Electrospinning Scaffold Fabrication of Polymer Nanofibers and Sensors for Tissue Engineering Applications
Edmondson, Dennis LeRoy
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Well-ordered one-dimensional nanostructures (nanofibers) are enabling important new applications in textile, energy, structural, environmental, and bioengineering applications such as sensors, transducers, and energy harvesters, due to their unique anisotropic properties. Through electrospinning, polymeric, ceramic or metallic solutions can be ejected from an electrically charged syringe needle spinneret at an appropriate flow rate, collection distance, and voltage to form nanofiber filaments. A substantial electrical field gradient of 6,000 to 25,000 volts is required, depending on solution type, in the space between the charged syringe needle spinneret and an electrically grounded collection electrode. As solution flows from the syringe needle, Coulomb forces created by electrical charge on fibers can extend the stream into a fine, continuous group of filaments. The short transition from spinneret blunt needle tip to the stream occurs through formation of a Taylor cone droplet at an angle of around 30 degrees to the needle tip axis, which is a common characteristic for charged fluids in motion within electric field gradients. The electric field gradient accelerates the filament stream onto a collecting target of opposite polarity between eight and thirty centimeters away. For best circumstances the polymeric solution should be dry upon reaching the grounded collecting electrode to yield a dry fibrous mat. Resultant fiber diameter (or shape), porosity, morphology, and other characteristics can be controlled through modifying electrospinning solution material composition, voltage, solution flow rate (natural or forced), electric field strength, and distance between spinneret needle and fiber collector geometry. The collector component is very significant to the electrospinning process as the fiber collector geometry can be altered to control fiber deposition orientation. Electrospinning fabrication equipment designed for manufacturing nanofibers, especially for highly aligned fibers, can be used to develop effective scaffolds for cell proliferation and chemical attachment. Many electrospinner variations have been tried in the effort to create a bio-cellular scaffold environment on which cells can proliferate. The ability to promote substantial cell growth on an artificial scaffold brings research closer to fabrication of difficult organs, such as heart, neural conduits, intestinal tissue, and skeletal structure, to name but a few. Current laboratory and clinical trials focused on bladder and kidney are searching for the ideal technique for creating a fully functioning replacement organ by identifying the most reliable polymer fiber-cell compatibility scaffold that will both support cell proliferation and be biodegradable. A key aspect is developing a consistent method of aligning fibers through electrospinning. However, the challenges in large-scale production of highly aligned and uniform nanofibers limit the scope of their applications and commercialization. This dissertation presents a powerful yet economical approach that integrates the concepts of stationary parallel-electrode gap method with centrifugal polymer dispersion to produce nanofibers with a high degree of alignment and uniformity at large scale. This approach was first demonstrated with polyvinylidene fluoride to illustrate how the experimental parameters regulate fiber production and piezoelectric response, leading to the production of aligned nanofibers up to four inches in length. Further work with chitosan and polyethylene oxide, a natural and a synthetic polymer, demonstrated the versatility of the system. The now-patented centrifugal electrospinning technology presented here has already opened new avenues of invention through mass production of aligned nanofibers, allowing development of novel sensors. One novel device under development is a spiral-coiled biosensor. Biotelemetry has become an important part of medical research for advancing patient care by remotely monitoring continuing biological processes and physiological functions. Current biotelemetry systems are complex and require multiple electronic components to function, for example, battery, sensor element, and transmitter circuit. Another significant concern of current biotelemetry devices is direct wire coupling of the in vivo portion to external supporting equipment. Without the need for a power supply, the spirally coiled sensors in the nanofiber bundle generate and transmit an electrical signal wirelessly in response to deflections. The sensor is encapsulated within a thin biocompatible polymer shell of poydimethylsiloxane (PDMS) providing device integrity and moisture isolation. The results suggest that such a sensor can potentially function as both mechanical and biotelemetry sensors for various in vitro and in vivo biomedical applications. The following chapters discuss how combining technologies of selected organic polymer materials and fiber electro-spinning apparatus with cell immobilization procedures determines electrospun fiber mat effectiveness. Several examples demonstrate how a solution of electrospun, biocompatible nanofibers composed of polymer(s) can be used to control time rate of degradation and produce a nanofiber scaffold structure supporting attachment and proliferation of cells of interest for in vitro application. They show in detail how cells combined with artificial extra-cellular matrix (ECM) of desirable biocompatible and biodegradable characteristics are important to tissue engineering research. Several polymers, both natural and synthetic, including alginate, chitosan, polyethylene oxide (PEO), polycaprolactone (PCL), and piezoelectric polyvinylidene fluoride (PVDF) compositions for electrospinning are presented. Piezoelectric nanofibers that could offer many useful applications were electrospun. Solutions having different composition ratios producing random and semi-aligned nanofibers through standard electrospinning collector electrode techniques are discussed, as is a newly developed aligned fiber electrospinner that uses centrifugal force with pneumatic assistance to produce a highly viscous solution rendering the fibers electrospinable. A novel biotelemetry sensor owes its embryonic beginnings to highly aligned nanofibers created through centrifugal electrospinning. Through these techniques, simple, highly aligned piezoelectric polyvinylidene fluoride and tetrafluoralethane nanofibers can be fabricated to function as a standalone power source, sensor, and transmitter.