| In recent years,microfluidics have shown broad applications in chemistry,analysis,biology,and medical detection because of their feature advantages such as reduced sample volume,shortened analysis time,high throughput,high sensitivity,miniaturization,portability,and little pollution.As a classical microfluidic fluid control method,passive microvalve can achieve fluid control by its own structures and properties.The passive microvalves with functional micro-nanostructures can achieve precise manipulation of fluids,which is due to the Gibbs energy effect of micro-nanostructures and the stability of the gas-liquid-solid three-phase line.Due to the limitation of preparation and modification methods and the lack of micro-nanostructures patterning design expansion,this kind of passive valve has been stuck in the basic control of fluid flow and flow direction,and further exploration of their applications is limitted.In this dissertation,we start from the design of functional micro-nanostructure arrays,and fabricate a series of anisotropic structure array surfaces with specific arrangement.These surfaces are used for stimulus-responsive microvalves,network microvalves,fluid flow switchings in microchips.We successfully achieve gas-liquid separation,capture and mixing of ultramicro-volume liquids,anisotropic nanoparticle synthesis,and liquid pressure sensing in microchips.The detailed research work is as follows:1.Thermal-Responsive Anisotropic Wetting Microstructures for Manipulation of Fluids in Microfluidics.A smart method was proposed for liquid control in microfluidics.With a two-step modification process,we fabricated N-isopropylacrylamide-modified Si stripes on silicon slides.The surfaces were employed as substrates for smart fluid manipulation in microchannels.By grafting N-isopropylacrylamide material on the morphological microstrip surface,we prepared a temperature-responsive wetting surface.The wettability of liquids can be switched between strong and weak anisotropic infiltration by adjusting temperatures.The temperature-sensitive surfaces were used as chip substrates to achieve smart regulation of liquid flow behavior in microchannels.The smart flow behavior of fluids is affected by liquid pressures,which can be achieved until the liquid pressure is greater than a certain value.We defined this pressure as the threshold pressure.In addition,we discussed the effects of microstripe structure and microchannel dimension on smart flow behavior.The results demonstrated that the threshold pressure was affected by the dimensional parameters.Through tuning the system temperature and adding the assistant gas,we realized successive“valve”function.This temperature-sensitive microvalve is easy to be integrated,and can be applied to the surface of a variety of microchips,which provides a repeatable continuous temperature-sensitive valve for microfluidic chips.2.Integrated obstacle microstructures for gas-liquid separation and flow switching in microfluidic networks.A method was proposed for gas-liquid separation and flow switching in microfluidic networks.In this work,based on the morphology microstripe structure,we have constructed passive valve arrays with specific arrangement,which greatly simplified the passive microvalve construction strategy,and liquid flow switching and gas-liquid separation were successfully achieved in network microchannels.Anisotropic structure array can be used for passive microvalve in network microchannels,and we found that what really played the role of microvalve function was the structure at the microchannel intersection.A single microstripe structure was prepared by the two-step method of photolithography and dry etching,and it could act as a microvalve in linear microchannel.Next,we discussed the effects of microchannel and microstripe dimensions on the threshold pressure of microvalves.In addition,we explained the entire liquid control process of the microstripe valve.The valve function is mainly due to the enlarged contact angle and the Gibbs energy imbalance effect at the edge of the microstripe.Next,we connected the microstripe valve at the intersection of a T-shaped channel,and continuous separation of gas-liquid two-phase fluids was realized.In addition,we found that when liquid pressures was greater than the threshold pressure,the time for liquids to flow through the microstripe valve was positively related to the number of valves.Based on this principle,microvalves with different microstripe numbers were arranged at the intersections of the microchannels to achieve independent control of the liquid flow state in network microchannels.The function usually needs to be realized through the use of multiple active microvalves.This research work can achieve precise control of fluids in network microchannels by specific arrangement of the passive microvalves,which greatly expanded the application of passive microvalves.The method provides an effective fluid control tool for microfluidic chips.3.Pressure-controlled microfluidic sub-picoliter ultramicro-volume syringes based on integrated micro-nanostructure arrays.A method for capturing ultramicro-volume liquids in microchips was proposed.Through the specific arrangement of microstripe structures in a trapezoidal microchannel,we have realized continuous acquisition of sub-picoliter ultramicro-volume liquids,and successfully synthesized a variety of gradient anisotropic nanocrystals.The threshold pressure of the microstripe microvalve has a negative correlation with the length of the structure.We designed a trapezoid-shaped microfluidic channel,which was separated into a series of chambers with pre-set volumes by gradient-length microstripe arrays.The volume of the captured liquids can be modified by changing the applied pressure of the inlets and the dimensions of microchannels.To prevent the deformation of PDMS microchannels and reduce the volume of available liquid,a glass-Si syinge was fabricated,and it could be used to obtain volume of liquid as low as 0.5 pL with 96%accuracy.In addition,the syrings also showed great applicability for fluids with wide surface tension?ST?ranges,including aqueous alcohol solutions,rapeseed oil,blood,and various salt solutions.To demonstrate the practical application of the syringe,a microchip consisting of three UVSs was designed,and Au nanoparticles and anisotropic Au nanorods with a gradient of sizes were successfully synthesized.The construction of the ultramicro-volume syringes expands the application of passive microvalves.This ultramicro-volume liquid volume capture method is easy to integrate into multiple applications of microchips,which provides a practical ultramicro-volume liquid volume capture tool for microfluidic chips.4.High-sensitivity microliter liquid pressure sensors based on patterned micro-nanostructure arrays.A method was proposed for preparing a highly sensitive,low cost,small volume liquid pressure sensor.Through the design of microchannels and the arrangement of microstripe positions,we have successfully prepared in-situ,highly sensitive liquid pressure sensors.The threshold pressure of microstripe valve has a negative correlation with its length in microchannels,which gradually increases in trapezoidal microchannels.In microfluidic channels,due to the existence of friction water layers between microchannels and fluids,there is pressure drop in the pressure driven liquid,and the pressure of liquid front is gradually reduced.Based on this principle,five metering microchannels which were embedded with MSNP arrays of varying lengths,were fabricated along the side of the primary fluidic microchannel.We found that the inlet pressure can then be measured based on the amount of microstripes that fluids flow through,within the five metering microchannels.By properly designing the dimensions of the microchannels and the positions of the microstrips,the liquid pressure sensor possess a measurement range ranging from 2 to 800 mbar,high sensitivity up to 16.7 mbar-1,trace sample volume of less than 1.3?L.In addition to water,this pressure sensor is suitable for the measurement of various surface tensions and types of liquid pressure,such as ethanol,rapeseed oil,and n-hexadecane.The pressure sensor was demonstrated to function well,with slight adaptations,for common liquids within an ultra-wide surface tension range,enabling its application for sensing environmental fluidic pressures in changing microchannels,detecting the central venous pressures,and diagnosing the morbidity of hypertension,hypotension and arterial thrombosis.The results can be recorded by a typical built-in cell phone camera,which provides a highly sensitive and low-cost measurement method for blood pressure detection. |
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