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A transparent, self-healing and high-κ dielectric for low-field-emission stretchable optoelectronics

Nature Materials volume 19, pages 182–188 (2020)Cite this article

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Abstract

Stretchable optoelectronic materials are essential for applications in wearable electronics, human–machine interfaces and soft robots. However, intrinsically stretchable optoelectronic devices such as light-emitting capacitors usually require high driving alternating voltages and excitation frequencies to achieve sufficient luminance in ambient lighting conditions. Here, we present a healable, low-field illuminating optoelectronic stretchable (HELIOS) device by introducing a transparent, high permittivity polymeric dielectric material. The HELIOS device turns on at an alternating voltage of 23 V and a frequency below 1 kHz, safe operating conditions for human–machine interactions. We achieved a brightness of 1,460 cd m−2 at 2.5 V µm−1 with stable illumination demonstrated up to a maximum of 800% strain. The materials also self-healed mechanically and electronically from punctures or when severed. We further demonstrate various HELIOS light-emitting capacitor devices in environment sensing using optical feedback. Moreover, our devices can be powered wirelessly, potentially enabling applications for untethered damage-resilient soft robots.

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Fig. 1: Design of the HELIOS optoelectronic device.
Fig. 2: Shape reconfigurability and low emission voltage of the conformable HELIOS optoelectronic device.
Fig. 3: Electro-mechanical properties of the HELIOS materials and the HELIOS LEC device.
Fig. 4: Self-healing of the HELIOS dielectric material.
Fig. 5: Self-healing of the HELIOS materials and the optoelectronic device.
Fig. 6: HELIOS-equipped soft robots.

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Data availability

The data that support the findings of this study are available from the authors on reasonable request.

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Acknowledgements

B.C.K.T. acknowledges support from the Singapore National Research Fellowship (NRFF2017-08), an NUS Start-up Grant and the Singapore National Robotics Programme (NRP 1822500053). J.Ho acknowledges support from National Research Foundation Singapore (NRFF2017-07) and an NUS Young Investigator Award. We thank Z. Goh for assistance with the illustration in Fig. 1a and W. Lee for photographs in Fig 1b. We thank A. Cheong, X. Tan and H. Li for access to testing equipment.

Author information

Author notes

  1. Yongqing Cai

    Present address: Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau, China

  2. These authors contributed equally: Hareesh Godaba, Ge Chen.

Authors and Affiliations

  1. Department of Materials Science and Engineering, National University of Singapore, Singapore, Singapore

    Yu Jun Tan, Hareesh Godaba, Ge Chen, Guanxiang Wan, Si Li & Benjamin C. K. Tee

  2. Institute of Innovation in Health Technology (iHealthtech), National University of Singapore, Singapore, Singapore

    Yu Jun Tan, Hareesh Godaba, Siew Ting Melissa Tan, Guojingxian Li, John S. Ho & Benjamin C. K. Tee

  3. Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore

    Pui Mun Lee, John S. Ho & Benjamin C. K. Tee

  4. Institute Of High Performance Computing, Agency for Science Technology and Research, Singapore, Singapore

    Yongqing Cai

  5. Mechanical & Aerospace Engineering, Cornell University, Ithaca, NY, USA

    Robert F. Shepherd

  6. N.1 Institute of Health, National University of Singapore, Singapore, Singapore

    John S. Ho & Benjamin C. K. Tee

  7. Institute of Materials Research and Engineering, Agency for Science Technology and Research, Singapore, Singapore

    Benjamin C. K. Tee

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Contributions

Y.J.T. and B.C.K.T conceived the idea. B.C.K.T. directed the research activities. Y.J.T. and B.C.K.T. designed and led the experiments (with input from H.G., G.C., S.T.M.T., G.X.W., G.L., P.M.L., Y.C., S.L., R.F.S. and J.H.). Y.J.T., H.G., G.C., G.X.W. and B.C.K.T. contributed to data analysis and interpretation. Y.J.T. and B.C.K.T. wrote the paper, and all authors provided feedback.

