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Robert Henderson, Nick Selock and Dr. Govind Rao^*
Center for Advanced Sensor Technology, and Department of Chemical, Biochemical, and Environmental Engineering, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
Email: grao[at]umbc.edu

Why is this useful?

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The use of HPLC fittings and tubing in microfluidics is becoming more commonplace as the use of microfluidics in the sciences increases. Even though acrylic or PMMA is one of the most common plastic substrates used in microfluidics, the ability to strongly and easily connect an HPLC fitting to a PMMA chip has until now been elusive. Although there are some commercially available HPLC-to-chip products from Idex, they require heating the microfluidic chip well above the glass transition temperature of PMMA to create a permanent epoxy bond between attachment and chip. For these reasons, we present a simple scheme to strongly connect a standard HPLC flat bottom nut to a PMMA microfluidic device through the creation and bonding of an easy-to-make ¼-28 PMMA microfluidic union, using common lab equipment.

What do I need?

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• Laser cutter or another implement to cut PMMA, such as a hacksaw or band saw
• An appropriate piece of equipment to anneal the PMMA (we use a convection toaster oven)
• 5.3mm thick acrylic sheet for the body of the PMMA microfluidic union
• A drill and drill bits
• Vise grips, or another way to hold the PMMA union in place while tapping
• A ¼-28 tap
• A few sheets of progressively finer sandpaper
• A 50-50% mixture of ethanol and water, along with an ultrasonic cleaner and lint-free wipes for cleaning purposes
• A strong solvent for acrylic, such as chloroform or methylene chloride
• A glass syringe with a blunt needle or needle blunted through filing

This connection scheme has been tested with good results when using the LT-115 and P-259 ¼-28 flat bottom HPLC nut and ferrule combination for 1/16″ OD tubing (Idex). This method was tested on microfluidic devices that were not bonded with the use of adhesives. This method is adaptable to other flat bottomed nuts as long as an appropriate tap and drill bit are used. Keep in mind that as the wall thickness of the PMMA union decreases, so does the strength of the connection between the HPLC nut and the device.

What do I do?

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1. Use a laser cutter to cut a circle with a diameter of 13mm out of the 5.3mm thick acrylic sheet. Make sure that you also either etch a center mark or make a small center hole using the laser cutter. If a laser cutter is not available, make a 13mm PMMA square using one of the other mentioned cutting implements. (Fig.1)
2. Using a small drill bit, drill a pilot hole for the larger drill bit to follow in the center of the circle that you cut out in Step 1.
3. Using the larger drill bit (a #3 or 0.213″ drill bit for the ¼-28 tap), drill the center hole in the cutout piece following the pilot hole drilled in Step 2.
4. Using the ¼-28 tap and appropriate technique, tap the hole you have been creating in Steps 1-3. Do not use oil to lubricate the plastic as it is unnecessary and difficult to clean.
5. After removing the plastic chips from the threads you just created, use progressively finer sandpaper to lap the surface that will make contact with your microfluidic device. Lapping the surface will ensure a very strong bond between the union and your device. We lap using a figure of eight motion to help guarantee a flat bonding surface. (Fig. 2)
6. Anneal both your created microfluidic union and your device in an appropriate fashion. Annealing will relieve the thermal stress present in the union due to laser cutting and the drilling/tapping steps. (We use a convection toaster oven, set at 85ºC for 90 minutes, 50ºC for 30 minutes and then off with the oven door closed for 30 minutes. We find it useful to put a smooth metal plate below and above the piece to be annealed, to prevent uneven heating in the oven.)
7. Thoroughly clean both the union and the microfluidic device to be bonded using a 50-50% mixture of ethanol and water. Clean only the union in an ultrasonic cleaner for 10 minutes. This mixture will not cause the union or the device to crack if they are both appropriately annealed beforehand and if the cleaning is performed within a few days of annealing.
8. Place the union, lapped side down, onto the microfluidic device’s surface where the connection is desired. It is essential to center the union over the hole which receives the fluid or gas from the HPLC tubing. We use a small jig composed of two different sized plastic dowels to help us in this step (the black object in the bottom right corner of Fig.3).
9. Pull some solvent into the glass syringe.While holding the union in place using your jig, gently drip some solvent from the glass syringe at the interface between the union and your microfluidic device (start with a single drop). The solvent will creep into the joint between the union you created and your microfluidic device due to capillary action. Add enough solvent to fill the gap between the union and your device, but be careful not to get any solvent into the fluid passages within your device.
10. Being careful not to disturb the union’s position, gently apply pressure on the union until the surfaces become lightly bonded using your gloved fingers (Fig. 3). Light bond is usually achieved in less than a minute. In this step, also attempt to add enough pressure to force out any bubbles at the interface.
11. Remove the alignment jig and add more solvent around the interface between the union and your device. This step reinforces the union’s connection to the microfluidic device.
12. Put a small mass directly on top of the union, and leave the bonding union overnight to achieve the strongest connection between the union and your device.
13. After the piece has dried overnight, remove the mass and then use the connection as you would any other flat bottom HPLC union. (Figure 4)

