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Background: We report the development and characterization of gelcore solid lipid nanoparticles (SLN) for the encapsulation of hydrophilic proteins, taking bovine serum albumin (BSA) as model compound. Methods: SLN were produced by multiple emulsion (w/o/w) method, while hydrogels were produced by dissolving Carbopol 971 in purified water under high-speed stirring. Physicochemical characterization followed the pH determination, fluorescence and scanning electron microscopy analyses, and the quantification of the drugs by spectrophotometric measurements (for the determination of encapsulation efficiency and release profile). Results: The stability of SLN was found to be dependent on the type of nanoparticle core. Gel-core SLN were more stable than aqueous-core SLN, both during production and during shelf life. It has been shown that an increase in the core volume is possible for gel-core SLN during their preparation. The hydrodynamic diameter of gelcore SLN formulated in the absence and presence of Bovine serum albumin was ca. 200 nm. To determine the encapsulation efficiency (EE) and in vitro release profile, two model compounds, i.e. hydrophilic dye (Methylene blue) and fluorescent probe (Rhodamine B) have been used. It has been shown that EE depends on the type of nanoparticles. EE of the Rhodamine B-loaded SLN was recorded between 80-90%, for both gel-core and aqueous-core SLN. The release of Rhodamine D from gel-core SLN was slower than that from aqueous-core SLN. The release profile was shown to be dependent on the swelling of the core of SLN. Conclusion: The optimized SLN formulation proposed in this study (with the internal gel phase composed of 0.2% Carbopol 971) was found to be more stable over time, depicting lower polydispersity index and slower release, than the aqueouscore SLN. Refereed/Peer-reviewed