sensors
Communication

Mouthguard-Type Wearable Sensor for Monitoring Salivary
Turbidity to Assess Oral Hygiene
Kenta Ichikawa 1,† , Kenta Iitani 1,† , Gentaro Kawase 2 , Koji Toma 1,3 , Takahiro Arakawa 1,4 ,Dzung Viet Dao 5,6 and Kohji Mitsubayashi 1,2, *
1

2

3

4
5

6

*
†

Citation: Ichikawa, K.; Iitani, K.;
Kawase, G.; Toma, K.; Arakawa, T.;
Dao, D.V.; Mitsubayashi, K.
Mouthguard-Type Wearable Sensor
for Monitoring Salivary Turbidity toAssess Oral Hygiene. Sensors 2024, 24,
1436. https://doi.org/10.3390/
s24051436
Academic Editor: Michele Norgia

Department of Biomedical Devices and Instrumentation, Institute of Biomaterials and Bioengineering,Tokyo Medical and Dental University (TMDU), Tokyo 101-0062, Japan; ichikawa.kenta@tmd.ac.jp (K.I.);i.bdi@tmd.ac.jp (K.I.); k-toma@shibaura-it.ac.jp (K.T.); arakawath@stf.teu.ac.jp (T.A.)
Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU),Tokyo 113-8510, Japan
Department of Electronic Engineering, College of Engineering, Shibaura Institute of Technology,
Tokyo 135-8548, Japan
Department of Electric and Electronic Engineering, Tokyo University of Technology, Tokyo 192-0982, Japan
School of Engineering and Built Environment, Griffith University, Southport 4222, Australia;
d.dao@griffith.edu.au
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan 4111, Australia
Correspondence: m.bdi@tmd.ac.jp; Tel.: +81-3-5280-8091; Fax: +81-3-5280-8094
These authors contributed equally to this work.

Abstract: Salivary turbidity is a promising indicator for evaluating oral hygiene. This study proposeda wearable mouthguard-type sensor for continuous and unconstrained measurement of salivary
turbidity. The sensor evaluated turbidity by measuring the light transmittance of saliva with an LED
and a phototransistor sealed inside a double-layered mouthguard. The sensor was also embeddedwith a Bluetooth wireless module, enabling the wireless measurement of turbidity. The mouthguard
materials (polyethylene terephthalate-glycol and ethylene-vinyl acetate) and the wavelength of the
LED (405 nm) were experimentally determined to achieve high sensitivity in salivary turbidity
measurement. The turbidity quantification characteristic of the proposed sensor was evaluatedusing a turbidity standard solution, and the sensor was capable of turbidity quantification over awide dynamic range of 1–4000 FTU (formazine turbidity unit), including reported salivary turbidity(400–800 FTU). In vitro turbidity measurement using a saliva sample showed 553 FTU, which isequivalent to the same sample measured with a spectrophotometer (576 FTU). Moreover, in vivo
experiments also showed results equivalent to that measured with a spectrophotometer, and wireless measurement of salivary turbidity was realized using the mouthguard-type sensor. Based onthese results, the proposed mouthguard-type sensor has promising potential for the unconstrained
continuous evaluation of oral hygiene.
Keywords: wearable sensor; mouthguard; oral hygiene; salivary turbidity; optical sensing

Received: 27 January 2024
Revised: 15 February 2024
Accepted: 20 February 2024
Published: 23 February 2024

Copyright: © 2024 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
Evaluation of oral hygiene is instrumental in diagnosing the health status of individuals [1,2]. Poor oral hygiene not only causes oral diseases such as dental caries [3,4]and periodontal disease [5], but also carries the risk of developing various systemic diseases [6]. For example, periodontitis has been reported to be correlated with diabetes andcardiovascular disease, and the mechanism by which periodontitis develops into such a
serious systemic disease has also been reported [7,8]. Therefore, continuous evaluation oforal hygiene is expected to contribute to risk assessment, early diagnosis, and preventive
medicine for these systemic diseases.
As an evaluation index of oral hygiene, quantification of oral bacteria is widely used [9].
A typical method for quantification of oral bacteria is to culture sampled dental plaque [10]Sensors 2024, 24, 1436. https://doi.org/10.3390/s24051436

