Small electronics, such as smartwatches or fitness trackers, are difficult to dismantle and recycle. The fast pace of technology innovations causes consumers to upgrade their devices often and leads to large amounts of electronic waste. Recycling can reduce the volume of e-waste and is mandatory in many places. However, it often is not worth the effort to recycle small consumer electronics because their parts must be salvaged by hand, and some processing steps, such as open burning and acid leaching, can cause health issues and environmental pollution. Dissolvable devices that break apart on demand could solve these problems.

Xian Huang, Tianjin University, China, and colleagues have developed a two-metal nanocomposite for circuits that disintegrates when submerged in water. Previously the team had developed a zinc-based nanocomposite that dissolves in water for use in temporary circuits, but it was not conductive enough for consumer electronics. They modified the zinc-based nanocomposite by adding silver nanowires, making it highly conductive. They screen-printed the desired circuits onto pieces of poly(vinyl alcohol), a polymer that degrades in water, and solidified the circuits using a water-sintering technique.

With this approach, the team made a dissolvable smartwatch, using multiple nanocomposite-printed circuit boards inside a 3D-printed poly(vinyl alcohol) case. The smartwatch has sensors that measure the heart rate, blood oxygen levels, and step count of the wearer and sends the information to a cellphone app via a Bluetooth connection. The outer shell holds up to sweat, but once the whole device was fully immersed in water, both the polymer case and circuits dissolve completely within 40 hours. All that was left behind were components such as an organic light-emitting diode (OLED) screen and microcontroller, as well as resistors and capacitors that had been integrated into the circuits.

• Water-Sintered Transient Nanocomposites Used as Electrical Interconnects for Dissolvable Consumer Electronics,
  Jiameng Li, Jiayin Liu, Wangwei Lu, Ziyue Wu, Jingxian Yu, Bangbang Wang, Zhe Ma, Wenxing Huo, Xian Huang,
  ACS Appl. Mater. Interfaces 2021, 13, 32136–32148.
  https://doi.org/10.1021/acsami.1c07102