing technology, this study constructed a personalized and large-scale bionic gel scaffold, which can offer a certain theoretical foundation for the scar-free healing of skin defects (Table 1) [33].

4. Application of novel fillers in treating scars

4.1. Strategies for treating scar with nanofat

Nanofat was introduced in 2013 by Tonnard et al. and represents the microscopic tissue remaining after the mechanical emulsification and purification of extracted granular fat [34]. Through mechanical emulsification, the mature fat cells containing large oil droplets, which are hard to survive after transplantation, are removed, thereby reducing the inflammatory response and absorption after transplantation. Additionally, as research on the composition and mechanism of nanofat deepens, it has been discovered that nanofat contains a significant number of adipocyte-derived stem cells, endothelial precursor cells, endothelial cells, macrophages, smooth muscle cells, lymphocytes, pericytes, as well as extracellular matrix and growth factors, with a high expression of CD34 and CD49d. These components have the potential to foster tissue regeneration by participating in the regulation of epithelial cell proliferation, lipolysis, innate immune response, coagulation, wound healing, cell migration and extracellular matrix production [35,36]. Particularly in the realm of skin repair, nanofat can not only fulfill a filling function but also facilitate wound healing, inhibit post-repair scars, enhance skin texture and color and even promote the repair of skin appendages such as hair.

The regenerative capacity of nanofat is associated with ADSCs. Some studies have demonstrated that ADSCs exert a regenerative effect mainly by secreting various growth factors. The ADSCs stem cell component within nanofat can stimulate the expression of scar-inhibiting genes, thereby suppressing scar formation. During skin repair, particularly in the process of wound or injury healing, fibroblasts will phenotypically differentiate into myofibroblasts and express α-SMA, a classical marker of smooth muscle. Under physiological conditions, myofibroblasts will contract the wound, reduce the tissue volume required for repair and secrete a substantial amount of Col-III to swiftly fill tissue defects. Nevertheless, if this process is not halted in a timely manner, prominent scars will occur. Studies have indicated that ADSCs treatment can considerably diminish the expression of α-SMA and Col-I in scars, reduce collagen accumulation and ameliorate scar hyperplasia [37,38]. Besides α-SMA, ADSCs can also inhibit the expression of TGF-β1, thus preventing scar formation [39]. Regarding the molecular mechanism, nanofat can mitigate hypertrophic scar fibrosis through the miR-192-5p/IL-17RA/Smad axis [40]. Its component hADSCs promotes skin wound healing via the Jagged1/Notch signaling pathway (Table 1) [41].

Bhooshan et al. utilized nanofat for local intra-scar injection therapy, and the findings demonstrated that nanofat scar injection could efficaciously enhance scar characteristics and symptoms, thereby contributing to scar restoration [42]. Uyulmaz et al. carried out intradermal injection of nanofat or direct injection into scar tissue in 52 patients presenting with scars, wrinkles or skin discoloration. They were followed up for (155 ± 49) days before and after treatment, evaluated skin quality in accordance with the scoring system, and recorded patient satisfaction. It was discovered that 40 patients (76%) were effectively treated for scars, 6 for wrinkles and 6 for skin discoloration. The quality of scars was significantly improved after treatment, and patient satisfaction was high. The results indicated that nanofat transplantation could assist in improving scars, wrinkles and skin discoloration [43]. Gu et al. employed concentrated nanofat in combination with fat transplantation technology to manage facial atrophic scars, and the results proved that the combination of nanofat and fat transplantation technology is a safe and effective approach for treating facial atrophic scars [44]. Huang et al. implemented nanofat filling treatment in 52 patients with sunken facial scars. At a follow-up visit 3 months after treatment, 91% of the patients showed improvement in scar appearance after treatment. Additionally, the incidence of complications at the injection site was low, which affirmed the stabilizing effect of nanofat injection in the treatment of depressed facial scars [45]. Rageh et al. administered a single injection of autologous nanofat to 30 patients with scarring resulting from various etiologies. The evaluation after 6 months of treatment indicated an increment in epidermal thickness at the treatment site, an increase in the quantity and density of collagen and elastic fibers, as well as neovascularization. Regarding patient satisfaction, one case (3.3%) showed mild improvement, five cases (16.7%) demonstrated moderate improvement, eight cases (26.7%) exhibited significant improvement and 16 cases (53.3%) manifested marked improvement. After injection with nanofat, scar tissue was notably ameliorated and complications were fewer. It has been clinically and pathologically verified that autologous nanofat injection is an efficacious approach to treat scars arising from various etiologies [46]. Regarding breast repair, especially bilateral breast asymmetry, cicatricial contracture and nipple deviation resulting from radical mastectomy, in recent years, it is prevalent to employ autologous fat granule transplantation or in combination with lATS and prostheses for breast reconstruction. Autologous fat can be filled in the quadrant or depression that is arduous to repair, particularly in the inner upper quadrant of the breast. This enables the reconstructed breast to appear more natural [46–48]. Kemaloglu et al. utilized fat and nanofat enriched fat transplantation to assess the effect of scar repair following breast reduction plasty. Nano-fat enriched fat transplantation is effective in enhancing scar quality and patient satisfaction and reported for the first time that nanofat treatment can impact the alterations of fresh scar pigmentation. Hence, nanofat transplantation can be a safe and promising management strategy for postoperative scar repair in breast surgery [49].

