J Mater Sci (2015) 50:1435–1445 DOI 10.1007/s10853-014-8703-4
Toughening of electrospun poly(L-lactic acid) nanofiber scaffolds with unidirectionally aligned halloysite nanotubes Ning Cai • Qin Dai • Zelong Wang • Xiaogang Luo • Yanan Xue • Faquan Yu
Received: 27 June 2014 / Accepted: 4 November 2014 / Published online: 13 November 2014 Ó Springer Science+Business Media New York 2014
Abstract The mechanical properties of the tissue engineering scaffold are important as they are tightly related the regeneration of structural tissue. The application of poly(L-lactic acid) (PLLA) nanofiber scaffolds in tissue engineering has been hindered by their insufficient mechanical properties. In the study, halloysite nanotubes (HNTs) were used to reinforce the mechanical properties of PLLA-based nanofibers. 4 wt% HNT/PLLA nanofiber membranes possess the best mechanical performance, which represents 61 % increase in tensile strength, 100 % improvement of Young’s modulus, 49 % augment of elongation to break, as well as 181 % elevation in energy to break compared with neat PLLA samples. The satisfactory enhancement effect of HNTs can be attributed to the effective dispersion and incorporation of HNTs in PLLA matrix, which have been confirmed by the analysis of SEM, TEM, and FTIR. The addition of HNTs also improves the degree of crystallization and thermal stability of PLLA-based nanofibers. HNT-incorporated PLLA nanofiber membranes possess higher protein adsorption from fetal bovine serum than the neat PLLA specimen. Therefore, the introduction of HNTs can effectively enhance the mechanical properties of PLLA nanofiber scaffolds. HNT/PLLA nanofiber scaffolds possess potential application in skin tissue engineering.
N. Cai Q. Dai Z. Wang X. Luo Y. Xue F. Yu (&) Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430073, China e-mail:
[email protected];
[email protected] Z. Wang Department of Research, Hunan Xiangjiang Kansai Paint Co., Ltd., Changsha 410003, China
Introduction One of the key objectives in tissue engineering is to develop a scaffold for supporting three-dimensional tissue regeneration [1]. To fulfill this goal, some specific criterions should be observed in designing tissue engineering scaffolds [2]. A high porosity with adequate pore size is necessary to facilitate cell seeding and nutrients’ diffusion [3]. Biodegradability is also an essential factor, since scaffolds should be absorbed by the surrounding tissues during the new tissue regeneration [4]. In addition, the sufficient mechanical strength and the structural integrity of the scaffolds are very important for handling an implant and maintaining the desired structure prior to the formation of new tissue [5]. Indeed, it has been found that the strength and deformability of scaffold influence in vitro cell migration, proliferation and differentiation, along with cell morphology [6]. Electrospinning biopolymer to generate nanometer to micrometer-scale fibers has emerged as a prominent method for fabricating 3D scaffolds with tissue-like microstructures. Numerous natural and synthetic biopolymers have been successfully electrospun to produce microor nanofibers for tissue engineering and other related applications [7]. Among them, poly(L-lactic acid) (PLLA) is one of the most extensively used biopolymers, as it is one of the few bioresorbable polymers that have been approved by the U.S. Food and Drug Administration for in vivo applications. However, electrospun PLLA nanofiber scaffolds normally have weak mechanical strength, partially resulted from high porosity and random alignment of fibers, which limit their biomedical applications, especially as tissue engineering scaffolds [8, 9]. To address this issue, different types of fillers, including inorganic particles, such as hydroxyapatite, carbon
