Performance of Laser Microjoints in Rat Brain

The stability of the laser bonded titanium coated glass/polyimide microjoints were studied in-vivo (by implanting on the rat brain surface for 10 days) and were compared with the earlier in-vitro (by soaking in artificial cerebrospinal fluid, CSF at 37°C for one week) data. In the current state, the strength of the joints were measured by a specially designed instrument called "pressure test" equipment where the samples were subjected to a variable pressure load (using high pressure nitrogen) controlled by a pressure regulator. The strength of the joints seems to degrade as a result of soaking in rat brain. The bond degradation in rat brain implants is similar compared to those soaked in CSF solution. Polyimide uptakes water through existing pores in it and also water gets in the joint region through the edges of the samples. Water might have caused oxidation of the chemical bonds which are thought to have formed by the laser fabrication process. Water availability at the joint region in implanted samples is less than that of the CSF soaked samples-which explains the better retention of the joint strength of the vivo samples. The average failure load from pressure test was found to be 1.4 N/mm, which is below the average tensile strength of the joint (7.3 N/mm) as published elsewhere by the authors. The difference between the joint strengths results from the difference in applied mechanical (joint peeling in pressure testing and joint shearing in tensile testing) loads and loading rates by the experiments.