Introduction

Advanced 3D printing has many advantages over conventional and unconventional manufacturing techniques, in terms of cost reduction, speed, freedom in designing complex parts, manufacturing in single step and sustainability. Additive manufacturing is a layer-by-layer manufacturing technique, which reduces the process of machining and removing the excess material from a larger stock. This process directly creates the components to their final shape, with less material wastage and attains required geometric dimensions. It is difficult to pick a desired additive manufacturing technique for a particular material and application due to availability of a wider range of 3D printing techniques (Ref 1,2,3,4). Based on the materials to be printed, various techniques offer difference in surface finish, high-dimensional accuracy and post-processing requirements. 3D printed components possess increased applications in the areas like aerospace, structural, biomedical, complex component fabrication, etc. Out of the said applications, 3D printing possesses higher attention in the biomedical field because of its flexible solution in producing cost-effective, quick surgical instruments and patient-specific bio-implants (Ref 5,6,7,8). It is not only used for the repair or fabrication of human body tissues like dental implants, artificial liver, artificial cardiovascular systems and orthopedic implants but also for the fabrication of medical electronic and micro-fluidic devices.

Due to the mismatch of mechanical properties of the metal implants and native body parts or organs, stress shielding would appear. This leads to the organ or part resorption and implant failure, repeatedly. The failure caused in the implants is due to the implants manufactured through conventional manufacturing processes. Hence, the development of 3D printing technique has been a viable alternative for the fabrication of prosthesis with controlled strength and porosity matching the properties of the native bone and organs, thus reduces the chances of stress shielding. The ability to produce customized products in 3D printing technique created an unprecedented opportunity for the fabrication of personalized medical implants that could suit for the specific body part complications (Ref 9). Since, AM being a demand-based manufacturing process and its ability to produce complex shaped implants has significantly made the process more economic and cost effective. There is no any tooling involved in fabrication of components and hence, the cost per component remains the same in AM processes. While in conventional machining process, tooling cost contributes more in deciding the cost of the AM components. The cost analysis has created a rationale for the use of 3D printing technique for the fabrication of bodily implants (Ref 10). The image of the biomedical implant fabricated through 3D printing technique is given in Fig. 1.

Fig. 1
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Image of the biomedical implant fabricated through 3D printing technique (a) cranial prosthesis (reprinted from Procedia CIRP, Vol 49, A.L. Jardini, M.A. Larosa, M.F. Macedo, L.F. Bernardes, C.S. Lambert, C.A.C. Zavaglia, R. Maciel Filho, D.R. Calderoni, E.Ghizoni, P. Kharmandayan, Improvement in Cranioplasty: Advanced Prosthesis Biomanufacturing, pages 203-208, Copyright 2016, with permission from Elsevier); (b) surgical guide (reprinted from The Journal of Prosthetic Dentistry, Vol 85, Dov M. Almog, Eduardo Torrado, Sean W. Meitner, Fabrication of imaging and surgical guides for dental implants, pages 504-508, Copyright 2001, with permission from Elsevier); (c) scapula prosthesis (reprinted from Journal of Bone Oncology, Vol 12, Dong Liu, Jun Fu, Hongbin Fan, Dichen Li, Enchun Dong, Xin Xiao, Ling Wang, Zheng Guo, Application of 3D-printed PEEK scapula prosthesis in the treatment of scapular benign fibrous histiocytoma: A case report, pages 78-82, Copyright 2018, with permission from Elsevier); (d) knee prosthesis (reprinted from Journal of Alloys and Compounds, Vol 714, Mahmoud Z. Ibrahim, Ahmed A.D. Sarhan, Farazila Yusuf, M. Hamdi, Biomedical materials and techniques to improve the tribological, mechanical and biomedical properties of orthopedic implants – A review article, pages 636-667, Copyright 2017, with permission from Elsevier); (e) dental implants (reprinted from The Journal of Prosthetic Dentistry, Vol 120, Michael Tischler, Claudia Patch, Avinash S. Bidra, Rehabilitation of edentulous jaws with zirconia complete-arch fixed implant-supported prostheses: An up to 4-year retrospective clinical study, pages 204-209, Copyright 2018, with permission from Elsevier); (f) interbody fusion cage (reprinted from Metallic Foam Bone, T. Matsushita, S. Fujibayashi, T. Kokubo, pages 111-130, Copyright 2017, with permission from Elsevier); (g) acetabular cup (reprinted from Tribology International, Vol 60, Vesa Saikko, Tiina Ahlroos, Hannu Revitzer, Oskari Ryti, Petri Kuosmanen, The effect of acetabular cup position on wear of a large-diameter metal-on-metal prosthesis studied with a hip joint simulator, pages 70-76, Copyright 2013, with permission from Elsevier); and (h) hip prosthesis (reprinted from Arthroplasty Today, Vol 2, Urban Hedlundh, Lars Karlsson, Combining a hip arthroplasty stem with trochanteric reattachment bolt and a polyaxial locking plate in the treatment of a periprosthetic fracture below a well-integrated implant, pages 141-145, Copyright 2016, with permission from Elsevier)

