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 (Rshell) that was then fit to a chain-length-dependent energy barrier model. Importantly, the pre-exponential factor was ∼10× higher and the microbubbles persisted ∼4× longer (from 15 μm 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

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 (Rshell) that was then fit to a chain-length-dependent energy barrier model. Importantly, the pre-exponential factor was ∼10× higher and the microbubbles persisted ∼4× longer (from 15 μm 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

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 (Rshell) that was then fit to a chain-length-dependent energy barrier model. Importantly, the pre-exponential factor was ∼10× higher and the microbubbles persisted ∼4× longer (from 15 μm 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

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 (Rshell) that was then fit to a chain-length-dependent energy barrier model. Importantly, the pre-exponential factor was ∼10× higher and the microbubbles persisted ∼4× longer (from 15 μm 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

Vesicles Tethered to Microbubbles by Hybridized DNA Oligonucleotides: Flow Cytometry Analysis of This New Drug Delivery Vehicle Design

Hybridization of complementary lipid-linked DNA oligonucleotides was used to tether small unilamellar vesicles (SUVs) to the lipid monolayer shells of air-microbubbles, a new attachment design for a drug delivery vehicle to be used in tandem with ultrasound imaging. Flow cytometry was used, and a novel analysis was developed, based upon light scattering and fluorescence intensity, to quantify the fraction of microbubbles of chosen size-ranges with oligonucleotide-tethered fluorescently labeled SUVs. Fluorescence microscopy was used to verify that our methodology results in successful high-density SUV tethering to a similar fraction of the microbubbles when compared to the flow cytometry statistics. The fraction of successful tetherings increased with the concentration of the complementary lipid-linked oligonucleotide as expected and decreased with the time that microbubbles were incubated with SUVs, which was not expected. Also unexpected, a large fraction of microbubbles had only background fluorescence levels while a much smaller fraction (at most one-eighth, for the shortest incubation and highest concentration of lipid-linked oligonucleotide) had oligonucleotide-tethered fluorescently labeled SUVs and, according to our fluorescence microscopy, that small fraction was densely covered with SUVs. Ejection of the lipid-linked oligonucleotide during high surface pressure compression of the monolayer shells of actively shrinking microbubbles subjected to the Laplace overpressure is speculated as a qualitative explanation for the statistics