Thermal-Responsive Anisotropic Wetting Microstructures for

Dec 21, 2016 - Borosilicate glass (SG-2506; with a 145 nm thick chrome film and a 570 nm thick S-1805-type positive photoresist, Changsha Shaoguang Ch...
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Thermal-Responsive Anisotropic Wetting Microstructures for Manipulation of Fluids in Microfluidics Nianzuo Yu,† Shuli Wang,† Yongshun Liu,‡ Peihong Xue,† Peng Ge,† Jingjie Nan,† Shunsheng Ye,† Wendong Liu,† Junhu Zhang,*,† and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Jilin 130012, P. R. China State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP), Chinese Academy of Sciences, Beijing 130033, P. R. China

‡

S Supporting Information *

ABSTRACT: We show morphology-patterned stripes modified by thermal-responsive polymer for smartly guiding flow motion of fluid in chips. With a two-step modification process, we fabricated PNIPAAm-modified Si stripes on silicon slides, which were employed as substrates for fluid manipulation in microchannels. When the system temperature switches between above and below the lower critical solution temperature (LCST) of PNIPAAm, the wettability of the substrates also switches between strong anisotropy and weak anisotropy, which resulted in anisotropic (even unidirectional) flow and isotropic flow behavior of liquid in microchannels. The thermal-responsive flow motion of fluid in the chip is influenced by the applied pressure, the thickness of PNIPAAm, and dimension of the microchannels. Moreover, we measured the feasible applied pressure scopes under different structure factors. Because of the excellent reversibility and quick switching speed, the chip could be used as a thermal-responsive microvalve. Through tuning the system temperature and adding the assistant gas, we realized successive “valve” function. We believe that the practical and simple chip could be widely utilized in medical detection, immunodetection, protein analysis, and cell cultures.

1. INTRODUCTION In recent years, microfluidics has become a hot research topic because of their superiorities in analytical chemistry and biological field.1−8 Pumping liquids, metering samples, and timing reaction require precise manipulation of the fluid in microchannels, which, in most cases, relies on microvalves. Passive microvalves play important roles in guiding and operating fluid in microchannels owing to their simple work structures.9−20 Compared with conventional microvalves,21,22 microfluidic channels that use underlying anisotropic (or directional) wetting surface to manipulate flow of liquid are promising alternatives.12−20 Several modification approaches have been used to fabricate the asymmetric patterns and manipulate fluid motion in microchannels which act as microvalves function, such as photolithography, laser writing, and plasma etching.23 Design of the microchannels is also an important procedure of the hybrid chip preparation process. Optimized microchannels could assist us in perfecting the chip to realize desired functions, such as mixing, extraction, separation, etc. Recently, as special wettability competitors, asymmetric microstructures have been developed and applied.24−32 Also, stimuli-responsive polymers act as a new platform for smart control of fluid in microfluidics.33 With the intersection of responsive asymmetric microstructures and microfluidics, a © XXXX American Chemical Society

series of responsive asymmetric microstructures have been fabricated and applied in microfluidics devices.16,19,39 For example, Chunder et al. fabricated thermally switchable superhydrophobic/hydrophilic valves and realized the regulation of fluid smartly.16 Regarding the photoresponsive wrinkle system using a azobenzene containing liquid crystalline film, Monobe and Ohzono et al. have successfully prepared micrometer-scale liquid filaments based on photoinduced wetting change. In this case, the wrinkle wavelength is immobilized by an underlying polyimide film.39 Wang et al. utilized the Janus silicon pillar arrays with thermal-responsive polymer to smartly manipulate flow motion of water.19 In these microdevices, the stimuli-responsive flow of liquid is regulated by the stimuli-responsive wetting surfaces, which smartly guides flow motion of liquid as desired. However, stacking fabrication process and high cost are still challenging at present. Stripes are classical asymmetric patterned surfaces based on its straightforward fabrication process and remarkable anisotropic wettability.27−32 Previously, we reported an original method to manipulate liquid in the microchannel upon the patterned silicon stripes structures.12 Straightforward fabricaReceived: October 26, 2016 Revised: December 17, 2016 Published: December 21, 2016 A

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atomic force microscopy. The contact-angle measurements (Dataphyscis OCA20) were conducted to study the wettability of liquid on these surfaces. The flow motions of fluid were recorded with an Olympus fluorescence microscope. The flow speed of and flow rate of liquid were obtained according to the recording video.

