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Biomimetic 4 D printing
| Content Provider | Semantic Scholar |
|---|---|
| Author | Gladman, A. Sydney Matsumoto, Elisabetta A. Nuzzo, Ralph G. Mahadevan, L. Lewis, Jennifer A. |
| Copyright Year | 2016 |
| Abstract | Shape-morphingsystemscanbe found inmanyareas, including smart textiles1, autonomous robotics2, biomedical devices3, drug delivery4 and tissue engineering5. The natural analogues of such systems are exemplified by nastic plant motions, where a variety of organs such as tendrils, bracts, leaves and flowers respond to environmental stimuli (such as humidity, light or touch) by varying internal turgor, which leads to dynamic conformations governed by the tissue composition and microstructural anisotropy of cell walls6–10. Inspired by these botanical systems, we printed composite hydrogel architectures that are encoded with localized, anisotropic swelling behaviour controlled by the alignment of cellulose fibrils along prescribed four-dimensional printing pathways. When combined with a minimal theoretical framework that allows us to solve the inverse problem of designing the alignment patterns for prescribed target shapes, we can programmably fabricate plant-inspired architectures that change shape on immersion in water, yielding complex three-dimensional morphologies. Plants exhibit hydration-trigged changes in their morphology due to differences in local swelling behaviour that arise from the directional orientation of stiff cellulose fibrils within plant cell walls6–10. Emerging pathways for mimicking these dynamic architectures incorporate materials that can respond to external stimuli, such as shape memory alloys11,12 and swellable hydrogel composites13,14, and are assembled by methods such as fourdimensional (4D) printing11,15 and self-folding origami16–18. For example, recent efforts to create plant-inspired, shape-changing structures10 have employed differential swelling in isotropic or composite bilayers and hinges8,13,14,16. However, none of these approaches enable shape change using a single material patterned in a one-step process, nor do they utilize a predictive model capable of tackling both the forward and inverse design problems (Supplementary Text and Supplementary Figs 1 and 2). Here, we develop a biomimetic hydrogel composite that can be 4D printed into programmable bilayer architectures patterned in space and time, which are encoded with localized swelling anisotropy that induces complex shape changes on immersion in water. Our hydrogel composite ink is composed of stiff cellulose fibrils embedded in a soft acrylamide matrix, which mimics the composition of plant cell walls. The composite architectures are printed using a viscoelastic ink that contains an aqueous solution of N ,N -dimethylacrylamide (or N -isopropylacrylamide for reversible systems), photoinitiator, nanoclay, glucose oxidase, glucose, and nanofibrillated cellulose (NFC). The constituents serve different purposes: the clay particles are a rheological aid, inducing the desired viscoelastic behaviour required for direct ink writing19 (Supplementary Fig. 3); glucose oxidase and glucose minimize oxygen inhibition during the ultraviolet curing process by scavenging ambient oxygen20, thereby improving polymerization in the printed filamentary features (∼100 μm to 1mm in diameter) to yield mechanically robust structures; the wood-derived cellulose fibrils, which bundle intomicrofibrils with high aspect ratio (∼100), serve as stiff fillers (E > 100GPa; ref. 21). After printing under ambient conditions, the acrylamide monomer is photopolymerized and physically crosslinked by the nanoclay particles, producing a biocompatible hydrogel matrix that swells readily in water22. (See Methods for further details.) The efficacy of our biomimetic 4D printing (bio-4DP) method relies on the ability to deterministically define the elastic and swelling anisotropies by local control of the orientation of cellulose fibrils within the hydrogel composite. During printing, these fibrils undergo shear-induced alignment23 as the ink flows through the deposition nozzle24, which leads to printed filaments with anisotropic stiffness, and, hence, swelling behaviour in the longitudinal direction (along the filament length, as defined by the printing path) compared to the transverse direction (Fig. 1a). Significant cellulose fibril alignment is directly observed in the printed samples compared to isotropic cast sheets of the same material (Fig. 1b–d). Fourier analysis quantifies the relative alignment between cast and printed specimens, which indicates a clear directionality peak in the latter case (Fig. 1e and Supplementary Fig. 4). Thus, the printed architectures exhibit a fourfold difference in longitudinal and transverse swelling strains of α‖ ∼ 10% and α⊥ ∼ 40%, respectively (Fig. 1f,g). Likewise, this signature of anisotropy is present in the elastic moduli, with longitudinal and transverse values of E‖∼40 kPa and E⊥∼20 kPa, respectively (Supplementary Figs 5 and 6). The extent of shearinduced alignment, and, hence, the magnitude of the anisotropic swelling, depends on the nozzle diameter and printing speed. For a fixed printing speed, the shear forces that align the cellulose fibrils scale inversely with nozzle size, as reflected in the observed longitudinal and transverse swelling strains (Fig. 1f). Harnessing anisotropic swelling allows precise control over the curvature in bilayer structures9,25. Quantifying this requires a mathematical model for the mechanics of anisotropic plates and shells, which combines aspects of the classical Timoshenko model for thermal expansion in bilayers26 with a tailored metricdriven approach9,25 that employs anisotropic swelling to control the embedding of a complex surface. In a bilayer system, differential swelling between the top and bottom layers induces curvature, because the layers are forced to remain in contact along the entire midplane. Thereby, the displacements, integrated from the swelling |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://www.seas.harvard.edu/softmat/downloads/2016-04.pdf |
| Alternate Webpage(s) | http://lewisgroup.seas.harvard.edu/files/lewisgroup/files/nature_mater_bio_4d_printing.pdf?m=1453820690 |
| Alternate Webpage(s) | https://www.seas.harvard.edu/softmat/downloads/2016-04.pdf |
| Alternate Webpage(s) | http://www.mrsec.harvard.edu/pubs/2016_IRG2.a.4_Gladman.Matsumoto.Nuzzo.Mahadevan.Lewis_Biomimetic%204D%20printing.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
