3D Printed Anatomical Nerve Regeneration Pathways

Biodegradable Bisvinyl Sulfonemethyl-crosslinked Gelatin Conduit Promotes Regeneration after Peripheral Nerve Injury in Adult Rats

In our previous study, we found that gelatin-based materials exhibit good conductivity and are non-cytotoxic. In this study, gelatin was cross-linked with bisvinyl sulfonemethyl (BVSM) to fabricate a biodegradable conduit for peripheral nerve repair. First, BVSM on the prepared conduit was characterized to determine its mechanical properties and contact angle. The maximum tensile strength and water contact angle of the gelatin-BVSM conduits were 23 ± 4.8 MPa and 74.7 ± 9°, which provided sufficient mechanical strength to resist muscular contraction; additionally, the surface was hydrophilic. Cytotoxicity and apoptosis assays using Schwann cells demonstrated that the gelatin-BVSM conduits are non-cytotoxic. Next, we examined the neuronal electrophysiology, animal behavior, neuronal connectivity, macrophage infiltration, calcitonin gene-related peptide localization and expression, as well as the expression levels of nerve regeneration-related proteins. The number of fluorogold-labelled cells and histological analysis of the gelatin-BVSM nerve conduits was similar to that observed with the clinical use of silicone rubber conduits after 8 weeks of repair. Therefore, our results demonstrate that gelatin-BVSM conduits are promising substrates for application as bioengineered grafts for nerve tissue regeneration.

3D neural tissue models: From spheroids to bioprinting

Three-dimensional (3D) in vitro neural tissue models provide a better recapitulation of in vivo cell-cell and cell-extracellular matrix interactions than conventional two-dimensional (2D) cultures. Therefore, the former is believed to have great potential for both mechanistic and translational studies. In this paper, we review the recent developments in 3D in vitro neural tissue models, with a particular focus on the emerging bioprinted tissue structures. We draw on specific examples to describe the merits and limitations of each model, in terms of different applications. Bioprinting offers a revolutionary approach for constructing repeatable and controllable 3D in vitro neural tissues with diverse cell types, complex microscale features and tissue level responses. Further advances in bioprinting research would likely consolidate existing models and generate complex neural tissue structures bearing higher fidelity, which is ultimately useful for probing disease-specific mechanisms, facilitating development of novel therapeutics and promoting neural regeneration.

Stem cell therapy for nerve injury

Peripheral nerve injury has remained a substantial clinical complication with no satisfactory treatment options. Despite the great development in the field of microsurgery, some severe types of neural injuries cannot be treated without causing tension to the injured nerve. Thus, current studies have focused on the new approaches for the treatment of peripheral nerve injuries. Stem cells with the ability to differentiate into a variety of cell types have brought a new perspective to this matter. In this review, we will discuss the use of three main sources of mesenchymal stem cells in the treatment of peripheral nerve injuries.

3D-Printed pHEMA Materials for Topographical and Biochemical Modulation of Dorsal Root Ganglion Cell Response

Understanding and controlling the interactions occurring between cells and engineered materials are central challenges toward progress in the development of biomedical devices. In this work, we describe materials for direct ink writing (DIW), an extrusion-based type of 3D printing, that embed a custom synthetic protein (RGD-PDL) within the microfilaments of 3D-hydrogel scaffolds to modify these interactions and differentially direct tissue-level organization of complex cell populations in vitro. The RGD-PDL is synthesized by modifying poly-d-lysine (PDL) to varying extents with peptides containing the integrin-binding motif Arg-Gly-Asp (RGD). Compositional gradients of the RGD-PDL presented by both patterned and thin-film poly(2-hydroxyethyl) methacrylate (pHEMA) substrates allow the patterning of cell-growth compliance in a grayscale form. The surface chemistry-dependent guidance of cell growth on the RGD-PDL-modified pHEMA materials is demonstrated using a model NIH-3T3 fibroblast cell line. The formation of a more complex cellular system-organotypic primary murine dorsal root ganglion (DRG)-in culture is also achieved on these scaffolds, where distinctive forms of cell growth and migration guidance are seen depending on their RGD-PDL content and topography. This experimental platform for the study of physicochemical factors on the formation and the reorganization of organotypic cultures offers useful capabilities for studies in tissue engineering, regenerative medicine, and diagnostics.

