Modulating hydrogel crosslink density and degradation to control bone morphogenetic protein delivery and in vivo bone formation

Tunable hydrogel networks by varying secondary structures of hydrophilic peptoids provide viable 3D cell culture platforms for hMSCs

Hydrogels have excellent ability to mimic the extracellular matrix (ECM) during 3D cell culture, yet it remains difficult to tune their mechanical properties without also changing network connectivity. Previously, we developed 2D culture platforms based on tunable hydrogels crosslinked by peptoids with various secondary structures: helical, non-helical, and unstructured, which allowed control over hydrogel mechanics independent of network connectivity. Here, we extend our strategy to 3D matrices by modifying the peptoids with piperazine and homopiperazine residues to enhance water solubility without altering their secondary structure. Hydrogels crosslinked with helical peptoids exhibited significantly higher stiffness compared to hydrogels crosslinked with non-helical or unstructured peptoids. Human mesenchymal stem cells (hMSCs) encapsulated within these hydrogels were assessed for viability, proliferation, and immunomodulatory potential. The stiffest hydrogels promoted the highest rates of proliferation and increased yes-associated protein (YAP) nuclear localization. Softer hydrogels, however, showed enhanced production of indoleamine 2,3-dioxygenase (IDO), both with and without interferon gamma (IFN-γ) stimulation, highlighting their potential in immunomodulatory applications. The biomimetic platform developed here enables the study of how matrix mechanics influence stem cell behavior without confounding factors from network connectivity, leading to insights for hMSC-mediated immunomodulation.

Peptide Hydrogel for Sustained Release of Recombinant Human Bone Morphogenetic Protein-2 In Vitro

This study aimed to investigate the impact of varying the formulation of a specific peptide hydrogel (PepGel) on the release kinetics of rhBMP-2 in vitro. Three PepGel formulations were assessed: (1) 50% v/v (peptides volume/total volume) PepGel, where synthetic peptides were mixed with crosslinking reagents and rhBMP-2 solution; (2) 67% v/v PepGel; (3) 80% v/v PepGel. Each sample was loaded with 12 µg of rhBMP-2 and incubated in PBS. Released rhBMP-2 was quantified by ELISA at 1 h, 6 h, and 1, 2, 4, 7, 10, 14, and 21 days. To explore how PepGel formulations influence rhBMP-2 release, the gel porosities, swelling ratios, and mechanical properties of the three PepGel formulations were quantitatively analyzed. The results showed that rhBMP-2 encapsulated with 50% v/v PepGel exhibited a sustained release over the 21-day experiment, while the 67% and 80% v/v PepGels demonstrated significantly lower rhBMP-2 release rates compared to the 50% formulation after day 7. Higher histological porosity of PepGel was significantly correlated with increased rhBMP-2 release rates. Conversely, the swelling ratio and elastic modulus of the 50% v/v PepGel were significantly lower than that of the 67% and 80% v/v formulations. In conclusion, this study indicates that varying the formulation of crosslinked PepGel can control rhBMP-2 release rates in vitro by modulating gel porosity, swelling ratio, and mechanical properties. Encapsulation with 50% v/v PepGel offers a sustained rhBMP-2 release pattern in vitro; if replicated in vivo, this could mitigate the adverse effects associated with burst release of rhBMP-2 in clinical applications.

Injectable hydrogels for bone regeneration with tunable degradability via peptide chirality modification

The degradability of hydrogels plays a pivotal role in bone regeneration, yet its precise effects on the bone repair process remain poorly understood. Traditional studies have been limited by the use of hydrogels with insufficient variation in degradation properties for thorough comparative analysis. Addressing this gap, our study introduces the development of matrix metalloproteinase (MMP)-responsive hydrogels engineered with a tunable degradation rate, specifically designed for bone regeneration applications. These innovative hydrogels are synthesized by integrating MMP-sensitive peptides, which exhibit chirality-transferred amino acids, with norbornene (NB)-modified 8-arm polyethylene glycol (PEG) macromers to form the hydrogel network. The degradation behavior of these hydrogels is manipulated through the chirality of the incorporated peptides, resulting in the classification into L, LD, and D hydrogels. Remarkably, the L hydrogel variant shows a significantly enhanced degradation rate, both in vitro and in vivo, which in turn fosters bone regeneration by promoting cell migration and upregulating osteogenic gene expression. This research highlights the fundamental role of hydrogel degradability in bone repair and lays the groundwork for the advancement of degradable hydrogel technologies for bone regeneration, offering new insights and potential for future biomaterials development.

An Injectable Hydrogel Composing Anti-Inflammatory and Osteogenic Therapy toward Bone Erosions Microenvironment Remodeling in Rheumatoid Arthritis

Healing bone erosions in rheumatoid arthritis (RA) remains greatly challenging via biomaterial strategies. Given the unsuccessful innate bone erosion healing due to an inflammatory disorder, over-activated osteoclasts, and impaired osteoblasts differentiation, RA pathogenesis-guided engineering of an innovative hydrogel platform is needed for remodeling osteoimmune and osteogenic microenvironment of bone erosion healing. Herein, in situ adaptable and injectable interpenetrating polymer network (IPN) hydrogel is developed through an ingenious combination of a bio-orthogonal reaction between hyaluronic acid (HA) and collagen, along with effective electrostatic interactions leveraging bisphosphonate (BP)-functionalized HA macromers (HABP) and nanorod shaped zinc (Zn)-doped biphasic calcium phosphate (ZnBCP). IPN hydrogel exhibits exceptional adaptability to the local shape complexity at bone erosions, and by integrating ZnBCP and HABP, a multi-stage releasing platform is engineered, facilitating controlled cargo delivery for remodeling more anti-inflammatory M2 cells and reducing over-activated osteoclastic activities, thereby reconstructing the bone regeneration microenvironment. Sustainedly co-delivering multiple ions (calcium and phosphate) can display excellent osteogenic properties and be conducive to the bone formation process, by effects of osteogenesis-associated cell differentiation. Overall, the introduced bioactive IPN hydrogel therapy remodels the osteoimmune environment by synergistic pro-inflammation-resolving, osteogenesis, and anti-osteoclastic activities, displaying excellent bone reconstruction in the collagen-induced arthritis rabbit model.

Dimethyl Fumarate-Loaded Gellan Gum Hydrogels Can Reduce In Vitro Chemokine Expression in Oral Cells

Dimethyl fumarate (DMF), originally proposed to treat multiple sclerosis, is considered to have a spectrum of anti-inflammatory effects that effectively control periodontitis, mainly when applied with a hydrogel delivery system. Chemokine expression by gingival fibroblasts is a significant driver of periodontitis; thus, hydrogel-based strategies to deliver DMF, which in turn dampen chemokine expression, are of potential clinical relevance. To test this approach, we have established a bioassay where chemokine expression is induced by exposing gingival fibroblast to IL1β and TNFα, or with saliva. We show herein that DMF effectively reduced the expression of CXCL8, CXCL1, CXCL2, and CCL2-and lowered the phosphorylation of ERK and JNK-without affecting cell viability. This observation was confirmed by immunoassays with CXCL8. Consistently, the forced chemokine expression in HSC2 oral squamous epithelial cells was greatly diminished by DMF. To implement our hydrogel-based delivery system, gingival fibroblasts were cocultured with gellan gum hydrogels enriched for DMF. In support of our strategy, DMF-enriched gellan gum hydrogels significantly reduced the forced chemokine expression in gingival fibroblasts. Our data suggest that DMF exerts its anti-inflammatory activity in periodontal cells when released from gellan gum hydrogels, suggesting a potential clinical relevance to control overshooting chemokine expression under chronic inflammatory conditions.