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

Sensors 2024, 24, 1436

2 of 10or tongue plaque [11]. However, the culturing process requires time and appropriate
equipment, and is not suitable for real-time evaluation of oral hygiene. As a simpler sample
for oral hygiene evaluation, mouth-rinsed water has been utilized [12,13]. Takeuchi et al.
reported that the turbidity of mouth-rinsed water is a potential indicator of several oral
health conditions, including oral hygiene [14]. On the other hand, sampling of mouthrinsed water requires the external water supply and rinsing process, making it difficult torealize a continuous oral hygiene evaluation system utilizing mouth-rinsed water closed
in the oral cavity. As with mouth-rinsed water, saliva is a notable oral biomarker thathas various health-related information [15]. Saliva also contains bacteria by retention in
the oral cavity. This bacteria contamination mechanism is similar to that of mouth-rinsed
water, and the high correlation between the bacteria in saliva and those in mouth-rinsed
water [12] indicates that salivary turbidity can be a promising indicator of oral hygiene.
Because saliva is naturally produced in daily life without any external operations, a closed
measurement system, such as an intra-oral wearable sensor [16], can be realized in theoral cavity.
In this study, we proposed a mouthguard (MG)-type wearable sensor that utilizes
salivary turbidity as an indicator for oral hygiene, which realizes continuous unconstrained
monitoring of oral hygiene in the oral cavity. In the following sections, we investigated
LED wavelengths and MG materials for optical measurement of salivary turbidity andconstructed an MG-type salivary turbidity sensor. We also evaluated the turbidity quantification characteristics of the fabricated sensor and demonstrated the measurement ofsalivary turbidity with in vitro and in vivo experiments.
2. Materials and Methods
2.1. Fabrication of Mouthguard-Type Sensor
Figure 1a shows the configuration of the MG-type salivary turbidity sensor developedin this study. This sensor consists of a phototransistor (PT, APS3227SP1C-P22, KingbrightElectronic Co, Ltd., Taiwan, 3.2 mm × 2.7 mm × 1.1 mm, peak sensitive wavelength 580 nm),
LED (SM1206UV-405-IL, Bivar Inc., Irvine, CA, USA, 3.2 mm × 1.4 mm × 1.6 mm), wireless module (customized, Discretek Inc., Shizuoka, Japan, 28 mm × 8.5 mm × 3.7 mm), andcoin cells (SR716SW, Panasonic Corporation, Osaka, Japan) sealed inside a double-layered
MG. The PT and LED were installed facing each other on the lingual surface of the incisor.
This sensor evaluated the salivary turbidity by measuring the transmittance of light emitted
from the LED through the saliva flowing in the gap between the LED and PT.
The MGs were vacuum-formed on plaster dental models obtained from subjects. First,
the inner layer MG, which is in contact with the dentition, was formed using a dental
vacuum-forming machine (Vacuum adapter I, Yamahachi Dental MFG., Co., Aichi, Japan).
To achieve the desired shape of the turbidity-sensing part and to create a space capableof housing sensor elements such as PT and LED, a 3D-printed dummy part with thedimensions shown in the enclosed view of Figure 1a was fabricated. The outer layer MG
was vacuum-formed on the plaster model with the dummy part, wireless module, andbatteries were attached with double-sided tape (NW-N30, Nichiban Co., Ltd., Tokyo, Japan)
at the designated position. Because the gap between the PT and LED in the turbiditysensing part of the outer layer MG was incompletely followed in shape by only vacuum
forming, the outer layer MG was locally reshaped by pressing a metal piece heated to about
100 ◦ C into the gap. After forming the inner and outer MGs, each sensor element wasconnected using enameled wire with a diameter of 0.1 mm and sandwiched between the
MGs at the appropriate position. Each sensor element was wired as shown in Figure 1b.
Two coin cells for lighting the LED were connected in series with a 10 kΩ resistor andplaced on the facial surface of the left molars. Finally, the edges of both MGs were heatedwith a heating gun (No. 880B, Hakko Corp., Osaka, Japan) and thermally welded to seal the
sensor elements inside the double-layered MG. The fabricated MG-type sensor is shown in
Figure 1c.

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Sensors 2024, 24, 1436

3 of 10gun (No. 880B, Hakko Corp., Osaka, Japan) and thermally welded to seal the sensor ele3 of 10
ments inside the double-layered MG. The fabricated MG-type sensor is shown in Figure
1c.

Figure 1.
1. (a)(a)Schematics
Schematicsofofconfiguration
configuration
proposed
MG-typesalivary
turbidity
sensorand enof of
proposed
MG-typesalivary
turbidity
sensorand enlarged
largedof turbidity-sensing
part;(b) circuit
diagramtransfersystemof MG-typesenview ofview
turbidity-sensing
part; (b)circuitdiagramand anddatadatatransfersystemof MG-type
sensor;sor;(c)photographoffabricatedMG-typeturbidity
sensor.
(c) photograph of fabricated MG-type turbidity sensor.

