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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX
Four-dimensional Printing of Liquid Crystal Elastomers Cedric P. Ambulo,† Julia J. Burroughs,† Jennifer M. Boothby,† Hyun Kim,† M. Ravi Shankar,‡ and Taylor H. Ware*,† †
Department of Bioengineering, The University of Texas at Dallas, Richardson, Texas 75080, United States Department of Industrial Engineering, The University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States
‡
S Supporting Information *
ABSTRACT: Three-dimensional structures capable of reversible changes in shape, i.e., four-dimensional-printed structures, may enable new generations of soft robotics, implantable medical devices, and consumer products. Here, thermally responsive liquid crystal elastomers (LCEs) are direct-write printed into 3D structures with a controlled molecular order. Molecular order is locally programmed by controlling the print path used to build the 3D object, and this order controls the stimulus response. Each aligned LCE filament undergoes 40% reversible contraction along the print direction on heating. By printing objects with controlled geometry and stimulus response, magnified shape transformations, for example, volumetric contractions or rapid, repetitive snap-through transitions, are realized.
KEYWORDS: 4D printing, additive manufacturing, liquid crystal elastomers, smart materials, actuator
1. INTRODUCTION
applications in which the material operates in the requisite aqueous environment.18 Liquid crystal elastomers (LCEs) are a class of stimuliresponsive polymers that undergo large, reversible, anisotropic shape change in response to a variety of stimuli, including heat and light.19−21 Unlike many materials that undergo reversible shape change, these materials require neither external loads nor aqueous environments, making them ideal candidates for many applications. For LCEs to undergo reversible shape change in the absence of load, LCEs should be cross-linked in an aligned state.22 In response to heat, aligned LCEs contract along the nematic director, the orientation direction of the liquid crystal (LC) molecules, and expand in the perpendicular directions.23 Commonly, partially cross-linked LCEs are fully reacted under a mechanical strain leading to the permanent orientation of the mesogens within the polymer network.24,25 With this process, it is difficult to program the stimulus response of the material in a spatially varied manner. As such, several methods have been developed to align monomeric or oligomeric LCE precursors. Using patterned surface treatments,26 LC monomers can be patterned with high spatial resolution.27,28 LCEs resulting from this process can be designed to undergo both in-plane and outof-plane patterned shape change. However, this technique is fundamentally limited to the production of relatively thin, planar films (1 mm) molecular orientation deteriorates. With an inner nozzle diameter of 0.31 mm, the viscosity of the material at 85 °C, ∼8 Pa s at 50 s−1, was identified to be acceptable for printing. After extrusion, the integrity of the geometry and the molecular orientation of the LC filament are of primary concern. In this system, three factors combine to stabilize the printed structure: shear-thinning behavior drives an increase in viscosity at low shear rates, cooling from print temperature to room temperature leads to an increase in the viscosity, and photopolymerization of the LC filament by UV LEDs on the print head cross-links the material. As a result, the modulus of the material becomes high enough to enable print paths that span gaps within the structure (Figure 1d). Controlled molecular orientation, generated by direct-write printing, leads to anisotropic optical properties, elastic modulus, and stimulus response. Birefringence associated with oriented nematic LCEs is evident when uniaxially printed LCEs are observed between crossed polarizers. Single-layer, printed LCE films are dark when the direction of extrusion (i.e., direction of molecular orientation) is parallel to the polarizer or analyzer, and films are bright when the extrusion direction is 45° to the polarizer (Figure 1e). The order parameter of the printed films was measured to be 0.26 using polarized UV−vis spectroscopy (Figure S4). This measurement confirms that extrusion from
A − A⊥ A + 2A⊥
(1)
Quasi-static tensile testing of 3D-printed rectangular samples is conducted at room temperature using a RSA-G2 Dynamic Analyzer (TA Instruments, New Castle, DE). Samples are printed in 15 mm × 5 mm × 0.25 mm rectangles. Samples are loaded along the long geometric axis with a deformation rate of 1 mm min−1 until failure. Thermal stimulus response is characterized by image analysis (ImageJ) of the printed LCE structures from room temperature to 160 or 200 °C. Each structure was immersed in a silicone oil bath on a hot plate. The hot plate is allowed to reach the desired temperature and equilibrate for 5 min before the sample is photographed for image analysis. The snap-through actuation occurs in the presence of a thermal gradient by placing the structure of the opposed Gaussian