UL fellow plays key role in bone discovery

Science Today last month described research at the University of Limerick on the widely used bone implant material, hydroxyapatite.

The report failed however to clarify the key role played in this research by post-doctoral research fellow Syed Tofail, of UL's Materials and Surface Science Institute.

The MSSI research group revealed the correct crystalline structure of hydroxyapatite, an accomplishment that had eluded scientists for some decades. More importantly Tofail and colleague, computational chemist Donncha Haverty confirmed that the valuable bone and dental implant material, worth €2.1bn globally, should display a piezoelectric effect similar to that seen in natural bone.

"This new structure allows the material to have piezoelectric properties, whereas the older, previously accepted structures for hydroxyapatite did not," explains Tofail.

The project arose in 2002 when materials lecturer Ken Stanton asked Tofail to model a temperature driven phase change in hydroxyapatite.

Tofail matched his background in metallurgy and physics with Haverty's computational expertise to produce a team with the skills to achieve this.

"We tried a few different ideas to approach the problem," says Tofail. "We spent the whole summer studying how we would do this phase transition study."

Haverty quickly recognised the flaw in existing models of the calcium phosphate material. The assumed structure was wrong, says Tofail. "The hydroxyl ions in that structure were too close."

As they came up with alternative structures they found other researchers had already been there. "There was a good deal of frustration," he says. "There were many false starts."

The following summer had them examining the structure in greater detail.

They were trying to assess its symmetry, looking at monoclinic (with three unequal axes of which one is at right angles to the other two) versus hexagonal symmetry.

"If you construct a structure with hydroxyls organised in the same direction, you get a polar structure," he says, in other words one with an electrical bias.

This got Tofail thinking about piezoelectric effects, the ability to produce an electric signal from the application of mechanical stress. It is well known that polar crystals frequently deliver a piezoelectric effect. Natural bone also has piezoelectric characteristics.

The MSSI team acquired pure hydroxyapatite crystals from the US, carrying out quantum mechanical simulations of the crystal structure of hydroxyapatite and X-ray crystallography in an effort to learn its true structure. They spotted poorly explained peaks in the x-ray diffraction pattern and were able to put forward a new correct crystal structure for the material. "We were very lucky that on our first go there were some diffuse peaks there," Tofail says.

The original theory held that the material had a mixed structure, part hexagonal and part monoclinic. They showed that in fact it is all monoclinic, with 78 per cent polar and 22 per cent non-polar monoclinic.

This strongly polar structure supports piezoelectricity, whereas earlier suggested structures did not reveal its polar nature, says Tofail. "That is why hydroxyapatite is bioactive, it is piezoelectric."

The team, including Haverty, Tofail, Stanton and Haverty's and Tofail's research mentor, Seamus McMonagle, published their findings this month in the journal Physical Review B.