The dialysis tube, often called Visking tubing, is a foundational tool in biological and chemical research. Its specialized function is separating dissolved substances based solely on their size. This flexible tube acts as a barrier that differentiates between molecules in a solution. Scientists use it to purify complex mixtures, such as those containing proteins or DNA, by removing smaller contaminants like salts or solvents. This selective movement principle governs both laboratory purification and life-saving medical procedures.
The Semi-Permeable Structure
The physical makeup of the dialysis tube grants it its unique separating ability, classifying it as a semi-permeable membrane. Laboratory-grade tubing is most commonly made from regenerated cellulose, a polymer derived from wood pulp. This material is extruded into a hollow cylinder, forming a film with an intricate network of microscopic pores.
The size of these pores determines which molecules pass through the membrane and which are retained. Manufacturers characterize this property using the Molecular Weight Cut-Off (MWCO), measured in Daltons (Da) or kilodaltons (kDa). The MWCO indicates the approximate molecular weight above which a solute is largely retained by the membrane. Common laboratory tubing often has an MWCO of 12,000 to 14,000 Daltons, which is sufficient to hold back most proteins while letting smaller ions pass freely.
Selective Molecular Separation
Separation relies on diffusion, a passive transport process driven by a concentration gradient. When a solution of large and small molecules is sealed inside the dialysis tube and immersed in a surrounding liquid, the small molecules begin to move. Small solutes, such as salt ions or urea, exhibit a net movement across the membrane from the area of higher concentration (inside the tube) to the area of lower concentration (the surrounding liquid).
This movement continues until the concentration of the small molecules is equalized on both sides of the membrane, a state called dynamic equilibrium. Large molecules, like proteins or nucleic acids, are physically blocked from passing through the pores because they exceed the MWCO. This mechanism acts like a molecular sieve, separating components based purely on their physical dimensions in solution. The rate of diffusion is inversely related to a molecule’s size (smaller molecules move faster) and directly proportional to the concentration difference.
Applications in Laboratory Purification
In the laboratory, the selective separation capability of the dialysis tube is indispensable for purifying biological samples. A frequent use is desalting or buffer exchange, often necessary after chemical reactions or purification steps. For example, a researcher may need to remove high concentrations of salt ions from a protein solution before a subsequent experiment.
By placing the protein solution inside the tubing and surrounding it with a salt-free buffer, the small salt ions diffuse out, while the large protein molecules remain trapped. This process exchanges the sample’s original environment for a new, desired buffer solution. Dialysis tubing is also used for sample concentration. This is achieved by submerging the filled tube in a highly concentrated solution of a large molecule, such as polyethylene glycol (PEG). The high external concentration of PEG causes water to move out of the tube via osmosis to balance the osmotic pressure, reducing the volume and increasing the concentration of the macromolecule inside.
Connecting Lab Function to Medical Dialysis
The underlying principle of the laboratory dialysis tube is the exact same mechanism employed in clinical medicine. An artificial kidney machine, or dialyzer, uses a semi-permeable membrane to treat patients with kidney failure. The dialyzer contains a vast number of hollow fibers, which are essentially high-performance dialysis tubes.
A patient’s blood flows through the inside of these fibers, while a specialized dialysis fluid, or dialysate, flows around the outside in the opposite direction. This arrangement maintains a steep concentration gradient, causing small, toxic waste products like urea and creatinine to diffuse out of the blood and into the dialysate. The membrane’s pores are sized to retain large components in the blood, such as blood cells and plasma proteins, ensuring they are not lost. Hemodialysis is a large-scale, controlled application of the size-selective diffusion demonstrated by the laboratory dialysis tube.