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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. FABRICATION METHOD
  5. EXPERIMENTAL
  6. RESULTS AND DISCUSSION
  7. CONCLUSIONS
  8. REFERENCES

In this study, a microfluidic chip prototype having circular microchannels was replicated by microinjection molding process, employing a modularized and sectioned micromold system (MSMS). In the viewpoint of microfluidic manipulation, a microchannel with circular cross-section shows several advantages over a conventional rectangular and/or square microchannel. To achieve a mass production of the microchannels with circular or round cross-sections, the micromold was designed and fabricated based on the concept of MSMS. It consisted of several micromold modules, each having half-circular cross-sectional microstructures on its one-side surface. The modules were precisely manufactured by a deep X-ray lithography using a synchrotron radiation and a subsequent nickel electroforming process. Then, the MSMS for a microinjection molding process was constructed by assembling the nickel modules. After the molding of plastic plate with open microchannels of half-circular cross-section, a thermal bonding of microinjection-molded plates was carried out to produce the microfluidic chip prototype including the circular microchannels. Observation of the surface quality, measurement of cross-sectional profiles, and microfluidic test were carried out, which verified the usefulness of the present fabrication process. POLYM. ENG. SCI., 54:42–50, 2014. © 2013 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. FABRICATION METHOD
  5. EXPERIMENTAL
  6. RESULTS AND DISCUSSION
  7. CONCLUSIONS
  8. REFERENCES

Recently, numerous microfluidic devices have been developed and applied in various areas. For instance, Ehrfeld et al. [1] provided key technologies for microfluidic devices for realizing several chemical reaction processes as well as practical examples. Madou et al. [2] reported their integrated microfluidic platform for a biomedical diagnostics, which was developed on a circular base like compact disk and driven by a centrifugal force. Takagi et al. [3] demonstrated an efficient and continuous separation of microscale particles utilizing a simple flow inside microchannels. Also, Andersson and van den Berg [4] showed that a precise control of cells could be efficient achieved in microfluidic systems, providing a new technology for cell biology, regenerative medicine, and tissue engineering.

Generally, because of a relatively small feature size of the components embedded in such micro-devices, there are several advantages, such as high throughput, high sensitivity, easy integration, and so on, over conventional devices. Among the components consisting of the microfluidic devices, a microchannel, which can be defined as a channel with a hydraulic diameter below 1 mm, is the essential and indispensable one. It can be employed not only for a stable interconnection of other components but also for a construction of complex microfluidic system itself. For instance, Nguyen and Wu [5] summarized various passive micromixers which essentially consisted of simple microchannels, resulting in the microfluidic network providing an efficient mixing and/or dispersion without any external power input. Harris et al. [6] demonstrated that a micro heat exchanger could enhance the convective heat transfer although the system was constructed based on simple microchannels. Therefore, it is considered to be so important to precisely fabricate microchannels with various kinds of cross-sections.

So far, the microchannels with square and/or rectangular cross-sections have been usually utilized in the microfluidic applications as they can be easily realized by conventional microfabrication technologies. Martin and Aksay [7] reported that the square or rectangular microchannels were simply obtained by a soft lithography-based casting of polydimethylsiloxane (PDMS) onto a master pattern. The conventional dry etching and/or photo-lithography techniques are commonly utilized to prepare the master structure for the PDMS replication process. Szalmas and Valougeorgis [8] showed that the microchannels with inclined sidewalls, such as triangular or trapezoidal cross-sections, could be manufactured by KOH etching of a patterned silicon substrate. In spite of the simple fabrication, however, these microchannels inherently have sharp corner edges, thereby showing some drawbacks in a precise microfluidic handling. In contrast, circular or round microchannels have advantages such as symmetric velocity profile, uniform diffusion property, no stagnation and retardation at edge corners, and so on. They can also effectively mimic a human vein, which increases the use of such microchannels for various biomedical devices in particular.

Therefore, there have been several efforts to efficiently realize the microchannels of circular and/or round cross-sections. As representative fabrication techniques, Yang et al. [9] demonstrated the microchannel with round corners in cross-section with the help of an isotropic wet etching of silicon or glass substrates. Also, Wang et al. [10] developed a simple reflow process of the prepatterned polymeric photoresist, making the initial sharp edges of the photoresist patterns to be round after a thermal treatment. However, via these fabrication methods, one cannot freely control the detailed shape of the round corners. Furthermore, although Yang et al. [11] developed a simple fabrication technique using circular optical fibers as sacrificial molds to form the circular microchannels, a time-consuming removal process of the embedded sacrificial structures is mandatory. Agarwal et al. [12] found that a combination of standard silicon microfabrication processes based on a reflow of silicon oxide could result in a self-sealed circular microchannel, but the applicable material was limited only to silicon. Qin and Li [13] utilized a laser micromachining process for a precise fabrication of a complex three-dimensional network of microchannels. Iga et al. [14] also investigated relevant phenomena during a microchannel manufacturing by a femtosecond laser ablation in water. Although these precision laser machining technologies can produce the microchannels having round edges, they have the limitations such as a relatively long fabrication time and a rough surface.

