Time-Dependent Properties of Human Umbilical Fascia

This investigation is devoted to the study of the viscoelastic behavior of human abdominal fascia from the umbilical region. Seventeen samples 10 mm wide and up to 70 mm long were cut along the primary fiber direction (group FL) or perpendicular to it (group FT) and subjected to relaxation tests. The viscoelastic response of the tissue at three different strain levels (4%, 5%, and 6%) was investigated. The relaxation curves were fitted using a two-stage decaying exponential form. The following parameters were determined: initial stress σ₀, relaxation times τ₁ and τ₂, stress reduction Δσ, initial relaxation modulus E and equilibrium relaxation modulus E_eq, as well as the ratio E/E_eq. Fiber orientation and strain levels were varied to determine their influence on the viscoelastic properties of fascia. The results highlight the inherent viscoelastic mechanical properties of umbilical fascia. The values of the viscoelastic parameters determined for the longitudinal and transverse directions varied markedly. Significant differences were found between the two groups FL and FT for the initial stress at 5% and 6% strain (p < 0.038) and for the initial and equilibrium moduli at the 6% strain level (p < 0.046). The stress reduction in samples from the FL group (45–55%) was less than that in samples from the FT group (37–54%), but this difference was not significant (p > 0.388). The influence of strain level on the parameter values was not statistically significant (p > 0.121). The nonlinear response of the tissue was demonstrated over the chosen strain range.     

Oligomodal metamaterials with multifunctional mechanics

Mechanical metamaterials are artificial composites that exhibit a wide range of advanced functionalities such as negative Poisson’s ratio, shape shifting, topological protection, multistability, extreme strength-to-density ratio, and enhanced energy dissipation. In particular, flexible metamaterials often harness zero-energy deformation modes. To date, such flexible metamaterials have a single property, for example, a single shape change, or are pluripotent, that is, they can have many different responses, but typically require complex actuation protocols. Here, we introduce a class of oligomodal metamaterials that encode a few distinct properties that can be selectively controlled under uniaxial compression. To demonstrate this concept, we introduce a combinatorial design space containing various families of metamaterials. These families include monomodal (i.e., with a single zero-energy deformation mode); oligomodal (i.e., with a constant number of zero-energy deformation modes); and plurimodal (i.e., with many zero-energy deformation modes), whose number increases with system size. We then confirm the multifunctional nature of oligomodal metamaterials using both boundary textures and viscoelasticity. In particular, we realize a metamaterial that has a negative (positive) Poisson’s ratio for low (high) compression rate over a finite range of strains. The ability of our oligomodal metamaterials to host multiple mechanical responses within a single structure paves the way toward multifunctional materials and devices.

Flexible metamaterials use carefully designed arrangements of deformable building blocks to achieve unusual and tunable mechanical functionalities (1). Such mechanical responses rely on on-demand deformation pathways that cost a relatively low amount of elastic energy. A useful and widely applicable paradigm for the design of such pathways leverages the limit in which their elastic energy is zero—these pathways then become mechanisms or zero-energy modes. Flexible metamaterials based on such principle are, so far, either monomodal (Fig. 1A) or plurimodal (Fig. 1C). On one hand, monomodal metamaterials feature a single zero-energy mode and a single functionality (2–8), which is typically robust and easy to control with a simple actuation protocol, that is, a protocol that requires a single actuator, for example, uniaxial compression. On the other hand, plurimodal metamaterials feature a large number of zero-energy modes, which increases with system size (9, 10). The presence of these multiple zero modes offers multiple possible functionalities in principle, but they are hard to control in practice; that is, they require complex actuation protocols—protocols that require more than one actuator—for successful execution (9). The challenge we address here is whether it is possible to find a middle ground between monomodal and plurimodal metamaterials. In other words, can we design and create metamaterials that have more than one zero-energy mode, but not a number that grows with system size? For convenience and clarity, we term such metamaterials oligomodal (Fig. 1B). Could oligomodal metamaterials be actuated in a robust fashion with a simple actuation protocol (Fig. 1B)? Could oligomodal metamaterials host distinct mechanical properties within a single structure?Open in a separate windowFig. 1.Oligomodal materials. (A) Monomodal materials have a single zero-energy mode, hence a single property, that can be obtained via a simple actuation protocol. (B) Oligomodal materials have a small but fixed number of zero-energy modes larger than one, hence a few distinct properties, that can be selected with a simple actuation protocol, for example, uniaxial compression. (C) Plurimodal materials have a large number of zero-energy modes that grows with system size, and hence are kinematically able to host a large number of properties, but they often require complex actuation protocols, for example, multiaxial loading.     

