3D Printed Neural Regeneration Devices

The combined application of stem cells and three-dimensional bioprinting scaffolds for the repair of spinal cord injury

Spinal cord injury is considered one of the most difficult injuries to repair and has one of the worst prognoses for injuries to the nervous system. Following surgery, the poor regenerative capacity of nerve cells and the generation of new scars can make it very difficult for the impaired nervous system to restore its neural functionality. Traditional treatments can only alleviate secondary injuries but cannot fundamentally repair the spinal cord. Consequently, there is a critical need to develop new treatments to promote functional repair after spinal cord injury. Over recent years, there have been several developments in the use of stem cell therapy for the treatment of spinal cord injury. Alongside significant developments in the field of tissue engineering, three-dimensional bioprinting technology has become a hot research topic due to its ability to accurately print complex structures. This led to the loading of three-dimensional bioprinting scaffolds which provided precise cell localization. These three-dimensional bioprinting scaffolds could repair damaged neural circuits and had the potential to repair the damaged spinal cord. In this review, we discuss the mechanisms underlying simple stem cell therapy, the application of different types of stem cells for the treatment of spinal cord injury, and the different manufacturing methods for three-dimensional bioprinting scaffolds. In particular, we focus on the development of three-dimensional bioprinting scaffolds for the treatment of spinal cord injury.

Advances in 3D tissue models for neural engineering: self-assembled versus engineered tissue models

Neural tissue engineering has emerged as a promising field that aims to create functional neural tissue for therapeutic applications, drug screening, and disease modelling. It is becoming evident in the literature that this goal requires development of three-dimensional (3D) constructs that can mimic the complex microenvironment of native neural tissue, including its biochemical, mechanical, physical, and electrical properties. These 3D models can be broadly classified as self-assembled models, which include spheroids, organoids, and assembloids, and engineered models, such as those based on decellularized or polymeric scaffolds. Self-assembled models offer advantages such as the ability to recapitulate neural development and disease processes in vitro, and the capacity to study the behaviour and interactions of different cell types in a more realistic environment. However, self-assembled constructs have limitations such as lack of standardised protocols, inability to control the cellular microenvironment, difficulty in controlling structural characteristics, reproducibility, scalability, and lengthy developmental timeframes. Integrating biomimetic materials and advanced manufacturing approaches to present cells with relevant biochemical, mechanical, physical, and electrical cues in a controlled tissue architecture requires alternate engineering approaches. Engineered scaffolds, and specifically 3D hydrogel-based constructs, have desirable properties, lower cost, higher reproducibility, long-term stability, and they can be rapidly tailored to mimic the native microenvironment and structure. This review explores 3D models in neural tissue engineering, with a particular focus on analysing the benefits and limitations of self-assembled organoids compared with hydrogel-based engineered 3D models. Moreover, this paper will focus on hydrogel based engineered models and probe their biomaterial components, tuneable properties, and fabrication techniques that allow them to mimic native neural tissue structures and environment. Finally, the current challenges and future research prospects of 3D neural models for both self-assembled and engineered models in neural tissue engineering will be discussed.

4D Printed Nerve Conduit with In Situ Neurogenic Guidance for Nerve Regeneration

Nerve repair poses a significant challenge in the field of tissue regeneration. As a bioengineered therapeutic method, nerve conduits have been developed to address damaged nerve repair. However, despite their remarkable potential, it is still challenging to encompass complex physiologically microenvironmental cues (both biophysical and biochemical factors) to synergistically regulate stem cell differentiation within the implanted nerve conduits, especially in a facile manner. In this study, a neurogenic nerve conduit with self-actuated ability has been developed by in situ immobilization of neurogenic factors onto printed architectures with aligned microgrooves. One objective was to facilitate self-entubulation, ultimately enhancing nerve repairs. Our results demonstrated that the integration of topographical and in situ biological cues could accurately mimic native microenvironments, leading to a significant improvement in neural alignment and enhanced neural differentiation within the conduit. This innovative approach offers a revolutionary method for fabricating multifunctional nerve conduits, capable of modulating neural regeneration efficiently. It has the potential to accelerate the functional recovery of injured neural tissues, providing a promising avenue for advancing nerve repair therapies.

Biomaterials targeting the microenvironment for spinal cord injury repair: progression and perspectives

Spinal cord injury (SCI) disrupts nerve pathways and affects sensory, motor, and autonomic function. There is currently no effective treatment for SCI. SCI occurs within three temporal periods: acute, subacute, and chronic. In each period there are different alterations in the cells, inflammatory factors, and signaling pathways within the spinal cord. Many biomaterials have been investigated in the treatment of SCI, including hydrogels and fiber scaffolds, and some progress has been made in the treatment of SCI using multiple materials. However, there are limitations when using individual biomaterials in SCI treatment, and these limitations can be significantly improved by combining treatments with stem cells. In order to better understand SCI and to investigate new strategies for its treatment, several combination therapies that include materials combined with cells, drugs, cytokines, etc. are summarized in the current review.

TPP-Based Microfluidic Chip Design and Fabrication Method for Optimized Nerve Cells Directed Growth

Microfluidic chips offer high customizability and excellent biocompatibility, holding important promise for the precise control of biological growth at the microscale. However, the microfluidic chips employed in the studies of regulating cell growth are typically fabricated through 2D photolithography. This approach partially restricts the diversity of cell growth platform designs and manufacturing efficiency. This paper presents a method for designing and manufacturing neural cell culture microfluidic chips (NCMC) using two-photon polymerization (TPP), where the discrete and directional cell growth is optimized through studying the associated geometric parameters of on-chip microchannels. This study involves simulations and discussions regarding the effects of different hatching distances on the mold surface topography and printing time in the Describe print preview module, which determines the appropriate printing accuracy corresponding to the desired mold structure. With the assistance of the 3D maskless lithography system, micron-level rapid printing of target molds with different dimensions were achieved. For NCMC with different geometric parameters, COMSOL software was used to simulate the local flow velocity and shear stress characteristics within the microchannels. SH-SY5Y cells were selected for directional differentiation experiments on NCMC with different geometric parameters. The results demonstrate that the TPP-based manufacturing method efficiently constructs neural microfluidic chips with high precision, optimizing the discrete and directional cell growth. We anticipate that our method for designing and manufacturing NCMC will hold great promise in construction and application of microscale 3D drug models.

