Hybrid magnetorheological elastomers enable versatile soft actuators

The appropriate combination of soft and hard magnetic particles within an ultra-soft polymeric matrix opens new routes to design versatile multifunctional actuators. These materials allow for, depending on an external magnetic stimulation, changing their stiffness and/or their shape significantly.
Hybrid magnetorheological elastomers enable versatile soft actuators

The engineering fields have experienced a change of paradigm with the incursion of intelligent materials that can mechanically respond to external stimuli. Among then, magneto-responsive materials stand out for soft robotics and biomedical applications. These materials react to external magnetic fields by changing their material properties (e.g., stiffness) or their shape. Interestingly, the human body has a low magnetic permeability (i.e., it is almost transparent to magnetic fields) so that, if these materials are placed inside it, we can control them remotely by using any kind of magnetic source. The mechanical response to such magnetic actuation is determined by the nature of the polymeric matrix and, especially, by the nature of the magnetic fillers. Thus, if soft magnetic particles are used, i.e., particles that do not present permanent magnetisation, the material presents great opportunities for changing its stiffness but, in turn, it presents certain limitations for programming complex shape-morphing. Contrary, the use of hard magnetic particles, i.e., particles that do present permanent magnetisation, allows for significant shape-morphing but limited stiffness modulation. Therefore, an open question is whether the combination of soft and hard magnetic fillers may provide versatile solutions with both material modulations (significant changes in stiffness and shape).

This question has recently been revisited by a collaborative team composed of active researchers in the magneto-mechanics field from Swansea University (UK), FAU Erlangen-Nuremberg (Germany) and Universidad Carlos III de Madrid (Spain). This work has been conducted within the framework of the ERC project 4D-BIOMAP. The authors demonstrate that soft and hard magnetic particles embedded within an ultra-soft elastomeric matrix provide materials whose mechanical properties and shape-morphing can be remotely controlled by external magnetic stimulation. To conceptualise these hybrid magnetorheological elastomers (MREs), a comprehensive experimental characterisation is performed, where the mechanical behaviour of the materials is analysed under different magnetic conditions. The results unravel that the magneto-mechanical performance of hybrid MREs can be optimised by selecting an adequate mixing ratio between particles. Interestingly, these materials not only allow for combining relevant stiffness modulation and shape-morphing, but also for reaching stronger modulation in both responses with respect to traditional soft and hard MRE solutions.  

To understand the underlying mechanisms that govern the magneto-mechanical behaviour of hybrid MREs, the authors conceptualised a multi-physics computational framework that simulates the problem at the microscale. The use of this virtual framework unravels synergistic magneto-mechanical interactions between the soft and hard particles. Hard particles contribute to torsional actuations whereas soft particles amplify the magnetisation by creating magnetic bridges that connect the particle network (Figure 1). The numerical results suggest that the effective response of hybrid MREs emerges from these intricate interactions.

Figure 1: Hybrid magnetorheological elastomer exposed to a vertical magnetic field. The magnetic stimulation creates interaction forces between the particles that are transmitted to the elastomeric matrix leading to an increase in stiffness and shape bending. The hard magnetic particles (bigger ones) are responsible to introduce microstructural torques whereas soft particles (smaller ones) amplify the resulting magnetic fields, in turn, amplifying the stiffening and shape-morphing effects.

Following these concepts, the authors developed a virtual testbed to design multifunctional actuators and evaluate computationally their performance. This virtual framework uncovers exciting possibilities to push the frontiers of MRE solutions. These are demonstrated by simulating a bimorph beam that provides actuation flexibility either enhancing mechanical bending or material stiffening, depending on the magnetic stimulation (Figure 2). This application may be useful in, for example, microfluidic systems to actuate in two modes: 1) the beam bending can be activated via a perpendicular magnetic stimulation and a parallel fluid flow can be penalised; 2) alternatively, the parallel magnetic stimulation would increase its structural stiffness opposing to the fluid flow perpendicular to the beam. Other existing possibilities are related to the bioengineering field, where these materials can be used to interact with cellular systems transmitting forces or stiffness gradients to them dynamically (see recent work by the authors, Moreno-Mateos et al. in Applied Materials Today, 2022). Overall, this work provides the experimental and computational bases to design multifunctional structures with a high flexibility from different perspectives, and opens other possibilities such as functionally graded materials to activate locally different actuation modes.

Figure 2: Bimorph beam conceptualised with a hybrid magnetorheological elastomer. When the beam is exposed to a parallel magnetic field, the structure presents an increase in effective stiffness. Moreover, when the beam is exposed to a perpendicular magnetic field, the structure experiences bending. The figure shows the magnetisation and magnetic flux density distributions within the beam. The microstructural magnetic bridges can be observed in both actuation modes.

More details are provided in the original version of the manuscript: https://doi.org/10.1038/s41524-022-00844-1

Miguel Angel Moreno-Mateos, Mokarram Hossain, Paul Steinmann, Daniel Garcia-Gonzalez, Hybrid magnetorheological elastomers enable versatile soft actuators. npj Comput Mater 8, 162 (2022).