Hydrogel ionic electronics is an emerging discipline that uses hydrogel as an ion conductor and hydrophobic elastomer as a dielectric. Integrating hydrogels and hydrophobic elastomers in various manufacturing processes and having strong, stretchable and transparent adhesion is the biggest challenge for hydrogel ion electronics. A few days ago, Academician Suo Zhigang of Harvard University/Southern University of Science and Technology Yang Canhui designed a multi-step dip coating process to realize a variety of configurations of hydrogel ion electronic devices. In doing so, the hydrophobic surface is primed to wet the hydrophilic precursors, and then the different layers of polymers are interconnected with covalent bonds. This process combines hydrogel and hydrophobic elastomer, which has strong adhesion without compromising stretchability and transparency. Related work was published on Advanced Materials with the title "Ionotronic Luminescent Fibers, Fabrics, and Other Configurations".
Ionized luminescent fiber
The research started from the ionized luminescent fiber with a coaxial structure, which is composed of a hydrogel core, a luminescent layer, a hydrogel layer and an elastomer layer. When the power source applies an AC voltage between the hydrogel core and the hydrogel layer, ions of opposite polarity will periodically accumulate on the hydrogel/elastomeric interface, generating alternating electric fields in the light-emitting layer, and in the phosphor Electron-hole pairs are generated in the particles. Elastomers and hydrogels are transparent, so they can emit light. Specifically, through a multi-step dip coating method, using polyacrylamide (PAAm) hydrogel containing lithium chloride as an ion conductor, poly(dimethyl dimethyl sulfide) particles dispersed with copper-doped zinc sulfide (ZnS: Cu) Siloxane) (PDMS) serves as the light-emitting layer, PDMS as the dielectric layer and silane as the coupling agent.
The researchers compared two steps: dip coating a layer of elastomer on the hydrogel and dip coating a layer of hydrogel on the elastomer. It was found that the elastomer precursor fluid can spontaneously spread on the surface of the hydrogel, while the water forms droplets on the surface of the untreated elastomer. But after the silane coupling agent primer process, water can also spread on the surface of the elastomer.
Performance of ionized luminescent fiber
The researchers connected two metal wires to the power source. The voltage of the power supply divided by the thickness of the light-emitting layer defines the nominal electric field. Under the action of the pulsed electric field, the optical fiber lights up and extinguishes light. The ionized luminescent fiber is stretchable and maintains its brightness under deformation. After stretching it to 1.5 times its original length, after 10,000 cycles, the fiber maintains its peak stress and brightness.
Various hydrogel ionized light emitting devices
The flexibility and stretchability of the hydrogel, the strong adhesion between the hydrogel and the elastomer, and the wide applicability of dip coating all together make the hydrogel ionized electroluminescent device of complex shape possible. Researchers create ionized light-emitting fabrics by assembling two types of fibers into rows and columns. Furthermore, a light-emitting ring is manufactured, which comprises a light-emitting layer, a closed cylindrical shell of PAAm hydrogel and PDMS elastomer, and a light-emitting handbag with a cage structure. Finally, by using different ZnS particles for the light-emitting layer, multicolor fibers can also be made.
Sum up
The researchers describe a multi-step dip coating method to create complex geometries of hydrogel ion electronics. This method achieves a strong, stretchable and transparent bond between the hydrogel and the hydrophobic elastomer. Demonstration of ionized luminescent fibers, fabrics and other configuration devices. The proposed method paved the way for the widespread application of hydrogel ion electronics.
Paper link:
https://onlinelibrary.wiley.com/doi/10.1002/adma.202005545