А. Pulver, A. Tselikovsky, N. Pulver, I. Artyuhov, V. I. Artyukhov, A. Peregudov.
Before describing the essence of our propositions, it wouldn't be inappropriate to recall some matters of common cryobiological knowledge.
Any means of cryopreservation generally relies upon vitrification, in which cells survive in glass between ice crystals. Even under slow programmable freezing cells are surrounded by unfrozen liquid between growing ice crystals [Mazur 1984].
Vitrification is a process whereby fluid becomes a solid during cooling (more precisely, exhibits most of the properties of solids) without any substantial change in molecular arrangement or thermodynamic state variables. Hence, no crystallization with the consequent heat release and volume increase. Proposals of a simultaneous vitrification of entire samples without the formation of ice [Luyet 1937] even predate the discovery of the first cryoprotectants, [Polge, Smith et al. 1949] in 1940s. However, the first practical implementation of vitrification (murine preimplantation embryos) was carried out only in 1985, by Gregory Fahy and William Rall [Rall, Fahy 1985].
The main condition for the onset of vitrification during cooling is an increase of liquid viscosity up to about ten to the thirteenth poise. On the molecular level it is expressed in loss of rotational and translational degrees of freedom, leaving only bond vibration within a fixed molecular structure, which leads to a decrease in the specific heat and thermal expansion coefficient [Wowk 2010].
Cryoprotectants lower the critical cooling rate (which is not less than ten to the seventh °С/min [Armitage 1991] for pure water) required for vitrification inversely proportional to their concentration within the cooling solution, reducing it up to quite available speeds.
Cryopreservation of individual cell suspensions requires nothing more than a slow programmable freezing (SPF), and the vitrification protocols for whimsical preimplantation embryos [Isachenko, Alabart et al. 2003] are even more easy to apply. However, when dealing with macroscopic objects, researchers are faced with a number of additional obstacles, namely:
Ice formation during warming happens faster than during cooling because ice nucleation occurs at lower temperatures than ice growth. This nucleation leads to extensive ice growth at warmer temperatures. That’s why the ‘‘critical warming rates” (minimum warming rates to avoid “devitrification”, or significant ice formation during warming from a vitrified state) are typically two or more orders of magnitude greater than critical cooling rates [Hopkins, Badeau et al. 2012].
The same stringent and even contradictory conditions are imposed on storage temperature and transport of macroscopic samples, because masses of vitrifiable tissues larger than a few cubic centimeters almost invariably develop large-scale fractures. Storing them at liquid nitrogen temperature leads to cracking due to shear stress relaxation, while keeping them close to the glass transition temperature (down to 15 degrees) results in the formation of nanoscale ice crystals due to lateral diffusion of water molecules, which increases the critical warming rate upon subsequent heating.
A detailed review of the entire spectrum of research requires a separate report, and goes beyond the scope of this work. The more so because no major success has been achieved yet – excluding successful vitrification of blood vessels (1996), peripheral nerves, pancreatic islets and so on. Although speaking of nerves and blood vessels, I personally believe it is rather a tissue engineering than cryobiology.
The most promising advances in this field were made by “21st Century Medicine”, a company led by Gregory Fahy and Brian Wowk. In 2005 they have reported on a rabbit kidney that survived vitrification and subsequent transplantation with immediate contralateral nephrectomy, successfully functioning for 9 days. However, due to the above-mentioned unresolved problems, this experiment remains anecdotal so far.
We know three possible ways of macroscopic biological objects accelerated warming: the dielectric warming [Wusteman, Robinson et al. 2004], vascular perfusion with inert fluids that remain liquid at cryogenic temperatures [Federowicz, Harris et al. 1998], and gas perfusion [Schimmel, Wajcner et al. 1964; Bickis & Henderson 1966; Hamilton, Holst et al. 1973; Van Sickle & Jones 2014].
Dielectric warming results in very uneven heating.
Cooling solutions to cryogenic temperatures directly by cryoprotectants is impossible, due to progredient solute viscosity and peripheral vascular resistance elevation. As a result, the perfusion rate decreases. From a certain moment, instead of passing through the microvasculature, refrigerants begin to "shunt" through the major vascular arcades. Respectively the heat exchange is complementarily weakened and cooling becomes irregular. With a further increase viscosity perfusion either stops or ruptures in the parenchyma occur. Liquid perfluorocarbon-based coolants are also unable to solve the problem of sufficient cooling rate for the same reasons, albeit at lower temperatures.
Gaseous coolants, despite their negligible heat capacity, have two to three orders of magnitude lower viscosity, and freely pass through the vascular bed even at cryogenic temperatures. The most popular in this area is helium, due to its low condensing temperature and the highest diffusion mobility.
Of course, the gas perfusion has its own shortcomings:
In view of the foregoing, for vitrification of isolated organs (and eventually up to intact organisms), suitable for transplantation after storage and rewarming, we plan the following:
In order to evaluate the cryoprotective potential of xenon we have conducted two series of experiments – with baker’s yeast (S. cerevisiae) and mammalian cell cultures (CHO-K1 and NIH-3T3) in custom-built bronze miniature hyperbaric chambers.
Yeast survival at all xenon pressures tested (3 to 7 at) after 2 hours of pressure chamber exposure at -20°C with subsequent chamber immersion in liquid nitrogen (68,5 plus or minus 6%) turned out to be better than in control (35 plus or minus 6%) and comparable or even higher to that of 5% DMSO or glycerol application (50 plus or minus 25%). Joint application of xenon + DMSO or glycerol showed 71 plus or minus 15% of cell survival.
However, experiments with mammalian cells at xenon partial pressures of 2.5 to 12 at upon slow programmable freezing (SPF) showed complete cells dissolution upon conventional rewarming. Even after designing a sophisticated decompression protocol we could not achieve survival upon thawing. This made us disappointedly conclude that xenon has no cryoprotective properties applying SPF. Nevertheless, 2,8 plus or minus 2,3% of control group cells frozen by the abovementioned “yeast” protocol of cells survived and, moreover, fastest achievable cooling speed (direct placing of pressurized hyperbaric chamber into liquid nitrogen, which we considered an absolute death sentence) in the other control experiment improved cells survival up to 22,5 plus or minus 13,4%.
Large scale molecular dynamics simulations of xenon clathrate hydrate and ice crystal growth (details are recently described in our report "Possible Mechanisms of Cryoprotective Effect of Xenon") indicate vitrification of water inside cells, especially under combined application of xenon and DMSO, as a possible explanation of these results.
The main thing for us is that the hydrophobic solubility of xenon in low temperature water increases with cooling faster than the thermal stability of clathrate hydrate [Artyukhov, Pulver et al. 2014].
As a result, it should turn out that traditional cryoprotectants, by suppressing xenon crystalline hydrate formation on cooling stage, would allow the solution to be saturated by xenon to a significant increase in liquid viscosity. This will enable vitrification using a much lesser amount of chemical cryoprotectants, and thus yield large decrease in toxicity. We have submitted a patent application.
Sounds so simple, even primitive. Why didn't anybody think of this before?..