Development and Characterization of Heparin-Containing Hydrogel/3D-Printed Scaffold Composites for Craniofacial Reconstruction

Regeneration of cartilage and bone tissues remains challenging in tissue engineering due to their complex structures, and the need for both mechanical support and delivery of biological repair stimuli. Therefore, the goal of this study was to develop a composite scaffold platform for anatomic chondral and osteochondral repair using heparin-based hydrogels to deliver small molecules within 3D-printed porous scaffolds that provide structure, stiffness, and controlled biologic delivery. We designed a mold-injection system to combine hydrolytically degradable hydrogels and 3D-printed scaffolds that could be employed rapidly (< 30 min) in operating room settings (~23 °C). Micro-CT analysis demonstrated the effectiveness of our injection system through homogeneously distributed hydrogel within the pores of the scaffolds. Hydrogels and composite scaffolds exhibited efficient loading (~94%) of a small positively charged heparin-binding molecule (crystal violet) with sustained release over 14 days and showed high viability of encapsulated porcine chondrocytes over 7 days. Compression testing demonstrated nonlinear viscoelastic behavior where tangent stiffness decreased with scaffold porosity (porous scaffold tangent stiffness: 70%: 4.9 MPa, 80%: 1.5 MPa, and 90%: 0.20 MPa) but relaxation was not affected. Lower-porosity scaffolds (70%) showed stiffness similar to lower ranges of trabecular bone (4-8 MPa) while higher-porosity scaffolds (80% and 90%) showed stiffness similar to auricular cartilage (0.16-2 MPa). Ultimately, this rapid composite scaffold fabrication method may be employed in the operating room and utilized to control biologic delivery within load-bearing scaffolds.

Conformal encapsulation of mammalian stem cells using modified hyaluronic acid

Micro- and nanoencapsulation of cells has been studied as a strategy to protect cells from environmental stress and promote survival during delivery. Hydrogels used in encapsulation can be modified to influence cell behaviors and direct assembly in their surroundings. Here, we report a system that conformally encapsulated stem cells using hyaluronic acid (HA). We successfully modified HA with lipid, thiol, and maleimide pendant groups to facilitate a hydrogel system in which HA was deposited onto cell plasma membranes and subsequently crosslinked through thiol-maleimide click chemistry. We demonstrated conformal encapsulation of both neural stem cells (NSCs) and mesenchymal stromal cells (MSCs), with viability of both cell types greater than 90% after encapsulation. Additional material could be added to the conformal hydrogel through alternating addition of thiol-modified and maleimide-modified HA in a layering process. After encapsulation, we tracked egress and viability of the cells over days and observed differential responses of cell types to conformal hydrogels both according to cell type and the amount of material deposited on the cell surfaces. Through the design of the conformal hydrogels, we showed that multicellular assembly could be created in suspension and that encapsulated cells could be immobilized on surfaces. In conjunction with photolithography, conformal hydrogels enabled rapid assembly of encapsulated cells on hydrogel substrates with resolution at the scale of 100 μm.

The ciliary protein C2cd3 is required for mandibular musculoskeletal tissue patterning

The mandible is composed of several musculoskeletal tissues including bone, cartilage, and tendon that require precise patterning to ensure structural and functional integrity. Interestingly, most of these tissues are derived from one multipotent cell population called cranial neural crest cells (CNCCs). How CNCCs are properly instructed to differentiate into various tissue types remains nebulous. To better understand the mechanisms necessary for the patterning of mandibular musculoskeletal tissues we utilized the avian mutant talpid2 (ta2) which presents with several malformations of the facial skeleton including dysplastic tendons, mispatterned musculature, and bilateral ectopic cartilaginous processes extending off Meckel's cartilage. We found an ectopic epithelial BMP signaling domain in the ta2 mandibular prominence (MNP) that correlated with the subsequent expansion of SOX9+ cartilage precursors. These findings were validated with conditional murine models suggesting an evolutionarily conserved mechanism for CNCC-derived musculoskeletal patterning. Collectively, these data support a model in which cilia are required to define epithelial signal centers essential for proper musculoskeletal patterning of CNCC-derived mesenchyme.

In vitro degradation of fibrous bilayer poly-L-lactic acid scaffolds

Polymer degradation and mechanical properties are of paramount importance in tissue engineering. The degradation rate of polymeric scaffolds is influenced by several important material and environmental factors. In particular, the mechanical support provided by the scaffold to the surrounding tissue during tissue regeneration is critical for that it directly impacts the cell behavior through mechanical signals sensed by mechanoreceptors on the cell surface. Consequently, the principal objective of the present study was to investigate the degradation behavior of electrospun poly-L-lactic acid (PLLA) bilayer microfibrous scaffolds in pH-neutral medium. Changes in the morphology, molecular weight, crystallinity, mass loss, and thermomechanical properties of the scaffolds over an extended period were studied by scanning electron microscopy, differential scanning calorimetry, gel permeation chromatography, and tensile testing. An interplay between chain scission and orderly chain rearrangement in the polymer scaffold commenced during degradation, leading to the decrease of the molecular weight and stiffness, a constant mass loss, and an increase in crystallinity, tensile strength, and glass transition temperature, with virtually constant yield strength and melting temperature. The unchanged structure morphology and adequate matrix stiffness after prolonged degradation illuminated the potential of the bilayer PLLA scaffolds for tissue engineering and drug delivery applications. Nonetheless, modifications to the scaffold structure or surface may be required to accordingly tune the degradation rate in these applications. The experimental methodology introduced in this study can be extended to potentially investigate material degradation in other fields, such as agriculture, packaging, and disposable products.

Dynamic modulation of matrix adhesiveness induces epithelial-to-mesenchymal transition in prostate cancer cells in 3D

Synthetic matrices with dynamic presentation of cell guidance cues are needed for the development of physiologically relevant in vitro tumor models. Towards the goal of mimicking prostate cancer progression and metastasis, we engineered a tunable hyaluronic acid-based hydrogel platform with protease degradable and cell adhesive properties employing bioorthogonal tetrazine ligation with strained alkenes. The synthetic matrix was first fabricated via a slow tetrazine-norbornene reaction, then temporally modified via a diffusion-controlled method using trans-cyclooctene, a fierce dienophile that reacts with tetrazine with an unusually fast rate. The encapsulated DU145 prostate cancer single cells spontaneously formed multicellular tumoroids after 7 days of culture. In situ modification of the synthetic matrix via covalent tagging of cell adhesive RGD peptide induced tumoroid decompaction and the development of cellular protrusions. RGD tagging did not compromise the overall cell viability, nor did it induce cell apoptosis. In response to increased matrix adhesiveness, DU145 cells dynamically loosen cell-cell adhesion and strengthen cell-matrix interactions to promote an invasive phenotype. Characterization of the 3D cultures by immunocytochemistry and gene expression analyses demonstrated that cells invaded into the matrix via a mesenchymal like migration, with upregulation of major mesenchymal markers, and down regulation of epithelial markers. The tumoroids formed cortactin positive invadopodia like structures, indicating active matrix remodeling. Overall, the engineered tumor model can be utilized to identify potential molecular targets and test pharmacological inhibitors, thereby accelerating the design of innovative strategies for cancer therapeutics.

