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Strain-invariant stretchable radio-frequency electronics

Abstract

Wireless modules that provide telecommunications and power-harvesting capabilities enabled by radio-frequency (RF) electronics are vital components of skin-interfaced stretchable electronics1,2,3,4,5,6,7. However, recent studies on stretchable RF components have demonstrated that substantial changes in electrical properties, such as a shift in the antenna resonance frequency, occur even under relatively low elastic strains8,9,10,11,12,13,14,15. Such changes lead directly to greatly reduced wireless signal strength or power-transfer efficiency in stretchable systems, particularly in physically dynamic environments such as the surface of the skin. Here we present strain-invariant stretchable RF electronics capable of completely maintaining the original RF properties under various elastic strains using a ‘dielectro-elastic’ material as the substrate. Dielectro-elastic materials have physically tunable dielectric properties that effectively avert frequency shifts arising in interfacing RF electronics. Compared with conventional stretchable substrate materials, our material has superior electrical, mechanical and thermal properties that are suitable for high-performance stretchable RF electronics. In this paper, we describe the materials, fabrication and design strategies that serve as the foundation for enabling the strain-invariant behaviour of key RF components based on experimental and computational studies. Finally, we present a set of skin-interfaced wireless healthcare monitors based on strain-invariant stretchable RF electronics with a wireless operational distance of up to 30 m under strain.

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Fig. 1: Strain-invariant stretchable wireless system enabled by dielectro-elastic composite.
Fig. 2: Fabrication and characterization of DEE.
Fig. 3: Design and characterization of strain-invariant stretchable RF devices.
Fig. 4: Skin-interfaced wireless system based on strain-invariant RF devices.
Fig. 5: Strain-invariant wearable bionic bands for whole-body physiological monitoring.

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Data availability

The data supporting the findings of this study are available in the article and the Supplementary Information. Further raw data may be obtained from the corresponding author on request. Source data are provided with this paper.

Code availability

The code that produced the findings of this study is available on GitHub (https://github.com/JooHwanS/nrf52832_BLE). Further information can be requested from one of the corresponding authors.

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Acknowledgements

This work was supported by the Institute of Information & Communications Technology Planning & Evaluation (IITP) under the artificial intelligence semiconductor support program to nurture the best talents IITP-(2024)-RS-2023-00253914 grant funded by the Ministry of Science and ICT (MSIT) of the Korean government, the IITP grant IITP-2022-0-00310 funded by the MSIT of the Korean government and National Research Foundation of Korea (NRF) grants NRF-2022R1C1C1003994 and NRF-2019M3C7A1032076 funded by the MSIT of the Korean government.

Author information

Authors and Affiliations

Authors

Contributions

S.H.K. and Y.H.J. conceived the idea and designed the research. S.H.K., A.B., H.Y. and Y.H.J. analysed the data and wrote the manuscript. A.B. and H.Y. designed the electromagnetic devices and characterization. S.H.K., A.B., J.L., S.W.H., G.C., J.J. and K.-S.C. carried out the measurement and fabrication. R.A. performed the permittivity and mechanical simulations. J.H.S. designed the graphical user interface for demonstrations. S.H.K., A.B., R.A., J.L., S.W.H., G.C., J.H.H., S.Y.P., J.J., C.L., J.C., B.L., K.-S.C., S.J., T.-i.K., H.Y. and Y.H.J. carried out the technical revisions of the manuscript.

Corresponding authors

Correspondence to Hyoungsuk Yoo or Yei Hwan Jung.

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Extended data figures and tables

Extended Data Fig. 1 Comparison of dielectric properties between dielectro-elastic materials.

a, Graph showing the permittivity change of the conventional elastomer and three kinds of BaTiO3 composites (BaTiO3 μP, BaTiO3 NP, DEE) under an applied strain at 13.56 MHz. b, Bar graph comparing the permittivity changes in each composite at 13.56 MHz. The DEE shows the best performance. c, Dielectric losses of the conventional elastomer and each composite at 13.56 MHz. The dielectric loss decreases when the particles are embedded in the elastomer. d,e, Permittivity decreases of the conventional elastomer and each composite at 2.4 GHz. f, The same trends at 13.56 MHz.

