Development and Application of Water-dispersible Nanogels

Objective: Nanogel polymeric particles have been employed as reactive additives to limit the polymerization shrinkage and stress in dental composite materials.  These hydrophobic nanogels have also been used to enhance the dentin bond strength of dental adhesive resins.  While both the dry and wet bulk mechanical strength of adhesive polymers was improved by nanogel addition, stability of the monomeric adhesive resin towards water-induced phase separation was decreased.  The goal is to develop a water compatible nanogel that maintains the ability to reinforce water saturated polymer networks or to serve as network precursors. 

Method: An amphiphilic nanogel was prepared by polymerization of ethoxylated bisphenol-A-dimethacrylate with ethoxylated hydroxyethyl methacrylate in the presence of mercaptoethanol.  To complete the reactive nanogel synthesis, isocyanatoethyl methacrylate was added to a fraction of the available hydroxyl groups. The nanogel was readily dispersible in water at either 10 or 50 wt% and with an added water soluble initiator, these transparent solutions were photopolymerized with reaction kinetics monitored by near-infrared.  The nanogel was also dispersed in HEMA at the same concentrations and photocured with the wet compressive modulus determined relative to the HEMA homopolymer control.

Result: The 50 wt% nanogel dispersed in water produced a flexible photopolymer construct (98% conversion) that remained clear upon water evaporation, which indicates a dense, continuous network.  The 10 wt% nanogel solution failed to produce any three-dimensional structure and exhibited limited conversion (26%), as expected for spatially isolated particles.  The wet modulus of the HEMA control was 5±3 MPa compared with either 22±8 or 28±6 MPa for the 10 and 50 wt% nanogel-modified HEMA samples, respectively.

Conclusion: Water dispersible nanogels can contribute to or even solely generate relatively strong, water-equilibrated polymer networks whether formed dry or in the presence of water.  This nano-materials approach appears well suited to the application of dental adhesives.