Structure and process relationship of electrospun bioabsorbable nanofiber membranes
Introduction
Linear aliphatic polyesters such as polyglycolide (PGA), polylactide (PLA) and their random copolymer poly(glycolide-co-lactide) (PGA-co-PLA), are often used as the base materials for implant devices, such as suture fibers and scaffolds for tissue engineering [1], [2], [3]. These materials meet several controlled release criteria, i.e. they are biocompatible and biodegradable, and they can provide high efficiency of drug loading. Many different techniques have been developed to produce nanostructured biodegradable articles such as microspheres, foams and films. It has been demonstrated that the molecular structure and morphology of PLA, PGA and their copolymers can play a major role in the degradation and mechanical properties of the final products [4], [5]. The release profiles of entrapped drugs can be fine-tuned by controlling the degradation rate of the polymer matrix through molecular weight and molecular weight distribution, material composition, and porosity of the carriers [6], [7].
Recently, the electrospinning method has attracted a great deal of attention to produce non-woven membranes of nanofibers. This process was first studied by Zeleny [8] in 1914 and patented by Formhals in 1934 [9]. In 1964, Taylor showed that at a critical voltage, the equilibrium shape of the suspended meniscus was a cone with a semi-vertical angle of 49.3° [10]. When the applied voltage exceeded this critical voltage, a stable jet of liquid could be ejected. The work of Taylor and others on electrically driven jets of the liquids inspired scientists to apply the same principle to polymeric systems [11], [12], [13], [14], [15]. Great efforts have been made to study the effects of processing parameters on the structure and morphology of electrospun fibers. For example, Doshi and Reneker correlated the electrospinning process and the physical properties of electrospun polymeric nanofibers [13]. They found that by reducing the surface tension, fibers could be produced without beads. A higher net charge density of the polymer solution could also yield thinner fibers with no beads. Studies were also made to investigate the formation mechanism of nanofibers during electrospinning. For example, Reneker et al. [14] characterized the bending instability in the charged liquid jet during electrospinning. Shin et al. [15] studied the instability of the electrically forced fluid jet and argued that the essential mechanism of electrospinning was a rapidly whipping fluid jet.
The most studied polymer system by electrospinning is perhaps polyethylene oxide (PEO). However, the relationships between processing parameters and microstructures in the electrospun PEO membrane are still not well understood. Recently, Deitzel et al. [16] pointed out that the crystalline structure of the electrospun PEO fibers was not well developed in spite of the good crystallization ability of pure PEO, especially when under elongational flow. Some unique microstructures have been observed in other types of polymers by electrospinning. For example, Buchko et al. [17] electrospun a silk-like polymer with fibronectin functionality (SLPF) and reported a shish–kebab structure in the filament morphology.
The electrospinning technology is well suited to process natural biomaterials and synthetic biocompatible or bioabsorbable polymers for biomedical applications. Potential applications of these non-woven nanostructured membranes include filtration, anti-adhesion membranes, wound dressing scaffolds, and artificial blood vessels. In the present study, we have investigated the effects of varying the processing parameters in electrospinning on the microstructure of biodegradable amorphous poly(d,l-lactide) (PDLA) and semi-crystalline poly(l-lactide) (PLLA) membranes. These two samples were chosen because the degree of crystallinity in the sample can significantly hinder the degradation rate, which is the subject of our research interest. The target application of these membranes is for the prevention of surgery induced-adhesions. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques were used to investigate the structure and morphology of the electrospun PDLA and PLLA nanofibers. We have also demonstrated that the electrospinning technique could be used to load medicines, such as antibiotics, into the membranes.
Section snippets
Experimental
The amorphous poly(d,l-lactic acid) (PDLA) sample with an inherent viscosity of 0.55–0.75 dl/g was purchased from Birmingham Polymers, Inc. (Birmingham, AL). This polymer contained a weight average molecular weight (Mw) of 1.09×105 g/mol and a polydispersity index (Mw/Mn) of 1.42. The semi-crystalline poly(l-lactic acid) (PLLA) sample was an experimental material produced from DuPont, having a weight average molecular weight (Mw) of 1.0×105 g/mol, a polydispersity index (Mw/Mn) of 2.0 and %R∼5.
Electrospinning processing parameters
The following variables including solution properties (concentration and ionic salt addition) and processing parameters (applied electric field and solution flow feeding rate) have been examined. Their relationships with the membrane microstructure are summarized below.
Conclusions
The electrospinning technique was used to fabricate nanostructured bioabsorbable membranes for biomedical applications. The effects of solution properties and processing parameters on the structure and morphology of the electrospun membranes were thoroughly investigated. Results demonstrated that the morphology of electrospun polymer fibers depended on the strength of the electric field, the solution viscosity (e.g. concentration), the charge density of the solution (by salt addition), and the
Acknowledgements
The authors wish to thank Professor D. Reneker for providing valuable inspiration to this project and are grateful to Mr Greg Roduman for the help in taking SEM images. The authors also thank Dr Je-Young Kim and Ms Sharon Cruz for technical assistance. Financial support of this work was provided by the Center of Biotechnology at Stony Brook, a National Institute of Health-SBIR grant (GM63283-02) administered by the Stonybrook Technology and Applied Research Inc., the SUNY-SPIR program and the
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