Anisotropic pH-Responsive Hydrogels Containing Soft or Hard Rod-Like Particles Assembled Using Low Shear

A simple and versatile low-shear approach for assembling hydrogels containing aligned rod-like particles (RLPs) that are birefringent and exhibit pH-triggered anisotropic swelling is developed. Anisotropic composite hydrogels are prepared by applying low shear (0.1 s–1) to mixtures of pH-responsive nanogels (NGs) and RLPs. The NGs, which contained high methacrylic acid contents, acted as both shear transfer vehicles and macro-cross-linkers for anisotropic gel formation. Three model RLP systems are investigated: (i) soft triblock copolymer worms, (ii) stiff self-assembled β-sheet peptide fibers, and (iii) ultrahigh modulus nanocrystalline cellulose fibers. RLP alignment was confirmed using polarized light imaging, atomic force microscopy, and small-angle X-ray scattering as well as modulus and anisotropic swelling experiments. Unexpectedly, the composite gel containing the soft copolymer worms showed the most pronounced anisotropy swelling. The copolymer worms enabled higher RLP loadings than was possible for the stiffer RLPs. For fixed RLP loading, the extent of anisotropic swelling increased with intra-RLP bonding strength. The facile and versatile approach to anisotropic gel construction demonstrated herein is expected to enable new applications for strain sensing or biomaterials for soft tissue repair.