Progress in Natural Science: Materials International 2013;23(2):183–189 Chinese Materials Research Society
Progress in Natural Science: Materials International www.elsevier.com/locate/pnsmi www.sciencedirect.com
ORIGINAL RESEARCH
Processing, microstructure and mechanical properties of biomedical magnesium with a specific two-layer structure Xue Zhanga,b, Xiaowu Lia,b,n, Jiguang Lib, Xudong Sunb a
Institute of Materials Physics and Chemistry, College of Sciences, Northeastern University, Shenyang 110004, PR China Key Laboratory for Anisotropy and Texture Engineering of Materials, Ministry of Education, Northeastern University, Shenyang 110004, China b
Received 10 December 2012; accepted 10 January 2013 Available online 6 April 2013
KEYWORDS Magnesium; Salt particle; Scaffold material; Porous structure; Mechanical property
Abstract A novel magnesium based scaffold with a two-layer structure was synthesized by powder metallurgical process using salt particles as space holder. The outer layer of the scaffold shows an interconnected porous structure and the inner layer presents a compact structure reinforced by the salt particles. Such a specific structure is introduced primarily for the purpose of a better combination of biocompatibility and mechanical compatibility. Experimental results demonstrate that the structural features and mechanical properties of the magnesium based scaffold with a salt content of 30 wt% prepared by the current method are quite compatible with the cancellous bone. Such a novel Mg-based scaffold has the potential to act as degradable implants for bone substitute application. & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.
1.
Introduction
The porous scaffold materials provide the well-accommodated environment for proliferation and differentiation of the osteoblasts n Corresponding author at: Institute of Materials Physics and Chemistry, College of Sciences, Northeastern University, Shenyang 110004, PR China. Tel.: þ86 24 83678479. E-mail address:
[email protected] (X. Li). Peer review under responsibility of Chinese Materials Research Society.
obtained from the patient's hard tissue, and the biological anchorage for the surrounding bony tissue via the ingrowth of mineralized tissue into the pore space [1,2]. The ideal bone scaffold should be osteoconductive and biocompatible together with good mechanical properties under load-bearing conditions after implanted into the human body. The traditional scaffolds including porous hydroxyapatite (HAp) [3–5], natural or synthesized polymers (e.g. chitin, collagen, etc.) [6–8] and porous biometals (e.g. Ti and its alloys, 316L stainless steels, etc.) [9] have been extensively studied for tissue engineering of bone and cartilage [10,11]. The ceramics and polymers have good bioactivity, but their unsuitable mechanical properties would lead to the premature failure of scaffold. Although porous metals possess excellent mechanical properties, their applications for tissue
1002-0071 & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.pnsc.2013.03.006
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engineering and orthopedic implants are still limited by low bioactivity and release of certain toxic elements into the human body [12]. Mg and its alloys have been used in orthopedic applications since the end of the 1930s [13]. Porous magnesium has been recognized as a very promising biodegradable metal for bone implant because of its density close to that of human bone, relatively low Young's modulus, and excellent biodegradability, biocompatibility [14] and bioresorbability [15]. Recently, there have been many methods available for producing porous magnesium, such as sintering-together of particles [16], laser perforation on a dense substrate [17], indirect solid free-form fabrication method [18], etc. However, in those studies, the mechanical properties (e.g., compressive flow stress and Young's modulus) of the porous magnesium become worse generally as porosity increases, and high porosity might lead to a rapid corrosion rate, thus negating its required load-bearing functions after implantation into the human body. Therefore, it is crucial to develop novel scaffold materials, which should have a porous structure resembling natural bone, and the Young's modulus and compressive strength comparable to those of natural bone. In view of this, a novel magnesium based scaffold with a specific two-layer structure showing both high biocompatibility and mechanical compatibility with natural bone was prepared by powder metallurgical process using salt particles as space holder in the present work. The effect of the space particle size on the microstructures and mechanical properties of the prepared materials were investigated.
2.
