Extrusion printing of ionic-covalent entanglement hydrogels with high toughness

Three-dimensional (3D) printing of hydrogels has recently been investigated for use in tissue engineering applications. One major limitation in the use of synthetic hydrogels is their poor mechanical robustness but the development of 'tough hydrogels' in conjunction with additive fabrication techniques will accelerate the advancement of many technologies including soft robotics, bionic implants, sensors and controlled release systems. This article demonstrates that ionic-covalent entanglement (ICE) gels can be fabricated through a modified extrusion printing process that facilitates in situ photopolymerisation. The rheological properties of alginate-acrylamide hydrogel precursor solutions were characterised to develop formulations suitable for extrusion printing. A range of these printed hydrogels were prepared and their mechanical performance and swelling behaviour evaluated. ICE gels exhibit a remarkable mechanical performance because ionic cross links in the biopolymer network act as sacrificial bonds that dissipate energy under stress. The printed ICE gels have a work of extension 260 ± 3 kJ m^-3. Swelling the hydrogels in water has a detrimental effect upon their mechanical properties, however swelling the hydrogels in a calcium chloride solution as a post-processing technique reduces the effects of swelling the hydrogels in water. The integration of the modified extrusion printing process with existing plastic 3D printing technologies will allow for the fabrication of functional devices.