Caged neuron MEA: a system for long-term investigation of cultured neural network connectivity

Design, synthesis, and characterization of novel nanowire structures for photovoltaics and intracellular probes

Semiconductor nanowires (NWs) represent a unique system for exploring phenomena at the nanoscale and are expected to play a critical role in future electronic, optoelectronic, and miniaturized biomedical devices. Modulation of the composition and geometry of nanostructures during growth could encode information or function, and realize novel applications beyond the conventional lithographical limits. This review focuses on the fundamental science aspects of the bottom-up paradigm, which are synthesis and physical property characterization of semiconductor NWs and NW heterostructures, as well as proof-of-concept device concept demonstrations, including solar energy conversion and intracellular probes. A new NW materials synthesis is discussed and, in particular, a new "nanotectonic" approach is introduced that provides iterative control over the NW nucleation and growth for constructing 2D kinked NW superstructures. The use of radial and axial p-type/intrinsic/n-type (p-i-n) silicon NW (Si-NW) building blocks for solar cells and nanoscale power source applications is then discussed. The critical benefits of such structures and recent results are described and critically analyzed, together with some of the diverse challenges and opportunities in the near future. Finally, results are presented on several new directions, which have recently been exploited in interfacing biological systems with NW devices.

Hippocampal networks on reliable patterned substrates

Toward the goal of reproducible live neuronal networks, we investigated the influence of substrate patterns on neuron compliance and network activity. We optimized process parameters of micro-contact printing for reproducible geometric patterns of 10 μm wide lines of polylysine with 4, 6, or 8 connections at a constant square array of nodes overlying the recording electrodes of a multielectrode array (MEA). We hypothesized that an increase in node connections would give the network more inputs resulting in higher neuronal outputs as network spike rates. We also chronically stimulated these networks during development and added astroglia to enhance network activity. Our results show that despite frequent localization of neuron somata over the electrodes, the number of spontaneously active electrodes was reduced 3-fold compared to random networks, independent of pattern complexity. Of the electrodes active, the overall spike rate was independent of pattern complexity, consistent with homeostasis of activity. Lower mean burst rates were seen with higher levels of pattern complexity; however, burst durations increased 1.6-fold with pattern complexity (n=6027 bursts, p<0.001). Inter-burst interval and percentage of active electrodes displaying bursts also increased with pattern complexity. The extra-burst (non-burst or isolated) spike rate increased 4-fold with pattern complexity, but this relationship was reversed with either chronic stimulation or astroglia addition. These studies suggest for the first time that patterns which limit the distribution of branches and inputs are deleterious to activity in a hippocampal network, but that higher levels of pattern complexity promote non-burst activity and favor longer lasting, but fewer bursts.

Introduction to the fractality principle of consciousness and the sentyon postulate

Recently, consciousness research has gained much attention. Indeed, the question at stake is significant: why is the brain not just a computing device, but generates a perception from within? Ambitious endeavors trying to simulate the entire human brain assume that the algorithm will do the trick: as soon as we assemble the brain in a computer and increase the number of operations per time, consciousness will emerge by itself. I disagree with this simplistic representation. My argument emerges from the "atomism paradox": the irreducible space of the consciously perceived world, the endospace is incompatible with the reducible and decomposable architecture of the brain or a computer. I will first discuss the fundamental challenges in current consciousness models and then propose a new model based on the fractality principle: "the whole is in each of its parts". This new model copes with the atomism paradox by implementing an iterative mapping of information from higher order brain structures to smaller scales on the cellular and molecular level, which I will refer to as "fractalization". This information fractalization gives rise to a new form of matter that is conscious ("bright matter"). Bright matter is composed of conscious particles or units named "sentyons". The internal fractality of these sentyons will close a loop (the "psychic loop") in a recurrent fractal neural network (RFNN) that allows for continuous and complete information transformation and sharing between higher order brain structures and the endpoint substrate of consciousness at the molecular level.

