Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 7381−7388
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3D Printing of Highly Stretchable, Shape-Memory, and Self-Healing Elastomer toward Novel 4D Printing Xiao Kuang, Kaijuan Chen, Conner K. Dunn, Jiangtao Wu, Vincent C. F. Li, and H. Jerry Qi* The George W. Woodruff School of Mechanical Engineering, Renewable Bioproduct Institute, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *
ABSTRACT: The three-dimensional (3D) printing of flexible and stretchable materials with smart functions such as shape memory (SM) and self-healing (SH) is highly desirable for the development of future 4D printing technology for myriad applications, such as soft actuators, deployable smart medical devices, and flexible electronics. Here, we report a novel ink that can be used for the 3D printing of highly stretchable, SM, and SH elastomer via UV-light-assisted direct-ink-write printing. An ink containing urethane diacrylate and a linear semicrystalline polymer is developed for the 3D printing of a semiinterpenetrating polymer network elastomer that can be stretched by up to 600%. The 3D-printed complex structures show interesting functional properties, such as high strain SM and SM -assisted SH capability. We demonstrate that such a 3D-printed SM elastomer has the potential application for biomedical devices, such as vascular repair devices. This research paves a new way for the further development of novel 4D printing, soft robotics, and biomedical devices. KEYWORDS: 3D printing, highly stretchable, shape memory, self-healing, semi-IPN
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and biomedical devices.19,20 Therefore, 3D printing of soft and elastomeric SMPs is highly desirable to satisfy the application demands of soft actuators, deployable smart medical devices, and flexible electronics, which is rarely reported.21 Self-healing (SH) materials are developed as a smart material to enable the structural restoration and function recovery upon damages, which can enhance material reliability and extend its lifetime.22−32 Generally, the SH polymer systems fall into two categories: extrinsic SH and intrinsic SH polymers. The former relies on the encapsulated healants in microcapsule33,34 or microvascular networks,35,36 which can be released and polymerized upon crack intrusion to heal microcracks. For example, in a recent innovative work, White et al. developed a vascular synthetic system in the polymer to restore mechanical performance in response to large-scale damages.37 However, the unpredictable damage repair may need a significant amount of vascular coverage, which may increase the difficulty in fabricating the vascular network and the material. Intrinsic SH polymers that use the intrinsic physical or chemical properties
INTRODUCTION The three-dimensional (3D) printing, also known as additive manufacturing (AM), has drawn tremendous attention among different research fields and gained considerable progress in recent years. The 3D printing finds myriad applications, including physical prototypes,1 tissue engineering,2 electronics devices,3 microfluidics,4 and high specific strength materials.5 More recently, the convergence of 3D printing and smart materials led to the new concept of four-dimensional (4D) printing, where the shape and property of a 3D-printed component can change as a function of time under external stimulus.6 The 4D printing can be achieved by using hydrogel,7 shape memory (SM) polymer (SMP),8−10 or internal stress developed during 3D printing.11 Among these methods, SMPs are a class of smart polymeric materials that have the ability to “memorize” a permanent shape, be programmed to fix a temporary shape, and later recover to its initial shape upon an external stimulus, such as heat, electricity, or light irradiation.12−16 A variety of glassy polymers have been fabricated by 3D printing to achieve shape-changing objects.17,18 Compared with the glassy SMPs, the elastomeric SMPs that are soft and more deformable are especially suitable for wearable electronics © 2018 American Chemical Society
Received: December 1, 2017 Accepted: February 5, 2018 Published: February 5, 2018 7381
DOI: 10.1021/acsami.7b18265 ACS Appl. Mater. Interfaces 2018, 10, 7381−7388
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustrations of the UV-assisted DIW-based 3D printing of semi-IPN elastomer composites containing crystalline linear chain and cross-linked network. (a) Chemical structures of the components in the semi-IPN elastomer composite resin. (B) DIW-based 3D printer equipped with heating elements prints each layer of the filament followed by shining UV light (50 mW/cm2) to cure the resin. (c) Structure evolution of the ink during printing at 70 °C and cooling down after printing.
