The influence of hyaluronic acid hydrogel crosslinking density and macromolecular diffusivity on human MSC chondrogenesis and hypertrophy

Tuning the Degradation Rate of Alginate-Based Bioinks for Bioprinting Functional Cartilage Tissue

Negative foreign body responses following the in vivo implantation of bioprinted implants motivate the development of novel bioinks which can rapidly degrade with the formation of functional tissue, whilst still maintaining desired shapes post-printing. Here, we investigated the oxidation of alginate as a means to modify the degradation rate of alginate-based bioinks for cartilage tissue engineering applications. Raw and partially oxidized alginate (OA) were combined at different ratios (Alginate:OA at 100:0; 75:25; 50:50; 25:75; 0:100) to provide finer control over the rate of bioink degradation. These alginate blends were then combined with a temporary viscosity modifier (gelatin) to produce a range of degradable bioinks with rheological properties suitable for extrusion bioprinting. The rate of degradation was found to be highly dependent on the OA content of the bioink. Despite this high mass loss, the initially printed geometry was maintained throughout a 4 week in vitro culture period for all bioink blends except the 0:100 group. All bioink blends also supported robust chondrogenic differentiation of mesenchymal stem/stromal cells (MSCs), resulting in the development of a hyaline-like tissue that was rich in type II collagen and negative for calcific deposits. Such tuneable inks offer numerous benefits to the field of 3D bioprinting, from providing space in a controllable manner for new extracellular matrix deposition, to alleviating concerns associated with a foreign body response to printed material inks in vivo.

Dynamic Nanocomposite Microgel Assembly with Microporosity, Injectability, Tissue-Adhesion, and Sustained Drug Release Promotes Articular Cartilage Repair and Regeneration

Owing to the lack of blood vessels, nerves, and lymph, articular cartilage defect is difficult to self-repair. Although several cartilage tissue engineering products have been authorized for clinical use, there are still some problems such as large surgical wounds, weak adhesion with the host tissue, and the limited source of autologous chondrocytes. In this paper, a novel dynamic nanocomposite microgel assembly with excellent microporosity, injectability, tissue-adhesion, and sustained kartogenin (KGN) release is reported. Specifically, KGN-loaded cyclodextrin nanoparticles are synthesized through nanoemulsification and incorporated into bone marrow mesenchymal stem cell (BMSCs)-laden microgels via droplet-based microfluidics and photo-crosslinking, which are then bottom-up assembled via dynamic crosslinking between dopamine-modified hyaluronic acid and phenylboronic acid groups on microgel surface. Results reveal that the microgel assembly can avoid the cell endocytosis of nanoparticles, ensure the high BMSC viability during the regular cell culture, cryopreservation and injection process, promote the chondrogenic differentiation of BMSCs. In addition, animal expriment proves the newborn cartilages present the typical characteristics of articular cartilage. In brief, this microgel assembly not only offers convenience for clinical use (injectability, tissue adhesion) but also provides good microenvironments for chondrogenesis (controlled drug release, interconnected micropores), indicative of its promising application for cartilage repair and regeneration.

Incorporation of a Collagen-Binding Chondroitin Sulfate Molecule to a Collagen Type I and II Blend Hydrogel for Cartilage Tissue Engineering

Adding chondroitin sulfate (CS) to collagen scaffolds has been shown to improve the outcomes for articular cartilage tissue engineering. Instead of physical entrapment or chemical crosslinking of CS within a scaffold, this study investigated the use of CS with attached collagen-binding peptides (termed CS-SILY). This method better recapitulates the aspects of native cartilage while retaining CS within a collagen type I and II blend (Col I/II) hydrogel. CS retention, average fibril diameter, and mechanical properties were altered by varying the number of SILY peptides attached to the CS backbone. When mesenchymal stromal cells (MSCs) were encapsulated within the scaffolds, the addition of CS-SILY molecules resulted in higher sulfated glycosaminoglycan production, and these results suggest that CS-SILY promotes MSC differentiation into chondrocytes. Taken together, our study shows the promise of adding a CS-SILY molecule to a Col I/II hydrogel with encapsulated MSCs to promote cartilage repair.

Role of Canonical Wnt/β-Catenin Pathway in Regulating Chondrocytic Hypertrophy in Mesenchymal Stem Cell-Based Cartilage Tissue Engineering

In the past 3 decades, the cartilage repair potential of mesenchymal stromal cells, or mesenchymal stem cells (MSCs), has been widely examined in animal studies. Unfortunately, the phenotype and physical properties of MSC-derived cartilage tissue are not comparable to native hyaline cartilage. In particular, chondrocytic hypertrophy, a phenotype that is not observed in healthy hyaline cartilage, is concomitant with MSC chondrogenesis. Given that hypertrophic chondrocytes potentially undergo apoptosis or convert into osteoblasts, this undesired phenotype needs to be prevented or minimized before MSCs can be used to repair cartilage injuries in the clinic. In this review, we first provide an overview of chondrocytic hypertrophy and briefly summarize current methods for suppressing hypertrophy in MSC-derived cartilage. We then highlight recent progress on modulating the canonical Wnt/β-catenin pathway for inhibiting hypertrophy. Specially, we discuss the potential crosstalk between Wnt/β-catenin with other pathways in regulating hypertrophy. Lastly, we explore future perspectives to further understand the role of Wnt/β-catenin in chondrocytic hypertrophy.

Multiple Crosslinking Hyaluronic Acid Hydrogels with Improved Strength and 3D Printability

Hyaluronic acid (HA) hydrogel is preferred for biomedicine applications, as it possesses biodegradability, biocompatibility, and cell-regulated capacity as well as high hydration nature similar to the native extracellular matrix. However, HA hydrogel fabricated via a 3D printing technique often faces poor printing properties. In this study, maleiated sodium hyaluronate (MHA) with a high substituted degree of the acrylate group (i.e., 2.27) and thiolated sodium hyaluronate (SHHA) were synthesized. By blending these modified HAs, the MHA/SHHA hydrogels were prepared via pre-crosslinking through thiol-acrylate Michael addition and subsequently covalent crosslinking using thiol-acrylate and acrylate-acrylate photopolymerization mechanisms. Rheological properties, swelling behaviors, and mechanical properties can be modulated by altering the molar ratio of the thiol group and acrylate group. The results showed that the MHA/SHHA hydrogel precursors have rapidly gelling capacity and improved compressive strength. Based on these results, high-resolution hydrogel scaffolds with good structural stability were prepared by extrusion-based 3D printing. This HA hydrogel is cytocompatible and capable of supporting adherence of L929 cells, indicating its great potential for tissue engineering scaffolds.

Advances in 3D printing of composite scaffolds for the repairment of bone tissue associated defects

The conventional methods of using autografts and allografts for repairing defects in bone, the osteochondral bone and the cartilage tissue have many disadvantages, like donor site morbidity and shortage of donors. Moreover, only 30% of the implanted grafts are shown to be successful in treating the defects. Hence, exploring alternative techniques such as tissue engineering to treat bone tissue associated defects is promising as it eliminates the above-mentioned limitations. To enhance the mechanical and biological properties of the tissue engineered product, it is essential to fabricate the scaffold used in tissue engineering by the combination of various biomaterials. Three-dimensional (3D) printing, with its ability to print composite materials and with complex geometry seems to have a huge potential in scaffold fabrication technique for engineering bone associated tissues.This review summarizes the recent applications and future perspectives of 3D printing technologies in the fabrication of composite scaffolds used in bone, osteochondral and cartilage tissue engineering. Key developments in the field of 3D printing technologies involves the incorporation of various biomaterials and cells in printing composite scaffolds mimicking physiologically relevant complex geometry & gradient porosity. Much recently, the emerging trend of printing smart scaffolds which can respond to external stimulus such as temperature, pH and magnetic field, known as 4D printing is gaining immense popularity and can be considered as the future of 3D printing applications in the field of tissue engineering. This article is protected by copyright. All rights reserved.

