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

Nanomagnetic Guidance Shapes the Structure-Function Relationship of Developing Cortical Networks

In this study, we implement large-scale nanomagnetic guidance on cortical neurons to guide dissociated neuronal networks during development. Cortical networks cultured over microelectrode arrays were exposed to functionalized magnetic nanoparticles, followed by magnetic field exposure to guide neurites over 14 days in vitro. Immunofluorescence of the axonal protein Tau revealed a greater number of neurites that were longer and aligned with the nanomagnetic force relative to nonguided networks. This was further confirmed through brightfield imaging on the microelectrode arrays during development. Spontaneous electrophysiological recordings revealed that the guided networks exhibited increased firing rates and frequency in force-aligned connectivity identified through Granger Causality. Applying this methodology across networks with nonuniform force directions increased local activity in target regions, identified as regions in the direction of the nanomagnetic force. Altogether, these results demonstrate that nanomagnetic forces guide the structure and function of dissociated cortical neuron networks at the millimeter scale.

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.

Patterning Networks of Grade IV Glioblastoma on Silicon Chip. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2023;2023:1-4. [PMID: 38083627 DOI: 10.1109/embc40787.2023.10340936] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Glioblastoma (GBM) is the most aggressive high-grade brain cancer with a median survival time of <15 months. Due to GBMs fast and infiltrative growth patient prognosis is poor with recurrence after treatment common. Investigating GBMs ability to communicate, specifically via Ca2+ signaling, within its functional tumour networks may unlock new therapeutics to reduce the rapid infiltration and growth which currently makes treatment ineffective. This work aims to produce patterned networks of GBM cells such that the Ca2+ communication at a network level can be repeatedly and reliably investigated.

A micropatterned thermoplasmonic substrate for neuromodulation of in vitro neuronal networks

Understanding how the spatial organization of a neural network affects its activity represents a leading issue in neuroscience. Thanks to their accessibility and easy handling, in vitro studies remain an essential tool to investigate the relationship between the structure and function of a neuronal network. Among all the patterning techniques, ink-jet printing acquired great interest thanks to its direct-write approach, which allows the patterned substrate realization without mold, leading to a considerable saving of both cost and time. However, the inks commonly used give the possibility to control only the structure of a neuronal network, leaving aside the functional aspect. In this work, we synthesize a photosensitive ink combining the rheological and bioadhesive properties of chitosan with the plasmonic properties of gold nanorods, obtaining an ink able to control both the spatial organization of a two-dimensional neuronal network and its activity through photothermal effect. After the ink characterization, we demonstrate that it is possible to print, with high precision, different geometries on a microelectrode array. In this way, it is possible obtaining a patterned device to control the structure of a neuronal network, to record its activity and to modulate it via photothermal effect. Finally, to our knowledge, we report the first evidence of photothermal inhibition of human neurons activity. STATEMENT OF SIGNIFICANCE: Patterned cell cultures remain the most efficient and simple tool for linking structural and functional studies, especially in the neuronal field. Ink-jet printing is the technique with which it is possible to realize patterned structures in the fastest, simple, versatile and low-cost way. However, the inks currently used permit the control only of the neuronal network structure but do not allow the control-modulation of the network activity. In this study, we realize and characterize a photosensitive bioink with which it is possible to drive both the structure and the activity of a neuronal network. Moreover, we report the first evidence of activity inhibition by the photothermal effect on human neurons as far as we know.

Dynamic and Functional Alterations of Neuronal Networks In Vitro upon Physical Damage: A Proof of Concept

There is a growing technological interest in combining biological neuronal networks with electronic ones, specifically for biological computation, human-machine interfacing and robotic implants. A major challenge for the development of these technologies is the resilience of the biological networks to physical damage, for instance, when used in harsh environments. To tackle this question, here, we investigated the dynamic and functional alterations of rodent cortical networks grown in vitro that were physically damaged, either by sequentially removing groups of neurons that were central for information flow or by applying an incision that cut the network in half. In both cases, we observed a remarkable capacity of the neuronal cultures to cope with damage, maintaining their activity and even reestablishing lost communication pathways. We also observed-particularly for the cultures cut in half-that a reservoir of healthy neurons surrounding the damaged region could boost resilience by providing stimulation and a communication bridge across disconnected areas. Our results show the remarkable capacity of neuronal cultures to sustain and recover from damage, and may be inspirational for the development of future hybrid biological-electronic systems.

Rich dynamics and functional organization on topographically designed neuronal networks in vitro

Neuronal cultures are a prominent experimental tool to understand complex functional organization in neuronal assemblies. However, neurons grown on flat surfaces exhibit a strongly coherent bursting behavior with limited functionality. To approach the functional richness of naturally formed neuronal circuits, here we studied neuronal networks grown on polydimethylsiloxane (PDMS) topographical patterns shaped as either parallel tracks or square valleys. We followed the evolution of spontaneous activity in these cultures along 20 days in vitro using fluorescence calcium imaging. The networks were characterized by rich spatiotemporal activity patterns that comprised from small regions of the culture to its whole extent. Effective connectivity analysis revealed the emergence of spatially compact functional modules that were associated with both the underpinned topographical features and predominant spatiotemporal activity fronts. Our results show the capacity of spatial constraints to mold activity and functional organization, bringing new opportunities to comprehend the structure-function relationship in living neuronal circuits.

