Macroporous methacrylated hyaluronic acid hydrogel with different pore sizes for in vitro and in vivo evaluation of vascularization

"Biological evaluation of 3D-Printed chitosan-based scaffolds for tissue engineering"

Chitosan is a natural polysaccharide with excellent biocompatibility, biodegradability, and antibacterial capabilities, making it a good candidate for tissue engineering. 3D printing has revolutionized biomaterial fabrication owing to its precision, customization, and ability to create complex structures. This article aims to provide an overview of the current advances in 3D printing techniques for fabricating scaffolds based on chitosan and its derivatives. It also describes various printing methods, including extrusion bioprinting technique, inkjet bioprinting, stereolithography, digital light processing, and indirect 3D printing for controlling porosity, mechanical strength, and biological characteristics of chitosan scaffolds for a variety of tissues, like bone, vascular, cardiac, cartilage, and skin. This review also examines the biological properties of 3D-printed chitosan scaffolds. The advancements in biological performance and 3D printing technology indicate a promising future for developing flexible, customized scaffolds consisting of chitosan.

In Situ-Formed Tissue-Adhesive Macroporous Scaffolds Enhance Cell Infiltration and Tissue Regeneration

Macroporous hydrogels have shown significant promise in tissue engineering and regenerative medicine. However, conventional macroporous scaffold fabrications are complex and incompatible with in situ customization and fabrication. Here, we propose a highly translational approach for the in situ formation of adhesive macroporous scaffolds through microfluidic homogenization of gas into a self-crosslinkable gelatin transglutaminase (TG) mixture using a double syringe system. Using this strategy, the tissue defect can be evaluated, and the precursor, with the desired composition and volume, foamed and administered in situ. The TG-induced crosslinking stabilizes the pores, leading to strong tissue adhesion and accurate defect geometry approximation. We demonstrate precise control over the porosity, by changing the foaming parameters, and crosslinking kinetics, by adjusting the concentration of gelatin and TG. The resulting foam scaffolds offer controlled pore distribution, flexibility, tissue adhesion, stability, sustained protein release profile, and cell permissibility, with a faster biodegradation profile compared to bulk hydrogel compartments. Consequently, enhanced cell infiltration and reduced fibrous capsule formation are observed upon subcutaneous injection of foams compared to bulk hydrogels. Finally, the scaffolds demonstrate significant improvements in the rate and quality of the healing compared to the bulk hydrogels for the treatment of full-thickness cutaneous wounds in mice. STATEMENT OF SIGNIFICANCE: A highly translational method is presented for the in situ formation of tissue-adhesive macroporous scaffolds through microfluidic homogenization of gas into a self-crosslinkable hydrogel precursor using a double syringe system. This approach allows precise control over porosity and pore size, facilitating cell infiltration, tissue integration, and improved wound healing compared to bulk hydrogels, highlighting their potential in regenerative medicine.

Angiogenesis: Biological Mechanisms and In Vitro Models

The translation of biomedical devices and drug research is an expensive and long process with a low probability of receiving FDA approval. Developing physiologically relevant in vitro models with human cells offers a solution to not only improving the odds of FDA approval but also to expand our ability to study complex in vivo systems in a simpler fashion. Animal models remain the standard for pre-clinical testing; however, the data from animal models is an unreliable extrapolation when anticipating a human response in clinical trials, thus contributing to the low rates of translation. In this review, we focus on in vitro vascular or angiogenic models because of the incremental role that the vascular system plays in the translation of biomedical research. The first section of this review discusses the most common angiogenic cytokines that are used in vitro to initiate angiogenesis, followed by angiogenic inhibitors where both initiators and inhibitors work to maintain vascular homeostasis. Next, we evaluate previously published in vitro models, where we evaluate capturing the physical environment for biomimetic in vitro modeling. These topics provide a foundation of parameters that must be considered to improve and achieve vascular biomimicry. Finally, we summarize these topics to suggest a path forward with the goal of engineering human in vitro models that emulate the in vivo environment and provide a platform for biomedical device and drug screening that produces data to support clinical translation.

