Droplet Polymer Bilayers for Bioelectronic Membrane Interfacing

Model membranes interfaced with bioelectronics allow for the exploration of fundamental cell processes and the design of biomimetic sensors. Organic conducting polymers are an attractive surface on which to study the electrical properties of membranes because of their low impedance, high biocompatibility, and hygroscopic nature. However, establishing supported lipid bilayers (SLBs) on conducting polymers has lagged significantly behind other substrate materials, namely, for challenges in membrane electrical sealing and stability. Unlike SLBs that are highly dependent on surface interactions, droplet interface bilayers (DIBs) and droplet hydrogel bilayers (DHBs) leverage the energetically favorable organization of phospholipids at atomically smooth liquid interfaces to build high-integrity membranes. For the first time, we report the formation of droplet polymer bilayers (DPBs) between a lipid-coated aqueous droplet and the high-performing conducting polymer poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS). The resulting bilayers can be produced from a range of lipid compositions and demonstrate strong electrical sealing that outcompetes SLBs. DPBs are subsequently translated to patterned and planar microelectrode arrays to ease barriers to implementation and improve the reliability of membrane formation. This platform enables more reproducible and robust membranes on conducting polymers to further the mission of merging bioelectronics and synthetic, natural, or hybrid bilayer membranes.

Simultaneous Electrophysiology and Imaging Reveal Changes in Lipid Membrane Thickness and Tension upon Uptake of Amphiphilic Gold Nanoparticles

Amphiphilic gold core nanoparticles (AmNPs) striped with hydrophilic 11-mercapto-1-undecanesulfonate (MUS) and hydrophobic 1-octanethiol (OT) ligands are promising candidates for drug carriers that passively and nondisruptively enter cells. Yet, how they interact with cellular membranes is still only partially understood. Herein, we use electrophysiology and imaging to carefully assess changes in droplet interface bilayer lipid membranes (DIBs) incurred by striped AmNPs added via microinjection. We find that AmNPs spontaneously reduce the steady-state specific capacitance and contact angle of phosphatidylcholine DIBs by amounts dependent on the final NP concentration. These reductions, which are greater for NPs with a higher % OT ligands and membranes containing unsaturated lipids but negligible for MUS-only-coated NPs, reveal that AmNPs passively embed in the interior of the bilayer where they increase membrane thickness and lateral tension through disruption of lipid packing. These results demonstrate the enhanced evaluation of nano-bio interactions possible via electrophysiology and imaging of DIBs.

Entrapment and Voltage-Driven Reorganization of Hydrophobic Nanoparticles in Planar Phospholipid Bilayers

Engineered nanoparticles (NPs) possess diverse physical and chemical properties, which make them attractive agents for targeted cellular interactions within the human body. Once affiliated with the plasma membrane, NPs can become embedded within its hydrophobic core, which can limit the intended therapeutic functionality and affect the associated toxicity. As such, understanding the physical effects of embedded NPs on a plasma membrane is critical to understanding their design and clinical use. Here, we demonstrate that functionalized, hydrophobic gold NPs dissolved in oil can be directly trapped within the hydrophobic interior of a phospholipid membrane assembled using the droplet interface bilayer technique. This approach to model membrane formation preserves lateral lipid diffusion found in cell membranes and permits simultaneous imaging and electrophysiology to study the effects of embedded NPs on the electromechanical properties of the bilayer. We show that trapped NPs enhance ion conductance and lateral membrane tension in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) bilayers while lowering the adhesive energy of the joined droplets. Embedded NPs also cause changes in bilayer capacitance and area in response to applied voltage, which are nonmonotonic for DOPC bilayers. This electrophysical characterization can reveal NP entrapment without relying on changes in membrane thickness. By evaluating the energetic components of membrane tension under an applied potential, we demonstrate that these nonmonotonic, voltage-dependent responses are caused by reversible clustering of NPs within the unsaturated DOPC membrane core; aggregates form spontaneously at low voltages and are dispersed by higher transmembrane potentials of magnitude similar to those found in the cellular environment. These findings allow for a better understanding of lipid-dependent NP interactions, while providing a platform to study relationships between other hydrophobic nanomaterials and organic membranes.

Homogeneous hybrid droplet interface bilayers assembled from binary mixtures of DPhPC phospholipids and PB-b-PEO diblock copolymers

Hybrid membranes built from phospholipids and amphiphilic block copolymers seek to capitalize on the benefits of both constituents for constructing biomimetic interfaces with improved performance. However, hybrid membranes have not been formed or studied using the droplet interface bilayer (DIB) method, an approach that offers advantages for revealing nanoscale changes in membrane structure and mechanics and offers a path toward assembling higher-order tissues. We report on hybrid droplet interface bilayers (hDIBs) formed in hexadecane from binary mixtures of synthetic diphytanoyl phosphatidylcholine (DPhPC) lipids and low molecular weight 1,2 polybutadiene-b-polyethylene oxide (PBPEO) amphiphilic block copolymers and use electrophysiology measurements and imaging to assess the effects of PBPEO in the membrane. This work reveals that hDIBs containing up to 15 mol% PBPEO plus DPhPC are homogeneously mixtures of lipids and polymers, remain highly resistive to ion transport, and are stable-including under applied voltage. Moreover, they exhibit hydrophobic thicknesses similar to DPhPC-only bilayers, but also have significantly lower values of membrane tension. These characteristics coincide with reduced energy of adhesion between droplets and the formation of alamethicin ion channels at significantly lower threshold voltages, demonstrating that even moderate amounts of amphiphilic block copolymers in a lipid bilayer provide a route for tuning the physical properties of a biomimetic membrane.

