Electrowetting-On-Dielectric Based Economical Digital Microfluidic

micromachines

Article Electrowetting-on-Dielectric Based Economical Digital Microﬂuidic Chip on Flexible Substrate by Inkjet Printing

He Wang 1,2 and Liguo Chen 1,*

1 Robotics and Microsystems Center, Soochow University, Soochow 215006, China; [email protected] 2 School of Mechanical Engineering, Henan University of Engineering, Zhengzhou 451191, China * Correspondence: [email protected]

Received: 27 November 2020; Accepted: 14 December 2020; Published: 16 December 2020

Abstract: In order to get rid of the dependence on expensive photolithography technology and related facilities, an economic and simple design and fabrication technology for digital microfluidics (DMF) is proposed. The electrodes pattern was generated by inkjet printing nanosilver conductive ink on the flexible Polyethylene terephthalate (PET) substrate with a 3D circuit board printer, food wrap film was attached to the electrode array to act as the dielectric layer and Teflon® AF was sprayed to form a hydrophobic layer. The PET substrate and food wrap film are low cost and accessible to general users. The proposed flexible DMF chips can be reused for a long time by replacing the dielectric film coated with hydrophobic layer. The resolution and conductivity of silver traces and the contact angle and velocity of the droplets were evaluated to demonstrate that the proposed technology is comparable to the traditional DMF fabrication process. As far as the rapid prototyping of DMF is concerned, this technology has shown very attractive advantages in many aspects, such as fabrication cost, fabrication time, material selection and mass production capacity, without sacrificing the performance of DMF. The flexible DMF chips have successfully implemented basic droplet operations on a square and hexagon electrode array.

Keywords: digital microfluidics; electrowetting-on-dielectric; inkjet printing; flexible substrate; dielectric wrap film; hexagon electrode

1. Introduction The digital microfluidics system has developed rapidly in last few years [1] in response to the need for handling and manipulating pico-to-microliter volume of fluids [2,3]. It has great potential for point-of-care applications such as in the field of biochemical analysis, clinical diagnosis, environmental monitoring and food safety analysis in recent years. In digital microfluidics (DMF) devices, discrete droplets containing different biochemical samples or reagents are typically manipulated for various analysis and detection by applying a series of electrical potentials to an array of electrodes coated with a dielectric layer and a hydrophobic layer, which is referred to as electrowetting-on-dielectric (EWOD), one of the most common actuation mechanisms. DMF have the capacity to address each droplet individually [4]. It can effectively avoid the cross-contaminations, greatly reduce reagent and sample consumption and the time of biochemical reactions, and achieve precise quantitative control. For the DMF chip, an array of patterned metal electrodes, the thin dielectric layer and the good hydrophobic layer must be processed. In the traditional fabrication technology, the patterned electrodes are fabricated by standard metal photolithography and sputtering on glass and silicon substrate, or etching on printed circuit boards based on copper substrate [5–9]. The former needs a clean-room

Micromachines 2020, 11, 1113; doi:10.3390/mi11121113 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 1113 2 of 11 facility while the latter adds to the complexity as it required additional chemical etching and the printed circuit board (PCB)-based DMF chip has the deep trenches between electrodes that maybe hinder the droplet movement [5]. The thin dielectric layer is deposited on the electrodes, which can be accomplished in different manners, depending on the dielectric material, such as vapor deposition of parylene or amorphous fluoropolymers, chemical vapor deposition of silicon nitride, spin coating of PDMS or SU-8, or thermal growth for silicon oxide [5,10–12]. The good hydrophobic layer is usually Teflon® AF by spin coating, but these generation methods of the dielectric and hydrophobic layer usually also require a clean-room facility. It is impossible for general users to manipulate the micromachining facilities required to fabricate microfluidic chips. There are only two channels for most users to obtain DMF chips in general. Either participate in relevant training courses provided by a few professional companies or research institutions, pass the assessment, and then manipulate the facilities to fabricate the chips by yourself, or entrust them to be fabricated by someone else. However, regardless of the methods, the services provided by these companies or institutions can easily become too expensive because they charge for equipment usage, chip design and fabrication, etc. Due to high material and fabrication cost, high time consumption, lack of rapid prototyping, stringent requirements on the clean-room facility, and difficulty of optical detection, the traditional fabrication process of a digital microfluidic device is a bottleneck for the DMF accessibility in laboratories, which decrease its superiority and in particular are not well fit for point-of-care testing (POCT) for low-resource settings, such as remote areas and developing nations. The development of a low cost DMF chip, selection and utilization of the simple material deposition and improvement of the droplet operation and detection performance are major challenges for the existing fabrication process. With the rapid development of printing technology, it has evolved from the office application to an important tool for industrial production. Printed electronics is widely gaining much attention and is increasingly used in flexible MEMS devices [13–16]. Therefore, the DMF chips on various flexible substrates via printing technology have recently emerged as a novel platform for simple, portable and low-cost DMF devices [17–20]. Two printing technologies, inkjet printing and screen printing, have been demonstrated to fabricate DMF Electrodes on flexible substrates [21–27]. In some reported literatures [21–24], although inkjet printing has replaced photolithography technology to process the electrode and got rid of the dependence on photolithography facilities and clean-rooms, high-cost dielectric material (parylene-C) and hydrophobic material (Teflon® AF) are used. The coating of the two materials still needs to rely on expensive facilities and time-consuming process technologies such as chemical vapor deposition and spin coating, so it is still expensive from the perspective of cost. In addition, with such dielectric layer and hydrophobic layer coating technologies, once dielectric breakdown, poor hydrophobic effect or other chip failure occurs, the chip is very likely to be scrapped and cannot be reused, which will lead to the increase in the use cost of the chip. In other studies [25–27], electrodes have been fabricated by screen printing, and cheap dielectric and hydrophobic material (e.g., parafilm, transparent adhesive tape, rain repellent and SI 7200) are used. However, the original thickness of parafilm and transparent adhesive tape is 120 µm and 30 µm, and the initial contact angle without applied voltage is small. At such a high dielectric layer thickness and small contact angle, the droplet operation either sacrifices its movement velocity or requires a high voltage (300 Vrms). Following the DMF fabrication principle based on “out of clean-room”, we introduce a simple, convenient and low-cost way to fabricate a DMF chip which is barely restricted by the harsh fabrication environment as the traditional processing technology, especially suitable for household use or low-resource settings. Inkjet printing replaces photolithography to form electrodes, cheap food wrap film replaces expensive traditional dielectric materials to form dielectric layer, and then the hydrophobic layer is generated by spray-coating Teflon® AF. Through the combination of the above three simplified material deposition methods, we successfully fabricated the economical flexible DMF chips (FDMFC). Micromachines 2020, 11, 1113 3 of 11 Micromachines 2020, 11, x FOR PEER REVIEW 3 of 11

