Microfluidic flow control is a key functionality for complex labs-on-chips. It ensures that the fluids in the chip arrive at the right time, in the right volume, at the right location. This is crucial in managing all the different on-chip assay or process steps. On-chip flow control enables the design of compact, autonomous microfluidic platforms (or chips), features that are especially vital in the growing market of Point-of-Care (POC) applications. In this article, we dive deeper into the possibilities of microfluidic flow control, active flow control, and passive flow control. Furthermore, we discuss capillary burst valves, electrostatic triggering, and materials that are used.

The possibilities of microfluidic flow control

Microfluidic flow control enlarges possibilities. Steps that are made possible by microfluidic flow control:

• Accurate volume sampling
• Spatial and temporal control over fluid flows
• Controlled mixing of specific volumes
• Controlled washing
• Incubation and heating

Active flow control

Active flow control means that the flow is driven by external forces. Active flow control typically makes use of a flexible membrane in the architecture of the valve and pump elements that are integrated in the microfluidic chip. This flexible membrane causes a channel to close, and there are different ways to achieve this. Our three most frequently used elements in this segment are:

• Pneumatic valve/pump
  The membrane is actuated by pneumatic pressure. Pressure differences cause the flexible membrane to deform, opening or closing the fluidic pathway. By placing several of these pneumatic valves in a row, a pumping system can be created. Such a pump can drive fluids forward of backward, depending on the sequence actuation of the valves.
• Pinch valve
  This membrane-based flow control works by ‘pinching’ the microfluidic channel until it closes. By mechanically pushing a pin against the membrane, the path of the fluid is obstructed. As soon as the pin is withdrawn, the flow can recommence. An advantage of this type of valving is that the actuation mechanism can easily be reused from one chip to another.
• Check-valve
  Microfluidic check-valves (or one-way valves) are the hydraulic equivalent of the electric diode, allowing fluid to flow in one direction while blocking it in the opposite direction. This functionality can be achieved by the use of a particular type of flap made using the flexible membrane. A fluid can easily bend downward the membrane to open a pathway, as long as it flows in the right direction. As soon as the flow is reversed, the membrane is pushed against the edges of the fluidic pathway, serving as a tight lid preventing beckfow.

Active flow control uses external actuators that determine whether and when the flow will go or stop. It is based on valves that are fully integrated in the microfluidic chip, that are controlled through pneumatic or mechanical control systems. By properly combining and controlling the valves, they can also perform on-chip pumping. This enables for example controlled mixing, washing and multiplexing of various inputs over one sensor area. We make use of visibly clear materials and provide reliable on-chip valve and pump systems in a selection of materials and hybrid combinations. This is the mechanically driven type of microfluidic flow control, that relies on an external input for actuation. This type of flow control offers dynamic flow actuation.

The active flow control systems Micronit provides, are based on fully integrated membrane valves and pumps. These are actuated through pneumatic control systems.

Electrostatic triggering

When a time component is of the essence in a multi-step assay, this can be induced by electrostatic triggering. When the flow has stopped at a capillary burst valve, applying a short voltage will retrigger the flow. The operation principle lies in applying a voltage between two electrodes which are placed in front of and behind the valve. Applying the voltage attracts the meniscus towards the second electrode and triggers the valve. Using electrostatic triggering, we can produce autonomous devices that require little operational power and are suitable for use in handheld instruments.

Passive flow control

Passive flow control makes use of the properties of components that are integrated into the microfluidic chip to control the flow. For example, integrating check valves can ensure that the flow can only go in the desired direction.

Capillary flow control

A specific type of passive flow control is capillary driven flow control. In this case, the flow is controlled because the interplay between the microfluidic geometry, surface properties and the properties of the liquid ensure that the liquid flows as desired. Incorporating changes in channel geometry and surface properties in the microfluidic design, allows us to control where flows stop, when they will continue, etc.

Our portfolio includes the two following families of elements:

• Liquid-triggered stop valves
  Capillary stop valves or burst valves are commonly used to regulate flow in a capillary driven system. These valves rely on an abrupt increase in the cross-section of a microfluidic channel, which causes the capillary filling to stop at the transition. The flow recommences as a result of a certain trigger, for example when a different fluidic stream passes the junction.
• Electrostatic valves
  In the case of an electrostatic valve, electric voltage pulses are used as the trigger, bursting the valve and recommencing the flow. The voltage is applied between to integrated electrodes: one placed in front of and the other behind the valve.

Materials

Microfluidic flow control is especially of the essence for Point-of-Care devices and workflow automation. These devices rely on a complete on-chip assay that can perform a specified (test) procedure without the help of medically skilled personnel. Point-of-Care tests usually make use of disposable microfluidic chips that are made of cost-efficient and easily accessible materials. At Micronit, we focus on developing our polymer substrate technology and are capable of integrating functional elements, capillary valves, and metal electrodes into a polymer-based microfluidic chip.

Our microfluidic flow control systems

We have experience in adding microfluidic flow control elements to different materials and various hybrid combinations:

• COC devices with elastomeric COC membranes
• Glass devices with PDMS membranes
• Full glass devices
• Polystyrene / SEBS membranes

Tell us your application-specific needs and we will make a design that meets your requirements. We have in-house manufacturing capabilities for all of these membrane-based valve solutions. To manufacture them, we make use of the following fabrication techniques:

• Milling
• Embossing
• Injection molding
• Lamination
• Laser ablation
• Thermal bonding