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There are various techniques and processes involved in fabricating the biomedical implants and surgical components. In the present article, the various processes like friction stir additive manufacturing (FS-AM), laser additive manufacturing, selective laser melting (SLM), paste extrusion deposition, (PED) are discussed. The methodology of the given processes, various works carried out so far in the said areas is discussed in detail.

Friction Stir Additive Manufacturing (FS-AM)

Additive manufacturing is classified mainly based on raw material’s physical state, as solid-based systems, powder-based systems and liquid-based systems. FS-AM is a solid-state additive manufacturing process for metals and alloys, developed to resolve the challenges like internal cavities, inclusions, shrinkage, internal porosity and microstructural in-homogeneity, and low mechanical properties due to its operation below the melting point (Ref 11,12,13,14). FS-AM is a multiple variant technique which deposits the materials layer by layer through the thermo-mechanical stirring of tool rotation which causes high temperature by significant plastic deformation. This involves (i) surface cladding, (ii) friction surfacing, (iii) modification of functionally graded composition, (iv) supplying innovative materials from wire/powder. The heat generation is obtained due to the effects of plastic deformation (45.6% of total heat) and friction (54.4% of total heat) (Ref 15). The components manufactured through FS-AM method provides low residual stress, refined microstructure, low distortion and higher building rate when compared with fusion-based AM techniques (Ref 16). The building rate of (i) metallic material (aluminum alloys) is 10 kg/hr, and (ii) harder material (nickel-based alloys) is lower than 0.5 kg/hr, and its accuracy is increased with the reduced building rate. Besides, the accuracy of the technique could be improved by controlling the process parameters like forging pressure, traverse velocity, rotational speed, diameter, type of feeder, etc.

Metallic Alloys for Biomedical Implants

Initially, the plates or sheets were prepared with appropriate size and shape and cleaned using acetone and are placed in overlap condition with appropriate build orientation. The schematic representation of FS-AM process is displayed in Fig. 2 (Ref 17). The suitable process parameters were selected, and the layer placement was confirmed. The first run of friction stir additive manufacturing should be checked for desired layer height. If the obtained height is not sufficient, then the subsequent layer was added by placing the plates or sheets over the fabricated plate.

Fig. 2
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Reprinted from Ref 17, with permission from Elsevier

Schematic representation of FS-AM and bonding layers with step by step.

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The metallic alloys are preferable for biomedical applications than polymers, composites and ceramics due to their higher biocompatibility, hardness and strength. The estimated contribution of FS-AM for the medical fields is illustrated in Fig. 3 (Ref 17). The bio-compatible materials such as cobalt (Co), titanium (Ti), magnesium (Mg), and stainless steel are most commonly used as biomedical implants which cannot react with human blood for release of ions and wear debris. The implant material used in human environment must have good mechanical properties such as compressive strength, tensile strength, toughness, microhardness, yield strength, modulus of elasticity and fatigue strength (Ref 8).

