Self-growing photonic composites with programmable colors and mechanical properties

Structural coloration, first observed on peacock feathers, arises from micro/nanostructures that interfere with light, producing brilliant, stable, non-toxic colors. Artificial structural color materials are fabricated via top-down microfabrication (costly, low throughput) or bottom-up self-assembly (simple, scalable). However, achieving multicolor, high-resolution, durable patterns remains challenging: many current methods yield monochrome or iridescent responses, require uniform building blocks, and often result in brittle, non-reprocessable materials. The peacock’s feather provides a biological paradigm where keratin growth modulates lattice spacing among melanin rods to tune photonic bandgaps and coloration. Motivated by this, the study asks whether a synthetic photonic composite can be designed to grow its polymer matrix selectively to modulate lattice parameters and photonic bandgaps, enabling programmable colors and mechanical properties, patterning, and self-healing—while preserving ordered photonic structures.

Top-down fabrication can produce 3D photonic crystals but suffers from high costs and low efficiency. Bottom-up self-assembly of colloids is scalable, yet uniform particle sizes typically yield single or pseudo photonic bandgaps, leading to monochrome or iridescent outputs. Various multicolor patterning strategies have been explored: confinement deposition/swelling, regioselective removal, and post-modification of inverse opals. Selective swelling can endow stimuli-responsiveness and reversibility but requires region-dependent swellability and suffers instability from liquid-containing systems. Photopolymerization enables direct patterning by immobilizing compounds to modulate bandgaps, but bandgap variation is limited by rigid substrates. Many artificial structural color materials are fragile and not reprocessable. Existing approaches to improve mechanics include adding flexibilizers (which can disrupt colloidal order) or making inverse opals (often requiring HF etching, yielding milky appearance and lower color saturation due to scattering). Biological systems grow robust photonic structures by controlling matrix growth, suggesting a sustainable route to tunable coloration and mechanics.

System design: A photonic composite comprising SiO₂ nanospheres embedded within a dynamic, crosslinked acrylate polymer matrix capable of growth via nutrient uptake and photopolymerization. Growth relies on swelling in a nutrient solution (monomer, crosslinker, photoinitiator, and benzenesulfonic acid (BZSA) transesterification catalyst), followed by UV-induced polymerization (365 nm). The resulting new-old double network is homogenized via catalyst-enabled chain exchange (transesterification), relaxing the stretched original network to allow repeated growth cycles and further nutrient uptake.

Materials: SiO₂ nanoparticles synthesized by a modified Stöber method (TEOS in ethanol, ammonia catalyst; 60 °C, 8 h; washed ≥6 times; collected by centrifugation). Polymer matrix monomers: 4-hydroxybutyl acrylate (HBA, elastomeric), PEGDA (used in initial sample; growth with PEGDA maintains composition), and 2-hydroxyethyl methacrylate (HEMA, rigid). Crosslinker: 1,6-hexanediol diacrylate (HDDA). Photoinitiator: 2-hydroxy-2-methylpropiophenone. Catalyst: benzenesulfonic acid (BZSA). Alcohol additives (e.g., ethyl decanoate as model ester/alcoholysis-related agents) used to soften via transesterification/alcoholysis. Solvents: ethanol; additional THF control as hydroxyl-free additive.

Initial film fabrication: SiO₂/PEGDA precursors prepared by mixing PEGDA (with 1 wt% photoinitiator) into SiO₂ ethanol suspensions, vortexed/sonicated, annealed at 90 °C for 2 h to evaporate solvent, yielding a supersaturated acrylate/SiO₂ solution. Films formed by sandwiching precursor between glass and hydrophobic silicon wafer and UV-curing (365 nm, 10 mW·cm⁻², 2 min). Resulting films are vivid, ~0.14 mm thick (thickness 0.1–0.2 mm does not affect swellability/color). SEM shows predominantly short-range ordered domains (angle-independent color) interspersed with minor long-range ordered domains (iridescence minimal).

Growth protocol: Initial purple films (typical polymer matrix mass fraction 40 wt%, denoted EGₙ) immersed in nutrient solutions: B (HBA-based), EG (PEGDA-based), or M (HEMA-based), each containing HDDA, photoinitiator, and BZSA as specified. Samples swell, then are UV-irradiated (365 nm, typically 10 mW·cm⁻²) to polymerize entrapped nutrients, fixing increased size/color and toughening the material. Photopolymerization is exothermic, heating samples to ~47.1 °C in 50 s, facilitating transesterification to relax tensions and enable subsequent swelling in fresh nutrient (repeated growth). Catalyst-free controls do not reswell after the first cycle, confirming the necessity of chain-exchange homogenization. Samples are denoted EGₙ–B/EG/Mₘ, where m is net mass increase (wt%) of added polymer relative to the original.

Optical characterization: SEM cross-sections to measure double interplanar spacing 2d; UV–Vis reflection spectroscopy to track peak wavelength λ. Analysis uses Bragg’s law (mλ = 2dn_eff; m=1) and volume fraction relations for colloidal composites, with n_eff ≈1.453 at 20 °C due to index matching (SiO₂ ~1.45; polymer ~1.47).

Mechanical characterization: Tensile tests to obtain stress–strain curves and E-moduli; bending/flexibility tests; effect of nutrient monomer type and crosslinker content; alcohol-induced softening and post-annealing stiffening (70 °C, hours). Reshaping by softening in alcohol-containing nutrient, deforming, and annealing to fix shape; process can be repeated.

Patterning and self-healing: Spatially selective growth via photomasks induces localized swelling/polymerization with sharp boundaries and surface relief due to nutrient transport gradients. Multicolor images created by sequential masked irradiations with re-swelling between steps. Self-healing of scratches achieved by soaking in nutrient, UV irradiation of damaged region (10 mW·cm⁻², 2 min), and annealing (70 °C, 2 h), forming new matrix and restoring ordered structure.

• Tunable photonic bandgap/color across visible: Growth increases lattice spacing while preserving order, shifting reflection peak λ from 481 nm (EG₄₀) to 690 nm (e.g., EG₄₀–EG₄5.4), with corresponding SEM-measured 2d increasing from 313 ± 7.5 nm to 473 ± 9.2 nm.
• Predictable structure–color relations: Experimental 2d follows theoretical scaling with polymer matrix volume fraction (2d ∝ 1/√(1−ϕ)); measured λ vs 2d fits Bragg relation with n_eff ≈ 1.453 (20 °C).
• Swelling/growth behavior: Equilibrium swelling ratios for initial EG₄₀ in nutrients: B 9.1 wt%, EG 6.2 wt%, M 7.1 wt%. Post-UV growth fixes mass/color and toughens samples. Without catalyst, reswelling after first cycle is negligible; with catalyst, repeated growth is enabled.
• Optical quality: Grown samples exhibit intense, relatively narrow reflection peaks; peaks broaden slightly with growth due to minor decrease in order. Compared to one-step-prepared samples of similar composition, grown samples have sharper reflection peaks.
• Mechanical tuning: With crosslinker-free nutrients, E-modulus can be decreased (softening with B), maintained (EG), or increased (stiffening with M). Example E-moduli (MPa): EG₄₀ 344.2; EG₄₀–B₉.₇ 329.9; EG₄₀–EG₆.₉ 348.8; EG₄₀–M₉.₁ 455.8. Increasing crosslinker content in nutrient increases stiffness independent of monomer type.
• Flexibility and toughness: Alcohol additives induce reversible softening via transesterification/alcoholysis of ester linkages between SiO₂ surface hydroxyls and polymer/within matrix, enabling bending, rolling, twisting, folding. Grown EG₄₀–EG₁₀.₆ achieved greater strain at similar strength than compositionally similar as-prepared EG₄₅.₁. SiO₂-free PEGDA control showed ~50% flexibility increase with alcohol-containing nutrient versus alcohol-free. Hydroxyl-free additives (e.g., THF) did not soften.
• Reshaping: Alcohol-induced softening followed by annealing at 70 °C (e.g., 4–5 h) removes alcohol and reforms linkages, fixing new shapes; reshaping can be repeated. Deformed samples maintain shape even when elastic.
• Spatially selective growth and patterning: Masked UV yields localized growth with sharp boundaries; surface profile shows ~9.5 µm height drop over ~1 µm (aspect ratio ~9.5). Sequential masked irradiations on an initially blue EG substrate produced multicolor high-resolution images (e.g., “Sichuan facebook”). Residual nutrients can be re-mobilized for subsequent patterning without removal.
• Self-healing: Scratches that do not heal by heating alone are repaired via localized growth, restoring continuous photonic structure and mechanical properties. Healed region self-healing efficiency ~80.1% (by work under tensile loading); unhealed scratched samples broke easily (~8.8%).

