Passive Flow Regulator Research Articles

Design and Characterization of an Adjustable Passive Flow Regulator and Application to External CSF Drainage

Passive valves that deliver a constant flow rate regardless of inlet pressure changes have numerous applications in research, industry, and medical fields. The present article describes a passive spring valve that can be adjusted manually to deliver the required flow rate. The valve consists of a movable rod with an engraved microchannel. The fluidic resistance of the device varies together with the inlet pressure to regulate the flow rate. A prototype was made and characterized. Flow-rate adjustment up to +/-30% of the nominal flow rate was shown. A simple numerical model of the fluid flow through the device was made to adapt the design to external ventricular drainage of cerebrospinal fluid (CSF). Some insights about the implementation of this solution are also discussed.

Oil-gated isoporous membrane with micro-apertures for controllable pressure-induced passive flow regulator.

The pressure-driven liquid flow controller is one of the key components in diverse applications including microfluidic systems, biomedical drug injection devices, and pressurized water supply systems. Electric feedback loop based flow controllers are fine-tunable but expensive and complex. The conventional safety valves based on spring force are simple and low cost, but their diverse application is limited due to their fixed pressure range, size, and shape. Herein, we propose a simple and controllable liquid-flowing system combining a closed liquid reservoir and an oil-gated isoporous membrane (OGIM). The ultra-thin and flexible OGIM acts as an immediately responsive and precisely controlled gas valve to maintain internal pneumatic pressure as designed to induce constant liquid flow. The oil filling apertures act as a gate for gas flow depending on the applied pressure and the threshold (gating) pressure of the gate is determined by the surface tension of the oil and the gate diameter. It is confirmed that the gating pressure is precisely controlled by varying the gate diameter, which agrees with the theoretically estimated pressures. Based on stably maintained pressure due to the function of OGIM, the constant liquid flow rate is achieved even with the high gas flow rate.

Series and parallel integration of flow regulators for precise and multiple-output fluid delivery

The passive flow regulator for accurate flow control is vital in miniaturized microfluidic systems. The reported regulators commonly have complex structures and fabrication processes, and could only deliver fixed flow rates. In this paper, the series and parallel integrations of single membrane flow regulators were reported to achieve the enhanced flow control and the multiple-output fluid delivery. To understand the flow adjustment characteristics, a series of prototype devices with different structural dimensions were systematically investigated. Our flow regulators were fabricated in low-cost polymer films using a quick nonclean room fabrication process. By adjusting the structural parameters, our flow regulators were able to output the stable flow rates of 100–2000 µL/min over a wide pressure range of 1000–4000 mbar. More importantly, the strategies of series and parallel integrations of single passive flow regulators were carried out to create the precise and multiple-output fluid delivery. The series-connected flow regulators could reduce the fluctuation of flow rates to be less than 1.5% under highly varied pressures. Through selectively turning off the branches, the parallel-arranged flow regulators could output various precise flow rates as demanded. Therefore, we envisioned that our integrated flow regulators held the potentials for controlling fluid with on-demand flow rates in portable microfluidic systems.

Theoretical modeling of the behavior of a parallel membrane passive flow regulator

We present a theoretical model for the non-linear pressure-flow relationship of a microvalve which exploits the non-linear behavior of two horizontal parallel membranes for autonomous flow regulation. The analysis, buttressed by numerical studies correctly illustrates the nonlinear behavior of the valve based on the experimental study presented in our earlier report. We study the elastic deformation of the PDMS membranes under varied pressure loads to the point of contact, and their resultant effects on flow restriction which counteractively changes - without any external energy input - to balance pressure variations, thereby ensuring a stable output flow. We demonstrate analytically how structural parameters and material properties influence the performance of the device in terms of saturation of flow and the minimum pressure required to kick-start the autonomous regulation. The outputs of our approach are in very good conformity with our experimental investigations, giving us all confidence to follow it for the delivery of designs with specific performance targets. We expect this model to provide insights and guidelines for optimized designs of microfluidic actuators for various lab-on-a-chip applications.

