Scientists in Japan are celebrating the birth of mice pups, conceived with freeze-dried sperm delivered on a "postcard".
A team from the University of Yamanashi preserved the mammalian sperm via extreme cold temperatures, storing it at -30°C (-22°F) for more than three months.
The sperm were initially transported via glass cases, which often broke in transit.
Writing in the journal iScience, the scientists have demonstrated freeze-dried mice sperm led to "healthy offspring" after being transported in plastic sheets attached to a postcard.
While this raises hopes for newfound fertility treatments, one scientist has stressed "a lot of experimentation and a guarantee of absolute success" is required before the technique can be used for humans.
"When I developed this method for preserving mouse sperm by freeze-drying it on a sheet, I thought it should be able to be mailed on a postcard and so when offspring were actually born after being mailed, I was very impressed," said study author Daiyu Ito.
"The postcard strategy was easier and cheaper compared to any other method. We think the sperm never expected the day would come when they would be in the mailbox."
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The preservation of mammalian sperm is important for infertility treatments, livestock production and the conservation of "genetic resources", particularly among endangered species.
After thawing, the sperm can be used via artificial insemination or in-vitro fertilisation (IVF).
The Yamanashi scientists previously transported frozen sperm in glass bottles, which were bulky and often broke, rendering the contents unusable.
The same team sent freeze-dried mice sperm to the space station to study the effects of space radiation on baby rodents.
Large volumes of sperm were needed for this research. The requirement of cushions to prevent breakages, however, meant only small amounts could be sent to the station.
With this setback in mind, the Yamanashi scientists discovered plastic sheets offered a compact method of transportation with no breakage risk.
The sheets were toxic for the sperm, however, leading the team to test various materials to go within the sheets. Weighing paper was found to be easiest to handle, with the highest offspring rate.
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This allowed thousands of mice sperm to be held in a so-called "sperm book", which was kept at -30°C.
The sperm was still viable after being attached to a postcard via a plastic sheet, with no protection, and mailed tens of thousands of miles.
The scientists are now working towards storing the sperm book for at least one month at room temperature.
"It is now recognised that genetic resources are an asset to humanity's future," added co-author Teruhiko Wakayama.
"Even though many genetic traits are not needed for survival, depending on the environmental context, it is necessary to preserve them.
"The plastic sheet preservation method in this study will be the most suitable method for the safe preservation of a large amount of valuable genetic resources because of the resistance to breakage and less space required for storage."
Freeze-dried sperm has also led to healthy rats, hamsters and rabbits.
"In the case of humans, the application of this technology will be in the distant future because it requires a lot of experimentation and a guarantee of absolute success for humans," Ito told Vice.
Watch: Sperm counts dropping in Western countries