ReviewTopological design and additive manufacturing of porous metals for bone scaffolds and orthopaedic implants: A review
Introduction
Bone is a complex tissue that continually undergoes dynamic biological remodelling, i.e., the coupled process whereby osteoclasts resorb mature bone tissue followed by osteoblasts that generate new bone to maintain healthy homeostasis of bone [1]. This unique feature of bone underpins its ability to remodel itself to repair damage. However, when a bone defect exceeds a critical non-healable size, external intervention is required to supplement self-healing if the defect is to be bridged [2]. Despite recent advances in biomaterials and tissue engineering, repair of such a critical-sized bone defect still remains a challenge. The optimal choice is to use autograft (patients' own tissue) [3]. However, harvesting autograft tissue creates the morbidity associated with a second surgical site. An alternative choice is allograft tissue (taken from another person), which carries the risk of transmissible disease and depends on logistic circumstances (limited availability). The insufficiencies of application of autograft and allograft tissue have led to greater research efforts to identify biomimetic materials and structures that are suitable for skeletal repair without the inherent problems.
Metals and alloys have a long history of application as bone implants [4], [5], [6], [7]. Among them, the use of stainless steels, cobalt (Co) based alloys (CoCrMo), and titanium (Ti) and its alloys are well established due to their good biocompatibility, satisfactory mechanical strength and superior corrosion resistance [5]. However, implants made of these materials are usually much stiffer than natural bones, leading to stress shielding - a major source for bone resorption and eventual failure of such implants [5]. Cortical bone (compact bone) has elastic moduli ranging from 3 to 30 GPa, while trabecular or cancellous bone has significantly lower elastic moduli of 0.02–2 GPa. Most current implant materials have much higher moduli than those of bones, e.g., Ti6Al4V has a modulus of around 110 GPa and CoCrMo alloys have a modulus of around 210 GPa [5], [6], [8]. Therefore, to avoid stress shielding at the bone-implant interface, the equivalent Young's modulus and yield stress have to be adjusted when using these bulk materials. An effective method is to introduce adjustable porosity or relative density as proposed by Gibson and Ashby [9] for isotropic materials.
Traditional methods for fabricating open-cell porous metals include liquid state processing (direct foaming, spray foaming, etc.), solid state processing (powder metallurgy, sintering of powders and fibres, etc.), electro-deposition and vapour deposition [10], [11]. Although the shape and size of the pores can be adjusted by changing the parameters of these manufacturing processes, only a randomly organized porous structure can be achievable. However, additive manufacturing (AM) technologies can fabricate porous metals with predefined external shape and internal architecture [2], [12], [13], [14]. Metal-based additive manufacturing (MAM) techniques, such as selective laser melting (SLM) and electron beam melting (EBM), are computer controlled fabrication process based on layer-wise manufacturing principles. SLM [15], [16], [17] and EBM [18], [19] are increasingly used for the fabrication of porous metals with complex architecture. Instead of using electron beam as the energy source in EBM, the SLM technology uses laser beam with adjustable wavelength. Therefore, EBM can only process conductive metals whereas SLM can process polymer or ceramics as well as metal. Furthermore, due to more diffuse energy (larger heat-affected zone), EBM process has larger minimum feature size, median powder particle size, layer thickness, resolution and surface finish [20]. The robust application of MAM technologies requires extensive material, process and design knowledge, specific to each MAM technology [21]. MAM system behaviour is subject to significant stochastic error and experimental uncertainties, requiring that “assumptions are necessary to simplify the problem” [22]. Sources of error include: complex and transient heat transfer phenomena [23], geometric effects [24] with poorly defined powder thermal properties [25]. MAM prediction error can lead to excess melt pool temperature [26], resulting in undesirable microstructure, residual stress, local porosity, and surface roughness. Understanding the effects of design decisions on temperature related process defects is critically important to the process control. Comprehensive reviews of AM technologies can be found elsewhere [27], [28].
