Self-growing photonic composites with programmable colors and mechanical properties

Structural coloration, unlike pigment-based coloration, arises from the interaction of light with microscopic structures. This phenomenon, observed in nature by scientists like Hooke and Newton centuries ago, has inspired the development of artificial structural color materials for various applications, including optoelectronics and anti-counterfeiting. Current methods for fabricating these materials often involve energy-intensive top-down or limited bottom-up approaches that restrict color versatility and mechanical properties. Top-down methods, such as microfabrication, are expensive and inefficient for large-scale production, while existing bottom-up self-assembly methods often result in monochrome or iridescent colors due to limitations in controlling the photonic bandgap. Multicolor patterning, crucial for many applications, relies on techniques like confinement deposition/swelling, regioselective removal, or post-modification, but these methods often result in ephemeral, low-resolution, dull, stiff, and poorly controllable images. Natural structural color systems, such as peacock feathers, demonstrate a superior approach: selective growth of the keratin matrix precisely controls the spacing of melanin rods, thus adjusting the photonic bandgap and producing diverse colors. Mimicking this biological process offers a pathway to creating more advanced synthetic structural color materials. This research aims to develop a self-growing photonic composite system that combines the advantages of both bottom-up self-assembly and controlled growth to produce vibrant, robust, and adaptable structural color patterns.

The field of artificial structural color materials has seen significant advancements, but challenges remain in achieving the versatility and robustness of natural systems. Existing methods for creating multicolor patterns, such as those involving swelling or photopolymerization, often suffer from limitations in control, resolution, and material properties. While top-down approaches provide precise control, they lack scalability and cost-effectiveness. Bottom-up self-assembly, while scalable, often produces limited color variations. Bio-inspired approaches, mimicking the structural color generation in organisms like peacocks and butterflies, offer a promising avenue for creating advanced materials. Studies have investigated the intricate microstructures found in nature, revealing how controlled growth processes lead to diverse colors and patterns. These studies highlight the potential of mimicking natural growth mechanisms for creating high-performance structural color materials.

The researchers developed a self-growing photonic composite system composed of SiO₂ nanospheres embedded in a polymer matrix. Acrylate-based polymers, known for their versatility, were selected for the matrix. The initial photonic composite films were fabricated through the self-assembly of SiO₂ nanospheres and poly(ethylene glycol) diacrylate (PEGDA) followed by UV curing. The resulting films exhibited angle-independent colors due to the presence of both long-range and short-range ordered domains. The growth process involved immersing the films in a nutrient solution containing monomers (HBA, PEGDA, HEMA), a crosslinker (HDDA), a photoinitiator, and a catalyst (BZSA). This solution causes the polymer matrix to swell. Subsequent UV irradiation triggers photopolymerization, creating a new-old double network structure. The catalyst facilitates chain exchange between the new and old networks, ensuring homogeneous growth and allowing for further swelling and growth cycles. The wavelength of the reflected light can be estimated using Bragg's diffraction equation, where the interparticle distance (d) is modulated by the growth process. Different monomers in the nutrient solution allow tuning the mechanical properties of the resulting material. For spatially selective growth, patterned photoirradiation was employed. This allowed creating multicolor patterns by selectively growing different regions of the film with varying compositions and colors. For self-healing, damaged regions were subjected to localized growth, restoring the original structure and color. The mechanical properties of the materials were characterized via tensile testing, while optical properties were analyzed using UV-Visible reflection spectroscopy and scanning electron microscopy (SEM).

The self-growing photonic composite system successfully demonstrated several key features: 1. **Programmable Coloration:** The system allowed for precise control over the photonic bandgap and hence the color of the material by changing the composition of the nutrient solution and the number of growth cycles. Colors were tunable across the entire visible spectrum, from purple to red. The reflection peaks observed in UV-Vis spectroscopy were sharp and intense, indicative of a well-ordered structure. 2. **Tunable Mechanical Properties:** By modifying the monomer type or crosslinker concentration in the nutrient solution, the researchers successfully modulated the elasticity modulus of the material. They also demonstrated a method for making the initially brittle material flexible using alcohol as an additive, which induces reversible alcoholysis of the ester linkages. This alcoholysis allowed the material to be reshaped and then fixed in the new shape by annealing to remove the alcohol. 3. **Spatial Selective Growth and Multicolor Patterning:** Localized growth using patterned UV irradiation was demonstrated to achieve sharp boundaries between regions of different colors. This spatial control allowed the creation of complex, high-resolution multicolor patterns, significantly advancing patterning capabilities compared to previous methods. 4. **Self-Healing:** The researchers demonstrated that damage caused to the film could be repaired by localized growth in the damaged area, completely restoring the structural color and mechanical properties. This self-healing mechanism differed from previous approaches, combining advantages of both intrinsic and extrinsic self-healing. The self-healing efficiency of 80.1% further validated the effectiveness of this method. The experimental results showed a strong correlation between the interplanar spacing and the reflection wavelength, confirming the predictability of the color modulation. The theoretical and experimental data closely aligned, suggesting precise control over the optical properties of the material through growth.

This research successfully demonstrates a novel, bio-inspired approach to fabricating photonic structural color composites with superior properties compared to existing technologies. The ability to precisely control color, mechanical properties, and patterning, combined with self-healing capabilities, addresses key limitations of current structural color materials. The simplicity, scalability, and cost-effectiveness of the self-growing method make it highly promising for various applications, offering significant advantages over traditional techniques. The ability to achieve high-resolution multicolor patterns with precise control over both optical and mechanical properties opens new possibilities for advanced applications in diverse fields.

This study presents a bio-inspired self-growing approach to produce photonic composites with exceptional control over color, mechanical properties, and patterning. The method offers a simple, scalable, and cost-effective way to create advanced structural color materials with desirable properties like flexibility, toughness, self-healing, and reshaping capabilities. Future research could explore the use of different polymer systems, expanding the range of achievable colors and properties. Investigating the long-term stability of the material and exploring its potential applications in various fields, such as flexible electronics, sensors, and biomedicine, are also promising avenues for future research.

While the study demonstrates significant advancements, there are certain limitations. The long-term stability of the self-healing properties under various environmental conditions needs further investigation. The current study focused on visible-light applications; exploring the potential of this methodology for other spectral ranges could broaden its applications. Furthermore, a more in-depth analysis of the influence of different monomers on the mechanical properties could provide further insights into material design and optimization.