Polymer Microchannel and Micromold Surface Polishing for Rapid, Low-Quantity Polydimethylsiloxane and Thermoplastic Microﬂuidic Device Fabrication

polymers

Article Polymer Microchannel and Micromold Surface Polishing for Rapid, Low-Quantity Polydimethylsiloxane and Thermoplastic Microﬂuidic Device Fabrication

Chia-Wen Tsao * and Zheng-Kun Wu Department of Mechanical Engineering, National Central University, Taoyuan City 32001, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-3-4267343; Fax: +886-3-4254501

Received: 12 October 2020; Accepted: 30 October 2020; Published: 2 November 2020

Abstract: Polymer-based micromolding has been proposed as an alternative to SU-8 micromolding for microﬂuidic chip fabrication. However, surface defects such as milling marks may result in rough microchannels and micromolds, limiting microﬂuidic device performance. Therefore, we use chemical and mechanical methods for polishing polymer microchannels and micromolds. In addition, we evaluated their performance in terms of removing the machining (milling) marks on polymer microchannel and micromold surfaces. For chemical polishing, we use solvent evaporation to polish the sample surfaces. For mechanical polishing, wool felt polishing bits with an abrasive agent were employed to polish the sample surfaces. Chemical polishing reduced surface roughness from 0.38 µm (0 min, after milling) to 0.13 µm after 6 min of evaporation time. Mechanical polishing reduced surface roughness from 0.38 to 0.165 µm (optimal pressing length: 0.3 mm). As polishing causes abrasion, we evaluated sample geometry loss after polishing. Mechanically and chemically polished micromolds had optimal micromold distortion percentages of 1.01% 0.76% and 1.10% 0.80%, ± ± respectively. Compared to chemical polishing, mechanical polishing could better maintain the geometric integrity since it is locally polished by computer numerical control (CNC) miller. Using these surface polishing methods with optimized parameters, polymer micromolds and microchannels can be rapidly produced for polydimethylsiloxane (PDMS) casting and thermoplastic hot embossing. In addition, low-quantity (15 times) polymer microchannel replication is demonstrated in this paper.

Keywords: polymer microfabrication; polymer microﬂuidics; microchannel; micromold; micromilling; polymer polishing; PDMS casting; thermoplastic hot embossing

1. Introduction Microfluidics is a highly integrated and multidisciplinary applied science that has been developing for over 30 years [1,2]. Microfluidics has been widely applied in various fields such as public health [3], biology [2–4], energy [4,5], chemical engineering [6,7], and environmental science [8,9]. Microfluidic devices are fabricated using numerous materials and approaches. In the early stages of development, microfluidic devices, derived from microelectromechanical systems (MEMSs), were mainly fabricated using silicon microfabrication techniques [10]. However, optical transmissivity is a fundamental challenge that limits optical observation or detection on silicon-based microfluidic platforms. Glass substrates, which have excellent optical performance, are suitable materials for microfluidic device fabrication [11]. However, glass microchannels are mainly generated through photolithography, followed by hydrofluoric acid etching, which is a hazardous process, and the isotropic etching behavior constrains researchers in the construction of various microchannel

Polymers 2020, 12, 2574; doi:10.3390/polym12112574 www.mdpi.com/journal/polymers Polymers 2020, 12, 2574 2 of 15 geometries. Around the 2000s, polymer materials with favorable mechanical, optical, and chemical properties were introduced for the construction of microfluidic systems. Polydimethylsiloxane (PDMS) elastomers [12,13] and thermoplastics such as polymethyl methacrylate (PMMA) and cyclic olefin copolymer (COC) [14,15] are widely used as substrate materials in microfluidics due to their simple and low cost advantages. Other materials such as paper [16] and wood [17] have recently been employed in microfluidics. Among all such materials, polymer materials are still the major substrates used in microfluidics in this stage. Polymer microfluidic devices can be fabricated through inexpensive and high-throughput manufacturing approaches. Various conventional plastic production processes have been employed for polymer microfluidic device fabrication. Rapid prototyping methods such as computer numerical control (CNC) milling [18], laser ablation [19], or 3D printing [20] can be used to rapidly produce microfluidic devices for a proof-of-concept study at low quantities. For larger quantity requirements, mass production approaches such as injection molding [21], hot embossing [22], or roller imprinting [23] can be used to create polymer microfluidic devices at a low cost. With these advantages, polymers are favorable disposable substrate materials for the microfluidics field and can meet academic research and commercialization requirements [24]. Micromold is a critical tooling technique for making polymer microfluidic device. For thermoplastic-based microfluidic devices, micromold is used to replicate the microchannel through hot embossing or injection molding. For PDMS-based microfluidics, micromold is an essential component in the soft lithography process [25] for casting PDMS microchannels from the mold. Currently, the micromolds are usually constructed through LIGA (Lithographie, Galvanoformung, Abformung), electroplating, dry/wet etching, pulse microelectroforming, or photolithography to generate silicon or metal micromolds [26,27]. SU-8 micromold is the most commonly used method for creating thermoplastic or PDMS microfluidic devices, as it is relatively straightforward and has lower facility costs than other micromold fabrication methods [28,29]. However, adhesion between SU-8 resins and silicon substrates is a potential issue that limits the micromold’s lifetime. The SU-8 layer may peel off during demolding. Furthermore, SU-8 micromold fabrication still requires a cleanroom and UV lithography facilities, limiting accessibility for researchers. Recently, polymer-based micromold has been proposed as a simple alternative to silicon or SU-8 micromold for microfluidic chip fabrication. Polymer micromolds are fabricated through CNC milling [30], laser ablation [31], or 3D printing [32,33]; these are straightforward methods with low-cost desktop fabrication facilities. Such facilities also require fewer operating skills, enabling broad usage [3,34]. However, surface defects, such as milling marks and those due to overheating, or poor printing resolution resulting from the use of related techniques may result in rough microchannels or micromolds or uneven substrate surfaces, limiting microfluidic device performance. Several approaches have been introduced to smooth the surface after milling. For example, Matellan et al. use acetone vapor treatment to smooth the PMMA surface as a cost-effective rapid prototyping method [35] to generate the microfluidic device; or they use cryogenic cooling to polish the acrylic-based polymer for optical lens application [36]. In addition to these approaches, abrasive polishing tools are commonly used to polish plastic parts in large scale [37,38]. This method can also be applied in microfluidics application, e.g., Szymborski et al. use polishing paste and felt to polish the Teflon bioreactor [39]. Although polishing methods have been reported to smooth the microchannel or polymer surfaces, less emphasis has been placed on their use in polymer replications for microfluidics. Therefore, in this study, we introduced a method for polishing polymer microchannels and micromolds for polymer microfluidics. A desktop CNC micromiller was used to generate microchannels and micromolds for rapid prototyping and replication in polymer microfabrication. We evaluated chemical and mechanical polishing performance for microchannel and micromold surfaces. Finally, we used a polished polymer micromold to imprint thermoplastic microfluidic devices and cast PDMS microfluidic devices to demonstrate that the proposed polishing method is simple, cost-effective, and enables the rapid fabrication of micromolds for related applications. Polymers 2020, 12, x FOR PEER REVIEW 3 of 16 Polymers 2020, 12, 2574 3 of 15 2. Experiment

