Using Printing Orientation for Tuning Fluidic Behavior in Microfluidic

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Using printing orientation for tuning fluidic behaviour in microfluidic chips made by fused deposition modelling (FDM) 3D printing Feng Li, Niall P. Macdonald, Rosanne M Guijt, and Michael C. Breadmore Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03228 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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Analytical Chemistry

Using printing orientation for tuning fluidic behaviour in microfluidic chips made by fused deposition modeling (FDM) 3D printing Feng Li 1, 3, Niall P. Macdonald 1,3, Rosanne M. Guijt 2, Michael C. Breadmore *1,3 1

Australian Centre for Research on Separation Science, School of Chemistry, University

of Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia 2

Centre for Rural and Regional Futures, Deakin University, Geelong, Private Bag

20000, 3220 Geelong, Australia 3

ARC Centre of Excellence for Electromaterials Science (ACES), School of Chemistry,

University of Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia

ABSRACT Fluidic behaviour in microfluidic devices is dictated by low Reynolds numbers, complicating mixing. Here, the effect of the orientation of the extruded filament on the fluidic behaviour is investigated in fused deposition modeling (FDM) printed fluidic devices. Devices were printed with filament orientations at 0°, 30°, 60°, 90° to the direction of the flow. The extent of mixing was observed when pumping yellow and blue solutions into the inlets of a y-shaped device, and measuring the extent of mixing of two coloured solutions under different angles and at flow rates of 25, 50, and 100 μL/min. Fluidic devices printed with filament extruded at 60° to the flow showed the highest mixing efficiency, but results obtained at 30° suggested more complex fluid movement, as the *

[email protected]

ACS Paragon Plus Environment


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measured degree of mixing decreased along the fluidic channel at higher flow rates. To explore this, a device with -37° filament orientation on top surface was designed to align with the direction of the first fluid input channel and +37 ° on bottom surface of the channel to align with the direction of the second fluidic input. Results indicated a rotational movement of the fluids down the microchannel, which were confirmed by computational fluid dynamics. These results demonstrate the impact of the filament extrusion direction on fluidic behaviour in microfluidic devices made by FDM printing. Two chips with laminar flow (0° filament direction) or mixing flow (corkscrew filament direction) were used to perform isotachophoresis and colorimetric detection of iron in river water, respectively, demonstrating the simplicity with which the same device can be tuned for different applications simply by controlling the way the device is printed.

Introduction Owing to their microscale dimensions, microfluidic devices are typically operated at low Reynolds numbers, and fluidically characterised by laminar flow behaviour. Under laminar flow, mixing between adjacent parallel streams is minimal and diffusion is the only mechanism of mixing. Laminar flow has been exploited in microfluidics for several interesting applications such as the creation of a microfluidic fuel cell

1,2,

devices for cell3-5, DNA6 and molecular7

analysis and for selecting high motility sperm8. There are also many applications where rapid mixing is desired, such as for biochemical assays9-12 and chemical reactions13-16, which require complete mixing of solutions17. Mixing in microfluidic devices is typically achieved by


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passive or active methods, with a full discussion provided in several excellent reviews17-20. Passive methods do not require external energy and rely on diffusion or chaotic advection9,21-24. Stroock and co-workers developed one of the first passive micromixers by placing two different groove patterns on the microchannel base, thus creating chaotic advection for mixing – the herringbone mixer – which works well at a Reynolds number from 1 to 100

21.

In contrast,

active methods utilize a disturbance generated by an external field such as pressure, temperature, electrokinetics, acoustics, etc, to mix

25-30.

