xtracellular ice crystal growth and recrystallization, biomaterials such as alginates, polyvinyl alcohol or chitosan can be used to impede ice crystal growth along with traditional small molecule cryoprotectants. That rate differs between cells of differing size and water permeability: a typical cooling rate of about 1 °C/minute is appropriate for many mammalian cells after treatment with cryoprotectants such as glycerol or dimethyl sulfoxide, but the rate is not a universal optimum. The 1 °C / minute rate can be achieved by using devices such as a rate-controlled freezer or a benchtop portable freezing container.
Several independent studies have provided evidence that frozen embryos stored using slow-freezing techniques may in some ways be 'better' than fresh in IVF. The studies indicate that using frozen embryos and eggs rather than fresh embryos and eggs reduced the risk of stillbirth and premature delivery though the exact reasons are still being explored.
Researchers Greg Fahy and William F. Rall helped to introduce vitrification to reproductive cryopreservation in the mid-1980s. As of 2000, researchers claim vitrification provides the benefits of cryopreservation without damage due to ice crystal formation. The situation became more complex with the development of tissue engineering as both cells and biomaterials need to remain ice-free to preserve high cell viability and functions, integrity of constructs and structure of biomaterials. Vitrification of tissue engineered constructs was first reported by Lilia Kuleshova, who also was the first scientist to achieve vitrification of oocytes, which resulted in live birth in 1999. For clinical cryopreservation, vitrification usually requires the addition of cryoprotectants prior to cooling. The cryoprotectants act like antifreeze: they decrease the freezing temperature. They also increase the viscosity. Instead of crystallizing, the syrupy solution becomes an amorphous ice—it vitrifies. Rather than a phase change from liquid to solid by crystallization, the amorphous state is like a "solid liquid", and the transformation is over a small temperature range described as the "glass transition" temperature.
Vitrification of water is promoted by rapid cooling, and can be achieved without cryoprotectants by an extremely rapid decrease of temperature (megakelvins per second). The rate that is required to attain glassy state in pure water was considered to be impossible until 2005.
Two conditions usually required to allow vitrification are an increase of the viscosity and a decrease of the freezing temperature. Many solutes do both, but larger molecules generally have a larger effect, particularly on viscosity. Rapid cooling also promotes vitrification.
For established methods of cryopreservation, the solute must penetrate the cell membrane in order to achieve increased viscosity and decrease freezing temperature inside the cell. Sugars do not readily permeate through the membrane. Those solutes that do, such as dimethyl sulfoxide, a common cryoprotectant, are often toxic in intense concentration. One of the difficult compromises of vitrifying cryopreservation concerns limiting the damage produced by the cryoprotectant itself due to cryoprotectant toxicity. Mixtures of cryoprotectants and the use of ice blockers have enabled the Twenty-First Century Medicine company to vitrify a rabbit kidney to −135 °C with their proprietary vitrification mixture. Upon rewarming, the kidney was transplanted successfully into a rabbit, with complete functionality and viability, able to sustain the rabbit indefinitely as the sole functioning kidney.
Blood can be replaced with inert noble gases and/or metabolically vital gases like oxygen, so that organs can cool more quickly and less antifreeze is needed. Since regions of tissue are separated by gas, small expansions do not accumulate, thereby protecting against shattering. A small company, Arigos Biomedical, "has already recovered pig hearts from the 120 degrees below zero", although the definition of "recovered" is not clear. Pressures of 60 at