Solid lipid nanoparticles and nanostructured lipid carriers of dual functionality at emulsion interfaces. Part II: active carrying/delivery functionality

The utilisation of lipid nanostructures that can in tandem act as Pickering emulsion stabilisers and as active carrier/delivery systems, could potentially enable the development of liquid (emulsion-based) formulations with the capacity for multi-active encapsulation and delivery. Part I of this work focused on the first aspect of this two-fold functionality by investigating the capacity of both solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) to act as effective Pickering particles in o/w emulsions. Herein, attention shifts to the secondary functionality, with part II of this study assessing both SLNs and NLCs in terms of their capacity to act as carriers and release regulators for curcumin, a model hydrophobic active. The previously established Pickering functionality and physical properties in terms of particle size, zeta potential and interfacial tension of the lipid particles remained unaffected after encapsulation of curcumin. In emulsions, loss of crystalline (solid lipid) matter and particle interfacial presence were specifically investigated, as these aspects can impact upon the particles’ active carrying and delivery performance. Low solid matter losses were recorded for all emulsions (ranging between 0% and 15%), with increasing liquid lipid fraction in the particles (SLNs to NLCs) resulting in relatively higher depletion of crystallinity. Removal of unadsorbed surfactant (remnant from the particle formation processing step) prior to emulsification led to higher particle interfacial occupancy. Despite said changes, the lipid particles’ curcumin carrying capacity, expressed as encapsulation efficiency and loading capacity, did not differ between an emulsion and dispersion setting. Although the active carrying capacity was retained, it was shown that the presence of the particles at the emulsion interfaces affects the curcumin release rate. Partial migration of curcumin to the oil droplet and creation of an additional release-inducing potential to the particles in close proximity to the droplet interface are proposed to be responsible for the overall faster active expulsion. What is more, the curcumin release profile from either SLNs or NLCs (also) stabilising an emulsion microstructure, was shown to persist after storage; either storage of the particles (up to 4 months) prior to emulsification, or storage of emulsions (up to 3 months) stabilised by ‘freshly’ formed lipid particles. Overall, the present study provides evidence that the two-fold functionality of the lipid particles can be indeed realised, markedly demonstrating that their concurrency does not compromise one another.