Biopolymer nanomaterials for growth factor stabilization and delivery
Date
2014
Authors
Place, Laura Walker, author
Kipper, Matt J., advisor
James, Susan, committee member
Popat, Ketul C., committee member
Miller, Benjamin, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Biopolymers are useful in tissue engineering due to their inherent biochemical signals, including interactions with growth factors. There are six biopolymers used in this work, the glycosaminoglycans (GAGs), heparin (Hep), chondroitin sulfate (CS), and hyaluronan (HA), chitosan (Chi), a GAG-like molecule derived from arthropod exoskeletons, a Chi derivative N,N,N¬-trimethyl chitosan (TMC), and an extracellular matrix (ECM)-derived material, demineralized bone matrix (DBM). The direct delivery of growth factors is complicated by their instability. GAG side chains of proteoglycans stabilize growth factors. GAGs also regulate growth factor-receptor interactions at the cell surface. The majority of proteoglycan function is derived from its GAG side chain composition. Here we report the development of nanoparticles, proteoglycan-mimetic graft copolymers, incorporation of nanoparticles into electrospun nanofibers, and processing methods for electrospinning demineralized bone matrix to fabricate bioactive scaffolds for tissue engineering. The nanoparticles were found to show similar size, composition, and growth factor binding and stabilization as the proteoglycan aggrecan. We use basic fibroblast growth factor (FGF-2) as a model heparin-binding growth factor, demonstrating that nanoparticles can preserve its activity for more than three weeks. Graft copolymers were synthesized with either CS or Hep as the side chains at four different grafting densities. Their chemistry was confirmed via ATR-FTIR and proton NMR. They were shown to increase in effective hydrodynamic diameter with grafting density, resulting in a size range from 90-500 nm. Graft copolymers were tested for their ability to deliver FGF-2 to cells. The CS conditions and the Hep 1:30 performed equally as well as when FGF-2 was delivered in solution. Preliminary dynamic mechanical testing demonstrated that hydrogels containing the copolymers exhibit changes in compressive modulus with cycle frequency. Two electrospinning techniques were developed, using an emulsion and a coaxial needle, for incorporating growth factor into electrospun nanofibers. We bound FGF-2 to aggrecan-mimetic nanoparticles for stabilization throughout electrospinning. The two techniques were characterized for morphology, nanoparticle and FGF-2 incorporation, cytocompatibility, and FGF-2 delivery. We demonstrated that both techniques result in nanofibers within the size range of collagen fiber bundles and dispersion of PCNs throughout the fiber mat, and exhibit cytocompatibility. We determined via ELISA that the coaxial technique is superior to the emulsion for growth factor incorporation. Finally, FGF-2 delivery to MSCs from coaxially electrospun nanofibers was assessed using a cell activity assay. We developed a novel method for tuning the nanostructure of DBM through electrospinning without the use of a carrier polymer. This work surveys solvents and solvent blends for electrospinning DBM. The effects of DBM concentration and dissolution time on solution viscosity are reported and correlated to observed differences in fiber morphology. We also present a survey of techniques to stabilize the resultant fibers with respect to aqueous environments. Glutaraldehyde vapor treatment is successful at maintaining both macroscopic and microscopic structure of the electrospun DBM fibers. Finally, we report results from tensile testing of stabilized DBM nanofiber mats, and preliminary evaluation of their cytocompatibility. The DBM nanofiber mats exhibit good cytocompatibility toward human dermal fibroblasts (HDF) in a 4-day culture.
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Subject
glycosaminoglycans
growth factors
nanostructure
polymers
proteoglycans
tissue engineering