Long-term maintenance of patterns of hippocampal pyramidal cells on substrates of polyethylene glycol and microstamped polylysine

A neuronal wiring platform through microridges for rationally engineered neural circuits

Precisely engineered neuronal circuits are promising for both fundamental research and clinical applications. However, randomly plating thousands of cells during neural network fabrication remains a major technical obstacle, which often results in a loss of tracking in neurons' identities. In this work, we demonstrated an accurate and unique neural wiring technique, mimicking neurons' natural affinity to microfibers. SU-8 microridges, imitating lie-down microfibers, were photolithographically patterned and then selectively coated with poly-l-lysine. We accurately plated Aplysia californica neurons onto designated locations. Plated neurons were immobilized by circular microfences. Furthermore, neurites regrew effectively along the microridges in vitro and reached adjacent neurons without undesirable crosstalks. Functional chemical synapses also formed between accurately wired neurons, enabling two-way transmission of electrical signals. Finally, we fabricated microridges on a microelectrode array. Neuronal spikes, stimulation-evoked synaptic activity, and putative synaptic adaption between connected neurons were observed. This biomimetic platform is simple to fabricate and effective with neurite pathfinding. Therefore, it can serve as a powerful tool for fabricating neuronal circuits with rational design, organized cellular communications, and fast prototyping.

A Versatile Protein and Cell Patterning Method Suitable for Long-Term Neural Cultures

Herein, we present an easy-to-use protein and cell patterning method relying solely on pipetting, rinsing steps and illumination with a desktop lamp, which does not require any expensive laboratory equipment, custom-built hardware or delicate chemistry. This method is based on the adhesion promoter poly(allylamine)-grafted perfluorophenyl azide, which allows UV-induced cross-linking with proteins and the antifouling molecule poly(vinylpyrrolidone). Versatility is demonstrated by creating patterns with two different proteins and a polysaccharide directly on plastic well plates and on glass slides, and by subsequently seeding primary neurons and C2C12 myoblasts on the patterns to form islands and mini-networks. Patterning characterization is done via immunohistochemistry, Congo red staining, ellipsometry, and infrared spectroscopy. Using a pragmatic setup, patterning contrasts down to 5 μm and statistically significant long-term stability superior to the gold standard poly(l-lysine)-grafted poly(ethylene glycol) could be obtained. This simple method can be used in any laboratory or even in classrooms and its outstanding stability is especially interesting for long-term cell experiments, e.g., for bottom-up neuroscience, where well-defined microislands and microcircuits of primary neurons are studied over weeks.

Temporal Characterization of Neuronal Migration Behavior on Chemically Patterned Neuronal Circuits in a Defined in Vitro Environment

Directed control of neuronal migration, facilitating the correct spatial positioning of neurons, is crucial to the development of a functional nervous system. An understanding of neuronal migration and positioning on patterned surfaces in vitro would also be beneficial for investigators seeking to design culture platforms capable of mimicking the complex functional architectures of neuronal tissues for drug development as well as basic biomedical research applications. This study used coplanar self-assembled monolayer patterns of cytophilic, N-1[3-(trimethoxysilyly)propyl] diethylenetriamine (DETA) and cytophobic, tridecafluoro-1,1,2,2-tetrahydrooctyl-1-trichlorosilane (13F) to assess the migratory behavior and physiological characteristics of cultured neurons. Analysis of time-lapse microscopy data revealed a dynamic procedure underlying the controlled migration of neurons, in response to extrinsic geometric and chemical cues, to promote the formation of distinct two-neuron circuits. Immunocytochemical characterization of the neurons highlights the organization of actin filaments (phalloidin) and microtubules (β-tubulin) at each migration stage. These data have applications in the development of precise artificial neuronal networks and provide a platform for investigating neuronal migration as well as neurite identification in differentiating cultured neurons. Importantly, the cytoskeletal arrangement of these cells identifies a specific mode of neuronal migration on these in vitro surfaces characterized by a single process determining the direction of cell migration and mimicking somal translocation behavior in vivo. Such information provides valuable additional insight into the mechanisms controlling neuronal development and maturation in vitro and validates the biochemical mechanisms underlying this behavior as representative of neuronal positioning phenomena in vivo.

Photopolymerized Microfeatures Guide Adult Spiral Ganglion and Dorsal Root Ganglion Neurite Growth

HYPOTHESIS

Microtopographical patterns generated by photopolymerization of methacrylate polymer systems will direct growth of neurites from adult neurons, including spiral ganglion neurons (SGNs).

BACKGROUND

Cochlear implants (CIs) provide hearing perception to patients with severe to profound hearing loss. However, their ability to encode complex auditory stimuli is limited due, in part, to poor spatial resolution caused by spread of the electrical currents in the inner ear. Directing the regrowth of SGN peripheral processes towards stimulating electrodes could help reduce current spread and improve spatial resolution provided by the CI. Previous work has demonstrated that micro- and nano-scale patterned surfaces precisely guide the growth of neurites from a variety of neonatal neurons including SGNs. Here, we sought to determine the extent to which adult neurons likewise respond to these topographical surface features.

METHODS

Photopolymerization was used to fabricate methacrylate polymer substrates with micropatterned surfaces of varying amplitudes and periodicities. Dissociated adult dorsal root ganglion neurons (DRGNs) and SGNs were cultured on these surfaces and the alignment of the neurite processes to the micropatterns was determined.

RESULTS

Neurites from both adult DRGNs and SGNs significantly aligned to the patterned surfaces similar to their neonatal counterparts. Further DRGN and SGN neurite alignment increased as the amplitude of the microfeatures increased. Decreased pattern periodicity also improved neurite alignment.

