Every complex human tool, from the first spear to the latest smartphone, 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 an entirely new toolbox – after all, there's no such thing as a soft screw.
Current methods for combining 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 is deposited during 3D printing.
But now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a new method for chemically bonding multiple soft materials independent of the manufacturing process. In principle, the method can be applied to any manufacturing process, including 3D printing and coating. This technique, which is reported in a paper in Nature Communications, opens the door to manufacturing more complex soft machines.
"This technique allows us to bond various hydrogels and elastomers in various manufacturing processes without sacrificing the properties of the materials," said Qihan Liu, a postdoctoral fellow at SEAS and co-first author of the paper. "We hope that this will pave the way for rapid-prototyping and mass-producing biomimetic soft devices for healthcare, fashion and augmented reality."
The researchers focused on the two most-used building blocks for soft devices, hydrogels (conductors) and elastomers (insulators). To combine these materials, the team mixed chemical coupling agents into the precursors of both the hydrogels and elastomers. These coupling agents look like molecular hands with small tails. As the precursors transform into material networks, the tails of the coupling agents attach to the polymer networks, while the hands remain 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 be used to bond two pieces of casted materials like glue but without applying a glue layer at 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 at high temperatures using a bonded coating, extending the temperature range over which hydrogel-based devices can be used. For example, a hydrogel-based wearable device can now be ironed without boiling.
"Several recent findings have shown that hydrogels can enable electrical devices well beyond previously imagined," said Zhigang Suo, a professor of mechanics and materials at SEAS and senior author of the paper. "These devices mimic the functions of muscle, skin and axon. Like integrated circuits in microelectronics, these devices function by integrating dissimilar materials. 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."
This story is adapted from material from Harvard SEAS, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
An unmodified hydrogel (left) peels off easily from an elastomer. A chemically-bonded hydrogel and elastomer (right) are tough to peel apart, leaving residue behind. Image courtesy of Suo Lab/Harvard SEAS.