Evaluation of Salivation Promotion by Odor, Pressure, and Thermal Stimulus for Designing Wearable Device to Increase Salivation

Abstract

Saliva is an important fluid related to the maintenance of oral and systemic health, and a decrease in saliva volume causes various adverse effects. Saliva volume should be increased to avoid the adverse effects caused by decreased saliva volume. One method to increase saliva volume is to use a wearable device daily. Wearable devices that increase saliva volume in daily life should have the following features: easy to put on and take off, automatic movement, and no need to put the device in the mouth. To the authors’ knowledge, no such device exists. Therefore, this study aims to design a wearable device that can increase saliva volume in daily life using stimulus. Several previous studies have shown that lemon odor, massage of the lower part of the jaw, and warming of the lower jaw can increase saliva volume. However, it is not known whether these three stimuli can increase saliva volume quickly and under any situation. Therefore, in this paper, we investigate suitable stimuli for increasing saliva volume under situations that may change the effect of the stimuli for the construction of a wearable device. First, we implemented a mask-type experimental wearable device that can provide odor, pressure, and thermal stimulus. Next, a questionnaire survey was administered to confirm the situations under which saliva volume decreases. The results of the questionnaire showed that saliva volume decreased after conversations, after exercise, and upon waking up. Therefore, we investigated which of the three stimuli increased saliva volume quickly under these three situations and the usual situation. The results show that an odor stimulus significantly increased saliva volume in all situations compared to other stimuli. Based on the experimental results, we propose an eyeglass-shaped device that can provide an odor stimulus. The odor stimulus of the proposed device also significantly increased saliva volume.

1. Introduction

Saliva is an important fluid related to the maintenance of oral and systemic health, and a decrease in saliva volume causes various adverse effects [1]. For example, decreased saliva volume causes glossitis, stomatitis, and halitosis [1]. In addition, decreased saliva volume is associated with dysphagia [2] and aspiration pneumonia [3]. These show that decreased saliva volume has a variety of adverse effects not only on oral disease but also on the body. The causes of decreased saliva volume include aging [4], changes in emotions [5], stress [6], etc. Therefore, decreased saliva volume can occur in anyone at any time.

There are two types of treatments to prevent the adverse effects of decreased saliva volume: causal therapy and symptomatic therapy. Although symptomatic therapy is preferable to causal therapy, it is difficult to solve the cause of saliva volume decrease because the cause of saliva volume decrease is often complex [7]. Therefore, symptomatic therapy is necessary to resolve the adverse effects of decreased saliva volume. Symptomatic therapy includes increasing saliva volume and artificial saliva. However, artificial saliva is unsatisfactory [8]. Therefore, to prevent the adverse effects of decreased saliva volume, a method to increase saliva volume is needed.

Several methods have been proposed to increase saliva volume. The existing methods are summarized in

Table 1. Existing methods to increase saliva volume.

Method Name	Stimulus	Wearable	Automatic	Stimulus Location	Reference
Sialogogue	Sialogogue	No	No	-	[8]
Lozenges	Lozenges	No	No	In the mouth	[9,10]
Chewing gum	Chewing	No	No	In the mouth	[9,10]
Tongue exercises	Tongue exercises	No	No	In the mouth	[11]
Oral exercises	Oral exercises	No	No	In the mouth	[11]
Odor	Odor	No	No	Nose	[12,13,14]
Thermal	Thermal	No	No	Jaw	[15,16,17]
Massage	Massage	No	No	Jaw	[18,19]
Salipen	Electrical	No	No	In the mouth	[20]
GenNarino	Electrical	Yes	Yes	In the mouth	[21]
Saliwell Crown	Electrical	Yes	Yes	In the mouth	[22]

. These methods are either non-wearable or devices that are placed in the mouth. Saliva can decrease at any time due to changes in emotions and other factors. In such cases, if saliva volume can be immediately increased to prevent bad breath and other adverse effects caused by decreased saliva volume, the quality of life can be improved. Therefore, a wearable device that can be used at any time is desirable as a method to increase saliva volume. In addition, the wearable device should be able to automatically increase saliva production without requiring user operation. Furthermore, a device that is placed in the mouth does not allow the user to talk or eat with the device in use. However, these methods are not wearable devices that can operate automatically without being placed in the mouth.

