冷冻保护剂对神经干细胞球导入过程的模拟分析

建立了以刚性球孔隙模型为基础的神经干细胞球的冷冻保护剂导入模型.在结合跨膜传质扩散的基础上,分别对细胞内空间和细胞外空间列传质扩散方程,并通过计算,得出了不同条件下细胞内外浓度以及细胞膜两侧浓差随时间和空间的变化曲线.从结果中可以看出,细胞外的保护剂浓度变化较快,在第5 s即可接近渗透平衡;细胞内的保护剂浓度变化则较为平缓.保护剂扩散系数对于导入过程影响不大,增加神经球尺寸及溶质胞膜渗透系数可以有效地减少导入过程中的细胞浓差.低温导入过程中细胞浓差较常温导入时更大,对细胞的渗透性损伤也更为强烈.研究结果对优化冷冻保护剂的导入程序有重要的指导意义.     

多组分冷冻保护剂导入神经干细胞球的传质模拟

基于Maxwell-Stefan方程和两参数模型构建了神经干细胞球多组分保护剂导入模型,并模拟了多组分保护剂导入过程中细胞内外浓度及细胞膜两侧浓差的时空分布。在此基础上系统分析了多组分浓度配比、神经球尺寸、保护剂导入方式对传质渗透过程的影响。结果表明,多组分保护剂导入神经球过程中出现"逆梯度扩散"现象,胞内浓度变化总是滞后于胞外浓度变化,细胞膜两侧浓差呈现先增大后减少的趋势。改变保护剂组分的浓度配比时,不同组分的渗透特性变化规律不同。此外,神经球尺寸对中心处细胞的浓度时空分布影响显著,分步导入和连续导入能够减缓保护剂扩散,降低细胞内外浓差。本研究结果可用于指导神经球冷冻保存的实验研究,优化保护剂导入程序。     

钒离子浓度对全钒液流电池极化分布的影响

基于物质传递方程、电荷传递方程和电化学动力学方程提出了浓差极化系数的概念,建立了全钒液流电池二维模型,利用有限元法研究了钒电池极化过程,并对电极区域极化的分布进行了定量评估。研究表明:增加钒离子浓度,电极表面和溶液本体浓度趋同,活化极化和浓差极化减小,这种现象在高电流密度下尤为明显;浓度的增加也使得浓差极化系数减小,物质传递的影响减小,浓差极化相对于整个极化影响越来越弱;从进口到出口,浓差极化系数逐渐增加;电极与集流体接触面的浓差极化系数比电极中间区域高得多。     

细粘物料分离新技术实验研究

中、低品位矿的磷酸料浆为细粘物料 ,其过滤性能较差。为提高过滤强度 ,将传统料浆过滤的混和加料改为分步加料 ,形成浓差过滤。浓差过滤改变了料浆滤饼的形成机理 ,改善了磷酸料浆的过滤性能 ,降低了过滤阻力 ,强化了过滤操作条件 ,使过滤强度比传统过滤显著提高。在采用浓差过滤时 ,比较合适的操作规程参数是 :液固比 2 .5～ 3.0 ,沉降时间不低于 2 0s,分步加料浓、稀浆体积比 5 5 ,操作温差不低于 70℃。     

正渗透膜分离的研究进展

正渗透是浓度驱动的膜技术,是指水通过选择性渗透膜从高水化学势区域向低水化学势区域的传递过程。本文介绍了正渗透的基本构成（驱动力、汲取液和正渗透膜材料）,指出膜两侧的浓差极化是水通量性能的最大障碍,采用通量模型说明了膜在两种放置方向下存在的内浓差极化和外浓差极化,内浓差极化对驱动力的减小起着重要的作用;论述了膜材料、原料液浓度、汲取液浓度对正渗透和压力延迟渗透水通量的影响;此外,评述了正渗透过程的膜污染和能耗。     

硝化菌发酵浓缩液保存条件及衰减因素分析

硝化菌是一类自养化能细菌,其对保存条件要求严格,硝化菌发酵浓缩液难以高效长时间储存。以硝化菌发酵浓缩液为研究对象,考察了保存温度、浓缩液中硝酸根浓度、添加保护剂等对保存时间的影响,探究最佳保存条件。结果表明,最佳保存温度为4℃,浓缩液中硝酸根浓度越低越有利于硝化菌的保存,添加保护剂可以延长硝化菌的保存时间;4℃下添加保护剂可以使硝化菌的保存时间延长到56 d以上。     

