Overview
In February 2011, Professor Skylar Tibbits, director of the Self-Assembly Laboratory at the Massachusetts Institute of Technology (MIT), in the United States, presented the concept of modeling through the self-assembly of materials during a TED talk (Technology, Entertainment, Design ). —a US-based non-profit organization.
In 2013, Professor Tibbits returned to the TED stage, introducing the concept of 4D printing to the world for the first time. His talk attracted widespread attention across industries, recognizing him as the inventor of 4D printing.
Since then, 4D printing has captured global interest, rapidly advancing technology and expanding into broader applications. Professor Tibbits and his Self-Assembly Lab, in collaboration with Stratasys, continued research and developed numerous physical 4D printing models, as shown in Figures 8-1 through 8-4.
Currently, 4D printing technology is still in the exploratory phase and has not yet reached large-scale production applications.
However, it has enormous potential to revolutionize production technology, attracting the attention of several countries around the world, especially the United States and China, with successive emerging research results.
In 2011, MIT began research into 4D printing technology; in 2014, the American company Nervous System launched the first 4D printed hollow dress; in 2017, NASA used 4D printing to create foldable metal fabrics, preparing them for use in the manufacture of various spacecraft; In 2016, a team of experts from Xijing Hospital of the Fourth Military Medical University of China, together with a local national laboratory, became the first to apply 4D-printed tracheal stents in treating babies with complex congenital heart diseases accompanied by severe bilateral tracheal stenosis.
In short, 4D printing is poised to shift paradigms, expand thinking, and inspire anticipation and excitement for the future.
The concept and implications of 4D printing.
Since Professor Skylar Tibbits first introduced the concept of 4D printing in 2013, the definition has been interpreted by an increasing number of scholars, leading to a variety of descriptions and some debate, without reaching a consensus. It is worth noting that as 4D printing technology continues to evolve, its conceptual content will also become more enriched.
First, this book provides a comprehensive explanation of 4D printing as described by Professor Skylar Tibbits, which is as follows: 4D printing is a new process that demonstrates a step change in additive manufacturing. It involves multi-material prints with the ability to transform over time, or a customized material system that can change from one format to another, directly from the print bed.
This technique offers a streamlined path from idea to reality with performance-oriented functionality built directly into materials.
The fourth dimension is described here as transformation over time, emphasizing that printed structures are no longer simply static, dead objects; rather, they are programmably active and can transform independently. 4D printing is a first glimpse into the world of evolutionary materials that can respond to user needs or environmental changes.
At the heart of this technology are three main capabilities: the machine, the material and the geometric “program”.
Professor Skylar Tibbits explains 4D printing as a process fundamentally distinct from 3D printing (additive manufacturing). 4D printing requires an evolving system of multiple or customized materials that, after being produced by 3D printing, can continue to evolve over time or transform from one form to another.
By incorporating performance-driven features directly into the materials system, 4D printing offers a shortcut from concept to reality. The fourth dimension of 4D printing, which can be described as evolution over time, emphasizes that the structures created by 3D printing are no longer static, but can be programmed for autonomous transformation.
There are three main technologies in 4D printing: the hardware, the materials and the geometric “programming”.
To fully understand the concept of 4D printing explained by professor Skylar Tibbits, it is necessary to understand the following nuances:
The so-called fourth dimension refers to the evolution of the evolutionary material system over time, which, according to Professor Tibbits, is equivalent to self-assembly – meaning that the material system can change into the desired form within a defined period of time. based on software-defined models. .
The fundamental distinction of 3D printing lies primarily in a transformative change in the production approach. 3D printing requires a pre-designed three-dimensional model and then uses corresponding materials to mold it, while 4D printing embeds the design of the three-dimensional model directly into the material system, streamlining the process from “design” to “physical object.” ”.
The three main technologies mentioned are the following: For the equipment, conventional 3D printers can normally be used directly or with small modifications; Regarding materials, they are generally multiphase composites or evolutionary materials, also known as smart materials or stimuli-responsive materials.
Refer to materials that can autonomously change their physical or chemical properties (including shape, density, color, elasticity, conductivity, optical characteristics, electromagnetic properties, etc.) under predetermined stimuli (such as immersion in water or exposure to heat, pressure, electricity, light, etc.).
The current focus is mainly on simple filamentous chains and thin sheet-shaped materials, with the next research goal being the creation of more complex three-dimensional structures.
Currently, 4D printing is particularly suitable for printing single filament chains. As for “geometric programming”, it refers to the response relationship between the material stimuli and the physical properties mentioned above, with the primary response mechanism being the generation of localized characteristic deformations (or mismatch deformations) within the printed object during or after the impression. process.
