Pressure driven flow control
Among pressure-driven flow control means a method with which in the microfluidic liquids through pressure be moved. The pressure is typically generated pneumatically by gases , but can also be generated by electrical , magnetic fields or by the gravitational field.
history
Even sucking in a liquid through a straw is a pressure-driven system, so the first applications cannot be clearly identified. An example of a pressure-driven flow from ancient times is the Heronsbrunnen . Here, pneumatic pressure is generated hydrostatically and converted back into the movement of a liquid ( fountain ). This is already a feedback and thus a regulated system.
Physical basics
From thermodynamics it is known that conjugate quantities differ in a systematic way with regard to the scale behavior. The quantities can be divided into two classes: Intensive quantities (temperature T , pressure p , chemical potential μ ) and extensive quantities (entropy S , volume V , amount of substance N ). The extensive sizes scale with the system size, the intensive ones are independent of the scale. The quantity pressure, for example, is defined in thermodynamics as the quotient of two extensive quantities: p = d E / d V (energy E and volume V ), and therefore independent of the scale, since a scaling factor is canceled out. In microsystems there is now the general problem that extremely small volumes can only be controlled with difficulty. The reason is that surface effects dominate, for example surface charges, van der Waals forces or entropic effects such as dewetting due to the entropically unfavorable localization of a fluid molecule penetrating a rough surface. In addition, control is always done on the macroscopic human scale and the translation factor is immense. Volume-based approaches are therefore fraught with problems, because the restriction required for the transition to microscopic scales transforms the shift by several orders of magnitude. As an example: In a square-shaped plunger of a syringe pump with an edge length of 10 mm there is 1 milliliter of liquid. This reservoir is connected to a microchannel measuring 10 μm x 10 μm. If you want to move the liquid in the microchannel by 10 micrometers / s, this corresponds to a flow rate of 1 picolitre / s. To do this, the piston must be moved by 10 femtometers / s. Such a precise drive is not possible with the best of today's technologies at a reasonable cost.
Advantages over other methods
Syringe pumps and peristaltic pumps have numerous disadvantages. Due to the permanent closure of the liquid volume and its enormous size (milliliters) compared to the volume that should be shifted in some applications (nanoliters and less), the slightest deformation (syringe, feed tube, microfluidic chip) or thermal expansion of the materials lead to strong Movements of the sample liquid in the chip. Gas inclusions that are difficult to avoid cause a substantial delay in the movement of the liquid, since the compression of a gas bubble is initially easier than forcing the liquid through the microchannel. This creates a large, non-reproducible hysteresis if you want to pump liquid back and forth. Due to the design, there are further disadvantages, since the precise and at the same time rapid piston movement requires complex and expensive precision mechanics.
Applications in microfluidics
Especially in small canals, pressure-driven flow control has proven to be superior to other methods in terms of speed, precision and long-term stability. Some typical applications are summarized below as examples:
- In the electrophoresis of long DNA molecules in microchannels, very coarse-pored microstructures are advantageous, since the optimal bandwidth of the collision rate of the DNA with the obstacles scales with the length of the DNA. The longer the DNA, the larger the pores should be (quote: de Gennes et al.). By choosing large-pored artificial or natural gels, one deals with the problem of parasitic flow. By applying pneumatic pressures to the openings of the microchannels, this flow can be stopped completely in seconds and for hours with a suitable pressure control.
- Another example is the generation of gradients by co-currents. Here, two different liquids are introduced into one channel at the same time. Due to the absence of turbulence - mainly laminar flows can be found on small length scales - the two liquids in the microchannel run parallel for a long time without significantly mixing. Diffusion alone then ultimately leads to a complete mixture. Before that, however, one finds stable gradients of different profiles that are used for automated cell migration assays. The stability of these gradients requires a high degree of constancy of the flow.
- The precise positioning of individual cells, vesicles, molecules or other micro-objects is further simplified considerably by pressure-driven flow control, since the reaction time is in the range of the diffusion time, based on the movement around an object diameter.
Individual evidence
- ↑ P2CS - Syringe Pump Comparison Calculator - Biophysical Tools
- ↑ C. Fütterer et al., Injection and flow control system for microchannels , Lab Chip , 2004, 4, pp. 351-356, doi: 10.1039 / B316729A , abstract