For example,

the magneto hydrodynamic effect has been used for enhancing mixing by Bau and co-workers. In the presence of a magnetic field, the coupling between the magnetic and electric fields induces Lorentz forces in the fluid, which in turn induces mixing movement in the chamber 30. Whilst effective in controlling fluid mixing, passive methods typically require complex geometries of ridges or grooves which increases the difficulty for fabrication, while the active methods require an external electrical source, complicating the experimental set-up. Recently 3D printing has been rapidly gaining popularity in microfluidic fabrication as a potential replacement for rapid prototyping through softlithography31,32. Literature reviews indicate that stereolithography (SL)33-35, fused deposition modelling (FDM)

36-38,

and photopolymer inkjet printing

(PolyJet)39-41 are the most common 3D printing approaches in microfluidics, with a

recent

experimental

comparison

highlighting

the

advantages

and

disadvantages of each for microfluidics42. A 3D micromixer based on a Baker’s transformation43 was fabricated using a Miicraft+ SL printer for mixing fluorescein dyes35 and to mix electrolytes for the rapid determination of the pKa33. Although 3D printing provided an easier and affordable way to fabricate



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this micromixer compared with the conventional femtosecond laser direct writing44, the mixing still required complex 3D geometries. A number of other 3D printed mixer devices have also been published

29,45-47.

Su et.al utilized a

stereolithographic

solid

phase

3D

printer

to

fabricate

a

extraction

preconcentrator for selective extraction trace elements from seawater, the 3D printed extraction channel was filled with ordered cuboids, which was able to enhance the liquid mixing45.

Rafeie and co-workers developed a 3D fine-

threaded lemniscate-shaped PDMS micromixer by curing the PDMS in a 3D printed removable wax structure, and this device showed a high mixing efficiency (>90%) over a wide range of flow rates47. In this paper, fluid mixing behaviour was examined using microfluidic devices printed with a FDM printer using various FDM printing orientations (0°, 30°, 60°, 90°) of the filament to the direction of the fluid flow. The performance of these devices was compared with devices fabricated by a SL and PolyJet printer. The FDM printed devices with 60° orientation showed the highest mixing efficiency, while 0° and 90° orientations printed chips with least mixing, thus more laminar fluidic behaviour. The presented results demonstrate that the FDM printing orientation provides a simple means to control fluidic behaviour in microfluidic devices. Isotochaphoresis and mixed based colorimetric measurement of iron level in environmental water sample were demonstrated using chips with different printing filament directions.

EXPERIMENTAL SECTION Materials and Chemicals. Crystal Clear acrylonitrile− butadiene−styrene (ABS) 1.75-mm-diameter filament was purchased from 3D Printing Systems


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(Melbourne, Australia). Veroclear-RGD810 print material and SUP707 watersoluble support were purchased from Stratasys, Ltd. (Eden Prairie, MN, USA). BV-007 photopolymer was purchased from MiiCraft (Young Optics, Inc., Hsinchu, Taiwan). Deionized water was provided by a Merck Millipore purification system (MA, USA). Colored food dyes were purchased locally and used as received. Tris

(hydroxymethyl)

aminomethane

(Tris),

4-(2-hydroxyethyl)-1-

piperazineethanesulfonic acid (HEPES), polyvinylpyrrolidone (PVP) (Mw=1,300 kDa), hydrochloride acid (HCl), ferric chloride (FeCl3) and potassium thiocyanate (KSCN) were purchased from Sigma-Aldrich Co (Missouri, USA). All solutions were prepared in Milli-Q water obtained from a Millipore (North Ryde, Australia) purification system. Environmental water samples were collected from Queen river (Tasmania, Australia). Instrumentation. The microhips were designed using AutoCAD 2016 (Student version) and were printed using a ROVA 3D FDM printer (ORD Solutions, Canada), Objet Eden 260VS professional 3D printer (Stratasys, Ltd., Eden Prairie, MN, USA), and a MiiCraft+ desktop DLP-SLA 3D printer (Young Optics, Inc., Hsinchu, Taiwan). Slicing of .stl files into G-code for the ROVA was completed using Simplify3D (www.simplify3d.com), and printing was controlled by Pronterface (www.pronterface.com). For the polyjet devices, Objet Suite v9.211.3626 (Stratasys Ltd., Eden Prairie, MN, USA), sliced and processed models for printing, according to manufacturer guidelines. For the MiiCraft+, devices were sliced using Creation Workshop (DataTree3D, Dallas, TX, USA), and edited in Photoshop Elements 14.1 (Adobe Systems Inc., San Jose, CA, USA) before printing on the MiiCraft+ controller software. A schematic of the device design is shown in Figure 1a, with channel dimensions of 500 × 500 μm for the inlet



