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Organogelators and their application in bioactive dental composites

Elizabeth A. Wilder and Joseph M. Antonucci

Polymers Division, Biomaterials Group

Concerns regarding the use of amalgam fillings in restorative dentistry have spurred an interest in alternative polymeric dental materials such as bioactive composites. However, current polymeric dental composites are inadequate for use in posterior dental restorations where they are subject to large stresses, and efforts to create remineralizing dental materials with wide utility have been thwarted by the inherent fragility of recently developed bioactive composites. Accordingly, the development of bioactive dental composites with sufficient mechanical strength remains a challenge to the field of dental materials research.

Dibenzylidene sorbitol (DBS) is an organic molecule capable of inducing physical gelation in a wide variety of organic solvents and polymer melts by forming a rigid three-dimensional network. While numerous studies have looked at the effect of DBS on the rheological properties of various polymer melts, little is known about the impact of DBS on the polymerization of monomers. Recent efforts have sought to determine the effect of DBS on common dental monomers such as ethoxylated bisphenol-A dimethacrylate (EBPADMA) and its composites with amorphous calcium phosphate as a bioactive filler. DBS, a crystalline powder at room temperature, was dissolved into the dental monomer where it was found to self-assemble to promote gelation. The gelled monomers were photoactivated and used as is or mixed with amorphous calcium phosphate. The materials were cured with visible light and the resultant polymers were characterized. FT-IR spectroscopy was employed to measure monomer conversion and a computer-controlled universal testing machine was used to measure the mechanical properties of the dental composites.

Results suggest that incorporation of DBS into dental monomers results in an increase in both conversion and mechanical strength. For the unfilled EBPADMA, adding 10 % by mass fraction DBS increases the conversion from roughly 75 % to 85 %, while increases for the filled systems were more modest. Biaxial flexural strength (BFS) tests indicate that the addition of 5 % DBS increases the biaxial flexural strength from 60 MPa to 80 MPa. Complementary fracture analyses show that DBS may improve filler dispersion and reduce the number and size of pores introduced during polymerization. These results suggest that DBS may be a useful additive for improving the structural integrity of polymeric dental composites.

Other efforts aimed at synthesizing DBS derivatives with a variety of functional groups also show promise. For example, DBS was reacted with isocyanatoethyl methacrylate (IEM) with the intention of substituting IEM groups for the hydroxyl groups normally found on DBS. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) spectroscopy indicated that the product consisted of trace amounts of unreacted DBS, and more substantial amounts of the mono- and di-substituted DBS, indicative of conversion of the hydroxyl groups to urethane groups. Such functionalized gelators are expected to lead to more stable, self-assembled network structures.

Elizabeth Wilder

Polymers Division

Biomaterials Group

Room B118 Building 224

Mail Stop 8545

(301) 975-6786 (phone)

(301) 975-4977 (fax)

elizabeth.wilder@nist.gov

Materials/Biotechnology