Reverse micelles produce hydroxyapatite nanoparticles as more efficient gene delivery carriers than regular micelles

Hydroxyapatite (HAp) is an effective inorganic gene delivery carrier due to its ability to transport genetic cargo across cell membranes, protect it from proteolysis, and enable escape from late endosomes via pH-controlled dissolution. However, its transfection efficiency remains lower than that of viral agents, prompting studies of hybrids with cationic molecules or phases to enhance the gene delivery performance. This study reports on the synthesis of HAp in regular and reverse micellar regions of a ternary microemulsion system composed of cetyltrimethylammonium bromide (CTAB), 1-hexanol and water. Spectroscopic characterization revealed that CTAB headgroups adopted more ordered supramolecular conformations in reverse micelles compared to regular ones. Similarly, water within reverse micelles exhibited more homogeneity and unexpected freedom, creating favorable entropic conditions for chemical reactions. CTAB showed strong electrostatic affinity for DNA and bound more effectively to HAp synthesized within the confined nanoscale environment of reverse micelles than to HAp produced in the aqueous continuum surrounding regular micelles. Also, reverse micelles produced narrowly dispersed, rod-shaped HAp nanoparticles, unlike the larger, macroporous particles formed in regular micelles. Both of these effects predisposed HAp from reverse micelles to exhibit a higher transfection efficiency in K7M2 osteosarcoma cells than its regular micelle counterpart. Despite these positive outcomes, HAp could only partially mitigate the cytotoxic effects of CTAB. Therefore, further exploration of advanced synthesis methods, biocompatible surfactants or strategies to preserve the synergy between HAp, CTAB and DNA while reducing CTAB toxicity is essential for enhancing the gene delivery performance of reverse micellar HAp. • HAp was synthesized in regular and reverse micelles of a cationic microemulsion. • The shape and connectivity of aqueous compartments determined the morphology of HAp. • The surfactant bound DNA and HAp more strongly in reverse micelles than in regular. • Reverse micelle HAp transfected osteosarcoma cells more effectively. • The use of biocompatible surfactants is imperative for future optimizations.