Cui Z F, Dykhuizen R C, Nerem R M, Sembanis A
Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, USA.
Biotechnol Prog. 2002 Mar-Apr;18(2):354-61. doi: 10.1021/bp0101886.
Long-term storage of engineered bio-artificial tissues is required to ensure the off-the-shelf availability to clinicians due to their long production cycle. Cryopreservation is likely the choice for long-term preservation. Although the cryopreservation of cells is well established for many cell types, cryopreservation of tissues is far more complicated. Cells at different locations in the tissue could experience very different local environmental changes, i.e., the change of concentration of cryoprotecting chemicals (CPA) and temperature, during the addition/removal of CPA and cooling/warming, which leads to nonuniformity in cell survival in the tissue. This is due to the limitation of mass and heat transfer within the tissue. A specific aim of cryopreservation of tissue is to ensure a maximum recovery of cells and their functionality throughout a tissue. Cells at all locations should be protected adequately by the CPA and frozen at rates conducive to survival. It is hence highly desirable to know the cell transient and final states during cryopreservation within the whole tissue, which can be best studied by mathematical modeling. In this work, a model framework for cryopreservation of one-dimensional artificial tissues is developed on the basis of solving the coupled equations to describe the mass and heat transfer within the tissue and osmotic transport through the cell membrane. Using an artificial pancreas as an example, we carried out a simulation to examine the temperature history, cell volume, solute redistribution, and other state parameters during the freezing of the spherical heterogeneous construct (a single bead). It is found that the parameters affecting the mass transfer of CPA in tissue and through the cell membrane and the freezing rate play dominant roles in affecting the cell volume transient and extracellular ice formation. Thermal conductivity and extracellular ice formation kinetics, on the other hand, have little effect on cell transient and final states, as the heat transfer rate is much faster than mass diffusion. The outcome of such a model study can be used to evaluate the construct design on its survivability during cryopreservation and to select a cryopreservation protocol to achieve maximum cell survival.
由于工程化生物人工组织的生产周期较长，因此需要进行长期储存，以确保临床医生能够随时取用。冷冻保存可能是长期保存的选择。尽管对于许多细胞类型来说，细胞的冷冻保存已经很成熟，但组织的冷冻保存要复杂得多。在添加/去除冷冻保护剂（CPA）以及冷却/升温过程中，组织中不同位置的细胞可能会经历非常不同的局部环境变化，即冷冻保护化学物质（CPA）浓度和温度的变化，这会导致组织中细胞存活的不均匀性。这是由于组织内质量和热传递的限制。组织冷冻保存的一个特定目标是确保整个组织中的细胞及其功能能够最大程度地恢复。所有位置的细胞都应受到CPA的充分保护，并以有利于存活的速率冷冻。因此，非常希望了解整个组织冷冻保存过程中的细胞瞬态和最终状态，这可以通过数学建模进行最佳研究。在这项工作中，基于求解耦合方程来描述组织内的质量和热传递以及通过细胞膜的渗透传输，开发了一种一维人工组织冷冻保存的模型框架。以人工胰腺为例，我们进行了模拟，以研究球形异质构建体（单个珠子）冷冻过程中的温度历史、细胞体积、溶质重新分布和其他状态参数。研究发现，影响CPA在组织中和通过细胞膜的质量传递以及冷冻速率的参数在影响细胞体积瞬态和细胞外冰形成方面起主导作用。另一方面，热导率和细胞外冰形成动力学对细胞瞬态和最终状态影响很小，因为热传递速率比质量扩散快得多。这种模型研究的结果可用于评估构建体在冷冻保存期间的生存能力设计，并选择冷冻保存方案以实现最大的细胞存活率。
