Annealing effects on flexible multi-layered parylene-based sensors

To mitigate long term, soaking-induced delamination failure of multi-layered Parylene C devices, a post-process annealing step can be employed to increase adhesion between the Parylene layers. However, it has been shown that annealing of Parylene thin films can alter the bulk properties of the polymer, and thus impact final device performance. To elucidate these effects, the mechanical and electrochemical properties, and sensing performance of untreated and annealed Parylene C-platinum electrochemical impedance-based force sensors were compared. Annealing reduced the height (~3%) and increased the stiffness of the Parylene C sensing channel structure (~1.6x), affecting the sensor´s mechanical response. Furthermore, the electrode surface was smoothed as built-in residual stresses were removed during annealing, altering the sensor´s electrochemical properties. Together, these phenomena resulted in a 24% reduction in sensor sensitivity. These results indicate that heat-based effects cannot be ignored for Parylene-metal device systems, including neural microelectrode implants, and that mechanical and electrochemical properties and performance must be determined after heat treatment, such as annealing and sterilization.