Wavelength
2.2. LED Wavelength
wavelength ofof the LED utilized in the MG-type sensor was determined
determined to be
The wavelength
suitable for salivary turbidity measurement. The
The light
light transmittancetransmittance ofof saliva
saliva at eachsuitablesolution(FSS,
KishidaChemicalwavelength was
was compared
comparedwith
withthatthatofofformazine
formazinestandard
standard
solution(FSS,
KishidaChemCo., Co.,
Ltd.,Ltd.,
Osaka,
Japan),
whichwhichis commonlyused asuseda turbidity
standardstandard
solutionsolution
becauseical
Osaka,
Japan),
is commonly
as a turbidityof its superior
and dispersibility[17]. Optical[17].
density(OD),density
representing
becauseof its particle
superioruniformity
particle uniformityand dispersibility
Optical(OD),
logarithmic
ratioofincident
light
intensityItotransmitted
light
intensityI
wasutilized
0 intensity 𝐼 to transmitted light intensity
representing logarithmic ratio of incident lightan indexof light
expressed as follows:𝐼aswasutilized
as antransmittance,
index of light transmittance,
expressed as follows:

OD==−log
log
(𝐼/𝐼
OD
( I/I
0) )

(1)
(1)

The OD spectrum of a saliva sample and FSS prepared to a turbidity of 553 FTU
The OD spectrum of a saliva sample and FSS prepared to a turbidity of 553 FTU(formazine
turbidityunit,
unit,where
where11FTU
FTUequals
equalsformazine
formazineatat1 1mg/L)
mg/L)waswasmeasured
measuredusing
usinga(formazine turbidity
amicrovolume
microvolumespectrophotometer
spectrophotometer(Nanodrop™
(Nanodrop™2000,
2000,Thermo
Thermo Fisher
Fisher Scientific,
Waltham,
Scientific, Waltham,MA,
MA, USA)
USA) with
with aa resolution
resolution ofof 11 nm
nm wavelength.
wavelength. The
The saliva
saliva samples
samples were
were collectedcollected using
using
aa saliva
collection
aid(#5016.02,
Salimetrics,
LLC,
Carlsbad,
CA,
USA).
In
addition,saliva collection aid (#5016.02, Salimetrics, LLC, Carlsbad, CA, USA). In addition, several
severalFSS
lighttransmittanceof of
FSSFSSwith
1–
FSS were
were preparedpreparedwith
withthethespecified
specifiedturbidity,
turbidity,and
andthethelighttransmittancewith
1–4000 FTU was evaluated especially at 660 nm, a wavelength widely used in industrial
turbidimeters [18,19], and at 405 nm, a wavelength used in dental treatment [20].

