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

Solid lipid nanoparticles and nanostructured lipid carriers of dual functionality at emulsion interfaces. Part I : Pickering stabilisation functionality

Solid lipid nanoparticles and nanostructured lipid carriers are two types of lipid nanoparticulate systems, that have been primarily studied for their capability to function as active carriers, and only more recently utilised in Pickering emulsion stabilisation. Unveiling the factors that impact upon the lipid particle characteristics related to their Pickering functionality could enable the development of a liquid formulation with tailored microstructure and potentially the capacity to display a two-fold performance. In part I, this work investigates how certain formulation characteristics, namely solid-to-liquid lipid mass ratio and presence of unadsorbed surfactant in the aqueous carrier phase, affect the structural properties of the lipid particles, and in turn how these influence their Pickering stabilisation capacity. The effect of the formulation parameters was assessed in terms of the wettability and physicochemical properties of the lipid particles, including particle size, crystallinity and interfacial behaviour. Lipid particles fabricated with higher liquid lipid content (70% w/w) were shown to be more hydrophilic and have lower surfactant decoration at their surface compared to particles containing lower or no liquid lipid in their crystalline matrix. The emulsion stabilisation ability through a Pickering mechanism was confirmed for all types of lipid particles using polarised microscopy. Increasing liquid lipid content and removal of excess surfactant did not compromise the particle stabilisation capacity, though emulsion droplets of larger sizes were initially acquired in the latter case. The particle-stabilised emulsions maintained their physical integrity, with particles retaining close association with the emulsion interface over a storage period of 12 weeks