Emerging Advances in 3-D & 4-D Printing of Polymeric Nanoarchitectures & Multifacet Applications
Christopher Igwe Idumah 1,* , Ifeanyi Emmanuel Okoye 2, Chioma Joan Ikebudu 3
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Department of Polymer Engineering, Faculty of Engineering, Nnamdi Azikiwe University, Awka, Nigeria
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Department of Material and Metallurgical Engineering, University of Nigeria, Nsukka, Nigeria
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Department of Sociology/Anthropology, Nnamdi Azikiwe University, Awka, Nigeria
* Correspondence: Christopher Igwe Idumah
Academic Editor: Seyed Ghaffar
Special Issue: Additive Manufacturing Technology in Construction
Received: September 05, 2024 | Accepted: January 02, 2025 | Published: February 02, 2025
Recent Progress in Materials 2025, Volume 7, Issue 1, doi:10.21926/rpm.2501005
Recommended citation: Idumah CI, Okoye IE, Ikebudu CJ. Emerging Advances in 3-D & 4-D Printing of Polymeric Nanoarchitectures & Multifacet Applications. Recent Progress in Materials 2025; 7(1): 005; doi:10.21926/rpm.2501005.
© 2025 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.
Abstract
3-Dimension printing (3-DPT) or additive manufacturing (AM) technology has been utilized for a while in the construction of customized 3-D objects utilizing computer software such as computer-aided design (CAD). However, the emergence of an innovative and new four-dimensional printing (4-DPT) has enabled the synergy of AM technology with programmable materials in transforming digital processes virtually into physical entities, thereby providing innovation and advanced functionalities. 4-DPT is a procedure facilitating 3-DPT components to undergo programming in order to enable the transformation of their shape with time on exposure to external stimuli such as physical stimuli, including light-responsivity, chemical responsivity, magnetic/electrical responsivity, pH response, thermo-sensitive as well as biological stimulation such as biomolecular responsivity. The inherent difference between 3-DPT and 4-DPT is premised on the shape-changing material (SCM) utilized during manufacturing, depicting the advanced material exhibiting the specified changes in responsivity to external parameters. Therefore, this elucidation presents emerging 4-DPT technology in manufacturing polymeric nanoarchitectures and applications. Furthermore, insight into 4-DPT techniques, including extrusion, direct ink writing (DIW), fused filament fabrication (FFF), and vat photo-polymerization strategies (digital light processing (DLP), stereo-lithography (SLA), and multi-photon polymerization (MPP), are also presented.
Graphical abstract
Keywords
4-D printing technology; polymeric nanoarchitectures; applications
1. Introduction
AM, also called 3-DPT, has garnered escalating attention from industrial engineers and academic investigators due to its enchanting attributes, including freedom of prototyping and susceptibility to achieving intricate architectures at an elevated resolution [1]. The 3-DPT of deformable and innovative materials has resulted in the construction of emerging 4-DPT, referring to the material propensity in altering their form post-printing with time in the fourth dimension regulated through designing dynamic architectures [2]. 4-DPT offers added functional features and performance-propelled uses in vast segments, such as automobile, aerospace, maritime, biomimetic, construction, biomedical, civil engineering, electronics, acoustics, industrial food, textiles, fashion, oil and gas, mechanical, defense, retail, and other industrial applications as elucidated in Figure 1 [3].
Figure 1 prospective application of 4-DPT materials and gadgets.
The acronym “4D” combined with “printing” ab initio emanated in a conference proceeding in 2012, where it was referred to as a 4-D barcode (color + 2D + time) [4]. In a subsequent publication, the initial term was formed by 3-DPT, whereas the programmed polymeric activity created a time-based configuration termed the 4-D perspective [5]. The 3-DPT component exhibits a single static architecture, whereas the 4-DPT bright part exhibits at least two smart phases vis-a-vis static and dynamic architectures. 4-DPT technology, as an emerging AM technique, facilitates further shape variation, features, or functionalities of printed objects on subjection to external stimuli [6].
Nowadays, 4-DPT of materials prone to programming and deforming, like thermo-responsive shape-memory polymers susceptible to varying shapes when exposed to heat, has garnered attention. 3-D objects premised on shape memory polymers (SMPs) have been propounded for varying prospective uses in differing segments, such as soft robotics, biomedical, electronics, and smart actuators. Thus, to facilitate the manufacturing of intricate multi-functional 3-D objects, SMPs are regularly synergized with other functionalized polymeric matrices or nano-fillers before or after-3-DPT [7].
