Aligned microcontact printing of micrometer-scale poly-L-lysine structures for controlled growth of cultured neurons on planar microelectrode arrays

Direct rapid prototyping of PDMS from a photomask film for micropatterning of biomolecules and cells

The soft lithographic technique is a collection of simple and cost-effective patterning techniques which applies an elastomeric stamp to transfer a nano/micro-scale pattern. Patterning biological materials using soft lithography provides procedurally simple control of the surface chemistry and the cell environments. However, conventional methods for generating microstructures on a substrate require expensive clean room facilities and skillful training. Here we report a simple and inexpensive clean-room free process using a conventional photomask film as a master to fabricate elastomeric stamps or microfluidic channels. This ultra rapid prototyping technique was applied to print FITC labeled poly-L-lysine with a 10 microm feature size on a glass substrate using soft lithographic processes, such as micro-contact printing and micromolding in capillaries, for patterning human hepatocellular carcinoma cells, human skin fibroblasts and hippocampal neurons from E-18 Sprague-Dawley rat. This novel technique using a photomask film as a master would be very useful 'hands-on' tool for the generation of micro-patterned chemical or biological assays using cells and proteins.

Surface immobilization of neural adhesion molecule L1 for improving the biocompatibility of chronic neural probes: In vitro characterization

Silicon-based implantable neural electrode arrays are known to experience failure during long-term recording, partially due to host tissue responses. Surface modification and immobilization of biomolecules may provide a means to improve their biocompatibility and integration within the host brain tissue. Previously, the laminin biomolecule or laminin fragments have been used to modify the neural probe's silicon surface to promote neuronal attachment and growth. Here we report the successful immobilization of the L1 biomolecule on a silicon surface. L1 is a neuronal adhesion molecule that can specifically promote neurite outgrowth and neuronal survival. Silane chemistry and the heterobifunctional coupling agent 4-maleimidobutyric acid N-hydroxysuccinimide ester (GMBS) were used to covalently bind these two biomolecules onto the surface of silicon dioxide wafers, which mimic the surface of silicon-based implantable neural probes. After covalent binding of the biomolecules, polyethylene glycol (PEG)-NH(2) was used to cap the unreacted GMBS groups. Surface immobilization was verified by goniometry, dual polarization interferometry, and immunostaining techniques. Primary murine neurons or astrocytes were used to evaluate the modified silicon surfaces. Both L1- and laminin-modified surfaces promoted neuronal attachment, while the L1-modified surface demonstrated significantly enhanced levels of neurite outgrowth (p<0.05). In addition, the laminin-modified surface promoted astrocyte attachment, while the L1-modified surface showed significantly reduced levels of astrocyte attachment relative to the laminin-modified surface and other controls (p<0.05). These results demonstrate the ability of the L1-immobilized surface to specifically promote neuronal growth and neurite extension, while inhibiting the attachment of astrocytes, one of the main cellular components of the glial sheath. Such unique properties present vast potentials to improve the biocompatibility and chronic recording performance of neural probes.

Reusable, reversibly sealable parylene membranes for cell and protein patterning

The patterned deposition of cells and biomolecules on surfaces is a potentially useful tool for in vitro diagnostics, high-throughput screening, and tissue engineering. Here, we describe an inexpensive and potentially widely applicable micropatterning technique that uses reversible sealing of microfabricated parylene-C stencils on surfaces to enable surface patterning. Using these stencils it is possible to generate micropatterns and copatterns of proteins and cells, including NIH-3T3 fibroblasts, hepatocytes and embryonic stem cells. After patterning, the stencils can be removed from the surface, plasma treated to remove adsorbed proteins, and reused. A variety of hydrophobic surfaces including PDMS, polystyrene and acrylated glass were patterned using this approach. Furthermore, we demonstrated the reusability and mechanical integrity of the parylene membrane for at least 10 consecutive patterning processes. These parylene-C stencils are potentially scalable commercially and easily accessible for many biological and biomedical applications.

A new instrument for automated microcontact printing with stamp load adjustment

An instrument for automated microcontact printing (microCP) on microscope slides is described. The movement of the stamp, which is actuated by a computer controlled pneumatic actuator, is precisely guided until it makes contact with the substrate. As a consequence, the absolute position of the microprinted patterns is reproducible over a series of substrates with 1 mum standard deviation. Exchange of substrates and stamps is a quick and simple procedure. This makes possible the microprinting of adjacent or superimposable patterns, with different products, in a reproducible manner. Furthermore, a novel approach is described for adjusting the load on the stamp during contact. Two adjustable screws are set up so that their length (with reference to the substrate holder) limits the stamp compression during contact. The load on the stamp is proportional to the stamp compression and from the experimental point of view, this is controlled by the operator adjusting the screws. This makes possible the microCP with stamps incorporating large surface features as well as stamps with isolated features raised on the surface. For proof of concept, automated microCP of a single parallelepiped polydimethylsiloxane feature, with a surface of 2 cm x 30 microm and a height of 25 mum, is demonstrated inside a microfluidic channel without roof collapse. A second example is provided with a single cross feature, possessing an overall surface of 140 x 140 microm(2) and a height of 14 microm. Potential applications of this versatile, inexpensive and compact instrument are discussed. The machine's potential for high throughput also makes it suitable for mass production applications.

