Structural and mechanical properties of the spines from Echinocactus grusonii cactus

It is intuitional that most cactus spines are hard and sharp, but their structures and properties have never been systematically studied. Here, we report a comprehensive study on the microstructures and mechanical properties of spines from Echinocactus grusonii cactus for the first time by combining optic and scanning electron microscope investigations, x-ray microanalysis, as well as nanoindentation and tensile tests. It is found that the cactus spine consists of fibres with diameter of 5-15 μm and length of hundreds of micrometres and sclerified epidermis, but contains no mineralisation component as some hard substance does. The spine fibres have high crystallinity and in high alignment. Nanoindentation experiments show that the spine has much higher longitude hardness than that of bamboo reported in the literatures, although their transverse hardness is nearly identical to that of woods, crops, and bamboo. The indentation modulus of the spine cell wall is also within the same order of magnitude as woods, crops and bamboo, with detailed value similar to that of bamboo and slightly lower than that of woods and crops. The tensile strength of the dry spine is measured to be 140 MPa within the range of tensile strength of bamboo reported in the literatures. It is also found that the fresh spine has certain toughness, but the dry spine becomes brittle. The high crystallinity and high alignment of fibres in the cactus spines as well as the very small multifibrillar angle within each fibre can explain the special mechanical properties of the spines.