Structure and properties of various hybrids fabricated by silk nanofibrils and nanohydroxyapatite

Nanosilk Template-Guided/Induced Construction of Brush-/Flower-like 3D Nanostructures

Biomaterials with natural hierarchical structures typically exhibit extraordinary properties because of their multilevel structural designs. They offer many templates and models as well as inspiration for material design, particularly for fabricating structure-regulated, performance-enhanced, and function-enriched materials. Biopolymer-based nanocomposites with ingenious nanostructures constructed through ecofriendly and sustainable approaches are highly desirable to meet the multifunctional requirements of developing bioinspired materials. Herein, an all-silk fibroin-based nanocomposite with a brush-like nanostructure was constructed for the first time using a nanotemplate-guided assembly approach in which dissolved silk assembled directly on a silk nanowhisker (SNW) backbone to form peculiar nanobrushes based on the classical micelle model. Three-dimensional spider-like or centipede-like silk nanobrushes (SNBs) were fabricated by varying the SNW backbone length from 0.16 to 6 μm. The branches with average lengths of 32-290 nm were also adjustable. SNBs were further designed to regulate and induce biomineralization of hydroxyapatite (HAP) to form interesting flower-like nanostructures, in which the HAP nanosphere (diameters ∼16 nm) "core" was covered by SNBs with branches extending to form a "shell" (∼101 nm in length). Based on such protein nanotemplate-guided formation of nanoscale structures, practical hollow conduits with remarkable mechanical properties, biocompatibility, shape memory behavior, and bone engineering potential were fabricated. This study inspires the design of polymorphous biopolymer-based nanostructures with enhanced performance at multiple length scales where the weaknesses of individual building blocks are offset.

Physico-mechanical and in-vivo evaluations of tri-layered alginate-gelatin/polycaprolactone-gelatin-β-TCP membranes for guided bone regeneration

Guided bone regeneration (GBR) membranes favor periodontal regrowth, but they still have certain limitations, such as improper biodegradation and poor mechanical property. To overcome these shortcomings, we have generated a unique multifunctional membrane. A polycaprolactone/gelatin/β-TCP and alginate/gelatin trilayered construction was fabricated through electrospinning and casting technology. The prepared membranes have suitable physicomechanical and in-vitro properties to confirm the compatibility of the product in the body. Phase analysis, functional groups, surface microstructure, and contact angle were measured as basic characteristics. For a mechanical performance evaluation, the tensile strength at suturing point was measured through pullout tensile strength test, and it showed the suture capability of bi-layered membranes. Highest tensile strength for A75G25 was recorded with 2.9 ± 0.15 MPa with 105% strain. Further, the osteoblast and fibroblast-type cell toxicity results showed that the electrospun membrane offered compatible environment to cells while the alginate sheet was found to be sufficiently capable to suppress the cellular attachment while also being a nontoxic material. Post-implantation, according to the in-vivo conclusions of the tri-layered membrane, there was appreciable bone formation. Compared to an implant without membrane covering, enhanced new bone formation can be identified after 8 weeks of implantation with P1G4β10 membranes-covered site.

Silk fibroin directs the formation of monetite nanocrystals and their assembly into hierarchical composites

Natural biominerals are usually composite materials produced through mineralization of inorganic crystals within an organic matrix. Silk fibroin is known to be capable of directing the nucleation and growth of hydroxyapatite crystals. Here, we used silk films as the substrate to induce the mineralization of calcium phosphate. We show that the silk fibroin in solution could induce the formation of monetite crystals with a hierarchical structure, which are assembled by well aligned single crystals of monetite. In addition, we show that silk fibroins are incorporated inside the crystals. Therefore, the self-assembly of silk fibroin during the crystallization process is critical for the formation of such hierarchical structures.

Features of a simvastatin-loaded multi-layered co-electrospun barrier membrane for guided bone regeneration

A novel tri-layer membrane consisting of polycaprolactone (PCL) fibrous sheets and structured nanofibers with a gelatin (Gt) shell and a simvastatin-containing PCL core (PCL-Gt/PCL-simvastatin membrane) was prepared. The soft external layer comprised of Gt/PCL-simvastatin, the external layer of PCL and the middle layer of both microfilaments, interwoven together. The membrane was designed to promote osteoinduction and act as a barrier against cells but not against water and molecules in order to promote guided bone regeneration. The structure of the membrane was characterized by scanning electronic microscopy. The in vitro release rates of simvastatin over 32 days were determined by high-performance liquid chromatography. For in vitro biological assays, bone marrow mesenchymal stem cells and human fibroblasts were cultured on the different surfaces of the membrane. Cell adhesion, proliferation, distribution, and differentiation were examined. For in vivo testing, cranial defects were created in rabbits to assess the amount of new bone formed for each membrane. The results revealed that membranes with multi-layered structures showed good cell viability and effective osteoinductive and barrier properties. These results suggest that the novel multi-layered PCL-Gt/PCL-simvastatin membranes have great potential for bone tissue engineering.