Nanofat 2.0 is a novel concept put forward by Lo Furno [50]. He contends that numerous ADSCs are lost during the process of preparing nanofat through the filtration method. Thus, the nylon gauze filtration step is omitted in the preparation of nanofat, and “nanofat 2.0” is fabricated. The findings demonstrate that nanofat 2.0 does contain a greater number of stem cells than the nanofat prepared by the classical approach. It not only possesses obvious proliferative ability but also reduces the likelihood of fat contamination in the production process and enhances the safety of fat transplantation. Due to the high density and proliferation rate of stem cells, nanofat 2.0 can yield faster and more efficient regeneration. Jan et al. injected unfiltered nano-fat into the facial scars of 49 patients. After 6 months, the pigmentation, flexibility of the affected area and scar quality were significantly improved [51]. The drawback is that nanofat 2.0, similar to nanofat, comprises excessive oil and cell debris. Therefore, in future clinical applications, nanofat 2.0 can be combined with purification techniques such as centrifugation to further concentrate ADSCs for better clinical services.

Nanofat transplantation not only can significantly enhance the appearance and texture of scars, alleviate local symptoms such as pain and itching caused by scars, but also can soften and loosen the adhesion between scar tissue and deep fascia, creating the possibility of further fat filling and tissue reconstruction.

4.2. Nanofat combined with other ingredients treatment strategy

Besides nanofat, the derived components of it have also attained favorable outcomes in the treatment of scars. At present, the adipose-derived Stromal vascular fraction (SVF) and platelet components have been utilized in clinical applications (Figure 3).

Figure 3.

Application strategy of nanofat in treating scar. Nanofat and the combination of nanofat with adipose-derived stromal vascular fraction (SVF) and platelet rich plasma (PRP) can reduce the expression of α-SMA, TGF-β1 and Smad3 etc. to inhibit the secretion of collagen I of keloid fibroblasts and inhibited cell proliferation, thereby reducing the formation of keloid.

Stromal vascular fraction (SVF) represents an active mixed cell component extracted from adipose tissue. SVF pertains to a heterogeneous cell population characterized by stem cell traits, encompassing various cell types and exhibiting a potent regenerative potential for immune regulation, angiogenesis and tissue reconstruction. SVF is fabricated by eliminating mature adipose cells from adipose tissue to obtain stromal vascular components, such as hematopoietic stem cells, endothelial progenitor cells, fibroblasts, lymphocytes, macrophages, red blood cells and the extracellular matrix, in addition to approximately 2% to 10% ADSCs. The composition of SVF is essentially equivalent to that of nanofat, but it incorporates fewer dead fat cell fragments and does not elicit a local inflammatory response. Currently, it has been utilized in the clinical treatment of numerous diseases, all of which have demonstrated favorable safety and therapeutic efficacy. Behrangi et al. evaluated the efficacy of SVF combined with nanofat in 7 patients with acne scars. They administered subcutaneous nanofat on one side of the patients' faces and a combination of subcutaneous nanofat and subcutaneous SVF on the other side. The results indicated that the combined treatment of SVF and nanofat for acne scars significantly augmented collagen content and dermal thickness and expedited the improvement of scar volume, area and depth compared with that of patients treated with nanofat alone. Hence, the combined treatment of SVF and nanofat can serve as a potential efficacious treatment for acne scar patients [52].

