Tough Hydrogels & Stretchable Ionics(Jeong-Yun Sun, Seoul Nat'l Univ.)

Jeong-Yun Sun
2014-05-22 ~ / EB1, 301-11
Hydrogels are used as scaffolds for tissue engineering, vehicles for drug delivery, actuators for optics and fluidics, and model extracellular matrices for biological studies. The scope of hydrogel applications, however, is often severely limited by their mechanical behaviour. Most hydrogels do not exhibit high stretchability; for example, an alginate hydrogel ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels have achieved stretches in the range 10–20, but these values are markedly reduced in samples containing notches. Most hydrogels are brittle, with fracture energies of about 10 Jm-2, as compared to 1,000 Jm-2 for cartilage and 10,000 Jm-2 for natural rubbers. Intense efforts are devoted to synthesizing hydrogels with improved mechanical properties; certain synthetic gels have reached fracture energies of 100–1,000 Jm-2.
We have reported the synthesis of a new hydrogel from polymers that form ionically and covalently crosslinked networks. Although such gels contain 90% water, they can be stretched beyond 20 times their initial length, and have fracture energies of 9,000 Jm-2. Even for samples containing notches, a stretch of 17 is demonstrated. We attribute the gels’ toughness to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed on unloading. The unzipped ionic crosslinks cause internal damage, but are healed by re-zipping. These gels serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications.
Furthermore, as a new application, we have proposed a class of devices enabled by hydrogel conductors that are highly stretchable, fully transparent to light of all colors, and capable of operation at frequencies beyond 10 kHz and voltages above 10 kV. We have created a transparent actuator that can generate large strains, and a transparent loudspeaker that produces sound over the entire audible range. The electromechanical transduction is achieved without electrochemical reactions.