Structure-Function Relationships in Biological Glass Fibers

Recent interest in the optical and mechanical properties of silica skeletal structures (spicules) made by living sponges, and the possibility of harnessing these mechanisms for the synthesis of advanced materials and devices, motivate our investigation of the micro- and nanoscale architecture of these remarkable biological materials. High resolution scanning electron and atomic force microscopic analyses of spicules isolated from five different sponge species reveals an unanticipated diversity of structural complexity characteristic of these unique skeletal systems. All spicules, measuring greater than a few mm in total length, exhibit a unique laminated architecture consisting of alternating layers of hydrated amorphous silica and organic that effectively halts crack propagation through these materials. In spicules that experience stresses not confined to a single axis (e.g. the anchor spicules from Euplectella aspergillum), there is a reduction in silica layer thickness as one travels from the spicule core to its periphery. In contrast, spicules that experience uniaxial loading exhibit a discrete graded architecture with the thickest silica layers found in regions of maximum compression and the thinnest layers in regions of maximum tension. Basic design principles learned from these studies are presented and may prove useful in a wide range of technologically-important applications including the design of more fracture-resistant optical fibers.