Significance
Shape memory photonic crystals (SMPCs) can create smart composite materials that can dynamically alter their structure and color in response to external stimuli which can be of industrial use in environmental sensing, ink-free printing, anti-counterfeiting, and biomedical devices. Despite the promising of SMPCs they have several challenges which limit their widespread application specially how to achieve precise control over the shape memory and color-changing mechanisms, enhance the response time to external stimuli, ensure durability through repeated cycles of deformation and recovery, and develop scalable manufacturing processes. Additionally, the integration of SMPCs into practical applications requires overcoming limitations related to environmental stability, biocompatibility for biomedical uses, and customization for specific functional requirements. To better understand the SMPCs, new paper published in Nano Research by postdoctoral fellow Dr. Yong Qi & Professor Shufen Zhang from the Dalian University of Technology provided an expert and comprehensive review of the state-of-the-art in SMPC technology where they identified key areas for future research and potential interdisciplinary applications. They also explored the recent progress in multifunctional SMPCs and focused on the mechanisms that drive their shape memory effects and structural color changes and investigated traditional and cold programmable SMPCs, to unlock new applications and inspire the creation of next-generation SMPCs with enhanced performance and broader utility.
Various methods have been developed to fabricate SMPCs which integrated photonic crystals within shape memory polymer matrices. Typical fabrication techniques include self-assembly, lithography, and 3D printing, which are employed to create 1D, 2D, and 3D photonic crystal structures. The authors discussed the studies that showed the fabrication methods can achieve precise control over the periodicity and uniformity of photonic crystals embedded within the polymer matrix. For example, SEM and TEM images have demonstrated the successful integration of photonic crystals, which ensured that the SMPCs exhibit consistent and predictable structural color changes upon deformation and recovery. Moreover, several experiments have focused on the thermal-responsive behavior of SMPCs. By subjecting SMPC samples to varying temperatures, researchers have observed shape memory effects and corresponding color changes and the technique of differential scanning calorimetry (DSC) is often used to determine the thermal transition temperatures of the SMP matrix. The reviewed studies by Dr. Yong Qi & Professor Shufen Zhang indicated that SMPCs can recover their original shapes when heated above their transition temperatures, accompanied by significant changes in structural color. DSC data has shown that the transition temperatures can be fine-tuned by modifying the polymer composition, allowing for customizable thermal responsiveness.
On the other hand, cold programmable SMPCs are designed to respond to lower temperatures, which expand the applicability and research has demonstrated that cold programmable SMPCs can exhibit shape memory effects and color changes at significantly lower temperatures compared to traditional SMPCs. Mechanical testing has confirmed that these materials retain flexibility and durability, making them suitable for applications where thermal activation is impractical. Additionally, solvent-responsive SMPCs was discussed and studies to understand their behavior when exposed to various solvents and the shape changes and color variations were monitored using spectroscopic analysis. According to the authors, the solvent-responsive SMPCs have shown rapid and reversible shape and color changes upon exposure to specific solvents and spectroscopic analysis demonstrated that the photonic bandgap shifts in response to solvent-induced swelling of the polymer matrix, which confirmed the high sensitivity of SMPCs to chemical environments. Moreover, the authors discussed the influence of mechanical stress on structural color and researchers used optical microscopy to record and analyze the resulting color changes. The studies reviewed have demonstrated that SMPCs can undergo significant color changes in response to mechanical deformation. Optical microscopy images have shown a clear correlation between the degree of mechanical stress and the shift in structural color, indicating the potential of SMPCs for stress sensing applications.
The authors also discussed the application of SMPCs in ink-free printing which has been explored by programming the shape and color of the materials to create patterns. These patterns were tested for stability and durability under various environmental conditions. They reported in their review that ink-free printing have successfully produced high-resolution, multicolor patterns that remain stable and durable. Indeed, the ability to create and erase patterns using SMPCs without traditional inks is more sustainable and cost-effective alternative for various printing applications. Another potential application is in SMPC-based security features which have been developed and tested for their effectiveness in anti-counterfeiting applications. The SMPC-based security features can exhibit dynamic color changes that are difficult to replicate, which demonstrate their effectiveness in preventing counterfeiting and provide visually striking and easily verifiable methods for securing documents and products.
In conclusion, the expert opinion review by Dr. Yong Qi and Professor Shufen Zhang highlighted the transformative potential of SMPCs in multiple fields and showcased SMPCs’ ability to respond to environmental stimuli with color changes makes them ideal for developing advanced sensors for industrial and environmental monitoring. Qi and Zhang discussed how SMPCs can create high-resolution, durable, and erasable patterns without the need for inks which reduces environmental impact and printing costs which can be applied in packaging, textiles, and electronics. Moreover, the unique, difficult-to-replicate color patterns of SMPCs can enhance security in documents and products. Furthermore, SMPCs’ biocompatibility and responsiveness can be of benefit in developing smart implants that respond to physiological conditions of temperature or pH changes, to release drugs or adjust their shape. Additionally, SMPC-based sensors can detect biomarkers and provide diagnostic information which will advance early disease detection and personalized medicine. Another promising application the authors highlighted is how SMPCs can play a role in environmental monitoring and protection because of their high sensitivity to pollutants and environmental changes which allows for the development of smart sensors that can monitor air and water quality. These sensors can provide valuable data for environmental management and help in detecting hazardous conditions promptly.
Reference
Qi, Y., Zhang, S. Recent advances in multifunctional shape memory photonic crystals and practical applications. Nano Res. 17, 79–96 (2024). https://doi.org/10.1007/s12274-023-5801-0