Large-scale topographical screen for investigation of physical neural-guidance cues

Interfacing with the Brain: How Nanotechnology Can Contribute

Interfacing artificial devices with the human brain is the central goal of neurotechnology. Yet, our imaginations are often limited by currently available paradigms and technologies. Suggestions for brain-machine interfaces have changed over time, along with the available technology. Mechanical levers and cable winches were used to move parts of the brain during the mechanical age. Sophisticated electronic wiring and remote control have arisen during the electronic age, ultimately leading to plug-and-play computer interfaces. Nonetheless, our brains are so complex that these visions, until recently, largely remained unreachable dreams. The general problem, thus far, is that most of our technology is mechanically and/or electrically engineered, whereas the brain is a living, dynamic entity. As a result, these worlds are difficult to interface with one another. Nanotechnology, which encompasses engineered solid-state objects and integrated circuits, excels at small length scales of single to a few hundred nanometers and, thus, matches the sizes of biomolecules, biomolecular assemblies, and parts of cells. Consequently, we envision nanomaterials and nanotools as opportunities to interface with the brain in alternative ways. Here, we review the existing literature on the use of nanotechnology in brain-machine interfaces and look forward in discussing perspectives and limitations based on the authors' expertise across a range of complementary disciplines─from neuroscience, engineering, physics, and chemistry to biology and medicine, computer science and mathematics, and social science and jurisprudence. We focus on nanotechnology but also include information from related fields when useful and complementary.

The Future of Biohybrid Regenerative Bioelectronics

Biohybrid regenerative bioelectronics are an emerging technology combining implantable devices with cell transplantation. Once implanted, biohybrid regenerative devices integrate with host tissue. The combination of transplant and device provides an avenue to both replace damaged or dysfunctional tissue, and monitor or control its function with high precision. While early challenges in the fusion of the biological and technological components limited development of biohybrid regenerative technologies, progress in the field has resulted in a rapidly increasing number of applications. In this perspective the great potential of this emerging technology for the delivery of therapy is discussed, including both recent research progress and potential new directions. Then the technology barriers are discussed that will need to be addressed to unlock the full potential of biohybrid regenerative devices.

Hijacking plant skeletons for biomedical applications: from regenerative medicine and drug delivery to biosensing

The field of biomedical engineering continually seeks innovative technologies to address complex healthcare challenges, ranging from tissue regeneration to drug delivery and biosensing. Plant skeletons offer promising opportunities for these applications due to their unique hierarchical structures, desirable porosity, inherent biocompatibility, and adjustable mechanical properties. This review comprehensively discusses chemical principles underlying the utilization of plant-based scaffolds in biomedical engineering. Highlighting their structural integrity, tunable properties, and possibility of chemical modification, the review explores diverse preparation strategies to tailor plant skeleton properties for bone, neural, cardiovascular, skeletal muscle, and tendon tissue engineering. Such applications stem from the cellulosic three-dimensional structure of different parts of plants, which can mimic the complexity of native tissues and extracellular matrices, providing an ideal environment for cell adhesion, proliferation, and differentiation. We also discuss the application of plant skeletons as carriers for drug delivery due to their structural diversity and versatility in encapsulating and releasing therapeutic agents with controlled kinetics. Furthermore, we present the emerging role played by plant-derived materials in biosensor development for diagnostic and monitoring purposes. Challenges and future directions in the field are also discussed, offering insights into the opportunities for future translation of sustainable plant-based technologies to address critical healthcare needs.

Nanomagnetic Guidance Shapes the Structure-Function Relationship of Developing Cortical Networks

In this study, we implement large-scale nanomagnetic guidance on cortical neurons to guide dissociated neuronal networks during development. Cortical networks cultured over microelectrode arrays were exposed to functionalized magnetic nanoparticles, followed by magnetic field exposure to guide neurites over 14 days in vitro. Immunofluorescence of the axonal protein Tau revealed a greater number of neurites that were longer and aligned with the nanomagnetic force relative to nonguided networks. This was further confirmed through brightfield imaging on the microelectrode arrays during development. Spontaneous electrophysiological recordings revealed that the guided networks exhibited increased firing rates and frequency in force-aligned connectivity identified through Granger Causality. Applying this methodology across networks with nonuniform force directions increased local activity in target regions, identified as regions in the direction of the nanomagnetic force. Altogether, these results demonstrate that nanomagnetic forces guide the structure and function of dissociated cortical neuron networks at the millimeter scale.

Creation of Grooved Tissue Engineering Scaffolds from Architectured Multilayer Polymer Composites by a Tuneable One-Step Degradation Process

The surface properties of biomaterials interact directly with biological systems, influencing cellular responses, tissue integration, and biocompatibility. Surface topography plays a critical role in cardiac tissue engineering by affecting electrical conductivity, cardiomyocyte alignment, and contractile function. Current methods for controlling surface properties and topography in cardiac tissue engineering scaffolds are limited, expensive, and lack precision. This study introduces a low-cost, one-step degradation process to create scaffolds with well-defined micro-grooves from multilayered 3D printed poly(lactic acid)/thermoplastic polyurethane composites. The approach provides control over erosion rate and surface morphology, allowing easy tuning of scaffold topographical cues for tissue engineering applications. The findings reported in this study provide a library of easily tuneable scaffold topographical cues. A strong dependence of neonatal rat cardiomyocyte (NRCM) contact guidance with the multilayers' dimension and shape in partially degraded polylactic acid (PLA)/thermoplastic polyurethane (TPU) samples is observed. NRCMs cultured on samples with a layer thickness of 13 ± 2 µm and depth of 4.7 ± 0.2 µm demonstrate the most regular contractions. Hence, the proposed fabrication scheme can be used to produce a new generation of biomaterials with excellent controllability determined by multilayer thickness, printing parameters, and degradation treatment duration.

Electrophysiological features of cortical 3D networks are deeply modulated by scaffold properties

Three-dimensionality (3D) was proven essential for developing reliable models for different anatomical compartments and many diseases. However, the neuronal compartment still poses a great challenge as we still do not understand precisely how the brain computes information and how the complex chain of neuronal events can generate conscious behavior. Therefore, a comprehensive model of neuronal tissue has not yet been found. The present work was conceived in this framework: we aimed to contribute to what must be a collective effort by filling in some information on possible 3D strategies to pursue. We compared directly different kinds of scaffolds (i.e., PDMS sponges, thermally crosslinked hydrogels, and glass microbeads) in their effect on neuronal network activity recorded using micro-electrode arrays. While the overall rate of spiking activity remained consistent, the type of scaffold had a notable impact on bursting dynamics. The frequency, density of bursts, and occurrence of random spikes were all affected. The examination of inter-burst intervals revealed distinct burst generation patterns unique to different scaffold types. Network burst propagation unveiled divergent trends among configurations. Notably, it showed the most differences, underlying that functional variations may arise from a different 3D spatial organization. This evidence suggests that not all 3D neuronal constructs can sustain the same level of richness of activity. Furthermore, we commented on the reproducibility, efficacy, and scalability of the methods, where the beads still offer superior performances. By comparing different 3D scaffolds, our results move toward understanding the best strategies to develop functional 3D neuronal units for reliable pre-clinical studies.