Innovative materials, also called programmable materials, wherein the printed shape or features are not the final state. When the final geometry is required, they can undergo shape-shifting into the pre-programmable form on exposure to external stimuli. Instances of environmental stimuli include temperature, light, pH, moisture and magnetic field [8,9]. These stimuli cause physically or chemically affiliated reactions like phase variations, stress relaxation, or molecular motions, which induce geometrical deformation or reformation. The materials programmed, thus, garner high attention in multifacet segments from up-scale manufacturing to micro/nanometric level for a vast range of applications in biomedicals or soft robotics [10]. Hence, materials susceptible to responding to differing external stimuli over time have been widely investigated for application in tissue regeneration procedures. Stimuli-responsive biomaterials have been utilized in conceptualizing 4-D bioprinting, in which 3-DPT scaffolds are constructed for transformation over time according to one or more environmental stimuli [9,10].
Therefore, this elucidation presents a novel 4-DPT of PNC with insight into AM techniques (extrusion, direct ink writing (DIW), fused filament fabrication (FFF), and vat photo-polymerization strategies (digital light processing (DLP), stereo-lithography (SLA), and multi-photon polymerization (MPP)), for 4-DPT of smart polymeric architectures and multifacet applications.
2. Insight into 4-D Printing
4-DPT utilizes AM strategies in synergy with innovative materials. These are known as materials undergoing variations in geometry, and at times functionalities on subjection to the suitable external stimuli (light, solvent, temperature, magnetic, pH, and so on) [11,12]. From this synergy, 4-DPT enables the construction of dynamic and adapting components in negative to inactive parts garnered via 3-DPT. Innovative materials capable of recovering their configuration (SME) on exposure to external stimuli are the most effortless preference when considering materials to be manufactured via 4-DPT [13].
The movement of a 3-D printer is three-dimensional and geared towards fabricating 3-D architectures. 4-DPT entails the integration of a 4th dimension and time, implying that 3-D printed items undergo transformation and shape variation in time on exposure to external stimuli like magnetic fields, water, light, pH, heat, electricity, and so on [14,15,16]. Garnered objects are termed 4-D printed entities printed using 3-D printers but utilizing 4-DPT materials [14]. Hence, a 4-D printing material can transform or change its geometry or configuration over time on exposure to external stimuli. This geometry transformation can be achieved by utilizing multiple materials within the printer, each representing differing responsiveness to external stimuli.
The printing step for a 4-D printed item is similar to that of a 3-D printed object. However, the key difference lies in the premise that materials utilized in 4-DPT technology are prone to be programmed and subjected to transformation in contact with external stimuli (heat, light, or moisture). These materials capable of responding to external/internal stimuli are termed innovative materials. Hence, 4-DPT uses a 3-D printer to print innovative materials for required objects prone to architectural or feature transformation on exposure to external/internal stimuli [15]. Polymeric matrices, metals, ceramics, and biomaterials are materials susceptible to being used in differing AM procedures to construct multi-facet materials. Adaptive and shape-morphing features cannot be achieved using rigid, pre-arranged architectures garnered via 3-DPT. This is where 4-DPT comes in as a channel for attaining these features. An enchanting 4-DPT material that is responsive to solvent interaction is water [16].
4-DPT is the construction procedure of 3-D objects with capability of changing their shape/configuration over time or as a response to an external environmental stimulus thereby demonstrating a sharp shifting in AM and providing a streamlined route from idea to reality with performance-focused functionality in-built directly within the materials [17]. Using this strategy, a vast range of actively programmable materials is fabricated with the capability of self-transformation from one configuration to another [18]. The main differences between 3-DPT and 4-DPT are schematically elucidated in Figure 2 [18].
Figure 2 Differences between 3-DPT and 4-DPT [18].
The main differences between 3-DPT and 4-DPT are ascribed to material depositing into a pre-ascertained static configuration. On the other hand, 4-DPT entails the precise depositing of an innovative material into a pre-ascertained, smart static architecture. When this brilliant static architecture interacts with an internal or external stimulus, the material will undergo inherent shape transformation, becoming an innovative and dynamic architecture [18].
3. AM Techniques for 4-DPT
The 3-DPT/AM technology is utilized in generating 3-D samples, wherein sheets of material are continually formed under a computer-regulated program to construct a physical sample. According to ISO/ASTM52900-15, there are seven sets of AM procedures: binder jetting, directed energy deposition, material extrusion, powder bed fusion, vat photo-polymerization, material jetting, and sheet lamination [19]. The key principles of commercially available AM technologies are presented in Figure 3 (a-g).
