A novel, visible light-induced, rapidly cross-linkable gelatin scaffold for osteochondral tissue engineering

Conducting osteochondral injury model in rabbit knee: Pearls and pitfalls

Osteochondral damage is a commonly encountered issue in the daily orthopedic practice and has been extensively researched across various areas, including tissue transplantations, tissue engineering products, stem cell applications, and cell culture studies. The absence of a universally accepted treatment as the gold standard for osteochondral damage indicates the necessity for further studies in this field in the future. Although the biomechanical characteristics of the rabbit knee do not perfectly mimic those of the human knee, experimental studies conducted on rabbit knees are considered the most practical experimental model for testing a well-constructed experimental hypothesis. Our article endeavors to impart our practical insights and experiences to researchers without experience whom seeking to design studies utilizing this model. We aim to offer valuable guidance for preoperative, operative, and postoperative considerations. •Rabbits used in osteochondral healing models should be at least 4 months old or older. Inducing damage in the trochlea is a well-established technique and relatively easy to apply.•Do not use pointy ended drills as it might create uneven damage. Do not place applied treatment agent in inappropriate level in relation to the surrounding cartilage surface.

Use of polyglycolic acid-hyaluronic acid/β-tricalcium phosphate scaffold with or without mesenchymal stem cells found ineffective in treating osteochondral lesions in rabbit knees

OBJECTIVE

In this experimental animal study, a novel bilayered scaffold used in the treatment of osteochondral defects in rabbit knees was evaluated. This novel scaffold's upper (cartilage) layer consists of polyglycolic acid and hyaluronic acid, and the lower (bone) layer consists of β-tricalcium phosphate. The purpose of this study was to evaluate the efficacy of this novel scaffold, combined with or without mesenchymal stem cells (MSCs), in the treatment of osteochondral defects in rabbit knees.

METHODS

Osteochondral defects were created in the left femoral trochlea of 30 rabbits. In group A, defects were treated with scaffold combined with MSCs; in group B, defects were treated with cell-free scaffolds; and group C was a control group with defects left untreated. In the 12th week, animals were sacrificed for macroscopic evaluation.

RESULTS

The mean International Cartilage Repair Society (ICRS) macroscopic scores were 4.95 for group A, 6.16 for group B, and 8.25 for group C. The mean Oswestry Arthroscopic Scores (OAS) were 1.65 for group A, 3.39 for group B, and 6.05 for group C. The macroscopic scores were significantly higher in group C than group A for ICRS scores and group A and group B for OAS (P < .001, P < .000, P < .022).

CONCLUSION

In essence, our findings indicate that the newly developed osteochondral scaffold, when tested in a rabbit model, is not as effective as expected in repairing full-thickness osteochondral defects, with or without the supplementation of MSCs. Further investigation is required to enhance the effectiveness of this novel combination.

Advances in Gelatin Bioinks to Optimize Bioprinted Cell Functions

Gelatin is a widely utilized bioprinting biomaterial due to its cell-adhesive and enzymatically cleavable properties, which improve cell adhesion and growth. Gelatin is often covalently cross-linked to stabilize bioprinted structures, yet the covalently cross-linked matrix is unable to recapitulate the dynamic microenvironment of the natural extracellular matrix (ECM), thereby limiting the functions of bioprinted cells. To some extent, a double network bioink can provide a more ECM-mimetic, bioprinted niche for cell growth. More recently, gelatin matrices are being designed using reversible cross-linking methods that can emulate the dynamic mechanical properties of the ECM. This review analyzes the progress in developing gelatin bioink formulations for 3D cell culture, and critically analyzes the bioprinting and cross-linking techniques, with a focus on strategies to optimize the functions of bioprinted cells. This review discusses new cross-linking chemistries that recapitulate the viscoelastic, stress-relaxing microenvironment of the ECM, and enable advanced cell functions, yet are less explored in engineering the gelatin bioink. Finally, this work presents the perspective on the areas of future research and argues that the next generation of gelatin bioinks should be designed by considering cell-matrix interactions, and bioprinted constructs should be validated against currently established 3D cell culture standards to achieve improved therapeutic outcomes.

Polymerization in living organisms

Vital biomacromolecules, such as RNA, DNA, polysaccharides and proteins, are synthesized inside cells via the polymerization of small biomolecules to support and multiply life. The study of polymerization reactions in living organisms is an emerging field in which the high diversity and efficiency of chemistry as well as the flexibility and ingeniousness of physiological environment are incisively and vividly embodied. Efforts have been made to design and develop in situ intra/extracellular polymerization reactions. Many important research areas, including cell surface engineering, biocompatible polymerization, cell behavior regulation, living cell imaging, targeted bacteriostasis and precise tumor therapy, have witnessed the elegant demeanour of polymerization reactions in living organisms. In this review, recent advances in polymerization in living organisms are summarized and presented according to different polymerization methods. The inspiration from biomacromolecule synthesis in nature highlights the feasibility and uniqueness of triggering living polymerization for cell-based biological applications. A series of examples of polymerization reactions in living organisms are discussed, along with their designs, mechanisms of action, and corresponding applications. The current challenges and prospects in this lifeful field are also proposed.

Smart biomaterials and their potential applications in tissue engineering

Smart biomaterials have been rapidly advancing ever since the concept of tissue engineering was proposed. Interacting with human cells, smart biomaterials can play a key role in novel tissue morphogenesis. Various aspects of biomaterials utilized in or being sought for the goal of encouraging bone regeneration, skin graft engineering, and nerve conduits are discussed in this review. Beginning with bone, this study summarizes all the available bioceramics and materials along with their properties used singly or in conjunction with each other to create scaffolds for bone tissue engineering. A quick overview of the skin-based nanocomposite biomaterials possessing antibacterial properties for wound healing is outlined along with skin regeneration therapies using infrared radiation, electrospinning, and piezoelectricity, which aid in wound healing. Furthermore, a brief overview of bioengineered artificial skin grafts made of various natural and synthetic polymers has been presented. Finally, by examining the interactions between natural and synthetic-based biomaterials and the biological environment, their strengths and drawbacks for constructing peripheral nerve conduits are highlighted. The description of the preclinical outcome of nerve regeneration in injury healed with various natural-based conduits receives special attention. The organic and synthetic worlds collide at the interface of nanomaterials and biological systems, producing a new scientific field including nanomaterial design for tissue engineering.

