Janus Vitrification of Droplet via Cold Leidenfrost Phenomenon

A randomized controlled trial comparing embryo vitrification with slush nitrogen to liquid nitrogen in women undergoing frozen embryo transfer: embryology and clinical outcomes

STUDY QUESTION

Does the use of slush nitrogen (SN) for embryo vitrification improve embryo transfer outcomes compared to liquid nitrogen (LN)?

SUMMARY ANSWER

SN is a safe method for embryo preservation and significantly improves post-warming survival rates during repeated vitrification-warming cycles; however, after a single freeze-thaw cycle, pregnancy outcomes are not improved when embryos are vitrified with SN compared to LN.

WHAT IS KNOWN ALREADY

SN is a combination of solid and LN, with a temperature lower than regular LN, and it is an alternative to conventional LN in achieving a faster cooling speed. Studies have shown that SN improves survival in non-human embryos and human oocytes. However, it is unknown whether the use of SN reduces blastocyst damage in humans during vitrification-as indicated by increased survival across multiple vitrification-warming cycles-or whether it enhances pregnancy outcomes in a single vitrification-warming cycle.

STUDY DESIGN, SIZE, DURATION

Following the pre-clinical trial assessing embryo survival after repeated freeze-thaw cycles using SN and LN on 50 donated embryos per group, a randomized controlled trial was performed, where 253 patients were enrolled between September 2020 and January 2022, and 245 underwent an IVF stimulation, which resulted in at least one blastocyst for cryopreservation. Of those, 121 were allocated to the SN (study), and 124 were allocated to the LN (control) group. Randomization occurred on the day of blastocyst biopsy using a computer-generated block schema. Groups were assigned via opaque envelopes, opened by the embryologist on vitrification day. The patient, physician, and clinical team were blinded to the intervention.

PARTICIPANTS/MATERIALS, SETTING, METHODS

All couples with female aged between 18 and 42 years old undergoing IVF stimulation at one university-affiliated infertility center, with plan for preimplantation genetic testing for aneuploidy and subsequent single, frozen embryo transfer (FET) were eligible for inclusion in this study.

MAIN RESULTS AND THE ROLE OF CHANCE

The pre-clinical trial demonstrated significant improvements in blastocyst survival, with the SN group achieving a mean of 7.5 survived vitrification-warming cycles (range: 3-22), significantly surpassing the mean of 3.0 cycles (range: 0-10) in the LN group (P < 0.0001). Following the pre-clinical trial, 223 patients randomized to SN or LN underwent single FET. Baseline characteristics were similar between groups, as were embryology outcomes, including the number of oocytes retrieved, mature oocytes, fertilization rate, and total blastocysts biopsied. No significant differences were observed between the two groups in pregnancy rate, clinical pregnancy rate, sustained implantation rate, or miscarriage rate (P = 0.16, 0.80, 0.49, and 0.74, respectively, using Student's t-test). A futility analysis indicated no value in continuing recruitment and therefore the study was closed.

LIMITATIONS, REASONS FOR CAUTION

Neonatal or birth outcomes were not assessed. Termination of the study based on futility analysis precludes a conclusion of equivalence between SN and LN.

WIDER IMPLICATIONS OF THE FINDINGS

This study demonstrates that SN is a safe alternative to traditional LN for vitrification; however, it did not demonstrate improvements in the reproductive potential of vitrified embryos.

STUDY FUNDING/COMPETING INTEREST(S)

The project was funded by the Foundation for Embryonic Competence.

TRIAL REGISTRATION NUMBER

NCT04496284.

TRIAL REGISTRATION DATE

3 August 2020.

DATE OF FIRST PATIENT’S ENROLLMENT

5 September 2020.

Unraveling the role of vaporization momentum in self-jumping dynamics of freezing supercooled droplets at reduced pressures

Supercooling of water complicates phase change dynamics, the understanding of which remains limited yet vital to energy-related and aerospace processes. Here, we investigate the freezing and jumping dynamics of supercooled water droplets on superhydrophobic surfaces, induced by a remarkable vaporization momentum, in a low-pressure environment. The vaporization momentum arises from the vaporization at droplet's free surface, progressed and intensified by recalescence, subsequently inducing droplet compression and finally self-jumping. By incorporating liquid-gas-solid phase changes involving vaporization, freezing recalescence, and liquid-solid interactions, we resolve the vaporization momentum and droplet dynamics, revealing a size-scaled jumping velocity and a nucleation-governed jumping direction. A droplet-size-defined regime map is established, distinguishing the vaporization-momentum-dominated self-jumping from evaporative drying and overpressure-initiated levitation, all induced by depressurization and vaporization. Our findings illuminate the role of supercooling and low-pressure mediated phase change in shaping fluid transport dynamics, with implications for passive anti-icing, advanced cooling, and climate physics.

