Transport of biological molecules in surfactant–alginate composite hydrogels

Obstructed transport of biological molecules can result in improper release of pharmaceuticals or biologics from biomedical devices. Recent studies have shown that nonionic surfactants, such as Pluronic® F68 (F68), positively alter biomaterial properties such as mesh size and microcapsule diameter. To further understand the effect of F68 (incorporated at concentrations well above the critical micelle concentration (CMC)) in traditional biomaterials, the transport properties of BSA and riboflavin were investigated in F68–alginate composite hydrogels, formed by both internal and external cross-linking with divalent cations. Results indicate that small molecule transport (represented by riboflavin) was not significantly hindered by F68 in homogeneously (internally) cross-linked hydrogels (up to an 11% decrease in loading capacity and 14% increase in effective diffusion coefficient, Deff), while protein transport in homogeneously cross-linked hydrogels (represented by BSA) was significantly affected (up to a 43% decrease in loading capacity and 40% increase in Deff). For inhomogeneously cross-linked hydrogels (externally cross-linked by CaCl2 or BaCl2), the Deff increased up to 50 and 83% for small molecules and proteins, respectively. Variation in the alginate gelation method was shown to affect transport through measurable changes in swelling ratio (30% decrease) and observable changes in cross-linking structure as well as up to a 3.6- and 11.8-fold difference in Deff for riboflavin and BSA, respectively. Aside from the expected significant changes due to the cross-linking method utilized, protein transport properties were altered due to mesh size restrictions (10–25 nm estimated by mechanical properties) and BSA–F68 interaction (DLS). Taken as a whole, these results show that incorporation of a nonionic surfactant at concentrations above the CMC can affect device functionality by impeding the transport of large biological molecules.