The effects of proteoglycan surface patterning on neuronal pathfinding

Protein micropatterning techniques are increasingly applied in cell choice assays to investigate fundamental biological phenomena that contribute to the host response to implanted biomaterials, and to explore the effects of protein stability and biological activity on cell behavior for in vitro cell studies. In the area of neuronal regeneration the protein micropatterning and cell choice assays are used to improve our understanding of the mechanisms directing nervous system during development and regenerative failure in the central nervous system (CNS) wound healing environment. In these cell assays, protein micropatterns need to be characterized for protein stability, bioactivity, and spatial distribution and then correlated with observed mammalian cell behavior using appropriate model system for CNS development and repair. This review provides the background on protein micropatterning for cell choice assays and describes some novel patterns that were developed to interrogate neuronal adaptation to inhibitory signals encountered in CNS injuries.

Three-dimensional direct writing of B35 neuronal cells

We have demonstrated two-dimensional and three-dimensional transfer of B35 neuronal cells onto and within polymerized Matrigel substrates, using matrix-assisted pulsed laser evaporation-direct write (MDW). The B35 cells were transferred from a quartz ribbon to depths of up to 75 microm by systematically varying the fluence emitted from the ArF (lambda = 193 nm) laser source. MDW-transferred cells were examined using terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL), 4',6-diamidino-2-phenylindole (DAPI), and alpha-tubulin staining. Confocal microscopy has shown that the transferred B35 cells extended their axons outward in three dimensions within the polymerized Matrigel substrate. The B35 cells made axonal connections and formed a three-dimensional neural network within 72 h after MDW transfer. In addition, TUNEL staining demonstrated that only 3% of the B35 cells underwent apoptosis after being transferred using the MDW process. MDW and other emergent direct write processes may provide unique approaches for creating layered, heterogeneous, three-dimensional cell-seeded scaffolds for use in peripheral nerve repair.

Rapid prototyping of patterned poly-L-lysine microstructures

For applications in cell biology, the ability to produce patterns of adhesion proteins for directing cell patterning is of particular interest. Often though, these patterns require extensive clean room facilities and intricate chrome masks to achieve very small feature sizes. We have developed a modified lift-off method for rapid prototyping of simple PLL structures that have features on a micron scale. The lift-off method is simple and easily adaptable to a variety of biological applications.

Electrical stimulation-induced cell clustering in cultured neural networks

Planar microelectrode arrays (MEAs) are widely used to record electrical activity from neural networks. However, only a small number of functional recording sites frequently show electrical activity. One contributing factor may be that neurons in vitro receive insufficient synaptic input to develop into fully functional networks. In this study, electrical stimulation was applied to neurons mimicking synaptic input. Various stimulation paradigms were examined. Stimulation amplitude and frequency were tailored to prevent cell death. Two effects of stimulation were observed when 3-week-old cultures were stimulated: (1) clusters of neural cells were observed adjacent to stimulating electrodes and (2) an increase in spontaneous neuronal activity was recorded at stimulating electrodes. Immunocytochemical analysis indicates that stimulation may cause both new neuron process growth as well as astrocyte activation. These data indicate that electrical stimulation can be used as a tool to modify neural networks at specific electrode sites and promote electrical activity.

Anisotropic growth of conducting polymers along heparin-modified surfaces

We have studied the possibility of making biocompatible, conductive patterns on a substrate by controlling the lateral growth rate of conducting polymers upon electropolymerization. Surface modification with heparin was found to enhance the lateral growth of polypyrrole, especially in the presence of dodecylbenzenesulfonate, and thus the micropatterning of heparin around electrodes leads to the formation of polypyrrole patterns.

Neuronal Network Morphology and Electrophysiologyof Hippocampal Neurons Cultured on Surface-Treated Multielectrode Arrays

Toward the development of biocompatible surfaces for implantable electrode arrays and the creation of patterned neuronal networks, the impact of select biochemical substrates [poly-D-lysine (PDL), polyornithine (PO), polyethylenimine (PEI), and a basement membrane extract (BM)] on network morphology and spontaneous electrophysiological activity of dissociated hippocampal neurons was investigated. Cultured in serum-free Neurobasal medium at 100 000 cells/cm(2), neurons attached to each substrate. PDL, PO, and PEI induced little or no neuronal clustering and process fasciculation, whereas the addition of BM promoted these features. The ratios of somas to processes, and axons to dendrites, as determined by immunohistochemical staining and image analysis were comparable across all substrates. Spontaneous firing was recorded using planar multielectrode arrays (MEAs) at the third week in vitro for the two most divergent morphologies according to Euclidian cluster analysis, namely those induced by PO + BM and PEI. Mean spike amplitude, mean firing rate, median interspike interval (ISI), mean burst rate, and correlation index were analyzed and compared to morphological features. Synchronized bursting was highly correlated with neuronal clustering and process fasciculation. Spike amplitude was negatively correlated with thin branching which was most evident in neurons grown on PEI. These data indicate that factors, which influence adherence of neurons to surfaces, can profoundly impact both neuronal network morphology and electrophysiological activity in vitro.

Electrodeposition of anchored polypyrrole film on microelectrodes and stimulation of cultured cardiac myocytes

The electrically conducting polymer polypyrrole (PPy) was electrochemically deposited onto Pt microelectrodes on a polyimide (PI) substrate. Pre-modification of the PI surface with a self-assembled monolayer of octadecyltrichlorosilane-induced anisotropic lateral growth of PPy along the PI surface and enhanced adhesive strength of the PPy film. The lateral growth of PPy film around the electrode anchored the whole film to the substrate. External stimulation of cultured cardiac myocytes was carried out using the PPy-coated microelectrode. The myocytes on the microelectrode substrate were electrically conjugated to form a sheet, and showed synchronized beating upon stimulation. The threshold charge for effective stimulation of a 0.8 cm(2) sheet of myocytes was around 0.2 microC, roughly corresponding to a membrane depolarization of 250 mV.