Figure 3 Varying Additive manufacturing (AM) technologies utilized in 3-DPT. (a) Photo-polymerization; (b) Power bed fusion; (c) Material extrusion; (d) Material jetting; (e) Binder jetting; (f) Direct energy deposition; and (g) Sheet lamination [19].
AM techniques presently utilized in 4-DPT includes fused deposition modeling (FDM); selective laser sintering (SLS); stereolithographic apparatus (SLA); and polyjet [19]. The key commercially available 4-D AM procedures have been mainly classified using their associated printing mechanisms; liquid solidification, powder solidification, and direct material extrusion. These strategies entail photo-polymeric light-curing; melt extrusion, and direct-ink printing [19].
The routes selected are premised on the innovative materials to be printed and the required features/functions of the end architecture. Parameters, including the speed of printing, frequency of the laser, and nozzle temperature, influence the preciseness of fabrication and are therefore susceptible to investigation and optimization to enable the feasibility of industrial scale-up [20]. The printing step can also be selected based on susceptibility to enhancing and facilitating the object shape-memory functionality. Despite the AM strategy utilized, constructing a 3-D architecture requires a comprehensive Computer Aided Design (CAD) model of the physical architecture [21]. The model design is primarily digitally cut into slim horizontal sheets, as the printer creates the architecture through sequential printing of individual layers of the material [22].
3.1 4-DPT of Mechanically Stable and Elevated Temperature Shape Memory Polymer with Good Irradiation Resistance
4D-printed shape memory polymers (SMPs) have aroused high attention in a vast range of segments. However, the development of elevated-temperature SMPs combining the elevated mechanical features and radiation inhibition remains daunting. Hence, in work, a novel photosensitive constitution (PSC) premised on elevated-performance heat stable poly-N, N'- (m-phenylene) isophthalamide (MPA), rigid photopolymerizable tris [2-(acryloyloxy) ethyl] isocyanurate (TAI), and a reactive solvent (N, N-dimethylacrylamide (DMAA)) was constructed [23]. The garnered PSC was used in fabricating elevated-temperature SMPs via a facile two-step LCD 3DPT and thermal post-curing step (Figure 4) [23].
Figure 4 Components of 4-DPT SMPs and garnered properties [23].
As a result of their vast prospects in varying segments (energy conversion, bio-construction, adsorption, and catalysis), the construction of 4-DPT hierarchically porously architectures from the molecular degree to the macroscopic level has garnered great attention. Hence, comprehending the structural-feature linkage of innovative materials in 4-DPT is essential in designing novel constructs with a broader scope. Hence, work has been conducted on the construction of heat-responsive macroporous polymerized-elevated internal phase emulsions (poly-HIPEs) via 3DPT of Pickering-type HIPEs [24]. Garnered 4-DPT objects displayed elevatedly interconnected porous architecture exhibiting thermo-responsivity and elevated mechanical strength with exceptional self-recovery behavior, thereby depicting prospects of constructing a heat-responsive macroporous structure exhibiting shape memory features at varying temperatures through regulation of emulsion formulation [24].
Nevertheless, the printing disposition of PPH-0 ink also offered a better comparison between this specimen and the superior ink (i.e., PPH-8). Figure 5 (bottom) depicts the printed architectures of a 1-D zigzag architecture, 3-D square lattice, 3-D DNA helix, and a flowery appearance bearing 5 petals. The shape recovery attribute of the printed PPH-8 entered on subjection to heat stimulus was studied in accordance with its quantitative shape-memory attributes exhibiting satisfactory printability (Figure 6) [24].
Figure 5 Sequential shape memory responsivity of 4-DPT PPH-8 figurines susceptible to temperature variations. (Top): The elevated transformation between permanency and temporary 3-D figurines of printed “butterfly” and (bottom): “5 petals flower”. Individual constructs include a thermometric graphic monitoring atmospheric temperature [24].
Figure 6 Printability of varying printed configurations from 1-D to 3-D constructed via PPH-0 and PPH-8 inking. (Up): FESEM images of (i) 1-D printed surface filaments, (ii) their significant cross-sectional rapture, (iii) 2-D square lattice, (iv) 3-D self-supporting scaffolds and (v) micro-architectures of 3-D self-supportive scaffolds. (Down): Printing quality of 3-D printed configurations constructed by PPH-8 inking. (a): 1-D zigzag pattern; (b): 3-D DNA helix; (c): 3-D square lattice, (d): 3-D flowery with 5 petals, and (e): magnified FESEM image of Figure 5 Up (v) for PPH-8 [24].
