Improvement of endothelial progenitor outgrowth cell (EPOC)-mediated vascularization in gelatin-based hydrogels through pore size manipulation

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.

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.

A chemically defined, mechanically tunable, and bioactive hyaluronic acid/alginate double-network hydrogel for liver cancer organoid construction

Liver cancer organoids replicate the pathophysiology of primary tumors, making them ideal for drug screening and efficacy evaluation. However, their growth in complex, variable, animal-derived matrices hinders practical application. Here, we designed an easily accessible, chemically defined, biocompatible double-network hydrogel (HADR) using methacrylated hyaluronic acid (HAMA), sodium alginate (SA), methacrylamide dopamine (DMA), and c(RGDFC) for liver cancer organoid culture. By optimizing critical extracellular matrix (ECM) parameters, the HADR hydrogel achieves compatibility with the physiological mechanics of the human liver and fosters the adhesion and proliferation of multiple cell types. In vitro drug efficacy tests showed that HepG2 cell line-derived liver cancer organoids exhibited higher IC50 values than 2D cultures, indicating greater drug resistance. Subcutaneous tumor models in nude mice revealed that HADR hydrogels created a microenvironment for HepG2 cells mirroring the natural tumor ECM, leading to increased tumor volume, denser cell arrangement, and concurrent microvascular development. In vivo drug efficacy evaluations indicated that DOX treatment downregulated Ki-67 and MMP-9 expression, inhibiting HepG2 cell proliferation, invasion, and metastasis. These findings demonstrate the potential of HADR hydrogels for liver cancer organoid culture, offering new strategies for personalized drug screening and efficacy evaluation.

Mechanically reinforced hydrogel vehicle delivering angiogenic factor for beta cell therapy

Type 1 diabetes mellitus (T1DM) is a chronic disease affecting millions worldwide. Insulin therapy is currently the golden standard for treating T1DM; however, it does not restore the normal glycaemic balance entirely, which increases the risk of secondary complications. Beta-cell therapy may be a possible way of curing T1DM and has already shown promising results in the clinic. However, low retention rates, poor cell survival, and limited therapeutic potential are ongoing challenges, thus increasing the need for better cell encapsulation devices. This study aimed to develop a mechanically reinforced vascular endothelial growth factor (VEGF)-delivering encapsulation device suitable for beta cell encapsulation and transplantation. Poly(l-lactide-co-ε-caprolactone) (PLCL)/gelatin methacryloyl (GelMA)/alginate coaxial nanofibres were produced using electrospinning and embedded in an alginate hydrogel. The encapsulation device was physically and biologically characterised and was found to be suitable for INS-1E beta cell encapsulation, vascularization, and transplantation in terms of its biocompatibility, porosity, swelling ratio and mechanical properties. Lastly, VEGF was incorporated into the hydrogel and the release kinetics and functional studies revealed a sustained release of bioactive VEGF for at least 14 days, making the modified alginate system a promising candidate for improving the beta cell survival after transplantation.

Hydrogel-Encapsulated Pancreatic Islet Cells as a Promising Strategy for Diabetic Cell Therapy

Islet transplantation has now become a promising treatment for insulin-deficient diabetes mellitus. Compared to traditional diabetes treatments, cell therapy can restore endogenous insulin supplementation, but its large-scale clinical application is impeded by donor shortages, immune rejection, and unsuitable transplantation sites. To overcome these challenges, an increasing number of studies have attempted to transplant hydrogel-encapsulated islet cells to treat diabetes. This review mainly focuses on the strategy of hydrogel-encapsulated pancreatic islet cells for diabetic cell therapy, including different cell sources encapsulated in hydrogels, encapsulation methods, hydrogel types, and a series of accessorial manners to improve transplantation outcomes. In addition, the formation and application challenges as well as prospects are also presented.

Fabrication of polymeric microspheres for biomedical applications

Polymeric microspheres (PMs) have attracted great attention in the field of biomedicine in the last several decades due to their small particle size, special functionalities shown on the surface and high surface-to-volume ratio. However, how to fabricate PMs which can meet the clinical needs and transform laboratory achievements to industrial scale-up still remains a challenge. Therefore, advanced fabrication technologies are pursued. In this review, we summarize the technologies used to fabricate PMs, including emulsion-based methods, microfluidics, spray drying, coacervation, supercritical fluid and superhydrophobic surface-mediated method and their advantages and disadvantages. We also review the different structures, properties and functions of the PMs and their applications in the fields of drug delivery, cell encapsulation and expansion, scaffolds in tissue engineering, transcatheter arterial embolization and artificial cells. Moreover, we discuss existing challenges and future perspectives for advancing fabrication technologies and biomedical applications of PMs.

