An integrated microfluidic platform to fabricate single-micrometer asymmetric giant unilamellar vesicles (GUVs) using dielectrophoretic separation of microemulsions

A Guide to Your Desired Lipid-Asymmetric Vesicles

Liposomes are prevalent model systems for studies on biological membranes. Recently, increasing attention has been paid to models also representing the lipid asymmetry of biological membranes. Here, we review in-vitro methods that have been established to prepare free-floating vesicles containing different compositions of the classic two-chain glycero- or sphingolipids in their outer and inner leaflet. In total, 72 reports are listed and assigned to four general strategies that are (A) enzymatic conversion of outer leaflet lipids, (B) re-sorting of lipids between leaflets, (C) assembly from different monolayers and (D) exchange of outer leaflet lipids. To guide the reader through this broad field of available techniques, we attempt to draw a road map that leads to the lipid-asymmetric vesicles that suit a given purpose. Of each method, we discuss advantages and limitations. In addition, various verification strategies of asymmetry as well as the role of cholesterol are briefly discussed. The ability to specifically induce lipid asymmetry in model membranes offers insights into the biological functions of asymmetry and may also benefit the technical applications of liposomes.

Assembly methods for asymmetric lipid and polymer-lipid vesicles

Asymmetric unilamellar vesicles are aqueous bodies surrounded by two dissimilar leaflets made from lipids, polymers, or both. They are great models for cell membranes and attractive vehicles in potential biomedicine applications. Despite their promise, asymmetric unilamellar vesicles are not widely studied or adopted in applications. This is largely due to the complexity in generating asymmetric membranes. Recent technical advances in microfluidics have opened doors to high throughput fabrication of asymmetric unilamellar vesicles. In this review, we focus on microfluidic methods for generating asymmetric lipid vesicles, with two dissimilar lipid leaflets, and asymmetric lipid-polymer vesicles, with one lipid leaflet and one polymer leaflet. We also review a few standard non-microfluidic methods for generating asymmetric vesicles. We hope to highlight the improved capability in obtaining asymmetric vesicles through a variety of methods and encourage the wider scientific community to adopt some of these for their own work.

Protocells: Milestones and Recent Advances

The origin of life is still one of humankind's great mysteries. At the transition between nonliving and living matter, protocells, initially featureless aggregates of abiotic matter, gain the structure and functions necessary to fulfill the criteria of life. Research addressing protocells as a central element in this transition is diverse and increasingly interdisciplinary. The authors review current protocell concepts and research directions, address milestones, challenges and existing hypotheses in the context of conditions on the early Earth, and provide a concise overview of current protocell research methods.