Recyclable electronic skin can be healed

February 13, 2018 // By Julien Happich
With a focus on recyclability, researchers from the University of Colorado Boulder have developed a malleable e-skin whose base chemistry is fully reversible into re-usable molecules and metal. Made up of polyimine for its dielectric parts, and polyimine doped with silver nanoparticles (AgNPs) for its conductive parts, the e-skin is able to detect touch, temperature, flow, and humidity.

The sensing skin derives its recyclability from the basic four ingredients of its chemistry, three commercially available compounds (terephthalaldehyde, diethylenetriamine, and tris(2-aminoethyl)amine) mixed in ethanol and cross-linking into polyimine. The paper "Rehealable, fully recyclable, and malleable electronic skin enabled by dynamic covalent thermoset nanocomposite" published in the Science Advances explains how the e-skin can be fully depolymerized and recycled because of the reversible bond exchange between these different compounds under certain chemical environments.

The e-skin can be fully recycled in a solution, yielding
dissolved oligomers/monomers and AgNPs at the bottom.
Both the solution and AgNPs can then be reused to
make a new e-skin.

Unlike conventional thermoset materials that cannot be reprocessed, reshaped, and recycled because of their highly cross-linked polymeric networks connected with irreversible covalent bonds, the links in the polyimine can be broken in solution and all its original constituents recycled. All it takes is to soak the e-skin into ethanol and diethylenetriamine. "The stoichiometric balance between aldehyde and amine groups (their reaction forming the imine linkage) within the polyimine network can be upset by introducing an excess of free primary amine groups (for example, excess diamine monomer)" reads the paper, explaining the depolymerization and showing a sample e-skin sensor being dissolved in a test tube, with the silver nanoparticles sinking to the bottom.

Going full circle, the researchers prove they are able to completely reuse the recycled solution and nanoparticles by simply adding and mixing the other two compounds (terephthalaldehyde, diethylenetriamine, and tris(2-aminoethyl)amine)) in stoichiometric proportions, together with additional silver nanoparticles. After polymerization, the conductive polyimine can be used to fabricate new devices.

Going the full circle, a conductive polyimine film connecting a simple LED circuit is being recycled. The LED light turns off as the polymer decomposes in its original oligomers/monomers (top right). The recycled solution is then cast into a new, square petri dish (bottom right). After polymerization, the film is conductive and the LED light turns on (bottom left).

The full circle demonstration performed at room temperature took the shape of a conductive polymer in a petri dish, completing a simple lighting circuit connected to a LED. As the recycling solution was poured into the petri dish, the polymer decomposed and the nanoparticles collapsed to the bottom of the dish, the LED light turned off. Adding the compounds and silver nanoparticles resulted in a novel polymerization, yielding a conductive film and a lit LED.

Healing a cracked device essentially consists in applying a drop of the initial mixture (doped with AgNPs if there is a conductive trace to be mended) onto the crack and heat pressing it so the new oligomers/polymers grow across the broken surfaces. The end result is indistinguishable from the original, with the exact same electrical properties.


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