Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Micropatterning of Highly Stretchable Tough Polymer Actuators for Multistage Detection of Acetone Vapors Jiang Wei, Fushun Wang, and Lidong Zhang* Department of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200241, People’s Republic of China
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S Supporting Information *
ABSTRACT: Micropatterning of soft polymer actuators is an emerging technique that overcomes many drawbacks of macroscopic patterning to trigger the shape-programmable deformations for various functional applications. We thus report a polymer composite actuator that combines micropatterning with high stretchability and toughness, whereupon it demonstrates shape-programmable deformation, and can be utilized in an electronic device for multistage detection of acetone vapors. The actuator is created by alignment of Fe(0) particles (FePs) into poly(vinylidene difluoride) (PVDF) matrix in a strong magnetic field, followed by stretching the PVDF/FePs film into necking structure. The necking induces more-directional alignments of FePs and further crystallization of PVDF, so that bringing PVDF/FePs actuator with anisotropic elastic tensors, resulting in controllable shape deformation upon sorption of acetone vapors. Assisted by magnetic field, acetone-driven deformation can be transferred to directional movement by rolling over a substrate. Micropatterned PVDF/FePs actuator is stretchable and tough, with maximum stress reaching 160 MPa at the rupture strain of 100%, making it capable of continuous deformation for several hours, or even longer, depending on the concentration of acetone vapors. This directional reversible fatigueless response is involved in a smart controller that exhibits promising potential for multistage detection of acetone vapors. KEYWORDS: micropatterning, smart materials, stretchable and tough actuators, detectors
1. INTRODUCTION
Alternatively, microscopic patterning overcomes these above drawbacks because it is designed with accompanying chemical reactions or molecular recombination that is capable of combining micropatterning structures with robust mechanical properties into one unit of SPA. Examples of the strong points of micropatterns include an ambient-driven actuator that takes advantage of the inherent nanoscale molecular channel patterns to realize a rapid response, self-adaptive, and exceptionally stable actuation,31 and stimuli-responsive actuators that are composed of microscopic alignments of liquid crystals to obtain shape-programmed mechanical actuations.32−36 We herein report a microscopic patterning technique to prepare acetone vapor-responsive SPAs with high flexibility, stretchability, and toughness. We aim at controlling the stimuli-responsive kinematics of SPAs yet not reducing their mechanical properties. The SPAs were designed by introducing Fe(0) particles (FePs, diameter ∼3 μm) into poly(vinylidene difluoride) (PVDF) matrix in a strong magnetic field. This operation induced anisotropic alignments of FePs.37 Further uniaxial stretching of PVDF/FePs film enhanced the orientation of FePs alignments and led to orientational
Soft polymer actuators (SPAs) that are capable of responding to light,1,2 temperature,3,4 gas,5,6 and other stimuli7−9 for biomimetic,10 robotic,11,12 and sensing13,14 applications require controllable responsiveness and accurate executive capacity.15,16 Thus, precise structure design is essential. The progress normally includes a macroscopic or microscopic structure design. Currently, the macroscopic structure design of the materials through the protocols such as surface patterning stands among the top choices.17,18 The surface patterning is effective in inducing SPAs with controllable responses to external stimuli. For example, surface patterning with ink printing for prestrained polymer sheets results in materials with a spatial selective light-sorption ability that discriminately releases the strain to induce controllable responses;19−22 replication of channeled structure of silicon template into a polymer bilayer leads to actuators with differential flexural modulus, triggering a controllable kinematics upon external stimuli.23,24 Therefore, macroscopic surface-patterning techniques demonstrated by previous reports indeed have exhibited high practicability and universality to design SPAs with controllable responsiveness and accurate executive capacity.25−30 However, the drawback is that the macroscopic surface patterns might weaken the mechanical properties of SPAs; as they suffer from large shape deformations, the patterning structures may be destroyed.5 © XXXX American Chemical Society
Received: June 12, 2018 Accepted: August 1, 2018 Published: August 1, 2018 A
DOI: 10.1021/acsami.8b09826 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Preparation and kinematics of PVDF/FePs actuator. (a) Schematic diagram showing the procedures for the preparation of structural PVDF/FePs film. PVDF/FePs solution-coated template was moved to magnetic field and dried at 70 °C to make oriented alignments of FePs inside PVDF matrix. Prepared PVDF/FePs film was stretched into a necking, resulting in microscopic patterning structures. (b) Stress−strain profile of PVDF/FePs film showing a high stretchability. Inset is the snapshot of stretching PVDF/FePs strip. (c) Top view of the necking of PVDF/FePs film showing well-defined structure orientation along the stretching direction. (d) Kinematics of a square necking of PVDF/FePs over an acetone-containing filter paper (acetone: 20−30 wt %). The square necking was cut out with patterning orientation parallel to one axis. Square size: 10 mm × 10 mm × 16 μm. (e) A long necking strip that was cut out with the patterning orientation at an angle of 45° with respect to the long axis. Strip size: 15 mm × 5 mm × 16 μm. (f) Kinematics of PVDF/FePs without patterning structures. Environmental temperature: 15 °C for kinematics demonstration.
