Review  www.afm-journal.de Soft Optomechanical Systems for Sensing, Modulation, and Actuation Ferran Pujol-Vila, Pau Güell-Grau, Josep Nogués, Mar Alvarez,* and Borja Sepúlveda* respond to one or more external stimuli Soft optomechanical systems have the ability to reversibly respond to optical (e.g., light, temperature, strain, humidity, and mechanical external stimuli by changing their own properties (e.g., pH or electromagnetic fields) by changing shape, size, viscosity, stiffness, color or transmittance). These systems typi- their own properties (e.g., shape, size, cally combine the optical properties of plasmonic, dielectric or carbon-based viscosity, stiffness, color, among others). In this framework, soft optomechanical nanomaterials with the high elasticity and deformability of soft polymers, materials have the capacity to merge the thus opening the path for the development of new mechanically tunable unique optical properties of plasmonic optical systems, sensors, and actuators for a wide range of applications. This and/or photonic structures, carbon-based review focuses on the recent progresses in soft optomechanical systems, nanomaterials (CNMs) or macromolecules which are here classified according to their applications and mechanisms of with the high tunability and elasticity of soft polymers. Given the high transpar- optomechanical response. The first part summarizes the soft optomechanical ency of many soft polymers in the visible systems for mechanical sensing and optical modulation based on the vari- and near-infrared (NIR) spectral ranges, ation of their optical response under external mechanical stimuli, thereby together with the possibility to incorporate inducing mechanochromic or intensity modulation effects. The second part such optical structures with high compli- describes the soft optomechanical systems for the development of light ancy in polymers that can withstand large induced mechanical actuators based on different actuation mechanisms, mechanical deformations, makes this combination of optical and mechanical such as photothermal effects and phase transitions, among others. The final features ideal for the development of func- section provides a critical analysis of the main limitations of current soft tional optomechanical devices.[1] optomechanical systems and the progress that is required for future devices. Soft optomechanical systems can be designed to change their optical proper- ties under mechanical stimuli induced, 1. Introduction for example, by strain, temperature, pH and so on, to develop cost-effective and highly sensitive sensors and optical modula- During the last decade, a large interest has arisen in the so- tors handling large strains, keeping their properties after many called smart materials, which have the ability to reversibly deformation cycles and adapting to complex and irregular sur- faces. Note that, for example, sensors detecting mechanical deformations (e.g., strain or displacements) are required in F. Pujol-Vila, P. Güell-Grau, M. Alvarez, B. Sepúlveda structures such as buildings, vehicles, packages or wearable Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC) devices, where it is necessary to determine the experienced Campus UAB, Bellaterra, 08193 Barcelona, Spain E-mail: mar.alvarez@csic.es; borja.sepulveda@csic.es mechanical stress. Conversely, mechanical sensors can be P. Güell-Grau, J. Nogués used as transducers to detect other kind of external stimuli Catalan Institute of Nanoscience and Nanotechnology (ICN2) (e.g., chemical, biochemical, optical, to mention a few) that are CSIC and BIST able to produce mechanical deformations in the material. This Campus UAB, Bellaterra, 08193 Barcelona, Spain category includes, e.g., some biosensors and micro-electro- J. Nogués mechanical systems (MEMS) that have been widely developed ICREA during the last decade. Soft mechanical sensors require a com- Pg. Lluís Companys 23, 08193 Barcelona, Spain pliant transducer, being the electrical and optical transduction M. Alvarez Networking Research Centre on Bioengineering, Biomaterials and methods the most widely developed. However, optical transduc- Nanomedicine (CIBER-BBN) tion offers several advantages, particularly, its wireless detec- 28029 Madrid, Spain tion nature, high sensitivity, easy readout, and the capacity to The ORCID identification number(s) for the author(s) of this article provide 2D strain mapping, even with sub-micron scale spatial can be found under https://doi.org/10.1002/adfm.202213109. resolution.[2–4] © 2023 The Authors. Advanced Functional Materials published by The high optical tunability of soft optomechanical systems Wiley-VCH GmbH. This is an open access article under the terms of the by external mechanical stimuli can also be exploited to build Creative Commons Attribution License, which permits use, distribution mechanical modulators to modify the color or transparency of and reproduction in any medium, provided the original work is properly cited. surfaces, having great potential in the development of smart windows,[5] rewritable optical displays, encryption, and anti- DOI: 10.1002/adfm.202213109 counterfeiting applications,[6] among others.