具有双特征时间参数的生物软组织黏弹性性质的理论研究

目的 探索描述生物软组织黏弹性特性的普遍行为或规律。方法 根据生物软组织的力学结构,构建由两个线性弹簧和两个黏壶的不同组合构成的四元件黏弹性结构模型;并通过弹性理论,结合不同黏弹性模型的几何构型推导其运动微分方程,利用其微分方程分析不同四元件模型的应力松弛和蠕变行为以及反映弹性和黏性相结合的应力松弛时间和蠕变推迟时间。结果 所有可能的四元件黏弹性模型都具有普遍的本构关系、应力松弛和蠕变函数形式。通过比较模型预测结果与主动脉瓣、韧带和脑动脉等生物软组织的实验数据发现,四元件黏弹性模型能够很好地描述生物软组织的力学行为。“快”和“慢”两个特征时间τ1和τ2对生物软组织的应力松弛具有显著的影响。“快”松弛时间τ1对应力达到稳定态所需时间有明显的影响,而“慢”松弛时间τ2对松弛率的影响不显著,但对应力松弛的稳定态有明显的影响。结论 生物软组织的时间依赖性行为可以通过两个特征时间尺度来表征,即“快”和“慢”时间;且具有两个特征时间的生物软组织的应力 应变关系、应力松弛和蠕变函数具有相同的数学形式,这与所选择的弹簧和黏壶的配置和排列无关,但为保证模型力学参数的合理性,针对不同的生物软组织应选取适合的模型。     

压缩载荷下猪腰椎间盘髓核摘除后力学行为的实验研究

为研究退变及摘除髓核后腰椎间盘的力学行为,对猪腰椎间盘进行压缩实验。髓核摘除并经胰蛋白酶处理后的椎间盘作为实验组,正常椎间盘作为对照组。考虑载荷大小及加载速率的影响,得到椎间盘应力和应变关系、瞬时弹性模量及蠕变性能,并建立了蠕变本构模型。结果发现:随着压缩载荷和加载速率的增大,实验组的应变及蠕变与对照组相比均明显增大,瞬时弹性模量与对照组相比则明显减小,这表明髓核摘除后,椎间盘的承载能力受压缩载荷大小及加载速率的影响明显大于正常椎间盘。实验组仍可以采用Kelvin三参量固体模型描述其蠕变性能,预测腰椎间盘去核后的蠕变行为。研究结果可为椎间盘疾病的临床治疗及术后康复提供理论基础。     

人工椎间盘及髓核假体的生物力学

背景：全椎间盘或髓核假体是否符合人的椎间盘的解剖特点、生物力学是目前关注的焦点。 目的：综述目前存在的人工椎间盘及髓核假体的类型及其生物力学特征。 方法：由第一作者用计算机检索万方数据库、中国期刊全文数据库和PubMed数据库,检索时间：1989/2011,检索词分别为“椎间盘、髓核、人工假体、生物力学”和“intervertebral diac,nucleus pulposus,artificial prosthesis,biomechanics”,语言分别设定为中文和英文。纳入与人工椎间盘及髓核假体的材料类型及其生物力学特性相关的研究,排除重复性研究。共检索到72篇文章,按纳入和排除标准对文献进行筛选,共纳入26篇文章进行分析。 结果与结论：目前应用于人工椎间盘的材料有钴铬合金、钛合金、不锈钢、陶瓷和超高分子量聚乙烯等。应用不同材料制备的人工椎间盘假体校多,其中Bryan假体在临床中最常用,体外实验、三维有限元分析及临床研究均证实其具有良好的生物力学特性,置换成功率高。目前存在的人工髓核假体有预制型和原位聚合型,由于其手术损伤小,是目前研究的热点,但仍很难达到人体髓核的生物力学功能。发掘新型材料和设计个性化假体是将来的发展方向。      

