Finite Element Analysis of Laser Fabricated Microjoint Performance in Cerebrospinal Fluid

Assessment of neural biocompatibility requires that materials be tested with exposure in neural fluids. Laser bonded microjoint samples made from titanium foil and polyimide film (TiPI) were evaluated for mechanical performance before and after exposure in artificial cerebrospinal fluid (CSF) for two, four and twelve weeks at 37°C. These samples represent a critical feature i.e., the microjoint — a major weakness in the bioencapsulation system. The laser microbonds showed initial degradation up to four weeks which then stabilized afterwards and retained similar strength until twelve weeks. To understand this bond degradation mechanism better, a finite element modeling approach was adopted. From the finite element results, it was revealed that the bond degradation was not owing to the hygroscopic expansion of polyimide. Rather, relaxation of the process induced residual stresses may have resulted in weakening of the bond strength as observed from experimental measurements.