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Metals and Materials International (2020) 26:564–585https://doi.org/10.1007/s12540-019-00441-w

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4D Printing: Future Insight in Additive Manufacturing

Muhammad Qasim Zafar1,2  · Haiyan Zhao1,2

Received: 25 February 2019 / Accepted: 2 September 2019 / Published online: 17 September 2019 © The Korean Institute of Metals and Materials 2019

AbstractDevelopment in additive manufacturing is exceptionally rapid than the expected forecast so far and it has traced out new dimensions in engineering applications. 3D printing technology becomes more glamorous when Skylar Tibbits incorporated the concept of “Time” as a fourth dimension by encapsulating smart materials in current additive manufacturing technique. Materials having an explicit response to external stimuli over a certain time span are designated as smart materials and addi-tive manufacturing of such time-dependent, programmable, and intelligent materials is termed as 4D printing. In 4D print-ing, primary 3D printed configuration switched exclusively into a transformed shape when exposed to an external stimuli, e.g. heat, light, water, chemical, electric current, magnetic field or pH. Perhaps, additive manufacturing technology seems to be superseded exclusively by this modern technology in forthcoming years, and much effort is demanding from every discipline to actualize this technology. A task-oriented entire landscape of 4D printing followed by a comprehensive smart material perspective is presented in this review. Graphical abstract set forth a route to the complete process comprehension. Moreover, other components of 4D technology like customary techniques, computational challenges, reversibility and cur-rent stature of 4D printing are probed through recent experimental and theoretical literature. Finally, potential applications of 4D printing are summarised with promising research directions and outlook.

Keywords 4D printing · Additive manufacturing · Smart materials · Stimulus

1 Introduction

The current twenty first century has become contemporary due to radical advancements in different cadres of science and technology. “4D printing” a new paradigm in additive manufacturing is reported as the most significant transfor-mation in existing 3D printing and traditional manufactur-ability. 4D printing is an encapsulation of another dimension “Time” in current additive manufacturing and blueprint of additional dimension as illustrated in graphical abstract. 3D printing or additive manufacturing of time-dependent, stim-uli-responsive, predictable self-evolving materials is termed as 4D printing. The technology was initiated and termed by MIT scientist Tibbit Skylar, Director of the self-assembly lab in 2013 [1]. Venus flytrap and the same plant sensational

behavior is an excellent manifestation of 4D printing con-cept. The leaves of a venus flytrap close in about 100 ms to capture insects, a response triggered when insects come into to contact with leaves skin, demonstrated in Fig. 1a. Similarly, in Fig. 1b shame plant responds to human touch and rearranges itself upon exposure to 430 nm LED light [2, 3]. Recently additive manufacturing of smart materials is in the spotlight, and active research is continued to overcome various challenges. The time-dependent sensational behavior of numerous materials making them dynamic upon exposing to external stimuli such as electricity, stress, temperature, moisture, pH, etc. [4].

4D printing possibility is categorically endorsed by sci-entists as a layer by layer fabrication of a physical structure by smart materials through an appropriate additive manu-facturing technique [5]. 4D printing has been envisioned for security [6], electronic devices and precise optical sur-faces [7], soft actuators [8] biomedical devices [9]. Where a specific structure must show a responsive behavior upon exposing to a stimulus over a particular domain of time [10]. There are three fundamental requirements to materialize the process of 4D printing; a significant one is the preparation

* Muhammad Qasim Zafar muhammadqasimzafar@gmail.com

1 State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, People’s Republic of China

2 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

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of stimulus-responsive composite material. Second is a spe-cific stimulus which triggers a particular response in blended material, and the final one is the length of time for response and transformation [11]. Shape transformation and shape-shifting phenomena in structure could be self-folding [12] bending, twisting, linear or nonlinear expansion/contraction, surface curling, and generation of surface topographical features [7]. Distinct structural shape transformation from any dimension deemed as 4D process e.g. transformation occurred from 1D to 1D shape-shifting or 2D to 3D or 3D to 3D conversion over time would be regarded as 4D printed structure. Transformation variation can be instigated by dis-tinct physical stimulations to fold, shrink, expand, and curl over a time domain. The idea was originated by using hydro-philic materials that can be activated when being immersed in water [1]. A self-folding structure fabricated by additive manufacturing of origami and kirigami can be automatically folded into required 3D shape by actuation triggering mecha-nisms such as heating [13].

The review presents a clear insight into four-dimensional printing, and all aspects from grain to bread are covered adequately. Paper structured in six sections where Sect. 1 for technological introduction while the fundamental concept of additive manufacturing for the foundation of 4D Printing described in Sect. 2. In Sect. 3, an extensive detail of smart material is sighted as input material. Section 4 is about 4D

Printing and manufacturing possibility of smart materials. Task-oriented modeling and simulation strategy and promis-ing 4D Printing applications are associated with Sects. 5 and 6 respectively. Finally, the conclusion is established about task-oriented 4D printing process feasibility followed by a promising research outlook.

2 Additive Manufacturing Landscape in 4D Printing

Additive manufacturing or 3D printing sometimes called as layered manufacturing is a fabrication of parts by adding successive layers of material via 3D printer. Additive manu-facturing is defined by ASTM (American Society for Testing and Materials) as “a process of joining materials to fabri-cate objects from 3D model data, ordinarily in layer by layer fashion, as opposed to subtractive manufacturing method-ologies” [14]. Additive manufacturing was introduced to the world in 1984 by Charles W. Hull and is rigorously explored in the last two decades [15]. Typically a computer-aided design (CAD) file is created and exported to stereolithog-raphy *.STL file format which is decoded by a 3D printer. A wide range of plastics, metals, ceramics and composites are used to render additive manufacturing or 3D printing [16] s. ASTM/F2921 differentiated additive manufacturing

Fig. 1 4D demonstration from nature a Sequence of leaves closing of Venus Flytrap, b Shameplant respond to a human touch which acts as a stimulus (opted from www.weliv ealot .com)

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technique into seven categories (Fig. 2), including mate-rial extrusion, binder jetting, material jetting, powder bed fusion directed energy deposition, sheet lamination, and vat photopolymerization. Additive manufacturing has an innate capacity to process smart and responsive materials [17].

Numerous advantages over traditional manufacturing regarded 3D printing into the future of production. On-demand components, manufacturing freedom, no need for special tooling like conventional machines, time and cost savings, complex geometrical attributes possibility, and lightweight novel structure can be built very conveniently. Healthcare, aerospace, pharmaceutical, automotive, tooling, fashion, and medical implantation industries have been mod-ernized end users according to Wohler report 2013. Wohler report 2017 revealed that 49, 62, and 97 companies were involved in manufacturing and selling additively manufac-tured components in 2014, 2015 and 2016 respectively. The rapidly growing interest in metallic additive manufactur-ing emerges the fact future belongs to metallic industrial components additive manufacturing. World demand for 3D printers, related materials and software’s were projected to rise 21 percent per year to $5.0 billion in 2017 as estimated by Fredonia group [18]. Wohler’s report 2018 shared over-whelming progress about AM equipment’s sales expedited by 80% just in the year 2016–2017 [19]. As far as the present stature of technology is concerned, researchers have made significant progress during the last 5 years to upgrade 3D printing facility. As far as input material is concerned, addi-tive manufacturing is classified into solid, liquid and pow-der material based techniques. Fused deposition modeling

is dedicated to solidus material printing while liquid materi-als are printed through Stereolithography, Direct Ink Writ-ing and Digital Light Processing, likewise powder materi-als processed through SLS(selective laser sintering), SLM (selective laser melting) and EBSM (electron beam selective melting) [20]. Increasing impact on development and com-mercialization is observed among additive manufacturing schemes during last few years. Since additive manufacturing provides bottom-line to materialize 4D printing technology, therefore 4D printing progress is associated with the matu-rity of additive manufacturing. Furthermore, the adaptation of a suitable printing scheme has become esssential in a broader spectrum accompanying material’s smart attitude.

