Archive for the ‘Connections’ Category

Connector-less manipulation of small liquid volumes in microchannels

Christopher Moraes, Yu Sun, and Craig A. Simmons
Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada

Why is this useful?


An often-touted advantage of microfluidic systems is the small volumes of reagents required.  However, the world-to-chip interconnects often require fluid volumes orders of magnitude greater than those used within the microfluidic channels themselves.  Moreover, commonly used interconnect schemes are either expensive; custom-manufactured or require substantial additional fabrication efforts; or are severely prone to failure, leaks and clogging.

This tip presents a simple, inexpensive and easily-accessible method to manipulate small volumes of fluid in standard microfabricated PDMS channels, without the use of a connector scheme.  Though it cannot be used to produce well-controlled or continuous long-term flow, we have found it ideal for applications such as device pre- and post-processing (e.g., chemical-based surface modification and immunostaining).  The technique is simple and robust, and has been used effectively both by experienced researchers and untrained, minimally supervised undergraduate students in a teaching lab.

What do I need?


  • Fully cured microfluidic PDMS device
  • Glass substrate
  • PDMS punch
  • 1 mL Pasteur pipette bulb (Sigma-Aldrich, product Z111589)
  • Pipettes and tips
  • Kimwipes

Figure 1

How do I do it?


1. Punch an access port into the PDMS device at the channel entry and exit points.  The size of the hole can be selected based on the reagent volume required by the device.  In this example, we used a 1/8″ diameter punch.

2. Bond the patterned PDMS layer to a glass slide, as per standard procedure for PDMS device fabrication.

3. Pipette a small quantity of reagent directly into the access port (Figure 1; blue dye used for this demonstration).  We have successfully used this technique with volumes as low as 4 mu.gifL.

4. Place the Pasteur pipette bulb (Figure 2) over the punched reservoir and gently hold it down so it forms a conformal seal around the reservoir (Figure 3).  Provided the area around the access port is fairly flat, maintaining a seal is typically not a problem.

Figure 2

Figure 3

5. Squeeze the bulb gently to apply positive pressure and cause the fluid to flow into the microchannels (Figure 4).

Figure 4

6. Channels can be cleared by first pipetting away excess fluid from the access port, and then using the Pasteur pipette bulb to force air through the channel, while wicking away the expelled liquid at the exit port with a Kimwipe.

The Pasteur pipette bulb can also be used to apply a negative pressure by squeezing it first, placing it over the channel access port, and gently releasing the bulb.  In Figure 5, we used this technique to drive flow in a standard microfluidic gradient generator system:  negative pressure is applied at the outlet channel port, drawing eight separated reagents through the mixing channels.

Figure 5

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Reusable, robust NanoPort connections to PDMS chips

Jesse Greener, Wei Li, Dan Voicu and Eugenia Kumacheva
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada

Why is this useful?


Typical fluidic connections to PDMS chips consist of tubing inserted into a punched inlet, held in place by glue, for example, a cured epoxy glue. These connections can fail at low pressures because of weak adhesion of the glue to PDMS. In addition, insertion of the tubing directly into the inlet may affect the flow if the tubing is pushed right to the bottom of the channel, thereby increasing hydrodynamic resistance. This is particularly a problem for microchips made from a thin layer of PDMS. Furthermore, even subtle changes to hydrodynamic resistance at the inlet can affect flow rates from pressure-driven sources or cause differential flow rates through inlets which are supplied from a manifold that is being driven from a single source.

Upchurch has devised a low fluidic resistance chip connection system made from the polymer PEEK called a NanoPort, which can easily accommodate new feeds via a threaded nut and ferrule system.[1] However, this device is designed as a single-use port which is sealed to the chip via a permanent bond at its base using an adhesive ring. The adhesive ring does not stick well to untreated PDMS but plasma treatment of the fully cured PDMS surface has been reported to enhance the bond between the NanoPort/adhesive ring assembly and PDMS.[2] This technique has the advantage of being able to handle working pressures of between 30-50 PSI, but suffers from the single use nature of the adhesive ring. For connections to PDMS chips, Upchurch recommends that the NanoPort be imbedded in the PDMS during microchip curing. This technique is not ideal because the sealing surface area is relegated to the bottom of the NanoPort only, and the semi-cured state of the PDMS is not conducive to creating a strong bond. Other groups have fabricated their own, low cost PDMS connectors, which may be a convenient substitute for the NanoPort in low pressure applications.[3]

This tip reviews an alternative approach; encasing the NanoPort in PDMS at the surface of a previously cured PDMS microchip. Benefits include NanoPort reusability (after liberating them from the PDMS encasing) and higher useable pressure ranges (we determined that up to 85 PSI can be achieved). This technique can be easily adapted to chips of any materials which make a strong bond with PDMS, including glass and polycarbonate.

