DISAPPEARING ACTS

When you think about electronics implanted in the body, a pacemaker—a solid, stable device meant to last a long time—is likely the first thing that comes to mind. But scientists at the University of Illinois at UrbanaChampaign, Northwestern University, and Tufts University are turning the notion of durability on its head by designing tiny electronic implants that dissolve in the body after a short period of time.

Called “transient” because of their temporary nature, these devices may one day help prevent infections at surgical incision sites, enhance wound-healing, deliver medication that is needed for only a few days, or monitor transplant patients’ reaction to their new organs.

“Surgical site infections are one of the leading causes for readmission to hospitals, and more and more of those infections are becoming resistant to antibiotics,” says John A. Rogers, PhD, Lee J. Flory-Founder professor of engineering at the University of Illinois. “So the thought here was that we might be able to use transient electronics in a form like a thin film appliqué that could be inserted before the patient is closed up. It could potentially be used to eliminate bacteria at the surgical site for the most critical risk period, which is about two to three weeks after surgery.”

In a recent issue of Science, the researchers described their first transient prototype, a tiny thermal electronic device designed to kill bacteria when implanted near a surgical incision. Rogers and his colleagues tested the prototype in mice and found that it dissolved within three weeks with no ill effects on the animals. The high-tech gadgets can be designed to disintegrate at controlled rates, perhaps lasting a day, a few weeks, or months before completely vanishing in body fl uids.

Transients in Practice

Like conventional integrated circuits, the transients are constructed of magnesium components and ultra-thin discs of silicon. The fragile electronics are then encapsulated in layers of silk protein from silk-worm cocoons that have been dissolved and recrystallized.

The prototype used in the mice is about the width of a nickel but only a fraction of the thickness of a human hair. Thickness is a critical aspect of transient electronics, said Yonggang Huang, PhD, Joseph Cummings professor of civil and mechanical engineering at Northwestern University. “The thinner the device, the shorter [time] it lasts; so thickness is very important when controlling dissolution time. You want the device to dissolve, but not too fast, because you need it to do its job.”

The thickness and structure of the silk also helps finetune reabsorption, he adds. “Each layer adds to the time. We can control the dissolution time quite precisely, from a few hours to a few months. If doctors tell us how long they want a device to last, we’ll be able to design it that way.”

The thought of silicon and silk dissolving in the body may give the squeamish pause, but Rogers says the materials have a long history of use in medical implants, particularly in the permanent stents that are sometimes inserted into arteries during angioplasty and in internal sutures that eventually melt away. Silk is known to be a good matrix for drugs and hormones, Rogers says, and it is already approved by the Food and Drug Administration (FDA) for absorbable sutures.

Although people tend be wary of silicon products, Rogers says they are exposed to more silicon when they take a dip in the ocean, where it occurs naturally, than they would be with a temporary implant. “The amount used for stents far exceeds anything [transient electronics] need for conducting,” he adds. As for magnesium, a multivitamin or a few handfuls of mixed nuts has more: “The recommended daily intake is much larger than the amount we need for integrated circuits.”

Numerous Applications

How the vanishing electronics’ structural materials react with human tissue is a critical hurdle, making widespread use of transient electronics in humans still a long way off. “Human body fl uid is a variable,” says Huang. “You can’t really control the pH value. Also, dissolution times at room temperature and body temperature are very different.” Huang notes that both pH and temperature can vary from person to person, and even from time to time for the same person, as with a fever.

Regulatory agencies like the FDA would, of course, require extensive testing and proof of safety. “There would be a full range of trials in animals and humans long before anything like this is available on a large scale,” Rogers adds.

Mass manufacturing poses another challenge, says Rogers. “We’re designing these devices to be soluble in water, but a lot of the conventional steps for fabricating them use water.”

Both Rogers and Huang defer to health professionals for determining the best use of transient electronics. “We’re just the engineers,” says Huang. But the pair envisions many potential medical applications for the technology. For example, a device like the antibacterial prototype may be useful in heaing diabetic foot ulcers. Transient electronics might also be used to target specific areas of the thyroid for iodine therapy in hyperthyroidism to zap “hot nodules” that produce too much thyroid hormone. Fertility treatments could be another area of consideration, with transient electronics delivering drugs that stimulate the production of eggs for in vitro fertilization, sparing women an uncomfortable series of injections.

For now, Rogers, Huang, and their colleagues plan to continue studying different materials—in their study they noted collagen, iron, and zinc as possibilities—and experimenting with different prototypes. Their work is not limited to medical devices, however.

Transient electronics might one day be used for environmental purposes, perhaps in wireless sensors that could detect or monitor oil or chemical spills without having an effect on the ocean itself. Th e technology could also be used to create biodegradable components for cell phones, MP3 players, or other portable devices, thus cutting the amount of waste generated when consumers upgrade their gear.

“Th e concept of designing electronics that don’t last forever is very new,” says Huang. “There are probably many uses and applications we haven’t even thought of yet.”

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