3D-Printed Objects for Multipurpose Applications

3D printing is a popular nonconventional manufacturing technique used to print 3D objects by using conventional and nonconventional materials. The application and uses of 3D printing are rapidly increasing in each dimension of the engineering and medical sectors. This article overviews the multipurpose applications of 3D printing based on current research. In the beginning, various popular methods including fused deposition method, stereolithography 3D printing method, powder bed fusion method, digital light processing method, and metal transfer dynamic method used in 3D printing are discussed. Popular materials utilized randomly in printing techniques such as hydrogel, ABS, steel, silver, and epoxy are overviewed. Engineering applications under the current development of the printing technique which include electrode, 4D printing technique, twisting object, photosensitive polymer, and engines are focused. Printing of medical equipment including artificial tissues, scaffolds, bioprinted model, prostheses, surgical instruments, COVID-19, skull, and heart is of major focus. Characterization techniques of the printed 3D products are mentioned. In addition, potential challenges and future prospects are evaluated based on the current scenario. This review article will work as a masterpiece for the researchers interested to work in this field.


Introduction
3D objects are created by the successive layers of material controlled digitally by three-dimensional printing. Complex parts which are time-consuming and expensive by traditional methods can be produced in less time and at low cost by 3D printers. Both prototype and functional parts are produced rapidly and accurately without making any waste. It is also considered sometimes as a new industrial revolution (Ref 1).
For the ease of consolidation, early additive manufacturing applications emphasized on the use of polymers either through a photopolymerization or thermal process ( Ref 2,3). Because of technological transmission, there has been an increased uptake of metal-based additive manufacturing used as a prototyping tool to make end products (Ref 4). The main technologies for 3D printing are direct metal laser sintering, electron beam melting, FDM using a metal-filled polymer filament, and directed energy deposition ( .
Various materials and techniques are utilized in the 3D printing process. The top-down lithography process is one of them that includes two-photon polymerization, focused ion beam, and electron beam -lithography, and therefore, it can produce precise patterning with the desired shape using a layerby-layer profile ( . Fused deposition of polymer is another type of popularly used 3D printing process where machines are not expensive and easy to operate ( Ref 12). Both conventional polymers and polymer composites containing solid particles can be used to print. Among the commonly used materials, hydrogel materials are commonly used for their capacity as bioinks in a 3D printing system for cell printability and encapsulation (Ref 13). Composite materials are employed in 3D printing for the fabrication of complex geometries because of having superior thermal, mechanical, and electrical properties (Ref [14][15][16]. Besides, having the ability to respond to external stimuli like water, temperature, and light smart materials has significant applications in 3D printing (Ref [17][18][19][20].
Applications of 3D printing in medical, industrial, automotive, construction, architecture, electronics, aerospace, and decorative sectors are abundant. Complex composite tissue constructs are created by 3D printing in a layer-by-layer fashion where cell-laden hydrogels are precisely placed (Ref 21,22). Battery components including a separator, electrodes, current collector, and solid polymer electrolyte are customized within the final optimized design 3D object through the direct incorporation of microbatteries and electronics (Ref [23][24][25][26][27]. Within a short time, complex-shaped microstructures and geometries can be built ( Ref 28,29). By using microelectromechanical systems microstructured metals are being manufactured from ceramics ( Ref 30,31). Conductive fillers made conductive nanocomposites manufactured by 3D printing have potential applications in the fields of robotics, tactile sensors, and microelectromechanical systems (Ref [32][33][34][35]. Application of additive manufacturing particularly in medical science and medicine shows tremendous improvement (Ref 36). Current research on additive manufacturing for medical applications is focused on four main areas, and the areas are: (1) pathological organ models manufacturing research to aid surgical treatment analysis and preoperative planning (Ref 37); (2) personalized manufacturing research to make permanent nonbioactive implants; (3) local bioactive and biodegradable scaffolds fabrication research; (4) organs and tissues printing research for complete life function (Ref [38][39][40]. To manufacture medical equipment, additive manufacturing makes the best use of raw material with minimum waste, make better mechanical integrity and the geometrical accuracy become satisfactory. The metals used in medical applications are typically titanium, titanium oxide, stainless steel, titanium nitride, zirconium oxide, carbon nitride, and cobalt chromium alloys (Ref 41, 42).