Polydopamine-modified konjac glucomannan scaffold with sustained release of vascular endothelial growth factor to promote angiogenesis

The fabrication of scaffolds capable of the sustained release of the vascular endothelial growth factor (VEGF) to promote angiogenesis for a long time remains a challenge in tissue engineering. Here, we report a facile approach for effectively fabricating a bioactive scaffold that gradually releases VEGF to promote angiogenesis. The scaffold was fabricated by coating polydopamine (PDA) on a konjac glucomannan (KGM) scaffold, followed by the surface immobilization of VEGF with PDA. The resulting VEGF-PDA/KGM scaffold, with a porous and interconnected microstructure (392 μm pore size with 84.80 porosity), combined the features of long-term biodegradability (10 weeks with 51 % degradation rate), excellent biocompatibility, and sustained VEGF release for up to 21 days. The bioactive VEGF-PDA/KGM scaffold exhibited multiple angiogenic activities over time, as confirmed by in vivo and in vitro experiments. For example, the scaffold significantly promoted the attachment and proliferation of human umbilical vein endothelial cells and the formation of vascular tubes in vitro. Moreover, the in vivo results demonstrated the formation and maturation of blood vessels after subcutaneous implantation in rats for four weeks. This promising strategy is a feasible approach for producing bioactive materials that can induce angiogenesis in vivo. These findings provide a new avenue for designing and fabricating biocompatible and long-term biodegradable scaffolds for sustained VEGF release to facilitate angiogenesis.

Injectable and Microporous Microgel-Fiber Granular Hydrogel Loaded with Bioglass and siRNA for Promoting Diabetic Wound Healing

Injectable hydrogels find extensive application in the treatment of diabetic wound healing. However, traditional bulk hydrogels are significantly limited due to their nano-porous structure, which obstructs cell migration and tissue infiltration. Moreover, regulating inflammation and matrix metalloproteinase -9 (MMP-9) expression in diabetic wounds is crucial for enhancing wound healing. This study marks the first instance of introducing an efficient, scalable, and simple method for producing microfiber-gel granules encapsulating bioceramics powders. Utilizing this method, an injectable microporous granular microgel-fiber hydrogel (MFgel) is successfully developed by assembling microgel-fibers made from hyaluronic acid (HA) and sodium alginate (SA) loaded with small interfering RNA (siRNA) and bioglass (BG) particles. Compared to traditional hydrogels (Tgel), MFgel possesses a highly interconnected network with micron-sized pores, demonstrating favorable properties for cell adhesion and penetration in in vitro experiments. Additionally, MFgel exhibits a higher compressive modulus and superior mechanical stability. When implanted subcutaneously in mice, MFgel promotes cellular and tissue infiltration, facilitating cell proliferation. Furthermore, when applied to skin defects in diabetic rats, MFgel not only effectively regulates inflammation and suppresses MMP-9 expression but also enhances angiogenesis and collagen deposition, thereby significantly accelerating diabetic wound healing. Taken together, this hydrogel possesses great potential in diabetic wound healing applications.

Construction of a Decellularized Multicomponent Extracellular Matrix Interpenetrating Network Scaffold by Gelatin Microporous Hydrogel 3D Cell Culture System

Interface tissue repair requires the construction of biomaterials with integrated structures of multiple protein types. Hydrogels that modulate internal porous structures provide a 3D microenvironment for encapsulated cells, making them promise for interface tissue repair. Currently, reduction of intrinsic immunogenicity and increase of bioactive extracellular matrix (ECM) secretion are issues to be considered in these materials. In this study, gelatin methacrylate (GelMA) hydrogel is used to encapsulate chondrocytes and construct a phase transition 3D cell culture system (PTCC) by utilizing the thermosensitivity of gelatin microspheres to create micropores within the hydrogel. The types of bioactive extracellular matrix protein formation by chondrocytes encapsulated in hydrogels are investigated in vitro. After 28 days of culture, GelMA PTCC forms an extracellular matrix predominantly composed of collagen type II, collagen type I, and fibronectin. After decellularization, the protein types and mechanical properties are well preserved, fabricating a decellularized tissue-engineered extracellular matrix and GelMA hydrogel interpenetrating network hydrogel (dECM-GelMA IPN) consisting of GelMA hydrogel as the first-level network and the ECM secreted by chondrocytes as the second-level network. This material has the potential to mediate the repair and regeneration of tendon-bone interface tissues with multiple protein types.

Accelerating Patterned Vascularization Using Granular Hydrogel Scaffolds and Surgical Micropuncture

Bulk hydrogel scaffolds are common in reconstructive surgery. They allow for the staged repair of soft tissue loss by providing a base for revascularization. Unfortunately, they are limited by both slow and random vascularization, which may manifest as treatment failure or suboptimal repair. Rapidly inducing patterned vascularization within biomaterials has profound translational implications for current clinical treatment paradigms and the scaleup of regenerative engineering platforms. To address this long-standing challenge, a novel microsurgical approach and granular hydrogel scaffold (GHS) technology are co-developed to hasten and pattern microvascular network formation. In surgical micropuncture (MP), targeted recipient blood vessels are perforated using a microneedle to accelerate cell extravasation and angiogenic outgrowth. By combining MP with an adjacent GHS with precisely tailored void space architecture, microvascular pattern formation as assessed by density, diameter, length, and intercapillary distance is rapidly guided. This work opens new translational opportunities for microvascular engineering, advancing reconstructive surgery, and regenerative medicine.

