Dermal Fibrosis and the Current Scope of Hydrogel Strategies for Scarless Wound Healing

After over a century of advancements, hydrogels have emerged as a pivotal therapeutic approach in the field of biomedical materials and tissue engineering. Hydrogels are created through covalent and/or non-covalent cross-linking of polymeric materials, leading to the formation of complex three-dimensional (3D) network structures [64,65,66]. Hydrogels typically exhibit high water absorption capabilities, excellent biocompatibility, flexibility, adhesiveness, and porosity. Due to their ability to retain moisture at wound sites, hydrogels are an appealing option for wound dressing applications. Furthermore, they can be tailored specifically to optimize their environmental responsiveness (to factors such as temperature, pH, and light) [67,68,69,70,71,72,73]. Controlled release of incorporated bioactive substances and nanomaterials from hydrogels is an effective means to achieve a prolonged treatment effect, ideal for maintaining an anti-inflammatory, antibacterial, and antifibrotic wound site during healing [74,75,76,77], thereby having the potential to realize attenuated scar formation. For example, Chen et al. disrupted mechanotransduction signaling in activated fibroblasts and myofibroblasts to mitigate hypertrophic scarring in split-thickness skin graft-treated wounds. By loading 1 mg small-molecule FAK inhibitor VS-6062 into a pullulan hydrogel patch that covered the skin grafts, early graft contracture was avoided, and the activation of wound site fibroblasts was attenuated (>75% released within 24 h, 100% released by 96 h) [78]. Hydrogels are made from both natural and synthetic materials. Natural polymers such as hyaluronic acid (HA), gelatin (Gel), alginate, and chitosan (CS) are commonly used, along with synthetic polymers like polyvinyl alcohol (PVA) and polyethylene glycol (PEG) [79,80,81,82,83,84,85,86,87]. The choice of polymer or polymer blend depends on the desired outcome. Natural polymers typically provide bioactivity, while synthetic polymers contribute structural stability. The wide range of available polymers and advancements in hydrogel fabrication techniques have expanded their applications beyond topical wound dressings. Additionally, many polymer backbones can be easily modified to include cross-linkable side groups, such as thiol/sulfhydryl or photoinducible methacrylate (MA). These modifications provide a straightforward way to improve gelation and enhance structural properties. Owing to bioactive and biomimetic properties resembling ECM, polymeric hydrogels may function as effective cell carrier materials that are also suitable for culturing organoids and specialized cell phenotypes. Additionally, hydrogel materials have a wide range of applications (Figure 3), including scaffold coatings, bioinks, biosensors, and more [68,88,89,90,91,92]. More recently, hydrogels have been utilized as sensors for physiological monitoring, further expanding their range of applications [93,94,95,96]. In the following sections of this review, we discuss recent advancements in hydrogel fabrication techniques and their applications in attenuating dermal scars. We classify hydrogels according to their construction principles. First, we summarize recent injectable hydrogels, then lyophilized hydrogel dressings/cryo-gels, spray-on/sprayable hydrogels, 3D printed hydrogels, and hydrogel-based microneedle patches. 3.1. Injectable Hydrogels Subcutaneous injection of hydrogels aims to achieve gelation in situ, forming a hydrogel network that regulates the surrounding tissue microenvironment and provides a reservoir for the controlled release of bioactive. Injectability enables therapeutic reach to deep tissues, and adhesive properties allow the filling of irregular wound spaces without the need for sutures, which traditional sheet-like hydrogels may struggle to achieve. Injectable hydrogels greatly reduce the need for invasive surgeries. Injectability requires certain key characteristics, such as self-healing and shear-thinning properties [97]. A variety of chemical and physical crosslinking techniques have been employed to create injectable hydrogels that gel under certain conditions. Crosslinking between the polymers can be induced by physical stimuli through noncovalent interactions like hydrophobic and ionic bonds. Chemical crosslinking of polymers is achieved through covalent bonds formed via coupling processes such as photoirradiation, Schiff base crosslinking, Michael-type addition, thiol exchange/disulfide crosslinking, and click chemistry. In most applications, injectable hydrogels are initially prepared in the solution state and eventually transition into the semi-solid gel state upon injection into the host due to external stimuli, of which a popularized stimulus is fast gelation at body temperature (>37 °C). After mixing or crosslinking of injectable hydrogel components, based on their unique shear thinning and