Unidirectional signal propagation in primary neurons micropatterned at a single-cell resolution

Network Bursts in 3D Neuron Clusters Cultured on Microcontact-Printed Substrates

Microcontact printing (CP) is widely used to guide neurons to form 2D networks for neuroscience research. However, it is still difficult to establish 3D neuronal cultures on the CP substrate even though 3D neuronal structures are able to recapitulate critical aspects of native tissue. Here, we demonstrate that the reduced cell-substrate adhesion caused by the CP substrate could conveniently facilitate the aggregate formation of large-scale 3D neuron cluster networks. Furthermore, based on the quantitative analysis of the calcium activity of the resulting cluster networks, the effect of cell seeding density and local restriction of the CP substrate on network dynamics was investigated in detail. The results revealed that cell aggregation degree, rather than cell number, could take on the main role of the generation of synchronized network-wide calcium oscillation (network bursts) in the 3D neuron cluster networks. This finding may provide new insights for easy and cell-saving construction of in vitro 3D pathological models of epilepsy, and into deciphering the onset and evolution of network bursts in developmental nerve systems.

Microfluidic cell engineering on high-density microelectrode arrays for assessing structure-function relationships in living neuronal networks

Neuronal networks in dissociated culture combined with cell engineering technology offer a pivotal platform to constructively explore the relationship between structure and function in living neuronal networks. Here, we fabricated defined neuronal networks possessing a modular architecture on high-density microelectrode arrays (HD-MEAs), a state-of-the-art electrophysiological tool for recording neural activity with high spatial and temporal resolutions. We first established a surface coating protocol using a cell-permissive hydrogel to stably attach a polydimethylsiloxane microfluidic film on the HD-MEA. We then recorded the spontaneous neural activity of the engineered neuronal network, which revealed an important portrait of the engineered neuronal network-modular architecture enhances functional complexity by reducing the excessive neural correlation between spatially segregated modules. The results of this study highlight the impact of HD-MEA recordings combined with cell engineering technologies as a novel tool in neuroscience to constructively assess the structure-function relationships in neuronal networks.

An Overview of In Vitro Biological Neural Networks for Robot Intelligence

In vitro biological neural networks (BNNs) interconnected with robots, so-called BNN-based neurorobotic systems, can interact with the external world, so that they can present some preliminary intelligent behaviors, including learning, memory, robot control, etc. This work aims to provide a comprehensive overview of the intelligent behaviors presented by the BNN-based neurorobotic systems, with a particular focus on those related to robot intelligence. In this work, we first introduce the necessary biological background to understand the 2 characteristics of the BNNs: nonlinear computing capacity and network plasticity. Then, we describe the typical architecture of the BNN-based neurorobotic systems and outline the mainstream techniques to realize such an architecture from 2 aspects: from robots to BNNs and from BNNs to robots. Next, we separate the intelligent behaviors into 2 parts according to whether they rely solely on the computing capacity (computing capacity-dependent) or depend also on the network plasticity (network plasticity-dependent), which are then expounded respectively, with a focus on those related to the realization of robot intelligence. Finally, the development trends and challenges of the BNN-based neurorobotic systems are discussed.

Nanoscale Patterning of In Vitro Neuronal Circuits

Methods for patterning neurons in vitro have gradually improved and are used to investigate questions that are difficult to address in or ex vivo. Though these techniques guide axons between groups of neurons, multiscale control of neuronal connectivity, from circuits to synapses, is yet to be achieved in vitro. As studying neuronal circuits with synaptic resolution in vivo poses significant challenges, we present an in vitro alternative to validate biophysical and computational models. In this work we use a combination of electron beam lithography and photolithography to create polydimethylsiloxane (PDMS) structures with features ranging from 150 nm to a few millimeters. Leveraging the difference between average axon and dendritic spine diameters, we restrict axon growth while allowing spines to pass through nanochannels to guide synapse formation between small groups of neurons (i.e., nodes). We show this technique can be used to generate large numbers of isolated feed-forward circuits where connections between nodes are restricted to regions connected by nanochannels. Using a genetically encoded calcium indicator in combination with fluorescently tagged postsynaptic protein, PSD-95, we demonstrate functional synapses can form in this region.