Additionally, the treatment involving the combination of nanofat and platelet components emerges as a potential therapeutic approach for scar management. As a multifunctional cell, platelets, upon activation, can release various growth factors that can enhance bone and soft tissue healing, even repair nerves, and play a crucial role in angiogenesis, wound repair and inflammation. Platelet-rich plasma (PRP), platelet-rich fibrin, concentrated growth factor and their derivatives have been extensively utilized in the realm of tissue damage repair. Among them, PRP is the most commonly employed in clinical practice. PRP is a human plasma extract replete with a significant number of growth factors, capable of promoting wound healing, collagen growth, antibacterial and other functions and containing abundant plasma nutrients that can also provide ample nourishment for the early survival of autologous fat cells. PRP has been employed in combination with large fat or microfat (conventional fat transplantation) for numerous years. However, with the deeper research on nanofat, it has been discovered that the combination of nanofat and PRP demonstrates superior outcomes in the treatment of facial scars. PRP is a platelet concentrate obtained from whole blood through concentration and separation. PRP can be prepared by collecting autologous blood. Its platelet content is 4–eight-times that of ordinary plasma, and it also has a high concentration of white blood cells and fibrin. Once activated by PRP, a multitude of growth factors such as PDGF, EGF, FGF and VEGF can be released, and a high concentration of white blood cells participates in the inflammatory response of the wound surface, augmenting the anti-infection capability of the wound surface. Fibronectin offers a scaffold for wound repair and can regulate the inflammatory response and tissue remodeling during wound healing [53]. Hoeferlin et al. discovered in their experiments that PRP could effectively stimulate the proliferation of skin fibroblasts and accelerate wound healing [54]. Pons et al. injected platelet-rich plasma (PRP) mixed with nanofat into the diseased dermis to treat and fill severe acne scars. After a one-year follow-up, it was found that the combination of PRP and Nanofat significantly improved skin elasticity, reduced scarring and reversed the inflammatory process. In this case, the outcomes of this combination therapy remained stable over time, with the effect remaining unchanged two years after the surgery and no recurrence. The combined technology of nanofat and PRP is applied as a treatment for other patients with acne scars. The combination of PRP and nanofat represents a promising technique for addressing inflammatory scarring in severe acne [55].

4.3. Application of exosomes & derivatives derived from mesenchymal stem cells in the treatment of scar

4.3.1. Mesenchymal stem cell-derived exosomes for scar treatment

Exosomes are a kind of polyvesicles with a bilayer membrane-like structure mainly formed by the invagination of intracellular lysosomal particles having a diameter ranging from 30 to 150 nm [56]. It has been discovered that exosomes encompass nucleic acids, lipids, proteins and other bioactive substances. Exosomes play a crucial role in the intercellular transportation of substances and the process of intercellular communication. Additionally, exosomes possess the advantages of low immunogenicity, no cytotoxicity, low molecular weight and high safety, which can guarantee that the carrier substances are not prematurely degraded and thereby lose their activity. Exosomes can be secreted by various cells and body fluids, including endothelial cells, immune cells, platelets, smooth muscle cells, mesenchymal stem cells (MSCs), etc. Among them, exosomes derived from MSCs, as paracrine products of mesenchymal stem cells, have similar functions to stem cells, can carry a variety of active substances and participate in the information exchange between tissues and cells and numerous physiological and pathological processes in the body [57] Recently, exosomes have emerged as a potentially effective treatment for keloids [58].

