September 28, 2012
Transient Electronics Disappear On Command
[ Watch the Video: Transient Electronic Circuit ]
April Flowers for redOrbit.com - Your Universe Online
The applications for these kinds of electronics are innumerable, including medicine, pharmaceuticals, environmental monitors and the military.
Conventional electronics have an indefinite shelf life. However, transient electronics would physically vanish over time in a well-controlled manner and at a prescribed time when exposed to water or body fluids. The electronic devices are encapsulated in a layer of magnesium oxide with a silk overcoat. The thickness of this encapsulation determines how long the system will take to disappear into its environment.
"We refer to this type of technology as transient electronics," said John A. Rogers, the Lee J. Flory-Founder Professor of Engineering at the University of Illinois at Urbana-Champaign, who led the multidisciplinary research team. "From the earliest days of the electronics industry, a key design goal has been to build devices that last forever — with completely stable performance. 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."
"These electronics are there when you need them, and after they've served their purpose they disappear," said Yonggang Huang, from Northwestern University. "This is a completely new concept."
"These devices are the polar opposite of conventional electronics whose integrated circuits are designed for long-term physical and electronic stability," says Fiorenzo Omenetto from Tufts University.
The possibilities for such electronics seem limitless. Transient electronics could be used for medical purposes, implanted inside the human body to monitor temperature, brain waves, heart and muscle tissue activity, apply thermal therapy or to deliver drugs. When their mission is completed, the body would absorb the electronics with no adverse effects. To date, implantation devices are not commonly used in medicine because of concerns about the long-term effects.
Transient electronics could be used as environmental monitors. They could be placed on buildings, roadways or military equipment to detect temperature change or structural deformation. They could also be used as wireless sensors dispersed after a chemical spill to eliminate any ecological impact. Because the device would dissolve when exposed to water, there would be no need for later retrieval.
A third possible application is in consumer electronic systems or sub-components. The devices would be compostable to reduce electronic waste streams.
The details of these biocompatible electronic devices are scheduled to be published in the journal Science.
"We selected materials familiar to the human body, such as magnesium," said Huang, the Joseph Cummings Professor of Civil and Environmental Engineering and Mechanical Engineering at Northwestern's McCormick School of Engineering and Applied Science. "We didn't want to use a material the body has no experience with."
The team tested a number of different biocompatible materials, including zinc and iron. They focused on silicon-based electronics with conductors made of magnesium. The key question to the whole exercise was; how long will it take the entire device to dissolve?
The encapsulation layers are the first to dissolve and dictate the first dissolution timescale. The second timescale is decided by the magnesium electrodes in the electronics. Combined, these two lengths of time determine the dissolution time for the entire system.
"The different applications that we are considering require different operating time frames," Rogers said. "A medical implant that is designed to deal with potential infections from surgical site incisions is only needed for a couple of weeks. But, for a consumer electronic device, you'd want it to stick around at least for a year or two. The ability to use materials science to engineer those time frames becomes a critical aspect in design."
The three universities divided the work. John Rogers at the University of Illinois lead the overall team and the group that focused on the experimental and fabrication work of the transient electronics. Huang's Northwestern team developed a model that can accurately predict how thick the encapsulation layers need to be for a specific dissolution time frame. The model was proven effective when tested against experimental evidence, which prevents the need to repeat experiments. Fiorenzo G. Omenetto, a professor of biomedical engineering, directed the Tufts team. They worked on biomaterials and chemistry and ran in-vivo experiments to demonstrate bio-reabsorption and biocompatibility.
The team built several working devices that are dissolvable, including field-effect transistors, resistors, diodes, a heater and a strain sensor. They also built a 64 pixel digital camera. All of the components of each of these disappeared completely and on schedule.
The devices can operate in both water and a phosphate buffered saline (PBS) liquid, which is very chemically similar to what is in the human body. They also implanted the electronics in a mouse model and showed that the heating device was effective and could kill bacteria.
Wireless power is supplied to the devices through induction coils.
"This way the devices in water or PBS liquid can have power without being physically connected to a power source, such as a battery," Huang said.
The fact that the materials used are biocompatible is very important for implantable devices. Other materials besides magnesium and silk include magnesium oxide and porous silicon, which are both biocompatible. The research team says that the materials, fabrication techniques and modeling tools can be used for component devices for almost any type of transient electronics.
"It's a new concept, so there are lots of opportunities, many of which we probably have not even identified yet" Rogers said. "We're very excited. These findings open up entirely new areas of application, and associated directions for research in electronics."
"Transient electronics offer robust performance comparable to current devices but they will fully resorb into their environment at a prescribed time–ranging from minutes to years, depending on the application," Omenetto explains. "Imagine the environmental benefits if cell phones, for example, could just dissolve instead of languishing in landfills for years."