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nanotubes and graphene oxide, have been co-electrospun into the polymer nanofibers for improving mechanical properties [10]. For instance, the improved mechanical stiffness has been shown on carbon nanotube-incorporated polymeric nanofibers [11]. However, these extensively used nanofillers do not possess satisfactory biocompatibility, which becomes the major hurdle in extending their potential applications to biomedical engineering [12]. The environmental friendliness and biocompatible nature make halloysite clay nanotubes [Al2Si2O5(OH)4nH2O] an attractive nanoreinformcent candidate. Halloysite clay nanotubes (HNTs) are aluminosilicate tubes with length of 100–1000 nm, diameter of ca 50 nm, and internal diameter of 15 nm [13]. Because of its good biocompatibility and low cost, HNTs have attracted increasing interests of researchers due to their unique advantages. HNTs have been demonstrated to be an ideal reinforcement agent for fabricating polymeric composites with improved mechanical performance [14]. Recently, several types of HNTfilled composite nanofibers have been successfully produced. However, there were no reported studies on the application of HNTs for improving mechanical performance of electrospun PLLA nanofibers. The properties of HNT/PLLA composite nanofibers were also not studied systematically. The aim of this work is to evaluate the effect of addition of biocompatible halloysite nanotubes (HNTs) on the mechanical properties of electrospun PLLA nanofiber membranes. The structure and morphology of the electrospun nanofiber membranes were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Crystallization behavior and thermal stability were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) methods, respectively. The effect of addition of HNTs on mechanical properties was examined by the tensile tests and the reinforcement mechanism was discussed. Protein adsorption of HNT/PLLA from fetal bovine serum (FBS) was also evaluated.
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10 wt% water solution of halloysite followed by the addition of 0.05 wt% sodium hexametaphosphate. The obtained solution was stirred for 30 min and left to stand for 20 min at room temperature. By filtration, the clay aggregate and impurities precipitated in the bottom were removed. The supernatant was collected and centrifuged to obtain HNTs. The purified HNTs were dried and stored for further use. Chloroform, dimethylformamide (DMF) and other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Preparation of electrospun HNT/PLLA nanofiber membranes Solution intercalation technique was employed for the fabrication of composites. Initially, HNTs were suspended in chloroform and DMF mixture solution (The volume ratio of chloroform to DMF was 6:1) by stirring for 12 h, followed by sonication of 1 h at room temperature to achieve good dispersion. PLLA pellets were gradually added into the HNTs suspension with slowly raising the temperature up to 60 °C. The suspension was continuously stirred at 60 °C for 2 h to dissolve PLLA. The electrical conductivity and viscosity of the HNT/PLLA solution were measured by an electrical conductivity meter (DDS 307A, Shanghai Rex Instrument, China) and an AR2000 rheometer (TA Instruments, United States) at 22 °C, respectively. The HNT/PLLA nanofibers were prepared by electrospinning solution using an electrospinning system (Beijing Kangsente Co., China), which comprised a syringe pump and a high voltage power supply generating positive DC voltage. A 10 mL syringe containing electrospinning solution was connected to a stainless steel needle with an inner diameter of 0.6 mm. The needle tip was set up horizontally. A vertical metal plate wrapped with aluminum foil was used to collect the electrospun nanofibers. The electrospinning parameters were fixed as follows: feeding rate, 2.0 mL/h; voltage, 20 kV; distance between needle tip and collector, 10 cm; humidity, 40–50 %; temperature, 25 °C. The nanofiber membranes were then peeled off from the aluminum foil for further characterization.
