By Neil Savage -
A new type of implantable electrode can get much more sensitive readings of brain waves and cost substantially less, according to a scientist studying the device. Organic electrochemical transistors (OECTs) consist of conductive polymers and liquid electrolytes, which could make for an easy interface between, say, the surface of the brain and conventional silicon electronics.
Device physicists have been studying OECTs since the early part of this century, says George Malliaras, head of the bioelectronics department at École Nationale Supérieure des Mines de Saint-Étienne, France, but they don’t yet fully understand how they work.
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Ref: High-performance transistors for bioelectronics through tuning of channel thickness. Science Advances (2015) | DOI: 10.1126/sciadv.1400251
Despite recent interest in organic electrochemical transistors (OECTs), sparked by their straightforward fabrication and high performance, the fundamental mechanism behind their operation remains largely unexplored. OECTs use an electrolyte in direct contact with a polymer channel as part of their device structure. Hence, they offer facile integration with biological milieux and are currently used as amplifying transducers for bioelectronics. Ion exchange between electrolyte and channel is believed to take place in OECTs, although the extent of this process and its impact on device characteristics are still unknown. We show that the uptake of ions from an electrolyte into a film of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) leads to a purely volumetric capacitance of 39 F/cm3. This results in a dependence of the transconductance on channel thickness, a new degree of freedom that we exploit to demonstrate high-quality recordings of human brain rhythms. Our results bring to the forefront a transistor class in which performance can be tuned independently of device footprint and provide guidelines for the design of materials that will lead to state-of-the-art transistor performance.