The process that caused the sodium polyacrylate-filled dialysis tubing to become filled with water is **osmosis**.
Osmosis is the movement of water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration. The goal of this movement is to equalize the solute concentration on both sides of the membrane.
In the case of sodium polyacrylate-filled dialysis tubing, the tubing acts as a selectively permeable membrane. Sodium polyacrylate is a superabsorbent polymer that can absorb and hold large amounts of water. When the dialysis tubing is placed in water, the water molecules move across the tubing's selectively permeable membrane into the region with a higher concentration of sodium polyacrylate. As a result, the dialysis tubing becomes filled with water.
This process continues until the solute concentration (sodium polyacrylate) inside the tubing and the surrounding water is equalized, or until the polymer reaches its maximum absorption capacity.