Through the integration of 3-DPT disposition with the shape-memory attribute of heat-stimulated printed PPH-8 architecture, varying printed figures with 3-D permanent configurations were garnered subject to structural restoration on exposure to temperature. A printed “butterfly” and “flowery” geometries were printed to entrench the transformation between permanency and temporary 3-D configurations (Figure 5).
3.2 4-DPT of Magnetic Responsive Shape Memory PLLA/TPU Blend
The combination of poly (L-lactic acid) (PLLA) with thermoplastic polyurethane elastomer (TPU) is capable of synergizing the benefits of outstanding biocompatibility with intrinsic shape memory capabilities of PLLA and elevated elasticity and exceptional shape memory features of TPU in use of minimally invasive surgery for bone tissue engineering. However, TPU creates a sea-island-appearing architecture in the PLLA matrix, decrementing shape memory features. Thus, in an investigation, a co-continual architecture of the TPU phase within the PLLA matrix was fabricated through the inclusion of Fe3O4 nanoparticles because of the varied interfacial tension and flowing disposition of TPU, which inculcated TPU/PLLA/Fe3O4 blend constructed via selective laser sintering (SLS) with outstanding shape memory features [25].
The construction steps of the composite blends are presented in Figure 7 (b). Lotus root-appearing blends were constructed via a self-formed SLS system composed of a computer-managing apparatus, a CO2 laser equipped with 35-W power, and a dynamic focusing optical system based on the 3D model of a lotus root (Figure 7) [25].
Figure 7 (a) Schematic elucidation of the construction of the T3P7F0, T3P7F5, T3P7F10, and T3P7F15 powders, (b-c) schematic illustration of the construction of the T3P7F0, T3P7F5, T3P7F10, and T3P7F15 lotus root-appearing blends, (d) FTIR spectra of the TPU, PLLA, T3P7F0, and T3P7F10 powders, (e) The contact angles of the TPU, PLLA, and Fe3O4 samples, (f) interfacial tension between differing pairs [25].
4. 4-DPT: Benefits and Disadvantages
AM has emerged as a prominent technology exhibiting great potential [26]. Despite 4DPT benefits similar to those of 3-DPT, 4-DPT performs better than 3-DPT relative to the dynamic features of fabricated products. Additionally, 4-DPT seems to be the following genre of AM [26]. The benefits entail energy conservation, time, money and materials [27]. Also, 4-DPT is advantageous for minimizing waste, errors, and loss of products in fabrication applications [27]. Differing reports depict the 4-DPT procedure as energy-proficient [27], with sustainability [27], and fast compared to other fabrication procedures.
Work has stated that 3-DPT entails object printing, while 4-DPT entails a creative procedure including topological modification or motion stimulation of the printed figurine [28]. These attributes are garnered by utilizing 4-DPT technology in fabricating the end product. Nevertheless, another work concluded that 4-DPT is a good material for biomedical scaffolds and micro-robotic construction premised on 4-DPT items affiliation with environmental variations in the system [27]. Inherent capabilities include elevated versatility as well as sensitivity, in addition to advanced designing and stimulation. Moreover, these parameters depict that 4-DPT facilitates the direct embedment of programming within a material devoid of external gadgets or systems usage [28].
4.1 Deficiencies of 4-DPT
Similar to its benefits, the deficiencies of 3-DPT may be identical to 4-DPT. Considering the added efforts essential for facilitating the mechanical behavior of printed figurines, 4D designing is more intricate than 3-D design [29]. The versatility of 4-DPT is still relatively limited, as 4-DPT shape-memory constructs are still within the early stages of maturation in comparison with 3-DPT [30]. A novel set of innovative materials has been constructed utilizing 4-DPT [31,32]. Conversely, a 4-DPT deficiency has been seen as an uphill task in maintaining its exactness due to instability garnered during optimizing parameters that regulate the functionalities of dimension and geometry [33].
4.2 Stimuli Affecting 4-DPT Procedures
The basic difference between 3-DPT and 4-DPT is the materials fabricating some figurines. The change in the value of the mechanical behavior of material from static to dynamic is gingered by external stimuli or actuators. Various stimuli applied to 3-D figurines to enable improvement of their functions and dispositions have been outlined including light, temperature, water, pH, magnetic, humidity, and electric [34,35,36].
