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Sensors and Materials, Vol. 35, No. 4 (2023) 1315–1325
MYU Tokyo
S & M 3248

Wireless Unconstrained Monitoring of Intra-oral Temperature
Using Thermistor and Telemeter Sealed in Mouthguard
Kenta Iitani,1 Kanako Kishima,2 Yuka Kasuga,2 Kumi Yokota,1 Hiroki Nitta,3Kohki Onchi,3 Gentaro Kawase,3 Kouta Umezawa,3 Koji Toma,1,4
Takahiro Arakawa,1,5 Dzung Viet Dao,6,7 and Kohji Mitsubayashi1,3*
1Department of Biomedical Devices and Instrumentation, Institute of Biomaterials and Bioengineering,Tokyo Medical and Dental University (TMDU), 2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, Japan
2Faculty of Dentistry, Tokyo Medical and Dental University (TMDU),
1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan
3Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU),
1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan
4Department of Electronic Engineering, College of Engineering, Shibaura Institute of Technology,
3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan
5Department of Electric and Electronic Engineering, Tokyo University of Technology,1404-1 Katakura, Hachioji City, Tokyo 192-0982, Japan
6School of Engineering and Built Environment, Griffith University,
Engineering Drive, Southport 4222, Australia
7Queensland Micro- and Nanotechnology Centre, Griffith University,
West Creek Road, Nathan 4111, Australia(Received October 31, 2022; accepted January 17, 2023)

Keywords: intra-oral temperature, cavitas sensors, thermistor, wearable, Bluetooth Low Energy

Monitoring body temperature has the potential to evaluate physical conditions. The oralcavity has a large accessible volume and exhibits a temperature close to the deep bodytemperature; the wireless monitoring of intra-oral temperature is also possible. However, devices
must be small enough to not interfere with tongue movement. Here, we fabricated a mouthguard(MG) thermometer that housed a telemeter, a thermistor, and a battery on the buccal side surfaceof the molar and premolar of the upper jaw. The devices were sealed between two layers of MG
material for waterproofing. Sealing the devices induced a longer response time to temperaturechanges (from 3.2 to 12.2 s), but the quantitative characteristics did not change. Rapid changes inintra-oral temperature were successfully monitored when ingesting water of various
temperatures by using the MG thermometer. Moreover, a high correlation between the differencein the intra-oral and water temperatures and the intra-oral temperature change was observed. In
addition, stable wireless measurement was possible during 3 h in an office with the wearer
engaged in normal activities. The effects of changes in ambient temperature and the wearing offace masks on intra-oral and skin temperatures were also investigated, where an effect of
ambient temperature on skin temperature but not on intra-oral temperature was observed. These
results suggest that wireless temperature monitoring in the oral cavity is feasible.
*Corresponding author: e-mail: m.bdi@tmd.ac.jp

https://doi.org/10.18494/SAM4218

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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1.

Sensors and Materials, Vol. 35, No. 4 (2023)

