Cryopreservation is a way to preserve germplasm with applications in agriculture, biotechnology, and conservation of endangered animals. Cryopreservation has been available for over a century, yet, using current methods, only around 50% of spermatozoa retain their viability after cryopreservation. This loss is associated with damage to different sperm components including the plasma membrane, nucleus, mitochondria, proteins, mRNAs, and microRNAs. To mitigate this damage, conventional strategies use chemical additives that include classical cryoprotectants such as glycerol, as well as antioxidants, fatty acids, sugars, amino acids, and membrane stabilizers. However, clearly current protocols do not prevent all damage. This may be due to the imperfect function of antioxidants and the probable conversion of media components to more toxic forms during cryopreservation.

The generation of reactive oxygen species during cryopreservation of human sperm has negative effects on the consistency of the thawed sperm. The antioxidant properties of cerium oxide nanoparticles (CeO ₂ NPs) may be useful for reducing cryodamage in thawed sperm. This research was conducted to determine the effects of CeO ₂ NPs on the quality and function of human sperm after thawing. Samples of semen obtained from 20 normozoospermic individuals were allocated to the following four groups: fresh, frozen control (sperm not treated with CeO ₂ NPs), and those exposed to 0.1 μg/mL CeO ₂ NPs (CeO ₂ -0.1), 1 μg/mL CeO ₂ NPs (CeO ₂ -1), and 5 μg/mL CeO ₂ NPs (CeO ₂ -5). Sperm parameters of motility, viability, membrane integrity, DNA fragmentation, protamination, malondialdehyde (MDA) levels, mitochondria membrane potential, and morphology were evaluated after the freezing–thawing process. The results showed that 0.1 μg/mL CeO ₂ NPs significantly ( p < 0.05) improved the following human sperm parameters after thawing: progressive (44.6% ± 1.14% vs. 36.2% ± 1.24%) and total motility (60.9% ± 2.5% vs. 51.3% ± 2.5%), viability (67.9% ± 1.5% vs. 58.1% ± 1.5%), membrane functionality (66.1% ± 1.85% vs. 55.4% ± 1.85%), DNA integrity (30.8% vs. 24.04%), and protamination (69.85% ± 2.09% vs. 57.2% ± 2.09%) compared with the frozen control group. We observed the lowest MDA levels in the CeO ₂ -0.1 (3.06 ± 0.25 nmol/mL), CeO ₂ -1 (3.1 ± 0.25 nmol/mL), and CeO ₂ -5 (3.08 ± 0.25 nmol/mL) groups compared with the frozen control group (3.72 ± 0.25). Different concentrations of CeO ₂ NPs did not significantly change sperm normal morphology and mitochondria activity ( p < 0.05).