Enhancing Dentin Bonding Durability Using a New Additive

Objectives: Improving the service life of dental restorations is a focus of developing next-generation dental materials. One of the major challenges is to decrease bonding failure within the adhesive layer. The objective of this study is the enhanced durability of dentin bonding by adding a new azo-type quaternary pyridinium salt (QPS), (E)-1-hexadecyl-4-((4-(methacryloyloxy)phenyl)diazenyl)-pyridinium bromide (named Azo-QPS-C16), into dental adhesives.
Methods: Azo-QPS-C16 has been added into two adhesives, an experimental adhesive containing 60wt% of 2-bis(4-(2-hydroxy-3-methacryl-oxypropoxy)-phenyl)-propane) (Bis-GMA) and 40wt% of 2-hydroxyethyl-methacrylate (HEMA), as well as a BisGMA/HEMA-based commercial adhesive, 3M™ Scotchbond™ Universal Adhesive. The degree of vinyl conversion (DC) and glass transition temperature (T_g) of adhesives with or without Azo-QPS-C16 were measured by Fourier transform infrared (FTIR) spectrometer and dynamic mechanical analyzer (DMA), respectively. The stability against enzymatic challenges of four adhesives was evaluated using a new atomic force microscopy (AFM)-based method. The adhesives’ bonding strength and bonding durability are determined and compared based on shear bond strength (SBS), micro-tensile bond strength (µTBS), and bonding durability under the challenges of thermal cycling.
Results: The enhancements in DC, T_g, and the stability against enzymatic degradation for both experimental and commercial adhesives upon adding 0.25-0.5wt% of Azo-QPS-C16 were shown by FTIR, DMA, and the AFM-based evaluation method, respectively. Both experimental and commercial adhesives with Azo-QPS-C16 achieved equivalent SBS (> 15 MPa) and µTBS (> 35 MPa) to their counterparts without Azo-QPS-C16. After 10,000 thermal cycles, both adhesives without Azo-QPS-C16 failed, while µTBS values of the two adhesives with Azo-QPS-C16 remained unchanged.
Conclusions: Azo-QPS-C16 was shown to be effective to enhance DC, T_g, and stability against enzymatic degradation in two adhesives. Furthermore, adding Azo-QPS-C16 can enhance the durability of dentin bonding without jeopardizing bonding strength. The current results suggest significant potential for further developing dental restorative materials with extended service life.