Precise and Stable Pressure-Driven Flow Control in Microchannels – P2CS!

Do you want to push and pull the liquid on a chip or in a capillary gently? Do you want to stop the flow at the place where you can observe it in an optimal way? Do you want to be independent of the volume control at the microscopic scale? Then try the pressure-driven approach in microfluidics.

P2CS is designed for controlling pressure and, thus, the fluid flow in microfluidic experiments and microsystems. Standard version of the pressure control system achieves flow stabilization (100 mbar step) after 17 ms and is free of overshootings.

Droplet Generation with P2CS

Pressure-driven (pressure-based) flow control for generating droplets about 35 µm.

Setup with P2CS

Setup P2CS

Droplet Generation with P2CS

Setup with P2CS

The depicted setup consists of:

Droplet generation with P2CS

The droplet generation with the pressure controller P2CS can be realised in typical manner with two-phase systems, e.g. oil-in-water,  air-in-water or vice versa. We typically use T-geometry, Y-geometry, cross-geometry or more complicated channel arrangements for cases, when additional media shall be added later on, e.g. injection of a substance into the droplet.

Applications of the Pressure Control System P2CS

✓ Pressure-driven control in microfluidic applications, e.g. lab-on-a-chip, organ-on-a-chip, point of care, but also capillaries or tubes.
✓ Droplet generation / droplet formation, e.g. in biphasic systems.
✓ Transportation of droplets, cells or other molecules on microfluidic chip or in capillaries.
✓ Implementation of the stop-flow or bidirectional flows in microfluidics.
✓ Experiments with low Reynolds numbers flows.
✓ Polymer and nanotube orientation on chips or in capillaries.
✓ Pressure-driven actuation of membranes and molecules.
✓ Dynamic perfusion during experiment (e.g. in electrophysiology), different mathematical functions are integrated: sine function, saw tooth function for the automated flow.

Advantages of the pressure control system P2CS

✓ Pressure control system is equipped with pressure AND vacuum in each channel, which allows to stop flows as fast as starting them.
✓ No contact with fluids, no contamination, no cleaning is required. All liquids reside externally on-chip.
✓ No slowing down of the system dynamics at extremely low flow rates (at low Reynolds numbers).
✓ Standard version of the pressure control system achieves flow stabilization (100 mbar step) after 17 ms, the fall time is also 17 ms.
P2CS Plus achieves 100 mbar steps in 5 ms.
✓ Pressure controller does not have overshooting or oscillatory settling.
✓ Pressure control system P2CS Plus is controlled with plain-text commands which allows to integrate it in almost all software solutions as LabVIEW, MatLab, scilab, own developments.
✓ Pressure control system P2CS is real time (e.g. pulses have exactly the same length).

Can the pressure control system be customized?
We can flexibly adapt the software of the system and implement a number of individual adaptations of our microfluidic devices.
Why are response, rise and falling times so important?
Fast reaction of the pressure control system results in a more stable and constant flow, as small perturbations are compensated more swiftly.
For example FACS, if you want to sweep cells through the channel, stop them in the focus of the observing microscope, take measurements and remove them and inject the next cell.
You strongly accelerate the screening with high and low pressures.
How can I characterize the transients of a dynamic system properly?
Why is it arbitrary/not correct to characterize the system with the settling time?
If you wish to change the flow speed you want to know how long this may take before the new value becomes “stable”. What does “stable” mean?
Maybe, once the measured value equals the desired value for the first or second time? This can easily be realized by tuning the system to be too “nervous”, i.e. with tendency to overshoot.

What is settling time?
Click here to find a good explanation on this very instructive wikipedia page

The settling time depends strongly on the tuning of the control loop and it is therefore quite arbitrary.
The engineer may tune the settling time to any value when disregarding the overshoot dynamics.

The ideal case would be that the observed quantity rises swiftly, but smoothly until it reaches the new value and settles there. We could realize that with the pressure control system P2CS. Therefore we claim that the dynamics of P2CS relies uniquely on quantities as the compressibility of materials and air and the dead volumes of the valves and manifolds. Otherwise the system is optimal limited only by physical laws (causality).

Now the problem is how to characterize the transition time in a well defined manner: Theoretically speaking, the settling time is infinite in this case and of no use.
The solution is to characterize the pressure control system with the rise and fall time (see the next FAQ).

How do we use rise/fall times to characterize the transient dynamics of the P2CS?
First: What is rise and fall time?
Click here to find a good explanation on this very instructive wikipedia page

The P2CS lacks any overshoot and the flow increases/decreases until it reaches the desired value.

The rise/fall time, however, gives the delay between 10% and 90% of the pressure and flow speed difference. These values are fixed by convention.

In physics 1/e is often used to characterize the time scale for the exponential approach to the final value.

These two measures can be converted by t1/e = t10% / ln(10) (numerical value of ln(10) ~2.3).

What does overshooting mean?
In conventional designs being on the market you observe a damped oscillation before the object stabilizes its position in the microscopy image.

This is a consequence of the classical control method called “PID” (For more information about PID check this very instructive wikipedia page) and non-linear response function of the system.
If you choose the PID-parameters too tight, overshoots appear. This is similar to a harmonic oscillator with a too small damping constant.
If choosing the parameters so that the overshoot disappears, the system’s settling time becomes much more slowly – no choice to get around this limitation – its the law of physics!
Therefore we developed a new control method, which removes such artifacts and improves strongly the response time, for rising as well as falling edges.

You will be surprised of the result because it gives a feeling as if you pushed the cells with your own fingers directly.

Why do I need two pressure levels?
You need it

  • if you want to have a very fine control an stop objects (e.g. cells or molecules) and at the next moment remove them fast and get the next object, or,
  • if you want to study the effect of shear flow over a flow range of 6 orders of magnitude
  • if you are working with small channels of very different lengths or sizes
  • if you mix large and very small channels (meso- and microfluidics)
  • if you need to remove debris of dead cells or dust from your channel and you do not want to remove the chip from the set-up

Do you have any further questions?
Don’t hesitate to contact us.