Medical Materials

Dr Ruth Cameron and Dr Serena Best from the Centre for Medical material at the University of Cambridge spoke to Chris earlier about how they are using Ceramics and polymers to help mend broken bones.

Ruth - Well what we're doing is to develop materials that are designed to replace body tissue such as bone after certain medical situations, for example if you've had a tumour or if you've had an accident and you've broken a bone, you may need to have bone replacement material to reinforce the bone you've got left.

Chris - And to help you heal up better and faster I should think?

Ruth - Exactly.

Chris - Serena, what sort of materials are you developing to make that happen?

Serena - Well we're looking at materials that are available at the moment and most of the hip implants at the moment, are made of metals and what we find is those materials are actually too stiff, so they don't flex quite as well as the bone does, so what we're trying to do is to actually have a look at the structure and the properties of bone and try and find some materials that we could replace those metals with to make them behave in a more similar manner and more chemically similar so you can get a direct bond between the bone and the implants.

Chris -   And what sorts of materials are you exploring?

Serena - We're interested in ceramics, and strangely, although ceramics make people think about tea cups and flower pots and things, the mineral part of your bone is a calcium phosphate and if you synthesis that chemically and if you then heat treat it, that will turn into a ceramic. But what we're interested in is using ceramics that are as similar as possible to the mineral component of bone, but we're also interested in using polymers and Ruth knows a lot more about the polymer side than me...

Chris -   Ruth, what sorts of polymers do you sue then?

Ruth -   There are a range of options, but one of the strategies we use is to tak polymer that would be degradable within the body, so once you get it in the body it gets wet and breaks down into its component parts and you can choose that such that it was something the body would be happy dealing with.

Chris -   So it won't irritate the body having that in there?

Ruth -   Exactly.

Chris -   And what does the breaking down?

Ruth -   Simply by getting it wet with a lot of the polymers that we use. So there's a chemical reaction that happens and the polymers, which are long string-like molecules, actually break down into much smaller molecules with that reaction and you can get the body to start to take over the function of your original implants.

Chris -   And you can control how long it takes for that to happen presumably?

Ruth -   Yes, there are parameters you can adjust and there are different time scales that are appropriate for different applications.

Chris -   And what sorts of polymers are you using?

Ruth -   One example is polylactic acid, which is lots of repeating units of lactic acid, and that gets wet slowly over weeks and months and years and that breaks down into lactic acid. Lactic acid is something the body has already, when you run very hard and get a stitch-that's a build up of lactic acid.

Chris -   So it should be very bio-compatible, the body shouldn't argue with it being there then, how are you hoping it will help the body by putting this stuff in?

Ruth -   Well the idea is that it can give you very strong, mechanical support initially but then after a period of time you're trying to stimulate the body to create its own new, natural body tissue.

Chris -   Can you, for instance, imbed cells in there and put those in or could you imbed factor that's would make cells grow more and therefore heal better in the polymers ad well?

Ruth -   Well both of those are strategies you can take. So you can put in drugs that will release slowly as the polymer degrades and as the material breaks down you're getting them in exactly the right place to be stimulating new tissue. But, the idea of tissue engineering, where you create a scaffold of degradable polymer and you put cells into it is also an idea that we use as well.

Chris -   And what sorts of body bits could you make to do this?

Ruth -   Well, often we're just replacing spaces in the body or you're just trying to approach a particular site in the body, so we're not out to make entire new organs or entire new bones, but really kind of repair situations.

Chris -   And Serena, presumably your ceramics don't get eaten away in the same way as the polymers Ruth has been talking about?

Serena -   Some of the ceramics do actually, there's a whole family of calcium phosphates which are, well your bone mineral is basically calcium phosphate an the one that's most similar to it is one called hydroxyapatite, and around that hydroxyapatite formula there are other calcium phosphates and depending on the number of calcium atoms in relation to the number of phosphorus atoms, then as you get fewer ad fewer calcium atoms the material becomes much more biodegradable, so what people sometimes want to do, just as Ruth has been describing, is to put something into the body which will serve a function and then it will disappear with time.

Other times we just want to put a material into the body, which will do its job but actually stay there with the bone having grown into it.

Chris -   So if I had a broken bone, for example, and I had  a bit of bone that was splintered and had to be removed, you could make a model bone with your ceramics, put that in, and it would act as a template for new bone growth?

Serena -   That would be an ideal situation. One of the problems that we have with these materials is that their mechanical properties are not great, so there are many ceramic materials that we know are very strong, but the calcium phosphates are a bit like chalk, they're a little bit brittle and not particularly strong in intention, and so what we're more likely to do with these materials is produce little granules and the granules are used to fill defects. For example, we use them to help people in spinal fusion (sticking two vertebrae together), and the little granules can be packed either side of the vertebrae and you might need to out some metal work in as well to hold everything in place. Or, you might use them at the end of the hole that's been created when somebody's had a hip operation, if they have to have that hip implant removed, then a big holes is left behind and sop we might pack the granules in too, to fill up those spaces.

Chris -   Is there any grounds Ruth, for combining a polymer with a ceramic and sort of mix the two technologies together?

Ruth -   Absolutely. This is where a lot of our research is going o at the moment so you can get the benefit of both worlds. As Serena has said, the ceramics tend to be very brittle on their own, it's like putting tea cup into your body, you can't expect it to be load bearing. So, if you can add the nice tough properties of a polymer to that, then you can get mechanical properties that are right, you can also get something that is resorbable over a period of time and you can get the bioactivity from choosing the right ceramic.

Chris -   So there's never been a better time to grow old then?

Ruth -   I guess so, yes.