情報処理学会 インタラクション 2024
IPSJ Interaction 2024

3B-42
2024/2/28

Wristband-type tactile sensors based onbioelectrical impedance measurements
Tianyi Deng1,2,a)Shin Takahashi2,b)Abstract: This study presents the development of a portable wristband-shaped touch sensor utilizing
bioimpedance technology. The system integrates a microcontroller (STM32F4), an impedance measurement
chip circuit (MAX30009), and a virtual keyboard interface implemented in PyQt. The impedance circuit
generates a sinusoidal current at 66.944 kHz, measuring voltage responses for impedance calculations. The
system detects impedance changes during hand contact, enabling ﬂexible touch interaction on the hands.

1.

Introduction

Bioimpedance measurement, which involves assessing theresistance of biological tissue to small electrical currents, offers a promising way for capturing physiological and biomechanical data. It can provide valuable information for touchdetection [1–5].
We propose a wristband-shaped portable hand-touch
sensor. It is a bioimpedance-based device that injects
a small current at 66.944 kHz to measure the human
impedance, which enables real-time detection of the change
in impedance that occurs when hands touch each other. We
also developed a simple touch-based virtual keyboard system to show the possible use of our wristband sensor.

2.

Related WorkRegarding tactile sensors, there exists a considerable bodyof prior research. Das et al. [5] proposed a wearable sensor for bioelectric monitoring. The developed ﬂexible conductive elastomer (CE) electrode can bend along the curvature of the body surface, allowing subjects to easily wear
the system on their ﬁngers for human-machine interaction.
Chen et al. [6] has developed a large-area ﬂexible tactile sensor based on Electrical Impedance Tomography (EIT) technology, achieving multi-touch and force detection. Liu etal. [7] presented a pressure-sensitive impedance-type touchscreen based on GPANI-PVB composite ﬁlm and glass substrate. This touchscreen is capable of measuring changes
in impedance and detecting touch positions as well as applied pressure. Corchia et al. [8] proposed two fully textile, chipless tags based on diﬀerent frequency signal technologies as wearable touch sensors. Both proposed devices,12a)b)

Hunan University
University of Tsukuba
deng@iplab.cs.tsukuba.ac.jp
shin@iplab.cs.tsukuba.ac.jp© 2024 Information Processing Society of Japan

when utilized as touch sensors, are capable of encoding four
diﬀerent conﬁgurations based on the distinct relative positions between the sensor and the hand. These studies haveeach oﬀered unique approaches to touch detection, achieving high accuracy and diverse touch modes. However, theyshare a common drawback: the touch range is heavily dependent on a small area covered by the speciﬁcally designed
electrodes. Our objective is to design a wristband-shaped
portable hand-touch sensor based on bioimpedance technology, allowing operation at any position on both hands, suchas situations where only one hand is available.

3.

Implementation of Impedance Measurement Device.

The device comprises three main components: a microcontroller based on STM32F4, an impedance measurement
circuit based on MAX30009, and a software interface basedon PyQt.
3.1 Hardware
The main function of the microcontroller is to control theMAX30009 (a bioimpedance analog front-end integrated circuit developed by ADI) via SPI communication and read themeasured impedance value in real time and then send it toa PC via Bluetooth at a baud rate of 115200.
The relevant circuit design for MAX30009 is illustrated
in Fig. 1. Following references [9, 10], it is conﬁgured tooutput a sinusoidal current of 45.25 uA at 66.944 kHz in afour-electrode setup and measures the voltage response tocalculate impedance.
We have also prepared a set of wrist-bands featuring copper dry electrodes, as illustrated in Fig. 2. This design enhances wearability, allowing users to easily put on the electrodes for seamless use.

1246

情報処理学会 インタラクション 2024
IPSJ Interaction 2024

Fig. 1Fig. 2

Impedance measurement circuit board design

Wristband-shaped electrodes and impedance measurement board

3.2 Software
To receive impedance information from the STM32F4, wedeveloped a Python-based supervisory software. After calibration, the software repetitively processes the measured
impedance information, and visualizes the results like an oscilloscope. For example, Fig. 3 shows the impedance change
when the user puts on the wristband and stands still forsome time and touches the lower arm area of the left handwith the right hand. The measured resistance and reactanceare indicated by the blue and red lines, respectively. In theleft half of the waveform, the current primarily ﬂows throughthe hand, passes through the chest tissue, and reaches theopposite electrode. When the hand touches the other hand,an additional path for the current is created, resulting in asigniﬁcant decrease in parallel resistance. In addition, due
to the change in contact area and force of the touch, theimpedance changes correspondingly. This property enables
ﬂexible sensing of the user’s various touch interaction on thehands.

