3D Printed Polymeric Hydrogels for Nerve Regeneration

Hydrogels for Neural Regeneration: Exploring New Horizons

Nerve injury can significantly impair motor, sensory, and autonomic functions. Understanding nerve degeneration, particularly Wallerian degeneration, and the mechanisms of nerve regeneration is crucial for developing effective treatments. This manuscript reviews the use of advanced hydrogels that have been researched to enhance nerve regeneration. Hydrogels, due to their biocompatibility, tunable properties, and ability to create a supportive microenvironment, are being explored for their effectiveness in nerve repair. Various types of hydrogels, such as chitosan-, alginate-, collagen-, hyaluronic acid-, and peptide-based hydrogels, are discussed for their roles in promoting axonal growth, functional recovery, and myelination. Advanced formulations incorporating growth factors, bioactive molecules, and stem cells show significant promise in overcoming the limitations of traditional therapies. Despite these advancements, challenges in achieving robust and reliable nerve regeneration remain, necessitating ongoing research to optimize hydrogel-based interventions for neural regeneration.

Application of Adipose Stem Cells in 3D Nerve Guidance Conduit Prevents Muscle Atrophy and Improves Distal Muscle Compliance in a Peripheral Nerve Regeneration Model

BACKGROUND

Peripheral nerve injuries (PNIs) represent a significant clinical problem, and standard approaches to nerve repair have limitations. Recent breakthroughs in 3D printing and stem cell technologies offer a promising solution for nerve regeneration. The main purpose of this study was to examine the biomechanical characteristics in muscle tissue distal to a nerve defect in a murine model of peripheral nerve regeneration from physiological stress to failure.

METHODS

In this experimental study, we enrolled 18 Wistar rats in which we created a 10 mm sciatic nerve defect. Furthermore, we divided them into three groups as follows: in Group 1, we used 3D nerve guidance conduits (NGCs) and adipose stem cells (ASCs) in seven rats; in Group 2, we used only 3D NGCs for seven rats; and in Group 3, we created only the defect in four rats. We monitored the degree of atrophy at 4, 8, and 12 weeks by measuring the diameter of the tibialis anterior (TA) muscle. At the end of 12 weeks, we took the TA muscle and analyzed it uniaxially at 10% stretch until failure.

RESULTS

In the group of animals with 3D NGCs and ASCs, we recorded the lowest degree of atrophy at 4 weeks, 8 weeks, and 12 weeks after nerve reconstruction. At 10% stretch, the control group had the highest Cauchy stress values compared to the 3D NGC group (0.164 MPa vs. 0.141 MPa, p = 0.007) and the 3D NGC + ASC group (0.164 MPa vs. 0.123 MPa, p = 0.007). In addition, we found that the control group (1.763 MPa) had the highest TA muscle stiffness, followed by the 3D NGC group (1.412 MPa), with the best muscle elasticity showing in the group in which we used 3D NGC + ASC (1.147 MPa). At failure, TA muscle samples from the 3D NGC + ASC group demonstrated better compliance and a higher degree of elasticity compared to the other two groups (p = 0.002 and p = 0.008).

CONCLUSIONS

Our study demonstrates that the combination of 3D NGC and ASC increases the process of nerve regeneration and significantly improves the compliance and mechanical characteristics of muscle tissue distal to the injury site in a PNI murine model.

Metal-based porous hydrogels for highly conductive biomaterial scaffolds

Multielectrode arrays are fabricated from thin films of highly conductive and ductile metals which cannot mimic the natural environment of biological tissues. These properties limit the conformability of the electrode to the underlying target tissue, and present challenges in developing seamless interfaces. By introducing porous, hydrogel materials that are embedded with metal additives, highly conductive hydrogels can be formed. Tuning the hydrogel composition, % volume and aspect ratio of different additive(s), and the processing conditions of these composite materials can alter the mechanical and electrical properties. The resulting materials have a high surface area, and can be used as biomaterial scaffolds to support the growth of macrophages for 5 days. Further optimization can enable the use of the materials for the electrodes in implantable arrays, or as living electrode platforms to study and modulate various cellular cultures. These advancements would benefit both in vivo and in vitro applications of tissue engineering.

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.

