The ultra-soft elastomer is fabricated by crosslinking bottlebrush polymers; it contains only cross-links (red chains) and no entanglements. Image: Li-Heng Cai, Harvard SEAS.
The ultra-soft elastomer is fabricated by crosslinking bottlebrush polymers; it contains only cross-links (red chains) and no entanglements. Image: Li-Heng Cai, Harvard SEAS.

Medical implants mimic the softness of human tissue by mixing liquids such as oil with long silicone polymers to create a squishy, wet gel. While implants have improved dramatically over the years, there is still a chance of the liquid leaking, which can be painful and sometimes dangerous.

Now, a team of polymer physicists and chemists led by David Weitz, professor of physics and applied physics at Harvard University and associate faculty member at the Wyss Institute for Biologically Inspired Engineering, has developed a way to create an ultra-soft dry silicone rubber. This new rubber features tuneable softness to match a variety of biological tissues, opening new opportunities in biomedical research and engineering. The material is described in a recent paper in Advanced Materials.

"Conventional elastomers are intrinsically stiff because of how they are made," explained lead author Li-Heng Cai, a postdoctoral fellow at Harvard. "The network strands are very long and are entangled, similar to a bunch of Christmas lights, in which the cords are entangled and form knots. These fixed entanglements set up an intrinsic lower limit for the softness of conventional elastomers."

In order to fabricate a soft elastomer, the team needed to eliminate the entanglements. To do this, they developed a new type of polymer that was fatter and less prone to entanglement than linear polymers. The polymers, nicknamed bottlebrushes, are easily synthesized by mixing three types of commercially available linear silicone polymers.

"Typically the fabrication of such bottlebrush molecules requires complex chemical synthesis," said co-author Thomas Kodger, now a postdoctoral fellow at University of Amsterdam. "But we found a very simple strategy by carefully designing the chemistry. This system creates soft elastomers as easily as silicone kits sold commercially."

The softness of the elastomers can be precisely controlled by adjusting the amount of cross-linked polymers, allowing them to mimic everything from soft brain tissue to relatively stiff cells. "If there are no crosslinks, all the bottlebrush molecules are mobile and the material will flow like a viscous liquid such as honey," said Cai. "Adding crosslinks connects the bottlebrush molecules and solidifies the liquid, increasing the material stiffness."

In addition to controlling the softness, the team also found a way to independently control the liquid-like behavior of the elastomer. "To make the conventional elastomer softer, one needs to swell it in a liquid," said co-author Michael Rubinstein, professor of chemistry at the University of North Carolina at Chapel Hill. "But now we can adjust the length of 'hairy' polymers on the bottlebrush molecules to tune the liquid-like behavior of soft elastomers – without swelling – allowing us to make these elastomers exceptionally non-adhesive yet ultra-soft."

These qualities make the material not only ideal for medical devices, such as implants, but also for commercial products such as cosmetics. "The independent control over both softness and liquid-like behavior of the soft elastomers will also enable us to answer fundamental questions in biomedical research," said Weitz. "For example, stem cell differentiation not only depends on the softness of materials with which they are in contact, but recent findings suggest that it is also affected by how liquid-like the materials are. This discovery will provide entirely new materials to study the cell behavior on soft substrates."

This story is adapted from material from Harvard University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.