Biomaterial Implants

24 September 2019

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Materials science is what it says on the tin – the science behind materials. Researchers are striving to engineer better materials that improve efficiency, reduce cost and reduce emissions and waste...

“You can always tell when a materials scientist has done their job properly - because their work is largely invisible,” says Chris Smith, the managing editor of the Naked Scientists. In this feature article, we will consider some examples of the type of material that are placed in our bodies and consider the challenges that materials scientists have faced – and continue to face – during their research.

Biomaterials comprise one of the largest industries in the materials science world. This class of material can be placed in the body as part of a medical device for a short amount of time – like a stitch – or a much longer amount of time – like a hip replacement.

The environment within a human body poses many challenges for materials scientists. Firstly, it can contain up to 60% water, which can promote the formation of oxide layers, like rust, on metals. Second, the pH levels – a measure of how acidic something is – can vary significantly. For example, the pH level in your stomach is much more acidic than lemon juice, vinegar and carbonated drinks.

What happens when you place a penny in a glass of carbonated drink or vinegar over night? It comes out shiny and clean as a result of the acid reacting with the duller surface of the penny. This simple example highlights the requirement for specialised materials that can be exposed to these potentially low pH levels without reacting adversely.

Knee and hip implants are some of the most common joint replacements used in patients. Most of these implants contain the plastic "polyethylene", which is the same type of plastic used in plastic bags. However, the polyethylene that is used in these implants is known as ultra-high molecular weight polyethylene. “That means it's really, really big, long polymer chains that are in that material and then really closely packed together,” says Sophie Williams from the University of Leeds.

The natural hip is a ball-and-socket joint, which allows the hip to rotate around multiple angles. The ball is attached to the top of the thigh bone and sits in a socket in the pelvis – known as an acetabulum. “Both of those are covered in cartilage that's really smooth and that is lubricated well,” explains Williams. This cartilage allows hip motion to be low friction, resulting in as little wear as possible.

Osteoarthritis is the most common reason in the UK for replacing a patient’s hip. “That's when your cartilage is effectively worn away, so you don't have that lovely soft cartilage layer there. You end up with bone against bone and that starts to cause pain,” says Williams.

Natural cartilage is difficult to mimic using artificial materials. Therefore, ultra-high molecular weight polyethylene is used to reduce the amount of wear that the replacement hip undergoes during hip motion. The additional links between polymer chains make the plastic harder and more difficult to wear.

Recently, the dental industry has required material scientists to design the best materials to use in fillings, crowns and dental implants. Our teeth are exposed to a variety of acidic foods that corrode our teeth and cause plaque to form on their surface unless they are brushed frequently. Often, when a tooth has been removed, a dental implant is used to replace the missing tooth. Materials scientists have designed a dental implant that is resistant to corrosion and will remain sturdy in the mouth, even when it is exposed to the relatively high forces experienced during chewing.

Dental implants are composed of a metallic screw – which secures the implant into the gum – and a ceramic crown – which sits on top of the screw and looks like a tooth. Titanium is the most common metal that is used for the screw due to its high strength, good corrosion-resistance and ductility – so it is easy to shape into a screw. A ceramic crown is used due to their toughness and resistance to wear, as well as the aesthetic considerations since they are often white. The crown is cut into the desired shape – which is modelled using the shape of the neighbouring teeth – using CNC (Computer Numerical Control) milling, a machining process that cuts the desired shape from a block of material using rotating blades.

CNC milling is used in many applications because it has allowed complex shapes to be cut using computer-aided design. However, the process produces a lot of waste material and can introduce unwanted surface stresses in the component – which can weaken it. Materials scientists are currently developing the process of 3D printing with metals and ceramics to produce components that are just as strong and quick and cheap to produce. “Imagine you've got a vat of powdered metal and you fire a laser beam at it and the metal will melt and fuse where the laser beam is focused,” explains Zoe Laughlin, from the Institute of Making. A thin layer of powder is added and blasted with the laser beam in a repetitive process until the desired shape is obtained. This process is attractive because it reduces the amount of waste material considerably. However, since this method is in its early stages, it is still relatively expensive compared with CNC milling and requires further research.

Materials scientists face many challenges when designing a new or improved material for a given application. All materials must meet the specified requirements, for example, a replacement tooth needs to be wear-resistant when exposed to acidic foods and high forces and hip implants must be strong enough to be subjected to thousands of load cycles every day without forming cracks or wearing. Additionally, the strength of a hip implant must not exceed the strength of natural bone otherwise, the surrounding bone might wear resulting in a loose implant.

This feature has considered only a few examples of the most common biomaterial implants that are used in humans, but there are many many more. So, the next time that you use a plaster, get a tooth filling, have stitches put in or even use a tampon, think about what materials that they are made from and the incredible amounts of scientific research that must have gone into them.

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