Effect of microstructure on molecular oxygen permeation through condensed phospholipid monolayers

Targeted Microbubbles for Drug, Gene, and Cell Delivery in Therapy and Immunotherapy

Microbubbles are 1-10 μm diameter gas-filled acoustically-active particles, typically stabilized by a phospholipid monolayer shell. Microbubbles can be engineered through bioconjugation of a ligand, drug and/or cell. Since their inception a few decades ago, several targeted microbubble (tMB) formulations have been developed as ultrasound imaging probes and ultrasound-responsive carriers to promote the local delivery and uptake of a wide variety of drugs, genes, and cells in different therapeutic applications. The aim of this review is to summarize the state-of-the-art of current tMB formulations and their ultrasound-targeted delivery applications. We provide an overview of different carriers used to increase drug loading capacity and different targeting strategies that can be used to enhance local delivery, potentiate therapeutic efficacy, and minimize side effects. Additionally, future directions are proposed to improve the tMB performance in diagnostic and therapeutic applications.

The Impact of Lipid Handling and Phase Distribution on the Acoustic Behavior of Microbubbles

Phospholipid-coated microbubbles are ultrasound contrast agents that can be employed for ultrasound molecular imaging and drug delivery. For safe and effective implementation, microbubbles must respond uniformly and predictably to ultrasound. Therefore, we investigated how lipid handling and phase distribution affected the variability in the acoustic behavior of microbubbles. Cholesterol was used to modify the lateral molecular packing of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)-based microbubbles. To assess the effect of lipid handling, microbubbles were produced by a direct method, i.e., lipids directly dispersed in an aqueous medium or indirect method, i.e., lipids first dissolved in an organic solvent. The lipid phase and ligand distribution in the microbubble coating were investigated using confocal microscopy, and the acoustic response was recorded with the Brandaris 128 ultra-high-speed camera. In microbubbles with 12 mol% cholesterol, the lipids were miscible and all in the same phase, which resulted in more buckle formation, lower shell elasticity and higher shell viscosity. Indirect DSPC microbubbles had a more uniform response to ultrasound than direct DSPC and indirect DSPC-cholesterol microbubbles. The difference in lipid handling between direct and indirect DSPC microbubbles significantly affected the acoustic behavior. Indirect DSPC microbubbles are the most promising candidate for ultrasound molecular imaging and drug delivery applications.

Shape morphology of dipolar domains in planar and spherical monolayers

We present a continuum theory for predicting the equilibrium shape and size of dipolar domains formed during liquid-liquid phase coexistence in planar and spherical monolayers. Our main objective is to assess the impact of the monolayer surface curvature on domain morphology. Following previous investigators, we base our analysis around minimizing the free energy, with contributions from line tension and electrostatic dipolar repulsions. Assuming a monodisperse system of circularly symmetric domains, we calculate self-energies and interaction energies for planar and spherical monolayers and determine the equilibrium domain size from the energy minima. We subsequently evaluate the stability of the circularly symmetric domain shapes to an arbitrary, circumferential distortion of the perimeter via a linear stability analysis. We find that the surface curvature generally promotes the formation of smaller, circularly symmetric domains instead of larger, elongated domains. We rationalize these results by examining the effect of the curvature on the intra- and inter-domain dipolar repulsions. We then present a phase diagram of domain shape morphologies, parameterized in terms of the domain area fraction and the monolayer curvature. For typical domain dimensions of 1-30 µm, our theoretical results are relevant to monolayers (and possibly also bilayers) in liquid-liquid phase coexistence with radii of curvature of 1-100 µm.