Methods
Manufacturing of 3D products by 3D printers is performed by various methods. The fused deposition is considered one of the most popular and economical methods where thermoplastic polymers are used for the preparation of filament suitable for a 3D printer ( Ref 43). In this method, cheap complicated items can be produced with reduced waste ( Ref 44). Thin layers of melted thermoplastic material are deposited in this process in many successive passes for making the desired 3D object where the computer controls a heated nozzle in the direction of the XYZ-axis. After extrusion, the thermoplastic material instantaneously cools down to be deposited and the material is heated only a few degrees above its melting temperature. Transparent parts with photo-induced layer can be produced allowing UV absorbers having the thickness typically around .
Due to the advantages of low cost, short cycle time, and high precision to manufacture complex-shaped ceramic parts stereolithography 3D printing is widely employed. Stereolithography 3D printing has produced SiO 2 , Al 2 O 3 , ZrO 2 , bioceramics, and lots more. In order to make optical mirrors in recent years, 3D printing has been applied for the manufacturing functions of ceramics and ceramics matrix composites. The stereolithography 3D printing process is rarely observed in the studies to make SiC ceramic and its optical components (Ref 46).
The laser beam is a subset of powder bed fusion where solid objects are created by heating solid particles. In the process, they are also fused together at their surfaces. Carl Deckard developed the technology based on a neodymium-doped yttrium aluminum garnet in 1984. The printer made many prototypes by acrylonitrile butadiene styrene (ABS) and thermoplastic polymer ( Ref 47).
Digital light processing is another popular 3D printing method where complex-shaped ceramic parts are fabricated. Personalized structured ZrO 2 ceramic teeth having a good mechanical property, biocompatibility, and strong potentiality in the field of oral restoration are prepared by this method. This method also produced fine lattice structural titanium dioxide ceramic and porous BN-SiO 2 ceramics ( Ref 48).
Metal transfer dynamics are investigated in depth in the wire feeding-based electron beam 3D printing process. Here the experiments are combined with novel modeling of the heat transfer and molten flow behaviors. Experimental and simulation results recognize different metal transfer modes and are revealed quantitatively. A simple theory determines the relationship between the mode good for forming quality and the process parameters (Ref 49).

Materials
Many different materials can be used in 3D printing to manufacture products. In medical science hydrogel materials for example collagen, gelatin, and alginate are commonly used as bioinks. Collagen hydrogel is the most abundant natural polymer found in mammalian tissue. It has the capability of providing a favorable microenvironment because of having a native extracellular matrix, and for this, it is commonly utilized for the regeneration of vasculature, bone, liver, and nerves (Ref To avoid degradation of the ABS 240°C temperature was kept constant in the extruder for 5 minutes to ensure complete melting and mixing. Maintaining the weight percentage 1, 3, 5, 7, and 10, the MWCNT/ABS nanocomposite samples were prepared. In order to satisfy the mechanical properties steel is reinforced with other materials to give higher strength. Li et al. (Ref 52) reinforced steel microcable with a geopolymer composite in his 3D printing research for the mechanical improvement of the product. 1.2 mm of diameter continuous steel microcable having 7 shares and each share had 19 strands was extruded from the print head along with the geopolymer. Steel microcable small curvature was ensured by using a 15mm-diameter round nozzle during printing. After attempting several times, the horizontal printing speed and the steel microcable entering speed are kept equal.
