This study described the rheological and mucoadhesive properties of one natural water-soluble polysaccharide from Bletilla striata (BSP). The rheological characteristics of BSP in aqueous solutions and BSP mixed with other polymers were investigated under various conditions, including concentration, temperature, pH, and salt addition. Viscometric studies and ex vivo mucoadhesion tests were also conducted to examine the mucoadhesive properties of BSP. Results indicated that BSP behaved as a shear-thinning fluid at various concentrations, and its viscosity decreased at high temperatures. The viscous flow properties of the BSP mixtures changed at high pH (>5.0). Conversely, the viscosity of the BSP solutions was slightly affected by electrolytes. The viscosities of the BSP mixtures with four other commonly used polymers (sodium alginate, sodium carboxymethyl cellulose, hypromellose, and chitosan) were enhanced. The synergistic viscosity of BSP/mucin mixtures increased as BSP concentrations increased, and the maximum value was observed in the SIF medium without enzymes. The adhesive abilities of 5.0% and 10.0% BSP were almost equivalent to that of 0.5% sodium alginate, suggesting that BSP exhibited a certain mucoadhesive property, although it was weaker than that of the other commonly used mucoadhesive materials. BSP showed potential for pharmaceutical excipient applications in bioadhesive drug delivery systems.
Conclusion
This study described the rheological and mucoadhesive properties of a natural water-soluble polysaccharide BSP [24]. It behaved as a shear-thinning fluid at different concentrations, and its viscosity was greatly influenced by temperature. Temperature should not be set too high to maintain the viscous behavior of the BSP solution. Instead, temperature should be set within 30 °C to ensure that a certain viscosity is maintained during adhesive preparations. The viscosities of the BSP solutions were similar between pH 1.0 and 5.0, but their viscosity was increased at pH ≥ 5.0, indicating that the BSP solution was fairly resistant to acidic pH variations. A more aggregated state was reached at high pH condition. The BSP mixture was less affected by electrolytes, but the effect of divalent cations (Ca2+) on the viscosity of BSP was more pronounced than that of monovalent cations (Na+) at a high salt concentration (0.3 mol/L). The viscosities of four kinds of mixtures (BSP/SA, BSP/CMC-Na, BSP/HPMC, and BSP/CS) were enhanced compared with their pure component solutions, and the mixture of BSP and an anionic polymer (SA) had a remarkable synergistic effect on rheology at low concentrations. Viscometric studies and ex vivo mucoadhesion tests revealed that BSP exhibited certain mucoadhesive properties, but these properties were relatively weaker than those of commonly used bioadhesive materials. Various methods, including premixed or chemical modification, could be used to enhance the mucoadhesive properties of BSP. For further research, the viscoelastic properties of BSP should be investigated through low-amplitude oscillatory experiments to explore its applications and understand its structure and functional properties. Dynamic oscillatory rheology, scattering techniques, and spectroscopy could be applied to examine the interactions between BSP and mucin.
