Hybrid Silicone-Fabric Sensor Takes Researchers One Step Closer to Creating Soft, Wearable Robots

Wearable technologies, ranging from heart rate monitors to virtual reality headsets, are gaining immense popularity in both the consumer and research spaces. However most of the electronic sensors are used for detecting and transmitting data from wearables are developed from inflexible, hard materials that limit both the accuracy of the data collected and the natural movements of the wearers.

The silicone-textile hybrid sensors are highly flexible and resilient, making them excellent candidates for sensing body movement. Credit: Wyss Institute at Harvard University.

Recently, Researchers at the Wyss Institute for Biologically Inspired Engineering and the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed a greatly sensitive soft capacitive sensor produced from silicone and fabric that moves and flexes with the human body in order to detect movement in an unobtrusive and accurate manner.

We’re really excited about this sensor because, by leveraging textiles in its construction, it is inherently suitable for integration with fabric to make ‘smart’ robotic apparel.

Conor Walsh, Ph.D., Corresponding Author and Core Faculty Member at the Wyss Institute and the John L. Loeb Associate Professor of Engineering and Applied Sciences, SEAS

“Additionally, we have designed a unique batch-manufacturing process that allows us to create custom-shaped sensors that share uniform properties, making it possible to quickly fabricate them for a given application,” states Co-Author Ozgur Atalay, Ph.D., Postdoctoral Fellow at the Wyss Institute. This research has been published in the current issue of Advanced Materials Technologies, and the protocol is available as part of the Harvard Biodesign Lab’s Soft Robotics Toolkit.

The Wyss team’s technology comprises of a thin sheet of silicone, a poorly conductive material, placed between two layers of conductive, silver-plated fabric, a highly conductive material, developing a capacitive sensor. Movement is registered by this type of sensor by measuring the change in capacitance, or the potential to hold electrical charge, of the electrical field between the two electrodes.

When we apply strain by pulling on the sensor from the ends, the silicone layer gets thinner and the conductive fabric layers get closer together, which changes the capacitance of the sensor in a way that’s proportional to the amount of strain applied, so we can measure how much the sensor is changing shape.

Daniel Vogt, Co-Author and Research Engineer at the Wyss Institute. Research Engineer at the Wyss Institute.

The superior performance of the hybrid sensor originates from its unique manufacturing process, in which the fabric is fixed to both sides of the silicone core with an extra layer of liquid silicone that is consequently cured. With this method, the silicone is allowed to fill some of the air gaps present in the fabric, mechanically locking it to the silicone and then increasing the surface area available for storing electrical charge and distributing strain. This silicone-textile hybrid enhances sensitivity to movement by way of capitalizing on the qualities of both materials: the sturdy, interlocking fabric fibers help in limiting the extent to which the silicone deforms while stretching, and the silicone then enables the fabric to return to its original shape after removal of the strain. Finally, thermal seam tape is used to permanently attach thin, flexible wires to the conductive fabric, allowing the transmission of electrical information from the sensor to a circuit without a bulky, hard interface.

The Researchers evaluated their new sensor design by carrying out strain experiments in which different measurements are taken as the sensor is made to stretch by an electromechanical tester. As an elastic material is pulled, its length usually increases while its width and thickness decrease, thus the total area of the material – and, therefore, its capacitance – remains constant. Surprisingly, the team discovered that the conductive area of their sensor increased as it was being stretched, resulting in greater capacitance that has ever been expected.

Silicone-based capacitive sensors have limited sensitivity based on the nature of material. Embedding the silicone in conductive fabric, however, created a matrix that prevented the silicone from shrinking as much width-wise, which improved sensitivity above that of the bare silicone we tested.

Asli Atalay, Lead Author and Postdoctoral Fellow, the Wyss Institute

The detected hybrid sensor increases in capacitance within 30 milliseconds of strain application and physical changes of less than half a millimeter, proving that it has the potential to capture movement on the scale of the human body. The Researchers tested the ability in a real-world scenario by incorporating a set of them into a glove in order to measure fine-motor hand and finger movements in real time. The sensors succeeded in detecting capacitance changes on individual fingers as they moved, thus highlighting their relative positions over time. “Our sensor’s greater sensitivity means it has the ability to distinguish smaller movements, like slightly moving one finger side-to-side rather than simply whether the whole hand is open or clenched in a fist,” explains Co-Author Vanessa Sanchez, a Graduate Student in the Biodesign Lab at SEAS.

The Researchers are excited about the number of future directions in which this technology could develop even as this study is a preliminary proof-of-concept. “This work represents our growing interest in leveraging textile technology in robotic systems, and we see promising applications for motion capture ‘in the wild,’ such as athletic clothing that tracks physical performance or soft clinical devices to monitor patients in their homes. In addition, when combined with fabric-based soft actuators, these sensors will enable new robotic systems that truly mimic apparel,” says Walsh.

“This technology opens up entirely new approaches to wearable diagnostics and coupled therapeutics that undoubtedly will pay a central role in the future of home healthcare.  It also reflects the power inherent in our focus on collaboration here at the Wyss Institute, as it draws insight and inspiration from both Conor Walsh’s Biodesign Lab and Rob Wood’s Microrobotics Lab, which are central to our Bioinspired Robotics platform,” states Wyss Founding Director Donald Ingber, M.D., Ph.D., who is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital, and Professor of Bioengineering at SEAS.

The National Science Foundation’s under Grant No. CBET-1454472, the Scientific and Technological Research Council of Turkey (TÜBİTAK) BIDEB-2219 Postdoctoral Research program, DARPA, and the Warrior Web Program under Award No. W911NF-14-C-0051 supported this research.

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