Aligned silk-based 3-D architectures for contact guidance in tissue engineering

Advances of regenerated and functionalized silk biomaterials and application in skin wound healing

The cocoon silk of silkworms (Bombyx mori) has multiple potential applications in biomedicine due to its good biocompatibility, mechanical properties, degradability, and plasticity. Numerous studies have confirmed that silk material dressings are more effective than traditional ones in the skin wound healing process. Silk material research has recently moved toward functionalized biomaterials and achieved remarkable results. Herein, we summarize the recent advances in functionalized silk materials and their efficacy in skin wound healing. In particular, transgenic technology has realized the specific expression of human growth factors in the silk glands of the silkworms, which lays the foundation for fabricating novel and low-cost functionalized materials. Without a green and safe preparation process, the best raw silk materials cannot be made into medically safe products. Therefore, we provide an overview of green and gentle approaches for silk degumming and silk sericin (SS) extraction. Moreover, we summarize and discuss the processing methods of silk fibroin (SF) and SS materials and their potential applications, such as burns, diabetic wounds, and other wounds. This review aims to enhance our understanding of new advances and directions in silk materials and guide future biomedical research.

Expanding the Potential of Self-Assembled Silk Fibroin as Aerogel Particles for Tissue Regeneration

A newly produced silk fibroin (SF) aerogel particulate system using a supercritical carbon dioxide (scCO2)-assisted drying technology is herein proposed for biomedical applications. Different concentrations of silk fibroin (3%, 5%, and 7% (w/v)) were explored to investigate the potential of this technology to produce size- and porosity-controlled particles. Laser diffraction, helium pycnometry, nitrogen adsorption-desorption analysis and Fourier Transform Infrared with Attenuated Total Reflectance (FTIR-ATR) spectroscopy were performed to characterize the physicochemical properties of the material. The enzymatic degradation profile of the SF aerogel particles was evaluated by immersion in protease XIV solution, and the biological properties by cell viability and cell proliferation assays. The obtained aerogel particles were mesoporous with high and concentration dependent specific surface area (203-326 m2/g). They displayed significant antioxidant activity and sustained degradation in the presence of protease XIV enzyme. The in vitro assessment using human dermal fibroblasts (HDF) confirm the particles' biocompatibility, as well as the enhancement in cell viability and proliferation.

Promotion of Wound Healing Using Nanoporous Silk Fibroin Sponges

Wound dressings are important for wound repair. The morphology of the biomaterials used in these dressings, and in particular, the pore structure affects tissue regeneration by facilitating attachment and proliferation of cells due to the hierarchical multiscale, water absorbance, and nutrient transport. In the present study, silk fibroin (SF) sponges with walls containing nanopores (SFNS) were prepared from SF nanoparticles generated during the autoclaving of SF solutions, followed by leaching the SF nanoparticles from the freeze-dried sponges of SF. The nano/microporous structure, biofluid absorbance, and porosity of the SF sponges with and without nanopores were characterized. In vitro cell proliferation, in vivo biocompatibility, and wound healing were evaluated with the sponges. The results demonstrated that SFNS had significantly increased porosity and water permeability, as well as cell attachment and proliferation when compared with SF sponges without the nanopores (SFS). Wound dressings were assessed in a rat skin wound model, and SFNS was superior to SFS in accelerating wound healing, supported by vascularization, deposition of collagen, and increased epidermal thickness over 21 days. Hence, such a dressing material with a hierarchical multiscale pore structure could promote cell migration, vascularization, and tissue regeneration independently without adding any growth factor, which would offer a new strategy to design and engineer better-performed wound dressing.

Chopped fibers and nano-hydroxyapatite enhanced silk fibroin porous hybrid scaffolds for bone augmentation

Chopped fiber (CF)- and nano-hydroxyapatite (n-HA)-enhanced silk fibroin (SF) porous hybrid scaffolds (SHCF) were prepared by freeze-drying for bone augmentation. Compared with pristine SF scaffolds, the incorporation of CF and n-HA can significantly enhance the mechanical properties of the composite scaffold. The results of cell experiments and mouse subcutaneous implantation indicated that the SHCF could alleviate foreign body reactions (FBR) led by macrophages and neutrophils, promote the polarization of RAW264.7 cells to anti-inflammatory M2 macrophages, and inhibit the secretion of pro-inflammatory cytokine TNF-α. A rat femoral defect repair model and bulk-RNA-seq analysis indicated that the CF- and n-HA-enhanced SHCF promoted the proliferation and osteogenic differentiation of bone mesenchymal stem cells (BMSCs) by the upregulation of Capns1 expression and regulated the calcium signaling pathway to mediate osteogenesis-related cell behavior, subsequently promoting bone regeneration.

