Second-Harmonic Generation Imaging of Humidity-Induced Salt Crystallization in Spider Aggregate Glue

Area of Science:

• Biomaterials Science
• Materials Chemistry
• Structural Biology

Background:

• Spider aggregate glue droplets are crucial for web adhesion, exhibiting complex physicochemical properties.
• Understanding the structural and compositional changes in these droplets is key to explaining web mechanics.
• Environmental factors like humidity significantly influence droplet behavior and adhesive properties.

Purpose of the Study:

• To investigate structural and compositional changes in spider aggregate glue droplets.
• To correlate these changes with surface attachment and controlled humidity conditions.
• To elucidate the role of crystallization in spider web adhesion.

Main Methods:

• Utilized second-harmonic generation (SHG) microscopy to probe droplet structure.
• Employed high-speed optical imaging and scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDS) for detailed analysis.
• Controlled experiments under varying relative humidity conditions.

Main Results:

• Observed spatially localized SHG signals correlated with potassium dihydrogen phosphate crystallization.
• Crystallization occurred upon droplet attachment or abrupt humidity reduction, while droplets remained amorphous under stable conditions.
• Crystallization onset and morphology were dependent on humidity history and dehydration rate, with rapid water loss favoring dendritic crystal formation.

Conclusions:

• Second-harmonic generation (SHG) microscopy selectively probes ordered, noncentrosymmetric inorganic domains.
• Humidity-driven phase behavior and kinetic perturbations play a significant role in spider web adhesive properties.
• Findings provide crucial insights into the mechanisms of spider silk adhesion.