Enhanced Ear Cartilage Regeneration with Dual-Network LT-GelMA/F127DA Hydrogel Featuring Nanomicelle Integration

Tissue-engineered cartilage, supported by advancements in photo-cross-linkable hydrogels, offers a promising solution for the repair and regeneration of damaged cartilage in anatomically complex and mechanically demanding sites. Low-temperature soluble GelMA (LT-GelMA) remains in a liquid state at room temperature, allowing for easier handling; however, it has limitations in mechanical strength and structural stability. To address these limitations, we developed a novel dual-network hydrogel combining LT-GelMA with Pluronic F127-diacrylate (F127DA). The resulting hydrogel uniquely integrates the low-temperature solubility of LT-GelMA with the enhanced mechanical strength provided by photo-cross-linkable F127DA nanomicelles. Additionally, the hydrogel exhibits controlled swelling and biodegradation rates. In vitro studies revealed a significant increase in chondrocyte viability by day 7 in formulations with higher F127DA concentrations. In vivo, the hydrogel demonstrated superior neo-cartilage formation in a subcutaneous nude mouse model, as indicated by increased deposition of cartilage-specific extracellular matrix components at 4 and 8 weeks. In summary, we developed a hydrogel with fluidity at room temperature and enhanced mechanical performance. These results indicate that the LT-GelMA/F127DA hydrogel effectively addresses the current gaps in cartilage tissue engineering. The hydrogel's superior performance, especially in promoting cartilage regeneration, positions it as a promising alternative for reconstructive surgery, representing a significant improvement over existing cartilage repair strategies.

Effects of protein conformational transition accompanied with crosslinking density cues in silk fibroin hydrogels on the proliferation and chondrogenesis of encapsulated stem cells

Silk fibroin (SF) hydrogels possess excellent biocompatibility and biomimetic properties of the extracellular matrix. Among them, the mild chemical crosslinked SF hydrogels show great application potential in the fields of 3D cell culture and tissue repairing and thus have attracted widespread attention. However, the mobility of hydrophobic chain segments of SF molecules in these chemical crosslinked hydrogels can easily cause the molecules to undergo a self-assembly process from random coil to β-sheet conformation due to its lower energy state, thus inducing an inevitable conformational transition process. This process further leads to dynamic changes of important material features, such as the hydrogel pore size and mechanical properties, which can probably bring some non-negligible and unknown impacts on cell behaviors and their biomedical applications. In this study, a typical mild crosslinking system composed of horseradish peroxidase and hydrogen peroxide was chosen to prepare SF hydrogels. A feasible protein conformational transition rate controlling strategy based on hydrogel crosslinking density regulation was also proposed. Our results demonstrate that the lower the hydrogel crosslinking density, the faster the conformational transition rate. Subsequently, SF hydrogels with different conformational transition rates were successfully constructed to investigate the impact of the protein conformational transition rate accompanied with initial crosslinking density on the proliferation and chondrogenic differentiation of encapsulated stem cells. Results comprehensively illustrated that the conformational transition process could effectively regulate cell behavior. The hydrogel with an appropriate conformational transition rate obviously promoted the proliferation and chondrogenesis of encapsulated stem cells, while too fast or too slow transition processes slowed down these cell activities. These findings are hopefully to provide valuable guidance for the development and efficient usage of SF hydrogels in the fields of 3D cell culture and tissue engineering.

Enhanced osteogenic differentiation in hyaluronic acid methacrylate (HAMA) matrix: a comparative study of hPDC and hBMSC spheroids for bone tissue engineering

Bone tissue engineering (BTE) seeks to overcome the limitations of traditional bone repair methods, such as autografts and allografts, which are often limited by availability, donor-site morbidity, immune rejection, and infection risks. Recent advancements have highlighted the potential of spheroids or microtissues as building blocks for BTE. This study aimed to investigate the osteogenic differentiation of spheroids formed from human periosteum-derived cells (hPDCs) and bone marrow-derived mesenchymal stromal cells (hBMSCs) in a hyaluronic acid methacrylate (HAMA) matrix, using encapsulation and extrusion bioprinting methods. Results showed significant morphological changes, high viability, and osteogenic differentiation of spheroids from hPDCs or hBMSCs in three-dimensional HAMA environments. Notably, hPDC spheroids demonstrated higher mineralization capabilities and superior hydrogel colonization than hBMSC spheroids. These findings reveal the potential of HAMA bioink containing hPDC spheroids to produce mineralized bone grafts using a bioprinting approach.

