4D printing of smart material-based structures

Abstract

4D printing technology is a manufacturing technology where a 3D printed object changes its shape, color, function, state, or other properties over time when external stimuli (heat, pH, electricity, humidity, etc.) are given. This technology offers many advantages over the spatial and temporal limitations of additive manufacturing or 3D printing. Self-deformation, self-assembly and multi-functionality is possible after 3D printing, and since the shape and other properties of the object can be dynamically changed according to the external environment. It also has benefits in terms of productivity compared to the conventional 3D printing technology. The cost can be lowered by the reduced need for supporting materials, while manufacturing time can be reduced with deformation. Furthermore, if the deformations are considered in the design stage, it is possible to manufacture objects larger than the size of the 3D printer bed. Additionally, it becomes possible to manufacture complex designs and control many mechanical properties compared with the traditional manufacturing methods of smart materials such as gas foaming, solution casting, molding and extrusion. Thus it has application potentials in many fields such as in biomedical engineering, military, aviation, and space, and attempts are being made to realize these.

In this dissertation, the deformation characteristics of smart materials (hydrogel and shape memory polymer) that are essential for their use in 4D printing technology were discussed. It includes experiments on how the amount of deformation changes with the design parameters and analysis of the results. Smart material-based structure models designed based on these studies of deformation characteristics were also presented for applications in various environment.

First, a dual-layer structure composed of hydrogel and non-hydrogel materials with a difference in expansion coefficients of ethanol was manufactured using a 3D printer. Then, changes of curvatures of bending deformation by design parameters (total thickness and thickness ratio) of the dual-layers were investigated. A starfish model based on the study of changes in the curvatures experimentally showed that arms bend in ethanol. Additionally, a model that transforms from a flat structure into a cylinder structure through time and retains the transformed shape even without ethanol was demonstrated. Guiders and stoppers are specially designed at both ends in consideration that a complete cylinder structure can be stably formed and retained. These proposed models are expected to be applicable to grippers, sensors, medical stents, and actuators.

Second, two kinds of kinetic components consisting of a shape memory polymer that recovers to its original shape when a higher than glass transition temperature is applied were presented. This can drive mechanical motions (rotation and torsion) and can be applied to models with complex motions or large shapes. The real recovery angles of the kinetic components were compared to the initially set angles. Motion data of the kinetic components, collected from experiments based on the parameters of each kinetic component, were uploaded to a kinetic library. The kinetic library was designed to predict the motion data of new parameters by using a neural network trained with uploaded data. These data in the kinetic library were used for simulation in the design stage of models that used kinetic components for checking collisions between the parts as well as aiding in choosing the right kinetic components for models. One of the models realized in this study is the icosahedron model, which inserted the rotational kinetic components to hinges between each of the 20 faces, was completely transformed from a flat structure. A solar panel model showed the unfolding of panels with high spatial efficiency by using the rotational kinetic components. The trajectory of the sun can also be tracked by using the torsional kinetic component to increase the incident solar radiation. Transformations without use of actuators or external power supplies can be made possible. In this study, these two models were subjected to transformations in hot water (about 60℃), but can be used in a variety of environments such as the desert, sea, space, etc., by adjusting parameters such as the glass transition temperature and rate of transformation.

This work described the whole process of manufacturing 4D printed smart material-based structures: from the design of the initial object, simulations of the changing object, manufacturing the initial object, up to the final desired shape prompted by an external stimulus. This work will serve as basis for working with smart materials for 4D printing.