Organic Bioelectronics: Conducting Polymers At The Interface With Biology

Abstract

Organic bioelectronics is a growing interdisciplinary research field that seeks to integrate organic electronic transducers (i.e., electrodes, electrochemical/field effect transistors) with biological systems (i.e., live cell cultures, biomaterials, and living organisms) for applications in basic research, medical diagnostics, and tissue engineering. These transducers can operate by either converting a biological signal into a measurable electrical signal (as in a biosensor), or conversely, by providing an electrical stimulus that affects the behaviour or function of a biological system. This latter approach is the focus of the work presented in this dissertation. First, a simple planar device is described based on thin films of the conducting polymer poly(3,4-ethylenedioxythiophene) doped with p-toluenesulfonate (PEDOT:TOS) or poly(styrenesulfonate) (PEDOT:PSS). When a linear potential gradient is applied to this device for one hour in the presence of cell culture medium, a gradient in oxidation state is established in the PEDOT:TOS film that remains after the bias is removed. When cells of different types are cultured on such surfaces, spatial gradients are established in two types of cell behaviour: 1) Cell adhesion is modulated, enabling gradients in cell density, and spatial control over cell adhesion; and 2) Cell motility is modulated, enabling continuum control over cell migration speed and persistence. Next, the interface mechanism is investigated by which the electrochemical oxidation/reduction of PEDOT:TOS thin films enables the observed modulation in cell behaviour. To this end, the important adhesion and signalling protein fibronectin (Fn) is studied, with regard to how its molecular conformation is directly altered by the local electrochemical oxidation potential of the under¨ lying PEDOT:TOS film. Conformation is assessed via Forster Resonance Energy Transfer (FRET) studies, which reveal that a linear electrical potential gradient establishes a smooth, monotonic gradient in the molecular conformation of surface-adsorbed Fn. The gradient in conformation varies from compact at the oxidized end of the film, to extended at the neutral part of the film, to partiallyunfolded at the reduced end of the film. Hence, the device is demonstrated to enable precise electrical control over the molecular conformation of adsorbed fibronectin. This device's ability to control protein conformation as an isolated parameter is then exploited to enable studies of cell secretory behaviour that are inaccessible to traditional "biology-only" experiments. The specific role of protein conformation (both fibronectin and serum proteins) in modulating the secretion of vascular endothelial growth factor (VEGF) is investigated. In particular, cells cultured on proteins in an unfolded conformation (the condition found in the tumour environment) secrete larger quantities of VEGF (with verified proangiogenic properties), implicating the role of altered fibronectin conformation in tumour angiogenesis. Finally, a three-dimensional macroporous PEDOT:PSS scaffold is introduced. This scaffold represents an electrically-active, three-dimensional environment for cell culture, offering tunable pore morphology and mechanical properties, in addition to the electrochemical tunability of conductivity, surface energy, and pH demonstrated in the two-dimensional thin-film system. This platform enables confirmation that the electrically-controllable adhesion and secretion behaviours are recapitulated in three-dimensions, and paves the way toward more complex future studies and applications with greater physiological relevance.