Structural and Mechanical Studies of the Individual Building Block of a Natural Transparent Armor of Placuna placenta
Ling Li (MIT), Christine Ortiz ()
Experimental Nanobiomechanics
Mon 10:45 - 12:15
Barus-Holley 163
A number of mollusks species possess mineralized transparent exoskeletons which combine both optical and mechanical functionalities. These biomaterials are composed of nanoscale building blocks which minimize the scattering and absorption of light, as well as resist penetration, dissipate energy, and localize fracture. Here, we investigate the nanostructure and mechanical behavior of the highly transparent shell of the mollusk, Placuna placenta. The nanocomposite shell (~99 wt% calcite) consists of elongated diamond-shaped laths with a length of 141.8 ± 43.4 um, width of 5.54 ± 1.36 um, thickness of 294 ± 84 nm, and tip angle of 10.45 ± 2.95°. The mechanical behavior of the individual laths was quantified using an atomic force microscopy (AFM)-based nano-three-point bending experiments. The testing specimens were prepared using focused ion beam (typical dimensions, length: 10 um, width: 0.8 um, height: 0.3 um). Customized AFM probes with a well-defined wedge geometry (angle: 60°, length: 1.1 um, and height: 5 um) were used for the bending test. Force/displacement curves were recorded using a 3D Molecular Force Probe (Asylum Research, CA), which maintains a closed loop X/Y/Z function on loading and unloading (maximum force: ~ 18 uN) in ambient conditions at displacement rates 0.1-5.0 um/s. The lack of any loading-unloading hysteresis and permanent deformation (verified by SEM) indicated a purely elastic response. The predictions of an isotropic, elastic 3D finite element analysis (FEA) of the beam exhibited an excellent fit to the experimental data (R-square > 0.99), and allowed for estimation of Young’s modulus as found to be 86 ± 9 GPa (n=5). This relatively high stiffness may be attributed to low organic content, which is also believed to enhance transparency by reducing scattering and absorption. The reported protocol can be potentially applied to investigate the mechanical properties of other hierarchical biological material systems at building block level.