Semiconductor diodes are basic building blocks of modern computation, communications and sensing1. As such, incorporating them into textile-grade fibres can increase fabric capabilities and functions2, to encompass, for example, fabric-based communications or physiological monitoring. However, processing challenges have so far precluded the realization of semiconducting diodes of high quality in thermally drawn fibres. Here we demonstrate a scalable thermal drawing process of electrically connected diode fibres. We begin by constructing a macroscopic preform that hosts discrete diodes internal to the structure alongside hollow channels through which conducting copper or tungsten wires are fed. As the preform is heated and drawn into a fibre, the conducting wires approach the diodes until they make electrical contact, resulting in hundreds of diodes connected in parallel inside a single fibre. Two types of in-fibre device are realized: light-emitting and photodetecting p–i–n diodes. An inter-device spacing smaller than 20 centimetres is achieved, as well as light collimation and focusing by a lens designed in the fibre cladding. Diode fibres maintain performance throughout ten machine-wash cycles, indicating the relevance of this approach to apparel applications. To demonstrate the utility of this approach, a three-megahertz bi-directional optical communication link is established between two fabrics containing receiver–emitter fibres. Finally, heart-rate measurements with the diodes indicate their potential for implementation in all-fabric physiological-status monitoring systems. Our approach provides a path to realizing ever more sophisticated functions in fibres, presenting the prospect of a fibre ‘Moore's law’ analogue through the increase of device density and function in thermally drawn textile-ready fibres.
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This work was supported in part by the MIT Materials Research Science and Engineering Center (MRSEC) through the MRSEC Program of the National Science Foundation under award number DMR-1419807 and in part by the US Army Research Laboratory and the US Army Research Office through the Institute for Soldier Nanotechnologies, under contract number W911NF-13-D-0001, with funding provided by the Air Force Medical Services. This work was also supported by the Assistant Secretary of Defense for Research and Engineering under Air Force Contract numbers FA8721-05-C-0002 and FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the Assistant Secretary of Defense for Research and Engineering. The authors express their gratitude to S. Maayani for discussions and simulations of the lensed fibre system; to D. Bono and C. Marcus for advice and support in building the fibre-based pulse measurement setup; to R. Yuan for illustration of the results presented in the manuscript; and to E. Simhon for discussions from research ideation through to its completion.
Nature thanks D. Richardson, M. Schmidt, M. Shtein and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, Light-emitting fibres with low device density. (i) Photograph of light-emitting fibres containing blue-colour LEDs. The devices appear with a periodicity of 2,000 ± 110 mm. Scale bar, 50 cm. (ii) Illustration of the device density in the polymeric layer, which is placed in the middle of the preform, with a device separation of 1.25 mm. b, Higher density of devices in the fibre. (i) Photograph of light-emitting fibres containing blue-colour LEDs, where the devices appear every 370 ± 110 mm. This is the maximum linear density available with the given draw-down ratio (40) and device size for the single-layer (plane) architecture. Scale bar, 50 cm. (ii) Illustration of the maximal linear density of devices in the preform. The devices are placed side by side in a single plane. c, An alternative approach to increasing device density in fibres. (i) Illustration of the structure of the fibre cross-section, where light-emitting devices (blue shapes) are placed in two layers on top of each other, connected to metallic electrodes (red circles) for current delivery. The + and − signs represent the polarity of the wires when connected to the power supply. (ii) Photograph of the resulting fibre, in which the devices appear every 173 ± 92 mm. Scale bar, 20 cm. (iii) Side view of the light-emitting fibre, showing the presence of three electrode wires. Scale bar, 600 μm. d, Distance between devices in the fibre as a function of the distance between devices in the preform for a draw-down ratio of 40 and using LEDs. Solid lines show calculation results; black curve, single device layer in the preform; blue curve, two device layers in the preform; green curve, three device layers in the preform. Red circles represent measurements of inter-device spacing in the fibres. The dashed red line corresponds to the minimal distance between devices in the preform, which is equal to the size of the devices (170 μm). e, Distance between devices in the fibre as a function of the draw-down ratio. Solid curves show calculation results: black curve, single device layer with a spacing of 230 μm between devices in the preform; green curve, two device layers with a spacing of 230 μm; blue curve, single device layer with a spacing of 1.25 mm. Red circles, measurements of inter-device spacing in the fibres. Error bars represent one standard deviation.
