Preparation and characterization of vancomycin releasing PHBV coated 45S5 Bioglass®-based glass–ceramic scaffolds for bone tissue engineering

Available online at www.sciencedirect.com

ScienceDirect Journal of the European Ceramic Society 34 (2014) 505–514

Preparation and characterization of vancomycin releasing PHBV coated 45S5 Bioglass®-based glass–ceramic scaffolds for bone tissue engineering Wei Li a , Patcharakamon Nooeaid a , Judith A. Roether b , Dirk W. Schubert b , Aldo R. Boccaccini a,∗ a

Institute of Biomaterials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Cauerstrasse 6, 91058 Erlangen, Germany Institute of Polymer Materials, Department of Materials Science and Engineering, University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany

b

Received 21 April 2013; received in revised form 13 August 2013; accepted 21 August 2013 Available online 27 September 2013

Abstract Porous 45S5 Bioglass® -based glass–ceramic scaffolds with high porosity (96%) and interconnected pore structure (average pore size 300 ␮m) were prepared by foam replication method. In order to improve the mechanical properties and to incorporate a drug release function, the scaffolds were coated with a drug loaded solution, consisting of PHBV and vancomycin. The mechanical properties of the scaffolds were significantly improved by the PHBV coating. The bioactivity of scaffolds upon immersion in SBF was maintained in PHBV coated scaffolds although the formation of hydroxyapatite was slightly retarded by the presence of the coating. The encapsulated drug in coated scaffolds was released in a sustained manner (99.9% in 6 days) as compared to the rapid release (99.5% in 3 days) of drug directly adsorbed on the uncoated scaffolds. The obtained drug loaded and bioactive composite scaffolds represent promising candidates for bone tissue engineering applications. © 2013 Elsevier Ltd. All rights reserved. Keywords: Scaffold; 45S5 Bioglass® ; PHBV; Mechanical properties; Drug release

1. Introduction Tissue engineering (TE) is a relatively recent scientific interdisciplinary field, involving cell biology, biomaterials science and medicine. TE aims to regenerate damaged tissues and/or promote new tissue growth using biomaterials, cells and growth factors alone or in combination.1 One important branch of tissue engineering involves the use of a highly porous biodegradable substrate, called scaffold, made of engineered materials, which should act as temporary 3D template for cell attachment, proliferation, migration, differentiation and ultimately to support the formation of new tissue. The relevant requirements for ideal scaffolds have been discussed frequently in the literature.2,3 45S5 Bioglass® , a silicate glass in the system SiO2 –Na2 O–CaO–P2 O5 , is receiving considerable attention as scaffold material for bone tissue repair and regeneration primarily because of its excellent bioactivity, biocompatibility, osteogenic and angiogenic effects.2,4,5 A convenient route to

∗

Corresponding author. Tel.: +49 9131 85 28601; fax: +49 9131 85 28602. E-mail address: [email protected] (A.R. Boccaccini).

0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.08.032

fabricate highly porous foam-like Bioglass® -based scaffolds has been developed by the foam replication method.6 However, due to the high porosity (>90%) and the need of keeping a balance between bioactivity (related to crystallization) and mechanical properties (related to densification), these Bioglass® -based scaffolds normally exhibit relative low strength and toughness.6 It has been shown that the addition of a polymer coating can enhance the mechanical properties of brittle bioceramic7–11 and bioactive glass10,12–15 based scaffolds. In addition, the polymer coating can be used as a carrier for in site drug delivery.9,14,16,17 In this case, the drug release profile can be tailored because different polymers exhibit different degradation rates, and several drug release mechanisms are possible.16,18 Indeed, there is increasing interest in optimizing the polymer coating approach to improve the mechanical competence and functionality of bioactive glass scaffolds14 and a review of previous related studies10 is available. PHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)) is a type of microbial polyester, which is biocompatible and biodegradable. As a copolymer of P(3HB), its mechanical properties, biocompatibility and biodegradability can be tailored by changing molar percentages of HV in the structure.19 PHBV has

