Hierarchical structures
The unique mechanical properties and behaviour of natural materials are a direct result of complex structures that are present from the molecular level. In nature the same materials are used in many different applications to produce an astounding range of structures and functions. The study of such systems has revealed much that could be applicable to modern engineering designs.
Several common characteristics of hierarchal materials include the recurring presence of bioelastomers (like collagen) and complex arrangements that result in non-linear mechanical behaviour. Furthermore many such structures are extremely durable and have regenerative abilities. This is because nature has a very limited range of materials to work with and so must optimise the use materials.
At present the creation of synthetic hierarchical materials is at a very early stage, partly due to lack of understanding of these structures but also due to technological constraints. However with the recent development of nanotechnology it is only a matter of time before these obstacles are overcome with the promise of high performance structures including low friction bearings, wear-resistant joints, composites and biomedical materials.
Tendons
Tendons are tough cords mainly composed of closely packed collagen fibres that attach muscles to internal structures such as bones or other muscles the purpose of the tendon is to enable the power of the muscle to transfer over a distance.
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The structure of tendons is highly hierarchal and display complex organization
on all levels that determines the mechanical behaviour. As a result it has a complicated
nonlinear, viscoelastic mechanical behaviour. Stretched to small amounts tendon
is easily deformed however if the stress grows the stress strain curve bends upwards
and finally the fibres show linear stress strain characteristics. Tendons are also
able to adapt to changes in their environment due to injury, disease or exercise.
This J-shaped stress-strain curve, so common in biomaterials is discussed by Gordon and Vincent and is known to contribute to the remarkable toughness of such materials.
The largest structure in the above diagram is the tendon that is then split in to smaller entities called fascicles. The fascicles contain the basic fibril and the fibroblasts that are the biological cells that produce the tendon. It can be seen that at this level the fibrils contain crimps. These contribute significantly to the non-linear stress strain relationship of tendons and basically made of collagen. It is easier to stretch out the crimp of the collagen fibrils. This part of the stress strain curve shows a relatively low stiffness as the collagen fibrils become un-crimped and this is then followed by the collagen fibril back bone itself is being stretched, which gives rise to a stiffer material.
The human skin can be considered to be one of the most amazing materials
on the planet with an immense amount of lessons that can be learned and applied
to engineering design. As with tendons the structure of skin is extremely complex
yet consists of only a few bioelastomers. The main function of skin is to hold
us together and stop our bodily fluids from seeping out. It also provides a
protective barrier against germs and guards against general wear and tear. It
is the secondary functions of the skin that display amazing properties that
are of interest to engineers. Firstly skin is exceptionally good at adapting
to external influences and has devised ways to change with the changing environment.
Take for example the changing levels in pigments in the skin depending on the
level of exposure to solar radiation or the varying toughness of skin on areas
such as the sole of the feet. At the same time skin works like a biological
temperature regulating device able to not only measure the surrounding temperature
but adapt in order to maintain a constant body heat. When it is too cold the
hairs stand up to provide extra insulation and if it is hot the skin will secrete
bodily fluids in the form of sweat. Another fascinating characteristic of skin
is its ability to heal. If the surface of the skin is damaged it will quickly
react and begin to repair the wound. It is also waterproof.
The composition of skin has two different layers called the epidermis and the dermis as seen in the diagram below. The outermost layer is called the epidermis and is made up of squamous cells with an underlying membrane. The dermis lies below the epidermis and contains a number of complex structures including blood vessels, nerves, hair follicles, smooth muscle, glands and lymphatic tissue. The main constituents of skin are collagen and elastin that give it its strong yet stretchable characteristics.
Skin is made of many thin sheets of layers of flat, stacked
cells that are formed through mitosis
at the innermost layers. These then move up the strata changing shape and composition
as they become filled with keratin.
Older cells are pushed to the surface by new cells and are eventually
rubbed away. In fact, every minute as much as 40,000 dead skin cells fall from
your body! This means that an entire new layer of skin is grown each month and
around 20kg of skin is shed in a lifetime.