Project

ambienBeat: Mobile tactile biofeedback for subliminal heart rate rhythmic regulation

Kyung Yun Choi

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We present a mobile heart rate regulator—ambienBeat—which provides closed-loop ambient biofeedback via subliminal tactile stimulus based on a user's heartbeat rate variability (HRV). We applied the principle of interpersonal physiological synchronization to achieve our goal of effortless regulation of HRV, which is tightly coupled with mental stress levels. ambienBeat provides various patterns of subliminal tactile stimuli, which mimics the feeling of a heartbeat pulse, to guide a user's HRV to resonate with its rhythmic, tactile patterns. The strength and rhythmic patterns of tactile stimulation are controlled to a level below the cognitive threshold of an individual's tactile sensitivity on their wrist so as to minimize task disturbance. Here we present an acoustically noiseless, soft voice-coil actuator to render the ambient tactile stimulus and present the system and implementation process. We evaluated our system by comparing it to ambient auditory and visual guidance. Results from the user study shows that the subliminal tactile stimulation was effective in guiding a user's HRV to resonate with ambienBeat to either calm or boost the heart rate using minimal cognitive load.

Published in ACM TEI '20 Fast Company 2019 Best Student Design Award - Honorable mention

Contributions

We present an overview of current technologies related to assisting self-awareness and regulating physiological states through biofeedback, and a theory of physiological synchronization via touch. We show how these studies can be used to design a tactile biofeedback system for guiding the users’ body to regulate their physiological state.
We describe and evaluate the development of a wearable closed-loop biofeedback system that integrates the capability of sensing and actuation for non-invasive regulation of users’ HR. Also, we introduce a fabrication method to develop an acoustically noise-less tactile soft actuator using a customized voice-coil with silicone membrane.
We evaluate 3 different ambient biofeedback modalities: auditory, visual, and tactile stimulation to verify the effectiveness in regulating HR with minimal task disruption. Also, we present the remarks from the user study showing the strength of tactile stimulation compared to other modalities.
We describe the design implications of this work, including promising directions for the development of closed-loop tactile biofeedback system and its potential to support self-awareness as well as an interface for ambient presence.

Mindfulness

By reading the distance between the peaks of the heart beat pulse (R-R interval), we can continuously estimate the changing HR, and derive a great deal of information about our cardiovascular state, mood, stress levels and anxiety. For this reason, one technique to regulate mental state is tightly coupled with cardiovascular afferents, which have numerous connections to multiple areas of the brain, and play an
important role in determining emotional experience [1]. Meditation is a form of mental training that has been practiced for thousands of years. It can be considered to be part of the family of complex emotional and attentional regulatory training regimens developed for various ends, including the cultivation of well-being and emotional balance [2]. Practicing meditation incorporates attention to internal body
sensations.

Mindfulness [E. J. Langer, 1989], which refers to a mental state of being conscious or aware of one's present moment, and accepting feelings, thoughts, and bodily sensations, has been called the ”heart” of Buddhist meditation. Over the past 20 years, researchers have focused on evaluating the effects of mindfulness on stress reduction [J. K. Zinn et al., 2009] through clinical tests and validated its working mechanisms [3].

Acoustically Noise-less Soft Actuator

We fabricated an electromagnetic field-driven soft actuator using a customized voice-coil and molded a silicone membrane with a neodymium magnet (Grade N52, 5233
Gauss Disc magnet) embedded, as this does not produce unwanted acoustic noise. We coiled a coated 33 AWG copper wire for 250 turns around a hollow aluminum cylinder (permeability k = 1.46) that had a diameter of 15 mm and height of 50 mm. To fabricate a magnet embedded silicone membrane, we 3D printed a plastic negative dome-shaped mold and a cavity to cast a silicone (EcoFlex 00-30, Smooth On). A barometric MEMS pressure sensor (MPL115A2, NXP) was embedded in the silicone for a touch sensitive pressure sensor. We placed it on the bottom part of the device. It is used for getting feedback of the force produced by the voice-coil soft actuator. We casted Ecoflex 00-50 (Smooth On) in 3D printed molds to make the pressure sensing pad. The main actuation part of the regular size has a diameter of 48 mm and a height of 25 mm. The smaller ambienBeat has a soft actuator with a diameter of 37.9 mm and a height of 15 mm.