Phospholipid Oxygen Microbubbles for Image-Guided Therapy

In recent work, oxygen microbubbles (OMB) have been shown to oxygenate hypoxic tumors, increase radio-sensitivity and improve tumor control by radiation therapy. Compared to intra-tumoral injection, intravenous delivery of adjuvant agents such as OMBs for radiotherapy offers an attractive means of achieving true theranostic function in a minimally invasive manner via contrast-enhanced ultrasound (CEUS), while reducing the risk of injury, infection or displacing tumor cells. However, short intravascular circulation times with conventional DSPC-lipid OMBs may lead to premature off-target dissolution of OMBs with an associated reduction in tumoral oxygen delivery. Prior work on microbubble stability and gas exchange suggests that increasing phospholipid acyl-chain length of the encapsulating shell and OMB size may increase circulation persistence, delivery and dissolved oxygen content. In the following studies, we investigate the effect of two phospholipid shell compositions, DSPC (C18:0) and DBPC (C22:0), as well as three size distributions (0.5-2 µm, 2-10 µm and polydisperse) on OMB circulation persistence utilizing CEUS in the kidneys of live C57B1/6 male and female mice, six weeks of age. DBPC OMB formulations demonstrated increased circulation half-lives versus DSPC formulations (2.4 ± 1.0 vs. 0.6 ± 0.5 s, p<0.01 for 2-10 µm), as well as an increased maximum intensity by over tenfold (p<0.01). Size-dependent effects remained consistent across both formulations with larger 2-10 µm microbubbles demonstrating significantly increased half-lives (2.4 ± 1.0 vs. 0.3 ± 0.2 s, p < 0.01) compared to smaller 0.5-2 µm formulations of DBPC. These studies indicate that DBPC 2-10 µm OMBs may be improved adjuvant agents for radiotherapy with significant potential for CEUS interrogation.

Intermolecular Forces Model for Lipid Microbubble Shells

Lipid-coated microbubbles are currently used clinically as ultrasound contrast agents for echocardiography and radiology and are being developed for many new diagnostic and therapeutic applications. Accordingly, there is a growing need to engineer specific formulations by employing rational design to guide lipid selection and processing. This approach requires a quantitative relationship between lipid chemistry and interfacial properties of the microbubble shell. Just such a model is proposed here on the basis of lateral Coulomb and van der Waals interactions between lipid head- and tailgroups, using previous coarse graining and force fields developed for molecular dynamics simulations. The model predicts with sufficient accuracy the monolayer permeability, the elasticity as a function of either lipid composition or temperature, and the equilibrium spreading surface tension of the lipid onto an air/water interface. In the future, the intermolecular forces model could be employed to elucidate more complex phenomena and to engineer novel microbubble formulations.

Crystalline Wax Esters Regulate the Evaporation Resistance of Tear Film Lipid Layers Associated with Dry Eye Syndrome

Dry eye syndrome (DES), one of the most common ophthalmological diseases, is typically caused by excessive evaporation of tear fluid from the ocular surface. Excessive evaporation is linked to impaired function of the tear film lipid layer (TFLL) that covers the aqueous tear film. The principles of the evaporation resistance of the TFLL have remained unknown, however. We combined atomistic simulations with Brewster angle microscopy and surface potential experiments to explore the organization and evaporation resistance of films composed of wax esters, one of the main components of the TFLL. The results provide evidence that the evaporation resistance of the TFLL is based on crystalline-state layers of wax esters and that the evaporation rate is determined by defects in the TFLL and its coverage on the ocular surface. On the basis of the results, uncovering the nonequilibrium spreading and crystallization of TFLL films has potential to reveal new means of treating DES.

Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications

Microbubbles are increasingly being used in biomedical applications such as ultrasonic imaging and targeted drug delivery. Microbubbles typically range from 0.1 to 10 µm in size and consist of a protective shell made of lipids or proteins. The shell encapsulates a gaseous core containing gases such as oxygen, sulfur hexafluoride or perfluorocarbons. This review is a consolidated account of information available in the literature on research related to microbubbles. Efforts have been made to present an overview of microbubble synthesis techniques; microbubble stability; microbubbles as contrast agents in ultrasonic imaging and drug delivery vehicles; and side effects related to microbubble administration in humans. Developments related to the modeling of microbubble dissolution and stability are also discussed.