Recent advances in microfluidic devices for single-cell cultivation: methods and applications

Single-cell analysis is essential to improve our understanding of cell functionality from cellular and subcellular aspects for diagnosis and therapy. Single-cell cultivation is one of the most important processes in single-cell analysis, which allows the monitoring of actual information of individual cells and provides sufficient single-cell clones and cell-derived products for further analysis. The microfluidic device is a fast-rising system that offers efficient, effective, and sensitive single-cell cultivation and real-time single-cell analysis conducted either on-chip or off-chip. Here, we introduce the importance of single-cell cultivation from the aspects of cellular and subcellular studies. We highlight the materials and structures utilized in microfluidic devices for single-cell cultivation. We further discuss biological applications utilizing single-cell cultivation-based microfluidics, such as cellular phenotyping, cell-cell interactions, and omics profiling. Finally, present limitations and future prospects of microfluidics for single-cell cultivation are also discussed.

Neurons-on-a-Chip: In Vitro NeuroTools

Neurons-on-a-Chip technology has been developed to provide diverse in vitro neuro-tools to study neuritogenesis, synaptogensis, axon guidance, and network dynamics. The two core enabling technologies are soft-lithography and microelectrode array technology. Soft lithography technology made it possible to fabricate microstamps and microfluidic channel devices with a simple replica molding method in a biological laboratory and innovatively reduced the turn-around time from assay design to chip fabrication, facilitating various experimental designs. To control nerve cell behaviors at the single cell level via chemical cues, surface biofunctionalization methods and micropatterning techniques were developed. Microelectrode chip technology, which provides a functional readout by measuring the electrophysiological signals from individual neurons, has become a popular platform to investigate neural information processing in networks. Due to these key advances, it is possible to study the relationship between the network structure and functions, and they have opened a new era of neurobiology and will become standard tools in the near future.

Efficient and Accurate Computational Model of Neuron with Spike Frequency Adaptation

Simplified models of neurons are widely used in computational investigations of large networks. One of the most important performance metrics of simplified models is their accuracy in reproducing action potential (spike) timing. In this article, we developed a simple, computationally efficient neuron model by modifying the adaptive exponential integrate and fire (AdEx) model [1] with sigmoid afterhyperpolarization current (Sigmoid AHP). Our model can precisely match the spike times and spike frequency adaptation of cortical pyramidal neurons. The accuracy was similar to a more complex two compartment biophysically realistic model of the same neurons. This work provides a simplified neuronal model with improved spike timing accuracy for use in modeling of large neural networks.Clinical Relevance- Accurate and computationally efficient single neuron model will enable large network modeling of brain regions involved in neurological and psychiatric disorders and may lead to a better understanding of the disorder mechanisms.

Microdevice for directional axodendritic connectivity between micro 3D neuronal cultures

Neuronal cultures are widely used in neuroscience research. However, the randomness of circuits in conventional cultures prevents accurate in vitro modeling of cortical development and of the pathogenesis of neurological and psychiatric disorders. A basic feature of cortical circuits that is not captured in standard cultures of dissociated cortical cells is directional connectivity. In this work, a polydimethylsiloxane (PDMS)-based device that achieves directional connectivity between micro 3D cultures is demonstrated. The device consists of through-holes for micro three-dimensional (μ3D) clusters of cortical cells connected by microtrenches for axon and dendrite guidance. The design of the trenches relies in part on the concept of axonal edge guidance, as well as on the novel concept of specific dendrite targeting. This replicates dominant excitatory connectivity in the cortex, enables the guidance of the axon after it forms a synapse in passing (an "en passant" synapse), and ensures that directional selectivity is preserved over the lifetime of the culture. The directionality of connections was verified morphologically and functionally. Connections were dependent on glutamatergic synapses. The design of this device has the potential to serve as a building block for the reconstruction of more complex cortical circuits in vitro.

