Synthesis and Properties of Gelatin Methacryloyl (GelMA) Hydrogels and Their Recent Applications in Load-Bearing Tissue

Preparation and In Vitro Characterization of Gelatin Methacrylate for Corneal Tissue Engineering

BACKGROUND

Corneal disease is second only to cataract considered as the leading cause of blindness in the world, with high morbidity. Construction of corneal substitutes in vitro by tissue engineering technology to achieve corneal regeneration has become a research hotspot in recent years. We conducted in-depth research on the biocompatibility, physicochemical and mechanical properties of rat bone marrow mesenchymal stem cells (rBM-MSCs)-seeded gelatin methacrylate (GelMA) as a bioengineered cornea.

METHODS

Four kinds of GelMA with different concentrations (7, 10, 15 and 30%) were prepared, and their physic-chemical, optical properties, and biocompatibility with rBM-MSCs were characterized. MTT, live/dead staining, cell morphology, immunofluorescence staining and gene expression of keratocyte markers were performed.

RESULTS

7%GelMA hydrogel had higher equilibrium water content and porosity, better optical properties and hydrophilicity. In addition, it is more beneficial to the growth and proliferation of rBM-MSCs. However, the 30%GelMA hydrogel had the best mechanical properties, and could be more conducive to promote the differentiation of rBM-MSCs into keratocyte-like cells.

CONCLUSION

As a natural biological scaffold, GelMA hydrogel has good biocompatibility. And it has the ability to promote the differentiation of rBM-MSCs into keratocyte-like cells, which laid a theoretical and experimental foundation for further tissue-engineered corneal stromal transplantation, and provided a new idea for the source of seeded cells in corneal tissue engineering.

Methacrylated Silk Fibroin Hydrogels: pH as a Tool to Control Functionality

The last decade has witnessed significant progress in the development of photosensitive polymers for in situ polymerization and 3D printing applications. Light-mediated sol-gel transitions have immense potential for tissue engineering applications as cell-laden materials can be crosslinked within minutes under mild environmental conditions. Silk fibroin (SF) is extensively explored in regenerative medicine applications due to its ease of modification and exceptional mechanical properties along with cytocompatibility. To efficiently design SF materials, the in vivo assembly of SF proteins must be considered. During SF biosynthesis, changes in pH, water content, and metal ion concentrations throughout the silkworm gland divisions drive the transition from liquid silk to its fiber form. Herein, we study the effect of the glycidyl-methacrylate-modified SF (SilkMA) solution pH on the properties and secondary structure of SilkMA hydrogels by testing formulations prepared at pH 5, 7, and 8. Our results demonstrate an influence of the prepolymer solution pH on the hydrogel rheological properties, compressive modulus, optical transmittance, and network swellability. The hydrogel pH did not affect the in vitro viability and morphology of human dermal fibroblasts. This work demonstrates the utility of the solution pH to tailor the SilkMA conformational structure development toward utility and function and shows the need to strictly control the pH to reduce batch-to-batch variability and ensure reproducibility.

Polyethylene glycol diacrylate scaffold filled with cell-laden methacrylamide gelatin/alginate hydrogels used for cartilage repair

Natural cartilage tissue has excellent mechanical properties and has certain cellular components. At this stage, it is a great challenge to produce cartilage scaffolds with excellent mechanical properties, biocompatibility, and biodegradability. Hydrogels are commonly used in tissue engineering because of their excellent biocompatibility; however, the mechanical properties of commonly used hydrogels are difficult to meet the requirements of making cartilage scaffolds. The mechanical properties of high concentration polyethylene glycol diacrylate (PEGDA) hydrogel are similar to those of natural cartilage, but its biocompatibility is poor. Low concentration hydrogel has better biocompatibility, but its mechanical properties are poor. In this study, two different hydrogels were combined to produce cartilage scaffolds with good mechanical properties and strong biocompatibility. First, the PEGDA grid scaffold was printed with light curing 3D printing technology, and then the low concentration GelMA/Alginate hydrogel with chondral cells was filled into the PEGDA grid scaffold. After a series of cell experiments, the filling hydrogel with the best biocompatibility was screened out, and finally the filled hydrogel with cells and excellent biocompatibility was obtained. Cartilage tissue engineering scaffolds with certain mechanical properties were found to have a tendency of cartilage formation in in vitro culture. Compared with the scaffold obtained by using a single hydrogel, this molding method can produce a tissue engineering scaffold with excellent mechanical properties on the premise of ensuring biocompatibility, which has a certain potential application value in the field of cartilage tissue engineering.

A 3D Bioprinted In Vitro Model of Pulmonary Artery Atresia to Evaluate Endothelial Cell Response to Microenvironment

Vascular atresia are often treated via transcatheter recanalization or surgical vascular anastomosis due to congenital malformations or coronary occlusions. The cellular response to vascular anastomosis or recanalization is, however, largely unknown and current techniques rely on restoration rather than optimization of flow into the atretic arteries. An improved understanding of cellular response post anastomosis may result in reduced restenosis. Here, an in vitro platform is used to model anastomosis in pulmonary arteries (PAs) and for procedural planning to reduce vascular restenosis. Bifurcated PAs are bioprinted within 3D hydrogel constructs to simulate a reestablished intervascular connection. The PA models are seeded with human endothelial cells and perfused at physiological flow rate to form endothelium. Particle image velocimetry and computational fluid dynamics modeling show close agreement in quantifying flow velocity and wall shear stress within the bioprinted arteries. These data are used to identify regions with greatest levels of shear stress alterations, prone to stenosis. Vascular geometry and flow hemodynamics significantly affect endothelial cell viability, proliferation, alignment, microcapillary formation, and metabolic bioprofiles. These integrated in vitro-in silico methods establish a unique platform to study complex cardiovascular diseases and can lead to direct clinical improvements in surgical planning for diseases of disturbed flow.

MechAnalyze: An Algorithm for Standardization and Automation of Compression Test Analysis

The mechanical properties of hydrogels, as well as native and engineered tissues are key parameters frequently assessed in biomaterial science and tissue engineering research. However, a lack of standardized methods and user-independent data analysis has impacted the research community for many decades and contributes to poor reproducibility and comparability of datasets, representing a significant issue often neglected in publications. In this study, we provide a software package, MechAnalyze, facilitating the standardized and automated analysis of force-displacement data generated in unconfined compression tests. Using comparative studies of datasets analyzed manually and with MechAnalyze, we demonstrate that the software reliably determines the compressive moduli, failure stress and failure strain of hydrogels, as well as engineered and native tissues, while providing an intuitive user interface that requires minimal user input. MechAnalyze provides a fast and user-independent data analysis method and advances process standardization, reproducibility, and comparability of data for the mechanical characterization of biomaterials as well as native and engineered tissues. Impact statement Mechanical properties of hydrogels are crucial parameters in the development of new materials for tissue engineering. However, manual assessment is tedious, not standardized and suffers under user-to-user bias. Hence, the here presented stand-alone software package provides analysis and statistics of force-displacement and material geometry data to determine the compressive moduli, failure stress, and failure strain in a standardized, robust, and automated fashion. MechAnalyze will substantially support biomechanical testing of hydrogels as well as engineered and native tissues and will thus, be of appreciable value to a broad target group in regenerative medicine, tissue engineering, but also life sciences and biomedicine.

