For a long time, heart preservation is a problem, the preservation time above zero degree is too short, and the human tissue below zero degree will be destroyed by ice crystals. Scientists have been trying to crack this dilemma. And now, they've found a way out.
Schematic diagram of containers that can be used for subcooling Image source: Anthony Consiglio, UC Berkeley
Written by | chestnut
Once the heart is detached from the human body, it usually does not last long. Using existing cold preservation techniques, the function of heart cells begins to fail after 4 hours. If such a heart is transplanted to a patient, the risk of abnormal organ function is greatly increased. Due to time constraints, most of the donated hearts cannot be transplanted in time to patients in need, only to be discarded.
But now, scientists at the University of California, Berkeley, have found a way to keep heart tissue alive for longer, allowing it to be successfully revived after 3 days of sub-zero storage. Their findings were published in the journal Communications Biology.
Although the temperature is low, it does not freeze
What is the way to keep objects in an environment below zero degrees? The traditional freezing method, also known as isobaric freezing, is to freeze the object in a place with a constant air pressure, such as a standard atmospheric pressure: when the temperature drops below zero, the water will begin to freeze.
However, ice crystals can destroy the cellular structure, causing damage to human tissues, which is not conducive to the preservation of living organs. So, how can we achieve a temperature drop below freezing without the tissue freezing?
Ice is less dense than water Image credit: Johnathon Choate
When water freezes, the volume expands. However, if you fill a hard container with water, drain the bubbles inside, and then put the container into the freezer, since there is no extra space for water to freeze, the water below the freezing point can also remain liquid. This prevents ice crystals from damaging human tissue.
This operation is called isochoric supercooling. You can do something similar, such as putting a shaken bottle of Coke in the freezer compartment of the refrigerator and getting a bottle of "super-frozen" Coke in a few hours.
"Super frozen" Coke Image source: The Sci Guys
It didn't originally freeze, but if you shake the bottle, you will find that the cola inside quickly solidifies: when the carbon dioxide bubble is released and in full contact with the liquid part, ice crystals form around the bubble.
Of course, berkeley scientists are not concerned about whether "ultra-frozen" cola can be made into smoothies, but whether the "ultra-frozen" heart can retain its original function.
What features are retained to be successful?
As a result, scientists induce human stem cells to form heart tissue, which can beat like real hearts, and the rhythm of beating is close to that of human heart rhythms. Such tissues are also known as the "heart microphysiological system" (MPS). At the same time, the rigid container was already filled with organ preservation fluid. Put the beating microphysiological system of the heart into the container and seal it, leaving almost no bubbles, and you can start the "waiting for overcooling" operation.
Supercool preservation of heart tissue, gray in the bottle for heart tissue, blue for preservation Solution Image source: doi: 10.1038/s42003-021-02650-9
They put the container in an environment of -3 °C, waited for 24 hours, 48 hours and 72 hours, and then took the tissue out in three batches and placed it in a 37 °C environment close to human body temperature to see how many pieces of heart tissue could re-beat. The results showed that most of the microphysiological systems of the heart that had stopped beating at -3 °C had resumed beating. Specifically, 65 percent of the heart tissue samples taken 24 hours later returned to normal spontaneous beats. Of the heart tissue removed after 48 hours and 72 hours later, 50% and 55% returned to normal spontaneous beats, respectively.
Blue is a sample of heart tissue that has returned to normal spontaneous beats, gray is a sample of partial recovery of spontaneous beats, and other samples have not recovered spontaneous beats Image source: doi: 10.1038/s42003-021-02650-9
Scientists believe that such a success rate is quite high in terms of cardiac re-beat. This result is all the more encouraging given the individual differences between heart tissues and the absence of cryoprotectants for the entire subcooling procedure.
However, spontaneous beating is only part of the function of the heart's tissue. In addition to this, the research team also tested the response of the preserved heart tissue to external electrical stimulation. If you've ever heard of a pacemaker, it works by stimulating the myocardium with electrodes and promoting myocardial contraction. This time, the scientists also inserted electrodes for the supercooled heart tissue to see how they behaved.
It was found that most (50%-70%) of the heart tissue inserted into the pacing electrodes responded normally. Normal here refers to these cardiac tissues and a control group that has not undergone cold, and behaves similarly when receiving electrical stimulation.
The second test ended, and there was a third. Qualified heart tissue should respond to some medications that treat the heart. Scientists wondered if the supercooled heart tissue could produce such a reaction. So they used 1 micromol/liter of isoproterenol, a drug used to increase heart rate.
Experiments have shown that the heart tissue removed after supercool preservation is exposed to the drug, and the heart rate has increased significantly. Although there was some discrepancy between the degree of heart rate improvement and the control group, the researchers could not determine whether this gap was caused by supercool preservation or related to individual differences between heart tissues.
Heart tissue removed after 24 hours and after 72 hours of overcooling, exposure to the drug, there is a significant increase in heart rate Image source: doi: 10.1038/s42003-021-02650-9
After various tests, scientists finally confirmed that the heart tissue that can tolerate excessive cold can still restore basic functions. In addition, immunofluorescence staining experiments have also confirmed that this cryopreservation method does not lose the structural integrity of heart tissue.
The team believes this is the first time that heart tissue (induced by stem cells) has been successfully revived after supercool preservation. Although the resuscitation is not really taken from the human heart tissue, it still brings more possibilities for heart transplantation.
Teleport the heart over long distances
Existing cardiac preservation techniques, as standard, are to put the stop-beating heart into a preservation solution to cool, usually only 4-6 hours. Therefore, for a long time, it has been difficult for the heart to be transplanted to withstand long-distance transmission, such as a donor on the southeast coast, whose donated heart may not be used on patients in the western provinces.
If one day, the waiting subcooling technology can store the heart for 24-72 hours and ensure the normal operation of basic functions, the time window of heart transplantation can be greatly extended, so that more patients in need can get the opportunity to live.
This study is a key proof of concept. However, a small piece of heart tissue can be revived, and it does not mean that an entire heart can also be revived. The next thing scientists have to do is to expand the scope of application of technology from tissues to organs, so that the heart that can tolerate overcooling can be put into clinical practice as soon as possible.
Reference Links:
https://www.nature.com/articles/s42003-021-02650-9
https://engineering.berkeley.edu/news/2021/09/cold-hearted-science/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5724041/
https://www.livescience.com/how-long-can-donated-organs-last-before-transplant.html
https://www.youtube.com/watch?v=cIiyuT-ogzo
Source: Global Science
Edited by: Dogcraft, yrLewis