sensors
ArticleReal-Time Continuous Monitoring of Oral Soft Tissue Pressure
with a Wireless Mouthguard Device for Assessing Tongue
Thrusting Habits
Hidekazu Matsumoto 1 , Keisuke Tomoto 2 , Gentaro Kawase 2 , Kenta Iitani 2 , Koji Toma 2,3 ,Takahiro Arakawa 2,4 , Kohji Mitsubayashi 2, * and Keiji Moriyama 1
1

2

3

4

*

Citation: Matsumoto, H.; Tomoto, K.;
Kawase, G.; Iitani, K.; Toma, K.;
Arakawa, T.; Mitsubayashi, K.;
Moriyama, K. Real-Time Continuous
Monitoring of Oral Soft Tissue
Pressure with a Wireless Mouthguard

Department of Maxillofacial Orthognathics, Graduate School of Medical and Dental Sciences,
Tokyo Medical and Dental University, Tokyo 113-8549, Japan; k-moriyama.mort@tmd.ac.jp (K.M.)
Department of Biomedical Devices and Instrumentation, Institute of Biomaterials and Bioengineering,Tokyo Medical and Dental University, Tokyo 101-0062, Japan
Department of Electronic Engineering, Shibaura Institute of Technology, College of Engineering,Tokyo 135-8548, Japan
Department of Electric and Electronic Engineering, Tokyo University of Technology, Tokyo 192-0982, Japan
Correspondence: m.bdi@tmd.ac.jp; Tel.: +81-3-5280-8091

Abstract: In orthodontics, understanding the pressure of oral soft tissues on teeth is important toelucidate the cause and establish treatment methods. We developed a small wireless mouthguard(MG)-type device that continuously and unrestrainedly measures pressure, which had previously
been unachieved, and evaluated its feasibility in human subjects. First, the optimal device components
were considered. Next, the devices were compared with wired-type systems. Subsequently, thedevices were fabricated for human testing to measure tongue pressure during swallowing. The
highest sensitivity (51–510 g/cm2 ) with minimum error (CV < 5%) was obtained using an MG device
with polyethylene terephthalate glycol and ethylene vinyl acetate for the lower and upper layers,respectively, and with a 4 mm PMMA plate. A high correlation coefficient (0.969) was observed
between the wired and wireless devices. In the measurements of tongue pressure on teeth during
swallowing, 132.14 ± 21.37 g/cm2 for normal and 201.17 ± 38.12 g/cm2 for simulated tongue thrust
were found to be significantly different using a t-test (n = 50, p = 6.2 × 10−19 ), which is consistent
with the results of a previous study. This device can contribute to assessing tongue thrusting habits.
In the future, this device is expected to measure changes in the pressure exerted on teeth during
daily life.

Device for Assessing Tongue
Thrusting Habits. Sensors 2023, 23,

Keywords: pressure sensor; wireless; mouthguard; oral soft tissue pressure; tongue thrust5027. https://doi.org/10.3390/
s23115027
Academic Editor: Giovanni Saggio
Received: 10 March 2023
Revised: 11 May 2023
Accepted: 18 May 2023
Published: 24 May 2023

Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms andconditions of the Creative Commons
Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

1. Introduction
Sensor devices are widely used in clinical practice to measure physical parametersthat serve as indicators of various human health conditions [1–4]. Currently, physicalparameters are measured by medical professionals at medical institutions within a limited
time period. However, since physical parameters are affected by circadian rhythms andbody position, they should be monitored continuously throughout the day to understanddiurnal fluctuations and time series data. Thus, in recent years, several studies havebeen conducted on the research and development of wearable (bio)sensors that enable
continuous measurement [5].
For instance, we developed an intraoral mouthguard (MG)-type biosensor for thecontinuous monitoring of the salivary glucose concentration, which reflects blood glucose
levels [6]. These devices would be beneficial for the simple monitoring of blood glucose
fluctuations caused by eating, sleeping, and taking medications. In addition, a contact
lens-type pressure sensor was developed to measure intraocular pressure, which is thegreatest risk factor for glaucoma in preventing blindness [7].

Sensors 2023, 23, 5027. https://doi.org/10.3390/s23115027

https://www.mdpi.com/journal/sensors

Sensors 2023, 23, 5027

levels [6]. These devices would be beneficial for the simple monitoring of blood gluc
fluctuations caused by eating, sleeping, and taking medications. In addition, a conlens-type pressure sensor was developed to measure intraocular pressure, which isgreatest risk factor for glaucoma in preventing blindness [7].
2 of 12
Measurements of several useful physical parameters can be acquired from thecavity, which has various functions, such as eating, swallowing, breathing, and speechorthodontics,
the “equilibrium
theory”
widely accepted.
This theory
explainsthat
Measurementsof several usefulphysicalisparameters
can be acquired
from theoral
cavity, which
has various
functions,such as eating,andarchspeech.
perioralmuscles
influencethe positionof theswallowing,
teeth andbreathing,
the dental
morpholo
In orthodontics,
the “equilibrium
is widelyThis theory
explains
thathas long
Therefore,
the pressure
exertedtheory”
on teethby theaccepted.
tongue andperioralmuscles
the perioral muscles influence the position of the teeth and the dental arch morphology.
tracted attention. Previously, it was reported that an imbalance among the pressure f
Therefore, the pressure exerted on teeth by the tongue and perioral muscles has longthelips, attention.
buccal mucosa,
andit tongue
induces
malocclusion[8–11].
attracted
Previously,
was reported
that animbalance among
theTongue
pressure thrustfrom is an
habit
that
aﬀectsthe balanceof inducesoral soft
tissue pressure.
Tulley thrust
definedtonguethe lips,
buccalmucosa,and tongue
malocclusion[8–11]. Tongue
is anoral thrus
habitthat affects
the balanceoralsoft tissuepressure. theTulleydefinedtheforwardmovementof ofthe
tonguetip between
upper
andtongue
lowerthrustteeth,asreaching
the forward
movementof the tongue
betweenupper1)andlower
teeth, reaching
thefrom relower
lip during
deglutition
and tip
speech(seetheFigure[12].
Althoughit varies
lower
lip
duringdeglutitionandspeech(see
Figure1)[12].
Althoughitvariesfromreportto report, tongue thrust is generally considered to potentially be associated with increa
to report, tongue thrust is generally considered to potentially be associated with increased
overjet, open bite, proclined maxillary anterior teeth, small maxillary dental arch wi
overjet, open bite, proclined maxillary anterior teeth, small maxillary dental arch width,
increased
upper
thickness,mouth
breathing,
lip incompetency
increased upper
liplipthickness,mouth
breathing,and lip andincompetency[13–16]. [13–16].