Noninvasive neuron pinning with nanopillar arrays

Cell migration in a cultured neuronal network presents an obstacle to selectively measuring the activity of the same neuron over a long period of time. Here we report the use of nanopillar arrays to pin the position of neurons in a noninvasive manner. Vertical nanopillars protruding from the surface serve as geometrically better focal adhesion points for cell attachment than a flat surface. The cell body mobility is significantly reduced from 57.8 μm on a flat surface to 3.9 μm on nanopillars over a 5 day period. Yet, neurons growing on nanopillar arrays show a growth pattern that does not differ in any significant way from that seen on a flat substrate. Notably, while the cell bodies of neurons are efficiently anchored by the nanopillars, the axons and dendrites are free to grow and elongate into the surrounding area to develop a neuronal network, which opens up opportunities for long-term study of the same neurons in connected networks.

Micro-scale and microfluidic devices for neurobiology

The precise spatial and temporal control afforded by microfluidic devices make them uniquely suited as experimental tools for cellular neuroscience. Micro-structures have been developed to direct the placement of cells and small organisms within a device. Microfluidics can precisely define pharmacological microenvironments, mimicking conditions found in vivo with the advantage of defined parameters which are usually difficult to control and manipulate in vivo. These devices are compatible with high-resolution microscopy, are simple to assemble, and are reproducible. In this review we will focus on microfluidic devices that have recently been developed for small, whole organisms such as C. elegans and dissociated cultured neurons. These devices have improved control over the placement of cells or organisms and allowed unprecedented experimental access, enabling novel investigations in neurobiology.

A study of the autofluorescence of parylene materials for microTAS applications

Parylene-C has been widely used as a biocompatible material for microfluidics and micro total analysis system (microTAS) applications in recent decades. However, its autofluorescence can be a major obstacle for parylene-C based devices used in applications requiring sensitive fluorescence detection. In this paper, Parylene-C was compared with other commonly used polymer and plastic materials in microTAS devices for their autofluorescence. We also report here an in-depth study of the behaviors and mechanisms of the autofluorescence of parylene-C, as well as several other commercialized members in the parylene family, including parylene-D, parylene-N and parylene-HT, using epifluorescence microscopy, fluorimeter and infrared spectroscopy. Strong autofluorescence was induced in parylene-C during short-wavelength excitation (i.e. UV excitation). Variation of autofluorescence intensity of parylene-C film was found to be related to both dehydrogenation and photo-oxidation. Moreover, the influence of microfabrication process on parylene-C autofluorescence was also evaluated. Parylene-HT, which exhibits low initial autofluorescence, decreasing autofluorescence behavior under UV excitation and higher UV stability, can be a promising alternative for microTAS applications with fluorescence detection.

Design and Implementation of Functional Nanoelectronic Interfaces With Biomolecules, Cells, and Tissue Using Nanowire Device Arrays

Nanowire FETs (NWFETs) are promising building blocks for nanoscale bioelectronic interfaces with cells and tissue since they are known to exhibit exquisite sensitivity in the context of chemical and biological detection, and have the potential to form strongly coupled interfaces with cell membranes. We present a general scheme that can be used to assemble NWs with rationally designed composition and geometry on either planar inorganic or biocompatible flexible plastic surfaces. We demonstrate that these devices can be used to measure signals from neurons, cardiomyocytes, and heart tissue. Reported signals are in millivolts range, which are equal to or substantially greater than those recorded with either planar FETs or multielectrode arrays, and demonstrate one unique advantage of NW-based devices. Basic studies showing the effect of device sensitivity and cell/substrate junction quality on signal magnitude are presented. Finally, our demonstrated ability to design high-density arrays of NWFETs enables us to map signal at the subcellular level, a functionality not enabled by conventional microfabricated devices. These advances could have broad applications in high-throughput drug assays, fundamental biophysical studies of cellular function, and development of powerful prosthetics.

Manipulating CD4+ T cells by optical tweezers for the initiation of cell-cell transfer of HIV-1

Cell-cell interactions through direct contact are very important for cellular communication and coordination - especially for immune cells. The human immunodeficiency virus type I (HIV-1) induces immune cell interactions between CD4(+) cells to shuttle between T cells via a virological synapse. A goal to understand the process of cell-cell transmission through virological synapses is to determine the cellular states that allow a chance encounter between cells to become a stable cell-cell adhesion. We demonstrate the use of optical tweezers to manipulate uninfected primary CD4(+) T cells near HIV Gag-iGFP transfected Jurkat T cells to probe the determinants that induce stable adhesion. When combined with fast 4D confocal fluorescence microscopy, optical tweezers can be utilized not only to facilitate cell-cell contact, but also to simultaneously track the formation of a virological synapse, and ultimately to probe the events that precede virus transfer.

