Mild preparation of hyaluronic acid/silk fibroin sponges by modified crosslinking method.

The composites composed of hyaluronic acid (HA) and silk fibroin (SF) exhibit great potential in diverse biomedical applications. However, the utilization of commercial crosslinkers such as 1,4-butanediol diglycidyl ether (BDDE) for crosslinking HA typically necessitates harsh conditions involving strong alkaline, which greatly limits its potential applications. In this study, a mild modified approach was developed to fabricate HA/SF blend sponges crosslinked by BDDE without alkaline conditions. The blend solutions were cryo-concentrated to induce crosslinking reactions. The mechanism of freezing crosslinking was elucidated by investigating the effects of ice crystal growth and HA molecular weight on the degree of crosslinking. The results revealed that HA achieved efficient crosslinking when its molecular weight exceeds 1000 kDa and freezing temperatures ranged from -40 °C to -20 °C. After introducing SF, multiple crosslinks were formed between SF and HA chains, producing water-stable porous sponges. The SEM results demonstrated that the introduction of SF effectively enhanced the interconnectivity between macropores through creating subordinate holes onto the pores wall. Raising the SF content significantly enhanced compression strength, resistance to enzymatic degradation and cell viability of blend sponges. This study provides a novel strategy for designing bioactive HA/SF blend sponges as substitutes for tissue repair and wound dressing.

High biocompatible bone screw enabled by a rapid and robust chitosan/silk fibroin composite material.

Hydrogel has attracted tremendous attentions due to its excellent biocompatibility and adaptability in biomedical field. However, it is challenging by the conflicts between inadequate mechanical properties and service requirements. Herein, a rapid and robust hydrogel was developed by interpenetrating networks between chitosan and silk fibroin macromolecules. Thanks to these unique networks, the chitosan-based hydrogel exhibited superior mechanical performances. The maximum breaking strength, Young's modulus and swelling ratio of the hydrogel were 1187.8 kPa, 383.1 MPa and 4.5 % respectively. The hydrogel also supported the proliferation of human umbilical vein endothelial cells for 7 days. Notably, the hydrogel was easily molded into bone screw, and demonstrated compressive strengths of 45.7 MPa, Young's modulus of 675.6 MPa, respectively. After 49-day biodegradation, the residual rate of the screw in collagenase I solution was up to 89.6 % of the initial weight. In vitro, the screws not only had high resistance to biodegradation, but also had outstanding biocompatibility of osteoblast. This study provided a promising physical-chemical double crosslinking strategy to build orthopedic materials, holding a great potential in biomedical devices.

Fabrication of multifunctional silk nanofibril/hyaluronic acid scaffold for spinal cord repair.

Bioactive scaffolds accurately mimicking the structure and composition of the extracellular matrix have garnered significant interest in tissue engineering. In this study, we developed a platform utilizing natural silk nanofibrils, hyaluronic acid, and basic fibroblast growth factor for the purpose of promoting spinal cord regeneration by creating an optimal microenvironment. The bioactive scaffold exhibited notable characteristics such as high porosity and hydrophilicity, attributed to its unique nanostructure, high connectivity, and polysaccharide composition. Furthermore, the pore size of the scaffold can be adjusted within the range of 90 µm to 120 µm by varying the content of hyaluronic acid. In vitro, human umbilical vein endothelial cells were seeded into the scaffold, demonstrating enhanced cell viability. The scaffold facilitated cell proliferation and migration. In vivo experiments on rats indicated that the scaffold had a beneficial impact on spinal cord regeneration, creating a conducive environment for motor function recovery of the rats. This effect may be attributed to the scaffold's ability to stimulate axon growth and neuronal survival, as well as inhibit the formation of glial scars, as evidenced by the decreased expression of growth associated protein-43, microtubule-associated protein 2, and neurofilament-200. This study presents a promising method to develop a feasible bioscaffold for the treatment of spinal cord injury.

Development of a bioactive silk fibroin bilayer scaffold for wound healing and scar inhibition.