tion process and excellent performance of the morphologypatterned stripes surface made it suitable for controlling the flow motion in microfluidics. Here, we combined the Si stripes with stimuli-responsive polymer brushes to intelligently control flow motion of fluid in a microchannel. Among the thermalresponsive polymers, the poly(N-isopropylacrylamide) (PNIPAAm) molecule is one of the most classical and practical polymers because of its applications in responsive surfaces.16,40−43 When the system temperature was above or below the LCST of PNIPAAm, the PNIPAAm-modified Si stripes exhibited strong anisotropic wettability and weak anisotropic wettability. Therefore, anisotropic (even unidirectional) flow and isotropic flow behavior of fluid in the microchannel upon the PNIPAAm-modified Si stripes could be realized. The thermal-responsive flow of fluid is influenced by the applied pressure, the thickness of PNIPAAm, dimension of the microchannels, and feasible applied pressure scopes under different structure factors were investigated. Considering the excellent reversibility and quick switching speed of the chip, we used it as a thermal-responsive microvalve and realized successive “valve” function through tuning the system temperature and adding the assistant gas.

3. RESULTS AND DISCUSSION 3.1. Thermal-Responsive Wetting Property of the PNIPAAm-Modified Si Stripes Surfaces. The morphologypatterned Si stripes structures are 10 μm in width, and the spacing between the stripes is also 10 μm (Figure 1a). It was

2. EXPERIMENTAL SECTION 2.1. Materials. Si substrates were cleaned by immersion in piranha solution (7:3 concentrated H2SO4/30% H2O2) for 5 h at 120 °C to create a hydrophilic surface. The photoresist (BP212-37 positive photoresist) was purchased from Kempur Microelectronics. Borosilicate glass (SG-2506; with a 145 nm thick chrome film and a 570 nm thick S-1805-type positive photoresist, Changsha Shaoguang Chrome Blank Co. Ltd.) was applied as the initial wafer for mold fabrication. A Sylgard 184 elastomer base and a curing agent for poly(dimethylsiloxane) (PDMS) were purchased from Dow Corning (Midland, MI). N-Isopropylacrylamide monomer (NIPAAm) was provided by J&K Chemical. 3-Aminopropyltrimethoxysilane (APTMS), 2-bromoisobutyryl bromide, copper(I) chloride (CuCl), and trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFS) were purchased from Aldrich. N,N,N′,N′,N″-Pentamethyldiethylenetriamine (PMDETA) was purchased from TCI. Dichloromethane, triethylamine, absolute ethanol, and methanol were used as received. 2.2. Fabrication of Morphology-Patterned Si Stripes with PNIPAAm via SI-ATRP. Traditional photolithography was utilized to fabricate the photoresist stripes.32 The inductively coupled plasma (ICP) dry etching method was used to produce Si stripes structure, and the prefabricated photoresist stripes were used as masks. ICP etching of Si was conducted using Alcatel 601E ICP with a gas mixture of C4F8, SF6, and O2. The radio frequency power was 1800 W. Then, we used ethanol to dissolve the remanent photoresist. Functionalization of the morphology-patterned Si stripes with PNIPAAm by the conventional SI-ATRP method.19,43 We fabricated PNIPAAmmodified Si stripes surfaces with different thicknesses of PNIPAAm under same conditions, and the polymerization is from 5 to 60 min. 2.3. Flow Motion of Liquid in Microchannels upon the PNIPAAm-Modified Si Stripes Surfaces. We fabricated the PDMS microchannels as published earlier.44 The PDMS microchannels were pressed onto the PNIPAAm-modified Si stripes surfaces underneath an Olympus fluorescence microscope and connected to a microfluidic flow control system (MFCS and FLOWELL, FLUIGENT) using a poly(tetrafluoroethylene) (PTFE) pipe. We actuated the fluid in microchannels using gas pressure, and the applied pressure was controlled by the MFCS. We used the recirculation system to control the system temperature. 2.4. Characterization. The surface morphology of the photoresist stripes were characterized by scanning electron microscopy (JEOL FESEM 6700F electron microscope). The chemical compositions were measured by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). The thickness of PNIPAAm was characterized by