Corresponding author

Correspondence to Benjamin C. K. Tee.

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Extended data

Extended Data Fig. 1 Thickness effect of dielectric-EL on the brightness of HELIOS 70EL at the same E and fac.

(a-b) The brightness of HELIOS 70EL increases with the increase in thickness in the dielectric-EL (t = 100 µm vs t = 1500 µm) when comparing with the same E and frequencies applied. The measurements were repeated on three samples and the error bars represent the standard deviation. The luminance and the operated fields on the HELIOS devices are expressed using the equation of L = Lo exp(−b/E0.5). Insets show the photographs of the oblique view of the HELIOS device. (c-d) Photographs showing that the HELIOS-thick 70EL device is significantly brighter than the HELIOS-thin 70EL device.

Extended Data Fig. 2 Mechanical tests.

(a) The stress-strain curve of the HELIOS dielectric stretched at different speeds. The modulus increases when stretching at higher speed, which is the typical behaviour of supramolecular polymers22,36. (b) Photograph showing tensile bar specimen (Type V) following ASTM standard D638 with 7.62 cm (gauge length) x 3.18 cm (width). (c) Dielectric-EL sample and (d) three-layered HELIOS device samples were subjected to ten loading-unloading cycles (loading at 1 mm s−1 and unloading at 1 mm min−1). These materials recovered to its original length when subjected to a strain of 50% over time.

Extended Data Fig. 3 Demonstration of HELIOS-thin (t = 67 µm) vs HELIOS-thick (t = 1 mm) LEC devices stretched to various strains.

(a) HELIOS-thin can stretch to ~1.6 times its original length. Scale bar: 5 mm. (b) Photographs showing that HELIOS-thick activated at fac of 200 V and 50 Hz (vaguely visible), and the device is bright when operated at 800 V (and 1000 V) and fac of 50 Hz when operated under daylight condition. Scale bar: 5 mm. (c) The HELIOS-thick device stretches with an increase in light emission and subsequently exhibited stable illumination up to 9 times its original length.

Supplementary information

Supplementary information

Supplementary Figures 1–21, Tables 1–2, Notes 1–5 and refs. 1–18.

Supplementary Video 1

Demonstration of low activation voltage and high brightness: brightness in daylight. This video demonstrates that the luminance from the HELIOS device can be perceived by human eyes under daylight conditions even at low activation voltages, while the control (Ecoflex-EL) requires a much higher activation voltage. The brightness of the HELIOS device is comparable to the maximum brightness of a smartphone display when activated at a Vac of 200 V. Towards the end of the video, the HELIOS device is illuminated at a Vac of 200 V in daylight conditions and twisted, manually demonstrating its softness and flexibility .

Supplementary Video 2

Demonstration of the stretchability of the dielectric material. This video shows that our dielectric layer is elastic and can recover to its original length upon the removal of forces .

Supplementary Video 3

Interfacial bonding between materials. First, we demonstrate that the HELIOS dielectric-EL does not stick to another HELIOS dielectric-EL. Then, we show that the HELIOS dielectric-EL adheres very well to a HELIOS electrode. We cannot peel the materials from each other. We also show that the HELIOS electrode does not stick to silicone (the control material). In the final part of the video, we show that the HELIOS dielectric-EL does not stick to the conventionally used electrode for EL devices, which is PET coated with ITO. The ITO/PET electrode is very rigid and not as transparent.

Supplementary Video 4

Demonstration of dielectric material properties: non-tacky surface. The dielectric is transparent and not tacky. The surface of the material is not self-sticking.

Supplementary Video 5

Demonstration of dielectric material properties: self-healing part A (tensile test). A dielectric material (~1 mm thick) was cut into a tensile coupon. The specimen was then cut in half, and the cut surfaces pressed back together with ~0.08 N force; the material was left to self-heal for 24 h. The video shows the healed material being pulled till fracture.

