View source for Three-dimensional microfluidic devices fabricated in layered paper and tape

Original entry:  Sorell Massenburg,  APPHY 226,  Spring 2009

Andres W. Martinez, Scott T. Phillips, and George M. Whitesides

=Soft Matter Keywords=
Capillary Force. Microfluidics

=Overview=
This article describes a method for fabricating 3D microfluidic devices by stacking layers of patterned paper and double-sided adhesive tape. Paper-based 3D microfluidic devices have capabilities in microfluidics that are difficult to achieve using conventional open-channel microsystems made from glass or polymers. In particular, 3D paper-based devices wick fluids and distribute microliter volumes of samples from single inlet points into arrays of detection zones (with numbers up to thousands). This capability makes it possible to carry out a range of new analytical protocols simply and inexpensively (all on a piece of paper) without external pumps. We demonstrate a prototype 3D device that tests 4 different samples for up to 4 different analytes and displays the results of the assays in a side-by-side configuration for easy comparison. Three-dimensional paper-based microfluidic devices are especially appropriate for use in distributed healthcare in the developing world and in environmental monitoring and water analysis.

=Soft Matters=
The authors here use SU8 photoresist (hydrophobic) to pattern paper with microchannels (basketweave pattern for example). A layer of tape cut with punched holes was then placed on top of the patterned paper. The holes were then filled cellulose paste in order to fill the voids created between adjacent paper layers by the tape's thickness. This process could be repeated many times over.
The fluid is transported through wicking action between the paper and fluid. The cellulose in the tape's holes provide the method for vertical fluid flow, a cavity would simply fill with water and remain stagnant with surface tension. A filling with cellulose paste would allow water to wick through this area (presumably assisted by gravity)
[[Image:upad_schem.png|thumb| Schematic of process to fabricate a <math>\mu PADs</math> with a basket weave pattern.]]

The figure to the left shows a sample 3-dimensional device that utilizes a basket weave pattern to simultaneously multiple streams that do not mix. In this particular device, the fluid requires about 5 min. to fully travel through the device. The materials for this particular device costs about $0.03US. To illustrate the ability to flow differing streams in three dimensions with high fidelity, the authors various arrays that can be created with this method (as denoted by the different colors).

[[Image:upad_arrays.png|thumb| Various arrays via <math>\mu PADs</math>.]]


The authors emphasize that this new technique can be a boon to the developing world. It is potentially a means of cheaply doing medical or other chemical detection with limited technology. The authors demonstrate that each detection zone can be designed to be visible by eye, or with a camera phone. Additionally the authors demonstrate assays of glucose and protein to demonstrate the technique's applicability.

[[Image:upad_assays.png|thumb| Assays of glucose and protein<math>\mu PADs</math>.]]

Supplemental figures can be found here:
[http://www.pnas.org/content/suppl/2008/12/08/0810903105.DCSupplemental/0810903105SI.pdf#nameddest=SF3 Supplemental figures for <math>\mu PADs</math> ]

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2nd Entry: Dariela Almeda, AP 225, Fall 2009 

=Keywords=

Microfluidic devices, diagnostic devices, patterned paper, capillarity, [[photoresist]].

=Summary=

The article presents a novel idea of creating microfluidic devices using simply paper and double-sided adhesive tape instead of the more common glass or polymeric microfluidic devices.  One-dimensional paper based assays had already been established prior to the publication of this article.  However, the idea presented by the authors involves creating three-dimensional (3-D) devices to aid in diagnostics.  The great advantages of this 3-D device are the extremely low cost of fabrication ($0.003 per square cm per layer of paper and tape) which allows developing countries to afford this technology and its ruggedness which makes it ideal to use in non-ideal environments.  To construct these paper and tape devices, each sheet of paper is patterened with SU-8 photoresist and each layer of tape is patterned with a laser cutter.  The device is then assembled by layering the patterned paper and tape one by one.  


=Soft Matter Connection=

Fluids are able to move through these hydrophilic channels and hydrophobic walls through wicking or capillary action.  Capillarity is defined as "the ability of a substance to draw another substance into it."  Here the photoresist (which is hydrophobic polymer) patterned on the paper defined the channels through which the fluid was moving horizontally.  The also hydrophobic double-sided tape separated these channels between the vertical layers and the holes cut into the tape allowed for the fluid to flow vertically.