A Medical Membrane Mimic

    Kids, have you ever heard of kidney dialysis? Kidneys are essential for keeping the proper chemical balance in our blood. They perform a wonderful balancing act of filtration and osmosis that is not only essential to life, but is extremely complicated. In very simple terms, the kidneys filter blood in order to pass some dissolved salts, any toxins, urea, and creatinine, along with a little water. When the body digests proteins, urea is formed. But if too many urea molecules build up due to faulty kidneys, they can cause serious diseases. So, doctors and chemists have worked together over the years to perfect a process called hemodialysis. Chemists have especially helped in creating the special artificial membranes now used (hollow polysulfone fibers), which filter out urea but leave all the other beneficial molecules behind (like red and white blood cells and essential nutrients).

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    This is all hard to imagine, so here we describe an experiment testing a simple membrane model. In dialysis, small molecules pass through a membrane by diffusion. It¹s like dust blowing through a screen window while keeping bugs out. Diffusion also predicts which direction the molecules will move, which is from more concentrated to more dilute areas. Here a plastic bag will be the membrane. Iodine, starch, and water are the molecules. Starch and iodine combine to make a dark blue product and this will be the visual test of your model.

    Make a thick cornstarch mixture of 1 tsp (3 gm) cornstarch in 1/8 cup (20 ml) water. Put 3/4 cup (180 ml) hot water in a thick clear glass (like a clear coffee mug or a 250 ml beaker). Slowly stir in the cornstarch slurry. Have an adult partner put just under 3/4 cup (150 ml) water and 1 tsp (5 ml) tincture of iodine in another clear glass. Cut off the top (ziploc side) of a ziploc sandwich bag (not a freezer or storage bag). Pour about 1/4 cup of the cornstarch solution into the bag (cleanly!). Close the bag tightly with a twist tie. Gently set the bag in the iodine solution without letting the twisted top get wet. Observe every 3 min for 15 min. Check for color changes. Iodine molecules in the solution (orange) will flow by diffusion through the pores in the bag to the inside, where there is no iodine yet (flow is from concentrated to dilute). When iodine bumps into cornstarch molecules, they react to make the dark blue complex. Which molecules were able to travel through the pores of the bag, and which were not? What does this tell you about the size of the molecules, the size of the pores, and whether this is a good model of kidney dialysis?


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    May 2001


    Reference: David Thielk, ChemMatters, ACS, April 2001
    Patient Handbook, the Kidney Transplant/Dialysis Assn.
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