Nanoparticle-Induced Disorder at Complex Liquid-Liquid Interfaces: Effects of Curvature and Compositional Synergy on Functional Surfaces

The self-assembly of surfactant monolayers at interfaces plays a sweeping role in tasks ranging from household cleaning to the regulation of the respiratory system. The synergy between different nanoscale species at an interface can yield assemblies with exceptional properties, which enhance or modulate their function. However, understanding the mechanisms underlying coassembly, as well as the effects of intermolecular interactions at an interface, remains an emerging and challenging field of study. Herein, we study the interactions of gold nanoparticles striped with hydrophobic and hydrophilic ligands with phospholipids at a liquid-liquid interface and the resulting surface-bound complexes. We show that these nanoparticles, which are themselves minimally surface active, have a direct concentration-dependent effect on the rapid reduction of tension for assembling phospholipids at the interface, implying molecular coassembly. Through the use of sum frequency generation vibrational spectroscopy, we reveal that nanoparticles impart structural disorder to the lipid molecular layers, which is related to the increased volumes that amphiphiles can sample at the curved surface of a particle. The results strongly suggest that hydrophobic and electrostatic attractions imparted by nanoparticle functionalization drive lipid-nanoparticle complex assembly at the interface, which synergistically aids lipid adsorption even when lipids and nanoparticles approach the interface from opposite phases. The use of tensiometric and spectroscopic analyses reveals a physical picture of the system at the nanoscale, allowing for a quantitative analysis of the intermolecular behavior that can be extended to other systems.

Author Correction: Artificial water channels enable fast and selective water permeation through water-wire networks

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Artificial water channels enable fast and selective water permeation through water-wire networks

Artificial water channels are synthetic molecules that aim to mimic the structural and functional features of biological water channels (aquaporins). Here we report on a cluster-forming organic nanoarchitecture, peptide-appended hybrid[4]arene (PAH[4]), as a new class of artificial water channels. Fluorescence experiments and simulations demonstrated that PAH[4]s can form, through lateral diffusion, clusters in lipid membranes that provide synergistic membrane-spanning paths for a rapid and selective water permeation through water-wire networks. Quantitative transport studies revealed that PAH[4]s can transport >109 water molecules per second per molecule, which is comparable to aquaporin water channels. The performance of these channels exceeds the upper bound limit of current desalination membranes by a factor of ~104, as illustrated by the water/NaCl permeability-selectivity trade-off curve. PAH[4]'s unique properties of a high water/solute permselectivity via cooperative water-wire formation could usher in an alternative design paradigm for permeable membrane materials in separations, energy production and barrier applications.

Insight into the Mechanisms Driving the Self-Assembly of Functional Interfaces: Moving from Lipids to Charged Amphiphilic Oligomers

Polymer-stabilized liquid/liquid interfaces are an important and growing class of bioinspired materials that combine the structural and functional capabilities of advanced synthetic materials with naturally evolved biophysical systems. These platforms have the potential to serve as selective membranes for chemical separations and molecular sequencers and to even mimic neuromorphic computing elements. Despite the diversity in function, basic insight into the assembly of well-defined amphiphilic polymers to form functional structures remains elusive, which hinders the continued development of these technologies. In this work, we provide new mechanistic insight into the assembly of an amphiphilic polymer-stabilized oil/aqueous interface, in which the headgroups consist of positively charged methylimidazolium ionic liquids, and the tails are short, monodisperse oligodimethylsiloxanes covalently attached to the headgroups. We demonstrate using vibrational sum frequency generation spectroscopy and pendant drop tensiometery that the composition of the bulk aqueous phase, particularly the ionic strength, dictates the kinetics and structures of the amphiphiles in the organic phase as they decorate the interface. These results show that H-bonding and electrostatic interactions taking place in the aqueous phase bias the grafted oligomer conformations that are adopted in the neighboring oil phase. The kinetics of self-assembly were ionic strength dependent and found to be surprisingly slow, being composed of distinct regimes where molecules adsorb and reorient on relatively fast time scales, but where conformational sampling and frustrated packing takes place over longer time scales. These results set the stage for understanding related chemical phenomena of bioinspired materials in diverse technological and fundamental scientific fields and provide a solid physical foundation on which to design new functional interfaces.