Post-Formation Shrinkage and Stabilization of Microfluidic Bubbles in Lipid Solution

Medical ultrasound imaging often employs ultrasound contrast agents (UCAs), injectable microbubbles stabilized by shells or membranes. In tissue, the compressible gas cores can strongly scatter acoustic signals, resonate, and emit harmonics. However, bubbles generated by conventional methods have nonuniform sizes, reducing the fraction that resonates with a given transducer. Microfluidic flow-focusing is an alternative production method which generates highly monodisperse bubbles with uniform constituents, enabling more-efficient contrast enhancement than current UCAs. Production size is tunable by adjusting gas pressure and solution flow rate, but solution effects on downstream stable size and lifetime have not been closely examined. This study therefore investigated several solution parameters, including the DSPC/DSPE-PEG2000 lipid ratio, concentration, viscosity, and preparation temperature to determine their effects on stabilization. It was found that bubble lifetime roughly correlated with stable size, which in turn was strongly influenced by primary-lipid-to-emulsifier ratio, analogous to its effects on conventional bubble yield and Langmuir-trough compressibility in existing studies. Raising DSPE-PEG2000 fraction in solution reduced bubble surface area in proportion to its reduction of lipid packing density at low compression in literature. In addition, the surface area was found to increase proportionately with lipid concentration above 2.1 mM. However, viscosities above or below 2.3-3.3 mPa·s seemed to reduce bubble size. Finally, lipid preparation at room temperature led to smaller bubbles compared to preparation near or above the primary lipid's phase transition point. Understanding these effects will further improve on postformation control over microfluidic bubble production, and facilitate size-tuning for optimal contrast enhancement.

The effect of lipid monolayer in-plane rigidity on in vivo microbubble circulation persistence

The goal of this study was to increase in vivo microbubble circulation persistence for applications in medical imaging and targeted drug delivery. Our approach was to investigate the effect of lipid monolayer in-plane rigidity to reduce the rate of microbubble dissolution, while holding constant the microbubble size, concentration and surface architecture. We first estimated the impact of acyl chain length of the main diacyl phosphatidylcholine (PC) lipid and inter-lipid distance on the cohesive surface energy and, based on these results, we hypothesized that microbubble stability and in vivo ultrasound contrast persistence would increase monotonically with increasing acyl chain length. We therefore measured microbubble in vitro stability to dilution with and without ultrasound exposure, as well as in vivo ultrasound contrast persistence. All measurements showed a sharp rise in stability between DPPC (C16:0) and DSPC (C18:0), which correlates to the wrinkling transition, signaling the onset of significant surface shear and gas permeation resistance, observed in prior single-bubble dissolution studies. Further evidence for the effect of the wrinkling transition came from an in vitro and in vivo stability comparison of microbubbles coated with pure DPPC with those of lung surfactant extract. Microbubble stability against dilution without ultrasound and in vivo ultrasound contrast persistence showed a monotonic increase with acyl chain length from DSPC to DBPC (C22:0). However, we also observed that stability dropped precipitously for all measurements on further increasing lipid acyl chain length from DBPC to DLiPC (C24:0). This result suggests that hydrophobic mismatch between the main PC lipid and the lipopolymer emulsifier, DSPE-PEG5000, may drive a less stable surface microstructure. Overall, these results support our general hypothesis of the role of in-plane rigidity for increasing the lifetime of microbubble circulation.

Microbubbles as biocompatible porogens for hydrogel scaffolds

In this study, we explored the application of lipid-shelled, gas-filled microbubbles as a method for creating on-demand microporous hydrogels for cartilage tissue engineering. The technique allowed for homogenous distribution of cells and micropores within the scaffold, increasing the absorption coefficient of large solutes (70kDa dextran) over controls in a concentration-dependent manner. The stability of the gas phase of the microbubbles depended on several factors, including the initial size distribution of the microbubble suspension, as well as the temperature and pressure during culture. Application of pressure cycles provided controlled release of the gas phase to generate fluid-filled micropores with remnant lipid. The resulting microporous agarose scaffolds were biocompatible, leading to a twofold increase in engineered cartilage properties (E(Y)=492±42kPa for the bubble group vs. 249±49kPa for the bubble-free control group) over a 42-day culture period. Our results suggest that microbubbles offer a simple and robust method of modulating mass transfer in cell-seeded hydrogels through mild pressurization, and the methodology may be expanded in the future to include focused ultrasound for improved spatio-temporal control.