Ultrafast Axial Freezing in a Liquid-Filled Capillary Tube

Liquid-filled capillary tubes are a kind of standard component in life science (e.g., blood vessels, interstitial pores, and plant vessels) and engineering (e.g., MEMS microchannel resonators, heat pipe wicks, and water-saturated soils). Under sufficiently low temperatures, the liquid in a capillary tube undergoes phase transition, forming an ice nucleus randomly on its inner wall. However, how an ice layer forms from the nucleus and then expands, either axially or radially to the tube inner wall, remains obscure. We demonstrated, both experimentally and theoretically, that axial freezing along the inner wall of a water-filled capillary tube occurs way ahead of radial freezing, at a nearly constant velocity 3 orders in magnitude faster than the latter. Rapid release of latent heat during axial freezing was identified as the determining factor for the short duration of recalescence, resulting in an exponential rise of the supercooling temperature from ice nucleation via axial freezing to radial freezing. The profile of the ice-water interface is strongly dependent upon the length-to-radius ratio of the capillary tube and the supercooling degree at ice nucleation. The results obtained in this study bridge the knowledge gap between the classical nucleation theory and the Stefan solution of phase transition.

Leidenfrost droplet jet engine by bubble ejection

HYPOTHESIS

Despite the flourishing studies of Leidenfrost droplet motion in its boiling regime, the droplet motion across different boiling regimes has rarely been focused on, where bubbles are generated at the solid-liquid interface. These bubbles are probable to dramatically alter the dynamics of Leidenfrost droplets, creating some intriguing phenomena of droplet motion.

EXPERIMENTS

Hydrophilic, hydrophobic, and superhydrophobic substrates with a temperature gradient are designed, and Leidenfrost droplets with diverse fluid types, volumes, and velocities travel from the hot end to the cold end of the substrate. The behaviors of droplet motion across different boiling regimes are recorded and depicted in a phase diagram.

FINDINGS

A special phenomenon of Leidenfrost droplets that resembles a jet engine is witnessed on a hydrophilic substrate with a temperature gradient: the droplet traveling across boiling regimes repulsing itself backward. The mechanism of repulsive motion is the reverse thrust from fierce bubble ejection when droplets meet nucleate boiling regime, which cannot take place on hydrophobic and superhydrophobic substrates. We further demonstrate that conflicting droplet motions can occur in similar conditions, and a model is developed to predict the occurring criteria of this phenomenon for droplets in diverse working conditions, which agrees well with the experimental data.

Droplet Generation, Vitrification, and Warming for Cell Cryopreservation: A Review

Cryopreservation is currently a key step in translational medicine that could provide new ideas for clinical applications in reproductive medicine, regenerative medicine, and cell therapy. With the advantages of a low concentration of cryoprotectant, fast cooling rate, and easy operation, droplet-based printing for vitrification has received wide attention in the field of cryopreservation. This review summarizes the droplet generation, vitrification, and warming method. Droplet generation techniques such as inkjet printing, microvalve printing, and acoustic printing have been applied in the field of cryopreservation. Droplet vitrification includes direct contact with liquid nitrogen vitrification and droplet solid surface vitrification. The limitations of droplet vitrification (liquid nitrogen contamination, droplet evaporation, gas film inhibition of heat transfer, frosting) and solutions are discussed. Furthermore, a comparison of the external physical field warming method with the conventional water bath method revealed that better applications can be achieved in automated rapid warming of microdroplets. The combination of droplet vitrification technology and external physical field warming technology is expected to enable high-throughput and automated cryopreservation, which has a promising future in biomedicine and regenerative medicine.