Microfluidic integrated circuits for signal processing using analogous relationship between pneumatic microvalve and MOSFET

In this paper, a new concept and potential demonstration of functional microfluidic integrated circuits using MEMS technology are presented. The fluidic integrated circuits were constructed utilizing analogous relationship between MOSFET and pneumatic microvalve with a diaphragm structure. The signal transmitted through the circuit is the fluidic signal, that is, the pressure or the flow-rate of the fluid. The pneumatic microvalve in this study is expressed by small-signal equivalent model similar to that of a MOSFET. Small signal behavior of microfluidic integrated circuits can be expected using the model, if the parameters in the model are extracted properly from fabricated microvalves. As an example of a fluidic circuit, pressure inverting amplifiers including integrated two microvalves were fabricated and evaluated. As a result, they showed sharp pressure transfer curves similar to MOS inverter circuits. A maximum pressure gain of 32.0 dB was obtained, and it can be used for pressure amplification in analog applications. In addition, they can be used as pressure inverter logic circuits for digital applications. Although the theory and design environment of the new microvalve circuit technology have not been established yet, multifunctional fluidic analog and digital circuits can be realized for special application fields different from electronic integrated circuits.     

Microfluidic phase change valve with a two-level cooling/heating system

A phase change (PC) microvalve with an integrated two-level cooling/heating system is developed for microfluidic applications in this article. This PC microvalve utilizes the liquid–solid PC of a small portion of the working medium in a microchannel to switch on/off the flow in the microchannel. The size of the working medium for the PC microvalve is 5-mm long, 50-μm high, and 80-μm wide (50 μm × 80 μm is the cross-sectional area of the channel) in this study. The switch is actuated by using a two-level cooling/heating system integrated on the chip. The first-level cooling/heating unit keeps the working medium in the valve area in the temperature range of supercooling state. Based on the supercooling state, the second-level cooling/heating unit either heats up or cools down the medium in the valve area to trigger its PC between liquid and solid for valving purposes. The proposed microfluidic PC microvalve is characterized experimentally in microfluidic chips. The thermal impact of one PC microvalve in one particular microchannel on its adjacent channels is discussed by establishing a preliminary analytical model and a numerical model. In addition to no leakage and no moving element, this PC microvalve with a two-level cooling/heating system can achieve a very short cooling time (i.e., 2.72 s).     

高聚物微流控芯片上集成化气动微阀的研制

报道了一种新型的聚甲基丙烯酸甲酯(PMMA)/聚二甲基硅氧烷(PDMS)复合芯片。该芯片采用PMMA-PDMS…PDMS-PMMA的四层构型,以在芯片上集成气动微阀。具有液路和控制通道网路的PMMA基片与PDMS弹性膜间采用不可逆封接,分别形成液路半芯片和控制半芯片,而2个半芯片则依靠PDMS膜间的粘性实现可逆封接,组成带有微阀的全芯片。这种制备方法解决了制备PMMA-PDMS-PMMA三层结构芯片的封接难题,封接过程简单可靠。其控制部分和液路部分可以单独更换,可进一步降低使用成本,尤其适合一次性应用场合。初步实验表明:该微阀具有良好的开关性能和耐用性。     

Printing 3D microfluidic chips with a 3D sugar printer

This study demonstrated how to quickly and effectively print two-dimensional (2D) and three-dimensional (3D) microfluidic chips with a low-cost 3D sugar printer. The sugar printer was modified from a desktop 3D printer by redesigning the extruder, so the melting sugar could be extruded with pneumatic driving. Sacrificial sugar lines were first printed on a base layer followed by casting polydimethylsiloxane (PDMS) onto the layer and repeating. Microchannels were then printed in the PDMS solvent, microfluidic chips dropped into hot water to dissolve the sugar lines after the PDMS was solidified, and the microfluidic chips did not need further sealing. Different types of sugar utilized for printing material were studied with results indicating that maltitol exhibited a stable flow property compared with other sugars such as caramel or sucrose. Low cost is a significant advantage of this type of sugar printer as the machine may be purchased for only approximately $800. Additionally, as demonstrated in this study, the printed 3D microfluidic chip is a useful tool utilized for cell culture, thus proving the 3D printer is a powerful tool for medical/biological research.     