Gelatin Methacryloyl Hydrogels for Musculoskeletal Tissue Regeneration

Musculoskeletal disorders are a significant burden on the global economy and public health. Hydrogels have significant potential for enhancing the repair of damaged and injured musculoskeletal tissues as cell or drug delivery systems. Hydrogels have unique physicochemical properties which make them promising platforms for controlling cell functions. Gelatin methacryloyl (GelMA) hydrogel in particular has been extensively investigated as a promising biomaterial due to its tuneable and beneficial properties and has been widely used in different biomedical applications. In this review, a detailed overview of GelMA synthesis, hydrogel design and applications in regenerative medicine is provided. After summarising recent progress in hydrogels more broadly, we highlight recent advances of GelMA hydrogels in the emerging fields of musculoskeletal drug delivery, involving therapeutic drugs (e.g., growth factors, antimicrobial molecules, immunomodulatory drugs and cells), delivery approaches (e.g., single-, dual-release system), and material design (e.g., addition of organic or inorganic materials, 3D printing). The review concludes with future perspectives and associated challenges for developing local drug delivery for musculoskeletal applications.

Chemical Bonding of Biomolecules to the Surface of Nano-Hydroxyapatite to Enhance Its Bioactivity

Hydroxyapatite (HA) is a significant constituent of bones or teeth and is widely used as an artificial bone graft. It is often used to replace the lost bones or in reconstructing alveolar bones before dental implantation. HA with biological functions finds its importance in orthopedic surgery and dentistry to increase the local concentration of calcium ions, which activate the growth and differentiation of mesenchymal stem cells (MSC). To make relevant use of HA in bone transplantation, the surfaces of orthopedic and dental implants are frequently coated with nanosized hydroxyapatite (nHA), but its low dispersibility and tendency to form aggregates, the purpose of the surface modification of bone implants is defeated. To overcome these drawbacks and to improve the histocompatibility of bone implants or to use nHA in therapeutic applications of implants in the treatment of bone diseases, various studies suggested the attachment of biomolecules (growth factors) or drugs through chemical bonding at the surface of nHA. The growth factors or drugs bonded physically at the surface of nHA are mostly unstable and burst released immediately. Therefore, reported studies suggested that the surface of nHA needs to be modified through the chemical bonding of biologically active molecules at the surface of bone implants such as proteins, peptides, or naturally occurring polysaccharides to prevent the aggregation of nHA and to get homogenous dispersion of nHA in solution. The role of irradiation in producing bioactive and antibacterial nHA through morphological variations in surfaces of nHA is also summarized by considering internal structures and the formation of reactive oxygen species on irradiation. This mini-review aims to highlight the importance of small molecules such as proteins, peptides, drugs, and photocatalysts in surface property modification of nHA to achieve stable, bioactive, and antibacterial nHA to act as artificial bone implants (scaffolds) in combination with biodegradable polymers.

3D Fiber Reinforced Hydrogel Scaffolds by Melt Electrowriting and Gel Casting as a Hybrid Design for Wound Healing

Emerging biomanufacturing technologies have revolutionized the field of tissue engineering by offering unprecedented possibilities. Over the past few years, new opportunities arose by combining traditional and novel fabrication techniques, shaping the hybrid designs in biofabrication. One of the potential application fields is skin tissue engineering, in which a combination of traditional principles of wound dressing with advanced biofabrication methods could yield more efficient therapies. In this study, a hybrid design of fiber-reinforced scaffolds combined with gel casting is developed and the efficiency for in vivo wound healing applications is assessed. For this purpose, 3D fiber meshes produced by melt electrowriting are selectively filled with photocrosslinkable gelatin hydrogel matrices loaded with different growth factor carrier microspheres. Additionally, the influence of the inclusion of inorganic bioactive glass particles within the composite fibrous mesh is evaluated. Qualitative evaluation of secondary wound healing criteria and histological analysis shows that hybrid scaffolds containing growth factors and bioactive glass enhances the healing process significantly, compared to the designs merely providing a fiber-reinforced bioactive hydrogel matrix as the wound dressing. This study aims to explore a new application area for melt electrowriting as a powerful tool in fabricating hybrid therapeutic designs for skin tissue engineering.

Gelatin-based electrospun and lyophilized scaffolds with nano scale feature for bone tissue engineering application: review

The rebuilding of the normal functioning of the damaged human body bone tissue is one of the main objectives of bone tissue engineering (BTE). Fabricated scaffolds are mostly treated as artificial supports and as materials for regeneration of neo bone tissues and must closely biomimetic the native extracellular matrix of bone. The materials used for developing scaffolds should be biodegradable, nontoxic, and biocompatible. For the resurrection of bone disorder, specifically natural and synthetic polymers such as chitosan, PCL, gelatin, PGA, PLA, PLGA, etc. meet the requirements for serving their functions as artificial bone substitute materials. Gelatin is one of the potential candidates which could be blended with other polymers or composites to improve its physicochemical, mechanical, and biological performances as a bone graft. Scaffolds are produced by several methods including electrospinning, self-assembly, freeze-drying, phase separation, fiber drawing, template synthesis, etc. Among them, freeze-drying and electrospinning are among the popular, simplest, versatile, and cost-effective techniques. The design and preparation of freeze-dried and electrospun scaffolds are of intense research over the last two decades. Freeze-dried and electrospun scaffolds offer a distinctive architecture at the micro to nano range with desired porosity and pore interconnectivity for selective movement of small biomolecules and play its role as an appropriate matrix very similar to the natural bone extracellular matrix. This review focuses on the properties and functionalization of gelatin-based polymer and its composite in the form of bone scaffolds fabricated primarily using lyophilization and electrospinning technique and their applications in BTE.

Novel Application of Photo-Crosslinked Urocanic-Acid-Modified Chitosan in Corneal Wounds

In the last few years, the use of tissue adhesives in corneal perforation has gained immense popularity in clinical practices. The present study aimed to devise a new application of urocanic-acid-modified chitosan (CS) with methylene blue (MB) as a photosensitizer for the development of a photo-crosslinked tissue adhesive. In particular, the curing time was controlled with the aid of a 650 nm red diode. Under the same irradiation condition, the mechanical properties were tuned using the photosensitizer at different concentrations. In vitro tests revealed that the gel was ductile and biocompatible. The application of the gel to a perforated cornea model stopped the leakage of aqueous humor, immediately after the gel was photo-crosslinked. The blue appearance of the gel provided high precision when applied to corneal wounds. Importantly, the crosslinked gel became transparent within 24 h, owing to the dissipation of MB from tears, and the gel spontaneously sloughed off without artificial removal. Altogether, the study reported the development of a novel photo-crosslinkable urocanic-acid-modified CS gel that exhibited significant potential to be utilized in the healing of corneal perforation.

