Evaluation of the effect of water-uptake on the impedance of dental resins

Electrical impedance spectroscopy (EIS) offers a quantitative method of measuring the stability of resin films in aqueous solution over time. Purpose: The purpose of this study was to measure the EIS of five experimental dental adhesive films (ca. 17 μm thick) of increasing hydrophilicity (ranked by their Hoy's solubility parameters), and how much these values change over 3 weeks in aqueous buffer. Methods: The resin films were placed in a U-shaped chamber and a pair of Ag-AgCl electrodes was used for EIS. The EIS results were confirmed by immersing the films in 50% AgNO₃ for 24 h to trace the distribution of any water absorption into the resins by TEM observations. Results: The resistance (R_r) of the resins 1-4 films increased most during the first day, and varied from 1×10¹¹ ohm for resin 1, to 40 Ω for resin 5 at day 1. The day 1 R_r values of resins 1-4 were inversely proportional to their Hoy's solubility parameter for hydrogen bonding forces. Electrical impedance values of resins 1-3 and 5 varied widely but were relatively constant over time, while those of resin 4 decreased more than 99% from day 1 to 21 (p<0.05). Capacitance (C_r) of films of resins 1-4 all increased over the first day and then were relatively unchanged over the 20 days (except for resin 4 that continued to increase) and were between 0.01 and 1 nF. Silver uptake by TEM revealed the development of water-filled branching structures that formed in resins 4 and 5 over time.