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Bioactive materials for enhanced preventative and restorative dental solutions

Final Report Summary - BIODENSOL (Bioactive materials for enhanced preventative and restorative dental solutions)

The Biodensol project proposes three PhD studies to address the problems of (i) Caries and (ii) enamel erosion by acidic foods (leading to sensitive teeth). In both cases, encouraging re-mineralisation (the formation of new calcium phosphate) can help avoid these problems. Where restorative work is required, re-mineralisation has potential to enhance the lifetime of restorations and so further reduce the number of visits to dentists. The PhD studies will represent a big challenge to the Early Stage Researchers (ESRs) selected, since knowledge of both materials chemistry and biology (cell testing for cytotoxicity and bioactivity) will be required. This knowledge resides, respectively, at Lucideon and University of Lyon respectively and the students will spend 3 cycles of 6 months at each organisation to allow for iterations.

Whilst the powder materials being developed by each student are versatile and could be used in a number of bone / tooth scenarios, each student has an end-goal product in mind. The picture below shows in yellow the areas within a tooth structure where their product will best be used. The orange area represents an associated light-cured restoration.

The 3 target composite products, each featuring novel powder developed, are:
- Product 1) An improved bonding layer product to strengthen and retain “restoration to tooth” interaction (benefit: longer duration restoration)
- Product 2) An improved glass ionomer cement restoration
- Product 3) A restoration material for deep dentine repair, where apatite growth via cells on the pulp-side is stimulated.

In the first 18 months the ESRs have focussed largely on development and characterisation of novel inorganic powder materials. Two students explored different sol-gel routes for powder production and one worked on melted glass formulations. Key characterisation techniques are XRD, XRF, SEM, ICP, zeta potential, FTIR, Surface Area / Porosity. All students made use of Factorial Experimental Design to accelerate their efforts to generate promising powders.

Also in months 1-18, work at the University of Lyon, focussed on the performance assessment of the developed powders (both mechanically and biologically). Specific formulations (for example as part of a novel glass-ionomer cement restoration) were evaluated. The following characterizations were performed:
- DSC/TGA: To find the polymer decomposition and crystallization temperature (thermal behaviour, type of kinetics and mass lost timing).
- XRD: to analyses the phase of the samples
- BET/BJH: to analyze the surface area, pore volume and pore size
- ICP: to study ion release profile at different time points
- FTIR: to study different chemical bonds in the glass
- TEM: to determine the pore features
- Optical microscopy analysis and SEM: to study the particle morphology and apatite (structural characterizations).

In addition, the influence of the elaborated biomaterials on our cell culture model were tested; human pulpal fibroblasts were cultured in direct contact with a biomaterial used as reference. The metabolic activity of cells has been then analysed with the Alamar Blue® assay.

Significant progress has been made on sol-gel development for product 1. The desired sol-gel powders need to be amorphous and have low solubility (but high porosity) to both stimulate re-mineralisation and allow adsorption (then release) of anti-bacterial or re-mineralisation agents. This has been delivered for high Ca / low Na formulations in Si-P-Ca-Na systems, following extensive studies into the role of chemistry and sintering conditions

For product 2, glass compositions within a B-Al-Ca oxide triaxial have been melt-quenched (left-hand photos in figure below) and then subsequently heat-treated (right-hand photos in figure below) to cause phase separation. Regions of high phase separation have been identified and analysis carried out to understand the chemistry and rate of ion leaching from the chemically weaker phase. Preferred compositions are being identified with a view to achieving both initial ion release (to set ionomer cements) AND latter ion release to stimulate re-mineralisation once the composite restoration weakens.
Project 3 has proved more challenging. The aim here is to make porous soluble P-Ca-Na sol-gels whereby dissolved ions stimulate a cellular response. Additional use of Sr to assist cellular response is also being investigated. Many proposed sol-gel routes employ toxic raw materials and / or have led to problems with unwanted crystallisation, carbon entrapment and loss of porosity. Some fused glass equivalent powder have been made and will be evaluated in cell tests first. Further attempts will be made to generate porous sol-gel equivalents in year 3.

Aside from project 3, where extra sol-gel investigations are needed, the second half of the project will see less effort on powder development and more on evaluating the powders (a) for cytotoxicity, bioactivity and then (b) for mechanical and ageing properties in the final composite product. Projects 1 and 2 are on track to deliver prototype composite materials for evaluation.

Months 18-36

Project 1
Project 1 progressed well to elegantly show how Ca to Na ratio and sintering temperatures impacted on porous sol-gel powder properties. The best compositions in terms of porosity, surface area and solubility were trialled in a liquid adhesive medium. No unwanted rise in viscosity (which would hamper application) was seen. Encouraging test data in terms of re-mineralisation (fresh apatite growth following immersion in phosphate buffer solution was demonstrated. Research has progressed to the point where more applied R&D with an end-user (to take the material towards commercialization) is justified. Scaled-up production of the Si-P-Ca-Na sol-gels is perhaps the biggest barrier to commercialisation. The student on project 1, Delihta Fernando has had two publications approved and successfully defended her PhD Lyon on 2nd May.

Project 2

This project has created novel aluminium-free, borate-containing glasses that perform very well in a GIC poly-acid liquid base. Interestingly the best performing glasses appear transparent after heat-treating to cause phase separation. The transparent glasses are phase-separated however, as shown in leaching / microscopy trials. The ESR has characterized which ions are preferentially leached from different compositions. The best glasses, when dispersed in the liquid polyacid base and cured show good strength, approaching that for Fuji IX. There is much that can be done to improve performance (glass particle size; polyacid chemistry) but it is clear that borate release from the glass encourages good cure. Leaching of other ions provides the remineralization ability. Like project 1, it is felt that this work could be progressed to a commercial product. A meeting between Lucideon and Universite de Lyon at the AIDR conference in London on 26th June sketched out a possible project. It was agreed that an industrial partner needs to be involved and Universite de Lyon have since gained interest from GC Europe. A meeting (with a signed NDA) is planned for September / October 2018 in Leuven with a view to creating a final project concept for submission under the Horizon 20/20 funding stream “Fast track to Innovation”. ESR student 2, Federico Lizzi also successfully defended his thesis in Lyon (3rd May 2018).

Project 3

As reported in the mid-term report, consistent production of phosphate (P-Ca-Na) glass powders via a sol-gel route proved impossible. Instead, melt-quenched P-Ca-Na oxide glasses were prepared before milling and assessing in collagen matrices. The glass powders were dispersed in (acidic) collagen solutions and subjected to freeze drying. A porous fibrous mass was generated which gave good results in terms of cell cytotoxicity and bioactivity. ESR student 3 has experienced delays in defending his research, largely due to delays in securing an authored paper. This has now happened and the PhD defence was successfully defended on 4th October 2018.

Project website :
Coordinator: Professor Brigitte Grosgogeat, Université Claude Bernard Lyon 1
Industrial Partner: Philip Jackson, Lucideon