Figure 1. (a) Tongue thrust type of swallowing; (b) mature type of swallowing.

Figure 1. (a) Tongue thrust type of swallowing; (b) mature type of swallowing.
Currently, several studies have been conducted on the measurement of pressure
on teeth from oral soft tissue pressure using sensor devices to elucidate the causes ofCurrently,
severalstudieshavemethods
been conductedon the measurementof pressure
malocclusions
andestablish
treatment[17–24]. However,to the best of ourteethfrom oralsoft tissue
using sensor
devicesto theelucidatethe under
causes of maknowledge,
previous
studiespressure
have not measured
pressure
insideoral cavity
naturalconditions.
Most
studiesusedpressure
sensors
that
were
wired
to
a
power
supplyclusions and establish treatment methods [17–24]. However, to the best of our knowled
and a recording
deviceoutside theoral cavity.
A few
communication
previous
studieshaveinstalled
not measured
pressure
insidethewirelessoral cavity
under natural co
devices
have
also
been
reported,but
thecomponents
in
the
mouth
were
large [25].
tions. Most studies used pressure sensors that were wired to a power supply
and a reco
The abovementioned methods have a few drawbacks. First, electrical wires and othering
device installed outside the oral cavity. A few wireless communication devices hcomponents of a device may affect the movement and measurement results of oral
also
been reported,but the wired
components
the mouth
were
large
[25].
The abovementio
soft tissuepressure. Second,
devices inrequirea power
supplyandconnection
tomethodsa few sodrawbacks.
First,must
electrical
wires
and
other
components
peripheralhave
equipment,
measurements
be takenundercertainconditions,such of a deas body
position.
The pressureof measurement
oral soft tissue resultschangesof
notoralonlysoft
duringbreathing,
may
aﬀect
the movement
andtissuepressure. Secochewing,and speaking
also insupply
response
to changes
in thetobody
and headposition. so m
wired
devicesrequirebut
a powerandconnection
peripheral
equipment,
Therefore, it is expected to change in a complex manner during daily activities [26–30].
urements
must be taken under certain conditions, such as body position. The pressur
In addition, since it is known that physiological conditions, such as respiratory dynamics
oral
soft tissue changes not only during breathing, chewing, and speaking but also inand muscle tone, differ between the sleep and awake periods [31], measurements takensponseto changes
in the bodyhead position.
Therefore,
it isoral
expected
to change
only under
specific conditions
areandconsideredinsufficient
to ascertain
soft tissuecomplexmannerduringdaily
activities
addition,since
is knownpressure on
the teeth.
Hence,
we wanted
to [26–30].
develop aInsensor
devicethat itwouldsolve that ph
both
of
these
problems
andenablethemeasurementofthecomplex
diurnalvariation
ological conditions, such as respiratory dynamics and muscle tone, diﬀer inbetween
oral soft
tissuepressureon theteeth.
sleep
and
awake
periods[31],
measurements taken only under specific conditions are c
The aim of this study was to develop an intraoral sensor device that solves thesesidered insuﬃcient to ascertain oral soft tissue pressure on the teeth. Hence, we wan
problems and enables real-time continuous measurement of oral soft tissue pressure withouttodevelopa sensor
devicewoulddevice
solvewasbothof theseinproblems
andenable the mrestraints.
The
feasibilityof thethat
developed
evaluateda study withhuman.

urement of the complex diurnal variation in oral soft tissue pressure on the teeth.
2. Materials
andofMethods
The aimthis study was to develop an intraoral sensor device that solves thThecomponentsof theoral sensor
device include
a pressureof
sensor,small
circuitpressure
toproblems and enables
real-time
continuous
measurement
oral soft
tissuew
acquiredata,
datatransmittertorealizeunrestrained
measurement,
andbatterytopower
out restraints. The feasibility of the developed device was evaluated in a study withthese components. When placed in the oral cavity, it is necessary to prevent inflammationman.

2. Materials and Methods
The components of the oral sensor device include a pressure sensor, small circuit toacquire data, data transmitter to realize unrestrained measurement, and battery to power
Sensors2023, 23,components.
5027
3 of 12
these
When placed in the oral cavity, it is necessary to prevent inflammationof the oral cavity, which may occur due to the presence of these components. Therefore,
all components were packed with mouthguard material to ensure safety. A schematic ofof the oral cavity, which may occur due to the presence of these components. Therefore,
the device is shownallin
Figure 2a.
methodfabrication
andtooverview
devicecomponents
wereThepackedwith mouthguard
material
ensure safety.ofA theschematicof areshown in Figure 2b.the device is shown in Figure 2a. The method fabrication and overview of the device areshown in Figure 2b.

