Specimen preparation
dentine
Human molars were selected from a biobank comprised of teeth extracted by dental practitioners in Switzerland. Before the extraction, the patients were informed about the use of their teeth for research purposes and their oral consent was retrieved. Because of using pooled teeth, the local ethics committee categorized the samples as “irreversibly anonymised”, and no previous ethical approval was necessary. The present experiment was carried out in conformity with the approved guidelines and regulations of the local ethical committee (Kantonale Ethikkommission: KEK).A total of 270 dentine squares (4x4mm) from the roots of human molars were embedded in acrylic resin (Paladur, Heraeus Kulzer GmbH). All specimens were ground flat with abrasive silicon carbide paper discs of decreasing grain size (18.3 μm to 5 μm) and polished with 3 μm grain size diamond paste (DPStick P, Struers, for 60 s) under constant tap water cooling (TegraPol-15, Struers). The specimens were rinsed with deionized water in an ultrasound bath (60 s) between each step. They were prepared shortly before the beginning of the experiment and kept in tap water at 4°C. Right before start of the experiment, final polishing was performed with 1 μm grain size diamond pastes (DP-Stick P, Struers, for 60s) under constant cooling (TegraPol-15, Struers) and rinsed with deionized water in an ultrasound bath for 60 s. An initial curvature measurement with an optical profilometer (MicroProf 100, FRT the art of metrology) was performed. Then, two thirds of the surface of each dentine specimen was protected with a tape to obtain two reference areas, while the middle third (width about 1.5mm) was let exposed to the treatments.