Financial support was provided by the National Science Foundation Grant CBET-1752197 and the Air Force Office of Scientific Research Grant FA9550-19-1-0213.

The authors have no conflicts of interests.

The protocol described herein provides instructions for assembling and operating an experimental system to control the temperature of the oil and droplets used to form DIBs. It is especially beneficial for enabling DIB formation using lipids that have melting temperatures above RT. Moreover, by precisely varying the temperature of the oil reservoir, the bilayer temperature can be manipulated to study the effects of elevated temperatures on various membrane properties and characteristics, including capacitance, area, thic...

Cellular membranes are selectively permeable barriers comprised of&#160;thousands of lipid types1, proteins, carbohydrates, and sterols that encapsulate and subdivide all living cells. Understanding how their compositions affect their functions and revealing how natural and synthetic molecules interact with, adhere to, disrupt, and translocate cellular membranes are, therefore, important areas of research with wide-reaching implications in biology, medicine, chemistry, physics, and materials engineering.
These aims for discovery directly benefit from proven techniques for assembling, manipulating, and studying model membranes&#8212;including lipid bilayers assembled from synthetic or naturally occurring lipids&#8212;that mimic the composition, structure, and transport properties of their cellular counterparts. In recent years, the droplet interface bilayer (DIB) method2,3,4 for constructing a planar lipid bilayer between lipid-coated water droplets in oil has received significant attention5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23, and has demonstrated practical advantages over other approaches for model membrane formation: the DIB method is simple to perform, requires no sophisticated fabrication or preparation (e.g., &#34;painting&#34;) of a substrate to support the membrane, consistently yields membranes with superior longevity, allows for standard electrophysiology measurements, and simplifies the formation of model membranes with asymmetric leaflet compositions3. Because the bilayer forms spontaneously between droplets and each droplet can be tailored in position and makeup, the DIB technique has also attracted considerable interest in developing cell-inspired material systems that build on the use of stimuli-responsive membranes18,24,25,26,27,28,29, balanced compartmentalization and transport14,30,31, and tissue-like materials17,23,32,33,34,35,36.
The majority of published experiments on model membranes, including those with DIBs, have been performed at room temperature (RT, ~20-25 &#176;C) and with a handful of synthetic lipids (e.g., DOPC, DPhPC, etc.). This practice limits the scope of biophysical questions that can be studied in model membranes and, based on observation, it can also restrict the types of lipids that can be used to assemble DIBs. For example, a synthetic lipid such as DPPC, which has a melting temperature of 42 &#176;C, does not assemble tightly-packed monolayers or form DIBs at RT37. DIB formation at room temperature has also proven difficult for natural extracts, such as those from mammals (e.g., brain total lipid extract, BTLE)38 or bacteria (e.g., Escherichia coli total lipid extract, ETLE)37, which contain many different types of lipids and originate from cells that reside at elevated temperatures (37 &#176;C). Enabling study of diverse compositions thus provides opportunities to understand membrane-mediated processes in biologically relevant conditions.
Raising the temperature of the oil can serve two purposes: it increases the kinetics of monolayer assembly and it can cause lipids to undergo a melting transition to reach a liquid disordered phase. Both consequences aid in monolayer assembly39, a pre-requisite for a DIB. In addition to heating for bilayer formation, cooling the membrane after the formation can be used to identify thermotropic transitions in single lipid bilayers38, including those in natural lipid mixtures (e.g., BTLE) that can be difficult to detect using calorimetry. Aside from assessing thermotropic transitions of lipids, precisely varying the temperature of the DIB can be used to study temperature-induced changes in membrane structure38 and examine how lipid composition and fluidity affect the kinetics of membrane-active species (e.g., pore-forming peptides and transmembrane proteins37), including mammalian and bacterial model membranes at a physiologically relevant temperature&#160;(37 &#176;C).