Polymeric scaffolds for three-dimensional culture of nerve cells: a model of peripheral nerve regeneration

Understanding peripheral nerve repair requires the evaluation of 3D structures that serve as platforms for 3D cell culture. Multiple platforms for 3D cell culture have been developed, mimicking peripheral nerve growth and function, in order to study tissue repair or diseases. To recreate an appropriate 3D environment for peripheral nerve cells, key factors are to be considered including: selection of cells, polymeric biomaterials to be used, and fabrication techniques to shape and form the 3D scaffolds for cellular culture. This review focuses on polymeric 3D platforms used for the development of 3D peripheral nerve cell cultures.

3D printed conformal microfluidics for isolation and profiling of biomarkers from whole organs

The ability to interface microfluidic devices with native complex biological architectures, such as whole organs, has the potential to shift the paradigm for the study and analysis of biological tissue. Here, we show 3D printing can be used to fabricate bio-inspired conformal microfluidic devices that directly interface with the surface of whole organs. Structured-light scanning techniques enabled the 3D topographical matching of microfluidic device geometry to porcine kidney anatomy. Our studies show molecular species are spontaneously transferred from the organ cortex to the conformal microfluidic device in the presence of fluid flow through the organ-conforming microchannel. Large animal studies using porcine kidneys (n = 32 organs) revealed the profile of molecular species in the organ-conforming microfluidic stream was dependent on the organ preservation conditions. Enzyme-linked immunosorbent assay (ELISA) studies revealed conformal microfluidic devices isolate clinically relevant metabolic and pathophysiological biomarkers from whole organs, including heat shock protein 70 (HSP-70) and kidney injury molecule-1 (KIM-1), which were detected in the microfluidic device as high as 409 and 12 pg mL^-1, respectively. Overall, these results show conformal microfluidic devices enable a novel minimally invasive 'microfluidic biopsy' technique for isolation and profiling of biomarkers from whole organs within a clinically relevant interval. This achievement could shift the paradigm for whole organ preservation and assessment, thereby helping to relieve the organ shortage crisis through increased availability and quality of donor organs. Ultimately, this work provides a major advance in microfluidics through the design and manufacturing of organ-conforming microfluidic devices and a novel technique for microfluidic-based analysis of whole organs.

3D bioprinting and the current applications in tissue engineering

Bioprinting as an enabling technology for tissue engineering possesses the promises to fabricate highly mimicked tissue or organs with digital control. As one of the biofabrication approaches, bioprinting has the advantages of high throughput and precise control of both scaffold and cells. Therefore, this technology is not only ideal for translational medicine but also for basic research applications. Bioprinting has already been widely applied to construct functional tissues such as vasculature, muscle, cartilage, and bone. In this review, the authors introduce the most popular techniques currently applied in bioprinting, as well as the various bioprinting processes. In addition, the composition of bioink including scaffolds and cells are described. Furthermore, the most current applications in organ and tissue bioprinting are introduced. The authors also discuss the challenges we are currently facing and the great potential of bioprinting. This technology has the capacity not only in complex tissue structure fabrication based on the converted medical images, but also as an efficient tool for drug discovery and preclinical testing. One of the most promising future advances of bioprinting is to develop a standard medical device with the capacity of treating patients directly on the repairing site, which requires the development of automation and robotic technology, as well as our further understanding of biomaterials and stem cell biology to integrate various printing mechanisms for multi-phasic tissue engineering.

3D Printed Stretchable Tactile Sensors

The development of methods for the 3D printing of multifunctional devices could impact areas ranging from wearable electronics and energy harvesting devices to smart prosthetics and human-machine interfaces. Recently, the development of stretchable electronic devices has accelerated, concomitant with advances in functional materials and fabrication processes. In particular, novel strategies have been developed to enable the intimate biointegration of wearable electronic devices with human skin in ways that bypass the mechanical and thermal restrictions of traditional microfabrication technologies. Here, a multimaterial, multiscale, and multifunctional 3D printing approach is employed to fabricate 3D tactile sensors under ambient conditions conformally onto freeform surfaces. The customized sensor is demonstrated with the capabilities of detecting and differentiating human movements, including pulse monitoring and finger motions. The custom 3D printing of functional materials and devices opens new routes for the biointegration of various sensors in wearable electronics systems, and toward advanced bionic skin applications.

Iodine and freeze-drying enhanced high-resolution MicroCT imaging for reconstructing 3D intraneural topography of human peripheral nerve fascicles

BACKGROUND

The precise annotation and accurate identification of the topography of fascicles to the end organs are prerequisites for studying human peripheral nerves.