2.1.2. Experiment Materials and Reagent 2.1. MaterialsPolymethylmethacrylate and Reagent (PMMA, Optical grade, CM-205X) is purchased from Chi-Mei CorporationPolymethylmethacrylate Co., Ltd. (Tainan, (PMMA,Taiwan). OpticalCyclic olefin grade, copolymer CM-205X) (COC, is purchased 8007) is purchased from Chi-Mei from TOPAS Advanced Polymers manufactures (Frankfurt am Main, Germany). Cyclic block copolymer Corporation Co., Ltd. (Tainan, Taiwan). Cyclic oleﬁn copolymer (COC, 8007) is purchased from TOPAS (CBC 010) is purchased from USI Corporation Co., Ltd. (Kaohsiung, Taiwan). Polydimethylsiloxane Advanced Polymers manufactures (Frankfurt am Main, Germany). Cyclic block copolymer (CBC 010) (PDMS, SYLGARD™ 184) is purchased from Dow Corning Inc. (Atlanta, GA, USA). End mills with is purchased from USI Corporation Co., Ltd. (Kaohsiung, Taiwan). Polydimethylsiloxane (PDMS, 200 μm diameter are purchased from Taiwan Microdrill Co., Ltd. (New Taipei, Taiwan). Wool felt SYLGARD™ 184) is purchased from Dow Corning Inc. (Atlanta, GA, USA). End mills with 200 µm bits (bullet type) are purchased from Dayuan Hardware Co., Ltd. (Taoyuan, Taiwan). PolyWatch diameter are purchased from Taiwan Microdrill Co., Ltd. (New Taipei, Taiwan). Wool felt bits (bullet (plastic polish) is purchased from EVI GmbH (Neuried, Germany). Acetone (ACE, 99.9%, electronic type) are purchased from Dayuan Hardware Co., Ltd. (Taoyuan, Taiwan). PolyWatch (plastic polish) is grade) is purchased from Mingyang Chemical Materials Co., Ltd (Taoyuan, Taiwan). purchased from EVI GmbH (Neuried, Germany). Acetone (ACE, 99.9%, electronic grade) is purchased from Mingyang Chemical Materials Co., Ltd (Taoyuan, Taiwan). 2.2. Micromold and Microchannel Fabrication through CNC Milling

2.2. MicromoldAs displayed and Microchannelin Figure 1a 6 Fabrication × 4 mm2 throughPMMA CNCsubstrate Milling was milled using a CNC micromilling machineAs displayed(Roland EGX-400, in Figure Roland1a 6 DGA4 mm Corporation,2 PMMA substrate CA, USA) was to milled generate using a microchannel a CNC micromilling (Figure × 1a)machine or micromold (Roland EGX-400, (Figure 1c) Roland with DGAdifferent Corporation, spin speeds CA, of USA) 10,000, to generate 20,000, aand microchannel 30,000 rpm (Figure and feed1a) ratesor micromold of 1, 3, 5, (Figure 7, and1 9c) mm/s. with di Thesefferent milling spin speeds parameters of 10,000, are pre-defined 20,000, and 30,000by refereeing rpm and related feed rates CNC of milling1, 3, 5, 7,investigations and 9 mm/s. These[40,41] milling and pilot parameters trails. We are selected pre-defined polymers by refereeing with favorable related CNCrigidity milling and thermalinvestigations properties [40, 41(i.e.,] and PMMA) pilot as trails. our micromold We selected substrate. polymers The with microchannel favorable rigidityhad a width, and thermal length, andproperties height (i.e., of 200 PMMA) μm, 4 as mm, our micromoldand 200 μm, substrate. respectively, The microchannel and it had two had reservoirs a width, length,(radius: and 500 height μm) connectedof 200 µm, at 4 mm,each and end. 200 Theµm, micromold respectively, surface and it was had twosubjected reservoirs to either (radius: mechanical 500 µm) connectedor chemical at polishingeach end. to The remove micromold milling surface marks was on subjectedthe polymer to either microchannel mechanical (Figure or chemical 1d) or polishingmicromold to remove(Figure 1d)milling surface. marks For on thechemical polymer polishing, microchannel we used (Figure solvent1d) or evaporation micromold (Figure to polish1d) surface.the surface, For chemical and for mechanicalpolishing, we polishing, used solvent we used evaporation wool felt to polishing polish the bits surface, with an and abrasive for mechanical agent (Polywatch) polishing, to we polish used thewool surface. felt polishing The polishing bits with process an abrasive is described agent (Polywatch) in detail in the to polish Discussion the surface. section. The After polishing polishing, process the PMMAis described substrate in detail was in cleaned the Discussion with deionized section. Afterwater polishing, in an ultrasonic the PMMA cleaner substrate (Delta was Ultrasonic cleaned withCo., Ltd.,deionized Delta waterD150). in Figure an ultrasonic 1e shows cleaner the experimental (Delta Ultrasonic setup Co.,for milling Ltd., Delta and D150). polishing. Figure For1e mechanical shows the polishing,experimental we setuptested for different milling andwool polishing. felt polishing For mechanicalbit pressing polishing, lengths (L). we testedThe pressing different length wool felt is defined as the initial position (Y0) subtracted by the polishing position (Y1); the related equation is polishing bit pressing lengths (L). The pressing length is defined as the initial position (Y0) subtracted illustrated in Figure 1e. by the polishing position (Y1); the related equation is illustrated in Figure1e.