Figure 2. (a) Schematic diagram of the MG-type pressure sensor device; (b) fabrication method andFigure 2. (a) Schematic
diagram of the MG-type pressure sensor device; (b) fabrication method andoverview of the MG-type pressure sensor devices.
overview of the MG-type pressure sensor devices.
The pressure sensor used in this system should have a dynamic range of 58.9–
154.12 g/cm2 [22,32,33], which is the pressure range reported so far for the lateral surThe pressure sensor
in thissystemshouldhave a dynamic
ofbe58.9–154.12
face of theused
maxillaryanterior
palate during
swallowing.
In addition, itrange
must notaffected
2
bytemperaturechangesandshouldnotcause
discomfort
whenplacedinsidethemouth.
g/cm [22,32,33], which is the pressure range reported so far for the lateral surfaceof theTherefore, a resistive pressure sensor [17,34] with a sufficiently thin thickness was emmaxillary anterior palate during swallowing. In addition, it must not be aﬀected by2temployed (FSR400 short, upper right of Figure 2a; measurable range: 102–10,710 g/cm ,perature changes andshouldnot
cause
when0.3placed
insidetheofmouth.
Therewidth:
6.35 mm,height:15.8 discomfort± 0.15 mm, thickness:± 0.03 mm,
and sizesensing area:
diameter sensor
of 5.08 mm,
Interlink
Electronics,
Camarillo, CA,USA).
fore, a resistive pressure[17,34]witha suﬃciently
thin
thickness was employed
2(FSR400 short, upper
right of Figure
measurable
range:
102–10,710
2.1. Examinationof Sensors2a;
Suitablefor Measuring
Oral Soft
Tissue Pressureg/cm , width: 6.35
of a Pressure 0.3
Transmission
Element
mm, height: 15.8 ± 2.1.1.
0.15Selection
mm, thickness:± 0.03 mm,
and size of sensing area: diameter
When the pressure
sensor is packed
MG material, the pressure is absorbed by theof 5.08 mm, Interlink Electronics,
Camarillo,
CA,withUSA).

MG material on the upper and lower surfaces of the pressure sensor, and the pressure is
not accurately transmitted to the sensor. Therefore, we investigated a method to improve
2.1. Examination of Sensors
Suitablefor Measuring
Tissuethe transmission
efficiency
of oral soft Oraltissue Softpressure
to thePressurepressure sensor by placing
a circular and high modulus material on the sensitive area of the pressure sensor. In this2.1.1. Selection of astudy,
Pressure
Transmission Elementa plate of polymethyl methacrylate (PMMA) was selected because it is not deformedby oral sensor
soft tissueisand
is easy withto mold.
PMMA plate
visually andmanually by
When the pressure
packedMGThematerial,
thewaspressureis absorbed
positioned so that the center of the sensor’s sensitive area was aligned with the center ofthe MG material onthethe
upper and lower surfaces of the pressure sensor, and the pressure
plate.

is not accurately transmitted to the sensor. Therefore, we investigated a method to improve the transmission eﬃciency of oral soft tissue pressure to the pressure sensor byplacing a circular and high modulus material on the sensitive area of the pressure sensor.
In this study, a plate of polymethyl methacrylate (PMMA) was selected because it is notSensors 2023, 23, 5027

4 of 12

The response of the pressure sensor varied depending on the size of the circular
PMMA plate (thickness: 1.0 mm) used in this method. The suitable diameters of the PMMA
plates for pressure transmission were selected as 2, 3, 4, and 5 mm. In the fabrication of
the experimental samples, the pressure sensor was placed on the MG material (Erkoflex,
ERKODENT, Pfalzgrafenweiler, Germany) made of ethylene vinyl acetate (EVA) on a flat
surface, and each circular PMMA plate was placed at the center of the sensitive area of thepressure sensor. Subsequently, the MG material made of EVA was heated and softenedin a vacuum-forming machine (Vacuum Adapter I, Yamahachi Dental, Aichi, Japan) andpressed from above (see Figure S1). To ensure that all of the MG materials were of equal
thickness, the vacuum-forming matinee was viewed from the side during heating, andthe MG materials were pressed when they were hanging down approximately 1.5 mm.
To evaluate the characteristics of the pressure sensor, various pressures were applied
to the experimental samples using a tensile and compression testing machine (SV-55C,
Imada Seisakusho, Aichi, Japan). The current generated by the change in the resistanceof the pressure sensor was measured by applying a constant potential of +400 mV using
a potentiostat (Model 1112, Husou Electrochemical System, Kanagawa, Japan), and thepressure transfer efficiency was compared.
2.1.2. Selection of MG Material for 2D Packaging
The composition of the MG material is considered to affect the response to the pressureof the packaged sensor. The MG material used for the upper and lower surfaces of thepressure sensor was made of EVA (thickness: 1.0 mm and modulus: 7.1 × 105 g/cm2 ) andpolyethylene terephthalate glycol (PETG) (thickness: 0.5 mm and modulus: 2.1 × 107 g/cm2 ).
Each of the four combinations was then compared. Each examination was conducted fivetimes and evaluated.
2.1.3. Selection of MG Materials for 3D Packaging
The three-dimensional formation of the MG was performed by pressing the heatsoftened MG material against the plaster dental model. Because the plaster dental model
has a complex morphology, the thickness of the MG after its formation may vary frompart to part, which may affect the response of the pressure sensor. Therefore, it wasdetermined that the MG material selected on the plane was appropriate for the model.
First, impressions of the maxillary dental model were obtained using alginate impression
material (HI-TECHNICOL, GC, Tokyo, Japan), and a hard plaster (ORTHO MAX, JM
ORTHO, Tokyo, Japan) was injected to create the plaster dental model. The MG materialon the underside of the pressure sensor was pressed against the model. Subsequently,
the pressure sensor and circular PMMA plate were fixed to the palatal surface of themaxillary left central incisor, and the MG material on the upper surface of the pressure
sensor was pressure welded over it (Figure 2b). The MG-type sensor device was attached
to a dental model, and currents were measured at various pressures on the palatal surfaceof the maxillary left central incisor palate. Each examination was conducted three times
and evaluated.
2.2. Pressure Measurement with MG-Type Sensor Device
2.2.1. Fabrication of MG-Type Sensor Device
The pressure signal was measured using a Bluetooth low-energy (BLE) communication
measurement device (size: 8.5 × 28 × 3.7 mm, sampling interval: 200 ms–30 s, weight:
0.92 g, radio frequency: 2.4 GHz, applied potential: 0–2.048 V, current consumption: 3.5 mA
with BLE communication and 2.2 mA without BLE communication, analogue to digitalconverter resolution: 22 bits, Discretek, Shizuoka, Japan), which had a commercial BLE
module (BYSGJNAWY-WX, size: 8.5 × 28 × 3.7 mm, TAIYO YUDEN Co., Tokyo, Japan).
A block diagram of the device is shown in Figure S2. The BLE communication measurement
device was worn in the oral cavity, and the communication range was measured to be
2 m (Figure S3b). The PC software included functions to search for the periphery ofSensors 2023, 23, 5027

