4D Printing

Abstract

With evolution and change in environment, Nature’s structures and its material system exists with products having multiple designs and dimensions. Inspired by Nature’s ability to develop structures with complexity, various research works are carried out to develop newer technology to build complex products with more design dimensions. In the process of bio mimicking nature’s fabrication process 3D printing has captured the imagination of everyone from industry to research experts. However, there are various challenges need to be addressed in the process of 3D printing related to material system and product functional dynamicity. Hence, to overcome the limitations of 3D printing in flexible product development, 4D printing was generated with one or more additional design dimensions. 4D printing invented by MIT research group relies on fast growth of smart materials, 3D printers, mathematical modelling and design, and shows advantages over 3D printing. This article presents a comprehensive overview of 4D printing concept, applications and future scope for research.

1 Introduction

Ever since the industrial revolution, manufacturing sectors of all domains exhibited factories with complex mechanisms for production. 3D printing in the past thirty odd years has changed production scenario without tooling, assembly lines or supply chains making it a classic riotous technology. Hence, seeing the current capabilities of advanced manufacturing, it can be ensured that conventional goods, engineering components and customized medical products will be developed with aid of 3D printing factories [6, 10].

Manufacturing sectors of all engineering disciplines are under the process of technological transformation promising the establishment of customizable and sustainable manufacturing environment for the betterment of the industries.

New forms of engineering such as Additive Manufacturing are giving raise to the new industries such as 3D printing, 4D printing or direct digital manufacturing. These set of industries can be used in all industrial sectors comprising of aerospace, civil, electronics, and medical industries. Especially in medical sector, these technologies can be used in development of customized surgical tools, human implants, replacement of tissues and organs and many more [12].

Decades back with the advent of 3DP, objects developed were only tested outside human body for medications and treatments. However, with introduction of 4DP researchers are developing samples of human organs for drug and functionality testing. This can support researchers to develop human organs with desirable capabilities as compared to human body natural limits.

The right implementation of 3D and 4D printing technologies can positively transform the world in the manufacturing industries domain rapidly leading to Third Industrial Revolution through elimination of limitations encountered in the period of first and second stage industrial revolution.

1.1 3D Printing

3D printing is an additive manufacturing process, where in, the products are developed by depositing layers using different 3D techniques for designed Computer Aided Design (CAD) model or 3D scanners used to scan a model and convert it into digital data. In here, the development of objects is controlled by optimizing position and adhesion of specimen in 3D space. 3D printing is a process which turns the digital data into physical product by depositing layer by layer. 4D printing is based on the 3D printing technology, it uses a special material and refined design that is planned to prompt 3D print to change its shape.

2 Advancement in 3D Printing

Additive manufacturing also known as 3D printing has been associated with Rapid prototyping from past 30 years [5]. Research in 3D printing technology has attracted unprecedented interest since 1980s [8]. Since then, 3D printing has become a multibillion dollar business rapidly growing in industries for development of prototypes. However, the assignment of properties of materials and distribution of the same along multiple dimension and direction cannot be modulated in 3D printing techniques though they develop accurate models and complex designs.

In order to overcome these limitations of 3D printing, a new technology was developed by Skylar Tibbits of Massachusetts Institute of Technology (MIT) in 2013 [14]. The research carried out at MIT focused on development of system smarter in solving the problems associated with reduction in wastage of energy, materials, money, time and increase in positive properties of the product. In order to achieve these, Tibbits proposed a combined concept of logic matter, materials with mathematical data programming known as 4D printing.

Integration of mathematical models with 3DP process functionally defines the functionality of 4DP components. Figure 1a illustrates the geometrical phenomenon of 4DP. During functional products development process, the minute dimension or the micro sized part of the product is initially printed, later the structure’s major dimensions are printed for enhancing the performance of the functionality of the product. Hence optimizing the process of the 4DP will lead to fast and low cost development of the component.

Fig. 1

a Simple illustration of the concept of 4D printing (P1) b REF T4 flat surface that self-folds into a closed cube developed using 4DP [12]

2.1 4D Printing: Reinventing Manufacturing

4D printing is the time targeted evolution of 3D printing. It is defined as 3D printed object where the shape, property and functional change occurs with respect to time. More comprehensively defined as per number of studies conducted on this study, it is a targeted evaluation of 3D printed object capable of performing self-assembly and multi functionality. In here, the targets are focused on development of objects having ability to memorize the structure shape, property and function before and after application of loads.