Lipid monolayer collapse and microbubble stability

Microbubbles are micrometer-size gaseous particles suspended in water, and they are often stabilized by a lipid monolayer shell. Natural microbubbles are found in freshwater and saltwater systems, and engineered microbubbles have a variety of applications in food sciences, biotechnology and medicine. Lipid-coated microbubbles are found to have remarkable stability and mechanical behavior owing to the resistance of the lipid monolayer encapsulation to collapse. The purpose of this review is to tie in recent observations of lipid-coated microbubble dissolution and gas exchange with current literature on the physics of lipid monolayer collapse in the context of lung surfactant. Based on this analysis, we conclude that microbubble shells collapse through the nucleation of microscopic folds, which then catalyze the formation and aggregation of new folds, leading to macroscopic folding events. This process results in a cyclic behavior of crumple-to-smooth transitions, which can be modulated through lipid composition. Eventually, the microbubbles stabilize at 1-2 μm diameter, regardless of initial size or lipid composition, and various mechanisms for this stabilization are postulated. Our ultimate goal is to inspire the reader to consider lipid monolayer collapse as the main long-term stabilizing mechanism for lipid-coated microbubbles, and to stimulate the use of microbubbles as a platform for studying monolayer collapse phenomena.

Coexisting phases in PEGylated phosphocholine membranes: a model study

Understanding the phase behavior of PEGylated phosphocholine membranes is becoming increasingly important in many biomedical applications. Here, we used binary mixtures of phosphocholines and PEG-phospholipids in monolayers on phosphate buffered saline as ideal models of PEGylated phosphocholine membranes. Several phase states and transitions between homogeneously mixed and completely immiscible phases have been visualized in these mixtures by epifluorescence microscopy, which is neither predicted nor easily explained by the existing interpretive schemes. The results of our study suggest that the phase state of PEGylated phosphocholine membranes may drastically vary depending on factors such as aliphatic chain length on phosphocholines and PEG-phospholipids, PEG content, and temperature. These findings are summarized in phase drawings and diagrams to demonstrate a striking variety of possible phases. The diagrams can also be instrumental in predicting the phase state of PEGylated phosphocholine membranes, in particular under physiological conditions.

Dioxygen transmembrane distributions and partitioning thermodynamics in lipid bilayers and micelles

Cellular respiration, mediated by the passive diffusion of oxygen across lipid membranes, is key to many basic cellular processes. In this work, we report the detailed distribution of oxygen across lipid bilayers and examine the thermodynamics of oxygen partitioning via NMR studies of lipids in a small unilamellar vesicle (SUV) morphology. Dissolved oxygen gives rise to paramagnetic chemical shift perturbations and relaxation rate enhancements, both of which report on local oxygen concentration. From SUVs containing the phospholipid sn-2-perdeuterio-1-myristelaidoyl, 2-myristoyl-sn-glycero-3-phosphocholine (MLMPC), an analogue of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), we deduced the complete trans-bilayer oxygen distribution by measuring (13)C paramagnetic chemical shifts perturbations for 18 different sites on MLMPC arising from oxygen at a partial pressure of 30 bar. The overall oxygen solubility at 45 °C spans a factor of 7 between the bulk water (23.7 mM) and the bilayer center (170 mM) and is lowest in the vicinity of the phosphocholine headgroup, suggesting that oxygen diffusion across the glycerol backbone should be the rate-limiting step in diffusion-mediated passive transport of oxygen across the lipid bilayer. Lowering of the temperature from 45 to 25 °C gave rise to a slight decrease of the oxygen solubility within the hydrocarbon interior of the membrane. An analysis of the temperature dependence of the oxygen solubility profile, as measured by (1)H paramagnetic relaxation rate enhancements, reveals that oxygen partitioning into the bilayer is entropically favored (ΔS° = 54 ± 3 J K(-1) mol(-1)) and must overcome an enthalpic barrier (ΔH° = 12.0 ± 0.9 kJ mol(-1)).