颅骨和硬脑膜随颅内压变化的粘弹性有限元分析

目的 颅内压对神经外科危重患者的手术时机起决定性作用,其测量方法的简单性、准确性、对患者较小创伤性都是神经外科临床和科研的重要研究方向.方法 提出一种通过测量颅骨应变可获得颅内压变化量的新思路,利用有限元软件ANSYS的粘弹性模型,对人体颅骨和硬脑膜随颅内压变化的相应应变进行了分析计算;分别建立了模拟颅腔的三维空心薄壳有限元模型,颅骨和硬脑膜的粘弹性模型;同时通过有限元软件分考虑与不考虑粘弹特性两种情况,对颅骨和硬脑膜随颅内压变化的应变进行对比.结果 考虑与不考虑粘弹性的颅骨和硬脑膜应变误差在14%左右,而颅骨和硬脑膜的粘性应变占总应变的40%.结论 通过在颅骨外表面粘贴应变片可以获得颅内压的变化量,微创颅内压应变电测法理论上可行,为进一步的临床应用提供了理论基础.     

Assessment of liver viscoelasticity using multifrequency MR elastography

MR elastography (MRE) allows the noninvasive assessment of the viscoelastic properties of human organs based on the organ response to oscillatory shear stress. Shear waves of a given frequency are mechanically introduced and the propagation is imaged by applying motion‐sensitive gradients. An experiment was set up that introduces multifrequency shear waves combined with broadband motion sensitization to extend the dynamic range of MRE from one given frequency to, in this study, four different frequencies. With this approach, multiple wave images corresponding to the four driving frequencies are simultaneously acquired and can be evaluated with regard to the dispersion of the complex modulus over the respective frequency. A viscoelastic model based on two shear moduli and one viscosity parameter was used to reproduce the experimental wave speed and wave damping dispersion. The technique was applied in eight healthy volunteers and eight patients with biopsy‐proven high‐grade liver fibrosis (grade 3–4). Fibrotic liver had a significantly higher (P < 0.01) viscosity (14.4 ± 6.6 Pa · s) and elastic moduli (2.91 ± 0.84 kPa; 4.83 ± 1.77 kPa) than the viscosity (7.3 ± 2.3 Pa · s) and elastic moduli (1.16 ± 0.28 kPa; 1.97 ± 0.30 kPa) of normal volunteers. Multifrequency MRE is well suited for the noninvasive differentiation of normal and fibrotic liver as it allows the measurement of rheologic material properties. Magn Reson Med 60:373–379, 2008. © 2008 Wiley‐Liss, Inc.     

A computational simulation of cyclic stretch of an individual stem cell using a nonlinear model

Many experiments have shown that mechanical stimuli like cyclic strains might be helpful in stem cell differentiation. To maximize such differentiations efficiency, it is imperative to detect the cellular mechanical responses to these stimuli. The purpose of this research was to show that a newly presented hyper‐viscoelastic model could correctly predict the level of stresses required to obtain a different response from a single mesenchymal stem cell cultured in a fibrin hydrogel block under a 10% cyclic strain at a frequency of 1 Hz, employing finite element method. One of the novelties of the research was the use of a model based on Simo's model. Another important feature of the research was the proposition of a multiscale model considering a layer of integrins. It was concluded that the forces exerted on the biological molecules had the maximum values of 24, 45, and 15 pN for the circumferential, radial, and shear forces, respectively. According to the results, the exerted forces within the cytoskeleton can lead to a different cellular response. These results might be a premise for interpreting events that lead to differentiation of stem cells into fibrochondrocytes. In addition, they can be beneficial in effective design of biological experiments as regards to this issue.     

