Abstract
Porcupine quills are composed of keratin proteins fabricated during quill growth into a cylindrical outer shell with an interior foam core. In the present research, the tensile and nanomechanical properties of quill shells from North American porcupine (Erethizon dorsatum) were tested in the axial and circumferential directions at relative humidities of 65% and 100%. At 65% relative humidity, the mean axial elastic modulus and strength of the shell were found to be significantly greater than the corresponding circumferential elastic modulus and strength. Increasing the relative humidity to 100% decreased the measured moduli and strengths and increased the fracture strains due to the plasticizing effects of the absorbed water molecules. Fracture morphologies after tensile testing revealed a three layer structure for the quill shells. The elastic modulus and hardness of the inner quill shell layer were found to be larger than the middle and outer layers by nanoindentation testing. An extensive amount of fibrous cortical cell structure was found aligned parallel to the growth direction of the quill and accounted for the higher moduli, strength and hardness measurements in the axial direction compared to the circumferential direction. Transmission electron microscopy revealed a fine structure of 3–4μm diameter cortical spindle cells composed of 7nm diameter intermediate filaments. The unfolding process of α-helices within the intermediate filaments was quantitatively measured by in-situ infrared spectroscopy technique.
Published Version
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