Herein, a description of how to assemble a modified DIB oil reservoir and operate a feedback-temperature controller to enable monolayer assembly and bilayer formation at temperatures higher than RT will be explained. Distinguished from a previous protocol40, explicit detail is included regarding the integration of instrumentation needed for measuring and controlling temperature in parallel to assembly and characterization of the DIB in the oil reservoir. The procedure will thus enable a user to apply this method for forming and studying DIBs across a range of temperatures in a variety of scientific contexts. Moreover, the representative results provide specific examples for the types of measurable changes in both membrane structure and ion transport that can occur as temperature is varied. These techniques are important additions to the many biophysical studies that can be designed and performed effectively in DIBs, including studying the kinetics of membrane-active species in different membrane compositions.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr >
			<td >25 mm x 40 mm x 1 mm insulative rubber (x2)</td>
			<td >Any</td>
			<td ></td>
			<td >Insulates the bottom of the aluminum fixture from the stage of the microscope</td>
		</tr>
		<tr >
			<td >25 mm x 40 mm x 6 mm insulative rubber (x2)</td>
			<td >Any</td>
			<td ></td>
			<td >Protects heating elements from being damaged by the microscope stage clips and insulates the top of the heating elements.</td>
		</tr>
		<tr >
			<td >3-(N-morpholino) propanesulfonic acid&#160;</td>
			<td >Sigma Aldrich</td>
			<td>M3183</td>
			<td >Buffering agent for lipid solution</td>
		</tr>
		<tr >
			<td >Acrylic substrate</td>
			<td >Fabricated in house</td>
			<td >HTD_STG_2</td>
			<td >~1000 uL acrylic well with a poka-yoke exterior profile to fix orientation</td>
		</tr>
		<tr >
			<td >Aluminum fixture</td>
			<td >Fabricated in house</td>
			<td >HTD_STG_1</td>
			<td >Base fixture with an oil well that holds the acylic fixture and includes two flat pads adjacent to the oil well for the heating elements&#160;</td>
		</tr>
		<tr >
			<td >Brain Total Lipid Extract</td>
			<td >Avanti</td>
			<td>131101C-100mg</td>
			<td >25 mg/mL porcine lipid extract&#160;</td>
		</tr>
		<tr >
			<td >Compact DAQ Chassis (cDAQ)</td>
			<td>National Instruments&#160;</td>
			<td>cDAQ-9174&#160;</td>
			<td >Chassis to house multiple types of sensor measurement or output modules</td>
		</tr>
		<tr >
			<td >Data Acquisition System (DAQ)</td>
			<td >Molecular Devices&#160;</td>
			<td>Digidata 1440A&#160;</td>
			<td >High resolution analog to digital converter</td>
		</tr>
		<tr >
			<td >Fixed gain amplifier/power supply</td>
			<td >Hewlitt Packard</td>
			<td >HP 6826A</td>
			<td >Amplifies DC voltage output from the voltage output module</td>
		</tr>
		<tr >
			<td >Glass Cover Slip</td>
			<td >Corning</td>
			<td>CLS284525</td>
			<td >Seals bottom of aluminum base and allows for optical characterization of the bilayer</td>
		</tr>
		<tr >
			<td >Heating element (x2)</td>
			<td >Omega</td>
			<td >KHLV-101/5</td>
			<td >25 mm x 25 mm polymide film kapton heating element with a 5 watt power limit.&#160;</td>
		</tr>
		<tr >
			<td >M3 Stainless Steel Screw</td>
			<td >McMaster Carr</td>
			<td >90116A150</td>
			<td >Secures thermocouple to aluminum fixture</td>
		</tr>
		<tr >
			<td >Patch clamp amplifier</td>
			<td >Molecular Devices&#160;</td>
			<td>AxoPatch 200B&#160;</td>
			<td >Measures current and outputs voltage to the headstage</td>
		</tr>
		<tr >
			<td >Personal computer</td>
			<td >Any</td>
			<td></td>
			<td >Computer with mulitiple high speed usb ports and a minimum of 6 Gb of ram</td>
		</tr>
		<tr >
			<td >Potassium Chloride</td>
			<td >Sigma Aldrich</td>
			<td>P3911</td>
			<td >Electrolyte solution of dissociated ions</td>
		</tr>
		<tr >
			<td >Temperature input module</td>
			<td >National Instruments&#160;</td>
			<td >NI 9211</td>
			<td >Enables open and cold junction thermocouple measurements for the cDAQ chassis</td>
		</tr>
		<tr >
			<td >Thermocouple</td>
			<td >Omega</td>
			<td>JMTSS-020U-6&#160;</td>
			<td >U-type thermocouple with a diameter of 0.02 inches and 6 inches in length</td>
		</tr>
		<tr >
			<td >UV Curable Adhesive</td>
			<td >Loctite</td>
			<td>19739</td>
			<td >Secures glass coverslip to aluminum base fixture</td>
		</tr>
		<tr >
			<td >Voltage output module</td>
			<td >National Instruments&#160;</td>
			<td >NI 9263</td>
			<td >Analog voltage output module for use with the cDAQ chassis</td>
		</tr>
		<tr >
			<td >Waveform generator</td>
			<td >Agilent</td>
			<td>33210A&#160;</td>
			<td >Used to output a 10 mV 10 Hz sinusoidal waveform</td>
		</tr>
	</tbody>
</table>

