Flexible organic field effect transistors for biomedical microimplants using polyimide and parylene C as substrate and insulator layers

Biomedical micro implants are used as neural prostheses to restore body functions after paraplegia by means of functional electrical stimulation (FES). Polymer electronic technology offers the potential to integrate flexible electronic circuits on microelectrodes in order to overcome the limit of traditional FES systems. This paper describes an approach of flexible organic transistors in order to develop a flexible biomedical micro implant for FES use. Polyimide shows excellent biocompatibility and biostability properties for flexible multi-channel microelectrodes in neural prosthetics application (Stieglitz et al 1997 Sensors Actuators A 60 240 - 3). Therefore, it was used as a flexible substrate on which polymer transistors have to be integrated. Gold or platinum was sputtered as the gate, drain and source. In this paper polyimide has been investigated as a gate isolator because of its high flexibility and biocompatibility. Polyimide was spin coated and imidized at different temperatures and times. Pentacene (C14H22) was evaporated at UHV and 75 degrees C substrate temperature as an active layer in an organic field effect transistor (OFET). Plasma activation and self-assembled monolayer surface modification were used to advance the electrical properties of organic transistors. The whole transistor was encapsulated in parylene C that was evaporated at room temperature using a standard Gorham system ( Gorham 1966 J. Polym. Sci. A-1 4 3027 - 39). Investigation of the electrical properties of the OFET using polyimide as the isolator led to promising results.