Microfluidics for Neuronal Cell and Circuit Engineering

The widespread adoption of microfluidic devices among the neuroscience and neurobiology communities has enabled addressing a broad range of questions at the molecular, cellular, circuit, and system levels. Here, we review biomedical engineering approaches that harness the power of microfluidics for bottom-up generation of neuronal cell types and for the assembly and analysis of neural circuits. Microfluidics-based approaches are instrumental to generate the knowledge necessary for the derivation of diverse neuronal cell types from human pluripotent stem cells, as they enable the isolation and subsequent examination of individual neurons of interest. Moreover, microfluidic devices allow to engineer neural circuits with specific orientations and directionality by providing control over neuronal cell polarity and permitting the isolation of axons in individual microchannels. Similarly, the use of microfluidic chips enables the construction not only of 2D but also of 3D brain, retinal, and peripheral nervous system model circuits. Such brain-on-a-chip and organoid-on-a-chip technologies are promising platforms for studying these organs as they closely recapitulate some aspects of in vivo biological processes. Microfluidic 3D neuronal models, together with 2D in vitro systems, are widely used in many applications ranging from drug development and toxicology studies to neurological disease modeling and personalized medicine. Altogether, microfluidics provide researchers with powerful systems that complement and partially replace animal models.

One-step harvest and delivery of micropatterned cell sheets mimicking the multi-cellular microenvironment of vascularized tissue

Techniques for harvest and delivery of cell sheets have been improving for decades. However, cell sheets with complicated patterns closely related to natural tissue architecture were hardly achieved. Here, we developed an efficient method to culture and harvest cell sheets with complex shape (noted as microtissues) using temperature-responsive hydrogel consisting of expandable polyethylene oxide polymer at low temperature. Firstly, a temperature-responsive hydrogel surface with honeycomb patterns (50 and 100 µm in width) were developed through microcontact printing of polydopamine (PD). The human dermal fibroblasts (HDFBs) and human umbilical vein endothelial cells (HUVECs) spontaneously formed honeycomb-shaped microtissues on the patterned hydrogel surface. The microtissues on the hydrogel were able to be harvested and directly delivered to the desired target through thermal expansion of the hydrogel at 4 °C with an efficiency close to 80% within 10 min which is faster than conventional method based on poly(N-isopropylacrylamide). The microtissues maintained their original honeycomb network and intact structures. Honeycomb-patterned cell sheets also were fabricated through serial seeding of various cell lines, including HDFBs, HUVECs, and human adipose-derived stem cells, in which cells were attached along the honeycomb pattern. The underlying honeycomb patterns in the cell sheets were successfully maintained for 3 days, even after delivery. In addition, patterned cell sheets were successfully delivered in vivo while maintaining an intact structure for 7 days. Together, our findings demonstrate that micropatterned temperature-responsive hydrogel is an efficient method of one-step culturing and delivery of complex microtissues and should prove useful in various tissue engineering applications. STATEMENT OF SIGNIFICANCE: Scaffold-free cell delivery techniques, including cell sheet engineering, have been developed for decades. However, there is limited research regarding culture and delivery of microtissues with complex architecture mimicking natural tissue. Herein, we developed a micro-patterned hydrogel platform for the culture and delivery of honeycomb-shaped microtissues. Honeycomb patterns were chemically engineered on the temperature-responsive hydrogel through microcontact printing of polydopamine to selectively allow for human dermal fibroblast or human umbilical vein endothelial cell adhesion. They spontaneously formed honeycomb-shaped microtissues within 24 hr upon cell seeding and directly delivered to various target area including in vivo via thermal expansion of the hydrogel at 4 °C, suggesting that the micro-patterned hydrogel can be an efficient tool for culture and delivery of complex microtissue.

The Flow of Axonal Information Among Hippocampal Subregions: 1. Feed-Forward and Feedback Network Spatial Dynamics Underpinning Emergent Information Processing

The tri-synaptic pathway in the mammalian hippocampus enables cognitive learning and memory. Despite decades of reports on anatomy and physiology, the functional architecture of the hippocampal network remains poorly understood in terms of the dynamics of axonal information transfer between subregions. Information inputs largely flow from the entorhinal cortex (EC) to the dentate gyrus (DG), and then are processed further in the CA3 and CA1 before returning to the EC. Here, we reconstructed elements of the rat hippocampus in a novel device over an electrode array that allowed for monitoring the directionality of individual axons between the subregions. The direction of spike propagation was determined by the transmission delay of the axons recorded between two electrodes in microfluidic tunnels. The majority of axons from the EC to the DG operated in the feed-forward direction, with other regions developing unexpectedly large proportions of feedback axons to balance excitation. Spike timing in axons between each region followed single exponential log-log distributions over two orders of magnitude from 0.01 to 1 s, indicating that conventional descriptors of mean firing rates are misleading assumptions. Most of the spiking occurred in bursts that required two exponentials to fit the distribution of inter-burst intervals. This suggested the presence of up-states and down-states in every region, with the least up-states in the DG to CA3 feed-forward axons and the CA3 subregion. The peaks of the log-normal distributions of intra-burst spike rates were similar in axons between regions with modes around 95 Hz distributed over an order of magnitude. Burst durations were also log-normally distributed around a peak of 88 ms over two orders of magnitude. Despite the diversity of these spike distributions, spike rates from individual axons were often linearly correlated to subregions. These linear relationships enabled the generation of structural connectivity graphs, not possible previously without the directional flow of axonal information. The rich axonal spike dynamics between subregions of the hippocampus reveal both constraints and broad emergent dynamics of hippocampal architecture. Knowledge of this network architecture may enable more efficient computational artificial intelligence (AI) networks, neuromorphic hardware, and stimulation and decoding from cognitive implants.