Phospholipid-stabilized microbubble foam for injectable oxygen delivery

A detailed study is presented on the synthesis and characterization of purely oxygen-filled microbubbles (OMBs) stabilized by phospholipids. Microbubbles with a diameter of less than 10 μm were generated and concentrated to >50 vol % in saline. The lipid acyl chain length had little effect on the size distribution but profoundly affected the foam stability. For example, OMBs stabilized by dipalmitoyl phosphatidylcholine (DPPC) degraded over 3 weeks, but OMBs stabilized with distearoyl phosphatidylcholine (DSPC) retained over half of their initially encapsulated gas. Interestingly, the polydisperse size distribution remained nearly constant as the foam slowly broke down. Injection into an undersaturated solution led to the immediate release of the oxygen gas core. Injectable gas delivery by OMBs may find use in a variety of medical and industrial fields.

Phospholipid-coated gas bubble engineering: key parameters for size and stability control, as determined by an acoustical method

We have recently reported the sampling of differently sized monomodal populations of microbubbles from a polydisperse lipid-coated bubble preparation. The microbubbles were coated with dimyristoylphosphatidylcholine (DMPC) and stabilized by perfluorohexane (PFH). Such microbubbles are useful as contrast agents and, potentially, for oxygen, drug, and gene delivery and as therapeutic devices. Monomodal populations of small bubbles (approximately 1.6 microm in radius) and large bubbles (approximately 5.4 microm) have been obtained, as assessed by acoustical measurement, static light scattering, and optical microscopy. In this paper, we have determined the influence of various preparation parameters on the initial size characteristics (mean radius and radii distribution) of the microbubbles and on their stability upon time. The bubble size was determined acoustically, with a homemade acoustic setup equipped with a low-power emitter, to avoid altering the bubble stability. We have focused on the effects of the bubble flotation time during the fractionation process and on the DMPC concentration. PFH was indispensable for obtaining stable bubbles. The nature of the buffer [Isoton II vs N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)] used as the continuous phase did not significantly impact the bubble characteristics and stability. In both buffers, the half-lives of small bubbles (approximately 1.6 microm in radius in Isoton II and approximately 2.1 microm in HEPES) were found to be longer than those of larger ones (approximately 5.4 and approximately 5.9 microm in Isoton II and HEPES, respectively). The bubble stability study revealed that in both buffers, the average radius of the population of large bubbles progressively increased with time. On the other hand, the average radius of the population of small bubbles decreased slightly in Isoton II and remained constant in HEPES. This suggests that the dissolution behavior of small and large bubbles is governed by different mechanisms.

Microbubble characterization through acoustically induced deflation

Ultrasound contrast agents (UCA) populations are typically polydisperse and contain microbubbles with radii over a given range. Although the behavior of microbubbles of certain sizes might be masked by the behavior of others, the acoustic characterization of UCA is typically made on full populations. In this paper, we have combined acoustic and optical methods to investigate the response of isolated lipid-shelled microbubbles to low-pressure (49 and 62 kPa peak negative pressure) ultrasound tone bursts. These bursts induced slow deflation of the microbubbles. The experimental setup included a microscope connected to a fast camera acquiring one frame per pulse transmitted by a single-element transducer. The behavior of each bubble was measured at multiple frequencies, by cyclically changing the transmission frequency over the range of 2 to 4 MHz during subsequent pulse repetition intervals. The bubble echoes were captured by a second transducer and coherently recorded. More than 50 individual microbubbles were observed. Microbubbles with radii larger than 3 mum did not experience any size reduction. Smaller bubbles slowly deflated, generally until the radius reached a value around 1.4 microm, independent of the initial microbubble size. The detected pressure amplitude backscattered at 2.5 cm distance was very low, decreasing from about 5 Pa down to 1 Pa at 2 MHz as the bubbles deflated. The resonant radius was evaluated from the echo amplitude normalized with respect to the geometrical cross section. At 2-MHz excitation, deflating microbubbles showed highest normalized echo when the radius was 2.2 microm while at higher excitation frequencies, the resonant radius was lower. The relative phase shift of the echo during the deflation process was also measured. It was found to exceed pi/2 in all cases. A heuristic procedure based on the analysis of multiple bubbles of a same population was used to estimate the undamped natural frequency. It was found that a microbubble of 1.7 microm has an undamped natural frequency of 2 MHz. The difference between this size and the resonant radius is discussed as indicative of significant damping.

