Over the past few years, there has been a great deal of interest in the development of hydrogel materials with tunable structural, mechanical, and rheological properties, which exhibit rapid and autonomous self-healing and self-recovery for utilization in a broad range of applications, from soft robotics to tissue engineering. However, self-healing hydrogels generally either possess mechanically robust or rapid self-healing properties but not both. Hence, the development of a mechanically robust hydrogel material with autonomous self-healing on the time scale of seconds is yet to be fully realized. Here, the current advances in the development of autonomous self-healing hydrogels are reviewed. Specifically, methods to test self-healing efficiencies and recoveries, mechanisms of autonomous self-healing, and mechanically robust hydrogels are presented. The trends indicate that hydrogels that self-heal better also achieve self-healing faster, as compared to gels that only partially self-heal. Recommendations to guide future development of self-healing hydrogels are offered and the potential relevance of self-healing hydrogels to the exciting research areas of 3D/4D printing, soft robotics, and assisted health technologies is highlighted.

A hydrogel–dielectric‐elastomer system, polyacrylamide and poly(dimethylsiloxane) (PDMS), is adapted for extrusion printing for integrated device fabrication. A lithium‐chloride‐containing hydrogel printing ink is developed and printed onto treated PDMS with no visible signs of delamination and geometrically scaling resistance under moderate uniaxial tension and fatigue. A variety of designs are demonstrated, including a resistive strain gauge and an ionic cable.

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Hydrogels were utilized as the active material in the 4D printed structures since these materials have been processed with printers [ 8 ] and some examples of hydrogels can drastically, and reversibly, alter their volume in response to changes within their environment. [ 9,10 ] We start with an ionic covalent entanglement (ICE) hydrogel [ 11 ] that can be 3D printed [ 12 ] and demonstrate high toughness. [ 12 ] The latter is important since we require thin sections that respond quickly to external stimuli but also need to be mechanically robust to support the internal and external mechanical loads. ICE gels are a type of an interpenetrating polymer network hydrogel that is made up of an entanglement of one polymer network crosslinked with metal cations and a second polymer network crosslinked with covalent bonds. [ 11,[13][14][15] This dual network structure is similar to that in double network hydrogels [ 16,17 ] and facilitates high toughness through large-scale crack-tip energy dissipation by unloading of the strands in the tight network, [ 18 ] in this case due to dissociation of the ionic crosslinks. [ 19 ] The loosely crosslinked covalent network serves to bridge the damage zones created by the loss of ionic bonds and prevents catastrophic crack propagation. Here, we use a thermally responsive covalent crosslinked network of poly( N -isopropylacrylamide) (PNIPAAm) to function as both the toughening agent and to provide actuation through reversible volume transitions. PNIPAAm is a widely studied temperature-sensitive hydrogel that exhibits a large reversible volume transition at a critical temperature, T C (≈32-35 °C). [ 20,21 ] The volume change is caused by a coil-globule transition of the polymer network strands [ 22 ] and results in a large decrease in the water content when the temperature is increased above T c . [ 20 ] Results and DiscussionAlginate/PNIPAAm ICE gel inks with various concentrations of NIPAAm were prepared for extrusion printing.

In this review hydrogel-forming polymers that are suitable for extrusion-based 3D printing are evaluated.

Production of stable polymer-nanotube composites depends on good wetting interaction between polymer and nanotube, which is polymer specific, and depends in particular on chain conformation. In this paper, we examine this interaction for a conjugated, semiconducting polymer by a range of microscopic and spectroscopic techniques, to gain a greater understanding of the binding. Several interesting effects are observed, including an order to the interaction between the polymer and nanotube, the tendency of defects in the nanotube structure to nucleate crystal growth, and substantial changes in the spectroscopic behavior of the polymer due to the effect of the nanotubes on polymer conformation. This is substantiated by computational modeling, which demonstrates that these conformational modifications are due to the interaction with the nanotubes.

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 m3. 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.Keywords printing, hydrogels, entanglement, extrusion, covalent, toughness, ionic, high Disciplines Engineering | Physical Sciences and Mathematics Publication DetailsBakarich, S., in het Panhuis, M., Beirne, S. T., Wallace, G. G. and Spinks, G. Maxwell. (2013) 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 10 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 15 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 ...

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