Flexible materials are highly desirable for various applications, particularly in flexible electronics and optoelectronics. While flexibility has been observed in some traditionally rigid crystalline materials like ice and diamond, the focus has shifted to organic crystals due to their potential in flexible devices. Many organic crystals, especially those with high aspect ratios, exhibit elasticity and deformability. Although the mechanisms behind this flexibility are still under investigation, their application prospects are clear, with potential in flexible wearable electronics and shape-shifting systems. Previous attempts to control the shape of these crystals have employed light, heat, mechanical force, and humidity gradients. However, these methods often suffer from drawbacks like internal structural changes (light and heat), mechanical damage and short lifespans (force), or slow response times (humidity). Furthermore, precise control over the degree of deformation remains a major obstacle. This research aims to address this limitation by exploring an alternative approach using magnetic fields for remote and precise control over the shape of flexible organic crystals. Magnetic materials offer advantages in terms of remote manipulation, dexterity, precision, speed, and robustness, surpassing other methods in microrobotics and biomedical applications. This study builds upon the success of magnetic field control in polymers and introduces a method to achieve precise, remote shape control of flexible organic crystals without direct contact or exposure to other stimuli.

The literature extensively documents the growing interest in flexible organic crystals and their potential applications in flexible electronics and optoelectronics. Studies have demonstrated the flexibility and elasticity of various organic crystals, utilizing techniques such as X-ray crystallography to analyze their structures and mechanical properties. Several methods for manipulating crystal shape have been explored, including photo-induced bending using light, thermally induced twisting and bending using heat, and mechanically induced deformation using force. However, limitations such as structural damage, slow response times, and lack of precise control over the degree of bending are discussed in existing literature. The existing literature also highlights the use of magnetic materials in controlling the shape and movement of other materials, such as polymers, demonstrating the potential of magnetic fields for precise and remote manipulation.

The researchers synthesized six organic small molecule compounds (1-6). Acicular crystals were grown via solvent evaporation, exhibiting elasticity when subjected to external force. The crystals were then coated with multiple alternating layers of positively charged poly(diallyldimethylammonium chloride) (PDDA) and negatively charged poly(sodium 4-styrenesulfonate) (PSS) polymers. Iron(III) oxide magnetic nanoparticles (MNPs) were then attached to a portion of the polymer-coated crystals, creating magnetic hybrid crystals (MNP//1-6). The thickness and length of the MNP layer were controlled by adjusting the concentration of the MNP suspension. Scanning electron microscopy confirmed the uniformity and thickness of the MNP coating. The mechanical properties of the hybrid crystals were assessed to ensure that the coating did not significantly alter the crystals’ original mechanical properties. Experiments to demonstrate precise shape control involved fixing one end of the MNP//1-6 crystal and manipulating the free end using a permanent magnet. Different shapes, letters, and numbers were formed, demonstrating 2D and 3D shape-shifting capabilities. The controlled movement of a crystal with MNPs on both ends was also achieved, demonstrating the potential for biomimetic actuator applications. To quantify the precise curvature control, crystals were fixed at one end, and a magnet was used to direct the free end to several specific points, achieving precise angular control. The durability and fatigue resistance of the crystals were evaluated by repeatedly bending the crystal between two set points for over 3000 cycles, demonstrating the robustness of the method. Optical waveguiding properties were tested by exciting the MNP//4 crystal with a UV laser, and the light output was controlled by manipulating the crystal's shape using a magnet. Optical loss coefficients (OLCs) were measured for both the original and hybrid crystals to assess the impact of the MNP coating on waveguiding properties. The relationship between magnetic field strength and the degree of bending was investigated by gradually approaching a magnet to a fixed crystal, measuring the magnetic field strength and corresponding bending angle. Various characterization techniques, including X-ray crystallography and scanning electron microscopy, were employed throughout the study.

The study successfully demonstrated a novel method for remote and precise control over the shape and movement of flexible organic crystals. The hybrid crystals, created by coating organic crystals with polymers and magnetic nanoparticles, exhibited highly precise shape control through magnetic fields. The method allowed for arbitrary shape manipulation in both two and three dimensions, creating various shapes, letters, and numbers with high accuracy. Microscopic motion of the crystals, akin to an inchworm-like motility, was also achieved. The precise curvature control was demonstrated by bending the crystals to specific angles with high accuracy and reproducibility. The hybrid crystals showed remarkable durability, maintaining precise shape control even after thousands of bending cycles. The OLC values for MNP//4 crystals showed minimal change compared to the original crystals, indicating that the MNP coating did not significantly affect optical transmission. The study established a clear relationship between magnetic field strength and the degree of crystal bending, opening avenues for further modulation of crystal deformation using variable magnetic fields. The method's ability to control crystal bending and movement was effective in both air and liquid media, broadening its applicability to underwater sensing applications. The successful use of the hybrid crystals as optical waveguides, with light output precisely controlled by magnetic fields, underscores their potential in flexible optoelectronics.

This research successfully addressed the significant challenge of precise and remote control over the shape of flexible organic crystals, a key limitation in harnessing their potential for flexible devices. The use of magnetic fields, combined with polymer-coated crystals and magnetic nanoparticles, proved superior to previous methods utilizing light, heat, or humidity gradients. The achieved level of control, along with the robustness and durability demonstrated, opens promising avenues for advanced applications. The minimal impact of the MNP coating on the crystals' optical properties validates their suitability for optoelectronic applications. The relationship between magnetic field strength and bending degree suggests further opportunities for refined control and modulation of crystal deformation. The ability to perform the shape manipulation in both air and water further broadens the potential applications of this technology. These results represent a notable advancement in crystal adaptronics and suggest significant potential for creating a new generation of flexible devices.

This study presents a groundbreaking non-destructive method for remotely and precisely controlling the shape and motion of flexible organic crystals using magnetic fields. The developed hybrid crystals exhibit superior control, robustness, and durability compared to existing techniques, expanding the applications of these crystals in flexible sensors, actuators, and optoelectronic devices. Future research could focus on exploring different types of magnetic nanoparticles and polymers to enhance performance, and further investigate the potential of this technology in creating complex micro-robotic systems and advanced biosensors.

While this study demonstrates a significant advance in controlling flexible organic crystals, some limitations exist. The current method requires attaching magnetic nanoparticles to the crystal surface, which might not be suitable for all types of crystals. Further research is needed to explore alternative attachment methods or investigate if other types of magnetic materials could be used. The study primarily focused on a specific set of organic crystals; further investigation is needed to determine the method's generalizability to a wider range of organic crystal materials. Although the hybrid crystals exhibited high durability, long-term stability studies under various environmental conditions are still necessary.