Synaptic connectivity of a low density patterned neuronal network produced on the poly-L-lysine stamped microelectrode array

Rectangular networks of rat hippocampal neurons have been produced on microelectrode arrays (MEAs). The crossing points of networks were located at the recording electrode sites by aligned microcontact printing (&#956;CP) technique. Polydimethysiloxane (PDMS) stamp was fabricated to print fine poly-L-lysine (PLL) patterns of 2 -width lines for neurites and 20 -diameter circles for cell bodies. Different densities of neurons were applied on the PLL-stamped MEAs to find how a low density of neurons still has the functional connectivity. From the neural network applied with a density of 200 cells/mm2, we could observe signal propagation among spontaneous activities. Electrical responses were also evoked by 200 current pulse stimulation with 50 pulse width. Immunocytostaining was employed to identify dendrites, synapses, and nuclei in the patterned neurons.

Neurite guidance on protein micropatterns generated by a piezoelectric microdispenser

In this study, we developed a microdispenser technique in order to create protein patterns for guidance of neurites from cultured adult mouse dorsal root ganglia (DRG). The microdispenser is a micromachined silicon device that ejects 100 picolitre droplets and has the ability to position the droplets with a precision of 6-8 microm. Laminin and bovine serum albumin (BSA) was used to create adhesive and non-adhesive protein lines on polystyrene surfaces (cell culture dishes). Whole-mounted DRGs were then positioned close to the patterns and neurite outgrowth was monitored. The neurites preferred to grow on laminin lines as compared to the unpatterned plastic. When patterns were made from BSA the neurites preferred to grow in between the lines on the unpatterned plastic surface. We conclude that microdispensing can be used for guidance of sensory neurites. The advantages of microdispensing is that it is fast, flexible, allows deposition of different protein concentrations and enables patterning on delicate surfaces due to its non-contact mode of operation. It is conceivable that microdispensing can be utilized for the creation of protein patterns for guiding neurites to obtain in vitro neural networks, in tissue engineering or rapid screening for guiding proteins.

Local presentation of L1 and N-cadherin in multicomponent, microscale patterns differentially direct neuron functionin vitro

The ability to pattern multiple bioactive cues on a surface is valuable for understanding how neurons interact with their complex extracellular environment. In this report, we introduce a set of methods for creating such surfaces, with the goals of understanding how developing neurons integrate multiple biologically relevant signals and as a tool for studying interactions between multiple neurons. Multiple microcontact printing steps are combined on a single surface to produce an array of polylysine nodes, interconnected by lines of proteins based on the extracellular domains of L1 or N-cadherin. Surprisingly, the N-cadherin protein could also be directly printed onto surfaces while retaining its biological activity. Rat hippocampal neurons selectively attached to the polylysine nodes, differentially extending axonal and dendritic processes along the patterns of L1 and N-cadherin, thus demonstrating control over neuron attachment and outgrowth. Combining these three biomolecules on a single surface revealed a highly complex pattern of protein recognition. Dendrites extended exclusively on N-cadherin patterns, while axons exhibited a very high degree of selectivity on L1 patterns, preferentially at distances greater than 55 mum from the cell body. At shorter distances, axonal processes recognized both L1 and N-cadherin, revealing a new aspect of neuron polarity and axon specification. This onset of L1 selectivity correlated with the establishment of intracellular L1 polarity, suggesting a functional outcome of the process of neuron polarization that has implications in development of neural tissues and creation of in vitro neuron networks.

Microtechnology: meet neurobiology

The field of neuroscience has always been attractive to engineers. Neurons and their connections, like tiny circuit elements, process and transmit information in a dramatic way that is intimately curious to researchers in the computer science and engineering fields. Of particular interest has been the recent push in applying microtechnology to the field of neuroscience. This review is meant to provide an overview of some of the subtle nuances of the nervous system and outline recent advances in lab on a chip applications in neurobiology. It also aims to highlight some of the challenges the field faces in the hopes of encouraging new engineering researchers to collaborate with neurobiologists to help advance our basic understanding of the nervous system and create novel applications based on neuroengineering principles.

Biological functionalization and surface micropatterning of polyacrylamide hydrogels

Hydrogels are useful for linking proteins to solid surfaces because their hydrophilic nature and porous structure help them to maintain these labile molecules in the native functional state. We have developed a method for creating surface-patterned, biofunctionalized hydrogels on glass or silicon, using polyacrylamide and the disulfide-containing polyacrylamide crosslinker, bis(acryloyl)cystamine. Treatment with a reducing agent created reactive sulfhydryl (-SH) groups throughout these hydrogels that were readily conjugated to iodoacetyl biotin and streptavidin (SA). Immobilization efficiency was approximately 1-2% of the total potential binding capacity of the hydrogel. Porosity of the hydrogel was not a limiting factor for SA immobilization, as determined using fluorescence confocal microscopy. Rather, steric hindrance due to the binding of SA decreased the effective porosity near the surface of the hydrogel, restricting access to the rest of the gel. Using microcontact printing, we indirectly patterned SA on the surface of the hydrogel, generating well-resolved feature sizes of 2 microm in width. Through repeated rounds of microcontact printing, multiple, adjacent protein patterns were generated on the surface of the hydrogel. Biotinylated immune complexes and lipid vesicles readily bound to SA-functionalized hydrogels, demonstrating the feasibility of using this hydrogel system to generate complex biofunctionalized surfaces.