Controlled Intracellular Polymerization for Cancer Treatment

Numerous prodrugs have been developed and used for cancer treatments to reduce side effects and promote efficacy. In this work, we have developed a new photoactivatable prodrug system based on intracellular photoinduced electron transfer-reversible addition-fragmentation chain-transfer (PET-RAFT) polymerization. This unique polymerization process provided a platform for the synthesis of structure-predictable polymers with well-defined structures in living cells. The intracellularly generated poly(N,N-dimethylacrylamide)s were found to induce cell cycle arrest, apoptosis, and necroptosis, inhibit cell proliferation, and reduce cancer cell motilities. This polymerization-based "prodrug" system efficiently inhibits tumor growth and metastasis both in vitro and in vivo and will promote the development of targeted and directed cancer chemotherapy.

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.

Bioactive Inks Development for Osteochondral Tissue Engineering: A Mini-Review

Nowadays, a prevalent joint disease affecting both cartilage and subchondral bone is osteoarthritis. Osteochondral tissue, a complex tissue unit, exhibited limited self-renewal potential. Furthermore, its gradient properties, including mechanical property, bio-compositions, and cellular behaviors, present a challenge in repairing and regenerating damaged osteochondral tissues. Here, tissue engineering and translational medicine development using bioprinting technology provided a promising strategy for osteochondral tissue repair. In this regard, personalized stratified scaffolds, which play an influential role in osteochondral regeneration, can provide potential treatment options in early-stage osteoarthritis to delay or avoid the use of joint replacements. Accordingly, bioactive scaffolds with possible integration with surrounding tissue and controlling inflammatory responses have promising future tissue engineering perspectives. This minireview focuses on introducing biologically active inks for bioprinting the hierarchical scaffolds, containing growth factors and bioactive materials for 3D printing of regenerative osteochondral substitutes.

Bioprinting of Stem Cells in Multimaterial Scaffolds and Their Applications in Bone Tissue Engineering

Bioprinting stem cells into three-dimensional (3D) scaffolds has emerged as a new avenue for regenerative medicine, bone tissue engineering, and biosensor manufacturing in recent years. Mesenchymal stem cells, such as adipose-derived and bone-marrow-derived stem cells, are capable of multipotent differentiation in a 3D culture. The use of different printing methods results in varying effects on the bioprinted stem cells with the appearance of no general adverse effects. Specifically, extrusion, inkjet, and laser-assisted bioprinting are three methods that impact stem cell viability, proliferation, and differentiation potential. Each printing method confers advantages and disadvantages that directly influence cellular behavior. Additionally, the acquisition of 3D bioprinters has become more prominent with innovative technology and affordability. With accessible technology, custom 3D bioprinters with capabilities to print high-performance bioinks are used for biosensor fabrication. Such 3D printed biosensors are used to control conductivity and electrical transmission in physiological environments. Once printed, the scaffolds containing the aforementioned stem cells have a significant impact on cellular behavior and differentiation. Natural polymer hydrogels and natural composites can impact osteogenic differentiation with some inducing chondrogenesis. Further studies have shown enhanced osteogenesis using cell-laden scaffolds in vivo. Furthermore, selective use of biomaterials can directly influence cell fate and the quantity of osteogenesis. This review evaluates the impact of extrusion, inkjet, and laser-assisted bioprinting on adipose-derived and bone-marrow-derived stem cells along with the effect of incorporating these stem cells into natural and composite biomaterials.

Effect of difference in fixation methods of tendon graft and the microfracture procedure on tendon-bone junction healing

Background

There are generally two methods of fixation for tendon grafts used in ligament reconstruction: bone tunnel fixation and anchor fixation. The microfracture (Mf) procedure is a technique to induce bleeding from the bone marrow, and the bleeding may contain cells with differentiation potential. However, few studies have compared the effects of the Mf procedure with those of the fixation methods. This study aimed to evaluate the effectiveness of the Mf procedure on two tendon graft fixation methods: histological, gene expression, tendon graft thickness, and mechanical. We especially focused our investigation on junction healing of tendon grafts and bone in the two fixation methods.

Methods

We used 20 rabbits to evaluate tendon and bone healing in a peroneal tendon graft model. The rabbit models were divided into five groups according to the combination of peroneal tendon graft fixation method and Mf technique as follows: control group (C, n = 4), bone tunnel fixation without Mf procedure group (BT − Mf, n = 4), bone tunnel fixation with Mf procedure group (BT + Mf, n = 4), anchor fixation without Mf procedure group (A − Mf, n = 4), and anchor fixation with Mf procedure group (A + Mf, n = 4). All animals were sacrificed at 4 weeks postoperatively. The specimens underwent histological evaluation, mRNA analysis, tendon graft thickness at the tendon-bone junction, and biomechanical testing.

Results

Histological evaluation of the BT + Mf and A + Mf groups showed healing with fibrocartilage formation at the tendon graft-bone junction. The mRNA expression showed significant increase in type 2 collagen, Scleraxis, and SRY-box9 in the BT + Mf and A + Mf groups. In biomechanical tests, the BT + Mf and A + Mf groups showed significantly increased tensile strength compared with the BT − Mf and A − Mf groups (BT + Mf group, 21.6 ± 1.7 N; A + Mf group, 22.5 ± 2.3 N vs. BT − Mf group, 12.3 ± 2.4 N; A − Mf group, 11 ± 2.3 N).

Conclusion

The Mf procedure resulted in fibrocartilage formation at the tendon-bone junction in the BT and anchor fixation and improved the fixation strength at 4 weeks.

Recent trends in gelatin methacryloyl nanocomposite hydrogels for tissue engineering

Gelatin methacryloyl (GelMA), a photocrosslinkable gelatin-based hydrogel, has been immensely used for diverse applications in tissue engineering and drug delivery. Apart from its excellent functionality and versatile mechanical properties, it is also suitable for a wide range of fabrication methodologies to generate tissue constructs of desired shapes and sizes. Despite its exceptional characteristics, it is predominantly limited by its weak mechanical strength, as some tissue types naturally possess high mechanical stiffness. The use of high GelMA concentrations yields high mechanical strength, but not without the compromise in its porosity, degradability, and three-dimensional (3D) cell attachment. Recently, GelMA has been blended with various natural and synthetic biomaterials to reinforce its physical properties to match with the tissue to be engineered. Among these, nanomaterials have been extensively used to form a composite with GelMA, as they increase its biological and physicochemical properties without affecting the unique characteristics of GelMA and also introduce electrical and magnetic properties. This review article presents the recent advances in the formation of hybrid GelMA nanocomposites using a variety of nanomaterials (carbon, metal, polymer, and mineral-based). We give an overview of each nanomaterial's characteristics followed by a discussion of the enhancement in GelMA's physical properties after its incorporation. Finally, we also highlight the use of each GelMA nanocomposite for different applications, such as cardiac, bone, and neural regeneration.

