Research ArticleMATERIALS SCIENCE

Bioinspired high-power-density strong contractile hydrogel by programmable elastic recoil

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Science Advances  18 Nov 2020:
Vol. 6, no. 47, eabd2520
DOI: 10.1126/sciadv.abd2520
  • Fig. 1 Conceptual scheme of the strong contractive materials based on mechanical energy storing method.

    (A) Frog leaping off a rock through prestoring and subsequent releasing of elastic potential energy. (Photo credit: Purchased from vjshi.com under royalty-free license.) (B) Elastic potential energy was initially stored (red area) in elastic structures by contraction of muscle via consuming chemical energy in the body and then instantaneously releasing based on releasing (blue area) of elastic potential energy. (C) Schematics of the design strategy of shrinking materials via storing energy during stretching and then releasing the energy: storing elastic potential energy in the material by mechanically stretching it and locking it at the stretched state through forming new chemical bonds under external stimulus-1 (i). Upon external stimulus-2, the newly formed bonds break, and the prestored energy is released, resulting in rapid and powerful contraction of the material, which generates large force (ii). The green images are confocal microscopic images of the hydrogel material.

  • Fig. 2 Materialization of elastic energy storing and releasing method based on reaction between carboxylic group and iron ion in p(AAm-co-AAc) hydrogels.

    (A) p(AAm-co-AAc) hydrogel network (left) can be locked by forming new carboxylic-Fe3+ coordination (middle) and subsequently unlocked by reduction of Fe3+ ions and protonation of carboxylic acid (right). (B) Prestretched hydrogel (left) after fixation in Fe3+ solution (EDG, middle) and after contraction in 1 M acid solution (right). (Photo credit: Yanfei Ma and Mutian Hua, University of California Los Angeles.) (C to E) Stress-stretch curves of the hydrogel before (C) and after Fe3+-treatment at different prestretched ratios (percentage of the original length) (D) and after subsequent acid treatment (E), showing reversible elastic-plastic switchability. (F) Stress-stretch curves of hydrogels during Fe3+ treatment and after five loading-unloading cycles. (G) Time profiles of the loading-unloading cycles in (F), in terms of Ls (prestretched length) and force (newton). (H) Extension of hydrogels with different PAAc contents as a function of soaking time in Fe3+ solution. (I) Programmable deformability: controllable multistage energy storing and releasing of hydrogels in Fe3+ solution and under UV radiation, respectively. (J) Fully reversible energy storing in 0.06 M Fe3+ solution and releasing in 1 M acid solution of hydrogel (0.03% BIS).

  • Fig. 3 Contractive properties of EDG.

    (A) Elastic energy density (WE) of EDG (0.04 wt % BIS) contraction as the function of prestretch ratio (λ). (B) EDG’s contraction strength versus UV radiation time for EDGs with different prestretch ratio. (C) Length ratio (L/L0) as a function of UV (320 to 390 nm, 225 mW/m2) radiation time was measured in water. (D and E) Contraction strength-time and length ratio-time curves of the EDG (200% energy storage, cross-linker: 0.04 wt % BIS) in 0.25, 0.5, and 1 M hydrochloric acid solution. (F) Stored energy and released energy of EDG, in comparison with the released energy of osmotic hydrogels. (G) Comparison of the contraction strength and elastic modulus between EDGs and typical osmotic hydrogels (1, 3, 14, 40, 41). (H) Photos of an EDG (300% energy storage, 0.04 wt % BIS) strip lifting a 20-g weight load upon UV light illumination. (Photo credit: Yanfei Ma and Shuwang Wu, University of California Los Angeles.)

  • Fig. 4 Programmable property of the EDG based on elastic energy storing and releasing method.

    (A to D) Schematic designs, the p(AAm-co-AAc) hydrogel was encoded with the applied mechanical force. (E to H) Confocal images of the hydrogel structures after stretching and fixed. (I to L) Optical photographs of edited EDG materials before and after exposure to the UV light. (I) Contracted in x-axis direction and extend in y axis. (J) Shrunken in y-axis direction under keeping constant length in x-axis direction. (K) Three vertices (left, top, and right) contracted but the bottom remained constant. (L) Isotropic shrinkage. (Photo credit: Yanfei Ma and Mutian Hua, University of California Los Angeles.) (M to P) EDG showed contractility from anisotropic to isotropic. (Q) EDG based on elastic energy storing and releasing method could achieve 3D contraction under UV light. (R) EDG could locally shrink after local exposure to UV light. The size of the grid unit in the photo background is 6.5 mm by 6.5 mm. Photo credit: Yanfei Ma, University of California Los Angeles.

  • Fig. 5 Applications of EDG materials via the strong contraction force and high elasticity modulus.

    (A) Contractive hydrogel tube prepared by elastic energy storing and releasing method could enlarge in the direction of tube diameter under UV light. L and D (length and diameter of the hydrogel tube). (B) Anisotropic EDG material was assembled on 3D printed arm and helps the arm realize lift. (C) Contractive material was adhered on a broken glass sheet loaded with 40-g weights and achieved the healing of the glass sheet depending on mechanical contraction. (D) Bending actuator made of EDG-paper (plastic material) bimorph structure showing strong driving force by lifting a weight (4 g), which is unachievable by the osmotic (PNIPAm) hydrogel as the control. (Photo credit: Yanfei Ma, University of California Los Angeles.)

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