CONCLUSION

Microscale surface topographic features direct the growth of adult SGN neurites. Topographical features could prove useful for guiding growth of SGN peripheral axons towards a CI electrode array.

Easy to Apply Polyoxazoline-Based Coating for Precise and Long-Term Control of Neural Patterns

Arranging cultured cells in patterns via surface modification is a tool used by biologists to answer questions in a specific and controlled manner. In the past decade, bottom-up neuroscience emerged as a new application, which aims to get a better understanding of the brain via reverse engineering and analyzing elementary circuitry in vitro. Building well-defined neural networks is the ultimate goal. Antifouling coatings are often used to control neurite outgrowth. Because erroneous connectivity alters the entire topology and functionality of minicircuits, the requirements are demanding. Current state-of-the-art coating solutions such as widely used poly(l-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) fail to prevent primary neurons from making undesired connections in long-term cultures. In this study, a new copolymer with greatly enhanced antifouling properties is developed, characterized, and evaluated for its reliability, stability, and versatility. To this end, the following components are grafted to a poly(acrylamide) (PAcrAm) backbone: hexaneamine, to support spontaneous electrostatic adsorption in buffered aqueous solutions, and propyldimethylethoxysilane, to increase the durability via covalent bonding to hydroxylated culture surfaces and antifouling polymer poly(2-methyl-2-oxazoline) (PMOXA). In an assay for neural connectivity control, the new copolymer's ability to effectively prevent unwanted neurite outgrowth is compared to the gold standard, PLL-g-PEG. Additionally, its versatility is evaluated on polystyrene, glass, and poly(dimethylsiloxane) using primary hippocampal and cortical rat neurons as well as C2C12 myoblasts, and human fibroblasts. PAcrAm-g-(PMOXA, NH₂, Si) consistently outperforms PLL-g-PEG with all tested culture surfaces and cell types, and it is the first surface coating which reliably prevents arranged nodes of primary neurons from forming undesired connections over the long term. Whereas the presented work focuses on the proof of concept for the new antifouling coating to successfully and sustainably prevent unwanted connectivity, it is an important milestone for in vitro neuroscience, enabling follow-up studies to engineer neurologically relevant networks. Furthermore, because PAcrAm-g-(PMOXA, NH₂, Si) can be quickly applied and used with various surfaces and cell types, it is an attractive extension to the toolbox for in vitro biology and biomedical engineering.

Biodegradable scaffolds promote tissue remodeling and functional improvement in non-human primates with acute spinal cord injury

Tissue loss significantly reduces the potential for functional recovery after spinal cord injury. We previously showed that implantation of porous scaffolds composed of a biodegradable and biocompatible block copolymer of Poly-lactic-co-glycolic acid and Poly-l-lysine improves functional recovery and reduces spinal cord tissue injury after spinal cord hemisection injury in rats. Here, we evaluated the safety and efficacy of porous scaffolds in non-human Old-World primates (Chlorocebus sabaeus) after a partial and complete lateral hemisection of the thoracic spinal cord. Detailed analyses of kinematics and muscle activity revealed that by twelve weeks after injury fully hemisected monkeys implanted with scaffolds exhibited significantly improved recovery of locomotion compared to non-implanted control animals. Twelve weeks after injury, histological analysis demonstrated that the spinal cords of monkeys with a hemisection injury implanted with scaffolds underwent appositional healing characterized by a significant increase in remodeled tissue in the region of the hemisection compared to non-implanted controls. The number of glial fibrillary acidic protein immunopositive astrocytes was diminished within the inner regions of the remodeled tissue layer in treated animals. Activated macrophage and microglia were present diffusely throughout the remodeled tissue and concentrated at the interface between the preserved spinal cord tissue and the remodeled tissue layer. Numerous unphosphorylated neurofilament H and neuronal growth associated protein positive fibers and myelin basic protein positive cells may indicate neural sprouting inside the remodeled tissue layer of treated monkeys. These results support the safety and efficacy of polymer scaffolds in a primate model of acute spinal cord injury. A device substantially similar to the device described here is the subject of an ongoing human clinical trial.

Neural Circuits on a Chip

Neural circuits are responsible for the brain's ability to process and store information. Reductionist approaches to understanding the brain include isolation of individual neurons for detailed characterization. When maintained in vitro for several days or weeks, dissociated neurons self-assemble into randomly connected networks that produce synchronized activity and are capable of learning. This review focuses on efforts to control neuronal connectivity in vitro and construct living neural circuits of increasing complexity and precision. Microfabrication-based methods have been developed to guide network self-assembly, accomplishing control over in vitro circuit size and connectivity. The ability to control neural connectivity and synchronized activity led to the implementation of logic functions using living neurons. Techniques to construct and control three-dimensional circuits have also been established. Advances in multiple electrode arrays as well as genetically encoded, optical activity sensors and transducers enabled highly specific interfaces to circuits composed of thousands of neurons. Further advances in on-chip neural circuits may lead to better understanding of the brain.

Creating cellular patterns using genetically engineered, gold- and cell-binding polypeptides

Patterning cells on material surfaces is an important tool for the study of fundamental cell biology, tissue engineering, and cell-based bioassays. Here, the authors report a simple approach to pattern cells on gold patterned silicon substrates with high precision, fidelity, and stability. Cell patterning is achieved by exploiting adsorbed biopolymer orientation to either enhance (gold regions) or impede (silicon oxide regions) cell adhesion at particular locations on the patterned surface. Genetic incorporation of gold binding domains enables C-terminal chemisorption of polypeptides onto gold regions with enhanced accessibility of N-terminal cell binding domains. In contrast, the orientation of polypeptides adsorbed on the silicon oxide regions limit the accessibility of the cell binding domains. The dissimilar accessibility of cell binding domains on the gold and silicon oxide regions directs the cell adhesion in a spatially controlled manner in serum-free medium, leading to the formation of well-defined cellular patterns. The cells are confined within the polypeptide-modified gold regions and are viable for eight weeks, suggesting that bioactive polypeptide modified surfaces are suitable for long-term maintenance of patterned cells. This study demonstrates an innovative surface-engineering approach for cell patterning by exploiting distinct ligand accessibility on heterogeneous surfaces.

