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Shape-Morphing Chromonic Liquid Crystal Hydrogels Ruvini S. Kularatne, Hyun Kim, Manasvini Ammanamanchi, Heather N. Hayenga, and Taylor H. Ware Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04553 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016
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Shape-Morphing Chromonic Liquid Crystal Hydrogels Ruvini S. Kularatne, Hyun Kim, Manasvini Ammanamanchi, Heather N. Hayenga and Taylor H. Ware* Department of Bioengineering The University of Texas at Dallas 800 West Campbell RD, Mailstop BSB 11, Richardson, TX 75080 chromonic liquid crystal, hydrogels, actuators, polymers, liquid crystals. ABSTRACT: Guided self-assembly of liquid crystals is a powerful approach for the design of active materials with dynamic shape or topography. However, these strategies are typically limited to hydrophobic, thermotropic materials. Here we report, the first example of stimuli-responsive hydrogels synthesized by the radical polymerization of chromonic liquid crystal monomers. Chromonic monomers are aligned using patterned surfaces and polymerized into mechanically-anisotropic hydrogels. By copolymerizing chromonics with n-isopropylacrylamide, these gels can be rendered stimuli-responsive to temperature changes near physiological conditions. This stimulus response is anisotropic and dictated by the molecular order of the gel. By patterning molecular order, we fabricate films that morph from flat to 3D shapes. Furthermore, the introduction of the chiral chromonic dopants allows the polymerization of chiral hydrogels with half pitch as small as 2.9 µm that undergo reversible changes in topography.
Stimuli-responsive hydrogels are a class of materials capable of changing shape and topography in response to changes in environmental conditions.1–3 Such smart materials may enable a new generation of active biomedical devices capable of controllable change in function in the physiological environment. These materials can change shape and function in response to a variety of stimuli such as pH, temperature, or light.1,2,4,5 For example, the change in hydrophilicity associated with the lower critical solution temperature (LCST) of poly(n-isopropylacrylamide) gives rise to volume changes in hydrogels.6 While this stimulus response is isotropic in nature, several strategies have emerged to enable the design of hydrogels capable of morphing into bent, curved, and or twisted geometries. Each of these approaches introduces anisotropy or spatial heterogeneity in the stimulus response of the gel. For example, spatially-controlled swelling, achieved by photopatterning crosslink density, can be used to design hydrogel films that morph from flat to Gaussian-curved surfaces.7 Another approach utilizes photolithographic patterning of stiff coatings on hydrogels. On swelling, the resulting bilayer structure bends and can be used to form Origamiinspired shapes.8,9 Anisotropy in the stimulus response can be achieved by aligning hydrogel composites using external forces such as shear or magnetic field.10,11 Each of these approaches uses top-down processing to create functional gel actuators. In addition to these top-down patterning approaches, guided self-assembly techniques can be used to pattern responsive materials. In hydrophobic polymers, directed self-assembly of thermotropic liquid crystalline monomers can be used to pattern stimulus response. For example,
directed self-assembly of molecular orientation in liquid crystal elastomers can be used to design actuators that respond to heat.12 Broer and coworkers, used selfassembly of chiral nematic liquid crystal monomers at the micron scale to create polymer films that reversibly morph from flat to textured in response to light, enabling control of gripping friction of a robotic finger.13 Despite the promise of this controllable stimulus response for creating smart devices, liquid crystalline polymer networks typically respond to stimuli that are unfavorable to living organisms, such as heat, UV light, or organic chemicals. One approach to overcome this limitation is to modify hydrophobic thermotropic liquid crystalline monomers to create more hydrophilic materials.14–16 Another approach is to template polymer structure with lyotropic liquid crystals, which exist in aqueous solutions.17,18 We describe for the first time a programmable hydrogel synthesized from monomers that exhibit lyotropic chromonic liquid crystal phases (chromonics). In comparison to amphiphilic lyotropics, chromonics are often formed in aqueous solutions of charged, plank-like molecules with a polyaromatic core and polar peripheral groups; a typical structure of drugs and textile dyes.19–26 These molecules stack into columns, and the columns form liquid crystalline phases in water at certain concentrations and temperatures. Furthermore, using patterned surfaces these liquid crystalline phases can be aligned macroscopically.27,28 Here we report that chromonic monomer mixtures can be aligned and then polymerized into morphing hydrogels with the chromonic liquid crystal order directing the shape change of the actuator.
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Figure 1. a) Chemical structure of ionic, methacrylatefunctionalized derivative of perylenediimide (PDI-DA), b) Phase diagram of PDI-DA, c) Schematic representation of the synthesis of anisotropic chromonic hydrogels, POM images indicating the macroscopic alignment of the PDI-DA/NIPAM mixture with (d) planar alignment and (e) twisted alignment.
The polymerizable chromonic monomer, an ionic, methacrylate-functionalized derivative of perylene diimide (PDI-DA), was synthesized according to a previously described method by Jeong and coworkers.29,30 (Supporting information and Figure 1a) This monomer has a perylene diimide core, which is a versatile synthetic intermediate for self-assembling materials.31,32 The monomer studied here exhibits lyotropic nematic and columnar phases typical of chromonic liquid crystals. This behavior is shown in the binary phase diagram of PDI-DA monomer in deionized water with respect to temperature, (Figure. 1b) as determined using textures observed in polarizing optical microscopy (POM). A typical nematic lyotropic chromonic liquid crystal phase is exhibited by the PDI-DA monomer in concentrations between 7 wt% and 17 wt%. Throughout this work, the nematic phase will be used as it can be aligned using programmed surfaces. For the synthesis of stimuli-responsive chromonic hydrogels, nisopropylacrylamide (NIPAM) was used as a comonomer. This monomer serves to reduce crosslink density of the gel and introduce an LCST transition. Nematic gels were synthesized from a monomer mixture of 20 wt% of PDI-DA, 10 wt% NIPAM, and 1 wt% potassium persulfate, a radical initiator, in deionized water. The addition of comonomers into the chromonic mixture alters the temperature and composition range of the liquid crystalline phase behavior.25,26 This mixture was selected as it exhibits the nematic phase near room temperature, while formulations with greater concentrations of NIPAM exhibit both isotropic and nematic phases at room temperature. (Supporting Figure 6) In order to program the mechanical response of the hydrogel, the monomer mixture must be aligned before polymerization. This monomer mixture can be macroscopically aligned following previously described methods, namely by simply rubbing two pieces of glass with a fine abrasive then assembling these two pieces of glass into a cell with a defined gap33 as shown in the schematic in Figure 1c. This rubbing generates grooves in the glass surface, which in turn align the liquid crystalline monomer mixture. If the rubbing directions are anti-parallel, homogeneous planar alignment is achieved (Figure. 1d). The characteristic bright and dark pattern observed while rotating
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the sample between crossed polarizers from the birefringent uniaxially oriented monomer is shown in Figure. 1d. If the rubbing directions are perpendicular, twisted nematic alignment is achieved, where the nematic director changes by 90° through the thickness of the cell. This orientation is confirmed through the bright appearance under crossedpolarizers and dark appearance under parallel polarizers (Figure 1e). As this monomer is achiral both left-handed and right-handed twist domains are observed. These domains are separated by the defect lines visible in Figure 1e. This order of this aligned monomer mixture can be trapped in a chromonic hydrogel through radical polymerization. This polymerization is triggered by exposure to visible light in the nematic state. While the precise mechanism of photopolymerization is unclear, visible light is known to induce radical anion formation on the dye molecule,34 and in the presence of K2S2O8, we observe photopolymerization.
Figure 2. a) AFM height image of PDI-DA monomer after drying, b) Normalized absorbance of the PDI-DA monomer and the PDI-DA/NNIPAM hydrogel as the sample is rotated with respect to the polarization of the incident light. c) Reversible anisotropic swelling of the chromonic hydrogel with varying solvent polarity, d) Anisotropic swelling and deswelling of the chromonic hydrogel with temperature parallel and perpendicular to the nematic director
By polymerizing the chromonic monomer mixtures in the ordered state, anisotropic hydrogels can be synthesized. To illustrate this point, we observed the morphology of the monomer mixture before and after polymerization. After drying the aligned monomer mixture, oriented aggregates could be observed using atomic force microscopy (AFM) (Figure 2a). A similar morphology was observed for the chromonic polymer network (Supporting Figure 7). The aggregation of the monomer mixture as well as the resulting polymer network was observed to be along the direction of rubbing. This was confirmed by the dichroic nature of both the monomer mixture and the resulting hydrogel (Supporting Figure 8). Both the monomer and polymer network showed the highest light absorption when irradiated with linear polarized light with the polarization perpendicular to the rubbing direction (Figure 2b). Thus, on average the long axis of the perylene core of the chromonic PDI-DA molecule is perpendicular to the rubbing direction and the PDI-DA columns are aligned along the rubbing direction.21 Critically, this molecular anisotropy also leads to hydrogels with anisotropic mechanical properties. As an indirect probe of mechanical properties,
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we evaluated the dimensional change of the film in solvents of increasing polarity. The chromonic hydrogel reversibly and anisotropically swells in all dimensions in the presence of polar solvents (Figure 2c and Supporting Figure 9). Parallel to the nematic director, a length change of approximately 50% was observed in response to water as compared to the length in chloroform. The swelling in the direction perpendicular to the nematic director, but parallel to the long axis of the molecule, was only 20% in length (Figure 2c). This mechanical anisotropy based on chromonic liquid crystalline order provides a programmable material property to control the deformation of hydrogels that swell in response to a given stimulus. The polymerization of the chromonic PDI-DA with NIPAM imparts stimuli-responsive behavior to the polymer network. Specifically, near the lower critical solution temperature (LCST) ~32 °C of the NIPAM comonomer, a reversible volume change is observed. The swelling and deswelling in the ordered chromonic hydrogel is anisotropic and dictated by the order of the hydrogel. On cooling from the deswollen state, the chromonic hydrogel expands ~25% along the nematic director and ~15% along the direction of molecular alignment (Figure 2d). In order to generate 3D shape change, this anisotropy must be patterned.
Figure 3. a) Chemical structure of the ionic, chiral derivative of perylene diimide (Chiral PDI). b) Schematic representation of the molecular orientation (left) and 3D actuation (right) of the twisted nematic chromonic hydrogel c) Reversible, 3D shape changes of the chromonic hydrogel with temperature.
Twisted nematic orientation in these stimuli-responsive gels leads to out-of-plane deformation. As demonstrated in Figure 1e, twisted orientation can be induced using orthogonally rubbed surfaces. However, this twist can occur in both the left and right-handed sense. To control the handedness of this twist, a chiral dopant was added. We synthesized a chiral-chromonic perylene diimde dopant (chiral PDI) according to a literature report (Figure 3a and Supporting Scheme 2).35 By adding 1 wt% of the chiral PDI to the PDI-DA/ NIPAM/ H2O mixture, a uniform twisted orientation in the chromonic hydrogel was obtained. Using the susceptibility of this monomer mixture to light, we polymerized twisted nematic hydrogels into shapes with sub-millimeter resolution. A mixture of 20% PDI-DA/ 10% NIPAM/ 1 w% chiral PDI/ 68 wt% water and 1 wt% K2S2O8 was first aligned in a twisted manner and was subsequently polymerized under a visible light through a mask to generate the cross shape shown in Figure 3b. In this case, the nematic director is oriented at ±45° to the long axis of each arm of the cross and rotates by 90° through the thickness of the gel. This patterned molecular structure causes the
flat film to morph into a controlled 3D shape (Figure 3b). Specifically, the gel undergoes a shape change from flat at 27 °C to 3D shape at 39 °C, with each arm in the cross twisting at an angle of 45° upon deswelling (Figure 3c). Here the twisted molecular orientation leads to orthogonal strains on the top and the bottom of the hydrogel. This deswelling mechanism qualitatively mimics the behavior of thermoresponsive thermotropic liquid crystal polymer networks, but notably occurs in response to temperature ranges that are compatible with living organisms.36
Figure 4. a) Schematic representation of the chiral chromonic hydrogel. b) POM image of the fingerprint texture observed for the chiral chromonic hydrogel. c) Surface morphology of the dried chiral chromonic hydrogel observed under the AFM. d) Dependence of the half pitch length of the fingerprint texture of hydrogels with various concentration of chiral dopant and the dependence of half pitch length on temperature.
This hierarchal self-assembly of molecular order in the chromonic hydrogel also enables shape programming at the micron-scale. Here we demonstrate free-standing hydrogels with dynamic topography that is not based on wrinkling or buckling. To achieve this small-scale shape change, chiral hydrogels were synthesized by the introduction of 7-15 wt% of chiral PDI to the PDI-DA/ NIPAM mixture. These mixtures exhibit a chiral nematic phase at room temperature (Supporting Figure 11). In unrubbed glass cells, a fingerprint texture is observed (Figure 4a). This texture corresponds to a rotation in the molecular alignment where the helical axis lies within the plane of the sample. After polymerization and removal of the nonpolymerizable chiral dopant, this texture remains imprinted in the polymer network (Figure 4b). The result is a surface which anisotropically deforms on deswelling leading to a surface topography, a series of valleys that correspond to the optically observed fingerprint texture (Figure 4c). Controlling the concentration of chiral dopant enables control of the pitch length of the fingerprint texture and the corresponding texture, with increasing pitch length observed with decreasing chiral dopant concentration (Figure 4d). A half pitch length (distance of periodicity) of 6.5 µm was observed upon the addition of 7.2 wt% chiral dopant, and the half pitch length was reduced to 2.9 µm by the addition of 14.2 wt% chiral dopant. Furthermore, the thermoresponsive nature of the resulting chromonic polymer network results in a dynamic change in pitch length of each chiral hydrogel with temperature (Figure 4d). The half pitch length was reduced by ~20% upon heating the hydrogel to 45 °C for the 14.2 wt% chiral dopant. The dy-
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namic pitch of these chiral hydrogels represents a distinct mechanism to introduce dynamic topography in hydrogels. In conclusion, we have synthesized chromonic liquid crystal hydrogels by the co-polymerization of ionic dimethacrylate derivative of perylene-3,4,9,10tetracarboxylic diimide (PDI-DA) with nisopropylacrylamide. This approach enables the powerful toolbox of liquid crystalline self-assembly to be used to design stimuli-responsive hydrogels. These materials retain the thermoresponsive behavior of NIPAM, but swell anisotropically as dictated by the molecular order of the chromonic liquid crystal. The anisotropic swelling in the chromonic hydrogel can be controlled to create actuators that morph into 3D shape at physiological temperatures. The introduction of the chiral perylene diimide derivative allowed the polymerization of chiral hydrogels that undergo reversible changes in topography. It is expected that this new strategy to synthesize responsive gels may be used to create materials for variety of biomedical applications from dynamic cell culture to self-cleaning surfaces.
ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details including materials, sample preparation, and characterization, UV-Vis spectra, AFM images, POM images of the swelling of hydrogel, fingerprint texture of the chiral chromonic hydrogel
AUTHOR INFORMATION Contributing Author * Taylor H. Ware. E-mail:
[email protected] ACKNOWLEDGMENT The authors would like to acknowledge funding from the University of Texas at Dallas.
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