Difference between revisions of "Assignment 8, Part 3: add flow control and test your device"

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(Build the DAQ interface circuits)
(Test your circuit)
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  <li> In MATLAB, initialize the OsmoticShocker  
 
  <li> In MATLAB, initialize the OsmoticShocker  
 
<pre>
 
<pre>
   foo = OsmotickShocker;
+
   foo = OsmoticShocker;
 
   foo.Initialize;
 
   foo.Initialize;
 
</pre>
 
</pre>
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  </li>
 
  </li>
* If your circuit is working properly, only one color will be on at a time (i.e. the command <t>foo.GreenOn</t> will both turn off blue and turn on green.)
+
* If your circuit is working properly, only one color will be on at a time (i.e. the command <tt>foo.GreenOn</tt> will both turn off blue and turn on green.)
 
  <li> Test your pinch valve control using:
 
  <li> Test your pinch valve control using:
 
<pre>
 
<pre>

Revision as of 15:38, 2 November 2018


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

In order to oscillate our flow at a particular rate, it would be nice to interface the valve with the computer so that we can use MATLAB to do the work for us. 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 card or (DAQ) allows us to send and receive electronic signals from the computer. These are powerful cards, however, we'll only use them to send a digital +5V signal to turn something on, and a 0 V signal to turn something 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 (this video goes through an awesome explanation if you're interested). When implemented in the following way, we can think of a transistor as a voltage controlled switch:

Using a BJT transistor as a switch.

We will build three of these circuits, one for each of the LEDs and one for the solenoid pinch valve. We can then use the +5 V DAQ signal to control each of their on/off states.

Build the DAQ interface circuits

On an electronics breadboard, build the following circuits:

Interfacing LEDs and pinch valve with DAQ

Note the following things:

  • Use a TIP120 transistor, found in the rightmost of the west drawers. Make sure to check the part number as we have several different transistor models floating around the lab that look the same but behave differently.
  • Each circuit will be connected to its own power supply. The blue and green LEDs will use the first and second channels on the lab power supply as usual, and 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
Blue LED P1.0 Orange
Green 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 (which has no fluorescence) and the other with a fluorescent dye called fluorescein (which is excited by blue light).

  1. Hook up tubing to your device
    1. Cut tubing lengths
    2. Connect to device
    3. Use a syringe and a blunt-tipped needle to pull through from outlet
  2. Open the high salt valve and turn on the blue LED.
  3. Set the exposure time and gain to get a bright (but not saturated) image of fluorescein that should be flowing through your device.
  4. 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.
  2. Record movies for several oscillation frequencies, including 2min, 1 min and 30 s.


Pencil.png
  1. Plot the average frame intensity vs. time for your step response movie. Fit the data to the function $ y = A (1- e^{-t/\tau})+ B $ to estimate the time constant, $ \tau $, of your fluidic system.
  2. If we will be measuring oscillations occurring on timescales of several minutes, can we assume an ideal step response?
  3. For each oscillation movie, plot the average frame intensity vs. time and estimate the amplitude gain and phase shift present between the input and output signals. (No need to do any fitting here, just estimate by eye from your plots.)


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 make it sharps waste). You will reuse the tubing in Assignment 7, 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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