16 research outputs found
Hydrostatic Pressurization of Lung Surfactant Microbubbles: Observation of a Strain-Rate Dependent Elasticity
The microbubble offers
a unique platform to study lung surfactant
mechanics at physiologically relevant geometry and length scale. In
this study, we compared the response of microbubbles (∼15 μm
initial radius) coated with pure dipalmitoyl-phosphatidylcholine (DPPC)
versus naturally derived lung surfactant (SURVANTA) when subjected
to linearly increasing hydrostatic pressure at different rates (0.5–2.3
kPa/s) at room temperature. The microbubbles contained perfluorobutane
gas and were submerged in buffered saline saturated with perfluorobutane
at atmospheric pressure. Bright-field microscopy showed that DPPC
microbubbles compressed spherically and smoothly, whereas SURVANTA
microbubbles exhibited wrinkling and smoothing cycles associated with
buckling and collapse. Seismograph analysis showed that the SURVANTA
collapse amplitude was constant, but the collapse rate increased with
the pressurization rate. An analysis of the pressure–volume
curves indicated that the dilatational elasticity increased during
compression for both shell types. The initial dilatational elasticity
for SURVANTA was nearly twice that of DPPC at higher pressurization
rates (>1.5 kPa/s), producing a pressure drop of up to 60 kPa across
the film prior to condensation of the perfluorobutane core. The strain-rate
dependent stiffening of SURVANTA shells likely arises from their composition
and microstructure, which provide enhanced in-plane monolayer rigidity
and lateral repulsion from surface-associated collapse structures.
Overall, these results provide new insights into lung surfactant mechanics
and collapse behavior during compression
Fluorocarbon Nanodrops as Acoustic Temperature Probes
This work investigated the use of
superheated fluorocarbon nanodrops
for ultrasound thermal imaging and the use of mixed fluorocarbons
for tuning thermal and acoustic thresholds for vaporization. Droplets
were fabricated by condensing phospholipid-coated microbubbles containing
C<sub>3</sub>F<sub>8</sub> and C<sub>4</sub>F<sub>10</sub> mixed at
various molar ratios. Vaporization temperatures first were measured
in a closed system by optical transmission following either isothermal
pressure release or isobaric heating. The vaporization temperature
was found to depend linearly on the percentage of C<sub>4</sub>F<sub>10</sub> in the droplet core, indicating excellent tunability under
these fluorocarbon-saturated conditions. Vaporization temperatures
were then measured in an open system using contrast-enhanced ultrasound
imaging, where it was found that the mixed droplets behaved like pure
C<sub>4</sub>F<sub>10</sub> drops. Additionally, the critical mechanical
index for vaporization was measured at the limits of therapeutic hyperthermia
(37 and 60 °C), and again the mixed droplets were found to behave
like pure C<sub>4</sub>F<sub>10</sub> drops. These results suggested
that C<sub>3</sub>F<sub>8</sub> preferentially dissolves out of the
droplet core in open systems, as shown by a simple mass transfer model
of multicomponent droplet dissolution. Finally, proof-of-concept was
shown that pure C<sub>4</sub>F<sub>10</sub> nanodrops can be used
as an acoustic temperature probe. Overall, these results not only
demonstrate the potential of superheated fluorocarbon emulsions for
sonothermetry but also point to the limits of tunability for fluorocarbon
mixtures owing to preferential release of the more soluble species
to the atmosphere
Thermal Activation of Superheated Lipid-Coated Perfluorocarbon Drops
This study explored the thermal conditions
necessary for the vaporization
of superheated perfluorocarbon nanodrops. Droplets C<sub>3</sub>F<sub>8</sub> and C<sub>4</sub>F<sub>10</sub> coated with a homologous
series of saturated diacylÂphosphatidylÂcholines were formed
by condensation of 4 μm diameter microbubbles. These drops were
stable at room temperature and atmospheric pressure, but they vaporized
back into microbubbles at higher temperatures. The vaporization transition
was measured as a function of temperature by laser light extinction.