3 Smart Material Perspective

Materials, which manifest their functions smartly called sense-able intelligent, smart, adaptive and multifunctional materials. An early definition of smart materials is the mate-rials, which respond to the environment over a time domain but afterward, it has been extended to the materials that pro-duce a useful reversible effect when brought into contact with an external stimulus like stress, strain, temperature, pH, electric or magnetic field. Their provoked result could be a color change, refraction, stress/strain development, or a shape or volume change [21]. Contrary to that material which can sense a change from the external environment and demonstrate a response either by changing properties, shape or structural configuration [4]. Unanimous definition of smart materials could be the materials with capacity to transform its configuration, properties, structure when sub-jected to an external stimulus. Recent advancements in mate-rial development and 4D printing characteristics put them in AM material cabinet. The smart materials expected to be applied rigorously in task-oriented intelligent structures manufacturing shortly. Printable smart materials are sur-veyed in this section.

3.1 Capabilities of Smart Materials

One dominant perspective is to focus on outcome response or behavior rather than the material itself. The forecasted response can lead to self-sensing, self-actuating, self-diag-nosing, self-healing, and shape-changing. Many products that use materials with smart behavior had been a part of our life since long, for example, photochromic spectacles. Smart or stimulus-responsive materials have the ability to react upon heat, chemical, and light. They are more sen-sitive and intelligent than ordinary materials. Most stim-ulus-responsive materials are limited to change in shape, physical and chemical properties of the structure. The pre-programmed response is the core strength of these

Fig. 2 Classification of AM processes based on additive manufactur-ing standards ISO/ASTM 52900:2015

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stimuli-responsive materials and best suited for intelligent and smart application [22, 23].

3.1.1 Self‑actuation Capacity

Several actuation mechanisms such as thermal expansion coefficient, liquid crystal gel phase transition, thermal con-ductivity discrepancies, dissimilar swelling and de-swelling ratio of bi-layer composite beams for temporal and spatial actuation can be realized through smart materials [24]. Term “self-actuation” closely related to shape-changing and asso-ciated with materials, which produce a massive displacement in response to external stimuli. Bodaghi et al. proposed a self-actuation mechanism through material expansion and shrinkage in multi-material 4D printing. Planer and tubu-lar structure design employed on glassy and rubbery phase tuned on Tg in subjected SMP and demonstrated actuation on low and high temperatures [25]. Composites comprised of piezoelectric and magnetostrictive elements, shape mem-ory alloys and electrorheological exhibits this property. The piezoelectric material is a pragmatic material which pro-duces an electric charge when subjected to applied stress. Technology is at the research stage and hopefully in a couple of years, self-actuators of piezoelectric materials would be fabricated through 4D printing.

3.1.2 Self‑healing Capacity

Materials with self-healing capacity are adequate to repair themselves and restore their intrinsic properties after dam-age, consequently enhance lifespan and extend service life. Three primary healing approaches such as intrinsic, cap-sule-based and vascular are mainly executed for imparting self-healing capacity in 4D printing. The intrinsic approach leads to damage recovery through so-called healing agent reversibility of bonding in material matrix. In a transforma-tion process, a healing agent is introduced in smart material printing to control damages automatically in printing struc-tures. Incorporation of healing microcapsules is another way to achieve self-recovery after damage. Healing agent intro-duces to defected area through these carrier capsules which rehabilitates material. Similar to the vascular system in the human body, microchannels are introduced for healing agent supply to concerned location in polymer matrix [26]. Presence of self-healing agent in a polymer matrix prob-ably affects its properties therefore, it becomes crucial to assess the changes and performance of the new composite. Recently significant work has been done to improve self-healing capacity of graphene-based self-healing materials. Self-healing coatings based on elastomers and thermosetting materials are applied to prevent corrosion and degradation of materials. Through powder coatings, this technology can be used in automobile, civil and aerospace structures. Product

performance, material safety, extending durability of struc-tures, and enhanced fatigue are the significant characteris-tics of self-healing materials dedicated to coatings. Recent progress is available in literature [27–29].

3.1.3 Self‑diagnose Ability

Self-diagnostic materials are used to quantify the loading factors when subjected to stress, load, etc. Self-diagnosis is an application of self-sensing, and most self-diagnostic tech-nologies involve the incorporation of sensory elements into a material or structure, and optical fibers have been inves-tigated extensively and can detect variables such as stress, strain, pressure, vibration, impact, corrosion, and tempera-ture. Self-sensing is an additional feature for advance cau-tion that can be incorporated in materials. This feature is actively researched in the Georgia Institute of technology for sensor development based on photobitronic effect [30]. Self-diagnose ability is active in research of targeted drug delivery system for cancer treatment. Drug delivery mecha-nism controlled by the stimulus is more reliable to avoid drug supply variation in drug delivery. Near-Infrared light is a potential stimulus with no medical implications. Pho-ton thermal effect, two-photon absorption, and upconverting nanoparticles are the most significant smart drug delivery mechanisms. Drug delivery based on photothermal effect has been under extensive investigation due to its tune-able features and carbon nanomaterials, indocyanine green and gold nanomaterials are frequently used photothermal agents [31].

3.1.4 Self‑Assembly

Self-assembly concept is the foundation of 4D printing as it has been used as interchangeability within smart structures. Tibbits demonstrated this ability with a flask containing separate parts are shaken and then self-assembled when indi-vidual pieces came into contact [32]. The term self-folding is related to self-assembly mechanism that demonstrated a shape transformation effect such as curving, folding or rolling using thin foils that are fabricated in spiral-shaped or cylindrical tubes. The sequence of instructions is prime design attribute for self-assembly 4D printing where instruc-tions might be in very simple fashion and algorithm descrip-tion is another part to construct 3D complex structure. It presumed a new type of experimental structure and instal-lation generated by computer code and realized through new technology and digital fabrication possible by new technol-ogies. It has been demonstrated that any given 1-D, 2-D or 3-D geometry can be described by a single sequence or folded line [33]. Folding mechanism and self-assembly are closely attached to each other and active in medical research on proteins and cell.