Figure 1.1. SchematicSchematic of of (a )( rapida) rapid microchannel microchannel prototyping prototyping through through direct milling direct formilling both ( bfor) mechanical both (b) mechanicaland chemical and polishing. chemical (c polishing.) shows polymer (c) shows micromold polymer replication micromold for replication both (d) mechanical for both (d and) mechanical chemical andpolishing chemical setup. polishing (e) displays setup. photographs (e) displays ofphotographs the experimental of the experimental the setup. the setup.

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2.3. Surface Roughness and Distortion Percentage Evaluation For a microchannel and micromold, we evaluated the polymer surface roughness (Ra) by using a surface roughness measuring instrument (SURFCOM 130A, Tokyo Seimitsu Co., Ltd., Tokyo, Japan; sample size: 6 6 0.2 mm3, method: Ra, cutoff (wavelength) λc: 0.8 mm, evaluation length: 3.2 mm, × × measuring length: 4 mm) and a microscope after both CNC milling and surface polishing. As polishing is either an abrasive or dissolution process, we also evaluated the microchannel and micromold geometry loss after surface polishing. The geometry distortion percentage is defined in Equation (1). Using an inverted fluorescence microscope (Eclipse Ti-U, Nikon Corporation, Tokyo, Japan), cross-sectional images were captured after razor blade cutting of the PDMS replica’s cast from the microchannel and micromolds; the cross-sectional areas were defined before and after polishing.

Cross sec tional area after polishing Distortion [%] = 1 − 100 % (1) − Cross sec tional area before polishing × − 3. Results and Discussion

3.1. Surface Roughness of Different Thermoplastic Surfaces after Milling In the fabrication of polymer microfluidic devices using a CNC miller, milling marks that remain on the microchannel surface are a critical concern. PMMA is among the most commonly used thermoplastics in polymer microfluidics due to its low cost and straightforward fabrication. In additional to PMMA, cyclic olefin-based polymers such as COC and cyclic olefin polymer (COP), which have high chemical resistance, are useful for microfluidic applications. Therefore, we first evaluated the surface roughness of PMMA and COC after milling at different spin speeds (10,000 ~30,000 rpm) and feed rates (1~9 mm/s). Both 200- and 500-µm-diameter end mills were selected for fabricating the 200- and 500-µm microchannels. It is noted that although end mills with a minimum diameter of 100 µm are available in the market, we found that end mills with smaller diameters (<100 µm) were easily broken. Thus, to achieve favorable production repeatability and tool lifetime, we chose 200 µm as the minimum end mill diameter in our experiments. The milling test results are displayed in Figure2. As detailed in the figure, low feed rates improved surface quality; the lowest feed rate of 1 mm/s resulted in optimal surface quality. A high spin speed enables the removal of unwanted materials from the substrate using end mills, resulting in smooth microchannel surfaces. These results agree with those of Chen et al., a high spin speed with a low feed rate reduced surface roughness [40,41]. For PMMA, as shown in Figure2a,b, lower surface roughness values were found at higher spin speeds, as predicted. However, for the COCs (Figure2c,d), surface roughness had a weak correlation with spin speed. Machining performance is primarily determined by the polymer’s glass transition temperature (Tg). PMMA, which has a Tg of 104 ◦C, has sufficient stiffness for machining. By contrast, in COC substrates, which have a low Tg (75 ◦C), due to frictional heat generated during machining at the end mill tip, the COC surfaces are softened or melted at the machining tip. This results in a “wavy” surface with high surface roughness. For the 500-µm end mill, at 30,000 rpm (Figure2c), we observed that the COC polymers were melt-clogging the end mills, resulting in broken tips. Therefore, the results for the 30,000-rpm experiment are not displayed in Figure2c. From the milling tests, we determined the optimum milling conditions for PMMA (Figure2b—Ø 0.2 mm, 1 mm/s, 30,000 rpm) and COC (Figure2d—Ø 0.2 mm, 1 mm /s, 20,000 rpm), with minimum surface roughness values of 0.380 µm (PMMA, Figure2b), 0.428 µm (COC, Figure2d), respectively. Although using an end mill with a 500-µm diameter generated rougher surfaces, surface roughness only increased by approximately 10%. Considering that the machining time was reduced by approximately 40% and the tool lifetime improved, using a larger end mill (500 µm) is a preferable choice for polymer micromold fabrication. However, in our tests, to determine how to obtain the smoothest surface with minimum dimension loss after polishing, we used 200-µm-diameter end mills for the following Polymers 2020, 12, 2574 5 of 15

Polymerstests. Microscope 2020, 12, x FOR images PEER REVIEW of the milled PMMA and COC surfaces are shown in Figures S1–S45 in of the 16 Supplementary Material. Since PMMA has better thermal stability and a higher Tg point than COC (104Tg point◦C vs.than 75 COC◦C), and(104 PMMA°C vs. 75 is ° alsoC), and easy-to-machine PMMA is also polymer, easy-to-machine it does not polymer, have the it does end-mill’s not have tip theclogged end-mill’s issues tip during clogged machining. issues during Thus, machining. for the following Thus, for polymer the following microchannel polymer and microchannel micromold andfabrication, micromold PMMA fabrication, was used PMMA as the was major used substrate as the major material substrate over material COCs. over In addition, COCs. In optimized addition, optimizedPMMA processing PMMA processing parameters parame (Ø 0.2 mm,ters (Ø 1 mm 0.2 /mm,s, 30,000 1 mm/s, rpm) 30,000 were usedrpm)to were generate used anto generate unpolished an unpolishedmicromold tomicromold investigate to polishinginvestigate performance polishing performance in the following in the discussion following sections. discussion sections.