Surface Engineering and Patterning Using Parylene for Biological Applications

Parylene is a family of chemically vapour deposited polymer with material properties that are attractive for biomedicine and nanobiotechnology. Chemically inert parylene “peel-off” stencils have been demonstrated for micropatterning biomolecular arrays with high uniformity, precise spatial control down to nanoscale resolution. Such micropatterned surfaces are beneficial in engineering biosensors and biological microenvironments. A variety of substituted precursors enables direct coating of functionalised parylenes onto biomedical implants and microfluidics, providing a convenient method for designing biocompatible and bioactive surfaces. This article will review the emerging role and applications of parylene as a biomaterial for surface chemical modification and provide a future outlook.

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

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

Sensory driven multi-neuronal activity and associative learning monitored in an intact CNS on a multielectrode array

The neuronal network controlling feeding behavior in the CNS of the mollusc Lymnaea stagnalis has been extensively investigated using intracellular microelectrodes. Using microelectrodes however it has not been possible to record from large numbers of neurons simultaneously and therefore little is known about the population coding properties of the feeding network. Neither can the relationships between feeding and neuronal networks controlling other behaviors be easily analyzed with microelectrodes. Here we describe a multielectrode array (MEA) technique for recording action potentials simultaneously from up to 60 electrodes on the intact CNS. The preparation consists of the whole CNS connected by sensory nerves to the chemosensory epithelia of the lip and esophagus. From the buccal ganglia, the region of the CNS containing the feeding central pattern generator (CPG), a rhythmic pattern of activity characteristic of feeding was readily induced either by depolarizing an identified feeding-command neuron (the CV1a) or by perfusing the chemosensory epithelia with sucrose, a gustatory stimulus known to activate feeding. Activity induced by sucrose is not restricted to the buccal ganglia but is distributed widely throughout the CNS, notably in ganglia controlling locomotion, a behavior that must be coordinated with feeding. The MEA also enabled us to record electrophysiological consequences of the associative conditioning of feeding behavior. The results suggest that MEA recording from an intact CNS enables distributed, multiple-source neural activity to be analyzed in the context of biologically relevant behavior, behavioral coordination and behavioral plasticity.

Three-dimensional bioprinting of rat embryonic neural cells

We present a direct cell printing technique to pattern neural cells in a three-dimensional (3D) multilayered collagen gel. A layer of collagen precursor was printed to provide a scaffold for the cells, and the rat embryonic neurons and astrocytes were subsequently printed on the layer. A solution of sodium bicarbonate was applied to the cell containing collagen layer as nebulized aerosols, which allowed the gelation of the collagen. This process was repeated layer-by-layer to construct the 3D cell-hydrogel composites. Upon characterizing the relationship between printing resolutions and the growth of printed neural cells, single/multiple layers of neural cell-hydrogel composites were constructed and cultured. The on-demand capability to print neural cells in a multilayered hydrogel scaffold offers flexibility in generating artificial 3D neural tissue composites.

Systematic analysis of synchronized oscillatory neuronal networks reveals an enrichment for coupled direct and indirect feedback motifs

MOTIVATION

Synchronized bursting behavior is a remarkable phenomenon in neural dynamics. So, identification of the underlying functional structure is crucial to understand its regulatory mechanism at a system level. On the other hand, we noted that feedback loops (FBLs) are commonly used basic building blocks in engineering circuit design, especially for synchronization, and they have also been considered as important regulatory network motifs in systems biology. From these motivations, we have investigated the relationship between synchronized bursting behavior and feedback motifs in neural networks.

RESULTS

Through extensive simulations of synthetic spike oscillation models, we found that a particular structure of FBLs, coupled direct and indirect positive feedback loops (PFLs), can induce robust synchronized bursting behaviors. To further investigate this, we have developed a novel FBL identification method based on sampled time-series data and applied it to synchronized spiking records measured from cultured neural networks of rat by using multi-electrode array. As a result, we have identified coupled direct and indirect PFLs.

CONCLUSION

We therefore conclude that coupled direct and indirect PFLs might be an important design principle that causes the synchronized bursting behavior in neuronal networks although an extrapolation of this result to in vivo brain dynamics still remains an unanswered question.