Defect-Engineered ZIF-8-Derived Nanoporous Carbon for Ultra-Stable Flexible Cryogenic Thermistors and Ultra-Miniature Physiological Sensors.

In cryogenic electronics and biomedicine, there is a growing need for flexible temperature sensors operating reliably down to liquid-nitrogen temperatures while conforming to complex, moving surfaces. However, state-of-the-art metal or ceramic thermistors are rigid and bulky, whereas polymer and hydrogel sensors lose conductivity or drift severely below -150°C, making robust cryogenic sensing extremely challenging. Herein, we design a ZIF-8-derived nanoporous carbon as a cryogenic thermistor material by rational defect engineering. By tuning the carbonization temperature, the heteroatom/vacancy defect landscape (N, O, Zn, and carbon vacancies) is programmed to balance percolated graphitic pathways and dense ionized impurity centers, leading to ionized-impurity-scattering-dominated transport at deep cryogenic temperatures. The printed carbon thermistors exhibit a wide operating window from 150 to -190°C, a high cryogenic temperature coefficient of resistance (TCR) up to -4.7%/°C, a temperature resolution of 0.05°C, and a response time of 0.58 s. After 100 days of immersion in liquid nitrogen, the TCR drift remains within ±0.2%, evidencing outstanding long-term cryogenic stability. Benefiting from the printable nature, 0.3 × 0.3 mm2 micro-thermistors are integrated into cryopreservation needles and oxygen tubes for real-time in vivo freeze-thaw monitoring and contactless sensing, demonstrating the promise of defect-engineered MOF-derived carbons for next-generation flexible cryogenics.