Shape-morphing active particles with invertible effective polarizability for configurable locomotion and steering

Domain: chemistry
Published: 2026-01-06
Editorial outcome: minor_revision

Abstract
Active particles are analogs of microorganisms in that they locally dissipate energy to propel in low Reynolds number fluids. However, most active particles lack the ability to undergo controlled shape transformations that change how they move in response to environmental cues. Here, we present a class of stimuli-responsive active particles that exhibit fully reversible, shape-dependent propulsion. The particles consist of a bilayer of a thermoresponsive hydrogel and a non-swelling glassy polymer, patterned into rectangular microscale prisms. Temperature changes near the phase transition of hydrogel cause large curvature shifts, from flat plates at 35 °C to crescent shapes at 20 °C, accompanied by changes in effective polarizability. When powered by AC electric fields, this coupling between geometry and polarizability enables programmable propulsion modes including linear and helical motions. Sequential temperature changes allow encoded in situ steering, establishing a design principle for microscale active systems capable of adaptive propulsion and reconfiguration.

Introduction
Microorganisms adapt their shape in response to their environment, enhancing their proliferation and survival1–4. For instance, Tetrahymena thermophila, a unicellular organism within the ciliated Protozoa group, swims upwards, forming dense swarms near the surface of water where oxygen, food, and light are more abundant. Initially, cultures of Tetrahymena thermophila exposed to infrared light grow normally, but as food becomes scarce, the cells elongate into spindles, swim 50% faster than their normal counterparts, and follow straighter paths toward food sources5. This increased motility is crucial for their formation, growth, and survival.

Active particles are synthetic analogs of microorganisms by mimicking some of their dissipative behaviors6–9. Particularly, self-phoretic active particles create local gradients of energy along their surfaces, driving asymmetric fluid flows at the single-particle level10–15. Their mechanism of motion is inherent in their design, as they asymmetrically draw and dispel energy, generating local gradients of force that propel them16–20. Active particles hold enormous promise for applications such as drug delivery21–24, biosensing25,26, and environmental remediation27,28. Despite their usefulness, most active particle systems developed to date are geometrically rigid, unable to change their shape in response to external stimuli due to difficulties in fabricating and controlling shape-morphing soft materials at the microscale29. We note one intriguing exception involving clusters of swelling and non-swelling microspheres that undergo electrokinetic propulsion16. Recently, stimuli-responsive polymers that change properties in response to environmental conditions have led to applications as smart materials30–34, including biomedical devices32,35–39, flexible electronics40–43, sensors44–46, and actuators47–50.

In this article, we present a class of shape-morphing active particles fabricated by layering a rigid polymer with a thermoresponsive hydrogel. Particles were fabricated by sequential spin-coating of the two materials followed by planar lithography, enabling fabrication of surface- and shape-anisotropic particles composed of polymers with different material properties. As a model system, we created bilayer particles that can morph from quasi-rectangular prisms to crescent- or donut-like shapes due to significant swelling of the hydrogel layer. We used AC electric fields to drive particle motion and studied how changes in temperature led to variations in effective polarizability, shape anisotropy, and resultant trajectories51–53. When an AC field is applied, the particles align with a preferred orientation to minimize energy between the induced polarization of the particles and the external field54,55. Distinguished from most reported active particles with a single, fixed powering layer—such as a metal patch of a Janus particle—we demonstrate that the powering surface can be dynamically switched by modulating the effective polarizability of the swelling layer, while the rigid layer remains unchanged.

By combining an asymmetric particle shape with controlled tuning of the effective polarizability between the two layers, we encode linear to nonlinear trajectories of the particle by modulating the temperature. At low Reynolds numbers, reciprocal shape changes alone cannot generate net motion due to time-reversal symmetry, as described by Purcell’s scallop theorem19,56,57. In our system, the applied AC electric field breaks this symmetry by inducing microscale fluid flows around the particles that are temperature-dependent. This coupling enables controlled propulsion and steering, distinguishing our approach from passive shape-morphing systems. We discuss several non-trivial findings relevant to configuring temperature-controlled propulsion in AC electric fields. While we propose that induced-charge electrophoresis (ICEP)58–60 plays a dominant role, other factors, such as concentration polarization electrophoresis (CPEP)61 and temperature-dependent ionic conductivity of the hydrogel layer62 may also contribute. While hydrogel-based shape-morphing has been extensively explored using diverse stimuli such as temperature, pH, light, and electric fields, these systems typically exploit static or reversible shape changes without propulsion. Unlike previous studies, including two recent works63,64, our system couples reversible hydrogel bending with electric field-driven active propulsion. This integration enables not only shape transformation but also tunable, directionally controlled motion and trajectory switching—advancing shape-morphing materials into the domain of microscale active matter. Our findings provide a fundamental basis to program the electrokinetic motions of shape-morphing particles for manipulating and steering at the single-particle level.

Results
Fabrication and propulsion of shape-morphing particles
A landmark example of particle propulsion in an AC electric field is linearly propelling metallodielectric Janus particles65. Fabrication of such particles typically involves formulating spherical particles through chemical methods, assembling them on a substrate, and vacuum deposition of metal patches25,55,66,67. In contrast, the methods used to create the non-spherical shape-morphing particles described in this study involve fewer steps. Bilayer particles were fabricated by spin-coating a glassy polymer, poly(p-methyl styrene-co-acrylamido-benzophenone (PpMS-BP), and a thermoresponsive polymer, poly(N,N-diethylacrylamide-acrylic acid-acrylamidobenzophenone-2-(methacryloyloxy)ethylrhodamine B) copolymer (PDEAM-BP), onto a substrate, followed by patterning via maskless lithography68. PpMS-BP, being a rigid, non-swelling polymer, fixes the dimension of one side of the bilayer system, while PDEAM-BP, a polymer with a volume phase transition temperature (VPTT), exhibits a temperature-dependent swelling response (Fig. 1) that drives particle curvature. The swelling behavior of PDEAM-BP is governed by hydrophobic interactions and water solvation. At temperatures above its VPTT, hydrophobic interactions between polymer chains dominate, causing the polymer chains to aggregate and water to be expelled, resulting in deswelling of the hydrogel. Conversely, below this temperature, the polymer chains interact favorably with water molecules, allowing the hydrogel to swell. This is because hydrated polymer chains form stable structures through hydrogen bonding and solvation with water molecules69. The VPTT can be modulated by copolymerization with hydrophilic or hydrophobic moieties within the polymer network70. Though PDEAM-BP contains both hydrophilic acrylic acid and hydrophobic BP at equal molar content (7 mol.%), the VPTT of PDEAM-BP is significantly shifted downward to approximately 22 °C, which is about 10 °C lower than that of neat PDEAM-BP. This pronounced lowering of the VPTT suggests that the hydrophobic effect of the BP moiety outweighs the hydrophilic contribution of acrylic acid. The BP moiety—with its two phenyl rings and bulky structure—likely promotes hydrophobic interactions, thereby driving the VPTT to a lower temperature. Though the VPTT behavior is not strongly influenced by crosslink density, the swelling ratio of PDEAM-BP does depend on the crosslink density, which can be controlled by the UV exposure dose. In this study, the PDEAM-BP exhibited a swelling ratio of ~1.8 (Supplementary Fig. 1).

To fabricate bilayer particles, we first spin-coated an aqueous solution of poly(vinyl alcohol) (PVA) as a water-soluble sacrificial layer on a substrate. Then, a solution of PpMS-BP in toluene was spin-coated to form a rigid layer (100 nm thickness). Subsequently, a solution of PDEAM-BP in 1-propanol was spin-coated to form the swelling layer (2 μm thickness). The bilayer film was photopatterned to create an array of 40 μm × 4 μm rectangles using maskless lithography (Fig. 1B). After unexposed regions were removed with a developing solution, the rectangular-shaped bilayer particles remained on the sacrificial layer of PVA (Supplementary Fig. 2).

The bilayer particles were dispersed in water, and their shape-morphing behaviors were monitored using a fluorescent microscope equipped with a temperature-controlled stage. Upon their release from the substrate and dispersion in water at 35 °C, the particles retained the shape of a quasi-rectangular prism. Decreasing the solution temperature to 15 °C, the particles morphed to a crescent shape. Further decreasing the temperature below 5 °C, the particles attained a donut-like shape where the two ends of each particle met. Bending curvature of each particle was calculated from the diameter of a circle drawn over the particle (d-1) to analytically express changes in shape (Fig. 1C). Upon decreasing the temperature below the VPTT of PDEAM-BP, the PDEAM-BP layer absorbs water and swells, while the PpMS-BP layer remains volumetrically stable, creating a mismatch in expansion. This swelling-induced strain progressively drives the shape transformation of the bilayer particles. Initially, the rod-shaped particles bend into a crescent shape due to differential swelling across two layers. As swelling continues and strain increases, the particles undergo further transformation, ultimately bending to a donut shape (Fig. 1D, Supplementary Movie 1). To evaluate fatigue resistance and interfacial stability of the bilayer microparticles, we subjected them to 10 repeated cycles of heating (35 °C) and cooling (5 °C). The shape morphing remained fully reversible with no observable loss in curvature. Additionally, no delamination or separation between the PDEAM-BP hydrogel and the PpMS layer was observed, confirming robust adhesion and mechanical integrity under cyclic thermal loads (Supplementary Fig. 3).

After the particles were released from the substrate, they were dispersed in water to form an aqueous suspension. This solution was introduced into the gap of a coplanar electrode setup (Fig. 2A). Upon applying an AC electric field, the preferred orientations of the particles and their active motions were observed and recorded. The observed propulsion occurs in the low-frequency regime (~300 Hz), consistent with reports of electrokinetically driven motion of asymmetric particles in similar conditions58. While the precise mechanism remains under investigation, our coplanar electrode geometry reduces the likelihood of significant electrohydrodynamic flows, which typically arise from steep field gradients near electrodes71–75. We observed that bilayer particles assumed different preferred orientations with respect to the electric field direction according to their shape (Fig. 2B) and exhibited unique active motions when subjected to AC electric fields (Fig. 2C–G, Supplementary Movie 2). At 35 °C, particles assumed a rod-shape, where their major axis aligned with the electric field, and the particles displayed no further motion. The lack of motion is believed to arise from the orientation of the particles, where the PpMS layer positions against the substrate. Upon cooling the solution to 25 °C, the PDEAM-BP layer swelled, inducing a bend in the particles to a boomerang-shape. Under an AC electric field, the particles are propelled longitudinally (i.e., perpendicular to the applied field direction). The PDEAM-BP layer was oriented toward the back, suggesting that this layer is more polarizable than PpMS-BP at higher temperatures, driving propulsion through ICEP, which we investigate in more detail as follows. At 17 °C, further swelling transformed the particles into a crescent-shape. In this conformation, particles exhibited 3D helical motions with their major axis aligned with the AC electric field, as the PDEAM-BP layer continued to push the particles forward.