基于超黏弹性的牙周膜本构模型构建及模拟

为了准确描述牙周膜的生物力学行为,基于大变形连续介质力学理论及不可压缩各向同性假设,以人体牙周膜平面剪切和应力松弛实验数据为基础,利用有限元软件ABAQUS的数据拟合功能,构建牙周膜的超黏弹性本构模型及参数。随后,通过对5组牙周膜平面剪切实验过程的模拟,验证牙周膜超黏弹性模型的正确性。最后,通过有限元计算对比分析牙周膜线弹性模型与超黏弹性模型对载荷的力学响应。结果表明在牙根位移量为0～0.06 mm时,牙周膜可近似用线弹性模型表示;当位移量大于0.06 mm,两种模型的差异显著,超黏弹性模型更加符合牙周膜的材料特性。研究结果为牙周膜提供一种实用性强的超黏弹性模型,为牙齿正畸的生物力学研究和治疗方案的精确设计提供理论依据。     

心血管系统体循环输入阻抗的几种集中参数模型的比较和应用

目的 分析比较几种常用的模拟心血管体循环左心室后负荷输入阻抗的集中参数模型,并将模型应用于心衰和高血压。方法 采用保证拟合精度的方法和指标,用最小二乘法拟合数据,以得到模型中元件参数值。结果 Burattini;Gnudi四元件模型和Liu ZR改进五元件模型拟合程度优于其它模型;这两种模型应用于心衰和高血压均得到了符合生理事实的结果。结论 四元件模型和改进五元件模型是比较理想的集中参数模型。     

模拟失重对雄性大鼠骨流变特性的测试

目的研究模拟失重对大鼠承重骨流变特性的影响,为航天医学提供模拟失重动物骨流变参数。方法30只Wistar雄性大鼠,随机分为正常对照组15只和头低位30°尾吊组(失重模型组)15只。实验第28天时取两组动物股骨进行压缩应力松弛、蠕变实验。结果得出了正常对照组和失重模型组大鼠股骨压缩应力松弛、蠕变数据和曲线,得出了两组大鼠股骨的归一化应力松弛函数,归一化蠕变函数与曲线,以一元线性回归分析的方法处理实验数据,得出了回归系数a、b与c、d值和归一化应力松弛函数G(t)、归一化蠕变函数J(t)表达式,还得出了两组大鼠股骨应力松弛、蠕变与时间的变化规律。结论失重模型组大鼠股骨应力松弛、蠕变量低于正常对照组。     

个性化 3D 打印钛合金短柄股骨假体的生物力学评价

目的 评估个性化 3D 钛合金打印短柄股骨假体的生物力学性能。 方法 采用 4 种不同的短柄股骨假体: 3D 打印 1 组(假体 A)、3D 打印 2 组(假体 B)、BE 组(假体 C)、SMF 组(假体 D),分别对 12 根成人尸体股骨标本实 行人工股骨头置换。 在万能材料力学试验机上通过初始稳定性测试和静力压缩测试,对比分析 4 组假体模型变形 量、最大压缩载荷、最大压缩位移和抗压缩刚度。 结果 初始稳定性测试结果显示,3D 打印 2 组假体变形量略低于 3D 打印 1 组,而明显低于 SMF 组和 BE 组,但差异无统计学意义(P>0. 05)。 3D 打印 2 组假体最大压缩载荷、抗压缩 刚度均大于其余 3 组,最大压缩位移小于其余 3 组,差异仍无统计学意义(P>0. 05)。 结论 个性化 3D 打印钛合金短 柄股骨假体力学性能与目前临床常用的股骨 SMF、BE 1 假体相当,力学稳定性较好。     