For professor Skylar Tibbits and his Self-Assembly Lab, the equipment and materials used for 4D printing are provided by Stratasys, while the geometric “programming” is provided by the Autodesk research and development team. They also developed new software called Cyborg. Figure 8-5 illustrates Professor Skylar Tibbits' explanation of 4D printing in a more intuitive way.
In 2014, Professor Li Dicheng of Xi'an Jiaotong University offered a concise concept of 4D printing: 4D printing refers to the additive manufacturing of smart materials, which means that structures made from 3D printing technology can change its shape and structure over time when stimulated by external environmental factors. The additional dimension that 4D printing technology adds to 3D printing is time.
Thus, the initial concept of 4D printing can be understood simply as “3D printing + time”, focusing on changing the shape of components over time with smart materials. It is believed that as research into 4D printing continues to deepen, its concept and essence will be further elevated.
4D printing materials
A key component of 4D printing is smart materials. In the late 1980s, inspired by certain capabilities found in nature, American and Japanese scientists first introduced the concept of intelligence into the field of materials and structures, proposing the innovative idea of intelligent material structures.
Also known as smart or responsive structures, these systems integrate sensing elements, actuators and associated signal processing and control circuits within the material structure. They are designed to respond to mechanical, thermal, optical, chemical, electrical and magnetic stimuli and controls.
These materials are not only able to withstand loads, but also have the ability to recognize, analyze, process and control, offering multifunctional capabilities such as self-diagnosis, self-adaptation, self-learning and self-repair.
Smart material structures represent an interdisciplinary frontier, encompassing a wide range of fields such as mechanics, materials science, physics, biology, electronics, control science, computer science and technology. A significant number of experts in these disciplines around the world are actively engaged in advancing these fields.
There are numerous classifications for smart materials, which can be broadly categorized based on their function and composition into shape memory materials, electroactive polymers, piezoelectric materials, electrorheological fluids, and magnetostrictive materials, with shape memory materials being the most widely used.
Shape memory materials include shape memory polymers (SMPs), shape memory alloys (SMAs), shape memory hydrogels (SMHs), shape memory ceramics (SMCs), and shape memory composites (SMCs). ).
Shape memory polymers (SMPs), also known as shape memory polymers, are polymeric materials that can change and fix their initial shape under certain conditions and then recover their original shape after stimulation by external conditions such as heat, electricity, light or chemicals. induction.
SMP technology utilizes modern polymer physics theory, along with polymer synthesis and modification techniques, to molecularly engineer and tune the molecular structure of common polymeric materials such as polyethylene, polyisoprene, polyester, copolyester, polyamide, copolyamide and polyurethane.
These materials are endowed with a specific shape under certain conditions (initial state), which can change and be fixed (deformed state) when external conditions vary. If the external environment changes again in a specific way, they can reversibly return to the initial state, completing the cycle of memorizing the initial state, fixing the deformed state, and returning to the initial state.
SMPs are categorized based on the type of stimulus to which they respond, including thermally induced SMPs, electrically induced SMPs, light-induced SMPs, and chemically induced SMPs.
Shape Memory Polymer, SMP
Thermally induced SMPs deform above room temperature and can correct deformation during long-term storage. When heated to a specific response temperature, components quickly recover their initial shape.
These polymers are widely used in various fields such as healthcare, sports, construction, packaging, automotive and scientific experiments, including medical devices, plastic foam, seat cushions, optical information storage media and alarms.
The shape memory function of thermally induced SMPs mainly originates from two incompatible phases within the material: the fixed phase that retains the shape of the molded product and the reversible phase that undergoes softening and hardening with temperature changes. The fixed phase is responsible for memorizing and recovering the original shape, while the reversible phase allows the product to change shape.
Based on the structural characteristics of the fixed phase, thermally induced SMPs can be divided into thermosetting and thermoplastic categories. In addition, there is a shape memory polymer called “cold strain molding”, which involves cold processing certain thermoplastic resins below temperature T to obtain high elastic deformation and then cooling to obtain a deformed state.
When reheated above temperature T g the material can also return to its original shape.
Electrically induced SMPs are composed of thermally induced shape memory polymeric materials with conductive substances such as conductive carbon black, metal powder, and conductive polymers. Its memory mechanism is identical to that of thermally induced shape memory polymers. The composite material uses heat generated from an electrical current to increase the temperature of the system and induce shape recovery.
Therefore, it has conductive properties and excellent shape memory functionality, mainly used in electronics, communications and instrumentation, such as electronic cathode ray tubes and electromagnetic shielding materials.
