Poly(ethylene glycol) lipid-shelled microbubbles: abundance, stability, and mechanical properties

Poly(ethylene glycol) (PEG) is widely used on the outside of biomedical delivery vehicles to impart stealth properties. Encapsulated gas microbubbles (MBs) are being increasingly considered as effective carriers for therapeutic intervention to deliver drug payloads or genetic vectors. MBs have the advantage that they can be imaged and manipulated by ultrasound fields with great potential for targeted therapy and diagnostic purposes. Lipid-shelled MBs are biocompatible and can be functionalized on the outer surface for tissue targeting and new therapeutic methods. As MBs become a key route for drug delivery, exploring the effect of PEG-ylation on the MB properties is important. Here, we systematically investigate the effect of PEG-lipid solution concentration ranging between 0 and 35 mol % on the formation of MBs in a microfluidic flow-focusing device. The abundance of the MBs is correlated with the MB lifetime and the whole MB mechanical response, as measured by AFM compression using a tipless cantilever. The maximal MB concentration and stability (lifetime) occurs at a low concentration of PEG-lipid (∼5 mol %). For higher PEG-lipid concentrations, the lifetime and MB concentration decrease, and are accompanied by a correlation between the predicted surface PEG configuration and the whole MB stiffness, as measured at higher compression loads. These results inform the rationale design and fabrication of lipid-based MBs for therapeutic applications and suggest that only relatively small amounts of PEG incorporation are required for optimizing MB abundance and stability while retaining similar mechanical response at low loads.