Mechanical Design of Spicule-Reinforced Connective Tissue: Stiffness

Abstract

Many animals from different phyla have, embedded in their pliable connective tissues, small bits of stiff material known as spicules. The tensile behaviour of spicule-reinforced connective tissues from various cnidarians and sponges, as well as of model spiculated ‘tissues’, is here investigated in order to elucidate the effects on mechanical properties of spicule size and shape, and of their packing density and orientation within a tissue. The main conclusions are: 1. Spicules increase the stiffness of pliable connective tissues probably by mechanisms analogous to those by which filler particles stiffen deformable polymers-local strain amplification, and interference with molecular re-arrangement in response to a load. 2. The greater the volume fraction of spicules, the stiffer the tissue. 3. The greater the surface area of spicules per volume of tissue, the stiffer the tissue. Thus, a given volume of spicules of high surface-area-to-volumeratio (S/V) have a greater stiffening effect than does an equal volume of spicules of low S/V. Furthermore, a high volume fraction of large spicules in a tissue can have the same stiffening effect as a lower volume fraction of smaller spicules. 4. Spicules that are anisometric in shape have a greater stiffening effect parallel to their long axes. 5. Spicules with very high aspect ratios appear to act like reinforcing fibres-stress is transferred by shearing from the pliable matrix to the stiff fibres, which thus bear in tension part of the load on the composite. 6. Spicule-reinforced tissues exhibit stress-softening behaviour, which is more pronounced in heavily spiculated tissues.