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Various materials are implanted within the human body as a result of of medical need. Dental implants, stabilizing plates, arterial stents, and orthopedic replacements are all common examples where a material is implanted within the body.

The typical response of the immune system to any foreign body is to attack the implant. If the immune system cannot eradicate the foreign body, then it attempts to encapsulate it instead. This self-preserving attack from the immune system can lead to implant failure, and the likelihood of failure is, in part, dependent upon the bio-reactivity of the material. For example, stainless steel has been evaluated to be less reactive than more base steel alloys, and titanium has been found to be less reactive than stainless steel.

How is bio-reactivity characterized, and how is it measured over the conditions of a long term implantation?

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Chronic animal experiment. This is not the only stage of evaluation of the material, but it's a mandatory stage. Chronic means that the animal is survived after implantation for a period of time and observed. Afterwards, the animal is sacrificed and autopsied.

Humans have a strong immune system, compared to other animals. So, you would want to use an animal that also has a strong immune system. Pigs become likely candidates1. Dogs too.

Indeed, since the immune response and repair functions in the body are so complicated it is not adequate to describe the biocompatibility of a single material in relation to a single cell type or tissue. Sometimes one hears of biocompatibility testing that is a large battery of in vitro test that is used in accordance with ISO 10993 (or other similar standards) to determine if a certain material (or rather biomedical product) is biocompatible. These tests do not determine the biocompatibility of a material, but they constitute an important step towards the animal testing and finally clinical trials that will determine the biocompatibility of the material in a given application, and thus medical devices such as implants or drug delivery devices.
[exceprt from wiki]

1 Pig's also weigh roughly the same as humans.

2 Nick worked on several projects developing electronics for surgical devices. As a part of my duties, I assisted with a handful of animal experiments. However, those devices weren't of an implanted sort. Biocompatibility was not one of my direct responsibilities; the mechanical design team was responsible for it.
An actual biocompatibility expert can provide this question deeper justice. I'm writing this answer only as a fallback.

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