What do I need?


  • PDMS SYLGARD 184 silicone elastomer base & curing agent (Dow Corning)
  • Upchurch NanoPort assembly
  • Plasma cleaning system for PDMS surface oxidation
  • A heat source (convection oven)
  • One 3 cm-long piece of tubing with 1.59 mm OD (1/16 inch) per inlet
  • Knife or 2 differently sized cork borers, the smallest of which should be at least 1.0 cm (we used 1.7 and 1.0 cm diameter)

What do I do?


1. Prepare the PDMS chip, ensuring that access holes with diameter of roughly 1.5 mm are punched into the surface.

2. Punch or create PDMS rings with thickness no less than approximately 5 mm.

Figure 1

3. Insert 1/16 inch OD tubing into the inlet holes to block PDMS from entering the device.

Figure 2

4. Plasma bond the PDMS rings centered around the inlet holes on the PDMS chip (we use 90 seconds at 600 mTorr). This serves as a reservoir for liquid PDMS for the following steps.

Figure 3

5. Coat the bottom of the reservoir with less than 1 mm of liquid PDMS, then place the NanoPort base over the tubing in the PDMS reservoir. Make sure at this stage that no air bubbles are trapped at the bottom of the NanoPort in the groove designed for the o-ring, which we are not using for this application. This can be accomplished by rotating the port’s base in the liquid PDMS within the reservoir while sliding it up and down slightly around the tubing to allow air to escape. Degassing under vacuum may also work.

6. Finish filling the reservoir with PDMS, being careful not to get any in the threaded area of the NanoPort or inside the tubing, and heat cure (we used 4 hours at 74ºC).

7. After curing, pull the tubing out of the inlet. If any PDMS leaked into the threaded part of the NanoPort base this can be removed with tweezers or by repeatedly connecting and disconnecting the base’s male threaded counterpart (nut). Clear debris with compressed air.

8. Connect the nut and ferrule (or coned nut) to 1/16 OD tubing, attach to the NanoPort base and supply liquid or gas to the chip.

Figure 4

What else should I know?


In this example, we apply this technique to mounting NanoPorts on to a PDMS microchip. In theory, other substrates can benefit from this technique as long as they can be bound to PDMS either by plasma treatment, or by curing liquid PDMS to it. Also, the PDMS ring provides a leak-proof reservoir for liquid PDMS which results in a clean finish when affixed to the surface via plasma treatment. However, rings can also be stuck to the surface via curing a thin coat of liquid PDMS between the two, though the finish may not look as nice.

Our pressure tests probed the strength of the bond between the NanoPort and PDMS. Therefore a flat piece of PDMS substrate was used instead of a microchip with channel structures. By progressively increasing the pressure of an inert gas (N2) being supplied to the NanoPort assembly it was determined that in every case the failure occurred between its base and the top of PDMS substrate. In our tests, failure occurred between 40 and 85 PSI. In theory, the assembly would benefit from more contact area between the PDMS/NanoPort assembly and the PDMS chip. A wider PDMS reservoir would accomplish this, though inlet spacing would have to be adjusted to account for the larger assembly footprint over each inlet. In addition, changing the ratio of silicone elastomer to curing agent and/or implementing this technique on a surface that has not yet fully cured or has been freshly plasma treated may also help to strengthen this bond. The same NanoPorts used in previous tests were then attached to a working microfluidic chip and subjected to increasing gas pressure. In this case, the chip itself separated before the reusable NanoPort connection failed.