Brain-on-a-Chip: Characterizing the next generation of advanced in vitro platforms for modeling the central nervous system

The complexity of the human brain creates significant, almost insurmountable challenges for neurological drug development. Advanced in vitro platforms are increasingly enabling researchers to overcome these challenges, by mimicking key features of the brain's composition and functionality. Many of these platforms are called "Brains-on-a-Chip"-a term that was originally used to refer to microfluidics-based systems containing miniature engineered tissues, but that has since expanded to describe a vast range of in vitro central nervous system (CNS) modeling approaches. This Perspective seeks to refine the definition of a Brain-on-a-Chip for the next generation of in vitro platforms, identifying criteria that determine which systems should qualify. These criteria reflect the extent to which a given platform overcomes the challenges unique to in vitro CNS modeling (e.g., recapitulation of the brain's microenvironment; inclusion of critical subunits, such as the blood-brain barrier) and thereby provides meaningful added value over conventional cell culture systems. The paper further outlines practical considerations for the development and implementation of Brain-on-a-Chip platforms and concludes with a vision for where these technologies may be heading.

Deciphering and reconstitution of positional information in the human brain development

Organoid has become a novel in vitro model to research human development and relevant disorders in recent years. With many improvements on the culture protocols, current brain organoids could self-organize into a complicated three-dimensional organization that mimics most of the features of the real human brain at the molecular, cellular, and further physiological level. However, lacking positional information, an important characteristic conveyed by gradients of signaling molecules called morphogens, leads to the deficiency of spatiotemporally regulated cell arrangements and cell–cell interactions in the brain organoid development. In this review, we will overview the role of morphogen both in the vertebrate neural development in vivo as well as the brain organoid culture in vitro, the strategies to apply morphogen concentration gradients in the organoid system and future perspectives of the brain organoid technology.

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.

Functionalization of TiO2 sol-gel derived films for cell confinement

The neuroscience field has increased enormously over the last decades, achieving the possible real application of neuronal cultures not only for reproducing neural architectures resembling in vivo tissues, but also for the development of functional devices. In this context, surface patterning for cell confinement is crucial, and new active materials together with new protocols for preparing substrates suitable for confining cells, guiding their processes in the desired configuration are extremely appreciated. Here, TiO₂ sol-gel derived films were selected as proof-of-concept materials to grow neurons in suitable confined configurations, taking advantage of the biocompatible properties of modified TiO₂ substrates. TiO₂ sol-gel derived films were made compatible with the growth of neurons thanks to a stable and controlled poly-lysine coating, obtained by silanization chemistry and streptavidin-biotin interactions. Moreover, a spotting protocol, here described and optimized, allowed the simple preparation of arrays of neurons, where cell adhesion was guided in specific areas and the neurites development driven in the desired arrangement. The resulting arrays were successfully tested for the growth and differentiation of neurons, demonstrating the possible adhesion of cells in specific areas of the film, therefore paving the way to applications such as the direct growth of excitable cells nearby electrodes of devices, with an evident enhancement of cell-electrodes communication.

Geometric micro-shapes facilitate trackless connections between human astrocytes

Objective.Cell patterning approaches commonly employed to direct the cytoplasmic outgrowth from cell bodies have been via chemical cues or biomaterial tracks. However, complex network designs using these approaches create problems where multiple tracks lead to manifold obstructions in design. A less common but alternative cell patterning modality is to geometrically design the nodes to project the cytoplasmic processes into a specific direction, thus, removing the need for tracks. Janget alperformed an in-depth study of how rodent neuron primaries could be directed accurately using geometric micro-shapes. In parallel and in contrast, to the work of Janget alwe investigate, for the first time, the effect that micro-shape geometry has on the cytoplasmic process outgrowth of human cells of astrocyte origin using the biomaterial parylene-C.Approach.We investigated eight different types of parylene-C micro-shape on SiO₂substrates consisting of the: circle, square, pentagon, hexagon, equilateral triangle and three isosceles triangles with top vertex angles of 14.2°, 28.8°, and 97.6°, respectively. We quantified how each micro-shape influenced the: cell patterning, the directionality of the cytoplasmic process outgrowth and the functionality for human astrocyte.Main results.Human astrocytes became equally well patterned on all different micro-shapes. Human astrocytes could discriminate the underlying micro-shape geometry and preferentially extended processes from the vertices of equilateral triangles and isosceles triangles where the vertex angle equal to 28.8° in a repeatable manner whilst remaining functional.Significance.We demonstrate how human astrocytes are extremely effective at directing their cytoplasmic process outgrowth from the vertices of geometric micro-shapes, in particular the top vertex of triangular shapes. The significance of this work is that it demonstrates that geometric micro-shapes offer an alternative patterning modality to direct cytoplasmic process outgrowth for human astrocytes, which can serve to simplify complex network design, thus, removing the need for tracks.

Electrophysiology Read-Out Tools for Brain-on-Chip Biotechnology

Brain-on-Chip (BoC) biotechnology is emerging as a promising tool for biomedical and pharmaceutical research applied to the neurosciences. At the convergence between lab-on-chip and cell biology, BoC couples in vitro three-dimensional brain-like systems to an engineered microfluidics platform designed to provide an in vivo-like extrinsic microenvironment with the aim of replicating tissue- or organ-level physiological functions. BoC therefore offers the advantage of an in vitro reproduction of brain structures that is more faithful to the native correlate than what is obtained with conventional cell culture techniques. As brain function ultimately results in the generation of electrical signals, electrophysiology techniques are paramount for studying brain activity in health and disease. However, as BoC is still in its infancy, the availability of combined BoC-electrophysiology platforms is still limited. Here, we summarize the available biological substrates for BoC, starting with a historical perspective. We then describe the available tools enabling BoC electrophysiology studies, detailing their fabrication process and technical features, along with their advantages and limitations. We discuss the current and future applications of BoC electrophysiology, also expanding to complementary approaches. We conclude with an evaluation of the potential translational applications and prospective technology developments.

