Archive for the ‘Fabrication’ Category

Simple and rapid fabrication of paper microfluidic devices utilizing Parafilm®

E. M. Dunfield, Y. Y. Wu, T. P. Remcho, M. T. Koesdjojo and V. T. Remcho
Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA

Why is this useful?


Paper-based microfluidics offers several advantages over conventional microfluidics, and has great potential to generate inexpensive, easy-to-use, rapid and disposable diagnostic devices. Unlike traditional microfluidics, which often requires pumps to move fluid through the microfluidic channels, paper microfluidics can be performed without such instrumentation due to the flow of fluid being driven by capillary action through the paper. Hence, paper-microfluidics is well-suited for use in point-of-care diagnostics and in developing countries where expensive instrumentation is not available. There have been several advancements in the fabrication of paper microfluidic chips. Fabrication of paper-based chips can be done by photolithography [1], wax printing [2, 3], plasma etching [4], inkjet etching [5], and use of a cutting plotter [6]. However, such techniques may require the use of organic solvents during the fabrication process, they can be labor-intensive and expensive, and may impose limits on the types of materials that may be used in the chip. A novel technique is presented that utilizes a hydrophobic film material, Parafilm® “M”, which is heated to above its melting temperature of 60οC, and pressed into a piece of paper. The channel mask is cut from polycarbonate (PC) film, and sandwiched between the paper and the Parafilm®. The PC film mask prevents the melted Parafilm® from penetrating into the paper in the channel region, and therefore defines the hydrophobic boundaries of the paper channel. This technique has been tested on a wide variety of paper materials including Whatman Grade 1 filter paper, VWR light-duty tissue wipes and Kimtec Kimwipes, among other papers. While in some cases the paper can be cut directly into the desired channels, thin paper can easily tear and complex patterns proved to be challenging when cut by a cutting plotter. In contrast, the more rigid PC film can easily be cut into simple or complex patterns with a cutting plotter. Furthermore, the process is inexpensive, rapid, and does not require the use of organic solvents during fabrication.

What do I need?


  • Paper, such as Whatman filter paper, Kimtech Kimwipe or VWR light-duty tissue paper
  • Polycarbonate film, thickness approximately 100 µm
  • Parafilm® “M”
  • Aluminium foil
  • Scissors or x-y cutting plotter (for more complex channel patterns)
  • Hot press

What do I do?


  1. Cut out the desired channel patterns in polycarbonate film. For better cutting precision and accuracy, an x-y cutting plotter can be used.
  2. Create an assembly consisting of paper, polycarbonate (PC) cutout, and Parafilm® “M” as shown in figure 1. The paper can be Whatman filter paper, Kimtech Kimwipes, VWR light-duty tissue wipes or other paper of desired properties.

    Paper, polycarbonate and Parafilm® stack

    Figure 1. Paper, polycarbonate and Parafilm® stack set-up before heating and pressing.

  3. Cover both sides of the paper, PC, Parafilm® stack with aluminium foil to prevent sticking of the Parafilm® to the hot press plates, and place the whole assembly into the hot press.
  4. Heat the hot press to above 60οC, and apply ~200 psi of pressure for 1 minute. Note: 200 psi is necessary when using Whatman Grade 1 filter paper. Applied pressure varies depending on the thickness and porosity of the paper used.
  5. Allow the aluminium packet to cool, and then remove the foil from the paper microfluidic chip.

    Completed paper microfluidic chips made using Parafilm®

    Figure 2. Completed paper microfluidic chips made using Parafilm®. (a.) Spiral design has a channel width of 1 mm. (b.) Design has circles of 4mm diameter and 8 straight channels of 2mm width and 10mm length. (c.) Blue dye added to the paper microfluidics chip shown in b.

What else should I know?


For heavier weight paper such as Whatman Grade 1 filter paper, it is necessary to apply higher pressure (~200 psi) such as in a hot press to produce the microchips. However, for lighter weight paper such as Kimtech Kimwipes and VWR light-duty tissue wipes, microchips can simply be made by heating the plates of a heating element such as a hair straightener, and then applying gentle pressure to the paper, PC, Parafilm® stack to produce the paper-based chips.

References


[1]  A. W. Martinez, S. T. Phillips, B. J. Wiley, M. Gupta, and G. M. Whitesides, Lab Chip, 2008, 8, 2146-2150.
[2] Y. Lu, W. Shi, L. Jiang, J. Qin, B. Lin, Electrophoresis, 2009, 30, 1497-1500.
[3] E. Carrilho, A. W. Martinez, G. M. Whitesides, Anal. Chem., 2009, 81, 7091-7095.
[4] X. Li, J. Tian, T. Nguyen, W. Shen, Anal. Chem., 2008, 80, 9131-9134.
[5] K. Abe, K. Suzuki, D. Citterio, Anal. Chem., 2008, 80, 6928-6934.
[6] E. M. Fenton, M. R. Mascareñas, G. P. Lopez, S. S. Sibbett, ACS Appl. Mater. Interfaces, 2009, 1, 124-129.

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Design of an inexpensive spin coater (with a touch-screen interface)

Gurucharan V. Karnad*a,b, R. N. Ninad*b and V. Venkataraman a
a Dept. of Physics, Indian Institute  of Science, Bangalore, India
b Dept. of Electronics and Communication Engineering, Amrita School of Engineering, Bangalore, India
* Corresponding authors: Gurucharan V. Karnad, R. N. Ninad
Email: gvkarnad[at]physics.iisc.ernet.in, rnninad[at]gmail.com

Why is this useful?


