Skin electronics from scalable fabrication of an intrinsically stretchable transistor array

Abstract

Skin-like electronics that can adhere seamlessly to human skin or within the body are highly desirable for applications such as health monitoring1,2, medical treatment3,4, medical implants5 and biological studies6,7, and for technologies that include human–machine interfaces, soft robotics and augmented reality8,9. Rendering such electronics soft and stretchable—like human skin—would make them more comfortable to wear, and, through increased contact area, would greatly enhance the fidelity of signals acquired from the skin. Structural engineering of rigid inorganic and organic devices has enabled circuit-level stretchability, but this requires sophisticated fabrication techniques and usually suffers from reduced densities of devices within an array2,10,11,12. We reasoned that the desired parameters, such as higher mechanical deformability and robustness, improved skin compatibility and higher device density, could be provided by using intrinsically stretchable polymer materials instead. However, the production of intrinsically stretchable materials and devices is still largely in its infancy13,14,15: such materials have been reported11,16,17,18,19, but functional, intrinsically stretchable electronics have yet to be demonstrated owing to the lack of a scalable fabrication technology. Here we describe a fabrication process that enables high yield and uniformity from a variety of intrinsically stretchable electronic polymers. We demonstrate an intrinsically stretchable polymer transistor array with an unprecedented device density of 347 transistors per square centimetre. The transistors have an average charge-carrier mobility comparable to that of amorphous silicon, varying only slightly (within one order of magnitude) when subjected to 100 per cent strain for 1,000 cycles, without current–voltage hysteresis. Our transistor arrays thus constitute intrinsically stretchable skin electronics, and include an active matrix for sensory arrays, as well as analogue and digital circuit elements. Our process offers a general platform for incorporating other intrinsically stretchable polymer materials, enabling the fabrication of next-generation stretchable skin electronic devices.

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Figure 1: Intrinsically stretchable transistor array as a core platform for functional skin electronics.
Figure 2: Platform for fabricating intrinsically stretchable transistor arrays.
Figure 3: Electrical performance and stretchability of the intrinsically stretchable transistor array.
Figure 4: Intrinsically stretchable circuits for skin electronics.

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Acknowledgements

This work is supported by Samsung Electronics. We thank L. Beker, N. Matsuhisa and S. Chen for helping with experiments, and L. Jin for discussions about mechanical simulation. J.L. acknowledges support by the National Science Foundation Graduate Research Fellowship Program under grant DGE-114747. S.-K.K. thanks NETEP and MOTIE of the Republic of Korea (grant 20173010013000).

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Contributions

S.W., J.X. and Z.B. designed the project and experiments. S.W., J.X. and W.W. fabricated the intrinsically stretchable transistor array and circuits, and carried out electrical characterizations. G.-J.N.W. synthesized the azide compound. R.R. carried out mechanical simulations. S.W. and F.M.-L. undertook the inkjet printing of a semiconducting polymer. V.R.F. carried out X-ray photoelectron spectroscopy characterizations. J.W.C., A.M.F. and A.E. helped to prepare schematics for the three-dimensional transistor array and carried out device photography. J.X. and J.L. did the mechanical characterizations. S.-K.K. and A.G. provided conjugated polymers. T.L. helped with development of semiconductor patterning. S.N., Y.K., Y.Y. and B.M. helped with circuit design and measurements. S.W., Z.B., J.X. and J.B.-H.T. wrote the manuscript. All authors reviewed and commented on the manuscript.

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Correspondence to Zhenan Bao.

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Extended data figures and tables

Extended Data Figure 1 A direct photo-patterning process for fabricating stretchable dielectrics.

Top, diagram of the stretchable dielectric undergoing ultraviolet-triggered azide crosslinking through a mask (grey). Bottom, detailed steps of the process.

Extended Data Figure 2 A photo-patterned SEBS film constitutes a solvent-resistant, stretchable dielectric layer for organic thin-film transistors.

a, Optical microscope images (left, bright field; top right, dark field) of a photo-patterned SEBS film. Bottom right, an image of the surface taken using an atomic-force microscope (AFM), showing the very small roughness value (Ra = 0.468 nm) produced after patterning. b, Top, diagram showing an azide-crosslinked SEBS film with semiconductor on top, constituting an organic field-effect transistor. Bottom, chemical structure of the semiconductor layer in the diagram above. We made these transistors to test the solvent resistance of the azide-patterned SEBS film, to ensure that it could allow direct spin-coating of semiconductor solution (with chlorobenzene as the solvent) on top. Channel length, 50 μm; channel width, 1,000 μm; gate dielectric capacitance, 1.75 nF cm−2. c, Transfer curve of a transistor that was obtained by transferring semiconductor film onto azide-patterned SEBS. In this case, the semiconductor film was obtained by spin-coating its solution onto an OTS-treated SiO2 surface. d, Transfer curve for a transistor that was obtained by directly spin-coating the semiconductor onto azide-patterned SEBS. The similar mobility (μ) in c and d indicates that the semiconductor can be spin-coated directly onto the solvent-resistant, azide-patterned SEBS. e, Transfer curve for a transistor with the semiconductor film transferred onto unpatterned (uncrosslinked) SEBS. This semiconductor film was obtained by spin-coating its solution onto an OTS-treated SiO2 surface. The similar mobility in c and e indicates that azide crosslinking does not change the ability of SEBS to function as a dielectric. f, Transfer curve for a transistor with the semiconductor film directly spin-coated onto unpatterned (uncrosslinked) SEBS, during which the SEBS dielectric is completely destroyed by the solvent.