Silver is extensively used in 3D printing to print dental materials. Liao et al. (Ref 53) utilized silver nanoparticles with zirconium oxide to strengthen dental base composites in 3D printing. In the process they utilized, desired materials were synthesized at two different stages. In the first stage, nanosilverloaded zirconium phosphate was salinized under an acidic condition with MPS. 4.0 g 6S-NP3 and 150.0 g deionized water were mixed in a clean beaker for ultrasonic dispersion for 1 h. Meanwhile, with constant speed injection, 1.0 g acetic acid, and 50.0 g deionized water were dropped together. The suspension was charged with a machine stirring (200 r/min) into a 500-mL four-neck flask and heated to 30°C followed by adding into the system in 1 h with the mixture of MPS and cyclohexane. The mixture temperature reached 80°C and maintained the reaction for 1.5 h. Using acetone M-6S-NP3 (methyl nanosilver-loaded zirconium phosphate) was created through extraction, and then, the solution was dried in the vacuum oven for 12 h at 80°C. At the next stage, M-6S-NP3 was grafted with PMMA where free radical polymerization was used. 1.0 g M-6S-NP3 was dispersed in a 250-mL four-neck flask in 50.0 g xylene with the help of a stirring machine and then dispersed ultrasonically for 0.5 h. Then, under nitrogen atmosphere, 10.0 g MMA and 0.1 g BPO initiator were added. In a water bath, the mixed suspension was kept stirring at 80°C. The P-6S-NP3 was obtained after 8.0-h polymerization through centrifugal separation and extraction with acetone and then dried for 12 h at 80°C in a vacuum oven.
Epoxy materials give desirable properties with improved mechanical, thermal, and chemical properties to the 3D-printed materials. For extrusion-based 3D printing, nanoclays can be used in epoxy or short fiber composite inks for the purpose of direct-write (DW) additive manufacturing (AM) so that it can be imparted in nearly ideal rheological properties. In general, significant effort is given so that polymer matrix composited can be used in 3D printing, and rapid progress is observed in epoxy-based composite materials. Hmeidat et al. (Ref 54) et al. used epoxy nanocomposites for making high-strength 3Dprinted materials. With appropriate amounts of nanoclay or fumed silica, the process inks were prepared by mixing the epoxy resin in 185-mL plastic containers using a centrifugal planetary Speed Mixer. Using 30 g of Epon 826 resin and 1.5 g of the curing agent (VS03) six formulations, plus the control, were prepared. Under vacuum at 0.1 atm for 60 s, the contents were mixed at 1700 rpm. After 60 seconds of mixing at 1700 rpm and 0.1 atm, the nanofiller was added. Then, they scraped the container with a spatula so that the nanoclay can be kept the mixture completely dispersed and bubble-free by mixing for an additional 60 s at 1800 rpm and 0.1 atm.

Printing Applications
Numerous applications are visible of 3D printing in printing applications. The 3D-printed electrode is one of them which can be printed both vertically and horizontally. The incorporation of smart materials or programmable materials that have the stimuli in temperature, water and light in 3D printing has brought a new dimension called 4D printing (Ref 55  Fig. 1. During the fabrication process, PLA pellets were first dried for 24 h at 80°C. 10 wt.% and 30 wt.% were used then as a plasticizer by melt-blending to PLA using a microcompounder for 10 min at 170°C and 100 RPM. They fabricated the 3Dprintable PLA/PEG filaments by attaching an automatic roller, after compounding at the end of the compounder. The 4D components were fabricated by a customized 3D FDM printer using a standard 0.3-mm nozzle keeping the nozzle temperature of 180°C. Reshapable arc-shaped components were printed later by compression force into a flat configuration. Products with rotational and twisting capabilities are manufactured with the help of 3D printing. Tuning elements having to twist and rotational bistable structures have been fabricated by Jeong et al. (Ref 57). In the fabrication procedure, they used the polyjet process in a Stratasys multi-material 3D printer where ultraviolet light was used for jetting the photopolymer ink droplets. They also created fine features with about 50 lm resolution in the plane and 15 lm in thickness by the process. Before importing into the Stratasys printer geometric design was first created using CAD software. Without postassembly and using a consolidated design they fabricated twisting and rotational bistable components. They used some special joints that allowed 3D printing of the whole components such as twisting components used ball joints, rotational used pin joints.
