Every complex human tool has contained multiple materials wedged, tied, screwed, glued, or soldered together. But the next generation of tools, from autonomous squishy robots to flexible wearables, will be soft. Combining multiple soft materials into a complex machine requires entirely new methods. Current methods are limited, relying on glues or surface treatments that can restrict the manufacturing process.
A new method to chemically bond multiple soft materials is independent of the manufacturing process. In principle, the method can be applied in any manufacturing process, including 3D printing and coating. This technique opens the door to manufacturing more complex soft machines, allowing bonding of hydrogels and elastomers in various manufacturing processes without sacrificing the properties of the materials.
To combine the two most-used building blocks for soft devices — hydrogels (conductors) and elastomers (insulators) — chemical coupling agents were mixed into the precursors of both hydrogels and elastomers. The coupling agents look like molecular hands with small tails. As the precursors form into material networks, the tail of the coupling agents attaches to the polymer networks, while the hand remains open. When the hydrogel and elastomer are combined in the manufacturing process, the free hands reach across the material boundary and shake, creating chemical bonds between the two materials. The timing of the “handshake” can be tuned by multiple factors such as temperature and catalysts, allowing different amounts of manufacturing time before bonding happens.
The method can bond two pieces of casted materials like glue, but without applying a glue layer on the interface. The method also allows coating and printing of different soft materials in different sequences. In all cases, the hydrogel and elastomer created a strong, long-lasting chemical bond.
Hydrogels — which are mostly water — can be made heat-resistant in high temperatures using a bonded coating, extending the temperature range in which hydrogel-based devices can be used; for example, a hydrogel-based wearable device could be ironed without boiling. Hydrogels can enable electrical devices that mimic the functions of muscle, skin, and axon. Like integrated circuits in microelectronics, these devices function by integrating dissimilar materials. These integrated soft materials could enable Spandex-like touchpads and displays that one can wear, wash, and iron.