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Magnetic movement under the spotlight

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Science Robotics  09 Dec 2020:
Vol. 5, Issue 49, eabf1503
DOI: 10.1126/scirobotics.abf1503

Abstract

Composite hydrogel robots can achieve programmable locomotion using light and magnetic fields.

The realization of synthetic analogs of natural systems, which can imitate the movement and functionalities exhibited by living organisms, has long been a source of intrigue for engineers and material scientists. Existing artificial systems lack the complexity and mechanical compliance found in nature, which results in a mismatch between the synthetic and the biological. In this respect, the field of soft robotics has seen a transition in recent years from engineering-led to biologically inspired concepts (13). The incompatibility between the synthetic and biological, in flexibility, adaptability, and self-repair, may be bridged by the use of soft polymers, in which actuation, control, and functionality can be integrated into a single material. Writing in Science Robotics, Li et al. (4) combine photoresponsive hydrogels with highly ordered ferromagnetic nanowires to deliver a magnetically driven hydrogel that can be programmed and reconfigured using light. They present a four-legged hydrogel robot that can walk, steer, climb, and deliver cargo to desired locations in an aqueous environment (Fig. 1).

Fig. 1 The photoactivated, magnetic hydrogel transporter.

(Left) Upon light irradiation from below, the cross-shaped flat hydrogel adopts an arch conformation, which allows for capture of an alginate bead. The robot is then transported in a rolling motion under magnetic fields, and the bead is released when light irradiation from above inverts the hydrogel’s curvature. (Right) Hydrogel robot walks with cargo from left to right, under a rotating magnetic field, and releases it through a fast spinning motion.

[CREDIT: FIGURE ADAPTED FROM FIGURE 6 (C AND D) IN (4)]

Hydrogels represent an exciting technological building block for the realization of soft robots in aqueous environments due to their hydrophilic polymer networks that can absorb large quantities of water. The addition of stimuli-responsive moieties results in hydrogels that can reversibly alter their water uptake in response to temperature, light, electric field, or local chemical environment. At the macroscale, hydrogel actuation is diffusion controlled, because it relies on recurring swelling/deswelling of the polymer network, under external stimulation. This in turn induces slow actuation kinetics, which represents a key challenge for the application of soft hydrogel robots.

It would be also remiss not to highlight the need to overcome weak mechanical properties and isotropic response, which represent two other notable, but surmountable, shortcomings of hydrogel actuators. Li et al. have elegantly overcome these challenges through the creation of an anisotropic hydrogel–metal hybrid material and the inclusion of aligned ferromagnetic nanowires. This is innovative for two reasons: the aligned nanowires give rise to improved mechanical properties and asymmetric hydrogel actuation, and the presence of a ferromagnetic material ensures a fast response under an external rotating magnetic field.

The hybrid robot has a simple geometrical design, starting as a flat, cross-shaped hydrogel slab. Under white light irradiation from beneath, it undergoes a macroscopic deformation, as if to rise to its four legs. In its newly adopted arched state, the hydrogel shows a nonuniform three-dimensional (3D) magnetization profile of the magnetic nanowires, enabling controlled, programmable actuation of the walker under a rotating magnetic field. Conversely, removal of the light source returns the hydrogel to its initial state of rest, where it remains unperturbed by magnetic stimulation. The interdependency of this photomagnetic response gives the robots, proposed by Li et al., their own in-built controller, making magnetic stimulation ineffective in the dark. High-intensity light irradiation from below causes the four-leg hydrogel walker to curl into a ball, similar to a hedgehog. This action can be used to wrap around external objects (exemplified here by an alginate bead), and the robots rolled to desired locations under a magnetic field, where their cargo could be released upon subsequent photoirradiation. By merging responsive hydrated soft matter with aligned magnetic materials and wireless actuation, the authors offer a potential platform for true imitation of biological motion and function.

In its current form, the hybrid hydrogel robot requires acidic conditions for operation, which does represent a notable limitation. Such conditions serve to protonate the spiropyran comonomer to its more hydrophilic form, which is essential for the generation of hydrogel expansion (5). Previous work in the field of spiropyran-based photoactuators has shown that copolymerization of an acidic comonomer (acrylic acid) can function as an internal proton source, thus drastically extending the operational window of such photoactuators (6, 7).

The findings presented by Li et al. are exciting because of the potential to miniaturize their technology and apply it to a variety of problems. For example, shrinking down to the microscale could yield vehicles capable of programmed navigation along narrow fluidic channels. Microfabrication techniques, such as direct laser writing by multiphoton polymerization, could potentially realize complex geometries, coupled with controlled and programmable actuation in 3D (8, 9). If these various elements can be achieved, the work presented may well find application in teams of microvehicles that nimbly weave their way through tortuous arterial networks.

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