Design and simulation of a normally closed glucose sensitive hydrogel based microvalve

In drug delivery systems microvalves are the key components that have been developed for active control of drugs. In this research a normally closed microvalve with a glucose sensitive hydrogel actuating system is designed and simulated. Swelling of the hydrogel forces a silicone rubber membrane to deflect and causes the valve to be opened. The component of the valve that can be opened because of the hydrogel pressure is a silicon nitride cantilever beam which is sealed with a parylene layer. Simulations have been done by FEM analysis and the results show that membrane deflection is large enough to enable the valve to be opened and the fluid to flow through the microchannel. For both rectangular and trapezoidal microchannels with various hydraulic diameters, output flow rates less than 50 μl/min to several hundreds of μl/min can be achieved. Final design has been optimized for the opening point of microvalve at glucose concentration of 15 mM. Overall investigation has been done for a microvalve with specific dimensions and with 4 kPa input pressure the output flow rate of 100 μl/min has been generated which is in the desired range.     

A microvalve for hybrid microfluidic systems

A hybrid valve for lab on chip applications is presented. The valve is assembled by bonding poly (methyl methacrylate), PMMA, and silicon-based elastomers. The process used to promote the hybrid bonding includes the deposition of an organosilane (TMSPM) on the thermoplastic polymer, PMMA to interface PMMA and elastomers. For this study, a membrane in ELASTOSIL^? is bonded in correspondence of the end of two microfluidic channels of a fabricated PMMA microfluidic chip. Prior the bonding, a plasma etching process has been used to remove the TMSPM in a confined circular area. This process made possible to bond selectively the edge of a membrane leaving free to move its central part. Actuating the membrane with an external positive pressure or vacuum is possible, respectively, to obstruct or to connect the microfluidic channels. The microvalve may be simply integrated in microfluidic devices and permits the control of microvolumes of fluid in processes such as transport, separation, and mixing. The deposition of the TMSPM, the bonding of the valve and its actuation has been characterized and tested. The flow rate control of liquids through the valve has been characterized. The results have been discussed and commented. The valve can stand up to 14 psi without showing leakages.     

A pneumatically actuated full in-channel microvalve with MOSFET-like function in fluid channel networks

A pneumatically actuated silicon microvalve applicable to integrated microfluidic systems is presented. All the ports of this microvalve are in-channel, and connectable to any surface fluid channels in microfluidic systems. This microvalve controls fluid flow by means of the controlled gap between glass and silicon diaphragm actuated by a control pressure. In addition, the diaphragm is also deformed by the outlet pressure of the microvalve. Due to the feature, this microvalve shows saturation of flow rate like MOSFETs operated at saturation region. The fabricated microvalve device was evaluated focusing on analogous relationship between MOSFET and the microvalve. Fluids such as air and DI-water were well controlled by the control pressure. Fluid starts to flow in the microvalve when the control pressure exceeds its "threshold pressure." Hysteresis due to sticking of diaphragm was not observed in the characteristics. Air flow rate of the microvalve was gradually saturated with the increasing of the outlet pressure as expected. Through the evaluation, analogous relationship between this microvalve and MOSFET has been experimentally demonstrated.     

Design and characterization of a platform for thermal actuation of up to 588 microfluidic valves

In this paper, we describe a large-scale microfluidic valve platform for thermally actuated phase change (PC) microvalves. PC microvalves can be actuated by heat sources such as ohmic resistors, which can be highly integrated resulting in dense arrays of individually addressable microfluidic valves. We present a custom-made electronic platform with custom-written control software that allows controlling a total of 588 individually addressable resistors each of which can be used as the actuator for a separate PC valve. The platform is demonstrated with direct PC microvalve (the simplest example of a PC valve) where working fluid and phase change material are the same media. We present experimental results for single valve setups as well as for a 24 microvalve setup showing the scalability of the system. Furthermore, we demonstrate that precise and individual ‘per-resistor’ temperature profiles are required for valve actuation in order to decrease thermal latency and ensure that the time required for switching the valve state is independent from the “thermal history” (i.e. the duration of the previous valve state) of the valve. To the best of our knowledge, there is no such platform described in the literature, which offers an equal potential for individual valve operation (potentially up to 588 individual valves) as presented in this work.     