Evolution of Retinal Neuron Fractality When Interfacing with Carbon Nanotube Electrodes

Exploring how neurons in the mammalian body interact with the artificial interface of implants can be used to learn about fundamental cell behavior and to refine medical applications. For fundamental and applied research, it is crucial to determine the conditions that encourage neurons to maintain their natural behavior during interactions with non-natural interfaces. Our previous investigations quantified the deterioration of neuronal connectivity when their dendrites deviate from their natural fractal geometry. Fractal resonance proposes that neurons will exhibit enhanced connectivity if an implant's electrode geometry is matched to the fractal geometry of the neurons. Here, we use in vitro imaging to quantify the fractal geometry of mouse retinal neurons and show that they change during interaction with the electrode. Our results demonstrate that it is crucial to understand these changes in the fractal properties of neurons for fractal resonance to be effective in the in vivo mammalian system.

Beyond a Transmission Cable-New Technologies to Reveal the Richness in Axonal Electrophysiology

The axon is a neuronal structure capable of processing, encoding, and transmitting information. This assessment contrasts with a limiting, but deeply rooted, perspective where the axon functions solely as a transmission cable of somatodendritic activity, sending signals in the form of stereotypical action potentials. This perspective arose, at least partially, because of the technical difficulties in probing axons: their extreme length-to-diameter ratio and intricate growth paths preclude the study of their dynamics through traditional techniques. Recent findings are challenging this view and revealing a much larger repertoire of axonal computations. Axons display complex signaling processes and structure-function relationships, which can be modulated via diverse activity-dependent mechanisms. Additionally, axons can exhibit patterns of activity that are dramatically different from those of their corresponding soma. Not surprisingly, many of these recent discoveries have been driven by novel technology developments, which allow for in vitro axon electrophysiology with unprecedented spatiotemporal resolution and signal-to-noise ratio. In this review, we outline the state-of-the-art in vitro toolset for axonal electrophysiology and summarize the recent discoveries in axon function it has enabled. We also review the increasing repertoire of microtechnologies for controlling axon guidance which, in combination with the available cutting-edge electrophysiology and imaging approaches, have the potential for more controlled and high-throughput in vitro studies. We anticipate that a larger adoption of these new technologies by the neuroscience community will drive a new era of experimental opportunities in the study of axon physiology and consequently, neuronal function.

Fractal Resonance: Can Fractal Geometry Be Used to Optimize the Connectivity of Neurons to Artificial Implants?

In parallel to medical applications, exploring how neurons interact with the artificial interface of implants in the human body can be used to learn about their fundamental behavior. For both fundamental and applied research, it is important to determine the conditions that encourage neurons to maintain their natural behavior during these interactions. Whereas previous biocompatibility studies have focused on the material properties of the neuron-implant interface, here we discuss the concept of fractal resonance - the possibility that favorable connectivity properties might emerge by matching the fractal geometry of the implant surface to that of the neurons.To investigate fractal resonance, we first determine the degree to which neurons are fractal and the impact of this fractality on their functionality. By analyzing three-dimensional images of rat hippocampal neurons, we find that the way their dendrites fork and weave through space is important for generating their fractal-like behavior. By modeling variations in neuron connectivity along with the associated energetic and material costs, we highlight how the neurons' fractal dimension optimizes these constraints. To simulate neuron interactions with implant interfaces, we distort the neuron models away from their natural form by modifying the dendrites' fork and weaving patterns. We find that small deviations can induce large changes in fractal dimension, causing the balance between connectivity and cost to deteriorate rapidly. We propose that implant surfaces should be patterned to match the fractal dimension of the neurons, allowing them to maintain their natural functionality as they interact with the implant.

Plant Cellulose as a Substrate for 3D Neural Stem Cell Culture

Neural stem cell (NSC)-based therapies are at the forefront of regenerative medicine strategies for various neural defects and injuries such as stroke, traumatic brain injury, and spinal cord injury. For several clinical applications, NSC therapies require biocompatible scaffolds to support cell survival and to direct differentiation. Here, we investigate decellularized plant tissue as a novel scaffold for three-dimensional (3D), in vitro culture of NSCs. Plant cellulose scaffolds were shown to support the attachment and proliferation of adult rat hippocampal neural stem cells (NSCs). Further, NSCs differentiated on the cellulose scaffold had significant increases in their expression of neuron-specific beta-III tubulin and glial fibrillary acidic protein compared to 2D culture on a polystyrene plate, indicating that the scaffold may enhance the differentiation of NSCs towards astrocytic and neuronal lineages. Our findings suggest that plant-derived cellulose scaffolds have the potential to be used in neural tissue engineering and can be harnessed to direct the differentiation of NSCs.

Sensitivity of CNN image analysis to multifaceted measurements of neurite growth

Quantitative analysis of neurite growth and morphology is essential for understanding the determinants of neural development and regeneration, however, it is complicated by the labor-intensive process of measuring diverse parameters of neurite outgrowth. Consequently, automated approaches have been developed to study neurite morphology in a high-throughput and comprehensive manner. These approaches include computer-automated algorithms known as 'convolutional neural networks' (CNNs)-powerful models capable of learning complex tasks without the biases of hand-crafted models. Nevertheless, their complexity often relegates them to functioning as 'black boxes.' Therefore, research in the field of explainable AI is imperative to comprehend the relationship between CNN image analysis output and predefined morphological parameters of neurite growth in order to assess the applicability of these machine learning approaches. In this study, drawing inspiration from the field of automated feature selection, we investigate the correlation between quantified metrics of neurite morphology and the image analysis results from NeuriteNet-a CNN developed to analyze neurite growth. NeuriteNet accurately distinguishes images of neurite growth based on different treatment groups within two separate experimental systems. These systems differentiate between neurons cultured on different substrate conditions and neurons subjected to drug treatment inhibiting neurite outgrowth. By examining the model's function and patterns of activation underlying its classification decisions, we discover that NeuriteNet focuses on aspects of neuron morphology that represent quantifiable metrics distinguishing these groups. Additionally, it incorporates factors that are not encompassed by neuron morphology tracing analyses. NeuriteNet presents a novel tool ideally suited for screening morphological differences in heterogeneous neuron groups while also providing impetus for targeted follow-up studies.

High-Performance Implantable Sensors based on Anisotropic Magnetoresistive La0.67Sr0.33MnO3 for Biomedical Applications

We present the design, fabrication, and characterization of an implantable neural interface based on anisotropic magnetoresistive (AMR) magnetic-field sensors that combine reduced size and high performance at body temperature. The sensors are based on La_0.67Sr_0.33MnO₃ (LSMO) as a ferromagnetic material, whose epitaxial growth has been suitably engineered to get uniaxial anisotropy and large AMR output together with low noise even at low frequencies. The performance of LSMO sensors of different film thickness and at different temperatures close to 37 °C has to be explored to find an optimum sensitivity of ∼400%/T (with typical detectivity values of 2 nT·Hz^-1/2 at a frequency of 1 Hz and 0.3 nT·Hz^-1/2 at 1 kHz), fitted for the detection of low magnetic signals coming from neural activity. Biocompatibility tests of devices consisting of submillimeter-size LSMO sensors coated by a thin poly(dimethyl siloxane) polymeric layer, both in vitro and in vivo, support their high suitability as implantable detectors of low-frequency biological magnetic signals emerging from heterogeneous electrically active tissues.