ink.43 Such a composition of the ink allows good printability as well as SM and SH behaviors after printing. As illustrated in Figure 1b, we achieved a 3D printing of semi-IPN elastomer composites via DIW printing, followed by UV curing. The DIW printer uses a heating syringe that writes inks on an x−y direction motion stage. The syringe is mounted on a z-direction stage so that the distance between the syringe nozzle and the x−y direction motion stage can be adjusted. The ink is pushed out of the syringe nozzle by the pressure controlled by a pneumatic apparatus. UV light-emitting diodes (LEDs) at a fixed location near the syringe were used to ex situ cure the ink. To facilitate successful printing, the syringe is wrapped by a heating element with the temperature of nearly 70 °C to avoid crystallization of PCL during printing (Figure S1, Movie S1). The ink in the heating syringe is consistently extruded through the syringe nozzle by a suitable pressure. Incorporating 4 wt % of SiO2 in the resin imparts obvious shear-thinning effect to enable stable ink extrusion and allow for shape holding after printing (Figure S2). The filament is consistently deposited on the silicone paper to prevent adhesion. After printing one layer, the x−y direction motion stage moves to the position under the UV light to cure the printed ink by photopolymerization. The polymerization was verified by Fourier transform infrared (FTIR) spectra, as indicated by the complete disappearance of vinyl characteristic peaks, such as the band of vinyl carbon−carbon double bond vibration at 1639 cm−1 (Figure S3). After photocuring, the PCL chains were confined in the cross-linked network. Finally, the semi-IPN elastomer with complicated 3D shape can be
of the polymer, such as chemical reactions in dynamic covalent bonds or secondary bonds, are also used for microcrack mending.38−40 These polymer systems usually suffer from relatively complex synthesis and lack of crack closure before healing.41 Here, we combine the attributes of SM and SH with 3D printing. Inspired by the SM-assisted SH concept in polymer composite,41,42 the 3D printing of an SM composite with large crack healing capability was achieved. A highly stretchable semiinterpenetrating polymer network (semi-IPN) elastomer with an embedded semicrystalline thermoplastic polymer was printed by using the direct-ink-write (DIW) approach. The embedded semicrystalline thermoplastic serves the dual roles of the switching phase for SM and the healant for SH behavior. The direct fabrication of 3D complex structure that can change its shape with time and has SH function provides a new capability for 4D printing.
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RESULTS AND DISCUSSION To facilitate DIW-based printing of semi-IPN elastomer composites, we prepared inks containing a photocurable resin and a semicrystalline thermoplastic polymer. The photocurable resin is made of aliphatic urethane diacrylate (AUD; containing 33 wt % of isobornyl acrylate) and n-butyl acrylate (BA) (Figure 1a). The semicrystalline polycaprolactone (PCL) was dissolved in the above acrylate. In addition, nanoparticles, such as fumed silica (200−300 nm), were added as a rheology modifier that imparts the shear thinning effect to the uncured 7382
DOI: 10.1021/acsami.7b18265 ACS Appl. Mater. Interfaces 2018, 10, 7381−7388
Research Article
ACS Applied Materials & Interfaces
Figure 2. Photographs and tension properties of 3D structures by DIW-based printing. Different architectures of (a) Archimedean spiral, (b) honeycomb, (c) hollow vase, and (d) Gumby model were printed. (e) Snapshots of stretching a printed Archimedean spiral up to 300% strain. (f) Highly stretchable stress−strain behavior of the printed semi-IPN elastomers with three different filament printing angle. (g) Young’s modulus, fracture strain, and fracture stress of printed dog bone-shaped samples with different printing angles.
