Creating biodegradable electronics using shellac

Scientists in Austria, Romania and Turkey have used the natural resin shellac to devise biocompatible organic field-effect transistors (OFETs), which could help make electronic gadgets biodegradable and allow easier use of OFETs in the body.

Together with their better known cousins – organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs) – OFETs are already revolutionising the electronics industry, bringing us flexible displays and light-weight solar-powered chargers. Switching the synthetic substrate material and dielectric layer to the naturally occurring shellac has a number of advantages such as low cost, low toxicity and low environmental impact.

Currently used in the fashion and beauty industry as a hard-wearing nail varnish, shellac has also been used to make gramophone records and as a furniture finish. The material is composed of a mixture of aliphatic and alicyclic hydroxy acids that are easily cross-linked by heating, resulting in a smooth, glassy substrate for the OFET devices to be built upon. It is also easy to process.

A team led by Mihai Irimia-Vladu, from Johannes Kepler University, Austria, spin coated a solution of shellac in ethanol onto an alumina–aluminium bilayer to make an inorganic–organic dielectric gate onto which an organic semiconductor – fullerene C₆₀ or pentacene – could be attached.

The shellac OFETs perform well in electrochemical tests. They display hysteresis-free behaviour, which indicates that the density of impurities or irregularities that would trap charges and cause a loss of performance is low. According to Irimia-Vladu, ‘the absence of hysteresis in the OFETs makes shellac on par with other dielectric materials of natural origin such as silk,’ with the main drawback being the brittleness of shellac. Alon Gorodetsky, an expert in biomolecular electronics at the University of California, Irvine, US, found the electronic behaviour surprising and impressive. ‘Shellac makes a welcome addition to the relatively small family of biologically compatible materials for green organic electronics,' comments Gorodetsky.

The next steps for the team are to develop flexible versions of these resins and to better understand the apparent absence of charge traps.