Recent advances in implantable sensors and electronics using printable materials for advanced healthcare

This review article focuses on the recent printing technological progress in healthcare, underscoring the significant potential of implantable devices across diverse applications. Printing technologies have widespread use in developing health monitoring devices, diagnostic systems, and surgical devices. Recent years have witnessed remarkable progress in fabricating low-profile implantable devices, driven by advancements in printing technologies and nanomaterials. The importance of implantable biosensors and bioelectronics is highlighted, specifically exploring printing tools using bio-printable inks for practical applications, including a detailed examination of fabrication processes and essential parameters. This review also justifies the need for mechanical and electrical compatibility between bioelectronics and biological tissues. In addition to technological aspects, this article delves into the importance of appropriate packaging methods to enhance implantable devices' performance, compatibility, and longevity, which are made possible by integrating cutting-edge printing technology. Collectively, we aim to shed light on the holistic landscape of implantable biosensors and bioelectronics, showcasing their evolving role in advancing healthcare through innovative printing technologies.

Beyond the limiting gap length: peripheral nerve regeneration through implantable nerve guidance conduits

Peripheral nerve damage results in the loss of sensorimotor and autonomic functions, which is a significant burden to patients. Furthermore, nerve injuries greater than the limiting gap length require surgical repair. Although autografts are the preferred clinical choice, their usage is impeded by their limited availability, dimensional mismatch, and the sacrifice of another functional donor nerve. Accordingly, nerve guidance conduits, which are tubular scaffolds engineered to provide a biomimetic environment for nerve regeneration, have emerged as alternatives to autografts. Consequently, a few nerve guidance conduits have received clinical approval for the repair of short-mid nerve gaps but failed to regenerate limiting gap damage, which represents the bottleneck of this technology. Thus, it is still necessary to optimize the morphology and constituent materials of conduits. This review summarizes the recent advances in nerve conduit technology. Several manufacturing techniques and conduit designs are discussed, with emphasis on the structural improvement of simple hollow tubes, additive manufacturing techniques, and decellularized grafts. The main objective of this review is to provide a critical overview of nerve guidance conduit technology to support regeneration in long nerve defects, promote future developments, and speed up its clinical translation as a reliable alternative to autografts.

Printable devices for neurotechnology

Printable electronics for neurotechnology is a rapidly emerging field that leverages various printing techniques to fabricate electronic devices, offering advantages in rapid prototyping, scalability, and cost-effectiveness. These devices have promising applications in neurobiology, enabling the recording of neuronal signals and controlled drug delivery. This review provides an overview of printing techniques, materials used in neural device fabrication, and their applications. The printing techniques discussed include inkjet, screen printing, flexographic printing, 3D printing, and more. Each method has its unique advantages and challenges, ranging from precise printing and high resolution to material compatibility and scalability. Selecting the right materials for printable devices is crucial, considering factors like biocompatibility, flexibility, electrical properties, and durability. Conductive materials such as metallic nanoparticles and conducting polymers are commonly used in neurotechnology. Dielectric materials, like polyimide and polycaprolactone, play a vital role in device fabrication. Applications of printable devices in neurotechnology encompass various neuroprobes, electrocorticography arrays, and microelectrode arrays. These devices offer flexibility, biocompatibility, and scalability, making them cost-effective and suitable for preclinical research. However, several challenges need to be addressed, including biocompatibility, precision, electrical performance, long-term stability, and regulatory hurdles. This review highlights the potential of printable electronics in advancing our understanding of the brain and treating neurological disorders while emphasizing the importance of overcoming these challenges.

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.

Anisotropic microtopography surface of chitosan scaffold regulating skin precursor-derived Schwann cells towards repair phenotype promotes neural regeneration

For repairing peripheral nerve and spinal cord defects, biomaterial scaffold-based cell-therapy was emerged as an effective strategy, requiring the positive response of seed cells to biomaterial substrate and environment signals. Previous work highlighted that the imposed surface properties of scaffold could provide important guidance cues to adhered cells for polarization. However, the insufficiency of native Schwann cells and unclear cellular response mechanisms remained to be addressed. Given that, this study aimed to illuminate the micropatterned chitosan-film action on the rat skin precursor-derived Schwann cells (SKP-SCs). Chitosan-film with different ridge/groove size was fabricated and applied for the SKP-SCs induction. Results indicated that SKP-SCs cultured on 30 μm size microgroove surface showed better oriented alignment phenotype. Induced SKP-SCs presented similar genic phenotype as repair Schwann cells, increasing expression of c-Jun, neural cell adhesion molecule, and neurotrophic receptor p75. Moreover, SKP-SC-secretome was subjected to cytokine array GS67 assay, data indicated the regulation of paracrine phenotype, a panel of cytokines was verified up-regulated at secreted level and gene expression level in induced SKP-SCs. These up-regulated cytokines exhibit a series of promotive neural regeneration functions, including cell survival, cell migration, cell proliferation, angiogenesis, axon growth, and cellular organization etc. through bioinformatics analysis. Furthermore, the effectively polarized SKP-SCs-sourced secretome, promoted the proliferation and migration capacity of the primarily cultured native rat Schwann cells, and augmented neurites growth of the cultured motoneurons, as well as boosted axonal regrowth of the axotomy-injured motoneurons. Taken together, SKP-SCs obtained pro-neuroregeneration phenotype in adaptive response to the anisotropic topography surface of chitosan-film, displayed the oriented parallel growth, the transition towards repair Schwann cell genic phenotype, and the enhanced paracrine effect on neural regeneration. This study provided novel insights into the potency of anisotropic microtopography surface to Schwann-like cells phenotype regulation, that facilitating to provide promising engineered cell-scaffold in neural injury therapies.

Histopathological and immunohistochemical investigation of the effect of Shilajit in rats with experimental spinal cord injury

BACKGROUND

This experimental study was designed to investigate the histopathological and immunohistochemical effects of Shilajit in rats with experimentally induced spinal cord injury (SCI).

METHODS

The rats were divided into three groups: Control group: The group in which spinal cord damage was created but no drug was administered. Low-dose group: This is the group in which intraperitoneal Shilajit is given at a dose of 150 mg/kg at the 1st h, 1st day, 2nd day, and 3rd day after spinal cord damage was induced. High-dose group: This is the group in which intraperitoneal Shilajit is given at a dose of 250 mg/kg at the 1st h, 1st day, 2nd day, and 3rd day after spinal cord damage was induced. Thin sections taken from the spinal cord after euthanasia were sent for histopathological and immunohistochemical examination.