temperature-controlled assembly and characterization of a droplet interface bilayer

The droplet interface bilayer (DIB) method for assembling lipid bilayers (i.e., DIBs) between lipid-coated aqueous droplets in oil offers key benefits versus other methods: DIBs are stable and often long-lasting, bilayer area can be reversibly tuned, leaflet asymmetry is readily controlled via droplet compositions, and tissue-like networks of bilayers can be obtained by adjoining many droplets. Forming DIBs requires spontaneous assembly of lipids into high density lipid monolayers at the surfaces of the droplets. While this occurs readily at room temperature for common synthetic lipids, a sufficient monolayer or stable bilayer fails to form at similar conditions for lipids with melting points above room temperature, including some cellular lipid extracts. This behavior has likely limited the compositions&#8212;and perhaps the biological relevance&#8212;of DIBs in model membrane studies. To address this problem, an experimental protocol is presented to carefully heat the oil reservoir hosting DIB droplets and characterize the effects of temperature on the lipid membrane. Specifically, this protocol shows how to use a thermally conductive aluminum fixture and resistive heating elements controlled by a feedback loop to prescribe elevated temperatures, which improves monolayer assembly and bilayer formation for a wider set of lipid types. Structural characteristics of the membrane, as well as the thermotropic phase transitions of the lipids comprising the bilayer, are quantified by measuring the changes in electrical capacitance of the DIB. Together, this procedure can aid in evaluating biophysical phenomena in model membranes over various temperatures, including determining an effective melting temperature (TM) for multi-component lipid mixtures. This capability will thus allow for closer replication of natural phase transitions in model membranes and encourage the formation and use of model membranes from a wider swath of membrane constituents, including those that better capture the heterogeneity of their cellular counterparts.

Figure 1&#160;shows how the aluminum fixture and acrylic oil reservoir are prepared on the microscope stage for DIB formation. Assembly steps 1.2-1.4 serve to thermally insulate the fixture from the stage for more efficient heating. Steps 1.5-1.7 show how to properly attach the thermocouple to the fixture and position the oil reservoir, and steps 1.8 -1.9 show recommended locations for dispensing oil into these pieces.
Figure 2 outlin...

Watch this Scientific Journal Video about Temperature-Controlled Assembly and Characterization of a Droplet Interface Bilayer at JoVE.com

Temperature-Controlled Assembly and Characterization of a Droplet Interface Bilayer

This protocol details the use of a feedback temperature-controlled heating system to promote lipid monolayer assembly and droplet interface bilayer formation for lipids with elevated melting temperatures, and capacitance measurements to characterize temperature-driven changes in the membrane.

The droplet interface bilayer (DIB) method for assembling lipid bilayers (i.e., DIBs) between lipid-coated aqueous droplets in oil offers key benefits ...

temperature-controlled-assembly-characterization-droplet-interface

Universidad de Cartagena UDEC

Research

JoVE Journal

Bioengineering

3.6K Views.  University of Tennessee. This protocol details the use of a feedback temperature-controlled heating system to promote lipid monolayer assembly and droplet interface bilayer formation for lipids with elevated melting temperatures, and capacitance measurements to characterize temperature-driven changes in the membrane.

