In my last project the service Ponoko was used to laser cut and engrave the enclosure. As I had some room left on the template, I decided to make some milli/microfluidics using the free space on the template and look at the possibility of using laser engraving and cutting for homemade microfluidics for simple lab-on-a-chip applications.
In the Photon Factory(a laser lab at the University of Auckland where I work) we build microfluidic devices using femtosecond laser machining as well as laser spectroscopy. We machine many different materials, some of which are not able to be machined by normal nanosecond lasers.
Figure 1 | Single cell traps machined in the Photon Factory for use in medical research.
The material used was clear acrylic 3mm. Two different types of chips were made, one engraved and one cut. This was very easy to set up using Inkscape where the lines were cut and the shapes with transparency were engraved. All that was needed was to install Inkscape an open source vector art drawing program and to download the template from the website. After waiting for a week the parts arrived and the chips were analysed using equipment at the University of Auckland.
Engraving
Figure 2 | Engraved milli/microfluidic devices.
Two different etched designs were created - a spiral and a shrinking channel. The engraving was found to be effective with the maximum engraving setting. Other engraving settings on acrylic did not provide a channel that was engraved all along the area and in some places a channel was not cut but produced a dotted line.
Figure 3 | Images of the shrinking channel design with the machined beam profile of 100 microns clearly seen.
Figure 4 | Edge of the chip showing ~100 microns of melting.
Figure 5 | Close up of the spiral design showing the overlap of the channels.
The images would suggest that the design is rastered. To design a chip it would therefore be preferable to use straight channels. The recommended distance between the features can be no smaller than 50 microns and 100 microns. The minimum channel size is around 100 microns.
Surface Profiling
The Dektak II from the Microfabrication Facility at the University of Auckland was kindly lent in order to profile the surface of the chip with the shrinking channel. The Dektak uses a small needle that is run along a surface to detect the roughness. The needle was run perpendicular to the direction of the laser to measure the undulations made by the laser.
Figure 6 | Dektak II used to analyse the surface of the engraved chips.
The uneven surface is visible (figure 7). This could be particularly good for mixing of fluids in the channel.
Figure 7 | Plot of the surface of the engraved chip (the height of the channel is around 400±10 microns).
The average height of the undulations was found to be 30.4 microns (stdev 8.3). The width between the peaks was 100 microns (stdev 5.7). The depth of the channel (found using Vernier calipers) was 400±10 microns. Around the edges the channel was quite curved with around 50 micron before the maximum depth was attained.
Laser cut
A laser cut flow cell and reaction chamber was cut. These chips need sandwiched plastic around them to hold the fluid in the channels.
Figure 8 | Flow cell to right and reaction chamber bottom left.
A single line (100±5 microns wide) was cut in one of the chips. It has to also be remembered that heat effects at the edge do create a slight beveled edge.
Figure 9 | Microscope image of 100 micron channel.
Possible Applications
These chips could be used to make some simple reaction chips which could be probed using a similar spectrometer to that in my previous post. We are also looking into using the flow cell to run spectra in the femtosecond laser system. The engraved chips could be used for making chips that mixed liquid. The chips could be used before chemicals go into smaller microfluidic channels as a premixer. As the channels are small enough they could be used for Flow Injection Analysis in which chemicals are mixed and allowed to react in a chamber and then the resulting products or reactants are detected, usually spectroscopically.
You could have a plant on a chip, in effect a micro-hydroponics system to study biological responses to different herbicides.
The ability to 3D print is not discussed but could be used to create some great experiments and may be reviewed in the future.
At the lab we will be sticking to our femtosecond system for microfluidic devices, however hopefully this discussion will give others outside the lab the means to invent their own milli/microfludic devices.
At the Photon Factory (a laser lab at the University of Auckland) I have just completed a summer studentship building the electronics for a nanosecond transient absorption spectrometer. This spectrometer will be able to excite molecules and then detect the excited state. Another laser will work out for how long the excited states are stable. This will be used to help research of photoactive molecules, made of nano particles that can convert sunlight into energy, such as those in Figure 1.
Figure 1 | Fullerene triad molecule in which light excites an electron in the porphyrin (to the left and right of the fullerene containing a metal). The electron is then taken up by the fullerene. The iron is oxidised at the top of the molecule and gives the electron to the porphyrin, resulted in charge being separated (like a battery separates charge). This can be used to turn sunlight into usable energy. In the spectrometer we work out how long these charge separated states last for in order to design efficient fullerene triad molecules.
