How do materials react in the body?

How does the immune system contribute to the body’s response to a material?

A biomaterial is implanted during surgery, and during this procedure, bacteria can get into the open wound. The immune system deals with this infection by first identifying the foreign cells as non-self. Macrophages recognize non-self cells by well conserved markers. These are called Pathogen Associated Molecular Patterns (PAMPs). the macrophages identify these PAMPs via transmembrane surface receptors which bind to a variety of PAMPs. Once the macrophage has identified the foreign pathogen, it sends out protrusions which bind to the PAMPs, and then 'pulls' the pathogen inside the macrophage. Now that the pathogen is inside the macrophage, it is digested.

Macrophages also interact with the biomaterial. When a biomaterial is implanted, proteins bind to the surface of it. The macrophages then bind to the surface proteins and then signal more macrophages to the area. The macrophages bind across the whole biomaterial, creating a foreign body giant cell (FBGC). The FBGC sends signals out to attract fibroblasts. These bind to the FBGC in layers, which creates a granuloma around the biomaterial. This hinders the biomaterials elasticity and function. The macrophages will also release proteins which gradually degrade the implant over time.

[15] Here is an image of a macrophage engulfing tuberculosis bacteria. You can clearly see the protrusions that come out of the macrophage and attach to the virus.

How do materials interact with the blood?

As biomaterials are implanted via surgery, blood usually makes first contact with the biomaterial. There are many proteins in the blood, which will effect how cells adhere to the biomaterial. A biomaterial is usually coated in proteins, and the proteins in the blood could effect these attached proteins, so that other cells do not adhere to the biomaterial as readily. Blood also contains the white blood cells which are involved in the immune response to a biomaterial.

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How does the blood interact with a cardiac stent?

A stent is small cylindrical mesh tube which is used to widen an artery which has been narrowed by a build up of fatty plaque. This will prevent the entire artery being blocked by the fatty plaque, which could prevent a heart attack.

The stents are usually made out of stainless steel, which prevents blood cells from adhering to the surface of the mesh as it is very smooth.

There is a small risk of a blood clot forming on the surface of the stent if the blood cells get stuck or caught up on the mesh, however this is not very common (around 1 in 50 in the first year).

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[16] The image above demonstrates how a stent is implanted into the body, and you can also see the structure of the stent.