3D Printing of Anisotropic Hydrogels with Bioinspired Motion

Abstract

Motion in biological organisms often relies on the functional arrangement of anisotropic tissues that linearly expand and contract in response to external signals. However, a general approach that can implement such anisotropic behavior into synthetic soft materials and thereby produce complex motions seen in biological organisms remains a challenge. Here, a bioinspired approach is presented that uses temperature-responsive linear hydrogel actuators, analogous to biological linear contractile elements, as building blocks to create three-dimensional (3D) structures with programmed motions. This approach relies on a generalizable 3D printing method for building 3D structures of hydrogels using a fugitive carrier with shear-thinning properties. This study demonstrates that the metric incompatibility of an orthogonally growing bilayer structure induces a saddle-like shape change, which can be further exploited to produce various bioinspired motions from bending to twisting. The orthogonally growing bilayer structure undergoes a transition from a stretching-dominated motion to a bending-dominated motion during its shape transformation. The modular nature of this approach, together with the flexibility of additive manufacturing, enables the fabrication of multimodular 3D structures with complex motions through the assembly of multiple functional components, which in turn consist of simple linear contractile elements.

Keywords: 3D printing; actuators; anisotropic hydrogels; bioinspired design; shape morphing.

Figure 1

3D printing of orthogonally growing bilayer structures of hydrogels with programmed motions. a) Schematic illustrating the 3D printing‐based process to create 3D structures with programmed motions using a bilayer structure of orthogonally oriented linear contractile elements. The arrows indicate the direction of anisotropic actuation, perpendicular to the PEG reinforcement direction. The angles θ shown in the legend (right figures) indicate the angle between the long axis of a bilayer structure and the direction of intrinsic curvature (Figure S2, Supporting Information). b) G′ and G″ of PNIPAM inks (10 wt% NIPAM and 1 wt% PEGDA) with different concentrations of the fugitive carrier (20–30 wt% as shown in the legend) on oscillatory strain sweeps (0.01%–1000%) at a frequency of 1 Hz. c) Step–strain measurement of a PNIPAM ink (10 wt% NIPAM, 1 wt% PEGDA, and 25 wt% fugitive carrier) with oscillatory strain steps between 0.5% and 250% at a frequency of 10 Hz. d) 3D printing of a multilayer lattice structure using a 200 µm nozzle. e) Temperature‐responsive reversible volume change of a PNIPAM structure. f–h) Optical microscope images (top view) of an as‐printed 3D structure (f) and the structure at the swelled state (25 °C) (g) and the shrunk state (40 °C) (h). i) Areal swelling (black) and shrinking (red) ratios (A _T/A ₀) of 3D structures of PNIPAM printed with different concentrations of the fugitive carrier. A _T and A ₀ are the areas of the top surface of the structures at temperature T and as‐printed structures, respectively. T _c is the volume phase transition temperature of PNIPAM (≈32.5 °C). j) G′ and G″ of DM (10 wt% DMA and 1 wt% PEGDA), PD (11 wt% PEGDA), and NA+B (10 wt% NIPAM and 1 wt% BIS) inks on oscillatory strain sweeps (0.01%–1000%) at a frequency of 1 Hz. k) Areal swelling (black) and shrinking (red) ratios of the inks. NA represents the PNIPAM ink (10 wt% NIPAM and 1 wt% PEGDA). Scale bars, 2 mm (d); 5 mm (e); 500 µm (f–h).

Figure 2

3D structures of hydrogels with anisotropic actuation. a) Hydrogel linear actuator with an as‐printed length and width of 6 mm. b–d) Hydrogel linear actuators with an as‐printed length and width of 12 and 6 mm, respectively, that actuate in the direction at 0° (b), 90° (c), and 45° (d) with respect to the long axis. The structures (a–d) consist of two layers of PNIPAM hydrogels, one layer of PEG pattern, and two layers of PNIPAM hydrogels from the bottom layer to the top layer and have an as‐printed thickness of 1 mm. The figures show the schematics of PEG patterns (dark blue lines) in PNIPAM hydrogels of as‐printed structures (left), the PNIPAM structures with PEG patterns (shown in dark blue lines) at the swelled state (middle), and the structures at the shrunk state (right). e) Changes in the relative length ΔL/ΔL ₀ of the structure shown in Figure 1e (black squares; as‐printed thickness of 2 mm) and the linear actuators shown in (b) (red squares) and (c) (blue squares) upon rapid increase in temperature from 24 to 50 °C. ΔL/ΔL ₀ = (L − L ₅₀)/(L ₂₄ − L ₅₀), where L, L ₅₀, and L ₂₄ are the lengths of the structures at the time of measurement, at the shrunk state (T = 50 °C), and at the swelled state (T = 24 °C), respectively. The dashed line indicates the time when the solution temperature reaches ≈35 °C (Figure S8, Supporting Information). The closed and open squares represent the relative lengths along the major actuation and transverse directions, respectively. Scale bars, 5 mm.

Figure 3

Orthogonally growing bilayer structures with saddle‐like shape change and bending motion. a) Saddle‐like shape of an orthogonally growing bilayer structure (as‐printed size: 12 mm × 12 mm) at the shrunk state. b) Bending motion of an orthogonally growing bilayer structure with a high aspect ratio (as‐printed size: 12 mm × 4.2 mm) along the long axis upon temperature increase from 25 to 40 °C. c) Bending of orthogonally growing bilayer structures with a low aspect ratio (left; as‐printed size: 12 mm × 7.8 mm) and a high aspect ratio (right; as‐printed size: 12 mm × 4.2 mm) at the shrunk state. The structures in (a), (b), and (c) have an as‐printed thickness of ≈1.6 mm. d,e) Curvature (d) and length (e) of the structure shown in (b) along the long axis as a function of 1/t. Scale bars, 5 mm.

Figure 4

Programming of complex motions. a) Twisting motion of an orthogonally growing structure. b–e) Twisting motions of orthogonally growing structures with an as‐printed length and thickness of 24 and 1.6 mm, respectively, and width of 4.2 mm (b), 5.4 mm (c), 6.6 mm (d), and 7.8 mm (e). The figures show the structures at the shrunk state (T > T _c). f–i) Hybrid bending and twisting motions of orthogonally growing bilayer structures (as‐printed size: 24, 4.2, and 1.4 mm in length, width, and thickness, respectively) with θ of 0° (f), 22.5° (g), 33.75° (h), and 45° (i). j) Pitch of the structures shown in (b–e) as a function of width. The dashed line shows a linear fitting (p = 4w). k) Pitch of the structures shown in (f–i) as a function of θ. The dashed line shows the theoretical prediction based on the seed pod model p = (2π/k ₀) sin2θ. l,m) Twisting configurations with θ of 22.5° (l) and 112.5° (m), showing reversed handedness. n) Multimodular 3D structure with multiple functional components, showing hybrid motions in response to temperature change from T < T _c (left) to T > T _c (right). Scale bars, 5 mm.