Characterization of Biomaterials
Scanning electron microscopy (SEM) and digital microscopy are useful tools for characterizing materials. The scanning electron microscope can be used to evaluate the wear properties of new generations of dental materials as they are being developed. Careful analysis of the wear process on these materials may show surface breakdown, micro-cracking, voids, localized particle fractures, tensile cracks, and smears on the wear surface, as well as changes occurring on the opposing enamel and on tooth micro-structure.
Shrinkage of restorative materials remains a major concern because it can cause residual stresses that have been associated with various clinical symptoms. Not all contraction during polymerization contributes to residual stresses.
An experimental method was developed by Dr. Antheunis Versluis and others in the MDRCBB to determine the post-gel shrinkage component (responsible for residual stress development) during polymerization using bi-axial strain gauges.
More recently, Dr Alex Fok and Dr Jianying Li developed a noncontact shrinkage strain measurement technique using digital image correlation (DIC). The new technique can provide more detailed, full-field strain distribution of dental restorative materials undergoing polymerization shrinkage.
Because repetitive loading is characteristic for the oral biomechanical environment, dental systems are subjected to fatigue. Failure mechanisms and crack propagation are studied through the measurement of progressive changes in structural compliance during fatigue loading in a dedicated servo-hydraulic MTS testing system.
The MDRCBB makes extensive use of the small rod methodology, which is appropriate to small amounts of synthesized materials for structures of tooth size dimensions. The methodology gives extensive information about failure mechanisms in brittle and visco-elastic materials. The methodology was refined by Dr. Chun-Pin Lin (currently dean of the National Taiwan University School of Dentistry) in 1993 and further developed by Dr. William Y. Cheng (Engineering Fracture Mechanics, 61: 343-353, 1998). Fracture resistance is a major requirement of dental hard tissues and their synthetic replacement. It is a major parameter in the development and optimization of new materials for clinical chairside use.
Wettability and Surface Free Energy Measurement
The Wilhelmy Balance is an instrument that measures the interaction between liquids and solids. It can be used to estimate the energy of solid surfaces - high energy surfaces are required for good adhesion and low energy surfaces may be better able to resist bacterial adhesion and plaque formation. The Wilhelmy balance is used as a tool to analyze the surface properties of newly developed materials.
Recently, Dr. Conrado Aparicio introduced a new Contact Angle Meter that measures the contact angle at which a liquid/vapor interface meets at a solid surface derived from interfacial tensions. Measuring the contact angle of a water droplet on a given surface is a quick and easy way to evaluate hydrophobicity and cleanliness. Other physical properties, such as surface energy, adhesion, wettability and repellency, may be determined.
The center has a versatile X-ray micro-computed tomography (micro-CT) system which can provide high-resolution digital imaging of the microstructures of dental biomaterials and restorations. The system is being used to nondestructively assess the quality of restorative procedures, monitor the development of artificial caries in tissues subjected to different demineralizing and remineralizing regimes, and characterize the architecture of scaffolding foam materials.