Tensile strength, often determined through tensile testing, quantifies the maximum load a material can endure while being stretched or pulled before failing. This is typically expressed in units of stress (e.g., Pascals or psi). Related properties include yield strength (the stress at which permanent deformation begins) and elongation at break (the percentage of stretching before fracture).

The elastic modulus is the ratio of stress to strain in the elastic region of a material's behavior. For biomaterials, matching the modulus to that of the surrounding tissue is often desirable to prevent stress shielding (in bone implants) or excessive deformation. For example, titanium alloys have a higher modulus than polymers like UHMWPE.

Hardness is typically measured using methods like the Rockwell, Brinell, or Vickers hardness tests. These tests involve pressing a standardized indenter into the material's surface under a specific load and measuring the depth or size of the resulting indentation. Materials with higher hardness generally exhibit better wear resistance.

Density is a fundamental physical property. For implantable devices, especially those intended for long-term use, lower density materials can lead to lighter implants, improving patient comfort and reducing the risk of mechanical complications. For instance, titanium alloys are significantly less dense than stainless steel.

Surface energy, often characterized by contact angle measurements, is a critical factor in biomaterial biocompatibility. High surface energy materials tend to adsorb more proteins, which can trigger inflammatory responses or promote unwanted cell adhesion. Conversely, carefully controlled surface energy can be used to encourage specific cellular interactions.

Metals used in implants, such as stainless steel, titanium alloys, and cobalt-chromium alloys, must exhibit excellent corrosion resistance. The physiological environment, with its electrolytes and pH variations, can induce electrochemical reactions leading to material breakdown, ion release, and potential toxicity. Passivation layers, like the oxide layer on titanium, are crucial for this resistance.

Biodegradable polymers, like polylactic acid (PLA) and polyglycolic acid (PGA), are designed to degrade over time, eliminating the need for removal surgery. The degradation rate can be controlled by factors such as molecular weight, crystallinity, and copolymer composition. This property is also exploited in controlled drug release systems.

XPS is particularly useful for characterizing the surface chemistry of biomaterials, which is critical for biocompatibility. It can reveal the presence of specific elements, oxidation states, and functional groups on the surface, providing insights into surface treatments, contamination, or degradation. It's often used to analyze the passivation layer on metals or the surface chemistry of polymer coatings.

SEM is essential for examining the surface texture, porosity, and microstructural features of biomaterials. It can reveal surface defects, the morphology of degradation, or how cells attach and interact with the material surface. Often, samples are coated with a thin layer of conductive material (like gold or platinum) to enhance imaging.

AFM is a powerful technique for visualizing surface features at the atomic and molecular level. It can measure surface roughness, identify nanoscale features, and even probe the mechanical properties of individual cells or surface molecules. This is crucial for understanding initial protein adsorption and cell adhesion events.

Viscoelastic materials, common in biomaterials (like hydrogels and some polymers), exhibit both viscous and elastic characteristics. DMA is used to determine properties such as the storage modulus (elastic component), loss modulus (viscous component), and tan delta (damping). This is crucial for understanding how these materials will behave under dynamic loading conditions, such as in artificial ligaments or soft tissue implants.

DSC is used to characterize the thermal properties of polymers and other materials. For biomaterials, it can help determine the melting point, degree of crystallinity, and glass transition temperature (Tg). The Tg is particularly important as it indicates the temperature range where a polymer transitions from a rigid, glassy state to a more flexible, rubbery state, influencing its mechanical properties and processing.