Overview of Silk Fibroin Use in Wound Dressings

LBL deposition of chitosan/heparin bilayers for improving biological ability and reducing infection of nanofibers

The complexity of cardiovascular disease requires that the materials for preparing vascular grafts possess good biocompatibility, high mechanical property, and even some excellent additional properties. In this study, polycaprolactone (PCL) with good mechanical property and natural source silk fibroin (SF) were electrospun into PCL/SF nanofibers to obtain the nanofibrous substrate. With the addition of SF, the mechanical property of PCL/SF nanofibrous mats was maintained to a certain extent. While, the hydrophilicity of PCL/SF nanofibrous mats was greatly improved which is more suitable for immersive layer-by-layer assembly (LBL). The oppositely charged heparin (Hep) and chitosan (CS) were alternatively deposited on the surface of PCL/SF nanofibers via LBL. After implanting human umbilical vein endothelial cells (HUVEC) on the LBL-structured nanofibrous mats for 48 h, it was confirmed that the CS/Hep bilayers enhanced the biocompatibility of the nanofibers. Furthermore, the results of the antibacterial test showed that the antibacterial effects of the LBL-structured nanofibrous mats for Escherichia coli and Staphylococcus aureus were both achieved 95% when the number of Hep/CS bilayer was 10. It can be demonstrated that the LBL-structured nanofibrous mats with improved biocompatibility and reduced infectivity had been prepared successfully, and can be potentially used in vascular grafts.

Nonmulberry Silk Based Ink for Fabricating Mechanically Robust Cardiac Patches and Endothelialized Myocardium-on-a-Chip Application

Bioprinting holds great promise towards engineering functional cardiac tissue constructs for regenerative medicine and as drug test models. However, it is highly limited by the choice of inks that require maintaining a balance between the structure and functional properties associated with the cardiac tissue. In this regard, we have developed a novel and mechanically robust biomaterial-ink based on non-mulberry silk fibroin protein. The silk-based ink demonstrated suitable mechanical properties required in terms of elasticity and stiffness (~40 kPa) for developing clinically relevant cardiac tissue constructs. The ink allowed the fabrication of stable anisotropic scaffolds using a dual crosslinking method, which were able to support formation of aligned sarcomeres, high expression of gap junction proteins as connexin-43, and maintain synchronously beating of cardiomyocytes. The printed constructs were found to be non-immunogenic in vitro and in vivo. Furthermore, delving into an innovative method for fabricating a vascularized myocardial tissue-on-a-chip, the silk-based ink was used as supporting hydrogel for encapsulating human induced pluripotent stem cell derived cardiac spheroids (hiPSC-CSs) and creating perfusable vascularized channels via an embedded bioprinting technique. We confirmed the ability of silk-based supporting hydrogel towards maturation and viability of hiPSC-CSs and endothelial cells, and for applications in evaluating drug toxicity.

Biopolymer Microparticles Prepared by Microfluidics for Biomedical Applications

Biopolymers are macromolecules that are derived from natural sources and have attractive properties for a plethora of biomedical applications due to their biocompatibility, biodegradability, low antigenicity, and high bioactivity. Microfluidics has emerged as a powerful approach for fabricating polymeric microparticles (MPs) with designed structures and compositions through precise manipulation of multiphasic flows at the microscale. The synergistic combination of materials chemistry afforded by biopolymers and precision provided by microfluidic capabilities make it possible to design engineered biopolymer-based MPs with well-defined physicochemical properties that are capable of enabling an efficient delivery of therapeutics, 3D culture of cells, and sensing of biomolecules. Here, an overview of microfluidic approaches is provided for the design and fabrication of functional MPs from three classes of biopolymers including polysaccharides, proteins, and microbial polymers, and their advances for biomedical applications are highlighted. An outlook into the future research on microfluidically-produced biopolymer MPs for biomedical applications is also provided.

Potential application of liposomal nanodevices for non-cancer diseases: an update on design, characterization and biopharmaceutical evaluation

Liposomes, lipid-based vesicular systems, have attracted major interest as a means to improve drug delivery to various organs and tissues in the human body. Recent literature highlights the benefits of liposomes for use as drug delivery systems, including encapsulating of both hydrophobic and hydrophilic cargos, passive and active targeting, enhanced drug bioavailability and therapeutic effects, reduced systemic side effects, improved cargo penetration into the target tissue and triggered contents release. Pioneering work of liposomes researchers led to introduction of long-circulating, ligand-targeted and triggered release liposomes, as well as, liposomes containing nucleic acids and vesicles containing combination of cargos. Altogether, these findings have led to widespread application of liposomes in a plethora of areas from cancer to conditions such as cardiovascular, neurologic, respiratory, skin, autoimmune and eye disorders. There are numerous review articles on the application of liposomes in treatment of cancer, which seems the primary focus, whereas other diseases also benefit from liposome-mediated treatments. Therefore, this article provides an illustrated detailed overview of liposomal formulations, in vitro characterization and their applications in different disorders other than cancer. Challenges and future directions, which must be considered to obtain the most benefit from applications of liposomes in these disorders, are discussed.

