Laser-Induced Graphitic Electrodes as Rapid Fabrication of Thin-Film Implantable Multielectrode Arrays

In the field of implantable electronics, thin-film electrodes – usually made of polymeric substrates – have attracted attention for their flexibility and versatility. Common insulating polymer layers include parylene-C and polyimide. While thin-film technology offers many attractive advantages – such as increased compliance and minimal footprint – it also brings new challenges. Firstly, their patterning or selective removal requires a series of additional lithographic steps. Secondly, a major hindering factor is the stability of the polymer-metal adhesion under mechanically-challenging conditions within electrolyte environments.<br/>Direct laser scribing of organic substrates has recently emerged as a rapid and straightforward method for patterning conductive tracks directly from insulating layers. Laser-induced graphite (LIG) electrodes are fabricated in a one-step process. A laser beam irradiation of suitable organic precursors thermally converts the insulating material into a multilayered graphite 3D structure with various degrees of porosity. Typically, CO₂ infrared lasers are employed for LIG fabrication over organic substrates with thickness down to 25 µm, but more commonly in the range of hundreds of µm. While LIG structures offer advantages such as direct fabrication, large charge storage for supercapacitor applications, and high effective surface area for biosensing, their higher feedline resistance compared to metals hinders their usability and advancement. Vomero et al. <i>[Sci. Rep. (2018) 8:14749]</i> fabricated parylene-based LIG electrodes over a 25 µm-thick metal foil, showing the possibility of employing LIG sensors in combination with metal feedlines.<br/>Typically, the conductive layers used in microelectrode arrays (MEAs) and flexible electronics are hundreds of µm thin. Here, we present a method to fabricate graphitic carbon electrodes by direct laser scribing of insulating polymers over such thin metal layers, effectively opening the electrode sites with graphitic carbon without damaging the underlying metal tracks. The MEAs are fabricated by depositing an insulating layer of 5 µm parylene-C over a detachable substrate, then sputtering and patterning a conductive layer of 100 nm gold, and insulating again with 5 µm parylene-C. Then, a UV (355 nm) nanosecond pulsed laser is used to carbonize the parylene-C locally over the gold to open the electrode sites and contact pads. The 10-µm-thin electrodes are then released from the substrate. Therefore, no dry etching and hard mask patterning are required to open the electrode insulation, and the conductive material is directly generated from the insulating layer.<br/>First, we test the fabricated electrodes by measuring the sheet resistance of the generated graphitic carbon material over gold, where we select the optimal laser parameters for best performance. Then, we test the electrodes in phosphate-buffered saline to measure the electrochemical impedance and the charge storage capacity, as well as the charge injection capabilities for neural stimulation applications. The mechanical stability of the adhesion between graphitic carbon and gold is also tested both in dry and aqueous environments, relevant to the contact pads and electrode sites, respectively. We also investigated the miniaturization limits of the fabricated electrode sites by laser scribing in different writing modes and with different line writing densities. In summary, we propose a new and rapid method for electrode fabrication applicable to thin-film and polymer-based electronics.