Recent developments in natural biopolymer based drug delivery systems

Targeted delivery of drug molecules to diseased sites is a great challenge in pharmaceutical and biomedical sciences. Fabrication of drug delivery systems (DDS) to target and/or diagnose sick cells is an effective means to achieve good therapeutic results along with a minimal toxicological impact on healthy cells. Biopolymers are becoming an important class of materials owing to their biodegradability, good compatibility, non-toxicity, non-immunogenicity, and long blood circulation time and high drug loading ratio for both macros as well as micro-sized drug molecules. This review summarizes the recent trends in biopolymer-based DDS, forecasting their broad future clinical applications. Cellulose chitosan, starch, silk fibroins, collagen, albumin, gelatin, alginate, agar, proteins and peptides have shown potential applications in DDS. A range of synthetic techniques have been reported to design the DDS and are discussed in the current study which is being successfully employed in ocular, dental, transdermal and intranasal delivery systems. Different formulations of DDS are also overviewed in this review article along with synthesis techniques employed for designing the DDS. The possibility of these biopolymer applications points to a new route for creating unique DDS with enhanced therapeutic qualities for scaling up creative formulations up to the clinical level.

Applications of Stimuli-Responsive Hydrogels in Bone and Cartilage Regeneration

Bone and cartilage regeneration is an area of tremendous interest and need in health care. Tissue engineering is a potential strategy for repairing and regenerating bone and cartilage defects. Hydrogels are among the most attractive biomaterials in bone and cartilage tissue engineering, mainly due to their moderate biocompatibility, hydrophilicity, and 3D network structure. Stimuli-responsive hydrogels have been a hot topic in recent decades. They can respond to external or internal stimulation and are used in the controlled delivery of drugs and tissue engineering. This review summarizes current progress in the use of stimuli-responsive hydrogels in bone and cartilage regeneration. The challenges, disadvantages, and future applications of stimuli-responsive hydrogels are briefly described.

Hydrogels from TEMPO-Oxidized Nanofibrillated Cellulose Support In Vitro Cultivation of Encapsulated Human Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are the most prominent type of adult stem cells for clinical applications. Three-dimensional (3D) cultivation of MSCs in biomimetic hydrogels provides a more physiologically relevant cultivation microenvironment for in vitro testing and modeling, thus overcoming the limitations of traditional planar cultivation methods. Cellulose nanofibers are an excellent candidate biomaterial for synthesis of hydrogels for this application, due to their biocompatibility, tunable properties, availability, and low cost. Herein, we demonstrate the capacity of hydrogels prepared from 2,2,6,6-tetramethylpiperidine-1-oxyl -oxidized and subsequently individualized cellulose-nanofibrils to support physiologically relevant 3D in vitro cultivation of human MSCs at low solid contents (0.2-0.5 wt %). Our results show that MSCs can spread, proliferate, and migrate inside the cellulose hydrogels, while the metabolic activity and proliferative capacity of the cells as well as their morphological characteristics benefit more in the lower bulk cellulose concentration hydrogels.

Interplay between biomaterials and the immune system: Challenges and opportunities in regenerative medicine

The use of biomaterials for tissue engineering and regenerative medicine applications has increased dramatically over recent years. However, the clinical uptake of a wide variety of biomaterials remains limited due to adverse effects commonly exhibited by patients, which are caused by the host immune response. Despite this, current in vitro evaluation standards (ISO-10993) for assessing the host response to biomaterials have limitations in predicting the likelihood of in vivo biomaterial acceptance. Furthermore, endotoxin contamination of biomaterials is rarely considered, despite its ability to significantly affect the performance of biomaterials and engineered tissues. This review highlights the importance of the immune response to biomaterials and discusses existing challenges and opportunities in the development and standardised assessment of the immune response to biomaterials, including the importance of endotoxin levels. In addition, the properties of biomaterials that impact the host immune response and the exploitation of immunomodulatory biomaterials in regenerative medicine are explored. Finally, a standardised in vitro pathway of evaluating the immune response to biomaterials (hydrogels) and their regenerative potential is proposed, aiming to ensure safety and consistency, while reducing costs and the use of animals in the biomaterials research for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: This review presents a critical analysis of the role of the interactions between the immune system and biomaterials in determining the therapeutic success of biomaterial-based approaches. No such review addressing the lack of understanding of biomaterial-immune system interactions during the developmental and pre-clinical stages of biomaterials, including the impact of the endotoxin levels of biomaterials on the immune response, is published. As there is a lack of in vitro regulations to evaluate the immune response to biomaterials, a standardised in vitro pathway to evaluate the immune response to biomaterials (hydrogels) and their immunomodulatory and regenerative potential for use in tissue engineering/regenerative medicine applications is presented. The aim of the proposed pathway of biomaterial evaluation is to ensure safety and consistency in the biomaterials research community, while reducing costs and animal use (through the concept of the 3R's - reduction, refinement, and replacement of animals).

Bone Formation on Murine Cranial Bone by Injectable Cross-Linked Hyaluronic Acid Containing Nano-Hydroxyapatite and Bone Morphogenetic Protein

New injection-type bone-forming materials are desired in dental implantology. In this study, we added nano-hydroxyapatite (nHAp) and bone morphogenetic protein (BMP) to cross-linkable thiol-modified hyaluronic acid (tHyA) and evaluated its usefulness as an osteoinductive injectable material using an animal model. The sol (ux-tHyA) was changed to a gel (x-tHyA) by mixing with a cross-linker. We prepared two sol−gel (SG) material series, that is, x-tHyA + BMP with and without nHAp (SG I) and x-tHyA + nHAp with and without BMP (SG II). SG I materials in the sol stage were injected into the cranial subcutaneous connective tissues of mice, followed by in vivo gelation, while SG II materials gelled in Teflon rings were surgically placed directly on the cranial bones of rats. The animals were sacrificed 8 weeks after implantation, followed by X-ray analysis and histological examination. The results revealed that bone formation occurred at a high rate (>70%), mainly as ectopic bone in the SG I tests in mouse cranial connective tissues, and largely as bone augmentation in rat cranial bones in the SG II experiments when x-tHyA contained both nHAp and BMP. The prepared x-tHyA + nHAp + BMP SG material can be used as an injection-type osteoinductive bone-forming material. Sub-periosteum injection was expected.

Pushing the Natural Frontier: Progress on the Integration of Biomaterial Cues toward Combinatorial Biofabrication and Tissue Engineering

The engineering of fully functional, biological-like tissues requires biomaterials to direct cellular events to a near-native, 3D niche extent. Natural biomaterials are generally seen as a safe option for cell support, but their biocompatibility and biodegradability can be just as limited as their bioactive/biomimetic performance. Furthermore, integrating different biomaterial cues and their final impact on cellular behavior is a complex equation where the outcome might be very different from the sum of individual parts. This review critically analyses recent progress on biomaterial-induced cellular responses, from simple adhesion to more complex stem cell differentiation, looking at the ever-growing possibilities of natural materials modification. Starting with a discussion on native material formulation and the inclusion of cell-instructive cues, the roles of shape and mechanical stimuli, the susceptibility to cellular remodeling, and the often-overlooked impact of cellular density and cell-cell interactions within constructs, are delved into. Along the way, synergistic and antagonistic combinations reported in vitro and in vivo are singled out, identifying needs and current lessons on the development of natural biomaterial libraries to solve the cell-material puzzle efficiently. This review brings together knowledge from different fields envisioning next-generation, combinatorial biomaterial development toward complex tissue engineering.

Pharmacological intervention of the FGF-PTH axis as a potential therapeutic for craniofacial ciliopathies

Ciliopathies represent a disease class characterized by a broad range of phenotypes including polycystic kidneys and skeletal anomalies. Ciliopathic skeletal phenotypes are among the most common and most difficult to treat due to a poor understanding of the pathological mechanisms leading to disease. Using an avian model (talpid2) for a human ciliopathy with both kidney and skeletal anomalies (Orofaciodigital syndrome 14), we identified disruptions in the FGF23-PTH axis that resulted in reduced calcium uptake in the developing mandible and subsequent micrognathia. While pharmacological intervention with the FDA-approved pan-FGFR inhibitor AZD4547 alone rescued expression of the FGF target Sprouty2, it did not significantly rescue micrognathia. In contrast, treatment with a cocktail of AZD4547 and Teriparatide acetate, a PTH agonist and FDA-approved treatment for osteoporosis, resulted in a molecular, cellular, and phenotypic rescue of ciliopathic micrognathia in talpid2 mutants. Together, these data provide novel insight into pathological molecular mechanisms associated with ciliopathic skeletal phenotypes and a potential therapeutic strategy for a pleiotropic disease class with limited to no treatment options.