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already been used to develop tissue scaffolds and drug delivery vehicles.20–22 The use of specific antibiotics incorporated in medical devices is a field of increasing interest because one of the major complications associated with the use of implants (or scaffolds) for bone disease treatment is the occurrence of infections.23 As a model drug, vancomycin is an attractive choice for bone tissue engineering related applications since it has a broad-spectrum antimicrobial effect and it is applied widely in bone and prosthetic devices mainly to protect against Gram-positive bacterial infections.9,24,25 Based on the facts discussed above, in the present investigation, 45S5 Bioglass® -based scaffolds were fabricated and further coated with PHBV for the first time in order to improve the mechanical properties of the scaffolds and to impart a local drug release function using vancomycin. These scaffolds are intended for applications in cancellous bone tissue regeneration and they belong to an expanding family of polymer coated bioceramic scaffolds being investigated currently by several research groups worldwide aiming at developing improved materials for bone tissue engineering.

2.3. Polymer coating procedure and drug loading The 5% (w/v) PHBV (PHV content 12 wt%, Goodfellow, UK) solution was prepared by dissolving PHBV in chloroform (Merck, Germany). The as sintered Bioglass® scaffolds of dimensions 10 mm × 5 mm × 5 mm were then completely immersed in the PHBV solution (20 mL) for 5 min; in the meantime the container was manually shaken to achieve homogeneous coatings. After that, the scaffolds were taken out and dried in air at room temperature for 24 h. In order to load the drug together with PHBV in the scaffolds, vancomycin hydrochloride (0.5%, w/v) was unltrasonicated for 30 s in the above PHBV-chloroform solution by using a probe sonicator (Branson Sonifier® S-250D, Emerson, USA). For comparison purposes, the selected concentration of vancomycin hydrochloride in the polymer solution was based on trial-and-error in order to obtain a comparable amount of drug loading in the coated and uncoated scaffolds. The coating procedure of scaffolds was the same as mentioned above. It has been shown in a previous study that vancomycin hydrochloride is compatible with chloroform.26 2.4. Characterization

2. Materials and methods 2.1. Fabrication of Bioglass® -based scaffolds The starting materials were melt-derived 45S5 Bioglass® powder (particle size ∼5 ␮m) and fully reticulated polyurethane (PU) foams (45 ppi, Eurofoam, Germany). The scaffolds were fabricated by the foam replication method, following a similar process as described elsewhere.6 Briefly, the slurry was prepared by dissolving 6% (w/v) polyvinyl alcohol (PVA) (MW ∼ 30,000, Merck, Germany) in deionized water at 80 ◦ C, and then 45S5 Bioglass® powder was added to the PVA solution up to a concentration of 50 wt%. PU foams (15 mm ×15 mm × 12 mm) were immersed in the slurry and rotated to ensure homogeneous slurry infiltration. Each procedure was carried out under vigorous stirring using a magnetic stirrer. The scaffolds were then extracted from the slurry, and the extra slurry was completely squeezed out. The samples were dried at room temperature for 24 h and then the procedure described above was repeated again. The samples were then heated at 400 ◦ C for 1 h in air to decompose the PU foam, and then at 1100 ◦ C for 2 h to densify the glass network. The heating and cooling rates used were 2 ◦ C/min and 5 ◦ C/min, respectively. In this way, Bioglass® -based scaffolds of high porosity (>90%) and of sufficient structural integrity to be handled safely without fracture were manufactured. 2.2. Drug loading in uncoated Bioglass® scaffolds In order to load drug in the uncoated scaffolds, vancomycin hydrochloride (AppliChem, Germany) was dissolved in deionized water at a concentration of 10 mg/mL. The scaffolds (5 mm × 5 mm × 10 mm) were immersed into the aqueous drug solution for 30 min at room temperature, followed by drying at room temperature for 24 h.