Bioinspired silk fibroin materials: From silk building blocks extraction and reconstruction to advanced biomedical applications

Silk fibroin has become a promising biomaterial owing to its remarkable mechanical property, biocompatibility, biodegradability, and sufficient supply. However, it is difficult to directly construct materials with other formats except for yarn, fabric and nonwoven based on natural silk. A promising bioinspired strategy is firstly extracting desired building blocks of silk, then reconstructing them into functional regenerated silk fibroin (RSF) materials with controllable formats and structures. This strategy could give it excellent processability and modifiability, thus well meet the diversified needs in biomedical applications. Recently, to engineer RSF materials with properties similar to or beyond the hierarchical structured natural silk, novel extraction and reconstruction strategies have been developed. In this review, we seek to describe varied building blocks of silk at different levels used in biomedical field and their effective extraction and reconstruction strategies. This review also present recent discoveries and research progresses on how these functional RSF biomaterials used in advanced biomedical applications, especially in the fields of cell-material interactions, soft tissue regeneration, and flexible bioelectronic devices. Finally, potential study and application for future opportunities, and current challenges for these bioinspired strategies and corresponding usage were also comprehensively discussed. In this way, it aims to provide valuable references for the design and modification of novel silk biomaterials, and further promote the high-quality-utilization of silk or other biopolymers.

Biomimetic Silk Macroporous Materials for Drug Delivery Obtained via Ice-Templating

Silk from Bombyx mori is one of the most exciting materials in nature. The apparently simple arrangement of its two major components─two parallel filaments of silk fibroin (SF) coated by a common sericin (SS) sheath─provides a combination of mechanical and surface properties that can protect the moth during its most vulnerable phase, the pupal stage. Here, we recapitulate the topology of native silk fibers but shape them into three-dimensional porous constructs using an unprecedented design strategy. We demonstrate, for the first time, the potential of these macroporous silk foams as dermal patches for wound protection and for the controlled delivery of Rifamycin (Rif), a model antibiotic. The method implies (i) removing SS from silk fibers; (ii) shaping SF solutions into macroporous foams via ice-templating; (iii) stabilizing the SF macroporous foam in a methanolic solution of Rif; and (iv) coating Rif-loaded SF foams with a SS sheath. The resulting SS@SF foams exhibit water wicking capacity and accommodate up to ∼20% deformation without detaching from a skin model. The antibacterial behavior of Rif-loaded SS@SF foams against Staphylococcus aureus on agar plates outperforms that of SF foams (>1 week and 4 days, respectively). The reassembly of natural materials as macroporous foams─illustrated here for the reconstruction of silk-based materials─can be extended to other multicomponent natural materials and may play an important role in applications where controlled release of molecules and fluid transport are pivotal.

Fabrication and characterization of an acellular annulus fibrosus scaffold with aligned porous construct for tissue engineering

Scaffolds mimicking the native annulus fibrosus (AF) extracellular matrix (ECM) structure are crucial to guide the seeding cells to regenerate aligned tissue, while fabricating such a scaffold by synthetic material is challengeable. Native acellular scaffolds derived from AF tissue certainly possess the advantages of natural structure and composition. Based on previous studies, we modified decellularization procedure and especially compared two drying methods, including gradient dehydration and freeze-drying. The decellularization process can effectively remove the host cells and antigens such as α-Gal, while maintaining the original ECM including GAG and collagen I. Compared with gradient dehydration, freeze-drying not only rendered the decellularized scaffold in dry state for storage but also gave the scaffold more aligned porous structure and hydrophilicity. And, the acellular porous scaffold manifested better capacity of supporting cell ingrowth when seeded human bone marrow mesenchymal stem cells (hBMSCs) or implanted in vivo. Furthermore, this optimized freeze-dried scaffold showed similar mechanical elastic modulus as native AF and demonstrated rare inflammatory granuloma and immune rejection as observed in HE staining and immunohistochemistry staining (IHC) of CD8 and MAC387 epitopes when implanted subcutaneously in vivo. To sum up, through our decellularization and freeze-drying procedure, an aligned porous three-dimensional scaffold derived from the natural AF ECM was successfully fabricated with good retention of ECM components and benign biocompatibility. It will be a promising scaffold for AF tissue engineering.

Ice Templating Soft Matter: Fundamental Principles and Fabrication Approaches to Tailor Pore Structure and Morphology and Their Biomedical Applications

Porous scaffolds are widely used in biomedical applications where pore size and morphology influence a range of biological processes, including mass transfer of solutes, cellular interactions and organization, immune responses, and tissue vascularization, as well as drug delivery from biomaterials. Ice templating, one of the most widely utilized techniques for the fabrication of porous materials, allows control over pore morphology by controlling ice formation in a suspension of solutes. By fine-tuning freezing and solute parameters, ice templating can be used to incorporate pores with tunable morphological features into a wide range of materials using a simple, accessible, and scalable process. While soft matter is widely ice templated for biomedical applications and includes commercial and clinical products, the principles underpinning its ice templating are not reviewed as well as their inorganic counterparts. This review describes and critically evaluates fundamental principles, fabrication and characterization approaches, and biomedical applications of ice templating in polymer-based biomaterials. It describes the utility of porous scaffolds in biomedical applications, highlighting biological mechanisms impacted by pore features, outlines the physical and thermodynamic mechanisms underpinning ice templating, describes common fabrication setups, critically evaluates complexities of ice templating specific to polymers, and discusses future directions in this field.

