Artificial Cell Membranes Interfaced with Optical Tweezers: A Versatile Microfluidics Platform for Nanomanipulation and Mechanical Characterization

Abstract

Cell lipid membranes are the site of vital biological processes, such as motility, trafficking, and sensing, many of which involve mechanical forces. Elucidating the interplay between such bioprocesses and mechanical forces requires the use of tools that apply and measure piconewton-level forces, e.g., optical tweezers. Here, we introduce the combination of optical tweezers with free-standing lipid bilayers, which are fully accessible on both sides of the membrane. In the vicinity of the lipid bilayer, optical trapping would normally be impossible due to optical distortions caused by pockets of the solvent trapped within the membrane. We solve this by drastically reducing the size of these pockets via tuning of the solvent and flow cell material. In the resulting flow cells, lipid nanotubes are straightforwardly pushed or pulled and reach lengths above half a millimeter. Moreover, the controlled pushing of a lipid nanotube with an optically trapped bead provides an accurate and direct measurement of important mechanical properties. In particular, we measure the membrane tension of a free-standing membrane composed of a mixture of dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC) to be 4.6 × 10–6 N/m. We demonstrate the potential of the platform for biophysical studies by inserting the cell-penetrating trans-activator of transcription (TAT) peptide in the lipid membrane. The interactions between the TAT peptide and the membrane are found to decrease the value of the membrane tension to 2.1 × 10–6 N/m. This method is also fully compatible with electrophysiological measurements and presents new possibilities for the study of membrane mechanics and the creation of artificial lipid tube networks of great importance in intra- and intercellular communication.

Hydrodynamic shear dissipation and transmission in lipid bilayers

“…The geometry of the freestanding lipid bilayer simplifies the problem, as out-of-plane membrane deformations do not need to be taken into account. Indeed, for our observations, we find that the ratio of membrane tension to viscous forces, characterized by the capillary number, is negligible, , with N previously measured for a DOPC/DPPC bilayer prepared using the same method ( 39 ). Therefore, we do not expect the bilayer to deform.…”

“…Freestanding lipid bilayers are formed in microfluidic devices following a previously described protocol ( 38 , 39 ) ( Materials and Methods ). The microfluidic devices consist of two parallel rectangular microchannels (100- m high and 500- m wide) with several apertures between the two channels, where lipid bilayers are formed ( Fig.…”

“…Here, we perform active, noncontact, two-point microrheology to characterize the viscous and frictional properties of the bilayer. Our approach uses a planar freestanding lipid bilayer ( 38 , 39 ). The planar geometry of the freestanding membranes allows us to decouple in- and out-of-plane deformations, which facilitates the interpretation of the measurements using hydrodynamic models ( 22 , 36 , 37 ).…”

Characterizing microfluidic approaches for a fast and efficient reagent exchange in single-molecule studies

“…We employed a pair of glass syringes (1 ml PTFE-PEEK thread, Setonic 2624016 and 10 ml PTFE-PEEK thread, Setonic 2624076) and PTFE tubing to connect them to the flow cells (PTFE 1.6 OD, Cetoni 701175). Alternative solutions controlling the pressure of a liquid reservoir; thus the velocity of the outlet, were also considered 35,36 . These have been widely employed by other authors in multiple studies [37][38][39][40] .…”

Biological lipid nanotubes and their potential role in evolution

“…In model systems, the retraction speeds are typically in the range between 1 and 10 µm/s [21]. Force induced nanotube formation has been studied with optical tweezers [25], hydrodynamic flows [21], microfluidics [26], or micro-needles [27,28] (Fig. 1B).…”