Male thermal contraception is based on a simple principle: raising the temperature of the testicles to 37 °C (instead of the physiological 33–34 °C) for at least 15 hours a day. This is sufficient to temporarily block sperm production.
Here are the main observed effects on sperm cells:
Collapsed concentration
Sperm production drops significantly (often below 1 million per milliliter), sometimes reaching partial or complete azoospermia. Currently, the contraceptive threshold is defined solely based on this concentration criterion.
Reduced motility (categories A and B, or progressive)
Progressive motile sperm—those capable of reaching the egg—are nearly absent or show ineffective movement.
Altered vitality
Most of the remaining sperm have almost no vitality.
Major morphological abnormalities
Sperm often exhibit significant deformities: abnormal head shape (globular, irregular, vacuolated), deformed or missing flagella, and thickened or fragmented midpieces.
Disrupted capacitation
Even the few remaining sperm are unable to undergo capacitation, the final maturation step required for fertilization. This impairment is caused by damaged cell membranes, dysfunction of CatSper ion channels, and increased oxidative stress. This means that even in the presence of residual sperm, the actual ability to fertilize is nearly zero, further reinforcing the overall contraceptive effectiveness of the method.
Chromatin alterations and aneuploidies
Prolonged heat exposure can induce temporary anomalies in chromatin condensation, as well as a slight increase in the frequency of aneuploidies (abnormal chromosome numbers) in sperm. These effects do not contribute directly to contraception but are observed during exposure to testicular heat.
And afterwards?
All of these effects are reversible: after stopping the method, sperm production, quality, fertilizing capacity, and genetic viability return to normal within a few months.
