Fiber-reinforced Hydogels for Soft Tissue Replacement
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Fiber-reinforced Hydrogels for Soft Tissue Replacement Animesh Agrawal and Paul Calvert Department of Materials and Textiles, University of Massachusetts Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02747 ABSTRACT Synthetic hydrogels have poor mechanical properties that limit their use in load bearing applications. In contrast, biological hydrogels are tough and strong due to reinforcement with nano to micron size fibers. Our work is focused on designing new class of hydrogel assemblies based on fiber reinforced hydrogel composites. In analogy to the spinning of a spider web, a pultrusion system was developed to spin micron-diameter polymer fibers from solution in order to build predefined three dimensional patterned fiber-reinforced hydrogel structures. The gel chemistry is based on epoxy-amine crosslinking. We have formulated various epoxy-amine gels with swellability ranges from 0% to 1000% in water. The fibrous reinforcement affects the swelling and mechanical properties of the gel. For mechanical characterization flat punch probe indentation technique was performed. The effect of fiber density and extent of swelling on the reinforcement of epoxy matrix is investigated. Based on a better understanding of fiber reinforcement of gels, we hope to be able to design strong gels for biomedical devices and soft machines.
INTRODUCTION In soft tissue engineering, hydrogels are the most favorable materials due to their biocompatibility, tunable biodegradability, hydrophilicity and mechanical properties. The most common application for high performance hydrogels is in contact lenses but implant coatings, wound dressings, microfluidic devices, sensors and actuators have been discussed [1, 2]. Hydrogels are essentially brittle and break under low stress due a lack of energy dissipation mechanisms to slow the crack propagation. The uneven distribution of polymer chain length between cross-links is thought to cause uneven stress distribution which results in crack initiation [3]. Many attempts have been made to improve the mechanical properties of hydrogel but their robustness still remains unsatisfactory [3-7]. In nature, strong connective tissue structures are fiber reinforced gels. For example articular cartilage is a chondroitin and keratan sulfate proteoglycan gel of about 10% weight reinforced with a type I collagen fibril network corresponding to about 10% of the total weight. The collagen fibers resist the swelling pressure generated by the proteoglycans [8]. Articular cartilage has a strength of 1.5MPa, ranging up to 30MPa in regions with high fiber content, and it has extension to break is about 100%. The large extension to break and large work of fracture permits cartilage to resist impact even though the average strength is not high. [2] In order to understand the contribution of fiber-gel interactions to toughness in these materials, we have used freeform fabrication techniques to make model fiber-reinforced gel materials.
MATERIALS AND PREPARATION The epoxy gel was formed from polyethyle
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