Cooling the solution further to 10 °C resulted in a more pronounced bend, creating a horseshoe-shape. Interestingly, the particles reoriented such that their minor axis aligned with the applied field, and the direction of propulsion changed laterally (i.e., parallel to the applied field direction), whereby the PpMS-BP layer was oriented toward the back, suggesting that this layer is more effectively polarizable than the PDEAM-BP at lower temperatures. Below 5 °C, the particles fully bent into a donut-shape, achieving a near-isotropic geometry. In this conformation, particles no longer exhibited preferential alignment or active motion in the electric field. For boomerang-shaped particles, the speed of the particles in the y-direction remained near zero, while the position of the particles in the x-direction increased linearly at a constant rate. For crescent-shaped particles, the position of the particles oscillated along the x-direction while steadily increasing in the y-direction, consistent with the characteristics of helical motions. This helical trajectory is particularly interesting, as it suggests that the motion arises from asymmetric fluid flow in both the x- and y-directions near the particle, indicating complex fluid interactions influenced by both the PDEAM-BP and PpMS-BP layers (discussed later)55. For horseshoe-shaped particles, the speed of particles in the x-direction was approximately zero, while the position of the particles in the y-direction increased linearly at a constant rate (Fig. 2H, I). For boomerang-shaped particles, we measured the speeds of the particles at different field strengths. The relationship between speed and the square of the applied field strength was fit well by a linear model (R2 = 0.992) (Fig. 2J).

To more closely investigate the mechanism of propulsion of the shape-morphing bilayer particles in AC electric fields, we fabricated non-shape-morphing block-shaped particles by spin-coating uniformly thick layers of PpMS-BP and PDEAM-BP, each approximately 0.5 μm thick (Supplementary Fig. 4). These particles retained a fixed geometry under varying temperature conditions, allowing us to isolate the effects of induced polarizability via the applied electric field from particle shape on propulsion dynamics. When subjected to an AC electric field (340 V cm−1 at 300 Hz) at 22 °C, the block-shaped particles exhibited 3D helical motions (Supplementary Fig. 5, Supplementary Movie 3), which we attribute to multiaxial asymmetric fluid flows around the particles (Fig. 3A). Tangential speeds of the block-shaped particles were measured as a function of the square of the applied electric field strength. The observed linear relationship between particle speed and the square of the field strength confirmed that ICEP is the primary mechanism driving particle locomotion (Fig. 3B)65. This characteristic dependence reflects the ability of the particles to polarize in an AC electric field, creating asymmetric ion flows that propel it through the surrounding fluid. To assess temperature-dependent behaviors, we measured propulsion velocities of both shape-morphing and non-shape-morphing bilayer particles at 10 °C and 25 °C. Shape-morphing particles exhibited a significantly higher propulsion speed at 25 °C, with an average increase of ~4 μm s−1 compared to their speed at 10 °C. In contrast, non-shape-morphing particles showed no significant change in propulsion velocity across the same temperature range (Supplementary Fig. 6). We note that PDEAM-BP was found to orient toward the rear at 25 °C and toward the front at 10 °C; however, for the non-shape-morphing bilayer particles, we could not visualize orientation during propulsion. This result may be attributed to the asymmetric bilayer structure, particularly the thicker PDEAM-BP hydrogel layer in the shape-morphing particles, which is expected to play a key role in enhancing propulsion through temperature-induced changes in dielectric properties and polarizability mismatch.

Invertible effective polarizability of PDEAM-BP
The observations above suggest that upon cooling of the sample from 35 °C to 5 °C, the bilayer shows ‘invertible polarizability’, a term we use to describe the temperature-dependent reversal in the relative effective polarizability between the two layers of the bilayer particle. This difference in effective polarizability—quantified by the real part of the Clausius–Mossotti (CM) factor—can change sign as the PDEAM-BP hydrogel layer swells or deswells. This results in an inversion in the direction of the induced dipole moment across the particle, relative to the non-responsive PpMS-BP layer. The effective polarizability of a polymer particle is typically governed not only by dielectric permittivity but also by transport of ions associated with the electrical double layer (EDL) at the particle surface76,77. At low frequencies, ion transport is sufficiently rapid to enable polarization across the particle dimensions, leading to a much larger dipole moment than due to dielectric polarization, and therefore an effective polarizability higher than that of the surrounding aqueous medium. At sufficiently high frequencies, ions do not have time to redistribute, and the response of the particle is dominated by dielectric polarization, leading to an effective polarizability lower than water.

To shed further light on these effects, we characterized the polarizability differences between the two materials at different temperatures. First, we fabricated cubic particles (2 μm × 2 μm × 2 μm) composed entirely of PDEAM-BP or PpMS-BP. Following the methods outlined78, we experimentally measured the real component of the CM factor, Re[CM] of each material, by analyzing the interactions between the induced dipole formed in the particle and the applied electric field. This was achieved by tracking the dielectrophoretic (DEP) response of each type of particle in a non-uniform electric field. DEP experiments were conducted using a trapping chamber mounted on a coplanar interdigitated electrode device (Fig. 3C, Supplementary Fig. 7). For experiments involving PDEAM-BP above and below its swelling-dependent critical polarizability solution temperature (CPST), revealing temperature-dependent changes in both the swelling ratio and polarizability of PDEAM-BP, the device was mounted on a temperature-controlled stage with continuous temperature monitoring. Samples were allowed to equilibrate for at least 5 min to enable particles to reach their approximate mechanical equilibrium height in the solution. After this settling period, a sinusoidal AC signal oscillating at 200 kHz was applied to the electrodes, generating an electric field strength of E0 = 200–500 V cm−1. While we note that such DEP measurements cannot be reliably performed at the lower frequencies relevant to propulsion due to background fluid flows, they provide meaningful insight into the temperature-dependent polarizability transitions. At a given frequency, particles more polarizable than the suspending medium migrated to regions of the highest electric field strength at the electrode edges, a phenomenon known as positive dielectrophoresis (pDEP)78. Conversely, particles less polarizable than the medium migrated to regions of the lowest electric field strength in the center of the electrode gap, known as negative dielectrophoresis (nDEP)78. These DEP responses, dictated by the Re[CM], depend on the frequency and the relative polarizabilities of the particle and suspending medium (Fig. 3D). A frequency of 200 kHz was chosen for these experiments because DEP dominates transport mechanisms at this frequency, whereas electro-osmosis interferes at lower frequencies (100 Hz < ω < 100 kHz)78,79. By observing the DEP behavior, we quantified the polarizability differences between the bilayer components, providing critical insights into the role of material properties in particle motion.

Particle trajectories were extracted from 10 min video recordings through image analysis. The positions of individual particles were normalized relative to their neighboring electrode positions, resulting in normalized trajectories within the electrode gap (Fig. 3E, Supplementary Fig. 8 and Supplementary Movie 4). The normalized trajectories were then used to generate histograms of particle positions, capturing the probabilities of sampling within the electrode gap for each type of particle (Fig. 3F, Supplementary Fig. 8). The Boltzmann equation was applied to relate the sampling probabilities to the potential energy at each position relative to a chosen reference point (Supplementary Text). The reference point was selected based on the theoretical position where potential energy is minimized (i.e., the center of the electrode gap for nDEP and the edge of the electrodes for pDEP)78. Transforming the sampling probabilities using the Boltzmann equation produced the relative potential energy landscape within the electrode gap (Fig. 3G).

The theoretical net potential energy of a single particle in a high-frequency, non-uniform electric field simplifies to the interaction between the induced dipole and the electric field78. To determine the Re[CM] for PpMS-BP at 22 °C, a PDEAM-BP at 5 °C, and a PDEAM-BP at 30 °C, a nonlinear regression function was employed. The theoretical potential energy expression was fitted to the transformed experimental data, with Re[CM] as the sole adjustable parameter. The individually fitted Re[CM] values were averaged for each type of particle and subsequently used to calculate the real component of the effective polarizability77,80.

Our findings show that the thermoresponsive behavior of PDEAM-BP is associated with significant changes in the effective polarizability of the bilayer particles across its swelling-dependent CPST (Fig. 3H). Above the CPST, PDEAM-BP shows higher effective polarizability than the surrounding medium, exhibiting positive dielectrophoresis (pDEP) with Re[CM] = 0.36 (Fig. 3I). In contrast, below the CPST, PDEAM-BP becomes less polarizable than the medium, transitioning to negative DEP (nDEP) with Re[CM] = –0.15. The PpMS-BP layer maintains consistent behavior across temperatures, showing pDEP with Re[CM] = 0.03. While the underlying polarizability change is likely continuous due to gradual water uptake or expulsion from the thermoresponsive hydrogel (PDEAM-BP), our measurements focused on two discrete temperatures (5 °C and 30 °C) due to the DEP trapping time required per condition to collect sufficient data for analysis (Supplementary Text). Future studies may explore this continuum more thoroughly at different temperature conditions.

Although the effective polarizabilities measured at 200 kHz cannot be directly mapped onto the swimming behaviors measured at 300 Hz, the temperature-dependent changes in the effective polarizability of PDEAM-BP correlate well with the observed differences in propulsion direction and mode. At 25 °C, just above the CPST, the bilayer particles adopt a boomerang shape due to reduced swelling. In this state, PDEAM-BP maintains a higher polarizability than both the medium and the PpMS-BP layer, likely generating asymmetric flows that drive linear motion perpendicular to the applied AC electric field and away from the PDEAM-BP side. Below 12 °C, just below the CPST, the PDEAM-BP becomes less effectively polarizable, and the particle bends further into a horseshoe shape, whereby propulsion switches direction, now occurring parallel to the field and away from the more polarizable PpMS-BP layer.

