A Comprehensive Review of 4D Printing: State of the Arts, Opportunities, and Challenges

Abstract

Over the past decade, 3D printing technology has been leading the manufacturing revolution. A recent development in the field of 3D printing has added time as a fourth dimension to obtain 4D printing parts. A fabricated design created by 3D printing is static, whereas a design created by 4D printing is capable of altering its shape in response to environmental factors. The phrase “4D printing” was introduced by Tibbits in 2013, and 4D printing has since grown in popularity. Different smart materials, stimulus, and manufacturing methods have been published in the literature to promote this new technology. This review paper provides a description of 4D printing technology along with its features, benefits, limitations, and drawbacks. This paper also reviews a variety of 4D printing applications in fields such as electronics, renewable energy, aerospace, food, healthcare, and fashion wear. The review discusses gaps in the research, the current challenges in 4D printing, and the future of 4D printing.

1. Introduction

1.1. Background

In May 1980, Dr. Kodama introduced an additive manufacturing (AM) method that was used for a range of applications at the time; the method was known as rapid prototyping [1]. Because of its possibilities, AM has been acknowledged and developed in all industry sectors. The AM approach has the capability to produce high-end complex parts in a timely manner that was previously unattainable using traditional manufacturing methods. The AM method are cost-effective by reducing the assembly time and environmentally friendly by minimizing waste material.

Aside from the Industry 4.0 revolution, there has recently been an increase in the demand for personalized items. Three-dimensional printing is one of many AM techniques that have gained popularity in recent years. As a result, innovation in 3D printing technology has expanded into manufacturing methods and materials. As a result of the expansion, the term “4D printing” was introduced in 2013 by Skyler Tibbits, the founder and codirector of the Self-Assembly Lab at MIT and an assistant professor of design research in the Department of Architecture. He introduced 4D printing techniques at TED Talks, proposing to add the time factor as a fourth dimension [2]. Since then, researchers have been investigating how to use this technique for a variety of research purposes. Therefore, the purpose of this paper is to synthesize the literature on this topic and provide a description of 4D-printing technology, along with its features, benefits, limitations, and drawbacks.

To determine the number of publications, the literature search process was based on the Google Scholar and ScienceDirect databases. Despite the fact that the majority of the literature used different databases to collect the number of publications, such as Web of Science and Scopus.com [3,4,5], the results in this paper are systematic with their statements in the relevant context. The search excluded duplicate publications using filters. Furthermore, the search criteria were set for publications between the years of 2013 and 2022. The search keywords used were “4D printing” and “polymer” for example, which were enclosed in parentheses. The number of publications provides a good indication of the focus of the research (Figure 1). It also indicates whether a specific topic has been enhanced in comparison to other components and applications. For this review, the search of the literature was conducted on 30 November 2022.

1.2. Overview of 4D Printing

Although several researchers have studied the modification of the shapes of items using various approaches such as shape-memory materials [6,7,8] and shape-morphing systems [9,10,11,12], the phrase “4D printing” was introduced in 2013. Four-dimensional printing refers to a 3D-printed object that can deform and alter its shape as a result of an external stimulus or interaction mechanism [13]. Thus, changes in the mechanical status of the 4D-printed object from static to dynamic can be observed (Figure 2). This technique has opened a new era of technology in the field of additive manufacturing.

To better understand how 4D printing operates, Momeni and Ni suggested three rules that serve as general design guidelines for prospective 4D-printed structures [14]. The first rule is that the majority of multi-material self-morphing behaviors involve the phenomenon of relative expansion. The second rule is that these behaviors are characterized by the following four physical properties: mass diffusion, thermal expansion, molecular change, and organic growth. The third rule is that the majority of 4D-printed structures have one active layer and one passive layer.

For the materialization of the 4D printing process, there are three principles [15]. The first principle involves creating stimulus-responsive composite materials. The second principle is to provide a stimulus for a particular response of a printed material. The third principle is that the time required to respond to and change a printed object is known.

An, Chua, and Mironov presented three approaches to fabricate a 4D object [16]. The first approach is that the smart material alters its behavior in response to a stimulus. The second involves implanting a 3D-printed device with polymer-based components to accommodate tissue growth. The third approach is to precisely place the object in a certain pattern to initiate self-organization. Subsequently, the 4D printing process is influenced by the following five factors: the manufacturing method, materials, stimulus, interaction between materials and stimuli, and programming model [17].

1.3. Advantages of 4D Printing

The industrial revolution is being led by additive manufacturing, which has great potential and involves promising technology [18]. Although the advantages of 4D printing technology are similar to those of 3D printing, 4D printing outperforms 3D printing in the dynamic status of produced items. In addition, 4D printing technology may be the next generation of additive manufacturing and may be revolutionary in this regard [19].

Table 1. Summary of Advantages and Disadvantages of 4D Printing.

Advantages	Disadvantages
Saves energy, material, cost and time	Complexity
Sustainable	Requires special programming to adjust the responsiveness
Rapid production	Lack of literature
Response to stimulus	Limited options for materials

summarizes the benefits of adopting 4D printing technology to produce products; the benefits include savings in energy, materials, time, and money [20]. In addition, 4D printing technology is beneficial for reducing waste, errors, and product loss in manufacturing applications [21]. Different studies described the 4D printing process as energy-efficient [22], sustainable [23], and rapid [24] in comparison to other production processes.

Pandian and Belavek stated that 3D printing is the act of printing an object; for 4D printing, a creative process should be introduced, such as modifying the topology or stimulating a motion of the printed object [25]. These features are offered by using 4D printing technology to fabricate the final product. Miao et al.’s study concluded that 4D printing is considered as a good candidate for fabricating biomedical scaffolds [26]. This argument was based on the capability of a 4D-printed object to respond to environmental changes in the body. Because of the capabilities of 4D printing, Adam et al. contend that 4D printing will be the next generation of production in the field of microrobots [27]. These capabilities include high versatility and sensitivity, along with advanced design and stimulation. Aside from these characteristics, Ahmed et al. claim that 4D printing enables us to directly embed programming in a material without the use of external devices or systems [28].

1.4. Disadvantages of 4D Printing

In the same manner as its advantages, 3D printing’s drawbacks may apply to 4D printing. Because of the additional steps required to accommodate the mechanically changing behavior of printed objects, 4D design is more complex than 3D design [29]. With such a brief history of innovation, 4D printing currently lacks a substantial body of literature. In this regard, Wang et al. stated that 4D printing with shape-memory materials is still in its early stages and is not yet mature enough in comparison to 3D printing scaffold technology [30]. Moreover, Chu et al. and Cai both stated that several alternative smart materials have been introduced by using 4D printing technology [31,32]. Furthermore, Raghavendra et al. recognized a disadvantage of 4D printing, which is the difficulty in ensuring its accuracy because of the fluctuations in the process of optimizing parameters to regulate the functionality of size and shape [33]. The advantages and disadvantages of using 4D printing technology are summarized in Table 1.

