1. 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, HW , 65A Nanyang Drive, Singapore
637333,
Singapore Centre for 3D Printing, School of Mechanical & Aerospace Engineering, Nanyang Technological University, HW , 2A
Nanyang Link, Singapore ,
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore
מערכות יצור
Presentation by:
Jomi Kramer (TZ ) and Tom Kertesz (TZ )

2. 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)

3. 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.

4. 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.

5. 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.

6. Additive manufacturing of Orthopedic Implants
Selective Laser Melting
Electron Beam Melting

7. 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

8. 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

9. 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

10. 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.

11. 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.

12. 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

13. 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.

14. Data Acquisition for Patient-Specific Design

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

16. 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.

17. 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

18. Cobalt-Chromium (CoCr)
Various conclusions about suitability for dental applications
Relative Density 99.94%
Strength and Hardness SLMUts : 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

19. 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

20. 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 : EBM Uts-960
New Materials

21. 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.

22. 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.

23. 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.

24. 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.

25. 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

26. Questions?