Research ArticleAPPLIED SCIENCES AND ENGINEERING

Mechanically transformative electronics, sensors, and implantable devices

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Science Advances  01 Nov 2019:
Vol. 5, no. 11, eaay0418
DOI: 10.1126/sciadv.aay0418
  • Fig. 1 Schematic illustrations and images of a TES that can shift its form from a conventional rigid electronic device to a soft, stretchable electronics format, or vice versa, through a temperature-dependent phase transition.

    (A) Conceptual illustration of the key feature of the system. A flat, rigid, and handheld electronic device becomes soft and stretchable upon integration onto the skin due to the effect of body temperature and thus converts into a wearable device that can measure physiological signals [e.g., electromyogram (EMG) and electrocardiogram]. Similarly, the device can recover its original rigidity when removed from the body to decrease its temperature. (B) Exploded-view schematic diagram illustrating the system architecture. (C) Images of a TES demonstrating its mechanical characteristics in the rigid (top) and soft (bottom) mode. The inset of the far left image in the rigid mode shows a magnified view of the region indicated by the red dashed box, highlighting a stretchable gold electrode integrated with the gallium-based transformative platform. Scale bars, 1 cm. Photo credit: Jae-Woong Jeong.

  • Fig. 2 Thermomechanical studies of the phase transition characteristics of the gallium-based transformative platform.

    (A) Images comparing the mechanical characteristics of a transformative LED device in a rigid (solid gallium) and soft (liquid gallium) state, powered through conductive gallium wires, when applied with a uniaxial strain by 300-g weights. (B to D) Effective elastic moduli (B) and stress-strain curves (C and D) of testbed platforms in the rigid (C) and soft state (D). Red, green, blue, and black lines in the plots indicate a testbed platform with the gallium thickness of 50, 320, 640, and 1500 μm, respectively. In all samples, the width of gallium frame is 790 μm, and the encapsulating polymer has dimensions of 5.81 mm (w) by 2.12 mm (t) (see fig. S2). The solid and dashed lines in (B) represent experimental data and theoretically calculated values, respectively. (E and F) Plot illustrating the phase transition of gallium-based transformative platform as it is thawed with a temperature of 45°C (E) and associated IR images at phases I, II-III, III-IV, and IV (F). The thawing process is divided into three phases: I-II, solid; II-III, solid-liquid transition; and III-IV, liquid. (G and H) Plot illustrating the phase transition of the gallium-based transformative platform during freezing with an applied temperature of 5°C (G) and the corresponding IR images at phases I, II*, II-III, and IV (H). The freezing process is divided into four phases: I-I*, liquid; I*-II*, supercooled liquid; II*-III, liquid-solid transition; and III-IV, solid. II* indicates a nucleation temperature. (I) Temperature changes at the gallium-silicone interface induced by the thawing (35°C; solid lines) and freezing temperatures (15°C; dashed lines) applied through different thicknesses of silicone encapsulants (red, 30 μm; green, 200 μm; and blue, 500 μm). (J) Time required for phase transition [i.e., duration for II-III in (E)] as a function of thawing temperature (Tth) for testbed platforms made with a 1.5-mm-thick gallium frame encapsulated in 30-μm-thick (red), 200-μm-thick (green), and 500-μm-thick (blue) silicone. (K) Degree of supercooling (∆T) of gallium as a function of Tth. (L) Characteristic time for gallium to reach the nucleation temperature (tc1) and time required for the phase transition (tc2) as a function of ∆T during the freezing process in (G). (M and N) Time required to switch the transformative platforms (gallium thickness, 50, 500, and 1500 μm; silicone encapsulant thickness, 30, 200, and 500 μm) from the rigid to soft state when applying Tth of 35°C (M) and 45°C (N). (O and P) Characteristic transition time (tc) for the transformative platform used in (M) and (N) to transform from the soft to rigid state when applying freezing temperatures (Tfr) of 15°C (O) and 5°C (P). Note that the soft-state platform in (O) and (N) was prepared by applying Tth of 35°C for 20 min before measuring tc in freezing.

  • Fig. 3 Application demonstration of transformative electronics that can convert between a rigid tabletop clock and a stretchable wearable sensor.