Feed-Forward Propagation of Temporal and Rate Information between Cortical Populations during Coherent Activation in Engineered In Vitro Networks

Transient propagation of information across neuronal assembles is thought to underlie many cognitive processes. However, the nature of the neural code that is embedded within these transmissions remains uncertain. Much of our understanding of how information is transmitted among these assemblies has been derived from computational models. While these models have been instrumental in understanding these processes they often make simplifying assumptions about the biophysical properties of neurons that may influence the nature and properties expressed. To address this issue we created an in vitro analog of a feed-forward network composed of two small populations (also referred to as assemblies or layers) of living dissociated rat cortical neurons. The populations were separated by, and communicated through, a microelectromechanical systems (MEMS) device containing a strip of microscale tunnels. Delayed culturing of one population in the first layer followed by the second a few days later induced the unidirectional growth of axons through the microtunnels resulting in a primarily feed-forward communication between these two small neural populations. In this study we systematically manipulated the number of tunnels that connected each layer and hence, the number of axons providing communication between those populations. We then assess the effect of reducing the number of tunnels has upon the properties of between-layer communication capacity and fidelity of neural transmission among spike trains transmitted across and within layers. We show evidence based on Victor-Purpura's and van Rossum's spike train similarity metrics supporting the presence of both rate and temporal information embedded within these transmissions whose fidelity increased during communication both between and within layers when the number of tunnels are increased. We also provide evidence reinforcing the role of synchronized activity upon transmission fidelity during the spontaneous synchronized network burst events that propagated between layers and highlight the potential applications of these MEMs devices as a tool for further investigation of structure and functional dynamics among neural populations.

Structure, Function, and Propagation of Information across Living Two, Four, and Eight Node Degree Topologies

In this study, we created four network topologies composed of living cortical neurons and compared resultant structural-functional dynamics including the nature and quality of information transmission. Each living network was composed of living cortical neurons and were created using microstamping of adhesion promoting molecules and each was "designed" with different levels of convergence embedded within each structure. Networks were cultured over a grid of electrodes that permitted detailed measurements of neural activity at each node in the network. Of the topologies we tested, the "Random" networks in which neurons connect based on their own intrinsic properties transmitted information embedded within their spike trains with higher fidelity relative to any other topology we tested. Within our patterned topologies in which we explicitly manipulated structure, the effect of convergence on fidelity was dependent on both topology and time-scale (rate vs. temporal coding). A more detailed examination using tools from network analysis revealed that these changes in fidelity were also associated with a number of other structural properties including a node's degree, degree-degree correlations, path length, and clustering coefficients. Whereas information transmission was apparent among nodes with few connections, the greatest transmission fidelity was achieved among the few nodes possessing the highest number of connections (high degree nodes or putative hubs). These results provide a unique view into the relationship between structure and its affect on transmission fidelity, at least within these small neural populations with defined network topology. They also highlight the potential role of tools such as microstamp printing and microelectrode array recordings to construct and record from arbitrary network topologies to provide a new direction in which to advance the study of structure-function relationships.

Probing neural cell behaviors through micro-/nano-patterned chitosan substrates

In this study, we describe the development of surface-modified chitosan substrates to examine topographically related Neuro-2a cell behaviors. Different functional groups can be modified on chitosan surfaces to probe Neuro-2a cell morphology. To prepare chitosan substrates with micro/nano-scaled features, we demonstrated an easy-to-handle method that combined photolithography, inductively coupled plasma reactive ion etching, Ag nanoparticle-assisted etching, and solution casting. The results show that Neuro-2a cells preferred to adhere to a flat chitosan surface rather than a nanotextured chitosan surface as evidenced by greater immobilization and differentiation, suggesting that surface topography is crucial for neural patterning. In addition, we developed chitosan substrates with different geometric patterns and flat region depth; this allowed us to re-arrange or re-pattern Neuro-2a cell colonies at desired locations. We found that a polarity-induced micropattern provided the most suitable surface pattern for promoting neural network formation on a chitosan substrate. The cellular polarity of single Neuro-2a cell spreading correlated to a diamond-like geometry and neurite outgrowth was induced from the corners toward the grooves of the structures. This study provide greater insight into neurobiology, including neurotransmitter screening, electrophysiological stimulation platforms, and biomedical engineering.

Universality, complexity and the praxis of biology: Two case studies

The phenomenon of biology provides a prime example for a naturally occurring complex system. The approach to this complexity reflects the tension between a reductionist, reverse-engineering stance, and more abstract, systemic ones. Both of us are reductionists, but our observations challenge reductionism, at least the naive version of it. Here we describe the challenge, focusing on two universal characteristics of biological complexity: two-way microscopic-macroscopic degeneracy, and lack of time scale separation within and between levels of organization. These two features and their consequences for the praxis of experimental biology, reflect inherent difficulties in separating the dynamics of any given level of organization from the coupled dynamics of all other levels, including the environment within which the system is embedded. Where these difficulties are not deeply acknowledged, the impacts of fallacies that are inherent to naive reductionism are significant. In an era where technology enables experimental high-resolution access to numerous observables, the challenge faced by the mature reductionist-identification of relevant microscopic variables-becomes more demanding than ever. The demonstrations provided here are taken from two very different biological realizations: populations of microorganisms and populations of neurons, thus making the lesson potentially general.