Tethered TGF-β1 in a Hyaluronic Acid-Based Bioink for Bioprinting Cartilaginous Tissues

In 3D bioprinting for cartilage regeneration, bioinks that support chondrogenic development are of key importance. Growth factors covalently bound in non-printable hydrogels have been shown to effectively promote chondrogenesis. However, studies that investigate the functionality of tethered growth factors within 3D printable bioinks are still lacking. Therefore, in this study, we established a dual-stage crosslinked hyaluronic acid-based bioink that enabled covalent tethering of transforming growth factor-beta 1 (TGF-β1). Bone marrow-derived mesenchymal stromal cells (MSCs) were cultured over three weeks in vitro, and chondrogenic differentiation of MSCs within bioink constructs with tethered TGF-β1 was markedly enhanced, as compared to constructs with non-covalently incorporated TGF-β1. This was substantiated with regard to early TGF-β1 signaling, chondrogenic gene expression, qualitative and quantitative ECM deposition and distribution, and resulting construct stiffness. Furthermore, it was successfully demonstrated, in a comparative analysis of cast and printed bioinks, that covalently tethered TGF-β1 maintained its functionality after 3D printing. Taken together, the presented ink composition enabled the generation of high-quality cartilaginous tissues without the need for continuous exogenous growth factor supply and, thus, bears great potential for future investigation towards cartilage regeneration. Furthermore, growth factor tethering within bioinks, potentially leading to superior tissue development, may also be explored for other biofabrication applications.

Synthesis, classification and properties of hydrogels: their applications in drug delivery and agriculture

Absorbent polymers or hydrogel polymer materials have an enhanced water retention capacity and are widely used in agriculture and medicine. The controlled release of bioactive molecules (especially drug proteins) by hydrogels and the encapsulation of living cells are some of the active areas of drug discovery research. Hydrogel-based delivery systems may result in a therapeutically advantageous outcome for drug delivery. They can provide various sequential therapeutic agents including macromolecular drugs, small molecule drugs, and cells to control the release of molecules. Due to their controllable degradability, ability to protect unstable drugs from degradation and flexible physical properties, hydrogels can be used as a platform in which various chemical and physical interactions with encapsulated drugs for controlled release in the system can be studied. Practically, hydrogels that possess biodegradable properties have aroused greater interest in drug delivery systems. The original three-dimensional structure gets broken down into non-toxic substances, thus confirming the excellent biocompatibility of the gel. Chemical crosslinking is a resource-rich method for forming hydrogels with excellent mechanical strength. But in some cases the crosslinker used in the synthesis of the hydrogels may cause some toxicity. However, the physically cross-linked hydrogel preparative method is an alternative solution to overcome the toxicity of cross-linkers. Hydrogels that are responsive to stimuli formed from various natural and synthetic polymers can show significant changes in their properties under external stimuli such as temperature, pH, light, ion changes, and redox potential. Stimulus-responsive hydrogels have a wider range of applications in biomedicine including drug delivery, gene delivery and tissue regeneration. Stimulus-responsive hydrogels loaded with multiple drugs show controlled and sustained drug release and can act as drug carriers. By integrating stimulus-responsive hydrogels, such as those with improved thermal responsiveness, pH responsiveness and dual responsiveness, into textile materials, advanced functions can be imparted to the textile materials, thereby improving the moisture and water retention performance, environmental responsiveness, aesthetic appeal, display and comfort of textiles. This review explores the stimuli-responsive hydrogels in drug delivery systems and examines super adsorbent hydrogels and their application in the field of agriculture.

Optimized Photoclick (Bio)Resins for Fast Volumetric Bioprinting

Volumetric printing (VP) is a light-mediated technique enabling printing of complex, low-defect 3D objects within seconds, overcoming major drawbacks of layer-by-layer additive manufacturing. An optimized photoresin is presented for VP in the presence of cells (volumetric bioprinting) based on fast thiol-ene step-growth photoclick crosslinking. Gelatin-norbornene (Gel-NB) photoresin shows superior performance, both in physicochemical and biocompatibility aspects, compared to (meth-)acryloyl resins. The extremely efficient thiol-norbornene reaction produces the fastest VP reported to date (≈10 s), with significantly lower polymer content, degree of substitution (DS), and radical species, making it more suitable for cell encapsulation. This approach enables the generation of cellular free-form constructs with excellent cell viability (≈100%) and tissue maturation potential, demonstrated by development of contractile myotubes. Varying the DS, polymer content, thiol-ene ratio, and thiolated crosslinker allows fine-tuning of mechanical properties over a broad stiffness range (≈40 Pa to ≈15 kPa). These properties are achieved through fast and scalable methods for producing Gel-NB with inexpensive, off-the-shelf reagents that can help establish it as the gold standard for light-mediated biofabrication techniques. With potential applications from high-throughput bioprinting of tissue models to soft robotics and regenerative medicine, this work paves the way for exploitation of VPs unprecedented capabilities.

Key considerations on the development of biodegradable biomaterials for clinical translation of medical devices: With cartilage repair products as an example

With the interdisciplinary convergence of biology, medicine and materials science, both research and clinical translation of biomaterials are progressing at a rapid pace. However, there is still a huge gap between applied basic research on biomaterials and their translational products - medical devices, where two significantly different perspectives and mindsets often work independently and non-synergistically, which in turn significantly increases financial costs and research effort. Although this gap is well-known and often criticized in the biopharmaceutical industry, it is gradually widening. In this article, we critically examine the developmental pipeline of biodegradable biomaterials and biomaterial-based medical device products. Then based on clinical needs, market analysis, and relevant regulations, some ideas are proposed to integrate the two different mindsets to guide applied basic research and translation of biomaterial-based products, from the material and technical perspectives. Cartilage repair substitutes are discussed here as an example. Hopefully, this will lay a strong foundation for biomaterial research and clinical translation, while reducing the amount of extra research effort and funding required due to the dissonance between innovative basic research and commercialization pipeline.

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To elucidate the chain of medical devices development and basic research process.

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To propose rationales of biomaterial research with mindset of clinical translation.

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To elaborate with established medical devices for cartilage repairs as examples.

Bioink Platform Utilizing Dual-Stage Crosslinking of Hyaluronic Acid Tailored for Chondrogenic Differentiation of Mesenchymal Stromal Cells

3D bioprinting often involves application of highly concentrated polymeric bioinks to enable fabrication of stable cell-hydrogel constructs, although poor cell survival, compromised stem cell differentiation, and an inhomogeneous distribution of newly produced extracellular matrix (ECM) are frequently observed. Therefore, this study presents a bioink platform using a new versatile dual-stage crosslinking approach based on thiolated hyaluronic acid (HA-SH), which not only provides stand-alone 3D printability but also facilitates effective chondrogenic differentiation of mesenchymal stromal cells. A range of HA-SH with different molecular weights is synthesized and crosslinked with acrylated (PEG-diacryl) and allylated (PEG-diallyl) polyethylene glycol in a two-step reaction scheme. The initial Michael addition is used to achieve ink printability, followed by UV-mediated thiol-ene reaction to stabilize the printed bioink for long-term cell culture. Bioinks with high molecular weight HA-SH (>200 kDa) require comparably low polymer content to facilitate bioprinting. This leads to superior quality of cartilaginous constructs which possess a coherent ECM and a strongly increased stiffness of long-term cultured constructs. The dual-stage system may serve as an example to design platforms using two independent crosslinking reactions at one functional group, which allows adjusting printability as well as material and biological properties of bioinks.

