Through a special organ on their snouts, pit vipers can sense the body heat of their prey. Scientists at Caltech and ETH Zurich have developed an artificial skin that uses a similar mechanism and could allow prosthetic limbs to detect changes in temperature, or make a smart bandage that can warn of infection.
A snake's pit organ contains a sensitive membrane that can react to the heat of an animal from up to 1 m (3.3 ft) away. When the membrane warms up, it opens an ion channel that allows calcium ions to flow through, and the viper's nerves pick these up as electrical signals that alert the animal to the presence of prey. The new material uses a similar kind of mechanism to detect very small changes in temperature.
In this case, the membrane is a flexible, transparent film just 20 micrometers thick made of water and pectin, a molecule commonly found in the cell walls of plants. Like the pit organ, an increase in temperature causes an increase in the flow of calcium ions, but here it's due to the heat breaking down the pectin's weak molecular bonds.
Using a multimeter hooked up to electrodes embedded in the film, the researchers detected an electrical response to the temperature changes. That effect comes as a result of the material's electrical resistance decreasing, thanks to the larger number of positively-charged calcium ions flowing through it, or their mobility increasing, or most likely, both.
This isn't the first artificial skin to boast the ability to detect temperature fluctuations. The feature was just one point on the checklist of KAUST's Paper Skin last year. But according to the researchers, the pectin film is far more precise, able to detect heat variations an order of magnitude smaller than existing devices, and it has an input-output ratio that's two orders of magnitude better than others. The electronic skin can currently work across a temperature range of between about 5º and 50º C (41º and 122º F).
Grafted onto prosthetics, the new skin could help alert a wearer to potentially-damaging heat, in a more efficient way than existing prototypes that build heat sensors into the limb itself. It might also lead to craft smart bandages that detect changes in body heat, which can be a sign of an infection developing in a wound. To top it off, pectin isn't exactly expensive or hard to come by.
"Pectin is widely used in the food industry as a jellifying agent; it's what you use to make jam," says Chiara Daraio, lead researcher on the project. "So it's easy to obtain and also very cheap."
But for the heat-sensing artificial skin to reach its full potential, the team wants to bump its maximum operating temperature up to 90º C (194º F). That would increase its practicality for use in robot skins and as thermal sensors for consumer devices. But before that can be done, the fabrication process – which currently leads to water being present – will need to be rethought.
The research is due to be published in the journal Science Robotics this week and the artificial skin is shown in the video below.
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