Pristine gelatin incorporation as a strategy to enhance the biofunctionality of poly(vinyl alcohol)-based hydrogels for tissue engineering applications

Synthetic polymers, such as poly(vinyl alcohol) (PVA), are popular biomaterials for the fabrication of hydrogels for tissue engineering and regenerative medicine (TERM) applications, as they provide excellent control over the physico-chemical properties of the hydrogel. However, their bioinert nature is known to limit cell-biomaterial interactions by hindering cell infiltration, blood vessel recruitment and potentially limiting their integration with the host tissue. Efforts in the field have therefore focused on increasing the biofunctionality of synthetic hydrogels, without limiting the advantages associated with their tailorability and controlled release capacity. The aim of this study was to investigate the suitability of pristine gelatin to enhance the biofunctionality of tyraminated PVA (PVA-Tyr) hydrogels, by promoting cell infiltration and host blood vessel recruitment for TERM applications. Pure PVA-Tyr hydrogels and PVA-Tyr hydrogels incorporated with vascular endothelial growth factor (VEGF), a well-known pro-angiogenic stimulus, were used for comparison. Incorporating increasing concentrations of VEGF (0.01-10 μg mL-1) or gelatin (0.01-5 wt%) did not influence the physical properties of PVA-Tyr hydrogels. However, their presence within the polymer network (>0.1 μg mL-1 VEGF and >0.1 wt% gelatin) promoted endothelial cell interactions with the hydrogels. The covalent binding of unmodified gelatin or VEGF to the PVA-Tyr network did not hamper their inherent bioactivity, as they both promoted angiogenesis in a chick chorioallantoic membrane (CAM) assay, performing comparably with the unbound VEGF control. When the PVA-Tyr hydrogels were implanted subcutaneously in mice, it was observed that cell infiltration into the hydrogels was possible in the absence of gelatin or VEGF at 1- or 3-weeks post-implantation, highlighting a clear difference between in vitro an in vivo cell-biomaterial interaction. Nevertheless, the presence of gelatin or VEGF was necessary to enhance blood vessel recruitment and infiltration, although no significant difference was observed between these two biological molecules. Overall, this study highlights the potential of gelatin as a standalone pro-angiogenic cue to enhance biofunctionality of synthetic hydrogels and provides promise for their use in a variety of TERM applications.

3D Bioprinting of Neurovascular Tissue Modeling with Collagen-Based Low-Viscosity Composites

In vitro neurovascular unit (NVU) models are valuable for investigating brain functions and developing drugs. However, it remains challenging to recapitulate the native architectural features and ultra-soft extracellular matrix (ECM) properties of the natural NVU. Cell-laden bioprinting is promising to prepare complex living tissues, but hard to balance the fidelity and cell growth. This study proposes a novel two-stage methodology for biomanufacturing functional 3D neurovascular constructs in vitro with low modulus of ECM. At the shaping stage, a low-viscosity alginate/collagen is printed through an embedded approach; at the culturing stage, the alginate is removed through targeted lysing. The low-viscosity and rapid crosslinking properties provide a printing resolution of ≈10 µm, and the lysis processing can decrease the hydrogels' modulus to ≈1 kPa and adjust the porosity of the microstructure, providing cells with an environment closing to the brain ECM. A 3D hollow coaxial neurovascular model is fabricated, in which the endothelial cells has expressed tight junction proteins and shown selective permeability, and the astrocytes outside of the endothelial layer are found to spread out with branches and directly interact with endothelial cells. The present study offers a promising modeling method for better understanding the NVU function and screening neuro-drugs.

Kartogenin-loaded hydrogel promotes intervertebral disc repair via protecting MSCs against reactive oxygen species microenvironment by Nrf2/TXNIP/NLRP3 axis

Intervertebral disc (IVD) degeneration (IDD) and the consequent low back pain present a major medical challenge. Stem cell-based tissue engineering is promising for the treatment of IDD. However, stem cell-based treatment is severely impaired by the increased generation of reactive oxygen species (ROS) in degenerative disc, which can lead to a high level of cell dysfunction and even death. In this study, a kartogenin (KGN)@PLGA-GelMA/PRP composite hydrogel was designed and used as a carrier of ADSCs-based therapies in disc repair. Injectable composite hydrogel act as a carrier for controlled release of KGN and deliver ADSCs to the degenerative disc. The released KGN can stimulate the differentiation of ADSCs into a nucleus pulposus (NP) -like phenotype and boost antioxidant capacity of ADSCs via activating Nrf2/TXNIP/NLRP3 axis. Furthermore, the composite hydrogel combined with ADSCs attenuated the in vivo degeneration of rat IVDs, maintained IVD tissue integrity and accelerated the synthesis of NP-like extracellular matrix. Therefore, the KGN@PLGA-GelMA/PRP composite hydrogel is a promising strategy for stem cell-based therapies of IDD.

