HydReminder-W: A Bottle Cap that Listens to Your Heart to

Remind You to Drink!

Nishiki Motokawa, Anna Yokokubo and Guillaume Lopez

Aoyama Gakuin University, Kanagawa, Japan

Keywords:

Hydration, System, Water Intake, Heart Rate, Wearable.

Abstract:

Hydration is essential for maintaining life. Consequently, a lack of awareness of the amount of water required

and the actual amount of water intake has been issued for all generations and may lead to diseases that result

in death. However, existing systems still have limited capabilities for tracking hydration reminder systems

in unconstrained, realistic environments. Therefore, this study employs personalized information based on

the user’s biometric information and environmental information. We proposed and developed a smart bottle

cap system that includes an environmental sensor, a barometric pressure sensor, an infrared sensor, the user’s

heart rate from a smartwatch, and a cap-shaped shell to create a compact system. Our proposed system,

‘HydReminder-W,’ promotes proper hydration using the individual’s biometric information, which teaches

individuals about the lack of hydration. A comparison of hydration with HydReminder-W showed that the

amount of water intake increased with the use of HydReminder-W. The average System Usability Scale (SUS)

score of all the subjects was 78.9 points, conﬁrming that HydReminder-W has excellent usability and is helpful

as a hydration promotion system.

1 INTRODUCTION

Water supports human life activities and is indispens-

able for maintaining life. Water in the body plays a vi-

tal role in providing nutrients, oxygen, and hormones

to each cell while circulating nutrients obtained from

food and other sources throughout the body and dis-

charging metabolic wastes that are no longer needed

in the body as urine. Lack of water in the body can

cause thirst and loss of mobility and heatstroke, cere-

bral infarction, myocardial infarction, and other dis-

eases that can lead to death (Kawaraban, 2019).

According to a report published by the Ministry

of Internal Affairs and Communications of Japan

(MIC) (of IMinistry of Internal Affairs and Commu-

nications, 2020), a total of 64,869 people were trans-

ported to emergency rooms due to heatstroke nation-

wide from June to September 2020, and 43.4% of

them developed the disease even though they were

at home. In addition, there is also data (Suntory,

2019) that ofﬁce workers consume less than 1L of

water in a day, which means that they cannot replen-

ish the amount of water they need in a day and are

chronically underhydrated. That is to say, we may be-

come water-deﬁcient at any time and in any location.

The Ministry of Health, Labour and Welfare of Japan

(MHLW) recommends that 1.2 liters of drinking wa-

ter is necessary to replenish the body’s daily water

intake. In addition, depending on the state of per-

spiration caused by exercise, consuming even more

drinking water may be necessary.

On the other hand, the human body cannot ab-

sorb a large amount of water at a time, with a limit

of 250 ml per hour. Therefore, in addition to the

hydration status of the user, it is necessary and ade-

quate to support the user with a system that compre-

hensively takes into account the user’s health status

related to perspiration, etc., and environmental infor-

mation such as the temperature and humidity of the

environment in which the user is located.

In this study, we propose and develop a hydration

promotion system, HydReminder-W, to notify users

of the lack of hydration, eliminate the lack of hydra-

tion at an appropriate time, and verify its usefulness.

We will also clarify the issues that need to be ad-

dressed in practical use from the results of evaluation

experiments. In this paper, we present HydReminder-

W, a smart bottle cap system that promotes proper

hydration using biometric information and veriﬁes

whether hydration can be promoted to replenish ﬂu-

ids regularly by conducting an evaluation experiment.

HydReminder-W measures the user’s hydration sta-

Motokawa, N., Yokokubo, A. and Lopez, G.

HydReminder-W: A Bottle Cap that Listens to Your Heart to Remind You to Drink!.

