(467b) Core-Shell Gallium/Polymer Microstructure for Thermal-Responsive Implantable Electrode

Implantable microelectrodes are essential tools for recording and stimulation of excitable tissues that have attracted great interest because of their potential to revolutionize the treatments of many diseases. Mechanical rigidity is essential to assist the device implantation process. However, high stiffness often causes tissue damage and ultimately induces a chronic inflammatory response, which generates gliosis around the implanted devices. Therefore, the ideal implantable electrode needs to be mechanically rigid during the tissue insertion procedure, then adapt to flexible mechanical properties in the physiological condition. Recently, stimuli-responsive materials, such as cellulose whisker and polymer gel, are researched to generate sharp mechanical property transition with water swollen. However, these mechanical transitions are confined solely for the substrate, not for the electrode material that causes delamination between those.

Here, Gallium (Ga) is introduced as an electrode material because it has a unique melting point at 29.36 °C. This indicates Ga will be a rigid solid at room temperature (Young`s modulus of 10 GPa) and a liquid (no mechanical strength) at body temperature. This provides an ideal sharp mechanical transition, rigid to soft, between 23°C to 37°C for implantable microelectrodes. In this study, we developed a novel manufacturing process to create a gallium/polymer core-shell structure. The shell can be used as not only a container to prevent leaking when Ga changed to a liquid state but an insulator. Polyether block amide (PEBAX) was selected as a shell material because of its flexibility and biocompatibility. Lastly, Poly(3,4-ethylene dioxythiophene) (PEDOT) was chosen for the ion detecting site, which helps stimulation property as well as recording. By the help of those characteristics of each material, we fabricated the 50 μm diameter thermal-responsive microelectrodes.

We evaluated the mechanical and electrochemical performance of this core-shell structure for implantable electrodes. In first, the microelectrodes have a stiffness enough to penetrate brain tissues. In addition, Young`s modulus is decreased by three orders of magnitudes from keeping at room temperature to body temperature. The result demonstrates that the microelectrodes have a lower stiffness than brain tissue, which means the devices are helpful to minimize implant history. Lastly, we confirmed that their electrochemical properties such as impedance and capacitance showed suitable performance to apply electrodes, even they have electrochemical stability for four weeks. All abilities including sharp mechanical transition in a temperature change may find uses in chronical neuro-prosthetic devices.