be adopted to design other highly flexible tough actuators with diverse responses to external stimuli.
crystallization of PVDF, resulting in the PVDF/FePs actuators with controllable responses to acetone vapors. PVDF was selected as a matrix because it is flexible, stretchable, and tough.38 Especially, after treatment with additives, it can be stretched into a “necking” to induce orientational crystallization (Figure S1 in the Supporting Information, SI).39 The neck structure is crucial in patterning PVDF/FePs at a microscopic level. On the other hand, PVDF is acetone sensitive and capable of capturing acetone vapors through noncovalent interactions between fluorine atoms and acetone molecules.23,40 Even though PVDF-based actuators have been reported, few of them have introduced micropatterning method in PVDF actuating system. Alignment of FePs into the PVDF actuator is another key point that generates film with anisotropic elastic tensors for controllable responses to acetone vapors. The interesting points of this work thus include the orientational alignment of FePs into stretchable PVDF matrix, the shape-programmed actuations in a single-layer film induced by structural orientation obtained from uniaxial stretching, and the involvement of high flexibility, stretchability, and toughness in stimuli-responsive actuators. Moreover, an electronic detector of acetone vapors was assembled to demonstrate an inspiration for future application. This method is expected to
2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinylidene difluoride) (model: Kynar HSV900) was purchased from a commercial supplier in China (Dongwan Fuqiao Plastic Technology Co., Ltd). It is a kind of additives-modified composite with high stretchability and toughness. Acetone, Fe(0) nanoparticles, and N,N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. All images and videos were recorded with smart phone (Smartisan T2). All necking films were obtained from electronic tensile testing machine (HY-0580, purchased from Shanghai Hengyi Precise Instrument Limited Company). 2.2. Preparation of PVDF/FePs Film. PVDF/DMF solution was prepared by dissolving PVDF powder (1 g) in DMF (15 mL) in a 100 mL beaker under vigorous stirring by magnetic stirring bar at 90 °C. After complete dissolution of the PVDF powder, Fe(0) particles (100 mg) were added and the mixture stirred for another 3 h to ensure that Fe(0) particles dispersed homogeneously. A glass slide (76 mm × 25 mm × 1 mm) was washed with ethanol and acetone and blow-dried by hot-air gun. The suspension was cast onto the glass slide and the sample was then moved to magnetic field, followed by drying at 70 °C to give rise to PVDF/FePs film. 2.3. Preparation of PVDF/FePs Film Necking. The PVDF/ FePs film was cut into rectangular strips (25 mm × 7 mm × 60 μm), with the FePs-aligned direction parallel to the long axis. The strips B
DOI: 10.1021/acsami.8b09826 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. Characterizations of micropatterning structures. (a) Zoom-out and (b, c) zoom-in images of heat-treated necking of PVDF/FePs taken by polarizing optical microscope. (d) Zoom-out and (e, f) zoom-in images of heat-treated necking of PVDF/FePs taken by scanning electron microscope. (g) X-ray diffraction (XRD) spectroscopy of unstretched PVDF film. (h) Snapshot showing the necking structure of PVDF/FePs film strip. (i) XRD spectroscopy of the PVDF/FePs necking. Four-layer filter papers were then located over the cotton. Acetone was absorbed into the filter paper and diffused to generate consistent gradients of acetone vapors over the filter paper (p = 19 kPa, 15 °C). The PVDF/FePs film was cut to various strips that were placed on the paper, and their motions were recorded by a smart phone. 2.9. In Situ Measurement of Deformation Stress. The acetone-induced deformation stress of the PVDF/FePs necking strips with micropatterns perpendicular and parallel to the long axis was measured on a tensile tester (model: HY-0580). A strip sample (20 mm × 5 mm × 16 μm) was gripped between two sample holders, and the gauge length was adjusted to 1 cm while keeping the strip straight by a prestretch at 0.02 N. The gauge length was constant throughout the measurement. Acetone-soaked filter paper was used to supply acetone vapor. When approached by the acetone-soaked filter paper, the strip absorbed acetone vapor and deformed with the generation of deformation stress. After the paper was removed, the strip quickly released the stress by evaporation of acetone.