Bioinspired synthesis of fiber-shaped silk fibroin-ferric oxide nanohybrid for superior elimination of antimonite

Bioinspired fibrous materials have emerged as a unique class of matrix for fabrication of fiber-shaped nanomaterial assemblies. Here, we report a novel functional fiber-shaped nanohybrid for efficient removal of antimonite via in situ synthesis of ferric oxides anchored to silk nanofibril. The silk nanofibril matrix played important roles in the growth of ferric oxides via metal-ligand interactions. The achieved nanocomposites had high surface areas and activity with more functional groups, contributing to superior antimonite elimination. The nanocomposite achieved a maximum removal capacity of 159.9 mg/g toward antimonite. And the common interfering ions of SO₄^2-, NO₃-, CO₃^2-, PO₄^3- and SiO₃^2- exhibited negligible influence on antimonite removal. The mechanism study point that two factors are closely involved: surface complexation and hydrogen bonding. Benefiting from the low cost and environmental-friendly nature of silk fibroin as well as excellent removal capacity and high selectivity, it suggests that the nanohybrids might be promising for antimonite extraction from contaminated water.

An interference screw made using a silk fibroin-based bulk material with high content of hydroxyapatite for anterior cruciate ligament reconstruction in a rabbit model

Upgradation is still in need for the clinically applied interference screws in anterior cruciate ligament reconstruction for more reliable fixation. Silk fibroin bulk materials offer a promising opportunity for this application except lacking osteoinductivity to some extent. Here we report a novel silk-based bulk material with high content of hydroxyapatite-silk fibroin (HA-SF) hybrid particles, which is prepared via a dual-network hydrogel. This composite bulk material possesses a compression modulus of 3.2 GPa, comparable to that of the natural compact bone, and presents satisfactory cytocompatibility and osteoinductivity in vitro when combined with the HA-SF nanoparticles particularly. This composite bulk material shaped into interference screws exhibits remarkable biomechanical properties and significant new-bone ingrowth in the host bone tunnel in a rabbit anterior cruciate ligament reconstruction (ACLR) model at 4 weeks and 12 weeks post-operatively. Moreover, considering that this "hydrogel method" allows the material to be formed in a mold, avoiding complicated post fabrication, it is a potential candidate for clinical translation.

A silk-based high impact composite for the core decompression of the femoral head

Core decompression of the femoral head is a recommended head-conserving strategy for early-stage osteonecrosis of the femoral head. However, no ideal filling material has been found so far. In this study, we fabricated a "solid core-porous coating" composite scaffold, which is a silk fibroin/hydroxypropyl methylcellulose (SF/HPMC) scaffold, by a "two-step" process. The solid core scaffold possesses a sufficient compression modulus (860 MPa) for support, while the porous coating scaffold with controllable pore size and porosity provides a suitable microenvironment for the osteoblast cell to adhere and proliferate. Moreover, the porous coating scaffold was mineralized by adding different contents of hydroxyapatite crystal to further enhance its osteoinductivity, according to the simulated body fluid (SBF) biomineralization assay. To demonstrate the biocompatibility and osteoinductivity of such composite scaffolds, a series of in vitro experiments were performed, indicating the MC3T3-E1 pre-osteoblast cells grew and differentiated well on the mineralized porous coating scaffolds. The mechanical testing results also proved that the mechanical property of the solid core scaffold varied (230-1600 MPa) with different solid contents of SF/HPMC, as expected. Furthermore, the rabbit femoral head core decompression model was adopted and confirmed the excellent mechanical performance of the solid core scaffolds, as well as the satisfied osteoinductivity of the porous coating scaffold, by inserting the composite scaffolds into the bone tunnel in vivo. All of the preliminary results implied that the novel biodegradable composite scaffold has an outstanding prospective for the clinical use of core decompression of the femoral head.