Experimental procedures
Spherical magnesium powders (purity: 99.9%, powder size: 100– 165 mm) were used as starting materials. Salt particles were chosen as space holder particles, and they were first sent into a mortar and ground down to finer powders with three kinds of particle sizes, i.e. 500–700, 350–500, and 250–350 mm, and then dried again at 200 1C for 24 h. Fig. 1 shows clearly the detailed processing steps for producing Mg-based scaffold materials. Pre-processed salts (30 wt%) and spherical magnesium powders (70 wt%) were firstly layered alternately to the benefit of an uniform distribution of salt particles in the magnesium matrix, and then hot-pressed at 640– 650 1C under a pressure of 40 MPa for 20 min into green compacts with a dimension of 10 mm 10 mm 30 mm. The compressive specimens with a dimension of 6 mm 6 mm 9 mm were cut from those green compacts using a wire-cut electrical discharge machine, After these specimens were polished with #180–#2000 emery papers, they were immersed into the mixed solution of glycerin and ethanol (volume ratio: 1:2) (abbreviated as GE solution) for 48 h to dissolve out the salt particles in the outer layer of the magnesium specimens.
Fig. 1
The immersion time was selected to be 48 h, since the salt particles in the outer layer of the magnesium based scaffold could not be removed completely, if the immersion time is lower than 48 h. After immersion in the GE solution, the specimens were polished again with #2000 emery papers, followed by ultrasonic cleaning for 10 min with ethanol, for morphological observations and compressive tests. One of the polished specimen surfaces was chemically etched in 75 wt% phosphoric acid for seconds for microstructural observations by scanning electron microscopy (SEM). Pore size, morphologies and fracture features of the magnesium specimens were also observed by SEM. The porosity of the magnesium specimens were measured by the Archimedes' method. The phase composition of the synthesized Mg-based materials was analyzed by X-ray diffraction (XRD) and energy dispersive X-ray spectrometer (EDS) in SEM. The XRD data were obtained over 2θ range of 201–901 at a step size of 0.251. The compressive strength and Young's modulus of the porous magnesium specimens were obtained using room-temperature compression tests with an Instron-1195 testing machine at an initial strain rate of 10−4 s−1. Due to the fact that there are generally some deviations in mechanical measurements for porous materials, the compressive test was repeated 4 times for each specimen.
3. 3.1.
Experimental results and discussion Microstructures
Fig. 2 gives the overall views of magnesium based scaffold with a salt content of 30 wt% and with different salt particle sizes before and after immersion in the GE solution for 48 h, as well as relevant magnified SEM micrographs. Before immersion, salt particles with different sizes can be clearly seen on the surface of three groups of specimens (Fig. 2a, d and g). After immersion, the salt particles were removed and pores with different sizes thus formed on the surface of those specimens (Fig. 2b, e and h). It can be more clearly seen in magnified SEM images that the immersed Mg-based scaffold materials have open-cell structures with three kinds of pore sizes on the outer layer of the scaffolds and some pores were interconnected (see Fig. 2c, f and i). Investigations have indicated that the appropriate pore size for attachments, differentiations, ingrowth of osteoblasts and vascularization is approximately 200–500 mm [19] or 300–400 mm [20] in porous bone substitute applications. Therefore, we selected salt powders with three kinds of particles sizes, i.e. 500–700, 350–500 and 250–350 mm, as space holder particles to study the effect of different particles sizes on mechanical properties of the Mg specimens, the porosity of which was measured to be around 6%, 10% and 8%, respectively.
Schematic illustration of the processing steps for fabricating Mg-based scaffold materials.
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Fig. 2 Overal views of magnesium specimens with a salt content of 30 wt% and with different particle sizes before (a, d and g) and after (b, e and h) immersion in the GE solution for 48 h as well as local magnified SEM micrographs of immersed specimens (c, f and i).
