Swelling and Evaporation Sculpt the Surface of Grafted Hydrogel Films

Langmuir 2025, 41, 4, 2400–2410


Soft and deformable materials are ubiquitous, ranging from living organisms such as bacteria to inanimate substances like clay. They are also found in everyday objects like sponges and in complex synthetic materials such as foams and polymer networks. These materials, known as poroelastic, combine elasticity with the ability to absorb solvents.

In a recent study, researchers [1] investigated the behavior of PNIPAM, a polymer that can swell up to four times its size in water to form a hydrogel. When grafted onto a rigid substrate, it is used in microfluidic valves, single-cell trapping, controlled drug release, and cell culture.

However, when attached to a rigid surface, the gel’s swelling is constrained, leading to the formation of surface patterns. Once formed, these patterns evolve during drying—a phenomenon that had not been well understood until now. The researchers conducted a systematic characterization of the gel’s structures and mechanical properties under both wet and dry conditions.

One of the challenges was studying these extremely soft surfaces, which have a Young’s modulus in the kilopascal range. By developing a specialized protocol using atomic force microscopy (AFM), the results showed that the distribution of the Young’s modulus follows the swollen-state surface topography, revealing a heterogeneous solvent distribution.

The study highlights a wide range of patterns that depend on the film thickness and drying conditions. By identifying the mechanisms behind these formations, the researchers pave the way for improved control of hydrogel materials for advanced applications in biomedicine and surface engineering.

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References
Swelling and Evaporation Determine Surface Morphology of Grafted Hydrogel Thin FilmsCl, Caroline Kopecz-Muller, Clémence Gaunand, Yvette Tran, Matthieu Labousse, Elie Raphaël, Thomas Salez, Finn Box, Joshua D. McGraw, Langmuir, 2025.
https://doi.org/10.1021/acs.langmuir.4c04025

Contact
Scientific communication from ESPCI Paris - PSL: Paul Turpault