Chips and Tips

Johnny J. Perez and Jonathan G. Shackman
Department of Chemistry, Temple University, Philadelphia, PA, USA 19122

Why is this useful?

A critical step in fabrication of microfluidic devices is coupling with the macroscopic world.  Reservoirs or pressure fittings are frequently adhered to devices by liquid epoxies or heat cured adhesive rings, such as supplied with commercial Upchurch Scientific (Oak Harbor, WA, USA) NanoPorts.  Liquid methods can potentially enter the device and clog channels.  Non-tacky resin rings make alignment of critical fittings and O-rings difficult, as they are inert until heat cured.  As an alternative to PDMS-based interfacing [1,2,], we describe a simplified method for forming customized, tacky adhesive rings using 3MT VHBT pressure-sensitive tape (Fig 1).  The rings are similar to the now discontinued NanoPort rings formed from 3MT 583 tape [3], but can cure at room temperature.  We have tested NanoPorts using these rings underwater on both polycarbonate and glass substrates in excess of 40 psi with no failures.  The cost per ring (assuming a 1 cm diameter) is a few US pennies.  Rings can easily be peeled from the ports and substrate after applying sufficient torque or sheer forces without leaving residues, facilitating aggressive cleaning (such as in piranha solution) of clogged microchannels.

What do I need?

The materials needed are shown in Figure 2 and include:

• 3MT VHBT tape.  We use 0.5″ wide VHBT 4926, but a variety of tapes (including 3MT 583 tape) could be used, with selectable adhesion qualities and solvent resistances.
• Gasket Punch or Cork Borers.  Cork borers (commonly used in chemistry labs) require two cuts while dual cutting punches, such as available from McMaster-Carr (Elmhurst, IL, USA), cut both inner and outer rings simultaneously.  We use 3/16″ inner and 7/16″ outer diameter punches for NanoPort rings.
• Hammer and pad.  A piece of scrap wood can be used for a cutting pad.
• Forceps.  Fine tips aid in ring manipulation and removing the adhesive backing.
• Ports and microchip.  Homemade or commercial ports and chips can be used with the method.  The glass device used for demonstration was made using previous protocols [4].
• Binder clip.  The tape requires at least 10 psi for strong bonds and pressure should be maintained during curing.  Mini-vises or C-clamps can also be used for hard to reach ports [3].
• Oven (optional).  VHBT tape can cure overnight at room temperature or in 1 hr at 60 to 90 °C.

What do I do?

The sequence is shown in Figure 3.  The following specifics can easily be modified for a given application.

1. Clean ports / O-rings in DI water followed by methanol or isopropanol.  It is assumed the microdevice is clean after fabrication but can also be carefully cleaned.
2. Cut a 1 cm length of tape and place the sticky side on the punch.
3. Place the end of the punch/tape on a pad or wood block and hit the opposite end of the punch with a hammer (Fig 3A).
4. The ring will likely remain in the punch, with the backing side facing out (Fig 3B).  Use forceps to remove the newly formed ring and attach the exposed tacky side to the port.  Pressing the port (prior to removing the backing) against a flat surface can help adhesion.
5. Remove the backing with forceps to expose the other tacky side (Fig 3C) and align the assembly with the microchip access hole.
6. Clamp the port and microfluidic device using a binder clip or clamps (Fig 3D).
7. Cure either overnight or for 1 hr in an oven at 60 to 90 °C.
8. Adhered ports can be removed by either twisting and tilting or tightening a nut longer than the port to lift the port off the substrate.  The remaining ring can be removed using forceps to lift an edge and peeling (Fig 4).

What else should I know?

Rings should be made as needed to prolong shelf life (VHBT on the roll lists a 2 yr life when stored at room temperature and 50% relative humidity) [5].  While suitable for aqueous solutions, prolonged use of organic solvents is not recommended with VHBT tape [6], and an alternative adhesive or method, such as described by Watson and Wheeler [1], is suggested.  Application of high torque will cause the rings to fail, as can occur when over-tightening pressure fittings.