Low-density neuronal networks cultured using patterned poly-l-lysine on microelectrode arrays

Synaptic activity recorded from low-density networks of cultured rat hippocampal neurons was monitored using microelectrode arrays (MEAs). Neuronal networks were patterned with poly-l-lysine (PLL) using microcontact printing (microCP). Polydimethysiloxane (PDMS) stamps were fabricated with relief structures resulting in patterns of 2 microm-wide lines for directing process growth and 20 microm-diameter circles for cell soma attachment. These circles were aligned to electrode sites. Different densities of neurons were plated in order to assess the minimal neuron density required for development of an active network. Spontaneous activity was observed at 10-14 days in networks using neuron densities as low as 200 cells/mm(2). Immunocytochemistry demonstrated the distribution of dendrites along the lines and the location of foci of the presynaptic protein, synaptophysin, on neuron somas and dendrites. Scanning electron microscopy demonstrated that single fluorescent tracks contained multiple processes. Evoked responses of selected portions of the networks were produced by stimulation of specific electrode sites. In addition, the neuronal excitability of the network was increased by the bath application of high K(+) (10-12 mM). Application of DNQX, an AMPA antagonist, blocked all spontaneous activity, suggesting that the activity is excitatory and mediated through glutamate receptors.

Brain-Controlled Interfaces: Movement Restoration with Neural Prosthetics

Brain-controlled interfaces are devices that capture brain transmissions involved in a subject's intention to act, with the potential to restore communication and movement to those who are immobilized. Current devices record electrical activity from the scalp, on the surface of the brain, and within the cerebral cortex. These signals are being translated to command signals driving prosthetic limbs and computer displays. Somatosensory feedback is being added to this control as generated behaviors become more complex. New technology to engineer the tissue-electrode interface, electrode design, and extraction algorithms to transform the recorded signal to movement will help translate exciting laboratory demonstrations to patient practice in the near future.

Detection, Stimulation, and Inhibition of Neuronal Signals with High-Density Nanowire Transistor Arrays

We report electrical properties of hybrid structures consisting of arrays of nanowire field-effect transistors integrated with the individual axons and dendrites of live mammalian neurons, where each nanoscale junction can be used for spatially resolved, highly sensitive detection, stimulation, and/or inhibition of neuronal signal propagation. Arrays of nanowire-neuron junctions enable simultaneous measurement of the rate, amplitude, and shape of signals propagating along individual axons and dendrites. The configuration of nanowire-axon junctions in arrays, as both inputs and outputs, makes possible controlled studies of partial to complete inhibition of signal propagation by both local electrical and chemical stimuli. In addition, nanowire-axon junction arrays were integrated and tested at a level of at least 50 "artificial synapses" per neuron.

Scanning force microscopy and fluorescence microscopy of microcontact printed antibodies and antibody fragments

Unlabeled primary immunoglobulin G (IgG) antibodies and its F(ab')2 and Fc fragments were attached to oxygen-plasma-cleaned glass substrates using either microcontact printing (MCP) or physical adsorption during bath application from dilute solutions. Fluorescently labeled secondary IgGs were then bound to surface-immobilized IgG, and the relative surface coverage was determined by measuring the fluorescence intensity. Results indicated that the surface coverage of IgG increased with increasing protein solution concentration for both MCP and bath-applied IgG and that a greater concentration of IgG was transferred to a glass substrate using MCP than during physisorption during bath applications. Scanning force microscopy (SFM) showed that patterned MCP IgG monolayers were 5 nm in height, indicating that IgG molecules lie flat on the substrate. After incubation with a secondary IgG, the overall line thickness increased to around 15 nm, indicating that the secondary IgG was in a more vertical orientation with respect to the substrate. The surface roughness of these MCP patterned IgG bilayers as measured by SFM was observed to increase with increasing surface coverage. Physisorption of IgG to both unmodified patterned polydimethylsiloxane (PDMS) stamps and plasma-cleaned glass substrates was modeled by Langmuir adsorption kinetics yielding IgG binding constants of K(MCP) = 1.7(2) x 10(7) M(-1) and K(bath) = 7.8(7) x 10(5) M(-1), respectively. MCP experiments involving primary F(ab')2 and Fc fragments incubated in fluorescently labeled fragment-specific secondary IgGs were carried out to test for the function and orientation of IgG. Finally, possible origins of MCP stamping defects such as pits, pull outs, droplets, and reverse protein transfer are discussed.

New ways to print living cells promise breakthroughs for engineering complex tissues in vitro

The ability to control the placement of cells and the assembly of networks in vitro has tremendous potential for understanding the regulation of development as well as for generating artificial tissues. To date, most engineering tools that can place materials with precision are not compatible with the requirements of living cells, and so approaches to tissue engineering have focused on patterning substrates as a way of controlling cell growth rather than patterning cells directly. In this issue of Biochemical Journal, however, Eagles et al. adapt electrohydrodynamic printing technology to 'print' living cells from a neuronal cell line on to a substrate. The importance of this approach is that it has the potential for unprecedented control over the position of cells in culture by directly placing them, thus allowing for the systematic assembly of cell networks.

Effects of substrate geometry on growth cone behavior and axon branching

At the leading edge of a growing axon, the growth cone determines the path the axon takes and also plays a role in the formation of branches, decisions that are regulated by a complex array of chemical signals. Here, we used microfabrication technology to determine whether differences in substrate geometry, independent of changes in substrate chemistry, can modulate growth cone motility and branching, by patterning a polylysine grid of narrow (2 or 5 microm wide) intersecting lines. The shape of the intersections varied from circular nodes 15 microm in diameter to simple crossed lines (nodeless intersections). Time-lapse recordings of cultured hippocampal neurons showed that simple variations in substrate geometry changed growth cone shape, and altered the rate of growth and the probability of branching. When crossing onto a node intersection the growth cone paused, often for hours, and microtubules appeared to defasciculate. Once beyond the node, filopodia and lamellipodia persisted at that site, sometimes forming a collateral branch. At nodeless intersections, the growth cone passed through with minimal hesitation, often becoming divided into separate areas of motility that led to the growth of separate branches. When several lines intersected at a common point, growth cones sometimes split into several subdivisions, resulting in the emergence of as many as five branches. Such experiments revealed an intrinsic preference for branches to form at angles less than 90 degrees . These data show that simple changes in the geometry of a chemically homogeneous substrate are detected by the growth cone and can regulate axonal growth and the formation of branches.