Spin coating is one  of  the  coating  techniques  used  to  apply  thin uniform  films  to  flat  substrates. A Spin Coater is a machine used for spin coating, and is one of the most ubiquitous instruments in any laboratory dealing with microfluidic devices.

Most commercial spin coaters are extremely expensive (≈ > US $1,995 [1]) and come with features and specifications not necessarily needed for fabricating and experimenting with polymer based microfluidic devices.

The cost of this instrument should not act as a deterrent for groups intending to venture into fabricating and experimenting with simple devices, hence the effort in this direction.

We have designed an inexpensive spin coater (with a touch-screen interface) costing less than US $350. The user input is through the touch-screen interface, where parameters such as spin duration and speed can be entered. Real time speed is also displayed alongside. A microcontroller forms the intelligence of the system and manages the inputs, display, and speed and duration control. The real time speed is sensed by the microcontroller using an optical encoder, and a control loop keeps it within acceptable error limits.

The substrate is mounted either using a double-side tape or a set of clamps. Improvements with regard to vacuum chuck, computer interfacing etc., can be done as per necessity.

What do I need?


  • PIC Microcontroller Programmer (with support for 18F4550)
  • PIC 18F4550 (≈ US $04.47 [2])
  • Graphic LCD 128×64( JHD 12864E or equivalent) (≈ US $16.80 [3])
  • TouchPanel (≈ US $07.00 [4])
  • TouchPanel Connector Board (≈ US $04.00 [5])
  • Infrared Distance Sensor (equivalent – Cytron IR01A Medium Range Infrared Sensor) – Optical Encoder-Sensor (≈ US $08.10 [6])
  • Crystal oscillator circuit (11 Mhz Crystal Oscillator, 22pf Capactitor (2) – connection as given in PIC 18F4550 Datasheet) (≈ US $01.00 [7])
  • IR 2110 (≈ US $07.25 [8])
  • IRFP 150N (≈ US $02.77 [9])
  • 1N 4744A (≈ US $00.21 [10])
  • BYQ 28E200 (≈ US $00.98 [11])
  • Brushed DC Motor (Como Drills 719RE850 or equivalent) (≈ UK £21.69 [12] ≈ US $35)
  • Any PC CPU Case (for Power Supply-SMPS and as instrument control box) (≈ US $50.00 [13])
  • PC (to program the microcontroller initially)
  • Software – MPLAB IDE (Free), HI TECH C Compiler for PIC 18 MCU (Free Version)
  • Machining, raw material, workshop access ( for chuck, optical encoder mount and motor cabinet) (≈ US $200.00)

What do I do?


  1. Interface and connect the PIC Microcontroller Programmer to the PC as per the instructions given in its manual
  2. Burn the Program01.hex file into the microcontroller with the help of the programmer
  3. Connect the microcontroller and other components as given in the circuit diagram (Figure 1). The circuit and display can be appropriately mounted in a PC CPU case
  4. Power up the circuit
  5. The Graphic LCD should display data similar to Figure 2. Figure 3 and Figure 4 will appear on the display if appropriate icons on the Menu “Selected Values” screen are touched
  6. There can be dissimilarities with each touch panel and hence it may not respond to input due to change in touch co-ordinates. Hence, there would be a need to modify the Program01.c. If there are no problems, jump to instruction 11
  7. Install the software mentioned above
  8. Modify the program, by finding out the new co-ordinates of the icons by following the instructions given in readme.txt
  9. Compile the modified Program01.c and program the microcontroller as per the instructions given in the programmer manual
  10. Repeat instruction 2 to 5
  11. Make an appropriate box to mount the motor
  12. An aluminium ( light and easy to clean) chuck (Figure 6, Figure 7) to fit the motor shaft  has to be machined
  13. A simple optical encoder set up  which includes mounting of a slim white acrylic piece on the bottom of the motor shaft and the IR optical sensor has to be made ( Figure  5)
  14. Put together all the components appropriately. The spin coater (Figure 8 ) should now be ready ( appropriate modification of  the chuck may result in a centrifuge too)
  15. Mount the wafer samples for spin coating either with a double-sided tape or a set of adjustable clamps

What else should I know?


  • Basic knowledge of C is essential to modify the code to suit individual requirements or specification
  • The spin coater has been set to receive input of speeds from 1000-7000 RPM ( limited only due the motor used, can be modified easily)
  • The maximum spin time is 999 seconds (increments of 1)
  • Due to nonlinear response of the brushed DC motors to voltage ( and hence varying PWM values),  the speed response of the motor to variation in PWM values has to be plotted and an appropriate equation has to be estimated. The equation in Program01.C has to be modified (see the readme.txt).  Note: This needs to be done only if a Brushed DC motor other than Como Drills 719RE850 is used, or if further fine tuning of response is required.
Spinner control schematic
Fig 1. Spinner control schematic
Menu display screen
Fig 2. Menu display screen

Runtime settings – spin duration (in seconds) screen
Fig. 3 Runtime settings – spin duration (in seconds) screen
RPM settings screen
Fig. 4 RPM settings screen
Optical encoder set-up
Fig. 5 Optical encoder set-up
Chuck with adjustable clamps
Fig.6 Chuck with adjustable clamps
Chuck without adjustable clamps – samples mounted using a double-sided tape
Fig.7 Chuck without adjustable clamps – samples mounted using a double-sided tape

Spin coater
Fig 8. Spin coater

References


[1] http://www.mtixtl.com/DeskTopSpinCoater0-5100rpmwithCompleteAccessoriesVTC-100P.aspx

[2] http://us.element14.com/microchip/pic18f4550-i-p/ic-8bit-mcu-pic18f-48mhz-dip-40/dp/74K8623?in_merch=Popular%20Products

