Conductive and injectable hyaluronic acid/gelatin/gold nanorod hydrogels for enhanced surgical translation and bioprinting

Extracellular Matrix-Surrogate Advanced Functional Composite Biomaterials for Tissue Repair and Regeneration

Native tissues, comprising multiple cell types and extracellular matrix components, are inherently composites. Mimicking the intricate structure, functionality, and dynamic properties of native composite tissues represents a significant frontier in biomaterials science and tissue engineering research. Biomimetic composite biomaterials combine the benefits of different components, such as polymers, ceramics, metals, and biomolecules, to create tissue-template materials that closely simulate the structure and functionality of native tissues. While the design of composite biomaterials and their in vitro testing are frequently reviewed, there is a considerable gap in whole animal studies that provides insight into the progress toward clinical translation. Herein, we provide an insightful critical review of advanced composite biomaterials applicable in several tissues. The incorporation of bioactive cues and signaling molecules into composite biomaterials to mimic the native microenvironment is discussed. Strategies for the spatiotemporal release of growth factors, cytokines, and extracellular matrix proteins are elucidated, highlighting their role in guiding cellular behavior, promoting tissue regeneration, and modulating immune responses. Advanced composite biomaterials design challenges, such as achieving optimal mechanical properties, improving long-term stability, and integrating multifunctionality into composite biomaterials and future directions, are discussed. We believe that this manuscript provides the reader with a timely perspective on composite biomaterials.

Neural Tissue-Like, not Supraphysiological, Electrical Conductivity Stimulates Neuronal Lineage Specification through Calcium Signaling and Epigenetic Modification

Electrical conductivity is a pivotal biophysical factor for neural interfaces, though optimal values remain controversial due to challenges isolating this cue. To address this issue, conductive substrates made of carbon nanotubes and graphene oxide nanoribbons, exhibiting a spectrum of conductivities from 0.02 to 3.2 S m-1, while controlling other surface properties is designed. The focus is to ascertain whether varying conductivity in isolation has any discernable impact on neural lineage specification. Remarkably, neural-tissue-like low conductivity (0.02-0.1 S m-1) prompted neural stem/progenitor cells to exhibit a greater propensity toward neuronal lineage specification (neurons and oligodendrocytes, not astrocytes) compared to high supraphysiological conductivity (3.2 S m-1). High conductivity instigated the apoptotic process, characterized by increased apoptotic fraction and decreased neurogenic morphological features, primarily due to calcium overload. Conversely, cells exposed to physiological conductivity displayed epigenetic changes, specifically increased chromatin openness with H3acetylation (H3ac) and neurogenic-transcription-factor activation, along with a more balanced intracellular calcium response. The pharmacological inhibition of H3ac further supported the idea that such epigenetic changes might play a key role in driving neuronal specification in response to neural-tissue-like, not supraphysiological, conductive cues. These findings underscore the necessity of optimal conductivity when designing neural interfaces and scaffolds to stimulate neuronal differentiation and facilitate the repair process.

Bioprinting: Mechanical Stabilization and Reinforcement Strategies in Regenerative Medicine