Sample patterning on NMR surface microcoils

Aligned microcontact printing for patterning the sample in areas of homogeneous RF-field on the highly sensitive surface of planar NMR microprobes is presented. We experimentally demonstrate that sample patterning allows drastic improvement of the spin excitation uniformity. The NMR microprobes are designed for cell analysis and characterized using lipid vesicles as cell substitutes. Lipid vesicles are advantageous as composition and concentration of the confined solution are precisely controlled and because of their similarity to living cells. Using aligned microcontact printing, a monolayer of lipid vesicles is immobilized on the surface of the planar NMR microprobe in a patterned way. 1H NMR spectra and CPMG spin echoes of sucrose solution confined within the lipid vesicles are successfully recorded. Nutation curves of the sample structured in different patterns demonstrate the impact of patterning on the spin excitation uniformity. The total detection volumes are between 1 and 2 nL and derived with help of a theoretic model based on 3D finite element simulation. This model predicts the signal-to-noise ratio and the progression of the nutation curves.

Constraining the connectivity of neuronal networks cultured on microelectrode arrays with microfluidic techniques: A step towards neuron-based functional chips

In vitro culture of small neuronal networks with pre-defined topological features is particularly desirable when the electrical activity of such assemblies can be monitored for long periods of time. Indeed, it is hoped that such networks, with pre-determined connectivity, will provide unique insights into the structure/function relationship of biological neural networks and their properties of self-organization. However, the experimental techniques that have been developed so far for that purpose have either failed to provide very long-term pattern definition and retention, or they have not shown potential for integration into more complex microfluidic devices. To address this problem, three-dimensional microfluidic systems in poly(dimethylsiloxane) (PDMS) were fabricated and used in conjunction with both custom-made and commercially available planar microelectrode arrays (pMEAs). Various types of primary neuronal cell cultures were established inside these systems. Extracellular electrical signals were successfully recorded from all types of cells placed inside the patterns, and this bioelectrical activity was present for several weeks. The advantage of this approach is that it can be further integrated with microfluidic devices and pMEAs to yield, for example, complex neuron-based biosensors or chips for pharmacological screening.

Integration of topographical and biochemical cues by axons during growth on microfabricated 3-D substrates

During embryonic neural development, axon tips ("growth cones") are guided through a dynamic three-dimensional (3-D) landscape by soluble chemotropic factors and by immobilized, growth-permissive or growth-inhibiting contact cues present in the extracellular matrix and on the surface of surrounding cells. It has been difficult to probe the search algorithms of growth cones in response to multiple contact cues during 3-D navigation using traditional two-dimensional (2-D) substrates. Here, we present an in vitro study in which the axons of murine embryonic cortical neurons are challenged with competing growth options, using 3-D substrates that feature variations in permissiveness and microtopography. As 3-D substrates, we used poly-D-lysine (PDL) coatings on microfabricated steps of polydimethylsiloxane (PDMS) and complementary features of Matrigel. We found that axons display a preference for PDL over Matrigel and for the straightest path within a distance consistent with the exploratory range of the growth cone. When these two preferences are in conflict, axons choose to grow straight into Matrigel; when the straight path is not permissive, the axon turns in the direction that minimizes the turning angle. These results suggest that growth cones make 3-D navigation decisions by integrating permissiveness and topographical cues.

Rapid Fabrication and Chemical Patterning of Polymer Microstructures and their Applications as a Platform for Cell Cultures

Much of the current knowledge regarding biological processes has been obtained through in-vitro studies in bulk aqueous solutions or in conventional Petri-dishes, with neither methodology accurately duplicating the actual in-vivo biological processes. Recently, a number of innovative approaches have attempted to address these shortcomings by providing substrates with controlled features. In particular, tunable surface chemistries and topographical micro and nanostructures have been used as model systems to study the complex biological processes. We herein report a versatile and rapid fabrication method to produce a variety of microstructured polymer substrates with precise control and tailoring of their surface chemistries. A poly(dimethylsiloxane) (PDMS) substrate, produced by replication over a master mold with specific microstructures, is modified by a fluoro siloxane derivative to enhance its anti-adhesion characteristics and used as a secondary replication mold. A curable material, deposited by spin coating on various substrates, is stamped with the secondary mold and crosslinked. The removal of the secondary mold produces a microstructured surface with the same topographical features as the initial master mold. The facile chemical patterning of the microstructured substrates is demonstrated through the use of microcontact printing methods and these materials are tested as a platform to guide cell attachment, growth and proliferation. The master mold and flexible fluorinated PDMS stamps can be used in a repeated manner without any degradation of the anti-adhesion characteristics opening the way to the development of high-throughput fabrication methods that can yield reliable and inexpensive microstructured and chemically patterned substrates.

