Imagine that from particleboard bookshelves to apartment buildings, the machines and structures we use every day can be assembled. No more IKEA hex wrenches, no cranes, only 3D printed materials "know" how to fold, curl and harden, just like plants growing in time-lapse video.

Can we print four-dimensional objects?

Technically, everything is four-dimensional-according to physicists, it is actually 10-dimensional or higher-but we mainly consider the real world in terms of length, width, and height. The fourth dimension, time, we regard as the enemy, and we will try our best to resist its influence.

Therefore, we build walls and pipes that are as strong as possible and continue to repair them as we age, because construction requires time, money, and energy, and we don’t want to do it over and over again. But what if time is not the enemy? Suppose a structure can be unfolded like origami. Imagine whether its walls will bend or harden due to changing loads, or whether the buried pipe can change shape to adapt to changing water flow. Or pump water through peristalsis like the digestive system. With 4D printing, nothing will fall on the stone unless you wish.

If researchers and manufacturers can use it, 4D printing can change our entire manufacturing philosophy. Companies can print bunkers, machines, and tools, pack them flat, and then transport them to where they are needed (perhaps in disaster areas), or prepare for harsh environments such as space or the ocean floor. There, environmental conditions harmful to humans may actually push the shape and characteristics of objects to change-not only once, but repeatedly.

The core is the basic physics, chemistry and geometry behind the most ordinary natural processes. Consider how the shape of the hair changes when a storm comes. This is a simple airborne problem that causes keratin to form a very high percentage of hydrogen bonds, which causes them to fold back instead of stretching.

Two-dimensional devices do not require humans to build them, nor are they robots that require microchips, servo systems, and armatures to work.

The Chromat Adrenaline dress made of 3D printed panels uses Intel's Curie module. What makes it 4-D?

The core of 4-D printing is the result of 3-D printing and self-assembly in another frontier field.

Self-assembly is exactly what it sounds like: the spontaneous arrangement of parts into a larger functional whole. This field is very popular in the field of nanotechnology for two very good reasons. First, self-assembly has already taken place at the nanoscale and provides the driving force behind the processes from protein folding to crystal formation.

However, if we can increase the scale of self-assembly in proportion to people, it can enable us to make current products cheaper and simpler, or to create new technologies that would otherwise be impossible. This is hard work and often frustrating. Even under ideal circumstances, it would need to break the assembly sequence, develop programmable parts and provide an energy source that can make your equipment run. It is not a bad idea to establish some error corrections. But, basically, you need the right tools and materials to do this job.

Enter 3-D printing. Although new methods continue to emerge, traditionally, 3D printing requires repeated laying of well-defined polymer layers on the printing bed. As each new layer hardens and merges with the layers below, a three-dimensional shape appears. Early models can only print one material at a time, but newer 3-D printers can use a wider range of printing media, and can print with more than one material at a time. For 4D printing, this is an important breakthrough because the different materials allow developers to build in areas that will harden, bend or swell, or "hope" to fold in some way. They can have areas that absorb water like sponges, or areas that generate electricity when exposed to light.

This is what MIT’s Self-Assembly Laboratory calls programmable matter. This is a scientific, engineering, and materials method that focuses on matter that can be coded to reshape itself or change its function. One application of programmable materials is 4-D printing.

Variable market

Market research firm Marketsandmarkets predicted in a 2015 report that by 2025, 4-D printing will constitute an industry worth $555.6 million annually. The report assumes that 4-D technology will be commercialized in the short term, but the initial progress is small (this conversion brings huge business opportunities). The report specifically mentions the aviation, defense and military sectors, but it believes that industries such as automobiles, textiles, healthcare, construction and utilities are also potential early adopters.

Programmable things: geometry is destiny

Skylar Tibbits, director of the MIT Self-Assembly Laboratory, and his team have been leading this innovation.

