Control of Reversible Self-Bending Behavior in Responsive Janus

Mar 14, 2016 - In this research, we explore a basic self-bendable “Janus microstrip” system consisting of a soft, pH-sensitive hydrogel microstrip...
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Control of Reversible Self-Bending Behavior in Responsive Janus Microstrips Myung Seok Oh,†,∥ Young Shin Song,‡,∥ Cheolgyu Kim,§ Jongmin Kim,‡ Jae Bem You,† Taek-Soo Kim,§ Chang-Soo Lee,*,‡ and Sung Gap Im*,† †

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Chemical Engineering, Chungnam National University, Yuseong-gu, Daejeon 305-764, Republic of Korea § Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: Here, we demonstrate a simple method to systematically control the responsive self-bending behavior of Janus hydrogel microstrips consisting of a polymeric bilayer with a high modulus contrast. The Janus hydrogel microstrips could be easily fabricated by a simple micromolding technique combined with an initiated chemical vapor deposition (iCVD) coating, providing high flexibility in controlling the physical and chemical properties of the microstrips. The fabricated Janus hydrogel microstrip is composed of a soft, pH-responsive polymer hydrogel layer laminated with a highly cross-linked, rigid thin film, generating a geometric anisotropy at a micron scale. The large difference in the elastic moduli between the two layers of the Janus microstrips leads to a self-bending behavior in response to the pH change. More specifically, the impact of the physical and chemical properties of the microstrip on the self-bending phenomena was systematically investigated by changing the thickness and composition of two layers of the microstrip, which renders high controllability in bending of the microstrips. The curvature of the Janus microstrips, formed by self-bending, highly depends on the applied acidity. A reversible, responsive self-bending/ unbending exhibits a perfect resilience pattern with repeated changes in pH for 5 cycles. We envision that the Janus microstrips can be engineered to form complex 3D microstructures applicable to various fields such as soft robotics, scaffolds, and drug delivery. The reliable responsive behaviors obtained from the systematic investigation will provide critical information in bridging the gap between the theoretical mechanical analysis and the chemical properties to achieve micron-scale soft robotics. KEYWORDS: Janus microstrip, self-bending, pH responsivity, micromolding, initiated chemical vapor deposition (iCVD)

1. INTRODUCTION

based on the advances in the development of theoretical studies. Many 3D structures have been demonstrated via harnessing a designed self-bending behavior triggered by controlling the environmental conditions.3−6 In particular, the reversibility and reproducibility are examples of the unique advantages of this approach,3,7,8 and the method has been applied to various fields requiring 3D structure architecting

Over the past decade, there has been a great deal of interest in the study of responsive self-transitions in novel functional materials as alternative ways to construct three-dimensional (3D) microstructures from two-dimensional (2D) objects.1,2 More specifically, responsive self-bending is a spontaneous behavior in which a matter undergoes a conformational change in response to external stimuli. Experimental, empirical, and theoretical investigations have been performed on various types of developed novel responsive materials. The primary goal is to understand and mimic the mechanics of biological systems © XXXX American Chemical Society

