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During the preparation of the microspheres by double emulsion method, the osmotic pressure difference between the internal and external water phase is very important.The objective of this study was researching the relationship between this difference and the porosity, burst release, particle size, external morphology, drug-loading rate, entrapment rate and yield.We changed the difference from 0 to 342, 684, 1368, 2051, 2735 mOsmol/kg of the internal and external water phase, and observed the affects to the above agents.When theres no difference of osmotic, the water-soluble drug cannot load in the microsphere efficiently, the entrapment and drug-loading rates are very low, about 1.05% and 4.5% individually, while the normals are about 20% and 80%.When the osmotic pressure difference is low, for example 342 and 684 mOsmol/kg, the burst release at the first day of the microsphere is high up to 83.07 and 68.67%.But if the osmotic pressure difference is larger, the burst release comes down fast.For example at the 2735 mOsmol/kg, it is as low as 4.2%.The porosity and the specific surface area were affected by this difference deeply.When the difference is very low, they are 3.03 cc/g and 30.59 m2/g individually, but 0.13cc/g and 0.63m2/g individually when 2735 mOsmol/kg.The external morphology of the microspheres at low difference covered with so many big holes and channels, and the higher ones were smooth and have hardly any hole.Fig 1 to 3 show the SEM photos of microspheres prepared at the osmotic pressure difference of 0, 684 and 2735 mOsmol/kg individually.The particle size is also influenced by this difference.It is about 77.3 um in 0 mOsmol/kg microspheres, and when the difference up to 2735 mOsmol/kg, it decreased to 41.0 um.Furthermore, the yield is lower in slight difference, which is about 56.0%, and it will be very high in significant osmotic pressure difference, that is about 82.4%.