Figure 2. Surface roughness of polymethyl methacrylate (PMMA) (a,b) and cyclic olefin copolymer (COC)Figure (2.c, dSurface) after millingroughness at di offferent polymeth feedyl rates, methacrylate spin speeds, (PMMA) and end (a mill,b) and diameters. cyclic olefin Error copolymer bars were (COC)obtained (c, basedd) after on milling three individualat different measurements. feed rates, spin speeds, and end mill diameters. Error bars were obtained based on three individual measurements. 3.2. Surface Polishing of Microchannels and Micromolds 3.2. SurfaceThis section Polishing provides of Microchannels details of our andevaluation Micromolds of surface roughness and polymer microstructure distortionThis section percentage provides obtained details using of our chemical evaluation and mechanical of surface roughness polishing methods. and polymer Both microstructure microchannels distortionand micromolds percentage were fabricated,obtained asusing displayed chemical in Figure and1 .mechanical Chemical polishing polishing is themethods. most widely Both microchannelsused method forand polishing micromolds related were plastic fabricated, parts. as Thus,displayed we first in Figure evaluated 1. Chemical the effects polishing of chemical is the polishingmost widely on aused blank method PMMA for surface polishing and within related the plastic microchannel parts. Thus, and we micromold first evaluated substrates. the Chemicaleffects of vaporchemical polishing polishing was on performed a blank by PMMA fixing thesurfac PMMAe and substrate within inthe an microchannel ACE-filled glass and beaker, micromold which wassubstrates. heated Chemical to vaporize vapor the polishing solvent for was polishing performed the by milling fixing marksthe PMMA on the substrate PMMA in surface. an ACE-filled In our glassinitial beaker, trials, wewhich used was evaporation heated to vaporize temperatures the solvent ranging for frompolishing room the temperature milling marks (~23 on◦ C)the to PMMA 80 ◦C. surface.Excessive Insurface our initial solvation trials, we was used observed evaporation at a high temperatures temperature. ranging Maintaining from room solvation temperature evaporation (~23 °atC) a to lower 80 °C. temperature Excessive surface offered solvation better controllability was observed of at the a polishinghigh temperature. process byMaintaining tuning evaporation solvation evaporationtimes. In a relatedat a lower study, temperature Matellan etoffered al. used better acetone controllability to polish of a laser-engravedthe polishing process microchannel by tuning at evaporation30 ◦C. They revealedtimes. In that a evaporationrelated study, at lowerMatellan temperatures et al. used can acetone avoid the to generation polish a oflaser-engraved visible cracks microchannelduring polishing at [3035 ].°C. Thus, They we revealed chose 40 ◦thatC as evaporation our solvent temperatureat lower temperatures and tested di canfferent avoid solvent the generationevaporation of times visible (1, cracks 2, 3, 4, during 5, and 6polishing min) to evaluate [35]. Thus, changes we chose in surface 40 °C roughnessas our solvent during temperature chemical polishing.and tested Asdifferent displayed solvent in Figure evaporation3, the surface times (1, roughness 2, 3, 4, 5, values and 6 min) of PMMA to evaluate were reduced changes as in solventsurface µ µ roughnessevaporation during time increased chemical from polishing 0.338. Asm (1displayed mins) to 0.13in Figurem (6 3, min). the surface roughness values of PMMA were reduced as solvent evaporation time increased from 0.338 μm (1 mins) to 0.13 μm (6 min).

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Figure 3. (a) Surface roughness of the PMMA surface after chemical polishing with solvent FigureFigureevaporation 3. 3.(a )( Surfacea) timesSurface roughnessof 1,roughness 2, 3, 4, of 5, the andof PMMA the 6 mi PMMAn. surface Microscope surface after chemical imagesafter chemical polishingof PMMA polishing with surfaces solvent withat ( evaporationb ) solvent1, (c) 2, (d) timesevaporation3, (e of) 4, 1, (f 2,) 5, 3,times and 4, 5, of( andg )1, 6 2, 6min. min.3, 4, Error 5, Microscope and bars 6 mi weren. imagesMicroscope obtained of PMMA based images on surfaces of three PMMA individual at (surfacesb) 1, (c )measurements. 2,at (db) 3,1, (ec) 2, 4, ( d f)) 5, and3, (e (g) )4, 6 ( min.f) 5, and Error (g) bars 6 min. were Error obtained bars were based obtained on three based individual on three measurements. individual measurements. We revealed that chemical polishing is effective at removing milling marks from sample surfaces.WeWe revealedrevealed Surface thatroughnessthat chemicalchemical was polishingpolishingreduced with is effective e ffanective increase at at removing removingin the solvent milling milling evaporation marks marks from time. from sample However, sample surfaces.surfaces.for microfluidic Surface Surface roughnessroughness applications, waswas reducedthe geometric with an an integrityin increasecrease in inof the themicrostructures solvent solvent evaporation evaporation must time. be time. retained However, However, after forforpolishing. microfluidic microfluidic Therefore,applications, applications, we further the the geometric evaluatedgeometric integrity the integrity micr ofostructural microstructures of microstructures geometric must be mustdistortions retained be retained after after polishing. polishing. after Therefore,polishing.Figure 4a,b we Therefore, furthershow the evaluatedwe furthermicrostructural the evaluated microstructural images the micr ofostructural geometric micromolds distortionsgeometric and microchannels distortions after polishing. after after polishing. Figure chemical4a,b showFigurepolishing. the 4a,b microstructural Asshow shown the inmicrostructural images the top of micromoldsand cross-sectionalimages and of microchannelsmicromolds (left-bottom) and after imagesmicrochannels chemical in the polishing. figure, after chemical Asthe shownoriginal inpolishing.rectangular the top and As shapeshown cross-sectional of in microstructures the top (left-bottom) and cross-sectional changed images to in(left-bottom)hi thell-like figure, geometries; images the original in the the hill rectangularfigure, shapes the originalwere shape more of microstructuresrectangularpronounced shape with changed ofa longermicrostructures to hill-like polishing geometries; changed time. As theto summarized hi hillll-like shapes geometries; in were Figure more the 4c, pronounced hillthe shapesdistortion withwere percentage a more longer polishingpronouncedfrom chemical time. with polishing As a summarizedlonger increased polishing in fromFigure time. the 4Asc, initial summarized the distortion value at in 0 percentage minFigure to 53.1%4c, the from for distortion micromolds chemical percentage polishing and 32.3% increasedfromfor microchannels chemical from thepolishing initial after increased value6 min atof 0 polishing.from min the to 53.1% initial These forvalue results micromolds at 0 indicated min to and 53.1% that 32.3% for although micromolds for microchannels chemical and 32.3%polishing after 6for mincould microchannels of polishing.effectively Theseafterremove 6 resultsmin milling of indicatedpolishing. marksthat Theseand although irreguresultslarities chemicalindicated from polishingthat surface although could surfaces, chemical effectively microstructure polishing remove could effectively remove milling marks and irregularities from surface surfaces, microstructure millingdistortion marks was and substantial. irregularities From from a surfacemicrochannel surfaces, fabrication microstructure point-of-view, distortion the was microchannel substantial. Fromshould distortion was substantial. From a microchannel fabrication point-of-view, the microchannel should a microchannelmaintain its fabricationgeometry integrity point-of-view, with theoriginal microchannel design shouldas close maintain as possible. its geometry For microfluidic integrity maintain its geometry integrity with original design as close as possible. For microfluidic withapplications, original design this factor as close should as possible. be considered For microfluidic when a applications,microchannel’s this geometric factor should stability be considered is a crucial applications, this factor should be considered when a microchannel’s geometric stability is a crucial whenissue. a microchannel’s geometric stability is a crucial issue. issue.