VEGF loading heparinized hyaluronic acid macroporous hydrogels for enhanced 3D endothelial cell migration and vascularization

The formation of robust vascular systems within voluminous scaffolds remains a formidable barrier in the realm of tissue engineering. There is a growing interest in the integration of biomaterial scaffolds with multiple physical and chemical stimuli to augment the process of vascularization. This study aims to investigate the combined impact of macroporous structures and vascular endothelial growth factor (VEGF) on cell migration and vascularization. Heparinized hyaluronic acid (HepHA) macroporous hydrogels with differing pore sizes, composed by methacrylated hyaluronic acid (HAMA) and methacrylated heparin (HepMA), were fabricated by a gelatin microspheres (GMS) template leaching method. After characterization of their physical properties, VEGF was immobilized on the HepHA hydrogels. The in vitro release study indicated that the HepHA hydrogels can provide sustained release of VEGF. Subsequently, cells migration of human umbilical vein endothelial (HUVECs) assessment indicated that HUVECs cultured on VEGF-loaded HepHA hydrogels with larger pores (VEGF@HepHA250) migrated the furthest. Finally, the hydrogels were implanted and evaluated using a dorsal subcutaneous model. The histological analyses conducted in vivo were consistent with the in vitro results, VEGF@HepHA250 hydrogels exhibited the most pronounced vascularization four weeks post-implantation, indicating that hydrogels with expanded pores and an enriched VEGF promoted angiogenesis within the hydrogels. This study sheds light on the synergistic effects of VEGF release on 3D cell migration and vascularization within hydrogels of differing pore sizes, thus providing novel insights into the strategic design and fabrication of tissue-engineered scaffolds that are amenable to vascularization.

Progress in Biomaterials-Enhanced Vascularization by Modulating Physical Properties

Sufficient vascular system and adequate blood perfusion is crucial for ensuring nutrient and oxygen supply within biomaterials. Actively exploring the optimal physical properties of biomaterials in various application scenarios has provided clues for enhancing vascularization within materials, leading to improved outcomes in tissue engineering and clinical translation. Here we focus on reviewing the physical properties of biomaterials, including pore structure, surface topography, and stiffness, and their effects on promoting vascularization. This angiogenic capability has the potential to provide better standardized research models and personalized treatment strategies for bone regeneration, wound healing, islet transplantation and cardiac repair.

Hyaluronic Acid Role in Biomaterials Prevascularization

Tissue vascularization is a major bottleneck in tissue engineering. In this review, the state of the art on the intricate role of hyaluronic acid (HA) in angiogenesis is explored. HA plays a twofold role in angiogenesis. First, when released as a free polymer in the extracellular matrix (ECM), HA acts as a signaling molecule triggering multiple cascades that foster smooth muscle cell differentiation, migration, and proliferation thereby contributing to vessel wall thickening. Simultaneously, HA bound to the plasma membrane in the pericellular space functions as a polymer block, participating in vessel formation. Starting with the HA origins in native vascular tissues, the approaches aimed at achieving vascularization in vivo are reviewed. The significance of HA molecular weight (MW) in angiogenesis and the challenges associated with utilizing HA in vascular tissue engineering (VTE) are conscientiously addressed. The review finally focuses on a thorough examination and comparison of the diverse strategies adopted to harness the benefits of HA in the vascularization of bioengineered materials. By providing a nuanced perspective on the multifaceted role of HA in angiogenesis, this review contributes to the ongoing discourse in tissue engineering and advances the collective understanding of optimizing vascularization processes assisted by functional biomaterials.

