Shape morphing of plastic films

Three-dimensional (3D) architectures are highly sought after for functional materials and electronic devices due to their ability to accommodate more functions and offer superior spatial resolution compared to their two-dimensional (2D) counterparts. Current fabrication techniques, however, often limit device substrates to 2D geometries or simple macroscale 3D shapes. Shape morphing presents a promising alternative, typically relying on internal stresses generated by heterogeneous expansion or compression of responsive materials. Existing shape-morphing methods are often limited to heterogeneous or bilayer responsive materials, excluding the use of free-standing, inert homogeneous plastic films such as polyethylene terephthalate (PET) and polyimide (PI), which are crucial substrates in flexible electronics. This research addresses this limitation by introducing a novel strategy to achieve shape morphing in these important materials.

The literature extensively documents the benefits of 3D architectures for enhanced functionality in materials and electronics, particularly regarding strain sensing and photodetection. Existing 3D fabrication methods, including spin coating, lithographic patterning, etching, and thin film deposition, mostly constrain substrates to 2D or simple 3D geometries. Shape morphing techniques utilizing responsive materials, such as bilayer polymers, patterned polymers, and liquid crystal polymers, have shown promise but are limited to heterogeneous materials or lack applicability to free-standing inert plastic films. Compressive buckling has been explored for transforming 2D micro/nanostructures into 3D devices, however, it's constrained to substrates and is unsuitable for free-standing inert plastic films. The lack of a general method for shape-morphing inert homogeneous plastic films motivated this research.

This study introduces a novel shape-morphing strategy based on the controlled peeling of 2D plastic film precursors from adhesive substrates. The process programs the distribution and direction of plastic strains within the film, leading to the formation of various free-standing 3D structures. A peeling model was developed using finite-element analysis (FEA) to understand the mechanism of shape morphing and predict resultant morphologies. The model considers key parameters such as adhesion energy, peeling speed, peeling angle (φ), and deviation angle (δ). The peeling angle dictates the asymmetry of plastic strain, while the deviation angle defines the strain orientation. FEA simulations were conducted using ABAQUS, employing a cohesive element model and an elastic-plastic model for the adhesive layer and plastic film, respectively. The model was validated against experimental results. The effects of various parameters, including adhesion energy, peeling speed, adhesive thickness, film thickness, and Young's modulus on the resulting curvature and chirality were investigated both theoretically and experimentally using PTFE, PI, and PET films with different adhesive layers (PDMS with varying crosslinking and commercial tapes). The fabrication of complex 3D structures was achieved by gradually changing peeling parameters during the peeling process, resulting in spiral-like, helical, and conical spiral structures. Polygonal shapes were fabricated by arranging adhesive layers with predetermined widths and intervals. A two-step peeling process was used to create hyperboloids. The application of this technique to 3D electronics (circuits and piezoelectric systems) and 4D structures (using responsive bilayer films) was demonstrated. Characterization techniques included optical microscopy, scanning electron microscopy (SEM), and resistance measurements.

The research demonstrates that peeling-induced shape morphing is a feasible and versatile method for creating various 3D structures from 2D plastic film precursors. The FEA model accurately predicts the morphologies obtained experimentally by adjusting the peeling parameters. The curvature of the peeled film was found to be significantly influenced by adhesion energy, peeling speed, adhesive thickness, film thickness, and Young's modulus. Higher adhesion energy, peeling speed, and thinner adhesive layers resulted in films with larger curvatures. Thicker films and films with higher Young's moduli exhibited smaller curvatures. The peeling angle had a substantial effect on the curvature, with higher angles leading to greater curvatures. The deviation angle controlled the chirality and pitch of the helical structures. By controlling these parameters, the researchers successfully created a wide range of 3D shapes, including tubes, helices, spirals, polygons, and hyperboloids. The applicability of the technique was further demonstrated by fabricating 3D circuits, piezoelectric systems, and 4D structures using responsive bilayer films. In 3D circuits, the resistance increase was minimal even with significant bending, demonstrating the robustness of the method. The 3D piezoelectric systems showed enhanced performance compared to their 2D counterparts, exhibiting broader frequency sensing and higher voltage generation. 4D shape transformation was achieved with bilayer PEO/PDMS films, where humidity changes induced further shape changes in pre-morphed 3D structures. The method was also extended to create 3D elastomer films using plastic films as templates, demonstrating the versatility of the approach.

This study successfully addresses the challenge of fabricating complex 3D structures from inert homogeneous plastic films, a significant limitation in current flexible electronics manufacturing. The peeling-induced shape-morphing strategy offers a simple, versatile, and scalable approach, applicable to a wide range of materials. The ability to predict and program the final 3D geometry by manipulating peeling parameters is a key advancement. The enhanced performance of the 3D circuits and piezoelectric systems demonstrates the practical value of this technique. The integration of responsive materials further expands the possibilities, opening avenues for 4D devices and advanced functionalities. The findings have significant implications for the fabrication of flexible electronics, sensors, actuators, and other functional 3D devices.

This research demonstrates a novel and versatile peeling-induced shape-morphing strategy for creating complex 3D structures from 2D plastic film precursors. The ability to precisely control the shape and chirality of the resulting structures, coupled with its applicability to diverse materials and the demonstration of enhanced performance in 3D electronics, highlights the significant potential of this technique for advancing the field of flexible electronics and functional materials. Future research could explore the integration of more complex materials and functionalities, further optimization of peeling parameters for even finer control over shape, and the application of this technique to large-scale manufacturing processes.

While the study demonstrates the effectiveness of the peeling-induced shape-morphing strategy for a variety of materials and structures, potential limitations exist. The accuracy of the theoretical model depends on the accuracy of the material parameters used in the simulations. The complexity of the peeling process and the interaction between the film, adhesive, and substrate could lead to variations in the final structure. The scalability of the technique to large-scale manufacturing requires further investigation. The current focus is on relatively simple structures; extending the method to create highly intricate 3D shapes might require more sophisticated control over the peeling process.