Figure 1. (a, b) SEM images of a patterned surface with Si stripes. WCA on a PNIPAAm-modified patterned surface with Si stripes viewed from (c) the perpendicular direction, (d) the parallel direction at 50 °C, above the LCST of PNIPAAm, and viewed from (d) the perpendicular direction and (f) the parallel direction at 20 °C, below the LCST of PNIPAAm.

shown in Figure 1b that the stripes structure is about 1.4 μm in depth. The Si stripes are functionalized with PNIPAAm via SIATRP, and the polymerization time and thickness of PNIPAAm are 10 min and 27.7 ± 1.4 nm. XPS data of the PNIAAM-modified Si stripes proves that PNIPAAm is successfully bonded onto the Si stripes surface (Figure S1). The water contact angle (WCA) values of the PNIPAAmmodified Si stripes surface were measured at different temperatures. Then we found that water exhibits strong anisotropic wettability and weak anisotropic wettability on the surface when the system temperature is above (50 °C) and below (20 °C) the LCST of PNIPAAm. The WCA that perpendicular to the stripes direction (CA∥ = 97.9°) was larger than the WCA that parallel to the stripes direction (CA∥ = 80.5°) at 50 °C (Figure 1c,d). Also, as shown in Figure 1e,f, the WCA was 75.1° (CA⊥) and 67.6° (CA⊥) at 20 °C, respectively. We know that ΔCA (= CA∥ − CA⊥) reflects the degree of anisotropic wetting, and the ΔCA (= 17.4°) at 50 °C was larger than that at 20 °C (ΔCA = 7.5°). We theorize that the degree of wetting anisotropy at 50 °C is stronger than that at 20 °C. B

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channels are inclined to obtain at 50 °C.12,27 The flow mechanism in the microchannels is shown in Figure S3a−c. In contrast, when the applied pressure was 60 mbar, as shown in Figure 3c, water flowed into outlet 1 and outlet 2 simultaneously at 20 °C. It showed isotropic flow motion in microchannels (Video S2), and the flow rate ratio of outlet 1 and outlet 2 is 7.4:1 ( 0.1). However, it showed isotropic flow behavior in the microchannel when the applied pressure was greater than 75 mbar. Therefore, the Pmax is 75 mbar. In other words, it shows unidirectional and anisotropic flow motion in microchannels as long as the applied pressure is smaller than 75 mbar, whereas the fluid flows isotropically if the applied pressure is larger than 75 mbar. Next, we conducted the experiment process at 20 °C, and the isotropic flow behavior was our desired result under this condition. Similarly, the flow speed and flow rate of water both increased along with the increase of the applied pressure. It showed anisotropic flow behavior (Figure S5b) in the microchannel until the applied pressure increased to 50 mbar (D2/D1 = 0.101 > 0.1).

Pmax [mbar] 78 75 77 78 76

± ± ± ± ±

0.3 0.4 0.2 0.5 0.2

CA∥ [deg] 81.8 75.1 79.1 74.5 76.4

± ± ± ± ±

0.6 0.2 1.0 0.7 0.9

CA⊥ [deg]

ΔCA [deg]

± ± ± ± ±

20.8 7.5 11.7 17.8 11.4

61.0 67.6 67.4 56.7 65.0

0.4 0.4 1.2 1.8 1.3

Pmin [mbar] 73 50 66 71 68

± ± ± ± ±

0.1 0.4 0.3 0.1 0.2

Therefore, the Pmin is 50 mbar. In conclusion, unidirectional or anisotropic flow behavior appears in the microchannel when the applied pressure is smaller than the Pmax at 50 °C, and water flows isotropically when the applied pressure surpasses the Pmin at 20 °C. The feasible applied pressure scope is from 50 to 75 mbar. In this pressure scope, smartly manipulation of the fluid motion can be realized by changing the system temperature. 3.3.2. Thickness of PNIPAAm Film on the Si Stripes. We modified the Si stripes with different PNIPAAm film thicknesses and investigated how the thickness affects the thermal-responsive flow motion of water in the microchannel. The thickness of PNIPAAm film was controlled by the polymerization time (Figure S4), and eight Si stripes with different thicknesses of PNIPAAm film were prepared under the same conditions except polymerization time. As illustrated in Table 2, WCAs, Pmax and Pmin of some representative Si stripes surfaces with different thicknesses of PNIPAAm film E

