Multi-material direct ink writing of photocurable elastomeric foams

Developments in additive manufacturing have enabled the fabrication of soft machines that can safely interface with humans, creating new applications in soft robotics, wearable technologies, and haptics. However, designing custom inks for the 3D printing of soft materials with Young’s modulus less than 100 kPa remains a challenge due to highly coupled structure-property-process relationship in polymers. Here, we show a three-stage material chemistry process based on interpenetrating silicone double networks and ammonium bicarbonate particles that decouples the transient behavior during processing from the final properties of the material. Evaporation of ammonium bicarbonate particles at the final stage creates gaseous voids to produce foams with a low effective Young’s modulus in the 25 kPa −90 kPa range. Our photoirradiation-assisted direct ink writing system demonstrates the ability to maintain high resolution while enabling controlled loading of ammonium bicarbonate particles. The resultant multi-material possesses programmed porosity and related properties such as density, stiffness, Shore hardness, and ultimate strength in a monolithic object. Our multi-hardness synthetic hand and self-righting buoyant structure highlight these capabilities.

Enzyme-induced mineralization of hydrogels with amorphous calcium carbonate for fast synthesis of ultrastiff, strong and tough organic–inorganic double networks

Hydrogels with good mechanical properties have great importance in biological and medical applications. Double-network (DN) hydrogels were found to be very tough materials. If one of the two network phases is an inorganic material, the DN hydrogels also become very stiff without losing their toughness. So far, the only example of such an organic–inorganic DN hydrogel is based on calcium phosphate, which takes about a week to be formed as an amorphous inorganic phase by enzyme-induced mineralization. An alternative organic–inorganic DN hydrogel, based on amorphous CaCO3, which can be formed as inorganic phase within hours, was designed in this study. The precipitation of CaCO3 within a hydrogel was induced by urease and a urea/CaCl2 calcification medium. The amorphous character of the CaCO3 was retained by using the previously reported crystallization inhibiting effects of N-(phosphonomethyl)glycine (PMGly). The connection between organic and inorganic phases via reversible bonds was realized by the introduction of ionic groups. The best results were obtained by copolymerization of acrylamide (AAm) and sodium acrylate (SA), which led to water-swollen organic–inorganic DN hydrogels with a high Young’s modulus (455 ± 80 MPa), remarkable tensile strength (3.4 ± 0.7 MPa) and fracture toughness (1.1 ± 0.2 kJ m−2). Graphical Abstract The present manuscript describes the method of enzymatic mineralization of hydrogels for the production of ultrastiff and strong composite hydrogels. By forming a double-network structure based on an organic and an inorganic phase, it is possible to improve the mechanical properties of a hydrogel, such as stiffness and strength, by several orders of magnitude. The key to this is the formation of a percolating, amorphous inorganic phase, which is achieved by inhibiting crystallization of precipitated amorphous CaCO3 with N-(phosphonomethyl)glycine and controlling the nanostructure with co polymerized sodium acrylate. This creates ultrastiff, strong and tough organic–inorganic double-network hydrogels.

Rigidity and fracture of biopolymer double networks

Tunable mechanics and fracture resistance are hallmarks of biological tissues whose properties arise from extracellular matrices comprised of double networks. To elucidate the origin of these desired properties, we study...