An international research team from Japan, Germany, and United States reported creating a flexible organic transistor that features good thermal stability at temperatures up to 150°C. The new transistor has been fabricated with a biocompatible polymeric substrate (Parylene), making it potentially useful for ECoG and other types of implants. Fabrication of many implantable devices involves some steps that have to be performed at elevated temperatures (e.g. parylene annealing). In addition, device sterilization is also commonly done at elevated temperatures of 130-170°C and (optionally) an elevated chamber pressure, in a process called autoclaving. The autoclaving, done at 150°C and atmospheric pressure, takes less than 3 hours, faster than room-temperature sterilization using ethylene oxide (24 hours).
The key technological achievement in the reported study is the use of an ultrathin (2 nm) heat-resistant monolayer film for insulation between the organic semiconductor and its gate. The monolayer is synthesized by a self-assembly of long-tailed phosphonic acids and has a densely packed crystalline (rather than amorphous) structure. Such ordered chemical structure prevents a formation of pinholes during heating. The use of ultrathin monolayer between the semiconductor and its gate instead of thicker dielectric films allowed the researchers to reduce the transistor driving voltage from 20V to 2V, making it more suitable for neuroprosthetic applications. Main limitation of the reported study is the short duration of the applied heat stress (20 sec), which does not evaluate a possibility of a slow heat-induced degradation of the self-assembled monolayer.