In addition to a material being certified as biocompatible, biomaterials must be engineered specifically to their target application within a medical device. This is especially important in terms of mechanical properties which govern the way that a given biomaterial behaves. One of the most relevant material parameters is the Youngs Modulus, E, which describes a materials elastic response to stresses. The Youngs Moduli of the tissue and the device that is being coupled to it must closely match for optimal compatibility between device and body, whether the device is implanted or mounted externally. Matching the elastic modulus makes it possible to limit movement and delamination at the biointerface between implant and tissue as well as avoiding stress concentration that can lead to mechanical failure. Other important properties are the tensile and compressive strengths which quantify the maximum stresses a material can withstand before breaking and may be used to set stress limits that a device may be subject to within or external to the body. Depending on the application, it may be desirable for a biomaterial to have high strength so that it is resistant to failure when subjected to a load, however in other applications it may be beneficial for the material to be low strength. There is a careful balance between strength and stiffness that determines how robust to failure the biomaterial device is. Typically, as the elasticity of the biomaterial increases, the ultimate tensile strength will decrease and vice versa. One application where a high-strength material is undesired is in neural probes; if a high-strength material is used in these applications the tissue will always fail before the device does (under applied load) because the Youngs Modulus of the dura mater and cerebral tissue is on the order of 500 Pa. When this happens, irreversible damage to the brain can occur, thus the biomaterial must have an elastic modulus less than or equal to brain tissue and a low tensile strength if an applied load is expected.
For implanted biomaterials that may experience temperature fluctuations, e.g., dental implants, ductility is important. The material must be ductile for a similar reason that the tensile strength cannot be too high, ductility allows the material to bend without fracture and also prevents the concentration of stresses in the tissue when the temperature changes. The material property of toughness is also important for dental implants as well as any other rigid, load-bearing implant such as a replacement hip joint. Toughness describes the material's ability to deform under applied stress without fracturing and having a high toughness allows biomaterial implants to last longer within the body, especially when subjected to large stress or cyclically loaded stresses, like the stresses applied to a hip joint during running.
For medical devices that are implanted or attached to the skin, another important property requiring consideration is the flexural rigidity, D. Flexural rigidity will determine how well the device surface can maintain conformal contact with the tissue surface, which is especially important for devices that are measuring tissue motion (strain), electrical signals (impedance), or are designed to stick to the skin without delaminating, as in epidermal electronics. Since flexural rigidity depends on the thickness of the material, h, to the third power (h), it is very important that a biomaterial can be formed into thin layers in the previously mentioned applications where conformality is paramount.