[7] Adv. Funct. Mater. 2023, 33, 2213109 2213109 (1 of 22) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 14, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202213109 by Readcube (Labtiva Inc.), Wiley Online Library on [07/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de On the other hand, light can be used in soft optomechanical wavelength, λ (i.e., the color) of the transmitted or reflected systems for exerting controlled local mechanical deformations light generated by mechanical deformations, also denoted as as base to create new powerful and versatile actuators. The mechanochromic effect, or ii) the variations in the transmitted interest to develop soft stimuli-responsive actuators has recently or reflected light intensity. grown considerably due to their advantages with respect to con- Mechanochromic detection relies on the interaction of light ventional hard actuators, such as their adaptability to changing with plasmonic and/or dielectric materials that are structured environments, resiliency to high loads, easy miniaturization at the sub-wavelength or wavelength scale. In the case of plas- and low-cost. Light-responsive actuators have the advantage of monic nanomaterials, their color is given by the interaction of offering wireless remote control with high precision and rapid light with the free electrons in the metal nanostructures, which modulation. Moreover, these actuators can be tailored to incor- can excite localized or propagating surface plasmon resonances. porate extra functionalities, such as biocompatibility or self- On the other hand, dielectric photonic structures, such as grat- sensing. These optomechanical actuators can be used in a wide ings and photonic crystals (PhCs), are wavelength-scale struc- range of applications, including wireless soft robots,[8–10] arti- tures, where diffraction and interference effects are predomi- ficial muscles,[11,12] self-regulated optical components,[13] exter- nant in the observed color. In contrast, optical detection based nally controlled switches[14] and energy harvesters.[15,16] on light intensity variations is typically based on structures with With the aim to cover all these alternatives, here we present dimensions larger than the light wavelength, such as textured an extensive literature review of the most representative soft surfaces, waveguides and microlenses.[1] optomechanical systems and their applications for sensing, modulation, and actuation, which are summarized in Figure 1. We first focus on the optomechanical systems for sensing and 2.1. Mechanochromic Systems Based on the Near-Field modulation, which we have grouped according to their dif- Interaction Between Nanoplasmonic Structures ferent detection mechanisms, and next we present the soft optomechanical actuators. One of the most widely used strategies to detect mechanical strains and deformations using nanoplasmonic materials is based on the strong near-field interaction between neighboring 2. Soft Optomechanical Systems for Sensing plasmonic nanostructures, which is extremely sensitive to the inter-particle distance. The near-field interaction is generated and Modulation by the electromagnetic field that light induces around the plas- In soft optomechanical systems, the application of an external monic nanostructures, which has a decay length in the range mechanical stimulus produces a variation in the optical prop- of few nanometers, which enables the efficient electromagnetic erties that can be used either as detector of the mechanical interaction between nearby particles. The phenomenology can stimulus or as modulator to tune its own optical properties. be understood as near-field interacting resonant dipoles. How- There are two main optical properties that can be exploited ever, due to the finite size of the nanoparticles (NPs) and their for detection or modulation applications: i) the changes in the very small separation distance, the dipole–dipole model does Figure 1. Schematic of the soft optomechanical systems for mechanical sensing, optical modulation and mechanical actuation, according to their different working principles. Adv. Funct. Mater. 2023, 33, 2213109 2213109 (2 of 22) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 14, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202213109 by Readcube (Labtiva Inc.), Wiley Online Library on [07/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de not provide correct quantitative spectral changes, thus requiring the simplest case of two identical nearly touching plasmonic numerical simulations, such as finite-difference time-domain NPs, the near-field interaction (Figure 2a left panel) produces (FDTD), to accurately model the electromagnetic response. In a blue-shift in their localized surface plasmon resonance when Figure 2. Strain sensors and modulators based on plasmonic near-field interaction. a) FDTD simulations of the near-field interaction between two identical nearly touching plasmonic NPs (left panel), and light extinction cross section (Cext) as a function of the inter-particle distance when the light polarization is parallel (middle panel) and perpendicular (right panel) to the axis of the particle dimer. b) Optomechanical response of bow-tie antennas of different separations to 25% strain. Reproduced with permission.