Wearable devices, which are highly conformable and intimately associated with human skin, are necessary for daily-using human/machine interface and healthcare monitoring system. Since the devices must be able to withstand the stress applied to the devices by the curvature of the skin or the movement of the human body, many researchers have focused on developing a method for implementing the existing electronic devices on flexible substrates. Our research group has manufactured a variety of wearable sensors and stretchable electrodes with high performance and stability on flexible/stretchable substrates by using nano/micro-structuring & patterning technology. Then, we also successfully demonstrated the application of those sensors and electrodes on human body and prosthesis.
Followings are the recent works of our research group :
1.1. Linearly and Highly Pressure-Sensitive Electronic Skin
Pressure-sensitive electronic skin applications generally require high sensitivity for applied pressures of up to ≈ 10 kPa, which corresponds to a gentle touch, most of previously reported pressure sensors did not sustain their high sensitivity to this upper pressure value. Because of the narrow sensing range, these sensors could not discriminate a subtle change in the relatively high-pressure regime. Furthermore, most of these sensors did not exhibit a wide range of linear sensitivity. In this study, we present a high-performance piezo-resistive pressure-sensor device with a linear relationship between applied pressure and output and with a high sensitivity of 8.5 kPa −1 for a wide range of pressures, specifically between 0 and 12 kPa, by using a bioinspired hierarchical structure consisting of PDMS covered with monolayer graphene. Our pressure sensor was shown to exhibit a high durability of 10,000 cycles and a low limit of detection of 1 Pa. In addition, the highly transparent and conductive monolayer graphene resulted in a transparent sensor with a low operating voltage of only 1 V.
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1.2. Pressure/Temperature Sensing Bimodal Electronic Skin with Stimulus Discriminability
Human skin imperfectly discriminates between pressure and temperature stimuli under mixed stimulation, and exhibits nonlinear sensitivity to each stimulus. Despite great advances in the field of electronic skin (E-skin), the limitations of human skin have not previously been overcome. For the first time, the development of a stimulus-discriminating and linearly sensitive bimodal E-skin that can simultaneously detect and discriminate pressure and temperature stimuli in real time is reported. By introducing a novel device design and using a temperature-independent material, near-perfect stimulus discriminability is realized. In addition, the hierarchical contact behavior of the surface-wrinkled microstructure and the optimally reduced graphene oxide in the E-skin contribute to linear sensitivity to applied pressure/temperature stimuli over wide intensity range. The E-skin exhibits a linear and high pressure sensitivity of 0.7 kPa−1 up to 25 kPa. Its operation is also robust and exhibits fast response to pressure stimulus within 50 ms. In the case of temperature stimulus, the E-skin shows a linear and reproducible temperature coefficient of resistance of 0.83% K−1 in the temperature range 22–70 °C and fast response to temperature change within 100 ms. In addition, two types of stimuli are simultaneously detected and discriminated in real time by only impedance measurements.
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II. Vibrational Sensor
2.1 Wearable throat microphone
Flexible and skin-attachable vibration sensors have been researched for wearable mechanosensors to recognize human voice, the most important bio-signal for communication. We have presented polymer-based wearable throat microphone that detects voice quantitatively. While commercial microphone could perceive human voice as distorted in ambient noise or use of mouth mask, our device detects the voices clearly in the same acoustic environment by detecting neck skin vibration when human speaks. Our device successfully demonstrated voice recognition application in human machine interface and Internet of Things area such as voice security authentication and voice-controlled system. In addition, the device enabled vocal healthcare monitoring conveniently even in noisy work places, by measuring quantitatively phonation time, voice frequency and pressure. This diagnostic application can provide a valuable monitoring approach for more than one-third of the working population, who use their voice as the primary tool.
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III. Stretchable Device
3.1 Stretchable electrode
Stretchable electrodes have applications in a large variety of flexible and stretchable electronics including displays, photovoltaics, batteries, and sensors. These applications require a range of stretchabilities at low (< 30%) to high (> 100%) strain levels. Existing stretchable electrodes are mainly based on one of two different wavy structures of metals: out-of-plane buckled thin metal films, or in-plane serpentine metal interconnects. Hybrid materials with both out-of-plane and in-plane wavy structures have the potential to exhibit greater stretchability than these systems. Our research group presents an omnidirectionally and highly stretchable electrode that consists of silver nanowire (AgNW) arrays containing non-coplanar (out-of-plane wavy) and zigzag (in-plane wavy) mesh structures transferred to an elastomeric substrate. A reversible wrinkling pattern was used as a template for the alignment of the NWs, and as a stamp for transferring the aligned NWs to the target elastomeric substrate because the amplitude of the wrinkles on the template can easily be controlled by varying the external strain. We systematically examined the stretchability of aligned AgNW arrays that had been transferred onto prestrained elastomeric substrates with precisely tuned stretching ratios and axes., which results in the improvement of the stretchable device performance.
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