Sensors 2024, 24, 1436

4 of 102.3. Mouthguard Material
The light emitted from the LED passes through the outer MG twice before it is detectedby the PT. To achieve high sensitivity, a reduction of light absorption at the MG is required,
so the MG material used for the sensor was investigated.
The light transmittance of commercially available MG sheets of different materials wasmeasured with a UV-visible spectrophotometer (V-530, Jasco Corp., Tokyo, Japan). As specimens, four kinds of MG materials were tested: polyethylene terephthalate-glycol (PETG,
Erkodur J80421, ERKODENT Erich Kopp GmbH, Pfalzgrafenweiler, Germany), ethylenevinyl acetate (EVA, Erkoflex J8041, ERKODENT Erich Kopp GmbH, Pfalzgrafenweiler,
Germany), polyester (PEs, Ortholy 70730000, GC Ortholy, Tokyo, Japan), and polypropylene(PP, Essix® type C 08100001, Dentsply Sirona K.K., Tokyo, Japan).
In addition, a three-point bending test was conducted using a tensile and compression
tester (SV-55C, Imada Seisakusyo Co., Ltd., Aichi, Japan) to obtain stress–strain curves of
MG materials.
2.4. Turbidity Quantification with Fabricated Mouthguard-Type Sensor
The fabricated MG-type sensor was immersed in FSS prepared at 1–4000 FTU, and theturbidity quantification characteristic of the sensor was evaluated from the output for eachturbidity. Also, tap water was used as the 0 FTU sample.
The wireless module for the MG-type sensor functions as an A/D converter, potentiostat, and wireless transmitter using Bluetooth Low Energy (BLE), similar to those in
the previous paper [21,22]. A constant voltage of 800 mV was applied to the PT using
the wireless module, and the output current was evaluated as an output representing
turbidity. In the potentiostat of the wire module, the voltage across a 10 kΩ shunt resistor
connected in series to the PT is amplified 1.52 times using a non-inverting amplifier. The
output current was measured in real-time after converting it to a digital signal with an A/D
converter. The nominal resolution of the current measurement was 8 pA. As the definitionin Section 2.2, OD was also used as an index of light transmittance. The intensity of theincident and transmitted light was represented by the output current for tap water (0 FTU)
and the output current at each turbidity, respectively. The output current was measured
by transmitting it via BLE wireless communication to a receiver connected to a PC. The
sampling time of the wireless measurement was one second.
2.5. In Vitro and In Vivo Measurement of Salivary Turbidity
The OD value of saliva samples collected from subjects was evaluated by using afabricated MG-type sensor and a microvolume spectrophotometer, respectively. For the MGtype sensor, 1 mL of saliva sample was dropped onto the sensitive part of the sensor, andthe output current was measured wirelessly. The salivary turbidity was estimated from thecalibration equation between OD and turbidity determined experimentally in advance. The
validity of salivary turbidity measurement by the fabricated MG-type sensor was evaluatedby comparing the estimated turbidity with that of the microvolume spectrometer.
After that, the MG-type sensor was attached to subjects, and in vivo turbidity measurement experiments were conducted. The subjects held tap water and saliva in their mouths,
and the output current was measured, respectively. By utilizing the output current withtap water as a baseline, salivary turbidity was calculated using the calibration equation.
The calculated results were compared with the results derived from the microvolume
spectrometer with saliva samples collected after the in vivo experiment.
These experiments were conducted with the approval of the ethics review committee
at Tokyo Medical and Dental University (TMDU) in accordance with the latest versionof the Declaration of Helsinki (approval number D2018-054). Before the experiments,
participants were provided with a detailed explanation of the methods and significanceof the study, and written informed consent was obtained from participants. The study
included a total of three participants in their 20s. Initially, participants were requested tocomplete a questionnaire regarding their gender, age, and body mass index.

Sensors 2024, 24, 1436

the Declaration of Helsinki (approval number D2018-054). Before the experiments, participants were provided with a detailed explanation of the methods and significance of thestudy, and written informed consent was obtained from participants. The study included
a total of three participants in their 20s. Initially, participants were requested to complete
5 of 10a questionnaire regarding their gender, age, and body mass index.
3.
3. Results
Results andand Discussion
Discussion
3.1. Optical Wavelength
Wavelength for Salivary
Salivary Turbidity
Turbidity Measurement
Figure 2a
sampleandFSS
preparedto
2a shows
shows thetheOD
ODspectrum
spectrumresults
resultsofofthe
thesaliva
salivasampleandFSS
prepared553
FTU,
bothbothmeasuredusingusing
a microvolume
spectrophotometer.
Both thesalivato 553
FTU,
measureda microvolume
spectrophotometer.
Boththesample
salivaand
FSSandexhibitedhigher OD
valuesthe low-wavelength
region. However,while thesampleFSS exhibited
higherODinvalues
in the low-wavelength
region. However,saliva thesampledemonstrated
a distinct peakin ODpeakaroundthe aroundwavelengthof 290 nm, nowhile
salivasample demonstrated
a distinct
in OD
the wavelengthof
290 nm,nowas
suchobserved
peak wasinobservedin FSS.
the case
FSS.represents
This peak therepresents
the absorbance
such
peak
the case of
Thisofpeak
absorbancecharactercharacteristicsof the proteinssaliva. Beyond
the wavelengthof 310OD spectrum
isticsof the proteinsin saliva.inBeyond
the wavelengthof 310 nm,
thenm,
ODthe
spectrumof theof the sample
saliva sample
wereagreement.
in good agreement.
These
results
indicate
FSS
salivaand FSSand
wereFSSin good
These results
indicatethat
FSS canthat
be usedcananbealternative
used as ansample
alternative
sampleto saliva measurement
for turbidity measurement
using
light
as
to saliva
for turbidity
by using lightby
witha wavewith
wavelength
310 nmor longer. Subsequently,
turbidity of
dependenceof OD
lengtha of
310 nm oroflonger.
Subsequently,
the turbidity thedependence
OD with specificwithspecific
wavelengths
was
evaluated
using
FSS
preparedatvaryingturbidity
wavelengths was evaluated using FSS prepared at varying turbidity levels (1–4000 levels
FTU).
(1–4000
FTU).
resultsare shown
2b for ofthe660wavelength
nm,
which
The results
areTheshown
in Figure2b forin
theFigurewavelength
nm, whichofis660
used
in indusis used
in industrialandturbidimeters,
and forof
thewavelength
which treatment.
is used intrial
turbidimeters,
for the wavelength
405
nm, whichof
is 405
usednm,in dental
dentalwavelengths
treatment. Bothwavelengths
showedgoodlinearity
of OD
Both
showedgood linearity
of OD
with
turbidity.
Aswithalsoturbidity.
indicatedAs
byalsotheindicated
the results
Figure2a, the lower
wavelengthof 405nm showed
higher
OD
results
in by
Figure2a, theinlower
wavelengthof 405nm showed
higherOD valuesat eachvaluesat eachlevel,12which
12 times
higherthatFromof 660these
nm. Fromtheseturbidity
level,turbidity
which weretimeswerehigherthan thatof than
660 nm.
results,anresults,
an
LED
with
a
wavelength
of
405
nm,
which
is
considered
to
be
more
sensitivefor
LED with a wavelength of 405 nm, which is considered to be more sensitive for turbidity
turbidity measurement,
wasas
selectedassource
the light
for the MG-type
measurement,
was selected
the light
forsource
the MG-type
sensor. sensor.

