Dual-Cross-Linked Superparamagnetic Hydrogels with Tailored Viscoelasticity for Soft Robotics

Abstract

Paving the way for advancements in soft robotics, this study divulges a novel and efficient method to synthesize highly magnetic hydrogels (MagGels) composed of double surfactant-coated MnFe 2 O 4 superparamagnetic nanoparticles (SPNPs) and poly(vinyl alcohol) (PVA). This synthesis leverages a dual cross-linking process that integrates glutaraldehyde addition, freeze−thaw (FT) cycles, and surface-engineered SPNPs, endowing the hydrogels with distinctive rheological properties that significantly diverge from those of conventional MagGels. Our findings reveal a notable decrease in viscosity and shear stress evident from shear induced yielding under steady-state rotational measurements, suggesting a transition toward a solid-like state when exposed to an external magnetic field. This behavior results from the enhanced cross-linking density and improved coupling between the SPNPs and the hydrogel matrix, achieved through the dual cross-linking process and thermal cycling. Moreover, MagGels subjected to dual cross-linking and thermal cycling demonstrate enhanced stability due to more efficient encapsulation of SPNPs within the hydrogel matrix. Dynamic oscillatory measurements further support these results, revealing solid-like behavior under the influence of external magnetic field and a significant increase in both storage modulus (G′) and magnetorheological effect in the dual cross-linked MagGels. To demonstrate the potential of these MagGels for soft robotics applications, a MagGel sheet has been fabricated with a gradient distribution of SPNP concentration. The edges of the sheet, containing a higher concentration of SPNPs, exhibited a more rapid and robust response to an external magnetic field. This gradient-induced variation in magnetic response enabled the development of a magnetic field-induced gripping mechanism, showcasing the practical utility of MagGels in soft robotics.