By Ahsan Mian, Associate Professor in the Department of Mechanical and Materials Engineering at Wright State University, Dayton OH
Laser bonded microjoints between two combinations of biocompatible dissimilar materials such as: (a) titanium/polyimide (TPI); and (b) titanium-coated borosilicate glass (BSG)/polyimide (GPI) are considered. Laser-fabricated joints of such materials have potential applications in encapsulation of miniature implantable biomedical devices. First, the laser joining of TPI system will be discussed. Such laser joints were characterized by means of mechanical failure (tensile) tests, optical and scanning electron microscopy, X-ray photoelectron spectroscopy (XPS). The results suggest that the formation of the joints is a result of the creation of strong chemical bonds between Ti-containing species and certain polymeric functional groups. Next, the laser joining of glass and polyimide will be presented. To facilitate bonding, the glass substrate in this case was coated with 200 nm titanium thin film. The titanium-coated BSG/polyimide laser joined interfaces can be expected to have similar or identical bonding mechanism of polyimide and titanium. To assess neural biocompatibility of these microjoints, samples were tested with exposure in neural fluids. Both the GPI and TPI samples were evaluated for mechanical performance before and after exposure in artificial cerebrospinal fluid (CSF) for two, four and twelve weeks at room temperature. Both material systems showed initial degradation up to four weeks which then stabilized afterwards and retained similar strength until twelve weeks. The TPI system appears to exhibit better overall performance with less degradation compared to its as-received strength. The stability of the laser bonded titanium coated glass/polyimide microjoints also were studied in-vivo (by implanting on a rat brain surface for 10 days) and were compared with the earlier in-vitro (by soaking in artificial cerebrospinal fluid, CSF at room temperature for one week) data. The bond degradation in rat brain implants is similar compared to those soaked in artificial CSF solution. A separate set of samples were created using same parameters for testing the hermeticity of the laser bonds. The samples were also exposed to rat brain CSF and were tested for hermiticity at the end of ten days exposure time. It was observed that the implanted samples retained their hermeticity even after some degradation of bond strength. The effect of laser type (diode vs. Yb-doped fiber laser), thin film deposition processes (sputtering vs. chemical vapor deposition), laser processing parameters (laser power and scanning speed) on the bond strength were also been studied briefly by using experimental and finite element methods. Results obtained from such studies will also be presented and briefly discussed.
The laser welding principle is currently being used in additive manufacturing (AM) method that is termed as the next industrial revolution in manufacturing. Direct metal laser sintering (DMLS) is a typical AM process that enables the quick production of complex shaped three-dimensional (3D) parts directly from metal powder. The DMLS process creates parts in a layer-by-layer fashion by selectively fusing and consolidation of thin layers of the loose powder with a scanning laser beam. Each scanned layer represents a two-dimensional (2D) cross section of the object’s mathematically sliced CAD model. After consolidation of one cross-section, a fresh layer of powder is deposited and the process is repeated until a three-dimensional part is completed. Current and future research in AM in general at WSU and MLPC will be briefly discussed.