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LDPE porous composites for intrauterine devices
Materials Letters 93 (2013) 275–277
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Influence of pore morphology on the mechanical properties of Cu/LDPE porous composites for intrauterine devices Cheng Xiao, Xianping Xia n, Changsheng Xie, Lian Xiao, Man Ge, Shuizhou Cai State Key Laboratory of Material Processing and Die & Mould Technology, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e i n f o
abstract
Article history: Received 30 August 2012 Accepted 17 November 2012 Available online 27 November 2012
Copper/low-density polyethylene (Cu/LDPE) porous composites are novel materials for copper intrauterine devices (Cu-IUDs) developed in our research group. Here the influence of pore morphology on the mechanical properties of Cu/LDPE porous composites is investigated. The results show that the mechanical properties of the porous composites vary with the changing of pore morphology. The mechanical properties of the porous composites with pipe-shaped pores (PCD-30) are the best, and those of the porous composites with polyhedral pores (PCP-30) are the worst among those experimental porous composites samples. Those differences might be attributed to the influences of different pore shapes on the occurrence of stress concentration and different progens on the fine-grain strengthening of LDPE matrix. These results are very important and they can be applied to guide the design of Cu/LDPE porous composites IUDs for the future clinical use. & 2012 Elsevier B.V. All rights reserved.
Keywords: Composite materials Porous materials Pore morphology Electron microscopy Mechanical properties
1. Introduction The copper intrauterine devices (Cu-IUDs) are one of the longacting, cheap, safe, convenient, reversible and culturally acceptable methods used widely in the world [1]. However, such side effects as bleeding and pain caused by the existing Cu-IUDs still exist in clinical use [2,3]. It is believed that the burst release of cupric ions, the direct contact between copper and endometrium, and the formation of adamant deposit can increase the occurrence of those side effects significantly [4–6]. To overcome those deficiencies of the existing Cu-IUDs that cause those side effects, a novel cupric ions control release system, the copper/low-density polyethylene (Cu/LDPE) non-porous composites, has been developed for use in Cu-IUDs [7,8]. It has already been verified that the Cu/LDPE composites IUDs not only can maintain the same excellent contraceptive efficacy but also can lessen those side effects of the existing Cu-IUDs [9,10]. Recently, a better Cu-IUDs material, the Cu/LDPE porous composites, has been developed in our research group successfully [11,12]. Similar to other Cu-IUDs, the Cu/LDPE porous composites IUDs also need to meet certain mechanical properties once they were used in clinical application. Many researches show that the morphology of porous structure has significant effect on the mechanical properties of porous composites [13,14]. However, apart from our previous study on the mechanical properties of Cu/
n
Corresponding author. Tel.: þ86 27 87559835; fax: þ 86 27 87543778. E-mail address: [email protected] (X. Xia).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.065
LDPE non-porous composites and their devices [15], no other report on the mechanical properties of the Cu/LDPE porous composites and their devices can be found. Thereby, the influence of pore morphology on the mechanical properties of Cu/LDPE porous composites has been studied in the present work. These results are very important and they can be used as guidance for the design of Cu/LDPE porous composites IUDs for the future clinical use.
2. Materials and methods The same LDPE, copper microparticles and 2,5-di-tert-butylhydroquinone (DTBHQ) as we used in one of our previous work [12] are used here. In addition, 2,20 -methylenebis (6-tert-butyl-4methylphenol) (antioxidant 2246) is also used in this study. Antioxidant 2246 is provided by Nanjing Milan Chemical Co., Ltd. (China) with a melting point of 125–133 1C and a purity of 99.8%. The experimental samples, which have the same size and shape as described in our previous work [15], were prepared by using injection molding and particulate leaching techniques in this study. Except for the preparation of homogeneous mixtures of LDPE powders, copper particles and porogens (i.e., DTBHQ and Antioxidant 2246 were mixed in various mass fraction ratios (100:0, 50:50, and 0:100, respectively) to obtain three different porogens mixtures at first. Then 30 wt% of each kind of porogens mixtures, 45 wt% of LDPE powders and 25 wt% of copper particles were mixed to obtain their homogeneous mixtures), the other
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C. Xiao et al. / Materials Letters 93 (2013) 275–277
processes used here are identical with those described in our previous work [12]. The prepared porous composites samples, which were obtained from samples with 30 wt% of DTBHQ, 15 wt% of antioxidant 2246 plus 15 wt% of DTBHQ, and 30 wt% of antioxidants 2246, were denoted as PCP-30, PCPD-30 and PCD30, respectively. For each kind of experimental samples, their morphologies before and after tensile testing were observed by using scanning electron microscopy (SEM), their mechanical properties were measured with five identical samples by using tensile testing, their crystallinity degrees (Xc) were investigated by using differential scanning calorimetry (DSC) and they were obtained by using the following equation. X c ¼ DHf ð1xyÞ=DH0f where DHf is the heat flow obtained from 60 to 110 1C, x is the content of copper particles, y is the content of porogen, and DH0f is the fusion heat of the purely crystalline form of PE, DH0f ¼ 289.9 J/g [16].