The induction of bone formation by 3D-printed PLGA microsphere scaffolds in a calvarial orthotopic mouse model: a pilot study

Polymeric biodegradable microspheres are readily utilized to support targeted drug delivery for various diseases clinically. 3D printed tissue engineering scaffolds from polymer filaments with embedded microspheres or nanoparticles, as well as bulk microsphere scaffolds, have been investigated for regenerative medicine and tissue engineering. However, 3D printed scaffolds consisting only of a homogenous microsphere size with an optimized architecture that includes a unique micro- and macroporosity, have been challenging to produce and hence, have not been assessed in the literature yet. Utilizing our recently established 3D-MultiCompositional Microsphere-Adaptive Printing (3D-McMap) method, the present study evaluated the effectiveness of 3D-printed poly (lactic-co-glycolic acid) (PLGA) microsphere scaffolds, consisting of microsphere sizes 50, 100, or 200 μm, on the induction of bone formation when implanted in the calvarial murine regeneration model. Our results showed that PLGA microsphere scaffolds possess unique properties that support bone regeneration by supporting osteoconduction and stimulating, in our opinion, true spontaneous osteoinduction. The study demonstrated that PLGA microsphere-based scaffolds support bone growth in the absence of additional growth factors and promote osteogenesis primarily via their unique geometric configuration. The larger the microspheres were, the greater de novo bone formation was. This proves that bone tissue engineering scaffolds 3D printed from microspheres, enabled by the 3D-McMap method, are superior over bulk material printed scaffolds, as they possess the unique capability of spontaneous induction of new bone formation. With the addition of encapsulated modulatory bone-forming biomolecules they can substantially improve the spatiotemporal control of tissue morphogenesis, potentially leading to new innovative clinical tissue repair therapies that regenerate bone in large defects correctly and fully.

Small molecular weight alginate gel porogen for the 3D bioprinting of microvasculature

In order to recreate the complexity of human organs, the field of tissue engineering and regenerative medicine has been focusing on methods to build organs from the bottom up by assembling distinct small functional units consisting of a biomaterial and cells. This bottom-up engineering requires bioinks that can be assembled by 3D bioprinting and that permit fast vascularization of the construct to ensure survival of embedded cells. To this end, a small molecular weight alginate (SMWA) gel porogen is presented herein. Alginate is a biocompatible biomaterial, which can be easily converted into small porogen gels with the procedure reported in this article. The SMWA porogen is mixed with photo-crosslinkable hydrogels and leached from the hydrogel post-crosslinking to increase porosity and facilitate vascularization. As a proof of concept, this system is tested with the commonly used biomaterial Gelatin Methacryloyl (GelMA). The SMWA porogen-GelMA blend is proven to be bioprintable. Incubating the blend for 20 min in a low concentration phosphate buffered saline and sodium citrate solution significantly reduces the remaining porogen in the hydrogel . The intent to completely leach the porogen from the hydrogel was abandoned, as longer incubation times and higher concentrations of phosphate and citrate were detrimental to endothelial proliferation. Nonetheless, even with remnants of the porogen left in the hydrogel, the created porosity significantly improves viability, growth factor signaling, vasculogenesis, and angiogenesis in 3D bioprinted structures. This article concludes that the usage of the SMWA porogen can improve the assembly of microvasculature in 3D bioprinted structures. This technology can benefit the bottom-up assembly of large scaffolds with high cell density through 3D bioprinting by improving cell viability and allowing faster vascularization.

Quantitative atlas of collagen hydrogels reveals mesenchymal cancer cell traction adaptation to the matrix nanoarchitecture

Collagen-based hydrogels are commonly used in mechanobiology to mimic the extracellular matrix. A quantitative analysis of the influence of collagen concentration and properties on the structure and mechanics of the hydrogels is essential for tailored design adjustments for specific in vitro conditions. We combined focused ion beam scanning electron microscopy and rheology to provide a detailed quantitative atlas of the mechanical and nanoscale three-dimensional structural alterations that occur when manipulating different hydrogel's physicochemistry. Moreover, we study the effects of such alterations on the phenotype of breast cancer cells and their mechanical interactions with the extracellular matrix. Regardless of the microenvironment's pore size, porosity or mechanical properties, cancer cells are able to reach a stable mesenchymal-like morphology. Additionally, employing 3D traction force microscopy, a positive correlation between cellular tractions and ECM mechanics is observed up to a critical threshold, beyond which tractions plateau. This suggests that cancer cells in a stable mesenchymal state calibrate their mechanical interactions with the ECM to keep their migration and invasiveness capacities unaltered. STATEMENT OF SIGNIFICANCE: The paper presents a thorough study on the mechanical microenvironment in breast cancer cells during their interaction with collagen based hydrogels of different compositions. The hydrogels' microstructure were obtained using state-of-the-art 3D microscopy, namely focused ion beam-scanning electron microscope (FIB-SEM). FIB-SEM was originally applied in this work to reconstruct complex fibered collagen microstructures within the nanometer range, to obtain key microarchitectural parameters. The mechanical microenvironment of cells was recovered using Traction Force Microscopy (TFM). The obtained results suggest that cells calibrate tractions such that they depend on mechanical, microstructural and physicochemical characteristics of the hydrogels, hence revealing a steric hindrance. We hypothesize that cancer cells studied in this paper tune their mechanical state to keep their migration and invasiveness capacities unaltered.