5 of 12

the wireless measuring device for MG-type sensors, receive data, graph data, save data,
set data acquisition intervals, and record events. A silver oxide button battery (size:
Φ7.9 × 1.65 mm, voltage: 1.55 V, and nominal capacity: 21 mAh, SR716SW, Panasonic,
Tokyo, Japan) was used as the power supply for the BLE device. The BLE device wasplaced on the buccal side of the molar to reduce discomfort while wearing the device. The
pressure sensor, circular PMMA plate, and BLE device were placed on the MG materialon the lower surface of the pressure sensor. The MG material on the upper surface of thepressure sensor was softened and pressure welded. Subsequently, the MG material was
trimmed and thermally welded over the entire circumference using a hot air gun to seal
and waterproof it (Figure 2b).
2.2.2. Evaluation of Sensor Characteristics
The fabricated MG device was attached to a dental model, and its response to pressure
was examined. In addition, continuous measurements at 1 s intervals were performed via
wireless communication to examine the response to various pressures. The examination
was conducted three times and evaluated.
2.3. Soft Tissue Pressure Measurement in the Mouth
2.3.1. Evaluation of Sensor Devices in the Mouth
Five Japanese male participants were selected for this study (23.5 ± 1.1 y.o.). This study
was conducted with the approval of the Ethics Review Committee of the School of Dentistryof Tokyo Medical and Dental University (approval no. D2018-054) in accordance withthe latest version of the Declaration of Helsinki. All participants provided their informed
consent with an understanding of the purpose and significance of the experiment. The
participants were selected based on the following criteria: The participants had permanentdentition, a class I molar relationship, good facial balance determined by visual inspection,and no oral habits. They had no history of orthodontic or surgical treatment, and they were
not taking any medications known to affect muscle activity.
First, wireless and wired measurements were performed simultaneously on one participant, and the outputs were compared to examine whether the frequency of the wirelessmeasurement was sufficient. The sensor was placed at the same locations as in the model
system described in Section 2.1. The sensor was connected to an extraoral data logger(PicoLog1216, Pico Technology, Cambridge, UK) and a wireless device to compare thewired and wireless outputs produced by the pressure applied from the tongue during
swallowing (Figure S4). The sampling interval for the wired measurement was set to 10 msbased on past reports. The sampling interval for wireless measurements was set to 200 ms,
the minimum interval for BLE devices.
Tongue pressure during swallowing was measured by swallowing 10 mL water. As
previous reports using electropalatography have indicated that the time of contact between
the tongue and palate during swallowing is 1.1–2.9 s [35], swallowing was performed every
10 s to ensure that the effect of each swallow on the output was fully eliminated. The
measurements were performed 15 times in succession. The participants sat in a chair andfixed their gaze in front with a natural head position, and the measurements were taken. To
reduce discomfort with the device and to stabilize the temperature of the sensor, the subjects
rested for 5 min after wearing the device before beginning the measurements in subsequent
experiments. The water temperature was set to 37 ◦ C to minimize the temperature change
during swallowing.
2.3.2. Tongue Pressure Measurement in the Mouth
The fabricated devices were calibrated for each participant before the experiment. The
pressure sensor was placed at the same position as in Section 2.3.1. The participants wore thedevice and performed 10 normal swallows and 10 swallows mimicking tongue thrust-typeswallowing, and the output of the device was measured. Tongue thrust swallowing was
explained to the participants before the experiment, and they practiced swallowing with

Sensors 2023, 22, x FOR PEER REVIEW

Sensors 2023, 23, 5027

6 ofthe simulated behavior was confirmed with visual inspection. The participants were aske
6 of 12
to perform a swallow mimicking tongue thrust and to maintain the tongue’s position athe end of the swallow. The tongue apex was then visually confirmed to be positione
betweenthe maxillary anterior teeth. The pressures of the two types of swallow were com
their tongue pressed between the maxillary anterior teeth. The accuracy of the simulated
pared
for eachusing
a t-test.
The otherexperimental
were the sam
behavior
was participant
confirmed withvisualinspection.
The participants
wereconditions
asked to perform
as athose
described
in Section
2.3.1. and to maintain the tongue’s position at the end ofswallow
mimickingtongue thrust
the swallow. The tongue apex was then visually confirmed to be positioned between the
maxillaryand
anterior
teeth. The pressures of the two types of swallow were compared for
3. Results
Discussion
each participant using a t-test. The other experimental conditions were the same as those
3.1.described
Suitable in
DeviceStructure
for Oral Soft Tissue Pressure Measurement
Section
2.3.1.