Stepwise neuronal network pattern formation in agarose gel during cultivation using non-destructive microneedle photothermal microfabrication

Conventional neuronal network pattern formation techniques cannot control the arrangement of axons and dendrites because network structures must be fixed before neurite differentiation. To overcome this limitation, we developed a non-destructive stepwise microfabrication technique that can be used to alter microchannels within agarose to guide neurites during elongation. Micropatterns were formed in thin agarose layer coating of a cultivation dish using the tip of a 0.7 [Formula: see text]-diameter platinum-coated glass microneedle heated by a focused 1064-nm wavelength infrared laser, which has no absorbance of water. As the size of the heat source was 0.7 [Formula: see text], which is smaller than the laser wavelength, the temperature fell to 45 [Formula: see text] within a distance of 7.0 [Formula: see text] from the edge of the etched agarose microchannel. We exploited the fast temperature decay property to guide cell-to-cell connection during neuronal network cultivation. The first neurite of a hippocampal cell from a microchamber was guided to a microchannel leading to the target neuron with stepwise etching of the micrometer resolution microchannel in the agarose layer, and the elongated neurites were not damaged by the heat of etching. The results indicate the potential of this new technique for fully direction-controlled on-chip neuronal network studies.

Role of Noise in Spontaneous Activity of Networks of Neurons on Patterned Silicon Emulated by Noise–activated CMOS Neural Nanoelectronic Circuits

Background noise in biological cortical microcircuits constitutes a powerful resource to assess their computational tasks, including, for instance, the synchronization of spiking activity, the enhancement of the speed of information transmission, and the minimization of the corruption of signals. We explore the correlation of spontaneous firing activity of ≈ 100 biological neurons adhering to engineered scaffolds by governing the number of functionalized patterned connection pathways among groups of neurons. We then emulate the biological system by a series of noise-activated silicon neural network simulations. We show that by suitably tuning both the amplitude of noise and the number of synapses between the silicon neurons, the same controlled correlation of the biological population is achieved. Our results extend to a realistic silicon nanoelectronics neuron design using noise injection to be exploited in artificial spiking neural networks such as liquid state machines and recurrent neural networks for stochastic computation.

Thermoplasmonic neural chip platform for in situ manipulation of neuronal connections in vitro

Cultured neuronal networks with a controlled structure have been widely studied as an in vitro model system to investigate the relationship between network structure and function. However, most cell culture techniques lack the ability to control network structures during cell cultivation, making it difficult to assess functional changes induced by specific structural changes. In this study, we present an in situ manipulation platform based on gold-nanorod-mediated thermoplasmonics to interrogate an in vitro network model. We find that it is possible to induce new neurite outgrowths, eliminate interconnecting neurites, and estimate functional relationships in matured neuronal networks. This method is expected to be useful for studying functional dynamics of neural networks under controlled structural changes.

Cultured neuron networks provide insight into network structure and function, but the ability to control network topology is a challenge. Here the authors develop a nanorod-mediated thermoplasmonics platform that enables the formation of new connections, the abolishment of existing connections, and the modulation of network activity during cultivation.

Suppression of hypersynchronous network activity in cultured cortical neurons using an ultrasoft silicone scaffold

The spontaneous activity pattern of cortical neurons in dissociated culture is characterized by burst firing that is highly synchronized among a wide population of cells. The degree of synchrony, however, is excessively higher than that in cortical tissues. Here, we employed polydimethylsiloxane (PDMS) elastomers to establish a novel system for culturing neurons on a scaffold with an elastic modulus resembling brain tissue, and investigated the effect of the scaffold's elasticity on network activity patterns in cultured rat cortical neurons. Using whole-cell patch clamp to assess the scaffold effect on the development of synaptic connections, we found that the amplitude of excitatory postsynaptic current, as well as the frequency of spontaneous transmissions, was reduced in neuronal networks grown on an ultrasoft PDMS with an elastic modulus of 0.5 kPa. Furthermore, the ultrasoft scaffold was found to suppress neural correlations in the spontaneous activity of the cultured neuronal network. The dose of GsMTx-4, an antagonist of stretch-activated cation channels (SACs), required to reduce the generation of the events below 1.0 event per min on PDMS substrates was lower than that for neurons on a glass substrate. This suggests that the difference in the baseline level of SAC activation is a molecular mechanism underlying the alteration in neuronal network activity depending on scaffold stiffness. Our results demonstrate the potential application of PDMS with biomimetic elasticity as cell-culture scaffold for bridging the in vivo-in vitro gap in neuronal systems.

