By stacking and connecting layers of stretchable circuits on top of one another, soft, pliable 3D stretchable electronics were fabricated that can pack a lot of functions while staying thin and small in size.
As a proof of concept, a stretchable electronic patch was made that can be worn on the skin like a bandage and used to wirelessly monitor a variety of physical and electrical signals, from respiration to body motion, temperature, eye movement, and 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.
The device consists of four layers of interconnected, stretchable, flexible circuit boards. Each layer is built on a silicone elastomer substrate patterned with an “island-bridge” design. Each “island” is a small, rigid electronic part (sensor, antenna, Bluetooth chip, amplifier, accelerometer, resistor, capacitor, inductor, etc.) 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: creating electrical connections between the circuits 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. VIAs are traditionally made using lithography and etching. While these methods work well on rigid electronic substrates, they don't work on stretchable elastomers.
The researchers mixed silicone elastomer with a black organic dye so 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, they filled in the VIAs with conductive materials to electrically connect the layers to one another. And a benefit of using lasers is that they are widely used in industry, so the barrier to transfer the technology is low.
A proof-of-concept 3D stretchable electronic device was built, which is 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 eye 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 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.