[3] http://www.ebay.com/itm/Graphic-Matrix-LCD-Module-LCM-JHD-12864-E-128X64-    /200392901987?pt=LH_DefaultDomain_0&hash=item2ea8590563#ht_4043wt_906

[4] http://www.mikroe.com/eng/products/view/277/various-components/

[5] http://www.mikroe.com/eng/products/view/207/touchpanel-connector-board/

[6] http://www.robotshop.com/2cm-10cm-infrared-distance-sensor-2.html

[7] http://us.element14.com/

[8] http://us.element14.com/international-rectifier/ir2110-1pbf/ic-mosfet-driver-high-low-side/dp/19K8414?in_merch=Popular%20Products

[9] http://us.element14.com/international-rectifier/irfp150npbf/n-ch-mosfet-100v-42a-to-247ac/dp/63J6848?Ntt=irfp+150n

[10] http://us.element14.com/multicomp/1n4744a/zener-diode-1w-15v-do-41/dp/90R9431?in_merch=Popular%20Products

[11] http://in.mouser.com/Search/ProductDetail.aspx?qs=kop3EGWU%252bIt67Q6mQhs7oA%3D%3D

[12] http://uk.rs-online.com/web/p/dc-motors/3213192/

[13] http://www.amazon.com/Computer-Cases-Add-Ons-Computers/b?ie=UTF8&node=572238#/ref=sr_nr_p_n_feature_keywords_0?rh=n%3A172282%2Cn%3A%21493964%2Cn%3A541966%2Cn%3A193870011%2Cn%3A572238%2Cp_n_feature_keywords_browse-bin%3A3606145011&bbn=572238&sort=price&ie=

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Easy and robust interconnection methods for PDMS-based microfluidics

Shuo Wang, Huaiqiang Yu, Wei Wang and Zhihong Li
National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, China

Why is this useful?


PDMS (polydimethylsiloxane) is one of the most important materials in microfluidics and is widely used because of its optical transparency, ease-in-fabrication, low cost and air permeability. A widely used interconnection approach for PDMS chips is the “press-fit” method [1]. However, the seal is only achieved by the compression of PDMS. An unexpected disturbance to the needle may damage the PDMS around it and produce small cracks leading to leakage around the needle. The mechanism of disturbance-caused leakage is shown in Fig. 1.

Fig. 1 Mechanism of disturbance-caused leakage

Here, we report two easy methods of fixing needles by a secondary PDMS fabrication. In these “cure and fix” methods, uncured PDMS is poured and cured to fix needles. Sectional views in Fig. 2 show the two schematic fabrication processes respectively. Cover packaging methods can be applied to produce covers in large number used as standard components. After PDMS chips are made, simply bond them with these standard covers to seal reservoirs. We can also employ whole packaging method to fabricate one specific device with lots of reservoirs.

Fig. 2 “Cure and fix” method

What do I need?


  • PDMS chip peeled off from Si mould
  • Uncured PDMS (base:cure = 10:1)
  • Silicon or glass substrate
  • Unmodified needles
  • Scalpel and tweezers
  • Hole puncher
  • Oxygen plasma etching machine or corona charging device

How do I do it?


Whole packaging method

  1. Punch holes for reservoirs on a PDMS chip and bond the chip with silicon or glass substrates using oxygen plasma or corona treatment [2].
  2. Plunge unmodified needles into reservoirs laterally using the “press-fit” method. Clear away PDMS scraps with a pair of tweezers.
  3. Seal all reservoirs by bonding PDMS blocks. Cast uncured PDMS onto the chip until lower half of the needle is submerged.
  4. After curing PDMS at 70°C for 1 hour, cut the chip into proper size.

Cover packaging method

  1. Punch a hole for the reservoir on a flat PDMS block and bond it with another PDMS block
  2. Plunge an unmodified needle into the reservoir laterally using the “press-fit” method. Clear away PDMS scraps with a pair of tweezers.
  3. Put the PDMS cover on a flat culture dish and cast uncured PDMS.
  4. After curing PDMS at 70°C for 1 hour, cut the cover into proper size.
  5. Bond the cover with a PDMS chip to seal reservoirs.

What else should I know?


In order to plunge the unmodified needle into reservoirs successfully, the PDMS cannot be too thin. The thickness should be larger than 3 mm.
Be careful in step 4 of whole packaging method because silicon and glass are brittle.

Fig. 3 A) Vertical view and B) side view of device using whole packaging method C) vertical view and D) side view of device using cover packaging method

References


[1] A. M. Christensen, D. A. Chang-Yen and B. K. Gale, Characterization of interconnects used in PDMS microfluidic systems. J. Micromech. Microeng., 2005, 15, 928-934.
[2] K. Haubert, T. Drier and D. Beebe, PDMS bonding by means of a portable, low-cost corona system, Lab Chip, 2006, 6, 1548-1549.

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Adding colour to PMDS chips for enhanced contrast

Marco A. Cartas-Ayala and Suman Bose
Department of Mechanical Engineering, Massachusetts Institute of Technology, USA.

Why is this useful?


Most materials used to fabricate microfluidic devices are transparent to facilitate sample visualization (e.g. PDMS), but this property has several drawbacks too. Alignment and visualization of the channels is difficult when the channels are completely transparent, making bonding of polymer devices difficult. Additionally, when multilayer polymer devices are manufactured, sometimes it is necessary to distinguish between different layers to easily evaluate functionality. Finally, having a way to add permanently colour to any kind of transparent channel can become really handy when creating permanent exhibitions displaying the devices created in the lab.

What do I need?