3. Results and discussion The microstructures of the prepared Cu/LDPE porous composites samples (PCP-30, PCPD-30 and PCD-30) before tensile testing are given in Fig. 1. It can be seen that pore morphology of the prepared porous composites has been tailored successfully. The pores in PCP-30 (Fig. 1a) are polyhedral pores with the same shapes and sizes as those of the DTBHQ particles, the pores in PCD-30 (Fig. 1c) are pipe-shaped pores, and the pores in PCPD-30 (Fig. 1b) are the mixtures of polyhedral pores and pipe-shaped pores. Their differences are attributed to the different melting points of the different porogen, the melting point of DTBHQ (212– 218 1C) is higher than the injection temperature (from hopper to die, 145, 180, 180 and 165 1C), and the DTBHQ particles have not been melted during the injection process and maintain their own polyhedral shapes in the porous composites; the melting point of antioxidant 2246 (125–133 1C) is lower than the injection temperature, and the antioxidant 2246 particles have been melted
during the injection process and recrystallized subsequently, resulting in their obtained pores are pipe-shaped pores. The mechanical properties of PCP-30, PCPD-30 and PCD-30 are demonstrated in Fig. 2. It can be seen that the pore morphology has great influence on both the elongation at break and the elastic modulus but little influence on the tensile strength of Cu/LDPE porous composites. Although these three kinds of porous composites have the same porosity, the elongation at break and the elastic modulus of PCP-30, PCPD-30 and PCD-30 increase in sequence. To explain these phenomena, both the SEM observation of tensile fracture surface and the DSC analysis of each kind of porous composites have been carried out. The morphologies of tensile fracture surface of PCP-30, PCPD30 and PCD-30 are presented in Fig. 3. It can be seen that the length of LDPE fibers in PCD-30 is the longest and that in PCP-30 is the shortest among PCP-30, PCD-30 and PCPD-30 . And the lengths of LDPE fibers of those experimental samples are consistent with their mechanical properties such as the elongation at break as shown in Fig. 2. It is known that different shape of pores has different influence on the stress concentration; and the larger the stress concentration is, the higher the occurrence of cracks is. As we mentioned above, the pores in PCP-30, PCD-30 and PCPD30 are very different. Among these pores, the shape of pores in PCD-30 is difficult and that in PCP-30 is easy to cause the occurrence of stress concentration, resulting in the mechanical properties of PCD-30 being the best and that of PCP-30 being the worst among those three porous composites. The non-isothermal crystallization exothermic curves of PCP-30, PCPD-30 and PCD-30 and the crystallinity degree obtained from these DSC curves are given in Fig. 4. It can be seen that the crystallinity degree of PCD-30 is the largest and that of PCP-30 is the lowest. Apparently, these differences are related closely to the influences of progens on the recrystallization of LDPE during the process of solidification. The melting point of antioxidant 2246 is lower than the injection temperature, and antioxidant 2246 would be melted firstly and then recrystallized. The sizes of recrystallized antioxidant 2246 particles are smaller than those of DTBHQ, thus the antioxidant 2246 could provide more nucleation sites for the crystallization of LDPE, resulting in
Fig. 1. SEM images of microstructure of four Cu/LDPE composites (a) PCP-30, (b) PCPD-30, and (c) PCD-30.
Fig. 2. Mechanical properties of the Cu/LDPE porous composites: (A) elongation at break, (B) tensile strength, and (C) elastic modulus.
C. Xiao et al. / Materials Letters 93 (2013) 275–277
277
Fig. 3. SEM images of tensile fracture surface morphology of the Cu/LDPE porous composites: (a) PCP-30, (b) PCPD-30, and (c) PCD-30.