On-chip fabrication of magnetic alginate hydrogel microfibers by multilayered pneumatic microvalves

Alginate hydrogel has widespread applications in tissue engineering, cancer therapy, wound management and drug/cell/growth factor delivery due to its biocompatibility, hydrated environment and desirable viscoelastic properties. However, the lack of controllability is still an obstacle for utilizing it in the fabrication of 3D tissue constructs and accurate targeting in mass delivery. Here, we proposed a new method for achieving magnetic alginate hydrogel microfibers by dispersing magnetic nanoparticles in alginate solution and solidifying the magnetic alginate into hydrogel fiber inside microfluidic devices. The microfluidic devices have multilayered pneumatic microvalves with hemicylindrical channels to fully stop the fluids. In the experiments, the magnetic nanoparticles and the alginate solution were mixed and formed a uniform suspension. No aggregation of magnetic nanoparticles was found, which is crucial for flow control inside microfluidic devices. By regulating the flow rates of different solutions with the microvalves inside the microfluidic device, magnetic hydrogel fibers and nonmagnetic hydrogel fibers were fabricated with controlled sizes. The proposed method for fabricating magnetic hydrogel fiber holds great potential for engineering 3D tissue constructs with complex architectures and active drug release.     

Bi-objective topology optimization of asymmetrical fixed-geometry microvalve for non-Newtonian flow

Fixed-geometry microvalves such as Tesla microvalves rely on the inertial forces of the fluid to allow flow in the desired direction while inhibiting flow in undesired direction. In the traditional topology optimization design methods of fixed-geometry microvalves, single objective function is used to minimize the energy dissipation of forward flow. And several previous studies have widely used diodicity to indicate the performance of fixed-geometry microvalves, which is defined as the ratio of the pressure drop of reverse flow to that of the forward flow. However, higher diodicity does not reflect the degree of forward energy dissipation, leading to a significant pumping power is required to drive flow. Therefore, treating the forward flow pressure drop and its performance independently by a bi-objective formulation is preferable to design fixed-geometry microvalve. This paper proposes a bi-objective topology optimization design method and uses the regularization constraint to design asymmetrical fixed-geometry microvalve for non-Newtonian flow. Several numerical examples with different bifurcation angles, Darcy number and weight coefficients of the bi-objective functions are studied and the validity of the topology optimization method presented in this paper is demonstrated.

     

A layered modular polymeric <Emphasis Type="Italic">μ</Emphasis>-valve suitable for lab-on-foil: design,fabrication, and characterization

Cost-effective fabrication of microfluidic networks require that all components have to be manufactured with up-scalable processes such as reel-to-reel fabrication of foil-based devices. A microvalve design must take into account functional requirements together with manufacturing feasibilities. Here we present the development of a modular polymeric laser structured microvalve. The complete valve structure is designed to be used in a bendable lab-in-foil system. The modular microvalve design consists of three layers: an actuator layer, an interfacing membrane, and a passive microchannel layer to be separately fabricated and then stacked. Different actuator layer concepts are compared out of which a thermal actuation scheme generating sufficient stroke using phase changing paraffin is chosen. The passive layer is designed with a shallow and sufficiently smooth spherical cavity that acts as the valve seat from which paraffin material can reliably retract during solidification. The shape and dimensions of the shallow cavity are derived from the natural membrane deflection and from the channel cross section. It is not essential that all the paraffin within the actuator cavity to be molten for valve closure allowing a high degree of assembly tolerance and inherent sealing of actuator cavity. All the module layers in the current prototype are structured using 3D laser fabrication processes but mass-fabrication methods like reel-to-reel hot-embossing are foreseen as well. A prototype microvalve stack was assembled with a thickness of 1.1 mm which could be further reduced to meet the requirements of extremely flexible lab-on-foil systems. The closed valve is tested up to a pressure of 3 kPa without any measurable leakage. The dynamics of valve closure is evaluated by a new optical characterization method based on image processing of color micrograph sequences taken from the transparent valve.     