3.1.1. Eﬀect of Diameter of Circular PMMA Plate

3. Results and Discussion
thepressure
sensorwas
packedwith theMG material, the pressure-sensitiv
3.1.When
Suitable
DeviceStructure
for OralSoft
Tissue Pressure
Measurement
area
did
not
deformuniformly.
This
leads
to
a
decreasein the sensitivity and reproduc
3.1.1. Effect of Diameter of Circular PMMA Platebility. Thechangein thesensor
outputvoltageunitchangeis defined as sensitivit
When
the pressure
waspackedperwiththe pressure
MG material,
the pressure-sensitive
Therefore,
a deform
circularuniformly.
PMMA This
plate
with
pressure-transfer
element
was placed on tharea did not
leads
to a adecreasein the sensitivity
and reproducibility.
The change
in thevoltageper unit
is definedsensitivity.
There- frompressure
sensorforoutput
robustsensing.
Thepressure
circularchange
PMMA
plates as
variedin diameterfore,acircular
PMMA
plate
with
a
pressure-transfer
element
wasplacedonthepressure2
to 5 mm, and various pressures (51–510 g/cm ) were applied for optimization. As show
sensor for robust sensing. The circular PMMA plates varied in diameter from 2 to 5 mm,
in Figure3, a smaller standard deviation was observed when a PMMA plate with a diamand various pressures (51–510 g/cm2 ) were applied for optimization. As shown in Figure 3,
eter
of
less
than 4 mm
was installed.
Whena PMMA
diameterofof
5 mm waa smaller standard
deviation
was observed
whena PMMAplate
platewith
with aa diameter
lessinstalled,
it wasdeemed When
inappropriate
due tothefact of its
sensitivity
than 4 mm
was installed.
a PMMA platewitha diameterof 5large
mm wasinstalled,but larg
standard
deviation.
Among themmfactplates,largestsensitivity
withthe minimum
it was deemed
inappropriate
due2–4to theof its thelarge
sensitivity
but large
standard
deviation.
Among the2–4
mm plates,
the largest
sensitivity
withthe minimum
standard
standard
deviation
wasobservedusing
a 4 mmdiameter
PMMA
plate. The
coeﬃcient odeviation
wasobserved
using
a
4
mm
diameter
PMMA
plate.
The
coefficientofvariation
variation (CV) that was calculated using the mean output value and standard deviatio(CV) that was calculated using the mean output value and standard deviation was less
was less than 5% over the entire measurement range.
than 5% over the entire measurement range.

Figure 3. Calibrations with different diameters of PMMA plates.

Figure 3. Calibrations with diﬀerent diameters of PMMA plates.
Considering the structure of the resistive pressure sensor, when a PMMA plate witha diameter of 4 mm or smaller was installed, the effective area of the pressure-sensitive
Considering the structure of the resistive pressure sensor, when a PMMA plate wit
part decreased as the PMMA plate became smaller. Consequently, the output decreaseda diameterof 4 mm or smaller was installed, the eﬀective area of the pressure-sensitiv(see Figure S5a). As the size of the circular PMMA plate became closer to the size of thepartdecreased
as thePMMA
platesmaller.
outputdecreasesensitive
area, thespacerinfluenced
thebecame
output (seeFigureConsequently,
S5b). The positiontheof thesensor(see
Figure
S5a).
As
the
size
of
the
circularPMMA
plate
becamecloser
to
the
size
of th
causes variations among sensors, and each sensor needs to be individually calibrated
against area,
the pressure.
To reduce
the variationand the(seeneedfor theS5b).
calibrationof individual
sensitive
the spacer
influenced
the output
FigureThe positionof the senso
sensors,
high-precision
positioning
may
beuseful.
We
would
like
to
introduce
quantitativecauses variations among sensors, and each sensor needs to be individually calibrate
positioning methods in future studies. Based on the above, we determined that the 4 mm
against the pressure. To reduce the variation and the need for the calibration of individu
diameter PMMA plate was optimal for transmitting the minute pressure generated in thesensors,
high-precision
positioning
may beWe would like to introduce quantita
oral soft
tissues to the pressure
sensor within
theuseful.
MG materials.

tive positioning methods in future studies. Based on the above, we determined that the
mm diameter PMMA plate was optimal for transmitting the minute pressure generate
in the oral soft tissues to the pressure sensor within the MG materials.