Those differences might be attributed to the influences of different pore shapes on the occurrence of stress concentration and different progens on the fine-grain strengthening of LDPE matrix.
Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 50671039).
References
Fig. 4. The non-isothermal crystallization exothermic curves and the obtained crystallinity degree (Xc) of each kind of Cu/LDPE porous composites.
the crystallinity degree of PCD-30 is higher that of PCP-30 . Additionally, PCPD-30 is obtained by using the mixture of DTBHQ and antioxidant 2246 as porogen, thus its crystallinity degree is between PCD-30 and PCP-30 . The higher the crystallinity degree is, the better the strength is. Therefore, the tensile strength and the elastic modulus of PCD-30 are the highest and those of PCP-30 are the lowest. Additionally, the elongation at break of PCD-30 is the best; this might be caused by fine-grain strengthening of LDPE matrix, because the recrystallized antioxidant 2246 particles could provide more nucleation sites for the crystallization of LDPE matrix.
4. Conclusions Pore morphology of the Cu/LDPE porous composites has been tailored by using different progens with different melting points successfully, and pore morphology has great influence on both the elongation at break and the elastic modulus but little influence on the tensile strength of Cu/LDPE porous composites. Among those experimental porous composites samples, the mechanical properties of PCD-30 are the best and those of PCP-30 are the worst.
[1] The ESHRE Capri workshop group. Intrauterine devices and intrauterine systems. Hum reprod update 2008; 14:197–208. [2] Blumenthal PD, Voedisch A, Gemzell-Danielsson K. Strategies to prevent unintended pregnancy: increasing use of long-acting reversible contraception. Hum Reprod Update 2011;17:121–37. [3] Hubacher D, Chen Pai-Lien, Park Sola. Side effects from the copper IUD: do they decrease over time? Contraception 2009;79:356–62. [4] Zhang C, Xu N, Yang B. The corrosion behavior of copper in simulated uterine fluid. Corros Sci 1996;38:635–41. [5] Mora N, Cano E, Mora EM, Bastidas JM. Influence of pH and oxygen on copper corrosion in simulated uterine fluid. Biomaterials 2002;23:667–71. [6] Patai K, De´ve´nyi L, Zelko´ R. Comparison of surface morphology and composition of intrauterine devices in relation to patient complaints. Contraception 2004;70:149–52. [7] Cai S Z, Xia XP, Xie C S. Corrosion behavior of copper/LDPE nano-composites in simulated uterine solution. Biomaterials 2005;26:2671–6. [8] Xia XP, Cai S Z, Xie CS. Preparation, structure and thermal stability of Cu/LDPE nanocomposites. Mater Chem Phys 2006;95:122–9. [9] Xia XP, Xie CS, Zhu CH, Cai SZ, Yang XL. Effect of implanted Cu/low-density polyethylene nanocomposite on the morphology of endometrium in the mouse. Fertil Steril 2007;88:472–8. [10] Liu H F, Liu ZL, Xie CS, Yu J, Zhu CH. The antifertility effectiveness of copper/ low-density polyethylene nanocomposite and its influence on the endometrial environment in rats. Contraception 2007;2:157–61. [11] Zhang WW, Xia XP, Qi C, Xie CS, Cai SZA. porous Cu/LDPE composite for copper-containing intrauterine contraceptive devices. Acta Biomater 2012;8:897–903. [12] Qi C, Xia XP, Zhang WW, Xie CS, Cai SZ. Indomethacin/Cu/LDPE porous composite for medicated copper intrauterine devices with controlled release performances. Compos Sci Technol 2012;72:428–34. [13] Gauvin R, Chen YC, Lee JW, Soman P, Zorlutuna P, Nichol JW, et al. Microfabrication of complex porous tissue engineering scaffolds using 3D projection stereolithography. Biomaterials 2012;33:3824–34. [14] Nouri A, Hodgson PD, Wen CE. Effect of process control agent on the porous structure and mechanical properties of a biomedical Ti–Sn –Nb alloy produced by powder metallurgy. Acta Biomater 2010;6:1630–9. [15] Tang Y, Xia XP, Wang Y, Xie CS. Study on the mechanical properties of Cu/ LDPE composite IUDs. Contraception 2011;83:255–62. [16] Olmos D, Domı´nguez C, Castrillo PD, Gonzalez-Benito J. Crystallization and final morphology of HDPE: effect of the high energy ball milling and the presence of TiO2 nanoparticles. Polymer 2009;50:1732–42.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Influence of pore morphology on the mechanical properties of Cu/LDPE porous composites for intrauterine devices Cheng Xiao, Xianping Xia n, Changsheng Xie, Lian Xiao, Man Ge, Shuizhou Cai State Key Laboratory of Material Processing and Die & Mould Technology, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e i n f o
abstract
Article history: Received 30 August 2012 Accepted 17 November 2012 Available online 27 November 2012
Copper/low-density polyethylene (Cu/LDPE) porous composites are novel materials for copper intrauterine devices (Cu-IUDs) developed in our research group. Here the influence of pore morphology on the mechanical properties of Cu/LDPE porous composites is investigated. The results show that the mechanical properties of the porous composites vary with the changing of pore morphology. The mechanical properties of the porous composites with pipe-shaped pores (PCD-30) are the best, and those of the porous composites with polyhedral pores (PCP-30) are the worst among those experimental porous composites samples. Those differences might be attributed to the influences of different pore shapes on the occurrence of stress concentration and different progens on the fine-grain strengthening of LDPE matrix. These results are very important and they can be applied to guide the design of Cu/LDPE porous composites IUDs for the future clinical use. & 2012 Elsevier B.V. All rights reserved.