Surface engineering within a microchannel for hydrodynamic and self-assembled cell patterning

The applications of cell patterning are widespread due to the high-throughput testing and different resolutions offered by these platforms. Cell patterning has aided in deconvoluting in vivo experiments to better characterize cellular mechanisms and increase therapeutic output. Here, we present a technique for engineering an artificial surface via surface chemistry to form large-scale arrays of cells within a microchannel by employing microstamping. By changing the approach in surface chemistry, H1568 cells were patterned hydrodynamically using immunoaffinity, and neutrophils were patterned through self-assembly via chemotaxis. The high patterning efficiencies (93% for hydrodynamic patterning and 68% for self-assembled patterning) and the lack of secondary adhesion demonstrate the reproducibility of the platform. The interaction between H1568 and neutrophils was visualized and quantified to determine the capability of the platform to encourage cell-cell interaction. With the introduction of H1568 cells into the self-assembled patterning platform, a significant hindrance in the neutrophils' ability to swarm was observed, indicating the important roles of inflammatory mediators within the nonsmall cell lung cancer tumor microenvironment.

Single-cell patterning technology for biological applications

Single-cell patterning technology has revealed significant contributions of single cells to conduct basic and applied biological studies in vitro such as the understanding of basic cell functions, neuronal network formation, and drug screening. Unlike traditional population-based cell patterning approaches, single-cell patterning is an effective technology of fully understanding cell heterogeneity by precisely controlling the positions of individual cells. Therefore, much attention is currently being paid to this technology, leading to the development of various micro-nanofabrication methodologies that have been applied to locate cells at the single-cell level. In recent years, various methods have been continuously improved and innovated on the basis of existing ones, overcoming the deficiencies and promoting the progress in biomedicine. In particular, microfluidics with the advantages of high throughput, small sample volume, and the ability to combine with other technologies has a wide range of applications in single-cell analysis. Here, we present an overview of the recent advances in single-cell patterning technology, with a special focus on current physical and physicochemical methods including stencil patterning, trap- and droplet-based microfluidics, and chemical modification on surfaces via photolithography, microcontact printing, and scanning probe lithography. Meanwhile, the methods applied to biological studies and the development trends of single-cell patterning technology in biological applications are also described.

Improvements of Microcontact Printing for Micropatterned Cell Growth by Contrast Enhancement

Patterned neuronal cell cultures are important tools for investigating neuronal signal integration, network function, and cell-substrate interactions. Because of the variable nature of neuronal cells, the widely used coating method of microcontact printing is in constant need of improvements and adaptations depending on the pattern, cell type, and coating solutions available for a certain experimental system. In this work, we report on three approaches to modify microcontact printing on borosilicate glass surfaces, which we evaluate with contact angle measurements and by determining the quality of patterned neuronal growth. Although background toxification with manganese salt does not result in the desired pattern enhancement, a simple heat treatment of the glass substrates leads to improved background hydrophobicity and therefore neuronal patterning. Thirdly, we extended a microcontact printing process based on covalently linking the glass surface and the coating molecule via an epoxysilane. This extension is an additional hydrophobization step with dodecylamine. We demonstrate that shelf life of the silanized glass is at least 22 weeks, leading to consistently reliable neuronal patterning by microcontact printing. Thus, we compared three practical additions to microcontact printing, two of which can easily be implemented into a workflow for the investigation of patterned neuronal networks.

