3.1. Microstructure and Film Thickness
Consistent with the previous study [
14], the surface of the control CMC/GEL film revealed the presence of GEL-rich microspheres with diameters ranging from 13.33 to 26.67 μm (
Figure 1). It confirms that CMC and GEL are two incompatible biopolymers [
8] that cannot form the homogeneous blend films at the micro-level. In turn, good compatibility (lack of aggregates or undissolved particles) was found for the other polysaccharide/GEL blends. In accordance with the previous studies [
7], the surfaces of WSSP-containing film were rough and uneven, which could be attributed to the weak dissolution of the polysaccharide. The AST-added films were more heterogeneous as compared to the controls (
Figure 1). Since starch is the component of the AST formulation (
Figure 2), it is easy to deduce that the observed round-shaped inequalities were the starch granules. The size of the starch granules in the AST powder and the dehydrated AST solution ranged from 5.71 to 15.71 (
Figure 2), which corresponds to the size of corn starch granules [
28].
It was found that the CMC75/GEL25 films with higher AST content (0.5–1%) exhibited higher thickness as compared to other carriers
(p < 0.05,
Table 2). This result may be explained by the fact that the combination of the phase-separated microspheres with the starch granules (from AST) was unable to form a compact film network (
Figure 1). However, regardless of the carrier type, no difference (
p > 0.05) in thickness was observed between the AST-added and control films.
3.2. Release of AST
Figure 3 and
Figure 4 show the cumulative amount (mg/cm
2) and percentage of AST released from the films, respectively. It was found that during 32 h dissolution test, only the CMC/GEL carrier offered complete release. Depending on the AST concentration, the 100% release occurred after 0.5–1 h (
Figure 4). This result could be easily explained by the fact that CMC dissolves rapidly after coming into contact with water [
8]. The t
25% values estimated for the CMC-based films ranged from 0.09 to 0.15 h (
Table 2). In the case of the other carriers, ~14–89% of the initial dose of AST was released. The slowest release of AST was found for the OSA/GEL (t
25% = from 13.34 to >27.26 h) and the WSSP/GEL (t
25% = 6.16–50.95 h) carriers (
Table 2). This result is likely associated with the partial solubility of these films at 25 °C [
7]. It should be noted here that since the release of AST from the 1% AST-added OSA-based film was very slow and incomplete, it was not possible to predict t
25% for this system.
Regardless of the AST concentration, the WSSP/GEL film ensured the longest time without AST release (
Tlag = 50 min,
Figure 4). It suggests a strong physicochemical entrapment of the carotenoid in this matrix. This result confirms our earlier observations, which showed that WSSP-based film offered longer
Tlag (thus slower release) than GAR-based carrier [
17]. As suggested before, the cause could be the highly branched structure of WSSP [
29]. Apart from the 1% AST-added WSSP-based system, the release profiles exhibited a sudden burst of AST. This phenomenon can be attributed to the erosion of the films.
Apart from the CMC-based carrier, the gradual rise in the AST concentration decreased the release rate from the carriers. It seems possible that this result is due to increasing amounts of the starch in the AST-added films; i.e., the starch granules (
Figure 1) inhibited access of water molecules to the polymeric matrix, which limited dissolution of AST [
17]. The highly erodible character of CMC probably meant that the presence of starch did not affect the dissolution behavior of the CMC/GEL carrier.