Experimental
Structural and thermal characterization
Materials
The morphology of HNTs and the electrospun HNT/PLLA nanofiber membranes was investigated using a SEM (JEOL JSM-5510LV) at an accelerating voltage of 10 kV. The nanofiber diameter distribution was obtained by analyzing at least three distinct images using ImageJ software. The dispersion of the HNTs within the composite nanofibers was examined by a TEM (FEI TecnaiG2 20 S-Twin). FTIR was recorded with a Nicolet 6700 FTIR spectrometer in the range of 4000–600 cm-1 to determine the chemical
PLLA with molecular weight of 7,3000 kD was obtained from Sigma-Aldrich, and its density was 1.25 g/cm3. HNTs were mined from the deposit of Hubei province of China, which had an average diameter of 60 nm, a length of about 1.2 microns, and a density of 2.5 g/cm3. Before usage, HNTs were purified according to a reported protocol [15]. In brief, dry halloysite was added into water to prepare
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signatures. XRD analysis was performed with a Bruker D8 ADVANCE X-ray diffractometer at a voltage of 40 kV with Ni-filtered Cu Ka radiation. The 2h scan data were collected from 10.0° to 50.0° at a scanning speed of 1.0°/ min. The thermal stability was studied with thermo gravimetric analysis (TGA) using a thermogravimeter (Netzsch, Germany). A 5 mg sample was placed in an aluminum pan and a heating rate of 10 °C/min was employed under nitrogen flow. DSC measurements were carried out in the 20–200 °C range using a SII DSC 6220 equipment apparatus (Seiko Instruments, Japan), at a heating rate of 10 °C/ min in nitrogen atmosphere. The degree of crystallinity was obtained by using the equation [16]. Xc ¼
DHm DHcc 100 %; DHm0 ð1 Wf Þ
where Xc (%) is crystallinity, DHm is the melting enthalpy, DHcc is the cold crystallization enthalpy, DH0m is the melting enthalpy of completely crystallized PLLA (93.7 J/ g according to Ref. [17]), and Wf is the weight fraction of HNTs in the composites.
Fig. 1 TEM image of halloysite nanotubes
calculated based on the difference of FBS solution concentration before and after adsorption.
Mechanical properties testing Statistical analysis The mechanical properties of electrospun fiber membranes were measured with an MTS (CMT 800, USA) tensile testing machine using a 200 N load cell. Rectangular specimens were cut to 60 mm 9 5 mm and extended at a constant speed of 10 mm/min with a 40 mm gauge length. Each specimen was tested for five times to acquire the mean value. The thickness of each specimen was the average of three measurements taken along the gauge length with a digital micrometer. The force displacement data were taken from the tensile machine and converted to engineering stress-engineering strain results. Engineering stress was defined as the ratio of force to the initial crosssectional area, and engineering strain was defined as the ratio of the change in length to the original gauge length. Protein adsorption onto HNT/PLLA nanofiber membranes Electrospun PLLA and HNT/PLLA fibrous membranes were cut into round pieces in a diameter of 15 mm. The samples were placed in a 24-well tissue culture plate and immersed in 0.01 M PBS. After being equilibrated with PBS overnight, the samples were incubated in 0.5 mL of FBS (10 %) for 24 h at 37 °C. The concentration of FBS solution before or after adsorption was determined by the absorbance read at 280 nm in a SpectraMax M2e spectrophotometer. Independent measurements were performed in four samples and the amount of the adsorbed protein was
All the data were shown as a mean ± standard deviation. A one-way analysis of variance (ANOVA) was performed to compare the mean values among different groups. Statistical significance was tested at p \ 0.05.