(a(a) )

Figure 4. Cont.

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(b)

(c)

Figure 4. Top and cross-sectional (left-bottom) microscope images showing (a) micromolds and Figure 4. Top and cross-sectional (left-bottom) microscope images showing (a) micromolds and (b) (b) microchannels immediately after milling (0 min) and after 6 min of chemical polishing. (c) Summary microchannels immediately after milling (0 min) and after 6 min of chemical polishing. (c) Summary of distortion percentage and surface roughness (Ra). of distortion percentage and surface roughness (Ra). Compared with chemical polishing, mechanical polishing using wool felt polishing bits is a more Compared with chemical polishing, mechanical polishing using wool felt polishing bits is a more effective and straightforward process; polishing bits are used instead of end mills in an identical CNC effective and straightforward process; polishing bits are used instead of end mills in an identical CNC machine. The polishing process can be automatically controlled using a CNC machine, with little setup machine. The polishing process can be automatically controlled using a CNC machine, with little required. For mechanical polishing using wool felt polishing bits, the distance between the wool bit setup required. For mechanical polishing using wool felt polishing bits, the distance between the and the PMMA surface (pressing length (L) shown in Figure1e) is highly correlated with the pressure wool bit and the PMMA surface (pressing length (L) shown in Figure 1e) is highly correlated with applied to the polymer surface. Figure5 shows the surface roughness related to pressing lengths of the pressure applied to the polymer surface. Figure 5 shows the surface roughness related to pressing 0.1, 0.2, 0.3, and 0.4 mm. We defined the original pressing length (L) to be 0 when the wool felt tip lengths of 0.1, 0.2, 0.3, and 0.4 mm. We defined the original pressing length (L) to be 0 when the wool touched the PMMA surface. As shown in the figure, surface roughness decreased from 0.380 µm (blue felt tip touched the PMMA surface. As shown in the figure, surface roughness decreased from 0.380 dashed line) to 0.351 µm (L = 0.1 mm) and then to 0.236 µm (L = 0.2 mm). For a pressing length of μm (blue dashed line) to 0.351 μm (L = 0.1 mm) and then to 0.236 μm (L = 0.2 mm). For a pressing 0.3 mm, the lowest surface roughness of 0.165 µm was obtained. However, for a pressing length of length of 0.3 mm, the lowest surface roughness of 0.165 μm was obtained. However, for a pressing 0.4 mm, surface roughness increased to 0.231 µm. This is due to wool felt excessively pressing the length of 0.4 mm, surface roughness increased to 0.231 μm. This is due to wool felt excessively PMMA surfaces; 5–10-µm dents were observed, indicative of higher surface roughness. For mechanical pressing the PMMA surfaces; 5–10-μm dents were observed, indicative of higher surface roughness. polishing, an optimal pressing length of 0.3 mm was revealed. For mechanical polishing, an optimal pressing length of 0.3 mm was revealed.

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FigureFigure 5. ((aa)) Roughness Roughness of of the the PMMA PMMA surface surface after after mechanical mechanical polishing polishing at at pressing pressing lengths lengths of of 0.1, 0.1, 0.2,0.2, 0.3, 0.3, and and 0.4 mm. Microscope Microscope images images of of P PMMAMMA surface surface within within for for pressing pressing lengths lengths of of ( (bb)) 0.1, 0.1, ( (cc)) 0.2,0.2, ( dd)) 0.3, 0.3, and ( e) 0.4 mm. Error Error bars were obtained base basedd on three measurements. For mechanical polishing using a computer computer numerical numerical control control (CNC) machine, machine, the the spin spin speed, speed, feed feed rate, rate, and and overlap overlap path were set as 8000 rpm, 1 mm/s, mm/s, and 25%, respectively.