Micropatterning of Substrates for the Culture of Cell Networks by Stencil-Assisted Additive Nanofabrication

The fabrication of in vitro neuronal cell networks where cells are chemically or electrically connected to form functional circuits with useful properties is of great interest. Standard cell culture substrates provide ensembles of cells that scarcely reproduce physiological structures since their spatial organization and connectivity cannot be controlled. Supersonic Cluster Beam Deposition (SCBD) has been used as an effective additive method for the large-scale fabrication of interfaces with extracellular matrix-mimicking surface nanotopography and reproducible morphological properties for cell culture. Due to the high collimation of SCBD, it is possible to exploit stencil masks for the fabrication of patterned films and reproduce features as small as tens of micrometers. Here, we present a protocol to fabricate micropatterned cell culture substrates based on the deposition of nanostructured cluster-assembled zirconia films by stencil-assisted SCBD. The effectiveness of this approach is demonstrated by the fabrication of micrometric patterns able to confine primary astrocytes. Calcium waves propagating in the astrocyte networks are shown.

Modulating electrophysiology of motor neural networks via optogenetic stimulation during neurogenesis and synaptogenesis

Control of electrical activity in neural circuits through network training is a grand challenge for biomedicine and engineering applications. Past efforts have not considered evoking long-term changes in firing patterns of in-vitro networks by introducing training regimens with respect to stages of neural development. Here, we used Channelrhodopsin-2 (ChR2) transfected mouse embryonic stem cell (mESC) derived motor neurons to explore short and long-term programming of neural networks by using optical stimulation implemented during neurogenesis and synaptogenesis. Not only did we see a subsequent increase of neurite extensions and synaptophysin clustering, but by using electrophysiological recording with micro electrode arrays (MEA) we also observed changes in signal frequency spectra, increase of network synchrony, coordinated firing of actions potentials, and enhanced evoked response to stimulation during network formation. Our results demonstrate that optogenetic stimulation during neural differentiation can result in permanent changes that extended to the genetic expression of neurons as demonstrated by RNA Sequencing. To our knowledge, this is the first time that a correlation between training regimens during neurogenesis and synaptogenesis and the resulting plastic responses has been shown in-vitro and traced back to changes in gene expression. This work demonstrates new approaches for training of neural circuits whose electrical activity can be modulated and enhanced, which could lead to improvements in neurodegenerative disease research and engineering of in-vitro multi-cellular living systems.

Nanogrooves for 2D and 3D Microenvironments of SH-SY5Y Cultures in Brain-on-Chip Technology

Brain-on-chip (BOC) technology such as nanogrooves and microtunnel structures can advance in vitro neuronal models by providing a platform with better means to maintain, manipulate and analyze neuronal cell cultures. Specifically, nanogrooves have been shown to influence neuronal differentiation, notably the neurite length and neurite direction. Here, we have drawn new results from our experiments using both 2D and 3D neuronal cell culture implementing both flat and nanogrooved substrates. These are used to show a comparison between the number of cells and neurite length as a first indicator for valuable insights into baseline values and expectations that can be generated from these experiments toward design optimization and predictive value of the technology in our BOC toolbox. Also, as a new step toward neuronal cell models with multiple compartmentalized neuronal cell type regions, we fabricated microtunnel devices bonded to both flat and nanogrooved substrates to assess their compatibility with neuronal cell culture. Our results show that with the current experimental protocols using SH-SY5Y cells, we can expect 200 – 400 cells with a total neurite length of approximately 4,000–5,000 μm per 1 mm² within our BOC devices, with a lower total neurite length for 3D neuronal cell cultures on flat substrates only. There is a statistically significant difference in total neurite length between 2D cell culture on nanogrooved substrates versus 3D cell culture on flat substrates. As extension of our current BOC toolbox for which these indicative parameters would be used, the microtunnel devices show that culture of SH-SY5Y was feasible, though a limited number of neurites extended into microtunnels away from the cell bodies, regardless of using nanogrooved or flat substrates. This shows that the novel combination of microtunnel devices with nanogrooves can be implemented toward neuronal cell cultures, with future improvements to be performed to ensure neurites extend beyond the confines of the wells between the microtunnels. Overall, these results will aid toward creating more robust BOC platforms with improved predictive value. In turn, this can be used to better understand the brain and brain diseases.

Long-Range Optogenetic Control of Axon Guidance Overcomes Developmental Boundaries and Defects

Axons connect neurons together, establishing the wiring architecture of neuronal networks. Axonal connectivity is largely built during embryonic development through highly constrained processes of axon guidance, which have been extensively studied. However, the inability to control axon guidance, and thus neuronal network architecture, has limited investigation of how axonal connections influence subsequent development and function of neuronal networks. Here, we use zebrafish motor neurons expressing a photoactivatable Rac1 to co-opt endogenous growth cone guidance machinery to precisely and non-invasively direct axon growth using light. Axons can be guided over large distances, within complex environments of living organisms, overriding competing endogenous signals and redirecting axons across potent repulsive barriers to construct novel circuitry. Notably, genetic axon guidance defects can be rescued, restoring functional connectivity. These data demonstrate that intrinsic growth cone guidance machinery can be co-opted to non-invasively build new connectivity, allowing investigation of neural network dynamics in intact living organisms.

