Bioinspired Smart Peristome Surface for Temperature-Controlled

Jan 20, 2017 - Unidirectional liquid spreading without energy input has attracted considerable attention due to various potential applications such as...
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Bioinspired Smart Peristome Surface for Temperature-Controlled Unidirectional Water Spreading Pengfei Zhang,† Huawei Chen,*,† Li Li,‡ Hongliang Liu,*,‡ Guang Liu,† Liwen Zhang,† Deyuan Zhang,† and Lei Jiang‡ †

School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

S Supporting Information *

ABSTRACT: Unidirectional liquid spreading without energy input has attracted considerable attention due to various potential applications such as biofluidics devices and self-lubrication. Introducing a surface wettable gradient or asymmetric nanostructures onto the surface has successfully harnessed the liquid to spread unidirectionally. However, these surfaces are still plagued with problems that restrict their practical applications: fixed spreading state for a fixed surface, and spreading slowly over a short distance. Herein, bioinspired from the fast continuous unidirectional water transport on the peristome of Nepenthes alata, we report a smart peristome with temperature-controlled unidirectional water spreading. The smart artificial peristome was fabricated by grafting the thermoresponsive material PNIPAAm onto the artificial PDMS peristome. Unidirectional water spreading on the smart peristome can be dynamically regulated by changing the surface temperature. Besides, the water spreading is demonstrated with a remarkable reversibility and stability. By investigating the relationship between liquid spreading distance and wettability, the underlying mechanism was revealed. This work gives a new way to achieve the control of unidirectional liquid spreading available for controllable microfluidics and medical devices. KEYWORDS: unidirectional liquid spreading, artificial peristome, smart surface, temperature-controlled water spreading, Nepenthes alata and so on.30 Recently, we discovered the fast continuous unidirectional water transport mechanism on the peristome surface of Nepenthes alata,31 in which gradient corner enhances Taylor capillary rise, and arc-shaped edge prevents backflow by pinning in the reverse direction. We have successfully fabricated surfaces with peristome-mimetic structures and made the liquid exhibit a fast unidirectional spreading.32,33 However, how to control the unidirectional liquid spreading and how to fabricate smart materials with dynamically controllable unidirectional liquid spreading ability are still big challenges to widen the applications. Herein, we present a smart artificial peristome of Nepenthes alata with dynamically temperature-controlled unidirectional water spreading by grafting the surface with thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm). The grafted surface shows two states of wettability, that is, hydrophilicity below the lower critical solution temperature (LCST) and hydrophobicity above the LCST.34−36 The temperature-responsive wettability

1. INTRODUCTION Unidirectional liquid spreading without energy input is of significant interest for a wide range of applications, such as microfluidic devices,1−3 self-lubrication,4,5 controllable chemical reaction,6,7 and biomedical devices.8,9 Studies of controlling surface chemistry with patterned or gradient wettability,10−12 and constructing structures of grooves,13−15 fibers,16,17 and asymmetric nanowire arrays have been conducted to achieve directional liquid spreading.18−24 Liquid in the uniform groove usually symmetrically spreads without unidirectionality.13,15 Although surfaces with wettable gradient or asymmetric nanowires can harness the liquid to spread unidirectionally, the spreading is remarkably slow over a short distance. In addition, for a fixed surface with a specific surface chemistry property, the spreading state cannot be changed.17,19,22,24 Inspirations from nature, particularly the spider silks and cactus spines,25,26 give new insights into the design of novel materials with unidirectional water transport ability by combining surface energy gradient and curvature gradient together.27−29 However, materials mimicking these properties are mainly one-dimensional fibers and are difficult to be applied for large-area surfaces anticipated in microfluidics devices, biomedical devices © XXXX American Chemical Society

Received: December 10, 2016 Accepted: January 20, 2017 Published: January 20, 2017 A

DOI: 10.1021/acsami.6b15802 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Design of smart artificial peristome of Nepenthes alata for temperature-controlled unidirectional water spreading. (a) Schematic showing reversible unidirectional water spreading on the smart artificial peristome mediated by reversible formation of intermolecular hydrogen bonding between PNIPAAm chains and water molecules (hydrophilicity, left) below the LCST and intramolecular hydrogen bonding between CO and NH groups in PNIPAAm chains above the LCST (hydrophobicity, right). (b) Time-lapsed photographs of unidirectional water spreading on the smart artificial peristome at ∼20 °C (above) and nearly no water spreading at ∼20 °C (below). Left images are the corresponding thermal images. Scar bar: 0.5 cm.