To better understand the temperature-induced inversion in polarizability, we next consider changes in the electric double layer (EDL) by fabricating cubic particles (2 μm × 2 μm × 2 μm) composed entirely of PDEAM-BP or PpMS-BP and measuring their zeta potentials across a temperature range encompassing the CPST of PDEAM-BP (Fig. 4A), as spherical particles are not readily achievable using 2D lithography. While geometric anisotropy can influence zeta potential at smaller length scales or with highly elongated shapes, the electrokinetic behavior of micron-sized cubes is expected to be comparable to that of spheres. Thus, we interpret zeta potential values from these cubes as representative of particles with similar surface chemistry and dimensions81. The zeta potential of PpMS-BP particles remained constant at ≈ − 25 mV across the tested temperature range studied (0–35 °C), indicating no significant changes occurred in their EDL. Conversely, the PDEAM-BP particles showed a temperature-dependent zeta potential, changing from −16 mV at 1 °C to −44 mV at 25 °C, after which it plateaued at higher temperatures. We observe that the magnitude of the zeta potential of PDEAM-BP increases at elevated temperatures with an enhanced surface charge for thermoresponsive polymers in their collapsed state, which agrees with prior reports82,83. While deswelling is typically expected to lower the local dielectric constant, the concurrent increase in the density of ionizable groups near the surface appears to dominate, leading to the observed enhancement in the magnitude of the zeta potential. We propose that the temperature-dependent zeta potential of PDEAM is responsible for the invertible effective polarizability seen via both low-frequency (300 Hz) measurements of particle swimming and high-frequency (200 kHz) dielectrophoretic trapping behavior. At high temperatures (above the CPST), the larger magnitude of zeta potential for PDEAM could suggest that a locally higher concentration of ions near the surface contributes to a higher polarization than PpMS, while the opposite would be true at low temperatures. The crossover in zeta potential (at ≈ 15 °C) is close to the range of CPST (15–20 °C) and in good agreement with the crossover in DEP behavior between 5–30 °C. Notably, the specific crossover temperature may depend on the frequency of the applied field, so the values do not necessarily align perfectly.

An additional effect that may contribute, given the use of a PDEAM hydrogel layer, is ion transport through the “bulk” of the particle, rather than simply motion of charges associated with the EDL. We sought to gain additional insight into this aspect by performing electrochemical impedance spectroscopy (EIS) on UV-crosslinked PDEAM films swollen to equilibrium on interdigitated electrodes (Supplementary Fig. 9). Fitting the impedance spectra to an equivalent circuit model yielded an ionic conductivity (presumably primarily of protons associated with acrylic acid groups in the PDEAM-BP copolymers) of (7.19 ± 0.03) × 10−8 S m−1 and a specific capacitance of (1.14 ± 0.002) × 10-4 F m−2. Together with a characteristic PDEAM layer thickness of ≈ 2 μm in the particles, these values correspond to an RC time constant of ~3 ms associated with field-driven migration of ions across the thickness of the PDEAM-BP film (Supplementary Table 1). The inverse of this time constant reveals a characteristic frequency of ~300 Hz. The proximity of this characteristic frequency and the frequency used in swimming experiments suggests that bulk ion transport may also play a role in the reported electrokinetic phenomena, although we expect these effects to be negligible at 200 kHz where DEP measurements were performed.

To isolate the role of particle shape, we examined the propulsion of: (1) shape-morphing bilayer particles, (2) non-shape-morphing crescent-shaped particles made from PDEAM-BP only, and (3) non-shape-morphing crescent-shaped particles made from PpMS-BP only (Fig. 4B). Under an AC electric field, bilayer particles exhibited active self-propulsion with an average linear speed of 12 μm s−1, moving perpendicular to the applied field as before (Fig. 1E). As discussed above, this motion was driven by asymmetric fluid flows generated between the PDEAM-BP and PpMS-BP layers (Fig. 3B). For the non-shape-morphing PDEAM-BP particles, the speed of propulsion was significantly slower and was primarily driven by the geometric and material asymmetry that likely promotes uneven surface polarization, driving weak but directional propulsion. In contrast, non-shape-morphing PpMS-BP particles exhibited nil motion, potentially due to the lack of polarization of their EDL. At 25 °C, the PDEAM-BP layer in bilayer particles is highly negatively charged, leading to a strongly polarized EDL compared to the PpMS-BP layer, whose zeta potential is smaller in magnitude. In the case of bilayer particles, the mismatch in polarization between the two materials generates ICEP flows that drive propulsion. By contrast, particles with asymmetric shape but homogeneous material properties (e.g., single-component PDEAM-BP) show little translation, with any residual motion likely arising from the relatively higher zeta potential of PDEAM and/or contributions from bulk ion transport. This comparison highlights that while geometric asymmetry alone is sufficient to generate propulsion, the material contrast is essential for the high swimming speeds observed. We note that rod-shaped bilayer particles did not exhibit measurable propulsion, which may be due to differences in orientation (e.g., PDEAM layer facing upward relative to the electrodes) that drive the symmetric fluid flows near the particle.

The temperature-dependent morphological transitions and associated changes in active motion of shape-morphing bilayer particles highlight the interplay between particle geometry, induced polarization, and field-driven dynamics. The application of an AC electric field to a polarizable surface suspended in fluid induces charge separation and generates localized fluid flows, a phenomenon known as induced-charge electro-osmosis (ICEO)53. For shape-morphing bilayer particles, the asymmetric ICEO flows around their surfaces provide the driving force for propulsion in AC electric fields. To visualize these flows, particles were immobilized at various temperatures within an experimental cell, and the induced fluid flows were visualized using isotropic polystyrene (PS) tracer spheres (diameter = 0.75 μm). Videos of tracer motion were recorded and analyzed to generate flow patterns by superimposing video frames. Additionally, particle image velocimetry (PIV) was applied to quantify the speeds of the tracer particles and study the flow dynamics across each layer of the immobilized particles.

The ICEO flow patterns at different temperatures and particle shapes are shown in Fig. 4C–E, Supplementary Movie 5. At 25 °C, corresponding to the boomerang-shaped particle, the fluid flow exhibited higher velocities near the PDEAM-BP layer compared to the PpMS-BP layer (Fig. 4C). This disparity arises from the higher polarizability of PDEAM-BP relative to PpMS-BP above the CPST, resulting in asymmetric flows primarily generated from the PDEAM-BP side. These flows drive linear propulsion of the particles perpendicular to the applied AC electric field. At 12 °C, the horseshoe-shaped particle exhibited more complex asymmetric flow patterns due to its reduced symmetry. Below the CPST of PDEAM-BP, the Re[CM] for PDEAM-BP falls below PpMs (Fig. 3H), indicating that the rigid layer is more polarizable than the swelling layer. This is consistent with the observation that tracer speeds are faster on the PpMS-BP layer (i.e., the left region of the immobilized particle) (Fig. 4D) as well as the oblique flows across both layers, resulting in nonlinear propulsion in solution. In contrast, at temperatures below 5 °C, the particles assumed a donut shape with an isotropic geometry (Fig. 4E). The ICEO flows around the particle were quasi-symmetric, lacking the necessary asymmetry to sustain directional motion. Consequently, no net propulsion was observed for the donut-shaped particles.

Temperature-modulated steering of shape-morphing particles
To show that particle trajectories can be encoded into the bilayers through programmed changes in shape and polarizability, we performed an experiment to illustrate in situ steering of particles at the single-particle level. Specifically, we wrote a program to drive a particle left or right, invert the direction of motion (i.e., right or left) by flipping84–86, and induce a final change to drive the particle up or down (Fig. 5A). We first observed a boomerang-shaped particle at 25 °C moving linearly in the positive longitudinal (x-direction) under the applied field. The field was then turned off, and the temperature was reduced to 17 °C to induce a shape change to a crescent. Utilizing the 3D helical motion of the particles at this temperature, we flipped the orientation of the particle using a 12 s exposure to the field. Subsequently, the temperature was increased back to 25 °C, causing the particle to resume linear motion in the negative longitudinal (x-direction). Last, the field was turned off again, and the temperature was reduced to 12 °C, resulting in a shape transition to a horseshoe configuration. At this temperature, the particle reoriented itself upon reapplying the field, and it moved linearly in the positive lateral (y-direction) back to its original position (Fig. 5B, C).

By controlling the orientation of individual particles during cluster formation, it may be possible to design and manipulate the trajectories of these assemblies with a higher degree of precision. Our findings demonstrate how temperature-dependent changes in particle shape and polarizability can be exploited to alter particle motion dynamically. Such control over particle trajectories holds promise for applications in soft robotics, targeted cargo delivery, and programmable active matter.

Methods
Materials
N,N-diethylacrylamide (DEAM) and acrylic acid (AAc) were purchased from TCI America, and inhibitors were removed using basic alumina. 1,4-dioxane was purchased from Thermo Fisher Scientific. p-methylstyrene (pMS), azobisisobutyronitrile (AIBN), and poly(vinyl alcohol) (PVA, 13-23 kg mol-1, 87-89% hydrolyzed) were purchased from Sigma-Aldrich. Acrylamidobenzophenone (AAmBP) and 2-(methacryloyloxy)ethylrhodamine B (RhBMA) were prepared according to a previously reported literature procedure87,88.

Synthesis of polymers
Polymers were synthesized by free-radical polymerization. Amounts of 2.80 g of DEAM, 450 mg of AAmBP, 130 mg of AAc, 17 mg of RhBMA, and 35 mg of AIBN were polymerized in 30 mL of 1,4-dioxane at 80 °C for 18 h under nitrogen, resulting in poly(DEAM-co-AAc-co-AAmBP-co-RhBMA) copolymer, shortly PDEAM-BP, containing 7 mol.% AAc, 7 mol.% AAmBP, and 0.1 mol.% RhBMA. The copolymer was purified by precipitation in hexane and dried overnight in a vacuum oven. Amounts of 2.65 g of pMS, 295 mg of AAmBP, and 15 mg of AIBN were polymerized in 30 mL of 1,4-dioxane at 80 °C for 15 h under nitrogen, yielding poly(pMS-co-AAmBP), shortly PpMS-BP, with 5 mol.% BP. The copolymer was purified by precipitation into stirring methanol and dried overnight in a vacuum oven.

Fabrication of shape-morphing bilayer particles
Bilayer structures of PpMS-BP and PDEAM-BP were prepared by sequential spin-coating, crosslinking, and developing the copolymer films. Prior to trilayer fabrication, a PVA film of 100 nm was coated on a silicon substrate as a sacrificial layer. The first layer was formed by spin-coating PDEAM-BP (120 mg mL−1 in 1-propanol) at 500 rpm for 50 s and dried on a 105 °C hotplate for 8 min. The second layer was formed by spin-coating PpMS-BP (15 mg mL−1 in toluene) at 2000 rpm for 50 s and dried on a 105 °C hotplate for 5 min. The thickness measured using a profilometer (XT Stylus Profilometer, Daktak) was 2 μm and 100 nm for the first and second layers, respectively. The film was patterned to 40 μm × 4 μm rectangles by maskless lithography (μMLA, Hidelberg Instruments) with a UV dose of 4000 mJ cm−2. Unexposed area was developed in 3:1 mixture of toluene and hexane, and the developer solution was mildly removed using an air gun. Finally, the substrate was immersed into MilIi-Q water to release the rectangular particles.