References

[1] M. W. L. Watson, A. R. Wheeler, Organic solvent compatible reservoirs for glass microfluidic chips, Chips & Tips, (Lab on a Chip), 12 December 2007
[2] J. Greener, W. Li, D. Voicu, E. Kumacheva, Reusable, robust NanoPort connections to PDMS chips, Chips & Tips, (Lab on a Chip), 24 October 2008
[3] C. Koch, J. Ingle, V. Remcho, Bonding Upchurch® NanoPorts to PDMS, Chips & Tips, (Lab on a Chip), 15 February 2008
[4] N.I. Davis, M. Mamunooru, C.A. Vyas, J.G. Shackman, Anal. Chem., 2009, 81, 5452-5459.
[5] 3MT VHBT Double Coated Acrylic Foam Tapes – June 2009 Data Sheet
[6] 3MT VHBT Durability – March 2001 Bulletin

Jian Hua Han, Shao Hua Li, Zhong Han Sheng and Long Jiang
Key Laboratory of Colloid and Interface, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing, China

Why is this useful?

We demonstrate a reversible bonding method to fabricate microscope slide glass chips. This method involves pre-curing a PDMS polymer layer to form a highly adhesive glue which strongly bonds glass chips. Traditional bonding methods[1-4] are either irreversible or not very strong; our approach solves this problem. The glass chips made by our approach can bear high fluid pressure without leakage and can easily be disassembled and reassembled when clogging happens. Our method is suitable for any ordinary labs that lack expensive equipment to fabricate microfluidic chips.

What do I need?

• Microscope slides and PDMS kits (SYLGARD®184), produced by Dow Corning. The silicone elastomer base and curing agent were mixed well at the ratio of 10:1 to form a paste before use.
• Craft knife, scalpel, plastic wrap, etc.
• Microscope slides chip with patterns and appropriately located inlet and outlet holes.

What do I do?

First, prepare a microscope slide chip using standard wet etching methods[5] and drill holes. Then execute the following 5 steps:

1. Pour the PDMS (elastomer mixed with curing agent and degassed) onto a glass slide, see figure 1.

2. Place another glass slide onto the PDMS of step 1 carefully, avoiding any bubbles during the process. Use a staple as a spacer, and wrap the sandwich in plastic wrap to avoid leakage of PDMS. See figure 2.

3. Cure the PDMS to a solid state, then pry up the upper slide. Use a scalpel to trim the PDMS layer to size, see figure 3.

4. Spin coat a fresh PDMS layer 1µm thick onto the PDMS layer of step 3 at the rate of 3000 RPM, then pre-cure for 20min at 80°C to form a hard glue, see figure 4.

5. Place the glass slide chip (with patterns and appropriately located inlet and outlet holes) onto the PDMS layer of step 4, then continue to cure the polymer to a solid state, and the microfluidic chip is formed, see figure 5.

What else should I know?

The method presented above is a useful method for rapid fabrication of microfluidic chips. The advantages of this method are as follows:

• does not need a strictly flat surface, very cheap microscope slides are available
• high contact bonding strength, does not rupture under high pressure (280kPa), while commonly used devices cannot withstand fluid pressures of more than 100kPa
• the chip can be opened with a craft knife when clogging happens and can be reused many times

The limitation of this method is that you must be careful when placing the etched glass slide onto the uncured PDMS layer – avoid too much pressure on it since this could lead to squeezing of PDMS into the channels and clogging of the chip.

Acknowledgements

This research is funded by the Chinese Academy of Sciences (grant number KJCX2-YW-H18) and the National Sci-Tech Special Item for Water Pollution Control and Management (2009ZX07 5287- 007 – 03).