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channels and 750 × 500 μm for the mixing channel. A rendered image of the device was shown in Figure 1b. A dual syringe pump system equipped with 10 mL disposable syringes was used for pumping two yellow and blue dye solutions into the devices (Harvard Apparatus, Inc., Holliston, MA, USA) at 25, 50, and 100 μL/min. A Nikon Eclipse E200 microscope was used to capture all images, and image processing as conducted with ImageJ for measurement of the size of the fabricated channels and extend of mixing based on the green colour. The parameters of FDM 3D printing process were: layer height, 100 μm; infill, 100%; extrusion width, 250 μm; print speed, 20mm/s; nozzle and bed temperature, 210 °C and 130 °C. Computational Fluid Dynamics: The multiphysics software COMSOL (version 4.3b) was used to simulate the microfluidic device with different filament direction both the top and bottom fluidic surfaces. The device consisted of two rectangular input channels (500 μm x 500 μm x 1 mm) and a main channel (700 μm x 500 μm x 30 mm), with cylindrical filament of 200 μm protruding regularly 100 μm into the channel. The top surface had the filament aligned 45° to the fluid flow through the main channel, while the bottom surface was aligned at an angle of -45°. The input flow from the two input channels had a velocity of 1 μm/s. RESULTS AND DISCUSSION Fabrication. FDM 3D printing involves melting and extruding thermoplastics through a heated nozzle, which is controlled by a precision motor. It is attractive for making microfluidic devices because it does not require a support material and multi-material printing can be achieved easily of which there are many materials with a diversity of properties as recently discussed by Salentijn


37.

It

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has been used to make a micro free-flow electrophoresis device 38, a microfluidic device for encapsulating dental pulp stem cells within alginate droplets48 and an integrated microfluidic device for direct analysis of nitrate level in soil36, however from the published works, several limitations have been noted. Of significance to this work is the ‘poor’ fluidic performance identified as an absence of laminar flow. This has been attributed to the uneven surface profile as a result of extrusion of the filaments during the printing process. Here, we studied the influence of the orientation of the filament relative to the flow direction on the fluidic behaviour inside an FDM printed device. A Y-shaped channel with two fluid inputs and one fluid output was used for these studies, with schematic and rendered images shown in Figure 1. The device contains two 500 μm x 500 μm microchannels which merge into a single 750 μm x 500 μm channel, with a wall thickness of 1000 μm. Figure 2 shows two different print paths to print this device (a and c) with photographs of the resulting printed chips shown in b and d, respectively. When the loop pass number was set at 2 in the software (as shown in c), the dimension of central channel was 500 μm, instead of 750 μm. If the loop number was set to 4 in the software (as shown in a), the central channel was 700 μm, which more closely represented the designed dimensions of 750 μm. This dimensional difference was due to the different nozzle movement. With the loop pass set to 4, the ‘channel’ was defined by continuous movement of the nozzle in 4 continuous loops around the structure to fill in the 1000 μm wall, with each loop being 250 μm wide. When only 2 loop passes were used, the 500 μm gap between the inner and outer loops was ‘in-filled’ which resulted in rapid movement of the nozzle, which resulted in the actual channel being smaller than the design. The main



