Bone is an anisotropic, hierarchically structured material, and as a result, its mechanical behavior is highly statistical in nature. It has been shown for other engineering materials that mechanical testing at the microscale enables characterization of individual microstructural components in an effort to understand their role in the macroscopic mechanical behavior. The application of such microscale testing to bone will permit modeling of the aggregate material to predict effects of age, disease, or injury on the mechanical properties, thus enabling a better understanding of the disease state.
In the present work, dual focused ion beam (FIB) and femtosecond (FS) laser micromachining techniques are employed to produce microscale mechanical test specimens of bovine cortical bone on the order of 10 – 30 µm. A FIB is advantageous for micromachining pillars as it is capable of producing small scale features by applying a Ga+ ion beam that penetrates and removes the surrounding material. A FS laser uses ultrashort laser pulses to ablate the material by locally heating it to its vaporization temperature, creating a plasma that is dissipated into a flowing gas. The FS laser is advantageous for micromachining of biological materials because it may be used in ambient, non-vacuum environments, making it a flexible tool for machining the bone surface while preserving its microstructure. The short pulse duration minimizes thermal diffusion and heating of the surrounding material. Prior research suggests that FS laser machining causes very little residual damage to the surrounding bone tissue. Processing parameters and feasible specimen geometries and dimensions are discussed.
The fabrication of such pillars allows for micromechanical compression testing of time independent behavior using a modified nanoindenter with a flat punch tip. By achieving successful fabrication of micron scale pillars, it is possible to test the constitutive mechanical properties of mineralized tissue that comprises bone. The present work analyzes the mechanical testing of 20- and 30 µm nominal diameter pillars to small and large strains. Pillars tested to small strains are selectively placed within regions of interest on the bone sample surface (i.e. osteons). Modulus, strength, and modes of deformation are compared between samples.