Keywords: Composite materials Porous materials Pore morphology Electron microscopy Mechanical properties
1. Introduction The copper intrauterine devices (Cu-IUDs) are one of the longacting, cheap, safe, convenient, reversible and culturally acceptable methods used widely in the world [1]. However, such side effects as bleeding and pain caused by the existing Cu-IUDs still exist in clinical use [2,3]. It is believed that the burst release of cupric ions, the direct contact between copper and endometrium, and the formation of adamant deposit can increase the occurrence of those side effects significantly [4–6]. To overcome those deficiencies of the existing Cu-IUDs that cause those side effects, a novel cupric ions control release system, the copper/low-density polyethylene (Cu/LDPE) non-porous composites, has been developed for use in Cu-IUDs [7,8]. It has already been verified that the Cu/LDPE composites IUDs not only can maintain the same excellent contraceptive efficacy but also can lessen those side effects of the existing Cu-IUDs [9,10]. Recently, a better Cu-IUDs material, the Cu/LDPE porous composites, has been developed in our research group successfully [11,12]. Similar to other Cu-IUDs, the Cu/LDPE porous composites IUDs also need to meet certain mechanical properties once they were used in clinical application. Many researches show that the morphology of porous structure has significant effect on the mechanical properties of porous composites [13,14]. However, apart from our previous study on the mechanical properties of Cu/
n
Corresponding author. Tel.: þ86 27 87559835; fax: þ 86 27 87543778. E-mail address: [email protected] (X. Xia).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.065
LDPE non-porous composites and their devices [15], no other report on the mechanical properties of the Cu/LDPE porous composites and their devices can be found. Thereby, the influence of pore morphology on the mechanical properties of Cu/LDPE porous composites has been studied in the present work. These results are very important and they can be used as guidance for the design of Cu/LDPE porous composites IUDs for the future clinical use.
2. Materials and methods The same LDPE, copper microparticles and 2,5-di-tert-butylhydroquinone (DTBHQ) as we used in one of our previous work [12] are used here. In addition, 2,20 -methylenebis (6-tert-butyl-4methylphenol) (antioxidant 2246) is also used in this study. Antioxidant 2246 is provided by Nanjing Milan Chemical Co., Ltd. (China) with a melting point of 125–133 1C and a purity of 99.8%. The experimental samples, which have the same size and shape as described in our previous work [15], were prepared by using injection molding and particulate leaching techniques in this study. Except for the preparation of homogeneous mixtures of LDPE powders, copper particles and porogens (i.e., DTBHQ and Antioxidant 2246 were mixed in various mass fraction ratios (100:0, 50:50, and 0:100, respectively) to obtain three different porogens mixtures at first. Then 30 wt% of each kind of porogens mixtures, 45 wt% of LDPE powders and 25 wt% of copper particles were mixed to obtain their homogeneous mixtures), the other
276
C. Xiao et al. / Materials Letters 93 (2013) 275–277
processes used here are identical with those described in our previous work [12]. The prepared porous composites samples, which were obtained from samples with 30 wt% of DTBHQ, 15 wt% of antioxidant 2246 plus 15 wt% of DTBHQ, and 30 wt% of antioxidants 2246, were denoted as PCP-30, PCPD-30 and PCD30, respectively. For each kind of experimental samples, their morphologies before and after tensile testing were observed by using scanning electron microscopy (SEM), their mechanical properties were measured with five identical samples by using tensile testing, their crystallinity degrees (Xc) were investigated by using differential scanning calorimetry (DSC) and they were obtained by using the following equation. X c ¼ DHf ð1xyÞ=DH0f where DHf is the heat flow obtained from 60 to 110 1C, x is the content of copper particles, y is the content of porogen, and DH0f is the fusion heat of the purely crystalline form of PE, DH0f ¼ 289.9 J/g [16].
