Putting the Heat on Hydrogels

Title: Controllable heterogeneity in a supramolecular hydrogel

Authors: Atsuomi Shundo*a, Keiko Mizuguchib, Misao Miyamotocd, Masahiro Gotoae and Keiji Tanaka*abe

Journal: Chemical Communications

Affiliation: aDepartment of Applied Chemistry, Kyushu University; bDepartment of Automotive Science, Kyushu University; cNissan Chemical Industries, Ltd.; dArt, Science and Technology Center for Cooperative Research, Kyushu University; eCenter for Future Chemistry, Kyushu University

Normally a gel is unable to transition back and forth between a solid and a liquid; however supramolecular gels (SMGs) are able to do so.  You can control their physical properties by changing conditions such as temperature, pH, and UV light exposure.  In this paper the authors investigate the supramolecular gel properties of a peptide amphilphile, which self assembles into micelle-like structures that form into a fibril network.  Specifically, they look at how the heterogeneity of the gel influences the properties of the gel.

The investigators use a N-Palmitoyl-Gly-Gly-Gly-His trifluoroacetate (PalG3H) as the material to make their hydrogel.  A 0.2 wt% (weight percent) aqueous dispersion of PalG3H was heated to 363 K for 10 minutes to yield a clear solution.  When this was cooled to room temperature and allowed to sit for 1 hour it formed a hydrogel.  However if the dispersion was heated to 333 K the solution did turn clear, but a hydrogel was never formed even after sitting for weeks.  Additionally when the gel was shaken it became a fluid, but reformed a gel after sitting for 72 hours.  These observations led the authors to further investigate the viscoelastic properties and local structures of the gel and sol forms of PalG3H.

Using optical tweezers the authors placed polystyrene particles in the PalG3H and investigated the movement of the particles as motion was induced via an infrared laser.  The amplitude of the particles were measured and found to vary widely in the gel, but they remained fairly constant in the sol.  This led the authors to suggest that the gel is heterogeneous.  The authors also determined the storage and loss moduli of the sol and gel.  They determined both to be elastic.  This is interesting, because it is unusual (but not unheard of) for sol states to be elastic.

They also used a fluorescent probe that has stronger emission in a hydrophobic environment (like the core of the micelles) than a hydrophilic environment (like the outside of the micelles).  They found that the sol state the fibrils were short and monodisperse, while the gel state contained both short and long fibrils.  The authors conclude that the heterogeneity of the gel is due to the heterogeneous network of the fibrils.  They state that further studies into the local viscoelastic properties will soon be reported.

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