Preparation of Electrospun Gelatin Mat with Incorporated Zinc Oxide/Graphene Oxide and Its Antibacterial Activity

Current wound dressings have poor antimicrobial activities and are difficult to degrade. Therefore, biodegradable and antibacterial dressings are urgently needed. In this article, we used the hydrothermal method and side-by-side electrospinning technology to prepare a gelatin mat with incorporated zinc oxide/graphene oxide (ZnO/GO) nanocomposites. The resultant fibers were characterized by field emission environment scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). Results indicated that the gelatin fibers had good morphology, and ZnO/GO nanocomposites were uniformly dispersed on the fibers. The loss of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) viability were observed to more than 90% with the incorporation of ZnO/GO. The degradation process showed that the composite fibers completely degraded within 7 days and had good controllable degradation characteristics. This study demonstrated the potential applicability of ZnO/GO-gelatin mats with excellent antibacterial properties as wound dressing material.

Hyaluronic Acid (HA)-Based Silk Fibroin/Zinc Oxide Core-Shell Electrospun Dressing for Burn Wound Management

Burn injuries represent a major life-threatening event that impacts the quality of life of patients, and places enormous demands on the global healthcare systems. This study introduces the fabrication and characterization of a novel wound dressing made of core-shell hyaluronic acid-silk fibroin/zinc oxide (ZO) nanofibers for treatment of burn injuries. The core-shell configuration enables loading ZO-an antibacterial agent-in the core of nanofibers, which in return improves the sustained release of the drug and maintains its bioactivity. Successful formation of core-shell nanofibers and loading of zinc oxide are confirmed by transmission electron microscopy, Fourier-transform infrared spectroscopy, and energy dispersive X-ray. The antibacterial activity of the dressings are examined against Escherichia coli and Staphylococcus aureus and it is shown that addition of ZO improves the antibacterial property of the dressing in a dose-dependent fashion. However, in vitro cytotoxicity studies show that high concentration of ZO (>3 wt%) is toxic to the cells. In vivo studies indicate that the wound dressings loaded with ZO (3 wt%) substantially improves the wound healing procedure and significantly reduces the inflammatory response at the wound site. Overall, the dressing introduced herein holds great promise for the management of burn injuries.

Silk biomaterials in wound healing and skin regeneration therapeutics: From bench to bedside

Silk biomaterials are known for biomedical and tissue engineering applications including drug delivery and implantable devices owing to their biocompatible and a wide range of ideal physico-chemical properties. Herein, we present a critical overview of the progress of silk-based matrices in skin regeneration therapeutics with an emphasis on recent innovations and scientific findings. Beginning with a brief description of numerous varieties of silks, the review summarizes our current understanding of the biological properties of silk that help in the wound healing process. Various silk varieties such as silkworm silk fibroin, silk sericin, native spider silk and recombinant silk materials have been explored for cutaneous wound healing applications from the past few decades. With an aim to harness the regenerative properties of silk, numerous strategies have been applied to develop functional bioactive wound dressings and viable bio-artificial skin grafts in recent times. The review examines multiple inherent properties of silk that aid in the critical events of the healing process such as cell migration, cell proliferation, angiogenesis, and re-epithelialization. A detailed insight into the progress of silk-based cellular skin grafts is also provided that discusses various co-culture strategies and development of bilayer and tri-layer human skin equivalent under in vitro conditions. In addition, functionalized silk matrices loaded with bioactive molecules and antibacterial compounds are discussed, which have shown great potential in treating hard-to-heal wounds. Finally, clinical studies performed using silk-based translational products are reviewed that validate their regenerative properties and future applications in this area. STATEMENT OF SIGNIFICANCE: The review article discusses the recent advances in silk-based technologies for wound healing applications, covering various types of silk biomaterials and their properties suitable for wound repair and regeneration. The article demonstrates the progress of silk-based matrices with an update on the patented technologies and clinical advancements over the years. The rationale behind this review is to highlight numerous properties of silk biomaterials that aid in all the critical events of the wound healing process towards skin regeneration. Functionalization strategies to fabricate silk dressings containing bioactive molecules and antimicrobial compounds for drug delivery to the wound bed are discussed. In addition, a separate section describes the approaches taken to generate living human skin equivalent that have recently contributed in the field of skin tissue engineering.