curvature directly on a hot plate. It is to be noted that the LCE structures undergo oxidation if exposed to high temperatures for long periods of time. Specific work and stroke of the opposing Gaussian curvature are determined through image analysis of the presnap, snapthrough, and postsnap actuation geometries imposed by heating the structure past 150 °C. Specific work was determined by multiplying the displacement of the center of mass of the external load by the weight of the external load. Stroke is calculated by measuring the displacement of the top of the opposing Gaussian curvature. Peak specific power was calculated by dividing the specific work by the time it takes to fully snap-through. The reported values represent an average of at least three samples. Polarized optical micrographs are taken with an Olympus BX51 microscope with an Olympus UC30 color camera equipped with two polarizers that are operated crossed with the sample in between. Scanning electron micrographs are acquired using the Zeiss SUPRA 40 SEM on gold-sputtered samples. Macroscopic images and videos are taken at 30 fps with a Nikon DSLR camera or at 240 fps using an Apple iPad Pro. Dimensional changes of the printed LCE structures are measured in ImageJ.
3. RESULTS AND DISCUSSION We formulate an LC ink that exhibits a printable viscosity in the nematic phase (allowing the director to be programmed) and can be rapidly polymerized into a lightly cross-linked elastomer C
DOI: 10.1021/acsami.7b11851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Print path schematic of a one-layer Archimedean chord print pattern. (b) Side-view images and a top-view inset image of a 3D-printed 20 mm diameter disk printed Archimedean chord pattern at room temperature and at 160 °C in silicone oil. (c) Representative curve of normalized height as a function of temperature of the cone that results from heating. (d) Schematic of a two-layer rectangular LCE film with the top and bottom layers printed at 90° to each other or ±45° with respect to the long axis of the film. (e) At room temperature each film is flat, and upon heating the films morph into either a helix or a helical ribbon. (f) Normalized twist as a function of temperature for several aspect ratios. As the width of the samples increases, the amount of twists decreases. (g) Print path schematic of a porous structure comprising rectilinear print paths alternating 90° with respect to the previous layer at 25% fill density. (h) The printed porous structure exhibits an in-plane contraction. (i) Pore size and volumetric dimensions as a function of temperature. The pore size curve is depicted as the red curve, and the volumetric dimensions are depicted as the blue curve.
original film thickness (Figure 2c). Out-of-plane deformation can be further programmed by varying molecular orientation through the thickness of printed LCEs. By printing structures two-layers thick, active bilayers are fabricated (Figure 2d). In rectangular structures with 90° differences in orientation between the top and bottom layers, heating causes incompatible strains and results in out-of-plane deformation.41 If print directions are offset by 45° to the long axis, twisting is observed upon heating (Figure 2e). The nature and degree of the twist is dependent upon the aspect ratio of the printed material, as previously described by Sawa et al.41 Films with widths above 4 mm transform from flat to helical ribbons. As the width decreases, the film’s geometry transitions, forming a tightly wound helix. Films with a width of 2 mm exhibit on average 45° mm−1 of twist per length, whereas those with a width of 5 mm exhibit 30° mm−1 (Figure 2f). This behavior qualitatively mimics results seen in twisted nematic LCEs, where molecular order twists by 90° through the thickness of the film, fabricated by surface alignment. In addition, additive manufacturing enables 3D structures that cannot be fabricated using surfacealigned cells or mechanical loading. Highly porous, thick LCEs with locally controlled molecular orientation are fabricated by direct-write printing. The schematic for a printed 10 mm × 10 mm × 3 mm, porous structure comprising 16 layers is shown in Figure 2g. Each layer is oriented at 90° to the underlying layer, and the overall
the printing syringe aligns the mesogens parallel to the computer-generated path. Anisotropic and nonlinear mechanical properties associated with uniaxially aligned LCE films28,38,39 are present in single-layered LCE prints (Figure S5). These materials have an elastic modulus of 18 MPa along the extrusion direction and a modulus of 4 MPa normal to the extrusion direction. It should be noted that some of the difference in mechanical behavior may be attributed to nonuniform microstructures resulting from printing (Figure 1e). Critically, printed LCE films are capable of shape change in response to temperature. A reversible 40% contraction along the director is observed on heating from room temperature to 200 °C (Figure 1f). This uniaxial actuator demonstrates that the direction of contraction within LCEs is controlled through direct-write printing. Printing LCEs with nonuniaxial print paths within the plane or through the thickness leads to materials that undergo complex deformations on heating. Directing the printer to extrude the LC ink in an Archimedean chord pattern results in LCE films programmed with the director pattern associated with a +1 topological defect, where orientation varies azimuthally around a single point (Figure 2a,b). As first predicted by Modes et al.40 and later realized experimentally in surface-aligned LC polymer networks,26 the printed +1 topological defect LCE morphs from flat (thicknesses of ∼80 μm) into a cone on heating, reaching heights up to 10 times the D
DOI: 10.1021/acsami.7b11851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) Printed LCE cylinder (zero Gaussian curvature) with an azimuthal print path. Upon heating, the cylinder undergoes radial contraction. Radial contraction is dependent on the thickness of the cylinder wall. (b) Printed LCE hemispherical shell (positive Gaussian curvature). Upon heating, the hemisphere undergoes an azimuthal contraction and an axial expansion forming a peaked hemisphere. Height increases continuously as a function of temperature (red point indicates height). (c) Printed LCE hemihyperboloid (negative Gaussian curvature). Upon heating, the negative Gaussian curved LCE undergoes an azimuthal contraction and an axial expansion leading to a continuous increase in height.
structure has a relative fill density of 25%. In this thick structure, bending is suppressed, and anisotropic contraction in each filament results in isotropic contraction in the X−Y plane (Figure 2h). The pores undergo apparent areal contraction of 77%. The structure’s in-plane contraction is greater than the corresponding thickness expansion, causing the structure to exhibit a reversible 36% volumetric contraction (Figure 2i). It should be noted that the intrinsic deformation of the LCE is isochoric, and the observed volumetric contraction is a structural effect enabled by direct-write printing. Local molecular orientation can also be programmed in 3D curved objects. To demonstrate this capability, we print hollow cylinders with azimuthal print paths that are 10 mm in diameter and 4 mm tall (Figure 3a). As predicted by Modes et al.,42 azimuthally aligned LCE cylinders contract radially and expand axially when heated (Figure S6). Varying the cylinder’s wall thickness allows the degree of radial contraction to be tuned, with thinner-walled cylinders producing larger degrees of contraction, up to 30% (Figure 3a). In addition to printing structures with zero Gaussian curvature, aligned LCEs with a positive or negative Gaussian curvature can be direct-write printed. LCEs with this combination of alignment and geometry have not been fabricated using previously described methods. A hollow LCE hemisphere programmed with a +1 defect pattern can be fabricated layer-by-layer (Figure 3b). Following theoretical predictions of spherical shells imposed
with +1 defect patterning, the hemisphere experiences azimuthal contraction and expansion in the axis orthogonal to the contraction when heated,42 morphing into a “peaked” hemisphere. The peaked portion of the hemisphere results from deformation of the sample and retention of the existing positive Gaussian curvature. Heating results in doubling of the height of the hemisphere. LCE structures with negative Gaussian curvatures are printed with the same topological pattern of molecular alignment (Figure 3c). In response to thermal stimulus, the negative Gaussian curvature LCE maintains its curvature while undergoing azimuthal contraction and orthogonal expansion. LCE structures containing regions of opposing Gaussian curvatures with the same +1 topological alignment patterns undergo reversible and rapid deformations on heating and cooling that are not observed in traditionally fabricated LCEs. Inspired by curved structures that exhibit snap-through deformations,43−46 we design LCE structures that exhibit a snap-through transition by printing modified hemitoroidal shells. This geometry contains a region of positive Gaussian curvature that connects to a region of negative Gaussian curvature (Figure 4a). When placed on a heated surface (as compared with uniform heating), the structure approaches an elastic instability at the region of opposing Gaussian curvature. When enough energy is stored to overcome the instability, snap-through actuation occurs, releasing the energy and E
DOI: 10.1021/acsami.7b11851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Structure comprising opposing negative and positive Gaussian curvatures is printed with an azimuthal print path. An asymmetric snapthrough transition occurs on heating during the reorientation of the positive Gaussian curvature. (b) Snap-through actuation from the combined Gaussian structure is capable of reversible snapping upon cyclical heating and cooling. (c) Time lapse images of the printed structure undergoing an asymmetric snap-through transition. The snap transition occurs over 16 ms, but releases enough energy to make the structure airborne for ∼64 ms. (d) Stroke and specific work for structures loaded with external loads.
