Fabricating PFPE Membranes for Microfluidic Valves and Pumps

This process contributes to development of “laboratory-on-a-chip” devices.

A process has been developed for fabricating membranes of a perfluoropolyether (PFPE) and integrating them into valves and pumps in “laboratory-on-a-chip” microfluidic devices. Membranes of poly(tetrafluoroethylene) [PTFE] and poly(dimethylsilane) [PDMS] have been considered for this purpose and found wanting. By making it possible to use PFPE instead of PTFE or PDMS, the present process expands the array of options for further development of microfluidic devices for diverse applications that could include detection of biochemicals of interest, detection of toxins and biowarfare agents, synthesis and analysis of proteins, medical diagnosis, and synthesis of fuels.

To be most useful, a membrane material for a microfluidic valve or pump should be a chemically inert elastomer. PTFE is highly chemically inert and is a thermoplastic and, therefore, subject to cold flow and creep. Also, procedures for fabricating PTFE membranes are excessively complex. PDMS is an elastomer that has been used in prior microfluidic devices but, undesirably, reacts chemically with some liquids (acetonitrile, acids, and fuels) that might be required to be handled by microfluidic devices in some applications. On the other hand, the PFPE in question has elastomeric properties similar to those of PDMS and a degree of chemical inertness similar to that of PTFE.

The specific membrane material to which the present process applies is a commercially available, ultraviolet-curable PFPE. A microfluidic device of the type to which the process applies consists mainly of this PFPE sandwiched between two plates of a highly chemically resistant, low-thermal-expansion borosilicate glass manufactured by the float method. Heretofore, there have been two obstacles to fabrication of microfluidic devices from this combination of materials: (1) The lack of chemical reactivity between the PFPE and the glass makes it impossible to form a lasting bond between them; and (2) such conventional membrane-fabrication techniques as spin coating yield membranes that are not sufficiently flat and not sufficiently resistant to curling upon themselves. The present process overcomes these obstacles.

The process consists mainly of the following steps:

1. A fluorocarbon-based polymer is formed on the glass plates by means of a plasma deposition subprocess.
2. The polymer is patterned by use of a photoresist and conventional photo-lithography.
3. The polymer is removed in the pattern by use of an O₂/Ar plasma.
4. The remaining polymer surface areas are cleaned and modified by use of a low-energy O₂ plasma.
5. The glass plates are spin-coated with a lift-off material, which is then cured by heating to a temperature of 150 °C for 5 minutes.
6. The liquid (uncured) PFPE material is pressed between the two lift-off-layer-coated glass plates, along with 250-μm-thick shims to define the desired thickness of the PFPE membrane.
7. The liquid PFPE is cured to a solid by exposure to ultraviolet light for 5 minutes.
8. The PFPE membrane is released from the glass plates by submersion in a developer solution and/or acetone.
9. The glass plates and the PFPE membrane are cleaned and activated for bonding by exposure to an O₂ plasma.
10. The glass plates and the membrane are aligned and sandwiched together at a temperature ≤100 °C and a pressure of 3 bar (0.3 MPa) for one hour. This combination of pressure and temperature is sufficient to cause a chemical reaction that results in bonding of the PFPE membrane to the polymer coats on the glass plates.

This work was done by Frank Greer, Victor E. White, Michael C. Lee, Peter A. Willis, and Frank J. Grunthaner of Caltech and Jason Rolland and Jake Sprague of Liquidia Technologies Inc. for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
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Refer to NPO-45725, volume and number of this NASA Tech Briefs issue, and the page number.