CE08 - Matériaux métalliques et inorganiques et procédés associés

StrUcturiNg and SETting processes of mineral cements for bone repair – SUN7

SUN7: Shedding light on the structuring and setting processes of mineral cements for bone repair

Minimally invasive surgery procedures for bone grafting are needed but they require biomaterials in an injectable viscous state for the injection, which then turn into a solid. This transition, known as setting reaction, has to be precisely controlled. During the setting process, the chemical, microstructural, and mechanical properties of the paste drastically evolve. SUN7 aims at investigating all of these aspects to achieve a thorough understanding of setting processes and their kinetics.

Systemic approach of the setting process: investigation of evolutive phenomena in complex conditions and development of quantitative characterization means, resolved both in time and in space.

SUN7 is a fundamental research project dedicated to Material Science. It aims at developing advanced experimental methods, characterization tools and protocols for the analysis of experimental data to in situ monitor and quantify the setting process of hydraulic binders. This setting process is experienced via a liquid-to-solid transition, which occurs in mineral pastes with various fields of applications: for instance, cements and plasters that are used for construction or as biomedical devices, but also for fabrication and 3D impression of ceramics.<br />Quantitative and complementary approaches are developed to assess et multiple length scales physico-chemical aspects (setting reaction, chemical evolution of the paste, and reaction rate), microstructural features (dissolution of reactive particles, nucleation, growth and entanglement of crystals) and rheological and mechanical properties (liquid-to-solid-transition). One of the objectives of SUN7 is to develop these protocols to characterize the evolution of mineral pastes in situ and in complex environment (humidity, temperature…), which requires a good enough temporal resolution to continuously monitor evolutive processes. It will also be necessary to quantify the possible influence of the experimental protocols on the setting processes and their kinetics.<br />The second objective is to benefit from the global approach carried out in the frame of the project to bring a comprehensive knowledge of the setting phenomenon, and of factors, which impact its kinetics. This will be made possible thanks to the coupling of experimental methods and the correlation of results obtained in different setting conditions, with various techniques (multi-physic approach), at the surface and within the bulk of materials and at multiple length scales.<br />Among all setting processes, a specific interest will be dedicated to the setting and hardening of biomedical cements for bone repair. The knowledge gained thanks to SUN7 will help improving the biomaterials, which are currently commercialized.

SUN7 is based on a multi-physics and multiscale innovative approach. Experimental methods will be developed to investigate:
- the physico-chemical evolution of setting pastes (X-Ray Diffraction, FT-IR spectrophotometry, calorimetry,…);
- the microstructural changes occurring at the surface and within the bulk of the setting pastes (Scanning Electron Microscopy, X-Ray microtomography…);
- the liquid-to-solid transition by assessing the evolution of rheological and mechanical properties (Oscillatory Shear Response, Dynamic Mechanical Analysis, Compression tests,…)
For all these aspects, a special care will be provided to develop experimental protocols in complex and specific environment (humidity, temperature) to assess in situ the setting process or in conditions as close as possible from the operating environments.
Another important aspect of SUN7 is the 3D monitoring of microstructural features of a setting paste, using electron and X-ray tomographies. This characterization in 4D (3 spatial + 1 time-related dimensions) will bring an unprecedented understanding of crystal growth and entanglements and of the formation of porosities during the setting process.