A new approach for investigating the response of lipid membranes to electrocompression by coupling droplet mechanics and membrane biophysics

A new method for quantifying lipid-lipid interactions within biomimetic membranes undergoing electrocompression is demonstrated by coupling droplet mechanics and membrane biophysics. The membrane properties are varied by altering the lipid packing through the introduction of cholesterol. Pendant drop tensiometry is used to measure the lipid monolayer tension at an oil-water interface. Next, two lipid-coated aqueous droplets are manipulated into contact to form a bilayer membrane at their adhered interface. The droplet geometries are captured from two angles to provide accurate measurements of both the membrane area and the contact angle between the adhered droplets. Combining the monolayer tension and contact angle measurements enables estimations of the membrane tension with respect to lipid composition. Then, the membrane is electromechanically compressed using a transmembrane voltage. Electrostatic pressure, membrane tension and the work necessary for bilayer thinning are tracked, and a model is proposed to capture the mechanics of membrane compression. The results highlight that a previously unaccounted for energetic term is produced during compression, potentially reflecting changes in the lateral membrane structure. This residual energy is eliminated in cases with cholesterol mole fractions of 0.2 and higher, suggesting that cholesterol diminishes these adjustments.

Lipid shape determination of detergent solubilization in mixed-lipid liposomes

The effects of lipid charge and head group size on liposome partitioning by detergents is an important consideration for applications such as liposomal drug delivery or proteoliposome formation. Yet, the solubilization of mixed-lipid liposomes, those containing multiple types of lipids, by detergents has received insufficient attention. This study examines the incorporation into and subsequent dissolution of mixed-lipid liposomes comprised of both egg phosphatidylcholine (ePC) and egg phosphatidic acid (ePA) by the detergent Triton-X100 (TX). Liposomes were prepared with mixtures of the two lipids, ePC and ePA, at molar ratios from 0 to 1, then step-wise solubilized with TX. Changes in turbidity, size distribution, and molar heat power at constant temperature throughout the solubilization process were assessed. The data suggest that the difference in lipid shapes (shape factors = 0.74 and 1.4 [1,2]) affects packing in membranes, and hence influences how much TX can be incorporated before disruption. As such, liposomes containing the observed ratios of ePA incorporated higher concentrations of TX before initiating dissolution into detergent and lipid mixed-micelles. The cause was concluded to be increased mismatching in the bilayer from the conical shape of ePA compared to the cylindrical shape of ePC. Additionally, the degree to which ePA is approximated as conical versus cylindrical was modulated with pH. It was confirmed that less conical ePA behaved more similarly to ePC than more conical ePA. The understanding gained here on lipid shape in liposome incorporation of TX enables research to use in vitro liposomes that more closely mimic native membranes.

Memristive plasticity in artificial electrical synapses via geometrically reconfigurable, gramicidin-doped biomembranes

It is now known that mammalian brains leverage plasticity of both chemical and electrical synapses (ES) for collocating memory and processing. Unlike chemical synapses, ES join neurons via gap junction ion channels that permit fast, threshold-independent, and bidirectional ion transport. Like chemical synapses, ES exhibit activity-dependent plasticity, which modulates the ionic conductance between neurons and, thereby, enables adaptive synchronization of action potentials. Many types of adaptive computing devices that display discrete, threshold-dependent changes in conductance have been developed, yet far less effort has been devoted to emulating the continuously variable conductance and activity-dependent plasticity of ES. Here, we describe an artificial electrical synapse (AES) that exhibits voltage-dependent, analog changes in ionic conductance at biologically relevant voltages. AES plasticity is achieved at the nanoscale by linking dynamical geometrical changes of a host lipid bilayer to ion transport via gramicidin transmembrane ion channels. As a result, the AES uniquely mimics the composition, biophysical properties, bidirectional and threshold-independent ion transport, and plasticity of ES. Through experiments and modeling, we classify our AES as a volatile memristor, where the voltage-controlled conductance is governed by reversible changes in membrane geometry and gramicidin channel density. Simulations show that AES plasticity can adaptively synchronize Hodgkin-Huxley neurons. Finally, by modulating the molecular constituents of the AES, we show that the amplitude, direction, and speed of conductance changes can be tuned. This work motivates the development and integration of ES-inspired computing devices for achieving more capable neuromorphic hardware.

