Slow Freezing and Thawing Dynamics of Human Ejaculate on Extremely Water-Repellent Carbon Soot Coatings–Implications to Sperm Cryopreservation

Abstract

Paradoxically, but the humans cannot survive at ultralow temperatures, while individual cells such as the spermatozoa can be stored at cryogenic conditions. This facilitates the in-vitro fertilization in cases of male infertility, but the success of assisted reproduction is not guaranteed due to cryodamage of part of the gametes. Recent innovations in carbon nanotechnologies provide perspectives to resolve the existing problems in reproductive medicine, since the flame deposition of rapeseed oil soot on the surfaces of freezing tools favors the cryopreservation of human semen. The water-repellent soot supports heat exchange rates allowing timely osmotic removal of the intracellular water and retained chemical equilibrium in the cells. It is unknown, however, whether the non-wettability of soot is responsible for the enhanced cryopreservation or the dynamics of sperm freezing and thawing influences the outcome. To understand this, 50 µL semen without and with 50 vol % cryoprotectant SpermFreeze^TM are frozen within twenty minutes on two types of soot coatings by simultaneously cooling all components of the cryogenic chamber, leading to ice-liquid content in the droplets that eliminates the singular tip, followed by uniform melting via thermocapillary convection. The pre-cooling of soot-coated substrates and the absence of cryoprotectant generates an abrupt upward-moving freezing front and increases the total ice mass in the semen, creating a cone tip—processes, presumably worsening the cells’ viability. These novel results reveal that the fraction of ice crystals and their spatial distribution could be adjusted by selecting appropriate carbon nanostructures and cooling regimes, targeting future harmless sperm freezing.