In-Situ Measurement of Local Shrinkage Strains in Photo-Polymerised RBC's

Objectives: Fibre Bragg grating (FBG) strain sensors are commonly used in biomechanical applications because of their resolution, biocompatibility, and small size. The study objective was to investigate the suitability and repeatability of FBG sensors in studying the volumetric shrinkage behavior of resin-based composites (RBC’s) during and following photo-polymerisation.
Methods: Preliminary experiments investigated the efficiency of strain transfer to the FBG sensors. Controlled volumes of Dentsply SureFil SDR Flow+ were photo-polymerised with lubricated fibres (n=3), silanated fibres (n=5) and native fibres (n=9) inserted (providing enhanced, reduced, and unaffected intimacy between RBC and fibre, respectively). Following characterisation of strain transfer to the FBG, simulated Class II cavities were manufactured out of 6061 aluminum at wall thicknesses of 2 and 4mm. The same RBC was used and a total of n=6 trials were conducted for each wall thickness. Using a manufactured jig to ensure consistency of irradiance parameters, photo-polymerisation was initiated using two periods of 20s of illumination with 15s of rest. Two FBGs were placed at the middle and bottom, and wavelength data were recorded at a frequency of 2.5kHz. Data were then averaged across the trials and coefficient of variance calculated to observe repeatability.
Results: Findings from preliminary testing showed no difference between using bond enhancement over the native fibre for strain measurements. In cavity simulation experiments, average strains were found to be -275µstrain (bottom, 2mm), -581µstrain (middle, 2mm), -319µstrain (bottom, 4mm), and -687µstrain (middle, 4mm) with coefficient of variance values all below 0.4.
Conclusions: Preliminary work showed that using the native fibre was sufficient for efficient strain transfer from the RBC material to the fibre. Cavity simulations resulted in expected and repeatable results at the interface between the RBC and cavity wall, and were consistent with known assumptions regarding cavity wall compliance and shrinkage strain generation.