Agarose-Based Substrate Modification Technique for Chemical and Physical Guiding of Neurons In Vitro

A new low cost and highly reproducible technique is presented that provides patterned cell culture substrates. These allow for selective positioning of cells and a chemically and mechanically directed guiding of their extensions. The patterned substrates consist of structured agarose hydrogels molded from reusable silicon micro templates. These templates consist of pins arranged equidistantly in squares, connected by bars, which mold corresponding wells and channels in the nonadhesive agarose hydrogel. Subsequent slice production with a standard vibratome, comprising the described template pattern, completes substrate production. Invertebrate neurons of locusts and pond snails are used for this application as they offer the advantage over vertebrate cells as being very large and suitable for cultivation in low cell density. Their neurons adhere to and grow only on the adhesive areas not covered by the agarose. Agarose slices of 50 μm thickness placed on glass, polystyrene, or MEA surfaces position and immobilize the neurons in the wells, and the channels guide their neurite outgrowth toward neighboring wells. In addition to the application with invertebrate neurons, the technique may also provide the potential for the application of a wide range of cell types. Long-term objective is the achievement of isolated low-density neuronal networks on MEAs or different culture substrates for various network analysis applications.

NeuroArray: a universal interface for patterning and interrogating neural circuitry with single cell resolution

Recreation of neural network in vitro with designed topology is a valuable tool to decipher how neurons behave when interacting in hierarchical networks. In this study, we developed a simple and effective platform to pattern primary neurons in array formats for interrogation of neural circuitry with single cell resolution. Unlike many surface-chemistry-based patterning methods, our NeuroArray technique is specially designed to accommodate neuron's polarized morphologies to make regular arrays of cells without restricting their neurite outgrowth, and thus allows formation of freely designed, well-connected, and spontaneously active neural network. The NeuroArray device was based on a stencil design fabricated using a novel sacrificial-layer-protected PDMS molding method that enables production of through-structures in a thin layer of PDMS with feature sizes as small as 3 µm. Using the NeuroArray along with calcium imaging, we have successfully demonstrated large-scale tracking and recording of neuronal activities, and used such data to characterize the spiking dynamics and transmission within a diode-like neural network. Essentially, the NeuroArray is a universal patterning platform designed for, but not limited to neuron cells. With little adaption, it can be readily interfaced with other interrogation modalities for high-throughput drug testing, and for building neuron culture based live computational devices.

Micropatterning neuronal networks

A simple and effective method for patterning primary neuronal networks and circuits.

Neural circuits with long-distance axon tracts for determining functional connectivity

The cortical circuitry in the brain consists of structurally and functionally distinct neuronal assemblies with reciprocal axon connections. To generate cell culture-based systems that emulate axon tract systems of an in vivo neural network, we developed a living neural circuit consisting of compartmentalized neuronal populations connected by arrays of two millimeter-long axon tracts that are integrated on a planar multi-electrode array (MEA). The millimeter-scale node-to-node separation allows for pharmacological and electrophysiological manipulations to simultaneously target multiple neuronal populations. The results show controlled selectivity of dye absorption by neurons in different compartments. MEA-transmitted electrical stimulation of targeted neurons shows ∼46% increase of intracellular calcium levels with 20 Hz stimulation, but ∼22% decrease with 2k Hz stimulation. The unique feature of long distance axons promotes in vivo-like fasciculation. These axon tracts are determined to be inhibitory afferents by showing increased action potential firing of downstream node upon selective application of γ-aminobutyric acid (GABA) to the upstream node. Together, this model demonstrates integrated capabilities for assessing multiple endpoints including axon tract tracing, calcium influx, network architecture and activities. This system can be used as a multi-functional platform for studying axon tract-associated CNS disorders in vitro, such as diffuse axonal injury after brain trauma.

Patterned neuronal networks using nanodiamonds and the effect of varying nanodiamond properties on neuronal adhesion and outgrowth

OBJECTIVE

Detonation nanodiamond monolayer coatings are exceptionally biocompatible substrates for in vitro cell culture. However, the ability of nanodiamond coatings of different origin, size, surface chemistry and morphology to promote neuronal adhesion, and the ability to pattern neurons with nanodiamonds have yet to be investigated.

APPROACH

Various nanodiamond coatings of different type are investigated for their ability to promote neuronal adhesion with respect to surface coating parameters and neurite extension. Nanodiamond tracks are patterned using photolithography and reactive ion etching.

MAIN RESULTS

Universal promotion of neuronal adhesion is observed on all coatings tested and analysis shows surface roughness to not be a sufficient metric to describe biocompatibility, but instead nanoparticle size and curvature shows a significant correlation with neurite extension. Furthermore, neuronal patterning is achieved with high contrast using patterned nanodiamond coatings down to at least 10 µm.

SIGNIFICANCE

The results of nanoparticle size and curvature being influential upon neuronal adhesion has great implications towards biomaterial design, and the ability to pattern neurons using nanodiamond tracks shows great promise for applications both in vitro and in vivo.

Three dimensional live cell lithography

We investigate holographic optical trapping combined with step-and-repeat maskless projection stereolithography for fine control of 3D position of living cells within a 3D microstructured hydrogel. C2C12 myoblast cells were chosen as a demonstration platform since their development into multinucleated myotubes requires linear arrangements of myoblasts. C2C12 cells are positioned in the monomer solution with multiple optical traps at 1064 nm and then encapsulated by photopolymerization of monomer via projection of a 512x512 spatial light modulator illuminated at 405 nm. High 405 nm sensitivity and complete insensitivity to 1064 nm was enabled by a lithium acylphosphinate (LAP) salt photoinitiator. These wavelengths, in addition to brightfield imaging with a white light LED, could be simultaneously focused by a single oil immersion objective. Large lateral dimensions of the patterned gel/cell structure are achieved by x and y step-and-repeat process. Large thickness is achieved through multi-layer stereolithography, allowing fabrication of precisely-arranged 3D live cell scaffolds with micron-scale structure and millimeter dimensions. Cells are shown to retain viability after the trapping and encapsulation procedure.