Exosomes can inhibit scar formation by modulating extracellular matrix remodeling and fibroblast apoptosis. During normal wound healing and proliferation, fibroblasts partially differentiate into myofibroblasts, which contract the wound through expressing alpha-smooth muscle actin (α-SMA). Subsequently, myofibroblast apoptosis occurs in the remodeling stage of wound healing and physiological scars form at the wound healing site. However, in the abnormal healing process, the persistence of myofibroblasts leads to excessive contraction of the wound tissue and the local overproduction of extracellular matrix dominated by collagen, ultimately resulting in the formation of hypertrophic scars and keloids [3]. Exosomes can inhibit collagen expression, reshape extracellular matrix and prevent scar formation (Figure 4). Moreover, during the formation of hypertrophic scars and keloids, fibroblasts often undergo excessive proliferation and differentiation and apoptosis is not prone to occur. Therefore, inducing fibroblast apoptosis may represent a new potential therapy after inhibiting fibroblast proliferation. In scar tissue, genes associated with apoptosis have been identified, including the genes of p53, Survivin and Bcl-2, etc. Exosomes induce fibroblast apoptosis by regulating apoptosis-related genes and inhibit or treat scar formation.

Figure 4.

Mechanism of mesenchymal stem cell-derived exosomes and their derivatives in the treatment of scars. By reducing the expression of α-SMA, TGF-β1 and Smad3 etc., exosomes derived from mesenchymal stem cells and their derivatives inhibited the secretion of collagen I of keloid fibroblasts and inhibited cell proliferation, thereby reducing the formation of keloid.

Bone marrow-derived mesenchymal stem cells (BMSCs), the earliest isolated type of MSCs, have undergone extensive research and been widely utilized in diverse therapeutic fields and have displayed considerable efficacy and safety in the majority of studies. It was discovered that following the subcutaneous injection of MSCs-Exos into the backs of rabbits surrounding the skin wounds, the synthesis efficiency of skin collagen was notably enhanced and it also exerted an effective role in promoting the angiogenesis process, accelerating the development and maturation of blood vessels and facilitating wound healing (Table 1). For instance, Zhang et al. discovered that exosomes derived from BMSC-Exos suppress the NF-κB signaling pathway through the inhibition of TNFSF13 and HSPG2, subsequently inhibiting the proliferation and migration of fibroblasts and alleviating hypertrophic scarring [59]. Zhu et al. isolated exosomes from hBMSC for their effect on keloid fibroblasts (KFs), and discovered that hBMSC-Exos could enhance the proliferation, migration and invasion of KFs, while facilitating the apoptosis of KFs. Furthermore, hBMSC-Exos is capable of decreasing the expression of α-SMA, Col-I and Col-III in KFs, and suppressing the formation of keloids. Additionally, lncRNA MEG3 was identified to play a crucial role in inhibiting keloid formation. The expression of lncRNA MEG3 can diminish the activity of KFs. hBMSC-Exos promotes the TP53 transcription of MCM5 in KFs by delivering MEG3 to KFs, leading to reduced KF activity and inhibition of keloid formation [60].

