Laser and Electron-Beam Powder-Bed Additive Manufacturing of Metallic Implants: A Review on Processes, Materials and Designs Article by: Swee Leong Sing, Jia An, Wai Yee Yeong, Florencia Edith Wiria NOTE ABOUT THE AUTHORS OF THE PAPER: Mr. Swee Leong Sing is the main author of the paper. His contribution includes the research for literature for the bulk of this review paper, inclusive of describing selective laser melting and electron beam melting, design aspects, material properties and biological response, as well as the overall outline and flow of the paper. Dr. Jia An contributes to the biological aspects in the paper and link them to the designs and material properties. Dr. Wai Yee Yeong and Dr. Florencia Edith Wiria contribute to various sections of the paper for material consistency and language. They also ensures that the paper is concise and easy to digest for readers. The authors hereby confirm that this manuscript is their original work and has not been published nor has it been submitted simultaneously elsewhere. Moreover, they confirm that all authors have checked the manuscript and have agreed to the submission. SIMTech-NTU Joint Laboratory (3D Additive Manufacturing), Nanyang Technological University, HW3-01-01, 65A Nanyang Drive, Singapore 637333, Singapore Centre for 3D Printing, School of Mechanical & Aerospace Engineering, Nanyang Technological University, HW1-01-05, 2A Nanyang Link, Singapore 637372, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075 מערכות יצור Presentation by: Jomi Kramer (TZ 328794003) and Tom Kertesz (TZ 308476142)

Key Words Additive manufacturing 3D printing Rapid prototyping Selective laser melting (SLM) Electron beam melting (EBM) Stainless Steel Titanium-6Aluminium-4Vanadium Cobalt-Chromium (CoCr) Stainless Steel Titanium-6Aluminium-4Vanadium Cobalt-Chromium (CoCr)

Additive manufacturing (AM) = 3D printing Additive manufacturing is a group of processes that join materials to make objects from 3D model data, usually layer-by-layer. Additive manufacturing (AM), also commonly known as 3D printing: allows the direct fabrication of functional parts with complex shapes from digital models. Numerous studies applying AM techniques in tissue engineering. advancement in AM techniques and materials Functionality (of AM )- extended to the orthopedic implants.

Additive manufacturing Key advantages: Fewer design constraints than conventional manufacturing techniques More complex geometries in similar building time No tooling or molds required Fabrications of several patients can be fabricated in the same batch Greater freedom of design to product Advantage over binder-based processes in direct part manufacturing.

Additive manufacturing of Orthopedic Implants Powder bed fusion processes use an energy source to melt and fuse selective regions of powder according to CAD data. When the selective melting of one layer is completed, the building platform is lowered by a predetermined and a next layer of powder is deposited on the platform. The process is then repeated until the required part is completely built. Additive manufacturing of Orthopedic Implants Powder bed fusion processes use an energy source to melt and fuse selective regions of powder according to computer aided design (CAD) data. Afterwards the building platform is lowered by a predetermined distance (usually 20–100 mm for SLM and 100mm for EBM) The next layer of powder is deposited on the platform. The process is repeated until the required part is completely built. The exclusion of sacrificial binders in the process enables near-full density parts to be built.

Additive manufacturing of Orthopedic Implants Selective Laser Melting Electron Beam Melting

Selective Laser Melting Selective melting of a powder layer based on the geometry defined by the CAD file is done by the laser. Energy source = a fibre laser source The process is carried out in an inert gas filled chamber ensures higher purity by minimizing the oxygen in the environment SLM— selective melting of the powder layer based on the geometry defined by the CAD file is done by the laser. energy source= a fibre laser source Process is carried out in an inert gas filled chamber which ensures higher purity by minimizing the oxygen in the environment This reduces the risk of hydrogen pick up. Parts: Fibre laser, which can operate up to 1 kW depending on the laser module installed The beam focus is controlled by the galvanometer and the movement of the beam on the build table is controlled by F-theta lens. Building a part: a powder layer of 20–100 mm thickness is spread over the table. The powder is carried and spread by the powder recoater across the build table. The build table can be preheated up to 200°C. Every layer of a part is built in two steps. The outer boundary of the part is built -referred as contouring the powder within the contour is melted subsequently to complete one layer. Process continues until the desired 3D part is fully completed