1.5. Differences between 3D Printing and 4D Printing

The differences between 3D and 4D printing technologies will be discussed in this section. The comparison between 3D and 4D printing technology will be based on seven criteria that are summarized in

Table 2. Comparison of 3D Printing and 4D Printing.

Criteria	3D Printing	4D Printing
Maturity	Sufficiently mature (4 decades)	Not mature
Body mechanics	Static	Dynamic
Printing a complex design	Easy and fast	Difficult to print
Materials	Normal materials	Smart materials
Manufacturing approach	Parts are printed with a layer-upon-layer technique	Printed part is exposed to a specific stimulus
Printer/software	3D printer/software	4D printer/software
Properties of programming	Not applied	Mathematical modeling must apply

. These criteria include maturity, body mechanics, printing of a complex design, materials, manufacturing approach, printers/software, and properties of the programming.

Regarding the maturity of the field, 3D printing is more mature and advanced than 4D printing, since 3D printing was introduced four decades earlier. Kodama was the first to introduce 3D printing in 1980 [34], while Tibbits was the first to introduce 4D printing in 2013 [2]. While 3D printing fabricates rigid bodies, 4D printing fabricates bodies that can be deformed. The mechanical function features of 4D printing allow this technology to build complex patterns; 3D printing technology experiences difficulties in printing such designs [35]. Regarding the materials used, 3D printing technology works with common materials such as polymers, plastic, ceramic, and metal. 4D printing technology, on the other hand, involves the use of smart materials to fabricate an object.

Manufacturing with 3D printing is based on printing objects layer by layer [36]. In addition to this approach, its exposure to a stimulus has a substantial impact on the formation of 4D objects. Four-dimensional printing technology needs no special hardware or software to operate. In fact, 3D printers can be used to fabricate a 4D object [37]. The only difference is that 4D technology uses special programmable properties of a smart material to change certain functionalities of the final printed product [38].

1.6. Market Size: Economic Value and Statistics

Four-dimensional printing technology, including its features of self-assembly [39,40] and self-expansion [41], has the potential to develop a new manufacturing model in which reducing the size and space used by a printed object is the ultimate goal. These features reduce the costs associated with inventory and transportation, leading to increased market efficiency. Additional improvements could be achieved by a self-repair feature [42]. These features may result in a superior approach for the supply chain and logistics system.

In 2019, the global market share for 4D printing technology development was estimated to be worth roughly USD 65 million [43]. By 2021, the market had risen to roughly USD 90 million [44]. With a compound annual growth rate (CAGR) of 40%, the worldwide market for 4D printing is expected to reach over USD 2 billion by 2030 [45].

2. Four-Dimensional Printing Stimuli

The fundamental distinction between 3D and 4D printing technologies is the materials used to fabricate certain objects. The transition in the material’s mechanical status from static to dynamic is triggered by an external actuator (stimuli). Various stimuli applied to 3D objects to adjust their function and behavior have been investigated in the literature. Based on the number of publications (Figure 3), the most investigated stimuli in the literature are as follows, in descending order: temperature, light, water, magnetic, pH, electric, and humidity. Table 3 shows a summary of the advantages, limitations, and applications of each of these stimuli in 4D printing technology.

Temperature is the most commonly stimulus have been used for 4D-printed objects [46,47,48]. Because of its capabilities, many studies have investigated using temperature as a stimulus [49,50,51]. An advantage of using temperature as a stimulus is the capability to control the adjustment in a fabricated object [52,53], the capability to perform mechanical manipulations [54,55], and the ease of operation [56]. The limitations associated with using temperature as a stimulus are cell damage [31,57], slow response [58], complexity of use [31], and structure deformation [59]. The majority of experimental studies focused on using temperature as an external trigger to generate shape-deforming bio-printed parts [48,60]. Temperature as a stimulus has also been used in an application for in vitro drug release [61] and in tissue engineering [62].

Light is the second most common stimulus found in the literature, and it attracts more researchers [63]. The advantages of using light as stimulus include, but are not limited to, rapid switching [48,64], precise focusing [65], biocompatibility, sustainable source [66], wireless and remote control [67], color modification [68], and control of mechanical properties [69,70]. Although light is an excellent source for modifying the structure of 4D-printed objects, it has some drawbacks, including potential toxicity [71], diminished shape transformation [48], heat generation [72], and complexity [31]. Unlike temperature, light does not damage cells, so light as a stimulus can be used in biomedicine and drug delivery applications in vivo [31]. Light-sensitive materials can also be used in optical devices and microcantilevers [73].

Several studies have used water as a stimulus to actuate 4D objects [74,75,76]. The advantages of using water for deformation in 4D-printed objects are that water is controllable [75], convenient [59], and has the power to reduce temperature [77]. Water has some disadvantages, including a slow reaction time [77] and is difficult to control for humidity-sensitive materials [31]. Water as a stimulus has been widely used to control drug delivery [78,79,80,81].

Magnetic fields are another type of stimulus that have been used in various studies [82,83,84]. The use of magnetic fields as actuator have several advantages, including a quick and rapid response [57,85], low safety risk [48], remote guidance [27,86] and ability to accelerate the speed of moving target 4D objects [87]. However, a magnetic field as a stimulus also has the following limitations: a highly reactive nature and aggregation affinity of magnetic-response materials in biomedical applications [48], complication with magnetic nanoparticles in living systems [17], low use temperature, and high density of traditional magnetic absorbents [88]. Under certain circumstances, a magnetic field may cause the temperature to rise during the experiment [89]. Materials that are sensitive to magnetic fields are used in drug-delivery applications [90] and for fastening purposes [91].

In chemistry, “pH” stands for the “power of hydrogen” or the “potential for hydrogen”, which is a good source of stimulus for a 4D-printed object because of its capability to cause swelling, shrinking, dissociation, or degradation [48]. pH can also activate expansion, contraction, and torsion functions [92]. Furthermore, pH allows improvements in responses [92], the control of drug release [93], self-healing [94], color changes [95,96], biodegradation, and biocompatibility [97]. Because of their biodegradability and biocompatibility, 3D and 4D printing of pH-responsive polymers demonstrate significant potential for biomedical applications; however, 3D and 4D printing of pH-responsive protein hydrogels have limited applicability due to non-physiological pH gelation [73]. pH is used in applications for drug delivery [98,99,100], soft robots [101], medical purposes [102], food packaging [103], spinal cord regeneration [104], and tissue engineering [105].

