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3D Stretchable Circuits
3D Stretchable Circuits
A team has built a stretchable "electronic patch," which can be worn on the skin and used to wirelessly monitor physical and electrical signals.
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As explained recently in the journal Nature Electronics, engineers have stacked and connected layers of stretchable circuits on top of one another, to build soft, pliable "3D stretchable electronics," which can pack a lot of functions while staying thin and small in size. As a proof of concept, a team led by the engineers at the University of California San Diego has built a stretchable "electronic patch," which can be worn on the skin like a bandage and used to wirelessly monitor a variety of physical and electrical signals, ranging from respiration to body motion and from skin temperature, eye movement, and heart activity to brain activity.

The device, which is as small and thick as a U.S. one-dollar coin, can also be used to wirelessly control a robotic arm.

The objective is to take stretchable electronics to the next level and to do this, the team is building upwards rather than outwards. As the team leader puts it, "Rigid electronics can easily be manufactured with as many as fifty layers of circuits that are all intricately connected, with a lot of chips and components packed densely inside. Our 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 (such as a sensor, antenna, Bluetooth chip, amplifier, accelerometer, resistor, capacitor, or inductor) which is 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 big technological roadblock to building stretchable electronics in 3D. The problem isn't stacking the layers. It's 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. VIAs are traditionally made using lithography and etching. While these methods work fine on rigid electronic substrates, they don't work on stretchable elastomers.

So, the UC San Diego team 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 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 (or 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.

Today, the researchers don't have a specific end-use in mind for all of those functions combined together, but the point is that we can integrate all these different sensing capabilities on the same small bandage. And the researchers did not sacrifice quality for quantity. This device is a 'master of all trades.' They picked high quality, robust subcomponents as well as the best strain sensor we 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.

At this point, 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. And the device runs on a total of 35.6 milliwatts.

The team will continue working with industrial partners to optimize and refine this technology. They hope to test it in clinical settings in the near future.

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