Research ArticleSENSORS

Sustainable manufacturing of sensors onto soft systems using self-coagulating conductive Pickering emulsions

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Science Robotics  26 Feb 2020:
Vol. 5, Issue 39, eaay3604
DOI: 10.1126/scirobotics.aay3604
  • Fig. 1 Illustration of SCP emulsion and the printing process.

    SCP emulsion consists of PDMS precursors stabilized by CB nanoparticles in an EtOH medium. Printing SCP emulsion onto polymer substrates initiates a cascade reaction that spontaneously forms the conductive composite. Using EtOH as the medium enables a sustainable printing process without swelling or degradation of the polymer substrate, a typical drawback when using conventional suspension inks with harsh, often toxic solvents. The nondestructive characteristic of SCP emulsions unlocks opportunities to integrate complaint sensors into soft systems.

  • Fig. 2 Material characteristics and rheological behavior of SCP emulsions.

    (A) Optical micrograph of CB/PDMS-precursor droplets within EtOH medium. (B) SEM images of a CB/PDMS droplet cured within EtOH medium and its freeze-fractured morphology. The cross-sectional image is false-colored to highlight the PDMS core (blue) and the CB shell (red). (C) Dynamic light scattering results for the size distribution of the emulsion at various CB concentrations. The average diameter for the droplets is around 2 μm. (D) Absorbance retention of the emulsions with varying CB concentrations, obtained from UV-vis measurements at 450 nm. The inset image shows SCP emulsions at 5 and 15 wt % after 2 weeks. (E) The rheological profile of a PDMS precursor alone, a CB/PDMS suspension, and SCP emulsions. EtOH thins out a CB/PDMS suspension to a printable viscosity, and the colloids in SCP emulsions exhibit a shear-thinning behavior. (F) Release and coalescence of PDMS precursors when printed onto a glass slide and EtOH evaporates. Coalesced PDMS precursors spontaneously polymerize in contact with atmospheric moisture.

  • Fig. 3 Characteristics of conductive composites directly printed onto bare polymer substrates.

    (A) Sheet resistance of the composites and their surface morphology. Wrinkles/cracks formed as EtOH evaporates become apparent with an increasing CB concentration. (B) SEM image of the composite surface at a 15 wt % CB concentration and TEM inset image revealing uniformly distributed CB nanoparticles. (C) Porosity of composites as a function of applied strains. Composite microstructure consisting of microscale cracks give rise to the stretchability (SCP emulsion), different from the crack-free composites that exhibit a catastrophic failure (control ink). (D) Measured crack opening ratio in composites and the normalized electrical conductance predicted from the measured data and the inset crack opening model. Error bars indicate 1 SD. Printing SCP emulsions onto a prestretched substrate (at ε = 100%) decreases the initial crack width (at ε = 0%), thereby increasing the working strain range from ca. 20 to 100%. (E) Hysteresis in mechanoelectrical responses of the composites produced on Ecoflex 00-30 substrates. Crack opening mechanism of the SCP emulsion sensor grants negligible hysteresis and high sensitivity to strain within the working strain range. (F) Long-term fatigue behavior of the composite sensor produced from SCP emulsions.

  • Fig. 4 Morphology of CB/PDMS composites infused into textiles.

    (A) Illustration of the direct printing of SCP emulsions into textiles. (B) Optical micrograph of the surface of a composite-infused textile. (C) SEM image of the infused composite that contains generic microcracks on the surface. (D) TEM images of a thin CB/PDMS composite layer bonded to the textile fiber. EDS result confirms the wet-out of the textile with the PDMS matrix. (E) Sheet resistance of composite-infused textiles and the tensile modulus increment due to the composite. Error bars indicate 1 SD. Composites from SCP emulsions add negligible stiffness to the textile, unlike that from a control ink. (F) Electrical resistance of composite-infused polyester/spandex textiles during uniaxial cyclic loading for the initial 100 cycles (N). Highlighted curves indicate N = 50. (G) Long-term fatigue behavior of the composite-infused polyester/spandex textile. The inset shows a detailed electrical resistance as a response to a sinusoidal uniaxial displacement. (H) Strain-responsive electrical resistances of composite-infused textiles with various substrate textiles.

  • Fig. 5 All-soft systems with directly integrated strain sensors.

    (A) A latex balloon with an integrated composite sensor. Volumetric expansion of the balloon is monitored at <10% of error from the sensor response. (B) A spherical latex balloon with SCP emulsion traces printed at varied printing directions (i.e., 0°, 45°, and 90°) to examine any directionality in the volumetric change. Error bars indicate 1 SD. (C) A silicone rubber four-legged gripper, onto which SCP emulsion traces are printed. The resistance (R) of the trace increases as the gripper inflates. (D) A fiber-reinforced silicone rubber actuator with a printed SCP emulsion trace to monitor the strain along the length. A strain-limiting layer is adhered onto the actuator at different locations (i.e., 90° and 180° with respect to the sensor) to create various bending strains.

