2.2.4. Matrix Systems Characterization

The tablets and filaments obtained were characterized as follows:

Physical dimensions. The tablet average weight and the standard deviation (SD) were obtained from six individually weighed tablets (Sartorius CP224S, Gottingen, Germany). The external dimensions (height and diameter) were determined as the mean of six tablets from each batch using the digital micrometer (VWR International, Leuven, Belgium). The multidimensional image analysis software Nis-elements BR (Nikon Corporation, Chiyoda-ku, Tokyo, Japan) was used to calculate the lateral surface area of the filaments.

Drug release assays were carried out in a USP apparatus 2 Sotax AT7 Smart, (Sotax, Allschwil, Switzerland) using 900 mL of deaerated water maintained at 37 ± 0.5 °C, during 24 h at 50 rpm. The percentage of drug released was measured with an UV–vis spectrophotometer Agilent 8453 (Agilent, CA, USA) at a wavelength of 272 nm. The test was performed in triplicate for each batch. Sink conditions were met throughout the dissolution test.

Kinetic adjustments. Drug release data (M_t/M_∞ ≤ 0.6) were analyzed according to zero order, Higuchi [29], Korsmeyer et al. [30], and Peppas and Sahlin [31]. Equations (1)–(4):

where M_t/M_∞ is the fraction of drug released at time t (the drug loading was considered as M_∞). k is the Higuchi’s release rate constant. k_k is the Korsmeyer’s kinetic constant, t the release time, n the release exponent that depends on the release mechanism and the shape of the matrix tested [32], k_d and k_r are, respectively, the diffusion and relaxation rate constants, and finally m, which is the purely Fickian diffusion exponent for a device of any geometrical shape that exhibits controlled release.

Estimation of the excipient efficiency. The excipient efficiency was calculated for the different batches by applying the following Equation (5) according to Caraballo [33]:

where ε is the total porosity of the matrices, b is the Higuchi’s release rate constant. The total porosity is calculated with the known values of volume and weight of tablets according to the following Equation (6):

where Vreal is the real volume of the tablets and Vtheor the theoretical volume of the tablets, obtained as the PBS_CL volume calculated from its true density obtained by Helium picnometry, Pentapycnometer 5200E (Quantachrome Instruments, Boynton, FL, USA). Drug volume was not taken into account due to its soluble behavior.

Scanning electron microscopy. The surface of samples, tablets obtained by USAC, and DC and HME filaments were evaluated at the Microscopy Service of the CITIUS in the University of Seville by using scanning electron microscopy (SEM) with a FEI TENEO electronic microscope (FEI Company, Hillsboro, OR, USA), operating at 5 kV. Tablets were previously coated with a 10 nm thin Pt layer with Leica EM SCD500 high vacuum sputter coater.