Effect of Polymeric Matrix Stiffness on Osteogenic Differentiation of Mesenchymal Stem/Progenitor Cells: Concise Review

Mesenchymal stem/progenitor cells (MSCs) have a multi-differentiation potential into specialized cell types, with remarkable regenerative and therapeutic results. Several factors could trigger the differentiation of MSCs into specific lineages, among them the biophysical and chemical characteristics of the extracellular matrix (ECM), including its stiffness, composition, topography, and mechanical properties. MSCs can sense and assess the stiffness of extracellular substrates through the process of mechanotransduction. Through this process, the extracellular matrix can govern and direct MSCs' lineage commitment through complex intracellular pathways. Hence, various biomimetic natural and synthetic polymeric matrices of tunable stiffness were developed and further investigated to mimic the MSCs' native tissues. Customizing scaffold materials to mimic cells' natural environment is of utmost importance during the process of tissue engineering. This review aims to highlight the regulatory role of matrix stiffness in directing the osteogenic differentiation of MSCs, addressing how MSCs sense and respond to their ECM, in addition to listing different polymeric biomaterials and methods used to alter their stiffness to dictate MSCs' differentiation towards the osteogenic lineage.

Three-Dimensional Printability of an ECM-Based Gelatin Methacryloyl (GelMA) Biomaterial for Potential Neuroregeneration

The current study introduces two novel, smart polymer three-dimensional (3D)-printable interpenetrating polymer network (IPN) hydrogel biomaterials with favorable chemical, mechanical, and morphological properties for potential applications in traumatic brain injury (TBI) such as potentially assisting in the restoration of neurological function through closure of the wound deficit and neural tissue regeneration. Additionally, removal of injury matter to allow for the appropriate scaffold grafting may assist in providing a TBI treatment. Furthermore, due to the 3D printability of the IPN biomaterials, complex structures can be designed and fabricated to mimic the native shape and structure of the injury sight, which can potentially assist with neural tissue regeneration after TBI. In this study, a peptide-only approach was employed, wherein collagen and elastin in a blend with gelatin methacryloyl were prepared and crosslinked using either Irgacure or Irgacure and Genipin to form either a semi or full IPN hydrogel 3D-printable neuromimicking platform system, respectively. The scaffolds displayed favorable thermal stability and were amorphous in nature with high full width at half-maximum values. Furthermore, no alteration to the peptide secondary structure was noted using Fourier transform infrared spectroscopy. The IPN biomaterials have a stiffness of around 600 Pa and are suitable for softer tissue engineering applications-that is, the brain. Scanning electron micrographs indicated that the IPN biomaterials had a morphological structure with a significant resemblance to the native rat cortex. Both biomaterial scaffolds were shown to support the growth of PC12 cells over a 72 h period. Furthermore, the increased nuclear eccentricity and nuclear area were shown to support the postulation that the IPN biomaterials maintain the cells in a healthy state encouraging cellular mitosis and proliferation. The Genipin component of the full IPN was further shown to exhibit antimicrobial properties and this suggests that Genipin can prevent the growth of pathogens associated with postsurgical brain infections. In addition to these findings, the study presents an anomaly, wherein the full IPN is found to be more brittle than the semi IPN, a finding that is in contradiction with the literature. This research, therefore, contributes to the collection of potential biomaterials for TBI applications coupled with 3D printing and can assist in the progression of neural treatments toward patient-specific scaffolds through the development of custom scaffolds.

In Situ Oxygen Generation Enhances the SCAP Survival in Hydrogel Constructs

Prolonged and severe hypoxia is the main cause of death of transplanted cells prior to the establishment of functional circulation. In situ generation of oxygen by oxygen-producing scaffolds-a unique solution that could produce and deliver oxygen to the adjacent cells independently of blood perfusion-has attracted considerable attention to enhance the survivability of the transplanted cells. However, the application of oxygen-generating scaffolds for facilitating cell survival in pulp-like tissue regeneration is yet to be explored. In this study, gelatin methacryloyl (GelMA)-a biocompatible scaffolding material that closely mimics the native extracellular matrix and is conducive to cell proliferation and differentiation-was used to fabricate oxygen-generating scaffolds by loading various concentrations of CaO₂. The CaO₂ distribution, topography, swelling, and pore size of CaO₂-GelMA hydrogels were characterized in detail. The release of O₂ by the scaffold and the viability, spreading, and proliferation of stem cells from apical papilla (SCAPs) encapsulated in the GelMA hydrogels with various concentrations of CaO₂ under hypoxia were evaluated. In addition, cellular constructs were engineered into root canals, and cell viability within the apical, middle, and coronal portions was assessed. Our findings showed that 0.5% CaO₂-GelMA was sufficient to supply in situ oxygen for maintaining the embedded SCAP viability for 1 wk. Furthermore, the 0.5% CaO₂-GelMA hydrogels improved the survivability of SCAPs within the coronal portion of the engineered cellular constructs within the root canals. This work demonstrated that 0.5% CaO₂-GelMA hydrogels offer a potential promising scaffold that enhances survival of the embedded SCAPs in endodontic regeneration.

Biofabrication of Cell-Laden Gelatin Methacryloyl Hydrogels with Incorporation of Silanized Hydroxyapatite by Visible Light Projection

Gelatin methacryloyl (GelMA) hydrogel is a photopolymerizable biomaterial widely used for three-dimensional (3D) cell culture due to its high biocompatibility. However, the drawback of GelMA hydrogel is its poor mechanical properties, which may compromise the feasibility of biofabrication techniques. In this study, a cell-laden GelMA composite hydrogel with a combination incorporating silanized hydroxyapatite (Si-HAp) and a simple and harmless visible light crosslinking system for this hydrogel were developed. The incorporation of Si-HAp into the GelMA hydrogel enhanced the mechanical properties of the composite hydrogel. Moreover, the composite hydrogel exhibited low cytotoxicity and promoted the osteogenic gene expression of embedded MG63 cells and Human bone marrow mesenchymal stem cells (hBMSCs). We also established a maskless lithographic method to fabricate a defined 3D structure under visible light by using a digital light processing projector, and the incorporation of Si-HAp increased the resolution of photolithographic hydrogels. The GelMA-Si-HAp composite hydrogel system can serve as an effective biomaterial in bone regeneration.

Fabrication of 3D-Printed Interpenetrating Hydrogel Scaffolds for Promoting Chondrogenic Differentiation

The limited self-healing ability of cartilage necessitates the application of alternative tissue engineering strategies for repairing the damaged tissue and restoring its normal function. Compared to conventional tissue engineering strategies, three-dimensional (3D) printing offers a greater potential for developing tissue-engineered scaffolds. Herein, we prepared a novel photocrosslinked printable cartilage ink comprising of polyethylene glycol diacrylate (PEGDA), gelatin methacryloyl (GelMA), and chondroitin sulfate methacrylate (CSMA). The PEGDA-GelMA-CSMA scaffolds possessed favorable compressive elastic modulus and degradation rate. In vitro experiments showed good adhesion, proliferation, and F-actin and chondrogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) on the scaffolds. When the CSMA concentration was increased, the compressive elastic modulus, GAG production, and expression of F-actin and cartilage-specific genes (COL2, ACAN, SOX9, PRG4) were significantly improved while the osteogenic marker genes of COL1 and ALP were decreased. The findings of the study indicate that the 3D-printed PEGDA-GelMA-CSMA scaffolds possessed not only adequate mechanical strength but also maintained a suitable 3D microenvironment for differentiation, proliferation, and extracellular matrix production of BMSCs, which suggested this customizable 3D-printed PEGDA-GelMA-CSMA scaffold may have great potential for cartilage repair and regeneration in vivo.