Constructing dual-readout logic operations based on the silk fibroin sol-gel transition

Inspired by an H₂O₂-induced chemical gelation of silk fibroin (SF) and biochemical computing, we demonstrated for the first time that the phase transformation of SF can serve as a visual output signal for constructing multiple Boolean logic operations. In our biological logic gates, three common enzymes, β-galactosidase (β-Gal), glucose oxidase (GOx) and catalase (CAT), were used as inputs and the SF state (sol or gel) was defined as an output. The designed logic gates could be observed by the naked eye and UV-Vis absorption change. Since a distinctive advantage of biocomputing is that different enzymatic reactions can take place smoothly when they work together, our logic system could be further scaled up to generate a multi-input logic network without any "crosstalk" and interference between them. We hope that such a constructed system might help us understand complex physiological processes in living systems, and have potential applications in digital bio-diagnosis.

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.

Protein Microgels from Amyloid Fibril Networks

Nanofibrillar forms of amyloidogenic proteins were initially discovered in the context of protein misfolding and disease but have more recently been found at the origin of key biological functionality in many naturally occurring functional materials, such as adhesives and biofilm coatings. Their physiological roles in nature reflect their great strength and stability, which has led to the exploration of their use as the basis of artificial protein-based functional materials. Particularly for biomedical applications, they represent attractive building blocks for the development of, for instance, drug carrier agents due to their inherent biocompatibility and biodegradability. Furthermore, the propensity of proteins to self-assemble into amyloid fibrils can be exploited under microconfinement, afforded by droplet microfluidic techniques. This approach allows the generation of multi-scale functional microgels that can host biological additives and can be designed to incorporate additional functionality, such as to aid targeted drug delivery.

Creating artificial Rhino Horns from Horse Hair

Demand for rhino horn is driving poaching with devastating effect for the few individuals left of the few species surviving from this once numerous, widespread and cosmopolitan clade of pachyderms. We bundled together tail hairs of the rhino’s ubiquitous near relative, the horse, to be glued together with a bespoke matrix of regenerated silk mimicking the collagenous component of the real horn. This approach allowed us to fabricate composite structures that were confusingly similar to real rhino horn in look, feel and properties. Spectral and thermal FT-IR, DSC and TGA analysis demonstrated the similar chemical composition and thermo-mechanical properties between the natural and the faux horns.

Bone Repair and Regenerative Biomaterials: Towards Recapitulating the Microenvironment

Biomaterials and tissue engineering scaffolds play a central role to repair bone defects. Although ceramic derivatives have been historically used to repair bone, hybrid materials have emerged as viable alternatives. The rationale for hybrid bone biomaterials is to recapitulate the native bone composition to which these materials are intended to replace. In addition to the mechanical and dimensional stability, bone repair scaffolds are needed to provide suitable microenvironments for cells. Therefore, scaffolds serve more than a mere structural template suggesting a need for better and interactive biomaterials. In this review article, we aim to provide a summary of the current materials used in bone tissue engineering. Due to the ever-increasing scientific publications on this topic, this review cannot be exhaustive; however, we attempted to provide readers with the latest advance without being redundant. Furthermore, every attempt is made to ensure that seminal works and significant research findings are included, with minimal bias. After a concise review of crystalline calcium phosphates and non-crystalline bioactive glasses, the remaining sections of the manuscript are focused on organic-inorganic hybrid materials.

Sericin Nerve Guidance Conduit Delivering Therapeutically Repurposed Clobetasol for Functional and Structural Regeneration of Transected Peripheral Nerves

Peripheral nerve injury often causes significant function loss. Autologous nerve grafting as a gold-standard repair strategy for treating such an injury is limited by donor nerve supply. Tissue-engineered nerve guidance conduits (TENGCs) as a promising alternative for autografting are challenged by large nerve gaps. Herein, we fabricate a glutaraldehyde-cross-linked sericin nerve guidance conduit (GSC) incorporated with clobetasol, a glucocorticoid receptor agonist, for repairing a 10 mm long sciatic nerve gap in a rat model. The GSC exhibits biocompatibility and regeneration-favorable physicochemical properties. GSC's degradation products promote the secretion of neurotrophic factors in Schwann cells. By repurposing clobetasol for peripheral nerve regeneration, our work uncovers clobetasol's previously unknown functions in promoting Schwann cell proliferation and upregulating the expression of myelin-related genes. Importantly, the implantation of this clobetasol-loaded GSC in vivo leads to successful regeneration of the transected sciatic nerve. Strikingly, the regeneration outcome is functionally comparable to that of autologous nerve grafting (evidenced by three parameters). Specifically, the static sciatic index (SSI), relative reaction time (RRT) and nerve conduction velocity (NCV) in Clobetasol/GSC group are -74.55, 1.30, and 46.4 mm/s at Week 12, respectively, while these parameters are -64.53, 1.23, and 49.8 mm/s in Autograft group. Thus, this work represents the first report unveiling clobetasol's potential in peripheral nerve regeneration, reveals the feasibility of applying a sericin conduit for repairing a large nerve defect, and demonstrates the effectiveness of the clobetasol-loaded-GSC based strategy in transected nerves' regeneration.