be subsequently polymerized into responsive elastomers. The interplay of molecular orientation and geometry enables an array of thermally driven structural adaptations. Along with dense planar structures, porous and 3D structures with complex director orientation and shape change can be fabricated. Fabrication of LCEs with regions of opposing Gaussian curvature leads to structures that undergo rapid, reversible snapping. Four-dimensional printing of LCEs may enable the application of these materials in smart devices from low-density machines to implantable medical devices.
resulting in an inversion of the positive Gaussian curvature portion of the structure (Figure 4a). This inversion of curvature is reminiscent of “toy poppers”, but notably occurs without requiring externally applied mechanical deformation. Characteristic of LCE systems, the structure reversibly snaps back into its original geometry on cooling (Figure 4b and Movie 1, Supporting Information). This reversible snapping event is often asymmetric. The LCE structure undergoes the transition from partially inverted to fully inverted over ∼16 ms. During this process, the entire structure is airborne for ∼64 ms before landing (Figure 4c). This snap-through actuation is capable of lifting external loads and performing useful work. At external loads up to 5 N N−1 (when normalized to the weight of the actuator), the LCE structure catapults the loaded mass (Movie 2, Supporting Information). For all loads tested below 20 N N−1, the opposing Gaussian curved structure exhibits a roughly constant stroke of 1 mm mm−1 (Figure 4d). As such, specific work done by the structure trends from 0.1 to 0.7 J kg−1 in a continuous manner as the load increases (Figure 4d). During the snapping transition, a specific peak power of 15.5 W kg−1 is exhibited against a normalized load 5 times heavier than the actuator. Under loads greater than 40 times the mass of the actuator, the snap-through actuation is no longer observed (Figure S7).
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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.7b11851.
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4. CONCLUSIONS Direct-write printing enables fabrication of 3D LCE structures with molecular order encoded during fabrication, and the resulting structures undergo programmed shape change. Shear forces intrinsic to direct-write printing processes can be used to orient LC reactive inks along the print path, and these inks can
Additional characterization data, figures, and movies of the LC ink and LCE printed structures (PDF) Combined Gaussian curvature LCE exhibiting reversible snap-through actuation in response to thermal stimulus (AVI) Combined Gaussian curvature LCE producing work and catapulting a mass (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Taylor H. Ware: 0000-0001-7996-7393 F
DOI: 10.1021/acsami.7b11851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Author Contributions
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C.P.A., M.R.S., and T.H.W. conceived the research. C.P.A., J.J.B., J.M.B., and H.K. performed experiments and analyzed the experimental results. C.P.A. composed the manuscript. T.H.W. oversaw the preparation of the manuscript. All authors reviewed the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS T.H.W., C.P.A., J.J.B., J.M.B., and H.K. acknowledge partial support from the University of Texas at Dallas. T.H.W. also acknowledges that this material is partially based upon work supported by the Air Force Office of Scientific Research under award number FA9550-17-1-0328. MRS acknowledges support from the Air Force Office of Scientific Research under award number FA9550-14-1-0229.
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DOI: 10.1021/acsami.7b11851 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