Author Correction: Dynamical nonlinear memory capacitance in biomimetic membranes

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Assembly and Characterization of Biomolecular Memristors Consisting of Ion Channel-doped Lipid Membranes

The ability to recreate synaptic functionalities in synthetic circuit elements is essential for neuromorphic computing systems that seek to emulate the cognitive powers of the brain with comparable efficiency and density. To date, silicon-based three-terminal transistors and two-terminal memristors have been widely used in neuromorphic circuits, in large part due to their ability to co-locate information processing and memory. Yet these devices cannot achieve the interconnectivity and complexity of the brain because they are power-hungry, fail to mimic key synaptic functionalities, and suffer from high noise and high switching voltages. To overcome these limitations, we have developed and characterized a biomolecular memristor that mimics the composition, structure, and switching characteristics of biological synapses. Here, we describe the process of assembling and characterizing biomolecular memristors consisting of a 5 nm-thick lipid bilayer formed between lipid-functionalized water droplets in oil and doped with voltage-activated alamethicin peptides. While similar assembly protocols have been used to investigate biophysical properties of droplet-supported lipid membranes and membrane-bound ion channels, this article focuses on key modifications of the droplet interface bilayer method essential for achieving consistent memristor performance. Specifically, we describe the liposome preparation process and the incorporation of alamethicin peptides in lipid bilayer membranes, and the appropriate concentrations of each constituent as well as their impact on the overall response of the memristors. We also detail the characterization process of biomolecular memristors, including measurement and analysis of memristive current-voltage relationships obtained via cyclic voltammetry, as well as short-term plasticity and learning in response to step-wise voltage pulse trains.

Memristive Ion Channel-Doped Biomembranes as Synaptic Mimics

Solid-state neuromorphic systems based on transistors or memristors have yet to achieve the interconnectivity, performance, and energy efficiency of the brain due to excessive noise, undesirable material properties, and nonbiological switching mechanisms. Here we demonstrate that an alamethicin-doped, synthetic biomembrane exhibits memristive behavior, emulates key synaptic functions including paired-pulse facilitation and depression, and enables learning and computing. Unlike state-of-the-art devices, our two-terminal, biomolecular memristor features similar structure (biomembrane), switching mechanism (ion channels), and ionic transport modality as biological synapses while operating at considerably lower power. The reversible and volatile voltage-driven insertion of alamethicin peptides into an insulating lipid bilayer creates conductive pathways that exhibit pinched current-voltage hysteresis at potentials above their insertion threshold. Moreover, the synapse-like dynamic properties of the biomolecular memristor allow for simplified learning circuit implementations. Low-power memristive devices based on stimuli-responsive biomolecules represent a major advance toward implementation of full synaptic functionality in neuromorphic hardware.

Evaporation-induced monolayer compression improves droplet interface bilayer formation using unsaturated lipids

In this article, we report on a new experimental methodology to enable reliable formation of droplet interface bilayer (DIB) model membranes with two types of unsaturated lipids that have proven difficult for creating stable DIBs. Through the implementation of a simple evaporation technique to condition the spontaneously assembled lipid monolayer around each droplet, we increased the success rates of DIB formation for two distinct unsaturated lipids, namely 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), from less than 10% to near 100%. Separately, using a pendant drop tensiometer, we learned that: (a) DOPC and POPC monolayers do not spontaneously assemble into their tightest possible configurations at an oil-water interface, and (b) reducing the surface area of a water droplet coated with a partially packed monolayer leads to a more tightly packed monolayer with an interfacial tension lower than that achieved by spontaneous assembly alone. We also estimated from Langmuir compression isotherms obtained for both lipids that the brief droplet evaporation procedure prior to DIB formation resulted in a 6%-16% reduction in area per lipid for DOPC and POPC, respectively. Finally, the increased success rates of formation for DOPC and POPC DIBs enabled quantitative characterization of unsaturated lipid membrane properties including electrical resistance, rupture potential, and specific capacitance.

Capacitive Detection of Low-Enthalpy, Higher-Order Phase Transitions in Synthetic and Natural Composition Lipid Membranes

In-plane lipid organization and phase separation in natural membranes play key roles in regulating many cellular processes. Highly cooperative, first-order phase transitions in model membranes consisting of few lipid components are well understood and readily detectable via calorimetry, densitometry, and fluorescence. However, far less is known about natural membranes containing numerous lipid species and high concentrations of cholesterol, for which thermotropic transitions are undetectable by the above-mentioned techniques. We demonstrate that membrane capacitance is highly sensitive to low-enthalpy thermotropic transitions taking place in complex lipid membranes. Specifically, we measured the electrical capacitance as a function of temperature for droplet interface bilayer model membranes of increasing compositional complexity, namely, (a) a single lipid species, (b) domain-forming ternary mixtures, and (c) natural brain total lipid extract (bTLE). We observed that, for single-species lipid bilayers and some ternary compositions, capacitance exhibited an abrupt, temperature-dependent change that coincided with the transition detected by other techniques. In addition, capacitance measurements revealed transitions in mixed-lipid membranes that were not detected by the other techniques. Most notably, capacitance measurements of bTLE bilayers indicated a transition at ∼38 °C not seen with any other method. Likewise, capacitance measurements detected transitions in some well-studied ternary mixtures that, while known to yield coexisting lipid phases, are not detected with calorimetry or densitometry. These results indicate that capacitance is exquisitely sensitive to low-enthalpy membrane transitions because of its sensitivity to changes in bilayer thickness that occur when lipids and excess solvent undergo subtle rearrangements near a phase transition. Our findings also suggest that heterogeneity confers stability to natural membranes that function near transition temperatures by preventing unwanted defects and macroscopic demixing associated with high-enthalpy transitions commonly found in simpler mixtures.

