Tensile Reinforcement of Silk Films by the Addition of Telechelic-Type Polyalanine

Tissue/Organ Adaptable Bioelectronic Silk-Based Implants

Implantable bioelectronic devices, designed for both monitoring and modulating living organisms, require functional and biological adaptability. Pure silk is innovatively employed, which is known for its excellent biocompatibility, to engineer water-triggered, geometrically reconfigurable membranes, on which functions can be integrated by Micro Electro Mechanical System (MEMS) techniques and specially functionalized silk. These devices can undergo programmed shape deformations within 10 min once triggered by water, and thus establishing stable bioelectronic interfaces with natively fitted geometries. As a testament to the applicability of this approach, a twining peripheral nerve electrode is designed, fabricated, and rigorously tested, demonstrating its efficacy in nerve modulation while ensuring biocompatibility for successful implantation.

Tunable Light-Actuated Interpenetrating Networks of Silk Fibroin and Gelatin for Tissue Engineering and Flexible Biodevices

Soft materials with tunable properties are valuable for applications such as tissue engineering, electronic skins, and human-machine interfaces. Materials that are nature-derived offer additional advantages such as biocompatibility, biodegradability, low-cost sourcing, and sustainability. However, these materials often have contrasting properties that limit their use. For example, silk fibroin (SF) has high mechanical strength but lacks processability and cell-adhesive domains. Gelatin, derived from collagen, has excellent biological properties, but is fragile and lacks stability. To overcome these limitations, composites of gelatin and SF have been explored. However, mechanically robust self-supported matrices and electrochemically active or micropatterned substrates were not demonstrated. In this study, we present a composite of photopolymerizable SF and photogelatin, termed photofibrogel (PFG). By incorporating photoreactive properties in both SF and gelatin, control over material properties can be achieved. The PFG composite can be easily and rapidly formed into free-standing, high-resolution architectures with tunable properties. By optimizing the ratio of SF to gelatin, properties such as swelling, mechanical behavior, enzymatic degradation, and patternability are tailored. The PFG composite allows for macroscale and microscale patterning without significant swelling, enabling the fabrication of structures using photolithography and laser cutting techniques. PFG can be patterned with electrically conductive materials, making it suitable for cell guidance and stimulation. The versatility, mechanical robustness, bioactivity, and electrochemical properties of PFG are shown for skeletal muscle tissue engineering using C2C12 cells as a model. Overall, such composite biomaterials with tunable properties have broad potential in flexible bioelectronics, wound healing, regenerative medicine, and food systems.

Silk fibroin films with embedded magnetic nanoparticles: evaluation of the magneto-mechanical stimulation effect on osteogenic differentiation of stem cells

We report about a biomaterial in the form of film ∼10 μm thick, consisting of a silk fibroin matrix with embedded iron oxide superparamagnetic nanoparticles, for prospective applications as bioactive coating in regenerative medicine. Films with different load of magnetic nanoparticles are produced (nanoparticles/silk fibroin nominal ratio = 5, 0.5 and 0 wt%) and the structural, mechanical and magnetic properties are studied. The nanoparticles form aggregates in the silk fibroin matrix and the film stiffness, as tested by nanoindentation, is spatially inhomogeneous, but the protein structure is not altered. In vitro biological tests are carried out on human bone marrow-derived mesenchymal stem cells cultured on the films up to 21 days, with and without an applied static uniform magnetic field. The sample with the highest nanoparticles/silk fibroin ratio shows the best performance in terms of cell proliferation and adhesion. Moreover, it promotes a faster and better osteogenic differentiation, particularly under magnetic field, as indicated by the gene expression level of typical osteogenic markers. These findings are explained in light of the results of the physical characterization, combined with numerical calculations. It is established that the applied magnetic field triggers a virtuous magneto-mechanical mechanism in which dipolar magnetic forces between the nanoparticle aggregates give rise to a spatial distribution of mechanical stresses in the silk fibroin matrix. The film with the largest nanoparticle load, under cell culture conditions (i.e. in aqueous environment), undergoes matrix deformations large enough to be sensed by the seeded cells as mechanical stimuli favoring the osteogenic differentiation.