TEMPO/TCC as a Chemo Selective Alternative for the Oxidation of Hyaluronic Acid

Hyaluronic acid (HA)-based hydrogels are very commonly applied as cell carriers for different approaches in regenerative medicine. HA itself is a well-studied biomolecule that originates from the physiological extracellular matrix (ECM) of mammalians and, due to its acidic polysaccharide structure, offers many different possibilities for suitable chemical modifications which are necessary to control, for example, network formation. Most of these chemical modifications are performed using the free acid function of the polymer and, additionally, lead to an undesirable breakdown of the biopolymer’s backbone. An alternative modification of the vicinal diol of the glucuronic acid is oxidation with sodium periodate to generate dialdehydes via a ring opening mechanism that can subsequently be further modified or crosslinked via Schiff base chemistry. Since this oxidation causes a structural destruction of the polysaccharide backbone, it was our intention to study a novel synthesis protocol frequently applied to selectively oxidize the C6 hydroxyl group of saccharides. On the basis of this TEMPO/TCC oxidation, we studied an alternative hydrogel platform based on oxidized HA crosslinked using adipic acid dihydrazide as the crosslinker.

Tunable metacrylated hyaluronic acid-based hybrid bioinks for stereolithography 3D bioprinting

Hyaluronic acid is a native extra-cellular matrix derivative that promises unique properties, such as anti-inflammatory response and cell-signaling with tissue-specific applications under its bioactive properties. Here, we investigate the importance of the duration of synthesis to obtain photocrosslinkable methacrylated hyaluronic acid (MeHA) with high degree of substitution. MeHA with high degree of substitution can result in rapid photocrosslinking and can be used as a bioink for stereolithographic (SLA) three dimensional 3D bioprinting. Increased degree of substitution results Our findings show that a ten-day synthesis results in an 88% degree of methacrylation (DM), whereas three-day and five-day syntheses result in 32% and 42% DM, respectively. The rheological characterization revealed an increased rate of photopolymerization with increasing DM. Further, we developed a hybrid bioink to overcome the non-cell-adhesive nature of MeHA by combining it with gelatin methacryloyl (GelMA) to fabricate 3D cell-laden hydrogel scaffolds. The hybrid bioink exhibited a 55% enhancement in stiffness compared to MeHA only and enabled cell-adhesion while maintaining high cell viability. Investigations also revealed that the hybrid bioink was a more suitable candidate for stereolithography (SLA) 3D bioprinting than MeHA because of its mechanical strength, printability, and cell-adhesive nature. This research lays out a firm foundation for the development of a stable hybrid bioink with MeHA and GelMA for first-ever use with SLA 3D bioprinting.

Chemically Modified Biopolymers for the Formation of Biomedical Hydrogels

Biopolymers are natural polymers sourced from plants and animals, which include a variety of polysaccharides and polypeptides. The inclusion of biopolymers into biomedical hydrogels is of great interest because of their inherent biochemical and biophysical properties, such as cellular adhesion, degradation, and viscoelasticity. The objective of this Review is to provide a detailed overview of the design and development of biopolymer hydrogels for biomedical applications, with an emphasis on biopolymer chemical modifications and cross-linking methods. First, the fundamentals of biopolymers and chemical conjugation methods to introduce cross-linking groups are described. Cross-linking methods to form biopolymer networks are then discussed in detail, including (i) covalent cross-linking (e.g., free radical chain polymerization, click cross-linking, cross-linking due to oxidation of phenolic groups), (ii) dynamic covalent cross-linking (e.g., Schiff base formation, disulfide formation, reversible Diels-Alder reactions), and (iii) physical cross-linking (e.g., guest-host interactions, hydrogen bonding, metal-ligand coordination, grafted biopolymers). Finally, recent advances in the use of chemically modified biopolymer hydrogels for the biofabrication of tissue scaffolds, therapeutic delivery, tissue adhesives and sealants, as well as the formation of interpenetrating network biopolymer hydrogels, are highlighted.

Efficacy of Combination Therapy with Apigenin and Synovial Membrane-Derived Mesenchymal Stem Cells on Knee Joint Osteoarthritis in a Rat Model

Background:

Osteoarthritis (OA) is a degenerative joint disease that causes a variety of adverse health effects. Considering the need to identify additional effective therapeutic options for OA therapy, we investigated the effect of co-injection of apigenin and synovial membrane-derived mesenchymal stem cells (SMMSCs) on OA in male rats’ knee joints.

Methods:

The study was performed in 2019 at the Department of Pharmacology, Shiraz University of Medical Sciences, Shiraz, Iran. Anterior cruciate ligament transection (ACLT) was used to induce OA. For three weeks, male Sprague-Dawley rats (eight groups, n=6 each) were treated once-weekly with intra-articular injections of apigenin alone or in combination with SMMSC (three million cells), phosphate-buffered saline, or hyaluronic acid. After three months, the interleukin 1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α), superoxide dismutase (SOD), and malondialdehyde (MDA) levels were measured in the cartilage homogenate. The expression of extracellular matrix (ECM) components including collagen 2a1, aggrecan, IL-1β, TNF-α, inducible nitric oxide synthase (iNOS), transcription factor SOX-9, and matrix metalloproteinases 3 and 13 were assessed using real-time polymerase chain reaction (RT-PCR) analysis. Radiological evaluation and histopathological assessment were used to evaluate the knees.

Results:

Levels of TNF-α (P=0.009), MDA (P>0.001), and IL-1β (P<0.001) decreased and the level of SOD increased (P=0.004) in the apigenin 0.3 µM with SMMSCs group. RT-PCR analysis indicated that IL-1β in the apigenin 0.3 µM with SMMSCs group reduced significantly (P<0.001). This group also exhibited increased expression levels of SOX-9, collagen 2a1, and aggrecan (P<0.001).

Conclusion:

Apigenin may have supplementary beneficial effects on cell therapy in a rat model of OA due to its possible effect on the reduction of oxidative stress, suppression of inflammation, and promotion of production of ECM components.

Tuning the Properties of PNIPAm-Based Hydrogel Scaffolds for Cartilage Tissue Engineering

Poly(N-isopropylacrylamide) (PNIPAm) is a three-dimensional (3D) crosslinked polymer that can interact with human cells and play an important role in the development of tissue morphogenesis in both in vitro and in vivo conditions. PNIPAm-based scaffolds possess many desirable structural and physical properties required for tissue regeneration, but insufficient mechanical strength, biocompatibility, and biomimicry for tissue development remain obstacles for their application in tissue engineering. The structural integrity and physical properties of the hydrogels depend on the crosslinks formed between polymer chains during synthesis. A variety of design variables including crosslinker content, the combination of natural and synthetic polymers, and solvent type have been explored over the past decade to develop PNIPAm-based scaffolds with optimized properties suitable for tissue engineering applications. These design parameters have been implemented to provide hydrogel scaffolds with dynamic and spatially patterned cues that mimic the biological environment and guide the required cellular functions for cartilage tissue regeneration. The current advances on tuning the properties of PNIPAm-based scaffolds were searched for on Google Scholar, PubMed, and Web of Science. This review provides a comprehensive overview of the scaffolding properties of PNIPAm-based hydrogels and the effects of synthesis-solvent and crosslinking density on tuning these properties. Finally, the challenges and perspectives of considering these two design variables for developing PNIPAm-based scaffolds are outlined.

