No match for nature? The amazing properties of bone

boneThere are some things that nature just gets right. Take bone, for example. This typically has an elastic modulus similar to concrete, but is 10 times stronger in compression and around 50 times stronger in tension. It has a compressive strength similar to stainless steel, but is three times lighter. Not only that, but as a living tissue, it can adapt to meet property requirements. Bones in the legs, such as the femur and tibia, are typically much stronger than bones found in the arm, for example. And its properties aren’t fixed: the graph below shows how bones change in behavior with age, as explored within Granta’s Human Biological Materials database. What’s more, bones adapt depending on external conditions – a constant challenge in space as bones weaken if they are not loaded (as happens in zero-gravity).

The age-dependence for a mechanical property (tensile strength) of femur compact bone (Granta’s Human Biological Materials data)

The age-dependence for a mechanical property (tensile strength) of femur compact bone (Granta’s Human Biological Materials data)

Nature’s outstanding achievements become particularly clear when we start trying to replace these design and material combinations in medical engineering applications.  In hip and knee replacements, for example, the natural materials found in the joint might wear out over 60-80 (or more) years, but are far superior to even the most up-to-date joint replacement technologies, which typically last around 15-20 years. These extraordinary capabilities of natural materials such as bone and cartilage fascinate me: so I want to ask what makes bone so successful in its role?

Bone is analogous to a composite material consisting of collagen fibers stiffened by an extremely dense filling and surrounding of calcium phosphate crystals (known as apatite), with other constituents including water, other proteins, living cells and blood vessels[i]. It forms the protective, load bearing framework of the body. In this role, bone experiences some significant loads – 3 times body weight on average during every day activities, with peak loads of 10 times body weight during walking and in excess of 20 times body weight during jumping[ii]. With the range of activities typically realized by a human, these loads can be compressive, torsional, shear, tensile and so on. It is the microstructure of bone that enables it to handle such complex forces and gives it its material properties.

So where do you start in trying to find alternative materials to use when these incredible natural materials have been damaged?  Bone’s properties, its ability to adapt to changing environments and its unique characteristics make it a very difficult material to replace. Research and innovation in this field continues to advance. Computer modeling, simulation, and the use of advanced technology are all helping. Even the humble lego brick has been brought into play: it’s used by a team at Cambridge University to support their investigations into the use of Hydroxyapatite-gelatin composites as scaffolds for bone regeneration[iii].

Despite these advances, the perfect material for a design often does not exist. Now, material selection and design must go hand in hand in order to meet the requirements for the desired application.

The key is, in fact, not to try and replicate the complete range of bone’s abilities. Instead, medical device designers focus on the most important properties for a given situation. When a bone graft is required (e.g., following the removal of a tumor) the main challenge is matching the chemistry and microstructure. Designers need to understand the biological response of the existing bone to the new material. For example, scientists at the University of London have shown[iv] how particles of calcium phosphate (the primary component of bone ash) have the ability to stimulate bone regrowth. They believe that biomaterial-based bone grafts can manipulate cell behavior – attracting stem cells and ‘growth factors’ to promote healing and the integration of the grafted tissue.

When considering joint replacements, on the other hand, the key focus is finding a material that has the appropriate performance given the specific loading condition. Age, gender, weight, lifestyle, and many other factors must also be considered. This is where a comprehensive resource such as the Human Biological Materials database can help. Materials used in implants have to be hard, tough, corrosion resistant, and bio-compatible. Popular choices include highly cross-linked polyethylene, cobalt-chromium alloy, or ceramics. The performance of the bearing surface between the device and the socket must also be considered. Unlike the natural articular cartilage, man-made material couplings experience more friction than natural joint surfaces, and therefore wear faster. Ongoing innovation in this area continues to produce better materials, improving the durability, corrosion resistance, and mechanical performance.

Bone is an amazing material, still far exceeding made-man equivalents in terms of overall performance.  But by considering the specific environment and conditions for each device, more appropriate substitutions are being made possible.  Getting this right, and pushing the boundaries of what’s possible, continues to provide an interesting challenge to medical device engineers!

 


[i] John D. Currey, Bones: Structure and Mechanics, Edition 2, Princeton University Press, 2002, ISBN 0691090963, 9780691090962

[ii] John J. Callaghan, The Adult Knee Vol 1, Lippincott Williams & Wilkins, 2003, ISBN 0781732476, 9780781732475

[iii] Strange DG, Oyen ML. Biomimetic bone-like composites fabricated through an automated alternate soaking process. Acta Biomater. 2011 Oct;7(10):3586-94. Epub 2011 Jun 21.

[iv] Yuan et al. Osteoinductive ceramics as a synthetic alternative to autologous bone grafting. Proceedings of the National Academy of Sciences, 2010; DOI: 10.1073/pnas.1003600107

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