NEW METHOD

In this study, we present a feasible imaging method that acquires 3D high-resolution (HR) topography of peripheral nerve fascicles using an iodine and freeze-drying (IFD) micro-computed tomography (microCT) method to greatly increase the contrast of fascicle images.

RESULTS

The enhanced microCT imaging method can facilitate the reconstruction of high-contrast HR fascicle images, fascicle segmentation and extraction, feature analysis, and the tracing of fascicle topography to end organs, which define fascicle functions.

COMPARISON WITH EXISTING METHODS

The complex intraneural aggregation and distribution of fascicles is typically assessed using histological techniques or MR imaging to acquire coarse axial three-dimensional (3D) maps. However, the disadvantages of histological techniques (static, axial manual registration, and data instability) and MR imaging (low-resolution) limit these applications in reconstructing the topography of nerve fascicles.

CONCLUSIONS

Thus, enhanced microCT is a new technique for acquiring 3D intraneural topography of the human peripheral nerve fascicles both to improve our understanding of neurobiological principles and to guide accurate repair in the clinic. Additionally, 3D microstructure data can be used as a biofabrication model, which in turn can be used to fabricate scaffolds to repair long nerve gaps.

Microphysiological Human Brain and Neural Systems-on-a-Chip: Potential Alternatives to Small Animal Models and Emerging Platforms for Drug Discovery and Personalized Medicine

Translational challenges associated with reductionist modeling approaches, as well as ethical concerns and economic implications of small animal testing, drive the need for developing microphysiological neural systems for modeling human neurological diseases, disorders, and injuries. Here, we provide a comprehensive review of microphysiological brain and neural systems-on-a-chip (NSCs) for modeling higher order trajectories in the human nervous system. Societal, economic, and national security impacts of neurological diseases, disorders, and injuries are highlighted to identify critical NSC application spaces. Hierarchical design and manufacturing of NSCs are discussed with distinction for surface- and bulk-based systems. Three broad NSC classes are identified and reviewed: microfluidic NSCs, compartmentalized NSCs, and hydrogel NSCs. Emerging areas and future directions are highlighted, including the application of 3D printing to design and manufacturing of next-generation NSCs, the use of stem cells for constructing patient-specific NSCs, and the application of human NSCs to 'personalized neurology'. Technical hurdles and remaining challenges are discussed. This review identifies the state-of-the-art design methodologies, manufacturing approaches, and performance capabilities of NSCs. This work suggests NSCs appear poised to revolutionize the modeling of human neurological diseases, disorders, and injuries.

Emerging Biofabrication Strategies for Engineering Complex Tissue Constructs

The demand for organ transplantation and repair, coupled with a shortage of available donors, poses an urgent clinical need for the development of innovative treatment strategies for long-term repair and regeneration of injured or diseased tissues and organs. Bioengineering organs, by growing patient-derived cells in biomaterial scaffolds in the presence of pertinent physicochemical signals, provides a promising solution to meet this demand. However, recapitulating the structural and cytoarchitectural complexities of native tissues in vitro remains a significant challenge to be addressed. Through tremendous efforts over the past decade, several innovative biofabrication strategies have been developed to overcome these challenges. This review highlights recent work on emerging three-dimensional bioprinting and textile techniques, compares the advantages and shortcomings of these approaches, outlines the use of common biomaterials and advanced hybrid scaffolds, and describes several design considerations including the structural, physical, biological, and economical parameters that are crucial for the fabrication of functional, complex, engineered tissues. Finally, the applications of these biofabrication strategies in neural, skin, connective, and muscle tissue engineering are explored.

Mechanically Oriented 3D Collagen Hydrogel for Directing Neurite Growth

Recent studies in the field of neuro-tissue engineering have demonstrated the promising effects of aligned contact guidance cue to scaffolds of enhancement and direction of neuronal growth. In vivo, neurons grow and develop neurites in a complex three-dimensional (3D) extracellular matrix (ECM) surrounding. Studies have utilized hydrogel scaffolds derived from ECM molecules to better simulate natural growth. While many efforts have been made to control neuronal growth on 2D surfaces, the development of 3D scaffolds with an elaborate oriented topography to direct neuronal growth still remains a challenge. In this study, we designed a method for growing neurons in an aligned and oriented 3D collagen hydrogel. We aligned collagen fibers by inducing controlled uniaxial strain on gels. To examine the collagen hydrogel as a suitable scaffold for neuronal growth, we evaluated the physical properties of the hydrogel and measured collagen fiber properties. By combining the neuronal culture in 3D collagen hydrogels with strain-induced alignment, we were able to direct neuronal growth in the direction of the aligned collagen matrix. Quantitative evaluation of neurite extension and directionality within aligned gels was performed. The analysis showed neurite growth aligned with collagen matrix orientation, while maintaining the advantageous 3D growth.