Natural Nanofiber Shuttles for Transporting Hydrophobic Cargo into Aqueous Solutions

Hydrophobic biomolecules realize their functions in vivo in aqueous environments, often through a delicate balance of amphiphilicity and chaperones. Introducing exogenous hydrophobic biomolecules into in vivo aqueous systems is a challenge in drug delivery and regenerative medicine, where labile linkers, carriers, and fusions or chimeric molecules are often designed to facilitate such aqueous interfaces. Here, we utilize naturally derived silk nanofiber shuttles with the capacity to transport hydrophobic cargos directly into aqueous solutions. These nanofibers disperse in organic solvents and in aqueous solutions because of their inherent amphiphilicity, with enriched hydrophobicity and strategically interspersed negatively charged groups. Hydrophobic molecules loaded on these shuttles in organic solvent-water systems separated from the solvent after centrifugation. These concentrated hydrophobic molecule-loaded nanofibers could then be dispersed into aqueous solution directly without modification. These shuttle systems were effective for different hydrophobic molecules such as drugs, vitamins, and dyes. Improved biological stability and functions of hydrophobic cargos after loading on these nanofibers suggest potential applications in drug delivery, cosmetology, medical diagnosis, and related health fields, with a relatively facile process.

Incorporation of ROS-Responsive Substance P-Loaded Zeolite Imidazolate Framework-8 Nanoparticles into a Ca2+-Cross-Linked Alginate/Pectin Hydrogel for Wound Dressing Applications

PURPOSE

Wound healing, especially of extensive full-thickness wounds, is one of the most difficult problems in clinical studies. In this study, we prepared a novel substance P (SP)-delivery system using zeolite imidazolate framework-8 (ZIF-8) nanoparticles.

METHODS

We synthesized ZIF-8 nanoparticles using a modified biomimetic mineralization method. We then coated SP-loaded ZIF-8 nanoparticles (SP@ZIF-8) with polyethylene glycol-thioketal (PEG-TK) to fabricate SP@ZIF-8-PEG-TK nanoparticles, and encapsulated them in injectable hydrogel composed of sodium alginate and pectin and cross-linked using calcium chloride. The final hydrogel wound dressing containing SP@ZIF-8-PEG-TK nanoparticles was called SP@ZIF-8-PEG-TK@CA.

RESULTS

The fabricated ZIF-8 nanoparticles had high SP-loading efficiency. SP-release assay showed that the SP@ZIF-8-PEG-TK nanoparticles maintained drug activity and showed responsive release under stimulation by reactive oxygen species. The SP@ZIF-8-PEG-TK nanoparticles promoted proliferation of human dermal fibroblasts, up-regulated expression levels of inflammation-related genes in macrophages, and exhibited favorable cytocompatibility in vitro. Full-thickness excision wound models in vivo confirmed that SP@ZIF-8-PEG-TK@CA dressings had excellent wound-healing efficacy by promoting an early inflammatory response and subsequent M2 macrophage polarization in the wound-healing process.

CONCLUSION

In conclusion, these findings indicated that SP@ZIF-8-PEG-TK@CA dressings might be useful for wound dressing applications in the clinic.

Wound healing and antimicrobial effect of active secondary metabolites in chitosan-based wound dressings: A review

Wound healing can lead to complex clinical problems, hence finding an efficient approach to enhance the healing process is necessary. An ideal wound dressing should treat wounds at reasonable costs, with minimal inconveniences for the patient. Chitosan is one of the most investigated biopolymers for wound healing applications due to its biocompatibility, biodegradability, non-toxicity, and antimicrobial activity. Moreover, chitosan and its derivative have attracted numerous attentions because of the accelerating wound healing, and easy processability into different forms (gels, foams, membranes, and beads). All these properties make chitosan-based materials particularly versatile and promising for wound dressings. Besides, secondary natural metabolites could potentially act like the antimicrobial and anti-inflammatory agents and accelerate the healing process. This review collected almost all studies regarding natural compounds applications in wound healing by focusing on the chitosan-based bioactive wound dressing systems. An accurate analysis of different chitosan formulations and the influence of bioactive compounds on their wound healing properties are reported.

Silk fibroin for skin injury repair: Where do things stand?

Several synthetic and natural materials are used in soft tissue engineering and regenerative medicine with varying degrees of success. Among them, silkworm silk protein fibroin, a naturally occurring protein-based biomaterial, exhibits many promising characteristics such as biocompatibility, controllable biodegradability, tunable mechanical properties, aqueous preparation, minimal inflammation in host tissue, low cost and ease of use. Silk fibroin is often used alone or in combination with other materials in various formats and is also a promising delivery system for bioactive compounds as part of such repair scenarios. These properties make silk fibroin an excellent biomaterial for skin tissue engineering and repair applications. This review focuses on the promising characteristics and recent advances in the use of silk fibroin for skin wound healing and/or soft-tissue repair applications. The benefits and limitations of silk fibroin as a scaffolding biomaterial in this context are also discussed. STATEMENT OF SIGNIFICANCE: Silk protein fibroin is a natural biomaterial with important biological and mechanical properties for soft tissue engineering applications. Silk fibroin is obtained from silkworms and can be purified using alkali or enzyme based degumming (removal of glue protein sericin) procedures. Fibroin is used alone or in combination with other materials in different scaffold forms, such as nanofibrous mats, hydrogels, sponges or films tailored for specific applications. The investigations carried out using silk fibroin or its blends in skin tissue engineering have increased dramatically in recent years due to the advantages of this unique biomaterial. This review focuses on the promising characteristics of silk fibroin for skin wound healing and/or soft-tissue repair applications.