We found that C<sub>3</sub>F<sub>8</sub> and C<sub>4</sub>F<sub>10</sub> drops experienced 90% vaporization at 40 and 75 °C, respectively,
near the theoretical superheat limits (80–90% of the critical
temperature). We therefore conclude that the metastabilty of these
phase-change agents arises not from the droplet Laplace pressure altering
the boiling point, as previously reported, but from the metastability
of the pure superheated fluid to homogeneous nucleation. The rate
of C<sub>4</sub>F<sub>10</sub> drop vaporization was quantified at
temperatures ranging from 55 to 75 °C, and an apparent activation
energy barrier was calculated from an Arrhenius plot. Interestingly,
the activation energy increased linearly with acyl chain length from
C14 to C20, indicating that lipid interchain cohesion plays an important
role in suppressing the vaporization rate. The vaporized drops (microbubbles)
were found to be unstable to dissolution at high temperatures, particularly
for C14 and C16. However, proper choice of the fluorocarbon and lipid
species provided a nanoemulsion that could undergo at least ten reversible
condensation/vaporization cycles. The vaporization properties presented
in this study may facilitate the engineering of tunable phase-shift
particles for diagnostic imaging, targeted drug delivery, tissue ablation,
and other applications
Condensation Phase Diagrams for Lipid-Coated Perfluorobutane Microbubbles
The goal of this study was to explore
the thermodynamic conditions
necessary to condense aqueous suspensions of lipid-coated gas-filled
microbubbles into metastable liquid-filled nanodrops as well as the
physicochemical mechanisms involved with this process. Individual
perfluorobutane microbubbles and their lipid shells were observed
as they were pressurized at 34.5 kPa s<sup>–1</sup> in a microscopic
viewing chamber maintained at temperatures ranging from 5 to 75 °C.
The microbubbles contracted under pressure, ultimately leading to
either full dissolution or microbubble-to-nanodrop condensation. Temperature–pressure
phase diagrams conveying condensation and stability transitions were
constructed for microbubbles coated with saturated diacylphosphatidylcholine
lipids of varying acyl chain length (C16 to C24). The onset of full
dissolution was shifted to higher temperatures with the use of longer
acyl chain lipids or supersaturated media. Longer chain lipid shells
resisted both dissolution of the gas core and mechanical compression
through a pronounced wrinkle-to-fold collapse transition. Interestingly,
the lipid shell also provided a mechanical resistance to condensation,
shifting the vapor-to-liquid transition to higher pressures than for
bulk perfluorobutane. This result indicated that the lipid shell can
provide a negative apparent surface tension under compression. Overall,
the results of this study will aid in the design and formulation of
vaporizable fluorocarbon nanodrops for various applications, such
as diagnostic ultrasound imaging, targeted drug delivery, and thermal
ablation
Monodispersity Increases Adhesion Efficiency and Specificity for Ultrasound-Targeted Microbubbles
Ultrasound molecular imaging with targeted microbubbles
(MBs) can
be used to noninvasively diagnose, monitor, and study the progression
of different endothelial-associated diseases. Acoustic radiation force
(Frad) can initiate and enhance MB adhesion
at the target site. The goal of this study was to elucidate the effects
of various MB parameters on Frad targeting.
Monodisperse or polydisperse MBs with the immune-stealth cloaked (buried)-ligand
architecture were conjugated with targeting RGD or nonspecific isotype
control RAD peptides and then pumped through an αvβ3 integrin-coated microvessel phantom at a wall
shear stress of 3.5 dyn/cm2. Targeting was assessed by
measuring MB attachment for varying Frad time and frequency, as well as MB concentration and size distribution.
We first confirmed that primary Frad is
necessary to target the cloaked-ligand MBs. MB targeting increased
monotonically with αvβ3 integrin
density and Frad time. MB attachment and,
to a lesser extent specificity, also increased when driven by Frad near resonance. MB targeting increased with
MB concentration, although a shift in behavior was observed with increasing
MB–MB interactions and aggregations forming from secondary Frad effects as MB concentration was increased.