DOI: 10.5220/0011800700003414

In Proceedings of the 16th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2023) - Volume 5: HEALTHINF, pages 199-208

ISBN: 978-989-758-631-6; ISSN: 2184-4305

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

199

Figure 1: HydReminder-W.

tus from the tube pressure sensor and infrared dis-

tance sensor inside the cap-shaped shell, the user’s

environment (temperature and humidity) from the en-

vironmental sensor also mounted inside the shell, and

heart rate from the smartwatch worn by the user. If the

user’s hydration status is insufﬁcient, an alarm sounds

to alert the user to the lack of hydration.

We illustrated our method in one distinct exam-

ple scenario to conﬁrm whether hydration is pro-

moted when using HydReminder-W. To evaluate the

impression of HydReminder-W, we measured 1-hour

ﬂuid intake for 10 subjects and 8-hour ﬂuid intake

for 10 subjects and evaluated the impression using

SUS(System Usability Scale). SUS is a question-

naire method to measure the evaluation of the us-

ability of a new system. The SUS is a Likert-based

evaluation to express the degree of agreement of the

participants with ten questions, ﬁve evaluating pos-

itive aspects and the other ﬁve evaluating negative

aspects of the system. Our ﬁndings showed that us-

ing the HydReminder-W resulted in more hydration.

Comparing the amount of hydration with and with-

out the HydReminder-W, we found that using the

HydReminder-W increased the amount of ﬂuid in-

take. The average SUS score for all participants was

78.5, conﬁrming that the HydReminder-W has excel-

lent usability and is helpful as a hydration promotion

system.

2 RELATED WORK

Here we review technology and systems related to hy-

dration assistance. We explore various techniques that

enable tracking of drinking water and existing system

structures created using different embedded systems.

2.1 Research on Hydration Assistance

The following are some of the studies that have fo-

cused on drinking water. Beddoe et al. (Beddoe et al.,

2020) developed a system based on Nudge Technol-

ogy, a mechanism or method to encourage users to

make appropriate choices and avoid risks. The sys-

tem overﬂows the bottle’s contents if the user is not

hydrated at the right time, reminding the user that he

or she is not fully hydrated and encouraging the user

not to spill anymore. Lessel et al. (Lessel et al., 2016)

developed a coaster-type device that measures water

content based on weight and proposed a smartphone

game that reﬂects the user’s hydration status. This

system was effective in promoting hydration by in-

creasing the amount of water consumed, but no sig-

niﬁcant difference was found by using the feedback

function of the smartphone game.

On the other hand, Nagata et al. (nagata, 2017) de-

veloped a system to encourage people to drink enough

water when taking medication by creating an intel-

ligent cup with sensors embedded in the cup. As a

result, the system measured the amount of water con-

sumed, and many subjects could drink the speciﬁed

amount of water. It was conﬁrmed that the system

could provide appropriate medication guidance to pa-

tients. Chiu et al. (Chiu et al., 2009) developed a

system to support hydration with two games by at-

taching a smartphone to a cup. The proposed system

uses an accelerometer, and a camera mounted on a

smartphone. The accelerometer is used to detect the

drinking motion. Zhou et al. (Zhou et al., 2021) de-

veloped a system that estimates the user’s hydration

state from a cup equipped with a tilt sensor and re-

ﬂects it in the state of moss on the tabletop. Although

positive impressions were obtained from the system,

which watered and dried the plants according to the

user’s hydration, the actual increase in water intake

was not veriﬁed.

Besides this, Kaner et al. (Kaner et al., 2018) de-

veloped a bottle with a design on a copper plate on the

bottle wall depending on the amount of water con-

sumed. Gouko et al. (Gouko and Arakawa, 2017)

created a sound-emitting coaster to explore the ap-

propriate feedback interval for promoting hydration.

Sanchez et al. (Sanchez and Gonzales, 2018) con-

ducted a study to promote hydration by mounting a

screen on a bottle. Nakano et al. (Nakano et al., 2006)

developed a system that uses a weight sensor to calcu-

late the fullness of a cup and notify colleagues when

the cup is empty to promote communication in a par-

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200

titioned ofﬁce. Beigl et al. (Beigl et al., 2001) de-

veloped a cup that can measure the temperature of a

beverage.

