**4.3 Ultrarapid vitrification**

 Ultrarapid vitrification is the modified and improved version of the conventional vitrification procedure. The concept of ultrarapid cooling for cryopreserving oocytes and embryos was introduced by Vajta and his co-workers with their invention of the open pulled straw [29, 30]. The viscosity of the vitrification medium and cooling rate are inversely related. Thus, a medium containing lesser concentration of cryoprotectants and other additives can be vitrified efficiently at higher cooling rate. Theoretically, vitrification can be achieved with a 1.5 M concentration of any cryoprotectant, providing a cooling rate of 15,000°C/min is employed [28]. Ultrarapid vitrification technique employs extremely high cooling and warming rates as compared to the slow freezing or conventional vitrification methods (**Table 4**). It allows vitrification of relatively low concentration of CPA solution using extremely high cooling rate and thus reduces the CPA-mediated toxicity and osmotic stress to the vitrified cells. The ultrarapid cooling rate is achieved by reducing the effective volume of the solution to be vitrified. At present, this method is considered to be the most superior, and high post freeze-thaw survival of oocytes and embryos has been demonstrated using this method in different mammalian species.
