(150d) Human Tear Production from Capillary Wicking Dynamics | AIChE

(150d) Human Tear Production from Capillary Wicking Dynamics


Radke, C. - Presenter, University of California-Berkeley
A Schirmer tear test (STT) commonly gauges human tear production, especially when dry-eye symptoms are present. In a STT, the rounded tip of a standardized paper strip (Whatman #41) is inserted into the lower fornix of the eye and the wetted length extending out from the lower lid is recorded after 5 min of eye closure. Longer wetted lengths suggest higher tear production rates. To date, however, there is no methodology to transform STT transient wetting lengths into basal tear production rates, e.g., in μL/min. We develop a physicochemical wicking model to elucidate wetting kinetics in a Schirmer strip. Careful consideration of the initial depletion of tear in the closed-eye tear prism reveals an initial fast increase in wetted length followed by slower growth. Evaporation from the exposed portion of the strip is carefully considered. Excellent agreement of the proposed model is achieved with available experimental observation.

Based on our proposed numerical model, we suggest a new clinical methodology in which the Schirmer strip is sheathed against evaporation by transparent tape and the wetted length of a distance-marked strip is measured at three time points near the current 5 min of insertion. Based the proposed wicking model, the slope of the resulting best-fit straight line gives basal tear production rate. Preliminary clinical evaluation of ten college-age subjects shows subject variation in tear production rates between 1 and 4 μL/min, all that lie within the range of currently accepted values. Our proposed modeling effort provides a simple new tool for diagnosing dry eye.