Guiding neuron development with planar surface gradients of substrate cues deposited using microfluidic devices

Engineering cortical neuron polarity with nanomagnets on a chip

Intra- and extracellular signaling play critical roles in cell polarity, ultimately leading to the development of functional cell-cell connections, tissues, and organs. In the brain, pathologically oriented neurons are often the cause for disordered circuits, severely impacting motor function, perception, and memory. Aside from control through gene expression and signaling pathways, it is known that nervous system development can be manipulated by mechanical stimuli (e.g., outgrowth of axons through externally applied forces). The inverse is true as well: intracellular molecular signals can be converted into forces to yield axonal outgrowth. The complete role played by mechanical signals in mediating single-cell polarity, however, remains currently unclear. Here we employ highly parallelized nanomagnets on a chip to exert local mechanical stimuli on cortical neurons, independently of the amount of superparamagnetic nanoparticles taken up by the cells. The chip-based approach was utilized to quantify the effect of nanoparticle-mediated forces on the intracellular cytoskeleton as visualized by the distribution of the microtubule-associated protein tau. While single cortical neurons prefer to assemble tau proteins following poly-L-lysine surface cues, an optimal force range of 4.5-70 pN by the nanomagnets initiated a tau distribution opposed to the pattern cue. In larger cell clusters (groups comprising six or more cells), nanoparticle-mediated forces induced tau repositioning in an observed range of 190-270 pN, and initiation of magnetic field-directed cell displacement was observed at forces above 300 pN. Our findings lay the groundwork for high-resolution mechanical encoding of neural networks in vitro, mechanically driven cell polarization in brain tissues, and neurotherapeutic approaches using functionalized superparamagnetic nanoparticles to potentially restore disordered neural circuits.

Polydimethylsiloxane SlipChip for mammalian cell culture applications

A polydimethylsiloxane (PDMS) SlipChip for in vitro mammalian cell culture applications, including multiple-treatment assays, cell co-culture, and cytokine detection assays.

Quantifying biased response of axon to chemical gradient steepness in a microfluidic device

Axons are very sensitive to molecular gradients and can discriminate extremely small differences in gradient steepness. Microfluidic devices capable of generating chemical gradients and adjusting their steepness could be used to quantify the sensitivity of axonal response. Here, we present a versatile and robust microfluidic device that can generate substrate-bound molecular gradients with evenly varying steepness on a single chip to precisely quantify axonal response. In this device, two solutions are perfused into a central channel via two inlets while partially flowing into two peripheral channels through interconnecting grooves, which gradually decrease the fluid velocity along the central channel. Molecular gradients with evenly and gradually decreased steepness can therefore be generated with a high resolution that is less than 0.05%/mm. In addition, the overall distribution range and resolution of the gradient steepness can be highly and flexibly controlled by adjusting various parameters of the device. Using this device, we quantified the hippocampal axonal response to substrate-bound laminin and ephrin-A5 gradients with varying steepnesses. Our results provided more detailed information on how and to what extent different steepnesses guide hippocampal neuron development during the initial outgrowth. Furthermore, our results show that axons can sensitively respond to very shallow laminin and ephrin-A5 gradients, which could effectively initiate biased differentiation of hippocampal neurons in the steepness range investigated in this study.

In situ generation of tunable porosity gradients in hydrogel-based scaffolds for microfluidic cell culture

Compared with preformed anisotropic matrices, an anisotropic matrix that allows users to alter its properties and structure in situ after synthesis offers the important advantage of being able to mimic dynamic in vivo microenvironments, such as in tissues undergoing morphogenesis or in wounds undergoing tissue repair. In this study, porous gradients are generated in situ in a hydrogel comprising enzymatically crosslinked gelatin hydroxyphenylpropionic acid (GTN-HPA) conjugate and carboxylmethyl cellulose tyramine (CMC-TYR) conjugate. The GTN-HPA component acts as the backbone of the hydrogel, while CMC-TYR acts as a biocompatible sacrificial polymer. The hydrogel is then used to immobilize HT1080 human fibrosarcoma cells in a microfluidic chamber. After diffusion of a biocompatible cellulase enzyme through the hydrogel in a spatially controlled manner, selective digestion of the CMC component of the hydrogel by the cellulase gives rise to a porosity gradient in situ instead of requiring its formation during hydrogel synthesis as with other methods. The influence of this in situ tunable porosity gradient on the chemotactic response of cancer cells is subsequently studied both in the absence and presence of chemoattractant. This platform illustrates the potential of hydrogel-based microfluidics to mimic the 3D in vivo microenvironment for tissue engineering and diagnostic applications.

Microfabrication and microfluidics for muscle tissue models

The relatively recent development of microfluidic systems with wide-ranging capabilities for generating realistic 2D or 3D systems with single or multiple cell types has given rise to an extensive collection of platform technologies useful in muscle tissue engineering. These new systems are aimed at (i) gaining fundamental understanding of muscle function, (ii) creating functional muscle constructs in vitro, and (iii) utilizing these constructs a variety of applications. Use of microfluidics to control the various stimuli that promote differentiation of multipotent cells into cardiac or skeletal muscle is first discussed. Next, systems that incorporate muscle cells to produce either 2D sheets or 3D tissues of contractile muscle are described with an emphasis on the more recent 3D platforms. These systems are useful for fundamental studies of muscle biology and can also be incorporated into drug screening assays. Applications are discussed for muscle actuators in the context of microrobotics and in miniaturized biological pumps. Finally, an important area of recent study involves coculture with cell types that either activate muscle or facilitate its function. Limitations of current designs and the potential for improving functionality for a wider range of applications is also discussed, with a look toward using current understanding and capabilities to design systems of greater realism, complexity and functionality.

