Research ArticleENGINEERING

Instant tough bonding of hydrogels for soft machines and electronics

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Science Advances  21 Jun 2017:
Vol. 3, no. 6, e1700053
DOI: 10.1126/sciadv.1700053
  • Fig. 1 Instant tough bonding of hydrogels to a wide range of materials.

    (A) Schematic illustration of the bonding method. An adhesive dispersion is used to instantly tough-bond (i) heterogeneous hydrogels, (ii) hydrogels and elastomers, and (iii) rigid to flexible foils to (prestretched) hydrogels in less than a minute. (B) The bonding agent links tough hydrogels with covalent and dissipative cross-links to elastomers, and the formed interface is instant and tough yet remains stretchable. (C) Instant healing of a conductive hydrogel rod used to light up a light-emitting diode (LED) circuit. Both mechanical and electrical properties are restored after complete incision. (D) Normalized resistance versus uniaxial strain before (blue trace) and after (red trace) healing a hydrogel conductor. (E to G) Hydrogel square (colored blue for visibility) bonded to a transparent elastomer. The soft hybrid is stretched more than 1000% in area without delamination. (H to J) A conducting Cu-coated poly(ethylene terephthalate) (PET) foil is tough-bonded to a prestretched hydrogel. Upon release of the prestrain, out-of-plane wrinkles form in the foil, creating a reversibly stretchable soft-hard hybrid.

  • Fig. 2 Interfacial toughness and optically transparent, stretchable bonds.

    (A) Illustration of the 90° peeling test, with the hydrogel instantly tough-bonded to the bulk substrate. A liner serves as stiff backing. Crack propagation in the hydrogel occurs perpendicular to the peeling direction (detail and photograph with colored hydrogel). (B) Measured interfacial toughness for PVA-isoprene (turquoise trace), PAAm/alginate–PET (light blue trace), and PHEMA-Ecoflex (purple trace) instant tough bonds. (C) Interfacial toughness of PHEMA (purple), PAAm/alginate (light blue), and PVA (turquoise) hydrogels instantly tough-bonded to plastics, elastomers, leather, bone, and chromium-coated metals (aluminum and copper) and glass. Mean values and variance for at least three individual peel tests are shown. PDMS, poly(dimethylsiloxane); PI, polyimide. (D) Transmission and absorption spectra of the acrylic elastomer VHB (yellow trace), PAAm hydrogel (blue trace), and PAAm hydrogel instantly bonded to VHB (red trace). Bonding does not deteriorate the optical properties of the soft hybrid. (E) Nearly strain-independent transmission (red trace) at 600 nm for a biaxially stretched VHB/PAAm stack up to 2000% areal expansion. Inset: Photograph of the sandwich at 1000% strain with a color wheel in the background. The dashed red line outlines the PAAm hydrogel.

  • Fig. 3 Soft adaptive lens and energy harvester with instantly tough-bonded hydrogel electrodes.

    (A) Working principle of the tunable lens. Transparent hydrogel electrodes were used to electrically deform the lens, defined by a liquid reservoir embedded within a VHB elastomer. (B) Photograph of the soft lens without and with applied voltage, demonstrating electrical tuning of focal length. (C) Focal length and change of focal length versus voltage, illustrating a large voltage-induced focal length change of 110% at 6.5 kV. (D and E) Visualization of the voltage-controlled focal length change by showing laser light traces in a basin filled with a diluted rhodamine/water solution. (F) Illustration of mechanical into electrical energy conversion with a deformable elastomer/hydrogel capacitor. (G) Photographs of the deformable balloon-shaped elastomer/hydrogel capacitor. (H) Normalized capacitance of the deformable capacitor versus stretch ratio λ, demonstrating a λ4 behavior. (I and J) Exemplary energy harvesting cycle in pressure-volume and voltage-charge work conjugate plots. The enclosed areas in the two diagrams illustrate the mechanical energy input (497 mJ) and electrical energy output (54 mJ) of the cycle, resulting in a conversion efficiency of more than 10%.

  • Fig. 4 Tough, stretchable battery and self-powered stretchable circuit.

    (A) Scheme of a stretchable battery in top configuration, enabled by the tough hydrogel separator instantly tough-bonded to the elastomer matrix. Imperceptible 1.4-μm-thick electrodes serve as current collector. Cu-coated PET foil with Zn paste is the anode, and MnO2 paste contacted with Au-coated PET is the cathode. (B) Nyquist plot for top (red dots) and lateral (blue dots) configuration of a stretchable battery, showing reduced internal resistance due to shorter ionic paths through the separator in top configuration. (C) Voltage versus discharge current at 0% (red squares) and 50% (blue triangles) strain. Internal resistance decreases from 8.9 to 6.7 ohms with stretching due to thinning of the hydrogel separator. (D) Illustration of the self-powered stretchable LED circuit with a 6-μm PET circuit board and SMD elements. A dc-dc converter boosts the voltage of the battery to power three LEDs. (E and F) Stretchable circuit atop a hydrogel-based battery in relaxed (E) and strained and twisted state (F), without impairing function.

  • Fig. 5 Hydrogel electronic skin.