Introduction

Measuring body temperature is one of the most common methods to objectively determine
human health status. Although body temperature is usually measured by an individual when
discomfort occurs in the body, further advantages arise from continuous body temperaturemonitoring, such as in menstrual cycle tracking,(1) improving the diagnosis of female
infertility,(2) early detection of infectious disease,(3) evaluation of circadian physiology,(4) andearly clinical intervention for febrile neutropenia.(5) Therefore, many wearable thermometers
have been developed to enable the continuous measurement of body temperature.(6) Han et al.
developed battery-free wireless skin temperature and pressure sensors based on near-fieldcommunication technology.(7) Also, the Earable smart device, which monitors tympanum
temperature using a thermopile IR sensor, was developed by Ota et al.(8)Body temperature shows different values depending on the measurement position of the
body. Although deep body temperature, which can be measured at internal organs and the brain,
is medically required, it is difficult to measure non-invasively. Thus, the temperature of body
cavities, such as the rectal temperature, tympanic temperature, and intra-oral temperature,
which is close to deep body temperature, is measured in clinical practice. Although skintemperature is easily measured, it is sensitive to environmental disturbances such as sunlightand wind. Recently, Matsunaga et al. have developed a skin patch device for the real-time
estimation of deep body temperature based on the zero-heat-flux method that enables theaccurate estimation of deep body temperature even in windy conditions.(9) However, its
applicability to extreme environments such as underwater remains questionable. Unlike the ear
canal, the oral cavity has a relatively large space in which sensors and peripheral circuits can beplaced. Moreover, unlike the rectum, it is not only easily accessible but can also be separated
from the outside by simply closing the mouth. Therefore, the oral cavity can be considered asuitable site for the continuous measurement of body temperature.
Although detailed studies have been conducted on intra-oral temperature measurement,(10,11)
less research has been carried out on continuous measurement for daily monitoring. Moore et al.
conducted an influential study in 1999 in which they installed thermocouples in an intra-oral
retainer and connected them to a data logger by a signal wire that emerged from the mouth torecord the intra-oral temperature every 5 s for 24 h.(12) In 2015, Choi et al. developed amouthguard (MG) device for the continuous measurement of intra-oral temperature and pH withwired sensors,(13) which was updated to a wireless measurement system by Farella et al. in
2016.(14) The MG-type device developed by Farella et al. had the advantage of the simultaneous
measurement of temperature and pH but was designed with measurement circuits placed on thehard palate because of its size. In this case, the movement of the tongue would be inhibited, andthere was a scope for the miniaturization of the device. Very small oral temperature measurement
units are used to monitor the wear time of oral appliances.(15,16) However, these devices have low
time resolution (measurement every 5–15 min) because they are designed to only check the wear
time.(17)Previously, we developed an intra-oral biosensor for the continuous measurement of salivary
glucose by sealing a tiny telemeter-cum-potentiostat and enzymatic biosensor inside an MG as a

Sensors and Materials, Vol. 35, No. 4 (2023)

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series of “cavitas sensors”, which are our proposed format of healthcare sensors.(18,19) In thisstudy, we fabricated an MG-type wireless thermometer that does not make the subject aware thatthey are wearing the sensor. First, the quantitative properties and response time of a wirelessthermometer and the effect of packing the wireless thermometer with MG material on the basic
properties were evaluated. The fabricated system was then used to monitor intra-oral
temperature. We then performed human subject experiments to investigate the feasibility ofmonitoring rapid changes in intra-oral temperature, the possibility of long-term measurements,
and the effects of environmental disturbances such as ambient temperature and wearing a facemask on intra-oral temperature.

2.

Materials and Methods

2.1

Fabrication and characterization of MG thermometer

The wireless thermometer consisted of three main components as shown in Fig. 1(a). The
button cell and telemeter were the same as those used in previous papers.(19,20) A silver oxidebutton cell (SR716SW, Panasonic, Japan) was selected because of the stability of the operating
voltage. The telemeter based on Bluetooth Low Energy was custom built. The thermistor (PartNo. 504GT-2, resistance at 25 ℃ = 500.0 kΩ ± 3%, Semitec, Japan) was selected by considering
the current measurement range (0.1–3276.7 nA) and stabilized potential (+400 mV) of thetelemeter to enable sensitive measurement in a normal intra-oral temperature range (36.0–
38.0 ℃). These components were combined to build the wireless thermometer before

(a)

(b)

(c)

Fig. 1. (Color online) (a) Components and integrated system of the wireless thermometer. (b) Schematic
illustration of the MG thermometer. (c) Fabricated MG thermometer.

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Sensors and Materials, Vol. 35, No. 4 (2023)