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3.2 Shape Memory Effect (SME)

Shape-changing materials are the widely explored and versa-tile materials with the capacity of remembering their original shape after deformation upon exposing to external stimuli. Material’s potential of recovering primary shape after defor-mation termed as shape memory effect (SME). This ther-moelastic effect was primarily observed by William J. Bue-hler, who was working at US Naval Ordnance Laboratory in White Oak, Maryland. SME enables materials to keep interim shape that can be virtually held permanently until a particular stimulus is applied to trigger the shape recovery. All shape memory materials (SMMs) are characterized by shape memory effect which empower them to recover the original shape after being quasi-plastically deformed [34]. Significant SMPs possess one-way SME, which is irrevers-ible when deformed to a temporary shape (which becomes permanent) on stimulus interaction. Two-way shape memory effect is the ability of some shape memory alloys to recover a shape on heating above transformation temperature and return to the alternative configuration on cooling. Shape memory effect is not an intrinsic material property that can transform shape themselves without external aid [35]. Dif-ference between a one-way, two-way and three-way SME is illustrated in (Fig. 3). Where three-way shape memory effect has one intermediate shape between its native and interim shape that can be engineered by embedding multiple two-way shape memory polymers with  different glass transition temperature (Tg) [35].

Specific stimuli response can be embedded in smart mate-rials through synthetic design and selective additives. Per-haps, the shape memory effect is not an intrinsic property of material however, this unique attribute can be engineered through polymeric molecular architecture by combining

other additives. A comprehensive investigation on design and chemistry is performed for stimuli responsive materi-als used in additive manufacturing recently by Shafranek et al. [22]. Shape fixity ratio and shape recovery ratio are the parameters to estimate shape memory performance. Shape recovery ratio describes the ability of original recovery of shape while fixity ratio indicates the strength of a material to fix temporary deformation.

3.3 Shape Memory Materials (SMMs)

Although different materials are termed as shape memory materials (Fig. 4) however, the shape memory alloys and shape memory polymers are the reputed materials among

Fig. 3 One-way, two way or three-way shape memory effect [119]

Fig. 4 Shape memory materials (SMMs) for 4D printing

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them because of exceptional shape memory and shape recov-ery characteristics.

3.3.1 Shape Memory Alloys (SMAs)

US Naval ordnance laboratory discovered shape memory alloys based on Nickle and Titanium materials referred to as nitinol (NiTi). Nitinol typically comprises 55% nickel and 45% titanium with different physical properties in phases. This alloy has unique shape memory behavior on account of thermoelastic martensitic transformation, which yields from the temperature-induced austenite-to-martensite phase transformation at atomic scale. Higher temperature phase is austenite, and the lower temperature phase is a martensitic phase. A martensitic phase is obtained by cooling the mate-rial from a higher temperature in alloys. Stimulus-responsive alloys fall into two categories thermo-responsive and mag-neto responsive. Shape memory alloys sensitive to heat are thermo-responsive smart alloys. Thermo-elastic martensitic transformation is a result of the need for the crystal lattice to maintain a minimum energy state at a given temperature. Reversible martensitic transformation in the crystal lattice is the driving mechanism behind shape recovery in both cat-egories of shape memory alloys [36]. Transition tempera-ture (Tg) for the austenite to martensitic phase is ranging from − 100 °C to 150 °C typically [37]. Pseudoelasticity or superelasticity retrieve original shape without heat encounter in shape memory alloys. High-temperature shape memory alloys operate > 100 °C however, they are very difficult to process [38]. Shape memory alloys, which change dimen-sions and configuration when subjected to a magnetic field, are called magneto-responsive or ferromagnetic shape mem-ory materials discovered in 1996 at MIT by Ullakko [39]. Actuation mechanism behind magnetic shape memory mate-rial is magnetically induced reorientation termed as (MIR) proposed by Faehler [40]. Twin boundary motion mecha-nism is the phenomena for magnetic shape memory alloys behavior and, also known as magneto plasticity [41]. The twin boundary motion of the martensitic structure provoked by the difference of magnetic energy, which overtakes the mechanical energy needed for the displacement of atoms near the boundary. The difference in magnetic power is a result of various orientations of magnetic moments in twin-related variants [42]. Actuation energy produced by magnet-ization is much higher than heat transfer, and an increase in magnetization leads to increase the actuation frequency [37]. Shape memory alloys are extensively applicable in a variety of physical applications, first was reported in the late 1960s.

3.3.2 Shape Memory Polymers (SMPs)

Shape memory polymers (SMPs) a class of polymeric mate-rials that can be programmed to memorize a predetermined

configuration and subsequently reverting that structure when subjected to external agent [43]. Shape memory polymers can retain two sometimes three shapes on exposure to a stimulus. Triple SMPs potentially adequate to memorize two temporary shapes and sequentially recover their origi-nal shape over heating. Triple shape memory effect can be introduced either by incorporation of two SMPs with vary-ing transition temperature into polymer network or introduc-ing an SMP with higher transition temperature [44]. Recent experimental study of triple SMP printed by FDM facility revealed hyperelastic response at high temperature and elas-toplastic response at low temperature in large strain regime [45]. Dual component mechanism, dual state mechanism, and partial transition mechanism drive shape memory effect in SMPs [46] The mechanism of shape recovery is depended on glass transition temperature (Tg) of a particular shape memory polymer which is entirely different as compared to shape memory alloys. Glass transition temperature (Tg) is the temperature below which moment of molecules is rela-tively low in the glassy state. Complete shape memory effect could be seen in thermally actuated shape memory polymer when heated above its (Tg) temperature as shown in (Fig. 5). Whereas elastic entropy of polymer chains drive the strain recovery mechanism in SMPs. Lower cost, lightweight, superior recoverability are the prime qualities of SMPs.

Heat responsive shape memory polymers are classified as thermoplastics and thermosets by physical and chemi-cal cross-linked morphology such as conventional poly-mers [47]. Both chemically and physically cross-linked shape memory polymers have been synthesized in more than twenty types. Simple manufacturability, lightweight, relatively high strain recovery, flexible programming, and biocompatibility are the significant advantages of SMPs. Capacity to resume the distorted shape can be utilized in broad applications primarily in the field of biomedical and soft robotics [48]. Poor shape recovery and low mechanical properties make them unsuitable for many mechanical appli-cations. Many efforts have been made in the form of shape memory composites to overcome its shortcomings so far. Shape recovery entirely depends on the material capability as well as the application of a particular stimulus in a speci-fied time domain. Heat, light, electricity, moisture, magnet-ism can work as a stimulus in SMPs structures to provoke shape memory effect (Fig. 6). Such as shape memory effect of thermally responsive shape memory polymers is triggered by Joule’s heating directly from a medium such as hot water and heated gas [49].

3.3.2.1 Thermoresponsive SMPs Heat is most frequently used stimuli in 4D printing so far, where temperature differ-ence triggers response to drive self-assembly, self-healing and shape memory in concerned material. Direct heating or indirect heating has been employed in certain applications

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Fig. 5 Shape recovery of a SMP cube when heated with an IR-lamp. A complete cycle of shape transformation is demonstrated in a time series from temporary (a, 0 sec) to permanent shape (f, 1202 sec)

Fig. 6 Stimuli and resulting response with dimensional change[53]

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SMPs triggered by light. Magnetism, ultrasound, microwave or electricity are intrinsically responded to heat through the additives used in matrix. Physical or chemical cross-linked thermoresponsive polymers may be regarded as dual, triple or multiple SMPs. A significant number of such polymers are supposed to recover their native shape while insignifi-cant demonstrate reversibility in temporary and permanent shape on stimulus interaction [50]. Shape-memory mecha-nism work on handling temperature above and below Tg or Tm, which is necessary for the mechanical disruption and subsequent formation of a temporary shape. Heat stimuli interaction facilitate to restore permanent shape through covalent bonding in such SMPs [51].