Small-molecule axon-polarization studies enabled by a shear-free microfluidic gradient generator

A deep understanding of the mechanisms behind neurite polarization and axon path-finding is important for interpreting how the human body guides neurite growth during development and response to injury. Further, it is of great clinical importance to identify diffusible chemical cues that promote neurite regeneration for nervous tissue repair. Despite the fast development of various types of concentration gradient generators, it has been challenging to fabricate neuron-friendly (i.e. shear-free and biocompatible for neuron growth and maturation) devices to create stable gradients, particularly for fast diffusing small molecules, which typically require high flow and shear rates. Here we present a finite element analysis for a polydimethylsiloxane/polyethylene glycol diacrylate (PDMS/PEG-DA) based gradient generator, describe the microfabrication process, and validate its use for neuronal axon polarization studies. This device provides a totally shear-free, biocompatible microenvironment with a linear and stable concentration gradient of small molecules such as forskolin. The gradient profile in this device can be customized by changing the composition or width of the PEG-DA barriers during direct UV photo-patterning within a permanently bonded PDMS device. Primary rat cortical neurons (embryonic E18) exposed to soluble forskolin gradients for 72 h exhibited statistically significant polarization and guidance of their axons. This device provides a useful platform for both chemotaxis and directional guidance studies, particularly for shear sensitive and non-adhesive cell cultures, while allowing fast new device design prototyping at a low cost.

Monitoring the differentiation and migration patterns of neural cells derived from human embryonic stem cells using a microfluidic culture system

Microfluidics can provide unique experimental tools to visualize the development of neural structures within a microscale device, which is followed by guidance of neurite growth in the axonal isolation compartment. We utilized microfluidics technology to monitor the differentiation and migration of neural cells derived from human embryonic stem cells (hESCs). We co-cultured hESCs with PA6 stromal cells, and isolated neural rosette-like structures, which subsequently formed neurospheres in suspension culture. Tuj1-positive neural cells, but not nestin-positive neural precursor cells (NPCs), were able to enter the microfluidics grooves (microchannels), suggesting that neural cell-migratory capacity was dependent upon neuronal differentiation stage. We also showed that bundles of axons formed and extended into the microchannels. Taken together, these results demonstrated that microfluidics technology can provide useful tools to study neurite outgrowth and axon guidance of neural cells, which are derived from human embryonic stem cells.

Measuring physical properties of neuronal and glial cells with resonant microsensors

Microelectromechanical systems (MEMS) resonant sensors provide a high degree of accuracy for measuring the physical properties of chemical and biological samples. These sensors enable the investigation of cellular mass and growth, though previous sensor designs have been limited to the study of homogeneous cell populations. Population heterogeneity, as is generally encountered in primary cultures, reduces measurement yield and limits the efficacy of sensor mass measurements. This paper presents a MEMS resonant pedestal sensor array fabricated over through-wafer pores compatible with vertical flow fields to increase measurement versatility (e.g., fluidic manipulation and throughput) and allow for the measurement of heterogeneous cell populations. Overall, the improved sensor increases capture by 100% at a flow rate of 2 μL/min, as characterized through microbead experiments, while maintaining measurement accuracy. Cell mass measurements of primary mouse hippocampal neurons in vitro, in the range of 0.1-0.9 ng, demonstrate the ability to investigate neuronal mass and changes in mass over time. Using an independent measurement of cell volume, we find cell density to be approximately 1.15 g/mL.

Micro-patterning of mammalian cells on suspended MEMS resonant sensors for long-term growth measurements

MEMS resonant mass sensors can measure the mass of individual cells, though long-term growth measurements are limited by the movement of cells off the sensor area. Micro-patterning techniques are a powerful approach to control the placement of individual cells in an arrayed format. In this work we present a method for micro-patterning cells on fully suspended resonant sensors through select functionalization and passivation of the chip surface. This method combines high-resolution photolithography with a blanket transfer technique for applying photoresist to avoid damaging the sensors. Cells are constrained to the patterned collagen area on the sensor by pluronic acting as a cell adhesion blocker. This micro-patterning method enables long-term growth measurements, which is demonstrated by a measurement of the change in mass of a human breast cancer cell over 18 h.

A microfluidic device mimicking acinar concentration gradients across the liver acinus

The acinus-mimicking microfluidic chip, which simulates the in vivo condition of the liver, was developed and reported in this paper. The gradient microenvironment of the liver acinus is replicated within this proposed microfluidic chip. The advantage of this acinus-mimicking chip is capable of adjusting the concentration gradient in a relatively short period of time at around 10 s. At the same instance the non-linear concentration gradient can be presented in the various zones within this microfluidic chip. The other advantage of this proposed design is in the convenience of allowing the direct injection of the cells into the chip. The environment within the chip is multi-welled and gel-free with high cell density. The multi-row pillar microstructure located at the entrance of the top and bottom flow channels is designed to be able to balance the pressure of the perfusion medium. Through this mechanism the shear stress experienced by the cultured cells can be minimized to reduce the potential damage flow from the perfusion process. The fluorescence staining and the observations of the cell morphology verify the life and death of the cells. The shear stress experienced by the cells in the various zones within the chip can be effectively mapped. The serum glutamic oxaloacetic transaminase (SGOT) collected from the supernatants was used to determine the effects of the degassing process and the shear stress of the medium flow on the cultured cells.

Surface-printed microdot array chips for the quantification of axonal collateral branching of a single neuron in vitro

Precise and quantitative control of extracellular signalling cues using surface-engineered chips has facilitated various neurobiological assays in vitro. Although the formation of axon collateral branches is important for the establishment and refinement of the neuronal connections during the development and regeneration, surface designs for controlling branch phenotypes have been rarely proposed. In this work, we fabricated a surface-printed microdot array for controlling axon branch formation. Following the culture of hippocampal neurons on a 5 μm dot array patterned by micro-contact printing of poly-d-lysine, we found that most axon collateral branches were initiated from axonal regions on a microdot and terminated on neighboring dots. In addition, the length of branches increased as the spacing between dots increased. Surprisingly, other morphological features were not significantly different from the neurons cultured on a conventional unpatterned surface. Further investigation of this phenomenon indicated that the branch-forming machineries, such as actin patches, were focused on the dot. According to these investigations, we concluded that discontinuous adhesion spots given by dot arrays arranged the branching formation on the expectable location and direction. Therefore, microdot arrays will be applicable as the surface design parameter of bio-chip platforms to reduce branching complexity and quantize branching formation for the simple and easy assay in neurobiological studies.