3D printing has the advantages of producing polymeric material over other methods. Transparent, light and photosensitive polymer material has been fabricated by Wang et al. (Ref 58). A rigid, nearly colorless material exhibiting dimensional stability with the trade name VeroClearRGD 810 resinous material has been chosen for the experiment. By using AutoCAD cylindrical and dog-bone-shaped specimens were created at first. The specimen was printed by the Object Studio program then.
Engines for optical communication are fabricated by 3D printers. Hybrid multi-chip assembly of optical communications engines has been fabricated by Bleicher et al. (Ref 59) by in situ 3D nanolithography. In the process, they fabricated all PWB structures where a modified commercial two-photon lithography system was utilized. An fs laser having a pulse length of 100 fs along with a repetition rate of 80 MHz has been used as a lithography light source. The proprietary control software was used for allowing the precise localization of coupling interfaces and automated the PWB fabrication with high shape fidelity used to equip the lithography system.

Medical Applications
The invention of 3D printing has brought a new era for medical science. Numerous materials are being printed for the applications in this field. In the field of tissue engineering, artificial tissues are being developed by controlled deposition and cells (Ref 60). Kang et al. (Ref 57) synthesized bioprinted tissue by depositing cell-laden hydrogels together with a synthetic biodegradable polymer. That was accomplished in the following ways: they designed multi-dispensing modules for delivering different cell types and polymers in a single construct, they made a carrier material to deliver cells in a liquid form to discrete locations in the 3D structure, they designed a sophisticated nozzle system where the resolution was 2 lm for biomaterials and 50 lm for cells, they linked together cell-laden hydrogels after passage through the nozzle system, they printed an outer sacrificial acellular hydrogel mold simultaneously and dissolved after acquiring enough rigidity by tissue construct to retain its shape, and they created a lattice of microchannels that could nutrient and diffuse oxygen into the printed tissue constructs.
Hybrid microscaffolds are being produced with the help of 3D printing. Tan et al. (Ref 61) applied a new biofabrication strategy called hybrid microscaffold-based 3D bioprinting of multicellular constructs that could produce products having compressive strength. High specific surface areas are provided by the highly porous microscaffolds to get the anchoragedependent cells capable of attaching infiltrate and grow before extrusion-based printing. That property will expand the cells seeded on the microspheres where it will be exploited in stirred or perfused culture and no passaging will take place from cellladen microspheres (CLMs). Together with thin hydrogel encapsulation, those CLMs could act as a bioink material for 3D bioprinting when the CLMs were lubricated by the printing hydrogel and glued after printing upon gelation shown in Fig. 2.
Bioprinted models are another product of 3D printing. In the fabrication process, an ionizable comonomer is prepared by cross-linking polyacrylamide microgels with 17 mol.% methacrylic acid. Another solution was prepared in 490 mL ethanol by 8% (w/w) acrylamide, 2% (w/w) methacrylic acid, 1% (w/w) poly(ethylene glycol) diacrylate (MW = 700 g mol À1 ), and 0.1% (w/w) azobisisobutyronitrile. The solution was placed into a preheated oil bath after sparging with nitrogen for 30 min at 60°C. The reaction mixture was heated after forming white precipitation for 4 h. Then the microparticles were triturated after filtration with 500 mL of ethanol overnight. The solids were collected again and dried. The purified microgel powder was dispersed, mixed, and neutralized. For each type, the microgel 3D printing is prepared along with the culture medium. Using 12-well plates of single 35-mm Petri dishes, fabrication of microbeams was enabled.