4-DPT figurines based on sensitivity to temperature have been constructed [37,38]. A notable advantage of utilizing temperature as a stimulus involves the capacity to control the enhancements in a fabricated figure [39], performance capacity for mechanical enhancements [40], and operational ease [41]. The hindrances linked with temperature use as a stimulus include damage to cells [42], poor responsivity [43], usage complexities [31], as well as structural failures [35]. Most reports of temperature influence as the stimulus are based on temperature usage as an external triggering agent in generating shape-deforming bio-printed components [42]. Temperature as a stimulus has additionally been utilized for in vitro drug release [43] and tissue engineering [44].
Light is another notable stimulus greatly reported [45,46]. Inherent benefits of light utilization as a stimulus include fast-switching [47,48], precision [49], sustainability and biocompatibility [50], controlling wirelessly and remotely [51], modification of color [52], as well as mechanical features manipulation [53]. On the other hand, the use of light as an external source of stimulus for modifying 4-DPT objects structural modification also exhibits inherent deficiencies such as reduced geometrical transformation [54], potential toxicity [55], generation of heat generation [56], as well as complexities [57]. Because light cannot deteriorate cells, it can be applied in optical tools, micro-cantilevers, and the segment of in vivo biomedicine and drug conveying uses [58].
Numerous investigations have utilized water as a stimulus in actuating 4-DPT figurines [59]. Water utility in 4-DPT figurines has inherent benefits, including convenience, manipulability, and temperature minimization [59,60]. Other inherent water benefits include gradual time of reaction and difficulty manipulating humidity-sensitive materials [61].
Magnetic fields utility in actuation has varying benefits such as fast response [62], inferior safety [63], remote guidance as well as capability of accelerating the mobility of specific 4-DPT figurines [64]. On the other hand, inherent deficiencies include elevatable reactive prospects and agglomeration affinity of magnetic-responsive materials, especially in biomedical applications [65], intricacies of magnetic nanoparticles within living structures [66], inferior temperature usage, and elevatable density of conventional magnetic absorbents [67].
pH is a veritable bank of stimulus for 4-DPT figures due to its susceptibility to causing swelling, shrinkage, degradation, and dissociation [68]. pH can also activate expansion, contraction, and torsion functionalities [69]. Moreover, pH facilitates biocompatibility, enhancements of responsivity, drug release controlling, self-mending, color variation and biodegradation [70,71,72,73]. Due to their inherent biodegradability as well as biocompatibility, 4-DPT of pH-responsivity polymeric systems displays notable prospects for biomedical uses. pH is utilized in drug delivery applications, soft robotics, medicine, food packaging, spinal cord regeneration, and tissue engineering applications [74,75,76,77,78].
The electric field is another notable form of stimulus investigated in various studies, and it exhibits capabilities including high speed and remote control [79]. Inherent deficiencies linked with electric field stimulus include membrane degradation, localized heating, or cell demise [80]. Electric stimulation is utilized for tissue regeneration, artificial muscle and drug conveyance. Another rarely utilized stimulus is humidity due to inherent prospects to swell, shrink, twist, bend, and expand. The advantages of humidity use as an actuator include low cost and harmlessness [81].
4.3 Materials for 4-DPT
Since the approach for 4-DPT is similar to 3-DPT, especially relative to productivity, the novelty in 4-DPT includes material selectivity and characterization for reactions to an active entity. Commonly prevalent 3-DPT substrates include plastics, ceramics, and metals. Nevertheless, these materials are not appropriate for 4-DPT [82]. Materials that have been utilized in constructing 4-DPT include shape-memory polymeric materials, shape-memory alloys, shape-memory metals, hydrogel-sensitive materials, shape-memory ceramics, and liquid crystal elastomeric substrates [82,83].
Smart shape-memory polymers (SMPs) are commonly utilized materials for 4-DPT technology attributable to inherent features (inexpensiveness, low density, satisfactory strain recovery, elevated ratio of shape recovery, universality sustainability, biodegradability, biocompatibility and ability to manipulate molecular weights [84,85]. Moreover, SMP features include their lightweight nature and capability to deform significantly [86]. Moreover, SMPs perform more than shape-memory alloys relative to the end-product quality as well as dimensional precision [87]. Inherent deficiencies of SMP materials include inferior modulus strength paucity, unwanted transitional temperature, gradual time responsivity, toxicity affinity, and lack of immune responsivity [88]. Application segments of 4-DPT SMP nanomaterials include biomedicals, aerospace, inner surfaces of automobiles, textiles, and industries [89,90].
