Nanoindentation to assess the effect of water uptake on different nanofilled resins-based composite mechanical properties

The purpose of this in vitro study was to evaluate the effect of water uptake on nanohardness and nanoindentation modulus measured through instrumented indentation of various nanofilled resins. 30 pressed and, disk shaped samples, characterized by a thickness of 2 mm, were prepared for each tested composite. Group 1: Admira Fusion (Voco, Germany); Group 2: Admira Fusion X-Tra (Voco, Germany); Group 3: Ceramix Universal (Dentsply, Germany); Group 4: Filtek Bulk Fill Posterior (3M, USA); Group 5: Grandioso (Voco, Germany). All prepared samples were dried at 37°C for 24 h. Then, their masses were measured, using a precision scale, before storage in artificial saliva. Samples were stored in artificial saliva for 1 day, 7 days, 14 days, 180 days and 365 days. After each storage period the weight of all samples was measured and then, each sample were tested through a Nanoindenter XP (MTS/Agilent, USA), equipped with a diamond Berkovich indenter and characterized by a theoretical force resolution of 50 nN and a theoretical displacement resolution lower than 0.01 nm. The loading-displacement (P-h) curves were analyzed by using the Oliver-Pharr method in way to obtain the nanoindentation modulus and the nanohardness. Statistical analyses was performed through the analysis of variance (ANOVA) test setting the significance level to 0.05 (or 5%). The results showed a difference in mass weight of all tested composites, but the statistical analysis did not shown any significant difference between the weight values measured during the year. In conclusion, the mechanical properties of the different dental composites, evaluated at the nanometric scale, are not affected by the storage in artificial saliva.