Adipose-derived stem cell-derived exosomes (ADSC-Exos), another type of exosomes derived from mesenchymal stem cells, have drawn widespread attention in disease treatment due to their relatively facile isolation and acquisition as well as relatively low operational cost. Recent studies have indicated that ADSC-Exos is more efficacious in promoting skin wound healing than BMSCs-Exos (Table 1). Through establishing a rabbit skin injury model, researchers conducted a comparative study on the efficacy of BMSCs-Exos and ADSC-Exos in promoting wound healing, and found that, in contrast to BMSCs-Exos, ADSC-Exos had a superior healing effect in treating skin wounds, and the healing effect of Exos in both groups was superior to that of the MSCs group from which they were derived. Wu et al. derived exosome ADSCs-exo from human adipose tissue and applied it to human keloid fibroblasts. They found that ADSCs-exo could inhibit the proliferation and migration of fibroblasts and significantly reduce the expression of α-smooth muscle actin (α-SMA), TGF-β1 and Smad3 [61]. Li et al. further confirmed that ADSCs-exo can suppress the expression of Col-I and Col-III, fibronectin (FN) and α-SMA in KFs. Additionally, ECM deposition in keloids can be inhibited through inhibiting the TGF-β2/Smad3 and Notch-1 signaling pathways [62]. Chen et al. discovered that the exosomes derived from ADSCs (ADSCs-Exo) could diminish the expression of scar-related molecules, such as Col-I, Col-III, α-SMA and miR-181a, in the skin tissue of mice subsequent to wound healing. Further investigations have demonstrated that ADSCs-Exo can repress the expression of miR-181a, activate the SIRT1 axis and exert an anti-scar effect. Despite the fact that ADSCs-Exo does not directly deliver miR-181a to fibroblasts to play a role, ADSCs-Exo can indirectly influence the miR181a/SIRT1 signal axis, significantly lowering the expression of Col-I, Col-III and α-SMA, thereby ameliorating scar formation. It was confirmed that exosomes originated from ADSCs could effectively inhibit scar formation and treat keloid [63].

Therefore, MSCs-Exo can influence the scar formation process through regulating angiogenesis, cell proliferation and migration, inducing fibroblast apoptosis and collagen deposition. Currently, exosomes are anticipated to constitute a novel approach in the clinical treatment of scars and exert a positive effect in inhibiting scar formation, reducing scar area and enhancing the clinical outcomes of scars.

4.3.2. Exosome-derived non-coding RNA is application of treating scars

Exosome (Exos)-derived non-coding RNAs have gained recognition as crucial regulators throughout biological processes, such as proliferation and angiogenesis. Non-coding RNA (ncRNA) has been discovered to fulfill numerous significant roles in cell functionality, and the mutation or abnormal manifestation of ncRNA is closely associated with the occurrence of numerous diseases. ncRNAs carried within exosomes mainly encompass microRNAs (miRNAs), circular RNAs (circRNAs) and long noncoding RNAs (lncRNAs). The utilization of exosome-derived non-coding RNA homologues or mimics can be employed as efficacious drugs for scar treatment (Figure 4).

Studies have demonstrated that miRNAs exhibit potent functions in regulating cellular processes associated with excessive scarring. The aberrant expression of miRNAs in keloid tissues is intimately correlated with TGF-β, MAPK, apoptosis and cell cycle signaling pathways. The functional disparities among different miRNAs might be ascribed to the distinct targets they impact. Specific miRNAs, serving as a predominant Exos-rich substance, can also influence the function of the Exos-carriers (Table 1). For instance, Wu et al. predicted that miR-7846-3p is an exosomal miRNA with dysregulated expression in keloids based on the analysis of the GSE113620 database. Subsequent experimental validation revealed that ADMSC-Exos-derived miR-7846-3p can downregulate the expression of Hedgehog pathway molecules SHH, SMO and GLI1 by targeting NRP2, thereby deactivating the Hedgehog signaling pathway and consequently attenuating fibroblast proliferation while promoting angiogenesis. This effectively inhibits scar formation [64]. Li et al. have demonstrated that miR-192-5p, derived from ADSC-Exo, is highly expressed in ADSC-Exo. It can lower the expression of Col-I, Col-III, α-SMA and p-Smad2/p-Smad3 by targeting the expression of IL-17RA. Moreover, it can enhance the expression of SIP1 in Hypertrophic scar-derived fibroblasts (HSFs), thereby alleviating Hypertrophic scar fibrosis. In vivo experiments confirmed that in mouse wounds treated with ADSC-Exo, ADSC-EXO-derived miR-192-5p mitigated collagen deposition, reverse differentiation of fibroblasts to myofibroblasts and the formation of hypertrophic scars by regulating the IL-17RA/Smad pathway. miRNAs derived from ADSC-exo may potentially represent a promising therapeutic strategy for the clinical management of hypertrophic scars [40].