Recent successes in orthopaedic regenerative medicine have promised an exciting future of AM technology. The world's first additively manufactured mandible was implanted in a patient by Dr. Jules Poukens and his team in 2012 in Belgium [29]. A full lower jaw implant (mandible in Fig. 1) was coated with hydroxyapatite and implanted in an 83 year old lady. The porous implant was slightly heavier than a natural jaw, and provided robust attachment of muscles and sufficient space for nerves [29]. Skull reconstructions with AM parts have been performed successfully by using digital design and AM. Mertens et al. [30] successfully reconstructed a class III defect using AM manufactured titanium implants, which provided both midfacial support and a graft fixture (midface defect in Fig. 1). Jardini et al. [31] in Brazil designed and AM fabricated a customized implant for the surgical reconstruction of a large cranial defect. In 2014, Prof. Peter Choong, an Australian surgeon from St Vincent's Hospital, together with scientists from the Commonwealth Scientific and Industrial Research Organization (CSIRO) and Anatomics, successfully implanted the world's first 3D-printed titanium heel bone into a patient [32].
Typical design and application approaches of porous metallic implants normally include the design of scaffold, AM and post-processing (heat-treatment and surface modification) as illustrated in Fig. 1. This review aims to identify the current status and the future directions of design-oriented AM technology in producing porous metallic structures for bone tissue repair, with a particular emphasis on topological design of internal architecture of porous metals for bone implants.
Section snippets
Structure of bone
Bone is a natural composite containing both organic components (mainly type-I collagen, but also type-III, type-IV collagen and fibrillin) and inorganic crystalline mineral (e.g., hydroxyapatite, HA) [1], [22], [33], [34], as illustrated in Fig. 2. The structure of bone is similar to reinforced concrete that is used in the building industry. The function of HA crystals and collagen molecules are like the steel rod and cement to concrete: one part provides flexibility and the other provides
Porous metallic implants and topology optimization techniques
As mentioned previously, bone is a 3D inhomogeneous structure with elaborate features from macro-to nano-scales. While it is impossible, and perhaps unnecessary, to recreate all details of natural bone in the porous metallic implant, ideally the implant should have similar hierarchical configurations on multiple scales. It is essential that the implant should possess properties similar to the host bone and the ambient tissue [60]. This calls for a well-established design methodology integrating
Current status of AM and topology optimization in producing porous metallic structures
AM technologies are superior to conventional fabrication techniques for producing porous metallic implants with complex and customized structures, as shown in Fig. 5. In addition to the geometric flexibility, composites with two or more phases can be manufactured. These advantages enable AM to become a promising tool for the production of biomedical implant devices, controlled drug delivery systems, and engineered tissues [124], [125], [126], [127], [128], [129], [130], [131], [132], [133],
Heat-treatment
The mechanical properties of AM produced materials depend heavily on the processing parameters, including building layer thickness, scan speed, energy density and focal offset distance [200], [201]. Usually AM (SLM or EBM) produced materials have relatively high yield stress (Ti6Al4V, ∼1000 MPa) and ultimate tensile strength (Ti6Al4V, ∼1150 MPa), but a relatively low ductility (Ti6Al4V, less than 10%) [200], [202], [203]. In order to improve the mechanical properties of AM produced porous
Challenges and future directions
Additive manufacturing provides unprecedented opportunities for producing customized medical implants as this technology can fabricate structures of complex external shapes and intricate internal architectures. Topology optimization has become a powerful digital tool for the design of optimal structures and materials. The integration of these two technologies sees a promising future in designing and manufacturing biocompatible orthopaedic implants with desired mechanical properties and minimal
Conclusions
In this paper, the current status of the topological design of porous metallic implants and the fabrication of such implants using additive manufacturing is reviewed. First the mechanical properties of human bones are discussed. Then it is demonstrated that topology optimization is a powerful digital tool that can be used to obtain optimal internal architectures for porous implants which not only satisfy multifunctional requirements but also mimic human bones. Furthermore it is shown that
Acknowledgements
This work was supported by the Australian Research Council (DP140100213 and LP140100607).
References (221)
- et al.
Bone tissue engineering using 3D printing
Mater. Today
(2013) - et al.
Biocompatibility of Ti-alloys for long-term implantation
J. Mech. Behav. Biomed. Mater.
(2013) - et al.
Titanium alloys in total joint replacement - a materials science perspective
Biomaterials
(1998) Recent research and development in titanium alloys for biomedical applications and healthcare goods
Sci. Tech. Adv. Mater.
(2003)- et al.
Surface modification of titanium, titanium alloys, and related materials for biomedical applications
Mater. Sci. Eng. R.
(2004) Manufacture, characterisation and application of cellular metals and metal foams
Prog. Mater. Sci.
(2001)- et al.
