We all need oxygen. Individuals exposed to inadequate oxygen (regardless of the cause) can be at high risk for organ dysfunction, cardiac arrest and death – all within minutes. So when the oxygen supply becomes too low, medical attention may be needed to raise the oxygen level quickly. But most treatments for hypoxia — low oxygen levels — require an intact and functional respiratory system.
Much research has focused on developing methods to safely deliver oxygen directly into the bloodstream, where it can be quickly distributed to tissues and organs in need. In a recent study published in PNASa team of researchers at Harvard Medical School has developed hollow polymer particles that deliver oxygen to the bloodstream.
Manufacturing of microparticles
The scientists developed a two-stage, emulsion-based manufacturing 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 an important chemical, a biodegradable polymer called poly(D,L-lactic acid-co-glycolic acid) or PLGA.
Then the emulsion was diluted with distilled water and allowed to “ripen” (their term, not ours). Solvent exchange takes place during the maturation process: part of the oil-based solution diffuses into the aqueous phase and eventually evaporates. As the concentration decreases, the relative concentrations of the remaining components will naturally increase. This shift in concentration drives a phase separation that produces a core-shell structure. The resulting cores were rich in the oil phase, while the shell consisted mainly of PLGA.
As the maturation process progresses, the water phase also begins to diffuse into the oil-based interior, forming small water-in-oil emulsions. The microparticles were then freeze-dried, a process that allows the frozen solvent phase to sublime into the environment. This process resulted in hollow, gas-filled microparticles composed of PLGA with an interconnected network of pores.
The researchers showed that they can manipulate the particle size, shell thickness, pore density and pore diameter by controlling various steps in this process. Depending on the speed of the different steps, the microparticle sizes ranged from less than a micron to about 50 µm in diameter.
The team found that the microparticles could absorb twice their mass in water. Because there was still gas in the interior, multiple gas-water interfaces formed within the shell’s porous network. Over time, this gas was released into the environment if the pore size was large enough (>1.4 µm).
The researchers confirmed that the oxygen delivery of the microparticles was diffusion-controlled by mixing them with donated human red blood cells. The progress of the reaction was monitored by monitoring the formation of the oxygen-hemoglobin complex. Depending on the exact formulation of the microparticles, most of the oxygen (76-85 percent) was released during the first minute after introduction into the red blood cells. Oxygen delivery continued for an additional 20 minutes at a reduced rate.
Next, the team characterized the gas-carrying ability of the microparticles and found that they could contain 1.3 to 0.88 milliliters of O2.2 per gram of microparticle, depending on the exact particle formulation. This level is much higher than human red blood cells, which can hold only 0.17 ml of oxygen per gram. Evaluation of the oxygen delivery revealed that the microparticles would give up 59 to 90 percent of their oxygen load, depending on their formulation.
The most promising microparticle formulation, which delivers 90 percent of the total oxygen load, was tested in rats to determine whether it affected resistance to blood flow in the vasculature of the lungs. It decreased during the injection, but then quickly returned to baseline. These findings suggest that the microparticles did not clog the fine blood vessels of the lungs. They can even open blood vessels because of the oxygen they release.
Microparticles were injected into the femoral vein of male rats and the blood oxygen content was measured continuously in the pulmonary artery. The findings indicated that the microparticles were efficient unidirectional oxygen carriers in vivo. The team also measured the cardiac index, which relates the cardiac output of the left ventricle in one minute to 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 retaining their size distribution and gas transport capacity.
This promising research describes an innovative oxygen delivery strategy that overcomes many of the shortcomings of previous technologies. However, further testing in living organisms and clinical studies are needed to determine the overall safety of this platform before it can be freely used in patients.
PNAS2016. DOI: 10.1073/pnas.1608438113 (About DOIs).