Engineering topography: effects on nerve cell behaviors and applications in peripheral nerve repair

There have been extensive studies on the application of topography in the field of tissue repair. A common feature of these studies is that the existence of topological structures in tissue repair scaffolds can effectively regulate a series of behaviors of cells and play a positive role in a variety of tissue repair and regeneration processes. This review focuses on the application of topography in the field of peripheral nerve repair. The integration of the topological structure and biomaterials to construct peripheral nerve conduits to mimic a natural peripheral nerve structure has an important role in promoting the recovery of peripheral nerve function. Therefore, in this review, we systematically analysed the structure of peripheral nerves and summarized the effects of topographic cues of different scales and shapes on the behaviors of nerve cells, including cell morphology, adhesion, proliferation, migration and differentiation. Furthermore, the application and performance of scaffolds with different topological structures in peripheral nerve repair are also discussed. This systematic summary may help to provide more effective strategies for peripheral nerve regeneration (PNR) and shed light on nervous tissue engineering and regenerative medicine.

A Paradigm Shift in Tissue Engineering: From a Top–Down to a Bottom–Up Strategy

Tissue engineering (TE) was initially designed to tackle clinical organ shortage problems. Although some engineered tissues have been successfully used for non-clinical applications, very few (e.g., reconstructed human skin) have been used for clinical purposes. As the current TE approach has not achieved much success regarding more broad and general clinical applications, organ shortage still remains a challenging issue. This very limited clinical application of TE can be attributed to the constraints in manufacturing fully functional tissues via the traditional top–down approach, where very limited cell types are seeded and cultured in scaffolds with equivalent sizes and morphologies as the target tissues. The newly proposed developmental engineering (DE) strategy towards the manufacture of fully functional tissues utilises a bottom–up approach to mimic developmental biology processes by implementing gradual tissue assembly alongside the growth of multiple cell types in modular scaffolds. This approach may overcome the constraints of the traditional top–down strategy as it can imitate in vivo-like tissue development processes. However, several essential issues must be considered, and more mechanistic insights of the fundamental, underpinning biological processes, such as cell–cell and cell–material interactions, are necessary. The aim of this review is to firstly introduce and compare the number of cell types, the size and morphology of the scaffolds, and the generic tissue reconstruction procedures utilised in the top–down and the bottom–up strategies; then, it will analyse their advantages, disadvantages, and challenges; and finally, it will briefly discuss the possible technologies that may overcome some of the inherent limitations of the bottom–up strategy.

Porous Silk Fibroin/Cellulose Hydrogels for Bone Tissue Engineering via a Novel Combined Process Based on Sequential Regeneration and Porogen Leaching

Scaffolds used for bone tissue engineering need to have a variety of features to accommodate bone cells. The scaffold should mimic natural bone, it should have appropriate mechanical strength, support cell differentiation to the osteogenic lineage, and offer adequate porosity to allow vascularization and bone in-growth. In this work, we aim at developing a new process to fabricate such materials by creating a porous composite material made of silk fibroin and cellulose as a suitable scaffold of bone tissue engineering. Silk fibroin and cellulose are both dissolved together in N,N-dimethylacetamide/LiCl and molded to a porous structure using NaCl powder. The hydrogels are prepared by a sequential regeneration process: cellulose is solidified by water vapor treatment, while the remaining silk fibroin in the hydrogel is insolubilized by methanol, which leads to a cellulose framework structure embedded in a silk fibroin matrix. Finally, the hydrogels are soaked in water to dissolve the NaCl for making a porous structure. The cellulose composition results in improving the mechanical properties for the hydrogels in comparison to the silk fibroin control material. The pore size and porosity are estimated at around 350 µm and 70%, respectively. The hydrogels support the differentiation of MC3T3 cells to osteoblasts and are expected to be a good scaffold for bone tissue engineering.

Silk fibroins in multiscale dimensions for diverse applications

Silk biomaterials in different forms such as particles, coatings and their assemblies, represent unique type of materials in multiple scales and dimensions. Herein, we provide an overview of multi-scale silk fibroin materials including silk particles, silk coatings and silk assemblies, each of which represents a unique type of material with wide range of applications. They feature tunable structures and mechanical properties with excellent biocompatibility, which are essentially required for various biomedical and drug delivery applications. The review focuses on bringing a new perspective on the utilization of regenerated silk fibroins in modern biomedicine by beginning with the fabrication of silk in multiscale dimensions and their state-of-the-art applications in various biomedical and bioelectronic fields. It covers the fundamentals of processing silk fibroins in multi-dimensions (sizes and shapes) with a specific emphasis on its structural tunability at various length scales (nano-micro) by using the latest fabrication methods/mechanisms and advanced fabrication technologies, followed by their recent applications in diverse fields of biomedicine.