In cases of deep skin defects, spontaneous tissue regeneration and excessive collagen deposition lead to hyperplastic scars. Conventional remedial action after scar formation is limited with a high recurrence rate. In this study, we designed a new artificial skin bilayer using silk fibroin nanofibers films (SNF) as the epidermis, and silk fibroin (SF) / hyaluronic acid (HA) scaffold as the dermal layer. The regenerated SF film was used as a binder to form a functional SNF-SF-HA bilayer scaffold. The bilayer scaffold showed high porosity, hydrophilicity, and strength, and retained its shape over 30 days in PBS. In vitro, human umbilical vein endothelial cells were seeded into the bilayer scaffold and showed superior cell viability. In vivo analyses using the rabbit ear hypertrophic scar (HS) model indicated that the bilayer scaffold not only supported the reconstruction of new tissue, but also inhibited scar formation. The scaffold possibly achieved scar inhabitation by reducing wound contraction, weakening inflammatory reactions, and regulating collagen deposition and type conversion, which was partly observed through the downregulation of type I collagen, transforming growth factor-ß, and α-smooth muscle actin. This study describes a new strategy to expand the application of silk-based biomaterials for the treatment of hyperplastic skin scars.

Engineering biomimetic scaffolds by combining silk protein nanofibrils and hyaluronic acid.

Nanofibrous scaffolds mimicking important features of the native extracellular matrix (ECM) provide a promising strategy for tissue regeneration. However, 3D scaffolds mimicking natural protein nanofibers and bioactive glycosaminoglycans remain poorly developed. In this study, a biomimetic nanofibrous scaffold composed of natural silk protein nanofibers and glycosaminoglycan hyaluronic acid (HA) was developed. HA functionalization significantly improved the hydrophilicity and bioactivity of silk nanofibers (SNFs). SNFs can be assembled into nanofibrous aerogel scaffolds with low density and desirable shapes on a large scale. More importantly, with the assistance of HA, the silk nanofibrous aerogel scaffolds with ultra-high porosity, natural bioactivity, and structural stability in aqueous environment can be fabricated. In the protease/hyaluronidase solution, the SNF scaffolds with 10.0 % HA can maintain their monolithic shape for >3 weeks. The silk nanofibrous scaffolds not only imitate the composition of ECM but also mimic the hierarchical structure of ECM, providing a favorable microenvironment for cell adhesion and proliferation. These results indicate that this structurally and functionally biomimetic system is a promising tissue engineering scaffold.

Silk fibroin scaffolds with stable silk I crystal and tunable properties.

It is crucial to develop a three-dimensional scaffold with tunable physical properties for the biomedical application of silk fibroin (SF). The crystallization of polymers dictates their bulk properties. The presence of two unique crystal types, silk I and silk II, provides a mechanism for controlling the properties of SF biomaterials. However, it remains challenging to manipulate silk I crystallization. In this study, we demonstrate the stability and tunability of SF scaffolds with silk I structure prepared by a freezing-annealing processing. The porous structure and mechanical properties of the scaffolds can be readily regulated by SF concentration. XRD results show that the typical peaks representing silk I do not shift when subjected to various post-treatments, such as ethanol soaking, heating, water vapor annealing, UV irradiation, and high-temperature/high-pressure, indicating the stability of silk I crystal type. Moreover, the crystallization kinetics can be regulated by changing annealing time. This physical process can regulate the transition from non-crystalline to silk I, in turn controlling the mechanical properties and degradation rate of the SF scaffolds. Our result show that this green, all-aqueous strategy provides new directions for the design of SF-based biomaterials with controllable properties.

Controllable Production of Natural Silk Nanofibrils for Reinforcing Silk-Based Orthopedic Screws.

As a natural high-performance material with a unique hierarchical structure, silk is endowed with superior mechanical properties. However, the current approaches towards producing regenerated silk fibroin (SF) for the preparation of biomedical devices fail to fully exploit the mechanical potential of native silk materials. In this study, using a top-down approach, we exfoliated natural silk fibers into silk nanofibrils (SNFs), through the disintegration of interfibrillar binding forces. The as-prepared SNFs were employed to reinforce the regenerated SF solution to fabricate orthopedic screws with outstanding mechanical properties (compression modulus > 1.1 GPa in a hydrated state). Remarkably, these screws exhibited tunable biodegradation and high cytocompatibility. After 28 days of degradation in protease XIV solution, the weight loss of the screw was ~20% of the original weight. The screws offered a favorable microenvironment to human bone marrow mesenchymal stem cell growth and spread as determined by live/dead staining, F-action staining, and Alamar blue staining. The synergy between native structural components (SNFs) and regenerated SF solutions to form bionanocomposites provides a promising design strategy for the fabrication of biomedical devices with improved performance.