Another type of stimulus that has been investigated in various studies [59,106] is electric fields, which demonstrate capabilities such as a high speed [31] and remote control [31,59]. Some potential complications associated with using an electric field as a source of stimulus are localized heating, membrane disruption, or cell death [107]. Electric stimulation is used for drug delivery [108], artificial muscle [109], and tissue regeneration [110].

Finally, based on the number of publications (Figure 3), the least commonly used stimulus is humidity. The humidity stimulus has received less attention than temperature and light [111]. Humidity was previously analyzed in some studies [112,113,114] because of its capability to perform swelling or shrinking [57], twisting [115], bending [116], and expanding [117]. The benefits of using humidity as an actuator are its low cost and harmless nature [113]. Furthermore, the humidity stimulus has been used in applications such as artificial muscles [118] and sensors [119]. Since the human body contains water, humidity is suitable for biomedical applications [37]. However, there are several barriers to using moisture as a stimulus in 4D printing technology, including the slow response time [31] and the need for precise control to ensure the structure’s safety [57].

Other stimuli that have been investigated for 4D-printed objects include, but are not limited to, microwaves [120,121], enzymes [46], glucose [122,123], and stress [124]. The majority of publications on 4D printing technology are limited to the investigation of a single stimulus; however, some researchers have explored responsiveness to multiple stimuli. Guo et al. studied the sensitivity of a hydrogel material to temperature and pH stimuli [125,126]. Other investigations with hydrogel materials used temperature and magnetic field stimuli [127], as well as magnetic and pH stimuli [128]. Studies have used temperature and water with polymer materials [75,129]. Kuksenok and Balazs captured the stimuli-response behavior of a composite material to temperature and light [130].

Table 3. Summary of Stimulus Factors Used in 4D Printing Technology.

Stimulus	Advantages	Limitations	Applications	References
Temperature	- Control the adjustment in a fabricated object - Perform mechanical manipulations - Ease of operation	- Damaged cells - Slow response - Complicated	- Biomedical - Drug delivery - Tissue engineering	[31,48,52,53,54,55,56,57,58,59,60,61,62]
Light	- Rapid switching - Precise focusing - Biocompatibility - Sustainability - Control of mechanical property	- Potential toxicity - Diminished shape transformation - Heat generation - Complexity	- Drug delivery - Optical devices	[31,48,64,65,66,67,68,69,70,71,72,73]
Water	- Controllable - Convenient - Reduced temperature	- Slow reaction times	- Drug delivery	[59,75,77,78,79,80]
Magnetic	- Quick response - Safety - Rapid response - Remote guide	- Highly reactive - Aggregation affinity - Complication with nanoparticles in living systems - High density	- Drug delivery - Fastening purposes	[17,27,48,57,85,86,88]
pH	- Achieve various structure behaviors - Improved response - Biodegradation - Biocompatibility	- Limited applicability	- Drug delivery - Soft robots - Medical - Food packaging - Spinal cord regeneration - Tissue engineering	[48,73,92,97,98,99,100,101,102,103,104,105]
Electric	- Speed - Remote control	- Localized heat - Membrane disruption - Cell death	- Drug delivery - Artificial muscle - Tissue regeneration	[31,59,107,108,109,110]
Humidity	- Perform various behaviors - Low cost - Environmentally friendly	- Slow response - Need for precise control	- Artificial muscles - Sensors - Biomedical	[31,37,57,113,115,116,117,118,119]

Table 3. Summary of Stimulus Factors Used in 4D Printing Technology.

Stimulus	Advantages	Limitations	Applications	References
Temperature	- Control the adjustment in a fabricated object - Perform mechanical manipulations - Ease of operation	- Damaged cells - Slow response - Complicated	- Biomedical - Drug delivery - Tissue engineering	[31,48,52,53,54,55,56,57,58,59,60,61,62]
Light	- Rapid switching - Precise focusing - Biocompatibility - Sustainability - Control of mechanical property	- Potential toxicity - Diminished shape transformation - Heat generation - Complexity	- Drug delivery - Optical devices	[31,48,64,65,66,67,68,69,70,71,72,73]
Water	- Controllable - Convenient - Reduced temperature	- Slow reaction times	- Drug delivery	[59,75,77,78,79,80]
Magnetic	- Quick response - Safety - Rapid response - Remote guide	- Highly reactive - Aggregation affinity - Complication with nanoparticles in living systems - High density	- Drug delivery - Fastening purposes	[17,27,48,57,85,86,88]
pH	- Achieve various structure behaviors - Improved response - Biodegradation - Biocompatibility	- Limited applicability	- Drug delivery - Soft robots - Medical - Food packaging - Spinal cord regeneration - Tissue engineering	[48,73,92,97,98,99,100,101,102,103,104,105]
Electric	- Speed - Remote control	- Localized heat - Membrane disruption - Cell death	- Drug delivery - Artificial muscle - Tissue regeneration	[31,59,107,108,109,110]
Humidity	- Perform various behaviors - Low cost - Environmentally friendly	- Slow response - Need for precise control	- Artificial muscles - Sensors - Biomedical	[31,37,57,113,115,116,117,118,119]

3. Four-Dimensional Printing Materials

Since the methodology for 4D printing follows procedures similar to 3D printing, particularly in terms of production, the novelty in 4D printing is the material selection and characterization for reactions to an active agent. Common 3D printing materials include plastic, ceramic, and metal; however, these materials are not suitable for 4D printing [131]. This section discusses the smart materials that have been investigated in the literature. Materials that have been used to fabricate an object using 4D printing technology include, but are not limited to, shape-memory polymers [132,133,134,135], shape-memory alloys [136,137,138], shape-memory metals [139,140], hydrogel-responsive materials [141,142], shape-memory ceramic [143,144,145,146], and liquid crystal elastomers [147,148]. Figure 4 shows the number of publications for various materials used in the application of 4D printing technology. Shape-memory polymers are the most prevalent in this context, followed by shape-memory metal, hydrogel, shape-memory alloys, shape-memory ceramic, and liquid crystal elastomers. Table 4 provides a comparison of all the materials mentioned in this review.