Ultra-Thin Flexible Encapsulating Materials for Soft Bio-Integrated Electronics

Recently, bioelectronic devices extensively researched and developed through the convergence of flexible biocompatible materials and electronics design that enables  more precise diagnostics and therapeutics in human health care and opens up the potential to expand into various fields, such as clinical medicine and biomedical research. To establish an accurate and stable bidirectional bio-interface, protection against the external environment and high mechanical deformation is essential for wearable bioelectronic devices. In the case of implantable bioelectronics, special encapsulation materials and optimized mechanical designs and configurations that provide electronic stability and functionality are required for accommodating various organ properties, lifespans, and functions in the biofluid environment. Here, this study introduces recent developments of ultra-thin encapsulations with novel materials that can preserve or even improve the electrical performance of wearable and implantable bio-integrated electronics by supporting safety and stability for protection from destruction and contamination as well as optimizing the use of bioelectronic systems in physiological environments. In addition, a summary of the materials, methods, and characteristics of the most widely used encapsulation technologies is introduced, thereby providing a strategic selection of appropriate choices of recently developed flexible bioelectronics.

Electrospun Silk Fibroin/kappa-Carrageenan Hybrid Nanofibers with Enhanced Osteogenic Properties for Bone Regeneration Applications

Simple Summary

Bone tissue engineering has recently been considered as a potential alternative approach to treating patients with bone disorders/defects caused by tumors, trauma, and infection. Scaffolds play a crucial role in the field because they can serve as a template that can provide optimal structural and functional support for cells. In this study, we prepared a series of electrospun silk fibroin/kappa-carrageenan nanofibrous membranes with the aim of mimicking bone extracellular matrix structure and composition and improving the biological properties of silk-fibroin-based nanofibers. Our research found that a combinational approach blending kappa-carrageenan and silk fibroin could enhance the biological properties of the nanostructured scaffold. kappa-carrageenan could also enhance the osteogenic potential and bioactivity properties of silk fibroin nanofibers, while genipin crosslinking preserved the mechanical strength of hybrid nanofibrous mats, indicating that the electrospun hybrid scaffolds could be a potential candidate for bone regeneration applications.

Abstract

In this study, a novel nanofibrous hybrid scaffold based on silk fibroin (SF) and different weight ratios of kappa-carrageenan (k-CG) (1, 3, and 5 mg of k-CG in 1 mL of 12 wt% SF solution) was prepared using electrospinning and genipin (GP) as a crosslinker. The presence of k-CG in SF nanofibers was analyzed and confirmed using Fourier transform infrared spectroscopy (FTIR). In addition, X-ray diffraction (XRD) analysis confirmed that GP could cause SF conformation to shift from random coils or α-helices to β-sheets and thereby facilitate a more crystalline and stable structure. The ultimate tensile strength (UTS) and Young’s modulus of the SF mats were enhanced after crosslinking with GP from 3.91 ± 0.2 MPa to 8.50 ± 0.3 MPa and from 9.17 ± 0.3 MPa to 31.2 ± 1.2 MP, respectively. Notably, while the mean fiber diameter, wettability, and biodegradation rate of the SF nanofibers increased with increasing k-CG content, a decreasing effect was determined in terms of UTS and Young’s modulus. Additionally, better cell viability and proliferation were observed on hybrid scaffolds with the highest k-CG content. Osteogenic differentiation was determined from alkaline phosphatase (ALP) activity and Alizarin Red staining and expression of osteogenic marker genes. To this end, we noticed that k-CG enhanced ALP activity, calcium deposition, and expression of osteogenic genes on the hybrid scaffolds. Overall, hybridization of SF and k-CG can introduce a promising scaffold for bone regeneration; however, more biological evaluations are required.

A silk composite fiber reinforced by telechelic-type polyalanine and its strengthening mechanism

A telechelic-type polyalanine was doped in silkworm silk fibroins to prepare reinforced composite fibers, which exhibited 42% and 51% higher mechanical properties than silk-only fibers in terms of tensile strength and toughness, respectively.