References

[1] N. Miki, Sensor Letters, 2005, 3, 263-273
[2] A. Sayah, D. Solignac, T. Cueni and M. A. M. Gijs, Sensors and Actuators A: Physical, 2000, 84, 103-108
[3] S. Shoji, H. Kikuchi and H. Torigoe, Sensors and Actuators A: Physical, 1998, 64, 95-100
[4] N. Chiem, L. Lockyear-Shultz, P. Andersson, C. Skinner and D. J. Harrison, Sensors and Actuators B: Chemical, 2000, 63, 147-152
[5] M. Castano-Alvarez, D. F. Pozo Ayuso, M. Garcia Granda, M. T. Fernandez-Abedul, J. Rodriguez Garcia and A. Costa-Garcia, Sensors and Actuators B: Chemical, 2008, 130, 436-448

Christopher Y. Wong*, Georgian Ionut Ciobanu*, Mohammad A. Qasaimeh, and David Juncker
Micro and NanoBioEngineering Laboratory, Biomedical Engineering Department, Faculty of Medicine, McGill University, Montreal, Canada.
* Authors contributed equally

Why is this useful?

Microfluidic and lab-on-a-chip (LOC) devices are typically connected to external sample reservoirs. A significant issue for LOC devices is interfacing multiple sample reservoirs and connecting them both to external pressure controllers or syringe pumps and to the microfluidic chip [1,2]. Researchers typically use home-made reservoirs and interfacing them to the chip and the controllers requires specialized skills and training and/or additional fabrication steps.[3,4] Often the connections are unreliable and leaky, and as a consequence much time is lost in setting them up and it is difficult to add or remove connections from the reservoirs.

Here we introduce a “plug-and-play” reservoir that allows for multiple connections to be simply plugged in. The reservoirs are made using off-the-shelf threaded microcentrifuge tubes and screw-on septum caps. Needles, capillaries and steel tubing can be inserted one at a time – or in combination – through the cap’s septum and used to connect the reservoir to a pressure source and to one or several inlets of a microfluidic chip. The connectivity between reservoir and pressure source(s) and chip(s) can be adapted to specific needs, for example to form a reservoir with a single input and a single output, or with multiple inputs and outputs. Reservoirs with single or multiple input/output lines can be switched easily by by unscrewing the microcentrifuge tube and screwing the cap with the capillaries on another microcentrifuge tube containing a different solution.

The reservoirs are cheap, disposable, easy to assemble, and robust enough to support pressures up to 345 kPa (and even higher pressures with the variant described in the supplementary material) while having almost no dead volume. The connection of reservoirs to microfluidic chips – or to other reservoirs so as to form complex circuits – can be performed in a matter of seconds. Arrays of off-chip reservoirs can be assembled within racks and many lines can be connected at high density to a microfluidic chip. These plug-and-play reservoirs are a versatile solution to the world-to-chip interface and should find widespread application for microfluidic experimentation.

What do I need?

For the reservoir:

• threaded screw cap with membrane/septum [5]
• threaded microcentrifuge tube (any volume, as long as it has the same thread type as the cap) [5]

For the connections:

• sharp hypodermic needle with Luer-Lock (any size from 23 to 16 gauge) [6]
• 1/16″ ID Tygon tubing or other preferred tubing [7]
• male Luer-lock to 1/16″ ID hose barbed tubing adapters [7]
• glass capillary (Note: OD of glass capillary needs to be smaller than ID of needle) [8]

Optional for other connection types (shown in supplementary materials):

• 0.0250″ OD hollow steel tubing [9]
• 0.0200″ ID microbore Tygon Tubing [10]

Fig. 1 shows the required parts. Fig. S1 in the supplementary materials shows the alternative options in our laboratory of microcentrifuge tube sizes and input/output connection types. (A) Threaded screw cap with membrane/septum [5]. (B) Threaded microcentrifuge tube [5]. (C) Sharp 20G1½ hypodermic needle with Luer-Lock [6]. (D) 1/16″ ID Tygon tubing [7]. (E) Male Luer-lock to 1/16″ ID hose barbed tubing adapter [7]. (F) Glass capillary of 350 µm OD [8].

What do I do?

1. In order to insert a glass capillary to serve as an output, use a sharp hypodermic needle with an inner diameter (ID) that is just larger than the capillary’s outer diameter (OD). Insert the needle into the membrane screw cap and then slide the capillary through the needle, as shown in Fig. 2.

