Research Highlights of Polymers Division

Current dental adhesives systems are characterized by the complexity of their compositions and bonding procedures. Bonding protocols for applying a variety of adhesive agents often are technique sensitive, labor intensive and multi-step in nature. In order to cleanse debris (smear layer) from the surfaces of prepared teeth and condition these substrates to accept adhesive agents, many dental adhesive systems use strongly acidic etchants, e.g., aqueous phosphoric acid. These aggressive conditioners have a propensity to over-etch mineralized tissues, especially dentin which is not as highly mineralized as enamel and, consequently, lacks the superior buffering capacity of the latter. Many bonding protocols require rinsing and drying of the conditioned surface. Conditioning dentin with strong acids removes the smear layer, demineralizes peritubular and intertubular dentin and creates a micro-porous surface into which adhesive agents can diffuse. However, these conditioners have the potential of also damaging subsurface dentin and reducing the strength and/or durability of the adhesive interface.

Recent studies at NIST have shown that N-phenyliminodiacetic acid (PIDAA), a unique imino diacid of moderate acidity (pH=2.5) with excellent chelation potential for calcium, can be used as a dentin conditioner. Not only does PIDAA remove the smear layer and create a micro-porous surface without excessive demineralization or the need for aqueous rinsing, it also has the ability to prime the surface for effective interfacial polymerization. Thus, PIDAA combines several functions in one adhesive agent: substrate conditioning for infiltration and surface activation for polymerization. Several, simplified, two-step dentin bonding systems based on PIDAA have been developed. However, there is a need for a self-etching primer that is effective not only with dentin but also with enamel, that is a “total” self-etching primer. PIDAA itself is only marginally effective as a self-etching primer for enamel. In a recently completed study we have shown that the addition of small quantities of strong acids such as nitric acid to PIDAA solutions results in effective “total” self-etching primers. SEM analysis of enamel treated with PIDAA primers showed that effective surface etching occurred (See Figures 1 and 2).

For assessing shear bond strength to bovine enamel and human dentin, a two-step bonding protocol was used: 1) aqueous acetone (1:1 by mass ratio) solutions of PIDAA (6.4 %) and nitric acid (NA) at two concentrations (2.5 % or 1.25 %) were applied to either bovine enamel or human dentin surfaces for either 30 s or 60 s, rinsed with distilled H2O and air dried. 2) Five coats of 20 % photoactivated PMGDM (a carboxylic acid containing monomer, pyromellitic glycerol dimethacrylate) in acetone, were then applied and light cured 20 s prior to placement of a photocurable composite (irradiation time=60 s). All % values represent mass fractions.

Shear Bond Strength (SBS) and Standard Deviation in MPa to Tooth Structure

Bovine Enamel

Human Dentin

A two-way ANOVA (á =0.05) indicated that there was no statistically significant interaction between application time and NA concentration (p=0.4 for enamel and p=0.74 for dentin).

For more information on this topic:

Antonucci JM, Bennett PA; Method and Composition for Promoting Improved Adhesion to Substrates. U.S. Patent 5,498,643, March 12, 1996; U.S. Patent 5,690,840, November 25, 1997; U.S. Patent 5,756,560, May 26, 1998.

Joseph M. Antonucci, Frederick C. Eichmiller and Gary E. Schumacher