Sensors 2023, 22, x FOR PEER REVIEW

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Sensors 2023, 23, 5027

7 of 123.1.2. Eﬀect of Material on the 2D Sheet-Type Sensor
eﬀect
of eachonMG
in contact
with the upper and lower surfaces of the
3.1.2.The
Effectof Material
thematerial
2D Sheet-Type
Sensorpressure sensor was evaluated. Figure 4a shows the response when the upper MG mateThe effect of each MG material in contact with the upper and lower surfaces of therial
was PETG.
Withevaluated.
a combination(lower)andwhen
PETGthe(upper),thematerialpressure senpressure
sensor was
Figure of
4a PETG
shows theresponse
upper MG
2. In addition, there was no signal at asor
wasinsensitivetoapressureof
51–510g/cm
was PETG. With a combination of PETG (lower) and PETG (upper), the pressure sensor
pressureof 51–357
g/cm2 whenusing
the2 .PETG(upper)and(lower)was insensitive
to a pressureof 51–510
g/cm
In addition,
there
wasEVAno signal
at a sensors.
pressure These2
of 51–357 g/cm
when using
PETG
and EVA measurements.
(lower) sensors. These
properties
properties
are unsuitable
fortheoralsoft(upper)
tissue pressure
In contrast,
when theareunsuitable
for
oral
soft
tissuepressuremeasurements.
In
contrast,
when
the
upper
MG
upper MG material was EVA, the sensor responded sensitively to pressure, regardless ofmaterial was
EVA, the sensor
sensitively
pressure,regardlessof 4b).
whether
theresults
whetherthe materialon theresponded
lower surface
was toPETG
or EVA(FigureThesematerialon the lower
surfacewas PETG or EVA(Figureresults
were observed
were
observedbecausethe deformation
distanceof 4b).
the These
MG on
the upper
surface of the
because the deformation distance of the MG on the upper surface of the sensor under
sensor under pressure load decreased owing to the large elastic modulus of the PETG.
pressure load decreased owing to the large elastic modulus of the PETG. Based on theseBasedon these results, EVA was used as the MG material for the upper surface of theresults, EVA was used as the MG material for the upper surface of the pressure sensor inpressure
sensorexperiments.
in the subsequent experiments.
the subsequent

Figure 4. Effect of MG material packing: (a) upper MG, PETG (on the plane); (b) upper MG, EVA (onFigure4. Eﬀectof MG
upper MG,
(onoftheplane);(b) upper MG, EVA
the plane);(c) upper
MG,material
EVA (on packing:model); (d)(a)calibrations
on PETGthe model
wireless
communication.
(on the plane); (c) upper MG, EVA (on model); (d) calibrations on the model of wireless communi3.1.3. Effect of Material on MG-Type Sensor
cation.

The response of a pressure sensor packed in MG material molded three-dimensionallyalongEﬀect
a dentalwas
A pressure sensor and PMMA plate were placed
3.1.3.
of modelMaterial
oninvestigated.
MG-Type Sensor
on the palatal surface of the maxillary left central incisor palate. As shown in Figure 4c,
The response of a pressure sensor packed in MG material molded three-dimensionthe variation at a 102 g/cm2 load was large (CV = 25.4%) when the MG material on theally along a dental model was investigated. A pressure sensor and PMMA plate werelower surface of the pressure sensor was EVA. In contrast, the CV decreased (3.1%) whenplacedon thepalatalof the formaxillaryleftsurface.
centralWhen
incisorthe
palate.
shown
in Figure
PETG wasused
as thesurface
MG material
the lower
lowerAs
MG
material
2
4c,thevariation
at a were
102 g/cm
loadfrom
wasthoselargeobtained(CV = 25.4%)
the MGwas
EVA,
the results
differentwith awhen
flat surface
andmaterial
showed on thelower surface of the pressure sensor was EVA. In contrast, the CV decreased (3.1%) when
PETG was used as the MG material for the lower surface. When the lower MG material
was EVA, the results were diﬀerent from those obtained with a flat surface and showedlarge variation. This may be due to the small gap created between the MG material andthe dental model during the three-dimensional formation. When the MG material on thelower part of the pressure sensor was EVA, which has a low elastic modulus, the MG
Sensors 2023, 23, 5027
8 of 12material deformed significantly during pressurization. As a result, there was a large variation in the responsiveness. From the results, we conclude that EVA and PETG are suitablevariation.sensors
This mayas
beupperdue to thesmall
gap createdbetween therespectively.
MG material and thefor the fabrication oflarge
MG-typeandlower
MG materials,
dental model during the three-dimensional formation. When the MG material on the lower
part of the pressure sensor was EVA, which has a low elastic modulus, the MG material
3.2. Wireless Measurement
deformed significantly during pressurization. As a result, there was a large variation in
the responsiveness.
the device
results, weconcludethat EVAPETG areasuitable
for theA wireless MG-type
pressure Fromsensorwasfabricatedbyandpackingpressure
fabricationofMG-typesensors
as
upper
and
lower
MG
materials,
respectively.
sensor and BLE telemeter connected to a pressure sensor. Suﬃcient water resistance wasconfirmed, with no3.2.
floodingof the device owing to the water immersion. The wirelessWireless Measurement
MG-type sensor showedA awirelessstepwise
outputin responseto thewas
appliedpressure,similartoMG-typepressure
sensor device
fabricatedby packing
a pressure
sensor(Figureand BLE 4d).
telemetera pressure
sensor.
Sufficient water
resistancethat of the wired system
The connected
output oftothe
wirelessmeasurement
wassmallerwas
confirmed,
withnofloodingofthedevice
owing
to
the
water
immersion.
Thethan that of the wired measurement. This was thought to be due to the diﬀerence inwirelesstheMG-type sensor showed a stepwise output in response to the applied pressure, similar toproperties of the MG
material caused by the three-dimensional formation.
that of the wired system (Figure 4d). The output of the wireless measurement was smaller
than that of the wired measurement. This was thought to be due to the difference in theof the MG material caused by the three-dimensional formation.
3.3. Oral Soft Tissue properties
Pressure Measurement

