Ultrathin Fiber‐Mesh Polymer Thermistors

Flexible sensors enable on‐skin and in‐body health monitoring, which require flexible thermal protection circuits to prevent overheating and operate the devices safely. Here, ultrathin fiber‐mesh polymer positive temperature coefficient (PTC) thermistors via electrospinning are developed. The fiber‐type thermistors are composed of acrylate polymer and carbon nanofibers. The fibrous composite materials are coated with a parylene to form a core–sheath structure, which improves the repeatability of temperature characteristics. Approximately 5 µm thick fiber‐type thermistors exhibit an increase in the resistance by three orders of magnitude within ≈2 °C and repeatable temperature characteristics for up to 400 cycles. The mesh structure enables the thermistor layer to be ultra‐lightweight and transparent; the mesh‐type thermistor operates with a fiber density of 16.5 µg cm(−2), whose fiber layer has a transmittance of more than 90% in the 400–800 nm region. By fabricating the mesh thermistor on a 1.4 µm thick substrate, the thermistor operates without degradation when wrapped around a 280 µm radius needle. Furthermore, the gas‐permeable property is demonstrated by fabricating the fibrous thermistor on a mesh substrate. The proposed ultrathin mesh polymer PTC thermistors form the basis for on‐skin and implantable devices that are equipped with overheat prevention.