MIT researchers are not the only researchers engaged in 4D printing, but the school’s self-assembly laboratory was the first laboratory that attracted attention, thanks in large part to the TED of its chief architect Skylar Tibbits speech.

Researchers in the laboratory first entered the world of self-assembly by creating simple large self-built robots. When they found that labor and expenditure were not feasible, they turned to shapes and materials with built-in logic.

In 2010, they created Logic Matter, a set of interlocking shapes that can solve computational problems using only their geometric shapes.

After all, computers use electronic gates that combine 1 and 0 to operate. These gates use Boolean algebra, and Boolean algebra will ask questions such as "both inputs are 1?". Or "Enter 1?" Tibbits Lab also asked the same question, just using complex polyhedrons instead of the usual electrical on/off states that represent 1 and 0. Input involves clicking the shape into place. This creates a new configuration that will allow the next shape (output) to be connected only in the up (true) or down (false) direction to provide an answer.

The logical matter has not risen to the level of self-assembly-the pieces required to be captured together by human hands-but it is indeed possible to build it by demonstrating the problem. In the following years, researchers in self-assembly laboratories increasingly turned to items that were consistent with their names: geometric shapes that would join together if rolled or shaken in a container, and chains of a specific shape when shaken. and many more.

This marks the next important step: to combine the built-in geometric trend with the input energy (or other environmental factors) to make it work.

But what is this geometric trend?

If you have ever tried to make something out of cardboard (or wood or metal), then you know it will be easier to fold if you score it first. Therefore, scoring is a kind of programming, a way to make the material more likely to behave the way you want. Now, imagine materials that replace cardboard, some of which can absorb moisture and grow, while others remain rigid. Throw it into the water and observe its shape change. Be smart enough by folding and scoring, and before you know it, you will find something truly special.

But first, you need a lot of precise control over the materials used and how the machine places the materials. This method will work better on a smaller scale. On a smaller scale, energy input and material differences will have a greater impact. Multi-material 3D printing can help provide researchers with the control they need, but they also need the right materials.

Self-folding origami

A team at Harvard University created an orchid that will take shape when placed in water.

When Tibbits mentioned his idea to people at Stratasys, a 3-D printing company based in Minnesota, they showed him a material that can grow 150% when immersed in water. Water provides a promising means for manipulating 4D objects, because nature provides working models of many objects that change shape as moisture changes. We call them plants.

Plants exhibit centripetal properties and grow in certain ways according to environmental factors, such as sunlight (phototropism), water (water-tropism), gravity, chemical substances (chemotaxis) and even physical contact (thixotropy). For example, plants tend to bend toward sunlight because sunlight kills hormones called auxins, thereby promoting growth. Therefore, the side of the plant facing away from the sun grows faster than the side facing the sun, causing the plant to bend toward the light. With a little imagination, it is easy to see how we similarly bend the physical principles that connect materials, environment and energy to bid.

In view of the inspiration provided by plants for 4D printing researchers, it is not surprising that the Harvard team produced "orchid" in 2016 by manufacturing 4D printed "orchid" (which has the shape of the same name when placed in water). The flower is printed with a hydrogel composite material, and then it is transported layer by layer through a pipe to the printing bed like a paper bag.

Two aspects of the printing process explain the behavior of the flowers. The first is to use hydrogel, which can absorb a lot of water. The second fact is that composite materials also contain cellulose fibrils-thin and strong fibers that are essential to plant structure. Because cellulose always flows in a known direction, the research team can carefully pattern it to control which parts of the flower may swell and which parts will remain rigid once exposed to water [source: McAlpine].

Undoubtedly, over time, we will see more experiments using a variety of other materials, such as conductors for flexible and dynamic circuits. However, we are also likely to see that the term "4-D printing" has its own life like most buzzwords and expands to cover a wider range of topics. For example, a company called Nervous System describes its novel technology for 3-D printing garments (that is, clothes made from nylon petals cleverly connected by joints) as "4-D printing"

Link to this article： 4D printing subverts the lifestyle of the future!

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