RESULTS

Histopathological examination of the high-dose group showed lower amounts of morphological findings compared to the low-dose group and control group. While a significant CD68 immune reaction was observed in the control group of rats with spinal injury, the positive immune reaction was found to be significantly decreased in the Shilajit-applied groups.

CONCLUSION

It is thought that the use of Shilajit in SCI will reduce the effects of secondary damage in SCI and that its administra-tion to such patients will have positive effects on the results.

Conformable Multifunctional Space Fabric by Metal 3D Printing for Collision Hazard Protection and Self-Powered Monitoring

The monitoring of space debris assumes paramount significance to ensure the sustainability and security of space activities as well as underground bases in outer space. However, designing a wide range monitoring system with easy fabrication, low power, and high precision remains an urgent challenge under the scarcity of materials and extreme environment conditions of outer space. Here, we designed a one-piece, robust, but flexible, and repairable 3D metal-printed triboelectric nanogenerator (FR-TENG) by incorporating the advantages of standardization and customization of outer space 3D metal printing. Inspired by the structure of hexagonal and pangolin scales, a curved structure is ingeniously applied in the design of 3D printed metal to adapt different curved surfaces while maintaining superior compressive strength, providing excellent flexibility and shape adaptability. Benefiting from the unique structural design, the FR-TENG has a minimum length of 1 cm with a weight of only 3.5 g and the minimum weight resolution detected of 9.6 g, with a response time of 20 ms. Furthermore, a multichannel self-powered collision monitoring system has been developed to monitor minor collisions, providing warnings to determine potential impacts on the space station and bases surfaces. The system may contribute to ensuring the successful completion of space missions and providing a safer space environment for the exploration of extraterrestrial life and the establishment of underground protective bases.

Low-intensity pulsed ultrasound enhances neurite growth in serum-starved human neuroblastoma cells

Introduction

Low-intensity pulsed ultrasound (LIPUS) is a recognized tool for promoting nerve regeneration and repair; however, the intracellular mechanisms of LIPUS stimulation remain underexplored.

Method

The present study delves into the effects of varying LIPUS parameters, namely duty cycle, spatial average-temporal average (SATA) intensity, and ultrasound amplitude, on the therapeutic efficacy using SK-N-SH cells cultured in serum-starved conditions. Four distinct LIPUS settings were employed: (A) 50 mW/cm2, 40%, (B) 25 mW/cm2, 10%, (C) 50 mW/cm2, 20%, and (D) 25 mW/cm2, 10%.

Results

Immunochemistry analysis exhibited neurite outgrowth promotion in all LIPUS-treated groups except for Group D. Further, LIPUS treatment was found to successfully promote brain-derived neurotrophic factor (BDNF) expression and enhance the phosphorylation of extracellular signal-regulated kinase (ERK)1/2, protein kinase B (Akt), and mammalian target of rapamycin (mTOR) signaling pathways, as evidenced by western blot analysis.

Discussion

The study suggests that the parameter combination of LIPUS determines the therapeutic efficacy of LIPUS. Future investigations should aim to optimize these parameters for different cell types and settings and delve deeper into the cellular response mechanism to LIPUS treatment. Such advancements may aid in tailoring LIPUS treatment strategies to specific therapeutic needs.

Development and Prospective Applications of 3D Membranes as a Sensor for Monitoring and Inducing Tissue Regeneration

For decades, tissue regeneration has been a challenging issue in scientific modeling and human practices. Although many conventional therapies are already used to treat burns, muscle injuries, bone defects, and hair follicle injuries, there remains an urgent need for better healing effects in skin, bone, and other unique tissues. Recent advances in three-dimensional (3D) printing and real-time monitoring technologies have enabled the creation of tissue-like membranes and the provision of an appropriate microenvironment. Using tissue engineering methods incorporating 3D printing technologies and biomaterials for the extracellular matrix (ECM) containing scaffolds can be used to construct a precisely distributed artificial membrane. Moreover, advances in smart sensors have facilitated the development of tissue regeneration. Various smart sensors may monitor the recovery of the wound process in different aspects, and some may spontaneously give feedback to the wound sites by releasing biological factors. The combination of the detection of smart sensors and individualized membrane design in the healing process shows enormous potential for wound dressings. Here, we provide an overview of the advantages of 3D printing and conventional therapies in tissue engineering. We also shed light on different types of 3D printing technology, biomaterials, and sensors to describe effective methods for use in skin and other tissue regeneration, highlighting their strengths and limitations. Finally, we highlight the value of 3D bioengineered membranes in various fields, including the modeling of disease, organ-on-a-chip, and drug development.

Advances in Biomimetic Nerve Guidance Conduits for Peripheral Nerve Regeneration

Injuries to the peripheral nervous system are a common clinical issue, causing dysfunctions of the motor and sensory systems. Surgical interventions such as nerve autografting are necessary to repair damaged nerves. Even with autografting, i.e., the gold standard, malfunctioning and mismatches between the injured and donor nerves often lead to unwanted failure. Thus, there is an urgent need for a new intervention in clinical practice to achieve full functional recovery. Nerve guidance conduits (NGCs), providing physicochemical cues to guide neural regeneration, have great potential for the clinical regeneration of peripheral nerves. Typically, NGCs are tubular structures with various configurations to create a microenvironment that induces the oriented and accelerated growth of axons and promotes neuron cell migration and tissue maturation within the injured tissue. Once the native neural environment is better understood, ideal NGCs should maximally recapitulate those key physiological attributes for better neural regeneration. Indeed, NGC design has evolved from solely physical guidance to biochemical stimulation. NGC fabrication requires fundamental considerations of distinct nerve structures, the associated extracellular compositions (extracellular matrices, growth factors, and cytokines), cellular components, and advanced fabrication technologies that can mimic the structure and morphology of native extracellular matrices. Thus, this review mainly summarizes the recent advances in the state-of-the-art NGCs in terms of biomaterial innovations, structural design, and advanced fabrication technologies and provides an in-depth discussion of cellular responses (adhesion, spreading, and alignment) to such biomimetic cues for neural regeneration and repair.

