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Cool Nanotechnology Keeps Cryopreserved Tissues Intact

Nanowarming (right) may offer a way to thaw cryogenically preserved tissues with less tissue damage than conventional (left) warming techniques. | Manuchehrabadi et al., Science Translational Medicine (2017)

One of the major obstacles to transplant medicine may soon melt away, thanks to a technique that uses nanoparticles to rapidly rewarm delicate frozen tissues.

The method , published in the 1 March issue of Science Translational Medicine , could help make organ banks a viable option for transplant surgeons, said John Bischof of the University of Minnesota, the senior author on the study.

"Currently there are not enough organs for transplantation and the need is growing because patients who need organs are far outstripping donors. One potential way to address this is to improve organ preservation," said Bischof.

More than 60% of the hearts and lungs donated for transplantation must be discarded annually, because these tissues cannot be kept on ice for longer than four hours. According to recent estimates, if only half of unused organs were successfully transplanted, transplant waiting lists could be eliminated within two years.

Long-term preservation methods could help establish organ and tissue banks, which would also reduce transplant rejection rates by facilitating the process to find matching donors when needed. One such sophisticated cryopreservation technique involves super-cooling biological samples to a glassy state — a process called vitrification. Although vitrification has yet to be successfully implemented for preserving entire organs, scientists are working to improve the process.

"Currently only very small and thin tissues in small volumes can be vitrified. Some examples include veins, arterial rings, heart valve segments, skin, cornea and cartilage," said Bischof.

Tissue banks are already used to store small amounts of skin material for evaluating the safety of new cosmetics, which reduces the need for animal testing. Being able to preserve larger samples also could alleviate social stigma for trauma victims by allowing surgeons to transplant skin grafts that closely match a patient's own skin.

The biggest stumbling block to preserving larger samples comes from tiny ice crystals that tend to form right around the transition point to or from a vitrified state. Crystallization damages biological materials, or even causes tissues to crack, during the thawing process.

"Typically ice crystals form, nucleate, in larger systems," Bischof explained. "During warming, rapid rates are needed to outstrip the growth of these ice crystals or the system will fail."

To quickly warm up larger volumes of frozen tissues without compromising their cellular viability, a team of researchers in Bischof's lab led by Navid Manuchehrabadi developed a unique approach based on nanotechnology. The scientist tested their set-up using frozen human skin cells, segments of pig heart tissue, and sections of pig arteries in volumes almost 20 times larger than previously attempted samples.

The researchers were able to rapidly generate uniform heat throughout frozen tissues by mixing tiny silica-coated iron oxide nanoparticles, each one with a diameter 150 times smaller than a red blood cell, into the cryopreservation solution used for vitrification. When an external magnetic field was applied to the tissues, the nanoparticles warmed up the samples from within.

The frozen tissues warmed up by over 120 oC (almost 250 oF) in one minute — ushering the samples safely from vitrification to a thawed state before ice crystals could form. After rewarming, none of the tissues displayed signs of damage, unlike control samples rewarmed slowly over ice, which is currently the standard thawing method.

More than 60% of the hearts and lungs donated for transplantation must be discarded annually, because these tissues cannot be kept on ice for longer than four hours. According to recent estimates, if only half of unused organs were successfully transplanted, transplant waiting lists could be eliminated within two years.

"We knew from previous work that the gold standard thawing method would fail," said Manuchehrabadi.

The scientists deployed several advanced imaging techniques to verify that the tissues thawed by nanowarming maintained their structural integrity.

What's more, the nanoparticles could be easily washed away from the samples following thawing.

"This opens the way for a distributed heat source from nanoparticles to be placed within and around any tissue or organ we hope to rewarm," said Bischof. "From my perspective, the ability to do tissue banking in the volumes from the paper exists now. As far as organs, we're not there yet, but we have extremely promising results."

Hearts are promising organs for the new technology because introducing nanoparticles into the wide-open spaces of the organ's atria and ventricles presents less of a challenge than incorporating the tiny magnets all throughout more solid structures like brains or livers, according to Kelvin Brockbank of Clemson University and Tissue Testing LLC, one of the authors of the paper who spoke to reporters during a 28 February teleconference.

"This is a huge landmark. We've got the missing link, we can see the road ahead for large sample size warming to get these organs into patients," Brockbank said.

Although scaling up the system to accommodate whole organs will require further study, the authors are applying some of the insights gained from this study to other large medical problems.

"Our results are related to the ability of magnetic nanoparticles to heat," said Manuchehrabadi. "We believe this technology will relate directly to both preservation and therapeutic approaches used for healthcare."

Bischof said his collaborators and many other labs are already pursuing the use of magnetically-heated nanoparticles for the treatment of various conditions including cancer, neural and other diseases.

Next, the scientists plan to assess whether nanowarming can thaw vitrified rabbit kidneys without causing tissue damage. For this, they will collaborate with Greg Fahy of 21st Century Medicine, one of the world's leading experts on organ vitrification. To assess the biological viability of thawed organs, the researchers will work with their transplant collaborator, Erik Finger at the University of Minnesota, and an industrial collaborator, Kelvin Brockbank at Tissue Testing Technologies LLC in South Carolina.

[Credit for associated image: Navid Manuchehrabadi]