Instrumented Microphysiological Systems for Real-Time Measurement and Manipulation of Cellular Electrochemical Processes

Recent advancements in electronic materials and subsequent surface modifications have facilitated real-time measurements of cellular processes far beyond traditional passive recordings of neurons and muscle cells. Specifically, the functionalization of conductive materials with ligand-binding aptamers has permitted the utilization of traditional electronic materials for bioelectronic sensing. Further, microfabrication techniques have better allowed microfluidic devices to recapitulate the physiological and pathological conditions of complex tissues and organs in vitro or microphysiological systems (MPS). The convergence of these models with advances in biological/biomedical microelectromechanical systems (BioMEMS) instrumentation has rapidly bolstered a wide array of bioelectronic platforms for real-time cellular analytics. In this review, we provide an overview of the sensing techniques that are relevant to MPS development and highlight the different organ systems to integrate instrumentation for measurement and manipulation of cellular function. Special attention is given to how instrumented MPS can disrupt the drug development and fundamental mechanistic discovery processes.

Hindbrain V2a Neurons Pattern Rhythmic Activity of Motor Neurons in a Reticulospinal Coculture

As the capacity to isolate distinct neuronal cell types has advanced over the past several decades, new two- and three-dimensional in vitro models of the interactions between different brain regions have expanded our understanding of human neurobiology and the origins of disease. These cultures develop distinctive patterns of activity, but the extent that these patterns are determined by the molecular identity of individual cell types versus the specific pattern of network connectivity is unclear. To address the question of how individual cell types interact in vitro, we developed a simplified culture using two excitatory neuronal subtypes known to participate in the in vivo reticulospinal circuit: HB9+ spinal motor neurons and Chx10+ hindbrain V2a neurons. Here, we report the emergence of cell type-specific patterns of activity in culture; on their own, Chx10+ neurons developed regular, synchronized bursts of activity that recruited neurons across the entire culture, whereas HB9+ neuron activity consisted of an irregular pattern. When these two subtypes were cocultured, HB9+ neurons developed synchronized network bursts that were precisely correlated with Chx10+ neuron activity, thereby recreating an aspect of Chx10+ neurons' role in driving motor activity. These bursts were dependent on AMPA receptors. Our results demonstrate that the molecular classification of the neurons comprising in vitro networks is a crucial determinant of their activity. It is therefore possible to improve both the reproducibility and the applicability of in vitro neurobiological and disease models by carefully controlling the constituent mixtures of neuronal subtypes.

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.

Evaluation of parylene derivatives for use as biomaterials for human astrocyte cell patterning

Cell patterning is becoming increasingly popular in neuroscience because it allows for the control in the location and connectivity of cells. A recently developed cell patterning technology uses patterns of an organic polymer, parylene-C, on a background of SiO₂. When cells are cultured on the parylene-C/SiO₂ substrate they conform to the underlying parylene-C geometry. Parylene-C is, however, just one member of a family of parylene polymers that have varying chemical and physical properties. In this work, we investigate whether two commercially available mainstream parylene derivatives, parylene-D, parylene-N and a more recent parylene derivative, parylene-HT to determine if they enable higher fidelity hNT astrocyte cell patterning compared to parylene-C. We demonstrate that all parylene derivatives are compatible with the existing laser fabrication method. We then demonstrate that parylene-HT, parylene-D and parylene-N are suitable for use as an hNT astrocyte cell attractive substrate and result in an equal quality of patterning compared to parylene-C. This work supports the use of alternative parylene derivatives for applications where their different physical and chemical properties are more suitable.

Non-contact monitoring of extra-cellular field potentials with a multi-electrode array

Developing tools to enable non-invasive, high-throughput electrophysiology measurements of large functional-networks of electrogenic cells used as in vitro disease models for the heart and brain remains an outstanding challenge for preclinical drug discovery, where failures are costly and can prove to be fatal during clinical trials. Here we demonstrate, for the first time, that it is possible to perform non-contact monitoring of extra-cellular field potentials with a multi-electrode array (MEA). To do this preliminary demonstration we built a prototype with a custom mechanical stage to micro-position cells grown on conventional glass coverslips over the recording surface of a MEA sensor. The prototype can monitor extra-cellular fields generated by multi-cellular networks in a non-contact configuration, enabling a single MEA sensor to probe different cultures in succession, without fouling or degrading its sensitive electronic surface. This first demonstration with easy to culture cardiomyocyte cells and a prototype device points to the exciting possibility for instrument development leading to more efficient and cost-effective drug screening paradigms for cardiovascular and neurological diseases.

Bundling of axons through a capillary alginate gel enhances the detection of axonal action potentials using microelectrode arrays

Microelectrode arrays (MEAs) have become important tools in high throughput assessment of neuronal activity. However, geometric and electrical constraints largely limit their ability to detect action potentials to the neuronal soma. Enhancing the resolution of these systems to detect axonal action potentials has proved both challenging and complex. In this study, we have bundled sensory axons from dorsal root ganglia through a capillary alginate gel (Capgel™) interfaced with an MEA and observed an enhanced ability to detect spontaneous axonal activity compared with two-dimensional cultures. Moreover, this arrangement facilitated the long-term monitoring of spontaneous activity from the same bundle of axons at a single electrode. Finally, using waveform analysis for cultures treated with the nociceptor agonist capsaicin, we were able to dissect action potentials from multiple axons on an individual electrode, suggesting that this model can reproduce the functional complexity associated with sensory fascicles in vivo. This novel three-dimensional functional model of the peripheral nerve can be used to study the functional complexities of peripheral neuropathies and nerve regeneration as well as being utilized in the development of novel therapeutics.