Bi-layered Tubular Microfiber Scaffolds as Functional Templates for Engineering Human Intestinal Smooth Muscle Tissue

Designing biomimetic scaffolds with in vivo-like microenvironments using biomaterials is an essential component of successful tissue engineering approaches. The intestinal smooth muscle layers exhibit a complex tubular structure consisting of two concentric muscle layers in which the inner circular layer is orthogonally oriented to the outer longitudinal layer. Here, we present a three-dimensional (3D) bi-layered tubular scaffold based on flexible, mechanically robust and well aligned silk protein microfibers to mimic native human intestinal smooth muscle structure. The scaffolds were seeded with primary human intestinal smooth muscle cells to replicate human intestinal muscle tissues in vitro. Characterization of the tissue constructs revealed good biocompatibility and support for cell alignment and elongation in the different scaffold layers to enhance cell differentiation and functions. Furthermore, the engineered smooth muscle constructs supported oriented neurite outgrowth, a requisite step to achieve functional innervation. These results suggested these microfiber scaffolds as functional templates for in vitro regeneration of human intestinal smooth muscle systems. The scaffolding provides a crucial step toward engineering functional human intestinal tissue in vitro, as well as for the engineering of many other types of smooth muscles in terms of their similar phenotypes. Such utility may lead to a better understanding of smooth muscle associated diseases and treatments.

Silk-Based Therapeutics Targeting Pseudomonas aeruginosa

Pseudomonas aeruginosa (P. aeruginosa) infections may lead to severe damage of the cornea, mucosa, and skin. The highly aggressive nature of P. aeruginosa and the rise in multi-drug resistance, particularly in nosocomial settings, lead to an increased risk for permanent tissue damage and potentially death. Thus, a growing need exists to develop alternative treatments to reduce both the occurrence of bacterial infection and biofilm development, as well as pathological progression post-infection. Silk derived from Bombyx mori silkworms serves as a unique biomaterial that is biocompatible with low immunogenicity and high versatility, and thereby ideal for stabilizing therapeutics. In this study, we assessed the cytotoxicity of P. aeruginosa on human corneal stromal stem cells and two mucosal cell lines (Caco-2 and HT29-MTX). To determine whether antibiotic-immobilized scaffolds can serve as alternative therapeutics to free, diffuse forms, we developed novel gentamicin-conjugated silk films as functional scaffolds and compared antimicrobial effects and free gentamicin. The advantages of generating a surface coating with a covalently-bound antibiotic may reduce potential side-effects associated with free gentamicin, as well as limit the diffusion of the drug. Our results suggest that gentamicin conjugated to native silk and carboxyl-enriched silk inhibits P. aeruginosa growth. Development of stabilized antibiotic treatments with surface toxicity selective against bacteria may serve as an alternative approach to treat active infections, as well as potential prophylactic use as coatings in high-risk cases, such as post-surgical complications or prolonged hospitalization.

Silk fibroin-alginate based beads for human mesenchymal stem cell differentiation in 3D

Lately silk fibroin has gained a lot of popularity as a tissue engineering scaffold due to its exceptional mechanical properties, negligible inflammatory reactions, remarkable biocompatibility, and tunable biodegradability. Nonetheless, 3 dimensional (3D) silk fibroin based scaffolds, which allow simultaneous formation of scaffolds and cell encapsulation with minimal damage to the cells, are unavailable, as most of the methods involve the use of some cell destructive techniques. Thus, cells have to be loaded after the scaffold formation and the study has to rely upon the ability of the cells to penetrate the scaffold to obtain a 3D microenvironment. Hence, these platforms do not allow for a true 3D system replicating the in vivo environment. Here silk fibroin-alginate based beads have been developed, and retain silk fibroin for a longer period of time and allow for simultaneous cell encapsulation as the crosslinking method is cell-compatible. It is demonstrated for the first time that these silk fibroin-alginate beads can be used to encapsulate the cells at varying cell densities depending on the desired application. These beads were further used to study the effect of functional groups on human mesenchymal stem cell (hMSC) differentiation in 3D, by utilizing carboxylic groups naturally present in alginate as well as introducing phosphate groups. The results showed that these beads were able to support the growth and proliferation of hMSCs and induced differentiation solely due to functional groups within 14 days. These beads were better in directing hMSC differentiation into osteogenic and chondrogenic lineages compared to 2D surfaces and differentiation media.

Water-insoluble amorphous silk fibroin scaffolds from aqueous solutions

Regenerated silk fibroin (RSF) is emerging as promising biomaterial for regeneration, drug delivery and optical devices, with continued demand for mild, all-aqueous processes to control microstructure and the performance. Here, temperature control of assembly kinetics was introduced to prepare the water-insoluble scaffolds from neutral aqueous solutions of RSF protein. Higher temperatures were used to accelerate the assembly rate of the silk fibroin protein chains in aqueous solution and during the lyophilization process, resulting in water-insoluble scaffold formation. The scaffolds were mainly composed of amorphous states of the silk fibroin chains, endowing softer mechanical properties. These scaffolds also showed nanofibrous structures, improved cell proliferation in vitro and enhanced neovascularization and tissue regeneration in vivo than previously reported silk fibroin scaffolds. These results suggest utility of silk scaffolds in soft tissue regeneration.

