Well, I hope you all are not surprised too much, but I’m going to talk about composites in medicine for a few posts here. This is sort of an expansion of the sports medicine post I wrote earlier, and I’m going to try to be more all encompassing here in talking about composites in medicine. This is one of my interests since I come from a long line of medical doctors on both sides. And, again, there’s lots to cover here, so here we go.
Far and away the two most prevalent areas of medicine that use composites in a big way are actually fairly disparate fields – diagnostic imaging and surgical implants. And while these may be disparate fields, the choice of composites over some other more traditional materials nearly always remains the same. Composites are lighter weight, there is an infinite number of possibilities once you understand how they work, and in general they can mimic mother nature far more closely than traditional engineering materials can.
In the diagnostic imaging market, composites are used almost exclusively in NMR (Nuclear Magnetic Resonance) imaging machines which have become the standard for non-invasive diagnostic imaging. Composite materials are non-magnetic, and about the same magnetic density as biologic tissue, so they make an excellent platform to use to lay the patient down and take the NMR image. The table where the patient that is being imaged is laying is transparent to the highly magnetic MR machine, so the image of the patient is a true 3D representation of what is inside of them. And at high resolution, the NMR operator can see the very fine detail that is needed to detect cancer at its earlier stages. Since cancer cells show up on an NMR image because of their very different metabolic processes from normal cells, getting a good 3D image of the entire area under investigation without the shadow of the table that the patient is on makes the NMR machine a very valuable tool. So, all of the patient tables in NMR machines are made using magnetically transparent carbon fiber / epoxy composites.
The dental composites business is yet again somewhat different. As it turns out, normal dental enamel is a biological composite of a protein-based glue called Keratin (part of what makes your hair stay together) and carbonized Apatite which is a calcium phosphate mineral found in all bony tissues in animals. So, it is a composite material to start with. In the past, when people had cavities that got so bad that they had to have a root canal, the cap that was put back on was either metal or porcelain. However, recently that has changed as the dental industry has been playing around and developing a polymer based composite that mimics natural tooth enamel. The formulations that the industry has come up with are usually a silica reinforcement with a resin carrier. One of the more common resins used is Bis-GMA which is a diglycidyl ether derived from methacrylic acid and DGEBA (Di Glycydyl Ether of Bisphenol-A) which is the backbone of our good friend Bis-Phenol A based epoxy. There are other similar organics that are used as well, most of which are related to epoxy in some form. And most commonly a photo-sensitive hardener is used so all the dentist has to do is to form the basic replacement enamel the way that is needed, and then stick a UV light into the patient’s mouth to cure the resin. So, dental composites, which are one of the largest markets in the medical composites field are basically fine sand and epoxy that is colored to match your teeth, stuck into your mouth and cured to form a tooth that looks just like the original.
And, finally, I added a couple of images to this post to give me a lead into the next couple of posts in the medical composites series. First, to the left, is a schematic image of the spine where the repair that you see was developed using composites in two different ways – both as a structural support (those bolts and the shaft are composite so that the body does not reject them) and also as a repair for the flexible disk between two vertebrae.
There is also a picture of a COPV (Composite Overwrapped Pressure Vessel) to the right here to demonstrate a very widely used application of composites in providing light weight pressure vessels for medical gasses, like portable oxygen tanks for people to carry around that need oxygen. These are two completely different fields, the composite material biological implants, and the portable medical equipment made possible through the use of composites.
I am going to expand on the Composite Overwrapped Pressure Vessel business in some future posts because it is a field that has effectively matured to the point that it is overtaking monolithic metallic pressure vessels. There is even a section of the ASME Boiler and Pressure Vessel Code specifically for COPVs. But I get ahead of myself again. Stay tuned.
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