Mechanotransducive Biomimetic Systems for Chondrogenic Differentiation In Vitro

Osteoarthritis (OA) is a long-term chronic joint disease characterized by the deterioration of bones and cartilage, which results in rubbing of bones which causes joint stiffness, pain, and restriction of movement. Tissue engineering strategies for repairing damaged and diseased cartilage tissue have been widely studied with various types of stem cells, chondrocytes, and extracellular matrices being on the lead of new discoveries. The application of natural or synthetic compound-based scaffolds for the improvement of chondrogenic differentiation efficiency and cartilage tissue engineering is of great interest in regenerative medicine. However, the properties of such constructs under conditions of mechanical load, which is one of the most important factors for the successful cartilage regeneration and functioning in vivo is poorly understood. In this review, we have primarily focused on natural compounds, particularly extracellular matrix macromolecule-based scaffolds and their combinations for the chondrogenic differentiation of stem cells and chondrocytes. We also discuss different mechanical forces and compression models that are used for In Vitro studies to improve chondrogenic differentiation. Summary of provided mechanical stimulation models In Vitro reviews the current state of the cartilage tissue regeneration technologies and to the potential for more efficient application of cell- and scaffold-based technologies for osteoarthritis or other cartilage disorders.

High-resolution lithographic biofabrication of hydrogels with complex microchannels from low-temperature-soluble gelatin bioresins

Biofabrication via light-based 3D printing offers superior resolution and ability to generate free-form architectures, compared to conventional extrusion technologies. While extensive efforts in the design of new hydrogel bioinks lead to major advances in extrusion methods, the accessibility of lithographic bioprinting is still hampered by a limited choice of cell-friendly resins. Herein, we report the development of a novel set of photoresponsive bioresins derived from ichthyic-origin gelatin, designed to print high-resolution hydrogel constructs with embedded convoluted networks of vessel-mimetic channels. Unlike mammalian gelatins, these materials display thermal stability as pre-hydrogel solutions at room temperature, ideal for bioprinting on any easily-accessible lithographic printer. Norbornene- and methacryloyl-modification of the gelatin backbone, combined with a ruthenium-based visible light photoinitiator and new coccine as a cytocompatible photoabsorber, allowed to print structures resolving single-pixel features (∼50 ​μm) with high shape fidelity, even when using low stiffness gels, ideal for cell encapsulation (1-2 ​kPa). Moreover, aqueous two-phase emulsion bioresins allowed to modulate the permeability of the printed hydrogel bulk. Bioprinted mesenchymal stromal cells displayed high functionality over a month of culture, and underwent multi-lineage differentiation while colonizing the bioresin bulk with tissue-specific neo-deposited extracellular matrix. Importantly, printed hydrogels embedding complex channels with perfusable lumen (diameter <200 ​μm) were obtained, replicating anatomical 3D networks with out-of-plane branches (i.e. brain vessels) that cannot otherwise be reproduced by extrusion bioprinting. This versatile bioresin platform opens new avenues for the widespread adoption of lithographic biofabrication, and for bioprinting complex channel-laden constructs with envisioned applications in regenerative medicine and hydrogel-based organ-on-a-chip devices.

Mini review: Biomaterials in repair and regeneration of nerve in a volumetric muscle loss

Volumetric muscle loss (VML) following a severe trauma or injury is beyond the intrinsic regenerative capacity of muscle tissues, and hence interventional therapy is required. Extensive muscle loss concomitant with damage to neuromuscular components overwhelms the muscles' remarkable regenerative capacity. The loss of nervous and vascular tissue leads to further damage and atrophy, so a combined treatment for neuromuscular junction (NMJ) along with the volumetric muscle regeneration is important. There have been immense advances in the field of tissue engineering for skeletal muscle tissue and peripheral nerve regeneration, but very few address the interdependence of the tissues and the need for combined therapies to repair and regenerate fully functional muscle tissue. This review addresses the problem and presents an overview of the biomaterials that have been studied for tissue engineering of neuromuscular tissues associated with skeletal muscles.

Functional Duality of Chondrocyte Hypertrophy and Biomedical Application Trends in Osteoarthritis

Chondrocyte hypertrophy is one of the key indicators in the progression of osteoarthritis (OA). However, compared with other OA indications, such as cartilage collapse, sclerosis, inflammation, and protease activation, the mechanisms by which chondrocyte hypertrophy contributes to OA remain elusive. As the pathological processes in the OA cartilage microenvironment, such as the alterations in the extracellular matrix, are initiated and dictated by the physiological state of the chondrocytes, in-depth knowledge of chondrocyte hypertrophy is necessary to enhance our understanding of the disease pathology and develop therapeutic agents. Chondrocyte hypertrophy is a factor that induces OA progression; it is also a crucial factor in the endochondral ossification. This review elaborates on this dual functionality of chondrocyte hypertrophy in OA progression and endochondral ossification through a description of the characteristics of various genes and signaling, their mechanism, and their distinguishable physiological effects. Chondrocyte hypertrophy in OA progression leads to a decrease in chondrogenic genes and destruction of cartilage tissue. However, in endochondral ossification, it represents an intermediate stage at the process of differentiation of chondrocytes into osteogenic cells. In addition, this review describes the current therapeutic strategies and their mechanisms, involving genes, proteins, cytokines, small molecules, three-dimensional environments, or exosomes, against the OA induced by chondrocyte hypertrophy. Finally, this review proposes that the contrasting roles of chondrocyte hypertrophy are essential for both OA progression and endochondral ossification, and that this cellular process may be targeted to develop OA therapeutics.

3D Bioprinting Photo-Crosslinkable Hydrogels for Bone and Cartilage Repair

Three-dimensional (3D) bioprinting has become a promising strategy for bone manufacturing, with excellent control over geometry and microarchitectures of the scaffolds. The bioprinting ink for bone and cartilage engineering has thus become the key to developing 3D constructs for bone and cartilage defect repair. Maintaining the balance of cellular viability, drugs or cytokines' function, and mechanical integrity is critical for constructing 3D bone and/or cartilage scaffolds. Photo-crosslinkable hydrogel is one of the most promising materials in tissue engineering; it can respond to light and induce structural or morphological transition. The biocompatibility, easy fabrication, as well as controllable mechanical and degradation properties of photo-crosslinkable hydrogel can meet various requirements of the bone and cartilage scaffolds, which enable it to serve as an effective bio-ink for 3D bioprinting. Here, in this review, we first introduce commonly used photo-crosslinkable hydrogel materials and additives (such as nanomaterials, functional cells, and drugs/cytokine), and then discuss the applications of the 3D bioprinted photo-crosslinkable hydrogel scaffolds for bone and cartilage engineering. Finally, we conclude the review with future perspectives about the development of 3D bioprinting photo-crosslinkable hydrogels in bone and cartilage engineering.

Comparison of chondro-inductivity between collagen and hyaluronic acid hydrogel based on chemical/physical microenvironment

Achieving chondrogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) successfully is crucial for cartilage regeneration. To date, various hydrogels with different chemical microenvironment have been used to modulate chondrogenic differentiation of BMSCs, especially collagen and hyaluronic acid hydrogel. However, the chondro-inductive ability of collagen and hyaluronic acid hydrogel has not been evaluated yet and the different chemical and physical microenvironment of these two hydrogels increase the difficulty of comparison. In this study, three different hydrogels based on collagen and hyaluronic acid (self-assembled collagen hydrogel (Col), self-assembled collagen hydrogel cross-linked with genipin (Cgp), and methacrylated hyaluronic acid hydrogel (HA)) were prepared and their chondro-inductive ability on the encapsulated BMSCs was evaluated. Col and Cgp have the same chemical composition and similar microstructure, but are different from HA, while Cgp and HA hydrogels have the same mechanical strength. It was found that chemical and physical microenvironments of the hydrogels combined to influence cell condensation. Thanks to cell condensation was more likely to occur in collagen hydrogels in the early stage, the cartilage-induced ability was in the order of Col > Cgp > HA. However, the severe shrinkage of Col and Cgp resulted in no enough space for cell proliferation within hydrogels in the later stage. In contrast, relatively stable physical microenvironment of HA helped to maintain continuous production of cartilage-related matrix in the later stage. Overall, these results revealed that the chondro-inductive ability of collagen and hyaluronic acid hydrogel with different chemical and physical microenvironment cannot be evaluated by a particular time period. However, it provided important information for optimization and design of the future hydrogels towards successful repair of articular cartilage.