  1. PDMS (Sylgard 184)
  2. SILC PIG. blue silicone pigment, from Smooth-On, Inc
  3. 3 mL Syringe
  4. Blunt pieces of stainless tube (1/2 inch long, diameter smaller than PDMS holes, from New England Small Tube)
  5. Tygon tubing that fits the blunt needles and the stainless pieces of tubing
  6. Blunt needles for 1 mL syringe (diameter selected accordingly to tygon tubing diameter)

What do I do?


  1. Mix PDMS (Sylgard 184) in the recommended 10:1 ratio
  2. Add to the mix 5% w/w of the rubber paint and mix completely. If the mixture is not mixed thoroughly, pockets of paint can be formed in the final mixture, if you have problems with the mix, reduce the paint ratio
  3. Degas the mixture for 30 minutes
  4. Load 0.1 mL of the sample into the syringe with the blunt needle and tubing
  5. Inject into the channels to visualize. Be careful to not introduce bubbles, while air in PDMS leaks out when enough pressure is applied, air has to be flown out from glass devices
  6. Cure PDMS at 70 C for 1 hour
  7. Devices are ready for display. Notice the enhanced contrast of the colour filled channels vs the empty channels for the same device in Figure 2. While channels are visible only from some directions when they reflect light, colour-PDMS devices can be observed from every direction. Additionally, different device layers or areas can be specified by colour. In the figure control layers are blue and flow layers are red

Fig. 1 Injection of PDMS through the channels. Air trapped inside the syringe provides a way to regulate the pressure applied to the device to minimize de-bonding. Compressing the air to 1/3 of original volume should provide enough pressure to drive the PDMS through.


Fig. 2 Enhanced channel contrast after injection, devices on the left side have empty channels and devices on the right have color PDMS inside.

Fig. 3 Different device zones can be identified by color. Here control layer is blue and flow layer is red. Secondary regulation channels are practically invisible when not filled.

References


[1] http://www.upchurch.com
[2] http://www.smooth-on.com

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Drilling inlet and outlet ports in brittle substrates

R.J. Shilton, L. Y. Yeo, and J. R. Friend
MicroNanophysics Research Laboratory, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, 3800, Australia

Purpose


It is often necessary to form millimeter order holes in glass (and similar) substrates to form inlet or outlet ports in microfluidic devices. The easiest way to do this is to simply drill a hole in the required position, however owing to the brittle nature of most materials used in these devices, this can often lead to a high failure rate where the devices crack during the drilling process. Outlined here is a simple procedure for drilling ports in microfluidic devices, which has been tested with glass, silicon, and lithium niobate, with a very high success rate.

Materials


  • ~ 1mm diamond drill bit (UKAM Industrial Superhard Tools, Valencia, CA)
  • Drill press
  • Double sided tape
  • Disposable plastic petri dish
  • Small piece of alumina (or other hard flat material)
  • Substrate to drill ports into

Procedure


1. Attach microfluidic device to alumina with double sided tape. We have used this method reliably to drill ports in glass, silicon, and lithium niobate, however in principle it should work on a range of similar substrates. Silicon is shown in all images.

2.  Stick alumina to bottom of petri dish with a small amount of double sided tape.

3. Fill petri dish with a generous amount of water to cool down the drill site, and to remove particles into the fluid while drilling the hole.

4. Attach diamond drill bit to drill press. We successfully used drill bits of diameter 0.75 and 1 mm, however others should work equally as well.

5. Drill at a high speed (~10,000 RPM). Quite a bit of force can be applied without cracking the substrate, as it is stuck to a rigid backing. Drilling through a 0.5 mm thick substrate should take about ten to fifteen seconds, if it is attached firmly.


What else should I know?


  • Release the downward force a little near the hole exit, to avoid a rough hole on the other side.
  • Keep device immersed in water after drilling until it is ready to be cleaned to avoid particles becoming stuck in device channels etc…
  • Change water regularly to remove particle build up

MicroNanophysics Research Laboratory,

Department of Mechanical and Aerospace Engineering,

Monash University, Clayton, Victoria, 3800, Australia

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A novel technique for aligning multiple microfluidic devices

Tiama Hamkins-Indik, Sandra Lam, Megan E. Dueck and Luke P. Lee
Department of Bioengineering, Berkeley Sensor and Actuator Center, Biomolecular Nanotechnology Center, University of California, Berkeley, CA 94720, US

Why is this useful?


Currently, there is no simple method for aligning multiple layers of PDMS microfluidic devices onto a glass slide.  We report a method for alignment that is easy, inexpensive, and has many relevant applications including printing proteins adjacently , flowing cells over previously printed proteins, and aligning two PDMS devices on top of each other for complex 3D geometries .  Currently, glass etching can be used to permanently mark glass, but this process is labor intensive and costly .  As a proof-of-concept, a 3-layered, 3 mm wide replica of Van Gogh’s Starry Night was created (Figure 5).

What do I need?


1.    Permanent marker (e.g. Sharpie™ marker)
2.    Syringe (1mL – 10mL) with needle tip (any gauge)
3.    Vacuum chamber
4.    Light microscope with 4X and 10X objectives
5.    Scotch™ Tape
6.    Alignment marker

The Alignment Marker:
While any alignment marker can be used between layers, we suggest using the one shown in Figure 1.  The teeth act as Vernier scales in the x and y directions, thus, the degree of misalignment can be measured.  This alignment marker was designed with 10 µm wide teeth.  On the first layer (Figure 1, left), there are 22 posts which are 10 µm apart.  On the second layer, there are 20 posts each 11µm apart (Figure 1, right), making both markers 430µm wide.  The interlocking geometry of the markers is shown in Figure 2.  An inlet must be placed on the first layer’s alignment marker so that ink may flow through and stain the glass.