Therefore, this research aims to propose a wearable device that automatically increases saliva volume without the need to place it in the mouth. The proposed device uses stimuli to increase saliva. Candidate stimuli used in the proposed device include odor [12,13,14], pressure [18,19], and thermal stimulus [15,16,17], which have already been shown to increase saliva volume.

The proposed device’s stimulus must increase saliva volume in unusual situations because the proposed device is used in daily life, including unusual situations. However, previous studies have only examined the effects of those stimuli just under usual situations. Stimulus effects may differ between usual and unusual situations.

Since the situation in which the proposed device is used is one of reduced saliva volume, we investigated the effects of three stimuli in a saliva-reduced situation. If the effective stimuli differ from situation to situation, we propose a wearable device that switches the stimuli for each situation.

This paper is a total reworking of the implementation and experiments of the authors’ previous conference paper [23]. In our previous study, we implemented two different devices. One device provides pressure, thermal, and odor stimuli, while the other device can only provide odor stimuli. However, the odor stimulus used in those devices was not evaluated. In addition, both odors are blown at the temples. Therefore, in this study, we redesigned the device so that the odor can be blown directly onto the tip of the nose. Based on the design, the experimental wearable device with three stimuli was implemented, and the stimuli of the device were evaluated. We also proposed a device based on the experimental results and evaluated the increase in saliva volume by odor stimulus of the proposed device.

The contributions of this study are as follows:

• Since no device could provide all three types of stimuli, we implemented an experimental wearable device that can provide odor, thermal, and pressure stimuli.
• Since the effects of the stimuli have not been compared under the same conditions previously, we compared the effects of three stimuli and found that odor stimuli increased saliva volume the most in our comparison.
• Since the effect of stimuli in situations of decreasing saliva volume has not been previously investigated, we applied three stimuli in three situations of decreasing saliva volume and showed that odor stimuli increased saliva volume the most in all three situations.
• We implemented an eyeglass-type wearable device that can provide odor stimulus and found that it can increase saliva volume.

2. Methods

The purpose of this study is to propose a wearable device that can increase saliva volume in daily life. Previous studies have shown that odor, pressure, and thermal stimuli increase saliva volume. Other stimuli, such as electrical stimulation [24,25], have also been shown to increase saliva volume, but are not suitable for constant use in daily life, so these stimuli are candidates for use in the proposed device.

However, the effects of the three stimuli have not been compared under the same conditions. It is also not known whether these stimuli increase saliva volume in unusual situations, such as after exercise. Thermal stimulus may be effective under normal situations, but after exercise, when the body is hotter, the effect of thermal stimulus may be smaller.

Therefore, in this study, we compared the effects of stimuli in different situations. It is not easy to investigate various situations in daily life comprehensively. Since the purpose of the proposed device is to increase saliva volume when saliva volume decreases, the situation in which the proposed device is most likely to be used is when saliva volume decreases. Therefore, we investigated various situations in daily life where saliva volume decreases. If the most suitable stimulus for different situations of reduced saliva volume is one, we propose a device with only one stimulus. If the most effective stimulus differs depending on the situation, the proposed device has multiple stimuli and switches between them depending on the situation.

In addition, the proposed device must be able to increase saliva volume quickly. For example, a user has bad breath due to decreased saliva volume after exercise. The time that bad breath continues is as long as the time taken to increase saliva volume. Therefore, the proposed wearable device should increase saliva volume quickly.

To compare the effects of stimuli in different situations, we addressed the following three points:

• Identify situations in daily life that decrease saliva volume.
• Implementation of an experimental wearable device that can provide three stimuli.
• Compare the effects of three stimuli under situations of decreasing saliva volume.