A smart shape-memory polymer (SMP) material is the most commonly used material in 4D technology [149] because of its low cost, low density, good recovery strain [47,150], high shape recovery ratio [151], biodegradability, sustainability [152], biocompatibility [59], universality [153], and ability to adjust molecular weights [154,155]. Moreover, SMP features also include their lightweight nature and their capability to significantly deform [156]. Shape-memory polymers outperform shape-memory alloys regarding final-product quality and dimensional accuracy [151]. The limitations of the SMP material include its relatively low modulus [148,149], lack of strength [157], undesirable transition temperature [158], and slow response time [159,160]. Furthermore, SMP materials are susceptible to toxicity or the absence of an immune response [161]. SMP materials are widely used in 4D printing applications such as biomedical applications [162,163], aerospace [164], automobile interior surfaces [15], textiles [165], and industry [166].

Metals are pure elements that have been investigated for the purest fabrication of 4D-printed parts. Shape-memory metals (SMM) are an excellent conductor of heat and electricity [167]. The disadvantages associated with metal materials include their hypersensitivities [168]. SMM applications have been explored in industrial fields such as automobiles [169] and biomedicine [155,170].

Shape-memory hydrogel is another material that has been used; it has great biocompatibility, transparency, stretchability, ionic conductivity [171], and simplicity of modification [150]. Hydrogels are well-known for their high water content and ability to absorb large amounts of water or other biological fluids [56,172]. Some limitations associated with hydrogels are their high cost, difficulty in handling and loading [151], low mechanical strength [15,173,174], slow response [56,159,160], and reduced properties due to the possibility of dryness in open air [175]. Some uses of smart hydrogels in 4D printing include applications in healthcare, agriculture, tissue engineering, and drug delivery [176].

A shape-memory alloy (SMA) is a blended composition of two or more metal elements. Alloys are excellent conductors of heat and electricity [167]. Additional advantages of an SMA for 4D printing are its strength [177] and compatibility [178]. Compared to SMP, SMAs have higher mechanical strength and enhanced recovery abilities from stress [151]. On the other hand, some SMAs demonstrate potential toxicity [168], limited ductility, brittleness [136], and slow responses [179]. Due to the strong capabilities of SMA materials, they are suitable for use in aerospace [177], neurosurgical [180], and biomedical [181] applications.

Shape-memory ceramic (SMC) materials are safe and non-toxic [168], have high mechanical strength and biocompatibility [182], and can withstand high operating temperatures [178]. It is difficult to fabricate 4D-printed objects with this type of material because of their extremely high melting points [183], difficulty with controlling the microstructure [177], and a lack of deforming ability [143]. SMC materials have prospective applications in biochemistry [184], architecture [185], aerospace [186] and bio-inspired hybrid materials [187].

Several researchers have investigated the production of a 4D product using liquid crystal elastomers (LCEs); nevertheless, when compared to other materials, these experiments were less remarkable [113]. A liquid crystal elastomer has high performance properties [188], low crosslinked density, flexible structure [173], repeatability [189], complex shape construction, and improved responding speed [173]. According to Ula et al., our lack of theoretical understanding of elastomers limits the development of applications for liquid crystal elastomer materials [189]. Furthermore, LCEs have a restricted ability to scale up production [189], a limited deformation mode [148], and a low resolution [190]. Liquid crystal material is used for the fabrication of biomedical devices [188] and soft robotics [189].

To overcome some limitations associated with single components, a composite material is proposed in the context of 4D printing. Various methods for producing parts with 4D printing have been examined, including multi-polymers [64,191,192], multi-hydrogels [193,194], and multi-metals [140]. The investigation was extended to include the use of a multi-material by combining polymers and hydrogels [195] or polymers and alloys [196].

Table 4. Summary of 4D Printing Technology Materials.

Material	Advantages	Limitations	Applications	References
Shape- memory polymer	- Low cost and density - Lightweight - High recovery strain - Biodegradability - Sustainability - Biocompatibility - High-quality product	- Low modulus - Lack of strength - Have undesirable temperatures - Slow response - Toxicity	- Biomedical - Aerospace - Automobile - Textiles - Industrial	[15,47,59,149,150,151,152,156,157,158,159,160,161,162,163,164,165,166]
Shape- memory metal	- Excellent conductor of heat and electricity	- Hypersensitivities	- Automobile - Biomedical	[155,167,168,169,170]
Smart hydrogel	- Biocompatibility - Transparency - Stretchability - Ionic conductivity - Simplicity of modification - Absorb large amounts of fluid	- Expensive - Difficult to handle and load - Low mechanical strength - Slow response - Possibility of dryness in open air	- Healthcare - Agriculture - Tissue engineering - Drug delivery	[15,56,150,159,160,171,172,173,174,175,176]
Shape- memory alloy	- Excellent conductor of heat and electricity - High strength and recovery stress - Compatibility	- Toxicity - Limited ductility - Brittleness - Slow response	- Aerospace - Neurosurgical - Biomedical	[136,151,167,168,177,178,179,180,181]
Shape- memory ceramic	- Non-toxic - Safe - High mechanical strength - Biocompatibility - Outstanding high temperature	- High melting point - Difficult to control - Lack of deforming ability	- Biochemical - Architectural - Aerospace - Bio-inspired materials	[143,168,177,178,182,183,184,185,186,187]
Liquid crystal elastomers	- High performance properties - Low crosslinked density - Flexible structure - Repeatability - Complex shape construction - Improved responding speed	- Lack of theoretical understanding - Restricted ability to scale up production - Limited deformation mode - Low resolution	- Biomedical devices - Soft robotics	[148,173,188,189,190]

Table 4. Summary of 4D Printing Technology Materials.

Material	Advantages	Limitations	Applications	References
Shape- memory polymer	- Low cost and density - Lightweight - High recovery strain - Biodegradability - Sustainability - Biocompatibility - High-quality product	- Low modulus - Lack of strength - Have undesirable temperatures - Slow response - Toxicity	- Biomedical - Aerospace - Automobile - Textiles - Industrial	[15,47,59,149,150,151,152,156,157,158,159,160,161,162,163,164,165,166]
Shape- memory metal	- Excellent conductor of heat and electricity	- Hypersensitivities	- Automobile - Biomedical	[155,167,168,169,170]
Smart hydrogel	- Biocompatibility - Transparency - Stretchability - Ionic conductivity - Simplicity of modification - Absorb large amounts of fluid	- Expensive - Difficult to handle and load - Low mechanical strength - Slow response - Possibility of dryness in open air	- Healthcare - Agriculture - Tissue engineering - Drug delivery	[15,56,150,159,160,171,172,173,174,175,176]
Shape- memory alloy	- Excellent conductor of heat and electricity - High strength and recovery stress - Compatibility	- Toxicity - Limited ductility - Brittleness - Slow response	- Aerospace - Neurosurgical - Biomedical	[136,151,167,168,177,178,179,180,181]
Shape- memory ceramic	- Non-toxic - Safe - High mechanical strength - Biocompatibility - Outstanding high temperature	- High melting point - Difficult to control - Lack of deforming ability	- Biochemical - Architectural - Aerospace - Bio-inspired materials	[143,168,177,178,182,183,184,185,186,187]
Liquid crystal elastomers	- High performance properties - Low crosslinked density - Flexible structure - Repeatability - Complex shape construction - Improved responding speed	- Lack of theoretical understanding - Restricted ability to scale up production - Limited deformation mode - Low resolution	- Biomedical devices - Soft robotics	[148,173,188,189,190]