3D printing also helps in a bone generation. Bioceramic scaffolds stimulate pediatric bone regeneration has been fabricated by Wang et al. (Ref 64) with the help of 3D printing. At the beginning of the process for the visualization of the rabbit's calvarium, a 13-mm skin incision was created. For exposing calvarial bone periosteum and soft tissue were dissected. Using 10 mm diameter trephine defects were created. Then the defects were repaired with either 3D-printed scaffolds loaded with 1000 lM DIPY or bone graft where a fit-and-fill method was used. Proper inset was ensured when the violation of the dura mater was avoided, and the primary stability of the scaffold was obtained. Then calvarial bone graft was created by the immersion of trephined calvarium in saline solution. On the right aspect of the midface, a 13-mm skin was created in the alveolus. The soft tissue, alveolar ridge, and the maxillary suture and periosteum were dissected for the visualization of the maxilla. 3D-printed template was produced by either 3Dprinted scaffold loaded with 1000 lM, DIPY or bone graft defects were repaired again with the help of a fit-and-fill method so that reconstruction primary stability and avoiding violation of the maxillary sinus membrane were ensured. Before creating the maxillary defect, an alveolar bone graft was created by harvesting radial bone from the right rabbit forearm and 10-mm longitudinal incisions were made. Innovative approaches like incorporating antibiotics into 3D-printed constructs are done by 3D printing. The common applications are medical implants, prostheses, and surgical instruments. Increased surface area for drug distribution, sequential layers of antibiotics and the ability to rapidly fabrication is permitted by 3D-printed antibiotic-impregnated devices. Table 1 shows the advantages and disadvantages of inkjet, fused deposition modeling, and stereolithography 3D printing techniques to incorporate antibiotics into 3D-printed constructs, and a short summary of recent bioprinting is shown in Table 2.
There has been a shortage of personal protective equipment (PPE) in many countries of the world due to the current COVID-19 pandemic where 3D printing is playing a good role. Breathing device is one of the most wanted devices for patience in many countries. As the normal production and the key pieces of PPEs cannot match the current demand, other means of manufacturing for these items are being practiced in many countries. In the field of cardiology, additive manufacturing is showing its excellence. For the communication of doctors with patients in some unique medical situations, patientÕs heart can be created by 3D printing. Additive manufacturing is relatively quick and cost-effective method to produce 3D anatomical heart models to develop surgical planning as well as treatment outcomes. Similar models can also be used in medical training program to train surgeon ( Ref 79). For the development of vascular stents, additive manufacturing can also be applied to manage blood flow obstruction. On-demand and custom-fit can be created by this method for each patient. Thicker or thinner design can be created by customizing of stents around stressed or unstressed region of a vessel in order to increase blood flow (Ref 80). 3D printing is also useful in cardiology to design scaffolds. Melt extrusion technique is applied to produce this type of scaffolds that enable heart regeneration following a heart attack ( Ref 81).
The production of orthopedic aids is now more accurate, more automated as well as less expensive due to the application of additive manufacturing in making these aids. A method has been described by Molnar and Morovi (Ref 82) where a custom-fit orthopedic corset has been produced for lower back support. In the method, a digital model was created by scanning the patient. CAD software has been used to prepare a custom-fit orthopedic corset using the data from the scan. Then, by applying the fused deposition method the corset was printed (Ref 83). Polylactic acid and polyethylene terephthalate glycol were used to make the corset because the materials needed to be both printable and biocompatible.

Simulation and Characterization of 3D Structure
3D-printed materials are simulated and characterized by different techniques. Park et al. (Ref 75) performed an SEM test to characterize 3D-printed electronics made of conductive cellulose composites with a low porcelain threshold. Embedded conductive fillers which are in an insulative polymer matrix are shown in images. Figure 3(a) with a higher volume fraction is compared with Fig. 3(b) having a lower volume fraction. Clusters forming the localized percolation networks are seen in Fig. 3(b). This shows a well-matched resistivity change result in the simulation. In between Fig. 3(a) and (b), comparatively higher resistivity is observed in Fig. 3(a).
