Dynamic Mechanical Conditioning of Collagen-Gel Blood Vessel Constructs Induces Remodeling In Vitro

Dynamic mechanical conditioning is investigated as a means of improving the mechanical properties of tissue-engineered blood vessel constructs composed of living cells embedded in a collagen-gel scaffold. This approach attempts to elicit a unique response from the embedded cells so as to reorganize their surrounding matrix, thus improving the overall mechanical stability of the constructs. Mechanical conditioning, in the form of cyclic strain, was applied to the tubular constructs at a frequency of 1 Hz for 4 and 8 days. The response to conditioning thus evinced involved increased contraction and mechanical strength, as compared to statically cultured controls. Significant increases in ultimate stress and material modulus were seen over an 8 day culture period. Accompanying morphological changes showed increased circumferential orientation in response to the cyclic stimulus. We conclude that dynamic mechanical conditioning during tissue culture leads to an improvement in the properties of tissue-engineered blood vessel constructs in terms of mechanical strength and histological organization. This concept, in conjunction with a proper biochemical environment, could present a better model for engineering vascular constructs. © 2000 Biomedical Engineering Society. PAC00: 8719Rr, 8714Ee, 8718-h, 8768+z     

Simultaneous electrical and mechanical recording in small isolated perfused arteries

A new method is described for simultaneous measurements of electrical and mechanical activity in an intact segment of a small artery (outside diameter 0·3–0·5 mm). A 7–9 mm segment of rabbit middle-cerebral artery was ligated on a Teflon tube connected to a perfusion circuit. The presence of a plug in the tube, flanked by two orifices, forced the physiological solution to flow in the annular space between the tube and the artery wall. Physiological pressures could thus be attained at low rates of flow, and the pressure, measured upstream, was significantly modifed by the slightest constriction or dilatation of the vascular segment. Electrical recording with glass microelectrodes was performed on a short immobilised portion of the artery. The artery and tube were bathed in a physiological solution at 38°C, but perfusing and incubating solutions did not mix. Spontaneous electrical and mechanical activity of middle cerebral arteries is described, together with modifications induced by vasoactive agents.     

Drawn gelatin films with improved mechanical properties

Chain anisotropic distribution in gelatin films has been obtained by uniaxial stretching at constant relative humidity, followed by air drying and successive cross-linking with glutaraldehyde. The drawn samples have been characterized by mechanical tests, differential scanning calorimetry and scanning electron microscopy. The Young’s modulus, E, and the stress at break, σ_b, increase linearly with the draw ratio and reach values which are about five times those characteristic of undrawn samples. Furthermore, on stretching the alignment of the gelatin strands along the direction of deformation increases while the thickness of the layers decreases significantly. The renaturation level, that is the fraction of gelatin in a collagen-like structure, has been calculated as the ratio between the melting enthalpy of gelatin samples and that of tendon collagen. The results indicate that the improvement of mechanical properties achieved by drawn gelatin is closely related to the renaturation level. The experimental approach utilized to induce segmental orientation in gelatin films, allows to obtain anisotropic materials with improved mechanical properties in the direction of deformation, and can be usefully applied in the preparation of biomaterials.     

Effects of local nerve cooling on conduction in vagal fibres shed light upon respiratory reflexes in the rabbit

In ten vagus nerves the effect of local cooling on the compound action potential was studied in the temperature range of 34 to 0 °C in spontaneously breathing, anaesthetized rabbits. The mean temperature at which the myelinated (A) fibres were completely blocked, was 10.2±2.4 °C (mean ± S.D.). In nine nerves, local vagus cooling to 0 °C failed to block all non-myelinated (C) fibres. In one nerve, total blocking occurred at 2.0 °C. We conclude that in the rabbit, the earlier found increase in tonic activity of the diaphragm following lung inflation or deflation during bilateral local vagus cooling to a temperature between 8 and 0 °C is due to afferent impulses in vagal C fibres.     

Efficiency of work performance and contraction velocity in isotonic tetani of frog sartorius

Mechanical work, ATP, ADP, PC, free creatine and lactate concentrations were determined on IAA poisoned frog sartorii tetanically stimulated in humidified N, at 10°C in isotonic conditions (0.25 or 0.45 P₀). Tetanus duration was 0.35 s, number of tetani was varied from 0 (rest) to 25 (exhaustion). The mechanical work performed per mole ATP+PC split (W _P ^* ) amounted on the average to 16.7 kJ/mol. It was observed, however, that w _p ^* increased from about 13 to about 24 kJ/mol with decreasing ATP concentration from about 2 (resting value) to about 1 mol/g and that this decrease in ATP was associated with a decrease of the shortening (and relaxation) speed of the muscle to about 30% of the values observed on the first tetanus. It is concluded that the thermodynamic efficiency of muscle contraction, calculated from the ratio of w _P ^* (measured) to the thermodynamic affinity (free energy change) of ATP hydrolysis (estimated) increases from about 0.3 to about 0.5 with decreasing ATP concentration and shortening speed.This work was supported by the Fonds National Suisse de la Recherche Scientifique, Grants 3.332.78 and 3.364.0.82     

