In [»] cycles flows, the pump is controlled back and forth such that the fluid is alternatively pumped in and out. This is an already interesting pumping pattern if you intend to analyse, for instance, the content of a reactor with a microscope. However, it takes all of its sense when you add two check valves to realize the flow circuit of Figure 1.
Check-valves are fluid elements that let the fluid passes in only one direction. It may consist of a small spring-loaded ball obtruding an orifice (“professional” check-valves), or a small hole made in a neoprene membrane that expands or shrinks depending on the fluid direction (often met in aquarium equipments). The idea of Figure 1 is that the inlet check-valve opens when the pump withdraw fluid and the outlet check-valve opens when the pump inject the fluid. This works fine but, in my experience, has some disadvantages:
Pro: It requires only passive elements (the check-valves).
Cons: It clog very easily with living cells and is very fragile to over-pressure, especially the model with a neoprene membrane.
A solution to these issues is to replace the passive check-valves with an active element: a 3-ways stop-cock/valve. This latter flow element is shown on Figure 2 and can be bought at [∞] Cole Parmer for about $6US a piece (tax and shipment included for a bulk quantity). It is made of polycarbonate and cannot then be used with aldehydes, amines, ketones, esters… More chemically resistant versions exist in polypropylene however.
A 3-ways stop cock is a special type of valve that turns 180° and has three inlets/outlets ports. The hole in the centre piece is too small to achieve proportional flows which is why it is called a stop-cock and not a valve although you may find both terms used by some vendors. When the centre piece is placed at 0° (neutral position), all the three ports are connected together. When the centre piece is placed at either -90°/+90° the centre bore is connected to either side bores depending on the selector position. In other terms, the centre bore is always connected to the flow circuit but the side bores can be connected or disconnected by the selector position. 4-ways stop cocks/valves differs from 3-ways versions by allowing the selector to turn 360° and so it is possible to connect the two side bores together without connecting the centre bore.
Here we will stick to 3-ways versions because it achieves all we need: simply connect the syringe to the centre bore and the side bores to either the inlet or the outlet of the fluid system. When the pump withdraws liquids, the stop-cock should be placed at -90° and +90° when the pump injects liquid. Because we do not want to turn the selector element ourselves, we will couple the valve to a servo-motor according to Figure 3. Be careful to order a servo-motor that actually goes from -90° to +90° because some models only achieve a ±60° range. At the end of the post, I have included the 3D models resources that operates with a HItec HS-635HB servo-motor that you can find at [∞] Robotshop for less than $20US. I have tested the actuator and it performs successfully with the valves ordered from Cole Parmer.
If you have build the [»] Arduino controller, all you have to do is to plug the servo-motor cable on the motor shield, just as shown on the assembly pictures of the [»] previous post. Also, in case you may need to work with 2-ways stop-cocks, the solution proposed here works as fine: just drive the servo-motor at ±90° to block the flow and set to neutral position to let the flow passes. I have used this technology during my PhD and it is very reliable with cell cultures so don’t hesitate!
Here you can download the 3D models required to build the valve coupler. They were designed to work with the HItec HS-635HB servo-motor and 2/3-ways Luer stop-cocks made by Cole Parmer. Don’t forget to set your 3D printer settings to 200 µm resolution to get a reproducible result. Additional M3 and M4 bolts are necessary as well as some glue to fix the Nylon shaft plate of the servo-motor kit; please refer to Figure 3 for assembly instructions.
You may also like: