Magnetically Actuated Open Microfluidics On Superlyophobic Surfaces

The droplet-based digital microfluidics technology has received widespread attention due to its advantages of high precision,low reagent dose,rapid analysis,excellent controllability,and high throughput,which is widely utilized in diverse applications including chemical reaction,biomedicine,single cell analysis,and drug synthesis.Conventional microchannel-based droplet manipulation techniques primarily manipulate droplets through the geometrical limitation of the microchannel structure,combining with the control of the micro-pumps and micro-valve units.However,integrating micro-pumps and micro-valve units requires expensive and complicated fabrication processes,which tend to increase their cost.Moreover,due to the limitation of the microchannel dimension,the size range of the droplets can be manipulated with limitations,which can not meet the demands of the multi-analysis complex systems.In contrast,the open-surface droplet manipulation techniques mainly utilize the wettability and adhesion of the superlyophobic surface to realize the manipulation of the droplets for completing the microdose,rapid and efficient biochemical reaction under its own gravity or external excitation(electric field,magnetic field,acoustic field,etc.).Among all the actuation methods,magnetically actuated droplet manipulation has attracted much attention because of its simple procedure,flexible operation,instantaneous response,good biocompatibility and magnetic beads separation technology.Therefore,the study of open-surface magnetically actuated droplet manipulation technology has potential scientific and engineering significance.In recent years,the research on open-surface magnetically actuated droplet manipulation mainly focuses on two aspects.There are magnetically actuated manner and the fabrication of the new superlyophobic surface.In the magnetically actuated manner,either permanent magnets or micro-coils are used to realize the droplet manipulation such as the transport and mixing of discrete droplets on the surface of the chip.Both approaches have their own limitations and can not be widely used.In the research of new superlyophobic surface,it mainly focuses on the fabrication of multi-functional and intelligent magnetically responsive surface to complete droplet manipulation with the excitation of magnetic field.Although some achievements have been realized in this field,simple,inexpensive methods for the fabrication of the magnetically responsive surface,the flexibility of droplet manipulation,and the controllability still present a challenge.Therefore,by virtue of the practical applications of open-surface magnetically actuated droplet manipulation technology and the frontier of development in superlyophobic surface,we exploit magnetically actuated droplet manipulation methods with excellent properties including simple procedure,low cost,high precision,flexible operation,instantaneous response,wide application range,automation,and intelligence.Based on these characteristics,we strive to make it more practical in the fields of point of care technology,analytical chemistry,bioengineering and precision medicine.This work focuses on the in-depth fundamental theoretical analysis and experimental research on magnetically actuated open microfluidics on superlyophobic surfaces,and the major work is provided in the following,(1)A novel magnetically actuated droplet manipulation strategy is proposed,which uses a superhydrophobic electromagnet needle(EMN)to manipulate droplets on an open surface.With the assist of the superhydrophobic surface treated electromagnet needle and chip,and based on the competition between magnetic force,surface tension and gravitational force,the platform can simply and flexibly perform a full range of droplet manipulations in three dimensions(3D),including droplet transport,fusion,mixing,magnetic beads(MBs)extraction,and droplet dispensing by switching the EMN power supply and the movement of the EMN.Meanwhile,a simple force-balance model of magnetic droplets in the presence of magnetic field is developed and the competitive relationship between magnetic force,surface tension and gravitational force is theoretically analyzed to predict the boundary conditions of different droplet manipulation forms.In addition,we also investigate the effects of the MBs concentrations,droplet volume and current amplitude applied to the EMN on the droplet kinematics.The experimental values exhibit a good agreement with theoretical values.(2)Based on the development of superhydrophobic EMN-based droplet manipulation method and for the purpose of broaden the range of droplet manipulation types,a droplet manipulation method is proposed,which uses the superoleophobic EMN instead of the superhydrophobic EMN.The superoleophobic coating based on silica/epoxy resin/fluorinated compounds is prepared by spraying method,which makes the EMN not only can manipulate the water droplets,but also can control the organic solvent and the oil droplets.Therefore,EMN-based magnetic droplet manipulation method will broaden the applicability of droplet manipulation platforms.(3)We present a novel programmable magnetically actuated droplet manipulation platform that combines a superhydrophobic magnetoelastic film(SHMF)with an electromagnetic pillar array(EMPA).Based on the properties of polydimethylsilane(PDMS)and magnetic particles,and with the computer numerically controlled(CNC)engraving process and carbon dioxide laser ablation technology,a multifunction SHMF with supehydrophobic,magnetic and elastic characteristics is prepared.Through the combination of SHMF with the EMPA,a programmable droplet manipulation platform is developed.Upon dynamic excitation of the EMPA under the SHMF,the surface topography of the SHMF was programmatically changed,which can direct and control the motion of aqueous droplets.The experimental results show that this new type droplet manipulation platform can realize automatic,precise,efficient and orderly droplet manipulation,including programmable droplet transport and mixing,and parallel manipulation of multiple droplets.(4)An intelligent droplet manipulation system based on a magnetically responsive superhydrophobic surface with controlled wettability and adhesion is developed.As the core component of the system,the magnetically responsive superhydrophobic surface was facilely prepared by combining spray coating and magnetic-field-directed self-assembly technique,which is comprised a dense array of magnetorheological elastomer micropillars(MREMPs).According to the magnetic field tunable stiffness of the MREMPs,the reversible switchability of the wetting and adhesive properties of the proposed magnetically responsive superhydrophobic surface can be achieved.Besides,we developed a magnetically active “mechanical hand” for no-loss water droplet transportation by combining the magnetically responsive superhydrophobic surface with an electromagnet rod.The experimental results show that this droplet manipulation system based on magnetically responsive superhydrophobic surface has advantages of simple procedure,flexible operation,instantaneous field responses and automation,which opens up opportunities for next generation smart microfluidic devices.