Keywords

1 Introduction

In modem society, the pace of life is fast and the competition is fierce. An increasing number of people are suffering from chronic stress in their daily life. Breathing techniques have been widely used in mind-body practices for stress reduction and relaxation [1]. Breathing techniques offer a simple tool to improve the balance of autonomic nervous system, strengthen its capability to adapt to stress and further mitigate the negative effects of stress on the health [2]. With a variety of breathing assistance devices, individuals learn to regulate their breathing into an optimal pattern with the aim of stress reduction. But this is not an easy task for most people to achieve a real relaxed state physically and mentally; because besides an improved breathing skills, the users’ psycho-physiologic relaxing experience also affects the stress mitigation. To put it simply, users should also “feel good” during a breathing exercise. Therefore, the interface of breathing assistance device should not only offer an effective breathing guidance, but also a “feeling good” relaxing experience.

In medical applications, breathing guidance is usually presented in graphic or numeric forms, which tend to be technical and performance-oriented. In recent years, several new audiovisual interfaces with more aesthetic qualities have been created for daily use. For instance, Yongqiang Qin et al. developed an immersive breathing training game “Balloon” to train the user to improve breathing pattern [3]. In [4], the authors presented “Sonic Respiration” to provide breathing feedback by modifying the quality of musical interface. However, very little work has explored the feasibility of tactile interface for breathing guidance.

The sense of touch enables us to interact with real objects around us, meanwhile, to perceive these interactions. In [5], the authors suggested that the characteristic bidirectional property of touch sense provides a basis to further enhance motor learning and somatic experience. Studies in [6] revealed that tactile feedback could also reduce the perceived workload in learning tasks and enhance the user’s feeling of presence. Moreover, stimulating the tactile sense can give people strong relaxing experiences or even emotional experiences, which are beneficial for stress mitigation and health. Therefore, tactile stimuli are often used as a way to reduce stress and make people feel better, such as in massaging techniques and physiotherapy. Tactile feedback is often integrated into multimodal interfaces to enhance the user experience. For example in [7] the authors developed a breathing guidance system to provide the users with an immersive experience through a multimodal interface of auditory, vibratile and light stimuli.

In this study, we aim to investigate whether the tactile interface would help users perform breathing exercise more effectively and enhance user experience during the exercise. We present the concept of “Breathe with Touch”: a tactile interface of breathing assistance device that provides breathing guidance through a shape-changing airbag. The airbag inflates and deflates at a specific rhythm to simulate the targeted respiratory pattern. We assume that the changes in the shape of airbag can be mirrored by the user resulting in a better and more relaxing breathing pattern. The tactile interface was evaluated from three aspects: the effect on stress reduction, the efficacy of breathing guidance and the usability. We synthesized the results with valuable qualitative responses from users.

2 Design and Implementation

The concept “Breathe with Touch” entails a tactile interface for breathing exercises. By touching the interface, the user follows the shape change of the interface to receive the feedback information. To design the proper form of tactile feedback, we observe the nature of human’s breathing movement. Breathing is accompanied by diaphragm fluctuation activities. As an individual inhale, the diaphragm contracts and flattens, causing the expansion of the lungs. Conversely, on exhale, the diaphragm relaxes and moves upward to reduce the space in the chest cavity (see Fig. 1). The lungs are like two air balloons inside of our body. The changes in its shape depend on the airflow.

Fig. 1.
figure 1

The shape change of the airbag might be naturally associated with breathing movements

Full size image

“Breathe with Touch” simulates human’s breathing movements through the shape changes of an inflatable airbag. The expansion and contraction of the lungs are mirrored by the inflation and deflation of the airbag. We assume that this kind of natural mapping between the targeted behavior and the interface could minimize cognitive workload thanks to the intuitive interpretation of the interface. Besides, we think the gradual shape-changing process of airbag could render the breathing guidance in a more soothing way, which helps users slow down their breathing and calm down as well. A hand-sized ellipsoid airbag was made of thermoplastic textile covered by a layer of soft velvet. The user rests her hand on the airbag to “feel” the targeted breathing rhythm (see Fig. 2b).

Fig. 2.
figure 2

(a). Touch Airbag schematic diagram. (b). The user’s right hand on the airbag

Full size image

The breathing guidance is mapped to the airbag in the following way: when the airbag inflates, an inhale activity is implied. When the airbag deflates, an exhale activity is implied. The maximum volume of airbag is around 120 ml that is the same size of a mouse approximately. This enables the user to identify subtle changes of the shape easily and accurately by hand. The sensitive tactile feelings from the hand and fingers ensure that users can receive the guidance precisely as well. An air pump and a solenoid vent valves implement the inflation and deflation of the airbag. During the inflation, the air-pump pumps air into the airbag and the valve is closed. During the deflation, the air-pump stops working and the valve turn opened. Then, with the hand own weight, the air will be pushed out of airbag gently.