Dynamic Tuning of Viscoelastic Hydrogels with Carbonyl Iron Microparticles Reveals the Rapid Response of Cells to Three-Dimensional Substrate Mechanics

Current methods to dynamically tune three-dimensional hydrogel mechanics require specific chemistries and substrates that make modest, slow, and often irreversible changes in their mechanical properties, exclude the use of protein-based scaffolds, or alter the hydrogel microstructure and pore size. Here, we rapidly and reversibly alter the mechanical properties of hydrogels consisting of extracellular matrix proteins and proteoglycans by adding carbonyl iron microparticles (MPs) and applying external magnetic fields. This approach drastically alters hydrogel mechanics: rheology reveals that application of a 4000 Oe magnetic field to a 5 mg/mL collagen hydrogel containing 10 wt % MPs increases the storage modulus from approximately 1.5 to 30 kPa. Cell morphology experiments show that cells embedded within these hydrogels rapidly sense the magnetically induced changes in ECM stiffness. Ca²⁺ transients are altered within seconds of stiffening or subsequent softening, and slower but still dynamic changes occur in YAP nuclear translocation in response to time-dependent application of a magnetic field. The near instantaneous change in hydrogel mechanics provides new insight into the effect of changing extracellular stiffness on both acute and chronic changes in diverse cell types embedded in protein-based scaffolds. Due to its flexibility, this method is broadly applicable to future studies interrogating cell mechanotransduction in three-dimensional substrates.

Polysaccharides on gelatin-based hydrogels differently affect chondrogenic differentiation of human mesenchymal stromal cells

Selection of feasible hybrid-hydrogels for best chondrogenic differentiation of human mesenchymal stromal cells (hMSCs) represents an important challenge in cartilage regeneration. In this study, three-dimensional hybrid hydrogels obtained by chemical crosslinking of poly (ethylene glycol) diglycidyl ether (PEGDGE), gelatin (G) without or with chitosan (Ch) or dextran (Dx) polysaccharides were developed. The hydrogels, namely G-PEG, G-PEG-Ch and G-PEG-Dx, were prepared with an innovative, versatile and cell-friendly technique that involves two preparation steps specifically chosen to increase the degree of crosslinking and the physical-mechanical stability of the product: a first homogeneous phase reaction followed by directional freezing, freeze-drying and post-curing. Chondrogenic differentiation of human bone marrow mesenchymal stromal cells (hBM-MSC) was tested on these hydrogels to ascertain whether the presence of different polysaccharides could favor the formation of the native cartilage structure. We demonstrated that the hydrogels exhibited an open pore porous morphology with high interconnectivity and the incorporation of Ch and Dx into the G-PEG common backbone determined a slightly reduced stiffness compared to that of G-PEG hydrogels. We demonstrated that G-PEG-Dx showed a significant increase of its anisotropic characteristic and G-PEG-Ch exhibited higher and faster stress relaxation behavior than the other hydrogels. These characteristics were associated to absence of chondrogenic differentiation on G-PEG-Dx scaffold and good chondrogenic differentiation on G-PEG and G-PEG-Ch. Furthermore, G-PEG-Ch induced the minor collagen proteins and the formation of collagen fibrils with a diameter like native cartilage. This study demonstrated that both anisotropic and stress relaxation characteristics of the hybrid hydrogels were important features directly influencing the chondrogenic differentiation potentiality of hBM-MSC.

Injectable Hyaluronic Acid/Poly(γ-glutamic acid) Hydrogel with Step-by-step Tunable Properties for Soft Tissue Engineering

Injectable hydrogels as an important class of biomaterials have gained much attention in tissue engineering. However, their crosslinking degree is difficult to be controlled after being injected into body. As we all know, the crosslinking degree strongly influences the physicochemical properties of hydrogels. Therefore, developing an injectable hydrogel with tunable crosslinking degree in vivo is important for tissue engineering. Herein, we present a dual crosslinking strategy to prepare injectable hydrogels with step-by-step tunable crosslinking degree using Schiff base reaction and photopolymerization. The developed hyaluronic acid/poly(γ-glutamic acid) (HA/γ-PGA) hydrogels exhibit step-by-step tunable swelling behavior, enzymatic degradation behavior and mechanical properties. Mechanical performance tests show that the storage moduli of HA/γ-PGA hydrogels are all less than 2000 Pa and the compressive moduli are in kilopascal, which have a good match with soft tissue. In addition, NIH 3T3 cells encapsulated in HA/γ-PGA hydrogel exhibit a high cell viability, indicating a good cytocompatibility of HA/γ-PGA hydrogel. Therefore, the developed HA/γ-PGA hydrogel as an injectable biomaterial has a good potential in soft tissue engineering.

Tuning the Phenotype of Cartilage Tissue Mimics by Varying Spheroid Maturation and Methacrylamide-Modified Gelatin Hydrogel Characteristics

In hybrid bioprinting of cartilage tissue constructs, spheroids are used as cellular building blocks and combined with biomaterials for dispensing. However, biomaterial intrinsic cues can deeply affect cell fate and to date, the influence of hydrogel encapsulation on spheroid viability and phenotype has received limited attention. This study assesses this need and unravels 1) how the phenotype of spheroid-laden constructs can be tuned through adjusting the hydrogel physico-chemical properties and 2) if the spheroid maturation stage prior to encapsulation is a determining factor for the construct phenotype. Articular chondrocyte spheroids with a cartilage specific extracellular matrix (ECM) are generated and different maturation stages, early-, mid-, and late-stage (3, 7, and 14 days, respectively), are harvested and encapsulated in 10, 15, or 20 w/v% methacrylamide-modified gelatin (gelMA) for 14 days. The encapsulation of immature spheroids do not lead to a cartilage-like ECM production but when more mature mid- or late-stage spheroids are combined with a certain concentration of gelMA, a fibrocartilage-like as well as a hyaline cartilage-like phenotype can be induced. As a proof of concept, late-stage spheroids are bioprinted using a 10 w/v% gelMA-Irgacure 2959 solution with the aim to test the processing potential of the spheroid-laden bioink.

The Effects of Crosslinking on the Rheology and Cellular Behavior of Polymer-Based 3D-Multilayered Scaffolds for Restoring Articular Cartilage

Polymer-based tri-layered (bone, intermediate and top layers) scaffolds used for the restoration of articular cartilage were prepared and characterized in this study to emulate the concentration gradient of cartilage. The scaffolds were physically or chemically crosslinked. In order to obtain adequate scaffolds for the intended application, the impact of the type of calcium phosphate used in the bone layer, the polymer used in the intermediate layer and the interlayer crosslinking process were analyzed. The correlation among SEM micrographs, physical-chemical characterization, swelling behavior, rheological measurements and cell studies were examined. Storage moduli at 1 Hz were 0.3-1.7 kPa for physically crosslinked scaffolds, and 4-5 kPa (EDC/NHS system) and 15-20 kPa (glutaraldehyde) for chemically crosslinked scaffolds. Intrinsic viscoelasticity and poroelasticity were considered in discussing the physical mechanism dominating in different time/frequency scales. Cell evaluation showed that all samples are available as alternatives to repair and/or substitute cartilage in articular osteoarthritis.

