Dialysis
Dialysis is a procedure for exchanging the solvent around a protein. In general the protein solution is placed inside a semi-permeable membrane (dialysis bag) which is suspended in a larger volume of buffered solution (see image to the right). The key to this procedure working is that the membrane has to be permeable to water and ions, but not to your protein of interest. Thus buffers & salts exchange until an equilibrium is established between the inside & outside of the membrane.
Naturally in medicine the types of dialysis you are likely to see are hemodialysis and peritoneal dialysis which remove wastes and excess water from the blood in different ways. Hemodialysis removes wastes and water by circulating blood outside the body through an external filter containing a semipermeable membrane. The blood flows in one direction and the dialysate flows in the opposite. The counter-current flow of the blood and dialysate maximizes the concentration gradient of solutes between the blood and dialysate, which helps to remove more urea and creatinine from the blood. The concentrations of solutes (for example potassium, phosphorus, and urea) are undesirably high in the blood, but low or absent in the dialysis solution, and constant replacement of the dialysate ensures that the concentration of undesired solutes is kept low on this side of the membrane. The dialysis solution has levels of minerals like potassium and calcium that are similar to their natural concentration in healthy blood.
In peritoneal dialysis, wastes and water are removed from the blood inside the body using the peritoneal membrane of the peritoneum as a natural semipermeable membrane. Wastes and excess water move from the blood, across the peritoneal membrane, and into a special dialysis solution, called dialysate, placed in the abdominal cavity. Diffusion and osmosis drive waste products and excess fluid through the peritoneum into the dialysate until the dialysate approaches equilibrium with the body's fluids. Then the dialysate is drained, discarded, and replaced with fresh dialysate often 4-5 times pr day.