3.3.2.2 Photoresponsive SMPs Light is another bewitch-ing stimulus that is easy to trigger with its intensity wave-length and polarization and facilitates non-contact control. SMPs with photosensitive functional groups or fillers, (e.g., azobenzenes or cinnamic acid) are potential candidates for this class of polymers. Integration of photosensitive mol-ecules into an engineered polymer surrounding is in com-bination with functionalization process is a matured strat-egy for embedding desired response [52]. Some polymers especially cinnamic category is capable of producing new interim configuration when exposed to ultraviolet (UV) light. The initial shape can be recovered at ambient tem-perature on different wavelengths of UV [53].

3.3.2.3 Electroactive Polymers Electro-active polymers (EAP) can alter their shape and size significantly in an elec-tric field. In electroactive SMPs heat generated due to Joul’s heating when current pass through polymer domain which trigger shape memory effect. Conductive additives used to tailor an electroactive SME in such SMPs. CNTs, graphene, CB, graphene, metal nanoparticles and carbon nanoparticles work on same principle. Bucky Gel, Ionic polymer-metal composites, and Dielectric elastomers are types of EAPs [54]. 3D Printed structures of electroactive polymers are

potentially research areas in the near future. Bucky gel: lat-est ionic EAPs is bucky gel due to its intelligent behavior. The sensing and driving theory of bucky gel is close to that of IPMC. It comprises of three layers: the middle layer is an electrolyte that incorporates a polymer and an ionic liquid while both sides of the base material are electrode material that includes carbon nanotubes, ionic solution and polymer [55]. Bucky gel gets bent on applying voltages on electrodes as cat-ion and an-ion of ionic liquid moves.

3.3.2.4 Chemoresponsive Polymers Moisture, pH, redox- and solvent-induced, are the chemical stimuli which used to trigger shape memory effect in structures. In 2012, Han et  al. proposed to use pH as a triggering agent for SMPs for its convenience and safety in medical implication. Plasticity is widely observed in polymers when subjected to appropriate stimuli chemical. Mechanisms for SME in chemo-responsive materials are softening, dissolving, and swelling. Hydrogels have been extensively used to achieve shape memory effect for chemo-responsive shape memory materials [56].

3.3.2.5 Shape Memory Polymer Composites Shape mem-ory composites SMPCs are composed with at least one shape memory material either SMA or SMP [57]. Shape memory bulk metallic glass composites are the example of shape memory composites [58]. Multi-shape PAC printed active composite with two SMP fibers has been reported in nature that energized with hot water. Glass transition temperature of blended SMP fibers of Grey 60 DM9895, DM8530 were 380 °C and 570 °C respectively. The matrix was prepared with Tango Black+ , having low glass tran-sition temperature − 20 among the materials available in the laboratory. Fibers were printed inside of the matrix of a two-layer composite as shown in Fig.  7. Due to differ-ent (Tg) glass transition temperatures of the matrix and the two fibers, the composites can withstand three temporary figures, and permanent configuration is recovered through

Fig. 7 Multi-shape memory effect a to e in SMPCs [59]

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heating above glass transition temperature [59]. The concept of PAC printed active composites fabricated with SMP fib-ers into the elastomeric matrix was reported in 2013 [60]. PACs (polymer active composites) are relatively soft com-posites consisting of glassy polymer fibers reinforcing elas-tomeric matrices, exhibit a shape memory effect. In which glassy polymer fibers act as a switch in shape transforma-tion. SMPCs have been extensively used in sports wears, Judo-suits and medical implantation like dental braces and cardiovascular stents [61].

3.3.3 Shape Memory Hybrids

Shape memory hybrids look similar to SMPs but differ in composition as matrix and inclusions can be deduced by different materials. They provide simple shape memory solu-tion as a synthesis of polymers for unique applications which require strong skills and chemical knowledge although prop-erties of SMPs are more easily tailorable as compared to SMAs. A similar example of SMH is mixing of elastomer ionomers with low mass fatty acid with appropriate salts, way to design an SMP is reported in the literature [62]. Metal, organic and inorganic inclusions, and matrix can be chosen for a required application to yield a shape memory hybrids. There is no chemical reaction between matrix and inclusions, so the properties of an individual participant may remain the same in a hybrid. A shape memory hybrid made of elastic sponge and cupric sulfate pentahydrate, a matrix and inclusions respectively demonstrates a quick water responsive shape recovery feature.

3.3.4 Shape Memory Gels

Smart gels are typically self-adoptable, macromolecular physically or chemically interconnected network of poly-mers demonstrate expansion and contraction to change the structure. These hydrogels are composed of synthetic polymers (PHEMA) and biological polymers (alginate) immersed in water [22]. These are extensively used in 3D bioprinting and medical applications due to their superior characteristics like dynamic response, functionality, and expansion in water [63]. Smart hydrogels matrixes with higher water contents respond to various environmental fac-tors such as light, pH, temperature, ionic strength, magnetic and electric field. Additive manufacturing of smart hydro-gels is a newly introduced paradigm in bio-fabrication of functional 3D organs and tissues. pH-responsive hydrogels reacted over environmental pH variation and comprised of polymeric backbones that can accept and donate protons with the change in medium pH [64]. Lee et al. developed electroactive hydrogels (EAH) which demonstrate bend-ing in electric field. This Gel is based on PAA cross-linked by PEG-DA (Mn = 700 g mol−1) and printed with micro

stereolithography (PμSL) process [65]. Smart hydrogels possess unique characteristics such as shape memory effect, self-healing. Perhaps, these qualities make them suitable for 3D organ and tissue development engineering.

3.3.5 Shape Memory Ceramics

Shape memory effect is observed in ceramics too, and MIT has taken the lead by reporting shape memory ceramics in 2013 [66]. Shape memory ceramics undergo a similar mar-tensitic transition in which microstructure changes from tetragonal to the monoclinic atomic structure. Lead zirconate titanate (Pb (Zr, Ti)O3, PZT) based ceramics demonstrated substantial isotropic volumetric shape memory behavior referred to as antiferroelectric shape memory (AFE) com-posite. Reverse electric field application can recover previ-ous AFE phase with a quick recovery speed of 2.5 ms. The electric field-induced phase transition to make it feasible for many smart actuation applications. Intelligent electro-mechanical actuators and micro-mechatronics are exciting applications in the forthcoming future [67].

