Composed of a thermoresponsive hydrogel and a rigid polymer, these tiny structures can switch between flat, curved, or donut-like configurations. Crucially, their motion in fluid isn't just a byproduct of these shape changes; it’s driven by asymmetric electrokinetic flows generated when the particles interact with alternating current (AC) electric fields.
Background: A Step Forward in Soft Microrobotics
Microscale robots that mimic swimming microorganisms have long fascinated researchers, particularly for their potential in medicine and environmental sensing. However, most synthetic swimmers are rigid and lack the adaptability seen in biological systems. While past efforts have produced soft, stimuli-responsive materials, integrating reversible shape changes with steerable propulsion at this scale remained a challenge.
This study bridges that gap by designing soft, bilayer particles that not only change shape reversibly with temperature but also exploit those changes to control propulsion, offering a programmable platform for adaptive motion.
Materials and Fabrication
The particles were engineered from two custom copolymers synthesized via free-radical polymerization: a thermoresponsive hydrogel that swells or contracts with temperature and a thin, rigid 'glassy' polymer layer. Both polymers contained benzophenone crosslinkers to allow precise patterning.
Fabrication involved spin-coating the two polymers onto a silicon wafer layered with sacrificial polyvinyl alcohol (PVA), followed by UV lithography to define rectangular particles (40?µm × 4?µm). After development, the particles were released into water by dissolving the PVA layer.
For propulsion experiments, researchers used a microfluidic chamber fitted with coplanar gold electrodes. Particles were introduced into the chamber and exposed to AC electric fields (typically 400?V/cm at 300?Hz), while a temperature-controlled microscope stage allowed precise thermal tuning.
High-resolution tracking captured particle trajectories, and complementary analyses, including dielectrophoresis (DEP), impedance spectroscopy, and particle image velocimetry (PIV), helped reveal the mechanisms behind motion.
Key Findings: Morphing Shapes, Configurable Motion
The core finding: these bilayer particles exhibit distinct propulsion modes depending on their temperature, and thus their shape and polarizability.
- At 35?°C (flat): Particles aligned with the field but remained stationary.
- At 25?°C (boomerang-like): They moved linearly, perpendicular to the electric field.
- At 17?°C (crescent): Motion became three-dimensional and helical.
- At 10?°C (horseshoe): Propulsion switched direction, moving parallel to the field.
- Below 5?°C (donut): Motion ceased due to isotropic shape and polarization symmetry.
This behavior hinges on a critical temperature (~22?°C) where the effective polarizability of the two layers flips. Above this threshold, the hydrogel is more polarizable; below it, the rigid layer dominates.
This temperature-dependent "polarizability inversion" creates asymmetric electro-osmotic flows (known as induced-charge electrophoresis or ICEP), which, coupled with the particle's shape, produce directional motion.
In one striking demonstration, the team programmed a single particle's path by cycling temperature, causing it to reverse direction, change orientation, and shift trajectories in real time. This illustrates complete two-dimensional control, achieved without altering the electric field.
Mechanistic Insights: Steering via Electrokinetic Coupling
The propulsion mechanism relies on ICEP, where the electric field induces charge separation at the particle-fluid interface, creating local fluid flows. The temperature-dependent shift in hydrogel properties (zeta potential and ionic conductivity) alters the direction of these flows.
The shape of the particle enhances this effect. When the structure bends asymmetrically, the flow fields become unbalanced, creating net motion. When the shape becomes symmetric (as in the donut form), motion stops, which is a clear sign that propulsion arises from coupled shape and field interactions rather than deformation alone.
This coupling between shape anisotropy and polarizability inversion provides a robust design rule for creating adaptive microswimmers capable of programmable movement.
Toward Reconfigurable, Responsive Microrobots
This work introduces a new class of soft, shape-morphing microscale particles with programmable propulsion and steering, controlled entirely by simple temperature changes. The key innovation lies in combining reversible mechanical deformation with field-driven motion governed by temperature-dependent electrokinetic properties.
These adaptive particles offer exciting possibilities for applications requiring responsive, on-demand control, such as targeted drug delivery, microscale sensing, and active materials that reconfigure in real time.
Journal Reference
Lee et al. (2026). Shape-morphing active particles with invertible effective polarizability for configurable locomotion and steering. Nature Communications, 17(1). DOI:10.1038/s41467-025-65482-9. https://www.nature.com/articles/s41467-025-65482-9
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