Microtechnologies to fuel neurobiological research with nanometer precision

The interface between engineering and molecular life sciences has been fertile ground for advancing our understanding of complex biological systems. Engineered microstructures offer a diverse toolbox for cellular and molecular biologists to direct the placement of cells and small organisms, and to recreate biological functions in vitro: cells can be positioned and connected in a designed fashion, and connectivity and community effects of cells studied. Because of the highly polar morphology and finely compartmentalized functions of neurons, microfabricated cell culture systems and related on-chip technologies have become an important enabling platform for studying development, function and degeneration of the nervous system at the molecular and cellular level. Here we review some of the compartmentalization techniques developed so far to highlight how high-precision control of neuronal connectivity allows new approaches for studying axonal and synaptic biology.

WITHDRAWN: Neural circuits with long-distance axon tracts for determining functional connectivity

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Biomimetic polymer brushes containing tethered acetylcholine analogs for protein and hippocampal neuronal cell patterning

This paper describes a method to control neuronal cell adhesion and differentiation with both chemical and topographic cues by using a spatially defined polymer brush pattern. First, biomimetic methacrylate polymer brushes containing tethered neurotransmitter acetylcholine functionalities in the form of dimethylaminoethyl methacrylate or free hydroxyl-terminated poly(ethylene glycol) units were prepared using the "grown from" method through surface-initiated atom transfer radical polymerization reactions. The surface properties of the resulting brushes were thoroughly characterized with various techniques and hippocampal neuronal cell culture on the brush surfaces exhibit cell viability and differentiation comparable to, or even better than, those on commonly used poly-l-lysine coated glass coverslips. The polymer brushes were then patterned via UV photolithography techniques to provide specially designed surface features with different sizes (varying from 2 to 200 μm) and orientations (horizontal and vertical). Protein absorption experiments and hippocampal neuronal cell culture tests on the brush patterns showed that both protein and neurons can adhere to the patterns and therefore be guided by such patterns. These results also demonstrate that, because of their unique chemical composition and well-defined nature, the developed polymer brushes may find many potential applications in cell-material interactions studies and neural tissue engineering.

Highly ordered large-scale neuronal networks of individual cells - toward single cell to 3D nanowire intracellular interfaces

The use of artificial, prepatterned neuronal networks in vitro is a promising approach for studying the development and dynamics of small neural systems in order to understand the basic functionality of neurons and later on of the brain. The present work presents a high fidelity and robust procedure for controlling neuronal growth on substrates such as silicon wafers and glass, enabling us to obtain mature and durable neural networks of individual cells at designed geometries. It offers several advantages compared to other related techniques that have been reported in recent years mainly because of its high yield and reproducibility. The procedure is based on surface chemistry that allows the formation of functional, tailormade neural architectures with a micrometer high-resolution partition, that has the ability to promote or repel cells attachment. The main achievements of this work are deemed to be the creation of a large scale neuronal network at low density down to individual cells, that develop intact typical neurites and synapses without any glia-supportive cells straight from the plating stage and with a relatively long term survival rate, up to 4 weeks. An important application of this method is its use on 3D nanopillars and 3D nanowire-device arrays, enabling not only the cell bodies, but also their neurites to be positioned directly on electrical devices and grow with registration to the recording elements underneath.

TISSUE ENGINEERING FOR NERVE REPAIR

Nerve regeneration is a complex biological phenomenon. Once the nervous system is impaired, its recovery is difficult and malfunctions in other parts of the body may occur because mature neurons don't undergo cell division. To increase the prospects of axonal regeneration and functional recovery, researches have focused on designing “nerve guidance channels” or “nerve conduits”. For developing tissue engineered nerve conduits, four components come to mind, including a scaffold for axonal proliferation, supporting cells such as Schwann cells, growth factors, and extracelluar matrix. This article reviews the nervous system physiology, the factors that are critical for nerve repair, and the advanced technologies that are explored to fabricate nerve conduits. Furthermore, we also introduce a new method we developed to create longitudinally oriented channels within biodegradable polymers, Chitosan and PLGA, using a combined lyophilizing and wire-heating process. This innovative method using Ni-Cr wires as mandrels to create nerve guidance channels. The process is easy, straightforward, highly reproducible, and could easily be tailored to other polymer and solvent systems. These scaffolds could be useful for guided regeneration after transection injury in either the peripheral nerve or spinal cord.

Controlled adhesion and growth of long term glial and neuronal cultures on Parylene-C

This paper explores the long term development of networks of glia and neurons on patterns of Parylene-C on a SiO₂ substrate. We harvested glia and neurons from the Sprague-Dawley (P1–P7) rat hippocampus and utilized an established cell patterning technique in order to investigate cellular migration, over the course of 3 weeks. This work demonstrates that uncontrolled glial mitosis gradually disrupts cellular patterns that are established early during culture. This effect is not attributed to a loss of protein from the Parylene-C surface, as nitrogen levels on the substrate remain stable over 3 weeks. The inclusion of the anti-mitotic cytarabine (Ara-C) in the culture medium moderates glial division and thus, adequately preserves initial glial and neuronal conformity to underlying patterns. Neuronal apoptosis, often associated with the use of Ara-C, is mitigated by the addition of brain derived neurotrophic factor (BDNF). We believe that with the right combination of glial inhibitors and neuronal promoters, the Parylene-C based cell patterning method can generate structured, active neural networks that can be sustained and investigated over extended periods of time. To our knowledge this is the first report on the concurrent application of Ara-C and BDNF on patterned cell cultures.

