A circuit made with ultrathin silicon dissolves in water, shown here with a standard pipet tip.
With the right materials and conditions, silicon-based implantable medical devices can be designed to be degraded and resorbed by the body, an international team of scientists reports (Science, DOI: 10.1126/science.1226325). Such devices could be used for short-term diagnostic or therapeutic applications and then dissolve, eliminating the need to retrieve them.
“The good news is that it’s silicon-based technology,” says team leader John A. Rogers, a materials chemist at the University of Illinois, Urbana-Champaign. “It immediately builds on an advanced industrial and scientific knowledge base.”
Silicon wafers normally undergo hydrolysis in the body at a rate of about 1 nm per day, Rogers says—too slow for degradation to occur on a meaningful timescale. But for silicon layers less than 100 nm thick, “even the very slow dissolution rates become relevant,” Rogers says.
Working with Fiorenzo G. Omenetto, a biomedical engineer at Tufts University, Rogers and coworkers exploit this dissolvability by using thin silicon layers to design resorbable integrated circuits on silk substrates. They constructed the devices using silicon nanomembranes for semiconductors, magnesium for conductors, and magnesium oxide or silicon dioxide for dielectrics. In addition to serving as the substrate, silk can encapsulate the entire device, like an electronic cocoon.
Because the materials involved break down into chemicals that can be assimilated by the body or simply rinsed away, toxicity concerns are minimal.
The electronic properties of conventional electronic devices depend on only the layers at or near the silicon surface. “Even if you slice the silicon very thin, you can retain the kind of electrical properties and performance you see in a conventional wafer-based device,” Rogers says.
Thus, the materials can be used to construct any electronic components that can be made with conventional silicon wafers, Rogers says, including transistors, diodes, and capacitors. The researchers built a digital camera from the materials.
In collaboration with cardiologist Marvin J. Slepian of the Sarver Heart Center at the University of Arizona, Rogers and coworkers developed a thermal device to kill bacteria in surgical wounds. They implanted the silk-encapsulated devices in mice. After three weeks, only faint residues of the silk, which takes longer to break down than the other portions, remained.
“This work is a significant advance in the use of silicon technology for implantable medical devices,” says Zhenan Bao, a materials chemist at Stanford University. Her group previously demonstrated biodegradable organic transistors, but she notes that “the complexity of device integration is much more advanced with silicon technology.”
Such devices could be used for applications beyond medicine, Rogers says, including environmental monitoring. For example, thousands of sensors could be dropped from airplanes to measure large oil spills without the need to retrieve the devices when finished.
In addition to the new devices, Rogers and coworkers have developed a set of models to predict how quickly a given combination of materials and device geometry will degrade. “Increasing the thickness of the packaging layer on top of the circuit increases the amount of time that needs to elapse before the circuit is exposed to body fluids,” Rogers says.
Rogers is also collaborating with technicians at a silicon wafer manufacturing facility to determine what modifications will be needed to produce the new devices. “The industry is already driving to smaller and thinner devices,” Rogers says. “Those scaling trends are of direct benefit to transient electronics because they reduce the volumes of material that need to dissolve, thereby opening up possibilities for higher levels of integration and increased sophistication in function.”