3.3.1. Comparison of
Wired
and
Wireless
3.3.
Oral Soft
TissuePressureSystems
Measurement
3.3.1.
Comparisonof Wiredand Wireless
Tongue pressure
againstthe palatal
surfaceof theSystems
maxillary left central incisor during
Tonguepressureagainstthepalatalsurfaceof theInmaxillary
incisorand
duringswallowing was measured using wired and wireless systems.
Figure left
5a,central
the upper
swallowing was measured using wired and wireless systems. In Figure 5a, the upper andlower graphs show lower
the time
profilesoftimetheprofiles
pressure
responses
obtainedusingusing
wired
andandgraphsshow theof thepressure responses
obtained
wired
wireless systems, respectively.
The
numberThe
ofnumberspikesofgenerated
by swallowing
wasthethewireless systems,
respectively.
spikes generated
by swallowing was
same
in
both
systems
(n
=
15).
The
averagerelative
error,
asper
Equation
(1),
for
the
maximumsame in both systems (n = 15). The average relative error, as per Equation (1), for the maxpressure during a swallowing event measured using the wired and wireless systems wasimum pressure during
a swallowing event measured using the wired and wireless sysnegative 7.5%. This suggests that the data sampling interval of the wireless system was nottems was negative 7.5%.
This suggeststhat thesampling
intervalofswallowing
the wirelesssysvery problematicin measuring
thedatapressure
generatedby humanevents.
tem was not very problematic in measuring the pressure generated by human swallowing
wireless output (µA) − wired output (µA)
events.
Relative error (%) =(1)
wired output (µA)

Figure 5. (a) Responses from wired and wireless systems; (b) relationship between output of wired
andwirelessmeasurements.
Figure 5. (a) Responses
from
wired
and wireless systems; (b) relationship between output of wired
and wireless measurements.

The relationship between the wired measurement output and wireless measurement
output at each time is shown in Figure 5b. The two measurement methods exhibited highcorrelation coefficients(R = 0.969).
This
indicates
thatoutput
the wirelesswireless
output(μA)− wired(μA) system can monitor
Relative
error
% =
(1)
not only
the maximum
pressure but also the change in the output during swallowing.
wired output (μA)
However, although it was short enough to measure tongue pressure during swallowing,a shorter
data theacquisition
may be needed
if thetargetof measurement
is an event
The relationshipbetween
wiredintervalmeasurement
outputandwirelessmeasurementthat requires even less time.

output at each time is shown in Figure 5b. The two measurement methods exhibited highcorrelation coeﬃcients (R = 0.969). This indicates that the wireless system can monitor not
only the maximum pressure but also the change in the output during swallowing. However, although it was short enough to measure tongue pressure during swallowing, aensors 2023, 22, x FOR PEER REVIEW

Sensors 2023, 23, 5027

3.3.2. Measurement of Tongue Pressure during Swallowing

9 of 12The mean maximum pressure on the palatal surface of the maxillary left cen
sor3.3.2.
duringswallowing
is shown
Figure
6. The pressure during swallowing wh
Measurementof Tongue
Pressureinduring
Swallowing
ickingThe
tonguewassignificantly
largerthanofthat
swallowi
mean thrustmaximumpressureon the palatal
surfacethe during
maxillarynormal
left central
incisorduring swallowing
is shown
Figure 6. deviation
The pressureofduring
swallowing pressure
while
five
participants.
The meanand instandard
the maximum
formimicking tongue thrust was significantly larger than that during normal2 swallowing in
mal swallow in the five participants was 132.14 ± 21.37 g/cm , and the mean of t
all five participants. The mean and standard deviation of the maximum pressure for the
2 , andthrust-typemumpressureforwhile
mimickingtongueswallownormalswallow intheswallowing
five participants
was 132.14± 21.37 g/cm
the mean of the2
201.17± 38.12g/cmfor. swallowing
This valuewhilewasmimicking
significantly
higherthanswallowingthat for normalswmaximumpressure
tonguethrust-type
was
2 . This value was significantly higher than that for normal swallowing
201.17±38.12
g/cm(p = 6.2 × 10−19).
−19(p = 6.2 × 10).

Figure 6. Maximum pressure values for normal swallowing and swallowing while mimicking
tongue thrust.

Figure 6. Maximum pressure values for normal swallowing and swallowing while mtongueHaroon
thrust. et al. reported that the mean pressure on the palatal surface of the max-

illary right central incisor during swallowing in eight males with Angle class I molar
2 [32], which is close to the mean pressure for normal
relationship139.61± 1.58 g/cm
Haroonwas
et al.
reported
that themean pressure on the palatal surface of the mswallowing in the present study. Sinem et al. showed that the tongue pressure on a tongue
rightcentral incisor during swallowing in eight males with Angle class I molar
crib appliance in open bite patients with the tongue thrust significantly decreased, from
2 to 142.95
2 , 10ship
was± 1.58g/cm2±[32],whichismonths
close tothepressurefornormal216.43± 139.61
65.79 g/cm
29.2 g/cm
afterthemeanset of theappliance[33].
tongue study.
crib wasSinem
an appliancejust behindthe maxillaryteeth
ingNotein thatthe thepresent
et al.placed
showedthat thetongue anteriorpressure
on a tonthat improves the tongue thrust habit by bringing the tongue into the correct position. Thus,
appliance in open bite patients with the tongue thrust significantly decreased, fro
the results of the study by Sinem suggest that tongue pressure during swallowing can be2
2 10 months after the set of the appliance [33]. N± 65.79g/cm
29.2 g/cm
differentwith toand142.95
without±tongue
thrust,inthe same participant, which is consistent withthethetongue
crib
anstudy.
appliance
placedjustresults
behindthe maxillaryanterior teeth
results of
the waspresent
Based onthe aboveand discussions,
it is assumed
that thethe
deviceused inthis study
canby
correctly
measurepressure.
provestonguethrusthabit
bringing
the tongue
tongueinto the correct position. T
A comparison of the oral soft tissue pressure measurement devices used in the previous
resultsof the study by Sinem suggest that tongue pressure during swallowing ca
studies is shown in Table 1. Our device is considered to be one of the smallest and mostferentwithofandwithout
tonguethrust in
the sameparticipant,is consist
powerful
the few
wirelessoral soft-tissuepressuremeasurement
deviceswhichavailable.
research
was conductedunder limited
Therefore,
is necessary it is
theHowever,resultsthisof thepresent
study. Basedon theconditions.
above results
anditdiscussions,infutureresearch
to
show
that
oral
soft
tissuepressurecanbemeasuredundervariousthat the device used in this study can correctly measure tongue pressure.
conditions, such as during pronunciation, exercise, and sleep. In the future, this device
A comparison
ofthe
thecomplex
oral soft
tissuepressure
is expected
to measurechangesin pressure
on measurement
the teeth over thedevices
course ofused in
vious
studies is shown in Table 1. Our device is considered to be one of the sma
the day.

