Potential of Agarose/Silk Fibroin Blended Hydrogel for in Vitro Cartilage Tissue Engineering

Overcoming the Dependence on Animal Models for Osteoarthritis Therapeutics - The Promises and Prospects of In Vitro Models

Osteoarthritis (OA) is a musculoskeletal disease characterized by progressive degeneration of osteochondral tissues. Current treatment is restricted to the reduction of pain and loss of function of the joint. To better comprehend the OA pathophysiological conditions, several models are employed, however; there is no consensus on a suitable model. In this review, different in vitro models being developed for possible therapeutic intervention of OA are outlined. Herein, various in vitro OA models starting from 2D model, co-culture model, 3D models, dynamic culture model to advanced technologies-based models such as 3D bioprinting, bioassembly, organoids, and organ-on-chip-based models are discussed with their advantages and disadvantages. Besides, different growth factors, cytokines, and chemicals being utilized for induction of OA condition are reviewed in detail. Furthermore, there is focus on scrutinizing different molecular and possible therapeutic targets for better understanding the mechanisms and OA therapeutics. Finally, the underlying challenges associated with in vitro models are discussed followed by future prospective. Taken together, a comprehensive overview of in vitro OA models, factors to induce OA-like conditions, and intricate molecular targets with the potential to develop personalized osteoarthritis therapeutics in the future with clinical translation is provided.

Nonmulberry silk proteins: multipurpose ingredient in bio-functional assembly

The emerging field of tissue engineering and regenerative medicines utilising artificial polymers is facing many problems. Despite having mechanical stability, non-toxicity and biodegradability, most of them lack cytocompatibility and biocompatibility. Natural polymers (such as collagen, hyaluronic acid, fibrin, fibroin, and others), including blends, are introduced to the field to solve some of the relevant issues. Another natural biopolymer: silkworm silk gained special attention primarily due to its specific biophysical, biochemical, and material properties, worldwide availability, and cost-effectiveness. Silk proteins, namely fibroin and sericin extracted from domesticated mulberry silkwormBombyx mori, are studied extensively in the last few decades for tissue engineering. Wild nonmulberry silkworm species, originated from India and other parts of the world, also produce silk proteins with variations in their nature and properties. Among the nonmulberry silkworm species,Antheraea mylitta(Indian Tropical Tasar),A. assamensis/A. assama(Indian Muga), andSamia ricini/Philosamia ricini(Indian Eri), along withA. pernyi(Chinese temperate Oak Tasar/Tussah) andA. yamamai(Japanese Oak Tasar) exhibit inherent tripeptide motifs of arginyl glycyl aspartic acid in their fibroin amino acid sequences, which support their candidacy as the potential biomaterials. Similarly, sericin isolated from such wild species delivers unique properties and is used as anti-apoptotic and growth-inducing factors in regenerative medicines. Other characteristics such as biodegradability, biocompatibility, and non-inflammatory nature make it suitable for tissue engineering and regenerative medicine based applications. A diverse range of matrices, including but not limited to nano-micro scale structures, nanofibres, thin films, hydrogels, and porous scaffolds, are prepared from the silk proteins (fibroins and sericins) for biomedical and tissue engineering research. This review aims to represent the progress made in medical and non-medical applications in the last couple of years and depict the present status of the investigations on Indian nonmulberry silk-based matrices as a particular reference due to its remarkable potentiality of regeneration of different types of tissues. It also discusses the future perspective in tissue engineering and regenerative medicines in the context of developing cutting-edge techniques such as 3D printing/bioprinting, microfluidics, organ-on-a-chip, and other electronics, optical and thermal property-based applications.

Bioactive Silk Fibroin-Based Hybrid Biomaterials for Musculoskeletal Engineering: Recent Progress and Perspectives

Musculoskeletal engineering has been considered as a promising approach to customize regenerated tissue (such as bone, cartilage, tendon, and ligament) via a self-healing performance. Recent advances have demonstrated the great potential of bioactive materials for regenerative medicine. Silk fibroin (SF), a natural polymer, is regarded as a remarkable bioactive material for musculoskeletal engineering thanks to its biocompatibility, biodegradability, and tunability. To improve tissue-engineering performance, silk fibroin is hybridized with other biomaterials to form silk-fibroin-based hybrid biomaterials, which achieve superior mechanical and biological performance. Herein, we summarize the recent development of silk-based hybrid biomaterials in musculoskeletal tissue with reasonable generalization and classification, mainly including silk fibroin-based inorganic and organic hybrid biomaterials. The applied inorganics are composed of calcium phosphate, graphene oxide, titanium dioxide, silica, and bioactive glass, while the polymers include polycaprolactone, collagen (or gelatin), chitosan, cellulose, and alginate. This article mainly focuses on the physical and biological performances both in vitro and in vivo study of several common silk-based hybrid biomaterials in musculoskeletal engineering. The timely summary and highlight of silk-fibroin-based hybrid biomaterials will provide a research perspective to promote the further improvement and development of silk fibroin hybrid biomaterials for improved musculoskeletal engineering.

