Abstract
Abstract The rapid development of additive manufacturing and advances in shape memory materials have fueled the progress of four-dimensional (4D) printing. With the right external stimulus, the need for human interaction, sensors, and batteries will be eliminated, and by using additive manufacturing, more complex devices and parts can be produced. With the current understanding of shape memory mechanisms and with improved design for additive manufacturing, reversibility in 4D printing has recently been proven to be feasible. Conventional one-way 4D printing requires human interaction in the programming (or shape-setting) phase, but reversible 4D printing, or two-way 4D printing, will fully eliminate the need for human interference, as the programming stage is replaced with another stimulus. This allows reversible 4D printed parts to be fully dependent on external stimuli; parts can also be potentially reused after every recovery, or even used in continuous cycles—an aspect that carries industrial appeal. This paper presents a review on the mechanisms of shape memory materials that have led to 4D printing, current findings regarding 4D printing in alloys and polymers, and their respective limitations. The reversibility of shape memory materials and their feasibility to be fabricated using three-dimensional (3D) printing are summarized and critically analyzed. For reversible 4D printing, the methods of 3D printing, mechanisms used for actuation, and strategies to achieve reversibility are also highlighted. Finally, prospective future research directions in reversible 4D printing are suggested.
Highlights
Three-dimensional (3D) printing, known as additive manufacturing or rapid prototyping, has been around for three decades, ever since its commercialization in 1987 with stereolithography from 3D Systems [1]
The development of smart materials has established a foundation for a faster pace in the development and improvement of 4D printing
This table indicates that more mechanisms are used in conventionally fabricated shape memory materials than in 4D printing
Summary
Three-dimensional (3D) printing, known as additive manufacturing or rapid prototyping, has been around for three decades, ever since its commercialization in 1987 with stereolithography from 3D Systems [1]. 4D printing is a combination of 3D printing and the fourth dimension, which is time [15,16,17] This technique allows a printed object to be programmed to carry out shape change while adapting to its surroundings. This breakthrough technology is mainly fueled by the rapid development of smart materials and the latest progression of multi-material printing. SMAs are usually subjected to a programming process between two transformation phases of metal alloys These phases depend on differences in temperature [20] or magnetic field [21]. To achieve 3D printing of SMAs and shape memory metals (SMMs), it is necessary to understand the functioning mechanisms of different SMAs and SMMs before reviewing their repeatability and current printability
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