2.2. Design, Design, Materials Materials and and FabricationFabrication Methods for for FDMFC FDMFC

2.1.2.1. Chip Chip Design Design TheThe FDMFC FDMFC are are shown shown inin FigureFigure1 1.. TwoTwo didifferentfferent chip chip structure structure diagrams diagrams are are designed. designed. Design Design I includesI includes 1 1 reservoir reservoir electrodes (2.4 (2.4 mm mm × 2.72.7 mm), mm), 12 square 12 square driving driving electrodes electrodes (1 mm (1 × mm1 mm) and1 mm) × × and4 dispensing 4 dispensing electrodes electrodes with with special special shapes, shapes, and and design design II features II features 32 32hexagon hexagon driving driving electrodes electrodes withwith a sidea side length length of 0.707 of 0.707 mm. mm. The gapThe betweengap between adjacent adjacent electrodes electrodes and the and width the ofwidth traces of connecting traces electrodesconnecting to contactelectrodes pads to incontact both designs pads in areboth both designs 180 µ m.are The both bottom 180 μm. plate The is bottom formed plate by inkjet-printing is formed anby array inkjet-printing of patterned an electrodes array of patterned with nanosilver electrodes conductive with nanosilver ink on PETconductive substrate. ink weon PET chose substrate. household foodwe wrapchose filmshousehold attached food towrap the films electrodes attached as theto the dielectric electrodes layer as the to simplydielectric the layer deposition to simply process; the deposition process; furthermore, although Teflon® AF has been commonly used for the deposition of furthermore, although Teflon® AF has been commonly used for the deposition of hydrophobic layer hydrophobic layer by spin coating, we tried a simpler and cheaper way to fabricate the hydrophobic by spin coating, we tried a simpler and cheaper way to fabricate the hydrophobic layer, that is, to spray layer, that is, to spray Teflon® AF on the dielectric layer with a common spray bottle. The top plate is Teflon® AF on the dielectric layer with a common spray bottle. The top plate is composed of an indium composed of an indium tin oxide (ITO) glass slide coated with Teflon® AF. These simplified material tin oxide (ITO) glass slide coated with Teflon® AF. These simplified material deposition methods allow deposition methods allow general users to easily fabricate a FDMFC, especially at home or in generallow-resource users to settings. easilyfabricate a FDMFC, especially at home or in low-resource settings.

FigureFigure 1. 1.Chip Chip design. design. (a ()a Photos) Photos of of ﬂexible flexible DMF DMF chips chips (FDMFC) (FDMFC) structure structure diagram diagram with with design design I I and designand design II drawn II bydrawn Microsoft by Microsoft Visio; (b) PhotosVisio; ( ofb) thePhotos chip printedof the chip on PET printed substrate on correspondingPET substrate to (a);corresponding (c) Side-view to schematics (a); (c) Side-view of a FDMFC. schematics of a FDMFC.

2.2.2.2. Materials Materials NanosilverNanosilver particle particle conductiveconductive ink and PET PET su substratesbstrates are are purchased purchased from from Beijing Beijing BroadTeko BroadTeko IntelligentIntelligent Technology Technology Co., Co., Ltd. Ltd. (Beijing, (Beijing, China). China). The The particle particle size, size, viscosity viscosity and and surface surface tensiontension ofof the conductivethe conductive ink are ink less are than less 50than nm, 50 5–20 nm, cP5–20 and cP 27–33 and 27–33 dyn/cm, dyn/cm, respectively. respectively. After After the ink the is ink printed, is printed, it needs to be sintered at 120° for 30 min. We selected a variety of food wrap films that are it needs to be sintered at 120◦ for 30 min. We selected a variety of food wrap films that are portable and easilyportable accessible and easily on the accessible internet on and the in internet the supermarket and in the supermarket as dielectric layers.as dielectric We brought layers. We four brought brands of PVDCfour wrapbrands films of PVDC (Saran wrap Wrap, films Tokyo, (Saran Japan; Wrap, Uncle Tokyo, Tom, Japan; Los Angeles, Uncle Tom, CA, USA;Los Angeles, Miaojie, CA, Wuxi, USA; China; Miaojie, Wuxi, China; Youmiao, JiaXing, China) and PMP wrap film (Miaojie, Wuxi, China). The Youmiao, JiaXing, China) and PMP wrap film (Miaojie, Wuxi, China). The Teflon® AF 1600 (Dupont, Teflon® AF 1600 (Dupont, Wilmington, NC, USA) are used as the hydrophobic layer. Silicon oil with Wilmington, NC, USA) are used as the hydrophobic layer. Silicon oil with a kinematic viscosity of a kinematic viscosity of 10 cst (Dow Corning, Midland, MI, USA) acts as lubrication and medium 10 cst (Dow Corning, Midland, MI, USA) acts as lubrication and medium filling in FDMFC. filling in FDMFC.