Fig. 3
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Reprinted from Ref 17, with permission from Elsevier

Estimated contribution of FS-AM for the medical fields.

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Yee-Hsien et al. (Ref 18) reported the bio-corrosion demeanor of AZ31B magnesium alloy fabricated through FS-AM in hydroxyapatite (bone mineral) environment considering it a suitable alternative for the natural bone because of its improved mechanical properties and a better bio-corrosion resistance due to the grain size refinement effects. Ni-Ti alloy is also used to make biomedical implants to withstand high toughness. But, the presence of Ni ion causes a toxic impact on human cells (Ref 19).

Metallurgical and Mechanical characterization of FS-AM Components

The microstructure of Inconel 625 fabricated through FS-AM technique exhibited a severe dynamic recrystallization, which leads to the development of fine equiaxed structure with better fatigue properties. The SEM microstructure of different staked layers fabricated through FS-AM is illustrated in Fig. 4 (Ref 16). Similarly, Ti-6Al-4V revealed refined grains with a fully dense structure, resulting in a considerable increase in mechanical properties (Ref 20). Dilip et al. investigated the FS-AM of AISI 304 SS rod over the mild steel with an axial force of 9.8 kN, and a rotational speed of 800 rpm. The author pointed out the axial reduction of consumable material was proportional to the built-up layer thickness. The microstructure reveals the fine equiaxed structure with an average grain size of 20 μm (Ref 21).

Fig. 4
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Reprinted from Ref 16, with permission from Elsevier

SEM microstructure of different staked layers fabricated through FS-AM.

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Palanivel et al. (Ref 22) reported six times increase in the ductility (17%) of the FS-AM fabricated Mg-based alloy (WE43) specimen over base metal (2.9%). Also, the strength of 400 MPa is obtained through the FS-AM, which is higher than the base metal (357 MPa) strength. The process improves the properties like ductility, mechanical property and strength of the selected work material, which are not achieved by conventional techniques (Ref 23, 24). Palanivel et al. (Ref 25) investigated the FS-AM of AA5083 and Mg-4Y-3Nd build. The average microhardness value of Mg-4Y-3Nd alloy build developed through FS-AM (120 HV) was relatively higher than the base metal (97 HV). Similarly, AA5083 build average microhardness (104 HV) was higher than base metal (88 HV). The ultimate tensile strength (362 MPa) and yield strength (267 MPa) of FS-AM are higher than the base metal strength of 336 MPa and 190 MPa, respectively. Hence, the results showed the potential benefit of FS-AM for both Al and Mg alloy build. The tensile strength and microhardness distribution of the different materials fabricated through FS-AM are illustrated in Fig. 5 and 6 (Ref 17).

Fig. 5
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Reprinted from Ref 17, with permission from Elsevier

Tensile strength of different materials fabricated through FS-AM.

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Fig. 6
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Reprinted from Ref 17, with permission from Elsevier

Microhardness distribution of different materials fabricated through FS-AM.

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Laser Additive Manufacturing (LAM)

At the present time, polymers and selected grades of metals (stainless steel, Inconel, aluminum and titanium) are predominantly used in 3D printing (Ref 26). Most aerospace materials consist of aluminum composites, which can be produced with near net shape and high-performance parts using laser additive manufacturing (LAM) (Ref 27). LAM is capable of printing in short period of time with controlling material’s mechanical properties and forming accuracy. The high energy density of the laser makes the material to fuse rapidly, resulting in improved mechanical properties with refined grain structure. The LAM is further classified based on partial and complete melting of powders, as shown in Fig. 7.