Biomaterial-Assisted Regenerative Medicine

This review aims to show case recent regenerative medicine based on biomaterial technologies. Regenerative medicine has arousing substantial interest throughout the world, with “The enhancement of cell activity” one of the essential concepts for the development of regenerative medicine. For example, drug research on drug screening is an important field of regenerative medicine, with the purpose of efficient evaluation of drug effects. It is crucial to enhance cell activity in the body for drug research because the difference in cell condition between in vitro and in vivo leads to a gap in drug evaluation. Biomaterial technology is essential for the further development of regenerative medicine because biomaterials effectively support cell culture or cell transplantation with high cell viability or activity. For example, biomaterial-based cell culture and drug screening could obtain information similar to preclinical or clinical studies. In the case of in vivo studies, biomaterials can assist cell activity, such as natural healing potential, leading to efficient tissue repair of damaged tissue. Therefore, regenerative medicine combined with biomaterials has been noted. For the research of biomaterial-based regenerative medicine, the research objective of regenerative medicine should link to the properties of the biomaterial used in the study. This review introduces regenerative medicine with biomaterial.

Development and Evaluation of Gellan Gum/Silk Fibroin/Chondroitin Sulfate Ternary Injectable Hydrogel for Cartilage Tissue Engineering

Hydrogel is in the spotlight as a useful biomaterial in the field of drug delivery and tissue engineering due to its similar biological properties to a native extracellular matrix (ECM). Herein, we proposed a ternary hydrogel of gellan gum (GG), silk fibroin (SF), and chondroitin sulfate (CS) as a biomaterial for cartilage tissue engineering. The hydrogels were fabricated with a facile combination of the physical and chemical crosslinking method. The purpose of this study was to find the proper content of SF and GG for the ternary matrix and confirm the applicability of the hydrogel in vitro and in vivo. The chemical and mechanical properties were measured to confirm the suitability of the hydrogel for cartilage tissue engineering. The biocompatibility of the hydrogels was investigated by analyzing the cell morphology, adhesion, proliferation, migration, and growth of articular chondrocytes-laden hydrogels. The results showed that the higher proportion of GG enhanced the mechanical properties of the hydrogel but the groups with over 0.75% of GG exhibited gelling temperatures over 40 °C, which was a harsh condition for cell encapsulation. The 0.3% GG/3.7% SF/CS and 0.5% GG/3.5% SF/CS hydrogels were chosen for the in vitro study. The cells that were encapsulated in the hydrogels did not show any abnormalities and exhibited low cytotoxicity. The biochemical properties and gene expression of the encapsulated cells exhibited positive cell growth and expression of cartilage-specific ECM and genes in the 0.5% GG/3.5% SF/CS hydrogel. Overall, the study of the GG/SF/CS ternary hydrogel with an appropriate content showed that the combination of GG, SF, and CS can synergistically promote articular cartilage defect repair and has considerable potential for application as a biomaterial in cartilage tissue engineering.

Recent Advancements in 3D Printing of Polysaccharide Hydrogels in Cartilage Tissue Engineering

The application of hydrogels coupled with 3-dimensional (3D) printing technologies represents a modern concept in scaffold development in cartilage tissue engineering (CTE). Hydrogels based on natural biomaterials are extensively used for this purpose. This is mainly due to their excellent biocompatibility, inherent bioactivity, and special microstructure that supports tissue regeneration. The use of natural biomaterials, especially polysaccharides and proteins, represents an attractive strategy towards scaffold formation as they mimic the structure of extracellular matrix (ECM) and guide cell growth, proliferation, and phenotype preservation. Polysaccharide-based hydrogels, such as alginate, agarose, chitosan, cellulose, hyaluronan, and dextran, are distinctive scaffold materials with advantageous properties, low cytotoxicity, and tunable functionality. These superior properties can be further complemented with various proteins (e.g., collagen, gelatin, fibroin), forming novel base formulations termed "proteo-saccharides" to improve the scaffold's physiological signaling and mechanical strength. This review highlights the significance of 3D bioprinted scaffolds of natural-based hydrogels used in CTE. Further, the printability and bioink formation of the proteo-saccharides-based hydrogels have also been discussed, including the possible clinical translation of such materials.

Investigation on the Composition of Agarose-Collagen I Blended Hydrogels as Matrices for the Growth of Spheroids from Breast Cancer Cell Lines

Three-dimensional (3D) cell culture systems mimic the structural complexity of the tissue microenvironment and are gaining increasing importance as they resemble the extracellular matrix (ECM)–cell and cell–cell physical interactions occurring in vivo. Several scaffold-based culture systems have been already proposed as valuable tools for large-scale production of spheroids, but they often suffer of poor reproducibility or high costs of production. In this work, we present a reliable 3D culture system based on collagen I-blended agarose hydrogels and show how the variation in the agarose percentage affects the physical and mechanical properties of the resulting hydrogel. The influence of the different physical and mechanical properties of the blended hydrogels on the growth, size, morphology, and cell motility of the spheroids obtained by culturing three different breast cancer cell lines (MCF-7, MDA-MB-361, and MDA-MB-231) was also evaluated. As proof of concept, the cisplatin penetration and its cytotoxic effect on the tumor spheroids as function of the hydrogel stiffness were also investigated. Noteworthily, the possibility to recover the spheroids from the hydrogels for further processing and other biological studies has been considered. This feature, in addition to the ease of preparation, the lack of cross-linking chemistry and the high reproducibility, makes this hydrogel a reliable biomimetic matrix for the growth of 3D cell structures.

Silk Biomaterials for Bone Tissue Engineering

Silk is a natural fibrous polymer with application potential in regenerative medicine. Increasing interest remains for silk materials in bone tissue engineering due to their characteristics in biocompatibility, biodegradability and mechanical properties. Plenty of the in vitro and in vivo studies confirmed the advantages of silk in accelerating bone regeneration. Silk is processed into scaffolds, hydrogels, and films to facilitate different bone regenerative applications. Bioactive factors such as growth factors and drugs, and stem cells are introduced to silk-based matrices to create friendly and osteogenic microenvironments, directing cell behaviors and bone regeneration. The recent progress in silk-based bone biomaterials is discussed and focused on different fabrication and functionalization methods related to osteogenesis. The challenges and potential targets of silk bone materials are highlighted to evaluate the future development of silk-based bone materials.