Growth and dissolution of an encapsulated contrast microbubble: effects of encapsulation permeability

Gas diffusion from an encapsulated microbubble is modeled using an explicit linear relation for gas permeation through the encapsulation. Both the cases of single gas (air) and multiple gases (perfluorocarbon inside the bubble and air dissolved in surrounding liquid) are considered. An analytical expression for the dissolution time for an encapsulated air bubble is obtained; it showed that for small permeability the dissolution time increases linearly with decreasing permeability. A perfluorocarbon-filled contrast microbubble such as Definity was predicted to experience a transient growth because of air infusion before it dissolves in conformity with previous experimental findings. The growth phase occurs only for bubbles with a critical value of initial mole fraction of perfluorocarbon relative to air. With empirically obtained property values, the dissolution time of a 2.5-micron diameter (same as that of Definity), lipid-coated octafluoropropane bubble, with surface tension 25 mN/m, is predicted to be 42 min in an air-saturated medium. The properties such as shell permeability, surface tension and relative mole fraction of octafluoropropane are varied to investigate their effects on the time scales of bubble growth and dissolution, including their asymptotic scalings where appropriate. The dissolution dynamics scales with permeability, in that when the time is nondimensioanlized with permeability, curves for different permeabilities collapse on a single curve. Investigation of bubbles filled with other gases (nonoctafluoropropane perfluorocarbon and sulfur hexafluoride) indicates longer dissolution time because of lower solubility and lower diffusivity for larger gas molecules. For such micron-size encapsulated bubbles, lifetime of hours is possible only at extremely low surface tension (<1 mN/m) or at extreme oversaturation.

Microbubbles as ultrasound triggered drug carriers

Originally developed as contrast agents for ultrasound imaging and diagnostics, in the past years, microbubbles have made their way back from the patients' bedside to the researcher's laboratory. Microbubbles are currently believed to have great potential as carriers for drugs, small molecules, nucleic acids, and proteins. This review provides insight into this intriguing new frontier from the perspective of the pharmaceutical scientist. First, basic aspects on the application of ultrasound-targeted microbubble destruction for drug delivery will be presented. Next, we will review the recently applied approaches for manufacturing and drug-loading microbubbles. Important quality issues and characterization techniques for advanced microbubble formulation will be discussed. Finally, we will provide an assessment of the prospects for microbubbles in drug and gene therapy, illustrating the problems and requirements for their future development.

Microbubbles coated with disaturated lipids and DSPE-PEG2000: phase behavior, collapse transitions, and permeability

