Recyclable electronic skin can be healed
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.
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.
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.
About the sensors
On this recycle e-skin, tactile sensing is based on the capacitance change between two conductive element arrays (in grey) separated by a dielectric polymer ring array (purple ring array in the illustration), so both force and position can be detected across the array.
A serpentine-shaped conductive track (made of AgNP-doped polyimine) can also be monitored to detect temperature (the track’s resistance changes linearly with temperature between 24° and 54°C). With a longer track, humidity can be sensed too. As water molecules diffuse into the sensor, the polymer network expands, leading to a measurable increase of the sensor resistance.
As for the water flow sensor, very similar in shape to the temperature sensor, it can sense flow rates between 0 and 10 m/s (due to the flow’s cooling effect on the resistor). Beyond 10ml/s, its resistance no longer changed with increasing flow rate.
All four sensors were made out of serpentine structures so they would flexibly conform to any shape when their substrate is slightly heated and pressed. The tactile sensor was characterized with a sensitivity of 0.0067 kPa−1 in the 0 to 14g range, the temperature sensor was sensitive down to 0.17%°C−1 with a detection limit below 60ºC, and the average sensitivity of the humidity sensor was 0.22%/% with an upper limit of detection about 80 to 90%.
The e-skin can be easily conformed onto curved surfaces by applying moderate heat and pressure, with serpentine structures adopted to minimize the influence of strains on the performance of the sensors.
According to the researchers, the recyclability of such an e-skin could greatly reduce electronic waste and environmental impact while potentially decreasing manufacturing cost for a wide range of applications in robotics, prosthetics, health monitoring, and biomedical devices.
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