As a result of evolutionary selection, biomaterials are well adapted for their functions, as will be discovered in this TLP. (Note. In this TLP the term biomaterials refers to the materials of living systems and not man-made materials with biomedical applications). Unlike man-made materials, a limited range of ingredients (never metal) is used to display a wide range of properties. Charles Darwin himself noted that, ‘As a general principle, natural selection is continually trying to economise every part of the organisation’. For instance, tendons and muscle are made of collagen, as are the cornea, skin and blood vessels. Whereas the manufacturing process determines engineering materials’ properties, in biomaterials the structure’s age and environment affect the materials properties, leading to variation in the properties of a specific biomaterial. Biomaterials tend to be strong, anisotropic and are usually composite materials. Biomaterials have many advantages, as they are sustainable, recyclable and biodegradable, unlike most common engineering materials. However, the hierarchical structure makes replicating the structure of biomaterials complicated and their use is restricted to ambient-temperature applications.
Similarly to engineering materials, biomaterials are classed into the following groups corresponding to shared characteristics:
- Natural ceramics and ceramic composites (e.g. enamel, bone, shell, antler).
- Natural polymers and polymer composites (e.g. proteins such as silk and polysaccharides such as cellulose).
- Natural elastomers (e.g. skin, artery, cartilage).
- Natural cellular materials (e.g. wood, cancellous bone, cork).
This TLP shows how materials-selection maps can be used to compare biomaterials with common engineering materials, looking specifically at using these maps in conjunction with merit indices. Four of the most commonly used materials-selection maps will be studied:
- Young’s modulus against density,
- Strength against density,
- Young’s modulus against strength,
- Toughness against Young’s modulus.
These are particularly important for choosing the most suitable material for a specific application, and looking at the interesting properties specific to certain biomaterials such as viscid silk found as the capture threads in spiders’ webs.