Comparative Study on Adhesive Performance of Functional Monomers

Self-etch adhesives contain functional monomers that to a large extent determine their actual adhesive performance. Up to now the interaction of functional monomers with dental tissues has seldom been characterized using chemical analytical techniques. Objectives: The hypothesis tested was that the bonding mechanism of "mild" self-etch adhesives involves chemical interaction of functional monomers with hydroxyapatite (HAp) in addition to micro-mechanical hybridization. Methods: The interaction of 3 functional monomers (10-MDP, 4-MET, phenyl-P) with synthetic hydroxyapatite (HAp) was characterized using XPS and AAS. The functional monomers originate from 3 representative two-step self-etch adhesives (Clearfil SE, Unifil Bond, Clearfil Liner Bond 2), of which their interaction with dentin was ultra-morphologically characterized as well. Results: Unstained, non-demineralized TEM sections revealed that all adhesives demineralized dentin only partially, leaving HAp around collagen within the submicron hybrid layer. XPS revealed that 10-MDP readily adhered to HAp. The intensity of the C 1s peak of untreated HAp considerably increased when HAp was exposed to 10-MDP for 30 sec, while it only slightly increased for 4-MET and did not increase for phenyl-P. The intensity of the C 1s peak did not further increase when the application time was extended to 30 min for 10-MDP, while it did for 4-MET. For phenyl-P, a slightly increased C 1s peak only appeared when phenyl-P was applied for 30 min. AAS revealed that the calcium salt of 10-MDP was hardly soluble in water, indicating that following the AD-concept (Yoshida et al., 2001) 10-MDP is capable of forming a stable bond to Ca of HAp. The bonding stability of 4-MET was substantially worse, whereas phenyl-P and its bond to HAp appeared not very hydrolytically stable. Conclusion: Besides self-etching dentin, specific functional monomers have an additional chemical bonding efficacy that is expected to contribute to its adhesive performance.