**4. Ultra-rapid freezing**

Ultra-rapid freezing can be considered a midway technique between slow freezing and Vitrification. It is quicker than the slow-freezing technique, does not involve the use of programmable machines and requires lower concentrations of cryoprotectant agents (CPA) than those used in vitrification.

Experimental results demonstrate that this technique has lower performances than slow freezing's and vitrification's ones (AbdelHafez et al., 2010).