Results and discussion Morphological and structural analysis HNTs consist of gibbsite octahedral sheet (Al–OH) groups on the internal surface and siloxane groups (Si–O–Si) on the external surface [18]. Figure 1 shows the TEM image of HNTs. As exhibited in Fig. 1, HNTs possess the tubular structures. Following the analysis by ImageJ, it is determined that the HNTs used in this study range in length from 0.5 to 1.2 lm and in diameter from 50 to 100 nm. The morphology of electrospun HNT/PLLA nanofiber membranes was examined through SEM inspection. It is shown in Fig. 2a that the neat PLLA nanofibers are smooth, and no broken ends as well as beads were found. The smooth surface of PLLA-based nanofibers is still maintained after the incorporation of low content of HNTs. However, C2 wt% addition of HNTs resulted in nanofibers with uneven surface. It is clear that HNT/PLLA nanofiber membranes containing 6 wt% loading of HNTs possess rather rough surfaces. In addition, the incorporation of
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1438 Fig. 2 SEM micrographs and the corresponding diameter distribution of electrospun HNT/PLLA nanofiber membranes containing 0 (a), 1 (b), 2 (c), 4 (d), and 6 wt% (e) loading of HNTs
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HNT particles into PLLA also induces the change of the nanofiber diameter. As shown in Fig. 2a–e, the composite nanofiber diameter gradually increases from 307 ± 82 to 917 ± 121 nm when the HNT content is elevated gradually up to 6 wt%. The influence of HNT particles on the morphology of PLLA nanofibers presumably results from the impaired stretching of the fiber during the electrospinning process. It is known that the viscosity and the charged density of electrospinning solution contribute to the change of diameter of electrospun nanofibers [19]. According to our measurement, the incorporation of HNTs induces the gradual increase of viscosities of electrospinning solution from 0.09 Pa s (neat PLLA) to 0.22 Pa s (6 wt% HNT/PLLA). High viscosity restricts the stretching of the liquid jet, beneficial for the formation of larger electrospun fibers [20]. With addition of HNTs, the electrical conductivity of electrospinning solution demonstrates a ascending trend with the augment of HNT contents (0.27 lS cm-1 for PLLA and 3.78 lS cm-1 for 6 wt % HNT/PLLA). Thus, the introduction of negatively charged HNTs into the electrospinning solution results in the increased charge density of electrospinning solution, promoting the formation of smaller nanofibers. As shown in Fig. 2, the diameter of HNT/PLLA nanofibers demonstrates an increasing trend with HNT content. Therefore, it is the elevation in viscosity following the addition of HNTs that induces the increase of the diameter with HNT content for electrospun HNT/PLLA nanofibers [21]. FTIR spectroscopy was utilized to study the chemical structures of the neat PLLA and HNT/PLLA composite nanofiber membranes. As illustrated in Fig. 3, FTIR spectra of HNT exhibits two characteristic bands at 3623 and 3695 cm-1 which are assigned to the stretching vibration of inner O–H and the O–H located at the inner-surface of the nanotubes, respectively [22]. A strong absorption peak centers at 1028 and 1008 cm-1, which is attributed to the stretching vibration band of in-plane Si–O–Si of HNT [23]. For HNT/PLLA nanocomposites, the peak at 912 cm-1 corresponding to deformation vibration of inner O–H of HNT [24] can be identified when HNT content is raised over 2 wt%. Under the influence of the incorporated HNTs, the strong peak attributed to the stretching vibration of C=O of PLLA gradually shifts to lower wavenumber, from 1750 cm-1 (neat PLLA) to 1746 cm-1 (6 wt% HNT/ PLLA). The shift may be mainly stemmed from hydrogenbonding interactions between the carbonyl groups (C=O) of PLLA and the hydroxyl groups of HNTs. In fact, the hydroxyl groups of PLLA can also interact with the Si–O– Si groups of HNT via hydrogen bonding interactions [25]. Liu et al. have reported that the peak of the C=O stretching vibration at 1756 cm-1 for neat PLLA is shifted to 1750 cm-1 for the 40 wt% HNT/PLLA nanocomposites [25]. Therefore, there is not conspicuous difference in the
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Fig. 3 FTIR spectra of electrospun HNT/PLLA composite nanofiber membranes containing 0 (a), 1 (b), 2 (c), 4 (d), and 6 wt% (e) loading of HNTs and pristine HNTs (f)
Fig. 4 XRD spectra of HNTs (a) and electrospun HNT/PLLA composite nanofiber membranes containing 0 (b), 1 (c), 2 (d), 4 (e) and 6 wt% (f) loading of HNTs
shift of the peak assigned to C=O stretching vibration between HNT/PLLA composites and electrospun HNT/ PLLA composite nanofibers. The influence of nanofillers on characteristic FTIR band shift of PLLA was also reported in other composite systems [26, 27]. Obviously, these interactions result in effective adhesion between HNTs and PLLA matrix, which probably affect the mechanical and thermal properties of PLLA-based composite nanofibers. To illustrate the interactions of HNTs with PLLA, XRD experiment was conducted. For neat PLLA, only a broad scattering reflection, locating at around 2h = 16°, is found in the XRD spectrum, indicating that it does not crystallize during the electrospinning in the sample preparation process [28]. The XRD pattern of the original halloysite sample, shown in Fig. 4, is in good agreement with a
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Fig. 5 DSC curves of electrospun HNT/PLLA composite nanofiber membranes
Table 1 Thermal properties of electrospun HNT/PLLA nanofiber membranes HNT content (wt%)
Tg (°C)
Tcc (°C)
Tm (°C)
DHm (J/g)
Xc (%)
T10wt% (°C)
T50wt% (°C)
0
63.9
82.5
165.5
30.2
20.1
314.8
353.6
1
62.4
77.6
166.9
39.2
25.3
328.1
357.3
2
61.6
77.5
166.9
36.8
32.3
334.9
361.4
4 6
61.8 61.8
77.1 80.5
166.5 165.8
41.1 37.6
40.8 36.7
343.9 342.6
366.1 367.0
previously published pattern for halloysite [29]. Three distinct XRD peaks at 2h = 12.1°, 20.0° and 24.9° in relation to reflection planes (0 0 1), (0 2 0), (1 1 0) and (0 0 2) are observed, which correspond to d = 0.732, 0.446 and 0.358 nm, respectively [30]. It should be pointed out that the reflection of HNTs at around 20.0° seems to disappear in all the HNT/PLLA nanocomposites, which may be attributed to the low intensity of XRD peak of HNT at 2h = 20.0°. Thermal analysis PLLA is semicrystalline polymer and its mechanical and physical properties are governed by the crystal microstructure. The introduction of nanosized HNTs may induces the formation of the interfacial interactions between PLLA and HNTs, affecting the crystallization behavior of composites. As shown in Fig. 5, there is a step-like change for all samples in the temperature range of 50–70 °C, which is assigned to the glass transition region of PLLA [31]. It can be seen in Fig. 5 and Table 1 that the glass transition temperature (Tg) of the HNT/PLLA nanocomposites decreases slightly with the increment of HNT content within 2 wt%.
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Incorporating isotropic nanoparticles including nanodiamonds [32] and nano-TiO2 [33] into the PLLA matrix usually induces the increase of Tg. Following the addition of nanofillers, polymer chain may interact with the fillers through intermolecular attractions, such as van der Waals force and hydrogen bonding [34]. Thus, the mobility of the polymer chain segment in the vicinity of the nanofiller surface may be inhibited by the inorganics [35]. For the anisotropic nanofillers, the influence of nanofillers on Tg of polymer matrix could be different, which were due to oversize in at least one dimension for the nanofillers [36, 37]. For instance, the diameter of HNTs ranges from 50 to 100 nm. Although the diameter of HNTs is on the nanoscale, their length is usually in the microscale ([1 lm), which far exceeds the typical gyration radii of polymer chains. Consequently, the packing of the polymer chains is prevented and the free volume near HNT surface increases [38]. With the increase of free volume, the increase mobility of the polymer chains is allowed [39]. Of course, the incorporated HNTs can still inhibit the motion of PLLA chains due to HNT-PLLA intermolecular interactions. However, the inhibition of motion of polymer segments is offset by the promotion of free movement resulted from the increased free volume [38]. Furthermore, the latter has become the major factor in determining the Tg. As a result, the incorporation of HNTs tends to lower the Tg of the PLLA-based nanocomposites. The decrease of Tg with the increase nanofiller content has also been reported in HNT/EPDM (Ethylene Propylene Diene Monomer) [37] and MWCNTs/PMMA [40] systems. It should be pointed out that the Tg maintains almost constant value in the range of HNT content from 2 to 6 wt%, which suggests the enhancing effect of newly generated free volume is just canceled out by the restraining effect of additional HNTs incorporated in PLLA matrix on the free motion of PLLA chain segments. The introduction of HNTs also induces the change of cold crystallization peak temperature (Tcc). It is shown in Table 1 that Tcc maintains a descending trend within the 4 wt% HNT content and then goes up with the increase of HNT content from 4 wt% upwards. This lowering of Tcc could be attributed to the nucleating effect of HNTs on polymer crystallization. Generally, small amount of HNT can serve as an effective nucleating agent for PLLA [41]. When HNT content is too high ([4 wt%), nucleating effect is on long pronounced and Tcc starts to go down with the increase of HNT content, which could be explained by the reduction of nucleation surface resulted from the aggravation of HNT agglomeration [42]. This explanation is consistent with TEM results (Fig. 8). Due to the nucleating effect of HNTs, the degree of crystallinity (Xc) of HNT/PLLA nanocomposites also makes change, which increases from 20.1 to 40.8 % within
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Fig. 6 TGA curves of electrospun HNT/PLLA composite nanofiber membranes
4 wt% of HNT content. The ascending trend of Xc is reversed when HNT content exceeds 4 wt%. Based on the analysis of Tg and Xc, 4 wt% as the critical concentration, can be supposed to an indicator which reflects the distribution status of HNTs in PLLA matrix. For melting temperature (Tm) of HNT/PLLA nanocomposite, no obvious change is observed, as halloysites are mineral fillers [43]. The thermal stability of the neat PLLA and HNT/PLLA nanocomposites were characterized by TGA. As shown in Fig. 6, neat PLLA begins to decompose at *254 °C. Comparatively, higher temperature is required to initiate the decomposition of HNT/PLLA nanocomposite. Following the decomposition of neat PLLA, there was almost no residue. In contrast, HNT/PLLA cannot decompose completely even under 600 °C, the residue of which is related to the HNTs. Comparing the thermal decomposition temperatures (Td) of neat PLLA and HNT/PLLA nanocomposites for 10 and 50 % weight loss, it is found that T10 and T50wt% of HNT/PLLA are higher than those of neat PLLA. Therefore, HNT/PLLA nanocomposites possess better thermal stability than neat PLLA. It was reported the improvement of thermal stability through introducing fillers was largely dependent on the dispersion of fillers in matrix. In our study, serious agglomeration of HNTs is not found in TEM observation. Therefore, relatively uniformly dispersed HNTs in the matrix act like a barrier to the passage of the volatile pyrolized products of PLLA, eventually retarding thermal decomposition of the HNT/PLLA [44]. The present TGA results are consistent with the previous reported thermal stability results of silica/ PLLA [45], and graphene/epoxy nanocomposites [46]. Mechanical properties The mechanical properties of the electrospun nanocomposite membranes were investigated by the tensile stress– strain testing method. Representative stress–strain curves
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of the HNT/PLLA nanofiber membranes are presented in Fig. 7a, and their tensile strength, Young’s modulus, elongation at break and toughness are summarized in Table 2. For all the samples, it is found that the improvement of tensile strength and Young’s modulus is achieved on HNT/PLLA samples. With the addition of 4 wt% HNT, the tensile strength and Young’s modulus demonstrate an elevation of 61 and 100 %, respectively, which indicates that the nanofiber membranes become stiffer and stronger. However, extra addition of HNT nanotubes no longer produces enhancing effects. In fact, both tensile strength and Young’s modulus start falling down with the increase of HNT content from 4 to 6 wt%, as demonstrated in Table 2. To quantitatively characterize the toughness of nanocomposite membranes, the energy to break was determined by integrating the stress–strain curves in Fig. 7a. It is shown in Table 2 that neat PLLA possesses the energy to break of 0.32 MJ/m3. Accompanied by the addition of HNTs, the energy to break for the nanocomposite is elevated up to 0.90 MJ/m3 (at 4 wt%). Therefore, the incorporation of HNTs induces conspicuous increase in toughness. According to the reported studies, the Young’s modulus of human skin ranges from 15 to 150 MPa, dependent to different age [47]. As shown in our study, Young’s modulus of PLLA-based nanofiber membranes can be elevated from 7.1 MPa (neat PLLA) to 14.2 MPa (4 wt% HNT/ PLLA), which is just lying in the aforementioned range. Of course, if higher Young’s modulus is required, the scaffolds fabricated in our study still need further processing including thermal annealing to achieve the desired mechanical properties. The mechanical reinforcement performance of fillers on composites is relied on the effective load transfer from the matrix to the fillers [48], which can be achieved when there are strong interactions at the nanofiller-matrix interface and the nanofillers are dispersed uniformly in the matrix [49]. There exist three main mechanisms for interactions between matrix and fillers, which are micromechanical interlocking, chemical bonding, and van der Waals force. There are effective interactions between HNT walls and PLLA chains due to the existence of hydrogen bonding. As-received HNTs are held together in bundles van der Waals force. Thus, it is crucial to disperse nanotubes well in polymer matrix to acquire satisfactory mechanical performance of the composites. Ultrasonication was employed in the fabrication of HNT/PLLA composite nanofibers. To monitor the distribution of HNT within the polymer matrix, TEM observation was performed. As shown in Fig. 8a, the incorporated nanotubes are straight and aligned along the fiber axis. In fact, agglomeration of HNTs in the 2 wt% HNT/PLLA nanofibers is seldom observed, indicating good dispersion of HNTs. Thus, applied external load can be
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Fig. 7 Stress-strain and Young’s modulus of electrospun HNT/PLLA nanofiber membranes
Table 2 Mechanical properties of electrospun HNT/PLLA nanofiber membranes HNT content (wt%)
Tensile strength (MPa)
Young’s modulus (MPa)
Elongation at break (%)
Toughness (MJ/m3)
0
0.75 ± 0.02
7.1 ± 0.6
59.6 ± 6.1
0.32 ± 0.03
1 2
0.92 ± 0.04 1.05 ± 0.02
12.0 ± 1.3 12.9 ± 1.2
74.9 ± 7.9 79.3 ± 8.2
0.52 ± 0.04 0.63 ± 0.05
4
1.21 ± 0.02
14.2 ± 1.3
88.7 ± 9.0
0.90 ± 0.07
6
1.13 ± 0.03
13.7 ± 1.3
71.6 ± 7.2
0.62 ± 0.05
effectively transferred to HNTs, inducing the improvement of mechanical properties for PLLA-based composite nanofibers. However, if HNT content is too high (e.g., 6 wt%), agglomeration becomes evident. As shown in Fig. 8b, the two HNTs in the fibers are assembled ‘‘shoulder by shoulder’’. Good particle–matrix interfacial adhesion cannot form in aggregative nanotubes, which
Fig. 8 TEM images of electrospun 2 wt% (a) and 6 wt% (b) HNT/PLLA nanofibers
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impairs the effective load transfer from the polymer matrix to the fillers. Thus, these aggregates act as defects, resulting in degraded mechanical performance [35]. In the results of DSC, 4 wt% is deduced to be the critical concentration to judge whether the dispersion of HNTs is aggravated. HNT/PLLA nanofiber membranes exhibit their best mechanical performance at 4 wt% of HNT content, which is consistent of DSC results. It should be emphasized that nanotubes are preferentially oriented along the longitude of fiber, which is induced by the shear force during electrospinning processing. Longitudinal alignment rather than random orientation of nanofillers in nanofibers is beneficial for the improvement of the mechanical properties of the HNT/ PLLA nanocomposite [25]. To gain better estimation of the enhancing effect of HNT particles, Reuss and Voigt models equation [46, 50] was used to estimate the reinforcement effect of the HNT particles on the Young’s modulus of the composite. The
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absorb more protein than PLLA samples without HNTs. The protein adsorption onto PLLA scaffolds with 1, 2 and 4 wt% HNT loading is 1.1–1.8 times higher than that of neat PLLA scaffolds. Therefore, the incorporation of HNT favors the protein adsorption, which could be related to the difference in surface composition of nanofibers due to the incorporation of HNTs [52]. In fact, rough surface of nanofiber with high HNT content may also make partial contribution to the higher protein absorption [53].
Conclusions
Fig. 9 Protein adsorption onto cover slips and electrospun HNT/ PLLA nanofiber membranes
lower and upper bounds of Young’s modulus deduced by Reuss and Voigt models are given by: ð1Þ Eclower ¼ Ef Em Ef 1 Vf þ Em Vf Ecupper ¼ Ef Vf þ Em 1 Vf ; ð2Þ and Eupper are lower- and upper-bound of where Elower c c composite’s modulus, Ef and Em are modulus of filler and matrix, and Vf is volume fraction of filler. Ef of HNT was assumed to be 300 GPa [51]. Here, 1200 and 60 nm are used as lf and df, respectively, from the SEM image (data not shown). The volume fraction of the HNT can be calculated according to Vf ¼
W f ; qf q Wf þ q q f Wf m
ð3Þ
HNTs were employed for enhancing the mechanical properties of PLLA-based nanofibers scaffolds. Substantial improvement of mechanical properties of HNT/PLLA composite nanofibers is achieved with the addition of HNTs. At 4 wt% of HNT content, HNT/PLLA nanofiber membranes possess the optimum mechanical performance, which represents 61 % increase in tensile strength, 100 % improvement of Young’s modulus, 49 % augment of elongation to break, as well as 181 % elevation in energy to break. The results of SEM and TEM demonstrate the effective dispersion of HNTs in PLLA matrix. Strong interactions between nanofillers and PLLA are confirmed by FTIR, XRD, and DSC. Therefore, efficient transfer of applied load from the matrix to HNTs is enabled, which explains the reinforcement effect of incorporated HNTs on PLLA nanofibers. The introduction of HNTs also improves the degree of crystallization and thermal stability of PLLA-based nanofibers. Furthermore, HNT-reinforced PLLA nanofiber membranes possessed higher protein adsorption from FBS than neat PLLA specimen, which possesses potential application in tissue engineering.
m
where Wf is the weight fraction of the HNT, and qf and qm are the densities of the HNT and the polymer matrix, respectively. HNT density and PLLA density are taken as 2.5 and 1.25 g/cm3, respectively. The experimental data, theoretical lower- and upper-bound of Young’s modulus are showed in Fig. 7b. It is found that the experimental data lie between these two bounds, confirming the reasonability of our results. Protein adsorption onto HNT-doped PLLA fibrous scaffolds An ideal scaffolding material should allow good protein adsorption onto the material surface, to provide sufficient nutrition to promote cell growth and migration. As demonstrated in Fig. 9, neat PLLA and HNT-incorporated PLLA nanofiber scaffolds have greater absorption of FBS than cover slips which are lack of porous fibrous structure. In addition, the HNT/PLLA nanofiber membranes can
Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No. 21071114), the Excellent Program of Activity of Science and Technology for Overseas-Returned Scientists founded by the Ministry of Human Resources and Social Security of the People’s Republic of China, the Program for Innovative Research Teams of Hubei Provincial Department of Education, the Scientific Research Foundation for Returned Overseas Chinese Scholars of State Education Ministry, Key Natural Science Foundation of Hubei Province (Grant No. 2012FFA100), the Innovative Team Incubation Program in High-Tech Industry of Wuhan City (Grant No. 2014070504020244) and Graduate Innovative Fund of Wuhan Institute of Technology (Grant No. CX2013010).
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