Shape-memory hydrogels (SMHs) are another set of materials used for 4-DPT objects due to inherent ionic conductivity, functionalization simplicity, biocompatibility, stretchability and transparency. Hydrogels are established for inherent elevated water content and capability of absorbing high water levels or other biological fluids [91]. Hydrogels' scope of expansion is hindered by some parameters, including elevated cost, constraints in loading handling, inferior mechanical strength, gradual responsivity, and minimized features attributable to the dryness in open-air feasibility [92]. Selected applications of smart hydrogels for 4-DPT include healthcare, tissue engineering, drug delivery and agriculture [93].
Other materials used for 4-DPT include shape-memory alloy (SMA), shape-memory composites (SMCs), and so on [94].
5. Manufacturing Technologies of 4-DPT
Notable manufacturing technologies used in 4-DPT figurines include fused deposition modeling (FDM), direct ink writing (DIW), stereo-lithography (SLA), digital light processing (DLP) and selective laser sintering (SLS) as depicted in Figure 8 [95,96,97,98,99,100]. These 4-DPT fabrication techniques are further elucidated in Figure 9 [96].
Figure 8 Schematic depictions of 4-DPT technologies. a) Fused Deposition Modeling (FDM). b) Direct Ink Writing (DIW). c) Stereo-lithography (SLA). d) Select Laser Sintering (SLS). e) Binder Jetting. f) Inkjet printing [95].
Figure 9 4-DPT techniques for polymeric nanoarchitectures [96].
FDM is commonly utilized in the fabrication of 4-D components, which are then followed by SLA, SLS, DLP, and DIW. Other techniques used in 4-DPT figure manufacturing include PolyJet printing, powder bed fusion, photolithography [101], directed energy deposition, and selective laser melting [101,102]. Fused deposition modeling (FDM) is the most prevalent construction route for fabricating a 4-DPT object [103].
The FDM technique has utilized various stimuli (temperature, pH, light, and water) [103]. This relatively inexpensive method produces high-quality, elevated-resolution products, involving rapid printing procedures and easy accessibility [103]. FDM flaws include elevatable fragility, complexity, low printing speed, rough surface finishing, and greater materials wastage compared with other techniques [104]. The FDM techniques have been utilized for vast industrial uses (biomedical gadgets, aerospace components, origami architectures, drug conveyance, as well as optical gadgets [105]. Figure 10 depicts a schematic elucidation of the FDM technique [105].
Figure 10 A schematic elucidation of the fused deposition modeling (FDM) technique [105].
Stereo-lithography (SLA) is another strategy utilized in fabricating 4-DPT objects. The benefits of SLA strategy utilization include rapid speed, high profiling, smooth surface finishing, as well as elevated resolution, and operating optimally with SMP at relatively low temperatures [106]. Despite SLA capability of printing polymeric components, the final component display inferior mechanical features [107]. Varying studies have been conducted utilizing SLA soft robotic segments, drug delivery, tracheal stents, biomedical scaffolds, and tissue engineering [108].
Selective laser sintering (SLS) facilitates component manufacturing without supporting materials [108]. Furthermore, this technique displays the benefits of elevated production volume and processing speed [108]. The SLS technique is utilized for biomedical gadgets, drug delivery, aerospace, and magnetism-responsive grippers [108].
The digital light processing (DLP) construction technique has been used to fabricate a 4-DPT figure. Texas Instruments found this strategy shortly after the SLA technology was invented in 1987 [107], offering benefits such as fast production time, elevated-quality resolution, and intricate structures [107]. DLP is limited by material selectivity, elevated cost of materials, and inferior mechanical features [108]. The DLP construction strategy has been utilized in several of applications including medication delivery, tissue engineering, and electronic gadgets [109].
Direct ink writing (DIW) is not as viable as other strategies. DIW displays minimal figurine precision of printed components in comparison with other printing strategies such as SLA [110]. However, the DIW technique is good for use in biomedicine and robotics, tissue engineering, electronic gadgets, and biomedical engineering [110].