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impact of the number of loops was on the dimensional accuracy of the fluidic channel, but it did not influence the fluid flow properties as the channels were printed by following the contour of the channel design. Each layer will follow the same design, thus while there may not be a vertical side wall for the fluid to follow, the surface of the wall itself was smooth and did not disturb the laminar flow. The situation was entirely different for the bottom and top surfaces of the microchannel, which are defined by the way in which the printer extrudes the filament to make these. To study the effect of the extrusion direction on the fluidic behaviour, the nozzle movement in the slicing software was adjusted such that the filament was extruded at angles of 0° (parallel), 30°, 60°, 90° (perpendicular) relative to the fluid flow in the main microchannel. Photographs of the devices shown in Figure 3 illustrate that different orientations produce different surface morphologies on the channels’ bottom and top surfaces. Fluid Mixing control. A Y shaped channel was used to evaluate the fluidic properties of the printed devices by the determination of the extent of fluid mixing for the different surface topographies. This was derived from the percentage of the channel width occupied by the green colour, indicative of mixing of the blue and yellow input fluids.

This was assessed in devices

fabricated with FDM printing orientation of 0°, 30°, 60°, 90° relative to the flow direction. Photographs of the channels at a flow rate of 25 μL/min are shown in Figure 4, showing the blue and yellow input fluids yielding the green colour when mixed. The extent of mixing was expressed as the mixing ratio and was plotted for the different print orientations at flow rates of 100, 50, 25 μL/min in Figure 4.


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As can be seen from Figure 4, FDM chips printed with an extrusion angle of 60° relative to the fluid flow showed the most mixing for all flow rates studied. Less mixing was observed when the filament was extruded parallel to the flow, with initial mixing only 0-13% (±4%) at 0 mm. FDM chips printed with the filament extruded perpendicular to the flow (90°) showed slightly more mixing to those where the filament was printed in parallel with the flow direction (0°), but the mixing in both of these was less than determined for the ones extruded at 30° and 60°. At 25 µL/min, complete mixing was realised at 10 mm and 20 mm for the 60° and 30° extrusion angle devices respectively, while the maximum mixing achieved at 20 mm was 84% (±5%) for parallel extruded filament (0°) and 96% (±3%) for perpendicular (90°) extruded filament. We hypothesize that a greater extent of mixing was obtained with the filament extruded at an angle to the flow direction due to fluid from one input channel initially penetrating under/over the flow from the second inlet as it follows the bottom/top surface topography, effectively decreasing the diffusion depths. Further interrogation of the results at higher flow rates indicated that this increased mixing derived from the green colour was not necessarily due to the shorter diffusion distances caused by a more complex fluid flow.

When

examining the extent of mixing observed in the 30° chip at a flow rate of 100 μL/min, the percentage green dropped from 78% (±4%) at 10 mm to 40%(±4%) at 20 mm, suggesting the blue and yellow dyes separated after being mixed. As unmixing is a physical impossibility, we hypothesized that the observed green colour taken as an indicator of mixing was caused by layering of the yellow and blue fluids on top of each other due to a rotation of the fluid within the microchannel away from the original parallel side-by-side arrangement.



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To examine this hyphothesis, a device was designed and printed to further enhance a rotating fluidic motion, extruding the top surface in-line with one of the inlet channels at an angle of +37°, and the bottom surface in-line with the other inlet channel at an angle of -37°. This is shown in Figure 5a. Again, blue and yellow dye solutions were used to study the fluid behaviour at a flow rate of 100 μL/min, as shown in Figure 5b. The mixing ratio was measured along the channel and plotted in Figure 5c. The percentage green colour reached a maximum at 15 mm, then decreased at 25 mm, increased again at 30 mm, and decreased at 45 mm. When looking at the photographs in Figure 5b recorded at 5 and 20 mm, the yellow colour moved from the bottom, to the top of the channel, before returning to the bottom at 40 mm. These observations support the hypothesis that the two fluids were not mixed, but had rotated in a corkscrewlike fluidic motion as they moved down the channel. To test this hypothesis theoretically, a CFD model was created. The CFD results shown in Figure 6 confirm the rotational movement of the two fluids along the channel. These results demonstrate that the extrusion angle relative to the flow direction is an important parameter in determining the fluidic behaviour of microfluidic devices made by FDM printing. To compare the performance of the FDM printer microfluidic devices, the mixing performance was compared with equivalent devices printed by polyjet and SLA printers, and the relevant mixing data is also shown in Figure 4. Microfluidic devices printed with the SLA printer showed the least amount of mixing among all the chips and showed more laminar flow than all of the FDM printed chips printed with any extrusion angle. Surprisingly the data shows that the flow was more laminar in fluidic devices printed with the FDM printer with filament