    (A) Exploded-view schematic diagram that illustrates a TES that includes a gallium-silicone composite material as a transformative platform, a heater as an actuator for accelerated phase transition from a rigid to soft state, a stretchable PCB consisting of a temperature sensor, a UV sensor, an OLED screen, capacitive touch sensors, and a microcontroller for environmental and physiological sensing. (B) Illustration of the assembled device. The top inset shows a cross-sectional view. (C) IR images showing the mechanical transformation of the device triggered by body temperature upon mounting on the skin, which leads to conversion from a rigid, flat to soft, flexible form that enables conformal contact on the curved surface of the skin. (D) Temperature at the bare heater surface (red), the heater-gallium interface (green), and the device-skin interface (blue) when operating the heater with a 3.3-V coin cell battery. The operation of the heater can accelerate the phase transition of gallium while maintaining a biologically safe temperature (<45°C) at the device-skin interface. (E) Bending stiffness of the device as a function of temperature. (F) Overlay of optical images and FEA results for the device in a soft state under uniaxial strain of 0% (left) and 26% (right). (G and H) Application demonstration of the transformative electronics as a tabletop clock in the rigid mode (G) and as a wearable sensor in the soft mode (H). The sensor on the contracted arm stretched 15% because of muscle volume expansion. (I and J) Application demonstration of the device in different scenarios (I) and use of integrated sensors for the measurement of room (in a tabletop clock mode) or body temperature (in a wearable mode) and UV index for associated situations indicated by numbers (J). Photo credit: Sang-Hyuk Byun (C, F, G, and I) and Raza Qazi (H).

  • Fig. 4 Application demonstration of transformative electronics for pressure sensing, tuning both the sensitivity and dynamic range for versatile operation.

    (A) Conceptual illustration of a tunable pressure sensor based on an elastomeric composite with gallium-microdroplet inclusion. In the rigid mode, the microporous dielectric elastomer filled with solid gallium undergoes small compressive deformation due to its high effective modulus leading to large dynamic range for pressure sensing. In the soft mode, the microporous elastomer with liquefied gallium becomes much more compliant, resulting in higher sensitivity and lower dynamic range. (B) Photographs of a gallium-elastomer composite in the rigid (top) and soft (bottom) states under compression by a fingertip. (C) Cross-sectional optical micrographs of the gallium-elastomer composite in the rigid (top) and soft (bottom) states under compression, corresponding to the dotted rectangular areas in (B). (D) Pressure versus strain curves of the composite in the soft (red) and rigid modes (blue) in comparison to that of a pure elastomer (black). The inset shows magnified plots for the pressure range between 0 and 12 kPa. (E) Relative capacitance changes of the tunable pressure sensor with applied pressure in the rigid and soft modes. Red, blue, and black lines are plots for the composite in the soft and rigid modes and the pure elastomer, respectively. (F) FEA simulation results demonstrating the stress distribution in the composite in the soft (left) and rigid modes (right) under a compressive pressure of 50 kPa. The diameter of the gallium microdroplets is assumed to be 125 ± 50 μm (mean ± SD) based on our measurements (fig. S7E). (G and K) Cyclic capacitive response of a sensor (red) to applied pressure [1 MPa for the rigid mode (G) and 0.22 MPa for the soft mode (K); black] as a function of time in the rigid (G) and soft modes (K). The speed of loading and unloading was 0.167 mm/s in (G) and (K). (H and L) Loading and unloading test in the rigid (H) and soft (L) modes for 1200 cycles. (I and J) Image (I) and measurement result (J) demonstrating the application of a pressure sensor in the rigid mode for foot stepping pressure sensing. (M and N) Image (M) and measurement result (N) demonstrating the application of a pressure sensor in the soft mode for monitoring of carotid arterial blood pressure. Photo credit: Sang-Hyuk Byun.

  • Fig. 5 Deep-tissue penetration with transformative electronics is better tolerated than rigid materials.

