A Novel Micro-Volume Viscosity Detection Device And Its Cytobiological Applications

Viscosity measurement is essential in biomedicine,chemical processes,materials and other fields.For example,both basic and clinical research have shown that the viscosity of body fluid is related to many diseases: blood viscosity of cardiovascular patients are significantly changed,synovial fluids viscosity are increased in degenerative joint disease and rheumatoid arthritis patients.Since it is difficult and expensive to collect several milliliters of the biological samples,to minimize the sample consumption is important.Furthermore,viscosity influences the adhesion behavior of cancer cell.Viscosity is a key factor on particles’ settling rating in dispersion systems.For multiphase fluid,settling occurs when the density of the particles is greater than that of dispersion medium.According to Stokes law,higher viscosity leads to lower settling rate and the more stable the particles in the dispersion system.Simplifying the adhesion behavior of cancer cells onto vascular wall as a “particle settlement – contact with tube wall-adhesion” process,the influence of viscosity is significant and therefore worth to be investigated.On the other hand,previous study has shown that the microfluidic devices has been widely used,owing to the miniaturization trend seen in the biotechnology,manufacturing and chemical processing industries.When controlling the flow of fluid at length scales of less than a millimeter,the dynamic properties of the fluid and fluid response should be considered.Viscosity is an essential parameter in the analysis of rheological behavior.A viscometer with low material consumption in an easy and efficient way,to meet the miniaturization trend,is required.Nevertheless,there are some challges in the microvolumelization study:(1)as the characteristic length-scale of the flow geometry approaches that of the fluid microstructure,physical confinement can alter the dynamical evolution of the microstructure and must be taken into account,rheological response may not be identical to the macroscopic counterparts.At such scales,the geometric dimension,interfacial tension,end effect can all begin to interact,and lead to remarkable error.(5%~24%)(2)microfluidic devices involving syringe pumps and connecting tubing typically require larger volumes in the range 100μl–10 ml in order to fill the system and establish steady flow conditions.3)As the scale decreases,disturbance influence caused by roughness of the viscometer surface could be dominant.Increasing requirement of surface roughness increases the manufactural cost.4)Measurement at decreased scale is extra sensitive to atmosphere temperature,since the total thermal capacity of whole system is low.Current viscometer does not yet have variation to solve such issue.Based on the above mentioned reasons,current microfluidic viscometers have not been successfully commercialized.Goal of this research is therefore set as to create such accurate,stable and mircovolumelization solution.We present a new accurate,efficient microfluidic viscometer to measure the viscosity of biological fluids.The geometric configurations of the micro-chip have been optimized by considering the end effect,surface tension and kinetic energy correction term.Both ends of the channel are joined by a symmetric microtube,which almost completely avoided the influence of surface tension;dimension of chip is designed following error assessment and the error is controlled below 0.5%;only 200μl sample is needed for each measurement.Methodology combine with microwire-molding and corresponding mold designed by the researcher,has been proved to decrease the roughness of channel inner surface.In order to ensure the constant temperature measurement environment,a special water bath has been designed.During the test,the micro-chip is placed in the water bath.The sample was loaded into the upstream reservoir and was driven by an external pressure control device.Then it passed through the main channel and flowed into the downstream reservoir.Flowing process is protocoled by CCD and transferred to numeric output by image processing program.Final viscosity value is automatically calculated.Operation is therefore simplified and error caused by human factors is reduced.Result of such method is highly repeatable.The microchip viscometer offers higher accurate of measurements on samples with low viscosity than other microfludic viscometers: result of our microchip viscometer were compared with the reference value given by the National Institute of Standards and Technology(NIST)and to data obtained from a traditional glass capillary viscometer and rotational rheometer.It shows a good agreement between our microchip viscomter,the traditional capillary viscometer and standard value,with an average relative error of less than 2%.The expanded relative standard uncertainty U95 =2.2%,according to the “Guide to the expression of uncertainty in measurements(GUM)”(published by International Organization for Standardization and International Electrotechnical Commission).The accuracy is 2 times higher than traditional viscometers.After build-up of the micro-chip viscometer prototype,an idealized adhesion model of HepG2 cells was designed and constructed to simulate the adhesion of tumor cells in human vascular environment.In this model,influences of viscosity and flow field on cancer cell adhesion behavior have been studied.Firstly,the finite element analysis has been performed to predict the flow field and distribution of turbulence in the chip.Then 3 groups of tests have been designed to investigate influences of viscosity,cell density and their interactions with fluid field on the adhesion amount of HepG2 cells.The test results have been compared with simulation.Result shows that larger bifurcation angle of vessel leads to higher turbulence intensity,which also increases the amount of adhesion HepG2 cells,but limited by capacity of the microchips.Therefore,the amount of adhesion cells become stable when turbulence achieves certain threshold.Increasing cell density means increasing the likelihood of contact with microchip surface and thus also leads to higher amount of adhesion.This effect is extremely significant at lower turbulence level.Influence of viscosity can be divided by 2 parts: On one hand,increasing viscosity enhances flow resistance and decreases moving velocity of HepG2 cells,which increases the likelihood of adhesion of the cells to the bottom.On the other hand,increasing viscosity decreases the settling velocity of HepG2,and more of HepG2 cells are suspended in the solution without contact with the bottom.Under weak turbulence condition,these two influences counteract with each other and have little effect on final adhesion amount.Under strong turbulence condition,the latter effect is dominant.This result consists with previous simulation.The microchip viscometer presented by this research is easier and cheaper to produce,cost-effective,simple to operate and maintenance.The chip was fabricated using a novel microwire-molding technique,and required no expensive equipment.Based on the microwire-molding technique,all the components(connection,microtubes,capillary)are integrated into one chip,which avoids the possibility of poor sealing at the junction of the components and extra lossing of the pressure drop.Every single microchip can be disposed of,avoiding the trivial cleaning process.Also,it can be reused after ultrasonic cleaning for cost saving.Because of the low material consumption and sensitivity to low viscosity fluids,the microchip viscometer provide a new approach to measure the biological fluids with small sample volume.And it can offer real parameters for a flow simulation model,which is a useful way to analyze and predict the shear stress in the channel of a microfluidic device.It could have applications in medicine(e.g.,viscosity of bodily fluids),chemical processes(e.g.,polymer solutions),biology(e.g.,cell suspensions,media)and other fields.