Mechanics of pH-responsive hydrogel capsules

Area of Science:

• Materials Science
• Biomaterials Engineering
• Nanotechnology

Background:

• Soft hydrogels are promising for therapeutic applications but challenging to characterize mechanically.
• Understanding hydrogel nanomechanics is crucial for in vivo and therapeutic uses.

Purpose of the Study:

• To investigate the influence of hydrogel architecture and thiol cross-linking on the mechanical properties of poly(methacrylic acid) (PMASH) hydrogels.
• To compare the mechanical behavior of supported hydrogel films and free-standing capsules.
• To explore the impact of pH on hydrogel properties.

Main Methods:

• Fabrication of single-component, supported hydrogel films using pendant-thiol-modified poly(methacrylic acid) (PMASH).
• Creation of free-standing hydrogel capsules.
• Nanomechanical characterization using colloidal-probe atomic force microscopy (CP-AFM).
• Investigation of pH-induced changes (7.4 to 4.0) on film properties.

Main Results:

• Mechanical properties, stability, and swelling of PMASH hydrogels can be tuned by controlling thiol modification.
• pH changes from 7.4 to 4.0 induce film densification via increased hydrogen bonding.
• Hydrogel capsules showed stiffness values from 0.9 to 16.9 mN m(-1), significantly higher than planar films (0.7-5.7 mN m(-1)).
• Substrate geometry had minimal effect on supported films, but membrane and tension effects are critical for capsule stiffness.

Conclusions:

• Tuning thiol modification and pH offers precise control over hydrogel mechanical properties.
• Free-standing hydrogel capsules exhibit distinct mechanical behaviors compared to supported films due to geometrical and tension effects.
• Accurate characterization of soft hydrogels requires consideration of architecture and environmental factors.