Lubricating biologic surfaces with hydroxypropyl guar and hyaluronic acid
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Excellent in-the-eye lubricity has been reported for new ophthalmic products containing hydroxypropyl guar galactomannan (HPGG), comparable to that of standard hyaluronic acid (HA) preparations, but the mechanistic details associated with their apparent reduction of tissue-on-tissue friction to values less than 0.1 have not been previously documented. This study utilized a reciprocating tissue-on-tissue friction test series, employing glutaraldehyde-preserved bovine pericardium at varying stiffness with lubricious HPGG and HA formulations at different concentrations in distilled water, ophthalmic buffer, normal saline, and calcium chloride solutions at pH ranging from 3-11, to monitor friction coefficients over time periods of seconds to hours at usual laboratory conditions of 20°C and 50%RH. Repeated tests with freshly harvested paraformaldehyde-preserved pig pericardium, having reduced self-fluorescence from the commercial glutaraldehyde-preserved bovine product, provided confirming frictional data with Texas-Red-labeled HPGG and allowed inspection of the lubricated surfaces by confocal microscopy. All test tissues and formulations were characterized by contact angle methods and multiple attenuated internal reflection infrared spectroscopy (MAIR-IR) for component identification, surface tension values, lubricant substantivity, and transferability to and from surface-modified germanium substrata having hydrophilic, high-surface-energy versus hydrophobic low-surface-energy properties. Added urea in some test formulations allowed testing of potential hydrogen-bonding versus hydrophobic-bonding mechanisms for lubricant retention. Light microscopy, scanning electron microscopy, and quantitative surface profilometry were used to assess surface texture and friction-induced surface damage to articulated tissue specimens. The combined data demonstrate the best lubricity results during articulation of tissue-bound HPGG-gel structured water-on-structured water interfaces, similar to ice-on-ice low-friction couples, diminishing tissue friction-induced superficial damage without interpenetration of the HPGG into the tissue surface layers. HPGG gels form hydrogen-bonding overlayers serving as therapeutic glycocalyxes having greatest lubricity associated with spontaneously formed superficial 0.7% w/v HPGG gel layers on both articulating surfaces. Further HPGG gelation stiffens the overlayers and promotes easier mechanical detachment, limiting longevity of the lubricant effect. In contrast, HA-based tissue lubrication does seem to include superficial tissue layer penetration, requiring higher formulation concentrations to obtain minimum friction values but also fostering greater persistence of the lubricious effect. Additional confocal microscopic studies will be required to confirm the described HA-tissue interactions.