Monomer-Solvent Phase-Separation in Contemporary One-step Self-etch Adhesives

One-step self-etch adhesives (1-SEA) have been associated with lower bonding effectiveness to tooth tissue than their multi-step versions. Objectives: To investigate that this lower bonding effectiveness must be attributed in part to phase-separation between adhesive ingredients. Methods: The micro-tensile bond strength (µTBS) of two experimental 1-SEAs, containing a carboxylate- and phosphate-based functional monomer dissolved in ethanol/water (exp-Eth) or in acetone/water (exp-Ac), of one commercial 1-SEA, iBond (Heræus-Kulzer), and of two commercial 2-SEAs, Unifil Bond (GC) and Clearfil SE (Kuraray) was determined to enamel/dentin. One additional exp. 2-SEA tested consisted of the application of exp-Eth followed by the application/curing of a non-solvent containing adhesive resin (Unifil Bond Bonding Agent, UFB). All adhesive solutions were examined for homogeneity by light-microscopy, and their interaction with enamel/dentin was imaged using Feg-SEM and TEM. Results:

µTBS exp-Eth exp -Ac exp-Eth+UFB iBond Unifil Bond Clearfil SE

Enamel 16.1±6.3^a,b*
n=33
15.4±8.6^a,b#
n=14
29.5±6.7^b
n=13
19.3±10.3^a,b*
n=12
14.5±6.0^a*
n=14
30.4±12.9^a,b
n=14

Dentin 20.2±6.2^c
n=29
20.7±4.9^c
n=14
27.6±11.5^c,d
n=11
11.1±8.6^e§
n=14
30.4±12.9^c,d
n=14
39.6±5.4^d
n=12

*1 pre-testing failure (ptf); ^#2 ptf; ^§4 ptf; Different superscript letters indicate significant difference; Kruskal-Wallis test, p<0.05.

The lower bond strength of the 1-SEAs was associated with light-microscopic observation of multiple droplets that disappeared relatively slowly with time. Interfacial analysis confirmed the entrapment of droplets within the adhesive layer, especially adjacent to the hybrid layer. Conclusion: The prompt disappearance of droplets upon application of a HEMA-containing bonding agent (exp-Eth+UFB), and upon adding a little drop of pure HEMA, as well as the absence of HEMA in the 1-SEA formulations and the absence of droplets at the interface of all HEMA-containing 2-SEAs, strongly suggest that upon evaporation of ethanol/acetone the adhesive monomers within the 1-SEAs separated from water. Water droplets were formed and got entrapped within the adhesive resin upon polymerization.