Tough, Strong Hydrogels with Elastomeric Fiber Reinforcement.
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Tough, Strong Hydrogels with Elastomeric Fiber Reinforcement. Paul Calvert, Animesh Agrawal, Nima Rahbar and Vijay Chalivendra University of Massachusetts Dartmouth, North Dartmouth, MA 02747
ABSTRACT In biological hydrogels, the gel matrix is usually reinforced with micro- or nano-fibers and the resulting composite is tough and strong. In contrast, synthetic hydrogels are weak and brittle, although they are highly elastic. Other than in food, the main structural application of hydrogels is as soft contact lenses. The developing interest in soft tissue engineering has exposed a need for strong synthetic hydrogels to act as scaffolds for tissue growth. In this work a new class of hydrogels based on fiber reinforced hydrogel composites with a cartilage-like structure is designed. A 3D rapid prototyping technique was used to form crossed “logpiles” of elastic fibers that are then impregnated with an epoxy-based hydrogel in order to form a fiber-reinforced gel structure. The fibrous construct improves the strength, modulus and toughness of the hydrogel and also constrains the swelling. By altering the construct geometry and studying the effect on mechanical properties we will develop the understanding needed to design strong hydrogels for biomedical devices and soft machines. INTRODUCTION In nature, hydrogels are found abundantly throughout the animal kingdom in the form of tissues, where they are frequently under tension, compression or shear forces. What makes biological hydrogel stronger is the fiber reinforcement. Biological hydrogels are soft composite, for example articular cartilage, which is a collagen fibril reinforced proteoglycan gel [1, 2]. Articular cartilage has strength of 1.5MPa and can go up to 30MPa in regions with high fiber content with an extension to break of approximately 100%. The large extension to break and large work of fracture permits cartilage to resist impact even though the average strength is not high [3]. When a gel swells up to its maximum capacity polymer chains become very elongated and stiff, so even a small crack can break the gel. Fibers in the reinforced gel not only improve the strength but also keep the gel partially swollen so when a force is applied, the gel is able to absorb the force or the gel has the flexibility to compress or expand. Hydrogels tend to swell under tension and lose water under compression, with change in water content gels mechanical properties varies. Other methods have been used to improve the mechanical properties of the hydrogel such as laponite clay reinforced nano-composite hydrogels [4-7], reinforcing the gel with short fibers, or woven or knitted fabrics reinforcement [8], cyclic freeze-thaw method to form polyvinylalcohol gel with a fine network of dense polymer fibrils and dilute zone [9] and double network (DN) hydrogel, a new class of hydrogel, is introduced by Gong et al. at Hokkaido University [10]. DN hydrogels can be synthesized by swelling a moderately crosslinked hydrogel in a less crosslinked hydrogel precursor and then polymerizing it.
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