Flexible Myoelectric Potential Measurement Sheet Formed With Organic Transistors
The University of Tokyo developed a flexible myoelectric (muscle) potential measurement sheet that is about 1μm thick by using an organic transistor technology and announced its details at ISSCC 2013 (lecture number: 6.4).
It can be attached to the surface of a human body to measure a myoelectric potential. In addition, because of its slimness, the university expects that it will be attached to parts having a complicated structure such as a brain to measure their electric potentials.
In general, for measuring a myoelectric potential, amplifiers need to be embedded in sensor electrodes to improve signal quality. As a result, a sensor sheet becomes inflexible, and it becomes difficult to improve electrode density.
This time, by forming an organic transistor circuit that operates with a voltage of 2V on a PEN (polyethylene naphthalate) substrate whose thickness is about 1μm, the university realized a flexible structure and improved electrode density.
The new myoelectric potential measurement sheet consists of two sheets. One is an electrode array of 64 (8 x 8) 0.7 x 0.7mm electrodes, and the other is an amplifier array of 16 (8 x 2) amplifiers. The measurement sheet was realized by attaching the two sheets together.
This time, however, for the electrode array, the university used commercially-available electrodes designed for electrocardiogram. And it announced the evaluation results of the amplifier array. It confirmed that myoelectric potential was drastically changed by opening and closing a hand equipped with the measurement sheet.
For the amplifier, a p-type organic transistor was employed. Its gate length is 20μm. Its channel material is DNTT (organic semiconductor material) with a mobility of 1cm2/Vs. 2nm-thick SAM (self assembled monolayer) and 4nm-thick AlOx are used as gate dielectric films. The gate electrode is made of aluminium (Al).
In the past, it was difficult to improve electrode density because an amplifier was formed for each electrode. But, this time, the university employed a circuit structure in which an amplifier is shared by four electrodes to improve electrode density.
Also, there is a significant variation in the properties of organic transistors. Therefore, it is necessary to uniform the properties of amplifiers.
To solve this problem, the university measured the properties of each organic transistor in advance and selected a combination that minimizes the variation in the properties of amplifiers. Specifically, the university formed the circuit by using an ink-jet technology after forming the transistors so that the transistors are connected to one another with an optimal combination.