Figure 2. Characterization of PNIPAAm-grafted artificial peristome. (a) SEM image of PDMS artificial peristome surface. (b) Enlarged image marked by the red rectangle in (a). (c) Cross-section SEM image of artificial peristome. The enlarged image in (c) shows the edge angle of ∼25°. (d) XPS wide-scan spectra of the PDMS surface. (e) XPS wide-scan spectra of the PNIPAAm-PDMS surface, with a peak component around ∼400 eV ascribing to N 1s appearing. The inset in (e) shows the high-resolution N 1s XPS spectrum of the PNIPAAm-PDMS surface. (f) ATR-FTIR spectra of PDMS artificial peristome (dot line) and PNIPAAm-grafted PDMS artificial peristome (solid line).

results in controllable water spreading on the PNIPAAmgrafted artificial peristome (Figure 1a). At ∼20 °C (below the LCST), the PNIPAAm-modified surface is hydrophilic, and water spreads just along one direction but is pinned in reverse direction (upper in Figure 1b and Movie S1, see Figure S1a for side view of the Supporting Information, SI). At ∼40 °C (above the LCST), the artificial peristome becomes hydrophobic, and water is easily pinned at the both directions (bottom in Figure

1b and Movie S1, see Figure S1b for side view). We believe our findings offer new opportunities to control liquid flow especially in the design of controllable microfluidics and medical devices.

2. RESULTS AND DISCUSSION A smart artificial peristome was fabricated by combining a simply modified replica molding method with surface-initiated B

DOI: 10.1021/acsami.6b15802 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces atom-transfer polymerization (SI-ATRP). The complicated three-dimensional structures on the peristome of Nepenthes alata make the fabrication very difficult using the traditional methods such as chemical etching and photolithography.33 To fabricate the artificial peristome with actual multiscale structures as on the natural surface, a simply modified replica molding method was employed (Figure S2a). In particular, we applied a freezing process to decrease the deformation of the negative replica induced by the high elasticity and surface stickiness of the casting material of polydimethylsiloxane (PDMS) (see Experimental Section for details). The obtained structures of the negative replica exhibit nearly no deformation and the protruding structures possess sharp edges (Figure S2b−d). The same surface morphology as the natural peristome, with two-order microgrooves and anisotropic microcavities scattering along the second-order grooves, is perfectly transferred onto the artificial peristome after the second replication (Figure 2a). Its enlarged image (Figure 2b) and cross-section image (Figure 2c) show that the sharp edges of microcavities are well replicated. Although the inner part of the microcavity is not replicated completely, that is, the depth of the artificial peristome is smaller than that on the natural peristome, overlapped wedges are perfectly formed at the sides of each microcavity. The edge angle of the microcavity is approximately of 25°. And then thermoresponsive PNIPAAm brushes were grafted onto the PDMS peristome surface by use of SI-ATRP (Figure S3). The existence of PNIPAAm brushes on artificial peristome is demonstrated by the appearance of a peak component at about 400 eV corresponding to N 1s specie in the wide scan X-ray photo electron spectroscopy (XPS) of the PNIPAAm units (Figure 2d,e). Attenuated total reflectionFourier-transform infrared (ATR-FTIR) spectra of the PNIPAAm-modified surface indicates the peak characteristic (1650 cm−1 of amide I and 1547 cm−1 of amide II) of PNIPAAm (Figure 2f). AFM images show that PNIPAAmgrafted PDMS (Ra = 12.142 nm) becomes much rougher than PDMS (Ra = 2.422 nm), owing to the introduction of grafted PNIPAAm brushes (Figure S4). We then investigated the water spreading behaviors on the prepared smart peristome surface with different temperatures to analyze the influence of temperature. The specific water spreading process was recorded by a high-speed camera, and Figure 3 shows the in situ observations of water spreading on the PNIPAAm-grafted artificial peristome at different temperatures (Movie S2). At 40 °C (Figure 3a), water just keeps the initial state and shows nearly no spreading in both directions. At 25 °C (Figure 3b), water gradually spreads toward the positive direction but is nearly pinned in the negative direction. When the surface temperature is decreased to 20 °C (Figure 3c), more evident unidirectional spreading occurs. The water shows a faster and longer spreading in the positive direction than that at 25 °C, and holds a similar pinning in the reverse direction as well. The results show the effective response of water spreading to the temperature of the PNIPAAm-grafted artificial peristome, and also indicate that the unidirectional water spreading on the smart peristome surface can be tuned by the temperature. This temperature-responsive effect of water spreading should be ascribed to the wettability change of the surface, caused by the thermoresponsive PNIPAAm molecules on the PDMS surface. Controllability of unidirectional water spreading on the PNIPAAm-grafed artificial peristome was further investigated by exploring its reversible performances. The artificial

