5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT Guwahati, Assam, India

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Studies on CO2 laser micromachining on PMMA to fabricate micro channel for microfluidic applications

KantRishi1,2, *, Gupta Ankur1,2, Bhattacharya S.1,2

1Microsystems fabrication laboratory, IIT Kanpur - 208016, 2Mechanical Engineering Department, IIT Kanpur - 208016,

*dsrishikant@gmail.com, drankur@iitk.ac.in, bhattacs@iitk.ac.in

Abstract Microfluidic devices are in great demand in the field of biomedical technology,point of care diagnostics and chemical analysis. The rapid and low cost manufacturing of these devices have been a challenge. CO2 laser micromachining plays an important role in machining although it renders the machined surfaces with high roughness.This study is an attempt to do process optimization of laser micromachining technique which may produce smooth machined surfaces. Herein, the impact of process parameters like raster speed ,laser power, print resolution etc. are optimized using two target functions ofdimensional precision and surface roughness on microchannels made in PMMA (Poly methyl metha acrylate) substrates. The laser machined PMMA samples are analyzed using 3D- profilometry and Field emission scanning electron microscope (FESEM) for surface quality and dimensional precision.To investigate optimum process parameters of CO2 laser for fabricating the microchannel on PMMA with dimensional accuracy and good surface quality, Analysis of variance (ANOVA) and regression analyses is conducted. It is found that optimum surface roughness of this process is around7.1µm at theoptimum values of the process parameters 7.5 mm /sec (50% of maximum machine limit)raster speed, 17.9Watt (51% of maximum machine limit) laser power and 1200 DPI (100% of maximum machine limit)printing resolution. The static contact angle of the micro-machined surface has also been observedfor analyzing the amenability of these channels to flow of water like fluids for micro-fluidic applications.

Keywords: PMMA, CO2 laser, micromachining, contact angle

1 Introduction Polymeric materials are utilized for fabricating microfluidic devices as applied to diagnostics and detection principally due to ease offabrication, biocompatibility, optical clarity and easy conformabilityto features of up to micro/nano-meter scale. Microfluidic devices find wide applications in the heath care sector. The apparent interest and participation of the industry in microfluidic research and development illustrate the commercial value of these devices for practical applications. With this commercial potential, microfluidics is poised to become one of the most dynamic segments of the MEMS/NEMS technology.

Microfluidic devices explore the application in confined space of several micron range. This micron sized space can be microchannel, micromixer, micro pumps, micro valves and several other active or passive deviceswhich can be fabricated using expensive and low throughput advanced lithography processes or other processes borrowed from microelectronic industry. Laser machining and some other non-traditional machining processes are being increasingly explored for high

throughput fabrication of microfluidic devices. Among all these processes the laser machining process holds a lot of promises due to its flexibility towards parameter selection, its applicability on a wide range of materials and its rapidity of manufacture. We have been successfully using CO2laser as a very effective meansof micromachining PMMA based devices. However, the post machined surface obtained throughthis process is quite rough and poses a variety of restrictions on the use of soft biological matter within the devices so fabricated. For example, channels used for detection and counting of mammalian cells may need a very low level of roughness to prevent cell lysis. Also, more roughness sometimes induces local eddies and vortices very close to the rough surface leading to non specific adsorption causing a lot of false positives in detection protocols. In addition cell adhesion, protein unselective adsorption, particle/cell aggregation, bubble generation, optical signal detection etc. are range of problems posed by rough surfaces associated with microfluidic devices.

In earlier work Lawrence et al (2001), Nayak et al (2008),Heng Q.et. al(2006),J. Paulo Davima et. al.(2008),DetlefSnakenborg et. al. (2004) and a variety of other authors have investigated the roughness of CO2

Studies on CO2 laser micromachining on PMMA to fabricate micro channel for microfluidic applications

 

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laser machined samples of PMMA and have been able to obtain roughness in the range of few microns[Lawrence J. et al]. While applying such methodology to fabrication of features and structures of micro-fluidic devices we were limited by the channel roughness hugely particularly in cases where biological soft matter was flown into samples Rishi Kant et. al.(2013)definitely for specific microfluidic applications this range of roughness as reported before is insufficient and therefore we wanted to perform process optimization to obtain less than 10 microns roughness and through thetechniques mentioned earlier we have been successfully achieving 7 microns average roughness and a high level of dimensional precision in our manufacture.

2 Experimental set-up Laser micromachining has been performed with the EPILOG WIN32 laser machine (M/S Epiloglaser,USA )keeping all environmental conditions same. All machining is done at (25°C) room temperature condition and a positive pressure environment. The laser has a high power of 35 Watts and hasa total working bedsize of 2ft x1ft.A design of the device that needs to be laser machined is drafted using a CAD package (Coral Draw) which is used to drive the various stepper motors with the lasing stage and also the working stage.The working stage is capable of movement in the ‘z’ direction and the lasing stage can operate in the raster (engraving) and vector (deep cutting) modes in the x-y plane. Figure 1 shows schematic representation of laser machining set up.