Saturated diacyl (disaturated) phosphatidylcholine (PC) mixed with the lipopolymer distearoylphosphatidylethanolamine (DSPE)-polyethyleneglycol molecular weight 2000 (PEG2000) self-assemble as a monolayer at the air-water interface of air-in-water micrometer-scale bubbles (microbubbles), similar to coatings (shells) on leading medical ultrasound contrast agents (UCAs). This system is characterized here to study the impact of the DSPE-PEG2000 species and PC chain-length on the monolayer coating phase behavior, collapse, shedding, and air transport resistance and microbubble dissolution rate and surface contour. Using fluorescence microscopy of dissolving microbubbles, we found that film microstructure and collapse behavior for all chain lengths (n = 14-20) was indicative of primarily condensed phase monolayers, unlike similar coatings containing polyethyleneglycol 40 stearate (PEG40S) that are either expanded phase or coexisting expanded-condensed phase monolayers. Additionally, we observed a new surface buckling type of behavior with all chain lengths, by bright field microscopy, where the air-water interface continuously appears rough (rather than cyclically rough and smooth), with this behavior most frequently observed for n = 16. In correlating the statistical frequency of this behavior with the monolayer microstructure, we propose that it arises from a slowed nucleation rate of collapse structures at condensed-condensed phase interfaces, not present in systems containing PEG40S. By modeling the dissolution (radius vs time) data, we obtained, for each chain length, the film air transport resistance (R(shell)) that was then fit to a chain-length-dependent energy barrier model. Importantly, the pre-exponential factor was approximately 10 x higher and the microbubbles persisted approximately 4 x longer (from 15 microm at a fixed dissolved oxygen content) in comparison to previously studied films containing PEG40S. We attribute the unique stability properties of microbubble coatings containing DSPE-PEG2000 to the propensity of this molecule to form a condensed-phase monolayer, such that the monolayer coatings approach the properties of one continuous condensed domain.

Influence of the dissolution rate on the collapse and shedding behavior of monostearin/monopalmitin-rich coated microbubbles

A low cost food grade emulsifier (a mixture of monoglycerides, diglycerides, and sodium stearoyl lactylate) in combination with polyethylene glycol-40 stearate (PEG-40S) was used as an alternative to pure saturated phospholipids to form the thin shell of a microbubble. To investigate the stability of these microbubbles in a water system over time, their dissolution behavior was studied at various degas factors and at two percentages of PEG-40S. It was found that the favored shell collapse/shedding mechanism switched, as the dissolution rate increased (degas factor decreased), from folding with a smooth surface contour to buckling accompanied by surface folding/expulsion with a cyclic buckled-smooth surface contour. The compositional change that we made played a more minor role, mainly controlling the resistance to mass transfer of the microbubble shell and again modifying the mechanism-determinant dissolution rate. The shell resistance behavior for these microbubbles varied from that of previous lipid/PEG-40S-coated microbubbles by the presence of a maximum in shell resistance during dissolution. We hypothesize that the dominance of one collapse mechanism over another for these compositions is related to the time scales of two competing processes, fold nucleation and area compression. For these mixtures at room temperature, we estimate that the maximum area compression rate for folding as the major collapse mechanism is approximately 0.2 s (-1), a rate unattainable in a traditional Langmuir trough but achievable by the use of a dissolving microbubble.

Liposomes in ultrasonic drug and gene delivery

Liposome-based drug and gene delivery systems have potential for significant roles in a variety of therapeutic applications. Recently, liposomes have been used to entrap gas and drugs for ultrasound-controlled drug release and ultrasound-enhanced drug delivery. Echogenic liposomes have been produced by different preparation methods, including lyophilization, pressurization, and biotin-avidin binding. Presently, significant in vivo applications of liposomal ultrasound-based drug and gene delivery are being made in cardiac disease, stroke and tumor therapy. Translation of these vehicles into the clinic will require a better understanding of improved physical properties to avoid rapid clearance, as well as of possible side effects, including those of the ultrasound. The aim of this review is to provide orientation for new researchers in the area of ultrasound-enhanced liposome drug and gene delivery.

Ultrasound microbubble contrast agents: fundamentals and application to gene and drug delivery

This review offers a critical analysis of the state of the art of medical microbubbles and their application in therapeutic delivery and monitoring. When driven by an ultrasonic pulse, these small gas bubbles oscillate with a wall velocity on the order of tens to hundreds of meters per second and can be deflected to a vessel wall or fragmented into particles on the order of nanometers. While single-session molecular imaging of multiple targets is difficult with affinity-based strategies employed in some other imaging modalities, microbubble fragmentation facilitates such studies. Similarly, a focused ultrasound beam can be used to disrupt delivery vehicles and blood vessel walls, offering the opportunity to locally deliver a drug or gene. Clinical translation of these vehicles will require that current challenges be overcome, where these challenges include rapid clearance and low payload. The technology, early successes with drug and gene delivery, and potential clinical applications are reviewed.