Cofunctionalization of Macroporous Dextran Hydrogels with Adhesive Peptides and Growth Factors Enables Vascular Spheroid Sprouting

Ensuring good definition of scaffolds used for 3D cell culture is a prominent challenge that hampers the development of tissue engineering platforms. Since dextran repels cell adhesion, using dextran-based materials biofunctionalized through a bottom-up approach allows for precise control over material definition. Here, we report the design of dextran hydrogels displaying a fully interconnected macropore network for the culture of vascular spheroids in vitro. We biofunctionalized the hydrogels with the RGD peptide sequence to promote cell adhesion. We used an affinity peptide pair, the E/K coiled coil, to load the gels with epidermal growth factor (EGF) and vascular endothelial growth factor (VEGF). Dual functionalization with adhesive and proliferative cues allows vascular spheroids to colonize naturally cell-repellant dextran. In supplement-depleted medium, we report improved colonization of the macropores compared to that of unmodified dextran. Altogether, we propose a well-defined and highly versatile platform for tissue engineering and tissue vascularization applications.

Modulation of macrophage polarization by secondary cross-linked hyaluronan-dopamine hydrogels

The inflammatory response plays a critical role in standard tissue repair processes, wherein active modulation of macrophage polarization is necessary for wound healing. Dopamine, a mussel-inspired bioactive material, is widely involved in wound healing, neural/bone/myocardial regeneration, and more. Recent studies indicated that dopamine-modified biomaterials can potentially alter macrophages polarization towards a pro-healing phenotype, thereby enhancing tissue regeneration. Nevertheless the immunoregulatory activity of dopamine on macrophage polarization remains unclear. This study introduces a novel interpenetrating hydrogel to bridge this research gap. The hydrogel, combining varying concentrations of oxidized dopamine with hyaluronic acid hydrogel, allows precise regulation of mechanical properties, antioxidant bioactivity, and biocompatibility. Surprisingly, both in vivo and in vitro outcomes demonstrated that dopamine concentration modulates macrophage polarization, but not linearly. Lower concentration (2 mg/mL) potentially decrease inflammation and facilitate M2 type macrophage polarization. In contrast, higher concentration (10 mg/mL) exhibited a pro-inflammatory tendency in the late stages of implantation. RNA-seq analysis revealed that lower dopamine concentrations induced the M1/M2 transition of macrophages by modulating the NF-κB signaling pathway. Collectively, this research offers valuable insights into the immunoregulation effects of dopamine-integrated biomaterials in tissue repair and regeneration.

A novel injectable sericin hydrogel with strong fluorescence for tracing

Hydrogel systems with strong fluorescence, as convenient tracers or bio-probes, have attracted much attention in biomedical engineering. Currently, most hydrogels endowed fluorescent properties due to modifying additional fluorophores. However, these fluorophores owing to photobleaching and toxicity limit the practical applications of hydrogels. Herein, we prepared a novel self-luminescence hydrogel through double crosslinking glutaraldehyde and hydrogen peroxide/horseradish peroxidase (H2O2/HRP) with sericin protein. The double cross-linked sericin hydrogel exhibits strong green and red intrinsic fluorescence which can be excited over a wide range of wavelengths. Moreover, this hydrogel with strong intrinsic fluorescence could penetrate thick pigskin tissue, which has potential application in implantable bio-tracer areas. In addition to the above unique properties, this sericin hydrogel possesses two types of micropore structures with high porosity, swelling properties, pH-responsive degradability, super elasticity, injectability, viscosity, and excellent biocompatibility. The investigation could significantly expand the scope of protein hydrogels in biomedical applications.

