2.3.2. Characterization of NCT Fast Dissolving Films

The morphology of NCT fast dissolving films was investigated by scanning electron microscopy (SEM, JEOL JSM-6610LV, Tokyo, Japan). A square-shaped film with a size of 5 mm × 5 mm was cut to investigate film surface, whereas a rectangle-shaped film with a size of approximately 1 mm × 5 mm was cut to investigate film thickness. The film was placed on a carbon tape, and the film surface was then coated with gold for 15 s using a 40 mA sputter coater (JEOL JFC-1100E, Tokyo, Japan). The SEM images were taken at an accelerating voltage of 15 kV with 350× and 500× magnifications.

The thickness of each 2 cm × 2 cm piece of film was measured at three points (left, middle, and right) in the same position on each film by an outside micrometer (3203-25A, Insize CO, Ltd., Suzhou New District, Jiangsu, China). Five replicates were conducted for each film, and the average film thickness (in mm) with standard deviation was calculated.

Three pieces of 2 × 2 cm² size were cut randomly from each film formulation. Films were weighed individually on an electronic balance (PA214, Ohaus Corporation, Parsippany, NJ, USA), and the mean weight was calculated.

The disintegration test method used in this study was modified from Preis et al. (2014) [14]. The film was clamped between the sample holder and the magnetic clip (attached to the bottom side of the film). The magnetic clip had a weight of 3 g (0.03 N), which represented the approximate minimal force applied by the human tongue. Then, the attached film was half-immersed (50%) in 65 mL of simulated salivary fluid with pH 6.8 at 37 ± 0.5 °C. The simulated salivary fluid was composed of sodium chloride 8.0 g/L, potassium phosphate monobasic 0.19 g/L, and sodium phosphate dibasic dihydrate 2.98 g/L. The pH was adjusted to 6.8 with 1 M hydrochloric acid [15]. The time required for the film to break and the magnetic clip to drop down was recorded visually and noted as in vitro disintegration time. All studies were performed in triplicate for each formulation. For the direct comparisons of in vitro disintegration times, the obtained disintegration times were normalized by thickness of each film.

Mechanical strength of the films was tested using a texture analyzer TX.TA plus (Stable Micro Systems, Surrey, UK). An individual sample holder was constructed to facilitate measurements of 2 cm × 2 cm sized film samples. The dry film was fixed on the plate with a cylindrical hole with a 9.0 mm diameter (area of the sample holder hole = 63.56 mm²). A cylindrical stainless probe (2 mm diameter) with a plane flat-faced surface was used (probe contact area = 3.14 mm²). The texture analyzer was adjusted for the probe’s forward movement at a velocity of 1.0 mm/s. Measurement started when the probe had contacted the sample surface (triggering force). The probe moved on at constant speed until the film was torn. The applied force and distance were recorded. All of the experiments were conducted at room temperature (25 ± 2 °C, 70% relative humidity). Five replicates were conducted for each film. The mechanical strength of the film was characterized by tensile strength, elongation at break, and Young’s modulus [16,17,18].