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Multidisciplinary Materials Chronicles

Home / Journals / Materials Science / Multidisciplinary Materials Chronicles

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Received: Jan. 11, 2024; Accepted: Jan. 30, 2024; Published Online: Feb. 27, 2024


Porous Titanium for Medical Implants

Walaa Abd-Elaziem1, Moustafa M. Mohammed*2, Hossam M. Yehia3, Tamer A Sebaey4 , Tabrej Khan4

1 Department of Mechanical Design and Production Engineering, Faculty of Engineering, Zagazig University, 44519, Egypt

2 Mechanical Department, Faculty of Technology and Education, Beni-Suef University, Beni Suef 62511, Egypt

3 Production Technology Department, Faculty of Technology and Education, Helwan University, Saray-El Qoupa, El Sawah Street, Cairo 11281, Egypt;

4 Department of Engineering Management, Faculty of Engineering, Prince Sultan University, Riyadh 11586, Saudi Arabia

 https://doi.org/10.62184/mmc.jmmc100020241


Keywords

Porous titanium, Titanium alloys, Biomedical implants, Additive manufacturing, Elastic modulus


Abstract

    Porous titanium and its alloys have shown immense promise as orthopedic and dental implant materials owing to their outstanding properties, namely tailorable porosity, the ability of blood vessels and bone ingrowth, the transport of nutrients and/or biofluids, and vascularization. The previously mentioned properties facilitate osseointegration, a crucial device integration and stability factor. The presented review investigates the influence of pore characteristics of porous titanium and its alloys (e.g., size, shape, interconnectivity, and gradients) on biological response, mechanical properties, and key considerations in scaffold design. Recent literature showed that the progress of porous titanium and its alloys is summarized in biomaterials, specifically the processing techniques utilized in fabricating porous. Accordingly, recent advances in the previously stated processing techniques are powder metallurgy, additive manufacturing, plasma spraying, etc., which are applied in constructing optimized porous architectures. Overall, porous titanium structures with controlled porosity and tailored pore networks can promote bone ingrowth and long-term stability, thereby overcoming the limitations of traditional dense titanium (Ti) implants.



1.  Introduction

    The organs and tissues in the human body have precise and complex functions. Our lives always depend on the performance of these organs versus their proper functioning. However, these organs and tissues are sometimes damaged and can fail due to diseases or accidents. There are many medical treatments available that can help treat damaged organs, but many of these treatments still lack the ability to repair the organ until it regains its full function. The field of regenerative medicine seeks to provide new tools that repair or replace damaged organs and tissues [1]. Regenerative medicine uses tissue engineering to develop new strategies for repairing damaged organs and tissues. Tissue engineering involves biology, chemistry, and engineering to make new biomaterials compatible with the human body and can be used to repair or replace damaged organs and tissues [2]. Biomaterials have improved significantly since their first development and are still changing as scientists try to better understand diseases and how these materials interact with the body. Biomaterials can take many forms and are produced from many different materials. Ideally, biomaterials should have a porous structure with small holes that allow air, fluids, and even cells to pass through, like the organs and tissues they are intended to treat [3]. Cells aiding in the healing process are loaded into these tiny holes that permeate the biomaterial [2]. In this way, a porous biomaterial can be used to transfer cells to damaged tissue. The biomaterial helps maintain new cells in tissues and, at the same time, is necessary to promote the healing process. Moreover, the porous structure of the biomaterial closely resembles the “exocellular matrix,” which resembles the hooks by which cells in the body are “grabbed” [2, 4].

    Titanium and its alloys are extensively used in biomedical implants because they offer a combination of high strength, lightness, resistance to corrosion, and compatibility with biological tissues. Currently, materials used in these applications include 316L stainless steel, cobalt-chromium (Co-Cr) alloys, and Ti-based alloys (specifically Ti-6Al-4V) [5, 6]. However, these materials sometimes fail after prolonged due to their stiffness relative to bone, inadequate wear and corrosion resistance, and insufficient compatibility with biological tissue. Achieving a critical stiffness matching between orthopedic implants and adjacent bone is crucial in avoiding stress shielding, bone resorption, and implant failure. Despite advances in biomaterials and tissue engineering, ongoing research focuses on developing durable metallic implants due to concerns about the long-term performance of current metallic biomaterials.

   Porous titanium structures have demonstrated immense potential to transform orthopedic and dental implant performance via tailorable interconnected pore networks facilitating bone infiltration and vascularization. It is reported that the elastic modulus of these porous titanium structures is significantly lower, ranging from 91% to 96%, compared to dense Ti alloys [7]. Moreover, porous titanium and its alloys have emerged as promising candidate materials for orthopedic and dental implants owing to their outstanding biocompatibility, corrosion resistance, and mechanical properties closer to natural bone [8-10]. Incorporating of controllable porosity in titanium implants facilitates bone ingrowth, transport of nutrients and/or biofluids, and vascularization, promoting osseointegration for device integration and stability. Subsequently, it can significantly enhance post-implantation healing outcomes and long-term implant lifespan performance. Accordingly, extensive research has focused on engineering Ti implant surfaces and bulk structures with optimized porous architectures. It includes the fabrication of porous coatings on conventionally dense implants as well as the printing of fully porous titanium components using additive manufacturing. Additionally, processing innovations, pore topography, interconnectivity, and gradients influence biological responses [11, 12].