4D printing can be applied in diverse industries/fields including aerospace, automotive, electronics, medical and education. The technology can revolutionize interdisciplinary fields of industries by benefitting the process of product development at lower processing costs. In 4D printing, the time, effort and cost to create complex designs are comparatively lower in comparison to conventional manufacturing processes. Hence 4D printing will possibly be the technology to redefine the currently existing manufacturing processes.

2.1.1 Working Principle-4D Printing

4D printing is a system having the ability of fabricating dynamic structures with adjustable shapes properties or functionality [14]. This capability can be obtained by having appropriate combination of smart materials in 3D space [13]. In order to design such complex structures mathematical modeling is desired. Hence, as shown in Fig. 2, the fundamental building blocks for effective working of a 4DP system are, 3D printing facility, stimulus, responsive material, interaction mechanism, and mathematical modeling. The optimal combination of these elements has contributed to effective evolution of 4D printed structures over time.

Fig. 2

4D printing with working bases

2.1.2 Elements for Effective Working of 4DP

• I. 3D printing facility: use of 3D printing apparatus is necessary for fabrication of multi material structures with simple and complex geometry having differences in material properties such as swelling ratio, coefficient of thermal expansion etc. this will enhance the property of the structure in shape-shifting behavior.

• II. Stimulus: on a need basis system, in order to trigger the change and regain of shape/property/functionality of a 4D printed structure stimulus are stipulated for a 4D printed structure. The stimulus include water, heat and light, and a combination of water and heat [12]. Stimulus are generally selected based on the specific application and the smart materials involved in the same.

• III. Smart or stimulus responsive materials: is the most important component of 4D printed structure. For effective working of the 4DP structure the material should fulfill properties such as, self-sensing, decision making, responsiveness, shape memory, self-adaptability, and multi functionality and self-repair [8]. Hence processing the same with the combination of 3DP facility and stimulus is of prior importance.

• IV. Interaction mechanism: in some cases application of stimulus in a predetermined sequence defines the effectiveness of a 4D printed structure with respect to the 4th dimension time.

• V. Mathematical modeling: mathematical modeling defines the design of material distribution and structure during printing by providing desired shape, property and functionality. Development of theoretical and numerical models will lead establishment of connections between 4 core elements namely; material structure, shape, material properties and stimulus properties.

4D printing is an invention made with effective combination of 3D printing and smart materials. The effectiveness can be achieved by exposing to the external stimulus through an interaction mechanism and through assistance of Mathematics.

3 Biomimetic

Biomimetic is the modern technological approach followed for replication of Nature’s way of behavior in synthetic product by fabrication processes through technology surveys knowing the limitations in functionality of the materials and the processes being used.

The behavior and organization of material with time is as shown in Fig. 3. It defines the response of a system to the environmental conditions and constraints based on the material structure. Hence, seeing the materials compositions of the material system, for replication of the same in the artificial system the process involved in the development of the system should be deliberate and preconceived in design of materials, dimensions, and shape to achieve necessary functions and necessary constraints. This is considered very crucial as anisotropy in natural materials is omnipresent with most materials exhibiting anisotropic behavior in accordance to their function and behavior.

Fig. 3

Composition and form in Nature’s material systems [15]

However, in technology associated with biomimetic and additive manufacturing, limitations always arise in prototyping with current methods of fabrication and materials. Hence, a new approach such as the 4D printing with use of one or more additional dimensions of materials grade, adaptation and response over time, along with control over volumetric anisotropy is being developed by research team to address such mismatch in biomimeting.

4 4D Printing Materials

Material homogeneity is omnipresent in nature and the functional material gradient determines the efficiency of a product through spatially varying compositions and properties [2]. However, industrially produced components comprise of homogeneously defined forms and materials of parts. Hence, development of such components though have manufacturing components and design tools, they may compromise with certain improvements in strength, weight, functionality and measurement. In order to overcome these limitations of material property after processing, a strategic control of material property density and directionality in the generation of complex smart structures known as digital anisotropy was introduced by researchers at MIT [14] to achieve controlled gradients of stiffness and elasticity. Table 1 presents some of the significant advantages of 4D printing over 3D printing.

