大鼠离体心肌粘性劲度定量评定的研究

本实验依据心肌工作模型和心肌顺应性的理论,从大鼠离体心肌静态、动态应力-应变指数曲线和应力松弛对数曲线中,求得反映心肌粘性劲度的指标η_1和η_2,并对心肌组织粘性成分的粘性劲度进行了定量评定。η_1和η_2值与心肌粘性劲度呈同向性变化、且重复性好。本工作提示,η_1和η_2可作为实用可靠的定量评定心肌顺应性的指标。     

人角膜的生物力学特性

以人角膜为研究对象,通过建立一套规范、系统的实验方法,揭示人角膜的生物力学特性。对人角膜标本进行了单轴拉伸实验,测定了其极限强度、应力最大时的变形能及其变形能密度;循环加载实验表明应力－应变的滞后环随加载次数的增加而逐渐减小,应力－应变曲线在应力为０．１５ＭＰａ以上时能用幂指数拟合;应力松弛实验测定了应力松弛谱各参数;蠕变实验表明归一化蠕变数据和时间对数呈线性关系     

阿霉素心肌病时左心室收缩性、舒张性和顺应性的动态演变及其对心泵贮备功能的影响

已有研究表明,在阿霉素心肌病时,心脏收缩性、顺应性明显降低的情况下,基础状态下的心泵功能无明显降低。近年来临床和基础研究均表明,在反映心功不全的指标中,基础状态下心泵功能的变化不如负荷状态下心泵功能的变化可靠。因而,目前临床多采用运动负     

切应力作用下与血管平滑肌细胞联合培养的内皮细胞PDGF-B含量的变化

目的：探讨切应力作用下联合培养的血管平滑肌细胞(VSMC)对内皮细胞(EC)PDGF-B含量变化的影响,为组织工程血管和预防血管移植物再缩窄研究提供一些实验资料。方法应用免疫细胞化学方法和图像分析技术,以静态条件下单独培养的EC和联合培养的EC为两对照组,研究了切应力作用下单独培养的EC和与VSMC联合培养EC的PDGFB含量的变化。结果：静态条件下联合培养EC的PDGF-B的含量比单独培养的EC减少;切应力作用下,联合培养EC的PDGF-B含量在切应力作用1h左右有瞬时上升,随后下降至低于联合培养条件下的静态水平,且瞬时上升的时间点比单独培养的EC提前。结论：切应力作用下,与VSMC联合培养EC的PDGF-B含量下降,这可能有利于抑制VSMC的增殖,提示在组织工程血管构建时VSMC与EC联合种植有利于预防血管移植后的再狭窄     

静态单轴拉伸应变刺激对细胞有丝分裂方向的影响

力学刺激对细胞发育具有重要意义,它如何对细胞分化及组织形态的发生产生影响是一个尚未完全阐明的问题.细胞的有丝分裂过程与细胞增殖、分化以及胚胎发育、组织器官形态形成和损伤组织的修复再生等特性密切相关,例如,细胞的有丝分裂方向就是影响细胞极性分化,乃至组织形态发生的因素之一.那么,力学刺激是否通过改变细胞有丝分裂方向从而影响细胞的分裂分化呢?以小鼠成骨细胞系MC3T3为模型,探讨了静态单轴拉伸应变刺激对细胞形态、应力纤维排布方向和有丝分裂方向的影响.结果显示,在4%及8%静态单轴拉伸应变条件下,48 h之内细胞形态发生明显变化,细胞呈梭状,长轴沿应变方向排列,细胞骨架微丝呈束状平行排列,方向与应变方向相关.统计学分析表明,4%应变刺激48 h后、8%应变6 h后、8%应变12 b后、8%应变24 h后,及8%应变48 h后,分别有49%,43%,54%,54%,和62%的细胞应力纤维排列方向与单轴拉伸应变方向的夹角在300以内,以及50%,48%,56%,53%和62%的细胞有丝分裂方向与单轴应变方向夹角在300以内.统计学分析表明,细胞形态、应力纤维排布及有丝分裂方向与拉伸方向相关,且应力纤维排列方向和有丝分裂方向之间呈现高的相关性,这种相关性在拉伸刺激48 h后表现很明显,由此推测,存在力学刺激影响细胞形态及细胞应力纤维排布方向,控制有丝分裂方向的机制.     