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2.3. Selection of Flexible Substrate As the carrier of electronic circuits, the substrate’s heat resistance, chemical resistance, flexibility and other properties directly determine its applicability in the field of printed electronics. At present, in related studies at home and abroad, the applicable substrates for inkjet-printing nano conductive inks are mainly focused on photo paper, plastic (such as PET, Polyimide (PI)), and glass. These substrates have their own characteristics. Photo paper is cheap, accessible, environmentally friendly, and bendable. It has great advantages in the fields of disposable printed electronics and flexible printed electronics, especially RC high glossy photo paper which is the preferred material for inkjet printing of conductive patterns [21]. However, its high temperature resistance is poor, and the surface is prone to cracks when the temperature is higher than 120 ◦C. There are many types of plastic, of which PET- and PI-substrate can be used for printed electronic technology. PET substrate is cheap, but its high temperature resistance is slightly poor. If it is heated at 150 ◦C for a long time, irreversible deformation will occur. However, the heating temperature is lower than 150 ◦C in the proposed DMF fabrication technology. PI substrate, as a promising engineering plastic, can withstand a high temperature of about 500 ◦C, but it is expensive. Glass is a rigid substrate and cannot be bent, which is contrary to our intention to choose a flexible substrate. Therefore, we finally chose PET as flexible substrate. Due to the smooth surface structure of PET substrate, it is difficult to obtain good ink adhesion in printing, which makes the conductive ink layer deposited on the substrate uneven, resulting in poor conductivity of the driving electrode [22,23]. Therefore, the surface of PET substrate was pretreated before printing. The substrate is coated with a layer of water-based coating which is a film-forming dispersion with good leveling, great compatibility, high stability and good film-forming property, and can be used on various substrate surfaces. The substrate is baked for about 15 min at 100 ◦C after coating.

2.4. Inkjet Printing of Patterned Electrode Array An array of patterned electrodes was designed by vector drawing software (Microsoft Visio 2016, Microsoft office, Redmond, WA, USA), and then converted into 1-bit TIF diagram. The electrode diagram was inkjet-printed by a 3D circuit board printer (BroadJET L3000, Beijing BroadTeko Intelligent Technology Co., Ltd.). The printer is equipped with 1440 industrial nozzles, the smallest ink drop is 3.5 pL, the mechanical repeatability is 0.01 mm, the maximum printing size is 297 mm 210 mm, × the printing accuracy, speed and resolution are 0.05–0.075 mm, 250 mm/s, 1440 1440 dpi, respectively. × Inkjet printing has a sponge adsorption effect, so the printed lines are often imprinted, especially for the conductive ink, which affects the conductivity of the lines [22]. The usual solution is a multi-layer printing mode to get sufficient conductivity. We printed the electrode diagram with different layers to study the effect of printing of silver traces on conductivity. For each printing, it was dried before printing the next time. After the last printing, the conductive ink on the substrate was sintered at 120◦ for 30 min to ensure the solidification of silver electrodes and reduce the resistance [24,25]. We used a scanning electron microscope (SEM) (Quanta 250, FEI, Hillsboro, OR, USA), atomic force microscope (AFM) (Dimension FastScan, Bruker, Karlsruhe, Germany) and digital multimeter to characterize the printed electrodes. Furthermore, the PET substrate is fixed onto a glass slide with double-sided adhesive tape after printing the electrodes to ensure the flatness of the FDMFC.

