Mussel-Inspired Tough Hydrogel with In Situ Nanohydroxyapatite Mineralization for Osteochondral Defect Repair

Gellan gum/alginate-based Ca-enriched acellular bilayer hydrogel with robust interface bonding for effective osteochondral repair

The treatment of osteochondral (OC) defects remains challenging because of the lack of economical and feasible therapeutic strategies for OC repair and reconstruction. In this study, we report an integrated bilayer hydrogel with robust interface binding force (40 kPa) by facilitating the diffusion of calcium ions to the secondary crosslink of the bilayer hydrogel, in which gellan gum and sodium alginate acted as the chondral layer, gellan gum and hydroxyapatite acted as subchondral layer. This integrated construct has high cytocompatibility, and can seed with mesenchymal stem cells (MSCs) related to different functional protein expression for cartilage and bone formation, respectively. Furthermore, in the rabbit critical-sized osteochondral defect model (4.0 mm in diameter and 8.0 mm in depth), the calcium enriched hydrogel act as a calcium reservoir, promote neovascularization at week 4, and repair the critical defect at week 8, demonstrating the feasible preparation of an acellular hydrogel for OC repair.

Conductive Hydrogels with Dynamic Reversible Networks for Biomedical Applications

Conductive hydrogels (CHs) are emerging as a promising and well-utilized platform for 3D cell culture and tissue engineering to incorporate electron signals as biorelevant physical cues. In conventional covalently crosslinked conductive hydrogels, the network dynamics (e.g., stress relaxation, shear shining, and self-healing) required for complex cellular functions and many biomedical utilities (e.g., injection) cannot be easily realized. In contrast, dynamic conductive hydrogels (DCHs) are fabricated by dynamic and reversible crosslinks. By allowing for the breaking and reforming of the reversible linkages, DCHs can provide dynamic environments for cellular functions while maintaining matrix integrity. These dynamic materials can mimic some properties of native tissues, making them well-suited for several biotechnological and medical applications. An overview of the design, synthesis, and engineering of DCHs is presented in this review, focusing on the different dynamic crosslinking mechanisms of DCHs and their biomedical applications.

Biomimetic epidermal sensors assembled from polydopamine-modified reduced graphene oxide/polyvinyl alcohol hydrogels for the real-time monitoring of human motions

Conductive hydrogel-based epidermal strain sensors can generate repeatable electrical changes upon mechanical deformations for indication of the skin's physiological condition. However, this remains challenging for many conductive hydrogel sensors due to biomechanical mismatch with skin tissues and an unstable resistance variation response, resulting in non-conformable deformations with the epidermis and dermis, and consequently generating inaccurate monitoring of human movements. Herein, a conductive hydrogel that highly matches the skin is fabricated from dynamically hydrogen-bonded nanocrystallites of polydopamine-modified reduced graphene oxide (PDA-rGO) nanosheets composited with polyvinyl alcohol, namely the PDA-rGO/PVA hydrogel. PDA-rGO provides a large number of dynamic hydrogen-bonding interactions in the hydrogel, resulting in a skin-matching modulus (78 kPa) and stretchability. Moreover, the resultant hydrogel possesses excellent cytocompatibility and conductivity (0.87 S m-1), high sensitivity (gauge factor of compression: 20) at low strain and outstanding linearity at high strain as well as a stable resistance variation response. These desirable properties enable the application of the PDA-rGO/PVA hydrogel as a skin-friendly wearable sensor for real-time and accurate detection of both large-scale joint movements and tiny physiological signals, including the bending and relaxing of fingers, the wrist, elbow and knee joints, and wrist pulse and swallowing. Moreover, this hydrogel is integrated into a 2D sensor array that monitors strains or pressures in two dimensions, which is promising for electronic skin, biosensors, human-machine interfaces, and wearable electronic devices.

Adhesive and tough hydrogels: from structural design to applications

In recent years, multifunctional hydrogels have garnered great interest. Usually, there is a contradiction between the toughness and interface adhesion of traditional hydrogels. In engineering and medical applications, hydrogels need to have good adhesive properties and toughness. The design of functional hydrogels with strong adhesion and high toughness is key to their application. In this review, the research progress of adhesive and tough hydrogels in recent years is outlined. Specifically, the structural design (such as integrated, layered, and gradient structures) and applications (such as cartilage repair, drug delivery, strain sensors, tissue adhesives, soft actuators, and supercapacitors) of adhesive and tough hydrogels are classified and discussed, providing new insights on their design and development.