Two-step method fabricating a 3D nerve cell model with brain-like mechanical properties and tunable porosity vascular structures via coaxial printing

Three-dimensional (3D) nerve cell models have been widely developed to understand the mechanisms and discover treatment methods of ischemic stroke and neurodegenerative disease. However, there is a contradiction in the production of 3D models that they should possess high modulus to ensure mechanical stability while low modulus to provide mechanical stimuli for nerve cells. In addition, it is challenging to maintain the long-term viability of 3D models when lacking vascular structures. Here, a 3D nerve cell model with brain-like mechanical properties and tunable porosity vascular structures has been fabricated. The matrix materials with brain-like low mechanical properties were favorable for promoting HT22 proliferation. The nerve cells could exchange nutrients and waste with the cultural environment through vascular structures. The vascular structures also played a supporting role, and model stability was enhanced by combining matrix materials with vascular structures. Furthermore, the porosity of vascular structure walls was adjusted by adding sacrificial materials to the tube walls during 3D coaxial printing and removing them after preparation, resulting in tunable porosity vascular structures. Finally, HT22 cells showed better cell viability and proliferation performance after culturing 7 days in the 3D models with vascular structures than in the 3D models with solid structures. All these results suggest that this 3D nerve cell model possesses good mechanical stability and long-term viability, which is expected to be used in pathological studies and drug screening for ischemic stroke and neurodegenerative diseases.

Adjusting Degree of Modification and Composition of gelAGE-Based Hydrogels Improves Long-Term Survival and Function of Primary Human Fibroblasts and Endothelial Cells in 3D Cultures

This study aimed to develop a suitable hydrogel-based 3D platform to support long-term culture of primary endothelial cells (ECs) and fibroblasts. Two hydrogel systems based on allyl-modified gelatin (gelAGE), G_1MM and G_2LH, were cross-linked via thiol-ene click reaction with a four-arm thiolated polyethylene glycol (PEG-4-SH). Compared to G_1MM, the G_2LH hydrogel was characterized by the lower polymer content and cross-linking density with a softer matrix and homogeneous and open porosity. Cell viability in both hydrogels was comparable, although the G_2LH-based platform supported better F-actin organization, cell-cell interactions, and collagen and fibronectin production. In co-cultures, early morphogenesis leading to tubular-like structures was observed within 2 weeks. Migration of fibroblasts out of spheroids embedded in the G_2LH hydrogels started after 5 days of incubation. Taken together, the results demonstrated that the G_2LH hydrogel fulfilled the demands of both ECs and fibroblasts to enable long-term culture and matrix remodeling.

Hydrogel: A Potential Material for Bone Tissue Engineering Repairing the Segmental Mandibular Defect

Free flap surgery is currently the only successful method used by surgeons to reconstruct critical-sized defects of the jaw, and is commonly used in patients who have had bony lesions excised due to oral cancer, trauma, infection or necrosis. However, donor site morbidity remains a significant flaw of this strategy. Various biomaterials have been under investigation in search of a suitable alternative for segmental mandibular defect reconstruction. Hydrogels are group of biomaterials that have shown their potential in various tissue engineering applications, including bone regeneration, both through in vitro and in vivo pre-clinical animal trials. This review discusses different types of hydrogels, their fabrication techniques, 3D printing, their potential for bone regeneration, outcomes, and the limitations of various hydrogels in preclinical models for bone tissue engineering. This review also proposes a modified technique utilizing the potential of hydrogels combined with scaffolds and cells for efficient reconstruction of mandibular segmental defects.

Scavenging ROS and inflammation produced during treatment to enhance the wound repair efficacy of photothermal injectable hydrogel

Injectable hydrogels with near infrared (NIR) photothermal ability show attractive application prospects in the treatment of wound infection and promoting skin defect repair. Nevertheless, excess reactive oxygen species (ROS) and inflammatory responses caused by bacterial infection and photothermal therapy (PTT) would delay tissue regeneration and wound healing. In this study, a novel NIR photothermal injectable hydrogel with anti-oxidation and anti-inflammation by incorporating α-lipoic acid modified palladium nanoparticles into calcium ions crosslinked sodium alginate hydrogel was developed. The resulting hydrogel facilitated to fill perfectly various irregular wounds, and could convert NIR light into local high-heat to kill >80 % of Escherichia coli and Staphylococcus aureus. Remarkably, the hydrogel exhibited excellent anti-oxidant and anti-inflammatory activity, which could scavenge >60 % of ROS in cells and decrease the relative expression level of tumor necrosis factor-alpha and interleukin-1β genes by 52.9 % and 53.3 % respectively. It was found that the NIR photothermal injectable hydrogel with anti-oxidation and anti-inflammation could effectively reduce ROS and inflammation caused by bacterial infection and PPT. Additionally, it could also enhance wound repair efficiency. The hydrogel is expected to be a potential wound dressing for the treatment of clinical skin defects.

Macropore Regulation of Hydroxyapatite Osteoinduction via Microfluidic Pathway

Macroporous characteristics have been shown to play a key role in the osteoinductivity of hydroxyapatite ceramics, but the physics underlying the new bone formation and distribution in such scaffolds still remain elusive. The work here has emphasized the osteoinductive capacity of porous hydroxyapatite scaffolds containing different macroporous sizes (200–400 μm, 1200–1500 μm) and geometries (star shape, spherical shape). The assumption is that both the size and shape of a macropore structure may affect the microfluidic pathways in the scaffolds, which results in the different bone formations and distribution. Herein, a mathematical model and an animal experiment were proposed to support this hypothesis. The results showed that the porous scaffolds with the spherical macropores and large pore sizes (1200–1500 μm) had higher new bone production and more uniform new bone distribution than others. A finite element analysis suggested that the macropore shape affected the distribution of the medium–high velocity flow field, while the macropore size effected microfluid speed and the value of the shear stress in the scaffolds. Additionally, the result of scaffolds implanted into the dorsal muscle having a higher new bone mass than the abdominal cavity suggested that the mechanical load of the host tissue could play a key role in the microfluidic pathway mechanism. All these findings suggested that the osteoinduction of these scaffolds depends on both the microfluid velocity and shear stress generated by the macropore size and shape. This study, therefore, provides new insights into the inherent osteoinductive mechanisms of bioceramics, and may offer clues toward a rational design of bioceramic scaffolds with improved osteoinductivity.