Recent advancements in the bioprinting of vascular grafts

Recent advancements in the bioinks and three-dimensional (3D) bioprinting methods used to fabricate vascular constructs are summarized herein. Critical biomechanical properties required to fabricate an ideal vascular graft are highlighted, as well as various testing methods have been outlined to evaluate the bio-fabricated grafts as per the Food and Drug Administration (FDA) and International Organization for Standardization (ISO) guidelines. Occlusive artery disease and cardiovascular disease are the major causes of death globally. These diseases are caused by the blockage in the arteries, which results in a decreased blood flow to the tissues of major organs in the body, such as the heart. Bypass surgery is often performed using a vascular graft to re-route the blood flow. Autologous grafts represent a gold standard for such bypass surgeries; however, these grafts may be unavailable due to the previous harvesting or possess a poor quality. Synthetic grafts serve well for medium to large-sized vessels, but they fail when used to replace small-diameter vessels, generally smaller than 6 mm. Various tissue engineering approaches have been used to address the urgent need for vascular graft that can withstand hemodynamic blood pressure and has the ability to grow and remodel. Among these approaches, 3D bioprinting offers an attractive solution to construct patient-specific vessel grafts with layered biomimetic structures.

Adipose-derived mesenchymal stromal cell-derived exosomes promote tendon healing by activating both SMAD1/5/9 and SMAD2/3

Background

The use of adipose-derived mesenchymal stromal cell-derived exosomes (ADSC-Exos) may become a new therapeutic method in biomedicine owing to their important role in regenerative medicine. However, the role of ADSC-Exos in tendon repair has not yet been evaluated. Therefore, we aimed to clarify the healing effects of ADSC-Exos on tendon injury.

Methods

The adipose-derived mesenchymal stromal cells (ADSCs) and tendon stem cells (TSCs) were isolated from the subcutaneous fat and tendon tissues of Sprague-Dawley rats, respectively, and exosomes were isolated from ADSCs. The proliferation and migration of TSCs induced by ADSC-Exos were analyzed by EdU, cell scratch, and transwell assays. We used western blot to analyze the tenogenic differentiation of TSCs and the role of the SMAD signaling pathways. Then, we explored a new treatment method for tendon injury, combining exosome therapy with local targeting using a biohydrogel. Immunofluorescence and immunohistochemistry were used to detect the expression of inflammatory and tenogenic differentiation after tendon injury, respectively. The quality of tendon healing was evaluated by hematoxylin-eosin (H&E) staining and biomechanical testing.

Results

ADSC-Exos could be absorbed by TSCs and promoted the proliferation, migration, and tenogenic differentiation of these cells. This effect may have depended on the activation of the SMAD2/3 and SMAD1/5/9 pathways. Furthermore, ADSC-Exos inhibited the early inflammatory reaction and promoted tendon healing in vivo.

Conclusions

Overall, we demonstrated that ADSC-Exos contributed to tendon regeneration and provided proof of concept of a new approach for treating tendon injuries.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02410-w.

Effect of Photoinitiator on Precursory Stability and Curing Depth of Thiol-Ene Clickable Gelatin

Recent advances highlight the potential of photopolymerizable allylated gelatin (GelAGE) as a versatile hydrogel with highly tailorable properties. It is, however, unknown how different photoinitiating system affects the stability, gelation kinetics and curing depth of GelAGE. In this study, sol fraction, mass swelling ratio, mechanical properties, rheological properties, and curing depth were evaluated as a function of time with three photo-initiating systems: Irgacure 2959 (Ig2959; 320–500 nm), lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP; 320–500 nm), and ruthenium/sodium persulfate (Ru/SPS; 400–500 nm). Results demonstrated that GelAGE precursory solutions mixed with either Ig2959 or LAP remained stable over time while the Ru/SPS system enabled the onset of controllable redox polymerization without irradiation during pre-incubation. Photo-polymerization using the Ru/SPS system was significantly faster (<5 s) compared to both Ig2959 (70 s) and LAP (50 s). Plus, The Ru/SPS system was capable of polymerizing a thick construct (8.88 ± 0.94 mm), while Ig2959 (1.62 ± 0.49 mm) initiated hydrogels displayed poor penetration depth with LAP (7.38 ± 2.13 mm) in between. These results thus support the use of the visible light based Ru/SPS photo-initiator for constructs requiring rapid gelation and a good curing depth while Ig2959 or LAP can be applied for photo-polymerization of GelAGE materials requiring long-term incubation prior to application if UV is not a concern.

Human Salivary Histatin-1-Functionalized Gelatin Methacrylate Hydrogels Promote the Regeneration of Cartilage and Subchondral Bone in Temporomandibular Joints

The avascular structure and lack of regenerative cells make the repair of osteochondral defects in the temporomandibular joint (TMJ) highly challenging in the clinic. To provide a viable treatment option, we developed a methacrylated gelatin (Gel-MA) hydrogel functionalized with human salivary histatin-1 (Hst1). Gel-MA is highly biocompatible, biodegradable, and cost-effective. Hst1 is capable of activating a series of cell activities, such as adhesion, migration, differentiation, and angiogenesis. To evaluate the efficacy of Hst1/Gel-MA, critical-size osteochondral defects (3 mm in diameter and 3 mm in depth) of TMJ in New Zealand white rabbits were surgically created and randomly assigned to one of the three treatment groups: (1) control (no filling material); (2) Gel-MA hydrogel; (3) Hst1/Gel-MA hydrogel. Samples were retrieved 1, 2, and 4 weeks post-surgery and subjected to gross examination and a series of histomorphometric and immunological analyses. In comparison with the control and Gel-MA alone groups, Hst1/Gel-MA hydrogel was associated with significantly higher International Cartilage Repair Society score, modified O’Driscoll score, area percentages of newly formed bone, cartilage, collagen fiber, and glycosaminoglycan, and expression of collagen II and aggrecan. In conclusion, Hst1/Gel-MA hydrogels significantly enhance bone and cartilage regeneration, thus bearing promising application potential for repairing osteochondral defects.

Additively Manufactured Gradient Porous Ti-6Al-4V Hip Replacement Implants Embedded with Cell-Laden Gelatin Methacryloyl Hydrogels

Laser additive manufacturing has led to a paradigm shift in the design of next-generation customized porous implants aiming to integrate better with the surrounding bone. However, conflicting design criteria have limited the development of fully functional porous implants; increasing porosity improves body fluid/cell-laden prepolymer permeability at the expense of compromising mechanical stability. Here, functionally gradient porosity implants and scaffolds designed based on interconnected triply periodic minimal surfaces (TPMS) are demonstrated. High local porosity is defined at the implant/tissue interface aiming to improve the biological response. Gradually decreasing porosity from the surface to the center of the porous constructs provides mechanical strength in selective laser melted Ti-6Al-4V implants. The effect of unit cell size is studied to discover the printability limit where the specific surface area is maximized. Furthermore, mechanical studies on the unit cell topology effects suggest that the bending-dominated architectures can provide significantly enhanced strength and deformability, compared to stretching-dominated architectures. A finite element (FE) model developed also showed great predictability (within ∼13%) of the mechanical responses of implants to physical activities. Finally, in vitro biocompatibility studies were conducted for two-dimensional (2D) and three-dimensional (3D) cases. The results of the 2D in conjunction with surface roughness show favored physical cell attachment on the implant surface. Also, the results of the 3D biocompatibility study for the scaffolds incorporated with a cell-laden gelatin methacryloyl (GelMA) hydrogel show excellent viability. The design procedure proposed here provides new insights into the development of porous hip implants with simultaneous high mechanical and biological responses.

Utilization of GelMA with phosphate glass fibers for glial cell alignment

Glial cell alignment in tissue engineered constructs is essential for achieving functional outcomes in neural recovery. While gelatin methacrylate (GelMA) hydrogel offers superior biocompatibility along with permissive structure and tailorable mechanical properties, phosphate glass fibers (PGFs) can provide physical cues for directionality of neural growth. Aligned PGFs were fabricated by a melt quenching and fiber drawing method and utilized with synthesized GelMA hydrogel. The mechanical properties of GelMA and biocompatibility of the GelMA-PGFs composite were investigated in vitro using rat glial cells. GelMA with 86% methacrylation degree were photo-crosslinked using 0.1%wt photo-initiator (PI). Photocrosslinking under UV exposure for 60 s was used to produce hydrogels (GelMA-60). PGFs were introduced into the GelMA before crosslinking. Storage modulus and loss modulus of GelMA-60 was 24.73 ± 2.52 and 1.08 ± 0.23 kN/m² , respectively. Increased cell alignment was observed in GelMA-PGFs compared with GelMA hydrogel alone. These findings suggest GelMA-PGFs can provide glial cells with physical cues necessary to achieve cell alignment. This approach could further be used to achieve glial cell alignment in bioengineered constructs designed to bridge damaged nerve tissue.