Electric field-driven propulsion setup
Coplanar propulsion chambers were prepared according to a previously reported literature procedure25,86 and thoroughly cleaned with acetone prior to each experiment. A hydrophobic barrier pen was used to draw a square outline, approximately 1.25 cm × 1.25 cm, overlapping both electrodes. Two strips of PTFE tape were positioned flat and perpendicular to the electrodes at the edges of the hydrophobic boundary. A small volume (10–20 μL) of particle solution was then pipetted into the space between the PTFE tape spacers and the gold electrodes, followed by placing a coverslip over the spacers. The particles were allowed to settle for at least 5 min to reach an approximate mechanical equilibrium height in the chamber, after which the AC electric field was applied. Each time the temperature was changed, particles equilibrated for at least 5 min before the AC electric field turned on again. The entire setup was mounted on a Zeiss AxioVert A1 TL/RL inverted fluorescence microscope, equipped with a Zeiss Axiocam 305 mono camera and a Linkam PE100 temperature-controlled stage. For electrical input, a 33210A waveform generator (Agilent) was connected to a 2340 high-voltage amplifier (Tegam), with the leads of the amplifier attached to Cu tape on the propulsion chamber. A 300 Hz square wave signal was applied to the propulsion chamber, forming an electric field with magnitudes of 340–540 V cm−1. Particle tracking was performed at the center of the electrode gap (2 mm wide region of interest), where the AC electric field is spatially uniform, to minimize field gradients and avoid dielectrophoretic contributions to propulsion84.

Microscopy and particle tracking
To characterize the lithographic patterns of block-shaped particles, the substrate was sputter-coated with a 20 nm layer of Pt and imaged using a SEM in secondary electron imaging mode at an accelerating voltage of 10 kV (Hitachi SU3500). The active motion of shape-morphing bilayer particles was captured using the built-in camera on the microscope. Particle coordinates during propulsion were extracted from the experimental videos using ImageJ software. Each video frame was processed into a binary image, and the particle’s position was determined using the “Centroid” function within the “Analyze Particles” tool21.

DEP experimental setup
DEP experiments were conducted in a trapping chamber fixed on a coplanar interdigitated electrode device (Supplementary Fig. 7) connected to a function generator by electrode leads. The chamber comprised of a thin silicone rubber spacer (USA Sealing) with a hole in the center created by a biopsy punch to confine the liquid particle sample over the electrodes. The electrode device was washed thoroughly with acetone, and the spacer was washed with DI water prior to each experiment. The spacer was then placed firmly on the electrode device, and 10 μL of a particle sample was pipetted into the center and covered by a coverslip. The electrode device was then situated on the stage of a fluorescent microscope equipped with a camera. For experiments investigating PDEAM-BP above and below the VPTT, the device was first situated on a temperature-controlled stage, and the temperature was monitored throughout the experiments. Particles were left to settle for at least 5 min to allow them to reach an approximate mechanical equilibrium height in the chamber prior to application of the AC electric field.

EIS experimental setup
To characterize the dielectric and conductive properties of PDEAM-BP, a thin film of PDEAM-BP was deposited onto an interdigitated electrode (IDE) substrate via drop-casting. Droplets of PDEAM dissolved in toluene (120 mg mL−1) were deposited and dried four consecutive times on an IDE, yielding an uneven film of ≈ 25 μm thickness. The film was exposed to UV light to initiate crosslinking. To simulate the hydrated state of PDEAM bilayers during electromotive propulsion, the substrate was placed in a sealed plastic chamber containing a water reservoir and allowed to equilibrate for 48 h at 22 °C. EIS measurements were performed in a two-electrode configuration on an interdigitated electrode (Micrux ED-IDRA-Au chip) using a potentiostatic frequency sweep (frequency range: 1 Hz to 1 MHz) with an AC amplitude of 40 mV on a Gamry Potensiostat (Reference 600+). The resulting Nyquist plot was fit to a standard equivalent circuit model composed of a capacitor, representing the high-frequency dielectric polarization, in parallel with a resistor and constant phase element, representing the ion conductivity and interfacial polarization, respectively (Supplementary Fig. 9A).

Flow pattern near bilayer particles
The fluid flow field surrounding an immobilized bilayer shape-morphing particle was analyzed using polystyrene spheres as tracer particles. These isotropic tracers (diameter = 0.75 μm, Magsphere Inc.) did exhibit self-propulsion but instead moved with the surrounding fluid. The tracer particles were functionalized with negatively charged carboxylate groups (zeta potential = −38 mV at pH 7), washed three times with deionized water, and dispersed in water at a final concentration of 0.005 wt.%. For a typical experiment, the bilayer particles were immobilized in the experimental cell between the electrodes by drying the particle suspension. Following immobilization, 10 μL of the tracer dispersion was added to the cell. Passive motion of the tracer particles around the immobilized particles under an applied AC electric field was recorded using a bright field microscope.