Bioprinting describes the printing of biomaterials and cell-laden or cell-free hydrogels with various combinations of embedded bioactive molecules. It encompasses the precise patterning of biomaterials and cells to create scaffolds for different biomedical needs. There are many requirements that bioprinting scaffolds face, and it is ultimately the interplay between the scaffold's structure, properties, processing, and performance that will lead to its successful translation. Among the essential properties that the scaffolds must possess-adequate and appropriate application-specific chemical, mechanical, and biological performance-the mechanical behavior of hydrogel-based bioprinted scaffolds is the key to their stable performance in vivo at the site of implantation. Hydrogels that typically constitute the main scaffold material and the medium for the cells and biomolecules are very soft, and often lack sufficient mechanical stability, which reduces their printability and, therefore, the bioprinting potential. The aim of this review article is to highlight the reinforcement strategies that are used in different bioprinting approaches to achieve enhanced mechanical stability of the bioinks and the printed scaffolds. Enabling stable and robust materials for the bioprinting processes will lead to the creation of truly complex and remarkable printed structures that could accelerate the application of smart, functional scaffolds in biomedical settings. Impact statement Bioprinting is a powerful tool for the fabrication of 3D structures and scaffolds for biomedical applications. It has gained tremendous attention in recent years, and the bioink library is expanding to include more and more material combinations. From the practical application perspective, different properties need to be considered, such as the printed structure's chemical, mechanical, and biological performances. Among these, the mechanical behavior of the printed constructs is critical for their successful translation into the clinic. The aim of this review article is to explore the different reinforcement strategies used for the mechanical stabilization of bioinks and bioprinted structures.

Wired for Success: Probing the Effect of Tissue-Engineered Neural Interface Substrates on Cell Viability

This study investigates the electrochemical behavior of GelMA-based hydrogels and their interactions with PC12 neural cells under electrical stimulation in the presence of conducting substrates. Focusing on indium tin oxide (ITO), platinum, and gold mylar substrates supporting conductive scaffolds composed of hydrogel, graphene oxide, and gold nanorods, we explored how the substrate materials affect scaffold conductivity and cell viability. We examined the impact of an optimized electrical stimulation protocol on the PC12 cell viability. According to our findings, substrate selection significantly influences conductive hydrogel behavior, affecting cell viability and proliferation as a result. In particular, the ITO substrates were found to provide the best support for cell viability with an average of at least three times higher metabolic activity compared to platinum and gold mylar substrates over a 7 day stimulation period. The study offers new insights into substrate selection as a platform for neural cell stimulation and underscores the critical role of substrate materials in optimizing the efficacy of neural interfaces for biomedical applications. In addition to extending existing work, this study provides a robust platform for future explorations aimed at tailoring the full potential of tissue-engineered neural interfaces.

3D bioprinting approaches for spinal cord injury repair

Regenerative healing of spinal cord injury (SCI) poses an ongoing medical challenge by causing persistent neurological impairment and a significant socioeconomic burden. The complexity of spinal cord tissue presents hurdles to successful regeneration following injury, due to the difficulty of forming a biomimetic structure that faithfully replicates native tissue using conventional tissue engineering scaffolds. 3D bioprinting is a rapidly evolving technology with unmatched potential to create 3D biological tissues with complicated and hierarchical structure and composition. With the addition of biological additives such as cells and biomolecules, 3D bioprinting can fabricate preclinical implants, tissue or organ-like constructs, andin vitromodels through precise control over the deposition of biomaterials and other building blocks. This review highlights the characteristics and advantages of 3D bioprinting for scaffold fabrication to enable SCI repair, including bottom-up manufacturing, mechanical customization, and spatial heterogeneity. This review also critically discusses the impact of various fabrication parameters on the efficacy of spinal cord repair using 3D bioprinted scaffolds, including the choice of printing method, scaffold shape, biomaterials, and biological supplements such as cells and growth factors. High-quality preclinical studies are required to accelerate the translation of 3D bioprinting into clinical practice for spinal cord repair. Meanwhile, other technological advances will continue to improve the regenerative capability of bioprinted scaffolds, such as the incorporation of nanoscale biological particles and the development of 4D printing.

Polysaccharides as a promising platform for the treatment of spinal cord injury: A review

Spinal cord injury is incurable and often results in irreversible damage to motor function and autonomic sensory abilities. To enhance the effectiveness of therapeutic substances such as cells, growth factors, drugs, and nucleic acids for treating spinal cord injuries, as well as to reduce the toxic side effects of chemical reagents, polysaccharides have been gained attention due to their immunomodulatory properties and the biocompatibility and biodegradability of polysaccharide scaffolds. Polysaccharides hold potential as drug delivery systems in treating spinal cord injuries. This article aims to present an extensive evaluation of the potential applications of polysaccharide materials in scaffold construction, drug delivery, and immunomodulation over the past five years so that offering new directions and opportunities for the treatment of spinal cord injuries.