Self-Aligned Functionalization Approach to Order Neuronal Networks at the Single-Cell Level

Despite significant progress, our knowledge of the functioning of the central nervous system still remains scarce to date. A better understanding of its behavior, in either normal or diseased conditions, goes through an increased knowledge of basic mechanisms involved in neuronal function, including at the single-cell level. This has motivated significant efforts for the development of miniaturized sensing devices to monitor neuronal activity with high spatial and signal resolution. One of the main challenges remaining to be addressed in this domain is, however, the ability to create in vitro spatially ordered neuronal networks at low density with a precise control of the cell location to ensure proper monitoring of the activity of a defined set of neurons. Here, we present a novel self-aligned chemical functionalization method, based on a repellant surface with patterned attractive areas, which permits the elaboration of low-density neuronal network down to individual cells with a high control of the soma location and axonal growth. This approach is compatible with complementary metal-oxide-semiconductor line technology at a wafer scale and allows performing the cell culture on packaged chip outside microelectronics facilities. Rat cortical neurons were cultured on such patterned surfaces for over one month and displayed a very high degree of organization in large networks. Indeed, more than 90% of the network nodes were settled by a soma and 100% of the connecting lines were occupied by a neurite, with a very good selectivity (low parasitic cell connections). After optimization, networks composed of 75% of unicellular nodes were obtained, together with a control at the micron scale of the location of the somas. Finally, we demonstrated that the dendritic neuronal growth was guided by the surface functionalization, even when micrometer scale topologies were encountered and we succeeded to control the extension growth along one-dimensional-aligned nanostructures with sub-micrometrical scale precision. This novel approach now opens the way for precise monitoring of neuronal network activity at the single-cell level.

Assembly and Connection of Micropatterned Single Neurons for Neuronal Network Formation

Engineering of neuronal network geometry by micropatterning technology is a key future technology for creating artificial brains on a chip. However, engineering of network geometry at the single-cell-level with functional morphology (axon/dendrite) and connectivity (synapses) is still challenging. Here, we describe a method for controlling the axon and dendrite morphology of single primary-cultured neurons and assembling a neural circuit using mobile microplates. The microplates enabled morphological control of neurons by their shapes and bringing their ends into contact caused the formation of physical connections. Functional synapse formation at the connection was indicated by immunostaining of synapse-related proteins and intracellular Ca²⁺ imaging of neural activity. We believe that the method will be useful in engineering neural circuits with selected neurons and defined morphology.

Biofunctionalized 3D Nanopillar Arrays Fostering Cell Guidance and Promoting Synapse Stability and Neuronal Activity in Networks

A controlled geometry of in vitro neuronal networks allows investigation of the cellular mechanisms that underlie neuron-to-neuron and neuron-extracellular matrix interactions, which are essential to biomedical research. Herein, we report a selective guidance of primary hippocampal neurons by using arrays of three-dimensional vertical nanopillars (NPs) functionalized with a specific adhesion-promoting molecule-poly-dl-ornithine (PDLO). We show that 90% of neuronal cells are guided exclusively on the combinatorial PDLO/NP substrate. Moreover, we demonstrate the influence of the interplay between nanostructures and neurons on synapse formation and maturation, resulting in increased expression of postsynaptic density-95 protein and enhanced network cellular activity conferred by the endogenous c-fos expression. Successful guidance to foster synapse stability and cellular activity on multilevel cues of surface topography and chemical functionalization suggests the potential to devise technologies to control neuronal growth on nanostructures for tissue engineering, neuroprostheses, and drug development.

Functionalized Thick Film Impedance Sensors for Use in In Vitro Cell Culture

Multi-electrode arrays find application in electrophysiological recordings. The quality of the captured signals depends on the interfacial contact between electrogenic cells and the electronic system. Therefore, it requires reliable low-impedance electrodes. Low-temperature cofired ceramic technology offers a suitable platform for rapid prototyping of biological reactors and can provide both stable fluid supply and integrated bio-hardware interfaces for recordings in electrogenic cell cultures. The 3D assembly of thick film gold electrodes in in vitro bio-reactors has been demonstrated for neuronal recordings. However, especially when dimensions become small, their performance varies strongly. This work investigates the influence of different coatings on thick film gold electrodes with regard to their influence on impedance behavior. PEDOT:PSS layer, titanium oxynitride and laminin coatings are deposited on LTCC gold electrodes using different 2D and 3D MEA chip designs. Their impedance characteristics are compared and discussed. Titanium oxynitride layers emerged as suitable functionalization. Small 86-µm-electrodes have a serial resistance R_s of 32 kOhm and serial capacitance C_s of 4.1 pF at 1 kHz. Thick film gold electrodes with such coatings are thus qualified for signal recording in 3-dimensional in vitro cell cultures.

Stiffness analysis of 3D spheroids using microtweezers

We describe a novel mechanical characterization method that has directly measured the stiffness of cancer spheroids for the first time to our knowledge. Stiffness is known to be a key parameter that characterizes cancerous and normal cells. Atomic force microscopy or optical tweezers have been typically used for characterization of single cells with the measurable forces ranging from sub pN to a few hundred nN, which are not suitable for measurement of larger 3D cellular structures such as spheroids, whose mechanical characteristics have not been fully studied. Here, we developed microtweezers that measure forces from sub hundred nN to mN. The wide force range was achieved by the use of replaceable cantilevers fabricated from SU8, and brass. The chopstick-like motion of the two cantilevers facilitates easy handling of samples and microscopic observation for mechanical characterization. The cantilever bending was optically tracked to find the applied force and sample stiffness. The efficacy of the method was demonstrated through stiffness measurement of agarose pillars with known concentrations. Following the initial system evaluation with agarose, two cancerous (T47D and BT474) and one normal epithelial (MCF 10A) breast cell lines were used to conduct multi-cellular spheroid measurements to find Young’s moduli of 230, 420 and 1250 Pa for BT474, T47D, and MCF 10A, respectively. The results showed that BT474 and T47D spheroids are six and three times softer than epithelial MCF10A spheroids, respectively. Our method successfully characterized samples with wide range of Young’s modulus including agarose (25–100 kPa), spheroids of cancerous and non-malignant cells (190–200 μm, 230–1250 Pa) and collagenase-treated spheroids (215 μm, 130 Pa).