How do I do it?


1. Incorporate alignment marker

  • Add the alignment markers into the device design. On the silicon master that will be used to cast PDMS molds, The the channel height of the alignment marker on the silicon wafer should be in the 5 – 200 µm range.  The PDMS channel height only needs to be tall enough for Sharpie™ ink to flow through.

Figure 1. Alignment markers. The figure on the left is the alignment marker of the first level and the figure on the right is the alignment marker for the second layer.
Figure 2. Alignment markers interlocked.

2. Sharpie™ Ink Extraction

  • Put a needle onto a 1 mL – 10 mL syringe.
  • Insert the needle into the felt tip of a Sharpie™ marker, and slowly pull the plunger.  Slow extraction is necessary to allow air to diffuse into the marker as the ink escapes into the syringe.  Repeat as necessary.  (Figure 3)
  • Dispense the Sharpie™ ink into a microcentrifuge tube.  Sharpie™ extraction should result in 0.5 – 1 mL of Sharpie™ ink.
  • Dilute Sharpie™ ink 3X in 100% ethanol.

Figure 3. Sharpie™ extraction technique, pierce tip of Sharpie™ maker with syringe needle and slowly pull plunger.

3. First Layer

  • Cut and punch desired PDMS device.
  • Clean a glass slide and the device with Scotch™ tape.
  • Reversibly bond the PDMS device onto a glass slide, by simply placing the cleaned PDMS onto the glass slide. (Figure 4a)
  • Load the Sharpie™ ink into the alignment marker channel.  This can be done by placing the device into a vacuum chamber for 5-10 minutes, removing the device from the vacuum, and placing a ~5 µL drop of Sharpie™ ink over the punch hole.  Only punch one entry hole for this method. (Figure 4b)
  • Allow the Sharpie™ ink to dry for 2 hours. (Figure 4c)
  • Remove PDMS. (Figure 4d)

Figure 4. Alignment technique schematic. a) place clean PDMS device on glass slide, b) load Sharpie™ ink, c) allow Sharpie™ ink to dry, d) remove PDMS device, e) place two pieces of scotch tape surrounding design, f) align second device to Sharpie™ ink alignment marker, g) remove scotch tape.

4. Second Layer

  • Cut and punch desired PDMS device.
  • Clean device with Scotch™ tape.
  • Place Scotch™ tape onto glass slide a few millimeters away from previous design.  (Figure 4e)
  • Under 4X or 10X magnification, bring the Sharpie™ alignment marker into the center of view and focus slightly above it.
  • Carefully place the cleaned second layer onto the scotch tape, but do not press down on the device so that the PDMS device is not in contact with the glass slide.
  • Still under the microscope, align the second layer with the Sharpie™ alignment marker by gently pushing the device along the Scotch™ tape.  The Scotch™ tape prevents the device from bonding with the glass slide.  (Figure 4f)
  • Attach the second layer by reversibly binding it to the glass slide by pressing down on the device.
  • Remove the scotch tape by holding the middle of the device down and pulling the scotch tape out from the edges of the PDMS device. (Figure 4g)
  • If additional layers are necessary, repeat second layer procedure.

What else should I know?


When using this technique by hand, the accuracy of the alignment between two layers can be down to 5 µm.  If a more precise alignment is necessary, a six axis alignment machine can be used.  As a proof-of-concept, we have reproduced Van Gogh’s Starry Night (Figure 5).  This design has two layers, and each layer was filled with Sharpie™ ink using vacuum loading.

Figure 5. 3 mm wide reproduction of Starry Night by Van Gough.

References


  1. Kane et al., Patterning proteins and cells using soft lithogrpahy, Biomaterials, 1999, 20, 2363-2376.
  2. Natarajan et al., Continuous-flow microfluidic printing of proteins for array-based applications including surface plasmon resonance imaging, Anal. Biochem., 2008, 373 (1), 141-146.
  3. Chiu et al., Patterned deposition of cells and proteins onto surfaces by using three-dimensional microfluidic systems, Proc. Natl. Acad. Sci. U. S. A., 2000, 97 (6), 2408 -2413.
  4. 3-Dimensional Molding for Making Microfluidic Devices, MicroDysis – Instrumentation Company with Micro- and Nano-fabrication, and Lab Automation, http://www.microdysis.com/TechMicrofab.aspx, 2010, accessed 16 April 2011.
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The fabrication of PDMS interconnecting interface assisted by tubing fixation

Pengfei Li, Wei Xue, Jie Xu*

Mechanical Engineering, School of Engineering and Computer Science, Washington State University, Vancouver, WA 98686, USA.  E-mail: jie.xu[at]wsu.edu

Why is this useful?


Even though PDMS microfluidic devices have been widely applied in various research areas, it is still challenging to create the precise macro-to-micro interconnecting interface. Here we describe a method to connect a micro PDMS channel to the peripheral systems. With this method, we can avoid attaching exterior tubing using glue which is difficult to work with at the micro scale. In an effort to increase the connecting precision, a pre-curing step on a hot plate is adopted to prevent the tubing sliding away from the desired locations.

What do I need?


  • BD Vacutainer winged blood collection sets. (Model: 23G⨉3/4” ⨉12”, Fisher Scientific, Pittsburgh, PA, U.S.A.)
  • Dow Corning Silastic laboratory tubing. (11-189-15 Series, Fisher Scientific, Pittsburgh, PA, U.S.A.)
  • Sylgard 184 silicone elastomer kit. (Dow Corning, Midland, MI, U.S.A.)
  • SU-8 3050 permanent epoxy negative photoresist. (MicroChem, Newton, MA, U.S.A.)
  • 4-inch silicon wafer, pipette, aluminum foil, hot plate, and oven.