Recent advances in 3D printable conductive hydrogel inks for neural engineering

Recently, the 3D printing of conductive hydrogels has undergone remarkable advances in the fabrication of complex and functional structures. In the field of neural engineering, an increasing number of reports have been published on tissue engineering and bioelectronic approaches over the last few years. The convergence of 3D printing methods and electrically conducting hydrogels may create new clinical and therapeutic possibilities for precision regenerative medicine and implants. In this review, we summarize (i) advancements in preparation strategies for conductive materials, (ii) various printing techniques enabling the fabrication of electroconductive hydrogels, (iii) the required physicochemical properties of the printed constructs, (iv) their applications in bioelectronics and tissue regeneration for neural engineering, and (v) unconventional approaches and outlooks for the 3D printing of conductive hydrogels. This review provides technical insights into 3D printable conductive hydrogels and encompasses recent developments, specifically over the last few years of research in the neural engineering field.

An injectable, self-healing, electroconductive hydrogel loaded with neural stem cells and donepezil for enhancing local therapy effect of spinal cord injury

BACKGROUND

Spinal cord injury (SCI) is a serious injury with high mortality and disability rates, and there is no effective treatment at present. It has been reported that some treatments, such as drug intervention and stem cell transplantation have positive effects in promoting neurological recovery. Although those treatments are effective for nerve regeneration, many drawbacks, such as low stem cell survival rates and side effects caused by systemic medication, have limited their development. In recent years, injectable hydrogel materials have been widely used in tissue engineering due to their good biocompatibility, biodegradability, controllable properties, and low invasiveness. The treatment strategy of injectable hydrogels combined with stem cells or drugs has made some progress in SCI repair, showing the potential to overcome the drawbacks of traditional drugs and stem cell therapy.

METHODS

In this study, a novel injectable electroactive hydrogel (NGP) based on sodium hyaluronate oxide (SAO) and polyaniline-grafted gelatine (NH₂-Gel-PANI) was developed as a material in which to load neural stem cells (NSCs) and donepezil (DPL) to facilitate nerve regeneration after SCI. To evaluate the potential of the prepared NGP hydrogel in SCI repair applications, the surface morphology, self-repairing properties, electrical conductivity and cytocompatibility of the resulting hydrogel were analysed. Meanwhile, we evaluated the neural repair ability of NGP hydrogels loaded with DPL and NSCs using a rat model of spinal cord injury.

RESULTS

The NGP hydrogel has a suitable pore size, good biocompatibility, excellent conductivity, and injectable and self-repairing properties, and its degradation rate matches the repair cycle of spinal cord injury. In addition, DPL could be released continuously and slowly from the NGP hydrogel; thus, the NGP hydrogel could serve as an excellent carrier for drugs and cells. The results of in vitro cell experiments showed that the NGP hydrogel had good cytocompatibility and could significantly promote the neuronal differentiation and axon growth of NSCs, and loading the hydrogel with DPL could significantly enhance this effect. More importantly, the NGP hydrogel loaded with DPL showed a significant inhibitory effect on astrocytic differentiation of NSCs in vitro. Animal experiments showed that the combination of NGP hydrogel, DPL, and NSCs had the best therapeutic effect on the recovery of motor function and nerve conduction function in rats. NGP hydrogel loaded with NSCs and DPL not only significantly increased the myelin sheath area, number of new neurons and axon area but also minimized the area of the cystic cavity and glial scar and promoted neural circuit reconstruction.

CONCLUSIONS

The DPL- and NSC-laden electroactive hydrogel developed in this study is an ideal biomaterial for the treatment of traumatic spinal cord injury.

