In vitro evaluation of the long-term stability of polyimide as a material for neural implants

In order to be used as a material for neural prostheses, polyimide has to withstand the body environment over years. To obtain more information about the long-term stability of this material, we tested three commercially available polyimides (PI2611 - HD-Microsystems (type: BPDA-PPD), U-Varnish-S - UBE (type: BPDA-PPD), Durimide 7510 - Fujifilm (type: information not provided)). Specimens were stored in phosphate buffered saline (PBS) at 37 degrees C to simulate body temperature and at elevated temperatures of 60 degrees C and 85 degrees C to accelerate aging. During the course of 20 months, stress-strain curves were measured monthly by tensile testing. From these curves the Young's modulus, the stress and strain at break, the stress at 10% strain as well as the fracture energy were extracted and used to characterize and to statistically evaluate the mechanical material properties. Mass loss was determined by weighing polyimide foils (Upilex25S - UBE) and optical measurements were conducted to examine changes in chemical structure and crystallinity. At 37 degrees C and 60 degrees C no change in material behavior could be observed, except for an increase of the Young's modulus of the BPDA-PPD type stored at 60 degrees C. This demonstrates the long-term stability of all tested polyimides with respect to PBS. All extracted quantities mentioned above, as well as the mass, decreased in specimens stored in PBS at 85 degrees C. As a comparison, BPDA-PPD type specimens stored at 85 degrees C in deionized water showed no change in any property but behaved similarly to the reference material.