Dually cross-linked single networks: structures and applications

Cross-linked polymers have attracted an immense attention over the years, however, there are many flaws of these systems, e.g. softness and brittleness; such materials possess non-adjustable properties and cannot recover from damage and thus are limited in their practical applications. Supramolecular chemistry offers a variety of dynamic interactions that when integrated into polymeric gels endow the systems with reversibility and responsiveness to external stimuli. A combination of different cross-links in a single gel could be the key to tackle these drawbacks, since covalent or chemical cross-linking serve to maintain the permanent shape of the material and to improve overall mechanical performance, whereas non-covalent cross-links impart dynamicity, reversibility, stimuli-responsiveness and often toughness to the material. In the present review we sought to give a comprehensive overview of the progress in design strategies of different types of dually cross-linked single gels made by researchers over the past decade as well as the successful implementations of these advances in many demanding fields where versatile multifunctional materials are required, such as tissue engineering, drug delivery, self-healing and adhesive systems, sensors as well as shape memory materials and actuators.

Mesenchymal stem cells loaded on 3D-printed gradient poly(ε-caprolactone)/methacrylated alginate composite scaffolds for cartilage tissue engineering

Cartilage has limited self-repair ability due to its avascular, alymphatic and aneural features. The combination of three-dimensional (3D) printing and tissue engineering provides an up-and-coming approach to address this issue. Here, we designed and fabricated a tri-layered (superficial layer (SL), middle layer (ML) and deep layer (DL)) stratified scaffold, inspired by the architecture of collagen fibers in native cartilage tissue. The scaffold was composed of 3D printed depth-dependent gradient poly(ε-caprolactone) (PCL) impregnated with methacrylated alginate (ALMA), and its morphological analysis and mechanical properties were tested. To prove the feasibility of the composite scaffolds for cartilage regeneration, the viability, proliferation, collagen deposition and chondrogenic differentiation of embedded rat bone marrow mesenchymal stem cells (BMSCs) in the scaffolds were assessed by Live/dead assay, CCK-8, DNA content, cell morphology, immunofluorescence and real-time reverse transcription polymerase chain reaction. BMSCs-loaded gradient PCL/ALMA scaffolds showed excellent cell survival, cell proliferation, cell morphology, collagen II deposition and hopeful chondrogenic differentiation compared with three individual-layer scaffolds. Hence, our study demonstrates the potential use of the gradient PCL/ALMA construct for enhanced cartilage tissue engineering.

Polymer-Protein Hybrid Network Involving Mucin: A Mineralized Biomimetic Template for Bone Tissue Engineering

Biomimetic matrices offer a great advantage to understand several biological processes including regeneration. The study involves the development of a hybrid biomimetic scaffold and the uniqueness lies in the use of mucin, as a constituent protein. Through this study, the role of the protein in bone regeneration is deciphered through its development as a 3D model. As a first step towards understanding the protein, the interactions of mucin and collagen are determined by in silico studies considering that collagen is the most abundant protein in the bone microenvironment. Both proteins are reported to be involved in bone biology though the exact role of mucin is a topic of investigation. The in silico studies of collagen-mucin suggest to have a proper affinity toward each other, forming a strong basis for 3D scaffold development. The developed 3D scaffold is a double network system comprising of mucin and collagen and vinyl end functionalized polyethylene glycol. In situ deposition of mineral crystals has been performed enzymatically. Biological evaluation of these mineral deposited scaffolds is done in terms of their bone regeneration potential and a comparison of the two systems with and without mineral deposition is presented.

Processing, mechanical properties and bio-applications of silk fibroin-based high-strength hydrogels

Hydrogels are an attractive class of materials that possess similar structural and functional characteristics to wet biological tissues and demonstrate a diversity of applications in biomedical engineering. Silk fibroin (SF) is a unique natural polymer due to its fibrous protein nature, versatile formats, biocompatibility, tunable biodegradation and is thus a good hydrogel candidate for bio-applications. Compared to synthetic polymer hydrogels, poor mechanical performance is still a fatal drawback that hinders the application of SF hydrogels as structural materials. Researchers have attempted to develop strategies to construct silk fibroin-based high-strength hydrogels (SF-HSHs). Herein, we firstly provide an overview of the approaches of processing SF-HSHs with a focus on the physical/non-covalent crosslinking mechanisms. The examples of SF-HSHs are discussed in detail under four categories, including physical-crosslinked, dual-crosslinked, double network and composite hydrogels respectively. A brief section follows to elucidate on the gelation mechanisms of SF-HSHs before a description of the utility of SF-HSHs in biomedicine and devices is presented. Finally, the potential challenges and future development of SF-HSHs are briefly discussed. This review aims to enhance our understanding of the structure-mechanical property-function relationships of soft materials made from natural polymers and guide future research of silk fibroin-based hydrogels for biomedical applications. STATEMENT OF SIGNIFICANCE: Silk fibroin (SF) extracted from silk fibres is increasingly applied in the biomedical field, and SF hydrogel has been an emerging area for frontier bio-research. Since SF biopolymer has an intrinsic tendency to form regular β-sheet stacks, it can be processed into purely physically crosslinked hydrogels, thus avoiding the use of chemical crosslinkers. Nevertheless, akin to other natural polymers, lab-produced SF is variable (i.e. the molecular weight and distribution), and the gelation of SF hydrogel is challenging to control. In addition, hydrogels made from SF are usually weak and brittle, which hinders the wide use of this biofriendly and biodegradable hydrogel. Recently, there is a pressing need for high strength hydrogels from natural polymers for biomedical applications, and SF is proposed as a strong candidate. Therefore, we have studied the literature in the past 10 years and would like to focus on the gelation mechanism and mechanical strength of SF hydrogels for the review.

