Brooks McKinney

Oct 9th 2020

4D Printing Promises Smarter Cities


Imagine bringing home a piece of “some-assembly-required” furniture from IKEA and discovering, to your utter delight, that your new table or stereo console actually assembles itself. Or that those new shades for your home windows autonomously close themselves at night and open themselves in the morning. Both of these fantasies may be coming soon to a city near you, thanks to 4D printing technology. And both may be part of a new paradigm of smart city technology.

Paradigm Shift

4D printing was first defined on the TED stage in 2013 by Skylar Tibbits, an assistant professor of design research at MIT and founder of the school’s Self Assembly Lab.

“4D printing is multi-material 3D printing plus a new capability called transformation,” Tibbits explained. “We program 3D materials to transform their shape, properties or even their function autonomously in response to external stimuli such as heat, moisture, light, vibration or electromagnetic energy.” These new materials, he added, can actually build themselves.

Tibbits emphasized, however, that 4D printing — the fourth dimension is time — is no longer strictly about printing. It includes many types of production techniques such as printing, lamination, injection molding and even knitting.

“The unifying concept is that we are integrating materials that change their behavior in response to external stimuli,” Tibbits said.

Material Differences

Tibbits is careful to distinguish the 4D work being done in his Self Assembly Lab from traditional smart city technology.

“The conventional approach to building ‘smart’ cars, ‘smart’ shoes, or ‘smart’ buildings has been to add a bunch of robotics — sensors, actuators, processors — to that application,” he said. “What we are proposing is eliminating all of the robotics, making the same ‘smart’ product, but doing it strictly through materials.” Eliminating those traditional electronics can reduce a product’s weight and cost, simplify its design and improve its reliability, he added.

Promising Apps

To date, Tibbits and his team have explored multiple applications of 4D printing to future smart cities, primarily in the areas of building materials, transportation and adaptable infrastructure.

For example, they developed a new paradigm for water delivery in which pipes — sans pumps or valves — can expand or contract on their own to change capacity or flow rate or even undulate to move water themselves. For cars, they have developed self-morphing skins that adapt their aerodynamic shape in response to increased airflow, air pressure, etc. And for coastal cities dealing with beach erosion caused by rising sea levels, they are working with researchers in the tropical island nation of Maldives to develop a system of underwater structures that use wave energy to create sand accumulation in strategic locations. This technology could be used to literally grow new islands or to rebuild beaches.

Industry Resistance

Unfortunately, 4D work remains largely a laboratory demonstration science. Its biggest limiting factor, as Tibbits sees it, is human adaptation, not the technology itself.

“4D printing is really just a different way of designing and making things,” he said. Historically, engineers have been focused on making things that do NOT move. By contrast, Tibbits’ team is focused on making highly active garments, buildings, cars and planes that are always moving, always transforming, always morphing and adapting to their environment.

“The limitation is not about the fundamental technology or material performance. It’s more about getting people to understand what’s possible,” said Tibbits.

Physics First

The notion of using 4D materials to protect buildings from earthquakes — imagine stiff structural materials that transform into resilient, shock-absorbing materials in response to vibration — remains tantalizing. The reality, however, is that the technology is still constrained by physics.

“4D objects need to be made from relatively soft materials such as polymers that can change shape rapidly in response to external stimuli,” explained Jerry Qi, professor of mechanical engineering at Georgia Tech. “Unfortunately, these softer materials do not perform well in applications such as buildings or highway structures that have to support a lot of weight.”

Stiffer materials such as stainless steel, concrete or iron are more practical for structural applications, he continued, but they generally do not respond quickly enough to external stimuli, which limits their usefulness as 4D materials.

A more achievable, near-term application in tomorrow’s smart city, suggests Qi, might be to embed 4D printed sensors in the exterior shell of electronic devices. “With 4D printing, you could integrate a thermometer, for example, in your bedside clock radio in a simple, elegant way while making that device more adaptive to its environment,” he said.

Scale Matters

Other issues to consider, said Howon Lee, assistant professor of mechanical and aerospace engineering at Rutgers University, are the scale of the object being produced and the capabilities of the 3D printing device. The key factor here is called resolution, i.e. the smallest feature you can print in detail.

“The larger the object you hope to print,” he explained, “the larger your smallest feature size will be.” Larger feature size means that molecules have to travel farther within the material to effect the desired transformation, which limits a 4D object’s ability to change quickly.

“If the length or thickness of a 4D object becomes 10 times larger, for example, its response time will become 100 times slower,” said Lee.

He equated this dilemma to trying to print a large-scale photograph using a digital file that contains only a small number of pixels, a situation that produces a coarse and “out-of-focus” image. The best approach is either to limit the size of your 4D printed object or to use a 3D printer that can print features with higher resolution.

Lee and his research team at Rutgers are trying to resolve this size limitation of 4D objects by decoupling the overall response time of 4D objects from their underlying physics. One way, he suggests, is to embed a series of smaller 4D objects with a characteristically shorter response time within a larger object. By placing these smaller objects strategically, Lee believes they can create larger objects that morph on the faster time scale of the smaller objects.

Fad or Future?

So will 4D printing become a ubiquitous part of the future or languish as a passing fad?

“The most important property for moving any research out of the lab into commercially viable products is reliability,” said Lee. “A product has to be proven to be functional, durable, and reliable for the long haul. If it is not reliable, no one will buy it.” Proving that reliability, he adds, will fall on the shoulders of investors and third-party testers.

Even more important to the future, however, will be industry awareness. For most people, it remains a very new concept.

“People tend to think of solutions to a problem only in terms of what they know,” said Lee. “Once they become aware of 4D printing, they realize that their problem could be solved easily with this technology. Our challenge, therefore, is to disseminate knowledge about the technology, get more people involved in the field, and encourage them to collaborate with 4D industry and university experts to discover smart solutions to their most urgent problems.”

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