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to 300 μm, the corresponding Pmax and Pmin increase at any temperature. We speculate that because the Si stripes in the beginning of outlet 2 is longer in a wider outlet channel, and the energy barrier in the perpendicular direction is larger. So, water is more difficult to flow through the beginning of the outlet 2 in a wider microchannel. However, Pmin is increasing faster than that of Pmax when the wide of microchannel is gradually increasing. Therefore, the microchannel of 100 μm has larger practical pressure scope (36−69 mbar). The PNIPAAm-modified Si stripes surfaces show a better manipulating ability for fluid in narrow and low microchannels. 3.4. Successive “Valve” Function: Controlling the Flow Path by Adjusting the System Temperature and the Applied Pressure of Gas. Water presented thermalresponsive flow behavior in the microchannels upon PNIPAAm-modified Si stripes surface, indicating that the PNIPAAm-modified Si stripes could act as passive microvalves in microfluidics.16 We could realize successive “valve” function by adjusting the system temperature and the applied pressure of gas. We added another inlet microchannel to the chip for injection of gas (Figure 5a). When the applied pressure was 55 mbar, water showed unidirectional flow motion in the microchannel at 44.6 °C, so outlet 1 was open but outlet 2 was “closed” (Figure 5b,c). When the system temperature decreased from 44.6 to 31.8 °C, water broke the initial gas− water interface (Figure 5d) and flowed through the end of outlet 2 when the system temperature decreased to 29.7 °C (Figure 5e), namely, outlet 1 and outlet 2 were both open. Then, the system temperature was increased to 44.7 °C, and the applied pressure of gas increased from 42 to 69 mbar. Nitrogen cut off the water plug at the junction of the water inlet and gas inlet and pushed the water in outlet 1 and outlet 2 away (Figure 5f−h). In other words, both outlets were “closed”. When the driving pressure of gas decreased to 42 mbar, the gas plug was cut off by the water at the junction of the water inlet and gas inlet and flowed through the outlet 1 but pinned at starting point of outlet 2 (Figure 5i,j), namely, outlet 1 was open for the second time. Then, water flowed through outlet 2 when the system temperature decreased to 30.1 °C, and outlet 2 was open again (Figure 5k). When the driving pressure of gas increased to 69 mbar, nitrogen pushed the water in outlet 1 and outlet 2 away, and both outlets were simultaneously “closed” for the second time (Figure 5l,m). The video that records the entire process is shown in Video S3. It is worthwhile noting that the reversibility and stability of the chip are excellent. We could realize on/off switch more than ten cycles continuously. Therefore, PNIPAAm-modified Si stripes surface could serve as a microvalve, and the flow motion can be controlled by adjusting the system temperature and the driving pressure of the gas.

were measured from multiple samples produced at the same conditions. Strong anisotropic wettability and weak anisotropic wettability were desired when the system temperature was at 50 and 20 °C, respectively. The wetting anisotropy ΔCA of these surfaces were all larger than 17.0° when the system temperature was 50 °C. Besides, all the Pmax were larger than 75 mbar, and the whole substrates achieve basic requirements of anisotropic flow motion of water at larger applied pressure. However, the wetting anisotropy ΔCA was distinct at 20 °C, and the weak anisotropic wettability performed best when thickness of PNIPAAm film was 27.7 ± 1.4 nm (the polymerization time of Si-ATRP is 10 min). The ΔCA was 7.5°, and the Pmin was 50 mbar (the minimum) under this thickness of PNIPAAm film. The Pmin of other substrates with different polymerizations time is larger; thus, it is not easy for them to realize isotropic flow motion of water at lower applied pressure. We assume that intermolecular hydrogen bonding between very short polymer chains and water molecules cannot formed easily, which contributes to excessive hydrophobicity of PNIPAAm film under short polymerization time, and PNIPAAm brushes are too thick to roll over easily when the system temperature is falling if polymerization time is prolonged. Therefore, we conclude that the thickness of PNIPAAm film affects the scope of feasible applied pressure, and PNIPAAm film with thickness of 27.7 ± 1.4 nm is suitable for manipulating the flow motion of water in the microchannels. The PNIPAAm-modified Si stripes with thickness of 27.7 ± 1.4 nm were chosen in the following experiments. 3.3.3. Dimension of the Microchannels. Different dimensions of the microchannels on the PNIPAAm-modified Si stripes may correspond to different Pmax and Pmin. Microchannels with the same width but different heights and the same height with different widths were fabricated. Flow motions of water in these microchannels were investigated at different temperatures. When the height of the microchannel increases, the homologous Pmax and Pmin decrease whatever the temperature is, but the decreasing velocity of Pmax is faster than that of Pmin (Table 3). Therefore, the feasible applied pressure Table 3. Pmax and Pmin in the Microchannels with Different Dimensions channel no.