Hierarchical and heterogeneous hydrogel system as a promising strategy for diversified interfacial tissue regeneration

A state-of-the-art review on the design and preparation of hierarchical and heterogeneous hydrogel systems for interfacial tissue regeneration.

Biomimetic hydrogels designed for cartilage tissue engineering

Cartilage-like hydrogels based on materials like gelatin, chondroitin sulfate, hyaluronic acid and polyethylene glycol are reviewed and contrasted, revealing existing limitations and challenges on biomimetic hydrogels for cartilage regeneration.

3D Printing of Bioinspired Biomaterials for Tissue Regeneration

Biological systems, which possess remarkable functions and excellent properties, are gradually becoming a source of inspiration for the fabrication of advanced tissue regeneration biomaterials due to their hierarchical structures and novel compositions. It would be meaningful to learn and transfer the characteristics of creatures to biomaterials design. However, traditional strategies cannot satisfy the design requirements of the complicated bioinspired materials for tissue regeneration. 3D printing, as a rapidly developing new technology that can accurately achieve multimaterial and multiscale fabrication, is capable of optimizing the fabrication of bioinspired materials with complex composition and structure. This review summarizes the recent developments in 3D-printed bioinspired biomaterials for multiple tissue regeneration, and especially highlights the progresses on i) traditional bioinspired designs for biomaterials fabrication, ii) biological composition inspired designs for the 3D-printed biomaterials, and iii) biological structure inspired designs for the 3D-printed biomaterials. Finally, the challenges and prospects for the development of 3D-printed bioinspired biomaterials are discussed.

Nanotextured silk fibroin/hydroxyapatite biomimetic bilayer tough structure regulated osteogenic/chondrogenic differentiation of mesenchymal stem cells for osteochondral repair

OBJECTIVES

Articular cartilage plays a vital role in bearing and buffering. Injured cartilage and subchondral bone repair is a crucial challenge in cartilage tissue engineering due to the peculiar structure of osteochondral unit and the requirement of osteogenic/chondrogenic bi-directional differentiation. Based on the bionics principle, a nanotextured silk fibroin (SF)-chondroitin sulphate (CS)/hydroxyapatite (HAp) nanowire tough bilayer structure was prepared for osteochondral repair.

METHODS

The SF-CS/HAp membrane was constructed by alcohol-induced β-sheet formation serving as the physical crosslink. Its osteochondral repairing capacity was evaluated by culturing bone marrow mesenchymal stem cells (BMSCs) in vitro and constructing a rat osteochondral defect model in vivo.

RESULTS

The bilayer SF-CS/HAp membrane with satisfactory mechanical properties similar to natural cartilage imitated the natural osteochondral unit structural layers and exerted the function of bearing and buffering timely after in vivo implantation. SF-CS layer upregulated the expression of chondrogenesis-related genes of BMSCs by surface nanotopography and sustained release CS. Meanwhile, nanotextured HAp layer assembled with nanowire endowed the membrane with an osteogenic differentiation tendency for BMSCs. In vivo results proved that the biomimetic bilayer structure dramatically promoted new cartilage formation and subchondral bone remodelling for osteochondral defect model after implantation.

CONCLUSIONS

The SF-CS/HAp biomimetic bilayer membrane provides a promising strategy for precise osteochondral repair.

A 3D, Dynamically Loaded Hydrogel Model of the Osteochondral Unit to Study Osteocyte Mechanobiology

Osteocytes are mechanosensitive cells that orchestrate signaling in bone and cartilage across the osteochondral unit. The mechanisms by which osteocytes regulate osteochondral homeostasis and degeneration in response to mechanical cues remain unclear. This study introduces a novel 3D hydrogel bilayer composite designed to support osteocyte differentiation and bone matrix deposition in a bone-like layer and to recapitulate key aspects of the osteochondral unit's complex loading environment. The bilayer hydrogel is fabricated with a soft cartilage-like layer overlaying a stiff bone-like layer. The bone-like layer contains a stiff 3D-printed hydrogel structure infilled with a soft, degradable, cellular hydrogel. The IDG-SW3 cells embedded within the soft hydrogel mature into osteocytes and produce a mineralized collagen matrix. Under dynamic compressive strains, near-physiological levels of strain are achieved in the bone layer (≤ 0.08%), while the cartilage layer bears the majority of the strains (>99%). Under loading, the model induces an osteocyte response, measured by prostaglandin E2, that is frequency, but not strain, dependent: a finding attributed to altered fluid flow within the composite. Overall, this new hydrogel platform provides a novel approach to study osteocyte mechanobiology in vitro in an osteochondral tissue-mimetic environment.

