- Controlling the piezoelectric dispenser
When working well, piezo dispensers are alot of fun to work with. Since they can dispense very small volumes (50pl to 1nl) and can be set up to do drop on demand, it is possible to design all sorts of miniaturized chemical reactions and experiments. In the biotech world, piezoelectric dispensers are commonly used in DNA, protein and cell based microarray fabrication and they are for manufacturing digital PCR biochips. In addition to biotech, we have systems in labs doing chemical synthesis, making small liquid crystal arrays and they are being used for making small conductive circuits.
The great thing about piezodispensers is that they conserve samples and can be instrumental in increasing the efficiency of experimental workflows. These dispensers are typically used in conjuction with a 3 way valve syringe pump that its possible to aspirate small (<10 microliter) volumes of material from microwell vessels which is achieved when the syringe pump draws sample into the tip when placed in the reagent reservoir. Another aspect that is nice which is overlooked is that the syringe pump can also dispense microliter volumes out of these tips so you have the benefit of aspirating and dispensing in the microliter range using the same nozzle that is used for dispensing in the picoliter range.
Despite the wide application potential, the performance of these dispensers are dependant on many variables. The tips work with a amplified waveform generator and the volume and shape of the droplets can vary signficantly when adjusting the voltage, pulse width and frequency the the waveform that is generated by this controller. Therefore it is important to be able to visualize and measure the droplet in flight using a stroboscope camera. A stroboscope is set up by synchronizing a flashed LED with the piezo dispensing trigger. This like the voltage, pulse width and frequency are adjustable variables used to optimize the dispensing conditions.
In the software there are two tools for controlling and optimizing the dispensing conditions. The first tool is the Piezo controller window, iot_piezocontroller_window.png. This tool lets you adjust the various piezo dispensing parametrs and there you can see the video stream of the droplet in flight in real time. This is displayed below. In addition to the dispensing parameters (voltage, pulse width, frequency and droplet number). It is also possible to adjust the strob LED flash delay from the trigger event and be able to adjust the pressure compensation liquid level. It is important to pay attention to the pressure compensation level since too much pressure (where the liquid level is too high with respect to the tip height) will cause drops to leak out of the orifice of the tip and too little pressure (where the level is too low) then you will see air form at the orifice. In both cases this will effect the dispensing.
In addition to this graphical interface it is worth mentioning that there is also a command line library for adjusting the piezoelectric dispensing parameters. Here is a list of the relevant commands:
- strobcam start - this starts the stroboscope camera feed (CLI commands)
- strobcam stop - this stops the stroboscope camera feed (CLI commands)
- savestrobimgpath (folder) - select folder for saving files, if webroot then "/" (CLI commands)
- remove (folder) - remove folder (CLI commands)
- mkdir (folder) - creates a folder (CLI commands)
- list - lists the folders (can be stroboscope image folders or headcam image folders (CLI commands)
- strobsnap - takes a stroboscope photo these are individual images calling CLI (CLI commands)
- analyzestrobimg - analyzes the stroboscope image, these are individual images calling CLI (CLI commands)
- strobcam snap (image number) (delay) - takes pictures (number of images passed) at a
(defined delay passed) and the file is saved based on the
timestamp,voltage and pulse width as a jpg, gearman function (CLI commands)
- analyze strobimages (sample) - quantifies the strobimages, sample id is called (note no
whitespace characters, please use _ instead) (CLI commands)
- wavesocket report - reports the waveform generator socket server settings (CLI commands)
- setledtime (~5) - sets led exposure time (CLI commands)
- setleddelay (~250) - sets led delay (CLI commands)
- setvolt (50-150) - sets the output waveform voltage (192.168.1.67) (CLI commands)
- setpulse (50-150) - sets the output waveform pulse width in microseconds (192.168.1.67) (CLI commands)
- setfrequency (20-1000) - sets the output waveform frequency in hertz (192.168.1.67) (CLI commands)
- FIRE - piezo nozzle dispenses (192.168.1.67) (CLI commands)
- settrigger on - dispenses upon sensing a 5V signal (192.168.1.67) (CLI commands)
- settrigger off - does not dispense upon sensing a 5V signal (192.168.1.67) (CLI commands)
- gearmangotostrob - positions to strob position by calling a gearman function
- gotostrob - goes to stroboscope position (CLI commands)
- stroboscope on - turns stroboscope on (CLI commands)
- stroboscope off - turns stroboscope off (CLI commands)
- strobon - turns stroboscope on (used in robot command list)
- stroboff - turns stroboscope off (used in robot command list)
- reportliquidlevel (number) - reports current level of pressure compensation vessel (CLI commands)
- setliquidlevel (number) - sets the liquid level and reports current level of pressure compensation vessel (CLI commands)
The second tool that is used for controlling the piezodispending conditions is the stroboscope analysis window, iot_stroboscopeanalysiswindow.png.
This tool allows you to visual stroboscope images that have been analyzed and it can be used to analyze individual images. Note: if you want to analyze a set of images then you need to run the schedular command, analyze strobimages (sample). This happens when you first take a series of photos using the command strobcam snap (image number) (delay). When this happens the list of images taken are saved in the imgdataset json file ("strobimglist":"strobimages\/1422472904_V100_P50_LD250.jpg, strobimages\/1422472908_V100_P50_LD250.jpg, strobimages\/1422472988_V100_P90_LD250.jpg">. When analyzing multiple images, this tool allows the possiblity of establishing threshholds for variation in Max Speed, Avg Speed, Volume and Deflection so if there is a higher variation then this cut off then the stroboscope test fails. This tool measures the speed (meters per second), volume in picoliters, deflection in microns and normalized signal intensity is collected.