Fast Self‐Assembly of Photonic Crystal Hydrogel for Wearable Strain and Temperature Sensor

Abstract

functional materials are used as additives to endow devices with functional properties. [7,8] Although with great success, readouts of the response behaviors of wearable devices generally require sophisticated instruments, which are usually expensive and inconvenient to be portable. To this end, visual colors, which are directly integrated into wearable devices, offer an emerging strategy to intuitively obtain the responsive signals of wearable devices. [9][10][11] Structure color, different from dye and pigment, is a result of a microscale or nanoscale periodic structural material, which commonly exists in natural structures such as butterfly wings and chameleon skins. [12,13] When these periodic distances are within a few hundreds of nanometers, a modulation of the incident white light occurs, resulting in a strengthening in the light with the specific wavelength determined by Bragg's law. [12] Benefiting from structure colors, numerous visual devices, such as actuators, [14] flexible electronics, [15,16] strain sensors, [17][18][19] and other sensors [20,21] have been developed to monitor human motions, [15,16,22] temperature, [23,24] pH, [24] chemical sensing, [25][26][27] and cell monitoring. [28] At present, structure colors are obtained mainly via two schemes, including top-down fabrications such as direct laser writing, [29] nanoimprinting, [30] solvent release, [31] and bottom-up methods such as the self-assembly of nanoparticles (NPs), [32][33][34][35] cellulose nanocrystals, [36][37][38] block copolymers, [24,39] under capillary force [40,41] or external forces. [42,43] However, the former methods generally require the assistance of sophisticated and expensive equipment, whereas the latter methods generally take a long period from days [14,44] to weeks [24,45,46] with precise environmental control. Therefore, the development of a fast, robust, and convenient fabrication method for photonic crystals (PCs) is highly desired in the field of wearable sensors.Herein, we introduce a fast self-assembly method for PCs based on horizontal precipitation of silica NPs. With a hydrophilic polydimethylsiloxane (PDMS) fence on the glass substrate, a meniscus liquid surface between the adjacent NPs was obtained horizontally, which was beneficial to achieve improved and high-quality periodic silica PC (Figure 1a,b). Moreover, this self-assembly process could be completed within 1-4 h due to the less silica dispersion to evaporate, which was much faster than conventional vertical deposition methods and had better quality than horizontal deposition. [47,48] Furthermore, a Structural colors from photonic crystals (PCs) have attracted emerging attention in the research area of wearable sensors. Conventional self-assembly of PC takes days to weeks. Here, a fast self-assembly method of PC with horizontal precipitation of silica nanoparticles (NPs) in a polydimethylsiloxane fence, which can be completed within 1-4 h depending on the fence parameters, is introduced. The resultant PC exhibits tunable structural colors in the e...

Responsive and self-healing supramolecular photonic crystal hydrogels based on host–guest interactions

“…The photonic bandgap of PC hydrogels can be manipulated by the hydrogel swelling and deswelling in response to external stimuli, such as the electric field, 7 humidity, 8 temperature, 9 organic solvent, 10 and so on. 11,12 PC hydrogels are widely used in various applications, such as the wearable electronics, 13 sensing, 14 anticounterfeit labels, 15 and so on. 16 In spite of the great progress in the research of PC hydrogels, most conventional covalently bonded cross-linked PC hydrogels are fragile without self-healing properties, which decreases their light reflection quality and shortens their lifespan.…”

Heterogeneous Structural Color Conductive Photonic Organohydrogel Fibers with Alternating Single and Dual Networks

“…Inspired by biological skin, biomimetic flexible sensors that can transform external information into digital electrical signals have been developed for accurately perceiving environmental stimuli, showing great vitality in the fields of healthcare, movement monitoring, and soft robots. − Attractively, the fast color-changing properties of chameleons , and cephalopods , are further leading researchers to develop next-generation interactive sensors with optical/electrical dual-response modes to accommodate the increasing intelligent and complex requirements. − Compared with the electrical signal alone, the introduction of optical signal allows the sensor to visually self-report the detailed information (including shape, position, gradient distribution, and category), providing a reliable signal feedback and interaction interface in the complex biological environment. A flexible photonic crystal with mechanically tunable ordered structures, which can dynamically reflect light at specific wavelengths to achieve color transduction of mechanical stimuli, provides a promising optical element for constructing interactive sensors. − Particularly, photonic crystal gels (i.e., photonic gels) perform mechanochromism in the full visible spectrum range through a large change in lattice spacing.…”

“…A flexible photonic crystal with mechanically tunable ordered structures, which can dynamically reflect light at specific wavelengths to achieve color transduction of mechanical stimuli, provides a promising optical element for constructing interactive sensors. − Particularly, photonic crystal gels (i.e., photonic gels) perform mechanochromism in the full visible spectrum range through a large change in lattice spacing. Meanwhile, conductive nanofillers or ions can be easily doped in the gel phase, ,, thus successfully obtaining electrical and optical dual-signal synchronous response metamaterials with uniform photonic structural colors. − By contrast, heterogeneous structural colors such as strips and spots have been widely found on the skin of organisms, which give more complex and diverse information. These patterns play a crucial role in their survival, camouflage, information communication, courtship, and so forth. − , Therefore, by fully simulating the above characteristics, the development of conductive heterogeneous structural colored photonic sensors is still highly anticipated for interactive sensing electronics.…”

A multifunctional structural coloured electronic skin monitoring body motion and temperature

“…15 Despite the recent realization of multimodal photonic wearables, discriminating the mechanical strain and temperature remains challenging as superimposed optical responses are inevitable while operating. [16][17][18] The current state-of-the-art structural coloured e-skins are based on temperature-responsive photonic elastomers, inducing a simultaneous mechanochromic and thermochromic response. 16,17 Thus, the creation of user-interactive photonic e-skins featuring optical feedback without disturbing the real-time electrical signal monitoring is still anticipated.…”

“…[16][17][18] The current state-of-the-art structural coloured e-skins are based on temperature-responsive photonic elastomers, inducing a simultaneous mechanochromic and thermochromic response. 16,17 Thus, the creation of user-interactive photonic e-skins featuring optical feedback without disturbing the real-time electrical signal monitoring is still anticipated. Additionally, the fabrication of structural coloured e-skins via scalable methods remains challenging.…”