4D printing: the next dimension with smart materials

Abstract

As regards the additive manufacturing process, based on the ISO/ASTM 529000 standard, AM methods can be classified into seven different categories, which are currently available commercially, each having its advantages and limitations. This graphic represents this classification based on the technology, feedstock, energy source, build volume, etc.
Whatever the type of the used process, the objective behind developing these different technologies is to reduce production time with no extra cost for manufacturing parts with complex morphology. From my point of view, we are still far from reaching this goal, but the flexibility offered by this technology makes us more tolerant and orients our focus on other advantages, such as the use of smart materials to manufacture new generations of parts with functionalities that are not easy to obtain with traditional processes such as machining or moulding.
In the past five years, there have been more and more new 3D printer suppliers and manufacturers. However, only a limited number of these printer enables the production of 3D parts with multiple materials. And most of them use their material to guarantee the success of the print jobs. The idea was to guide users on the optimal printing conditions and parameters, but in research to test new materials, these limitations reduce the potential development of innovative products.

While we are still trying to understand the possibilities and limits of three-dimensional printing and additive manufacturing, a new term has emerged in our vocabulary. 4D printing is nothing more than a digital manufacturing technology -3D printing- which includes a new dimension: the temporal. This means that the printed material, once ready, will be able to modify, transform or move autonomously due to its intrinsic properties that respond to environmental stimuli.
The concept was popularized by researcher Skylar Tibbits, who coordinates the Massachusetts Institute of Technology (MIT) Self-Assembly Lab in collaboration with Stratasys and Autodesk. The technology is still relatively new, but it is expected to be used in many fields, from construction, infrastructure, automobile and aeronautics and even for healthcare, combined with bioprinting.

4D printing is directly dependent on the materials used to create the object. So-called intelligent materials, as shown in the green part of this diagram, are materials that have a property, their shape, dimensions or colour that changes in response to an external stimulus, such as heat, light, humidity, pressure or magnetism, simple due to its internal material properties. So, the difference between 3D and 4D parts is that the 3D parts are made from passive, inanimate materials, whereas 4D structures are made from active, animated, which move autonomously, swell, shrink or bend in reaction to a stimulus, which adds a new dimension to the additive manufacturing process.
It's important to mention that the type of stimulus depends on the kind of material. This dependency and this characteristic ultimately become our reference in the design of the final application as well as in the selection of the process to be used to print our object.
As an emerging new technology, 4D printing is an attractive field of research. The number of scientific papers published each year is growing exponentially. For instance, from 2017 to 2021, articles published on sciencedirect.com increased from 167 to 621. They cover different aspects, such as the printing techniques, the development of the shape memory polymers, and their applications.

4D printing is futuristic and has an exciting pack of wonders in the future. Applications of 4D printing are in various fields, such as metamaterials. This technology allows us to print reconfigurable, deployable and mechanically tunable metamaterial. Shape memory polymers can also be used to print reconfigurable metamaterial, which can carry a load 300 times its weight. It's also able to regain its original shape upon heating. The same materials can also print deployable metamaterial or small structures that can navigate through a narrow channel.
After its commercialization, 4D technology and equipment are expected to increase in the healthcare, aerospace, defense, and automotive industries. 4D printing could create a range of products from automotive parts to human organs. Benefits of 4D printing include increased capabilities of the printed products, new applications from adaptive materials, added manufacturing efficiency, and reduced manufacturing cost and carbon footprint.
The healthcare market could use the technology for tissue engineering, self-assembling biomaterials, and creating nanorobots for chemotherapy. 4D printing could lead to coatings that adapt to the automotive industry's sun, wind, and rain. The aerospace market might benefit from 4D printing by self-repairing parts or planes and printed solar panels to power satellites. Potential innovations in the military and defense sector include textiles with camouflage capabilities and self-healing materials for bridges and temporary roads.
The major challenge in the short term for 4D parts is the high initial costs, partially due to there being only a few companies currently developing techniques that support 4D. However, after commercialization, the cost of 4D printing should come down. Long-term, 4D parts will need to meet regulatory and performance standards in various industries before it becomes widespread.
Regarding potential application, one question about the use of 4D printing in fashion: Is it possible to print clothes with 3D printers? The answer is, Yes, but how would clothes made by 4D printers be different from those made by 3D technology? The response to this question is in this video about the Kinematics Dress, made up of more than 200 unique triangular panel pieces connected with over more than 3300 hinges to get the world's first 4D printed dress.

