Suddenly there is a stabbing in the chest. The pain spreads into the left arm, the lower jaw, the stomach. Fear spreads, you are exhausted, sweating and pale as chalk: It is a heart attack. If the coronary arteries become blocked, the heart muscle is no longer supplied with sufficient blood – and therefore also no longer with oxygen.
In order to supply hypoxic, i.e. very low-oxygen blood, with fresh oxygen, a new type of material could be used in the future: “porous water”. Researchers from Harvard and Northwestern Universities report on this in a recent publication in Nature published work.
Porous water is a solution of finely distributed nanoparticles, each of which forms a cavity – practically a loophole – for gas molecules due to their crystal structure. The microporous particles enable aqueous fluids to absorb many times the amount of gas that would otherwise be possible. If such a gas-enriched suspension is added to a less gaseous liquid, such as hypoxic blood, the gas escapes from the loopholes and enriches the other liquid. The possible applications of porous liquids – as the relatively young field of research is commonly known – range from the biomedical field to the capture of carbon dioxide.
Blood can absorb ten times more oxygen than water
The hemoglobin in the red blood cells is actually responsible for carrying oxygen in the blood, through which the blood can absorb around ten times more oxygen than water. So far, it has not been possible to produce liquids with a similarly high oxygen content that are also biologically compatible. There is no lack of attempts to produce synthetic hemoglobin or microbubbles filled with oxygen from fats or polymers. The experiments often failed because of the absorption capacity of the gas carrier, or the method was technically too complex.
The authors of the study, led by Harvard researcher Daniel Erdosy, write that the gas absorption capacity of the porous water exceeds that of all previously known oxygen-carrying liquids. Other porous liquids are made by dissolving particles in them that have such a small cavity that only gas particles but no liquid particles can slip into them. However, this method is not practical for water: H₂O molecules are small enough to also enter cages intended for gas.
Instead, Erdosy’s researchers exploit the fact that it is thermodynamically unfavorable for water to penetrate the pores of certain crystals, since these are hydrophilic on the outside, i.e. water-attracting, but hydrophobic on the inside – and remain dry there. Such porous materials are on the one hand silicalite-1, an inorganic compound from the zeolite group of substances, and on the other hand so-called metal-organic framework compounds, MOFs for short (metal-organic frameworks).
“Ultimately, a zeolite is a porous stone that the researchers produce as a powder and then distribute in the water,” explains chemist Alexander Knebel, who researches porous materials at the University of Jena. “The colloidal solution, the fine distribution of the nanoparticles that float in the water and don’t sink, that’s the highlight.” Due to the dispersion of the microporous crystals, concentrations of up to 40 percent can be achieved – and a correspondingly high gas absorption capacity.
Nevertheless, Knebel still sees a lot of development work for the US scientists: “We are still miles away from medical applications for direct injection into the bloodstream.” The reason for this is that clinical approval procedures generally take a very long time and that silicon, which occurs in the silicalite-1 used, is only broken down to a small extent by the body. However, it is realistic to supply an artificial lung with oxygen using a porous liquid.
Porous liquids could also trap CO₂
Apart from the biomedical field, Knebel considers other areas of application for porous liquids to be promising: “I could imagine applications in gas separation.” Knebel’s working group is therefore working on MOFs, among other things, which are intended to make separation processes in chemistry more efficient. In addition, the porous materials could help capture CO₂ from industrial processes. To date, the exhaust gases have usually been passed through a solution containing chemicals such as alkali hydroxides or organic ammonia compounds that react with the CO₂ and thus wash it out.
A more efficient solution, which Knebel and other researchers have long pinned high hopes for, could be to force the exhaust gases through a porous fluid instead. The MOFs dissolved in it would only sieve out carbon dioxide, which would then remain trapped in the micropores of the MOFs – and could be stored underground.