EDS spectrum analysis is used to show the chemical composition of the 3D-printed material. EDS analysis of the additive manufactured 3D nanoarchitect metals is performed by Andrey  Fig. 4(b).
Different mechanical tests are also performed in the 3Dprinted structures. Bending with bare hand and simple radial  The 3D-printed hydrogel network with microporous structure and interconnected channels is stable and acquired high cell viability (more than 90%).
The developed HCECs exhibited a greater cytokeratin 3 (CK3) and higher proliferation, signifying that newly developed technique may help to enhance the alginate bioink system for the application of 3D printing in tissue engineering. direction compression are among them. The shape was recovered after applying compression and bending force to the original state for the 3D-printed 2L-P MFT construct without breaking down as seen in Fig. 5 A straight microchannel was fabricated in order to investigate the bonding quality having dimensions of 50 lm height, 200 lm width, and 4 cm length, and it was tested accordingly. For the appearance and growth of Saffman-Taylor fingers, the device performance has been monitored until it becomes stable, known as ''inflation stability'' (Fig. 6a). To identify the channel behavior the results are presented in a 2D diagram at a given pressure, as shown in Fig. 6(c). According to the results, the interface between the 3D-printed part and PMMA sheet became leakproof because of achieving holding strength of the doublecoated adhesive tape. Shear rate distribution was also evaluated across a line parallel to the channel width as Fig. 6(b). The safe zone for performing inertial microfluidic experiments is seen in the green area in Fig. 6(c). Experiments show that Saffman-Taylor fingers begin to appear when pressure is more than 82.6 psi. When it was run at high pressure in channels, any delamination or deformation was not observed (Ref 87).

Potential Challenges in Future
Future studies in 3D printing will likely involve the development of new printable metals for structural elements. Printing of large and bulk components can be focused. In order to enable strength, flexibility, and safety further research is required for the evaluation of the mixtures of materials and printing techniques. Comparatively less-expensive 3D printers have lower resolution and poorer surfaces that can lead to increased cost due to post-processing requirements. Besides printers with lower tolerance are not suitable for manufacturing products for assembly works. Energy consumption, space, and setting requirement result in more cost in industrial machining. As a result, large production becomes more expensive in 3D printing compared to traditional machining ( Ref 88). For the support of new applications in medicine, new printable and biocompatible material may be involved in the research. Research may also associate with the cost reduction in printing.
The field of 3D printing is still far away for the mass production of products to meet the demand of average customers. It has passed a long way in the last 2 decades through sophisticated printers that are too expensive to attract the nonspecialist. The environmental standards are not set to the equipment technology properly ( Ref 89).
3D printing has several environmental impacts associated with the printing process. It has greater energy demand compared to the traditional machining processes. Volatile organic compounds, solvents, nanoparticles may pollute the environment. Toxicity emission from materials might harm human health. Recycling of some materials is beyond processing. Some prospects are as well compared to other machining processes. It has greater raw materials efficiency and required less cutting fluid than milling. Green materials can be used as raw materials and thus protect the environment. Time consumption has decreased in prototype construction. Lower CO 2 emission is observed during lightweight construction. Spare parts and tools have a greater lifetime and adaptability to climate change. Clinical sectors are highly benefited because of bioprinting (Ref 90).

Conclusions
3D printing is a supportive advancement and probably the prime manufacturing technique in the future for different disciplines of engineering and medical science. It has shown dramatic development in recent years of using smart materials popularly known as 4D printing with conventional other materials. Incorporating nanomaterials to give strength and achieve desired mechanical properties is another milestone. Advancement in synthesizing biomaterials is another achievement probably beyond other conventional techniques. Though 3D printing has to pass a long way to be economical in industrial sectors, it has shown great advancement in a short period.