Covalent-supramolecular hybrid polymers as muscle-inspired anisotropic actuators

Abstract

Skeletal muscle provides inspiration on how to achieve reversible, macroscopic, anisotropic motion in soft materials. Here we report on the bottom-up design of macroscopic tubes that exhibit anisotropic actuation driven by a thermal stimulus. The tube is built from a hydrogel in which extremely long supramolecular nanofibers are aligned using weak shear forces, followed by radial growth of thermoresponsive polymers from their surfaces. The hierarchically ordered tube exhibits reversible anisotropic actuation with changes in temperature, with much greater contraction perpendicular to the direction of nanofiber alignment. We identify two critical factors for the anisotropic actuation, macroscopic alignment of the supramolecular scaffold and its covalent bonding to polymer chains. Using finite element analysis and molecular calculations, we conclude polymer chain confinement and mechanical reinforcement by rigid supramolecular nanofibers are responsible for the anisotropic actuation. The work reported suggests strategies to create soft active matter with molecularly encoded capacity to perform complex tasks.

Fig. 1

Preparation of hybrid actuators. a PA1 and PA2 are codissolved in hexafluoroisopropanol (HFIP) to ensure complete dissolution and molecular mixing. Removal of HFIP and redissolution in aqueous buffer gives b, coassembled peptide amphiphile (PA) nanofibers. Subsequent annealing at 80 ˚C for 30 min gives a LC solution of high aspect-ratio PA nanofibers. c Cryogenic transmission electron microscopy shows long nanofibers (scale bar is 200 nm), and polarized optical microscopy (inset, scale bar is 200 μm) shows birefringent domains. d To fabricate the PA tubes, rotational shear force is applied to PA solution to circumferentially align the nanofibers within a tubular mold. The central rod is then retracted to allow influx of CaCl₂ solution, gelling the PA as shown in e, maintaining nanostructure orientation. Covalent polymer chains are grafted from nanofibers with atom-transfer radical polymerization in a polymerization bath. f The resulting hybrid contains covalent chains grafted radially from the nanofiber surface and shows a distinct opacity change from the unpolymerized state (photographic insets, scale bars are 1 cm), while maintaining fibrous morphology (scanning electron microscopy insets, scale bars are 10 μm)

Fig. 2

Anisotropic actuation in hybrids. a Circumferentially aligned hybrid polymer (left) shows anisotropic contraction along the length of the tube upon heating (center), as normalized to original tube dimensions (right) over the course of 1800 s. b Covalent polymer tube. c Supramolecular polymer tube. d Axially aligned hybrid polymer. (scale bars are 3 mm). A representative heating curve is depicted in Supplementary Fig. 7. Statistical analysis was performed using an unpaired two samples Student´s t-test; *p < 0.05, ** p < 0.01, ***p < 0.001; (Data are presented as mean ± s.d., n = 3)

Fig. 3

PA alignment in hybrid materials. Circumferentially aligned hybrid polymer shows birefringence in cross-sectional slices (a) as well as along the tube wall (d) under cross-polarized light. Axially aligned hybrid polymer shows birefringence only in the tube wall (b, e). Covalent polymer shows negligible birefringence (c, f). 2D small-angle x-ray scattering patterns show angle dependent intensity maxima in the aligned hybrids but not in the covalent polymer (g-i). Insets show integrated radial intensity versus azimuthal angle. Scale bars in images of cross-sectional slices (a–c) are 1 mm and 400 μm in images of the tube walls (d–f)

Fig. 4

Mechanical reinforcement of PA nanofibers. a PA nanofibers with high persistence length provide mechanical reinforcement along the PA axis, preventing contraction in the direction of alignment. b Experimental results of heating a circumferentially aligned composite material. Statistical analysis was performed using an unpaired two samples Student´s t-test; *p < 0.05, **p < 0.01, ***p < 0.001; (Data are presented as mean ± s.d., n = 3). c Schematic of finite element analysis model of covalent-noncovalent system. d Finite element analysis results of shrinkage due to mechanical reinforcement with varying interfiber distance

Fig. 5

Confinement of polymer chains. a Schematic representation of confinement effect in grafted polymer chains below (extended) and above (collapsed) the transition temperature leading to pronounced volume changes in the material perpendicular to the supramolecular nanofiber. b Polymer volume fraction versus radial distance away from the PA nanofiber. Polymer is a linear alternating copolymer (EG₂-a-HB)₆₆-EG₂. c Height of end-tethered polymer as function of temperature for different copolymers with increasing amounts of hydrophobic monomers. The polymer is a linear alternating copolymer (EG_n-a-HB₁)_m. Number of segments is N_p = 200, the surface coverage is 0.07 chains nm⁻², and radius of nanofiber equals 5 nm

Fig. 6

3D printing of hybrid sheets. a. Schematic of 3D printing process. Aligned sheets before (b) and after (e) heating. Square spiral printed sheets before (c) and after (f) heating. Cross-hatched sheets before (d) and after (g) heating. Insets show printing patterns of each sheet. (scale bars are 5 mm)