3D printed magnetic polymer composite hydrogels for hyperthermia and magnetic field driven structural manipulation

Magnetic hydrogels and soft composites have fueled the development of next generation biomimetic soft robotics due to their precise control and non-cytotoxic nature. Bare magnetic nanoparticles are difficult to regulate via remote controlling whereas, when these nanoparticles are arrested inside polymeric matrices, the whole system become an artificial soft mussel like integrated system. Concurrently, these polymeric magnetic soft materials are also prone to response of external magnetic field (static or oscillatory). Additive manufacturing via spatial assembly of polymeric precursors followed by actuation like behavior is quite a new manufacturing technique to fabricate magnetic soft materials. In this review, we focused on the magnetic nanoparticles and their entrapment into polymeric matrices and assessing their applicability in clinical (hyperthermia) as well as shape morphing behaviours. Both the behaviors are also amassed together to form dual responsive soft microbots. The techniques and their applications are elaborated in this review.

3D printed magnetic polymer composite hydrogels for hyperthermia

3D printed magnetic polymer composite hydrogels for hyperthermia

Magnetic hydrogelsSoft roboticsHyperthermiaExternal magnetic fieldAdditive manufacturing

Abbreviations
Magnetic hydrogels composed of hydrogel matrices and magnetic nanomaterials have attracted widespread interests. Thereinto, magnetic hydrogels with ordered structure possessing enhanced functionalities and unique architectures, show tremendous advantages in biomedical fields. The ordered structure brought unique anisotropic properties and excellent physical properties. Furthermore, the anisotropic properties of magnetic ordered hydrogels are more analogous to biological tissues in morphology and mechanical property, showing better biocompatibility and bioinducibility. Thus, we aim to systematically describe the latest advances of magnetic hydrogels with ordered structure. Firstly, this review introduced the synthetic methods of magnetic hydrogels focus on constructing ordered structure. Then, their functionalities and biomedical applications are also summarized. Finally, the current challenges and a compelling perspective outlook of magnetic ordered hydrogel are present.

Construction of magnetic hydrogels with ordered structure
In general, magnetic hydrogels are composed of a hydrogel matrix and a magnetic component. A variety of magnetic nanomaterials have been incorporated into hydrogel networks, such as γ-Fe2O3, Fe3O4, and transition metal ferrite nanoparticles (CoFe2O4, MnFe2O4, etc.) (Lima-Tenorio et al., 2015; Naderi and Azizian, 2018; Ganguly and Margel, 2021). Among these magnetic materials, superparamagnetic iron oxide nanoparticles (γ-Fe2O3, Fe3O4) were the most promising candidate for clinical applications due to their good chemical stability, high magnetization ability and great biocompatibility (Meenach et al., 2009; Ling and Hyeon, 2013; Bustamante-Torres et al., 2022).

Many endeavors have been conducted to fix magnetic materials inside polymer networks (Zhang et al., 2016b; Li et al., 2021). As systematically reviewed previously, the main strategies for fabrication of magnetic hydrogels were developed including the blending method, in situ precipitation method and the grafting-onto method, as shown in Figure 2 (Liu et al., 2020; Ganguly and Margel, 2021). Besides altering the type of hydrogel matrices and magnetic nanoparticles, the properties of magnetic hydrogels can also be readily modulated by the concentration, size and distribution of magnetic particles within the hydrogels (Li et al., 2013; Ganguly and Margel, 2021). Many biological tissues exhibit well-defined ordered structures, thus introducing ordered structures into hydrogel is necessary and significant which can mimic biological tissues more better and enhance performance (Liu et al., 2015). A variety of fabrication strategies have been developed to fabricate magnetic hydrogels with ordered structure (Sano et al., 2018; Li K. et al., 2022). We can categorize them as magnetic-field-induced assembly, microfluidics and 3D printing (Table 1). Magnetic-field-induced assembly is a controllable and easy method for construction of magnetic ordered materials by arrangement of magnetic nanoparticles under magnetic field (Shi et al., 2020a). Here, orderly manufacturing-based construction is another strategy, including microfluidics and three-dimensional (3D) printing (Daly et al., 2020; Cai et al., 2021; Yang et al., 2022b). This strategy offers the ability to prepare microscale hydrogels and unique control over the fabrication of complex structures (Zhang et al., 2021). In addition, magnetic-field-induced assembly can be integrated into 3D printing to fabricate anisotropic composites (Wang et al., 2017). In this section, we focus on how to introduce magnetic nanoparticles into the hydrogel matrices with ordered structure, and meanwhile illustrating the properties of magnetic hydrogels.

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