Selective Laser Melting Parts of SLM: Fibre laser: The beam focus is controlled by the galvanometer The movement of the beam on the build table is controlled by F-theta lens. SLM— selective melting of the powder layer based on the geometry defined by the CAD file is done by the laser. energy source= a fibre laser source Process is carried out in an inert gas filled chamber which ensures higher purity by minimizing the oxygen in the environment This reduces the risk of hydrogen pick up. Parts: Fibre laser, which can operate up to 1 kW depending on the laser module installed The beam focus is controlled by the galvanometer and the movement of the beam on the build table is controlled by F-theta lens. Building a part: a powder layer of 20–100 mm thickness is spread over the table. The powder is carried and spread by the powder recoater across the build table. The build table can be preheated up to 200°C. Every layer of a part is built in two steps. The outer boundary of the part is built -referred as contouring the powder within the contour is melted subsequently to complete one layer. Process continues until the desired 3D part is fully completed

Selective Laser Melting Steps to Building a part: A powder layer of 20–100 mm thickness is spread over the table. The powder is carried and spread by the powder recoater across the build table. The build table can be preheated up to 200°C. Every layer of a part is built in two steps: The outer boundary of the part is built -referred as contouring The powder within the contour is melted subsequently to complete one layer. SLM— selective melting of the powder layer based on the geometry defined by the CAD file is done by the laser. energy source= a fibre laser source Process is carried out in an inert gas filled chamber which ensures higher purity by minimizing the oxygen in the environment This reduces the risk of hydrogen pick up. Parts: Fibre laser, which can operate up to 1 kW depending on the laser module installed The beam focus is controlled by the galvanometer and the movement of the beam on the build table is controlled by F-theta lens. Building a part: a powder layer of 20–100 mm thickness is spread over the table. The powder is carried and spread by the powder recoater across the build table. The build table can be preheated up to 200°C. Every layer of a part is built in two steps. The outer boundary of the part is built -referred as contouring the powder within the contour is melted subsequently to complete one layer. Process continues until the desired 3D part is fully completed

Electron Beam Melting EBM is a metal additive manufacturing technique which is believed to revolutionize the implant manufacturing industry. Energy source =electron beam in a vacuum chamber The process utilizes electron beam energy to melt the metal powder EBM is a metal additive manufacturing technique which is believed to revolutionize the implant manufacturing industry. Process – conceptualized and patented by Arcam AB based in Sweden. energy source =electron beam in a vacuum chamber The process utilizes electronbeam energy to melt the metal powder. building the part: elevated temperature of about 700°C is maintained in the chamber to reduce residual stresses EBM system electron gun similar to an electron gun in a scanning electron microscope or an electron beam welding machine operates at a power of 60kW to generate a focusedbeam of energy density above 100 kW/cm2. The beam focus is controlled by the electromagnetic lenses and the movement of the beam on the build table is controlled by deflection coils. Building a part: a powder layer of 100m thickness is spread over the table. The powder is supplied from two hoppers kept inside the build chamber. A moving rake fetches powder from both sides and spread over the table. electron beam first pre-heats the powder layer with a higher scan speed, followed by melting the powder layer based on the geometry defined by the CAD file. every layer of a part is built in two steps. outer boundary of the part is built first which is referred as contouring and the powder within the contour is melted subsequently to complete one layer. This process continues until the desired three-dimensional part is fully completed. EBM has been used to produce orthopaedic components such as knee, hip, jaw replacements, and maxillofacial plates.14–17 EBM produced implants, such as acetabular cups, have also gained approval from United States Food and Drug Administration (FDA) and are CE-certified since 2010 and 2007, respectively.