Alginate hydrogel enriched with Ambystoma mexicanum epidermal lipoxygenase-loaded pectin nanoparticles for enhanced wound healing

Epidermal lipoxygenase enzyme extracted from Ambystoma mexicanum (AmbLOXe) is known to accelerate the wound-healing process. AmbLOXe as a protein suffers from inactivation and losing its activity during formulation. Therefore, a delivery system that protects AmbLOXe from inactivation and preserves its activity is needed. We prepared AmbLOXe-loaded pectin nanoparticles (AmbLOXe Pec-NPs) and placed them into an alginate hydrogel. AmbLOXe Pec-NPs incorporation into the alginate hydrogel provides a means for controlled and sustained delivery of AmbLOXe to the wound site. Furthermore, the suitable swelling behavior and mechanical properties of AmbLOXe Pec-NPs alginate hydrogel make it feasible for clinical use. AmbLOXe Pec-NPs alginate hydrogel significantly enhanced the wound-healing process on the rat full-thickness excisional wounds, increased the rate of wound closure, enhanced the re-epithelialization and decreased the incidence of abnormal scarring. AmbLOXe Pec-NPs alginate hydrogel can be proposed as an effective wound hydrogel for improving wound healing with minimal scarring.

Silver crosslinked injectable bFGF-eluting supramolecular hydrogels speed up infected wound healing

Topical wound dressings with various silver compositions that exhibit effective bacterial inhibition properties are often used to treat infected wounds. However, a silver dressing with no bioactive functionality will typically delay subsequent wound repair processes. Therefore, development of a simple wound dressing containing silver and loaded with a bioactive drug is a very attractive solution. Herein, we developed a silver crosslinked injectable chitosan-silver hydrogel as a silver immobilization matrix, loaded with basic fibroblast growth factor (bFGF) as its cargo (namely, bFGF@CS-Ag) for treatment of both acute and infected wounds. The in vivo results showed that bFGF@CS-Ag significantly enhanced infectious wound regeneration compared to that of acute wounds. Further investigation demonstrated that the improved wound repair by bFGF@CS-Ag was ascribed to the effectiveness of bacterial inhibition, the promotion of granulation formation, collagen deposition, neovascularization and re-epithelization, and to the reduction of the inflammatory response through promotion of M2 macrophage polarization. These results proved that the immobilization of silver in the hydrogel not only reduced the side effects of silver on the bioactivity of bFGF but also allowed elution of bFGF in a controlled release manner. Thus, this novel system has promising therapeutic potential for topical treatment of wounds.

Bioengineered Skin Substitutes: the Role of Extracellular Matrix and Vascularization in the Healing of Deep Wounds

The formation of severe scars still represents the result of the closure process of extended and deep skin wounds. To address this issue, different bioengineered skin substitutes have been developed but a general consensus regarding their effectiveness has not been achieved yet. It will be shown that bioengineered skin substitutes, although representing a valid alternative to autografting, induce skin cells in repairing the wound rather than guiding a regeneration process. Repaired skin differs from regenerated skin, showing high contracture, loss of sensitivity, impaired pigmentation and absence of cutaneous adnexa (i.e., hair follicles and sweat glands). This leads to significant mobility and aesthetic concerns, making the development of more effective bioengineered skin models a current need. The objective of this review is to determine the limitations of either commercially available or investigational bioengineered skin substitutes and how advanced skin tissue engineering strategies can be improved in order to completely restore skin functions after severe wounds.

Silk Fibroin-Based Biomaterials for Biomedical Applications: A Review

Since it was first discovered, thousands of years ago, silkworm silk has been known to be an abundant biopolymer with a vast range of attractive properties. The utilization of silk fibroin (SF), the main protein of silkworm silk, has not been limited to the textile industry but has been further extended to various high-tech application areas, including biomaterials for drug delivery systems and tissue engineering. The outstanding mechanical properties of SF, including its facile processability, superior biocompatibility, controllable biodegradation, and versatile functionalization have allowed its use for innovative applications. In this review, we describe the structure, composition, general properties, and structure-properties relationship of SF. In addition, the methods used for the fabrication and modification of various materials are briefly addressed. Lastly, recent applications of SF-based materials for small molecule drug delivery, biological drug delivery, gene therapy, wound healing, and bone regeneration are reviewed and our perspectives on future development of these favorable materials are also shared.