Received: December 27, 2015 Accepted: March 14, 2016

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DOI: 10.1021/acsami.5b12704 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces such as in soft microrobotic,9 cell scaffold,10−13 actuator,6,14 and drug delivery applications.7,15−18 Several types of potential driving forces, including temperature,5,12,18,19 solvents,6,13 magnetism,20 ionic strength,11 and pH4,7,8,17 have been employed to induce the responsive self-bending/unbending. The driving force to initiate the transformation of 2D objects to form 3D microstructures is mainly attributed to a residual internal stress programmed in the responsive 2D objects with separate swelling properties. By harnessing this responsively generated internal stress, the desired transformational shapes can be designed. Selective patterns with characteristic features can also be employed in the responsive 2D object in order to prescribe the modulation of the mechanical properties thereof. In several cases, the modulation change provided to the 2D object could result in a large difference in the final 3D shape of the object. However, only a few studies are available to demonstrate the quantitative relationship between the physical/ chemical properties of the responsive object and its selfresponsive behavior at micron scale. Moreover, the lack of understanding of the mechanism driving shape transformations through self-responsive behavior seriously limits further advancement in the guided transformation of self-responsive soft matter. Furthermore, understanding the role of the magnitude of the strength difference between the passive and active parts in the 2D soft matter is an important step in developing the basic design rules for more elegant matter, and it paves the way for producing programmable soft matter on demand. In this research, we explore a basic self-bendable “Janus microstrip” system consisting of a soft, pH-sensitive hydrogel microstrip as the “active layer” and a rigid, nonreactive thin film as the “passive layer” under the action of biaxial stresses. The influence of the thickness and composition of each layer on the pH-responsive self-bending behavior of the synthesized Janus microstrips is investigated systematically. The active layer part of the Janus microstrip is composed of a hydrogel cross-linked from a mixture of photocurable monomers, 2-hydroxyethyl methacrylate (HEMA), poly(ethylene glycol diacrylate) (PEGDA), and acrylic acid (AA). A micromolding technique is employed to define the dimensions of the Janus microstrips on demand and precisely attenuate the chemical properties through a simple change of the chemical composition of the monomer mixture.21,22 Then, a rigid, nonreactive poly(divinylbenzene) (PDVB) passive layer is deposited on the polymerized hydrogel active layer via initiated chemical vapor deposition (iCVD) to complete the Janus microstrip. The iCVD polymer film grows from the surface, which enabled the spreading out of the highly cross-linked polymer film on the soft surface, maintaining the polymer film ultrathin, which is challenging to achieve with conventional liquid-phase methods. Moreover, the surface growing mechanism can warrant high adhesion between the cross-linked polymer film with the PEGDA-based soft segment via the surface reaction between the residual un-cross-linked acrylate moiety in PEGDA and the vinyl group in DVB monomers. The AA-containing microstrip readily swells in response to the change of pH due to the electrostatic repulsion formed by the deprotonation of the acrylic acid moiety in basic conditions. The degree of swelling of the active layer (pH-responsive microstrip) can be precisely tuned by changing the pH; meanwhile, the passive layer of the PDVB thin film maintains the initial dimensions during the same pH changes. The difference in the swelling behavior generated between the active

and passive layers results in a directional self-bending of the “Janus microstrip”. The degree of the self-bending behavior of the Janus microstrip could be programmed initially during the fabrication step, by architecting the geometrical dimension, and altering the chemical composition of the active layer of the microstrip. We find that the thickness ratio of the active layer to the passive film greatly influences the self-bending behavior. The fraction of AA in the active layer is also found to be a critical engineering parameter for controlling the electrostatic repulsion force in the polymeric network to achieve reliable responsive self-bending behavior. Unlike inorganic or metal materials, the polymeric Janus microstrip shows a high degree of reversibility of the bending/unbending behavior in responsive to the repeated cycles of pH change. The Janus microstrips developed in this study will serve as a novel tool for controlling the self-bending behavior by independently tuning the parameters at the micron scale. The obtained results will also contribute to the establishment of a framework to fabricate responsive 3D microstructures derived from simple 2D microstructures. We believe that the findings shown here can have a significant impact on the design of environmentally responsive materials in biomechanics, soft robotics, healing materials, tissue engineering, and medicine.

2. EXPERIMENTAL SECTION 2.1. Chemicals Used to Fabricate Janus Microstrips. 2Hydroxyethyl methacrylate (HEMA, 97%), poly((ethylene glycol) diacrylate) (PEGDA, Mn = 700), acrylic acid (AA), and 2-hydroxy-2methylpropiophenone (Darocure 1173, 97%) were purchased from Sigma-Aldrich Chemicals (St. Louis, MO). The SU-8 3005 (photoresist) and developer solution were purchased from Micro Chem (Newton, MA). Poly(dimethylsiloxane) (PDMS, Sylgard 184) was obtained from Dow Corning (Midland, MI). The iCVD monomer, divinylbenzene (DVB), and the initiator, tert-butyl oxide (TBPO), were all purchased from Sigma-Aldrich Chemicals. All chemicals were used as purchased without further purification. 2.2. Procedure to Fabricate Janus Microstrips. Active microstrips were synthesized using the micromolding technique.21,22 The micromold was fabricated using a conventional soft lithography technique. A mixture of PDMS prepolymer and its curing agent (10:1 ratio) was mixed to form the PDMS micromold. The mixture was poured on a prepatterned Si master (300 × 100 × 8 μm), and the air bubble in the PDMS mixture was removed in a vacuum desiccator for 30 min. Next, the mixture was cured at 65 °C for 8 h. After the curing, the PDMS replica was peeled off from the Si master. The ultraviolet (UV)-curable precursor solution for the active part was composed of 38% HEMA, 28.5% AA, 28.5% PEGDA and 5% photoinitiator. A droplet of the UV-curable solution in the molds was exposed to UV (100 W HBO mercury lamp) over a wavelength range of 330−380 nm (UV-2A filter, Nikon, Japan) under N2 atm. This mixture was polymerized under UV irradiation for 2 min. After the fabrication of the active microstrips, the Janus property was assigned on the top surface of the microstrips using iCVD polymeric film, poly(divinylbenzene) (PDVB). A PDVB layer was deposited in an iCVD reactor (Daeki Hi-Tech Co., Ltd., Korea). The DVB vaporized monomer and TBPO initiator were introduced into the iCVD chamber at a flow rate of 6 and 1 sccm, respectively. To vaporize the reactants, the TBPO was kept at room temperature while the DVB was heated to 40 °C. The process pressure of the chamber was set to 400 mTorr. The temperature of the filament was 180 °C, and the substrate was kept at 23 °C. The deposition rate of PDVB is approximately 100 nm/15 min. The thickness of the polymer layer was monitored in situ using a He−Ne laser (JDS Uniphase). To induce self-bending of fabricated Janus microstrip in acidic and basic condition, we tuned the pH of the solution precisely by addition of 1 M NaOH and 1 M HCI solution into DI water while monitoring the pH of the solution in situ using pH meter (Mettler toledo). For acidic solutions, we added 100 B

