New method for bonding soft materials will enable more complex wearable smart devices

Combining multiple soft materials into a complex machine requires many new innovations.

Current methods to combine soft materials are limited, relying on glues or surface treatments that can restrict the manufacturing process. For example, it doesn’t make much sense to apply glue or perform surface treatment before each drop of ink falls off during a 3D printing session.

New bonding methods can join various hydrogels and elastomers in various manufacturing processes without sacrificing the properties of the materials.

Nature Communications – Bonding dissimilar polymer networks in various manufacturing processes

A hydrogel and elastomer are separately molded and then placed in contact with a thin film of paraffin sandwiched in between. After curing, the contact region between the hydrogel and the elastomer forms bonds while the paraffin region does not. The bonding remains intact as a nozzle inflates the hydrogel into a balloon.

The researchers focused on the two most-used building blocks for soft devices, hydrogels (conductors) and elastomers (insulators). To combine the materials, the team mixed chemical coupling agents 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 researchers showed that 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.

“The manufacturing of soft devices involves several ways of integrating hydrogels and elastomers, including direct attachment, casting, coating, and printing,” said Canhui Yang, a postdoctoral fellow at SEAS and co-first author of the paper. “Whereas every current method only enables two or three manufacturing methods, our new technique is versatile and enables all the various ways to integrate materials.”

The researchers also demonstrated that hydrogels — which as the name implies are mostly water — can be made heat resistant in high temperatures using a bonded coating, extending the temperature range that hydrogel-based device can be used. For example, a hydrogel-based wearable device can now be ironed without boiling.

This work enables strong adhesion between soft materials in various manufacturing processes. It is conceivable that integrated soft materials will enable spandex-like touchpads and displays that one can wear, wash, and iron.”

An integrated circuit achieves its function by integrating dissimilar components, and so does a living organ. A family of recently demonstrated devices mimics the functions of neuromuscular and neurosensory systems—actuating, sensing, and signaling—by integrating hydrogels and elastomers. The hydrogels function as stretchable, transparent, ionic conductors. The elastomers function as stretchable, transparent dielectrics. The elastomers also function as seals to retard dehydration when the devices are in the open air, or to retard the exchange of solutes when the devices are in an aqueous environment. To function as dielectrics and seals, the elastomers must be hydrophobic, with low solubility and diffusivity of water. Demonstrated devices include transparent loudspeakers, ionic skins, ionic cables, stretchable electroluminescent displays, soft touchpads, soft actuators and triboelectric generators. In particular, a salt-containing and elastomer-coated hydrogel fiber mimics the myelinated axon as a fast conduit for electrical signals, and endures the wearing and washing conditions commonly associated with textiles. Such an artificial axon may be used to develop soft touchpads and soft displays for wearable and washable smart clothes.

Recently developed devices mimic neuromuscular and neurosensory systems by integrating hydrogels and hydrophobic elastomers. While different methods are developed to bond hydrogels with hydrophobic elastomers, it remains a challenge to coat and print various hydrogels and elastomers of arbitrary shapes, in arbitrary sequences, with strong adhesion. Here we report an approach to meet this challenge. We mix silane coupling agents into the precursors of the networks, and tune the kinetics such that, when the networks form, the coupling agents incorporate into the polymer chains, but do not condensate. After a manufacturing step, the coupling agents condensate, add crosslinks inside the networks, and form bonds between the networks. This approach enables independent bonding and manufacturing. We formulate oxygen-tolerant hydrogel resins for spinning, printing, and coating in the open air. We find that thin elastomer coatings enable hydrogels to sustain high temperatures without boiling.