were clipped on universal stretching machine and stretched along the FePs-aligned direction at a speed of 3 mm/min. As the necking stage ran out, the stretching was stopped; at this stage, the film did not break. To eliminate the internal stress generated during stretching, the necking film was collected and fixed between two glass slides. They were then heated at 70 °C for 4 h before being used for kinematic tests. 2.4. Polarized Optical Microscope (POM). The necking was cut into a rectangle (25 mm × 5 mm) and observed by POM (Carl Zeiss, Axio Scope.A1). The observation was done under light-reflective and light-transmissive modes to analyze the surface and inner structures. 2.5. X-ray Diffraction (XRD). Microscopic patterning structure was characterized by XRD spectroscope (model: Ultima IV). Unstretched PVDF/FePs film and necking film were cut into rectangular strips (25 mm × 8 mm) for the diffraction tests. The final spectra were obtained by deducting the background of the template. 2.6. Scanning Electron Microscope (SEM). The SEM images were recorded on an electron microscope (S-4800, SYST TA PRO 1156) with a primary electron energy of 2 kV. The film samples were attached to silicon wafer with adhesive carbon tape and coated with a 5 nm thick gold layer before SEM observation. The micrographs were recorded at room temperature and pressure of 8.8 × 10−7 Pa. 2.7. Atomic Force Microscope (AFM). The AFM measurements were performed on a microscopic system (Multimode 8 DI, Brucker, Germany) in alternating current (“tapping”) mode. Cantilever tips with a nominal tip radius of less than 7 nm were used at the resonant frequency of ∼90 kHz. Feedback control parameters were optimized for each scan. Optical images were taken with a 20× objective lens. PVDF/FePs necking film was cut into rectangles (5 mm × 5 mm) and attached on a mica plate using an adhesive tape. The testing results were processed by software of nanoscope analysis 1.5. 2.8. Responsive Kinematics to Acetone Vapors. A Petri dish (outer diameter 12 cm) was fully filled with acetone-saturated cotton.
3. RESULTS AND DISCUSSION 3.1. Preparation and Kinematics of Micropatterned PVDF/FePs Film Actuator. We prepared a homogeneous PVDF/FePs suspension solution in DMF (see details in the Experimental Section). We cast the solution on a glass template and moved it to magnetic field (density: 13 mT), followed by drying at 70 °C (Figure 1a). Before drying, the FePs were locally movable, and could be induced into directional ordered alignments by the magnetic field. After drying, the orientationally aligned FePs were fixed inside the PVDF matrix. The PVDF/FePs film was then cut into strips with FePs aligned parallel to the long axis and stretched into a necking structure by universal stretching machine at a speed of 3 mm/min (Figure 1a and Movie S1, in the SI). The necking C
DOI: 10.1021/acsami.8b09826 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. Micropatterning effects on mechanical properties and kinematics. (a) Stress−strain profiles of PVDF/FePs necking strips showing patterning orientation-dependent mechanical strength, demonstrating the anisotropic elastic tensors. (b, c) Stress−time profiles indicating the deformation stress generated in PVDF/FePs necking strips upon acetone vapor ad/desorption; (b) the strip having micropatterns perpendicular to its long axis and (c) parallel to the long axis. (d) Time-bending angle profiles of PVDF/FePs necking strips driven by acetone vapors. After removal of FePs, the strip responded with a faster bending motion. Inset defines the bending angle. (e) Sketch of the PVDF/FePs necking strip rolling forward driven by the combination of acetone vapors and magnetic field. (f) Snapshots of the rolling strip of micropatterned PVDF/FePs.