(b)

(a)
405 nm

1

660 nmsalivaFSS (553 FTU)

0
200

0.8

optical density (OD)

optical density (OD)2405 nm
0.6
0.4
0.2
0

400

600

wavelength (nm)

660 nm

0

2000

4000

turbidity (FTU)

Figure2. (a)
Figure 2.
(a) ODOD spectrum
spectrum ofof aa saliva
saliva sample
sample andand FSSFSS preparedprepared to
to 553553 FTU;
FTU; (b)(b) turbidity-dependent
turbidity-dependent
characteristics
of
OD
for
wavelengths
of
405
and
660
nm
on
FSS.
characteristics of OD for wavelengths of 405 and 660 nm on FSS.

3.2. Mouthguard Material Appropriate for the Proposed Sensor
investigatedto toselectthetheapproThe light
light transmissivity
transmissivityofofvarious
variousMG
MGmaterials
materialswas
wasinvestigatedselectappriatematerialfortheproposed
MG-typesensor.
Figure
3a
shows
the
light
transmittance
propriate material for the proposed MG-type sensor. Figure 3a shows the light transmitspectrumof eachcommerciallyavailable
MG MG
material.
EachMG
material
exhibitedtance
spectrumof eachcommerciallyavailablematerial.
EachMG
material
exhibiteda
declineintransmittanceatshorter
wavelengths.
Notably,attheselected
wavelengtha decline in transmittance at shorter wavelengths. Notably, at the selected wavelength ofof
405 nm for the sensor, the order of transmittance was as follows: EVA (79.9%), PETG
(78.6%), PEs (77.6%), and PP (31.8%). To achieve the high-sensitivity turbidity measurement with the proposed MG-type sensor, the light emitted from the LED must pass theturbidity sensing part without being absorbed by the MG material. Therefore, PETG and
EVA, which exhibited particularly high transmittance, were evaluated for their mechanicalcharacteristics. Figure 3b shows the stress–strain characteristics of MG materials obtained

EVA, which exhibited particularly high transmittance, were evaluated for their mechanical characteristics. Figure 3b shows the stress–strain characteristics of MG materials obtained from the three-point bending test. The stiﬀness of PETG and EVA were 0.58 N/mm
and 0.079 N/mm, respectively, with PETG exhibiting 7.4 times stiﬀer than EVA.
6 of 10
According to these results, EVA was used for the inner layer MG, which is in contact
with the gingiva and not involved in the optical path for turbidity sensing, because of its
flexibility leading to low discomfort even in long-term wearing. On the other hand, for thefrom thethree-point
test.
The stiffnessof PETG
andtransmittance
EVA were 0.58
N/mm
and
outer
layer
MG, PETGbending
was usedbecauseof its excellent
light
and
high stiﬀ0.079durable
N/mm,enoughrespectively,
with occlusion.
PETG exhibiting 7.4 times stiffer than EVA.
ness
for dental

Sensors 2024, 24, 1436

(a)

PETG (Erkodur)PEs (Duran®)

EVA (Erkoflex)

PP (Essix®)

(b)

100

load (N)80transmittance (%)

PETG(Erkodur)

2

60
40

1

20
0
300

EVA (Erkoflex)

400

500
600
wavelength (nm)

700

0

0

2

4

displacement (mm)6Figure 3.
3. (a)(a) TransmittanceTransmittancespectrum
spectrumofofsome
somecommercial
MGmaterials;
mechanicalcharactercommercial
MGmaterials;(b) (b)mechanicalcharacteristics
isticsMG materials
and EVA)
obtainedfrom three-point
bendingof MGofmaterials(PETG(PETG
and EVA)
obtainedfrom three-point
bending
test. test.

