dental materials 25 ( 2 0 0 9 ) 188–197 available at www.sciencedirect.com journal homepage: www.intl.elsevierhealth.com/journals/dema The mechanical properties of nanofilled resin-based composites: The impact of dry and wet cyclic pre-loading on bi-axial flexure strength Andrew R. Curtis a , William M. Palin a,∗ , Garry J.P. Fleming b , Adrian C.C. Shortall a , Peter M. Marquis a a Biomaterials Unit, School of Dentistry, University of Birmingham, St. Chads Queensway, Birmingham B4 6NN, UK b Materials Science Unit, Division of Oral Biosciences, Dublin Dental School and Hospital, Trinity College Dublin, Dublin 2, Ireland article info Article history: Received 1 February 2008 Received in revised form 25 April 2008 Accepted 2 June 2008 Keywords: Cyclic fatigue Bi-axial flexure strength Nanofillers Resin-based composite abstract Objectives. To determine the influence of nano-sized filler particles and agglomerates of nanoparticles (‘nanoclusters’) in resin-based composite (RBC) materials on the bi-axial flex- ure strength (BFS) following cyclic pre-loading and storage in a ‘dry’ or ‘wet’ environment. Method. Seven commercially available RBC restoratives, Heliomolar (Ivoclar Vivadent, Schaan, Liechtenstein), Z100 MP Restorative, Filtek TM Z250, Filtek TM Supreme (3M ESPE, St. Paul, MN, USA) in Body (FSB) and Translucent (FST) shades, Grandio and Grandio Flow (VOCO, Cuxhaven, Germany), containing differing filler particle types and morphologies were inves- tigated. Specimens were pre-loaded at 20, 50 or 100 N for 2000 cycles and stored in a ‘dry’ or ‘wet’ environment prior to BFS testing. Results. A general linear model analysis of variance highlighted a reduction in the BFS following pre-loading, however, individual RBC materials responded differently. The RBCs containing agglomerated nano-sized particles or ‘nanoclusters’ (Filtek TM Supreme) demon- strated distinctive and unique patterns of response to pre-loading. Cyclic pre-loading at 20 and 50N significantly increased the Weibull modulus of both FSB (8.53 ± 1.91 and 10.23 ± 2.29) and FST (16.89 ± 3.78 and 10.91 ± 2.45) compared with FSB and FST control (no pre-cyclic load) specimens (5.98 ± 1.34 and 7.99 ± 1.78, respectively). BFS of FSB and FST was maintained or significantly increased compared with the other materials following 20 and 50 N cyclic pre-load (P < 0.05). Significance. The ‘nanoclusters’ provided a distinct reinforcing mechanism compared with the microhybrid, microfill or nanohybrid RBC systems resulting in significant improvements to the strength and reliability, irrespective of the environmental storage and testing condi- tions. Silane infiltration within interstices of the nanoclusters may modify the response to pre-loading induced stress, thereby enhancing damage tolerance and providing the potential for improved clinical performance. © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. ∗ Corresponding author. E-mail address: w.m.palin@bham.ac.uk (W.M. Palin). 0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2008.06.003