Cell-laden interpenetrating network hydrogels formed from methacrylated gelatin and silk fibroin via a combination of sonication and photocrosslinking approaches

Development of multilayered artificial urethra graft for urethroplasty

To enhance the treatment of patients' urethral defects, such as strictures and hypospadias, we investigated the potential of using artificial urethral tissue. Our study aimed to generate this tissue and assess its effectiveness in a rabbit model. Two types of bioprinted grafts, based on methacrylated gelatin-silk fibroin (GelMA-SF) hydrogels, were produced: acellular, as well as loaded with autologous rabbit stem cells. Rabbit adipose stem cells (RASC) were differentiated toward smooth muscle in the GelMA-SF hydrogel, while rabbit buccal mucosa stem cells (RBMC), differentiated toward the epithelium, were seeded on its surface, forming two layers of the cell-laden tissue. The constructs were then reinforced with polycaprolactone-polylactic acid meshes to create implantable multilayered artificial urethral grafts. In vivo experiments showed that the cell-laden tissue integrated into the urethra with less fibrosis and inflammation compared to its acellular counterpart. Staining to trace the implanted cells confirmed integration into the host organism 3 months postsurgery.

Biological Functions of Macromolecular Protein Hydrogels in Constructing Osteogenic Microenvironment

Irreversible bone defects resulting from trauma, infection, and degenerative illnesses have emerged as a significant health concern. Structurally and functionally controllable hydrogels made by bone tissue engineering (BTE) have become promising biomaterials. Natural proteins are able to establish connections with autologous proteins through unique biologically active regions. Hydrogels based on proteins can simulate the bone microenvironment and regulate the biological behavior of stem cells in the tissue niche, making them candidates for research related to bone regeneration. This article reviews the biological functions of various natural macromolecular proteins (such as collagen, gelatin, fibrin, and silk fibroin) and highlights their special advantages as hydrogels. Then the latest research trends on cross-linking modified macromolecular protein hydrogels with improved mechanical properties and composite hydrogels loaded with exogenous micromolecular proteins have been discussed. Finally, the applications of protein hydrogels, such as 3D printed hydrogels, microspheres, and injectable hydrogels, were introduced, aiming to provide a reference for the repair of clinical bone defects.

Cross-Linking Methods of the Silk Protein Hydrogel in Oral and Craniomaxillofacial Tissue Regeneration

BACKGROUND

Craniomaxillofacial tissue defects are clinical defects involving craniomaxillofacial and oral soft and hard tissues. They are characterized by defect-shaped irregularities, bacterial and inflammatory environments, and the need for functional recovery. Conventional clinical treatments are currently unable to achieve regeneration of high-quality oral craniomaxillofacial tissue. As a natural biomaterial, silk fibroin (SF) has been widely studied in biomedicine and has broad prospects for use in tissue regeneration. Hydrogels made of SF showed excellent water retention, biocompatibility, safety and the ability to combine with other materials.

METHODS

To gain an in-depth understanding of the current development of SF, this article reviews the structure, preparation and application prospects in oral and craniomaxillofacial tissue regenerative medicine. It first briefly introduces the structure of SF and then summarizes the principles, advantages and disadvantages of the different cross-linking methods (physical cross-linking, chemical cross-linking and double network structure) of SF. Finally, the existing research on the use of SF in tissue engineering and the prospects of using SF with different cross-linking methods in oral and craniomaxillofacial tissue regeneration are also discussed.

CONCLUSIONS

This review is intended to show the advantages of SF hydrogels in tissue engineering and provides theoretical support for establishing novel and viable silk protein hydrogels for regeneration.

Recent advances of silk fibroin materials: From molecular modification and matrix enhancement to possible encapsulation-related functional food applications

Silk fibroin materials are emergingly explored for food applications due to their inherent properties including safe oral consumption, biocompatibility, gelatinization, antioxidant performance, and mechanical properties. However, silk fibroin possesses drawbacks like brittleness owing to its inherent specific composition and structure, which limit their applications in this field. This review discusses current progress about molecular modification methods on silk fibroin such as extraction, blending, self-assembly, enzymatic catalysis, etc., to address these limitations and improve their physical/chemical properties. It also summarizes matrix enhancement strategies including freeze drying, spray drying, electrospinning/electrospraying, microfluidic spinning/wheel spinning, desolvation and supercritical fluid, to generate nano-, submicron-, micron-, or bulk-scale materials. It finally highlights the food applications of silk fibroin materials, including nutraceutical improvement, emulsions, enzyme immobilization and 3D/4D printing. This review also provides insights on potential opportunities (like safe modification, toxicity risk evaluation, and digestion conditions) and possibilities (like digital additive manufacturing) in functional food industry.