Defined Physicochemical Cues Steering Direct Neuronal Reprogramming on Colloidal Self-Assembled Patterns (cSAPs)

Direct neuronal reprogramming of somatic cells into induced neurons (iNs) has been recently established as a promising approach to generating neuron cells. Previous studies have reported that the biophysical cues of the in vitro microenvironment are potent modulators in the cell fate decision; thus, the present study explores the effects of a customized pattern (named colloidal self-assembled patterns, cSAPs) on iN generation from human fibroblasts using small molecules. The result revealed that the cSAP, composed of binary particles in a hexagonal-close-packed (hcp) geometry, is capable of improving neuronal reprogramming efficiency and steering the ratio of the iN subtypes. Cells exhibited distinct cell morphology, upregulated cell adhesion markers (i.e., SDC1 and ITGAV), enriched signaling pathways (i.e., Hippo and Wnt), and chromatin remodeling on the cSAP compared to those on the control substrates. The result also showed that the iN subtype specification on cSAP was surface-dependent; therefore, the defined physicochemical cue from each cSAP is exclusive. Our findings show that direct cell reprogramming can be manipulated through specific biophysical cues on the artificial matrix, which is significant in cell transdifferentiation and lineage conversion.

High-Content Screening and Analysis of Stem Cell-Derived Neural Interfaces Using a Combinatorial Nanotechnology and Machine Learning Approach

A systematic investigation of stem cell-derived neural interfaces can facilitate the discovery of the molecular mechanisms behind cell behavior in neurological disorders and accelerate the development of stem cell-based therapies. Nevertheless, high-throughput investigation of the cell-type-specific biophysical cues associated with stem cell-derived neural interfaces continues to be a significant obstacle to overcome. To this end, we developed a combinatorial nanoarray-based method for high-throughput investigation of neural interface micro-/nanostructures (physical cues comprising geometrical, topographical, and mechanical aspects) and the effects of these complex physical cues on stem cell fate decisions. Furthermore, by applying a machine learning (ML)-based analytical approach to a large number of stem cell-derived neural interfaces, we comprehensively mapped stem cell adhesion, differentiation, and proliferation, which allowed for the cell-type-specific design of biomaterials for neural interfacing, including both adult and human-induced pluripotent stem cells (hiPSCs) with varying genetic backgrounds. In short, we successfully demonstrated how an innovative combinatorial nanoarray and ML-based platform technology can aid with the rational design of stem cell-derived neural interfaces, potentially facilitating precision, and personalized tissue engineering applications.

Delivering Mechanical Stimulation to Cells: State of the Art in Materials and Devices Design

Biochemical signals, such as growth factors, cytokines, and transcription factors are known to play a crucial role in regulating a variety of cellular activities as well as maintaining the normal function of different tissues and organs. If the biochemical signals are assumed to be one side of the coin, the other side comprises biophysical cues. There is growing evidence showing that biophysical signals, and in particular mechanical cues, also play an important role in different stages of human life ranging from morphogenesis during embryonic development to maturation and maintenance of tissue and organ function throughout life. In order to investigate how mechanical signals influence cell and tissue function, tremendous efforts have been devoted to fabricating various materials and devices for delivering mechanical stimuli to cells and tissues. Here, an overview of the current state of the art in the design and development of such materials and devices is provided, with a focus on their design principles, and challenges and perspectives for future research directions are highlighted.

Submicron Topographically Patterned 3D Substrates Enhance Directional Axon Outgrowth of Dorsal Root Ganglia Cultured Ex Vivo

A peripheral nerve injury results in disruption of the fiber that usually protects axons from the surrounding environment. Severed axons from the proximal nerve stump are capable of regenerating, but axons are exposed to a completely new environment. Regeneration recruits cells that produce and deposit key molecules, including growth factor proteins and fibrils in the extracellular matrix (ECM), thus changing the chemical and geometrical environment. The regenerating axons thus surf on a newly remodeled micro-landscape. Strategies to enhance and control axonal regeneration and growth after injury often involve mimicking the extrinsic cues that are found in the natural nerve environment. Indeed, nano- and micropatterned substrates have been generated as tools to guide axons along a defined path. The mechanical cues of the substrate are used as guides to orient growth or change the direction of growth in response to impediments or cell surface topography. However, exactly how axons respond to biophysical information and the dynamics of axonal movement are still poorly understood. Here we use anisotropic, groove-patterned substrate topography to direct and enhance sensory axonal growth of whole mouse dorsal root ganglia (DRG) transplanted ex vivo. Our results show significantly enhanced and directed growth of the DRG sensory fibers on the hemi-3D topographic substrates compared to a 0 nm pitch, flat control surface. By assessing the dynamics of axonal movement in time-lapse microscopy, we found that the enhancement was not due to increases in the speed of axonal growth, but to the efficiency of growth direction, ensuring axons minimize movement in undesired directions. Finally, the directionality of growth was reproduced on topographic patterns fabricated as fully 3D substrates, potentially opening new translational avenues of development incorporating these specific topographic feature sizes in implantable conduits in vivo.

Insight and Recent Advances into the Role of Topography on the Cell Differentiation and Proliferation on Biopolymeric Surfaces

It is well known that surface topography plays an important role in cell behavior, including adhesion, migration, orientation, elongation, proliferation and differentiation. Studying these cell functions is essential in order to better understand and control specific characteristics of the cells and thus to enhance their potential in various biomedical applications. This review proposes to investigate the extent to which various surface relief patterns, imprinted in biopolymer films or in polymeric films coated with biopolymers, by utilizing specific lithographic techniques, influence cell behavior and development. We aim to understand how characteristics such as shape, dimension or chemical functionality of surface relief patterns alter the orientation and elongation of cells, and thus, finally make their mark on the cell proliferation and differentiation. We infer that such an insight is a prerequisite for pushing forward the comprehension of the methodologies and technologies used in tissue engineering applications and products, including skin or bone implants and wound or fracture healing.

Biophysical Regulations of Epigenetic State and Notch Signaling in Neural Development Using Microgroove Substrates

A number of studies have recently shown how surface topography can alter the behavior and differentiation patterns of different types of stem cells. Although the exact mechanisms and molecular pathways involved remain unclear, a consistent portion of the literature points to epigenetic changes induced by nuclear remodeling. In this study, we investigate the behavior of clinically relevant neural populations derived from human pluripotent stem cells when cultured on polydimethylsiloxane microgrooves (3 and 10 μm depth grooves) to investigate what mechanisms are responsible for their differentiation capacity and functional behavior. Our results show that microgrooves enhance cell alignment, modify nuclear geometry, and significantly increase cellular stiffness, which we were able to measure at high resolution with a combination of light and electron microscopy, scanning ion conductance microscopy (SICM), and atomic force microscopy (AFM) coupled with quantitative image analysis. The microgrooves promoted significant changes in the epigenetic landscape, as revealed by the expression of key histone modification markers. The main behavioral change of neural stem cells on microgrooves was an increase of neuronal differentiation under basal conditions on the microgrooves. Through measurements of cleaved Notch1 levels, we found that microgrooves downregulate Notch signaling. We in fact propose that microgroove topography affects the differentiation potential of neural stem cells by indirectly altering Notch signaling through geometric segregation and that this mechanism in parallel with topography-dependent epigenetic modulations acts in concert to enhance stem cell neuronal differentiation.