How do I do it?


1.    Prepare the SU-8 mold on a 4-inch silicon wafer with photolithography techniques. Based on the processes from the SU-8 data sheet supplied by MicroChem, SU-8 microstructures with approximately 50 µm in thickness are fabricated for this demonstration.

2.    Mix the elastomer base and the curing agent (mass ratio 10:1) to form PDMS, remove the air bubbles thoroughly using a desiccator or a centrifuge [1].

3.    Place the silicon wafer with the SU-8 channel molds on a hot plate. The temperate is set as 60oC under which the PDMS is able to cure in 10~20 minutes.

4.    Cut the Silastic tubing into short pieces with proper length. Align the tubing section with the channel molds.

5.    Apply a small amount of PDMS to fix the tubing sections. Due to the relative high temperature of 60oC, the PDMS is nearly cured in 10~20 minutes. This step ensures that the tubing sections stay in the intended places where they are well aligned with channel molds. The tubing is still filled with air, though both ends of the tubing are sealed by PDMS.

6.    Use aluminum foil to wrap the silicon wafer and create a container for storing PDMS. Extra PDMS is then quickly poured into the container and later cured in an oven [2].

7.    Peel the cured PDMS off the channel molds and cut the PDMS into channel devices.

8.    Punch a small hole on each end of the channel. The tubing sections can be connected to the channels through the holes.

9.    Bond the PDMS channel to a glass substrate. Then bond another layer of PDMS as the top cover to seal the holes.

10.    Insert needles into the tubing sections. This creates the connection for the PDMS microchannel.

References


[1]  C. N. LaFratta, Degas PDMS in two minutes, Chips & Tips (Lab on a Chip), 17 August 2010.
[2] A. O’Neill, J. Soo Hoo and G. Walker, Rapid curing of PDMS for microfluidic applications, Chips and Tips , 23 October 2006.

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Rapid, inexpensive and stress free drilling in glass substrates or thermally bonded glass chips using electrochemical spark erosion method

Arun Arora
KIST Europe, Korea Institute of Science and Technology, Campus E7.1, 66123 Saarbrücken, Germany

Background


An electrochemical spark erosion method is presented to drill holes in glass substrates or thermally bonded glass chips. Shohi and co-workers [1] reported this method in 1990. We have modified the method by using  a Pt wire 0.5 mm diameter anode instead of a steel needle cathode as a drilling needle and 50 V (instead of 35 V) was applied from a variable unregulated dc power supply to produce a constant spark in 8 M sodium hydroxide solution. When a steel needle cathode was used as reported in reference Shohi’s method, a large amount of brown precipitate was formed, which entered into the channel and created blocks.
Figure 1 shows a schematic diagram of the complete process and Figure 2 is a photo of the setup showing the process in operation.

Figure 1: Electro chemical spark erosion method of drilling holes in a thermally bonded glass device
Figure 2: Photo of the electro chemical spark erosion method for drilling holes in a thermally bonded glass device

The conventional methods to drill holes in glass include laser ablation or powder blasting. Both methods require expensive setup and trained staff therefore these methods are expensive.  The dentist drill also used to drill holes in bonded glass devices which can cause stress to the glass or can damage the channel walls. Sometimes glass particles can also clog the channel. A simple method for drilling access hole in a glass substrate or a thermally bonded glass chip is presented.

What do I need?


  • Pt wire (0.5 mm diameter)
  • Sodium Hydroxide solution (8M) (Caution)
  • Variable DC Power supply (100 V)
  • Petri dish

What do I do?


  1. Fixed a pt wire cathode on the side of the Petri dish as shown in schematic diagram in figure 1 or the photo in figure 2. Make sure that Pt wire reaches to the bottom of the petridish so it can make contact to the sodium hydroxide solution.
  2. Wear gloves and safety goggles and then place the Petri dish on a dry table. Fill it with sodium hydroxide (8M) solution (Caution: sodium hydroxide solution can cause severe burn if it comes in contact with skin. If any part of the body comes in contact, immediately wash it off with plenty of water.)
  3. Mark the position for the hole using a glass marker pencil on other side of the glass substrate where you want to drill the hole. Place it in the Petri dish facing the mark downward and immersed it approximately 5 mm deep into the sodium hydroxide solution.
  4. Connect the PVC covered anode and Pt wire cathode to the DC power supply.
  5. Hold the PVC covered Pt electrode precisely over the marked position on the glass substrate. Switch the power supply on and increase the voltage until the orange spark become visible. After 5 to 10 seconds of constant spark check the progress of drilling by taking out the glass substrate and viewing it with a hand held magnifying lens or a microscope (Figure 3).

Figure 3
Figure 3: Photo of the hole drilled by electrochemical spark erosion; a) Hole across a 60 µm wide channel. b) a 500 µm diameter hole across a 60 µm wide channel in a pre-bonded capillary electrophoresis glass device

References


[1] Shohi, S.; Esashi, M. Photoetching and Electrochemical Discharge Drilling of Pyrex Glass, Technical Digest of the 9th Sensor Symposium, Japan, 1990, 27.

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Optimal protocol for moulding PDMS with a PDMS master

Jiaxing Wang1, Mingxin Zheng2, Wei Wang3,4*, and Zhihong Li3,4

1 Basic Medicine School, Peking University Health Science Center, Beijing, 100191, China
2 YuanPei College, Peking University, Beijing, 100871, China
3 Institute of Microelectronics, Peking University, Beijing, 100871, China
4 National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Beijing, 100871, China
* E-mail: w.wang[at]pku.edu.cn

Why is this useful?