Electon Beam Melting Parts of EBM: Fibre laser Steps to Building a part: A powder layer of 20–100 mm thickness is spread over the table. Powder is carried and spread by the powder recoater across the build table The build table can be preheated up to 200°C. Every layer of a part is built in two steps. The outer boundary of the part is built -referred as contouring The powder within the contour is melted subsequently to complete one layer. Process continues until the desired 3D part is fully completed EBM is a metal additive manufacturing technique which is believed to revolutionize the implant manufacturing industry. Process – conceptualized and patented by Arcam AB based in Sweden. energy source =electron beam in a vacuum chamber The process utilizes electronbeam energy to melt the metal powder. building the part: elevated temperature of about 700°C is maintained in the chamber to reduce residual stresses EBM system electron gun similar to an electron gun in a scanning electron microscope or an electron beam welding machine operates at a power of 60kW to generate a focused beam of energy density above 100 kW/cm2. The beam focus is controlled by the electromagnetic lenses and the movement of the beam on the build table is controlled by deflection coils. Building a part: a powder layer of 100m thickness is spread over the table. The powder is supplied from two hoppers kept inside the build chamber. A moving rake fetches powder from both sides and spread over the table. electron beam first pre-heats the powder layer with a higher scan speed, followed by melting the powder layer based on the geometry defined by the CAD file. every layer of a part is built in two steps. outer boundary of the part is built first which is referred as contouring and the powder within the contour is melted subsequently to complete one layer. This process continues until the desired three-dimensional part is fully completed. EBM has been used to produce orthopaedic components such as knee, hip, jaw replacements, and maxillofacial plates.14–17 EBM produced implants, such as acetabular cups, have also gained approval from United States Food and Drug Administration (FDA) and are CE-certified since 2010 and 2007, respectively.

Design Considerations for Orthopedic Implants using Additive Manufacturing Design constraints for an optimal implant Design dependent porosity for osteo-inductive implants Surface topology of implants Reduction of stress-shielding Data acquisition The human skeleton consists of two types of bone, namely, cortical, and trabecular bones. These two types of bone differ in terms of proportions of organic and inorganic materials, degree of porosity and organization. Thus orthopaedic implants require precise designs of pores and porosities in order to mimic the bone properties closely

Data Acquisition for Patient-Specific Design Manufacturing of custom made implants The manufacturing of custom made implants necessitates the recording of the patient’s anatomical data by scanning processes and medical imaging such as magnetic resonance imaging (MRI) and radiography (X-ray). The patient’s anatomical data The patient’s anatomical data is then reconstructed in 3D through medical image processing and implants geometry modelling such as addition of features that are specific and achievable by AM.

Data Acquisition for Patient-Specific Design

Additive Manufactured Materials Stainless Steel Titanium-6Aluminium-4Vanadium Cobalt-Chromium (CoCr)

Stainless Steel Low-cost and easily available Can be individualized Very low customization costs Relative Density 99.90/5% Strength and Hardness Uts : 316L 316L Stainless Steel This material is low-cost and easily available and thus suitable in the medical industry as a biocompatible metal bone implant. it is well suited for applications as implants or prostheses can be individualized very low customization costs. Various research by the authors showed: Yang et al. investigated the optimization of building accuracy and density of orthodontic products using a self-developed SLM machine and they were able to achieve the required surface quality and mechanical properties. Li et al. studied the possibility of making SLM 316L stainless steel parts with gradient porosity where the dense portion is designed for strength and the porous part is designed to enhance tissue growth in biocompatible implants. Bibb et al. reported SLM denture frameworks with the same material. presented 4 case studies on surgical guides in different maxillofacial (jaw and face) surgeries. Kruth et al. published on a biocompatible metal framework for dental prostheses Wehmoller et al. reported body implants of cortical bone, mandibular canal segment and support structures or tubular bone made from SLM 316L stainless steel.

