Temporary Tattoo pH Sensor with pH‐Responsive Hydrogel via Initiated Chemical Vapor Deposition

In the expanding field of physiological pH skin sensors, thin temporary tattoo‐based devices have gained attention because of ultra‐conformal adhesion while providing excellent water vapor transmission. Here, a pH sensor tattoo made of temporary tattoo paper, screen‐printed poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate electrodes, and pH‐responsive hydrogel deposited via initiated chemical vapor deposition, is proposed. The tattoo sensor is easily transferred on skin, maintaining full functionality, and shows excellent conformability to topographical features of epidermis. The investigation of the morphology of all layers within the sensor verifies full control of the desired width of electrodes, thickness of hydrogel, and deposition shielded areas. The hydrogel layer exhibits a reversible pH‐responsive swelling of 24.7 ± 0.3% to 38 ± 1%, with respect to the dry state, at pH 4 to pH 6, respectively. Impedance spectroscopy identifies the phase shift at 10 and 1000 Hz as an excellent pH‐related property of the sensor, which can be only ascribed to the presence of the hydrogel. The sensors ability to be operated with non‐sophisticated read‐out hardware and software is also demonstrated.