Manufacturing solid lipid nanoparticle (SLN) dispersions using a microwave-assisted microemulsion approach

As a drug delivery system, solid lipid nanoparticles (SLNs) hold a great deal of promise thanks to their stability and biocompatibility. SLNs are solid particles in the nanometre range, synthesised from fatty acid and dispersed in a liquid medium with the aid of surfactants. The solid nature of SLNs imparts stability to the dispersion, while the use of lipids allows for solubilisation of hydrophobic drugs. The nanoparticle size gives them access across biological barriers that may block other, larger particles. A recent advance in SLN technology has simplified the synthesis of SLNs by the use of microwave-assisted microemulsion formulations. This is a single-pot synthesis method. Typical ingredients consist of a lipid, a surfactant, perhaps a co-surfactant, water and a drug of interest. There is scope to design the formulation to allow for requirements of SLN size, for example, or length of time before the drug is released. In the current study, several SLN formulations fabricated from different lipids (fatty acids) were compared for their size and stability. Stearic acid was the main focus, with an investigation into any factors that may have had an influence on SLN synthesis, such as manufacturer, purity and age. A secondary focus was on how lipid chain length might affect SLNs, and to this end SLNs synthesised from lauric acid, palmitic acid and behenic acid were compared with those synthesised from stearic acid. The findings of the stearic acid assay suggest that the age of the lipid, the purity and the manufacturer from whom they are sourced make little difference. By contrast, chain length was found to have a significant effect on SLN size. The lipid with the longest chain, behenic acid, produced the smallest SLN, and the most stable. Palmitic acid, at a similar length to stearic acid, produced SLNs of a similar size, though with a reduced stability. Lauric acid, the smallest lipid investigated, produced the largest and least stable SLNs. Microwave-assisted microemulsion synthesis is a fast and facile method of SLN production, with scope for formulation design. Further investigation could be of value into the best way of ensuring that drugs are incorporated into the SLN in the optimum manner for the required delivery time; for example, in the shell for a fast delivery or in the core for a delayed timescale.