Immobilization of bromelain and ZnO nanoparticles on silk fibroin nanofibers as an antibacterial and anti-inflammatory burn dressing

Burns is a critical fatal event due to the risk of infection and complex inflammatory cascades. This study aimed to fabricate and characterize a new antibacterial and anti-inflammatory dressing for second-degree burns by the immobilization of bromelain and zinc oxide nanoparticles on silk fibroin nanofibers. Thus, electrospun silk nanofibers with an average fiber diameter of 345 nm were prepared and then grafted with acrylic acid after exposure to O₂ plasma. Next, bromelain was immobilized on the modified SF nanofibers (SF-Br). Subsequently, different amounts of ZnO NPs coated with polydopamine were immobilized on the SF-Br nanofibers. The successful immobilization of bromelain and ZnO NPs on the SF nanofibers was proved by SEM, EDS, and FTIR analysis. The loading efficiency of bromelain was 85.63%, and activity ranged between 88% and 92%. The crystallinity of SF nanofibers decreased after the addition of bromelain and ZnO NPs, which increased the bromelain and zinc ions released from the dressing. Antibacterial activity has improved with the addition of ZnO NPs. The amounts of bromelain released from the dressings are not toxic to fibroblasts. Moreover, fibroblast attachment and proliferation enhanced at lower ZnO amounts, while there was an inverse trend at high doses of ZnO NPs. In vivo studies showed that treating the burn with silk fibroin-bromelain-ZnO NPs enhanced the healing process and considerably lowered the inflammatory response at the wound. Overall, the dressing presented here offers excellent potential for burn management.

Robust and double-layer micro-patterned bioadhesive based on silk nanofibril/GelMA-alginate for stroma tissue engineering

We develop a robust micro-patterned double-layer film that can adhere firmly to the tissue and provide a sustained release of ascorbic acid (AA) for corneal regeneration. This double-layer film consists of a AA reservoir sodium alginate (SA) adhesive and an anisotropic layer made of micro-patterned silk nanofibrils (SNF) incorporated gelatin methacrylate (GelMA) (S/G). The S/G layer facilitates the adhesion and orientation of corneal stroma cells, depending on the pattern sizes (50 μm (P1) and 100 (P2) μm). Results reveal that more than 90% and 80% of the cells are located at angles close to the vertical axis (0-20°) in the sample with the smaller and larger pattern size, respectively. The mechanical robustness and 90% light transmission of this hybrid film originate from the micro-patterned S/G layer. However, the micro-pattern size does not show a significant role in the mechanical properties of hybrid films (tensile strength of S/G-SA, S/G-SA(P₁), and S/G-SA(P₂) is 3.4 ± 0.1 MPa, 3.6 ± 0.6 MPa and 3.3 ± 0.2 MPa, respectively). In addition, the strong adhesion to the tissue of this double-layer film is related to the alginate layer. AA can release in a controlled manner, which can significantly promote corneal stroma cells' attachment, alignment, and proliferation compared to the control (AA-free micro-patterned film). Our results reveal that this innovative multifunctional S/G-SA + AA film can be a proper candidate for use in stroma tissue engineering of the human cornea.

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.

Autonomous Self-Healing Silk Fibroin Injectable Hydrogels Formed via Surfactant-Free Hydrophobic Association

Many natural materials, such as silk, animal bone, nacre, and plant fibers, achieve outstanding strength and toughness through the rupture of sacrificial bonds between chain segments in the organic phase. In this work, we present a bioinspired strategy to fabricate silk fibroin-based hydrophobic-association (HA) hydrogels by incorporating the hydrophobic interaction as a sacrificial bond into the alginate ionic network, which not only enhanced the mechanical extensibility, strength, and toughness of the hydrogels but also enabled self-recovery and self-healing properties via reversible hydrophobic interactions without external stimuli at room temperature. The hydrophobic interaction system consisted of the hydrophobic monomer stearyl methacrylate (C18M) and an amphiphilic regenerated silk fibroin (RSF) solution. The mechanical tests and rheometry indicated that the hydrophobic interaction served as the sacrificial bond that preferentially ruptures prior to the alginate ionic network under an external load, which dissipated enormous amounts of energy and conferred an improved mechanical performance. Moreover, the structure of HA gels could be quickly recovered after injection due to the existence of hydrophobic interactions. In addition, the degradability of the HA gels in a protease XIV solution was strongly dependent upon the C18M component, which significantly promoted the degradation rate of HA gels. The biomimetic mineralization process of HA gels within a simulated body fluid (SBF), mimicking the inorganic composition of human blood plasma, was performed and the calcium phosphate nanoparticles on the hydrogel were observed. Importantly, in vivo experiments illustrated that the HA gels exhibited satisfactory biocompatibility, and the mouse osteoblasts (MC3T3-E1) could attach and spread on the hydrogels. Overall, the self-healing, biocompatibility, and high mechanical properties of the HA gels render them potentially suitable for load-bearing applications in drug delivery or other soft tissue-engineering applications.