Hydrodynamic trapping for rapid assembly and in situ electrical characterization of droplet interface bilayer arrays

The droplet interface bilayer (DIB) is a modular technique for assembling planar lipid membranes between water droplets in oil. The DIB method thus provides a unique capability for developing digital, droplet-based membrane platforms for rapid membrane characterization, drug screening and ion channel recordings. This paper demonstrates a new, low-volume microfluidic system that automates droplet generation, sorting, and sequential trapping in designated locations to enable the rapid assembly of arrays of DIBs. The channel layout of the device is guided by an equivalent circuit model, which predicts that a serial arrangement of hydrodynamic DIB traps enables sequential droplet placement and minimizes the hydrodynamic pressure developed across filled traps to prevent squeeze-through of trapped droplets. Furthermore, the incorporation of thin-film electrodes fabricated via evaporation metal deposition onto the glass substrate beneath the channels allows for the first time in situ, simultaneous electrical interrogation of multiple DIBs within a sealed device. Combining electrical measurements with imaging enables measurements of membrane capacitance and resistance and bilayer area, and our data show that DIBs formed in different trap locations within the device exhibit similar sizes and transport properties. Simultaneous, single channel recordings of ion channel gating in multiple membranes are obtained when alamethicin peptides are incorporated into the captured droplets, qualifying the thin-film electrodes as a means for measuring stimuli-responsive functions of membrane-bound biomolecules. This novel microfluidic-electrophysiology platform provides a reproducible, high throughput method for performing electrical measurements to study transmembrane proteins and biomembranes in low-volume, droplet-based membranes.

Reversible, voltage-activated formation of biomimetic membranes between triblock copolymer-coated aqueous droplets in good solvents

Biomimetic membranes assembled from block copolymers attract considerable interest because they exhibit greater stability and longetivity compared to lipid bilayers, and some enable the reconstitution of functional transmembrane biomolecules. Yet to-date, block copolymer membranes have not been achieved using the droplet interface bilayer (DIB) method, which uniquely allows assembling single- and multi-membrane networks between water droplets in oil. Herein, we investigate the formation of poly(ethylene oxide)-b-poly(dimethyl siloxane)-b-poly(ethylene oxide) triblock copolymer-stabilized interfaces (CSIs) between polymer-coated aqueous droplets in solutions comprising combinations of decane, hexadecane and AR20 silicone oil. We demonstrate that triblock-coated droplets do not spontaneously adhere in these oils because all are thermodynamically good solvents for the hydrophobic PDMS middle block. However, thinned planar membranes are reversibly formed at the interface between droplets upon the application of a sufficient transmembrane voltage, which removes excess solvent from between droplets through electrocompression. At applied voltages above the threshold required to initiate membrane thinning, electrowetting causes the area of the CSI between droplets to increase while thickness remains constant; the CSI electrowetting response is similar to that encountered with lipid-based DIBs. In combination, these results reveal that stable membranes can be assembled in a manner that is completely reversible when an external pressure is used to overcome a barrier to adhesion caused by solvent-chain interactions, and they demonstrate new capability for connecting and disconnecting aqueous droplets via polymer-stabilized membranes.

Droplet immobilization within a polymeric organogel improves lipid bilayer durability and portability

The droplet interface bilayer (DIB) is a promising technique for assembling lipid membrane-based materials and devices using water droplets in oil, but it has largely been limited to laboratory environments due to its liquid construction. With a vision to transform this lab-based technique into a more-durable embodiment, we investigate the use of a polymer-based organogel to encapsulate DIBs within a more-solid material matrix to improve their handling and portability. Specifically, a temperature-sensitive organogel formed from hexadecane and poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) triblock copolymer is used to replace the liquid solvent that surrounds the lipid-coated droplets to establish a novel liquid-in-gel DIB system. Through specific capacitance measurements and single-channel recordings of the pore forming peptide alamethicin, we verify that the structural and functional membrane properties are retained when DIBs are assembled within SEBS organogel. In addition, we demonstrate that organogel encapsulation offers improved handling of droplets and yields DIBs with a near 3× higher bilayer durability, as quantified by the lateral acceleration required to rupture the membrane, compared to liquid-in-liquid DIBs in oil. This encapsulated DIB system provides a barrier against contamination from the environment and offers a new material platform for supporting multilayered DIB-based devices as well as other digital microfluidic systems that feature water droplets in oil.