    (A) Conceptual illustration of the fabrication scheme (left) and photographs (right) of the TES neural probe in rigid and soft modes. (B) Photographs of deep brain injection of the TES-based probe in rigid (left and center) and soft (right) modes. (C and D) Fluorescence images of 35-μm horizontal slices stained for astrocytes [glial fibrillary acidic protein (GFAP), purple] and activated microglia (Iba1, yellow) from tungsten (C) and TES (D) neural probes. Scale bars, 50 μm. (E to G) Cross sections through the tissue show reduced lesion area (E) and astrocytic (F) and microglial (G) responses from the TES compared to tungsten implants following 4 weeks of implantation (data are means ± SEM, n = 3 to 5 animals per implant; *P < 0.05, **P < 0.01, ***P < 0.001, unpaired two-tailed t test). a.u., arbitrary units. (H) Cartoon of optogenetic and TES device targeting strategy. (I) Fluorescent micrograph of PVH section showing ChR2-eYFP expression (green) and TES probe implantation site (yellow dashed outline). Scale bars, 10 μm. (J) One hour of 20-Hz photostimulation of ARCAgRP-PVH increases food consumption compared to 1 hour before stimulation [PVH: n = 6 mice, 4 AgRP-Cre × Ai32 and 2 AgRP-Cre with viral ChR2 transduction; non-PVH: n = 4; one-way analysis of variance (ANOVA) with Tukey’s multiple comparisons: PVH OFF versus PVH ON, **P < 0.01; non-PVH ON versus PVH OFF, **P < 0.01]. (K) Example heatmap of locomotor activity near the food container (hexagon) before (top) and during (bottom) photostimulation. (L) Photos of a functional TES neural probe retrieved 6 weeks after implantation. Scale bars, 500 μm. Photo credit: Sang-Hyuk Byun (A), Kyle E. Parker (I), Graydon B. Gereau (C and D), and Jordan G. McCall (B and L).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/11/eaay0418/DC1

    Note S1. Theoretical analysis of phase transition time for thawing

    Note S2. Fabrication of transformative electronics with flexible, stretchable electrode arrays

    Note S3. Fabrication of transformative electronics integrated with a stretchable PCB

    Note S4. Fabrication of neural probe with variable stiffness

    Fig. S1. Design and application of a transformative electronics that can convert between an EMG sensor and a handheld touch sensor.

    Fig. S2. Design of testbed transformative platforms for mechanical studies.

    Fig. S3. Characterization of the degree of supercooling.

    Fig. S4. Thermal characterization of gallium-based transformative platform using IR camera during thawing process and freezing process.

    Fig. S5. Design of transformative electronics that can convert between a rigid tabletop clock and a stretchable wearable sensor with characterization.

    Fig. S6. Mechanical simulation of bending stiffness of the transformative electronics that appeared in Fig. 3E.

    Fig. S7. Fabrication process of a transformative pressure sensor.

    Fig. S8. Transformative resistive pressure sensor built with a gallium frame.

    Fig. S9. Response time of the pressure sensor in rigid and soft mode.

    Fig. S10. Design and implementation of TES optical neural probe for behavioral experiments with wireless control.

    Movie S1. Movie of a transformative electronics transforming between a rigid tabletop clock and a wearable sensor.

    Movie S2. Movie of a transformative pressure sensor with variable deformability.

    Movie S3. Movie of a transformative pressure sensor: Application demonstration in rigid and soft modes.

    Movie S4. Movie of a transformative neural probe penetrating a mouse brain in rigid mode.

  • Supplementary Materials

    The PDF file includes:

    • Note S1. Theoretical analysis of phase transition time for thawing.
    • Note S2. Fabrication of transformative electronics with flexible, stretchable electrode arrays.
    • Note S3. Fabrication of transformative electronics integrated with a stretchable PCB.
    • Note S4. Fabrication of neural probe with variable stiffness.
    • Fig. S1. Design and application of a transformative electronics that can convert between an EMG sensor and a handheld touch sensor.
    • Fig. S2. Design of testbed transformative platforms for mechanical studies.
    • Fig. S3. Characterization of the degree of supercooling.
    • Fig. S4. Thermal characterization of gallium-based transformative platform using IR camera during thawing process and freezing process.
    • Fig. S5. Design of transformative electronics that can convert between a rigid tabletop clock and a stretchable wearable sensor with characterization.
    • Fig. S6. Mechanical simulation of bending stiffness of the transformative electronics that appeared in Fig. 3E.
    • Fig. S7. Fabrication process of a transformative pressure sensor.
    • Fig. S8. Transformative resistive pressure sensor built with a gallium frame.
    • Fig. S9. Response time of the pressure sensor in rigid and soft mode.
    • Fig. S10. Design and implementation of TES optical neural probe for behavioral experiments with wireless control.
    • Legends for movies S1 to S4

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Movie of a transformative electronics transforming between a rigid tabletop clock and a wearable sensor.
    • Movie S2 (.mp4 format). Movie of a transformative pressure sensor with variable deformability.
    • Movie S3 (.mp4 format). Movie of a transformative pressure sensor: Application demonstration in rigid and soft modes.
    • Movie S4 (.mp4 format). Movie of a transformative neural probe penetrating a mouse brain in rigid mode.

    Files in this Data Supplement:

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