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Y. Cui, R. H. Pelton, H. Ketelson, D. Meadows; Characterization of a Novel Polymeric Artificial Tear Delivery System. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4643.
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To characterize physico-chemical properties of hydroxypropyl guar (HPG) under conditions relevant to application in artificial tear formulations.
The binding of borate groups to hydroxypropyl guar (HPG) was followed by polyelectrolyte titrations. In this procedure, poly(diallyldimethylammonium bromide) (PDADMAC) was titrated into a HPG boric acid mixture. The net charge of polymer species was measured with a Mütek streaming current detector. For flocculation studies, HPG-borate mixtures were added to a cationic surfactant-free polystyrene latex dispersion and aggregation was followed by dynamic light scattering and by changes in turbidity.
Negatively charged HPG-borate chains form polyelectrolyte complexes with PDADMAC, much like classical polyelectrolytes. The unusual feature is that during complex formation, additional borate ions bind to HPG until the carbohydrate is saturated. The significance is that HPG-borate may increase charge density in the presence of positively charged tear fluid components, such as lysozyme. The polyelectrolyte nature of HPG-borate is further illustrated by the ability of HPG-borate to flocculate cationic latex. Electrostatic attraction drives HPG adsorption onto latex surfaces and the extended HPG chain conformation promotes bridging flocculation. These subtle, secondary interactions between HPG-borate and charged surfaces may help explain the specific efficacy associated with HPG in artificial tear formulations.
HPG-borate is an anionic polyelectrolyte bearing labile borate groups. Thus, the density of charges on HPG-borate is increased by raising pH or by the presence of neighboring cationic copolymers or surfaces. Like conventional anionic polyelectrolytes, HPG-borate adsorbs onto positively charged particle surfaces, inducing coagulation. Unlike conventional polyelectrolytes, the anionic charge density of HPG-borate varies remarkably in response to small changes in the local environment.
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