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 study on a hybrid structure flexible electro-rheological microvalve for soft microactuators

In order to realize a fluidic soft microactuator with a built-in control valve, this paper presents a cantilever type flexible electro-rheological microvalve (FERV) with a hybrid flow channel structure made from polydimethylsiloxane (PDMS) and SU-8. The hybrid structure provides high flexibility with the PDMS structure while only slight expansion occurred under high pressure with the SU-8 structure. In addition, its flexible electrodes are realized by UV-curable PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) that is a flexible conductive polymer and can be fabricated by simple and fast fabrication process without high-cost equipment. The proposed FERV can control the flow rate of the electro-rheological fluid (ERF) through the flow channel by changing its apparent viscosity with an applied electric field. FEM simulations were conducted to demonstrate the flexural rigidity of the designed FERV and compare it with the previous FERVs. Developing micro-electro-mechanical systems (MEMS) processes using the photolithography technique, the FERV was successfully fabricated and its characteristics were experimentally clarified. The results showed the feasibility of the proposed FERV in the soft microactuator application.

     

Development of a low cost hybrid Si/PDMS multi-layered pneumatic microvalve

We report on the design and fabrication of a low cost active microvalve with a soft elastomer membrane driven by pneumatic actuation. The valve was made in two separate parts, a fluidic part in biocompatible and optically transparent material (PDMS) and a robust pneumatic interface in silicon, which were assembled together. The main issue of alignment and localized selective bonding of the PDMS parts to preserve the membrane mobility, hence the valving function, is described. In this work we also investigated two types of silicon moulds for PDMS casting, made by KOH anisotropic wet etching or DRIE.     

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     

Electromagetically driven microvalve

We discuss photolithographic fabrication techniques and experimental results for a prototype electromagnetically driven microvalve. The valve is constructed on a silicon substrate, using a magnetic suspension spring with a valve cap, a valve plate with a 30 μm diameter bore, and an external coil for driving the valve cap. The external electromagnetic drive approach was chosen for its ease of use and practicality in controlling the valve actuator. The valve cap, made of soft magnetic material (NiFe) and supported by the spring, moves vertically as a result of the magnetic field applied by the external coil. To precisely adjust both the valve cap and the valve plate bore and to minimize fluid leakage, a new self-alignment process was developed. The valve is controlled by a 0.1–100 Hz rectangular magnetic field applied by the external coil. The resulting minimum gas flow rate can be controlled to within the neighborhood of 3 × 10⁻⁵ torr·l/s.     

Surface micromachined paraffin-actuated microvalve

Normally-open microvalves have been fabricated and tested which use a paraffin microactuator as the active element. The entire structure with nominal dimension of /spl phi/600 /spl mu/m /spl times/ 30 /spl mu/m is batch-fabricated by surface micromachining the actuator and channel materials on top of a single substrate. Gas flow rates in the 0.01-0.1 sccm range have been measured for several devices with actuation powers ranging from 50 to 150 mW on glass substrates. Leak rates as low as 500 /spl mu/sccm have been measured. The normally-open blocking microvalve structure has been used to fabricate a precision flow control system of microvalves consisting of four blocking valve structures. The control valve is designed to operate over a 0.01-5.0 sccm flow range at a differential pressure of 800 torr. Flow rates ranging from 0.02 to 4.996 sccm have been measured. Leak rates as low as 3.2 msccm for the four valve system have been measured.     

A liquid-triggered liquid microvalve for on-chip flow control

This work introduces a novel surface tension and geometry based liquid-triggered liquid microvalve for on-chip liquid flow control. The simultaneous presence of two liquid plugs at the uncomplicated valve junction triggers the further movement of the liquids and overcomes the stop valve function of the device, thereby providing a precise means of timing liquid movement on-chip. The generic structure was shown to successfully function and forms the basis for several novel and useful functions, including fluidic AND gates, contactless on-chip liquid sample control, timing of independent processes on the same microchip, bubble-free joining of liquids, all of which pose great challenges in the area of microfluidics. The device may be applied to chemical analysis, drug discovery, medical diagnostics and biochemistry.     

A hydrogel-actuated environmentally sensitive microvalve for active flow control

This paper reports on the fabrication and test of a hydrogel-actuated microvalve that responds to changes in the concentration of specific chemical species in an external liquid environment. The microvalve consists of a thin hydrogel, sandwiched between a stiff porous membrane and a flexible silicone rubber diaphragm. Swelling and deswelling of the hydrogel, which results from the diffusion of chemical species through the porous membrane is accompanied by the deflection of the diaphragm and hence closure and opening of the valve intake orifice. A phenylboronic-acid-based hydrogel was used to construct a smart microvalve that responds to the changes in the glucose and pH concentrations. The fastest response time (for a pH concentration cycle) achieved was 7 min using a 30-/spl mu/m-thick hydrogel and a 60-/spl mu/m-thick porous membrane with 0.1 /spl mu/m pore size and 40% porosity.     