Fig. 7
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Classification of laser-metal additive manufacturing process

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Selective Laser Melting (SLM)

Among the entire laser-metal additive manufacturing processes, selective laser melting (SLM) tends to be a selective area for repeated cooling and heating cycles of the material, making the powder to solid (Ref 28). A laser beam focused on powder layers consecutively according to the 3D CAD model to build a 2D cross-sectional profile with ultra-thinness (Ref 29). The laser beam diameter used in SLM is typically 0.03 mm and can build a step thickness of 0.05 mm, which is sufficient to build complex metal parts. The process starts with roller makes a metallic powder layer upon the piston fabrication; the focused laser beam melts the powder bed in selective areas as a weld bead. Once the laser focusing is completed on selected areas on the first layer of the powder bed, piston fabrication is lowered in the Z direction with the prescribed length for next powder bed deposition (Ref 30). The second powder bed layer when focused with laser, the powder melts and merges with the previous melted areas. In this approach, the parts are fabricated, layer-upon-layer with ultra-thinness. The process is the interaction of laser radiation and metal powder, which may include the phase transformation, heat transfer and chemical reactions. The process has building speed of 5 cm/h−1 offering completely homogeneous and dense with no pores and mechanical properties of the elementary powder. The schematic view of selective laser melting process is given in Fig. 8.

Fig. 8
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Reprinted from Ref 29, with permission from Elsevier

Schematic view of selective laser melting process.

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A huge number of process parameters and their levels like powder layer thickness, laser power density, scanning speed, scanning strategy, powder bed temperature and powder properties of SLM have its combined effect on fabricated parts properties (Ref 31). Changes made in the process parameters have unfavorable results in surface morphology and mechanical properties of fabricated parts. SLM has its beneficial applications in biomedical implants and tissue engineering for printing dental prostheses with Ti6Al4V (Ref 32). The laser energy supplied by the laser beam to the unit volume of Ti6Al4V is 195 J/mm with a scanning speed of 1.8 cm3/h that is high when compared with Co-Cr-Mo alloys when checked with the diode-pumped Nd:Yag laser (Ref 33).

SLM is capable of creating fine porous structures with a variety of shapes; this made the process preferable in making implants. For biomedical implants, parts should have high porosity compared with their volume that can be controlled with the SLM scanning strategy parameter. One can achieve more than 450 channels and holes per 1 cm3 (Ref 34). The porous microstructure helps in improving the mechano-transduction between the human bone and implanted part. Adopting the sheet-based gyroid microarchitecture fulfills the criteria in offering high permeability, surface area and negligible mean curvature (Ref 35). Scaffolds with thicker wall size of microarchitecture and unit cell size within the range of trabecular bone showed improved tensile and compressive deformations. Tailoring the SLM parameters with thicker-walled gyroid construction resulted in further dominated results of tensile fatigue testing. Heat treatment of Co-Cr removable particle denture (RPD) frameworks has a significant role during fabrication through SLM (Ref 36). Heat-treated SLM fabricated framework showed that the formation of γ-face-centered with submicron scale grains exhibits highest retentive forces and elongation compared with SLM and casted fabrications (Ref 37). Build orientation during the SLM process will affect grain growth direction, surface roughness and Von Mises stress accumulation of fabricated clasps. SLM build Ti6Al4V at 90° orientation showed co-mixture of α+β phase with β grains sandwiched between acicular α grains. Among all the tested build directions, SLM90 clasps have good fatigue resistance and low surface roughness (Ref 36).

A series of diamond-based Pentamode material (PM) scaffolds fabricated with SLM technique showed an adjustable deformation with a change in strut slandering ratio. Application of double cone strut topology enhanced the cell migration, waste removal and supplement of nutrients to scaffold region as well as circumambient bone tissue. The permeability values and mechanical properties of SLM fabricated PM’s are in the range of trabecular bone (Ref 38).

