Active optics with silk

3.1 Reconfigurable optics

Silk’s polymorphic transitions have been recently harnessed to design silk-based three-dimensional (3D) photonic lattices which can undergo non-reversible conformational changes that affect the construct’s optical response. The combination of protein self-assembly with colloidal assembly has been utilized to transform silk aqueous solutions into 3D photonic crystals [9], [23]. The latter is a facile, scalable, and cost-effective manufacturing technique that is widely adopted for the bottom-up fabrication of 3D photonic crystals [37], [38]. The resulting colloidal crystals can be further used as templates for the formation of inverted photonic lattices, which are obtained by template removal through solvent etching. Silk solutions can readily infiltrate such templates and self-assemble into a freestanding silk film through control over the dynamics of water evaporation. The fabricated silk inverse opals (SIOs) present bright structural colors because of the diffraction of incident light induced by the periodicity of the nanostructures. SIOs can be either amorphous or crystalline depending on the protein’s conformation. Since the protein chains in crystalline SIOs are physically crosslinked, this type of photonic lattice is robust and difficult to tune once the structure is fabricated [23]. On the contrary, amorphous SIOs possess a photonic lattice that can be easily modulated under external stimuli, allowing for the fabrication of materials with variable structural color.

Large-scale, highly-ordered, and reconfigurable SIOs have been recently fabricated by using polystyrene sphere multilayers as templates [9]. Instead of using conventional deposition techniques, such templates were prepared by scooping transfer of the floating monolayers at the water/air interface and by stacking them layer-by-layer (Figure 2A). Uniform structural colors were observed as a result of the favorable material characteristics of silk protein, including robust mechanical properties, nanoscale processability, and conformability (Figure 2B). Such amorphous SIOs are stable at ambient conditions; upon exposure to water vapor or UV light, SIOs’ photonic lattice can be controllably compressed along the vertical direction of the film, leading to reconfiguration of the photonic bandgap. This mechanism allows for the generation of structural color over the entire visible spectrum. Multiple multicolor patterns can be easily generated by selectively exposing masked SIOs to water vapor or UV light for different durations (Figure 2C). The underlying mechanism that enables the fabrication of patterned SIOs is the stimuli-induced controllable conformational changes through the molecular rearrangement of silk fibroin.

Figure 2:

(A) Fabrication schematic of polystyrene sphere multilayer templates on flat and patterned surfaces. (B, C) Schematic (left) and photograph (right) of SIO (B) and patterned SIO (C). (A, B, C) Reproduced from Ref. [9]. Copyright 2017, WILEY-VCH Verlag GmbH & Co. (D) Schematic of hierarchical inverse opal assembled on a diffraction grating; (E) reflected diffraction patterns (left) of hierarchical opals assembled on pattern generators with different layers of inverse colloidal crystals (from 0 to 5) and corresponding relative diffraction and absolute reflection intensity as a function of layers number (right). (F) Photographs of hierarchical opal with a multicolor tree pattern observed at 0^o and 45°. (D, E, F) Reproduced from Ref. [31]. Copyright 2018, WILEY-VCH Verlag GmbH & Co. (G) Fabrication schematic (left) of patterned polystyrene multilayer template by depositing ethyl acetate on the structure using inkjet printing and photograph (right) of a multispectral SIO QR code. (H) Fabrication schematic (left) of patterned SIO by depositing MeOH/water mixture on SIO using inkjet printing and photograph (right) of SIO with a butterfly pattern. (G, H) Reproduced from Ref. [20]. Copyright 2019, WILEY-VCH Verlag GmbH & Co.

