Assignment 8, Part 3: add flow control and test your device

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Overview

The next step is to set up the reservoirs, tubing, and valves to control the flow through our microfluidic device. The fluid from two large reservoirs will be connected to the PDMS device with thin tubing. We also need a mechanism to drive fluid flow through our device. While some devices use carefully regulated air pressure or syringe pumps to drive flow, we will take advantage of gravity. By raising the fluid reservoirs higher than the outlet tubing of our device, we create a difference in potential energy that will push the fluid from the reservoir, through the tubing and device, then out into the waste. A pinch valve (that does exactly what you think) will allow us to choose which reservoir will provide flow to the device.

Schematic of flow setup

The solenoid pinch valve has two slots for tubing – one is normally open (NO), which will be used for the regular (low salt) medium, and one is normally closed (NC), which will be used for tubing connected to the high salt medium. Applying 12V to the solenoid will switch the flow from low to high salt. We will use a short length of flexible silicon tubing that can be easily pinched by the valves, and connect it to the device and reservoirs using a stiff less-expensive tubing made by Tygon.

Pinch valve operation schematic

Assemble the fluidics caddy

  1. Grab one of the L-shaped black acrylic fluidics caddys and secure it to your vertical P14 post.
  2. Secure two 50 mL conical tubes to the leftmost side of the acrylic using two routing clamps, a 1/4-20 screw and a wing nut.
  3. Secure the pinch valve to the acrylic nearest the P14 post using a two line routing clamp.
Fluidics caddy

Using a data acquisition card to control pinch valve and LEDs

To oscillate the flow between two fluid reservoirs, we need to switch on and off the solenoid valve at a particular rate. Let's get MATLAB to do the work for us by interfacing the valves with a lab computer. While we're at it, wouldn't it also be nice to turn on and off your LEDs using a computer signal? A data acquisition or (DAQ) card allows us to send and receive electronic signals from the computer. These are powerful cards, however, we'll only use them to provide a digital output of either 0 or +5V to control whether something is on or off.

Unfortunately the +5 V signal from the computer cannot provide enough power to run our LEDs at 1A of current, nor can it provide the +12 V necessary to control the solenoid pinch valve. Should we give up and go home? No! Let me introduce you to the cornerstone of digital electronics: the Transistor. We won't go into detail about how these incredibly useful and versatile circuit elements work, but this video goes through an awesome explanation if you're interested. For our purposes, we'll implement a transistor in the following way, where it will act like a voltage controlled switch:

Using a transistor as a switch.

By sending a 5V signal to the gate, the transistor effectively "closes the switch" and allows current to flow from the drain to the source, turning the circuit ON. Sending a 0V signal in turn, prevents current from flowing from the drain to the source, effectively acting like an "open switch", turning the circuit OFF. We will implement three of these circuits: one for each LED and one for the solenoid valve.

LED control circuits

LEDcontrolCircuit.png

The LED control circuits are implemented as shown in the above figure. Note that:

  • P1.0 and P1.1 are the signals generated by the DAQ.
  • When the DAQ signal is +5V, the LED will turn on. 0 V from the DAQ turns the LED off.
  • The 1k resistor connecting the gate to ground is there to prevent the transistor from switching on accidentally when the DAQ is disconnected.
  • Each LED is connected to its own channel on the lab power supply, so the blue and green brightnesses can be controlled independently.

Solenoid valve control and power management

To control flow through our microfluidic device, we will use essentially the same circuit to open and close the solenoid pinch valve as we did to turn on and off the LEDs. The circuit is shown below.

Solenoid pinch valve control circuits.

There are two small modifications that we need to make compared to the LED control circuits. Namely:

  1. There is a diode in parallel with the solenoid valve. This prevents voltage spikes caused by switching and inductive load on and off.
  2. We'll use pulse-width modulation (PWM) to reduce the power consumed by the valve. Supplying the solenoid valve with +12V for long periods of time will cause it to heat up significantly. This can have adverse effects like heating the media in the tubing which can cause bubbles. We want to avoid bubbles at all costs. The valve requires +12 V to turn on initially, but requires much less to stay on. Our solution is to use the DAQ switch the circuit on for 100 ms (to engage the solenoid valve), then switch on and off the power to the valve at 10kHz with a 17% duty cycle (to keep the solenoid engaged, but with lower power).
Solenoid valve PWM.