These secondary Frad effects reduced targeting
specificity. Finally, after having validated our approach by testing
different parameters with the appropriate controls, we then determined
the effects of monodispersity on adhesion efficiency and specific
targeting. We observed that both MB targeting efficiency and specificity
were greatly enhanced for monodisperse vs polydisperse
MBs. Analysis of videomicroscopy images indicated that secondary Frad effects may have disproportionally inhibited
targeting of polydisperse MBs. In conclusion, our in vitro results indicate that monodisperse MBs driven near resonance and
at a low concentration (∼106 MB/mL) can be used
to maximize the adhesion efficiency (up to 88%) and specificity of
RGD–MB targeting
Single Microbubble Measurements of Lipid Monolayer Viscoelastic Properties for Small-Amplitude Oscillations
Lipid
monolayer rheology plays an important role in a variety of
interfacial phenomena, the physics of biological membranes, and the
dynamic response of acoustic bubbles and drops. We show here measurements
of lipid monolayer elasticity and viscosity for very small strains
at megahertz frequency. Individual plasmonic microbubbles of 2–6
μm radius were photothermally activated with a short laser pulse,
and the subsequent nanometer-scale radial oscillations during ring-down
were monitored by optical scatter. This method provided average dynamic
response measurements of single microbubbles. Each microbubble was
modeled as an underdamped linear oscillator to determine the damping
ratio and eigenfrequency, and thus the lipid monolayer viscosity and
elasticity. Our nonisothermal measurement technique revealed viscoelastic
trends for different lipid shell compositions. We observed a significant
increase in surface elasticity with the lipid acyl chain length for
16 to 20 carbons, and this effect was explained by an intermolecular
forces model that accounts for the lipid composition, packing, and
hydration. The surface viscosity was found to be equivalent for these
lipid shells. We also observed an anomalous decrease in elasticity
and an increase in viscosity when increasing the acyl chain length
from 20 to 22 carbons. These results illustrate the use of a novel
nondestructive optical technique to investigate lipid monolayer rheology
in new regimes of frequency and strain, possibly elucidating the phase
behavior, as well as how the dynamic response of a microbubble can
be tuned by the lipid intermolecular forces
Monodispersity Increases Adhesion Efficiency and Specificity for Ultrasound-Targeted Microbubbles
Ultrasound molecular imaging with targeted microbubbles
(MBs) can
be used to noninvasively diagnose, monitor, and study the progression
of different endothelial-associated diseases. Acoustic radiation force
(Frad) can initiate and enhance MB adhesion
at the target site. The goal of this study was to elucidate the effects
of various MB parameters on Frad targeting.
Monodisperse or polydisperse MBs with the immune-stealth cloaked (buried)-ligand
architecture were conjugated with targeting RGD or nonspecific isotype
control RAD peptides and then pumped through an αvβ3 integrin-coated microvessel phantom at a wall
shear stress of 3.5 dyn/cm2. Targeting was assessed by
measuring MB attachment for varying Frad time and frequency, as well as MB concentration and size distribution.
We first confirmed that primary Frad is
necessary to target the cloaked-ligand MBs. MB targeting increased
monotonically with αvβ3 integrin
density and Frad time. MB attachment and,
to a lesser extent specificity, also increased when driven by Frad near resonance. MB targeting increased with
MB concentration, although a shift in behavior was observed with increasing
MB–MB interactions and aggregations forming from secondary Frad effects as MB concentration was increased.
These secondary Frad effects reduced targeting
specificity. Finally, after having validated our approach by testing
different parameters with the appropriate controls, we then determined
the effects of monodispersity on adhesion efficiency and specific
targeting. We observed that both MB targeting efficiency and specificity
were greatly enhanced for monodisperse vs polydisperse
MBs. Analysis of videomicroscopy images indicated that secondary Frad effects may have disproportionally inhibited
targeting of polydisperse MBs. In conclusion, our in vitro results indicate that monodisperse MBs driven near resonance and
at a low concentration (∼106 MB/mL) can be used
to maximize the adhesion efficiency (up to 88%) and specificity of
RGD–MB targeting
Experimental procedures.
<p>a) Schematic of the experimental set-up used for in-vivo hypoxia modulation measurements using the Zenascope system. b) Schematic of tumor volume assessment via B-mode ultrasound imaging. Two cross-sectional images were acquired and lengths a, b and c were used to calculate tumor volume. c) Radiotherapy pre and post-imaging experimental protocol.</p
A single oxygen microbubble administration alone does not influence tumor control.
<p>No significant difference was found between the no treatment and oxygen microbubble group in the absence of any radiotherapy (n = 4 per group). Box-and-whisker plots represent all data from the No treatment and OMB alone controls.</p
Individual datapoints for radiotherapy tumor control times (in days) stratified by matched initial tumor size for each treatment group, showing RT effect size depends on initial tumor volume.
<p>Individual datapoints for radiotherapy tumor control times (in days) stratified by matched initial tumor size for each treatment group, showing RT effect size depends on initial tumor volume.</p