Neural cell alignment by patterning gradients of the extracellular matrix protein laminin

Anisotropic orientation and accurate positioning of neural cells is achieved by patterning stripes of the extracellular matrix protein laminin on the surface of polystyrene tissue culture dishes by micromoulding in capillaries (MIMICs). Laminin concentration decreases from the entrance of the channels in contact with the reservoir towards the end. Immunofluorescence analysis of laminin shows a decreasing gradient of concentration along the longitudinal direction of the stripes. The explanation is the superposition of diffusion and convection of the solute, the former dominating at length scales near the entrance (characteristic length around 50 μm), the latter further away (length scale in excess of 900 μm). These length scales are independent of the channel width explored from about 15 to 45 μm. Neural cells are randomly seeded and selectively adhere to the pattern, leaving the unpatterned areas depleted even upon 6 days of incubation. Cell alignment was assessed by the orientation of the long axis of the 4',6-diamidino-2-phenylindole-stained nuclei. Samples on patterned the laminin area exhibit a large orientational order parameter. As control, cells on the unpatterned laminin film exhibit no preferential orientation. This implies that the anisotropy of laminin stripes is an effective chemical stimulus for cell recruiting and alignment.

A review of microfabrication and hydrogel engineering for micro-organs on chips

This review highlights recent trends towards the development of in vitro multicellular systems with definite architectures, or "organs on chips". First, the chemical composition and mechanical properties of the scaffold have to be consistent with the anatomical environment in vivo. In this perspective, the flourishing interest in hydrogels as cellular substrates has highlighted the main parameters directing cell differentiation that need to be recapitulated in artificial matrix. Another scaffold requirement is to act as a template to guide tissue morphogenesis. Therefore specific microfabrication techniques are required to spatially pattern the environment at microscale. 2D patterning is particularly efficient for organizing planar polarized cell types such as endothelial cells or neurons. However, most organs are characterized by specific sub units organized in three dimensions at the cellular level. The reproduction of such 3D patterns in vitro is necessary for cells to fully differentiate, assemble and coordinate to form a coherent micro-tissue. These physiological microstructures are often integrated in microfluidic devices whose controlled environments provide the cell culture with more life-like conditions than traditional cell culture methods. Such systems have a wide range of applications, for fundamental research, as tools to accelerate drug development and testing, and finally, for regenerative medicine.

Superimposed topographic and chemical cues synergistically guide neurite outgrowth

Guidance of neuronal extensions is a complex process essential for linking neurons into complex functional networks underlying the workings of the neural system. Decades of research have suggested the ability of neuronal growth cones to integrate multiple types of cues during the extension process, but also have raised numerous still unanswered questions about synergy or antagonism between the superimposed chemical and mechanical signaling inputs. In this study, using a novel microfabricated analysis platform, we investigate the response of primary mouse embryonic hippocampal neurons to superimposed topographic and soluble chemical cues. We find that an optimal spatial frequency of topographic cues exists, maximizing the precision of the neurite extension. This optimal frequency can help the extending neurites navigate a topographically complex environment, providing pronounced directional selectivity. We also demonstrate that this cue can synergistically enhance attractive and suppress repulsive guidance by the bi-functional soluble cue Netrin-1, and eliminate the repulsive guidance by a chemorepellent Semaphorin3A (Sema3A). These results suggest that topographic cues can provide optimal periodic input into the guidance signaling processes involved in growth cone chemoattraction and can synergistically interact with chemical gradients of soluble guidance cues, shedding light on complex events accompanying the development of the functional nervous system.

Simultaneous generation of gradients with gradually changed slope in a microfluidic device for quantifying axon response

Over the past decades, various microfluidic devices have been developed to investigate the role of the molecular gradient in axonal development; however, there are very few devices providing quantitative information about the response of axons to molecular gradients with different slopes. Here, we propose a novel laminar-based microfluidic device enabling simultaneous generation of multiple gradients with gradually changed slope on a single chip. This device, with two asymmetrically designed peripheral channels and opposite flow direction, could generate gradients with gradually changed slope in the center channel, enabling us to investigate simultaneously the response of axons to multiple slope gradients with the same batch of neurons. We quantitatively investigated the response of axon growth rate and growth direction to substrate-bound laminin gradients with different slopes using this single-layer chip. Furthermore, we compartmented this gradient generation chip and a cell culture chip by a porous membrane to investigate quantitatively the response of axon growth rate to the gradient of soluble factor netrin-1. The results suggested that contacting with a molecular gradient would effectively accelerate neurites growth and enhance axonal formation, and the axon guidance ratio obviously increased with the increase of gradient slope in a proper range. The capability of generating a molecular gradient with continuously variable slopes on a single chip would open up opportunities for obtaining quantitative information about the sensitivity of axons and other types of cells in response to gradients of various proteins.

Microfluidic platforms for mechanobiology

Mechanotransduction has been a topic of considerable interest since early studies demonstrated a link between mechanical force and biological response. Until recently, studies of fundamental phenomena were based either on in vivo experiments with limited control or direct access, or on large-scale in vitro studies lacking many of the potentially important physiological factors. With the advent of microfluidics, many of the previous limitations of in vitro testing were eliminated or reduced through greater control or combined functionalities. At the same time, imaging capabilities were tremendously enhanced. In this review, we discuss how microfluidics has transformed the study of mechanotransduction. This is done in the context of the various cell types that exhibit force-induced responses and the new biological insights that have been elucidated. We also discuss new microfluidic studies that could produce even more realistic models of in vivo conditions by combining multiple stimuli or creating a more realistic microenvironment.

Control of neural network patterning using collagen gel photothermal etching

Two-dimensional (2D) micropatterning techniques have been developed to guide dissociated neurons into predefined distributions on solid substrates, such as glass and plastic. Micropatterning methods using three-dimensional (3D) substrates or scaffolds that reproduce aspects of the in vivo microenvironment could facilitate the engineering of functional tissues for transplantation or more robust experimental models. We developed a 3D collagen gel photothermal etching method using an infrared laser that precisely controls the area of cell adhesion and neurite projection by etching a small targeted section of the collage gel. It was then possible to guide neural network formation under microscopic observation. After conventional cell seeding, we succeeded in creating isolated 3D networks, while controlling (1) the number of each neural subtype (neurons, glia, and fluorescently-labeled neurons) and (2) the direction of neurite elongation. Neurons seeded on a 10-μm-thick collagen gel survived longer and projected greater numbers of neurites than neurons growing on 2D culture substrates. Intracellular Ca(2+) imaging revealed both synchronous and discordant oscillations in different neuronal populations that suggested the pattern and strength of synaptic connectivity. This photothermal etching technique allows for the creation of designed 3D neural networks during cultivation for use in studies of synaptic transmission, neuron-glial signaling, pathogenesis, and drug responses.