Nowadays, microfabricated silicon has been widely used as a master in PDMS-based soft lithography. It carries the merits of the microelectromechanical system (MEMS) technique, such as being able to construct complex 3D or high aspect ratio micro/nanostructures. However, the silicon master is easy to break during the PDMS moulding process. Due to the lag effects existed in deep reactive ion etching (DRIE) for silicon microfabrication, there will be a high risk of leakage between these two channels after PDMS moulding and bonding, if two parallel microchannels were designed too close, as shown in Fig. 1a. More seriously, the bonding surface of the PDMS replica will follow the morphology of the etched surface (Fig. 1a). Usually the etched silicon surface has high roughness, especially after a deep etching of silicon by DRIE (Fig. 1b), which makes the subsequent PDMS bonding being difficult, sometimes even impossible. To overcome the fragileness of the silicon master, it has been reported that moulding epoxy with PDMS replica, which is prepared by using the original microfabricated silicon as the master, to achieve a durable master in the PDMS moulding process. [1] However, this approach still suffers the aforementioned lag-induced leakage and bonding problems.

Moulding PDMS with a PDMS master replicated from the microfabricated silicon directly can overcome the aforementioned problems, as illustrated in Fig. 1c. It has been developed by silanizing the PDMS master with tridecafluoro- 1,1,2,2- tetrahydrooctyl- 1- trichlorosilane (in vacuo, 8h[2] or overnight[3]). In this Tip, we optimized the PDMS moulding process with the PDMS replica as the master. The optimal protocol is simple and easy to establish in any labs.

a

b

c

Fig. 1 Moulding PDMS with microfabricated silicon as master. (a) Schematic illustration of the lag effect induced leakage and bonding problems existed in the traditional soft-lithography when the PDMS is replicated from the microfabricated silicon master directly. (b) SEM photo of the microfabricated silicon master used in the present work (with the tilting angle of 45o). Inserted SEM photos give the morphology comparison between the etched silicon surface and the original one. (c) Schematic illustration of the moulding PDMS process with a PDMS master.

What do I need?


Microfabricated silicon, aluminum foil (Select HORECATM, Jinweiyuan Hotel Supplies Co. ltd.), mould release reagent (Micro90®, International Products Corporation, USA), PDMS (Sylgard 184; Dow Corning Co., MI), detergent (commercial available detergent, White CATTM, White Cat co. ltd.), ethanol (MOS grade, Beijing Chemical Reagent Institute).

What else should I know?


Process optimization

The present protocol contains two PDMS moulding processes: The first one is to construct the PDMS master from the microfabricated silicon while the second to make the final PDMS replica directly from the so-prepared PDMS master.

Considering the Corona-triggered PDMS-PDMS bonding technique [4], which will be adopted in the subsequent PDMS-microfluidic device construction, curing at 60oC for an hour of the PDMS pre-polymer prepared following the manufacture guideline was selected as the second PDMS moulding recipe. In this chips and tips, the first PDMS moulding process and the surface treatment of the PDMS master were optimized.

1) Optimization of the first PDMS moulding process

Table 1 Optimization of the first PDMS molding process

The first PDMS moulding process was optimized for the PDMS master preparation as shown in Table 1. Based on the above experiments, we found the optimal recipe for the first PDMS moulding process was curing the PDMS pre-polymer at 120oC for an hour with the mass ratio (Base to Curing agent) of 5:1. (Condition No.4)

2) Optimization of the surface treatment of the PDMS master

Table 2 Optimization of the surface treatment of the PDMS master

Surface treatment is another important factor for the PDMS moulding and was optimized as listed in Table 2. The results indicated that detergent dissolved in 75% ethanol (about 10 w.t.%) was the optimal recipe for the second PDMS moulding process. (Condition No.3) Herein, we name it the modified mold release agent. In later work we also use it for pre-treatment of the microfabricated silicon master.


How do I do it?


1. Make a holder with aluminium foil, according to the shape of the microfabricated silicon master. [5]
2. Mix the PDMS according to the manufacturer’s procedure, but with a mass ratio of the Base to the Curing agent of 5:1.
3. Pre-treat the microfabricated silicon master with the modified mold release agent.
4. Dry the microfabricated silicon master with nitrogen and put it into the holder.
5. Pour the PDMS pre-polymer into the holder, covering the whole microfarbicated silicon master, and then place the holder in an oven at 120oC for an hour.
6. Gently peel the PDMS replica (the PDMS master in the second PDMS molding process) off from the microfabricated silicon master. (Fig.2)

Fig. 2 The PDMS master, the PDMS replica from the microfabricated silicon. (The scale bar is 100μm.)

7. Treat the PDMS master with the modified mould release agent. For the deep holes (e.g. some holes with the depth of 100?m in Fig.2), treat the master with the modified mould release agent in the vacuum.
8. Place this PDMS master in a Petri dish, with the moulded-side upwards.
9. Mix PDMS prepolymer again according to the standard manufacturer’s procedure with a mass ratio of the base to the curing agent of 10:1.
10. Pour the PDMS mixture into the Petri dish, covering the whole PDMS master, and then cure at 60oC for an hour.
11. Carefully peel the PDMS replica (Fig.3) off from the PDMS master for the subsequent experiments.

Fig. 3 The final PDMS replica from the PDMS master. (The scale bar is 100μm).

Acknowledgements


This work was financially supported by the 973 Program (Grant No. 2009CB320300).