A chondrogenesis induction system based on a functionalized hyaluronic acid hydrogel sequentially promoting hMSC proliferation, condensation, differentiation, and matrix deposition

Hydrogel scaffolds are widely used in cartilage tissue engineering as a natural stem cell niche. In particular, hydrogels based on multiple biological signals can guide behaviors of mesenchymal stem cells (MSCs) during neo-chondrogenesis. In the first phase of this study, we showed that functionalized hydrogels with grafted arginine-glycine-aspartate (RGD) peptides and lower degree of crosslinking can promote the proliferation of human mesenchymal stem cells (hMSCs) and upregulate the expression of cell receptor proteins. Moreover, grafted RGD and histidine-alanine-valine (HAV) peptides in hydrogel scaffolds can regulate the adhesion of the intercellular at an early stage. In the second phase, we confirmed that simultaneous use of HAV and RGD peptides led to greater chondrogenic differentiation compared to the blank control and single-peptide groups. Furthermore, the controlled release of kartogenin (KGN) can better facilitate cell chondrogenesis compared to other groups. Interestingly, with longer culture time, cell condensation was clearly observed in the groups with RGD and HAV peptide. In all groups with RGD peptide, significant matrix deposition was observed, accompanied by glycosaminoglycan (GAG) and collagen (Coll) production. Through in vitro and in vivo experiments, this study confirmed that our hydrogel system can sequentially promote the proliferation, adhesion, condensation, chondrogenic differentiation of hMSCs, by mimicking the cell microenvironment during neo-chondrogenesis.

Silk Hydrogels with Controllable Formation of Dityrosine, 3,4-Dihydroxyphenylalanine, and 3,4-Dihydroxyphenylalanine-Fe3+ Complexes through Chitosan Particle-Assisted Fenton Reactions

The oxidation of tyrosine residues of silk fibroin involves the generation of dityrosine and 3,4-dihydroxyphenylalanine (DOPA). However, it remains a challenge to selectively control the reaction pathway to produce dityrosine or DOPA in a selective fashion. Here, silk hydrogels with controllable formation of not only dityrosine and DOPA but also DOPA-Fe3+ complexes within the cross-linked networks were developed. The use of chitosan particles in the Fenton reaction allowed the interaction of Fe3+ ions with silk fibroin to be limited through the adsorption of Fe3+ ions onto chitosan particles by manipulating contact time between the reaction medium and chitosan particles. This led to significant suppression of the premature formation of β-sheet structures that cause steric hindrance to the collisions between tyrosyl radicals and thus enabled higher selectivity toward the formation of dityrosine than DOPA. Remarkably, the addition of ethylenediaminetetraacetic acid (EDTA) to the chitosan particle-assisted Fenton reactions resulted in hydrogels that significantly favored the formation of DOPA over dityrosine due to the increase in the hydroxylation of phenol in the presence of EDTA. Despite the existence of Fe3+-EDTA complexes, Raman spectra indicated the DOPA-Fe3+ complexation in the hydrogels. Mechanistically, the hydrogel networks with small-sized and uniformly distributed β-sheet structures as well as the abundance of DOPA appear to make non-EDTA-chelated Fe3+ ions more accessible to complexation with DOPA. These findings have important implications for understanding the oxidation of tyrosine residues of silk fibroin by metal-catalyzed oxidation systems with potential benefits for future studies on silk protein-based hydrogels capable of generating intrinsic adhesive features as well as for exploring dual-cross-linked silk hydrogels constructed by chemical cross-linking and metal-coordinate complexation.

Ad-Dressing Stem Cells: Hydrogels for Encapsulation

Regenerative medicine is a novel scientific field that employs the use of stem cells as cell-based therapy for the regeneration and functional restoration of damaged tissues and organs. Stem cells bear characteristics such as the capacity for self-renewal and differentiation towards specific lineages and, therefore, serve as a backup reservoir in case of tissue injuries. Therapeutically, they can be autologously or allogeneically transplanted for tissue regeneration; however, allogeneic stem cell transplantation can provoke host immune responses leading to a host-versus-transplant reaction. A probable solution to this problem is stem cell encapsulation, a technique that utilizes various biomaterials for the creation of a semi-permeable membrane that encases the stem cells. Stem cell encapsulation can be accomplished by employing a great variety of natural and/or synthetic hydrogels and offers many benefits in regenerative medicine, including protection from the host’s immune system and mechanical stress, improved cell viability, proliferation and differentiation, cryopreservation and controlled and continuous delivery of the stem-cell-secreted therapeutic agents. Here, in this review, we report and discuss almost all natural and synthetic hydrogels used in stem cell encapsulation, along with the benefits that these materials, alone or in combination, could offer to cell therapy through functional cell encapsulation.

Biodegradable materials for bone defect repair

Compared with non-degradable materials, biodegradable biomaterials play an increasingly important role in the repairing of severe bone defects, and have attracted extensive attention from researchers. In the treatment of bone defects, scaffolds made of biodegradable materials can provide a crawling bridge for new bone tissue in the gap and a platform for cells and growth factors to play a physiological role, which will eventually be degraded and absorbed in the body and be replaced by the new bone tissue. Traditional biodegradable materials include polymers, ceramics and metals, which have been used in bone defect repairing for many years. Although these materials have more or fewer shortcomings, they are still the cornerstone of our development of a new generation of degradable materials. With the rapid development of modern science and technology, in the twenty-first century, more and more kinds of new biodegradable materials emerge in endlessly, such as new intelligent micro-nano materials and cell-based products. At the same time, there are many new fabrication technologies of improving biodegradable materials, such as modular fabrication, 3D and 4D printing, interface reinforcement and nanotechnology. This review will introduce various kinds of biodegradable materials commonly used in bone defect repairing, especially the newly emerging materials and their fabrication technology in recent years, and look forward to the future research direction, hoping to provide researchers in the field with some inspiration and reference.

Metabolic Labeling to Probe the Spatiotemporal Accumulation of Matrix at the Chondrocyte-Hydrogel Interface

Hydrogels are engineered with biochemical and biophysical signals to recreate aspects of the native microenvironment and to control cellular functions such as differentiation and matrix deposition. This deposited matrix accumulates within the pericellular space and likely affects the interactions between encapsulated cells and the engineered hydrogel; however, there has been little work to study the spatiotemporal evolution of matrix at this interface. To address this, metabolic labeling is employed to visualize the temporal and spatial positioning of nascent proteins and proteoglycans deposited by chondrocytes. Within covalently crosslinked hyaluronic acid hydrogels, chondrocytes deposit nascent proteins and proteoglycans in the pericellular space within 1 d after encapsulation. The accumulation of this matrix, as measured by an increase in matrix thickness during culture, depends on the initial hydrogel crosslink density with decreased thicknesses for more crosslinked hydrogels. Encapsulated fluorescent beads are used to monitor the hydrogel location and indicate that the emerging nascent matrix physically displaces the hydrogel from the cell membrane with extended culture. These findings suggest that secreted matrix increasingly masks the presentation of engineered hydrogel cues and may have implications for the design of hydrogels in tissue engineering and regenerative medicine.

A high molecular weight hyaluronic acid biphasic dispersion as potential therapeutics for interstitial cystitis

Interstitial cystitis (IC) is a progressive bladder disease characterized by increased urothelial permeability, inflammation of the bladder with abdominal pain. While there is no consensus on the etiology of the disease, it was believed that restoring the barrier between urinary solutes and (GAG) urothelium would interrupt the progression of this disease. Currently, several treatment options include intravesical delivery of hyaluronic acid (HA) and/or chondroitin sulfate solutions, through a catheter to restore the urothelial barrier, but have shown limited success in preclinical, clinical trials. Herein we report for the first time successful engineering and characterization of biphasic system developed by combining cross‐linked hyaluronic acid and naïve HA solution to decrease inflammation and permeability in an in vitro model of interstitial cystitis. The cross‐linking of HA was performed by 4‐arm‐polyethyeleneamine chemistry. The HA formulations were tested for their viscoelastic properties and the effects on cell metabolism, inflammatory markers, and permeability. Our study demonstrates the therapeutic effects of different ratios of the biphasic system and reports their ability to increase the barrier effect by decreasing the permeability and alteration of cell metabolism with respect to relative controls. Restoring the barrier by using biphasic system of HA therapy may be a promising approach to IC.