Self-adaptive flexible valve as passive flow regulator

Valves are widely used in fluidic channel systems to regulate flows. In many applications, maintaining a constant flow rate under varying input conditions is critical. However, current active or passive valves suffer from several drawbacks, such as complex structures, complicated fabrication processes, and complex operational procedures. This paper presents a novel design concept for fluidic channels using a simple structure that can serve as self-adaptive passive valves to regulate flow under external pressure. The design relies on embedding a cantilever-like flap inside a channel to provide a self-regulated closed-loop control of the flow. The flap can passively deform to adapt to the variation of the fluid pressure. A theoretical model is developed and validated by finite-element simulations to understand the flow characteristics of this design. Based on the theoretical model, we provide the design rules for obtaining valves with desired pressure-flow rate responses. Furthermore, by integrating the quantitative design tool and 3D printing, a valve that can maintain a constant flow rate over a certain pressure range is designed, fabricated, and characterized. With support from systematic analysis and rapid manufacturing techniques, the valve design proposed here can open routes for applications in microfluidic systems and drug infusion systems.

Design and Characterization of 3-Stack MEMS-Based Passive Flow Regulators for Implantable and Ambulatory Infusion Pumps

Passive implantable or portable infusion pumps comprise an energy source to pressurize a drug and a fluidic restriction to determine the delivery rate. MEMS technology enables the design of microfluidic chips that can passively regulate the flow rate and therefore limit the impact of pressure variations on flow accuracy. The device is a triple-stack structure made of silicon and borosilicate with valves associated in parallel. Flow regulation is obtained from the deflection of a silicon membrane that progressively obstructs the valves as the reservoir pressure increases. Samples dedicated to the infusion at 1 ml/h and 1 ml/day have been designed, manufactured and tested. Flow regulation in the pressure range 200–1000mbar was obtained, with an overall flow rate variability of +/−10% with respect to nominal targets. By comparison to commercial devices, these data showed a reduction of the flow variability up to a factor 2.4 and the lowering of the reservoir pressure by a factor 5. A passive flow regulator coupled with an active valve would match the flow control capabilities of the state-of-the-art programmable implantable pumps. Furthermore, these microfluidic chips could be adapted to disposable pumps to improve the flow accuracy during the infusion of potent drugs having short half-lives. [2019-0161]

Design of a Passive Flow Regulator Using a Genetic Algorithm

Passive flow regulators are usually intended to deliver or drain a fluid at a constant rate independently from pressure variations. Microfluidic devices made of a stack of two plates are considered here: the first plate comprises a flexible silicon membrane having through holes while the second plate is a rigid substrate with a cavity, an outlet hole and pillars aligned with the through holes of the membrane. The liquid flows through the holes etched in the membrane and through the small gap between the bottom of the membrane and the pillars: each hole can therefore be considered as a valve which progressively closes as the pressure increases, thus leading to a non-linear fluidic behaviour. FEM simulations have been performed to ensure a constant flow rate in the specified range of pressure. To make the design reliable, the device characteristics have been optimized using an evolutionary algorithm. The fitness function notably takes into account machining and alignment tolerances. Typical designs dedicated to drug delivery and hydrocephalus treatment are discussed.

Passive flow regulator for precise high-throughput flow rate control in microfluidic environments

In this paper, we propose a passive flow regulator with a five-layer structure for high-throughput flow-rate control in microfluidic environments.

A passive flow regulator with low threshold pressure for high-throughput inertial isolation of microbeads.