Quantitative reflection imaging of fixed Aplysia californica pedal ganglion neurons on nanostructured plasmonic crystals

Studies of the interactions between cells and surrounding environment including cell culture surfaces and their responses to distinct chemical and physical cues are essential to understanding the regulation of cell growth, migration, and differentiation. In this work, we demonstrate the capability of a label-free optical imaging technique-surface plasmon resonance (SPR)-to quantitatively investigate the relative thickness of complex biomolecular structures using a nanoimprinted plasmonic crystal and laboratory microscope. Polyelectrolyte films of different thicknesses deposited by layer-by-layer assembly served as the model system to calibrate the reflection contrast response originating from SPRs. The calibrated SPR system allows quantitative analysis of the thicknesses of the interface formed between the cell culture substrate and cellular membrane regions of fixed Aplysia californica pedal ganglion neurons. Bandpass filters were used to isolate spectral regions of reflected light with distinctive image contrast changes. Combining of the data from images acquired using different bandpass filters leads to increase image contrast and sensitivity to topological differences in interface thicknesses. This SPR-based imaging technique is restricted in measurable thickness range (∼100-200 nm) due to the limited plasmonic sensing volume, but we complement this technique with an interferometric analysis method. Described here simple reflection imaging techniques show promise as quantitative methods for analyzing surface thicknesses at nanometer scale over large areas in real-time and in physicochemical diverse environments.

Microfluidic systems for studying neurotransmitters and neurotransmission

Neurotransmitters and neuromodulators are molecules within the nervous system that play key roles in cell-to-cell communication. Upon stimulation, neurons release these signaling molecules, which then act at local or distant locations to elicit a physiological response. Ranging from small molecules, such as diatomic gases and amino acids, to larger peptides, these chemical messengers are involved in many functional processes including growth, reproduction, memory and behavior. Understanding signaling molecules and the conditions that govern their release in healthy or damaged networks promises to deliver insights into neural network formation and function. Microfluidic devices can provide optimal cell culture conditions, reduced volume systems, and precise control over the chemical and physical nature of the extracellular environment, making them well-suited for studying neurotransmission and other forms of cell-to-cell signaling. Here we review selected microfluidic approaches that are suitable for monitoring cell-to-cell signaling molecules. We highlight devices that improve in vivo sample collection as well as compartmentalized devices designed to isolate individual neurons or co-cultures in vitro, including a focus on systems used for studying neural injury and regeneration, and devices that allow selective chemical stimulations and the characterization of released molecules.

Microfluidic investigation of BDNF-enhanced neural stem cell chemotaxis in CXCL12 gradients

In vivo studies have suggested that gradients of CXCL12 (aka stromal cell-derived factor 1α) may be critical for neural stem cell (NSC) migration during brain development and neural tissue regeneration. However, traditional in vitro chemotaxis tools are limited by unstable concentration gradients and the inability to decouple cell migration directionality and speed. These limitations have restricted the reproducible and quantitative analysis of neuronal migration, which is required for mechanism-based studies. Using a microfluidic gradient generator, nestin and Sox-2 positive human embryonic NSC chemotaxis is quantified within a linear and stable CXCL12 gradient. While untreated NSCs are not able to chemotax within CXCL12 gradients, pre-treatment of the cells with brain-derived neurotrophic factor (BDNF) results in significant chemotactic, directional migration. BDNF pre-treatment has no effect on cell migration speed, which averages about 1 μm min(-1). Quantitative analysis determines that CXCL12 concentrations above 9.0 nM are above the minimum activation threshold, while concentrations below 14.7 nM are below the saturation threshold. Interestingly, although inhibitor studies with AMD 3100 revealed that CXCL12 chemotaxis requires receptor CXCR4 activation, BDNF pre-treatment is found to have no profound effects on the mRNA levels or surface presentation of CXCR4 or the putative CXCR7 scavenger receptor. The microfluidic study of NSC migration within stable chemokine concentration profiles provides quantitative analysis as well as new insight into the migratory mechanism underlying BDNF-induced chemotaxis towards CXCL12.

Advances in microfluidics-based experimental methods for neuroscience research

The application of microfluidics to neuroscience applications has always appealed to neuroscientists because of the capability to control the cellular microenvironment in both a spatial and temporal manner. Recently, there has been rapid development of biological micro-electro-mechanical systems (BioMEMS) for both fundamental and applied neuroscience research. In this review, we will discuss the applications of BioMEMS to various topics in the field of neuroscience. The purpose of this review is to summarise recent advances in the components and design of the BioMEMS devices, in vitro disease models, electrophysiology and neural stem cell research. We envision that microfluidics will play a key role in future neuroscience research, both fundamental and applied research.

Microfluidic devices for cell cultivation and proliferation

Microfluidic technology provides precise, controlled-environment, cost-effective, compact, integrated, and high-throughput microsystems that are promising substitutes for conventional biological laboratory methods. In recent years, microfluidic cell culture devices have been used for applications such as tissue engineering, diagnostics, drug screening, immunology, cancer studies, stem cell proliferation and differentiation, and neurite guidance. Microfluidic technology allows dynamic cell culture in microperfusion systems to deliver continuous nutrient supplies for long term cell culture. It offers many opportunities to mimic the cell-cell and cell-extracellular matrix interactions of tissues by creating gradient concentrations of biochemical signals such as growth factors, chemokines, and hormones. Other applications of cell cultivation in microfluidic systems include high resolution cell patterning on a modified substrate with adhesive patterns and the reconstruction of complicated tissue architectures. In this review, recent advances in microfluidic platforms for cell culturing and proliferation, for both simple monolayer (2D) cell seeding processes and 3D configurations as accurate models of in vivo conditions, are examined.