ForFor mechanical polishing, polishing, because because the the wool wool felt felt polishing polishing path path was was controlled controlled by by the the CNC machine,machine, milling milling marks marks were were locally polished and and removed without causing excessive damage to polymer microstructures. Figure Figure 6 shows toptop andand cross-sectionalcross-sectional imagesimages ofof PMMAPMMA micromoldsmicromolds (Figure(Figure 66a)a) andand microchannelsmicrochannels (Figure(Figure6 6b)b) after after mechanical mechanical polishing. polishing. The The microstructure microstructure distortion distortion percentage was only moderately increased for microchannel and micromold fabrication (5.70% and 7.80%,7.80%, respectively) respectively) for for a a pressing pressing length length of of 0.1 0.1 mm. Wool Wool felt pressing pressure increased with the pressing length. This This resulted resulted in in lower lower surface surface roughness, roughness, as as discussed discussed previously. previously. This This also also led led to thethe increased increased removal removal of of polymer polymer microstructures. microstructures. In In microchannels, microchannels, for for pressing pressing lengths lengths of of 0.2, 0.2, 0.3, 0.3, andand 0.4 0.4 mm, mm, the the distortion distortion percentage percentage increased increased to to13 13.90%.90% (L (L= 0.2),= 0.2), 14.50% 14.50% (L = (L 0.3),= 0.3),and 17.70% and 17.70% (L = 0.4),(L = respectively.0.4), respectively. In micromold In micromold polishing, polishing, for pressing for pressing lengths of lengths 0.2, 0.3, of and 0.2, 0.4 0.3, mm, and the 0.4 distortion mm, the percentagedistortion percentage increased increasedto 10.80%, to 11.80%, 10.80%, 11.80%,and 16. and40%, 16.40%, respectively. respectively. In mechanical In mechanical polishing, polishing, the highestthe highest distortion distortion percentage percentage (53.1%) (53.1%) was was obtained. obtained. Compared Compared with with the the chemical chemical approach, approach, mechanicalmechanical polishingpolishing by by wool wool felts felts was was more more capable capable of maintaining of maintaining the geometric the geometric integrity integrity of samples. of samples.The cross-sectional The cross-sectional images in Figureimages6 inshow Figure that 6 both show microchannels that both microchannels and micromolds and retained micromolds their rectangular-shaped geometries after mechanical polishing. retained1 their rectangular-shaped geometries after mechanical polishing.

(a) (a)

Figure 6. Cont.

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(c)

3 Figure 6. Top and cross-sectional (left-bottom) microscope images of a (a) micromold and (b) 4 microchannel after milling (0 min) and after mechanical polishing at a pressing length of 0.4 mm. (c) 5 Summary of distortion percentage and surface roughness (Ra).

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(b)

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Figure 6. Top and cross-sectional (left-bottom) microscope images of a (a) micromold and 2 (b) microchannel after milling (0 min) and after mechanical polishing at a pressing length of 0.4 mm. (c) Summary of distortion percentage and surface roughness (Ra). 3 Figure 6. Top and cross-sectional (left-bottom) microscope images of a (a) micromold and (b) 4 microchannel after milling (0 min) and after mechanical polishing at a pressing length of 0.4 mm. (c) To further evaluate the surface quality and condition, we performed surface proﬁlometer 5 Summary of distortion percentage and surface roughness (Ra). (DektakXT, Bruker Corporation, Billerica, MA, USA) analysis and collected 0.2 mm 1 mm × surface scanning data for 3D maps of the mechanical polished micromold surfaces as displayed 6 in Figure7. For the PMMA micromold after CNC milling (Figure7a), it can be clearly observed that milling marks/defects are uniformly distributed on the PMMA surface. With wool felt bit polishing (L = 0.1 mm), the milling marks were gradually removed (Figure7b). For increased pressing length (L = 0.2 and 0.3 mm), the milling marks were ﬂattened, generating smoother polymer surfaces (Figure7c,d) compared to the original condition after milling (Figure7a). For excessive wool felt bit pressing (L = 0.4), dents may be generated for a rougher surface. Therefore, the moderate pressing length of L = 0.3 is preferred for mechanical polishing.

Polymers 2020, 12, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2020, 12, 2574 10 of 15 Polymers 2020, 12, x FOR PEER REVIEW 10 of 16

Figure 7. The 3D map of the mechanical polishing micromold analyzed by a surface profilometer. (Figurea) After 7. millingThe 3D and map polished of the mechanical with wool feltpolishing bit with micr pressingomold length analyzed (b)L by= a0.1, surface (c)L =profilometer.0.2, (d)L = 0.3,(a) andAfter (e milling)L = 0.4. and polished with wool felt bit with pressing length (b) L = 0.1, (c) L = 0.2, (d) L = 0.3, and (e) L = 0.4. 3.3. Thermoplastic and PDMS Microfluidic Device Fabrication Using a Polished Polymer Micromold 3.3. Thermoplastic and PDMS Microfluidic Device Fabrication Using a Polished Polymer Micromold Different from SU-8 or other photolithography-based micromolds, polymer-based micromolds fabricatedDifferent through from CNC SU-8 milling or other are photolithography a bulk material because-based themicromolds, PMMA micromold polymer-based is directly micromolds removed fromfabricated a single through substrate. CNC Therefore, milling are no a microstructure bulk material peel-o becauseff occurs, the PMMA and such micromold micromolds is directly have a longerremoved lifetime from thana single photolithography-based substrate. Therefore, micromolds.no microstructure However, peel-off under occurs, excessive and heatsuch andmicromolds pressure applicationhave a longer during lifetime polymer than photolithography-based replication, the microstructures micromolds. of a polymer However, micromold under excessive may be distorted,heat and whichpressure potentially application aff duringects the polymer micromold’s replication, lifetime. the microstructures Thus, we further of investigateda polymer micromold the geometric may stabilitybe distorted, of micromolds which potentially under heataffects and the pressure micromold’s application lifetime. during Thus, thermoplastic we further investigated hot embossing the andgeometric PDMS stability casting. of As micromolds shown in Figure under8a, heat the polished and pressure PMMA application polymer micromold during thermoplastic and COC were hot sandwichedembossing and between PDMS glasscasting. substrates As shown under in Figure a hot 8a, embossing the polished machine PMMA (Ray polymer Cheng micromold Enterprise and Co., 2 Ltd.,COC HT-1).were sandwiched The hot embosser between plate glass was substrates heated to 95under◦C at a 20 hot kg /cmembossingfor 3 min machine to emboss (Ray the Cheng COC thermoplastic.Enterprise Co., The Ltd., hot HT-1). plate wasThe thenhot cooledembosse tor 65plate◦C, andwas COCheated replicas to 95 were°C at released 20 kg/cm from2 for the 3 PMMAmin to micromold.emboss the TheCOC COC thermoplastic. replicas were The tape-bonded hot plate was [42] withthen anothercooled polycarbonateto 65 °C, and (PC)COC cover replicas substrate were toreleased complete from microchannel the PMMA fabrication. micromold. For The PDMS COC microchannel replicas were casting, tape-bonded as presented [42] with in Figure another8b, PDMSpolycarbonate (10:1 PDMS (PC) base cover/curing substrate reagent) to complete was poured microchannel onto the PMMA fabrication. micromold For PDMS and heated microchannel at 60 ◦C forcasting, 5 h. Then,as presented the PDMS in replicaFigure was8b, PDMS released (10:1 from PDMS the PMMA base/curing micromold reagent) and was bonded poured with onto another the PDMSPMMA cover micromold layer using and heated an oxygen at 60 plasma°C for 5 process. h. Then, the PDMS replica was released from the PMMA micromold and bonded with another PDMS cover layer using an oxygen plasma process.