Recent Advances in Cell and Functional Biomaterial Treatment for Spinal Cord Injury

Spinal cord injury (SCI) is a devastating central nervous system disease caused by accidental events, resulting in loss of sensory and motor function. Considering the multiple effects of primary and secondary injuries after spinal cord injury, including oxidative stress, tissue apoptosis, inflammatory response, and neuronal autophagy, it is crucial to understand the underlying pathophysiological mechanisms, local microenvironment changes, and neural tissue functional recovery for preparing novel treatment strategies. Treatment based on cell transplantation has become the forefront of spinal cord injury therapy. The transplanted cells provide physical and nutritional support for the damaged tissue. At the same time, the implantation of biomaterials with specific biological functions at the site of the SCI has also been proved to improve the local inhibitory microenvironment and promote axonal regeneration, etc. The combined transplantation of cells and functional biomaterials for SCI treatment can result in greater neuroprotective and regenerative effects by regulating cell differentiation, enhancing cell survival, and providing physical and directional support for axon regeneration and neural circuit remodeling. This article reviews the pathophysiology of the spinal cord, changes in the microenvironment after injury, and the mechanisms and strategies for spinal cord regeneration and repair. The article will focus on summarizing and discussing the latest intervention models based on cell and functional biomaterial transplantation and the latest progress in combinational therapies in SCI repair. Finally, we propose the future prospects and challenges of current treatment regimens for SCI repair, to provide references for scientists and clinicians to seek better SCI repair strategies in the future.

3D Printed Personalized Nerve Guide Conduits for Precision Repair of Peripheral Nerve Defects

The treatment of peripheral nerve defects has always been one of the most challenging clinical practices in neurosurgery. Currently, nerve autograft is the preferred treatment modality for peripheral nerve defects, while the therapy is constantly plagued by the limited donor, loss of donor function, formation of neuroma, nerve distortion or dislocation, and nerve diameter mismatch. To address these clinical issues, the emerged nerve guide conduits (NGCs) are expected to offer effective platforms to repair peripheral nerve defects, especially those with large or complex topological structures. Up to now, numerous technologies are developed for preparing diverse NGCs, such as solvent casting, gas foaming, phase separation, freeze-drying, melt molding, electrospinning, and three-dimensional (3D) printing. 3D printing shows great potential and advantages because it can quickly and accurately manufacture the required NGCs from various natural and synthetic materials. This review introduces the application of personalized 3D printed NGCs for the precision repair of peripheral nerve defects and predicts their future directions.

Advanced approaches to regenerate spinal cord injury: The development of cell and tissue engineering therapy and combinational treatments

Spinal cord injury (SCI) is a central nervous system (CNS) devastate event that is commonly caused by traumatic or non-traumatic events. The reinnervation of spinal cord axons is hampered through a myriad of devices counting on the damaged myelin, inflammation, glial scar, and defective inhibitory molecules. Unfortunately, an effective treatment to completely repair SCI and improve functional recovery has not been found. In this regard, strategies such as using cells, biomaterials, biomolecules, and drugs have been reported to be effective for SCI recovery. Furthermore, recent advances in combinatorial treatments, which address various aspects of SCI pathophysiology, provide optimistic outcomes for spinal cord regeneration. According to the global importance of SCI, the goal of this article review is to provide an overview of the pathophysiology of SCI, with an emphasis on the latest modes of intervention and current advanced approaches for the treatment of SCI, in conjunction with an assessment of combinatorial approaches in preclinical and clinical trials. So, this article can give scientists and clinicians' clues to help them better understand how to construct preclinical and clinical studies that could lead to a breakthrough in spinal cord regeneration.

3D printing of functional nerve guide conduits

Nerve guide conduits (NGCs), as alternatives to nerve autografts and allografts, have been widely explored as an advanced tool for the treatment of peripheral nerve injury. However, the repairing efficiency of NGCs still needs significant improvements. Functional NGCs that provide a more favorable microenvironment for promoting axonal elongation and myelination are of great importance. In recent years, 3D printing technologies have been widely applied in the fabrication of customized and complex constructs, exhibiting great potential for tissue engineering applications, especially for the construction of functional NGCs. In this review, we introduce the 3D printing technologies for manufacturing functional NGCs, including inkjet printing, extrusion printing, stereolithography-based printing and indirect printing. Further, we summarize the current methods and strategies for constructing functional NGCs, such as designing special conduit architectures, using appropriate materials and co-printing with different biological cues. Finally, the challenges and prospects for construction of functional NGCs are also presented.