Silk fibroin/methacrylated gelatine/hydroxyapatite biomimetic nanofibrous membranes for guided bone regeneration

By mimicking in vivo bionic microenvironment and promoting osteogenic differentiation, the hybrid organic-inorganic nanofibrous membranes provide promising potential for guided bone regeneration (GBR) in the treatment of clinical bone defects. To develop a degradable and osteogenic membrane for GBR by combining the natural biomacromolecule silk fibroin (SF) and gelatine with the bioactive nano hydroxyapatite (nHA), the anhydride-modified gelatine-nano hydroxyapatite (GelMA-nHA) composites were synthesized in situ and introduced into silk fibroin to prepare nanofibrous membranes with different ratios using electrospinning and photocrosslinking. The nanofibrous membranes, particularly those with a mass ratio of 7:2:1, were found to exhibit satisfactory elongation at break up to 110 %, maintain the nanofibrous structure for up to 28 days, and rapidly form bone-like apatite within 3 days, thus offering advantages when it comes to guided bone regeneration. In vitro cell results showed that the SF/GelMA/nHA membranes had excellent biocompatibility and enhanced osteogenic differentiation of hBMSCs. In vivo studies revealed that the hybrid composite membranes can improve bone regeneration of critical-sized calvarial defects in rat model. Therefore, the novel hybrid nanofibrous membrane is proposed to be a alternative candidate for creating a bionic microenvironment that promotes bone regeneration, indicating their potential application to bone injury treatment.

Towards superior biopolymer gels by enabling interpenetrating network structures: A review on types, applications, and gelation strategies

Gels derived from single networks of natural polymers (biopolymers) typically exhibit limited physical properties and thus have seen constrained applications in areas like food and medicine. In contrast, gels founded on a synergy of multiple biopolymers, specifically polysaccharides and proteins, with intricate interpenetrating polymer network (IPN) structures, represent a promising avenue for the creation of novel gel materials with significantly enhanced properties and combined advantages. This review begins with the scrutiny of newly devised IPN gels formed through a medley of polysaccharides and/or proteins, alongside an introduction of their practical applications in the realm of food, medicine, and environmentally friendly solutions. Finally, based on the fact that the IPN gelation process and mechanism are driven by different inducing factors entwined with a diverse amalgamation of polysaccharides and proteins, our survey underscores the potency of physical, chemical, and enzymatic triggers in orchestrating the construction of crosslinked networks within these biomacromolecules. In these mixed systems, each specific inducer aligns with distinct polysaccharides and proteins, culminating in the generation of semi-IPN or fully-IPN gels through the intricate interpenetration between single networks and polymer chains or between two networks, respectively. The resultant IPN gels stand as paragons of excellence, characterized by their homogeneity, dense network structures, superior textural properties (e.g., hardness, elasticity, adhesion, cohesion, and chewability), outstanding water-holding capacity, and heightened thermal stability, along with guaranteed biosafety (e.g., nontoxicity and biocompatibility) and biodegradability. Therefore, a judicious selection of polymer combinations allows for the development of IPN gels with customized functional properties, adept at meeting precise application requirements.

Comparison of Silk Hydrogels Prepared via Different Methods

Silk fibroin (SF) hydrogels have garnered extensive attention in biomedical materials, owing to their superior biological properties. However, the challenges facing the targeted silk fibroin hydrogels involve chemical agents and shortfalls in performance. In this study, the silk fibroin hydrogels were prepared in different ways: sonication induction, chemical crosslinking, photopolymerization, and enzyme-catalyzed crosslinking. The SF hydrogels derived from photopolymerization exhibited higher compressive properties, with 124 Kpa fracture compressive stress and breaks at about 46% compression. The chemical crosslinking and enzyme-catalyzed silk fibroin hydrogels showed superior toughness, yet sonication-induced hydrogels showed brittle performance resulting from an increase in silk II crystals. The chemical-crosslinked hydrogel demonstrated lower thermostability due to the weaker crosslinking degree. In vitro, all silk fibroin hydrogels supported the growth of human umbilical vein endothelial cells, as the cell viability of hydrogels without chemical agents was relatively higher. This study provides insights into the formation process of silk fibroin hydrogels and optimizes their design strategy for biomedical applications.

Weaving the next generation of (bio)materials: Semi-interpenetrated and interpenetrated polymeric networks for biomedical applications

Advances in polymer science have led to the development of semi-interpenetrated and interpenetrated networks (SIPN/IPN). The interpenetration procedure allows enhancing several important properties of a polymeric material, including mechanical properties, swelling capability, stimulus-sensitive response, and biological performance, among others. More interestingly, the interpenetration (or semi-interpenetration) can be achieved independent of the material size, that is at the macroscopic, microscopic, or nanometric scale. SIPN/IPN have been used for a wide range of applications, especially in the biomedical field, including tissue engineering, delivery of chemical compounds or biological macromolecules, and multifunctional systems as theragnostic platforms. In the last years, this fascinating field has gained a great interest in the area of polymers for therapeutics; therefore, a comprehensive revision of the topic is timely. In this review, we describe in detail the most relevant synthetic approaches to fabricate polymeric IPN and SIPN, ranging from nanoscale to macroscale. The advantages of typical synthetic methods are analyzed, as well as novel and promising trends in the field of advanced material fabrication. Furthermore, the characterization techniques employed for these materials are summarized from physicochemical, thermal, mechanical, and biological perspectives. The applications of novel (semi-)interpenetrated structures are discussed with a focus on drug delivery, tissue engineering, and regenerative medicine, as well as combinations thereof.

