Session: 04-23-01: Mechanics and Materials of Soft/Flexible/Stretchable Electronics

Paper Number: 120318

120318 - Printed Liquid Metal Sensory System for Wearable Applications and Boxing Training 

Advances in high-performance electronics with flexibility and stretchability have enabled novel applications in bio-integrated health-monitoring devices, curvilinear electronics, and human-machine interfaces, etc. Liquid metals (LMs) are highly desired in skin-mounted, deformable electronics due to its distinct combination of excellent electrical conductivity comparable to that of metals and exceptional deformability derived from its liquid state. Stretchable LM-based flexible electronics enable precise perception of complex strain and pressure stimuli, thus holds great promise in health monitoring, advanced wearable electronics, and human-machine interfaces. However, achieving sophisticated applications with LMs remains a daunting challenge, which requires overcoming the high surface tension of LMs and improving the sensitivity of sensors.

Recent advances in LM modification paved the way for creating better flexible electronics with improved processes. By modifying the wetting characteristics and viscosity of LMs, the fabrication processes that originally involved lithography and reduction procedures can be simplified. Some efforts have been invested to incorporate metal particles into LMs. To ensure homogeneous dispersion, both LMs and metal particles were acid treated to eliminate oxidation,  which allowed the particles to be wetted and suspended in the LMs to improve adhesion for directly writing on various substrates. Ultrasonic treatment has also been adopted to improve device manufacturing. In this approach, the accelerated oxides were distributed inside LMs during sonication, to increase the viscosity of LMs. However, due to the limited viscosity improvement effect, it remains a challenge to pattern the LMs with rapid prototyping capability.

In addition to the necessity to develop an efficient preparation process, the LM-based sensors still suffer from relatively low sensitivities. Most LM-based strain sensors are based on piezoresistive effect, due to the change in electrical resistance upon deformation. The gauge factor of a piezoresistive strain sensor attributes to two effects, the geometry change due to deformation and the resistivity change. Prior results reported that the resistivity of LMs remained unchanged due to their excellent conductivities, thus resistance change of LMs only depends on their geometry changes during deformation. The reported gauge factors of LM-based strain sensors are usually in the range of 2-6, thereby limiting their application in areas high sensitivity is required, such as skin prosthetics, humanoid robotics, and wearable health monitoring. Therefore, it is highly desirable to develop novel sensory systems based on LMs with high sensitivity.

Here, we demonstrate a modification method to modify the rheological properties of LMs, and to enhance the sensitivity and robustness of LM-based strain and pressure sensors. By mixing SiO₂ micro-particles into LM, we have effectively increased the viscosity, modulus, and yield stress, enabling the manufacturing of LM sensors through 3D printing technology. The strain redistribution mechanics tuned by SiO₂ micro-particles allows the printed sensor to achieve high sensitivity (gauge factor is 5.72 for strain range within 100%, 11.36 for 100-200%, and 23.91 for 200-300%) and excellent robustness. The sensory arrays are further integrated into a tactile glove to exploit the capability of monitoring clenching postures and punching strength in real time. Combined with a trained convolutional neural network algorithm, the multifunctional tactile glove can classify the various boxing motions, such as jab, swing, uppercut, and combination punches, with an accuracy of up to 90.5%, demonstrating its capability of recognizing personal activities in boxing training.

Presenting Author: JIANLIANG XIAO University of Colorado Boulder

Presenting Author Biography: Jianliang Xiao is an Associate Professor in the Department of Mechanical Engineering at University of Colorado Boulder. Before joining CU-Boulder, he was a Postdoctoral Research Associate at the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign. He obtained his Ph.D. degree in Mechanical Engineering in 2009 from Northwestern University, M.S. and B.S. degrees in Engineering Mechanics in 2006 and 2003 from Tsinghua University, respectively. His research interests include stretchable/flexible electronics, and mechanics of soft materials, thin films and nanomaterials.

Authors:

JIANLIANG XIAO University of Colorado Boulder