Research ArticleSOFT ROBOTS

Soft phototactic swimmer based on self-sustained hydrogel oscillator

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Science Robotics  21 Aug 2019:
Vol. 4, Issue 33, eaax7112
DOI: 10.1126/scirobotics.aax7112
  • Fig. 1 Light-driven hydrogel oscillator.

    (A) Schematic representation of the gel oscillator and the setup. Initially, the gel-made pillar is placed vertically on the ground in water. As the light is switched on, the gel pillar bends toward the light (step i) and then starts the oscillation (step ii). (B) Superimposed frames indicating the tracking followed with oscillation. The gel dimensions are 14 mm (L) by 0.9 mm (d). (C) Mechanism of the oscillation: The out-of-equilibrium actuation is controlled by a self-shadowing–enabled negative feedback loop inherent in the dynamic stimuli-material interactions. Photothermal effect of AuNP induces the temperature increase over the LCST of the PNIPAAm gel, leading to the local shrinkage of the gel and pillar tracking toward the light. Among the oscillation cycles, the tip displacement, temperature, and specific volume of the gel experience periodically change over time. (D) Time-resolved tip displacement and local temperature of the hinge for the system with the LCST of 32°C. There is a 90° phase shift of two curves. Dark columns represent the upstroke of the tip, where the temperature is lower than LCST and gel locally recovers. Bright columns, however, represent the downstroke of the tip, where the temperature is higher than LCST and gel locally deswells.

  • Fig. 2 Realization of omnidirectional oscillation.

    (A) Schematic of zenith scenario, where the pillar is originally placed upright at the bottom with light approaching at different zenith angles. (B) Schematic of azimuthal scenario, where the pillar responds to the light approaching at different azimuthal angles. (C) Zenith angle test for a pillar with a diameter of 0.9 mm. Under light exposure of 500 mW, the oscillation could be initialized ranging from 90° to 59°. Deflection angle ≤56° resulted in tracking. (D) As the diameter was reduced to 0.36 mm, oscillations with 90° to 0° deflection angles and a large oscillation amplitude were observed. Each frame represents 1.3 s for all four different angles. (E) Symmetric, upper-biased, and lower-biased oscillations at 0° deflection angle. (F) Superimposed snapshots of a pillar in response to different azimuthal angles of light. The frequency dependence of oscillation at (G) different zenith angles and (H) different azimuthal angles. Error bars indicate SD. (To show the relative positions, all images are superimposed photos of pillars of different configurations at different time points during oscillation.)

  • Fig. 3 The frequency of the oscillator dependent on the geometry.

    (A) Frequency as a function of arm length. The fitting curve with water damping consideration matches better with the experimental data (red curve, R2 = 0.987) compared with no water damping (black dashed curve, R2 = 0.983). (B) Comparison of the experimental (upper) and computer simulation (lower) result of the time-resolved tip displacement. The dimensions of pillars are as follows: d = 0.56 mm; L = 13 mm (blue), 17 mm (red), and 22 mm (gray). (C) Frequency as a function of diameter. (D) Comparison of the experimental and theoretical results. The dimensions of pillars are as follows: d = 0.56 mm (purple), 0.78 mm (green), and 1.03 mm (brown); L = 17 mm. Input power, 300 mW. Scale bars (in A and C insets), 1 cm. Error bars indicate SD.

  • Fig. 4 Input energy dependency and long-term stability of oscillation.

    (A and B) Amplitude and frequency as a function of input power. When the input power was higher than the threshold energy, the amplitude increased as input power increased. The frequency, however, maintained a relatively similar value. The thinner pillar could also oscillate with larger amplitude at the same power input. (C) Given the same power input of 300 mW, a thinner pillar oscillated at larger amplitude because of a smaller diffusion time scale t2/D. Tip displacement (D) and frequency (E) upon irradiation of 200 mW over a long period. The dimensions of the pillar are as follows: L = 22 mm and d = 0.56 mm. Once reaching steady oscillation, the long-lasting oscillation could be maintained stably over a long period with little frequency and amplitude fluctuation. Error bars indicate SD.

  • Fig. 5 Self-sustained oscillator-based soft swimming robot (OsciBot) powered and controlled by visible light.

    (A) Scheme of the soft swimmer and (B) real oscillation in water. The green arrows denote the light direction. (C) Sequential snapshots of swimmer while shining constant light. Input power was 450 mW. (D) Velocity versus mass of swimmer in comparison with previous reports. A represents a PDMS-cardiomyocyte biohybrid swimmer (35); B represents a ciliate-inspired swimmer (29); C represents a flagellum-inspired swimmer (30); D represents a hydrogel swimmer under water (36); and E represents an eel-inspired microrobot (37). (E) Comparisons of the fully swollen, thoroughly dried, and reswollen soft swimmer, showing on-demand encapsulating and deployable abilities. Scale bars, 1 cm. (F) On/off control of soft swimmer. While the light was switched on, the swimmer kept moving forward. Once the light was switched off, the swimmer stopped in response. During every cycle of the oscillation, mainly the downward stroke (via deswelling) pushed the swimmer forward, resulting in the step-by-step movement over time.