3.3. Output
Dependence
Turbidity
Accordingto theseonresults,
EVA was used for the inner layer MG, which is in contact
with Figure
the gingiva
andnotinvolvedin thepath forMG-typeturbiditysensor
sensing,becauseof its4 shows the output current
ofoptical
the fabricatedwhen
immersed
flexibility
leadingtolow
discomfort
eveninlong-term
wearing.
On
the
other
hand,
for
in FSS prepared at each turbidity level. The average output current was 12.8 µA when the
the
outer
layer
MG,
PETG
was
used
becauseofits
excellent
light
transmittanceandhighsensor was immersed in tap water (0 FTU), serving as a baseline, and the output current
stiffness durable
enough forturbidity.
dental occlusion.
decreasedwith increasing
The OD values for each turbidity level were calculated by substituting the obtained output current values into Equation (1). From the ex3.3. Output Dependence on Turbidity
perimental results, the following calibration equation for turbidity and OD values was
Figure 4 shows the output current of the fabricated MG-type sensor when immersed
determined:in FSS prepared at each turbidity level. The average output current was 12.8 µA when the(2)(turbidity) . and the output current
ODwater= 4.0(0×FTU),
10 ×serving
sensor was immersed in tap
as a FTU
baseline,decreasedwith increasing
turbidity.
The ODfor eachturbidity
level were
calculated
The calibration
equation
elucidates
the values
correlation
betweenturbidity
and OD
acrossby substituting the obtained output current values into Equation (1). From the experimental
a wide turbidity range, covering the turbidity range of saliva (400–800 FTU). The coeﬃresults, the following calibration equation for turbidity and OD values was determined:
cient of variation (C.V.) of the five measurements at 500 FTU was 6.4%. These results suggest that the proposed MG-type turbidity
sensor has excellent reproducibility and turbidOD = 4.0 × 10−4 × (turbidity[FTU])0.78(2)
ity calibration capability and is promising for salivary turbidity measurement.
The calibration equation elucidates the correlation between turbidity and OD across awide turbidity range, covering the turbidity range of saliva (400–800 FTU). The coefficientof variation (C.V.) of the five measurements at 500 FTU was 6.4%. These results suggestthat the proposed MG-type turbidity sensor has excellent reproducibility and turbidity
calibration capability and is promising for salivary turbidity measurement.

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7 7ofof1010
10
7 of

(a)
(a)

(b)
(b)

12
12

10
10

2000
2000

88
30
30

time(s)(s)time

salivary turbidity
salivary turbidity(400 - 800 FTU)
(400 - 800 FTU)

0.2
0.2

0.1
0.1

4000
4000

00

n=5
n=5

optical(OD)
density(OD)
opticaldensity

(FTU)
(FTU)
0–100
0–100
200
200
500
500
1000
1000

00

60
60

100 0
10

101 1
10

102 2
10

103 3
10

turbidity(FTU)(FTU)
turbidity

104 4
10

Figure(a)
Outputcurrents
MG-typesensorFSS
for various
various turbidity
turbidity levels;(b)identified
Figure4.4.
4.(a)(a)Output
Outputcurrents
currentsofof
ofMG-typeMG-typesensor
sensortoto
toFSS
FSSforforlevels;(b)(b)identified
identified
Figurevarious turbidity
levels;correspondence
between
outputOD
ofthe
MG-typesensorandturbidity
level.
correspondencebetween
betweenoutput
outputOD
ODofofthe
theMG-type
MG-typesensor
sensorand
andturbidity
turbiditylevel.
level.
correspondence