In this work, we present a novel passive flow regulator fabricated by stacking five functional layers. To understand the flow-rate regulation characteristics, a series of prototype devices with different structural dimensions are investigated. The experimental results show that our regulator can exhibit a constant delivery flow rate of up to 4.38 ± 0.1 ml min(-1) with variations less than 5%, and the minimum threshold pressure for achieving a constant flow rate is only 10 kPa. As compared with previously reported regulators, our regulator offers a much wider flow-rate regulation range under lower threshold pressures. To validate the practical function of our regulator, a low pressure gas-driven flow system integrated with two passive flow regulators and a Dean flow fractionation chip is constructed to achieve high-throughput inertial isolation of differently sized microbeads. The isolation performance is found to be totally independent of the inlet pressure, which permits the use of portable and low-cost flow-driving apparatus for accurate flow control. Therefore, the passive flow regulator proposed in our work is potentially useful for the steady injection and accurate control of the sample fluid in low cost, miniaturized microfluidic systems.

A MEMS-based passive air flow regulator for handheld breath diagnostics

This paper reports on a passive MEMS-based flow regulator designed to maintain a steady flow during asthma diagnostics. A prototype consisting of six in-plane moving pistons that restrict the flow through six flow orifices has been fabricated from three wafers using standard silicon micromachining. The in-plane design enables relatively large flows and tuning of the flow and pressure range to specific application requirements by changing a wafer thickness. In particular, for FENO asthma monitoring, regulatory guidelines specifies that measurements should be made at steady flow of approximately 50ml/s and within a pressure range of 1–2kPa. Experimental evaluation of the prototype shows that the flow rate is controlled within a dynamic pressure range of 770Pa compared to only 430Pa for a dummy structure and that it can be achieved on a chip measuring only 2mm×2mm×4mm. The evaluation also showed that condensation of exhaled air that expectedly occurs in the flow regulator at room temperature can be eliminated by local heating of the device to 40°C.

A Planar Compliance-Based Self-Adaptive MicrofluidVariable Resistor

This paper presents a self-adaptive microfluidic variable resistor that accomplishes passive control of liquid flows by exploiting the large compliance of elastomeric polymers such as polydimethylsiloxane. The device features a compliant microstructure embedded in a microchannel that consists of a flexible thin flap and a stiff stopper located in close proximity. The shape and size of the gap between the flap and stopper vary with the applied pressure, resulting in small resistance with respect to forward flow and large resistance with respect to reverse flow. The variable flow resistor shows an interesting diode behavior because its flow resistance is drastically different for different directions of applied pressures and is self-adaptive in the sense that its flow resistance varies with reverse pressure in such a way that the resulting flow rate remains a constant. That is, the flow resistor can be used as a check valve and, more importantly, a passive flow regulator. Prototype devices have demonstrated regulation of nearly constant water flow rates from 0.21 to 1.2 ml/min, with variations less than 3%, under driving pressures that vary significantly from 100 to over 200 kPa. Three-dimensional fluid-structure interaction simulations have been performed to investigate the interactions between the fluid flow and flap deflection. The simulation results agree with the experimental data and provide insight into the device characteristics. Based on its single-layer planar configuration, passive operation over a large pressure range, and use of a flexible and inexpensive polymer, this self-adaptive variable flow resistor is well suited to flow control in lab-on-a-chip systems

Superquadratic Behaviour of Vortex Diodes

The flow/pressure-drop characteristics of most hydraulic and pneumatic elements are approximately quadratic. Deviations from quadraticity are known to occur, particularly at lower Reynolds numbers, due to wall friction. This effect decreases the exponent of characteristic to a value lower than 2. On the other hand, obtaining an exponent higher than 2 has been so far possible only with mechanically variable area in a device employing a negative feedback. A new, purely fluidic solution was discovered by this author. It is based on properties of vortex diodes until now apparently not discussed in literature. Fluid mechanics of the phenomenon is as yet not understood. In systematic series of experiments, the dependence of the value of the exponent on various parameters was established - the results are shown in this paper. A spectrum of interesting applications of the discovered effect seems to exist and this paper gives several examples, ranging from flowmeters to gagging of boiler tubes or shock absorbers. Probably most important is a proposed passive flow regulator (stabiliser).