Biomicrofluidics. 2011 Mar 17;5(1):14108. doi: 10.1063/1.3567888.
In this study, we show the importance of extensional rheology, in addition to the shear rheology, in the choice of blood analog solutions intended to be used in vitro for mimicking the microcirculatory system. For this purpose, we compare the flow of a Newtonian fluid and two well-established viscoelastic blood analog polymer solutions through microfluidic channels containing both hyperbolic and abrupt contractions∕expansions. The hyperbolic shape was selected in order to impose a nearly constant strain rate at the centerline of the microchannels and achieve a quasihomogeneous and strong extensional flow often found in features of the human microcirculatory system such as stenoses. The two blood analog fluids used are aqueous solutions of a polyacrylamide (125 ppm w∕w) and of a xanthan gum (500 ppm w∕w), which were characterized rheologically in steady-shear flow using a rotational rheometer and in extension using a capillary breakup extensional rheometer (CaBER). Both blood analogs exhibit a shear-thinning behavior similar to that of whole human blood, but their relaxation times, obtained from CaBER experiments, are substantially different (by one order of magnitude). Visualizations of the flow patterns using streak photography, measurements of the velocity field using microparticle image velocimetry, and pressure-drop measurements were carried out experimentally for a wide range of flow rates. The experimental results were also compared with the numerical simulations of the flow of a Newtonian fluid and a generalized Newtonian fluid with shear-thinning behavior. Our results show that the flow patterns of the two blood analog solutions are considerably different, despite their similar shear rheology. Furthermore, we demonstrate that the elastic properties of the fluid have a major impact on the flow characteristics, with the polyacrylamide solution exhibiting a much stronger elastic character. As such, these properties must be taken into account in the choice or development of analog fluids that are adequate to replicate blood behavior at the microscale.
在这项研究中，我们展示了在选择用于体外模拟微循环系统的血液类似物溶液时，除了剪切流变学之外，拉伸流变学的重要性。为此，我们比较了牛顿流体和两种经过充分验证的粘弹性血液类似物聚合物溶液通过含有双曲和突然收缩/扩张的微流道的流动。选择双曲形状是为了在微通道的中心线处施加几乎恒定的应变速率，并实现在人类微循环系统的特征中经常发现的准均匀且强的拉伸流动，例如狭窄。使用的两种血液类似物流体是聚丙烯酰胺（125ppm w∕w）和黄原胶（500ppm w∕w）的水溶液，它们在稳态剪切流中使用旋转流变仪和在拉伸流中使用毛细管束断裂拉伸流变仪（CaBER）进行了流变学特性表征。两种血液类似物都表现出与全人类血液相似的剪切稀化行为，但它们的弛豫时间（从 CaBER 实验中获得）却大不相同（相差一个数量级）。使用条纹摄影术可视化流动模式，使用微粒子图像测速法测量速度场，并在广泛的流速范围内进行压降测量，进行了实验测量。实验结果还与牛顿流体和具有剪切稀化行为的广义牛顿流体的流动数值模拟进行了比较。我们的结果表明，尽管两种血液类似物溶液具有相似的剪切流变学，但它们的流动模式却有很大的不同。此外，我们证明了流体的弹性特性对流动特性有重大影响，聚丙烯酰胺溶液表现出更强的弹性特征。因此，在选择或开发适合在微观尺度上复制血液行为的类似物流体时，必须考虑这些特性。