3. Results and discussion The microstructures of the prepared Cu/LDPE porous composites samples (PCP-30, PCPD-30 and PCD-30) before tensile testing are given in Fig. 1. It can be seen that pore morphology of the prepared porous composites has been tailored successfully. The pores in PCP-30 (Fig. 1a) are polyhedral pores with the same shapes and sizes as those of the DTBHQ particles, the pores in PCD-30 (Fig. 1c) are pipe-shaped pores, and the pores in PCPD-30 (Fig. 1b) are the mixtures of polyhedral pores and pipe-shaped pores. Their differences are attributed to the different melting points of the different porogen, the melting point of DTBHQ (212– 218 1C) is higher than the injection temperature (from hopper to die, 145, 180, 180 and 165 1C), and the DTBHQ particles have not been melted during the injection process and maintain their own polyhedral shapes in the porous composites; the melting point of antioxidant 2246 (125–133 1C) is lower than the injection temperature, and the antioxidant 2246 particles have been melted
during the injection process and recrystallized subsequently, resulting in their obtained pores are pipe-shaped pores. The mechanical properties of PCP-30, PCPD-30 and PCD-30 are demonstrated in Fig. 2. It can be seen that the pore morphology has great influence on both the elongation at break and the elastic modulus but little influence on the tensile strength of Cu/LDPE porous composites. Although these three kinds of porous composites have the same porosity, the elongation at break and the elastic modulus of PCP-30, PCPD-30 and PCD-30 increase in sequence. To explain these phenomena, both the SEM observation of tensile fracture surface and the DSC analysis of each kind of porous composites have been carried out. The morphologies of tensile fracture surface of PCP-30, PCPD30 and PCD-30 are presented in Fig. 3. It can be seen that the length of LDPE fibers in PCD-30 is the longest and that in PCP-30 is the shortest among PCP-30, PCD-30 and PCPD-30 . And the lengths of LDPE fibers of those experimental samples are consistent with their mechanical properties such as the elongation at break as shown in Fig. 2. It is known that different shape of pores has different influence on the stress concentration; and the larger the stress concentration is, the higher the occurrence of cracks is. As we mentioned above, the pores in PCP-30, PCD-30 and PCPD30 are very different. Among these pores, the shape of pores in PCD-30 is difficult and that in PCP-30 is easy to cause the occurrence of stress concentration, resulting in the mechanical properties of PCD-30 being the best and that of PCP-30 being the worst among those three porous composites. The non-isothermal crystallization exothermic curves of PCP-30, PCPD-30 and PCD-30 and the crystallinity degree obtained from these DSC curves are given in Fig. 4. It can be seen that the crystallinity degree of PCD-30 is the largest and that of PCP-30 is the lowest. Apparently, these differences are related closely to the influences of progens on the recrystallization of LDPE during the process of solidification. The melting point of antioxidant 2246 is lower than the injection temperature, and antioxidant 2246 would be melted firstly and then recrystallized. The sizes of recrystallized antioxidant 2246 particles are smaller than those of DTBHQ, thus the antioxidant 2246 could provide more nucleation sites for the crystallization of LDPE, resulting in
Fig. 1. SEM images of microstructure of four Cu/LDPE composites (a) PCP-30, (b) PCPD-30, and (c) PCD-30.
Fig. 2. Mechanical properties of the Cu/LDPE porous composites: (A) elongation at break, (B) tensile strength, and (C) elastic modulus.
C. Xiao et al. / Materials Letters 93 (2013) 275–277
277
Fig. 3. SEM images of tensile fracture surface morphology of the Cu/LDPE porous composites: (a) PCP-30, (b) PCPD-30, and (c) PCD-30.
Those differences might be attributed to the influences of different pore shapes on the occurrence of stress concentration and different progens on the fine-grain strengthening of LDPE matrix.
