Spontaneous Formation of Oil-Rich Polymer-Only Droplet Interface Bilayers Assembled from PB-PEO Block Copolymers

Droplet interface bilayers (DIBs) are versatile membrane models that have traditionally relied on phospholipids for mimicking the structure and transport properties of biological membranes. However, lipid-based DIBs are often limited by their mechanical fragility and restricted compositions. Here, we report the spontaneous formation and characterization of polymer-only DIBs (pDIBs) assembled from amphiphilic polybutadiene-<i>block</i>-poly(ethylene oxide) (PB-PEO) diblock copolymers in hydrocarbon oils. Using a combination of interfacial tensiometry, electrophysiology, and imaging, we show that PB-PEO amphiphiles form stable monolayers and thick interfacial bilayer membranes in both hexadecane and squalene without the need for external bias, additional lipids, or cosolvents. These pDIBs exhibit high values of membrane resistance (10-50 MΩ·cm²), negligible permeability to small hydrophilic solutes, and substantially larger hydrophobic thicknesses (18-23 nm) than their lipid analogs. Under applied voltage, PB-PEO membranes undergo reversible thinning and lateral expansion, with extracted electrocompression and electrowetting coefficients exceeding those reported for lipid membranes by up to 1000-fold. These characteristics indicate a significant amount of hexadecane or squalene remains within the hydrophobic core of the membrane upon bilayer formation. Incorporation of valinomycin and alamethicin resulted in voltage-responsive ion transport, demonstrating that even thick polymer membranes can support bioinspired ion conduction. These findings demonstrate PB-PEO-based pDIBs as an alternative option for constructing model membranes and motivate continued work to examine a wider variety of amphiphilic block copolymers and oils necessary for achieving thinner polymer membranes useful in diverse applications such as synthetic biology, programmable biomimetic materials, and bioinspired tissue-like assemblies.