Gradient Hydrogels for Optimizing Niche Cues to Enhance Cell-Based Cartilage Regeneration

Hydrogels have been widely used for cell delivery to enhance cell-based therapies for cartilage tissue regeneration. To better support cartilage deposition, it is imperative to determine hydrogel formulation with physical and biochemical cues that are optimized for different cell populations. Previous attempts to identify optimized hydrogels rely mostly on testing hydrogel formulations with discrete properties, which are time-consuming and require large amounts of cells and materials. Gradient hydrogels encompass a range of continuous changes in niche properties, therefore offering a promising solution for screening a wide range of cell-niche interactions using less materials and time. However, harnessing gradient hydrogels to assess how matrix stiffness modulates cartilage formation by different cell types in vivo have never been investigated before. The goal of this study is to fabricate gradient hydrogels for screening the effects of varying hydrogel stiffness on cartilage formation by mesenchymal stem cells (MSCs) and chondrocytes, respectively, the two most commonly used cell populations for cartilage regeneration. We fabricated stiffness gradient hydrogels with tunable dimensions that support homogeneous cell encapsulation. Using gradient hydrogels with tunable stiffness range, we found MSCs and chondrocytes exhibit opposite trend in cartilage deposition in response to stiffness changes in vitro. Specifically, MSCs require soft hydrogels with Young's modulus less than 5 kPa to support faster cartilage deposition, as shown by type II collagen and sulfated glycosaminoglycan staining. In contrast, chondrocytes produce cartilage more effectively in stiffer matrix (>20 kPa). We chose optimal ranges of stiffness for each cell population for further testing in vivo using a mouse subcutaneous model. Our results further validated that soft matrix (Young's modulus <5 kPa) is better in supporting MSC-based cartilage deposition in three-dimensional, whereas stiffer matrix (Young's modulus >20 kPa) is more desirable for supporting chondrocyte-based cartilage deposition. Our results show the importance of optimizing niche cues in a cell-type-specific manner and validate the potential of using gradient hydrogels for optimizing niche cues to support cartilage regeneration in vitro and in vivo. Impact statement The present study validates the utility of gradient hydrogels for determining optimal hydrogel stiffness for supporting cartilage regeneration using both chondrocytes and stem cells. We demonstrate that such gradient hydrogels can be used for fast optimizing matrix stiffness for specific cell type to support optimal cartilage regeneration. To our knowledge, this is the first demonstration of applying gradient hydrogels for assessing optimal niche cues that support tissue regeneration in vivo and may be used for assessing optimal niche cues for different cell types to regeneration of different tissues.

Effects of the bonding intensity between hyaluronan and gelatin on chondrogenic phenotypic maintenance

Although there have been many reports on the use of crosslinked hyaluronic acid and gelatin derivatives as injectable hydrogels in cartilage tissue engineering, however, almost no reports have analyzed the kinds of bonding intensity that were most conducive for the maintenance of cartilage phenotypes. Herein, the biomimetic composite hydrogels based on thiolated hyaluronic acid and modified gelatin derivatives with physical mixed, weak, and strong bonding intensity were fabricated, wherein the thiolated hyaluronic acid ensured the basic network structure of composite hydrogels, and gelatin derivatives endowed the bioactivity to hydrogels. These physicochemical properties of composite hydrogels implied that strong bonding intensity (HA-GSH) contributed to the maintenance of a more uniform pore structure, and increased the ability of water retention and resistance to degradation. Further immunohistochemical and RT-PCR analyses demonstrated that the HA-GSH hydrogel greatly improved the expression level of the associated cartilage matrix and the possibility of hyaline cartilage formation in comparison to the physically blended HA-Gel gel and weak bonding crosslinked HA-GMA gel. Overall, all results proved that strong bonding intensity of the disulfide bonds in the HA-GSH hydrogel was more beneficial for the proliferation of chondrocytes and the maintenance of the hyaline cartilage phenotype, which might provide valuable inspiration for designing cartilage repair scaffolds.

Effects of biophysical cues of 3D hydrogels on mesenchymal stem cells differentiation

For stem cell research, three-dimensional (3D) hydrogels are increasingly recognized as more physiological systems than two-dimensional culture plates due to bidirectional and 3D interaction of stem cells and surrounding matrix. Among various stem cells, mesenchymal stem cells (MSCs) are one of the most widely applied from bench to bedside. In 3D hydrogels, MSCs are allowed to actively remodel the surrounding matrix through proteolytic degradation and cell-exerted force, which highly resembles in vivo situation. Notably, factors affecting hydrogel modifiability including matrix viscoelasticity and matrix degradability have been found to regulate adhesion, morphology, and fate decision of MSCs. In addition, MSCs within 3D hydrogels have been found to employ multiple mechanotransduction mechanisms including not only the classic integrin-actomyosin cytoskeleton system but also ion channels, microtubule cytoskeleton, and self-secreted proteinaceous matrix. This review summarizes the effects of biophysical cues on MSCs differentiation in 3D hydrogels and underlying mechanobiology in a hope to update our readers' understanding of stem cell biology and guide tissue engineering.

Engineering the cellular mechanical microenvironment to regulate stem cell chondrogenesis: Insights from a microgel model

Biophysical cues (especially mechanical cues) embedded in cellular microenvironments show a critical impact on stem cell fate. Despite the capability of traditional hydrogels to mimic the feature of extracellular matrix (ECM) and tune their physicochemical properties via diverse approaches, their relatively large size not only induces biased results, but also hinders high-throughput screening and analysis. In this paper, a microgel model is proposed to recapitulate the role of 3D mechanical microenvironment on stem cell behaviors especially chondrogenesis in vitro. The small diameter of microgels brings the high surface area to volume ratio and then the enlarged diffusion area and shortened diffusion distance of soluble molecules, leading to uniform distribution of nutrients and negligible biochemical gradient inside microgels. To construct ECM-like microenvironment with tunable mechanical strength, three gelatin/hyaluronic acid hybrid microgels with low, medium and high crosslinking densities, i.e., Gel-HA(L), Gel-HA(M) and Gel-HA(H), are fabricated in microfluidic devices by Michael addition reaction between thiolated gelatin (Gel-SH) and ethylsulfated hyaluronic acid (HA-VS) with different substitution degrees of vinyl sulfone groups. Our results show that mouse bone marrow mesenchymal stem cell (BMSC) proliferation, distribution and chondrogenesis are all closely dependent on mechanical microenvironments in microgels. Noteworthily, BMSCs show a clear trend of differentiating into hyaline cartilage in Gel-HA(L) and fibrocartilage in Gel-HA(M) and Gel-HA(H). Whole transcriptome RNA sequencing reveals that mechanical microenvironment of microgels affects BMSC differentiation via TGF-β/Smad signaling pathway, Hippo signaling pathway and Integrin/YAP/TAZ signaling pathway. We believe this microgel model provides a new way to further explore the interaction between cells and 3D microenvironment. STATEMENT OF SIGNIFICANCE: In recent years, hydrogels have been frequently used to construct 3D microenvironment for cells. However, their relatively large size not only brings biased experimental results, but also limits high-throughput screening and analysis. Herein we propose a gelatin/hyaluronic acid microgel model to explore the effects of 3D cellular mechanical microenvironment (biophysical cues) on BMSC behaviors especially chondrogenesis, which can minimize the interference of biochemical gradients. Our results reveal that BMSC differentiation into either hyaline cartilage or fibrocartilage can be regulated via tailoring the mechanical properties of microgels. Whole transcriptome RNA sequencing proves that "TGF-β/Smad signaling pathway", "Hippo signaling pathway" and "Integrins/YAP/ TAZ signaling pathway" are activated or inhibited in this process.

