4D Printing

Research and Article by Helia Rastegar
B.Sc. Student in Polymer Engineering
Amirkabir University of Technology- Tehran

Source: Sanat e Chaap Magazine

4D printing is one of the newest technologies in additive manufacturing. By adding the dimension of time to printed structures, it enables the design and production of dynamic structures. A component produced using this method is an active system capable of responding to environmental stimuli such as light, humidity, pH, heat, and more. It can change its function, properties, and shape, exhibiting pre-programmed behavior.

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4D printing technology is inherently interdisciplinary, combining materials science, polymer chemistry, mechanical engineering, structural dynamics, and additive manufacturing. The use of smart materials makes it possible to exploit properties that were previously observed only in nature. Moreover, the ability to control the behavior of responsive materials and precisely print complex geometries has transformed 4D printing from a laboratory concept into an emerging industrial technology with high potential.

Background of 3D Printing

3D printing forms the foundation of 4D printing, and understanding its history and principles is essential for properly grasping 4D printing. 3D printing first emerged in the 1980s with the introduction of stereolithography (SLA). In this method, a focused light source was used to cure photopolymer resin layer by layer. Initially developed as a rapid prototyping tool, it evolved—through technical advances and new material development—into one of the most important manufacturing methods in modern industry.

In 3D printing, the goal is to convert a digital model into a physical, layer-by-layer structure. Designs created in CAD software are fabricated by printers using powdered materials, thermoplastics, or resins, deposited in micrometer-thick layers. Its special role in aerospace, medicine, art, architecture, mechanical engineering, and even biotechnology stems from its ability to produce complex geometries, hollow parts, cellular networks, and structures impossible to manufacture with conventional methods.

In recent years, various 3D printing technologies have been introduced:

• FDM: Uses thermoplastic extrusion; economical and suitable for engineering parts.
• SLS: Uses lasers to fuse polymer or metal powders; produces highly durable parts.
• SLA and DLP: Utilize photopolymers; offer the highest dimensional accuracy and finest geometric details.

These developments laid the groundwork for the emergence of 4D printing.

Photopolymers and Their Role in Additive Manufacturing

Photopolymers revolutionized precision and quality in 3D printing and are among the most important materials used in the field. These light-sensitive resins polymerize when exposed to suitable wavelengths (usually UV or visible light), solidifying at very small scales. This makes them ideal for producing complex geometries with fine details and high surface quality.

Their chemical composition includes monomers, reactive oligomers, initiators, and modifying additives. Engineering these components allows the production of resins with tailored mechanical, thermal, and reactive properties. One major advantage of photopolymers is molecular design flexibility—mechanical behavior, flexibility, curing speed, hardness, softness, and even sensitivity to specific stimuli can be precisely controlled.

Recently, a new generation of photopolymers has emerged. Some can change shape in response to light, heat, or moisture. Others exhibit self-healing properties or structural rearrangement under secondary light exposure. As a result, photopolymers have evolved from simple printing materials into key tools for 4D printing.

Light-based technologies such as DLP and SLA, due to their high accuracy, speed, and ability to fabricate multi-material components, provide the best platform for developing smart materials and dynamic structures. Thus, the relationship between photopolymers and 4D printing is very strong.

Definition of 4D Printing and Operating Principles

4D printing refers to processes in which a printed structure transforms and reorganizes over time under environmental stimuli. Unlike conventional 3D-printed parts, 4D-printed components are adaptive systems capable of responding to their surroundings, changing shape, or achieving new functionality.

In its simplest definition, 4D printing consists of three main components:

1. Smart materials
2. High-precision additive manufacturing
3. Time-based behavioral design and modeling

The fourth dimension is active time—the moment of stimulus or energy change.

Possible stimuli include heat, light (UV, visible, IR), humidity or water absorption, pH changes, ions and salts, electric or magnetic fields, and mechanical stress.

These stimuli cause changes in chemical bonds, polymer chain rearrangement, solvent absorption or release, phase transitions, thermal expansion coefficients, or release of stored energy—manifesting as movement, unfolding, rotation, bending, or twisting.

