Supported Lipid Monolayer with Improved Nanomechanical Stability: Effect of Polymerization

Nanomechanical Properties of Artificial Lipid Bilayers Composed of Fluid and Polymerizable Lipids

Polymerization enhances the stability of a planar supported lipid bilayer (PSLB) but it also changes its chemical and mechanical properties, attenuates lipid diffusion, and may affect the activity of integral membrane proteins. Mixed bilayers composed of fluid lipids and poly(lipids) may provide an appropriate combination of polymeric stability coupled with the fluidity and elasticity needed to maintain the bioactivity of reconstituted receptors. Previously (Langmuir, 2019, 35, 12483-12491) we showed that binary mixtures of the polymerizable lipid bis-SorbPC and the fluid lipid DPhPC form phase-segregated PSLBs composed of nanoscale fluid and poly(lipid) domains. Here we used atomic force microscopy (AFM) to compare the nanoscale mechanical properties of these binary PSLBs with single-component PSLBs. The elastic (Young's) modulus, area compressibility modulus, and bending modulus of bis-SorbPC PSLBs increased upon polymerization. Before polymerization, breakthrough events at forces below 5 nN were observed, but after polymerization, the AFM tip could not penetrate the PSLB up to an applied force of 20 nN. These results are attributed to the polymeric network in poly(bis-SorbPC), which increases the bilayer stiffness and resists compression and bending. In binary DPhPC/poly(bis-SorbPC) PSLBs, the DPhPC domains are less stiff, more compressible, and are less resistant to rupture and bending compared to pure DPhPC bilayers. These differences are attributed to bis-SorbPC monomers and oligomers present in DPhPC domains that disrupt the packing of DPhPC molecules. In contrast, the poly(bis-SorbPC) domains are stiffer and less compressible relative to pure PSLBs; this difference is attributed to DPhPC filling the nm-scale pores in the polymerized domains that are created during bis-SorbPC polymerization. Thus, incomplete phase segregation increases the stability of poly(bis-SorbPC) but has the opposite, detrimental effect for DPhPC. Overall, these results provide guidance for the design of partially polymerized bilayers for technological uses.

Physicochemical Analysis of DPPC and Photopolymerizable Liposomal Binary Mixture for Spatiotemporal Drug Release

The development of a spatiotemporal drug delivery system with a long release profile, high loading efficiency, and robust therapeutic effects is still a challenge. Liposomal nanocarriers have secured a fortified position in the biomedical field over decades. Herein, liposomal binary mixtures of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and photopolymerizable 1,2-bis(10,12-tricosadiynoyl)- sn-glycero-3-phosphocholine (DC_8,9PC) phospholipids were prepared for drug delivery applications. The diacetylenic groups of DC_8,9PC produce intermolecular cross-linking following UV irradiation. Exposure of the liposomal mixture to 254 nm radiation induces a pore within the lipid bilayer, expediting the release of its entrapped 5,6-carboxyfluorescein dye. The dosage and rate of the released content are highly dependent on the number and size of the induced pore. Photochemical cross-linking studies at different exposure times were reported through the analysis of UV-visible spectrophotometry, nano differential scanning calorimetry, Fourier transform infrared spectroscopy, and Raman spectroscopy. The optimal irradiation time was established after 8 min of exposure, inducing lipid cross-linking with minimal oxidative degradation, which plays an essential role in the pathogenesis of numerous diseases due to the formation of primary and secondary oxidation products, accordingly reducing the encapsulated drug therapeutic level.

Nanomechanical Characterization of Micellar Surfactant Films via Atomic Force Microscopy at a Graphite Surface

In this work, we study the mechanical properties of sodium dodecyl sulfate (SDS) and dodecylamine hydrochloride (DAH) micellar films at a graphite surface via atomic force microscopy (AFM). Breakthrough forces for these films were measured using silicon nitride cantilevers and were found to be 1.1 ± 0.1 nN for a 10 mM DAH film and 3.0 ± 0.3 nN for a 10 mM SDS film. For 10 mM SDS films, it was found that the addition of 1.5 mM of NaCl, Na₂SO₄, or MgCl₂ produced a 50-70% increase in the measured breakthrough force. Similar results were found for 10 mM DAH films when NaCl and MgCl₂ were added. A model was developed on the basis of previous work on lipid films and CMC data gathered via spectrofluorometry measurements to predict the change in normalized breakthrough forces with added salt concentrations for SDS and DAH films. Using this model, it was found that the activation volume required to initiate the breakthrough was roughly 0.4 nm³ for SDS and 0.3 nm³ for DAH, roughly the volume of a single molecule. Normalized breakthrough force data for SDS films with added MgCl₂ showed an unexpected dip at low added salt concentrations. The model was adapted to account for changing activation volumes, and a curve of activation volume versus magnesium concentration was obtained, showing a minimum volume of 0.21 nm³. The addition of 0.2 mM SDS to a 10 mM DAH solution was found to double the measured breakthrough force of the film. Images taken of the surface showed a phase change from cylindrical hemimicelles to a planar film that may have produced the observed differences. The pH of the bulk solution was varied for both 10 mM SDS and DAH films and was found to have little effect on the breakthrough force.