Over a century of neuron culture: from the hanging drop to microfluidic devices

The brain is the most intricate, energetically active, and plastic organ in the body. These features extend to its cellular elements, the neurons and glia. Understanding neurons, or nerve cells, at the cellular and molecular levels is the cornerstone of modern neuroscience. The complexities of neuron structure and function require unusual methods of culture to determine how aberrations in or between cells give rise to brain dysfunction and disease. Here we review the methods that have emerged over the past century for culturing neurons in vitro, from the landmark finding by Harrison (1910) - that neurons can be cultured outside the body - to studies utilizing culture vessels, micro-islands, Campenot and brain slice chambers, and microfluidic technologies. We conclude with future prospects for neuronal culture and considerations for advancement. We anticipate that continued innovation in culture methods will enhance design capabilities for temporal control of media and reagents (chemotemporal control) within sub-cellular environments of three-dimensional fluidic spaces (microfluidic devices) and materials (e.g., hydrogels). They will enable new insights into the complexities of neuronal development and pathology.

Generating multiplex gradients of biomolecules for controlling cellular adhesion in parallel microfluidic channels

Here we present a microfluidic platform to generate multiplex gradients of biomolecules within parallel microfluidic channels, in which a range of multiplex concentration gradients with different profile shapes are simultaneously produced. Nonlinear polynomial gradients were also generated using this device. The gradient generation principle is based on implementing parrallel channels with each providing a different hydrodynamic resistance. The generated biomolecule gradients were then covalently functionalized onto the microchannel surfaces. Surface gradients along the channel width were a result of covalent attachments of biomolecules to the surface, which remained functional under high shear stresses (50 dyn/cm(2)). An IgG antibody conjugated to three different fluorescence dyes (FITC, Cy5 and Cy3) was used to demonstrate the resulting multiplex concentration gradients of biomolecules. The device enabled generation of gradients with up to three different biomolecules in each channel with varying concentration profiles. We were also able to produce 2-dimensional gradients in which biomolecules were distributed along the length and width of the channel. To demonstrate the applicability of the developed design, three different multiplex concentration gradients of REDV and KRSR peptides were patterned along the width of three parallel channels and adhesion of primary human umbilical vein endothelial cell (HUVEC) in each channel was subsequently investigated using a single chip.

A bio-inspired neural environment to control neurons comprising radial glia, substrate chemistry and topography

Achieving alignment of cells is key to the success of regenerative strategies of neural tissue. We report a high-throughput method to investigate neural cell response to surface chemistry overlaid orthogonally onto a gradient of gradually changing groove widths. Using a bio-inspired approach wherein radial glial cells, which naturally guide neurons in the developing brain, enhance the attachment and directional outgrowth of neurons, we show the differences in the interaction and cellular response of glia, neurons and co-cultured cells. Radial glia were found to preferentially reside in grooves of width 6-35 μm with greater alignment to grooves <10 μm on the hydrophobic and hydrophilic extremes of chemistry. When neurons were sequentially cultured after radial glia, they showed enhanced alignment compared to when they were cultured alone, for all chemistries and groove widths. This is not dependent on co-localisation of the neurons with glia suggesting the radial glial cells pre-condition the substrate giving rise to enhanced attachment and alignment of subsequently cultured neurons. The results indicate a dependence of both primary radial glia and neuron responses on surface chemistry and micro-groove width. Grooved surfaces (width 5-10 μm) of mid-range wettability show the greatest potential to significantly enhance axonal alignment and, therefore, potential regeneration, when pre-conditioned by radial glia, highlighting the importance of surface engineering for neural scaffolds.

New perspectives on neuronal development via microfluidic environments

Understanding the signals that guide neuronal development and direct formation of axons, dendrites, and synapses during wiring of the brain is a fundamental challenge in developmental neuroscience. Discovery of how local signals shape developing neurons has been impeded by the inability of conventional culture methods to interrogate microenvironments of complex neuronal cytoarchitectures, where different subdomains encounter distinct chemical, physical, and fluidic features. Microfabrication techniques are facilitating the creation of microenvironments tailored to neuronal structures and subdomains with unprecedented access and control. The design, fabrication, and properties of microfluidic devices offer significant advantages for addressing unresolved issues of neuronal development. These high-resolution approaches are poised to contribute new insights into mechanisms for restoring neuronal function and connectivity compromised by injury, stress, and neurodegeneration.

Multi-compartment neuron-glia co-culture platform for localized CNS axon-glia interaction study

Formation of myelin sheaths by oligodendrocytes (OLs) in the central nervous system (CNS) is essential for rapid nerve impulse conduction. Reciprocal signaling between axons and OLs orchestrates myelinogenesis but remains largely elusive. In this study, we present a multi-compartment CNS neuron-glia microfluidic co-culture platform. The platform is capable of conducting parallel localized drug and biomolecule treatments while carrying out multiple co-culture conditions in a single device for studying axon-glia interactions at a higher throughput. The "micro-macro hybrid soft-lithography master fabrication" (MMHSM) technique enables a large number of precisely replicated PDMS devices incorporating both millimeter and micrometer scale structures to be rapidly fabricated without any manual reservoir punching processes. Axons grown from the neuronal somata were physically and fluidically isolated inside the six satellite axon/glia compartments for localized treatments. Astrocytes, when seeded and co-cultured after the establishment of the isolated axons in the satellite axon/glia compartments, were found to physically damage the established axonal layer, as they tend to grow underneath the axons. In contrast, oligodendrocyte progenitor cells (OPCs) could be co-cultured successfully with the isolated axons and differentiated into mature myelin basic protein-expressing OLs with processes aligning to neighboring axons. OPCs inside the six axon/glia compartments were treated with a high concentration of ceramide (150 μM) to confirm the fluidic isolation among the satellite compartments. In addition, isolated axons were treated with varying concentrations of chondroitin sulfate proteoglycan (CSPG, 0-25 μg ml(-1)) within a single device to demonstrate the parallel localized biomolecular treatment capability of the device. These results indicate that the proposed platform can be used as a powerful tool to study CNS axonal biology and axon-glia interactions with the capacity for localized biomolecular treatments.