Video: Temperature-Controlled Assembly and Characterization of a Droplet Interface Bilayer

Synthesis, Assembly, and Characterization of Monolayer Protected Gold Nanoparticle Films for Protein Monolayer Electrochemistry

Colloidal gold nanoparticles protected with alkanethiolate ligands called monolayer protected gold clusters (MPCs) are synthesized and subsequently incorporated into film assemblies that serve as adsorption platforms for protein monolayer electrochemistry (PME). PME is utilized as the model system for studying electrochemical properties of redox proteins by confining them to an adsorption platform at a modified electrode, which also serves as a redox partner for electron transfer (ET) reactions. Studies have shown that gold nanoparticle film assemblies of this nature provide for a more homogeneous protein adsorption environment and promote ET without distance dependence compared to the more traditional systems modified with alkanethiol self-assembled monolayers (SAM).1-3 In this paper, MPCs functionalized with hexanethiolate ligands are synthesized using a modified Brust reaction4 and characterized with ultraviolet visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), and proton (1H) nuclear magnetic resonance (NMR). MPC films are assembled on SAM modified gold electrode interfaces by using a "dip cycle" method of alternating MPC layers and dithiol linking molecules. Film growth at gold electrode is tracked electrochemically by measuring changes to the double layer charging current of the system. Analogous films assembled on silane modified glass slides allow for optical monitoring of film growth and cross-sectional TEM analysis provides an estimated film thickness. During film assembly, manipulation of the MPC ligand protection as well as the interparticle linkage mechanism allow for networked films, that are readily adaptable, to interface with redox protein having different adsorption mechanism. For example, Pseudomonas aeruginosa azurin (AZ) can be adsorbed hydrophobically to dithiol-linked films of hexanethiolate MPCs and cytochrome c (cyt c) can be immobilized electrostatically at a carboxylic acid modified MPC interfacial layer. In this report, we focus on the film protocol for the AZ system exclusively. Investigations involving the adsorption of proteins on MPC modified synthetic platforms could further the understanding of interactions between biomolecules and man-made materials, and consequently aid the development of biosensor schemes, ET modeling systems, and synthetic biocompatible materials.5-8

Alkanethiolate stabilized gold colloids known as monolayer protected clusters (MPCs) are synthesized, characterized, and assembled into thin films as an adsorption interface for protein monolayer electrochemistry of simple redox protein like Pseudomonas aeruginosa azurin (AZ) and cytochrome c (cyt c).

Colloidal gold nanoparticles protected with alkanethiolate ligands called monolayer protected gold clusters (MPCs) are synthesized and subsequently ...

Lipid Bilayer Vesicle Generation Using Microfluidic Jetting

Bottom-up synthetic biology presents a novel approach for investigating and reconstituting biochemical systems and, potentially, minimal organisms. This emerging field engages engineers, chemists, biologists, and physicists to design and assemble basic biological components into complex, functioning systems from the bottom up. Such bottom-up systems could lead to the development of artificial cells for fundamental biological inquiries and innovative therapies1,2. Giant unilamellar vesicles (GUVs) can serve as a model platform for synthetic biology due to their cell-like membrane structure and size. Microfluidic jetting, or microjetting, is a technique that allows for the generation of GUVs with controlled size, membrane composition, transmembrane protein incorporation, and encapsulation3. The basic principle of this method is the use of multiple, high-frequency fluid pulses generated by a piezo-actuated inkjet device to deform a suspended lipid bilayer into a GUV. The process is akin to blowing soap bubbles from a soap film. By varying the composition of the jetted solution, the composition of the encompassing solution, and/or the components included in the bilayer, researchers can apply this technique to create customized vesicles. This paper describes the procedure to generate simple vesicles from a droplet interface bilayer by microjetting.

Microfluidic jetting against a droplet interface lipid bilayer provides a reliable way to generate vesicles with control over membrane asymmetry, incorporation of transmembrane proteins, and encapsulation of material. This technique can be applied to study a variety of biological systems where compartmentalized biomolecules are desired.