A network analysis of angiogenesis/osteogenesis-related growth factors in bone tissue engineering based on in-vitro and in-vivo data: A systems biology approach

The principal purpose of tissue engineering is to stimulate the injured or unhealthy tissues to revive their primary function through the simultaneous use of chemical agents, cells, and biocompatible materials. Still, choosing the appropriate protein as a growth factor (GF) for tissue engineering is vital to fabricate artificial tissues and accelerate the regeneration procedure. In this study, the angiogenesis and osteogenesis-related proteins' interactions are studied using their related network. Three major biological processes, including osteogenesis, angiogenesis, and angiogenesis regulation, were investigated by creating a protein-protein interaction (PPI) network (45 nodes and 237 edges) of bone regeneration efficient proteins. Furthermore, a gene ontology and a centrality analysis were performed to identify essential proteins within a network. The higher degree in this network leads to higher interactions between proteins and causes a considerable effect. The most highly connected proteins in the PPI network are the most remarkable for their employment. The results of this study showed that three significant proteins including prostaglandin endoperoxide synthase 2 (PTGS2), TEK receptor tyrosine kinase (TEK), and fibroblast growth factor 18 (FGF18) were involved simultaneously in osteogenesis, angiogenesis, and their positive regulatory. Regarding the available literature, the results of this study confirmed that PTGS2 and FGF18 could be used as a GF in bone tissue engineering (BTE) applications to promote angiogenesis and osteogenesis. Nevertheless, TEK was not used in BTE applications until now and should be considered in future works to be examined in-vitro and in-vivo.

Engineered cell-laden alginate microparticles for 3D culture

Advanced microfabrication technologies and biocompatible hydrogel materials facilitate the modeling of 3D tissue microenvironment. Encapsulation of cells in hydrogel microparticles offers an excellent high-throughput platform for investigating multicellular interaction with their surrounding microenvironment. Compartmentalized microparticles support formation of various unique cellular structures. Alginate has emerged as one of the most dominant hydrogel materials for cell encapsulation owing to its cytocompatibility, ease of gelation, and biocompatibility. Alginate hydrogel provides a permeable physical boundary to the encapsulated cells and develops an easily manageable 3D cellular structure. The interior structure of alginate hydrogel can further regulate the spatiotemporal distribution of the embedded cells. This review provides a specific overview of the representative engineering approaches to generate various structures of cell-laden alginate microparticles in a uniform and reproducible manner. Capillary nozzle systems, microfluidic droplet systems, and non-chip based high-throughput microfluidic systems are highlighted for developing well-regulated cellular structure in alginate microparticles to realize potential drug screening platform and cell-based therapy. We conclude with the discussion of current limitations and future directions for realizing the translation of this technology to the clinic.

Hybrid Gelatin Hydrogels in Nanomedicine Applications

Gelatin based hydrogels are often incorporated with supporting materials such as chitosan, poly(vinyl alcohol), alginate, carbon nanotubes, and hyaluronic acid. These hybrid materials are specifically of interest in diversified nanomedicine fields as they exhibit unique physicochemical properties, antimicrobial activity, biodegradability, and biocompatibility. The applications include drug delivery, wound healing, cell culture, and tissue engineering. This paper reviews the various up-to-date methods to fabricate gelatin-based hydrogels, including UV photo-cross-linking, electrospinning, and 3D bioprinting. This paper also includes physical, chemical, mechanical, and biocompatibility characterization studies of several hybrid gelatin hydrogels and discusses their relevance in nanomedicine based applications. Challenges associated with the fabrication of hybrid materials for nanotechnology implementation, specifically in nanomedicine development, are critically discussed, and some future recommendations are provided.

Review of Design Considerations for Brain-on-a-Chip Models

In recent years, the need for sophisticated human in vitro models for integrative biology has motivated the development of organ-on-a-chip platforms. Organ-on-a-chip devices are engineered to mimic the mechanical, biochemical and physiological properties of human organs; however, there are many important considerations when selecting or designing an appropriate device for investigating a specific scientific question. Building microfluidic Brain-on-a-Chip (BoC) models from the ground-up will allow for research questions to be answered more thoroughly in the brain research field, but the design of these devices requires several choices to be made throughout the design development phase. These considerations include the cell types, extracellular matrix (ECM) material(s), and perfusion/flow considerations. Choices made early in the design cycle will dictate the limitations of the device and influence the end-point results such as the permeability of the endothelial cell monolayer, and the expression of cell type-specific markers. To better understand why the engineering aspects of a microfluidic BoC need to be influenced by the desired biological environment, recent progress in microfluidic BoC technology is compared. This review focuses on perfusable blood-brain barrier (BBB) and neurovascular unit (NVU) models with discussions about the chip architecture, the ECM used, and how they relate to the in vivo human brain. With increased knowledge on how to make informed choices when selecting or designing BoC models, the scientific community will benefit from shorter development phases and platforms curated for their application.

Toward Designing of Anti-infective Hydrogels for Orthopedic Implants: From Lab to Clinic

An alarming increase in implant failure incidence due to microbial colonization on the administered orthopedic implants has become a horrifying threat to replacement surgeries and related health concerns. In essence, microbial adhesion and its subsequent biofilm formation, antibiotic resistance, and the host immune system's deficiency are the main culprits. An advanced class of biomaterials termed anti-infective hydrogel implant coatings are evolving to subdue these complications. On this account, this review provides an insight into the significance of anti-infective hydrogels for preventing orthopedic implant associated infections to improve the bone healing process. We briefly discuss the clinical course of implant failure, with a prime focus on orthopedic implants. We identify the different anti-infective coating strategies and hence several anti-infective agents which could be incorporated in the hydrogel matrix. The fundamental design criteria to be considered while fabricating anti-infective hydrogels for orthopedic implants will be discussed. We highlight the different hydrogel coatings based on the origin of the polymers involved in light of their antimicrobial efficacy. We summarize the relevant patents reported in the prevention of implant infections, including orthopedics. Finally, the challenges concerning the clinical translation of the aforesaid hydrogels are described, and considerable solutions for improved clinical practice and better future prospects are proposed.

Effect of Different Additives on the Mechanical Properties of Gelatin Methacryloyl Hydrogel: A Meta-analysis

Gelatin methacryloyl (GelMA) hydrogel has adjustable physicochemical properties and a three-dimensional network structure for cell growth and hence a hot issue in the field of tissue engineering. However, its poor mechanical properties limit the application in the scaffold, especially as a bone scaffold. To date, many research studies have been carried out by adding some additives into GelMA to construct GelMA-based composites to improve the mechanical properties. However, there is a controversy as to whether the additives can improve the mechanical properties of GelMA. Herein, meta-analysis was used to evaluate the influence of the additives on the mechanical properties of GelMA-based composites, which can provide reference for the further enhancement of mechanical properties of GelMA. In this study, meta-analysis was adopted to investigate the influence of additives on the mechanical properties of GelMA composites; composites with different concentrations of GelMA, that is, ≥10% (w/v), 5-10% (w/v), and ≤5% (w/v) were found in 23 literatures and heterogeneity could be found among these references. Accordingly, it is found that additives can improve the mechanical properties in each concentration.