肢体延长下软组织的生物力学特性

对羊前肢延长后和对照侧的正中神经、正中动脉、正中静脉进行应力－应变、应力松弛实验,确定其应力－应变函数和连续松弛谱的各参数。从应力－应变曲线和归一化连续松弛曲线来看,在延长停止的初期,延长后试样相对于其对照侧试样的力学特性差异较大,随着恢复时间的增加,延长后试样相对于其对照侧试样的力学特性又趋于接近,正中神经、正中动脉、正中静脉都表现出这样的规律     

猪眼角膜的生物力学特性

对猪眼角膜进行了有系统的单轴拉伸实验,通过实验来确定其极限强度,断裂能、显著非线性的应力-应变关系和滞后环.在不同的应变水平下进行了应为松弛实验并确定了连续松弛谱的各参数.从极限强度、断裂能和应力松弛来看,角膜的纵向和横向之间无重大的差异,据此,各向同性假定可用于初步的角膜力学模型.     

髋骨术后仿真生物力学分析与临床研究

目的:探讨坐耻骨肿瘤切除术后髋骨生物力学变化及其与恢复期并发症产生的关系,指导术后骨盆重建.方法:构建单侧完整髋骨及坐耻骨肿瘤切除术后髋骨有限元模型,在相同约束和负载条件下,计算髋骨相同部位位移、应力及应变值,比较与分析模拟手术前后力学变化,结合临床评价意义.结果:手术前后髋骨节点位移、应力及应变分布区域相似,手术前后骶髂关节节点位移及应变、髋臼顶节点应力及应变有显著性差异;手术前后骶髂关节节点应力、坐骨大切迹应力及应变无显著性差异.结论:坐耻骨肿瘤切除术后主要影响同侧骶髂关节、髋臼顶的生物力学状态,恢复期并发症发生的原因由骶髂关节、髋臼顶生物力学改变及耻骨联合与耻骨上支连接与稳定作用的消失所致,三者相互作用,相互影响.     

应力作用下软骨细胞信号传导的研究进展

研究显示应力刺激对软骨细胞生长及基质代谢具重要作用。软骨正常结构形态以及应力下的软骨细胞形态和基质代谢的变化是力-生物信号转化的基础,信号分子及信号通路则是应力信号传导的核心,二者是对软骨细胞应力下信号传导过程深入了解不可或缺的信息组成,了解应力对软骨细胞的作用方式及作用机制有助于软骨相关疾病诊治、组织工程等领域的研究,本文就这两个方面研究进展做一综述。     

应力作用下软骨细胞信号传导的研究进展

研究显示应力刺激对软骨细胞生长及基质代谢具重要作用。软骨正常结构形态以及应力下的软骨细胞形态和基质代谢的变化是力-生物信号转化的基础,信号分子及信号通路则是应力信号传导的核心,二者是对软骨细胞应力下信号传导过程深入了解不可或缺的信息组成,了解应力对软骨细胞的作用方式及作用机制有助于软骨相关疾病诊治、组织工程等领域的研究,本文就这两个方面研究进展做一综述。     

A nonlinear viscoelastic finite element model of polyethylene

A nonlinear viscoelastic finite element model of ultra-high molecular weight polyethylene (UHMWPE) was developed in this study. Eight cylindrical specimens were machined from ram extruded UHMWPE bar stock (GUR 1020) and tested under constant compression at 7% strain for 100 sec. The stress strain data during the initial ramp up to 7% strain was utilized to model the "instantaneous" stress-strain response using a Mooney-Rivlin material model. The viscoelastic behavior was modeled using the time-dependent relaxation in stress seen after the initial maximum stress was achieved using a stored energy formulation. A cylindrical model of similar dimensions was created using a finite element analysis software program. The cylinder was made up of hexahedral elements, which were given the material properties utilizing the "instantaneous" stress-strain curve and the energy-relaxation curve obtained from the experimental data. The cylinder was compressed between two flat rigid bodies that simulated the fixtures of the testing machine. Experimental stress-relaxation, creep and dynamic testing data were then used to validate the model. The mean error for predicted versus experimental data for stress relaxation at different strain levels was 4.2%. The mean error for the creep test was 7% and for dynamic test was 5.4%. Finally, dynamic loading in a hip arthroplasty was modeled and validated experimentally with an error of 8%. This study establishes a working finite element material model of UHMWPE that can be utilized to simulate a variety of postoperative arthroplasty conditions.     

