Gelatin diodes? From ace_alejandre’s photostream

Semiconductor opens our age. We have grown up with micro-scale technologies and seem to enter the phase of nano- or even a angstrom-scale (molecular) technologies. The down-sizing trend of electronic devices is said to cater the desire of higher density and efficiency of processors and memories, and to pave the way to smarter computers/robots. Anxiousness prevails, however, among futurists, that the development of artificial intelligence will one day threaten human being. Many movies (e.g. The Matrix I, II, III) have visualized how hard, dry, silicon-based intelligence will rule over soft, wet, and carbon-based one.

So why not we soft and blood-circulating organisms be ourselves semiconducting? A hydrogel p-n junction that shows perfect rectifying property may be the first step to biocompatible electronic device (J. Am. Chem. Soc. 2007. DOI: 10.1021/ja072449z).

P-n junction is the basic of the whole semiconductor industry. It consists of an electron-rich (n-type) and a hole-rich (p-type) materials that contact each other at an interface (junction). The two opposite charges in this two regions form an equilibrium at the interface where the electrons travel to the p-region and the holes travel to the n-region so as to recombine with their counterparts. The separated charges accumulate at either side and form a local electric field that prevent more traveling. This electric field covers a thin layer at the interface and is called the depletion layer. When an outer forward bias is applied at the two end of the junction, given the bias is larger than that of the depletion layer, both the charge carrier of p- and n-region can move freely and the junction is conductive. When a backward bias is applied, however, the depletion layer becomes thicker and serves as a barrier, thus there is very low current occur in the junction like an insulator. By manipulating this nonconductive layer, p-n junctions are commonly used as diodes/rectifiers: electrical switches that allow a flow of electricity in one direction but not in the other (opposite) direction.

To copy this mechanism in a soft and wet environment, a pair of polyelectrolytes of opposite charges, namely sodium poly-
(styrene sulfonate) (n-type) and poly(diallyl dimethylammonium) chloride (p-type), are immobilized separately in agarose-based gels. The polyelectrolytes automatically create a small electric field between them serving as a depletion layer, while their counterions move freely in water confined in the hydrogel, serving as charge carriers. A typical rectifying curve is recorded with this system.

So what’s next? Gel transistors, gel processors, or maybe you can play chess with one of your semiconducting organs?