Recent Advances of Taxol-Loaded Biocompatible Nanocarriers Embedded in Natural Polymer-Based Hydrogels

The discovery of paclitaxel (PTX) has been a milestone in anti-cancer therapy and has promoted the development and marketing of various formulations that have revolutionized the therapeutic approach towards several malignancies. Despite its peculiar anti-cancer activity, the physico-chemical properties of PTX compromise the administration of the compound in polar media. Because of this, since the development of the first Food and Drug Administration (FDA)-approved formulation (Taxol®), consistent efforts have been made to obtain suitable delivery systems able to preserve/increase PTX efficacy and to overcome the side effects correlated to the presence of some excipients. The exploitation of natural polymers as potential materials for drug delivery purposes has favored the modulation of the bioavailability and the pharmacokinetic profiles of the drug, and in this regard, several formulations have been developed that allow the controlled release of the active compound. In this mini-review, the recent advances concerning the design and applications of natural polymer-based hydrogels containing PTX-loaded biocompatible nanocarriers are discussed. The technological features of these formulations as well as the therapeutic outcome achieved following their administration will be described, demonstrating their potential role as innovative systems to be used in anti-tumor therapy.

Recent Advances on Bioprinted Gelatin Methacrylate-Based Hydrogels for Tissue Repair

Bioprinting of body tissues has gained great attention in recent years due to its unique advantages, including the creation of complex geometries and printing the patient-specific tissues with various drug and cell types. The most momentous part of the bioprinting process is bioink, defined as a mixture of living cells and biomaterials (especially hydrogels). Among different biomaterials, natural polymers are the best choices for hydrogel-based bioinks due to their intrinsic biocompatibility and minimal inflammatory response in body condition. Gelatin methacryloyl (GelMA) hydrogel is one of the high-potential hydrogel-based bioinks due to its easy synthesis with low cost, great biocompatibility, transparent structure that is useful for cell monitoring, photocrosslinkability, and cell viability. Furthermore, the potential of adjusting properties of GelMA due to the synthesis protocol makes it a suitable choice for soft or hard tissues. In this review, different methods for the bioprinting of GelMA-based bioinks, as well as various effective process parameters, are reviewed. Also, several solutions for challenges in the printing of GelMA-based bioinks are discussed, and applications of GelMA-based bioprinted tissues argued as well. Impact statement Bioprinting has been demonstrated as a promising and alternative approach for organ transplantation to develop various types of living tissue. Bioinks, with great biological characteristics similar to the host tissues and rheological/flow features, are the first requirements for the successful bioprinting approach. Gelatin methacryloyl (GelMA) hydrogel is one of the high-potential hydrogel-based bioinks. This review provides a comprehensive look at different methods for the bioprinting of GelMA-based bioinks and applications of GelMA-based bioprinted tissues for tissue repair.

Graphene-based hybrid materials as promising scaffolds for peripheral nerve regeneration

Peripheral nerve injury (PNI) is a serious clinical health problem caused by the damage of peripheral nerves which results in neurological deficits and permanent disability. There are several factors that may cause PNI such as localized damage (car accident, trauma, electrical injury) and outbreak of the systemic diseases (autoimmune or diabetes). While various diagnostic procedures including X-ray, magnetic resonance imaging (MRI), as well as other type of examinations such as electromyography or nerve conduction studies have been efficiently developed, a full recovery in patients with PNI is in many cases deficient or incomplete. This is the reason why additional therapeutic strategies should be explored to favor a complete rehabilitation in order to get appropriate nerve injury regeneration. The use of biomaterials acting as scaffolds opens an interesting approach in regenerative medicine and tissue engineering applications due to their ability to guide the growth of new tissues, adhesion and proliferation of cells including the expression of bioactive signals. This review discusses the preparation and therapeutic strategies describing in vitro and in vivo experiments using graphene-based materials in the context of PNI and their ability to promote nerve tissue regeneration.

3D-Printed Gelatin Methacryloyl-Based Scaffolds with Potential Application in Tissue Engineering

The development of materials for 3D printing adapted for tissue engineering represents one of the main concerns nowadays. Our aim was to obtain suitable 3D-printed scaffolds based on methacrylated gelatin (GelMA). In this respect, three degrees of GelMA methacrylation, three different concentrations of GelMA (10%, 20%, and 30%), and also two concentrations of photoinitiator (I-2959) (0.5% and 1%) were explored to develop proper GelMA hydrogel ink formulations to be used in the 3D printing process. Afterward, all these GelMA hydrogel-based inks/3D-printed scaffolds were characterized structurally, mechanically, and morphologically. The presence of methacryloyl groups bounded to the surface of GelMA was confirmed by FTIR and ¹H-NMR analyses. The methacrylation degree influenced the value of the isoelectric point that decreased with the GelMA methacrylation degree. A greater concentration of photoinitiator influenced the hydrophilicity of the polymer as proved using contact angle and swelling studies because of the new bonds resulting after the photocrosslinking stage. According to the mechanical tests, better mechanical properties were obtained in the presence of the 1% initiator. Circular dichroism analyses demonstrated that the secondary structure of gelatin remained unaffected during the methacrylation process, thus being suitable for biological applications.

Natural Biomaterials and Their Use as Bioinks for Printing Tissues

The most prevalent form of bioprinting-extrusion bioprinting-can generate structures from a diverse range of materials and viscosities. It can create personalized tissues that aid in drug testing and cancer research when used in combination with natural bioinks. This paper reviews natural bioinks and their properties and functions in hard and soft tissue engineering applications. It discusses agarose, alginate, cellulose, chitosan, collagen, decellularized extracellular matrix, dextran, fibrin, gelatin, gellan gum, hyaluronic acid, Matrigel, and silk. Multi-component bioinks are considered as a way to address the shortfalls of individual biomaterials. The mechanical, rheological, and cross-linking properties along with the cytocompatibility, cell viability, and printability of the bioinks are detailed as well. Future avenues for research into natural bioinks are then presented.

Modeling the Mechanobiology of Cancer Cell Migration Using 3D Biomimetic Hydrogels

Understanding how cancer cells migrate, and how this migration is affected by the mechanical and chemical composition of the extracellular matrix (ECM) is critical to investigate and possibly interfere with the metastatic process, which is responsible for most cancer-related deaths. In this article we review the state of the art about the use of hydrogel-based three-dimensional (3D) scaffolds as artificial platforms to model the mechanobiology of cancer cell migration. We start by briefly reviewing the concept and composition of the extracellular matrix (ECM) and the materials commonly used to recreate the cancerous ECM. Then we summarize the most relevant knowledge about the mechanobiology of cancer cell migration that has been obtained using 3D hydrogel scaffolds, and relate those discoveries to what has been observed in the clinical management of solid tumors. Finally, we review some recent methodological developments, specifically the use of novel bioprinting techniques and microfluidics to create realistic hydrogel-based models of the cancer ECM, and some of their applications in the context of the study of cancer cell migration.

Electrospun GelMA fibers and p(HEMA) matrix composite for corneal tissue engineering

The development of biocompatible and transparent three-dimensional materials is desirable for corneal tissue engineering. Inspired from the cornea structure, gelatin methacryloyl-poly(2-hydroxymethyl methacrylate) (GelMA-p(HEMA)) composite hydrogel was fabricated. GelMA fibers were produced via electrospinning and covered with a thin layer of p(HEMA) in the presence of N,N'-methylenebisacrylamide (MBA) as cross-linker by drop-casting. The structure of resulting GelMA-p(HEMA) composite was characterized by spectrophotometry, microscopy, and swelling studies. Biocompatibility and biological properties of the both p(HEMA) and GelMA-p(HEMA) composite have been investigated by 3D cell culture, red blood cell hemolysis, and protein adsorption studies (i.e., human serum albumin, human immunoglobulin and egg white lysozyme). The optical transmittance of the GelMA-p(HEMA) composite was found to be approximately 70% at 550 nm. The GelMA-p(HEMA) composite was biocompatible with tear fluid proteins and convenient for cell adhesion and growth. Thus, as prepared hydrogel composite may find extensive applications in future for the development of corneal tissue engineering as well as preparation of stroma of the corneal material.