a, Optical micrograph of a commercial GaAs photodetecting device element. The central part is the device aperture, surrounded by two metallic contacts. Scale bar, 275 µm. b, Illustration of the preform drawing process for the photodetecting fibres. The contact to the devices is established on the same side of the detectors, keeping the apertures of the devices uncovered by wires, whereas the third wire is placed behind the devices to prevent them from rotating during fibre drawing. c, Optical micrograph of the photodetecting fibres, showing a device embedded in the fibre. Scale bar, 600 µm.
The fibres are fully operational when immersed in water.
a, A bunch of light-emitting fibres is placed in a water-permeable protective sack. b, The protective sack with the fibres is placed in a household washing machine. c, Fibres and sack after a washing cycle. d, Fibre operation and light emission after the washing cycle.
Extended Data Fig. 5 Measurement of current registered by the photodetecting fibre as a function of the distance between the photoemitting and photodetecting fibres.
a, Illustration of the experimental setup. Red rectangle, photodetector; blue circle, LED point source; grey square, PC cladding. b, Current registered by the photodetecting fibre versus its distance from the light-emitting fibre, obtained with the photodetecting fibre placed in front of a light-emitting fibre while varying the distance between them. c, Current versus the inverse distance squared. The plot shows a linear dependence between the current and the inverse distance squared, which corresponds to the inverse-square law, at distances larger than 1 mm between the fibres. At shorter distances, deviation from the inverse-square law is observed. Several factors could contribute to this deviation, such as the finite sizes of the emitter and detector, the Lambertian profile of the emission and contact between the fibres at lower distances, which may have distorted the distance measurements between the fibres.
a, Illustration of the fibre structure. Blue, LED; grey, PC cladding. Fibre size, 500 µm × 500 µm. The black dot shows the centre of the fibre. The radiation pattern was assumed to be Lambertian. b, Results of the ray-optics simulations, showing collimation of the light from the LED when the device is placed at 190 µm from the centre of the fibre. The fibre structure is outlined by a black curve, and a general photodetector is plotted on the right-hand side of the figure. Blue lines represent optical rays emitted by the LED in the fibre.
a, Illustration of the fibre structure. Red, photodetector; grey, PC cladding. Fibre size, 500 µm × 500 µm. The centre of the fibre is denoted by a black dot. b, Results of the ray-optics simulations, showing focusing of the collimated external light on the photodetecting device. The results were obtained with the device placed 190 µm away from the centre of the fibre. Axis units, μm. c, The intensity of the illumination as a function of the location of the device in the fibre. The maximal intensity is achieved at 190 µm from the centre of the fibre.
Extended Data Fig. 8 Simulation of a lensed communication system containing a light-emitting fibre and a photodetecting fibre.
a, Illustration of the fibre system structure. Red, photodetector; blue, light-emitting device; grey, PC cladding. Fibre size, 500 µm × 500 µm. The black dot denotes the centre of the fibre. b, Results of the ray-optics simulations that show collimation of the emitted light and focusing of the light on the photodetecting device, with the devices placed 190 µm from the centre of the fibre.
a, Measured distance between adjacent LEDs in the drawn fibres. The diodes were arranged in two parallel arrays in the preform, which was drawn with a draw-down ratio of 33. b, Optical power characterization of the LEDs in the drawn fibre. The power was normalized with the power of the brightest diode, which was located adjacent to the power source. Non-operational LEDs are marked by a red cross. The emitted power decays as the voltage drops on the wires in the fibre for devices located away from the power source.
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Scientific Reports (2018)