The team worked on creating soft, solvent-free poly(dimethylsiloxane) elastomers by a one-step process via crosslinking bottlebrush polymers.
The bottlebrush architecture prevents the formation of entanglements, resulting in soft and more controllable elastomers enabling their negligible adhesiveness compared to commercially available silicone products of similar stiffness.
A team led by David A. Weitz, a professor at the School of Engineering and Applied Sciences (SEAS) at Harvard, and Associate Faculty member at the Wyss Institute for Biologically Inspired Engineering, published their findings on ‘Soft Poly(dimethylsiloxane) Elastomers from Architecture-Driven Entanglement Free Design’ in the journal Advanced Materials.
They found that this new rubber features tunable softness to match a variety of biological tissues, opening new opportunities in biomedical research and engineering.
However, due to the softness of the elastomers, and by being able to independently control the liquid–like behavior of the elastomer, the team says the material is also ideal for commercial products such as cosmetics.
Polydimethylsiloxane is used in a variety of cosmetics products for skin protection and also as a conditioner in hair products. The Cosmetic Ingredient Review's (CIR) Expert Panel, concludes that dimethicone and related polymers are ‘safe as used in cosmetic formulations.’
The study
In order to fabricate a soft elastomer in their study, the team needed to eliminate the entanglements from the beginning by developing a new type of polymer, nicknamed bottlebrushes, which are easily synthesized by mixing three types of commercially available linear silicone polymers.
"Conventional elastomers are intrinsically stiff because of how they are made," says lead author Li–Heng Cai, a postdoctoral fellow at the Harvard Paulson School.
"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."
The scientists explain that they found a very simple strategy to control the softness of the elastomers by adjusting the amount of cross–linked polymers to mimic everything from soft brain tissue and 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," says 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 behaviour of the elastomer.
"The independent control over both softness and liquid–like behaviour of the soft elastomers will also enable us to answer fundamental questions in biomedical research," adds 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 behaviour on soft substrates."