Characterization of Gelatin Hydrogels Cross-Linked with Microbial Transglutaminase as Engineered Skeletal Muscle Substrates

Engineered in vitro models of skeletal muscle are essential for efficiently screening drug safety and efficacy. However, conventional culture substrates poorly replicate physical features of native muscle and do not support long-term culture, which limits tissue maturity. Micromolded gelatin hydrogels cross-linked with microbial transglutaminase (gelatin-MTG hydrogels) have previously been shown to induce C21C2 myotube alignment and improve culture longevity. However, several properties of gelatin-MTG hydrogels have not been systematically characterized, such as changes in elastic modulus during incubation in culture-like conditions and their ability to support sarcomere maturation. In this study, various gelatin-MTG hydrogels were fabricated and incubated in ambient or culture-like conditions. Elastic modulus, mass, and transmittance were measured over a one- or two-week period. Compared to hydrogels in phosphate buffered saline (PBS) or ambient air, hydrogels in Dulbecco’s Modified Eagle Medium (DMEM) and 5% CO₂ demonstrated the most stable elastic modulus. A subset of gelatin-MTG hydrogels was micromolded and seeded with C2C12 or primary chick myoblasts, which aligned and fused into multinucleated myotubes with relatively mature sarcomeres. These data are important for fabricating gelatin-MTG hydrogels with predictable and stable mechanical properties and highlight their advantages as culture substrates for engineering relatively mature and stable muscle tissues.

Hydrogels and Dentin-Pulp Complex Regeneration: From the Benchtop to Clinical Translation

Dentin-pulp complex is a term which refers to the dental pulp (DP) surrounded by dentin along its peripheries. Dentin and dental pulp are highly specialized tissues, which can be affected by various insults, primarily by dental caries. Regeneration of the dentin-pulp complex is of paramount importance to regain tooth vitality. The regenerative endodontic procedure (REP) is a relatively current approach, which aims to regenerate the dentin-pulp complex through stimulating the differentiation of resident or transplanted stem/progenitor cells. Hydrogel-based scaffolds are a unique category of three dimensional polymeric networks with high water content. They are hydrophilic, biocompatible, with tunable degradation patterns and mechanical properties, in addition to the ability to be loaded with various bioactive molecules. Furthermore, hydrogels have a considerable degree of flexibility and elasticity, mimicking the cell extracellular matrix (ECM), particularly that of the DP. The current review presents how for dentin-pulp complex regeneration, the application of injectable hydrogels combined with stem/progenitor cells could represent a promising approach. According to the source of the polymeric chain forming the hydrogel, they can be classified into natural, synthetic or hybrid hydrogels, combining natural and synthetic ones. Natural polymers are bioactive, highly biocompatible, and biodegradable by naturally occurring enzymes or via hydrolysis. On the other hand, synthetic polymers offer tunable mechanical properties, thermostability and durability as compared to natural hydrogels. Hybrid hydrogels combine the benefits of synthetic and natural polymers. Hydrogels can be biofunctionalized with cell-binding sequences as arginine-glycine-aspartic acid (RGD), can be used for local delivery of bioactive molecules and cellularized with stem cells for dentin-pulp regeneration. Formulating a hydrogel scaffold material fulfilling the required criteria in regenerative endodontics is still an area of active research, which shows promising potential for replacing conventional endodontic treatments in the near future.

Gelatin methacryloyl-based tactile sensors for medical wearables

Gelatin methacryloyl (GelMA) is a widely used hydrogel with skin-derived gelatin acting as the main constituent. However, GelMA has not been used in the development of wearable biosensors, which are emerging devices that enable personalized healthcare monitoring. This work highlights the potential of GelMA for wearable biosensing applications by demonstrating a fully solution-processable and transparent capacitive tactile sensor with microstructured GelMA as the core dielectric layer. A robust chemical bonding and a reliable encapsulation approach are introduced to overcome detachment and water-evaporation issues in hydrogel biosensors. The resultant GelMA tactile sensor shows a high-pressure sensitivity of 0.19 kPa-1 and one order of magnitude lower limit of detection (0.1 Pa) compared to previous hydrogel pressure sensors owing to its excellent mechanical and electrical properties (dielectric constant). Furthermore, it shows durability up to 3000 test cycles because of tough chemical bonding, and long-term stability of 3 days due to the inclusion of an encapsulation layer, which prevents water evaporation (80% water content). Successful monitoring of various human physiological and motion signals demonstrates the potential of these GelMA tactile sensors for wearable biosensing applications.

Applications of Gelatin Methacryloyl (GelMA) Hydrogels in Microfluidic Technique-Assisted Tissue Engineering

In recent years, the microfluidic technique has been widely used in the field of tissue engineering. Possessing the advantages of large-scale integration and flexible manipulation, microfluidic devices may serve as the production line of building blocks and the microenvironment simulator in tissue engineering. Additionally, in microfluidic technique-assisted tissue engineering, various biomaterials are desired to fabricate the tissue mimicking or repairing structures (i.e., particles, fibers, and scaffolds). Among the materials, gelatin methacrylate (GelMA)-based hydrogels have shown great potential due to their biocompatibility and mechanical tenability. In this work, applications of GelMA hydrogels in microfluidic technique-assisted tissue engineering are reviewed mainly from two viewpoints: Serving as raw materials for microfluidic fabrication of building blocks in tissue engineering and the simulation units in microfluidic chip-based microenvironment-mimicking devices. In addition, challenges and outlooks of the exploration of GelMA hydrogels in tissue engineering applications are proposed.

Fabrication of Tapered Fluidic Microchannels Conducive to Angiogenic Sprouting within Gelatin Methacryloyl Hydrogels

INTRODUCTION

The transplantation of stem cells/tissue constructs into root canal space is a promising strategy for regenerating lost pulp tissue. However, the root canal system, which is cone shaped with a taper from the larger coronal end to the smaller apical end, limits the vascular supply and, therefore, the regenerative capacity. The current study aimed to fabricate built-in microchannels with different tapers to explore various approaches to endothelialize these microchannels.

METHODS

The fluidic microchannels with varying tapers (parallel, 0.04, and 0.06) were fabricated within gelatin methacryloyl (GelMA) hydrogel (with or without stem cell from the apical papilla [SCAP] encapsulation) of different concentrations (5%, 7.5%, and 10% [w/v]). Green fluorescent protein-expressing human umbilical vein endothelial cells (HUVECs-GFP) were seeded alone or with SCAPs in coculture into these microchannels. Angiogenic sprouting was assessed by fluorescence and a confocal microscope and ImageJ software (National Institutes of Health, Bethesda, MD). Immunostaining was conducted to illustrate monolayer formation. Data were statistically analyzed by 1-way/2-way analysis of variance.

RESULTS

HUVEC-only inoculation formed an endothelial monolayer inside the microchannel without angiogenic sprouting. HUVECs-GFP/SCAPs cocultured at a 1:1 ratio produced the longest sprouting compared with the other 3 ratios. The average length of the sprouting in the 0.04 taper microchannel was significantly longer compared with that in the parallel and 0.06 taper microchannels. Significant differences in HUVEC-GFP sprouting were observed in 5% GelMA hydrogel. Encapsulation of SCAPs within hydrogel further stimulated the sprouting of HUVECs.

CONCLUSIONS

The coculture of SCAPs and HUVECs-GFP at a ratio of 1:1 in 0.04 taper fluidic microchannels fabricated with 5% (w/v) GelMA hydrogel with SCAPs encapsulated was found to be the optimal condition to enhance angiogenesis inside tapered microchannels.