[19] Copyright 2018, Royal Society of Chemictry. c) Strain-responsive split-ring resonators on PDMS. Reproduced with permission.[22] Copyright 2010, American Chemical Society. d) Optomechanical response of 2D arrays of Au nanoparticle on PDMS to unidirectional and bidirectional strain. Reproduced with permission.[27] Copyright 2012, AIP publishing. e) Polarization-dependent response of Ag nano- disks on PDMS. Reproduced with permission.[28] Copyright 2015. American Chemical Society. f) Self-assembled arrays of Au nanobipyramids combining near-field and particle orientation changes under strains, reproduced with permission.[32] Copyright 2021, American Chemical Society. g) 3D near-field interaction in arrays of plasmonic AuNPs self-assembled in nanocellulose matrices. Reproduced with permission.[41] Copyright 2010, Wiley-VCH GmbH. Adv. Funct. Mater. 2023, 33, 2213109 2213109 (3 of 22) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 14, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202213109 by Readcube (Labtiva Inc.), Wiley Online Library on [07/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de the interparticle distance increases and the light polarization NPs in the elongation direction and contraction in the per- is parallel to the axis of the particle dimer (Figure 2a middle pendicular direction as a consequence of the Poisson ratio of panel), and red-shift when the polarization is perpendicular to the elastic material. These effects generated spectral shifts in the axis, being later effect much weaker (Figure 2a right panel). opposite directions and reduced the overall observed blue-shift Therefore, quantification of the polarization dependent spectral in the plasmonic resonance (sensitivity of 1.15 nm/%). In con- shifts of the plasmonic resonances provides an extremely sen- trast, biaxial stretching was able to separate the distance in the sitive method (i.e., plasmonic ruler) to determine the changes two dimensions, thereby maximizing the observed blue-shifts in the separation distance between the nanostructures, as the (1.8 nm/%). Similar mechanochromic effects were observed in ones produced by the stretching of an elastomeric substrate. the case of monolayers of self-assembled silver (Ag) nanodiscs This strategy was initially exploited by F. Huang et al.,[17] in the using Langmuir−Blodgett (LB) technique and deposited on case of isolated dimers of spherical gold nanoparticles (AuNPs) PDMS substrates (Figure 2e).[28] The plasmo-mechanical color deposited on a stretchable polydimethylsiloxane (PDMS) sub- modulation was also demonstrated in dense arrays of Ag nano- strate. In this case, the color change of the particle dimer cubes by biaxial stretching, which enabled active tuning the generated by the PDMS film stretching could be detected by light transmitted color of unpolarized light from magenta to dark-field spectroscopy. A similar tendency in the optical reso- orange to yellow by stretching the substrate up to 20%.[29] In a nance was shown for Au nanodiscs dimers under stretching,[18] different fabrication approach, a drying-mediated self-assembly with a peak-shift sensitivity of 3.4 nm/% of linear deformation method could generate highly ordered close-packed arrays of for tensile strain, and of 16 nm/% for compressive strain. With plasmonic particles.[30] Based on this method, periodic arrays the aim to enhance the near field interaction between the plas- of Au nanocubes in PDMS were fabricated, and the influence monic nanostructures and the optomechanical response under on strain sensitivity exerted by different types of polymeric elongations, bow-tie antennas and nanorods (NRs) fabricated ligands, interparticle distance, and size, was studied revealing by stencil lithography directly on the elastomer substrates or by a dramatic effect of these factors.[31] Fu. et al. reported PDMS- e-beam lithography and transferred to PDMS[19,20] were devel- supported self-assembled close-packed arrays of Au nanobipy- oped (Figure 2b). ramids with different orientation angles (Figure 2f).[32] Under Large optomechanical effects were also shown by Pryce and strain, the material exhibited simultaneous changes in particle coworkers for a much more complex nanoplasmonic system orientations and interparticle distance displaying highly sensi- composed of arrays of non-interacting dimers formed by asym- tive optomechanical responses, which depended on the initial metric U-shaped Au nanostructures in elastomer substrates.