Regulatory Effects of Gradient Microtopographies on Synapse Formation and Neurite Growth in Hippocampal Neurons

Among approaches aiming toward functional nervous system restoration, those implementing microfabrication techniques allow the manufacture of platforms with distinct geometry where neurons can develop and be guided to form patterned connections in vitro. The interplay between neuronal development and the microenvironment, shaped by the physical limitations, remains largely unknown. Therefore, it is crucial to have an efficient way to quantify neuronal morphological changes induced by physical or contact guidance of the microenvironment. In this study, we first devise and assess a method to prepare anisotropic, gradient poly(dimethylsiloxane) micro-ridge/groove arrays featuring variable local pattern width. We then demonstrate the ability of this single substrate to simultaneously profile the morphologcial and synaptic connectivity changes of primary cultured hippocampal neurons reacting to variable physical conditons, throughout neurodevelopment, in vitro. The gradient microtopography enhanced adhesion within microgrooves, increasing soma density with decreasing pattern width. Decreasing pattern width also reduced dendritic arborization and increased preferential axon growth. Finally, decreasing pattern geometry inhibited presynaptic puncta architecture. Collectively, a method to examine structural development and connectivity in response to physical stimuli is established, and potentially provides insight into microfabricated geometries which promote neural regeneration and repair.

Application and prospects of high-throughput screening for in vitro neurogenesis

Over the past few decades, high-throughput screening (HTS) has made great contributions to new drug discovery. HTS technology is equipped with higher throughput, minimized platforms, more automated and computerized operating systems, more efficient and sensitive detection devices, and rapid data processing systems. At the same time, in vitro neurogenesis is gradually becoming important in establishing models to investigate the mechanisms of neural disease or developmental processes. However, challenges remain in generating more mature and functional neurons with specific subtypes and in establishing robust and standardized three-dimensional (3D) in vitro models with neural cells cultured in 3D matrices or organoids representing specific brain regions. Here, we review the applications of HTS technologies on in vitro neurogenesis, especially aiming at identifying the essential genes, chemical small molecules and adaptive microenvironments that hold great prospects for generating functional neurons or more reproductive and homogeneous 3D organoids. We also discuss the developmental tendency of HTS technology, e.g., so-called next-generation screening, which utilizes 3D organoid-based screening combined with microfluidic devices to narrow the gap between in vitro models and in vivo situations both physiologically and pathologically.

Multivariate analysis reveals topography dependent relationships amongst neurite morphological features from dorsal root ganglia neurons

Objective.Nerve guidance scaffolds containing anisotropic architectures provide topographical cues to direct regenerating axons through an injury site to reconnect the proximal and distal end of an injured nerve or spinal cord. Previousin vitrocultures of individual neurons revealed that fiber characteristics such as fiber diameter and inter-fiber spacing alter neurite morphological features, such as total neurite length, the longest single neurite, branching density, and the number of primary neurites. However, the relationships amongst these four neurite morphological features have never been studied on fibrous topographies using multivariate analysis.Approach.In this study, we cultured dissociated dorsal root ganglia on aligned, fibrous scaffolds and flat, isotropic films and evaluated the univariate and multivariate differences amongst these four neurite morphological features.Main results.Univariate analysis showed that fibrous scaffolds increase the length of the longest neurite and decrease branching density compared to film controls. Further, multivariate analysis revealed that, regardless of scaffold type, overall neurite length increases due to a compromise between the longest extending neurite, branching density, and the number of primary neurites. Additionally, multivariate analysis indicated that neurite branching is more independent of the other neurite features when neurons were cultured on films but that branching is strongly related to the other neurite features when cultured on fibers.Significance.These findings are significant as they are the first evidence that aligned topographies affect the relationships between neurite morphological features. This study provides a foundation for analyzing how individual neurite morphology may relate to neural regeneration on a macroscopic scale and provide information that may be used to optimize nerve guidance scaffolds.

The Bumpy Road to Stem Cell Therapies: Rational Design of Surface Topographies to Dictate Stem Cell Mechanotransduction and Fate

Cells sense and respond to a variety of physical cues from their surrounding microenvironment, and these are interpreted through mechanotransductive processes to inform their behavior. These mechanisms have particular relevance to stem cells, where control of stem cell proliferation, potency, and differentiation is key to their successful application in regenerative medicine. It is increasingly recognized that surface micro- and nanotopographies influence stem cell behavior and may represent a powerful tool with which to direct the morphology and fate of stem cells. Current progress toward this goal has been driven by combined advances in fabrication technologies and cell biology. Here, the capacity to generate precisely defined micro- and nanoscale topographies has facilitated the studies that provide knowledge of the mechanotransducive processes that govern the cellular response as well as knowledge of the specific features that can drive cells toward a defined differentiation outcome. However, the path forward is not fully defined, and the "bumpy road" that lays ahead must be crossed before the full potential of these approaches can be fully exploited. This review focuses on the challenges and opportunities in applying micro- and nanotopographies to dictate stem cell fate for regenerative medicine. Here, key techniques used to produce topographic features are reviewed, such as photolithography, block copolymer lithography, electron beam lithography, nanoimprint lithography, soft lithography, scanning probe lithography, colloidal lithography, electrospinning, and surface roughening, alongside their advantages and disadvantages. The biological impacts of surface topographies are then discussed, including the current understanding of the mechanotransductive mechanisms by which these cues are interpreted by the cells, as well as the specific effects of surface topographies on cell differentiation and fate. Finally, considerations in translating these technologies and their future prospects are evaluated.

Omics Technologies for High-Throughput-Screening of Cell-Biomaterial Interactions

Recent research effort in biomaterial development has largely focused on engineering bio-instructive materials to stimulate specific cell signaling. Assessing the biological performance of these materials using time-consuming and trial-and-error traditional...

A review on 3D printing functional brain model

Modern neuroscience increasingly relies on 3D models to study neural circuitry, nerve regeneration, and neural disease. Several different biofabrication approaches have been explored to create 3D neural tissue model structures. Among them, 3D bioprinting has shown to have great potential to emerge as a high-throughput/high precision biofabrication strategy that can address the growing need for 3D neural models. Here, we have reviewed the design principles for neural tissue engineering. The main challenge to adapt printing technologies for biofabrication of neural tissue models is the development of neural bioink, i.e., a biomaterial with printability and gelation properties and also suitable for neural tissue culture. This review shines light on a vast range of biomaterials as well as the fundamentals of 3D neural tissue printing. Also, advances in 3D bioprinting technologies are reviewed especially for bioprinted neural models. Finally, the techniques used to evaluate the fabricated 2D and 3D neural models are discussed and compared in terms of feasibility and functionality.

