Stretchable Solar Cells Could Power Artificial Skin for Robots

A flexible solar cell that converts sunlight into electricity even when stretched is the latest breakthrough in the work of Stanford University’s Zhenan Bao, a professor of chemical engineering who has created an artificial electronic "super skin" that also has applications in prosthetics, robotics and detection of harmful diseases.

Essentially the skin is a carbon-based flexible transistor comprised of grids containing millions of tiny inverted pyramids imprinted on a thin rubber sheet. When pressed, the pyramids rearrange themselves, altering the strength of an electric current running through the skin.

It is this ability to sense pressure and changes to its surrounding environment that allows the super skin to “think,” sending information back to a central processing unit, such as a computer - or a human brain. But it is the development of a "stretchable" polymer solar cell, used to power the artificial skin, that has the team really excited, and opens up a whole new world of applications. Unencumbered by grid power, or reliance on battery packs, solar energy has brought Bao’s experimental super skin into the light.

A recent research paper by Bao, describing the stretchable solar cells, will appear in an upcoming issue of Advanced Materials. The paper details the ability of the cells to be stretched in one direction, but she said her group has since demonstrated that the cells can be designed to stretch along two axes. The sensors have from several hundred thousand to 25 million pyramids per square centimetre, corresponding to the desired level of sensitivity.

The solar cells have a concertina-like structure and can stretch by up to 30 percent then snap back into shape without losing the ability to convert sunlight into electricity. To sense a particular biological molecule, the surface of the transistor has to be coated with another molecule to which the first one will bind when it comes into contact.  The coating layer only needs to be a nanometre or two thick.

"Depending on what kind of material we put on the sensors and how we modify the semiconducting material in the transistor, we can adjust the sensors to sense chemicals or biological material”. Bao's team has successfully demonstrated the concept by detecting a certain kind of DNA.  The researchers are now working on extending the technique to detect proteins, which could prove useful for medical diagnostics purposes.

"For any particular disease, there are usually one or more specific proteins associated with it – called biomarkers – that are akin to a 'smoking gun,' and detecting those protein biomarkers will allow us to diagnose the disease," Bao said.

The same approach would allow the sensors to detect chemicals, she said. By adjusting aspects of the transistor structure, the super skin can detect chemical substances in either vapour or liquid environments.

"One of the applications where stretchable solar cells would be useful is in fabrics for uniforms and other clothes," said Darren Lipomi, a graduate student and member of Bao’s research team. Bao’s team have successfully shown that the super skin can detect chemical substances in vapour or liquid environments, a useful function for armed and emergency services uniforms. "You can imagine a robot hand that can be used to touch some liquid and detect certain markers or a certain protein that is associated with some kind of disease and the robot will be able to effectively say, 'Oh, this person has that disease,'" she said. "Or the robot might touch the sweat from somebody and be able to say, 'Oh, this person is drunk.'"

Bao said the team had only recently discovered a way to stretch the solar cells across two axes, meaning they could successfully be fitted to cloth for uniforms, or incorporated into the surface structure of a robot providing autonomous solar power to the flexible super skin.

Bao said she sees the super skin as much more than a super mimic of human skin; it could allow robots or other devices to perform functions beyond what human skin can do. Bao has figured out how to replace the materials used in earlier versions of the transistor with biodegradable materials. Now, not only will the super skin be more versatile and powerful, it will also be more eco-friendly.

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