DOI: 10.1021/acsami.5b12704 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces and 1 μL of HCl solution to titrate to achieve pH 3 and 5, respectively. For base solutions, we added 1 and 100 μL of NaOH solution to titrate to achieve pH 9 and 11, respectively. To obtain pH 7, we added the about 0.1 μL of NaOH solution to DI water, which had initial pH around 6. The recovered Janus microstrips were rinsed with isopropyl alcohol (IPA) and redispersed in IPA for further analyses. 2.3. Image Analysis (Optical Microscopy and Scanning Electron Microscopy). An inverted fluorescence microscope (TE2000, Nikon, Japan) equipped with a CCD camera (Coolsnap, Photometrics, Tucson, AZ) was used to observe the synthesized Janus microstrips. The degree of bending of the Janus microstrips was characterized by measuring the bending curvature of each Janus microstrip using a fluorescence microscope. Image analyses of the selfbending Janus microstrips were performed using the ImageJ program (http://rsb.info.nih.gov/ij/). The morphology of the Janus microstrip structures was also imaged using scanning electron microscopy (SEM; JEOL, JSM-7000F, Japan). 2.4. Measurement of Elastic Modulus (Active Microstrip and Passive PDVB Layer). The elastic moduli of the active microstrips and of the passive films were measured in each pH solution because the active microstrip responds to a pH condition. A dynamic mechanical analyzer (DMA8000, PerkinElmer, Waltham, MA) was used to measure the modulus of the active microlayer. Specimens were prepared with a cylindrical shape with a diameter of 2.5 mm and a height of 3 mm. The storage modulus of the active microstrips was measured using the compressive DMA mode with a submerged chamber. The measured storage modulus was considered to be an elastic modulus in the condition of 0.1 Hz for low strain rates.20 The modulus of the passive PDVB layer could not be measured using a conventional tensile tester due to its extremely small thickness ( δp,h). To explain further, the resistance force against deformation can be introduced as a concept of “stiffness”. The stiffness stands for the rigidity of an object or the resistance to deformation, which

Figure 2. (a) Schematic illustration showing the radius of curvature, R, and the bending curvature, κ, of the bent Janus microstrips. (b) A low thickness ratio between the passive PDVB film and active microstrip. (c) A high thickness ratio between the passive PDVB film and active microstrip. (δ is the change in length of the layer after the forces equilibrate; F is the force acting on the layer; and a and p stand for the active microstrip and passive film, respectively.).

Figure 3. (a) The dependency of the bending curvature (κ) of Janus microstrips with respect to the thickness of the passive PDVB thin film at pH 11. (b) OM images of self-bent Janus microstrips with different dimensions of microstrips. The text inside the images indicates the thicknesses of the active layer (μm) and the passive thin film (nm). The scale bar in each figure indicates 300 μm.

is defined as k = F/δ, where F is the force applied to the material, and δ is the displacement along the same direction as the applied force. For the case of tension or compression forces, stiffness can be expressed as k = EA/δ, where E is the Young’s modulus of the material, and A is the cross-sectional area. Therefore, to create the same displacement, materials with low stiffness require a smaller force than those with high stiffness. At the final equilibrium states with the consideration of bending, the lower active layer causes the system to expand and D