Extended Data Fig. 2 Permittivity change under biaxial strain.

a, Permittivity decrease of conventional elastomer and DEE under applied biaxial strain of 30%. b, Bar graph showing that the permittivity changes are 0.02, 1.95 and 2.5 in conventional elastomer, DEE at uniaxial strain and DEE at biaxial strain, respectively.

Extended Data Fig. 3 Simulation showing the geometrical effect on permittivity decrease.

a, Simulation graph showing the permittivity decrease of nanocomposites in two cases: (1) composite embeds rigid particles and (2) composite embeds deformable clusters. Compared with rigid particles, deformable clusters, which show geometrical deformation from sphere to ellipsoid, reduce the permittivity of nanocomposite substantially under applied strain. b, Schematic of the simulation results showing capacitance change of each case under applied strain. Simulation graph (c) and schematic results (d,e) showing the permittivity change of various substrates, including Ecoflex, BaTiO3 μP, BaTiO3 NP and DEE.

Extended Data Fig. 4 Performance of the RF devices as the dissipation factor of the substrate material.

a, Simulated return losses of a microstrip patch antenna with respect to frequency plotted for increasing loss tangent. b, Calculated normalized radiation efficiency at 2.4 GHz with respect to loss tangent of the substrate. The blue dot indicates our work, showing normalized efficiency of 90% at a loss tangent value of 0.007. c, Calculated peak gain of the antenna at 2.4 GHz with respect to the loss tangent of the substrate. The blue dot indicates our work, showing antenna gain of 2.1 dBi at a loss tangent value of 0.007. d, The wireless power-transfer efficiency of the near-field coil as a function of the loss tangent of the substrate. The blue dot indicates our work, showing power-transfer efficiency of 48% at a loss tangent value of 0.007. e, The wireless power-transfer efficiencies of the near-field coil with respect to frequency plotted for increasing loss tangent. f, Transmission coefficient of the transmission line with respect to frequency plotted for increasing loss tangent.

Extended Data Fig. 5 Stereomicroscopic images and dielectric properties of various DEE composites.

Graphs showing permittivity change (a) and dielectric loss (b) of the conventional elastomer and DEE with diverse high-κ NPs under an applied strain. Effective permittivity (c) and dielectric loss (d) in far-field frequencies ranging from 2 to 3 GHz. Optical microscopic images showing DEE with SrTiO3 (e) and Al2O3 (f). Spherical clusters are shown in the polymer matrix before stretching (top) and the geometric change of the clusters occur under the applied strain (bottom). Scale bars, 100 μm.

Extended Data Fig. 6 Mechanical simulation showing stress distribution under strain in different cluster moduli.

a, Simulation showing the expected Young’s modulus of a nanocomposite embedded with high-modulus rigid inclusions in different volume fractions. b, Contour mapping of simulation results according to modulus and volume fraction of inclusions. c, Simulation showing distribution of stress in two cases: (1) viscoelastic inclusion (top) and (2) rigid inclusion (bottom) in nanocomposite under an applied strain of 30%. d, Stress distributions of various composites in different cases, including BaTiO3 µP, BaTiO3 NP, DEE and Ecoflex. The stress applied to the polymer matrix is expected to decrease as the particles become smaller or form clusters.

Extended Data Fig. 7 Effect of volume fraction of NPs on permittivity change under strain.

a, Graph showing the decrease in permittivity under an applied strain at different volume fractions. b, Graph showing that the stretchability decreases and the initial permittivity increases as the volume fraction of the NPs increases. Optical microscopic images of DEE before and after stretching at 1 vol% (c) and 10 vol% (d). The BaTiO3 clusters are deformed from spherical to ellipsoidal shapes under an applied strain of 30%.

Extended Data Fig. 8 Performance comparison of different composites in terms of five standards.

