A team of researchers have developed a method to build soft, pliable, 3D stretchable electronics, that can pack a lot of functions while staying thin and small in size. The team achieved this by stacking and connecting layers of stretchable circuits on top of one another, as mentioned in their work published in Nature Electronics.
As a proof of concept, the team, led by the University of California San Diego has built a stretchable electronic patch that can be worn on the skin like a bandage and wirelessly monitor a variety of physical and electrical signals, from respiration, to body motion, to temperature, to eye movement, to heart and brain activity. The device, which is as small and thick as a U.S. dollar coin, can also be used to wirelessly control a robotic arm.
According to Senior Author of the paper and Professor in the Department of NanoEngineering and the Center for Wearable Sensors, both at the UC San Diego Jacobs School of Engineering, Sheng Xu, the team’s vision is to make 3D stretchable electronics that are as multifunctional and high-performing as today’s rigid electronics. Xu was named in the MIT Technology Review’s 35 Innovators Under-35 list in 2018 for his work in this area.
To take stretchable electronics to the next level, Xu and his colleagues are building upwards rather than outwards. Rigid electronics can offer a lot of functionality on a small footprint - they can easily be manufactured with as many as 50 layers of circuits that are all intricately connected, with a lot of chips and components packed densely inside. The goal is to achieve that with stretchable electronics.
The new device developed in this study consists of four layers of interconnected stretchable, flexible circuit boards. Each layer is built on a silicone elastomer substrate patterned with what’s called an island-bridge design. Each “island” is a small, rigid electronic part (sensor, antenna, Bluetooth chip, amplifier, accelerometer, resistor, capacitor, inductor, etc.) that’s attached to the elastomer. The islands are connected by stretchy “bridges” made of thin, spring-shaped copper wires, allowing the circuits to stretch, bend and twist without compromising electronic function.
This work overcomes a technological roadblock to building stretchable electronics in 3D. According to Xu, the problem isn’t stacking the layers but creating electrical connections between them so they can communicate with each other. These electrical connections, known as vertical interconnect accesses or VIAs, are essentially small conductive holes that go through different layers on a circuit. They are traditionally made using lithography and etching. While these methods work fine on rigid electronic substrates, they don’t work on stretchable elastomers.
So Xu and his colleagues turned to lasers. They first mixed silicone elastomer with a black organic dye so that it could absorb energy from a laser beam. Then they fashioned circuits onto each layer of elastomer, stacked them, and then hit certain spots with a laser beam to create the VIAs. Afterward, the researchers filled in the VIAs with conductive materials to electrically connect the layers to one another. And a benefit of using lasers, notes Xu, is that they are widely used in industry, so the barrier to transfer this technology is low.
The team built a proof-of-concept 3D stretchable electronic device, which they’ve dubbed a “smart bandage”. A user can stick it on different parts of the body to wirelessly monitor different electrical signals. When worn on the chest or stomach, it records heart signals like an electrocardiogram (ECG). On the forehead, it records brain signals like a mini EEG sensor, and when placed on the side of the head, it records eyeball movements. When worn on the forearm, it records muscle activity and can also be used to remotely control a robotic arm. The smart bandage also monitors respiration, skin temperature and body motion.
The researchers did not sacrifice quality for quantity even. The device is like a ‘master of all trades, according to co-first author Yang Li, a NanoEngineering graduate student at UC San Diego in Xu’s research group. The team picked high quality, robust subcomponents—the best strain sensor they could find on the market, the most sensitive accelerometer, the most reliable ECG sensor, high quality Bluetooth, etc.—and developed a clever way to integrate all these into one stretchable device.
So far, the smart bandage can last for more than six months without any drop in performance, stretchability or flexibility. It can communicate wirelessly with a smartphone or laptop up to 10 meters away. The device runs on a total of about 35.6 milliwatts, which is equivalent to the power from 7 laser pointers.
The team will be working with industrial partners to optimize and refine this technology. They hope to test it in clinical settings in the future. This work is supported by the Center for Wearable Sensors, Center of Healthy Aging and the Contextual Robotics Institute, all at UC San Diego, and the National Institutes of Health (Grant UL1TR001442).
