Using a combination of lasers, gold particles, and antifreeze, scientists have successfully reanimated frozen zebrafish embryos. This new technique could one day help biologists bank the embryos of species at risk for extinction or preserve the genetically modified fish that scientists use to study human diseases. But first, researchers have to make sure more of the embryos can actually survive this new process.

Deep-freezing eggs, sperm, and embryos to save them for later is called cryopreservation. We’ve been cryopreserving sperm since the 1950s and human embryos since the 1980s. But fish embryos have been frustrating scientists for almost 60 years. With sea life threatened by warming and acidifying oceans, figuring out how to safeguard that genetic diversity is becoming more urgent.

You can’t just pop an embryo in the freezer or defrost it in the microwave

The problem is that if you want your frozen embryo to eventually grow into a living creature, you can’t just pop it in the freezer or defrost it in the microwave. You need both the freezing and thawing processes to be fast. Otherwise, ice crystals can grow inside that can shred the embryo’s water balloon-like cells. Since fish embryos can run large, they’re especially difficult to heat up quickly or evenly the typical way — by popping them into a bath of warm water. It’s like trying to defrost an entire chicken compared to a chicken cutlet in your sink.

So scientists led by John Bischof at the University of Minnesota came up with a new thawing method. Before freezing zebrafish embryos, the researchers injected them with tiny gold cylinders coated in antifreeze. Then, when the scientists were ready to thaw the gold-filled balls of cells, the researchers lit them up with a laser. The laser’s beam heated up the gold particles, warming the embryos from the inside out, according to results published today in the journal ACS Nano.

It’s a hot new technique (sorry). And the gold particles or antifreeze didn’t seem to cause birth defects in the zebrafish they revived this way — at least, in the ones that actually survived. More than two-thirds of the embryos died within an hour of being thawed. And by the next day, 90 percent were dead. Still, it’s an improvement on conventional methods, which killed every last one of the embryos frozen and thawed the typical way. With a little work, this new method might help the scientists of the future retrieve some of the oceans’ lost biodiversity.