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.

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

Vascularization of thick hydrogel scaffolds is still a big challenge, because the submicron- or nano-sized pores seriously restrict endothelial cells adhesion, proliferation and migration. Therefore, porous hydrogels have been fabricated as a kind of promising hydrous scaffolds for enhancing vascularization during tissue repairing. In order to investigate the effects of pore size on vascularization, macroporous methacrylated hyaluronic acid (HAMA) hydrogels with different pore sizes were fabricated by a gelatin microspheres (GMS) template method. After leaching out GMS templates, uniform and highly interconnected macropores were formed in hydrogels, which provided an ideal physical microenvironment to induce human umbilical vein endothelial cells (HUVECs) migration and tissue vascularization. In vitro results revealed that macroporous hydrogels facilitated cells proliferation and migration compared with non-macroporous hydrogels. Hydrogels with middle pore size of 200-250 μm (HAMA250 hydrogels) supported the best cell proliferation and furthest 3D migration of HUVECs. The influences of pore sizes on vascularization were then evaluated with subcutaneous embedding. In vivo results illustrated that HAMA250 hydrogels exhibited optimum vascularization behavior. Highest number of newly formed blood vessels and expression of CD31 could be found in HAMA250 hydrogels rather than in other hydrogels. In summary, our results concluded that the best pore size for endothelial cells migration and tissue vascularization was 200-250 μm. This research provides a new insight into the engineering vascularized tissues and may find utility in designing regenerative biomaterial scaffolds.

Tunable Biomaterials for Myocardial Tissue Regeneration: Promising New Strategies for Advanced Biointerface Control and Improved Therapeutic Outcomes

Following myocardial infarction (MI) and the natural healing process, the cardiac mechanostructure changes significantly leading to reduced contractile ability and putting additional pressure on the heart muscle thereby increasing the...

Development and Characterization of Gelatin-Norbornene Bioink to Understand the Interplay between Physical Architecture and Micro-Capillary Formation in Biofabricated Vascularized Constructs

The principle challenge for engineering viable, cell-laden hydrogel constructs of clinically-relevant size, is rapid vascularization, in order to moderate the finite capacity of passive nutrient diffusion. A multiscale vascular approach, with large open channels and bulk microcapillaries may be an admissible approach to accelerate this process, promoting overall pre-vascularization for long-term viability of constructs. However, the limited availability of bioinks that possess suitable characteristics that support both fabrication of complex architectures and formation of microcapillaries, remains a barrier to advancement in this space. In this study, gelatin-norbornene (Gel-NOR) is investigated as a vascular bioink with tailorable physico-mechanical properties, which promoted the self-assembly of human stromal and endothelial cells into microcapillaries, as well as being compatible with extrusion and lithography-based biofabrication modalities. Gel-NOR constructs containing self-assembled microcapillaries are successfully biofabricated with varying physical architecture (fiber diameter, spacing, and orientation). Both channel sizes and cell types affect the overall structural changes of the printed constructs, where cross-signaling between both human stromal and endothelial cells may be responsible for the reduction in open channel lumen observed over time. Overall, this work highlights an exciting three-way interplay between bioink formulation, construct design, and cell-mediated response that can be exploited towards engineering vascular tissues.

3D hydrogel models of the neurovascular unit to investigate blood-brain barrier dysfunction

The neurovascular unit (NVU), consisting of neurons, glial cells, vascular cells (endothelial cells, pericytes and vascular smooth muscle cells (VSMCs)) together with the surrounding extracellular matrix (ECM), is an important interface between the peripheral blood and the brain parenchyma. Disruption of the NVU impacts on blood-brain barrier (BBB) regulation and underlies the development and pathology of multiple neurological disorders, including stroke and Alzheimer's disease (AD). The ability to differentiate induced pluripotent stem cells (iPSCs) into the different cell types of the NVU and incorporate them into physical models provides a reverse engineering approach to generate human NVU models to study BBB function. To recapitulate the in vivo situation such NVU models must also incorporate the ECM to provide a 3D environment with appropriate mechanical and biochemical cues for the cells of the NVU. In this review, we provide an overview of the cells of the NVU and the surrounding ECM, before discussing the characteristics (stiffness, functionality and porosity) required of hydrogels to mimic the ECM when incorporated into in vitro NVU models. We summarise the approaches available to measure BBB functionality and present the techniques in use to develop robust and translatable models of the NVU, including transwell models, hydrogel models, 3D-bioprinting, microfluidic models and organoids. The incorporation of iPSCs either without or with disease-specific genetic mutations into these NVU models provides a platform in which to study normal and disease mechanisms, test BBB permeability to drugs, screen for new therapeutic targets and drugs or to design cell-based therapies.

