4D Printing as a New Paradigm for Advanced Manufacturing

Abstract

4D printing is a new manufacturing paradigm that combines stimuli-responsive materials, mathematics, and multi-material additive manufacturing to yield encoded 3D structures with intelligent behavior over time. This field has received growing interests from various disciplines such as space exploration, renewable energy, bioengineering, textile industry, infrastructures, soft robotics, etc. Here, after a review of 4D printing, three substantial gaps are identified. First, the main difference between 3D and 4D printed structures is one extra dimension that is smart evolution over time. However, currently, there is no general formula to model and predict this extra dimension. This gap pertains to the design aspect of 4D printing. Second, 3D printing is a well-known manufacturing process with its unique attributes. Now, 4D printing needs to be underpinned as a manufacturing process and its unique attributes should also be proved. This gap pertains to the manufacturing aspect of 4D printing. Third, various shape-morphing 4D printed structures have been illustrated in the literature. However, real applications and products, where 4D printing can provide unique features still need to be demonstrated. This gap pertains to the product development aspect of 4D printing. To address the first gap (design), we delve into the fourth dimension and reveal three general laws that govern the shape-shifting behaviors of almost all (photochemical-, photothermal-, solvent-, pH-, moisture-, electrochemical-, electrothermal-, ultrasound-, etc.-responsive) multi-material 4D structures. By starting from fundamental concepts, we derive and validate a universal bi-exponential formula that is required to model and predict the fourth dimension of 4D multi-materials. Our results, starting from the most fundamental concepts and ending with governing equations, can serve as general design principles for future research in 4D printing, where the time-dependent behaviors should be understood, modeled, and predicted correctly. Future 4D printing software and hardware developments can also benefit from these results. To address the second gap (manufacturing), first, we underpin 4D printing as a new manufacturing process and identify its unique attributes. Then, we specifically focus on the energy-saving attribute of 4D printing. We obtain the theoretical limit of energy consumption in 4D printing and prove that 4D printing can be the most energy-efficient manufacturing process. To address the third gap (product development), we demonstrate two real applications, where 4D printed products can provide unique features. First, we demonstrate a novel wind turbine blade based on 4D printing that provides several advantages in one blade, simultaneously. Scientists reported that leaf veins grow in a manner not only to facilitate their biological and physiological functions but also to sustain the environmental loads. Researchers showed that plant-leaf-mimetic blades could always have better structural properties compared with the conventional structures. However, the plant-leaf-mimetic blade has remained at the level of simulations. We demonstrate the plant-leaf-mimetic blade in practice that simultaneously has the capability of bend-twist-coupling. Second, we introduce the concept of smart solar concentrators inspired by nature and enabled by 4D printing. We found that diurnal flowers mainly have parabolic and nocturnal flowers mainly have hyperbolic petals. Based on this inspiration, we propose a smart solar concentrator that can increase the overall optical efficiency more than 25 percent compared with its non-smart counterparts.