[21] orientation of the plasmonic elements (maximum strain sen- In these particular structures, film stretching produced both sitivitity in ≈45° oriented samples). Cataldi and co-workers changes in the interparticle distance and geometrical deforma- studied Au nanoparticle-coated PDMS plasmomechanical tions in the nanostructures. The combination of both effects sensors[33–36] fabricated by chemical growth on the substrate, yielded up to 400 nm red-shift (i.e., even larger than the plas- which allowed accurate control of the inter-particle gap with the monic resonance linewidth) when the light polarization was number of growth cycles. Exploiting this effect, they analyzed perpendicular to the dimer, and enabled tuning the Fano- how this material could mechanically tune the photogenerated resonance of the structures. Moreover, they also analyzed the heat by the AuNPs upon laser excitation.[37] In the case of small optomechanical response of split-ring resonators of different 2D arrays, such as Au nanodiscs heptamers embedded on a geometries under stretching, yielding wavelength tunabili- PDMS membrane,[38] dynamic tuning and symmetry lowering ties up 400 nm (Figure 2c).[21,22] The shape changes caused by of the Fano resonances under uniaxial stress was shown by polymer stretching were also the basis of the optomechanical Cui et al. response in metallic electromagnetic meta-atoms, which were The optomechanical changes could be amplified by fabri- fabricated using MEMS-based stencil lithography, showing cating dense arrays of plasmonic nanodisks on low-modulus, mechanically tunable optical resonances in the mid-infrared.[23] high-elongation elastomeric substrates under extreme defor- The optomechanical changes observed in particle dimers mations.[39] These structures showed tunable optical response, could be enhanced in the case of particle chains. Linear chains with reversible shifts over nearly 600 nm, due to the genera- embedded in polymer films were applied as stress-respon- tion of nonlinear buckling processes, which transformed the sive colorimetric films that could memorize the experienced initially planar arrays into 3D configurations with rotated nano- stress.[24,25] The film compression induced an increase in the discs out of the plane and forming linear arrays with a wavy lateral dimensions of the film, which enlarged the interpar- geometry. ticle distance in the chains and produced intense blue-shifts The previously described soft optomechanical systems were of their plasmonic resonances (working range up to 160 MPa). based on the near-field interaction in one or two dimensions. Exploiting this effect, a pressure sensor was developed, in However, mechanically responsive soft plasmonic materials which the color change of the composite material could be can also be fabricated with interactions in three dimensions. easily analyzed using a conventional smartphone camera.[26] For example, densely packed AuNPs embedded in a spherical The optomechanical response is somewhat more complex in silica crust and embedded in polymer film could act as mecha- the case of 2D arrays of plasmonic nanostructures that experi- nochromic strain sensors.[40] When the films were stretched, ence simultaneous near-field interaction in the two dimensions. the microcapsules were deformed into elongated ellipsoidal This was the case of self-assembled AuNPs in a hexagonal close- shapes and the distance between the AuNPs embedded in packed structure deposited on an elastomer film (Figure 2d).[27] their shells increased, thus generating the expected resonant The uniaxial film stretching induced the separation of the blue-shifts. Another 3D alternative was achieved by bacterial Adv. Funct. Mater. 2023, 33, 2213109 2213109 (4 of 22) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 14, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202213109 by Readcube (Labtiva Inc.), Wiley Online Library on [07/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de nanocellulose matrices in which different types of plasmonic nance experienced a redshift and a steep decrease on the ampli- NPs were self-assembled. These 3D matrixes exhibited large tude upon increasing strain. The scalability of the fabrication spectral variations in response to compression,[41] going from enabled the fabrication of large arrays in which the strain was the typical absorption bands of the isolated NPs to nearly per- translated into macroscopic colorimetric changes that could be fect light absorption in a broadband range, which resulted in a easily analyzed, e.g., for 2D strain mapping. color change from red to almost black (Figure 2g). These nano- Another interesting example involving propagating sur- composites also exhibited improved refractometric biosensing face plasmons was reported by Yoo et al.