4.

Touch-based Virtual Keyboard Interface

Fig. 4, shows the virtual keyboard interface. The interface
consists of three parts, a text input box located at the top,a touch impedance prompt bar in the middle, and a current
impedance prompt bar at the bottom.
When there is a rapid change in the slope of the resistance(indicating a quick contact between the hands), the system
detects this change and records the post-change value. Sub-© 2024 Information Processing Society of Japan

3B-42
2024/2/28

Fig. 3The impedance changes when one hand touches the otherhandsequently, the current value is continuously compared withthe recorded value and mapped to a progress bar for display. When there is a swift separation between the hands,
the system determines the current selection, and associates
it with one of seven diﬀerent choices, corresponding to ”asdfg[Cap]”, ”yuiop[Del]”, ”qwert[Num]”, ”[Null]”, ”hjkl;[]”,”zxcvb[Space]”, ”nm,.?[Shift]”. Upon the second touchselect-release sequence, the system outputs a speciﬁc character or command associated with the chosen option. For
example, the ﬁrst operation selects the corresponding ”asdfg[Cap]” in the yellow block, and the second time again
selects the corresponding character ’d’ in the yellow block
to complete the keying of a letter.

Fig. 4

5.

Virtual Keyboard Interface

Summary

We have developed a portable wristband-shaped touch
sensor based on bioimpedance technology.
The device includes a microcontroller (STM32F4), an
impedance measurement chip circuit (MAX30009), and agraphical user interface (GUI) implemented in PyQt. We designed wristbands with copper dry electrodes for easy wear.
The system detects impedance changes when hands touch,enabling ﬂexible touch interaction on the hands.

References[1] Chi Cuong Vu, Sang Jin Kim, and Jooyong Kim. Flexible wearable sensors - an update in view of touchsensing. Science and Technology of Advanced Materials, 22(1):26–36.
[2] Guojin Liang, Zhaoheng Ruan, Zhuoxin Liu,
Hongfei Li, Zifeng Wang, Zijie Tang, Funian Mo,Qi Yang, Longtao Ma, Donghong Wang, and ChunyiZhi. Toward Multifunctional and Wearable Smart
Skins with Energy-Harvesting, Touch-Sensing, andExteroception-Visualizing Capabilities by an All-1247

情報処理学会 インタラクション 2024
IPSJ Interaction 2024

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

3B-42
2024/2/28

Polymer Design. Advanced Electronic Materials, 5(10):1900553.
Ngoc Tan Nguyen, Mirza Saquib Sarwar, Claire Preston, Aziliz Le Goﬀ, Cédric Plesse, Frédéric Vidal,
Eric Cattan, and John D. W. Madden. Transparent stretchable capacitive touch sensor grid using ionicliquid electrodes. Extreme Mechanics Letters, 33:100574.
Oliver Ozioko, William Navaraj, Marion Hersh, andRavinder Dahiya. Tacsac: A Wearable Haptic Device
with Capacitive Touch-Sensing Capability for Tactile
Display. Sensors, 20(17):4780.
Partha Sarati Das and Jae-Yeong Park. A ﬂexible
touch sensor based on conductive elastomer for biopotential monitoring applications. Biomedical Signal
Processing and Control, 33:72–82.
Haofeng Chen, Xuanxuan Yang, Peng Wang, Jialu
Geng, Gang Ma, and Xiaojie Wang. A Large-Area
Flexible Tactile Sensor for Multi-Touch and Force
Detection Using Electrical Impedance Tomography.
IEEE Sensors Journal, 22(7):7119–7129.
Shi-Yu Liu, Jin Pan, Jian-Gang Lu, and Han-Ping D.
Shieh. A Pressure-Sensitive Impedance-Type Touch
Panel With High Sensitivity and Water-Resistance.
IEEE Electron Device Letters, 39(7):1061–1064.
LauraCorchia,
Giuseppina
Monti,Egidio
De Benedetto, and Luciano Tarricone. Low-Cost
Chipless Sensor Tags for Wearable User Interfaces.
IEEE Sensors Journal, 19(21):10046–10053.
Manoj Jose, Marijn Lemmens, Seppe Bormans,
Ronald Thoelen, and Wim Deferme. Fully printed,
stretchable and wearable bioimpedance sensor on textiles for tomography. Flexible and Printed Electronics,
6(1):015010.
Mehmood Nawaz, Russell W. Chan, Anju Malik, TariqKhan, and Peng Cao. Hand Gestures Classiﬁcation Using Electrical Impedance Tomography Images.
IEEE Sensors Journal, 22(19):18922–18932.

© 2024 Information Processing Society of Japan

1248