Next-generation flexible neural and cardiac electrode arrays

The electrical activities of the brain and heart have been recorded and analyzed for diverse clinical and pathological purposes. To construct an implantable system for monitoring the electrical activity effectively, flexible and stretchable electrode arrays that are capable of making conformal contacts on the curvilinear, soft, and dynamic surfaces of the target organs have been extensively researched. Among many strategies, the most representative approach is to fabricate electrode arrays on plastic substrates to achieve more intimate and conformal contact with the target organs. Further optimizations are along with the development of ultrathin and stretchable electronics. Advanced structural modifications, such as thinning the overall profile or applying a mesh-like electrode network, have shown the greatly enhanced conformability and deformability of the device, providing improved signal-to-noise ratios (SNRs). Furthermore, brittle but high-performance silicon transistors have been successfully incorporated in flexible forms by virtue of mechanics-based active electronics designs, enabling the construction of high-density arrays comprising hundreds of multiplexed electrodes that can be individually addressed by only a few external wires. This review summarizes these strategies and describes their strengths and weaknesses, and it suggests possible technologies for nextgeneration electrode arrays.