Multiphysics Investigation of Thermomechanics and Ice Formation in Organ Cryopreservation

<p dir="ltr">As part of an ongoing efforts to shorten the long organ transplant waitlist, organ cryopreservation techniques can increase the shelf life of donated organs and help fill the shortage of transplantable organs – a public health benefit that has been recognized and sought after for decades. This dissertation discusses two cryopreservation approaches at the contrasting ends of preservation time scale: vitrification for virtually indefinite period of preservation, and partial freezing for extending organ shelf life by a few days.</p><p dir="ltr">Vitrification has been considered to be the only viable alternative for long-term, ice-free preservation of large organs, where a specimen perfused with cryoprotective agents (CPAs) is rapidly cooled down while suppressing ice formation, to be stored below the glass transition temperature of CPAs. By virtue of the exponential increase in CPA viscosity with the decreasing temperature, this process facilitates the specimen storage in a glassy or vitreous state, a process which can theoretically be applied to specimen of any size. While vitrification success depends on overcoming a host of challenges related to CPA toxicity, ice crystallization, and thermomechanical stress, this dissertation mainly focuses on thermomechanical stress (or thermal stress) during vitrification (Chapters 2 and 3), reduction of which is crucial for maintaining structural integrity of specimen. Using computational tools, Chapter 2 describes how selection of specimen packaging can improve heat transfer and reduce thermal stress during vitrification of human popliteal artery. Additionally, the benefits of volumetric heating by exciting nanoparticles in a radio-frequency (RF) electromagnetic field, or nanowarming, are discussed in Chapter 2, while exploring effects of various boundary conditions during rewarming of vitrified artery segments. An intriguing effect observed during vitrification of large size specimens is the deformation of free surface of the CPA, which is caused by thermal gradients in the domain, continuous contraction of the CPA during cooling and exponential increase in CPA viscosity. Such a large deformation can cause localized stress in the domain, which may exceed the strength of material, leading to structural damage such as fractures. Chapter 3 presents a simplified multiphysics model to analyze surface deformation in CPA, while validating simulation results with experimental observations obtained during scanning cryomacroscopy. The outcomes discussed in Chapter 2 and 3 can help improve the design of vitrification protocols for successful cryopreservation of large organs. </p><p dir="ltr">The method of partial freezing for organ preservation has been recently developed, where the organ is stored at high subzero (HSZ) temperatures. In this nature-inspired approach, the aim is to control and confine ice formation to the extracellular spaces and vasculature, while preserving remaining organ in a non-frozen state. Here, Snomax is used as an ice nucleating agent when mixed with the CPA, in order to modulate ice formation in the vicinity of the melting temperature of the CPAs. Chapters 4 and 5 of this dissertation present a multiscale exploration of ice nucleation and crystallization in CPAs mixed with Snomax: using differential scanning calorimetry (DSC) for the microscale analysis (Chapter 4), and scanning cryomacroscopy for macroscale observations (Chapter 5). It is demonstrated in this dissertation that the outcomes from a microscale investigation cannot be extrapolated to the macroscale but require additional modeling tools. The results from this multiscale analysis of ice formation using Snomax enhance our understanding of the effect of using ice nucleating agents and help design the partial freezing protocols for improved tissue cryopreservation outcomes.</p>