Review of Polymeric Materials in 4D Printing Biomedical Applications

The purpose of 4D printing is to embed a product design into a deformable smart material using a traditional 3D printer. The 3D printed object can be assembled or transformed into intended designs by applying certain conditions or forms of stimulation such as temperature, pressure, humidity, pH, wind, or light. Simply put, 4D printing is a continuum of 3D printing technology that is now able to print objects which change over time. In previous studies, many smart materials were shown to have 4D printing characteristics. In this paper, we specifically review the current application, respective activation methods, characteristics, and future prospects of various polymeric materials in 4D printing, which are expected to contribute to the development of 4D printing polymeric materials and technology.

Current Applications of Biopolymer-based Scaffolds and Nanofibers as Drug Delivery Systems

BACKGROUND

The high surface-to-volume ratio of polymeric nanofibers makes them an effective vehicle for the release of bioactive molecules and compounds such as growth factors, drugs, herbal extracts and gene sequences. Synthetic polymers are commonly used as sensors, reinforcements and energy storage, whereas natural polymers are more prone to mimicking an extracellular matrix. Natural polymers are a renewable resource and classified as an environmentally friendly material, which might be used in different techniques to produce nanofibers for biomedical applications such as tissue engineering, implantable medical devices, antimicrobial barriers and wound dressings, among others. This review sheds some light on the advantages of natural over synthetic polymeric materials for nanofiber production. Also, the most important techniques employed to produce natural nanofibers are presented. Moreover, some pieces of evidence regarding toxicology and cell-interactions using natural nanofibers are discussed. Clearly, the potential extrapolation of such laboratory results into human health application should be addressed cautiously.

Fabrication and Characterization of Silk Fibroin/Curcumin Sustained-Release Film

In the present work, a sustained-release film composed of silk fibroin (SF), curcumin (Cur), glutaraldehyde (GA), and glycerol (Gly) was prepared successfully for wound dressings. Features relevant to wound dressings of SF/Gly/GA/Cur film were assessed. Physical and chemical properties of the fabricated materials were also characterized. The results showed that the prepared SF/Gly/GA/Cur film demonstrated a good sustained-release performance, flexibility, and gas permeability. In addition, it was found that the prepared SF/Gly/GA/Cur film possessed the capability to effectively inhibit the growth of bacteria and prevent bacterial penetration with a suitable water vapor transmission rate. Furthermore, the prepared composite film was non-cytotoxic, which makes it an ideal material for wound dressings.

Hydrogel Bioink with Multilayered Interfaces Improves Dispersibility of Encapsulated Cells in Extrusion Bioprinting

One of the challenges for extrusion bioprinting using low-viscosity bioinks is the fast gravity-driven sedimentation of cells. Cells in a hydrogel bioink that features low viscosity tend to settle to the bottom of the bioink reservoir, and as such, their bioprintability is hindered by association with the inhomogeneous cellularized structures that are deposited. This is particularly true in cases where longer periods are required to print complex or larger tissue constructs. Increasing the bioink's viscosity efficiently retards sedimentation but gives rise to cell membranolysis or functional disruption due to increased shear stress on the cells during the extrusion process. Inspired by the rainbow cocktail, we report the development of a multilayered modification strategy for gelatin methacryloyl (GelMA) bioink to manipulate multiple liquid interfaces, providing interfacial retention to retard cell sedimentation in the bioink reservoir. Indeed, the interfacial tension in our layer-by-layer bioink system, characterized by the pendant drop method, was found to be exponentially higher than the sedimental pull (Δ_{Gravity-Buoyancy} = ∼10^-9 N) of cells, indicating that the interfacial retention is crucial for preventing cell sedimentation across the adjacent layers. It was demonstrated that the encapsulated cells displayed better dispersibility in constructs bioprinted using the multilayered GelMA bioink system than that of pristine GelMA where the index of homogeneity of the cell distribution in the multilayered bioink was 4 times that of the latter.

Injectable Silk Nanofiber Hydrogels for Sustained Release of Small-Molecule Drugs and Vascularization

Strategies to control neovascularization in damaged tissues remain a key issue in regenerative medicine. Unlike most reported desferrioxamine (DFO)-loaded systems where DFO demonstrates a burst release, here we attain zero-order release behavior above 40 days. This outcome was achieved by blending DFO with silk nanofibers with special hydrophilic-hydrophobic properties. The special silk nanofibers showed strong physical binding capacity with DFO, avoiding chemical cross-linking. Using these new biomaterials in vivo in a rat wound model suggested that the DFO-loaded silk nanofiber hydrogel systems stimulated angiogenesis by the sustained release of DFO, but also facilitated cell migration and tissue ingrowth. These features resulted in faster formation of a blood vessel network in the wounds, as well improved healing when compared to the free DFO system. The DFO-loaded systems are also suitable for the regeneration of other tissues, such as nerve and bone, suggesting universality in the biomedical field.