Fabrication and testing of embedded microvalves within PMMA microfluidic devices

The integration of a PDMS membrane within orthogonally placed PMMA microfluidic channels enables the pneumatic actuation of valves within bonded PMMA–PDMS–PMMA multilayer devices. Here, surface functionalization of PMMA substrates via acid catalyzed hydrolysis and air plasma corona treatment were investigated as possible techniques to permanently bond PMMA microfluidic channels to PDMS surfaces. FTIR and water contact angle analysis of functionalized PMMA substrates showed that air plasma corona treatment was most effective in inducing PMMA hydrophilicity. Subsequent fluidic tests showed that air plasma modified and bonded PMMA multilayer devices could withstand fluid leakage at an operational flow rate of 9 μl/min. The pneumatic actuation of the embedded PDMS membrane was observed through optical microscopy and an electrical resistance based technique. PDMS membrane actuation occurred at pneumatic pressures of as low as 10 kPa and complete valving occurred at 14 kPa for ~100 μm by 100 μm channel cross-sections.     

A pneumatic micropump incorporated with a normally closed valve capable of generating a high pumping rate and a high back pressure

This study reports on a new pneumatic micropump integrated with a normally closed valve that is capable of generating a high pumping rate and a high back pressure. The micropump consists of a sample flow microchannel, three underlying pneumatic air chambers, resilient polydimethylsiloxane (PDMS) membrane structures and a normally closed valve. The normally closed valve of the micropump is a PDMS-based floating block structure located inside the sample flow microchannel, which is activated by hydraulic pressure created by the peristaltic motion of the PDMS membranes. The valve is used to effectively increase pumping rates and back pressures since it is utilized to prevent backflow. Experimental results indicate that a pumping rate as high as 900 μL/min at a driving frequency of 90 Hz and at an applied pressure of 20 psi (1.378 × 10⁵ Nt/m²) can be obtained. The back pressure on the micropump can be as high as 85 cm-H₂O (8,610.5 Nt/m²) at the same operation conditions. The micropump is fabricated by soft lithography processes and can be easily integrated with other microfluidic devices. To demonstrate its capability to prevent cross contamination during chemical analysis applications, two micropumps and a V-shape channel are integrated to perform a titration of two chemical solutions, specifically sodium hydroxide (NaOH) and benzoic acid (C₆H₅COOH). Experimental data show that mixing with a pH value ranging from 2.8 to 12.3 can be successfully titrated. The development of this micropump can be a promising approach for further biomedical and chemical analysis applications.     

A novel bulk micromachined electrostatic microvalve with acurved-compliant structure applicable for a pneumatic tactile display

Recent success of microelectromechanical systems (MEMS) in projection displays have raised similar expectation for an efficient, low power, affordable, full-page and pneumatic tactile display. Such design has not been achieved by the conventional technology but could bring significant improvement to current refreshable Braille displays. This paper demonstrates a novel bulk-micromachined electrostatic microvalve suitable for a pneumatic tactile display. The microvalve, a silicon perforated diaphragm juxtaposed to a silicon inlet orifice, requires relatively low closing voltage against a large supply differential pressure and flow rate, i.e., 72.9 V-rms for 19.3 kPa and 85 mi/min. Such an attractive characteristic is due to its unique curved-compliant structure that has, unlike other electrostatic microvalves, no tolerance for any initial air gap between its electrodes. As a design tool, a mechanical model of the microvalve is introduced based on the lubrication theory and large plate deflection theory. The model is established on a steady-state coupled field problem of fluid-solid mechanics. Reynolds and von-Karman equations were simultaneously solved for the microvalve geometry by finite difference approximation and double Fourier series expansion. The results of the model and experiments are compared and found to be in good agreement with a relative error less than 10%     

Design of shrinkage microvalve in microchannel with hydrophilic and hydrophobic walls

This paper presents a simple design of shrinkage microvalves which can be used to effectively stopping capillary flow inside a microchannel with hydrophilic and hydrophobic walls. Based on the relationship between capillary pressure and cross-section geometry of a microchannel, the microvalve is designed with a critical ratio of rectangular section. In order to verify the feasibility of the design rule, a couple of shrinkage microvalves with different aspect ratios of cross-section are fabricated by using PDMS bonded with glass wafer. The experiment demonstrates the stopping effect of the proposed design of shrinkage microvalve.     

