Researchers at the University of California at Santa Barbara have detailed a method of 3D printing soft, elastic materials — resembling human skin in their mechanical properties — at room temperatures and without solvents.
"We start with long polymers that are not crosslinked," explains first author Renxuan Xie of the team's work. "That allows them to flow like a fluid. But, after you shine the light on them, the small molecules between the polymer chains react and are linked together into a network, so you have a solid, an elastomer that, when stretched, will return to its original shape."
The approach involves self-assembling bottlebrush polymers, which begin as a soft butter-like solid but liquefy under sufficient pressure — making them suitable for use in a 3D printer using direct ink writing (DIW). Post-printing, the material is exposed to ultraviolet light — transforming it into a permanent solid with a softness similar to that of human skin.
"The modulus of our material is a thousand times smaller than that of a rubber band," Xie notes. "It is super-soft — it feels very much like human tissue — and very stretchy. It can stretch about three to four times its length."
Interestingly, the material was discovered entirely by accident during a separate project by co-author Sanjoy Mukherjee to develop materials to increase the storable charge in an actuator. "I could see right away that it was different," Mukherjee adds, "because it could hold its shape so well" — a key property for the 3D printing process, to prevent sagging prior to the application of UV light which would warp the parts.
"When we saw this really well-defined yield stress, it dawned on everyone collectively that we could 3D print it," says Assistant Professor Christopher Bates, "and that would be cool, because none of the 3D-printable materials we know of have this super-soft property.
"These super-soft elastomers might be applicable as implants. You may be able to reduce inflammation and rejection by the body if the mechanical properties of an implant match native tissue."
The paper has been published under open-access terms in the journal Science Advances.