    A prime determinant of in-vivo device integration is the effective pore volume fraction. Studies showed that ≥ 50% porosity enables sufficient bone ingrowth [13]. Wang et al. [14] found that in the context of early-stage osteonecrosis of the femoral head (ONFH) after core decompression, a biogenic trabecular porous Ti rod that is manufactured through the selective laser melting technique exhibited notable quantitative advantages over core decompression alone. The rod group demonstrated significantly higher ratios of bone volume to total volume (BV/TV) at both 3 months (890.0% increase) and 6 months (438.1% increase) compared to the core decompression (CD) group. The histological analysis supported these findings, showing substantial improvements in BV/TV in the rod group, with increases of 881.0% at 3 months and 413.3% at 6 months. Woodard et al. [15] found that hydroxyapatite (HA) scaffolds with multi-scale porosity, specifically microporous (MP) scaffolds with both microporosity (250 - 350 μm) and microporosity (2 - 8 μm), demonstrated superior osteoconductivity, drug-carrying efficacy, and mechanical properties compared to non-microporous (NMP) scaffolds. Thus, it is showcases the importance of scaffold microporosity for bone ingrowth and mechanical behavior in HA implant materials. The profound influence of porous titanium surface topographies on modulating osteoblast cell morphology, adhesion, differentiation, and mineralization also underlines the role of multiple hierarchical porosity. The presented study provides a short review of the current advances in porous titanium-based biomaterials for medical implants. 

    The review explores the impact of pore characteristics on biological and mechanical aspects and summarizes recent progress in processing techniques for constructing optimized porous architecture.

2.Titanium Properties

   The widespread use of Ti in medical implants is grounded in its exceptional biocompatibility, with attributes inertness, a stable oxide layer, and the potential for osseointegration. Its strength-to-weight ratio and flexibility, resembling human bone's elastic modulus, result in robust yet lightweight implants, essential for load-bearing applications such as joint replacements. The adsorption of proteins onto Ti surfaces plays a crucial role in osseointegration, influencing cellular responses and promoting implant integration [16]. Beyond its neutral interaction with the body, Ti actively fuses with bone, ensuring the stability and durability of implants, particularly in dental and orthopedic applications. With minimal ion release, Ti exhibits corrosion resistance inside the human body, avoiding potential harm or allergic reactions. Its non-magnetic nature makes it ideal for patients requiring MRI scans, ensuring safety and compatibility.

    The innate oxide layer grants passive corrosion protection while permitting biofunctionalization that is essential for integration. However, elastic moduli still exceed natural bone, leading to stress-shielding long-term failures. Figure 1 shows the failure causes of implants leading to revision surgery.

    Thus, ongoing research scrutinizes the absolute biocompatibility of Ti alloys, leading to the development of new beta Ti (β-Ti) alloys with non-toxic elements (e.g., Tantalum (Ta), Niobium (Nb), and Zirconium (Zr)). These alloys show promise in improved biocompatibility, strength, and wear resistance, addressing concerns associated with traditional Ti alloys such as Ti-6Al-4V. The quest for materials devoid of cytotoxic elements and with a low elastic modulus continues, fueled by the need for enhanced biomaterials in medical applications. Hence, porous titanium structures better emulate the cellular architecture of bone through an interconnected, open-cell network with adjustable porosity and pore sizes while harnessing the advantages of dense titanium/alloy compositions.

                                                                                

Figure 1. The causes for failure of implants leading to revision surgery.


3.Types of Titanium-Based Alloys

    The biocompatibility and mechanical properties of titanium and its alloys, vital for medical implants, stem from their crystalline structure, transitioning between hexagonal close-packed (hcp) α-and body-centered cubic (bcc) β-phases at an allotropic phase transformation temperature [17-19].