Interrelation of creep and relaxation: a modeling approach for ligaments

Experimental data (Thornton et al., 1997) show that relaxation proceeds more rapidly (a greater slope on a log-log scale) than creep in ligament, a fact not explained by linear viscoelasticity. An interrelation between creep and relaxation is therefore developed for ligaments based on a single-integral nonlinear superposition model. This interrelation differs from the convolution relation obtained by Laplace transforms for linear materials. We demonstrate via continuum concepts of nonlinear viscoelasticity that such a difference in rate between creep and relaxation phenomenologically occurs when the nonlinearity is of a strain-stiffening type, i.e., the stress-strain curve is concave up as observed in ligament. We also show that it is inconsistent to assume a Fung-type constitutive law (Fung, 1972) for both creep and relaxation. Using the published data of Thornton et al. (1997), the nonlinear interrelation developed herein predicts creep behavior from relaxation data well (R > or = 0.998). Although data are limited and the causal mechanisms associated with viscoelastic tissue behavior are complex, continuum concepts demonstrated here appear capable of interrelating creep and relaxation with fidelity.     

The relation between collagen fibril kinematics and mechanical properties in the mitral valve anterior leaflet

We have recently demonstrated that the mitral valve anterior leaflet (MVAL) exhibited minimal hysteresis, no strain rate sensitivity, stress relaxation but not creep (Grashow et al., 2006, Ann Biomed Eng., 34(2), pp. 315-325; Grashow et al., 2006, Ann Biomed. Eng., 34(10), pp. 1509-1518). However, the underlying structural basis for this unique quasi-elastic mechanical behavior is presently unknown. As collagen is the major structural component of the MVAL, we investigated the relation between collagen fibril kinematics (rotation and stretch) and tissue-level mechanical properties in the MVAL under biaxial loading using small angle X-ray scattering. A novel device was developed and utilized to perform simultaneous measurements of tissue level forces and strain under a planar biaxial loading state. Collagen fibril D-period strain (epsilonD) and the fibrillar angular distribution were measured under equibiaxial tension, creep, and stress relaxation to a peak tension of 90 N/m. Results indicated that, under equibiaxial tension, collagen fibril straining did not initiate until the end of the nonlinear region of the tissue-level stress-strain curve. At higher tissue tension levels, epsilonD increased linearly with increasing tension. Changes in the angular distribution of the collagen fibrils mainly occurred in the tissue toe region. Using epsilonD, the tangent modulus of collagen fibrils was estimated to be 95.5+/-25.5 MPa, which was approximately 27 times higher than the tissue tensile tangent modulus of 3.58+/-1.83 MPa. In creep tests performed at 90 N/m equibiaxial tension for 60 min, both tissue strain and epsilonD remained constant with no observable changes over the test length. In contrast, in stress relaxation tests performed for 90 min epsilonD was found to rapidly decrease in the first 10 min followed by a slower decay rate for the remainder of the test. Using a single exponential model, the time constant for the reduction in collagen fibril strain was 8.3 min, which was smaller than the tissue-level stress relaxation time constants of 22.0 and 16.9 min in the circumferential and radial directions, respectively. Moreover, there was no change in the fibril angular distribution under both creep and stress relaxation over the test period. Our results suggest that (1) the MVAL collagen fibrils do not exhibit intrinsic viscoelastic behavior, (2) tissue relaxation results from the removal of stress from the fibrils, possibly by a slipping mechanism modulated by noncollagenous components (e.g. proteoglycans), and (3) the lack of creep but the occurrence of stress relaxation suggests a "load-locking" behavior under maintained loading conditions. These unique mechanical characteristics are likely necessary for normal valvular function.     

Mechanical properties of the heart muscle in the passive state

The viscoelastic behaviour of the heart muscle (papillary muscle) in the passive unstimulated) state is studied by such methods as stress relaxation, creep, vibration and stress-strain testing. The tests are conducted on a newly developed electromechanical muscle testing device which is suitable for conducting active and passive tests on biological materials.     