  • Fig. 6 Self-sensing McKibben actuator.

    (A) Schematic of the self-sensing McKibben actuator, enabled by printing SCP emulsion onto an inflated balloon. Pin and Pout indicate input pressure and output pressure, respectively, and R denotes the electrical resistance of the sensor. (B) Photographs of the actuator that elicits the contraction as the internal balloon inflates. With the input pressure (Pin), the initial length (l) of the actuator is decrease by Δl. (C) Sensor responses collected from the actuator at various contraction states and the calculated error on the state estimation (predicted length calculated from state estimation compared to ground truth length). The error is quantified as εrms, a root mean square of the error. (D) Closed-looped control of the self-sensing McKibben actuator with the controller matching the sensor output to a target value.

  • Fig. 7 Application of composite-infused textile sensors.

    (A) Composite-infused kinesiology tape attached to a pneumatic fabric gripper to monitor the grip opening ratio (Φ). This self-adhesive textile sensor can conform to the surface contours and measure the curvature upon gripping motion. (B) Active clothes that are easy to don/doff and elucidate a specific motion artifact present in normal human activities. Ready-made garments are also transformed into strain-sensing wearables (see fig. S10 for details). (C) OmniSkin, a hybrid of sensorized soft systems where composite sensors are printed onto bare polymer substrates and textiles, respectively. This robotic textile can monitor bidirectional strains applied to a passive foam during its inchworm motion via integrated self-sensing McKibben actuator (εx) and textile sensor (εy).

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/5/39/eaay3604/DC1

    Fig. S1. Rheological profile of the SCP emulsion at different EtOH concentrations.

    Fig. S2. Crack opening mechanism for strain sensing.

    Fig. S3. Optical images of composite-infused polyester/spandex textile.

    Fig. S4. Separation of EtOH from CB/PDMS-precursor droplets on a textile.

    Fig. S5. Cross-sectional SEM image of SCP emulsion–printed textile.

    Fig. S6. Tensile moduli of composite-infused textiles.

    Fig. S7. Fatigue resistance of composite-infused polyester/spandex textiles.

    Fig. S8. Shape estimation of an inflatable latex balloon.

    Fig. S9. Fabric gripper with a stand-alone sensor and a self-adhesive sensor.

    Fig. S10. Strain sensing of composite-infused textiles fabricated from ready-made garments.

    Fig. S11. Comparison with existing manufacturing processes for composite sensors.

    Movie S1. Direct comparison of SCP emulsion (EtOH) and conventional suspension (cyclohexane) on a latex balloon.

    Movie S2. Sensorized latex balloon using an SCP emulsion.

    Movie S3. Fiber-reinforced actuator with an SCP emulsion–printed trace and a self-adhesive strain-limiting layer.

    Movie S4. Closed-loop control of a self-sensing McKibben actuator.

    Movie S5. SCP emulsion–printed kinesiology tape to measure elbow flexion.

    Movie S6. Hybrid robotic textile (OmniSkin) giving rise to an inchworm motion.

    References (5070)

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Rheological profile of the SCP emulsion at different EtOH concentrations.
    • Fig. S2. Crack opening mechanism for strain sensing.
    • Fig. S3. Optical images of composite-infused polyester/spandex textile.
    • Fig. S4. Separation of EtOH from CB/PDMS-precursor droplets on a textile.
    • Fig. S5. Cross-sectional SEM image of SCP emulsion–printed textile.
    • Fig. S6. Tensile moduli of composite-infused textiles.
    • Fig. S7. Fatigue resistance of composite-infused polyester/spandex textiles.
    • Fig. S8. Shape estimation of an inflatable latex balloon.
    • Fig. S9. Fabric gripper with a stand-alone sensor and a self-adhesive sensor.
    • Fig. S10. Strain sensing of composite-infused textiles fabricated from ready-made garments.
    • Fig. S11. Comparison with existing manufacturing processes for composite sensors.
    • References (5070)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Direct comparison of SCP emulsion (EtOH) and conventional suspension (cyclohexane) on a latex balloon.
    • Movie S2 (.mp4 format). Sensorized latex balloon using an SCP emulsion.
    • Movie S3 (.mp4 format). Fiber-reinforced actuator with an SCP emulsion–printed trace and a self-adhesive strain-limiting layer.
    • Movie S4 (.mp4 format). Closed-loop control of a self-sensing McKibben actuator.
    • Movie S5 (.mp4 format). SCP emulsion–printed kinesiology tape to measure elbow flexion.
    • Movie S6 (.mp4 format). Hybrid robotic textile (OmniSkin) giving rise to an inchworm motion.

    Files in this Data Supplement:

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