Research ArticleChemistry

Designing self-propelled, chemically active sheets: Wrappers, flappers, and creepers

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Science Advances  21 Dec 2018:
Vol. 4, no. 12, eaav1745
DOI: 10.1126/sciadv.aav1745
  • Fig. 1 Chemomechanical response of catalse-coated sheet.

    (A) Top-down view of a fluidic chamber containing a catalase-coated flower-like, elastic sheet composed of nodes (indicated by green dots) and connected by bonds (black lines) and (B) schematic of the solutal convection due to the catalytic reaction on the sheet. (C to E) Sequential wrapping and unwrapping of a passive sphere (in yellow) by a catalase-coated sheet (Embedded Image mol m−2 s−1), positioned in parallel to the bottom surface at a distance h = 0.19 mm. Catalase decomposes hydrogen peroxide to less dense products, oxygen and water, which rise upward to produce an inward convective flow at the bottom of the microchamber. Black arrows indicate the directionality and magnitude of the flow field, and the color bar indicates the concentration of H2O2 in the solution. Inset in (E): Relative distances between the tips of opposite pairs of petals. As H2O2 is consumed, the convective flow is diminished and the sheet gradually opens up to return to the flat state. (F to I) Repetitive wrapping and unwrapping of petals with regular influxes of hydrogen peroxide. (F) H2O2 introduced periodically through the side walls at a rate R = 5.4 × 10−9 mol s−1. (G) Concentration of the reactant decreases as it is decomposed into products, only to increase with the next input of H2O2. (H) Mean velocity reaches a maximum with each influx of H2O2. (Average is taken over the velocities in the central, vertical plane that cuts through the center of the decorative sphere.) (I) Distance between the tips of respective pairs of petals, showing the periodic opening and closing.

  • Fig. 2 Top and side views of the bending and unbending of the respective catalase-coated (green) and GOx-coated (pink) petals.

    Coordinated behavior of the petals involves the reaction cascade in Eqs. 8 and 9. (A) Inward flow generated by the decomposition of d-glucose and O2 drives the pink petals to fold and join together. (B) With the consumption of these reactants, the pink petals unfold and the concentration of H2O2 increases in the medium. The decomposition of H2O2 on the green petals drives an inward flow that causes the bending of the green petals. (C) Temporal variation of concentrations of chemicals participating in the reaction cascade given by Eqs. 8 and 9. As expected, the concentration of hydrogen peroxide [the product of the first reaction (Eq. 9) and the reactant for the second reaction (Eq. 8)] initially increases and then decreases. (D) Temporal delay between the folding of the pairs of catalase- and GOx-coated petals is characterized by the difference in time between the minima of the plots. To obtain an appreciable time delay in the relative dynamic behavior of the green and pink petals, we specified the following areal concentrations of the enzymes on the petals and chamber walls: Embedded Image, Embedded Image, Embedded Image, Embedded Image, and Embedded Image.

  • Fig. 3 Schematic describing temporal regulation of petals.

    To achieve the temporal delay between the closing of the opposing petals, the side walls adjacent to GOx-coated petals are coated with catalase, and the side walls adjacent to catalase-coated petals are coated with GOx. Consumption of H2O2 happens not only on catalase-coated green petals but also on the appropriate side walls. Consumption of H2O2 at these side walls creates an outward flow along GOx-coated pink petals that speeds up the movement of these petals to the horizontally flat state.

  • Fig. 4 Chemical logic gates, or flappers.

    Coating the petals with catalase (green) and AP (yellow) allows the system to perform different logic operations, such as XOR and AND [see corresponding truth tables (A) and (B)], as well as NOT. The XOR operation, defined when at least one of the opposing pair of petals is up, is achieved through introduction of either (C) PNPP or (D) H2O2 in the solution. (In this case, the “1” in the truth table indicates that two petals are bent upward.) (E) In the presence of both H2O2 and PNPP, all four flaps come together and serve as the AND operation. (In this case, “1” indicates that four petals are bent upward.) The corresponding reaction rates at the petals and on the walls for this simulation are set at Embedded Image, Embedded Image, Embedded Image, and Embedded Image. The NOT operation is achieved in the absence of both H2O2 and PNPP and is represented by all four flappers staying horizontally flat above the bottom surface.

  • Fig. 5 Tumbling motion of active sheets.

    Motion of passive (A) and active (B to E) sheets on the bumpy surface due to an influx of H2O2 (with a rate R = 4.5 × 10−10 mol s−1) through the right wall of the chamber. (A) The passive, noncoated sheet becomes arrested at the first bump. (B to E) Tumbling motion of the catalase-coated (Embedded Image mol m−2 s−1) sheet. As the active sheet slides along the surface (B), the left end is temporarily arrested by the bump (C), while the right end is dragged upward by the flow due to the solutal force and pushed forward due to the influx of dense reactant. Consequently, the right end of the sheet tumbles over the bump (D). The sheet is then flattened by the flow field and slides along the surface to the next bump (E), at which point the process is repeated.

  • Fig. 6 Creeping motion of active sheets.

    Response of passive (A) and active (B to G) sheets to the sequential influxes of H2O2 (at a rate R = 1.8 × 10−9 mol s−1). Initially, H2O2 is introduced at the right end; in subsequent cycles, it is injected from both side walls at regularly spaced positions in x. (A) The passive sheet simply slides along the surface as H2O2 is introduced into the chamber. (B to G) Inchworm-like motion of the catalase-coated, active sheet with a nonuniformly distributed mass. The sheet is lighter in the center and heavier at the left and right edges. With an influx of the dense, reactant-rich solution, the active sample moves away from the regions where H2O2 is introduced. Simultaneously, catalase on the sample (Embedded Image mol m2 s−1) decomposes H2O2 to less dense products, which flow upward. The lighter, central portion of the sheet is pushed upward by this flow, but the heavier edges (head and tail) remain on the bottom surface; inset in (C) shows a blowup of this configuration. As the reactant is depleted and reaction ceases to produce the less dense products, the sheet flattens out. This cycle of motion is repeated, with each influx of H2O2, thereby allowing the sheet to effectively creep along the surface.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/12/eaav1745/DC1

    Movie S1. Wrapping and unwrapping of a catalase-coated flower-like sheet around a capsule.

    Movie S2. Periodic wrapping and unwrapping of a catalase-coated flower-like sheet around a capsule.

    Movie S3. Temporal delay in bending of catalase-coated green and GOx-coated pink petals of the flower.

    Movie S4. Crawling motion of a catalase-coated rectangular sheet over a bumpy terrain.

    Movie S5. Creeping motion of a catalase-coated rectangular sheet over the bottom surface of the channel.

  • Supplementary Materials

    The PDF file includes:

    • Legends for movies S1 to S5

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Wrapping and unwrapping of a catalase-coated flower-like sheet around a capsule.
    • Movie S2 (.mp4 format). Periodic wrapping and unwrapping of a catalase-coated flower-like sheet around a capsule.
    • Movie S3 (.mp4 format). Temporal delay in bending of catalase-coated green and GOx-coated pink petals of the flower.
    • Movie S4 (.mp4 format). Crawling motion of a catalase-coated rectangular sheet over a bumpy terrain.
    • Movie S5 (.mp4 format). Creeping motion of a catalase-coated rectangular sheet over the bottom surface of the channel.

    Files in this Data Supplement:

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