1 Biological materials
Materials engineers have long recognised the impressive range and combination of properties offered by biological materials. Figure 1 shows some representative examples of the combination of tensile strength and toughness (measured by Young's modulus, or elastic modulus for polymers) offered by natural materials, with some more common engineering materials included for comparison.
I'm using the term ‘biological material’ here to describe materials of natural origin. The more general term ‘biomaterial’ can include both natural and synthetic materials that are used to replace or interact with part of a living system.
Many biological materials approach or exceed the strength of steel: in fact, some forms of spider silk exceed it by a good margin. If density is taken into account, the specific strengths (strength per unit mass) are even more impressive. Biological materials frequently combine high strength with relatively low stiffness, and this leads to high values for the energy needed to break the material, as indicated by the shading on the diagram. You will see that collagen, the main component of skin and tendons, is particularly impressive in this respect. Biomaterials tend to have a specific combination of properties, closely suited to their function, which can be difficult to match with manufactured alternatives.
If we look closely at natural materials we find that their properties are controlled right down to the molecular level, with each material uniquely adapted to suit a particular purpose and to interact in complex ways with its neighbours in response to changes in circumstances. Consider bone, for example. Natural bone is a composite, combining polymeric protein chains with crystals of ceramic calcium phosphate. In the body it responds to stress levels, being absorbed when low stress levels indicate that it is not needed, growing thicker when the stress levels are high and receding in situations where the stress levels are so high that damage could occur. Given this intelligent response, together with the mismatch in mechanical properties of bone and steel evident from Figure 1, it is no wonder that the design of effective hip replacements, for example, is so technologically challenging.
One of the reasons that it is so difficult to match the properties of biological materials is that they are built within cells from the bottom up, unit by unit, to a strict specification. If, by studying their structure and method of production, we can learn to copy the design principles used in nature, then the scope for producing new, smarter materials outside the cell is immense.
A particular spider of mass 50 mg can spin dragline silk with a tensile strength of 1.2 GPa ± 0.2 GPa. This compares to 0.55 GPa ± 0.05 GPa for a fibre of nylon 6 (a common form of nylon). The tensile strength is the force per unit area needed to break the fibre.
(a) What diameter must the dragline exceed to ensure that it can support the weight of the spider?
(b) What diameter fibre would be necessary to support a 60 kg human? What is the comparable value for nylon 6?
(c) Spider silk and nylon 6 have similar densities of about 1200 kg m−3, while that of steel is 7800 kg m−3. Assuming the steel to have the same tensile breaking stress as the spider silk, calculate the mass per metre of a fibre of sufficient diameter to support the 60 kg human, for all three materials.
(a) Use the bottom-of-the-range value of the tensile strength to find the minimum diameter, i.e. 1.0 GPa for dragline silk, 0.5 GPa for nylon.
We know that the tensile strength is the force per unit area needed to break the fibre; thus:
where r is the minimum radius, in m.
Therefore, at breaking point:
The minimum diameter of the dragline that will support the weight of the spider is 0.78 µm.
(b) Using the same method as in (a), but with the weight equal to (60 × 9.8) N, gives values for the minimum diameter required to support a 60 kg human of 0.86 mm for dragline silk and 1.2 mm for nylon 6.
(c) We know that:
The volume of a metre of fibre of radius r is equal to . For dragline silk, using the value for r calculated in part (b):
The equivalent value for nylon 6 is 1.4 g m−1, and for steel 4.5 g m−1
Several of the materials in Figure 1, including collagen, keratin and silk, are proteins. Proteins fulfil many important roles in cells and make up much of their mass. You probably think of proteins in terms of foodstuffs like eggs, fish and meat; in fact, looking at the contents of the food that we eat gives a pretty good guide to the range of materials found in cells. For example, a chicken curry from my fridge lists the following composition (by mass): 15% protein, 9% fat, 5% carbohydrate, 0.8% salt (and the rest mainly water). Compare this to the chemical composition of a typical mammalian cell (shown in Table 1) and you'll see that they are remarkably similar.
Table 1 Chemical composition of a typical mammalian cell
|Compound||Mass / % of total|
|Lipids (fats and oils)||5|
|Small molecules and ions||4|
Despite their complexity, living cells are constructed almost entirely from four basic classes of chemical compound:
lipids, which are used for energy storage and to form cell membranes
proteins, which fulfil a very wide range of functions from structural materials to nanoscale motors
sugars and polysaccharides, which are important for energy storage and as structural materials in plants
nucleic acids, DNA and RNA, which although less prolific are hugely important as they allow information to be stored and read within cells.
In this unit, I'll introduce lipids and proteins as the materials that give structure to cells.