Synthesis and Characterization of Multi-Modal Phase-Change Porphyrin Droplets
October 15th, 2021
In this protocol, methods for synthesizing and characterizing multi-modal phase-change porphyrin droplets are outlined.
This protocol is significant because it provides the complete procedure for creating droplets from the lipid film to lipid solution to microbubbles and then to droplets. The main advantage of this technique is the simplicity for condensing microbubbles into droplets. Just by cooling and swirling, droplets can be made without the need for pressurization.
A visual demonstration is critical because a simple test to see if the microbubbles have been successfully condensed is by visually checking the change in translucency. Demonstrating the procedure will be Kimoon Yoo, a master's student in the Gang Zheng laboratory. Using a decapper, remove the aluminum seal on the serum vial and transfer one milliliter of the lipid solution to a 1.85 milliliter borosilicate glass sample vial with a phenol screw cap by letting the lipid solution flow down the interior wall without creating bubbles.
With the 1.85 milliliter sample vial prepare to flow in decafluorobutane gas into the sample vial headspace using the gas exchanger. Ensure that all the valves on the gas exchanger are properly closed and the pump is turned off. Open the manifold valve and carefully unsheathe the corresponding needle from the manifold.
Open pressure valve A and pressure valve B and turn the gas cylinder valve approximately 1/16 to 1/8 counter-clockwise to partially open it. Open the T handle valve and uncapped the sample vial with the lipid solution. Place the manifold needle above the vial's liquid air interface, slowly turn the air regulator valve clockwise until the air regulator gauge needle moves slightly from its resting position and let the perfluorocarbon gas gently flow into the vial headspace for 30 seconds, taking care to not create bubbles.
Adjust the air regulator valve if necessary. After 30 seconds, carefully and quickly kept the sample vial without moving the vial too much. Both the gas cylinder valve, the T handle valve, air regulator valve, pressure valve A, pressure valve B, and manifold valve.
Carefully sheath the needle, then labeled the sample vial and seal the neck with wax film going clockwise. Store the sample vial in the dark and at four degrees Celsius for at least 10 minutes or up to 24 hours. Place about 100 grams of dry ice in an insulated container in place regular ice in another insulated container.
Retrieve a previously prepared decafluorobutane serum vial to 1.5 inch 20 gauge needles, a one milliliter plastic syringe, a 200 milliliter container, metal tongs, and a thermometer. Place the sample vial with the lipid solution in the mechanical agitator and agitate for 45 seconds. There should be a change in color and translucency.
After the mechanical agitation, stand the sample vial right side up shielded from light and start a 15 minute countdown to cool down the vial and size select the microbubbles. When the countdown has reached 10 minutes, fill a container with about 200 milliliters of isopropanol and cool it to minus 20 degrees Celsius with dry ice using metal tongs. After the microbubbles have been sized selected for 15 minutes, look for the size selected partition inside the sample vial.
Keeping the sample vial right side up, carefully uncapped the sample vial and withdraw about 0.7 milliliters of the bottom partition with a 1.5 inch 20 gauge needle attached to a one milliliter plastic syringe. Ensure none of the top partition is withdrawn and do not flick the syringe to remove air pockets. Aiming at the center circle on the rubber stopper, insert a different 20 gauge needle into a decafluorobutane serum vial while keeping the needle near the top of the serum vial to vent and then insert the needle with the size selected microbubbles.
Slowly transfer the size selected microbubbles. Let the liquid gently slide down the interior wall of the decafluorobutane serum vial. Once all the size selected microbubble solution has been transferred, remove the needle with the syringe, but keep the venting needle in to relieve negative pressure.
Add small amounts of dry ice or room temperature isopropanol to the isopropanol bath to ensure the bath temperature is between minus 15 to minus 17 degrees Celsius. With the 20 gauge venting needle inserted near the top of the serum vial, place the serum vial and the isopropanol bath, keeping the microbubble level below the level of the isopropanol, but the vial neck above it and intermittently swirled the serum vial for two minutes to condense the microbubbles. Do not swirl the serum vial continuously in the isopropanol and do not let the solution freeze.
Swirl for about five seconds and lift the serum vial out of the isopropanol. Check for ice nucleation, then resume swirling in isopropanol. If there is ice formation, swirl the serum vial in the air until it dissipates.
After the two minute condensation remove the serum vial from the isopropanol bath and remove the venting needle. The microbubbles should have condensed into droplets as indicated by the change in translucency. Wipe the serum vial, label it, and place it on regular ice in a dark insulated container until ready for use.
Unopened droplets with an intact aluminum seal should be stable for up to six hours as long as the melted ice gets replaced as needed. When ready to use, remove the aluminum seal with the decapper. After using this protocol, pre-condensed, size selected microbubbles and post-condensed droplets were sized on a Coulter counter with a 10 micron aperture.
The sizing data for the 30%pyro-lipid formulation is shown. The statistics based on the sizing data are shown here. Using a ratio of pre-and post-condensed mean diameters, the results showed that as the pyro-lipid content increased, the concentration decreased, and the mean diameter increased.