不同降解周期下Ⅱ型胶原-丝素蛋白复合软骨支架的应力松弛行为

目的 利用有限元分析与理论模型相结合的方法,研究软骨支架在不同降解周期下的应力松弛行为。方法 基于已建立的降解本构模型计算不同降解周期下支架弹性模量;建立软骨支架有限元模型,并进行应力松弛仿真,分析支架松弛应力随时间的变化。建立应力松弛本构模型,对支架力学性能进行预测。结果 软骨支架降解14、28、42、56 d时,弹性模量分别为32.35、31.12、29.91、28.74 kPa。软骨支架各层的应力分布表现为上层受力最大;支架整体松弛应力随着时间增加先快速下降,然后趋于平稳;降解56 d时,支架能承受的应力仍在软骨的生理载荷范围内。应力松弛本构模型预测结果与有限元模拟结果吻合较好。结论 随降解时间的增加,支架弹性模量逐渐减小。降解时间越长,支架能承受的应力越小。相同降解时间下,施加的压缩应变越大,支架受力越大。有限元仿真和应力松弛本构模型可以有效预测支架降解时的应力变化。     

人体椎间盘的一个力学模型

对人体椎间盘力学性质的深入研究,牦极大地有助于对椎间盘脱出进行治疗和预防。众所周知,椎间盘表现出粘弹性性质,椎间盘中的含水量对于控制材料的松弛特性起重要作用。本文采用从人体椎间盘中得到的试件,进行应力松弛实验。试件取自髓核和两个不同方向纤维环的外层,在不同的湿度下进行实验,得到了松弛控制曲线。根据实验数据,提出了一种定性椎间盘模型。这种模型可以考虑椎间盘的粘弹性性质,并可研究年龄,材料结构变化和含水量等对其性质的影响。结果发现:髓核     

模拟全髋关节置换后股骨假体的稳定性

背景：国内外关于髋关节置换后股骨的压缩力学实验研究较多,因此研究髋关节置换后股骨的扭矩、扭转角、载荷-位移关系非常重要。对比分析传统型假体和解剖型假体的压缩、扭转力学特性,对于髋关节置换及人工假体的稳定性研究具有重要意义。 目的：通过模拟髋关节置换后股骨的轴向压缩和扭转实验,对比分析传统型和解剖型人工假体的稳定性,为临床提供生物力学参数。 方法：取股骨左右侧标本共12个,其中左侧6个标本保留股骨颈作为解剖型钛合金人工关节假体组,右侧6个标本去除股骨颈作为传统型钴铬钼人工关节假体组。分别将两组标本置于电子万能试验机工作台上,以     5 mm/min的实验速度对标本施加压应力,读取20,40,60,80,100 N时所对应的位移值。之后取两组标本,将标本两端置于扭转试验机夹头内,以1 (°)/s的实验速度对标本施加扭矩,读取5,10,15,20 N•m扭矩时所对应的扭转角值。 结果与结论：在100 N外力作用下,传统型假体位移为(2.03±0.06) mm,解剖型假体位移为(1.83±0.05) mm;在20 N•m扭矩作用下,传统型假体扭转角为(21.7±0.7)°,解剖型假体扭转角为(13.2±0.4)°。解剖型假体在100 N作用下的位移和在20 N•m扭矩作用下的扭转角均小于传统型假体组,差异有显著性意义(P < 0.05)。提示解剖型假体和传统型假体具有不同的压缩和扭转力学特性,解剖型假体置入股骨后具有较好的稳定性。中国组织工程研究杂志出版内容重点：人工关节;骨植入物;脊柱;骨折;内固定;数字化骨科;组织工程     

Mechanical properties of a novel PVA hydrogel in shear and unconfined compression

Poly(vinyl alcohol) (PVA) hydrogels have been proposed as promising biomaterials to replace diseased or damaged articular cartilage. A critical barrier to their use as load-bearing tissue replacements is a lack of sufficient mechanical properties. The purpose of this study was to characterize the functional compressive and shear mechanical properties of a novel PVA hydrogel. Two formulations of the biomaterial were tested, one with a lower water content (75% water), and the other with higher water content (80% water). The compressive tangent modulus varied with biomaterial formulation and was found to be statistically strain magnitude and rate dependent. Over a strain range of 10-60%, the compressive modulus increased from approximately 1-18 MPa, which is within the range of the modulus of articular cartilage. The shear tangent modulus (0.1-0.4 MPa) was also found to be strain magnitude dependent and within the range of normal human articular cartilage, but it was not statistically dependent on strain rate, This behavior was attributed to the dominance of fluid flow and related frictional drag on the viscoelastic behavior. Compressive failure of the hydrogels was found to occur between 45 and 60% strain, depending on water content.     

