Researchers have developed the first 3D-printable “bottlebrush” elastomer that results in printed objects with unusual softness and elasticity — mechanical properties that closely resemble those of human tissue.
Conventional elastomers, such as rubbers, are stiffer than many biological tissues due to the size and shape of their constituent polymers, which are long, linear molecules that easily entangle like cooked spaghetti. In contrast, bottlebrush polymers have additional polymers attached to the linear backbone, leading to a structure more akin to a bottle brush you might find in a kitchen. The bottlebrush polymer structure imparts the ability to form extremely soft elastomers.
The ability to 3D print bottlebrush elastomers makes it possible to leverage these unique mechanical properties in applications that require careful control over the dimensions of objects ranging from biomimetic tissue to high-sensitivity electronic devices such as touch pads, sensors, and actuators.
The bottlebrush polymer material is categorized as a yield-stress fluid, meaning it begins as a semi-soft solid that holds its shape, like butter or toothpaste, but when sufficient pressure is applied, it liquefies and can be squeezed through a syringe. The researchers can tune the material to flow under various amounts of pressure to match the desired processing conditions.
Once the object is printed, UV light is shined onto it to activate crosslinkers that were synthesized and included as a part of the ink formulation. The crosslinkers can link up nearby bottle-brush polymers, resulting in a super-soft elastomer. At that point, the material becomes a permanent solid — it will no longer liquefy under pressure — and exhibits extraordinary properties.
The softness of a material is measured in terms of its modulus and for most elastomers, it is rather high, meaning their stiffness and elasticity are similar to those of a rubber band. The modulus of the new material is 1,000 times smaller than that of a rubber band. The material can also stretch about three to four times its length.
The super-soft elastomers might be applicable as implants; for example, to reduce inflammation and rejection by the body if the mechanical properties of an implant match native tissue. Another important element of the material is that it is pure polymer — there is no water or other solvent to artificially make it softer.