UCLA

Morphogenesis is the generation process of specific morphologies of living organisms depending on complex biological, biochemistry and mechanical process. Researchers have put effort on these interesting morphology formation based on artificial materials and applied them to the electrical devices, clinical applications and etc. However, few researcher achieved longitudinal buckling formation on the 3D surface which is similar to the patterns appeared on cucurbitaceous fructification (such as pumpkin). Recently, theoretical calculation and simulation model, for example spherical core-shell structure and tubular structure, are employed to explain the mechanism, yet they are lack of experimental validation. The heterogeneities of growth rate and mechanical property in these researches are considered as the most significant factors in fabricating distinct morphologic formations. In this work, we demonstrate that longitudinal buckles of pumpkins can be reproduced by a barrel core-shell structure using 4D printing, which takes advantages of digital light processing (DLP) 3D printing and stimulus-responsive materials. The mechanical mismatch between the stiff core and compliant shell results in buckling instability on the surface. The initiation and formation of buckles are governed by the ratio of core/shell radius and the differences in swelling ratio and stiffness of the core and shell. We are capable of control the number of buckles increasing from 4 to 44 by simply tuning the swelling ratio of the shell. This heterogeneous structure with controllable buckling geometrically and structurally resembles the morphology of cucurbitaceous fructification and can be further applied to 3D anti-counterfeiting technologies. The heterogeneity mechanism in this work is also used to achieve different other morphologies like bending, helix and curving.