Prospects of Using Chitosan-Based Biopolymers in the Treatment of Peripheral Nerve Injuries

Peripheral nerve injuries are common neurological disorders, and the available treatment options, such as conservative management and surgical repair, often yield limited results. However, there is growing interest in the potential of using chitosan-based biopolymers as a novel therapeutic approach to treating these injuries. Chitosan-based biopolymers possess unique characteristics, including biocompatibility, biodegradability, and the ability to stimulate cell proliferation, making them highly suitable for repairing nerve defects and promoting nerve regeneration and functional recovery. Furthermore, these biopolymers can be utilized in drug delivery systems to control the release of therapeutic agents and facilitate the growth of nerve cells. This comprehensive review focuses on the latest advancements in utilizing chitosan-based biopolymers for peripheral nerve regeneration. By harnessing the potential of chitosan-based biopolymers, we can pave the way for innovative treatment strategies that significantly improve the outcomes of peripheral nerve injury repair, offering renewed hope and better prospects for patients in need.

Recent advances and future directions of 3D to 6D printing in brain cancer treatment and neural tissue engineering

The field of neural tissue engineering has undergone a revolution due to advancements in three-dimensional (3D) printing technology. This technology now enables the creation of intricate neural tissue constructs with precise geometries, topologies, and mechanical properties. Currently, there are various 3D printing techniques available, such as stereolithography and digital light processing, and a wide range of materials can be utilized, including hydrogels, biopolymers, and synthetic materials. Furthermore, the development of four-dimensional (4D) printing has gained traction, allowing for the fabrication of structures that can change shape over time using techniques such as shape-memory polymers. These innovations have the potential to facilitate neural regeneration, drug screening, disease modeling, and hold tremendous promise for personalized diagnostics, precise therapeutic strategies against brain cancers. This review paper provides a comprehensive overview of the current state-of-the-art techniques and materials for 3D printing in neural tissue engineering and brain cancer. It focuses on the exciting possibilities that lie ahead, including the emerging field of 4D printing. Additionally, the paper discusses the potential applications of five-dimensional and six-dimensional printing, which integrate time and biological functions into the printing process, in the fields of neuroscience.

How 3D scaffolds with different mechanical properties affect the activity of neuronal networks in in vitro models. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2023;2023:1-4. [PMID: 38083594 DOI: 10.1109/embc40787.2023.10340624] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Three-dimensionality has been proven extensively to be critical in the development of a reliable model for different anatomical compartments and for many diseases. Currently, we can produce implantable structures that help in the regeneration of different tissues such as bone and heart. Different is the situation when we consider the neuronal compartment. As it is still difficult to understand exactly how the brain computes, to conceive how the complex chain of neuronal events can generate conscious behavior, a comprehensive and workable model of neuronal tissue still has to be found. In this perspective, in the present work, we developed and compared different 3D scaffolds to understand the effects produced by the mechanical and material properties of four different scaffolds on a 3D neuronal network. To help in preclinical testing procedure, the scalability and ease-of-use of the different approaches were also taken into consideration.Clinical Relevance- By comparing different 3D scaffolds for the creation of neuronal constructs, the results in this paper move towards understanding the best strategy to develop functional 3D neuronal units for reliable pre-clinical studies.

Micron track chitosan conduit fabricated by 3D-printed model topography provides bionic microenvironment for peripheral nerve regeneration

The micron track conduit (MTC) and nerve factor provide a physical and biological model for simulating peripheral nerve growth and have potential applications for nerve injury. However, it has rarely been reported that they synergize on peripheral nerves. In this study, we used bioderived chitosan as a substrate to design and construct a neural repair conduit with micron track topography using threedimensional (3D) printing topography. We loaded the MTC with neurotrophin-3 (NT-3) to promote the regeneration of sensory and sympathetic neurons in the peripheral nervous system. We found that the MTC@NT3 composite nerve conduit mimicked the microenvironment of peripheral nerves and promoted axonal regeneration while inducing the targeted growth of Schwann cells, which would promote functional recovery in rats with peripheral nerve injury. Artificial nerve implants with functional properties can be developed using the strategy presented in this study.

3D and 4D Bioprinting Technologies: A Game Changer for the Biomedical Sector?

Bioprinting is an innovative and emerging technology of additive manufacturing (AM) and has revolutionized the biomedical sector by printing three-dimensional (3D) cell-laden constructs in a precise and controlled manner for numerous clinical applications. This approach uses biomaterials and varying types of cells to print constructs for tissue regeneration, e.g., cardiac, bone, corneal, cartilage, neural, and skin. Furthermore, bioprinting technology helps to develop drug delivery and wound healing systems, bio-actuators, bio-robotics, and bio-sensors. More recently, the development of four-dimensional (4D) bioprinting technology and stimuli-responsive materials has transformed the biomedical sector with numerous innovations and revolutions. This issue also leads to the exponential growth of the bioprinting market, with a value over billions of dollars. The present study reviews the concepts and developments of 3D and 4D bioprinting technologies, surveys the applications of these technologies in the biomedical sector, and discusses their potential research topics for future works. It is also urged that collaborative and valiant efforts from clinicians, engineers, scientists, and regulatory bodies are needed for translating this technology into the biomedical, pharmaceutical, and healthcare systems.

Collagen for neural tissue engineering: Materials, strategies, and challenges

Neural tissue engineering (NTE) has made remarkable strides in recent years and holds great promise for treating several devastating neurological disorders. Selecting optimal scaffolding material is crucial for NET design strategies that enable neural and non-neural cell differentiation and axonal growth. Collagen is extensively employed in NTE applications due to the inherent resistance of the nervous system against regeneration, functionalized with neurotrophic factors, antagonists of neural growth inhibitors, and other neural growth-promoting agents. Recent advancements in integrating collagen with manufacturing strategies, such as scaffolding, electrospinning, and 3D bioprinting, provide localized trophic support, guide cell alignment, and protect neural cells from immune activity. This review categorises and analyses collagen-based processing techniques investigated for neural-specific applications, highlighting their strengths and weaknesses in repair, regeneration, and recovery. We also evaluate the potential prospects and challenges of using collagen-based biomaterials in NTE. Overall, this review offers a comprehensive and systematic framework for the rational evaluation and applications of collagen in NTE.

