Material science is constantly developing new functional components and conformation strategies. In the last decades, additive manufacturing (also known as 3D printing) has emerged and allowed the fabrication of complex geometries. This technique is based on overlapping layers of a certain material from a CAD model.
It cannot be denied that 3D printing has created an enormous impact in almost all industries, including aerospace, biomedical or construction. In comparison with subtractive manufacturing, this technology entails great advantages in terms of flexibility in design as well as in material savings. Therefore, it is not surprising that forecasts suggest a significant investment into the 3D print market in the next 10 years.
However, the structures in 3D printing are pre-determined and rigid, which can be a drawback for some applications, which require more flexibility. A new disruptive technology has appeared to solve it: 4D printing.
But what is 4D printing?
4D printing is a promising technology based on the formation of 3D structures, pre-programmed to change its shape or other properties when subjected to an environmental stimulus. The “code” is added to the material and provides instructions to the 3D printed part. As declared by Bastien E. Rapp, president of the Laboratory Process Technology Neptun Lab: “4D printing is the functional form of 3D printing. Instead of printing only physical structures, we can now print functions. It’s like incorporating a snippet into a material: once activated, it manages to do what you’ve programmed”.
But a picture is worth than a thousand of words, so in the next video you will understand the concept better:
The above-mentioned stimulus can be either of physical or chemical nature such as temperature, pH, light intensity or moisture, among others. Consequently, materials used for 4D printing can be classified according to the stimuli they react with:
- Thermo-responsive materials exhibit a deformation caused by one of two mechanisms: the shape memory effect (SME) or the shape change effect (SCE). Those materials based on the SME are: shape memory alloys (SMA), shape memory polymers (SMP), shape memory hybrids (SMH), shape memory ceramics (SMC) and shape memory gels (SMG). Materials which obey SME mechanism have two or three discrete states, with no intermediate stable shapes. In contrast, SCE materials’ shape is proportional to the stimulus applied. It usually takes place in bilayer structures, with enough difference in their thermal coefficient expansion.
- Moisture-responsive materials react in water or moisture conditions. Hydrogels are widely used in this category, as they have a notorious increase in its size when in contact with water (they can expand up to 200% of their original volume). The behavior of hydrogels can be programmed taking into account the anisotropy to swelling.
- Photo/Electro-responsive materials produce a response to indirect stimulus. The first one absorbs light as heat while the second responds to the heat generated by the application of a current.
- Magneto-responsive materials respond to magnetic fields. It is possible to incorporate magnetic nanoparticles with other materials in solution.
3D printing technologies can be adapted and improved for 4D, but a high understanding of materials is required. Stereolithography, binder jetting (for multiple materials) or cast materials are some of these techniques. The printing process may imply the parallel printing of two materials or even heating and cooling the material. It depends on the complexity of the programmed fourth dimension.
Which are 4D potential applications?
4D printing has an immense potential in many fields in industry. Some of them will be mentioned next:
In the medical field, 4D printing will become an essential technology to fabricate smart, customized and implantable medical devices with a high flexibility. It is expected that 4D printing will be able of printing different organs and tissues by using smart materials.
By using 3D printing, a person with tissue or organ damages can only have the static tissue printed. With 4D printing, it will be possible to design the tissue expanding with their age, for instance.
Another possible application is a 4D printed drug, which can release its functional substance depending on patient’s body temperature.
In aerospace industry, 4D printing offers great opportunities in terms of manufacturing costs’ reduction, improvement of aerodynamic characteristics or minimization of component’s number and assembly time.
In fact, some companies have already got down to it: Airbus and MIT researchers are testing shape-shifting carbon composites materials. These can change between two or more shapes in response to heat and air pressure, among other factors. This could make aircraft simpler and lighter, potentially saving fuel.
Another example is NASA, which has developed an intelligent metallic fabric with 4D printing. The piece is able to reflect heat and light, acting like a strong shield that can protect astronauts and aircrafts against meteorites.
Traditional robotics are made of rigid materials, which limits the performance of organic operations, such as the grip of a human hand. Soft robotics has appeared to solve this drawback, by using certain soft materials (mainly elastomers) interacting with their environment.
In textile industry, 4D printing allows the manufacture of clothing that adapts to the body’s shape and movement. For example, U.S. Army is testing camouflageable uniforms that can alter their color depending on the environment or regulate perspiration depending on the outside temperature.
Which is the future of 4D printing?
Currently, 4D printing is still in the research stage, but it appears as a promising technology. It has the capability to solve many real-world problems. The global 4D printing market is estimated to grow at a CAGR superior to 33.2% from 2020 to 2025, due to its rising demand on defense, aerospace and healthcare industries, mainly.
Nevertheless, there are several technical challenges related to its implementation. This includes printing technology advances in order to simultaneously print different materials group. It must also be considered the “slow and inaccurate actuation” of some materials, together with an insufficient control of the changing phases of deformations.
Moreover, it must be considered the high cost of development and those issued related to intellectual property rights and potential safety hazards. However, it is expected that the potential benefits of 4D printing overcome the mentioned restraints. Let’s see how this amazing technology evolve in the next years!
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Zhang, Z., Demir, K. G., & Gu, G. X. (2019). Developments in 4D-printing: a review on current smart materials, technologies, and applications. International Journal of Smart and Nano Materials, 10(3), 205-224.
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