Table 1 4D printing over 3D printing [3]

4D printing materials are intelligent materials which do active functions from the external atmosphere and make a useful response, but this would include physical sensing materials such as piezoelectric or magnetostrictive compounds are categorize as “intelligent”. The material provides an active response in a product that would otherwise be lacking and have the potential to yield a multitude of enhanced capabilities and functionalities [9]. The intelligent materials capabilities are reported in Table 2.

Table 2 The intelligent material capabilities

4.1 Shape Memory Polymers (SMPs)

Shape memory polymers are the evolving active class of polymers that can be used in a wide range of application in biomedical devices and microsystems [7]. SMP have an ability to alter the shape in a predefined way to form a temporary transformation to permanent transformation when it exposed to appropriate stimuli-triggered dynamic processes. This characteristics of SMP helps in application of 4D printed materials as a shape changes with respect to time dependent. These materials have high elastic deformability, low cost, light weight and extremely biocompatible and biodegradable. These materials functions are largely depend on the glass transitions temperature (T_g). Thermoresponsive SMPs the temporary shape is presented by heating the polymer to a glass transition temperature (T_g) and then reforming into temporary shape by the physical force. Among all the SMPs thermoresponsive are the most widely applied derivatives, which demonstrates a broad tuneable range of mechanical, thermal and optical properties [1]. Also among many other applications, these polymers are used to generate medical support devices such as stents and catheters. Since such complex applications demand multiple functionality shape memory effect, polymers such as acrylates, polyurethanes and other multiple polymer blends are used. Also, in order to achieve multiple functionality within the developed device, SMP’s are pre and post processed to achieve shape memory mechanisms. To achieve this, indirect heating methods using infrared light of electronic triggers is generally performed to enhance the shape recovery mechanism property. However, direct heating of SMP’s is generally considered in processing SMP’s.

4.2 Thermoset Shape Memory Polymers

Thermoset SMP’s are the blend of thermoset resins or polymers of similar monomers of designed composition leading to shape memory system. The monomer combinations involve polyethylene glycol or polybutadiene to form shape memory cyanate system. With aid of synthesizing/polymerizing materials such as soya oil epoxidized acrylate these polymers can be made use in fabrication of scaffolds with different printing techniques. Figure 4 represents 3D printed article developed using table top stereolithography system. Similarly UVLED digital light processing printer can also be used for such developments. Hence, with controlled UV light and LASER light projecting systems, networks can be developed via printing through high resolution projection systems.

Fig. 4

Microstereolithogrphy 3D printed thermoset polymer [11]

4.3 Shape Memory Thermoplastic Polymers

Thermo plastic polymer is a polymer generally used as printing ink in 3D printers. Polyurethane elastomer was used as TPSM material by [16]. Study showed commercially available pallets of DiAPLEXMM4520 from SMP technologies having a crystallanity of 350wt% was found to be feasible for processing in 3DP. Also, shape memory filaments processable with 3DP were found in bulk. However, in order to process such materials the process parameters such as nozzle temperature, scanning speed and part cooling in 3DP initially needs to be essentially optimized in a controlled environment for establishing high quality structures. Based on optimization of process parameters of 3DP, 3D structures were fabricated with good shape memory properties.

4.4 Self-healing Hydrogel Materials

Polymeric materials are being synthesized with self-healing properties for application in 3D bio mimicking of tissue such as external skin. Hydrogel monomers/polymers are considered to be the effective raw materials due to its outstanding properties of 3D network structure and water retention capabilities fulfilling the conditions of extracellular matrix. Thus, soft hydrogels are preferred as ideal materials for preparation of scaffolds in tissue engineering applications. Applications of hydrogels vary from wound dressings, personal hygiene products and contact lenses. Currently, hydrogels are also extensively researched for biologically active compounds for use in drugs, antibodies and development of implants. Hydrogels motivate 4DP as they can be used as self-healing materials in normalizing wound tissues having the dynamic process behaviour of 4DP.

5 Applications

4D printing technology has the ability to upgrade current manufacturing business environment. Study on materials with self-changing properties will increase the application areas of 4D printed articles.

Currently researchers are exploring 4D printed structures in various fields of applications due to material’s anisotropic and shape memory property. Beyond the simple regular applications, complex shelf life structures are investigated. Figure 5 shows development of prosthetic fingers for humans. Also, multicellular matrices can be made to grow into functional organs along with replacement tissues. Figure 6 shows the droplets network of tissue engineering substrate used as a support for the failing tissues functionality. Using 4DP technology dynamic reversible functional behavior with in a product can be achieved. Hence, if the shape shifting cycles of 4D printed structures can be equalized in comparison to the natural element capability, this technology can be used with multiple disciplines for various applications.

