Tasuku Nakajima, Hokkaido University
Advanced functional hydrogels based on reversible sacrificial bonds.
Written by Devesh Mistry
Hydrogels are an exciting category of soft materials with many potential applications. However, current hydrogels lack strength; that is, they fail at low stresses. As such, hydrogels are not currently suitable as structural materials. Tasuku Nakajima of Hokkaido University has been exploring the use of double networks to strengthen hydrogels and introduce further properties such as self-healing.
Nakajima has used a two-step process to produce hydrogels with two separate networks. The first network is formed from a dilute polymer which is densely crosslinked. A second network is then formed within the first. This network is much more condensed, but has a far lower crosslink density. On application of a stress, the energy is dissipated by the brittle bonds of the first network, preserving the shape of the hydrogel. In a dramatic series of videos, Nakajima showed how the hydrogel could be driven over with a truck with no apparent damage. This is remarkable for a material which is 90% water!
The breaking of the brittle bonds is an irreversible process, so subsequent stresses destroy the hydrogel. To produce a self-healing hydrogel, Nakajima replaced the brittle bonds with ionic bonds capable of reforming. Other hydrogel systems involving lamellar structures displayed tunable Bragg reflections.