Bottom-up synthetic biology presents a novel approach for investigating and reconstituting biochemical systems and, potentially, minimal organisms. This ...

Biomembrane Fabrication by the Solvent-assisted Lipid Bilayer (SALB) Method

In order to mimic cell membranes, the supported lipid bilayer (SLB) is an attractive platform which enables in vitro investigation of membrane-related processes while conferring biocompatibility and biofunctionality to solid substrates. The spontaneous adsorption and rupture of phospholipid vesicles is the most commonly used method to form SLBs. However, under physiological conditions, vesicle fusion (VF) is limited to only a subset of lipid compositions and solid supports. Here, we describe a one-step general procedure called the solvent-assisted lipid bilayer (SALB) formation method in order to form SLBs which does not require vesicles. The SALB method involves the deposition of lipid molecules onto a solid surface in the presence of water-miscible organic solvents (e.g., isopropanol) and subsequent solvent-exchange with aqueous buffer solution in order to trigger SLB formation. The continuous solvent exchange step enables application of the method in a flow-through configuration suitable for monitoring bilayer formation and subsequent alterations using a wide range of surface-sensitive biosensors. The SALB method can be used to fabricate SLBs on a wide range of hydrophilic solid surfaces, including those which are intractable to vesicle fusion. In addition, it enables fabrication of SLBs composed of lipid compositions which cannot be prepared using the vesicle fusion method. Herein, we compare results obtained with the SALB and conventional vesicle fusion methods on two illustrative hydrophilic surfaces, silicon dioxide and gold. To optimize the experimental conditions for preparation of high quality bilayers prepared via the SALB method, the effect of various parameters, including the type of organic solvent in the deposition step, the rate of solvent exchange, and the lipid concentration is discussed along with troubleshooting tips. Formation of supported membranes containing high fractions of cholesterol is also demonstrated with the SALB method, highlighting the technical capabilities of the SALB technique for a wide range of membrane configurations.

Biomembrane Fabrication by the Solvent-assisted Lipid Bilayer SALB Method

We present an experimental protocol to form a supported lipid bilayer on solid substrates without using lipid vesicles. We demonstrate a one-step method to form a lipid bilayer on silicon dioxide and gold as well as supported membranes with cholesterol-enriched domain for various biological applications.

In order to mimic cell membranes, the supported lipid bilayer (SLB) is an attractive platform which enables in vitro investigation of membrane-related ...

Automated Lipid Bilayer Membrane Formation Using a Polydimethylsiloxane Thin Film

An artificial lipid bilayer, or black lipid membrane (BLM), is a powerful tool for studying ion channels and protein interactions, as well as for biosensor applications. However, conventional BLM formation techniques have several drawbacks and they often require specific expertise and laborious processes. In particular, conventional BLMs suffer from low formation success rates and inconsistent membrane formation time. Here, we demonstrate a storable and transportable BLM formation system with controlled thinning-out time and enhanced BLM formation rate by replacing conventionally used films (polytetrafluoroethylene, polyoxymethylene, polystyrene) to polydimethylsiloxane (PDMS). In this experiment, a porous-structured polymer such as PDMS thin film is used. In addition, as opposed to conventionally used solvents with low viscosity, the use of squalene permitted a controlled thinning-out time via slow solvent absorption by PDMS, prolonging membrane lifetime. In addition, by using a mixture of squalene and hexadecane, the freezing point of the lipid solution was increased (~16 &#176;C), in addition, membrane precursors were produced that can be indefinitely stored and readily transported. These membrane precursors have reduced BLM formation time of &#60; 1 hr and achieved a BLM formation rate of ~80%. Moreover, ion channel experiments with gramicidin A demonstrated the feasibility of the membrane system.

We demonstrate a storable, transportable lipid bilayer formation system. A lipid bilayer membrane can be formed within 1 hr with over 80% success rate when a frozen membrane precursor is brought to ambient temperature. This system will reduce laborious processes and expertise associated with ion channels.

An artificial lipid bilayer, or black lipid membrane (BLM), is a powerful tool for studying ion channels and protein interactions, as well as for ...