Numerical and experimental study of critical roof collapse conditions in soft lithography

One of the major issues during soft lithographic processes is that, if the pressing force on the stamp becomes too high, the stamp may erroneously come into contact with the substrate in zones where contact is not intended. This decreases the patterning accuracy and may lead to badly or nonperforming electronic devices and is therefore undesired. Design rules, available at an early stage in the design phase, are desired to speed-up the development of this technique. Ultimately, these rules should give an indication of the critical pressure that can safely be applied on the stamp thereby avoiding unwanted contact between the stamp and the substrate. To obtain these critical pressures, numerical analyses of the deformation behavior of two characteristic configurations in the microstructured surface pattern of the rubber stamp are performed. The deformation behavior of the rubber is modeled according to a Gaussian and a non-Gaussian approach, leading to a neo-Hookean and Arruda-Boyce constitutive model, respectively. Besides these material nonlinearities, geometrical nonlinearities are taken into account as well. The calculated pressure at which undesired contact takes place (the roof collapse pressure) is compared to experimentally obtained values for two particular types of structures, and the results are in agreement within the error margins of the experiments and those ensuing from the assumptions of the numerical simulations.

Investigating neuronal activity with planar microelectrode arrays: achievements and new perspectives

Neuronal networks underlie memory storage and information processing in the human brain, and ultimately participate in what Eccles referred to as "the creation of consciousness". Moreover, as physiological dysfunctions of neurons almost always translate into serious health issues, the study of the dynamics of neuronal networks has become a major avenue of research, as well as their response to pharmacological tampering. Planar microelectrode arrays represent a unique tool to investigate such dynamics and interferences, as they allow one to observe the activity of neuronal networks spread in both space and time. We will here review the major results obtained with microelectrode arrays and give an overview of the latest technological developments in the field, including our own efforts to develop the potential of this already powerful technology.

Oxygen plasma-treatment effects on Si transfer

Oxygen plasma-treatment is commonly used to increase the hydrophilicity of poly(dimethylsiloxane) (PDMS) stamps used for microcontact printing (muCP) aqueous-based inks. Review of the literature reveals that a wide range of plasma parameters are currently employed to modify stamp surfaces. However, little is known about the effect of these parameters (e.g., power, chamber pressure, duration) on the undesirable transfer of low-molecular-weight silicon-containing fragments from the stamps that commonly occurs during muCP. To study the effect of oxygen plasma-treatment on Si transfer, unpatterned PDMS stamps were treated with oxygen plasma under various conditions and used to stamp deionized water on plasma-activated poly(methyl methacrylate) (PMMA) substrates. Once stamped, the PMMA substrates were analyzed with X-ray photoelectron spectroscopy (XPS) to quantify and characterize silicon present on the substrate surface. In addition, used PDMS stamps were analyzed with scanning electron microscopy (SEM) to observe topographical changes that occur during oxygen plasma-treatment. XPS results show that all plasma treatments studied significantly reduced the amount of Si transfer from the treated stamps during muCP as compared to untreated PDMS stamps and that the source of transfer is residual PDMS fragments not removed by oxygen plasma. SEM results show that, although the treated stamps undergo a variety of topographical changes, no correlation exists between stamp topography and extent of Si transfer from the stamps.

Building In Vitro Models of Organs

Tissue-engineering techniques are being used to build in vitro models of organs as substitutes for human donor organs for transplantation as well as in vitro toxicology testing (as alternatives to use of animals). Tissue engineering involves the fabrication of scaffolds from materials that are biologically compatible to serve as cellular supports and microhabitats in order to reconstitute a desired tissue or organ. Three organ systems that are currently the foci of tissue engineering efforts for both transplantation and in vitro toxicology testing purposes are discussed. These are models of the cornea, nerves (peripheral nerves specifically), and cardiovascular components. In each of these organ systems, a variety of techniques and materials are being used to achieve the same end results. In general, models that are designed with consideration of the developmental and cellular biology of the target tissues or organs have tended to result in morphologically and physiologically accurate models. Many of the models, with further development and refinement, have the potential to be useful as functional substitute tissues and organs for transplantation or for in vitro toxicology testing.

Extracellular recordings from patterned neuronal networks using planar microelectrode arrays

Neuronal cell networks have been reconstructed on planar microelectrode arrays (MEAs) from dissociated hippocampal pyramidal neurons. Microcontact printing (microCP) and a photoresist-liftoff method were used to selectively localize poly-L-lysine (PLL) on the surface of MEAs. Haptotaxis led to the organization of the neurons into networks localized adjacent to microelectrodes. Various grids of PLL with 2-25-microm-wide lines spaced by 50-200 microm with 15-25-microm nodes at intersection points were used to guide cell body attachment and neurite outgrowth. Bursting activity with spike amplitude attenuation was observed, and multichannel recordings detected instances of coincident firing activity. Finally, we present here an extracellular recording from a approximately 2 microm bundle of guided neurites.

Modification of a neuronal network direction using stepwise photo-thermal etching of an agarose architecture

Control over spatial distribution of individual neurons and the pattern of neural network provides an important tool for studying information processing pathways during neural network formation. Moreover, the knowledge of the direction of synaptic connections between cells in each neural network can provide detailed information on the relationship between the forward and feedback signaling. We have developed a method for topographical control of the direction of synaptic connections within a living neuronal network using a new type of individual-cell-based on-chip cell-cultivation system with an agarose microchamber array (AMCA). The advantages of this system include the possibility to control positions and number of cultured cells as well as flexible control of the direction of elongation of axons through stepwise melting of narrow grooves. Such micrometer-order microchannels are obtained by photo-thermal etching of agarose where a portion of the gel is melted with a 1064-nm infrared laser beam. Using this system, we created neural network from individual Rat hippocampal cells. We were able to control elongation of individual axons during cultivation (from cells contained within the AMCA) by non-destructive stepwise photo-thermal etching. We have demonstrated the potential of our on-chip AMCA cell cultivation system for the controlled development of individual cell-based neural networks.