bottom width [μm]

top width [μm]

height [μm]

50 °C Pmax [mbar]

20 °C Pmin [mbar]

feasible applied press. scope [mbar]

1 2 3 4 5 6

200 200 200 200 100 300

146 129 100 75 47 250

25 35 48 63.5 25 25

75 52 48 35 69 80

50 49 48 42 36 56

50−75 49−52 48 36−69 56−80

4. CONCLUSIONS In summary, we demonstrate that the PNIPAAm-modified Si stripes can be utilized to regulate the flow behavior of fluid smartly in microchannels. The thermal-responsive flow motion of fluid in a microchannel originates from the thermalresponsive wettability of the stripes surfaces. Flow motions could switch between anisotropic (even unidirectional) flow and isotropic flow in microchannels upon the PNIPAAmmodified Si stripes. The thermal-responsive flow behavior of water is influenced by the applied pressure, the thickness of PNIPAAm, and dimension of the microchannels. Feasible applied pressure scopes under different conditions were also

scope is decreased gradually. The chip could smartly manipulate water flow motion in the microchannel as before at 48 mbar (Pmax = Pmin) when the height of microchannel increases to 48 μm. Considering the practical application, we deem that the chip could not realize smart microfluidic manipulation when the height of microchannel is greater than 48 μm (Pmax < Pmin). When the height of the microchannel increases, the ratio of the Si stripes for the whole microchannels wall decreases, and the energy barrier of outlet 2 also decreases relatively whatever the temperature is. This results in a weakening of the smart microfluidic manipulation ability of the chip. When the width of microchannels increases from 100 F

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Figure 5. “Valve” function by changing the system temperature. (a) Schematic illustration of the microchannel. (b, c) The water plug arrived at the Y-junction and flowed through outlet 1 at 44.6 °C. (d, e) The water plug flowed through outlet 2 along with the reduction of the system temperature. (f−h) The air plug reached the junction and flowed through outlet 1 and outlet 2 at 44.7 °C. (i, j) The water plug arrived at the Yjunction and flowed through outlet 1 again when the system temperature increased to 44.7 °C. (k) The water plug flowed through outlet 2 along with the reduction of the system temperature. (l, m) The air plug reached the junction and flowed through outlet 1 and outlet 2 again when the system temperature was 44.8 °C. The red and blue arrows represent the flow direction of water and air, respectively. The red line represents the position of water in the two outlet channels. The scale bar is 500 μm, and the driving pressure of water was 55 mbar.

investigated. Furthermore, the PNIPAAm-modified Si stripes surface could be used as microvalve in microchannels. Reversibility of the microvalve is excellent, and we realized successive valve on/off switching processes. We believe that the practical, cost-effective, and simple chip could be used for flow control and multiphase chemical reaction. Additionally, to realize multiresponse control of the chip, other stimuliresponsive polymers may be modified to the morphologypatterned surfaces, such as photo- or pH-responsive materials, which may show wide applications in future microfluidics systems.

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shaped microchannels on PNIPAAm-modified Si stripes surface under different conditions (PDF) Video S1 (AVI) Video S2 (AVI) Video S3 (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Z.). ORCID

Junhu Zhang: 0000-0001-9100-6608 Bai Yang: 0000-0002-3873-075X

ASSOCIATED CONTENT

S Supporting Information *

Notes

The authors declare no competing financial interest.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03896. X-ray photoelectron spectroscopy of the PNIPAAmmodified Si stripes and pure Si surface; optical microscope image of a patterned surface with Y-shaped photoresist stripes and microchannel; mechanism of the flow motion of water in the microchannel when the stripes direction is perpendicular and parallel to the microchannel at 50 and 20 °C; detailed description of the larger apparent contact angle; thickness of PNIPAAm film varied with the polymerization time of SI-ATRP; optical microscopy images of water flowing distance in Y-

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant no. 21474037), Doctoral Fund of Ministry of Education of China (20130061110019), and Natural Science Foundation of China (51505456).

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DOI: 10.1021/acs.langmuir.6b03896 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b03896 Langmuir XXXX, XXX, XXX−XXX