    Titanium alloys are classified into α-, near-α, (α+β), and β-types based on their microstructure and alloying elements [20]. The α-type includes commercially pure titanium (CP-Ti) and Ti alloys, known for exceptional corrosion resistance but limited mechanical strength at room temperature [21, 22]. Near-α Ti alloys, featuring minor β phases, share similar characteristics with α-type alloys but have not been extensively utilized in biomedical applications. (α+β)-type Ti alloys, exemplified by Ti-6Al-4V, dominate biomedical applications due to their superior strength, corrosion resistance, and osteointegration capabilities [20, 23]. Despite their prevalence, concerns about toxic elements such as vanadium have led to the development of alternatives, for instance, Ti-6Al-7Nb and Ti-5Al-2.5Fe [24–26]. However, high moduli in (α+β)-type alloys may pose challenges (see Table. 1), prompting the exploration of β-type Ti alloys with non-toxic β-stabilizers such as molybdenum (Mo), tantalum (Ta), and Zr for improved biocompatibility and suitable elastic modulus [27, 28]. Figure 2 compares the elasticity modulus of different biomedical Ti alloys with human bone, stainless steel, and Co alloys. When comparing (α+β)- and β-Ti alloys to 316L stainless steels and Co alloys, their ultimate strength values are comparable with those of 316L stainless steels but lower than those of Co alloys. However, their yield strengths are also comparable with those of 316L stainless steels but closer to the lower side of the range for Co alloys, as shown in Table. 1.

    However, dense forms of these Ti biomaterials with a misfit Young's modulus can induce stress-shielding, which in turn causes instability at the bone-implant interface. Thus, it results in fibrous tissue ingrowth, disruption of osseointegration, implant mobility, and an inflammatory response necessitating revision surgery. Porous materials have been developed to overcome these issues associated with bulk materials such as Ti. These materials aim to decrease Young's modulus, approaching values similar to bone, thereby improving stress distribution patterns and creating favorable conditions for bone remodeling.

 

Table 1. A comparison of mechanical properties of metallic implants titanium alloys for biomedical applications with human bone, stainless steels, and Co-alloys.

Alloy

Elastic Modulus

(GPa)

Yield Strength

(MPa)

Ultimate strength

(MPa)

Engineering strain

(%)

Ref.
Cortical bone 5 − 23 30 − 70 194 − 195 - [29]
SS 316L 200 200–700 500–1350 10–40 [30]
Co-alloys 240 500–1500 900–1800 10–50 [30]
Ti (unalloyed) 105 692 785 - [31]
Ti-20Ag ~103 - - - [32]
Ti-6Al-2Sn-4Zr-2Mo-0.1Si 114 990 1010 - [33]
Ti-6Al-4V 113 999 1173 6 [34]
Porous- Ti-6Al-4V 5.1 - 171.86 - [35]
Ti-6Al-7Nb 114 880-950 900-1050 8-15 [36]
Ti-5Al-2.5Fe 112 895 1020 15 [36]
Ti-6Al-2Sn-4Zr-6Mo 114 1000-1100 1100-1200 - [33]
Ti-5Al-2Sn-2Zr-4Mo-4Cr 112 1050 1100-1250 - [33]
Ti–5Al–1.5B 107 820–930 925–1080 15–17 [37]
Ti-10Cr-20Nb 17 1180 1580 - [38]
Ti-20 Cr-10Nb 50 980 1590 - [38]
Ti-20Cr-20Nb 28 1015 1700 - [38]
Ti–12V–6Sn 57 897 1024 10 [39]
Ti-4.5Al-3V-2Mo-2Fe 110 900 960 - [33]
Ti-35Nb-7Zr-4Cu 57 1062 1374 - [40]
Ti-35Nb-7Zr-7Cu 63 1205 1602 - [40]
Ti-35Nb-7Zr-10Cu 74 1469 1856 - [40]
Ti-35Nb-7Zr-13Cu 79 1216 1571 - [40]
Ti- 20Nb 74 - - - [41]
Ti-45Nb 64.3 438 527 - [42]
Ti-42Nb 60.5 674 683 11.7 [43]
Ti-42Nb 47.9 715 718 17.8 [44]
Ti-27.5Nb 70 800 820 10 [45]
Ti-15Mo 84 745 921 25 [34]
Ti-15Mo 78 544 874 21 [37]
Ti-35Nb-6Ta 50 - 820 ~10-12 [46]
Ti-35Nb-15Zr 72.82 1185.18 1199.39 6.7 [47]
Ti–15Zr–10Cr 78 1038 - - [48]
Ti-7Mn-10Nb 87 842 1842 ~34 [49]
Ti-16Nb-10Hf 81 730–740 740–850 10 [30]
Ti-25Zr-25Nb 60 1025 1588 32.9 [50]
Ti-24Nb-0.5O 54 665 810 - [51]
Ti-24Nb-0.5N 43 665 665 - [51]
Ti–35Nb–4Sn 55 - ~470 - [52]
Ti-26Nb-8Mo 54.5 663 - - [53]
Ti–20Nb–1.0Ru 65 920 960 ~22 [54]
Ti-15Mo–2.8Nb–0.2Si 83 945–987 979–999 16–18 [37]
Ti-15Mo-2.8Nb-3Al 82 771 812 - [55]
Ti–15Mo–3Nb–0.3O 82 1020 1020 - [55]
Ti-23Nb-0.7Ta-2Zr 55 280 400 - [51]
Ti-25Zr-20Nb-5Ag 61 1544 2184 22.4 [50]
 (50Ti-50Zr)77-15Nb-8Mo 96 545 - - [56]
Ti-25Nb-5Sn-2Cr 68 314 - - [57]
Ti–5Cr–3Au–1Cu ~ 11.2 ~520 ~670 31 [58]
Ti–25Nb–6Zr–1.7Fe 61 598 1256 15.7 [59]
Ti–35Nb–5Ta–7Zr–0.4O 66 976 1010 - [55]
Ti-23Nb-0.7Ta-2Zr-1.2O 60 830 880   [51]