In addition, some exosome-derived non-coding RNAs, functioning as negative regulators, can facilitate the proliferation of keloid fibroblasts and be involved in the formation and development of keloids. They can also serve as potential therapeutic targets to inhibit keloid formation. For instance, Li et al. discovered that keloid fibroblasts released more exosome miR-21 than normal skin fibroblasts. Furthermore, within keloid fibroblasts, miR-21 is capable of enhancing the proliferation of keloid fibroblasts and the formation of collagen by activating the TGF-β-Smad2/3 signaling pathway, thereby facilitating scar formation. However, miR-21 can be transferred to target keloid fibroblasts via exosomes and promote the proliferation of neighboring keloid fibroblasts. Suppressing the level of miR-21 in exosomes can up-regulate the expression of Smad7 protein, and reduce the levels of Smad2 and Smad3 proteins in target keloid fibroblasts. It also decreased the expression of collagen I and collagen III in keloid fibroblasts, increased the proportion of apoptotic cells and decreased cell proliferation, thereby inhibiting the formation of keloid. The results verified that exosome-derived miR-21 is implicated in the formation and development of keloid and assists keloid fibroblasts in promoting the proliferation of neighboring keloid fibroblasts. miR-21 could serve as a potential target for the treatment of keloids [65]. Dai et al. discovered that hypoxic macrophage-derived exosomes (HMDE) exert a crucial regulatory role in the genesis and progression of keloids. The hypoxic environment is a distinctive feature of keloid formation, and hypoxic-induced macrophage-derived exosomes play a significant role in facilitating the migration, invasion and proliferation of keloid fibroblasts. Dai et al. revealed that exosome-derived miR-26b-5p produced by hypoxic macrophages can directly target PTEN and promote extracellular matrix synthesis, fibrosis and epithelial-mesenchymal transition (EMT) of keloid fibroblasts via the PTEN-PI3K/AKT pathway. Furthermore, exosomes can transfer miR-26b-5p from macrophages to keloid fibroblasts, which might render keloids more prone to proliferation, migration and invasion, thereby causing keloids to appear aggressive. Consequently, regulating the expression of miR-26b-5p might offer a potential therapeutic strategy for keloid treatment [66].

Abnormal expression of circular RNAs (circRNAs) has been identified in keloid tissues [67]. Additionally, exosomes are also abundant in various circRNAs, which are closely associated with the formation and progression of keloids [68]. circRNA is a sort of non-coding RNAs with a closed ring structure and regulatory function. Laden with miRNA-binding sites, circRNA can serve as a “sponge” to prevent the interaction between miRNA and mRNA by adsorbing miRNA, and play a significant role in mediating cell proliferation, migration and invasion. For instance, Hu et al. discovered that plasma-derived exosomes in patients with keloid were abundant in hsa_circ_0020792, but had decreased levels of miR-193a-5p. The hsa_circ_0020792 derived from plasma exosomes in keloid patients can facilitate fibroblast proliferation, migration, collagen synthesis and fibrosis by activating the TGF-β1/Smad2/3 signaling pathway. Downregulating the expression of hsa_circ_0020792 can significantly diminish the viability, migration and fiber formation in fibroblasts. Further investigations revealed that the plasma exosome-derived hsa_circ_0020792 in keloid patients influenced the progression of keloid through targeting miR-193a-5p function inhibition. miR-193a-5p inhibitors reversed the inhibitory effects of hsa_circ_0020792 knockdowns on fibroblast proliferation, migration, collagen synthesis, and fibrosis. Therefore, it was affirmed that the plasma-derived exosome hsa_circ_0020792 in keloid patients can enhance fibroblast proliferation, migration and fibrosis by regulating miR-193a-5p and activating the TGF-β1/Smad2/3 signaling pathway. Studies have demonstrated that these exosome-derived circRNAs can act as key targets for the effective treatment of keloids [69].