悬浮和贴壁成骨细胞渗透压耐受极限及膜渗透性质的研究

在优化组织工程骨低温保存程序时,需要了解体外培养的种子细胞成骨细胞的渗透特性.尤其对于高浓度的玻璃化保护剂,更有必要设计合适的保护剂导入、洗脱方案来减小高浓度保护剂的渗透效应损伤及其化学毒性作用.利用显微镜图像分析法研究了用于构建组织工程骨的种子细胞—-成骨细胞的渗透特性,其中包括细胞的等渗体积、细胞在高渗及低渗溶液中...     

聚合物载药微球活性组分控释性能的动力学模拟

将聚合物载药微球的溶解扩散模型与药代动力学模型相结合,计算了载药微球的药物控释性能的浓度时间曲线.采用叠加法计算了多次投药所产生的稳态药物浓度特征,探讨了释放速率、给药时间间隔对药物浓度曲线的影响.相对于口服药物溶液,微球中的药物通过载体的控制释放,血药浓度峰值有很大的降低,多次给药时载药微球的药物浓度波动也有大幅度的减小;适当的溶解速率可减小药物浓度波动,同时也满足治疗所要求的浓度值.     

铜基片静态气液界面腐蚀规律的实验

气液两相界面腐蚀广泛存在于日常工业中。因气液相界面存在不稳定特性,迄今对于相界面处腐蚀的研究多针对不断变化的界面位置附近,分析一段时间内的累积腐蚀效果。本文向液相内注入一定体积的气泡,维持气液界面在金属铜壁面的稳定以及气液相界面和液相主体浓度近似恒定;在含不同Cl^-浓度的溶液中,通过电化学方法对比有无相界面的腐蚀规律,并根据数据深入分析相界面处的腐蚀机理。结果表明：引入气液界面导致铜试样腐蚀电位升高、腐蚀电流增大,腐蚀更严重;气液界面处由于引入微观界面能差腐蚀、宏观氧浓差电池腐蚀的作用显著增大了试样的腐蚀电流,使得在整个溶液浓度范围内含界面的铜腐蚀电流均高于不含界面的腐蚀电流,同时在铜表面留下清晰的界面腐蚀线;通过对界面腐蚀电流的剥离,包含液相主体腐蚀部分时无论有无界面的铜腐蚀速率均随Cl^-浓度的升高呈现先增大后减小的趋势,而去除液相部分影响的单独界面腐蚀电流则随Cl^-浓度升高而增大。     

膜厚度和流动状况对渗透蒸发过程的浓差极化影响

浓差极化对渗透蒸发过程的渗透速率和分离因子有一定负面影响。文中利用渗透蒸发过程中的分步阻力传质模型分析浓差极化,以含乙醇质量分数2%的乙醇/水为进料体系,观察进料流速对渗透蒸发过程中分离因子和通量的影响。分别估算了边界层阻力和膜阻力以及流速对边界层阻力的影响,并考察了膜厚度对浓差极化和分离因子的影响。     

果蔬渗透脱水过程动力学研究

结合植物组织结构与流体传输过程机理建立了渗透脱水过程的一维质量传递数学模型。模型以植物细胞为传输过程的基本单元，考虑了各组分在细胞内、细胞外、通过细胞膜及胞间连丝的质量扩散，和由于体积收缩而导致的集流传输。以土豆为实验物料，在40℃恒温条件下，采用40％（质量百分比）的蔗糖溶液作为渗透液，进行渗透脱水实验，得到的实验结果与模拟结果十分接近，验证了模型的有效性。通过数值模拟可详细描述渗透脱水过程中土豆细胞内外水和蔗糖的质量浓度分布。并就能源与生产效率方面对“渗透-干燥”与“无预处理干燥”过程作了比较。     