Toward cell-inspired materials that feel: measurements and modeling of mechanotransduction in droplet-based, multi-membrane arrays

The droplet interface bilayer (DIB) was recently used to show that a 5 nm thick lipid membrane placed near a vibrating synthetic hair could transduce hair motion into electrical current. Herein, we study for the first time mechanoelectrical transduction of hair motion using multi-membrane DIB arrays formed with more than 2 droplets connected in series, and we introduce a transduction model to investigate how airflow across the hair generates current in a membrane-based hair cell. Measurements of sensing currents across every membrane in serial chains of up to 5 connected droplets demonstrate that perturbation of a single hair creates vibrations that propagate across several droplets, allowing for membranes that are not directly attached to the hair to still transduce its motion. Membranes positioned closest to the hair generate the largest currents, while those farther away produce less current due to energy loss from fluid damping. Inserting multiple hairs of different lengths into different droplets in the array yields sensing currents that exhibit multiple characteristic frequencies in addition to location specific current intensities, features that can be used to spatially localize mechanical perturbations. We also develop a transduction model that provides an order-of-magnitude approximation of the sensing current generated by a membrane in response to airflow across the hair. This model provides physical insights into how membrane-based materials can be used for sensing mechanical stimuli--just like nature does.

Adsorption Kinetics Dictate Monolayer Self-Assembly for Both Lipid-In and Lipid-Out Approaches to Droplet Interface Bilayer Formation

The droplet interface bilayer (DIB)--a method to assemble planar lipid bilayer membranes between lipid-coated aqueous droplets--has gained popularity among researchers in many fields. Well-packed lipid monolayer on aqueous droplet-oil interfaces is a prerequisite for successfully assembling DIBs. Such monolayers can be achieved by two different techniques: "lipid-in", in which phospholipids in the form of liposomes are placed in water, and "lipid-out", in which phospholipids are placed in oil as inverse micelles. While both approaches are capable of monolayer assembly needed for bilayer formation, droplet pairs assembled with these two techniques require significantly different incubation periods and exhibit different success rates for bilayer formation. In this study, we combine experimental interfacial tension measurements with molecular dynamics simulations of phospholipids (DPhPC and DOPC) assembled from water and oil origins to understand the differences in kinetics of monolayer formation. With the results from simulations and by using a simplified model to analyze dynamic interfacial tensions, we conclude that, at high lipid concentrations common to DIBs, monolayer formation is simple adsorption controlled for lipid-in technique, whereas it is predominantly adsorption-barrier controlled for the lipid-out technique due to the interaction of interface-bound lipids with lipid structures in the subsurface. The adsorption barrier established in lipid-out technique leads to a prolonged incubation time and lower bilayer formation success rate, proving a good correlation between interfacial tension measurements and bilayer formation. We also clarify that advective flow expedites monolayer formation and improves bilayer formation success rate by disrupting lipid structures, rather than enhancing diffusion, in the subsurface and at the interface for lipid-out technique. Additionally, electrical properties of DIBs formed with varying lipid placement and type are characterized.

Direct in situ measurement of specific capacitance, monolayer tension, and bilayer tension in a droplet interface bilayer

Thickness and tension are important physical parameters of model cell membranes. However, traditional methods to measure these quantities require multiple experiments using separate equipment. This work introduces a new multi-step procedure for directly accessing in situ multiple physical properties of droplet interface bilayers (DIB), including specific capacitance (related to thickness), lipid monolayer tension in the Plateau-Gibbs border, and bilayer tension. The procedure employs a combination of mechanical manipulation of bilayer area followed by electrowetting of the capacitive interface to examine the sensitivities of bilayer capacitance to area and contact angle to voltage, respectively. These data allow for determining the specific capacitance of the membrane and surface tension of the lipid monolayer, which are then used to compute bilayer thickness and tension, respectively. The use of DIBs affords accurate optical imaging of the connected droplets in addition to electrical measurements of bilayer capacitance, and it allows for reversibly varying bilayer area. After validating the accuracy of the technique with diphytanoyl phosphatidylcholine (DPhPC) DIBs in hexadecane, the method is applied herein to quantify separately the effects on membrane thickness and tension caused by varying the solvent in which the DIB is formed and introducing cholesterol into the bilayer. Because the technique relies only on capacitance measurements and optical images to determine both thickness and tension, this approach is specifically well-suited for studying the effects of peptides, biomolecules, natural and synthetic nanoparticles, and other species that accumulate within membranes without altering bilayer conductance.

Control of membrane permeability in air-stable droplet interface bilayers

Air-stable droplet interface bilayers (airDIBs) on oil-infused surfaces are versatile model membranes for synthetic biology applications, including biosensing of airborne species. However, airDIBs are subject to evaporation, which can, over time, destabilize them and reduce their useful lifetime compared to traditional DIBs that are fully submerged in oil. Here, we show that the lifetimes of airDIBs can be extended by as much as an order of magnitude by maintaining the temperature just above the dew point. We find that raising the temperature from near the dew point (which was 7 °C at 38.5% relative humidity and 22 °C air temperature) to 20 °C results in the loss of hydrated water molecules from the polar headgroups of the lipid bilayer membrane due to evaporation, resulting in a phase transition with increased disorder. This dehydration transition primarily affects the bilayer electrical resistance by increasing the permeability through an increasingly disordered polar headgroup region of the bilayer. Temperature and relative humidity are conveniently tunable parameters for controlling the stability and composition of airDIB membranes while still allowing for operation in ambient environments.