Mechanically Strong Silica-Silk Fibroin Bioaerogel: A Hybrid Scaffold with Ordered Honeycomb Micromorphology and Multiscale Porosity for Bone Regeneration

Due to the synergic feature of individual components in hybrid (nano)biomaterials, their application in regenerative medicine has drawn significant attention. Aiming to address all the current challenges of aerogel as a potent scaffold in bone tissue engineering application, we adopted a novel synthesis approach to synergistically improve the pore size regime and mechanical strength in the aerogel. The three-dimensional aerogel scaffold in this study has been synthesized through a versatile one-pot aqueous-based sol-gel hybridization/assembly of organosilane (tetraethyl orthosilicate) and silk fibroin (SF) biopolymer, followed by unidirectional freeze-casting of the as-prepared hybrid gel and supercritical drying. The developed ultralight silica-SF aerogel hybrids demonstrated a hierarchically organized porous structure with interesting honeycomb-shaped micromorphology and microstructural alignment (anisotropy) in varied length scales. The average macropore size of the hybrid aerogel lied in ∼0.5-18 μm and was systematically controlled with freeze-casting conditions. Together with high porosity (91-94%), high Young's modulus (∼4-7 MPa, >3 order of magnitude improvement compared to their pristine aerogel counterparts), and bone-type anisotropy in the mechanical compressive behavior, the silica-SF hybrid aerogel of this study acted as a very competent scaffold for bone tissue formation. The results of in vitro assessments revealed that the silica-SF aerogel is not only cytocompatible and nonhemolytic but also acted as an open porous microenvironment to trigger osteoblast cell attachment, growth, and proliferation on its surface within 14 days of incubation. Moreover, to support the in vitro results, in vivo bone formation within the aerogel implant in the bone defect site was studied. The X-ray radiology and microcomputed tomography analyses confirmed that a significant new bone tissue density formed in the defect site within 25 days of implantation. Also, in vivo toxicology studies showed a zero-toxic impact of the aerogel implant on the blood biochemical and hematological parameters. Finally, the study clearly shows the potential of aerogel as a bioactive and osteoconductive open porous cellular matrix for a successful osseointegration process.

Aligned Scaffolds with Biomolecular Gradients for Regenerative Medicine

Aligned topography and biomolecular gradients exist in various native tissues and play pivotal roles in a set of biological processes. Scaffolds that recapitulate the complex structure and microenvironment show great potential in promoting tissue regeneration and repair. We begin with a discussion on the fabrication of aligned scaffolds, followed by how biomolecular gradients can be immobilized on aligned scaffolds. In particular, we emphasize how electrospinning, freeze drying, and 3D printing technology can accomplish aligned topography and biomolecular gradients flexibly and robustly. We then highlight several applications of aligned scaffolds and biomolecular gradients in regenerative medicine including nerve, tendon/ligament, and tendon/ligament-to-bone insertion regeneration. Finally, we finish with conclusions and future perspectives on the use of aligned scaffolds with biomolecular gradients in regenerative medicine.

Macromolecular crowding tunes 3D collagen architecture and cell morphogenesis

Collagen I is the primary extracellular matrix component of most solid tumors and influences metastatic progression. Collagen matrix engineering techniques are useful for understanding how this complex biomaterial regulates cancer cell behavior and for improving in vitro cancer models. Here, we establish an approach to tune collagen fibril architecture using PEG as an inert molecular crowding agent during gelation and cell embedding. We find that crowding produces matrices with tighter fibril networks that are less susceptible to proteinase mediated degradation, but does not significantly alter matrix stiffness. The resulting matrices have the effect of preventing cell spreading, confining cells, and reducing cell contractility. Matrix degradability and fibril length are identified as strong predictors of cell confinement. Further, the degree of confinement predicts whether breast cancer cells will ultimately undergo individual or collective behaviors. Highly confined breast cancer cells undergo morphogenesis to form either invasive networks reminiscent of aggressive tumors or gland and lobule structures reminiscent of normal breast epithelia. This morphological transition is accompanied by expression of cell-cell adhesion genes, including PECAM1 and ICAM1. Our study suggests that cell confinement, mediated by matrix architecture, is a design feature that tunes the transcriptional and morphogenic state of breast cancer cells.

Irradiation-Induced Compositional Effects on Human Bone After Extracorporeal Therapy for Bone Sarcoma

The present study investigates Raman scattering of human bone irradiated with 50 Gy single dose during therapeutic treatment of Ewing and Osteosarcoma. Bone quality was evaluated via mineral-to-matrix ratio, degree of crystallinity, change in amount of calcium, and carbonate substitution. Alteration in collagen and its cross-links was quantified through second-derivative deconvolution of Amide I peak. A dose of 50 Gy radiation leads to almost 50% loss of mineral content, while maintaining mineral crystallinity, and small changes in carbonate substitution. Deconvolution of Amide I suggested modifications in collagen structure via increase in amount of enzymatic trivalent cross-linking (p < 0.05). Overall irradiation led to detrimental effect on bone quality via changes in its composition, consequently reducing its elastic modulus with increased plasticity. The study thus quantifies effect of single-dose 50 Gy radiation on human bone, which in turn is necessary for designing improved radiation dosage during ECRT and for better understanding post-operative care.

Silk Fibroin-Based Scaffold for Bone Tissue Engineering

Regeneration of diseased or damaged skeletal tissues is one of the challenge that needs to be solved. Although there have been many bone tissue engineering developed, scaffold-based tissue engineering complement the conventional treatment for large bone by completing biological and functional environment. Among many materials, silk fibroin (SF) is one of the favorable material for applications in bone tissue engineering scaffolding. SF is a fibrous protein mainly extracted from Bombyx mori. and spiders. SF has been used as a biomaterial for bone graft by its unique mechanical properties, controllable biodegradation rate and high biocompatibility. Moreover, SF can be processed using conventional and advanced biofabrication methods to form various scaffold types such as sponges, mats, hydrogels and films. This review discusses about recent application and advancement of SF as a biomaterial.

Silk scaffolds in bone tissue engineering: An overview

Bone tissue plays multiple roles in our day-to-day functionality. The frequency of accidental bone damage and disorder is increasing worldwide. Moreover, as the world population continues to grow, the percentage of the elderly population continues to grow, which results in an increased number of bone degenerative diseases. This increased elderly population pushes the need for artificial bone implants that specifically employ biocompatible materials. A vast body of literature is available on the use of silk in bone tissue engineering. The current work presents an overview of this literature from materials and fabrication perspective. As silk is an easy-to-process biopolymer; this allows silk-based biomaterials to be molded into diverse forms and architectures, which further affects the degradability. This makes silk-based scaffolds suitable for treating a variety of bone reconstruction and regeneration objectives. Silk surfaces offer active sites that aid the mineralization and/or bonding of bioactive molecules that facilitate bone regeneration. Silk has also been blended with a variety of polymers and minerals to enhance its advantageous properties or introduce new ones. Several successful works, both in vitro and in vivo, have been reported using silk-based scaffolds to regenerate bone tissues or other parts of the skeletal system such as cartilage and ligament. A growing trend is observed toward the use of mineralized and nanofibrous scaffolds along with the development of technology that allows to control scaffold architecture, its biodegradability and the sustained releasing property of scaffolds. Further development of silk-based scaffolds for bone tissue engineering, taking them up to and beyond the stage of human trials, is hoped to be achieved in the near future through a cross-disciplinary coalition of tissue engineers, material scientists and manufacturing engineers.

STATEMENT OF SIGNIFICANCE

The state-of-art of silk biomaterials in bone tissue engineering, covering their wide applications as cell scaffolding matrices to micro-nano carriers for delivering bone growth factors and therapeutic molecules to diseased or damaged sites to facilitate bone regeneration, is emphasized here. The review rationalizes that the choice of silk protein as a biomaterial is not only because of its natural polymeric nature, mechanical robustness, flexibility and wide range of cell compatibility but also because of its ability to template the growth of hydroxyapatite, the chief inorganic component of bone mineral matrix, resulting in improved osteointegration. The discussion extends to the role of inorganic ions such as Si and Ca as matrix components in combination with silk to influence bone regrowth. The effect of ions or growth factor-loaded vehicle incorporation into regenerative matrix, nanotopography is also considered.

Enhanced Directional Migration of Cancer Stem Cells in 3D Aligned Collagen Matrices

Directed cell migration by contact guidance in aligned collagenous extracellular matrix (ECM) is a critical enabler of breast cancer dissemination. The mechanisms of this process are poorly understood, particularly in 3D, in part because of the lack of efficient methods to generate aligned collagen matrices. To address this technological gap, we propose a simple method to align collagen gels using guided cellular compaction. Our method yields highly aligned, acellular collagen constructs with predictable microstructural features, thus providing a controlled microenvironment for in vitro experiments. Quantifying cell behavior in these anisotropic constructs, we find that breast carcinoma cells are acutely sensitive to the direction and extent of collagen alignment. Further, live cell imaging and analysis of 3D cell migration reveals that alignment of collagen does not alter the total motility of breast cancer cells, but simply redirects their migration to produce largely one-dimensional movement. However, a profoundly enhanced motility in aligned collagen matrices is observed for the subpopulation of carcinoma cells with high tumor initiating and metastatic capacity, termed cancer stem cells (CSCs). Analysis of the biophysical determinants of cell migration show that nuclear deformation is not a critical factor associated with the observed increases in motility for CSCs. Rather, smaller cell size, a high degree of phenotypic plasticity, and increased protrusive activity emerge as vital facilitators of rapid, contact-guided migration of CSCs in aligned 3D collagen matrices.

Surgical repair of annulus defect with biomimetic multilamellar nano/microfibrous scaffold in a porcine model

Annulus defect is associated with reherniation and disc degeneration after discectomy; currently there is no effective treatment that addresses this problem. The annulus is a hierarchical lamellar structure, where each lamella consists of aligned collagen fibres, which are parallel and tilted at 30° to the spinal axis. In this study, a biomimetic biodegradable scaffold consisting of multilamellar nano/microfibres, sharing nanotopography and microporosity similar to the native lamellar structure, was assessed in a porcine model, aided by sealing with fascia and medical glue and subsequent suture fixation. After 6- and 12-week observation, we found that this treatment restored nucleus volume and slowed down disc degeneration, as indicated by magnetic resonance imaging of T1/T2-weighted, T2-mapping, T1-ρ imaging. Histological analysis showed aligned collagen fibres organized in the scaffold and integrated with surrounding native annulus tissue. The autologous bone marrow concentrate-seeded scaffolds showed slightly earlier collagen fibre formation at 6 weeks. This novel treatment could efficiently close the annulus defect with newly formed, organized and integrated collagen fibres in a porcine model. Copyright © 2016 John Wiley & Sons, Ltd.