Tuning Polymer Hydrophilicity to Regulate Gel Mechanics and Encapsulated Cell Morphology

Mechanically tunable hydrogels are attractive platforms for 3D cell culture, as hydrogel stiffness plays an important role in cell behavior. Traditionally, hydrogel stiffness has been controlled through altering either the polymer concentration or the stoichiometry between crosslinker reactive groups. Here, an alternative strategy based upon tuning the hydrophilicity of an elastin-like protein (ELP) is presented. ELPs undergo a phase transition that leads to protein aggregation at increasing temperatures. It is hypothesized that increasing this transition temperature through bioconjugation with azide-containing molecules of increasing hydrophilicity will allow direct control of the resulting gel stiffness by making the crosslinking groups more accessible. These azide-modified ELPs are crosslinked into hydrogels with bicyclononyne-modified hyaluronic acid (HA-BCN) using bioorthogonal, click chemistry, resulting in hydrogels with tunable storage moduli (100-1000 Pa). Human mesenchymal stromal cells (hMSCs), human umbilical vein endothelial cells (HUVECs), and human neural progenitor cells (hNPCs) are all observed to alter their cell morphology when encapsulated within hydrogels of varying stiffness. Taken together, the use of protein hydrophilicity as a lever to tune hydrogel mechanical properties is demonstrated. These hydrogels have tunable moduli over a stiffness range relevant to soft tissues, support the viability of encapsulated cells, and modify cell spreading as a consequence of gel stiffness.

Bone Augmentation for Implant Placement: Recent Advances

There are various advancements in biomaterials and methods for bone augmentation. This article aims to review the recent advances in bone augmentation for dental implants. Relevant articles on bone augmentation for dental implants were searched in PubMed/Medline, Scopus, Google Scholar, and Science Direct published in English literature published between January 1996 and March 2021. Relevant studies on bone grafts for dental implants were included and critically analyzed in this review. Various biomaterials can be used to augment bone for implant placement. Each graft procedure has advantages and disadvantages in each clinical application and needs to choose the graft material with a high success rate and less morbidity.

Recent advances in smart stimuli-responsive biomaterials for bone therapeutics and regeneration

Bone defects combined with tumors, infections, or other bone diseases are challenging in clinical practice. Autologous and allogeneic grafts are two main traditional remedies, but they can cause a series of complications. To address this problem, researchers have constructed various implantable biomaterials. However, the original pathological microenvironment of bone defects, such as residual tumors, severe infection, or other bone diseases, could further affect bone regeneration. Thus, the rational design of versatile biomaterials with integrated bone therapy and regeneration functions is in great demand. Many strategies have been applied to fabricate smart stimuli-responsive materials for bone therapy and regeneration, with stimuli related to external physical triggers or endogenous disease microenvironments or involving multiple integrated strategies. Typical external physical triggers include light irradiation, electric and magnetic fields, ultrasound, and mechanical stimuli. These stimuli can transform the internal atomic packing arrangements of materials and affect cell fate, thus enhancing bone tissue therapy and regeneration. In addition to the external stimuli-responsive strategy, some specific pathological microenvironments, such as excess reactive oxygen species and mild acidity in tumors, specific pH reduction and enzymes secreted by bacteria in severe infection, and electronegative potential in bone defect sites, could be used as biochemical triggers to activate bone disease therapy and bone regeneration. Herein, we summarize and discuss the rational construction of versatile biomaterials with bone therapeutic and regenerative functions. The specific mechanisms, clinical applications, and existing limitations of the newly designed biomaterials are also clarified.

Design and clinical application of injectable hydrogels for musculoskeletal therapy

Musculoskeletal defects are an enormous healthcare burden and source of pain and disability for individuals. With an aging population, the proportion of individuals living with these medical indications will increase. Simultaneously, there is pressure on healthcare providers to source efficient solutions, which are cheaper and less invasive than conventional technology. This has led to an increased research focus on hydrogels as highly biocompatible biomaterials that can be delivered through minimally invasive procedures. This review will discuss how hydrogels can be designed for clinical translation, particularly in the context of the new European Medical Device Regulation (MDR). We will then do a deep dive into the clinically used hydrogel solutions that have been commercially approved or have undergone clinical trials in Europe or the United States. We will discuss the therapeutic mechanism and limitations of these products. Due to the vast application areas of hydrogels, this work focuses only on treatments of cartilage, bone, and the nucleus pulposus. Lastly, the main steps toward clinical translation of hydrogels as medical devices are outlined. We suggest a framework for how academics can assist small and medium MedTech enterprises conducting the initial clinical investigation and post‐market clinical follow‐up required in the MDR. It is evident that the successful translation of hydrogels is governed by acquiring high‐quality pre‐clinical and clinical data confirming the device mechanism of action and safety.

Enhanced osteogenic effect in reduced BMP-2 doses with siNoggin transfected pre-osteoblasts in 3D silk scaffolds

Bone morphogenetic proteins (BMPs), especially BMP-2, are being increasingly used in bone tissue engineering due to its osteo-inductive effects. Although recombinant human BMP-2 (rhBMP-2) was approved by Food and Drug Administration (FDA) to use for bone repair, its high doses cause undesired side effects. In order to reduce the BMP-2 dose for enhanced osteogenic differentiation, in this study we decided to suppress the synthesis of Noggin protein, the primary antagonist of BMP-2, on the MC3T3-E1 cells using Noggin targeted small interfering RNA (siRNA). Unlike other studies, Noggin siRNA (siNoggin) transfected cells were seeded on silk scaffolds, and osteogenic differentiation was investigated for a long-term period (21 days) with MTT, qPCR, SEM/EDS, and histological analysis. Besides, siNoggin transfected MC3T3-E1 cells were evaluated as a new cell source for tissue engineering studies. It was determined that Nog gene expression was suppressed in the siNoggin group and Ocn gene expression increased 5-fold compared to the control group (*p < 0.05). The osteogenic effect of BMP-2 was clearly observed in siNoggin transfected cells. According to the SEM/EDS analysis, the siNoggin group has mineral structures clustered on cells, which contain intense Ca and P elements. Histological staining showed that the siNoggin group has a more intense mineralized area than that of the control group. In conclusion, this study indicated that Noggin silencing by siRNA induces osteogenic differentiation in reduced BMP-2 doses for scaffold-based bone regeneration. This non-gene integration strategy has as a safe therapeutic potential to enhance tissue regeneration.