2.4.1. Porosity The density of the scaffolds (ρscaffold ) was determined from the mass and volume of the scaffolds before and after coating with PHBV. The porosities before (p1 ) and after (p2 ) coating were calculated by Eqs. (1) and (2): p1 = 1 −

W1 ρBG V1

(1)

p2 = 1 −

W1 /ρBG + (W2 − W1 )/ρPHBV V2

(2)

where W1 and W2 are the weight of the scaffolds before and after coating with PHBV, respectively; V1 and V2 are the volume of the scaffolds before and after coating, respectively; ρBG (2.7 g/cm3 ) is the theoretical density of Bioglass® ,27 and ρPHBV (1.26 g/cm3 ) is the density of PHBV.20 2.4.2. Surface morphology The microstructure of the scaffolds was characterized in a LEO 435 VP scanning electron microscope (SEM). Samples were carbon coated and observed at an accelerating voltage of 10 kV. 2.4.3. Structural analysis FTIR spectra were collected using a Nicolet 6700 FTIR spectrometer in transmittance mode in the mid infrared region (4000–400 cm−1 ). All samples were characterized before and after immersion in simulated body fluid (SBF) (see below) by FTIR. Selected scaffolds were also characterized using XRD (Siemens Kristalloflex D500, Bragg-Brentano, 30 kV/30 mA, Cu K␣) analysis with the aim to determine the crystallinity after sintering and HA formation after immersion in SBF. Data were collected over the 2θ range from 15◦ to 70◦ using a step

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size 0.02◦ . For FTIR spectroscopy and XRD measurement, the samples were ground and measured in powder form.

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3. Results and discussion 3.1. Microstructure characterization

2.4.4. Contact angle measurement Static contact angle measurements were carried out on disk samples using a DSA30 contact angle measuring instrument (Kruess, Germany). The disks were made of sintered Bioglass® and had a diameter of 15 mm. The Bioglass® powder was uniaxially pressed in a cylindrical die, and the obtained pellets were sintered using the same heat treatment as for the scaffolds. After sintering, the Bioglass® disks were coated with the same PHBV solution using the same procedure described for coating the scaffolds. PHBV films were also prepared by casting the above PHBV solution into glass Petri dish. Water (3 ␮L) was added by a motor-driven syringe at room temperature. Reported data were obtained by averaging the results of five measurements. 2.4.5. Mechanical testing The compressive strength of Bioglass® scaffolds before and after coating with PHBV was measured using a Zwick/Roell Z050 mechanical tester equipped with a 50 N loading cell at a crosshead speed of 0.5 mm/min. The samples were prismatic in shape, with nominal dimensions: 10 mm × 5 mm × 5 mm. During compressive strength test, the load was applied until the strain reached 70%. The compressive strength was determined from the maximum load of the obtained stress–strain curve. The work of fracture (Wwof ) of the uncoated and coated scaffolds is related to the energy necessary to deform a sample to a certain strain and was calculated from the area under the stress–strain curve at a given strain.9 Ten samples were tested for each condition and results were averaged. 2.4.6. Assessment of bioactivity in simulated body fluid This part of the study was carried out using the standard in vitro procedure described by Kokubo et al.28 The scaffolds were immersed in 50 mL of SBF and maintained at 37 ◦ C, 90 rpm in a shaking incubator (KS 4000 i control, IKA, Germany). The pH of the solution was maintained constant at 7.4. The nominal size of all samples for the tests was 10 mm × 10 mm × 10 mm. Samples were collected after 1, 3, 7, 14, 28 days of incubation, respectively. The SBF was replaced twice a week during the experiments. Once removed from the incubator, the samples were rinsed with deionized water and left to dry at room temperature in a desiccator for further examination. 2.4.7. Drug release study In order to determine the vancomycin release profile, the uncoated and PHBV coated scaffolds were immersed in glass vials containing 10 mL phosphate buffered saline solution (pH 7.4, Sigma–Aldrich, USA) and placed in a shaking incubator (90 rpm, 37 ◦ C). At pre-determined time intervals, 2 mL medium from each sample was extracted and replenished with fresh PBS solution. The amount of vancomycin was measured using the UV–vis spectrophotometer (Specord® 40, Analytik Jena, Germany) at a wavelength of 281 nm. The drug release study was performed in triplicate for up to 10 days.

