As a concept proposed by Skylar Tibbits of MIT in 2013, 4D printing is a field with many new possibilities. 4D printing produces programmable material that responds to external stimuli and changes its structure or properties into the pre-designed shape through 3D printing. 1] One of the advantages of 4D printing is that a movement can be realized in a simple constitution so that a certain material can move even without a complicated drive mechanism like a car.
1] This paper first explains 3D printing and 4D printing and then deals with 4D printing using the properties of Poly-lactic acid (PLA), which is a shape memory polymer, and 4D printing which is a bistable structure used. In the case of 4D printing using PLA, we applied the characteristics of the shape recovery to the original shape at a condition higher than glass temperature to 3D printing and tested what structure of 4D printing will be possible in the future . In the case of the bistable structure, we made the basic elements of a mechanical bistable structure so that 4D printing using Shape Memory Polymer (SMP) can produce more efficiently, and prepare the ground that can realize the diversified and complicated mechanical movements .
Introduction
Background
Before explaining the contents of the study, in this part we will explain the concepts of the contents treated in the paper. And, we'll show some additional types of examples of what 4D printing has to offer.
2-1. 3D printing
Material extrusion is a method that ejects semi-liquid materials from a nozzle to create successive layers of objects. VAT photopolymerization is a method that hardens the base or surface of the container containing the liquid photopolymer by laser or other light sources in order to produce successive layers of objects. Casting is a method in which the print head casts the liquid build material like a normal inkjet printer and produces it by curing it with UV or heat.
Binder Jetting is a method that produces by solidifying the powder by partially spraying the binder in the process of applying the powder layers one by one. Powder Bed Fusion is a method that produces by partial solidification with a laser or heat source as the powder is deposited layer by layer. Directed energy deposition is a method that directly cures with a UV or heat source when applying a powder building material.
Sheet lamination is a manufacturing method by stacking cut paper, plastic or metal in one layer after another. FDM is a method that deposits one layer after another by melting thermoplastics at a temperature higher than the melting temperature and pulling them through a die.
2-2. 4D printing
For example, we must consider limiting the bending angle with a rigid material so that the parts to be bent can be bent at an appropriate angle. In other words, we must design what results will come out when we respond to external stimuli before we produce in the beginning. As 4D printing is in the initial stage of study, there are the following issues to be resolved.
The design part primarily concerns dynamic components with a design application such as CAD, when creating a digital object for the material to be manufactured. For materials, we need to find the materials with multifunctional properties necessary for 4D printing. With adhesions there is the problem that they easily stick together when printing many different materials.
And finally, 4D printing must solve the problems of how it will make, store and use the energy needed to respond to external stimuli. 8] Despite these problems, researchers will solve the problems in the future and enable the transformative digital information in the virtual world to materialize in the real world. To explain this part a little more precisely, we will be able to design much more diversely than the current 3D printing, since it can easily realize many movements when designing the products.
Since it means that the obstacles that exist to produce certain goods eventually disappear, we can produce the goods with more diversified forms and functions. In other words, it means that we can easily produce complicated goods at a cheap price in the future. If it is possible to realize movements for the goods, this means that functions such as self-assembly can be easily realized.
For example, when we transport goods, we can seriously reduce the cost of transport by manufacturing them to fold well with each other, and we can also make them turn into finished goods later. 9] We can fabricate manufactured goods to self-optimize for users as well. In 3D printing, we have to produce items one by one by changing the conditions when making prototypes, etc.
2-3. programmable matter
Nature
Programmable Matter (or Programmable materials)
2-4. Comparison between 3D printing and 4D printing
2-5. Examples of 4D printing
3. 4D printing based on SMP
3-1. Shape memory polymer
As a material that reaches glass temperature of around 35 degrees Celsius, it can be used for 4D printing that responds to body temperature.
3-2. 4D printing process
3-3. test samples
For the simulation conditions in COMSOL, we used the interpretation of electric currents and flowed the applied voltage from bottom right to top right, and the input voltage was 30V. 30V has been applied to the bottom right source, and the top right is the drain section and 0V has been applied there.
3-4. Method to overcome SMP disadvantages
Mechanical Bistable structure
In this study, we conducted an experiment using a bistable structure to make programmable matter that responds to external forces. We can commonly find this structure in our environment like the cradles of smartphones and also find examples like the Flytrap of Venus [21] in the natural world. For this experiment, we made a bistable mechanical structure that responds linearly [22], rotationally [23] and torsionally to external forces.
In other words, we came to realize various complicated movements from the mechanical bistable structure we made this time. Moreover, we expect that we can also make more efficient devices if we adapt them to 4D printing using SMP, as was performed earlier.
4-1. Background of bistable structure
4-2. Bistable basic elements
Summary
In this study, we conducted a survey to find the direction 4D printing will be developed in the future and to check what things are possible to happen. Through the experiment performed first, we checked what kind of programmable fabric we can produce with 4D printing using SMP. When using SMP, we could verify that transformation of different forms is possible, such as expansion of material, and confirm the possibility of applying this to other different areas.
However, we also found that there are limitations such as slow reaction speed and weak power. To solve the disadvantages of SMP material like this, we produced a structure that acts by different forces such as the elastic force, and SMP only plays the role of trigger. Starting with the flipping bistable structure that exerts linear motions, we made the rotation structure and the twisting structure in this study.
It is expected that if we coordinate these three types of structures with each other, we can realize more complex movements. In addition, if we combine it with the aforementioned SMP, we can provide more login options. Finally, to tell the significance of this study, we have conducted a basic study for the advancement of 4D printing which is combined with SMP and bistable structure.
![Figure 4.1 Venus flytrap [21]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10524178.0/46.892.118.803.99.1143/figure-4-1-venus-flytrap-21.webp)
