Transient electronics that can disintegrate on demand could help stem the rising tide of electronic waste, but their performance is often inferior to conventional devices. Now scientists in China have developed what may be the first dissolvable smartwatch, a prototype that performs much like a conventional smartwatch.
Advances in electronics over the decades have led to explosive growth in their use, with global sales of consumer electronics reaching more than $1 trillion in 2019. However, continuously replacing outdated devices with new versions also generates more than 53.6 million tons of electronic waste per year.
Recycling can help deal with some e-waste, but small electronics such as smartwatches and fitness trackers are not easily dismantled and recycled. As such, electrical engineer Xian Huang at Tianjin University in China and his colleagues explored creating transient small electronics that could safely dispose of themselves.
Previously the researchers developed a composite material made using zinc nanoparticles that dissolved in water for use in temporary circuits. However, it was not electrically conductive enough for consumer electronics.
In the new study, the scientists modified their zinc-based nanocomposite by adding silver nanowires, which boosted its electrical conductivity. They next printed the metallic solution onto pieces of a polymer that degrades in water. They then sintered the circuits together by applying small droplets of water that support chemical reactions and then evaporate.
Using this approach, the researchers created a smartwatch with multiple nanocomposite, printed circuit boards inside a 3D-printed water-degradable polymer case. The device had sensors that accurately measured heart rates, blood oxygen levels and step counts, and transmitted this data to a mobile phone app via a Bluetooth connection, just like conventional smartwatches. Its organic light-emitting diode (OLED) screen could also display data such as the date, time and messages from linked mobile phones, as well as the wearer life signs it monitored.
The smartwatch's exterior could resist sweat, but once the whole device was fully immersed in water, the case and circuits dissolved completely within 40 hours. All that remained were the watch's components, such as the OLED screen and microcontroller, as well as resistors and capacitors formerly integrated into the circuits.
"This finding demonstrates techniques to yield high-performance electronic circuits that can be easily recycled," Huang says.
The researchers also noted the new composites are bioresorbable, meaning they can dissolve in the body, and display much better electrical and mechanical performance than other bioresorbable inks. As such, they may lead "to printable and implantable devices that can disappear in the human body after completing their functions," Huang says.
Most bioresorbable electronics devices are fabricated using complementary metal–oxide–semiconductor (CMOS) processes, "which are very time-consuming and demand many special processes," Huang says. In contrast, devices made with these nanocomposites can get mass-produced via printing with much less cost and energy and much higher yields and throughputs, suggesting they could readily get incorporated into existing production lines of electric circuits, he notes.
In the future, Huang and his colleagues plan on creating more kinds of transient devices using their nanocomposites. "I hope more and more electronics companies can use these techniques for their products to help them reduce the cost of manufacturing and recycling," Huang says.
One challenge in creating practical transient electronics is optimizing the properties of their packaging materials to match the degradation rates of the circuits. "But the introduction of extra packaging materials is not a big issue, as wearable and portable electronics need to be packaged anyway to improve their reliability," Huang says.
The scientists recently detailed their findings in the journal ACS Applied Materials & Interfaces.
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