5.1 Fabrication of Multiwalled Carbon Nanotubes/Halloysite Nanotubes Filled Heat Responsive Shape Memory Polymeric Nanocomposites for 4-DPT Applications
The shape-recovery and mechanical features of 4-DPT heat-responsive shape-memory polyurethane (SMPU) filled with multiwalled carbon nanotubes (MWCNTs) and Halloysite nanotubes (HNTs) were studied [110]. The HNTS-filled sample (1 wt.%) yielded elevated tensile, flexural, and impact strengths. However, the MWCNT-filled sample (1 wt.%) displayed rapid shape recovery.
Summarily, improved mechanical features were observed with HNT-filled samples, and a rapid shape recovery was observed with MWCNT-filled samples [110]. Garnered results revealed potential use for 4-DPT shape-memory polymeric nano architectures for continual cycles even after an extended bending deformation [111].
Figure 11 depicts the overall step of filament extrusion and 3-DPT. Parameters used (screw speed = 40 rpm, melting temperature = 210 C) utilized for filament extrusion in twin screws [111].
Figure 11 Elucidation of the procedure for 4-DPT of pristine SMPU and SMPU using nano-fillers: (a) SMPU pellets and MWCNT nano-fillers; (b) extraction of desired nanocomposite filaments constitution; (c) extruded filaments of pristine SMPU and filled with MWCNTs; (d) specimen 3-DPT; (e) tensile test specimens made of pristine SMPU and SMPU filled with MWCNTs [111].
The SMPU samples (125 × 15 × 5 mm3) underwent printing for shape recovery examination. After heating above Tg (in hot water), they deformed to a U shape (as shown in Figure 12) [111].
Figure 12 (a) Deformed and (b) permanent models of samples presented for shape recovery examination; (c) temporary deformed shape; (d) kept in a water bath above Tg; (e) shape recovery in a water bath [112].
6. Applications of 4-DPT
4-DPT is a potential technology capable of fabricating printed architecture at a minimal cost and labor duration [113]. 4-DPT has been vastly utilized for a vast range of applications (aerospace, military, healthcare, fashion, electronics, automobiles, agriculture, building and renewable energy) as elaborated in Figure 13 (a, b) [114].
Figure 13 Premised on 4-DPT in stimulation by varying conditions, 3-DPT objects can undergo fabrication into varying biomedical uses via differing 3-DPT techniques. The parameters and the classes involved in 4-DPT are presented. (1) 3-DPT: FFF, DIW, SLS, DLP, SLA, and inkjet; (2) 4-DPT stimulus (thermal, light, chemical, pH, water, and magnetic field); (3) biomedical uses of 4-DPT: vascular stents, tracheal stents, cell scaffolds, bone scaffolds, cardiac stents, occlusion devices and other applications (a). Multi-purpose applications of 4-DPT structures [114].
6.1 Healthcare
4-DPT uses in healthcare applications have garnered great interest globally. As the healthcare segment is a vast area, this section is subdivided into the following six sub-sections: dentistry, orthopedics, pharmaceuticals, drug delivery, biomedical devices, and tissue engineering [104,105,106,107,108,109,110,111,112,113,114,115].
6.1.1 Dentistry
4-DPT is a potential material for fabricating components that function in an oral environment, with humidity and temperature variation [116]. Certain challenges linked with this technology in improving the dentistry industry include not required dimensional variations, an unstable environment, and behavioral changes [117].
6.1.2 Orthopedics
The quest for artificial bones as well as orthopedic implantations has escalated and 4-DPT offers a dynamic architecture and stimulus responsivity imperative in the orthopedic sector [118]. Advancements in the field of orthopedics using 4-DPT have been reported with the usage of materials including hydrogels, PLA, SMP, and SMA for the construction of knee prostheses, intricate composite springs for fabricating parts such as the ankle, lower leg, and foot prosthetics [119,120,121].
6.1.3 Pharmaceuticals
Reports have insinuated that 4-DPT is still evolving, requiring more studies before usage as consumable medication [122]. It is perceived that further investigation will be conducted in this regard for materials investigation, manufacturing, and so on [123]. Investigations into polymeric chemical architectures for oral medication have garnered attention for varying SMP research and uses within the pharmaceutical segment [124].
6.1.4 Drug Delivery
In previous years, distinct enhancements in drug-delivery strategies have been achieved [125]. Here, enhancements in drug efficacy are intended to be attained via a drug-delivery approach with minimal aftermath effects. 4-DPT has exhibited potential for accelerating the development of drug-delivery mechanisms. 4-DPT’s self-folding and unfolding attributes facilitate carefully manipulated encapsulation and the effective release of medication via a programmable approach [126]. Nevertheless, a school of thought has postulated that innovative drug delivery via 4-DPT is presently constrained and biomedically unsuitable as applications require high external stimuli [127].