4.3.3. Engineering exosomes

Safe and effective drug delivery constitutes an efficacious approach for treating scars. Exosomes not only possess a nanoscale volume but also have the ability to traverse the blood-brain barrier, serving as an effective drug delivery vector. Nevertheless, the clinical application of natural exosomes is restricted due to inadequate targeting capability, a short circulating half-life and uncertainty regarding the composition and content of functional substances. Natural exosomes undergo modification through bioengineering technology, with substances such as targeted peptides, exogenous molecules, drugs, proteins, lipids and nucleic acids being loaded into the exosomal cavity or onto its surface, thereby enhancing the targeting and stability of exosomes and imparting them with specific therapeutic effects to fulfill the objective of treating diseases (Table 1, Figure 4). Currently, the techniques employed for preparing engineered exosomes mainly comprise surface modification and content modification.

The surface modification of exosomes has the potential to enhance their targeting and stability. For instance, Wu et al. incorporated fat MSCS-derived exosomes (ADMSC-Exo) into the chitosan composite hydrogel by the physical mixing approach to fabricate a composite wound dressing, Exo/Gel and achieved scarless, full-layer skin wound restoration by continuously releasing ADMSC-Exo in situ. Exosomes are gradually released from the exosome/gel dressing as the chitosan hydrogel degrades. ADMSC-Exo is capable of regulating the immune response, promoting angiogenesis, expediting the proliferation and re-epithelialization of skin cells, regulating collagen remodeling and suppressing scar hyperplasia. Moreover, the free amine groups in the chitosan hydrogels modulate the formation of factors related to scar formation, such as α-SMA, MMP-1 and TGF-β. Exo/Gel demonstrated enhanced cell migration and angiogenesis properties in vitro. Exo/Gel facilitates normal collagen deposition, angiogenesis and hair follicle chimeric regeneration in rat skin trauma models. Thus, this engineered exosome modified by hydrogels can serve as a novel strategy for wound healing dressings [70].

Additionally, the application of bioengineering technology for loading drug molecules into exosomes can endow exosomes with specific therapeutic effects. For instance, by loading specific miRNAs into exosomes to fabricate engineered exosomes, miRNAs can be delivered to keloid fibroblasts via the functionality of exosome carriers to inhibit the migration and metastasis of keloid fibroblasts, thereby effectively treating keloid. For instance, Yuan et al. incorporated miR-29a into hADSC exosomes and discovered that miR-29a could suppress the TGF-β2 and TGF-β2/Smad3 signaling pathway through targeted inhibition, thereby reducing the expression of Col-I and Col-III collagen in hypertrophic scar fibroblasts (HSFBs) and inhibiting the fibrosis and scar hyperplasia of HSFBs after scalding in mice [71]. Zhao et al. demonstrated that mesenchymal stem cell-derived exosomes (MSC-Exo) loaded with miR-138-5p suppressed the expression of SIRT1 by targeted inhibition. Reduced proliferation, migration and the expression of NF-κB, α-SMA and TGF-β1 in human skin fibroblasts (HSFs), thereby mitigating pathological scar formation. The potential function of MSC-Exo as a miRNA delivery vector in alleviating pathological scarring was verified [72]. Meng et al. encapsulated miR-141-3p into exosomes derived from adipose-derived mesenchymal stem cells to produce engineered exosomes (miR-141-3p^OE-Exos), which suppressed the TGF-β2/Smad signaling pathway by specifically targeting TGF-β2. This led to reduced expression of α-SMA, Col-I and FN in HSFs. Furthermore, the combined application of engineered exosome miR-141-3P^OE-Exos and dissolving microneedle arrays (DMNAs) technology facilitated deep penetration of miR-141-3P^OE-Exos into the skin. This approach effectively decreased hypertrophic scar thickness, improved fibroblast distribution and collagen fiber arrangement and inhibited keloid formation. Thus, miR-141-3P^OE-Exos holds significant promise as a novel therapeutic agent for keloid treatment [73].

Engineered exosomes modified through bioengineering techniques can compensate for the drawbacks of natural exosomes, such as inadequate targeting ability, a short cycle half-life and the uncertain composition and content of functional substances, thereby meeting the requirements of clinical treatment. Nevertheless, the industrial production of exosomes is still at an initial stage and it is imperative to establish large-scale production as well as separation and purification technology.