More natural more better: triple natural anti-oxidant puerarin/ferulic acid/polydopamine incorporated hydrogel for wound healing

Background

During wound healing, the overproduction of reactive oxygen species (ROS) can break the cellular oxidant/antioxidant balance, which prolongs healing. The wound dressings targeting the mitigation of ROS will be of great advantages for the wound healing. puerarin (PUE) and ferulic acid (FA) are natural compounds derived from herbs that exhibit multiple pharmacological activities, such as antioxidant and anti-inflammatory effects. Polydopamine (PDA) is made from natural dopamine and shows excellent antioxidant function. Therefore, the combination of natural antioxidants into hydrogel dressing is a promising therapy for wound healing.

Results

Hydrogel wound dressings have been developed by incorporating PUE or FA via PDA nanoparticles (NPs) into polyethylene glycol diacrylate (PEG-DA) hydrogel.

This hydrogel can load natural antioxidant drugs and retain the drug in the gel network for a long period due to the presence of PDA NPs. Under oxidative stress, this hydrogel can improve the activity of superoxide dismutase and glutathione peroxidase and reduce the levels of ROS and malondialdehyde, thus preventing oxidative damage to cells, and then promoting wound healing, tissue regeneration, and collagen accumulation.

Conclusion

Overall, this triple antioxidant hydrogel accelerates wound healing by alleviating oxidative injury. Our study thus provides a new way about co-delivery of multiple antioxidant natural molecules from herbs via antioxidant nanoparticles for wound healing and skin regeneration.

Graphic Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00973-7.

Drawing Inspiration from Developmental Biology for Cardiac Tissue Engineers

A sound understanding of developmental biology is part of the foundation of effective stem cell-derived tissue engineering. Here, the key concepts of cardiac development that are successfully applied in a bioinspired approach to growing engineered cardiac tissues, are reviewed. The native cardiac milieu is studied extensively from embryonic to adult phenotypes, as it provides a resource of factors, mechanisms, and protocols to consider when working toward establishing living tissues in vitro. It begins with the various cell types that constitute the cardiac tissue. It is discussed how myocytes interact with other cell types and their microenvironment and how they change over time from the embryonic to the adult states, with a view on how such changes affect the tissue function and may be used in engineered tissue models. Key embryonic signaling pathways that have been leveraged in the design of culture media and differentiation protocols are presented. The cellular microenvironment, from extracellular matrix chemical and physical properties, to the dynamic mechanical and electrical forces that are exerted on tissues is explored. It is shown that how such microenvironmental factors can inform the design of biomaterials, scaffolds, stimulation bioreactors, and maturation readouts, and suggest considerations for ongoing biomimetic advancement of engineered cardiac tissues and regeneration strategies for the future.

Biodegradable ZnLiCa ternary alloys for critical-sized bone defect regeneration at load-bearing sites: In vitro and in vivo studies

A novel biodegradable metal system, ZnLiCa ternary alloys, were systematically investigated both in vitro and in vivo. The ultimate tensile strength (UTS) of Zn0.8Li0.1Ca alloy reached 567.60 ± 9.56 MPa, which is comparable to pure Ti, one of the most common material used in orthopedics. The elongation of Zn0.8Li0.1Ca is 27.82 ± 18.35%, which is the highest among the ZnLiCa alloys. The in vitro degradation rate of Zn0.8Li0.1Ca alloy in simulated body fluid (SBF) showed significant acceleration than that of pure Zn. CCK-8 tests and hemocompatibility tests manifested that ZnLiCa alloys exhibit good biocompatibility. Real-time PCR showed that Zn0.8Li0.1Ca alloy successfully stimulated the expressions of osteogenesis-related genes (ALP, COL-1, OCN and Runx-2), especially the OCN. An in vivo implantation was conducted in the radius of New Zealand rabbits for 24 weeks, aiming to treat the bone defects. The Micro-CT and histological evaluations proved that the regeneration of bone defect was faster within the Zn0.8Li0.1Ca alloy scaffold than the pure Ti scaffold. Zn0.8Li0.1Ca alloy showed great potential to be applied in orthopedics, especially in the load-bearing sites.

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The first research work of ZnLiCa alloys to be used as biodegradable metals.

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The ultimate tensile strength (UTS) of Zn0.8Li0.1Ca alloy reached 567.60 ± 9.56 MPa, which is comparable to pure Ti, one of the most common material used in orthopedics.

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Porous scaffolds made of Zn0.8Li0.1Ca showed superior bone-defect-treating effects to pure Ti scaffolds in New Zealand rabbits.