Solvent-free surface modification by initiated chemical vapor deposition to render plasma bonding capabilities to surfaces

A versatile solvent-free method for surface modification of various materials including both metals and polymers is described. Strong irreversible bonds were formed when substrates modified by initiated chemical vapor deposition (iCVD) of poly(1,3,5-trivinyltrimethylcyclotrisiloxane) or poly(V3D3) and exposed to an oxygen plasma were brought into contact with plasma-treated poly(dimethylsiloxane) (PDMS). The strength of these bonds was quantified by burst pressure testing microfluidic channels in the PDMS. The burst pressures of PDMS bonded to various coated substrates were in some cases comparable to that of PDMS bonded directly to PDMS. In addition, porous PTFE membrane coated with poly(V3D3) was successfully bonded to a PDMS microfluidic device and withstood pressures of over 300 mmHg. Bond strength was shown to correlate with surface roughness and quality of the bond between the coating and substrate. This work paves a methodology to fabricate microfluidic devices that include a specifically tailored membrane. Furthermore, the bonded devices exhibited hydrolytic stability; no dramatic change was observed even after immersion in water at room temperature over a period of 10 days.     

A Latchable Phase-Change Microvalve With Integrated Heaters

This paper presents a latchable phase-change microvalve with integrated microheaters, which is suitable for lab-on-a-chip systems where minimal energy consumption is desired. The microvalve exploits low-melting-point paraffin wax, whose solid–liquid phase changes allow switching of fluid flow through deformable microchannel ceiling. Switching is initiated by melting of paraffin through an integrated microheater, with an additional pneumatic pressure used for the open-to-close switching. The valve consumes energy only during initiation of valve switching. When paraffin solidifies, the switched state is maintained passively. The microvalve was fabricated from polydimethylsiloxane through multilayer soft lithography techniques. Experiments show that the valve can switch flow within 4–8 s due to the small thermal mass and localized melting of paraffin wax; when closed, the valve can passively withstand an inlet pressure over 50 kPa without leakage. Time response of the valve can be further improved with improved heater and wax chamber designs, while the latching ability can be improved by optimizing the wax chamber/membrane design. Compared to existing latchable phase-change valves, the microvalve has no risk of cross-contamination. In addition, the improved sealing offered by the compliant membrane makes the valve robust and flexible in operation, allowing large ranges of initiation pressure from various actuation schemes. $hfill$[2008-0303]      

Design, fabrication and characterization of an externally actuated ON/OFF microvalve

A magnetic microfluidic valve has been designed, fabricated and tested. Operation relies on the use of a permanent magnet which interacts with an electrodeposited layer of Co–Ni (soft magnetic material) on a V-shaped cantilever beam. The deflection caused by the magnetic forces opens or closes the fluid flow. The microvalve performance has been optimized by means of finite element analysis (FEA). The FEA model has been experimentally validated using confocal microscopy and used to improve the magnetic circuit. Then, a fluidic cell has been built and the microvalve has been demonstrated to work as a check-valve or as ON/OFF valve when being magnetically actuated. Fabricated prototypes were evaluated in a flow of N₂at the flow rate of 20 sccm. The operational applied pressure was 50 mbar. The microvalve has a leaking rate in the order of 1.75 sccm at 50 mbar.     

A microvalve matrix using piezoelectric actuators

 A microvalve matrix is proposed for controlling gas flow. Mircovalves in the matrix are controlled independently and each one handles a very small gas flow. The microvalve matrix can thus control gas flows precisely in very small steps, over the range from zero flow to fully-open flow, by digitally opening and closing the appropriate number of microvalves. The microvalve proposed for this matrix has a compact and simple design for a high degree of integration. This valve is normally closed and is opened by using the deformation of the port caused by the piezoelectric effect. Calculations show that a microvalve smaller than 1×1 mm can handle a maximum gas flow rate of the order of 10^-4 Pa m³/s (Air, 20 °C). It is easy to reduce the flow rate. Therefore these results indicate that a microvalve matrix can achieve wide dynamic-range flow-control in small flow steps. Received: 1 November 1996/Accepted: 14 November 1996