Fabrication and characterization of poly(lactic-co-glycolic acid)/polyvinyl alcohol blended hollow fibre membranes for tissue engineering applications
Introduction
Tissue engineering is emerging as a clinically viable means of replacing diseased or damaged tissues and, ultimately, organs with functional, laboratory-grown substitutes. However, in order to achieve this, large numbers of cells, in the order of 1010, are required per application and such an expansion in cell number is not practical using standard two-dimensional cell culture flasks. Hollow fibre membrane bioreactors offer a viable alternative to traditional culture flasks for the expansion of large numbers of cells because they can offer a large surface area to volume ratio to culture high cell numbers in relatively small volumes of media [1], [2], [3]. In addition, these membranes act as a solid barrier between the cells and the media, allowing high media flow rates to be selected for appropriate mass transfer rates of nutrients to and waste products from the cells, without any cellular damage [1], [3]. The widest use of hollow fibre membranes in tissue engineering is found in bioartificial organ assist devices, such as bioartificial liver [4], [5], [6], [7], bioartificial pancreas [8], [9], [10], [11], and bioartificial kidney [12], [13], [14], [15], [16]. However, hollow fibres have also been employed as scaffolds in bone tissue engineering [2], [17], [18], [19], [20] and nerve tract guidance channel applications [21].
The optimal characteristics of hollow fibre membranes for use as a tissue engineering scaffold can be summarised as biocompatibility, biodegradability, high porosity to obtain high hydraulic permeability, narrow pore size distribution to obtain a sharp molecular weight cut-off, mechanical stability to withstand the pressure created by the stream, chemical and thermal stability to withstand sterilization and incubation at 37 °C for cell culture [22], and a maximum pore size of 5 μm to prevent the pores becoming blocked by cell infiltration. Materials used to fabricate hollow fibres for tissue engineering applications are mainly cellulosic or synthetic polymers including polysulfone, polyethersulfone, polyvinylpyrrolidone, poly(lactic-co-glycolic acid) (PLGA), polyethylene vinyl alcohol and polymethyl methacrylate [22]. PLGA is one of the most widely used synthetic polymers for tissue engineering as it is biodegradable, biocompatible and FDA-approved [23], [24], [25]. However, one of the major limitations in the use of PLGA for tissue engineering scaffolds is its hydrophobic character, resulting in sub-optimal adhesion, spreading and growth of cells on the surface of the material [26], and also resulting in low hydraulic permeability through the scaffold. Some techniques to make PLGA more hydrophilic include pre-wetting the material with ethanol before cell seeding [27], [28], hydrolysis with NaOH [27], [29], protein-coating [30] and plasma treatment [31]. In our research group there have been attempts to improve the hydrophilicity and permeability of PLGA membranes by ethanol pre-wetting or hydrolysis with NaOH, but both the methods resulted in distortion and/or breakage of the fibres after a very limited period of time [27]. Blending of polymers will result in a structure with a combination of properties, and of particular interest blending with a hydrophilic polymer can improve the hydrophilicity and permeability [32], [33], [34], [35]. For example, poly(ethyleneglycol) [36], [37], polyvinylpyrrolidone [38], [39], [40], [41], [42] and polyvinyl alcohol (PVA) [39], [43], [44], [45], [46], have been reported to improve the permeability of membranes. Due to its hydrophilicity, excellent chemical resistance (e.g., long term temperature and pH stability), non-toxicity and biodegradability, PVA has been used in various pharmaceutical, medical, cosmetic and food products [47], [48], [49]. For the same reasons, PVA is an attractive polymer for tissue engineering applications. For example, Oh et al. have demonstrated how blended PVA–PLGA sponge scaffolds, made by thermal compression molding, improved porosity, hydrophilicity and wettability of the material, resulting in better cell adhesion and growth [26].
In this paper, blended PVA–PLGA hollow fibre membranes were fabricated by wet spinning to allow wetting without the use of a wetting agent, and improve protein permeation. Morphology, surface chemistry, mechanical properties, hydrophilicity and permeability of the blended fibres were studied to determine their potential suitability for tissue engineering applications.
Section snippets
Hollow fibre membrane fabrication
The spinning dope was prepared from poly(lactic-co-glycolic acid) acid (Resomer RG 756 S, Boehringer Ingelheim) and polyvinyl alcohol (0, 1.25, 2.5, 5%, w/w) (Sigma) in N-methyl-2-pyrrolidone (NMP, Acros Organics) at a total concentration of 20% (w/w). The spinning dope was degassed under a vacuum for 1 h to remove air bubbles and hollow fibres were generated by wet spinning as described elsewhere [1]. The resulting fibres were soaked in water for 3 days, with the water changed twice a day, to
Morphology and mean pore size of hollow fibre membranes
Pure PLGA and blended PVA–PLGA hollow fibre membranes (PVA content: 1.25%, 2.5% and 5%) were fabricated by wet spinning. The phase inversion process resulted in anisotropic membranes with skin layers on both the inner and outer surfaces, with large finger-like macrovoids in the sublayer, extending from the skin layers to the centre of the fibre wall (Fig. 1). With increasing PVA content, the central core diminished and for the 5% PVA–PLGA hollow fibre membranes were completely replaced with
Conclusion
The addition of PVA to PLGA spinning dopes resulted in asymmetric porous hollow fibre membranes with mean pore sizes increasing linearly from 0.54 μm for PLGA to 1.1 μm for 5% PVA–PLGA, and porosity increasing from 0.46 to 0.77. An exponential decrease in water contact angle from 64° to 50° indicates that PVA improves hydrophilicity of PLGA hollow fibre membranes. Pure water flux was only obtained with 5% PVA–PLGA fibres, and these fibres showed no solute rejection of BSA, reduced flux or pore
Acknowledgements
The authors wish to acknowledge financial support from the European Union Marie Curie Early Stage Training Fellowship for G.M. We also thank Mr. Fernando Acosta, Mr. Richard Weston and Mr. Frank Hammett for technical assistance.
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