obtained, where PCL chains and small crystals are dispersed in the acrylate elastomer network matrix after cooling down (Figure 1c). It is noted that the content of PCL plays an important role in mechanical properties of the semi-IPN elastomer. Both of the Young’s modulus (Figure S4) and fracture stress (Figure S5) increase with the PCL content, as the crystallinity increases with the PCL content (Figure S6). The small PCL crystals can retard the propagation of microcracks in the elastomer network at large strain, which can improve the fracture toughness of semi-IPN.19,44 Thus, a highly stretchable semi-IPN elastomer composite can be obtained. Considering proper mechanical properties, the resin containing 20 wt % of PCL was utilized for printing in this paper. Taking advantage of the UV-assisted DIW-based 3D printing, a variety of 3D architectures, such as Archimedean spiral, honeycomb, hollow vase, and Gumby toy, can be directly printed (Figure 2a−d). The printed parts are expected to be stretchable. As shown in Figure 2e, the printed Archimedean spiral was stretched to over 300% of strain. We found that the in-plane anisotropy of printed parts is negligible. As shown in Figure 2f, the tensile stress−strain curves of the directly printed dog bone samples in different orientations relative to the longitudinal direction (0°, 45°, and 90°) nearly overlap. The fracture strain is 500−600% for all of the printed samples. The Young’s modulus, fracture strength, and fracture strain are comparable for the three printing angles (Figure 2g). The small deviation in mechanical properties may be attributed to the edge effects for different printing angles (Figure S7). After deposition, the filaments were chemically bonded to the neighbor ones by UV curing, resulting in an excellent
interfilament adhesion. The hydrogen-bond in urethane group may also do some help to enhance the interfacial bond between adjacent filaments. It is also noted that the excellent interlayer adhesion enables good contact between different layers; then the diffusion would help in the further enhancement of the interfacial adhesion. As a result, both good surface finish and inplane isotropic properties were achieved for the printed semiIPN elastomer composites. As mentioned above, the small PCL crystals act as excellent reinforcement phase that contributes to large fracture strain and fracture stress for the elastomer at the room temperature. The cyclic loading−unloading tests at room temperature show some plastic deformation, which is typical for semicrystalline polymers because of the plastic deformation of the crystals (Figure S8). At the temperature above Tm, the deformation becomes elastic (as shown in Figure S9). Figure S9 also shows that the elastomer composites can still be stretched up to 400% strain after complete melting of PCL crystals at 70 °C. The semi-IPN elastomer contains a crystalline component in the network, which can be used as a switching phase for SM effect. The thermomechanical properties and thermal transition of the materials were investigated before performing the SM tests. DMA shows only one glass transition temperature (Tg) at about 1.5 °C and one melting transition at 68 °C for the elastomer, as indicated by the sharp decrease in modulus (Figure S10). These results suggest relatively good compatibility with only microscale phase separation between the two components. As the PCL chains are confined to the acrylate network, it is reasonable to assume that the PCL chains are immersed in the elastomer network with small crystals homogeneously dispersed. Above the melting temperature of 7383
DOI: 10.1021/acsami.7b18265 ACS Appl. Mater. Interfaces 2018, 10, 7381−7388
Research Article
ACS Applied Materials & Interfaces
Figure 3. SM actions of printed architectures from temporary shapes to permanent shapes upon heating by a heat gun. SM cycles of semi-IPN elastomer where each sample was stretched at 70 °C and fixed at 0 °C, followed by recovery at 70 °C in a stress-controlled mode (a) and in a straincontrolled mode (b) by dynamic mechanical analysis (DMA). (c) Compressed plate recovers to its original shape of a standing hollow vase. (d) Long string recovers to its original shape of Gumby toy. All of the scale bars are 6 mm.
Figure 4. SM-assisted SH behavior of printed semi-IPN elastomer composites. (a) SEM micrographs of scratched and healed surface at different scales. (b) Optical micrograph indicating obvious crack regions on the notched sample can be totally healed with an invisible scar. (c) Tension stress−strain curves for the virgin, notched and healed state of semi-IPN elastomer samples. Inserted are the micrographs of deformation and crack growth of notched sample clamped in the MTS tensile tester. (d) Pictures showing SM-assisted SH process for printed Archimedean spiral structure. Inserted pictures are the optical micrograph showing the surface morphology with a scale bar of 1 mm. (e) Printed strip was cut and healed in three healing generations. The healing condition is the same by heating the sample at 80 °C for 20 min and then cooling down in the air.
