Incorporation of Carbon Electrodes on Flexible Microelectrode Arrays
Allison Yorita | 20-LW-038
Implantable chemical sensors track minute changes in the brain chemistry to better understand both brain function and dysfunction. By being able to record chemical dynamics in real time, neuroscientists can begin to elucidate how neurotransmitters come into play for issues such as neurodegeneration as a step towards developing treatments. In particular, carbon has proven to be an electrode material that is effective at detecting a class of neurotransmitters called catecholamines (examples of which include dopamine or epinephrine). However, current implantable technologies that utilize carbon rely on single carbon fiber electrodes, meaning creating arrays of these fibers for greater spatial resolution is difficult and time-consuming. Additionally, typical methods to create carbon material require high temperatures. Because our flexible implantable arrays are made of a polyimide (PI) polymer, exposure to these high temperatures is not possible. Thus, this project sought to leverage microfabrication techniques to incorporate carbon materials onto arrays of microelectrodes while preventing exposure of the polymer substrate to high temperatures. We focused on two carbon materials: carbon nanotubes (CNT) and laser-pyrolyzed carbon, each with different microfabrication approaches.
Prototype sensors were designed and fabricated out of both carbon materials. For CNT-based electrodes, the CNT material was grown and patterned prior to deposition of the PI in a "bottom-up" approach to avoid PI temperature limitations. For laser-pyrolyzed carbon, a layer of PI was deposited, which was then selectively pyrolyzed in an array pattern to create the carbon material. The microfabrication process, which involves the deposition and patterning of subsequent metal trace materials and the PI, was refined and tested to incorporate both of these materials into microelectrode arrays. Additionally, electrochemical characterization methods were established to evaluate the differences in the two electrode materials. As a starting point, microelectrode arrays with gold electrodes were used to test the system, demonstrating clear ability to discern between varying concentrations of dopamine. Finally, practice probes were successfully implanted in a rat model. Very little tissue damage was noted as a result of the implantation, demonstrating the ability for flexible probes to cause little tissue damage when surgically implanted. Overall, these results demonstrate incorporation of a new electrode material that had not been previously introduced on our flexible microelectrode arrays, along with the ability to characterize the probes both in vitro and in vivo. This new sensing capability adds additional modes of sensing to the implantable sensor platform, giving neuroscientists more advanced tools by which to better understand the brain.
By adding additional modalities of sensing to the implantable sensor platform, this research contributes to Lawrence Livermore National Laboratory (LLNL) and NNSA missions by further developing technologies to address future national security challenges. Most of the sensing technology for which the flexible microelectrode arrays are used are for detecting electrophysiological signals. With this new sensing capability, there is opportunity to expand the type of sensing possible. Carbon materials can be used not only to detect catecholamines, but also as a new electrode substrate by which other types of chemical sensing can be done. This work aims to leverage the benefits of the previously developed LLNL flexible microelectrode array (long-lifetime, softer materials, better biocompatibility) with the benefits of using carbon as an electrode material to push into addressing new challenges relevant to national security. Looking into real-time chemical sensing in other areas of the body outside of the brain, the technology developed through this project has the potential to assist in detection of chemical or biological threats.