一种模拟气液相变过程的相变模型

提出了一种基于VOF（volume of fluid）方法的相变模型，用于计算气液相变过程的控制方程中的传热传质源项。在单位时间步长上相界面附近发生的传热以瞬态热扩散来考虑，并假设该传热导致了相变的发生。相变模型能够通过相界面单元的温度、流体热物性以及时间步长来计算传热传质源项。通过一维Stefan问题和二维水平膜沸腾问题对该相变模型进行验证，对比了相界面位置以及温度分布，结果与理论解吻合良好。进一步探讨了相变模型中的时间步长对计算精度的影响。结果表明时间步长越小，本相变模型模拟得到的结果与理论解的偏差越小。     

VOLTAGE-MEDIATED CONFORMATIONAL CHANGES OF A POLYELECTROLYTE IN A LINEAR ELASTIC MEMBRANE

The wall of the cylindrically-shaped outer hair cell is piezoelectric, which allows the cell to function as an electromechanical transducer in the mammalian inner ear. Polyelectrolytes, which are located in the cell wall, may be responsible for the cell's piezoelectric properties. The polyelectrolytes can change conformation in response to a change in the external electric field; this conformational change can cause the cell to change its length. We have developed a model to predict the voltage response of a cylindrical cell whose wall contains conformationally-mobile polyelectrolytes. The cell wall is modeled as a rectangular lattice of polyelectrolytes connected by springs. The springs represent the (non-piezoelectric) elastic portion of the cell wall. The polyelectrolytes can exist in one of two possible conformations. The energy of the polyelectrolyte is a function of both the trans-wall electric potential difference and the forces which are applied by the surrounding elastic material in the wall. The fraction of polyelectrolytes in a given conformation is determined by the relative energies of the two conformations, by means of a Boltzmann distribution. If the electric potential difference and the pressure difference between the inside and outside of the cell are known, the model can predict the cell's radius, the length, the fraction of polyelectrolytes in a given conformation, and the electrical capacitance of the cell wall (due to the polyelectrolyte activity). We also propose an experimental scheme which would provide data that could be used to evaluate the parameters in our model. Our simulations predict that the electrical capacitance vs. voltage function should exhibit a peak which simply shifts its location (without a significant change in height) when the intracellular pressure is changed. In addition, we demonstrate that electrical stimulation of the cell, under conditions of constant cell volume, could result in a significant change in the intracellular pressure.     

Roles of Enteric Neural Stem Cell Niche and Enteric Nervous System Development in Hirschsprung Disease

The development of the enteric nervous system (ENS) is highly modulated by the synchronized interaction between the enteric neural crest cells (ENCCs) and the neural stem cell niche comprising the gut microenvironment. Genetic defects dysregulating the cellular behaviour(s) of the ENCCs result in incomplete innervation and hence ENS dysfunction. Hirschsprung disease (HSCR) is a rare complex neurocristopathy in which the enteric neural crest-derived cells fail to colonize the distal colon. In addition to ENS defects, increasing evidence suggests that HSCR patients may have intrinsic defects in the niche impairing the extracellular matrix (ECM)-cell interaction and/or dysregulating the cellular niche factors necessary for controlling stem cell behaviour. The niche defects in patients may compromise the regenerative capacity of the stem cell-based therapy and advocate for drug- and niche-based therapies as complementary therapeutic strategies to alleviate/enhance niche-cell interaction. Here, we provide a summary of the current understandings of the role of the enteric neural stem cell niche in modulating the development of the ENS and in the pathogenesis of HSCR. Deciphering the contribution of the niche to HSCR may provide important implications to the development of regenerative medicine for HSCR.     

Dimethyl Sulfoxide-Free Cryopreservation of Human Umbilical Cord Mesenchymal Stem Cells Based on Zwitterionic Betaine and Electroporation

The effective cryopreservation of mesenchymal stem cells (MSCs) is indispensable to the operation of basic research and clinical transplantation. The prevalent protocols for MSC cryopreservation utilize dimethyl sulfoxide (DMSO), which is easily permeable and able to protect MSCs from cryo-injuries, as a primary cryoprotectant (CPA). However, its intrinsic toxicity and adverse effects on cell function remain the bottleneck of MSC cryopreservation. In this work, we cryopreserved human umbilical cord mesenchymal stem cells (UCMSCs) using zwitterionic betaine combined with electroporation without any addition of DMSO. Betaine was characterized by excellent compatibility and cryoprotective properties to depress the freezing point of pure water and balance the cellular osmotic stress. Electroporation was introduced to achieve intracellular delivery of betaine, intending to further provide comprehensive cryoprotection on UCMSCs. Compared with DMSO cryopreservation, UCMSCs recovered from the protocol we developed maintained the normal viability and functions and reduced the level of reactive oxygen species (ROS) that are harmful to cell metabolism. Moreover, the in vivo distribution of thawed UCMSCs was consistent with that of fresh cells monitored by a bioluminescence imaging (BLI) system. This work opens a new window of opportunity for DMSO-free MSC cryopreservation using zwitterionic compounds like betaine combined with electroporation.     

