title: Effect of cations on the microstructure and in-vitro drug release of κ- and ι-carrageenan liquid and semi-solid aqueous dispersions.
abstract: OBJECTIVES The main objective of this study was to determine the effect of potassium and calcium ions on the microstructure and release dynamics of kappa (κ) and iota (ι) carrageenan.
  
  
  METHODS
  The microstructure of the dispersions was imaged using a cryogenic scanning electron microscope. Franz-cell diffusion apparatus was used to determine the release kinetics of a model hydrophilic drug, sodium fluorescein, incorporated in selected polymer dispersions. Release profiles were analysed using Higuchi, Korsmeyer-Peppas and dual first-order models.
  
  
  RESULTS
  Cryogenic scanning electron microscope images showed that κ-carrageenan forms hexagonal structures, whereas ι-carrageenan forms rectangular pores at low cation concentrations. In-vitro release studies showed sustained release profiles for all carrageenan systems; however the model drug, fluorescein, diffusion from ι-carrageenan with 0.06% w/v calcium was significantly higher than other ι-carrageenan systems. This may be attributed to improved tortuosity of this system. However further increase in cation concentration led to a reduction in fluorescein release from the matrices. The dual first-order release model illustrated two distinct release rates, an initial rapid release followed by a slow diffusion of fluorescein from the carrageenan matrices.
  
  
  CONCLUSIONS
  The observed microstructural differences may account for the well known variation in mechanical properties of κ- and ι-carrageenan gels. The dual first order release model adds a new tool in the elucidation of release mechanisms from polymer matrices, where parallel processes contribute to drug release.
authors.affiliation: Drug Delivery Research Unit (DDRU), School of Pharmacy, Faculty of Medical and Health Sciences, The University of Auckland, Auckland, New Zealand. t.thrimawithana@auckland.ac.nz
authors.forename: Thilini Rasika
authors.initials: TR
authors.lastname: Thrimawithana
authors.forename: Simon
authors.initials: S
authors.lastname: Young
authors.forename: Raid G
authors.initials: RG
authors.lastname: Alany
mesh_terms: D002118:Calcium / Q000737:chemistry; D002351:Carrageenan / Q000737:chemistry*; D003692:Delayed-Action Preparations; D005079:Excipients / Q000737:chemistry*; D019793:Fluorescein / Q000008:administration & dosage* / Q000737:chemistry; D007700:Kinetics; D008855:Microscopy, Electron, Scanning / Q000379:methods; D008962:Models, Theoretical; D011188:Potassium / Q000737:chemistry
chemical_list: D003692:Delayed-Action Preparations; D005079:Excipients; D002351:Carrageenan; D011188:Potassium; D002118:Calcium; D019793:Fluorescein