2.5. Preparation of Food Wrap Film as the Dielectric Layer At present, there are three types of food wrap films commonly used in the market, namely polyethylene (PE), polyvinylidene chloride (PVDC) and polymethylpentene (PMP). Teflon® AF, as the hydrophobic layer, is coated on the food wrap film and heated at high temperature to remove all solvent, which can give a smoother coating and improves the polymer’s adherence to the surface. The heat resistance temperature of PE, PVDC and PMP wrap films are 110 ◦C, 140 ◦C and 180 ◦C, respectively. Therefore, we chose PVDC and PMP wrap films as the dielectric layer. The thickness of the four brands of PVDC films we purchased are all about 12 µm, while that of PMP film is slightly Micromachines 2020, 11, 1113 5 of 11 thinner, about 10.5 µm. To ensure a close attachment between the dielectric film and the electrode array without air bubbles, dielectric film is tightened and adhered to the glass slide with double-sided tape until there are no wrinkles, and a thin layer of silicone oil coats the electrode array and PET substrate before the dielectric film is adhered to the glass slide. Micromachines 2020, 11, x FOR PEER REVIEW 5 of 11 2.6. Coating of Hydrophobic Layer thinner, about 10.5 μm. To ensure a close attachment between the dielectric film and the electrode Takingarray intowithout account air bubbles, the characteristics dielectric film of dielectricis tightened wrap and films,adhered spray-coating to the glass isslide used with instead of spin-coatingdouble-sided to coat tape a layer until of there Teflon are® noAF wrinkles, on the and dielectric a thin layer films. of Placesilicone the oil dielectric coats the electrode film vertically, array spray and PET substrate before the dielectric film is adhered to the glass slide. Teflon® AF evenly from up to down and from left to right with a common spray bottle, wait for a few ® minutes2.6. to Coating allow excessof Hydrophobic Teflon LayerAF to flow away along its surface and heat to form a hydrophobic film. ® The contactTaking angle ofinto the account droplet the on characteristics PVDC and of PMP dielectr wrapic wrap film coatedfilms, spray-coating with Teflon is usedAFwas instead measured of by the freespin-coating software Image to coat J a through layer of Teflon the droplet® AF on images the dielectric collected films. by Place a CCD the dielectric camera film (HV315UC, vertically, Daheng, Beijing,spray China). Teflon® AF evenly from up to down and from left to right with a common spray bottle, wait for a few minutes to allow excess Teflon® AF to flow away along its surface and heat to form a 3. Resultshydrophobic and Discussions film. The contact angle of the droplet on PVDC and PMP wrap film coated with Teflon® AF was measured by the free software Image J through the droplet images collected by a 3.1. PerformanceCCD camera of the(HV315UC, Electrodes Daheng, and Silver Beijing, Traces China). Inkjet-Printed on PET Substrate

Electrical3. Results conductivity and Discussions and resolution are critical to the performance of inkjet printing FDMFC. High-quality conductivity and resolution can effectively avoid chip failure. Based on the characteristics of inkjet3.1. printing, Performance silver of the traces Electrodes with and a small Silver width Traces willInkjet-Printed increase on the PET probability Substrate of electrical disconnection and small gapsElectrical between conductivity electrodes and resolution may easily are critical overlap to the with performance each other. of inkjet However, printing a FDMFC. large gap will increaseHigh-quality the difficulty conductivity for the droplet and resolution to cross the can adjacent effectively electrode. avoid chip The 3Dfailure. circuit Based board on printer the used here hascharacteristics two printing of modes: inkjet printing, large ink silver drop traces and wi smallth a inksmall drop. width Comparing will increase the the two probability different of printing electrical disconnection and small gaps between electrodes may easily overlap with each other. modes, it is found that the conductivity of silver traces printed by small ink drops and the gap between However, a large gap will increase the difficulty for the droplet to cross the adjacent electrode. The the traces3D significantlycircuit board printer outperform used here those has by two large printing ink drops. modes: After large the ink small drop inkand dropletssmall ink aredrop. dried and solidified,Comparing the ink the dots two are different smaller, printing and the modes, coverage it is found between that inkthe dropletsconductivity is moreof silver uniform, traces which effectivelyprinted improves by small the ink conductivity; drops and the the gap di betweenffusion the of smalltraces inksignificantly drops on outperform the PETsubstrate those by large is relatively lower. Whenink drops. printing After the parallel small silverink droplets traces, are the dried small and ink solidified, dots scattered the ink dots between are smaller, the traces and reducethe the connectioncoverage probability between ofink the droplets traces. is Therefore,more uniform, the which small effectively ink drop improves printing the mode conductivity; is more suitablethe for diffusion of small ink drops on the PET substrate is relatively lower. When printing parallel silver FDMFCtraces, in terms the small of electrical ink dots conductivity scattered between and resolution.the traces reduce The qualitythe connection of the patternedprobability electrodesof the by inkjet printingtraces. Therefore, with small the small ink drop ink drop was printing evaluated mode by is more SEM suitable and AFM for FDMFC images in in terms Figure of electrical2. The images show thatconductivity cracks and and small resolution. pits occur The quality after sinteringof the pattern in singleed electrodes printing, by inkjet which printing makes with the small silver ink electrode layer discontinous,drop was evaluated and thereby SEM are and some AFM irregular images in scatteredFigure 2. The particles images show of ~50 thatµ mcracks dia. and We small speculated that thepits particles occur after are sintering silver-salt in single precipitates printing, formed which makes during the thesilver sintering electrode process,layer discontinous, inevitably and resulting there are some irregular scattered particles of ~50 μm dia. We speculated that the particles are in the surface of the electrode layer being uneven; however, after multi-layer printing, there are no silver-salt precipitates formed during the sintering process, inevitably resulting in the surface of the cracks,electrode pits or scattered layer being silver uneven; particles, however, forming after bettermulti-layer surface printing, uniformity there are of theno silvercracks, electrodepits or layer. Therefore,scattered the multi-layer silver particles, printing forming mode better can surface effectively uniformity avoid of thethe surfacesilver electrode defects layer. of the Therefore, silver electrode layer inthe the multi-layer single printing. printing mode AFM can images effectively are used avoid to the assess surface the defects surface of the roughness silver electrode of silver layer in electrode layer. Thethe multi-layersingle printing. printing AFM images mode are reduces used to the assess surface the surface roughness roughness (Ra) of to silver about electrode 20 nm. layer. The multi-layer printing mode reduces the surface roughness (Ra) to about 20 nm.

(a)

Figure 2. Cont.