3.4 Advantages of SMPs over SMAs

Shape memory polymers (SMPs) have a lower density as compared to shape memory alloys (SMAs) that result in lightweight structures and can be employed in aerospace and automobile interior surfaces. Raw material and process-ing cost of SMPs is much lower than the fabrication cost of SMAs. In traditional or advanced manufacturing systems, complex shape with SMPs can be easily accomplished with high quality and dimensional accuracy [68]. Wide range 100–700 °C of glass transition temperature (Tg) conceived SMPs more reasonable as compared to SMAs in 4D print-ing. In addition, strain recovery is 400% higher than SMAs [35]. Thermo-mechanical properties can be easily tailorable using fillers with different compositions [68]. More than one stimuli may be used to trigger the SMPs shapes in distinct task-oriented assignments [69]. Within the transition range, the damping ratio is notably higher in SMPs [70]. Shape memory polymers and shape memory hydrogels are deemed as best suited input materials for 4D printing [56]. In hydro-gel-based 4D printing landscape, hydrogels are assimilated with a non-swelling polymer or filament. When the printed structure is immersed in a solvent, the hydrogel swells and it creates mismatch strains between two materials that lead to overall shape change. Programming requirement for activa-tion in this technique can be overlooked entirely. However, the strength of hydrogels is not substantially good, and stiff-ness is relatively inferior too. Composite strategy in which soft gel is combined with stiff SMP can reduce this drawback effectively [71].

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4 4D Printing Features and Capabilities

4.1 4D Printing Fundamentals

Additive manufacturing of smart and responsive materials is termed as 4D printing followed by a transformation mecha-nism, which haps just off the printing bed. In Atlantic Coun-cil of United States, 4D printing is defined as 3D printing of such smart structures, which can turn in shape or function when experienced to a predetermined stimulus including heat, current, osmotic pressure, ultraviolet light or another medium [72]. Time-dependent shape changing after manu-facturing is the primary characteristic of 4D printed chunks. Variation in configuration and attributes can be influenced by different external stimuli to induce expansion, shrink-age, and folding of printed objects [13]. Fundamental build-ing blocks of 4D printing are 3D printer, printable smart material and structure design, stimulus and mathematical modeling.

4.1.1 Printing Device

3D Printer is an integral part of 4D printing squad that oper-ates in a layer by layer fashion using computer’s instructions to build an object with materials such as polymers, ceram-ics, and metals. Stratasys, ExOne, Materialise and Proto Labs are the leading 3D printer manufacturing companies. 4D printed parts are created through additive manufactur-ing of single or combination of several smart materials that can evolve over a specified time[73]. The differences in the material properties like shape memory, thermal expansion, contraction, squeezing, etc. would lead to the shape-shifting mechanism of the whole structure.

4.1.2 Stimulus

Stimulus essentially required for triggering the functionali-ties, properties, and shape of the printed structure. Stimuli

can be divided into physical, chemical, and biological fami-lies. Physical stimuli refer to temperature, moisture, light, magnetism, and electric energy. Chemical stimuli based on pH, ionic strength and chemicals. whereas glucose and enzymes considered as biological stimuli. In 4D printing, scientists have used a number of stimulus for provoking the printed parts including water [64, 71, 74], heat [66, 75], a combination of heat and light [76] and the merger of water and temperature [77]. Humidity stimulus-based 3D printed aperture is developed by the Institute of Computational Design (Fig. 8) [78].

4.1.3 Material and Design

Responsive materials and geometrical design are the most critical components of 4D printing. Required class of materi-als is discussed with capabilities of shape memory, decision-making, self-sensing, self-healing, self-adoptability, and multifunctionality. Design of printed components showed a substantial impact on shape memory capacity of the mate-rials, so the optimum design is also required for profound shape recovery. JEM Teoh et al. claimed that response can be precisely controlled through the thickness of the printed structure [79]. Moreover, a design-based approach is pre-sented for the optimum shape recovery in their research [80].

4.1.4 Mathematical Modelling

Mathematical modeling is needed for the dissemination design of multiple materials in the structure. As a matter of fact, 4D printing based on the arrangement of active and passive materials distribution to acquire desired behavior. In 4D printed structure there are two or three stable states and the whole structure may change its state when subjected to specific stimuli. Mathematical modeling in 4D printing primarily for three reasons. (I) Prediction of time-dependent shape-shifting. (II) Prevention of collusion between parts during self-assembly. (III) Reduction of numerous experi-ments [5]. Sometime the interaction mechanism is also used

Fig. 8 3D printed aperture closing a and opening b at high and low relative humidity respectively [78]

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to induce a particular movement in the printed structure. The swelling effect embeds in shape memory polymers with the right stimuli interaction through modeling [81].

4.2 4D Route and Printing Mechanism

Self-assembly, bi-stability, deformation mismatch, and shape memory effect are the ways to 4D printing. Design and reshape the product based on these peculiar approaches leads to the 4D printing [82]. Self-evolving structures and self-assembly of elements and components are fascinating approaches to fabricate sophisticated smart structures [32]. Swelling ratio and different thermal expan-sion coefficients or physical change in smart materials are the driving mechanism for 4D structures established on deformation mismatch. A bi-layered beam tends to bend upon heating if the layer has a different thermal expansion capability. Bi-stability means printed structural demon-strates stability on multi-degree of freedom, which pro-ceeds toward reversibility in structure when exposed to suitable stimuli. Certain other conditions for 4D printing process are also considered and may be applied precisely to attain geometrical dimensions on individual compo-nents in a smart structure [82]. 4D printing is an elegant combination of science and technology  and envisage

with a new variety of materials. Novel and emerging 4D technology will lead us to fabricate smart devices and structures with a dynamic capacity of evolving over time (Fig. 9). Living engineering mechanisms can be developed with a variety of materials, stimulus, and mathematical modeling through 4D printing, which was truely a dream with conventional engineering processes.

An alternative way is proposed by Ding et al. [83] where a temporary shape is printed first with Stratasys multi-material J750 printer, then heat stimulus is applied to real-ize a permanent shape. The difference between 4D printing of single material and multiple materials depends upon their degree of change manifestation by the structure. In single material 4D, printing process the degree of change is the reaction of the smart materials while applying stimu-lus quantitatively. The degree of change is the factor that determines how promptly one material component change configuration upon activation. Four-dimensional printing of multiple materials assess the changes in the multi-mate-rial components, especially individual shape and structural change. Task-oriented actuation can be computed entirely with the design of the activation mode, folding, bending, compression, stretching, twisting and so on along with the intricateness of the basic design of these essential com-ponents [84].

Fig. 9 Multi-material 4D printed SMPs gripper working on heat stimulus from a to b then c  [113]

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4.3 Reversibility in 4D Printing

Reversibility in 4D structures has been relatively unexplored, particularly for those applications that consist of folding and unfolding cycles, including wetting and drying. Two-way 4D printing has been proven to be possible with an improved design for the 3D printing process with the latest understand-ing of shape memory effect. A substantial factor in conven-tional 4D printing is human interaction in the programming phase during the processing. Human interaction can be negated as programming is adequately replaced by another stimulus that makes this process wholly dependent on external engagement. A continues cycle of reusing 4D printed struc-tures becoming feasible in future especially for industrial applications [85]. SMPs showed much potential in reversibil-ity of 4D printing. The possibility of reversible and assisted shape transformation behavior of a hydrogel hybrid (PEO‐PU polymers into UV curable monomer solution) structure with substantial material properties is developed by printing [86].