4. Four-Dimensional Printing Manufacturing Methods

In the literature, researchers have used the following five methods for printing 4D objects: fused deposition modeling (FDM) [197,198,199,200]; direct ink writing (DIW) [201,202]; stereolithography (SLA) [203,204,205]; digital light processing (DLP) [206,207,208]; and selective laser sintering (SLS) [209,210,211]. The search approach used here for the estimated number of publications based on the fabrication method for 4D printing was slightly different. The search method contained the abbreviation of the manufacturing method as a keyword. For example, the phrase “4D printing” “fused deposition modeling” OR “FDM” was used in a search for the manufacturing process for fused deposition modeling. The “OR” function was used to ensure that all the studies that used an abbreviation were included. Figure 5 shows the number of publications from 2013 to November 2022 based on manufacturing methods; FDM is the most common fabrication method for printing 4D parts, followed by SLA, SLS, DLP, and DIW. Table 5 summarizes the manufacturing methods of the 4D printing technology covered in this review paper. The manufacturing of 4D-printed objects also involves other methods, such as PolyJet printing [212,213], powder bed fusion [214], photolithography [215], directed energy deposition [136], and selective laser melting [216].

The most common fabrication method for manufacturing a 4D-printed object is fused deposition modeling (FDM). The FDM technique has been used with a variety of stimuli such as temperature [199], pH [217], light [218], and water [219]. This method is relatively inexpensive, produces high-quality, high-resolution products [15], involves a fast printing process [220], and is easily accessible [221]. Although FDM was originally limited to thermoplastic polymer materials for printing, studies have expanded FDM to composite polymers for improved results [221]. Some limitations of FDM are its high complexity, fragility [222], low printing speed [220], and rough surface finish [223]; in addition, it wastes more material than other methods [224]. The FDM technique has been used for industrial applications [225], biomedical devices [226], aerospace products [227], origami structures [228], drug delivery [97], and optical devices [229].

Stereolithography (SLA) is another method that has been used to fabricate 4D-printed objects. The advantages of using the SLA method include its fast speed [223,230], sophistication [15], smooth surface finish [231], and high resolution [232]. Stereolithography is best known to operate with SMP [15] at relatively low temperatures [233]. Even though SLA is capable of printing parts using polymer materials, the final parts have low mechanical properties [234]. A disadvantage of SLA is the long response time [234]; in addition, SLA requires a support structure, and consequently requires post-processing [37]. Many studies have been carried out using SLA in the fields of soft robotics [235], drug delivery [236], tracheal stents [237], biomedical scaffolds [26], and tissue engineering [238].

Selective laser sintering (SLS) enables the manufacturing of components without the need for support material [17]. In addition, this technology has the advantages of a high production volume and high process speed [168]. Even though SLS is capable of printing with a variety of materials [179], it lacks suitable materials for printing 4D parts [206]. The disadvantages of this approach include health risks [168] and high cost [231], along with poor surface and dimensional accuracy [13]. Some applications that benefit from the SLS approach include biomedical devices [26], drug delivery [239], magnetism-responsive grippers [210], and aerospace [240].

A digital light processing (DLP) manufacturing method has been used to fabricate a 4D-printed object. Texas Instruments invented this technique shortly after the SLA technology was invented in 1987 [241]. This technique offers the following advantages: rapid production times [224], high-quality resolution, and complex structures [206]. DLP is constrained by material choices [168], high material costs [220], and poor mechanical properties [242]. The DLP manufacturing approach has been used and investigated in a variety of applications, including, but not limited to, medication delivery [243], tissue engineering [244], and electronic devices [245].

Direct ink writing (DIW) is the least commonly used method in the literature. DIW has a lower dimensional precision of printed parts when compared to other printing technologies such as SLA [93]. Furthermore, the impact of printing parameters on dimensional accuracy and roughness is largely unclear due to insufficient investigations and publications regarding DIW [246]. DIW 4D printing offers a diverse range of materials at a low cost [224], and the DIW process has minimal material waste of printed parts [220]. DIW is used in a wide range of material states, including solid, liquid, and gas states [223,240]. The DIW approach is appropriate for use in biomedicine and robotics [224]. It has also been used in a variety of domains, including tissue engineering [247], electronic devices [248], and biomedical engineering [249].

Table 5. Summary of Materials Used in 4D Printing Applications.

Manufacturing Method	Advantages	Limitations	Applications	References
Fused deposition modeling (FDM)	- Low cost - High resolution - Fast - Easy	- Complex - Fragile parts - Low printing speed - Rough surface - Waste material	- Industry - Biomedical devices - Aerospace - Drug delivery - Optical devices	[15,97,220,221,222,223,224,225,226,227,229]
Stereolithography (SLA)	- Fast - Sophisticated - Smooth surface - High resolution	- Poor material properties - Long response time - Requires support structure	- Soft robotics - Drug delivery - Tracheal stents - Biomedical scaffolds - Tissue engineering	[15,26,37,223,230,231,232,234,235,236,237,238]
Selective laser sintering (SLS)	- High production volume - Fast - No need for support material	- Lacks suitable materials - Health risks - High cost - Poor surface and dimensional accuracy	- Biomedical devices - Drug delivery - Magnetism-responsive grippers - Aerospace	[13,17,26,168,206,210,231,239,240]
Digital light processing (DLP)	- Fast - High resolution	- Limited options for materials - High material cost - Poor mechanical properties	- Drug delivery - Tissue engineering - Electronic devices	[168,206,220,224,242,243,244,245]
Direct ink writing (DIW)	- Low cost - Variety of options for materials - Low material waste	- Low resolution - Lack of literature - Lack of clarity about the effects of parameters	- Biomedicine - Robotics - Tissue engineering - Electronic devices - Biomedical engineering	[94,220,224,246,247,248,249]

Table 5. Summary of Materials Used in 4D Printing Applications.

