Ultrafine-fiber thermistors for microscale biomonitoring

Abstract

We have prepared spinel Mn_1.4Co_0.9Ni_0.5Cu_0.2O₄ (MCNC) film thermistors on ultrafine aramid (poly-p-phenylene-terephthalamide) fibers using a photocrystallization technique with KrF laser irradiation at room temperature. We coated a ϕ15 μm diameter aramid fiber using an MCNC nanoparticle dispersion, and we employed KrF laser irradiation to crystallize the MCNC film on the fiber surface. The resulting ultrafine-fiber thermistor showed a good semiconducting behavior with a high thermistor constant of 2767.3 K. It also exhibited a very rapid response time of only ca. 100 ms, and the rise and drop rates were almost equivalent. These rapid responses were observed due to the small heat capacity that resulted from the minuscule cross-sectional area (S) of the fiber substrate. Thermal-transfer simulations showed that the ϕ15 μm-core fiber thermistor can detect temperature variations of microscopic objects more reliably than a sheet-type thermistor with a substrate thickness of 60 μm. Using temperature simulations of both fiber- and sheet-type thermistors, we also confirmed that the equivalence of the rise and drop timescales (τ_r and τ_d) of the response was not influenced by undesirable heat flow to/from the substrates. It is very important for τ_d/τ_r to approach 1.0 and for the value of S to be reduced as much as possible, as τ_d/τ_r is proportional to log S. The ϕ15 μm-core fiber thermistor achieved a small τ_d/τ_r value of only 1.12. The rapid and accurate temperature responses of the ultrafine MCNC fiber thermistors make them promising candidates for microscopic temperature sensors.