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

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

Tailoring the Swelling-Shrinkable Behavior of Hydrogels for Biomedical Applications

Hydrogels with tailor-made swelling-shrinkable properties have aroused considerable interest in numerous biomedical domains. For example, as swelling is a key issue for blood and wound extrudates absorption, the transference of nutrients and metabolites, as well as drug diffusion and release, hydrogels with high swelling capacity have been widely applicated in full-thickness skin wound healing and tissue regeneration, and drug delivery. Nevertheless, in the fields of tissue adhesives and internal soft-tissue wound healing, and bioelectronics, non-swelling hydrogels play very important functions owing to their stable macroscopic dimension and physical performance in physiological environment. Moreover, the negative swelling behavior (i.e., shrinkage) of hydrogels can be exploited to drive noninvasive wound closure, and achieve resolution enhancement of hydrogel scaffolds. In addition, it can help push out the entrapped drugs, thus promote drug release. However, there still has not been a general review of the constructions and biomedical applications of hydrogels from the viewpoint of swelling-shrinkable properties. Therefore, this review summarizes the tactics employed so far in tailoring the swelling-shrinkable properties of hydrogels and their biomedical applications. And a relatively comprehensive understanding of the current progress and future challenge of the hydrogels with different swelling-shrinkable features is provided for potential clinical translations.

3D printed elastic hydrogel conduits with 7,8-dihydroxyflavone release for peripheral nerve repair

Nerve guide conduit is a promising treatment for long gap peripheral nerve injuries, yet its efficacy is limited. Drug-releasable scaffolds may provide reliable platforms to build a regenerative microenvironment for nerve recovery. In this study, an elastic hydrogel conduit encapsulating with prodrug nanoassemblies is fabricated by a continuous 3D printing technique for promoting nerve regeneration. The bioactive hydrogel is comprised of gelatin methacryloyl (GelMA) and silk fibroin glycidyl methacrylate (SF-MA), exhibiting positive effects on adhesion, proliferation, and migration of Schwann cells. Meanwhile, 7,8-dihydroxyflavone (7,8-DHF) prodrug nanoassemblies with high drug-loading capacities are developed through self-assembly of the lipophilic prodrug and loaded into the GelMA/SF-MA hydrogel. The drug loading conduit could sustainedly release 7,8-DHF to facilitate neurite elongation. A 12 ​mm nerve defect model is established for therapeutic efficiency evaluation by implanting the conduit through surgical suturing with rat sciatic nerve. The electrophysiological, morphological, and histological assessments indicate that this conduit can promote axon regeneration, remyelination, and function recovery by providing a favorable microenvironment. These findings implicate that the GelMA/SF-MA conduit with 7,8-DHF release has potentials in the treatment of long-gap peripheral nerve injury.

Tissue-Engineered Injectable Gelatin-Methacryloyl Hydrogel-Based Adjunctive Therapy for Intervertebral Disc Degeneration

Gelatin-methacryloyl (GelMA) hydrogels are photosensitive with good biocompatibility and adjustable mechanical properties. The GelMA hydrogel composite system is a prospective therapeutic material based on a tissue engineering platform for treating intervertebral disc (IVD) degeneration (IVDD). The potential application value of the GelMA hydrogel composite system in the treatment of IVDD mainly includes three aspects: first, optimization of the current clinical treatment methods, including conservative treatment and surgical treatment; second, regeneration of IVD cells to reverse or repair IVDD; and finally, IVDD instead of injury plays a biomechanical role. In this paper, we summarized and analyzed the preparation of GelMA hydrogels and their excellent biological characteristics as carriers and comprehensively demonstrated the research status and prospects of GelMA hydrogel composite systems in IVDD treatment. In addition, the challenges facing the application of GelMA hydrogel composite systems and the progress of research on new hydrogels modified by GelMA hydrogels are presented. Hopefully, this study will provide theoretical guidance for the future application of GelMA hydrogel composite systems in IVDD.

Fabrication of Hydrogel-Based Composite Fibers and Computer Simulation of the Filler Dynamics in the Composite Flow

Fibrous structures with anisotropic fillers as composites have found increasing interest in the field of biofabrication since they can mimic the extracellular matrix of anisotropic tissues such as skeletal muscle or nerve tissue. In the present work, the inclusion of anisotropic fillers in hydrogel-based filaments with an interpenetrating polymeric network (IPN) was evaluated and the dynamics of such fillers in the composite flow were analyzed using computational simulations. In the experimental part, microfabricated rods (200 and 400 μm length, 50 μm width) were used as anisotropic fillers in extrusion of composite filaments using two techniques of wet spinning and 3D printing. Hydrogels such as oxidized alginate (ADA) and methacrylated gelatin (GelMA) were used as matrices. In the computational simulation, a combination of computational fluid dynamics and coarse-grained molecular dynamics was used to study the dynamics of rod-like fillers in the flow field of a syringe. It showed that, during the extrusion process, microrods are far from being well aligned. Instead, many of them tumble on their way through the needle leading to a random orientation in the fiber which was confirmed experimentally.

