Controlled release properties of zein powder filled into hard gelatin capsules

This study aimed to investigate the relationship between controlled release behaviour, swelling and initial water sorption of zein powder filled into hard gelatin capsules. Release, release rate and dissolution efficiency of various physical mixtures of zein and propranolol HCl filled into capsules were measured by dissolution testing. As the gelatin shell dissolved, zein rapidly formed a matrix, providing prolonged drug release, after an initial drug burst. Drug loading and ionic strength of the medium significantly influenced drug release. Imaging and gravimetric analysis were used to investigate matrix swelling and erosion. Zein matrices swelled like hydrophilic matrices, yet were practically non-erodible. Moreover, a strong significant positive correlation was found between matrix swelling and drug release (Pearson's correlation coefficient, r ≥ 0.9657, p-value ≤ 0.0017), suggesting that the degree of swelling governed drug release. The tensiometry test revealed that the initial ingress of media through the pores of zein powder was significantly limited by a reduction of the capillary radii of these pores, as a result of rapid polymer hydration. In conclusion, the rapid hydration of zein powder could explain the matrix formation and the controlled release observed with the capsules in the dissolution studies. Initial hydration, swelling and release behaviour were dependent on the ionic strength of the medium.