4.4 Printing Process

Single or multi-material printing of non-metallic shape memory material has been attempted through several addi-tive manufacturing techniques like Micro extrusion, Stereo-lithography, and PolyJet technology. Selection of appropriate printing techniques largely depends on a particular shape memory material and required utmost configuration. Differ-ent shape memory materials have been printed with existing additive manufacturing techniques including Polyjet, Ste-reolithography and Fused Deposition Modeling (Fig. 10). Micro-extrusion printers use pneumatic or mechanical (piston or screw) dispensing systems to extrude continuous beads of material or cells. Renewable soybean oil epoxi-dized acrylate solidified into smart composite capable of

supporting the growth of multipotent human bone marrow mesenchymal stem cells (hMSCs) was laser printed for the biomedical scaffoldings [87]. Similarly, a shape memory stent based on polycaprolactone dimethacrylate is fabricated with UV-LED digital DLP printer. As far as metallic materi-als are concerned, direct energy deposition (DED), selective laser melting (SLM) and electron beam selective melting (EBSM) techniques are viable for 4D printing [88].

4.4.1 Stereolithography

Stereolithography is considered as most rigorous 3D print-ing technique especially when SMPs are to be additively manufactured; based on photopolymerization of liquid photopolymers upon ultraviolet radiation. SLA is used for prototypes to validate design due to its numerous advan-tages such as speed and sophistication. Photopolymers are synthesized from oligomers, monomers, and photoinitiators and UV-Polyurethane pre-polymer is primly synthesized and printed through SLA technique. Stereolithography process is much feasible to print intricated structures with SMPU. Shape memory cycle and fold-deploy test was performed to evaluate shape memory performance quantitatively and qualitatively in SLA printed structure. SMPU structures printed with stereolithography demonstrated prompt recov-ery, superior shape memory performance and good shape persistence, followed by adequate strength [89]. Substantial shape memory performance with full recovery using SLA is obtained in research experimented by Choong et al. [90]. Photopolymer resin tBA-co-DEGDA network based on a dual-component phase switching mechanism is used to form a single 4D printable SMPs. SLA technique is used for the printing of SMPs with modified bottom-up SLA printer (Fig. 11) [90]. 3D polymer printer (MakerBot Replicator 2, MakerBot, Brooklyn, NY 11201 USA) is used to fabricate

Fig. 10 3D printing systems for smart material manufacturing 1—stereolithography, 2—Polyjet printing, 3—Fused Metal Deposition Process [120]

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composite sheets by PLA filament [91]. Origami and kiri-gami based 2D part is additively manufactured first using mask image projection stereolithography (MIP-SL) then exposed to the stimulus.

Moreover, the part is converted to a targeted 3D con-figuration using certain stimulus conditions like heating and magnetizing [13]. Form 2, 3D systems ProJet 1200, EnvisionTec Aria are available printers in market for ste-reolithography (SLA), two-photon polymerization (2PP) and digital light processing (DLP) additive manufacturing. Multi-functional footwear comprised of SMPs heal is fabri-cated by DLP technology. The 4D printed shoe heal (Fig. 16) is built on PICO Plus39 (385 nm UV source) and PICO2 (385 and 405 nm UV source) printers (Aria, Australia), each equipped with a custom temperature-controlled resin bath. Aluminum-based resin baths are outfitted with internal fluid channels for temperature control, and the printable resin is heated up to its melting temperature.

4.4.2 Fused Deposition Modeling

Fused deposition modeling (FDM) and direct Ink writing are most suitable extrusion based AM processes for 4D print-ing. Material is mechanically extruded through a nozzle on build platform in these processes. In a typical FDM process, printers use a thermoplastic filament, which is heated to its melting point and then extruded, layer by layer, to create a 3D object. FDM printed structures can display shapeshift-ing phenomena via heating mechanism [92]. MakerBot replicator, 3D printer, Ultimaker 2 and 3D Bioscaffolder are available 3D printers based on FDM technology. FDM

has been proven expedient for multi-material 3D printing with a separate extrusion head in MakerBot and Ultimaker FDM systems. Recently a novel multi-material 3D printing system is introduced with low cost and high-resolution suf-ficiency including a multi-material library [93]. The material is drawn through a nozzle, where it is heated and then depos-ited layer by layer. The nozzle can move horizontally, and a platform moves up and down vertically after each new layer is deposited [94]. Fused deposition modeling may be used for additive manufacturing of graphene blended polymers in future. Graphene is a single-atomic-layer of carbon atoms arranged in a hexagonal lattice, much alike with arrange-ments of a chicken wire fence. Perhaps, it would become attractive material with excellent strength and electrical con-ductivity when blended with an SMP.

4.4.3 PolyJet Printing

Material is rendered through a nozzle and deposited in drop-lets in this additive manufacturing process. Polymer ceram-ics and metals can be printed through this technology. Pol-yJet technology for 4D printing by Stratasys Ltd. becomes feasible with recent provisions in multi-material printing. CAD interfaced 3D PolyJet printing is quite similar to con-ventional inkjet printing. PolyJet printers’ deposit liquid pol-ymer in a layered manner instead of spreading inks in tradi-tional additive manufacturing. Advance PolyJet technology can deposit multi-material liquid to fabricate components made up of different materials. Recent experimental study shown, smart and conventional materials can be printed precisely together with PolyJet Printer [95]. Insect model

Fig. 11 Stereolithography printed heat-sensitive SMP Buckminsterfullerene a to b demonstrates unfolding and recovering its printed pattern at different stages from c to h at 65 ℃ [90]

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multi-shape memory active composite is additively manu-factured by Object 260 an Object 3D Printer [59]. PolyJet technology is more concerned with those applications where precise, accurate, finished, intricated and sophisticated struc-tures are to be built (Fig. 12). MIT self-assembly lab is in collaboration with Stratasys and Autodesk to develop a prac-tical solution for modeling and simulation as well as a new printing technique for smart structures.

4.4.4 Powder Bed Fusion and Direct Energy Deposition

Several additive manufacturing techniques for metallic print-ing has been developed so far however, powder bed and direct energy deposition are persuasive from 4D printing outlook. Selective laser melting is a powder-based printing method-ology in which high energy laser melt fine powder material to fabricate a 3D object in layer by layer fashion [96]. SLM technology is used in 4D printing process when smart metal-lic materials like SMAs are used to layered-up dense struc-tures (Fig. 13a). Fabrication of shape memory alloys is not straightforward like other materials. For example, NiTi is one of the most popular SMA. Despite many functional proper-ties and unusual shape memory behavior of NiTi is not very simple due to certain reasons. Most significant arguments are