3D bioprinting of molecularly engineered PEG-based hydrogels utilizing gelatin fragments

Three-dimensional (3D) bioprinting is an additive manufacturing process in which the combination of biomaterials and living cells, referred to as a bioink, is deposited layer-by-layer to form biologically active 3D tissue constructs. Recent advancements in the field show that the success of this technology requires the development of novel biomaterials or the improvement of existing bioinks. Polyethylene glycol (PEG) is one of the well-known synthetic biomaterials and has been commonly used as a photocrosslinkable bioink for bioprinting; however, other types of cell-friendly crosslinking mechanisms to form PEG hydrogels need to be explored for bioprinting and tissue engineering. In this work, we proposed micro-capillary based bioprinting of a novel molecularly engineered PEG-based bioink that transiently incorporates low molecular weight gelatin (LMWG) fragments. The rheological properties and release profile of the LMWG fragments were characterized, and their presence during hydrogel formation had no effect on the swelling ratio or sol fraction when compared to PEG hydrogels formed without the LMWG fragments. For bioprinting, PEG was first functionalized with cell-adhesive RGD ligands and was then crosslinked using protease-sensitive peptides via a Michael-type addition reaction inside the micro-capillary. The printability was assessed by the analysis of extrudability, shape fidelity, and printing accuracy of the hydrogel filaments after the optimization of the gelation conditions of the PEG-based bioink. The LMWG fragments supplemented into the bioink allowed the extrusion of smooth and uniform cylindrical strands of the hydrogel and improved shape fidelity and printing accuracy. Encapsulated cells in both bioprinted and non-bioprinted PEG-based hydrogels showed high viability and continued to proliferate over time in culture with a well-defined cell morphology depending on the presence of the cell adhesive peptide RGD. The presented micro-capillary based bioprinting process for a novel PEG-based bioink can be promising to construct complex 3D structures with micro-scale range and spatiotemporal variations without using any cytotoxic photoinitiator, UV light, or polymer support.

Molecular, Structural, and Rheological Characterization of Camel Skin Gelatin Extracted Using Different Pretreatment Conditions

Optimum conditions for high-quality gelatin recovery from camel skin and its molecular, structural, and rheological characterization were carried out in this study. Increased yield and gel strength were recorded, with an increase in camel skin pretreatment times of 6 to 42 h and 0.50 and 0.75 M-NaOH. Gelatin from skin pretreated with 0.75 and 0.5 M-NaOH for 42 h showed the highest yield (22.60%) and gel strength (365.5 g), respectively. Structural characterization by Fourier transformation infrared spectra, X-ray diffraction, and nuclear magnetic resonance indicated that all gelatins possessed major peaks in the amide region, and diffraction peaks around 22° were basically amorphous. The temperatures for gelling and melting ranged from 20.9 °C to 25.8 °C and 27.34 °C to 30.49 °C. Microstructure revealed loose network with more voids in gelatin from skin pretreated with 0.5 and 0.75 M-NaOH for 6 h, while a highly cross-linked network and less voids were observed in those pretreated with 0.75 M-NaOH for 24 h and 0.5 M-NaOH for 42 h. The results reveal that great potential exists in producing halal gelatin with excellent quality and functionality from camel skin.

Osteochondral Tissue Engineering: The Potential of Electrospinning and Additive Manufacturing

The socioeconomic impact of osteochondral (OC) damage has been increasing steadily over time in the global population, and the promise of tissue engineering in generating biomimetic tissues replicating the physiological OC environment and architecture has been falling short of its projected potential. The most recent advances in OC tissue engineering are summarised in this work, with a focus on electrospun and 3D printed biomaterials combined with stem cells and biochemical stimuli, to identify what is causing this pitfall between the bench and the patients' bedside. Even though significant progress has been achieved in electrospinning, 3D-(bio)printing, and induced pluripotent stem cell (iPSC) technologies, it is still challenging to artificially emulate the OC interface and achieve complete regeneration of bone and cartilage tissues. Their intricate architecture and the need for tight spatiotemporal control of cellular and biochemical cues hinder the attainment of long-term functional integration of tissue-engineered constructs. Moreover, this complexity and the high variability in experimental conditions used in different studies undermine the scalability and reproducibility of prospective regenerative medicine solutions. It is clear that further development of standardised, integrative, and economically viable methods regarding scaffold production, cell selection, and additional biochemical and biomechanical stimulation is likely to be the key to accelerate the clinical translation and fill the gap in OC treatment.

Synthesis of pH responsive, photocrosslinked gelatin-based hydrogel system for control release of ceftriaxone

Stimuli-responsive or smart nanocarriers are emerging class of materials used for drug delivery and tissue engineering applications. The objective of this study was to develop environment friendly, economic and smart gelatin based hydrogel for controlled delivery of ceftriaxone. Gelatin was modified with furfuryl amine and converted to hydrogel via photo oxidation reaction, initiated with visible light in the presence of crosslinking agent (eosin Y and riboflavin) to enhance the mechanical properties and increase structural integrity of the hydrogel. The conjugated gelatin-furfuryl amine (GFA) was characterized by ¹H NMR and FTIR techniques. An antimicrobial drug (ceftriaxone) was encapsulated in hydrogels systems (HG-E and HG-R) and its encapsulation efficiency was found as 88 % and 74 %, respectively. The in-vitro swelling property and release profile of the drug reveals that the behavior of hydrogel was pH dependent for both the hydrogels systems. Antibacterial study performed on HG-R hydrogel system via disc diffusion method revealed a greater zone of inhibition for Staphylococcus aureus. Findings of the presented work revealed the formation of an excellent, smart hydrogel system using Eosin-Y as cross-linker having greater encapsulation efficiency that can potentially be used for drug delivery application of ceftriaxone to enhance its therapeutic effects.

A Rabbit Femoral Condyle Defect Model for Assessment of Osteochondral Tissue Regeneration

Osteochondral tissue repair represents a common clinical need, with multiple approaches in tissue engineering and regenerative medicine being investigated for the repair of defects of articular cartilage and subchondral bone. A full thickness rabbit femoral condyle defect is a clinically relevant model of an articulating and load bearing joint surface for the investigation of osteochondral tissue repair by various cell-, biomolecule-, and biomaterial-based implants. In this protocol, we describe the methodology and 1.5- to 2-h surgical procedure for the generation of a reproducible, full thickness defect for construct implantation in the rabbit medial femoral condyle. Furthermore, we describe a step-by-step procedure for osteochondral tissue collection and the assessment of tissue formation using standardized histological, radiological, mechanical, and biochemical analytical techniques. This protocol illustrates the critical steps for reproducibility and minimally invasive surgery as well as applications to evaluate the efficacy of cartilage and bone tissue engineering implants, with emphasis on the usage of histological and radiological measures of tissue growth. Impact statement Although multiple surgical techniques have been developed for the treatment of osteochondral defects, repairing the tissues to their original state remains an unmet need. Such limitations have thus prompted the development of various constructs for osteochondral tissue regeneration. An in vivo model that is both clinically relevant and economically practical is necessary to evaluate the efficacy of different tissue engineered constructs. In this article, we present a full thickness rabbit femoral condyle defect model and describe the analytical techniques to assess the regeneration of osteochondral tissue.