Research toward developing Ti6Al4V alloy interbody fusion cages for lumbar spine made with SLM process acquires attention. It is evident that the average compressive modulus of SLM fabricated Ti6Al4V is 3 GPa, which falls between trabecular and cortical bone compressive module (0.5–15 GPa). SLM process facilitates in fabricating Ti6Al7Nb multi-spiked periarticular trabecular bone implants that offer cementless fixation (Ref 39). This implant has shown minimal invasive resurfacing even with multi-spiked scaffold. SLM is potential in fabricating the customized Mg-based composites for advanced corrosion resistance and cytocompatibility. Mg-based composite with KZ30 as matrix and 10 wt.% of bioactive glass 45S5 as reinforcement has shown more cytocompatible in in vitro cell viability test (Ref 40). Novel face and body centered structured β Ti35Zr28Nb alloy fabricated with SLM process showed elastic modulus and plateau strength as 1 GPa and 58 MPa, respectively, and are in the range of trabecular bone mechanical properties (Ref 41). The SLM process structure exhibits healthy growth, spreading and attaching osteoblast cells on a surface when cultured for 28 days.

Paste Extrusion Deposition

The above-discussed techniques on additive manufacturing of various materials ultimately depends on melting and deposition of material over the base substrate. The strength and integrity of melting powder and depositing on the substrate are found to be outstanding. But, this stands challenging during fabrication of high functioning magnesium alloy implants, which is very risky on melting and depositing. Magnesium is one of the viable alternatives for biomedical implants; the material should resemble bone tissue structure for effective growth of the blood tissues. The material used for bone tissue development is collagen type I and hydroxyapatite (Ref 42). Since the material is an organic- and inorganic-based substance; it could not withstand the temperatures generated during melting and deposition seen in previous AM-based processes. Hence, the paste extrusion deposition (PED) could be the best technique for fabricating such biomedical implants (Ref 43). Though the implants produced through this technique possess lesser strength when compared with other AM-based processes, the paste material used for the fabrication of magnesium implants has drugs which could contribute more in quick healing of the wounded part of the body. Another advantage of using magnesium as an implant in the human body is that removing implants from the body after the tissue gets healed is completely eradicated. The elastic modulus of the magnesium alloys (41-45 GPa) is almost closer to that of natural bone elastic modulus (3-20 GPa), while other metals (Fe and Zn) possess 211.4 GPa and 90 GPA as elastic modulus, respectively (Ref 6).

Paste extrusion deposition (PED) is a process of extruding paste from the syringe over the base plate, resulting in required 3D profile generation. After the deposition, the extruded component is allowed to get dried and made hardened. Throughout the entire fabrication process, the application of heat is completely eluded, thus the fabrication process does not affect the organic material present in the paste used for the extrusion process (Ref 44). During the PED of magnesium alloys, different percentages of gelatin are added to the Mg powders, which adds strength to the scaffolds. The layout of the PED process is shown in Fig. 9 (Ref 45).

Fig. 9
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Reprinted from Ref 45, with permission from Elsevier

Layout of PED process.

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Classification of Extrusion Methods

The PED method possesses two different phases of extrusion, namely steady-state and transient phase. In the steady-state phase of extrusion, the paste is extruded from the ram at a uniform rate for printing continuous filament, while in a transient phase of extrusion, the rate of extrusion varies during the process, especially at the start and stop of printing process (Ref 46).

The PED process is classified into three different methods based on the method of extrusion, namely ram extruder, shutter valve-based extruder and Auger extruder. The classification of the PED process is given in Fig. 10. Out of three different extrusion processes, the ram type paste extrusion deposition is commonly used for fabricating components used for various applications like biomedical, structural, semiconductors, etc. The ram extruder consists of the ram-driven plunger and a syringe, in which the flow of paste is controlled and regulated by the plunger movement (Ref 47). The flow rate of the paste, start and stop of the extrusion process completely depends on the plunger velocity and the force applied by the plunger on the paste material. The various types of extrusion mechanism are shown in Fig. 11 (Ref 47).

Fig. 10
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Classification of paste extrusion deposition

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Fig. 11
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Source: Extrusion-on-demand methods for high solids loading ceramic paste in free-form extrusion fabrication, Wenbin Li, Amir Ghazanfari, et al., Virtual & Physical Prototyping, Jul 3, 2017, reprinted by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com)

(a) Ram extruder, (b) shutter valve-based extruder and (c) Auger extruder.