Rapid and efficient immunomagnetic isolation of endothelial cells from human peripheral nerves

Endothelial cells (ECs) have gained an increased scientific focus since they were reported to provide guidance for Schwann cells and subsequently following axons after nerve injuries. However, previous protocols for the isolation of nerve-derived ECs from human nerves are ineffective regarding time and yield. Therefore, we established a novel and efficient protocol for the isolation of ECs from human peripheral nerves by means of immunomagnetic CD31-antibody conjugated Dynabeads and assessed the purity of the isolated cells. The easy-to-follow and time-effective isolation method allows the isolation of > 95% pure ECs. The isolated ECs were shown to express highly specific EC marker proteins and revealed functional properties by formation of CD31 and VE-cadherin positive adherens junctions, as well as ZO-1 positive tight-junctions. Moreover, the formation of capillary EC-tubes was observed in-vitro. The novel protocol for the isolation of human nerve-derived ECs allows and simplifies the usage of ECs in research of the human blood-nerve-barrier and peripheral nerve regeneration. Additionally, a potential experimental application of patient-derived nerve ECs in the in-vitro vascularization of artificial nerve grafts is feasible.

The interaction of stem cells and vascularity in peripheral nerve regeneration

The degree of nerve regeneration after peripheral nerve injury can be altered by the microenvironment at the site of injury. Stem cells and vascularity are postulated to be a part of a complex pathway that enhances peripheral nerve regeneration; however, their interaction remains unexplored. This review aims to summarize current knowledge on this interaction, including various mechanisms through which trophic factors are promoted by stem cells and angiogenesis. Angiogenesis after nerve injury is stimulated by hypoxia, mediated by vascular endothelial growth factor, resulting in the growth of pre-existing vessels into new areas. Modulation of distinct signaling pathways in stem cells can promote angiogenesis by the secretion of various angiogenic factors. Simultaneously, the importance of stem cells in peripheral nerve regeneration relies on their ability to promote myelin formation and their capacity to be influenced by the microenvironment to differentiate into Schwann-like cells. Stem cells can be acquired through various sources that correlate to their differentiation potential, including embryonic stem cells, neural stem cells, and mesenchymal stem cells. Each source of stem cells serves its particular differentiation potential and properties associated with the promotion of revascularization and nerve regeneration. Exosomes are a subtype of extracellular vesicles released from cell types and play an important role in cell-to-cell communication. Exosomes hold promise for future transplantation applications, as these vesicles contain fewer membrane-bound proteins, resulting in lower immunogenicity. This review presents pre-clinical and clinical studies that focus on selecting the ideal type of stem cell and optimizing stem cell delivery methods for potential translation to clinical practice. Future studies integrating stem cell-based therapies with the promotion of angiogenesis may elucidate the synergistic pathways and ultimately enhance nerve regeneration.

Natural-Based Biomaterials for Peripheral Nerve Injury Repair

Peripheral nerve injury treatment is a relevant problem because of nerve lesion high incidence and because of unsatisfactory regeneration after severe injuries, thus resulting in a reduced patient's life quality. To repair severe nerve injuries characterized by substance loss and to improve the regeneration outcome at both motor and sensory level, different strategies have been investigated. Although autograft remains the gold standard technique, a growing number of research articles concerning nerve conduit use has been reported in the last years. Nerve conduits aim to overcome autograft disadvantages, but they must satisfy some requirements to be suitable for nerve repair. A universal ideal conduit does not exist, since conduit properties have to be evaluated case by case; nevertheless, because of their high biocompatibility and biodegradability, natural-based biomaterials have great potentiality to be used to produce nerve guides. Although they share many characteristics with synthetic biomaterials, natural-based biomaterials should also be preferable because of their extraction sources; indeed, these biomaterials are obtained from different renewable sources or food waste, thus reducing environmental impact and enhancing sustainability in comparison to synthetic ones. This review reports the strengths and weaknesses of natural-based biomaterials used for manufacturing peripheral nerve conduits, analyzing the interactions between natural-based biomaterials and biological environment. Particular attention was paid to the description of the preclinical outcome of nerve regeneration in injury repaired with the different natural-based conduits.

Machine intelligence for nerve conduit design and production

Nerve guidance conduits (NGCs) have emerged from recent advances within tissue engineering as a promising alternative to autografts for peripheral nerve repair. NGCs are tubular structures with engineered biomaterials, which guide axonal regeneration from the injured proximal nerve to the distal stump. NGC design can synergistically combine multiple properties to enhance proliferation of stem and neuronal cells, improve nerve migration, attenuate inflammation and reduce scar tissue formation. The aim of most laboratories fabricating NGCs is the development of an automated process that incorporates patient-specific features and complex tissue blueprints (e.g. neurovascular conduit) that serve as the basis for more complicated muscular and skin grafts. One of the major limitations for tissue engineering is lack of guidance for generating tissue blueprints and the absence of streamlined manufacturing processes. With the rapid expansion of machine intelligence, high dimensional image analysis, and computational scaffold design, optimized tissue templates for 3D bioprinting (3DBP) are feasible. In this review, we examine the translational challenges to peripheral nerve regeneration and where machine intelligence can innovate bottlenecks in neural tissue engineering.

