Biological signals such as brainwaves and heartbeats are measured using organic electrochemical transistors (OECTs). Signals are carried either by electrons or their positively charged counterparts, known as holes. Materials that use primarily hole-driven transport are called 'p-type' materials, while those that use primarily electron-driven transport are known as 'n-type'.
Ambipolar materials can transport both electrons and holes, but so far nobody has been able to develop such a material for use in the body, as materials with n-type capability tend to break down in water-based environments. Imperial’s work, published in Nature Communications, describes what the team claims is the first ambipolar OECT that can conduct electrons as well as holes with high stability in water-based solutions.
"Proving that an n-type organic electrochemical transistor can operate in water paves the way for new sensor electronics with improved sensitivity,” said lead author Alexander Giovannitti, a PhD student at Imperial’s Department of Chemistry and Centre for Plastic Electronics.
According to the team, the problem of n-type instability in water was solved by designing new structures that prevent electrons from engaging in side-reactions. The material’s ambipolarity means it can detect positively charged sodium and potassium ions that play an important role in brain activity, and which can’t be measured with today’s OECT technology.
"It will allow new applications, particularly in the sensing of biologically important positive ions, which are not feasible with current devices,” said Giovannitti. “For example, these materials might be able to detect abnormalities in sodium and potassium ion concentrations in the brain, responsible for neuron diseases such as epilepsy."
The researchers are hopeful they can build on the study to create materials tuned to particular ions, which will allow an even wider range of biological signals to be detected.