Ko et al. (Ko et al., 2007) proposed a system

that estimates the state of hydration from the tilt of a

mug and provides feedback through a water-drinking

game using a digital photo frame. Hamatani et

al. (Hamatani et al., 2017) proposed a method for es-

timating the amount of water consumed in milliliters

based on the recognition of drinking motions from a

smartwatch’s built-in sensor. Amft et al. (Amft et al.,

2010) proposed a method to recognize the type of

container and the amount of water consumed by es-

timating the posture of the upper body while drink-

ing from a wrist-mounted accelerometer. Cohen et

al. (Cohen et al., 2021) summarized a state-of-the-art

solution for automatic monitoring of water intake.

2.2 Policy of this Study Based on

Related Research

In general, the above-mentioned studies have many

limitations. For example, if the bottle falls over, the

contents will overﬂow, and using such a system in an

actual ofﬁce environment or carrying it around may

lead to the failure of surrounding electronic devices,

which is not practical. On the other hand, preparing

and operating devices such as smartphones is chal-

lenging to use such systems. Also, staying away from

the device makes the work more difﬁcult. As well as

users may overlook feedback at the same time. It is

also necessary to verify to what extent the system in-

creases the amount of water intake.

Regardless of the smartphone games (Lessel et al.,

2016), capabilities of sensing in the cup (nagata,

2017) or sound emitting systems, it is also possible

to acknowledge the users about the lack of hydration

and promote it. However, those mentioned existing

systems still have limited capabilities for tracking hy-

dration reminder systems in unconstrained, realistic

environments to enable tracking. In addition, few sys-

tems take into account whether the environment in

which the user is located is hot or cold or the user’s

health status. Therefore, to solve the above problems,

this study employs personalized information based on

the user’s biometric information and the environment

in which the user is located. We believe that person-

alized information presentation is helpful because it

allows the user to replenish the water depleted from

the user’s body without his/her knowledge, such as

through perspiration caused by exercise, etc.

Figure 2: HydReminder-W.

3 DESIGN SPACE FOR

HydReminder-W

HydReminder-W is a bottle cap system that promotes

hydration. HydReminder-W is a combination of the

words Hydration, Reminder, and Wearable. Focus-

ing on the ‘timing’ of hydration, HydReminder is a

portable system that can be used anywhere and pro-

vides personalized assistance using heart rate infor-

mation and information about the user’s environment.

If the user’s hydration status is inadequate, the bottle

cap-type device emits an audible tone and a message

on the smartwatch screen that says, It is time to drink.

The HydReminder-W device is shown in ﬁgure 1, and

the system conﬁguration is in ﬁgure 3. Details are de-

scribed in the next section.

3.1 Hardware

HydReminder-W consists of an environmental sensor,

a barometric pressure sensor, an infrared sensor, and

a cap-shaped shell to make a compact system. Also,

in the system, we use a smartwatch (heart rate sensor)

and a control unit (M5AtomLite) to computer-control

all of these with a drinking straw. The HydReminder-

W obtains air pressure data in the straw and dis-

tance data from the water surface in the container

from the air pressure sensor and infrared sensor to

check whether the user is hydrated or not. The mea-

sured air pressure data and infrared data are sent to

a preferentially connected control unit, which uses

them to recognize whether the user has rehydrated

and whether the user has ingested enough water. Sim-

ilarly, environmental sensors are mounted to measure

the environment in which the user is located, and the

data is sent to the control unit. The control unit is

wired to various sensors. It also communicates with

the smartwatch via the Firebase real-time database, a

cloud-hosted NoSQL database. The control unit can

be powered from an electrical outlet and turned on

with a switch on the side. To integrate these parts,

HydReminder-W: A Bottle Cap that Listens to Your Heart to Remind You to Drink!