Biocompatibility of silk methacrylate/gelatin-methacryloyl composite hydrogel and its feasibility as a vascular tissue engineering scaffold

Silk methacrylate (SilMA) has been studied extensively due to its ability to modify Silk fibroin (SF) by increasing the water solubility and enhancing the mechanical properties of SF hydrogels. However, SilMA hydrogels are generally soft with weak mechanical properties. In order to enhance the mechanical properties of hydrogel scaffolds, we used liquid nitrogen to modify SilMA to obtain a novel N₂-SilMA/gelatin-methacryloyl (GelMA) composite hydrogel. N₂-SilMA was successfully detected by Fourier transform infrared (FTIR) spectroscopy and ¹H nuclear magnetic resonance. Scanning electron microscope showed that the composite hydrogel still had certain arrangement characteristics of SF and dense pores which met the necessary conditions for the cell scaffold. The mechanical tests showed that the mechanical properties of SilMA were greatly enhanced after modification at ultra-low temperature. We evaluated its cytocompatibility and biocompatibility, and the results showed that the composite scaffold promoted the growth of cells. Different types of composite hydrogels were injected into ICR mice and the results showed a stable scaffold structure in vivo, suggesting their ability to promote angiogenesis. In conclusion, the N₂-SilMA/GelMA composite hydrogel had better mechanical properties, excellent cytocompatibility, and biological properties compared to the other groups.

MSCs-laden silk Fibroin/GelMA hydrogels with incorporation of platelet-rich plasma for chondrogenic construct

Repair of osteochondral defects and regeneration of cartilage is a major challenge. In this work, the mesenchymal stem cells (MSCs)-laden hydrogel was designed using silk fibroin (SF) and gelatin methacrylate (GelMA), to encapsulate platelet-rich plasma (PRP). Initially, GelMA was synthesized, and SF was prepared using silkworm cocoon, then MSCs-laden SF/GelMA (SG) hydrogel was fabricated. The physicochemical properties of the hydrogels were evaluated using Fourier-transform infrared spectroscopy, scanning electron microscope, and rheometry. After hydrogel preparation, the viability of MSCs in the hydrogels was investigated via CCK-8 analysis and fluorescent images. The MSCs-laden SG hydrogel containing PRP was subsequently injected into the cartilage defect area in Sprague Dawley rats. Hematoxylin and eosin (H&E), Masson staining, and Mankin scores evaluation confirmed the new cartilage formation in 8 weeks. The results presented in the study, therefore, showed that the prepared MSCs-laden SG hydrogel loaded with PRP has the potential for cartilage reconstruction, which is crucial to the treatment of knee osteoarthritis.

BMSCs-Seeded Interpenetrating Network GelMA/SF Composite Hydrogel for Articular Cartilage Repair

Because of limited self-healing ability, the treatment of articular cartilage defects is still an important clinical challenge. Hydrogel-based biomaterials have broad application prospects in articular cartilage repair. In this study, gelatin methacrylate (GelMA)and silk fibroin (SF) were combined to form a composite hydrogel with an interpenetrating network (IPN) structure under ultraviolet irradiation and ethanol treatment. Introducing silk fibroin into GelMA hydrogel significantly increased mechanical strength as compressive modulus reached 300 kPa in a GelMA/SF-5 (50 mg/mL silk fibroin) group. Moreover, composite IPN hydrogels demonstrated reduced swelling ratios and favorable biocompatibility and supported chondrogenesis of bone mesenchymal stem cells (BMSCs) at day 7 and day 14. Additionally, significantly higher gene expressions of Col-2, Acan, and Sox-9 (p < 0.01) were found in IPN hydrogel groups when compared with the GelMA group. An in vivo study was performed to confirm that the GelMA-SF IPN hydrogel could promote cartilage regeneration. The results showed partial regeneration of cartilage in groups treated with hydrogels only and satisfactory cartilage repair in groups of cell-seeded hydrogels, indicating the necessity of additional seeding cells in hydro-gel-based cartilage treatment. Therefore, our results suggest that the GelMA/SF IPN hydrogels may be a potential functional material in cartilage repair and regeneration.

Injectable, stretchable, toughened, bioadhesive composite hydrogel for bladder injury repair†

The bladder is exposed to constant internal and external mechanical forces due to its deformation and the dynamic environment in which it is placed, which can hamper its repair after an injury. Traditional hydrogel materials have limitations regarding their use in the bladder owing to their poor mechanical and tissue adhesion properties. In this study, a composite hydrogel composed of methacrylate gelatine, methacrylated silk fibroin, and Pluronic F127 diacrylate was developed, which combines the characteristics of natural and synthetic polymers. The mechanical properties of the novel hydrogel, such as stretchability, viscoelasticity, and toughness, were improved by virtue of a particular molecular design strategy whereby covalent and non-covalent bond interactions create a cross-linking effect. In addition, the composite hydrogel has important usability properties; it can be injected in liquid format and rapidly transformed into a gel via photo-initiated crosslinking. This was demonstrated on an isolated porcine bladder where the hydrogel closed arbitrarily-shaped tissue defects within 90 s of its application, verifying its effective bioadhesive and sealing properties. This composite hydrogel has great potential for application in bladder injury repair as a tissue-engineering scaffold.

An injectable, stretchable, toughened, bioadhesive composite hydrogel offers a new application strategy for sutureless repair and tissue regeneration of injured bladders.

