Sodium Polyacrylate (SPA) is a super-absorbent polymer that expands in the presence of water. The expansion is caused by the osmosis of sodium ions found in the polyacrylate structure. These ions break off and form a osmosis gradient, but subsequently makes reuse of polymer difficult as many of the sodium ions are no longer present in the structure after one evaporates all the water. Each expansion/evaporation cycle should greatly reduce the rate of absorption. Furthermore, one can modify absorption by adding salt to the polymer so that it is harder for the polymer to develop a gradient.
Currently I am exploring using the collapse of polyacrylate around molecules.I have shown success collapsing SPA around red dye. The experiment expanded SPA in water with a small quantity of red dye and drying that mixture. Then I rinsed the dried polyacrylate with acetone and ran UV spec on the results to determine the % of dye retained.
The next stages of my experimentation involve masking the SPA coatings and applying salt solution to the exposed area. The goal is to create areas where the SPA does not hold only the solution as readily and allows spreading into the shape produced by the mask.
This photo vaguely illustrates my point. There is a visible line between the grey left side and the whiter right side. The left side is treated with salt and thus the red dye spreads much further.
This is a painted line of brine over the SPA chip. The red dye only fills the line.
This is the 3rd generation chip. It uses a paper stencil to produce salted regions and then covers the surface with a piece of packing tape. The tape cover allows for capillary action to occur but only in the salted regions.