3.4.InIn
VitroTurbidity
TurbidityMeasurement
MeasurementofofSaliva
SampleInVitroSalivaSample
3.4.
turbidity
measurement
thethesalivasamplecollected
froma subject
was
Theturbidity
turbiditymeasurement
measurementresult
resultwhen
whensalivasamplecollected
fromsubject
The
resultwhen
the saliva
samplecollected
fromaasubject
droppedand andheldheldin theturbidity-sensing
partpartof theMG-typesensorwaswas553553
FTU,
as
was
droppedintheturbidity-sensingoftheMG-typesensorFTU,
was dropped and held in the turbidity-sensing part of the MG-type sensor was 553 FTU,
shown
in in
Figure
5. 5.
Also,
substituting
the
OD
value
of
the
same
salivasample measured
asshown
FigureAlso,
substituting
the
OD
value
of
the
same
salivaas shown in Figure 5. Also, substituting the OD value of the same saliva sample measured
with the microvolume spectrophotometer
into the following equation
equation derived
derived from thespectrophotometerinto
with the microvolume spectrophotometerthe following equation derived
from theindicatedby
bythe
theblack
blackline
lineinin
Figureturbidityof thesalivasamplewasresultsindicatedindicated
Figure2b,2b,
thetheturbidity
thesalivasamplewascalcalresultsby theblack line
in Figure2b,
theturbidity
ofofthesalivasamplewascalculated
to be
576
FTU:
culated
to
be
576
FTU:
culated to be 576 FTU:
−4−4 × (turbidity [FTU]) + 1.66 × 10
−3−3
(3)
OD=
2.78×
10
OD
==2.78×turbidity[FTU
(3)
(3)
OD
2.78
××10
10
× ((turbidity
FTU])) ++ 1.66
1.66 ×× 10
10
Theresults
results wereininclose
close agreementwith
with eachother,other, indicatingthatthat thefabricatedfabricatedThe
The resultswere
were in closeagreementagreement witheacheach other,indicating
indicating thatthe
the fabricatedMG-typesensorcanmeasuretheturbidityofsalivasamples.
MG-typeMG-typesensor
sensorcan
canmeasure
measurethe
theturbidity
turbidityofofsaliva
salivasamples.
samples.

0.3
0.3

optical(OD)
density(OD)
opticaldensity

output(µA)
current(µA)
outputcurrent

14
14

0.2
0.2

spectrophotometer
spectrophotometer
576 FTU
576 FTU
salivasaliva

0.1
0.1

00 0
0

400
400

MG-typeMG-type553 FTU
553 FTU
800
800

turbidity(FTU)(FTU)
turbidity
Figure 5. Result of in vitro measurements of saliva sample with microvolume spectrophotometer
FigureFigure5.5.Result
Resultofofininvitro
vitromeasurements
measurementsofofsaliva
salivasample
samplewith
withmicrovolume
microvolumespectrophotometer
spectrophotometerand MG-type sensor.
andMG-typesensor.
and MG-type sensor.

3.5. In Vivo Turbidity Measurement of Saliva

Sensors 2024, 24, 1436

The MG-type sensors were attached to subjects, and the output current was wirelessly measured when saliva and tap water were held in the mouth, respectively, as shownin Figure 6a. In this experiment, MG-type sensors were fabricated for each of the three
8 of 10subjects based on their dentition. The MGs were thin and fabricated to follow each dentition, contributing to less discomfort when it was worn. To eliminate the eﬀect of fabrication errors, the calibration equations for each MG-type sensor were respectively identified.
3.5.
In Vivo
Turbidity
Measurementof Saliva
Figure6b shows
an exampleof theoutput current measurement results from the experiment.The
In thiscase, forthe first
20attacheds shown toin subjects,
the figure,there
no liquid
the wirelessly
turbidityMG-type
sensorswere
andthe wasoutputcurrentinwas
measuredwhentapwithwater
in thethemouth,
respectively,
as shown
sensing part;aftersaliva
it wasandfilledtapwerewaterheldor saliva,
outputcurrent changed.
The
in
Figure6a. Inwas
this11.9
experiment,
MG-typesensors
werefor each of theoutputcurrentµA for tap
water and10.8 µA
forfabricatedsaliva, corresponding
to 0three
FTU
subjectson their dentition.
MGs wereinto
thintheandfabricatedto follow eachdentition,and 374 based
FTU, respectively,
whenThesubstituted
identified
calibration
equation.
Thecontributing
to less
discomfort
when
it was worn.
To andeliminate
the effectof fabrication
salivary turbidity
measuredwith
the MG-type
sensors
microvolume
spectrophotomerrors,
equationsfor ineach
MG-typesensor wererespectively
identified.
eter forthe
the calibration
three subjects
are shown
Table
1. The salivary
turbidity
measuredwith theFigure6bshows
anexampleoftheoutputcurrent
measurement
resultsfromtheexperiment.
MG-type sensors showed appropriate values with the reported salivary turbidity levels
In
thiscase, for theresults
first 20withs shownin the figure, there The
was averageno liquidsalivaryin the turbidityand
measurement
the spectrophotometer.
turbidity
sensing
part;
aftersubjects
it was filledwithtap water or saliva, the output current changed. The
among thethree
was 457
FTU.
output current was 11.9 µA for tap water and 10.8 µA for saliva, corresponding to 0 FTU
andFTU, respectively,when
substituted
into thesensor
identified
calibration equation. The
Table374
1. Salivary
turbidity levels
measuredwith MG-type
and spectrophotometer.
salivary turbidity measured with the MG-type sensors and microvolume spectrophotometer
MG-TypeSensor(FTU)turbidity
Spectrophotometer(FTU)
for the threeSubject
subjects are shown in
Table 1. The
salivarymeasured with theMG#1
374 the reported salivary turbidity
435 levels andtype sensors showedappropriate values with
2
414
430
measurement#resultswith the spectrophotometer.
The average salivary turbidity
among
#
3
583
620
the three subjects was 457 FTU.