Figure 3. In situ observations of water spreading on the PNIPAAmgrafted artificial peristome with temperature of 40 °C (a), 25 °C (b), and 20 °C (c). At 40 °C, water nearly keeps still; at 25 and 20 °C, water spreads in the positive direction and the spreading distance at 20 °C is larger than that at 25 °C. Black arrow in (a) shows the positive directions. Scale bar: 500 μm.

peristome was repeatedly heated to 40 °C and cooled to 20 °C, and the variation of water spreading state was recorded. Figure 4a shows the final spreading state of two cycles on the same sample. Both the unidirectional water spreading and no spreading nearly keep the same performance. The water droplet spreading length was also measured after each cycle (Figure 4b), and the results show excellent reversibility even the sample is cycled for 10 times. Actually, the reversibility remains after the sample has been evaluated for dozens of times. We also evaluated the response to temperature of the smart artificial peristome after storage in air for 100 days, and the surface still shows the successful control of water spreading. The ATRFTIR spectra after grafting of PNIPAAm on PDMS for 100 days indicates the still existence of the PNIPAAm, as shown in Figure S5. These results demonstrated the remarkable stability of the smart artificial peristome. Even though unidirectional liquid spreading has been achieved in some studies, one specific surface in them only shows one kind of liquid spreading state, and dynamic control of the unidirectional liquid spreading state on one specific surface still plagues the researches.17,19 We demonstrate that water spreading on our prepared surface can be dynamically controlled. As shown in Figure 4c (Movie S3), water droplet shows no spreading on the surface at the temperature of ∼40 °C. When the temperature of the sample is gradually decreased C

DOI: 10.1021/acsami.6b15802 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) Reversible unidirectional water spreading and pinning on the PNIPAAm-grafted artificial peristome. The water droplet was repeatedly dropped on the surface after the surface temperature was changed. (b) Spreading distance of water spreading on the surface of cycles. In one specific cycle, the smart artificial peristome was first heated up to 40 °C and then cooled down to 20 °C. The spreading distance at 40 °C (s1) and at 20 °C (s2) is defined in the inset (right). (c) In situ observations of water spreading on the PNIPAAm-grafted artificial peristome with temperature decreasing from 40 to 20 °C. Water spreading state gradually varies from no spreading to unidirectional spreading. Black arrows in (a) and (c) show the positive directions. Scale bar: 500 μm.

to ∼20 °C, the water droplet gradually spreads toward the positive direction and finally shows a unidirectional spreading state just like on the surface at the temperature of ∼20 °C. This results demonstrate the dynamically controllable unidirectional water spreading ability on the PNIPAAm-grafted artificial peristome. To understand the underlying mechanism, we first revealed the relationship between unidirectional liquid spreading and surface wettability on the peristome-like structure. We explored the water droplet spreading on the artificial PDMS peristome with different intrinsic contact angles (CAs), obtained by adjusting storage time in air after O2 plasma treatment. The results show that unidirectional water spreading distance decreases with the increase of storage time in air, as shown in Figure 5a with the final spreading state (see Figure S6a for details). By measuring CAs of water on the treated flat PDMS surface after the corresponding time (Figure S6b), we can obtain the corresponding wettability states of the specific spreading states on the surface (Figure 5b, in which the spreading in the positive direction and the negative direction is colored by black and white, respectively). The water spreading occurs just when the CA decreases to ∼62° (transition CA of water spreading), essentially in agreement with our previous study.31 And the water spreading distance gradually increases as the intrinsic CA decreases. Because of the size limitation (width of the artificial perstome is approximately 6 mm to 8 mm), liquid spreading reaches the margin after the CA decreases to ∼26°. We also dropped different ethanol aqueous solutions with different CAs on the PDMS surface and found the similar results (Figure S7). The liquid spreading occurs just when the contact angle decreases to ∼64°, and liquid spreading distance also increases with the decrease of the intrinsic CAs until reaching the margin after the contact angle decreased to ∼22°.