Perspectives on 3D Bioprinting of Peripheral Nerve Conduits

The peripheral nervous system controls the functions of sensation, movement and motor coordination of the body. Peripheral nerves can get damaged easily by trauma or neurodegenerative diseases. The injury can cause a devastating effect on the affected individual and his aides. Treatment modalities include anti-inflammatory medications, physiotherapy, surgery, nerve grafting and rehabilitation. 3D bioprinted peripheral nerve conduits serve as nerve grafts to fill the gaps of severed nerve bodies. The application of induced pluripotent stem cells, its derivatives and bioprinting are important techniques that come in handy while making living peripheral nerve conduits. The design of nerve conduits and bioprinting require comprehensive information on neural architecture, type of injury, neural supporting cells, scaffold materials to use, neural growth factors to add and to streamline the mechanical properties of the conduit. This paper gives a perspective on the factors to consider while bioprinting the peripheral nerve conduits.

Review of Polymeric Materials in 4D Printing Biomedical Applications

The purpose of 4D printing is to embed a product design into a deformable smart material using a traditional 3D printer. The 3D printed object can be assembled or transformed into intended designs by applying certain conditions or forms of stimulation such as temperature, pressure, humidity, pH, wind, or light. Simply put, 4D printing is a continuum of 3D printing technology that is now able to print objects which change over time. In previous studies, many smart materials were shown to have 4D printing characteristics. In this paper, we specifically review the current application, respective activation methods, characteristics, and future prospects of various polymeric materials in 4D printing, which are expected to contribute to the development of 4D printing polymeric materials and technology.

Additive Manufacturing of Nerve Decellularized Extracellular Matrix-Contained Polyurethane Conduits for Peripheral Nerve Regeneration

The nervous system is the part of our body that plays critical roles in the coordination of actions and sensory information as well as communication between different body parts through electrical signal transmissions. Current studies have shown that patients are likely to experience a functional loss if they have to go through a nerve repair for >15 mm lesion. The ideal treatment methodology is autologous nerve transplant, but numerous problems lie in this treatment method, such as lack of harvesting sites. Therefore, researchers are attempting to fabricate alternatives for nerve regeneration, and nerve conduit is one of the potential alternatives for nerve regeneration. In this study, we fabricated polyurethane/polydopamine/extracellular matrix (PU/PDA/ECM) nerve conduits using digital light processing (DLP) technology and assessed for its physical properties, biodegradability, cytocompatibility, neural related growth factor, and proteins secretion and expression and its potential in allowing cellular adhesion and proliferation. It was reported that PU/PDA/ECM nerve conduits were more hydrophilic and allowed enhanced cellular adhesion, proliferation, expression, and secretion of neural-related proteins (collagen I and laminin) and also enhanced expression of neurogenic proteins, such as nestin and microtubule-associated protein 2 (MAP2). In addition, PU/PDA/ECM nerve conduits were reported to be non-cytotoxic, had sustained biodegradability, and had similar physical characteristics as PU conduits. Therefore, we believed that PU/PDA/ECM nerve conduits could be a potential candidate for future nerve-related research or clinical applications.

Imaging in the repair of peripheral nerve injury

Surgical intervention followed by physical therapy remains the major way to repair damaged nerves and restore function. Imaging constitutes promising, yet underutilized, approaches to improve surgical and postoperative techniques. Dedicated methods for imaging nerve regeneration will potentially provide surgical guidance, enable recovery monitoring and postrepair intervention, elucidate failure mechanisms and optimize preclinical procedures. Herein, we present an outline of promising innovations in imaging-based tracking of in vivo peripheral nerve regeneration. We emphasize optical imaging because of its cost, versatility, relatively low toxicity and sensitivity. We discuss the use of targeted probes and contrast agents (small molecules and nanoparticles) to facilitate nerve regeneration imaging and the engineering of grafts that could be used to track nerve repair. We also discuss how new imaging methods might overcome the most significant challenges in nerve injury treatment.