[50] by using tun- resolution, as well as amplified 2-photon absorption and photo- able plasmonic arrays of Au-coated asymmetric PDMS thermal effects in the compressed samples. On the other hand, micropyramids, in which film stretching yielded a change in decorating AuNPs on poly(N-isopropylacrylamide) (PNIPAM) the angle between the pyramid facet and the incident light microgels exhibited reversible color shifts (between red and (Figure 3d). Due to the high angular sensitivity in the exci- grayish violet) in response to a temperature changes,[42] due to tation of the propagating surface plasmon via total internal the mechanical deformation of the polymer particles (due to reflection on the pyramidal facets, this material could thermal expansion) and the induced resonance red-shifts. This achieve an outstanding sensitivity of 15.22 nm for a 1% strain optomechanical concept was applied as thermometers with a (i.e., 15.22 nm/% sensitivity), thus showing the largest strain temperature resolution of 0.2 °C, and freestanding colorimetric spectral response reported to date. patches were utilized as spatial temperature scanners, which Interestingly, mechanical engineering can be used to amplify could potentially be applied to wearable sensors. Following this the strain in specific regions and, therefore to enhance the opto- trend, pH-sensitive hydrogels containing embedded AuNPs mechanical response of plasmonic metamaterials.[51] This effect were demonstrated,[43] as well hydrogel nanofiber waveguides was achieved by a reconfigurable metasurface constructed by a containing Au NRs for humidity sensing.[44] plasmonic lattice array in the gap between a pair of symmetric microrods that served to locally amplify the strain created on the elastomeric substrate by an external mechanical stim- 2.2. Mechanochromic Systems Based on Propagating ulus. The strain on the metasurface could be amplified up to Surface Plasmons 15.9-fold with respect to the external strain by tailoring the microrod geometry, thereby enhancing the mechano-sensitivity In contrast to near-field interacting small plasmonic NPs, peri- of the optical responses. odic arrays of plasmonic nanostructures whose size is in the same range of the light wavelength can excite propagating plasmon modes, and their excitation condition can be tuned 2.3. Mechanochromic Systems Based on Other when the arrays are mechanically deformed. Nanoplasmonic Interactions This approach was used in the case of Au-coated nanovoids (diameter 900 nm) in PDMS, in which the plasmonic periodic In addition to the near-field interaction between localized nanostructures supported localized and propagating plasmon plasmonic resonances, the optical anisotropies in elon- modes.[45] This system exhibited a broad resonance and it could gated plasmonic nanostructures can be also combined be stretched up to 5% strain, yielding resonance redshifts with with elastic matrices to generate enhanced optomechanical a sensitivity of 3 nm/%. functionalities. In addition, arrays of hexagonal close-packed polysty- Xu et al.[52] reported a Au NRs/hydrogel composite that rene nanospheres capped by Ag and Au layers on PDMS displayed a polarization-dependent light absorption upon were reported by Zhu X et al.[46,47] (Figure 3a). In this case, stretching due to the alignment of the plasmonic elements the proximity between particles and their periodicity enabled (randomly dispersed in the unstretched composite) (Figure 4a). the generation of delocalized propagating plasmonic modes Additionally, the non-aligned configuration could be recovered that were strain- and angle sensitive. When the material was by photolithographic patterning, with potential applications in stretched, two properties changed simultaneously: the dis- polarized displays and encryption. tance and near-field interaction between the metallic caps and On the other hand, the catalytic activity of Au nanostructures lattice pitch in the elongation directions, which produced a was used to generate the first wrinkled plasmonic enhanced redshift and broadening of the plasmon resonance (sensitivity Fabry-Perot (FP) elastic cavities.[53] The cavities were sponta- 1.33 nm/%). neously formed by thermally curing the PDMS on arrays of Strain-sensitive Au nanolines on flexible substrates con- plasmonic nanodomes, thereby inducing their penetration at stituted an example of plasmonic periodic structures in one controlled depths inside the polymer matrix and generating a dimension (Figure 3b).[48] Instead of conventional lithographic stiffness gradient. The combination of both effects yielded tun- techniques, in this work, AuNPs were self-assembled in ridges able corrugated FP cavities, which simultaneously exhibited and annealed to form the nanolines. As in the previous cases, large wavelength and intensity variations in the FP optical res- the films stretching produced a redshift of the plasmonic res- onances (Figure 4b). The analysis of both parameters enabled onance. In another example, periodic arrays of asymmetric achieving a high sensitivity (2.21 nm/%) and an unprecedented “U-shaped” aluminum-coated polyurethane nanowires showing strain detection limit of 0.006%. Mechanically tunable FP cavi- Fano resonances were investigated by Lütolf et al. (Figure 3c).[49] ties could also be obtained by elastomeric dielectric films sand- In these structures, the electromagnetic field could be guided wiched between two metal layers, which could have potential in the direction perpendicular to the nanowires and their reso- for structural health monitoring.[54] Adv. Funct. Mater. 2023, 33, 2213109 2213109 (5 of 22) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 14, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202213109 by Readcube (Labtiva Inc.), Wiley Online Library on [07/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 3. Strain sensors and modulators based on surface propagating plasmons. A) Array of Au-capped polystyrene nanospheres on PDMS. Adapted from.