Build the DAQ interface circuits

On an electronics breadboard, build the following circuits to control your green and blue LEDs and your pinch valve:

Interfacing LEDs with the DAQ

Note the following things:

  • Use a N03HDL transistor, found in the rightmost of the west drawers. Check the part number since we have several different transistor models floating around the lab that look the same but behave differently.
  • Use the two channels of the lab power supply to power each LED circuit (in independent mode).
  • The pinch valve will use the 12 V output from the Diablotek computer power supply (below). Also shown below is a Molex connector that you can use to connect the Diablotek power supply to the breadboard.
  • Use the cable provided (see figure to the right) to connect your breadboard to the DAQ. Each wire should be labeled based on the pin it is connected to in the table below.
DAQ connection cable
Signal Name Signal Location Pin wire color
Green LED P1.0 Orange
Blue LED P1.1 Red
Digital GND DGND Black (bundled with Red/Orange)
Solenoid pinch valve P1.2 +Green / -Blue or -Black
Digital GND DGND Blue or Black (bundled with Green)

Test your circuit

  1. In MATLAB, initialize the OsmoticShocker
      foo = OsmoticShocker;
      foo.Initialize;
    
  2. Test your control of blue and green LEDs using the commands:
      foo.BlueOn
      foo.GreenOn
    
    • If your circuit is working properly, only one color will be on at a time (i.e. the command foo.GreenOn will both turn off blue and turn on green.)
  3. Test your pinch valve control using:
      foo.OpenHighSalt
      foo.OpenLowSalt
    
    • You should be able to hear the valve switch on and off, and see the plunger move.
    • Make sure the Diablotek power supply is switched on.
    • It's best practice to leave the valve in the off (low salt) state when not in use.

Test your flow using water and fluorescein

Before measuring the osmotic shock response of yeast, we need to test that our microfluidic device is functioning like we expect it to. To test our flow, we'll fill one fluid reservoir with water and the other with a fluorescent dye called fluorescein (which is excited by blue light).

  1. Use the 50 ml conical tubes to hold 15 ml fluid reservoirs: one with DI water and one with fluorescein.
  2. Mount your device onto a microscope stage, and insert the inlet tubing into the reservoirs.
  3. Slowly pull fluid from both reservoirs through the device from the outlet using a syringe and a blunt-tipped needle.
  4. Use a 50 ml tube and tube rack as a waste container for the outlet. Check that a droplet forms on the outlet indicating fluid flow.
  5. Insert the flexible silicone tubing into the pinch valve making sure that the fluorescein is connected to the 'high salt' valve.
  6. Use Matlab to open the high salt valve and turn on the blue LED. Increase the LED current to (but not more than!) 1 A.
  7. Use the function:
    foo.SetBlueImageParameters( gain, exposure ) 
    to set the exposure time and gain to get a bright (but not saturated) image.
  8. Open the low salt valve. You should see the fluorescence go away. If you can't see a difference between high and low, it's time to do some debugging! Some good questions to ask are: can you see droplets forming at the outlet tubing (demonstrating that there is flow) when either valve is open? Are there any bubbles blocking your tubing? Are your exposure settings sufficient?

Record some test data

Once you're sure everything is working, take some test data.

  1. Record a step response movie (water first for 10 s, then fluorescein for 20 s) using the following commands:
    foo.SetStepResponseParameters
    foo.StartStepExperiment
  2. In addition, record movies for several oscillation frequencies (between water and fluorescein), including 2 min, 1 min, and 30 s.
    foo.SetOscillationParameters
    foo.StartOscillationExperiment
    

  3. Pencil.png
    1. For each oscillation movie, plot the average frame intensity vs. time, as well as the valve state (1 or 0) vs time. Normalize your average frame intensity so that it oscillates between 0 and 1.
    2. Did your fluidic device behave as expected? Why or why not? Reflect on any challenges you faced in the lab.
    3. Plot the average frame intensity vs. time for your step response movie. Fit the data to the function $ y = A (1- e^{-(t-t_0)/\tau})+ B $ to estimate the time constant, $ \tau $, of your fluidic system.
      1. What physical aspect of your experiment do each of the above fit parameters represent?
      2. In Assignment 10, we will be measuring oscillations with periods of several minutes. Can we assume an ideal step response in that case? Why or why not?


    Clean up

    1. Connect a syringe to the outlet tubing.
    2. Remove the tubing from the fluorescein reservoir and insert it into the water.
    3. Disconnect the tubing from the high salt valve (so both are open) and draw water through the entire device using the syringe. Once you have washed the tubing sufficiently, remove the inlets from the water, and withdraw the remaining fluid from the system.


    Global Tree.gif Make sure to dispose of liquid and solid waste as follows:
    1. Disconnect the tubing and dispose of the device (PDMS, tape and coverslip) in the sharps waste container (the glass coverslip makes it sharps waste). You will reuse the tubing in Assignment 10, keep it (relatively) clean by storing it it a petri dish that you keep with your microscope.
    2. Used needles (even blunt ones) and connected syringes go in the sharps waste.
    3. Any unused fluorescein from the reservoir can be kept for other groups, but you may empty the diluted fluorescent dye from the waste container down the drain.


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