The above described reconfigurable 3D photonic nanostructures can be further coupled with 2D microscale patterns to fabricate hierarchical photonic structures, where the reconfigurability allows additional functionality by providing extra spectral selectivity. In particular, the combination of silk protein self-assembly, colloidal assembly, and top-down topographical templating has been exploited to integrate 2D diffractive micropatterns with 3D inverse colloidal crystals (Figure 2D–F) [31]. Such hierarchical opals not only embody the convergence of diffusion and diffraction with structural color in a single matrix, but also present a suite of unique optical functions that leverage the interplay of the individual responses. On the one hand, the existence of a 3D nanoscale photonic lattice modulates the diffraction of the 2D optical element by tuning its photonic bandgap; for instance, this can be achieved by controlling the number of layers and the lattice constant of the assembled colloidal crystal or by reconfiguring the photonic lattice by water vapor or UV light treatment. The variation of the reflected (or transmitted) diffraction intensity is consistent with the change of the reflection (or transmission) intensity because of the photonic nanostructure modulation (Figure 2E). On the other hand, the 2D microscale topography affects the overall color appearance of the structures, resulting in uniform structural color over a broad range of viewing angles. Such feature, combined with the reconfigurability of the nanoscale lattice and with the design of multicolor patterns, allows these structures to be used for wide-angle multicolor pattern displays with spatially dependent iridescence (Figure 2F).

The ability to reconfigure photonic lattices provides additional levels of functionality to these engineered photonic crystals, which become structures that not only display multispectral responses but can also be used for information encoding. This has been recently demonstrated by creating multispectral QR codes through the combination of inkjet printing and water vapor treatment (Figure 2G, H) [20]. Inkjet printing was used to modify preassembled photonic crystal lattices to create patterned templates (e.g., a QR code pattern) for silk infiltration and subsequent fabrication of patterned inverse opals. In particular, ethyl acetate was chosen as the ink to selectively remove the photonic lattice in the printed areas, therefore, creating a structure which displayed inverse opal lattices only in the non-printed regions. The resulting SIO QR code was further programmed by exposing specific lattice-bearing regions (colored squares in Figure 2G micrograph) to water vapor for different times, leading to a multispectral QR code with double encryption layers, namely, (i) the scanned message from the QR code, and (ii) a three-digit key defined by the RGB intensity of the reconfigured colors. A reconfigurable SIO itself is an ideal optical platform for pattern writing, which can be achieved by direct integration with inkjet printing and by using a solvent that can trigger the protein’s conformational transition (Figure 2H). Different volumetric changes of the silk matrix, and thus distinct differences in the lattice constant and in the reflected colors, can be easily generated by controlling the ink composition or volume. For example, light-blue and dark-blue colors were obtained, respectively, by inkjet printing MeOH/water mixtures with water ratios of 3 and 18% on a 10-layer SIO substrate. This strategy provides great opportunities for the fabrication of arbitrary, complex, and multispectral photonic patterns by manipulating the protein’s initial conformation, the lattice’s configuration, and the ink’s formulation.

In addition to forming homogeneous inverse opal structures, silk-based reconfigurable photonic crystal superlattices can also be fabricated by stacking layers of 3D nanolattices with different periodicity [39]. Multispectral structural colors can be easily achieved by reconfiguring the lattices of the formed heterostructures by water vapor exposure.

3.2 Reversibly tunable optics

Silk protein materials are also capable of reversibly responding to external stimuli, allowing for the fabrication of dynamic tunable optical devices. Crystalline silk offers an ideal platform for the generation of reversible stimuli-responsiveness, due to its molecular chains being anchored through physical crosslinking. The general strategy for implementing dynamic tunable optical function relies primarily on the reversible volume change of the silk matrix in response to various environmental fluctuations such as humidity, aqueous solvents, mechanical strain, and temperature. Among these, humidity is the most widely utilized stimulus given its ease and speed in inducing swelling of the silk matrix. Silk’s high sensitivity to humidity makes silk-based optical devices suitable for sensing applications as demonstrated by the several silk-based optical platforms used to generate dynamic tunable responses to humidity, such as inverse opals [40], thin-film and multilayer interference structures [41], [42], optical fibers [43], and metamaterials [44], [45]. As an example, silk-coated terahertz metamaterials can be programmed to display a dynamic response to humidity changes (Figure 3A), whereby the frequency of the devices is gradually blue-shifted upon humidity increase due to the variation of the dielectric constant of the silk’s film induced by the swelling [45]. Another type of reversible responsiveness that relies on the physical swelling of silk matrix is achieved through aqueous solvent infiltration; in this case, the water content in the solvent determines the degree of swelling of the silk constituent within the optical device and thus its optical response. Examples of such tunable devices include metal–insulator–metal resonators using silk protein as the insulator layer [46], [47], and silk plasmonic devices [48]. Mechanically induced tunable optics are based on the continuous and reversible variation of optical structures upon changes in the tensile strain applied to them. In this scenario, silk hydrogels provide an ideal material format to transform a mechanical stimulus into an optical response. A typical example is deformable silk hydrogel inverse opals (Figure 3B) [49], in which the elastic deformation of the hydrogel induces a change in the photonic lattice constant of the SIO resulting in reversible changes in the reflected structural color. Further, the large thermal expansion coefficient of silk allows for the development of thermally responsive optical devices. For instance, silk fibroin whispering gallery microresonators showed a significant resonant wavelength shift as a function of temperature, making them suitable for high-precision thermal sensors [50].

