TITLE:
Cross-Linked Alginate Film Pore Size Determination Using Atomic Force Microscopy and Validation Using Diffusivity Determinations
AUTHORS:
Cheryl Simpliciano, Larissa Clark, Behrokh Asi, Nathan Chu, Maria Mercado, Steven Diaz, Michel Goedert, Maryam Mobed-Miremadi
KEYWORDS:
Alginate; Atomic Force Microscopy; Pore Size; Stokes’ Radius; Diffusivity; Cross-linking; Differential Scanning Calorimetry; Reflection Coefficient
JOURNAL NAME:
Journal of Surface Engineered Materials and Advanced Technology,
Vol.3 No.4A,
October
22,
2013
ABSTRACT: The deficit of organ donors has fueled
the need for advances in tissue engineering and regenerative medicine. Microencapsulation in alginate immuno-isolation membranes has been used to
treat many disabling metabolic disorders, namely, phenylketonuria, kidney
failure and diabetes mellitus. Systematic nutrient flux determinations are
hindered by the lack of experimental data on alginate-based membrane topography and the pore size thus preventing the full
therapeutic potential
of the bio-membranes to be reached. In this study, samples of cross-linked
alginate membranes were subjected to the following analytical characterization:
1) pore size characterization using atomic force microscopy operated in contact mode to detect and measure pore
size; 2) differential
scanning calorimetry to confirm biopolymer cross-linking; and 3) diffusivity measurements using
spectrophotometry and fluorescence microscopy to confirm the presence of
through pores and to calculate reflection coefficients. The pore sizes for the
pre-clinical standard
formulation of 1.5% (w/v) medium viscosity alginate cross-linked with 1.5% CaCl2 and 0.5% (w/v) alginate and chitosan cross-linked with 20% CaCl2 are 5.2 nm ± 0.9 nm and 7.0 nm ± 3.1 nm, respectively. An increase in the glass
transition temperatures as a function of cross-linker concentration was
observed. Diffusivity values obtained from the inward diffusivity of creatinine
into macrocapsules (d =
1000 μm ± 75 μm) and the outward diffusivity of FITC dextrans from macrocapsules (d = 1000 μm ± 75 μm) and microcapsules
(d = 40 μm ± 5 μm)
were shown to correlate strongly (R2 = 0.9835) with the ratio of solute to
pore sizes, confirming the presence of through pores. Reflection coefficients
approaching and exceeding unity correlate with the lack of permeability of the
membranes to MW markers that are 70 kDa and greater.