Emerging progress on the mechanism and technology in wound repair

Normal wound repair is a dynamic and complex process involving multiple coordinated interactions between growth factors, cytokines, chemokines, and various cells. Any failure during the repair process may cause chronic wounds or scar formation, which increase the financial burden of patients due to repetitive treatments and considerable medical expenditures, and affect their quality of life. Nowadays, extensive efforts have been made to develop novel therapeutics for wound repair. Genetic engineering technology, tissue engineering technology, stem cell-based therapy, physical and biochemical technology, and vacuum-assisted closure technique have been proposed to be beneficial for wound repair, and shown considerable potential for improving the rate and quality of wound healing and skin regeneration. However, challenges remain as applying these techniques. As the development of cell biology and molecular biology, the understanding of the mechanism under wound repair has gradually deepened. As the growth of interdisciplinary research on physics, chemistry, biology, tissue engineering, and materials, the concept and technique relating wound repair for clinical application have rapidly developed. This article reviews the latest progress on the mechanism and technique in wound repair.

Naturally-derived electrospun wound dressings for target delivery of bio-active agents

Electrospun nanofibers are known as the advanced means for wound dressing. They have represented remarkable potency to encapsulate and deliver biomolecules promoting the wound healing process. Compared to synthetic polymers, naturally derived polymers (NDP) are more qualified candidates for fabrication of biomedical electrospun scaffolds. Not only nanofibers of NDP illustrate higher biocompatibility and biodegradability rates, but also they mimic the native extracellular matrix more closely, which leads to the wound closure acceleration by enhancing tissue regeneration. Aside, incorporation of bioactive molecules and therapeutic agents into the nanofibers can generate innovative bioactive wound dressings with significantly improved healing potentials. This paper starts with a brief discussion on the steps and factors influencing the wound healing process. Then, the recent applications of electrospun nanofibers as wound dressing with healing accelerating properties are reviewed. Further, the various healing agents and alternative strategies for modification and functionalization of bioactive naturally-derived electrospun nanofibers are discussed.

Electrospun Gelatin Fibers Surface Loaded ZnO Particles as a Potential Biodegradable Antibacterial Wound Dressing

Traditional wound dressings require frequent replacement, are prone to bacterial growth and cause a lot of environmental pollution. Therefore, biodegradable and antibacterial dressings are eagerly desired. In this paper, gelatin/ZnO fibers were first prepared by side-by-side electrospinning for potential wound dressing materials. The morphology, composition, cytotoxicity and antibacterial activity were characterized by using Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), particle size analyzer (DLS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), thermogravimetry (TGA) and Incucyte™ Zoom system. The results show that ZnO particles are uniformly dispersed on the surface of gelatin fibers and have no cytotoxicity. In addition, the gelatin/ZnO fibers exhibit excellent antibacterial activity against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) with a significant reduction of bacteria to more than 90%. Therefore, such a biodegradable, nontoxic and antibacterial fiber has excellent application prospects in wound dressing.

Tunable drug release from blend poly(vinyl pyrrolidone)-ethyl cellulose nanofibers

The management of pain and inflammation arising from wounds is essential in obtaining effective healing rates. The application of a wound dressing loaded with an anti-inflammatory drug would enable both issues to be ameliorated, and the aim of this work was to fabricate such a dressing by electrospinning. Fibers comprising ethyl cellulose (EC) and poly(vinyl pyrrolidone) (PVP) loaded with naproxen (Nap) were developed to be used in the early stages of wound care. A family of PVP/EC/Nap systems was prepared by varying the PVP: EC ratio. In all cases, the products of electrospinning comprise non-woven mats of fibers which generally have smooth and cylindrical morphologies. The formulations exist as amorphous solid dispersions, and there appear to be intermolecular interactions between the three components. Adjusting the polymer ratios results in tunable drug release, and formulations have been produced which give zero-order drug release over 20 and 80 h. The fiber mats generated in this work thus have great potential to be used as dressings for the treatment of wound pain and inflammation.