Photoresponsive SMPs incorporate specific photochromic groups (PCGs) into the polymer backbone and side chains. Upon exposure to UV light, PCGs undergo a photoisomerization reaction, causing a significant change in the state of the molecular chain.
Macroscopically, the material exhibits a light-induced shape transformation. When exposure to light ceases, the PCGs react reversibly, reverting the molecular state and material to their original form. These materials are used in printing, optical recording, light-activated molecular valves, and controlled drug release systems.
Chemically induced SMPs are activated by changes in the surrounding medium to allow material deformation and shape recovery. Common chemical stimuli include pH changes, ion exchange equilibrium, chelation reactions, phase transitions, and redox reactions.
These materials include partially saponified polyacrylamide, polyvinyl alcohol, and polyacrylic acid mixtures in film form and are used in specialized fields such as separation membranes for proteins or enzymes and chemical motors.
Shape Memory Alloy, SMA
Shape memory alloys (SMAs) are materials composed of two or more metallic elements that exhibit shape memory effect (SME) through martensitic thermoelastic phase transformations and their reverse.
SMAs offer the best shape memory performance among shape memory materials. Thermoelastic martensite, once formed, continues to grow as the temperature decreases and decreases upon heating, disappearing in a completely reversible process. The difference in free energy acts as the driving force for the phase transformation.
SMAs are categorized into three classes based on their deformation characteristics:
Unidirectional shape memory effect: SMAs deform at lower temperatures and recover their original shape upon heating, exhibiting a shape memory effect only during the heating process.
Bidirectional shape memory effect: Certain alloys recover their high-temperature phase form upon heating and revert to their low-temperature phase form upon cooling.
Full shape memory effect: After heating, the material recovers its high-temperature phase shape, and after cooling, it transitions to a low-temperature phase shape with the same geometry but opposite orientation.
To date, more than 50 types of alloys with shape memory effects have been discovered. In 1969, the shape memory effect of nickel-titanium alloy was first applied industrially, leading to the creation of a unique tube coupling device.
By adding other elements to nickel-titanium alloy, new nickel-titanium-based shape memory alloys such as nickel-titanium-copper, nickel-titanium-iron and nickel-titanium-chromium have been developed and researched. In addition, there are other types of shape memory alloys, including copper-nickel, copper-aluminum, copper-zinc, and iron-based alloys (Fe-Mn-Si, Fe-Pd).
SMAs are widely used in various fields such as aerospace, mechanical electronics, biomedical engineering, bridge construction, automotive industry and in everyday life.
Shape Memory Hydrogels (SMH)
Hydrogels are a type of highly hydrophilic three-dimensional network gel that can swell quickly in water and retain a significant volume of water without dissolving in the swollen state. Water absorption is closely related to the degree of crosslinking; the greater the crosslinking, the less water absorption. Hydrogels can be categorized into traditional hydrogels and environmentally responsive hydrogels based on their response to external stimuli.
Traditional hydrogels are not sensitive to environmental changes such as temperature or pH; they adapt by altering the cross-linking of macromolecules to capture and release water (providing stimulation), thereby achieving contraction and expansion to facilitate structural transitions.
Environmentally responsive hydrogels, on the other hand, are able to detect small changes or stimuli in the external environment (such as temperature, pH, light, electricity, pressure, etc.) and respond with significant changes in physical and chemical properties, even abrupt transformations . . The characteristic of these hydrogels is the remarkable change in their swelling behavior in response to environmental factors.
Based on the different response mechanisms of these two types of hydrogels, shape memory hydrogels have been developed and can be used as sensors, control switches, etc.
Shape Memory Ceramics (SMC)
SMCs exhibit shape memory effects that differ from SMPs and SMAs in the following ways: First, SMCs have a lower deformation capacity; second, SMCs experience varying degrees of irreversible deformation with each shape memory and recovery cycle, and as the number of cycles increases, the cumulative deformation increases, ultimately leading to crack formation.
SMCs can be classified based on the generation mechanism of shape memory effect into viscoelastic shape memory ceramics, martensitic phase transformation shape memory ceramics, ferroelectric shape memory ceramics, and ferromagnetic shape memory ceramics.
Viscoelastic shape memory ceramics include cobalt oxide, aluminum oxide, silicon carbide, silicon nitride and mica glass ceramics. When these materials are heated to a certain temperature, they are deformed under load, with the external force maintaining the deformation. After cooling and subsequent reheating to a specific temperature, the deformation of the ceramic recovers to its original state.
Studies suggest that viscoelastic shape memory ceramics contain two structures – crystalline and glassy – and that the elastic energy that drives shape recovery is stored in one of these structures, while deformation occurs in the other.