Development and characterization of a synthetic PVC/DEHP myocardial tissue analogue material for CT imaging applications

A simple myocardial analogue material has great potential to help researchers in the creation of medical CT Imaging phantoms. This work aims to outline a Bis(2-ethylhexyl) phthalate (DEHP) plasticizer/PVC material to achieve this. DEHP-PVC was manufactured in three ratios, 75, 80, and 85% DEHP by heating at 110 °C for 10 min to promote DEHP-PVC binding followed by heating at 150 °C to melt the blend. The material was then tested utilizing FTIR, tensile testing, dynamic mechanical analysis and imaged with computed tomography. The FTIR testing finds the presence of C-CL and carbonyl bonds that demonstrate the binding required in this plasticized material. The tensile testing finds a modulus of 180–20 kPa that increases with the proportion of plasticizer. The dynamic mechanical analysis finds a linear increase in viscoelastic properties with a storage/loss modulus of 6/.5–120/18 kPa. Finally, the CT number of the material increases with higher PVC content from 55 to 144HU. The 80% DEHP-PVC ratio meets the mechanical and CT properties necessary to function as a myocardial tissue analogue.     

扩张皮肤移植后的生物力学变化

背景：应用皮肤软组织扩张器植入正常的皮肤软组织下,通过增加扩张器内的容量,对皮肤表面软组织产生压力,使其扩张产生“额外”的皮肤软组织,已成为皮肤组织工程研究领域的基本技术手段。 目的：通过狗皮肤扩张移植模型了解扩张皮肤在移植后的近期和远期的变化规律。 方法：随机抽取12只成年狗,在其背部置入150 mL的4个扩张器,其中2个不扩张进行对照。测试扩张不同时间后皮肤应力-应变、应力松弛、极限抗拉强度的结果,并对比分析测定结果。 结果与结论：经过扩张,皮肤的黏弹性、强度发生明显降低,黏弹性可在移植后6个月恢复,强度恢复较慢。在行皮肤移植后,生物力学的变化仅为暂时现象,可以恢复至正常,临床应给予关注。     

In vivo magnetic resonance elastography of human brain at 7 T and 1.5 T

Purpose:

To investigate the feasibility of quantitative in vivo ultrahigh field magnetic resonance elastography (MRE) of the human brain in a broad range of low‐frequency mechanical vibrations.

Materials and Methods:

Mechanical vibrations were coupled into the brain of a healthy volunteer using a coil‐driven actuator that either oscillated harmonically at single frequencies between 25 and 62.5 Hz or performed a superimposed motion consisting of multiple harmonics. Using a motion sensitive single‐shot spin‐echo echo planar imaging sequence shear wave displacements in the brain were measured at 1.5 and 7 T in whole‐body MR scanners. Spatially averaged complex shear moduli were calculated applying Helmholtz inversion.

Results:

Viscoelastic properties of brain tissue could be reliably determined in vivo at 1.5 and 7 T using both single‐frequency and multifrequency wave excitation. The deduced dispersion of the complex modulus was consistent within different experimental settings of this study for the measured frequency range and agreed well with literature data.

Conclusion:

MRE of the human brain is feasible at 7 T. Superposition of multiple harmonics yields consistent results as compared to standard single‐frequency based MRE. As such, MRE is a system‐independent modality for measuring the complex shear modulus of in vivo human brain in a wide dynamic range. J. Magn. Reson. Imaging 2010;32:577–583. © 2010 Wiley‐Liss, Inc.     

The Effect of Side Chain Length and Hydrogen Bonding on the Viscoelastic Property of Isobutene/Maleimide Copolymers

The viscoelastic properties of the alternating copolymers of N‐substituted maleimides with n‐alkyl and ω‐carboxy‐n‐alkyl groups as the N‐substituent with isobutene were investigated by dynamic mechanical analysis. The transition temperatures in the α‐relaxation region depended on the alkyl chain length and the presence or absence of a terminal carboxy group in the side chain. β‐Relaxation due to side chain dynamics was also observed in a lower temperature region without merging the α‐ and β‐relaxation processes for all the copolymers. The molecular motions of the semiflexible main chain and the flexible side alkyl groups were investigated as well as the intermolecular interaction by hydrogen bonding of the carboxyl groups.