encapsulating it between two layers of MG material. Encapsulation was performed using a heatsealing method, the same as in the previous study of pressure sensors.(20) The following is a brief
explanation of the method. First, dental stone models were made by taking dental impressions
from study participants. Second, the first layer of an MG material (product name: Erkodur,material: polyethylene terephthalate glycol (PETG), thickness: 0.5 mm, Erkodent, Germany) was
formed by vacuum forming in the dental stone mold. Third, the first layer was cut out to leave abase area of the wireless thermometer, and then vacuum forming was performed with the second
layer of the MG material with the wireless thermometer between the two layers [Fig. 1(b)].
Finally, the wireless thermometer was securely encapsulated by heat sealing both MG layers
using a hot-air gun. Figure 1(c) shows a visual image of the fabricated MG thermometer. The
thermistor was placed on the buccal side surface of the second molar of the upper jaw.
In the characterization of the device, we first evaluated the response and quantitative
character of the wireless thermometer before encapsulation. After that, the MG thermometer
was characterized. The responses of both the wireless thermometer and the MG thermometer
were acquired by changing the peripheral temperature of the devices from laboratory room
temperature to a constant temperature of water in the range of 30–50 ℃. The sampling rate of
the wireless thermistor and the MG thermistor was set at 1 Hz. Note that the working time of thedevice depends on the sampling rate and communication rate. When that rate was 1 Hz, thedevice will stop within 6 h. The working time reached 15 h with the rate settings of 0.03 Hz. In
the future, issues in the trade-off of working time and measurement rate will be solved byenergy-harvesting devices, wireless powering technology, safe and high-energy cells, or biofuel
cells.
2.2

Monitoring intra-oral temperature using MG thermometer

To demonstrate the feasibility of the MG thermometer, three types of human subject
experiments were carried out. This study was authorized by the Institutional Review Board at
Tokyo Medical and Dental University (numbers 2015-05 and D2018-054) in accordance with thelatest version of the Declaration of Helsinki. Study participants were enrolled in this study after
receiving explanations about the study and giving written informed consent.
The first experiment involved monitoring the dynamic changes in intra-oral temperaturecaused by drinking water of various temperatures (20–50 ℃). Each study participant was
equipped with their own MG thermometer, then seated on a chair, after which they took 30 mL
of water at the time indicated by the experimenter. Then, the participant swallowed the water
after 10 s of holding it in the oral cavity. The second experiment involved monitoring theintra-oral temperature during office work. In this experiment, we aimed to establish whether
study participants would experience any discomfort when working while wearing the MG
thermometer and whether there would be any problems with the stability of the wirelessmeasurements over long periods. Each study participant was asked to engage in normal activities
such as breathing, talking, drinking water, and so forth, while wearing the MG thermometer.
Measurements were taken continuously for 3 h. In the third experiment, the MG thermometer, askin temperature sensor (Model number GSP-6, Elitech, USA), a heart rate monitor (ModelSensors and Materials, Vol. 35, No. 4 (2023)

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number H1, Polar, Finland), an environmental temperature and humidity sensor (GSP-6), and anaxillary thermometer (Product number ET-C231P, Terumo, Japan) were used simultaneously[Fig. 2(a)] similarly to in a previous paper.(21) The experimental protocol involved wearing a nonwoven mask for a predetermined period while working with the room temperature adjusted to 20
or 30 ℃, as shown in Fig. 2(b). The sampling rate of the MG thermometer in all the experiments
described in this section was 1 Hz, as in the experiment described in Sect. 2.1.

3.

Results and Discussion

3.1

Basic characteristics of MG thermometer

Figure 3(a) shows the response curves of the fabricated wireless thermometer for the water ofvarious temperatures. When the wireless thermometer was moved from air to water, theresistance of the thermistor decreased rapidly in response to the temperature change and theoutput current increased. The average 90% rise time (T90 rise) and 90% recovery time
(T90recovery) of the wireless thermometer were 3.2 and 13.8 s, respectively. T90recovery was 4.3
times longer than T90rise, which was considered to be largely due to the difference in thermal
conductivity between water and air. The output currents of the wireless thermometer at eachtemperature were in good agreement with the theoretical values [Fig. 3(b)]. Figure 3(c) shows theresults of the same experiment as for Fig. 3(a), except with the wireless thermometer sealed
inside the MG layers. Dull responses were observed for both average T90rise (12.2 s) and average

(a)

(b)
Fig. 2. (Color online) (a) Instruments for evaluation of effects of environmental disturbances on body, skin, andoral temperature. (b) Experimental protocol.