The assembly of silk fibroin and graphene-based nanomaterials with enhanced mechanical/conductive properties and their biomedical applications

The assembly of silk fibroin and graphene-based nanomaterials would present fantastic properties and functions via optimizing the interaction between each other, and can be processed into various formats to tailor specific biomedical applications.

Design, Fabrication, and Function of Silk-Based Nanomaterials

Animal silks are built from pure protein components and their mechanical performance, such as strength and toughness, often exceed most engineered materials. The secret to this success is their unique nanoarchitectures that are formed through the hierarchical self-assembly of silk proteins. This natural material fabrication process in sharp contrast to the production of artificial silk materials, which usually are directly constructed as bulk structures from silk fibroin (SF) molecular. In recent years, with the aim of understanding and building better silk materials, a variety of fabrication strategies have been designed to control nanostructures of silks or to create functional materials from silk nanoscale building blocks. These emerging fabrication strategies offer an opportunity to tailor the structure of SF at the nanoscale and provide a promising route to produce structurally and functionally optimized silk nanomaterials. Here, we review the critical roles of silk nanoarchitectures on property and function of natural silk fibers, outline the strategies of utilization of these silk nanobuilding blocks, and we provide a critical summary of state of the art in the field to create silk nanoarchitectures and to generate silk-based nanocomponents. Further, such insights suggest templates to consider for other materials systems.

Fabrication of In Situ Nanofiber-Reinforced Molecular Composites by Nonequilibrium Self-Assembly

Although the concept of molecular composites (MCs) is very promising, there are major obstacles arising from the immiscibility of the rigid-rod with the random-coil polymers. Here, we developed a novel method for fabricating an in situ reinforced MC system with nonequilibrium self-assembled nanofibrous structures based on bisphenol A epoxy resin, 4,4'-diaminodiphenylsulfone, bismaleimide, and a polyphenylene ether (PPO) oligomer. A variety of spectroscopic and morphological techniques were used to probe the structural evolution from the emergence of nanofibrils, to growth and aggregation of nanofibers, and then to the formation of in situ reinforced MC with strong interfacial interactions. The in situ nanofibers within the polymer matrix could be formed by the polymerization force extruding the PPO phase through the interspaces within the simultaneous interpenetrating network polymers during the cure process of the thermosetting resin system. Compared to the control sample, the in situ nanofiber-reinforced MC exhibited better thermal properties and flame retardancy. In particular, the obtained MC showed a significant improvement in glass transition temperature and mechanical properties, which were mainly attributed to the restriction of high thermal stability of PPO on the segmental motion of polymer chains, the toughening and reinforcement behaviors of PPO nanofibers on the matrix, and the chemical interaction at the PPO/matrix interface.

Biopolymer nanofibrils: structure, modeling, preparation, and applications

Biopolymer nanofibrils exhibit exceptional mechanical properties with a unique combination of strength and toughness, while also presenting biological functions that interact with the surrounding environment. These features of biopolymer nanofibrils profit from their hierarchical structures that spun angstrom to hundreds of nanometer scales. To maintain these unique structural features and to directly utilize these natural supramolecular assemblies, a variety of new methods have been developed to produce biopolymer nanofibrils. In particular, cellulose nanofibrils (CNFs), chitin nanofibrils (ChNFs), silk nanofibrils (SNFs) and collagen nanofibrils (CoNFs), as the four most abundant biopolymer nanofibrils on earth, have been the focus of research in recent years due to their renewable features, wide availability, low-cost, biocompatibility, and biodegradability. A series of top-down and bottom-up strategies have been accessed to exfoliate and regenerate these nanofibrils for versatile advanced applications. In this review, we first summarize the structures of biopolymer nanofibrils in nature and outline their related computational models with the aim of disclosing fundamental structure-property relationships in biological materials. Then, we discuss the underlying methods used for the preparation of CNFs, ChNFs, SNF and CoNFs, and discuss emerging applications for these biopolymer nanofibrils.