self-healing properties, the generation of high strain through high shear force during extrusion from the syringe temporarily fluidizes the hydrogel before it returns to a gel state again within a short time after injection [98,99,100]. Selection of pre-gelling components is necessary to achieve the injectability of hydrogels with wet tissue adhesiveness, rapid gelling, and the capacity to achieve timely hemostasis. For example, Zhao et al. prepared injectable hydrogel adhesives by combining poly(citric acid-co-polyethylene glycol)-g-dopamine prepolymers and aminocapsule-terminated Pluronic F127 micelles loaded with astragaloside IV (<30% released within 24 h; sustained release reached 80% within 300 h). The researchers used an H₂O₂/horseradish peroxidase system to cross-link catechol groups in polydopamine (PDA)/dopamine (DA) via oxidative coupling. This system also facilitated the chemical crosslinking of catechol to the micelle amino groups. In a methicillin-resistant Staphylococcus aureus (MRSA)-infected mouse model of full-thickness skin defects, the treatment group showed less inflammatory cell infiltration, a relatively mild inflammatory response, with low levels of TNF-α and high levels of IL-10. The regeneration of skin tissue by the injected hydrogel adhesives exhibited dense collagen I/III deposition with optimal directional alignment and timely revascularization of the wound site, thereby achieving scar-reduced wound repair. Notably, hydrogels containing astragaloside IV accelerated healing with minimal scarring, while also promoting the regeneration of functional skin appendages [101]. In another study, Zhang et al. used an innovative approach wherein the imine-based photo-induced crosslinking of injectable hydrogels could be realized as a delayed or pulsatile drug release platform. HA/o-nitrobenzene and HA/carbohydrazide were used as the pre-gelling polymers to deliver o-nitrobenzene modified poly(lactic-co-glycolic) acid (PLGA) capsules loaded with 0.01% w/w TGF-β inhibitors. In a mouse model of full-thickness skin defect, wounds showed accelerated closure and the delayed release of TGF-β inhibitor from PLGA capsules (none released <6–8 days, 100% released <8–10 days depending on molecular weight). The result was significantly reduced collagen deposition, decreased inflammation levels, and inhibited tissue fibrosis, compared to a non-pulsatile HA hydrogel control. The team went on to demonstrate attenuated scar formation in a rabbit ear full-thickness wound model and a porcine full-thickness excision wound model [102]. The inherent advantage of hydrogels is their capacity to serve as cytocompatible carrier materials. Zhang et al. prepared injectable microgels composed of aligned silk nanofibers that enhanced the paracrine secretion of beneficial factors from the loaded bone-marrow mesenchymal stromal cells (BMSCs; 4 × 10⁶ cells/mL) to influence the surrounding tissue microenvironment. Scarless healing with accelerated hair follicle recovery was successfully achieved in a rat excisional wound model. Treatment promoted angiogenesis and regulated the inflammatory response to facilitate skin regeneration [103]. A potential weakness of injectable hydrogels is their potentially rapid degradation in vivo, accompanied by fast release of payloads. To address this issue, Griffin et al. prepared microporous annealed particle (MAP) microgel scaffolds composed of crosslinked L- and D-peptides to study the influence of chirality on material degradation. In murine wound models, there were no differences in wound closure time between mixed L/D-MAP, but MAP doped with D-chiral peptides showed anti-degradation properties. Interestingly, D-MAP promoted the formation of hair follicles and enhanced tissue tensile strength, akin to embryonic-like tissue regeneration. Furthermore, D-MAP was shown to trigger antigen-specific responses while enhancing bone marrow cell recruitment through adaptive humoral immunity, a mechanism that induced skin regeneration depends on [65]. Endowing polymers with bioadhesion, such as by modification with PDA, remains a popular strategy to achieve hydrogel crosslinking and rapidly form stress-resistant wound seals. Deng et al. designed an injectable multifunctional hydrogel loaded with therapeutic PDA nanoparticles encapsulating 0.182 ± 0.043% w/w asiaticoside. By using dynamic Schiff base crosslinking between oxidized dextran and quaternary ammonium chitosan, reinforced by crosslinking between DA-modified oxidized dextran and DA-modified reduced graphene oxide, the researchers engineered a multifunctional hydrogel that exhibited tissue adhesion, self-healing, antibacterial and antioxidant properties. Furthermore, electrical conductivity granted by the incorporated graphene oxide promoted cell migration and proliferation. In a rat full-thickness skin defect model, the developed hydrogel effectively blocked fibrosis while stimulating collagen synthesis, and this antifibrotic effect was further enhanced by asiaticoside mitigation of myofibroblast activity. The combination of reduced oxidative stress, accelerated cell proliferation and neoangiogensis, and reduced inflammation, promoted scarless skin tissue with skin appendages [104]. 