Scaffold-Free Bioprinter Utilizing Layer-By-Layer Printing of Cellular Spheroids

Free from the limitations posed by exogenous scaffolds or extracellular matrix-based materials, scaffold-free engineered tissues have immense clinical potential. Biomaterials may produce adverse responses, interfere with cell–cell interaction, or affect the extracellular matrix integrity of cells. The scaffold-free Kenzan method can generate complex tissues using spheroids on an array of needles but could be inefficient in terms of time, as it moves and places only a single spheroid at a time. We aimed to design and construct a novel scaffold-free bioprinter that can print an entire layer of spheroids at once, effectively reducing the printing time. The bioprinter was designed using computer-aided design software and constructed from machined, 3D printed, and commercially available parts. The printing efficiency and the operating precision were examined using Zirconia and alginate beads, which mimic spheroids. In less than a minute, the printer could efficiently pick and transfer the beads to the printing surface and assemble them onto the 4 × 4 needles. The average overlap coefficient between layers was measured and found to be 0.997. As a proof of concept using human induced pluripotent stem cell-derived spheroids, we confirmed the ability of the bioprinter to place cellular spheroids onto the needles efficiently to print an entire layer of tissue. This novel layer-by-layer, scaffold-free bioprinter is efficient and precise in operation and can be easily scaled to print large tissues.

3D Printing of Amino Resin-based Photosensitive Materials on Multi-parameter Optimization Design for Vascular Engineering Applications

Cardiovascular diseases are currently the most common cause of death globally and of which, the golden treatment method for severe cardiovascular diseases or coronary artery diseases are implantations of synthetic vascular grafts. However, such grafts often come with rejections and hypersensitivity reactions. With the emergence of regenerative medicine, researchers are now trying to explore alternative ways to produce grafts that are less likely to induce immunological reactions in patients. The main goal of such studies is to produce biocompatible artificial vascular grafts with the capability of allowing cellular adhesion and cellular proliferation for tissues regeneration. The Design of Experimental concepts is employed into the manufacturing process of digital light processing (DLP) 3D printing technology to explore near-optimal processing parameters to produce artificial vascular grafts with vascular characteristics that are close to native vessels by assessing for the cause and effect relationships between different ratios of amino resin (AR), 2-hydroxyethyl methacrylate (HEMA), dopamine, and curing durations. We found that with proper optimization of fabrication procedures and ratios of materials, we are able to successfully fabricate vascular grafts with good printing resolutions. These had similar physical properties to native vessels and were able to support cellular adhesion and proliferation. This study could support future studies in exploring near-optimal processes for fabrication of artificial vascular grafts that could be adapted into clinical applications.

3D Bioprinted Nanocellulose-Based Hydrogels for Tissue Engineering Applications: A Brief Review

Nanocellulosic materials, such as cellulose nanocrystals, cellulose nanofibers, and bacterial nanocellulose, that display high surface area, mechanical strength, biodegradability, and tunable surface chemistry have attracted great attention over the last decade for biomedical applications. Simultaneously, 3D printing is revolutionizing the field of biomedical engineering, which enables the fast and on-demand printing of customizable scaffolds, tissues, and organs. Nanocellulosic materials hold tremendous potential for 3D bioprinting due to their printability, their shear thinning behavior, their ability to live cell support and owing to their excellent biocompatibility. The amalgamation of nanocellulose-based feedstocks and 3D bioprinting is therefore of critical interest for the development of advanced functional 3D hydrogels. In this context, this review briefly discusses the most recent key developments and challenges in 3D bioprinting nanocellulose-based hydrogel constructs that have been successfully tested for mammalian cell viability and used in tissue engineering applications.

Alginate/Gelatin Hydrogels Reinforced with TiO₂ and β-TCP Fabricated by Microextrusion-based Printing for Tissue Regeneration

Three-dimensional (3D) printing technologies have become an attractive manufacturing process to fabricate scaffolds in tissue engineering. Recent research has focused on the fabrication of alginate complex shaped structures that closely mimic biological organs or tissues. Alginates can be effectively manufactured into porous three-dimensional networks for tissue engineering applications. However, the structure, mechanical properties, and shape fidelity of 3D-printed alginate hydrogels used for preparing tissue-engineered scaffolds is difficult to control. In this work, the use of alginate/gelatin hydrogels reinforced with TiO₂ and β-tricalcium phosphate was studied to tailor the mechanical properties of 3D-printed hydrogels. The hydrogels reinforced with TiO₂ and β-TCP showed enhanced mechanical properties up to 20 MPa of elastic modulus. Furthermore, the pores of the crosslinked printed structures were measured with an average pore size of 200 μm. Additionally, it was found that as more layers of the design were printed, there was an increase of the line width of the bottom layers due to its viscous deformation. Shrinkage of the design when the hydrogel is crosslinked and freeze dried was also measured and found to be up to 27% from the printed design. Overall, the proposed approach enabled fabrication of 3D-printed alginate scaffolds with adequate physical properties for tissue engineering applications.