An anisotropic hydrogel with electrostatic repulsion between cofacially aligned nanosheets

Abstract

Machine technology frequently puts magnetic or electrostatic repulsive forces to practical use, as in maglev trains, vehicle suspensions or non-contact bearings. In contrast, materials design overwhelmingly focuses on attractive interactions, such as in the many advanced polymer-based composites, where inorganic fillers interact with a polymer matrix to improve mechanical properties. However, articular cartilage strikingly illustrates how electrostatic repulsion can be harnessed to achieve unparalleled functional efficiency: it permits virtually frictionless mechanical motion within joints, even under high compression. Here we describe a composite hydrogel with anisotropic mechanical properties dominated by electrostatic repulsion between negatively charged unilamellar titanate nanosheets embedded within it. Crucial to the behaviour of this hydrogel is the serendipitous discovery of cofacial nanosheet alignment in aqueous colloidal dispersions subjected to a strong magnetic field, which maximizes electrostatic repulsion and thereby induces a quasi-crystalline structural ordering over macroscopic length scales and with uniformly large face-to-face nanosheet separation. We fix this transiently induced structural order by transforming the dispersion into a hydrogel using light-triggered in situ vinyl polymerization. The resultant hydrogel, containing charged inorganic structures that align cofacially in a magnetic flux, deforms easily under shear forces applied parallel to the embedded nanosheets yet resists compressive forces applied orthogonally. We anticipate that the concept of embedding anisotropic repulsive electrostatics within a composite material, inspired by articular cartilage, will open up new possibilities for developing soft materials with unusual functions.

Bioinspired Nanocomposite Hydrogels with Highly Ordered Structures

“…[60][61][62][63][64] Recently, Liu et al proposed the concept of embedding anisotropic electrostatic repulsions within nanocomposite hydrogels, generating new possibilities for fabricating anisotropic soft materials with highly ordered structures. [62] In their aligned hydrogel systems, negatively charged unilamellar TINSs aligned coaxially in a strong magnetic field, which maintained maximum electrostatic repulsion and thereby induced a quasicrystalline structural ordering over macroscopic length scales and with uniformly large face-to-face nanosheet separation (Figure 3a). This transiently induced structural order was rigidified by using light-triggered in situ vinyl polymerization.…”

“…[62] Researchers discovered that the resultant hydrogel deformed easily under shear forces that were applied parallel to the embedded nanosheets yet resisted compressive forces that were applied orthogonally, thereby enabling excellent directional isolation of vibrations. This reinforcement of hydrogels by using intrinsic ordered nanocomposite structures can make for exceptionally strong and elastomeric nanocomposites with properties that closely mimic those of articular cartilage.…”

High‐Performance Nanocomposites Inspired by Nature

“…This is because the load transfer efficiency could be greatly enhanced by the nacre-like layered structure. In addition, Liu et al [67] demonstrated an anisotropic nanocomposite hydrogel with a cartilage-like layered architecture. A magnetic field was first applied to the N,N-dimethylacrylamidecan suspension with titanate nanosheets (TiNSs).…”

Supramolecular hydrogels cross-linked by preassembled host–guest PEG cross-linkers resist excessive, ultrafast, and non-resting cyclic compression

“…In addition to their pioneering study on double-network hydrogels, Gong et al developed ionic bond-based polyampholyte hydrogels that possess excellent robustness and viscoelasticity 6,[18][19][20][21] . Aida et al developed mechanically robust hydrogels by incorporating various nanocomposites [22][23][24] . The aforementioned research demonstrates the great potential of supramolecular chemistry to improve the mechanical properties and fatigue resistance of hydrogels.…”