References


[1] http://www.upchurch.com/PDF/I-Cards/N4.PDF
[2] C. Koch, J. Ingle, and V. Remcho,  Bonding Upchurch® NanoPorts to PDMS, Chips & Tips (Lab on a Chip), 12 February 2008.
[3] S. Mohanty, D. J. Beebe and G. Mensing, PDMS connectors for macro to microfluidic interfacing, Chips & Tips (Lab on a Chip), 23 October 2006.

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Integrated reservoirs for PDMS microfluidic chips

T. Liu, E. V. Moiseeva, and C. K. Harnett
Electrical and Computer Engineering, University of Louisville, Louisville, KY 40208, USA

Purpose


Reservoirs are sometimes preferable to tubing connections, especially for pressurized or gravity-driven flow, for installing electrode wires into open ports, or for storing a PDMS chip pre-loaded with water to preserve its hydrophilicity after plasma treatment. However, gluing individual reservoirs onto a finished chip can be messy and prone to clogging ports and channels with silicone, and the assembly may be fragile. These reservoirs, made from inexpensive nylon spacers, are embedded in the chip by PDMS casting, eliminating several of these problems. They can run with up to 30 psi gas pressure-driven flow when provided with connections for pressurized tubing.

Materials


Consumables:

  • Flanged nylon shoulder spacers (McMaster Carr part 91145A154, shown in Figure 1)
  • PDMS compound (Dow Corning Sylgard 182 or 184)

Figure 1: Flanged nylon shoulder spacers for use as reservoirs

For optional gas pressure-driven flow:

  • Epoxy (Scotch-Weld DP-420)
  • Hose barb connectors (McMasterCarr part 51525K211)

Tools:

  • A mold for casting chips (Figure 2)
  • PDMS weighing and degassing equipment
  • A punching tool for creating ports in PDMS, such as thin-walled stainless hypodermic Tubing (Small Parts Inc., part HTX-22X or similar)
  • Tweezers
  • X-Acto knife

Figure 2: A microfabricated mold on a 4-inch silicon wafer

Procedure


1. Make a mold for your design, with reservoir locations 1 cm apart or more. This method was tested on SU-8 molds on silicon wafers, with resist thickness from 60 to 170 microns, without any special mold release treatment.

Pour a thin (~1 mm) layer of PDMS over the mold, and cure until solid. Place the nylon standoffs on top of the PDMS layer, over the ends of your channels, flange side down. Later you will punch to connect the reservoir with the channel on the underside of the chip. These reservoirs easily stay upright if the PDMS is still a bit sticky. (Figure 3)

Figure 3: Reservoirs are placed on a thin layer of PDMS cured over the mold

Figure 4: A second layer of PDMS holds the reservoirs in place

2. Pour a second 2 mm or thicker layer of PDMS around the reservoirs, covering the flanges to anchor them. (Figure 4) It doesn’t matter if PDMS seeps into some of the reservoirs. Let the PDMS level out, then cure until solid. The nylon reservoirs are OK at temperatures up to 80ºC.

3. Cut out individual devices with the X-Acto knife and peel off devices plus embedded reservoirs. It’s best to grip the devices at the PDMS using tweezers, rather than pulling on the reservoirs. (Figure 5)

Figure 5: Reservoirs and channels are removed from the mold together

4. Use the punching tool to create a port in each reservoir that connects to the microfluidic channel, making sure to remove a core of PDMS from each port. This can be done from the channel side (Figure 6) or from the reservoir side.

Figure 6: Punched ports connect reservoirs to channels

5. Bond the punched chips to a glass, PDMS or other substrate using plasma activation or other method of choice. Figure 7 is a side view of a bonded chip, showing anchored reservoirs and punched ports on a molded PDMS chip, attached to a blank PDMS slab on a glass slide.

Figure 7: Side view of a sealed chip showing sunken reservoirs and ports

6. (Optional, for gas pressure-driven flow) Use epoxy to attach hose-barb connectors to the tops of any reservoirs you want to pressurize. Gas can be provided to each reservoir using Tygon tubing. The 1/16″ hose barb to Luer connectors, and 1/16″ ID Tygon tubing are a good match to the size of these reservoirs. Gluing adjacent Luer connectors together can help strengthen the assembly when several gas connections are anticipated.

7. Fill reservoirs through the open tops or through the bonded-on hose barbs. Dispensing from a syringe with a long needle or a needle threaded into a small diameter piece of tubing helps prevent trapped air by filling from the bottom up. Figure 8 shows a filled chip with barbs for gas tubes.

