The
bonding performance of a glycerol phosphate dimethacrylate
(GPDM)-based, two-step, self-etch (SE) adhesive was experimentally
compared to that of 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based
universal adhesives in different application modes for enamel bonding.
Microtensile bond strength (μTBS) for adhesives bonded to enamel
was measured initially (24 h water storage) and after 10 000
thermocycles plus water storage for 30 days. A GPDM-based, two-bottle,
two-step, self-etch adhesive (Optibond Versa, OV) and three one-bottle
MDP-based universal adhesives, one self-etching (Tetric N Bond Universal,
TNBU) and two with etch-and-rinse (E&R) processing (Single Bond
Universal (SBU); Clearfil Universal Bond Quick (CUBQ)), were tested.
Scanning electron microscopy (SEM) evaluated nanoleakage at the bonding
interfaces. A profilometer determined roughnesses of enamel surfaces
after phosphoric acid etching, OV priming, or TNBU conditioning. SEM
observed the corresponding surface morphology. NMR and X-ray photoelectron
spectroscopy (XPS) characterized chemical bonding in hydroxyapatites
(HAps) conditioned with the adhesives. Etch-and-rinse samples had
significantly stronger bonding than self-etch samples (
p
< 0.05) irrespective of aging. The μTBS values for initial
and aged OV were significantly higher than those of TNBU (
p
< 0.05). Aging did not significantly decrease μTBS
for any sample except TNBU (
p
< 0.05), but it
significantly aggravated nanoleakage. Etch-and-rinse processing resulted
in less nanoleakage than self-etching; the OV samples leaked less
than TNBU, both before and after aging. Phosphoric acid etching achieved
the highest enamel surface roughness, followed by OV primer. Ca–O–P
bonds in hydroxyapatite conditioned with TNBU, SBU, and CUBQ were
confirmed by NMR, which showed similar results to XPS observations
of conditioned hydroxyapatite powders except OV primer. The GPDM-based,
two-step, self-etch adhesive can provide higher micromechanical retention
potential, bond strength, and durability than the MDP-based universal
adhesive in self-etch mode but lower performance than the MDP-based
universal adhesive in etch-and-rinse mode. None of the tested adhesives
could avoid nanoleakage after aging.
To
investigate the effects of 3-aminopropyltriethoxysilane (APTES)-
or (3-mercaptopropyl)trimethoxysilane (MPTS)-conditioned nanozirconia
fillers on the mechanical properties of Bis-GMA-based resin composites.
The conditioned fillers were characterized by Fourier transform infrared
(FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermodynamic
calculations. They were then used to prepare Bis-GMA-based resin composites,
whose flexural strength and elastic modulus were evaluated. The Cell
Counting Kit-8 (CCK-8) assessed the composites’ cytotoxicity.
The FTIR spectra of the conditioned fillers showed new absorption
bands at 1569 and 1100 cm
–1
, indicating successful
grafting of APTES or MPTS onto nanozirconia. XPS confirmed the Zr–O–Si
bonds in the APTES- or MPTS-conditioned fillers at contents of 2.02
and 6.98%, respectively. Thermodynamic calculations reaffirmed the
chemical binding between the two silanes and nanozirconia fillers.
Composites containing the conditioned nanozirconia fillers had significantly
greater flexural strengths (APTES, 121.02 ± 8.31 MPa; MPTS, 132.80
± 15.80 MPa; control, 94.84 ± 9.28 MPa) and elastic moduli
(8.76 ± 0.52, 9.24 ± 0.60, and 7.44 ± 0.83 GPa, respectively)
than a control with untreated fillers. The cytotoxicity assay identified
no significant cytotoxicity by composites containing the conditioned
fillers. Silanes were previously considered to be unable to chemically
condition zirconia to bond with resin. Inclusion of APTES- or MPTS-conditioned
nanozirconia fillers can improve the mechanical properties of Bis-GMA-based
resin composites without obvious cytotoxicity in this study.
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