Bridging Gaps in Peripheral Nerves: From Current Strategies to Future Perspectives in Conduit Design

In peripheral nerve injuries (PNI) with substance loss, where tensionless end-to-end suture is not achievable, the positioning of a graft is required. Available options include autografts (e.g., sural nerve, medial and lateral antebrachial cutaneous nerves, superficial branch of the radial nerve), allografts (Avance^®; human origin), and hollow nerve conduits. There are eleven commercial hollow conduits approved for clinical, and they consist of devices made of a non-biodegradable synthetic polymer (polyvinyl alcohol), biodegradable synthetic polymers (poly(DL-lactide-ε-caprolactone); polyglycolic acid), and biodegradable natural polymers (collagen type I with/without glycosaminoglycan; chitosan; porcine small intestinal submucosa); different resorption times are available for resorbable guides, ranging from three months to four years. Unfortunately, anatomical/functional nerve regeneration requirements are not satisfied by any of the possible alternatives; to date, focusing on wall and/or inner lumen organization/functionalization seems to be the most promising strategy for next-generation device fabrication. Porous or grooved walls as well as multichannel lumens and luminal fillers are the most intriguing options, eventually also including the addition of cells (Schwann cells, bone marrow-derived, and adipose tissue derived stem cells) to support nerve regeneration. This review aims to describe common alternatives for severe PNI recovery with a highlight of future conduits.

3D culture of fibroblasts and neuronal cells on microfabricated free-floating carriers

3D cell culture is a relatively recent trend in biomedical research for artificially mimicking in vivo environment and providing three dimensions for the cells to grow in vitro, particularly with regard to surface-adherent mammalian cells. Different cells and research objectives require different culture conditions which has led to an increase in the diversity of 3D cell culture models. In this study, we show two independent on-carrier 3D cell culture models aimed at two different potential applications. Firstly, micron-scale porous spherical structures fabricated from poly (lactic-co-glycolic acid) or PLGA are used as 3D cell carriers so that the cells do not lose their physiologically relevant spherical shape. Secondly, millimetre-scale silk fibroin structures fabricated by 3D inkjet bioprinting are used as 3D cell carriers to demonstrate cell growth patterning in 3D for use in applications which require directed cell growth. The L929 fibroblasts demonstrated excellent adherence, cell-division and proliferation on the PLGA carriers, while the PC12 neuronal cells showed excellent adherence, proliferation and spread on the fibroin carriers without any evidence of cytotoxicity from the carriers. The present study thus proposes two models for 3D cell culture and demonstrates, firstly, that easily fabricable porous PLGA structures can act as excellent cell carriers for aiding cells easily retain their physiologically relevant 3D spherical shape in vitro, and secondly, that 3D inkjet printed silk fibroin structures can act as geometrically-shaped carriers for 3D cell patterning or directed cell growth in vitro. While the 'fibroblasts on PLGA carriers' model will help achieve more accurate results than the conventional 2D culture in cell research, such as drug discovery, and cell proliferation for adoptive cell transfer, such as stem cell therapy, the 'neuronal cells on silk fibroin carriers' model will help in research requiring patterned cell growth, such as treatment of neuropathies.

Nervous tract-bioinspired multi-nanoyarn model system regulating neural differentiation and its transcriptional architecture at single-cell resolution

Bioinspired by native nervous tracts, a spinal cord-mimicking model system that was composed of multiple nanofibrous yarns (NYs) ensheathed in a nanofibrous tube was constructed by an innovative electrospinning-based fabrication and integration strategy. The infilling NYs exhibited uniaxially aligned nanofibrous architecture that had a great resemblance to spatially-arranged native nervous tracts, while the outer nanofibrous tubes functioned as an artificial dura matter to provide a stable intraluminal microenvironment. The three-dimensional (3D) NYs were demonstrated to induce alignment, facilitate migration, promote neuronal differentiation, and even phenotypic maturation of seeded neural stem and progenitor cells (NSPCs), while inhibiting gliogenesis. Single-cell transcriptome analysis showed that the NSPC-loaded 3D NY model shared many similarities with native spinal cords, with a great increase in excitatory/inhibitory (EI) neuron ratio. Curcumin, as a model drug, was encapsulated into nanofibers of NYs to exert an antioxidant effect and enhanced axon regeneration. Overall, this study provides a new paradigm for the development of a next-generation in vitro neuronal model system via anatomically accurate nervous tract simulation and constructs a blueprint for the research on NSPC diversification in the biomimetic microenvironment.

Classification, processing, and applications of bioink and 3D bioprinting: A detailed review

With the advancement in 3D bioprinting technology, cell culture methods can design 3D environments which are both, complex and physiologically relevant. The main component in 3D bioprinting, bioink, can be split into various categories depending on the criterion of categorization. Although the choice of bioink and bioprinting process will vary greatly depending on the application, general features such as material properties, biological interaction, gelation, and viscosity are always important to consider. The foundation of 3D bioprinting is the exact layer-by-layer implantation of biological elements, biochemicals, and living cells with the spatial control of the implantation of functional elements onto the biofabricated 3D structure. Three basic strategies underlie the 3D bioprinting process: autonomous self-assembly, micro tissue building blocks, and biomimicry or biomimetics. Tissue engineering can benefit from 3D bioprinting in many ways, but there are still numerous obstacles to overcome before functional tissues can be produced and used in clinical settings. A better comprehension of the physiological characteristics of bioink materials and a higher level of ability to reproduce the intricate biologically mimicked and physiologically relevant 3D structures would be a significant improvement for 3D bioprinting to overcome the limitations.

The application of 3D-bioprinted scaffolds for neuronal regeneration after traumatic spinal cord injury - A systematic review of preclinical in vivo studies

BACKGROUND

The implantation of 3D-bioprinted scaffolds represents a promising therapeutic approach for traumatic Spinal Cord Injury (SCI), currently investigating in preclinical in vivo studies. However, a systematic review of the relevant literature has not been performed to date. Hence, we systematically reviewed the outcomes of the application of 3D-bioprinted implants in the treatment of SCI based on studies conducted on experimental animal models.

METHODS

We searched PubMed, Scopus, Web of Science, and Cochrane Library databases. Manuscripts in other designs than in vivo preclinical study and written in other languages than English were excluded. A risk of bias assessment was performed using SYRCLE's tool. The quality of included articles was assessed by ARRIVE guidelines. Extracted data were synthesized only qualitatively because the data were not suitable for conducting the meta-analysis.

RESULTS

Overall, eleven animal studies reporting on the transection SCI rat model were included. Six of included studies investigated 3D-bioprinted scaffolds enriched with stem cells, two studies - 3D-bioprinted scaffolds combined with growth factors, and three studies - stand-alone 3D-bioprinted scaffolds. In all included studies the application of 3D-bioprinted scaffolds led to significant improvement in functional scores compared with no treated SCI rats. The functional recovery corresponded with the changes observed at the injury site in histological analyses. Seven studies demonstrated medium, three studies - high, and one study - low risk of bias. Moreover, some of the included studies were conducted in the same scientific center. The overall quality assessment ratio amounted to 0.60, which was considered average quality.

