High speed underwater hydrogel robots with programmable motions powered by light

The study addresses a core limitation of hydrogel soft actuators for underwater robotics: slow actuation speed governed by water diffusion during swelling/deswelling, which typically yields minute- to hour-scale cycles and constrains complex motions. Existing approaches to accelerate actuation (thinning films, adding porosity, or employing anisotropic nanofillers) often trade off mechanical output, design freedom, or require complex external fields (e.g., magnetic alignment). Moreover, converting isotropic hydrogel volume changes into programmable nonlinear motions generally relies on fixed spatial heterogeneity set during synthesis, limiting reconfigurability. The authors propose a thermoresponsive hydrogel network with dynamic disulfide crosslinks enabling photo-mechanical programming of anisotropy, thereby switching the actuation mechanism from mass-diffusion-driven swelling to rapid heat-transport-limited chain conformational changes. The objective is to realize high-speed, reprogrammable, complex underwater motions powered remotely by light.

Prior work has accelerated hydrogel actuation by reducing diffusion paths (thin films, porous structures) and by designing organohydrogels to shorten water pathways, achieving up to ~0.1 Hz, but with constraints on geometry and decreased actuation output in porous systems. Nonmonolithic strategies using cofacially oriented electrolyte nanosheets allow diffusion-independent actuation within ~1 s by temperature-modulated electrostatic interactions; however, generating and controlling required magnetic fields for complex motions is challenging. Introducing spatial heterogeneity in crosslinking density can yield complex deformations, but the heterogeneity is typically fixed during synthesis, preventing reprogramming. Some thermoresponsive systems with additional crystalline/melting transitions enable reprogrammability, yet suffer slow actuation, especially in cooling steps due to hydrophobic associations slowing chain relaxation. Overall, there is a need for actuators that combine fast, diffusion-independent response with programmable, complex deformation modes and practical remote powering.

Materials and network design: A thermo-responsive poly(N-isopropylacrylamide) (PNIPAM/NIPAM) hydrogel is crosslinked with a disulfide-containing crosslinker (BISS) to enable UV-triggered dynamic bond exchange. Poly(vinyl alcohol) (PVA) is incorporated (0–5 wt% in precursor, typically 2 wt%) to stabilize the network, build water tension, and prevent blister formation. Post-synthesis, multivalent metal ion complexation (Fe3+, Al3+, Cu2+) is used to enhance mechanics; Al3+ is selected as a balance between stiffness and actuation amplitude. Photothermal capability is introduced by doping carbon black particles (0.005 g; ~150 µm diameter) into the precursor for NIR-driven heating. Synthesis of programmable hydrogels: NIPAM (0.25 g) and BISS (10–80 wt% relative to NIPAM; optimized at 40 wt%) were dissolved in 2.5 mL PVA solution (0–5 wt%; optimized at 2 wt%). Initiation used 5.0 µL of 4 wt% APS aqueous solution and 10 µL TEMED. The mixture was cast in a mold (two glass slides with a 0.5 mm silicone spacer) and polymerized at 4 °C for 24 h. Cured gels were immersed for 24 h in an aqueous ionic solution to introduce counter-crosslinking (Al3+ preferred; Fe3+ and Cu2+ also evaluated). Complexation decreased LCST (e.g., from ~34 °C to ~27 °C at the studied BISS content). All samples had strains at break >100%. Bilayer reference hydrogel: A passive PAAm layer was first polymerized (details: PAAm with initiator APS and TEMED) at 25 °C for 24 h (0.25 mm), followed by casting the PNIPAM/BISS layer on top and polymerization at 4 °C for 24 h (0.25 mm). The bilayer was ionically crosslinked in 0.01 M Al3+ solution for 24 h. Curvature κ was used to quantify shape-shifting; κ was defined positive during cooling and negative during heating. Photo-mechanical programming: Hydrogel strips (3 × 10 × 0.3 mm unless otherwise stated) were mechanically deformed (e.g., folded/bent) and irradiated with UV light (80 mW/cm²; CEL-HXF300) for 0–7 min. Optimized irradiation time was 300 s, maximizing shape retention and actuation amplitude. UV triggers disulfide bond exchange under load to rearrange network topology, fix a new permanent shape with a through-thickness gradient in chain orientation due to light attenuation. Spatial patterning was achieved by masking unexposed regions with thin foils. Chronological programming allowed sequential addition of actuation zones. Actuation testing and metrics: After programming, samples were equilibrated at 25 °C; the remaining angle θ0 defined shape retention: (180° − θ0)/180° × 100%. Upon equilibration at 60 °C, θfin was measured; actuation angle θ was θmax − θfin. For kinetics, actuation degree α was defined as (xt − x0)/(xmax − x0) × 100%, where x is curvature κ for the programmed actuator and an equivalent actuation metric for the bilayer. Heating at 60 °C and cooling at 25 °C were used unless otherwise noted. Tests in oil vs water probed diffusion independence. Thickness dependence was examined (0.5, 1, 2 mm). Cycling durability was evaluated over repeated thermal cycles. Characterization: Swelling ratio measured as mwet/mdry (n=5). LCST assessed by DSC (TA Q200; 5 °C/min). Tensile testing in water bath (Care Test IPBF-300; 5 mm/min; n=5). Photothermal heating used 808 nm IR source (45 W; LSR8038H), with sample-to-source distance 4 cm; temperature monitored by Fortic 237 thermometer. Polarized optical microscopy (ECLIPSE E600N POL) verified chain orientation. Robot demonstrations: NIR-driven oscillatory actuators were tested at imposed light modulation frequencies from 0.3 to 1.7 Hz. Multimodal robots (swimming, step-wise walking, rotating) were fabricated from single hydrogel samples via spatio-selective programming and integrated photothermal fillers; light was steered to specific regions to drive motions.

