Biomaterials: from old to new world

Biomaterials is the term used to indicate a special class of materials, expressly used for medical applications. The ways they can be used can be directing, replacing or supporting a particular function of human body. In biomedical engineering we have always used materials like METALS, POLYMERS or CERAMICS. During the last years engineers have started to think that it's not about how devices can be designed in order to follow needs and limits of traditional materials, but how we can process materials in order to have a particular kind of device responding to our requests. This is the aim of composite materials, born to provide alternative choices to overcome failure and limits of traditional materials.

Biomaterials are expected to perform in human body, which is a very aggressive environment, many parameters vary in a large scale in different tissues, for example pH that varies from 1 to 9, or stress on bones, varying from 4 MPa to 40-80 MPa. More than this, these stresses are repetitive and fluctuating depending on the activities.

Polymers are used in many applications, mainly because they can be engineered in a wide variety of properties or forms or compositions. Anyway the problem is about the loss of mechanical properties in some applications, furthermore it's difficult choosing the sterilization processes that may affect the properties.

Ceramics are known for high compression resistance,high stiffness and good biocompatibility; on the other hand they have low fracture strength, low mechanical reliability and are difficult to fabricate, sometimes very expensive.

Metals are widely used for their high strength, resistance to wear and ductility. Drawbacks of metals include corrosion, low compatibility, release of metal ions which may cause allergic reactions and high stiffness compared to tissues.

Composite materials are the most innovative, containing in macroscopic or microscopic scale, two or more phases, usually fibers in a polymeric matrix. They can provide better performance compared to homogeneous materials. They can control the macroscopic and microscopic properties of the material through the variation of volume fractions and interfaces (grain size, porosity, size of phases).