PhD student Casey Kraft successfully defends dissertation

May 2, 2026 — University of Minnesota Biomedical Engineering PhD student Casey Kraft successfully defended his dissertation, "Advances in Cardiac Cryopreservation" in March. He was advised by Professor John Bischof.

Following is the dissertation abstract:

Heart disease is a leading cause of death worldwide. Donor hearts (for transplant) and in vitro tissue (for research and development) are valuable cardiac systems for treating heart disease. However, the preservation time of cardiac tissue is limited to ~4 hours using 4°C storage, which is too short for stockpiling and on-demand supply. To enable a cold supply chain of human cardiac systems, we have made advances towards cryopreservation, which offers months of preservation in the stored state (-150°C). Our chosen cryopreservation approach, vitrification, involves using a high concentration of cryoprotective agents (CPAs) to avoid ice damage. At the CPA concentration necessary to avoid ice formation, however, vitrification solutions can cause toxicity due to CPA exposure alone. Therefore, we conducted a CPA screening (no cryo) in hiPSC-derived cardiomyocytes (CMs) and found that formamide, a CPA commonly used in vitrification solutions, caused significantly worse recovery of contractility than CPA formulations without formamide. Therefore, we developed a formamide-free CPA formulation, DEP, which we applied in our engineered heart tissue (EHT) cryopreservation protocol. We found that the common approach of liquid nitrogen dunking caused EHT cracking and an adjacent approach, -190°C cooling, caused worse EHT recovery than -150°C. By designing an EHT cryopreservation protocol for limited thermal strain, we achieved recovery of EHT force, electrically active area, and sarcomere organization similar to the 2 hr 4°C controls, which exhibited 40% lower force than the fresh control. As a first step towards applying our process to donor hearts, we conducted CPA perfusion experiments (no cryo) in adult rat hearts and found that the CPA’s carrier solution played a substantial role in the CPA toxicity threshold. At 100% of the CPA concentration needed to avoid ice, Celsior caused recovery of rat heart beating following CPA whereas other carriers required concentrations as low as 25%. To date, the two published cardiac tissue cryopreservation successes have been limited to immature constructs (with embryonic CMs or < D30 hiPSC-CMs as in our EHT). Modulating CM metabolism using carrier solution may be the key to achieving nondamaging CPA exposure and therefore the first cryopreservation success in adult cardiac tissue.