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Materials Science of Supported Lipid Membranes www.mrs.org/bulletin assert that the creation of new materials capable of performing advanced techno- logical functions, for example, complex Materials Science of and cooperative processes for memory, self-replication, and self-repair, will ulti- mately rely upon understanding and con- trolling dynamic self-assembly in both Supported Lipid natural and synthetic systems. We empha- size that understanding the interrelations between structure, assembly, dynamics, and function in lipid membranes con- Membranes tributes toward these broad objectives of contemporary materials science. Furthermore, there are many direct and Atul N. Parikh and Jay T. Groves, Guest Editors practical benefits of learning to devise synthetic materials that mimic the key properties of cellular membranes. Such Abstract materials may provide new classes of Supported membranes represent an elegant route to designing well-defined fluid biosensors, diagnostic tools, biocompati- interfaces which mimic many physical–chemical properties of biological membranes. ble materials, and high-throughput char- acterization platforms for rapid and early Recent years have witnessed rapid growth in the applications of physical and materials detection of interactions between cells and science approaches in understanding and controlling lipid membranes. Applying these their environment. Examples include sen- approaches is enabling the determination of their structure–dynamics–function relations sors and assays for detecting toxins and and allowing the design of membrane-mimetic devices. The collection of articles pathogens (e.g., in national security appli- presented in this issue of MRS Bulletin illustrates the breadth of activity in this growing cations), air and water quality monitoring, partnership between materials science and biophysics. Together, these articles highlight and drug screening platforms. They also some of the key challenges of cellular membranes and exemplify their utility in promise tools for life science research, for fundamental biophysical studies and technological applications. The topics covered example, in developing a molecular-level also confirm the importance of lipid membranes as an exciting example of soft understanding of cell surface mechanisms condensed matter. We hope that this issue will serve readers by highlighting the during cellular homeostasis and diseases. intellectual scope and emerging opportunities in this highly interdisciplinary area of ma- Within this framework, one ap- terials research. proach that is proving powerful involves interfacing lipid bilayer membranes to Keywords: biological, biomimetic, cellular. solid substrates via self-assembly and directed self-assembly methods. These membrane-mimicking architectures are Lipid bilayer membranes are the uni- materials. Important as they are, the range collectively referred to as supported mem- versal material of choice for defining and of functions in these materials is con- branes7 (Figure 1). controlling cellular organization in living strained by their static structure. In contrast, Supported membranes are typically systems. They are a major constituent of lipid bilayer membranes exploit their formed at the solid–liquid interface when the biological membranes that serve as the chemical heterogeneity, phase behavior, vesicular microphases of lipids spontane- outer boundary of cells and organelles.1 and dynamics to produce an impressive ously rupture and spread on hydrophilic As such, they serve as a means to com- set of time-dependent functions for the surfaces (Figures 1a and 1b). Alternative partmentalize, juxtapose, regulate, and biological membrane. Examples include strategies employ Langmuir–Blodgett generally mediate biomolecular interac- stimuli-induced protein clustering, lipid troughs to successively transfer two li- tions. The astounding complexity of life reorganization, and topographical changes pidic monolayers from the air–water in- is intimately associated with the diverse that regulate broad classes of biological terface onto planar surfaces3,8 (Figure 1c). physical–chemical properties of these mem- functions of membranes4,5 including sig- Recent studies reveal that single lipid bi- branes,2 which include two-dimensional naling, molecular recognition, and trans- layers can also form when dried lipids fluidity, material elasticity, thermal fluctu- port. This spatio-temporal mode of lipid stamped onto a hydrophilic surface expe- ations, chemical diversity, and rich phase organization, or dynamic self-assembly, rience hydration.9 In all of these cases, behavior.3 In this regard, developing a in biological membranes6 exemplifies a single lipid bilayers are separated from materials-science-based understanding of major shift in