Nanorods bring fish back to life


Rebecca Pool

Monday, July 17, 2017 - 13:45
Image: Live and developing embryo after nano-warming [University of Minnesota/Smithsonian Conservation Biology Institute]
In a breakthrough for cryo-preservation and wildlife conservation, US-based researchers have successfully frozen and thawed zebrafish embryos, releasing images of the live embryos, after successful thawing.
Using gold nanorods and laser pulses to rapidly 'nano-warm' the frozen embryos after cryo-preservation, researchers from the University of Minnesota and the Smithsonian Conservation Biology Institute (SCBI) have, as they say, developed a reliable tool to 'switch life back on'.
"We have taken a unique approach... to do what has been impossible previously: to successfully freeze and thaw a fish embryo so that the embryo begins to develop, rather than falls apart,” highlights Dr Mary Hagedorn from SCBI. “There’s no doubt that the use of this technology, in this way, marks a paradigm shift for cryopreservation and the conservation of many wildlife species,
While researchers have successfully cryopreserved the embryos of many mammal species and the sperm of many species of fish, freezing fish embryos has proven infinitely more complicated. 
Successful cryopreservation of an embryo requires cooling the embryo to a cryogenically stable state, then warming it at a rate faster than it was cooled, and using an antifreeze, or cryoprotectant, to prevent ice-crystal growth.
Fish embryos, however, are very large, making it difficult to thaw them quickly and avoid ice crystal development.
In addition, because aquatic animals need to survive harsh environments, their embryonic membranes are mostly impenetrable, blocking the cryoprotectants out.
With this in mind, the researchers micro-injected the cryoprotectant, propylene glycol into zebrafish embryos, along with gold nanorods, and then cooled the samples at a rate of 90 000 °C/min in liquid nitrogen.
They then irradiated the embryos with a pulsed infrared laser, warming the embryo from -196°C to 20°C in just one thousandth of a second.
Crucially, the rapid warming rate twinned with the cryoprotectant, prevented the formation of ice crystals, which can damage the embryos.
Researchers have addressed the zebrafish cryopreservation problem by using gold nanorods to assist in the warming process. [American Chemical Society]
As the researchers highlights, embryos that underwent this process went on to develop at least to the 24-hour stage where they develop a heart, gills, tail musculature and moved, proving post-thaw viability.
“Lasers have the exciting ability to act like a “light switch” that can turn biological activity on and off within gold nanoparticle laden biomaterials,” asserts Professor John Bischof from Mechanical Engineering at Minnesota.
“In this case, by careful engineering and deployment of gold nanoparticles within a cryogenically stored and biological inactive embryo, we can use a laser pulse to quickly warm the embryo back to ambient temperatures and switch biological activity, and therefore life, back on,” he adds.
Because the embryos of other aquatic animals - fish, amphibians and coral - are very similar to those of zebrafish, this technology is directly applicable to the cryopreservation of many species’ embryos.
The researchers reckon the technology may also be customised to cryopreserve reptile and bird embryos and enhance the process of cryopreserving mammalian embryos, including giant pandas and large cats.
And, the technology could help aquaculture farms become more efficient and cost effective, putting less pressure on wild populations.
Research is published in ACS Nano.
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