Manufacturing Method	Advantages	Limitations	Applications	References
Fused deposition modeling (FDM)	- Low cost - High resolution - Fast - Easy	- Complex - Fragile parts - Low printing speed - Rough surface - Waste material	- Industry - Biomedical devices - Aerospace - Drug delivery - Optical devices	[15,97,220,221,222,223,224,225,226,227,229]
Stereolithography (SLA)	- Fast - Sophisticated - Smooth surface - High resolution	- Poor material properties - Long response time - Requires support structure	- Soft robotics - Drug delivery - Tracheal stents - Biomedical scaffolds - Tissue engineering	[15,26,37,223,230,231,232,234,235,236,237,238]
Selective laser sintering (SLS)	- High production volume - Fast - No need for support material	- Lacks suitable materials - Health risks - High cost - Poor surface and dimensional accuracy	- Biomedical devices - Drug delivery - Magnetism-responsive grippers - Aerospace	[13,17,26,168,206,210,231,239,240]
Digital light processing (DLP)	- Fast - High resolution	- Limited options for materials - High material cost - Poor mechanical properties	- Drug delivery - Tissue engineering - Electronic devices	[168,206,220,224,242,243,244,245]
Direct ink writing (DIW)	- Low cost - Variety of options for materials - Low material waste	- Low resolution - Lack of literature - Lack of clarity about the effects of parameters	- Biomedicine - Robotics - Tissue engineering - Electronic devices - Biomedical engineering	[94,220,224,246,247,248,249]

5. Four-Dimensional Printing Application

Four-dimensional printing technology has been widely used for a variety of applications. The capabilities of this technology have attracted the interest of academics, companies, and scientists. Four-dimensional printing is a promising technology that can fabricate printed structures at a reasonable cost and with reduced labor time [250]. A number of studies have highlighted the potential of 4D printing applications in disciplines such as healthcare, aerospace, electronics, food, renewable energy, fashion, and military. Figure 6 displays the number of publications of 4D printing applications in several fields. These estimates may not accurately reflect the advancements in this domain, as some research advances are not published for a variety of reasons, including commercial purposes such as fashion or apparel, or military classified security concerns. Other applications of 4D printing that have been investigated but were not included in this paper involve construction [251], agriculture [58], and automobiles [252].

5.1. Healthcare

Healthcare applications in 4D printing have received the most attention in terms of publications. With a CAGR of 26.7%, the 4D printing in healthcare market size is predicted to be approximately USD 10 million in 2021 [253], rising to roughly USD 32 million in 2026 [254], and USD 36 million in 2028 [255]. Because healthcare is such a broad field, this section is divided into the following six subsections: dentistry, orthopedics, pharmaceuticals, drug delivery, biomedical devices, and tissue engineering.

5.1.1. Dentistry

Four-dimensional printing is a strong candidate for producing parts that function in an oral environment, with humidity and temperature changes [256]. Some issues associated with this technology in advancing the dentistry industry include undesirable dimensional changes, an unstable environment, and behavioral transformation. Many papers have covered the advancements of using 4D printing in dentistry applications, such as prosthetics [257], orthodontics [40,258], implanted teeth [259,260], and maxillofacial surgery [168].

5.1.2. Orthopedics

There is a need for artificial bones and orthopedic implants that can grow, especially in children, and 4D printing provides a dynamic structure and stimulus response that is useful in the orthopedic field [261]. Besides enhancing performance and functionality, Javaid and Haleem argued that 4D-printed implants lower the risk associated with post-implanting processes [262]. With the use of materials such as PLA [226], SMP [263], SMA [264], and hydrogels [265], the orthopedics field has advanced using 4D printing. Wang et al. developed an artificial hand with five fingers made of PLA material that can perform specific functions [266]. The study investigated the influence of various process parameters on the properties of shape memory. Grinberg et al. used the fused filament technique to fabricate a knee prosthesis with an active sensor made of piezoelectric composites [267]. Different factors were applied in the study to trigger the dynamics of the printed piezoelectric composite. The research contributed to the development of a high-performance, multifunctional part that is inexpensive and lightweight. Hoa created complicated composite springs with the help of 4D printing technology [268]. The study’s novelty was the ability to construct the springs without the use of complex curved molds. This concept could be a fundamental approach for manufacturing springs for lower leg, ankle, and foot prosthetics. Ploszajski et al. created a lightweight chainmail cloth with SLS and magnetic stimulation [269]. This research procedure advanced the development of a fabric for orthotics applications, including an active stiffening wrist brace.

5.1.3. Pharmaceuticals

According to Basit and Gaisford, 4D printing is still in its infancy and requires further investigation before it can be used for consumable medications [270]. Melocchi et al. agreed with the previous argument and believed that there will be extensive efforts made in this area to investigate materials, manufacturing, and behaviors [157]. The exploration of polymer chemical structures for consumable medication has been the center of numerous SMP research applications in the pharmaceutical industry [156].

5.1.4. Drug Delivery

Over the past few decades, an extraordinary improvement in drug-delivery technologies has been made [271]. The drug-delivery method is intended to improve drug efficiency while minimizing side effects. Four-dimensional printing is a novel technology with advanced features that has the potential to accelerate the development of drug-delivery systems. Four-dimensional printing’s self-folding and self-unfolding features allow for carefully controlled encapsulation and medication release through the use of a programmed method [272]. However, Toit et al. argued that drug delivery innovation via 4D printing is currently limited and not yet biomedically suitable because the present applications require strong external stimuli [273].

Melocchi et al. developed a 4D-printed gastric retention device based on shape-memory behavior [274]. The device was built using the FDM method and exposed to aqueous fluids and temperature during the study. When subjected to these stimuli, the device demonstrated good mechanical properties that are appropriate for the stomach environment. Moreover, a 4D-printed intravesical drug delivery device was created using hot-melt extrusion and the FDM manufacturing technology, along with SMP materials [275]. The device was programmable to function following a certain shape deformation and return to its original shape when exposed to the external stimulus. In addition, this device is appropriate for administration in developing retentive drug-delivery treatment of bladder diseases. Villar et al. developed encapsulated multisomes (small oil drops in water) that are released when stimulated by pH or temperature [276].

5.1.5. Biomedical Devices

The biomedical industry has shown increased interest in the application of 4D printing (4D bioprinting). Studies in the field of biomedical engineering have investigated materials such as SMPs [83,277,278] and hydrogels [249]. Lai et al. demonstrated shape morphing with using hydrogel base structure that had excellent printability feature towards the advancement of biomedical devices [141]. However, the authors argued that because of the roughness of the surface, fabricating a 4D printable product with a hydrogel structure remains challenging. The manufacturing methods that have been used to advance the fabrication of biomedical devices include FDM [279,280,281], DIW [282], and SLA [26].