The Artificial Synapse Chip: a flexible retinal interface based on directed retinal cell growth and neurotransmitter stimulation

The Artificial Synapse Chip is an evolving design for a flexible retinal interface that aims to improve visual resolution of an electronic retinal prosthesis by addressing cells individually and mimicking the physiological stimulation achieved in synaptic transmission. We describe three novel approaches employed in the development of the Artificial Synapse Chip: (i) micropatterned substrates to direct retinal cell neurite growth to individual stimulation sites; (ii) a prototype retinal interface based on localized neurotransmitter delivery; and (iii) the use of soft materials to fabricate these devices. By patterning the growth of cells to individual stimulation sites, we can improve the selectivity of stimulation and decrease the associated power requirements. Moreover, we have microfabricated a neurotransmitter delivery system based on a 5- micro m aperture in a 500-nm-thick silicon nitride membrane overlying a microfluidic channel. This device can release neurotransmitter volumes as small as 2 pL, demonstrating the possibility of chemical-based prostheses. Finally, we have fabricated and implanted an equivalent device using soft flexible materials that conform to the retinal tissue more effectively. As many of the current retinal prosthesis devices use hard materials and electrical excitation at a lower resolution, our approach may provide more physiologic retinal stimulation.

Pattern modification of a neuronal network for individual-cell-based electrophysiological measurement using photothermal etching of an agarose architecture with a multielectrode array

A new type of individual-cell-based on-chip multielectrode array (MEA) cell-cultivation system with an agarose microchamber (AMC) array for topographical control of the network patterns of a living neuronal network has been developed. The advantages of this system are that it allows control of the cell positions and numbers for cultivation using AMCs, as well as easy and flexible control of the pattern of connections between the AMCs through photothermal etching where a portion of the agarose layer is melted with a 1480 nm infrared laser beam. With adequate laser power, narrow micrometer-order grooves (microchannels) can easily be fabricated that can be used to combine neighbouring AMCs to enable topographical control of the neural network pattern. Using this system, an individual-cell-based neural network pattern was formed of rat hippocampal cells within the AMC array without cells escaping from the electrode positions in the microchamber during an eight-day cultivation, and could record cell firing in response to 1.5 V, 500 kHz stimulation through an electrode. This demonstrated the potential of the on-chip AMCMEA cell cultivation system for long-term single-cell-based electrophysiological measurement of a neural network system.

Pharmacological characterization of micropatterned cardiac myocytes

The cardiac myocytes patterned on a single cell level were prepared by microcontact printing, and the response to chemical stimuli was studied using confocal fluorescent Ca(2+) imaging. The patterned myocytes were found to conjugate by forming gap junction. It was confirmed for the patterned myocytes that gap junction communication was reversibly inhibited by 1-octanol, but activated by caffeine. Localized stimulation with chemicals was also attempted using a microinjection system for the myocyte patterns formed by single cell alignment. This research was carried out with the objective of developing a bioassay system based on a cellular network.

Methods for the topographical patterning and patterned surface modification of hydrogels based on hydroxyethyl methacrylate

Hydrogels have gained broad acceptance as a class of biocompatible materials. In this paper, we report the topographic patterning and regiospecific functionalization of hydrogel surfaces. Both photolithography and soft lithography are combined in a hybrid process to form these topographic features. By functionalization of a base layer surface followed by lithographic patterning steps, it is possible to introduce chemical functions to specific regions of the patterned surface. The model systems investigated were based on 2-hydroxyethyl methacrylate (HEMA), which is well-known for its low toxicity and widespread use in biomedical applications. Tests of Ni-NTA modified hydrogel surfaces showed successful binding of fluorescently labeled proteins to selected regions of the patterned hydrogel surface. These processes can be expanded to a wide range of monomer systems.

A modified microstamping technique enhances polylysine transfer and neuronal cell patterning

Macromolecular microstamping with polydimethylsiloxane (PDMS) stamps has been demonstrated to transfer proteins onto glassy substrates for antigen or antibody detection and for cell patterning. For many applications, including neuronal cell patterning, it is important to assure reliable transfer of sufficient quantity of protein. Research has shown that protein transfer is enhanced with the selection of the proper protein-stamp-substrate combination. In addition, detergent studies have shown that detergent-protein complexes detach from surfaces to a greater extent than proteins alone. Therefore, we hypothesized that stamp surface modification (termed here a release layer) can enhance polylysine transfer and benefit cell growth on microstamped substrates. We found unmodified stamps to transfer insufficient polylysine to support good cell survival of hippocampal neurons in a widely used serum-free, reduced-glia cell culture system. However, with modified stamps neuronal growth was reliably good. This enhanced cell growth can be attributed to the increased polylysine transfer due to the release layer rather than increased loading onto the stamp. This enhancement was found to be even greater for two-month old stamps that were stored in water. Furthermore, the physicochemical properties of the release layer can modulate the loading process. Thus, our data supports the conclusions that the release layer: (1) modulates polylysine loading, (2) enhances polylysine transfer, (3) enhances cellular growth on microstamped substrates, and (4) extends the durability (defined as the number of times a stamp can be reused) of PDMS microstamps.