201

Figure 3: System Conﬁguration.

a 3D printer was used to create a spherical cap that

meets the requirements of being comfortable in a liv-

ing space and being able to contain all sensors and

control units.

3.2 Software

3.2.1 Roles of Various Sensors

HydReminder-W obtains data from an environmen-

tal sensor, a barometric pressure sensor, an infrared

distance sensor, a smartwatch (heart rate sensor), and

a control unit (M5AtomLite) that computer controls

these sensors. The control unit acquires the user’s

heart rate from the smartwatch via the Firebase Real-

time Database, a cloud-hosted NoSQL database. The

acquired heart rate data is used as the user’s health

and exercise status. In addition, the temperature and

humidity of the user’s environment are acquired from

the environmental sensor connected by wire and are

used to determine the degree of risk of heat stroke or

extreme dryness of the environment. The user’s hy-

dration status is based on the air pressure in the straw

and the distance from the cap device to the surface of

the water in the bottle. If there is a signiﬁcant change

in the air pressure inside the straw, the system deter-

mines that the user is hydrated and, a short time af-

ter the air pressure returns to normal, uses an infrared

distance sensor to measure the distance between the

device and the water surface inside the bottle to con-

ﬁrm that the user has been sufﬁciently hydrated. If the

user is not sufﬁciently hydrated, the cap-type device

emits an audible warning, and the smartwatch screen

displays a message to remind the user to rehydrate.

The relationship between infrared distance and mois-

ture content is discussed in 3.2.2.

3.2.2 Relationship between Water Content in

the Bottle and Infrared Distance

An experiment was conducted to verify whether the

amount of water ingested could be measured by in-

Figure 4: Relationship between water content in the bottle

and infrared distance.

frared distance. Using the infrared distance sensor in

this study, we measured whether the distance to the

water surface could be measured correctly. Details of

the experiment are shown below.

1. Prepare a commercially available 500 ml bottle.

2. We poured water into the bottles in 20 ml incre-

ments from 100 ml to 460 ml and measured the

infrared distance to the water’s surface in each in-

crement.

3. This data was measured ten times for each ml.

The results are shown in ﬁgure 4. The results show

that the distance to the water surface is measured cor-

rectly to some extent, but the deviation increases as

the distance between the sensor and the water surface

increases. In addition, there is a slight deviation de-

pending on the measurement time. These results indi-

cate that obtaining detailed water content in units of 1

ml is difﬁcult. On the other hand, the infrared distance

sensor is sufﬁcient to measure a difference of about

20 ml, which is necessary to determine whether a per-

son has drunk enough water in one hydration session.

Based on the above, this system uses the infrared dis-

tance sensor to determine whether a person has drunk

enough water in a single hydration session.

3.2.3 Algorithm for Diagnosing a User’s

Hydration Needs

Describes the algorithm for diagnosing the need for

hydration. The system workﬂow diagram is shown

in ﬁgure 5. The degree of need for hydration is di-

vided into three primary levels based on external fac-

tors (temperature and humidity).

1. Necessity level 1: necessity 1 is deﬁned as a tem-

perature of 25

◦

C or lower and humidity of 40%-

60%, which is considered comfortable for hu-

mans. In this environment, there is a slight de-

crease in body water due to external factors, but

moderate rehydration is necessary. It is desirable

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to rehydrate at a pace of about once every 30 min-

utes.

2. Necessity level 2: necessity 2 is a slightly hot

space (temperature 25

◦

C-30

◦

C) or dry space (hu-

midity 30%-40%). Due to perspiration and dry-

ness associated with hot temperatures, more hy-

dration is considered necessary than in comfort-

able spaces. It is desirable to rehydrate at a pace

of about once every 20 minutes.

3. Necessity level 3: necessity 3 is when the envi-

ronment is sweltering (temperature above 30

◦

and extremely dry (humidity below 30%). The

body loses much water in this environment due to

profuse sweating and extreme dryness. Therefore,

the amount of water intake must be very high. It

is desirable to rehydrate at a pace of about once

every 10 minutes.

The degree of need also considers information on the

user’s heart rate. If the user’s heart rate is 60bpm-

90bpm, we assume that the user is not exercising in

particular and is in good physical condition, and we

do not change the level of need. For example, if the

degree of need due to external factors were 2, it would

remain at 2. On the other hand, if the user’s heart rate

is above 90bpm, the user is assumed to be doing light

exercise or in some physical condition, and the degree

of need is raised by 1. For example, if the need level

due to external factors were 2, it would be changed to

Based on these Necessity levels, the system deter-

mines how often the user should hydrate. The system

determines if the user is hydrating at a set pace based

on data obtained from the pressure sensor in the tube

and the infrared distance sensor. If the user is not fully

hydrated, the system will display a message on the

smartwatch screen and emit a warning tone from the

device to remind the user to rehydrate.

If the user intentionally or unintentionally ignores

this feedback and does not hydrate, the system will

again prompt the user to hydrate at half the pace ac-

cording to the need (15 minutes, half of 30 minutes

for a need of 1), with an on-screen message on the

smartwatch and an alert tone on the device.

3.2.4 Feedback Method

Identify user hydration through the algorithm de-

scribed above and provide feedback when inadequate

trends are observed. There are two types of feedback.

The ﬁrst is an audible tone from the bottle cap de-

vice, and the second is a message on the screen of the

smartwatch. The message is shown in ﬁgure 6. There

are three types of messages depending on the user’s

hydration status. A graph of the user’s hydration and

the messages displayed is shown in table 1.

Figure 5: HydReminder-W system transition diagram.

4 PILOT STUDY OF USING

HydReminder-W SYSTEM

In this study, two different experiments were con-

ducted to conﬁrm whether hydration was promoted

when using the HydReminder-W and to evaluate the

impression of the HydReminder-W. Below, we will

explain the pilot study experiment method, details of

the system used in general/existing methods, and our

ﬁndings elaborately.

4.1 Method of Pilot Study

Experiment-A: Veriﬁcation of the

Effectiveness of the System when

Used for Short Periods of Time

A usability validation experiment was conducted to

verify the following three points of the proposed

HydReminder-W. The experiment was conducted in

an ofﬁce that the participants usually use, and the en-

vironment was kept free from extreme dryness and

humidity. Therefore, we believe the surrounding en-

vironment does not affect the hydration process. The

point are:

1. Conﬁrmation that HydReminder-W promotes hy-

dration and ensures that hydration is above the

minimum required.

2. Evaluate the ease of use and user experience of

HydReminder-W.

There were 10 subjects in the study. The subjects

were divided into the following two groups:

HydReminder-W: A Bottle Cap that Listens to Your Heart to Remind You to Drink!

203

Figure 6: Example of feedback on a smartwatch.

Table 1: The hydration requirement algorithm.

Environment/Necessity level Level 1 Level 2 Level 3

-25

◦

C / 40-60% Let’s drink It’s about time. Drink and rest now.

25-30

◦

C / -40% Let’s drink It’s about time. Drink and rest now.

◦

C- / -30% It’s about time. Drink and rest now. Drink and rest now.

Group A: Day 1 without HydReminder-W; Day 2

with HydReminder-W

Group B: Day 1 with HydReminder-W; Day 2 with-

out HydReminder-W

The experiment is shown in Figure 7. The exper-

imenter explained the use of the HydReminder-W to

each participant for about 3 minutes. Each participant

then worked at his or her desk for one hour. The sys-

tem monitors whether the user is hydrating regularly

using the tube pressure sensor and infrared sensor

data from each sensor mounted in the HydReminder-

W, which measures the amount of water present in

the bottle. If the user is under-hydrated compared to

the hydration needs of the environment in which he

or she is located, the bottle cap device will emit an

audible sound, and the smartwatch screen will dis-

play a message encouraging the user to hydrate. Af-

ter one hour of work, participants completed a ques-

tionnaire on the usability of HydReminder-W, and the

amount of water consumed by the experimenter dur-

ing the desk work was measured with a digital scale.

In addition, the number of times they rehydrated was

measured from the videos taken. The system usability

scale (SUS) was included in the questionnaire to eval-

uate the system. The SUS is a Likert-based evaluation

to express the degree of agreement (from 1—strongly

disagree to 5—strongly agree) of the participants with

ten questions, ﬁve evaluating positive aspects, and the

other ﬁve evaluating negative aspects of the system:

1. I think that I would like to use HydReminder-W

frequently.

2. I found HydReminder-W unnecessarily complex.

3. I thought HydReminder-W was easy to use.

4. I think that I would need the support of a technical

person to be able to use HydReminder-W.

5. I found the various functions in HydReminder-W

were well integrated.

6. I thought there was too much inconsistency in

HydReminder-W

7. I would imagine that most people would learn to

use HydReminder-W very quickly.

8. I found HydReminder-W very cumbersome to

use.

9. I felt very conﬁdent using HydReminder-W.

10. I needed to learn a lot of things before I could get

going with HydReminder-W.

4.2 Pilot Study Experiment Method-B:

Validation of the Effectiveness of the

System During Prolonged Use

To validate the proposed HydReminder-W, an ex-

periment was conducted to verify its effectiveness

over a longer time period. The experiment was con-

ducted in the participants’ daily lives, including their

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204

Figure 7: Scene of the experiment.

usual commuting environment. The environments

ranged in temperature from 12

◦

C to 32

◦

C and hu-

midity from 40% to 76%, and it is believed that

these conditions would change users’ attitudes to-

ward hydration. The experiment’s purpose is to con-

ﬁrm that HydReminder-W promotes hydration and

ensures that hydration is above the minimum re-

quired and evaluate the ease of use and user experi-

ence of HydReminder-W, the same as in Pilot Study

Experiment-A. There were ten subjects. The ex-

perimenter explained to participants how to use the

hyd-reminder for 3 minutes the day before the ex-

periment. The next day, participants worked their

usual commute for 8 hours (8:00 am-12:00 pm, 2:00-

6:00 pm). After the 8 hours of work, participants

completed a questionnaire about the usability of the

HydReminder-W, and a digital scale measured the

amount of water the experimenter consumed while

working at his/her desk. The questionnaire content

was the same as that used in Pilot Study Experiment-

4.3 Results

In this study, experiments were conducted to ver-

ify the usefulness of HydReminder-w and to evaluate

the impression of HydReminder-w. This section de-

scribes the results obtained from the experiment.

4.3.1 Results Comparing Water Intake and

Frequency of Water Intake with and

without HydReminder-W for 1 Hour

The amount of water intake and the number of

water intakes were compared with and without

HydReminder-W.The results of the comparison of the

amount of water ingested are shown in ﬁgure8, and

the results of the comparison of the number of times

water was ingested are shown in ﬁgure 9. The results

show that 9 of the 10 subjects increased their water in-

take. In addition, hydration frequency increased by 7

out of 10. Therefore, it is thought that HydReminder-

W makes it possible for users to be informed of the

Figure 8: Comparison of the amount of water consumed by

each subject (ml).

Figure 9: Comparison of water intake frequency per sub-

ject(times).

need for hydration at the appropriate time when there

is a lack of water and promotes hydration.

4.3.2 Results of Impression Evaluation Using

SUS - A

In the usability veriﬁcation experiment of

HydReminder-W, we conducted a questionnaire

using SUS. A score out of 100 was calculated for

each item to quantify the usability. The results are

shown in Table 2. The results show that seven out

of ten users scored higher than the ‘ease of use’

standard score of 68. The average score of the 10

users was 78.5. Therefore, it was conﬁrmed that

HydReminder-W has excellent usability.

