Mechanical Characterization of Biomaterials

The design of biomedical devices almost always involves some form of mechanical characterization of biomaterials. This chapter provides a broad overview of experimental methods and important considerations for mechanical characterization of biomaterials, with special attention to the practical needs of engineers and scientists who encounter a need to characterize the mechanical properties of a biomaterial but may not know where to begin or what the key considerations should be. Many details are necessarily omitted from this broad overview, but numerous references are provided for greater technical depth on a particular topic, standardized methodologies, and exemplary studies. Fundamental concepts are introduced, beginning with stress and strain versus force and displacement. The mechanical properties measured from a stress–strain curve, different types of stress–strain curves, and corresponding constitutive models are reviewed, including differences in material classes and anisotropy. Three primary methods of analysis for fracture mechanics are introduced, including stress concentrations, energy criteria for crack initiation and propagation (fracture toughness), and statistical methods for the probability of fracture. The mechanical characterization of biomaterials begins with selection and preparation of standardized test specimens, which are critical to obtaining accurate and reproducible measurements of material properties. Practical considerations are outlined for selection and preparation of the specimen size, geometry, surface finish, and precracking. The mechanical characterization of biomaterial test specimens always involves the application and measurement of load and deformation. Practical considerations are outlined for the selection and use of load frames, load cells, load fixtures, extensometers, and strain gauges. A number of common loading modes are introduced and compared: uniaxial tension, uniaxial compression, biaxial tension, torsion, diametral compression, three-point bending, four-point bending, and in-plane shear (including biomaterial-tissue interfacial shear strength). Strain-rate sensitivity or time-dependent behavior can profoundly influence stress–strain behavior and thus measured mechanical properties. The effects of high strain rates may be characterized by impact testing using a pendulum, drop tower, or split Hopkinson pressure bar. The effects of low strain rates may be characterized by creep deformation or creep rupture tests. The time-dependent behavior of viscoelastic materials is introduced, including creep, stress relaxation, common constitutive models, and practical considerations for testing. The frequency of loading, or cyclic loading, is another aspect of time-dependent behavior, which is critical for mechanical characterization of biomaterials, leading to fatigue deformation and failure or viscoelastic creep and stress relaxation. Practical considerations are described for selecting the waveform, frequency, cyclic stress/strain levels, loading mode, and test duration. Common methods are introduced for fatigue lifetime testing (including S-N curves, notch factors, and fatigue damage), fatigue crack propagation, and dynamic mechanical analysis (DMA). Nondestructive tests are particularly useful for sampling small volumes of a biomaterial (e.g., implant retrieval or biopsy) or characterizing spatial heterogeneity in mechanical properties. Various indentation tests and indenter geometries are introduced and compared, including classic hardness (Brinell and Rockwell), microhardness (Knoop and Vickers), and instrumented nanoindentation (Berkovich, cube corner, etc.). Methods and limitations are described for characterizing the reduced modulus, viscoelasticity, and fracture toughness using indentation. Ultrasonic wave-propagation methods are also introduced with an emphasis on methods for characterizing anisotropic elastic constants. Biomaterials are typically subjected to various sterilization methods prior to service and an aqueous physiological environment in service. Therefore, the effects of temperature, pressure, various aqueous media (water, phosphate buffered saline (PBS), media, foetal bovine serum (FBS), lipids, etc.), and irradiation on mechanical characterization of biomaterials are considered, including the degradation of mechanical properties by various mechanisms involving water uptake, hydrolysis, and oxidation. Finally, methods and guidelines are provided for data acquisition from transducers and data analysis, including an introduction to some basic statistical methods.