Interaction of micropatterned topographical and biochemical cues to direct neurite growth from spiral ganglion neurons

Functional outcomes with neural prosthetic devices, such as cochlear implants, are limited in part due to physical separation between the stimulating elements and the neurons they stimulate. One strategy to close this gap aims to precisely guide neurite regeneration to position the neurites in closer proximity to electrode arrays. Here, we explore the ability of micropatterned biochemical and topographic guidance cues, singly and in combination, to direct the growth of spiral ganglion neuron (SGN) neurites, the neurons targeted by cochlear implants. Photopolymerization of methacrylate monomers was used to form unidirectional topographical features of ridges and grooves in addition to multidirectional patterns with 90^o angle turns. Microcontact printing was also used to create similar uni- and multi-directional patterns of peptides on polymer surfaces. Biochemical cues included peptides that facilitate (laminin, LN) or repel (EphA4-Fc) neurite growth. On flat surfaces, SGN neurites preferentially grew on LN-coated stripes and avoided EphA4-Fc-coated stripes. LN or EphA4-Fc was selectively adsorbed onto the ridges or grooves to test the neurite response to a combination of topographical and biochemical cues. Coating the ridges with EphA4-Fc and grooves with LN lead to enhanced SGN alignment to topographical patterns. Conversely, EphA4-Fc coating on the grooves or LN coating on the ridges tended to disrupt alignment to topographical patterns. SGN neurites respond to combinations of topographical and biochemical cues and surface patterning that leverages both cues enhance guided neurite growth.

Bionic microenvironment-inspired synergistic effect of anisotropic micro-nanocomposite topology and biology cues on peripheral nerve regeneration

Anisotropic topographies and biological cues can simulate the regenerative microenvironment of nerve from physical and biological aspects, which show promising application in nerve regeneration. However, their synergetic influence on injured peripheral nerve is rarely reported. In the present study, we constructed a bionic microenvironment-inspired scaffold integrated with both anisotropic micro-nanocomposite topographies and IKVAV peptide. The results showed that both the topographies and peptide displayed good stability. The scaffolds could effectively induce the orientation growth of Schwann cells and up-regulate the genes and proteins relevant to myelination. Last, three signal pathways including the Wnt/β-catenin pathway, the extracellular signal-regulated kinase/mitogen-activated protein pathway, and the transforming growth factor-β pathway were put forward, revealing the main path of synergistic effects of anisotropic micro-nanocomposite topographies and biological cues on neuroregeneration. The present study may supply an important strategy for developing functional of artificial nerve implants.

Understanding, engineering, and modulating the growth of neural networks: An interdisciplinary approach

A deeper understanding of the brain and its function remains one of the most significant scientific challenges. It not only is required to find cures for a plethora of brain-related diseases and injuries but also opens up possibilities for achieving technological wonders, such as brain-machine interface and highly energy-efficient computing devices. Central to the brain's function is its basic functioning unit (i.e., the neuron). There has been a tremendous effort to understand the underlying mechanisms of neuronal growth on both biochemical and biophysical levels. In the past decade, this increased understanding has led to the possibility of controlling and modulating neuronal growth in vitro through external chemical and physical methods. We provide a detailed overview of the most fundamental aspects of neuronal growth and discuss how researchers are using interdisciplinary ideas to engineer neuronal networks in vitro. We first discuss the biochemical and biophysical mechanisms of neuronal growth as we stress the fact that the biochemical or biophysical processes during neuronal growth are not independent of each other but, rather, are complementary. Next, we discuss how utilizing these fundamental mechanisms can enable control over neuronal growth for advanced neuroengineering and biomedical applications. At the end of this review, we discuss some of the open questions and our perspectives on the challenges and possibilities related to controlling and engineering the growth of neuronal networks, specifically in relation to the materials, substrates, model systems, modulation techniques, data science, and artificial intelligence.

Patient-Specific Organoid and Organ-on-a-Chip: 3D Cell-Culture Meets 3D Printing and Numerical Simulation

The last few decades have witnessed diversified in vitro models to recapitulate the architecture and function of living organs or tissues and contribute immensely to advances in life science. Two novel 3D cell culture models: 1) Organoid, promoted mainly by the developments of stem cell biology and 2) Organ-on-a-chip, enhanced primarily due to microfluidic technology, have emerged as two promising approaches to advance the understanding of basic biological principles and clinical treatments. This review describes the comparable distinct differences between these two models and provides more insights into their complementarity and integration to recognize their merits and limitations for applicable fields. The convergence of the two approaches to produce multi-organoid-on-a-chip or human organoid-on-a-chip is emerging as a new approach for building 3D models with higher physiological relevance. Furthermore, rapid advancements in 3D printing and numerical simulations, which facilitate the design, manufacture, and results-translation of 3D cell culture models, can also serve as novel tools to promote the development and propagation of organoid and organ-on-a-chip systems. Current technological challenges and limitations, as well as expert recommendations and future solutions to address the promising combinations by incorporating organoids, organ-on-a-chip, 3D printing, and numerical simulation, are also summarized.

Directing osteoblastic cell migration on arrays of nanopillars and nanoholes with different aspect ratios

To realize highly directional guidance for cell migration, both micro- and nano-scale topographies were studied to better understand and mimic the complex extracellular matrix environment. Polydimethylsiloxane-based platforms with micro- and nano-topographies were developed to systematically study their guidance effects on cell migration behaviors. Compared to microtopography such as flat surface or grating, nanotopographies such as nanoholes and nanopillars could promote the generation of filopodia and extension of long protrusions with increased migration speed for MC3T3-E1 cells. Although cells on the grating structures showed lower migration speed, more directional cell migration was achieved due to their anisotropic topography compared to nanohole or nanopillar arrays with isotropic structures. To further enhance the cell migration directionality, the nanotopographies were patterned in grating arrangements and the results showed that both nanoholes and nanopillars in grating arrangements introduced more directional cell migration compared to gratings. The effects of physical dimensions of the nanotopographies on cell migration were studied and the results showed that there was less cell elongation and less directional migration of the nanoholes in grating arrangements with increasing depth of nanoholes. However, the nanopillars in grating arrangements showed more cell elongation, more directional migration, and higher migration speed with increasing height of the nanopillars. Platforms with nanopillars in grating arrangements and large height could be used to control cell migration speed and directionality, which could potentially lead to cell sorting and screening.

