**4.3 Lipids and phospholipids**

Phospholipid material didecanoyl-L-α-phosphatidyl choline was used as an absorption enhancer to develop the nasal delivery system for insulin. However, the development was halted due to low bioavailability in diabetic patients [151]. A nasal insulin formulation with a bioavailability of 35% has been reported. Irritation problems were also reported.
