Using the milk-skin effect to make smart biomaterials

The film that forms on heated milk – the “skin” – is displaying a clever chemical reaction that could be used to make materials from solid-state batteries to wearable electronics sensors.

An international team of researchers has adapted the milk-skin reaction into a quick and simple way to make flexible gels from sustainable biological materials.

“In the milk-skin effect, a film forms at the outer layer of milk when it is heated,” says Professor Guihua Yu, a researcher at the University of Texas, US, and co-author on a paper describing the research, published in Nature Synthesis.

Yu says the team was inspired by this phenomenon.

When milk is heated, the fats and proteins begin to stretch out and line themselves up at the surface. This causes the skin on the top: a thin membrane of neatly organised molecules.

The researchers copied this process by dipping a thin material (similar to cellulose) into a liquid solvent (in this case, acetonitrile).

Molecules in the solvent naturally line themselves up on the edge of the material, forming a thin film.

The researchers could pull the film off the material with a pair of tweezers.

Clear gel being pulled off clear material labelled 'colloid' with tweezers
The gel being pulled off the material it forms on. Credit: The University of Texas at Austin / Cockrell School of Engineering

The films made are called “ionogels”: gels made from thin layers of polymers, filled liquid that contains electrically charged ions.

This makes the gels highly electrically conductive, as well as very sensitive.

Their sensitivity and conductivity makes them ideal candidates for wearable electronics, like heart sensors or motion trackers. In their paper, the researchers demonstrate prototype gels that work as thin electronic circuit chips and sensors which sit comfortably on fingertips or arms.

The gels would also be a good electrolyte in solid-state batteries, which could be safer and more powerful than modern lithium-ion batteries.

Ionogel drawing
Credit: The University of Texas at Austin/Cockrell School of Engineering

The so-called “dip and peel” process can make gels of varying levels of thickness, and from a variety of different sources. As well as cellulose, the researchers have used silk fibroin, chitosan and guar gum, to make their ionogels.

The international research team is hoping that their quick, easy method will be adopted by other scientists for making new materials.

Next, they’re planning to see if they can optimise the process and use it to make materials for wearable electronics, smart robots or artificial intelligence.

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