Degree of conversion and residual monomer elution of 3D-printed, milled and self-cured resin-based composite materials for temporary dental crowns and bridges

Abstract

The aim of this work was to investigate the elution of residual monomers as a function of the manufacturing process, which are CAD/CAM manufacturing, self-curing and 3D printing. The experimental materials used consisted of the base monomers TEGDMA, Bis-GMA and Bis-EMA and 50 wt.% fillers. Additionally, a 3D printing resin without fillers was tested. The elution of the base monomers into the different media (water, ethanol and ethanol/water (75/25 vol. %)) at 37 °C over a period of up to 120 d as well as the degree of conversion (DC) by FTIR were investigated. No monomer elution could be detected in water. Most residual monomers in both other media were released from the self-curing material whereas the 3D printing composite released relatively little. The CAD/CAM blanks released hardly any quantitatively detectable amounts of monomers. Relative to the base composition, TEGDMA eluted less than Bis-GMA and Bis-EMA. DC did not correlate with residual monomer release; thus, leaching was determined not only by the amount of residual monomers present but by further factors as possibly network density and structure. The CAD/CAM blanks and the 3D printing composite showed similar high DC but lower residual monomer release from the CAD/CAM blank, likewise the self-curing composite and the 3D printing resin exhibited similar DC but different monomer elution. In terms of residual monomer elution and DC, the 3D printing composite seems promising as a new material class for the use as temporary dental crowns and bridges.

Keywords: Additive manufacturing; CAD/CAM; FTIR; HPLC.

Fig. 1

Chromatograms recorded by HPLC-DAD at 205 nm (A) of the pure monomers TEGDMA (Peak 1), Bis-GMA (Peak 2), and Bis-EMA (Peaks 3) with a solvent peak (S) at the time of injection. And (B) of eluates from the samples CAD/CAM blank (red), self-curing composite (green), 3D printing resin (blue) and 3D printing composite (yellow) stored in ethanol after 21 d. In addition to the peaks of the monomers (1–3), unknown substances labeled with (I-V) elute

Fig. 2

Cumulated release of the residual monomers TEGDMA (a), Bis-GMA (b) and Bis-EMA (c) over a period of 10 d normalized to the specimen’s surface area [µg/mm²]. The mean value with standard deviation of monomer release from two 3D printing materials and a self-curing composite in ethanol and ethanol/water as elution media are shown (n = 5). Significant differences (p ≤ 0.05) between the amount of elution in the two media are marked with a *

Fig. 3

The amount of all released monomer per specimen’s surface area [µg/mm²] (mean value with standard deviation, n = 5) as a cumulation of the release of TEGDMA, Bis-GMA and Bis-EMA from two 3D printing materials and a self-curing composite in ethanol over a period of 120 d

Fig. 4

a Mean value and standard deviation (n = 5) of the Degree of conversion (DC) [%] of the cured samples (see “Sample preparation”). Non-significant differences (p > 0.05) between DC are marked with “ns”. b Degree of conversion (DC) [%] over time [min] during curing of the two 3D printing materials and the self-curing composite

Fig. 5

Chromatograms recorded by HPLC-DAD at 205 nm of co-eluting E-bisPA (red), MA (blue) and Ethanol (green) in comparison to an eluate from 3D printing composite (black)