Printability and Shape Fidelity of Bioinks in 3D Bioprinting

Three-dimensional bioprinting uses additive manufacturing techniques for the automated fabrication of hierarchically organized living constructs. The building blocks are often hydrogel-based bioinks, which need to be printed into structures with high shape fidelity to the intended computer-aided design. For optimal cell performance, relatively soft and printable inks are preferred, although these undergo significant deformation during the printing process, which may impair shape fidelity. While the concept of good or poor printability seems rather intuitive, its quantitative definition lacks consensus and depends on multiple rheological and chemical parameters of the ink. This review discusses qualitative and quantitative methodologies to evaluate printability of bioinks for extrusion- and lithography-based bioprinting. The physicochemical parameters influencing shape fidelity are discussed, together with their importance in establishing new models, predictive tools and printing methods that are deemed instrumental for the design of next-generation bioinks, and for reproducible comparison of their structural performance.

Crosslinking Strategies for 3D Bioprinting of Polymeric Hydrogels

Three-dimensional (3D) bioprinting has recently advanced as an important tool to produce viable constructs that can be used for regenerative purposes or as tissue models. To develop biomimetic and sustainable 3D constructs, several important processing aspects need to be considered, among which crosslinking is most important for achieving desirable biomechanical stability of printed structures, which is reflected in subsequent behavior and use of these constructs. In this work, crosslinking methods used in 3D bioprinting studies are reviewed, parameters that affect bioink chemistry are discussed, and the potential toward improving crosslinking outcomes and construct performance is highlighted. Furthermore, current challenges and future prospects are discussed. Due to the direct connection between crosslinking methods and properties of 3D bioprinted structures, this Review can provide a basis for developing necessary modifications to the design and manufacturing process of advanced tissue-like constructs in future.

Hyaluronic acid as a bioink for extrusion-based 3D printing

Biofabrication is enriching the tissue engineering field with new ways of producing structurally organized complex tissues. Among the numerous bioinks under investigation, hyaluronic acid (HA) and its derivatives stand out for their biological relevance, cytocompatibility, shear-thinning properties, and potential to fine-tune the desired properties with chemical modification. In this paper, we review the recent advances on bioinks containing HA. The available literature is presented based on subjects including the rheological properties in connection with printability, the chemical strategies for endowing HA with the desired properties, the clinical application, the most advanced preclinical studies, the advantages and limitations in comparison with similar biopolymer-based bioinks, and future perspectives.

Hydrogel to guide chondrogenesis versus osteogenesis of mesenchymal stem cells for fabrication of cartilaginous tissues

The ideal combination of hydrogel components for regeneration of cartilage and cartilaginous interfaces is a significant challenge because control over differentiation into multiple lineages is necessary. Stabilization of the phenotype of stem cell derived chondrocytes is needed to avoid undesired progression to terminal hypertrophy and tissue mineralization. A novel ternary blend hydrogel composed of methacrylated poly(ethylene glycol) (PEG), gelatin, and heparin (PGH) was designed to guide chondrogenesis by bone marrow derived mesenchymal stem cells (BMSCs) and maintenance of their cartilaginous phenotype. The hydrogel material effects on chondrogenic and osteogenic differentiation by BMSCs were evaluated in comparison to methacrylated gelatin hydrogel (GEL), a conventional bioink used for both chondrogenic and osteogenic applications. PGH and GEL hydrogels were loaded with goat BMSCs and cultured in chondrogenic and osteogenic mediums in vitro over six weeks. The PGH showed no sign of mineral deposition in an osteogenic environment in vitro. To further evaluate material effects, the hydrogels were loaded with adult human BMSCs (hBMSCs) and transforming growth factor β-3 and grown in subcutaneous pockets in mice over eight weeks. Consistent with the in vitro results, the PGH had greater potential to induce chondrogenesis by BMSCs in vivo compared to the GEL as evidenced by elevated gene expression of chondrogenic markers, supporting its potential for stable cartilage engineering. The PGH also showed a greater percentage of GAG positive cells compared to the GEL. Unlike the GEL, the PGH hydrogel exhibited anti-osteogenic effects in vivo as evidenced by negative Von Kossa staining and suppressed gene expression of hypertrophic and osteogenic markers. By nature of their polymer composition alone, the PGH and GEL regulated BMSC differentiation down different osteochondral lineages. Thus, the PGH and GEL are promising hydrogels to regenerate stratified cartilaginous interfacial tissues in situ, such as the mandibular condyle surface, using undifferentiated BMSCs and a stratified scaffold design.

Human articular cartilage repair: Sources and detection of cytotoxicity and genotoxicity in photo-crosslinkable hydrogel bioscaffolds

Three-dimensional biofabrication using photo-crosslinkable hydrogel bioscaffolds has the potential to revolutionize the need for transplants and implants in joints, with articular cartilage being an early target tissue. However, to successfully translate these approaches to clinical practice, several barriers must be overcome. In particular, the photo-crosslinking process may impact on cell viability and DNA integrity, and consequently on chondrogenic differentiation. In this review, we primarily explore the specific sources of cellular cytotoxicity and genotoxicity inherent to the photo-crosslinking reaction, the methods to analyze cell death, cell metabolism, and DNA damage within the bioscaffolds, and the possible strategies to overcome these detrimental effects.

Engineering and Functionalization of Gelatin Biomaterials: From Cell Culture to Medical Applications

Health care and medicine were revolutionized in recent years by the development of biomaterials, such as stents, implants, personalized drug delivery systems, engineered grafts, cell sheets, and other transplantable materials. These materials not only support the growth of cells before transplantation but also serve as replacements for damaged tissues in vivo. Among the various biomaterials available, those made from natural biological sources such as extracellular proteins (collagen, fibronectin, laminin) have shown significant benefits, and thus are widely used. However, routine biomaterial-based research requires copious quantities of proteins and the use of pure and intact extracellular proteins could be highly cost ineffective. Gelatin is a molecular derivative of collagen obtained through the irreversible denaturation of collagen proteins. Gelatin shares a very close molecular structure and function with collagen and thus is often used in cell and tissue culture to replace collagen for biomaterial purposes. Recent technological advancements such as additive manufacturing, rapid prototyping, and three-dimensional printing, in general, have resulted in great strides toward the generation of functional gelatin-based materials for medical purposes. In this review, the structural and molecular similarities of gelatin to other extracellular matrix proteins are compared and analyzed. Current strategies for gelatin crosslinking and production are described and recent applications of gelatin-based biomaterials in cell culture and tissue regeneration are discussed. Finally, recent improvements in gelatin-based biomaterials for medical applications and future directions are elaborated. Impact statement In this study, we described gelatin's biochemical properties and compared its advantages and drawbacks over other extracellular matrix proteins and polymers used for biomaterial application. We also described how gelatin can be used with other polymers in creating gelatin composite materials that have enhanced mechanical properties, increased biocompatibility, and boosted bioactivity, maximizing its benefits for biomedical purposes. The article is relevant, as it discussed not only the chemistry of gelatin, but also listed the current techniques in gelatin/biomaterial manufacturing and described the most recent trends in gelatin-based biomaterials for biomedical applications.