Exploiting maleimide-functionalized hyaluronan hydrogels to test cellular responses to physical and biochemical stimuli

Currentin vitrothree-dimensional (3D) models of liver tissue have been limited by the inability to study the effects of specific extracellular matrix (ECM) components on cell phenotypes. This is in part due to limitations in the availability of chemical modifications appropriate for this purpose. For example, hyaluronic acid (HA), which is a natural ECM component within the liver, lacks key ECM motifs (e.g. arginine-glycine-aspartic acid (RGD) peptides) that support cell adhesion. However, the addition of maleimide (Mal) groups to HA could facilitate the conjugation of ECM biomimetic peptides with thiol-containing end groups. In this study, we characterized a new crosslinkable hydrogel (i.e. HA-Mal) that yielded a simplified ECM-mimicking microenvironment supportive of 3D liver cell culture. We then performed a series of experiments to assess the impact of physical and biochemical signaling in the form of RGD peptide incorporation and transforming growth factorß(TGF-ß) supplementation, respectively, on hepatic functionality. Hepatic stellate cells (i.e. LX-2) exhibited increased cell-matrix interactions in the form of cell spreading and elongation within HA-Mal matrices containing RGD peptides, enabling physical adhesions, whereas hepatocyte-like cells (HepG2) had reduced albumin and urea production. We further exposed the encapsulated cells to soluble TGF-ßto elicit a fibrosis-like state. In the presence of TGF-ßbiochemical signals, LX-2 cells became activated and HepG2 functionality significantly decreased in both RGD-containing and RGD-free hydrogels. Altogether, in this study we have developed a hydrogel biomaterial platform that allows for discrete manipulation of specific ECM motifs within the hydrogel to better understand the roles of cell-matrix interactions on cell phenotype and overall liver functionality.

Systematic optimization of visible light-induced crosslinking conditions of gelatin methacryloyl (GelMA)

Gelatin methacryloyl (GelMA) is one of the most widely used photo-crosslinkable biopolymers in tissue engineering. In in presence of an appropriate photoinitiator, the light activation triggers the crosslinking process, which provides shape fidelity and stability at physiological temperature. Although ultraviolet (UV) has been extensively explored for photo-crosslinking, its application has been linked to numerous biosafety concerns, originated from UV phototoxicity. Eosin Y, in combination with TEOA and VC, is a biosafe photoinitiation system that can be activated via visible light instead of UV and bypasses those biosafety concerns; however, the crosslinking system needs fine-tuning and optimization. In order to systematically optimize the crosslinking conditions, we herein independently varied the concentrations of Eosin Y [(EY)], triethanolamine (TEOA), vinyl caprolactam (VC), GelMA precursor, and crosslinking times and assessed the effect of those parameters on the properties the hydrogel. Our data showed that except EY, which exhibited an optimal concentration (~ 0.05 mM), increasing [TEOA], [VA], [GelMA], or crosslinking time improved mechanical (tensile strength/modulus and compressive modulus), adhesion (lap shear strength), swelling, biodegradation properties of the hydrogel. However, increasing the concentrations of crosslinking reagents ([TEOA], [VA], [GelMA]) reduced cell viability in 3-dimensional (3D) cell culture. This study enabled us to optimize the crosslinking conditions to improve the properties of the GelMA hydrogel and to generate a library of hydrogels with defined properties essential for different biomedical applications.

Bone Regeneration Using MMP-Cleavable Peptides-Based Hydrogels

Accumulating evidence has suggested the significant potential of chemically modified hydrogels in bone regeneration. Despite the progress of bioactive hydrogels with different materials, structures and loading cargoes, the desires from clinical applications have not been fully validated. Multiple biological behaviors are orchestrated precisely during the bone regeneration process, including bone marrow mesenchymal stem cells (BMSCs) recruitment, osteogenic differentiation, matrix calcification and well-organized remodeling. Since matrix metalloproteinases play critical roles in such bone metabolism processes as BMSC commitment, osteoblast survival, osteoclast activation matrix calcification and microstructure remodeling, matrix metalloproteinase (MMP) cleavable peptides-based hydrogels could respond to various MMP levels and, thus, accelerate bone regeneration. In this review, we focused on the MMP-cleavable peptides, polymers, functional modification and crosslinked reactions. Applications, perspectives and limitations of MMP-cleavable peptides-based hydrogels for bone regeneration were then discussed.

Collagen- and hyaluronic acid-based hydrogels and their biomedical applications

Hydrogels have been widely investigated in biomedical fields due to their similar physical and biochemical properties to the extracellular matrix (ECM). Collagen and hyaluronic acid (HA) are the main components of the ECM in many tissues. As a result, hydrogels prepared from collagen and HA hold inherent advantages in mimicking the structure and function of the native ECM. Numerous studies have focused on the development of collagen and HA hydrogels and their biomedical applications. In this extensive review, we provide a summary and analysis of the sources, features, and modifications of collagen and HA. Specifically, we highlight the fabrication, properties, and potential biomedical applications as well as promising commercialization of hydrogels based on these two natural polymers.

Structurally Dynamic Hydrogels for Biomedical Applications: Pursuing a Fine Balance between Macroscopic Stability and Microscopic Dynamics

Owing to their unique chemical and physical properties, hydrogels are attracting increasing attention in both basic and translational biomedical studies. Although the classical hydrogels with static networks have been widely reported for decades, a growing number of recent studies have shown that structurally dynamic hydrogels can better mimic the dynamics and functions of natural extracellular matrix (ECM) in soft tissues. These synthetic materials with defined compositions can recapitulate key chemical and biophysical properties of living tissues, providing an important means to understanding the mechanisms by which cells sense and remodel their surrounding microenvironments. This review begins with the overall expectation and design principles of dynamic hydrogels. We then highlight recent progress in the fabrication strategies of dynamic hydrogels including both degradation-dependent and degradation-independent approaches, followed by their unique properties and use in biomedical applications such as regenerative medicine, drug delivery, and 3D culture. Finally, challenges and emerging trends in the development and application of dynamic hydrogels are discussed.

A Gelatin-Hyaluronic Acid Double Cross-Linked Hydrogel for Regulating the Growth and Dual Dimensional Cartilage Differentiation of Bone Marrow Mesenchymal Stem Cells

Owing to its unique physiochemical properties similar to the extracellular matrix (ECM), three-dimensional (3D) crosslinked hydrogels are widely studied materials for tissue engineering. In this study, to mimic the ECM microenvironment, a two-step covalent cross-linking with hyaluronic acid and gelatin was performed to form an interpenetrating polymer network structure. Gelatin as the first network greatly improved the mechanical strength of the hydrogels, while a hyaluronic acid network as the second network improved the tenacity and biological activity. Compared with a single network hydrogel, the interpenetrating hydrogel system can further regulate the mechanical properties of the hydrogel by adjusting the ratio of the two components, thereby changing the proliferation, activity, and direction of cartilage differentiation of bone marrow mesenchymal stem cells (BMSCs). Not only that, with two culture methods for BMSCs on the surface and 3D wrapped in the double cross-linked hydrogels, they exhibited their potential to induce BMSCs to cartilage differentiation under the condition of 3D encapsulation of BMSCs and contact with BMSCs on its surface. As a scaffold material for cartilage tissue engineering, this double cross-linked hydrogel demonstrated its high feasibility and applicability in delivering BMSCs in vivo and repairing defects.

Electrospun hydrogels for dynamic culture systems: advantages, progress, and opportunities

The extracellular matrix (ECM) is a water-swollen, tissue-specific material environment in which biophysiochemical signals are organized and influence cell behaviors. Electrospun nanofibrous substrates have been pursued as platforms for tissue engineering and cell studies that recapitulate features of the native ECM, in particular its fibrous nature. In recent years, progress in the design of electrospun hydrogel systems has demonstrated that molecular design also enables unique studies of cellular behaviors. In comparison to the use of hydrophobic polymeric materials, electrospinning hydrophilic materials that crosslink to form hydrogels offer the potential to achieve the water-swollen, nanofibrous characteristics of endogenous ECM. Although electrospun hydrogels require an additional crosslinking step to stabilize the fibers (allowing fibers to swell with water instead of dissolving) in comparison to their hydrophobic counterparts, researchers have made significant advances in leveraging hydrogel chemistries to incorporate biochemical and dynamic functionalities within the fibers. Consequently, dynamic biophysical and biochemical properties can be engineered into hydrophilic nanofibers that would be difficult to engineer in hydrophobic systems without strategic and sometimes intensive post-processing techniques. This Review describes common methodologies to control biophysical and biochemical properties of both electrospun hydrophobic and hydrogel nanofibers, with an emphasis on highlighting recent progress using hydrogel nanofibers with engineered dynamic complexities to develop culture systems for the study of biological function, dysfunction, development, and regeneration.