Shape memory martensitic ceramics such as ZrO 2 BaTiO 3 KNbO 3 PbTiO 3 are mainly employed in energy storage actuator elements and specialized functional materials.
Ferroelectric shape memory ceramics refer to ceramics that exhibit shape memory characteristics when their orientation changes under an external electric field. The phase regions of ferroelectric shape memory ceramics include paraelectric, ferroelectric, and antiferroelectric substances, with phase transition types such as paraelectric-ferroelectric and antiferroelectric-ferroelectric transformations.
These phase transitions can be induced by an electric field or by the switching or reorientation of polar magnetic domains. Although ferroelectric shape memory ceramics have smaller deformations compared to shape memory alloys, they have fast response times.
They can also undergo reversible transitions, such as paramagnetic-ferromagnetic, paramagnetic-antiferromagnetic, or from ordered to disordered orbital states, typically accompanied by recoverable lattice deformations.
4D printing applications
Objects manufactured through 4D printing are intelligent products that have adaptive and self-healing capabilities. They are widely applicable in various industries, including artificial tissues and organs, medical devices, automotive transportation, precision machinery, aerospace, defense industry, as well as fashion, furniture and construction. Here are seven specific application examples:
(1) 4D Printed Lattice Dress
Nervous System, founded in the United States in 2007 by Jessica and Jessie, both graduated from MIT, Jessica having a degree in architecture from the same institution as professor Skylar Tibbits, began using a special fabric to create dresses through 4D printing in 2014. The dress, as shown in Figure 8-6, consists of a lattice structure made up of 2,279 triangles and 3,316 articulation points, as illustrated in Figure 8-7.
The tension between the triangles and hinges adjusts to the wearer's body shape, ensuring the dress fits well even with changes in weight. This dress not only addresses fit issues but also adapts to the wearer's body shape. The dress is created using SLS 3D printing technology, letting unsintered powder fall after printing, resulting in an interconnected fiber structure.
Nervous System has also developed an app that allows users to perform a 3D scan of their body, choose fabric size and shape, and customize a unique 4D printed dress. Currently, this 4D printed dress is permanently collected by four museums or galleries.
4D Printed Space Metal Fabric
In 2017, a research team led by Raul Polit Casillas of NASA's Jet Propulsion Laboratory reported the creation of a foldable metallic fabric using 4D printing technology, as shown in Figure 8-8. The fabric features small silver metal squares on the front and black metal threads on the back, as shown in Figure 8-9.
This structure significantly increases its ability to resist external impacts and is also conveniently designed for application to spacecraft surfaces or astronaut spacesuits. The structure can reflect light (metal square side) and absorb heat (metal wire side), encompassing five capabilities: physical impact resistance, fabric-like folding ability, steel-like tensile strength, intense light refraction, and management passive thermal.
Passive thermal management allows the spacecraft to maintain a minimum temperature differential with the external environment, achieving dynamic balance when this material is used as a cover.
NASA envisions this metallic fabric to be used in several domains, including large foldable and shape-changing antennas, thermal insulation for spacecraft visiting cold and icy planets/moons, as well as flexible insulation mats for astronauts, miniature meteorites for spacecraft and spacesuits.
Additionally, this innovative material could be used in aircraft on ice moons/planets, creating bendable “feet” that adapt to rough planetary surfaces, helping to prevent certain physical damage and facilitating sample collection.
4D printed biodegradable tracheal stent
On March 28, 2016, thoracic surgeons at Tangdu Hospital, affiliated with the Fourth Military Medical University, used cutting-edge 4D printing technology to alleviate the suffering of a patient with tracheomalacia caused by tracheal endobronchial tuberculosis. The affected tracheal segment exceeded the maximum length allowed for resection, making it impossible to remove.
Traditional stent implantation can lead to complications such as difficulty in expectoration. Internationally, the University of Michigan reported a similar case in The New England Journal of Medicine, where experts developed an external stent suspension for a patient with left bronchial disease spanning just 1-1.5 cm, while this patient had a 6 cm lesion. cm in the trachea, representing a greater challenge.
After careful analysis of the characteristics of the disease, Dr. Li Xiaofei, deputy director Huang Lijun and Dr. Medical University, created a 3D printed tracheal model.
After a thorough evaluation, they decided to proceed with the external stent suspension surgery. They also partnered with Professor He Jiankang's team from Xi'an Jiaotong University to manufacture a 4D-printed biodegradable tracheal stent for the patient, as shown in Figure 8-10.