Linear and nonlinear viscoelastic modeling of aorta and carotid pressure-area dynamics under in vivo and ex vivo conditions

A better understanding of the biomechanical properties of the arterial wall provides important insight into arterial vascular biology under normal (healthy) and pathological conditions. This insight has potential to improve tracking of disease progression and to aid in vascular graft design and implementation. In this study, we use linear and nonlinear viscoelastic models to predict biomechanical properties of the thoracic descending aorta and the carotid artery under ex vivo and in vivo conditions in ovine and human arteries. Models analyzed include a four-parameter (linear) Kelvin viscoelastic model and two five-parameter nonlinear viscoelastic models (an arctangent and a sigmoid model) that relate changes in arterial blood pressure to the vessel cross-sectional area (via estimation of vessel strain). These models were developed using the framework of Quasilinear Viscoelasticity (QLV) theory and were validated using measurements from the thoracic descending aorta and the carotid artery obtained from human and ovine arteries. In vivo measurements were obtained from 10 ovine aortas and 10 human carotid arteries. Ex vivo measurements (from both locations) were made in 11 male Merino sheep. Biomechanical properties were obtained through constrained estimation of model parameters. To further investigate the parameter estimates, we computed standard errors and confidence intervals and we used analysis of variance to compare results within and between groups. Overall, our results indicate that optimal model selection depends on the artery type. Results showed that for the thoracic descending aorta (under both experimental conditions), the best predictions were obtained with the nonlinear sigmoid model, while under healthy physiological pressure loading the carotid arteries nonlinear stiffening with increasing pressure is negligible, and consequently, the linear (Kelvin) viscoelastic model better describes the pressure–area dynamics in this vessel. Results comparing biomechanical properties show that the Kelvin and sigmoid models were able to predict the zero-pressure vessel radius; that under ex vivo conditions vessels are more rigid, and comparatively, that the carotid artery is stiffer than the thoracic descending aorta; and that the viscoelastic gain and relaxation parameters do not differ significantly between vessels or experimental conditions. In conclusion, our study demonstrates that the proposed models can predict pressure–area dynamics and that model parameters can be extracted for further interpretation of biomechanical properties.     

The effect of polyethylene glycol on the stability of pores in polyvinyl alcohol hydrogels during annealing

Poly(vinyl alcohol) (PVA) hydrogels are candidate biomaterials for cartilage resurfacing or interpositional arthroplasty devices requiring high-creep resistance and high water content to maintain lubricity. Annealing of PVA improves creep resistance but also reduces the water content. We hypothesized that maintaining poly(ethylene glycol) (PEG) within PVA during annealing would prevent the collapse of the pores and thus would result in high equilibrium water content (EWC). Our hypothesis tested positive. The PVA hydrogels containing PEG maintained their opacity through annealing and exhibited large pores under confocal imaging while hydrogels not containing PEG turned translucent and no pores were visible after annealing. The EWC of gels annealed with PEG (83 +/- 1.0%) was higher than that of the gels processed without PEG (55 +/- 4.8). The crystallinity of the former was 8.0 +/- 1.7% and the latter was 27.5 +/- 8.7%. The hydrogels processed in the presence of PEG exhibited a significantly higher total creep strain (69 +/- 3.4%) when compared to the PEG-free hydrogels (17 +/- 3.7) under an initial contact stress of 0.45 MPa. EWC appeared to be strongly related to the creep resistance of annealed PVA theta-gels.     

