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Title:Direct-write assembly of 3D microperiodic scaffolds for tissue engineering applications
Author(s):Parker, Sara T.
Director of Research:Lewis, Jennifer A.
Doctoral Committee Chair(s):Lewis, Jennifer A.
Doctoral Committee Member(s):Nuzzo, Ralph G.; Braun, Paul V.; Cheng, Jianjun
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):direct-write assembly
tissue engineering
silk fibroin
3D scaffolds
Abstract:Three-dimensional (3D) microperiodic scaffolds have been fabricated by direct-write assembly for tissue engineering. Such scaffolds are necessary to accurately mimic the complex environment of real tissues. Three distinct inks have been developed: (1) a hydrogel ink that contains high molecular weigh chains of poly(2-hydroxyethyl methacrylate) (pHEMA) in a photopolymerizable solution, (2) a silk fibroin ink, regenerated from Bombyx mori silkworm cocoons, that can be induced to change conformation in a coagulating reservoir, and (3) a hydroxyapatite (HA) particle-filled silk fibroin ink. By carefully controlling their rheological properties, these both flow through nozzles without clogging and form continuous filaments that span gaps present in underlying layers without deformation. The hydrogel ink contains pHEMA chains (300,000 and 1,000,000 g/mol) along with monomer, comonomer, and photoinitiator that polymerize upon exposure to UV light after printing. The viscosity and shear elastic modulus of this physically entangled, viscoelastic ink can be tailored by blending long chain pHEMA species of different molecular weights to create a physically entangled network. Once exposed to UV light, the monomer and comonomer components of the ink polymerize to yield an interpenetrating physical and chemical gel network. By comparing the ink viscoelasticity to that of the cured scaffolds, we find that the chemical gel provides the dominant contribution to the mechanical properties. 3D pHEMA scaffolds are patterned with varied architectures are rendered biocompatible by absorption of polylysine. They are then cultured with primary neuronal cells, which form intricately branched networks. Confocal laser scanning microscopy reveals that both cell distribution and extent of neuronal process alignment depend upon scaffold architecture. Silk fibroin inks are prepared by regenerating the proteins from Bombyx mori silkworm cocoons and dialyzing the silk solution to form a concentrated ink (~28-30 wt% silk). Upon printing into a methanol-rich reservoir, the ink transforms from a random coil to a β-sheet conformation, thereby inducing rapid solidification of the printed filaments. The mechanical properties of the 3D silk scaffolds are probed by atomic force microscopy nanoindentation. These scaffolds are cultured with human mesenchymal stem cells (hMSCs) and support cell adhesion, growth, and chondrogenic differentiation. To produce 3D silk scaffolds at larger length scales for bone tissue engineering, hydroxyapatite (HA) nanoparticles are added to the silk fibroin inks to form a HA-filled silk ink. The HA particle network imparts significant elasticity to the ink, such that 3D structures can be patterned in air using a 200 μm nozzle. After printing is completed, the scaffolds are immersed in a methanol-rich solution to transform the silk proteins to β-sheets, thereby further strengthening their structure. The mechanical properties of the printed filaments are measured by dynamic mechanical analysis. The scaffolds support both hMSCs and mammary microvascular endothelial cells (MMECs) as well as osteogenic differentiation of the hMSCs. In summary, direct-write assembly is a facile method for fabrication of 3D microperiodic scaffolds that model real tissues. As demonstrated, this technique is suitable for a broad array of ink compositions, characteristic feature sizes, 3D architectures, and cell types. This research provides a foundation for future efforts based on biocompatible polymer and composite scaffolds in three-dimensional motifs.
Issue Date:2010-08-31
Rights Information:Copyright 2010 Sara Tien-Mei Parker
Date Available in IDEALS:2010-08-31
Date Deposited:2010-08

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