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(b)

Figure 2. SilverFigure electrode2. Silver electrode printed printed on PET on substrate PET substrate for difoffr erentdifferent numbers numbers of of inkjet inkjet printing.printing. (a (a,b,)b ) scanning scanning electron microscope (SEM) and atomic force microscope (AFM) images of electrode at one, electron microscope (SEM) and atomic force microscope (AFM) images of electrode at one, two and two and three printing. three printing. The output spatial resolution and conductivity of the electrodes and silver traces can be The outputobserved spatial in Figure resolution 3. It shows and the conductivitycontrast and clarity of the of electrodessilver traces and with silver different traces width can and be observed in Figure3electrodes. It shows with the different contrast gaps. and When clarity the gap of between silver tracessilver traces with is dismall,fferent there width are small and scattered electrodes with different gaps.ink droplets. When It is the notgap until betweenthe gap increases silver to traces 150 μm is that small, the gap there in both are horizontal small scattered and vertical ink droplets. directions can be clearly distinguished. The actual width is 40% larger and the actual gap is 40% It is not untilsmaller the than gap the increases design value. to 150Figureµm 3b that demonstrat the gapes the in bothresistance horizontal of silver traces and verticalwith different directions can be clearlywidth distinguished. for different printing The actual numbers. width With is the 40% increase larger in trace and width the actual (from 80 gap μm isto 40%300 μ smallerm), the than the design value.resistance Figure of the3b silver demonstrates trace gradually the resistancedecreases and of the silver silver traces trace with with the di largerfferent width width shows for different printing numbers.better continuity With and the conductivity. increase inMoreover, trace widthregardless (from of trace 80 µwidth,m to the 300 averageµm), resistance the resistance of of the silver traces decreases with the increase in number of prints. The resistance was greatly reduced for silver tracetwo gradually printings and decreases was slightly and lower the silver for three trace printings. with the When larger printing width once, shows since betterless silver continuity is and conductivity.deposited Moreover, on the substrate, regardless the silver of trace film width, is not uniform the average and the resistance silver traces of may silver be broken, traces decreases with the increaseresulting in in high number resistance. of prints. The main The reason resistance for the sharp was greatlydecrease reducedin trace resistance for two for printings the two and was slightly lowerprintings for is three that the printings. second printi Whenng constitutes printing a multi-layer once, since printi lessng silvermode, depositing is deposited more on silver, the substrate, which improves the compactness of the microstructure of traces and strengthens the connectivity the silverbetween film is not silver uniform particles andto form the a silvercontinuous traces cond mayuctive be path. broken, When resulting the number in of high prints resistance. continues The main reason forto theincrease, sharp although decrease the increase in trace in silver resistance helps the forformation the two of clear printings and complete is that silver the traces, second the printing constitutesamount a multi-layer of resin in printingthe ink also mode, increases. depositing Too much more resin silver, can affect which the contact improves between the the compactness of the microstructureconductive fillers, of so traces there is and no significant strengthens reduct theion in connectivity average resistance between for the third silver printing. particles The to form a average resistance of the silver traces with 80 μm width processed by photolithography is about continuous40.19 conductive Ω while that path. of two When and three the printing number are of41.38 prints Ω and continues 33.54 Ω, respectively, to increase, which although shows that the increase in silver helpsthe conductivity the formation in multilayer of clear printing and mode complete is comparable silver to traces, that in thephotolithography amount of technology. resin in the ink also increases.Although Too much the resinincrease can in aprintingffect the number contact can between improve the conductivity, conductive it fillers, will reduce so there the output is no significant reductionspatial in average resolution, resistance which increases for the thirdthe probability printing. of Theoverlap average between resistance electrodes of in thethe silverprinted traces with pattern of FDMFCs, thereby affecting the performance of droplet operations and resulting in 80 µm widthmisoperation. processed Therefore, by photolithography the three printing ismode about is used 40.19 in allΩ thewhile following that ofFDMFC two andexperiments. three printing are 41.38 Ω andThe 33.54thicknessΩ, of respectively, the silver trace whichin this printing shows mode that is the about conductivity 110 μm. in multilayer printing mode is comparable to that in photolithography technology. Although the increase in printing number can improve the conductivity, it will reduce the output spatial resolution, which increases the probability of overlap between electrodes in the printed pattern of FDMFCs, thereby affecting the performance of droplet operations and resulting in misoperation. Therefore, the three printing mode is used in all the following FDMFC experiments. The thickness of the silver trace in this printing mode is about 110 µm. Micromachines 2020, 11, x FOR PEER REVIEW 7 of 11

Figure 3.FigureCharacterization 3. Characterization of resolution of resolution and and conductivity conductivity of of the the inkjet-printed inkjet-printed FDMFCs. FDMFCs. (a) Photo (a) of Photo of printed silver traces showing gradients of trace widths and gap between traces (from 80 to 300 μm) in printed silver traces showing gradients of trace widths and gap between traces (from 80 to 300 µm) horizontal and vertical directions. (b) average resistance of silver traces as a function of trace width in horizontalfor different and vertical numbers directions. of inkjet printing (b) average on PET substrate. resistance of silver traces as a function of trace width for diﬀerent numbers of inkjet printing on PET substrate. 3.2. The Properties of Dielectric Film and Hydrophobic Layer

