**2.8 Microspheres**

Microspheres (**Figure 11**) can be prepared from polymers using various methods such as double emulsification, spray drying, complexation of macromolecules with opposite charges. Microspheres have sizes ranging from 1 μm to 1 mm. Much research has been performed on the use of microspheres for the oral delivery of protein or peptide drugs [59]. Microsphere-based delivery systems can protect the protein or peptide of interest from the harsh environment of the GIT such as enzymatic hydrolysis, acidic pH, etc. Microspheres can also enhance the absorption of the protein or peptide of interest, mainly through the paracellular pathway. The microsphere-based delivery system also allows the controlled release of the protein or peptide of interest at a specific area in the GIT by using pH-sensitive polymers. Insulin was loaded into the microsphere formed by the polymer poly (methacrylic-g-ethylene-glycol) [60]. This microsphere prevents the enzymatic degradation of insulin in the acidic environment of the stomach. However, swelling of the microsphere and consequent release of the insulin occurred in the basic environment of the intestine.