Fig. 3 shows the microstructures of the Mg specimens with the salt particle size of 500–700 mm after dissolving out of salt particles. It seems that there are two types of pores forming in the surface layer of the Mg specimens. One is macropore, which forms due to dissolving out of the salt particles in the outer layer of the Mg-based scaffold (Fig. 3a), and the size of these pores could be controlled by the salt particle size. The other one is small pore, which forms by the sintering process of the spherical magnesium powders, as shown in Fig. 3(a) and a magnified image of Fig. 3(b). These small pores distribute in the wall of the macropores, and the pore size ranges from several to dozens of microns. From the cross-section morphology of the Mg-based scaffold (Fig. 3c), it can be clearly seen that the depth of porous outer layer of the Mg-based scaffold is around 600 mm, and there exist interconnected pores and isolated pores for these two types of pores. Undoubtedly, the interconnected pores can meet the demands for the fresh fluid to be easily sent into the scaffold materials, and allow the ingrowth of new bone tissue. Meanwhile, the formation of these pores could greatly enlarge the surface area and surface roughness, favoring the connection of the scaffold material with the host bone. Such a porous structure in the outer layer is schematically depicted in Fig. 3(d). Fig. 4 shows the X-ray diffraction patterns of the magnesium powder, magnesium specimens with particle sizes of 350–500 mm before and after dissolving out of salt particles, and salt powder. It
is found that the diffraction peaks of the magnesium specimen surface after immersion in the GE solution for 48 h well match those for magnesium powder, indicating that the space holder salt particles in the outer layer of the magnesium specimens have been dissolved out completely after immersion in the GE solution. The EDS pattern detected on the cell wall of the porous magnesium specimen with particle sizes of 350–500 mm after immersion in the GE solution for 48 h is shown in Fig. 5. It reveals that no salt was observed after immersion and only very little carbon and oxygen were observed on the cell wall. This result implies that the residue of the mixed solution consisting of carbon and oxygen remains on the surface of the cell wall. Zhuang et al. [21] observed the similar experimental results on the cell wall of the porous magnesium after sintering, and they thought that the residue was caused by the decomposed carbamide. Further research efforts are needed to understand whether there is an influence of the residue on the bio-properties of the porous magnesium.
3.2.
Mechanical properties
The nominal compressive stress–strain curves of the magnesium specimens with different salt particle sizes (500–700, 350–500, and 250–350 mm) before and after immersion are shown in Fig. 6.
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Fig. 3 (a) The SEM micrographs of microstructure of the magnesium specimens with a salt content of 30 wt% and with salt particle sizes of 500–700 mm after immersion in the GE solution for 48 h, (b) a magnified SEM image of the local area in (a) revealed by chemical etching in phosphoric acid, (c) the cross-section view of the macropore in the outer layer of the magnesium based scaffold and (d) the corresponding schematic drawing of the surface microstructure of the magnesium specimens.
Fig. 4 XRD patterns of salt powder (a), and magnesium specimens with a salt content of 30 wt% and with particle sizes of 350–500 mm before (b) and after (c) being immersed in the GE solution for 48 h, as well as of Mg powder (d).
Apparently, the curves exhibit three regions for all the specimens, i.e. an elastic region at the beginning of deformation, plastic flowing gradually to a peak stress after plastic yielding, and deformation softening to a final failure, as shown in Fig. 6. The addition of the salt particles decreases the compressive yield stress and compressive strength (peak stress) of the magnesium (Fig. 6a).
From the experimental data in Fig. 6, the changes in compressive strength and Young's modulus of the magnesium specimens before and after immersion as a function of the salt particle size can be quantitatively indicated in Fig. 7. It can be seen that the compressive yield stress and Young's modulus of the magnesium specimen increase with increasing salt particle size. Specifically, with increasing the salt particle size from 250–350 mm to 500– 700 mm, the compressive strength of the immersed magnesium specimens goes up markedly from 5575 MPa to 7976 MPa, and the Young's modulus also increases from 2.570.3 GPa to 3.170.4 GPa. The SEM images of fracture surface features of the magnesium specimens with salt particle sizes of 500–700 mm after immersion compressed at a strain rate of 10–4 s−1 are shown in Fig. 8, from which it can be seen that cleavage cracking takes place in undissolved salt particles inside the specimen, and it is presumed that these salt particles could restrain the crack propagation efficiently in the process of compression deformation. Therefore, the magnesium matrix reinforced by the inside salt particles can exhibit a higher strength, as compared to that for purely porous magnesium [17,21]. As one knows, load bearing is one of the major function for bone scaffold materials after they are implanted into the human body [22]. Table 1 gives some representative comparions of mechanical properties of various scaffold biomaterials, and the relevant properties of human bones are also listed here. In general, a high porosity could be induced in the processed porous HAp, polymer and titanium, but the major problems of these implant
Processing, microstructure and mechanical properties of biomedical magnesium with a specific two-layer structure
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Fig. 5 EDS pattern detected on the cell wall of the porous magnesium specimen with a salt content of 30 wt% and with particle sizes of 350–500 mm after immersion in the GE solution for 48 h.