In this study, we focus on the design and evaluation of the tactile interface. A feed forward system was built with a pre-set breathing rate of 6 breathing cycles per minute (c/min). According to literature [8], most people could achieve a high HRV level under a respiration of 6 c/min. The airbag starts from the deflated state to inflated state then return to deflated state again. The duration of this inflation/deflation process is 10 s and this process repeats for the whole training session. As shown in Fig. 2(a), the proposed tactile interface in this work can be embedded into a complete closed-loop biofeedback breathing assistance system in our future research.

3 Study One: Evaluation of the Effects on Stress Reduction

We administered the first user study to investigate whether the proposed tactile interface would enhance breathing training and reduce stress effectively. 12 subjects (six females and six males, age range: 25 to 35) participated in the study. Each participant performed two stress-induced tasks (mathematical test) before and after breathing training. For each task, the physiological data (pulse signals and respiration data) and subjective stress reports were collected (Fig. 3).

Fig. 3.
figure 3

(a). The experiment set up (b). Right hand on the airbag

Full size image

For each participant, A PPG sensor was placed on the left index finger and a respiratory sensor was placed at the abdominal level. Pulse signals were measured by a data acquisition unit developed by our lab, and then beat-to-beat intervals (RR intervals) were calculated and transmitted to a Processing program for data storage. The standard deviation of the beat-to-beat intervals (SDNN) was calculated as the index of HRV. Respiration data was recorded by the ANT systemFootnote 1 with a sampling rate of 256 Hz. The participant’s stress level were measured by the state component of a Spielberger State-Trait Anxiety Inventory (STAIS) [9].

The experiment followed a procedure as shown in Fig. 4. On arrival at the laboratory, the participants were instructed how to use the breathing assistance system. The pulse sensor and respiration sensor were applied to the participants. Then participants were instructed to relax with their eyes closed for 5 min. After the resting period and without moving, participants were instructed to open their eyes and complete a pre-training mathematical task which lasted 10 min during which time measurements of HRV and respiratory rate were calculated. After the task, participants completed a pre-training STAIS questionnaire. After further 5-min rest with eyes closed, then participants completed a 10-min breathing training session with “Breath with touch”. The instructions given to participants were: “Please follow the changes of the airbag to breathe, when the airbag inflates, you should breathe in. When airbag begins to deflate, you should breathe out. This session will last for 10 min.” After the training, participants completed a further 10-min mathematical task. Pulse signal and respiratory data were also recorded throughout this period. Then, a post-training STAIS questionnaire was completed.

Fig. 4.
figure 4

The experiment procedure

Full size image

The HRV, respiration rate and STAIS were calculated in Pre-training and Post-training periods separately. Differences were analyzed using an independent t-test. All data are described as means ± standard deviation (SD). A p-value of < 0.05 was considered to be statistically significant.

Physiological data were missing from two participants because of technical problems. As the index of HRV, SDNN showed different degrees of increase among all participants after the breathing training, as shown in Fig. 5. The SDNN values of post-training period were significantly higher than pre-training period (49.5 ± 14.2 vs. 66.3 ± 20.4; Pre-training vs. Post-training, p < 0.05). Regarding the results of respiration data, seven participants showed a slower respiration rate during post-training task. However, there were no significant differences before or after training (18 ± 6 vs. 15 ± 2 circles; Pre-training vs. Post-training).

Fig. 5.
figure 5

HRV (SDNN) and respiration rate before and after the breathing training.

Full size image

Psychometric data from the STAIS questionnaires are shown in Fig. 6. Nine participants reported a lower anxiety level during the post-training mathematics task; however, there were three participants showed a higher anxiety levels (subject #1, #4, and #5). The STAIS scores were not significantly different between Pre-training period and Post-training period (46 ± 11 vs. 39 ± 9 points; Pre-training vs. Post-training).

Fig. 6.
figure 6

Scores of STAIS questionnaires pre- and post- the breathing training (N = 12)

Full size image

4 Study Two: Evaluation of Interface Usability

As tactile interfaces are seldom used in breathing assistance, we conducted another study to investigate the user’s perception on the usability of tactile interface. To gain more insightful feedback and recommendations for further design, ten students with interaction design background (four females and six males, age range: 20 to 35) were recruited for this study. All participants complete three 10-min breathing training sessions using the same breathing guidance, but with three different interfaces: visual, auditory and tactile interfaces. The experiment follows a within-subjects design with counter balancing to avoid carry-over effects.

