Deception between Orthopedic Implants and the Human Body

In our last blog post, we discussed how nanocages were formed through manipulations of intermolecular forces. A compound that deals with intermolecular forces as well is known as hydroxyapatite. It is well-known for its applications towards its improvements to prosthetic implants. Being chemically similar to the mineral component of bones and hard tissues, it is one of the few materials classified as bioactive; this means it supports living tissue, bone ingrowth, and the direct structure and functional connection between living bone and the surface of a load-bearing artificial implant. Before we can descend further into its contributions to the world of medicine, we need to have a more in-depth understanding of the properties of hydroxyapatite.

So, what is hydroxyapatite?   

Hydroxyapatite (Ca10(PO4)6(OH)2) is a form of calcium phosphate with a broad range of applications. It can separate/purify proteins, assist bone implants, and can also be applied in drug delivery systems (like the nanocages in our previous post).

Figure 1: Unit cell of Hydroxyapatite

Part of what makes hydroxyapatite (HA) so useful in implants is that it is readily accepted by the body. The body tends to reject foreign bodies implanted into it. HA, however, is actually a major component of bone. Because the body is already used to seeing HA, it doesn’t react violently at all with it.

However, it is not so easy to create the HA coatings for marketable use. The only method commercially acceptable is plasma spraying. The process involves a number of variable intricacies, small changes to which can drastically affect the final outcome. This is especially troublesome given that, at the temperatures at which plasma spraying operates (Over 800°C) HA begins to decompose. Until other, more manageable methods for the synthesis of HA become commercially acceptable, the widespread use of the coating in the medical field is still a lofty goal.

How does it work?

First of all, layers of more than 20 µm of HA have to be applied to the implant. Simply coating, however, exposes the risk of bacterial infection, easy exfoliation of the coating layer, and non-homogeneous coating thickness and chemical compositions. This can be avoided through the creation of a hydrophilic surface. Promimic, a biomaterial company, developed a way to transform a surface to a super hydrophilic surface. Using a special coating procedure, they were able to produce a uniform layer of only 20 nm. Because this is so thin, it does not risk exfoliation. Furthermore, the presence of this surface produces an osteoconductive surface, and is thus used on titanium implants.

So how does HA actually work? In the late 1980’s, new synthesis procedures yielded HA in hexagonal-cross section microrods, which then could be sintered at high temperature with spheres of similar diameter to bond their structures. The functional groups of HA consist of positively charged calcium ions and negatively charged oxygen atoms. When brought near alkaline protein or acidic protein, the HA interacts with the protein. This is done through ionic and hydrogen bonding. The exchange between cation and crystal phosphates, and anion and crystal calcium ions promote these bonding and help the bioactive material serve as the connection between living bone and the artificial implant.

Ionic and hydrogen bonding are important concepts in usage of HA, because it’s imperative that the intermolecular forces between the HA and protein clusters overcome the electrostatic repulsion they feel between the cations and cations, and anions and anions. Calculations must be done in order to ensure the interaction is successful in keeping the structures together.

Applications of Hydroxyapatite

Although nanosized HA is not yet commercially available as a competitive material with respect to other forms of HA, nanosized HA shows promise for many future applications. This includes faster implant surface turnover, bone replacement, provide scaffolding properties required in tissue engineering applications, drug delivery systems like intestinal delivery of insulin, and use in genetic therapy for certain types of tumors. Some of these examples will be discussed more in depth in our next blog post.

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