Affinity precipitation. In recent years, affinity interactions have been widely used for precipitation, since it reduces the number of steps needed for downstream processing. In the past, the affinity ligands used were expensive and needed conjugation with solid matrices, making the technology expensive. Leakage of affinity ligand was another problem. These problems were solved at least in some cases, by using carbohydrate polymers, and textile dyes, which were the first cheap and robust class of affinity ligands. Following are some of the approaches used for designing or improving affinity ligands for separation and purification: (i) Metal ions are used as pseudoaffinity ligands, which bind proteins via some of the surface residues. In some cases, an immobilized metal chelator like iminodiacetic acid (IDA) or nitrilotriacetic acid (NTA) can also be charged with metal ions like Cu²⁺, Ni²⁺ and Zn²⁺, and then used as ligands.