We introduced visual and auditory interfaces for breathing assistance as shown in Fig. 7. In visual interface, an ellipse with varying radius represents the breathing guidance. When the ellipse grows, an inhale process is implied. In auditory feedback, we used the changes in sound volume to present breathing guidance. An increasing sound volume prompted user to breathe in, and the fading sound implied breathing out. The usability of the interface was measured using an adapted Lund’s USE Questionnaire [10]. The questionnaire was designed for three dimensions: ease of use, ease of learning and satisfaction. All the questions used a seven-point Likert scale (1 = strongly disagree, 7 = strongly agree).

Fig. 7.
figure 7

The results of evaluation of interface usability

Full size image

The results are shown in Fig. 7. In terms of “Ease of Use”, all of three interfaces received high scores; there were no significant differences between tactile, auditory and visual interfaces (19 ± 2 vs. 17 ± 2 vs. 17 ± 2; Tactile vs. Auditory vs. Visual). In terms of “Ease of Learning”, there were also no significant differences between three interfaces (19 ± 2 vs. 18 ± 2 vs. 17 ± 2; Tactile vs. Auditory vs. Visual). In terms of “Satisfaction”, the score of visual interface was significant lower than tactile and auditory interfaces (15 ± 3, 14 ± 3 vs. 11 ± 2; Haptic, Auditory vs. Visual, respectively, p < 0.05). From the open-ended interview, we got more positive feedback about tactile interface. More than 70 % participants chose the tactile interface as their favorite interface. Specifically, they expressed a strong interest in working with the tactile interface and emphasized that it was more comfortable due to the touch experience.

5 General Discussion

During a breathing training, users need to recognize the breathing guidance and follow it to regulate their breathing. In other words, the breathing training is a learning process, in which the users should take an active role, rather than just being exposed to it. Therefore, in breathing training, although the learning tasks are very simple (i.e. to regulate breathing to a specific rate), some participants still regard it as a serious task, which keep them away from a pure relaxation. We think this might be an interesting “paradox” in the design of self-training system for relaxation. It appeals to us and drive us to think about the interaction design in these products or systems that aim to promote relaxation. The training session requires user’s mental effort to achieve optimal training effect, such as improved breathing skills, positive imagination or more concentration. However, such mental effort of learning might leads to new stress.

In this design, we tried to lower the workload and enhance the relaxing experience by using a tactile interface. The touch airbag was used as a tangible communication media between the training system and the user. It is assumed that the user can adapt her breathing movements to an optimal pattern more effortlessly by mirroring the shape changes of the tangible interface. The results of user study confirm that the proposed “natural-mapping” interface could help the users perform breathing exercise more spontaneously. The users do not need to put much effort into understanding the instructions, and the transition between inflation and deflation of airbag might naturally trigger a slow and smooth transition between inhalation and exhalation.

The effectiveness of tactile breathing guidance in aiding relaxation and reducing stress was shown by the results of SAITS self-report. However, there is no significant reduction of stress level during the post-training mathematics task. There might be two possible explanations. For most participants, it was their first time to use breathing assistance system; the unfamiliarity with the system brings about new stresses. We also have reservations about the acute effect of 10-min breathing exercise on stress reduction. Although in some clinical use [11], the minimum time of each training session could be 5 min; a short-period exercise may be repeated several times throughout a day to achieve a greater effect on stress mitigation. From physiological measurements, the results suggested that there was a significant improvement in HRV during the post-training task, which suggests that tactile breathing training could enhance the users’physical ability to adapt to stress.

The users’ ratings on the usability of visual, auditory and lighting interface were shown in Fig. 7. The tactile interface shows the potential in improving user experience during the relaxation exercise as seen from significantly higher user ratings on satisfaction. Improvements in user experience were also evidenced by the participants’ feedback from the interviews at the end of the experiment. During a breathing training process, repeated breathing instructions become boring easily. This is a major problem of most breathing guidance systems for long-term use. To some extent, the touch airbag shifts the focus from the superficial interface to users’ own regulating behavior, which helps to relieve users of tedium. Besides, the users thought tactile feedback offers a condition for them to perform breathing training with eyes closed, which also helped relax. They also gave us a lot of insightful recommendations for further design of the interface, for instance, changing the size or position of airbag, integrating the airbag into regular items, and combining tactile and auditory feedback.