In a new study in Nature Communications, scientists have developed a polymer that undergoes a dramatic blue-to-red colour shift on exposure to minute amounts of moisture. Materials that display this behavior are called "hydrochromics" (hydro = water, chromic = colour) and often find applications in sensing, switching and memory devices.

The researchers used a polymer based on polydiacetylene, which is formed by self-assembly of monomers into a neat stack thanks to pi-orbital interactions (don't know what those are? don't worry, it doesn't really matter). When water is present the stack is disrupted, causing the system to absorb light at a different wavelength and thus display a different colour. All of which happens in a matter of microseconds.

What is so useful about this new material is it reponds to really tiny amounts of water — drops on the order of a billionth of a liter. That allows it to readily map out the sweat pores on human skin in incredible detail and, like the ridges and whorls of our finger tips, their associated sweat pore patterns are also unique.


As it happens, there are many surfaces that will hold a unique sweat pore pattern without leaving behind an identifiable finger print. Further, you can identify a person based on their fingers' sweat pores even when you have only a partial fingerprint. In fact, a primary reason we use fingerprints instead of sweat pore analysis for identification is the lack of an inexpensive and readily available method for capturing sweat pore patterns. This work starts to change all that.

Aside from the obvious crime fighting applications, the authors also suggest this hydrochromic polymer could also find use in clinical diagnosis of malfunctioning sweat pores. The moral of the story is of course that if you're going to commit a crime, gloves are still your best option.

Read the full story (which is open source!), Hydrochromic conjugated polymers for human sweat pore mapping, here.