Multichannel activity propagation across an engineered axon network

OBJECTIVE

Although substantial progress has been made in mapping the connections of the brain, less is known about how this organization translates into brain function. In particular, the massive interconnectivity of the brain has made it difficult to specifically examine data transmission between two nodes of the connectome, a central component of the 'neural code.' Here, we investigated the propagation of multiple streams of asynchronous neuronal activity across an isolated in vitro 'connectome unit.'

APPROACH

We used the novel technique of axon stretch growth to create a model of a long-range cortico-cortical network, a modular system consisting of paired nodes of cortical neurons connected by axon tracts. Using optical stimulation and multi-electrode array recording techniques, we explored how input patterns are represented by cortical networks, how these representations shift as they are transmitted between cortical nodes and perturbed by external conditions, and how well the downstream node distinguishes different patterns.

MAIN RESULTS

Stimulus representations included direct, synaptic, and multiplexed responses that grew in complexity as the distance between the stimulation source and recorded neuron increased. These representations collapsed into patterns with lower information content at higher stimulation frequencies. With internodal activity propagation, a hierarchy of network pathways, including latent circuits, was revealed using glutamatergic blockade. As stimulus channels were added, divergent, non-linear effects were observed in local versus distant network layers. Pairwise difference analysis of neuronal responses suggested that neuronal ensembles generally outperformed individual cells in discriminating input patterns.

SIGNIFICANCE

Our data illuminate the complexity of spiking activity propagation in cortical networks in vitro, which is characterized by the transformation of an input into myriad outputs over several network layers. These results provide insight into how the brain potentially processes information and generates the neural code and could guide the development of clinical therapies based on multichannel brain stimulation.

Three Dimensional Conjugation of Recombinant N-Cadherin to a Hydrogel for In Vitro Anisotropic Neural Growth

Living cells are extensively being studied to build functional tissues that are useful for both fundamental and applied bioscience studies. Increasing evidence suggests that cell-cell adhesion controlled by intercellular cadherin junction plays important roles in the quality of the resulting engineered tissue. These findings prompted efforts to interrogate biological effects of cadherin at a molecular scale; however, few efforts were made to harness the effects of cadherin on cells cultured in an in vivo-like three dimensional matrix. To this end, this study reports a hydrogel matrix three dimensionally functionalized with a controlled number of Fc-tagged recombinant N-cadherins (N-Cad-Fc). To retain the desired conformation of N-Cad, these cadherins were immobilized and oriented to the gel by anti-Fc-antibodies chemically coupled to gels. The gels were processed to present N-Cad-Fc in uniaxially aligned microchannels or randomly oriented micropores. Culturing cortical cells in the functionalized gels generated a large fraction of neurons that are functional as indicated by increased intracellular calcium ion concentrations with the microchanneled gel. In contrast, direct N-Cad-Fc immobilization to microchannel or micropore walls of the gel limited the growth of neurons and increased the glial to neuron ratio. The results of this study will be highly useful to organize a wide array of cadherin molecules in a series of biomaterials used for three-dimensional cell culture and to regulate phenotypic activities of tissue-forming cells in an elaborate manner.

Live-Cell, Label-Free Identification of GABAergic and Non-GABAergic Neurons in Primary Cortical Cultures Using Micropatterned Surface

Excitatory and inhibitory neurons have distinct roles in cortical dynamics. Here we present a novel method for identifying inhibitory GABAergic neurons from non-GABAergic neurons, which are mostly excitatory glutamatergic neurons, in primary cortical cultures. This was achieved using an asymmetrically designed micropattern that directs an axonal process to the longest pathway. In the current work, we first modified the micropattern geometry to improve cell viability and then studied the axon length from 2 to 7 days in vitro (DIV). The cell types of neurons were evaluated retrospectively based on immunoreactivity against GAD67, a marker for inhibitory GABAergic neurons. We found that axons of non-GABAergic neurons grow significantly longer than those of GABAergic neurons in the early stages of development. The optimal threshold for identifying GABAergic and non-GABAergic neurons was evaluated to be 110 μm at 6 DIV. The method does not require any fluorescence labelling and can be carried out on live cells. The accuracy of identification was 98.2%. We confirmed that the high accuracy was due to the use of a micropattern, which standardized the development of cultured neurons. The method promises to be beneficial both for engineering neuronal networks in vitro and for basic cellular neuroscience research.

Human astrocytic grid networks patterned in parylene-C inlayed SiO2 trenches

Recent literature suggests that glia, and in particular astrocytes, should be studied as organised networks which communicate through gap junctions. Astrocytes, however, adhere to most surfaces and are highly mobile cells. In order to study, such organised networks effectively in vitro it is necessary to influence them to pattern to certain substrates whilst being repelled from others and to immobilise the astrocytes sufficiently such that they do not continue to migrate further whilst under study. In this article, we demonstrate for the first time how it is possible to facilitate the study of organised patterned human astrocytic networks using hNT astrocytes in a SiO2 trench grid network that is inlayed with the biocompatible material, parylene-C. We demonstrate how the immobilisation of astrocytes lies in the depth of the SiO2 trench, determining an optimum trench depth and that the optimum patterning of astrocytes is a consequence of the parylene-C inlay and the grid node spacing. We demonstrate high fidelity of the astrocytic networks and demonstrate that functionality of the hNT astrocytes through ATP evoked calcium signalling is also dependent on the grid node spacing. Finally, we demonstrate that the location of the nuclei on the grid nodes is also a function of the grid node spacing. The significance of this work, is to describe a suitable platform to facilitate the study of hNT astrocytes from the single cell level to the network level to improve knowledge and understanding of how communication links to spatial organisation at these higher order scales and trigger in vitro research further in this area with clinical applications in the area of epilepsy, stroke and focal cerebral ischemia.

