As nanoscientists, we often become so engrossed in the task of shrinking our devices that we neglect to pursue ideas that involve relatively large components. At our worst, our attitude can be summed up as, “If it’s visible to the naked eye, then it is too simple to bother with.”
So a recent paper that demonstrates, for the first time, “electronic tattoos” for biomedical sensing applications comes as a surprise and something of a wake-up call to the nanoscience community. The paper, published last week in the journal Science and written by a team led by John Rogers of the University of Illinois, is notable for its lack of nanotubes, quantum dots, scanning electron micrographs, or any of the other hallmarks of a modern scientific paper about electronic devices. Instead, the majority of the figures in the paper are photographs taken through the same optical microscopes that you’d find in a typical middle school classroom. Heck, a skilled surgeon could probably have pieced together the device by hand. And yet, despite its low degree of difficulty from a technological standpoint, their work has taken the blogosphere by storm and is one of the most exciting results I personally have seen all year.
If it’s not “nano”, what did they do that’s so special? Well, they came up with a killer (figuratively) solution to a killer (literally) problem, for one. Gathering real-time medical data from patients outside of a clinical setting is, in many cases, impossible as it requires bulky electronics and an uncomfortable interface between the body and measurement device like electrodes or needles. As a result, patients with conditions such as diabetes suffer from a lack of convenient and timely access to information critical to their health. What Rogers and his co-authors came up with is a method for printing lightweight electronic sensor circuits onto a temporary tattoo, which can then be transferred onto the skin in a manner that is no more invasive or uncomfortable than the tattoo itself. The circuits can flex, bend, and stretch just like the skin it rests on without being damaged and can receive power and transmit information wirelessly.
The device is not pretty. The wires are squiggly and look like the border around a small child’s stick-figure drawing. But what the circuits lack in style they make up for in functionality: the curvy shape of the wires is the only way to allow them to stretch, compress, and bend without breaking or delaminating from the skin. A large square array of the serpentine wires – the most easily visible feature of the electronic tattoo – serves as an antenna that is used to remotely power the circuit.
The sensors themselves cannot be made from simple circuit components like wires alone; they also require, among other things, silicon transistors. The problem is silicon is a crystal – essentially a rock – and is not normally known for its flexibility. However, the group has found that when the silicon slabs are sliced thin enough, they are no longer the rigid, brittle material we are used to but rather flexible and stretchable just like skin. Trend watchers out there might take note that a similar technique involving ultra-thin semiconductor layers recently made waves for its application to cheap, high-efficiency solar cells.
As the paper makes clear, electronic tattoos as a practical technology are not out of the woods yet. In particular, providing continuous power to the circuits is challenging as they have no energy storage mechanism built in (at least not the ones the team has reported; Rogers founded a start-up company to commercialize their results that may privately hold more answers than they are publicly releasing). Then, there is the issue of longevity: a temporary tattoo normally begins to chip away from the skin within hours after it is applied, so how long is the useful life of the electronic tattoo, with all its sensitive circuitry? Can it be counted on in a moment of need?
These questions certainly do not downplay the impact of the group’s work. Hey, one of their tattoos may save my life someday. But they do serve to remind us of the uphill battle facing emerging electronic devices as they try to make their way from laboratory to market. After all, in this case it’s not even “nano” – it should be easy!
Kim DH, Lu N, Ma R, Kim YS, Kim RH, Wang S, Wu J, Won SM, Tao H, Islam A, Yu KJ, Kim TI, Chowdhury R, Ying M, Xu L, Li M, Chung HJ, Keum H, McCormick M, Liu P, Zhang YW, Omenetto FG, Huang Y, Coleman T, & Rogers JA (2011). Epidermal electronics. Science (New York, N.Y.), 333 (6044), 838-43 PMID: 21836009