Particle image velocity (PIV) analysis
PIV analysis was conducted using PIVlab version 3.0289. Briefly, the videos taken for the tracer particle experiments were imported into the PIVlab graphical user interface in MATLAB. Firstly, videos were cropped in a 300 × 300 pixel region around the particle of interest. A mask was then manually drawn around the particles to exclude them from analysis. After, the videos were pre-processed in PIVlab using a contrast-limited adaptive histogram equalization (CLAHE) with a window size of 64 pixels, intensity capping, Wiener 2 denoising with a window size of 3 pixels, and were auto-contrasted. The videos were then analyzed with the FFT window deformation correlation algorithm, with a first pass interrogation area of 64 pixels with a step size of 32 pixels and a second pass interrogation area of 32 pixels with a step size of 16 pixels. The velocity data was averaged over all frames of the video, undefined values were interpolated with a bilinear interpolation scheme, and data was smoothed at full strength prior to visualization. Velocity data was converted from pixel frame−1 units to m s−1 using a frame rate of 1.35 frames−1 and a distance calibration of 3.06 pixel μm−1. The x- and y-components of velocity are represented via a quiver plot, and the velocity magnitude is represented via a contour plot.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript introduces shape-morphing active particles that can invert their polarizability, enabling reconfigurable motion in AC electric fields. The authors combine direct observation of temperature-induced shape changes with particle-tracking of locomotion. The manuscript is well-written and presents intriguing behavior, but the interpretation relies primarily on qualitative observations. Below are comments intended to help clarify the mechanisms and improve the scientific rigor of the study:\n1) The authors attribute the observed motion reversal and steering to shape-induced inversion of electrical polarizability. While plausible, shape-morphing particles could, in principle, produce motion through other mechanisms—such as fluid flows generated by deformation or mechanical kicks associated with non-reciprocal shape changes. A brief discussion or reference to the scallop theorem would strengthen the interpretation, emphasizing that reciprocal shape changes alone cannot produce net locomotion at low Reynolds number, and that the applied field is essential to break time-reversal symmetry and achieve propulsion.\n2) A crucial control would be to test particle behavior in a spatially uniform AC electric field, where dielectrophoretic forces should vanish, and only torques (if any) would act on polarizable particles. If the shape-morphing particles still exhibit net propulsion in a uniform field, alternative mechanisms such as self-electrophoresis or Marangoni-type flows may be responsible. Likewise, repeating experiments in media of different permittivity or conductivity—thus shifting the crossover frequency—could provide further support for the proposed invertible polarizability mechanism.\n3) The manuscript introduces the term invertible polarizability to describe a switch in the sign of the particle’s induced dipole response. As this is not standard terminology, a clearer explanation of what is being inverted—e.g., the sign of the Clausius–Mossotti factor or the direction of induced dipole alignment—would be beneficial. It would also be helpful to address why the system exhibits a discrete two-state switching behavior (e.g., attracted vs. repelled), rather than a continuous field- or frequency-dependent transition.\n4) While the correlation between shape and motion direction is compelling, the claims would be significantly strengthened by direct measurements of particle polarizability. Quantitative estimates of dielectrophoretic forces, alignment torques, or induced dipole moments (e.g., via orientation in the field, light scattering, or theoretical modeling) would provide stronger evidence for the proposed mechanism. As it stands, the interpretation relies on qualitative proxies (e.g., motion direction) rather than direct physical measurements.\n5) The manuscript would benefit from the inclusion of statistical data—such as particle-to-particle variability, error bars on speed or trajectory measurements, and consistency over multiple trials—to support the generality and robustness of the reported behavior.", "reviewer_2": "The manuscript from Shields and coworkers reports on fabrication and shape-morphing of the microparticles through layering a thermoresponsive hydrogel on a glassy polymer, and investigation of their motions responded to environmental temperature and AC electric field. The most interesting findings include that these microparticles conducted different shape-morphing according to their shape anisotropy, leading to different induced polarization and trajectory under the AC electric field. Overall, this work was nicely done, and the conclusions are well-supported by the results. Therefore, I recommend accepting it for publication in NC. However, prior to its publication, authors should pay attention to the following points for revision:\n(1) Shape morphing of hydrogels have been reported in a number of papers through various stimuli in different fashions (such as a new paper from Zhao and coworkers appeared on NC 2025, 16, 2830). It’s necessary to make more clear statement on the novelty contribution of the present work to this area.\n(2) Transition of hydrogels from collapsed to swollen state is driven by balance between the hydrophobic interactions among the polymer chains and the solvation (hydration) of the polymer chains with water. This should be clearly stated on page 3 to avoid misleading. In addition, although crosslink density of the hydrogels can show some influence on their swelling ratios due to the flexibility of the network, it doesn’t often exhibit influence on phase transitions of the hydrogels.\n(3) Solid content plays a pivotal role in swelling and deswelling of the hydrogels according to temperature around their VPTT. In present work, shape-morphing of the double-layered microparticles is also dependent on the swelling of the hydrogel portion, and therefore, all data reported here should be correlated to the solid contents of the hydrogel part. Swelling ratios of the hydrogel part should also been included in the figures. These can be done to add another axis such as in Figure 1C.\n(4) Since heating or cooling rate have been found to exhibit influences greatly on selling and deswelling of hydrogels, how about the effects of heating or cooling rate on the shape-morphing of the microparticles within the AC electric field?\n(5) How about the shape-morphing differences of these microparticles after several cyclic heating and cooling processes? This is important to exhibit fatigue resistance of the microparticles in response to the external stimuli. In addition, since the hydrogel layer was spinning-coated onto the glassy polymer layer, how about the adhesive stability of these two polymer layers, especially within AC electric field after multiple actions?\n(6) Authors found thermoresponsive behavior of PDEAM-BP showed significant influence on effective polarizability of the bilayer particles, leading to different motion dynamically, and therefore, whether there are effects of the solid contents from the hydrogel layer on their polarizability within AC electric field?", "reviewer_3": "The article by Lee et al. presents a study of shape morphing particles, with configurable locomotion. The authors synthesized shape-morphing materials that can propel under electric fields, and that change shape as well as their locomotion with temperature. Thus, temperature can be used to steer particles. First, the authors synthesized particles from a spin-coated bilayer containing a thermoresponsive hydrogel and a non-swelling glassy polymer. The layers were then patterned into rectangular prisms using maskless lithography. Due to changes in hydration of the hydrogel, particles experience changes in mechanical and electrical properties that allow dual control with temperature and electric fields. This is an important contribution because, at the moment, most micromotors are unable to respond to multiple stimuli.\n\nA key contribution of this work is in the design and synthesis of the materials, which allows the authors to create surface and shape anisotropic particles. This contribution will open a new parameter space for the design of active particles. The experiments were performed with adequate controls for the most part, to support the following claims:\n• Locomotion is due to asymmetric flows, Fig. 4.\n• That changes in locomotion with temperature are not due solely to changes in shape, but also due to changes in electrical properties (polarizability) of the particles, Fig. 3.\n• Polarizability changes at different temperatures. Zeta potential of particles also changes.\n\nWeakness:\n• Interpretation and Proposed mechanisms: My assessment is that the authors do not know what the mechanism of propulsion is. It is unlikely that the mechanism of propulsion is ICEP. But this should not preclude publication of this work because, not knowing the mechanism of propulsion does not take away from the fact that they have created thermoresponsive shape morphing particles. The authors have made interesting observations. However, I strongly suggest the authors to exercise more caution in their assertions and acknowledge the limits of what they know. Perhaps the authors should change the tone in key parts of the text to recognize that revealing the details of the mechanism will require further study---which is ok in my view. I will provide more details below. See, for example, page 9, 11 (par. 1)\n• For example, experiments used to measure DEP response, and therefore make an argument about changes in polarizability, were performed at 200 kHz. This is a difference in three orders of magnitude where locomotion was observed, 200 Hz. Given the significant changes, the observations at such different time scales (200 Hz vs 200 kHz) cannot be directly compared. Therefore, until measurements of polarizability are performed at lower frequencies (200 Hz), the results in Figure 3E are just a proxy, a potential clue, a suggestion, and nothing more.\n• Page 6. Continuing with the low frequency in the locomotion experiments, this is a frequency region where electrohydrodynamic flows are likely to occur. The dependence of the field on square of electric field does not confirm that ICEP is the mechanism. Electrohydrodynamic flows also have a E2 dependence. A relevant reference is:\no Yang, X., Johnson, S. & Wu, N. The Impact of Stern‐Layer Conductivity on the Electrohydrodynamic Flow Around Colloidal Motors under an Alternating Current Electric Field. Adv Intelligent Syst 1, 1900096 (2019).\nAnother reference that might be relevant, since it links propulsion to polarizability, is:\nBehdani, B., Lumpkins, J. & Batista, C. A. S. Impact of the Electric Polarizability on the Transport and Collective Dynamics of Metallodielectric Janus Particles. Langmuir 39, 9025–9034 (2023).\n• 200 Hz is not considered a high frequency (page 9) in colloidal electrokinetics. However, the authors repeatedly refer to 200 Hz as a high frequency. If so, in comparison to what?\n• On Page 9. The authors make the following statement: “Above the CPST, PDEAM-BP is less hydrated and more polarizable than the surrounding medium, exhibiting pDEP with Re[CM] = 0.36 (Fig. 3I). In contrast, below the CPST, PDEAM-BP becomes highly swollen and less polarizable than the surrounding medium, transitioning to nDEP with Re[CM] = -0.15.”\nI find this statement dubious. Water is a highly polar molecule, whose addition is likely to increase the polarizability of a material. Therefore, by dehydrating, I would expect the polarizability of the material to decrease, not increase. Therefore, hydration usually means an increase in dielectric constant of a material, particularly in comparison to organic molecules, such as the polymers in this study. Also, hydration helps the separation of charges at the interface, while dehydration would preclude such separation of charges. However, measurements indicate that magnitude of zeta potential increases for PDEAM at 25 C. My understanding is that dehydration would change the Bjerrum length unfavorably for charge separations to increase at the interface (and therefore increase magnitude of zeta potential) when the surface is dehydrated.\n• How was zeta potential measured? Also, what is the dielectric constant of the polymers? And how was the deviation from spherical shape considered in the measurements of zeta potential?\n• What is propulsion velocity for the particles at different temperatures for the shape-morphing and non-shape morphing particles?"}, "author_response": "Response to reviewers’ comments for NCOMMS-25-19580-T\nReviewer #1 (Remarks to the Author):\nGeneral comment: The manuscript introduces shape-morphing active particles that can invert their polarizability, enabling reconfigurable motion in AC electric fields. The authors combine direct observation of temperature-induced shape changes with particle-tracking of locomotion. The manuscript is well-written and presents intriguing behavior, but the interpretation relies primarily on qualitative observations. Below are comments intended to help clarify the mechanisms and improve the scientific rigor of the study:\nAuthor reply: We thank the reviewer for their thoughtful evaluation of our work. Below we provide a point-by-point response to the concerns raised.\nComment: 1) The authors attribute the observed motion reversal and steering to shape-induced inversion of electrical polarizability. While plausible, shape-morphing particles could, in principle, produce motion through other mechanisms—such as fluid flows generated by deformation or mechanical kicks associated with non-reciprocal shape changes. A brief discussion or reference to the scallop theorem would strengthen the interpretation, emphasizing that reciprocal shape changes alone cannot produce net locomotion at low Reynolds number, and that the applied field is essential to break time-reversal symmetry and achieve propulsion.