Polystyrene microspheres in the size range 50 nm to 3 microns were fed by gavage to female Sprague Dawley rats daily for 10 days at a dose of 1.25 mg/kg-1. Previous histological evidence of the uptake of these particles and their absorption across the gastrointestinal tract and passage via the mesentery lymph supply and lymph nodes to the liver and spleen was confirmed by analysis of tissues for the presence of polystyrene by gel permeation chromatography. Measurement of radioactivity of tissues following administration of 100 nm and 1 micron I125-labelled polystyrene latex particles for 8 days was corroborative although less secure because of the potential lability of the labelled particles. The extent of absorption of 50 nm particles under the conditions of these experiments was 34% and of the 100 nm particles 26% (as measured by determination of polystyrene content), of which total, about 7% (50 nm) and 4% (100 nm), was in the liver, spleen, blood and bone marrow. Particles larger than 100 nm did not reach the bone marrow, and those larger than 300 nm were absent from blood. No particles were detected in heart or lung tissue.

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Vesicles were prepared on hydration of a mixture of a single or double alkyl-chain, non-ionic surfactant with cholesterol. These vesicles, or 'niosomes', are capable of entrapping and retaining water soluble solutes such as carboxyfluorescein, are osmotically active and can be formulated to release entrapped solute slowly. The physical characteristics of the vesicles were found to be dependent on the method of production and three such methods, based on liposome technology, are described. The vesicles have been characterized by photon correlation spectroscopy, freeze fracture electron micrography, measurement of solute entrapment efficiency, and solute release rates. Vesicular forms of the single chain surfactant which could be formed under certain conditions in the absence of cholesterol are also described.

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