Micropatterning and Assembly of 3D Microvessels

In vitro platforms to study endothelial cells and vascular biology are largely limited to 2D endothelial cell culture, flow chambers with polymer or glass based substrates, and hydrogel-based tube formation assays. These assays, while informative, do not recapitulate lumen geometry, proper extracellular matrix, and multi-cellular proximity, which play key roles in modulating vascular function. This manuscript describes an injection molding method to generate engineered vessels with diameters on the order of 100 &#181;m. Microvessels are fabricated by seeding endothelial cells in a microfluidic channel embedded within a native type I collagen hydrogel. By incorporating parenchymal cells within the collagen matrix prior to channel formation, specific tissue microenvironments can be modeled and studied. Additional modulations of hydrodynamic properties and media composition allow for control of complex vascular function within the desired microenvironment. This platform allows for the study of perivascular cell recruitment, blood-endothelium interactions, flow response, and tissue-microvascular interactions. Engineered microvessels offer the ability to isolate the influence from individual components of a vascular niche and precisely control its chemical, mechanical, and biological properties to study vascular biology in both health and disease.

This manuscript presents an injection molding method to engineer microvessels that recapitulate physiological properties of endothelium. The microfluidic-based process creates patent 3D vascular networks with tailorable conditions, such as flow, cellular composition, geometry, and biochemical gradients. The fabrication process and examples of potential applications are described.

In vitro platforms to study endothelial cells and vascular biology are largely limited to 2D endothelial cell culture, flow chambers with polymer or glass ...

Ligand Nano-cluster Arrays in a Supported Lipid Bilayer

Currently there is considerable interest in creating ordered arrays of adhesive protein islands in a sea of passivated surface for cell biological studies. In the past years, it has become increasingly clear that living cells respond, not only to the biochemical nature of the molecules presented to them but also to the way these molecules are presented. Creating protein micro-patterns is therefore now standard in many biology laboratories; nano-patterns are also more accessible. However, in the context of cell-cell interactions, there is a need to pattern not only proteins but also lipid bilayers. Such dual proteo-lipidic patterning has so far not been easily accessible. We offer a facile technique to create protein nano-dots supported on glass and propose a method to backfill the inter-dot space with a supported lipid bilayer (SLB). From photo-bleaching of tracer fluorescent lipids included in the SLB, we demonstrate that the bilayer exhibits considerable in-plane fluidity. Functionalizing the protein dots with fluorescent groups allows us to image them and to show that they are ordered in a regular hexagonal lattice. The typical dot size is about 800 nm and the spacing demonstrated here is 2 microns. These substrates are expected to serve as useful platforms for cell adhesion, migration and mechano-sensing studies.

We present a protocol to functionalize glass with nanometric protein patches surrounded by a fluid lipid bilayer. These substrates are compatible with advanced optical microscopy and are expected to serve as platform for cell adhesion and migration studies.

Currently there is considerable interest in creating ordered arrays of adhesive protein islands in a sea of passivated surface for cell biological ...

Fabrication of Custom Agarose Wells for Cell Seeding and Tissue Ring Self-assembly Using 3D-Printed Molds

Engineered tissues are being used clinically for tissue repair and replacement, and are being developed as tools for drug screening and human disease modeling. Self-assembled tissues offer advantages over scaffold-based tissue engineering, such as enhanced matrix deposition, strength, and function. However, there are few available methods for fabricating 3D tissues without seeding cells on or within a supporting scaffold. Previously, we developed a system for fabricating self-assembled tissue rings by seeding cells into non-adhesive agarose wells. A polydimethylsiloxane (PDMS) negative was first cast in a machined polycarbonate mold, and then agarose was gelled in the PDMS negative to create ring-shaped cell seeding wells. However, the versatility of this approach was limited by the resolution of the tools available for machining the polycarbonate mold. Here, we demonstrate that 3D-printed plastic can be used as an alternative to machined polycarbonate for fabricating PDMS negatives. The 3D-printed mold and revised mold design is simpler to use, inexpensive to produce, and requires significantly less agarose and PDMS per cell seeding well. We have demonstrated that the resulting agarose wells can be used to create self-assembled tissue rings with customized diameters from a variety of different cell types. Rings can then be used for mechanical, functional, and histological analysis, or for fabricating larger and more complex tubular tissues.

