We all need oxygen. Individuals who are subjected to low enough levels of oxygen (whatever the cause) can be at major risk for organ dysfunction, cardiac arrest, and death—all within minutes. So when oxygen supply gets too low, medical attention can be needed to increase oxygen levels quickly. But most treatments for hypoxia—low oxygen levels—require an intact and functional respiratory system.

A great deal of research has focused on developing methods to safely administer oxygen directly into the bloodstream, where it can be quickly distributed to tissues and organs in need. In a recent investigation published in PNAS, a team of researchers at Harvard Medical School have developed hollow polymer particles that deliver oxygen to the bloodstream.

Microparticle Fabrication

The scientists developed a two-stage, emulsion-based fabrication process to make these oxygen-carrying polymers. In the first step, a two-phase oil-in-water emulsion was prepared—droplets of an oil-based phase are suspended in a water-based solution. In this case, the oil phase contained a key chemical, a biodegradable polymer called poly(D,L-lactic-co-glycolic) acid, or PLGA.

Next, the emulsion was diluted with distilled water and allowed to "ripen" (their term, not ours). During the ripening process, solvent exchange occurs: part of the oil-based solution diffuses into the aqueous phase and eventually evaporates out. As its concentration decreases, the relative concentrations of the remaining components will obviously increase. This shift in concentration drives a phase separation that produces a core-shell structure. The resultant cores were rich in the oil phase, while the shell was composed primarily of PLGA.

As the ripening process continues, the water-phase also begins to diffuse into the oil-based interior, forming tiny water-in-oil emulsions. The microparticles were then freeze-dried, a process that allows the frozen solvent phase to sublimate into the environment. This process resulted in hollow, gas-filled microparticles composed of PLGA with an interconnected network of pores.

The researchers demonstrated that they can manipulate particle size, shell thickness, pore density, and pore diameter through control of various steps in this process. Depending on the rate of different steps, the microparticle sizes ranged from less than a micron to about 50 µm in diameters.

Oxygen Delivery

The team found that the microparticles could absorb twice their mass in water. Since there was still gas in the interior, multiple gas-water interfaces were formed within the shell’s porous network. With time, this gas was released into the environment if the pore size was large enough (>1.4 µm).

The researchers confirmed that the oxygen release from the microparticles was diffusion-controlled by mixing them with donated human red blood cells. Progress of the reaction was monitored by tracking the formation of the oxygen-hemoglobin complex. Depending on the exact microparticle formulation, the majority of the oxygen (76-85 percent) was released during the first minute after introduction to the red blood cells. The oxygen release continued for an additional 20 minutes at a decreased rate.

Next, the team characterized the gas carrying capacity of the microparticles, finding they could hold from 1.3 to 0.88 milliliters of O 2 per gram of microparticle, depending on the exact particle formulation. This level is much higher than human red blood cells, which can only hold 0.17 ml oxygen per gram. Evaluation of oxygen delivery revealed that the microparticles would give up from 59 to 90 percent of their oxygen payload, depending on their formulation.

The most promising microparticle formulation, which released 90 percent total oxygen payload, was tested in rats to determine whether it affected the resistance to blood flow in the vasculature of the lung. It decreased during injection but then returned quickly to baseline. These findings suggest that the microparticles did not clog up the fine blood vessels of the lungs. In fact, they may open up blood vessels due to the oxygen they deliver.

Microparticles were injected into the femoral vein of male rats and the blood oxygen levels was measured continuously in the pulmonary artery. The findings indicated that the microparticles were efficient one-way oxygen carriers in vivo. The team also measured the cardiac index, which relates the cardiac output from the left ventricle in one minute to the body surface area. They found that the cardiac index increased during injection, which is desirable for clinical settings.

The microparticles could be stored dry for two months while maintaining their size distribution and gas carrying capacity.

This promising investigation details an innovative oxygen-delivery strategy that overcomes many of the shortcomings of previous technologies. However, further tests in living organisms and clinical studies are necessary to determine the overall safety of this platform before it can be freely used in patients.

PNAS, 2016. DOI: 10.1073/pnas.1608438113 (About DOIs).