Design Challenges in Polymeric Scaffolds for Tissue Engineering

Numerous surgical procedures are daily performed worldwide to replace and repair damaged tissue. Tissue engineering is the field devoted to the regeneration of damaged tissue through the incorporation of cells in biocompatible and biodegradable porous constructs, known as scaffolds. The scaffolds act as host biomaterials of the incubating cells, guiding their attachment, growth, differentiation, proliferation, phenotype, and migration for the development of new tissue. Furthermore, cellular behavior and fate are bound to the biodegradation of the scaffold during tissue generation. This article provides a critical appraisal of how key biomaterial scaffold parameters, such as structure architecture, biochemistry, mechanical behavior, and biodegradability, impart the needed morphological, structural, and biochemical cues for eliciting cell behavior in various tissue engineering applications. Particular emphasis is given on specific scaffold attributes pertaining to skin and brain tissue generation, where further progress is needed (skin) or the research is at a relatively primitive stage (brain), and the enumeration of some of the most important challenges regarding scaffold constructs for tissue engineering.

Matrix Metalloproteinase Inspired Therapeutic Strategies for Bone Diseases

Matrix Metalloproteinases (MMPs), as a family of zinc-containing enzymes, show the function of decomposing Extracellular Matrix (ECM) and participate in the physiological processes of cell migration, growth, inflammation, and metabolism. Clinical and experimental studies have indicated that MMPs play an essential role in tissue injury and repair as well as tumor diagnosis, metastasis, and prognosis. An increasing number of researchers have paid attention to their functions and mechanisms in bone health and diseases. The present review focuses on MMPs-inspired therapeutic strategies for the treatment of bone-related diseases. We introduce the role of MMPs in bone diseases, highlight the MMPs-inspired therapeutic options, and posit MMPs as a trigger for smart cell/drug delivery.

Biomaterials for protein delivery for complex tissue healing responses

Tissue repair requires a complex cascade of events mediated by a variety of cells, proteins, and matrix molecules; however, the healing cascade can be easily disrupted by numerous factors, resulting in impaired tissue regeneration. Recent advances in biomaterials for tissue regeneration have increased the ability to tailor the delivery of proteins and other biomolecules to injury sites to restore normal healing cascades and stimulate robust tissue repair. In this review, we discuss the evolution of the field toward creating biomaterials that precisely control protein delivery to stimulate tissue regeneration, with a focus on addressing complex and dynamic injury environments. We highlight biomaterials that leverage different mechanisms to deliver and present proteins involved in healing cascades, tissue targeting and mimicking strategies, materials that can be triggered by environmental cues, and integrated strategies that combine multiple biomaterial properties to improve protein delivery. Improvements in biomaterial design to address complex injury environments will expand our understanding of both normal and aberrant tissue repair processes and ultimately provide a better standard of patient care.

Hyaluronic Acid Hydrogel with Adjustable Stiffness for Mesenchymal Stem Cell 3D Culture via Related Molecular Mechanisms to Maintain Stemness and Induce Cartilage Differentiation

The stemness and differentiation characteristics of bone marrow mesenchymal stem cells (BMSCs) in three-dimensional (3D) culture are of great significance for stem cell therapy and cartilage tissue engineering repair. Moreover, due to their mechanical sensitivity, scaffold materials play important roles in various cell behaviors in 3D culture. In this study, the mechanical strength of hydrogel scaffolds was adjusted by changing the molecular weight of hyaluronic acid (HA). It was proven that BMSCs in a low-strength hydrogel could maintain stemness properties by activating the Wnt/β-catenin pathway for 1 week, while the high-molecular-weight hydrogel with a higher mechanical strength had the potential to promote the direction of cartilage differentiation of BMSCs by opening transient receptor potential vanilloid 4 (TRPV4)/Ca²⁺ molecular channels, also increasing the expression of type II collagen and SOX9 in BMSCs. This research has a certain reference value for the design of biomaterials for BMSCs' delivery in vivo, as well as the formulation of cartilage repair drug delivery programs based on molecular mechanisms.

A tough act to follow: collagen hydrogel modifications to improve mechanical and growth factor loading capabilities

Collagen hydrogels are among ​the most well-studied platforms for drug delivery and in situ tissue engineering, thanks to their low cost, low immunogenicity, versatility, biocompatibility, and similarity to the natural extracellular matrix (ECM). Despite collagen being largely responsible for the tensile properties of native connective tissues, collagen hydrogels have relatively low mechanical properties in the absence of covalent cross-linking. This is particularly problematic when attempting to regenerate stiffer and stronger native tissues such as bone. Furthermore, in contrast to hydrogels based on ECM proteins such as fibronectin, collagen hydrogels do not have any growth factor (GF)-specific binding sites and often cannot sequester physiological (small) amounts of the protein. GF binding and in situ presentation are properties that can aid significantly in the tissue regeneration process by dictating cell fate without causing adverse effects such as malignant tumorigenic tissue growth. To alleviate these issues, researchers have developed several strategies to increase the mechanical properties of collagen hydrogels using physical or chemical modifications. This can expand the applicability of collagen hydrogels to tissues subject to a continuous load. GF delivery has also been explored, mathematically and experimentally, through the development of direct loading, chemical cross-linking, electrostatic interaction, and other carrier systems. This comprehensive article explores the ways in which these parameters, mechanical properties and GF delivery, have been optimized in collagen hydrogel systems ​and examines their in vitro or in vivo biological effect. This article can, therefore, be a useful tool to streamline future studies in the field, by pointing researchers into the appropriate direction according to their collagen hydrogel design requirements.

A Rapid Crosslinkable Maleimide-Modified Hyaluronic Acid and Gelatin Hydrogel Delivery System for Regenerative Applications

Hydrogels have played a significant role in many applications of regenerative medicine and tissue engineering due to their versatile properties in realizing design and functional requirements. However, as bioengineered solutions are translated towards clinical application, new hurdles and subsequent material requirements can arise. For example, in applications such as cell encapsulation, drug delivery, and biofabrication, in a clinical setting, hydrogels benefit from being comprised of natural extracellular matrix-based materials, but with defined, controllable, and modular properties. Advantages for these clinical applications include ultraviolet light-free and rapid polymerization crosslinking kinetics, and a cell-friendly crosslinking environment that supports cell encapsulation or in situ crosslinking in the presence of cells and tissue. Here we describe the synthesis and characterization of maleimide-modified hyaluronic acid (HA) and gelatin, which are crosslinked using a bifunctional thiolated polyethylene glycol (PEG) crosslinker. Synthesized products were evaluated by proton nuclear magnetic resonance (NMR), ultraviolet visibility spectrometry, size exclusion chromatography, and pH sensitivity, which confirmed successful HA and gelatin modification, molecular weights, and readiness for crosslinking. Gelation testing both by visual and NMR confirmed successful and rapid crosslinking, after which the hydrogels were characterized by rheology, swelling assays, protein release, and barrier function against dextran diffusion. Lastly, biocompatibility was assessed in the presence of human dermal fibroblasts and keratinocytes, showing continued proliferation with or without the hydrogel. These initial studies present a defined, and well-characterized extracellular matrix (ECM)-based hydrogel platform with versatile properties suitable for a variety of applications in regenerative medicine and tissue engineering.