Improved antifouling properties of PVA hydrogel via an organic semiconductor graphitic carbon nitride catalyzed surface-initiated photo atom transfer radical polymerization

An innovative g-C₃N₄ catalyzed surface-initiated photo atom transfer radical polymerization (SI-photoATRP) has been developed to construct MEDSAH zwitterionic polymer brushes on PVA hydrogel surface. g-C₃N₄ catalyzed SI-photoATRP is temporal and spatial control. As a heterogeneous reaction system, it can solve the catalyst residues problem. After grafting with MEDSAH, surface chemical composition and morphology of PVA-g-pMEDSAH hydrogel confirmed that MEDSAH was successfully grafted onto PVA hydrogel. Thermal property of PVA-g-pMEDSAH hydrogel decreased and hydrophilicity increased. No statistically significant differences between PVA and PVA-g-pMEDSAH were observed on mechanical properties. Cytotoxicity in vitro of PVA-g-pMEDSAH hydrogel could be considered as no cytotoxicity for L929 and NDHF cells. The antifouling properties of PVA-g-pMEDSAH hydrogel were significantly improved due to the enhancement of the surface hydration and steric repulsion effects caused by pMEDSAH polymer brushes. In addition, g-C₃N₄ is easier to modify to enhance the photocatalyst property. Thus, the heterogeneous reaction system of g-C₃N₄ catalyzed SI-photoATRP has huge potential applied in biomaterials surface modification.

Methacrylated pullulan/polyethylene (glycol) diacrylate composite hydrogel for cartilage tissue engineering

Pullulan hydrogels are widely used in tissue engineering and drug delivery. However, these hydrogels do not meet the requirements of articular cartilage repair because of their fast degradation rate and poor mechanical strength. Herein, we fabricated a hybrid hydrogel system by combining pullulan with synthetic polymers polyethylene (glycol) diacrylate (PEGDA). In this study, pullulan was modified with methacrylic anhydride (MA) to obtain photo-crosslinkable methacrylated pullulan (PulMA). Moreover, the lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate (LAP) was used as a water-soluble UV photoinitiator to form the PulMA/PEGDA hydrogel by photopolymerization strategy. Compared with the pure PulMA hydrogel, the increase of PEGDA concentration led to a slower degradation rate and an increase of residual mass from 63.9% to 86.8%. There was about 8-fold increase in storage modulus (G') (reach to 16.0 × 10³ Pa) and 13-fold increase in compressive modulus (reach to 1.17 ± 0.17 MPa) with increasing the concentration of PEGDA to 15% (w/v) in the hydrogel. In cell culture in vitro, the rabbit's mesenchymal stem cells (MSCs) encapsulated in the PulMA/PEGDA hydrogel could adhere and proliferate, indicating that the PulMA/PEGDA hydrogel had a good biocompatibility. Furthermore, the hydrogels supported glycosaminoglycan (GAG) synthesis, and chondrogenic phenotype of MSCs with TGF-β3-containing chondrogenic medium. This study demonstrated that the photo-crosslinking PulMA/PEGDA hydrogels, with good mechanical properties and slow degradation rate are promising scaffolds for cartilage repair and regeneration.

Injectable in situ forming hydrogels incorporating dual-nanoparticles for chemo-photothermal therapy of breast cancer cells

Chemo-photothermal therapy (chemo-PTT) mediated by nanomaterials holds a great potential for cancer treatment. However, the tumor uptake of the systemically administered nanomaterials was recently found to be below 1%. To address this limitation, the development of injectable tridimensional polymeric matrices capable of delivering nanomaterials directly into the tumor site appears to be a promising approach. In this work, an injectable in situ forming ionotropically crosslinked chitosan-based hydrogel co-incorporating IR780 loaded nanoparticles (IR/BPN) and Doxorubicin (DOX) loaded nanoparticles (DOX/TPN) was developed for application in breast cancer chemo-PTT. The produced hydrogels (IR/BPN@Gel and IR/BPN+DOX/TPN@Gel) displayed suitable physicochemical properties and produced a temperature increase of about 9.1 °C upon exposure to Near Infrared (NIR) light. As importantly, the NIR-light exposure also increased the release of DOX from the hydrogel by 1.7-times. In the in vitro studies, the combination of IR/BPN@Gel with NIR light (photothermal therapy) led to a reduction in the viability of breast cancer cells to 35%. On the other hand, the non-irradiated IR/BPN+DOX/TPN@Gel (chemotherapy) only diminished cancer cells' viability to 85%. In contrast, the combined action of IR/BPN+DOX/TPN@Gel and NIR light reduced cancer cells' viability to about 9%, demonstrating its potential for breast cancer chemo-PTT.