Polymers 2020, 12, x FOR PEER REVIEW 2 of 2 Polymers 2020, 12, 2574 11 of 15 7

(a)

(b)

Figure 8. (a) Thermoplastic microfluidic device fabrication by hot embossing. (a-i) Imprinted 8 COCFigure substrate 8. (a from) Thermoplastic a PMMA micromold, microfluidic (a-ii device) drill fabrication inlet/outlet by reservoirs, hot embossing. (a-iii) dry (a-i) adhesive Imprinted tape COC 9 appliedsubstrate to the from cover a substratePMMA micromold, (a-iv), bond (a-ii) to COC drill inlet/outlet layer, and ( a-vreservoirs,), completed (a-iii) hot dry embossing adhesive tape process; applied 10 (b) polydimethylsiloxaneto the cover substrate(PDMS) (a-iv), bond microfluidic to COC device layer, obtained and (a-v), by completed casting. (b-i hot) PDMS embossing from a process; PMMA (b) 11 micromoldpolydimethylsiloxane (b-ii), drill inlet (PDMS)/outlet reservoirsmicrofluidic (b-iii device) oxygen obtained plasma by treatmentcasting. (b-i) (b-iv PDMS), bond from to PDMS a PMMA 12 device,micromold and (b-v (b-ii),) completed drill inlet/outlet casting process. reservoirs (b-iii) oxygen plasma treatment (b-iv), bond to PDMS 13 device, and (b-v) completed casting process. Figure9 displays the COC microchannel hot embossing results obtained using a polymer micromold. Both optimized mechanically polished (0.3-mm pressing length) and chemically polished (4-min solvent polishing time) polymer micromolds were compared with polymer micromolds after milling (without polish, control sample). We measured the COC microchannel surface roughness and microchannel distortion percentage after imprinting, and 15 microchannel replications (imprint runs) were created to examine micromold production repeatability. Although the hot embossing temperature was lower than the PMMA micromold Tg, the micromold could still distort under 2 heat (95 ◦C) and pressure (20 kg/cm ) application during hot embossing because thermoplastic is a semicrystalline material. As shown in Figure9a, the shape of the mechanically polished micromold changed (distortion percentage: 4.4%) after 15 imprint runs. The distortion percentage of the chemically polished micromold was distorted 2.4% after 15 imprint runs. For the surface roughness, for the micromold without surface polishing, the roughness reduced from 0.43 to 0.35 µm. With surface polishing micromold, since surface defects were already removed during the polishing step, surface roughness remained almost constant after 15 imprint runs for both mechanically (0.14–0.15 µm) and chemically polished (0.17–0.18 µm) micromolds. Thus, compared with polymer micromolds after milling (without polishing, dark-blue bars in Figure9), both the mechanically and chemically polished micromolds had better repeatability and smoother surfaces after more polymer imprint runs. This also indicates that residue stress or surface defects on the CNC-milled micromold can effectively be removed by the proposed polishing methods for higher geometric stability.

PolymersPolymers2020, 202012, 2574, 12, x FOR PEER REVIEW 1212 of of 16 15

(c)

FigureFigure 9. (a )9. PMMA (a) PMMA micromold micromold distortion distortion percentage percentage and and (b ()b surface) surface roughness roughness afterafter 1515 imprint runs. runs. (c) Cross-sectional(c) Cross-sectional images images of micromold of micromold surfaces surfaces after after 1, 5,1, 10,5, 10, and and 15 15 imprint imprint runs. runs.

PDMSPDMS casting casting is another is another widely widely used used method method for for manufacturing manufacturing polymer polymer microﬂuidicmicrofluidic devices; it requiresit requires the usethe ofuse micromolds of micromolds to replicateto replicate PDMS PDMS microchannels. microchannels. For For PDMS PDMS casting,casting, becausebecause no pressure is applied in moderate thermal curing (60 °C), polymer micromolds have better repeatability pressure is applied in moderate thermal curing (60 ◦C), polymer micromolds have better repeatability and higher geometrical stability for PDMS casting procedures than other materials. Figure 10 shows and higher geometrical stability for PDMS casting procedures than other materials. Figure 10 shows the PMMA micromold distortion percentage based on five casting measurements (each individual the PMMA micromold distortion percentage based on ﬁve casting measurements (each individual measurement and corresponding photographs are displayed in Figure S5 of the Supporting measurement and corresponding photographs are displayed in Figure S5 of the Supporting Information). Information). Mechanically and chemically polished micromolds had minimum micromold Mechanicallydistortion and percentages chemically of polished1.01% ± micromolds0.76% and 1.10% had minimum ± 0.80%, respectively. micromold distortion This demonstrates percentages that of 1.01% 0.76% and 1.10% 0.80%, respectively. This demonstrates that polishing PMMA micromolds ± ± could improve their stability for casting compared with a no-polish condition (2.33% 2.66%). ±