emphasis from thermody- the substrate surface through an interven- lipid membranes is an exciting endeavor. namic to kinetic regimes. Here, equilib- ing layer of hydration water (dark-blue Traditional approaches to materials rium structures (global free-energy layer in Figure 1d, variously estimated at synthesis have largely relied on uniform, minima) are replaced by higher-order 6–15 Å in thickness at bilayer–silica inter- equilibrated phases leading to static “con- organizational states representing facile faces) and exhibit two-dimensional conti- densed matter” structures, for example, transitions between various local meta- guity and fluidity resembling that of lipid monolithic single crystals. Over the past stable free-energy minima of different membranes of vesicles and living cells.10 several decades, these approaches have structures. Examples of synthetic mate- Each of the two lipid layers in the lipid bi- led to the development of a wide range of rials deliberately fabricated using such a layer is called a “leaflet.” In cells, the outer technologically useful materials including dynamic self-assembly approach are rare, leaflet is in contact with the extracellular semiconducting, ferroelectric, nonlinear reflecting the current lack of a fundamen- matrix, and the inner leaflet faces the cell optical, superconducting, and piezoelectric tal understanding of such processes. We cytoplasm. The precursor phases in these MRS BULLETIN • VOLUME 31 • JULY 2006 507 Materials Science of Supported Lipid Membranes issues and offers a strategy to cushion the membrane–substrate interface via ultra- thin polymer supports. He further demon- strates how the use of intercalated polymers may offer a materials route to modeling the generic role of the extracel- lular matrix and glycocalyx. The term gly- cocalyx refers to a variety of glycosylated proteins, glycolipids, and polymeric struc- tures that often terminate cell surfaces. An important structural feature of lipid membranes in biology is their ability to bend. There is a growing recognition that membrane curvature is an active regulator of membrane microphase separation and a concentrator of important biological functions. Sasaki and Stevens emphasize the issue, survey a physical-science-based understanding of the formation of these submembrane structural features, and highlight associated materials issues. The perspective by Zhang and Granick illustrates the importance of advanced characterization methods in characteriz- ing the lateral dynamics in supported membranes. A long-standing issue, one with many inconsistencies, relates to the issue of complex translational dynamics. Figure 1. Schematic illustration of basic supported-membrane formation processes. Structural, chemical, and phase hetero- (a) Vesicles adsorb, fuse, and rupture at solid surfaces; (b) lipid stacks spontaneously geneities within membranes exist both spread upon hydration; and (c) two successive transfers of Langmuir–Blodgett monolayers statically and dynamically (such as in re- deposit pre-equilibrated lipid mixtures to form (d) single, supported bilayers on hydrophilic sponse to external stimuli, e.g., protein substrates. The dark-blue area is the intervening layer of hydration water between the binding). Moreover, these heterogeneities substrate and the lipid. often emerge independently in the two leaf- lets of the bilayer architecture. Complex approaches can consist of purified and provide an experimental test bed for ques- dynamics attending such asymmetries re- synthetically tailored lipids and their tions in soft condensed matter, interfacial quires advanced characterization tools ca- complex mixtures in predetermined com- confinement, low-dimensional phases, pable of measuring molecular diffusivities positions, providing molecular-level chemistry in two dimensions, reaction- at a range of length and time scales, also control over their structure, chemical com- diffusion systems, and wetting. discriminating the two leaflets. position, phase state, and lateral fluidity. Second, it demonstrates how the appli- The article by Wu et al. is focused on the Moreover, because these constructs inter- cation of principles, tools, and methodolo- permeability characteristics of cellular face membrane architecture with solid sub- gies developed extensively in one area of membranes. Using supported membranes strates, they are amenable to a broad suite science can benefit the development of an- and lipid monolayers, the authors illus- of surface analytical tools for molecular- other, disparate discipline. We emphasize trate how synthetic molecules (e.g., tri- scale characterization of their structure that the two-way traffic between funda- block copolymers) can be used to repair and dynamics and enable chip-based bio- mental studies
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