Acknowledgments The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 50671039).
References
Fig. 4. The non-isothermal crystallization exothermic curves and the obtained crystallinity degree (Xc) of each kind of Cu/LDPE porous composites.
the crystallinity degree of PCD-30 is higher that of PCP-30 . Additionally, PCPD-30 is obtained by using the mixture of DTBHQ and antioxidant 2246 as porogen, thus its crystallinity degree is between PCD-30 and PCP-30 . The higher the crystallinity degree is, the better the strength is. Therefore, the tensile strength and the elastic modulus of PCD-30 are the highest and those of PCP-30 are the lowest. Additionally, the elongation at break of PCD-30 is the best; this might be caused by fine-grain strengthening of LDPE matrix, because the recrystallized antioxidant 2246 particles could provide more nucleation sites for the crystallization of LDPE matrix.
4. Conclusions Pore morphology of the Cu/LDPE porous composites has been tailored by using different progens with different melting points successfully, and pore morphology has great influence on both the elongation at break and the elastic modulus but little influence on the tensile strength of Cu/LDPE porous composites. Among those experimental porous composites samples, the mechanical properties of PCD-30 are the best and those of PCP-30 are the worst.
[1] The ESHRE Capri workshop group. Intrauterine devices and intrauterine systems. Hum reprod update 2008; 14:197–208. [2] Blumenthal PD, Voedisch A, Gemzell-Danielsson K. Strategies to prevent unintended pregnancy: increasing use of long-acting reversible contraception. Hum Reprod Update 2011;17:121–37. [3] Hubacher D, Chen Pai-Lien, Park Sola. Side effects from the copper IUD: do they decrease over time? Contraception 2009;79:356–62. [4] Zhang C, Xu N, Yang B. The corrosion behavior of copper in simulated uterine fluid. Corros Sci 1996;38:635–41. [5] Mora N, Cano E, Mora EM, Bastidas JM. Influence of pH and oxygen on copper corrosion in simulated uterine fluid. Biomaterials 2002;23:667–71. [6] Patai K, De´ve´nyi L, Zelko´ R. Comparison of surface morphology and composition of intrauterine devices in relation to patient complaints. Contraception 2004;70:149–52. [7] Cai S Z, Xia XP, Xie C S. Corrosion behavior of copper/LDPE nano-composites in simulated uterine solution. Biomaterials 2005;26:2671–6. [8] Xia XP, Cai S Z, Xie CS. Preparation, structure and thermal stability of Cu/LDPE nanocomposites. Mater Chem Phys 2006;95:122–9. [9] Xia XP, Xie CS, Zhu CH, Cai SZ, Yang XL. Effect of implanted Cu/low-density polyethylene nanocomposite on the morphology of endometrium in the mouse. Fertil Steril 2007;88:472–8. [10] Liu H F, Liu ZL, Xie CS, Yu J, Zhu CH. The antifertility effectiveness of copper/ low-density polyethylene nanocomposite and its influence on the endometrial environment in rats. Contraception 2007;2:157–61. [11] Zhang WW, Xia XP, Qi C, Xie CS, Cai SZA. porous Cu/LDPE composite for copper-containing intrauterine contraceptive devices. Acta Biomater 2012;8:897–903. [12] Qi C, Xia XP, Zhang WW, Xie CS, Cai SZ. Indomethacin/Cu/LDPE porous composite for medicated copper intrauterine devices with controlled release performances. Compos Sci Technol 2012;72:428–34. [13] Gauvin R, Chen YC, Lee JW, Soman P, Zorlutuna P, Nichol JW, et al. Microfabrication of complex porous tissue engineering scaffolds using 3D projection stereolithography. Biomaterials 2012;33:3824–34. [14] Nouri A, Hodgson PD, Wen CE. Effect of process control agent on the porous structure and mechanical properties of a biomedical Ti–Sn –Nb alloy produced by powder metallurgy. Acta Biomater 2010;6:1630–9. [15] Tang Y, Xia XP, Wang Y, Xie CS. Study on the mechanical properties of Cu/ LDPE composite IUDs. Contraception 2011;83:255–62. [16] Olmos D, Domı´nguez C, Castrillo PD, Gonzalez-Benito J. Crystallization and final morphology of HDPE: effect of the high energy ball milling and the presence of TiO2 nanoparticles. Polymer 2009;50:1732–42.