(180at) Tribological and Mechanical Characterization of κ-Carrageenan Hydrogels

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.