Mussel-Inspired Electroactive and Antioxidative Scaffolds with Incorporation of Polydopamine-Reduced Graphene Oxide for Enhancing Skin Wound Healing

Wound repair and tissue regeneration are complex processes that involve many physiological signals. Thus, employing novel wound dressings with potent biological activity and physiological signal response ability to accelerate wound healing is a possible solution. Herein, inspired by mussel chemistry, we developed a polydopamine (PDA)-reduced graphene oxide (pGO)-incorporated chitosan (CS) and silk fibroin (SF) (pGO-CS/SF) scaffold with good mechanical, electroactive, and antioxidative properties as an efficient wound dressing. First, pGO with good dispersibility and cell affinity was obtained upon reduction by PDA under alkali conditions. Second, pGO was dispersed into a CS/SF mixture, and then CS and SF chains were dual-cross-linked by poly(ethylene glycol) diglycidyl ether and glutaraldehyde to obtain a pGO-incorporated gel. Finally, the gel underwent a freeze-dry process to obtain the pGO-CS/SF scaffold. Owing to PDA reduction and functionalization, pGO in the scaffold plays important roles for the performances of the scaffolds. First, the pGO acts as nanoreinforcement to enhance the mechanical properties of the scaffold by combining the dual-cross-linked CS/SF network. Second, the uniformly distributed pGO in the scaffolds comprises a well-connected electric pathway, which can provide a channel for the transmission of electrical signals in the scaffold. Moreover, pGO in the scaffolds serves as an antioxidant agent to scavenge reactive oxygen species (ROS) and therefore terminates excessive ROS oxidation. In vitro studies show that electroactive pGO-CS/SF scaffolds can respond to electrical signals and promote cytological behavior. In addition, the pGO-CS/SF scaffolds can reduce cellular oxidation by removing excessive ROS. The in vivo full-thickness skin defect model demonstrates that the electroactive and antioxidative pGO-CS/SF scaffold can efficiently enhance wound healing. In summary, the pGO-CS/SF scaffold is a promising wound dressing because of its ability to promote physiological electrical signal transmission for cell growth and reduce ROS oxidation, resulting in an improved wound regeneration effect.

Modulation of Hypertrophic Scar Formation Using Amniotic Membrane/Electrospun Silk Fibroin Bilayer Membrane in a Rabbit Ear Model

Hypertrophic scarring is a dermal disorder resulting from collagen and other extra cellular matrix protein depositions following the deep trauma, severe burn injury, and surgery incisions. A variety of therapeutic procedures are currently available, however, achieving an ideal treatment method remains a challenge. In our recently published report, a 3D bilayered decellularized human amniotic membrane/electrospun silk fibroin membrane was fabricated and characterized for regenerative medical applications. To obtain a solid bind between two layers, the samples were immersed in 70% ethanol. In this study, the effects of amniotic membrane/electrospun silk fibroin on minimizing the postinjury hypertrophic scar formation were determined in the rabbit ear model. In vivo experiments were carried out to assess the bilayer membrane characteristics on full thickness hypertrophic scar at days 28 and 50 postimplantations. A significant decrease in collagen deposition and expression and increased expression and deposition of MMP1 in the wound bed were observed on the wounds dressed with bilayered membrane when compared to the amniotic membrane alone and controls (wound with no implant). The current study shows that our fabricated construct has potential as an efficient antiscarring wound dressing material and may also serve for the subsequent soft tissue engineering needs.

Remodeling of inherent antimicrobial nanofiber dressings with melamine-modified fibroin into neoskin

A melamine-modified fibroin was synthesized and fabricated into electrospun nanofiber films with broad-spectrum antimicrobial activity, sustained water retention, and fast reepithelialization and revascularization.

Anisotropic Biomimetic Silk Scaffolds for Improved Cell Migration and Healing of Skin Wounds

Improved and more rapid healing of full-thickness skin wounds remains a major clinical need. Silk fibroin (SF) is a natural protein biomaterial that has been used in skin repair. However, there has been little effort aimed at improving skin healing through tuning the hierarchical microstructure of SF-based matrices and introducing multiple physical cues. Recently, enhanced vascularization was achieved with SF scaffolds with nanofibrous structures and tunable secondary conformation of the matrices. We hypothesized that anisotropic features in nanofibrous SF scaffolds would promote cell migration, neovascularization, and tissue regeneration in wounds. To address this hypothesis, SF nanofibers were aligned in an electric field to form anisotropic porous scaffolds after lyophilization. In vitro and in vivo studies indicated good cytocompatibility, and improved cell migration and vascularization than nanofibrous scaffolds without these anisotropic features. These improvements resulted in more rapid wound closure, tissue ingrowth, and the formation of new epidermis, as well as higher collagen deposition with a structure similar to the surrounding native tissue. The new epidermal layers and neovascularization were achieved by day 7, with wound healing complete by day 28. It was concluded that anisotropic SF scaffolds alone, without a need for growth factors and cells, promoted significant cell migration, vascularization, and skin regeneration and may have the potential to effectively treat dermal wounds.