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Stress, MPa
Stress, MPa
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0 0.00
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Fig. 6 The nominal compressive stress–strain curves of the magnesium specimens with a salt content of 30 wt% and with different salt particles sizes (500–700, 350–500, and 250–350 mm) before (a) and after (b) immersion in the GE solution for 48 h. Note that the result of pure magnesium is also included in (a) for comparison.
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Fig. 7 Changes of the compressive strength (a) and Young's modulus (b) of the magnesium specimens before and after immersion in the GE solution for 48 h as a function of the salt particle size.
materials in medical use are the high brittleness and low degradation rate of porous HAp [23–25], poor mechanical properties of the porous polymer [26,27], and non-biodegradability of the porous titanium. The current magnesium based scaffold with a
specific two-layer structure might be able to avoid the above shortcomings. From Table 1 one can see that there is nearly no change in the mechanical properties of the current Mg specimens with a salt
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content of 30 wt% before and after dissolving out of the salt particles in the outer layer. According to the work by Gibson [28], the compressive strength and Young's modulus of the cancellous bone are known to be 0.2–80 MPa and 0.01–2 GPa, respectively. Apparently, the mechanical properties of the magnesium scaffold with porous structure in the outer layer (i.e. after immersion) prepared by the current method are fairly comparable to those of cancellous bone (see Table 1), and it has a superior mechanical properties to those of purely porous magnesium [17,21], primarily because the interior salt particles of the current Mg-based scaffold material could restrain the crack propagation during the process of compression deformation. Besides, the porosity and pore size are also important function for scaffold materials [28,30]. Actually, for the current Mg-based scaffold material, an appropriate porosity can be adjusted by the interconnected porous structure of the outer layer of the scaffold and the newly formed pores during the degradation process after the scaffold is implanted into the human body, while the pore size can be controlled by the selection of the mass fraction and size of salt particles. The specific two-layer structure of the current Mg-based scaffold material with a salt content of 30 wt% can thus be summarized schematically in Fig. 9. A porous structure with interconnected pores in the outer (or surface) layer of the scaffold is mainly aimed for a better biocompatibility, namely, with such an outer structure, the fresh body fluid can be easily sent into the
Fig. 8 SEM images of fracture surface features of the magnesium specimens with a salt content of 30 wt% and with salt particle sizes of 500–700 mm after immersion in the GE solution for 48 h compressed at a strain rate of 10−4 s−1.
Table 1
scaffold material, allowing the ingrowth of new bone tissue. The interior compact structure reinforced by the salt particles can effectively increase the compressive strength of the material, enhancing its mechanical compatibility. Therefore, it is believed that such-prepared magnesium based materials with a specific twolayer structure, i.e., a porous structure in the outer layer and a compact reinforced structure in the inner layer, might be one of promising degradable scaffold materials for bone substitute applications. 4.
Conclusions
(1) A novel Mg-based scaffold material with a specific two-layer structure was successfully prepared by powder metallurgical process using salt particles as space holder. The outer layer of the scaffold presents a porous structure with interconnected pores showing a better biocompatibility, while the inner compact structure reinforced by the salt particles possesses excellent mechanical properties under loadbearing conditions. (2) The compressive yield stress and Young's modulus of the Mg-based scaffold material increase with increasing salt particle size. With increasing the salt particle size from 250–350 mm to 500–700 mm, its compressive strength goes up markedly from 5575 MPa to 7976 MPa, and its Young's modulus increases from 2.570.3 GPa to 3.17 0.4 GPa. The mechanical properties of the Mg-based scaffold are exactly in a range of those of cancellous bone. (3) Structural characterizations and mechanical tests strongly demonstrate that the Mg-based scaffold with a salt content of
Fig. 9 Schematic structure of the magnesium specimens with a salt content of 30 wt% after immersion in the GE solution for 48 h.
Summary of mechanical properties of various scaffold biomaterials and natural bones.