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.

Signal Propagation between Neuronal Populations Controlled by Micropatterning

The central nervous system consists of an unfathomable number of functional networks enabling highly sophisticated information processing. Guided neuronal growth with a well-defined connectivity and accompanying polarity is essential for the formation of these networks. To investigate how two-dimensional protein patterns influence neuronal outgrowth with respect to connectivity and functional polarity between adjacent populations of neurons, a microstructured model system was established. Exclusive cell growth on patterned substrates was achieved by transferring a mixture of poly-l-lysine and laminin to a cell-repellent glass surface by microcontact printing. Triangular structures with different opening angle, height, and width were chosen as a pattern to achieve network formation with defined behavior at the junction of adjacent structures. These patterns were populated with dissociated primary cortical embryonic rat neurons and investigated with respect to their impact on neuronal outgrowth by immunofluorescence analysis, as well as their functional connectivity by calcium imaging. Here, we present a highly reproducible technique to devise neuronal networks in vitro with a predefined connectivity induced by the design of the gateway. Daisy-chained neuronal networks with predefined connectivity and functional polarity were produced using the presented micropatterning method. Controlling the direction of signal propagation among populations of neurons provides insights to network communication and offers the chance to investigate more about learning processes in networks by external manipulation of cells and signal cascades.

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

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

Effects of Morphology Constraint on Electrophysiological Properties of Cortical Neurons

There is growing interest in engineering nerve cells in vitro to control architecture and connectivity of cultured neuronal networks or to build neuronal networks with predictable computational function. Pattern technologies, such as micro-contact printing, have been developed to design ordered neuronal networks. However, electrophysiological characteristics of the single patterned neuron haven't been reported. Here, micro-contact printing, using polyolefine polymer (POP) stamps with high resolution, was employed to grow cortical neurons in a designed structure. The results demonstrated that the morphology of patterned neurons was well constrained, and the number of dendrites was decreased to be about 2. Our electrophysiological results showed that alterations of dendritic morphology affected firing patterns of neurons and neural excitability. When stimulated by current, though both patterned and un-patterned neurons presented regular spiking, the dynamics and strength of the response were different. The un-patterned neurons exhibited a monotonically increasing firing frequency in response to injected current, while the patterned neurons first exhibited frequency increase and then a slow decrease. Our findings indicate that the decrease in dendritic complexity of cortical neurons will influence their electrophysiological characteristics and alter their information processing activity, which could be considered when designing neuronal circuitries.

Generation of highly enriched V2a interneurons from mouse embryonic stem cells

Challenges in parsing specific contributions to spinal microcircuit architecture have limited our ability to model and manipulate those networks for improved functional regeneration after injury or disease. While spinal interneurons (INs) have been implicated in driving coordinated locomotor behaviors, they constitute only a small percentage of the spinal cord and are difficult to isolate from primary tissue. In this study, we employed a genetic strategy to obtain large quantities of highly enriched mouse embryonic stem cell (ESC)-derived V2a INs, an excitatory glutamatergic IN population that is defined by expression of the homeodomain protein Chx10 during development. Puromycin N-acetyltransferase expression was driven by the native gene regulatory elements of Chx10 in the transgenic ESC line, resulting in positive selection of V2a INs after induction and treatment with puromycin. Directly after selection, approximately 80% of cells are Chx10(+), with 94% Lhx3(+); after several weeks, cultures remain free of proliferative cell types and mature into normal glutamatergic neurons as assessed by molecular markers and electrophysiological methods. Functional synapses were observed between selected ESC-derived V2a INs and motor neurons when co-cultured, demonstrating the potential of these cells to form neural networks. While ESC-derived neurons obtained in vitro are not identical to those that develop in the spinal cord, the transgenic ESCs here provide a unique tool to begin studying V2a INs in isolation or for use in in vitro models of spinal microcircuits.

Towards the fabrication of artificial 3D microdevices for neural cell networks

This work reports first steps towards the development of artificial neural stem cell microenvironments for the control and assessment of neural stem cell behaviour. Stem cells have been shown to be found in specific, supportive microenvironments (niches) and are believed to play an important role in tissue regeneration mechanisms. These environments are intricate spaces with chemical and biological features. Here we present work towards the development of physically defined microdevices in which neural and neural stem cells can be studied in 3-dimensions. We have approached this challenge by creating bespoke, microstructured polymer environments using both 2-photon polymerisation and soft lithography techniques. Specifically, we have designed and fabricated biodegradable microwell-shaped devices using an in house synthetized polymer (4-arm photocurable poly-lactid acid) on a bespoke 2-photon polymerisation (2PP) set-up. We have studied swelling and degradation of the constructs as well as biocompatibility. Moreover, we have explored the potential of these constructs as artificial neural cell substrates by culturing NG108-15 cells (mouse neuroblastoma; rat glioma hybrid) and human neural progenitor cells on the microstructures. Finally, we have studied the effects of our artificial microenvironments upon neurite length and cell density.

Emergence of critical dynamics in large-scale in vitro cortical networks

In vitro neuronal networks coupled to Micro-Electrode Arrays (MEAs) represent a valid experimental framework to study neuronal dynamics. This preparation is free of chemical or physical constraints and allows neurons to self-organize during development, creating networks that exhibit complex spatio-temporal patterns of activity. Starting from this experimental evidence, here we address the question whether a particular network architecture can drive the network dynamics towards a sub-, super-, or critical state.