Typical SEM micrographs of the scaffolds without and with PHBV coating are shown in Fig. 1. The uncoated scaffold, which was obtained by foam replication method, exhibits a highly interconnected pore structure. The porosity and average pore size were calculated to be ∼96% and 300 ␮m, respectively. The pore size was in the range 200–550 ␮m as assessed by SEM images, which is suitable for bone tissue engineering applications.2,4 After coating, the open pore structure was maintained, as confirmed by close examination of SEM images (Fig. 1(b)). Only few pores were blocked by the polymer coating. The porosity of the coated scaffolds was ∼94%, and the weight percentage of PHBV in coated scaffolds was 15% on average. As shown in the high magnification images [Fig. 1(c) and (d)], the struts of the scaffolds were only partly covered by the polymer. The valley-like areas were mostly coated while hills-like protrudent areas were still uncoated. Coating thickness values of max. ∼1.5 ␮m were estimated from SEM observations. 3.2. Surface hydrophilicity measurement It has been reported that surface hydrophilicity significantly affects biological performance of materials, such as protein adsorption, cell attachment, migration and spreading.29,30 Therefore, water contact angle was measured to evaluate the surface hydrophilicity of the samples in the present study. Table 1 shows the contact angles of the uncoated Bioglass® disk, PHBV coated Bioglass® disk and PHBV film. The contact angle of Bioglass® disk was increased in the presence of PHBV coating. However, it was still obviously lower than that of the pure PHBV film, indicating that the Bioglass® disk was only partly covered by the PHBV coating which is in agreement with the SEM micrograph of the scaffold strut (Fig. 1(c)). It is anticipated that the relatively hydrophilic surface of the scaffolds, which was induced by unevenly polymer coating, could have adequate cell–material interaction and would maintain the bioactivity of the scaffolds (see also SBF results below). 3.3. Mechanical properties Typical compressive stress–strain curves of uncoated and PHBV coated scaffolds are shown in Fig. 2. The compressive strength of the scaffolds was significantly increased by PHBV coating. The average compressive strengths of uncoated and coated scaffolds were determined to be 0.02 ± 0.01 MPa and 0.10 ± 0.02 MPa, respectively. The area under the stress–displacement curve of PHBV coated scaffolds was calculated to be ∼8.7 N mm, whereas it was only ∼1.2 N mm for the uncoated scaffolds. In addition, it was observed that uncoated scaffolds completely crumbled into powder during compressive strength test, while the PHBV coated scaffolds partly retained the configuration and did not collapse

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Fig. 1. SEM micrographs of Bioglass® scaffolds (a) before and (b–d) after coating with PHBV at different magnifications. (d) shows also a fractured strut indicating the interaction of the polymer on the crack surfaces. Table 1 Contact angle of uncoated Bioglass® disk, PHBV coated Bioglass® disk and PHBV film. Sample Contact

angle/◦

Uncoated Bioglass® disk

PHBV coated Bioglass® disk

PHBV film

14 ± 3

51 ± 2

100 ± 4

(see supplementary information: videos 1 and 2). As discussed in previous studies7,8,10,12 it is suggested that the polymer coating covers the struts and fills microcracks on the strut surface, which improves the mechanical stability of the flaw sensitive materials

Fig. 2. Compressive stress–strain curves of uncoated and PHBV coated scaffolds.

and makes the original weak and fragile struts into stronger and tougher composite struts. This behavior is in broad agreement with what has been known in the field of polymer-coated brittle scaffolds.7,8,10,12,14,30 The strengthening and toughening effects in the composites could be explained by the micronscale crack-bridging mechanism described by Peroglio et al.7 and investigated by Pezzotti et al.,31 which, in the present case, was evidenced by the polymer ligaments that were stretched upon crack opening along the crack wake (see Fig. 1(d)). Similar toughening evidence called uncracked-ligament bridging, which involves two-dimensional un-cracked regions that can bridge the crack on opening, was observed in specimens made from cadaveric human humerus.32 It suggests that the polymer–glass composite struts obtained in the present study mimic the fracture behavior of human bones to a certain extent. Taking into account the high porosity of the present coated scaffolds (∼94%), the compressive strength (0.10 MPa) falls close to the lower bound of the values for cancellous bone (0.15 MPa, porosity ∼90%).33 According to previous experiences with this class of materials, the compressive strength achieved with the present scaffolds is sufficient for safe handling in the laboratory and for manipulation in in vivo studies.6

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Fig. 3. XRD spectra of uncoated scaffolds and coated scaffolds before and after immersion in SBF for different times.