Preparation and characterization of a soluble eggshell membrane/agarose composite scaffold with possible applications in cartilage regeneration

Articular hyaline cartilage is an extremely hydrated, not vascularized tissue with a low-cell density. The damage of this tissue can occur after injuries or gradual stress and tears (osteoarthritis), minor damages can be self-healed in several weeks, but major injuries may eventually require surgery. In fact, in this case, because of nature of the cartilage (the absence of cells and vascularization) it is difficult to expect its natural regeneration in a reasonable amount of time. In recent years, cell therapy, in which cells are directly transplanted, has attracted attention. In this study, a scaffold for implanting chondrocytes was prepared. The scaffold was made as a sponge using the eggshell membrane and agarose. The eggshell membrane is structurally similar to the extracellular matrix and nontoxic due to its many collagen components and has good biocompatibility and biodegradability. However, scaffolds made of collagen only has poor mechanical properties. For this reason, the disulfide bond of collagen extracted from the insoluble eggshell membrane was cut, converted into water-soluble, and then mixed with agarose to prepare a scaffold. Agarose is capable of controlling mechanical properties, has excellent biocompatibility, and is suitable for forming a hydrogel having a three-dimensional porosity. The scaffold was examined for Fourier-transform infrared, mechanical properties, biodegradability, and biocompatibility. In in vitro experiment, cytotoxicity, cell proliferation, and messenger RNA expression were investigated. The study demonstrated that the agarose/eggshell membrane scaffold can be used for chondrocyte transplantation.

Systematic Review of Silk Scaffolds in Musculoskeletal Tissue Engineering Applications in the Recent Decade

During the past decade, various novel tissue engineering (TE) strategies have been developed to maintain, repair, and restore the biomechanical functions of the musculoskeletal system. Silk fibroins are natural polymers with numerous advantageous properties such as good biocompatibility, high mechanical strength, and low degradation rate and are increasingly being recognized as a scaffolding material of choice in musculoskeletal TE applications. This current systematic review examines and summarizes the latest research on silk scaffolds in musculoskeletal TE applications within the past decade. Scientific databases searched include PubMed, Web of Science, Medline, Cochrane library, and Embase. The following keywords and search terms were used: musculoskeletal, tendon, ligament, intervertebral disc, muscle, cartilage, bone, silk, and tissue engineering. Our Review was limited to articles on musculoskeletal TE, which were published in English from 2010 to September 2019. The eligibility of the articles was assessed by two reviewers according to prespecified inclusion and exclusion criteria, after which an independent reviewer performed data extraction and a second independent reviewer validated the data obtained. A total of 1120 articles were reviewed from the databases. According to inclusion and exclusion criteria, 480 articles were considered as relevant for the purpose of this systematic review. Tissue engineering is an effective modality for repairing or replacing injured or damaged tissues and organs with artificial materials. This Review is intended to reveal the research status of silk-based scaffolds in the musculoskeletal system within the recent decade. In addition, a comprehensive translational research route for silk biomaterial from bench to bedside is described in this Review.

Silk Fibroin as a Functional Biomaterial for Tissue Engineering

Tissue engineering (TE) is the approach to combine cells with scaffold materials and appropriate growth factors to regenerate or replace damaged or degenerated tissue or organs. The scaffold material as a template for tissue formation plays the most important role in TE. Among scaffold materials, silk fibroin (SF), a natural protein with outstanding mechanical properties, biodegradability, biocompatibility, and bioresorbability has attracted significant attention for TE applications. SF is commonly dissolved into an aqueous solution and can be easily reconstructed into different material formats, including films, mats, hydrogels, and sponges via various fabrication techniques. These include spin coating, electrospinning, freeze drying, physical, and chemical crosslinking techniques. Furthermore, to facilitate fabrication of more complex SF-based scaffolds with high precision techniques including micro-patterning and bio-printing have recently been explored. This review introduces the physicochemical and mechanical properties of SF and looks into a range of SF-based scaffolds that have been recently developed. The typical TE applications of SF-based scaffolds including bone, cartilage, ligament, tendon, skin, wound healing, and tympanic membrane, will be highlighted and discussed, followed by future prospects and challenges needing to be addressed.

Aminocellulose-grafted-polycaprolactone coated gelatin nanoparticles alleviate inflammation in rheumatoid arthritis: A combinational therapeutic approach

Rheumatoid arthritis (RA) is a chronic autoimmune disorder and serious cause of disability. Despite considerable advances in RA management, challenges like extensive drug metabolism and rapid clearance causes poor bioavailability. Core-shell nanocarriers for co-delivery of glycyrrhizic acid (GA) and budesonide against RA were developed. GA-loaded gelatin nanoparticles (NPs) were synthesized and coated with budesonide encapsulated aminocellulose-grafted polycaprolactone (PCL-AC). GA- and budesonide-loaded PCL-AC-gel NPs had diameter of 200-225 nm. Dual drug-loaded (DDL) NPs reduced joint swelling and erythema in rats while markedly ameliorating bone erosion evidenced by radiological analysis, suppressed collagen destruction, restored synovial tissue, bone and cartilage histoarchitecture with reduced inflammatory cells infiltration. NPs also reduced various inflammatory biomarkers such as TNF-α, IL-1β, COX-2, iNOS. Results of this study suggest that dual NPs exerted superior therapeutic effects in RA compared to free drugs which may be attributed to slow and sustained drug release and NPs' ability to inhibit inflammatory mediators.

A review of the properties and applications of bioadhesive hydrogels

Due to their outstanding properties, bioadhesive hydrogels have been extensively studied by researchers in recent years.