Ti-27.5Nb-8.5Ta-3.5Mo-2.5Zr–5Sn

70 826 846 - [60]
 

 Figure 2. Elasticity modulus of various dense titanium alloys with a comparison of porous Ti-6Al- 4V.

 

4. Importance of Porosity in Medical Implants

    Porosity is a critical property of biomaterials used for medical implants that enables tissue integration, vascularization, and diffusion of nutrients and waste products [11, 61-63]. Implants made from metals, ceramics, or polymers often aim to mimic the porous architecture of natural bone through engineered surface modifications that create microscopic pores and channels. The size, density, interconnectivity, and orientation of pores within an implanted biomaterial dramatically impact its performance and determine outcomes in osseointegration, infection resistance, and long-term viability.

    For bone implants such as joint replacement prosthetics, dental implants, and fracture fixation devices, porosity facilitates bone ingrowth during osseointegration. Pore sizes between 100-400 microns allow osteoblasts and mesenchymal stem cells to penetrate the implant surface, differentiate, and begin secreting new bone matrix [64 - 66]. Moreover, it has been noted that for an implant to effectively encourage the growth of bone, having an optimal porosity level ranging from 20% to 50% is essential [67]. Highly porous surfaces with three-dimensional (3D) interconnected networks of pores enable more rapid bone integration across the entire implant rather than isolated regions. The degree of porosity can be tuned during manufacturing through extrusion, injection molding, or 3D printing of the bulk biomaterial. Beyond osseointegration, porosity imparts critical advantages in infection prevention and antibiotic delivery for orthopedic implants. Studies found that porous surfaces help protect bones from competing bacterial colonization while allowing the migration of macrophages, lymphocytes, and nutrients to fight infection [68, 69]. Local antibiotic elution is also enhanced by porous channels and reservoirs that increase drug loading capacity [69]. For hip and knee arthroplasties, the built-in porosity serves as a safeguard, where chronic infection can necessitate implant removal.

     The porous architecture that assists short-term bone ingrowth can also determine the long-term stability of an implant by allowing continued diffusion of nutrients and waste transport. Densely calcified, avascular interfaces between bone and implants often lead to fibrous encapsulation, isolating the implant over time. Such starvation causes cell death, loosening, and a potential fracture around the affected region. Implant designs and surfaces that facilitate highly vascular integration through interconnected porosity help prevent this scenario.

5. Titanium Scaffolds Design

    Porous titanium scaffolds for bone implants and tissue engineering applications require careful design considerations across multiple vital parameters. The pore geometry, including overall topology (e.g., spherical, cubical, etc.) and pore size distributions, needs optimization based on mechanical requirements and intended bone and vascular ingrowth behavior. According to Van Bael et al. [70], Ti-6Al-4V scaffolds featuring hexagonal pores exhibited the most significant cell growth, which decreased in scaffolds with rectangular pores and further diminished in those with triangular pores, as illustrated in Figure 3 Hence, such discrepancy is attributed to the higher number of corners and the shorter distance between the two arches in the corners, especially noticeable in hexagonal pores. Consequently, cell bridging occurs more rapidly in hexagonal pores than in rectangular and triangular pores, where the struts are more widely spaced. Despite this, it was observed that the regulation of osteogenic differentiation in the cells was independent of their proliferation, and alkaline phosphatase (ALP) activity increased in triangular pores [70].

     Manufacturing technique constraints, including solid-state foaming, powder sintering, electrodeposition, or additive manufacturing, inform achievable pore geometries [71]. For a given production method, fine-tuning processing factors specifically applied stresses, sintering profiles, laser scanning patterns, and post-treatments enable tailoring final porous structure metrics such as density, surface-area-to-volume ratios, interconnectivity, and anisotropy based on application needs. Robust characterization using scanning electron microscopy (SEM), micro-computed tomography, and related image analysis quantifies the resulting pore morphology down to the micron scale. Then, it feeds back into subsequent design revisions and process parameter improvements for the Ti scaffold architecture. Following these steps in scaffold design, optimized application-specific porous titanium implants can be constructed to promote bone ingrowth.

    (a)                                                                                                                                   (b)