Including fluorescent nanoparticle probes within injectable gels for remote strain measurements and discrimination between compression and tension

The ability to remotely and non-invasively monitor and measure the strain within injectable gels used to augment soft tissue is highly desirable. Such information could enable real-time monitoring of gel performance and bespoke gel design. We report progress towards this goal using two fluorescent particle probe systems included within two different injectable gels. The two injectable gels have been previously studied in the contexts of intervertebral disc repair and stretchable gels for cartilage repair. The two fluorophore particle probes are blue or near-infrared (NIR) emitting and are present at very low concentrations. The normalised photoluminescence (PL) intensity from the blue emitting probe is shown to equal the compressive deformation ratio of the gels. Furthermore, the normalised ratio of the PL intensities for the blue and NIR probes varies linearly with deformation ratio over a wide range (from 0.2 to 3.0) with a seamless transition from compression to tension. Hence, PL can discriminate between compression and tension. The new approach established here should apply to other gels and enable remote detection of whether a gel is being compressed or stretched as well as the extent. This study may provide an important step towards remotely and minimally invasively measuring the strain experienced by load-supporting gels in vivo.

Role of Hydrogels in Bone Tissue Engineering: How Properties Shape Regeneration

Bone defect that resulted from trauma, tumors, and other reasons is believed as a common clinical problem, which exists mainly in post-traumatic healing. Additionally, autologous/allogeneic transplantation, bone tissue engineering attracts increasing attention due to the existing problem of the limited donor. The applications of biomaterials can be considered as a rising and promising strategy for bone regeneration. Especially, hydrogel is featured with hydrophilic characteristic, good biocompatibility, and porous structure, which shows unique properties for bone regeneration. The main properties of hydrogel such as surface property, adhesive property, mechanical property, porosity, and degradation property, generally present influences on the migration, proliferation, and differentiation of mesenchymal stem cells exclusively or in combination, which consequently affect the regeneration of bones. This review mainly focuses on the theme: "how properties of hydrogel shape bone regeneration." Moreover, the latest progress achieved in the above mentioned direction is further discussed. Despite the fascinating advances researchers have made, certain potential challenges continue to exist in the research field, which need to be addressed for accelerating the clinical translation of hydrogel in bone regeneration.

Microfluidic Encapsulation of Single Cells by Alginate Microgels Using a Trigger-Gellified Strategy

Microfluidics-based alginate microgels have shown great potential to encapsulate cells in a high-throughput and controllable manner. However, cell viability and biological functions are substantially compromised due to the harsh conditions for gelation, which remains a major challenge for cell encapsulation. Herein, we presented an efficient and biocompatible method by on-chip triggered gelation to generate microfluidic alginate microgels for single-cell encapsulation. Two calcium complexes of calcium–ethylenediaminetetraacetic acid (Ca-EDTA) and calcium–nitrilotriacetic (Ca-NTA) as crosslinkers for triggered gelation of alginate were compared and investigated for feasible application. By triggered release of Ca²⁺ ions from the calcium complex via adding acetic acid in the oil phase, the alginate precursor in the aqueous droplets can be crosslinked to form alginate microgels. Although using Ca-EDTA and Ca-NTA both achieved on-chip gelation, Ca-NTA led to significantly higher cell viability since the dissociation of Ca²⁺ ions from Ca-NTA can be obtained using less concentration of acid compared to Ca-EDTA. We further demonstrated the functionality of encapsulated mesenchymal stem cells (MSCs) in alginate microgels prepared using Ca-NTA, as evidenced by the osteogenesis of encapsulated MSCs upon inductive culture. In summary, our study provided a biocompatible strategy to prepare alginate microgels for single-cell encapsulation which can be further used for applications in tissue engineering and cell therapies.

On-demand and tunable dual wavelength release of antibody using light-responsive hydrogels

There has been an increased interest in the use of protein therapeutics, especially antibodies, for the treatment of a variety of diseases due to their high specificity to tissues and biological pathways of interest. However, the use of antibodies can be hindered by physical aggregation, degradation, and diffusion when injected in vivo leading to the need for antibody-releasing depots for the controlled and localized delivery within tissues of interest. Here, we investigated photolabile hydrogel chemistries for creating on-demand and tunable antibody release profiles. Innovative, scalable synthetic procedures were established and applied for fabricating hydrogels with nitrobenzyl (NB) and coumarin (CMR) photolabile crosslinks that responded to clinically relevant doses of long-wavelength UV and short-wavelength visible light. This synthetic procedure includes a route to make a CMR linker possessing two functional handles at the same ring position with water-stable bonds. The photocleavage properties of NB and CMR crosslinked hydrogels were characterized, as well as their potential for translational studies by degradation through pig skin, a good human skin mimic. The mechanism of hydrogel degradation, bulk versus surface eroding, was determined to be dependent on the wavelength of light utilized and the molar absorptivity of the different photolabile linkers, providing a facile means for altering protein release upon hydrogel degradation. Further, the encapsulation and on-demand release of a model monoclonal antibody was demonstrated, highlighting the ability to control antibody release from these hydrogels through the application of light while retaining its bioactivity. In particular, the newly designed CMR hydrogels undergo surface erosion-based protein release using visible light, which is more commonly used clinically. Overall, this work establishes scalable syntheses and relevant pairings of formulation-irradiation conditions for designing on-demand and light-responsive material systems that provide controlled, tunable release of bioactive proteins toward addressing barriers to preclinical translation of light-based materials and ultimately improving therapeutic regimens.

Biomimetic Cell-Laden MeHA Hydrogels for the Regeneration of Cartilage Tissue

Methacrylated hyaluronic acid (MeHA) and chondroitin sulfate (CS)-biofunctionalized MeHA (CS-MeHA), were crosslinked in the presence of a matrix metalloproteinase 7 (MMP7)-sensitive peptide. The synthesized hydrogels were embedded with either human mesenchymal stem cells (hMSCs) or chondrocytes, at low concentrations, and subsequently cultured in a stem cell medium (SCM) or chondrogenic induction medium (CiM). The pivotal role of the synthesized hydrogels in promoting the expression of cartilage-related genes and the formation of neocartilage tissue despite the low concentration of encapsulated cells was assessed. It was found that hMSC-laden MeHA hydrogels cultured in an expansion medium exhibited a significant increase in the expression of chondrogenic markers compared to hMSCs cultured on a tissue culture polystyrene plate (TCPS). This favorable outcome was further enhanced for hMSC-laden CS-MeHA hydrogels, indicating the positive effect of the glycosaminoglycan binding peptide on the differentiation of hMSCs towards a chondrogenic phenotype. However, it was shown that an induction medium is necessary to achieve full span chondrogenesis. Finally, the histological analysis of chondrocyte-laden MeHA hydrogels cultured on an ex vivo osteochondral platform revealed the deposition of glycosaminoglycans (GAGs) and the arrangement of chondrocyte clusters in isogenous groups, which is characteristic of hyaline cartilage morphology.

The effect of BMP-mimetic peptide tethering bioinks on the differentiation of dental pulp stem cells (DPSCs) in 3D bioprinted dental constructs

The goal of this study was to use 3D bioprinting technology to create a bioengineered dental construct containing human dental pulp stem cells (hDPSCs). To accomplish this, we first developed a novel bone morphogenetic protein (BMP) peptide-tethering bioink formulation and examined its rheological properties, its printability, and the structural stability of the bioprinted construct. Second, we evaluated the survival and differentiation of hDPSCs in the bioprinted dental construct by measuring cell viability, proliferation, and gene expression, as well as histological and immunofluorescent analyses. Our results showed that the peptide conjugation into the gelatin methacrylate-based bioink formulation was successfully performed. We determined that greater than 50% of the peptides remained in the bioprinted construct after three weeks in vitro cell culture. Human DPSC viability was >90% in the bioprinted constructs immediately after the printing process. Alizarin Red staining showed that the BMP peptide construct group exhibited the highest calcification as compared to the growth medium, osteogenic medium, and non-BMP peptide construct groups. In addition, immunofluorescent and quantitative reverse transcription-polymerase chain reaction analyses showed robust expression of dentin sialophosphoprotein and osteocalcin in the BMP peptide dental constructs. Together, these results strongly suggested that BMP peptide-tethering bioink could accelerate the differentiation of hDPSCs in 3D bioprinted dental constructs.

