Material of the month – Bone

Materials World magazine
3 Aug 2015

The toe bone’s connected to the foot bone, the foot bone’s connected to the heel bone – you know how it goes. Anna Ploszajski explores bone.

Bone is a natural material, synthesised by all living vertebrates to form skeletal structures that support and protect the body’s organs. It is a composite material, combining an inorganic component (70%) and an organic component (30%) to form a strong, lightweight and hard tissue. The inorganic part is mineral calcium hydroxyapatite, which provides compressional strength and rigidity, and the organic part is comprised mainly of collagen – an elastic protein that provides pliancy and overall fracture resistance. Synergistically, the composite has a high compressive strength of 179MPa, with good elasticity.

Bone is produced by cells called osteoblasts. These cells secrete collagen fibres and ground substance – an amorphous, gel-like material. The collagen fibres quickly polymerise to form collagen strands, onto which calcium and phosphate precipitate, eventually forming hydroxyapatite crystals over a period of days to weeks. 

The particular arrangement of the collagen fibres determines the mechanical strength of the bone – haphazardly organised fibres in woven bone give a mechanically weak material, although its advantage is that it can be produced quickly. Woven bone is subsequently replaced by lamellar bone, as parallel sheets of collagen form much stronger materials at a rate of 1–2 micrometers a day. The strength of lamellar bone comes from its plywood-like structure – collagen fibres run in opposite directions in alternate layers, providing torsional robustness.

There are two different structures that make up the macroscopic bone assembly. Cortical bone constitutes the outer layer – hard, smooth and white. It consists of many microscopic columns housing osteoblasts (which make bone) and osteocytes (which reabsorb bone). The interior of bone is made of cancellous bone. It is an open cell porous network, which makes a spongy mass in which bone marrow and stem cells exist, producing platelets, red blood cells and white blood cells. 

Bone serves multiple functions in vertebrates, not just to hold the body together. Every day the bone marrow produces a staggering 2.5 billion red blood cells and platelets in the human body. Bone also acts as a repository, storing minerals, growth factors and fatty acids, as well as foreign bodies, such as potentially harmful heavy metals. It regulates blood sugar, pH changes, calcium and phosphate metabolism. The ossicles are three small bones in the middle ear, responsible for our hearing.

A bone replacement 

Around 10% of the adult skeleton is replaced annually, to repair damaged bone tissue and regulate calcium homeostasis. Recurrent stress on bones occurring, for example, in weight bearing exercise, causes the bone to thicken – this remodelling is a manifestation of Wolff’s Law, which observes that bone will grow as a result of a stress, adapting to manage the loads it experiences. It occurs because of a phenomenon called mechanotransduction, where mechanical signals are converted into biochemical signals. This results in a cell response to remodel the bone and optimise the skeleton to the external loads it experiences.

The skeletal system 

Although strong and elastic, bone is not completely indestructible. Ordinarily, bone is self-healing, but in instances where large parts of the bone are missing or in complex fractures, it may be necessary to employ a bone graft. This is a natural or synthetic material with matched mechanical properties to the host bone, which aids bone growth and is eventually replaced entirely by the body with new bone. Natural bone grafts come from the patient’s own body or from a bone bank. Synthetic grafts are made from hydroxyapatite or other biocompatible materials such as tricalcium phosphate, bioglass, calcium sulphate or polymers like PMMA. These can be doped with growth factors such as strontium to increase biological activity and hasten recovery.

With old age, bone density can significantly decrease to render bones vulnerable to fracture. Individuals suffering from osteoporosis may experience breakages with only minor stresses, most commonly in areas of high load, such as the spine, hip, shoulder, wrist and forearm. It is most common in post-menopausal women due to decreased levels of oestrogen – testosterone deficiency in men has a comparable but less pronounced effect. Prevention methods include regular weight-bearing exercise, a diet rich in calcium and vitamin D, and lifestyle changes, such as giving up smoking and limiting alcohol consumption. 

The other ‘bone’ 

Bone as a raw material has been used for millennia by hunter-gatherer civilisations – the oldest known example dates back from African peoples living 1.5 million years ago and bone tools are known to have been used by Neanderthal and human antecedents. Artefacts from across the globe made from bone include hunting tools such as spear and arrow tips, fishhooks and knives and domestic products such as cutlery, pins, needles, pendants and combs. Bone was also used to fashion musical instruments – a European vulture bone flute dates back as long as 40,000 years. This coincides with the time it is believed modern humans were settling in the area, and it is possible that instruments like this aided the formation of social bonds between people, bonds which proved to be advantageous over their Neanderthal counterparts. In more recent centuries, buttons, cutlery handles, and ornaments made out of bone have made attractive and aesthetic artefacts.

Bone is a crucial ingredient in the aptly named bone china, a porcelain-like ceramic material used for making fine crockery and ornaments. Invented in 1748, by Thomas Frye at Bow Porcelain Factory, and subsequently developed by Josiah Spode in Stoke-on-Trent, bone china contains six parts bone ash, four parts china stone and 3.5 parts china clay or kaolin. The bone ash is used as a source of calcium phosphate and Frye, in East London originally developed the mixture due to its proximity to the cattle markets in the area, and therefore plentiful supply of bone. Bone ash itself is made by crushing cattle bones, followed by the removal of gelatine and calcination at 1,250°C. The ash is milled into fine particles, ready for incorporation into the ceramic mix.

The bone ash is mixed with china stone and kaolin, which supplies plasticity to the unfired product so that it can be shaped. The article is set by firing at 1,200°C. Despite the cheapness of the raw materials, the labour-intensive processing route makes bone china a luxury and expensive material. Bone china tableware is known for its translucent whiteness, high strength and resistance to chipping. These properties permit objects to be made with narrower cross-sections than alternative porcelain materials.

In the English language, bones are often used to represent the core of the self and one’s feelings – ‘chilled to the bone’, ‘close to the bone’ or even ‘I feel it in my bones’. Indeed, our skeleton represents a fundamental, central part of our bodies. Without bone, life as we know it on Earth would simply not exist, and the crucial role played by bone for the living animal becomes no less critical as a raw material after its owner is deceased.