Background. Freezing of biological tissue emerges as an innovative biotechnology to treat diseased tissues, and to manufacture and preserve tissues and biomaterials. These applications include cryotherapy for cancers and cardiovascular diseases, and cryopreservation of native/engineered cells, tissues and pharmaceutical products. In spite of its preliminary success, the development of a freezing protocol with desired outcome is still extremely challenging. This is primarily due to limited understanding of tissue-level biophysical phenomena during freezing. This NSF-supported research led by Professor Han at Purdue University in Indiana helps to understand underlying mechanisms to determine the characteristics of the tissue after thawing. Outcomes. The Han's Team developed a new microscopic imaging technique to study ice formation and subsequent events during freezing of biological tissues. Using this technique, they established a biophysical mechanism to predict the characteristics of the tissue after thawing. During freezing of biological tissue, the interstitial fluid (mainly water) freezes and expands due to the density difference between water and ice. Since biological tissue has complex architecture of cells, interstitial fluid and extracellular matrix (Figure 1), this expansion causes the movement of the interstitial fluid and subsequent interactions with the extracellular matrix (Figure 2). They found that these interactions result in local swelling and shrinking of the tissues, and determine the characteristics of the tissue after thawing (Figure 3). This understanding provides mechanistic prediction of the microstructure of the tissue after thawing, and helps to design a freezing protocol for various tissues with desired outcomes. This research finding has also been used to create teaching modules for challenge-based undergraduate and graduate courses for mechanical and biomedical engineering students. Intellectual Merit. This research provides a quantitative understanding of freezing-induced cell-fluid-matrix interactions, which is a missing link to bridge macro-scale freezing protocols to cellular (micro-scale), and tissue (meso-scale) level heat and mass transport phenomena. This understanding significantly enhances our knowledge on freezing-induced biotransport phenomena. In addition, the multi-scale framework of this research substantially advance our capability to process biological materials. Broader Impacts. The most manifest benefit is improving the quality of healthcare based on freezing. The research outcome will also support the tissue engineering industry by providing reliable long-term storage methods for native/engineered tissues so as to reduce the manufacturing cost and improve product quality control. This project has provided research and education opportunities to both graduate and undergraduate students from underrepresented groups – two Hispanic and five female students.