2. Screw the cap onto a reservoir of the desired size and then remove the needle while holding the capillary in place, as shown in Fig.3. Repeat steps 1 & 2 for every additional glass capillary connection required.

3. Insert the input pressure needle to appropriate depth (see next section for details) and connect to the Tygon tubing to the needle using the Luer-lock to tubing adapters, as shown in Fig.4. Repeat this step for every additional Tygon tubing connection required.

4. The reservoir is completed. Unscrew the cap from the container and fill it with your reagent and screw the cap back on.

5. (Optional step) Instead of using a glass capillary, a steel tube connected to Tygon microbore tubing can be used. We used 0.0250″ OD steel tubing, inserted into 0.0200″ ID Tygon tubing and then forced the pin through the septum, as seen in Fig. 5. Multiple steel-Tygon tubing connections can be connected to one septum as shown in Fig. S2.

6. Connect the 1/16″ Tygon tubing to the pressure source and the capillary (or the microbore Tygon tubing) to the chip. Make sure to create slightly smaller sized holes for the input connections on the chip to create a tight seal. When using multiple reservoirs, they can be placed into microcentrifuge tube racks for easy handling (Microtube Storage Racks), as shown in Fig. 6.

7. Everything is now ready to use!

What else should I know?

This setup is simple, easy to build and can be done in less than a minute. Depending on the cap, the membrane/septum provides an airtight seal, but after inserting and removing needles more than a few times, especially with larger needles, leakage becomes apparent. Since the system is pressure controlled, a small leakage rate relative to the supply flow rate can be tolerated. Only a limited number of connections, up to 10 in our experience, can be connected to a single cap or else leakage becomes excessive. The reservoirs work well up to 345 kPa, and for higher pressure applications solid caps can be used as described in the supplementary material.

It is important to maintain the needle connected to the pressure source above the liquid surface in order to prevent the formation of bubbles. Be careful of the sharp points and remember to dispose of the needles properly.

Supplementary materials

Figure S1. The different available options (excluding glass capillaries) in our laboratory of microcentrifuge tubes sizes, membrane caps, steel tubing, and input/output connections.

The presented setup above was not tested at pressures higher than 345 kPa, and at pressures above 140 kPa, there was minimal gas leakage. However, we have developed another setup that works without any leakage, even at 345 kPa, but requires several minutes to make and a precision micro-drill. This setup does not use a membrane cap, but a conventional screw cap without a membrane. Using suitable micro-sized drill bits, different sized holes can be made in the micro tube or the cap. After inserting the output capillaries or steel tubing and the input syringe needle, Instant Krazy glue [10] is applied to affix the inputs and outputs at the precise place. Dual melt glue [11] is then applied to seal the terminals from air/liquid leaks. The final setup is shown in Fig. S3.

We mention here that this setup is permanent and should be robust at high pressure applications. At the same time, it requires a precision micro drill, more time to make, and it is less flexible in the sense that it is harder to modify the interface.

References

[1] S. Li and S. Chen, IEEE Trans. Adv. Packag., 2003, 26, 242-247.
[2] R. Lo and E. Meng, Sens. Actuators, B, 2008, 132, 531-539.
[3] T. Liu, E. V. Moiseeva and C. K. Harnett, Integrated reservoirs for PDMS microfluidic chips, Chips & Tips (Lab on a Chip), 22 April 2008.
[4] T. Das et al., Interfacing of microfluidic devices, Chips & Tips (Lab on a Chip), 27 February 2009.
[5] Sarstedt AG & Co.: http://www.sarstedt.com/
[6] BD: http://www.bd.com/
[7] McMaster-Carr Supply Company: http://www.mcmaster.com/
[8] Polymicro Technologies: http://www.polymicro.com/
[9] New England Small Tube: http://www.nesmalltube.com/
[10] Small Parts, Inc.: http://www.smallparts.com/
[11] Krazy Glue: http://www.krazyglue.com/
[12] The Stanley Works: http://www.stanleyworks.com/

Jean-Christophe Baret
Institut de Science et d’Ingénierie Supramoléculaires (ISIS),
Université de Strasbourg, CNRS UMR 7006, Strasbourg, France
jc.baret[at]unistra.fr

Why is this useful?