At the Photon Factory, we have an ultrafast transient absorption spectrometer that can send pulses femtoseconds (1*10^-15 seconds) across to excite and probe very fast chemical reactions. This system is limited to the femto- to pico- time scales due to the delay being set by making one laser pulse travel a slightly further distance than the other. To probe molecules that have excited states in the nanosecond to millisecond range a new laser system was built that could delay the pulses electronically. Delay generators can be purchased for thousands of dollars, however an in-house Arduino-based delay generator was constructed and computer-controlled using Arduino in order to set the delay.
Figure 2 | Animation of transient absorption spectroscopy for simple formaldehyde C=O group. (Click on the image to see the animation in your browser.)
My part of the project was to construct the electronic delay that could
delay a pulse coming from the pumping laser by nanoseconds to milliseconds in order to fire the probing
laser at varying times to look at the excited state as it decays. Digital delay generators are common in laser applications where timing
down to the nanoseconds is needed to look at processes that happen in
the nanosecond scale and below. Most commercial delay generators cost around $US1600 and up. These digital delays use oscillating clocks to do the
timing but in the lab we only wanted resolution down to a nanosecond scale. For this, I used the Arduino nanosecond delay generator which is capable of delaying a
signal by 4 ns to 10 ms.
Electronics
Analog
The delay was set up using monostable multivibrator chips (TC74HC123(F) $NZ1.56). These chips used two timers whose time constants were determined through a capacitor and resistor. When a pulse is detected the output becomes high via a Schmitt trigger. This discharges the capacitor and then charges it again. When the voltage in the capacitor is 66% the output pulse is toggled low. This gives the delay time in seconds as the product of the resistor and capacitor values. I used three dual timing ICs. The first two have four delays which are variable between 4 ns to 10 ms by using four different capacitor values and 100kOhm variable resistors. The output pulse then triggers the next timer, which has a PCB mounted pot for referencing between the lasers. Finally, the pulse is lengthened in a fixed delay, giving the laser the right pulse length for it to trigger. This can be seen in figure 3. The impedance was not taken into account and a simple signal generator was used to provide the needed current for the high speed electronics which uses input impedances of 50 Ohms. As I understand it (and you can test this yourself) a lot of electronics have a long decay after a pulse has passed through due to the current having nowhere to discharge. Using drop down resistors from the signal line to ground it gives the current a place to quickly discharge. However, this usually leads to a voltage drop due to the load this resistor places on the circuit. In a lot of high speed systems impedance (another way of thinking about resistance if taken in the context of DC circuits) is very low. This leads to no trailing of the pulse as the current is quickly removed, however this requires that enough current is outputted current to keep the voltage above the triggering threshold. There are other things to take into account, like reflection caused by ill-matched impedance. To put it simply, I did not plan for it in the design so I used a circuit which had built-in impedance.
Figure 3 | Timing and delay of the pulse in the various chips.
Digital
To vary the four variable timing chips, digital dual chips' potentiometers were used (MCP42100 $NZ2.56). These were used as variable resistors with 8-bit resolution, giving 256 different values between 0-100kOhm which it could be switched to. The capacitor values chosen were 10pF, 100pF, 1nF and 100nF. This gave a range of 4 ns to 10 ms by using one of the four delay timers. The chips were switched off and on using the clear pins and the Arduino.
An Arduino clone, the Etherten, from Freetronics was used to set the value of the digipots using simple SPI interface. The chip select pin was toggled low and two bytes were sent. The first byte was a command byte that told the chip which pot to set. The second was the 8-bit number 0-255 to set the pot. The clear pin on the vibrator chips are used to turn on and off the chips when the pin is set low.
Figure 4 | Schematic of the nanosecond signal delaydone on Eagle CAD.
Figure 5 | PCB design done on Eagle CAD. Surface-mounted capacitors and resistors were used for cost and also space efficiency.
Software
Arduino
The code to run the Arduino is attached in the link at the bottom of the page. Serial commands set the pot values and turn on the various timing chips. The Arduino can also sum the number of pulses that have passed and disable the chips when a certain shot number has been reached.