Porous shape memory scaffold of dextran and hydroxyapatite for minimum invasive implantation for bone tissue engineering applications

Minimally invasive implantation of a porous scaffold of large volume into bone defect site remains a challenge. Scaffolds based on shape memory polymer (SMP) show potential to be delivered in the compact form via minimally invasive surgery. The present study chooses poly (ε-caprolactone)-diols (PCL-diols) as the SMP to cross-link carboxyl dextran via ester bonds together with particle leaching method to yield a porous SMP scaffold. The inner surfaces of porous SMP scaffold are then mineralized via in situ precipitation to yield mineralized porous SMP-hydroxyapatite (SMP-HA) scaffold. The porous SMP-HA scaffold possesses pore size of 400-500 μm, with HA particles uniformly distributed and orientationally aligned on the inner surfaces of scaffold. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) are carried out to identify the HA deposition. The phase transition temperature of the scaffold is adjusted to 38°C via changing the dosage of PCL (molecule weight: 2800) to endow the scaffold with shape deformation and fixed properties, as well as well-performed shape recovery property under body temperature. Bone marrow mesenchymal stem cells (BMSCs) adhere on the inner surfaces of SMP-HA scaffold, exhibiting larger spreading area when compared to cells adhered on SMP scaffold without HA, promoting its osteogenesis. In vivo degradation showed that the scaffold degrades completely after 6 months post-implantation. At the same time, significant tissue and capillary invasion indicated that the present porous SMP-HA scaffold hold great promise towards bone tissue engineering applications.

Hyaluronic Acid and Regenerative Medicine: New Insights into the Stroke Therapy

Stroke is known as one of the very important public health problems that are related to societal burden and tremendous economic losses. It has been shown that there are few therapeutic approaches for the treatment of this disease. In this regard, the present therapeutic platforms aim to obtain neuroprotection, reperfusion, and neuro recovery. Among these therapies, regenerative medicine-based therapies have appeared as new ways of stroke therapy. Hyaluronic acid (HA) is a new candidate, which could be applied as a regenerative medicine-based therapy in the treatment of stroke. HA is a glycosaminoglycan composed of disaccharide repeating elements (N-acetyl-Dglucosamine and D-glucuronic acid). Multiple lines of evidence demonstrated that HA has critical roles in normal tissues. It can be a key player in different physiological and pathophysiological conditions such as water homeostasis, multiple drug resistance, inflammatory processes, tumorigenesis, angiogenesis, and changed viscoelasticity of the extracellular matrix. HA has very important physicochemical properties i.e., availability of reactive functional groups and its solubility, which make it a biocompatible material for application in regenerative medicine. Given that HAbased bioscaffolds and biomaterials do not induce inflammation or allergies and are hydrophilic, they are used as soft tissue fillers and injectable dermal fillers. Several studies indicated that HA could be employed as a new therapeutic candidate in the treatment of stroke. These studies documented that HA and HA-based therapies exert their pharmacological effects via affecting stroke-related processes. Herein, we summarized the role of the extracellular matrix in stroke pathogenesis. Moreover, we highlighted the HA-based therapies for the treatment of stroke.

A redox-responsive hyaluronic acid-based hydrogel for chronic wound management

Polymer-based hydrogels have been widely applied for chronic wound therapeutics, due to their well-acclaimed wound exudate management capability. At the same time, there is still an unmet clinical need for simple wound diagnostic tools to assist clinical decision-making at the point of care and deliver on the vision of patient-personalised wound management. To explore this challenge, we present a one-step synthetic strategy to realise a redox-responsive, hyaluronic acid (HA)-based hydrogel that is sensitive to wound environment-related variations in glutathione (GSH) concentration. By selecting aminoethyl disulfide (AED) as a GSH-sensitive crosslinker and considering GSH concentration variations in active and non-self-healing wounds, we investigated the impact of GSH-induced AED cleavage on hydrogel dimensions, aiming to build GSH-size relationships for potential point-of-care wound diagnosis. The hydrogel was also found to be non-cytotoxic and aided L929 fibroblast growth and proliferation over seven days in vitro. Such a material offers a very low-cost tool for the visual detection of a target analyte that varies dependent on the status of the cells and tissues (wound detection), and may be further exploited as an implant for fibroblast growth and tissue regeneration (wound repair).

State-of-the-Art Strategies for the Vascularization of Three-Dimensional Engineered Organs

Engineering three-dimensional (3D) implantable tissue constructs is a promising strategy for replacing damaged or diseased tissues and organs with functional replacements. However, the efficient vascularization of new 3D organs is a major scientific and technical challenge since large tissue constructs or organs require a constant blood supply to survive in vivo. Current approaches to solving this problem generally fall into the following three major categories: (a) cell-based, (b) angiogenic factor-based, and (c) scaffold-based. In this review, we summarize state-of-the-art technologies that are used to develop complex, stable, and functional vasculature for engineered 3D tissue constructs and organs; additionally, we have suggested directions for future research.

Skin wound repair: Results of a pre-clinical study to evaluate electropsun collagen-elastin-PCL scaffolds as dermal substitutes

The gold standard treatment for severe burn injuries is autologous skin grafting and the use of commercial dermal substitutes. However, resulting skin tissue following treatment usually displays abnormal morphology and functionality including scarring, skin contracture due to the poor elasticity and strength of existing dermal substitutes. In this study, we have developed a triple-polymer scaffold made of collagen-elastin-polycaprolactone (CEP) composite, aiming to enhance the mechanical properties of the scaffold while retaining its biological properties in promoting cell attachment, proliferation and tissue regeneration. The inclusion of elastin was revealed to decrease the stiffness of the scaffold, while also decreasing hysteresis and increasing elasticity. In mice, electrospun collagen-elastin-PCL scaffolds promoted keratinocyte and fibroblast proliferation, tissue integration and accelerated early-stage angiogenesis. Only a mild inflammatory response was observed in the first 2 weeks post-subcutaneous implantation. Our data indicates that the electrospun collagen-elastin-PCL scaffolds could potentially serve as a skin substitute to promote skin cell growth and tissue regeneration after severe burn injury.