Shrinking the biologic world--nanobiotechnologies for toxicology

Although toxicologic effects need to be considered at the organismal level, the adverse events originate from interactions and alterations at the molecular level. Cellular structures and functions can be disrupted by modifications of the nanometer structure of critical molecules; therefore, devices used to assess biologic and toxicologic processes at the nanoscale will allow important new research pursuits. In order to properly assess alterations at these dimensions, nanofabricated tools are needed to detect, separate, analyze, and manipulate cells or biologic molecules of interest. The emergence of laser tweezers, surface plasmon resonance (SPR), laser capture microdissection (LCM), atomic force microscopy (AFM), and multi-photon microscopes have allowed for these assessments. Micro- and nanobiotechnologies will further advance biologic, clinical, and toxicologic endeavors with the aid of miniaturized, more sensitive devices. Miniaturized table-top laboratory equipment incorporating additional innovative technologies can lead to new advances, including micro total analysis systems (microTAS) or "lab-on-a-chip" and "sentinel sensor" devices. This review will highlight several devices, which have been made possible by techniques originating in the microelectronics industry. These devices can be used for toxicologic assessment of cellular structures and functions, such as cellular adhesion, signal transduction, motility, deformability, metabolism, and secretion.

Intracellular Ca2+ imaging for micropatterned cardiac myocytes

The patterning of cardiac myocytes on a micron scale ( approximately 5 microm) was achieved by microcontact printing of fibronectin onto a hydrophobically pretreated glass substrate. The patterned cardiac myocytes conjugated with each other by forming a gap junction, as judged from the synchronized Ca(2+) transition over the pattern, and thus simultaneously contracted. The dynamic change of the Ca(2+) concentration within the patterned tissue was analyzed quantitatively during successive contraction and relaxation using a Nipkow-type high-speed confocal microscope.

Biology on a chip: microfabrication for studying the behavior of cultured cells

The ability to culture cells in vitro has revolutionized hypothesis testing in basic cell and molecular biology research and has become a standard methodology in drug screening and toxicology assays. However, the traditional cell culture methodology--consisting essentially of the immersion of a large population of cells in a homogeneous fluid medium--has become increasingly limiting, both from a fundamental point of view (cells in vivo are surrounded by complex spatiotemporal microenvironments) and from a practical perspective (scaling up the number of fluid handling steps and cell manipulations for high-throughput studies in vitro is prohibitively expensive). Microfabrication technologies have enabled researchers to design, with micrometer control, the biochemical composition and topology of the substrate, the medium composition, as well as the type of neighboring cells surrounding the microenvironment of the cell. In addition, microtechnology is conceptually well suited for the development of fast, low-cost in vitro systems that allow for high-throughput culturing and analysis of cells under large numbers of conditions. Here we review a variety of applications of microfabrication in cell culture studies, with an emphasis on the biology of various cell types.

Synergistic effects of physical and chemical guidance cues on neurite alignment and outgrowth on biodegradable polymer substrates

This article demonstrates that directional outgrowth of neurites is promoted by applying a combination of physical and chemical cues to biodegradable polymer substrates. Films of poly-D,L-lactic acid and poly(lactide-co-glycolide) were micropatterned to form grooves on substrate surfaces, using novel indirect transfer techniques developed specifically for biodegradable polymers that cannot be micropatterned directly. Laminin was selectively adsorbed in the grooves. Whole and dissociated dorsal root ganglia were seeded on the substrates and neurite outgrowth and alignment along the microgrooves were measured. The microgrooves provide physical guidance, whereas laminin provides chemical cues to the neurons. The groove depth and spacing were found to significantly influence neurite alignment. The presence of laminin was found to promote neurite adhesion and outgrowth along the grooves. Using a combination of optimized physical and chemical cues, excellent spatial control of directional neurite outgrowth, with up to 95% alignment of neurites, was obtained. The synergistic effect of physical and chemical guidance cues was found to be more effective than individual cues in promoting directional outgrowth of neurites.

Extracellular potentials in low-density dissociated neuronal cultures

The detection of extracellular potentials by means of multi-electrode arrays (MEA) is a useful technique for multi-site long-term monitoring of cultured neuronal activity with single-cell resolution. To optimize the geometry of the MEA it is advantageous to localize the cellular compartments that constitute the generators of these signals. For this purpose, an in vitro technique for the detection of extracellular signals with subcellular resolution has been developed. It makes use of easy-to-manufacture large-tip pipettes, monitoring of electrode-cell gap resistance for precise electrode positioning and low-density (100 cells/mm(2)) dissociated hippocampal cultures. Negative monophasic extracellular spikes, typically 60 microV, were measured over putative axonal processes and monophasic, biphasic and triphasic signals were recorded over the soma. A compartmental simulation suggests that different somatic conductance densities of Na(+) (1-10 mS/cm(2)) and K(+) (5-10 mS/cm(2)) channels can produce characteristic somatic extracellular potentials, with a variety of shapes similar to those observed experimentally.

Topographical and physicochemical modification of material surface to enable patterning of living cells

Precise control of the architecture of multiple cells in culture and in vivo via precise engineering of the material surface properties is described as cell patterning. Substrate patterning by control of the surface physicochemical and topographic features enables selective localization and phenotypic and genotypic control of living cells. In culture, control over spatial and temporal dynamics of cells and heterotypic interactions draws inspiration from in vivo embryogenesis and haptotaxis. Patterned arrays of single or multiple cell types in culture serve as model systems for exploration of cell-cell and cell-matrix interactions. More recently, the patterned arrays and assemblies of tissues have found practical applications in the fields of Biosensors and cell-based assays for Drug Discovery. Although the field of cell patterning has its origins early in this century, an improved understanding of cell-substrate interactions and the use of microfabrication techniques borrowed from the microelectronics industry have enabled significant recent progress. This review presents the important early discoveries and emphasizes results of recent state-of-the-art cell patterning methods. The review concludes by illustrating the growing impact of cell patterning in the areas of bioelectronic devices and cell-based assays for drug discovery.