(1) change in composition can modify transition temperature [97]. (2) Shape memory behavior makes NiTi difficult for machining. (3) Finally, heat treatment such as annealing may affect phase transformation behavior [98]. Additive manufac-turing using selective laser melting technique is a possible solution for NiTi printing. However, there are some compli-cations in NiTi printed parts yielded by SLM process. Printed components have reduced Ni content due to evaporation dur-ing the process because of low evaporation temperature of Ni as compared to Ti which increases in phase transformation temperature. Introducing more Ni contents can be a possible solution during formation of NiTi alloys [99]. Another criti-cal problem noticed in the conventional processing of NiTi alloys is substantially increasing level of impurities. That can be resolved in selective laser melting process as an inert gas is used in the chamber during processing [100]. Besides NiTi other shape memory alloys also have been investigated for SLM additive manufacturing. Fabrication of copper-based shape memory alloy through a similar process laser melting deposition (LMD) with shape memory properties has been exercised previously [101]. Electron beam selective melting is another metallic printing technique in which a high energy electron beam is used instead of a laser and powder is lay-ered with a moving rake (Fig. 13 b). Porous NiTi implants

Fig. 12 Biomimetic 4D printing of composite hydrogel architectures. a Alignment of cellulose fibers during direct ink printing b its effects on anisotropic stiffness c and d demonstrated time-lapse sequence during swelling process in printed flower [121]

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for bone fixation and minimizing invasive surgery have been additively manufactured using EBSM for medical application based on the assumption that NiTi has a similar stress–strain curve to famous stainless steel [97]. DED process involved a multi-axis nozzle arm that can deposit melted material on bed surface. Material melting is accomplished with a laser or electron beam heat source. A critical understanding of the specific additive manufacturing process for target application is prevalent for implementation [16].

5 Computational Solution in 4D Printing

5.1 4D Modeling and Simulation

Modeling and simulation of respective design and mate-rial has become an integral part of every engineering design. Modeling programmes for additive manufacturing are not remarkably optimized with the current progress of technology. CAD system used for additive manufactur-ing cannot demonstrate micro-scale material properties especially smart material properties. Deep understanding of material behavior is a pre-requisite for accurate mod-eling and simulation. 4D printing simulation solution can be devised through two way, simulations using FEA soft-ware and development of software for simulation of par-ticular response (Fig. 14). Furthermore, 4D printing has been successfully simulated using finite element software ABAQUS in research published in Nature by Mao et al. [102] at Georgia Institute of Technology. FEA COMSOL software is used to simulate the band structure of uniform composite beams made with PLA metamaterial. In which plain strain triangular element is chosen with maximum size [91]. COMSOL Multiphysics module has been reported for the simulation of deformation in shape memory materials

at ETH Zürich [103]. Subject material was adopted as lin-ear elastic solids with the hygroscopic swelling module in software. Design of shoe and the smart ring is prepared on Rhino 3D and solid works (Fig. 16) [104].

5.2 CAD File Format

CAD system converts input data into triangular facet model *.STL format and send instructions to printers. *.STL format represents only homogenous objects which might not cover other aspects of materials [105]. Mostly commercial AM printers still incorporated with *.STL file format, which is incapable to demonstrate all material and process information. Additive manufacturing format AMF and 3D manufacturing format is designed to overcome shortcomings in *.STL file. These format files can gener-ate small files configuring detailed curves and illustrate complex surface materials. AMF and 3MF are based on XML extensible markup language. For future perspective, file format need to be review with respect to simulation of 4D printing attributes [106]. Other approaches such as finite element, use of voxels, particle system elements for lattice generation demonstrated in modeling are hard to edit due to lack in robust methodology. All the production requirements including geometry, process, part orienta-tion, tool path and, tolerances should be contained in a sin-gle file to obtain an effective and task oriented printings.

5.3 Software Development for 4D Printing

“Project Cyborg” a 4D simulation software is developed by MIT Self-Assembly Lab and Stratasys, Autodesk mutu-ally. Project Cyborg is a design program spanning multiple application from mesoscale to the nanoscale. The software

Fig. 13 Metallic 3D Printing process feasible for shape memory alloys a Selective Laser Melting process illustration, b Electron Beam Selective Melting setup with a sequence of process [122]

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provides simulation for programmable materials, self-assembly along with optimization for design constraints and joints [1]. Another simulation software for 4D printing pro-cess, named “4D Modeller” is designed by Gwangju Insti-tute of Science and Technology South Korea [107]. Research in 4D printing software development is minimal as this tech-nology is not mature up to a certain level yet. Extensive work is needed to be done to simulate shape memory effect in shape memory materials so that it would help research-ers working in 4D printing domain [23]. Moreover, the researcher proposed an idea to adopt six software solution to cover all the stages during 4D printing. Subsequently, each software will deal with a particular printing stage including simulation, modeling, slicing, host/firmware, monitoring, and printing management software (PMS), to cover all the required operations in 4D Printing [108].

6 Potential Applications

Owing to distinct advantages and complicated design free-dom probably 4D printing will play an unbelievable role in near future. Printed structures can be easily transported with lower cost and reduced labor [12]. Self-actuators printed with additive manufacturing part can allow researchers to invent smart devices without batteries and energy sources. Applications of 4D printing have been reported in various fields, such as organ and tissue engineering, biomedical devices, electronic devices, security [6], fabrication of pre-cisely patterned surfaces for optics and soft actuators, smart valves, regenerative design, electromechanical switches and smart clothing [8]. Seemingly various additional

applications might be conceivable with this outrageous tech-nology on maturity in the immediate future.

6.1 4D Printing in Renewable Energy

The 4D printing has been reported in the renewable energy field recently by Momeni and Ni [109]. The inspiration for researchers is a geometrical difference of flower’s petals and they developed a smart solar concentrator created by 4D printing that can increase the outstanding optical effi-ciency in solar applications [109]. Recently wind turbine blades have been fabricated through 4D printing adhering leave structure by Farhang et al. [110]. They demonstrated that blades based on plant leave network has better struc-tural properties and declared 4D printing is a viable solution because the complexity of the plant leaves network could not be achieved through traditional manufacturing processes. Eco-friendly wind turbines based on biodegradable materi-als possess bend-twist coupling (BTC) which can be accom-plished with negligible post-processing [110].

6.2 4D Printed Soft Actuators and Soft Robotics

Soft robot manufacturing has become very demanding due to its flexibility and human safety in stiff working surround-ings. SMP and hydrogels can play an exciting role in the development of soft elements capable of performing com-plex and recoverable movements. All 4D printing structures require adequate material and printing technique that enable accurate control of the mechanical response while interact-ing with the stimulus. Existing systems and mechanisms developed so far in 3D printing are capable of fabricating

Fig. 14 a and b ABAQUS simulation of recovery acti-vated sequential self-folding strip is programmed at high temperature. Similarly, c and d shows subsequent folding process of helical segment with varying material properties at different hinged sections [123]

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smart soft actuators efficiently as shown in Fig. 15. Soft actuators fabricated by additive manufacturing enables new possibilities in biomedical applications. Soft actuators applied in human body prosthetics can ensure a healthy and active life in adulthood [8].

