Highly sensitive non-classical strain gauge using organic heptazole thin-film transistor circuit on a flexible substrate

A non-classical organic strain gauge as a voltage signal sensor is reported, using an inverter-type thin-film transistor (TFT) circuit, which is able to sensitively measure a large quantity of elastic strain (up to ≈2.48%), which approaches an almost folding state. Novel heptazole-based organic TFTs are chosen to be incorporated in this gauge circuit; organic solid heptazole has small domain size in general. While large crystal domain-pentacene TFTs seldom show sufficient current variation upon mechanical bending for tensile strain, these heptazole TFTs demonstrate a significant variation for the same strain condition as applied to pentacene devices. In addition, the pentacene channel does not recover to its original electric state after bending but heptazole channels are very elastic and reversible, even after going through serious bending. More interesting is that the heptazole TFTs show only a little variation of signal current under horizontal direction strain, while they make a significant amount of current decrease under vertical direction strain. Utilizing the anisotropic response to the tensile bending strain, an ultrasensitive voltage output strain gauge composed of a horizontally and vertically oriented TFT couple is demonstrated. An ultrasensitive organic strain gauge as an inverter-type thin-film transistor (TFT) circuit, which uses heptazole-based organic TFTs, is reported. On a plastic substrate, this non-classical organic strain gauge sensitively measures 2.48% tensile strain by bending, which leads to a minimum radius of 1 mm. Both strain sensitivity and sensing speed of our strain gauge are good enough to be used as a muscle motion sensor attached on a human arm.