Epoxy casting used as nonhermetic encapsulation technique for implantable electronic devices

Implantable medical applications experienced a rapid growth since starting in the late 1950s with the first pacemakers. However, there are only two main packaging strategies available to protect the implanted electronics from the body environment. While hermetic packaging oftentimes seems to be more feasible, nonhermetic encapsulation can be an alternative under certain conditions. Previous studies using commercially available USB flash drives (UFD) pointed out the feasibility of using epoxy resins as encapsulant material. Based on this study, the water uptake was measured according to ISO 294-3 to determine the water diffusion rate of three EPO-TEK (ET) epoxies. The average water uptake Me for ET 301 was 3.18 %mass, for ET 301-2 it was 1.81 %mass and for ET 302-3M it was 2.34 %mass. The calculated diffusion constant D for ET 301 was 3.61E-13 m2/s, for ET 301-2 it was 3.63E-13 m2/s and for ET 302-3M it was found at 1.98E-13 m2/s. Next, we developed a setup in which eight microcontrollers were fully casted into two selected types of epoxies, four respectively. The casted microcontrollers where immersed in phosphate buffered saline (PBS, pH=7.4) and incubated at a temperature of 37 °C corresponding to the human body core temperature. The electronics were tested for 78 days without malfunctioning.