Fig. 5

Self-bending prosthetic finger [12]

Fig. 6

a Osmosis effect between two droplets, b macroscopic deformation arising from osmosis effect [4, 12]

6 SWOT Analysis

SWOT is a process that shall lead to strategic planning for future of industries implementing 4DP in product development cycle. The factors of SWOT are as defined in the following:

• 1. Strength:

  • Raw materials and the manufacturing process programming used have high rate of efficacy due to Programmable Materials (PM).

  • Mathematical modeling has the ability to control structure functionality.

  • Materials can be programmed based on the need with multiple compositions.

  • Structure with varying functionality can processed using cuing currently available 3D printers.

• 2. Weakness:

  • Requires extensive domain knowledge of 3DP and Mathematical modeling as the 3DP and the material used for the 4DP system is completely mathematical program controlled.

  • Hard to define accuracy through variation in optimization of process parameters for controlling functionality of size and shape.

  • Requires experts to be processing 4DP in a controlled environment.

• 3. Opportunities:

  • Can be highly useful in development of artificial skin and organs especially in warzone and space.

  • Helpful in development of biocompatible medical implants of complex size and shape.

  • 4DP can be exclusive made useful in development of smart structures.

• 4. Threats:

  • Since new categories of raw materials are defined for process based on the functionality requirement, health effects on users and manufacturers are of major concern.

  • Ethical and technical issues relating to the application of 4D Printed products is also a major issue.

7 Conclusions

• All technologies in the modern manufacturing world are generally inspired from the nature.

• Natural systems comprising natural materials continuously adopt and evolve over a period of time leading to changes in working pattern. In order to replicate such elements processing of structures is highly dependent on materials and processing technology. One such process involved in replication of natural systems is 4DP.

• The current technology of 4DP revolutionized by Skylar Tibbits of MIT-USA is an updated version of 3DP started by Hull 30 years ago. In 4DP various technological enhancements as compared to 3DP were implemented relating to material processing and printing technology.

• 4DP is one of the technologies developed to replicate natural elements showcasing anisotropic properties. 4DP still is in superficial stage of exploration in the modern manufacturing domain. However, in order to fulfill this requirement Programmable Matter (PM) has been significantly researched in design, control and modulation of property and behavior across spatial scales for higher efficiency and effectiveness during additive manufacturing. Programmable matter and 4DP has the ability to revolutionize medical implant and other manufacturing sectors. However, the technology needs to grow exponentially in real engineering and science research to compete with the current state of the art technology.

• With complete optimization of 4DP technology implants of medical industries will reach higher stature. Hence 4DP can be the future of manufacturing sector especially in the medical industries if proper care is taken to enhance the technological aspects of the same and there by produce cheaper products to the market.

• Hence, it can be concluded that 4DP shows a very high potential to become a new platform for bio inspired additive manufacturing technology that can perform as efficient and effective as natural systems and contribute to the welfare of human beings through evolution of stable medical implants.

8 Future Scope

In the coming years the several challenges needs to be addressed in the technological areas of 4DP, which are as following:

• 1. Design: suitable CAD software to support development of Programmable Materials (PM) based technological/scientific products.

• 2. Materials: optimization of material properties during production to create PM wit multifunctional properties.

• 3. Adhesion of particulates: extensive study on behavior of materials when embedded to achieve required property.

• 4. Energy: energy optimization to activate the PM according to the design requirement.

• 5. Electronics: control of electronic systems for effectiveness and efficiency.

• 6. Programming: digital and physical communications of voxels through programs as the defined product will be anisotropic in nature.

• 7. Adaptability to different environments: design and programming the product for different environmental conditions.

• 8. Assembly: micro and macro scale assembly during manufacturing and accidents.

• 9. Standardization: development of standards to incorporate 4DP in mass production.

• 10. Certification: certification of 4D printed products and raw materials.

• 11. Cost: initial cost of the 4DP system and raw materials.

• 12. Testing and characterization techniques for proving the effectiveness of product developed by 4DP system.

• 13. Technology for recycling 4DP systems and its products developed and reuse of raw materials.