UC Santa Barbara

Surfactants and lipids are used in a broad range of applications from basic science to industrial hygiene and food products. These molecules are versatile building blocks for self-assembling materials, with tunable surface properties and nanostructure via the interplay of different forces. Further, surface modification of membranes with specialized lipids can allow for formation of complex, functional materials with hierarchical assembly. This work discusses two different surfactant-based systems and their tunable features.

Cationic liposomes (CLs) are a common synthetic carrier of nucleic acids (NA) for gene delivery and silencing. Optimization of NA delivery and expression requires understanding of the interactions between the lipid nanoparticles (NPs) and cellular membranes, affecting NP binding, uptake, endocytic trafficking, and endosomal escape. PEG(polyethylene glycol)-lipid molecules can also be distally modified with peptide binding groups in order to target to different tissue types via specific protein-ligand interactions. In our first study, we modulate the specific and nonspecific binding interactions between peptide-targeted NPs and cells and use flow cytometry to measure the uptake of targeted NPs by several cancer cell lines in vitro. Several optimized formulations were subsequently evaluated in vivo for tumor selectivity, with promising results.

After CL-NA NPs bind to targeted cells, surface properties of the NP are still critical for endosomal escape and NA cargo delivery. In two studies, we tuned NP surface properties via addition of cationic lipids or PEG-lipid molecules modified with peptide or hydrophobic binding groups, which can tether the NPs to cellular membranes via polymer bridges, with the goal of promoting endosomal escape. The study on hydrophobically-modified PEG-lipids showed that small variations in chemical structure and membrane composition significantly altered the binding properties of CLs and CL-NA NPs, likely due to the equilibrium balance of different conformations of the PEG-lipid. Formulations were subsequently evaluated in vitro using confocal microscopy and particle localization software to demonstrate the effect of surface properties on cell binding and endosomal trafficking, with GFP-labeled Rab proteins utilized to identify different endocytic pathways.

Finally, we explore inter-membrane interactions in a system of single-chain surfactants, similar to those used in the commercial beauty products (specifically, hair conditioner). This system is composed of two fatty alcohols, a monovalent cationic surfactant, and water. Small-angle x-ray scattering and polarized optical microscopy were used to determine structure and phase behavior across composition space. We observed at least four different phases as a function of membrane charge density (σ), including a chain-ordered lamellar phase (LCO) at low σ and a second lamellar phase (Lx) of intermediate order at high σ that coexists with a fiber-like phase of tubular morphology. This study led to the discovery of new structural features and unexpected membrane interactions that better informed our understanding of how composition determines structure and, ultimately, the viscoelastic properties of interest for this application.