For a droplet on the printed electrode with dielectric film and Teflon® AF, the contact angle (CA) was measured as a function of the applied voltage by the EWOD device (Figure 4a) to explore the dielectric properties of PVDC and PMP wrap film coated Teflon® AF. The spreading images of the droplet was collected by a digital camera and processed by ImageJ software. As shown in Figure 4b,c, the initial CA under no applied voltage is close to 120 °C, which is consistent with the results in previous literatures [26–28]. The change trend of CA of the droplet on two dielectric films versus the applied AC voltage at 400 Hz is very similar and both of CAs reach saturation when the applied voltage is higher than 180 Vrms. Through droplet movement experiments, it was found that for the printed chips of two dielectric films, the droplet started to move only when the applied voltage reached 70 Vrms, but it is extremely slow; it is not until the applied voltage increases to 90 Vrms that the droplet can move smoothly, as shown in Figure 5a. Such a voltage is higher than that required on the chips fabricated by the traditional photolithography technology. It is due to the excessive thickness of PVDC and PMP wrap film (both greater than 10 μm) that a higher voltage is required to drive the droplets. Under the same voltage, the velocity of droplets on two kinds of FDMFC is equivalent. Therefore, the two kinds of food wrap films are suitable for FDMFC in term of CA and the velocity of the droplet.

Figure 4. (a) electrowetting-on-dielectric (EWOD) device, (b) the contact angle of 3 μL potassium permanganate droplet on polyvinylidene chloride (PVDC) and polymethylpentene (PMP) wrap film coated Teflon® AF 1600 versus the applied AC voltages at 400 Hz, (c) the spreading photos of 3 μL potassium permanganate droplet on PVDC and PMP wrap film coated Teflon® AF 1600 under the applied AC voltage.

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Figure 3. Characterization of resolution and conductivity of the inkjet-printed FDMFCs. (a) Photo of printed silver traces showing gradients of trace widths and gap between traces (from 80 to 300 μm) in horizontal and vertical directions. (b) average resistance of silver traces as a function of trace width Micromachinesfor different2020, 11numbers, 1113 of inkjet printing on PET substrate. 7 of 11

3.2. The Properties of Dielectric Film and Hydrophobic Layer 3.2. The Properties of Dielectric Film and Hydrophobic Layer For a droplet on the printed electrode with dielectric film and Teflon® AF, the contact angle For a droplet on the printed electrode with dielectric film and Teflon® AF, the contact angle (CA) (CA) was measured as a function of the applied voltage by the EWOD device (Figure 4a) to explore was measured as a function of the applied voltage by the EWOD device (Figure4a) to explore the the dielectric properties of PVDC and PMP wrap film coated Teflon® AF. The spreading images of dielectric properties of PVDC and PMP wrap film coated Teflon® AF. The spreading images of the the droplet was collected by a digital camera and processed by ImageJ software. As shown in Figure droplet was collected by a digital camera and processed by ImageJ software. As shown in Figure4b,c, 4b,c, the initial CA under no applied voltage is close to 120 °C, which is consistent with the results in the initial CA under no applied voltage is close to 120 C, which is consistent with the results in previous literatures [26–28]. The change trend of CA of the◦ droplet on two dielectric films versus the previous literatures [26–28]. The change trend of CA of the droplet on two dielectric films versus applied AC voltage at 400 Hz is very similar and both of CAs reach saturation when the applied the applied AC voltage at 400 Hz is very similar and both of CAs reach saturation when the applied voltage is higher than 180 Vrms. Through droplet movement experiments, it was found that for the voltage is higher than 180 V . Through droplet movement experiments, it was found that for the printed chips of two dielectricrms films, the droplet started to move only when the applied voltage printed chips of two dielectric films, the droplet started to move only when the applied voltage reached reached 70 Vrms, but it is extremely slow; it is not until the applied voltage increases to 90 Vrms that the 70 V , but it is extremely slow; it is not until the applied voltage increases to 90 V that the droplet dropletrms can move smoothly, as shown in Figure 5a. Such a voltage is higher than thatrms required on the can move smoothly, as shown in Figure5a. Such a voltage is higher than that required on the chips chips fabricated by the traditional photolithography technology. It is due to the excessive thickness fabricated by the traditional photolithography technology. It is due to the excessive thickness of PVDC of PVDC and PMP wrap film (both greater than 10 μm) that a higher voltage is required to drive the and PMP wrap film (both greater than 10 µm) that a higher voltage is required to drive the droplets. droplets. Under the same voltage, the velocity of droplets on two kinds of FDMFC is equivalent. Under the same voltage, the velocity of droplets on two kinds of FDMFC is equivalent. Therefore, Therefore, the two kinds of food wrap films are suitable for FDMFC in term of CA and the velocity ofthe the two droplet. kinds of food wrap films are suitable for FDMFC in term of CA and the velocity of the droplet.

Figure 4. ((aa)) electrowetting-on-dielectric (EWOD) (EWOD) device, device, ( (bb)) the the contact contact angle angle of of 3 μµL potassium permanganate droplet on polyvinylidene chloride (PVDC) and polymethylpentene (PMP) wrap film film ® coated Teflon Teflon® AFAF 1600 1600 versus versus the the applied applied AC AC voltages voltages at at 400 400 Hz, Hz, ( (cc)) the the spreading spreading photos photos of of 3 3 μµL ® potassium permanganate droplet on PVDCPVDC and PMP wrap filmfilm coatedcoated TeflonTeflon® AF 1600 under the applied AC voltage.