关节软骨细胞和细胞周基质的压缩特性及其对软骨细胞代谢的影响

软骨细胞和细胞周基质(pericellular matrix, PCM)的力学特性对于关节软针的生理功能具有重要的意义。软骨细胞在压缩应力下表现出黏弹性固体材料特性，其压缩特性具有各向异性和多相性。PCM对软骨细胞具有明显的力学保护作用。压缩应力影响软骨细胞的代谢活动。软骨细胞和PCM的力学特性有许多仍未澄清，尚需进一步的研究。     

Correlating Material Properties with Tissue Composition in Enzymatically Digested Bovine Annulus Fibrosus and Nucleus Pulposus Tissue

Aging and degeneration of the intervertebral disk are accompanied by decreases in water and proteoglycan contents, and structural alterations. The aim of this study was to determine the impact of compositional changes on the material properties of intervertebral disk tissues. Confined compression stress-relaxation experiments were applied to bovine caudal annulus fibrosus and nucleus pulposus tissue specimens that were separated into three experimental groups: in situ, free-swelling control (PBS), and digestion (chondroitinase-ABC). Measurements of glycosaminoglycan (GAG) and water content, as well as nonlinear finite deformation biphasic theory and multiple linear regression analyses were performed. The compressive modulus H _A0 and permeability k ₀ of in situ specimens were 0.37±0.06 MPa and 0.49±0.08×10⁻¹⁵ m⁴ N⁻¹ s⁻¹ for nucleus, and 0.74±0.13 MPa and 0.42±0.05×10⁻¹⁵ m⁴ N⁻¹ s⁻¹ for annulus, respectively. There was a larger effect of swelling and digestion on the material properties and biochemical composition of nucleus pulposus than for annulus fibrosus. Alterations in proteoglycan and water content affected the compressive modulus and permeability, although the permeability was somewhat more strongly affected by water content than by proteoglycan content. Correlation coefficients r≤0.75 for the multiple regression indicated water and GAG content can moderately predict material properties, however other compositional and structural factors must be considered.     

Evaluation of novel injectable hydrogels for nucleus pulposus replacement

Branched copolymers composed of poly(N-isopropylacrylamide) (PNIPAAm) and poly(ethylene glycol) (PEG) are being investigated as an in situ forming replacement for the nucleus pulposus of the intervertebral disc. A family of copolymers was synthesized by varying the molecular weight of the PEG blocks and molar ratio of NIPAAm monomer units to PEG branches. Gel swelling, dissolution, and compressive mechanical properties were characterized over 90 days and stress relaxation behavior over 30 days immersion in vitro. It was found that the NIPAAm to PEG molar ratio did not affect the equilibrium swelling and compressive mechanical properties. However, gel elasticity exhibited a dependency on both the PEG block molecular weight and content. The equilibrium gel water content increased and compressive modulus decreased with increasing PEG block size. While all of the branched copolymers showed significant increases in stress relaxation time constant compared to the homopolymer (p < 0.05), the high PEG content PNIPAAm-PEG (4600 and 8000 g/mol) exhibited the maximum elasticity. Because of its high water content, requisite stiffness and high elastic response, PNIPAAm-PEG (4600 g/mol) will be further evaluated as a candidate material for nucleus pulposus replacement.     

兔髌韧带压缩力学性能

目的 研究不同应变率下韧带的压缩应力-应变关系,并构建本构模型,为韧带的损伤预估及替代材料的研 发提供参考。 方法 通过万能拉伸试验机测试兔髌韧带在不同应变率(0. 001、0. 01、0. 1、1 s-1 )下的压缩力学性能 和压缩松弛响应,并构建相应的本构方程。 结果 单轴无侧限压缩实验表明,随着应变率增加,30% 、40% 应变下的 应力和切线模量均明显增大。 相比于 Gent 模型,Fung 和 Ogden 模型更适用于拟合韧带压缩曲线(R2 >0. 99);采用 4 项 Prony 级数更适用于拟合韧带的松弛曲线(R2>0. 99)。 结论 兔髌韧带的压缩力学性能有显著的黏弹性响应, Fung 和 Ogden 模型可用于拟合韧带的压缩响应,4 项 Prony 级数可用于拟合兔髌韧带的压缩松弛响应。     