Natural silk nanofibers as building blocks for biomimetic aerogel scaffolds.

Protein nanofibers offer great promise for tissue engineering scaffolds owing to biomimetic architecture and exceptional biocompatibility. Natural silk nanofibrils (SNFs) are promising but unexplored protein nanofibers for biomedical applications. In this study, the SNF-assembled aerogel scaffolds with ECM-mimicking architecture and ultra-high porosity are developed based on a polysaccharides-assisted strategy. The SNFs exfoliated from silkworm silks can be utilized as building blocks to construct 3D nanofibrous scaffolds with tunable densities and desirable shapes on a large scale. We demonstrate that the natural polysaccharides can regulate SNF assembly through multiple binding modes, endowing the scaffolds with structural stability in water and tunable mechanical properties. As a proof of concept, the biocompatibility and biofunctionality of the chitosan-assembled SNF aerogels were investigated. The nanofibrous aerogels have excellent biocompatibility, and their biomimetic structure, ultra-high porosity, and large specific surface area endow the scaffolds with enhanced cell viability to mesenchymal stem cells. The nanofibrous aerogels were further functionalized by SNF-mediated biomineralization, demonstrating their potential as a bone-mimicking scaffold. Our results show the potential of natural nanostructured silks in the field of biomaterials and provide a feasible strategy to construct protein nanofiber scaffolds.

Silk fibroin/chitosan coating with tunable catalytic nitric oxide generation for surface functionalization of cardiovascular stents.

Developing a functional coating for vascular stents with sustainable and tunable NO release remains challenging. In this work, we report a silk fibroin/chitosan-based biopolymer coating incorporating copper ions as a catalyst for NO generation and demonstrate its potential for the surface functionalization of cardiovascular stents. Based on the differences in silk fibroin and chitosan coordinating with copper ions, the loading, bonding, and release of copper ions could be precisely regulated over a wide range by controlling the ratio of silk fibroin and chitosan. This system shows good cytocompatibility for endothelial cells and tunable catalytic activity to decompose S-nitroso-N-acetyl-D-penicillamine (SNAP) for NO generation. Consequently, a functionalized coating with sustainable and tunable NO catalysis generation was developed on the metallic stent. Based on good biocompatibility, tunable NO release, and simple processing, the coating is expected to have great promise in the field of intervention therapy of cardiovascular disease.

Biomimetic Natural Silk Nanofibrous Microspheres for Multifunctional Biomedical Applications.

Silk nanofibrils (SNFs) extracted from natural silkworm silk represent a class of high-potential protein nanofiber material with unexplored biomedical applications. In this study, a SNF-assembled microsphere with extracellular matrix (ECM)-mimicking architecture and high specific surface area was developed. The SNFs were exfoliated from silkworm silks through an all-aqueous process and used as the building blocks for constructing the microspheres. Inspired by the structure and bioactive composition of ECM, hyaluronic acid (HA) was used as a bio-glue to regulate SNF assembly. With the assistance of HA, the SNF microspheres with stable fluffy nanofibrous structures were synthesized through electrospray. The biomimetic structure and nature derived composition endow the microspheres with excellent biocompatibility and enhanced osteogenic differentiation-inducing ability to mesenchymal stem cells. As proof of versatility, the SNF microspheres were further functionalized with other molecules and nanomaterials. Taking the advantages of the excellent blood compatibility and modifiability from the molecular level to the nanoscale of SNF microspheres, we demonstrated their versatile applications in protease detection and blood purification. On the basis of these results, we foresee that this natural silk-based nanofibrous microsphere may serve as a superior biomedical material for tissue engineering, early disease diagnosis, and therapeutic devices.

