Chips and Tips

Tamal Das
Department of Biotechnology, IIT Kharagpur-721302, India

Debapriya Chakraborty, Suman Chakraborty*
Department of Mechanical Engineering, IIT Kharagpur-721302, India
* e-mail: suman[at]mech.iitkgp.ernet.in

Why is this useful?

The integration of PDMS chips, with pumps or connections to reservoirs, has been found to pose serious difficulties because of its associated problems in leaking, interfacing with tubing, etc. Here, we present a simple approach for making reservoirs and also connecting devices with tubing for interfacing with pumps, without the aid of commercially available connectors and using commonly available consumables in the laboratory.

The PDMS mould is bonded with glass in two ways – using hydrophilic and hydrophobic means. Hydrophilic bonding involves using a plasma cleaner for oxidation. Hydrophobic bonding is done using a PDMS mixture. Here, we report easy and cost effective hydrophilic and hydrophobic bonding protocols. For example, the hydrophilic bonding can also be done without the use of conventional plasma oxidation.

What do I need?

• PDMS base and curing agent (SYLGARD 184 silicone elastomer kit) in 10:1 wt/wt ratio
• micropipette tip (2-200 µl pipette tips)
• 20 gauge (outer diameter) needles
• tubing (20 gauge inner diameter)
• Piranha solution (H₂O₂ and H₂SO₄ solution in 1:1 vol/vol ratio)

What do I do?

1. Cut the syringe needles to the desired length (approximately 1.5 cm), preferably using wire electrical discharge machining. Any other needle cutter can be used if it can cut sharply the ends of the needles.

2. Punch holes in the PDMS mould using these needles at the inlet/outlet.

3. Bonding the PDMS mould to the substrate (Figure 2):

• Hydrophilic bond:

1. Oxidize the glass slide and the PDMS mould in freshly prepared Piranha solution.
2. Dip the glass slide in the solution for 5 mins, and the PDMS mould for 10 secs. Dipping the PDMS mould for larger times may lead to damage to the mould because of strong oxidation
3. Rinse them in deionised water at least three times. This step is required to remove any residual acid as well as to prevent hydrophobic recovery.
4. Dry both the glass slide and the mould using a compressed stream of dry nitrogen gas.
5. Place the mould over the glass slide and heat it at 70°C for 30 minutes.

• Hydrophobic bond:

1. Coat the glass slide with a thin layer (~ 10 µm thickness) of PDMS mixture (preferably by spin coater).
2. Heat it at 70°C for 7 mins (this makes the PDMS hardened but still sticky)
3. Place the PDMS mould on the sticky PDMS surface. In order to avoid wrinkles and air gaps, lay the mould down from one end to the other.
4. Heat it for at least 30 mins at 70°C.

4. After bonding the PDMS mould to the glass, place the needles in the holes. Apply a small amount of PDMS mixture to the cylindrical surface of the needles. The PDMS mixture slowly settles down near the junction. Applying larger quantities of PDMS mixture may lead to seepage into the channel through the junction. Heat it immediately for 10-15 mins at 70°C. (Figure 3)

5. Cut a small piece of micropipette tip with at least one planar surface. Put a bit of PDMS mixture around the periphery of the planar surface. Place it centering the needles and heat it for 20 mins at 70°C. This makes the reservoirs. (Figure 4)

6. Pour PDMS mixture into the reservoirs around the syringe needle tips and heat it at 70°C for 25-30 mins until the supports become rigid. This provides the support for the needles. (Figure 5)

The tubing can be connected to these cut out needles, and the other ends connected to a syringe or peristaltic pump (Figure 6). If the connection to the pump is not desired and only the reservoirs are required then steps (1, 4, 6) are not required.

Alexander Iles
Dept. Chemistry, University of Hull, Cottingham Rd, Hull, HU6 7RX, UK

Problem

In microfluidics it is often necessary to interface chips using capillaries or plastic tubing. This can introduce a number of problems. For example, with glass chips it is rarely the case that the tubing or capillary is a perfect match for the reservoir hole that has been drilled into the device. Hence when using adhesives to attach tubing, the glue can seep down into the microdevice and clog its channels. Alternatively, with PDMS it is difficult to find anything that will stick to it and also securely hold a connecting tube in place. Nanoports can be used to interface devices, but they are expensive [1]. Small pieces of PDMS can be plasma bonded to a chip to create ports [2], but this involves extra fabrication steps and a plasma oven.
The solution:

To use fumed silica to create thixotropic adhesives. Adding fumed silica to epoxy resins or to PDMS creates adhesives that are extremely viscous and will not flow. Hence it can be added to epoxy resins to prevent seepage between access holes and tubing with glass chips and also it can be added to PDMS to make highly effective ports for PDMS chips.

Materials

• A small quantity of premixed epoxy resin or PDMS
• Fumed silica. Fumed silica is manufactured by Degussa and can be obtained as a free sample by visiting their website[3]
• A container and a spatula for mixing

Procedure

1. Take a small quantity of your resin of choice (epoxy or PDMS). Place this in your mixing container.

2. To the resin, add a small amount of fumed silica, and using a spatula, mix it into the resin. A good starting point is 1 part fumed silica to 11 parts pre-mixed PDMS (by weight). Warning! Always wear a dust mask when handling fumed silica, it is like very fine dust. One sneeze and it will go everywhere! Bear in mind the working life of your epoxy resin, some resins set in 5 minutes or less, which will not give much time for mixing in fumed silica.

3. Once the fumed silica has been incorporated into the resin, add more as necessary to obtain the required thixotropy. Then it is ready for application.

4. For glass chips, always wipe down the surface of the device with acetone and allow it to evaporate before applying epoxy, to ensure a good bond.

