Effect of polytyrosine on the hydrophobicity of hydroxyethyl starch microspheres

The polypeptide, poly-l-tyrosine (PTYR) was incorporated into hydroxyethyl starch (HES) microspheres to modify the surface hydrophobicity in order to allow subsequent radioiodination and assessment of uptake into the intestinal mucosa following oral administration. The HES microspheres were prepared by a crosslinking polymerization process and characterized for particle size, peptide content and surface morphology. The hydrophobicity of the characterized microspheres was assessed by two techniques-water uptake (swelling) of the microspheres and contact angle measurement using the sessile drop method. Spherical microspheres containing PTYR were prepared with a greater than 80% loading efficiency in the 1-10 microns size range. The surface morphology suggested localization of the PTYR at the surface proportionate to the loading. The percent water uptake of the blank HES microspheres was 87%, whereas that of the 5, 10 and 20% PTYR/HES microspheres was 84%, 61% and 55%, respectively. HES/PTYR was polymerized as films to facilitate contact angle measurement. A monotonical increase in the contact angle was observed as the concentration of the PTYR was increased to 20%. Contact angle of pure HES (0% PTYR) films was 16 degrees and for the PTYR-containing films was 29 degrees for 5% PTYR, 39 degrees for 10% and 57 degrees for 20%. The hydrophobicity of the HES microsphere system could be increased by the incorporation of polytyrosine thereby paving the way to radioiodinate the microspheres and perform uptake studies in an animal model.