Zhang, X., Huang, J., Chang, P., Li, J., Chen, Y., Wang, D., Yu, J., & Chen, J. (2010). Structure and properties of polysaccharide nanocrystal-doped supramolecular hydrogels based on Cyclodextrin inclusion Polymer, 51 (19), 4398-4407 DOI: 10.1016/j.polymer.2010.07.025

Last year I unsubscribed the RSS feeds of several journals, including JACS, Adv. Mater., Angew. Chem. Int. Ed.. I do so because I am not quite interested in “inventing” things. Instead I appreciate the careful identification of the casual relationship of the physical world, although my knowledge is limited in the narrow range of polymeric and colloidal world.

I’m tired of beautiful graphical abstracts of stimuli response “smart” materials esp. supramolecular ones. My master degree project was also related to supramolecular chemistry — cyclodextrin threading on block copolymer in aqueous solution. This little field of study was pioneer by Prof. A. Harada with the legendary “molecular necklace” paper on Nature, and later crowded by followers all over the world creating various “supramolecular hydrogels” base on this strategy, the trend even lasting till now. This old trick can still buy a place in a IF>2 journal nowadays. How extraordinary!

A typical hydrogel research should include the following must-haves:

• Gel preparation, of course, whose imaginary mechanism should be represented by colorful artworks, but the proof is hidden in the supporting information.
• Photos of the gel and the sol, in several inverted or tilted tubes. Of course, a fluid only viscous enough th withstand its own weight can also be call a “physical” hydrogel
• Dynamic mechanical tests on a rotational rheometer, especially for the “physical” hydrogels. Because these pasty muds have little or no tensile strength at all!
• TEM, SEM photo plus free interpretation
• Drug release
• Cell culture

Rheologically, most of these “hydrogels” are only yield stress fluids consisting of a condensed suspension of the mesoscale aggregates of the allegedly self-assembled structures, i.e. merely a precipitation! They have no recoverability at all under visible deformation, contrary to the common sense of a gel, but best resembling the concept of pastes. Of course all of them show a storage modulus magnitudes of order higher than the loss modulus under dynamical tests, but this is only the necessary condition for a real hydrogel which can undergo recoverable deformation.

Moreover, what makes sense when using pasty muds for drug release and cell culture applications? They are only solid suspensions at high volume fractions. So the drugs are only dissolved in the aqueous phase as simple solution, or not. Maybe a simple extension of a diffusive model is enough to describe this physical process, so what’s the contribution in the repeated reports of experiments on the same process in these physically identical “novel” samples?

I doubt if there is someone really choose these hydrogels against conventional biomaterials for drug matrix or cell culture? Maybe. But this definitely helps under the publish or perish pressure.