\nAuthor reply: We thank the reviewer for the thoughtful suggestion. At low Reynolds numbers, reciprocal shape changes alone cannot generate net motion without an external asymmetry, as articulated by Purcell’s scallop theorem. In our system, the applied AC electric field plays a critical role in this regard. The observed propulsion and trajectory changes arise only when the field is present, and the particles undergo temperature-induced shape changes that modulate their electrical polarizability. Additionally, the timescale of shape change (tens of seconds) is much slower than the timescales of propulsion (microns per second). These findings support our conclusion that motion arises primarily from the shape-directed inversion of polarizability, which leads to unbalanced electrohydrodynamic flows around the particles. While other mechanisms, such as flows from mechanical deformation, do exist in principle, they do not account for the directional propulsion observed exclusively under combined thermal and electrical inputs. We have now added a brief discussion of this point in the manuscript to clarify the necessity of the field in breaking time-reversal symmetry.\n“At low Reynolds numbers, reciprocal shape changes alone cannot generate net motion due to time-reversal symmetry, as described by Purcell’s scallop theorem.19,56,57 In our system, the applied AC electric field breaks this symmetry by inducing microscale fluid flows around the particles that are temperature-dependent. This coupling enables controlled propulsion and steering, distinguishing our approach from passive shape-morphing systems.” (Page 2)\nAuthor reply: We appreciate the reviewer’s keen suggestion. In our experimental setup, the electric field is spatially uniform at the center of the parallel-plate electrodes, as validated in prior work (DOI: 10.1038/s41467-019-10579-1). All particle tracking and propulsion experiments were conducted in this central region of 2 mm gap to minimize dielectrophoretic effects. We have now clarified this in the Methods section.\n“Particle tracking was performed at the center of the electrode gap (2 mm wide region of interest), where the AC electric field is spatially uniform, to minimize field gradients and avoid dielectrophoretic contributions to propulsion.84” (Page 17)\nComment: 3) The manuscript introduces the term invertible polarizability to describe a switch in the sign of the particle’s induced dipole response. As this is not standard terminology, a clearer explanation of what is being inverted—e.g., the sign of the Clausius–Mossotti factor or the direction of induced dipole alignment—would be beneficial. It would also be helpful to address why the system exhibits a discrete two-state switching behavior (e.g., attracted vs. repelled), rather than a continuous field- or frequency-dependent transition.\nAuthor reply: We thank the reviewer for this opportunity for clarification. The term \"invertible polarizability\" refers not simply to a change in the sign of a single material’s polarizability, but rather to a temperature-dependent reversal in the relative effective polarizability between the 2\ntwo layers of the bilayer particle. This difference in effective polarizability can change sign as the PDEAM-BP hydrogel layer swells or deswells. This results in an inversion in the direction of the induced dipole moment across the particle, relative to the non-responsive PpMS-BP layer. Such inversion in interfacial polarization is central to induced-charge electrophoresis (ICEP), where propulsion arises from the asymmetry in induced dipoles. We have clarified the meaning of invertible polarizability by explicitly defining it through the following:\n“…the bilayer shows ‘invertible polarizability’, a term we use to describe the temperature-dependent reversal in the relative effective polarizability between the two layers of the bilayer particle. This difference in effective polarizability—quantified by the real part of the Clausius–Mossotti (CM) factor—can change sign as the PDEAM-BP hydrogel layer swells or deswells. This results in an inversion in the direction of the induced dipole moment across the particle, relative to the non-responsive PpMS-BP layer.” (Page 8)\nRegarding the reviewer’s observation about discrete vs. continuous behavior, we agree that the underlying polarizability transition is continuous, as the hydrogel (PDEAM) gradually swells or deswells with temperature. However, due to the experimental requirement of recording multiple 10-minute videos at each condition (5°C and 30°C), totaling at least 30 minutes of observation per temperature for DEP trapping per condition, we performed measurements only at 5°C and 30°C, which limited our sampling over this region. We have now clarified this in the main text and acknowledged that future studies with finer temperature resolution would help to more accurately map the continuum of polarizability changes.\n“While the underlying polarizability change is likely continuous due to gradual water uptake or expulsion from the thermoresponsive hydrogel (PDEAM-BP), our measurements focused on two discrete temperatures (5°C and 30°C) due to the DEP trapping time required per condition to collect sufficient data for analysis (Supplementary Text). Future studies may explore this continuum more thoroughly at different temperature conditions.” (Page 11)\nComment: 4) While the correlation between shape and motion direction is compelling, the claims would be significantly strengthened by direct measurements of particle polarizability. Quantitative estimates of dielectrophoretic forces, alignment torques, or induced dipole moments (e.g., via orientation in the field, light scattering, or theoretical modeling) would provide stronger evidence for the proposed mechanism. As it stands, the interpretation relies on qualitative proxies (e.g., motion direction) rather than direct physical measurements.\nAuthor reply: We thank the reviewer for this suggestion. We agree that direct measurements of particle polarizability would provide stronger evidence for our proposed mechanism. While we did not measure induced dipole moments or torques directly due to limitations in capabilities we have available, we have extensively reworked the manuscript to more clearly explain how the 3\nmeasurements of high frequency polarizability, low frequency fluid flows, and zeta potential can be combined to elucidate a likely mechanism. In lieu of more direct measurements, we added electrochemical impedance spectroscopy (EIS) measurements of swollen PDEAM films to better understand the rate of ion migration within the bulk of the PDEAM. A discussion of these results is provided:\n“…these values correspond to an RC time constant of ~ 3 ms associated with field-driven migration of ions across the thickness of the PDEAM-BP film (Supplementary Table 1). The inverse of this time constant reveals a characteristic frequency of ~300 Hz. The proximity of this characteristic frequency and the frequency used in swimming experiments suggests that bulk ion transport may also play a role in the reported electrokinetic phenomena, although we expect these effects to be negligible at 200 kHz where DEP measurements were performed.” (Page 13)\nComment: 5) The manuscript would benefit from the inclusion of statistical data—such as particle-to-particle variability, error bars on speed or trajectory measurements, and consistency over multiple trials—to support the generality and robustness of the reported behavior.\nAuthor reply: We appreciate the reviewer’s suggestion and have made the following revisions to improve clarity regarding the statistical treatment of our data. We have added error bars representing particle-to-particle variability to Figure 1C and clarified in the caption of Figure 4A that error bars are included there as well. We also increased the thickness of error bars. For the trajectory plots shown in Figures 2H, 2I, and 5C, we note that these are representative single-particle trajectories included to illustrate the qualitative differences in motion modes under different conditions. While these figures are not intended to convey statistical trends, the behaviors shown were consistently observed across multiple particles and trials. We now clarify this point in the figure captions:\n“Some error bars are smaller than the symbol size and not fully visible.\" (Figure 4A)\nReviewer #2 (Remarks to the Author):\nGeneral comment: The manuscript from Shields and coworkers reports on fabrication and shape-morphing of the microparticles through layering a thermoresponsive hydrogel on a glassy polymer, and investigation of their motions responded to environmental temperature and AC electric field. The most interesting findings include that these microparticles conducted different shape-morphing according to their shape anisotropy, leading to different induced polarization and trajectory under the AC electric field. Overall, this work was nicely done, and the conclusions are well-supported by the results. Therefore, I recommend accepting it for publication in NC. However, prior to its publication, authors should pay attention to the following points for revision:\nAuthor reply: We thank the reviewer for their helpful feedback and the excellent questions. 4\nComment: (1) Shape morphing of hydrogels have been reported in a number of papers through various stimuli in different fashions (such as a new paper from Zhao and coworkers appeared on NC 2025, 16, 2830). It’s necessary to make more clear statement on the novelty contribution of the present work to this area.\nAuthor reply: We thank the reviewer for this important comment. We agree that shape morphing in hydrogels has been widely studied through various stimuli and mechanisms, as exemplified by the recent work from Zhao and coworkers (DOI: 10.1038/s41467-025-58041-9). To clarify the novelty of our work, we revised the Introduction to emphasize that, in contrast to prior studies focused primarily on static or reversible shape changes, our system uniquely combines thermoresponsive shape morphing with electric field-driven active propulsion. This integration enables programmable and reversible control over both particle shape and motion.\n“While hydrogel-based shape-morphing has been extensively explored using diverse stimuli such as temperature, pH, light, and electric fields, these systems typically exploit static or reversible shape changes without propulsion. Unlike previous studies, including two recent works,63,64 our system couples reversible hydrogel bending with electric field-driven active propulsion. This integration enables not only shape transformation but also tunable, directionally controlled motion and trajectory switching—advancing shape-morphing materials into the domain of microscale active matter.” (Page 3)\nComment: (2) Transition of hydrogels from collapsed to swollen state is driven by balance between the hydrophobic interactions among the polymer chains and the solvation (hydration) of the polymer chains with water. This should be clearly stated on page 3 to avoid misleading. In addition, although crosslink density of the hydrogels can show some influence on their swelling ratios due to the flexibility of the network, it doesn’t often exhibit influence on phase transitions of the hydrogels.\nAuthor reply: We thank the reviewer for this helpful suggestion. We agree that the volume phase transition of hydrogels is fundamentally governed by the balance between hydrophobic interactions among polymer chains and their solvation by water. We have now revised the text on Page 3 to clearly reflect this mechanism. Additionally, we acknowledge the reviewer’s point that while crosslink density can influence the extent of swelling due to network flexibility, it does not typically alter the intrinsic phase transition behavior. We have corrected this distinction in the revised manuscript.\n“PpMS-BP, being a rigid, non-swelling polymer, fixes the dimension of one side of the bilayer system, while PDEAM-BP, a polymer with a volume phase transition temperature (VPTT), exhibits a temperature-dependent swelling response (Fig. 1) that drives particle curvature. The swelling behavior of PDEAM-BP is governed by hydrophobic interactions and water solvation. At temperatures above its VPTT, hydrophobic interactions between polymer chains dominate, causing the polymer chains to aggregate and water to be expelled, resulting in deswelling of the hydrogel. Conversely, below this temperature, the 5\npolymer chains interact favorably with water molecules, allowing the hydrogel to swell. This is because hydrated polymer chains form stable structures through hydrogen bonding and solvation with water molecules.69 The VPTT can be modulated by copolymerization with hydrophilic or hydrophobic moieties within the polymer network.69 Though PDEAM-BP contains both hydrophilic acrylic acid and hydrophobic BP at equal molar content (7 mol.%), the VPTT of PDEAM-BP is significantly shifted downward to approximately 22°C, which is about 10°C lower than that of neat PDEAM-BP. This pronounced lowering of the VPTT suggests that the hydrophobic effect of the BP moiety outweighs the hydrophilic contribution of acrylic acid. The BP moiety—with its two phenyl rings and bulky structure—likely promotes hydrophobic interactions, thereby driving the VPTT to a lower temperature. Though the VPTT behavior is not strongly influenced by crosslink density, the swelling ratio of PDEAM-BP does depend on the crosslink density, which can be controlled by the UV exposure dose. In this study, the PDEAM-BP exhibited a swelling ratio of ≈1.8 (Supplementary Fig. 1).” (Page 3)\nComment: (3) Solid content plays a pivotal role in swelling and deswelling of the hydrogels according to temperature around their VPTT. In present work, shape-morphing of the double-layered microparticles is also dependent on the swelling of the hydrogel portion, and therefore, all data reported here should be correlated to the solid contents of the hydrogel part. Swelling ratios of the hydrogel part should also been included in the figures. These can be done to add another axis such as in Figure 1C\nAuthor reply: We thank the reviewer for this insightful comment. We agree that the solid content and corresponding swelling behavior of the hydrogel layer are critical factors influencing the shape morphing of the bilayer particles. To address this, we fabricated 500 µm-diameter, 4 µm-thick PDEAM hydrogel disks and measured their diameter changes in response to temperature to estimate the swelling ratio. As expected, the diameter increased with rising temperature, consistent with the hydrogel swelling behavior. We have now added the measured swelling ratio data as a secondary Y-axis in Figure 1C, as suggested by the reviewer, to directly correlate the particle curvature response with the swelling behavior of the hydrogel.