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Mechanical Characterization of Electrospun Laminated Hydrogel/apatite Nanocomposite

Weijie Xu and Esmaiel Jabbari. Chemical Engineering, University of South Carolina, Dept. of Chemical Eng., Rm. 2C11, Swearingen Engineering Center, Columbia, SC 29208

Introduction and Objective: Bone exhibits many orders of structures from macroscopic to microscopic, sub-microscopic, nanoscale, and sub-nanostructure length scales. These levels of organization include the osteons and single trabeculae microstructures (10-500 ým), the lamellae sub-microstructures (1-10 Ám), the fibrillar collagen and embedded mineral nanostructures (100-1000 nm), and the molecular structure of constituent elements like mineral, collagenous, and non-collageneous soluble proteins. The mechanical properties of the bone are determined at the nanoscale with calcium phosphate crystals embedded in collagen fibrils. The objective of this work was to design and measure the mechanical properties of bone-mimetic laminated composites. The composites consists of eletrospun poly(lactide-co-glycolide) (PLGA) nanofibers embedded in a crosslinked hydrogel-apatite nanocomposite.

Methods: PLGA nanofibers were produced by electrospinning. A programmable syringe pump was used to inject the polymer solution (12% PLGA in 1,1,1,3,3,3 hexafluoro-2-propanol) in an electric field (25 kV) at a flow rate of 7.0 ml/h. An aluminum plate connected to the ground electrode was used as the collector. The morphology of bone marrow stromal (BMS) cells on the fibers, stained with Alexa 488 phalloidin and DAPI, was imaged with a confocal scanning laser microscope. The fiber morphology was measured with SEM. The fiber mesh were dipped in a hydrogel precursor mixture containing poly(lactide-co-ethylene oxide fumarate) (PLEOF) macromer, N-Vinyl-2-pyrrolidinone (NVP) crosslinker, water-soluble photoinitiator, and nanoapatite particles (50 nm diameter) in PBS. The dipped layer were stacked and pressed against each other, and the multi-layer structure was crosslinked by UV polymerization to form a degradable bone-mimetic composite network. The tensile strength of the laminated structure was measured by a Dynamic Mechanical Analyzer (DMA; TA Instruments).

Results and Conclusion: The elastic modulus of crosslinked PLEOF hydrogel was 0.47▒0.07 MPa. The elastic modulus increased by 65-fold to 31▒6 MPa with the addition of 4 layers of electrospum PLGA fibers. Furthermore, the elastic modulus increased to 140 MPa with the addition of 4 PLGA electrospun layers and 10% by weight apatite nanocrystals. As the number of electrospun layers was increased from 3 to 4 and 5, the elastic modulus of the composite structure increased from 145▒50 to 195▒47 and 560▒20 MPa, respectively. confocal microscopy results demonstrated that BMS cells elongated along the direction of PLGA fibers. These results demonstrate that laminated apatite nanocomposites are potentially useful as a synthetic bone-mimetic matrix in tissue engineering.