Titanium-6Aluminium-4Vanadium (Ti6Al4V) Biocompatibility Superior corrosion resistance High mechanical strength High specific strength Elastic moduli closer to bone Relative Density 99.98% Strength and Hardness depends on final microstructure Most research on titanium and its alloys is driven by its potential application as body prostheses due to their biocompatibility Ti6Al4V are of high interest because: applications in both aerospace and biomedical industries. This group of metallic materials has been widely used for various orthopaedic implants good biocompatibility Superior corrosion resistance and high mechanical strength high specific strength and elastic moduli closer to bone than CoCr alloys and stainless Steel Performance requirements for implants made in Ti6Al4V alloy are specified by ASTM standards and US FDA. Ti6Al4V components can be produced with variety of microstructures depending on the method used for processing the alloy: casting, wrought ingots and powder metallurgy give three different microstructures for Ti6Al4V. -- because for pure Ti, the microstructure is completely Whenpure Ti is alloyed with a and b stabilizers phase forms along the grain boundary. The percentage of a and b phases varies depending upon the processing conditions such as the temperature, cooling rates and degree of mechanical working. Research into Ti6Al4V body implants using SLM have been done by several groups. Lin et al. studied the structure and mechanical properties of a Ti6Al4V cellular inter-body fusion cage, Murr et al. focused their attention on the microstructure and mechanical properties of SLM Ti6Al4V for biomedical applications Warnke et al. conducted cell experiments and showed that SLM Ti6Al4V porous scaffolds allow total overgrowth of osteoblasts (bone cells). Vandenbroucke & Kruth examined the dimensional accuracy of the SLM process for fabrication of dental frameworks. Biemond et al. examined the bone in-growth potential of trabecular-like implant surfaces produced by SLM of Ti6Al4V in goats and concluded that the SLM produced parts showed good bone in-growth characteristics after 15 weeks

Cobalt-Chromium (CoCr) Various conclusions about suitability for dental applications Relative Density 99.94% Strength and Hardness SLMUts : 562-884 EBM Uts-960 CoCr has been studied by various groups for implant applications. Oyague et al. and Kim et al. separately evaluated the fit of dental prostheses produced by SLM They reached different conclusions about the suitability of SLM technology in producing dental prostheses. Qualities: hardness, elastic modulus and strength, Ayyildiz et al. concluded that CoCr produced by laser AM is suitable for dental applications

New Materials Other titanium based alloys Magnesium and its alloys fully bioresorbable mechanical properties aligned to bone induce no inflammatory response osteo-conductive encourage bone growth have a role in cell attachment higher corrosion resistance and bio-tolerance

Additive Manufactured Materials Stainless Steel Relative Density 99.90/5% Strength and Hardness Uts : 316L Surface Roughness: 5.82mm Titanium-6Aluminium-4Vanadium Relative Density 99.98% Strength and Hardness depends on final microstructure Cobalt-Chromium (CoCr) Relative Density 99.94% Strength and Hardness SLMUts : 562-884 EBM Uts-960 New Materials

Challenges in Fabrication of Cellular Lattice Structures Stochastic lattice structures – random variations in the shape and size of the cells Periodic lattice structures – repeating lattice structures that can be categorized by their shapes and sizes Development through research: CASTS (computer-aided system for tissue scaffolds) – a software that can assemble 13 designed unit cells into porous structures which can be modified easily to adapt to particular patient needs Cellular lattice structures can be classified into structures with stochastic and non-stochastic geometries. Stochastic lattice structures have random variations in the shape and size of the cells, whereas, non-stochastic or periodic lattice structures have repeating lattice structures and can be categorized by their shapes and sizes. There are many ongoing researches on the fabrication of cellular lattice structures by SLM and EBM, focusing on the dimensional accuracy of the fabricated structures, mechanical properties and biocompatibility of these structures. Researches have also been done on development of automated algorithm for cellular lattice structures. One of the in-house developed software, called the “computer-aided system for tissue scaffolds” (CASTS), can assemble 13 designed unit cells 2 into porous structures which can be modified easily. CASTS allows the designs with size and shape adapted to particular patient to be produced by AM. Examples show that porous structures with controlled porosity can be generated directly using CASTS.

Challenges in Fabrication of Cellular Lattice Structures Examples show that porous structures with controlled porosity can be generated directly using CASTS Cellular lattice structures samples clearly demonstrated the capability of powder bed fusion AM in producing cellular lattice structures from CAD models in different shapes Cellular lattice structures can be classified into structures with stochastic and non-stochastic geometries. Stochastic lattice structures have random variations in the shape and size of the cells, whereas, non-stochastic or periodic lattice structures have repeating lattice structures and can be categorized by their shapes and sizes. There are many ongoing researches on the fabrication of cellular lattice structures by SLM and EBM, focusing on the dimensional accuracy of the fabricated structures, mechanical properties and biocompatibility of these structures. Researches have also been done on development of automated algorithm for cellular lattice structures. One of the in-house developed software, called the “computer-aided system for tissue scaffolds” (CASTS), can assemble 13 designed unit cells 2 into porous structures which can be modified easily. CASTS allows the designs with size and shape adapted to particular patient to be produced by AM. Examples show that porous structures with controlled porosity can be generated directly using CASTS.

