Soft microelectrode arrays for 3D in vitro brain models

Neural organoids and spheroids are three-dimensional (3D) in vitro brain models that mimic key aspects of human brain cellular composition, organization, and function. These models have become essential for studying brain development and neurological disorders, as their spontaneous neural activity serves as a key indicator of electrophysiological maturation associated with morphological and cellular growth. Recording this activity has enabled insights into early brain network formation and provided a platform for investigating traumatic brain injury via electrophysiological analyses following mechanical trauma. Despite significant advancements in the development of neural organoids, their functional analysis is limited by reliance on planar and rigid microelectrode arrays (MEAs), which fail to accommodate the three-dimensional structure and maintenance requirements. This study introduces advanced MEA platforms tailored for brain spheroids, combining flexibility and mechanical actuation with the recording of neural activity.

The first approach, the e-Flower, is a flower-shaped MEA inspired by soft microgrippers. It envelops submillimeter brain spheroids upon activation by cell culture medium, leveraging the swelling properties of a polyacrylic acid hydrogel grafted to a polyimide substrate. The e-Flower achieves tunable curvature and enables comprehensive recording of spontaneous neural activity across the spheroid surface, demonstrating compatibility with standard electrophysiology systems without additional equipment. Comprehensive mechanical and electrical characterization of the e-Flower, coupled with predictive finite element modeling, demonstrated its potential for 3D electrophysiological recordings, as validated through a proof-of-concept experiment.

The second approach integrates electrophysiology with mechanical stimulation to study traumatic brain injury in vitro. A novel free-standing, stretchable, and perforated MEA was designed to monitor neural activity under static and dynamic mechanical loads. Finite element modeling and experimental validation identified key parameters for optimizing stretchability, failure resistance, and electrochemical stability. Incorporating a conductive polymer coating further reduced electrode impedance and enhanced mechanical performance under strain. These free-standing structures were integrated into a modular biochip capable of supporting tissue viability for up to 11 days, enabling simultaneous electrophysiological monitoring and mechanical stimulation. This system captured neural responses to low-velocity stretching and high-velocity impacts, mimicking sport-related concussions.

Overall, this work advances MEA design for neural organoids and spheroids, providing improved surface coverage, mechanical probing capabilities, and compatibility with 3D tissue requirements. It highlights the potential of combining thin-film technologies with innovative materials to develop next-generation platforms for s