This second post in my composites in medicine series, I’m going to I intend to provide some insights in to the tremendous amount of work, both in research and clinically, using composite materials for medical implants. Some of these composites are biomaterials themselves so compatibility with human tissue can be quite good.
As it turns out, a lot of the research and application of composite materials to medical implants has moved from the traditional bone implants – hips, knees, shoulder joints, etc. – toward more soft tissue support. If you take a look at the the pic above and the one to the right, those are implantable composite devices specifically engineered to be soft tissue scaffold materials that a surgeon can use to close a wound, bridge across an open gap, and a number of other things to hold the patient together while the wound is healing. Consequently, quite a bit of work has gone in to surface modifications of these textile composites to make them so that the body not only does not reject them, but in some instances can use them and build on them. In fact, some of these devices are coated with a rich nitric oxide layer that promotes healing and bonding of the tissue to the implant.
The pic to the left here is one that I thought I should add to this, primarily because it is an artists rendering of an actual device – a graphene based implantable, replaceable trachea. The company that makes this device also makes several other customizable implants, from heart bypass grafts to tear ducts, ears, noses, etc. using a graphene scaffold. Graphene is the strongest two dimensional material known to man, and since it is a sheet of pure carbon, it is very flexible yet extremely strong.
However, in very small amounts, graphene can be either toxic or deadly to humans. The coal miner’s plague – black lung – is primarily small graphene particulate from the coal dust that gets into the miner’s lungs and starts killing lung cells. And, graphene by itself has been shown to be toxic to humans if ingested. But, if it is turned into a composite material by coating the graphene sheet with a biocompatible plastic, the body will not only accept it, but if it is done right the body can actually use the plastic and the graphene. So, given the right surface treatment, graphene scaffolding can be made to be completely biodegradable within the body.
In the pic to the right, I show some micrographs of another biocompatible composite material, only this time the reinforcement rather than being glass or carbon or graphene, is a ceramic. These pics are of a bioplastic reinforced with calcium citrate for use in orthopedic medicine. These materials are starting to overtake the traditional metal (mostly titanium) inserts for hip and knee joints for a number of reasons. First, the stiffness and strength of these materials more closely matches the native bone. This is especially true when the implant has some hollow pockets like you see in the lower picture to the right. This structure is a good mimic of cancellous bone – the porous sponge like structure that makes up the interior of your bones and makes them light weight. Metals are extremely difficult to make into these spongy structures, whereas by using porous plastics reinforced with calcium citrate crystals researchers have been able to make a pretty good match to natural bone.
Another very clear benefit to these materials is that they have a similar x-ray and ultrasonic density as natural bone, so when these patients come in after their implant has been in for long enough that the bone has healed a bit around the implant, the surgeon can see how well the bone healing is coming, because the implant is about the same brightness on an x-ray as the bone is. With a metal implant, the metal in the image is so bright that it has a tendency to wash out the fine detail of the bone at the interface between bone and implant.
Finally, what does a neurosurgeon do when they have to take away a large part of someone’s skull, or if that skull bone is damaged so much that it needs to be replaced with something. That something is increasingly a composite material. There are bio-based glass fiber reinforced plastics that can be molded into very complex shapes and that will not be rejected by the body when implanted. Facial reconstruction happens this way as well, where the surgeon can implant a custom made piece of this material that not only matches the bone but that the bone will knit with quite well.
Implantable medical devices have come quite a long way in the last 10 to 15 years, and composite materials have enabled most of the progress in this very important field of medicine. There are literally thousands of people walking around with these implants in them, living quite ordinary lives because of these advancements in medical technology.
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