Combining Innovative Bioink and Low Cell Density for the Production of 3D-Bioprinted Cartilage Substitutes: A Pilot Study

3D bioprinting offers interesting opportunities for 3D tissue printing by providing living cells with appropriate scaffolds with a dedicated structure. Biological advances in bioinks are currently promising for cell encapsulation, particularly that of mesenchymal stem cells (MSCs). We present herein the development of cartilage implants by 3D bioprinting that deliver MSCs encapsulated in an original bioink at low concentration. 3D-bioprinted constructs (10 × 10 × 4 mm) were printed using alginate/gelatin/fibrinogen bioink mixed with human bone marrow MSCs. The influence of the bioprinting process and chondrogenic differentiation on MSC metabolism, gene profiles, and extracellular matrix (ECM) production at two different MSC concentrations (1 million or 2 million cells/mL) was assessed on day 28 (D28) by using MTT tests, real-time RT-PCR, and histology and immunohistochemistry, respectively. Then, the effect of the environment (growth factors such as TGF-β1/3 and/or BMP2 and oxygen tension) on chondrogenicity was evaluated at a 1 M cell/mL concentration on D28 and D56 by measuring mitochondrial activity, chondrogenic gene expression, and the quality of cartilaginous matrix synthesis. We confirmed the safety of bioextrusion and gelation at concentrations of 1 million and 2 million MSC/mL in terms of cellular metabolism. The chondrogenic effect of TGF-β1 was verified within the substitute on D28 by measuring chondrogenic gene expression and ECM synthesis (glycosaminoglycans and type II collagen) on D28. The 1 M concentration represented the best compromise. We then evaluated the influence of various environmental factors on the substitutes on D28 (differentiation) and D56 (synthesis). Chondrogenic gene expression was maximal on D28 under the influence of TGF-β1 or TGF-β3 either alone or in combination with BMP-2. Hypoxia suppressed the expression of hypertrophic and osteogenic genes. ECM synthesis was maximal on D56 for both glycosaminoglycans and type II collagen, particularly in the presence of a combination of TGF-β1 and BMP-2. Continuous hypoxia did not influence matrix synthesis but significantly reduced the appearance of microcalcifications within the extracellular matrix. The described strategy is very promising for 3D bioprinting by the bioextrusion of an original bioink containing a low concentration of MSCs followed by the culture of the substitutes in hypoxic conditions under the combined influence of TGF-β1 and BMP-2.

Mesenchymal stem cells for cartilage regeneration

Cartilage injuries are typically caused by trauma, chronic overload, and autoimmune diseases. Owing to the avascular structure and low metabolic activities of chondrocytes, cartilage generally does not self-repair following an injury. Currently, clinical interventions for cartilage injuries include chondrocyte implantation, microfracture, and osteochondral transplantation. However, rather than restoring cartilage integrity, these methods only postpone further cartilage deterioration. Stem cell therapies, especially mesenchymal stem cell (MSCs) therapies, were found to be a feasible strategy in the treatment of cartilage injuries. MSCs can easily be isolated from mesenchymal tissue and be differentiated into chondrocytes with the support of chondrogenic factors or scaffolds to repair damaged cartilage tissue. In this review, we highlighted the full success of cartilage repair using MSCs, or MSCs in combination with chondrogenic factors and scaffolds, and predicted their pros and cons for prospective translation to clinical practice.

Recent trends in protein and peptide-based biomaterials for advanced drug delivery

Engineering protein and peptide-based materials for drug delivery applications has gained momentum due to their biochemical and biophysical properties over synthetic materials, including biocompatibility, ease of synthesis and purification, tunability, scalability, and lack of toxicity. These biomolecules have been used to develop a host of drug delivery platforms, such as peptide- and protein-drug conjugates, injectable particles, and drug depots to deliver small molecule drugs, therapeutic proteins, and nucleic acids. In this review, we discuss progress in engineering the architecture and biological functions of peptide-based biomaterials -naturally derived, chemically synthesized and recombinant- with a focus on the molecular features that modulate their structure-function relationships for drug delivery.

A Visible Light-Cross-Linkable, Fibrin-Gelatin-Based Bioprinted Construct with Human Cardiomyocytes and Fibroblasts

In this study, fibrin was added to a photo-polymerizable gelatin-based bioink mixture to fabricate cardiac cell-laden constructs seeded with human induced pluripotent stem cell-derived cardiomyocytes (iPS-CM) or CM cell lines with cardiac fibroblasts (CF). The extensive use of platelet-rich fibrin, its capacity to offer patient specificity, and the similarity in composition to surgical glue prompted us to include fibrin in the existing bioink composition. The cell-laden bioprinted constructs were cross-linked to retain a herringbone pattern via a two-step procedure including the visible light cross-linking of furfuryl-gelatin followed by the chemical cross-linking of fibrinogen via thrombin and calcium chloride. The printed constructs revealed an extremely porous, networked structure that afforded long-term in vitro stability. Cardiomyocytes printed within the sheet structure showed excellent viability, proliferation, and expression of the troponin I cardiac marker. We extended the utility of this fibrin-gelatin bioink toward coculturing and coupling of CM and cardiac fibroblasts (CF), the interaction of which is extremely important for maintenance of normal physiology of the cardiac wall in vivo. This enhanced "cardiac construct" can be used for drug cytotoxicity screening or unraveling triggers for heart diseases in vitro.

3D Bioprinting of cardiac tissue and cardiac stem cell therapy

Cardiovascular tissue engineering endeavors to repair or regenerate damaged or ineffective blood vessels, heart valves, and cardiac muscle. Current strategies that aim to accomplish such a feat include the differentiation of multipotent or pluripotent stem cells on appropriately designed biomaterial scaffolds that promote the development of mature and functional cardiac tissue. The advent of additive manufacturing 3D bioprinting technology further advances the field by allowing heterogenous cell types, biomaterials, and signaling factors to be deposited in precisely organized geometries similar to those found in their native counterparts. Bioprinting techniques to fabricate cardiac tissue in vitro include extrusion, inkjet, laser-assisted, and stereolithography with bioinks that are either synthetic or naturally-derived. The article further discusses the current practices for postfabrication conditioning of 3D engineered constructs for effective tissue development and stability, then concludes with prospective points of interest for engineering cardiac tissues in vitro. Cardiovascular three-dimensional bioprinting has the potential to be translated into the clinical setting and can further serve to model and understand biological principles that are at the root of cardiovascular disease in the laboratory.