3.3. Droplet Operations on FDMFC According to Figure 1a,c, we fabricated FDMFCs economically and easily. The single FDMFC costs approximately $3, which includes material and technician time, and is much lower than that fabricated by traditional photolithography technology. Such a low cost and simple technology enable users to get rid of the dependence on expensive photolithography technology and related facilities, which is the first advantage of DMF fabrication technology proposed in this paper. Moreover, it takes only one hour to fabricate a chip by the proposed technology. In addition, the FDMFC is not disposable, but can be used repeatedly. If necessary, just replace the dielectric wrap film coated with hydrophobic layer, and the printed FDMFC can continue to be used. This is far Micromachinesdifferent from2020 ,the11, 1113traditional dielectric coating method in which the dielectric material (such8 of 11as Parylene-C) is attached to the electrodes. This feature is the second advantage of the new DMF fabrication technology. The FDMCs processed by this technology can realize stable and smooth Thisdroplet feature movement. is the second Droplet advantage operat ofions the newcan DMFbe performed fabrication successf technology.ully Theon square FDMCs and processed hexagon by thiselectrode technology arrays can on realizePVDC- stable and PMP-based and smooth FDMFC. droplet movement. Droplet operations can be performed successfully on square and hexagon electrode arrays on PVDC- and PMP-based FDMFC. 3.3.1. Droplet Operations on Square Electrodes 3.3.1. Droplet Operations on Square Electrodes Figure 5a shows the velocity of a droplet on PVDC- and PMP-based FDMFC. The velocity of the Figure5a shows the velocity of a droplet on PVDC- and PMP-based FDMFC. The velocity of the droplets on two chips is similar, which indicates that the performances of PVDC- and PMP-based droplets on two chips is similar, which indicates that the performances of PVDC- and PMP-based FDMFC are equivalent. With the AC voltage of 100 Vrms at 400 Hz, a 1 μL droplet was actuated along FDMFC are equivalent. With the AC voltage of 100 V at 400 Hz, a 1 µL droplet was actuated along square electrode array on a PVDC-based FDMFC, asrms shown in Figure 5b (Video S1). One of the main square electrode array on a PVDC-based FDMFC, as shown in Figure5b (Video S1). One of the main functions of DMF chips is to be used for biochemical analysis. Mixing plays an important role in functions of DMF chips is to be used for biochemical analysis. Mixing plays an important role in biochemical analysis because the time required for mixing is dominant [29]. Figure 5c demonstrates biochemical analysis because the time required for mixing is dominant [29]. Figure5c demonstrates a complete droplet mixing operation at the same AC voltage. Two droplets of different sizes were a complete droplet mixing operation at the same AC voltage. Two droplets of diﬀerent sizes were simultaneously actuated to move along their respective paths and merge at the intermediate simultaneously actuated to move along their respective paths and merge at the intermediate electrode. electrode. The merged droplet moved back and forth on the liner electrode array mixer to accelerate The merged droplet moved back and forth on the liner electrode array mixer to accelerate the mixing the mixing process until it was completely mixed (Video S2). In the whole process of droplet process until it was completely mixed (Video S2). In the whole process of droplet operation, even when operation, even when the AV voltage is increased to 300 Vrms, there is no dielectric breakdown, the AV voltage is increased to 300 V , there is no dielectric breakdown, which shows that the problem which shows that the problem of dielectricrms breakdown is not prone to happen on this kind of DMF of dielectric breakdown is not prone to happen on this kind of DMF chip. chip.

FigureFigure 5.5.Demonstration Demonstration of of droplet droplet operations operations on theon squarethe square electrode electrode array ofarray FDMFC. of FDMFC. (a) the velocity (a) the ofvelocity droplet of on droplet PVDC- on and PVDC- PMP-based and PMP-based FDMFCs. FDMFCs. (b,c) Series (b of,c) videoSeries frames of video displaying frames displaying transport, mergingtransport, and merging mixing and of dropletsmixing of on droplets a liner square on a liner electrode square array electrode of PVDC-based array of PVDC-based FDMFC under FDMFC the rms ACunder voltage the AC of 100voltage Vrms ofat 100 400 V Hz. at 400 Hz.

3.3.2. Droplet Operations on Hexagon Electrodes The hexagon is more similar to the droplet shape in the closed DMF than the square, so the hexagon electrode array can oﬀer many advantages over the square electrode array in droplet movement, mixing and splitting operations and velocity, etc. [30–32]. For example, the movement of the droplet on the former is more ﬂexible than that on the latter, since the droplet theoretically has six movement directions on the former while it only has four movement directions on the latter, as shown in Figure6a. Figure6b demonstrates that the droplet moved freely in zigzag mode or along parallel direction on the hexagon electrode array at the AC voltage of 100 Vrms (Video S3). Micromachines 2020, 11, x FOR PEER REVIEW 9 of 11

3.3.2. Droplet Operations on Hexagon Electrodes The hexagon is more similar to the droplet shape in the closed DMF than the square, so the hexagon electrode array can offer many advantages over the square electrode array in droplet movement, mixing and splitting operations and velocity, etc. [30–32]. For example, the movement of the droplet on the former is more flexible than that on the latter, since the droplet theoretically has six movement directions on the former while it only has four movement directions on the latter, as Micromachinesshown in 2020Figure, 11, 11136a. Figure 6b demonstrates that the droplet moved freely in zigzag mode or along9 of 11 parallel direction on the hexagon electrode array at the AC voltage of 100 Vrms (Video S3).