    (A) Concept of a hydrogel smart skin, with a flexible unit bearing power supply, control, readout and communication units, and a stretchable transducer batch. PCB, printed circuit board. (B) Photograph of an untethered electronic hydrogel with four stretchable heating elements and adjoined temperature sensors strongly bonded to a PVA hydrogel. Battery, control, readout, and Bluetooth low energy communication electronics are hosted on a flexible circuit board. (C) Finite element (FE) simulation of the transducer batch with four heating elements enabled. (D) Corresponding infrared (IR) thermography image of the freestanding device. Dashed lines in (C) and (D) outline the heater and sensor metal traces. (E) Autonomous hydrogel electronic skin controlled and read out continuously via mobile phone, worn on a human wrist with all heaters activated. (F) Corresponding IR image after ~450 s with all heater elements activated. (G) Measured temperature evolution of the heater elements measured via IR thermography and (H) the sensor elements. Switching of the heating elements is indicated by vertical lines. Recorded temperature traces of the sensor elements (solid lines) are in excellent agreement with data taken from IR images (open squares). (I) Concept of thermally triggered drug delivery where the substance is enclosed in a lipid shell. Temperature increase melts the shell, releasing the drug into the hydrogel. (J) Thermally triggered diffusion of a green food colorant throughout the hydrogel matrix.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/6/e1700053/DC1

    Supplementary Materials

    Supplementary Methods

    fig. S1. Raman spectroscopy of an instantly tough-bonded compound of a 6-μm-thick PET and PHEMA hydrogel using a 1:4 ethyl cyanoacrylate/2,2,4-trimethylpentane dispersion.

    fig. S2. Diffusion of bonding agent into PDMS and VHB.

    fig. S3. Biaxial stretching of instantly tough-bonded hydrogel.

    fig. S4. Uniaxial stretching of physically attached hydrogel versus instantly tough-bonded hydrogels.

    fig. S5. Interfacial toughness measurement of hydrogels bonded to various substrates and varying dispersion compositions.

    fig. S6. Single-edge notch test of PHEMA and PVA hydrogels.

    fig. S7. Model of a human lumbar vertebrae.

    fig. S8. Instant tough bonding of PAAm/alginate hydrogel to PMMA and aluminum and instant loading of the bond with more than 1-kg weight.

    fig. S9. Reflectance R of PAAm, VHB, and toughly bonded PAAm-VHB sandwich sheets.

    fig. S10. Transmission and absorption of PHEMA-PET heterostructures.

    fig. S11. Stretchable battery and self-powered stretchable circuit.

    fig. S12. Schematic top view and cross section of the stretchable battery in top configuration.

    fig. S13. Details for the electronic circuit board of the hydrogel electronic unit.

    fig. S14. Illustrating the stretchability of the complex electronic system.

    fig. S15. Illustrating the smartphone control of our hydrogel electronic patch.

    fig. S16. Interfacial toughness and cell viability studies.

    fig. S17. Heat-activated diffusion of a model drug.

    video S1. Instant healing of conductive hydrogels.

    video S2. Bulk rupture of a tough PAAm/alginate hydrogel undergoing 90° peeling.

    video S3. Assembly of a 3D-printed human lumbar vertebrae model.

    video S4. Instant tough bonding and instant loading of tough PAAm/alginate hydrogel.

    video S5. Visualization of the voltage-controlled focal length change in a soft hydrogel lens.

    video S6. Hydrogel electronic skin.

    References (56, 57)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials
    • Supplementary Methods
    • fig. S1. Raman spectroscopy of an instantly tough-bonded compound of a 6-μm-thick PET and PHEMA hydrogel using a 1:4 ethyl cyanoacrylate/2,2,4-trimethylpentane dispersion.
    • fig. S2. Diffusion of bonding agent into PDMS and VHB.
    • fig. S3. Biaxial stretching of instantly tough-bonded hydrogel.
    • fig. S4. Uniaxial stretching of physically attached hydrogel versus instantly tough-bonded hydrogels.
    • fig. S5. Interfacial toughness measurement of hydrogels bonded to various substrates and varying dispersion compositions.
    • fig. S6. Single-edge notch test of PHEMA and PVA hydrogels.
    • fig. S7. Model of a human lumbar vertebrae.
    • fig. S8. Instant tough bonding of PAAm/alginate hydrogel to PMMA and aluminum and instant loading of the bond with more than 1-kg weight.
    • fig. S9. Reflectance R of PAAm, VHB, and toughly bonded PAAm-VHB sandwich sheets.
    • fig. S10. Transmission and absorption of PHEMA-PET heterostructures.
    • fig. S11. Stretchable battery and self-powered stretchable circuit.
    • fig. S12. Schematic top view and cross section of the stretchable battery in top configuration.
    • fig. S13. Details for the electronic circuit board of the hydrogel electronic unit.
    • fig. S14. Illustrating the stretchability of the complex electronic system.
    • fig. S15. Illustrating the smartphone control of our hydrogel electronic patch.
    • fig. S16. Interfacial toughness and cell viability studies.
    • fig. S17. Heat-activated diffusion of a model drug.
    • Legends for videos S1 to S6
    • References (56, 57)

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    Other Supplementary Material for this manuscript includes the following:

    • video S1 (.mp4 format). Instant healing of conductive hydrogels.
    • video S2 (.mp4 format). Bulk rupture of a tough PAAm/alginate hydrogel undergoing 90° peeling.
    • video S3 (.mp4 format). Assembly of a 3D-printed human lumbar vertebrae model.
    • video S4 (.mp4 format). Instant tough bonding and instant loading of tough PAAm/alginate hydrogel.
    • video S5 (.mp4 format). Visualization of the voltage-controlled focal length change in a soft hydrogel lens.
    • video S6 (.mp4 format). Hydrogel electronic skin.

    Download Videos S1 to S6 PDF

    Files in this Data Supplement:

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