HydReminder-W: A Bottle Cap that Listens to Your Heart to Remind You to Drink!

205

Table 2: SUS score per subject(points).

Subject 1 2 3 4 5 6 7 8 9 10 Ave

Score 50 77.5 67.5 82.5 92.5 87.5 95 90 87.5 55 78.5

Figure 10: Comparison of the amount of water consumed

by each subject (ml).

Figure 11: Comparison of water intake frequency per sub-

ject(times).

4.3.3 Results Comparing Water Intake and

Frequency of Water Intake with and

without HydReminder-W for 8 Hours

To validate the proposed HydReminder-W, an exper-

iment was conducted to verify its effectiveness over

a longer time period. Fluid intake and frequency of

ﬂuid intake with and without the HydReminder-W

were compared during an 8-hour period in a normal

daily routine. The results of the comparison of the

amount of water ingested are shown in Figure 10, and

the results of the comparison of the number of times

water was ingested are shown in Figure 11. The re-

sults show that 10 of the 10 subjects increased their

water intake. In addition, the frequency of water in-

take increased for 7 out of 10. Therefore, it is thought

that HydReminder-W makes it possible for users to

be informed of the need for hydration at the appropri-

ate time when there is a lack of water and promotes

hydration.

4.3.4 Results of Impression Evaluation Using

SUS - B

In the effectiveness validation experiment of

HydReminder-W, we conducted a questionnaire us-

ing SUS. The usability was quantiﬁed by calculating

a score of 100 points for each item. The usability

of each item was quantiﬁed. The results are shown

in Table 3. The results show that 8 out of 10 users

scored higher than the standard score of 68 for ‘good

usability. The average score of the 3 users was 79.2.

Therefore, it was conﬁrmed that HydReminder-W

has excellent usability.

5 DISCUSSION

This section discusses the system based on data ob-

tained through veriﬁcation experiments.

5.1 Verify Data from Effectiveness

Testing Experiments

After 1 hour of use of the HydReminder-W, 7 of 10

subjects increased their ﬂuid intake and the mean ﬂuid

intake increased by approximately 80 ml. After 8

hours of use of the HydReminder-W, 10 out of 10

subjects increased their ﬂuid intake and the average

ﬂuid intake increased by approximately 500ml. We

believe this is mainly due to two factors: (1) Many

people forget to hydrate when they are concentrating,

such as during work. (2) Support for small changes

in physical condition and temperature that users are

not aware of In this system, we provided simple no-

tiﬁcation support in the form of warning tones and

smartwatch messages. However, some people do not

want to touch their cell phones or other electronic de-

vices while at work, so further study is needed to de-

termine how to provide the support that makes the

user aware of hydration and does not interfere with

the user’s activities. In the free-text section of the sur-

vey of subjects who used the HydReminder-W for one

hour, they said that they did not know that they needed

to drink so much in an hour and that the amount

they drank was not that different from what they nor-

mally drank. In addition, in the free response sec-

tion of the questionnaire of the subjects who used the

HydReminder-W for 8 hours, there were comments

such as ‘I now understand that the amount I usually

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206

Table 3: SUS score per subject(points).

Subject 1 2 3 4 5 6 7 8 9 10 Ave

Score 55 87.5 92.5 77.5 85 90 77.5 80 67.5 85 79.2

drink is too little’ and ‘I now understand the need to

hydrate before I get thirsty. We believe that using the

HydReminder-W for a longer period will help the par-

ticipants feel the system’s effectiveness. In addition,

one respondent commented, ‘I was surprised to re-

ceive a notiﬁcation when I was thinking it was time

to rehydrate.’ suggesting that personalized support

based on biometric information and the user’s envi-

ronment may be adequate. As shown in Fig. 2 and

Section4.3.2, the average SUS score was 78.5, which

is higher than the standard score of 68 for ‘ease of

use. In particular, the item ‘Did you have to learn a

lot of things before using the system?’ received a very

high score, indicating that the system is easy to use.

