Effect of micro-/nano-hybrid hydroxyapatite rod reinforcement in composite resins on strength through thermal cycling

This study was based on the hypothesis that mechanical strength and ion release from bioactive restorative composites containing hydroxyapatite (HA) nanometer scale fillers lead to superior precipitation ability, which could facilitate load transfer, bridge, and toughening mechanisms. The prepared specimens included the respective composite resins with singular filler form of synthetic 50 wt% microrod HA (abbreviated as 50Mr-HA) and nanorod HA (designated as 50Nr-HA) and micro-/nanorod hybrid fillers with 50 wt% microrod HA mixing with 5 and 10 wt% nanorod HA (55 and 60 wt% fillers in composites), which were designated as 50M5Nr-HA and 50M10Nr-HA groups, respectively. The measured properties were as follows: flexural strength and re-precipitation after specimen immersion for 24 h and further exposure to thermal fatigue between 5 degrees C and 55 degrees C for 600 and 2,400 cycles. The groups of micro-/nanorod hybrid resins reinforced with nanorods showed a significant increase in microhardness, modulus, and the capacity to re-precipitate. The 50M5Nr-HA sample showed the highest flexural strength after thermal fatigue among the four types of fillers. The 50Nr-HA sample showed the lowest flexural bending strength and modulus. The 50Mr-HA filler showed better re-mineralization capability after immersion. However, the micro-/nanorod hybrid 50M5Nr-HA exhibited an advantage after thermal fatigue and could avoid the aggregate problem that occurred in 50Nr-HA or 50M10Nr-HA. With proper hybrid nanorods and microrod HA-reinforced composites exhibited superior potential application as for dental caries restoration. POLYM. COMPOS., 40:3703-3710, 2019. (c) 2019 Society of Plastics Engineers