* Fabrication and Characterization of Biphasic Silk Fibroin Scaffolds for Tendon/Ligament-to-Bone Tissue Engineering

Tissue engineering is an attractive strategy for tendon/ligament-to-bone interface repair. The structure and extracellular matrix composition of the interface are complex and allow for a gradual mechanical stress transfer between tendons/ligaments and bone. Thus, scaffolds mimicking the structural features of the native interface may be able to better support functional tissue regeneration. In this study, we fabricated biphasic silk fibroin scaffolds designed to mimic the gradient in collagen molecule alignment present at the interface. The scaffolds had two different pore alignments: anisotropic at the tendon/ligament side and isotropic at the bone side. Total porosity ranged from 50% to 80% and the majority of pores (80-90%) were <100-300 μm. Young's modulus varied from 689 to 1322 kPa depending on the type of construct. In addition, human adipose-derived mesenchymal stem cells were cultured on the scaffolds to evaluate the effect of pore morphology on cell proliferation and gene expression. Biphasic scaffolds supported cell attachment and influenced cytoskeleton organization depending on pore alignment. In addition, the gene expression of tendon/ligament, enthesis, and cartilage markers significantly changed depending on pore alignment in each region of the scaffolds. In conclusion, the biphasic scaffolds fabricated in this study show promising features for tendon/ligament-to-bone tissue engineering.

Anisotropic forces from spatially constrained focal adhesions mediate contact guidance directed cell migration

Directed migration by contact guidance is a poorly understood yet vital phenomenon, particularly for carcinoma cell invasion on aligned collagen fibres. We demonstrate that for single cells, aligned architectures providing contact guidance cues induce constrained focal adhesion maturation and associated F-actin alignment, consequently orchestrating anisotropic traction stresses that drive cell orientation and directional migration. Consistent with this understanding, relaxing spatial constraints to adhesion maturation either through reduction in substrate alignment density or reduction in adhesion size diminishes the contact guidance response. While such interactions allow single mesenchymal-like cells to spontaneously 'sense' and follow topographic alignment, intercellular interactions within epithelial clusters temper anisotropic cell-substratum forces, resulting in substantially lower directional response. Overall, these results point to the control of contact guidance by a balance of cell-substratum and cell-cell interactions, modulated by cell phenotype-specific cytoskeletal arrangements. Thus, our findings elucidate how phenotypically diverse cells perceive ECM alignment at the molecular level.

Biocompatible, Ultralight, Strong Hydroxyapatite Networks Based on Hydroxyapatite Microtubes with Excellent Permeability and Ultralow Thermal Conductivity

In the past decade, ultralight materials such as aerogels have become one of the hottest research topics owing to their unique properties. However, most reported ultralight materials are bioinert. In this work, by using biocompatible, monodisperse, single-crystalline hydroxyapatite (HAP) microtubes as the building blocks, ultralight, strong, highly porous, three-dimensional (3-D) HAP networks have been successfully fabricated through a facile freeze-drying method and subsequent sintering at 1300 °C for 2 h. The as-prepared ultralight, strong, highly porous 3-D HAP microtube networks exhibit superior properties, such as ultrahigh porosity (89% to 96%), low density (94.1 to 347.1 mg/cm³), high compressive strength that can withstand more than 6400 times of their own weight without any fracture and is higher than aerogels with similar densities, and ultralow thermal conductivity (0.05 W/mK). Owing to their high porosity, ultralight, and good mechanical properties and high biocompatibility, the HAP microtube networks reported herein are promising for applications in various fields.

Robust silk fibroin/bacterial cellulose nanoribbon composite scaffolds with radial lamellae and intercalation structure for bone regeneration

Biomimetic scaffolds with a gradient gap distance and robust mechanical properties were prepared using silk fibroin and bacterial cellulose.

Carbon nanotube (CNT) and nanofibrillated cellulose (NFC) reinforcement effect on thermoplastic polyurethane (TPU) scaffolds fabricated via phase separation using dimethyl sulfoxide (DMSO) as solvent

Although phase separation is a simple method of preparing tissue engineering scaffolds, it suffers from organic solvent residual in the scaffold. Searching for nontoxic solvents and developing effective solvent removal methods are current challenges in scaffold fabrication. In this study, thermoplastic polyurethane (TPU) scaffolds containing carbon nanotubes (CNTs) or nanofibrillated cellulose fibers (NFCs) were prepared using low toxicity dimethyl sulfoxide (DMSO) as a solvent. The effects of two solvent removal approaches on the final scaffold morphology were studied. The freeze drying method caused large pores, with small pores on the pore walls, which created connections between the pores. Meanwhile, the leaching and freeze drying method led to interconnected fine pores with smaller pore diameters. The nucleation effect of CNTs and the phase separation behavior of NFCs in the TPU solution resulted in significant differences in the microstructures of the resulting scaffolds. The mechanical performance of the nanocomposite scaffolds with different morphologies was investigated. Generally, the scaffolds with a fine pore structure showed higher compressive properties, and both the CNTs and NFCs improved the compressive properties of the scaffolds, with greater enhancement found in TPU/NFC nanocomposite scaffolds. In addition, all scaffolds showed good sustainability under cyclical load bearing, and the biocompatibility of the scaffolds was verified via 3T3 fibroblast cell culture.

