Immobilization and electroactive switching of bovine serum albumin on polypyrrole functionalized bioelectroactive surfaces†

Hematopoietic stem cells (HSCs) are rare cells residing in the bone marrow and give rise to millions of new blood cells daily throughout life. Because of their multipotent, self-renewing nature, they have also been used for several decades to treat hematological disorders. However, HSCs are scarce and difficult to maintain ex vivo, demonstrating the need for developing novel in vitro methods to expand HSCs that mimic the complex in vivo microenvironment in suitable culture tissue plates, in extracellular matrix scaffolds, or on a biochip. One component to include in such an artificial microenvironment is HSC-related growth factors (GFs) immobilized on surfaces that mimic membrane-bound GFs in vivo. In this paper, we have initiated the development of an ex vivo system to study the immobilization of growth factors that sustain HSC maintenance and possibly expansion. However, since HSC-related GFs are expensive we have developed a proof-of-concept model using bovine serum albumin (BSA) as an alternative. Polypyrrole (PPy) was electrochemically synthesized in the presence of dicarboxylic acids with different hydrocarbon chain lengths and polycarboxylic acids with different molecular weights as dopants. BSA was immobilized on the PPy surface using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) to couple BSA to the carboxylic acid dopant of PPy. These PPy films with different dopants showed different abilities to immobilize BSA using EDC/NHS coupling and also different surface properties. In addition, owing to the interesting switchable properties of PPy upon alteration of the oxidation/reduction potential, the immobilized BSA could change its presentation on the PPy surface depending on the redox state. To characterize the PPy surfaces and to study the different immobilization results of BSA on these PPy variants with different dopants and different presentation behavior upon redox switching, the electrochemical properties, hydrophobicity, thickness, roughness, surface COOH density and fluorescence labeling were investigated. The results indicate that the polycarboxylic acid dopants could immobilize more BSA on the PPy surface. Moreover, the BSA in the as-fabricated state shows a “collapsed” presentation on the PPy surface, a “less collapsed” presentation in the oxidized state and an “erected” presentation in the reduced state. Cell viability studies using hematopoietic cells showed that the developed PPy-BSA surfaces did not negatively alter cell viability or cell proliferation compared to the control.