  • Fig. 6 The effects of oscillation parameters on swimming performance of the OsciBot.

    OsciBot velocity as the function of (A) the amplitude, (B) the frequency, and (C) the beam tip velocity of the oscillation.

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/4/33/eaax7112/DC1

    Materials and Methods

    Section S1. General

    Section S2. Fabrication of materials

    Section S3. Characterization of materials

    Section S4. Characterization of oscillation

    Section S5. Theory and simulations

    Fig. S1. UV-visible absorption spectrum of AuNPs.

    Fig. S2. The schematic of the measurement of hydrogel deswelling/swelling ratio and rate.

    Fig. S3. Deswelling/swelling kinetics of oscillating hydrogel and tracking hydrogel.

    Fig. S4. Scanning electron microscope images of hydrogels.

    Fig. S5. The stress-strain curves of hydrogels with different cross-linking densities.

    Fig. S6. Photo-tracking versus photo-oscillation.

    Fig. S7. Comparisons of the bending and unbending kinetics of oscillating pillar and tracking pillar.

    Fig. S8. Switch from tracking to oscillation by tuning light power.

    Fig. S9. Fishhook-shaped oscillator and position independency.

    Fig. S10. Schematic of lower-biased (case I), upper-biased (case II), and symmetric flapping (case III).

    Fig. S11. Range of operation input correlated to the dimension and the photothermal properties.

    Fig. S12. Realization of oscillation under ambient white light.

    Fig. S13. The hydrogel oscillator floating on the surface of water in the container.

    Fig. S14. Maneuverability of the OsciBot.

    Table S1. Summary of effect of cross-linking density on materials properties and oscillation performance.

    Movie S1. Hydrogel-based light-driven oscillator.

    Movie S2. Comparison of light-induced tracking and oscillation.

    Movie S3. Fishhook-shaped hydrogel oscillator.

    Movie S4. Position independency.

    Movie S5. Realization of omnidirectional oscillation.

    Movie S6. Oscillation frequency as a function of geometry.

    Movie S7. Initialization: From tracking to oscillation.

    Movie S8. Long-term stability of the oscillation.

    Movie S9. OsciBot: Continuous swimming.

    Movie S10. OsciBot: Controllable motion.

    Movie S11. Oscillation under ambient white light.

    References (3840)

  • Supplementary Materials

    The PDF file includes:

    • Materials and Methods
    • Section S1. General
    • Section S2. Fabrication of materials
    • Section S3. Characterization of materials
    • Section S4. Characterization of oscillation
    • Section S5. Theory and simulations
    • Fig. S1. UV-visible absorption spectrum of AuNPs.
    • Fig. S2. The schematic of the measurement of hydrogel deswelling/swelling ratio and rate.
    • Fig. S3. Deswelling/swelling kinetics of oscillating hydrogel and tracking hydrogel.
    • Fig. S4. Scanning electron microscope images of hydrogels.
    • Fig. S5. The stress-strain curves of hydrogels with different cross-linking densities.
    • Fig. S6. Photo-tracking versus photo-oscillation.
    • Fig. S7. Comparisons of the bending and unbending kinetics of oscillating pillar and tracking pillar.
    • Fig. S8. Switch from tracking to oscillation by tuning light power.
    • Fig. S9. Fishhook-shaped oscillator and position independency.
    • Fig. S10. Schematic of lower-biased (case I), upper-biased (case II), and symmetric flapping (case III).
    • Fig. S11. Range of operation input correlated to the dimension and the photothermal properties.
    • Fig. S12. Realization of oscillation under ambient white light.
    • Fig. S13. The hydrogel oscillator floating on the surface of water in the container.
    • Fig. S14. Maneuverability of the OsciBot.
    • Table S1. Summary of effect of cross-linking density on materials properties and oscillation performance.
    • Legends for movies S1 to S11
    • References (3840)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Hydrogel-based light-driven oscillator.
    • Movie S2 (.mp4 format). Comparison of light-induced tracking and oscillation.
    • Movie S3 (.mp4 format). Fishhook-shaped hydrogel oscillator.
    • Movie S4 (.mp4 format). Position independency.
    • Movie S5 (.mp4 format). Realization of omnidirectional oscillation.
    • Movie S6 (.mp4 format). Oscillation frequency as a function of geometry.
    • Movie S7 (.mp4 format). Initialization: From tracking to oscillation.
    • Movie S8 (.mp4 format). Long-term stability of the oscillation.
    • Movie S9 (.mp4 format). OsciBot: Continuous swimming.
    • Movie S10 (.mp4 format). OsciBot: Controllable motion.
    • Movie S11 (.mp4 format). Oscillation under ambient white light.

    Files in this Data Supplement:

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