3.2. Cryo-Gels Cryogels, formed by repeated freezing and thawing of polymer solutions or hydrogels, address the need for sponge-like polymer scaffolds with adjustable pore sizes and tailored porosity [105]. Additionally, based on the appropriate selection of polymer base material, cryo-gels can have excellent compressibility, elasticity, and swelling, thereby meeting ideal properties for wound dressing materials [106]. Liu et al. constructed a double network cryo-gel consisting of PVA and agarose loaded with 1% w/w hyperbranched polylysine and 0.8% w/w tannic acid, in which PVA formed a primary physical crosslinking network after repeated freezing and thawing, and agarose formed a secondary physical crosslinking network through hydrogen bonding. In a rabbit ear MRSA-infected wound repair model, the released components from cryo-gels (~50% hyperbranched polylysine within 24 h, total release <70%; ~30% tannic acid within 24 h, total release <50%) significantly reduced local tissue inflammation, reduced collagen deposition, modulated collagen type I:III ratios, down-regulated α-SMA production, and decreased scar thickness [107]. Following their design of an antifibrotic peptide that competes with integrins to bind EDA-fibronectin and prevent mechanotransduction signaling [108], Zhang et al. synthesized genipin-crosslinked hydrogel networks consisting of carboxymethyl CS, poly-γ-glutamic acid, and the antifibrotic peptide (~20 μg loaded per dressing). Lyophilization of the hydrogel resulted in a porous dressing biomaterial that exhibited ideal wound dressing properties such as hemostasis, antibacterial activity, and wound exudate absorption. Swelling and subsequent degradation of cryo-gel dressings achieved the controlled release of the antifibrotic peptide (~40% within 8 h; after which the release rate matched the dressing degradation rate) to mitigate hypertrophic scar formation and promote the regeneration of auxiliary skin structures in a rabbit ear model of full-thickness skin defects [109]. Notably, the hydrogel components selected for their wound-healing properties also demonstrated a mild antifibrotic effect by disrupting integrin signaling. This effect was further enhanced by the synergistic inhibitory actions of the released peptide. Fan et al. constructed a hybrid fibrous scaffold with vertically aligned Gel-MA cryo-gel fibers and randomly aligned fibroin fibers, to which 1% w/w anti-inflammatory peptide 1 (AP-1) was coupled. In a mouse back wound model, the treated group had a reduced inflammatory state and epidermal thickness, whereas collagen composition and densities were similar to normal skin. Additionally, a higher number of regenerated hair follicles were present in the treatment groups [110]. The base material of Gel-MA with fibroin also demonstrated an antifibrotic effect, offering potential routes of investigation into optimizing antifibrotic effects based on material design. fibers Ying et al. prepared a multifunctional aloin-arginine-alginate cryo-gel that transforms back into a hydrogel upon absorbance of wound exudate or blood for cutaneous regeneration. In a mouse model of S. aureus infected dorsal wounds, the cryogel treatment group demonstrated antibacterial and anti-inflammatory properties, accelerated wound healing by promoting angiogenesis, and achieved scar-attenuated healing with evidence of skin appendage regeneration [111]. 3.3. Spray-on/Paste-on Hydrogels Spray-on and paste-on hydrogels represent an innovative advancement, offering ease of use for treating wounds in challenging or hard-to-reach locations. Chen et al. constructed HA-modified and verteporfin (VP)-loaded polylactide nanogels (2 µg/mL VP per 4 mg/mL polylactide) to promote scarless wound healing after paste-on application to the wound site. The nanogel release of HA and lactic acid (within first 6 h) accelerated wound re-epithelialization, and the released VP (~28.3% within 12 h) inhibited YAP to suppress myofibroblast driven fibrosis without hindering the proliferation and migration potential of skin fibroblasts, which ultimately limited scar formation in a rabbit ear injury model [112]. Spray-on hydrogels can be applied by syringe and/or jet devices to rapidly form in situ protective layers on irregular and large deformation of wounds [113]. Sprayable hydrogels have gained attention in the field of wound healing and anti-scarring due to their ease of application and potential therapeutic properties. One study successfully prepared a sprayable hydrogel containing approximately 128 μg/mL Ni₃(HITP)₂ nanorods with antioxidant and anti-inflammatory properties, accelerating wound healing [114]. Spray-on hydrogels have shown promise in the field of anti-fibrosis topical