Figure 3:

(A) Schematic of humidity sensors based on silk-coated terahertz metamaterials. Reproduced from Ref. [45]. Copyright 2018, OSA publishing, CC BY. (B) Photographs of silk hydrogel inverse opal before (left) and after (right) stretching. Reproduced from Ref. [49]. Copyright 2017, National Academy of Sciences. (C) Fabrication schematic of a silk-based dynamic wrinkle pattern. (D) AFM images showing the reversible transition between wrinkled and wrinkle-free state of the silk-PDMS bilayer film. (E) Schematics (top) and corresponding photographs (bottom) of wrinkle patterns showing the switching between transparent and opaque by respectively exposure to WV and voltage application. (C, D, E) Reproduced from Ref. [51]. Copyright 2019, National Academy of Sciences. (F) Photographs of a magnetic silk film (top) and a silk sponge (bottom) with 50 wt% CrO₂ relative to silk fibroin. (G) Schematic of the actuation mechanism of a cantilever actuator based on heat-induced demagnetization and photothermal effect. (H) Schematics (top) and photographs (bottom) of a Curie engine rotating at 2 rpm under fixed local light illumination and immobile magnet. (F, G, H) Reproduced from Ref. [52]. Copyright 2018, National Academy of Sciences.

Dynamic, tunable optical functions can also be obtained through the combination of two independent stimuli-responsive behaviors. As a demonstration, the polymorphic transition of silk fibroin has been combined with its thermal responsiveness to develop reversible and dynamic wrinkling micropatterns [51]. These wrinkling systems consist of a bilayer structure composed of a stiff silk thin layer and a soft PDMS substrate. The mechanical mismatch between these two materials induces the formation of a wrinkled surface after application of thermal stimuli (Figure 3C). This wrinkle pattern accumulates compressive stress after its formation and can be tuned or erased through molecular rearrangement of silk with either water vapor, methanol vapor, or exposure to UV light (Figure 3D); these treatments cause the continuous disturbance of the localized stress field and the subsequent release of the compressive stress within the bilayer system. The wrinkles’ dynamics depend on the initial conformation of the silk film, showing that wrinkle tuning is governed by the interplay between silk protein chains and external stimuli. The switchable wrinkle morphology (Figure 3E), together with the versatility of silk protein, enables applications such as wearable optics, information coding, and thermal management.

Furthermore, reversible stimuli-responsiveness can be achieved by doping silk solutions with optically active components, adding functionality to mechanically robust and biocompatible platforms. One demonstration of this strategy is the fabrication of magnetic composites by incorporating chromium dioxide (CrO₂) in multiple silk formats such as films and sponges (Figure 3F) [52]. The low Curie temperature of CrO₂ along with its high photothermal conversion efficiency allows for the generation of a tunable magnetic response as a function of light, leading to light-controlled and dynamically tunable motions activated by fixed light source and magnetic field (Figure 3G, H). Devices fabricated from these composites show multiple complex locomotions, such as oscillation, gripping, and heliotactic movements, along with continuous rotation (Figure 3H). In addition to solution mixing, optical functionality can also be embedded in silk-based materials by chemical modification of the silk protein by taking advantage of its abundant surface chemistry. For instance, the incorporation of azobenzene moieties into silk structures creates a photo-responsive biomaterial (Azosilk) [53], [54], [55], whose optical absorption can be tuned by leveraging the reversible photo-switching between the trans and cis geometric isomers of azobenzene molecules. Optically switchable diffractive gratings have been developed by integrating an Azosilk grating with an elastomeric substrate, where the diffraction properties can be reversibly modulated by switching the light on and off [55].

