New materials produce soft, elastic objects that feel like human tissue

Researchers in the laboratories of Christopher Bates, an assistant professor of materials at UC Santa Barbara, and Michael Chabinyc, a professor of materials and chair of the department, collaborated on the first 3-D-printable “bottle brush” elastomer to develop. The new material results in printed objects with unusual softness and elasticity – mechanical properties that are very similar to those of human tissue.

Conventional elastomers, ie rubbers, are stiffer than many biological tissues. This is due to the size and shape of their ingredients, which are long, linear molecules that easily entangle like cooked spaghetti. In contrast, bottle brush polymers have additional polymers attached to the linear backbone, resulting in a structure more similar to a bottle brush that you can find in your kitchen. The bottle brush polymer structure confers the ability to form extremely soft elastomers.

The ability to print 3-D bottle brush elastomers makes it possible to utilize these unique mechanical properties in applications that require careful control over the dimensions of objects, ranging from biomimetic tissues to high-sensitivity electronic devices, such as touch pads. sensors and drives.

Two postdoctoral researchers – Renxuan Xie and Sanjoy Mukherjee – played key roles in the development of the new material. Their findings were published in the journal Scientific progress.

Xie and Mukherjee’s most important discovery involves the self-assembly of bottle-brush polymers on the nanometer length scale, causing a solid-to-liquid transition in response to applied pressure. This material is categorized as a yield stress liquid, which means that it starts as a semi-soft solid that retains its shape, such as butter or toothpaste, but when sufficient pressure is applied, it becomes liquid and can be pushed through a syringe. word. The team utilizes this property to create ink in a 3D printing process called Direct Ink Writing (DIW).

The researchers can adjust the material to flow under different amounts of pressure to suit the desired processing conditions. “Maybe you want the polymer to keep its shape under a different level of stress, like when there is vibration,” says Xie. “Our material can hold its shape for hours. This is important because if the material sinks during printing, the printed portion will have poor structural stability.”

After the object is printed, UV light is shone on it to activate cross-links that Mukherjee synthesized and included as part of the ink formulation. The crosslinks can bond nearby bottle brush polymers, resulting in a super-soft elastomer. At that point, the material becomes a permanent solid – it will no longer become liquid under pressure and exhibits extraordinary properties.

“We start with long polymers that are not crosslinked,” Xie said. “It allows them to flow like a liquid. But when you shine the light on it, the small molecules between the polymer chains react and are linked together in a network, so you have a solid, an elastomer that , when stretched, will return to its original shape. “

The softness of a material is measured at the modulus, and for most elastomers it is quite high, which means that their stiffness and elasticity are similar to those of an elastic band. “The modulus of our material is a thousand times smaller than that of an elastic band,” notes Xie. “It’s super soft – it feels a lot like human tissue – and very stretchy. It can stretch about three to four times its length.”

A random ink

Mukherjee accidentally discovered the material while trying to develop a material for another project, one that increases the amount of charge that can be stored by an actuator. When the elastomer came to Xie for characterization, he immediately knew it was special. “I could immediately see that it was different, because it could hold its shape so well,” he recalls.

“When we saw this very well-defined yield voltage, it all came together that we could 3D-print it,” Bates said, “and it would be cool, because none of the 3-D printable materials we know of this super-soft property has. ‘

Bottle brush polymers have been around for over twenty years. But Bates said: ‘The field has exploded over the past ten years thanks to advances in synthetic chemistry that provide excellent control over the size and shape of these unique molecules.

“These super-soft elastomers may be applicable as implants,” he added. “You can potentially reduce inflammation and rejection by the body if the mechanical properties of an implant match the native tissue.”

Another important element of the new material is that it is pure polymer, Chabinyc noted.

“There is no water or other solvent in it that makes them softer,” he said.

To understand how important it is to have no water in the polymer, it is helpful to think of Jell-O, which is mostly water and can hold its shape, but only as long as the water stays inside. “If the water disappeared, you would just have a shapeless pile of material,” Chabinyc said. “With a conventional polymer, you have to figure out how to keep the right amount of water in it to maintain the structure, but this new material is solid, so it will never change.”

In addition, the new material can be 3-D-printed and processed without solvent, which is also unusual. “People regularly add solvent to liquefy a solid so that it can be squeezed out of a nozzle,” Xie said, “but if you add solvent, it must evaporate after it is pressed, so that the object forms change or crack. “

Mukherjee added: ‘We wanted the material and the printing process to be as clean and as easy as possible, and therefore we played a chemical trick with solubility and self-assembly, which made the solvent-free process possible. The fact that we do not use solvent is a huge advantage. ”