A new agarose-based microsystem to investigate cell response to prolonged confinement

Emerging evidence suggests the importance of mechanical stimuli in normal and pathological situations for the control of many critical cellular functions. While the effect of matrix stiffness has been and is still extensively studied, few studies have focused on the role of mechanical stresses. The main limitation of such analyses is the lack of standard in vitro assays enabling extended mechanical stimulation compatible with dynamic biological and biophysical cell characterization. We have developed an agarose-based microsystem, the soft cell confiner, which enables the precise control of confinement for single or mixed cell populations. The rigidity of the confiner matches physiological conditions and its porosity enables passive medium renewal. It is compatible with time-lapse microscopy, in situ immunostaining, and standard molecular analyses, and can be used with both adherent and non-adherent cell lines. Cell proliferation of various cell lines (hematopoietic cells, MCF10A epithelial breast cells and HS27A stromal cells) was followed for several days up to confluence using video-microscopy and further documented by Western blot and immunostaining. Interestingly, even though the nuclear projected area was much larger upon confinement, with many highly deformed nuclei (non-circular shape), cell viability, assessed by live and dead cell staining, was unaffected for up to 8 days in the confiner. However, there was a decrease in cell proliferation upon confinement for all cell lines tested. The soft cell confiner is thus a valuable tool to decipher the effects of long-term confinement and deformation on the biology of cell populations. This tool will be instrumental in deciphering the impact of nuclear and cytoskeletal mechanosensitivity in normal and pathological conditions involving highly confined situations, such as those reported upon aging with fibrosis or during cancer.

Advanced hydrogels for the repair of cartilage defects and regeneration

Cartilage defects are one of the most common symptoms of osteoarthritis (OA), a degenerative disease that affects millions of people world-wide and places a significant socio-economic burden on society. Hydrogels, which are a class of biomaterials that are elastic, and display smooth surfaces while exhibiting high water content, are promising candidates for cartilage regeneration. In recent years, various kinds of hydrogels have been developed and applied for the repair of cartilage defects in vitro or in vivo, some of which are hopeful to enter clinical trials. In this review, recent research findings and developments of hydrogels for cartilage defects repair are summarized. We discuss the principle of cartilage regeneration, and outline the requirements that have to be fulfilled for the deployment of hydrogels for medical applications. We also highlight the development of advanced hydrogels with tailored properties for different kinds of cartilage defects to meet the requirements of cartilage tissue engineering and precision medicine.

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The biotechnology of developing hydrogels for cartilage defects repair is promising.

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The principle for cartilage regeneration using hydrogels and requirements for clinical transformation are summarized.

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Advanced hydrogels with tailored properties for different kinds of cartilage defects are discussed.

Controlled Cryogelation and Catalytic Cross-Linking Yields Highly Elastic and Robust Silk Fibroin Scaffolds

Silk biomaterials with tunable mechanical properties and biological properties are of special importance for tissue engineering. Here, we fabricated silk fibroin (SF, from Bombyx mori silk) scaffolds from cryogelation under controlled temperature and catalytic cross-linking conditions. Structurally, the cryogelled scaffolds demonstrated a greater β-sheet content but significantly smaller β-sheet domains compared to that without chemical cross-linking and catalyst. Mechanically, the cryogelled scaffolds were softer and highly elastic under tension and compression. The 120% tensile elongation and >85% recoverable compressive strain were among the best properties reported for SF scaffolds. Cyclic compression tests proved the robustness of such scaffolds to resist fatigue. The mechanical properties, as well as the degradation rate of the scaffolds, can be fine-tuned by varying the concentrations of the catalyst and the cross-linker. For biological responses, in vitro rat bone mesenchymal stem cell (rBMSC) culture studies demonstrated that cryogelled SF scaffolds supported better cell attachment and proliferation than the routine freeze-thawed scaffolds. The in vivo subcutaneous implantation results showed excellent histocompatibility and tissue ingrowth for the cryogelled SF scaffolds. This straightforward approach of enhanced elasticity of SF scaffolds and fine-tunability in mechanical performances, suggests a promising strategy to develop novel SF biomaterials for soft tissue engineering and regenerative medicine.

Gradient Biomineralized Silk Fibroin Nanofibrous Scaffold with Osteochondral Inductivity for Integration of Tendon to Bone

Enthesis injury repair remains a huge challenge because of the unique biomolecular composition, microstructure, and mechanics in the interfacial region. Surgical reconstruction often creates new bone-scaffold interfaces with mismatched properties, resulting in poor osseointegration. To mimic the natural interface tissue structures and properties, we fabricated a nanofibrous scaffold with gradient mineral coating based on 10 × simulated body fluid (SBF) and silk fibroin (SF). We then characterized the physicochemical properties of the scaffold and evaluated its biological functions both in vitro and in vivo. The results showed that different areas of SF nanofibrous scaffold had varying levels of mineralization with disparate mechanical properties and had different effects on bone marrow mesenchymal stem cell growth and differentiation. Furthermore, the gradient scaffolds exhibited an enhancement of integration in the tendon-to-bone interface with a higher ultimate load and more fibrocartilage-like tissue formation. These findings demonstrate that the silk-based nanofibrous scaffold with gradient mineral coating can regulate the formation of interfacial tissue and has the potential to be applied in interface tissue engineering.

