Archive for July, 2009

How to prevent sagging during the bonding or lamination of chips with large aspect ratio chambers

Jie Xu and Daniel Attinger
Laboratory for Microscale Transport Phenomena, Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA

Why is this useful?


Assembling multiple layers by bonding or lamination is a simple way to manufacture complex multilayer microfluidic chips [1, 2].  However, bonding or lamination of chambers with large aspect ratio, i.e. wide and shallow, sometimes fails because of sagging. Figure 1 illustrates a sagging problem, which resulted in the top chamber wall being accidentally bonded to the bottom wall.  Here, we describe a tip to prevent sagging by using regular cooking salt.

Figure 1: Accidental bonding due to sagging

What do I need?


  • Cooking salt (grain size: normally 300 microns, can be further ground to less than 100 micron; melting temperature: 801 °C, good for lamination)
  • Precision tweezers
  • Stereomicroscope

What do I do?


1. Carefully pave the bottom of the chamber with ordinary salt as in Figure 2. Try to perform this action using fine tweezers under a stereomicroscope, if the chamber is too small.

Figure 2

2. Plasma bond or laminate the top layer. Be careful during handling, so that the salt does not end up in your DRIE machine.

Figure 3

3. After bonding is done, flush the microfluidic system with deionized water for several minutes to dissolve and remove salt particles. As figure 4 shows, the bonded chamber does not exhibit adhesion between the top and bottom wall.

Figure 4: No accidental bonding

References


1. J. Xu and D. Attinger, Drop on demand in a microfluidic chip, J. Micromech. Microeng., 2008, 18, 065020.

2. P. J. Hung, P. J. Lee, P. Sabounchi, N. Aghdam, R. Lin, and L. P. Lee, A novel high aspect ratio microfluidic design to provide a stable and uniform microenvironment for cell growth in a high throughput mammalian cell culture array, Lab Chip, 2005, 5, 44-48.

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Quick assessment of the stability of flow generated by a syringe pump in a microfluidic device

Rachel Green and Siva A. Vanapalli
Department of Chemical Engineering, Texas Tech University, Lubbock, TX, USA

Why is this useful?


Syringe pumps are often used in a variety of microfluidic applications because of their portability and the ease with which flow rates can be changed. The syringe pump characteristics (type, age and wear), compliance in the tubing, a mismatch between the size of syringe used and the flow rate desired can generate pulsations in microfluidlic flows. These pulsations could be undesirable for lab-on-a-chip applications in which steady fluid flows are needed. The method described here is a quick means to assess the degree of pulsations present in flows driven by syringe pumps. The basic principle relies on using a microfluidic comparator [1,2] to detect small pressure fluctuations in fluid flows.

What do I need?


  • Microfluidic device with design as shown in Figure 2.
  • Ring stand
  • Ring stand clamp
  • Syringe pump
  • Distilled water
  • Dyed distilled water (refill ink)
  • Syringe (30 or 60 mL)
  • Tubing
  • Stereomicroscope

What do I do?


1. Set up ring stand with a wide-mouth syringe in the ring stand clamp (Figure 1). This is your hydrostatic head. The distilled water will go in this syringe.

Figure 1: Setup of the ring stand and syringe pump with the microfluidic device.

2. Place a syringe with the dyed distilled water into the syringe pump.

3. As shown in Figure 2, connect the syringe pump to the chip (labeled as flow rate Q). Connect the hydrostatic head, P, from the ring stand to the chip.

Figure 2: Microfluidic chip integrated with the comparator.

The outlets are open to atmospheric pressure. The device consists of two identical channels connected downstream to form a comparator region. At one inlet we impose a constant hydrostatic pressure (P) to generate steady flow and in the other inlet we use a syringe pump to introduce fluid admixed with a dye at a flow rate, Q.

4. Start the syringe pump at the desired flow rate. Vary the height of the hydrostatic head until the two fluid flows meet at the symmetry line of the comparator (see the white line in Figure 3a).

5. Because the hydrodynamic resistances of the two channels are equal, if any pulsations are present in the syringe-pump driven flow then the dyed fluid will be displaced above or below the symmetry line (shown in white) in the comparator, as shown in Figure 3b and 3c.

Figure 3: Images of the fluid-fluid interface in the comparator at different times.

(a) The flows are steady and balanced. (b) After a time interval of 2 seconds, the dyed fluid is below the symmetry line indicating the syringe pump flow rate is lower than the flow rate corresponding to the imposed hydrostatic pressure. (c) After 6.8 seconds, the dyed fluid flow is above the symmetry line, indicating the syringe pump flow rate is greater than the flow rate corresponding to the imposed hydrostatic pressure.

What else should I know?


The method described is a quick assessment of the pulsations generated by syringe pump driven flow. For a more in depth study, we recommend using a precision hydrostatic head rather than a ring stand. We built a stand that can hold a syringe on a precision linear translation stage (Edmund Optics, Part # NT56-796). The stage allows the hydrostatic head to be adjusted at 0.1 mm intervals. Using this hydrostatic head and a video camera, one can precisely determine the number of fluctuations per unit time for a specified pump, flow rate, syringe size and tubing.

References


1. M. Abkarian, M. Faivre and H. A. Stone, Proc. Natl. Acad. Sci., 2006, 103, 538-542
2. S. A. Vanapalli, D. van den Ende, M. H. G. Duits and F. Mugele, Appl. Phys. Lett., 2007, 90, 114109

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