A 3D-engineered porous conduit for peripheral nerve repair

End-to-end neurorrhaphy is the most commonly used method for treating peripheral nerve injury. However, only 50% of patients can regain useful function after treating with neurorrhaphy. Here, we constructed a 3D-engineered porous conduit to promote the function recovery of the transected peripheral nerve after neurorrhaphy. The conduit that consisted of a gelatin cryogel was prepared by molding with 3D-printed moulds. Due to its porous structure and excellent mechanical properties, this conduit could be collapsed by the mechanical force and resumed its original shape after absorption of normal saline. This shape-memory property allowed a simply surgery process for installing the conduits. Moreover, the biodegradable conduit could prevent the infiltration of fibroblasts and reduce the risk of scar tissue, which could provide an advantageous environment for nerve regeneration. The efficiency of the conduits in assisting peripheral nerve regeneration after neurorrhaphy was evaluated in a rat sciatic nerve transected model. Results indicated that conduits significantly benefitted the recovery of the transected peripheral nerve after end-to-end neurorrhaphy on the static sciatic index (SSI), electrophysiological results and the re-innervation of the gastrocnemius muscle. This work demonstrates a biodegradable nerve conduit that has potentially clinical application in promoting the neurorrhaphy.

Biomimetic nanofibrous scaffolds for neural tissue engineering and drug development

Neural tissue engineering aims to develop functional substitutes for damaged tissues, creating many promising opportunities in regeneration medicine and drug discovery. Biomaterial scaffolds routinely provide nerve cells with a physical support for cell growth and regeneration, yielding 3D extracellular matrix to mimic the in vivo cellular microenvironment. Among the various types of cellular scaffolds for reconstruction, biomimetic nanofibrous scaffolds are recognized as appropriate candidates by precisely controlling morphology and shape. Here, we review the current techniques in fabricating biomimetic nanofibrous scaffolds for neural tissue engineering, and describe the impact of nanofiber components on the properties of scaffolds and their uses in therapeutic models and drug development. We also discuss the current challenges and future directions of applying 3D printing and microfluidic technologies in neural tissue engineering.

Biomaterials and cells for neural tissue engineering: Current choices

The treatment of nerve injuries has taken a new dimension with the development of tissue engineering techniques. Prior to tissue engineering, suturing and surgery were the only options for effective treatment. With the advent of tissue engineering, it is now possible to design a scaffold that matches the exact biological and mechanical properties of the tissue. This has led to substantial reduction in the complications posed by surgeries and suturing to the patients. New synthetic and natural polymers are being applied to test their efficiency in generating an ideal scaffold. Along with these, cells and growth factors are also being incorporated to increase the efficiency of a scaffold. Efforts are being made to devise a scaffold that is biodegradable, biocompatible, conducting and immunologically inert. The ultimate goal is to exactly mimic the extracellular matrix in our body, and to elicit a combination of biochemical, topographical and electrical cues via various polymers, cells and growth factors, using which nerve regeneration can efficiently occur.

Establishing a reliable gait evaluation method for rodent studies

BACKGROUND

CatWalk is one of the most popular tools for evaluating gait recovery in preclinical research, however, there is currently no consensus on which of the many gait parameters captured by CatWalk can reliably model recovery. There are conflicting interpretations of results, along with many common but seldom reported problems such as heel walking and poor compliance.

NEW METHOD

We developed a systematic manual classification method that overcomes common problems such as heel walking and poor compliance. By correcting automation errors and removing inconsistent gait cycles, we isolated stretches of recordings that are more reliable for analysis. Recovery outcome was also assessed by quantitative histomorphometric analysis of myelinated axons.

RESULTS

While 40-60% of runs were erroneously classified without manual intervention, we corrected all errors with our new method, and showed that Stand Time, Duty Cycle, and Swing Speed are able to track significant differences over time and between experimental groups (all p<0.05). The usability of print area and intensity parameters requires further validation beyond the capabilities of CatWalk.