Silk/Natural Rubber (NR) and 3,4-Dihydroxyphenylalanine (DOPA)-Modified Silk/NR Composites: Synthesis, Secondary Structure, and Mechanical Properties

Silk composites with natural rubber (NR) were prepared by mixing degummed silk and NR latex solutions. A significant enhancement of the mechanical properties was confirmed for silk/NR composites compared to a NR-only product, indicating that silk can be applied as an effective reinforcement for rubber materials. Attenuated total reflection Fourier transform infrared (ATR-FTIR) and wide-angle X-ray diffraction (WAXD) analysis revealed that a β-sheet structure was formed in the NR matrix by increasing the silk content above 20 wt%. Then, 3,4-dihydroxyphenylalanine (DOPA)-modified silk was also blended with NR to give a DOPA-silk/NR composite, which showed superior mechanical properties to those of the unmodified silk-based composite. Not only the chemical structure but also the dominant secondary structure of silk in the composite was changed after DOPA modification. It was concluded that both the efficient adhesion property of DOPA residue and the secondary structure change improved the compatibility of silk and NR, resulting in the enhanced mechanical properties of the formed composite. The knowledge obtained herein should contribute to the development of the fabrication of novel silk-based elastic materials.

Facile terminal functionalization of peptides by protease-catalyzed chemoenzymatic polymerization toward synthesis of polymeric architectures consisting of peptides

Terminal functionalized polypeptides were synthesized in one-pot chemoenzymatic polymerization using protease for constructing special polymeric architectures.

Protein Polymer-Based Nanoparticles: Fabrication and Medical Applications

Nanoparticles are particles that range in size from about 1⁻1000 nanometers in diameter, about one thousand times smaller than the average cell in a human body. Their small size, flexible fabrication, and high surface-area-to-volume ratio make them ideal systems for drug delivery. Nanoparticles can be made from a variety of materials including metals, polysaccharides, and proteins. Biological protein-based nanoparticles such as silk, keratin, collagen, elastin, corn zein, and soy protein-based nanoparticles are advantageous in having biodegradability, bioavailability, and relatively low cost. Many protein nanoparticles are easy to process and can be modified to achieve desired specifications such as size, morphology, and weight. Protein nanoparticles are used in a variety of settings and are replacing many materials that are not biocompatible and have a negative impact on the environment. Here we attempt to review the literature pertaining to protein-based nanoparticles with a focus on their application in drug delivery and biomedical fields. Additional detail on governing nanoparticle parameters, specific protein nanoparticle applications, and fabrication methods are also provided.

Spider dragline silk composite films doped with linear and telechelic polyalanine: Effect of polyalanine on the structure and mechanical properties

Spider dragline silks have attracted intensive attention as eco-friendly tough materials because of their excellent mechanical property and biomass-based origin. Composite films based on a recombinant spider dragline silk protein (ADF3) from Araneus diadematus were prepared by doping with linear or telechelic poly(L-alanine) (L- or T-polyA, respectively) as a reinforcing agent. Higher tensile strength and toughness of the composite films were achieved with the addition of polyA compared with the tensile strength and toughness of the silk-only film. The difference in the reinforcing behavior between L- and T-polyA was associated with their primary structures, which were revealed by wide angle X-ray diffraction analysis. L-polyA showed a tendency to aggregate in the composite films and induce crystallization of the inherent silk β-sheet to afford rigid but brittle films. By contrast, T-polyA dispersion in the composite films led to the formation of β-sheet crystal of both T-polyA and the inherent silk, which imparted high strength and toughness to the silk films.

Chemoenzymatic Synthesis of Polypeptides for Use as Functional and Structural Materials

Polypeptides inspired by the natural functional and structural proteins present in living systems are promising materials for various fields in terms of their versatile functionality and physical properties. Designing and synthesizing mimetic sequences of specific peptide motifs in proteins are important for exploring the functionality of natural proteins. Chemoenzymatic polymerization, which utilizes aminolysis (i.e., the reverse reaction of hydrolysis catalyzed by proteases), is a useful technique for synthesizing artificial polypeptide materials and has several advantages, including facile synthesis protocols, environmental friendliness, scalability, and atom economy. In this review, recent progress in chemoenzymatic polypeptide synthesis for the production of functional and structural materials for various applications is summarized in conjunction with the current status of technical challenges in the field.