2.5. Characterization of the Solid SASD

The physical morphology of the MTX powder, carriers, and MTX-loaded SASD formulation were assessed using a Hitachi S-4800 SEM (Tokyo, Japan). These samples were affixed onto a brass disk with double-sided adhesive tape, and were then made electrically conductive by being coated with platinum (6 nm/min) using an EMI Tech Ion Sputter system (K 575 K) under vacuum (8 × 10⁻³ mbar) for 4 min at 15 mA. Microscopic images were then taken from different angles.

The particle size distributions were measured using a laser diffraction particle size analyzer (Mastersizer 3000, Malvern Instruments, Malvern, UK). The particle size was expressed as Dx10, Dx50, and Dx90, taken from the cumulative distribution, where Dx10, Dx50, and Dx90 are the particle diameters calculated respectively at the 10th, 50th and 90th percentiles of undersized particles. The measurements were carried out in triplicate.

The thermal characteristics of the free MTX powder, carriers, PM, and SA-SD formulation were analysed using a DSC Q20 (TA Instruments; New Castle, DE, USA). In brief, 5 mg of each sample were sealed in the Tzero pan and lid, and heated from 60 °C to 175 °C at a constant rate of 10 °C/min with a nitrogen purge of 50 mL/min. The PM was prepared by mixing the drug and carriers in the same weight ratio as that of the optimized formulation.

The crystallinity of each sample was investigated at room temperature using a PXRD instrument (D/MAX-2500, Rigaku, Japan) with monochromatic Cu˗Kα radiation (λ = 1.54178 Å, 40 kV, and 100 mA) in the region of 5° ≤ 2θ ≤ 45°, and with an angular rise of 0.02°/s.

The FTIR patterns of the pure MTX, PM, and the formulation were obtained by FTIR spectrophotometer (FTIR-4100, JASCO, Pittsburgh, PA, USA). All of the samples were appropriately loaded onto the sample disk, and the resulting peaks were analysed using Spectra Manager II software in the 4000–400 cm⁻¹ spectral range.