Biomechanical study using fuzzy systems to quantify collagen fiber recruitment and predict creep of the rabbit medial collateral ligament

In normal daily activities, ligaments are subjected to repeated loads, and respond to this environment with creep and fatigue. While progressive recruitment of the collagen fibers is responsible for the toe region of the ligament stress-strain curve, recruitment also represents an elegant feature to help ligaments resist creep. The use of artificial intelligence techniques in computational modeling allows a large number of parameters and their interactions to be incorporated beyond the capacity of classical mathematical models. The objective of the work described here is to demonstrate a tool for modeling creep of the rabbit medial collateral ligament that can incorporate the different parameters while quantifying the effect of collagen fiber recruitment during creep. An intelligent algorithm was developed to predict ligament creep. The modeling is performed in two steps: first, the ill-defined fiber recruitment is quantified using the fuzzy logic. Second, this fiber recruitment is incorporated along with creep stress and creep time to model creep using an adaptive neurofuzzy inference system. The model was trained and tested using an experimental database including creep tests and crimp image analysis. The model confirms that quantification of fiber recruitment is important for accurate prediction of ligament creep behavior at physiological loads.     

A device for testing mechanical properties of biological materials--the "Biodyne".

An electromechanical servo-controlled device has been developed. This device can be used to test the mechanical behavior of a wide variety of biological soft tissues. Control and execution of material testing procedures such as stress-strain, vibration, relaxation, creep etc. can be performed by manual operation of the device or by interfacing it with a laboratory type minicomputer. Experiments on excitable tissues such as muscle can also be executed. The design details and system performance are discussed.     

Stress relaxation of heterogeneous myocardial tissue. Numerical experiments with 3D model

We present here the simple block model of a heterogeneous system describing the processes of stress relaxation in spatial and mechanically heterogeneous myocardial tissue. In numerical experiments by the model it is established that to obtain the same level of tension as in the case of a uniform model, less stiff and less viscous heterogeneous blocks are required. It is shown that in the model of heterogeneous myocardial tissue one observes not only stress relaxation but also strain relaxation—creep. Notably, step-by-step adjustment of the neighbor structural blocks in response to the whole model deformation takes place. Furthermore, stationary deformation properties of separated blocks become nonlinear.     

A comparison of biomechanical properties between human and porcine cornea

Due to the difficulty in obtaining human corneas, pig corneas are often substituted as models for cornea research. The purpose of this study is to find the similarities and differences in the biomechanical properties between human and porcine corneas. Uniaxial tests were conducted using an Instron apparatus to determine their tensile strength, stress-strain relationship, and stress-relaxation properties. The tensile strength and stress-strain relation were very similar but significant differences between the two tissues were observed in the stress-relaxation relationship. Under the same stretch ratio lambda=1.5, porcine cornea relaxed much more than human cornea. If tensile strength and the stress-strain relation are the only mechanical factors to be investigated, porcine cornea can be used as a substitute model for human cornea research. However, when stress relaxation is a factor, porcine corneas cannot be used as an appropriate model for human corneas in mechanical property studies. It is very difficult to get enough specimens of human cornea, so we did the experiments for stress-strain relationship at a specific value of strain rate (corresponding to the velocity of loading 10mm/min), and for stress relaxation at a specific stretch ratio lambda=1.5.     

A computer simulation of the creep process of the cell wall using stress relaxation parameters

The stress relaxation of the cell wall of pea plants was measured and viscoelastic parameters were obtained according to the stress-relaxation analysis developed by Yamamoto et al. (Plant & Cell Physiol. 1970). The creep process of the cell wall was simulated by a numerical integration using stress-relaxation parameters, because the direct conversion of the stress relaxation process to the creep is impractical. In the conversion, a personal computer was programmed for the Maxwell viscoelastic model with a compiler language. Cell wall creep was measured with a specially constituted apparatus and compared with that calculated by the simulation process. The results suggested that the creep can be reproduced by a computer simulation using the stress-relaxation parameters. Both creep and stress-relaxation properties of plant cell walls can be analyzed by using a single model.