Application of bioactive hydrogels combined with dental pulp stem cells for the repair of large gap peripheral nerve injuries

Due to the limitations in autogenous nerve grafting or Schwann cell transplantation, large gap peripheral nerve injuries require a bridging strategy supported by nerve conduit. Cell based therapies provide a novel treatment for peripheral nerve injuries. In this study, we first experimented an optimal scaffold material synthesis protocol, from where we selected the 10% GFD formula (10% GelMA hydrogel, recombinant human basic fibroblast growth factor and dental pulp stem cells (DPSCs)) to fill a cellulose/soy protein isolate composite membrane (CSM) tube to construct a third generation of nerve regeneration conduit, CSM-GFD. Then this CSM-GFD conduit was applied to repair a 15-mm long defect of sciatic nerve in a rat model. After 12 week post implant surgery, at histologic level, we found CSM-GFD conduit could regenerate nerve tissue like neuron and Schwann like nerve cells and myelinated nerve fibers. At physical level, CSM-GFD achieved functional recovery assessed by a sciatic functional index study. In both levels, CSM-GFD performed like what gold standard, the nerve autograft, could do. Further, we unveiled that almost all newly formed nerve tissue at defect site was originated from the direct differentiation of exogeneous DPSCs in CSM-GFD. In conclusion, we claimed that this third-generation nerve regeneration conduit, CSM-GFD, could be a promising tissue engineering approach to replace the conventional nerve autograft to treat the large gap defect in peripheral nerve injuries.

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A novel 3^rd generation nerve conduit was successfully constructed and applied for repairing peripheral nerve injuries (PNI).

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Dental pulp stem cells (DPSCs) was optimized as an ideal seeding cells for nerve regeneration.

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A bioactive system combining GelMA with human bFGF and DPSCs could reconstruct the long gap PNI within 12 weeks in vivo.

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Our system could achieve the same outcome in nerve repair as that of autografting, a routine treatment for PNI.

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The proposed bioactive system may trigger an evolutional change into the current clinical practice in managing PNI.

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The majority of the regenerated nerve tissue was originated from the donor’s dental pulp stem cells.

Anisotropic Fiber-Reinforced Glycosaminoglycan Hydrogels for Heart Valve Tissue Engineering

This study investigates polymer fiber-reinforced protein-polysaccharide-based hydrogels for heart valve tissue engineering applications. Polycaprolactone and gelatin (3:1) blends were jet-spun to fabricate aligned fibers that possessed fiber diameters in the range found in the native heart valve. These fibers were embedded in methacrylated hydrogels made from gelatin, sodium hyaluronate, and chondroitin sulfate to create fiber-reinforced hydrogel composites (HCs). The fiber-reinforced gelatin glycosaminoglycan (GAG)-based HC possessed interconnected porous structures and porosity higher than fiber-only conditions. These fiber-reinforced HCs exhibited compressive modulus and biaxial mechanical behavior comparable to that of native porcine aortic valves. The fiber-reinforced HCs were able to swell higher and degraded less than the hydrogels. Elution studies revealed that less than 20% of incorporated gelatin methacrylate and GAGs were released over 2 weeks, with a steady-state release after the first day. When cultured with porcine valve interstitial cells (VICs), the fiber-reinforced composites were able to maintain higher cell viability compared with fiber-only samples. Quiescent VICs expressed alpha smooth muscle actin and calponin showing an activated phenotype, along with a few cells expressing the proliferation marker Ki67 and negative expression for RUNX2, an osteogenic marker. Our study demonstrated that compared with the hydrogels and fibers alone, combining both components can yield durable, reinforced composites that mimic heart valve mechanical behavior, while maintaining high cell viability and expressing positive activation as well as proliferation markers.

FLASH: Fluorescently LAbelled Sensitive Hydrogel to monitor bioscaffolds degradation during neocartilage generation

Regenerative therapies based on photocrosslinkable hydrogels and stem cells are of growing interest in the field of cartilage repair. Cell-mediated degradation is critical for the successful clinical translation of implanted hydrogels. However, characterising cell-mediated degradation, while simultaneously monitoring the deposition of a distinct new matrix, remains a major challenge. In this study we generated a Fluorescently LAbelled Sensitive Hydrogel (FLASH) to correlate the degradation of a hydrogel bioscaffold with neocartilage formation. Gelatine Methacryloyl (GelMA) was covalently bound to the FITC fluorophore to generate FLASH and bioscaffolds were produced by casting different concentrations of FLASH GelMA, with and without human adipose-derived stem cells (hADSCs) undergoing chondrogenesis. The loss of fluorescence from FLASH bioscaffolds was correlated with changes in mechanical properties, expression of chondrogenic markers and accumulation of a cartilaginous extracellular matrix. The ability of the system to be used as a sensor to monitor bioscaffold degradability during chondrogenesis was evaluated in vitro, in a human ex vivo model of cartilage repair and in a full chondral defect in vivo rabbit model. This study represents a step towards the generation of a high throughput monitoring system to evaluate de novo cartilage formation in tissue engineering therapies.

Advances and trends of hydrogel therapy platform in localized tumor treatment: A review

Due to limitations of treatment and the stubbornness of infiltrative tumor cells, the outcome of conventional antitumor treatment is often compromised by a variety of factors, including severe side effects, unexpected recurrence, and massive tissue loss during the treatment. Hydrogel-based therapy is becoming a promising option of cancer treatment, because of its controllability, biocompatibility, high drug loading, prolonged drug release, and specific stimuli-sensitivity. Hydrogel-based therapy has good malleability and can reach some areas that cannot be easily touched by surgeons. Furthermore, hydrogel can be used not only as a carrier for tumor treatment agents, but also as a scaffold for tissue repair. In this review, we presented the latest researches in hydrogel applications of localized tumor therapy and highlighted the recent progress of hydrogel-based therapy in preventing postoperative tumor recurrence and improving tissue repair, thus proposing a new trend of hydrogel-based technology in localized tumor therapy. And this review aims to provide a novel reference and inspire thoughts for a more accurate and individualized cancer treatment.

Rheological Properties of Coordinated Physical Gelation and Chemical Crosslinking in Gelatin Methacryloyl (GelMA) Hydrogels

Synthetically modified proteins, such as gelatin methacryloyl (GelMA), are growing in popularity for bioprinting and biofabrication. GelMA is a photocurable macromer that can rapidly form hydrogels, while also presenting bioactive peptide sequences for cellular adhesion and proliferation. The mechanical properties of GelMA are highly tunable by modifying the degree of substitution via synthesis conditions, though the effects of source material and thermal gelation have not been comprehensively characterized for lower concentration gels. Herein, the effects of animal source and processing sequence are investigated on scaffold mechanical properties. Hydrogels of 4-6 wt% are characterized. Depending on the temperature at crosslinking, the storage moduli for GelMA derived from pigs, cows, and cold-water fish range from 723 to 7340 Pa, 516 to 3484 Pa, and 294 to 464 Pa, respectively. The maximum storage moduli are achieved only by coordinated physical gelation and chemical crosslinking. In this method, the classic thermo-reversible gelation of gelatin occurs when GelMA is cooled below a thermal transition temperature, which is subsequently "locked in" by chemical crosslinking via photocuring. The effects of coordinated physical gelation and chemical crosslinking are demonstrated by precise photopatterning of cell-laden microstructures, inducing different cellular behavior depending on the selected mechanical properties of GelMA.

