In Vivo Materials Testing

Clearly one can implant microtensile specimens in animals such as rats or rabbits for several years, characterizing the materials as a function of periodic explant time. However, thin samples tend to curl or fold in vivo, affecting the reliability of the test. This can be dealt with by using thicker samples, but the rate of measurable degradation decreases as the thickness increases. Thus, we have abandoned this form of testing since it rarely gives the necessary answers in a reasonable implant time. It is more appropriate to implant the devices or subassemblies and remove characterization samples from them. 

Mineralization is a phenomenon where the body deposits calcium salts, such as hydroxy apatite on/within the device (intrinsic) or in the fibrotic encapsulation (extrinsic). Note that all implanted devices have been reported to be subject to mineralization, regardless of the materials used. At present there is really no good, predictive accelerated test method, although implant in adolescent rats appears to be about as good as is available. Explants are evaluated by SEM and EDS for calcium deposition. 

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To this day, we still hear people claim that in vitro testing of materials alone shows that they are suitable for use in chronically implanted devices. Others continue to say that I proved the materials are biocompatible and biostable, so I don t have to do any device testing. This statement can be very far from the truth. In vitro testing has its place, primarily to screen materials and processes for further testing. In some cases where no suitable in vitro test exists, one may be forced to develop accelerated in vivo materials tests. Once the preliminary testing is accomplished, however, one must test the device per se in animals. A biocompatible material does not necessarily make a biocompatible device. The same may be said about biostability. These statements are true because shape, size, surface finish, interactions between the materials in the device, etc., all can affect its biocompatibility and biostability. But even well-performed animal studies may not unveil previously unknown mechanisms, because animals do not perfectly mimic the human in vivo environment. An excellent example of this is the subclavian crush in humans (clamping a lead between the clavicle and first rib), which is impossible to discover in animals with no clavicles. With the right protocol for the device in question, only postmarket surveillance appropriate for the device in question can determine for certain that the device does or does not meet expectations. 

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