Last time, I wrote about the reverse-engineering of natural processes to develop more efficient solar cells. It turns out that photovoltaics research is not the only field being guided by nature. This month, the journal Nature Materials published two reports describing a pair of successful attempts to fabricate artificial skin – flexible, stretchable arrays of highly sensitive pressure sensors that produce electrical signals in response to contact. The so-called “e-skin” can be used in applications such as robotics and manufacturing to provide a softer touch during manipulation delicate objects.
Both of the e-skin designs required significant technological advances in the fabrication of flexible electronics. The first technique, developed by a group at Berkeley led by EECS Professor Ali Javey, uses a novel nanowire printing process to lay down an array of sensors on a sheet of plastic. Unlike conventional sensors, which are rigid, the group’s nanowire arrays are as flexible as the piece of plastic they are mounted to. Once the sensors are laid out, they’re coated with a layer of pressure-sensitive rubber to provide a touch response. The sensor arrays are connected electronically by a circuit similar to the ones that drive laptop displays, except instead of writing a value into a pixel (i.e. the color to be displayed), it reads a value out (the applied pressure on the rubber layer).
The second paper, from Professor Zhenan Bao’s lab at Stanford, describes a pressure sensor array built from a more traditional type of flexible electronics: organic semiconductors. The group developed a new method to increase the mechanical response of the rubber layer by patterning it into uniform micron-sized features. The gaps between polymer-coated regions allows the film to deform under even the slightest amount pressure. As a result, the Stanford group’s devices are up to five times more sensitive than unpatterned rubber films, which remain rigid at low pressures.
Who makes better e-skin, Berkeley or Stanford? I might be biased, but my vote right now is for the Berkeley version. Nanowire-based e-skin is a remarkable application of an entire body of research on nanowire electronics that has been years in the making. In particular, it exploits both the high quality electronic properties and exotic physical properties of nanowires that have been widely studied but, until now, sparingly applied. Stanford’s breakthrough, while clever, just doesn’t capture the imagination in the same way. Admittedly, it also doesn’t hurt that Javey, a Stanford grad before becoming a Berkeley professor, managed to incorporate some Cal spirit into his experiment (see picture). Go Bears!
Takei K, Takahashi T, Ho JC, Ko H, Gillies AG, Leu PW, Fearing RS, & Javey A (2010). Nanowire active-matrix circuitry for low-voltage macroscale artificial skin. Nature materials, 9 (10), 821-6 PMID: 20835235
Mannsfeld SC, Tee BC, Stoltenberg RM, Chen CV, Barman S, Muir BV, Sokolov AN, Reese C, & Bao Z (2010). Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers. Nature materials, 9 (10), 859-64 PMID: 20835231