Enhanced neurite outgrowth on electrically conductive carbon aerogel substrates in the presence of an external electric field

Previous works from our laboratory have firmly established that aerogels are a suitable substrate to elicit accelerated neurite extension. On non-conducting aerogels, in the presence of an externally-applied DC bias, neurons extended neurites which were preferentially aligned towards the anode. In this investigation, we sought to determine whether electrically-conductive carbon aerogels elicited a more robust alignment of neurites toward the anode than non-conductive aerogels due to the capacity of conductive aerogels to sustain a current, thereby providing a direct interface between neurons and the external electrical stimulus. To determine if this was the case, we plated PC12 neuronal cells on electrically conductive carbon aerolges derived from acetic acid-catalized resorcinol formaldehyde aerogels (ARF-CA) and subjected them to an external electric field. The voltages applied at the electrodes of the custom-built electro-stimulation chamber were 0 V, 15 V, and 30 V. For each voltage, the directionality and length of the neurites extended by PC12 cells were determined and compared to those observed when PC12 cells were plated on non-conductive aerogels subjected to the same voltage. The results show that the directionality of neurite extension was similar between conductive and non-conductive aerogels. A higher neurite length difference was observed on conductive aerogels with increasing voltage, 43% and 106% for 0-15 V and 0-30 V respectively, compared to non-conductive aerogels, 12% and 20%. These findings indicate that conductive carbon aerogels have a greater potential as scaffolds for nerve regeneration than non-conductive ones.

Novel Concept of Micro Patterned Micro Titer Plates Fabricated via UV-NIL for Automated Neuronal Cell Assay Read-Out

The UV-nanoimprint lithography(UV-NIL) fabrication of a novel network of micron-sized channels, forming an open channel microfluidic system is described. Details about the complete manufacturing process, from mastering to fabrication in small batches and in high throughput with up to 1200 micro titer plates per hour is presented. Deep insight into the evaluation of a suitable UV-curable material, mr-UVCur26SF is given, presenting cytotoxic evaluation, cell compatibility tests and finally a neuronal assay. The results indicate how the given pattern, in combination with the resist, paves the way to faster, cheaper, and more reliable drug screening.

Neural tissue engineering: the influence of scaffold surface topography and extracellular matrix microenvironment

Strategies using surface topography, contact guidance and biomechanical cues in the design of scaffolds as an ECM support for neural tissue engineering.

Nerve Response to Superelastic Shape Memory Polyurethane Aerogels

We have previously shown the suitability of aerogels as scaffolds for neuronal cells. Here, we report on the use of superelastic shape memory polyurethane aerogels (SSMPA). SSMPA have a distinctly different stiffness than previously reported aerogels. The soft and deformable nature of SSMPA allowed for radial compression of the aerogel induced by a custom designed apparatus. This radial compression changed the pore diameter and surface roughness (Sa) of SSMPA, while maintaining similar stiffness. Two varieties of SSMPA were used, Mix-14 and Mix-18, with distinctly different pore diameters and Sa. Radial compression led to a decreased pore diameter, which, in turn, decreased the Sa. The use of custom designed apparatus and two types of SSMPA allowed us to examine the influence of stiffness, pore size, and Sa on the extension of processes (neurites) by PC12 neuronal cells. PC12 cells plated on SSMPA with a higher degree of radial compression extended fewer neurites per cell when compared to other groups. However, the average length of the neurites was significantly longer when compared to the unrestricted group and to those extended by cells plated on SSMPA with less radial compression. These results demonstrate that SSMPA with 1.9 µm pore diameter, 1.17 µm Sa, and 203 kPa stiffness provides the optimum combination of physical parameters for nerve regeneration.

Neurite guidance and neuro-caging on steps and grooves in 2.5 dimensions

Directed guidance of neurites is a pre-requisite for tailor-made designs of interfaces between cells and semiconducting components. Grayscale lithography, reactive ion etching, and ultraviolet nanoimprint lithography are potent semiconductor industry-compatible techniques for a cost- and time-effective fabrication of modulated surfaces. In this work, neurite outgrowth of murine cerebellar neurons on 2.5D pathways produced with these methods is studied. Structures of micron-sized steps and grooves serve as cell culture platforms. The effects of contact guidance through topography and chemical guidance through selective poly-d-lysine coating on these platforms are analyzed. As a consequence, the herein presented fabrication approach can be utilized to cultivate and to study low-density neuronal networks in 2.5D configuration with a high degree of order.

Toward Brain-on-a-Chip: Human Induced Pluripotent Stem Cell-Derived Guided Neuronal Networks in Tailor-Made 3D Nanoprinted Microscaffolds

Brain-on-a-chip (BoC) concepts should consider three-dimensional (3D) scaffolds to mimic the 3D nature of the human brain not accessible by conventional planar cell culturing. Furthermore, the essential key to adequately address drug development for human pathophysiological diseases of the nervous system, such as Parkinson's or Alzheimer's, is to employ human induced pluripotent stem cell (iPSC)-derived neurons instead of neurons from animal models. To address both issues, we present electrophysiologically mature human iPSC-derived neurons cultured in BoC applicable microscaffolds prepared by direct laser writing. 3D nanoprinted tailor-made elevated cavities interconnected by freestanding microchannels were used to create defined neuronal networks-as a proof of concept-with two-dimensional topology. The neuronal outgrowth in these nonplanar structures was investigated, among others, in terms of neurite length, size of continuous networks, and branching behavior using z-stacks prepared by confocal microscopy and cross-sectional scanning electron microscopy images prepared by focused ion beam milling. Functionality of the human iPSC-derived neurons was demonstrated with patch clamp measurements in both current- and voltage-clamp mode. Action potentials and spontaneous excitatory postsynaptic currents-fundamental prerequisites for proper network signaling-prove full integrity of these artificial neuronal networks. Considering the network formation occurring within only a few days and the versatile nature of direct laser writing to create even more complex scaffolds for 3D network topologies, we believe that our study offers additional approaches in human disease research to mimic the complex interconnectivity of the human brain in BoC studies.

Reduced Graphene Oxide-Encapsulated Microfiber Patterns Enable Controllable Formation of Neuronal-Like Networks

Scaffold-guided formation of neuronal-like networks, especially under electrical stimulation, can be an appealing avenue toward functional restoration of injured nervous systems. Here, 3D conductive scaffolds are fabricated based on printed microfiber constructs using near-field electrostatic printing (NFEP) and graphene oxide (GO) coating. Various microfiber patterns are obtained from poly(l-lactic acid-co-caprolactone) (PLCL) using NFEP and complexity is achieved via modulating the fiber overlay angles (45°, 60°, 75°, 90°), fiber diameters (15 to 148 µm), and fiber spatial organization (spider web and tubular structure). Upon coating GO onto PLCL microfibers via a layer-by-layer (L-b-L) assembly technique and in situ reduction into reduced GO (rGO), the obtained conductive scaffolds, with 25-50 layers of rGO, demonstrate superior conductivity (≈0.95 S cm-1 ) and capability of inducing neuronal-like network formation along the conductive microfibers under electrical stimulation (100-150 mV cm-1 ). Both electric field (0-150 mV cm-1 ) and microfiber diameter (17-150 µm) affect neurite outgrowth (PC-12 cells and primary mouse hippocampal neurons) and the formation of orientated neuronal-like networks. With further demonstration of such guidance to neuronal cells, these conductive scaffolds may see versatile applications in nerve regeneration and neural engineering.