Above results demonstrate that wettability directly determines the final liquid spreading state, and the undirectional liquid spreading ability tends to be gradually enhanced when the intrinsic CA decreases below the transition CA of approximately from 62° to 64°. And then we studied the wettability of PNIPAAm-modified smooth PDMS at different temperatures, as shown in Figure 6a. The water CAs increase from ∼40° to ∼90° with an increase in temperature from below 20 °C to above 40 °C. The CAs scatter on the two sides of the transition CA of liquid spreading on the peristome-like structure, and indicates that there should be different water spreading performances when the temperature is changed, corresponding to previous results. To deeply understand how the wettability can determine the liquid spreading and the subsequent thermoresponsive unidirectional spreading, we study the edge effect of microcavity on the liquid propagation, as shown in Figure 6b,c. The unidirectional liquid spreading is determined by spreading in two directions, that is, negative direction (−s) and positive direction (+s). In the negative direction, liquid spreading tends to be prevented by the sharp edge of microcavity, i.e., pinning effect. According to the Gibbs inequality,37 the pinning occurs as the receding contact angle θr measured through the liquid at the edge satisfies the following: θo ≤ θr ≤ (180° − Φ) + θo

(1)

where θo is the intrinsic contact angle of liquid on the flat surface, Φ is the edge angle. From eq 1, the critical value of θr at the moment the liquid contact line just crosses the edge can be obtained as follows: θrc = (180° − Φ) + θo D

(2) DOI: 10.1021/acsami.6b15802 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

direction (Figure S8a), while it will be pinned when θover ≤ θo. Because θover has a positive correlation with V/S, i.e., θover ≈ V/ S,37 liquid spreading distance will increase with θover decreasing. And thus, liquid spreading distance in positive direction increases with the intrinsic CA θo decreases, corresponding to the results in the experiments. When the surface is highly wetted by the liquid, that is, the θo is relatively small, the liquid spreading distance is large, explaining the phenomenon of the liquid spreading reaching the margin when θo is below a certain value. Comparing θr with θa, both of which correspond to the same critical value, θa much nore easily exceeds the critical value, generating the significant unidirectionality of liquid spreading on the peristome-like structure. After exceeding the edge, the liquid will spread in the wedge of microcavity owing to the capillary effect and finally fully fill the microcavity.31 After fully filling the microcavity, liquid exceeding its edge will begin to produce the next cycle. And therefore, due to the wettability response to temperature, the PNIPAAm-grafted artificial perstiome shows water spreading varying from no spreading to unidirectional spreading when the temperature decreases from ∼40 °C to ∼20 °C.

3. CONCLUSIONS We have successfully fabricated a smart artificial peristome by grafting thermoresponsive PAIPAAm on artificial PDMS peristome with the complete structure property like the natural peristome of Nepenthes alata. Wettability on the PNIPAAmgrafted PDMS surface showed a significant response to temperatures, leading to a successful dynamic control of unidirectional water spreading by regulating the surface temperature. Moreover, the controllability of water spreading is also demonstrated with remarkable reversibility and stability. By exploring liquid spreading with different intrinsic CAs, we demonstrated that liquid spreading on the peristome-like structure is mainly determined by the wettability on the surface. Through analyzing the edge effect on the water spreading with different intrinsic CAs, the underlying mechanism was revealed. The present work gives new insights into the design of materials with unidirectional liquid spreading ability and offers new strategies to achieve the control of liquid spreading from the perspectives of both structures and stimulus-responsive materials.

Figure 5. (a) Final spreading state of water on artificial PDMS peristome with different intrinsic contact angles (CAs). (b) The spreading distance in the positive direction (+Δs) and negative direction (−Δs) as a function of intrinsic CA of liquid on the corresponding surface (defined in the inset). ▲ and ▼ represent the experimental results of ethanol solutions on intact PDMS peristome and water on O2 plasma treated PDMS peristome, respectively. The black and white colors represent the spreading in positive and negative direction, respectively. The spreading distance increases when the intrinsic CA decrease below that transition CA (approximately 62°− 64°).