Biomaterials for bone tissue engineering scaffolds: a review

Bone tissue engineering has been continuously developing since the concept of "tissue engineering" has been proposed. Biomaterials that are used as the basic material for the fabrication of scaffolds play a vital role in bone tissue engineering. This paper first introduces a strategy for literature search. Then, it describes the structure, mechanical properties and materials of natural bone and the strategies of bone tissue engineering. Particularly, it focuses on the current knowledge about biomaterials used in the fabrication of bone tissue engineering scaffolds, which includes the history, types, properties and applications of biomaterials. The effects of additives such as signaling molecules, stem cells, and functional materials on the performance of the scaffolds are also discussed.

Gelatin/Hyaluronic Acid Content in Hydrogels Obtained through Blue Light-Induced Gelation Affects Hydrogel Properties and Adipose Stem Cell Behaviors

Composite hydrogels of hyaluronic acid and gelatin attract great attention in biomedical fields. In particular, the composite hydrogels obtained through processes that are mild for cells are useful in tissue engineering. In this study, hyaluronic acid/gelatin composite hydrogels obtained through a blue light-induced gelation that is mild for mammalian cells were studied for the effect of the content of each polymer in the precursor solution on gelation, properties of resultant hydrogels, and behaviors of human adipose stem cells laden in the hydrogels. Control of the content enabled gelation in less than 20 s, and also enabled hydrogels to be obtained with 0.5-1.2 kPa Young's modulus. Human adipose stem cells were more elongated in hydrogels with a higher rather than lower content of hyaluronic acid. Stem cell marker genes, Nanog, Oct4, and Sox2, were expressed more in the cells in the composite hydrogels with a higher content of hyaluronic acid compared with those in the hydrogel composed of gelatin alone and on tissue culture dishes. These results are useful for designing conditions for using gelatin/hyaluronic acid composite hydrogels obtained through blue light-induced gelation suitable for tissue engineering applications.

Visible Light Cross-Linking of Gelatin Hydrogels Offers an Enhanced Cell Microenvironment with Improved Light Penetration Depth

In this study, the cyto-compatibility and cellular functionality of cell-laden gelatin-methacryloyl (Gel-MA) hydrogels fabricated using a set of photo-initiators which absorb in 400-450 nm of the visible light range are investigated. Gel-MA hydrogels cross-linked using ruthenium (Ru) and sodium persulfate (SPS), are characterized to have comparable physico-mechanical properties as Gel-MA gels photo-polymerized using more conventionally adopted photo-initiators, such as 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one (Irgacure 2959) and lithium phenyl(2,4,6-trimethylbenzoyl) phosphinate (LAP). It is demonstrated that the Ru/SPS system has a less adverse effect on the viability and metabolic activity of human articular chondrocytes encapsulated in Gel-MA hydrogels for up to 35 days. Furthermore, cell-laden constructs cross-linked using the Ru/SPS system have significantly higher glycosaminoglycan content and re-differentiation capacity as compared to cells encapsulated using I2959 and LAP. Moreover, the Ru/SPS system offers significantly greater light penetration depth as compared to the I2959 system, allowing thick (10 mm) Gel-MA hydrogels to be fabricated with homogenous cross-linking density throughout the construct. These results demonstrate the considerable advantages of the Ru/SPS system over traditional UV polymerizing systems in terms of clinical relevance and practicability for applications such as cell encapsulation, biofabrication, and in situ cross-linking of injectable hydrogels.

The applicability of furfuryl-gelatin as a novel bioink for tissue engineering applications

Three-dimensional bioprinting is an innovative technique in tissue engineering, to create layer-by-layer structures, required for mimicking body tissues. However, synthetic bioinks do not generally possess high printability and biocompatibility at the same time. So, there is an urgent need for naturally derived bioinks that can exhibit such optimized properties. We used furfuryl-gelatin as a novel, visible-light crosslinkable bioink for fabricating cell-laden structures with high viability. Hyaluronic acid was added as a viscosity enhancer and either Rose Bengal or Riboflavin was used as a visible-light crosslinker. Crosslinking was done by exposing the printed structure for 2.5 min to visible light and confirmed using Fourier transform infrared spectroscopy and rheometry. Scanning electron microscopy revealed a highly porous networked structure. Three different cell types were successfully bioprinted within these constructs. Mouse mesenchymal stem cells printed within monolayer and bilayer sheets showed viability, network formation and proliferation (∼5.33 times) within 72 h of culture. C2C12 and STO cells were used to print a double layered structure, which showed evidence of the viability of both cells and heterocellular clusters within the construct. This furfuryl-gelatin based bioink can be used for tissue engineering of complex tissues and help in understanding how cellular crosstalk happens in vivo during normal or diseased pathology. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 314-323, 2019.

Dynamic PEG-Peptide Hydrogels via Visible Light and FMN-Induced Tyrosine Dimerization

Photoresponsive hydrogels have become invaluable 3D culture matrices for mimicking aspects of the extracellular matrix. Recent efforts have focused on using ultraviolet (UV) light exposure and multifunctional macromers to induce secondary hydrogel crosslinking and dynamic matrix stiffening in the presence of cells. This contribution reports the design of a novel yet simple dynamic poly(ethylene glycol)-peptide hydrogel system through flavin mononucleotide (FMN) induced di-tyrosine crosslinking. These di-tyrosine linkages effectively increase hydrogel crosslinking density and elastic modulus. In addition, the degree of stiffening in hydrogels at a fixed PEG macromer content can be readily tuned by controlling FMN concentration or the number of tyrosine residues built-in to the peptide linker. Furthermore, tyrosine-bearing pendant biochemical motifs can be spatial-temporally patterned in the hydrogel network via controlling light exposure through a photomask. The visible light and FMN-induced tyrosine dimerization process produces a cytocompatible and physiologically relevant degree of stiffening, as shown by changes of cell morphology and gene expression in pancreatic cancer and stromal cells. This new dynamic hydrogel scheme should be highly desirable for researchers seeking a photoresponsive hydrogel system without complicated chemical synthesis and secondary UV light irradiation.