Zarek et al. created a tracheal stent using the SLA production process and SMP material [237]. The manufactured stent is able to overcome some of the disadvantages of the current proposed stents, such as the position of the cartilaginous ring and injury during deployment. In this regard, Marco et al. proposed an indirect printing technique that overcomes the current technical limitations of soft robotic structure development [283]. The researchers designed different stents at the microscale (5 m) using DIW and polymers. Cheng et al. fabricated 4D-printed vascular stents using a genetic algorithm to optimize the structure towards the progression of vascular stenosis treatment [284]. The stent was fabricated using the FDM technique with PLA material. Mechanical testing, finite element analysis, and in vitro feasibility tests were performed on the shape-memory polymer vascular stent, demonstrating the stent’s potential to function satisfactorily in the treatment of vascular stenosis.

5.1.6. Tissue Engineering

Extensive research has been carried out over the last decade to advance tissue engineering techniques. Three-dimensional printing has helped to improve this discipline; however, there are several challenges and limitations associated with it [285,286]. The advanced capabilities of 4D printing technology can help overcome these obstacles. These features include, but are not limited to, the ability to place implants in inaccessible locations [245] and minimally invasive surgery procedures [287].

Cui et al. expressed that the proposed 4D printing model can control cell–cell structures [288]. The authors developed an on-demand, user-friendly 4D inkjet printing technology for the production of a cell-encapsulated 3D scaffold. Aside from assisting with 2D cell seeding, the printed scaffold demonstrated significant potential and ability to encapsulate 3D cells. Luo et al. constructed a 4D cell-loaded structure that maintained good cell viability for at least two weeks, and they discovered that NIR laser irradiation had no discernible effect on burdened cells [224]. As a result, they hypothesized that printed shape-morphing materials with bioactivity may be exploited in tissue engineering and regenerative medicine, where bending curvatures are necessary.

Lai, Li, and Wang proposed a novel approach to progress in the field of tissue engineering [289]. Their proposed scaffold has the ability to self-fold, encapsulate, and control the release of growth factors. Moreover, Toit et al. argued that 4D printing can enhance the innate qualities of tissues and organs, ultimately restoring their natural capabilities [273].

To advance the development of the fabrication of tissue engineering by using 4D printing, several researchers have investigated manufacturing methods such as FDM [290] SLA [26], and DLP [291]; stimuli such as temperature [292], light [203,293], and magnetism [294]; and materials such as SMP [295], hydrogel [296], and SMA.

5.2. Aerospace

One concern associated with the aerospace industry is the complexity of components and the difficulties in assembling them [297]. Another point of concern is the high cost of airplane replacement parts [298], as well as the disruption of the global supply chain [299]. Four-dimensional printing technology mitigates these difficulties by creating lower assembly parts and saving time [300]. Several researchers have studied the accelerated development of aerospace applications, such as airplanes, wings, and aircraft spare parts [301,302]. Various researchers manufactured a bionic butterfly and analyzed the wing swung under magnetic stimulation [82,303]. When the magnetic field was far away, the wing-flap process decreased and the butterfly returned to its original position.

Li et al. demonstrated the usefulness of using 4D printing in airplane manufacturing by creating a model with outstanding shape-memory performance [304]. The authors proposed that shape-memory polyimides should be used as a material. The following two manufacturing approaches were evaluated: DLP, and extrusion molding. The 4D-printed model demonstrated good mechanical strength and the capacity to regain its original shape.

5.3. Electronics

With its promising features, 4D printing has been used in a variety of electronics applications, such as circuits [171,305], sensors [306,307], actuators [196,308], magnetics [303], robotics [309,310], energy storage devices [311,312], and antennas [310,313]. The printing processes used included, but were not limited to, FDM [74,314], SLS [210,211], SLA [315], DIW [248], and DLP [93]. The most common materials associated with electronic applications were shape-memory polymers [316,317], shape-memory hydrogels [318], and shape-memory alloys [33]. The stimuli proposed for the fabrication of 4D-printed electronic devices were heating [319], light [320], magnetic [248], and water [321].

Zhang et al. used shape-memory polymer material to create a 3D electrical circuit [322]. The study showed that the circuit was capable of stretching when exposed to external heating stimuli and achieved uniform heating performance. Another study focused on the self-folding fabrication of a circuit using the DIW method and heating stimuli [37]. According to Deng et al., the proposed technology is capable of manufacturing customized circuit designs in a simple, rapid manner. Chen et al. devised a multi-function sensor/actuator capable of self-sensing via thermal and mechanical actuation [323].

Using the DIW method, Mu et al. attempted to discover the effects of electro-mechanical behaviors on fabricated soft sensors that can stretch by 45% [306]. A new strategy to manufacture a reversible actuator was proposed by Wang and Li [324]. The strategy that followed was based on a bilayer structure design that controls the voltage to sense strain. The goal of Wang and Li’s work was to produce a multi-function intelligent actuator. A rotational multi-stable structure design was introduced by Jeong et al. by adjusting the thickness of the SMP beam to be used for various actuators [64]. By responding to external inputs, the suggested architecture allows for the control of the activation time. The study concluded that the programmable structure is capable of responding based on material properties, without the need for a power supply.

5.4. Food

Many studies have been conducted on food preparation using 4D printing technologies [12,47,151]. The most proposed production methods in the literature associated with the food sector are FDM, SLA, DIW, DLP, and SLM [325]. The printing materials that were used included, but were not limited to, chocolate [326], cheese [327], dough [328], starch [329], and potato [330].

A study by Phuhongsung, Zhang, and Bhandari examined the evolution of flavor changes in 4D-printed materials [331]. The printing material used was a combination of soybean protein isolate, k carrageenan, and vanilla flavor. The researchers used microwave heating as a stimulus, with varying power ranges to demonstrate the alterations by conducting electric nose, tongue, and GC MS analyses. According to the study results, the smell changed after heating for 20 min, but the taste did not change. The study found that the proposed material composite is a promising choice for 4D printing fabrication, while keeping an ideal heating level in mind.

A study by Shanthamma et al. investigated the development of color change in a 4D-printed product [96]. The study sought to evaluate the capabilities of different concentrations of turmeric powder and sago flour incorporated into a solution of sodium bicarbonate. The results showed that the color change from yellow to orange/red was time dependent. The study concluded that the proposed materials showed the capabilities toward the advancement of color transformation.