电化学共沉积制备有机高聚物/钙磷复合陶瓷膜层--Ⅱ XPS、SIMS表征及力学性能研究

通过电化学共沉积方法制备具有生物活性的有机高聚物／钙磷陶瓷复合膜层。用XPS、SIMS等对复合膜层的化学组分进行表征，证明少量有机高聚物可能在分子层次上掺杂形成有机高聚物／羟基磷灰石复合膜层。对电沉积HAP陶瓷膜层进行微刮痕实验表明，陶瓷膜层与金属基体的结合力得到显著改善。     

Mechanical Characterization of an In Vitro Device Designed to Quantitatively Injure Living Brain Tissue

Due to the nonlinear, viscoelastic material properties of brain, its mechanical response is dependent upon its total strain history. Therefore, a low strain rate, large strain will likely produce a tissue injury unique from that due to a high strain rate, moderate strain. Due to a lack of current understanding of specific in vivo physiological injury mechanisms, a priori assumptions cannot be made that a low strain rate injury induced by currently employed in vitro injury devices is representative of clinical, nonimpact, inertial head injuries. In the present study, an in vitro system capable of mechanically injuring cultured tissue at high strain rates was designed and characterized. The design of the device was based upon existing systems in which a clamped membrane, on which cells have been cultured, is deformed. However, the present system incorporates three substantial improvements: (1) noncontact measurement of the membrane deflection during injury; (2) precise and independent control over several characteristics of the deflection; and (3) generation of mechanical insults over a wide range of strains (up to 0.65) and strain rates (up to 15s^–1). Such a system will be valuable in the elucidation of the mechanisms of mechanical trauma and determination of injury tolerance criteria on a cellular level utilizing appropriate mechanical injury parameters.     

工程化肌腱修复肌腱缺损后力学特性的组织学基础

探讨组织工程化肌腱修复肌腱缺损后体内愈合过程中力学特性的组织学基础。取罗曼鸡肌腱细胞 ,经体外培养、扩增 ,与可降解生物材料聚羟基乙酸筛网构建工程化肌腱 ;将其植入修复 2 0只罗曼鸡第二趾深屈肌腱0 .5～ 0 .8cm缺损。术后第 2、4、6、8周取材 ,对标本进行大体、组织学及生物力学测定。植入 2、4、6、8周 ,新生肌腱在大体形态、细胞及胶原纤维排列方式上与正常肌腱相似 ,但新生肌腱的胶原纤维束并未形成较多的沿肌腱长度方向的致密结构 (“塑形”) ,导致其最大张力增加缓慢 ,到 8周时为 15 .4 0± 10 .6 3N,仅达正常肌腱的 2 3 ;8周时最大张应变为 2 2 .4 9± 10 .2 1 ,比正常肌腱大 10。结果表明 ,单纯聚羟基乙酸作支架 ,材料降解过快 ,新生肌腱失去了正常的力学刺激 ,“塑形”能力差 ,其生物力学强度低。提示 ,保持新生肌腱形成过程中正常的力学刺激对新生肌腱的“塑形”可能是至关重要的。     

脊髓型颈椎病对周围神经传导的影响

目的：探讨颈脊髓损伤对周围神经功能的影响。方法：对３２例脊髓型颈椎病患者分别于术前、术后１～５天及３个月三次检测周围神经传导速度。结果：术前下肢远段感觉神经传导速度和波幅异常者占８１％，于术后３个月内复查发现已明显恢复。结论：颈脊髓损伤对下肢远段感觉神经传导功能有影响     

Adaptation to destabilizing dynamics by means of muscle cocontraction

Adaptive control of wrist mechanics was investigated by means of destabilizing dynamics created by a torque motor. Subjects performed a 20 degrees movement to a 3 degrees target under the constraint that no motion should occur outside of the target zone once 800 ms had elapsed from movement onset. This constraint served as the minimum acceptable level of postural stability. The ability of subjects to modify their muscle activation patterns in order to successfully achieve this stability was investigated by creating three types of destabilizing dynamics with markedly different features: negative stiffness, negative damping, and square-wave vibration. Subjects performed sets of trials with the first type of destabilizing dynamics and were then required to adapt to the second and third. The adaptive response was quantified in terms of the rms electromyographic (EMG) activity recorded during various phases of the task. Surface EMG activity was recorded from three muscles contributing to wrist flexion and three muscles contributing to wrist extension. With negative stiffness, a significant compensatory increase in cocontraction of wrist flexor and extensor muscles was observed for slow movements, but there was little change in the muscle activity for rapid movements. With negative damping, muscle cocontraction was elevated to stabilize rapid movements, declining only gradually after the target was reached. For slow movements, cocontraction occurred only when negative damping was high. The response to square-wave vibration (10 Hz, +/-0.5 Nm), beginning at movement onset, was similar to that of negative damping, in that it resulted in elevated cocontraction. However, because the vibration persisted after the target was reached, there was no subsequent decrease in muscle activity. When the frequency was reduced to 5.5 Hz, but with the same torque impulse, cocontraction increased. This is consistent with greater mechanical instability. In summary, agonist-antagonist cocontraction was adapted to the stability of the task. This generally resulted in less of a change in muscle activity during the movement phase, when the task was performed quickly compared with slowly. On the other hand, the change in muscle activity during stabilization depended more on the nature of the instability than the movement speed.