Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99318

99318 - Understanding the Diffusivity of Water Through Plant-Inspired, Osmotically-Active, Elastomeric Membranes 

Selectively-permeable plant cell membranes allow fluid flow both in and out of plants’ cells, while remaining impermeable to osmolytes, or salt ions, that populate the intracellular fluid.  Water flow through the membrane is induced by an osmotic driving force, due to an imbalance in osmolyte concentration between the solution encapsulated in the cell and the surrounding aqueous media.   Water flows from regions of low osmolyte concentration to high osmolyte concentration, to establish chemical or osmotic equilibrium across the membrane.  This capability allows for cell membranes to actuate and deform in response to the increasing hydrostatic pressure, or turgor pressure, resulting from the increase in fluid volume in the cell.  The collective deformation of individual cell membranes in turn, enables larger bulk cell structures and tissues to deform and actuate, without the aid of muscle.  The aim of this study is to understand the diffusive behavior of water through polydimethylsiloxane (PDMS) membranes, by studying the osmosis-driven actuation behavior of synthetic plant cell mimetic structures, containing PDMS membranes.  We fabricate closed-cell, fluid-filled, osmosis-driven micro-actuators by adhering selectively-permeable PDMS membranes to stiff cylindrical PDMS micro-well structures, while submerged in salt water solutions, effectively encapsulating salt solutions within the actuator chambers.  After fabrication, the structures are submerged in pure water, allowing water to diffuse through the PDMS membrane due to osmotic potential differences across the membrane.  The membrane stretches and bulges outward, in response to increasing volume in the chamber and increasing turgor pressure in the membrane.  We model this response through the development of an ordinary differential equation (ODE) that describes the change in actuator chamber volume, due to the water flow through the membrane, with respect to time.  This ODE is dependent on the initial osmotic driving force due to the encapsulated solution concentration, varying chamber geometry and material mechanical properties, and can be cast to both linear-elastic and hyper-elastic mechanical models.  Increasing the initial osmotic concentration or decreasing the size or stiffness of the membrane, will lead the membrane to actuate faster due to an increased driving force.   We verify this by studying the actuation behavior of structures fabricated with varying the initial osmotic potential, chamber radius, and membrane thickness. The change in volume is mediated by the diffusivity rate, a pre-factor to the ODE, of water through the PDMS.  By experimentally measuring the volume change of the fabricated actuators through image analysis, this volume change data is compared to the analytically modeled ODE to determine the diffusive behavior of water through the PDMS membranes, including the diffusion rate and its dependence on the membrane’s stretch.  Understanding the time-dependent and tunable osmosis-driven actuation behavior of devices containing PDMS membrane, achievable in aqueous environments, may prove useful in future biomedical applications. 

Presenting Author: Alexandra Spitzer University of Illinois Urbana Champaign

Presenting Author Biography: Alexandra Spitzer is a 4th year PhD student in the Materials Science & Engineering department at the University of Illinois Urbana-Champaign. She studies the time-dependent actuation behavior of osmosis-driven, fluid-filled, elastomeric composites, under advisor Dr. Shelby Hutchens. She is currently a Mavis Future Faculty Fellow within the Grainger College of Engineering at UIUC, which is a fellowship program developed to facilitate the training of graduate students with the goal of becoming future engineering faculty.

Authors:

Alexandra Spitzer University of Illinois Urbana Champaign
Shelby Hutchens University of Illinois Urbana-Champaign