necking film was cut into a square with the FePs aligned at an angle of 90° to its axis (Figure 1d). Upon contacting the acetone-soaked filter paper substrate, the lower face expanded by absorbing more acetone than the upper face, inducing the film actuator to curl along the FePs-aligned direction; when the top surface was turned down, it was still curled along the FePsaligned direction (Figure 1d and Movie S2, in the SI). The reversible curling was capable of continuous proceeding over 100 times without apparent fatigue (Figure S2, in the SI). To further verify this kinematics, we cut the necking film into a strip with alignment of FePs at an angle of 45° to its long axis. As it contacted the acetone-soaked substrate, the lower face absorbed more acetone vapors and expanded to induce the strip twisting in the right-hand direction toward the top surface; after turning it over, it twisted upward, but in the lefthand direction (Figure 1e and Movie S3, in the SI). The above motions clearly demonstrate that the deformations always
appeared as the stress reached 15 MPa and was continuously generated until the strain reached 300% (Figure 1b). This process was enhanced by stretching the film strip to the strain of 550%. In the necking strip, alignment of FePs became more regular, and PVDF element was orientationally crystallized as well (see discussion below); the thickness was decreased to 16 μm. The edge of necking of PVDF/FePs became unsmooth due to residual stress from stretching. To remove the residual stress and make it flatter, the necking was clipped between two glass slides and heated at 70 °C for 4 h. Figure 1c shows the heat-treated necking of PVDF/FePs with a well-defined micropatterning structure. The orientational alignment of FePs is expected to affect the elastic tensor of the necking, which might direct the shape deformation upon exposure to acetone vapors, whereupon the deformation should be only related to the direction of stretching and not to the shape of the film itself. To test this hypothesis, the D
DOI: 10.1021/acsami.8b09826 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. Application of micropatterned PVDF/FePs strips. (a) Sketch of the device used to detect acetone vapors. Acetone-driven curling of micropatterned PVDF/FePs strips blocked infrared light of emitter into receiver to turn on the LEDs; as the blocking was retracted by uncurling, the LEDs were turned off. (b) Photograph of the detector. (c) Snapshot showing the LED that was turned on as acetone was added at P-1. (d, e) With diffusion of acetone, the LEDs at P-2 and P-3 were tuned on. (f, g, h) Snapshots of the working device showing that the LEDs were turned off one by one owing to the evaporation of acetone.