Supplementary Video 6

Demonstration of dielectric material properties: self-healing part B. After a thick dielectric material (~2 mm thick) is cut in half and the cut surfaces are pressed back together, the material starts to self-heal in one minute. The material formed at the interface is observed when we pull the two ends of the bifurcated and healed sample. After the cut-and-pressed sample is left long enough, the material heals completely. The healed sample does not fail at the cut interface when stretched until it is stretched to the original point of failure, indicating that the healed interface recovers the original toughness.

Supplementary Video 7

Punctures and repeatable healing of a HELIOS LEC device. We show that when using a non-self-healing dielectric-EL (PDMS is used as the dielectric here), optical healing is not achieved. The operating voltage and frequency for this device were Vac = 3,000 V and fac = 800 Hz. When punctured by a needle (27 gauge), the current through the needle is large and quickly generates heat to deteriorate the dielectric, causing sparks. The sample burns and breaks down. In contrast to the control, the HELIOS device can be illuminated at Vac = 1,700 V and fac = 800 Hz with the same brightness as the control. The HELIOS device can also remarkably self-heal to regain its optical performance. When punctured by a needle, the luminance dims around the pierced region. However, immediately after removing the needle, the optical healing effect is observed. We further puncture the same sample and repeat the same procedure a few times with thicker needles (26 gauge, 20 gauge). The HELIOS device self-heals immediately (with no time lag for healing) from every puncture. Then, we illuminate the HELIOS device at the same electrical conditions as the control, that is, Vac = 3,000 V and fac = 800 Hz. The specimen that is punctured multiple times still works, with brighter illuminance, as expected. After being punctured by a needle (27 gauge), the sample can still illuminate at the same brightness. In the HELIOS device, even though there is a short circuit between the electrodes, the amount of current flowing through the needle and the subsequent heat generated due to the flow of current are small enough not to damage the device. Moreover, the HELIOS device self-heals from damage. These reasons show the robust performance when punctured with a needle.

Supplementary Video 8

Soft gripper with a HELIOS device: light sensing of objects in the dark. In the first part of the video, the bottom part of the soft gripper is shown. There are three HELIOS devices and one photodetector at the bottom of the gripper. When the gripper is searching for objects, it lights up at Vac = 200 V and fac = 50 Hz; when it is ready to grasp an object, it will blink at Vac = 200 V and fac = 5 Hz. Later in the video, we demonstrate that the soft gripper can seek and subsequently grasp an apple.

Supplementary Video 9

Soft gripper with puncture-resistant HELIOS device. A HELIOS device embedded in a soft gripper can also self-heal in optical performance. When punctured by a needle, the luminance dims around the pierced region. Immediately after removing the needle, the optical healing effect is observed, with the sample recovering its initial brightness. Later in the video, we show the soft gripper grasping a balloon. We pierce the gripper through the HELIOS device until we burst the balloon being grasped. The puncture-resistant soft gripper and HELIOS device are both functional after the perforation.

Supplementary Video 10

Wirelessly operated HELIOS soft gripper. Silicone-EL (left) and HELIOS (right) LEC devices were embedded inside the bottom of a gripper. A “wireless disc” is on top of the gripper. A power harvester connected to a control circuit was all fabricated and soldered on flexible PCBs, subsequently encapsulated in the wireless disc. A yellow LED is connected in the circuit to indicate that wireless power is being transmitted to the LEC devices. When an external radiofrequency coil is brought close to the silicone-EL gripper, the yellow LED lights up, indicating that power was transferred to the silicone-EL. However, the voltage provided is not enough to illuminate the LEC. Conversely, the HELIOS device can be illuminated.

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Tan, Y.J., Godaba, H., Chen, G. et al. A transparent, self-healing and high-κ dielectric for low-field-emission stretchable optoelectronics. Nat. Mater. 19, 182–188 (2020). https://doi.org/10.1038/s41563-019-0548-4

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