m can help increase heat exchange rates. Gaseous oxygen perfusion/persufflation can enhance organ preservation relative to static cold storage or hypothermic machine perfusion, since the lower viscosity of gases, may help reach more regions of preserved organs and deliver more oxygen per gram tissue.
Generally, cryopreservation is easier for thin samples and small clumps of individual cells, because these can be cooled more quickly and so require lesser doses of toxic cryoprotectants. Therefore, cryopreservation of human livers and hearts for storage and transplant is still impractical.
Nevertheless, suitable combinations of cryoprotectants and regimes of cooling and rinsing during warming often allow the successful cryopreservation of biological materials, particularly cell suspensions or thin tissue samples. Examples include:
- Semen in semen cryopreservation
- Tissue samples like tumors and histological cross sections
- Eggs (oocytes) in oocyte cryopreservation
- Embryos at cleavage stage (that are 2, 4 or 8 cells) or at blastocyst stage, in embryo cryopreservation
- Ovarian tissue in ovarian tissue cryopreservation
- Plant seeds or shoots may be cryopreserved for conservation purposes.
Additionally, efforts are underway to preserve humans cryogenically, known as cryonics. For such efforts either the brain within the head or the entire body may experience the above process. Cryonics is in a different category from the aforementioned examples, however: while countless cryopreserved cells, vaccines, tissue and other biological samples have been thawed and used successfully, this has not yet been the case at all for cryopreserved brains or bodies. At issue are the criteria for defining "success".
Proponents of cryonics claim that cryopreservation using present technology, particularly vitrification of the brain, may be sufficient to preserve people in an "information theoretic" sense so that they could be revived and made whole by hypothetical vastly advanced future technology. Not only is there no guarantee of its success, many people[who?] argue that human cryopreservation is unethical. According to certain views[which?] of the mind body problem, some philosophers[who?] believe that the mind, which contains thoughts, memories, and personality, is separate from the brain. When someone dies, their mind leaves the body. If a cryopreserved patient gets successfully resuscitated, no one knows if they would be the same person that they once were or if they would be an empty shell of the memory of who they once were.[improper synthesis?]
Right now scientists are trying to see if transplanting cryopreserved human organs for transplantation is viable, if so this would be a major step forward for the possibility of reviving a cryopreserved human.
Cryopreservation for embryos is used for embryo storage, e.g., when in vitro fertilization (IVF) has resulted in more embryos than is currently needed.
Pregnancies have been reported from embryos stored for 16 years. Many studies have evaluated the children born from frozen embryos, or “frosties”. The result has been uniformly positive with no increase in birth defects or development abnormalities. A study of more than 11,000 cryopreserved human embryos showed no significant effect of storage time on post-thaw survival for IVF or oocyte donation cycles, or for embryos frozen at the pronuclear or cleavage stages. Additionally, the duration of storage did not have any significant effect on clinical pregnancy, miscarriage, implantation, or live birth rate, whether from IVF or oocyte donation cycles. Rather, oocyte age, survival proportion, and number of transferred embryos are predictors of pregnancy outcome.
Cryopreservation of ovarian tissue is of interest to women who want to preserve their reproductive function beyond the natural limit, or whose reproductive potential is threatened by cancer therapy, for example in hematologic malignancies or breast cancer. The procedure is to take a part of the ovary and perform slow freezing before storing it in liquid nitrogen whilst therapy is unde