5. Conclusion

The pathological mechanism of pathological scars (including hypertrophic scars and keloids) is intricate, encompassing the aberration of multiple cytokines, signaling pathways, regulatory factors, etc. In the quest for superior scar treatment options, a multitude of diverse approaches have been utilized clinically, such as surgery, medication and radiation therapy. Nevertheless, these treatments entail certain side effects and are prone to relapse. Currently, scar treatment remains a challenge in clinical practice, particularly for hypertrophic scars and keloids, which urgently require safe, minimally invasive and less recurrent treatment methods. Currently, novel nanotechnologies such as nanomicroneedles, photodynamics mediated by novel photosensitizers, bioelectrical stimulation and 3D printed dressings have been developed for the effective treatment of pathological scars. Additionally, innovative nanoscale fillers, including nano-fat and engineered exosomes, can serve as novel therapeutic agents for the efficient treatment of pathological scars. The intervention of nanomaterials can enhance drug absorption, stabilize and safeguard the active ingredients of drugs, delay or control drug release and enhance bioavailability. This article reviews these new treatment strategies for scar to explore novel approaches for efficient and safe for keloid treatment.

6. Future perspective

The pathological mechanism of pathological scars is intricate. Nevertheless, the relevant molecular mechanism remains insufficient. It is generally acknowledged that the overactivation of fibroblast and the excessive deposition of collagen constitute the main mechanisms of pathological scar formation. TGF-β assumes a dominant role in the activation of fibroblast and other cytokines cannot be disregarded during scar formation. Thus, clarifying the mechanisms through which these cytokines regulate fibroblast activation, collagen formation and degradation will facilitate the development of specific scar treatments. Due to the dearth of representative and recognized animal models of hypertrophic scarring in humans, the knowledge regarding the cellular and molecular mechanisms underlying the development of these fibroproliferative diseases remains relatively meager. However, with the maturation of technologies such as 3D tissue culture and single-cell sequencing, it is believed that these difficulties will be overcome one after another.

To enhance the therapeutic outcome, it is indispensable to further investigate the pathogenesis of pathological scar based on comprehensive intervention in multiple manners. Exploring the cellular, molecular and genetic aspects of wound healing and scar inhibition will provide new therapeutic targets. New drug delivery systems, novel therapeutic drugs and therapeutic strategies can effectively permeate the skin, offer an extended therapeutic effect at the application site, minimize side effects and enhance the efficacy of drugs. It might even be feasible to self-administer drugs painlessly.

However, at present, some of the new therapies still lack relatively standardized treatment plans. In the future, it is necessary to further conduct multi-center clinical trials for confirmation and formulate more scientific treatment plans for pathological scar under different circumstances to achieve the optimal treatment effect. The future research direction should be a more in-depth comprehension of the formation mechanism of pathological scar and focus on the interaction between cytokines and matrix molecules at the gene level, molecular level and signaling pathway level. Secondly, the development of new precise targeted therapy should be given attention. Greater attention should be paid to the study of individual differences, skin size, age of onset, site and other factors on pathological scar formation and response to treatment, so as to achieve individualized and multi-mode combined treatment. In conclusion, the above mentioned novel therapeutic drugs and therapeutic strategies exhibit great potential in the treatment of scars and can be extensively developed and applied in the future.

Funding Statement

This work was supported by funding from Natural Science Foundation of Guangxi Zhuang Autonomous Region (No. 2024GXNSFAA010018).

Author contributions

Z Chen: investigation, methodology, formal analysis, writing – original draft, writing – review and editing; J Gao: investigation, methodology, validation, visualization; L Li: investigation, methodology, writing – review and editing. All authors contributed to the writing of the paper.

Financial disclosure

This work was supported by funding from Natural Science Foundation of Guangxi Zhuang Autonomous Region (No. 2024GXNSFAA010018). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Competing interests disclosure

The authors have no competing interests or relevant affiliations with any organization or entity with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Writing disclosure

No writing assistance was utilized in the production of this manuscript.