A millimeter scale flexural testing system for measuring the mechanical properties of marine sponge spicules

Abstract

Many load bearing biological structures (LBBSs)—such as feather rachises and spicules—are small (<1 mm) but not microscopic. Measuring the flexural behavior of these LBBSs is important for understanding the origins of their remarkable mechanical functions. We describe a protocol for performing three-point bending tests using a custom-built mechanical testing device that can measure forces ranging from 10-5 to 101 N and displacements ranging from 10-7 to 10-2 m. The primary advantage of this mechanical testing device is that the force and displacement capacities can be easily adjusted for different LBBSs. The device's operating principle is similar to that of an atomic force microscope. Namely, force is applied to the LBBS by a load point that is attached to the end of a cantilever. The load point displacement is measured by a fiber optic displacement sensor and converted into a force using the measured cantilever stiffness. The device's force range can be adjusted by using cantilevers of different stiffnesses. The device's capabilities are demonstrated by performing three-point bending tests on the skeletal elements of the marine sponge Euplectella aspergillum. The skeletal elements—known as spicules—are silica fibers that are approximately 50 μm in diameter. We describe the procedures for calibrating the mechanical testing device, mounting the spicules on a three-point bending fixture with a ≈1.3 mm span, and performing a bending test. The force applied to the spicule and its deflection at the location of the applied force are measured.