A novel integrable microvalve for refreshable Braille display system

We introduce a novel integrable and electrostatic microvalve for the purpose of enabling a pneumatic refreshable Braille display system (RBDS). Physical design parameters of the microvalve such as orifice size, beam length, number of beams and beam profile are experimentally explored and found promising for use with the RBDS. Particularly, one design with an orifice of 70 /spl mu/m/spl times/70 /spl mu/m, beam length of 665 /spl mu/m, and beam count of 20 is electrostatically closed against a differential pressure of 82.7 kPa with an applied voltage of 68 V-rms. Also introduced is a steady-state mechanical model of the microvalve established on a coupled solution of fluid and solid domains. The model and experimental test results have been used to calculate the unknown discharge coefficient, elastic deflection, and entrance pressure. The model reveals that some of the designs have remarkably low discharge coefficient and entrance pressure, implying that pressure loss occurs mostly through and around the inlet port even at fairly large supply pressures. Experimental observations concerning the practical use of the microvalve are discussed.     

A thermally actuated microvalve using paraffin composite by induction heating

Microsystem Technologies - A new induction-heating-based microvalve using paraffin composite is successfully demonstrated in this paper. The microvalve consists of a polydimethylsiloxane (PDMS)...     

A high-flow thermopneumatic microvalve with improved efficiency and integrated state sensing

This paper reports a thermopneumatic microvalve featuring a corrugated diaphragm. A sealed cavity below the diaphragm contains a volatile fluid, the vapor pressure of which can be increased by resistive heating to deflect the diaphragm, thus closing the valve. Silicon heater grids are elevated 9 /spl mu/m above the cavity floor, and the cavity is only partially filled with fluid, to increase thermal efficiency. A vacuum-sealed, capacitive pressure sensor on the floor allows direct monitoring of the cavity pressure. Pentane-filled actuators sustain a 2070 torr pressure rise above atmospheric with 500 mW input power. A device tested in situ closes with 350 mW at 1000 torr inlet pressure (venting to vacuum) and maintains closure with 30 mW input. Valves conduct 400 sccm under 1500 torr differential pressure, while maintaining leak rates as low as 10/sup -3/ sccm, yielding a dynamic range of 10/sup 5/. A thermodynamic model has been developed that matches experimental power, pressure, and transient response data to within a few percent. This model is used to suggest an optimized structure capable of a 2000 torr pressure rise with 50 mW input and a 1 s response time. The glass-and-silicon valve structure is suitable for integration into complete microfluidic systems.     

A seat microvalve nozzle for optimal gas-flow capacity at large-controlled pressure

Seat microvalves are the most common microvalve type for gas-flow control. This paper presents a general method for optimizing the flow capacity of a seat valve nozzle and diminishing the requirements on the valve actuator's stroke length. Geometrical analysis and finite element (FE) simulations show that for controlling large gas flow at elevated pressure, the optimal nozzle design in terms of flow capacity for a given actuator performance is a multiple-orifice arrangement with miniaturized circular nozzles. Experimental results support the design introduced in this paper.     

Shape memory alloy based controllable multi-port microvalve

This paper describes an innovative miniature multi-port valve with a thin foil of shape memory alloy (SMA) as actuator for switching and dosing gaseous and liquid media. The normally closed (NC) microvalve has two structured SMA actuators that are switched independently of each other and either two inputs and one output or one input and two outputs. In addition to switching the media in the 3/4-way arrangement, it can also be used with a flow sensor in a closed loop control for dosing. Furthermore, the valve design is layer-based so that individual components can be manufactured according to given requirements or using different manufacturing technologies depending on the batch size. The SMA multi-port microvalve showed flow rates of about 2300 ml/min (nitrogen gas) and about 45 ml/min (water) for an applied pressure difference of 200 kPa and a heating current of about 400 mA. For flow regulation a closed loop control was realized and evaluated for a pressure difference of 100 kPa and a setpoint value of 900 ml/min.

     

A piezoelectric microvalve for compact high-frequency, high-differential pressure hydraulic micropumping systems

A piezoelectrically driven hydraulic amplification microvalve for use in compact high-performance hydraulic pumping systems was designed, fabricated, and experimentally characterized. High-frequency, high-force actuation capabilities were enabled through the incorporation of bulk piezoelectric material elements beneath a micromachined annular tethered-piston structure. Large valve stroke at the microscale was achieved with an hydraulic amplification mechanism that amplified (40/spl times/-50/spl times/) the limited stroke of the piezoelectric material into a significantly larger motion of a micromachined valve membrane with attached valve cap. These design features enabled the valve to meet simultaneously a set of high frequency (/spl ges/1 kHz), high pressure(/spl ges/300 kPa), and large stroke (20-30 /spl mu/m) requirements not previously satisfied by other hydraulic flow regulation microvalves. This paper details the design, modeling, fabrication, assembly, and experimental characterization of this valve device. Fabrication challenges are detailed.     

