NSF Research Centers Collaborate to Unveil Refrigerant Selection Framework for Next-Generation Organ Cryopreservation

During an organ transplant procedure, there are only a few hours of time that donated tissue can remain viable between the donor and the recipient. As a result of this short window of time, it is estimated that nearly 20 percent of donated organs go unused, as suitable recipients are not always found fast enough for successful transplants. But two research teams are collaborating to expand this window by not only hours, but perhaps months or even years through the proposed use of cryogenic refrigerants that can safely preserve a donated organ until an ideal patient match is identified.

Two multi-institutional U.S. National Science Foundation (NSF) Engineering Resource Centers (ERCs) are bringing their respective areas of expertise to facilitate the development of a donated organ bank system that can extend the life of organs once they are received from donors. Researchers from the NSF ERCs for Advanced Technologies for the Preservation of Biological Systems (ATP-Bio), led by the University of Minnesota (UMN), and the Environmentally Applied Refrigerant Technology Hub (EARTH), led by the University of Kansas (KU), have recently published research in Cryobiology, the official journal of the Society for Cryobiology, that establishes a refrigerant selection framework for the cooling, storage, and shipment of cryopreserved organs, a translational step towards building an organ bank in future.

“Preserving donated organs is fascinating,” said Mark B. Shiflett, Foundation Distinguished Professor of Chemical Engineering at the University of Kansas and Director of the NSF ERC EARTH Engineering Research Center. “I learned about the organ preservation research ATP-Bio was doing, and I realized that there was a need for specialized cooling. For this research, ATP-Bio leads the cryobiology and the rewarming process via nanowarming, and we in EARTH bring refrigerant engineering for effective cryogenic cooling and potential scale-up.” 

Cryogenic refrigerants can cool organs and other biological substances to extremely low, or “cryo” temperatures—as low as -200℃—which halts biological activity. As there are a myriad of such refrigerants with varying thermal performance attributes, toxicity, and eco-friendly benefits, this research establishes a framework by which ideal refrigerants, blends of refrigerants, or yet-to-be-discovered refrigerants can be selected for the various stages of organ cryopreservation, including cooling, transport, and storage. 

This cryogenic refrigerant framework research is the first step in the long process of designing an organ bank system, which is envisioned to be placed in hospitals that recover organ donations and those that specialize in organ transplant procedures. Shiflett said, “A surgeon who is removing an organ from a donor could put it into a machine, where it can be cooled down to the right cryogenic storage temperature. From there, the organ can be placed into a cryogenic freezer for long-term storage.”

“Later, when a suitable recipient is found, they can select a particular organ and put it back into another machine to quickly and uniformly bring it back up to the right temperature utilizing nanowarming technique. Then a surgeon can transplant it into a new patient,” added John C. Bischof, Director of NSF ERC ATP-Bio and Distinguished McKnight University Professor, Mechanical Engineering at UMN.

In the refrigeration framework, there are three phases or applications: Faster cooling, stable transport, and sustainable storage, each of which has different requirements and considerations that will contribute to effective and safe organ transplants. 

Faster Cooling

This phase considers the ability of the organ to cool and thaw with the least amount of stress to the tissue. Thermal performance is most important in this phase for the minimization of ice crystal formations, cell damage, organ fractures, and other stressors, as well as future uniform warming.

“We are figuring out how to rapidly cool to get around ice crystal formation and thermal stress on the organs,” said Dorothy Haggard, M.D./Ph.D. candidate at KU and one of two lead researchers of the framework. “By using cryoprotective agents, which are specialized preservation chemical compounds, we ‘perfuse’ or circulate these into an organ before cooling to prevent ice formation.”

“Then we use a process called ‘vitrification,’ which transforms the organ into glass enabled by rapid cooling. This minimizes thermal stress on the organ and allows the organ to have a theoretically indefinite shelf life,” said Lakshya Gangwar, co-lead researcher on the team and a postdoctoral scholar in the Department of Mechanical Engineering at UMN.

Stable Transport

Liquid nitrogen (LN₂) remains the dominant refrigerant for the transport of cryopreserved large tissues and organs. But because of its greater likelihood of contributing to organ fractures, there is pressure to develop redesigned systems for transport. 

Researchers determined that safety and volatility are the top priorities for stable transport, so refrigerants that rate higher in flammability and toxicity by ASHRAE standards need to be avoided. 

For both stable transport and sustainable storage of organs for transplant, the ongoing use of LN₂ in large quantities increases the environmental carbon footprint, a by-product of nitrogen liquefaction plants. As such, a transition to low global-warming-potential (GWP) and zero ozone-depletion-potential (ODP) refrigerants is prescribed in these phases of the framework.

Sustainable Storage

For long-term storage of cryopreserved donor organs, environmental concerns become a priority, as this is the longest period of time an organ is in the cryo-supply chain. Mechanical freezers commonly used to store vitrified samples, typically operate on HFC refrigerant blends which have high GWP, thus leaving a considerable carbon footprint. Alternatively, hydrofluoroolefins (HFOs) are a promising new option to replace HFCs. 

However, Haggard noted, “One problem is that these HFOs are so new that we don't have enough cryogenic physical property data available, so we cannot fully model the system with them yet. Measuring viscosity, density, and heat capacity at  -150℃ is really challenging.”

In the development of their framework, the research team reviewed and organized more than 100 current refrigerants for their viability in the cryopreservation framework, based on the targets developed for each phase. In developing their various technical parameters and selection criteria, they are allowing space for the development of new refrigerant options that may surface.

“The framework stays the same. We have target heat capacities and thermal conductivities that we want our refrigerant fluids in each application to have,” said Haggard. “So the addition of new refrigerants that may meet those targets will be a really good thing. This could be blends of refrigerants, such as two different HFOs, or blending an HFO and an HFC, and  modeling new combinations to see if they help us reach the property values that we want for this system.”

“If an organ bank system can be created, it could save so many more lives,” said Shiflett. “Fewer donor organs would be lost. This is something that is so important to humanity.” 

Rounding out the research team from UMN is Michael L. Etheridge, principal research engineer in the Department of Mechanical Engineering. This research was funded by grants from the NSF and National Institutes of Health. To learn more about NSF EARTH, please visit https://erc-earth.ku.edu

Please direct queries regarding this study to Mark B. Schiflett, mark.b.shiflett@ku.edu.

EARTH Engineering Research Center

The Environmentally Applied Refrigerant Technology Hub (EARTH) is an NSF- and corporate-funded Engineering Research Center (ERC). EARTH is dedicated to revolutionizing how refrigerants are formulated, manufactured, applied, monitored, and recycled to dramatically reduce the environmental footprint of the global cooling sector. Led by the University of Kansas, this consortium of partner research universities includes the University of Notre Dame, University of Maryland, Lehigh University, University of South Dakota, and the University of Hawai′i.