Bonding soft materials to manufacture the next generation of devices

The future is soft. As technologies advance in flexible robotics and wearables, industry increasingly needs new methods to fasten different materials together in manufacturing. Researchers from the Harvard John A Poulsen School of Engineering and Applied Sciences (SEAS) have devised a method using chemical additives that do not change the properties of soft materials such as elastomers and hydrogels, but which lock materials to which they have been added together.

Currently, soft materials can only be attached to each other using glues or surface treatments. This, however, can restrict the manufacturing process, adding multiple stages and difficult-to-handle materials. The SEAS method, described in Nature Communications, builds bonding into the materials rather than using a separate agent to bond.

The researchers concentrated on elastomers and hydrogels because these are the most common building blocks for soft devices; hydrogels often being electrically conductive and elastomers being insulators. Both types of materials are built up from chemical precursors, which are liquids that are polymerised to make a soft solid. The researchers mixed chemical coupling agents into the precursors. Resembling molecular hands with small tails, the agents embed themselves into the polymer network, with the tail fixed into the bulk of the polymer and the hand remaining open at the surface. When the hydrogel and elastomer are combined in a manufacturing process, the free hands “reach across” the boundary between the two materials and “shake”, creating chemical bond at the interface.