Creasing instability of surface-attached hydrogels.

Area of Science:

• Materials Science
• Polymer Science
• Mechanical Engineering

Background:

• Surface-attached polymer gels expand unidirectionally when swelling.
• This expansion generates biaxial compressive stress, potentially leading to mechanical instability.
• This instability, characterized by surface creasing, limits topographical feature formation in gels.

Purpose of the Study:

• To systematically investigate the critical compression required for creasing in polymer gels.
• To determine the relationship between gel properties (modulus, thickness) and creasing onset.
• To understand the reversibility and memory effects associated with gel creasing.

Main Methods:

• Utilized poly(acrylamide-co-sodium acrylate) hydrogels as a model system.
• Experimentally determined the onset of surface creasing during controlled swelling.
• Measured gel modulus and thickness across a range of experimental conditions.

Main Results:

• Established that creasing initiates at an effective linear compressive strain of approximately 0.33 (thickness change factor of ~2).
• Found that the critical strain for creasing is largely independent of gel modulus (0.6–24 kPa) and thickness (3 μm–1 mm).
• Observed that creasing is reversible upon reducing swelling, with surfaces retaining a memory of crease locations.

Conclusions:

• The study quantifies the critical compressive strain for creasing in surface-attached polymer gels.
• Findings align with theoretical predictions and provide a fundamental limit for gel swelling without topographical changes.
• The reversible nature and location memory of creases have implications for designing responsive biomaterials and sensors.