Cu(II)-functionalized silk fibroin films for the catalytic generation of nitric oxide.

In situ release of nitric oxide (NO) has been suggested to be a potential functionalization strategy for blood-contacting implants. In this study, the NO generation capability catalyzed by the copper ion-incorporated silk fibroin (SF) films in the presence of S-nitroso-N-acetyl-dl-penicillamine (SNAP) is demonstrated. Cu(II) is effectively bound to the surface of the SF film based on metal-protein coordination. The x-ray photoelectron spectroscopy results indicate that copper ions may exist on the surface of the SF film in the form of Cu(II)/Cu(I) coexistence. The degradation behavior showed that the bound copper ions on the surface of the SF films can maintain a slow release in phosphate-buffered saline (PBS) or collagenase IA solution for 7 days. There was no significant difference in the release of copper ions between PBS degradation and enzyme degradation. The loading of copper ions significantly improved the release of NO from SNAP through catalysis. Based on the biological effects of copper ions and the ability to catalyze the release of NO from S-nitrosothiols, copper ion loading provides an option for the construction of bioactive SF biomaterials.

Engineering vascularized dermal grafts by integrating a biomimetic scaffold and Wharton's jelly MSC-derived endothelial cells.

Tissue engineering aims to generate functional tissue constructs with the necessary scaffold properties for cell colonization and the establishment of a vascular network. However, treatment of tissue defects using synthetic scaffolds remains a challenge mainly due to insufficient and slow vascularization. Our previous study developed a macroporous silk fibroin scaffold with a nanofibrous microstructure, and demonstrated that the nanofibrous structure can promote the viability of endothelial cells (ECs) and guide cell migration. Further studies are needed to clarify the effect of scaffold microstructures on cell-mediated vascularization. Here, we investigated the efficacy of EC-seeded nanofibrous scaffolds in improving vascularization in vivo. ECs derived from induced human Wharton's Jelly mesenchymal stem cells served as a potential source for cell transplantation. The cell-seeded scaffolds were implanted into dermal defects of SD rats, demonstrating that the multiscale hierarchical design significantly improved the capacity of transplanted cells to promote and accelerate neovascularization and dermal reconstruction via enhancing cell infiltration, collagen deposition and growth factor expression. Our findings provide new insight into the development of degradable macroporous composite materials with 3D microstructures as tissue engineering scaffolds with enhanced vascularization functions, and also provide new treatment options for cell transplantation.

Natural Silk Nanofibril Aerogels with Distinctive Filtration Capacity and Heat-Retention Performance.

Nanofibrous aerogels have been extensively developed as multifunctional substrates in a wide range of fields. Natural silk nanofibrils (SNFs) are an appealing biopolymer due to their natural abundance, mechanical toughness, biodegradability, and excellent biocompatibility. However, fabricating 3D SNF materials with mechanical flexibility remains a challenge. Herein, SNF-based aerogels with controlled structures and well mechanical resilience were prepared. SNFs were extracted from silkworm silks by mechanical disintegration based on an all-aqueous system. The nanofibrils network and hierarchical cellular structure of the aerogels were tuned by the assembly of SNFs and foreign poly(vinyl alcohol) (PVA). The SNF aerogels exhibited an ultralow density (as low as 2.0 mg·cm-3) and well mechanical properties with a structure allowing for large deformations. These SNF aerogels demonstrated a reversible compression and stress retention after 100 cycles of compression. Furthermore, the resulting aerogels were used for air filtration and showed efficient filtration performance with a high dust-holding capacity and low resistance. Moreover, an extremely low thermal conductivity of 0.028 W·(m·K)-1 was achieved by the aerogel, showing its potential for use in heat-retention applications. This study provides a useful strategy for exploring the use of natural silks in 3D aerogels and offers options for developing filtration materials and ultralight heat-retention materials.

Electrochemically deposition of catechol-chitosan hydrogel coating on coronary stent with robust copper ions immobilization capability and improved interfacial biological activity.