anisotropic elastic tensors in the film and accelerated acetone sorption/desorption, making the shape-programmable deformation possible. The matrix of PVDF was stretched into an oriented microstructure by strain-induced crystallization,39 which has a synergistic contribution to the shape-programmable deformation (Figure 2f). The atomic force microscopy further reveals this patterning structure at the microscopic scale (Figure S4, in the SI). The X-ray diffraction spectroscopies indicate that this microscopic patterning structure has reached a molecular scale. Before the PVDF/FePs film was stretched, it did not give rise to a unidirectional diffraction pattern (Figure 2g,h). After stretching, a sharp single peak appeared at 20°, indicative of an oriented microstructure of the PVDF matrix (Figure 2h,i). 3.3. Mechanical Characterizations of Micropatterned PVDF/FePs Film Actuator. Micropatterning-induced shapeprogrammed deformations resulted from the anisotropic elastic tensors of the necking strips. To quantitatively confirm the anisotropic elastic tensors, we stretched the necking strips along the direction perpendicular or parallel to the microscopic patterns. As shown in Figure 3a, the stretching along the direction perpendicular to the micropatterns proceeded with lower tensile stress, indicating a lower elastic tensor in
proceeded along the FePs-aligned direction, no matter which way the acetone sorption happened. The vapomechanical kinematics of the PVDF/FePs necking films is different from that of pure PVDF film, where it deformed in random direction (Figure 1f and Movie S4, in the SI). 3.2. Structural Characterizations of Micropatterned PVDF/FePs Film Actuator. We first examined the PVDF/ FePs necking structure by polarized optical microscope under the light-transmissive mode. The alignments of FePs reached a high orientation in heat-treated necking film (Figure 2a). In the zoom-in view, we can see that the spacing between the alignments of the PsFe particles varied from 20 to 50 μm (Figure 2b,c). The observation under reflective mode confirms the same results (Figure S3, in the SI). The alignment spacing is of great significance to generate the film with different elastic tensors for shape-programmed deformation.23 To get more insights into the microscopic patterning structure, the necking film was characterized by scanning electron microscope. Figure 2d shows that the FePs were located indeed along a linear region instead of random dispersion inside the matrix. During the stretching process, the FePs were separated from the matrix, resulting in elliptical pores; however, the FePs did not come off (Figure 2e). The porous structure further induced E
DOI: 10.1021/acsami.8b09826 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
on the LEDs. We used micropatterned PVDF/FePs strip as a controller that could block or unblock the infrared light by acetone vapor-induced reversible deformation. The partial surface of the controller was covered with aluminum foil for better blocking of the light. At the initial state, the controller was flat and did not block the infrared light, so that the LEDs were turned off (Figure 4b, and Movie S6 in the SI). As we dropped acetone at position 1 (P-1), the controller absorbed the acetone vapors and curved up to block the infrared light, turning on the LED (Figure 4c). With the diffusion of acetone into P-2 and P-3, the LEDs were turned on one after another (Figure 4d,e). After acetone evaporated, the controller recovered its flat shape and the block was retracted; the LEDs were turned off again (Figure 4f−h). This device provides a simple yet effective method to visually detect the leakage of acetone, demonstrating the potential of micropatterned PVDF/FePs materials in electronics and industrial applications.
comparison to the necking strip that was stretched along direction parallel to the FePs alignments. The lower elastic tensor allows for a stronger deformation upon the sorption of acetone vapors, which was testified by monitoring the deformation stress generated in PVDF/FePs necking strips on a mechanical tester. A heat-treated PVDF/FePs necking strip (25 mm × 5 mm × 16 μm) with micropatterns perpendicular to its long axis was clamped and preloaded with a force of 0.02 N and held on for 5 min to keep the strip tight and straight in a stable state. Asymmetric sorption of acetone vapors for 2 s caused the necking strip to deform with the generation of a deformation stress of 0.4 MPa (Figure 3b). As acetone vapor was terminated, the strip desorbed acetone molecules and the stress was recovered. In contrast, when this operation was done for the necking strip having the micropatterns parallel to the long axis, the deformation stress reached only 0.1 MPa. This result provides more evidence for the micropattern-dependent shape-programmed deformation. 