Native-mimicking in vitro microenvironment: an elusive and seductive future for tumor modeling and tissue engineering

Human connective tissues are complex physiological microenvironments favorable for optimal survival, function, growth, proliferation, differentiation, migration, and death of tissue cells. Mimicking native tissue microenvironment using various three-dimensional (3D) tissue culture systems in vitro has been explored for decades, with great advances being achieved recently at material, design and application levels. These achievements are based on improved understandings about the functionalities of various tissue cells, the biocompatibility and biodegradability of scaffolding materials, the biologically functional factors within native tissues, and the pathophysiological conditions of native tissue microenvironments. Here we discuss these continuously evolving physical aspects of tissue microenvironment important for human disease modeling, with a focus on tumors, as well as for tissue repair and regeneration. The combined information about human tissue spaces reflects the necessities of considerations when configuring spatial microenvironments in vitro with native fidelity to culture cells and regenerate tissues that are beyond the formats of 2D and 3D cultures. It is important to associate tissue-specific cells with specific tissues and microenvironments therein for a better understanding of human biology and disease conditions and for the development of novel approaches to treat human diseases.

A Comparative Study of a 3D Bioprinted Gelatin-Based Lattice and Rectangular-Sheet Structures

3D bioprinting holds great promise in the field of regenerative medicine as it can create complex structures in a layer-by-layer manner using cell-laden bioinks, making it possible to imitate native tissues. Current bioinks lack both high printability and biocompatibility required in this respect. Hence, the development of bioinks that exhibit both properties is needed. In our previous study, a furfuryl-gelatin-based bioink, crosslinkable by visible light, was used for creating mouse mesenchymal stem cell-laden structures with a high fidelity. In this study, lattice mesh geometries were printed in a comparative study to test against the properties of a traditional rectangular-sheet. After 3D printing and crosslinking, both structures were analysed for swelling and rheological properties, and their porosity was estimated using scanning electron microscopy. The results showed that the lattice structure was relatively more porous with enhanced rheological properties and exhibited a lower degradation rate compared to the rectangular-sheet. Further, the lattice allowed cells to proliferate to a greater extent compared to the rectangular-sheet, which initially retained a lower number of cells. All of these results collectively affirmed that the lattice poses as a superior scaffold design for tissue engineering applications.

Cartilage tissue formation through assembly of microgels containing mesenchymal stem cells

Current clinical approaches to treat articular cartilage degeneration provide only a limited ability to regenerate tissue with long-term durability and functionality. In this application, injectable bulk hydrogels and microgels containing stem cells can provide a suitable environment for tissue regeneration. However insufficient cell-cell interactions, low differentiation efficiency and poor tissue adhesion hinder the formation of high-quality hyaline type cartilage. Here, we have designed a higher order tissue-like structure using injectable cell-laden microgels as the building blocks to achieve human bone marrow-derived mesenchymal stem cell (hBMSC) long-term maintenance and chondrogenesis. We have demonstrated that a 4-arm poly(ethylene glycol)-N-hydroxysuccinimide (NHS) crosslinker induces covalent bonding between the microgel building blocks as well as the surrounding tissue mimic. The crosslinking process assembles the microgels into a 3D construct and preserves the viability and cellular functions of the encapsulated hBMSCs. This assembled microgel construct encourages upregulation of chondrogenic markers in both gene and glycosaminoglycan (GAG) expression levels. In addition, the regenerated tissue in the assembled microgels stained positively with Alcian blue and Safranin O exhibiting unique hyaline-like cartilage features. Furthermore, the immunostaining showed a favourable distribution and significantly higher content of type II collagen in the assembled microgels when compared to both the bulk hydrogel and pellet cultures. Collectively, this tissue adhesive hBMSC-laden microgel construct provides potential clinical opportunities for articular cartilage repair and other applications in regenerative medicine.

STATEMENT OF SIGNIFICANCE

A reliable approach to reconstruct durable and fully functional articular cartilage tissue is required for effective clinical therapies. Here, injectable hydrogels together with cell-based therapies offer new treatment strategies in cartilage repair. For effective cartilage regeneration, the injectable hydrogel system needs to be bonded to the surrounding tissue and at the same time needs to be sufficiently stable for prolonged chondrogenesis. In this work, we utilised injectable hBMSC-laden microgels as the building blocks to create an assembled construct via N-hydroxysuccinimide-amine coupling. This crosslinking process also allows for rapid bonding between the assembled microgels and a surrounding tissue mimic. The resultant assembled microgel-construct provides both a physically stable and biologically dynamic environment for hBMSC chondrogenesis, leading to the production of a mature hyaline type cartilage structure.

Injectable hydrogels: a new paradigm for osteochondral tissue engineering

Osteochondral tissue engineering has become a promising strategy for repairing focal chondral lesions and early osteoarthritis (OA), which account for progressive joint pain and disability in millions of people worldwide. Towards improving osteochondral tissue repair, injectable hydrogels have emerged as promising matrices due to their wider range of properties such as their high water content and porous framework, similarity to the natural extracellular matrix (ECM), ability to encapsulate cells within the matrix and ability to provide biological cues for cellular differentiation. Further, their properties such as those that facilitate minimally invasive deployment or delivery, and their ability to repair geometrically complex irregular defects have been critical for their success. In this review, we provide an overview of innovative approaches to engineer injectable hydrogels towards improved osteochondral tissue repair. Herein, we focus on understanding the biology of osteochondral tissue and osteoarthritis along with the need for injectable hydrogels in osteochondral tissue engineering. Furthermore, we discuss in detail different biomaterials (natural and synthetic) and various advanced fabrication methods being employed for the development of injectable hydrogels in osteochondral repair. In addition, in vitro and in vivo applications of developed injectable hydrogels for osteochondral tissue engineering are also reviewed. Finally, conclusions and future perspectives of using injectable hydrogels in osteochondral tissue engineering are provided.

Bioglass-Incorporated Methacrylated Gelatin Cryogel for Regeneration of Bone Defects

Cryogels have recently gained interest in the field of tissue engineering as they inherently possess an interconnected macroporous structure. Considered to be suitable for scaffold cryogel fabrication, methacrylated gelatin (GelMA) is a modified form of gelatin valued for its ability to retain cell adhesion site. Bioglass nanoparticles have also attracted attention in the field due to their osteoinductive and osteoconductive behavior. Here, we prepare methacrylated gelatin cryogel with varying concentration of bioglass nanoparticles to study its potential for bone regeneration. We demonstrate that an increase in bioglass concentration in cryogel leads to improved mechanical property and augmented osteogenic differentiation of mesenchymal cells during in vitro testing. Furthermore, in vivo testing in mice cranial defect model shows that highest concentration of bioglass nanoparticles (2.5 w/w %) incorporated in GelMA cryogel induces the most bone formation compared to the other tested groups, as studied by micro-CT and histology. The in vitro and in vivo results highlight the potential of bioglass nanoparticles incorporated in GelMA cryogel for bone regeneration.