8. While these reservoirs have been pressurized up to 30 psi before delaminating, they are easily removed from the chip by bending them sideways. Reservoirs can be peeled out of old chips for re-use if needed.

Figure 8: A filled chip ready for gas pressure-driven flow
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PDMS connectors for macro to microfluidic interfacing

Swomitra Mohanty, David J. Beebe
University of Wisconsin Department Biomedical Engineering
Glennys Mensing
University of Illinois Department of Mechanical & Industrial Engineering

Why is this useful?


The purpose of this Tip is to show a simple method to connect the microfluidic world to the macro world.  Microfluidic devices are constructed from a variety of materials including glass, silicon, and polymers.

No matter how microfluidic devices are constructed, one common problem has been employing a user friendly method to connect standard fluidic equipment such as syringes, to the microfluidic device.  Often researchers attempt to use commercially available micro-tube fittings by gluing them to ports on a microfluidic device.  These fittings are small in size and difficult to work with and the glue can clog the port.  To solve this problem we employ an in house fabricated PDMS connector that consists of a hole through the center and uses double-sided adhesive to connect to the microfluidic device.

What do I need?


  • PDMS SYLGARD 184 silicone elastomer base & curing agent (Dow Corning)
  • 3″ Petri dish
  • One utility knife (McMaster Carr Supply Company No.:35575A71)
  • Grace Bio-Labs Secure-Seal Adhesive (double sided) (SA-S-1L)
  • Whatman 42 filter paper (9 cm Whatman No.:1442-09 )

The purpose of the filter paper is to help bond the PDMS to the double sided adhesive.  However the pores in the filter must be large enough for PDMS to penetrate.  Pore sizes between 2 -8 µm are appropriate

  • One blunt 16 gauge reusable steel needle (Integrated Dispensing Solutions Part # 9991279-2)

The blunt needles are used to core through the PDMS.  Care needs to be taken when coring otherwise tears may occur in the hole causing the PDMS connector to leak.  The inside of the blunt needle can be sharpened  using a round file.  Simply insert the file into the tip of the needle and file the inner edge till it is sharp.  This will make a cleaner hole through the PDMS.

The size of the needle will depend on the tubing size you are using to connect to your device.  Choose a needle size that is slightly smaller than the tubing you are using.  The PDMS will flex around the tubing and provide a good seal.

What do I do?


1. Trace Petri dish on adhesive and cut out circle  Typically 6 Petri dishes can be traced on a single Grace-Biolabs sheet.  The template provided can be used to help cut out circles.

2. Peel off the paper side of the adhesive and place the sticky side onto the filter paper. Apply pressure over the entire area to remove any air bubbles

Figure 1.

3. Place the filter/adhesive into the lid of the Petri dish.  Make sure the filter paper faces up and that the whole filter/adhesive assembly lies flat against the dish. Do not worry if the edge of the filter/adhesive curls up a little on the side. This will result in thinner PDMS connectors on that side.
Adhesive cicle in petri dish plus uncured PDMS


Figure 2.

They will still be functional. Mix 25 g of PDMS with 2.5 g (10%) hardening agent and pour it into the dish (Figure 2).  Place on a hot plate at 50 degrees Celsius and cure for 4 h.

4. Remove the dish from the hot plate and let cool for 5 min.  Using the knife cut the PDMS around the edges of the dish to loosen it.  Then pry the PDMS from the dish (Figure 3).


Figure 3.

5. Take the 16 gauge blunt needle and place it on the surface of the PDMS.  Turn the needle back and forth while applying pressure to create a through-hole.  Make several holes in a row (Figure 4).


Figure 4.

6. Take the knife and cut squares around each of the holes.  After cutting out the row, each hole can then be separated into individual connectors (Figure 5).

Figure 5.

7. Clean the surface of your microfluidic device with some ethanol.  (The surface must be clean for good adhesion.)  Peel the adhesive off the bottom of the PDMS connector. This should remove the PDMS core made with the blunt needle. If not, you may need to remove it with fine tweezers.

8. Attach the connector to your microfluidic device.  Insert tubing into the connector and attach the other end to a fluidic source such as a syringe (Figure 6).

Figure 6.

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