High Density, Transparent, Flexible, Microelectrode Array for Multimodal Cardiac Monitoring
Open Access DepositedAbstract High Density, Transparent, Flexible, Microelectrode Array for Multimodal Cardiac Monitoring The use of microelectrode arrays (MEAs) has revolutionized the study, diagnosis, and treatment of cardiac disease. The observation of electrical wave propagation through heart tissue and analysis of the individual cell populations that contribute to this phenomenon requires a robust network of high-density recording electrodes. While MEAs facilitate the study of electrical wave propagation on this scale, electrical behavior alone is no longer sufficient to fully understand the complex mechanisms underlying cardiac disease. There is also a wealth of information to be gained from optical studies of the heart, including optogenetics, or traditional dye-based fluorescence studies. The acknowledgement of cardiac disease as a complex, multiparametric issue has resulted in a desire to combine the electrical recording properties of an MEA with optical studies to achieve co-localized, simultaneous optical and electrical recording. However, the opacity of flat metal electrodes traditionally used in MEA fabrication proves to be a significant challenge in this effort. Opaque metal electrodes obscure the tissue beneath the electrode measurement site and create optical interference artifacts which result in skewed, incomplete optical data sets. The solution to this problem has been found in the form of transparent conductive materials, such as indium tin oxide and graphene. While still conductive, these materials are highly transparent, allowing electrical measurements to be made at the same time as optical measurements, with little fear of obstruction. However, these materials suffer from poor mechanical performance, leading to defects and damage when attached to the constantly mobile tissue of the heart, and are often too expensive to be practical in many applications. This paper investigates silver nanowire as a low cost, high electrical performance, high transmittance, highly flexible alternative to other transparent conductors. We present a 64 channel silver nanowire based MEA which achieves both an impedance of 29.5 kΩ at 1 kHz and an average transmittance of 88.5%, indicating strong potential for optoelectronic applications. Moreover, the use of hyper flexible substrates to support the device allowed it to withstand 5000 bending cycles around a 5 mm radius with no significant degradation of its electrical performance, indicating its ability to withstand use on beating cardiac tissue. Gold electroplating of silver nanowire electrodes was also investigated as a way to protect tissue from the cytotoxic effects of silver and protect the electrodes themselves against oxidation without sacrificing the transmittance of the device. Electroplating silver nanowire with gold for 10 seconds was found to protect the film from oxidation, demonstrating complete gold coverage without a significant sacrifice in transmittance. This paper presents a simple, versatile approach for the fabrication of a high density, flexible, transparent microelectrode array suitable for optoelectronic integration and a broad range of cardiac applications.
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