The representative absorbents measurements of the 30%pyro-lipid droplets sample is shown. This showed that the intact assemblies have different optical properties as reflected by different peak compared to the individual unassembled lipid components. Representative florescence measurements of the pre-condensed microbubble, post-condensed droplet sample with 30%pyro-lipid are shown here, demonstrating different fluorescence peaks for intact samples in the disrupted form.
Representative ultrasound images of the 30%pyro-lipid droplets sample imaged at different pressures are shown. At low pressures, only background signal from air bubbles stuck from the agar synthesis was observed. At a slightly higher power, a few microbubbles were generated, which is demonstrated by the appearance of bright speckles.
More microbubbles were generated as the pressure increased. It is important to remember that the condensation temperature here is optimized for this specific droplet shell formulation. Different shell formulations may require a different temperature.
Overview
This protocol outlines methods for synthesizing and characterizing multi-modal phase-change porphyrin droplets. The technique emphasizes simplicity in condensing microbubbles into droplets without the need for pressurization.
Key Study Components
Area of Science
- Neuroscience
- Biophysics
- Material Science
Background
- Phase-change porphyrin droplets have potential applications in various fields.
- Understanding the synthesis process can enhance their utility in research.
- Microbubble condensation is a critical step in droplet formation.
- Visual checks for translucency are essential for confirming successful condensation.
Purpose of Study
- To provide a comprehensive protocol for creating phase-change porphyrin droplets.
- To demonstrate the ease of the condensation process.
- To ensure reproducibility in droplet synthesis for future research.
Methods Used
- Preparation of lipid solution and microbubbles.
- Use of decafluorobutane gas for droplet formation.
- Mechanical agitation to size-select microbubbles.
- Cooling techniques to condense microbubbles into droplets.
Main Results
- Successful condensation of microbubbles into droplets was achieved.
- Changes in translucency indicated successful droplet formation.
- Sizing data showed a correlation between pyro-lipid content and droplet characteristics.
- Optical properties of droplets varied based on assembly integrity.
Conclusions
- The protocol is effective for synthesizing phase-change porphyrin droplets.
- Visual assessments are crucial for confirming droplet formation.
- Future applications can benefit from the established methods.
Frequently Asked Questions
Executive Industry Relevance
Multi-modal phase-change porphyrin droplets address a key limitation in ultrasound contrast agent development by enabling direct quantification of bio-distribution without companion diagnostics. This capability supports preclinical evaluation of delivery efficiency and therapeutic targeting, reducing reliance on indirect measurement approaches. The method enhances predictive confidence in early-stage vascular targeting and drug delivery programs by providing orthogonal readouts for agent behavior in complex biological environments.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of vascular targeting hypotheses through direct acoustic and optical tracking of agent accumulation.
- Operational Value: Reduces need for fluorescent or absorbent companion particles, simplifying experimental workflows.
- Predictive Value: Provides quantitative readouts for assessing target engagement and biodistribution patterns in disease-relevant models.
Screening & Assay Development
- Scientific Value: Generates size-controlled, acoustically active droplets with tunable optical properties for standardized assay inputs.
- Operational Value: Supports reproducible preparation of contrast agents via condensation-based sizing selection.
- Assay Readiness: Enables consistent microbubble-to-droplet conversion for dose-response and pressure-response screening.
Translational & Preclinical Research
- Scientific Value: Facilitates correlation of acoustic vaporization thresholds with drug release or vascular effects in vivo.
- Operational Value: Allows ex vivo validation of agent retention and clearance using intrinsic porphyrin fluorescence.
- Translational Continuity: Bridges discovery-phase screening with preclinical validation through multimodal readouts.
Pipeline & Workflow Integration
The method integrates into discovery workflows by enabling size-selected microbubble production followed by condensation into trackable droplets, supporting progression from hypothesis testing to lead optimization.
- Discovery Biology: Supports vascular target validation through quantifiable agent accumulation in tissue models.
- Screening: Enables pressure-dependent microbubble generation assays for evaluating acoustic sensitivity.
- Analytics: Provides sizing, absorbance, and fluorescence readouts to assess formulation stability and disruption kinetics.
- Translational Research: Connects in vitro characterization to ex vivo biodistribution assessment via intrinsic porphyrin signaling.
- Enterprise Reuse: Establishes a modular platform for porphyrin-lipid droplet synthesis applicable across multiple therapeutic areas.
Operational & Enterprise Impact
- Scientific Value: Enhances target validation confidence through direct, multimodal tracking of contrast agent behavior.
- Operational Value: Improves reproducibility via standardized condensation protocol and visual translucency check.
- Strategic Value: Reduces late-stage attrition risk by enabling early assessment of delivery efficiency.
- Portfolio Impact: Supports go/no-go decisions based on quantitative biodistribution and acoustic response profiles.
Implementation Considerations
- Requires expertise in lipid handling, gas exchange, and low-temperature manipulation.
- Depends on mechanical agitator, Coulter counter, and temperature-controlled isopropanol bath for sizing and condensation.
- Necessitates standardization of condensation timing and swirling technique across operators.
- Requires adaptation of cooling rates for different porphyrin-lipid formulations to avoid ice nucleation.
- Limited by six-hour stability window post-condensation, necessitating same-day use for time-sensitive experiments.