Nanotechnology translation in vascular diseases: From design to the bench

Atherosclerosis is a systemic pathophysiological condition contributing to the development of majority of polyvascular diseases. Nanomedicine is a novel and rapidly developing science. Due to their small size, nanoparticles are freely transported in vasculature, and have been widely employed as tools in analytical imaging techniques. Furthermore, the application of nanoparticles also allows target intervention, such as drug delivery and tissue engineering regenerative methods, in the management of major vascular diseases. Therefore, by summarizing the physical and chemical characteristics of common nanoparticles used in diagnosis and treatment of vascular diseases, we discuss the details of these applications from cellular, molecular, and in vivo perspectives in this review. Furthermore, we also summarize the status and challenges of the application of nanoparticles in clinical translation. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Cardiovascular Disease Implantable Materials and Surgical Technologies > Nanomaterials and Implants Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Hydrogel with ROS scavenging effect encapsulates BR@Zn-BTB nanoparticles for accelerating diabetic mice wound healing via multimodal therapy

The strategies for eliminating excess reactive oxygen species (ROS) or suppressing inflammatory responses on the wound bed have proven effective for diabetic wound healing. In this work, a zinc-based nanoscale metal-organic framework (NMOF) functions as a carrier to deliver natural product berberine (BR) to form BR@Zn-BTB nanoparticles, which was, in turn, further encapsulated by hydrogel with ROS scavenging ability to yield a composite system of BR@Zn-BTB/Gel (denoted as BZ-Gel). The results show that BZ-Gel exhibited the controlled release of Zn²⁺ and BR in simulated physiological media to efficiently eliminated ROS and inhibited inflammation and resulted in a promising antibacterial effect. In vivo experiments further proved that BZ-Gel significantly inhibited the inflammatory response and enhanced collagen deposition, as well as to re-epithelialize the skin wound to ultimately promote wound healing in diabetic mice. Our results indicate that the ROS-responsive hydrogel coupled with BR@Zn-BTB synergistically promotes diabetic wound healing.

Resorbable Biomaterials Used for 3D Scaffolds in Tissue Engineering: A Review

This article provides a thorough overview of the available resorbable biomaterials appropriate for producing replacements for damaged tissues. In addition, their various properties and application possibilities are discussed as well. Biomaterials are fundamental components in tissue engineering (TE) of scaffolds and play a critical role. They need to exhibit biocompatibility, bioactivity, biodegradability, and non-toxicity, to ensure their ability to function effectively with an appropriate host response. With ongoing research and advancements in biomaterials for medical implants, the objective of this review is to explore recently developed implantable scaffold materials for various tissues. The categorization of biomaterials in this paper includes fossil-based materials (e.g., PCL, PVA, PU, PEG, and PPF), natural or bio-based materials (e.g., HA, PLA, PHB, PHBV, chitosan, fibrin, collagen, starch, and hydrogels), and hybrid biomaterials (e.g., PCL/PLA, PCL/PEG, PLA/PEG, PLA/PHB PCL/collagen, PCL/chitosan, PCL/starch, and PLA/bioceramics). The application of these biomaterials in both hard and soft TE is considered, with a particular focus on their physicochemical, mechanical, and biological properties. Furthermore, the interactions between scaffolds and the host immune system in the context of scaffold-driven tissue regeneration are discussed. Additionally, the article briefly mentions the concept of in situ TE, which leverages the self-renewal capacities of affected tissues and highlights the crucial role played by biopolymer-based scaffolds in this strategy.