COMPARISON WITH EXISTING METHOD(S)

There is currently no strategy that addresses problems such as heel walking and poor compliance, and therefore no standard set of parameters that researchers can rely on to report their findings.

CONCLUSION

Manual classification is a crucial step to generate reliable CatWalk data, and Stand Time, Duty Cycle, and Swing Speed are suitable parameters for evaluating gait recovery. Static parameters such as print area and intensity should be used with extreme caution.

Stem Cell Transplantation for Peripheral Nerve Regeneration: Current Options and Opportunities

Peripheral nerve regeneration is a complicated process highlighted by Wallerian degeneration, axonal sprouting, and remyelination. Schwann cells play an integral role in multiple facets of nerve regeneration but obtaining Schwann cells for cell-based therapy is limited by the invasive nature of harvesting and donor site morbidity. Stem cell transplantation for peripheral nerve regeneration offers an alternative cell-based therapy with several regenerative benefits. Stem cells have the potential to differentiate into Schwann-like cells that recruit macrophages for removal of cellular debris. They also can secrete neurotrophic factors to promote axonal growth, and remyelination. Currently, various types of stem cell sources are being investigated for their application to peripheral nerve regeneration. This review highlights studies involving the stem cell types, the mechanisms of their action, methods of delivery to the injury site, and relevant pre-clinical or clinical data. The purpose of this article is to review the current point of view on the application of stem cell based strategy for peripheral nerve regeneration.

3D Bioprinting for Organ Regeneration

Regenerative medicine holds the promise of engineering functional tissues or organs to heal or replace abnormal and necrotic tissues/organs, offering hope for filling the gap between organ shortage and transplantation needs. Three-dimensional (3D) bioprinting is evolving into an unparalleled biomanufacturing technology due to its high-integration potential for patient-specific designs, precise and rapid manufacturing capabilities with high resolution, and unprecedented versatility. It enables precise control over multiple compositions, spatial distributions, and architectural accuracy/complexity, therefore achieving effective recapitulation of microstructure, architecture, mechanical properties, and biological functions of target tissues and organs. Here we provide an overview of recent advances in 3D bioprinting technology, as well as design concepts of bioinks suitable for the bioprinting process. We focus on the applications of this technology for engineering living organs, focusing more specifically on vasculature, neural networks, the heart and liver. We conclude with current challenges and the technical perspective for further development of 3D organ bioprinting.

Magnetic resonance imaging-three-dimensional printing technology fabricates customized scaffolds for brain tissue engineering

Conventional fabrication methods lack the ability to control both macro- and micro-structures of generated scaffolds. Three-dimensional printing is a solid free-form fabrication method that provides novel ways to create customized scaffolds with high precision and accuracy. In this study, an electrically controlled cortical impactor was used to induce randomized brain tissue defects. The overall shape of scaffolds was designed using rat-specific anatomical data obtained from magnetic resonance imaging, and the internal structure was created by computer-aided design. As the result of limitations arising from insufficient resolution of the manufacturing process, we magnified the size of the cavity model prototype five-fold to successfully fabricate customized collagen-chitosan scaffolds using three-dimensional printing. Results demonstrated that scaffolds have three-dimensional porous structures, high porosity, highly specific surface areas, pore connectivity and good internal characteristics. Neural stem cells co-cultured with scaffolds showed good viability, indicating good biocompatibility and biodegradability. This technique may be a promising new strategy for regenerating complex damaged brain tissues, and helps pave the way toward personalized medicine.

Three-dimensional cell-based bioprinting for soft tissue regeneration

Three-dimensional (3D) bioprinting technologies have been developed to offer construction of biological tissue constructs that mimic the anatomical and functional features of native tissues or organs. These cutting-edge technologies could make it possible to precisely place multiple cell types and biomaterials in a single 3D tissue construct. Hence, 3D bioprinting is one of the most attractive and powerful tools to provide more anatomical and functional similarity of human tissues or organs in tissue engineering and regenerative medicine. In recent years, this 3D bioprinting continually shows promise for building complex soft tissue constructs through placement of cell-laden hydrogel-based bioinks in a layer-by-layer fashion. This review will discuss bioprinting technologies and their applications in soft tissue regeneration.