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extruded parallel (0°) and perpendicular (90°) to the flow than was obtained using the polyjet printer, despite the average roughness of the FDM devices (Ra 10.97 m) being considerably greater than the polyjet (Ra 0.99 m)42. We believe that this is due to the average roughness of the FDM devices measured by the optical profiler including both the surface roughness and the lay – the predominant direction of the surface roughness. This was confirmed with Ra of the FDM devices decreasing from 32.41 m perpendicular to the lay to a value of 11.41 m when measured parallel to the lay. This average roughness was still considerably greater than the devices printed with the polyjet printer, and therefore does not correlate with the observed mixing trends. The increased mixing in the polyjet may be explained by increased channel wall roughness due to the interplay between the build and support materials, a phenomenon previously used to create a textured surface for TLC

49.

The side walls have an

average roughness of 9.81 m, which is similar to that of the extruded FDM devices, and may explain why there is greater mixing in these devices. A clearer understanding of roughness, waviness and lay is required to better design fluidic devices by 3D printing. Analytical applications. To demonstrate the importance of controlling fluid flow in a fluidic device, two examples have been chosen, one in which mixing is critical, and one in which mixing is undesired. ITP is a well-established electrophoresis method for sample preconcentration and purification 50,51, and the mechanism has been clearly investigated by several groups

52-54.

Due to the unique mechanism of ITP, analyte species above a

particular concentration are separated as adjacent blocks, while analytes below this concentration migrate as a peak. In both cases, mixing is undesired. ITP of



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fluorescein was performed in chips printed with 0° and 60° orientation, respectively, with the results shown in figure 7 a). The chip with 0° printing orientation showed a sharper and narrowed zone, which was 5 times narrower than the same fluorescein band in the device printed with a 60° orientation. The increase in peak width is due to the non-uniform fluid flow path and the electric field, which does not allow the fluorescein to migrate through the device without mixing with the adjacent leading and terminating electrolyte zones. Rapid and comprehensive mixing is required for many colorimetric reactions, and can be used to detect a range of heavy metals and inorganic ions. Iron is not considered to pose a direct threat to our health, but still causes some issues as it stain laundry and plumbing fittings, and also block irrigation systems used in agricultural and industrial applications

55.

The recommended level of iron in

drinking water is less than 0.3 ppm in Australia, excess of 3 ppm would cause water to taste objectionable56. Again, two fluidic devices (0° and the corkscrew device) were evaluated in terms of their ability to quantify iron in environmental water samples. Water sample (or standard solution FeCl3) and KSCN were pumped into different inlets, and the red intensity of iron thiocyanate used as a measure of mixing efficiency.

As shown in figure 7 b), the red color was

observed in the corkscrew chip, while no red color was observed in the 0° chip. A water sample collected from Queen river (Tasmania) was collected and analysed for Iron, with a value of 380 ppm using an external calibration series (0-1000 ppm FeCl3, R2=0.99), demonstrating the potential of the 3D printed device to perform simple colorimetric assays.


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CONCLUSIONS We demonstrated that in microfluidic devices made by FDM, the extrusion orientation relative to the flow direction is an important parameter in determining the fluidic behaviour. In devices where the filament was extruded at a 60° angle relative to the flow direction, fast mixing was observed, enabling these devices to be used as micromixer. Compared with other passive micromixers, no complex channel geometry or surface patterning were required while still avoiding the external instrumentation typical for active mixers. When the filament was extrude at 0° and 90° relative to the flow direction, significantly less mixing was observed, and the flows could be considered laminar at higher flow rates (320 µm/min).