Traversing behavior of tumor cells in three-dimensional platforms with different topography

Three-dimensional polydimethylsiloxane platforms were developed to mimic the extracellular matrix with blood vessels by having scaffolds with micropatterns, porous membrane and trenches. Precisely controlled physical dimensions, layouts, and topography as well as different surface chemical treatments were applied to study their influences on nasopharyngeal carcinoma cell (10-15 μm in diameter) migration in mimicked platforms over 15-hour of time-lapse imaging. By placing the pores at different distance from the edges of the trenches, pores with different trench sidewall exposures and effective sizes were generated. Pores right next to the trench sidewalls showed the highest cell traversing probability, most likely related to the larger surface contact area with cells along the sidewalls. Straight grating oriented perpendicular to trenches below the top layer increased cell traversing probability. Pore shape as well as pore size influenced the cell traversing probability and cells could not traverse through pores that were 6 μm or less in diameter, which is much smaller than the cell size. Trench depth of 15 μm could induce more cells to traverse through the porous membrane, while shallower trenches impeded cell traversing and longer time was needed for cells to traverse because 3 and 6 μm deep trenches were much smaller than cell size which required large cell deformation. Hydrophobic surface coating on the top layer and fibronectin in pores and trenches increased the cell traversing probability and reduced the pore size that cells could traverse from 8 to 6 μm, which indicated that cells could have larger deformation with certain surface coatings.

Laser-Induced Periodic Surface Structure Enhances Neuroelectrode Charge Transfer Capabilities and Modulates Astrocyte Function

The brain machine interface (BMI) describes a group of technologies capable of communicating with excitable nervous tissue within the central nervous system (CNS). BMIs have seen major advances in recent years, but these advances have been impeded because of a temporal deterioration in the signal to noise ratio of recording electrodes following insertion into the CNS. This deterioration has been attributed to an intrinsic host tissue response, namely, reactive gliosis, which involves a complex series of immune mediators, resulting in implant encapsulation via the synthesis of pro-inflammatory signaling molecules and the recruitment of glial cells. There is a clinical need to reduce tissue encapsulation in situ and improve long-term neuroelectrode functionality. Physical modification of the electrode surface at the nanoscale could satisfy these requirements by integrating electrochemical and topographical signals to modulate neural cell behavior. In this study, commercially available platinum iridium (Pt/Ir) microelectrode probes were nanotopographically functionalized using femto/picosecond laser processing to generate laser-induced periodic surface structures (LIPSS). Three different topographies and their physical properties were assessed by scanning electron microscopy and atomic force microscopy. The electrochemical properties of these interfaces were investigated using electrochemical impedance spectroscopy and cyclic voltammetry. The in vitro response of mixed cortical cultures (embryonic rat E14/E17) was subsequently assessed by confocal microscopy, ELISA, and multiplex protein array analysis. Overall LIPSS features improved the electrochemical properties of the electrodes, promoted cell alignment, and modulated the expression of multiple ion channels involved in key neuronal functions.

Guided growth with aligned neurites in adult spiral ganglion neurons cultured in vitro on silicon micro-pillar substrates

OBJECTIVE

Assessment of the relationship between the topographical organization of silicon micro-pillar surfaces (MPS) on guidance and neural alignment of adult spiral ganglion neurons (SGN) and use of the otosurgical approach as an alternative for the extraction and isolation of SGNs from adult guinea pigs.

APPROACH

SGNs from adult guinea pigs were isolated using conventional and otosurgical approach for in vitro cell culturing on MPS of various micro-pillar widths (1-5.6 µm) and spacing (0.6-15 µm). Cell cultures were compared morphologically with neuronal cultures on control glass coverslips.

MAIN RESULTS

We found enhanced SGN in vitro cultures in MPS areas with small and intermediate inter-pillar spacing (from 0.6 µm to 3.2 µm) as well as in MPS areas with wider pillars (from 1.8 µm to 4 µm) compared to MPS flat zones and control glass coverslips. Scanning electron microscopy (SEM) images highlighted how neurites of SGNs follow straight lines by growing on top and between micro-pillars. Only micro-pillars with small and intermediate pillar spacings favor neurite alignment along preferred angles (30°, 90°, and 150°), while pillars with wider spacing produced less aligned neurites. We found propensity of adult SGNs grown on MPSs to attain more bipolar and multipolar morphologies. Additionally, we observed reduced interaction between neuronal and glial cells compared to control glass coverslips. Finally, we found that the otosurgical approach was more beneficial for SGN survival on glass coverslips and MPS flat surfaces than the conventional method.

SIGNIFICANCE

MPS with specific architecture supports the guided growth of adult SGNs in vitro and controls adult SGN development and behavior.

High-Aspect-Ratio Semiconducting Polymer Pillars for 3D Cell Cultures

Hybrid interfaces between living cells and nano/microstructured scaffolds have huge application potential in biotechnology, spanning from regenerative medicine and stem cell therapies to localized drug delivery and from biosensing and tissue engineering to neural computing. However, 3D architectures based on semiconducting polymers, endowed with responsivity to visible light, have never been considered. Here, we apply for the first time a push-coating technique to realize high aspect ratio polymeric pillars, based on polythiophene, showing optimal biocompatibility and allowing for the realization of soft, 3D cell cultures of both primary neurons and cell line models. HEK-293 cells cultured on top of polymer pillars display a remarkable change in the cell morphology and a sizable enhancement of the membrane capacitance due to the cell membrane thinning in correspondence to the pillars' top surface, without negatively affecting cell proliferation. Electrophysiology properties and synapse number of primary neurons are also very well preserved. In perspective, high aspect ratio semiconducting polymer pillars may find interesting applications as soft, photoactive elements for cell activity sensing and modulation.

Directing Neuronal Outgrowth and Network Formation of Rat Cortical Neurons by Cyclic Substrate Stretch

Neuronal mechanobiology plays a vital function in brain development and homeostasis with an essential role in neuronal maturation, pathfinding, and differentiation but is also crucial for understanding brain pathology. In this study, we constructed an in vitro system to assess neuronal responses to cyclic strain as a mechanical signal. The selected strain amplitudes mimicked physiological as well as pathological conditions. By subjecting embryonic neuronal cells to cyclic uniaxial strain we could steer the direction of neuronal outgrowth perpendicular to strain direction for all applied amplitudes. A long-term analysis proved maintained growth direction. Moreover, stretched neurons showed an enhanced length, growth, and formation of nascent side branches with most elevated growth rates subsequent to physiological straining. Application of cyclic strain to already formed neurites identified retraction bulbs with destabilized microtubule structures as spontaneous responses. Importantly, neurons were able to adapt to the mechanical signals without induction of cell death and showed a triggered growth behavior when compared to unstretched neurons. The data suggest that cyclic strain plays a critical role in neuronal development.

Hierarchically aligned gradient collagen micropatterns for rapidly screening Schwann cells behavior

To penetrate the effect of protein gradient micropattern on peripheral nerve regeneration, the hierarchically aligned gradient collagen micropattern was prepared by micromoulding method and the influence on Schwann cells growth behavior was studied. The morphology, wettability, stability and component variation of the micropatterns were firstly characterized. Then, Schwann cells were cultured and the related mechanism was penetrated. The results showed that the gradient collagen micropattern could be well fabricated. The surface wettability varied with the change of collagen concentration, and the prepared gradient micropattern showed a good stability after PBS immersion for 15 days. The results of Schwann cells culture and morphological index analysis displayed that the prepared gradient collagen micropatten could well regulate the orientation growth of Schwann cells, while a much better cell alignment growth was obtained on the gradient micropattern with higher collagen concentration and wider pattern size. PCR and WB showed that the micropattern structure could effectively up-regulate the key specific genes for axon regeneration and myelination process. Overall, the study provides a systematic and facile method for understanding the effect of various sized micropatterns on cell behavior, which may have a great significance for the development of artificial implants for tissue engineering and regenerative medicine.

