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

Paper Number: 119835

119835 - A Highly Sensitive, Stretchable and Robust Strain Sensor Based on Crack Advancing and Opening 

Soft and stretchable strain sensors have been attracting significant attention. However, the tradeoff between the sensitivity (gauge factor) and the sensing range has been a major challenge. In this work, we report a novel soft stretchable strain sensor with an unusual combination of high sensitivity, large sensing range, and high robustness. The sensor is made of metal nanowire network embedded below the surface of an elastomeric matrix (e.g., polydimethylsiloxane). Periodic mechanical cuts are applied to the top surface of the sensor. Under the applied strain, the resistance increases as the crack propagates (the linear sensing range) but remains constant as the crack reaches the cut length and only opens (the reversible range).

To study the electrical performance of the sensors for different geometrical designs, we defined three major geometrical parameters – ratio of the cut depth to the sample thickness d_c/t, ratio of the pitch between the cracks to the specimen width p/w, and ratio of the cut length to the specimen width l_c/w. The resistance change curve can be divided into two regions, a sensing region where the resistance increases linearly with the increasing strain, and a plateau region. The crack propagation/opening process was observed in-situ under an optical microscope, while the resistance was measured simultaneously. 1,000 stretching/unloading cycles were applied to show the excellent repeatability of the sensor for long term use. This sensor overcame the limitation of most existing strain sensors and offered an unprecedented combination of GF, strain sensing range and robustness (under over-strain and 1,000 repeated loading cycles). A large GF of 290.1 was achieved with a sensing range over 22%. FEA was conducted to validate the electrical performance and predict the mechanical damage, agreeing very well with the experimental results.

To demonstrate the versatile applicability of our strain sensors for monitoring human motions, we first applied the sensors on the wrist to detect the pulse wave, which represents one of the most delicate strain signals on human skin. The pulse wave captured from the radial artery on the wrist. and the brachial artery on the arm. By measuring the distance between the two pulse areas and taking the average of the time gap between peaks of the two pulse waves, the averaged pulse wave velocity (PWV) can be measured. The other demonstration aimed to monitor the large strains on the lower back, which is a critical signal for metabolic syndrome and spine issues. Finally, besides wearable personal health monitoring, the sensor can be applied for human-machine interfaces and robotics. A 3-D touch controller has been developed for real-time control of an airplane in a video game. The sensor was also integrated on the fingertip of a glove to capture the shear and normal strains when grabbing and lifting objects.

Presenting Author: Shuang Wu North Carolina State University

Presenting Author Biography: Postdoctoral researcher at North Carolina State University. Research focused on soft and stretchable devices and soft robots.

Authors:

Shuang Wu North Carolina State University
Katherine Moody North Carolina State University
Abhiroo Kollipara North Carolina State University
Yong Zhu North Carolina State University