Notes

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The following YouTube video ‘Power Tools & Carpentry Skills : How to Use a Tap & Die Set’ by Expert Village helps explain the use of taps (Step 4): http://www.youtube.com/watch?v=6DDhw191MLo

Acknowledgements

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The authors would like to acknowledge the work of Dr. Yordan Kostov, Mike Tolosa and Mike Frizzell.

Boyang Zhang¹, Milica Radisic¹, Shashi Murthy^2
1Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON, Canada
²Department of Chemical Engineering, Northeastern University, Boston, MA, USA

Background

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The operation of microfluidic systems often requires sequential injection of more than one type of solution with a syringe pump. For instance, many adhesion-based cell-separation devices require additional injection of washing buffer or/and cell-releasing buffer after cell injection [1, 2]. Minimal flow disturbance to the system during switching to the next solution is critical for proper operation of such devices. Switching syringes during pumping will inevitably stop the flow and risk introducing bubbles to the device. Simple four-way valves are traditionally used in such cases [3]. However, many commercially available four-way valves have large dead volumes and long residence times that make them incompatible with microfluidic systems.

Why is this useful?

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Here we present a simple, three-chamber microfluidic system with interconnected tubings (Figure 1) as an alternative to the traditional 4-way valve used for switching between solutions during experiments. For microfluidics applications, our system has the advantages of (1) allowing for switching between two solutions with minimal disturbance to the flow, (2) greatly reduced risk of introduction of unwanted air bubbles into the system, and (3) greatly reduced dead volume. The total volume of our system is two orders of magnitude less than a typical, commercially available 4-way valve (~1 µL as compared to ~1 mL) [5]. Our valve system is simple and cost-effective, as it utilizes clamps (binder clips) to block selected tubings. We have also shown that the tubings are strong enough to withstand several iterations of clamping and release. Lastly, this method can be scaled up to control the flow of more than two types of solutions, simply by adding more chambers and clamps.

What do I need?

• Clamps (Binder Clips)
• Tygon tubing (any size)
• PDMS silicone elastomer base and curing agent (Sylgard 184, Dow Corning)
• Glass slides, pre-cleaned (Fisher Scientific, 75mm x 50mm x 1mm, Cat. No. 12-550-C)
• Scotch tape (3M Scotch® Transparent Tape 600)

What do I do?

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(1)  Fabricate the master for the device. The device includes three triangular shaped chambers. The exact dimensions of these chambers are not critical and should be tailored to the specific experiment. In this case, triangular shaped channel with height of 40 μm and edge-width of 3 mm is used for flow rate up to 80 μl/min. The master can be fabricated with scotch tape which would greatly speed up the fabrication step especially for design with only large features [4].

(1)  Pour PDMS over the master and cure to create a PDMS-based device.

(2)  Drill holes as indicated in Figure 1 and bond the device to a glass slide.

(3)  Insert the tubing into the holes, again following Figure 1.

(4)  Fill the device with selected solution and clamp shut the selected tubing.

(5)  To switch solutions during an experiment, simply switch the position of the clamps as shown in Figure 1.

Figure 1. Device schematic: (A) Microfluidic 4 way valve with color dyes to demonstrate the function of the clamps to stop and re-route flows. (B) Schematic of the device in operation. (C) Traditional concept of 4 way valves.