These angles are recommended for use for

applications where convective mixing is undesirable, for example those used for electrophoretic separation or diffusive extraction. Corkscrew like fluidic behaviour was observed when printing the top and bottom surfaces at the same angle of the inlet and outlet channel, respectively, which was at angles of +37 and -37 relative to the flow direction in the devices presented. These experimental results were confirmed by CFD simulation, and demonstrate the influence of the print orientation on the fluidic behaviour in 3D printed micorlfuidic devices made by FDM. The importance of controlling fluidic behaviour was demonstrated by an ITP separation of fluorescein and the colorimetric determination of iron in water samples.

ACKNOWLEDGMENTS F.L. acknowledges the University of Tasmania for the provision of a scholarship. M.C.B. acknowledges an Australian Research Council Future Fellowship award



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(FT130100101). R.M.G. acknowledges the Alexander von Humboldt Foundation for the award of a fellowship for Experienced Researchers. Support from the ARC Centre of Excellence for electromaterials Science (ACES) (Grant CE140100012) for funding is also acknowledged.

Author Contribution F.L. conceived, designed, fabricated, performed FDM chips experiments, collected data and prepared the manuscript. N.P.M. designed, fabricated, and performed the Eden and Miicraft+ experiments. M.C.B. performed the CFD simulation. N.P.M., R.G.M., and M.C.B. conceived and supervised the research and edited the paper. All authors have given approval to the final version of the manuscript.

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Figure legend Figure 1. CAD illustrations of the microchip design. a) CAD drawing showing the dimensions of two inlet channels and the mixing channel, all dimensions shown on the drawing are in mm. b) 3D CAD render of the microchip. Figure 2. Comparison of the printing using 4 loops (a, b), and with 2 loops (c, d) in fabrication. a) and c) are schematic showing the movement path of printing nozzle, b) and d) are microscopic images of microchip filled with dyes showing the channel dimension difference, the scale bar is 500 μm. Figure 3. Microscopic images of microchannels fabricated using different printing orientations. Scale bar is 500 μm. Figure 4. Microscopic images of laminar flow within 500 μm × 500 μm channels into 750 μm × 500 μm channels, visualized with yellow and blue food dye at 25 μL/min for FDM 0°, 30°, 60°, 90°, Eden, and Miicraft+ respectively. Plots of distance vs mixing ratio, demonstrating diffusion through the laminar flow channel at 25, 50, and 100 μL/min are also shown below the microscopic images. N=3, scale bar =500 μm.


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Figure 5. a) Schematic demonstration of the printing way of crock-screw flow chip. b) Microscopic images of crock-flow chip, visualized with yellow and blue food dye at various distance 0, 5, 15, 20, 30, 40 mm from the intersection, and the flow rate is 100 μL/min. The scale bar is 500 μm. c) Plots of distance vs mixing ratio through the chip at 100 μL/min. N=3. Figure 6: a) CFD model used to examine the fluid flow in channels with different direction filament on the top and bottom channel surfaces and b) particular trajectory through the microchannel from the left (blue) and right (red) channels. Figure 7: a) ITP of 0.5 ppm fluorescein in 0° (left) and 60° (right) chips. LE is 12/6 mM Tris/HCl with 1% PVP, TE is 5/2.5 mM Tris/HEPES with 1% PVP, and 1000 V was applied. b) Calibration of various concentrations of FeCl3 (0, 100, 20, 500, 1000 ppm), N=3, and comparison of iron level detection from water sample with 0° and corkscrew flow chips. The averagely color intensity was measured over a measurement window specified with the rectangle in corkscrew chip. Flow rate is 500μL/min. Scale bar=500 μm.



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Using Printing Orientation for Tuning Fluidic Behavior in Microfluidic

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