Functional hydrogels for the repair and regeneration of tissue defects

Tissue defects can be accompanied by functional impairments that affect the health and quality of life of patients. Hydrogels are three-dimensional (3D) hydrophilic polymer networks that can be used as bionic functional tissues to fill or repair damaged tissue as a promising therapeutic strategy in the field of tissue engineering and regenerative medicine. This paper summarises and discusses four outstanding advantages of hydrogels and their applications and advances in the repair and regeneration of tissue defects. First, hydrogels have physicochemical properties similar to the extracellular matrix of natural tissues, providing a good microenvironment for cell proliferation, migration and differentiation. Second, hydrogels have excellent shape adaptation and tissue adhesion properties, allowing them to be applied to a wide range of irregularly shaped tissue defects and to adhere well to the defect for sustained and efficient repair function. Third, the hydrogel is an intelligent delivery system capable of releasing therapeutic agents on demand. Hydrogels are capable of delivering therapeutic reagents and releasing therapeutic substances with temporal and spatial precision depending on the site and state of the defect. Fourth, hydrogels are self-healing and can maintain their integrity when damaged. We then describe the application and research progress of functional hydrogels in the repair and regeneration of defects in bone, cartilage, skin, muscle and nerve tissues. Finally, we discuss the challenges faced by hydrogels in the field of tissue regeneration and provide an outlook on their future trends.

Study of EGCG composite hydrogel for the treatment of radiation-induced skin injuries

With the rising incidence of cancer, radiotherapy has become an increasingly popular treatment modality. However, radiation-induced skin injuries (RSIs) have emerged as a commonly reported side effect of radiotherapy, thereby presenting a significant challenge in the field of radiotherapy. In this study, we report the successful synthesis of a photosensitive hydrogel via amide reaction for grafting the photosensitive group, methacrylate anhydride (MA), onto chitosan (CHI) and gelatin (GEL), with subsequent physical incorporation of epigallocatechin gallate (EGCG). The resulting composite photosensitive hydrogels exhibited favorable swelling properties, rheological properties, and biocompatibility, which promote angiogenesis and demonstrate notable therapeutic efficacy against RSIs. These findings provide valuable insights into the clinical utility of EGCG composite hydrogels for the effective management of RSIs.

Annealing High Aspect Ratio Microgels into Macroporous 3D Scaffolds Allows for Higher Porosities and Effective Cell Migration

Growing millimeter-scaled functional tissue remains a major challenge in the field of tissue engineering. Therefore, microporous annealed particles (MAPs) are emerging as promising porous biomaterials that are formed by assembly of microgel building blocks. To further vary the pore size and increase overall MAP porosity of mechanically stable scaffolds, rod-shaped microgels with high aspect ratios up to 20 are chemically interlinked into highly porous scaffolds. Polyethylene glycol based microgels (width 10 µm, lengths up to 200 µm) are produced via in-mold polymerization and covalently interlinked into stable 3D scaffolds via epoxy-amine chemistry. For the first time, MAP porosities can be enhanced by increasing the microgel aspect ratio (mean pore sizes ranging from 39 to 82 µm, porosities from 65 to 90%). These porosities are significantly higher compared to constructs made from spherical or lower aspect ratio rod-shaped microgels. Rapid filling of the pores by either murine or primary human fibroblasts is ensured as cells migrate and grow extensively into these scaffolds. Overall, this study demonstrates that highly porous, stable macroporous hydrogels can be achieved with a very low partial volume of synthetic, high aspect ratio microgels, leading to large empty volumes available for cell ingrowth and cell-cell interactions.

Designing functional hyaluronic acid-based hydrogels for cartilage tissue engineering

Damage to cartilage tissues is often difficult to repair owing to chronic inflammation and a lack of bioactive factors. Therefore, developing bioactive materials, such as hydrogels acting as extracellular matrix mimics, that can inhibit the inflammatory microenvironment and promote cartilage repair is crucial. Hyaluronic acid, which exists in cartilage and synovial fluid, has been extensively investigated for cartilage tissue engineering because of its promotion of cell adhesion and proliferation, regulation of inflammation, and enhancement of cartilage regeneration. However, hyaluronic acid-based hydrogels have poor degradation rates and unfavorable mechanical properties, limiting their application in cartilage tissue engineering. Recently, various multifunctional hyaluronic acid-based hydrogels, including alkenyl, aldehyde, thiolated, phenolized, hydrazide, and host–guest group-modified hydrogels, have been extensively studied for use in cartilage tissue engineering. In this review, we summarize the recent progress in the multifunctional design of hyaluronic acid-based hydrogels and their application in cartilage tissue engineering. Moreover, we outline the future research prospects and directions in cartilage tissue regeneration. This would provide theoretical guidance for developing hyaluronic acid-based hydrogels with specific properties to satisfy the requirements of cartilage tissue repair.