Functionally Graded Materials Composite materials formed from two or more constituent phases with a continuously variable composition or structures: Reduction in stress shielding on surrounding bones Improved biocompatibility with bone tissues Meeting biomechanical requirements at all bone regions Enhancing bone re-modelling Maintaining the bone health Functionally Graded Materials FGM are composite materials formed from two or more constituent phases with a continuously variable composition or structures. In addition, FGM has the potential to eliminate the problem of bone re-modeling due to mismatch in mechanical properties. FGM signifies a new class of composites, which consists of a graded pattern of material compositions, microstructures or macrostructures, allowing better matching of corresponding mechanical properties. The main advantages of using FGM implant are reduction in stress shielding effect on the surrounding bones, improvement of biocompatibility with bone tissues and meeting the biomechanical requirements at each region of the bone while enhancing bone re-modelling and maintaining the bone health. SLM and EBM permit the creation of very complex geometries with a gradient of porosity perpendicular to the longitudinal axis of the implant which in turn allows the choice of property distribution to achieve required functions. Cheng et al. fabricated cellular structures that have comparable compressive strength similar to those of trabecular and cortical bone and studied them. The study concluded that gradient functions in both biochemical affinity to osteogenesis and the mechanical properties with stress relaxation effect could contribute to efficient biocompatibility. It also demonstrated that tissue reaction changed in response to the changes in gradient composition or structure of materials, which implied that tissue response could be controlled using FGM. Lin et al. studied design of dental implants using FGM. The study found out that by lowering the FGM gradient, a better performance in bone turnover could be achieved, however, this would reduce the stiffness of implantation which would lead to higher risk of damage during the early healing stage.

Multi-Material Processing Due to the varying requirement at different parts of the implants, metallic implants require the use of multi-material design in a single structure. Different alloy mixtures experimented by Liu and Sing using SLM showed good characteristics, but some showed problematic ones as well: undesirable thermal stresses formation of brittle intermetallic phases These studies open up the opportunity to explore other alloy systems that are suitable for orthopaedic applications Multi-Material Processing SLM and EBM built structures are usually restricted to one type of material. However, for metallic implants, due to the varying requirement at different parts of the implants, there is often a need for multi-material design in a single structure. Multi-material processing has been carried out by AM systems that use directed energy deposition process such as Optomec’s laser engineered near net shape (LENS) system and selective laser sintering (SLS) systems. Very little research of the subject has been done so the properties of such parts are not fully understood. Recently, Liu and sing et al. experimented with multi-material processing in SLM and achieved good characteristics using a few different mixtures of alloys, but undesirable thermal stresses, due to differences in physical properties, and formation of brittle intermetallic phases upon joining of both materials were observed in some cases. Nonetheless, these studies open up the opportunity to explore other alloy systems that are suitable for orthopaedic applications.

Conclusion and Outlook Advancement in AM technologies allows for fully functional parts to be manufactured directly. SLM and EBM provide the orthopaedic field the opportunities to mass customize implants at a lower cost Studies show the immense potential of EBM and SLM, however, there are challenges in some areas that need to be overcome: Material Research Biocompatibility of SLM and EBM Produced Orthopaedic Implants Designed of Additively Manufactured Orthopaedic Implants CONCLUSION AND OUTLOOK With the advancement in AM technologies, it is now able to fabricate fully functional parts directly. In particular, SLM and EBM provide the orthopaedic field the opportunities to mass customize implants at a lower cost due to their ability to fabricate parts with complex and intrinsic designs that are specific to individual patients. To summarize, all the above studies show the immense potential of EBM and SLM to become the more preferred method for producing orthopaedic implants in future. However, there are still research challenges that need to be overcome, in the following areas: Material Research Biocompatibility of SLM and EBM Produced Orthopaedic Implants Designed of Additively Manufactured Orthopaedic Implants

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