Nanosilver: new ageless and versatile biomedical therapeutic scaffold

Silver nanotechnology has received tremendous attention in recent years, owing to its wide range of applications in various fields and its intrinsic therapeutic properties. In this review, an attempt is made to critically evaluate the chemical, physical, and biological synthesis of silver nanoparticles (AgNPs) as well as their efficacy in the field of theranostics including microbiology and parasitology. Moreover, an outlook is also provided regarding the performance of AgNPs against different biological systems such as bacteria, fungi, viruses, and parasites (leishmanial and malarial parasites) in curing certain fatal human diseases, with a special focus on cancer. The mechanism of action of AgNPs in different biological systems still remains enigmatic. Here, due to limited available literature, we only focused on AgNPs mechanism in biological systems including human (wound healing and apoptosis), bacteria, and viruses which may open new windows for future research to ensure the versatile application of AgNPs in cosmetics, electronics, and medical fields.

Peptide-Based Physical Gels Endowed with Thixotropic Behaviour

Thixotropy is one of the oldest documented rheological phenomenon in colloid science and may be defined as an increase of viscosity in a state of rest and a decrease of viscosity when submitted to a constant shearing stress. This behavior has been exploited in recent years to prepare injectable hydrogels for application in drug delivery systems. Thixotropic hydrogels may be profitably used in the field of regenerative medicine, which promotes tissue healing after injuries and diseases, as the molten hydrogel may be injected by syringe and then self-adapts in the space inside the injection site and recovers the solid form. We will focus our attention on the preparation, properties, and some applications of biocompatible thixotropic hydrogels.

The fabrication of an ICA-SF/PLCL nanofibrous membrane by coaxial electrospinning and its effect on bone regeneration in vitro and in vivo

GBR is currently accepted as one of the most effective approaches for bone defect regeneration relating to dental implant. Icariin is the main active ingredient in the extraction of total flavonoids from the Chinese traditional herb Epimediumbrevicornum Maxim. In this study, ICA was successfully incorporated into the nanofibers barrier membrane (ICA-SF/PLCL) as osteoinduction factor by coaxial electrospinning and was released in a sustained and controlled manner. The entire release period included two stages: an initial burst stage (47.54 ± 0.06% on 5 d) and a decreasing and constant stage (82.09 ± 1.86% on 30 d). The membrane has good biocompatibility with BMMSCs anchored and significantly promoted its osteogenic activity. Moreover, in vivo experiment, bone defect covered by ICA-SF/PLCL membrane in rat cranium were statistically repaired compare to other groups. 12 weeks after implantation, in the test group, the new bone formation spread to cover most of the defect region with volume and density of approximately 15.95 ± 3.58 mm³ and 14.02 ± 0.93%. These results demonstrated that ICA-SF/PLCL nanofibrous membrane could be a promising barrier applicated for GBR.

Biosynthesis and Characterization of AgNPs-Silk/PVA Film for Potential Packaging Application

Bionanocomposite packaging materials have a bright future for a broad range of applications in the food and biomedical industries. Antimicrobial packaging is one of the bionanocomposite packaging materials. Silver nanoparticle (AgNP) is one of the most attractive antimicrobial agents for its broad spectrum of antimicrobial activity against microorganisms. However, the traditional method of preparing AgNPs-functionalized packaging material is cumbersome and not environmentally friendly. To develop an efficient and convenient biosynthesis method to prepare AgNPs-modified bionanocomposite material for packaging applications, we synthesized AgNPs in situ in a silk fibroin solution via the reduction of Ag⁺ by the tyrosine residue of fibroin, and then prepared AgNPs-silk/poly(vinyl alcohol) (PVA) composite film by blending with PVA. AgNPs were synthesized evenly on the surface or embedded in the interior of silk/PVA film. The prepared AgNPs-silk/PVA film exhibited excellent mechanical performance and stability, as well as good antibacterial activity against both Gram-negative and Gram-positive bacteria. AgNPs-silk/PVA film offers more choices to be potentially applied in the active packaging field.