References

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1. B. D. Plouffe, M. A. Brown, R. K. Iyer, M. Radisic and S. K. Murthy, Controlled capture and release of cardiac fibroblasts using peptide-functionalized alginate gels in microfluidic channels, Lab Chip, 2009, 9, 1507-10.
2. B. D. Plouffe, M. Radisic and S. K. Murthy, Microfluidic depletion of endothelial cells, smooth muscle cells, and fibroblasts from heterogeneous suspensions, Lab Chip, 2008, 8, 462-72.
3. L. Kim, M. D. Vahey, H.-Y. Lee and J. Voldman, Microfluidic arrays for logarithmically perfused embryonic stem cell culture, Lab Chip, 2006, 6, 394-406.
4.  A. B. Shrirao and R. Perez-Castillejos, Simple Fabrication of Microfluidic Devices by Replicating Scotch-tape Masters, Chips & Tips (Lab on a Chip), 17 May 2010.
5. 4 WAY ACTUATION VALVES, Parker Hannifin Corp, 04 October 2011.

Sopan M. Phapal, Tuhina Vijay and P. Sunthar
Chemical Engineering Department, Indian Institute of Technology Bombay, Powai-400 076, India.

Why is this useful?

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There are several ways to connect macro to micro interfaces (e.g., from syringe pump to microfluidic chip) in the miniaturization field. The ideal connector should withstand very high pressure and not leak. Commercially available Nanoports are often used as connectors but they are expensive, sometimes become clogged when glue is used, and cannot be reused. In this tip we present an easy and inexpensive way to produce a macro to micro interface connector which can withstand high pressure and is glue free.

What do I need?

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•    2.5 ml plastic BD syringe with Luer-Lok (Luer tip ID at back 2.33 mm/0.095”; front ID 1.75 mm/0.07”)
•    16G SS needles (OD= 1.65  mm/0.065”)
•    Grinder with hard abrasive disk to cut/ blunt the needles
•    16G PTFE tubing, Hamilton, Cat No.20916 ; OD=1.9 mm/0.075”; ID=1.2 mm/0.047”[1]
•    Surgical knife/cutter to cut the Luer part of the syringe
•    Glue (Araldite).

A syringe contains a male Luer part and the needle head is its counter female Luer part. There are two types of Luer fittings, one Luer-slip which when pressed together, fits in each other by friction; and the other is Luer-Lok which has threads in the male part and the female part (needle head) screws in to it (Figure 1). We used both Luer-slip and Luer-Lok syringe tips, but the Luer-Lok is better for high pressure application.

What do I do?

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1. Make the needles blunt with the help of grinder (Fig. 2A & B). Cut the Luer-Lok tip of the plastic syringe with a sharp blade/cutter (Fig. 2C).

2. Prepare the male Luer part. Insert the PTFE tube (OD= 1.9mm) into Luer-Lok tip (back ID= 2.3 mm and front ID is 1.75 mm) as shown in Fig. 3A. Now heat the 16G SS blunt needle and insert it into the PTFE tube up to some mm and remove it (Fig. 3B). Because of this, the tip of the tube becomes wider than Luer-Lok ID, fitting tight mechanically. Pull the Luer-Lok up to the wide tip of PTFE tube and place a drop of araldite at the back side of it (Fig. 3C). This will cause a tight fit of the Luer-Lok tip with the PTFE tube.

3. Preparation of the female Luer part. Heat the 16G SS blunt needle (OD= 1.65 mm) and insert it into 16G PTFE tube (ID=1.2mm), as shown in Fig. 4. As the needle OD is bigger than tube ID, it will permanently fit into the PTFE tube upon cooling.

4. Examples of connecting the male Luer part of tubing into a needle head (female Luer part) which is acting as an inlet of micro channel assembly (Fig. 5). Example of the use of macro to micro interface tubing to connect a syringe to a microchip: screw fit the needle head (female Luer) into the male Luer part on the syringe placed on syringe pump and the male Luer-Lok part will screw fit into the inlet needle (counter fitting female part) of microfluidic chip (Fig. 6).

References

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[1]. http://www.hamiltoncompany.com

Acknowledgements

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We would like to acknowledge a grant provided by the Department of Science and Technology, Government of India (07-DS-032).