CONCLUSION

The results of our systematic review suggest that 3D-bioprinted scaffolds may be a feasible therapeutic approach for the treatment of SCI. Further evidence obtained on other experimental SCI models is necessary before the clinical translation of 3D-bioprinted scaffolds.

In Vitro 3D Modeling of Neurodegenerative Diseases

The study of neurodegenerative diseases (such as Alzheimer's disease, Parkinson's disease, Huntington's disease, or amyotrophic lateral sclerosis) is very complex due to the difficulty in investigating the cellular dynamics within nervous tissue. Despite numerous advances in the in vivo study of these diseases, the use of in vitro analyses is proving to be a valuable tool to better understand the mechanisms implicated in these diseases. Although neural cells remain difficult to obtain from patient tissues, access to induced multipotent stem cell production now makes it possible to generate virtually all neural cells involved in these diseases (from neurons to glial cells). Many original 3D culture model approaches are currently being developed (using these different cell types together) to closely mimic degenerative nervous tissue environments. The aim of these approaches is to allow an interaction between glial cells and neurons, which reproduces pathophysiological reality by co-culturing them in structures that recapitulate embryonic development or facilitate axonal migration, local molecule exchange, and myelination (to name a few). This review details the advantages and disadvantages of techniques using scaffolds, spheroids, organoids, 3D bioprinting, microfluidic systems, and organ-on-a-chip strategies to model neurodegenerative diseases.

Role of 3D Printing in the Development of Biodegradable Implants for Central Nervous System Drug Delivery

Increased life expectancy has led to a rise in age-related disorders including neurological diseases such as Alzheimer's disease and Parkinson's disease. Limited progress has been made in the development of clinically translatable therapies for these central nervous system (CNS) diseases. Challenges including the blood-brain barrier, brain complexity, and comorbidities in the elderly population are some of the contributing factors toward lower success rates. Various invasive and noninvasive ways are being employed to deliver small and large molecules across the brain. Biodegradable, implantable drug-delivery systems have gained lot of interest due to advantages such as sustained and targeted delivery, lower side effects, and higher patient compliance. 3D printing is a novel additive manufacturing technique where various materials and printing techniques can be used to fabricate implants with the desired complexity in terms of mechanical properties, shapes, or release profiles. This review discusses an overview of various types of 3D-printing techniques and illustrative examples of the existing literature on 3D-printed systems for CNS drug delivery. Currently, there are various technical and regulatory impediments that need to be addressed for successful translation from the bench to the clinical stage. Overall, 3D printing is a transformative technology with great potential in advancing customizable drug treatment in a high-throughput manner.

Human stem cell-derived neurons and neural circuitry therapeutics: Next frontier in spinal cord injury repair

Spinal cord injury (SCI) remains a life-altering event that devastates those injured and the families that support them. Numerous laboratories are engaged in preclinical and clinical trials to repair the injured spinal cord with stem cell-derived therapeutics. A new developmental paradigm reveals early bifurcation of brain or trunk neurons in mammals via neuromesodermal progenitors (NMPs) relevant to therapies requiring homotypic spinal cord neural populations. Human-induced pluripotent stem cell (hiPSC) NMP-derived spinal motor neurons generated ex vivo following this natural developmental route demonstrate robust survival in vivo when delivered as suspension grafts or as in vitro preformed encapsulated neuronal circuitry when transplanted into a rat C4-C5 hemicontusion injury site. Use of in vitro matured neurons avoids in vivo differentiation challenges of using pluripotent hiPSC or multipotent neural stem cell (NSC) or mesenchymal stem cell therapeutics. In this review, we provide an injury to therapeutics overview focusing on how stem cell and developmental fields are merging to generate exquisitely matched spinal motor neurons for SCI therapeutic studies. The complexity of the SCI microenvironment generated by trauma to neurons and vasculature, along with infiltrating inflammatory cells and scarring, underlies the challenging cytokine microenvironment that therapeutic cells encounter. An overview of evolving but limited stem cell-based SCI therapies that have progressed from preclinical to clinical trials illustrates the challenges and need for additional stem cell-based therapeutic approaches. The focus here on neurons describes how NMP-based neurotechnologies are advancing parallel strategies such as transplantation of preformed neuronal circuitry as well as human in vitro gastruloid multicellular models of trunk central and peripheral nervous system integration with organs. NMP-derived neurons are expected to be powerful drivers of the next generation of SCI therapeutics and integrate well with combination therapies that may utilize alternate biomimetic scaffolds for bridging injuries or flexible biodegradable electronics for electrostimulation.

Micropattern-based nerve guidance conduit with hundreds of microchannels and stem cell recruitment for nerve regeneration

Guiding the regrowth of thousands of nerve fibers within a regeneration-friendly environment enhances the regeneration capacity in the case of peripheral nerve injury (PNI) and spinal cord injury (SCI). Although clinical treatments are available and several studies have been conducted, the development of nerve guidance conduits (NGCs) with desirable properties, including controllable size, hundreds of nerve bundle-sized microchannels, and host stem-cell recruitment, remains challenging. In this study, the micropattern-based fabrication method was combined with stem-cell recruitment factor (substance P, SP) immobilization onto the main material to produce a size-tunable NGC with hundreds of microchannels with stem-cell recruitment capability. The SP-immobilized multiple microchannels aligned the regrowth of nerve fibers and recruited the host stem cells, which enhanced the functional regeneration capacity. This method has wide applicability in the modification and augmentation of NGCs, such as bifurcated morphology or directional topographies on microchannels. Additional improvements in fabrication will advance the regeneration technology and improve the treatment of PNI/SCI.

Electroceuticals for peripheral nerve regeneration

Electroceuticals provide promising opportunities for peripheral nerve regeneration, in terms of modulating the extensive endogenous tissue repair mechanisms between neural cell body, axons and target muscles. However, great challenges remain to deliver effective and controllable electroceuticals via bioelectronic implantable device. In this review, the modern fabrication methods of bioelectronic conduit for bridging critical nerve gaps after nerve injury are summarized, with regard to conductive materials and core manufacturing process. In addition, to deliver versatile electrical stimulation, the integration of implantable bioelectronic device is discussed, including wireless energy harvesters, actuators and sensors. Moreover, a comprehensive insight of beneficial mechanisms is presented, including up-to-date in vitro, in vivo and clinical evidence. By integrating conductive biomaterials, 3D engineering manufacturing process and bioelectronic platform to deliver versatile electroceuticals, the modern biofabrication enables comprehensive biomimetic therapies for neural tissue engineering and regeneration in the new era.