Heating-enabled formation of droplet interface bilayers using Escherichia coli total lipid extract

Droplet interface bilayers (DIBs) serve as a convenient platform to study interactions between synthetic lipid membranes and proteins. However, a majority of DIBs have been assembled using a single lipid type, diphytanoylphosphatidylcholine (DPhPC). The work described herein establishes a new method to assemble DIBs using total lipid extract from Escherichia coli (eTLE); it is found that incubating oil-submerged aqueous droplets containing eTLE liposomes at a temperature above the gel-fluid phase transition temperature (Tg) promotes monolayer self-assembly that does not occur below Tg. Once monolayers are properly assembled via heating, droplets can be directly connected or cooled below Tg and then connected to initiate bilayer formation. This outcome contrasts immediate droplet coalescence observed upon contact between nonheated eTLE-infused droplets. Specific capacitance measurements confirm that the interface between droplets containing eTLE lipids is a lipid bilayer with thickness of 29.6 Å at 25 °C in hexadecane. We observe that bilayers formed from eTLE or DPhPC survive cooling and heating between 25 and 50 °C and demonstrate gigaohm (GΩ) membrane resistances at all temperatures tested. Additionally, we study the insertion of alamethicin peptides into both eTLE and DPhPC membranes to understand how lipid composition, temperature, and membrane phase influence ion channel formation. Like in DPhPC bilayers, alamethicin peptides in eTLE exhibit discrete, voltage-dependent gating characterized by multiple open channel conductance levels, though at significantly lower applied voltages. Cyclic voltammetry measurements of macroscopic channel currents confirm that the voltage-dependent conductance of alamethicin channels in eTLE bilayers occurs at lower voltages than in DPhPC bilayers at equivalent peptide concentrations. This result suggests that eTLE membranes, via composition, fluidity, or the presence of subdomains, offer an environment that enhances alamethicin insertion. For both membrane compositions, increasing temperature reduces the lifetimes of single channel gating events and increases the voltage required to cause an exponential increase in channel current. However, the fact that alamethicin insertion in eTLE exhibits significantly greater sensitivity to temperature changes through its Tg suggests that membrane phase plays an important role in channel formation. These effects are much less severe in DPhPC, where heating from 25 to 50 °C does not induce a phase change. The described technique for heating-assisted monolayer formation permits the use of other high transition temperature lipids in aqueous droplets for DIB formation, thereby increasing the types of lipids that can be considered for assembling model membranes.

Air-stable droplet interface bilayers on oil-infused surfaces

Droplet interface bilayers are versatile model membranes useful for synthetic biology and biosensing; however, to date they have always been confined to fluid reservoirs. Here, we demonstrate that when two or more water droplets collide on an oil-infused substrate, they exhibit noncoalescence due to the formation of a thin oil film that gets squeezed between the droplets from the bottom up. We show that when phospholipids are included in the water droplets, a stable droplet interface bilayer forms between the noncoalescing water droplets. As with traditional oil-submerged droplet interface bilayers, we were able to characterize ion channel transport by incorporating peptides into each droplet. Our findings reveal that droplet interface bilayers can function in ambient environments, which could potentially enable biosensing of airborne matter.

Dynamic morphologies of microscale droplet interface bilayers

Droplet interface bilayers (DIBs) are a powerful platform for studying the dynamics of synthetic cellular membranes; however, very little has been done to exploit the unique dynamical features of DIBs. Here, we generate microscale droplet interface bilayers (μDIBs) by bringing together femtoliter-volume water droplets in a microfluidic oil channel, and characterize morphological changes of the μDIBs as the droplets shrink due to evaporation. By varying the initial conditions of the system, we identify three distinct classes of dynamic morphology. (1) Buckling and fission: when forming μDIBs using the lipid-out method (lipids in oil phase), lipids in the shrinking monolayers continually pair together and slide into the bilayer to conserve their mass. As the bilayer continues to grow, it becomes confined, buckles, and eventually fissions one or more vesicles. (2) Uniform shrinking: when using the lipid-in method (lipids in water phase) to form μDIBs, lipids uniformly transfer from the monolayers and bilayer into vesicles contained inside the water droplets. (3) Stretching and unzipping: finally, when the droplets are pinned to the wall(s) of the microfluidic channel, the droplets become stretched during evaporation, culminating in the unzipping of the bilayer and droplet separation. These findings offer a better understanding of the dynamics of coupled lipid interfaces.

Validity of a new respiratory resistance measurement device to detect glottal area change

OBJECTIVE

To determine the correlation between respiratory resistance (Rr) values measured with the Airflow Perturbation Device (APD) to laryngoscopic images of glottal area (GA) in feigned paradoxical vocal fold motion (PVFM), also known as vocal cord dysfunction.