Myra T. Koesdjojo*, Jintana Nammoonnoy, and Vincent T. Remcho

Department of Chemistry, Oregon State University, Corvallis, OR 97331
*Corresponding author: Myra T. Koesdjojo
Fax: (541) 737-2062
Email: koesdjom[at]onid.orst.edu

Why is this useful?

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Microfluidic systems, also referred to lab-on-a-chip or micro total analysis systems (μTAS) have been developing at a rapid pace in the last decade and offers promising analytical tools that may transform routine chemical analysis in the future. Microfluidic devices typically require multiple interconnects.¹ Consequently, reliable microfluidic interconnections have become one of the basic necessities in integrated fluidic and on-chip systems. The lack of an efficient interface or interconnect between microfluidic devices and the macroscale world has historically been a major challenge and one of the greatest limiters on acceptability and the application of μTAS into the broad world market. Clearly, there is a need for a low-cost, flexible interconnects for microfluidic devices to be successful in the long term.² There are a variety of current products available and techniques that have been used to provide interfacing between microchannels to external devices.^3-6 The most common and simplest approach is the direct integration of tubing or a syringe needle into the microchip inlet using epoxy glue. The drawback is it often leads to clogging of the microchannels and these types of interconnections are impractical to remove and are not reusable. This tip presents a simple method to making reusable quick release magnetic-based fluidic connectors. It is an alternative approach that provides a simple, cost effective universal interconnects in the microfluidic applications. The magnetic-based connectors was developed using two permanent magnets that form a compression seal against the tubing line and the surface of the microfluidic device. The magnetic connector also allow for standard tubing to be interfaced with the microchips. With this approach, interconnects can be easily assembled and reconfigured numerous times without causing damage to the microfluidic device. And since connection is established though a magnetic based approach, clogging of microchannels from adhesive or epoxy can be avoided.

What do I need?

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• Permanent magnets (design and dimension shown in Figure 2)
• Magnetic base
• Elastic such as PDMS compression seals or vacuum cup
• PEEK or Teflon tubing (not limited to the materials of the tubing)

What do I do?

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1. Custom made permanent magnet shown in Figure 2. The first component was a permanent magnet that allowed for a standard tubing to be attached to a microfluidic device via a flexible compression seal. It was manufactured with the dimensions shown below to provide a tight fitting to the flexible seal when sandwiched between two magnets, which in turns compressing the tubing inside. The magnets are Neodymium magnets which are over ten times stronger than the ceramic magnets. These magnets are ideal for use as interconnects since they provide much greater holding forces. They can be purchased from Indigo Instruments or K&J Magnetics.

2. Rubber cup compression seals The second component was a flexible rubber cup or compression seals. The dimension should be optimized so that the inner diameter of the seal should provide a tight fitting for a tubing (a in Figure 3b) and the outer diameter (b) is slightly larger than the magnets core so when it is sandwiched in between the two magnets, the tubing is tightly held. The last parameter is the diameter of the lip base (c), which should be larger than the magnet’s core so it can be compressed to the magnetic base and tightly secured against the microchip surface to prevent leaking. A variety selection of compression seal (vacuum cup) with different sizes can be purchased from McMaster-Carr.

3. A magnetic base was used which applied pressure between the microfluidic chip and the interconnects (Figure 4). The base is particularly useful for microchip and devices having different dimensions and port locations, as the interconnects could easily be relocated.

4. The magnetic interconnects were placed on each of the reservoir holes of the microchip to test for leaking.

References

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[1] T. Das, D. Chakraborty and S. Chakraborty, Interfacing of microfluidic devices, Chips & Tips (Lab on a Chip), 27 February 2009.
[2] IEEE Trans. On Adv. Packaging., 2003, 26 (3), 242-247.
[3] J. Greener, W. Li, D. Voicu and E. Kumacheva, Reusable, robust NanoPort connections to PDMS chips, Chips & Tips (Lab on a Chip), 8 October 2008.
[4] http://www.upchurch.com/
[5] www.labsmith.com/microfluidicsinterconnects.html
[6] J. Micromech. Microeng., 2005, 15, 928-934.