Photocross-linked silk fibroin/hyaluronic acid hydrogel loaded with hDPSC for pulp regeneration

The construction of suitable biomaterials for pulp regeneration has always been a major challenge in the field of stomatology. Considering the complex and irregular anatomy of the root canal system, injectable hydrogels have received extensive attention as cell carriers in dental pulp regeneration. Here, we developed an injectable photocrosslinked methacrylylated silk fibroin (RSFMA)/methacrylylated hyaluronic acid (MeHA) composite hydrogel and characterized its physicochemical properties. The biocompatibility of encapsulated human dental pulp stem cells (hDPSCs) was subsequently investigated. With the addition of RSFMA, the pore size of the scaffolds became more regular with negligible change in porosity and exhibited excellent mechanical properties. Furthermore, the low concentration of RSFMA hydrogel in the composite hydrogel had higher cross-linking efficiency. In contrast to MeHA hydrogels, hDPSCs were encapsulated in hydrogels either in the absence or presence of high concentrations of RSFMA. The results indicated that cells in low-concentration RSFMA composite gel presented better growth ability, proliferation ability and osteogenic differentiation ability. This injectable photocrosslinked silk fibroin/hyaluronic acid hydrogel shows great potential in the field of dental pulp tissue engineering.

Interpenetrating polymer network hydrogels as bioactive scaffolds for tissue engineering

Tissue engineering, after decades of exciting progress and monumental breakthroughs, has yet to make a significant impact on patient health. It has become apparent that a dearth of biomaterial scaffolds that possess the material properties of human tissue while remaining bioactive and cytocompatible has been partly responsible for this lack of clinical translation. Herein, we propose the development of interpenetrating polymer network hydrogels as materials that can provide cells with an adhesive extracellular matrix-like 3D microenvironment while possessing the mechanical integrity to withstand physiological forces. These hydrogels can be synthesized from biologically-derived or synthetic polymers, the former polymer offering preservation of adhesion, degradability, and microstructure and the latter polymer offering tunability and superior mechanical properties. We review critical advances in the enhancement of mechanical strength, substrate-scale stiffness, electrical conductivity, and degradation in IPN hydrogels intended as bioactive scaffolds in the past five years. We also highlight the exciting incorporation of IPN hydrogels into state-of-the-art tissue engineering technologies, such as organ-on-a-chip and bioprinting platforms. These materials will be critical in the engineering of functional tissue for transplant, disease modeling, and drug screening.

Consumer Acceptance and Production of In Vitro Meat: A Review

In vitro meat (IVM) is a recent development in the production of sustainable food. The consumer perception of IVM has a strong impact on the commercial success of IVM. Hence this review examines existing studies related to consumer concerns, acceptance and uncertainty of IVM. This will help create better marketing strategies for IVM-producing companies in the future. In addition, IVM production is described in terms of the types of cells and culture conditions employed. The applications of self-organising, scaffolding, and 3D printing techniques to produce IVM are also discussed. As the conditions for IVM production are controlled and can be manipulated, it will be feasible to produce a chemically safe and disease-free meat with improved consumer acceptance on a sustainable basis.

Gelatin Methacrylate Hydrogel for Tissue Engineering Applications—A Review on Material Modifications

To recreate or substitute tissue in vivo is a complicated endeavor that requires biomaterials that can mimic the natural tissue environment. Gelatin methacrylate (GelMA) is created through covalent bonding of naturally derived polymer gelatin and methacrylic groups. Due to its biocompatibility, GelMA receives a lot of attention in the tissue engineering research field. Additionally, GelMA has versatile physical properties that allow a broad range of modifications to enhance the interaction between the material and the cells. In this review, we look at recent modifications of GelMA with naturally derived polymers, nanomaterials, and growth factors, focusing on recent developments for vascular tissue engineering and wound healing applications. Compared to polymers and nanoparticles, the modifications that embed growth factors show better mechanical properties and better cell migration, stimulating vascular development and a structure comparable to the natural-extracellular matrix.

Nitric Oxide-Releasing Gelatin Methacryloyl/Silk Fibroin Interpenetrating Polymer Network Hydrogels for Tissue Engineering Applications

Bacterial infection is one of the principal reasons for the failure of tissue engineering scaffolds. Therefore, the development of multifunctional scaffolds that not only are able to guide tissue regeneration but also can inhibit bacterial colonization is of great importance for tissue engineering applications. In this study, a highly antibacterial, biocompatible, and biodegradable scaffold based on silk fibroin (SF) and gelatin methacryloyl (GelMA) was prepared. Sequential cross-linking of GelMA and SF under UV irradiation and methanol treatment, respectively, resulted in the formation of interpenetrating network (IPN) hydrogels with a porous structure. In addition, impregnation of the hydrogels with a nitric oxide (NO) donor molecule, S-nitroso-N-acetylpenicillamine (SNAP), led to the development of NO-releasing scaffolds with strong antibacterial properties. According to the obtained results, the addition of SF to GelMA hydrogels caused an enhancement in the mechanical properties and NO release kinetics and prevented their rapid enzymatic degradation in aqueous media. Furthermore, swelling the GelMA-SF scaffolds with SNAP resulted in a bacteria reduction efficiency of >99.9% against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. The scaffolds also showed great cytocompatibility in vitro by increasing the proliferation and supporting the adhesion of 3T3 mouse fibroblast cells. Overall, GelMA-SF-SNAP showed great promise to be used as a scaffold for tissue engineering and wound healing applications.