DOI: 10.1021/acsami.5b12704 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the upper passive film resists the expansion. Because the acting lines of force are not in the same position, a torque will be generated by the tensile force from the active layer; meanwhile, the contracting force is attributed to the passive film. In the case of a low thickness ratio, the active layer expands the passive film almost uniformly without large bending due to the relatively low stiffness of the passive film (expansion dominant). On the other hand, the active layer near the interface undergoes a greater resisting force than the freely expanding active layer farther away from the interface at a high thickness ratio. This mechanism results in a force imbalance, and a torque is generated, making the system bent upward (bending dominant). For this reason, the bending phenomenon was greater when the thickness of the passive film became comparable to that of the active layer. If the thickness of the passive film was too small or that of the active layer was too thick, the passive film would follow the expansion of the active layer. In this work, for example, the Janus microstrips with 8 μm thickness show that the increase from 6.15 to 21.50 mm−1 in the bending curvature is proportional to the increase from 100 to 400 nm in the thickness of the passive PDVB film. Analogously, the bending curvature increased with the decrease in the thickness of the active microstrip. In the case of a 100 nm thick passive film, the bending curvature decreased from 15.24 to 0.86 mm−1 for an increase in the thickness of the active microstrip from 4 to 10 μm. Figure 3b shows the optical microscope images of self-bent Janus microstrips exposed in pH 11. By increasing the thickness ratio of the passive film to the active microstrip, a higher degree of self-bending was observed. In particular, in the case of a ratio of 4 μm (active microstrip thickness) to 400 nm (passive film thickness), the Janus microstrips were fully rolled up to form a complete ring-shaped structure. The significant degree of self-bending observed from the Janus microstrip can be mainly ascribed to the high contrast in the elastic moduli between the active microstrip and the passive film. In this regard, the bending behavior of the micronthick Janus microstrips could be successfully controlled by the passive films whose thicknesses are on the order of only a few hundreds of nanometers. The bending curvature could also be tuned by changing the AA composition in the active microstrip to cause the variation of the induced electrostatic repulsion force in the pHresponsive AA molecules. The increased AA fraction in the microstrip induced more amount of electrostatic repulsion, leading to more pronounced swelling of the active microstrips, inducing a larger deformation of the active microstrips, increasing δa, and thus increased the bending curvature of the Janus microstrip. By increasing the AA content from 0% to 30%, the swelling ratio increased from 23% to 38% (Figure S2, Supporting Information) due to the increased electrostatic repulsion among the carboxylic anions in the AA molecules in high pH (pH = 11). The bending curvature also increased accordingly from 8.66 to 21.51 mm−1 as the AA concentration increased from 0 to 30% (Figure 4a). Note that a 23% swelling of the active microstrip was also observed even without the AA contents, which is most likely due to the hydrophilic HEMA content in the active layer because HEMA is also well-known for its high swelling properties. Figure 4b shows the optical microscope images of the self-bent Janus microstrips with varying AA compositions. In the cases of 10−20% of AA content, the Janus microstrips formed a half-pipe-like structure with a bending curvature ranging from 12.01 to 12.74 mm−1.

Figure 4. (a) The bending curvature, κ, of the self-bent Janus microstrip vs the AA concentration in the active microstrip. The thickness of each active microstrip and passive film was fixed to 8 μm and 400 nm, respectively. (b) Optical microscope images of self-bent Janus microstrips with various AA concentrations in pH 11. The scale bars indicate 300 μm.

The Janus microstrips formed a complete cylindrical shape with an increased bending curvature of 21.51 mm−1 when the AA content was increased up to 30%. It follows from this observation that the self-bending tendency can be controlled by tuning the AA concentration in the active microstrips. Along with the variation of thickness of the passive film, changing the AA concentration to control electrostatic repulsion also enables the systematic tuning of the self-bending behavior. The independent tunability of the degree of self-bending is highly desirable to form various 3D supra-structures made of the Janus microstrips presented in this study. The variation in the pH of the medium applied to the Janus microstrips also influences the degree of self-bending of a microstrip substantially. As mentioned above, the swelling of the active microstrip is mainly ascribed to the electrostatic repulsion among the deprotonated, negative carboxyl ions in the AA molecules, which are generated exclusively in high pH conditions. In Figure 5a, the change in the bending curvature with respect to the pH variation is illustrated. In pH 3, the Janus microstrips were practically flat with a 0.72 mm−1

Figure 5. (a) The pH-dependency variation of the bending curvature, κ, of the Janus microstrips. The thickness of the active microstrips and the passive films were fixed at 8 μm and 400 nm. (b) Optical microscope images of the self-bent Janus microstrips with each separate pH. The scale bars indicate 300 μm. E

