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Published online 10 September 2008 | Nature | doi:10.1038/news.2008.1097
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Tiny synthetic tree pumps water
'Tree on a chip' mimics water transport in plants.
A 'microtree' created from a synthetic gel used to make contact lenses has replicated water transport in plants. The design could be tweaked to improve extraction of water from dry soils, or to create more efficient cooling systems, researchers say.
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Some of the results and details of the methods in this very interesting paper, and its supplementary material, immediately stimulate questions which should, at the very least, have been commented on ? if not answered ? by Wheeler and Stroock. We are informed that methacrylic acid (MAA) and n-vinyl pyrrolidone (nVP) co-monomers were used in addition to the HEMA and EGDMA to form the hydrogel. Yet nowhere are we told why. If these monomers were omitted, would the experiment (model) have worked? If not, why not? The optical micrographs of ?bubbles? are confusing. Were they mislabeled? I?m a cell biologist, microscopist, coinventor, with the late B.J. Davis, of polyacrylamide electrophoresis and inventor of the Irristat. The Irristat is a moisture-sensitive, self-regulating irrigation valve, placed in the root-zone, that uses very slightly-crosslinked polyacrylamide gel as the sensor that swells and shrinks, in response to soil matric potential (water activity), to open and close the valve. So I?m especially familiar with properties of synthetic hydrogels and microscopic images. 1) The presence of 1% (vol.) MAA in the authors hydrogel would have the following consequence: At pH?s above about 4.0, virtually all the carboxyl groups of the MAA would be negatively charged. In the ABSENCE of salt cations, the mutual repulsion of those negative charges would generate CONSIDERABLE internal swelling pressure, that would only be opposed by the 6 v% EGDMA crosslinks. So such a turgid gel should be stiffer than the same gel without MAA comonomer. Does this swelling pressure have anything to do with the inhibition of cavitation? Is the activity of onset (0.85) of cavitation sensitive to salt? And if so, is such a charge-induced swelling pressure perhaps the ?explanation? of the inhibition of cavitation in the xylem of 100-meter-tall redwoods and eucalyptus? 2) Prolonged boiling would be expected to result in some hydrolysis of HEMA ester bonds, removing ethanol and producing even more pendant negatively-charged acrylate groups ? and dry-weight loss. Therefore the measurements of polymerization efficiency may be underestimated? 3) Polyvinyl pyrrolidone (PVP) is a polymer of the cyclic amide, nVP. Like polyacrylamide, it is neutral, and non-responsive to salts. However it has the well-known property of ?segregating? at hydrophobic/hydrophilic interfaces, aiding in the ?compatibility? of dissimilar chemical substances. Was the addition of nVP necessary for proper ?wetting? of the silicone polymer template or was it ?just? a convenient solvent for the photoinitiator? 4) The refractive index of the hydrated hydrogel is greater than that of water. Therefore, with partially coherent illumination (e.g., with the microscope condenser ?racked down?) water-filled ?bubbles? within the gel will exhibit some (less than a half-wavelength) phase-contrast with respect to the surrounding gel (Figure 2Bb). Since the refractive index of gas (e.g., air or water vapor) is much lower than even liquid water, gas-filled bubbles will exhibit even higher contrast, with a typical bulls-eye appearance (Figures 2Bc, 2Bd and 2C upper photograph-Day 0). Therefore some of these particular micrographs seem to have been interchanged or mislabeled? Leonard Ornstein
A small correc tion to my comment, above: "would be expected to result in some hydrolysis of HEMA ester bonds, removing ETHANOL" should read "would be expected to result in some hydrolysis of HEMA ester bonds, removing ETHYLENE GLYCOL" Leonard Ornstein