Materials
Porosity (%)
Compressive strength (MPa)
Young's modulus (GPa)
Reference
Compact bone Cancellous bone Porous HAp Porous polymers Porous Ti Porous Mg (C.) Porous Mg (L.P.) Mg-based scaffold (30 wt% salt, before immersion) Mg-based scaffold (30 wt% salt, after immersion)
– – 50%–77% 58%–80% 80% 36%–55% 43%–51% – 6%–10%
88–164 0.2–80 1.2–17.4 2.7–11 40 15–31 8–13 72–79 55–79
3.9–11.7 0.01–2 0.12–7 0.05–1.2 2.87 3.6–18.1 0.4–0.65 2.7–3.45 2.5–3.05
[29] [28] [23–25] [26,27] [9] [21] [17] Present work Present work
Note: C.—carbermide used as space holder particles; L.P.—laser perforation.
Processing, microstructure and mechanical properties of biomedical magnesium with a specific two-layer structure 30 wt% having a two-layer structure prepared by the current powder metallurgical process has the potential to serve as degradable implants for bone substitute applications.
Acknowledgments This work was financially supported by the Fundamental Research Funds for the Central University of China under Grant nos. N110605003 and N110105001, and partially by the National Natural Science Foundation of China (NSFC) under Grant no. 51271054. Prof. X.W. Li is grateful for these supports. References [1] J.P. Vacanti, C.A. Vacanti, The History and Scope of Tissue Engineering, Principles of Tissue Engineering, 2nd ed. Academic Press, New York, 2000 3–8. [2] D.W. Hutmacher, Scaffold in tissue engineering bone and cartilage, Biomaterials 21 (24) (2000) 2529–2543. [3] T. Nagashima, Y. Ohshima, H. Takeuchi, Osteoconduction in porous hydroxyapatite ceramics grafted into the defect of the lamina in experimental expansive open-door laminoplasty in the spinal canal, Nippon Seikeigekagakkai Zasshi 69 (4) (1995) 222–230. [4] B.S. Chang, C.K. Lee, K.S. Hong, H.J. Youn, H.S. Ryu, S.S. Chung, K.W. Park, Osteoconduction at porous hydroxyapatite with various pore configurations, Biomaterials 21 (12) (2000) 1291–1298. [5] N. Tamai, A. Myoui, T. Tomita, T. Nakase, J. Tanaka, T. Ochi, H. Yoshikawa, Novel hydroxyapatite ceramics with an interconnective porous structure exhibit superior osteoconduction in vivo, Journal of Biomedical Materials Research 59 (1) (2002) 110–117. [6] M.E. Gomes, A.S. Ribeiro, P.B. Malafaya, R.L. Reis, A.M. Cunha, A new approach based on injection moulding to produce biodegradable starch-based polymeric scaffolds: morphology, mechanical and degradation behaviour, Biomaterials 22 (9) (2001) 883–889. [7] C. Du, G.J. Meijer, C.Van.de. Valk, R.E. Haan, J.M. Bezemer, S.C. Hesseling, F.Z. Cui, K.de. Groot, P. Layrolle, Bone growth in biomimetic apatite coated porous Polyactive 1000PEGT70PBT30 implants, Biomaterials 23 (23) (2002) 4649–4656. [8] G. Ciapetti, L. Ambrosio, L. Savarino, D. Granchi, E. Cenni, N. Baldini, S. Pagani, S. Guizzardi, F. Causa, A. Giunti, Osteoblast growth and function in porous poly epsilon-caprolactone matrices for bone repair: a preliminary study, Biomaterials 24 (21) (2003) 3815–3824. [9] C.E. Wen, Y. Yamada, K. Shimojima, Y. Chino, T. Asahina, M. Mabuchi, Processing and mechanical properties of autogenous titanium implant materials, Journal of Materials Science: Materials in Medicine 13 (4) (2002) 397–401. [10] S.J. Simske, R.A. Ayers, T.A. Bateman, Porous materials for bone engineering, Materials Science Forum 250 (1997) 151–182. [11] M. Borden, S.F. El-Amin, M. Attawia, C.T. Laurencin, Structural and human cellular assessment of a novel microsphere-based tissue engineered scaffold for bone repair, Biomaterials 24 (4) (2003) 597–609. [12] R. Narayanan, S.K. Seshadri, Phosphoric acid anodization of Ti–6Al– 4V structural and corrosion aspects, Corrosion Science 49 (2) (2007) 542–558.
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