References


[1] A. Estévez-Torres, A. Yamada and L. Wang,  An inexpensive and durable epoxy mould for PDMS, Chips & Tips (Lab on a Chip), 22 April 2009.
[2] J. R. Anderson, D. T. Chiu, R. J. Jackman, O. Cherniavskaya, J. C. McDonald, H. Wu, S. H. Whitesides and G. M. Whitesides, Fabrication of Topologically Complex Three-dimensional Microfluidic Systems in PDMS by Rapid Prototyping, Anal. Chem., 2000, 72, 3158-3164.
[3] J. L. Tan, J. Tien, D. M. Pirone, D. S. Gray, K. Bhadriraju, and C. S. Chen, Cells lying on a bed of microneedles: An approach to isolate mechanical force, Proc. Natl. Acad. Sci. U. S. A., 2003, 100(4), 1484-1489.
[4] C. Yang, W. Wang, and Z. Li, Optimization of Corona-triggered PDMS-PDMS Bonding Method, in Proceedings of the 4th IEEE International Conference on Nano/Micro Engineered and Molecular Systems, 319-322.
[5] A. O’Neill, J. Soo Hoo and G. Walker, Rapid curing of PDMS for microfluidic applications, Chips & Tips (Lab on a Chip), 23 October 2006.

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Simple fabrication of microfluidic devices by replicating Scotch-tape masters

Anil B. Shrirao1 and Raquel Perez-Castillejos1,2
1 Department of Electrical and Computer Engineering, 2 Department of Biomedical Engineering, New Jersey Institute of Technology, Newark NJ, USA

Why is this useful?


We present a method for fabricating PDMS microfluidic devices based on replicating a master made of Scotch tape.  Often the fabrication of microfluidic devices by soft lithography is restricted to those who have access to a cleanroom that allows the fabrication of a master with micrometric features.  Here we demonstrate that patterned Scotch tape can be used (without the need of any chemical treatment) as a master for soft lithography yielding microfluidic devices with a uniform height of ~ 60 µm.  As a difference to previous Chips & Tips on rapid and easy prototyping techniques [1, 2], in this method the Scotch tape does not remain as part of the microfluidic device after fabrication.  Here we used a handheld cutting tool (scalpel) to pattern the Scotch tape.  A laser cutting machine could be used, instead, for masters requiring higher precision.

What do I need?


  • Glass slides, pre-cleaned from Manufacturer (Fisher Scientific, 75mm x 50mm x 1mm, Cat. No. 12-550-C)
  • Scotch tape (3M Scotch® Transparent Tape 600[6])
  • Stainless steel Scalpel or surgical blade with (Feather Safety Razor Co., LTD, Cat. No. 2976#11)
  • Polystyrene Petri dish (Fisher Scientific, 100mm x 15mm, Cat. No. 08-757-12)
  • Tweezers
  • Oven or hot plate (to work at 65°C)
  • Gloves (do not use latex gloves)
  • PDMS silicone elastomer base and curing agent (Sylgard 184, Dow Corning)

What do I do?


1. Attach a strip of Scotch tape to the glass slide.  The thickness of the Scotch tape will determine the height of the microchannel.  (To increase the height of the channel, attach additional strips of Scotch tape.)
2. Print the layout of the microchannel on regular paper.  Place the printout on a flat surface.
3. Place the glass slide on the printout, with the Scotch tape facing up.  Align the glass slide to the microchannel layout.  Fix the glass slide to the printout with a piece of Scotch tape on the corner of the slide.
4. Use the scalpel to cut the tape on the glass slide according to the layout.  (For cutting, we used another glass slide as a ruler)
5. Remove the Scotch tape from all regions of the glass slide except those in the layout of the microchannel.
6. Place the glass slide with patterned Scotch tape in a heating oven at 65°C for 2-3min; this improves the adhesion of the edges of patterned Scotch tape to the glass substrate.  At this point, the Scotch-tape pattern does not need any further treatment in order to be used as a master for soft lithography [1].
7. Mix the base and curing components of PDMS as recommended by the manufacturer [2].  Place the glass slide in a Petri dish, with the patterned Scotch tape facing up.  Pour the PDMS mixture in the Petri dish until the master (i.e., glass slide and Scotch tape) is covered completely.  Degas PDMS in vacuum (if needed) and allow it to cure for 1 hour at 65°C.
8. Use the scalpel to cut the slab of PDMS containing the microchannel.  Peel off the PDMS replica.  (The Scotch-tape master can be used again by repeating step 7.)
9. Punch holes in the PDMS replica for the inlets and outlets of the microfluidic device.
10. Seal the PDMS replica to a substrate, either (i) conformally-by bringing the PDMS replica in contact with a smooth, clean substrate-or (ii) irreversibly-by bringing the PDMS replica in contact with the substrate after oxidizing both parts in an oxygen plasma [3]).


Acknowledgements


R. P.-C. acknowledges the support of the New Jersey institute of Technology through starting faculty funds.

References


[1] R. J. Holmes and N. J, Goddard, Rapid prototyping of microfluidics, Chips & Tips (Lab on a Chip), 15 February 2007.
[2] R, Kumar, R. L. Smith, and M. G. Pappas, A method for rapid fabrication of microfluidic devices, Chips & Tips (Lab on a Chip), 30 June 2009.
[3] Y. Xia and G. M. Whitesides, Annu. Rev. Sci., 1998, 28, 153-184.
[4] Dow Corning Product Information, “Information about Dow Corning® brand Silicone Encapsulants”.
[5] M. K. Chaudhury and G. M. Whitesides, Langmuir, 1991, 7, 1013-1025.
[6] Scotch tape

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