3.3 Physically transient optics

Physically transient devices are programmed to either display an on/off signal or a modulation of their response as a function of time. Silk has been recently demonstrated to be a compelling material for the fabrication of both bio-electronic [56], [57], [58] and optically transient devices [15], [33], [59], [60] mostly due the ability to modulate its degradation. By properly tuning the cross-linking degree of the silk construct, its functional lifetime can be precisely programmed into the material so that it retains its structural integrity, and therefore, its form and optical functions, for a precise amount of time (from minutes to years) behaving as an intrinsically temporally transient device [24], [25]. The correlation between the silk structural degradation and the device’s optical response variation enables the fabrication of devices which can display an intrinsic, self-reporting, and continuous monitoring system. Silk-based transient devices find applications in drug delivery, resorbable implants and information security.

Silk’s high cytocompatibility [61], [62], and ability to stabilize labile compounds [63] have been exploited to use silk as a platform for drug-delivery devices. Therefore, silk-based transient optical devices can be particularly useful for the fabrication of comprehensive biomedical devices which simultaneously feature drug stabilization, controlled drug release and optical feedback (indirect drug release monitoring). The optical signature of these devices depends on their structural integrity and, if present, on the dopant type and loading [30]. Typically, silk fibroin is first mixed with therapeutics [19], and then molded into an optical element, such as microprism arrays [3], diffraction optical elements (DOEs) [15], [64], or fluorescent fibers [33], and finally annealed through water/methanol treatments to tune its degradation time. The progressive proteolytic cleaving of silk promotes the drug release while affecting the device’s form and, therefore, its optical response (Figure 4A). In particular, the amount of drug released can be correlated to changes in reflectivity for prism arrays [3] and to variations of the photoluminescent intensity and spectral position for fluorescent silk fibers [33]. For monochromatic DOEs, the signal-to-noise ratio (SNR) of the diffracted signal is used, instead, for the optical feedback: the simultaneous dissolution of the device and the release of the drug induce physical and chemical deterioration of the DOE (Figure 4B), leading to a progressive disappearance of the diffracted pattern and, therefore, to a decrease of the SNR [15, 18] (Figure 4C). The SNR variation can be correlated to the amount of drug released and, therefore, used as a self-monitoring system. Multiple drug release can also be monitored at the same time by embedding each one in a different component of multichromatic DOEs [64]. Depending on the silk conformation state, these devices can be programmed not only to lose their optical imprint within a specific timeframe but also to fully dissolve. This kind of transient devices provides monitoring of the therapeutic release while eliminating the need for retrieval of the drug-support system after completion of their functional lifetimes.

Figure 4:

(A) Effect of the annealing time (3, 6, 12, and 24 h) on silk DOEs structure degradation (top row, SEM images) and on the corresponding optical response (bottom row, diffracted pattern) as quantified after 48 h in vivo. (B) Reflected diffraction patterns of a drug-doped silk DOE at different times showing the progressive degradation of the diffracted signal. (C) Variation of the surface-to-noise ratio (SNR) of the DOE as a function of time. Inset: picture of the DOE implanted on the skin of a rat on an infection site. (D) Diffracted pattern (top) and schematics of the structure (bottom) demonstrating information concealment and reappearance obtained by combining a glass DOE (green) with a silk coating (yellow). (A, B, C, D) Adapted from Ref. [15]. Copyright 2017, WILEY-VCH Verlag GmbH & Co. (E) Silk multi-colored DOE with multiple crystallinity degrees: the blue DOE starts degrading after 3 min, the red DOE after 10 min and the blue DOE after 15 min leading to gradual destruction of the information. (F) Glass multi-colored DOE showing information concealment and reveal due to, respectively, the application and the removal of a silk coating. (E, F) Adapted from Ref. [64]. Copyright 2018, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS). Published by WILEY-VCH Verlag GmbH & Co. (G) Multi-metal metamaterials on silk substrates immersed in deionized water for 0, 5, and 10 min. (H) Schematic of multi-metal metamaterials on silk substrates showing the variation of the split ring resonator unit (top) and of the corresponding resonance (bottom) as a function of metals solubility. (G, H) Reproduced from Ref. [65]. Copyright 2020, WILEY-VCH Verlag GmbH & Co. (I) Doped fluorescent silk nanofiber strips after exposure to HCl vapors and after water rinsing. Adapted from Ref. [33]. Copyright 2017, The Authors.

