Stretchable and reflective displays: materials, technologies and strategies

Abstract

Displays play a significant role in delivering information and providing visual data across all media platforms. Among displays, the prominence of reflective displays is increasing, in the form of E-paper, which has features distinct from emissive displays. These unique features include high visibility under daylight conditions, reduced eye strain and low power consumption, which make them highly effective for outdoor use. Furthermore, such characteristics enable reflective displays to achieve high synergy in combination with wearable devices, which are frequently used for outdoor activities. However, as wearable devices must stretch to conform to the dynamic surfaces of the human body, the issue of how to fabricate stretchable reflective displays should be tackled prior to merging them with wearable devices. In this paper, we discuss stretchable and reflective displays. In particular, we focus on reflective displays that can be divided into two types, passive and active, according to their responses to stretching. Passive displays, which consist of dyes or pigments, exhibit consistent colors under stretching, while active displays, which are based on mechanochromic materials, change their color under the same conditions. We will provide a comprehensive overview of the materials and technologies for each display type, and present strategies for stretchable and reflective displays.

Keywords: Active display; Passive display; Reflective display; Stretchable display.

Fig. 1

Schematic concept of reflective display. Reflective displays have advantages in outdoor usage due to their superior readability in bright condition. They consume less power than emissive display as energy for light generation won’t be needed. Reflective displays exhibit natural light with balanced wavelength spectrum, which is not enriched with short wavelength. Stretchable reflective displays can be classified into two groups, passive and active. Passive displays do not change their color when deformed, while active displays sensitively change their color due to deformations

Fig. 2

Technologies for electrophoretic display with their diagrams. a Cross sectional schematic of a microencapsulated electrophoretic imaging film and material of each part. b An electrophoretic display with hybridized vertical and horizontal movements of pigment particles. Microscopic images of display pixels are shown below. c An electrophoretic color display with color filter array. d An electrophoretic color display with three different pigment particles in one microcup. e An electrophoretic display produced by E Ink Holdings Inc. Pixel density of 150 ppi in 20 inch diagonal was demonstrated (Figure reproduced from a [107], Copyright 2012, The Society for Information Display; b [12, 13], Copyright 2012, The Society for Information Display and The Korean Information Display Society; c [20], Copyright 2008, Optical Society of America; d [21], Copyright 2018, IEEE; e [34], Copyright E Ink Holdings Inc.)

Fig. 3

Technologies for electrowetting and electrofluidic display. a Structure and principle of electrowetting display. (i) Without any applied voltages, a colored oil film covers the pixel. (ii) When a voltage (~ 10 V) is applied, the oil film is contracting and makes the pixel transparent. b Schematic of an electrofluidic cell without a top plate. c Pixel array of electrowetting display with yellow dye based on alkylated pyrazole azo structure. d Images of electrofluidic pixels (square type and hexagonal type). Time-lapse images of a 500-um-square pixel is displayed. e Electrowetting based E-paper display by GR8 Optoelectronics Ltd. f Electrowetting based display demo by Liquavista (Figure reproduced from a [23], Copyright 2003, Springer Nature; b [25], Copyright 2012, The Society for Information Display; c [26], Copyright 2018, The Society for Information Display; d [24], Copyright 2009, Springer Nature; e Copyright 2017, Gr8; f Copyright Liquavista)

Fig. 4

Technologies for electrochromic displays. a Assembly configuration of an electrochromic display and material for each part. b Photographs of an electrochromic glasses at − 1.0 V and + 1.0 V with brown blend of orange and periwinkle. c (i) Photographs for three different states of an electrochromic gel: bleached at 0.00 V, blue at − 0.70 V and red at − 0.80 V. (ii) Transient profiles in optical properties with constant applied voltages (Figure reproduced from a [28], Copyright 2015, The Author(s); b [30], Copyright 2015, American Chemical Society; c [31], Copyright 2016, American Chemical Society)

