Over the past two decades, the development of hydrogels with high mechanical strength and toughness shows both fundamental and practical significances. It is challenging yet highly desired to process these tough hydrogels into structural elements with desired architectures, which are important for real applications. Three-dimensional (3D) printing, as an emerging additive manufacturing strategy, demonstrates great potentials for engineering tough hydrogels with versatile applications. This paper aims to review the recent progress on 3D printing of tough hydrogels and their relevant applications. We give an overview on the advantages and requirements of tough hydrogel systems for specific 3D printing technologies, followed by detailed summary of the network structures, mechanical performances, toughening mechanisms, and representative applications of the printed tough hydrogels. Finally, recent advances of 4D printing of tough hydrogels are discussed as a prospective direction to devise soft actuators and robots. 

Recent advances in 3D printing of tough hydrogels: A review

Smart windows with tunable optical properties that respond to external environments are being developed to reduce energy consumption in buildings. In the present study, we introduce a new type of 3D printed hydrogel with amazing flexibility and stretchability (as large as 1500%), as well as tunable optical performance controlled by surrounding temperatures. The hydrogel on a PDMS substrate shows transparent-opaque transition with high solar modulation (ΔT sol) up to 79.332% around its lower critical solution temperature (LCST) while maintaining a high luminous transmittance (T lum) of 85.847% at 20 °C. In addition, selective transparent-opaque transition above LCST can be achieved by patterned hydrogels which are precisely fabricated via a projection micro-stereolithography based 3D printing technique. Our hydrogel promises great potential applications for the next generation of soft smart windows.

https://iopscience.iop.org/article/10.1088/2631-7990/ac5ae3/pdf

3D printed hydrogel for soft thermo-responsive smart window

Most current hydrogel actuators suffer from either poor mechanical properties or limited responsiveness. Also, the widely used thermo-responsive poly-(N-isopropylacrylamide) (PNIPAM) homopolymer hydrogels have a slow response rate. Thus, it remains a challenge to fabricate thermo-responsive hydrogel actuators with both excellent mechanical and responsive properties. Herein, ultrafast thermo-responsive VSNPs-P(NIPAM-co-AA) hydrogels containing multivalent vinyl functionalized silica nanoparticles (VSNPs) are fabricated. The ultrafast thermo-responsiveness is due to the mobile polymer chains grafted from the surfaces of the VSNPs, which can facilitate hydrophobic aggregation, inducing the phase transition and generating water transport channels for quick water expulsion. In addition, the copolymerization of NIPAM with acrylic acid (AA) decreases the transition temperature of the thermo-responsive PNIPAM-based hydrogels, contributing to ultrafast thermo-responsive shrinking behavior with a large volume change of as high as 72.5%. Moreover, inspired by nature, intelligent hydrogel actuators with gradient structure can be facilely prepared through self-healing between the ultrafast thermo-responsive VSNPs-P(NIPAM-co-AA) hydrogel layers and high-strength VSNPs-PAA-Fe3+ multibond network (MBN) hydrogel layers. The obtained well-integrated gradient hydrogel actuators show ultrafast thermo-responsive performance within only 9 s in 60 °C water, as well as high strength, and can be used for more practical applications as intelligent soft actuators or artificial robots.

Biomimetic Gradient Hydrogel Actuators with Ultrafast Thermo-Responsiveness and High Strength.

Development of conductive hydrogels: from design mechanisms to frontier applications

Advances in 3D/4D printing of mechanical metamaterials: From manufacturing to applications

Solvent-Cast-Assisted Printing of Biomimetic Morphing Hydrogel Structures with Solvent Evaporation-Induced Swelling Mismatch

Hydrogels are widely used in applications such as soft robots and flexible sensors due to their sensitivity to environmental stimuli. It is highly demanded to develop multiple-responsive hydrogel structures. In this work, we employ the 3D printing technique to fabricate a PNIPAM–PEGDA hydrogel bilayer that can change shape through controlling the temperature, solvent mixture and magnetic field. The PNIPAM gel is a typical thermo-responsive gel, showing a decrease in swelling ratio with increasing temperature. Meanwhile, the PNIPAM gels also exhibit the cononsolvency effect in ethanol–water mixtures with a smaller swelling ratio in the mixture compared with that in each pure solvent. In comparison, the swelling ratio of PEGDA gels is insensitive to changes in both the temperature and solvent composition. Thus, the bilayer structure of PNIPAM–PEGDA can bend in different directions and with different angles with changing the temperature and solvent composition. Finally, Fe3O4 nanoparticles are incorporated into the matrix of PEGDA gels, endowing the whole structure with deformation and motion in response to an external magnetic field.

Fengrui Liu

Papers

Solvent-Cast-Assisted Printing of Biomimetic Morphing Hydrogel Structures with Solvent Evaporation-Induced Swelling Mismatch

Multi-responsive PNIPAM-PEGDA hydrogel composite.