Eight mathematical equations (
Table 1) were used to the quantitative interpretation of the data obtained from the ASTA release assay. The R
2adjusted and the parameter values estimated for the models are presented in
Table 3,
Table 4,
Table 5 and
Table 6. It was impossible to fit one optimal model to describe the migration of AST from the particular carrier types, however, based on R
2adjusted averages the
Hop Tlag model provided the best fit for all release series data (R
2adjusted mean = 0.9242). It was found that the
Hig Tlag model was the least useful for AST release prediction (R
2adjusted mean = 0.6650). Among the tested models, the
F-O Fmax,
K-P Tlag,
Hop Tlag, Log 2, and
Wb Fmax were quite suitable for the description of AST release from the CMC-based carrier (R
2adjusted = 0.9410–0.9890). The
Wb Fmax model ensured the best fit (R
2adjusted = 0.9610–0.9890). The shape parameter (
β) obtained from the
Wb Fmax model, showed that the AST release profile of the 0.25–0.5% AST-added CMC-based films was sigmoid (
β > 1), while for the 1% AST-added carrier the release was more parabolic (
β < 1). Nevertheless, the differences in the over mentioned shapes of the release curves were barely visible (
Figure 4 and
Figure 5).
The
M-B Tlag model was the best to predict the release of AST from the GAR-based (R
2adjusted = 0.9341–0.9380), OSA-based (R
2adjusted = 0.8861–0.9461) and WSSP-based films (R
2adjusted = 0.9187–0.9967). Interestingly, since the 1% AST-added WSSP/GEL film offered the most linear release profile (
Figure 4 and
Figure 5), all mathematical models described fairly well the release from this system (R
2adjusted = 0.9281–0.9967). The
K-P model is often used to analyse the release of the active substance from polymeric dosage forms, when the release mechanism is not well known, or when more than one type of release phenomena could be involved [
30]. In planar (thin films) geometry, the released mechanism is described as: (i) quasi-Fickian diffusion (
n < 0.5), (ii) Fickian diffusion (
n = 0.5), (iii) non-Fickian diffusion (0.5 <
n < 1), (iv) case II transport (zero-order release) (
n = 1), and (v) super case II transport (
n > 1) [
31]. It was found that in the case of all CMC-based films, the
n values were below 0.5 (
Table 5), which implies that release mechanism was non-swellable matrix-diffusion [
8,
32]. It shows that the very small release exponent (
n = 0.007–0.055) is characteristic for the burst release of AST (
Figure 5). This result may be explained by the fact that despite the CMC/GEL film has short initial ability to hold water (swelling ≈ 500%), it rapidly dissolves (due to high content of easily soluble polysaccharide fraction) [
8], which results in a rapid dissolution of AST in the aqueous medium. This result supports evidence from previous observations [
8], which showed that the release of potassium salts of iso-α-acids from the films based on CMC/GEL blends was beyond the limits of the
K-P “power law” model (
n < 0.5).
The release of AST from the 0.25–0.5% AST-added WSSP-based carrier and was followed by a non-Fickian mechanism. It shows that the AST migration was governed by diffusion and controlled swelling, whose rate was similar. The rearrangement of the polymeric chains occurred slowly, and the diffusion simultaneously caused the time-depended anomalous release [
33].
For the GAR- and OSA-based films (regardless of AST concentration), the
n value was above 1, which is an indicator of the extreme form of transport (super case II model), i.e., more than one mechanism (swelling, polymer chain disentanglement (relaxation), erosion) was involved in the AST release kinetics. Regarding the GAR-based carrier, the obtained result complies with the previous findings [
34,
35], which suggested that gelling properties of GAR (at high polymer concentrations) ensure the sustained drug release. Thus, it can be concluded that the water entrapped in the gel strongly affected the diffusional behavior of AST in the GAR-based films. In turn, limited release of AST from the OSA/GEL carrier (~19–46%, depending on the AST concentration) could be attributed to the encapsulation of ASX in the OSA/GEL complexes (coacervates) [
36].
As for the 1% AST-added WSSP-based film, it is possible that the bulky amounts of starch granules (from AST) acted as a release modifier. It is known that native starch has multifunctional uses in the different physical forms of carriers serving as the binder, disintegrant, diluents, glidant, and lubricant [
37]. Therefore, it is possible that the granules anchored in the film matrix (
Figure 1), hindered the contact of dissolution media with WSSP/GEL matrix, which consequently limited erosion of the carrier, thus ASX release was predominately controlled by diffusion (
n < 0.5,
Table 5).