3.4. Bioactivity assessment Hydroxyapatite formation on the surface of scaffolds upon immersion in SBF, as a measure of the scaffold bioactivity, was investigated by using XRD, FTIR and SEM. Fig. 3 shows the XRD spectra of both uncoated scaffolds and PHBV coated scaffolds before and after immersion in SBF. The major peaks in both uncoated scaffolds and coated scaffolds correspond to the Na4 Ca4 (Si6 O18 ) crystalline phase. Together with major Na4 Ca4 (Si6 O18 ) phase, Na2 Ca4 (PO4 )2 SiO4 as a minor second

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phase was observed. This major6,34–36 and second34,37–39 crystalline phases have also been found in previous studies on sintered 45S5 Bioglass® . Taking into consideration the compositions of 45S5 Bioglass® and the Na4 Ca4 (Si6 O18 ) phase, the present Bioglass® -based scaffolds are in fact a glass–ceramic material.6 HA peaks were observed on both uncoated scaffolds and PHBV coated scaffolds after immersion in SBF for different times. Therefore, the bioactivity of the scaffolds was maintained in the PHBV coated scaffolds, which is the result of the polymer covering only partially the struts as discussed above (Fig. 1(c)). FTIR spectra of both uncoated and PHBV coated scaffolds before and after immersion in SBF for 1, 3, 7, 14 and 28 days are presented in Fig. 4. All the FTIR spectra of the uncoated scaffolds after 3, 7, 14 and 28 days of immersion in SBF present dual peaks at 567 cm−1 and 602 cm−1 corresponding to the bending vibration of the P O bond.40–42 Furthermore, the peak at 873 cm−1 and the dual broad peak at 1420–1480 cm−1 can be assigned to the stretching vibration of C O bond, suggesting the formed HA was carbonated hydroxyapatite (HCA).42,43 The FTIR spectra of PHBV coated scaffolds showed similar peaks corresponding to P O and C O bonds (see Fig. 4(b)). However, these peaks were only visible after 7 days of immersion in SBF. The FTIR characterization confirmed that both uncoated and PHBV coated scaffolds exhibited formation of HCA. Therefore, the PHBV coating slightly retarded but it did not inhibit the formation of HCA, which indicates that bioactivity was still well maintained in the PHBV coated scaffolds. The surface morphologies of uncoated and PHBV coated scaffolds after immersion in SBF for different times are shown

Fig. 4. FTIR spectra of (a) uncoated scaffolds and (b) PHBV coated scaffolds before and after immersion in SBF for 1, 3, 7, 14 and 28 days.

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Fig. 5. SEM micrographs of HCA formation on the surfaces of uncoated scaffolds (a–c) and PHBV coated scaffolds (d–f) after immersion in SBF for 3 days, 7 days and 28 days. The insets in (c) and (f) indicate the globular and cauliflower shape of HCA crystals.

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Fig. 6. Cumulative percentage of drug released from the uncoated and PHBV coated scaffolds.

in Fig. 5(a)–(f). HCA-like crystalline apatite on the surface of uncoated scaffolds was clearly observed by SEM after immersion in SBF for 3 days. HCA crystals can be recognized by their well-known globular, cauliflower shape (see insets in Fig. 5(c) and (f)). As mentioned above and shown in Fig. 1, the PHBV coating was not homogeneous due to the surface roughness of the original struts. The uncoated areas of struts are exposed to SBF, providing paths for SBF to penetrate the area underneath the coating, thus establishing direct contact with the bioactive glass surface. After 3 days immersion, the strut surface was unevenly covered by apatite precipitates. However, after 7 days, the HCA layer on both the uncoated and PHBV coated scaffolds had grown fairly homogeneously throughout the struts. Therefore, the HCA crystals seem to grow not only on the uncoated areas but also on the PHBV coating confirming the maintenance of bioactive behavior of the coated scaffolds. 3.5. Drug release profile Uncoated and PHBV coated scaffolds were both loaded with vancomycin. The amount of loaded vancomycin was determined by measuring the total amount of vancomycin released from the scaffolds using UV–vis spectrophotometry. The cumulative percentage of drug release was normalized to the total amount of vancomycin. Fig. 6 shows the cumulative percentage of drug