therapies. Tan et al. developed a spray-on hydrogel containing 0.01% w/w insulin-like growth factor-1 (IGF1). The hydrogel was composed of caffeic acid-modified CS, hydroxypropyl CS, and oxidized dextran. Under the acidic conditions of the wound site, the pH responsive dynamic Schiff base network released IGF1 (~88% within 12 h, ~98% within 48 h) to achieve therapeutic effects. In a mouse model of bacterial infection with full-thickness skin defects, the treatment group exhibited a more mature collagen deposition and alignment, an abundance of hair follicle regeneration, reduced inflammation levels, and the promoted formation of new blood vessels, which convened to realize scar free healing [115]. Chen et al. designed a HA combined with lyotropic liquid crystal (LLC)-based spray dressing loaded with the antifibrotic drug pirfenidone (PFD, 0.5% w/w). Based on the properties of LLC, after the spray comes into contact with the wound, a phase change is triggered, causing the spray to gel and prolonging the release time of PFD (~75% of total drug release over 48 h). In a mouse burn wound model, the treatment group showed decreases in inflammation and interleukin (IL)-6 expression, whereas increases in neovascularization and vascular endothelial growth factor (VEGF) expression, together with stimulated collagen synthesis at appropriate collagen ratios, cumulating in scar prevention [116]. Spray-on hydrogels form thin, rapidly gelling layers, enabling the flexible application of multiple layered films to cover the wound site and provide distinct, targeted actions. Yang et al. were inspired by the structure and function of natural skin and prepared a sprayable biomimetic double mask (BDM) with rapid autophasing and hierarchical programming. The sprayable BDM consists of a bottom layer including hydrophilic Gel-MA hydrogel with calcium ion (Ca²⁺) incorporation and a top layer of hydrophobic poly (lactide-co–propylene glycol–co-lactide) dimethacrylate incorporating the antimicrobial drug triclosan (5% w/w). Following spray-on application, the two layers rapidly autophase into bilayered structures and are then bonded and solidified by photocrosslinking. The bottom GelMA layer releases Ca²⁺ to achieve hemostasis, while the top PLD layer maintains a wet, gas-permeable, and antimicrobial environment. In both full-thickness rat skin wound models and the full-thickness porcine skin wound models, the prolonged release of triclosan (over 14 days) promoted neovascularization, hair follicle regeneration, and modulated collagen deposition to enhance scar-free wound healing [117]. Zhong et al. prepared a biodegradable combined multi-functional spray-on hydrogel from select raw materials: a CS-MA (antibacterial) and 0.8% w/v ferulic acid (adhesive) layer, and an oxidized Bletilla striata polysaccharide (hemostasis, anti-inflammatory) layer. The spray-on hydrogel was prepared by dynamic Schiff bond cross-linking and cured by UV-photocrosslinking. Although the extent of dermal fibrosis quality was not assessed in detail, in both a rat model of S. aureus infected skin defect and miniature pig trauma wound model, the spray-on hydrogel system released ~50% ferulic acid over 24 h to achieve improved wound closure, reduced inflammation levels, and enhanced regeneration of skin appendages [118]. 3.4. 3D Bioprinting of Hydrogels In addition to the aforementioned methods of composition and use, hydrogels still have many possibilities. Attempts to combine hydrogels with unconventional construction methods such as 3D printing or microneedles have breathed new life into them. 3D printing technology enables the precise fabrication of various materials to meet diverse needs, fulfilling the high demands for geometric complexity in tissue engineering and clinical customization. Appropriate hydrogel materials can be produced through different 3D printing techniques (extrusion, photopolymerization, ultrasound, etc.) and applied as bionic/artificial skin, hierarchical wound dressings, tissue bonding adhesives, bioelectronic interfaces, and cell-laden scaffold materials, or used to study microtissue/organoid formation or disease model construction [89,119,120,121,122,123,124,125]. Taking advantage of the capacity of 3D bioprinting to produce hierarchical and multilayered structural hydrogels, Chen et al. developed a 3D-printed bilayer hydrogel using a dermal ECM-mimicking upper layer of Gel-MA and SF-MA doped with 100 μg/mL copper-epigallocatechin gallate as a lower layer and an epidermal-mimicking Gel-MA/HA-MA upper layer seeded with keratinocytes (1 × 10⁶ cells/mL). In a rat model of burn injury, copper-epigallocatechin gallate release from the 3D printed hydrogels (~55% within 3 days, ~75% within 6 days) attenuated the inflammatory response, and in combination with the seeded keratinocytes, improved regenerated dermal thickness with a type I:III collagen ratio resembling healthy skin after 2 weeks [126]. Liang et al. used 3D printing to develop antibacterial and piezoelectric scaffolds comprising zinc oxide (ZnO, 5% w/w) nanoparticle-modified polyvinylidene fluoride (PVDF)/sodium alginate. The constructs offered piezoelectric release models from both vertical swelling and horizontal friction, facilitating bioelectrical stimulation signals that enhanced cell migration, neovascularization, collagen alignment, and growth factor release that promoted scarless healing in a rat model of full thickness skin dorsal defects [127]. Notably, PVDF/sodium alginate control hydrogels also exhibited scar-reduced wound healing, suggesting that such a polymer blend may influence pro-fibrotic cell phenotypes. 