Diffusion MR in Biological Systems: Tissue Compartments and Exchange

The processes of diffusion in multi-compartmental tissue models are discussed using an analytical model of diffusion with exchange. Diffusion in the intracellular compartment has been found to be highly restricted, with the apparent diffusion coefficient (ADC₁) dependent on the experimental parameters. Extracellular diffusion mediated by tortuosity, depends on tissue geometry, but not on the experimental conditions. Both the intra- and extracellular diffusion can be anisotropic and exhibit non-monoexponential behavior. The cell membrane permeability is shown to significantly influence the diffusion measurement in tissues. In consequence, the phenomenological, bi-exponential models of diffusion cannot directly estimate the intrinsic characteristics of intra- and extracellular water. The mechanisms of the ADC decrease after stroke are also discussed.     

Master curves for mass transfer in bidisperse adsorbents for pressure‐swing and volume‐swing frequency response methods

A general model has been developed for pressure‐swing and volume‐swing frequency response methods with analytical solutions derived to analyze various mass transfer resistances in biporous adsorbents. The model can consider a combination of distributed mass transfer resistances acting independently or in series including macropore diffusion, micropore diffusion, and a surface barrier resistance at the entrance of micropores. Heat effects are included for nonisothermal systems. Temperature variations are shown to be less for pressure‐swing compared with volume‐swing systems. The general model is developed in such a way that information extracted from it is independent of whether a system is batch volume swing or flow‐through pressure swing. Simulation results demonstrate that mass transfer mechanisms can be easily discriminated by the master curves. Isothermal models exhibit one‐step curves, whereas nonisothermal effects are represented by two‐step curves. Examples are given to demonstrate the use of the master curves to describe experimental data. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

PERFORMANCE OF A MULTISTAGE FERMENTOR WITH CELL RECYCLE FOR CONTINUOUS ETHANOL PRODUCTION

The unsteady and steady-state performance of a multistage fermentor with cell recycle is investigated numerically for continuous production of ethanol. The tanks-in-series model is employed to describe the flow behaviour of culture in multiple tanks. In establishing mass balances, the cell volume fraction and the equilibrium partition coefficient for intracellular and extracellular ethanol concentrations are taken into account. Kinetic expressions are taken from the literature. Compared to a single tank fermentor with recycle, multi-staging and cell recycle are shown to bring high cell and product concentrations and low substrate concentration in the final tank during unsteady-state operation. Solutions to the steady state model are presented graphically, showing concentrations as a function of dilution rate for different numbers of tanks. Results are given for the maximum ethanol productivity as a function of the optimal dilution rate, with parameters of tank number, bleed ratio and recycle ratio. Also, the effects of the fractional tank volume and the saturation constant on the productivity and the substrate conversion are examined.     

Effectiveness factor for spatial gradient sensing in living cells

We consider the steady-state pattern of messenger molecules produced in the membrane of a cell perceiving and responding to an extracellular gradient of chemoattractant, which directs cell movement towards the chemoattractant source. Specifically, we analyze the undesirable effect of lateral diffusion in blurring the intracellular messenger profile. The concept of an effectiveness factor, akin to the analysis of reactions in porous catalysts, is applied to the spatial gradient sensing problem, with the distinction that slow, not fast, diffusion is required for effective gradient sensing. Analytical effectiveness factor expressions are derived for ideal geometries and then generalized to arbitrary cell shapes. In the case of mouse fibroblasts responding to gradients of platelet-derived growth factor, we conclude that the cell morphology and orientation with respect to the gradient can dictate whether messenger diffusion obliterates gradient sensing or has very little effect. The analysis outlined here allows the effect of intracellular messenger diffusion on spatial gradient sensing to be quantified for individual cells.