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released from the uncoated and PHBV coated scaffolds. Compared to the initial burst release (as high as 77%) from uncoated scaffolds, the PHBV coated scaffolds showed a much lower initial burst release of 33%. The vancomycin release was found to occur in a controlled manner over a period of 6 days from the PHBV coated scaffolds (99.9%), which is a more preferable result than that of uncoated scaffolds which exhibited 99.5% release over a short period of 3 days. Such a release profile observed in the PHBV coated scaffolds could provide a rapid release of the drug to give initial antibacterial effects and a further sustained release to aid long term healing. For the uncoated scaffolds, the initial burst release could be due to the drug being loosely bound on the struts of the scaffold, and the followed gradual release resulted from the drug being tightly bound on the struts or entrapped within the micropores present on the struts. The encapsulation of drug within the PHBV coating significantly reduced the initial burst release which was observed in the uncoated scaffolds (77%). It is interesting to highlight the slightly different drug release behavior of the uncoated scaffolds in the present study with that of the previous investigation (ref. [14]) which can be attributed to the different microstructure of the struts as the two scaffolds were fabricated from different powders and different slurries. The initial burst release (33%) in the coated scaffolds was probably due to the hydrophilic drug not being efficiently encapsulated inside the hydrophobic PHBV coating, as observed in similar previous studies.44,45 The drugs in the polymer coating could be released both by drug diffusion and polymer degradation. However degradation of PHBV is very slow in the present experimental conditions, and thus this mechanism can be neglected within the drug release period in this study.19,46 Therefore, it is highly likely that the drug release is mainly controlled by diffusion in the present PHBV coated scaffolds. There are very few reports available on vancomycin release from porous scaffolds.9,14,24,47,48 Table 2 summarizes the characteristics of porous scaffolds with vancomycin release function developed earlier, including information on porosity, compressive strength and drug release profile after coating (except for the study of Ref. [24] in which the scaffolds had no coating). In previous studies, PCL, alginate, PCL/chitosan and PLGA/P(NIPAM-co-AAc) microgel coatings were investigated. It is unwise to directly compare the compressive strength of scaffolds with different porosities. In general, higher porosity will lead to lower compressive strength. Therefore, it is not surprising

Table 2 Overview of porous scaffolds with vancomycin release function. Scaffold

Bioglass®

45S5 45S5 Bioglass® 45S5 Bioglass® HA ␤-TCP/agarose PDLLA/BCP a

Coating

PHBV PCL + Chitosan PLGA/micro-gel + PLGA PCL/HA – Cross-linked alginate

Porosity/%

94 – – 83 50–80 53–85

Compressive strength/MPa 0.10 0.20 – 0.45 1.7–17.7 0.58–1.12

Initial burst releasea /%

Cumulative release/%

Uncoated

Coated

1d

3d

77 71 – 70–80 35–70 –

33 63 13–77 44 – 7–27

74 85 91–96 70 75–90 20–57

95 95 97–98 85 100 40–85

Initial burst release was determined as the cumulative percentage of vancomycin release at 1 h.

Ref.