which indicates that the limitation for liquid crossing the edge increases as the intrinsic contact angle increases. Because θrc is pretty large (the edge angle Φ is approximate 25° in Figure 2c, meaning 180 − Φ is approximately 165°), the negative spreading is easily pinned at the edge (pinning state: θr ≤ θrc, Figure S8b). Therefore, there is nearly no spreading in the negative regardless of the wettability varying from hydrophobicity to hydrophilicity. In the positive direction, the liquid spreading can be described as the cycles of two typical liquid movements: liquid exceeding the sharp edge and overflowing around the secondorder ridge, and then liquid filling the microcavity along the wedge.31 The liquid exceeding the edge is also a process of liquid contact line crossing the edge of microcavity. We define the liquid surface level over the horizontal line as θover, and the liquid advancing contact angle measured through the liquid at the edge can be described as follows: θa = (180° − Φ) + θover

4. EXPERIMENTAL SECTION 4.1. Materials. Mature pitcher plants, Nepenthes alata, were purchased commercially from Kunjiyuanyi Corporation (Guangdong province, China). Polydimethylsiloxane (PDMS, sylgard 184), purchased from Dow Corning, America and was used as-received. Deionized water was obtained from a Millipore Simplicity 185 system, with a resistance of 18.2 MW cm−1. N-Isopropylacrylamide (NIPAAm, > 98%) and CuBr were supplied by J&K Scientific and Sigma-Aldrich, respectively. 2-Bromo-2-methyl-N-(3-(trimethoxysilyl) propyl) propanamide was purchased from Beijing ATM Biopharmaceutical technique Co., Ltd. Ethanol (EtOH), methanol (CH 3 OH), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), and acetic acid (CH3COOH, > 99.5%) were analytical reagent grade from Beijing Chemical Works and were used as-received. 4.2. Fabrication of Artificial PDMS Peristome. The partial peristome was cut apart from a pitcher and thoroughly cleaned by deionized water before use. After drying at room temperature, the partial peristome is directly used as the template for replica molding. The schematic of replication process is shown in Figure S2a, the prepolymer was formed by mixing PDMS and curing agent at a mass ratio of 10:1. Before the prepolymer was casted onto the peristome surface, it was first well stirred for 15 min, and then degassed in a

(3)

Because of the consistent structure, the critical value of θa at the moment when the liquid contact line just exceeds the edge is also as follows: θac = (180° − Φ) + θo

(4)

Comparing eq 3 with eq 4, we can see that the condition of liquid exceeding the edge is θa > θac, that is, θover > θo. The initial liquid produced a large θover, which may be larger than θo so that liquid exceeds the edge and spreads in the positive E

DOI: 10.1021/acsami.6b15802 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 6. Mechanism of unidirectional liquid spreading on the peristome-like structure. (a) Temperature dependence of water CA on the PNIPAAm-grafted PDMS flat surface. The CA increases from ∼40° to ∼90° with the temperature increasing from 10 to 60 °C. Parts (b) and (c) are the final liquid spreading states with the intrinsic contact angle marked by the insets in (b) and (c), respectively. Liquid spreading in the both directions corresponds to exceeding sharp edge of the microcavity. In the negative direction (−s), liquid spreading tends to be pinned by the sharp edge regardless of the wettability variation. In the positive direction (+s), the pinning is determined by the intrinsic CA θo with final spreading state condition of θo ≥ θover. The liquid spreading distance increases with the intrinsic CA decreasing from θo1 to θo2. vacuum oven for 15 min. Full curing was performed by heating the sample at the temperature of 80 °C for 45 min. Before demolding, the sample was freezed at −60 °C for 10 min to decrease the elasticity and surface stickiness of PDMS. The demolding was conducted as the freezed sample began to become soft at room temperature, and then the negative replica of peristome was obtained. A second replication of the negative replica resulted in the artificial peristome. Before the second replication, the negative replica was treated by CF4 plasma to form an antisticker layer. The replication process of the second replication was the same as the first replication without the freezing step. The artificial PDMS peristome was obtained by demolding. 4.3. Surface-Initiated Atom-Transfer Radical Polymerization of PNIPAAm on Artificial PDMS Peristome. In experiments, first, PDMS peristome samples were treated with plasma for 10 min to generate enough hydroxyl groups. Second, initiator coated PDMS peristome samples were prepared by immersing PDMS peristome samples in solution 95% EtOH 20 mL, with CH3COOH (200 μL) and 2-bromo-2-methyl-N-(3-(trimethoxysilyl) propyl) propanamide (3%− 6%, v%) added in, at room temperature for 1 h, and then annealed at 110 °C for 30 min. Finally, in the polymerization step, NIPAAm (3.2 g, 28.3 mmol) was mixed with solvent CH3OH/H2O (1:1, v/v) 20 mL, and then N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) (560 μL, 2.56 mmol) was added to the mixture via syringe. Next, the initiator coated PDMS peristome samples were submerged in the mixed solution. Thereafter the mixed solution was degassed by passing a continuous stream of nitrogen gas through the solution at 21 °C for 30 min. Then, CuBr (0.128 g, 0.87 mmol) was added into the solution which is protected under nitrogen. After certain polymerization time of 4 h, the samples were removed and washed with deionized water thoroughly, and then dried with a flow of nitrogen gas. 4.4. Investigation of Relationship between Intrinsic Contact Angle and Unidirectional Water Spreading. The unidirectional liquid spreading ability was investigated by depositing liquids on the horizontally placed artificial peristome, and the spreading process was