Carboxybetaine methacrylate polymers offer robust, long-term protection against cell adhesion

Films of poly(carboxybetaine methacrylate), poly(CBMA), grafted onto microetched gold slides are effective in preventing nonspecific adhesion of cells of different types. The degree of adhesion resistance is comparable to that achieved with the self-assembled monolayers, SAMs, of oligo(ethylene glycol) alkanethiolates. In sharp contrast to the SAMs, however, substrates protected with poly(CBMA) can be stored in dry state without losing their protective properties for periods up to 2 weeks.

Ring-shaped neuronal networks: a platform to study persistent activity

Persistent activity in the brain is involved in working memory and motor planning. The ability of the brain to hold information 'online' long after an initiating stimulus is a hallmark of brain areas such as the prefrontal cortex. Recurrent network loops such as the thalamocortical loop and reciprocal loops in the cortex are potential substrates that can support such activity. However, native brain circuitry makes it difficult to study mechanisms underlying such persistent activity. Here we propose a platform to study synaptic mechanisms of such persistent activity by constraining neuronal networks to a recurrent loop like geometry. Using a polymer stamping technique, adhesive proteins are transferred onto glass substrates in a precise ring shape. Primary rat hippocampal cultures were capable of forming ring-shaped networks containing 40-60 neurons. Calcium imaging of these networks show evoked persistent activity in an all-or-none manner. Blocking inhibition with bicuculline methaiodide (BMI) leads to an increase in the duration of persistent activity. These persistent phases were abolished by blockade of asynchronous neurotransmitter release by ethylene glycol tetraacetic acid (EGTA-AM).

Cell patterning using molecular vapor deposition of self-assembled monolayers and lift-off technique

This paper reports a precise, live cell-patterning method by means of patterning a silicon or glass substrate with alternating cytophilic and cytophobic self-assembled monolayers (SAMs) deposited via molecular vapor deposition. Specifically, a stack of hydrophobic heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane SAMs and a silicon oxide adhesion layer were patterned on the substrate surface, and a hydrophilic SAM derived from 3-trimethoxysilyl propyldiethylenetriamine was coated on the remaining non-treated areas on the substrate surface to promote cell growth. The primary characteristics of the reported method include: (i) single-cell resolution; (ii) easy alignment of the patterns with the pre-existing patterns on the substrate; (iii) easy formation of nanoscale patterns (depending on the exposure equipment); (iv) long shelf life of the substrate pattern prior to cell culturing; (v) compatibility with conventional, inverted, optical microscopes for simple visualization of patterns formed on a glass wafer; and (vi) the ability to support patterned cell (osteoblast) networks for at least 2 weeks. Here, we describe the deposition technique and the characterization of the deposited layers, as well as the application of this method in the fabrication of multielectrode arrays supporting patterned neuronal networks.

Induction of cell-cell connections by using in situ laser lithography on a perfluoroalkyl-coated cultivation platform

This article describes a novel laser-directed microfabrication method carried out in aqueous solution for the organization of cell networks on a platform. A femtosecond (fs) laser was applied to a platform culturing PC12, HeLa, or normal human astrocyte (NHA) cells to manipulate them and to facilitate mutual connections. By applying an fs-laser-induced impulsive force, cells were detached from their original location on the plate, and translocated onto microfabricated cell-adhesive domains that were surrounded with a cell-repellent perfluoroalkyl (R(f)) polymer. Then the fs-laser pulse-train was applied to the R(f) polymer surface to modify the cell-repellent surface, and to make cell-adhesive channels of several μm in width between each cell-adhesive domain. PC12 cells elongated along the channels and made contact with others cells. HeLa and NHA cells also migrated along the channels and connected to the other cells. Surface analysis by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) confirmed that the R(f) polymer was partially decomposed. The method presented here could contribute not only to the study of developing networks of neuronal, glial, and capillary cells, but also to the quantitative analysis of nerve function.

Precise cell patterning using cytophobic self-assembled monolayer deposited on top of semi-transparent gold

This paper reports a simple and effective method for cell patterning by using a self-assembled monolayer (SAM)-treated glass surface which is surrounded by semi-transparent gold coated with another type of SAM. Specifically, a hydrophobic SAM, derived from 1-hexadecanethiol (HDT), was coated on the gold surface to prevent cell growth, and a hydrophilic SAM, derived from 3-trimethoxysilyl propyl-diethylenetriamine (DETA), was coated on the exposed glass surface to promote cell growth. The capabilities of this technique are as follows: 1) single-cell resolution, 2) easy alignment of the cell patterns to the structures already existing on the substrate, 3) visualization and verification of the predefined cytophobic/cytophilic pattern prior to cell growth, and 4) convenient monitoring cell growth at the same location for an extended long term period of time. Whereas a number of earlier techniques have demonstrated the single cell resolution, or visualization and verification of the cytophobic/cytophilic patterns prior to cell growth, we believe that our technique is unique in possessing all of these beneficial qualities at the same time. The distinguishing characteristic of our technique is, however, that the use of semi-transparent Cr/Au film allows for convenient brightfield pattern visualization and offers an advantage over previously developed methods which require fluorescent imaging. We have successfully demonstrated the patterning of four different kinds of cells using this technique: immortalized mouse hypothalamic neurons (GT1-7), mouse osteoblast cells (MC3T3), mouse fibroblast cells (NIH3T3) and primary rat hippocampal neurons. This study was performed with a specific ultimate application-the creation of a multi electrode array (MEA) with predefined localization of cell bodies on top of the electrodes, as well as predefined patterns for cell extensions to grow in between the electrodes. With that goal in mind, we have also determined critical parameters for patterning of each of these cell types, such as the minimum size of a cell-adherent island for exclusively anchoring one cell or two cells, as well as the width of the cytophilic pathway between two islands that enables cell extensions to grow, while preventing the anchoring of the cell bodies. Additionally, we have provided statistical analysis of the occupancy for various sizes and shape of cell-anchoring islands. As demonstrated here, we have developed a novel and reliable cell patterning technique, which can be utilized in various applications, such as biosensors or tissue engineering.

In vitro model on glass surfaces for complex interactions between different types of cells

This report establishes an in vitro model on glass surfaces for patterning multiple types of cells to simulate cell-cell interactions in vivo. The model employs a microfluidic system and poly(ethylene glycol)-terminated oxysilane (PEG-oxysilane) to modify glass surfaces in order to resist cell adhesion. The system allows the selective confinement of different types of cells to realize complete confinement, partial confinement, and no confinement of three types of cells on glass surfaces. The model was applied to study intercellular interactions among human umbilical vein endothelial cells (HUVEC), PLA 801 C and PLA801 D cells.

Nanotechnology applications and approaches for neuroregeneration and drug delivery to the central nervous system

Nanotechnology is the science and engineering concerned with the design, synthesis, and characterization of materials and devices that have a functional organization in at least one dimension on the nanometer (i.e., one billionth of a meter) scale. The potential impact of bottom up self-assembling nanotechnology, custom made molecules that self-assemble or self-organize into higher ordered structures in response to a defined chemical or physical cue, and top down lithographic type technologies where detail is engineered at smaller scales starting from bulk materials, stems from the fact that these nanoengineered materials and devices exhibit emergent mesocale and macroscale chemical and physical properties that are often different than their constituent nanoscale building block molecules or materials. As such, applications of nanotechnology to medicine and biology allow the interaction and integration of cells and tissues with nanoengineered substrates at a molecular (i.e., subcellular) level with a very high degree of functional specificity and control. This review considers applications of nanotechnology aimed at the neuroprotection and functional regeneration of the central nervous system (CNS) following traumatic or degenerative insults, and nanotechnology approaches for delivering drugs and other small molecules across the blood-brain barrier. It also discusses developing platform technologies that may prove to have broad applications to medicine and physiology, including some being developed for rescuing or replacing anatomical and/or functional CNS structures.

Designing Neural Networks in Culture: Experiments are described for controlled growth, of nerve cells taken from rats, in predesigned geometrical patterns on laboratory culture dishes

Technology has advanced to where it is possible to design and grow-with predefined geometry and surprisingly good fidelity-living networks of neurons in culture dishes. Here we overview the elements of design, emphasizing the lithographic techniques that alter the cell culture surface which in turn influences the attachment and growth of the neural networks. Advanced capability in this area makes it possible to design networks of desired complexity. Other issues addressed include the influence of glial cells and media on activity and the potential for extending the designs into three dimensions. Investigators are advancing the art and science of analyzing and controlling through stimulation the function of the neural networks, including the ability to take advantage of their geometric form in order to influence functional properties.

Triangular neuronal networks on microelectrode arrays: an approach to improve the properties of low-density networks for extracellular recording

Multi-unit recording from neuronal networks cultured on microelectrode arrays (MEAs) is a widely used approach to achieve basic understanding of network properties, as well as the realization of cell-based biosensors. However, network formation is random under primary culture conditions, and the cellular arrangement often performs an insufficient fit to the electrode positions. This results in the successful recording of only a small fraction of cells. One possible approach to overcome this limitation is to raise the number of cells on the MEA, thereby accepting an increased complexity of the network. In this study, we followed an alternative strategy to increase the portion of neurons located at the electrodes by designing a network in confined geometries. Guided settlement and outgrowth of neurons is accomplished by taking control over the adhesive properties of the MEA surface. Using microcontact printing a triangular two-dimensional pattern of the adhesion promoter poly-D-lysine was applied to the MEA offering a meshwork that at the same time provides adhesion points for cell bodies matching the electrode positions and gives frequent branching points for dendrites and axons. Low density neocortical networks cultivated under this condition displayed similar properties to random networks with respect to the cellular morphology but had a threefold higher electrode coverage. Electrical activity was dominated by periodic burst firing that could pharmacologically be modulated. Geometry of the network and electrical properties of the patterned cultures were reproducible and displayed long-term stability making the combination of surface structuring and multi-site recording a promising tool for biosensor applications.

Macroporous hydrogels based on 2-hydroxyethyl methacrylate. Part 6: 3D hydrogels with positive and negative surface charges and polyelectrolyte complexes in spinal cord injury repair

Macroporous hydrogels are artificial biomaterials commonly used in tissue engineering, including central nervous system (CNS) repair. Their physical properties may be modified to improve their adhesion properties and promote tissue regeneration. We implanted four types of hydrogels based on 2-hydroxyethyl methacrylate (HEMA) with different surface charges inside a spinal cord hemisection cavity at the Th8 level in rats. The spinal cords were processed 1 and 6 months after implantation and histologically evaluated. Connective tissue deposition was most abundant in the hydrogels with positively-charged functional groups. Axonal regeneration was promoted in hydrogels carrying charged functional groups; hydrogels with positively charged functional groups showed increased axonal ingrowth into the central parts of the implant. Few astrocytes grew into the hydrogels. Our study shows that HEMA-based hydrogels carrying charged functional groups improve axonal ingrowth inside the implants compared to implants without any charge. Further, positively charged functional groups promote connective tissue infiltration and extended axonal regeneration inside a hydrogel bridge.

Patterning of 293T fibroblasts on a mica surface

Controllable cell growth on the defined areas of surfaces is important for potential applications in biosensor fabrication and tissue engineering. In this study, controllable cell growth was achieved by culturing 293 T fibroblast cells on a mica surface which had been patterned with collagen strips by a microcontact printing (microCP) technique. The collagen area was designed to support cell adhesion and the native mica surface was designed to repel cell adhesion. Consequently, the resulting cell patterns should follow the micro-patterns of the collagen. X-ray photoelectron spectroscopy (XPS), water contact angle (WCA) measurement, atomic-force microscope (AFM) observation, and force-curve measurement were used to monitor property changes before and after the collagen adsorption process. Further data showed that the patterned cells were of good viability and able to perform a gene-transfection experiment in vitro. This technique should be of potential applications in the fields of biosensor fabrication and tissue engineering.

Action potential recording from dielectrophoretically positioned neurons inside micro-wells of a planar microelectrode array

To organise in vitro neural networks at the cellular level and study their electrical patterns, we have fabricated 4 x 4 planar microelectrode arrays using conventional photolithography. The electrode sites of these arrays are located inside micro-wells, for confining the neurons, which are connected with neighbouring wells via micro-trenches capable of guiding the outgrowth of neurites. In order to load a single neuron inside each micro-well, a simple system has been developed that utilises the phenomenon of dielectrophoresis. It operates by moving neurons towards each electrode site of an array using a dielectrophoretic force, checking for the presence of a neuron inside each micro-well using image processing, and stopping the dielectrophoretic force when detecting a neuron inside a micro-well in order to prevent more cells from being trapped. This system provides a fast, effective and inexpensive way to assemble neural grids consisting of contacts between electrodes and single neurons, as the use of micromanipulator guided micropipettes can be avoided. Spontaneous and evoked action potentials from trapped neurons were successfully recorded using a 16-channel acquisition/stimulation unit.

Patterning of diverse mammalian cell types in serum free medium with photoablation

Integration of living cells with novel microdevices requires the development of innovative technologies for manipulating cells. Chemical surface patterning has been proven as an effective method to control the attachment and growth of diverse cell populations. Patterning polyelectrolyte multilayers through the combination of layer-by-layer self-assembly technique and photolithography offer a simple, versatile, and silicon compatible approach that overcomes chemical surface patterning limitations, such as short-term stability and low-protein adsorption resistance. In this study, direct photolithographic patterning of two types of multilayers, PAA (poly acrylic acid)/PAAm (poly acryl amide) and PAA/PAH (poly allyl amine hydrochloride), were developed to pattern mammalian neuronal, skeletal, and cardiac muscle cells. For all studied cell types, PAA/PAAm multilayers behaved as a cytophobic surface, completely preventing cell attachment. In contrast, PAA/PAH multilayers have shown a cell-selective behavior, promoting the attachment and growth of neuronal cells (embryonic rat hippocampal and NG108-15 cells) to a greater extent, while providing little attachment for neonatal rat cardiac and skeletal muscle cells (C2C12 cell line). PAA/PAAm multilayer cellular patterns have also shown a remarkable protein adsorption resistance. Protein adsorption protocols commonly used for surface treatment in cell culture did not compromise the cell attachment inhibiting feature of the PAA/PAAm multilayer patterns. The combination of polyelectrolyte multilayer patterns with different adsorbed proteins could expand the applicability of this technology to cell types that require specific proteins either on the surface or in the medium for attachment or differentiation, and could not be patterned using the traditional methods.

Surface-displayed antibodies as a tool for simultaneously controlling the arrangement and morphology of multiple cell types with microscale precision

Cell-cell interactions are considered to play critical roles in the development and physiology of most tissues. However, it is not straightforward to analyze cell-cell interactions with conventional cell culture in which cells are randomly distributed. To overcome this limitation, we employed here an antibody display to sort different cell types onto separate regions on a single substrate with microscale precision, taking advantage of the specific recognition of cell surface markers by surface-displayed antibodies. The results obtained with two sets of cell combinations, T cell/myelomonocytoid cell and neuron/astrocyte, demonstrate that antibody displays are feasible to establish a site-addressable coculture.

Causal measures of structure and plasticity in simulated and living neural networks

A major goal of neuroscience is to understand the relationship between neural structures and their function. Recording of neural activity with arrays of electrodes is a primary tool employed toward this goal. However, the relationships among the neural activity recorded by these arrays are often highly complex making it problematic to accurately quantify a network's structural information and then relate that structure to its function. Current statistical methods including cross correlation and coherence have achieved only modest success in characterizing the structural connectivity. Over the last decade an alternative technique known as Granger causality is emerging within neuroscience. This technique, borrowed from the field of economics, provides a strong mathematical foundation based on linear auto-regression to detect and quantify “causal” relationships among different time series. This paper presents a combination of three Granger based analytical methods that can quickly provide a relatively complete representation of the causal structure within a neural network. These are a simple pairwise Granger causality metric, a conditional metric, and a little known computationally inexpensive subtractive conditional method. Each causal metric is first described and evaluated in a series of biologically plausible neural simulations. We then demonstrate how Granger causality can detect and quantify changes in the strength of those relationships during plasticity using 60 channel spike train data from an in vitro cortical network measured on a microelectrode array. We show that these metrics can not only detect the presence of causal relationships, they also provide crucial information about the strength and direction of that relationship, particularly when that relationship maybe changing during plasticity. Although we focus on the analysis of multichannel spike train data the metrics we describe are applicable to any stationary time series in which causal relationships among multiple measures is desired. These techniques can be especially useful when the interactions among those measures are highly complex, difficult to untangle, and maybe changing over time.