Scientists are making skin for robots

A team at UNC Chapel Hill discovers that by rearranging silicone molecules, they can make a gel that performs a lot like skin and muscle, paving the way for new medical devices and possibly life-like skin for robots.

The chemistry lab at the University of North Carolina at Chapel Hill is buzzing. Small machines spin beakers of clear liquid. Bubbles continually move up and explode in other containers.

“We’re taking monomers, which are single molecules, and reacting them together to make these huge molecules,” said Andrew Keith, a graduate researcher in the lab. “And that’s really what polymer chemistry is about; making these huge molecules which allows us to make really interesting materials."

The bubbles continue to appear and disappear. “So I design our materials to be electro-active, in other words, if you think of something that is like muscle, I think about my hand moving and I can move my hand,” explains Ben Morgan, as he wiggles his fingers on his right hand. Morgan is also a graduate researcher in the lab. “So I use our soft materials to make devices where you can flip a switch, apply a voltage, apply a certain electric field to this material, and it will change shape and it can do work, just like my hand.”

It soon becomes clear that all of the bubbles and all of the lab equipment and all of the energy are focused on a clear gel that is sitting in a petri dish on a lab table. “The softness of the material comes from its molecular architecture,” said Daixuan Zhang, another graduate researcher in the lab as he picks up the clear, soft, putty like material. If you imagine jello on steroids you have the idea.

“There is no solvent, no water inside, so if I put this material in this bowl of dry ice, it will stay in the same property."

Zhag puts on gloves and then lays the material in the dry ice. Sure enough, within 30 seconds the material begins to feel cold, but it doesn’t harden or even become brittle. “It will stay the same, flexible yet firm, even at negative 70 Celsius,” Zhang says smiling. “That’s because there is no water in the gel so it won’t freeze and it also won’t evaporate.” What is sitting in the dry ice is a new type of silicone gel that could revolutionize medicine and robotics.

The team in Dr. Sergei Sheiko’s chemistry lab at the University of North Carolina at Chapel Hill created the gel. The secret lies in polymer chemistry. Researchers discovered manipulating the molecular structure controls the mechanical properties of the gel. It’s the architecture of how the silicon molecules are arranged, not the chemical composition, that’s most important.

“The goal is to make material that is soft as gel but without water,” said Sheiko, PhD. and a Distinguished Professor of Chemistry at UNC Chapel Hill. He picks up a bottle brush, which has a long handle with bristles arranged in a spoke like design at the end. “Each silicone polymer molecule has brush like architecture and there are a lot of side chains,” adds Sheiko. “And when another polymer sits next to it they don’t entangle, so it is all chemically the same material.” The gel is soft and flexible but when it is stretched it becomes pretty strong. It performs, mechanically, a lot like living tissue. "Think skin and muscle.

Researchers also found that making the polymer chains longer makes the material feel softer while increasing the distance between the molecules makes the material feel stiffer. Once again, it’s not the chemical composition that is changed; it is how the molecules are arranged. It’s the same silicone just a different softness.

“We can cover the range from the softness of brain tissue to the toughness of skin, all without changing the chemical composition,” said Sheiko, who calls it the Home Depot approach to chemistry. “If you want to build a house you go to Home Depot and buy the same lumber,” Sheiko smiles as he explains how the discovery works. “What makes the houses different is how you connect the lumber and how you design the architecture. It’s the same wood, just different ways of putting it together.”

Changing the molecular structure means the gel could be customized for different human tissues. And because there is no water in the gel, it could withstand a wide temperature range.

The silicone gel has been in development since 2001. And the creation is drawing a lot of attention because the material mimics living tissue: it is soft and flexible but also strong and durable. And it can withstand wide temperature swings. That means the gel could be customized to create different human tissues by simply changing the molecular structure of the gel.

“We could make implants for reconstructive surgery, for cardio vascular surgery,” Sheiko said with confidence. “If you give me any type of tissue I could give you a piece of silicone that mimics the tissue." That means that soft, sphere of silicone sitting in that dish in Chapel Hill could revolutionize medicine as well as robotics. Because it is not just a new type of molecule or material, it is a new material design platform.

 —Frank Graff 

Frank Graff is a producer/reporter with UNC-TV, focusing on Sci NC, a weekly science series. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!

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