Minimum Arduino code to set the pots and turn on one of the delays is below.
#include SPI.h
int Aselect = 3; //Assign variables
int Bselect = 2;
int Cselect = 5;
int Dselect = 4;
int varcalselect = 7;
int Outputselect = 6;
digitalWrite (Aselect, LOW); //Write high or low digital value to pins
digitalWrite (Bselect, LOW);
digitalWrite (Cselect, LOW);
digitalWrite (Dselect, LOW);
digitalWrite (varcalselect, HIGH);
digitalWrite (Outputselect, HIGH);
digitalWrite (10, HIGH);
digitalWrite (9, HIGH);
SPI.begin();
digitalWrite(10, LOW); //Set the chip select for the first pot
SPI.transfer(B00010011); //B00010001 writes pot 0 B00010010 writes pot 1 B00010011 writes both pots
SPI.transfer(200); //transfer the byte that sets the value
digitalWrite(10, HIGH);
digitalWrite(9, LOW);
SPI.transfer(B00010011);
SPI.transfer(100);
digitalWrite(9, HIGH);
digitalWrite(Dselect, HIGH); //Turn on the correct
}
void loop(){
}
Labview
A very simple program was built in Labview to control the Arduino through serial interface to send the commands and receive the number of shots. Labview was used as the equipment in the lab all runs on Labview and the program is quick and easy to set up. However, the serial communication makes it easy to control the delay using other programs such as any serial terminal or a simple Python program using pyserial.
Figure 6 | Labview VI for the sending of serial commands to the Arduino to control the delay.
Figure 7 | The front panel for the Labview program.
Finished project
The enclosure was designed in Inkscape, a free vector art drawing program and printed using the Ponoko laser cut service in New Zealand. The cost of the laser cutting was NZ20 dollars including the postage. There was some room on the template so I put in some micro/milli-fluidics which I will post about soon. The PCB board was built as described in the previous blog entry.
The Etherten was already in the lab when I started the project and can be bought for around NZ81 dollars in New Zealand, however an Arduino Uno is all that is needed and would be under NZ50 dollars. The rest of the components cost NZ49.47 dollars, bought from RS New Zealand.
Figure 8 | Electronics mounted in the laser cut enclosure.
Figure 9 | The shield with the two digital pots as well as the three timing chips can be seen mounted onto the Etherten, an Arduino clone.
Testing of the device using a signal generator and an oscilloscope found the jitter of the unit to be below 3 ns and the overall delay precision 4 ns. This gives us the needed 10 ns timing which is our limit with the laser pulse width. The main limitation is the jitter which at high resistor values of above approximately 70 kOhms can increase to ~25 ns. More delays would be needed in order to get the resolution at the longest delay. The reference delay was kept at 1kOhm and the signal generator that the signal went into to match the impedance has a simple timer that can be used to set the pump and probe to zero - it basically lines the laser pulses up so that they both trigger at the same moment.
Figure 10 | Testing of the nanosecond signal delay at the Photon Factory.
Figure 11 | Presentation for summer studentship at the University of Auckland put together with Powerpoint and Prezi.
Links
Photon Factory
I have added a sourceforge project page if anyone wants to find the files
and the code to build your own or if anyone wants to improve the delay; possibly make a completely digital solution with much greater resolution and precision.
Acknowledgements
Dr. Cather Simpson for the opportunity to work in the lab; Julie Kho for her help, in particular with the optics; Peter Hosking for his help with the electronics, the PCB making and the purchase of the Etherten Arduino clone; William Parker for his advice on some signal problems; Sarah Thompson for her assistance with the optics and basic
knowledge on the TrA systems and Matthew Edmund for his help with reviewing the Ponoko laser service. Ron Byrant and Roger Vanryn, the Department of Chemistry technicians who helped me work out problems in the circuit and supplied me with components.
PCBs or printed circuit boards are used in almost every electronic device to mount electronics and to connect different components with metal tracks. Most PCB's that are made commercially use a subtractive process where a laminate with a thin layer of copper on top is used. A plastic coating in the shape of the circuit prevents the copper from being etched while the rest of the unwanted copper is removed using an acid. The most common way to place the protecting plastic in the desired circuit shape is by using a photoresist. The design is printed out onto a transparent plastic sheet and placed on top of the board. Ultraviolet light is then used to break down the plastic coating where the mask is not covering. The board is then etched leaving the circuit design.
