PhD student George Washington University McLean, Virginia, United States
Introduction:: Recording cellular activity in soft tissue or organ systems with microelectrodes is an important tool for biomedical research and clinical applications. Traditional microelectrodes made of opaque metals such as gold, platinum, and iridium cannot read important functional parameters such as intracellular calcium dynamics, metabolic activity, or target specific cell types. Recently, optical techniques like high-speed fluorescence imaging and optogenetics have been used to measure and manipulate cells or circuits with cell-type specificity through the incorporation of light-sensitive proteins.
Combining electrical and optical modalities can provide complementary tools that leverage the advantages of both techniques. However, conventional electrophysiological studies rely heavily on opaque metal microelectrodes, which prevent direct optical probing of cells at the same sites. Also, the large mechanical mismatch between conventional rigid microelectrodes and soft tissue is the cause of many issues, such as tissue damage, inflammation, and degradation of microelectrode performance over time.
We developed optically transparent, mechanically stretchable, and chemically stable microelectrode arrays (MEAs) to address these issues. Local tissue activation times and conduction velocities during normal sinus rhythm and electrical pacing were the same when measured using the MEAs or optical mapping. Additionally, we precisely determined the voltage-calcium activation delay using MEA signals and optically mapped intracellular calcium transients through the MEAs, demonstrating faithful colocalized recording of cardiac excitation-contraction coupling.
Materials and Methods:: The fabrication process involved the spin-coating of a PMMA layer and SU-8 photoepoxy layer on a glass substrate, followed by the spin-coating of Ag NW/IPA solution on SU-8. Photolithography was used to define the Ag NW/SU-8 MEA and interconnect patterns, and another SU-8 photoepoxy layer was spin-coated to define the MEA windows. A 6 nm Au layer was electroplated on the unencapsulated Ag NW surface, and the PMMA layer was dissolved in acetone. WSTs were used to pick up the MEAs, which were then coated with a 50 nm thick transparent SiO2 layer and attached to a receiving PDMS substrate. The morphology and roughness of the Au-Ag NW networks were characterized using SEM and AFM, while optical transmission spectra were measured using a spectrophotometer. The electrical properties of the MEAs were measured using a four-point probe, and electrochemical impedance spectroscopy (EIS) was used to measure the impedance of the NW microelectrodes. The mechanical performance of the MEAs was assessed by stretching the device up to 40% of the initial length using a motorized test stand. Finite element analysis (FEA) simulations were performed using Abaqus/CAE 2018 to simulate the stretching of the MEAs. Male Sprague-Dawley rats were used in an approved animal experiment where their hearts were excised and retrograde perfused with a modified Tyrode's solution for electrical recording, with Au-Ag NW MEAs and needle electrodes, and for optical mapping with potentiometric or calcium-sensitive dyes and a CCD camera, with data analyzed using MATLAB algorithms.
Results, Conclusions, and Discussions:: Our work demonstrates Au–Ag NW-based stretchable and transparent MEAs that overcome the limitations of conventional rigid and opaque metal MEAs for multimodal electrophysiological and optical biointerfacing under mechanically active conditions, which represents a significant technological advancement in bioelectronics. Electroplating of an ultrathin Au layer on Ag NW surface simultaneously improves the chemical stability and electrochemical performance without significantly sacrificing optical transparency. The resulting Au–Ag NW microelectrodes exhibit excellent optical transparency >80%, low electrochemical impedance of 0.80–8.6 kΩ at 1 kHz, superior oxidation resistance under oxygen plasma treatment for 5 min, and chronic stability under a soak test in a PBS for >1 month, and durable mechanical performance under cyclic stretching of 600 cycles at 20% tensile strain. Successful proof-of-concept demonstrations in cardiac electrophysiology experiments demonstrate that the Au–Ag NW MEAs enable high-fidelity colocalized extracellular electrical and action potential/calcium transient optical mapping of excised perfused hearts during sinus rhythm and pacing. Our results greatly expand the landscape for MEA technologies and present high-performance mechanically compliant metal nanowires-based MEAs as promising tools to interrogate soft biological systems via multiple measurement modalities.