- Programming mechanism: UV-triggered disulfide bond exchange under mechanical deformation creates spatio-selective anisotropy and permanent shape reconfiguration with a gradient in chain orientation through thickness, enabling reversible nonlinear actuation upon thermal cycling. - Speed mechanism: Actuation is dominated by heat transport and chain conformational change, not water diffusion. Programmed samples showed ~80% actuation degree within 10 s, whereas a classical bilayer hydrogel reached only ~13% under the same conditions. During cooling (25 °C), the programmed actuator achieved ~100% actuation degree within 30 s vs ~23% for the bilayer reference. - High-frequency operation: With photothermal fillers and NIR irradiation, light-powered oscillations up to 1.7 Hz were realized underwater, enabling continuous swimming, step-wise walking, and rotation. - Composition optimization: Optimal BISS content was 40 wt% for maximal shape retention and actuation amplitude; too low BISS reduced exchange points and orientation; too high (>60 wt%) impeded exchange due to dense crosslinking. Optimal UV programming time was 300 s. PVA at 2 wt% yielded the largest actuation; Al3+ post-complexation balanced mechanical stiffness and actuation (Fe3+ gave highest stiffness but lowest actuation; Cu2+ the opposite trend). LCST decreased from ~34 °C to ~27 °C after ionic complexation at the studied composition. - Durability: After a slight decrease in the first cycle (stress relaxation reducing orientation), actuation stabilized at ~75° over subsequent cycles. - Diffusion independence: In oil, programmed samples completed fast shape-shifting within 10 s on heating and recovered within 10 s on cooling, while non-programmed PNIPAM showed negligible volume change at 10 s and required ~10 min heating for noticeable shrinkage. - Thickness effects: Actuation kinetics were characterized for 0.5, 1, and 2 mm samples, consistent with heat conduction dominance and anisotropic mechanism. - Programmability: The same sample could be programmed to exhibit different actuation modes by altering applied deformation, by sequential programming, and via spatio-selective exposure to define active regions.

The findings demonstrate that embedding photo-responsive dynamic disulfide bonds into a PNIPAM network enables photo-mechanical programming of anisotropy, switching the actuation mechanism from slow, diffusion-limited swelling to fast, heat-transport-limited chain conformational changes. This results in orders-of-magnitude faster actuation than conventional hydrogels that can produce similar nonlinear deformations. Photothermal fillers facilitate remote NIR powering, allowing high-frequency underwater motions. The programmable anisotropy offers extensive design freedom: actuation regions and modes can be customized post-synthesis by controlling deformation during UV exposure and spatial masking. Comparative tests against a bilayer hydrogel confirm substantially faster kinetics in both heating and cooling, consistent with the proposed mechanism. These attributes expand the functional space for soft robotic applications, particularly in aquatic environments where rapid response and reconfigurability are advantageous.

This work introduces a programmable hydrogel actuator that achieves high-speed, complex underwater motions by leveraging UV-triggered disulfide bond exchange to impose network anisotropy and by using thermally driven chain conformational changes for actuation. Coupled with photothermal fillers for NIR heating, the system enables oscillations up to 1.7 Hz and diverse locomotion modes (swimming, walking, rotating). Key material optimizations include 40 wt% BISS, 2 wt% PVA, Al3+ complexation, and 300 s UV programming. The approach overcomes diffusion-limited kinetics and allows reprogrammable, spatially selective actuation. Future work should (i) engineer region-specific photothermal domains to enable floodlight actuation without moving light spots, (ii) balance speed and amplitude to meet applications demanding both, and (iii) explore integration with other stimuli or energy sources for untethered operation and enhanced control.

Two main limitations are identified: (1) Light powering requires focusing and moving NIR irradiation across specific regions to produce desired motions, which is challenging to control in practice; incorporating spatially localized photothermal domains could enable uniform illumination while maintaining selective actuation. (2) Achieving very high speeds entails a trade-off with actuation amplitude, as full cycles cannot complete on very short timescales; this may limit applications requiring both large deformation and high frequency. Additionally, initial cycles show slight performance reduction due to stress relaxation diminishing programmed orientation.