Recent Advances in Macroporous Hydrogels for Cell Behavior and Tissue Engineering

Hydrogels have been extensively used as scaffolds in tissue engineering for cell adhesion, proliferation, migration, and differentiation because of their high-water content and biocompatibility similarity to the extracellular matrix. However, submicron or nanosized pore networks within hydrogels severely limit cell survival and tissue regeneration. In recent years, the application of macroporous hydrogels in tissue engineering has received considerable attention. The macroporous structure not only facilitates nutrient transportation and metabolite discharge but also provides more space for cell behavior and tissue formation. Several strategies for creating and functionalizing macroporous hydrogels have been reported. This review began with an overview of the advantages and challenges of macroporous hydrogels in the regulation of cellular behavior. In addition, advanced methods for the preparation of macroporous hydrogels to modulate cellular behavior were discussed. Finally, future research in related fields was discussed.

A Beginner’s Guide to the Characterization of Hydrogel Microarchitecture for Cellular Applications

The extracellular matrix (ECM) is a three-dimensional, acellular scaffold of living tissues. Incorporating the ECM into cell culture models is a goal of cell biology studies and requires biocompatible materials that can mimic the ECM. Among such materials are hydrogels: polymeric networks that derive most of their mass from water. With the tuning of their properties, these polymer networks can resemble living tissues. The microarchitectural properties of hydrogels, such as porosity, pore size, fiber length, and surface topology can determine cell plasticity. The adequate characterization of these parameters requires reliable and reproducible methods. However, most methods were historically standardized using other biological specimens, such as 2D cell cultures, biopsies, or even animal models. Therefore, their translation comes with technical limitations when applied to hydrogel-based cell culture systems. In our current work, we have reviewed the most common techniques employed in the characterization of hydrogel microarchitectures. Our review provides a concise description of the underlying principles of each method and summarizes the collective data obtained from cell-free and cell-loaded hydrogels. The advantages and limitations of each technique are discussed, and comparisons are made. The information presented in our current work will be of interest to researchers who employ hydrogels as platforms for cell culture, 3D bioprinting, and other fields within hydrogel-based research.

Spatial patterning of PCL µ-scaffolds directs 3D vascularized bio-construct morphogenesis in vitro

Modular tissue engineering (mTE) strategies aim to build three-dimensional tissue analogues in vitro by the sapient combination of cells, micro-scaffolds (μ-scaffs) and bioreactors. The translation of these newly engineered tissues into current clinical approaches is, among other things, dependent on implant-to-host microvasculature integration, a critical issue for cells and tissue survival in vivo. In this work we reported, for the first time, a computer-aided modular approach suitable to build fully vascularized hybrid (biological/synthetic) constructs (bio-constructs) with micro-metric size scale control of blood vessels growth and orientation. The approach consists of four main steps, starting with the fabrication of polycaprolactone μ-scaffs by fluidic emulsion technique, which exhibit biomimetic porosity features. In the second step, layers of μ-scaffs following two different patterns, namely ordered and disordered, were obtained by a soft lithography-based process. Then, the as obtained μ-scaff patterns were used as template for human dermal fibroblasts and human umbilical vein endothelial cells co-culture, aiming to promote and guide the biosynthesis of collagenous extracellular matrix and the growth of new blood vessels within the mono-layered bio-constructs. Finally, bi-layered bio-constructs were built by the alignment, stacking and fusion of two vascularized mono-layered samples featuring ordered patterns. Our results demonstrated that, if compared to the disordered pattern, the ordered one provided better control over bio-constructs shape and vasculature architecture, while minor effect was observed with respect to cell colonization and new tissue growth. Furthermore, by assembling two mono-layered bio-constructs it was possible to build 1-mm thick fully vascularized viable bio-constructs and to study tissue morphogenesis during 1 week of in vitro culture. In conclusion, our results highlighted the synergic role of μ-scaff architectural features and spatial patterning on cells colonization and biosynthesis, and pay the way for the possibility to create in silico designed vasculatures within modularly engineered bio-constructs.