In this regard, a development of a novel micromanufacturing process, which overcomes the drawbacks of the previous methods, is required to fabricate precise microchannels of various cross-sections. Also, it can enable us to manufacture microfluidic devices conducting more versatile functions. This study describes a new micromanufacturing process for circular microchannels and a plastic microfluidic chip with the various circular microchannels. Recently, a novel micromold technology, so-called the modularized and sectioned micromold system (MSMS), was developed and applied to microinjection molding of various microstructured surfaces and multistep microgears [15, 16]. In this study, the MSMS was employed to obtain a nickel mold to replicate circular microchannels with a smooth surface. Then, a plastic microinjection molding process using the fabricated micromold and a subsequent thermal bonding were carried out to produce the microfluidic chip prototype containing the circular microchannels. The cross-sectional shape of the replicated microchannels and their performance were investigated to verify the usefulness of the present method.

FABRICATION METHOD

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. FABRICATION METHOD
  5. EXPERIMENTAL
  6. RESULTS AND DISCUSSION
  7. CONCLUSIONS
  8. REFERENCES

To achieve a mass production of precise plastic microchannels, a microinjection molding process, which is the most productive one among various micromolding technologies, was carried out using a micromold of half-circular microstructures. Figure 1 schematically shows the fabrication method of the micromold based on the MSMS concept and its utilization in a replication process. As the first step, a deep X-ray lithography (DXRL) process using a synchrotron radiation (Fig. 1a) is performed to define a polymeric mother structure of half-circular microchannel structures (Fig. 1b). The straight microchannel with an excellent surface quality can be efficiently realized through the DXRL process. The subsequent metal electroforming onto the mother structure (Fig. 1c) produces a metallic micromold module (in other words, sectioned micromold module, SMM) having the convex microstructures, which is the reverse pattern of the mother structure (Fig. 1d). The half-circular microchannels in the original mother structure can be replicated by a micromolding process using the fabricated metal SMM (Fig. 1e).

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Figure 1. Fabrication process of circular microchannels using MSMS: (a) DXRL process, (b) PMMA mother structure, (c) nickel electroforming process, (d) electroformed SMM, and (e) replicated plastic circular microchannels. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The present method can produce the precise microchannels with the half-circular cross-section only. Also, the length of microchannels is inherently determined according to the height of the mother structure fabricated in the DXRL process. Hence, the metal SMMs with the identical features are assembled to construct the micromold (MSMS) into a mold core as shown in Fig. 2a. The reservoirs can also be simultaneously formed together with the half-circular microchannels by adding two SMMs with a different height. The micromolding process with the constructed MSMS fabricates the half-circular microchannel array as well as the reservoirs (Fig. 2b). As the open microchannel array with a half-circular cross-section is formed on the replicated plastic plate, a bonding process of two replicated plates follows, consequently resulting in an enclosed circular microchannel array (Fig. 2c).

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Figure 2. Fabrication of plastic microfluidic chip prototype with circular microchannels: (a) mold inset with MSMS, (b) replicated plate of the microfluidic chip prototype, and (c) plastic microfluidic chip prototype by bonding two replicated plates.

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EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. FABRICATION METHOD
  5. EXPERIMENTAL
  6. RESULTS AND DISCUSSION
  7. CONCLUSIONS
  8. REFERENCES

Design of Microfluidic Chip Prototype

To verify the micromanufacturing process in this study, a plastic microfluidic chip prototype with a circular microchannel array was designed and fabricated. Figure 3 shows a CAD model of a microfluidic network in the designed microfluidic chip prototype. It consisted of 10 circular microchannels and two reservoirs. Each microchannel had a different diameter varying from 50 to 500 μm. The center-to-center distance for adjacent microchannels was 1.4 mm. Two rectangular reservoirs were located in both ends of the circular microchannel array for the purpose of inlet and outlet for a working fluid. As shown in Fig. 2, the microfluidic chip prototype was fabricated by the micromolding of two identical plastic plates with the half-circular microchannel array and the subsequent bonding of them.