Design in 4D printing occurs at three levels:

• Molecular design: Adjusting chemical structure for desired responsiveness
• Geometric design: Using lattice patterns, heterogeneous layering, thickness variation, or print orientation to control deformation
• Architectural design: Employing complex structures such as lattices, origami, and metamaterials

Thus, 4D printing is not merely a manufacturing method but a form of behavioral design.

Smart Materials in 4D Printing

Smart materials are the core of 4D printing performance and include a wide range of polymers, composites, and multi-phase materials.

Shape Memory Polymers (SMPs)

SMPs are among the most well-known materials in 4D printing. They have two states: a programmed temporary shape and a recovered original shape. When heated above their glass transition temperature (Tg), they become deformable. After cooling, the new shape is fixed. Upon reheating, stored energy is released and the material returns to its original form.

Features:

• High energy storage capacity
• Large deformation capability
• Reversible behavior
• Adjustable activation temperature
• Good printability (FDM, SLA, DLP)

Applications include medical stents, surgical clips, self-deploying space structures, foldable antennas, and self-opening components.

Hydrogels

Hydrogels are three-dimensional polymer networks capable of absorbing large amounts of water. Their response to pH, temperature, or biochemical stimuli causes dramatic changes in volume, flexibility, or shape.

Features:

• Expansion/contraction several times their original volume
• High biocompatibility
• Environmental sensitivity
• Suitable for microscopic scales

Applications include smart drug delivery systems, soft bio-robots, sensors, and artificial responsive tissues.

Liquid Crystal Elastomers (LCEs)

These materials combine elastomer elasticity with molecular order of liquid crystals. In response to heat or light, molecular order changes, causing unidirectional contraction or expansion—similar to biological muscles.

Features:

• Fast and precise motion
• Continuous responsiveness
• Significant length change
• Suitable for thermal and optical stimuli

Applications include soft robotics, microfluidic pumps, artificial vision systems, and morphing surfaces.

Multi-Stimuli Composite Materials

These composites combine two or more responsive materials and can detect multiple stimuli simultaneously or selectively. They enable complex behaviors such as twisting, multi-stage motion, and rotational movements.

Factors Affecting 4D Printing Performance

Final performance depends not only on materials but also on process and environmental factors.

Printing method influences layer accuracy, orientation, bonding continuity, and response speed. For example:

• SLA (homogeneous curing) produces more uniform movement.
• FDM (directional layering) enables anisotropic behaviors.

Stimulus intensity, direction, application speed, and spatial distribution greatly affect behavior.

Structural parameters such as wall thickness, lattice pattern, density, zoning, and metamaterial architecture are critical in motion control.

Structures such as:

• Origami (foldable)
• Kirigami (expanded opening)
• Auxetic (lateral expansion)
• Lattice (lightweight, behavior control)

Accurate modeling is essential. Mechanical, thermal, optical, chemical, and polymer behaviors must be analyzed simultaneously. Software such as Abaqus, COMSOL Multiphysics, ANSYS Mechanical, and Rhino/Grasshopper + Kangaroo are used for time-based behavioral design.

Current Challenges and Limitations

• Limited smart material stability
• Slow response speed in some materials
• Complex multi-stimuli modeling
• Scalability issues
• High material and equipment costs

Future Outlook of 4D Printing

4D printing is moving toward integration with computational technologies and artificial intelligence. In data-driven design, machine learning predicts behavior, generative design creates dynamic geometries, and AI selects optimal materials.

In personalized medicine, implants will adapt to patient bodies, and responsive tissues and in-body bio-robots will emerge. In soft robotics and smart actuator systems, more complex and multi-stage motions will be engineered. In engineering structures, morphing aerodynamic wings, self-deploying space components, and dynamic architectural systems will become possible.

Conclusion

4D printing marks the beginning of the next generation of additive manufacturing, enabling dynamic and intelligent structures. With advances in smart materials, improved printing technologies, and AI integration, it has the potential to revolutionize medicine, aerospace, soft robotics, architecture, and intelligent systems.