Functionalized hydrogels in neural injury repairing

Repairing injuries to the nervous system has always been a prominent topic in clinical research. Direct suturing and nerve displacement surgery are the primary treatment options, but they may not be suitable for long nerve injuries and may require sacrificing the functionality of other autologous nerves. With the emergence of tissue engineering, hydrogel materials have been identified as a promising technology with clinical translation potential for repairing nervous system injuries due to their excellent biocompatibility and ability to release or deliver functional ions. By controlling their composition and structure, hydrogels can be Functionalized and almost fully matched with nerve tissue and even simulate nerve conduction function and mechanical properties. Thus, they are suitable for repairing injuries to both the central and peripheral nervous systems. This article provides a review of recent research progress in functionalized hydrogels for nerve injury repair, highlighting the design differences among various materials and future research directions. We strongly believe that the development of functionalized hydrogels has great potential for improving the clinical treatment of nerve injuries.

Hyaluronic Acid as Bioink and Hydrogel Scaffolds for Tissue Engineering Applications

Bioprinting is an additive manufacturing technique that focuses on developing living tissue constructs using bioinks. Bioink is crucial in determining the stability of printed patterns, which remains a major challenge in bioprinting. Thus, the choices of bioink composition, modifications, and cross-linking methods are being continuously researched to augment the clinical translation of bioprinted constructs. Hyaluronic acid (HA) is a naturally occurring polysaccharide with the repeating unit of N-acetyl-glucosamine and d-glucuronic acid disaccharides. It is present in the extracellular matrix (ECM) of tissues (skin, cartilage, nerve, muscle, etc.) with a wide range of molecular weights. Due to the nature of its chemical structure, HA could be easily subjected to chemical modifications and cross-linking that would enable better printability and stability. These interesting properties have made HA an ideal choice of bioinks for developing tissue constructs for regenerative medicine applications. In this Review, the physicochemical properties, reaction chemistry involved in various cross-linking strategies, and biomedical applications of HA have been elaborately discussed. Further, the features of HA bioinks, emerging strategies in HA bioink preparations, and their applications in 3D bioprinting have been highlighted. Finally, the current challenges and future perspectives in the clinical translation of HA-based bioinks are outlined.

Matching mechanical heterogeneity of the native spinal cord augments axon infiltration in 3D-printed scaffolds

Scaffolds delivered to injured spinal cords to stimulate axon connectivity often match the anisotropy of native tissue using guidance cues along the rostral-caudal axis, but current approaches do not mimic the heterogeneity of host tissue mechanics. Although white and gray matter have different mechanical properties, it remains unclear whether tissue mechanics also vary along the length of the cord. Mechanical testing performed in this study indicates that bulk spinal cord mechanics do differ along anatomical level and that these differences are caused by variations in the ratio of white and gray matter. These results suggest that scaffolds recreating the heterogeneity of spinal cord tissue mechanics must account for the disparity between gray and white matter. Digital light processing (DLP) provides a means to mimic spinal cord topology, but has previously been limited to printing homogeneous mechanical properties. We describe a means to modify DLP to print scaffolds that mimic spinal cord mechanical heterogeneity caused by variation in the ratio of white and gray matter, which improves axon infiltration compared to controls exhibiting homogeneous mechanical properties. These results demonstrate that scaffolds matching the mechanical heterogeneity of white and gray matter improve the effectiveness of biomaterials transplanted within the injured spinal cord.

