Tomohito Sekine 1
, Ryo Sugano 1
, Tashiro Tomoya 1
, Jyun Sato 1
, Daisuke Kumaki 1
, Fabrice Santos 2
, Atsushi Miyabo 3
, Shizuo Tokito 1 1
, Yamagata University, Yonezawa Japan, 2
, Piezotech, Pierre-Benite France, 3
, ARKEMA K. K., Kyoto Japan
Flexible printed pressure sensors possess great potential advantages for wearable and disposable healthcare applications such as a thermometer. Recently, we reported a flexible printed pressure sensor that was fabricated using a ferroelectric polymer, Poly (vinylidenefluoride-trifluoroethylene) [P(VDF-TrFE)], by printing methods . Here, we demonstrate a newly developed flexible printed pressure sensor consisting of the ferroelectric polymer layer and an organic thin-film transistor (OTFT) on a plastic film substrate in order to monitor the human pulse rate.
The P(VDF-TrFE) layer was formed as a pressure detector and the printed OTFT was fabricated as a voltage amplifier for the pressure detector by inkjet printing and spin-coating methods. The printed OTFT and the P(VDF-TrFE) layer were fabricated on a same film substrate. This P(VDF-TrFE) layer could generate the voltage of 10 mV by applied pressure of 15 N mm-2. The characteristics of the OTFT was measured at VDS = −10 V, VGS = +10 to −10 V. The estimated field-effect mobility μeff in the saturation region for the OTFT was 0.5 cm2 V-1 s-1 and the threshold voltage was −0.17 V . We estimated the ability of the printed OTFT as the amplifier by using a simulation-program-software (LT-Spice), and found an amplification factor of 1.7. The relationship between applied pressure and output voltage in our sensor was measured. The pressure was applied to the P(VDF-TrFE) layer by using a pressure tester. The output voltage of the pressure sensor linearly displayed a clear correlation with the applied pressure and the output voltage was 17 mV in the pressure of 15 N mm-2, which is almost consistent with the simulated value. This results indicate that the output voltage generated in the P(VDF-TrFE) layer was obviously amplified by the printed OTFT. Further improvement in the pressure sensitivity would be expected by using multiple the OTFTs.
Finally, we demonstrated the monitoring of the human pulse rate using the printed pressure sensor. In the printed sensor operation, VDS and VGS of the OTFT were fixed at −3 V. That sensor was attached to the skin near the neck of a volunteer using a skin-compatible adhesive patch. The sensor clearly detected the human pulse rate and the monitored rate was 55 pulse per minute. This study demonstrates that the employment of printed OTFTs for the pressure sensor is effective for improving the sensitivity in healthcare monitoring.
The detection of the pulse rate from human is authorized by the Ethics Committee of Yamagata University (authorization code: 29-2).
 T. Sekine et al., Japanese Journal of Applied Physics, 55, 10TA18 (2016).
 R. Shiwaku et al., Scientific Reports, 6, 34723 (2016).