3D bio-printed biphasic scaffolds with dual modification of silk fibroin for the integrated repair of osteochondral defects

Repair of osteochondral defects is still a challenge, especially the regeneration of hyaline cartilage. Parathyroid hormone (PTH) can inhibit the hypertrophy of chondrocytes to maintain the phenotype of hyaline cartilage. Here, we aimed to construct a bio-printed biphasic scaffold with a mechanical gradient based on dual modification of silk fibroin (SF) for the integrated repair of osteochondral defects. Briefly, SF was grafted with PTH (SF-PTH) and covalently immobilized with methacrylic anhydride (SF-MA), respectively. Next, gelatin methacryloyl (GM) mixed with SF-PTH or SF-MA was used as a bio-ink for articular cartilage and subchondral bone regeneration. Finally, the GM + SF-PTH/GM + SF-MA osteochondral biphasic scaffold was constructed using 3D bioprinting technology, and implanted in a rabbit osteochondral defect model. In this study, the SF-PTH bio-ink was synthesized for the first time. In vitro results indicated that the GM + SF-MA bio-ink had good mechanical properties, while the GM + SF-PTH bio-ink inhibited the hypertrophy of chondrocytes and was beneficial for the production of hyaline cartilage extracellular matrix. Importantly, an integrated GM + SF-PTH/GM + SF-MA biphasic scaffold with a mechanical gradient was successfully constructed. The results in vivo demonstrated that the GM + SF-PTH/GM + SF-MA scaffold could promote the regeneration of osteochondral defects and maintain the phenotype of hyaline cartilage to a large extent. Collectively, our results indicate that the integrated GM + SF-PTH/GM + SF-MA biphasic scaffold constructed by 3D bioprinting is expected to become a new strategy for the treatment of osteochondral defects.

Modeling and Fabrication of Silk Fibroin-Gelatin-Based Constructs Using Extrusion-Based Three-Dimensional Bioprinting

Robotic dispensing-based 3D bioprinting represents one of the most powerful technologies to develop hydrogel-based 3D constructs with enormous potential in the field of regenerative medicine. The optimization of hydrogel printing parameters, proper geometry and internal architecture of the constructs, and good cell viability during the bioprinting process are the essential requirements. In this paper, an analytical model based on the hydrogel rheological properties was developed to predict the extruded filament width in order to maximize the printed structure's fidelity to the design. Viscosity data of two natural hydrogels were imputed to a power-law model to extrapolate the filament width. Further, the model data were validated by monitoring the obtained filament width as the output. Shear stress values occurring during the bioprinting process were also estimated. Human mesenchymal stromal cells (hMSCs) were encapsulated in the silk fibroin-gelatin (G)-based hydrogel, and a 3D bioprinting process was performed to produce cell-laden constructs. Live and dead assay allowed estimating the impact of needle shear stress on cell viability after the bioprinting process. Finally, we tested the potential of hMSCs to undergo chondrogenic differentiation by evaluating the cartilaginous extracellular matrix production through immunohistochemical analyses. Overall, the use of the proposed analytical model enables defining the optimal printing parameters to maximize the fabricated constructs' fidelity to design parameters before the process execution, enabling to achieve more controlled and standardized products than classical trial-and-error approaches in the biofabrication of engineered constructs. Employing modeling systems exploiting the rheological properties of the hydrogels might be a valid tool in the future for guaranteeing high cell viability and for optimizing tissue engineering approaches in regenerative medicine applications.

3D printed hybrid bone constructs of PCL and dental pulp stem cells loaded GelMA

Fabrication of scaffolds using polymers and then cell seeding is a routine protocol of tissue engineering applications. Synthetic polymers have adequate mechanical properties to substitute for some bone tissue, but they are generally hydrophobic and have no specific cell recognition sites, which leads to poor cell affinity and adhesion. Some natural polymers, have high cell affinity but are mechanically weak and do not have the strength required as a bone supporting material. In the present study, 3D printed hybrid scaffolds were fabricated using PCL and GelMA carrying dental pulp stem cells (DPSCs), which is printed in the gaps between the PCL struts. This cell loaded GelMA was shown to support osteoinductivity, while the PCL provided mechanical strength needed to mimic the bone tissue. 3D printed PCL/GelMA and GelMA scaffolds were highly stable during 21 days of incubation in PBS. The compressive moduli of the hybrid scaffolds were in the range of the compressive moduli of trabecular bone. DPSCs were homogeneously distributed throughout the entire hydrogel component and exhibited high cell viability in both scaffolds during 21 days of incubation. Upon osteogenic differentiation DPSCs expressed two key matrix proteins, osteopontin and osteocalcin. Alizarin red staining showed mineralized nodules, which demonstrates osteogenic differentiation of DPSCs within GelMA. This construct yielded a very high cell viability, osteogenic differentiation and mineralization comparable to cell culture without compromising mechanical strength suitable for bone tissue engineering applications. Thus, 3D printed, cell loaded PCL/GelMA hybrid scaffolds have a great potential for use in bone tissue engineering applications.

Three-dimensional bioprinting of artificial ovaries by an extrusion-based method using gelatin-methacryloyl bioink

PURPOSE

The aim of this study was to design and fabricate a three-dimensional (3D) printed artificial ovary.

METHODS

We first compared the printability of gelatin-methacryloyl (GelMA), alginate and GelMA-alginate bioinks, of which GelMA was selected for further investigation. The swelling properties, degradation kinetics and shape fidelity of GelMA scaffolds were characterized by equilibrium swelling/lyophilization, collagenase processing and micro-computed tomography evaluation. Commercial ovarian tumor cell lines (COV434, KGN, ID8) and primary culture ovarian somatic cells were utilized to perform cell-laden 3D printing, and the results were evaluated by live/dead assays and TUNEL detection. Murine ovarian follicles were seeded in the ovarian scaffold and their diameters were recorded every day. Finally, in vitro maturation was performed, and the ovulated oocytes were collected and observed.