Fabrication and desired properties of conductive hydrogel dressings for wound healing

Conductive hydrogels are platforms recognized as constituting promising materials for tissue engineering applications. This is because such conductive hydrogels are characterized by the inherent conductivity properties while retaining favorable biocompatibility and mechanical properties. These conductive hydrogels can be particularly useful in enhancing wound healing since their favorable conductivity can promote the transport of essential ions for wound healing via the imposition of a so-called transepithelial potential. Other valuable properties of these conductive hydrogels, such as wound monitoring, stimuli-response etc., are also discussed in this study. Crucially, the properties of conductive hydrogels, such as 3D printability and monitoring properties, suggest the possibility of its use as an alternative wound dressing to traditional dressings such as bandages. This review, therefore, seeks to comprehensively explore the functionality of conductive hydrogels in wound healing, types of conductive hydrogels and their preparation strategies and crucial properties of hydrogels. This review will also assess the limitations of conductive hydrogels and future perspectives, with an emphasis on the development trend for conductive hydrogel uses in wound dressing fabrication for subsequent clinical applications.

High-stiffness, fast-crosslinking, cartilage matrix bioinks

Scaffolds derived from cartilage extracellular matrix may contain intrinsic chondroinductivity and have promise for cartilage regeneration. Cartilage is typically ground into devitalized particles (DVC) and several groups have pioneered innovative methods to rebuild the DVC into a new scaffold. However, challenges remain regarding the fluid and solid biomechanics of cartilage-based scaffolds in achieving 1) high mechanical performance akin to native cartilage and 2) easy surgical delivery/retention. Fortunately, photocrosslinking bioinks may benefit clinical translation: paste-like/injectable precursor rheology facilitates surgical placement, and in situ photocrosslinking enables material retention within any size/shape of defect. While solubilized DVC has been modified with methacryloyls (MeSDVC), MeSDVC is limited by slow crosslinking times (e.g., 5-10 min). Therefore, in the current study, we fabricated a pentenoate-modified SDVC (PSDVC), to enable a faster crosslinking reaction via a thiol-ene click chemistry. The crosslinking time of the PSDVC was faster (∼1.7 min) than MeSDVC (∼4 min). We characterized the solid and fluid mechanics/printabilities of PSDVC, pentenoate-modified hyaluronic acid (PHA), and the PHA or PSDVC with added DVC particles. While the addition of DVC particles enhanced the printed shape fidelity of PHA or PSDVC, the increased clogging decreased the ease of printing and cell viability after bioprinting, and future refinement is needed for DVC-containing bioinks. However, the PSDVC alone had a paste-like rheology/good bioprintability prior to crosslinking, the fastest crosslinking time (i.e., 1.7 min), and the highest compressive modulus (i.e., 3.12 ± 0.41 MPa) after crosslinking. Overall, the PSDVC may have future potential as a translational material for cartilage repair.

The Rheology and Printability of Cartilage Matrix-Only Biomaterials

The potential chondroinductivity from cartilage matrix makes it promising for cartilage repair; however, cartilage matrix-based hydrogels developed thus far have failed to match the mechanical performance of native cartilage or be bioprinted without adding polymers for reinforcement. There is a need for cartilage matrix-based hydrogels with robust mechanical performance and paste-like precursor rheology for bioprinting/enhanced surgical placement. In the current study, our goals were to increase hydrogel stiffness and develop the paste-like precursor/printability of our methacryl-modified solubilized and devitalized cartilage (MeSDVC) hydrogels. We compared two methacryloylating reagents, methacrylic anhydride (MA) and glycidyl methacrylate (GM), and varied the molar excess (ME) of MA from 2 to 20. The MA-modified MeSDVCs had greater methacryloylation than GM-modified MeSDVC (20 ME). While GM and most of the MA hydrogel precursors exhibited paste-like rheology, the 2 ME MA and GM MeSDVCs had the best printability (i.e., shape fidelity, filament collapse). After crosslinking, the 2 ME MA MeSDVC had the highest stiffness (1.55 ± 0.23 MPa), approaching the modulus of native cartilage, and supported the viability/adhesion of seeded cells for 15 days. Overall, the MA (2 ME) improved methacryloylation, hydrogel stiffness, and printability, resulting in a stand-alone MeSDVC printable biomaterial. The MeSDVC has potential as a future bioink and has future clinical relevance for cartilage repair.