This indicates that the HydReminder-W has excellent

usability. When asked if the presence of the device

interfered with their work, all subjects responded that

the device was small enough that it did not interfere

with their work. This suggests that the devices do not

get in the way when working at the desk and can be

integrated into the ofﬁce environment. On the other

hand, in response to hydration through a straw em-

ployed in this system, some subjects commented that

‘drinking through a straw is a little uncomfortable’

and ‘it takes time to drink a large amount of water.

In addition, users who had used the system for a long

time commented that it was a hassle to recharge the

battery. Therefore, the method of acquiring water in-

take and device design should be improved. We be-

lieve that designing a system that does not require

recharging for longer periods of time will gain more

positive feedback. As mentioned above, we believe

that the HydReminder-W can be used to notify users

of the need for hydration at the appropriate time and

encourage them to hydrate when they are short of wa-

ter.

5.2 Consideration of Temperature and

Humidity and User’s Heart Rate

after 8 Hours of Use of

HydReminder-w

The effects of temperature, humidity, and user’s heart

rate in an experiment in which the HydReminder-w

was used for 8 hours are discussed. Table 4 shows

the temperature and humidity at which the system

prompted hydration during the experiment. From

this table, we can see that there are very few data at

temperatures above 30

◦

C or at humidity below 30%.

Table 4: The temperature and humidity at which the system

prompted hydration during the experiment(number of sam-

ples).

Temp(

◦

C)/Humi(%) -30% 30%-40% 40%-

-25

◦

C 8 17 38

25-30

◦

C 0 3 25

◦

C- 0 0 15

Table 5: Data on the distribution of heart rate during the ex-

periment when hydration was prompted during the experi-

ment. (Number of samples).

60bpm - 90bpm 90bpm - 110bpm 110bpm-

73 21 12

Therefore, in the future, we would like to acquire ex-

perimental data under these environments to further

verify the usefulness of the system.

Table 5 also shows the distribution of users’ heart

rates when the system prompted them to hydrate dur-

ing the experiment. The data at high heart rates is

very small, as is the data at high temperatures. There-

fore, we would like to acquire data in situations where

the user’s heart rate increases due to exercise, etc., to

verify the system’s usefulness.

6 CONCLUSION

This paper developed a smart bottle cap system that

promotes proper hydration using biometric informa-

tion ‘HydReminder-W’ and conducted an evaluation

experiment to verify whether it can promote hydration

at appropriate timing. HydReminder-W measures the

user’s hydration status from the tube pressure sen-

sor and infrared distance sensor inside the cap-shaped

shell, the user’s environment (temperature and hu-

midity) from the environmental sensor also mounted

inside the shell, and heart rate from the smartwatch

worn by the user. If the user’s hydration status is

insufﬁcient, an alarm sounds to alert the user to the

lack of hydration. A comparison of hydration with

and without the HydReminder-W, per hour and per 8

hours that in each case, the amount of water intake

increased with the use of HydReminder-W. We be-

lieve that this is due to the effect of feedback. In addi-

tion, the results of the questionnaire survey using the

SUS showed that the average score was 78.9 points,

conﬁrming that the HydReminder-W has excellent us-

ability and is helpful as a hydration promotion sys-

HydReminder-W: A Bottle Cap that Listens to Your Heart to Remind You to Drink!

207

tem. On the other hand, some users commented that

‘drinking through a straw is a hassle/embarrassing’

and ‘charging is a hassle’, and there are many oppor-

tunities for improvement in device design and opera-

tion. Therefore, we would like to improve the device

design to make it more familiar to more users and im-

prove the device system to make it easier to use and

more habit-forming.

ACKNOWLEDGMENTS

This work was supported by SECOM Science and

Technology Foundation research grant.

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