Spider silk softening by water uptake: an AFM study

Area of Science:

• Materials Science
• Biomaterials
• Mechanics of Materials

Background:

• Spider dragline silk is a remarkable natural polymer known for its exceptional mechanical properties.
• Understanding the influence of environmental factors like humidity on silk's mechanics is crucial for its applications.
• Previous studies have highlighted humidity's effect on tensile properties, but compressional properties remain less explored.

Purpose of the Study:

• To investigate the mechanical properties of Nephila spider dragline fibers under varying relative humidity.
• To compare the Young's modulus (E) obtained through atomic force microscopy (AFM) indentation with results from single fiber stretching experiments.
• To elucidate the anisotropic elastic behavior and water-induced softening of spider silk.

Main Methods:

• Atomic force microscopy (AFM) was employed to collect force maps and derive Young's modulus (E) using a modified Hertz model for indentation.
• Single fiber tensile stretching experiments were conducted to measure mechanical properties.
• Experiments were performed across a range of relative humidity conditions.

Main Results:

• Young's modulus (E) decreased by an order of magnitude upon water immersion, indicating significant softening.
• Both indentation and tensile tests demonstrated a strong dependence of mechanical properties on relative humidity.
• Absolute E values from tensile tests were considerably higher than those from AFM indentation, revealing elastic anisotropy.
• AFM force maps revealed a sub-micron surface structure on the spider silk fibers.

Conclusions:

• Spider dragline silk exhibits highly anisotropic elastic properties.
• Water uptake significantly softens spider silk under both tensile and compressional (indentation) strain.
• The findings provide critical insights into the environmental sensitivity and complex mechanical behavior of spider silk, relevant for biomimetic material design.