Wafer-Scale Multilayer Fabrication for Silk Fibroin-Based Microelectronics

Silk fibroin is an excellent candidate for biomedical implantable devices because of its biocompatibility, controllable biodegradability, solution processability, flexibility, and transparency. Thus, fibroin has been widely explored in biomedical applications as biodegradable films as well as functional microstructures. Although there exists a large number of patterning methods for fibroin thin films, multilayer micropatterning of fibroin films interleaved with metal layers still remains a challenge. Herein, we report a new wafer-scale multilayer microfabrication process named aluminum hard mask on silk fibroin (AMoS), which is capable of micropatterning multiple layers composed of both fibroin and inorganic materials (e.g., metal and dielectrics) with high-precision microscale alignment. To the best of our knowledge, our AMoS process is the first demonstration of wafer-scale multilayer processing of both silk fibroin and metal micropatterns. In the AMoS process, aluminum deposited on fibroin is first micropatterned using conventional ultraviolet (UV) photolithography, and the patterned aluminum layer is then used as a mask to pattern fibroin underneath. We demonstrate the versatility of our fabrication process by fabricating fibroin microstructures with different dimensions, passive electronic components composed of both fibroin and metal layers, and functional fibroin microstructures for drug delivery. Furthermore, because one of the crucial advantages of fibroin is biocompatibility, we assess the biocompatibility of our fabrication process through the culture of highly susceptible primary neurons. Because the AMoS process utilizes conventional UV photolithography, the principal advantages of our process are multilayer fabrication with high-precision alignment, high resolution, wafer-scale large area processing, no requirement for chemical modification of the protein, and high throughput and thus low cost, all of which have not been feasible with silk fibroin. Therefore, the proposed fabrication method is a promising candidate for batch fabrication of functional fibroin microelectronics (e.g., memristors and organic thin film transistors) for next-generation implantable biomedical applications.

Aligning Synthetic Hippocampal Neural Circuits via Self-Rolled-Up Silicon Nitride Microtube Arrays

Directing neurons to form predetermined circuits with the intention of treating neurological disorders and neurodegenerative diseases is a fundamental goal and current challenge in neuroengineering. Until recently, only neuronal aggregates were studied and characterized in culture, which can limit information gathered to populations of cells. In this study, we use a substrate constructed of arrays of strain-induced self-rolled-up membrane 3D architectures. This results in changes in the neuronal architecture and altered growth dynamics of neurites. Hippocampal neurons from postnatal rats were cultured at low confluency (∼250 cells mm^-2) on an array of transparent rolled-up microtubes (μ-tubes; 4-5 μm diameter) of varying topographical arrangements. Neurite growth on the μ-tubes was characterized and compared to controls in order to establish a baseline for alignment imposed by the topography. Compared to control substrates, neurites are significantly more aligned toward the 0° reference on the μ-tube array. Pitch (20-60 and 100 μm) and μ-tube length (30-80 μm) of array elements were also varied to investigate their impact on neurite alignment. We found that alignment was improved by the gradient pitch arrangement and with longer μ-tubes. Application of this technology will enhance the ability to construct intentional neural circuits through array design and manipulation of individual neurons and can be adapted to address challenges in neural repair, reinnervation, and neuroregeneration.

Cells on hierarchically-structured platforms hosting functionalized nanoparticles

In this work, we report on a novel approach to develop hierarchically-structured cell culture platforms incorporating functionalized gold nanoparticles (AuNPs). In particular, the hierarchical substrates comprise primary pseudo-periodic arrays of silicon microcones combined with a secondary nanoscale pattern of homogeneously deposited AuNPs terminated with bio-functional moieties. AuNPs with various functionalities (i.e. oligopeptides, small molecules and oligomers) were successfully attached onto the microstructures. Experiments with PC12 cells on hierarchical substrates incorporating AuNPs carrying the RGD peptide showed an impressive growth and NGF-induced differentiation of the PC12 cells, compared to that on the NP-free, bare, micropatterned substrates. The exploitation of the developed methodology for the binding of AuNPs as carriers of specific bio-functional moieties onto micropatterned culture substrates for cell biology studies is envisaged.

Curvotaxis directs cell migration through cell-scale curvature landscapes

Cells have evolved multiple mechanisms to apprehend and adapt finely to their environment. Here we report a new cellular ability, which we term “curvotaxis” that enables the cells to respond to cell-scale curvature variations, a ubiquitous trait of cellular biotopes. We develop ultra-smooth sinusoidal surfaces presenting modulations of curvature in all directions, and monitor cell behavior on these topographic landscapes. We show that adherent cells avoid convex regions during their migration and position themselves in concave valleys. Live imaging combined with functional analysis shows that curvotaxis relies on a dynamic interplay between the nucleus and the cytoskeleton—the nucleus acting as a mechanical sensor that leads the migrating cell toward concave curvatures. Further analyses show that substratum curvature affects focal adhesions organization and dynamics, nuclear shape, and gene expression. Altogether, this work identifies curvotaxis as a new cellular guiding mechanism and promotes cell-scale curvature as an essential physical cue.

The effect that microscale surface curvature has on cell migration has not been evaluated. Here the authors fabricate sinusoidal 3D surfaces and show that the cell nucleus and cytoskeleton cooperate to guide cells to concave valleys in a process they coin curvotaxis.

Injectable Nanoreinforced Shape-Memory Hydrogel System for Regenerating Spinal Cord Tissue from Traumatic Injury

Traumatic injury in the central nervous system can lead to loss of functional neurons. Transplantation of neural progenitors is a promising therapeutic strategy. However, infusion of dissociated cells often suffers from low viability, uneven cell distribution, and poor in vivo engraftment that could be reinforced by a better cell delivery system. Here, we develop an injectable composite hydrogel system for use as a minimally invasive treatment of spinal cord injury (SCI) using motor neurons (MNs) derived from embryonic stem cells (ESCs). The composite hydrogel is based on a modified gelatin matrix integrated with shape-memory polymer fibers. The gelatin matrix creates a local microenvironment for cell assembly and also acts as a lubricant during injection through a fine catheter. Notably, shape-memory fiber scaffolds are able to recover to maintain the microstructures even after dramatic deformation from injection operation, providing the necessary support and guidance for motor neuron differentiation. We find that the composite hydrogel with an aligned fiber scaffold greatly improves the viability of ESCs and their differentiation toward MNs both in vitro and in vivo. When transplanted to SCI animals by injection, the ESC-loaded composite hydrogels are identified to significantly enhance tissue regeneration and motor function recovery in mice. With this proof-of-concept study, we believe that the injectable composite hydrogel system provides a promising solution for in vivo cell delivery with minimum invasiveness and can be readily extended to other stem-cell-based regenerative treatments.