Physioxia Expanded Bone Marrow Derived Mesenchymal Stem Cells Have Improved Cartilage Repair in an Early Osteoarthritic Focal Defect Model

Focal early osteoarthritis (OA) or degenerative lesions account for 60% of treated cartilage defects each year. The current cell-based regenerative treatments have an increased failure rate for treating degenerative lesions compared to traumatic defects. Mesenchymal stem cells (MSCs) are an alternative cell source for treating early OA defects, due to their greater chondrogenic potential, compared to early OA chondrocytes. Low oxygen tension or physioxia has been shown to enhance MSC chondrogenic matrix content and could improve functional outcomes of regenerative therapies. The present investigation sought to develop a focal early OA animal model to evaluate cartilage regeneration and hypothesized that physioxic MSCs improve in vivo cartilage repair in both, post-trauma and focal early OA defects. Using a rabbit model, a focal defect was created, that developed signs of focal early OA after six weeks. MSCs cultured under physioxia had significantly enhanced in vitro MSC chondrogenic GAG content under hyperoxia with or without the presence of interleukin-1β (IL-1β). In both post-traumatic and focal early OA defect models, physioxic MSC treatment demonstrated a significant improvement in cartilage repair score, compared to hyperoxic MSCs and respective control defects. Future investigations will seek to understand whether these results are replicated in large animal models and the underlying mechanisms involved in in vivo cartilage regeneration.

Spatiotemporally Controlled Photoresponsive Hydrogels: Design and Predictive Modeling from Processing through Application

Photoresponsive hydrogels (PRHs) are soft materials whose mechanical and chemical properties can be tuned spatially and temporally with relative ease. Both photo-crosslinkable and photodegradable hydrogels find utility in a range of biomedical applications that require tissue-like properties or programmable responses. Progress in engineering with PRHs is facilitated by the development of theoretical tools that enable optimization of their photochemistry, polymer matrices, nanofillers, and architecture. This review brings together models and design principles that enable key applications of PRHs in tissue engineering, drug delivery, and soft robotics, and highlights ongoing challenges in both modeling and application.

Effect of different aged cartilage ECM on chondrogenesis of BMSCs in vitro and in vivo

Extracellular matrix (ECM)-based biomaterials are promising candidates in cartilage tissue engineering by simulating the native microenvironment to regulate the chondrogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) without exogenous growth factors. The biological properties of ECM scaffolds are primarily depended on the original source, which would directly influence the chondrogenic effects of the ECM materials. Despite the expanding investigations on ECM scaffolds in recent years, the selection of optimized ECM materials in cartilage regeneration was less reported. In this study, we harvested and compared the articular cartilage ECM from newborn, juvenile and adult rabbits. The results demonstrated the significant differences in the mechanical strength, sulphated glycosaminoglycan and collagen contents of the different aged ECM, before and after decellularization. Consequently, different compositional and mechanical properties were shown in the three ECM-based collagen hydrogels, which exerted age-dependent chondrogenic inducibility. In general, both in vitro and in vivo results suggested that the newborn ECM promoted the most chondrogenesis of BMSCs but led to severe matrix calcification. In contrast, BMSCs synthesized the lowest amount of cartilaginous matrix with minimal calcification with adult ECM. The juvenile ECM achieved the best overall results in promoting chondrogenesis of BMSCs and preventing matrix calcification. Together, this study provides important information to our current knowledge in the design of future ECM-based biomaterials towards a successful repair of articular cartilage.

Hyaluronic Acid-Based Bioink Composition Enabling 3D Bioprinting and Improving Quality of Deposited Cartilaginous Extracellular Matrix

In 3D bioprinting, bioinks with high concentrations of polymeric materials are frequently used to enable fabrication of 3D cell-hydrogel constructs with sufficient stability. However, this is often associated with restricted cell bioactivity and an inhomogeneous distribution of newly produced extracellular matrix (ECM). Therefore, this study investigates bioink compositions based on hyaluronic acid (HA), an attractive material for cartilage regeneration, which allow for reduction of polymer content. Thiolated HA and allyl-modified poly(glycidol) in varying concentrations are UV-crosslinked. To adapt bioinks to poly(ε-caprolactone) (PCL)-supported 3D bioprinting, the gels are further supplemented with 1 wt% unmodified high molecular weight HA (hmHA) and chondrogenic differentiation of incorporated human mesenchymal stromal cells is assessed. Strikingly, addition of hmHA to gels with a low polymer content (3 wt%) results in distinct increase of construct quality with a homogeneous ECM distribution throughout the constructs, independent of the printing process. Improved ECM distribution in those constructs is associated with increased construct stiffness after chondrogenic differentiation, as compared to higher concentrated constructs (10 wt%), which only show pericellular matrix deposition. The study contributes to effective bioink development, demonstrating dual function of a supplement enabling PCL-supported bioprinting and at the same time improving biological properties of the resulting constructs.

Mechanotransduction and Stiffness-Sensing: Mechanisms and Opportunities to Control Multiple Molecular Aspects of Cell Phenotype as a Design Cornerstone of Cell-Instructive Biomaterials for Articular Cartilage Repair

Since material stiffness controls many cell functions, we reviewed the currently available knowledge on stiffness sensing and elucidated what is known in the context of clinical and experimental articular cartilage (AC) repair. Remarkably, no stiffness information on the various biomaterials for clinical AC repair was accessible. Using mRNA expression profiles and morphology as surrogate markers of stiffness-related effects, we deduced that the various clinically available biomaterials control chondrocyte (CH) phenotype well, but not to equal extents, and only in non-degenerative settings. Ample evidence demonstrates that multiple molecular aspects of CH and mesenchymal stromal cell (MSC) phenotype are susceptible to material stiffness, because proliferation, migration, lineage determination, shape, cytoskeletal properties, expression profiles, cell surface receptor composition, integrin subunit expression, and nuclear shape and composition of CHs and/or MSCs are stiffness-regulated. Moreover, material stiffness modulates MSC immuno-modulatory and angiogenic properties, transforming growth factor beta 1 (TGF-β1)-induced lineage determination, and CH re-differentiation/de-differentiation, collagen type II fragment production, and TGF-β1- and interleukin 1 beta (IL-1β)-induced changes in cell stiffness and traction force. We then integrated the available molecular signaling data into a stiffness-regulated CH phenotype model. Overall, we recommend using material stiffness for controlling cell phenotype, as this would be a promising design cornerstone for novel future-oriented, cell-instructive biomaterials for clinical high-quality AC repair tissue.

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.

Combination of IKVAV, LRE, and GPQGIWGQ Bioactive Signaling Peptides Increases Human Induced Pluripotent Stem Cell Derived Neural Stem Cells Extracellular Matrix Remodeling and Neurite Extension

Extracellular matrix (ECM) remodeling is emerging as a modulator of neural maturation and axon extension. Most studies have used rodent cells to develop matrices capable of manipulating extracellular matrix remodeling for regenerative applications. However, clinically relevant human induced pluripotent stem cell derived neural stem cells (hNSC) do not always behave in a similar manner as rodent cells. In this study, hNSC response to a hyaluronic acid matrix with laminin derived IKVAV and LRE peptide signaling that has previously shown to promote ECM remodeling and neurite extension by mouse embryonic stem cells is examined. The addition of enzymatically degradable cross linker GPQGIWGQ to the IKVAV and LRE containing hyaluronic acid matrix is necessary to promote neurite extension, hyaluronic acid degradation, and gelatinase expression over hyaluronic acid matrices containing GPQGIWGQ, IKVAV and LRE, or no peptides. Changes in peptide content alters a number of matrix properties that can contribute to the cellular response, but increases in mesh size are not observed with cross linker cleavage in this study. Overall, these data imply a complex interaction between IKVAV, LRE, and GPQGIWGQ to modulate hNSC behavior.