A Novel Technique for Micro-patterning Proteins and Cells on Polyacrylamide Gels

Spatial patterning of proteins (extracellular matrix, ECM) for living cells on polyacrylamide (PA) hydrogels has been technically challenging due to the compliant nature of the hydrogels and their aqueous environment. Traditional micro-fabrication process is not applicable. Here we report a simple, novel and general method to pattern a variety of commonly used cell adhesion molecules, i.e. Fibronectin (FN), Laminin (LN) and Collagen I (CN), etc. on PA gels. The pattern is first printed on a hydrophilic glass using polydimethylsiloxane (PDMS) stamp and micro-contact printing (μCP). Pre-polymerization solution is applied on the patterned glass and is then sandwiched by a functionalized glass slide, which covalently binds to the gel. The hydrophilic glass slide is then peeled off from the gel when the protein patterns detach from the glass, but remain intact with the gel. The pattern is thus transferred to the gel. The mechanism of pattern transfer is studied in light of interfacial mechanics. It is found that hydrophilic glass offers strong enough adhesion with ECM proteins such that a pattern can be printed, but weak enough adhesion such that they can be completely peeled off by the polymerized gel. This balance is essential for successful pattern transfer. As a demonstration, lines of FN, LN and CN with widths varying from 5-400 μm are patterned on PA gels. Normal fibroblasts (MKF) are cultured on the gel surfaces. The cell attachment and proliferation are confined within these patterns. The method avoids the use of any toxic chemistry often used to pattern different proteins on gel surfaces.

Quantitative analysis of axonal transport by using compartmentalized and surface micropatterned culture of neurons

Mitochondria, synaptic vesicles, and other cytoplasmic constituents have to travel long distance along the axons from cell bodies to nerve terminals. Interruption of this axonal transport may contribute to many neurodegenerative diseases including Alzheimer's disease (AD). It has been recently shown that exposure of cultured neurons to β-amyloid (Aβ) resulted in severe impairment of mitochondrial transport. This Letter describes an integrated microfluidic platform that establishes surface patterned and compartmentalized culture of neurons for studying the effect of Aβ on mitochondria trafficking in full length of axons. We have successfully quantified the trafficking of fluorescently labeled mitochondria in distal and proximal axons using image processing. Selective treatment of Aβ in the somal or axonal compartments resulted in considerable decrease in mitochondria movement in a location dependent manner such that mitochondria trafficking slowed down more significantly proximal to the location of Aβ exposure. Furthermore, this result suggests a promising application of microfluidic technology for investigating the dysfunction of axonal transport related to neurodegenerative diseases.

Label-free quantitation of peptide release from neurons in a microfluidic device with mass spectrometry imaging

Microfluidic technology allows the manipulation of mass-limited samples and when used with cultured cells, enables control of the extracellular microenvironment, making it well suited for studying neurons and their response to environmental perturbations. While matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) provides for off-line coupling to microfluidic devices for characterizing small-volume extracellular releasates, performing quantitative studies with MALDI is challenging. Here we describe a label-free absolute quantitation approach for microfluidic devices. We optimize device fabrication to prevent analyte losses before measurement and then incorporate a substrate that collects the analytes as they flow through a collection channel. Following collection, the channel is interrogated using MS imaging. Rather than quantifying the sample present via MS peak height, the length of the channel containing appreciable analyte signal is used as a measure of analyte amount. A linear relationship between peptide amount and band length is suggested by modeling the adsorption process and this relationship is validated using two neuropeptides, acidic peptide (AP) and α-bag cell peptide [1-9] (αBCP). The variance of length measurement, defined as the ratio of standard error to mean value, is as low as 3% between devices. The limit of detection (LOD) of our system is 600 fmol for AP and 400 fmol for αBCP. Using appropriate calibrations, we determined that an individual Aplysia bag cell neuron secretes 0.15 ± 0.03 pmol of AP and 0.13 ± 0.06 pmol of αBCP after being stimulated with elevated KCl. This quantitation approach is robust, does not require labeling, and is well suited for miniaturized off-line characterization from microfluidic devices.

Microfluidic approaches for gene delivery and gene therapy

Recent advances in microfluidics have created new and exciting prospects for gene delivery and therapy. The micro-scaled environment within microfluidic systems enables precise control and optimization of multiple processes and techniques used in gene transfection and the production of gene and drug transporters. Traditional non-viral gene transfection methods, such as electroporation, microinjection and optical gene transfection, are improved from the use of innovative microfluidic systems. Additionally, microfluidic systems have also made the production of many viral and non-viral vectors controlled, automated, and reproducible. In summary, the development and application of microfluidic systems are producing increased efficiency in gene delivery and promise improved gene therapy results.

Pattern analysis and spatial distribution of neurons in culture

The nervous system is a complex, highly-ordered, integrated network of cells. Dispersed cultures of neurons enable investigations into intrinsic cellular functions without the complexities inherent in the intact nervous system. This culture process generates a homogeneously dispersed population that is assumed to be spatially random. Despite the vast number of studies utilizing dispersed neurons, few studies address the spatial distribution of large populations of neurons, in vitro. We used ink-jet printing and surface chemistry to define patterned areas of poly-lysine adhesion (∼50 μm spots) juxtaposed against a fluorinated-silane background. We quantitatively analysed populations of patterned neurons on printed protein spots, and unpatterned neurons. Using a microarray scanner, we acquired large images (72 mm × 22 mm) of patterns, and neurons with and without patterns. Fast Fourier transformation (FFT) image analysis was used to determine global alignment of neurons to patterns. Through point pattern analysis, we described the spatial organization of dispersed neurons with, or without, patterned substrates. Patterned neurons show spatial organization characteristics reminiscent of printed patterns, with spatial distributions representative of unpatterned neurons. Most notably, both patterned and unpatterned neurons show departure from null models of complete spatial randomness (CSR; a homogeneous Poisson process) at shorter distances with conformity to CSR occurring at longer distances. Cellular morphometrics show that when compared to their unpatterned counterparts, spot-patterned neurons exhibit a significant increase (p < 0.0001) in the mean dendritic circularity and an increase in the number of more circular neurons. Through neurite tracing, we show that dendritic processes are also highly confined to patterned areas, and that they are on average 58% shorter than dendrites of neurons without patterns. Our findings show that patterned areas change the spatial organization of the somata and dendrites of cultured neurons, and that traditional neuronal cultures deviate from CSR.

