Emeritus scientist develops novel dosimeter through enamel research
June 03, 2016
— Dr. Larry Chow has been granted five patents and has written 11 articles for scientific publications in the last three years.
It’s a laudable haul, especially considering that three years ago, Dr. Chow officially retired.
But he still comes into the office most days at the ADA Foundation Volpe Research Center on the campus of the National Institute of Standards and Technology, working on research that ranges from bioactive nanomaterials to caries-preventing chewing gum.
One of Emeritus Chief Research Scientist Dr. Chow’s latest research projects should draw significant attention once it gains patent approval: a novel new dosimeter that measures radiation exposure over long periods of time. He calls it an improvement over currently used film badge dosimeters.
“Current film badge dosimeters can only be used once because the film needs to be developed to determine the radiation exposure,” Dr. Chow said. “In contrast, the CHA (carbonated hydroxyapatite) cement-based dosimeters are simple and inexpensive devices that can be used for an indefinite period.”
Periodic EPR (electron paramagnetic resonance) spectroscopy measurements, which do not destroy the dosimeter, Dr. Chow said, will log the cumulative exposure at various time points. “Other dosimeters that have the same functionality are either significantly more complex, expensive and fragile in more extreme environments.”
Therefore, he said, “We expect the CHA cement dosimeters will find wide biomedical and industrial applications.”
Dr. Thomas Hart, senior director of the VRC, agreed. “This has many broad applications,” he said. “This is an exciting area.”
The development of the dosimeter — which received notable contributions from Dr. Shozo Takagi of VRC and Dr. Marc Desrosiers of NIST — is an offshoot of research done over several decades on calcium phosphate cements. These cements have repaired bone defects since their approval by the FDA in 1996. CHA is the principal form of calcium phosphate found in human bone and teeth. About 65 percent of bone consists of CHA, and tooth enamel is almost 97 percent CHA.
“It has been known for some time that tooth enamel produces free radicals as a result of being exposed to ionizing radiation,” Dr. Chow explained. “The amount of the radicals is proportional to the dose of the radiation received. These radicals, which are very stable and do not disappear for decades, can be quantitatively measured by a method known as EPR. Based on this phenomenon, EPR measurements of tooth enamel are sometimes used to determine the amount of radiation absorbed by the subject.
“The mineral component of teeth and bone is an impure CHA. The carbonate ions in the CHA structure in enamel is the source of the free radicals formed by the radiation, and synthetic CHA was found to have the same property upon irradiation. Thus, synthetic CHA materials that have been exposed to known dosages of irradiation can be used as references for calibrating EPR biodosimentry instruments.
“We discovered that CHA materials prepared through a calcium phosphate cement system are significantly better for use as EPR measurements than mere CHA powders. Further, the CHA cements themselves can be used as dosimeters,” he concluded.
The dosimeter is another feather in the cap for Dr. Chow, who received his doctorate in physical chemistry at Georgetown University. He is the co-inventor of 34 issued U. S. patents and recipient of the 1998 Basic Research in Biological Mineralization Award, the highest honor the International Association for Dental Research can bestow. His research in the areas of calcium phosphate chemistry, calcium phosphate-based bone graft materials and tooth remineralization has spanned four decades since he was first employed on the NIST campus in 1969.
For more information on the ADA Foundation Volpe Research Center, visit ADAFoundation.org