Establishing a facile and versatile strategy to confer coronary stent with improved interfacial biological activity is crucial for novel cardiovascular implants. Developing a coating with NO release ability catalyzed by metal ions, such as copper, will be highly advantageous for the functionalized surface modification of metal stents. However, most available strategies involve drawbacks of low efficiency, complex processes, and toxic chemicals. Therefore, in this study, we report a green and facile electrobiofabrication method to construct the bioactive hydrogel coating by combining chitosan, catechol groups and copper ions on coronary stent and titanium surfaces. Experimental results demonstrated that the chitosan hydrogel coating can be precisely controlled synthesis via electrochemical deposition and serves as a versatile platform for copper ions immobilization. Additionally, mussel-inspired catechol groups could be chemically grafted on chitosan chains to further enhance the film mechanical properties and binding abilities of copper ions. Moreover, in vitro cell biocompatibility and catalyzed NO-generation activity have also been accessed and which suggesting great possibilities for biomedical applications. Therefore, by coupling the electrobiofabrication approach and multi-functionalities of the hybrid film, this report would advance the development of biomimetic hydrogel coating for vascular engineering (e.g., coronary stent) and other biomedical devices.

Natural silk nanofibrils as reinforcements for the preparation of chitosan-based bionanocomposites.

Nanofibrils derived from natural biopolymers have received extensive interest due to their exceptional mechanical properties and excellent biocompatibility. To fabricate biocompatible chitosan nanocomposites with high mechanical performance, silkworm silks were deconstructed into nanofibrils as structural and mechanical reinforcement of chitosan. After dispersing silk nanofibrils in chitosan solution, a set of nanocomposites, including film, porous scaffold, filament, and nanofibrous sponge, could be fabricated from the blended solutions. Silk nanofibrils could be uniformly dispersed in chitosan solution, and formed multi-dimensional nanocomposites. The nanocomposites exhibited enhanced mechanical strength and thermal stability, and provided a biomimetic nanofibrous structure for biomaterial applications. The enhancement in mechanical properties can be attributed to the interaction between the nanofibril phase and the chitosan matrix. As the polysaccharide/protein bionanocomposites derived from natural biopolymers, these materials offer new opportunities for biomaterial application by virtue of their biocompatibility and biodegradability, as well as enhanced mechanical properties and controllable mesoscopic structure.

Silk as templates for hydroxyapatite biomineralization: A comparative study of Bombyx mori and Antheraea pernyi silkworm silks.

Silk is extensively investigated in bone tissue engineering due to its extraordinary mechanical properties and ability to regulate biomineralization. Protein templates regulate biomineralization process through chemical interaction with ions. However, the effect of structural differences in silk fibroin on biomineralization has not been studied in detail. In this study, Antheraea pernyi silk fibroin (ASF) and Bombyx mori silk fibroin (BSF) fibers were used as templates to study the effect of silk species on biomineralization. The results showed that silk fibroin could induce the formation of calcium-deficient hydroxyapatite in simulated body fluid (SBF), and the SBF treatment resulted in the formation of silk I crystals. Compared with BSF, ASF exhibited a higher ability to induce mineralization, which may depend on the differences in hydrophilic amorphous fractions between ASF and BSF. The amorphous fractions of ASF contain more acidic amino acids, which can provide more nucleation sites in the initial stage of mineralization, resulting in faster mineralization process and more mineral deposits. This study decodes the key role of silk fibroin fractions on biomineralization, and provides deeper insights for the study of silk fibroin as biomineralization template and bone repair materials.

Physically crosslinked silk fibroin/hyaluronic acid scaffolds.

Combining the properties of natural protein and polysaccharide is a promising strategy to generate bioactive biomaterials with controlled structure. Here, a new method of preparing water-insoluble silk fibroin/hyaluronic acid (SF/HA) scaffolds with tunable performances using an all-aqueous process is reported. Freezing-induced assembly was used to form silk I crystallization in the SF/HA blends. Silk I crystallization enhanced the stability of SF/HA scaffolds in water by forming silk I crystal networks to entrap blended HA without chemical cross-linking. Increasing HA content significantly enhanced the flexibility and water binding capacity of porous scaffolds, but high amount of HA reduced the water-stability of porous scaffolds due to insufficient silk I crystal cross-links. The enzymatic degradation behavior of the SF/HA scaffolds was investigated, revealing that the regulation ability of HA in the SF scaffolds. This novel nonchemically cross-linked protein/polysaccharide scaffold may be useful for soft tissue engineering due to excellent biocompatibility and tunable performances.