5. Place the tube or capillary into the hole. Use a pipette tip or small applicator to apply a small blob of adhesive around the interface. The resin mixture will not flow, so it will be necessary to smooth it around the tube or capillary with the applicator. The procedure is similar for PDMS devices, except that a hole must first be bored into the chip, the tube inserted and then the thixotropic PDMS applied. In both cases, cure times can be reduced and adhesion strengths improved if an oven at 60-70 ºC is used for curing. Refer to Figures 1 and 2 for step by step photographic explanations and further details.

Figure 1. Procedure for making thixotropic epoxy for interfacing glass chips: (a) Mix the epoxy in a suitable container and have some fumed silica ready for use. (b) Add the fumed silica and mix in. (c) Insert a capillary or tube into a hole on the glass chip, after wiping it with acetone. (d) Apply thixotropic epoxy to the interface between the chip and the connector and leave to cure.

Figure 2. Procedure for making thixotropic PDMS for interfacing PDMS chips: (a) Mix the PDMS in a suitable container and have some fumed silica ready for use. (b) Add the fumed silica and mix in. (c) The PDMS should now look like this. (d) Cut a hole using a boring tool in your PDMS chip. (e) Insert a tube (e.g. PTFE) of a suitable size into the hole on the PDMS chip. (f) Apply thixotropic PDMS to the interface between the chip and the connector tube. (g) Leave it to cure, either at room temperature or at 60-70 ºC. (h) The completed interface. A silica capillary from, for example, a syringe pump, can now be securely inserted into the PTFE tubing and removed again as often as required without leaking. The PTFE tubing has an I.D. of 300 µm, which makes a good interference fit with standard 375 µm O.D. silica capillaries.

Conclusion

Adding fumed silica to PDMS or epoxy resins allows the researcher to fine tune the viscosity of their system as desired. Thixotropic PDMS is probably one of the best materials for interfacing PDMS devices, since few things will stick better to PDMS than PDMS itself!

References

[1] C. Koch, J. Ingle, and V. Remcho,  Bonding Upchurch® NanoPorts to PDMS, Chips & Tips (Lab on a Chip), 12 February 2008.

[2]  S. Mohanty, D. J. Beebe and G. Mensing, PDMS connectors for macro to microfluidic interfacing, Chips & Tips (Lab on a Chip), 23 October 2006.

[3] http://www.aerosil.com/aerosil/en/default

Arash Noori and P. Ravi Selvaganapathy
Department of Mechanical Engineering, McMaster University, Hamilton, Ontario, Canada

Why is this useful?

Sealing and bonding of components in PDMS (poly-dimethylsiloxane) molds using PDMS as glue is as challenging as it is effective because of the ability of PDMS glue to conform to nanometer sized features.  An ever present problem is the flow of the uncured PDMS into undesired locations because of the deposition of large drops and the low viscosity of PDMS.  This can lead to clogging of interconnects, channels and on-chip components in the assembly process thus damaging the device or necessitating repair.

This tip presents a method for the deposition of microdrops of uncured PDMS using a micropipette and probe station micromanipulator to allow for highly controlled and localized sealing and bonding.  This process can be used to seal interconnects in critical regions, to repair cracks and to bond components into microfluidic devices.  It can be used for any process that requires highly localized deposition of PDMS to act as glue or sealant.  For demonstration purposes this article will describe the process to embed and seal a 360 µm OD/20 µm ID fused silica capillary into a PDMS microfluidic device without clogging the opening.

What do I need?

• Fully cured PDMS microfluidic device
• Sylgard 184 silicone elastomer kit (Dow Corning)
• Probe station with microscope and micromanipulator probe
• Micropipette puller and borosilicate glass tubes (1 mm OD/0.5 mm ID, Sutter Instrument Company, California, USA) to fabricate micropipettes (may also be purchased from WPI Inc, Florida, USA)
• Syringe (1 ml), needle (20G1½) and tubing (Tygon 2.4 mm OD/0.7 mm ID) (Figure 1)
• Epoxy
• Source of heat (hotplate or oven)

What do I do?

1. Connect the un-pulled end of the micropipette and the needle to opposite ends of Tygon tubing and seal using epoxy. (Figure 2)  The tubing should be long enough to allow for pumping of the PDMS without disturbing the micropipette tip.

2. Carefully attach the micropipette to the probe station micromanipulator tip using adhesive tape.  Make sure that it is firmly attached to the probe. (Figure 3)

3. Place the PDMS device onto the probe station and locate the place where deposition of the microdrops of PDMS is desired.  Refocus and move the micropipette tip to the centre of the viewing area of the microscope.

4. Withdraw a small amount (<1 ml) of 1:30 ratio PDMS into the syringe and connect to needle.

5. Very slowly begin injecting PDMS into the tubing and micropipette, monitoring the movement of the PDMS in the micropipette until a drop of PDMS is visible at the micropipette tip.

6. Refocus the microscope to the PDMS substrate below and bring the deposition area in alignment with the micropipette tip.  Slowly start moving the probe station micromanipulator tip down until both the PDMS substrate and the micropipette are visible.  Readjust the micropipette tip to the desired location using the micro adjusters.  Slowly move the micropipette tip down.  Once the micropipette tip makes contact with the PDMS substrate pause and let the PDMS drop flow into the desired region.  More PDMS can be deposited by gently applying pressure to the syringe to cause more flow of PDMS to the deposition area. (Figure 4)

7. After deposition is complete, place the covered PDMS substrate onto a hotplate at ~95 ºC and cure for 10 minutes.

References

http://www.tygon.com/
http://www.dowcorning.com/
http://www.sutter.com/
http://www.wpiinc.com/