Novel Biodegradable Polymeric Microparticles Facilitate Scarless Wound Healing by Promoting Re-epithelialization and Inhibiting Fibrosis

Despite decades of research, the goal of achieving scarless wound healing remains elusive. One of the approaches, treatment with polymeric microcarriers, was shown to promote tissue regeneration in various in vitro models of wound healing. The in vivo effects of such an approach are attributed to transferred cells with polymeric microparticles functioning merely as inert scaffolds. We aimed to establish a bioactive biopolymer carrier that would promote would healing and inhibit scar formation in the murine model of deep skin wounds. Here we characterize two candidate types of microparticles based on fibroin/gelatin or spidroin and show that both types increase re-epithelialization rate and inhibit scar formation during skin wound healing. Interestingly, the effects of these microparticles on inflammatory gene expression and cytokine production by macrophages, fibroblasts, and keratinocytes are distinct. Both types of microparticles, as well as their soluble derivatives, fibroin and spidroin, significantly reduced the expression of profibrotic factors Fgf2 and Ctgf in mouse embryonic fibroblasts. However, only fibroin/gelatin microparticles induced transient inflammatory gene expression and cytokine production leading to an influx of inflammatory Ly6C+ myeloid cells to the injection site. The ability of microparticle carriers of equal proregenerative potential to induce inflammatory response may allow their subsequent adaptation to treatment of wounds with different bioburden and fibrotic content.

Silk Fibroin Microparticles with Hollow Mesoporous Silica Nanocarriers Encapsulation for Abdominal Wall Repair

Therapeutic vascularization appears to be an effective way of repairing abdominal wall defects. Attempts to implement this treatment tend to focus on the generation of featured drug carriers with the ability effectively to encapsulate the angiogenesis-stimulating agents and control their release to maintain an appropriate concentration at the injured area. Here, a new type of composite microparticle (CM) composed of silk fibroin (SF) and hollow mesoporous silica nanocarriers (HMSNs) is presented for therapeutic agent delivery. The CMs are generated by drying microfluidic emulsion templates of HMSN-dispersed SF solution. The resultant CMs have a distinctive micro-nanostructure, in which two barriers control the drug release. The encapsulated HMSNs increase the drug-carrying capacity of the CMs, and also form the first barrier via physical absorption. The microfluidic SF microparticles not only provide a shell with excellent monodispersity and biocompatibility but also form the second barrier via efficient encapsulation. Because of these superior properties of the CMs, the loaded drugs can be delivered with a satisfactory activity at the required rate, making them ideal for implementing therapeutic vascularization and repairing abdominal wall defects.

Prospects of nonmulberry silk protein sericin-based nanofibrous matrices for wound healing - In vitro and in vivo investigations

Recently, the progress in biomaterials for biomedical applications brings the focus of the research community toward nanomaterials. The nanofibrous matrices offer certain advantages (structural similarity to extracellular matrix, high surface area-to-volume ratio, increased elasticity, biostability, and strength) compared to other prevalent type of materials. This affirms their superiority and flexibility to be used in regenerative medicine. We have fabricated nonmulberry (Antheraea mylitta) silk protein sericin-based nanofibrous matrices (fiber thickness; 80-400 nm) with improved mechanical strength and desired stability (>4 weeks) as required for tissue reconstruction. These matrices support the adhesion, proliferation, and cellular interconnection of human keratinocytes. These are minimally hemolytic, nonimmunogenic, and capable of wound healing in vivo. Antibiotic (cephalexin hydrate [CH])-loaded nanofibrous matrices accelerate the full-thickness wound repair with minimal inflammation and without any signs of infection. The histological analysis authenticates skin restoration with re-epithelialization, generation of associated skin appendages, and synthesis of dense collagen fibrils. In addition, analysis of inflammatory genes and immunohistochemical assays have proved their biocompatibility and wound healing potential. Angiogenesis is also prevalent in the animal tissue treated with nanofibrous matrices. The results of in vitro and in vivo experimentations indicate a clear prospect of the fabricated sericin-based nanofibrous matrices to be used for skin regeneration.

STATEMENT OF SIGNIFICANCE

Nonmulberry silk protein sericin-based nanofibrous matrix is a useful biomaterial for wound healing, collagen production, and skin tissue repair. It has been used in different formulations including hydrogels and nanofibrous membranes with chitosan (CS) and polyvinyl alcohol (PVA). No experiments have been carried out to evaluate sericin-based nanofibrous membranes for skin tissue engineering application. The present study shows that the nanofibrous matrices fabricated by electrospinning nonmulberry silk protein sericin with CS and PVA mimic the architectural environment of the extracellular matrix fibrils. These matrices are minimally hemolytic, are nonimmunogenic, and support better growth of human keratinocytes in vitro and wound healing in vivo with re-epithelialization of the skin tissue and angiogenesis. This work indicates that these nonmulberry sericin-based nanofibrous matrices with CS may be used as an ideal physical environment and biological cues for the promotion of skin tissue reconstruction and repair.