Topographic cues reveal filopodia-mediated cell locomotion in 3D microenvironment.

In cell-material interactions, the formation and functioning of filopodia have been demonstrated to be very sensitive to topographic cues. However, substrate-exploring functions of filopodia in a 3D microenvironment remain elusive. In this study, the silk fibroin film with a micropillar structure was prepared to reveal a filopodial-mediated cell response to 3D topographic cues. The micropillars provided a confined space for cell spreading by a simplified 3D structure, allowing initial cells to settle on the bottom of substrates rather than on the top of micropillars. Shortly after cell adhesion, the authors describe how cells transform from a filopodia-rich spherical cell state to a lamellipodia-dominated state that enables cell to climb along micropillars and spread on the top of the micropillars. The authors found that filopodia not only served as sensors for pathfinding but also provided nucleation scaffolds for the formation and orientation of minilamellipodia on the micropillar substrate. On the route of long filopodial extension following micropillars, all three functional filopodial adhesions have the ability to form veil-like minilamellipodium, simply by tethering the filopodium to the micropillars. Stable filopodia contacts consistently stimulated the local protrusion of a lamellipodium, which ultimately steered cell migration. Their results suggest the filopodia-mediated cell locomotion in the 3D microenvironment using a filopodia-to-minilamellipodium transformation mechanism.

Cu(II) ion loading in silk fibroin scaffolds with silk I structure.

Metal ions play important roles in the diverse biochemical reactions associated with many cell signalling pathways. The modification of biomaterials with metal ions may offer a promising approach to stimulate cellular activity for improving tissue regeneration. Here, copper ion loading as a potential therapeutic agent in silk fibroin (SF) scaffolds was investigated. Freezing-annealing was used to induce silk I crystallization for forming water-insoluble SF scaffolds. Cu(II) ions were entrapped into SF scaffolds with different ratios by forming silk I crystal networks when copper chloride dihydrate was less than 5.0 wt%, producing water-stable materials. Moreover, it was found that copper ion chelation further enhanced SF stability when a low amount copper chloride was loaded. Increasing copper chloride content weakened silk I crystallization and Cu(II) ion chelation, rendering SF scaffolds unstable in water. Above 5.0 wt% copper chloride dihydrate, silk I crystallization was prevented. Finally, silk I scaffold with 1.5 wt% copper chloride dihydrate showed the strongest water-stability and highest loading efficiency. The results provide valuable data for understanding the effect of metal ions in freezing-induced SF crystallization, and also offer options for preparing novel Cu(II)-functionalized SF scaffolds.

Multichannel Bioactive Silk Nanofiber Conduits Direct and Enhance Axonal Regeneration after Spinal Cord Injury.

After a spinal cord injury, axonal regeneration over long distances is challenging due to the lack of physical guidance cues and bioactive signals. In this study, a multichannel bioactive silk fibroin nanofiber conduit was fabricated to improve spinal cord injury repair by enhancing axonal regeneration. The conduit was composed of longitudinally oriented silk fibroin nanofibers and then functionalized with laminin. In vitro, the bioactive conduits could promote neuron-like development and directional neurite extension of PC12 cells by providing a bioactive stimulus and physical guidance. In a spinal cord injury model in Sprague-Dawley rats, the biofunctionalized conduits displayed superior integration with the host tissue due to enhanced cell infiltration and tissue ingrowth. The glial scar was significantly reduced, allowing axonal ingrowth along with the channel direction. Compared to a single-channel conduit, the multichannel conduit improved spinal cord regeneration by boosting tissue ingrowth and axonal regeneration, indicating that the conduit architectures play critical roles in spinal cord regeneration. These silk fibroin conduits, along with the multichannel architecture, nanoscale cues, and the ability to bind bioactive compounds, represent promising candidates for spinal cord regeneration.