Mechanical reinforcement of granular hydrogels

Abstract

Granular hydrogels are composed of hydrogel-based microparticles, so-called microgels, that are densely packed to form an ink that can be 3D printed, injected or cast into macroscopic structures. They are frequently used as tissue engineering scaffolds because microgels can be made biocompatible and the porosity of the granular hydrogels enables a fast exchange of reagents, waste products, and if properly designed even the infiltration of cells. Most of these granular hydrogels can be shaped into appropriate macroscopic structures, yet, these structures are mechanically rather weak. The poor mechanical properties prevent the use of these structures as load-bearing materials and hence, limit their field of applications. The mechanical properties of granular hydrogels depend on the composition of microgels and the interparticle interactions. In this review, we discuss different strategies to assemble microparticles into granular hydrogels and highlight the influence of inter-particle connections on the stiffness and toughness of the resulting materials. Mechanically strong and tough granular hydrogels have the potential to open up new fields of their use and thereby to contribute to fast advances in these fields. In particular, we envisage them to be well-suited as soft actuators and robots, tissue replacements, and adaptive sensors.

Fig. 1. (a) Microgels are assembled and cured to form granular hydrogels. (b) W-shaped stripe of granular hydrogel composed of microgels with two different Young's moduli. The microgels are cured through electrostatic interactions. Adapted with permission. (c) Stripe of double network granular hydrogel holding a 1 kg weight. Adapted with permission.

Fig. 2. Elastic mechanical properties of granular hydrogels cured through various interparticle binding strategies. Ashby plots of the storage (a), compressive (b), and tensile (c) modulus as a function of polymer content. Schematic representation of the interparticle interaction mechanisms: covalent interaction (d), binder and additives (e), coordination interactions (f), charge interactions (g), interpenetrating percolating network (h) and other supramolecular interactions (i).

Fig. 3. Granular hydrogels are cured through different inter-particle interactions. (a) Micrograph of an injected, and covalently cured granular hydrogel. (b) Micrograph of an extruded filament of jammed microgels cured into a granular hydrogel using a binder. (c) Image and micrograph of a granular hydrogel cured using coordination chemistry. (d) Images of a granular hydrogel cured by charged interactions. (e) Micrographs and image of a double network granular hydrogel. Scale bars are (a) 100 μm, (b) 500 μm, (c) 100 μm, (d) 2 mm and 10 mm, (e) 500 μm, 1 mm and 5 mm. Adapted with permission.