A microvalve for lab-on-a-chip applications based on electrochemically actuated SU8 cantilevers

Increasing the complexity and functionality of Lab-on-a-Chip devices requires integrated valves for internal flow regulation. The device introduced in this work is an electrochemical SU8 microvalve, featuring a cantilevered structure that seals an adjacent channel under the action of a growing electrolysis bubble. The valve is based on previously reported movable microcantilevers embedded in microchannels. As opposed to membrane valves moving perpendicular to the substrate, the cantilever valve is patterned next to the channel and closes it by moving horizontally, dramatically reducing footprint. Its compact, monolithic and 2D construction reduces assembly problems and results in negligible increase in dead volume (14.5 nl). Besides, its location in the way of the channels avoids the need for auxiliary filling ports or separate working media for actuation. Effective sealing with negligible leakage at 20 kPa has been achieved. The valve also displays passive flow regulation induced by the pressure differential generated across the valve chamber under flow conditions (42-72% flow decrease).     

A passive through hole microvalve for capillary flow control in microfluidic systems

A passive through hole microvalve is proposed to stop the capillary-driven flow in microchannels with small static contact angle (θ_s < 45°). Its gating condition on regulating flow is derived based on contact line theory. Using numerical simulations in certain limits and some experiments, we investigated the valve performance of a few different valve designs. A kind of converging through hole microvalve is found which can stop the relative faster capillary flow and is easier to fabricate and integrate. It is shown that allowable flow velocity for DI water could reach 0.5 m/s, and the height of microvalve could be as short as to 20 μm.     

Leak-tight piezoelectric microvalve for high-pressure gas micropropulsion

This paper describes the results of our development of a leak-tight piezoelectric microvalve, operating at extremely high upstream pressures for microspacecraft applications. The device is a normally closed microvalve assembled and fabricated primarily from micromachined silicon wafers. The microvalve consists of a custom-designed piezoelectric stack actuator bonded onto silicon valve components (such as the seat, boss, and tether) with the entire assembly contained within a stainless steel housing. The valve seat configurations include narrow-edge seating rings and tensile-stressed silicon tethers that enable the desired, normally closed, leak-tight operation. Leak testing of the microvalve was conducted using a helium leak detector and showed leak rates of 5/spl times/10/sup -3/ sccm at 800 psi (5.516 MPa). Dynamic microvalve operation (switching rates of up to 1 kHz) was successfully demonstrated for inlet pressures in the range of 0/spl sim/1000 psi. The measured static flow rate for the microvalve under an applied potential of 10 V was 52 sccm at an inlet pressure of 300 psi. The measured power consumption, in the fully open state, was 3 mW at an applied potential of 30 V. The measured dynamic power consumption was 180 mW for 100 Hz continuous operation at 100psi.     

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 fabrication of an electrochemically actuated microvalve

A microfluidic valve based on electrochemical (ECM) actuation was designed, fabricated using UV-LIGA microfabrication technologies. The valve consists of an ECM actuator, polydimethylsiloxane (PDMS) membrane and a micro chamber. The flow channels and chamber are made of cured SU-8 polymer. The hydrogen gas bubbles were generated in the valve microchamber with Pt black electrodes (coated with platinum nanoparticles) and filled with 1 M of NaCl solution. The nano particles coated on the working electrode helps to boost the surface-to-volume ratio of the electrode for faster reversible electrolysis and faster valve operation. To test the functionality of the microvalve, a simple micropump based on ECM principle was also integrated in the system to deliver a microscopic volume of fluid through the valve. The experimental results have showed that an approximately 300 μm deflection of valve membrane was achieved by applying a bias voltage of ?1.5 V across the electrodes. The pressure in the valve chamber was estimated to be about 200 KPa. Experimental results proved that the valve can be easily operated by controlling the electrical signals supplied to the ECM actuators.     

Improvement of dynamic characteristics of polydimethylsiloxane based microvalve

Microfluidics is an indispensable part of micro total analysis system (μ-TAS) and lab-on-a-chip analysis systems. While most of the work in this area has focused on MEMS based actuation with micropumps and microvalves, polymer based Paraffin actuator is an attractive alternative in terms of ease in fabrication and low cost. While we made previous attempts in fabricating polydimethylsiloxane (PDMS) based devices, it suffered a drawback of low flow rates in the microchannel due to adherence of PDMS to the channel substratum. In the current work, we focused on improvement of mechanical properties of the PDMS membrane by altering prepolymer to crosslinker ratio. We found that a ratio of 100:15 produced sufficient tensile strength to the membrane and also enhanced actuation characteristics of microvalve fabricated with it.