[47] b) Optomechanical response of Au nanolines on PDMS. Reproduced with permission.[48] Copyright 2014, Springer Nature. c) Optomechanical response of U-shaped, aluminium-coated polyurethane nanowires. Reproduced with permission.[49] Copyright 2016, Wiley-VCH GmbH. d) Array of Au-coated micropyramids in PDMS and corresponding optomechanical response. Scale bar 20 µm. Reproduced with permission.[50] Copyright 2015, American Chemical Society. 2.4. Mechanochromic Systems Based on Light Interference periodicity and the angle of incidence, following the diffraction and Diffraction equation:[55] Light diffraction and interference generated by a periodic con- mλ = d ( sin α + sin β ) (1) trast of refractive index at the wavelength scale, as in diffraction gratings and photonics crystals, produce a structural coloration where α is the incident angle, β is the reflection angle, d is the in the material. Mechanical deformation in the ordered struc- periodicity, m is the diffraction order (m = 0, ± 1, ± 2, ± 3, …), ture generates a mechanochromic effect, which can be used for and λ is the wavelength. Nanoimprint lithography (NIL) offers developing sensors to detect mechanical deformations caused an easy, low-cost methodology to produce large area diffraction by strain or other stimuli capable to change their periodicity gratings,[56,57] and its use for mechanochromic sensing have (e.g., swelling). been reported by several authors. For example, colorimetric In the case of diffraction gratings, the diffracted light by pressure sensors were proposed,[58–60] which could be applied the periodic grooves at the surface of the material causes a in non-contact intraocular pressure sensing.[59] Diffraction grat- wavelength separation of the transmitted and reflected light ings have also been exploited as self-sensing mechanism to in different angles, which is very sensitive to variations in the quantify the mechanical actuation in soft-robotic structures, Adv. Funct. Mater. 2023, 33, 2213109 2213109 (6 of 22) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 14, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202213109 by Readcube (Labtiva Inc.), Wiley Online Library on [07/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 4. Strain sensors and modulators based on other plasmonic interactions. a) Polarized display composed of Au NRs embedded on elastomeric films that align the NRs under film stretching. Reproduced with permission.[52] Copyright 2020, Wiley-VCH GmbH. b) Elastic plasmonic enhanced wrin- kled FP cavities obtained by self-embedded Au nanodomes in PDMS films, showing intense blue-shifts and intensity increase in the cavity resonances under film stretching. Reproduced with permission.[53] Copyright 2022, Wiley-VCH GmbH. driven by pneumatic pressure[61] or temperature (by using from their differential deformation. The generation of periodic shape memory alloy wires).[62] Diffraction-based coloration was surface wrinkles on PDMS by an oxygen plasma treatment in also exploited to wirelessly detect the deflections of cantilever- an initially stretched PDMS was proposed for the design of shaped soft optomagnetic actuators.[63] This opto-magneto- mechanochromic devices featuring bright structural colors and mechanical system was composed of a nanostructured-iron programmable colorimetric responses.[68–70] Further advancing (Fe) film mechanically linked to a periodically corrugated thin in this concept, Xu et al. demonstrated the fabrication of micro- PDMS film, thereby enabling synergistic optical and magnetic prism arrays and diffraction gratings of different geometries actuation and simultaneous colorimetric detection (Figure 5a), using shape memory polymers and its posterior optomechanical with a detection limit of 1.8 mW cm−2 and 0.34 mT respectively, analysis to build shape-memorizing micro-optics[71] (Figure 5b). by a simple color imaging analysis in a conventional smart- Diffraction induced colored materials were also produced phone camera. A similar colorimetric detection method was by using soft 2D arrays of plasmonic nanostructures, where used in PDMS cantilevers with one grating surface to monitor far-field interference among individual nanostructures of the reversible molecular conformational changes of an azobenzene lattice suppressed all the scattered light outside the specified monolayer when exposed to Ultraviolet (UV) light, showing wavelength window.[72] Using this mechanism, vivid colors that similar sensitivity to the traditional beam detection method can be tuned across the entire visible spectrum were produced (4.6 × 10–3 N m−1).[64] Following this trend, versatile strain sen- by stretching in its two dimensions an array of Al plasmonic sors based on elastomeric 2D arrays of cone-shaped structures nanodiscs on top of a soft substrate. Similarly, a square array enabled measuring biaxial and shear strains with high perfor- of Au caped PDMS nanopillars on an elastomer substrate was mance.