most powerful of the few wireless oral soft-tissue pressure measurement devices
However, this research was conducted under limited conditions. Therefore, it is n
in future research to show that oral soft tissue pressure can be measured undeconditions, such as during pronunciation, exercise, and sleep. In the future, thisexpected to measure the complex changes in pressure on the teeth over the courday.

Sensors 2023, 23, 5027

10 of 12

Table 1. Comparison of reported oral soft tissue pressure measurement sensors.
Wire/
Wireless

Size

Dynamic
Range

Sampling
Interval

Reference

Takada et al.

Wire

Small

None stated−510 g/cm2

5 msec[19]

Kato et al.

Wireless

Large

None stated−90 g/cm2

1.4 msec[25]

Our device

Wireless

Small

51–510 g/cm2

200 msec

This work

4. Conclusions
In this study, we developed an MG-type device to measure the pressure on teeth fromoral soft tissue. A PMMA plate with a diameter of 4 mm was optimal for transmitting theminute pressure generated by the soft tissues in that oral cavity to the pressure sensor in the
MG material. EVA and PETG were suitable for fabricating MG-type sensors as materials
for the upper and lower MGs, respectively. In addition, a high correlation coefficient(0.969) was observed when comparing wired and wireless devices in human studies. In
the measurements of tongue pressure on the maxillary anterior teeth during swallowing,
132.14 ± 21.37 g/cm2 for normal and 201.17 ± 38.12 g/cm2 for simulated tongue thrust
were found to be significantly different using a t-test (n = 50, p = 6.2 × 10−19 ).
There are also several limitations to this study. First, the study was only validated
under the limited condition of swallowing in the sitting position. Future studies should
demonstrate that oral soft tissue pressure can be measured under a variety of conditions,
including during phonation, exercise, and sleep. Second, the measurement range of thedeveloped device is 51–510 g/cm. This is a sufficient measurement range when the targetof measurement is tongue pressure during swallowing. However, in order to measure lip
pressure and resting pressure, it is necessary to be able to measure even smaller forces,
and the device needs to be improved. Third, the continuous use time is limited due to thebattery capacity. Research on wireless power supplies and power harvesting devices could
help solve this problem.
This MG-type pressure-sensing device is expected to measure oral soft tissue pressure
on teeth, which changes in complex ways throughout the day, under more physiologicalconditions and over longer periods with further developments.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/s23115027/s1, Figure S1: Packing of a pressure sensor and a circular
PMMA plate; Figure S2: Algorithms for wireless measurement spacer; Figure S3: Performance of
BLE communication measurement device; Figure S4: Protocol for simultaneous wired and wirelessmeasurement; Figure S5: The influence of the pressure sensor.
Author Contributions: Conceptualization, H.M. and K.M. (Kohji Mitsubayashi); methodology, H.M.,
K.T. (Keisuke Tomoto), G.K., K.I., K.T. (Koji Toma), T.A. and K.M. (Kohji Mitsubayashi); software, K.T.
(Keisuke Tomoto), G.K. and K.I.; validation, H.M., K.T. (Keisuke Tomoto) and G.K.; formal analysis,
H.M., K.I. and K.M. (Kohji Mitsubayashi); investigation, H.M.; resources, K.I., K.T. (Koji Toma),
T.A. and K.M. (Kohji Mitsubayashi); data curation, H.M.; writing—original draft preparation, H.M.;
writing—review and editing, K.I., K.M. (Kohji Mitsubayashi) and K.M. (Keiji Moriyama); supervision,
K.M. (Kohji Mitsubayashi), and K.M. (Keiji Moriyama); project administration, K.I. and K.M. (Kohji
Mitsubayashi); funding acquisition, T.A. and K.M. (Kohji Mitsubayashi). All authors have read andagreed to the published version of the manuscript.
Funding: This research was funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI (grant number: JP19KK0259) and the Cooperative Research Project of the Research Center for
Biomedical Engineering.
Institutional Review Board Statement: The study was conducted in accordance with the Declarationof Helsinki and approved by Ethics Committee of the School of Dentistry of Tokyo Medical and
Dental University (protocol code: D2018-054; date of approval: 18 December 2018).

Sensors 2023, 23, 5027

11 of 12

Informed Consent Statement: Informed consent was obtained from all subjects involved in the
study. Written informed consent to publish this paper was obtained from the subjects.
Data Availability Statement: The data that support the findings of this study are available from thecorresponding author (K.M.) upon reasonable request.
Conflicts of Interest: The authors declare no conflict of interest.

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