267936 Tribological and Mechanical Characterization of κ-Carrageenan Hydrogels

Monday, October 29, 2012
Hall B (Convention Center )
Andrew Kozbial, Cristian Riley and Lei Li, Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA

Polymer hydrogels are known to exhibit unique frictional properties that are not observed in solids and liquids.  The ability of polymer hydrogels to exhibit low frictional forces when slid against one another or against a solid substrate allows them to be used in applications where the use of solids and liquids are not possible.  One application of polymer hydrogels is its use as artificial cartilage in joint replacements.  Animal cartilage exhibits extremely low friction coefficients at loads as high as 18 MPa.  This behavior cannot be replicated by a solid or liquid; therefore, current joint replacement materials can cause pain and discomfort.  Another application of hydrogels is for use as renewable lubricants.  Most current lubricants are petroleum based and therefore not renewable.  Carrageenan is a renewable polymer derived from red algae and may allow current lubricants to be replaced with renewable hydrogel based lubricants.  The objective of this study was to fabricate k-carrageenan hydrogels at various polymer concentrations and characterize their frictional and mechanical properties.

Three k-carrageenan hydrogels were fabricated for characterization: 2%, 2% with 0.01% KCl added for mechanical enhancement (2% KCl), and 3% polymer concentration.  Frictional characterization was performed on a dual beam cantilever nanotribometer and mechanical characterization was performed with a texture analyzer.  Each hydrogel was tested multiple times for reproducibility under both aerobic (air) and aqueous (water) conditions.

The Young's modulus of the 2%, 2% with KCl, and 3% k-carrageenan gels was determined to be 52 kPa, 85 kPa, and 79 kPa, respectively.  The mechanical strength of the 2% gel was less than the 3% gel.  In order to elucidate the effect of mechanical strength, KCl was added to the 2% gel during fabrication to increase the hydrogel's cross-linking density.  This resulted in a gel with a mechanical strength greater than the 3% k-carrageenan gel.  This implies that the addition of a small amount of cross-linking agent to the gel can have significant consequences on its physical properties.

The frictional characterization was performed at 1) constant velocity (increasing normal load) and 2) constant pressure (increasing velocity) in both aerobic and aqueous conditions.  At constant velocity, the coefficient of friction (COF) was lowest for the 2% gel and greatest for the 2% KCl and 3% gels in air and water conditions.  There was no discernible difference in the friction of the 2% KCl and 3% gel.  The friction of the gels was greater in air than water and the COF remained constant in water at all normal loads.  In air, the COF was greater at low normal load and decreased with an increase in load.  This trend was observed for all three gels. At constant pressure, the COF in air was lowest for the 2% gel and greatest for the 2% KCl gel; the COF of the 3% gel was intermediate.  In water, 2% and 3% gels showed similar frictional coefficients at all velocities with the COF of the 2% KCl gel significantly greater.  The friction coefficients of the gels were similar at low velocity, while at high velocity the difference was significant.

This in-depth characterization of k-carrageenan hydrogels focused on determining the frictional properties of the gels and elucidating the effect of mechanical strength on friction.  The major consequence of increasing mechanical strength was the direct increase in friction, observed for all three gels in aerobic and aqueous conditions.  The additional lubrication at the testing interface in the aqueous condition resulted in a consistent decrease of friction compared to the "dry" aerobic condition.  This effect was more significant at low loads than at high loads.  At high load, more water is forced away and the testing interface becomes more similar to the dry condition; whereas, at low load, the pressure at the testing interface is low and water is not "squeezed" from the interface allowing for sufficient lubrication. The direct consequence of a fast testing velocity is a significant increase in friction coefficient at aerobic conditions and a subtle increase in friction coefficient at aqueous conditions.  At increasingly fast testing velocities, the surface of the gel has less time to recover between testing cycles.  This results in less time for water to be transported from the bulk gel to the surface, effectively decreasing lubrication and increasing friction.

This tribological and mechanical characterization of k-carrageenan hydrogels creates a strong foundation for additional research on the effects of mechanical strength on friction and determining the friction of polymer gels.  Hydrogel design parameters for applications such as synthetic cartilage and renewable lubricants significantly rely on accurate and repeatable friction data to ensure proper function.  Further research on the surface properties of the gels and the gelation mechanism during fabrication combined with this study will establish the necessary characterization of k-carrageenan hydrogels.

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