| The nucleic acid extraction process based on microfluidic chip is an important link in the preparation and purification of nucleic acid detection samples,which is widely used in immunodetection,drug delivery,biosensors,and targeted therapy,etc.Magnetic particle method is a mainstream nucleic acid extraction with the advantages of high biocompatibility,large unit surface area and strong particle manipulation.As the mass transfer carriers of nucleic acid molecules,magnetic particles are repeatedly separated and mixed in the reagent under an external magnetic field.However,magnetic particles agglomerate due to the magnetic force between particles,causing a decrease in the quality of nucleic acid extraction.In the current researches,the separation and mixing of magnetic particles based on magnetic control are conducted independently,so the integral extraction process cannot be completed in a single chip.In addition,magnetic particles are used for stirring liquids in micro-mixing applications exclusively,and there are rarely solid-liquid mixing studies when magnetic particles are used as carriers for mass transfer.Therefore,the separation and mixing of magnetic particles in the same chamber are studied,and a microfluidic system is proposed to complete the whole processes of nucleic acid extraction on a single chip which integrates temperature,gas circuit and magnetic field control devices.In this study,a microfluidic chip for repeatable magnetic particles separationmixing process is developed based on the principle of nucleic acid extraction by magnetic particles.Sequential delivery of reagents is realized by pneumatic microvalve,micropump and external air source control system.The temperature control system is built to keep a constant temperature in each chamber.A mathematical model is established to analysis the motion characteristics of magnetic particle under the magnetic field and flow field.The effects of fluid flow rate,chamber height,and magnet position on the distribution of magnetic particles in the chamber with abrupt crosssection are studied through numerical simulation method.Subsequently,two methods are proposed to improve the separation characteristics of magnetic particles by suppressing the vortex,namely increasing an inlet baffle and reducing the size of the inlet.The improvement of separation characteristics in a fan-shaped chamber is verified through experiments.To solve the problem of magnetic particle agglomeration at low flow rates with a permanent magnet,the circumferentially symmetric integrated soft magnets are designed which matches the circular chamber.The magnetic field distribution model combining the integrated soft magnets and the external permanent magnet is established to verify the feasibility of suppressing the separation of magnetic particles near the inlet.Furthermore,the influence of different structural parameters of bar-shaped soft magnets and arc-shaped soft magnets on the magnetic force distribution in the chamber is analyzed,and the final structure and parameters of the combined soft magnets are established.The trajectory,motion characteristics,distribution position and separation characteristics of magnetic particles in the separation chamber are studied by numerical simulation at low inlet velocity,and the improvement of the separation characteristics in the micro chamber integrated soft magnets is verified.The mixing effect of mass transfer particles and solution in the micro chamber is significant to the quality of nucleic acid extraction.Magnetic micropillars are integrated in the chamber for solid-liquid mixing by the rotating magnetic field,and the mixing characteristics of non-magnetic particles are studied.The deformation of the magnetic micropillar with different influencing factors is analysis based on the large deformation model of the flexible beam,which is verified by the experiments.The Particle Image Velocimetry(PIV)method is used to analyze the influence of the arrangement of the magnetic micropillars and the rotation of the external magnetic field on the flow field.Through the fluorescent particle mixing,the surface powder cleaning and the particle precipitation-mixing experiments,it is verified that the integrated micropillars enhance the solid-liquid mixing effect in the micro chamber.Magnetic particles in micro chamber have agglomeration problems when mixed under external magnetic field.In this study,dual electromagnets are used to mix magnetic particles in the reaction chamber.According to the motion characteristics of the magnetic particles in different electromagnet coil current,it is verified that this structure realizes the small range and large displacement movement of the magnetic particles in a short time.Furthermore,the formation mechanism of the magnetic particle chains in the separation-mixing chamber by the permanent magnets are studied through the magnetic dipole method,which is the theoretical basis of the decomposition and reaggregation principles.The mixing effect of magnetic micropillars on the magnetic particles is verified by the experiments.Finally,the nucleic acid extraction experiments of the lactic acid bacteria are carried out and observed by the gel electrophoresis,which verify that the purity and yield of extracted nucleic acid are sufficient for the detection. |