deformed temporary shape was fixed after releasing the external load at the low temperature. Upon cooling, recrystallization of PCL crystals enables the fixing of the temporary shape. After reheating, the crystal phase melts and the permanent shape can be recovered because of the entropic elasticity of network. High-strain SM effect can be achieved for this material (Figure S12, Movie S2). The quantitative SM action was evaluated by DMA. With the programing strain over 230% using stresscontrolled testing mode, the shape fixity (Rf) was over 93% and
PCL (Tm), the material shows a rubbery plateau with a modulus of about 0.4 MPa. Differential scanning calorimetry (DSC) tests show that the Tm of PCL is about 55 °C and the crystallization temperature (Tc) is 14 °C (Figure S11). It is noted that the transition temperatures obtained by DMA are usually higher than by that obtained by DSC.16 In an SM cycle, the sample was first deformed at the temperature above the Tm of PCL and then cooled down to a temperature below Tm without releasing the external load. The 7384
DOI: 10.1021/acsami.7b18265 ACS Appl. Mater. Interfaces 2018, 10, 7381−7388
Research Article
ACS Applied Materials & Interfaces
Figure 5. DIW printing of semi-IPN SM tube and potential biomedical application for vascular repair. (a) Photograph (left) and optical microscopic images (right) of 3D-printed tubing with a different diameter. (b) SM action of a tube from left and right: initial shape, fixed temporary shape, and recovered initial shape. (c) Potential application demonstration of the SM tubing for rapid vascular reconnection and repair: The “blood vessel” (1) was cut for surgery or under damages (2); the severe “bleeding” was temporarily stopped by clamps (3); the broken “vessel” was implanted with the SM elastomer in the crack position (4). After heating, the SM tubing recovers to initial shape and attached to inner side of “vessel” (5). The broken vessel was connected regaining blood circulation (6). All of the scale bars are 2 mm.
the shape recovery ratio (Rr) was over 95% in the first cycle (Figure 3a). The slight shift of the curves and the decreasing Rf value may be attributed to the deformation of a large fraction of thermoplastic chains during high strain stretching in nine testing cycles (Figure S13). Using the strain-controlled testing mode (200% stretching strain), Rf was calculated to be about 99% during four cycles (Figure 3b), whereas Rr would decrease from 90% to 83% over four cycles (Figure S14). On the basis of these results, we demonstrated the SM behaviors of several printed 3D structures. A printed hollow vase (36 mm in height) was first heated and compressed into a plate followed by cooling to yield the temporary plate shape (∼3 mm in height) (Figure 3c). During heating (by a heat gun), with the continuous melting of crystals in the elastomer network, the entropic elasticity of the elastomer drove the recovery to its initial hollow vase shape. Finally, the original shape of a standing hollow vase was recovered. Besides using the heat gun, heating can be achieved by immersing the sample in hot water, which enables a relative uniform recovery (Movie S3). Other complex 3D-printed structures also show good SM properties. For example, the honeycomb structure was compacted into a cylinder as the temporary shape, which gradually recovered to its original shape in about 1 min using a heat gun (Movie S4). To further demonstrate the highly deformable SM properties, the printed Gumby toy was stretched and twisted into a long string as a temporary shape (Figure 3d). During heating, the twisted string shape rolls, shrinks, and gradually recovers to its initial shape. Using hot water as a heating media leads to much faster recovery than that by the heat gun because of uniform heating capability (Movie S5). Besides the SM effect, SH can be achieved for the semi-IPN elastomer. We first investigated the healing of microcracks. The
sample was scratched with a sharp blade as shown by SEM images in Figure 4a, showing a surface scratch of 3 mm long and 30 μm wide. After treating the sample in an oven at 80 °C for 20 min followed by cooling down, the surface crack disappeared with a slight scar. Larger cracks such as notched gap can be also healed. As shown in Figure 4b, the notched strip was stretched and released leaving a visible gap in the notch position. After treating at 80 °C for 20 min, the gap was closed and mended without obvious scars in the macro scale. For the notched sample, the initial crack of the notch edge easily propagates after reaching its yield point. The crack propagation leads to a sample fracture at a strain near 100%. After healing, the notched sample can be stretched over 160% strain (Figure 4c). Compared with the virgin-printed samples, the healing efficiency evaluated from the fracture strain is relatively low (