Polymers 2020, 12, x FOR PEER REVIEW 13 of 16

polishing PMMA micromolds could improve their stability for casting compared with a no-polish Polymerscondition2020, 12 (2.33%, 2574 ± 2.66%). 13 of 15

Figure 10. PMMA micromold distortion percentage for PDMS casting in no-polish, mechanical polishing, Figure 10. PMMA micromold distortion percentage for PDMS casting in no-polish, mechanical and chemical polishing conditions. The error bars were obtained based on five individual measurements. polishing, and chemical polishing conditions. The error bars were obtained based on five individual 4. Conclusionsmeasurements. 4.Polymer Conclusions materials with advantages of high performance, low cost, and disposability have been widelyPolymer applied materials in thewith field advantages of polymer of high microfluidics. performance, low CNC cost, milling and disposability is a straightforward have been and effectivewidely method applied forin the fabricating field of polymer polymer microfluidics. microfluidic CNC devices. milling Microchannels is a straightforward can and be directly effective milled formethod prototyping for fabricating to rapidly polymer fabricate microfluidic microchannels devices. orMicrochannels use micromolds can be to directly manufacture milled for polymer microchannelprototyping replicas to rapidly in large fabricate quantities. microchannels However, or post use milling micromolds surface to marks manufacture may affect polymer microfluidic devicemicrochannel performance. replicas In this in study, large we quantities. introduced However, chemical post and mechanicalmilling surface surface marks polishing may approachesaffect for removingmicrofluidic surface device defects performance. on microchannels In this study, and we micromolds introduced afterchemical milling. and mechanical The results surface showed that polishing approaches for removing surface defects on microchannels and micromolds after milling. through chemical polishing, surface roughness values decreased from 0.38 µm (0 min, after milling) The results showed that through chemical polishing, surface roughness values decreased from 0.38 to 0.13 µm after 6 min of evaporation time. However, after 6 min of solvent polishing, the distortion μm (0 min, after milling) to 0.13 μm after 6 min of evaporation time. However, after 6 min of solvent percentagespolishing, of the microchannels distortion percentages and micromolds of microcha reachednnels and 32.3% micromolds and 53.1%, reached respectively. 32.3% and For 53.1%, mechanical polishing,respectively. because For themechanical polishing polishing, area was because locally the controlled polishing area by awas CNC locally router, controlled mechanical by a CNC polishing usingrouter, wool mechanical felt polishing polishing bits using provided wool favorablefelt polishing surface bits provided roughness favorable with smallersurface roughness geometric loss comparedwith smaller to chemical geometric approach. loss compared Surface to chemical roughness approach. was reduced Surface to roughness 0.165 µm was for reduced the optimal to 0.165 pressing lengthμm offor 0.3the mm.optimal After pressing wool length felt polishing,of 0.3 mm. After the microchannelwool felt polishing, distortion the microchannel percentage distortion was 14.50%, andpercentage micromold was distortion 14.50%, percentage and micromold was 11.80%, distorti whichon percentage successfully was demonstrates11.80%, which that successfully the mechanical polishingdemonstrates method that can bethe usedmechanical to polish polishing microchannel method and can micromold be used to with polish lower microchannel distortion percentageand micromold with lower distortion percentage compared to the chemical method. compared to the chemical method. Finally, we evaluated unpolished, chemically polished, and mechanically polished PMMA micromoldsFinally, we for evaluated thermoplastic unpolished, (COC) hot chemically embossing polished,and PDMS and casting mechanically replication. polishedThe results PMMA micromoldsrevealed distortion for thermoplastic percentages (COC) of 2.4% hot and embossing 4.4% for and chemically PDMS castingand mechanically replication. polished The results revealedmicromolds, distortion respectively, percentages after of 15 2.4% COC and imprint 4.4% runs. for chemically For PDMS ca andsting, mechanically because the related polished materials micromolds, respectively,are subject afterto less 15heat COC and pressure imprint application runs. For compared PDMS with casting, hot embossing because process, the related the geometric materials are subjectintegrity to less of the heat PMMA and pressuremicromold’s application structure was compared maintained with (with hot ~1% embossing variation) process, after each the casting. geometric integrityThe results of the presented PMMA micromold’s herein demonstrate structure that was by maintainedusing surface (with polishing ~1% methods variation) to aftersatisfy each low- casting. Thequantity results presented requirements, herein polymer demonstrate micromolds that by usingcan be surface rapidly polishing produced methods for PDMS to satisfy casting low-quantity and requirements,thermoplastic polymer hot embossing. micromolds can be rapidly produced for PDMS casting and thermoplastic hot embossing.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/11/2574/s1, Figure S1: Microscope images of PMMA surface using Ø500 µm diameter end mills with different spin speeds and feed rates; Figure S2: Microscope images of PMMA surface using Ø200 µm diameter end mills with different spin speeds and feed rates; Figure S3: Microscope images of COC surface using Ø500µm diameter end mills with different spin speeds and feed rates; Figure S4: Microscope images of COC surface using Ø200 µm diameter end mills with different spin speeds and feed rates; Figure S5: Cross-sectional images of PMMA micromold after 5 PDMS casting runs. Polymers 2020, 12, 2574 14 of 15

Author Contributions: C.-W.T. performed project administration, supervision, funding acquisition, design methodology, validation data, writing—review and editing. Z.-K.W. performed the experiments, data curation and analysis, plot table and figures. All authors have read and agreed to the published version of the manuscript. Funding: Ministry of Science and Technology (MOST), Taiwan, Grant No. MOST 107-2221-E-008-029 and 108-2221-E-008-028-MY2. Acknowledgments: The authors would like to thank the Ministry of Science and Technology (MOST), Taiwan, for financially supporting this project. In addition, Mr. You-Shan Zheng for assisting the surface profilometer measurements. Conflicts of Interest: The authors declare no conflict of interest.

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