Fig. 5

Passive reflective displays with flexibility. a Schematic diagram of the top-gate organic thin film transistors (OTFT) device structure. Flexibility of a 6′’ electrophoretic display is demonstrated. b A flexible electrofluidic display fabricated on a PET backplane and its pixels in the off and on states. c (i) Schematic illustration of fabrication processes for a flexible, patterned, multicolored electrochromic display on plastic sheet by using the ‘cut-and-stick’ method. (ii) Its bleached (at 0.00 V) and colored (at − 0.70 V) states under a bending deformation. d (i) PEDOT-PSS based electrochromic reaction. (ii) A schematic diagram of an electrochromic device on a fabric. (iii) An image of patterned electrochromic fabric in neutral (top) and oxidized (bottom) states (Figure reproduced from a [33], Copyright 2015, The Society for Information Display; b [25], Copyright 2010, The Society for Information Display; c [31], Copyright 2016, American Chemical Society; d [39], Copyright 2010, American Chemical Society)

Fig. 6

Mechanochromic spiropyran based polymers that change color after stretching. a Spiropyran(SP) embedded poly(methyl acrylate) exhibits color change from yellow to red by ring opening reaction induced by applied strain. b Indolinospiropyran-poly(e-caprolactone) films turned to blue colored merocyanine form by mechanical force-induced bonds rearrangements. c Spiropyran embedded PDMS elastomer is patterned on a PDMS substrate and show a word “STOP” after a deformation (Figure reproduced from a [50], Copyright 2009, Springer Nature; b [51], Copyright 2010, American Chemical Society; c [52], Copyright 2018)

Fig. 7

1D Photonic crystals for mechanochromic applications a Structural color from morpho butterfly wings. b Morpho ridges and lamella structures with alternating layers of air and natural material. c 1D photonic crystals with a PS-b-P2VP block copolymer. d Mechanochromic color changes of the block copolymer as a function of compression. e Block copolymer embedded PDMS elastomer changes color from red to blue by applied stretch. f Lamella structure of poly(dodecyl glycidyl itaconate) lipid bilayer/poly acrylamide hydrogel. g Mechanochromic demonstration of the hydrogel (Figure reproduced from a [53], Copyright 2003, Springer Nature; b [54], Copyright 2017, Springer Nature; c, d [59], Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA; e [62], Copyright 2018, The Author(s), f,g [65], Copyright 2011, American Chemical Society)

Fig. 8

3D photonic crystals for mechanochromic applications. a Scheme of 3D photonic crystal with reversible lattice of polystyrene beads in a PDMS matrix. The lattice is controlled by stretching or releasing PDMS matrix. b Peak positions of reflected wavelength are shifted by elongating the photonic crystal embedded PDMS. c An illustration of silica particle arrays under deformation. Optical microscope images of 3D photonic crystal embedded PEGMA gel under pushing or pulling. d 3 × 3 pixels with the PEGMA photonic crystal gel (Figure reproduced from a, b [67], Copyright 2006, American Chemical Society; c, d [69], Copyright 2014, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fig. 9

Electrochemical swelling driven color changes in photonic crystal systems. a Bio-inspired electroactive color changing 1D photonic crystal display. (i) Digital image of a cephalopod, (ii) protein based multilayer, (iii) A simple electrochemical cell is fabricated by introducing an electrolyte and lamella photonic gel between two ITO coated glass substrates. (iv) a lamella structure of PS-b-P2VP block copolymer. b Mechanism of the electrochemical color change, and optical images of the device with various electric potentials. c (i) Schematic of an electrochemical cell with 3D opal photonic crystals in polyferrocenylsilane (PFS) matrix. (ii) Electrochemical induced swelling leads to color change in the photonic crystal. d (i) Preparation process of PFS-based inverse opal structure. (ii) The inverse opal based photonic crystal is operated electrochemically (Figure reproduced from a, b [70], Copyright 2009, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; c [73], Copyright 2007, Springer Nature; d [74], Copyright 2009, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fig. 10

Electrokinetic driven 3D photonic crystals a Schematic representation of the changes in an array of charged particles under different electric fields. b Changes in reflective colors exhibited by a single device with various voltage bias. c Profiles of peak reflectance as a function of time under different DC bias excitation. d (i) A digital photo of CeO₂ particles. (ii) a X-ray diffraction pattern of CeO₂ particles. (iii) a TEM image of CeO₂ particles. e (i) Working mechanism of the electrically tunable photonic crystals prepared from propylene carbonate solution of CeO₂, SiO₂, and SiO₂ coated CeO₂ colloidal particles. (ii) Optical microscope images for the photonic crystal under various voltages. (iii) CIE color space showing tunable range of photonic colors in different electric fields (Figure reproduced from a [76], Copyright 2013, The Royal Society of Chemistry; b, c [75], Copyright 2010, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; d, e [77], Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fig. 11