RESULTS

Our results indicated that GelMA was suitable for 3D printing fabrication. Its scaffolds performed well in terms of hygroscopicity, degradation kinetics and shape fidelity. The viability of ovarian somatic cells was lower than that of commercial cell lines, suggesting that extrusion-based 3D culture fabrication is not suitable for primary ovarian cells. Nevertheless, the GelMA-based 3D printing system provided an appropriate microenvironment for ovarian follicles, which successfully grew and ovulated in the scaffolds. Metaphase II oocytes were also observed after in vitro maturation.

CONCLUSIONS

The GelMA-based 3D printing culture system is a viable alternative option for follicular growth, development and transfer. Accordingly, it shows promise for clinical application in the treatment of female endocrine and reproductive conditions.

Highly Ordered 3D Tissue Engineering Scaffolds as a Versatile Culture Platform for Nerve Cells Growth

Tissue engineering scaffolds provide an encouraging alternative for nerve injuries due to their biological support for nerve cell growth, which can be used for neuronal repair. Nerve cells have been reported to be mostly cultured on 2D scaffolds that cannot mimic the native extracellular matrix. Herein, highly ordered 3D scaffolds are fabricated for nerve cell culture by melt electrospinning writing, the microstructures and geometries of the scaffolds could be well modulated. An effective strategy for scaffold surface modification to promote nerve cell growth is proposed. The effects of scaffolds with different surface modifications, viz., plasma treatment, single poly-D-lysine (PDL) coating after plasma treatment, single laminin (LM) coating after plasma treatment, double PDL and LM coatings after plasma treatment, on PC12 cell growth are evaluated. Experiments show the scaffold modified with double PDL and LM coatings after plasma treatment facilitated the growth of PC12 cells most effectively, indicating the synergistic effect of PDL and LM on the growth of nerve cells. This is the first systematic and quantitative study of the effects of different scaffold surface modifications on nerve cell growth. The above results provide a versatile culture platform for growing nerve cells, and for recovery from peripheral nerve injury.

Crosslinking strategies for silk fibroin hydrogels: promising biomedical materials

Due to their strong biomimetic potential, silk fibroin (SF) hydrogels are impressive candidates for tissue engineering, due to their tunable mechanical properties, biocompatibility, low immunotoxicity, controllable biodegradability, and a remarkable capacity for biomaterial modification and the realization of a specific molecular structure. The fundamental chemical and physical structure of SF allows its structure to be altered using various crosslinking strategies. The established crosslinking methods enable the formation of three-dimensional (3D) networks under physiological conditions. There are different chemical and physical crosslinking mechanisms available for the generation of SF hydrogels (SFHs). These methods, either chemical or physical, change the structure of SF and improve its mechanical stability, although each method has its advantages and disadvantages. While chemical crosslinking agents guarantee the mechanical strength of SFH through the generation of covalent bonds, they could cause some toxicity, and their usage is not compatible with a cell-friendly technology. On the other hand, physical crosslinking approaches have been implemented in the absence of chemical solvents by the induction of β-sheet conformation in the SF structure. Unfortunately, it is not easy to control the shape and properties of SFHs when using this method. The current review discusses the different crosslinking mechanisms of SFH in detail, in order to support the development of engineered SFHs for biomedical applications.

Ultrahigh-water-content biocompatible gelatin-based hydrogels: Toughened through micro-sized dissipative morphology as an effective strategy

Fabrication of simultaneously robust and superabsorbent gelatin-based hydrogels for biomedical applications still remains a challenge due to lack of locally dissipative points in the presence of large water content. Here, we apply a synthesis strategy through which water absorbency and energy dissipative points are separated, and toughening mechanism is active closely at the crack tip. For this, gelatin-based microgels (GeMs) were synthesized in a way that concentrated supramolecular interactions were present to increase the energy necessary to propagate a macroscopic crack. The microgels were interlocked to each other via both temporary hydrophobic associations and permanent covalent crosslinks, in which the sacrificial binds sustained the toughness due to the mobility of the junction zones and particles sliding. However, chemical crosslinking points preserved the integrity and fast recoverability of the hydrogel. Hysteresis increased strongly with increasing supramolecular interactions within the network. The prepared hydrogels showed energy loss and swelling ratio up to 3440 J. m^-3 and 830%, respectively, which was not achievable with conventional network fabrication methods. The microgels were also assessed for their in vivo biocompatibility in a rat subcutaneous pocket assay. Results of hematoxylin and eosin (H&E) staining demonstrated regeneration of the tissue around the scaffolds without incorporation of growth factors. Also, vascularization within the scaffolds was observed after 4 weeks implantation. These results indicate that our strategy is a promising method to manipulate those valuable polymers, which lose their toughness and applicability with increasing their water content.