In suspension, cells, beads and particles tend to sediment at a speed given by the balance of viscous drag and gravity force. In classical biological experiments, the most straight-forward solution to keep cells in suspension is to shake large volumes. This type of system is relevant at high Reynolds number but is not very compatible with the classical systems used to inject small amounts of liquid in confined geometries, such as microfluidic systems.

Another common way to prevent sedimentation is to match the densities of the particles and the carrier fluid, or to increase the viscosity of the carrier fluid to increase sedimentation times. However, these solutions are not always suitable due to the biological or physical constraints of a microfluidic experiment. For example, increasing viscosities increases the pressure drop in microchannels, and additives for density matching are not always biocompatible.

Previously in the Chips and Tips section a system has been described for cell injection [1]. With a similar system, we observed that sedimentation along the tubing binding the syringe reservoir to the chip hinders good mixing of the cells. This tubing should therefore be as short as possible, which is sometimes incompatible with the large footprint of the syringe pumps or other equipment around the microfluidic chip. Interestingly, in another Chips and Tips section, a system for temperature control of syringe pumps has been described [2]. The idea there was to use a syringe pump to pump liquid in order to move the piston of another syringe placed in a temperature-controlled bath. This system still showed problems related to the length of the connection tubing to the chip.

Here we combine both systems [1,2] to inject cells, beads and particles efficiently into microfluidic devices using small amounts of liquids (typically ~1 mL). The cells are initially placed in a syringe close to the chip and are automatically shaken inside the syringe using a small motor and a magnet. This syringe is then remotely actuated by another syringe mounted on a syringe pump [2] which reduces the footprint of the injection system in the vicinity of the chip. The tubing length from the syringe containing the cells to the chip can then be as small as 5 cm, which reduces the residence time of cells in the tubing and therefore reduces sedimentation.

What do I need?

• Syringes: BBraun Injekt (2 mL or 5 mL)
• Syringe needles: Terumo NEOPLUS 23G
• Tubing: Fisher Scientific PTFE tubing (i.d. 0.56 mm, o.d. 1.07 mm)
• Small magnets: Supermagnete, e.g. S-03-01-N
• Teflon magnets: Fisher Scientific, length 6 mm, diameter 3 mm
• Motor: Radiospares, e.g. 20GN steel 50:1 gear DC motor, 175rpm 12V
• Power supply: Radiospares, e.g. LASCAR – PSU 130
• Small equipment: stands and support, steel wire

What do I do?

1) The first part of the procedure deals with preparing the syringes. This system is very close to the one presented in [1]. Cut the plungers of 2 syringes, and insert a magnet in one (Fig 1).

2) Bind them together with the metal wire (Fig 2).

3) Fill the syringe that does not contain the magnet with water and hold it vertical (Fig 3).

4) Insert the needles in the tubing, fill a third syringe with water, place the needle in position, flush the tubing with water and connect the syringe with the free needle (Fig 4). Make sure no air bubbles are present in the water phase: they will increase the response time of the system to flow rate modifications.

5) Invert the syringe to have the magnet in the upper part and fill this part with the liquid containing the cells, beads or particles (Fig 5) and connect a short needle with tubing (Fig 6).

6) Rotate the syringe and mount the motor with the magnets. Turn on the power supply (Fig 7). The speed of the motor is simply controlled by the voltage to guarantee gentle agitation of cells.

7) The system is now ready to be connected to a chip on one hand and to a syringe pump on the other (see video below). More sophisticated systems can probably be adapted to allow a temperature control of the syringe.

References

[1] R. Cooper and L. Lee, Preventing suspension settling during injection, Chips & Tips (Lab on a Chip), 21 August 2007.
[2] L. Capretto, S. Mazzitelli and C. Nastruzzi, An easy temperature control system for syringe pumps, Chips & Tips (Lab on a Chip), 22 April 2008.