Extended Data Figure 3 Etching-based patterning for stretchable semiconductors.

a, Detailed patterning steps. b, Diagram of the CONPHINE semiconducting film, which is composed of 70 wt% SEBS (x/y = 12:88) and 30 wt% semiconductor phase, with both chemical structures shown on the right. c, Optical microscopic image of the patterned CONPHINE film aligned on top of SEBS patterns; scale bar, 700 μm.

Extended Data Figure 4 Test of the etching-based patterning method on different stretchable semiconductors.

a, CONPHINE film. b, c, Its electrical performance in a thin-film transistor before (b) and after (c) patterning. d, A conjugated polymer (DPP2TTVT-PDCA) with hydrogen bonding that is built from 2,6-pyridine dicarboxamide (PDCA) moieties. e, f, Its electrical performance in in a thin-film transistor before (e) and after (f) patterning. All the thin-film transistors for testing have the device structure shown in Extended Data Fig. 2b.

Extended Data Figure 5 Inkjet-printed polymer semiconductor pattern on an azide-crosslinked SEBS dielectric.

a, Optical microscope images of a typical printed pattern of 29-DPP-SVS-(2). i, Bright field. ii, Dark field. iii, iv, Cross-polarized images. b, Transfer characteristics of a transistor made from such an inkjet-printed semiconductor pattern, with thermally evaporated gold for the top contact electrodes. The transistor gives ideal transfer behaviour with no hysteresis. The obtained average mobility is 0.072 cm2 V−1 s−1. c, Corresponding output curve for this transistor. Here, even though its drop-casting-like deposition and crystallization of the semiconductor polymer may not give the best electrical performance (or, possibly, stretchability), the inkjet printing process is convenient as it involves fewer steps. d, Optical microscopic image of printed 29-DPP-SVS-(2) patterns aligned on top of SEBS patterns. Scale bar, 700 μm. e, Initial mobility and stretchability (the latter reflected by mobility at 100% strain along the charge-transport direction) of 29-DPP-SVS-(2) patterns obtained by inkjet printing, compared with patterns obtained by spin-coating with copper-protected etching. Inkjet printing gives slightly lower initial mobility than spin-coating, but slightly better stretchability.

Extended Data Figure 6 Morphology and performance optimization of a CONPHINE film directly spin-coated on an azide-crosslinked SEBS surface, through surface modification with OTS.

a, b, Contact angles of water on unmodified azide-crosslinked SEBS (a) and OTS-modified azide-crosslinked SEBS (b). c, d, Surface morphology (left, optical microscopic images; right, AFM images) of CONPHINE film on unmodified azide-crosslinked SEBS (c) and OTS-modified azide-crosslinked SEBS (d). e, f, XPS characterization of sulfur/carbon peak height ratios, showing the vertical distribution of the semiconductor phase in CONPHINE films obtained on unmodified azide-crosslinked SEBS (e) and on OTS-modified azide-crosslinked SEBS (f). Given that sulfur appears only in conjugated polymers, the ratio of the sulfur 2p to carbon 1s peaks qualitatively reflects the amount of conjugated polymer at different depths. g, h, Performance of transistor arrays fabricated on Si/SiO2 substrates with CNTs as source/drain electrodes, spin-coated CONPHINE film as the semiconductor layer, and unmodified azide-crosslinked SEBS (g) or OTS-modified azide-crosslinked SEBS (h) as the dielectric layer.

Extended Data Figure 7 Fabrication and structure of an intrinsically stretchable, active-matrix tactile sensor array.

a, Fabrication steps. b, Two-dimensional device structure. c, Three-dimensional device structure. d, Optical microscope image and circuit diagram of one pixel in the stretchable active-matrix tactile sensor array, showing the connection between the transistor and the corresponding tactile sensor. With the tactile sensors connected between the drain lines and the transistor drain electrodes through the dielectric’s via holes, their resistance change from contact with conductive objects (including human skin) can be sampled by the drain currents through the transistors.

Extended Data Figure 8 Circuit diagrams of the intrinsically stretchable circuits.

a, Inverter. b, NAND. c, Amplifier.

Extended Data Table 1 Comparison of our intrinsically stretchable transistor array with previous milestone works on stretchable transistors

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Wang, S., Xu, J., Wang, W. et al. Skin electronics from scalable fabrication of an intrinsically stretchable transistor array. Nature 555, 83–88 (2018). https://doi.org/10.1038/nature25494

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