3D Printing of Metal-Reinforced Double Network Granular Hydrogels

Advances in tissue engineering and soft robotics require a soft material able to bear a significant load while having a controlled microstructure, locally varying properties and responsiveness. Due to their potential biocompatibility and ability to retain large amounts of water, hydrogels are a good candidate. However, their ability to bear a significant load is limited by the intrinsic trade-off between stiffness and toughness. Inspired by nature, hydrogels with high mechanics can be designed controlling both microstructure and local composition. To achieve this, double network granular hydrogels (DNGHs) are introduced. They are composed of jammed micrometric-sized hydrogel, held together by a percolating second network. The energy dissipation mechanism of the double network causes an improvement in the mechanical performance. The granular hydrogels used for the first network have rheological and physiochemical properties that allow the 3D printability, therefore improving the structural control up to the micrometric scale. Emulsion-based methods are commonly used to obtain spherical polydisperse microgels. However, this technique has a limited throughput and presents batch-to-batch variations. To overcome these limitations, cryogenic mechanical fragmentation is introduced. Cryomilling is a scalable, clean process that allows a wide material choice and high reproducibility to create random-shaped microfragments. Jammed microfragments exhibit rheological characteristics and printing resolution that are comparable to those of inks made of jammed spherical microgels. The mechanical performance of the DNGHs can be further improved for load bearing applications with ionic coordination. The selective binding of metal ions with carboxyl groups is leveraged to enable a selective reinforcement of the material. Choosing where the responsive polymer is introduced, the double network can be reinforced either in the microfragments or in the second percolating network. Reinforcing the microfragments, it is obtained a homogeneous, stiff material, whose Young's modulus increases from 0.12 MPa to 8 MPa. Reinforcing the second network, a heterogeneous, core-shell structure is created. Its work of fracture increases from 0.05 MJ/m^3 to 12 MJ/m^3 after reinforcement, while maintaining a high stiffness of 9 MPa. The mechanical behaviour can be adjusted with the ionic concentration and by introduction of a competitive ligand, which can also be used to reverse the formation of ionic crosslinks. The main advantage of the ionically reinforced DNGHs is the combination of the outstanding mechanical properties, the printability imparted by the granular structure, and stimuli responsiveness to metal ions. Shape-morphing structures can be created by printing a reactive pattern within an unreactive gel. When exposed to an ion-containing solution, the structure can be selectively reinforced in the patterned areas. The shrinkage and reinforcement establish internal tensions within the structure which trigger the shape morphing properties. The development of a 3D printable, ionically reinforced DNGH enables the creation of responsive, smart soft materials that are strong enough to bear significant loads, which can be used for soft robots and tissue replacements. The soft load bearing synthetic tissues produced with this method may locally modify their properties in response to external stimuli, therefore closely resembling their natural counterparts.