Person:

Yang, Canhui

Hydrogel-based ionic devices represent an alternative approach to stretchable electronics through use of soft ionic conductors that are both highly stretchable and transparent. However, these devices require the integration of dissimilar materials, dielectric elastomers and hydrogels, into a single system; a process thus far achieved primarily via the combination of several different manufacturing techniques. We have developed a 3D extrusion printing technique capable of fabricating an entire ionic circuit that integrates a LiCl-doped poly(acrylamide) (PAAm) hydrogel with a poly(dimethylsiloxane) (PDMS) dielectric elastomer. By incorporating hygroscopic salts such as LiCl into the hydrogel, we are able to prepare an ionically conductive hydrogel with excellent water-retaining properties. For printing reliability, we have optimized the rheological properties of a high ionic-strength hydrogel precursor and the interfacial energy between PDMS and hydrogel. Printed ionic devices that consist of PAAm and PDMS exhibit outstanding mechanical and electrical stability when tested with up to 1000 cycles of uniaxial tension. Moreover, we successfully demonstrate functionality in terms of signal transmission and as a soft sensor by fabricating and characterizing an ionic cable and several strain gauges.

A fully organic liquid-crystal device is enabled by ionic conductors. The device uses a liquid crystal as voltage-driven light shutter, an elastomer as transparent dielectric, and a hydrogel as transparent conductor. A voltage switches the device from an opaque state to a transparent state without electrolyzing the hydrogel. The device maintains electrooptical performance under a biaxial stretch of 1.5.

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.

Hydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress-stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels, or even fatigue-free hydrogels. Such a hydrogel typically contains a covalent polymer network, together with some non-covalent, reversible interactions. Here we show that self-recovery hydrogels are still susceptible to fatigue fracture. We study a hydrogel containing both covalently crosslinked polyacrylamide and uncrosslinked polyvinyl alcohol. For a sample without pre-cut crack, the stress-stretch curve recovers after thousands of loading cycles. For a sample with a pre-cut crack, however, the crack extends cycle by cycle. The threshold for fatigue fracture depends on the covalent network, but negligibly on non-covalent interactions. Above the threshold, the non-covalent interactions slow down the extension of the crack under cyclic loads.