To start with, experimental protocols have been developed to monitor the setting process of gypsum plaster (based on calcium sulfate). 3 formulations of plaster pastes have been optimized to get adequate for the monitoring of their setting processes: optimization of the liquid-to-powder weight ratio used to prepare the paste (L/P=0.6, close to the ratio used in operando) and optimization of 3 formulations which set more or less rapidly, thanks to the use of citric acid as a retardant. Indeed, when no retardant is used, the paste sets very rapidly (“knife setting time” around 4.5 minutes), which is in some cases too quick to be assessed by in situ experimental approaches (both because of the time required to get ready before being able to launch the acquisition, and because of the temporal resolution between two consecutive measurements).
A thorough characterization of the initial powder of calcium sulfate hemihydrate has been carried out (physico-chemical composition, microscopic observations…).
To help correlating the results obtained thanks to various characterization methods, it is necessary to get reference samples, for which the setting process has been stopped at different stages. As for now, we managed to develop an efficient protocol to stop the setting phenomenon by grinding the setting paste in absolute ethanol. The as-obtained powder is then filtrated to be characterized with all experimental techniques used for the in situ investigations. However, this protocol is destructive and work is on-going to manage to completely stop the setting process without grinding the samples: keeping the samples intact will enable us to carry out mechanical tests (compression tests for instance) and microstructural analyses (X-ray microtomography) on samples at different stages of the setting process.
In situ experimental protocols have been developed and tested on the 3 different formulations of gypsum plaster, monitored by X-Ray Diffraction, FT-IR spectrophotometry, calorimetry, rheology and Scanning Electron Microscopy (thanks to the use of specific cells). For all these techniques, the developed protocols (both for the continuous measurement of the evolving properties and for data analysis) were proven to be reproducible. It is indeed important to acquire quantitative results (for instance by developing image analysis protocols in the case of microstructural investigations). In all cases, the setting kinetics is characterized by a sigmoid evolution. Sigmoid curves will be modelled in the weeks to come to correlate the physico-chemical, microstructural and mechanical evolutions.

Current prospects:
1 – the investigation of the setting process of gypsum plaster is still on-going. The characterization of reference samples (setting process stopped) will help us to correlate setting kinetics obtained thanks to various characterization methods (multiscale and multi-physical approaches) to bring a comprehensive knowledge of the setting phenomenon.
2 – in situ protocols developed to monitor the setting process of gypsum plaster will be transferred to other types of hydraulic binders, and more particularly to study the setting processes of biomedical cements for bone regeneration.

Scientific articles are currently being written: stay tuned!

SUN7 is a fundamental research project, dedicated to Material Science. It aims at developing advanced experimental methods and characterization tools to monitor in-situ the phenomena occurring during setting and hardening of calcium phosphate cements for bone repair. This liquid-tosolid transition is indeed the key-point for successful biomedical applications, since it governs all functional properties, including the ease of handling by surgeons and mechanical and biological properties of set cements. In this framework, we intend to precisely characterize the setting process at different scales and with complementary quantitative methods. This global approach will permit to assess the chemical evolution and its reaction rate, the structuring of the paste, as well as the progression of its rheological and mechanical properties. Another important aspect of SUN7 is the 3D monitoring of microstructural features of a setting paste, using electron and X-ray tomography. This characterization in 4D (3 spatial + 1 time-related dimensions) will bring an unprecedented understanding of crystal growth and entanglements and of the formation of porosities during the setting process. Then, all results will be correlated to draw a global multiphysics and multiscale picture of setting and hardening processes. To do so, the impact of the different experimental methodologies on the setting process will be assessed; complementary in-situ approaches will be developed and, when possible, coupled. Finally, SUN7 will permit a thorough understanding of the whole process and of its kinetics. All experimental methods will first be developed and validated using gypsum plaster as a model material. Thus, this project will not only bring a comprehensive knowledge on setting of cements for bone repair, but will also be useful for a wide range of applications, including plaster and cements used in civil engineering. More generally, SUN7 will shed light on all studies related to the structuring of a mineral paste. Such a liquid-to-solid transition occurs, for instance, during the fabrication of ceramics or is sought for 3D impression. The development of advanced characterization methods in complex and specific environment (humidity, temperature) is another expected outcome of the project. Finally, the knowledge gained thanks to SUN7 will then have a strong impact to improve the biomaterials, which are currently commercialized.

Project coordinator

Madame Solène Tadier (Matériaux : Ingénierie et Science)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

MATEIS Matériaux : Ingénierie et Science

Help of the ANR 238,191 euros
Beginning and duration of the scientific project: February 2020 - 48 Months

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