Silver fractal dendrites for highly sensitive and transparent polymer thermistors

Effective temperature measurement using non-invasive sensors finds applications in virtually every field of human life. Recently, significant efforts have been made toward developing polymer positive temperature coefficient (PTC) thermistors because they have advantages including flexibility, conformability, and biocompatibility. However, most polymer PTC thermistors still have issues such as low sensitivity, low optical transparency, and poor operational durability because of low electrical conductivity and inefficient hopping transport of conventional conductive filler. Here, a highly sensitive and transparent polymer thermistor composed of silver fractal dendrites (AgFDs) and a polyacrylate (PA) matrix has been successfully demonstrated. A AgFDs-PA composite film exhibits a superior PTC effect (about 10⁴ Ω °C^-1) around 35 °C because of the high electrical conductivity of the AgFDs and the quantum tunneling effect among them. A thermistor based on the AgFDs-PA composite shows excellent sensitivity, PTC intensity (∼10⁷), and sensing resolution through dramatic resistance changes from thousands to billions of ohms in the human body temperature range (34-37 °C). Moreover, it exhibits excellent optical transparency (82.14%), mechanical flexibility, and operational durability. An electrical impedance spectroscopy analysis shows that the distance between the AgFDs increases with temperature, which implies that the quantum tunneling effect amplified by the branches of the AgFDs has a significant influence on the changes in resistance. This characteristic makes the thermistor immediately suitable for monitoring body temperature. We anticipate that the new thermistor based on the AgFDs-PA composite can be a key component of various sensing applications.