DOI: 10.1021/acsami.5b12704 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

the Janus microstrips. In this study, by adopting micromolding and iCVD methods, we could reliably fabricate polymeric Janus microstrips. The chemical composition of the active microstrips and the nanoscale thickness of the passive PDVB layer could be varied during the fabrication step. The strong adhesion between the active and passive layers improved the pH-responsiveness and reproducibility of the Janus microstrips, even after the repetitive pH cycling transitions. By changing the thickness of both active microstrip and passive PDVB layer, the degree of the self-bending behavior could be controlled systematically. The self-bending curvature increased as the ratio of the passive layer thickness to the active microstrip increased. In this system, only a few percent change in the passive layer dimension was sufficient to induce a complete bending structure. Analogously, changing the AA composition of the active microstrip could also control the electrostatic repulsion force in a high pH solution, resulting in the controllable self-bending behavior of the Janus microstrips. The external stimulus (pH of the environment) could also change the self-bending behavior with high reproducibility. A reversible transition of the bending/ unbending behavior of the Janus microstrips between pH 3 and 11 was successfully observed for up to five cycles of pH variation. We believe the developed system in this work will be utilized widely to develop guided pH-responsive 3D suprastructures with precise controllability.

bending curvature resulting from a lack of electrostatic repulsive forces responsible for the swelling of the active microstrip. The bending curvature increased up to 21.51 mm−1 as the pH increased from 3 to 11. Figure 5b shows the optical microscope images of the self-bent Janus microstrip for each separate pH of 3, 7, and 11, respectively. In pH 3, the Janus microstrips were mostly flat. In pH 7, the Janus microstrips were self-bent and formed a half-ring shape, which turned into a complete ring shape in pH 11. These results clearly demonstrate the possibility of controlling the degree of self-bending not only by tuning the preset factors, such as the thickness of the layers or the AA composition, but also by tuning an external stimulus: the pH of the medium. The self-bending behavior of the Janus microstrips with pH change was fully reversible, as shown in Figure 6. The bending

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ASSOCIATED CONTENT

S Supporting Information *

Figure 6. Variation in the bending curvature, κ, with respect to the repetitive pH switching cycles between 3 and 11. The thicknesses of the active microstrip and the passive film were fixed at 8 μm and 200 nm, respectively.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12704. Elastic moduli of active microstrip and passive PDVB film, swelling ratio of active microstrip. (PDF)

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curvature of the Janus microstrips was monitored with a cyclic switching of the pH from 3 to 11. In acid conditions (pH 3), the Janus microstrips maintained their flat geometry and the measured bending curvature was 0.42 mm−1, indicating practically no self-bending of the Janus microstrips. Switching the pH to 11 induced the bending of the microstrips, resulting in a bending curvature of 6.59 mm−1, in which the Janus microstrips formed into complete cylindrical structures. We repeated the pH swelling test for four more cycles to confirm the reversibility of the self-bending behavior. Over the four pHswitching cycles, the bending curvatures of the Janus microstrips remained between 0.56 ± 0.22 mm−1 in pH 3 and 6.49 ± 0.33 mm−1 in pH 11. From this result, we can confirm the outstanding reversible self-bending/unbending property of the Janus microstrip. The reliable reversibility of the bending/ unbending behavior of the Janus microstrips is due to the chemical and mechanical stability of the highly cross-linked PDVB passive layer against the severe pH change. In addition, the strong adhesion between the PDVD passive layer and the hydrogel microstrip is also critical to achieve the excellent reversibility of the pH-responsiveness. The abrupt but stable transition of the bending curvature can further be employed for pH-responsive microstructure engineering with high durability.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +82-42-821-5896. *E-mail: [email protected]. Fax: +82-42-350-3910. Author Contributions ∥

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported in part by the Graphene Materials and Components Development Program of MOTIE/KEIT (10044412, Development of basic and applied technologies for OLEDs with graphene), by an Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korea government (MSIP) (B0101-15-0133, the core technology development of light and space adaptable energysaving I/O platform for future advertising service), and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by MSIP (No. NRF-20110017322).

4. CONCLUSION In conclusion, we developed a pH-responsive self-bending system consisting of bilayered Janus microstrips. The selfbending aspect could be controlled independently by preset parameters, such as the film thickness and the concentration of AA, and an external stimulus, the pH environment applied to

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DOI: 10.1021/acsami.5b12704 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX