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SPECIAL ISSUE ON NANOMANUFACTURING

# Process Robustness of Hot Embossing Microfluidic Devices

[+] Author and Article Information
Thor Eusner, Melinda Hale, David E. Hardt

Laboratory for Manufacturing and Productivity, Massachusetts Institute of Technology, Cambridge, MA 02139

J. Manuf. Sci. Eng 132(3), 030920 (Jun 14, 2010) (8 pages) doi:10.1115/1.4001421 History: Received September 24, 2009; Revised March 05, 2010; Published June 14, 2010; Online June 14, 2010

## Abstract

Polymeric substrates have significant advantages over silicon and glass for use in microfluidics. However, before polymer microfluidic devices can be mass produced, it must be shown that the manufacturing method used to create these devices is robust and repeatable. For this paper, a polymer manufacturing process, hot embossing, was used to produce microsized features in polymethylmethacrylate (PMMA) chips. A design of experiments that varied two factors during the hot embossing process (temperature and pressure), was conducted to determine the robustness of hot embossing microsized channels in PMMA. The channel height and width were measured at three sites on each chip, and the results were analyzed in two ways: response surface modeling (RSM) and nested variance analysis. For the RSM analysis, two separate ANOVA tests and regressions were performed on both channel width and channel height to obtain the response surface models between temperature, pressure and the channel width and height. Furthermore, the variance of channel width and height at each design point was determined and then two ANOVA tests and two separate regressions were performed to obtain the response surface models between temperature, pressure and the variance of channel height and channel width. This analysis was used to determine if hot embossing microfluidic devices is a robust process capable of producing quality parts at different operating conditions. The nested variance analysis was used to determine the primary source of the variation in channel height and width. For the nested variance analysis, two separate calculations were performed in order to determine whether the variance of channel width and height is mostly caused by within-chip variance or chip-to-chip variance. The analysis showed that the channel widths and heights were statistically equal across the four different operating points used (the low-temperature, low-pressure point was omitted). The variance of channel width and the variance of channel height remained constant in the desired operating region. Based on this analysis, it was concluded that hot embossing is a robust process for features on the order of $50 μm$. Furthermore, the nested variance analysis showed that the variance of channel width and height is mostly caused by site-to-site measurements on a chip rather than between-chip variance. Therefore, it was determined that hot embossing microfluidic devices are repeatable and consistent from chip-to-chip.

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## Figures

Figure 1

Hot embossing process: (a) polymer is heated, (b) tool is pressed into the hot polymer, and (c) tool is separated from polymer and polymer is cooled

Figure 2

PMMA compression tests at a strain rate of 0.1 /s and temperatures ranging from 90°C to 170°C(15)

Figure 3

Experimental hot embossing equipment

Figure 4

Bulk metallic glass tool

Figure 5

Hot embossed PMMA microfluidic device (micromixer)

Figure 6

Diagram of microfluidic micromixer and SEM micrograph of the channel. The numbered sites represent the measured channel locations.

Figure 7

Average channel height measurements (micrometer)

Figure 8

Average channel width measurements (micrometer)

Figure 9

Height residuals versus the predicted values of channel height

Figure 10

Normality plot of channel height residuals

Figure 11

The red ellipse shows the desired operating range for the RSM analysis

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