Figure 1: Schematic diagram of CO2 laser micromachining process

2.1 Interaction of CO2 laser with PMMA substrate:

PMMA is highly absorptive at the infra red region (wavelength=10.6µm). Laser machining operates on the physics of photon to phonon conversion where beam matter interaction results in rapid molecular vibrations leading to generation of thermal energy. The local

intensity posed by the laser is so intense (being in a very small zone) that it results in energy that is sufficient to melt and vaporize the PMMA material.The machining parameters for the laserare highly dependent on the overall feature size and correct choice of parameters leads togood dimensional accuracy. The various combination of parameters resulting into different surface finish levelscan be easily achieved using Laser processes [Nayak N. C. (2008) et.al.,Heng Q. (2006)] .

We have conducted around 30 trials with a combination of multiple parameters designed by Design Expert software. Although several trials did not scribe PMMA substrate, hence its surface roughness is not reported here and is denoted by N/A.Table 1 shows the various combinations that are proposed and the measured.

2.2Characterization of micro-machined PMMA substrate:

The surface roughnesscharacterization is performed on 3D-optical profilometer[NanoMap-D, AEP Technology,USA].Various channel widths varying from 10-100 microns have been evaluated for roughness. The interferometer has optical resolution in nanometer range. The minimum area that is scanned by the system is 0.064516 mm2. The interferogram is used to reconstruct the topology of the surface being scanned and the 3-D imaging is performed across the different micro-channel widths. The average roughness ‘Ra’ is reported to vary with the combination of different machining parameters as illustrated in figures 2(a)~(j).

Table1: Details of designed parameters and measured roughness values 

Resolution (DPI) 

Power (%)  Speed(%)  Roughness(µm) 

600  50  51  11.113 75  10  100  N/A* 

1200  10  10  N/A 600  50  51  11.113 75  10  10  N/A 600  10  51  N/A 600  50  51  11.113 600  50  51  11.113 75  10  10  N/A 

1200  100  10  22.003 75  100  100  N/A 600  50  51  11.113 1200  100  100  8.7607 600  50  51  11.113 600  50  100  14.461 600  50  10  N/A 75  100  10  N/A 

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT Guwahati, Assam, India

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600  50  51  11.113 75  100  100  N/A 

1200  10  100  N/A 600  100  51  22.014 75  10  100  N/A 

1200  10  100  N/A 75  50  51  N/A 

1200  50  51  7.0647 1200  10  10  N/A 75  100  10  N/A 600  50  51  11.113 1200  100  10  N/A 1200  100  100  8.7607 

Where N/A * Value not available, as laser did not scribedthe PMMA substrate

Figure 2: (a)/(b)Plan view of the reconstructed image of the microchannel/ 3-Dimesional view(Cutting parameters 1200DPI, 17.5Watts power and 7.65 mm/sec rastering speed, Ra= 7.0647µm (c)/(d) Plan view of the reconstructed image of the microchannel

/ 3-Dimnesional view (Cutting parameters 1200DPI, 35Watts power and 15 mm/sec rastering speed, Ra= 8.7607µm (e)/(f) Plan view of the reconstructed image of the microchannel / 3-Dimesional view (Cutting parameters 600 DPI, 17.5Watts power and 7.65 mm/sec rastering speed, Ra= 11.113 µm (g)/(h) Plan view of the reconstructed image of the microchannel / 3-Dimesional view (Cutting parameters 600 DPI, 17.5Watts power and 15 mm/sec rastering speed, Ra= 22.0114 µm(i)/(j) Plan view of the reconstructed image of the microchannel / 3-Dimesional view (Cutting parameters 600 DPI, 17.5Watts power and 15 mm/sec rastering speed, Ra= 14.463 µm).

3.Optimization of CO2 laser process parameters for smooth surface:

Optimization of laser machining parameters as discussed above has been performed with design of experiments (DOE) in which a central composite design (CCD) is used to fit a model. An optimum solution is extracted from the different machining parameters, including raster speed, resolution and power. Optimization was first carried out with respect to laser power and printing resolution, then with respect to speed and printing resolution and finally for speed and power. Figure 3(a)~(c) shows the output of this optimization process as contour plots. As can be seen in Figure 3(a) the contour corresponding to a roughness level of 7.0647µm starts at a base power of around 50% and the value 7.0647µm happens while printing at a resolution of 1200 dpi. We could not really explore a resolution more than 1200 DPI as we were limited by the machine specifications. There are of course more precise electromechanical stages particularly those used in scan jet printers which maybe able to print at a resolution of as high as 5000 DPIand it intuitively makes sense that if the resolving power of the lasing head is higher than it would be able to handle higher power values while sacrificing the roughness level not too much. Similarly from Figure 3(b) the minimum roughness attainable at 1200 dpi resolution limit of the system is corresponding to a speed that is 50% of the maximum limit of the system if a similar roughness level were to be contained in Figure 3(a) earlier for a combinatorial of the power as well. This gets further validated by Figure 3(c) which reports a contour of 7.0647µm average roughness corresponding to a power value of 51% of the maximum limit of the system while maintaining the 1200DPI level. Therefore, we find out the optimized average surface roughness as approximately 7.06 microns and also through analysis report the speed to be 50% of machine limit and power to be 51% of the machine limit.

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