recorded by the high-speed camera (I-speed LT, Olympus, Japan) equipped with the microscope (BX51, Olympus, Japan). For investigation of spreading of water and ethanol solutions on the liquid spreading, the same volume of liquid droplet (0.4 μL) was approximately deposited into a single first-order groove using a microliter syringe with a soft capillary tube. In the experiments of changing surface energy of PDMS, water of the same volume was deposited on the O2 plasma treated PDMS after a certain time. The plasma treatment was conducted using RF plasma (P8C, Schwarze, China) at an RF power of 100 W for 5 min, a system pressure of 100 μbar, and a flow rate of 20 sccm. The corresponding contact angles of water on the treated PDMS were performed on the flat PDMS with the same treatment after the corresponding time. In the experiments of changing ethanol aqueous solutions, a series of ethanol solutions with different volume percentage were deposited on the same artificial peristome. The corresponding contact angles of ethanol solutions were measured on the intact flat PDMS. 4.5. Observation of Unidirectional Liquid Spreading on PNIPAAm-Grafted Artificial Peristome. In experiments, the smart artificial was heated and cooled on a hot plate and on a semiconductor chilling plate, respectively. Approximate 0.5 μL dyed water (by rhodamine) was deposited on the artificial peristome surface and the spreading process was monitored by a digital camera (600D, Canon, Japan). For the in situ observations of water droplets (0.4 μL) on the temperature-controlled artificial peristome, the spreading process was recorded by the high-speed camera with samples being placed under the microscope lens. 4.6. Characterization. The microstructures of the artificial surface were observed using a SEM (CamScan-3400, CamScan Corp., Oxford, England) operating at a 20 kV acceleration voltage. An atomic force microscope (AFM, NanoScope, Veeco, America) was employed to scan the three-dimensional surface morphologies. Wettability was investigated by depositing a liquid droplet (approximately 4 μL) on the flat surface by an optical angle measuring system (SL200B, Solon, F

DOI: 10.1021/acsami.6b15802 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces China). X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300W Al Kα radiation. The base pressure was about 3 × 10−9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. Attenuated total refection-Fouriertransform infrared (ATR-FTIR) spectroscopy using an FTIR spectrometer (iN10MX, Nicolet, America). The thermal image and surface temperature were recorded by a thermal imager (Infra Tec, Germany).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15802. Side view of water spreading on the smart artificial peristome (Figure S1), schematic of fabrication process of artificial peristome and the SEM images of negative replicas (Figure S2), modification of artificial peristome with PNIPAAm brush (Figure S3), AFM images of PDMS and PNIPAAm-grafted PDMS (Figure S4), ATRFTIR spectra of PNIPAAm-grafted PDMS with different storage time (Figure S5), water spreading states on the O2 plasma treated PDMS artificial peristome after different storage time (Figure S6), ethanol solutions with different intrinsic contact angles spreading states on the PDMS artificial peristome (Figure S7), and in situ observations of water spreading on the hydrophilic PDMS artificial peristome (Figure S8) (PDF) Movie of water spreading on the smart peristome at temperature of ∼40 °C and ∼20 °C (Movie S1) (AVI) In situ observations of water spreading on the PNIPAAm-grafted artificial peristome at different temperatures (Movie S2) (AVI) Dynamically water spreading change with the surface temperature of smart peristome decreasing from ∼40 °C to ∼20 °C (Movie S3) (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.C.). *E-mail: [email protected] (H.L.). ORCID

Pengfei Zhang: 0000-0002-3038-3474 Lei Jiang: 0000-0003-4579-728X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51290292), Academic Excellence Foundation of BUAA for PhD Students, and was also supported by Youth Innovation Promotion Association, CAS (2016026).

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REFERENCES

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DOI: 10.1021/acsami.6b15802 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX