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Nanocomposites of poly(l -lactide) and surface modified magnesia nanoparticles: Fabrication, mechanical property and biodegradability
Journal of Physics and Chemistry of Solids 72 (2011) 111–116
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Nanocomposites of poly(L-lactide) and surface modified magnesia nanoparticles: Fabrication, mechanical property and biodegradability Fuqiu Ma, Xili Lu n, Zhaomin Wang, Zhijie Sun, Fengfa Zhang, Yufeng Zheng Center for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 150001, People’s Republic of China
a r t i c l e in f o
abstract
Article history: Received 17 May 2010 Received in revised form 14 October 2010 Accepted 23 November 2010
A novel nanocomposite based on biodegradable poly(L-lactide) (PLLA) was prepared by the incorporation of surface modified magnesia (g-MgO) nanoparticles using a solution casting method. The mechanical properties, biodegradable properties and protein adhesion behavior of the g-MgO/PLLA nanocomposites were investigated. Scanning electron microscopy (SEM) results showed that g-MgO nanoparticles could comparatively uniformly disperse in PLLA matrix. The addition of g-MgO nanoparticles to PLLA matrix improved the tensile strength and elastic modulus, whereas reduced the elongation at break. The mass loss results showed that the nanocomposites with higher g-MgO content had faster degradation rates. The in vitro pH value determination results indicated that the g-MgO nanoparticles could neutralize effectively the lactic acid resulting from the degradation of PLLA. The g-MgO/PLLA nanocomposites exhibited enhanced protein adsorption, i.e., with the increase of g-MgO content, the amount of protein adsorption increased. The (5 wt%)g-MgO/PLLA nanocomposites adsorbed 33% more protein than the pure PLLA. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Polymers C. Infrared spectroscopy D. Mechanical properties
1. Introduction Poly(L-lactide) (PLLA) has attracted wide attention due to its biodegradability and biocompatibility. It has been widely applied to biomedical applications such as bone screws, surgical sutures, tissue engineering and controlled drug delivery [1,2]. However, PLLA has several disadvantages such as not enough strength, lack of desired bioactivity, etc. Moreover, when PLLA is implanted in the body, it is reported that the inflammation arises which has been attributed to the formation of lactic acid during the PLLA degradation [3]. In order to overcome these disadvantages, the introduction of inorganic nanoparticles into the PLLA matrix is a prevalent method. In the previous works, many kinds of inorganic fillers have been added to the PLLA matrix, such as hydroxyapatite (HAP) [4], silica nanoparticles [5], titanium dioxide [6], carbon nanotube [7] and many inspiring results can be obtained. For example, the introduction of the HA nanoparticles into PLLA matrix not only enhances the mechanical properties but also provides satisfactory osteoconductivity. The introduction of the silica or titanium dioxide into the PLLA matrix can improve the mechanical properties and cell attachment. In this paper, magnesia nanoparticles are selected as the inorganic filler basing on the following reasons. Firstly, magnesia nanoparticles are alkaline oxide, which can reduce the inflammatory
n
Corresponding author. Tel.: + 86 451 82518173; fax: + 86 451 82518644. E-mail address: [email protected] (X. Lu).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.11.008
reaction by buffering or neutralizing the acidic by-product. Secondly, the additions of magnesia nanoparticles may be improve the mechanical properties of PLLA. Lastly, magnesium is the essential element of human body, and magnesium ion is nontoxic [8,9]. Although the magnesia has been considered better choice as inorganic filler, to our knowledge, few reports about the MgO/PLLA nanocomposites can be found. In the present work, we modify the surface of magnesia nanoparticles in order to improve their dispersibility in PLLA matrix. The surface grafted magnesia nanoparticles were characterized by Fourier transformation infrared (FTIR) and thermal gravimetric analysis (TGA). The mechanical properties and biodegradability of the g-MgO/PLLA nanocomposites were investigated by tensile test and in vitro degradation tests. Furthermore, the protein adhesion of the g-MgO/PLLA nanocomposites was evaluated.
2. Experimental 2.1. Materials
e-caprolactone (99%, Aldrich) was distilled prior to use. Stannous octanoate was obtained from Sigma. Magnesia nanoparticles (average diameter of 20 nm) were purchased from Shanghai Huijinya nanomaterials company (China). PLLA (Mw 250,000) was prepared in our own laboratory using ring opening polymerization of L-lactide according to the literature [10]. All other agents were of analytical grade and used as received.
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F. Ma et al. / Journal of Physics and Chemistry of Solids 72 (2011) 111–116
2.2. Surface modified by the ring opening polymerization of e-caprolactone The PCL was grafted onto the surface of MgO nanoparticles through ring opening polymerization of e-caprolactone (e-CL) using stannous octanoate as catalyst. 3 g MgO nanoparticles was added to 50 g freshly distilled e-caprolactone. Stannous octanoate was then added as catalyst (the mole ratio of catalyst: e-CL ¼1:2000) and the reaction mixture was stirred at 140 1C for 8 h under nitrogen protect. The PCL-grafted MgO nanoparticles (g-MgO) were separated by centrifugation and purified by washing with excessive amount of chloroform for five times. Finally the g-MgO was dried in vacuo at 60 1C for at least 24 h. 2.3. Prepared the g-MgO/PLLA nanocomposites The g-MgO/PLLA nanocomposites with different contents (1, 2.5 and 5 wt%) of g-MgO nanoparticles were prepared by solution casting method. The detailed procedure can be seen in our previous report [6]. 2.4. Characterization of surface modified magnesia nanoparticles and the g-MgO/PLLA nanocomposites FTIR spectra were examined using a Perkin-Elmer spectrum 100 spectrophotometer. The samples were prepared by mixing the nanoparticles with KBr powders and pressing the mixture into disks. Thermogravimetric analysis was carried out on a thermogravimetric analyzer (Perkin-Elmer 7 series thermal analysis system) at a heating rate of 5 1C/min from room temperature to 700 1C under nitrogen gas flow. The surface morphology of the g-MgO/PLLA nanocomposites was observed using a Hitachi S4700 scanning electron microscope (SEM) at accelerating voltages in the range 5–15 KV. The surfaces of specimens were coated with gold prior to SEM observation. 2.5. Mechanical properties The tensile tests were conducted using electromechanical universal testing machine (Changchun Chaoyang WDS-5, China) with a loading rate of 10 mm/min. Dumbbell shaped samples have effective dimensions of 16 4 0.3 mm3. All the experimental data were obtained from the average value of three samples for each nanocomposite.
measured by UV spectrophotometer (Shimadzu UV-2550, Japan). The concentration of the protein solutions prepared for the standard curve is 0.02, 0.05, 0.1, 0.2, 0.4 and 0.6 g/l, respectively.
3. Results and discussion 3.1. Characterization of surface modified MgO nanoparticles In order to characterize whether the PCL grafted on the surface of the MgO particles or not, the FTIR spectra of MgO nanoparticles, PCL-grafted MgO nanoparticles and pure PCL polymer were investigated as shown in Fig. 1. It can be seen from Fig. 1(a) that the absorption peak at 1445 cm 1 can be ascribed to the characteristic vibration of Mg–O bond. The absorption peak at 3701 cm 1 is attributed to the vibration of hydroxyl groups on the MgO particle’s surface. The observed broad absorption peak at 3436 cm 1 is assigned to some varying interactions between hydroxyl groups on the surface of MgO particles. The absorption peaks at 1631 and 1122 cm 1 may be ascribed to the vibration of H-bond, because the MgO nanoparticles surface can absorb the water in the air. Compared with the spectra of MgO nanoparticles and pure PCL polymer (Fig. 1(c)), the characteristic absorption peaks of PCL are clearly observed from Fig. 1(b). The absorption peaks at about 2935 and 2861 cm 1 are corresponding to the stretching vibrations of C–H bond. The absorption peak at 1732 cm 1 is attributed to the stretching vibration of the carbonyl group (CQO) of PCL. Based on these results of FTIR spectra, it may be concluded that the PCL has been successfully grafted onto the surface of MgO particles. Fig. 2 shows the TGA curves of MgO and PCL-grafted MgO particles. The total weight loss for MgO nanoparticles is about 0.7 wt% when heated from room temperature to 700 1C, which is mainly ascribed to the volatilization of the absorbed water. The PCL-grafted MgO nanoparticles shows appreciable weight loss, which is attributed to the decomposition of the grafted PCL and thus the amounts of PCL grafted on the MgO nanoparticles’s surface were calculated as follows: the grafting ratio¼weight loss%(g-MgO) weight loss%(MgO). The results show that the grafting ratio is 3.06%. 3.2. Characterization of g-MgO/PLLA composites FTIR spectra of the g-MgO/PLLA nanocomposites were used to analyze the relationship between the inorganic particles and
2.6. Degradation test
1631 1445 1122 Transmittance/a.u.
The g-MgO/PLLA nanocomposites films were cut into 1 1 cm2 pieces and then were accurately weighed. The pieces were immersed in phosphate buffer saline (PBS, pH 7.4) and incubated at 37 1C. The mass loss of the pieces and the pH values of the solution were measured every week. The mass loss percentage was used to represent the degradation rate of the nanocomposites and the pH values were used to monitor the variation of pH induced by the release of L-lactic acid and the dissolution of g-MgO nanoparticles.
(a)
(b) 3701 3436 2935 3695
2.7. Evaluation of protein adhesion The film samples (area 1 mm 1 mm) were immersed in bovine serum albumin (BSA) solution (1 g/l) and thermostated at 37 1C for 4 h, then the samples were removed from the protein solution and washed by 5 ml PBS (pH 7.4, 0.01 M) for 3 times. The three washing solutions were pooled in one tube. The amount of protein adhesion was calculated by product of BSA concentration of the lotions
2861 (c)
1732
2500
1741 2000 1500
2943
4000
3500
3000
1000
500
Wavenumber/cm-1 Fig. 1. FTIR spectra of (a) MgO, (b) surface modified MgO nanoparticles and (c) pure PCL polymer.
F. Ma et al. / Journal of Physics and Chemistry of Solids 72 (2011) 111–116
increases to 2.5 wt%, agglomeration of g-MgO nanoparticles can be observed, as shown in Fig. 4(c). Further increase in the g-MgO content, serious agglomeration of g-MgO nanoparticles appears, as shown in Fig. 4(d). Besides, it can be seen that the surface morphology of g-MgO/ PLLA nanocomposite becomes rough and some pits appear on the surface. The amount and the size of pits increase with the increase of gMgO nanoparticles content.
101
Weight loss/%
100
113
a
99
98
3.3. Mechanical properties of g-MgO/PLLA nanocomposites
b 97
96
95 0
100
200
300
400
500
600
700
Temperature/°C Fig. 2. TG curves of (a) MgO and (b) surface modified MgO nanoparticles.
(e)
1755
Transmittance/a.u.
(d)
(c)
1748
(b)
1748
(a)
1748 1732
4000
3500
3000
2500
2000
Wavenumber/cm
1500
1000
500
-1
Fig. 3. FTIR spectra of (a) g-MgO, (b) (1 wt%)g-MgO/PLLA, (c) (2.5 wt%)g-MgO/PLLA, (d) (5 wt%)g-MgO/PLLA nanocomposites and (e) PLLA.
polymeric matrix. The FTIR spectra of pure PLLA, g-MgO nanoparticles and g-MgO/PLLA nanocomposites with different MgO contents are exhibited in Fig. 3. It can be seen that characteristic peaks of pure PLLA and g-MgO nanoparticles are still be observed in the spectra of all g-MgO/PLLA nanocomposites. It may be confirmed that the chemical structure of PLLA cannot be affected by the presence of the g-MgO nanoparticles. It is important to note that some changes occur in several absorption bands. The strong absorption band corresponding to the CQO groups of PLLA in the nanocomposites shifts to 1748 cm 1. Moreover, the absorption peaks of MgO nanoparticles at 3701 and 3436 cm 1 disappear. These results reveal that there exists some weak chemical interaction between the hydroxyl bonds on the surface of g-MgO nanoparticles and PLLA matrix. The surface morphology of SEM micrographs for the g-MgO/PLLA nanocomposites with different g-MgO content is shown in Fig. 4. From Fig. 4(a), it can be seen that the surface of pure PLLA is relatively smooth. From Fig. 4(b) it is apparent that the g-MgO nanoparticles are dispersed uniformly in the PLLA matrix. When the g-MgO nanoparticles content
Typical stress–strain curves of the g-MgO/PLLA nanocomposites films are shown in Fig. 5. It can be seen that all the stress–strain curves exhibit the same style. Initially, a uniform elastic deformation occurs and then a yield phenomenon appears. Subsequently, stress hardening happens due to the orientation of the molecular chains along the direction of the applied force and then the sample breaks in finally. Fig. 6(a) shows the tensile strength of the nanocomposites with different g-MgO contents. It is observed that the tensile strength increases with an increasing amount of g-MgO nanoparticles. The maximum tensile strength is obtained with the g-MgO content of 2.5 wt%. When the g-MgO content increases further to 5 wt%, the tensile strength begins to decrease. Fig. 6(b) shows the plots of the elastic modulus versus g-MgO content. It can be seen that the elastic modulus of the g-MgO/PLLA nanocomposites strongly depended on the g-MgO content. The elastic modulus increases with the increase of g-MgO content and possesses a maximum value for the g-MgO loading 2.5 wt%. When continuing to increase the g-MgO contents, the elastic modulus of the g-MgO/PLLA nanocomposites starts to decrease. The elongation at break of the g-MgO/PLLA nanocomposites decreases with the increase of g-MgO content, as shown in Fig. 6(c). In comparison with the pure PLLA, the elongation at break of 2.5% g-MgO/PLLA nanocomposite decreases about 50%. As we known, the mechanical properties of nanocomposites depend on many factors, mainly including the aspect of the ratio of filler, the degree of dispersion and the adhesion between the filler and the matrix [11]. Therefore, the experimental results mentioned in Fig. 6 can be explained as follows: firstly, it is considered that the higher strength of g-MgO nanoparticles offers the reinforcement effects in the g-MgO/PLLA nanocomposites. Secondly, the PCL grafted onto the MgO nanoparticles surfaces enhances the interfacial adhesion between g-MgO nanoparticles and PLLA matrix. Lastly, the dispersibility of the MgO nanoparticles in PLLA matrix can be improved due to PCL grafted onto the MgO nanoparticles’ surfaces. As the combination of these aspects, it is reasonable that the mechanical properties such as tensile strength and elastic modulus of g-MgO/PLLA nanocomposites increase with the increase of g-MgO content when the g-MgO content is low. However, when the g-MgO content exceeds 2.5 wt%, the strong agglomeration of g-MgO nanoparticles occurs, which leads to the deterioration of mechanical properties for g-MgO/PLLA nanocomposites. According to SEM results, the amount and the size of pits existing in the composites increase with the increase of g-MgO nanoparticles content, which can do harm to the strength and the elongation at break. Moreover, the decrease of elongation at break of the g-MgO/PLLA nanocomposites can be attributed to the decreases of the PLLA chain mobility due to the addition of g-MgO nanoparticles.
3.4. Degradable properties of g-MgO/PLLA nanocomposites The mass loss of g-MgO/PLLA nanocomposites during the degradation period is shown in Fig. 7. It can be seen that the degradation rate of the nanocomposites is higher than that of pure
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Fig. 4. SEM micrographs of (a) PLLA, (b) (1 wt%)g-MgO/PLLA, (c) (2.5 wt%)g-MgO/PLLA and (d) (5 wt%)g-MgO/PLLA.
70 60 (c)
Stress/MPa
50 40
(b)
(a)
(d)
30 20 10 0 0
50
100
150
200
Strain/% Fig. 5. The stress–strain curves of (a) PLLA, (b) (1 wt%)g-MgO/PLLA, (c) (2.5 wt%) g-MgO/PLLA and (d) (5 wt%)g-MgO/PLLA.
PLLA. With the increase of the g-MgO nanoparticles content, the degradation rate of the nanocomposites increases obviously. It is known that the degradation occurs by the hydrolysis of ester bonds for biodegradable PLLA polyester. Therefore, the degradation rate is associated with the ability of water absorption for the g-MgO/PLLA nanocomposites. According to the results of SEM, with a higher content of g-MgO, the water absorption of the g-MgO nanocomposites is comparatively higher, which accounts for the fast hydrolytic degradation. Moreover, the mass loss of the g-MgO/ PLLA nanocomposites is higher than that of pure PLLA, indicating a preferential dissolution of the g-MgO nanoparticles.
Fig. 8 shows the pH decrement value of the solution of g-MgO/ PLLA nanocomposites during the hydrolysis process. It can be seen that with the increasing of the degradation days, the pH decrement value initially increases and then decreases. At the early degradation stage, the pH decrement value firstly increases and then decreases with an increase of g-MgO content on the same degradation duration. However, when the degradation time is 21 days, it is worth noting that the pH decrement value of (5 wt%) g-MgO/PLLA nanocomposite is lower than that of pure PLLA. It is well known that the degradation of PLLA produces the lactic acid which could decrease the pH value of the medium. However, accompanying with the degradation of PLLA, the dissolution of the g-MgO nanoparticles could increase the pH value by alkalizing the medium. At the early stage of incubation period, the degradation rate of pure PLLA is the slowest, accordingly the pH decrement value is the lowest. For the g-MgO/PLLA nanocomposites, with the increase of g-MgO content, on the one hand, the pH decrement value increases due to the faster degradation rate. On the other hand, the neutralizing effect coming from the g-MgO can be improved which leads to the decrease of the pH decrement value. Comprehensively considered with the two factors, the variation trend of pH decrement value with g-MgO content can be illuminated. These results suggest that the g-MgO nanoparticles can neutralize effectively the lactic acid in the medium. 3.5. Evaluation of protein adhesion Fig. 9 shows the UV-spectrum of washing solution from g-MgO/ PLLA nanocomposites after protein adhesion tests. It can be seen that an apparent absorption peak is present at 280 nm, which indicates that the BSA is absorbed on the surfaces of all samples. The Abs is proportional to the concentration of protein in the sample. Therefore, the protein concentration can be calculated. The standard curve of the Abs (Y) versus the concentration of protein (X) eliminate that there exist
F. Ma et al. / Journal of Physics and Chemistry of Solids 72 (2011) 111–116
115
56
Modulus of elasticity/MPa
Tensile Strength/Mpa
750 52
48
44
700
650
600
40 550 0
1
2
3
4
0
5
1
2
3
4
5
Content of MgO/wt%
Content of MgO/wt% 200
Elongation at break/%
180 160 140 120 100 80 0
1
2 3 Content of MgO/wt%
4
5
Fig. 6. Dependence of tensile properties on g-MgO content for g-MgO/PLLA nanocomposites: (a) tensile strength, (b) modulus of elasticity and (c) elongation at break.
30
0.14
0% 1% 2.5% 5%
Mass loss/%
20
o% 1% 2.5% 5%
0.12
Decrement of pH value
25
15 10
0.10 0.08 0.06 0.04 0.02
5
0.00
0
0
0
5
10 15 Degradations/days
20
25
Fig. 7. Dependence of mass loss of g-MgO/PLLA nanocomposites on degradation time under pH 7.4 at 37 1C.
good linear relationship between the Abs and the concentration of protein, as shown in Fig. 10. The standard curve equation is as follows Y ¼ 0:51X þ0:017
ð1Þ
5
10
15
20
25
Degradation time/days Fig. 8. The change of the pH decrement value of the g-MgO/PLLA nanocomposites with degradable time.
According to the standard curve equation, the protein concentration can be calculated, as shown in Fig. 10. It is seen that the protein adsorption increases with the increase of g-MgO content. The (5 wt%)g-MgO/PLLA nanocomposites adsorbed 33% more
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F. Ma et al. / Journal of Physics and Chemistry of Solids 72 (2011) 111–116
4. Conclusions
0.10 a(0%) b(1%) c(2.5%) d(5%)
Absorbtance/%
0.08
0.06 d c b
0.04
a 0.02
0.00 220
240
260
280
300
320
340
Wavenumber/nm Fig. 9. UV-spectrum of g-MgO/PLLA composites after protein adhesion tests.
Protein concentration/ (g/l)
0.060
In this study, ring opening polymerization method is adopted to graft the PCL onto the MgO surface and these surface modified MgO nanoparticles are used to fabricate novel g-MgO/PLLA nanocomposites. FTIR and TGA analyses show that the PCL can be successfully grafted onto the MgO surface. The SEM morphology of the g-MgO/PLLA nanocomposites shows that the g-MgO nanoparticles can be comparatively homogenously dispersed in PLLA matrix. Furthermore, the mechanical properties, biodegradability and protein adhesion of the nanocomposites are investigated and their dependence on the g-MgO content is assessed. The tensile strength and elastic modulus exhibit a maximum at 2.5 wt% of the MgO nanoparticle content. Beyond this content, the tensile strength and elastic modulus decrease rapidly with increasing MgO nanoparticle content. The degradation rate of the nanocomposites increases with the increase of g-MgO content. The g-MgO nanoparticles could effectively neutralize the lactic acid resulting from the degradation of PLLA. The g-MgO/PLLA nanocomposites exhibited the enhanced protein adsorption i.e., with the increase of g-MgO content, the amount of protein adsorption increased. On the whole, the experiment results reveal that the g-MgO/PLLA nanocomposites possess improved mechanical properties, enhanced protein adhesion and a suitable degradation property. Therefore, the g-MgO/PLLA nanocomposites are advantageous for biomedical applications.
0.055 0.050
Acknowledgments
0.045
This project was supported by the National Natural Science Foundation of China (No. 50903023) and China Postdoctoral Science Foundation (20080440836).
0.040 0.035
References
0.030 0.025 0
1
2 3 The content of MgO/wt%
4
5
Fig. 10. The effect of g-MgO content on the amount of adsorbed BSA.
protein than the pure PLLA. Pore surface roughness of g-MgO/PLLA nanocomposites may be contributed to the characteristics of protein adsorption. It is known that protein plays a role in matrix-induced cell survival signaling [12]. These results indicate that the g-MgO/PLLA nanocomposites provide more favorable micro-environment for cell survival through enhancing protein adhesion.
