Fewer polymer chains but higher adhesion: How gradient-stiffness hydrogel layers mediate adhesion through network...

Area of Science:

• Materials Science
• Biophysics
• Polymer Chemistry

Background:

• Gradient softer outer layers are common in biological tissues and synthetic hydrogels.
• These layers influence interactions with opposing surfaces and contact mechanics.
• Polymer distribution in gradient layers may enhance stretch and adhesion through network adaptability.

Purpose of the Study:

• Investigate depth-dependent adhesion in gradient-stiffness polyacrylamide hydrogels.
• Assess the impact of varying gradient layer thicknesses on adhesion characteristics.
• Understand the role of polymer network reorganization in adhesion phenomena.

Main Methods:

• Utilized Atomic Force Microscopy (AFM) nanoindentation.
• Fabricated polyacrylamide hydrogels with gradient stiffness using glass and polyoxymethylene (POM) molds.
• Analyzed depth-dependent adhesion, load-unload hysteresis, and work of separation.

Main Results:

• POM-molded hydrogels (thicker gradient layer) showed greater adhesion than glass-molded hydrogels (thinner gradient layer).
• Indentation within the gradient layer increased hysteresis due to fluid transport in the sparse outer network.
• Enhanced adhesion and work of separation were observed at shallow depths, attributed to the outer network's stretching capability.

Conclusions:

• Gradient-stiffness hydrogels possess tunable adhesion properties influenced by layer thickness and polymer distribution.
• The sparse outer polymer network plays a key role in promoting adhesion through enhanced stretching.
• Findings support the application of gradient hydrogels in health and environmental science.