新型人工髓核植入对腰椎稳定性影响的生物力学研究

目的: 评价一种新型果胶/聚乙烯醇复合水凝胶人工髓核的生物力学性能, 为其临床应用提供科学依据。方法: 采用6具新鲜人体标本的 L4/5脊柱功能单元进行生物力学实验, 在轴向压缩、前屈后伸和左右侧弯等运动工况下, 观察正常椎间盘、髓核摘除以及新型人工髓核CoPP植入三种状态下活动度(ROM)、中性区(NZ)的变化以及在中立位轴向加载下椎间隙高度变化。结果: 髓核摘除后, L4/5椎间屈伸、旋转、侧弯的ROM和 NZ较正常组显著增加(P<0.05), 植入CoPP人工髓核后, L4/5椎间屈伸、侧弯、旋转的 ROM和 NZ与完整椎间盘无明显差异, 除右侧弯外其他方向的ROM和 NZ较髓核摘除组明显下降　(P<0.05)。髓核摘除组在 0和500N的负荷下椎间隙高度较相同情况下正常组平均下降1.08 mm和 1.77 　mm; CoPP人工髓核植入组较相同情况下髓核摘除组分别增加1.23 mm和1.95 mm。结论: 新型果胶/聚乙烯醇复合水凝胶人工髓核植入椎间隙可维持腰椎节段正常的三维运动稳定性,恢复椎间隙高度,可望进入临床应用。     

Effect of Stenosis Asymmetry on Blood Flow and Artery Compression: A Three-Dimensional Fluid-Structure Interaction Model

A nonlinear three-dimensional thick-wall model with fluid-structure interactions is introduced to simulate blood flow in carotid arteries with an asymmetric stenosis to quantify the effects of stenosis severity, eccentricity, and pressure conditions on blood flow and artery compression (compressive stress in the wall). Mechanical properties of the tube wall are measured using a thick-wall stenosis model made of polyvinyl alcohal hydrogel whose mechanical properties are close to that of carotid arteries. A hyperelastic Mooney–Rivlin model is used to implement the experimentally measured nonlinear elastic properties of the tube wall. A 36.5% pre-axial stretch is applied to make the simulation physiological. The Navier–Stokes equations in curvilinear form are used for the fluid model. Our results indicate that severe stenosis causes critical flow conditions, high tensile stress, and considerable compressive stress in the stenosis plaque which may be related to artery compression and plaque cap rupture. Stenosis asymmetry leads to higher artery compression, higher shear stress and a larger flow separation region. Computational results are verified by available experimental data. © 2003 Biomedical Engineering Society. PAC2003: 8719Uv, 8710+e     

Molecular dynamics simulation study of P (VP-co-HEMA) hydrogels: Effect of water content on equilibrium structures and mechanical properties

Poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) (P(VP-co-HEMA)) hydrogel system with a composition of VP:HEMA = 37:13 was studied using molecular dynamics simulations in order to investigate the effect of the water content on the equilibrium structures and the mechanical properties. The degree of randomness of the monomer sequence for the random and the blocky copolymers, were 1.170 and 0.104, respectively, and the degree of polymerization was fixed at 50. The equilibrated density of the hydrogel was found to be larger for the random sequence than for the blocky sequence at low water contents (<40 wt%), but this density difference decreased with increasing water content. The pair correlation function analysis shows that VP is more hydrophilic than HEMA and that the random sequence hydrogel is solvated more than the blocky sequence hydrogel at low water content, which disappears with increasing water content. Correspondingly, the water structure is more disrupted by the random sequence hydrogel at low water content but eventually develops the expected bulk water-like structure with increasing water content. From mechanical deformation simulations, stress–strain analysis showed that the VP is found to relax more efficiently, especially in the blocky sequence, so that the blocky sequence hydrogel shows less stress levels compared to the random sequence hydrogel. As the water content increases, the stress level becomes identical for both sequences. The elastic moduli of the hydrogels calculated from the constant strain energy minimization show the same trend with the stress–strain analysis.