Innovators in the 3D printing space are taking on the fourth dimension, time. This week, researchers at Harvard University announced the development of a new, bio-inspired 4D-printing method to create objects that respond to environmental triggers much like plants. Four-dimensional printing is an intriguing fabrication method that has gained traction since MIT research scientist Skylar Tibbits helped to popularize the term in his 2013 TED talk. In its most basic form, the process requires the combination of a smart material and a source of energy for its activation. For example, a long strand of material, like plastic, can transform into a predetermined shape, such as a cube, when submerged in water or heated.
Following Tibbits, researchers at institutions including the University of Colorado Boulder, the Singapore University of Technology and Design, and now Harvard have developed their own approaches to 4D printing. Their research is evidence that the still-nascent technology is moving closer, albeit slowly, to finding commercial application, with a noticeable shift in the nature of its output from frame-like, geometric constructions to fluid, organic forms. As intriguing as the idea of integrating time into the 3D-printing equation may be, however, it’s important to ask whether this technology is relevant to today’s design processes and, if so, how. Below, I describe three of the most promising 4D-printing methodologies developed to date, and their likely impact on the designed environment.
As director of the Self-Assembly Lab at MIT, Tibbits has published and lectured frequently about 4D printing. His approach centers on the fabrication of multi-material prints that are typically composed of a rigid, inert polymer, and a softer, activated component. The two materials are arranged to create segmented structures that transform when water or another energy source is introduced to the activated component, which is usually a hydrophilic polymer. In some cases, this component can expand up to 150 percent of its original size when activated. Printed as a joint between two rigid sections, this component can bend when activated, with its design dictating the direction. In this way, 4D prints are predictive structures with strategic, unidirectional hinges. The overall geometry can be rectilinear, as in Tibbits' self-folding strands, or planar, like his self-folding surface cubes. Other variants, such as discs or truncated octahedrons, exhibit different geometries yet still rely on the same activated joint principle.
The Colorado–Singapore group's method centers on a heat-activated process with shape memory polymers that incorporates activated materials throughout the entire print, instead of Tibbits’ binary model of rigid elements and activated hinges. Thin plates are imbued with glassy shape-memory polymer fibers to reinforce an elastomeric matrix. The desired activated geometries are a function of the fibers' printed patterns within the soft composite plates, a phenomenon that the scientists call “programmed lamina and laminate architecture.” In other words, the lamina microstructure (shown above) is used to incorporate material intelligence throughout the print. By embedding activation materials in such a way, the researchers hope to create complex 3D geometries such as folded shapes, coils, twisted strips, and other contoured forms whose non-uniform curvatures vary spatially.
The Harvard scientists take a third approach, which uses hydrogel printing. The researchers—hailing from the Wyss Institute for Biologically Inspired Engineering and the John A. Paulson School of Engineering and Applied Sciences—use a composite gel containing cellulose microfibers derived from wood, known as hydrogel-cellulose fibril ink. This approach is based on an attempt to emulate the natural microstructures in plants, particularly those that allow shape changes like thermonasty (movement in response to temperature) or mimetic polymorphism (shapeshifting to match nearby foliage—yes, plants practice biomimicry, too). The Harvard team developed precise swelling behaviors in strategic areas of the hydrogel structures that are similar to the tissue microstructures of flowers and plants. The cellulose fibers within the hydrogel ink are carefully aligned, so their grain stiffness and directional swelling are directionally dependent, or anisotropic. Anisotropy is a familiar property of cellulosic materials, like wood, that exhibit a grain and therefore behave differently in one direction than in another. Most modern products are treated to reduce or eliminate anisotropy (for example, plywood), but in this case the researchers sought to use the property as a responsive mechanism to encourage the material to bend in a certain direction. When the completed 4D prints, resembling flowers with petals, are submerged in water, the petals curl up in a predetermined direction towards the center.
Like the Colorado–Singapore team’s technology, Harvard’s method creates uniform fields of shapeshifting materials. The latter, however, results in a single bio-composite gel that closely resembles natural organisms, as opposed to a synthetic, petroleum-derived hybrid. The Harvard version also invites complex, imaginative forms within the printed templates. “It enables the design of almost any arbitrary, transformable shape from a wide range of available materials with different properties and potential applications, establishing a new platform for printing self-assembling, dynamic microscale structures that could be applied to a broad range of industrial and medical applications,” said Donald Ingber, founding director of the Wyss Institute, in a press release.
The idea of self-assembly has long been associated with robotics. A decade ago, “robotics were the solution to programmable matter, and I think over time all of us have shifted into a much softer, more adaptive, more responsive vision of what programmable matter is,” Tibbits told the journal Research-Technology Management. Four-dimensional printing has nonetheless borrowed from robotics, namely motion and autonomic behavior, but it uses smart materials instead of mechanical parts, motors, and batteries. The three examples discussed in this article outline 4D printing’s general trajectory from geometric, frame-like constructions towards soft organic structures. They also show that there can be many different approaches to 4D printing, all based on the simple ingredients of a smart material and an activation source.
Several manufacturers representing a range of industries are interested in the technology, including aircraft manufacturer Airbus, carbon fiber composite–fabricator Carbitex, and auto maker Briggs Automotive, according to Research-Technology Management. Other potential applications include footwear, apparel, furniture, electronics, and building materials. While 4D printing remains a nascent technology requiring more research and development before it is ready for commercial use, the dream for a highly tunable, bio-receptive, and resilient designed environment is beginning to come into focus.
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