Advanced hydrogels for the repair of cartilage defects and regeneration

Cartilage defects are one of the most common symptoms of osteoarthritis (OA), a degenerative disease that affects millions of people world-wide and places a significant socio-economic burden on society. Hydrogels, which are a class of biomaterials that are elastic, and display smooth surfaces while exhibiting high water content, are promising candidates for cartilage regeneration. In recent years, various kinds of hydrogels have been developed and applied for the repair of cartilage defects in vitro or in vivo, some of which are hopeful to enter clinical trials. In this review, recent research findings and developments of hydrogels for cartilage defects repair are summarized. We discuss the principle of cartilage regeneration, and outline the requirements that have to be fulfilled for the deployment of hydrogels for medical applications. We also highlight the development of advanced hydrogels with tailored properties for different kinds of cartilage defects to meet the requirements of cartilage tissue engineering and precision medicine.

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The biotechnology of developing hydrogels for cartilage defects repair is promising.

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The principle for cartilage regeneration using hydrogels and requirements for clinical transformation are summarized.

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Advanced hydrogels with tailored properties for different kinds of cartilage defects are discussed.

Strategies for Using Polydopamine to Induce Biomineralization of Hydroxyapatite on Implant Materials for Bone Tissue Engineering

In the field of tissue engineering, there are several issues to consider when designing biomaterials for implants, including cellular interaction, good biocompatibility, and biochemical activity. Biomimetic mineralization has gained considerable attention as an emerging approach for the synthesis of biocompatible materials with complex shapes, categorized organization, controlled shape, and size in aqueous environments. Understanding biomineralization strategies could enhance opportunities for novel biomimetic mineralization approaches. In this regard, mussel-inspired biomaterials have recently attracted many researchers due to appealing features, such as strong adhesive properties on moist surfaces, improved cell adhesion, and immobilization of bioactive molecules via catechol chemistry. This molecular designed approach has been a key point in combining new functionalities into accessible biomaterials for biomedical applications. Polydopamine (PDA) has emerged as a promising material for biomaterial functionalization, considering its simple molecular structure, independence of target materials, cell interactions for adhesion, and robust reactivity for resulting functionalization. In this review, we highlight the strategies for using PDA to induce the biomineralization of hydroxyapatite (HA) on the surface of various implant materials with good mechanical strength and corrosion resistance. We also discuss the interactions between the PDA-HA coating, and several cell types that are intricate in many biomedical applications, involving bone defect repair, bone regeneration, cell attachment, and antibacterial activity.

Hydroxyapatite-Incorporated Composite Gels Improve Mechanical Properties and Bioactivity of Bone Scaffolds

Reinforcing polymeric scaffolds with micro/nanoparticles improve their mechanical properties and render them bioactive. In this study, hydroxyapatite (HA) is incorporated into 5% (w/v) gelatin methacrylate (GelMA) hydrogels at 1, 5, and 20 mg mL^-1 concentrations. The material properties of these composite gels are characterized through swelling, degradation, and compression tests. Using 3D cell encapsulation, the cytocompatibility and osteogenic differentiation of preosteoblasts are evaluated to assess the biological properties of the composite scaffolds. The in vitro assays demonstrate increasing cell proliferation and metabolic activity over the course of 14 d in culture. Furthermore, the scaffolds support osteogenic differentiation of the microencapsulated preosteoblasts. For the in vivo study, the composite scaffolds are subcutaneously implanted in rats for 14 d. The histological staining of the explanted in vivo samples exhibits the functional advantages of the scaffold's biocompatibility, biodegradability, and integration into the existing host tissue. This work demonstrates the enhanced mechanical and biological performance of HA-gelatin composite hydrogels for bone tissue engineering applications.