Manipulating location, polarity, and outgrowth length of neuron-like pheochromocytoma (PC-12) cells on patterned organic electrode arrays

In this manuscript, we describe a biocompatible organic electrode system, comprising poly(3,4-ethylenedioxythiophene) (PEDOT) microelectrode arrays on indium tin oxide (ITO) glass, that can be used to regulate the neuron type, location, polarity, and outgrown length of neuron-like cells (PC-12). We fabricated a PEDOT microelectrode array with four different sizes (flat; 20, 50, and 100 μm) through electrochemical polymerization. Extracellular matrix proteins absorbed well on these organic electrodes; cells absorbed selectively on the organic electrodes when we used polyethylene oxide/polypropylene oxide/polyethylene oxide triblock copolymers (PEO/PPO/PEO, Pluronic™ F108) as the anti-adhesive coating. In this system, the neurite polarities and neuron types could be manipulated by varying the width of the PEDOT microelectrode arrays. On the unpatterned PEDOT electrode, PC-12 cells were randomly polarized, with approximately 80% having multi-polar cell types. In contrast, when we cultured PC-12 cells on the 20 μm wide PEDOT line array, the neurites aligned along the direction of the organic electrodes, with the percentage of uni- and bipolar PC-12 cells increasing to greater than 90%. The outgrowth of neurites on the microelectrodes was promoted by ~60% with an applied electrical stimulation. Therefore, these electroactive PEDOT microelectrode arrays have potential for use in tissue engineering related to the development and regeneration of mammalian nervous systems.

Synergistic NGF/B27 gradients position synapses heterogeneously in 3D micropatterned neural cultures

Native functional brain circuits show different numbers of synapses (synaptic densities) in the cerebral cortex. Until now, different synaptic densities could not be studied in vitro using current cell culture methods for primary neurons. Herein, we present a novel microfluidic based cell culture method that combines 3D micropatterning of hydrogel layers with linear chemical gradient formation. Micropatterned hydrogels were used to encapsulate dissociated cortical neurons in laminar cell layers and neurotrophic factors NGF and B27 were added to influence the formation of synapses. Neurotrophic gradients allowed for the positioning of distinguishable synaptic densities throughout a 3D micropatterned neural culture. NGF and B27 gradients were maintained in the microfluidic device for over two weeks without perfusion pumps by utilizing a refilling procedure. Spatial distribution of synapses was examined with a pre-synaptic marker to determine synaptic densities. From our experiments, we observed that (1) cortical neurons responded only to synergistic NGF/B27 gradients, (2) synaptic density increased proportionally to synergistic NGF/B27 gradients; (3) homogeneous distribution of B27 disturbed cortical neurons in sensing NGF gradients and (4) the cell layer position significantly impacted spatial distribution of synapses.

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.

Quantitative photochemical immobilization of biomolecules on planar and corrugated substrates: a versatile strategy for creating functional biointerfaces

Methods for the generation of substratespresenting biomolecules in a spatially controlled manner are enabling tools for applications in biosensor systems, microarray technologies, fundamental biological studies and biointerface science. We have implemented a method to create biomolecular patterns by using light to control the direct covalent immobilization of biomolecules onto benzophenone-modified glass substrates. We have generated substrates presenting up to three different biomolecules patterned in sequence, and demonstrate biomolecular photopatterning on corrugated substrates. The chemistry of the underlying monolayer was optimized to incorporate poly(ethylene glycol) to enable adhesive cell adhesion onto patterned extracellular matrix proteins. Substrates were characterized with contact angle goniometry, AFM, and immunofluorescence microscopy. Importantly, radioimmunoassays were performed to quantify the site density of immobilized biomolecules on photopatterned substrates. Retained function of photopatterned proteins was demonstrated both by native ligand recognition and cell adhesion to photopatterned substrates, revealing that substrates generated with this method are suitable for probing specific cell receptor-ligand interactions. This molecularly general photochemical patterning method is an enabling tool for the creation of substrates presenting both biochemical and topographical variation, which is an important feature of many native biointerfaces.

Patterning small-molecule biocapture surfaces: microcontact insertion printing vs. photolithography

Chemical patterns prepared by self-assembly, combined with soft lithography or photolithography, are directly compared. Pattern fidelity can be controlled in both cases but patterning at the low densities necessary for small-molecule probe capture of large biomolecule targets is better accomplished using microcontact insertion printing (μCIP). Surfaces patterned by μCIP are used to capture biomolecule binding partners for the small molecules dopamine and biotin.

Device for co-culture of sympathetic neurons and cardiomyocytes using microfabrication

Rat superior cervical ganglion (SCG) neurons and ventricular myocytes (VMs) were co-cultured separately in a minichamber placed on a microelectrode-array (MEA) substrate. The minichamber, fabricated photolithographically using polydimethylsiloxane (PDMS), had 2 compartments, 16 microcompartments and 8 microconduits. The SCG neurons were seeded into one of the compartments and all of the microcompartments using a glass pipette controlled by a micromanipulator and a microinjector. The VMs were seeded into the other compartment. Three days after seeding of the VMs, the SCG neurons were still confined to one compartment and all of the microcompartments, and the neurites of the SCG neurons had connected with the VMs via the microconduits. Constant-voltage stimulation, using a train of biphasic square pulses (1 ms at +1 V, followed by -1 ms at 1 V), was applied to the SCG neurons in the microcompartments using 16 electrodes. Evoked responses were observed in several electrodes while electrical stimulation was applied to the SCG neurons. Two-way analysis of variance (ANOVA) revealed that the frequency of the stimulation pulses had significant effects in increasing the beat rate of the VMs, and that the interaction between the frequency and the number of the pulses also had a significant effect on the ratio. No significant increases in the beat rate were observed when propranolol, a β-adrenergic receptor antagonist, was added to the culture medium. These results suggest that synaptic pathways were formed between the SCG neurons and the VMs, and that this co-culture device can be utilized for studies of network-level interactions between sympathetic neurons and cardiomyocytes.