[1] V. Grigoriou, I.M. Shapiro, E.A. Cavalcanti-Adam, R.J. Composto, P. Ducheyne, C.S. Adams, J. Biol. Chem. 280 (3) (2005) 1733–1741. [2] J.C. Middleton, A.J. Tipton, Biomaterials 21 (2000) 2335–2342. [3] B.L. Seal, T.C. Otero, A. Panitch, Mater. Sci. Eng.: R 34 (2001) 147–153. [4] S.S. Kim, K.M. Ahn, M.S. Park, J.H. Lee, C.Y. Choi, B.S. Kim, J. Biomed. Mater. Res. 80A (2007) 206–214. [5] S. Yan, J. Yin, Y. Yang, Z. Dai, J. Ma, X. Chen, Polymer 48 (2007) 1688–1694. [6] X.L. Lu, X.Q. Lv, Z.J. Sun, Y.F. Zheng, Eur. Polym. J. 44 (2008) 2476–2481. [7] J.T. Feng, W. Cai, J.H. Sui, Z.G. Li, J.Q. Wan, A.N. Chakoli, Polymer 49 (2008) 4989–4994. [8] E. Zhang, D.S. Yin, L.P. Xu, L.Y. Yang, K. Yang, Mater. Sci. Eng.: C 29 (2009) 987–993. [9] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (9) (2006) 1728–1735. [10] S.H. Rhee, Biomaterials 23 (2002) 4915–4923. [11] S. Kumar, T. Rath, R.N. Mahaling, C.S. Reddy, C.K. Das, K.N. Pandey, R.B. Srivastava, S.B. Yadaw, Mater. Sci. Eng. B 141 (2007) 61–67. [12] K.M. Woo, J.H. Seo, R.Y. Zhang, P.X. Ma, Biomaterials 28 (2007) 2622–2629.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Nanocomposites of poly(L-lactide) and surface modified magnesia nanoparticles: Fabrication, mechanical property and biodegradability Fuqiu Ma, Xili Lu n, Zhaomin Wang, Zhijie Sun, Fengfa Zhang, Yufeng Zheng Center for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 150001, People’s Republic of China
a r t i c l e in f o
abstract
Article history: Received 17 May 2010 Received in revised form 14 October 2010 Accepted 23 November 2010
A novel nanocomposite based on biodegradable poly(L-lactide) (PLLA) was prepared by the incorporation of surface modified magnesia (g-MgO) nanoparticles using a solution casting method. The mechanical properties, biodegradable properties and protein adhesion behavior of the g-MgO/PLLA nanocomposites were investigated. Scanning electron microscopy (SEM) results showed that g-MgO nanoparticles could comparatively uniformly disperse in PLLA matrix. The addition of g-MgO nanoparticles to PLLA matrix improved the tensile strength and elastic modulus, whereas reduced the elongation at break. The mass loss results showed that the nanocomposites with higher g-MgO content had faster degradation rates. The in vitro pH value determination results indicated that the g-MgO nanoparticles could neutralize effectively the lactic acid resulting from the degradation of PLLA. The g-MgO/PLLA nanocomposites exhibited enhanced protein adsorption, i.e., with the increase of g-MgO content, the amount of protein adsorption increased. The (5 wt%)g-MgO/PLLA nanocomposites adsorbed 33% more protein than the pure PLLA. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Polymers C. Infrared spectroscopy D. Mechanical properties
1. Introduction Poly(L-lactide) (PLLA) has attracted wide attention due to its biodegradability and biocompatibility. It has been widely applied to biomedical applications such as bone screws, surgical sutures, tissue engineering and controlled drug delivery [1,2]. However, PLLA has several disadvantages such as not enough strength, lack of desired bioactivity, etc. Moreover, when PLLA is implanted in the body, it is reported that the inflammation arises which has been attributed to the formation of lactic acid during the PLLA degradation [3]. In order to overcome these disadvantages, the introduction of inorganic nanoparticles into the PLLA matrix is a prevalent method. In the previous works, many kinds of inorganic fillers have been added to the PLLA matrix, such as hydroxyapatite (HAP) [4], silica nanoparticles [5], titanium dioxide [6], carbon nanotube [7] and many inspiring results can be obtained. For example, the introduction of the HA nanoparticles into PLLA matrix not only enhances the mechanical properties but also provides satisfactory osteoconductivity. The introduction of the silica or titanium dioxide into the PLLA matrix can improve the mechanical properties and cell attachment. In this paper, magnesia nanoparticles are selected as the inorganic filler basing on the following reasons. Firstly, magnesia nanoparticles are alkaline oxide, which can reduce the inflammatory
n
Corresponding author. Tel.: + 86 451 82518173; fax: + 86 451 82518644. E-mail address: [email protected] (X. Lu).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.11.008
reaction by buffering or neutralizing the acidic by-product. Secondly, the additions of magnesia nanoparticles may be improve the mechanical properties of PLLA. Lastly, magnesium is the essential element of human body, and magnesium ion is nontoxic [8,9]. Although the magnesia has been considered better choice as inorganic filler, to our knowledge, few reports about the MgO/PLLA nanocomposites can be found. In the present work, we modify the surface of magnesia nanoparticles in order to improve their dispersibility in PLLA matrix. The surface grafted magnesia nanoparticles were characterized by Fourier transformation infrared (FTIR) and thermal gravimetric analysis (TGA). The mechanical properties and biodegradability of the g-MgO/PLLA nanocomposites were investigated by tensile test and in vitro degradation tests. Furthermore, the protein adhesion of the g-MgO/PLLA nanocomposites was evaluated.
2. Experimental 2.1. Materials
e-caprolactone (99%, Aldrich) was distilled prior to use. Stannous octanoate was obtained from Sigma. Magnesia nanoparticles (average diameter of 20 nm) were purchased from Shanghai Huijinya nanomaterials company (China). PLLA (Mw 250,000) was prepared in our own laboratory using ring opening polymerization of L-lactide according to the literature [10]. All other agents were of analytical grade and used as received.
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F. Ma et al. / Journal of Physics and Chemistry of Solids 72 (2011) 111–116
2.2. Surface modified by the ring opening polymerization of e-caprolactone The PCL was grafted onto the surface of MgO nanoparticles through ring opening polymerization of e-caprolactone (e-CL) using stannous octanoate as catalyst. 3 g MgO nanoparticles was added to 50 g freshly distilled e-caprolactone. Stannous octanoate was then added as catalyst (the mole ratio of catalyst: e-CL ¼1:2000) and the reaction mixture was stirred at 140 1C for 8 h under nitrogen protect. The PCL-grafted MgO nanoparticles (g-MgO) were separated by centrifugation and purified by washing with excessive amount of chloroform for five times. Finally the g-MgO was dried in vacuo at 60 1C for at least 24 h. 2.3. Prepared the g-MgO/PLLA nanocomposites The g-MgO/PLLA nanocomposites with different contents (1, 2.5 and 5 wt%) of g-MgO nanoparticles were prepared by solution casting method. The detailed procedure can be seen in our previous report [6]. 2.4. Characterization of surface modified magnesia nanoparticles and the g-MgO/PLLA nanocomposites FTIR spectra were examined using a Perkin-Elmer spectrum 100 spectrophotometer. The samples were prepared by mixing the nanoparticles with KBr powders and pressing the mixture into disks. Thermogravimetric analysis was carried out on a thermogravimetric analyzer (Perkin-Elmer 7 series thermal analysis system) at a heating rate of 5 1C/min from room temperature to 700 1C under nitrogen gas flow. The surface morphology of the g-MgO/PLLA nanocomposites was observed using a Hitachi S4700 scanning electron microscope (SEM) at accelerating voltages in the range 5–15 KV. The surfaces of specimens were coated with gold prior to SEM observation. 2.5. Mechanical properties The tensile tests were conducted using electromechanical universal testing machine (Changchun Chaoyang WDS-5, China) with a loading rate of 10 mm/min. Dumbbell shaped samples have effective dimensions of 16 4 0.3 mm3. All the experimental data were obtained from the average value of three samples for each nanocomposite.
measured by UV spectrophotometer (Shimadzu UV-2550, Japan). The concentration of the protein solutions prepared for the standard curve is 0.02, 0.05, 0.1, 0.2, 0.4 and 0.6 g/l, respectively.
3. Results and discussion 3.1. Characterization of surface modified MgO nanoparticles In order to characterize whether the PCL grafted on the surface of the MgO particles or not, the FTIR spectra of MgO nanoparticles, PCL-grafted MgO nanoparticles and pure PCL polymer were investigated as shown in Fig. 1. It can be seen from Fig. 1(a) that the absorption peak at 1445 cm 1 can be ascribed to the characteristic vibration of Mg–O bond. The absorption peak at 3701 cm 1 is attributed to the vibration of hydroxyl groups on the MgO particle’s surface. The observed broad absorption peak at 3436 cm 1 is assigned to some varying interactions between hydroxyl groups on the surface of MgO particles. The absorption peaks at 1631 and 1122 cm 1 may be ascribed to the vibration of H-bond, because the MgO nanoparticles surface can absorb the water in the air. Compared with the spectra of MgO nanoparticles and pure PCL polymer (Fig. 1(c)), the characteristic absorption peaks of PCL are clearly observed from Fig. 1(b). The absorption peaks at about 2935 and 2861 cm 1 are corresponding to the stretching vibrations of C–H bond. The absorption peak at 1732 cm 1 is attributed to the stretching vibration of the carbonyl group (CQO) of PCL. Based on these results of FTIR spectra, it may be concluded that the PCL has been successfully grafted onto the surface of MgO particles. Fig. 2 shows the TGA curves of MgO and PCL-grafted MgO particles. The total weight loss for MgO nanoparticles is about 0.7 wt% when heated from room temperature to 700 1C, which is mainly ascribed to the volatilization of the absorbed water. The PCL-grafted MgO nanoparticles shows appreciable weight loss, which is attributed to the decomposition of the grafted PCL and thus the amounts of PCL grafted on the MgO nanoparticles’s surface were calculated as follows: the grafting ratio¼weight loss%(g-MgO) weight loss%(MgO). The results show that the grafting ratio is 3.06%. 3.2. Characterization of g-MgO/PLLA composites FTIR spectra of the g-MgO/PLLA nanocomposites were used to analyze the relationship between the inorganic particles and
2.6. Degradation test
1631 1445 1122 Transmittance/a.u.
The g-MgO/PLLA nanocomposites films were cut into 1 1 cm2 pieces and then were accurately weighed. The pieces were immersed in phosphate buffer saline (PBS, pH 7.4) and incubated at 37 1C. The mass loss of the pieces and the pH values of the solution were measured every week. The mass loss percentage was used to represent the degradation rate of the nanocomposites and the pH values were used to monitor the variation of pH induced by the release of L-lactic acid and the dissolution of g-MgO nanoparticles.
(a)
(b) 3701 3436 2935 3695
2.7. Evaluation of protein adhesion The film samples (area 1 mm 1 mm) were immersed in bovine serum albumin (BSA) solution (1 g/l) and thermostated at 37 1C for 4 h, then the samples were removed from the protein solution and washed by 5 ml PBS (pH 7.4, 0.01 M) for 3 times. The three washing solutions were pooled in one tube. The amount of protein adhesion was calculated by product of BSA concentration of the lotions
2861 (c)
1732
2500
1741 2000 1500
2943
4000
3500
3000
1000
500
Wavenumber/cm-1 Fig. 1. FTIR spectra of (a) MgO, (b) surface modified MgO nanoparticles and (c) pure PCL polymer.
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increases to 2.5 wt%, agglomeration of g-MgO nanoparticles can be observed, as shown in Fig. 4(c). Further increase in the g-MgO content, serious agglomeration of g-MgO nanoparticles appears, as shown in Fig. 4(d). Besides, it can be seen that the surface morphology of g-MgO/ PLLA nanocomposite becomes rough and some pits appear on the surface. The amount and the size of pits increase with the increase of gMgO nanoparticles content.
101
Weight loss/%
100
113
a
99
98
3.3. Mechanical properties of g-MgO/PLLA nanocomposites
b 97
96
95 0
100
200
300
400
500
600
700
Temperature/°C Fig. 2. TG curves of (a) MgO and (b) surface modified MgO nanoparticles.
