Ultrafine-fiber thermistors for microscale biomonitoring

Biomonitoring devices are electronic devices that are designed to monitor and record physiological signals or biochemical markers in living organisms. These devices are commonly used in medical research, clinical diagnostics, and personal health monitoring. There are many different types of biomonitoring devices, such as wearable fitness trackers, continuous glucose monitors, electrocardiogram monitors, blood pressure monitors, pulse oximeters and breath analyzers. Recently, the application of microscopic sensing devices in precise medical treatments and single-cell analysis in new cell engineering continue to attract growing research interest. Both cases require detecting temperatures at cell sizes around 5 – 25 µm. Additionally, contact thermometry and image sensing are commonly used for monitoring microscopic temperature. Both methods are beneficial in obtaining two-dimensional (2D) temperature image data in real time. However, both methods face optical-calibration problems while fluorescent imaging is an invasive technique, making them unsuitable for precise temperature monitoring. Moreover, conventional methods fail to accurately detect microscale temperature variations.

In order to overcome these shortcomings, it is recommendable to prepare sensor elements on substrates that are adequately small to avoid unnecessary disturbance of the true heat variation of tiny objects. While this can be achieved by fabricating ceramic-film thermistor sensors on resin-fiber substrates, efficient preparation of ceramic materials on resin substrates presents another challenge since substrates cannot withstand high temperatures. More recently, photocrystallization (PC) techniques-based solution-deposition processes have emerged as promising alternatives for fabricating stable and flexible sensor films. This technique has several advantages, including allowing low-temperature fabrication of ceramic films on free-form resin substrates, otherwise known as free-form film ceramics.

Herein, Dr. Tomohiko Nakajima and Dr. Tetsuo Tsuchiya from the National Institute of Advanced Industrial Science and Technology fabricated a polycrystalline Mn-based spinel Mn_1.4Co_0.9Ni_0.5Cu_0.2O₄ (MCNC) film thermistors on ultrafine aramid (poly-pphenylene-terephthalamide) fibers for microscale biomonitoring. The MCNC was prepared using a photocrystallization technique through KrF laser irradiation at room temperature. Additionally, ∅15 μm-diameter aramid fiber was coated with MCNC nanoparticle dispersion, and KrF laser was also used to crystallize MCNC film on the fiber surface. Their work is currently published in the peer-reviewed Journal of Materials Chemistry C.

The research team showed the resulting ultrafine-fiber thermistor exhibited remarkable semiconducting behavior, a high thermistor constant of 2767.3 K as well as a very rapid response time of only ca. 100 ms with almost equivalent rise and drop rates. The rapid responses were attributed to the small heat capacity due to the minuscule cross-section of the fiber substrate. The feasibility of this approach in monitoring the temperature of microscopic objects was demonstrated by sufficiently reducing the thermal influence thermistor substrate. The thermal-transfer simulation results revealed the capability of ∅15 μm-core fiber thermistor for accurately detecting temperature variations of microscopic objects.

Nakajima, & Tsuchiya conducted temperature simulations of both sheet- and fiber-type thermistors. Results confirmed that the equivalence of drop and rise timescales of the responses was not influenced by the unwanted heat flow between the substrates. To enhance the accuracy, it was important for the rise/drop time scales to approach 1.0 and the value of S to be reduced significantly as possible because drop/rise time scales are proportional to log S.

In summary, the authors demonstrated the feasibility of the new ultrafine-fiber thermistors for microscale biomonitoring. Ultrafine-fiber thermistors exhibited superior performance than sheet sheet-type thermistors with 60 µm-thick substrates owing to their numerous advantages, like rapid and accurate temperature responses. In a statement to Advances in Engineering, the authors explained that the numerous performance advantages of the present thermistors make the ultrafine MCNC fiber thermistor a promising candidate for microscopic biomonitoring applications.