## 2 Materials and Methods
### Curing Protocols, Sample Preparation, and DC Measurements  
The specimens were irradiated using an LED curing unit (Bluephase PowerCure; Ivoclar, Schaan, Liechtenstein). Two curing protocols were applied: (1) a conventional protocol consisting of two consecutive 10 s exposures at 1100 mW/cm\({}^{2}\) (radiant exposure: 22 J/cm\({}^{2}\)) and (2) a rapid high-intensity protocol involving two consecutive 3 s exposures at 2900 mW/cm\({}^{2}\) (radiant exposure: 17.4 J/cm\({}^{2}\)). FTIR-ATR spectroscopy was used to determine DC at 2, 6, 10 min, and 24 h.  
Samples in the CG were cured directly above the material. Samples in the MBG were cured for 10 s from the mesial side and 10 s from the distal side of the bracket for conventional curing, and 3 s mesial and 3 s distal to the bracket for rapid curing, under a 45\({}^{\circ}\) angle [23]. Sample preparation and DC measurements were carried out following the methodology described in a previous study [31]. For short-term DC, a thin layer (-0.2 mm) of unpolymerized orthodontic adhesive, approximately matching the bracket base size (4 \(\times\) 4 mm), was applied to the ATR crystal. A polyethylene terephthalate (PET) foil was placed over the sample, followed by positioning a metal bracket with gentle pressure to replicate clinically relevant thickness. An upper incisor bracket was selected due to its flat base. Excess material was removed with a dental \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material Name** & **Manufacturer** & **Resin Matrix Composition** & **Filler Load and Composition** \\ \hline \multirow{2}{*}{Enlight} & Ormco, Brea, CA, USA & Dimethacrylate monomer & \multirow{2}{*}{70–80\% silane-treated silica} \\  & (9708681) & 20–30\% & \\ \hline \multirow{2}{*}{Heliosit} & Ivoclar, Schaan, Liechtenstein & Bis-GMA, UDMA, decandiol & \multirow{2}{*}{14\% silane-treated silica} \\  & (Z04 SHW) & dimethacrylate(85\%) & \\ \hline \multirow{2}{*}{Transbond XT} & 3M Unitek, Monrovia, CA, USA & 10–20\% Bis-GMA, 5–10\% & \\  & (9478429) & Bis-EMA & \\ \hline \hline \end{tabular}  
* Bis-MA: bisphenol A digliyicidylmethacrylate; Bis-EMA: bisphenol A digliyicidylmethacrylate ethoyalted; TECDMA: triethylene glycol dimethacrylate; UDMA: urethane dimethacrylate.  
\end{table} 
Table 1: Composition of resin-based orthodontic adhesive systems tested in this study. Figure 1: Study design. probe. For CG samples, the bracket was removed before curing, while for MBG samples, the bracket remained in place. For 24 h DC, the unpolymerized orthodontic adhesive was placed on a PET foil on top of a microscope slide, with two laterally positioned cover glasses, to attain a \(\sim\)0.2 mm thickness and planar dimensions of 4 \(\times\) 4 mm. A second PET foil was placed over the sample and pressed with a glass slide, which was then removed. For MBG samples, a bracket was placed on the upper PET foil before curing. After curing, the PET foil (and bracket, for MBG) was removed, and the samples were gently detached, placed in dark boxes in an incubator at 37 \({}^{\circ}\)C, and stored for 24 h. The surface of the specimen opposite to the light-exposed side was positioned on the ATR accessory and pressed using a load-controlled spectrometer press. FTIR spectra were collected as described below. Spectra of unpolymerized materials (n = 5) were also recorded for use in DC calculations. Short-term and 24 h DC were measured using Fourier-transform infrared (FTIR) spectroscopy coupled to an attenuated total reflectance (ATR) module (Nicolet iS50, Thermo Fisher, Madison, WI, USA). Spectra covering 500-3500 cm\({}^{-1}\) were collected at a resolution of 8 cm\({}^{-1}\) for short-term measurements and 4 cm\({}^{-1}\) for 24 h measurements. For the short-term measurements, two scans were collected at a rate of two spectra per second, starting when the light curing unit was activated and continuing for 10 min. For the 24 h measurements, a single spectrum comprising 30 co-added scans was acquired 24 h after curing. A new background spectrum was recorded before each specimen.