This journal is © The Royal Society of Chemistry 2015 Soft Matter, 2015, 11, 8253--8261 | 8253 Cite this: Soft Matter, 2015, 11, 8253 Exceptionally tough and notch-insensitive magnetic hydrogels† Hussain Haider,‡ ab Can Hui Yang,‡ a Wen Jiang Zheng, ab Jian Hai Yang, ab Mei Xiang Wang, ab Sen Yang, b Miklo ´ s Zrı ´ nyi, c Yoshihito Osada, d Zhigang Suo, e Qiqing Zhang, f Jinxiong Zhou* a and Yong Mei Chen* ab Most existing magnetic hydrogels are weak and brittle. The development of strong and tough magnetic hydrogels would extend their applications into uncultivated areas, such as in actuators for soft machines and guided catheters for magnetic navigation systems, which is still a big challenge. Here a facile and versatile approach to fabricating highly stretchable, exceptionally tough and notch-insensitive magnetic hydrogels, Fe 3 O 4 @Fe-alginate/polyacrylamide (PAAm), is developed, by dispersing alginate-coated Fe 3 O 4 nanoparticles into the interpenetrating polymer networks of alginate and PAAm, with hybrid physical and chemical crosslinks. A cantilever bending beam actuator as well as a proof-of-concept magnetically guided hydrogel catheter is demonstrated. The method proposed in this work can be integrated into other strong and tough magnetic hydrogels for the development of novel hydrogel nanocomposites with both desirable functionality and superior mechanical properties. 1. Introduction As a representative soft intelligent material combining the properties of magnetism and viscoelasticity, 1–6 magnetic hydrogels have attracted intensive attention due to their unique features including biocompatibility, low friction, fast response, and spatial and temporal control manipulation, as well as non-invasive and remote actuation. 7–13 Unfortunately, most existing magnetic hydrogels are brittle and fragile. 14,15 Some magnetic hydrogels achieve good mechanical strength (B2 MPa tensile strength, and B400% stretchability), 16 but these values are markedly decreased when the samples contain small cracks or notches. These hydrogels are very sensitive to notches, meaning they are not tough. 17 Mechanical weakness confines current magnetic hydrogels to limited low-level loading areas such as drug delivery and release, 18,19 hyperthermia cancer therapy, 20 3D cell culture, 21 and enzyme immobilization. 22 By endowing magnetic hydrogels with superior mechanical properties, tough magnetic hydrogels are expected to lead to breakthrough applications beyond the horizons of current hydrogels, where toughness, stretchability and fault-tolerance are the first priorities. Examples include actuators or artificial muscles for soft robotics, pumps or valves for fluidic control, and switches for micro-machines, as well as magnetic catheters for remote magnetic manipulation systems. In this contribution, we firstly propose a facile and versatile strategy to prepare a tough and notch-insensitive magnetic hydrogel, designated Fe 3 O 4 @Fe-alginate/PAAm, based on our previously developed hybrid crosslinked hydrogel. 23 This is realized by uniformly dispersing alginate-coated Fe 3 O 4 nano- particles into the interpenetrated polymer networks of alginate and PAAm. Alginate is one of the most abundant naturally derived polysaccharides possessing many favourable properties, such as gelation in mild conditions, controllable degradation, and excellent biocompatibility. 24 PAAm is a popular synthetic polymer with excellent flexibility, biocompatibility and hydrophilicity. 25,26 Coating alginate polymers on the surface of Fe 3 O 4 nanoparticles is vital to achieve tough and notch-insensitive magnetic hydrogels. The coated alginate polymers have two functions, i.e., uniformly dispersing the magnetic nanoparticles into the hydrogel matrix via electrostatic repulsion, and stabilizing the nanoparticles into the polymer networks via a physical interaction. It is noteworthy a State Key Laboratory for Strength and Vibration of Mechanical Structures, International Center for Applied Mechanics and School of Aerospace, Xi’an Jiaotong University, Xi’an 710049, China. E-mail: chenym@mail.xjtu.edu.cn, jxzhouxx@mail.xjtu.edu.cn b School of Science, State Key Laboratory for Mechanical Behaviour of Materials, Collaborative Innovation Center of Suzhou Nano Science and Technology, Xi’an Jiaotong University, Xi’an, 710049, China c Laboratory of Nanochemistry, Department of Biophysics and Radiation Biology, Semmelweis University, Nagyva ´radte ´r 4, H-1084 Budapest, Hungary d RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan e School of Engineering and Applied Science, Kavli Institute of Bionano Science and Technology, Harvard University, Cambridge, Massachusetts 02318, USA f Institute of Biomedical Engineering, Chinese Academy of Medical Science & Peking Union Medical College, Tianjin, 300192, China † Electronic supplementary information (ESI) available: Fracture energy calculation, FTIR spectra, size distribution of nanoparticles, magnetic hysteresis loop, compres- sion hysteresis curves, table of mechanical properties of the tough magnetic hydrogels and four movies. See DOI: 10.1039/c5sm01487e ‡ Hussain Haider and Can Hui Yang contributed equally to this work. Received 16th June 2015, Accepted 27th August 2015 DOI: 10.1039/c5sm01487e www.rsc.org/softmatter Soft Matter PAPER Published on 27 August 2015. Downloaded by Harvard University on 11/01/2016 19:46:22. View Article Online View Journal | View Issue