Five standards that are essential properties for stretchable wireless electronics include permittivity change (Δε), loss tangent, stretchability, thermal conductivity and Young’s modulus. The DEE composite exhibits excellent properties in all aspects.

Extended Data Fig. 9 Wireless power-transfer efficiency (η) of near-field coils fabricated on DEE and Ecoflex.

a, Schematic of a strain-invariant near-field coil-based wireless power-transfer system. b, Power-transfer efficiency (η) of the wireless power-transfer system based on conventional elastomer in its unstretched (0% strain) state at different separation distances. The efficiency decreases with increasing separation distances. c, Transmission coefficients (S21) of the wireless power-transfer system on conventional elastomer at different separation distances at 0% strain. d,e, Comparison of the wireless power-transfer efficiency (η) and transmission coefficients (S21) of the wireless power-transfer system on conventional elastomer analysed at different separation distances at 30% strain. The wireless power-transfer efficiency (η) and transmission coefficients (S21) decrease with increasing separation distances. f,g, Comparison of wireless power-transfer efficiencies (η) and transmission coefficients (S21) of the wireless power-transfer system on DEE substrate analysed at different separation distances at 0% strain. The wireless power-transfer efficiency (η) and transmission coefficients (S21) decrease with increasing separation distances. h,i, Comparison of wireless power-transfer efficiencies (η) and transmission coefficients (S21) of the wireless power-transfer system on DEE substrate analysed at different separation distances at 30% strain. The wireless power-transfer efficiency (η) and transmission coefficients (S21) decrease with increasing separation distances. However, unlike the wireless power-transfer system on conventional elastomer, the wireless power-transfer efficiency (η) and transmission coefficients (S21) of the wireless power-transfer system on DEE substrate remain strain-invariant under strains (0% and 30%).

Extended Data Fig. 10 RF characteristics of stretchable wireless devices under biaxial strain.

a, Schematic of the biaxial strain of strain-invariant stretchable patch antennas fabricated on DEE. b,c, S-parameters (S11) of the stretchable patch antennas fabricated on the conventional elastomer and DEE substrates at applied biaxial strains of 0, 15 and 30%. The resonance of the Ecoflex-based stretchable antenna shifts from 2.4 GHz to 1.8 GHz. However, the DEE-based stretchable patch antenna resonance is strain-invariant. d, Schematic of biaxial stretching of the near-field coil. e,f, Examination of S-parameters (specifically S11) of stretchable near-field coils based on conventional elastomers and DEE substrates at 0, 15 and 30% biaxial strains. The resonance frequency of the stretchable near-field coil made with Ecoflex material shifts from 13.56 MHz to 10.80 MHz under strain. However, the resonance frequency of the DEE-based stretchable near-field coil remains unchanged despite the applied strain. g, Schematic of the biaxial stretching of stretchable coplanar transmission lines. h,i, Comparison of S-parameters (S11 and S21) of stretchable coplanar transmission lines created with conventional elastomer and DEE substrates at 0, 15 and 30% biaxial strains. The resonance frequency of the stretchable transmission line that is created with the Ecoflex material shifts S11 from 4 GHz to 2.5 GHz under biaxial strains ranging from 0% to 30%. By contrast, the change in S-parameters (S11 and S21) of the DEE-based stretchable transmission lines is negligible at 0, 15 and 30% biaxial strains.

Supplementary information

Supplementary Information

This file contains Supplementary Notes 1–14; Supplementary Figs. 1–42; legends to Supplementary Videos and Supplementary References.

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Supplementary Video 1

Agglomeration of BaTiO3 NPs in DCM solution by adding polar solvent.

Supplementary Video 2

Wireless disconnection of Ecoflex-based antenna under an applied strain of 30%.

Supplementary Video 3

Continuous wireless connection of DEE-based antenna under an applied strain of 30%.

Supplementary Video 4

Real-time operation of the stretchable wireless system.

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Kim, S.H., Basir, A., Avila, R. et al. Strain-invariant stretchable radio-frequency electronics. Nature 629, 1047–1054 (2024). https://doi.org/10.1038/s41586-024-07383-3

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