Figure 6.
6. (a)(a) Schematics
Schematics ofof inin vivomeasurement; (b)(b) output
output current
current
Figurevivo experimentexperiment ofof wirelesswireless turbidity
turbidity measurement;ofMG-typesensor
for
tap
water
and
salivaintheoralcavity.
of MG-type sensor for tap water and saliva in the oral cavity.

4. Conclusions

Table 1. Salivary turbidity levels measured with MG-type sensor and spectrophotometer.

In this study, we developed the MG-type salivary turbidity sensor to realize a conSubject
MG-Type of
Sensor(FTU)turbidity,
Spectrophotometer(FTU) oftinuous andunconstrained measurement
salivary
which is an indicatororal hygiene.#To
turbidity
1 realize the high-sensitivity
374 optical measurement of salivary
435
#2
430
with the MG-type
sensor, the wavelength 414
of the light source and MG material
were ex3583
perimentally #optimized.
The turbidity quantification
characteristic of the620
MG-type sensor
4. Conclusions
In this study, we developed the MG-type salivary turbidity sensor to realize a continuous and unconstrained measurement of salivary turbidity, which is an indicator of oral
hygiene. To realize the high-sensitivity optical measurement of salivary turbidity with the
MG-type sensor, the wavelength of the light source and MG material were experimentally
optimized. The turbidity quantification characteristic of the MG-type sensor was evaluatedSensors 2024, 24, 1436

9 of 10

using FSS, and a dynamic range of 1–4000 FTU was achieved, which covers the typicalsalivary turbidity levels.
In vitro and in vivo turbidity measurement results of saliva samples showed similar
results with the MG-type sensor as with optical measurements with a spectrophotometer,
indicating that the MG-type sensor is capable of measuring salivary turbidity. In addition, the in vivo measurement experiments demonstrated wireless unconstrained with thefabricated MG-type sensor.
The proposed MG-type sensor is a wearable device that can measure salivary turbidity
in the oral cavity, contributing to the realization of real-time continuous evaluation of oral
hygiene. In future work, we will evaluate the feasibility of long-term continuous salivary
turbidity measurement with the MG-type sensor, and the responsibility of the MG-type
sensor for real-time changes in turbidity.
Author Contributions: Conceptualization, K.I. (Kenta Iitani), G.K., K.T., T.A., D.V.D. and K.M.;
methodology, K.I. (Kenta Iitani), G.K., K.T. and T.A.; validation, K.I. (Kenta Ichikawa) and G.K.; formal analysis, K.I. (Kenta Ichikawa), K.I. (Kenta Iitani) and G.K.; investigation, K.I. (Kenta Ichikawa),
K.I. (Kenta Iitani) and G.K.; data curation, K.I. (Kenta Ichikawa) and G.K.; writing—original draft
preparation, K.I. (Kenta Ichikawa); writing—review and editing, K.I. (Kenta Iitani), K.T., T.A., D.V.D.
and K.M.; visualization, K.I. (Kenta Ichikawa), G.K., K.I. (Kenta Iitani), K.T., T.A., D.V.D. and K.M.; supervision, K.T., T.A., D.V.D. and K.M.; project administration, K.I. (Kenta Ichikawa), K.I. (Kenta Iitani),
K.T., T.A., D.V.D. and K.M.; funding acquisition, K.I. (Kenta Iitani), K.T., T.A., D.V.D. and K.M. All
authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the Japan Society for the Promotion of Science (JSPS) Grantsin-Aid for Scientific Research (KAKENHI) grant number 19KK0259.
Institutional Review Board Statement: This study was conducted according to the guidelines of
the Declaration of Helsinki and approved by the Ethics Committee of Tokyo Medical and Dental
University (approval number: D2018-054).
Informed Consent Statement: Written informed consent was obtained from all participants involvedin this study.
Data Availability Statement: The data supporting this study’s findings are available from thecorresponding author (K.M.) upon reasonable request.
Conflicts of Interest: The authors declare no conflicts of interest.

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