A team led by UCSD has built a stretchable electronic patch that can be worn on the skin like a bandage and used to wirelessly monitor a variety of physical and electrical signals, from respiration and body motion, to temperature and eye movement, to heart and brain activity. The device is as small and thick as a U.S. dollar coin.

Tech Briefs: What interested you in this project?

Dr. Sheng Xu: Your car has sensors to give you information about the onset and location of impending failures so you can initiate repairs before a catastrophic failure occurs. How about such a system for the human body, which is much more sophisticated than a car? For a human, being able to have a prognosis can be life-saving. Most diseases are silent, so you can get a fatal diagnosis like final-stage cancer without prior warning. We need sensors to put on our bodies for sensing all kinds of vital signs to make informed decisions about our health status.

Tech Briefs:What is unique about your approach?

Dr. Xu: Conventional rigid electronics are usually multilayered — the PC board can have 30 or more layers. This layering is enabled by electrical pathways called VIAs (vertical interconnect access) for communication between each layer. Currently, however, most soft devices are based on single layers. The challenge is that the existing fabrication strategies for rigid surfaces do not work on softer ones.

We identified an approach that uses laser ablation combined with controlled soldering that allows us to build distributed VIAs in soft electronics. We demonstrated a four-layer device built upon a silicone elastomer matrix that supports a series of sensor circuits. We used an “island-bridge” layout on each layer, in which rigid components such as transistors and capacitors are mounted on an island. Stretchable conductive bridges that buckle in response to deformation provide the interconnections among the island components. The VIAs for interlayer electrical connections are created from conductive fillings in craters in the silicone that are created using laser ablation. Between the layers, an ultra-low-modulus silicone is used to encapsulate the components for mechanical robustness and to minimize constraints on the stretchability of the overall device.

Tech Briefs:What kinds of sensors were incorporated and how did you transmit the signals?

Dr. Xu: The device has five different types of sensors to record electrophysiological signals from the human body. We measure motion and acceleration in the X, Y, and Z directions. A goniometer measures orientation of the patch along the X and Y directions. We measure strain (bending curvature), temperature, and electrical potential. The system can simultaneously record respiration, skin temperature, and body motion when mounted on the chest. A field potential amplifier can be applied to record various electrophysiological signals; for example, to function as an electromyograph to record grasping forces, an electrocardiograph for the heart, or an electroencephalograph for the brain. The system remains stable under stretching and sends the signals wirelessly via Bluetooth to a smartphone or a laptop at distances of up to 10 meters.

Tech Briefs:What are your next steps?

Dr. Xu: We can use this set of techniques to integrate many different types of sensors into a soft electronic device. The sensors can acquire a range of different signals that are of interest for different problems.

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