The role of surface science in dental research.

Dental adhesives of the future will work to maximize both physical and chemical interactions.

Modern surface science has expanded well beyond the two areas which are most responsible for its growth to date (catalysis and semiconductor processing), although these continue to be very active

fields. Some of the areas that are receiving considerable attention these days include electrochemistry, adhesion, friction, and biosurfaces. One will find these topics represented at most Surface Canada Meetings, a biannual get-together of Canadian and international surface scientists organised by the joint Surface Science Division of the CSC and CAP. The Canadian surface science community has always been particularly open to "non-traditional" applications. For example, in the area of biosurfaces, the Centre for Biomaterials at the University of Toronto has a long tradition in surface and interface science.

Many modern surface science techniques can be applied to problems of importance to the biomedical community in a straightforward manner. These include: X-ray photoelectron spectroscopy (xPs), scanning probes such as atomic force microscopy (AFM), infrared and Raman spectroscopies, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to name but a few. For example, such surface science techniques are presently playing an important role in the analysis of medical implant surfaces and in the development of surface pre-treatments that can lead to improved biocompatability.

A second example may be found in the study of the surface properties of naturally occurring materials such as bones and teeth. In Montreal, the Biomaterials Interfaces Group (BIG) was initially formed to study adhesion to human dentin. Dentin is a structurally strong composite material, composed primarily of type I collagen fibres encased inside a matrix of polycrystalline hydroxyapatite. The adhesion of polymeric resins to dentin is an important area of study, particularly with respect to the search for alternatives to mercury-containing dental amalgams.

Adhesion to dentin is the result of physical (micromechanical) and chemical interactions at the interface. Although much of the dental research community has downplayed the role of the latter to date, our group has maintained that future generations of dental adhesives will be those that optimize both types of interactions. As such, surface science techniques are required to determine the structure and chemical composition of dentin surfaces. Figure 1 shows an AFM image of human dentin after it has been demineralized in acid to remove the mineral matrix at the surface. (Acid demineralization is a common step in most commercial dentin bonding systems, being used to remove the debris left behind after mechanical cutting). The collagen fibres are easily recognized in this image by their 67 nm periodic banding perpendicular to the long axis of the fibre. These are the first published AFM images of collagen in dentin, and they permit us to identify the effects that these acid treatments have on the fibrilar structure. Acid demineralization and other pretreatments are further being monitored using photoacoustic Fourier transform infrared spectroscopy. Such multi-technique studies are aimed at optimizing the structure and chemistry ofdentin surfaces for improved bonding.

The Biomaterials Interfaces Group now represents research efforts at four different Montreal research institutions, and has expanded its scope to include composite materials and dental implants. In all cases, a surface chemistry approach is being used with the goal of developing an understanding of biomaterial surfaces and interfaces at the molecular level. It has been exciting to see how a training in ultrahigh vacuum (UHV) surface science can be applied to complex biological systems. The transition would have been impossible without the active role played by Ivan Stangel of the Faculty of Dentistry at McGill University. Our group feels fortunate to have been able to form a good quality between surface science and dental research.

Thomas Ellis is a Professor of Chemistry at Universite de Montreal, Montreal, QC. Other members of BIG include Ed Sacher (Ecole Polytechnique), Ivan Stangel (McGill) and Erik Kruus (UQAM).