Using the 4D printed biodegradable tracheal stent to wrap around the weakened trachea and suture it in place, the collapsed trachea was supported and the narrowed airway was opened. Complete preoperative communication was carried out with the patient and his family, as illustrated in Figure 8-11.
The surgery was a success and the patient recovered well post-operatively.
The period of stent degradation can be regulated by controlling the type and molecular weight of the biomaterial, allowing it to gradually degrade and be absorbed by the body over the next 2 to 3 years, saving the patient the pain of a second surgery to remove the stent. .
This surgery was the first of its kind internationally, involving the suspension of an external stent into an extremely elongated soft segment of the trachea. Furthermore, in September of that year, doctors from Xijing Hospital of the Fourth Military Medical University joined the He Jiankang team from Xi'an Jiaotong University.
Using a similar 4D-printed absorbable tracheal stent, they performed stent suspension surgery on a 5-month-old child suffering from complex congenital heart disease combined with severe bilateral bronchial stenosis, successfully curing the disease – another world first.
4D printed SMP occluders
In 2019, Professor Liu Liwu of Harbin Institute of Technology collaborated with clinical experts from the First Affiliated Hospital of Harbin Medical University to embed magnetic Fe3O4 particles into a shape memory polylactic acid matrix. They designed and 4D printed a biodegradable and customizable SMP occluder that could be deployed remotely and in a controlled manner under a specific magnetic field strength.
They also conducted in vitro viability experiments with the 4D-printed SMP occluder to test the simplicity of its implantation process, as shown in Figures 8-12. The SMP occluder can be easily packaged, delivered and released via a catheter, with the deployment process completing in 16 seconds.
4D printed self-assembly robotic systems
At the 2013 IEEE International Conference on Robotics and Automation, Samuel M. Felton of the Institute for Biologically Inspired Engineering at the Harvard School of Engineering and Applied Sciences presented a self-assembling robotic system made using 4D printing technology.
The field of robotics, which demands high structural performance, automation and intelligence, often sees surprising effects when combining shape memory polymers, with the self-assembly robotic system being particularly significant. This 4D printed self-assembly system merges rigid flat materials with SMPs, achieving sequential bending, angle control and grooving actions upon external stimulation.
Figures 8-13 (a) depict a worm-like robot equipped with a 4D printed self-assembly robotic system capable of folding into a functional form that moves under suitable electrical current; Figure 8.13(b) shows the same worm-like robot demonstrating self-propulsion at a speed of 2 µm/s. These self-assembling robots can reduce material, processing and transportation costs and have promising prospects for exploring confined areas.
(a) Worm-like robot with 4D printed self-assembly robotic system
(b) Deployed structure and progression diagram of the worm-like robot
(6) 4D Printed Moisture Sensitive Sportswear
In 2017, Professor Zhao Xuanhe of the Massachusetts Institute of Technology deposited genetically tractable microbes onto a moisture-inert material using a 3D printing process, creating a biohybrid film with a multilayer structure of uneven microbial density. Utilizing the hygroscopic and bioluminescent behaviors of living cells, this biohybrid film can respond to environmental humidity gradients in seconds.
It reversibly changes the shape of the multilayer structure and the intensity of bioluminescence, forming open, ventilation flaps in high humidity environments, as illustrated in Figure 8-14. This biohybrid film was transformed into moisture-sensitive sportswear to enhance the athletic experience, as illustrated in Figure 8-15.
Since the deposition of this multilayer structure constitutes an additive production of moisture-sensitive smart materials, this 3D printing process can also be referred to as 4D printing.
(7) Large Deformation, High Modulus Self-Transformed Structures
In 2020, Professor H. Jerry Qi's team at the Georgia Institute of Technology demonstrated a method for designing and manufacturing self-transforming structures capable of large deformations and high modulus. They printed the designed structures using multi-material DIW processes with composite inks consisting of a high-volume fraction of solvent, light-curing resins, short glass fibers, and gaseous silica.
During printing, the glass fibers were aligned through shear-induced orientation through the nozzle, resulting in highly anisotropic mechanical properties. The solvent was then evaporated, causing anisotropic shrinkage of the glass fibers aligned in the parallel and perpendicular directions. A subsequent post-curing step further increased the stiffness of the composite material from approximately 300 MPa to approximately 4.8 GPa.
The printing and deformation process, illustrated in Figure 8-16, is described above. A finite element analysis model was developed to predict the effects of solvent, fiber content, and fiber orientation on shape changes.
The results confirmed that anisotropic volumetric shrinkage could act as an active hinge, enabling the self-transformation of complex structures with large deformation and high modulus. These structures show potential applications in lightweight structures with load-bearing capabilities.