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Sensors and Materials, Vol. 35, No. 4 (2023)

(a)

(b)

(c)

(d)

Fig. 3. (Color online) (a) Response curves of the wireless thermometer. (b) Comparison between measured temperatures and theoretical curve of the wireless thermometer. (c) Response curves of the MG thermometer. (d)Relationship between error caused by current reading noise and temperature.

T90recovery (60.0 s) after sealing the thermistor. The thermal conductivity of PETG is lower thanthat of water. Even with a PETG layer as thin as 0.5 mm, T90rise and T90recovery were about fourtimes longer after sealing the thermistor. Moreover, the output current values for any given
temperature before and after sealing the thermistor showed a proportional relationship with aslope of 1.01 and a correlation coefficient of R = 0.999. Thus, the sealing of the wirelessthermometer did not affect the temperature quantification. On the other hand, Bueno et al.
reported that the steady-state error for a thermometer embedded in an MG was 0.2 ℃, asobtained in a water bath test.(22) In addition, it was reported that the steady-state error is notaffected by the embedding materials.(17)Sensors and Materials, Vol. 35, No. 4 (2023)

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As a result of curve fitting of the steady-state values of the sensor response at each watertemperature, as shown in Fig. 3(c), a calibration curve of the temperature for the MG thermometer
was obtained [Eq. (1)]. The average standard deviation of the current reading by the MG
thermometer was 1.4 nA in an environment with a stable temperature. As shown in Fig. 3(d), thevariation of the temperature values calculated using Eq. (1) caused by a current change of 1.4 nA
at 30 to 50 ℃ was in the temperature range of 0.01 to 0.03 ℃. Therefore, the indicatedtemperature value included an error range of at least ±0.03 ℃. This number did not include theeffect of temperature drift from the circuit, humidity, external pressure, etc. MG thermistors
were calibrated before every experiment; thus, we considered that biases because of heat fromthe measurement circuit or by self-heating of the thermistor were canceled. In addition, since thethermistor element was sealed in the MG layers by vacuum forming, humidity around thethermistor was considered stable. Also, because of the sensor position, the thermistor element
was pressurized by only the oral soft tissue such as the buccal mucosa. That pressure wasconsidered much smaller than the pressure given from vacuum forming. Therefore, the pressure
effect could be negligible. Note that to eliminate the effects of individual differences, every
fabricated sensor needs to be calibrated before use in this method.
Temperature (℃) = 290.73e0.0421×output current (nA)
3.2

(1)

Changes in intra-oral temperature upon drinking cold and hot water

Figure 4 shows the result of intra-oral temperature monitoring before and after ingestingwater of different temperatures. The intra-oral temperature at rest (baseline) was 34.5 ℃. A
change in intra-oral temperature occurred upon drinking both cold and hot water. The results
suggest that the MG thermometer can monitor rapid changes in intra-oral temperature caused byshort events. We observed a longer response time for both T90positive recovery (changing fromlower temperature to baseline) and T90negative recovery (changing from higher temperature tobaseline) than the response time seen in Fig. 3(c). Newman et al. reported that drinking an icecold beverage had a greater effect than drinking a very hot beverage on intra-oral temperatureand that the effect was sustained for 10 min.(23) Therefore, it was considered that the long
recovery time obtained in this experiment did not originate from the device but rather
represented the characteristics of the temperature dynamics of the oral cavity. As shown in
Fig. 4(b), a proportional relationship was observed between the difference between the intra-oral
and water temperatures and the change in intra-oral temperature caused by ingesting water (ΔT).
We hypothesized that when the temperature difference between the oral temperature and water
was zero, ΔT would also be zero. However, there was a y-intercept of –2.01 ℃ as can be seenfrom Eq. (2). A possible cause of this negative shift was mouth breathing during the ingestion ofwater, which decreased the intra-oral temperature.(24)ΔT (℃) = 0.519 × diff. of baseline and water temp. (℃) − 2.01

(2)

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Sensors and Materials, Vol. 35, No. 4 (2023)

(a)

(b)

Fig. 4. (Color online) (a) Time course of intra-oral temperature upon drinking water of various temperatures. ΔT
represents the change in intra-oral temperature caused by ingesting water. (b) Relationship between difference ofbaseline and drinking water temperature and ΔT.

