Observations from the Accelerated Ageing of Polyimide Substrates for Use in Neural Implants

The functionality of neural implants relies on their capability to operate in a wet, chemically reactive, and dynamic environment over extended time frames, which determines their biostability. Failure mechanisms such as delamination, insulation breakage, or metal corrosion are common examples that must be prevented. In this study, we investigate the effects of reactive oxygen species (ROS) and physiological saline solution on polyimide (PI), a widely used substrate in neural applications that directly interfaces with the host tissue. To understand the processes that could influence PI degradation over time, we subjected PI-based samples to accelerated ageing conditions and observed not only the effects of storage media but also the impact that handling conditions could potentially have on the outcome. Scanning Electron Microscopy (SEM) indicated a possible connection between degradation of the PI substrate and experimental conditions rather than chemical variables alone. Furthermore, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Fourier Transform Infrared Spectroscopy (FTIR) confirmed the infiltration of chemicals at the molecular level without any significant unexpected results. This research offers valuable insights into how PI films can deteriorate over time under the influence of different storage conditions and emphasizes the necessity of standardized protocols within the scientific community to accurately distinguish degradation originating from different factors.