3.4. Controllable Kinematics of Micropatterned PVDF/FePs Film Actuator. As demonstrated in Figure 2d, stretching led to the formation of pores in the necking strips that might accelerate sorption of acetone vapors and increase the kinematic sensitivity. To examine this point, we designed a device in which a Petri dish (outer diameter: 12 cm) was fully filled with acetone-saturated cotton and covered with fourlayered filter papers (Figure S5, in the SI). This device allows generating consistent gradients of acetone vapors over the filter paper (p = 19 kPa, 15 °C). When a heat-treated PVDF/FePs necking strip (20 mm × 5 mm × 16 μm) with micropatterns perpendicular to its long axis was placed on the filter paper, it curled up with the bending angle reaching 40° in 95 s (Figure 3d), and the curvature changed to 0.9 mm−1 in this curling course (Figure S6, in the SI). After the removal of FePs by HCl solution, the porous structure became more visible (Figure S7, in the SI); as a result, it exhibited a more rapid curling motion with the bending angle reaching 40° in less than 5 s, which is 19 times the curling speed of the necking strip before FePs were removed (Figure 3d). Acetone vapor driven shape-programmed deformation of PVDF/FePs necking strips can be regulated into directional movements through the manipulation of magnetic field. As shown in Figure 3e, the strip (15 mm × 5 mm × 0.16 μm) was placed over an acetone-soaked paper (acetone: 20−30 wt %) and a small magnetic bar (100 mm × 10 mm × 5 mm) was located below (density: 9 mT); as the strip absorbed acetone vapors and curved up, the magnetic bar moved forward. Because of weak magnetic field, the deformed strip became mechanically unstable and thus rolled over toward the magnet bar at a speed of 1.3 cm/min (Figure 3f and Movie S5 in the SI). This is a demonstration of the capability of FePs element that is able to not only pattern the composite film for shapeprogrammed deformation but also generate weak interaction with the magnetic field for directional movements over the acetone-contained substrate. 3.5. Application of Micropatterned PVDF/FePs Film Actuator. To explore potential applications, the directional shape deformation of PVDF/FePs was utilized in a photoelectric device to detect acetone molecules. Figure 4a shows the schematic of the photoelectric device that was assembled with electronics and micropatterned PVDF/FePs strips. An emitter was capable of generating an infrared light into a receptor to turn off light-emitting diodes (LEDs); as the infrared light was blocked, the receptor made contact to turn
4. CONCLUSIONS A shape-programmed acetone vapors-responsive PVDF/FePs actuator was developed through a simple yet useful micropatterning technique. The micropatterning endows the actuator with controllable kinematics while maintaining its high stretchability and toughness, which is superior to the macroscopic patterning that normally breaks mechanical integrity during shape deformation. Upon exposure to acetone vapors, the PVDF/FePs actuator responded in an exactly controlled manner, where it always deformed along the orientation of the micropatterns induced by the anisotropic elastic tensors. As we combined acetone vapors with magnetic field at the same time in the actuating system, micropatterned PVDF/FePs actuator was capable of continuous movements at a speed of 1.3 cm/min, and the motion direction was controlled by the magnetic field. This directional controllable shape deformation was applied in a photoelectronic device, demonstrating the multistage-detecting capability of acetone vapors.
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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.8b09826. Stress−strain curve of pure PVDF film; cyclic testing on reversible and repeated actuation of the PVDF/FePs film; surface topology structures; curvature−time profile; SEM images (PDF) Stretching of PVDF/FePs into necking structure on a universal stretching machine; the video was shown at 3 times the real speed (MPG) Kinematics of a square necking of PVDF/FePs over the acetone-containing filter paper; square size: 1.0 cm × 1.0 cm × 16 μm; the video was shown at 4 times the real speed (MPG) Kinematics of a necking strip of PVDF/FePs over the acetone-containing filter paper; strip size: 1.5 cm × 0.5 cm × 16 μm; the video was shown at 8 times the real speed (MPG) Kinematics of pure PVDF film over the acetonecontaining filter paper (MPG) Directional movements of the strip of micropatterned PVDF/FePs driven by acetone vapors and magnetic F
DOI: 10.1021/acsami.8b09826 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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field; the video was shown at 8 times the real speed (MPG) Multistage-detecting device that provides a simple yet effective method to visually detect the leakage of acetone; the video was shown at 15 times the real speed (MPG)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Lidong Zhang: 0000-0002-0501-6162 Author Contributions
J.W. contributed to all aspects of this study. F.W. contributed to the part characterizations of the materials. L.Z. conceived the study. The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of Shanghai (Grant No. 17ZR1440600) and the National Natural Science Foundation of China (Grant No. 51603068).
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DOI: 10.1021/acsami.8b09826 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