This section is organized as follows: Section 2.1 describes the survey research on the situation of decreased saliva volume. In this subsection, we describe how we investigated the situation and how many people were investigated. Section 2.2 describes the structure of the experimental wearable device, how it can be worn, and what kind of stimuli it can provide. Section 2.3 describes the experimental design. In this section, we describe the situations, stimuli, subject information, and sample size for the experiment. Section 2.4 describes how the experiment was conducted, and Section 2.5 describes how saliva volume was measured.

2.1. Survey Research

The proposed device needs to increase saliva volume in situations where saliva volume is decreasing. Although the effect of the stimulus in usual situations has been investigated, it has not been investigated in situations with decreasing saliva volume.

However, to the best of our knowledge, there are no studies on whether saliva volume decreases under various situations of daily life, so we investigated these situations. Therefore, we conducted a questionnaire to investigate the situations in which saliva volume is likely to decrease.

In the questionnaire, respondents were asked to select the situations in which they felt their oral cavity was dry from the 14 situations. The 14 situations were upon waking up, after teeth brushing, before a meal, during a meal, after a meal, during exercise, after exercise, during conversations, when drinking alcohol, when drunk, when bathing, after bathing, before sleeping, and others. First, respondents spent one week before completing the questionnaire. During that week, the respondent spent time trying to figure out when their oral cavity was dry. After one week, the respondents answered all the situations in which they felt their oral cavity was dry.

Figure 1 shows the results of the questionnaire. The results show that among the various situations, subjects felt their saliva volume was most decreased upon waking up. The second was after exercise, the third was during exercise, and the fourth was during conversation. According to some studies, saliva volume is likely to decrease during sleep due to decreased oral activities such as swallowing [5,26]. In addition, saliva volume may decrease after exercise due to a lack of fluids [27,28]. Although we could not find any study on saliva levels after conversation, these three situations are likely to result in decreased saliva volume. Therefore, in our experiments, we chose the situations upon waking up, after exercise, and after conversations as situations of decreasing saliva volume.

2.2. Experimental Wearable Device

We compared the effects of the three stimuli using an experimental wearable device that can provide these stimuli, although several methods exist to compare the three stimuli. One way of comparison is to only prepare stimuli without implementing a wearable device. For example, we prepare lemon aroma and subjects smell the lemon aroma directly. However, this method is not adopted in this study because it may not be possible to implement the stimuli as a wearable device. Another way to compare three stimuli is to prepare three wearable devices, each with one stimulus. However, if the shapes of the three devices are different, it is not clear whether saliva volume is affected by the different shapes or stimuli. Therefore, a single experimental wearable device should provide all three stimuli.

We implemented an experimental wearable device with three stimuli, as shown in Figure 2. The device provides the wearer with three stimuli in various situations. Three stimuli are controlled by a microcontroller module that contains a battery (M5Stack, Shenzhen, China, M5StickC).

The microcontroller module has a button and is capable of Bluetooth and TCP/IP communication. The microcontroller module manipulates the stimuli of the experimental wearable device when the button is pressed or when it receives a signal from the communication destination. In this experiment, after confirming that the subject was wearing the experimental wearable device, the experimenter used a PC to send signals to the microcontroller module via TCP/IP communication to provide stimuli to the subject.

The odor stimulus increases saliva volume by blowing air toward the aroma stone and sending odors to the tip of the nose. The components used for odor stimulus are shown in Figure 3a. The aroma stone (Amazing Craft Co., Ltd., Shenzhen, China, CERAMIC AROMA BEADS 25 g) was soaked in approximately 0.2 mL lemon aroma (Amazing Craft Co., Ltd., Osaka, Japan, Lemon Essential oil) because the lemon odor increases saliva volume [12]. The microblower (Murata Manufacturing Co., Ltd., Kyoto, Japan, MZB1001T02) was activated when the microcontroller module applied a voltage of 5 V to it.