6.1.5 Biomedical Gadgets
The biomedical segment has displayed escalated attention to the use of 4-DPT or bioprinting. Investigations in biomedical engineering segments have revealed that materials such as hydrogels, SMPs, and so on are suitable for biomedical applications [128]. The fabrication strategies for construction of biomedical gadgets include DIW, FDM and SLA [129,130,131].
6.1.6 Tissue Engineering
Deficiencies inherent in 3-DPT can be mitigated using 4-DPT technologies. A vast range of research has been conducted in previous decades on ginger tissue engineering strategies [132,133]. 3-DPT has facilitated improvement in tissue engineering. Nevertheless, there are varying challenges and hindrances linked with 3-DPT [134]. The emergence of 4-DPT has shown potential for mitigating these challenges, which include the capability of implant placement in hitherto inaccessible positions and minimally invasive surgeries [135].
Various investigators have utilized different techniques for constructing 4-DPT gadgets, including FDM, SLA, and DLP, using stimuli such as temperature, light, and magnetism with materials such as SMP, hydrogel, and SMA [136,137].
6.2 Aerospace
An issue linked with the aerospace segment is the construction intricacies of components as well as the challenges faced in assembling these components [138]. Another notable issue is the expense of airplane replacement components, as well as intermittent disruption of the global supply network [139]. 4-DPT technology resolves these challenges by creating lower assembly components and saving time [140]. Varying investigations have reported escalated developments in aerospace components such as wings, aircraft spare parts and airplanes [141].
6.3 Electronics
Due to its inherent attributes, 4-DPT has been utilized in a vast range of electronic uses, including sensors, circuits, actuators, magnets, robotics, energy storage gadgets, antennas, and so on. The printing steps include SLS, FDM, DIW, DLP and SLA [142,143,144]. Commonly prevalent materials linked with electronic uses are shape-memory polymeric matrices, shape-memory hydrogels, and shape-memory alloys [145]. The stimuli propounded for constructing 4-DPT electronic gadgets are heating, light, magnetic, and water [146,147,148,149,150].
6.4 Food
Varying investigations have been reported on food fabrication utilizing 4-DPT strategies [151]. Commonly propounded strategies in the literature linked with the food segment include FDM, SLA, DIW, DLP, and SLM [152,153].
6.5 Renewable Energy
4-DPT is capable of overcoming renewable energy hindrances of low efficiency and expensiveness [154]. Though prospects of 4-DPT to enhance energy efficiency have been elucidated, 4-DPT usage in the renewable energy segment remains underdeveloped in comparison with other applications.
7. Challenges and Future Outlook
The manufacturing procedure and designing of 4-DPT objects are similar to 3-DPT technology. Thus, it has been opined that a notable restrain in designing and constructing 4-DPT architectures includes limited levels of 3-D printers capable of printing 4-D components and the paucity of bespoke software. Hence, in order to escalate the manufacturing of 4-DPT figurines, the use of advanced and bespoke printers and software has become imperative. 4-DPT technology design and formulation present obstacles to the advancement of this technology [155]. Hence, a quest exists for mathematical modeling capable of predicting 4-DPT components [156]. On the other hand, varying research has been conducted on a vast range of PNCs for multifunctional applications [157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244].
8. Conclusion
Though varying investigations have been conducted to study the fabrication of 4-D components utilizing varying construction strategies, features, stimuli, vast materials, and programmable figurines, a paucity of expanded comprehension of design factors facilitating further studies exists. It was opined that most inverse challenges are based on unsure steps. Here, it is anticipated that a systematic and outlined strategy would enhance the efficiency of the 4-DPT design procedure and architecture. It has been opined that hindrances in 4-DPT involve comprehension of the features of 4-DPT components. Hence, the inclusion of varying parameters as well as their impact on 4-DPT designing would escalate advances in 4-DPT technology.
Acknowledgments
UNIZIK Nigeria is acknowledged.
Author Contributions
Engr. Dr. Christopher Igwe Idumah conceptualized, wrote and supervised the paper. Ifeanyi Emmanuel Okoye contributed in project development while Chioma Joan Ikebudu supervised the project development.
Competing Interests
The authors have declared that no competing interests exist.
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