Another simple yet elegant application of transient silk DOEs is for information security, in which the variation of the refractive index contrast is used to create transient patterns. Silk DOEs can display three operation modes: information concealment, information reappearance, and information destruction (Figure 4D–F). The information is first encoded by designing the structure of the DOE, it can then be revealed by illuminating the DOE with the specific wavelength it has been optimized for, and it can be further destructed by degrading the microstructure of the DOE through silk dissolution. Information concealment is achieved by using a combination of silk and another material with similar refractive index and by exploiting the variation of the refractive index contrast. For instance, a DOE pattern fabricated on glass (n_glass ∼ 1.52) can be clearly visualized when exposed to air and illuminated with a laser beam; its message can then be temporarily hidden by covering it with a silk coating (n_silk ∼ 1.54) that creates optical continuity by strongly decreasing the refractive index contrast and, therefore, it nullifies the DOE effect (Figure 4D); the pattern can, then, be further revealed by removal of the silk layer, which can, again, be intrinsically programmed by tuning the silk crystallinity [15].

Multichromatic silk-based DOEs enable multilevel encryption by containing multiple DOEs, each one optimized to work at a specific wavelength (Figure 4E, F) [64]. Notably, the degradation rate and sequence can be independently tuned for each monochromatic DOE, therefore enabling individual functional lifetimes for each diffracting pattern. This vanishing DOE system can store both temporally and chromatic dependent information. Multichromatic DOEs can also be used for quantitively monitoring chemical reactions by embedding solid reagents in silk DOEs and by immersing them in a liquid reagent to trigger the reaction; the monitoring of the variation of the SNR and of the optical density of the DOEs as the reaction progresses allows to obtain quantitative information on the formed products, provided prior calibration [64]. In addition, transient silk-based distributed feedback (DFB) lasers can be fabricated by repeated application and dissolution of a dye-doped silk solution on diffraction gratings [60].

Based on the same operation principle of controlled silk degradation, metamaterial devices able to work as transient, implantable and drug-delivery systems have been fabricated by patterning split ring resonators made of electrically conductive, biocompatible, and biodegradable metals on flexible silk substrates [65]. Both the metamaterial and the silk substrate can be programmed to independently affect the time-dependent resonance frequency response, respectively, by controlling the initial conformation of the silk matrix, as previously discussed (Figure 4G), and by using metals with different dissolution rates (Figure 4H). The ability to selectively remove structural components of the actual resonating structures (for instance achieved by using metals with different solubilities) allows for further complexity in the overall metamaterial response and in the structure’s degradation path. Notably, small changes in the structure of the metamaterial induce significant variations in the inductance and capacitance, therefore, inducing a shift in the resonant response and making these devices suitable for high precision monitoring. Though these devices are currently operating in the terahertz regime, they could be fabricated to display an optical response in the visible range useful, for instance, for in vivo sensing and for information encryption.

The low cost of silk-based transient optical devices makes them ideal for the fabrication of, for instance, disposable acid vapor sensors. Fluorescent electrospun silk fibers doped with sodium fluorescein have been, indeed, reported as high precision, disposable, low cost and conformable sensors for highly toxic acid vapors, such as HCl (Figure 4I) [33]. Fluorescent silk fiber patches can detect HCl in a broad range of concentrations (5–300 ppm) inducing a deterioration in the photoluminescent signal and a variation of its spectral position upon exposure to the vapor. Due to silk’s biocompatibility and tunability, it is possible to apply these sensors directly on the skin and to program them to vanish upon water rinsing without creating environmental hazards [33].