(e)
1755
Transmittance/a.u.
(d)
(c)
1748
(b)
1748
(a)
1748 1732
4000
3500
3000
2500
2000
Wavenumber/cm
1500
1000
500
-1
Fig. 3. FTIR spectra of (a) g-MgO, (b) (1 wt%)g-MgO/PLLA, (c) (2.5 wt%)g-MgO/PLLA, (d) (5 wt%)g-MgO/PLLA nanocomposites and (e) PLLA.
polymeric matrix. The FTIR spectra of pure PLLA, g-MgO nanoparticles and g-MgO/PLLA nanocomposites with different MgO contents are exhibited in Fig. 3. It can be seen that characteristic peaks of pure PLLA and g-MgO nanoparticles are still be observed in the spectra of all g-MgO/PLLA nanocomposites. It may be confirmed that the chemical structure of PLLA cannot be affected by the presence of the g-MgO nanoparticles. It is important to note that some changes occur in several absorption bands. The strong absorption band corresponding to the CQO groups of PLLA in the nanocomposites shifts to 1748 cm 1. Moreover, the absorption peaks of MgO nanoparticles at 3701 and 3436 cm 1 disappear. These results reveal that there exists some weak chemical interaction between the hydroxyl bonds on the surface of g-MgO nanoparticles and PLLA matrix. The surface morphology of SEM micrographs for the g-MgO/PLLA nanocomposites with different g-MgO content is shown in Fig. 4. From Fig. 4(a), it can be seen that the surface of pure PLLA is relatively smooth. From Fig. 4(b) it is apparent that the g-MgO nanoparticles are dispersed uniformly in the PLLA matrix. When the g-MgO nanoparticles content
Typical stress–strain curves of the g-MgO/PLLA nanocomposites films are shown in Fig. 5. It can be seen that all the stress–strain curves exhibit the same style. Initially, a uniform elastic deformation occurs and then a yield phenomenon appears. Subsequently, stress hardening happens due to the orientation of the molecular chains along the direction of the applied force and then the sample breaks in finally. Fig. 6(a) shows the tensile strength of the nanocomposites with different g-MgO contents. It is observed that the tensile strength increases with an increasing amount of g-MgO nanoparticles. The maximum tensile strength is obtained with the g-MgO content of 2.5 wt%. When the g-MgO content increases further to 5 wt%, the tensile strength begins to decrease. Fig. 6(b) shows the plots of the elastic modulus versus g-MgO content. It can be seen that the elastic modulus of the g-MgO/PLLA nanocomposites strongly depended on the g-MgO content. The elastic modulus increases with the increase of g-MgO content and possesses a maximum value for the g-MgO loading 2.5 wt%. When continuing to increase the g-MgO contents, the elastic modulus of the g-MgO/PLLA nanocomposites starts to decrease. The elongation at break of the g-MgO/PLLA nanocomposites decreases with the increase of g-MgO content, as shown in Fig. 6(c). In comparison with the pure PLLA, the elongation at break of 2.5% g-MgO/PLLA nanocomposite decreases about 50%. As we known, the mechanical properties of nanocomposites depend on many factors, mainly including the aspect of the ratio of filler, the degree of dispersion and the adhesion between the filler and the matrix [11]. Therefore, the experimental results mentioned in Fig. 6 can be explained as follows: firstly, it is considered that the higher strength of g-MgO nanoparticles offers the reinforcement effects in the g-MgO/PLLA nanocomposites. Secondly, the PCL grafted onto the MgO nanoparticles surfaces enhances the interfacial adhesion between g-MgO nanoparticles and PLLA matrix. Lastly, the dispersibility of the MgO nanoparticles in PLLA matrix can be improved due to PCL grafted onto the MgO nanoparticles’ surfaces. As the combination of these aspects, it is reasonable that the mechanical properties such as tensile strength and elastic modulus of g-MgO/PLLA nanocomposites increase with the increase of g-MgO content when the g-MgO content is low. However, when the g-MgO content exceeds 2.5 wt%, the strong agglomeration of g-MgO nanoparticles occurs, which leads to the deterioration of mechanical properties for g-MgO/PLLA nanocomposites. According to SEM results, the amount and the size of pits existing in the composites increase with the increase of g-MgO nanoparticles content, which can do harm to the strength and the elongation at break. Moreover, the decrease of elongation at break of the g-MgO/PLLA nanocomposites can be attributed to the decreases of the PLLA chain mobility due to the addition of g-MgO nanoparticles.
3.4. Degradable properties of g-MgO/PLLA nanocomposites The mass loss of g-MgO/PLLA nanocomposites during the degradation period is shown in Fig. 7. It can be seen that the degradation rate of the nanocomposites is higher than that of pure
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Fig. 4. SEM micrographs of (a) PLLA, (b) (1 wt%)g-MgO/PLLA, (c) (2.5 wt%)g-MgO/PLLA and (d) (5 wt%)g-MgO/PLLA.
70 60 (c)
Stress/MPa
50 40
(b)
(a)
(d)
30 20 10 0 0
50
100
150
200
Strain/% Fig. 5. The stress–strain curves of (a) PLLA, (b) (1 wt%)g-MgO/PLLA, (c) (2.5 wt%) g-MgO/PLLA and (d) (5 wt%)g-MgO/PLLA.
PLLA. With the increase of the g-MgO nanoparticles content, the degradation rate of the nanocomposites increases obviously. It is known that the degradation occurs by the hydrolysis of ester bonds for biodegradable PLLA polyester. Therefore, the degradation rate is associated with the ability of water absorption for the g-MgO/PLLA nanocomposites. According to the results of SEM, with a higher content of g-MgO, the water absorption of the g-MgO nanocomposites is comparatively higher, which accounts for the fast hydrolytic degradation. Moreover, the mass loss of the g-MgO/ PLLA nanocomposites is higher than that of pure PLLA, indicating a preferential dissolution of the g-MgO nanoparticles.