Cell preservation methods and its application to studying rare disease

The ability to preserve and transport human cells in a stable medium over long distances is critical to collaborative efforts and the advancement of knowledge in the study of human disease. This is particularly important in the study of rare diseases. Recently, advancements in the understanding of renal ciliopathies has been achieved via the use of patient urine-derived cells (UDCs). However, the traditional method of cryopreservation, although considered as the gold standard, can result in decreased sample viability of many cell types, including UDCs. Delays in transportation can have devastating effects upon the viability of samples, and may even result in complete destruction of cells following evaporation of dry ice or liquid nitrogen, leaving samples in cryoprotective agents, which are cytotoxic at room temperature. The loss of any patient sample in this manner is detrimental to research, however it is even more so when samples are from patients with a rare disease. In order to overcome the associated limitations of traditional practices, new methods of preservation and shipment, including cell encapsulation within hydrogels, and transport in specialised devices are continually being investigated. Here we summarise and compare traditional methods with emerging novel alternatives for the preservation and shipment of cells, and consider the effectiveness of such methods for use with UDCs to further enable the study and understanding of kidney diseases.

Biomimetic Membranes of Methacrylated Gelatin/Nanohydroxyapatite/Poly(l-Lactic Acid) for Enhanced Bone Regeneration

Nanofibrous poly(l-lactic acid) (PLLA) membrane-simulated extracellular matrices (ECMs) can be used in the biomedical field. However, the hydrophobic nature and poor osteoinductive property of PLLA limit its application in guided bone regeneration (GBR). In this work, a methacrylated gelatin/nano-HA (GelMA/nHA) complex was first synthesized in situ and then introduced into PLLA to fabricate biomimetic GelMA/nHA/PLLA membranes, mimicking the nanofibrous architecture and composition of ECMs by electrospinning and photocrosslinking. Compared to PLLA and GelMA/PLLA membranes, the novel GelMA/nHA/PLLA membranes demonstrated better tensile, hydrophilic, water sorption, and degradation properties. An in vitro biological evaluation indicated that the membranes promoted human bone marrow-derived mesenchymal stem cell (hBMSC) proliferation, adhesion, and osteogenic differentiation. Critical-sized defects in rat models were used to evaluate the bone regeneration performances of the three kinds of membranes in vivo, and the GelMA/nHA/PLLA membranes demonstrated excellent osteogenic regeneration potential. Therefore, GelMA/nHA/PLLA membranes have wide application prospects in bioengineering applications such as GBR treatment.

Effect of Bone Morphogenic Protein-2-Loaded Mesoporous Strontium Substitution Calcium Silicate/Recycled Fish Gelatin 3D Cell-Laden Scaffold for Bone Tissue Engineering

Bone has a complex hierarchical structure with the capability of self-regeneration. In the case of critical-sized defects, the regeneration capabilities of normal bones are severely impaired, thus causing non-union healing of bones. Therefore, bone tissue engineering has since emerged to solve problems relating to critical-sized bone defects. Amongst the many biomaterials available on the market, calcium silicate-based (CS) cements have garnered huge interest due to their versatility and good bioactivity. In the recent decade, scientists have attempted to modify or functionalize CS cement in order to enhance the bioactivity of CS. Reports have been made that the addition of mesoporous nanoparticles onto scaffolds could enhance the bone regenerative capabilities of scaffolds. For this study, the main objective was to reuse gelatin from fish wastes and use it to combine with bone morphogenetic protein (BMP)-2 and Sr-doped CS scaffolds to create a novel BMP-2-loaded, hydrogel-based mesoporous SrCS scaffold (FGSrB) and to evaluate for its composition and mechanical strength. From this study, it was shown that such a novel scaffold could be fabricated without affecting the structural properties of FGSr. In addition, it was proven that FGSrB could be used for drug delivery to allow stable localized drug release. Such modifications were found to enhance cellular proliferation, thus leading to enhanced secretion of alkaline phosphatase and calcium. The above results showed that such a modification could be used as a potential alternative for future bone tissue engineering research.

Enhanced Proliferation and Differentiation of Human Mesenchymal Stem Cell-laden Recycled Fish Gelatin/Strontium Substitution Calcium Silicate 3D Scaffolds

Cell-encapsulated bioscaffold is a promising and novel method to allow fabrication of live functional organs for tissue engineering and regenerative medicine. However, traditional fabrication methods of 3D scaffolds and cell-laden hydrogels still face many difficulties and challenges. This study uses a newer 3D fabrication technique and the concept of recycling of an unutilized resource to fabricate a novel scaffold for bone tissue engineering. In this study, fish-extracted gelatin was incorporated with bioactive ceramic for bone tissue engineering, and with this we successfully fabricated a novel fish gelatin methacrylate (FG) polymer hydrogel mixed with strontium-doped calcium silicate powder (FGSr) 3D scaffold via photo-crosslinking. Our results indicated that the tensile strength of FGSr was almost 2.5-fold higher as compared to FG thus making it a better candidate for future clinical applications. The in-vitro assays illustrated that the FGSr scaffolds showed good biocompatibility with human Wharton jelly-derived mesenchymal stem cells (WJMSC), as well as enhancing the osteogenesis differentiation of WJMSC. The WJMSC-laden FGSr 3D scaffolds expressed a higher degree of alkaline phosphatase activity than those on cell-laden FG 3D scaffolds and this result was further proven with the subsequent calcium deposition results. Therefore, these results showed that 3D-printed cell-laden FGSr scaffolds had enhanced mechanical property and osteogenic-related behavior that made for a more suitable candidate for future clinical applications.