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QUANTIFYING EXTRACELLULAR MASS TRANSPORT USING DIGITAL HOLOGRAPHY.  M. R. Benoit¹, D. M. Klaus¹ and R. B. Owen².  ¹Aerospace Engineering Sciences Department, University of Colorado, Boulder.  ²Owen Research Inc., Boulder, CO.

     It has been hypothesized that many of the cellular level physiological effects observed to occur in space result, at least in part, from a reduction of extracellular mass transport in the absence of gravity-driven sedimentation and convection.  Empirical data are needed to more fully elucidate this hypothesis and to calibrate computer models designed to simulate these phenomena.  The objective of this current study was to evaluate a technique using double-exposure digital holographic interferometry for monitoring real-time changes in fluid density. 

     Initial experiments were conducted using a device called the Digital Holographic Monitor (DHM) to measure density gradients formed by adding a drop of 0.1% saline to distilled water. Relative shifts in refractive index on the order of 10’s of microns resolution were observed as density gradients formed.  The imaged data were then correlated to absolute saline percentages. 

     Based on these initial findings, experiments were designed to observe boundary layer density gradients formed by metabolically active bacterial cells growing on a tip of agar submersed in a liquid growth medium. Buoyant streaks of less dense fluid rising from the cells were imaged, but determined to originate from a combination of agar constituents and cellular metabolic byproducts. Differentiating between these two contributing factors and quantifying the absolute density changes from the phase map data remains as a challenge to be resolved. Observing density gradients surrounding growing protein crystals represents another potential application of this technology. Measurements of lysozyme/acetate buffer solutions indicated the ability to quantify concentration differences of 0.1%.    

     Successful demonstration of double-exposure digital holography is expected to lead to adaptation for use in space to further quantify the effects of gravity on density gradients created by submicron particles in a fluid environment.  

     (Supported by NASA: NAS8-99087)