HYPOTHESIS

There is a strong inverse relationship between Rr and GA such that laryngeal constriction can be detected and quantified by APD-measured Rr.

STUDY DESIGN

Prospective, single subject study.

METHODS

A healthy adult feigned breathing that was characteristic of PVFM. Rr and GA were simultaneously recorded, synchronized, and analyzed for three complete breathing cycles with significant glottal constriction occurring during inspiration.

RESULTS

Cross-correlation analysis revealed a strong negative correlation (-0.824) between GA and Rr during feigned PVFM breathing such that Rr increased when GA decreased.

CONCLUSION

APD-measured Rr appears to be a viable noninvasive method for diagnostic screening and monitoring of treatment outcomes for individuals presenting with dyspnea related to PVFM.

Bilayer formation between lipid-encased hydrogels contained in solid substrates

Solidified biomolecular networks that incorporate liquid-supported lipid bilayers are constructed by attaching lipid-encased, water-swollen hydrogels contained in oil. Poly(ethylene glycol) dimethacrylate (PEG-DMA) and a free-radical photoinitiator are added to an aqueous lipid vesicle solution such that exposure to ultraviolet light results in solidification of neighboring aqueous volumes. Bilayer formation can occur both prior to photopolymerization with the aqueous mixture in the liquid state and after solidification by using the regulated attachment method (RAM) to attach the aqueous volumes contained within a flexible substrate. In addition, photopolymerization of the hydrogels can be performed in a separate mold prior to placement in the supporting substrate. Membranes formed across a wide range of hydrogel concentrations [0-80% (w/v); MW=1000 g/mol PEG-DMA] exhibit high electrical resistances (1-10 GΩ), which enable single-channel recordings of alamethicin channels and show significant durability and longevity. We demonstrate that just as liquid phases can be detached and reattached using RAM, reconfiguration of solid aqueous phases is also possible. The results presented herein demonstrate a step toward constructing nearly solid-state biomolecular materials that retain fluid interfaces for driving molecular assembly. This work also introduces the use of three-dimensional printing to rapidly prototype a molding template used to fabricate polyurethane substrates and to shape individual hydrogels.

Regulated attachment method for reconstituting lipid bilayers of prescribed size within flexible substrates

A new method called the regulated attachment method (RAM) for reproducibly forming lipid bilayers within flexible substrates has been developed that enables precise control over the size of the bilayer. This technique uses a deformable flexible substrate to open and close an aperture that subdivides aqueous volumes submersed in an organic solvent. Phospholipids incorporated as vesicles in the aqueous phase self-assemble at the oil/water interface to form lipid monolayers that encapsulate each aqueous volume. Controlled attachment of opposing lipid monolayers is achieved by regulating the dimensions of the aperture in the substrate that separates the adjacent aqueous volumes. In this manner, the size of a lipid bilayer formed within a flexible substrate is a function of the substrate and aperture dimensions, and not determined by the sizes or shapes of the aqueous volumes. Lipid bilayers formed within the prototype flexible substrate exhibit DC resistances consistently higher than 10 GOmega and can survive 20-30x changes in area without rupture. Furthermore, RAM permits lipid bilayers to be completely unzipped after thinning by applying sufficient force to fully close the dividing aperture and even allows the introduction of species, such as alamethicin channels, into preformed lipid bilayers via controlled injection through an intersecting channel within the substrate. Controlling the size of the interface through indirect interactions with the supporting substrate offers a new platform for assembling durable lipid bilayers. We envision that this technology can be scaled to higher dimensions consisting of multiple apertures required for creating aqueous networks partitioned by functional lipid bilayers and to smaller length scales to produce very small lipid bilayers capable of hosting single proteins.

Physical encapsulation of droplet interface bilayers for durable, portable biomolecular networks

Physically-encapsulated droplet interface bilayers are formed by confining aqueous droplets encased in lipid monolayers within connected compartments of a solid substrate. Each droplet resides within an individual compartment and is positioned on a fixed electrode built into the solid substrate. Full encapsulation of the network is achieved with a solid cap that inserts into the substrate to form a closed volume. Encapsulated networks provide increased portability over unencapsulated networks by limiting droplet movement and through the integration of fixed electrodes into the supporting fixture. The formation of encapsulated droplet interface bilayers constructed from diphytanoyl phosphocoline (DPhPC) phospholipids is confirmed with electrical impedance spectroscopy, and cyclic voltammetry is used to measure the effect of alamethicin channels incorporated into the resulting lipid bilayers. The durability of the networks is quantified using a mechanical shaker to oscillate the bilayer in a direction transverse to the plane of the membrane and the results show that single droplet interface bilayers can withstand 1-10g of acceleration prior to bilayer failure. Observed failure modes include both droplet separation and bilayer rupturing, where the geometry of the supporting substrate and the presence of integrated electrodes are key contributors. Physically-encapsulated DIBs can be shaken, moved, and inverted without bilayer failure, enabling the creation of a new class of lab-on-chip devices.