For the current study, Xu and her team simplified the process even further. Instead of expensive purified enzymes, the biodegradable printed circuits rely on cheaper, shelf-ready BC lipase “cocktails.” This significantly reduces costs, facilitating the printed circuit’s entry into mass manufacturing, Xu said.
By doing so, the researchers advanced the technology, enabling them to develop a printable “conductive ink” composed of biodegradable polyester binders, conductive fillers such as silver flakes or carbon black, and commercially available enzyme cocktails. The ink gets its electrical conductivity from the silver or carbon black particles, and the biodegradable polyester binders act as a glue.
The researchers supplied a commercial 3D printer with the conductive ink to print circuit patterns onto various surfaces such as hard biodegradable plastic, flexible biodegradable plastic, and cloth. This proved that the ink adheres to a variety of materials, and forms an integrated device once the ink dries.
To test its shelf life and durability, the researchers stored a printed circuit in a laboratory drawer without controlled humidity or temperature for seven months. After pulling the circuit from storage, the researchers applied continuous electrical voltage to the device for a month and found that the circuit conducted electricity just as well as it did before storage.
Next, the researchers put the device’s recyclability to test by immersing it in warm water. Within 72 hours, the circuit materials degraded into its constituent parts – The silver particles completely separated from the polymer binders, and the polymers broke down into reusable monomers, allowing the researchers to easily recover the metals without additional processing. By the end of this experiment, they determined that approximately 94% of the silver particles can be recycled and reused with similar device performance.
That the circuit’s degradability continued after 30 days of operation surprised the researchers, suggesting that the enzymes were still active. “We were surprised that the enzymes ‘lived’ for so long. Enzymes aren’t designed to work in an electric field,” Xu said.
Xu attributes the working enzymes’ longevity to the biodegradable plastic’s molecular structure. In their previous study, the researchers learned that adding an enzyme protectant called random heteropolymer, or RHP, helps to disperse the enzymes within the mixture in clusters a few nanometers (billionths of a meter) in size. This creates a safe place in the plastic for enzymes to lie dormant until they’re called to action.
The circuit also shows promise as a sustainable alternative to single-use plastics used in transient electronics – devices such as biomedical implants or environmental sensors that disintegrate over a period of time, said lead author Junpyo Kwon, a Ph.D. student researcher from the Xu Group at UC Berkeley.
Now that they’ve demonstrated a biodegradable and recyclable printed circuit, Xu wants to demonstrate a printable, recyclable, and biodegradable microchip.
“Given how sophisticated chips are nowadays, this certainly won’t be easy. But we have to try and give our level best,” she said.