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Figure 3. Design of plastic microfluidic chip prototype with circular microchannel array. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Fabrication of MSMS

For the manufacturing of the plastic plates including the microchannel array of half-circular cross-sections, the metal SMMs, as a basic mold element for constructing the MSMS, were prepared by the DXRL process and the subsequent nickel electroforming. After the precise mother structure was fabricated by the DXRL process, the nickel SMMs were produced by electroforming onto the mother structure. As the robust metal mold system was utilized in the microinjection molding process, more reliable mass replication could be achieved.

As the first step, an X-ray mask was prepared by a conventional UV-photolithography and Au-electroplating processes. A polyimide film of 175 μm thick was used as a membrane of the X-ray mask. Thin metal layers of Cr (thickness, 20 nm) and Au (thickness, 100 nm) were deposited by E-beam evaporation prior to the UV-photolithography. By the photolithography process using a negative-tone photoresist (SU-8 2025, MicroChem), the photoresist micropatterns were fabricated, which corresponded to the SMMs as shown in Fig. 1a. Then, Au was electroplated onto the opened area to form an X-ray absorber layer. A constant current density of 1.0 mA/cm2 was applied during the Au-electroplating, resulting in the Au layer of 20 μm thickness.

Polymethyl methacrylate (PMMA) sheet of 2.0 mm thick (Goodfellow Cambridge) was used as an X-ray photoresist. After an annealing for dehydration and relaxation of residual stress, the PMMA sheet was bonded onto a Ti substrate. A solvent bonding using liquid PMMA and methyl methacrylate (MMA) was used in this study.

The synchrotron X-ray radiation was then selectively exposed onto the thick PMMA sheet through the fabricated X-ray mask. The X-ray exposure was carried out using the synchrotron facility of Pohang Light Source. An Al membrane filter of 54 μm thick was introduced during the X-ray exposure to cut off low-energy photons, which enhances a dimensional accuracy of the fabricated microstructures. At the bottom side of the PMMA sheet located underneath the opened area without the Au-absorber in the X-ray mask, the total energy density of about 3.0 kJ/cm3 was accumulated by the X-ray exposure. The exposed PMMA parts were clearly removed by a development process using a GG-developer, which is a typical etchant in the DXRL process. Thin organic residuals in the obtained PMMA microstructure were then rinsed away using deionized (DI) water.

The patterned PMMA sheet fabricated by the DXRL process was used as the mother structure in the subsequent nickel electroforming. In this study, the electroforming was carried out using a commercial nickel sulfamate solution. During the electroforming, a constant current density of 7.5 mA/cm2 was applied together with a reciprocating striking motion for the removal of hydrogen bubbles. Temperature and pH of the electrolyte were also carefully controlled at 55°C and 4.0, respectively. The electroforming was continued until the whole opened area in the patterned PMMA was filled completely. After finishing the electroforming, both top and bottom sides of the nickel parts in the PMMA sheet were polished out to have a precise surface and a thickness of 1.8 mm. Then, a wet cleaning was performed for the removal of the surrounding PMMA sheet, consequently resulting in the nickel SMMs.

Fabrication of Microfluidic Chip Prototype

To obtain the microfluidic chip prototype including the circular microchannel array, the plastic plates with the half-circular microchannel array were replicated by microinjection molding. The several nickel SMMs with the convex half-circular microchannels were assembled into a mold core. Two flat SMMs, which were produced by a precision machining, were also inserted at both sides of the assembled nickel SMMs for the formation of two reservoirs as mentioned earlier. Then, the mold core was firmly installed into a conventional mold base for the microinjection molding experiment.

All electric injection molding machine (SE-50D, Sumitomo) was utilized in the present microinjection molding process. Cyclic olefin copolymer (COC; Topas® 5013, Ticona) was chosen as a molding material owing to its high fluidity and optical transparency. The important processing conditions used in this study are summarized in Table 1. The mold temperature was controlled using an external chiller system (TT-157E, Tool-TEMP AG) with an oil coolant.

Table 1. Processing conditions used in the microinjection molding process
Processing parametersConditions
Filling time (s)0.15
Packing pressure (MPa)75.0
Packing time (s)1.00
Melt temperature (°C)240.0
Mold temperature (°C)120.0

Through microinjection molding, the square COC plates of 2 mm thick were obtained. Two COC plates, each having the identical half-circular microchannel array, were then thermally bonded to produce the microfluidic chip prototype including the circular microchannel array and the reservoirs [17].

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. FABRICATION METHOD
  5. EXPERIMENTAL
  6. RESULTS AND DISCUSSION
  7. CONCLUSIONS
  8. REFERENCES

Figure 4a shows a part of a CAD design used in the photomask fabrication for the UV-photolithography. Ten half-circular microfeatures, each having a different diameter, were placed on one side of the base part of the SMM. The same micropatterns were repeated on the 4-in scale photomask. The white portion in Fig. 4a defines the SU-8 microstructure on the polyimide substrate by the UV-photolithography process. In contrast, in the black portion, the metal layer of Cr/Au in the substrate appeared, thereby forming the X-ray absorber layer after the Au-electroplating. The fabricated X-ray mask is shown in Fig. 4b, which includes the electroplated X-ray absorber as well as the opened membrane. As the exposed hard X-ray radiation was sufficiently blocked in the X-ray absorber, the thick PMMA microstructure having its plane image identical to the absorber layer could be obtained by the DXRL process as shown in Fig. 4c. The fabricated PMMA sheet had the opened cavity including half-circular microstructures on one sidewall. It should be noted that the DXRL process can produce a microfeature with an excellent sidewall quality, thereby resulting in a smooth surface of the microchannel array in this study. The appeared Ti surface acted as the conductive seed layer in the subsequent nickel electroforming process.

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Figure 4. (a) CAD design for SMM unit in the X-ray mask; Photographs of (b) the fabricated X-ray mask, and (c) PMMA mother structure manufactured by DXRL process. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The nickel electroforming onto the fabricated PMMA sheet was carried out as shown in Fig. 5a. The patterned PMMA sheet on the Ti substrate was carefully installed on the electroforming jig. The whole jig part was immersed in the electrolyte bath and also oscillated up and down to achieve a bubble-removal striking motion during the electroforming. In this manner, the cavities surrounded by the patterned PMMA sheet were filled with a nickel. As mentioned, the polishing of top and bottom sides and the cleaning of the PMMA sheet were carried out after the nickel electroforming. Figure 5b shows the fabricated nickel SMMs. The convex microstructures with half-circular cross-section are shown in Fig. 5c and d. It should be noted that the thickness of SMM was reduced to 1.8 mm because of the polishing although the PMMA sheet used in the DXRL process had a thickness of 2.0 mm.

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Figure 5. Photographs of (a) nickel electroforming apparatus used in this study and (b) electroformed nickel SMMs. (c and d) Microscopic views of the half-circular microstructures in the SMM. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Figure 6a shows the mold core used in the microinjection molding experiment. In this example, three electroformed nickel SMMs were assembled in the center of the mold core. Also, two SMMs for the formation of inlet/outlet reservoirs were inserted at both sides of the assembled nickel SMMs. In this manner, both the various convex microstructures with half-circular cross-sections and the protruded rectangular parts were constructed on the surface of the mold core. The prepared mold core was installed on the stationary platen of the mold base as shown in Fig. 6b. The moving platen of the mold base was designed to have another mold core with a square cavity of 35 mm × 35 mm × 2 mm. During the microinjection molding process, two platens were closed to form the square cavity having the convex microstructures and the rectangular parts on its one surface. The closed cavity was filled with a molten polymer of COC under the prescribed processing conditions. After one cycle of microinjection molding such as filling, packing, cooling, and demolding, the microinjection-molded square plate could be obtained as shown in Fig. 6c. As the reverse features of the cavity surface were transcribed on the molded part, the half-circular microchannel array with different diameters and two rectangular reservoir parts were formed on the surface.

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Figure 6. Photographs of (a) mold core with the assembled SMMs, (b) mold base for microinjection molding, and (c) microinjection-molded square plate having the half-circular microchannel array. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The surface of the replicated microchannels was observed using a scanning electron microscope (SEM). As shown in Fig. 7a, the open half-circular microchannel was clearly produced on the plastic plate. The convex surface of the microchannel was also found to be very smooth, verifying the advantage of the DXRL process utilized in this study. Figure 7b shows the SEM image of the microchannels of 400, 450, and 500 μm diameters. It should be mentioned that tiny witness marks resulted from the interface between the assembled SMMs were imprinted. Although a minute witness mark including a parting line is usually inevitable in a conventional injection molding process using a mold with complex inserted blocks, they could be further minimized by assembling of the SMMs more carefully, changing polymeric material, and/or calibrating the relevant processing parameters. The surface profiles of the molded microchannel were measured by a surface profiler (Alpha Step-IQ, KLA-Tencor) as shown in Fig. 7c. The measurement indicated that the microchannels with convex half-circular shape, each showing the same values of depth and radius, were fabricated. For all cases, the round edges between the microchannel and the top surface were observed, which resulted from both a volumetric shrinkage of the molded polymeric material and a measurement error from the contact-type surface profiler. The shrinkage might be minimized by applying higher packing pressure and/or more sufficient packing time.

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Figure 7. SEM images of (a) concave surface of the half-circular microchannel of 500 μm diameter and (b) surface of the microinjection-molded plate; (c) surface profile of the molded microchannels. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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To observe the cross-sectional images of the enclosed circular microchannels, which could be achieved by the thermal bonding of two microinjection-molded plates, the center area of the bonded microfluidic chip prototype was carefully sliced by a low-speed diamond saw (IsoMet, BUEHLER) as shown in Fig. 8a. Figure 8b and c shows the tilted SEM images of the sliced face with the circular microchannels of 500 and 50 μm diameters, respectively. From the SEM images, it was found that the present thermal bonding produced perfectly enclosed circular microchannels. Although the sliced surface was not clean and some burrs or residues generated during the slicing remained in the larger microchannel, the enclosed microchannels were successfully obtained. It might be noted that the round edges in the replicated open microchannel were minimized in the final structure as the bonding process produced a slight thermal deformation at the bonded interface.

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Figure 8. SEM images of (a) circular microchannels in the bonded microfluidic prototype chip, (b) circular microchannel of 500 μm diameter, and (c) that of 50 μm diameter.

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Figure 9 shows a result of a simple microfluidic test to verify the fabricated microfluidic chip prototype. Two holes for the purpose of inlet and outlet of a working fluid (DI water mixed with a red ink in this study) were formed on the reservoirs by a mechanical drilling. After connecting silicone tubing into each hole, the working fluid was introduced from the inlet hole into the inlet reservoir by a syringe pump system. As indicated in the snapshot of the feasibility experiment in Fig. 9, no leakage of the working fluid was observed and the introduced working fluid could flow through the circular microchannels, verifying the usefulness of the present fabrication method.

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Figure 9. Snapshot of the feasibility test using the microfluidic chip prototype of circular microchannels. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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It should be mentioned that as the present method utilizes the DXRL process for a manufacture of micromold units (SMMs), the smooth surface in the molded microchannel is fabricated. Furthermore, because the cross-sectional image of the microchannel is transcribed onto the X-ray photoresist in the SMM manufacturing step, microchannels with various cross-sectional shapes can be easily produced. By applying the fabricated SMMs in micromolding technologies, a mass production of precise microchannels is realized with higher productivity. A simple insertion procedure of SMMs into a mold core can make it possible to achieve an easy integration of various microchannels into the complex microfluidic system, thereby enabling us to manufacture more versatile microfluidic systems for various application fields.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. FABRICATION METHOD
  5. EXPERIMENTAL
  6. RESULTS AND DISCUSSION
  7. CONCLUSIONS
  8. REFERENCES

In this study, the fabrication process for a circular microchannel with a smooth surface was proposed and demonstrated by microinjection molding of plastic microfluidic chip including the circular microchannel array. The present method utilized the DXRL process to manufacture the precise micromold, which was employed in a mass replication of the plastic plates having the open circular microchannel array. By bonding two identical microinjection-molded plates, the microfluidic chip prototype with the circular microchannel array was successfully fabricated. Through the surface observations and the profile measurement, the replicated microchannel showed a smooth surface as well as a dimensional accuracy. Also, a performance of the fabricated prototype was verified from the simple microfluidic feasibility test, showing no leakage of the working fluid.

The fabrication process in this study shows several advantages such as excellent surface quality of the microchannels, high-dimensional accuracy, design flexibility for the cross-sectional shape, high productivity, and possibility of using versatile molding materials. It can be efficiently employed as a manufacturing tool in the development of various microfluidic systems including microchannels with circular and/or other curved cross-sections. Although only the straight microchannels are produced within one SMM in the present fabrication method, a simple integration of SMMs into a mold core and its utilization in the various molding process enable us to realize the versatile microfluidic systems. Continuous-flow reaction/synthesis systems, cartridge-type microfluidic nozzle arrays, and efficient heat exchangers might be the appropriate candidates for the practical application of the present circular microchannels.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. FABRICATION METHOD
  5. EXPERIMENTAL
  6. RESULTS AND DISCUSSION
  7. CONCLUSIONS
  8. REFERENCES