Production and Characterization of an Integrated Multi-Layer 3D Printed PLGA/GelMA Scaffold Aimed for Bile Duct Restoration and Detection

We successfully fabricated artificial bile duct via 3D printing technique which was composed of poly (lactic-co-glycolic acid) (PLGA) and gelatin methacrylate (GelMA). The PLGA-inner layer provided sufficient strength to support the bile duct contraction, the GelMA-outer layer possessed good biocompatibility to provide a good living environment for the cells. Moreover, IKVAV laminin peptide (Ile-Lys-Val-Ala-Val) and ultrasmall superparamagnetic iron oxide (USPIO) were used to regulate scaffold cell adhesion and magnetic resonance imaging (MRI) detection, respectively. After BMSCs co-culture with IKVAV at a certain concentration, the survival rate and adhesion of BMSCs was increased obviously. Meanwhile, the fabricated scaffold exhibited the tensile modulus in the range of 17.19 - 29.05 MPa and the compressive modulus in the range of 0.042 - 0.066 MPa, which could meet the needs of human implantation. In an animal experiment in vivo pig bile duct regeneration, PLGA/GelMA/IKVAV/USPIO duct conduits could promote bile duct regeneration and enhance cytokeratin 19 (CK19) expression. In summary, the composite bile duct scaffold with excellent MRI imaging function and biocompatibility could be used to develop bioactive artificial bile ducts.

Injectable hydrogels for tendon and ligament tissue engineering

The problem of tendon and ligament (T/L) regeneration in musculoskeletal diseases has long constituted a major challenge. In situ injection of formable biodegradable hydrogels, however, has been demonstrated to treat T/L injury and reduce patient suffering in a minimally invasive manner. An injectable hydrogel is more suitable than other biological materials due to the special physiological structure of T/L. Most other materials utilized to repair T/L are cell-based, growth factor-based materials, with few material properties. In addition, the mechanical property of the gel cannot reach the normal T/L level. This review summarizes advances in natural and synthetic polymeric injectable hydrogels for tissue engineering in T/L and presents prospects for injectable and biodegradable hydrogels for its treatment. In future T/L applications, it is necessary develop an injectable hydrogel with mechanics, tissue damage-specific binding, and disease response. Simultaneously, the advantages of various biological materials must be combined in order to achieve personalized precision therapy.

Development of a Dual Hydrogel Model System for Vascularization

Numerous hydrogel-based culture systems are used to create in vitro model for prevascularization. Hydrogels used to induce a microenvironment conducive to microvessel formation are typically soft and fast degradable, but often suffer from maintaining a lasting perfusable channel in vitro. Here, a dual hydrogel system that consists of photo-crosslinkable gelatin methacrylate (GelMA) and polyethylene glycol dimethacrylate (PEGDMA) is reported. GelMA hydrogels present soft and rapidly degradable properties and show microporous structures while PEGDMA is relatively stiff, almost nondegradable in vitro, and less porous. The dual hydrogel system is sequentially photo-crosslinked to construct an endothelial cell (EC)-lined perfusable PEGDMA channel and surrounding GelMA for endothelial vascular networks. Such dual hydrogel system exhibits seamless integration of the stiff PEGDMA channel and the surrounding soft GelMA, and facilitates rapid EC sprouting and extensive microvessel formation from a stable endothelium on the PEGDMA channel into the GelMA. Furthermore, diffusivity of biomolecules in the perfusable dual hydrogel system is affected by both the structural and physicochemical properties of the hydrogel system and the microvascular networks formed in the system. The establishment of the dual hydrogel system for vascularization holds great promise as an in vitro angiogenesis model and prevascularization strategy of large tissue constructs.

Advanced Hydrogels as Wound Dressings

Skin is the largest organ of the human body, protecting it against the external environment. Despite high self-regeneration potential, severe skin defects will not heal spontaneously and need to be covered by skin substitutes. Tremendous progress has been made in the field of skin tissue engineering, in recent years, to develop new skin substitutes. Among them, hydrogels are one of the candidates with most potential to mimic the native skin microenvironment, due to their porous and hydrated molecular structure. They can be applied as a permanent or temporary dressing for different wounds to support the regeneration and healing of the injured epidermis, dermis, or both. Based on the material used for their fabrication, hydrogels can be subdivided into two main groups—natural and synthetic. Moreover, hydrogels can be reinforced by incorporating nanoparticles to obtain “in situ” hybrid hydrogels, showing superior properties and tailored functionality. In addition, different sensors can be embedded in hydrogel wound dressings to provide real-time information about the wound environment. This review focuses on the most recent developments in the field of hydrogel-based skin substitutes for skin replacement. In particular, we discuss the synthesis, fabrication, and biomedical application of novel “smart” hydrogels.

Nanoparticle-hydrogel superstructures for biomedical applications

The incorporation of nanoparticles into hydrogels yields novel superstructures that have become increasingly popular in biomedical research. Each component of these nanoparticle-hydrogel superstructures can be easily modified, resulting in platforms that are highly tunable and inherently multifunctional. The advantages of the nanoparticle and hydrogel constituents can be synergistically combined, enabling these superstructures to excel in scenarios where employing each component separately may have suboptimal outcomes. In this review, the synthesis and fabrication of different nanoparticle-hydrogel superstructures are discussed, followed by an overview of their use in a range of applications, including drug delivery, detoxification, immune modulation, and tissue engineering. Overall, these platforms hold significant clinical potential, and it is envisioned that future development along these lines will lead to unique solutions for addressing areas of pressing medical need.

Neuromuscular disease modeling on a chip

Organs-on-chips are broadly defined as microfabricated surfaces or devices designed to engineer cells into microscale tissues with native-like features and then extract physiologically relevant readouts at scale. Because they are generally compatible with patient-derived cells, these technologies can address many of the human relevance limitations of animal models. As a result, organs-on-chips have emerged as a promising new paradigm for patient-specific disease modeling and drug development. Because neuromuscular diseases span a broad range of rare conditions with diverse etiology and complex pathophysiology, they have been especially challenging to model in animals and thus are well suited for organ-on-chip approaches. In this Review, we first briefly summarize the challenges in neuromuscular disease modeling with animal models. Next, we describe a variety of existing organ-on-chip approaches for neuromuscular tissues, including a survey of cell sources for both muscle and nerve, and two- and three-dimensional neuromuscular tissue-engineering techniques. Although researchers have made tremendous advances in modeling neuromuscular diseases on a chip, the remaining challenges in cell sourcing, cell maturity, tissue assembly and readout capabilities limit their integration into the drug development pipeline today. However, as the field advances, models of healthy and diseased neuromuscular tissues on a chip, coupled with animal models, have vast potential as complementary tools for modeling multiple aspects of neuromuscular diseases and identifying new therapeutic strategies.

Summary: Modeling neuromuscular diseases is challenging due to their complex etiology and pathophysiology. Here, we review the cell sources and tissue-engineering procedures that are being integrated as emerging neuromuscular disease models.

Oxygen-generating smart hydrogels supporting chondrocytes survival in oxygen-free environments

Cartilage is one of our body's tissues which are not repaired automatically by itself. Problems associated with cartilage are very common worldwide and are considered the leading cause of pain and disability. Smart biomaterial or "Four dimensional" (4D) biomaterials has started emerging as a suitable candidate, which are principally three dimensional (3D) materials that change their morphology or generate a response measured at space and time to physiologic stimuli. In this context, the release of oxygen through hydrogels in contact with water is considered as 4D biomaterials. The objective of this study is to develop strategies to release oxygen in a sustainable and prolonged manner through hydrogels systems to promote chondrocytes survival in oxygen-free environment. The 4D biomaterials are engineered from gelatin methacryloyl (GelMA) loaded with calcium peroxide (CPO), which have the ability to generate oxygen in a controlled and sustained manner for up to 6 days. The incorporation of CPO into the hydrogel system provided materials with enhanced mechanical and porosity properties. Furthermore, the hydrogels promoted chondrocyte survival and reduced cell death under oxygen-free conditions.