Co-culture of human umbilical vein endothelial cells and human bone marrow stromal cells into a micro-cavitary gelatin-methacrylate hydrogel system to enhance angiogenesis

Vascular tissue engineering seeks to develop functional blood vessels that comprise of both endothelial cells and pericytes for translational medicine and is often faced with numerous challenges such as nutrients and wastes diffusion problem in the centre of the scaffolds. Various strategies have been adopted to solve the diffusion problem in thick engineered scaffolds. Typically, microchannels or dissolvable microspheres are introduced into three-dimensional (3D) scaffolds as an alternative way to improve the infiltration of scaffolds and endothelial cells are usually incorporated into the biomaterials. While some research groups now focus on finding supporting cells to build further vascularized structures in the scaffolds. In this study, a bioinspired 3D gelatin-methacrylate (Gel-MA) hydrogel with dissolvable microspheres was created to encapsulate human bone marrow stromal cells (HMSCs) and human umbilical vein endothelial cells (HUVECs) which was used to investigate whether HMSCs could play a pericytes-like role and enhance vascularization within the engineered scaffolds. The results showed co-culture of HMSCs and HUVECs demonstrated significantly improved vascularization when compared to either HUVECs or HMSCs monoculture. Angiogenic genes were expressed significantly higher in co-culture group. Moreover, when implanting the pre-vascularized scaffolds in vivo, co-culture system integrated more successfully with host tissue and showed higher host tissue invasion than any other groups. More importantly, both the qPCR and immunofluorescence results indicated MSCs differentiated towards pericytes to enhance vascularization in this study. This paper highlights the enhanced capability of 3D micro-cavitary Gel-MA hydrogel for co-culturing HUVECs and HMSCs to promote vascularization which presents a potential strategy for future tissue repair and regeneration.

Regenerative Medicine Therapies for Targeting Neuroinflammation After Stroke

Inflammation is a major pathological event following ischemic stroke that contributes to secondary brain tissue damage leading to poor functional recovery. Following the initial ischemic insult, post-stroke inflammatory damage is driven by initiation of a central and peripheral innate immune response and disruption of the blood-brain barrier (BBB), both of which are triggered by the release of pro-inflammatory cytokines and infiltration of circulating immune cells. Stroke therapies are limited to early cerebral blood flow reperfusion, and whilst current strategies aim at targeting neurodegeneration and/or neuroinflammation, innovative research in the field of regenerative medicine aims at developing effective treatments that target both the acute and chronic phase of inflammation. Anti-inflammatory regenerative strategies include the use of nanoparticles and hydrogels, proposed as therapeutic agents and as a delivery vehicle for encapsulated therapeutic biological factors, anti-inflammatory drugs, stem cells, and gene therapies. Biomaterial strategies-through nanoparticles and hydrogels-enable the administration of treatments that can more effectively cross the BBB when injected systemically, can be injected directly into the brain, and can be 3D-bioprinted to create bespoke implants within the site of ischemic injury. In this review, these emerging regenerative and anti-inflammatory approaches will be discussed in relation to ischemic stroke, with a perspective on the future of stroke therapies.

Effects of large dimensional deformation of a porous structure on stem cell fate activated by poly(l-glutamic acid)-based shape memory scaffolds

The effects of mechanostructural stimuli on stem cell fate in 3D structures have been investigated in a poly(l-glutamic acid)-based shape memory porous scaffold; the results indicate the scaffold a potential cell carrier.

In Situ Forming Gelatin Hydrogels-Directed Angiogenic Differentiation and Activity of Patient-Derived Human Mesenchymal Stem Cells

Directing angiogenic differentiation of mesenchymal stem cells (MSCs) still remains challenging for successful tissue engineering. Without blood vessel formation, stem cell-based approaches are unable to fully regenerate damaged tissues due to limited support for cell viability and desired tissue/organ functionality. Herein, we report in situ cross-linkable gelatin-hydroxyphenyl propionic acid (GH) hydrogels that can induce pro-angiogenic profiles of MSCs via purely material-driven effects. This hydrogel directed endothelial differentiation of mouse and human patient-derived MSCs through integrin-mediated interactions at the cell-material interface, thereby promoting perfusable blood vessel formation in vitro and in vivo. The causative roles of specific integrin types (α₁ and αvβ₃) in directing endothelial differentiation were verified by blocking the integrin functions with chemical inhibitors. In addition, to verify the material-driven effect is not species-specific, we confirmed in vitro endothelial differentiation and in vivo blood vessel formation of patient-derived human MSCs by this hydrogel. These findings provide new insight into how purely material-driven effects can direct endothelial differentiation of MSCs, thereby promoting vascularization of scaffolds towards tissue engineering and regenerative medicine applications in humans.