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Fabrication through PED

Based on the advantages of PED process of additive manufacturing and various mechanisms used for extrusion processes, various researchers had experimented on different materials fabrication. The alumina components produced through free-form extrusion fabrication technique depend upon the rate of extrusion, stability and 3D motion of the ram used for extrusion. Other parameters which affect the quality of the deposition is the minimum deposition angle, which is helpful in fabricating the overhanging features of the component without any support structures. Also, the defect obtained during fabrication is the under-filling of the alumina paste during extrusion, which would reduce the strength of the fabricated component (Ref 48). Another group of researchers studied the filament printing quality of extrusion-on-demand method using three different extrusion mechanisms. Based on the filament line quality, repeatability and process robustness, the auger valve and needle valve-based process showed better results than the ram-based extruder. As for as continuity of the filament line is concerned, the ram type and needle valve type extruder show flow rate fluctuations in the paste used, compared with the auger type extrusion process (Ref 49). The materials like barium titanate (BaTiO3), N, N‐dimethylformamide and polyvinylidene fluoride are mixed through a simple mixing process and introduced into PED setup extrusion. This has wide applications in the areas of sensors, harvesting and energy storage. Hence, this simple mixing and PED combination help produce customizable and design flexible piezoelectric and dielectric devices for the future generation (Ref 50).

Apart from fabricating components for various structural applications through the PED process, it is very well suited for fabricating biomedical implants. Magnesium implants being a suitable alternative for bodily implants could be fabricated through this technique effectively. 1-9% gelatin, 1 wt.% hydroxypropyl methylcellulose and magnesium phosphate powder are mixed together and made as a paste. The prepared paste is taken in the syringe at 10-40°C and is used scaffold extrusion. The computer-controlled 3D porous scaffolds were fabricated at room temperature. The prepared scaffolds are immersed in the di-ammonium hydrogen phosphate for 3 hours duration under vacuum condition, to harden it (Ref 51). The glycerol gelatin mixed with quinine and ethanol is made as paste and introduced into the syringe for extrusion and are made to get dried to increase the strength of the component (Ref 52).

Thus, the PED process is found to be a viable and suitable process for fabricating biomedical implants with higher strength and excellent adaptability. The extruded components structure and material are found adaptable for the growth of cells or tissues in the body. This is one of the best methods of fabricating the implants for excellent human body adaptability. The magnesium material could be easily fabricated through the PED process because of its superior surface and printing quality and feasibility. Lightweight, biocompatibility, biodegradability, reduced stress shielding capability and low cost have made magnesium a suitable and viable choice for biomedical implants.

Future Scope of the Biomedical Implants Fabrication Through 3D Printing Technique

Based on the technologies available in today’s biomedical implants fabrication, additive manufacturing has gained a lot of interest in the bioimplants manufacturing sector which is mostly segmented into four categories like laser or electron beam melting, paste extrusion technology, friction stir processing and photo-polymerization. In future, additively manufactured medical products are mostly categorized as Prosthesis & implants, surgical instruments, surgical guides and tissue engineering products. Additive manufacturing is one of the most advanced techniques used in the growing health care industry for the ease to develop customized implants. Due to its advanced nature, the implants fabricated through this technique have better biocompatibility than conventional techniques and play an important role in bioprinting of complex organs. Several additive manufacturing companies like Envision TEC, 3D Systems, Inc., EOS GmbH Electro Optical Systems and others are now concentrated on launching innovative high-speed bioprinters, especially for dental laboratories. Increasing adaptation of additive manufactured surgical implants has been continuing to account for more than 60% of market revenue. In less than 20 years, we can expect a fully functioning heart printed through additive manufacturing. In future, it could be possible to collect stem cells from infant’s teeth and use it as a tool kit for his entire life for organs and tissues replacements through additive manufacture.