Effect of compressed bovine lipid extract surfactant films on oxygen transfer

In the lungs, oxygen transfer from the inspired air to the capillary blood needs to cross the surfactant lining layer of the alveoli. Therefore, the gas transfer characteristics of lung surfactant film are of fundamental physiological interest. However, previous in vitro studies-most relied on the Langmuir-type balance-fail to cover the low surface tension range (i.e., less than the equilibrium surface tension of approximately 25 mJ/m2) due to film leakage. We have recently developed a novel in vitro experimental strategy, the combination of axisymmetric drop shape analysis and captive bubble technique (ADSA-CB), in studying the effect of surfactant films on interfacial gas transfer (Langmuir 2005, 21, 5446). In the present work, ADSA-CB is used as a micro-film-balance to study the effect of compressed bovine lipid extract surfactant (BLES) films on oxygen transfer. A low surface tension ranging from approximately 25 mJ/m2 to 2 mJ/m2 is studied. The experimental results suggest that lung surfactant films at a low surface tension near 2 mJ/m2 provide resistance to oxygen transfer, as indicated by a decrease of 30-50% in the mass transfer coefficient (kL) of oxygen in BLES suspensions with respect to water. At higher surface tension (i.e., >6 mJ/m2), the resistance to oxygen transfer is only modest, i.e., the decrease in kL is less than 20% compared to water. The experimental results suggest that lung surfactant plays a role in oxygen transfer in the pulmonary system.

Scanning electrochemical microscopy: principles and applications to biophysical systems

This review highlights numerous and wide ranging biophysical and biochemical applications of scanning electrochemical microscopy (SECM). SECM instrumentation and theoretical modelling, necessary for experimental interpretation, are outlined, followed by a detailed discussion of the diverse applications of this technique. These include the measurement of flow through membranes, the determination of kinetic parameters of reactions, the investigation of the permeability of small molecules in tissues and monitoring biological processes, such as the production of oxygen or nitric oxide by cells. The significant impact of micro-electrochemical techniques on our understanding of basic physicochemical processes at biologically relevant interfaces is also considered. Studies reviewed include transport across and within bilayers and monolayers. Recent advances in SECM include the combination of SECM with other techniques, such as atomic force microscopy and optical microscopy. These developments are highlighted, along with prospects for the future.

Collapse and shedding transitions in binary lipid monolayers coating microbubbles

We report on a fluorescence microscopy study of the monolayer collapse and shedding behavior due to shell compression during the dissolution of air-filled, lipid-coated microbubbles in degassed media. The monolayer shell was comprised of saturated diacyl phosphatidylcholine (C12:0 to C22:0) and an emulsifier, poly(ethylene glycol)-40 stearate. The morphologies of monolayer collapse structures and shed particles were monitored as a function of phospholipid acyl chain length (n) and temperature. The two components formed a single miscible phase when the phospholipid was near or above its main phase transition temperature, and collapse occurred via suboptical particles to vesicles (both were shed) and tubes as chain length increased. Conversely, two-phase coexistence was observed when the lipid was below its main phase transition temperature. For these bubbles, a transition from primary collapse to secondary collapse was observed. Primary collapse was observed as a loss of expanded phase due to vesiculation. Secondary collapse involved the rapid propagation of monolayer folds and simultaneous deformation. For very rigid monolayers, we observed substantial surface buckling with simultaneous nucleation and growth of folds. The folds merged at a single point or region, providing a conduit for the entire excess lipid to shed in a single event, and the bubble smoothed and became more spherical. These results are discussed in the context of general binary phospholipid collapse behavior, microbubble dissolution behavior, medical applications, and the dissolution behavior of natural microbubbles.