Resumen de Colloidal lipid gels: a novel approach to structure dispersions of lipids
Lipids are essential components in most living organisms whose functionality varies with their chemical structure. In an aqueous environment, diluted dispersions of phospholipids tend to form vesicles and other bilayered structures widely employed in the pharmaceutical field. These nanostructures present excellent biocompatibility and work as drug delivery systems for poorly soluble drugs.
Despite the remarkable properties of phospholipid nanostructures, those are mainly used in the liquid state; soft materials (with solid-like properties) tend to be made of other molecules, like polymers, able to form gels. The formation of phospholipid systems with a gel-like behaviour typically requires stabilising agents and increasing the phospholipids concentration above 40%. In these conditions, lipids can form cream-like dispersions of vesicles by mechanical procedures, or lyotropic phases by self-assembly.
It has been reported that polymeric or clay colloidal systems can form diluted gels by using strategies to control particle aggregation into structured networks. Although diluted suspensions of lipid vesicles behave like a colloidal system, structured networks of vesicles leading to a gel have barely been described.
This thesis aimed to develop an aqueous colloidal gel using a low concentration of lipids (mainly phospholipids) without requiring gelling agents.
We started studying aqueous lipid systems composed of phospholipids (hydrogenated soy phosphatidylcholines) and oleic acid at different pH and molar ratios. These mixtures, ranging from 0.2% to 6% lipid concentration, resulted in suspensions of vesicles with a fluid behaviour. Results showed how particle size and membrane phase transition temperature decreased at high pH due to the ionisation of oleic acid. Further assays revealed that dispersions of phospholipids and fatty acids with a specific pH (5.5 – 7.5) could form structured gels. Gelation was achieved employing a temperature-induced procedure: After the hydration of the lipids, the resulting dispersion of vesicles was frozen at -20 ºC, heated above the main phase transition temperature, and cooled back to room temperature, where gelation was completed. These results were the first report of a colloidal gel formed exclusively by lipid membranes and vesicles in a high-water content (95%). The unique properties of such colloidal phospholipid/fatty acid gels granted this new type of system with the requirements of novelty and inventive step to be patented.
Confocal microscopy illustrated that the microstructure of the gel was made of big vesicle aggregates connecting across the aqueous phase in a sponge-like matrix. The gel formation was attributed to the effect of charge in the lipid membranes. Rheological assays revealed that a lack of repulsion facilitated the rupture of the microstructure under deformation. In contrast, a high interparticle repulsion resulted in small, interconnected aggregates that deformed easier without yielding. This data led to a second patent, providing the details to form a broader range of gels stable in diverse conditions. Besides, we included preliminary experiments that tested the applicability of the gels as a topical system for permeation or wound healing, and for gastric application.
Finally, we assessed the distinct phases of the gelation mechanism through transmission electron microscopy, differential scanning calorimetry, and synchrotron X-ray scattering. These studies revealed a synergetic effect between freezing and heating that allows vesicles to fuse into tubular structures. The eventual cooling of the sample results in branched vesicle aggregates that end the formation of the gel.
With this thesis, we have contributed to the development and study of a novel colloidal gel formed exclusively by lipids. The high water content and the phospholipid composition provide the gel with excellent biocompatibility. The ability to be formed with different lipids makes these gels a platform to design products with diverse pharmaceutical and biomedical applications. Future works should address a significant focus on their practical application.