The pressure stimulus uses two linear actuators to massage the lower part of the jaw to increase saliva volume. The components used for pressure stimulus are shown in Figure 3b. The microcontroller module controls the extension and retraction of the actuators (Actuonix Motion Devices Inc., Saanichton, BC, Canada, L12-R Micro Linear Servos for RC & Arduino). The position of the pressure stimulus is shown in Figure 4. The experimental wearable device is in contact with the center of the neck, and the linear servo is located 3.5 cm away from the front in the horizontal direction. The actuator is positioned to extend and retract 1.5 cm every 4 s to stimulate the submandibular gland.

The length of extension and contraction was chosen to be closest to the finger strength. When the length of the extension varied between 1 cm, 1.5 cm, and 2 cm, the lower part of the first author’s jaw was massaged, and the length closest to the finger force was 1.5 cm. Therefore, the length of the extension was determined to be 1.5 cm. It applied a force of 40 N.

The thermal stimulus increases the saliva volume by warming the lower part of the jaw with a Peltier module (Thermonamic Electronics (Jiangxi) Co., Ltd., Nanchang, China, TEC1-12706). The components used for thermal stimulus are shown in Figure 3b. The LiPo battery applies a voltage of 3.7 V to the Peltier module, which reaches a temperature of approximately 40 degrees Celsius. The Peltier module heats a 4 cm × 4 cm area under the chin, as shown in Figure 4. The microcontroller module controls whether the LiPo battery applies voltage or not.

To make the device design suitable for individuals with different face shapes and sizes, the experimental device can be positioned and fixed with a strap so that people can adjust it to the appropriate position by themselves. Also, the experimental device is made slightly larger so that people with larger faces can wear it.

2.3. Experimental Design

Based on the questionnaire results, experiments were conducted in 16 conditions (4 stimuli × 4 situations) to identify which stimuli were effective in which situations. In this experiment, we recruited 12 subjects (all males in their 20s). All subjects were measured for these 16 conditions, so a total of 192 data points (12 subjects × 16 conditions) were collected.

The following four stimuli were used in this experiment to increase saliva volume:

• No stimulus: Wearing the device without a stimulus.
• Thermal: Warming the lower jaw to approximately 40 °C.
• Pressure: Applying pressure on the salivary glands.
• Odor: Blowing lemon odor to the tip of the nose.

The following four situations were defined based on the results of the questionnaire:

• Usual: After 30 min of desk work.
• Upon waking up: After 30 min of sleep.
• After exercise: After 30 min of jogging.
• After conversation: After 30 min of conversation with four people.

The usual situation is one in which the saliva volume is not affected by the situation. Therefore, we defined a “usual situation” as follows: the usual situation is the situation after 30 min or more of eating, drinking, smoking, talking, exercising, or sleeping. In addition, the location is the subject’s usual habitat, and there are no special odors compared to usual.

Eating and drinking were prohibited during desk work, jogging, sleeping, and conversation. The participants were also instructed not to look at pictures of food or drink while performing their desk work. Furthermore, jogging was performed at approximately 5 km/h, and speaking was prohibited during jogging. During the conversation, subjects were instructed to maintain an equal opportunity for everyone to speak.

2.4. Experimental Procedure

We experimented with 4 conditions per day, for a total of 16 conditions over 4 days. Figure 5 shows how the four conditions of the day were performed. In the one-day experiment, subjects first gathered at 1:00 p.m. for an explanation of the experiment. After the explanation, they spent 30 min in Situation A. After that, the subjects put on the device and were stimulated for 30 s. In addition, saliva volume was measured during the 30 s of stimulus. After that, the subject rested for 5 min and then moved to Situation B. During the rest, the subject drank a cup of water.

Since saliva volume varies with time during the day [29], it is an important factor to consider when to experiment. The order of the stimulus and the order of the situations could also affect saliva volume. Therefore, the orders of situations and stimuli were counterbalanced among the participants.

The experiment started at 1:00 p.m. To avoid the influence of lunch on saliva volume, subjects were allowed to finish their lunch by 12:00 a.m. Subjects were also asked to refrain from eating spicy foods or other stimulating foods for lunch.

