Everything from food aspiration to an asthma attack to heart failure can cause a patient to die from asphyxia, or lack of oxygen. For more than a decade, the Translational Research Laboratory (TRL) of Boston Children’s Hospital’s Heart Center has been pursuing a dream: tiny, oxygen-filled bubbles that can be safely injected directly into the blood, resuscitating patients who can’t breathe.
The lab’s first generation of bubbles were made with a fatty acid, but the lipid shells weren’t stable enough for long-term storage or clinical use. The bubbles popped open too easily.
“When gas bubbles break and coalesce into larger bubbles in the bloodstream, they can create lethal obstructions,” Brian Polizzotti, PhD told Vector in 2016. Polizzotti, a biomaterials engineer, co-directs the TRL with cardiologist John Kheir, MD.
To stabilize the tiny bubbles and make them safer, the team tried introducing a polymer shell. “Our second generation of bubbles had superior stability profiles compared to the lipid bubbles,” says Polizzotti. “But they behaved like rigid spheres and caused microvascular obstructions when infused at high rates.”
Another key design concern for oxygen bubbles was the possibility of the oxygen not diffusing out gradually, as it should, and instead creating an air embolism. That’s an air bubble that can travel to the brain, heart or lungs, causing stroke, respiratory failure or heart attack.
Perfecting the oxygen bubble
The TRL went back to the drawing board. In its latest publication, the team reports key improvements in the technology, resulting in microbubbles whose properties – like diameter, porosity and thickness of the outer shell — are easier to control.
“We have made it so that these bubbles spontaneously dissolve when they come into contact with blood, but remain stable for months when stored on the shelf,” says Polizzotti, who is also an assistant professor of pediatrics at Harvard Medical School. “Our current formulation has a large oxygen-carrying capacity, is stable at room temperature for more than three months, is mechanically robust and spontaneously dissolves once in contact with blood, thus eliminating the risk of lethal pulmonary emboli.”
To create their product, Polizzotti, Kheir and colleagues used a special production method called interfacial nanoprecipitation.
“People have been struggling to find effective ways to manipulate the properties of microbubbles to have them perform desired tasks, in part because the shells of bubbles are so thin and fragile to handle,” says materials chemist Yifeng Peng, PhD. Peng contributed to the technology’s development and is the current paper’s first author.
The team started with dextran, a polymer made of units of glucose. They then added different chemicals to alter the polymer structure in various ways. Next, they dissolved the polymers in an organic solvent and added water, in which the polymers aren’t soluble. This caused the polymers to form three-dimensional structures known as micelles. Finally, they exposed the liquid mixture to air. The result was a self-assembled foam containing air bubbles, each bearing an outer shell made up of micelles.
When these tiny oxygen bubbles are added to blood, they react chemically to the blood’s pH in a way that enables water to penetrate the shell and release the oxygen. The shells then fall apart and their components dissolve completely, leaving no detectable debris. (The researchers term this “pH-triggered self-elimination.”)
In tests, the researchers blocked the airways of rodents to induce asphyxia leading to cardiac arrest — modeling a condition that affects more than 100,000 hospital patients a year, and which only about 10 percent survive.
Half the animals received rapid, repeated IV injections of oxygen-loaded microbubbles; the control group received a simple dextrose (sugar) solution. After 10 minutes, when their airways were reopened, all animals receiving the microbubble injections had survived, versus none of the controls. The treated animals’ partial pressure of oxygen in arterial blood (PaO2) returned to normal within two minutes, indicating that the oxygen was continually consumed. None of the treated animals showed signs of embolism.
Bubbles for medicine and industry
The investigators note that varying the polymer’s structure with different chemicals produces different kinds of shells — thick or thin, textured or smooth. This could expand their potential medical uses — like drug delivery, imaging, cancer therapy and tissue engineering. The technology could also be used in industry, to produce cosmetics, inks, paints and coatings, lightweight materials and more.
“Our technique gives chemists greater power to achieve much better control over the properties of microbubbles through a very easy way — chemical modification of polymers. That is something chemists are already very good at doing,” says Peng. “This method can help materials scientists to design and engineer new ‘smart’ microbubbles with more interesting properties and functions for various uses.”
The next step toward making oxygen bubbles available for clinical use will be to demonstrate their safety during long term follow-up periods and in large animals. The team then plans a study to show that animals receiving oxygen bubble infusions have improved brain function.
“Once these steps are complete, the team will ready the drug for human trials in as soon as three to four years,” says Kheir, who first conceived of injectable oxygen bubbles more than a decade ago.
Kheir described his vision in this 2013 TED talk:
The fine print
The study’s coauthors were Raymond Seekell, Alexis Cole, Jemima Lamothe, Andrew Lock, Sarah van den Bosch, Xiaoqi Tang and John Kheir, all of the Translational Research Laboratory in Boston Children’s Hospital’s Department of Cardiology.
The work was funded by the Office of the Assistant Secretary of Defense for Health Affairs (W81XWH-15-1-0544 and W81XWH- 11-2-0041), a DRIVE grant from the Boston Biomedical Innovation Center (NIH U54HL119145), the Smith Family President’s Innovation Fund, three Technology Development Grants from Boston Children’s Hospital and the Hess Family Foundation.
More details in this press release from the journal.