3.3

Monitoring intra-oral temperature during office work

Figure 5 shows the time series data obtained by a participant wearing the MG thermometer
while performing standard work in an office. Since the MG thermometer was easily detachable,
the participant took lunch with it detached, then attached it after lunch to begin measurements.
Such ease of detachment and reattachment cannot be achieved with implantable devices. In
addition, stable intra-oral temperature monitoring was possible without interruption over 3 h,
despite the telemeter being placed in the water-rich oral cavity. This result indicates that there
were no problems with wireless communication inside and outside the oral cavity and that the
MG thermometer was securely sealed. In a post-experiment questionnaire, participants reported
no difficulty in working while wearing the MG thermometer. Particularly, subjects did notexperience any problems with speech, swallowing, or other oral movements, but somecommented that they were aware that they were wearing a device. Such discomfort is expected
to lessen with longer wearing time. According to the results, the developed MG thermometer canbe used for the unconstrained measurement of intra-oral temperature.
3.4

Effect of environmental temperature on intra-oral temperature

Figures 6(a) and 6(b) show the intra-oral and axillary temperature, skin temperature,heart rate, room temperature, and humidity from top to bottom with air conditioning set at
20 and 30 ℃, respectively. There were no differences in intra-oral temperature, axillarytemperature, skin temperature, and heart rate with or without wearing a non-woven mask at both
environmental temperatures. Although the results of this experiment were limited to 30 min, therisk of heat stroke was considered low for office work while wearing a non-woven mask in a hot
and humid environment.
Table 1 summarizes the average values at each environmental temperature. A large effect ofenvironmental temperature on heart rate and skin temperature was observed. Several studies

Sensors and Materials, Vol. 35, No. 4 (2023)

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Fig. 5. (Color online) Result of intra-oral temperature monitoring in an office work situation. RT represents room
temperature.

(a)

(b)

Fig. 6. (Color online) Intra-oral temperature (OT) and axillary temperature (AT), skin temperature (ST), heart rate(HR), room temperature (RT), and humidity (from top to bottom) at (a) 20 ℃ and (b) 30 ℃.

have reported that ambient temperatures lower or higher than 22 ℃ increase the heart rate evenfor a person at rest.(25,26) However, it would be difficult to conclude that the reasons for the large

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Table 1
Summary of average values at different environmental temperatures.
Intra-oral temperature (℃) Axillary temperature (℃) Skin temperature (℃)
20 ℃
36.4
36.7
30.6
30 ℃
37.1
36.8
34.9

Heart rate (bpm)
72.8
87.7

changes in heart rate observed in this experiment depend solely on environmental factors. The
skin temperature is known to decrease due to perspiration and is also sensitive to environmental
influences.(27) Hence, the accurate monitoring of body temperature based on skin temperature
measurement will require additional sensors and algorithms that cancel fluctuating disturbances.
In contrast, the intra-oral temperature was not affected by the ambient temperature. Therefore,
the continuous monitoring of body temperature by the MG thermometer can be realized with asimpler configuration than that for skin temperature measurement.

4.

Conclusions

We fabricated a wireless MG thermometer with continuous measurement capability.
Although the wireless thermometer was encapsulated between two layers of MG material, no
noticeable effects were observed that would be problematic for the response time andquantification to temperature changes. The MG thermometer successfully followed rapidchanges in intra-oral temperature when a participant ingested water, provided stable wirelessmeasurement in a real-world setting, and monitored the intra-oral temperature without being
affected by environmental disturbances. An MG device has also been developed that takes
advantage of the oral cavity as an entry point for food to estimate what is being eaten throughthe use of an inertial sensor and a thermometer.(28) The wireless monitoring of intra-oral
temperature demonstrated in this study has potential applications in various research areas, suchas evaluating the performance of athletes, assessing the quality of sleep, and detecting infectious
diseases and heat stroke early as part of preventive medicine.

Acknowledgments
This work was supported by the Japan Society for the Promotion of Science (JSPS)
KAKENHI Grant Number JP19KK0259 and the Cooperative Research Project of the ResearchCenter for Biomedical Engineering.

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