Present study 14 48 9 24 47

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that the obtained scaffolds in the present study, which possess the highest porosity, exhibit also the lowest compressive strength. Compared with the results from Refs. [14,9], the polymer coating in the present study seems to be more effective in reducing the initial burst release, which could be attributed to the suitable dispersion of the drug in the PHBV coating and the negligible degradation and swelling of PHBV in the early stages of drug release. By contrast, in a coating strategy using chitosan (as in Ref. [14]) the effect of swelling and rapid dissolution of this polymer when immersed in PBS solution must be taken into account and it will determine a different drug release behavior than in the present scaffolds. These properties of chitosan are likely the reason for the relatively high initial burst release of the PCL/chitosan coated scaffolds in which vancomycin was loaded within the chitosan layer.14 As also seen in Table 2, the initial burst release from the ␤-TCP/agarose scaffold24 was much higher than that in the present study even if the scaffolds possessed a lower porosity, while the cross-linked alginate coated PDLLA/BCP scaffold with a lower porosity47 had a lower initial burst release than the one measured in present study. The reduction of the initial burst release is of great importance in controlled drug release systems, because excessive initial burst release can be pharmacologically dangerous and economically inefficient.49 Taking into account the porosity differences between scaffolds investigated, the vancomycin release profile reported in previous studies9,47 and in this investigation should be fairly similar. From the viewpoint of the simplicity of the preparation process, a sustained release trend has been achieved in the present scaffolds by a simple one-step dip coating process, in comparison to the related vancomycin loaded PCL/chitosan or PLGA/P(NIPAMco-AAc) microgel coated scaffolds in which more complex two-step coating processes and combinations of polymers were involved.14,48 As stated above, the key purpose of the polymer coating on inorganic scaffolds is to improve the mechanical properties and also to incorporate a drug release function.10 At the same time, the intrinsic bioactivity of the bioceramic (or bioactive glass) should be maintained after polymer coating.7–14 Therefore, it is useful to suggest a “figure of merit” to evaluate the key characteristics of polymers for scaffold coating, which should capture three key markers of the scaffold performance: mechanical properties (improvement of compressive strength and work of fracture), drug release profile (long-term or sustained drug release) and bioactivity (rapid formation of HA). Generally speaking, natural polymers (such as chitosan and alginate) are preferable to maintain the bioactivity because of their strong swelling and rapid dissolution/degradation which however may lead to short-term drug release and rapid loss of the mechanical properties in body fluid environment. Therefore, natural polymers may not be the optimal choice for coating scaffolds at least in its original state if the time-dependent structural integrity of the scaffold is a key criterion of selection. Further treatment of natural polymers, such as appropriate cross-linking, could, on the other hand, improve the performance of natural polymers as scaffold coating systems. Synthetic polymers (such as PHBV, PCL and PLGA) are favorable to achieve long-term drug release due to their mild swelling and slow degradation rate, which,

however, might impair the bioactivity of the scaffold when the scaffold struts have been completely covered. Furthermore, the bioactivity is more likely to be delayed or even inhibited by the presence of a thick synthetic polymer coating of low degradability which, on the other hand, may be required to achieve a higher improvement of the time-dependent mechanical properties. Based on the facts discussed here, in the present study, the relatively low initial burst release and long-term sustained drug release were achieved by suitable dispersion of the drug in the synthetic polymer (PHBV) coating, and the bioactivity was maintained by a coating strategy in which the struts of the scaffolds were only partially covered by a single layer of PHBV. It was also confirmed that this single PHBV layer could significantly improve the mechanical properties of the scaffold. 4. Conclusions Bioglass® -based glass–ceramic scaffolds with high porosity and interconnected pore structure were prepared by foam replication method. The application of a PHBV coating significantly improved the compressive strength and mechanical stability of the scaffolds. The bioactivity of the scaffolds in SBF was well maintained after coating with PHBV, although the formation of hydroxyapatite was slightly retarded to 7 days when compared with the uncoated scaffolds (which showed HCA formation within 3 days). The antibiotic drug vancomycin was loaded within the PHBV coating to impart a controlled drug delivery function. The encapsulated drug within the PHBV coated scaffold was released in a more sustained and controlled manner as compared to the drug directly adsorbed on the scaffold without the PHBV coating. The obtained PHBV coated scaffolds, exhibiting well maintained bioactivity, significantly improved mechanical properties and preferable drug release function, represent a promising candidate for bone tissue engineering applications. The present scaffolds belong to a rapidly enlarging family of polymer coated bioceramic scaffolds being developed for bone tissue engineering. Criteria for the selection of suitable coating polymers in order to achieve the key objectives of desired scaffold performance, namely time dependent mechanical stability, high bioactivity and sustained drug release behavior, were discussed. Acknowledgements Wei Li would like to acknowledge the China Scholarship Council (CSC) (no. 2011628002) for financial support. The authors thank Ms. Alina Gruenewald, Ms. Yaping Ding and Ms. Qingqing Yao for experimental support, and also thank Dr. Menti Goudouri for discussing FTIR results. Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.jeurceramsoc.2013.08.032.

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