4D printing is the next generation of 3D printing technology, which adds an additional dimension to 3D printing. 4D printing technology requires the printed object to be composed of two parts: one part can be operated by external stimuli (or "triggers"), and the other part serves as a carrier, allowing the first part to perform 3D printing. So, what exactly is 4D printing? How to trigger 4D printing? What is the current development?


Background


△The function of 4D printing is surprising

In the initially accepted examples of 4D printing, researchers utilized the rapid prototyping capabilities of 3D printing to create responsive materials. In the late 1990s, a large number of research projects were carried out around the development of responsive or intelligent materials. In 2003, Yasuyuki Sakai of the University of Tokyo used a technology similar to existing stereo lithography (SLA) printers to 3D print polymers that undergo changes (dissolution) when exposed to ultraviolet light. Previously, in the 1990s, other hydrogels had been made to respond to pH, light, charge and temperature, but the above case was the first one made with a self-made 3D printer.

The term "4D printing" was created in 2012 as a strategic use of bioprinting, a technology developed for hydrogel printing after the availability of 3D printers increased. Nowadays, 4D printing is commonly understood as intelligent materials created on 3D printers. The popularity of this definition is largely attributed to Skylar Tibbits, the founder of the self-assembly laboratory at MIT.


△What do you hope 3D printing can do?

You may have experienced unexpected changes in your 3D printing, such as when using ABS for printing, parts begin to deform during the printing process. Over time, this change will occur as the ambient temperature or printing platform board temperature becomes too low. The concept of 4D printing is to control when and how this change occurs.

In FDM and resin printers, the entire process is usually based on changes in the response of the printing material to heat and ultraviolet radiation. These stimuli, also known as triggering mechanisms, maintain the desired shape of the material at ambient temperature. In FDM, changes caused by heat can be reversed, so parts can be "programmed" to soften at a certain triggering temperature (such as glass transition point) or melt at the melting point.

Generally speaking, the result of 4D printing is an action, but researchers have also designed other responses, such as:

● Chemical release

● Opening or closing of circuits

● Changes in surface area or density

● Changes in color and transparency

● Strengthening or weakening of materials

In summary, the programming objects generated by 4D printing execute their 3D printing "code" when exposed to triggers, and can observe and quantify changes in programming over time.

Trigger and Materials


△When the self-assembly line is immersed in water, it will fold into a cube

Every 4D printing process requires intelligent materials that can react to triggers, and these materials can even be classified based on their triggering factors. Water, heat, light, and current are common triggering factors, but there are other triggering factors. The most common triggers will be discussed below.
Water
Water is considered one of the reasons for the existence of life, and some substances (such as fats and oils) do not mix with water when in contact, but instead form droplets or spheres. This spherical shape allows for contact, but the surface amount and potential energy between water and other substances are minimal, which has inspired researchers to develop intelligent materials that can be used in daily life. Water also has a greater heat capacity than air, so most temperature sensitive smart materials are used in water.

A group of 3D printing materials that exhibit this reaction to water is called hydrogel. Hydrogel is a polymer with additional bonds, which can change the chemical structure from chain to network. This network structure is insoluble in water, but full of liquid, so that the hydrogel has liquid and solid like behavior. You may have used hydrogels in your daily life: for example, contact lenses, sanitary products, or wound dressings. Most hydrogels expand or contract when placed in water, which can be used to design responses. One of the most prominent uses of 4D printing hydrogel is to use this expansion and contraction effect to open and close actuators or valves.

Hydrogels have been used in smart materials research for more than 30 years. They have been printed in 4D as deformable and shape memory objects. In some projects, they are designed to not only react to water. However, water is an important part of hydrogel, and its existence can change the reaction mode of hydrogel in many other situations.

Ultraviolet (UV) light


△Several usage methods of ultraviolet radiation
When applied to smart materials, ultraviolet light can serve as an energy source, and this wavelength of light can be targeted to generate heat, stimulate laser initiators, or create cross-linking within the material. It is a very practical trigger because it only requires exposing 4D prints to sunlight.

UV reactive gel, epoxy resin and liquid crystal elastomer (LCE) are the most widely studied materials for UV based 4D printing applications. In most cases, when they are included in 4D printing, their response to ultraviolet radiation is a chemical reaction within the mixed photoinitiator, allowing them to establish bridges between other liquid parts. When gel, resins or elastomers are joined together, their molecular weight increase will reduce their melting point and make them become solid. In these cases, the structure created by the bridge can serve as a memory form; When bent or twisted, the object can be restored to this memory state.

Changing the intensity or wavelength of light can be used to generate heat, thereby damaging or replacing these bridges, thereby altering the shape of 4D printed objects. This has been used to create complex intelligent objects, such as flowers that automatically open and close. The heat generated by ultraviolet radiation can also be used to raise the temperature of polymers commonly used in FDM 3D printing to the softening or melting temperature mentioned above.

