Characterizing surface viscoelastic integrity of ultra-fast photo-polymerized composites: Methods development.OBJECTIVE
 Resin-Composites are now available designed for polymerization using 3 s of intense light irradiation.  The aim was to develop an experimental method to probe their surface viscoelastic integrity immediately following such rapid photo-cure via macroscopic surface indentation under constant stress as a function of time.


 Two bulk-fill composites (Ivoclar AG) were studied: Tetric PowerFill (PFill) and PowerFlow (PFlow).  Split molds were used to fabricate cylindrical {4 mm (dia) × 4 mm} paste specimens, irradiated at 23 °C at 0 mm from the top surface with a BluephasePowerCure LED-LCU, with 3 s or 5 s modes, emitting 3 and 2 W/cm2, respectively.  Post-irradiation specimens were immediately transferred to an apparatus equipped with a flat-ended indentor of 1.5 mm diameter.  14 MPa compressive stress at the indentor tip was applied centrally in < 2 min and maintained constant for 2 h.  Indentation (I) magnitudes were recorded in real-time (t), with I(t) data re-expressed as % indentation relative to the 4 mm specimen height.  After 2 h, the indentor was unloaded and indentation recovery was monitored for a further 2 h.  Further measurements were made with more conventional composites: EvoCeram Bulk Fill (ECeram) and Tetric EvoFlow Bulk Fill (EFlow).  These were irradiated for 20 s at 1.2 W/cm2.  Kinetic data were curve-fitted to exponential growth functions and key parameters analyzed by ANOVA and post-hoc tests (α = 0.05).


 I(t) plots looked initially similar to bulk creep/recovery: rapid deformation plus viscoelastic response; then, upon unloading: rapid (elastic) recovery followed by partial viscoelastic recovery.  However, unlike multiply irradiated and stored bulk-creep specimens, the present specimens were exposed to only 3 or 5 s "occlusal" irradiation; generating "hard" surfaces.  Subsequently, during the 2 h indentation, the polymer matrix network continued to harden and consolidate.  Upon initial loading, I(t) reached 2-3% indentation, depending upon the formulation.  Upon unloading at 2 h, elastic recovery was only ca.  Delayed loading for 24 h, generated I(t) plots of significantly reduced magnitude.  Most importantly, however, the I(t) plots for ECeram and EFlow, after 20 s irradiation, showed I(t) magnitudes quite comparable to the PFill and PFlow rapid-cure composites.


 SIGNIFICANCE
 Macroscopic indentation creep has been shown to be a workable procedure that can be applied to rapid-cure materials to assess their immediate surface integrity and developing viscoelastic characteristics.  The applied stress of 14 MPa was relatively severe and the indentation/recovery profiles of PowerFill materials with only 3 or 5 s irradiation demonstrated comparability with their established 20 s cure siblings, evidencing the suitability of the PowerCure system for clinical application.
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