Chips and Tips

Shuo Wang, Huaiqiang Yu, Wei Wang and Zhihong Li
National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, China

Why is this useful?

PDMS (polydimethylsiloxane) is one of the most important materials in microfluidics and is widely used because of its optical transparency, ease-in-fabrication, low cost and air permeability. A widely used interconnection approach for PDMS chips is the “press-fit” method [1]. However, the seal is only achieved by the compression of PDMS. An unexpected disturbance to the needle may damage the PDMS around it and produce small cracks leading to leakage around the needle. The mechanism of disturbance-caused leakage is shown in Fig. 1.

Fig. 1 Mechanism of disturbance-caused leakage

Here, we report two easy methods of fixing needles by a secondary PDMS fabrication. In these “cure and fix” methods, uncured PDMS is poured and cured to fix needles. Sectional views in Fig. 2 show the two schematic fabrication processes respectively. Cover packaging methods can be applied to produce covers in large number used as standard components. After PDMS chips are made, simply bond them with these standard covers to seal reservoirs. We can also employ whole packaging method to fabricate one specific device with lots of reservoirs.

Fig. 2 “Cure and fix” method

What do I need?

• PDMS chip peeled off from Si mould
• Uncured PDMS (base:cure = 10:1)
• Silicon or glass substrate
• Unmodified needles
• Scalpel and tweezers
• Hole puncher
• Oxygen plasma etching machine or corona charging device

How do I do it?

Whole packaging method

1. Punch holes for reservoirs on a PDMS chip and bond the chip with silicon or glass substrates using oxygen plasma or corona treatment [2].
2. Plunge unmodified needles into reservoirs laterally using the “press-fit” method. Clear away PDMS scraps with a pair of tweezers.
3. Seal all reservoirs by bonding PDMS blocks. Cast uncured PDMS onto the chip until lower half of the needle is submerged.
4. After curing PDMS at 70°C for 1 hour, cut the chip into proper size.

Cover packaging method

1. Punch a hole for the reservoir on a flat PDMS block and bond it with another PDMS block
2. Plunge an unmodified needle into the reservoir laterally using the “press-fit” method. Clear away PDMS scraps with a pair of tweezers.
3. Put the PDMS cover on a flat culture dish and cast uncured PDMS.
4. After curing PDMS at 70°C for 1 hour, cut the cover into proper size.
5. Bond the cover with a PDMS chip to seal reservoirs.

What else should I know?

In order to plunge the unmodified needle into reservoirs successfully, the PDMS cannot be too thin. The thickness should be larger than 3 mm.
Be careful in step 4 of whole packaging method because silicon and glass are brittle.

Fig. 3 A) Vertical view and B) side view of device using whole packaging method C) vertical view and D) side view of device using cover packaging method

References

[1] A. M. Christensen, D. A. Chang-Yen and B. K. Gale, Characterization of interconnects used in PDMS microfluidic systems. J. Micromech. Microeng., 2005, 15, 928-934.
[2] K. Haubert, T. Drier and D. Beebe, PDMS bonding by means of a portable, low-cost corona system, Lab Chip, 2006, 6, 1548-1549.