Whether the experimental device provided appropriate stimulation was confirmed by the first author’s visual inspection and oral interview with the subject. In particular, the location of the pressure stimulus had to be in an appropriate position. The experimental device was held in place by a strap at the top of the head, and the position of the experimental device could be adjusted by changing the strength and position of the strap. Therefore, the subject and the first author could change the strength and position of the strap to position the experimental device appropriately.

2.5. Measurement Method

We used a cotton method to measure the saliva volume [30,31]. Figure 6 shows how subjects measured saliva volume using the cotton method. In this method, a cotton roll, approximately 10 mm wide and 30 mm long, was placed under the subject’s tongue, with the mouth lightly closed. After 30 s, the cotton roll was removed. Then, the saliva volume was measured from the change in the weight of the cotton roll. Before the 30-s measurement, the subject swallowed any saliva remaining in the mouth. During the 30-s measurement, the subject was instructed to not move their face or swallow.

3. Results and Discussion

We performed ANOVA on the saliva volumes for the four stimuli and four situations. It was a two-factor within-subject ANOVA. The ANOVA results showed significant differences in the main effects of the stimulus and situation (stimulus: $F(3,33)=9.07,p<0.01,$ partial ${\eta }^2=0.292$; situation: $F(3,33)=4.55,p<0.01,$ partial ${\eta }^2=0.452$). No interaction was observed.

Figure 7 shows the average saliva volume for each situation. Error bars indicate standard errors. Here, ** represents p < 0.01. As shown in Figure 7, the saliva volume after conversation was significantly higher than that after exercise and upon waking up.

There was no situation in which there was a significant difference compared to the usual situation. This means that we could not create a situation in which saliva volume was significantly reduced. However, in this study, we only prepared situations in which saliva volume might decrease to set up a variety of situations. Therefore, there is no problem with the design of the experiment, which aimed to check the effect of the stimuli in situations where the saliva volume was likely to decrease.

Figure 8 shows the average saliva volume for each stimulus. Error bars indicate standard errors. Here, ** represents p < 0.01. As shown in Figure 8, odor stimulus significantly increased saliva volume compared to no stimulus and other stimuli. Therefore, 30 s of odor stimulus can be said to increase saliva volume.

Comparing the effects of the stimuli across individuals, the results for the 12 individuals can be divided into four patterns, as shown in Figure 9.

The first pattern shows that only odor stimuli increased saliva volume, with the same trend as the average saliva volume of the 12 subjects. In addition, the saliva volume in that pattern was almost the same for the following three: after no stimulus, thermal stimulus, and pressure stimulus. Further, 8 of the 12 subjects showed this pattern, and the typical saliva volume of 1 subject is shown in Figure 9a.

The second pattern is the highest saliva volume for the thermal stimulus compared to the other stimuli and the no stimulus condition. This pattern indicates that the thermal stimulus may be the most suitable stimulus to increase saliva volume in this subject. This pattern was only seen in 1 out of the 12 subjects, and the saliva volume for each stimulus for this subject is shown in Figure 9b.

The third pattern is that all stimuli increase saliva volume. Compared to the no-stimulus saliva volume, all the stimuli increased saliva volume. This pattern was observed in 1 of the 12 subjects. The saliva volume for each stimulus is shown in Figure 9c.

The fourth pattern is the one with the highest non-stimulated saliva volume. In addition, when stimuli were present, the saliva volume was highest with odor stimulus. This pattern of saliva volume decreased when a stimulus was received. It is necessary to examine whether other stimuli, such as electrical stimulation, increase saliva volume in subjects with this pattern. This pattern was observed in 2 out of 12 subjects, and the saliva volume for each stimulus of a typical subject is shown in Figure 9d.

The subjects were divided into four patterns as described above. In all patterns except for pattern d, saliva secretion was higher when odor stimuli were applied compared to no stimuli. In pattern d, saliva secretion was higher when subjects were given the odor stimulus than when they were given the other two stimuli. In addition, all subjects produced significantly more saliva when exposed to the odor stimuli than when not exposed to the odor stimuli. Therefore, we conclude that the odor stimulus is effective.

4. Proposed Device

We propose the device shown in Figure 10. The experimental results show that the odor stimulus increases saliva volume, so the proposed device can provide odor stimulus through the experimental wearable device. In addition, the proposed system does not require the ability to provide thermal and pressure stimuli. To reduce its weight and make it smaller, the proposed system can only provide odor stimuli and was changed from a mask type to a glasses-type device. The weight of the experimental device is 211 g, and the proposed system weighs 48 g, which reduces the burden of daily use compared to the experimental device.

The user wears the proposed device at all times, and when the amount of saliva secretion is insufficient, odor stimulation is provided. The amount of saliva secretion is measured using a method shown in previous studies [32,33], and when it is insufficient, a signal is sent to the microcontroller module, which automatically provides odor stimulation. Therefore, the proposed device is always worn, but odor stimulation is provided only when necessary.

The odor stimulus components of the proposed device are similar to those of the experimental wearable device. When the proposed device provides an odor stimulus, the microcomputer module applies a voltage of 5 V to the microblower. The microblower then blows air toward the aroma stone. The aroma stone is soaked in lemon aroma, so the odor of lemon reaches the wearer’s nose.

The odor stimuli used in the proposed device are the same as those used in the experimental wearable device. Therefore, the odor stimuli of the proposed device can significantly increase saliva volume in the four experimental situations.

We investigated whether the odor stimulus of the proposed device increased saliva volume. The odor stimulus of the proposed device and that of the experimental wearable device have similar mechanisms. The location of the odor stimulus is the same, as is the fact that the devices increase saliva volume by applying the odor stimulus for 30 s. However, the shape of the device was changed from a mask to an eyeglass. The difference in shape alters the wearability of the device. For example, the mask-type device presses on the jaw area, while the eyeglass-type does not. Therefore, we evaluated whether the odor stimulus also increases saliva volume in the eyeglass-shaped device, which has a different shape.

The proposed device was only evaluated under the usual situation. The odor stimulus of the experimental wearable device significantly increased the saliva volume in the usual, upon waking up, after exercise, and after a conversation situations. Therefore, if the odor stimulus of the proposed device can increase saliva volume in the usual situation, it can also increase saliva volume in the other three situations. Therefore, we investigated whether the odor stimulus of the proposed device increases saliva volume under the usual situation.

The experimental procedure was the same as in Section 2.4. However, while 16 conditions were conducted in Section 2.4, the evaluation experiment was conducted under two conditions. Another difference is that the subjects continued to wear the proposed device during the evaluation experiment. First, subjects gathered at 1:00 p.m. and we explained the experiment to them. After the explanation, subjects wore the proposed device and worked at a desk for 30 min. Then, 30 min later, the saliva volume was measured using the cotton roll method. After the measurement of saliva volume, the subjects took a 5-min break, during which they drank a glass of water. After that, the subjects worked at a desk for 30 min again. Finally, the saliva volume was measured for 30 s. In this evaluation experiment, the saliva volume was measured twice. In one measurement, the proposed device stimulated the participants with an odor stimulus, and in the other measurement, the proposed device did not stimulate the participants. The presence or absence of stimulus was counterbalanced among the participants. In this evaluation experiment, five subjects were recruited (all males in their 20s).

To see if the odor stimulus in our proposed device increases saliva volume, we performed a one-tailed t-test with correspondence against saliva volume with and without the stimulus. The t-test results show that the odor stimulus significantly increased the saliva volume compared to without odor stimulus ($t\left(4\right)=2.03,p<0.1,d=0.156$). Figure 11 shows the average saliva volume of all subjects without and with stimulus. Error bars indicate standard errors. Here, * represents p < 0.05.

The experimental results show that the odor stimulus in the proposed device significantly increased saliva volume. Therefore, it can be said that the proposed device has the same effect as the experimental wearable device. The odor of the experimental wearable device increased saliva volume even in situations where saliva volume was considered to decrease, such as after waking up. Therefore, the proposed device is also expected to increase saliva volume in situations where saliva volume is decreased.

In this experiment, we identified problems in the daily use of the proposed device by continuing to wear the device. After approximately one hour of the experiment, the subjects freely wrote about their impressions of wearing the proposed device. The results of the impressions are summarized as follows.

