RESUMEN

Thermoresponsive colloidal chitosan systems forming the polymer structure in situ are an example of promising solutions in tissue engineering as an injectable scaffolds or drug carriers. Their application method, and thus shearing, may affect the aggregation process in accordance with the colloidal engineering approach. The aim of the study is to compare the kinetics of chitosan aggregation in the perikinetic regime (limited by Brownian motions) with the orthokinetic process carried out under the influence of an external shear field. The research was carried out using static multiple light scattering (S-MLS) and rheometric measurement techniques coupled with small-angle light scattering (Rheo-SALS). It has been found that the introduction of an external shear field (orthokinetic regime) accelerates the aggregation of chitosan systems. Simultaneously, the rotational measurements can even lead to spontaneous gelation, most likely caused by changes in the conformation of chitosan molecules, their deformation and ordering along the shear field.

RESUMEN

Positively charged liposomes were coated with the negatively charged and temperature sensitive poly(N-isopropylacrylamide-co-methacrylic acid) by electrostatic deposition. Too low or too high polymer concentrations lead to unstable suspensions. However, intermediate polymer concentrations (0.05-0.2 wt.%) result in relatively stable suspensions of polymer-coated liposomes. At elevated temperatures the thickness of the polymer layer around the coated liposomes increased sharply at 40 °C, due to the formation of polymer multilayers. At higher temperatures, a contraction of the adsorbed polymer layer was observed. The uncoated liposomes exhibited an interesting transition in size and intensity of the scattered light when heated, attributed to the transition from the gel to liquid crystalline phase. Rheo-SALS (small angle light scattering under shear conditions) measurements demonstrated that the polymer coating was stable under shear at physiological temperature. It also revealed an anomalous high scattered intensity of the uncoated liposomes compared to the coated liposomes. This discrepancy was diminished at higher temperatures, and can probably be attributed to the change from a non-spherical, polyhedron-like conformation of the uncoated liposomes in the gel phase to a spherical shape above the phase transition.

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RESUMEN

Liposomes coated with polymers may have a great potential in drug delivery. In this study, adsorption of the non-ionic hydroxyethyl cellulose (HEC) onto non-charged phospholipid vesicles was investigated. Both unmodified and hydrophobically modified (HM) HEC were included in the study. Possible interactions between the liposomes and the polymers were determined by changes in the size and the size distribution. Rheo-SALS measurements were carried out to verify the successfulness of the coating process. The stability was investigated by zeta potential measurements, UV-analysis and HPTLC. Mixing unmodified HEC (Mw 90,000 and 300,000) with the liposomes yielded no increase in the particle size. HM-HEC, however, was adsorbed onto both the fluid phase egg-PC liposomes and the gel phase DPPC liposomes. The Rheo-SALS measurements confirmed the successful coating of the liposomes. Complete coating resulted in increased chemical stability of the dispersion and in addition prevented aggregation. This study has shown that the non-ionic HM-HEC can be used to form polymer coated liposomes with neutral surface charge for enhanced stability.

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