Single Hydrogel Particle Mechanics and Dynamics Studied by Combining Capillary Micromechanics with Osmotic...

Area of Science:

• Materials Science
• Polymer Science
• Soft Matter Physics

Background:

• Hydrogels exhibit complex responses to stimuli and rich mechanical behavior.
• Previous studies primarily focused on static hydrogel mechanics, neglecting time-dependent responses due to measurement limitations.
• Understanding dynamic mechanical properties is crucial for hydrogel applications.

Purpose of the Study:

• To investigate both static and time-dependent mechanical responses of polyacrylamide (PAAm) hydrogel particles.
• To explore the influence of osmotic stress from dextran solutions on hydrogel particle mechanics.
• To develop theoretical explanations for observed static and dynamic behaviors.

Main Methods:

• Utilized Capillary Micromechanics to apply direct contact forces to single PAAm particles within a tapered capillary.
• Applied osmotic forces using a high molecular weight dextran solution to alter particle hydration and internal concentration.
• Measured static compressive and shear elastic moduli, and dynamic deformation response times.

Main Results:

• Static elastic moduli (compressive and shear) were significantly higher for particles in dextran solutions (KDex≈63 kPa, GDex≈16 kPa) compared to water (Kwater≈36 kPa, Gwater≈7 kPa).
• Observed slower dynamic deformation in dextran solutions (τDex≈90 s) than in water (τwater≈15 s), contrary to poroelastic expectations.
• This surprising dynamic behavior was theoretically explained by the diffusion of dextran molecules, dominating compression dynamics.

Conclusions:

• Increased internal polymer concentration due to osmotic stress stiffens PAAm hydrogel particles statically.
• Dextran molecule diffusion significantly influences and slows down the dynamic mechanical response of hydrogel particles.
• The findings highlight the importance of considering solute diffusion in the dynamic mechanics of hydrogels.