Fabrication and characterization of poly(lactic-co-glycolic acid)/polyvinyl alcohol blended hollow fibre membranes for tissue engineering applications

Abstract

Hollow fibre membrane bioreactors are ideal for tissue engineering applications because they can offer a large surface area to volume ratio in order to culture large numbers of cells in relatively small volumes of media. Poly(lactic-co-glycolic acid) (PLGA) hollow fibres have great potential for use in tissue engineering due to their biocompatibility and biodegradability, but their hydrophobicity results in low hydraulic permeability and limited cell adhesion. To overcome this, PLGA hollow fibres have been previously used immediately after fabrication, while still wet, or have been dried for storage and subsequently wetted with ethanol. These methods both present problems since the fibres cannot be stored wet as they degrade, while ethanol causes deformation such that they distort in the bioreactor and stick together. Here we present an alternative method to allow wetting of dried fibres by water alone: the addition of polyvinyl alcohol (PVA) to the spinning dope. Blending of PLGA and PVA resulted in a concentration-dependent increase of the mean pore size, porosity, contact angle, and a corresponding decrease in mechanical strength. To determine whether membranes were amenable to transport of culture medium and its constituents, permeation of water and a model protein, BSA, was assessed. No pure water or protein permeation was observed in PLGA hollow fibre membranes that had been dried for storage or PVA–PLGA blends with 1.25 and 2.5% PVA. However, fibres with 5% PVA that had been dried for storage allowed permeation of both water and BSA, without rejection of, or fouling by the protein. It was concluded that 5% PVA was sufficient to obtain wetting with water alone, and transport properties suitable for the permeation of media proteins without a significant or detrimental effect on mechanical strength. Therefore a PVA–PLGA blend by the addition of 5% (w/w) PVA to 20% (w/w) PLGA in N-methyl-2-pyrrolidone spinning dope is recommended when preparing PLGA hollow fibre membrane scaffolds.

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.