6.3 4D Bioprinting for Healthcare Applications

4D bioprinting differs with 3D bioprinting as printed organs perform to evolve over time in a programmed path. Moreo-ver, 4D Bioprinting is a group of self-assembly, self-actua-tion technology with a set of three main features including man-made programmable design, printing process and pro-grammable evolving mechanism in printed organs triggered by stimulus [111]. 4D technology can be used to print heart valves, liver and kidney implants with suitable smart materi-als. Cardiologists can design a precise heart as per patient requirement based on the data acquired from CT and MRI. Smart materials capacity would govern opening and closing to control blood flow in subjected valves [112]. A cardiovas-cular stent fabricated by 4D printing of thermoresponsive SMPs is reported in research conducted by Ge et al. [113]. Traditional fabrication of stents consumes more time and experience difficulties due to complex geometrical features and high resolution. Another potential application in sur-gery through 4D printing, stents can be deformed into the small interim configuration, inserted into the body through a smaller surgical incision and after recovering its origi-nal shape through a liquid or thermal stimulus [114]. 4D printing in medical sciences is an emerging insight, and it can resolve many of the problems currently facing in organ development. Although printing of biological organs and tissues development through bio additive manufacturing is still infancy stage and so far 4D printing shows three approaches in biomanufacturing. The 1st approach involves the fundamental concept of 4D printing in which smart bio-polymer or responsive hydrogel fold into 3D shape upon exposing to a stimulus [115]. The 2nd approach is a kind of in vivo 4D Bioprinting in which 3D printed polymer

medical device specifically an airway splint for treatment of severe tracheobronchomalacia (TBM) is implanted first and then it accommodates the growth of tissue or organ over the postsurgical period. When the tissues become stronger, the medical devices dissolved in the body. 4D printing ena-bles tissue growth through designed mechanical behavior over time where implantation is sterilized before use [116]. TBM is a condition of the excessive collapse of the air-ways during respiration that can lead to life-threatening cardiopulmonary arrests [117]. 3rd approach is on-demand self-assembly or self-organization. Microdroplets of cells precisely accumulated in a pattern and the pattern changes over preprogrammed shape due to cell communication and self-organization. 3D printed active biological organs are going to replace the human organ in the near future. Heart valves like soft tissue organs could be printed through active cells in bioprinting.

6.4 4D Printing in Fashion

Applications of 4D printing technology have been extended to revolutionized fashion and jewelry industry. Previously only design and material aspects are discov-ered in jewelry, fabrics, and shoes. An adequate attempt has been made by researchers to incorporate dynamic features in finger ring and heal of shoes by using meth-acrylate polycaprolactone SMP in DLP based 3D print-ing process. Heal of shoes in demonstrate two states flat and high when encountered with heat as illustrated in (Fig. 16). Thermally-induced shape memory polymer is used in the heal of 3D printed shoes. Where so ever user wants to change the healing state it can be transformed to high heal with ordinary hand dryer [104]. The smart material could be printed into textiles which can not only change the shape but also give a textured look according to the new environment. 3D printed fabrics for standard size can be printed with SMPs by applying the same principle. External stimuli may change fabric colors and its configu-ration for a smart and bulky customer [118].

Fig. 15 3D Printing mechanism of soft actuators in 4D bioprinting [8]

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6.5 Material Handling and Transportation

4D printed structures can be transported easily as com-pared to raw material and activation on-site may be pos-sible in future. 4D printing technology might be the next possible paradigm of industrial revolution because 4D printing does not need much heavy machinery and other equipment during fabrication. The shipping volume could be dramatically reduced in a flat-pack manner, which can be activated after delivery to resume its 3D dimension and function. This might lead the market in a new direction with reduced inventories, spare parts, time to market and ultimately optimize business efficiency. Looking forward to an explosive research growth in 4D printing, where mar-ket size is expected to be 0.5 trillion $ by 2025 [30]. The significant advantage is manufacturing on-demand, which consequently reduced storage facilitation requirements. In addition to self-assembly capability of 4D printing nature,

it will also reduce manufacturing time, time to delivery and labor cost substantially.

6.6 Defence Applications

However, industrial and economic growth is necessary for every country and adopting latest research boost GDP rap-idly, but defence products can also be optimized utilizing the new concept of 4D printing. State of the art technology would also contribute to the defence sector that is yet unex-plored so far from a 4D printing perspective. Stimuli-respon-sive materials in armad uniform, arms, ordnance, tanks, and submarines would inevitably change the current perception of wars and revolutionized the weapon industry. Probably you will see soldiers equipped with smart light-weighted weapons with defensive shields produced by 4D printing that will activate only whenever need.

Fig. 16 a 3D printed SMP flower-themed rings is presented where upper row display permanent shape, and metastable tempory shape is shown in a 2nd row triggered by heat stimulus. b 3D printed smart heal attached with a shoe

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7 Concluding Remarks

4D printing established on additive manufacturing technol-ogy is widely explored through latest research database in this review. This research aims to investigate the capability of additive manufacturing techniques for smart materials print-ing and furnish a route to task-oriented printing scheme for diverse applications. Experimental approach with analytical knowledge followed by a software solution is reviewed com-prehensively for students and researchers. 4D printing, an absolute breakthrough in technology is not limited to additive manufacturing only. Indeed, it would open new horizons in entire manufacturing as outcomes could not be possible with other manufacturing processes. Task-oriented combination of additive manufacturing and smart materials envisioned in future applications specifically in medical implantations, aero-space, fashion, renewable energy, automobile, soft robots, and smart actuators. Selection of favorable additive manufacturing technique for a particular smart response and shape memory effect is a challenging task. Though Polyjet, stereolithogra-phy, and fused deposition modeling techniques are considered suitable for nonmetallic 4D printing chunks. It is obvious that 4D printing process largely depends upon the further develop-ment of smart and intelligent materials and shall remain the primary focus in-process research in the foreseeable future. No doubt, to date high-performance smart material availabil-ity is the prominent confrontation in the advancement of 4D printing. Perhaps, increasing research on material development might lead this technology to a glance. Despite new material development, exploring contemporary printing strategies for specific tasks and structures is also need of the day. For sure, 4D printing technology will implore new paradigms and probe new dimensions in every field of life. Especially it would pro-vide a manufacturing platform in tissue engineering, medical implantations and organ development in the near future. A new dimension in research can be introduced with the development of multi stimuli-responsive structure in which at the same time we could achieve collision-free movement in different sections of 3D printed structure. Perhaps, it would be the most classical improvement in existing technology.

7.1 Research Outlook

4D printing is in its infancy stage, and significant research is required in multiple segments of additive manufactur-ing significantly printing of large smart structures. An entirely different design-based approach is suggested by considering all the dimensions of this state of the art tech-nology. A dedicated 4D printing technique and printers may develop for some critical applications, e.g. artificial cardiac organs and a cardiovascular stent. Biocompatible and biodegradable smart materials may be printed with

a new and dedicated printing setup in organ and tissue engineering followed by cell culturing and implantations. Dedicated printers for specified applications adopting a suitable strategy and dedicated software solution can work in the fabrication of medical implantations.

Acknowledgements The author would like to express sincere gratitude to Tsinghua University and Chinese Scholarship Council for financial aid during his Ph.D.

Funding The Nat ional Key R&D Program of China (2017YFB1103300), State Key Laboratory of Tribology Tsinghua Uni-versity China (SKLT2018B06) and National Natural Science Founda-tion of China (51975320) supported this work.

Compliance With Ethical Standards

Conflict of interest It is solemnly declared that there is no conflict of interest between authors.

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