In a solution of detergent-solubilized membrane protein (MP), detergent molecules are in a constant equilibrium between monomers, micelles, and the surfactant layer that covers the transmembrane region of the protein and makes it hydrophilic.
To prevent MPs from aggregating, they must be handled above the cmc of the detergent, i.e. in the presence of free micelles. The latter act as a sink for hydrophobic and amphipathic molecules, e.g. lipids, which is a major cause of MP inactivation.
This could in principle be alleviated by designing molecules with such a high affinity for the transmembrane surface of MPs that they would never dissociate. Thus, traces of free surfactant would suffice to keep the protein soluble.
This concept led to the creation of 'amphipols' (APols), namely short amphipathic polymers that are able to keep individual MPs soluble under the form of small hydrophilic complexes.
A complete description of amphipol design and applications is available on Amphipol Website.