Vertically Aligned Nanopillar Electrodes: Engineered Interfaces for Electrophysiology and Cell-Electrode Coupling

Vertically aligned nanopillar structures have been extensively studied and exploited in numerous applications across various fields due to their unique features, including high surface area and aspect ratio, small size, superior electrical, mechanical, and optical characteristics. Of particular interest are their interesting interfacial properties, which make them ideal for use as electrodes for biophysiological measurements. This comprehensive review explores the growing applications and fabrication techniques of vertically aligned nanopillar electrodes, emphasizing their functions in electrophysiological sensing particularly. The review begins with an overview of the biophysical measurements offered by these electrodes, including electrocardiogram (ECG), electroencephalography (EEG), electromyography (EMG), electrooculography (EOG), and temperature monitoring. Invasive and non-invasive vertically aligned nanopillar electrodes are classified and evaluated based on characteristics such as conductivity, adhesion, breathability, biocompatibility, biodegradability, and flexibility. Various material systems are considered, including metal-based, carbon-based, and polymer-based electrodes, each contributing distinct advantages to overall performance. Fabrication methods, particularly lithography and template-assisted synthesis using porous anodic alumina (PAA) and porous silicon (pSi), are highlighted for their precision in controlling geometry and surface properties. The review concludes with an assessment of practical applications, emphasizing the enhanced capabilities of vertically aligned nanopillar electrodes in electrophysiological measurements.