“Oral health is an integrated part of the public wellbeing, and does not only affect the quality of life, but also the healthcare system through related economic costs. Despite great global progress in oral health related issues, dental caries is still a major problem that affects both children and adults. Indeed, dental restorative work in industrialized countries is very costly, thus developing and improving restorative materials can beneficially impact the public health system."
Marcella C Berg, Niels Bohr Institute, University of Copenhagen and the European Spallation Source ESS.
Despite their widespread use we spare little thought to the materials that fill our teeth. Advanced dental restorative materials, such as biologically active glass ionomer cements (GICs), possess a range of beneficial properties. These materials not only expand and contract just like real teeth but they chemically bond to enamel and dentin in the tooth. In addition, glass ionomer cements release fluoride, which can prevent further tooth decay.
There is an ongoing need to improve the strength and longevity of glass ionomer cements. In their ACS Applied Materials & Interfaces paper researchers found neutrons were the perfect tool to investigate this important material. The team conducted experiments at not one but four different neutron sources around the world, including ISIS Neutron and Muon Source.
Making a dental restorative material
Glass ionomer cements are made from a glass powder, formed of different cations, combined with an aqueous solution of polyacrylic acid (PAA). Water is needed to initially set the cement and to hydrate the cement matrix allowing it to react further. In water PAAs dissociate, leaving a main PAA chain with negative charges and ionized hydrogen ions in the bulk solution. Previous investigations have shown that these distinct populations of water influence the properties of glass ionomer cements.
Just like ordinary cement, the structure and hence long term strength of dental cement evolves with time, and is directly linked to liquid mobility and liquid confinement. In their investigation researchers studied how the mobility of aqueous PAA solution changes when confined in the cement matrix.
A global neutron study
Data was collected using four neutron instruments: VISION at Oak Ridge USA, PELICAN at ANSTO Australia, IN16B at ILL France and IRIS at ISIS Neutron and Muon Source UK, utilising both inelastic and quasielastic neutron scattering (QENS). Neutrons are the perfect tool for this study as they scatter strongly from hydrogenous materials and can penetrate deep into materials without altering their composition.
This research was the first to fully map hydrogen dynamics in the aqueous PAA solution to its behaviour when confined in the cement matrix.
“By exploring that the system is dynamically heterogeneous and thereby that different types of hydrogen motions can be superimposed independently we were able to separate distinct motions within the liquid and in the GIC's. The QENS analysis revealed that the self-diffusion translational motion identified in the liquid is also visible in the GIC. However, as a result of the formation of the cement matrix and its setting, both translational diffusion and residence time differed from the PAA solution. When comparing the local diffusion obtained for the selected GIC, the only noticeable difference was observed for the slow dynamics associated to the polymer chain originating from PAA solution. Additionally, over short-term ageing, progressive water binding to the polymer chain occurred in one of the investigated GIC. Finally, further analysis showed a considerable change in the density of the GIC without progressive water binding indicates an increased polymer crosslinking."
Marcella C Berg, Niels Bohr Institute, University of Copenhagen and the European Spallation Source ESS.
These results suggested that accurate and deep understanding of water-polymer binding, polymer cross-linking, as well as material density changes that occur during the glass ionomer cement maturing process are necessary for the development of advanced dental restorative materials.
In spite of great advancements in our oral health, tooth decay continues to impact a large portion of the population. Whilst neutron scattering won't cure your fear of the dentist, studies like this could help improve the strength and durability of the dental materials so many of us reply upon.
Further information
Marcella C. Berg et al. Nanoscale Mobility of Aqueous Polyacrylic Acid in Dental Restorative Cements ACS Applied Materials & Interfaces 2018 10 (12), 9904-9915 DOI: 10.1021/acsami.7b15735
Further information on our backscattering spectrometer can be found on the IRIS webpage.
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