Tao et al. investigated the shape alteration of 4D-printed dough using hydration and dehydration triggers [332]. When exposed to the stimuli, the proposed material displayed a geometric behavior of swelling and shrinking. Participants in this study stated that compared to regular pasta, manufactured pasta had a better structure, a more savory taste, and did not cook properly.

5.5. Fashion and Apparel

Several 4D-printed fashion and apparel products are displayed on commercial business websites, although studies have not been published in the literature for commercial reasons [333,334,335,336,337]. In this regard, 4D printing technology has the potential to produce a wide range of products such as shoes, clothing, jewelry, necklaces, bracelets, earrings, and rings [336,338,339].

According to Biswas et al., 4D printing technology may be suitable for the production of professional clothes that conform to specific functions [35]. The researchers proposed using 4D printing technology to fabricate gloves that used for surgical applications. Nespoli et al. fabricated personalized 4D-printed rings using SLM and shape-memory alloys as materials [340]. Although the rings were exposed to environmental changes, the printed rings fit and were comfortable, showing an improvement in the adaptability of jewelry. Furthermore, Zarek et al. fabricated a dynamic ring and footwear heel using a combination of the DLP process and SMP material [339].

5.6. Renewable Energy

Four-dimensional printing technology could overcome the renewable energy obstacles of low efficiency [341] and high cost [342]. Even though Momeni and Ni demonstrated the capability of 4D printing technology to improve energy efficiency [22], 4D printing in the field of renewable energy is underdeveloped compared to other applications (Figure 6). Momeni and Ni proposed a new design to set limits on the consumption of energy using thermodynamic analysis. The authors expanded their demonstration in this area by using 4D printing to develop a solar concentrator [343] and wind turbine blades [344].

5.7. Military

Even though military-related research in 4D printing has received a large share of funding, the research has been classified, so there are a limited number of studies in the literature. According to Allied Market Research, the global military 4D printing industry is estimated to reach USD 16 million by 2030 [345]. The estimated market size in 2040 is USD 673 million, with a CAGR of 45% from 2030 to 2040. In his master’s thesis, Hamel emphasized that an investment in developing a military operation by using 4D printing will keep the current dominance of American airpower well into the future [346]. This gives clear images of the future capabilities and potential of 4D printing technology. Moreover, the U.S. army research office funded three universities (e.g., Harvard University) with almost USD 900,000 to work on 4D printing technology projects [347].

6. Gaps in Research and Outlook for the Future

This paper listed the benefits and drawbacks of using 4D printing for a variety of applications, listing studies that proposed various smart materials, manufacturing processes, and stimuli for the construction of 4D-printed parts. However, 4D printing technology is immature, and there are still gaps in the field of 4D printing technology. This section discusses the future of 4D printing in terms of scaling up the manufacturing process, predictive modeling, next-generation, and other newly discovered technologies.

6.1. Scaling Up the Manufacturing Process

The current manufacturing process and design of 4D printed objects are similar to those of 3D printing technology. According to Sahafnejad-Mohammadi et al., a significant constraint in terms of designing and fabricating 4D structures is the limited number of 3D printers that print 4D parts and the lack of specialized software [37]. To scale up the 4D production process, 4D printing technology will benefit from specialized printers and software.

6.2. Predictive Model for 4D Printing Manufacturing

Although several studies have been conducted to investigate the creation of 4D parts using various manufacturing methods, stimuli, materials, behaviors, and programmable shapes, there is a lack of detailed understanding of the design parameters that warrants further investigation. Chen et al. emphasized that most of inverse problem are address with relying on trial-and-error process [348]. For this, a systematic and defined method would boost the effectiveness of the 4D printing design process and structure.

Kantareddy noted that design guidelines that relate fundamental forms to temporary shapes are still essential to enable designers to construct advanced SMP-based structures [349]. Furthermore, a lack of understanding of the design formulation of 4D printing technology presents barriers to the advancement of this technology [350]. In addition, Nikkanen argued that one of the obstacles in 4D printing is a lack of understanding of the behavior of 4D-printed parts; consequently, there is a need for a mathematical model that can predict the behavior of 4D-printed parts [351]. For this, the introduction of several parameters and their effects on the 4D printing design would accelerate the advancement of the field.

6.3. The Next Generation of 4D Printing

Despite the fact that several studies have presented various smart materials and methods for fabricating 4D objects, these proposed materials are limited and require further investigation. According to Farid et al., material availability limits the capability of 4D printing technology [350]. Furthermore, due to the limited studies on 4D printing methods and materials, constructing a complex structure with remarkable mechanical qualities remains a challenge [352]. According to Ma Quanjin et al., developing unique smart materials with advanced properties will assist in advancing 4D printing technology [353].

With its other characteristics under investigation, such as self-assembly and self-expansion, 4D printing could aid the logistics sector by lowering manufacturing and transportation costs. This would contribute to the advancement of the global supply chain system. On the other hand, Ghi and Rossetti argued that 4D printing might be the next generation of lean manufacturing because of its potential to self-repair and self-assemble parts from materials [21].

Finally, the recently proposed additive manufacturing technologies such as 5D printing [354] and 6D printing [355,356] could advance 4D printing technology. To speed up the building process and strengthen the printed output, 5D printing uses the movement of both the print head and the printed object to print parts with curved surfaces without the requirement for a support system. In addition, 6D printing is 4D printing combined with the capabilities of 5D printing to create smart-material structures, with features such as curved surfaces. These two technologies will open opportunities for further exploration.

7. Conclusions

In comparison to other methods such as 3D printing, 4D printing is still in its infancy and will require more time to evolve. Despite the limitations and concerns shown in this review, there is a bright future for 4D printing technology with advanced shape deformation features. This review outlines several features of 4D printing technology. Due to these benefits, extensive studies and research have been undertaken in the production of 4D-printed products. The current limitations of this technology include its complexity, the need for special programming, the lack of literature, and limited choices for materials. When exposed to environmental stimuli such as temperature, water, light, humidity, electricity, or magnetic fields, the 4D-printed object altered and modified the 4D-printed shape, mechanical properties, and physical characteristics. This paper reviews a variety of stimuli, smart materials, and manufacturing methods. Various applications have benefited from the use of 4D printing technology in fields such as healthcare, aerospace, renewable energy, and fashion apparel. Although a variety of innovative 4D printed products have been produced, this technology has not yet reached maturity, and this review listed some of the current challenges, from creating new materials to creating mathematical models to enhance the predictability of printed parts and ultimately scale up the 4D printing production process. The recent advancements in additive manufacturing technology such as 5D and 6D printing offer a great opportunity to usher in a new era of manufacturing processes.