Overcoming the warming bottleneck in animal vitrification: Volumetric heating and enabling technologies for...

Reproductive cryopreservation via vitrification is vital for livestock breeding and biodiversity conservation, as it enables ice-free storage of gametes and embryos. However, success increasingly depends on achieving rapid, uniform warming to avoid devitrification: the critical warming rate (CWR) required is often orders of magnitude higher than the critical cooling rate (CCR). Conventional convective thawing (e.g., water baths) produces edge-to-core thermal gradients that can lead to lethal ice formation in larger or more complex samples. Suboptimal warming disrupts cellular ultrastructure, leading to meiotic spindle collapse, mitochondrial depolarization, reactive oxygen species production, DNA damage, and apoptosis. These changes manifest as impaired embryo development and the formation of necrotic tissue cores. Notably, lipid-rich porcine oocytes and embryos are particularly susceptible to recrystallization during slow warming, with higher fragmentation and lower viability than their bovine and ovine counterparts. This review synthesizes thermophysical principles underlying the CWR requirement and biological evidence of the warming bottleneck across animal systems. This thermophysical imbalance means that rewarming, rather than cooling, is the decisive barrier to successful vitrification. We then discuss emerging volumetric rewarming technologies that uniformly deliver energy: magnetic nanoparticle-induced nanowarming, laser-driven photothermal heating, dielectric (radiofrequency/microwave) rewarming, and ultrafast Joule (ohmic) heating. These methods have demonstrably exceeded CWR thresholds in embryos, tissues, and organs, improving cell survival and function. We also highlight enabling tools such as microfluidic cryoprotectant (CPA) handling, automated vitrification platforms, artificial intelligence (AI)-guided protocol optimization, and isochoric (constant-volume) vitrification, which collectively enhance reproducibility and scalability of cryopreservation workflows. In conclusion, integrating volumetric heating modalities with these engineering innovations promises to transform animal cryopreservation: uniformly rapid warming will improve immediate post-thaw viability and preserve biological integrity, enabling routine, large-scale germplasm banking for livestock production and conservation.