Silk scaffolds for musculoskeletal tissue engineering

The musculoskeletal system, which includes bone, cartilage, tendon/ligament, and skeletal muscle, is becoming the targets for tissue engineering because of the high need for their repair and regeneration. Numerous factors would affect the use of musculoskeletal tissue engineering for tissue regeneration ranging from cells used for scaffold seeding to the manufacture and structures of materials. The essential function of the scaffolds is to convey growth factors as well as cells to the target site to aid the regeneration of the injury. Among the variety of biomaterials used in scaffold engineering, silk fibroin is recognized as an ideal material for its impressive cytocompatibility, slow biodegradability, and excellent mechanical properties. The current review describes the advances made in the fabrication of silk fibroin scaffolds with different forms such as films, particles, electrospun fibers, hydrogels, three-dimensional porous scaffolds, and their applications in the regeneration of musculoskeletal tissues.

Silk fibroin as biomaterial for bone tissue engineering

Silk fibroin (SF) is a fibrous protein which is produced mainly by silkworms and spiders. Its unique mechanical properties, tunable biodegradation rate and the ability to support the differentiation of mesenchymal stem cells along the osteogenic lineage, have made SF a favorable scaffold material for bone tissue engineering. SF can be processed into various scaffold forms, combined synergistically with other biomaterials to form composites and chemically modified, which provides an impressive toolbox and allows SF scaffolds to be tailored to specific applications. This review discusses and summarizes recent advancements in processing SF, focusing on different fabrication and functionalization methods and their application to grow bone tissue in vitro and in vivo. Potential areas for future research, current challenges, uncertainties and gaps in knowledge are highlighted.

STATEMENT OF SIGNIFICANCE

Silk fibroin is a natural biomaterial with remarkable biomedical and mechanical properties which make it favorable for a broad range of bone tissue engineering applications. It can be processed into different scaffold forms, combined synergistically with other biomaterials to form composites and chemically modified which provides a unique toolbox and allows silk fibroin scaffolds to be tailored to specific applications. This review discusses and summarizes recent advancements in processing silk fibroin, focusing on different fabrication and functionalization methods and their application to grow bone tissue in vitro and in vivo. Potential areas for future research, current challenges, uncertainties and gaps in knowledge are highlighted.

Influence of different surface modification treatments on silk biotextiles for tissue engineering applications

Biotextile structures from silk fibroin have demonstrated to be particularly interesting for tissue engineering (TE) applications due to their high mechanical strength, interconnectivity, porosity, and ability to degrade under physiological conditions. In this work, we described several surface treatments of knitted silk fibroin (SF) scaffolds, namely sodium hydroxide (NaOH) solution, ultraviolet radiation exposure in an ozone atmosphere (UV/O3) and oxygen (O2) plasma treatment followed by acrylic acid (AAc), vinyl phosphonic acid (VPA), and vinyl sulfonic acid (VSA) immersion. The effect of these treatments on the mechanical properties of the textile constructs was evaluated by tensile tests in dry and hydrated states. Surface properties such as morphology, topography, wettability and elemental composition were also affected by the applied treatments. The in vitro biological behavior of L929 fibroblasts revealed that cells were able to adhere and spread both on the untreated and surface-modified textile constructs. The applied treatments had different effects on the scaffolds' surface properties, confirming that these modifications can be considered as useful techniques to modulate the surface of biomaterials according to the targeted application.

PLA-poloxamer/poloxamine copolymers for ligament tissue engineering: sound macromolecular design for degradable scaffolds and MSC differentiation

The treatment of anterior cruciate ligament (ACL) failures remains a current clinical challenge. The present study aims at providing suitable degradable scaffolds for ligament tissue engineering. First, we focus on the design and the evaluation of poly(lactide)/poloxamer or poly(lactide)/poloxamine multiblock copolymers selected and developed to have suitable degradation and mechanical properties to match ACL repair. In the second part, it is shown that the copolymers can be processed in the form of microfibers and scaffolds consisting of a combination of twisted/braided fibers to further modulate the mechanical properties and prepare scaffold prototypes suitable for ligament application. Finally, after assessment of their cytocompatibility, the polymer scaffolds are associated with mesenchymal stem cells (MSCs). MSC differentiation toward a ligament fibroblast phenotype is promoted by a dual stimulation including an inductive culture medium and cyclic mechanical loads. RT-qPCR analyses confirm the potential of our scaffolds and MSCs for ACL regeneration with upregulation of some differentiation markers including Scleraxis, Tenascin-C and Tenomodulin.

Poly(ester-ether)s: III. assessment of cell behaviour on nanofibrous scaffolds of PCL, PLLA and PDX blended with amorphous PMeDX

SEM images of HDF cells on scaffolds: (a) PCL/PMeDX: 93/7-good adhesion and proliferation, (b) PDX/PMeDX: 98/2-good adhesion, proliferation & infiltration and (c) PLLA/PMeDX: 85/15-good proliferation and infiltration.