3.5. Microneedle Patch Hydrogels Microneedle patches, consisting of arrays of microneedles typically ranging from 100 to 2000 μm in length, serve as effective tools for dermal or in vivo drug delivery and monitoring. For dermal treatment, as an emerging tool for painless penetration of the stratum corneum, microneedles are non-invasive with diverse loading capabilities. They can be widely used in a variety of applications such as transdermal drug delivery, stimuli-triggered degradation, disease monitoring, physiological sensing, tissue fluid sampling, and vaccination [128,129,130,131,132]. Microneedles patches that use hydrogels as the main body have emerged as a popular strategy for topical pathologies. Hydrogel compositions allow for diversity in terms of preparation materials, microneedle structural design (core-shell structure, bilayer structure, bio-inspired, etc.), and payload release choices, with the focus on suiting the needs of the corresponding applications [133,134,135]. For example, Yang et al. sought to improve the controlled release of drugs with poor bioavailability into the hypertrophic scar tissue microenvironment to promote its remodeling. They synthesized microneedle patches from Gel-MA and 100 mM 5-Fluorouracil acetic acid (5-FuA) precursors that responded to high levels of reactive oxygen species (ROS) and matrix metalloproteinase (MMP)-2/9 overexpression associated with activated fibroblasts in the wound and scar sites to achieve the controlled release of approximately 75% 5-FuA within 24 h of patch application [136]. In a rabbit ear model of an established hypertrophic scar, 5-FuA delivery promoted the apoptosis of activated fibroblasts and myofibroblasts, while promoting inflammatory and keratinocyte associated pathways and their interactions with fibroblasts, which controlled excessive collagen deposition during remodeling of the dermal scar. Targeting of YAP in fibroblast activation and differentiation into myofibroblasts has been demonstrated to be a promising strategy in the temporal control over scar development. Zhang et al. prepared microneedles with a core-shell structure consisting of a ROS-sensitive PVA shell loaded with 3 μg/mL VP and a crosslinked heparin core in combination with photodynamic therapy for inhibition of YAP and promoting scar-free healing and scar repair of back wounds in mice. The release of ~90% VP within the first 24 h resulted in a decreased dermal thickness, reduced collagen deposition, and a lower proportion of type I/III collagen in the tissue [137]. Liu and colleagues developed a reactive oxygen species (ROS)-responsive, oxygen-generating microneedle patch using a glucose-sensitive sericin/hyaluronic acid (HA) hydrogel loaded with VP. In the highly oxidative infected wound site microenvironment, dopamine functionalized sericin depletes ROS to generate oxygen to alleviate hypoxia, promote angiogenesis, and attenuate the expression of proinflammatory cytokines. Additionally, total VP release within the first 20 h provided sustained inhibition of the YAP/Engrailed-1 axis to assist in the enhanced scarless re-epithelization of diabetic wound models [138]. Other groups capitalized on the incorporation of cell-inspired therapeutic strategies into their fabricated microneedle patch materials. Li et al. developed a novel microneedle patch from 5% w/w adipose-derived stem cell (ADSC) secretome conditioned medium cross-linked with keratin and loaded with 1.5% w/w triamcinolone acetonide (TA). The differential and dual release of ADSC secretome (50% over 5 days) and TA (80% in the first 16 h) synergistically reduced inflammation and ROS while regulating myofibroblast and hyperplastic scar fibroblast behavior. In a rabbit ear scar model, the microneedle patches effectively reduced scar formation and facilitated the regeneration of healthy skin [139]. Liu et al. loaded CAR-TREM2-macrophages targeting DPP4⁺ fibroblasts (1 × 10⁷ cells/mL) into microporous PLGA/PEG-DA/CaCO₃ hydrogel microneedle patches. Detailed analysis revealed that the microneedle patches attenuated EndMT processes by suppressing leucine-rich-α2-glycoprotein 1 (LRG1). Additionally, CAR-TREM2-macrophages phagocytosed DPP4⁺ fibroblasts and suppressed TGF-β secretion, demonstrating effective scar therapy in microneedle patch-treated mouse dorsal scars [140].