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Assembly, morphology, diffusivity and indentation of hydrogel-supported lipid bilayers Tooba Shoaib, Prathima Chandra Nalam, Yichen He, Yuting Chen, and Rosa M. Espinosa-Marzal Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01062 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017
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Assembly, morphology, diffusivity and indentation of hydrogel-supported lipid bilayers Tooba Shoaib † ‡, Prathima C. Nalam†, Yichen He†, Yuting Chen†, Rosa M. Espinosa-Marzal†,* †
Department of Civil and Environmental Engineering, University of Illinois at Urbana-
Champaign, Urbana 61801, Illinois, USA ‡
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign,
Urbana 61801, Illinois, USA * Corresponding Author:
[email protected] KEYWORDS: Cell membrane, hydrogel, lipid bilayer, polyelectrolyte multilayer, atomic force microscopy, hydrogel-supported membrane
ABSTRACT: Recognizing the limitations of solid-supported lipid bilayers to reproduce the behavior of cell membranes, including bendability, transmembrane protein inclusion and virus entry, this study describes a novel biomimetic system for cell membranes with the potential to overcome these and other limitations. The developed strategy utilizes a hydrogel with tunable mechanical behavior that resembles those of living cells as the soft support for the phospholipid bilayer, while a polyelectrolyte multilayer film serves as an intermediate layer to facilitate the self-assembly of the lipid bilayer on the soft cushion. Quartz Crystal Microbalance studies show
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that, upon coming into contact with the polyelectrolyte film, vesicles fuse and rupture to yield a robust lipid bilayer. Fluorescence recovery after photobleaching confirms the formation of a membrane, while Atomic Force Microscopy shows a low adhesion between the indenting probe and the bilayer. More importantly, in comparison to the solid-supported lipid bilayer, the response of this biomimetic system to nanoindentation demonstrates its increased mechanical stability and bendability when assembled on a soft cushion. Hence, the developed hydrogelsupported lipid bilayers can mimic biomechanical properties of cell membranes, which will enable to study and to understand biophysicochemical interactions between cell membranes and extracellular entities.
1. Introduction Lipid bilayer, the basic structural unit of cell biological membranes, is the nature’s unique artwork to provide an effective protection for the cell components. Its composition, elasticity and 2D-fluidity plays an important role in enabling the cell to perform crucial and complex transmembrane signaling functions, as well as in maintaining the required mechanical stability of the cell. The membrane of the living cell is highly intricate due to its heterogeneous and complex composition resulting in convoluted biophysical membrane interactions1. With the advent of nanomedicine and targeted drug delivery, there is a need for better understanding of the membrane interactions with the newly developed drugs in order to enhance the endocytosis of the drug2 and to reduce the cytotoxic effects of drugs and toxicants on living cells3, 4. A wellaccepted and effective approach to systematically evaluate the biophysicochemical interactions of cell membranes with the targeted drugs relies on the use of simplified cell membrane systems that mimic the behavior of natural membranes5.
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Solid-supported lipid bilayers (SLBs) deposited on smooth, solid substrates, such as silica, gold or mica, are widely used as model systems to study cell membrane processes6, 7, 8, 9. In spite of being transferred to a hard and impermeable substrate, SLBs have shown high mobility due to the presence of a thin water layer that remains between the hydrophilic substrate and the bottom lipid monolayer10, 11. Further, these model systems are readily prepared either by lipid vesicle fusion12,
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or by lipid layer transfer methods such as Langmuir-Blodgett/Schaefer14,
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techniques. However, SLBs suffer several limitations including the inability to incorporate transmembrane proteins and glycoproteins in their native state, due to both the proximity of the membrane to the substrate17 and the influence of the underlying substrate‘s surface forces on the permeation of solutes and drugs. More importantly, the mechanical response of the lipid bilayer when supported on a hard substrate differs significantly from that of the membrane on its native soft cytoskeleton18. The cytoskeleton of the cell is composed of a crosslinked network of filamentous proteins such as actin, microtubules and intermediate filaments, which are highly hydrated, characterized by a low elastic modulus (