Figure 6. Demonstration of droplet operations on the hexagon electrode array of FDMFC. (a) Figure 6. Demonstration of droplet operations on the hexagon electrode array of FDMFC. (a) direction direction of the droplet movement on square and hexagon electrode array. (b) the movement of the of the droplet movement on square and hexagon electrode array. (b) the movement of the droplet in droplet in zigzag mode or along parallel direction on the hexagon electrode array at the AC voltage zigzag mode or along parallel direction on the hexagon electrode array at the AC voltage of 100 Vrms. of 100 Vrms. 4. Conclusions 4. Conclusions We introduced a simplified technology for the fabrication of DMF in which the electrodes We introduced a simplified technology for the fabrication of DMF in which the electrodes were were generated by inkjet printing nanosilver conductive ink on the flexible PET substrate, PVDC generated by inkjet printing nanosilver conductive ink on the flexible PET substrate, PVDC− and − and PMP food wrap film was used as the dielectric layer and Teflon® AF was sprayed to form a PMP−food− wrap film was used as the dielectric layer and Teflon® AF was sprayed to form a hydrophobic layer. As long as the patterned electrodes can be drawn by drawing software, and within hydrophobic layer. As long as the patterned electrodes can be drawn by drawing software, and the resolution range of the printer, they can be printed out by inkjet printing. Therefore, the inkjet within the resolution range of the printer, they can be printed out by inkjet printing. Therefore, the printing method facilitates electrodes with variable and complicate shapes which undoubtedly extends inkjet printing method facilitates electrodes with variable and complicate shapes which theundoubtedly application rangeextends of the DMF. application The replacement range of ofDM traditionalF. The replacement dielectric materialof traditional with dielectric food wrap filmmaterial greatly with increases food wrap the utilizationfilm greatly rate increases of DMF the chips utilization and further rate of DMF reduces chips the and fabrication further reduces and use ® costs.the fabrication In addition, and spray-coating use costs. In addition, Teflon AFspray-coating with a common Teflon® spray AF with bottle a common can break spray away bottle from can the dependencebreak away on from expensive the dependence spin-coating on expensive facilities. Thespin-coating proposed facilities. design The and proposed fabrication design technology and of FDMFCsfabrication is technology superior toof theFDMFCs reported is superior rapid prototyping to the reported technology rapid prototyping of DMF intechnology term of cost,of DMF time, materialin term selection, of cost, chiptime, utilizationmaterial selection, and production chip utilization capacity, and which production is more accessiblecapacity, which to general is more users of DMF.accessible FDMFCs to general was demonstrated users of DMF. to performFDMFCs variouswas demonstrated droplet operations to perform on the various electrode droplet arrays withoperations different on shapes the electrode by EWOD, arrays such with as different transport, shapes merging by EWOD, and mixing. such as We transport, infer that merging the FDMFCs and designmixing. and We fabrication infer that technology the FDMFCs will design become and an fabrication important developmenttechnology will direction become in an the important future and actdevelopment as a promising direction alternative in the to future the traditional and act as technology.a promising alternative to the traditional technology.

SupplementarySupplementary Materials: Materials:The The following following are are available available online online at http:at www.mdpi.com/xxx/s1,//www.mdpi.com/2072-666X Video /S1:11/ 12Droplet/1113/ s1, Videotransport S1: Droplet on thetransport square electrode on the squarearray, Video electrode S2: Merging array, Video and mixing S2: Merging of twoand droplets mixing on ofthe two square droplets electrode on the squarearray, electrode Video S3: array, Droplet Video transport S3: Droplet on the transport hexagon electrode on the hexagon array. electrode array.

AuthorAuthor Contributions: Contributions:Conceptualization, Conceptualization, H.W.;H.W.; methodology, H.W.; H.W.; software, software, H.W.; H.W.; validation, validation, H.W.; H.W.; investigation, H.W.; resources, L.C.; data curation, H.W.; writing the original draft, H.W.; writing, review investigation, H.W.; resources, L.C.; data curation, H.W.; writing the original draft, H.W.; writing, review and and editing, H.W. and L.C.; visualization, H.W.; supervision, L.C.; project administration, L.C.; funding acquisition, L.C.editing, All authors H.W. haveand L.C.; read visualization, and agreed toH.W.; the publishedsupervision, version L.C.; project of the manuscript.administration, L.C.; funding acquisition, L.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China,Funding: grant This number research 17KJA460008. was funded by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China, grant number 17KJA460008. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Shen, H.H.; Fan, S.K.; Kim, C.J. EWOD microfluidic systems for biomedical applications. Microfluid. Nanofluid. 1. Shen, H.H.; Fan, S.K.; Kim, C.J. EWOD microfluidic systems for biomedical applications. Microfluid. 2014, 16, 965–987. [CrossRef] Nanofluid. 2014, 16, 965–987. 2. Miller, E.M.; Wheeler, A.R. Digital bioanalysis. Anal. Bioanal. Chem. 2009, 393, 419–426. [CrossRef][PubMed] 3. Cho, S.K.; Moon, H. Electrowetting on dielectric (EWOD): New tool for bio/micro fluids handling. BioChip J. 2009, 2, 79–96. 4. Mugele, F.; Baret, J.C. Electrowetting: From basics to applications. J. Phys. Condens. Matter. 2005, 17, R705–R774. [CrossRef] 5. Abdelgawad, M.; Wheeler, A.R. Low-cost, rapid prototyping of digital microfluidics devices. Microfluid. Nanofluid. 2007, 4, 349–355. [CrossRef] 6. Samiei, E.; Tabrizian, M.; Hoorfar, M. A review of digital microfluidics as portable platforms for lab-on a-chip applications. Lab Chip 2016, 16, 2376–2396. [CrossRef] Micromachines 2020, 11, 1113 10 of 11

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