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Description
Sap transport in vascular plants is ensured by a complex network of two "microfluidic channels", xylem and phloem, coupled by a biological membrane. Evapotranspiration drives a flow of almost pure water through the xylem, from the roots to the leaves. In parallel, the sugars produced in the leaves by photosynthesis generate a large turgor pressure allowing their transport in the phloem towards the roots, shoots and fruits. The size of this system makes it well-suited for investigation in controlled systems, such as those provided by microfluidics. This work investigates the osmotic driven transport of sugars in the phloem, known as the Münch mechanism. A microfluidic chip was engineered, consisting of two parallel channels coupled by a hydrogel membrane permeable to water, with a molecular weight cut-off (MWCO) of a round $1$ kg/mol and able to resist up to $10$ bar of hydrostatic pressures [1]. The hydrogel was photo-crosslinked in situ using a maskless photolithography device projecting collimated UV patterns. Solutes like dextran ($10$ kg/mol) induce stable osmotic flows in the channels (M>MWCO). In contrast, transient flows are observed with solutes of smaller molecular weights because of their permeation through the membrane. We show that the dynamics of this transport can be described by a model coupling forward osmosis and solute permeation under dilute conditions, which was verified experimentally to and allowed to estimate the membrane reflection and diffusion coefficients for a wide range of solutes [2]. In the future, our objective is to expand this study to higher solute concentrations, as found in the phloem of plants, and couple the current device to an artificial xylem in which the water is at negative pressure, involving the integration of supplementary hydrogel membranes.
[1] Nguyen, H-T., Massino, M., Keita, C., Salmon,J-B. Microfluidic Dialysis using photo-patterned hydrogel membranes in PDMS chips. Lab Chip 2020, DOI : 10.1039/D0LCOO279H
[2] Renaudeau, J., Lidon, P., Salmon, J-B. Transient osmotic flows in a microfluidic channel: measurements of solute permeability and reflection coefficients of hydrogel membranes. Lab Chip 2025, DOI : 10.1039/D5LCOO276A