Imagine an electronic medical implant that, like dissolvable stitches, could disintegrate after it is no longer needed. An innovative combination of silk and silicon have now been used to create just such ephemeral but effective devices, including diodes, transistors, mini heaters and stress sensors.
A flexible device that is just nanometers thick can fight post-surgical infections or even capture images—until its work is done, when it vanishes right on cue. "These electronics are there when you need them, and after they've served their purpose, they disappear," Yonggang Huang, an engineer at Northwestern University, said in a prepared statement. These so-called transient electronics have already been demonstrated to work—and disappear—in rats. These new electronics can be powered wirelessly via induction coils.
Stable, silicon-based electronics might not seem like the most biodegradable objects. "While silicon may appear to be impermeable, eventually it dissolves in water," Fiorenzo Omenetto, a biomedical engineer at Tufts School of Engineering, said in a prepared statement. He and his colleagues increased the rate of dissolution by using only nanomembranes of silicon. To this super-thin silicon, they added small amounts of magnesium for electrical conduction—and wrapped everything in silk protein from silkworm cocoons to support the circuit. But the silk was not used straight from the cocoon. First, the researchers dissolved the naturally made silk. They then controlled the reformation of the crystals to determine their structure. Different silk crystal structures result in different dissolution times. So by selecting the precise silk crystals they were creating, the researchers could generate a substance that would fade away after a particular interval. They used this specific silk to coat the whole device.
The levels of silicon and magnesium contained in the circuits are below maximum safety levels, so they should be suitable for the body. "We selected materials familiar to the human body," Huang said. "We didn't want to use a material the body has no experience with."
The circuits "will fully reabsorb into their environment at a prescribed time—ranging from minutes to years, depending on the application," depending on the silk recrystalization structure, Omenetto said. And because the electronic components can simply disappear, it would obviate the need for additional surgical procedures to remove them.
This approach to electronic constructions is a 180-degree shift from traditional engineering philosophy. "From the earliest days of the electronics industry, a key design goal has been to build devices that last forever," John Rogers, an engineer at the University of Illinois at Urbana-Champaign, said in a prepared statement. "But if you think about the opposite possibility—devices that are engineered to physically disappear in a controlled and programmed manner—then other, completely different kinds of application opportunities open up."
The innocuousness of these sensors means that they could be used both in the body as well as in the environment and perhaps even, someday in cell phones and other consumer electronics that are currently piling up in landfills. "It's a new concept, so there are probably lots of opportunities, many of which we probably have not even identified yet," Rogers said. "We're very excited."
Rogers and his colleagues had previously designed a flexible electronic circuit that could adhere to the skin like a temporary tattoo.
The dissolving circuits were described in a new study published online September 27 in Science.