Heat Induced Changes in the Mechanics of Soft Tissue

热引起的软组织力学变化

• 批准号: 9634109
• 负责人: Jay Humphrey
• 金额: $ 15.86万
• 依托单位: University of Maryland Baltimore County
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1996
• 资助国家: 美国
• 起止时间: 1996-09-01 至 1998-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9634109&HistoricalAwards=false
• 关键词: Heat; Induced; Changes; Mechanics; Soft

9634109 Humphrey Advances in laser, microwave, radio-frequency, and similar technologies continue to promote new applications of heat to treat various pathologies. yet, most clinical studies are motivated by the advances in device technology, not a fundamental understanding of the underlying thermo- biomechanics, which at present remains poorly understood. Exploiting the potential of heat-therapy will, therefore, require quantification of the effects of clinically controllable parameters such as the temperature level, heating history, and the mechanical load state during heating. In general, there are two primary irreversible effects of heat treatment, cell necrosis and denaturation of structural proteins. Thus, to design and eventually optimize a heat-therapy, one must be able to predict (spatially and temporally) the concomitant irreversible effects on cell viability and connective tissue integrity for a given heating device. Rather than studying a specific clinical application, the overall goal of this work is to establish a theoretical basis for quantifying irreversible heat-induced changes in the nonlinear, anistropic behavior of soft tissue -- our specific aim is to develop and validate phenomenological thermomechanical damage relations for three different tissues that will be exposed to a broad range of temperatures (from 45 to 85oC), both isothermally and non-isothermally), hearing periods (from seconds to hours), and mechanical load states during heating (isometric, isotonic, and cyclic stretching). The proposed tissues are bovine chordae tendineae, epicardium, and endocardium. In addition to a gel matrix, chordae consists primarily of uniaxially oriented collagen, epicardium of a 2- D network of collagen, and endocardium of a 2-D network of collagen and elastin. These three tissues were selected because they have increasingly more constituents and microstructural complexity, and they admit rigorously performed and interpreted uniaxial or in-pl ane biaxial testing -- this will allow the development of 1-D and then 2- D relations of increasing complexity. There are no comparable data or constitutive relations for thermal-damage in the literature. The PIs recently showed that the effects of temperature level and heating period are coupled in heat-induced collagen denaturation: there exists a "time-temperature equivalency" in the form of an exponential relation in traction free, isothermal shrinkage tests on chordae. A similar relation exists in heat-induced cell death. The PIs hypothesize that an analogous equivalency exists in tissues under more general conditions (e.g., multiaxial, non- isothermal loading), which will simplify quantification in terms of a load and temperature-dependent scaled heating time. Concepts from finite strain damage mechanics and irreversible continuum thermodynamics will be extended to determine the dependence of the Helmholtz free energy and the damage evolution equation on the current finite strain and prior thermal-damage, the latter of which likely depends on an accumulative scaled heating time. Such quantification will allow tissue-level engineering analysis of potential heat-therapies, and hopefully optimization thereof. ***

9634109汉弗莱在激光、微波、射频和类似技术方面的进步继续推动热能在治疗各种疾病方面的新应用。然而，大多数临床研究都是由设备技术的进步推动的，而不是对潜在的热生物力学的基本理解，这一点目前仍然知之甚少。因此，开发热疗的潜力将需要量化临床可控参数的影响，如温度水平、加热历史和加热期间的机械负荷状态。一般来说，热处理有两种主要的不可逆效应，即细胞坏死和结构蛋白变性。因此，要设计并最终优化热疗，必须能够(在空间和时间上)预测给定加热装置对细胞活性和结缔组织完整性的不可逆影响。这项工作的总体目标不是研究特定的临床应用，而是建立一个理论基础，用于量化热引起的软组织非线性各向异性行为的不可逆变化--我们的具体目标是建立和验证三种不同组织的现象学热机械损伤关系，这些组织将暴露在广泛的温度范围(从45到85摄氏度，包括等温和非等温)、听力周期(从几秒钟到几小时)和加热期间的机械负荷状态(等轴测、等张和循环拉伸)。建议的组织是牛腱索、心外膜和心内膜。除了凝胶基质，脊索主要由单轴取向的胶原蛋白、二维胶原网络的心外膜以及二维胶原和弹性蛋白网络的心内膜组成。这三种组织之所以被选中，是因为它们的成分越来越多，显微结构也越来越复杂，它们承认严格执行和解释单轴或平行双轴测试--这将使一维和二维关系的发展变得越来越复杂。文献中没有热损伤的可比性数据或本构关系。PI最近显示，在热诱导的胶原变性过程中，温度水平和加热时间的影响是耦合的：在无牵引力的、恒温收缩的脊索实验中，存在着以指数关系形式的“时间-温度当量”。类似的关系也存在于热诱导细胞死亡中。PI假设在更一般的条件下(例如，多轴、非等温载荷)组织中存在类似的等效性，这将简化根据载荷和依赖于温度的定标加热时间的量化。有限应变损伤力学和不可逆连续介质热力学的概念将被扩展，以确定Helmholtz自由能和损伤演化方程对当前有限应变和先前热损伤的依赖关系，后者可能取决于累积定标加热时间。这样的量化将允许对潜在的热疗进行组织级工程分析，并有望对其进行优化。***