Impact of modular organization on dynamical richness in cortical networks

As in many naturally formed networks, the brain exhibits an inherent modular architecture that is the basis of its rich operability, robustness, and integration-segregation capacity. However, the mechanisms that allow spatially segregated neuronal assemblies to swiftly change from localized to global activity remain unclear. Here, we integrate microfabrication technology with in vitro cortical networks to investigate the dynamical repertoire and functional traits of four interconnected neuronal modules. We show that the coupling among modules is central. The highest dynamical richness of the network emerges at a critical connectivity at the verge of physical disconnection. Stronger coupling leads to a persistently coherent activity among the modules, while weaker coupling precipitates the activity to be localized solely within the modules. An in silico modeling of the experiments reveals that the advent of coherence is mediated by a trade-off between connectivity and subquorum firing, a mechanism flexible enough to allow for the coexistence of both segregated and integrated activities. Our results unveil a new functional advantage of modular organization in complex networks of nonlinear units.

Dependence and Homeostasis of Membrane Impedance on Cell Morphology in Cultured Hippocampal Neurons

The electrical impedance of cell membranes is important for excitable cells, such as neurons, because it strongly influences the amount of membrane potential change upon a flow of ionic current across the membrane. Here, we report on an investigation of how neuronal morphology affects membrane impedance of cultured hippocampal neurons. Microfabricated substrates with patterned scaffolding molecules were used to restrict the neurite growth of hippocampal neurons, and the impedance was measured via whole-cell patch-clamp recording under the inhibition of voltage-dependent ion channels. Membrane impedance was found to depend inversely on the dendrite length and soma area, as would be expected from the fact that its electrical property is equivalent to a parallel RC circuit. Moreover, we found that in biological neurons, the membrane impedance is homeostatically regulated to impede changes in the membrane area. The findings provide direct evidence on cell-autonomous regulation of neuronal impedance and pave the way towards elucidating the mechanism responsible for the resilience of biological neuronal networks.

Engineering Highly Interconnected Neuronal Networks on Nanowire Scaffolds

Identifying the specific role of physical guidance cues in the growth of neurons is crucial for understanding the fundamental biology of brain development and for designing scaffolds for tissue engineering. Here, we investigate the structural significance of nanoscale topographies as physical cues for neurite outgrowth and circuit formation by growing neurons on semiconductor nanowires. We monitored neurite growth using optical and scanning electron microscopy and evaluated the spontaneous neuronal network activity using functional calcium imaging. We show, for the first time, that an isotropic arrangement of indium phosphide (InP) nanowires can serve as physical cues for guiding neurite growth and aid in forming a network with neighboring neurons. Most importantly, we confirm that multiple neurons, with neurites guided by the topography of the InP nanowire scaffolds, exhibit synchronized calcium activity, implying intercellular communications via synaptic connections. Our study imparts new fundamental insights on the role of nanotopographical cues in the formation of functional neuronal circuits in the brain and will therefore advance the development of neuroprosthetic scaffolds.

Unidirectional signal propagation in primary neurons micropatterned at a single-cell resolution

The structure and connectivity of cultured neuronal networks can be controlled by using micropatterned surfaces. Here, we demonstrate that the direction of signal propagation can be precisely controlled at a single-cell resolution by growing primary neurons on micropatterns. To achieve this, we first examined the process by which axons develop and how synapses form in micropatterned primary neurons using immunocytochemistry. By aligning asymmetric micropatterns with a marginal gap, it was possible to pattern primary neurons with a directed polarization axis at the single-cell level. We then examined how synapses develop on micropatterned hippocampal neurons. Three types of micropatterns with different numbers of short paths for dendrite growth were compared. A normal development in synapse density was observed when micropatterns with three or more short paths were used. Finally, we performed double patch clamp recordings on micropatterned neurons to confirm that these synapses are indeed functional, and that the neuronal signal is transmitted unidirectionally in the intended orientation. This work provides a practical guideline for patterning single neurons to design functional neuronal networks in vitro with the direction of signal propagation being controlled.