The Best of Both

Titanium is a very popular material for bone implants on account of many of its properties, but there are times where titanium implants can cause problems as well. Let’s take a look into just what is good and what is bad about titanium.

What’s good about titanium?

Titanium is a biocompatible material and exhibits an interesting phenomenon called osseointegration. Initially observed by Per-IngvarBrånemark where he observed whilst using a titanium implant as a channel through which a microscope can travel, to study blood flow in a rabbit bone. After a certain amount of time the bone had fused so completely with the implant that he could no longer remove it. This discovery gave scientists a glimpse at the possibilities in titanium.

Osseointegration Histology

Half Reaction

Standard Electron Potential (V)

Ti2+ + 2 e- → Ti(s)

-1.63[3]

Ti3+ + 3 e- → Ti(s)

-1.21[4]

TiO2+ + 2 H+ + 4 e- → Ti(s) +  H2O

-0.86[5]

2 TiO2(s) + 2 H+ + 2 e- → Ti2O3(s) +  H2O

-0.56[5]

Ti2+(aq)/M3+(aq)

-0.36[4]

 Solid titanium, while in the body, corrodes slightly due to a spontaneous redox reaction. The solid titanium gradually forms a thin film of titanium oxide, which becomes polarized due to the environment it is in. Increased adsorption of hydroxyl groups, lipoproteins, and glycolipids are factors that change during the exposure. This can greatly improve biocompatibility. Whether titanium can be formed into titanium oxide can be determined using electrochemistry.

As can be seen in the table above, reduction of titanium oxide to titanium has standard electron potential of -0.86V and that of synthesis of Ti2O3 from TiO2 has potential of -0.56V. If the first half reaction is flipped to  Ti(s) +  H2O→ TiO2+ + 2 H+ + 4 e-, and combined with the other half reaction, 4TiO2(s) + 4H+ + 4e- → 2Ti2O3(s) +  2H2O, a new redox reaction of Ti(s) + 4TiO2 + 2H+ → TiO2+ + 2Ti2O3(s) + H2O and standard potential of -(-0.86) + (-0.56) = 0.30V. This calculation shows that this phenomenon is indeed spontaneous and thus titanium can be used to replace parts of bones.

Another desirable property of titanium is that it is a near bioinert material, meaning it has very little interaction with the biological components of the human body. This is good because this means there are few complications that arise in terms of integration/rejection by the body of titanium implants.

What’s bad about titanium?

Now, even though titanium has great mechanical properties and a higher level of biocompatibility than other materials, it is not an ideal material for major bone replacements. TheYoung’s modulus of elasticityshows that titanium has a much greater tensile strength than hydroxyapatite, the main component of bone (as we mentioned in our last post). Because of this difference, the bone around the area of the titanium implant carries a much lighter load, and can begin to deteriorate throughstress shielding: a reduction in bone density due to a reduction in the amount of stress placed on bone (usually due to implant). Because the bone can become less dense, this region will be more prone to injury and fracture.

And, though being bioinert has its pros, on the other side of the coin, this lack of interaction also means that there is a limit to how much bone can integrate with an implant. Although osseointegration does take place to a certain extent, it does not imply that titanium will perfectly integrate into the bone. Conversely, synthetic hydroxyapatite would integrate very well with bone as it IS the main component of bone. However, synthesized hydroxyapatite has low mechanical strength, and cannot act as an implant by itself.

So why not both?

It’s funny, hydroxyapatite will integrate well with bone and won’t cause stress shielding, but it’s too mechanically weak to be used. Titanium on the other hand has great mechanical strength, but induces stress shielding and has limited integration with bone. So, why not take the best of both? In order to compensate for the weaknesses of each material individually, some researchers have looked into combining the two to form a superior composite material for bone implants.

810f.jpg
The New Jersey Regional Science Fair that Isabella Grabski attended

Isabella Grabski, currently a senior attending Bergen County Academies in Hackensack, NJ, recently won the chemistry division in the North Jersey Regional Science Fair with her project: “Low-Temperature Fabrication of Hydroxyapatite-Titanium Nanocomposites for Bone Replacements”. In it she devised a facile method of synthesization of hydroxyapatite, and incorporated it in a composite material with titanium. Unlike other more expensive methods of synthesization, Isabella’s final method “was much better because it could be carried out at low temperatures and only needed common reactants.” Although as a highschool senior it is difficult for her to continue her research, when asked about her plans for future research, she commented that “if I had more time (and more money), I’d want to explore 3D printing. Essentially, the material could be used in a 3D printer so that any bone implant of any desired specifications could be printed.”

Hydroxyapatite and titanium are able to cover for each other’s weaknesses as a composite material, making it more ideal than either alone. Furthermore, if fashioning the implants can be done easily through 3D printing, the costs of production would be much less than what they are using conventional methods. In the future, the advent of such facilitated medical processes may be more than a pretty ideal.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s