Electromechanically driven mechanochromic polymers. a Structure of electromechanical photonic crystals using dielectric elastomer actuator. b Photonic organogel is electrically operated with Maxwell stress induced by a DC bias, which results in areal expansion of the photonic gel. c Digital images show various colors from red to blue. d The electromechanical photonic device can generate sound in the audible frequency regime. e Piano notes to be programmed as an input signal to the device and recorded sound waves from a microphone. f A short-time Fourier transformation data, allowing for visualization of the frequencies over time. g Schematic structures for electro-mechano-chemically responsive(EMCR) color change displays. h Mechanism for the device using mechanochromic spiropyran materials. The applied voltages induce a large deformation in the elastomer, which causes ring open reaction of spiropyran resulting in color change (Figure reproduced from a–f [82], Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; g, h [83], Copyright 2014, Springer Nature)

Fig. 12

Polymeric substrates for stretchable displays. a Flexible E-paper display(QR-LPD^®) fabricated on PET substrate. b Flexible electrochromic display on ITO coated PET substrate during dynamic bending test. c Picture of a poly amide-imide film with low CTE(~ 4 ppm/ °C) and high transparency. d Schematic structure of photonic crystal fiber and color change at given strain. Polystyrene particles are coated on PDMS core. e Stretchable electrochromic device fabricated on polyurethane(PU) substrate (Figure reproduced from a [18], Copyright 2006, Society for Information Display; b [35], Copyright 2019, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; c [85], Copyright 2018, The Author(s); d [90], Copyright 2015, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; e [91], Copyright 2017, American Chemical Society)

Fig. 13

Stretchable electrodes and structure for stretchable electrochromic displays. a A SEM image of the AgNWs on a glass substrate. b Stretchability of a AgNW electrode is demonstrated with a light-emitting diode. c Electrical and optical properties of stretchable electrodes. (i) Normalized resistance as a function of applied strain. (ii) Transmittance versus sheet resistance. d (i) Possible structure of the stretchable electrochromic device. (ii) An example with AgNW/PDMS and WO₃. e Patterned electrochromic device in bleached and colored states at 0 and 50% strain (Figure reproduced from a–c [92], Copyright 2012, IOP Publishing Ltd; d(ii) and e [94], Copyright 2013, America Chemical Society)

Fig. 14

Ionic conductors for stretchable reflective displays. a Transparent ionic conductors are exploited for DEAs without electrochemical reaction. b Performance of hydrogel ionic conductor exhibiting relatively insensitive resistance change upon stretching with a high transparency comparing to other electrodes. c A design for stretchable reflective display with ionic conductors. d A similar structure of electroluminescent display using ionic conductors. e Luminescent behavior of the display under uniaxial stretching. f Patterned luminescent displays showing various colors under a mechanical deformation (Figure reproduced from a, b [96], Copyright 2013, American Association for the Advancement of Science; d, e [96], Copyright 2016, American Association for the Advancement of Science; f [97], Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fig. 15

Display fabrication processes and pixel structures. a Schematic process of layer-to-layer fabrication of the reflective display, reproduced with permission. b Roll-to-roll process for liquid powder display fabrication. c ITO grid patterned by lithography. d Structure of electrophoretic display with spacers (Figure reproduced from a [99], Copyright 2006, Society for Information Display; b [18], Copyright 2006, Society for Information Display; c [101], Copyright 2010, American Chemical Society; d [101], Copyright 2005, John Wiley & Sons, Ltd)

Fig. 16

Adhesion and sealing issues. a PEDOT: PSS and PDMS is chemically linked by poly ethyleneglycol diacrylate (PEGDA). b CNT dispersion into PDMS by flow stress. c (i) Bilayer of VHB 4910 (3 M) elastomer and hydrogel with silica nanoparticles, before and after debond. (ii) Nanoparticles absorb chains between hydrogel and elastomer. (iii) Debond energies between VHB elastomer and various hydrogels are increased by silica nanoparticles. d water permeability and elastic modulus of variety of materials (Figure reproduced from a [102], Copyright 2017, The Author(s); b [103], Copyright 2014, The Author; c [104], Copyright 2016, The Royal Society of Chemistry; d [105], Copyright 2017, American Chemical Society)