Magnetic force, current, and electromagnetics


△ Conductive and deformable metal gel made by inkjet printing

Due to the fact that most natural rare earth magnets lose their strength when heated, they cannot come into contact with the resin during FDM printing or curing. But they can be attached or placed into 3D printed objects, causing them to be attracted or repelled when in contact with metal or metal objects. If the printing material is made of metal injection or alloy wire, the printed piece has a unique ability to interact with itself.

If conductive materials and thermal responsive materials are properly combined, 4D printed objects can generate heat when applying current. Conductive FDM wires containing materials such as graphene or carbon fibers have sufficiently high internal resistance, and they release heat when applying current. If these filaments fuse with other FDM filaments with low glass transition temperature (such as PLA), the softening goal can be achieved.
Materials that can carry current also generate orthogonal magnetic fields related to the direction of the current. This phenomenon is called electromagnetic phenomenon, which is a method to avoid magnetic loss of rare earth magnets while achieving the same effect. The ability to create 3D printing electromagnets opens up possibilities for 3D printing relays and other simple electronic devices.

Research and Business Applications


△ 4D printed hydrogel can be implanted into human body

Research laboratories and commercial developers focus on "programming" the required shapes of objects into the microstructure of standard materials. The fields that seem to have made the greatest progress in the commercial production of 4D printing methods include hydrogels, smart silk and various functional equipment, some of which may even appear in daily life.

● Hydrogel

Hydrogel is one of the earliest materials designed for intelligent or programmable applications. Hydrogels are still at the forefront of research and have been applied to solve real-world problems. The Wyss Institute at Harvard University is dedicated to developing biomedical and healthcare applications, while Wollongong University is the location of the first 4D printing water valve.

One of the most promising applications of 4D printing hydrogel is the heart patch in applied medicine. Researchers at George Washington University have collaborated to create a 4D printed patch that can be surgically implanted to replace damaged tissue and help the heart self repair after a heart attack. They used beam scanning stereolithography printers, methacrylate gelatin, and polyethylene glycol diacrylate to create scaffolds for human induced pluripotent stem cell derived cardiomyocytes, which have shape regulation functions and participate in muscle contraction, while helping the heart repair nearby tissues.

The development and research of hydrogels for 4D printing are common in most universities. 3D printing giants such as 3D Systems and Stratasys are promoting the continuous research of healthcare applications. Even with all this support, there is still a lack of commercial products produced using 4D printing technology in the market. This is ironic because 3D printing is used to manufacture molds or final versions of retainers, dental implants, stents, and functional restorations in the healthcare field, but 4D printing itself has not yet been converted.

●Intelligent filament


△ Hatchbox's light triggered color changing filament

Scion Research has developed a flexible color changing 4D filament that can change from black to yellow with temperature. Many filament manufacturers offer similar products, and the filaments provided by Hatchbox change color due to exposure to sunlight.

Researchers at the University of Stuttgart have designed a humidity responsive filament for 4D printing. Prints made from this filament can be stretched, extended, and moved according to the design based on the humidity in the air. Purdue University is conducting a patent pending study that allows for the addition of sensor particles acting as embedded strain gauges to fine wires.

●Functional devices, printers, and strategies


△A material with programmed response to external forces

In summary, most innovations in the field of 4D printing are based on existing 3D printing methods. Whether using biological spraying, inkjet, laser sintering, FDM, or resin printing, researchers are looking for ways to modify and combine multiple technologies to enable existing 3D printing materials to be used for 4D printing. The following is a summary of the project being developed:

Singapore University of Technology and Design: Researchers have developed a new process that integrates multiple 3D printing methods into one. To this end, researchers used a commercially available multi material 3D printer, which has been enhanced to include a wider range of materials in 4D printing.

George Washington University: Researchers have made progress in 4D bioprinting technology, which can create multi response "intelligent structures" for nerve regeneration. This study used a mixture of SLA printers and graphene to transmit signals using stress induced shape transformation.

Dresden University of Technology: Scientists have developed a method to 3D print TPU filaments mixed with iron particles in the presence of a magnetic field at the hot end of the extruder, thereby endowing the printed piece with magnetic behavior.

North Carolina: Researchers use conductive gel to make conductive 4D devices for inkjet printing.

Eindhoven University of Technology: Perflex was developed as software to create 3D printed expandable fabrics to reduce clothing waste. This technology has been adopted in software such as Fabricx.

MIT's self-assembly laboratory: Many people are waiting for this influential research laboratory to release some form of 4D printing design software or 4D simulation software.

US Army Research Center: The US Army Research Center collaborated with the University of Illinois, Harvard University's Weiss Institute, and the University of Pittsburgh to create a proprietary fabric that can change color based on light, become more breathable based on temperature, and harden the structure when subjected to external forces.