Micro-CT-based improvement of geometrical and mechanical controllability of selective laser melted Ti6Al4V porous structures
Mater. Sci. Eng. A
(2011) - et al.
Sintering of commercially pure titanium powder with a Nd:YAG laser source
Acta Mater.
(2003) - et al.
Bone tissue engineering using polycaprolactone scaffolds fabricated via selective laser sintering
Biomaterials
(2005) - et al.
Cellular Ti-6Al-4V structures with interconnected macro porosity for bone implants fabricated by selective electron beam melting
Acta Biomater.
(2008)
Mechanical evaluation of porous titanium (Ti6Al4V) structures with electron beam melting (EBM)
J. Mech. Behav. Biomed. Mater.
Image data based reconstruction of the midface using a patient-specific implant in combination with a vascularized osteomyocutaneous scapular flap
J. Cranio. Maxill. Surg.
The elastic moduli of human subchondral, trabecular, and cortical bone tissue and the size-dependency of cortical bone modulus
J. Biomech.
Young's modulus of trabecular and cortical bone material: ultrasonic and microtensile measurements
J. Biomech.
Mechanical properties and the hierarchical structure of bone
Med. Eng. Phys.
Whole bone mechanics and mechanical testing
Vet. J.
Future directions in biomaterials
Biomaterials
Accurate measurement of cortical bone elasticity tensor with resonant ultrasound spectroscopy
J. Mech. Behav. Biomed. Mater.
Porosity of 3D biomaterial scaffolds and osteogenesis
Biomaterials
Peri- and intra-implant bone response to microporous Ti coatings with surface modification
Acta Biomater.
Effect of pore size on bone ingrowth into porous titanium implants fabricated by additive manufacturing: an in vivo experiment
Mater. Sci. Eng. C
Hierarchical tailoring of strut architecture to control permeability of additive manufactured titanium implants
Mater. Sci. Eng. C
Current trends in the design of scaffolds for computer-aided tissue engineering
Acta Biomater.
Creation of a unit block library of architectures for use in assembled scaffold engineering
Comput. Aided Des.
Investigation of the mechanical properties and porosity relationships in selective laser-sintered polyhedral for functionally graded scaffolds
Acta Biomater.
Bio-CAD modeling and its applications in computer-aided tissue engineering
Comput. Aided Des.
Optimal design and fabrication of scaffolds to mimic tissue properties and satisfy biological constraints
Biomaterials
Minimal surface scaffold designs for tissue engineering
Biomaterials
Schwarz meets Schwann: Design and fabrication of biomorphic and durataxic tissue engineering scaffolds
Med. Image Anal.
Porous scaffold design using the distance field and triply periodic minimal surface models
Biomaterials
Properties of solid core and porous surface Ti-6Al-4V implants manufactured by powder metallurgy
J. Alloys Compd.
The COC algorithm, Part II: topological, geometrical and generalized shape optimization
Comput. Methods Appl. Mech. Eng.
A simple evolutionary procedure for structural optimization
Comput. Struct.
Materials with prescribed constitutive parameters: an inverse homogenization problem
Int. J. Solids Struct.
Tailoring materials with prescribed elastic properties
Mech. Mater
Design of materials with extreme thermal expansion using a three-phase topology optimization method
J. Mech. Phys. Solids
A novel method for biomaterial scaffold internal architecture design to match bone elastic properties with desired porosity
J. Biomech.
Design of maximum permeability material structures
Comput. Methods Appl. Mech. Eng.
Optimizing multifunctional materials: design of microstructures for maximized stiffness and fluid permeability
Int. J. Solids Struct.
On stiffness of scaffolds for bone tissue engineering-a numerical study
J. Biomech.
Bones: Structure and Mechanics
3D modeling, custom implants and its future perspectives in craniofacial surgery
Ann. Maxillofac. Surg.
The effect of spinal implant rigidity on vertebral bone density: a canine model
Spine
Cellular Solids: Structure and Properties
Fabrication methods of porous metals for use in orthopaedic applications
Biomaterials
Porous scaffold design for tissue engineering
Nat. Mater.
Next-generation biomedical implants using additive manufacturing of complex cellular and functional mesh arrays
Phil. Trans. R. Soc. A
Bioactive Ti metal analogous to human cancellous bone: fabrication by selective laser melting and chemical treatments
Acta Biomater.
Additive Manufacturing Technologies
Redesign and cost estimation of rapid manufactured plastic parts
Rapid Prototyp. J.
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