ABSTRACT
Diabetes is characterized by hyperglycemia, resulting from insulin deficiency or resistance, or both. Insulin plays an irreplaceable role in the treatment of diabetes. Subcutaneous injection is the main route of insulin administration, but usually leads to poor compliance and many side effects. Oral insulin is safer and more convenient, which has always been the Holy Grail for people to explore. After oral administration, insulin is absorbed into the hepatic portal vein and transported to the liver, which can activate the normal physiological functions and reduce the risk of hypoglycemia, insulin resistance, and improve patient compliance. However, the gastrointestinal tract has multiple absorption barriers such as chemical barrier, enzyme barrier, and permeation barrier. Due to the physical and chemical properties of insulin, it is difficult to achieve desired oral bioavailability. This article reviews the recent attempts and progress in the field of oral administration of insulin driven by innovative drug delivery technologies and biomaterials, including structural modification, enzyme inhibitors, absorption enhancers, various nanoparticles, liposomes, microspheres, and even microorganisms. Some clinical researches on oral insulin are also introduced.