Biomimetic hydrogels with spatial- and temporal-controlled chemical cues for tissue engineering

Biomimetic hydrogels have emerged as the most useful tissue engineering scaffold materials. Their versatile chemistry can recapitulate multiple physical and chemical features to integrate cells, scaffolds, and signaling molecules for tissue regeneration. Due to their highly hydrophilic nature hydrogels can recreate nutrient-rich aqueous environments for cells. Soluble regulatory molecules can be incorporated to guide cell proliferation and differentiation. Importantly, the controlled dynamic parameters and spatial distribution of chemical cues in hydrogel scaffolds are critical for cell-cell communication, cell-scaffold interaction, and morphogenesis. Herein, we review biomimetic hydrogels that provide cells with spatiotemporally controlled chemical cues as tissue engineering scaffolds. Specifically, hydrogels with temporally controlled growth factor-release abilities, spatially controlled conjugated bioactive molecules/motifs, and targeting delivery and reload properties for tissue engineering applications are discussed in detail. Examples of hydrogels that possess clinically favorable properties, such as injectability, self-healing ability, stimulus-responsiveness, and pro-remodeling features, are also covered.

Bioactive hydrogel coatings of complex substrates using diffusion-mediated redox initiation

Hydrogels have long been established as materials with tunable stiffness and chemistry that enable controlled cellular interactions. When applied as coatings, hydrogels can be used to introduce biofunctionality to medical devices with minimal effect on bulk properties. However, it remains challenging to uniformly apply hydrogel coatings to three dimensional geometries without substantially changing the manufacturing process and potentially affecting device function. Herein, we report a new redox-based crosslinking method for applying conformable hydrogel coatings with tunable thickness and chemistry. This new diffusion-mediated strategy of redox initiation and hydrogel crosslinking enabled coating of a variety of three dimensional substrates without changing the primary fabrication process. Following adsorption of the reducing agent to the construct, hydrogel coating thickness was readily controlled by immersion time with desorption and diffusion of the reducing agent initiating hydrogel crosslinking from the surface. The process was used to generate a range of hydrogel properties by varying the macromer molecular weight and concentration. In addition, we demonstrated that these coatings can be applied sequentially to generate multilayered constructs with distinct features. Finally, incorporation of proteins into the bulk of the hydrogel coating or as a final surface layer permitted the controlled introduction of bioactivity that supported cell attachment. This work provides a versatile method for assembling bioactive coatings with a simple post-fabrication process that is amenable to diverse geometric substrates and chemistries.

Advances of Stem Cell-Laden Hydrogels With Biomimetic Microenvironment for Osteochondral Repair

Osteochondral damage from trauma or osteoarthritis is a general joint disease that can lead to an increased social and economic burden in the modern society. The inefficiency of osteochondral defects is mainly due to the absence of suitable tissue-engineered substrates promoting tissue regeneration and replacing damaged areas. The hydrogels are becoming a promising kind of biomaterials for tissue regeneration. The biomimetic hydrogel microenvironment can be tightly controlled by modulating a number of biophysical and biochemical properties, including matrix mechanics, degradation, microstructure, cell adhesion, and intercellular interactions. In particular, advances in stem cell-laden hydrogels have offered new ideas for the cell therapy and osteochondral repair. Herein, the aim of this review is to underpin the importance of stem cell-laden hydrogels on promoting the development of osteochondral regeneration, especially in the field of manipulation of biomimetic microenvironment and utilization growth factors with various delivery methods.

Hyaluronic Acid Macromer Molecular Weight Dictates the Biophysical Properties and in Vitro Cellular Response to Semisynthetic Hydrogels

In situ-forming hydrogels present a promising approach for minimally invasive cell transplantation and tissue regeneration. Among prospective materials, hyaluronic acid (HyA) has displayed great potential, owing to its inherent biocompatibility, biodegradation, and ease of chemical modification. However, current studies in the literature use a broad range of HyA macromer molecular weights (MWs) from <100 kDa to 1 MDa with no consensus regarding an optimal MW for a specific application. We investigated the effects of different HyA macromer MWs on key biophysical properties of semisynthetic hydrogels, such as viscosity, gelation time, shear storage modulus, molecular diffusion, and degradation. Using higher-MW HyA macromers leads to quicker gelation times and stiffer, more stable hydrogels with smaller mesh sizes. Assessment of the potential for HyA hydrogels to support network formation by encapsulated vascular cells derived from human-induced pluripotent stem cells reveals key differences between HyA hydrogels dependent on macromer MW. These effects must be considered holistically to address the multifaceted, nonmonotonic nature of HyA MW on hydrogel behavior. Our study identified an intermediate HyA macromer MW of 500 kDa as providing optimal conditions for a readily injectable, in situ-forming hydrogel with appropriate biophysical properties to promote vascular cell spreading and sustain vascular network formation in vitro.

Dextran-based tube-guides for the regeneration of the rat sciatic nerve after neurotmesis injury

In this work, dextran-based nerve tube-guides were prepared, characterized and used in a standardized animal model of neurotmesis injury. Non-porous and porous transparent tube-guides were obtained by photocrosslinking of two co-macromonomers based on dextran and poly(ε-caprolactone) (PCL). Swelling capacity of the tube-guides ranged from 40-60% with no visible constriction of their inner diameter. In vitro hydrolytic degradation tests showed that the tube-guides maintained their structural integrity up to 6 months. The in vivo performance of the tube-guides was evaluated by entubulation of the rat sciatic nerve after a neurotmesis injury, with a 10 mm-gap between the nerve stumps. The results showed that the tube-guides were able to promote the regeneration of the nerve in a similar manner to what was observed with conventional techniques (nerve graft and end-to-end suture). Stereological analysis proved that nerve regeneration occurred, and both tube-guides presented fibre diameter and g-ratio closer to healthy sciatic nerves. The histomorphometric analysis of Tibialis anterior (TA) skeletal muscle showed decreased neurogenic atrophy in the porous tube-guides treated group, presenting measurements that are similar to the uninjured control.

A collagen mimetic peptide-modified hyaluronic acid hydrogel system with enzymatically mediated degradation for mesenchymal stem cell differentiation

We have successfully designed and synthesized a biomimetic hydrogel system with maleimide-modified hyaluronic acid (HA) as the backbone and conjugated it to the collagen mimetic peptide (GPO)₈-CG-RGDS. The matrix metalloproteinase (MMP)-sensitive peptide GCRDGPQGI↓WGQDRCG was the cross-linker. HA has high biocompatibility, low immunogenicity, and the capacity to interact with extracellular molecules. Recent studies have found that matrix metalloproteinases (MMPs) are involved in regulating the differentiation of bone mesenchymal stem cells and play a pivotal role in cartilage formation. (GPO)₈-CG-RGDS has a natural collagen partial structure that follows the (Gly-Xaa-Yaa)n sequence, which is controllable in quality and can mimic the structure and biological activity of natural collagen. We found that combining this CMP with a MMP-sensitive peptide may have the potential to induce the differentiation of BMSCs into cartilage and inhibit the hypertrophic phenotype during differentiation. This design allows HA hydrogels to not only bind RGD sequences but also graft other functional peptide sequences to achieve a highly flexible platform with potential for multiple biomedical applications.