• The proposed device was heavy. The nose was particularly heavy and unbalanced.
• The proposed device limited the field of vision. It was especially difficult to see the lower part of the field of vision.
• It was shameful to wear the proposed device in daily life.
• Lemon smelled good.

From the above, it was found that the component providing odor stimulus imposes an unbalanced weight, limited visibility, and interferes with the user’s actions. In addition, the design of the proposed device is not good, and some users felt that it was shameful to use the device in daily life. Therefore, it is necessary to reduce the size and weight of the component that provides odor stimulus and to improve the design of the device. We continue to improve our devices and design them to be more ergonomic.

5. Limitation

In this study, we compared three stimuli in four situations with male subjects in their 20s. However, we also need to investigate other genders, age groups, situations, and stimuli.

We recruited men in the experiment. However, Percival et al. have shown that saliva volume differs between males and females [34]. Therefore, it is necessary to conduct the same experiment not only on males but also on females.

All subjects were in their 20s. However, Fenoli et al. showed that saliva volume decreases with age [4]. Therefore, it is necessary to confirm whether the proposed device can increase saliva volume in the elderly as well. It is also necessary to investigate the efficacy of the proposed device in long-term use throughout the user’s lifetime.

The stimuli used in this experiment were the odor of lemon, massage to the lower part of the jaw, and heat, but other stimuli should be evaluated. Since it is known that the odors of chocolate and beef increase saliva volume [13], it is necessary to use chocolate or beef as a candidate for the odor. Pressure stimulus was also applied to the lower part of the jaw, but a method to apply pressure to the parotid gland is also possible [35]. In addition, since it has also been shown that thermal stimulus increases saliva volume at cold temperatures [15], it may be possible to increase saliva volume using cold temperatures.

Although this study showed that an odor stimulus for 30 s increased saliva volume, it is necessary to investigate whether longer stimulus increase saliva volume. Odors are known to habituate [14], and habituation may make longer stimulations less effective. In other words, there may be a suitable stimulus time for odor stimulus. In addition, 30 s of pressure stimulation did not significantly increase saliva volume, but 1 min of pressure stimulation may increase saliva volume because 1 min of massage to the lower part of the jaw region is known to increase saliva volume [18].

It is also necessary to investigate the case in which all three stimuli are given simultaneously. Since the purpose of this study was to compare the stimuli and select the best stimulus, we did not investigate the case where all three stimuli were given at the same time. However, it is possible that some users, such as group d, may find all stimuli less effective. Therefore, it is necessary to investigate whether the simultaneous application of all stimuli can increase saliva volume in such users.

The situations prepared for this experiment were usual, upon waking up, after the conversation, and after exercise. There was no significant decrease in saliva volume compared to usual situations. On the other hand, saliva volume was significantly lower upon waking up and after exercise compared to saliva volume after conversation. Therefore, it cannot be said that we were able to evaluate the saliva volume in the situation where saliva volume decreased when compared to the usual situation. However, it was possible to compare the saliva volume in situations where there was a difference in saliva volume.

It is necessary to investigate whether the proposed device increases saliva volume during sleep, exercise, bathing, and drinking. In addition, it is necessary to examine situations in which saliva volume is even further reduced and to examine the stimuli in those situations as well. Furthermore, it is necessary to confirm the increase in saliva volume and to examine the safety of the proposed device by observing individuals actually wearing the device in daily life, not in a set situation as in the present experiment. Other issues include evaluation of the efficacy and safety of the proposed device in long-term use.

6. Conclusions

In this study, we compared the increases in saliva volume produced by the three stimuli in four different situations. A mask-type experimental wearable device was implemented for the experiment. The experimental results show that odor stimulus significantly increased saliva volume compared to no stimulus and other stimuli. In other words, the odor stimulus increased the saliva volume quickly in the three situations. In addition, we proposed, implemented, and evaluated an eyeglass-shaped device that can provide odor stimulus. The evaluation results show that the proposed device increased saliva volume.