Many researchers believe that 4D printing could significantly improve space missions. Researchers from the Georgia Institute of Technology used 4D printing to create objects which expand when exposed to heat. The goal is to find a way to deploy a large thing that initially takes up little space. With heat, the 3D printed objects turn into tensegrity structures. This system relies on struts, or floating polymer rods, held together by cables. After printing, the flat structures are put into 149 degrees Fahrenheit (or 65 degrees Celsius) water and begin to unfurl. This happens as the struts are 3D printed using shape memory polymers. It's then the "memory" of the struts which makes this happen, and researchers can control the speed.
So, 4D printing could be used to build something like an antenna that initially is compressed and takes up little space, but once it's heated, say just from the sun's heat, it would fully expand. 4D print gives an accurate solution in environments and conditions where access to manual repair is limited or impossible or where damage may not be detected. This potential will be more relevant by using Self-healing polymers to achieve high healing efficiency in a controlled environment. However, Scientific Research doesn't have an answer yet as to how self-healing materials will perform under long-term environmental exposure. In contrast, accelerated environment testing of self-healing systems is critically needed.

From the same principle of foldable structure, the aerospace industry seems to experience a radical change in aircraft wing design philosophy. 4D printing will allow overcoming limitations of current flight technology by adapting the geometry of lifting surfaces to pilot input and different flight conditions. The wing materials can be reshaped for a variety of purposes. They can be temporarily transformed into any deformed shape and then returned to their original shape on-demand when heated.
So, 4D printing will create a new pathway to a wide range of exciting aerospace applications that will improve technology, health, safety and quality of transportation.

One of the most critical actuators in engineering is dielectric elastomer actuators. A dielectric elastomer is a group of electroactive polymers that can generate strains under an electric field and operates as a capacitor with a sheet of elastomer sandwiched between two compliant electrodes, causing deformation under the electric field.
Dielectric elastomer actuators are typically called "artificial muscle" to mimic mammalian muscles and their large strain, lightweight and high energy density. Natural muscle uses many complex mechanisms involving protein motion and regeneration from neural signals, while artificial muscle changes shape when external stimuli are applied. Research about Dielectric elastomer actuators and applications started in the 1990s, such as robotics, active vibration control of structures, and energy harvesting.

As soft robotics developed over the past decades, manufacturing methods evolved from traditional machining, casting, and forging to more advanced approaches. Although some soft robots were fabricated using conventional manufacturing processes, the demand for more advanced soft robotics has encouraged the development of new manufacturing methods, such as 3D and 4D print. These advanced manufacturing processes provide freedom in designing new types of 3D structured soft robots. Of the multitude of soft actuation technologies, dielectric elastomer actuators, also known as artificial muscles', exhibit fast response, large deformation and high energy density and can simply be actuated with electric voltage. Soft actuators, such as electroactive polymer actuators, shape memory polymer actuators, and hydraulic or pneumatic actuators, have opened many opportunities for bioinspired robots.
In conclusion, 4D printing technology is expected to significantly increase the efficiency of the manufacturing process and increase the capability to produce complex parts and products for different industrial sectors. Expected to create many potential applications in diverse industrial sectors; for example, aerospace, defense, automotive, health care, infrastructure, manufacturing, and packaging.
4D printing technology is expected to be adopted by various industrial sectors. Research laboratories, universities, and companies are also likely to increase their 4D printing research activities, further enabling convergence between industries and increasing the breadth of applications of 4D printing technology.
4D printing technology, software, hardware, and materials, are still in the early phase. Dominant hardware/software architecture is yet to be established. Intellectual property on 4D printing smart materials is building up. 4D technology will become increasingly popular as the trends toward its integration with giant industries like manufacturing and healthcare have increased.