3D Printed Microfluidic Device with Microporous Mn2O3-Modified Screen Printed Electrode for Real-Time Determination of Heavy Metal Ions

Fabricating portable devices for the determination of heavy metal ions is an ongoing challenge. Here, a 3D printing approach was adopted to fabricate a microfluidic electrochemical sensor with the desired shape in which the model for velocity profiles in microfluidic cells was built and optimized by the finite element method (FEM). The electrode in the microfluidic cell was a flexible screen-printed electrode (SPE) modified with porous Mn₂O₃ derived from manganese containing metal-organic framework (Mn-MOF). The microfluidic device presented superior electrochemical detection properties toward heavy metal ions. The calibration curves at the modified SPE for Cd(II) and Pb(II) covered two linear ranges varying from 0.5 to 8 and 10 to 100 μg L^-1, respectively. The limits of detection were estimated to be 0.5 μg L^-1 for Cd(II) and 0.2 μg L^-1 for Pb(II), which were accordingly about 6 and 50 times lower than the guideline values proposed by the World Health Organization. Furthermore, the microfluidic device was connected to iPad via a USB to enable real-time household applications. Additionally, the sensing system exhibited a better stability and reproducibility compared with traditional detecting system which offered a promising prospect for the detection of heavy metal ions especially in household and resource-limited occasions.

Advances and Future Applications of Augmented Peripheral Nerve Regeneration

Peripheral nerve injuries remain a significant source of long lasting morbidity, disability, and economic costs. Much research continues to be performed in areas related to improving the surgical outcomes of peripheral nerve repair. In this review, the physiology of peripheral nerve regeneration and the multitude of efforts to improve surgical outcomes are discussed. Improvements in tissue engineering that have allowed for the use of synthetic conduits seeded with neurotrophic factors are highlighted. Selected pre-clinical and available clinical data using cell based methods such as Schwann cell, undifferentiated, and differentiated stem cell transplantation to guide and enhance peripheral nerve regeneration are presented. The limitations that still exist in the utility of neurotrophic factors and cell-based therapies are outlined. Strategies that are most promising for translation into the clinical arena are suggested.

3D-engineering of Cellularized Conduits for Peripheral Nerve Regeneration

Tissue engineered conduits have great promise for bridging peripheral nerve defects by providing physical guiding and biological cues. A flexible method for integrating support cells into a conduit with desired architectures is wanted. Here, a 3D-printing technology is adopted to prepare a bio-conduit with designer structures for peripheral nerve regeneration. This bio-conduit is consisted of a cryopolymerized gelatin methacryloyl (cryoGelMA) gel cellularized with adipose-derived stem cells (ASCs). By modeling using 3D-printed “lock and key” moulds, the cryoGelMA gel is structured into conduits with different geometries, such as the designed multichannel or bifurcating and the personalized structures. The cryoGelMA conduit is degradable and could be completely degraded in 2-4 months in vivo. The cryoGelMA scaffold supports the attachment, proliferation and survival of the seeded ASCs, and up-regulates the expression of their neurotrophic factors mRNA in vitro. After implanted in a rat model, the bio-conduit is capable of supporting the re-innervation across a 10 mm sciatic nerve gap, with results close to that of the autografts in terms of functional and histological assessments. The study describes an indirect 3D-printing technology for fabricating cellularized designer conduits for peripheral nerve regeneration, and could lead to the development of future nerve bio-conduits for clinical use.

Collagen and Elastin Biomaterials for the Fabrication of Engineered Living Tissues

Collagen and elastin represent the two most predominant proteins in the body and are responsible for modulating important biological and mechanical properties. Thus, the focus of this review is the use of collagen and elastin as biomaterials for the fabrication of living tissues. Considering the importance of both biomaterials, we first propose the notion that many tissues in the human body represent a reinforced composite of collagen and elastin. In the rest of the review, collagen and elastin biosynthesis and biophysics, as well as molecular sources and biomaterial fabrication methodologies, including casting, fiber spinning, and bioprinting, are discussed. Finally, we summarize the current attempts to fabricate a subset of living tissues and, based on biochemical and biomechanical considerations, suggest that future tissue-engineering efforts consider direct incorporation of collagen and elastin biomaterials.