\nAuthor reply: We thank the reviewer for raising these important points. Swelling and deswelling of hydrogels can be relatively slow due to the diffusion of water into and out of the polymer network, however the hydrogel layers here are also thin. In our experiments, the shape morphing process proceeded on a timescale of approximately one minute; however, this may be limited by the heating/cooling rate of the stage, and the intrinsic response time of the particles could be faster. After each temperature change, we waited at least 5 minutes until the system reached an equilibrium, where no further changes in shape were observed, before applying the AC electric field. For the purposes of this study, we constrained our focus to understanding the relationship between particle equilibrium shapes and responses to AC electric fields, particularly how induced polarization relates to trajectory. We believe that introducing kinetic factors would require detailed investigation in future work. Therefore, we have added the following statement in the revised manuscript:\n“The particles were allowed to settle for at least 5 min to reach an approximate mechanical equilibrium height in the chamber, after which the AC electric field was applied. Each time the temperature was changed, particles equilibrated for at least 5 min before the AC electric field turned on again.” (Page 17)\nComment: (5) How about the shape-morphing differences of these microparticles after several cyclic heating and cooling processes? This is important to exhibit fatigue resistance of the microparticles in response to the external stimuli. In addition, since the hydrogel layer was spinning-coated onto the glassy polymer layer, how about the adhesive stability of these two polymer layers, especially within AC electric field after multiple actions?\nAuthor reply: We thank the reviewer for raising this important point regarding the fatigue resistance and interfacial stability of the bilayer microparticles. In response, we performed additional experiments to evaluate the effect of repeated thermal cycling and exposure to electric fields. Specifically, we subjected the microparticles to 10 cycles of heating (35°C) and cooling (5°C) and observed that the shape-morphing behavior remained fully reversible with no evidence of structural fatigue. Furthermore, no separation between the PDEAM hydrogel layer and the PpMS glassy polymer layer was observed, even after repeated actuation. These results confirm the robustness of the bilayer interface and the fatigue resistance of the system. We have added these experimental details and results to the Supplementary Information:\n7\n“To evaluate fatigue resistance and interfacial stability of the bilayer microparticles, we subjected them to 10 repeated cycles of heating (35°C) and cooling (5°C). The shape morphing remained fully reversible with no observable loss in curvature. Additionally, no delamination or separation between the PDEAM-BP hydrogel and the PpMS layer was observed, confirming robust adhesion and mechanical integrity under cyclic thermal loads (Supplementary Fig. 3).” (Page 5)\nComment: (6) Authors found thermoresponsive behavior of PDEAM-BP showed significant influence on effective polarizability of the bilayer particles, leading to different motion dynamically, and therefore, whether there are effects of the solid contents from the hydrogel layer on their polarizability within AC electric field?\nAuthor reply: We thank the reviewer for raising this insightful question. We agree that the solid content of the hydrogel layer—which varies with temperature due to the thermoresponsive behavior of PDEAM—likely influences the effective polarizability in a continuous rather than binary fashion. While we did not directly measure the polarizability of PDEAM at multiple intermediate solid contents (i.e., over a temperature sweep) as justified in our response to Reviewer 1 (second half of Comment 3), we anticipate that as the polymer expels water with increasing temperature, its polarizability changes gradually. Based on insight from new experiments (discussed later in our response to Reviewer 3), this change is likely due to an increased concentration of ions along the particle surface as the PDEAM transitions from a highly swollen hydrogel to a more compact polymer with a greater density of surface charges. Given the challenges of measuring polarizability across multiple hydration states, we focused our experiments on two representative states: the swollen state at lower temperatures and the dehydrated state at elevated temperatures. We performed DEP measurements at 200 kHz to isolate dielectric effects. Under these conditions, the swollen PDEAM exhibited lower polarizability (negative Re[CM]), and the dehydrated PDEAM exhibited higher polarizability (positive Re[CM]), agreeing with our zeta potential measurements that that the magnitude of 8\nsurface charges increases on PDEAM as it expels water, increasing the effective polarizability and thus supporting our interpretation.\nWe have clarified in the revised manuscript that this polarizability shift is expected to occur gradually with temperature (and thus solid content) and is influenced by both the dielectric properties and surface charge of PDEAM. Please also see our detailed responses to Reviewer 3 for additional data and context (Page 13 of this document).\nReviewer #3 (Remarks to the Author):\nGeneral comment: The article by Lee et al. presents a study of shape morphing particles, with configurable locomotion. The authors synthesized shape-morphing materials that can propel under electric fields, and that change shape as well as their locomotion with temperature. Thus, temperature can be used to steer particles. First, the authors synthesized particles from a spin-coated bilayer containing a thermoresponsive hydrogel and a non-swelling glassy polymer. The layers were then patterned into rectangular prisms using maskless lithography. Due to changes in hydration of the hydrogel, particles experience changes in mechanical and electrical properties that allow dual control with temperature and electric fields. This is an important contribution because, at the moment, most micromotors are unable to respond to multiple stimuli.\n\nA key contribution of this work is in the design and synthesis of the materials, which allows the authors to create surface and shape anisotropic particles. This contribution will open a new parameter space for the design of active particles. The experiments were performed with adequate controls for the most part, to support the following claims:\n• Locomotion is due to asymmetric flows, Fig. 4.\n• That changes in locomotion with temperature are not due solely to changes in shape, but also due to changes in electrical properties (polarizability) of the particles, Fig. 3.\n• Polarizability changes at different temperatures. Zeta potential of particles also changes.\nAuthor reply: We thank the reviewer for their helpful feedback, which have helped us clarify and improve aspects of our work related to experiments and interpretation.\nWeakness:\nComment: Interpretation and Proposed mechanisms: My assessment is that the authors do not know what the mechanism of propulsion is. It is unlikely that the mechanism of propulsion is ICEP. But this should not preclude publication of this work because, not knowing the mechanism of propulsion does not take away from the fact that they have created thermoresponsive shape morphing particles. The authors have made interesting observations. However, I strongly suggest the authors to exercise more caution in their assertions and acknowledge the limits of what they know. Perhaps the authors should change the tone in key parts of the text to recognize that revealing the details of the mechanism will require further 9\nstudy---which is ok in my view. I will provide more details below. See, for example, page 9, 11 (par. 1)\nAuthor reply: We appreciate the reviewer’s thoughtful and fair assessment. We agree that while our study provides compelling evidence for temperature-responsive propulsion and shape-morphing behavior, the exact mechanism of propulsion remains to be definitively established. Based on additional experiments (described below), our current working hypothesis is that the propulsion is most consistent with a combination of induced-charge electrophoresis (ICEP), ion conductivity, and concentration polarization electrophoresis (CPEP), particularly given the use of coplanar electrodes, the observed temperature-dependent reversal in motion direction, and the absence of bulk electrohydrodynamic flows. We note that CPEP refers to propulsion driven by field-induced ion gradients near polarizable interfaces—often enhanced under AC fields—leading to asymmetric ionic environments and slip flows even in symmetric particle geometries (DOI: 10.1038/s41467-023-41923-1).\nIn response to the reviewer’s suggestion, we have carefully revised the main text to adopt a more cautious tone. We acknowledge the limits of our current understanding and emphasize that confirming the dominant propulsion mechanism will require further experiments, including flow visualization and quantitative mapping of field-induced charge and ion gradients, for example:\n“While we propose that induced-charge electrophoresis (ICEP)58–60 plays a dominant role, other factors such as concentration polarization electrophoresis (CPEP)61 and temperature-dependent ionic conductivity of the hydrogel layer62 may also contribute.” (Page 2)\nand\n“Our results suggest that the deswelling-induced crossover in dielectrophoretic behavior of PDEAM-BP is driven largely by an increase in surface charge density, and therefore effective polarizability due to ion motion. Future studies should explore the frequency and temperature dependence of Re[CM] across various material compositions to further decouple the roles of dielectric permittivity and ionic conduction in particle actuation and propulsion.” (Page 16)\nWe made multiple smaller changes throughout the manuscript to relax the tone around our descriptions of potential mechanisms to clarify that additional work is needed to prove a mechanism.\nComment: For example, experiments used to measure DEP response, and therefore make an argument about changes in polarizability, were performed at 200 kHz. This is a difference in three orders of magnitude where locomotion was observed, 200 Hz. Given the significant changes, the observations at such different time scales (200 Hz vs 200 kHz) cannot be directly compared. Therefore, until measurements of polarizability are performed at lower frequencies (200 Hz), the results in Figure 3E are just a proxy, a potential clue, a suggestion, and nothing more. 10\nAuthor reply: We thank the reviewer for pointing out the difference in frequencies between the dielectrophoresis (DEP) and propulsion experiments. We agree that polarizability recordings at 200 kHz cannot be directly equated to behavior at 200 or 300 Hz. Our choice of 300 Hz for propulsion studies was guided by empirical observations—this frequency produced the most robust and consistent particle locomotion. While this is lower than the typical ICEP frequency range used for metallodielectric Janus particles, our system is composed of polymeric bilayers, which likely have longer charge-relaxation times and thus a different polarization response regime. Thus, we agree that the results shown in Figure 3 serve as a proxy rather than a definitive measurement of polarizability under propulsion-relevant conditions. We have revised the text to expound on these differences and offer a new potential explanation based on the data provided in the manuscript:\n“Although the effective polarizabilities measured at 200 kHz cannot be directly mapped onto the swimming behaviors measured at 300 Hz, the temperature-dependent changes in the effective polarizability of PDEAM-BP correlate well with the observed differences in propulsion direction and mode. At 25°C, just above the CPST, the bilayer particles adopt a boomerang shape due to reduced swelling. In this state, PDEAM-BP maintains a higher polarizability than both the medium and the PpMS-BP layer, likely generating asymmetric flows that drive linear motion perpendicular to the electric field and away from the PDEAM-BP side. Below 12°C, just below the CPST, the PDEAM-BP becomes less effectively polarizable, and the particle bends further into a horseshoe shape, whereby propulsion switches direction, now occurring parallel to the field and away from the more polarizable PpMS-BP layer.” (Page 11)\nComment: Page 6. Continuing with the low frequency in the locomotion experiments, this is a frequency region where electrohydrodynamic flows are likely to occur. The dependence of the field on square of electric field does not confirm that ICEP is the mechanism. Electrohydrodynamic flows also have a E2 dependence. A relevant reference is:\no Yang, X., Johnson, S. & Wu, N. The Impact of Stern‐Layer Conductivity on the Electrohydrodynamic Flow Around Colloidal Motors under an Alternating Current Electric Field. Adv Intelligent Syst 1, 1900096 (2019).\nAnother reference that might be relevant, since it links propulsion to polarizability, is:\nBehdani, B., Lumpkins, J. & Batista, C. A. S. Impact of the Electric Polarizability on the Transport and Collective Dynamics of Metallodielectric Janus Particles. Langmuir 39, 9025–9034 (2023).\nAuthor reply: We thank the reviewer for raising this important point. We agree that the observed propulsion at low frequencies could involve multiple electrokinetic mechanisms, and we appreciate the opportunity to refine our interpretation. In our revised manuscript, we now take greater care to acknowledge other factors besides ICEP may contribute to the observed results. We also end the manuscript with a call for further mechanistic studies in the future, as discussed in an earlier response. 11\nWe also agree with the reviewer that electrohydrodynamic (EHD) flows often appear at low frequencies. However, given our coplanar electrode geometry, the generation of strong, EHD flows—such as those seen near vertical electrodes—is less likely. This geometry significantly limits the field gradients perpendicular to the substrate that would normally drive EHD flows. Therefore, while we cannot completely exclude all electrokinetic contributions, EHD flow is unlikely to dominate propulsion in our setup. To further support this interpretation, we now cite recent work by Velev and colleagues (DOI: 10.1016/j.jcis.2024.07.141), which shows that particle propulsion due to electrokinetic mechanisms can occur in the low-frequency regime (<1 kHz), consistent with our observations. Their findings suggest that complex interactions—such as ICEP, ion relaxation effects, and surface charge-driven flows—can give rise to propulsion at these frequencies, reinforcing the notion that ICEP remains a plausible contributor in our system.