Probing dynamic cell-substrate interactions using photochemically generated surface-immobilized gradients: application to selectin-mediated leukocyte rolling

Model substrates presenting biochemical cues immobilized in a controlled and well-defined manner are of great interest for their applications in biointerface studies that elucidate the molecular basis of cell receptor-ligand interactions. Herein, we describe a direct, photochemical method to generate surface-immobilized biomolecular gradients that are applied to the study of selectin-mediated leukocyte rolling. The technique employs benzophenone-modified glass substrates, which upon controlled exposure to UV light (350-365 nm) in the presence of protein-containing solutions facilitate the generation of covalently immobilized protein gradients. Conditions were optimized to generate gradient substrates presenting P-selectin and PSGL-1 (P-selectin glycoprotein ligand-1) immobilized at site densities over a 5- to 10-fold range (from as low as ∼200 molecules μm(-2) to as high as 6000 molecules μm(-2)). The resulting substrates were quantitatively characterized via fluorescence analysis and radioimmunoassays before their use in the leukocyte rolling assays. HL-60 promyelocytes and Jurkat T lymphocytes were assessed for their ability to tether to and roll on substrates presenting immobilized P-selectin and PSGL-1 under conditions of physiologically relevant shear stress. The results of these flow assays reveal the combined effect of immobilized protein site density and applied wall shear stress on cell rolling behavior. Two-component substrates presenting P-selectin and ICAM-1 (intercellular adhesion molecule-1) were also generated to assess the interplay between these two proteins and their effect on cell rolling and adhesion. These proof-of-principle studies verify that the described gradient generation approach yields well-defined gradient substrates that present immobilized proteins over a large range of site densities that are applicable for investigation of cell-materials interactions, including multi-parameter leukocyte flow studies. Future applications of this enabling methodology may lead to new insights into the biophysical phenomena and molecular mechanism underlying complex biological processes such as leukocyte recruitment and the inflammatory response.

A microfluidic device to establish concentration gradients using reagent density differences

Microfabrication has become widely utilized to generate controlled microenvironments that establish chemical concentration gradients for a variety of engineering and life science applications. To establish microfluidic flow, the majority of existing devices rely upon additional facilities, equipment, and excessive reagent supplies, which together limit device portability as well as constrain device usage to individuals trained in technological disciplines. The current work presents our laboratory-developed bridged μLane system, which is a stand-alone device that runs via conventional pipette loading and can operate for several days without need of external machinery or additional reagent volumes. The bridged μLane is a two-layer polydimethylsiloxane microfluidic device that is able to establish controlled chemical concentration gradients over time by relying solely upon differences in reagent densities. Fluorescently labeled Dextran was used to validate the design and operation of the bridged μLane by evaluating experimentally measured transport properties within the microsystem in conjunction with numerical simulations and established mathematical transport models. Results demonstrate how the bridged μLane system was used to generate spatial concentration gradients that resulted in an experimentally measured Dextran diffusivity of (0.82 ± 0.01) × 10(-6) cm(2)/s.

A high-throughput microfluidic assay to study neurite response to growth factor gradients

Studying neurite guidance by diffusible or substrate bound gradients is challenging with current techniques. In this study, we present the design, fabrication and utility of a microfluidic device to study neurite guidance under chemogradients. Experimental and computational studies demonstrated the establishment of a steep gradient of guidance cue within 30 min and stable for up to 48 h. The gradient was found to be insensitive to external perturbations such as media change and movement of device. The effects of netrin-1 (0.1-10 µg mL(-1)) and brain pulp (0.1 µL mL(-1)) were evaluated for their chemoattractive potential on neurite turning, while slit-2 (62.5 or 250 ng mL(-1)) was studied for its chemorepellant properties. Hippocampal or dorsal root ganglion (DRG) neurons were seeded into a micro-channel and packed onto the surface of a 3D collagen gel. Neurites grew into the matrix in three dimensions, and a gradient of guidance cue was created orthogonal to the direction of neurite growth to impact guidance. The average turning angle of each neurite was measured and averaged across multiple devices cultured under similar conditions to quantify the effect of guidance cue gradient. Significant positive turning towards gradient was measured in the presence of brain pulp and netrin-1 (1 µg mL(-1)), relative to control cultures which received no external guidance cue (p < 0.001). Netrin-1 released from transfected fibroblasts had the most positive turning effect of all the chemoattractive cues tested (p < 0.001). Slit-2 exhibited strong chemorepellant characteristics on both hippocampal and DRG neurite guidance at 250 ng mL(-1) concentration. Slit-2 also showed similar behavior on DRG neuron invasion into 3D collagen gel (p < 0.01 relative to control cultures). Taken together, the results suggest the utility of this microfluidic device to generate stable chemogradients for studying neurobiology, cell migration and proliferation, matrix remodeling and co-cultures with other cell lines, with potential applications in cancer biology, tissue engineering and regenerative medicine.

Distance dependence of neuronal growth on nanopatterned gold surfaces

Understanding network development in the brain is of tremendous fundamental importance, but it is immensely challenging because of the complexity of both its architecture and function. The mechanisms of axonal navigation to target regions and the specific interactions with guidance factors such as membrane-bound proteins, chemical gradients, mechanical guidance cues, etc., are largely unknown. A current limitation for the study of neural network formation is the ability to control precisely the connectivity of small groups of neurons. A first step in designing such networks is to understand the "rules" central nervous system (CNS) neurons use to form functional connections with one another. Here we begin to delineate novel rules for growth and connectivity of small numbers of neurons patterned on Au substrates in simplified geometries. These studies yield new insights into the mechanisms determining the organizational features present in intact systems. We use a previously reported atomic force microscopy (AFM) nanolithography method to control precisely the location and growth of neurons on these surfaces. By examining a series of systems with different geometrical parameters, we quantitatively and systematically analyze how neuronal growth depends on these parameters.

An effective lift-off method for patterning high-density gold interconnects on an elastomeric substrate

High-resolution, high-density gold interconnects are effectively patterned on an elastomeric substrate. A 3cm cable of ten gold wires with 10μm width and 20μm pitch is achieved, successfully demonstrating density increases of more than one order of magnitude from previously established work. Many applications in the fields of stretchable electronics and conformable neural interfaces will benefit from these fabrication developments.