UKH: A Three-Layer Hybrid Microelectrode Array Architecture for Volumetric Recording of Brain Organoids — A Quantitative Coverage Model

Conventional microelectrode array (MEA) platforms for brain organoid electrophysiology are geometrically limited to surface recordings, accessing at most 10–15% of neurons in a 2 mm organoid. Recent advances in 3D MEA design — including self-folding structures, mesh electronics, and circumferential probe arrays — have improved surface coverage but remain fundamentally two-dimensional or fail to achieve simultaneous volumetric access across all depth layers. Here we propose UKH (Three-Layer Hybrid), a conceptual MEA architecture combining three complementary electrode modalities: (K1) a high-density surface MEA (~5,000 electrodes); (K2) a flexible SU-8 mesh layer for in-growth during organogenesis (~100 electrodes); and (K3) a central ultra-flexible tungsten mini-probe (~200 electrodes). All layers are synchronized via PTP hardware clock. We survey the existing 3D MEA landscape, identify the residual gap, propose the UKH architecture, outline a four-phase development roadmap, and describe integration with the open-source Axon analysis pipeline. UKH is a conceptual framework; no prototype has been constructed. Version 2 adds a physics-based quantitative coverage model, derivation, and sensitivity analysis to the original qualitative proposal.