On-Chip Octanol-Assisted Liposome Assembly for Bioengineering

Summary

The present protocol describes octanol-assisted liposome assembly (OLA), a microfluidic technique to generate biocompatible liposomes. OLA produces monodispersed, micron-sized liposomes with efficient encapsulation, allowing immediate on-chip experimentation. This protocol is anticipated to be particularly suitable for synthetic biology and synthetic cell research.

Abstract

Microfluidics is a widely used tool to generate droplets and vesicles of various kinds in a controlled and high-throughput manner. Liposomes are simplistic cellular mimics composed of an aqueous interior surrounded by a lipid bilayer; they are valuable in designing synthetic cells and understanding the fundamentals of biological cells in an in vitro fashion and are important for applied sciences, such as cargo delivery for therapeutic applications. This article describes a detailed working protocol for an on-chip microfluidic technique, octanol-assisted liposome assembly (OLA), to produce monodispersed, micron-sized, biocompatible liposomes. OLA functions similarly to bubble blowing, where an inner aqueous (IA) phase and a surrounding lipid-carrying 1-octanol phase are pinched off by surfactant-containing outer fluid streams. This readily generates double-emulsion droplets with protruding octanol pockets. As the lipid bilayer assembles at the droplet interface, the pocket spontaneously detaches to give rise to a unilamellar liposome that is ready for further manipulation and experimentation. OLA provides several advantages, such as steady liposome generation (>10 Hz), efficient encapsulation of biomaterials, and monodispersed liposome populations, and requires very small sample volumes (~50 µL), which can be crucial when working with precious biologicals. The study includes details on microfabrication, soft-lithography, and surface passivation, which are needed to establish OLA technology in the lab. A proof-of-principle synthetic biology application is also shown by inducing the formation of biomolecular condensates inside the liposomes via transmembrane proton flux. It is anticipated that this accompanying video protocol will facilitate the readers to establish and troubleshoot OLA in their labs.

Introduction

All cells have a plasma membrane as their physical boundary, and this membrane is essentially a scaffold in the form of a lipid bilayer formed by the self-assembly of amphiphilic lipid molecules. Liposomes are the minimal synthetic counterparts of biological cells; they have an aqueous lumen surrounded by phospholipids, which form a lipid bilayer with the hydrophilic head groups facing the aqueous phase and the hydrophobic tails buried inward. The stability of liposomes is governed by the hydrophobic effect, as well as the hydrophilicity between the polar groups, van der Waals forces between the hydrophobic carbon tails, and the hydrogen bonding between water molecule....

Protocol

1. Fabricating the master wafer

1. Take a 4 in (10 cm) diameter clean silicon wafer (see Table of Materials). Clean it further using pressurized air to remove any dust particles.
2. Mount the wafer on a spin coater, and gently dispense ~5 mL of a negative photoresist (see Table of Materials) in the center of the wafer. Try to avoid air bubbles, as they might interfere with the downstream printing process of the wafer.
3. To obtain a 10 µm thick photoresist layer, spin-coat the wafer at 500 rpm for 30 s with an acceleration of 100 rpm/s for initial spreading, followed by a 60 s spin at 3,000....

Results

This study demonstrates the formation of membraneless condensates via the process of liquid-liquid phase separation (LLPS) inside liposomes as a representative experiment.

Sample preparation
The IA, OA, ES, and feed solution (FS) are prepared as follows:

IA: 12% glycerol, 5 mM dextran, 150 mM KCl, 5 mg/mL poly-L-lysine (PLL), 0.05 mg/mL poly-L-lysine-FITC labeled (PLL-FITC), 8 mM adenosine triphosphate (ATP), 15 mM citrate-HCl (pH 4)<.......

Discussion

Cellular complexity makes it extremely difficult to understand living cells when studied as a whole. Reducing the redundancy and interconnectivity of cells by reconstituting the key components in vitro is necessary to further our understanding of biological systems and create artificial cellular mimics for biotechnological applications^22,23,24. Liposomes serve as an excellent minimal system to understand cellular phenomena. A no.......

Materials

Name	Company	Catalog Number	Comments
1-Octanol	Sigma-Aldrich	No. 297887
1.5 mL tubes	Fisher scientific	10451043	Eppendorf 3810X Polypropylene microcentrifuge tubes
ATP	Sigma-Aldrich	No. A2383
Biopsy punch	Darwin microfluidics	PT-T983-05	0.5 mm and 3 mm diameter
Citrate-base	Sigma-Aldrich	No. 71405
Dextran	Sigma-Aldrich	No. 31388	Mr~6,000
Direct-write optical lithography machine	Durham Magneto Optics Ltd	MicroWriter ML3 Baby	setup and software
DOPC lipid	Avanti	SKU:850375C
F68	Sigma-Aldrich	No. 24040032
Glass cover slip	Corning	#1, 24 x 40 mm
Glycerol	Sigma-Aldrich	No. G2025
Hydrochloric acid	Thermo Scientific Acros	No. 124630010
Liss Rhod PE lipid	Avanti	SKU:810150C
Parafilm	Sigma-Aldrich	No. P7793
Photoresist	Micro resist technology GmbH	EpoCore 10
Photoresist developer	micro resist technology GmbH	mr-Dev 600
Plasma cleaner	Harrick plasma	PDC-32G
Polydimethylsiloxane	Dow	Sylgard 184	PDMS and curing agent
Poly-L-lysine	Sigma-Aldrich	No. P7890
Poly-L-lysine–FITC Labeled	Sigma-Aldrich	No. P3543
Polyvinyl alcohol	Sigma-Aldrich	no. P8136	molecular weight 30,000–70,000, 87%–90% hydrolyzed
Pressure controller	Elveflow	OBK1 Mk3+	Flow controller
Scotch tape	Magic Tape Invisible Matt Tape
Silicon wafer	Silicon Materials	0620R16002
Spin coater	Laurell Technologies Corporation	Model WS-650MZ-23NPP
Stainless Steel 90° Bent PDMS Couplers	Darwin microfluidics	PN-BEN-23G
Tris-base	Sigma-Aldrich	No. 252859
Tygon tubing	Darwin microfluidics	1/16" OD x 0.02" ID
UV laser			365 nm wavelength