Recent advances in 3D-printed polylactide and polycaprolactone-based biomaterials for tissue engineering applications

The three-dimensional printing (3DP) also known as the additive manufacturing (AM), a novel and futuristic technology that facilitates the printing of multiscale, biomimetic, intricate cytoarchitecture, function-structure hierarchy, multi-cellular tissues in the complicated micro-environment, patient-specific scaffolds, and medical devices. There is an increasing demand for developing 3D-printed products that can be utilized for organ transplantations due to the organ shortage. Nowadays, the 3DP has gained considerable interest in the tissue engineering (TE) field. Polylactide (PLA) and polycaprolactone (PCL) are exemplary biomaterials with excellent physicochemical properties and biocompatibility, which have drawn notable attraction in tissue regeneration. Herein, the recent advancements in the PLA and PCL biodegradable polymer-based composites as well as their reinforcement with hydrogels and bio-ceramics scaffolds manufactured through 3DP are systematically summarized and the applications of bone, cardiac, neural, vascularized and skin tissue regeneration are thoroughly elucidated. The interaction between implanted biodegradable polymers, in-vivo and in-vitro testing models for possible evaluation of degradation and biological properties are also illustrated. The final section of this review incorporates the current challenges and future opportunities in the 3DP of PCL- and PLA-based composites that will prove helpful for biomedical engineers to fulfill the demands of the clinical field.

Development of a polyacrylamide/chitosan composite hydrogel conduit containing synergistic cues of elasticity and topographies for promoting peripheral nerve regeneration

Substrate elasticity and topographical guidance are crucial factors for regulating tissue regeneration, but the synergistic effects of both cues on peripheral nerve regeneration are still unclear. In this paper, polyacrylamide/chitosan (PAM/CS) composite hydrogels with synergistic characteristics of elasticity and morphology were prepared using in situ free-radical polymerization and micro-molding. The physicochemical properties of hydrogels were characterized, and the effect on peripheral nerve regeneration was systematically evaluated via in vitro and in vivo experiments, respectively. The in vitro experiments showed that on a PAM/CS composite hydrogel with an elastic modulus of 5.822 kPa/8.41 kPa and a surface groove width of 30 μm, the dorsal root ganglion (DRG) neurite had a strong growth ability and better-oriented status. The samples were taken from each group at 2 and 12 weeks after bridging rabbit sciatic nerve defects with a PAM/CS composite hydrogel conduit. General observation of the rabbit body and transplanted nerve, nerve electro-physiological examination, muscle wet weight recovery rate detection and comparison, observation of sciatic nerve frozen section immunofluorescence staining and myelinated nerve fiber recovery rate comparison were used to evaluate the effect of nerve transplantation. The elastic modulus of 8.41 kPa and groove width of 30 μm were similar to those of the autograft group. At the same time, the signaling pathways, including the focal adhesion markers vinculin, p-FAK, and Rho A protein, referring to axon adhesion and extension, were initially revealed. In summary, our developed hydrogel implants containing synergistic cues of elasticity and topographies may provide a new and effective strategy for the treatment of peripheral nerve injury in the future.

Application and Prospects of Hydrogel Additive Manufacturing

Hydrogel has become a commonly used material for 3D and 4D printing due to its favorable biocompatibility and low cost. Additive manufacturing, also known as 3D printing, was originally referred to as rapid prototyping manufacturing. Variable-feature rapid prototyping technology, also known as 4D printing, is a combination of materials, mathematics, and additives. This study constitutes a literature review to address hydrogel-based additive manufacturing technologies, introducing the characteristics of commonly used 3D printing hydrogel methods, such as direct ink writing, fused deposition modeling, and stereolithography. With this review, we also investigated the stimulus types, as well as the advantages and disadvantages of various stimulus-responsive hydrogels in smart hydrogels; non-responsive hydrogels; and various applications of additive manufacturing hydrogels, such as neural catheter preparation and drug delivery. The opportunities, challenges, and future prospects of hydrogel additive manufacturing technologies are discussed.

Alginate-Based Hydrogels and Tubes, as Biological Macromolecule-Based Platforms for Peripheral Nerve Tissue Engineering: A Review

Unlike the central nervous system, the peripheral nervous system (PNS) has an inherent capacity to regenerate following injury. However, in the case of large nerve defects where end-to-end cooptation of two nerve stumps is not tension-free, autologous nerve grafting is often utilized to bridge the nerve gaps. To address the challenges associated with autologous nerve grafting, neural guidance channels (NGCs) have been successfully translated into clinic. Furthermore, hydrogel-based drug delivery systems have been extensively studied for the repair of PNS injuries. There are numerous biomaterial options for the production of NGCs and hydrogels. Among different candidates, alginate has shown promising results in PNS tissue engineering. Alginate is a naturally occurring polysaccharide which is biocompatible, non-toxic, non-immunogenic, and possesses modifiable properties. In the current review, applications, challenges, and future perspectives of alginate-based NGCs and hydrogels in the repair of PNS injuries will be discussed.

3D Bioprinting for Spinal Cord Injury Repair

Spinal cord injury (SCI) is considered to be one of the most challenging central nervous system injuries. The poor regeneration of nerve cells and the formation of scar tissue after injury make it difficult to recover the function of the nervous system. With the development of tissue engineering, three-dimensional (3D) bioprinting has attracted extensive attention because it can accurately print complex structures. At the same time, the technology of blending and printing cells and related cytokines has gradually been matured. Using this technology, complex biological scaffolds with accurate cell localization can be manufactured. Therefore, this technology has a certain potential in the repair of the nervous system, especially the spinal cord. So far, this review focuses on the progress of tissue engineering of the spinal cord, landmark 3D bioprinting methods, and landmark 3D bioprinting applications of the spinal cord in recent years.

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.

Strategies for effective neural circuit reconstruction after spinal cord injury: use of stem cells and biomaterials

Spinal cord injury (SCI), a serious disease of the central nervous system, often with irreversible loss of motor or sensory functions. Failure of axon connection and inhibition of microenvironment after SCI severely hinder the regeneration of damaged tissue and neuron function. Therefore, the new perspective of treatment of spinal cord injury is the reconstruction of neural circuit. Stem cells are a kind of cells with differentiation potential. They reconstruct local circulation by differentiating into neurons to replace damaged cells. It can also secrete various factors to regulate the host microenvironment and play a therapeutic role. Biomaterials can fill the cavity at the site of spinal cord injury, load therapeutic drugs, provide adsorption sites for transplanted cells and play a bridging role. In this review, the therapeutic role of stem cells and biomaterials is discussed, together with their properties, advantages, limitations, and future perspectives, providing a reference for basic and clinical research on SCI treatment.