Another method to place the protecting layer onto the copper is by using the toner transfer method. This is a much simplier method. A laser printed is used to print the curcuit design onto a piece of shiny paper (magazine paper works well). The ink is then transfered to the copper covered laminate and fused on using an iron or a laminator. The paper is then wet and rubbed off leaving the ink on the paper. The metal not covered with the ink can then be etched away leaving the circuit.
At the Photon Factory in University of Auckland we decided to use this method to make our own PCBs in the lab. We initially attempted using a cheap iron but the results were not very good with the ink not transferring over to the copper and the ink spreading in some places.
The laminator was the next option however as the laser printer ink melts at around 150 degrees C we had to modify the laminator to run at higher temperatures. The laminator had two thermostats that kept the temperature around 145 degrees C by cutting power to the heaters when the temperatures went above 145 degrees C. The other thermostat was used to light a green LED to tell the user when the laminator was hot enough to use. The simplest hack would be to replace the thermostat with a higher temperature thermostat (see link). However we wanted to be able to finely adjust the temperature of the laminator so we bought a temperature switch control board from Jaycar. This allowed us to control the temperature up to 245 degrees C. We set the laminator to sit between 160 and 180 degrees C. (Jaycar temperature switch)
Figure 2 | Modified laminator mounted onto a piece of MDF
Figure 3 | Temperature control switch with red and black cables turning the heater rails on and off and the grey cable going to a thermistor to measure the temperature.
Figure 4 | Top of the laminator was cut and covered the electronics. This meant we could put the PCB straight into the rollers and were able to see what was going on.
The laminator cost NZ$40 and
the temperature switch cost the same. So the whole system cost NZ$80.
This was a very quick and easy hack that will let us now make our own
PCB's in the lab.
Steps to making a PCB
1. Design circuit on the computer. I have used the free version of Eagle CAD a very good program for doing circuit diagrams and then easily transferring the design to a PCB .
Figure 5 | PCB design on Eagle CAD ready to be printed
2. Print the circuit board onto a magazine we used our trusty student magazine Cracuum. The best paper needs to be very shiny and is easily wet. So most magazines will do the trick. Print the design using a normal laser printer and make sure the toner density is set to its highest value no eco-saving mode. Then cut out the design ready to transfer the toner. As this will flip the design (mirror the design) you can print the circuit as is on the computer if you have designed the pads and circuit on the top layer they will be on the bottom of the pcb when transferred.
Figure 6 | PCB design laser printed
3. Clean the PCB using a metal cleaner we used Brasso. A scrubbing brush is used to clean the PCB. This is very important as even a finger print can stop the toner from transferring.
Figure 7 | Brasso and scrubbing cloth to clean copper on the PCB.
4. Put the paper onto the PCB and run it through the laminator. We found doing this 10 times worked well switching between length ways and width ways. Be careful to catch the PCB as it will shoot out the back due to the pressure from the rollers.
Figure 8 | Copper board and magazine with design going through the laminator
5. You want to put the board straight into water to start wetting the magazine. It takes around 5 minutes for the water to soak in. Then the paper can be gently rubbed off exposing the toner transferred to the copper. If you have done it right the toner should be bonded strongly with the copper.
Figure 9 | Toner bonded to the copper on the board.
6. A solution of one part potassium persulfate to 5 parts water was used to etch the copper around the toner. The hot water tap was run making a hot water bath which is needed to speed up the chemical reaction.
Figure 10 | Etching solution in hot water bath
The solution needs to be stirred and after around 5 minutes you will see a blue solution of copper 2+ forming as the acid dissolves the copper into aqueous ions.
Figure 11 | Copper being etched leaving the fiber glass underneath
7. Clean off the toner using acetone or some similar solvent and you are left with a copper printed circuit.
Figure 12 | Printed circuit board this board is for doing nanosecond to microsecond delays of nanosecond lasers more on this in another post. One hundred micron features were able to be made on this board.
Figure 13 | Header pins drilled and mounted onto an Arduino clone. Nano delay shield.
So in summary a PCB laminator was constructed to transfer laser printer toner to a copper board. This was found to produce features down to around 100 microns. I will post soon about the circuit that is being constructed when I have drilled and mounted all the components.