3D Printed Bionic Nanodevices

The ability to three-dimensionally interweave biological and functional materials could enable the creation of bionic devices possessing unique and compelling geometries, properties, and functionalities. Indeed, interfacing high performance active devices with biology could impact a variety of fields, including regenerative bioelectronic medicines, smart prosthetics, medical robotics, and human-machine interfaces. Biology, from the molecular scale of DNA and proteins, to the macroscopic scale of tissues and organs, is three-dimensional, often soft and stretchable, and temperature sensitive. This renders most biological platforms incompatible with the fabrication and materials processing methods that have been developed and optimized for functional electronics, which are typically planar, rigid and brittle. A number of strategies have been developed to overcome these dichotomies. One particularly novel approach is the use of extrusion-based multi-material 3D printing, which is an additive manufacturing technology that offers a freeform fabrication strategy. This approach addresses the dichotomies presented above by (1) using 3D printing and imaging for customized, hierarchical, and interwoven device architectures; (2) employing nanotechnology as an enabling route for introducing high performance materials, with the potential for exhibiting properties not found in the bulk; and (3) 3D printing a range of soft and nanoscale materials to enable the integration of a diverse palette of high quality functional nanomaterials with biology. Further, 3D printing is a multi-scale platform, allowing for the incorporation of functional nanoscale inks, the printing of microscale features, and ultimately the creation of macroscale devices. This blending of 3D printing, novel nanomaterial properties, and 'living' platforms may enable next-generation bionic systems. In this review, we highlight this synergistic integration of the unique properties of nanomaterials with the versatility of extrusion-based 3D printing technologies to interweave nanomaterials and fabricate novel bionic devices.

3D printed nervous system on a chip

Bioinspired organ-level in vitro platforms are emerging as effective technologies for fundamental research, drug discovery, and personalized healthcare. In particular, models for nervous system research are especially important, due to the complexity of neurological phenomena and challenges associated with developing targeted treatment of neurological disorders. Here we introduce an additive manufacturing-based approach in the form of a bioinspired, customizable 3D printed nervous system on a chip (3DNSC) for the study of viral infection in the nervous system. Micro-extrusion 3D printing strategies enabled the assembly of biomimetic scaffold components (microchannels and compartmented chambers) for the alignment of axonal networks and spatial organization of cellular components. Physiologically relevant studies of nervous system infection using the multiscale biomimetic device demonstrated the functionality of the in vitro platform. We found that Schwann cells participate in axon-to-cell viral spread but appear refractory to infection, exhibiting a multiplicity of infection (MOI) of 1.4 genomes per cell. These results suggest that 3D printing is a valuable approach for the prototyping of a customized model nervous system on a chip technology.

Remote Magnetic Orientation of 3D Collagen Hydrogels for Directed Neuronal Regeneration

Hydrogel matrices are valuable platforms for neuronal tissue engineering. Orienting gel fibers to achieve a directed scaffold is important for effective functional neuronal regeneration. However, current methods are limited and require treatment of gels prior to implantation, ex-vivo, without taking into consideration the pathology in the injured site. We have developed a method to control gel orientation dynamically and remotely in situ. We have mixed into collagen hydrogels magnetic nanoparticles then applied an external magnetic field. During the gelation period the magnetic particles aggregated into magnetic particle strings, leading to the alignment of the collagen fibers. We have shown that neurons within the 3D magnetically induced gels exhibited normal electrical activity and viability. Importantly, neurons formed elongated cooriented morphology, relying on the particle strings and fibers as supportive cues for growth. The ability to inject the mixed gel directly into the injured site as a solution then to control scaffold orientation remotely opens future possibilities for therapeutic engineered scaffolds.

Repopulating Decellularized Kidney Scaffolds: An Avenue for Ex Vivo Organ Generation

Recent research has shown that fully developed organs can be decellularized, resulting in a complex scaffold and extracellular matrix (ECM) network capable of being populated with other cells. This work has resulted in a growing field in bioengineering focused on the isolation, characterization, and modification of organ derived acellular scaffolds and their potential to sustain and interact with new cell populations, a process termed reseeding. In this review, we cover contemporary advancements in the bioengineering of kidney scaffolds including novel work showing that reseeded donor scaffolds can be transplanted and can function in recipients using animal models. Several major areas of the field are taken into consideration, including the decellularization process, characterization of acellular and reseeded scaffolds, culture conditions, and cell sources. Finally, we discuss future avenues based on the advent of 3D bioprinting and recent developments in kidney organoid cultures as well as animal models of renal genesis. The ongoing mergers and collaborations between these fields hold the potential to produce functional kidneys that can be generated ex vivo and utilized for kidney transplantations in patients suffering with renal disease.