This protocol describes a platform for fabricating self-assembled tissue rings in variable sizes using a customized 3D-printed plastic mold. PDMS negatives are cured in the 3D-printed mold; then agarose is cast in the cured PDMS negatives. Cells are seeded into the resulting agarose wells where they aggregate into tissue rings.

Engineered tissues are being used clinically for tissue repair and replacement, and are being developed as tools for drug screening and human disease ...

Fabrication and Characterization of Optical Tissue Phantoms Containing Macrostructure

The rapid development of new optical imaging techniques is dependent on the availability of low-cost, customizable, and easily reproducible standards. By replicating the imaging environment, costly animal experiments to validate a technique may be circumvented. Predicting and optimizing the performance of in vivo and ex vivo imaging techniques requires testing on samples that are optically similar to tissues of interest. Tissue-mimicking optical phantoms provide a standard for evaluation, characterization, or calibration of an optical system. Homogenous polymer optical tissue phantoms are widely used to mimic the optical properties of a specific tissue type within a narrow spectral range. Layered tissues, such as the epidermis and dermis, can be mimicked by simply stacking these homogenous slab phantoms. However, many in vivo imaging techniques are applied to more spatially complex tissue where three dimensional structures, such as blood vessels, airways, or tissue defects, can affect the performance of the imaging system. 
This protocol describes the fabrication of a tissue-mimicking phantom that incorporates three-dimensional structural complexity using material with optical properties of tissue. Look-up tables provide India ink and titanium dioxide recipes for optical absorption and scattering targets. Methods to characterize and tune the material optical properties are described. The phantom fabrication detailed in this article has an internal branching mock airway void; however, the technique can be broadly applied to other tissue or organ structures.

Optical tissue phantoms are essential tools for calibration and characterization of optical imaging systems and validation of theoretical models. This article details a method for phantom fabrication that includes replication of tissue optical properties and three-dimensional tissue structure.

The rapid development of new optical imaging techniques is dependent on the availability of low-cost, customizable, and easily reproducible standards. By ...

Detection of Endotoxin in Nano-formulations Using Limulus Amoebocyte Lysate (LAL) Assays

When present in pharmaceutical products, a Gram-negative bacterial cell wall component endotoxin (often also called lipopolysaccharide) can cause inflammation, fever, hypo- or hypertension, and, in extreme cases, can lead to tissue and organ damage that may become fatal. The amounts of endotoxin in pharmaceutical products, therefore, are strictly regulated. Among the methods available for endotoxin detection and quantification, the Limulus Amoebocyte Lysate (LAL) assay is commonly used worldwide. While any pharmaceutical product can interfere with the LAL assay, nano-formulations represent a particular challenge due to their complexity. The purpose of this paper is to provide a practical guide to researchers inexperienced in estimating endotoxins in engineered nanomaterials and nanoparticle-formulated drugs. Herein, practical recommendations for performing three LAL formats including turbidity, chromogenic and gel-clot assays are discussed. These assays can be used to determine endotoxin contamination in nanotechnology-based drug products, vaccines, and adjuvants.

Detection of Endotoxin in Nano-formulations Using Limulus Amoebocyte Lysate LAL Assays

Detection of endotoxins in engineered nanomaterials represents one of the grand challenges in the field of nanomedicine. Here, we present a case study that describes the framework composed of three different LAL formats to estimate potential endotoxin contamination in nanoparticles.

When present in pharmaceutical products, a Gram-negative bacterial cell wall component endotoxin (often also called lipopolysaccharide) can cause ...

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.

Soft, low-power, biomolecular memristors leverage similar composition, structure, and switching mechanisms of bio-synapses. Presented here is a protocol to assemble and characterize biomolecular memristors obtained from insulating lipid bilayers formed between water droplets in oil. The incorporation of voltage-activated alamethicin peptides results in memristive ionic conductance across the membrane.

The ability to recreate synaptic functionalities in synthetic circuit elements is essential for neuromorphic computing systems that seek to emulate the ...