Extended Abstracts of the Second Asian Conference on Thermal Sciences, 2nd ACTS
October 3-7, 2021, Online
ACTS-1032

DEVELOPMENT OF GEL-TYPE TEMPERATURE SENSOR USINGFLUORESCENCE POLARIZATION METHOD
Aoi Shiomi1, Kazuya Tatsumi1*, Ryoya Toma1, Reiko Kuriyama1, Kazuyoshi Nakabe1,Hidetoshi Kotera2
1

2

Kyoto University, Nishikyo-ku, Kyoto, 615-8540, Japan
Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama, 351-0198, Japan

KEY WORDS: Temperature measurement, Fluorescence polarization, Gel, Heat transfer, Evaporation
1. INTRODUCTIONIn this study, we are developing a gel-type sensor that can measure the surface temperature of human bodyand the biochemicals contained in the sweat and urine of human to diagnose health and stress conditions [1].
To obtain the gel temperature, we mix the fluorescence probe in the gel and measure the fluorescencepolarization characteristics. Linearly polarized excitation light is irradiated to the gel, and the polarization
degree of the fluorescence is measured to evaluate the gel temperature [2]. Fluorescence polarization (FP) isless affected by the intensity variation of the excitation light and ambient light, and is believed to give thetemperature of high accuracy. On the other hand, the temperature distribution in the gel and evaporation canaffect the performance of the temperature measurement of the surface to which the gel is attached. In this study,
we measured the FP of the gel attached to the isothermal wall, and analized the time distribution of the FP. We
evaluated the effects of the gel thickness and evaporation, and developed a model based on the heat transfer
characteristic of the gel to improve the measurement accuracy and also reduce the measurement time.

2. MEASUREMENT PRINCIPLES AND METHODS
2.1 Fluorescence polarization method As shown in Fig. 1(a), when fluorescent molecules are irradiated withlinearly polarized excitation light, molecules with absorption moments parallel to the polarization direction areselectively excited. The excited fluorescent molecules emit fluorescence polarized in the direction parallel to theabsorption moment. If the fluorescent molecules are dissolved, the molecules experience rotational Brownian
motion resulting in depolarization. The Brownian motion of fluorescent molecules varies with the solution

(b)

(a)

Photomultiplier 2

Photomultiplier 1
Polarizing prism

Fluorescence filter

Excitation filter
LED

Half mirror
Objective lensCollimator
Polarizing plateGelStage

Degree of polarization P [-]

(c)
0.21
0.20
0.19
0.18
0.17
0.16
0.15
0.14
0.13

Measurement
Model(d)

34°C
42°C

050

100

150

200

250300Time t [s]Fig. 1 Schematic images of (a) principle of fluorescence polarization of molecule with rotational
Brownian motion, (b) photograph of the gel sample during measurement, (c) measurement system,(d) relationship between polarization degree P of fluorescence and time t.
*Corresponding Author: tatsumi@me.kyoto-u.ac.jp

ACTS-1032

temperature, including the effect of changes in the viscosity of the solution. Therefore, the solution temperature
can be measured by measuring FP. Here, let the polarization degree of the fluorescence be P, and the fluorescenceintensities in the parallel and perpendicular directions to the polarization direction of the excitation light be 𝐼∥ and𝐼⊥ , respectively: P can be defined as 𝑃 = (𝐼∥ − 𝐼⊥ )⁄(𝐼∥ + 𝐼⊥ ).
2.2 Experimental apparatus and conditions Figures 1(b) and (c) show the sample and the optical system of themeasurement, respectively. The light from the LED is linearly polarized. The 𝐼∥ and 𝐼⊥ components of thefluorescence emitted from the gel are measured by two photomultipliers. The gel consisted of 2.5 wt%
hydroxyethyl cellulose (HEC), 50 wt% sucrose, and 1.0 × 10-6 mol/L uranin. A droplet of the gel was attached
on a cover glass, which was set on the temperature-controlled stage for measurement. In order to obtain thecalibration equation for the relationship between P and the temperature of the gel, preliminary measurements
were performed using a gel-sealed preparation glass. Preparation is made by placing a 1-mm silicon sheet on
both ends of a cover glass and sandwiching the solution between the cover glasses.

3. EXPERIMENTAL RESULTS AND DISCUSSION
The temperature of the gel changes over time due to heat transfer with the stage surface and the ambient air after
it is placed on the stage. Therefore, we need to estimate the wall temperature from the distribution of thepolarization degree. Figure 1(d) shows the distribution of the polarization degree P measured after the cover glasswith gel droplet was placed on the stage (stage temperature was set to 34 °C). Polarization degree decreases withtime, and the time P reaches a minimum is when the average temperature of the gel is closest to Tw. We obtain thewall temperature from this P applying the calibration formula for P and T. In the range of wall temperature 34 °C
to 42 °C, the maximum error of the measurement from the wall temperature was 4.67%.
It should be noted that the reason P is not constant after the minimum value in Fig. 1(d) is attributed to water
evaporation. The evaporation increases the sucrose concentration, which leads to increase of viscosity and P.
However, the time when the polarization degree reached its minimum, the effect of evaporation was 0.46 °C.
To model the time variation of the P, we used Eq. (1) and fitting it to the region of decreasing P as shown in Fig.
1(d). The minimum value of P was obtained from the asymptotic value (t → ∞). cp, k, ρ, and d are the specificheat capacity, thermal conductivity, density, and the thickness of gel, respectively.  is the correction factor.
𝛼𝑘(1)𝑃(𝑡) = 𝑃(∞) + (𝑃(0) − 𝑃(∞)) exp (−𝑡)𝜌𝑐p 𝑑2The error of the temperature obtained from P (t → ∞) was 2.24 % showing that the model can accurately predict
the wall temperature. It can also help reducing the measurement time.
However, the correction factor  was dependent on d. We measured the relationship between P and d using thepreparation with gel of thickness of 0.5, 1, 1.5, 2, 2.5 and 3 mm. In this case, the solution in the preparation is notaffected by evaporation and the effect of heat transfer only can be considered. The relationship between  and dwas  = 0.94 d. In addition to this, the minimum value of P was also affected by d. This error was due to thetemperature gradient generated in the gel even in steady state. Therefore, thickness should be evaluated from thetime distribution of P and be applied to correct the measured surface temperature. We will discuss this effect andthe applied model in the presentation.

4. CONCLUSIONSTo improve the accuracy and speed of wall temperature measurement by fluorescence polarization method, aheat transfer model was applied to the time distribution of the polarization degree. The correction factor in themodel varied linearly with the thickness of the gel. We needed to apply the effect of the thickness to also
correct the measurement error due to the temperature gradient generated in the gel. From these modifications,
the wall surface temperature was measured reasonably well.

ACKNOWLEDGMENT
This research was supported by the Center of Innovation (COI) Program of JST.

REFERENCES[1][2]

Research Promotion Institution for COI Site, Kyoto University, The last 5X innovation R＆D Center for a smart, Happy, andResilient Society. July 16, 2021, from http://www.coi.kyoto-u.ac.jp/, (2014).
Tatsumi, K., Hsu, C.H., Suzuki, A., Nakabe, K., “Liquid temperature measurement method in microchannels by using
fluorescence polarization,” Heat Mass Transfer, 54, pp. 2607-2616, (2018).