Figure 1.

Overview and history of chemical engineering on the EXOs. (A) Classification of chemical engineering on the EXOs. Apart from genetic engineering, theranostic agents for diagnosis/treatment can be selectively conjugated on the surface of EXOs by many kinds of chemical molecules or functional groups such as saponin, carboxyl, amino, azide, alkyne, maleimide, thiol, disulfide, biotin, and many others. The chemical engineering of surface modification on the EXOs includes the uses of original groups and foreign moieties. Usually, the foreign moieties are intentionally embedded into membranes by metabolic engineering. (B) The history and evolution of chemical engineering on the EXOs. Since the discovery of EXOs in 1987, there was an enormous gap in chemical engineering on the EXOs until 2014.