Triborheological Analysis of Reconstituted Gastrointestinal Mucus/Chitosan:TPP Nanoparticles System to Study Mucoadhesion Phenomenon under Different pH Conditions

Abstract

Polymeric nanoparticles have attracted much attention as pharmaceutical delivery vehicles to prolong residence time and enhance the bioavailability of therapeutic molecules through the mucoadhesive phenomenon. In this study, chitosan:TPP nanoparticles were synthetized using the ionic gelation technique to analyze their mucoadhesive interaction with reconstituted porcine gastrointestinal mucus from a triborheological point of view under different pH conditions (pH = 2.0, 4.0, 6.0 and 7.0). The triborheological profile of the reconstituted mucus was evaluated at different pH environments through the oscillation frequency and the flow sweep tests, demonstrating that the reconstituted mucus exhibits shear thinning behavior regardless of pH, while its viscoelastic properties showed a change in behavior from a polymeric solution performance under neutral pH conditions to a viscoelastic gel under acidic conditions. Additionally, a rheological synergism analysis was performed to visualize the changes that occur in the viscoelastic properties, the viscosity and the coefficient of friction of the reconstituted mucus samples as a consequence of the interaction with the chitosan:TPP nanoparticles to determine or to discard the presence of the mucoadhesion phenomenon under the different pH values. Mucoadhesiveness evaluation revealed that chitosan:TPP exhibited strong mucoadhesion under highly acidic pH conditions, below its pKa value of 6.5. In contrast, at neutral conditions or close to its pKa value, the chitosan:TPP nanoparticles' mucoadhesiveness was negligible.

Keywords: chitosan:TPP nanoparticles; gastrointestinal pH; mucoadhesion; reconstituted mucus; rheological synergism.

Figure 1

Evaluation of the viscoelastic behavior of reconstituted mucus at 37 °C under different pH conditions. At pH 2 (black dots), mucus exhibited an elastic behavior with a dominant elastic modulus (G′ > G′′). Similarly, at pH 4 (red squares), mucus kept an elastic behavior (G′ < G′′) but with less separation among the modulus and a crossover at high angular velocity (~82 rad/s). Instead, at pH 6 (green triangles) and pH 7 (blue diamonds), mucus showed an initial viscous behavior (G′′ > G′) until a crosslink around 1–1.5 rad/s for a characteristic polymeric solution performance. Emphasizing that at pH 7, mucus exhibited a significant decrease in its modulus. N = 5.

Figure 2

Evaluation of the viscous behavior of reconstituted mucus at 37 °C under different pH conditions: pH 2 (black dots), pH 4 (red squares), pH 6 (green triangles) and pH 7 (blue diamonds). Regardless of the pH, apparent viscosity (η) exhibited similar performances with a negative slope showing characteristic shear thinning properties. N = 5.

Figure 3

Stribeck curve to evaluate the coefficient of friction (CoF) of reconstituted mucus at 37 °C under an applied pressure of 1 N and different pH conditions: pH 2 (black), pH 4 (red), pH 6 (green) and pH 7 (blue). Independently of the pH, all the samples exhibited a similar performance during the boundary and mixed lubrication regimes. Instead, during the hydrodynamic regime, the more acidic sample showed a critical increase in its CoF in comparison with the other samples as a result of the conformational change that mucus undergoes around its isoelectric point (inside the pH range 2.0–3.0). N = 5.

Figure 4

SEM images of (a) freeze-dried chitosan:TPP nanoparticles (under high vacuum conditions, applied voltage of 15 kV, magnification of 5000×, working distance 47 mm, spot size 50 nm and secondary electrons detector) and (b) Dispersed chitosan:TPP nanoparticles (under high vacuum conditions, applied voltage of 20 kV, magnification of 1600×, working distance 11 mm, spot size 55 nm and secondary electrons detector).

Figure 5

Elastic modulus (G′) of reconstituted mucus (black dots); chitosan:TPP nanoparticles (red squares); summation of chitosan:TPP nanoparticles—reconstituted mucus (green triangles); mixture of chitosan:TPP nanoparticles—reconstituted mucus (blue diamonds) for evaluating chitosan:TPP nanoparticles mucoadhesivity through rheological synergism (ΔG′) under different pH conditions: (a) pH 2, (b) pH 4, (c) pH 6 and (d) pH 7. Mixtures under acidic pH conditions (pH = 2, 4 and 6) exhibited an elastic modulus much larger than summations in a clear performance of a strong polymeric mucoadhesion. Instead, the mixture at neutral pH (pH = 7) exhibited no significant difference with summation in negligible or absent mucoadhesion. N = 5.

Figure 6

Apparent viscosity (η) of reconstituted mucus (black dots); chitosan:TPP nanoparticles (red squares); summation of chitosan:TPP nanoparticles—reconstituted mucus (green triangles); and mixture of chitosan:TPP nanoparticles—reconstituted mucus (blue diamonds) for evaluating chitosan:TPP nanoparticles mucoadhesivity through rheological synergism (ΔG′) under different pH conditions: (a) pH 2, (b) pH 4, (c) pH 6 and (d) pH 7. Mixtures under highly acidic pH conditions (pH = 2 and 4) exhibited a larger viscosity than summations in an evident mucoadhesive performance. Mixture at slightly acidic (pH = 6) conditions displayed a minimum mucoadhesive behavior. Moreover, the mixture under a neutral environment (pH = 7) expressed no significant difference with summation in insignificant or null mucoadhesion. N = 5.

Figure 7

Comparison between the coefficients of friction (CoF) of the reconstituted mucus and the mixture of reconstituted mucus–chitosan:TPP nanoparticles at 37 °C under different pH conditions. (a) Stribeck curve at pH 2. (b) Stribeck curve at pH 4. (c) Stribeck curve at pH 6. (d) Stribeck curve at pH 7. All the mixtures exhibited improved lubricant properties regarding mucus samples. N = 3.