The study demonstrates that controlled, uniform growth of a polymer matrix within SiO₂-based photonic composites enables precise modulation of lattice spacing and thus photonic bandgaps, directly addressing the challenge of tunable, on-demand coloration in structurally colored materials. The catalyst-enabled transesterification homogenization step is key to repeated growth cycles, permitting continuous tuning across the visible spectrum while preserving ordered colloidal architectures. Because refractive indices are nearly matched, strong multiple scattering in sufficiently thick films produces vivid colors; the structure–property relationships (2d–ϕ and λ–2d) are quantitatively predictive, enabling rational post-modulation. Beyond optics, the approach decouples mechanical property tuning from initial self-assembly constraints: composition can be adjusted post-assembly (monomer type, crosslinker level, alcohol additives), achieving flexibility, toughness, self-healing, and reshaping without compromising microstructural order. Spatially selective photogrowth affords high-resolution, multicolor patterning via simple photolithography with reusability due to retained nutrients. Collectively, these capabilities bring synthetic photonic materials closer to biological analogs (e.g., peacock feathers) that grow robust, tunable photonic structures, and open practical routes for scalable, functional structural color devices.

A self-growing photonic composite platform was established by embedding SiO₂ nanospheres in a dynamic acrylate network capable of nutrient-driven swelling, UV-initiated polymerization, and catalyst-enabled homogenization. Growth precisely and predictably tunes interplanar spacing and photonic bandgaps to span purple-to-red colors while maintaining ordered structures. The same mechanism allows programmable mechanical properties (softening or stiffening), substantial improvements in flexibility and toughness, reversible softening for reshaping, high-resolution multicolor patterning via spatially selective growth, and efficient self-healing that restores optical and mechanical integrity. The method is simple, low-cost, versatile, and scalable, and is compatible with common polymer systems, suggesting broad applicability in optoelectronics, anti-counterfeiting, sensors, and aesthetic materials. Future work can extend tuning beyond the visible, optimize refractive index contrasts without sacrificing order, and integrate responsive functionalities for dynamic color/mechanical responses.

• Low swelling ratios (6.2–9.1 wt% depending on nutrient) due to the rigid, low-polymer-fraction composite; although sufficient for tuning, this limits single-cycle magnitude of bandgap shift.
• Small refractive index contrast between SiO₂ and polymer leads to inherently weak reflection; vivid colors rely on thick films and multiple scattering.
• Slight decrease in structural order upon repeated growth is evidenced by peak broadening in reflection spectra.
• Catalyst-free systems cannot undergo repeated growth due to lack of homogenization; the approach depends on transesterification chemistry.
• Study focused on visible-range tuning; while cycles can, in principle, continue indefinitely, performance beyond visible was not explored.
• Patterned growth induces surface relief; while beneficial for some applications, it may be undesirable for strictly planar optical surfaces without additional processing.