Stretchable, printable, and tunable PDMS-CaCl2 microcomposite for capacitive humidity sensors on textiles

In this research, a stretchable, printable, and tunable humidity sensing microcomposite composed of poly-dimethylsiloxane (PDMS) and CaCl2 for humidity sensing on a textile substrate was developed. In the microcomposite, deliquescent CaCl2 is encapsulated within independent microcavities in the PDMS. Based on theoretical calculations for this system, the low Young modulus of PDMS is found to be important for high water absorption capacity and high sensitivity. The dielectric properties of the microcomposite were experimentally measured, and the increased permittivity of the microcomposite with increasing relative humidity exhibited the trend predicted by theoretical calculations. For the most sensitive microcomposite, the permittivity increased by 10.2% from 30%RH (relative humidity) to 60%RH, corresponding to a sensitivity that is 2.7-fold greater than that of a conventional polyimide humidity sensor. A humidity sensor printed on a textile substrate was also fabricated with this microcomposite by screen printing.