Optimal nano-silica filler concentration to optimize kinetics, rheology and bonding of self-adhesive composites

To determine how silica nanoparticle (NanoSi) content modulates polymerization kinetics, dentin bonding, and interfacial adaptation in self-adhesive flowable resin composites (SAFRCs). Five UDMA/PPGDMA/10-MDP SAFRCs containing 0, 2.5, 5, 7.5, or 10 wt% NanoSi were compared to a commercial reference (Vertise Flow, VF). Real-time ATR-FTIR provided t_0.5, Rp_max, delay time, and DC_max; viscosity was measured under controlled shear. Shear bond strength (SBS) was tested at 24 h. Masson's trichrome and environmental SEM (E-SEM) assessed interfaces. Two-way MANOVA examined formulation and exposure time (20 vs 40 s) effects on kinetics; one-way ANOVA analyzed µSBS (ɑ = 0.05). Formulation and time significantly affected the multivariate kinetic response (Wilks' λ_formulation = 0.1905, p = 0.0099; Wilks' λ_time = 0.4296, p = 0.0045), with no interaction. NanoSi_2.5 polymerized fastest (Rp_max 3.17 ± 0.43%·s⁻¹); NanoSi_5 was slowest (1.61 ± 0.26%·s⁻¹). DC_max increased with filler, peaking at NanoSi_10 (78.77 ± 2.64%); extending exposure from 20 to 40 s raised DC_max by 10.23 ± 2.16% without altering kinetics. All pastes were shear-thinning, with viscosity rising monotonically with NanoSi. SBS differed among materials (ANOVA F = 3.14, p = 0.02): VF was highest (8.6 ± 2.2 MPa); NanoSi_0 reached 6.0 ± 1.1 MPa (NS vs VF); NanoSi_2.5-5 clustered lower (~ 3-4 MPa) and NanoSi_7.5 reached the minimum. Trichrome and E-SEM showed thin, continuous interfaces at 0-2.5 wt% and increasing porosity/thin separations at 7.5-10 wt%. Nanosilica altered cure, flow and interfacial quality trade-offs; faster early kinetics and higher final conversion did not translate into higher SBS, emphasizing the need to optimize interfacial wetting/adaptation alongside mechanical parameters in SAFRC design. A formulation window may exist in which flow, cure, and bonding are balanced to enhance the clinical potential of self-adhesive composites.