Berberine/Ag nanoparticle embedded biomimetic calcium phosphate scaffolds for enhancing antibacterial function

In the past decade, biomimetic calcium phosphate (CaP) ceramics have been considered as practicable grafts and biomaterial substitutes in repairing jaw bone defect after tumor resection or traffic accident. Nowadays, increasing incidence of biomedical material-associated infection has raised a concern when applying these materials. In this work, a new porous CaP scaffold with antibacterial coating was proposed. This biomimetic scaffold was composited with berberine (BBR), Ag nanoparticles (nAg), and silk fibroin (SF). The microstructures and phase composition of the scaffolds were analyzed. The cytocompatibility and osteogenic potential of the prepared samples were evaluated in vitro. The scaffolds held hierarchical structure: the first-level porous CaP ceramic with micron pores ranged from 250 to 600 µm; the second-level spongy-like structure with abundant capillary pores ranged from 500 nm to 10 µm; and the third-level structure was achieved by filling BBR, nAg, and SF gel coatings into the above porous structures. The experimental results showed that the antimicrobial capability of single BBR coating is inconspicuous. However, the introduction of nAg could significantly promote the antibacterial effect of scaffolds. At the same time, such scaffolds showed improved osteoinductivity. This new biomimetic CaP scaffold with antibacterial and osteoinductive properties may be a promising candidate for bone tissue engineering.

Agarose-Based Biomaterials: Opportunities and Challenges in Cartilage Tissue Engineering

The lack of adequate blood/lymphatic vessels as well as low-potential articular cartilage regeneration underlines the necessity to search for alternative biomaterials. Owing to their unique features, such as reversible thermogelling behavior and tissue-like mechanical behavior, agarose-based biomaterials have played a key role in cartilage tissue repair. Accordingly, the need for fabricating novel highly efficient injectable agarose-based biomaterials as hydrogels for restoration of injured cartilage tissue has been recognized. In this review, the resources and conspicuous properties of the agarose-based biomaterials were reviewed. First, different types of signals together with their functionalities in the maintenance of cartilage homeostasis were explained. Then, various cellular signaling pathways and their significant role in cartilage tissue engineering were overviewed. Next, the molecular structure and its gelling behavior have been discussed. Eventually, the latest advancements, the lingering challenges, and future ahead of agarose derivatives from the cartilage regeneration perspective have been discussed.

Natural biopolymer-based hydrogels for use in food and agriculture

Hydrogels are important materials that are of high scientific interest and with numerous applications. Natural polymer-based hydrogels are preferred to synthetic ones due to their safety, biocompatibility, and ecofriendly properties. They have been studied extensively and implemented in various fields, such as medicine, cosmetics, personal-care products, water purification, and more. This review focuses on the applications of nature-sourced polymer-based hydrogels in food and agriculture. Different types of biopolymers and crosslinking agents, and various methods for hydrogel formation are described. The physicomechanical properties and applied activities of the resulting materials are also comprehensively discussed. Biodegradable synthetic polymers are outside the scope of this review. © 2020 Society of Chemical Industry.

Biofunctionalized chondrogenic shape-memory ternary scaffolds for efficient cell-free cartilage regeneration

Cartilage defect repair remains a great clinical challenge due to the limited self-regeneration capacity of cartilage tissue. Surgical treatment of injured cartilage is rather difficult due to the narrow space in the articular cavity and irregular defect area. Herein, we designed and fabricated chondrogenic and physiological-temperature-triggered shape-memory ternary scaffolds for cell-free cartilage repair, where the poly (glycerol sebacate) (PGS) networks ensured elasticity and shape recovery, crystallized poly (1,3-propylene sebacate) (PPS) acted as switchable phase, and immobilized bioactive kartogenin (KGN) endowed the scaffolds with chondrogenic capacity. The resultant scaffolds exhibited shape-memory properties with shape-memory fixed ratio of 98% and recovered ratio of 97% at 37°C for PPS/PGS/KGN-100, indicating a good potential for minimally invasive implantation. The scaffolds gradually degraded in Dulbecco's phosphate-buffered saline and released KGN up to 12 weeks in vitro. In addition, the scaffolds promoted chondrogenic differentiation while inhibiting osteogenic differentiation of bone marrow-derived mesenchymal stem cells in a concentration-dependent manner and cartilage regeneration in full-thickness defects of rat femoropatellar groove for 12 weeks. Consequently, the PPS/PGS/KGN-100 scaffolds stimulated the formation of an overlying layer of neocartilage mimicking the characteristic architecture of native articular cartilage even in the absence of exogenous growth factors and seeded cells. This study provides much inspiration for future research on cartilage tissue engineering. STATEMENT OF SIGNIFICANCE: There are two crucial challenges for cartilage defect repair: the lack of self-regeneration capacity of cartilage tissue and difficult scaffold implantation via traditional open surgery due to space-limited joints. Herein, bioactive body-temperature-responsive shape memory scaffolds are designed to simultaneously address the challenges. The scaffolds can be readily implanted by minimally invasive approach and recover by body-temperature of patient. The integration of kartogenin endows scaffolds the bioactivity, leading to the first example of bulk shape-memory scaffolds for cell-free cartilage repair. These characteristics make the scaffolds advantageous for clinical translation. Moreover, our developed material is easy to be functionalized due to the presence of extensive free hydroxyl groups and provides a versatile platform to design diverse functional shape memory biomaterials.

Preparation and characterization of methacrylated gelatin/bacterial cellulose composite hydrogels for cartilage tissue engineering

Methacrylated gelatin (GelMA)/bacterial cellulose (BC) composite hydrogels have been successfully prepared by immersing BC particles in GelMA solution followed by photo-crosslinking. The morphology of GelMA/BC hydrogel was examined by scanning electron microscopy and compared with pure GelMA. The hydrogels had very well interconnected porous network structure, and the pore size decreased from 200 to 10 µm with the increase of BC content. The composite hydrogels were also characterized by swelling experiment, X-ray diffraction, thermogravimetric analysis, rheology experiment and compressive test. The composite hydrogels showed significantly improved mechanical properties compared with pure GelMA. In addition, the biocompatility of composite hydrogels were preliminarily evaluated using human articular chondrocytes. The cells encapsulated within the composite hydrogels for 7 days proliferated and maintained the chondrocytic phenotype. Thus, the GelMA/BC composite hydrogels might be useful for cartilage tissue engineering.