Fig. 8 shows the pH decrement value of the solution of g-MgO/ PLLA nanocomposites during the hydrolysis process. It can be seen that with the increasing of the degradation days, the pH decrement value initially increases and then decreases. At the early degradation stage, the pH decrement value firstly increases and then decreases with an increase of g-MgO content on the same degradation duration. However, when the degradation time is 21 days, it is worth noting that the pH decrement value of (5 wt%) g-MgO/PLLA nanocomposite is lower than that of pure PLLA. It is well known that the degradation of PLLA produces the lactic acid which could decrease the pH value of the medium. However, accompanying with the degradation of PLLA, the dissolution of the g-MgO nanoparticles could increase the pH value by alkalizing the medium. At the early stage of incubation period, the degradation rate of pure PLLA is the slowest, accordingly the pH decrement value is the lowest. For the g-MgO/PLLA nanocomposites, with the increase of g-MgO content, on the one hand, the pH decrement value increases due to the faster degradation rate. On the other hand, the neutralizing effect coming from the g-MgO can be improved which leads to the decrease of the pH decrement value. Comprehensively considered with the two factors, the variation trend of pH decrement value with g-MgO content can be illuminated. These results suggest that the g-MgO nanoparticles can neutralize effectively the lactic acid in the medium. 3.5. Evaluation of protein adhesion Fig. 9 shows the UV-spectrum of washing solution from g-MgO/ PLLA nanocomposites after protein adhesion tests. It can be seen that an apparent absorption peak is present at 280 nm, which indicates that the BSA is absorbed on the surfaces of all samples. The Abs is proportional to the concentration of protein in the sample. Therefore, the protein concentration can be calculated. The standard curve of the Abs (Y) versus the concentration of protein (X) eliminate that there exist
F. Ma et al. / Journal of Physics and Chemistry of Solids 72 (2011) 111–116
115
56
Modulus of elasticity/MPa
Tensile Strength/Mpa
750 52
48
44
700
650
600
40 550 0
1
2
3
4
0
5
1
2
3
4
5
Content of MgO/wt%
Content of MgO/wt% 200
Elongation at break/%
180 160 140 120 100 80 0
1
2 3 Content of MgO/wt%
4
5
Fig. 6. Dependence of tensile properties on g-MgO content for g-MgO/PLLA nanocomposites: (a) tensile strength, (b) modulus of elasticity and (c) elongation at break.
30
0.14
0% 1% 2.5% 5%
Mass loss/%
20
o% 1% 2.5% 5%
0.12
Decrement of pH value
25
15 10
0.10 0.08 0.06 0.04 0.02
5
0.00
0
0
0
5
10 15 Degradations/days
20
25
Fig. 7. Dependence of mass loss of g-MgO/PLLA nanocomposites on degradation time under pH 7.4 at 37 1C.
good linear relationship between the Abs and the concentration of protein, as shown in Fig. 10. The standard curve equation is as follows Y ¼ 0:51X þ0:017
ð1Þ
5
10
15
20
25
Degradation time/days Fig. 8. The change of the pH decrement value of the g-MgO/PLLA nanocomposites with degradable time.
According to the standard curve equation, the protein concentration can be calculated, as shown in Fig. 10. It is seen that the protein adsorption increases with the increase of g-MgO content. The (5 wt%)g-MgO/PLLA nanocomposites adsorbed 33% more
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4. Conclusions
0.10 a(0%) b(1%) c(2.5%) d(5%)
Absorbtance/%
0.08
0.06 d c b
0.04
a 0.02
0.00 220
240
260
280
300
320
340
Wavenumber/nm Fig. 9. UV-spectrum of g-MgO/PLLA composites after protein adhesion tests.
Protein concentration/ (g/l)
0.060
In this study, ring opening polymerization method is adopted to graft the PCL onto the MgO surface and these surface modified MgO nanoparticles are used to fabricate novel g-MgO/PLLA nanocomposites. FTIR and TGA analyses show that the PCL can be successfully grafted onto the MgO surface. The SEM morphology of the g-MgO/PLLA nanocomposites shows that the g-MgO nanoparticles can be comparatively homogenously dispersed in PLLA matrix. Furthermore, the mechanical properties, biodegradability and protein adhesion of the nanocomposites are investigated and their dependence on the g-MgO content is assessed. The tensile strength and elastic modulus exhibit a maximum at 2.5 wt% of the MgO nanoparticle content. Beyond this content, the tensile strength and elastic modulus decrease rapidly with increasing MgO nanoparticle content. The degradation rate of the nanocomposites increases with the increase of g-MgO content. The g-MgO nanoparticles could effectively neutralize the lactic acid resulting from the degradation of PLLA. The g-MgO/PLLA nanocomposites exhibited the enhanced protein adsorption i.e., with the increase of g-MgO content, the amount of protein adsorption increased. On the whole, the experiment results reveal that the g-MgO/PLLA nanocomposites possess improved mechanical properties, enhanced protein adhesion and a suitable degradation property. Therefore, the g-MgO/PLLA nanocomposites are advantageous for biomedical applications.
0.055 0.050
Acknowledgments
0.045
This project was supported by the National Natural Science Foundation of China (No. 50903023) and China Postdoctoral Science Foundation (20080440836).
0.040 0.035
References
0.030 0.025 0
1
2 3 The content of MgO/wt%
4
5
Fig. 10. The effect of g-MgO content on the amount of adsorbed BSA.
protein than the pure PLLA. Pore surface roughness of g-MgO/PLLA nanocomposites may be contributed to the characteristics of protein adsorption. It is known that protein plays a role in matrix-induced cell survival signaling [12]. These results indicate that the g-MgO/PLLA nanocomposites provide more favorable micro-environment for cell survival through enhancing protein adhesion.
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