# Reproducible Human Neural Circuits Printed with Single-Cell Precision Reveal the Functional Roles of Ephaptic Coupling

## TL;DR

Scientists created precise human neural circuits in the lab to study how electric fields between neurons affect brain function and disease.

## Contribution

A novel platform for engineering human neuronal networks with single-cell precision and reproducibility, enabling the study of ephaptic coupling mechanisms.

## Key findings

- Engineered circuits allowed quantification of ephaptic coupling effects, confirming theoretical predictions like increased synchronization.
- The platform supports high-throughput production of circuits for studying nonsynaptic interactions and disease modeling.

## Abstract

Although in vitro neuronal models are accessible and
versatile
systems for functional electrophysiological studies, the spontaneous
and random formation of neural circuits often compromises the structural
control and reproducibility. Here, we introduce a robust method for
engineering human neuronal networks in vitro with single-cell precision
and reproducibility. Our integrated platform combines direct laser-written
microstructure templates and soft lithography-based fabrication of
microscaffolds with functional multielectrode array recordings. This
system enables high-throughput production of diverse circuit designs
and allows for the exact placement of neurons within confined microenvironments.
The system enables precise recording of spontaneous neuronal activity,
as well as electrical and optogenetic stimulations. Using this approach,
we constructed reproducible, bottom-up neuronal circuits composed
of a defined number of human neurons. As a proof of principle, we
employed these circuits to investigate ephaptic coupling, which refers
to the modulation of neuronal activity by endogenous electric fields.
Although it is believed to play a role in neural computations and
cardiac conduction and is associated with epilepsy and arrhythmia,
its mechanisms are unclear due to limitations in experimental models,
both in vivo and in vitro. By controlling axonal proximity within
microchannels and the number of neurons in the engineered circuits,
we can quantify ephaptic coupling at different strengths, which validates
theoretical predictions, including reduced action potential velocity,
increased activity synchronization, and lower stimulation thresholds.
Furthermore, the platform has broad potential for studying synaptic
and nonsynaptic interactions, myelination processes, advancing disease
modeling, and fundamental neuroscience research.