Construction and imaging of a neurovascular unit model

In 2001, the concept of the neurovascular unit was introduced at the Stroke Progress Review Group meeting. The neurovascular unit is an important element of the health and disease status of blood vessels and nerves in the central nervous system. Since then, the neurovascular unit has attracted increasing interest from research teams, who have contributed greatly to the prevention, treatment, and prognosis of stroke and neurodegenerative diseases. However, additional research is needed to establish an efficient, low-cost, and low-energy in vitro model of the neurovascular unit, as well as enable noninvasive observation of neurovascular units in vivo and in vitro. In this review, we first summarize the composition of neurovascular units, then investigate the efficacy of different types of stem cells and cell culture methods in the construction of neurovascular unit models, and finally assess the progress of imaging methods used to observe neurovascular units in recent years and their positive role in the monitoring and investigation of the mechanisms of a variety of central nervous system diseases.

Structural alignment guides oriented migration and differentiation of endogenous neural stem cells for neurogenesis in brain injury treatment

Radial glia (RG) cells that align in parallel in the embryonic brain are found to be able to guide the directed migration of neurons in response to brain injury. Therefore, biomaterials with aligned architectures are supposed to have positive effects on neural migration and neurogenic differentiation for brain injury repair that are rarely addressed, although they have been widely demonstrated in spinal cord and peripheral nerve system. Here, we present a highly biomimetic scaffold of aligned fibrin hydrogel (AFG) that mimics the oriented structure of RG fibers. Through a combination of histological, behavioral, imaging, and transcriptomic analyses, we demonstrated that transplanting the AFG scaffold into injured cortical brains promotes effective migration, differentiation, and maturation of endogenous neural stem cells, resulting in neurological functional recovery. Therefore, this study will light up a new perspective on applying an aligned scaffold to promote cortical regeneration after injury by inducing endogenous neurogenesis.

Materials Chemistry of Neural Interface Technologies and Recent Advances in Three-Dimensional Systems

Advances in materials chemistry and engineering serve as the basis for multifunctional neural interfaces that span length scales from individual neurons to neural networks, neural tissues, and complete neural systems. Such technologies exploit electrical, electrochemical, optical, and/or pharmacological modalities in sensing and neuromodulation for fundamental studies in neuroscience research, with additional potential to serve as routes for monitoring and treating neurodegenerative diseases and for rehabilitating patients. This review summarizes the essential role of chemistry in this field of research, with an emphasis on recently published results and developing trends. The focus is on enabling materials in diverse device constructs, including their latest utilization in 3D bioelectronic frameworks formed by 3D printing, self-folding, and mechanically guided assembly. A concluding section highlights key challenges and future directions.

STEER: 3D Printed Guide for Nerve Regrowth Control and Neural Interface in Non-Human Primate Model

OBJECTIVE

Peripheral neural interface (PNI) with a stable integration of synthetic elements with neural tissue is key for successful neuro-prosthetic applications. An inevitable phenomenon of reactive fibrosis is a primary hurdle for long term functionality of PNIs. This proof-of-concept study aimed to fabricate and test a novel, stable PNI that harnesses fibro-axonal outgrowth at the nerve end and includes fibrosis in the design.

METHODS

Two non-human primates were implanted with Substrate-guided, Tissue-Electrode Encapsulation and Integration (STEER) PNIs. The implant included a 3D printed guide that strove to steer the regrowing nerve towards encapsulation of the electrodes into a fibro-axonal tissue. After four months from implantation, we performed electrophysiological measurements to test STEERs functionality and examined the macro and micro- morphology of the outgrowth tissue.

RESULTS

We observed a highly structured fibro-axonal composite within the STEER PNI. A conduction of intracranially generated action potentials was successfully recorded across the neural interface. Immunohistology demonstrated uniquely configured laminae of myelinated axons encasing the implant.

CONCLUSION

STEER PNI reconfigured the structure of the fibro-axonal tissue and facilitated long-term functionality and stability of the neural interface.

SIGNIFICANCE

The results point to the feasibility of our concept for creating a stable PNI with long-term electrophysiologic functionality by using simple design and materials.

3D Bioprinting of Neural Tissues

The human nervous system is a remarkably complex physiological network that is inherently challenging to study because of obstacles to acquiring primary samples. Animal models offer powerful alternatives to study nervous system development, diseases, and regenerative processes, however, they are unable to address some species-specific features of the human nervous system. In vitro models of the human nervous system have expanded in prevalence and sophistication, but still require further advances to better recapitulate microenvironmental and cellular features. The field of neural tissue engineering (TE) is rapidly adopting new technologies that enable scientists to precisely control in vitro culture conditions and to better model nervous system formation, function, and repair. 3D bioprinting is one of the major TE technologies that utilizes biocompatible hydrogels to create precisely patterned scaffolds, designed to enhance cellular responses. This review focuses on the applications of 3D bioprinting in the field of neural TE. Important design parameters are considered when bioprinting neural stem cells are discussed. The emergence of various bioprinted in vitro platforms are also reviewed for developmental and disease modeling and drug screening applications within the central and peripheral nervous systems, as well as their use as implants for in vivo regenerative therapies.

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.

Nerve Guidance Conduits with Hierarchical Anisotropic Architecture for Peripheral Nerve Regeneration

Nerve guidance conduits with multifunctional features could offer microenvironments for improved nerve regeneration and functional recovery. However, the challenge remains to optimize multiple cues in nerve conduit systems due to the interplay of these factors during fabrication. Here, a modular assembly for the fabrication of nerve conduits is utilized to address the goal of incorporating multifunctional guidance cues for nerve regeneration. Silk-based hollow conduits with suitable size and mechanical properties, along with silk nanofiber fillers with tunable hierarchical anisotropic architectures and microporous structures, are developed and assembled into conduits. These conduits supported improves nerve regeneration in terms of cell proliferation (Schwann and PC12 cells) and growth factor secretion (BDNF, brain-derived neurotrophic factor) in vitro, and the in vivo repair and functional recovery of rat sciatic nerve defects. Nerve regeneration using these new conduit designs is comparable to autografts, providing a path towards future clinical impact.