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.

Progress in Articular Cartilage Tissue Engineering: A Review on Therapeutic Cells and Macromolecular Scaffolds

Repair and regeneration of articular cartilage lesions have always been a major challenge in the medical field due to its peculiar structure (e.g., sparsely distributed chondrocytes, no blood supply, no nerves). Articular cartilage tissue engineering is considered as one promising strategy to achieve reconstruction of cartilage. With this perspective, the articular cartilage tissue engineering has been widely studied. Here, the recent progress of articular cartilage tissue engineering is reviewed. The ad hoc therapeutic cells and growth factors for cartilage regeneration are summarized and discussed. Various types of bio/macromolecular scaffolds together with their pros and cons are also reviewed and elaborated.

An overview of advanced biocompatible and biomimetic materials for creation of replacement structures in the musculoskeletal systems: focusing on cartilage tissue engineering

Tissue engineering, as an interdisciplinary approach, is seeking to create tissues with optimal performance for clinical applications. Various factors, including cells, biomaterials, cell or tissue culture conditions and signaling molecules such as growth factors, play a vital role in the engineering of tissues. In vivo microenvironment of cells imposes complex and specific stimuli on the cells, and has a direct effect on cellular behavior, including proliferation, differentiation and extracellular matrix (ECM) assembly. Therefore, to create appropriate tissues, the conditions of the natural environment around the cells should be well imitated. Therefore, researchers are trying to develop biomimetic scaffolds that can produce appropriate cellular responses. To achieve this, we need to know enough about biomimetic materials. Scaffolds made of biomaterials in musculoskeletal tissue engineering should also be multifunctional in order to be able to function better in mechanical properties, cell signaling and cell adhesion. Multiple combinations of different biomaterials are used to improve above-mentioned properties of various biomaterials and to better imitate the natural features of musculoskeletal tissue in the culture medium. These improvements ultimately lead to the creation of replacement structures in the musculoskeletal system, which are closer to natural tissues in terms of appearance and function. The present review article is focused on biocompatible and biomimetic materials, which are used in musculoskeletal tissue engineering, in particular, cartilage tissue engineering.

A three-dimensional printed silk-based biomimetic tri-layered meniscus for potential patient-specific implantation

Employing tissue engineering principles aided by three-dimensional (3D) printing strategies to fabricate meniscus tissue constructs could help patients with meniscus injury regain mobility, improve pain management and reduce the risk of development of knee osteoarthritis. Here we report a 3D printed meniscus scaffold that biomimics the internal and bulk architecture of the menisci. A shear-thinning novel silk fibroin-gelatin-based bioink with high print fidelity was optimized for the fabrication of scaffolds to serve as potential meniscus implants. Physicochemical characterization of the fabricated scaffolds shows optimum swelling, degradation and mechanical properties. Further, the scaffolds were seeded with meniscus fibrochondrocytes to validate their bioactivity. Fibrochondrocytes seeded on the scaffolds maintained their phenotype and proliferation, and enhanced glycosaminoglycan and total collagen synthesis was observed. Gene expression profile, biochemical quantification and histological studies confirmed the ability of the scaffolds to form meniscus-like tissue constructs. The scaffolds were found to possess amenable immunocompatibility in vitro as well as in vivo. Due to their excellent biological and physicochemical characteristics, these 3D printed scaffolds may be fine-tuned into viable alternatives to the present clinical treatment approaches to meniscus repair.

3D Bioprinting Using Cross-Linker-Free Silk-Gelatin Bioink for Cartilage Tissue Engineering

Cartilage tissue is deprived of intrinsic self-regeneration capability; hence, its damage often progresses to a chronic condition which reduces the quality of life. Toward the fabrication of functional tissue substitutes, three-dimensional (3D) bioprinting has progressed vastly over the last few decades. However, this progress is challenged by the difficulty in developing suitable bioink materials as most of them require toxic chemical cross-linking. In this study, our goal was to develop a cross-linker-free bioink with optimal rheology for polymer extrusion, aqueous, and nontoxic processing and offers structural support for cartilage regeneration. Toward this, we use the self-gelling ability of silk fibroin blends (Bombyx mori and Philosamia ricini) along with gelatin as a bulking agent. Silk and gelatin interact with each other through entanglement and physical cross-linking. The ink was rheologically and structurally optimized for printing efficiency in printing grid-like structures. The printed 3D constructs show optimal swelling capability, degradability, and compressive strength. Further, the construct supports the growth and proliferation of encapsulated chondrocytes and formation of the cartilaginous extracellular matrix as indicated by the increased sulfated glycosaminoglycan and collagen contents. This was further corroborated by the upregulation of chondrogenic gene expression with minimal hypertrophy of chondrocytes. Additionally, the construct demonstrates in vitro and in vivo biocompatibility. Notably, the ink demonstrates good print fidelity for printing anatomical structures such as the human ear enabled by optimized extrudability at adequate resolution. Altogether, the results indicate that the developed cross-linker-free silk-gelatin polymer-based bioink demonstrated high potential for its 3D bioprintability and application in cartilage tissue engineering.