[65] Another alternative to develop diffraction gratings used to study the inverse optical behavior between polarized is the use of 2D arrays of self-assembled polystyrene micro- light parallel and perpendicular to the stretch axis.[73] spheres as a template. This strategy was followed to fabricate In contrast to diffraction gratings, PhCs in 1D, 2D, and 3D periodic 2D structured elastomeric membranes as colorimetric can prevent the propagation of light at certain wavelengths pressure sensor for optofluidic applications.[66] A sensitivity (photonic bandgaps), presenting an intrinsic structural color of 0.17 kPa−1 was obtained by measuring the diffracted color defined by Bragg’s and Snell’s law:[74] changes induced by the membrane deformation. Alternatively, the linear buckling theory[67] has also been 2 λmax = 2 d neff − sin 2 α (2) exploited to fabricate periodic textured surfaces on elastic poly- m mers, based on the mechanical strain between two contacting materials of different Young’s modulus, which spontaneously where λmax is the wavelength at which maximum reflec- produce surface buckles to minimize the surface energy arising tion occurs, α is the incident angle, m is the diffraction order Adv. Funct. Mater. 2023, 33, 2213109 2213109 (7 of 22) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 14, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202213109 by Readcube (Labtiva Inc.), Wiley Online Library on [07/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de Figure 5. Optomechanical mechanochromic systems based on diffraction gratings and PhCs. a) Colorimetric detection of opto-magnetic and peri- odically corrugated cantilevers enabling optical and magnetic actuation. Reproduced with permission.[63] Copyright 2021, American Chemical Society. b) SEM image of different micropatterned shape-memory polymers for programmable micro-optics. Reproduced with permission.[71] Copyright 2013, Wiley-VCH GmbH. c) Graphene-infused block co-polymer PhCs. Reproduced with permission.[94] Copyright 2020, Wiley-VCH GmbH. d) Thermally- adjustable photochromic hydrogel spectrometry and representative images. Reproduced with permission.[96] Copyright 2007, Wiley-VCH GmbH. e) Mechanochromic elastic fiber based on a bio-inspired multilayer of PDMS/PSPI. Reproduced with permission.[115] Copyright 2013, Wiley-VCH GmbH. f) Bioinspired structural color patch. Reproduced with permission.[121] Copyright 2020, American Association fo the Advacement of Science. (m = 0, ± 1, ± 2, ± 3, …), d is the lattice constant, and neff is the effect with a sensitivity over 6 nm/% strain.[76] By embedding effective refractive index of the crystal. a 1D PhC formed by periodically arranged dielectric NRs in a This effect enables generating selective macroscopic colors deformable PDMS substrate, different planar strains could by regulating the center wavelength of the bandgap, thereby be identified, including their application direction, type, and being very sensitive to changes in the refractive index and peri- value.[77] odicity.[75] Soft 1D PhCs, consisting of alternating high and low 2D PhCs composed of periodic cylinder-shaped air holes in refractive index materials, have demonstrated mechanochromic a triangular lattice, made in PDMS by NIL could distinguish Adv. Funct. Mater. 2023, 33, 2213109 2213109 (8 of 22) © 2023 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2023, 14, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202213109 by Readcube (Labtiva Inc.), Wiley Online Library on [07/07/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.afm-journal.de four strain ranges below 50% of strain, from 0% to 6.7% (red), swelling/shrinking induced by chemical interactions.[86] For 6.7%-13% (yellow), 13%-30% (green), and 30%-50% (blue).[78] example, using pH-sensitive hydrogels, high-performance pH On the other hand, 2D PhCs based on 2D arrays of self- determinations were achieved,[101] as well as multi-responsive assembled particles were also applied to develop strain-sensors photonic nanochains for microenvironment analysis.[102] Going and other mechanochromic devices. For example, color writing further, chemical modifications of photonic hydrogels enabled in photonic “papers” with totally transparent materials was selective detection of specific molecules such as glucose,[103–105] proposed by Fudouzi et al.,[79] which were based on 2D PhCs and proteins,[106] among others. whose interparticle distance and coloration varied due to the On the other hand, hydrogel sensors based on thin-film swelling induced by the local application of an organic solvent. interference have also been showcased, in which the fast The majority of the strain sensitive soft 3D PhCs, also hydrogel swelling is translated into changes in the film thick- referred as opals, are based on the 3D self-assembly of polysty- ness and thus into a shift in its reflected color.[107] In thin- rene or silica nanospheres,[80] followed by the infiltration of the film interference, the constructive interference becomes elastic polymer matrix (such as PDMS) and a posterior etching mλ = 2n2dcos θ2 for n1