\nIn response to the reviewer’s concerns, we have revised the manuscript to: (i) remove language that attributes propulsion to EHD flows; (ii) clearly state that our observations are consistent with ICEP-based propulsion but that additional mechanisms may be involved; and (iii) emphasize the need for future studies to disentangle these effects and directly probe flow fields and particle charge distributions. See the quoted text earlier and the modified text below as examples:\n“The observed propulsion occurs in the low-frequency regime (~300 Hz), consistent with reports of electrokinetically driven motion of asymmetric particles in similar conditions.58 While the precise mechanism remains under investigation, our coplanar electrode geometry reduces the likelihood of significant electrohydrodynamic flows, which typically arises from steep field gradients near electrodes.71–75” (Page 5)\nComment: 200 Hz is not considered a high frequency (page 9) in colloidal electrokinetics. However, the authors repeatedly refer to 200 Hz as a high frequency. If so, in comparison to what?\nAuthor reply: We thank the reviewer for this pointing out. We agree that 300 Hz is considered a low frequency in the context of colloidal electrokinetics. We have revised the manuscript to remove references to 300 Hz as a \"high frequency\" to avoid confusion.\nComment: On Page 9. The authors make the following statement: “Above the CPST, PDEAM-BP is less hydrated and more polarizable than the surrounding medium, exhibiting pDEP with Re[CM] = 0.36 (Fig. 3I). In contrast, below the CPST, PDEAM-BP becomes highly swollen and less polarizable than the surrounding medium, transitioning to nDEP with Re[CM] = -0.15.”\nI find this statement dubious. Water is a highly polar molecule, whose addition is likely to increase the polarizability of a material. Therefore, by dehydrating, I would expect the polarizability of the material to decrease, not increase. Therefore, hydration usually means an increase in dielectric constant of a material, particularly in comparison to organic molecules, such as the polymers in this study. 12\nAuthor reply: We thank the reviewer for this insightful point. We agree that, in general, hydration increases the dielectric constant of a polymeric material, and thus one would expect that dehydration reduces the dielectric contribution to polarizability. Our original description attributing the observed crossover solely to dielectric mismatch was therefore an oversimplification.\nIn the revised manuscript, we now frame the behavior in terms of the temperature-dependent zeta potential of PDEAM-BP. Specifically, while deswelling of PDEAM-BP is expected to lower its local dielectric constant, it simultaneously increases the density of ionizable groups near the surface. This effect appears to dominate, leading to a higher magnitude of recorded zeta potentials under dehydrated conditions. The resulting enhancement in surface charge density is now hypothesized to facilitate stronger polarization via ion motion, which we propose underlies the observed “invertible polarizability” in both low-frequency (300 Hz) swimming and high-frequency (200 kHz) dielectrophoresis experiments.\nConsistent with this interpretation, the crossover in zeta potential (~15°C) aligns closely with the CPST (15–20°C) and the observed temperature range for the crossover in DEP behavior (5–30°C). We also acknowledge that the precise crossover temperature is likely frequency-dependent, since surface charge dynamics and bulk ion mobility operate on different characteristic timescales, and the relative importance of ion conductivity diminishes at higher frequencies.\n“We observe that the magnitude of the zeta potential of PDEAM-BP increases at elevated temperatures with an enhanced surface charge for thermoresponsive polymers in their collapsed state, which agrees with prior reports.82,83 While deswelling is typically expected to lower the local dielectric constant, the concurrent increase in the density of ionizable groups near the surface appears to dominate, leading to the observed enhancement in the magnitude of the zeta potential. We propose that the temperature-dependent zeta potential of PDEAM is responsible for the invertible effective polarizability seen via both low-frequency (300 Hz) measurements of particle swimming and high-frequency (200 kHz) dielectrophoretic trapping behavior. At high temperatures (above the CPST), the larger magnitude of zeta potential for PDEAM could suggest that a locally higher concentration of ions near the surface contribute to a higher polarization than PpMS, while the opposite would be true at low temperatures. The crossover in zeta potential (at ≈ 15°C) is close to the range of CPST (15-20°C) and in good agreement with the crossover in DEP behavior between 5-30°C. Notably, the specific crossover temperature may depend on frequency of the applied field, so the values do not necessarily align perfectly.” (Page 13)\nAdditionally, our new electrochemical impedance spectroscopy (EIS) experiments on swollen PDEAM films revealed an ionic conductivity of ~7×10⁻⁸ S m⁻¹ and a capacitance corresponding to an RC time constant of ~3 ms. This value is comparable to an inverse frequency near the 300 13\nHz fields used in the swimming experiments, suggesting that bulk ion transport through the PDEAM hydrogel layer may also contribute to the propulsion mechanism near the CPST.\nHowever, these bulk transport effects are expected to be negligible at the higher frequency (200 kHz) DEP measurements, where surface polarization is likely the dominant contribution.\nTaken together, we have revised the manuscript to clarify that the observed DEP crossover does not reflect a simple dielectric mismatch but rather arises from a complex interplay between dielectric polarization, zeta potential, and ion transport. We believe this updated interpretation better reconciles the reviewer’s concern with our experimental observations.\n“An additional effect that may contribute, given the use of a PDEAM hydrogel layer, is ion transport through the “bulk” of the particle, rather than simply motion of charges associated with the EDL. We sought to gain additional insight into this aspect by performing electrochemical impedance spectroscopy (EIS) on UV-crosslinked PDEAM films swollen to equilibrium on interdigitated electrodes (Supplementary Fig. 9). Fitting the impedance spectra to an equivalent circuit model yielded an ionic conductivity (presumably primarily of protons associated with acrylic acid groups in the PDEAM-BP copolymers) of (7.19 ± 0.03) ×10-8 S m-1 and a specific capacitance of (1.14 ± 0.002) x 10-4 F m-2. Together with a characteristic PDEAM layer thickness of ≈ 2 m in the particles, these values correspond to an RC time constant of ~ 3 ms associated with field-driven migration of ions across the thickness of the PDEAM-BP film (Supplementary Table 1). The inverse of this time constant reveals a characteristic frequency of ~333 Hz. The proximity of this characteristic frequency and the frequency used in swimming experiments suggests that bulk ion transport may also play a role in the reported electrokinetic phenomena, although we expect these effects to be negligible at 200 kHz where DEP measurements were performed.” (Page 13)\nSupplementary Table 1. Extracted electrical properties of swollen PDEAM-BP at 25°C from electrochemical impedance spectroscopy (EIS) measurements. Values in the table were calculated assuming a characteristic PDEAM-BP thickness of ≈ 2 µm in the bilayer particles from a ≈ 25 µm thick film on the EIS setup.\nResistance Conductivity Capacitance Specific RC time (s)\n(Ω) (S m-1) (F) Capacitance\n(F m-2)\n6.96 × 108 ± 7.19×10-8 ± 2.29×10-11 ± 1.14×10-4 ± ≈ 3×10-3\n2.1×106 3.2×10-10 4.65×10-14 2.3×10-7\nAs the reviewer points out, a more rigorous understanding of the relationship between polymer hydration, polarizability, and dielectrophoretic behavior require further experiments. Future studies could perform impedance measurements on PDEAM at controlled hydration levels and temperatures to quantify dielectric constant and ionic conductivity independently. We believe these are exciting directions that can further advance understanding of thermo-responsive polymeric materials in non-uniform electric fields and plan to pursue them in future work.\n“Future studies should explore the frequency and temperature dependence of Re[CM] across various material compositions to further decouple the roles of dielectric permittivity and ionic conduction in particle actuation and propulsion.” (Page 16)\nAuthor reply: We agree with the premise that hydration influences interfacial charge separation and that dehydration can reduce charge screening. In our system, as the temperature increases and the PDEAM layer expels water, the polymer becomes more collapsed and exposes more ionizable groups at the surface. This change reduces the local dielectric environment, which can increase the Bjerrum length and enhance charge separation at the polymer-electrolyte interface. These effects are consistent with our zeta potential measurements, which show an increase in the magnitude of the surface potential with temperature. We have clarified this interpretation in the revised manuscript.\n“We observe that the magnitude of the zeta potential of PDEAM-BP increases at elevated temperatures with an enhanced surface charge for thermoresponsive polymers in their collapsed state, which agrees with prior reports.82,83 While deswelling is typically expected to lower the local dielectric constant, the concurrent increase in the density of ionizable groups near the surface appears to dominate, leading to the observed enhancement in the magnitude of the zeta potential.” (Page 13)\nComment: How was zeta potential measured? Also, what is the dielectric constant of the polymers? And how was the deviation from spherical shape considered in the measurements of zeta potential?\nAuthor reply: We thank the reviewer for providing this opportunity for clarification. While particle shape can influence zeta potential measurements, we chose 2 µm cubic particles because they are the closest approximation to spheres achievable using our 2D maskless photolithography process. Although geometric anisotropy may influence electrokinetic properties, such effects are generally small for micron-sized cubes. We therefore assume that any deviation in zeta potential between our cubic particles and hypothetical spheres of similar size is negligible for the purposes of this study. We have revised the manuscript to acknowledge the particle geometry and justify our interpretation accordingly.\n“To better understand the temperature-induced inversion in polarizability, we next consider changes in the electric double layer (EDL) by fabricating cubic particles (2 μm × 2 μm × 2 μm) composed entirely of PDEAM-BP or PpMS-BP and measuring their zeta potentials across a temperature range encompassing the CPST of PDEAM-BP (Fig. 4A), as spherical particles are not readily achievable using 2D lithography. While geometric anisotropy can influence zeta potential at smaller length scales or with highly elongated shapes, the electrokinetic behavior of micron-sized cubes is expected to be 15\ncomparable to that of spheres. Thus, we interpret zeta potential values from these cubes as representative of particles with similar surface chemistry and dimensions.81” (Page 12)\nComment: What is propulsion velocity for the particles at different temperatures for the shape-morphing and non-shape morphing particles?\nAuthor reply: We thank the reviewer for this insightful question. We conducted additional experiments to quantify propulsion velocities for both shape-morphing and non-shape-morphing bilayer particles at two different temperatures: 10°C and 22°C. Our results show that the shape-morphing bilayer particles propel faster at 22°C than at 10°C, with an average increase of ~4 µm/s. In contrast, non-shape-morphing bilayer particles (with symmetric layer thicknesses) show no appreciable difference in propulsion velocity between the two temperatures. We hypothesize that this behavior arises due to the asymmetry in the layer structure. For shape-morphing particles, the PpMS layer is thin (~100 nm), while the PDEAM hydrogel layer is thicker (~500 nm). This asymmetry enhances polarizability mismatch between the two layers as PDEAM dehydrates and becomes more dielectric at elevated temperatures. For non-shape-morphing particles, both PpMS and PDEAM layers are ~500 nm thick, which may reduce this mismatch and, consequently, the temperature-dependent changes in propulsion speed. We have included this new data in the revised SI and added clarification in the main text.\n“To assess temperature-dependent behaviors, we measured propulsion velocities of both shape-morphing and non-shape-morphing bilayer particles at 10°C and 25°C. Shape-morphing particles exhibited a significantly higher propulsion speed at 25°C, with an average increase of ~4 µm s-1 compared to their speed at 10°C. In contrast, non-shape-morphing particles showed no significant change in propulsion velocity across the same temperature range (Supplementary Fig. 6). We note that PDEAM-BP was found to orient toward the rear at 25°C and toward the front at 10°C; however, for the non-shape-morphing bilayer particles, we could not visualize orientation during propulsion. This result may be attributed to the asymmetric bilayer structure, particularly the thicker PDEAM-BP hydrogel layer in the shape-morphing particles, which is expected to play a key role in enhancing propulsion through temperature-induced changes in dielectric properties and polarizability mismatch.” (Page 7)\n16"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have done a nice job addressing the prior concerns and strengthening the manuscript. The revisions clarify the fabrication methods, expand on the mechanistic interpretation of propulsion, and provide additional data to support the claims regarding temperature-dependent polarizability and shape–motion coupling. I find the revisions to be satisfactory, and I have no further substantive concerns.", "reviewer_2": "The authors have revised the manuscript accordingly. I feel quality of the manuscript has been greatly improved and can be accepted for publication in its present state.", "reviewer_3": "I appreciate the effort the authors demonstrated in addressing my comments. My previous concerns have been addressed, therefore I recommend the manuscript for publication."}, "author_response": ""}]

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