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Research Papers

Modular Microfluidic Filters Based on Transparent Membranes

[+] Author and Article Information
E. Archibong, H. Tuazon, H. Wang, J. Winskas

Innovative Biomedical Instruments
and Systems (IBIS) Laboratory,
Department of Chemical and Biomedical Engineering,
University of South Florida,
4202 E. Fowler Avenue,
Tampa, FL 33620

A. L. Pyayt

Innovative Biomedical Instruments
and Systems (IBIS) Laboratory,
Department of Chemical and Biomedical Engineering,
University of South Florida,
4202 E. Fowler Avenue,
Tampa, FL 33620
e-mail: pyayt@usf.com

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received March 12, 2016; final manuscript received August 1, 2016; published online August 19, 2016. Assoc. Editor: Satish Chaparala.

J. Electron. Packag 138(4), 041002 (Aug 19, 2016) (6 pages) Paper No: EP-16-1049; doi: 10.1115/1.4034369 History: Received March 12, 2016; Revised August 01, 2016

We propose a new approach to the modular packaging of microfluidic components, in which different functional components are not only fabricated separately but are also designed to be individually removable for the purposes of replacement or subsequent analysis. In this paper, we demonstrate one such component: a stand-alone microfluidic filter that can be custom-fabricated and then connected, disconnected, and replaced on a microfluidic chip as needed. This filter is also designed such that particles captured on the filter can be further analyzed or processed directly on the filter itself—for example, for microscopic examination or cell culturing. The filter is a thin (1 μm) transparent silicon nitride membrane that can be designed and fabricated according to specifications for different applications. This material is suitable for microscale fabrication; filtration of a variety of solutions, including biological samples; and subsequent particle imaging and processing. The porous nature of the thin filter allows for particle separation under relatively low pressures, thus protecting the particles from rupture or membrane damage. We describe two methods for integrating the filter apparatus onto a microfluidic chip such that it can be inserted, removed, and replaced. To demonstrate the utility of this approach, we fabricated custom-designed silicon-based filters, incorporated them onto microfluidic systems then filtered microparticles and live cells from test solutions, and finally removed the filters to image the microparticles and culture the cells directly on the filter membranes.

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Figures

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Fig. 1

Scanning electron microscope (SEM) image of a microfabricated filtering membrane device. The large square is a silicon membrane on the top (upstream) side of the wafer. It has a periodic array of holes. The smaller square, visible through the top membrane, is a membrane on the bottom (downstream) side of the wafer. This lower membrane has one circular hole in the middle.

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Fig. 2

A 3-in. silicon wafer with multiple microfabricated filtering membranes of different dimensions and with different pore sizes

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Fig. 3

Fabrication process for the transparent, reconfigurable microstrainer. (a) and (b) Low stress silicon nitride is deposited on both sides of a double-sided polished wafer. (c) Photoresist is spin-coated and exposed with photolithography to pattern pores in the membrane, (d) then followed by etching through the nitride layer using a reactive ion etcher. (e) The wafer is flipped, and the back side is similarly patterned using photolithography and etching to create a single large hole (rather than the pore array of topside). (f) To complete the fabrication, KOH is used to release the structure.

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Fig. 4

Water contact angle measured on the surface of the microfabricated membrane

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Fig. 5

Two methods of integrating the exchangeable filter device into a microfluidic filter system: (a) In the first method, the filter (circled) is sandwiched between PDMS adapters with supporting glass slides with capillary tubes attached. (b) Close-up view of the filter membrane, showing its periodic array of pores, each 6 μm in diameter. (c) In the second method of integration, the whole microfluidic structure, including microfluidic channels, is moulded in PDMS. In this photograph, the fluid-carrying channels are oriented left-to-right. The membrane filter would be inserted into the insertion slot perpendicular to the channels. (d) A closer view of the membrane filter in place.

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Fig. 6

Three-dimensional model of one method of integrating the silicon membrane into a microfluidic filter. Left panel: The membrane is sandwiched between two protective polymer (PDMS) adapters. Right panel: Glass slides are attached to the outside ends of the polymer blocks, oriented perpendicular to the direction of fluid flow. Here, the filtering of a blood sample is shown, with whole blood entering from the left and cell-free plasma exiting to the right.

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Fig. 7

Demonstration of microfluidic filtration and imaging. (a) A concentrated solution of brown microspheres in distilled water flows in from the left, and clear filtrate flows out to the right. (b) Human lung fibroblast cells on a membrane, filtered from a buffer solution. Panels (c) and (d) show 20 μm microbeads captured on a membrane with 10 μm pores, with the microscope focused first on the beads (c) and then the membrane (d). Panels (e) and (f) are focused above and below the transparent membrane, respectively, to show that small particles can pass through the membrane.

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Fig. 8

Human lung fibroblast cells, captured and then grown for 3 days on the surface of the microfiltering membrane. (a) Optical microscopy image of the membrane with cells. (b) Green fluorescent image of the cells, demonstrating that all are alive.

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