Abstract
The heart is an incredibly efficient biological pump, which contracts as a whole from the coordinated effort of millions of excitable cells called cardiac myocytes. These cells are considered excitable since they possess the ability to respond to a threshold activation signal, but only if sufficient time has passed and they have recovered from their previous excitation. Excitable cells also have the capacity to propagate an activation signal to their neighboring cells, and collectively are referred to as an excitable medium. Numerous examples of excitable media exist in nature, including the heart, forest fires, the intestine, nerves, and certain kinds of chemical reactions. Engineered artificial excitable media, however, have never been developed before.
The long-term goal of this project, therefore, is to develop the first such artificial excitable medium from an array of artificial excitable cells, called gel-cells. The conceptual design of the gel-cell consists of a thin electroactive ion-gating membrane (polypyrrole), sandwiched between two layers of hydrated polymer (hydrogel) loaded with different concentrations of potassium chloride (KCl(aq)) electrolytes. The voltage controlled artificial membrane initially confines ions to the upper chamber of the gel-cell, and releases them upon membrane opening into the lower chamber, a process similar to membrane activation in cardiac myocytes. Potassium sensing electrodes embedded throughout the gel-cell provide feedback on the location and concentration of ions within the gel-cell, becoming markers for its activation. The scope of the research presented here was to construct the biomimetic cell membrane portion of the gel-cell, and study ion transport through its assembly.
First, the electrochemical response of polypyrrole (PPy) was characterized via cyclic voltammetry, and its membrane morphology and thickness were observed via scanning electron microscopy. Experimentally obtained membrane thickness values were then compared to model predictions, which were subsequently improved by updating membrane density and structure assumptions. Hydrogel pore size approximations were correlated to equilibrium water content calculations, and KCl(aq) concentrations were quantified using electrochemical impedance spectroscopy. Time-resolved KCl(aq) flux experiments were performed on PPy membranes in solution, providing real-time concentration profiles of KCl(aq) as it passes through the membrane. From the parameter space tested, the optimal membrane preparation and gating potentials were selected which produced the highest flux rates. Time-lapse flux experiments (providing piece-wise concentration profiles for KCl(aq)) were then performed on PPy membranes in both solution and hydrogel. Ion transport through two nearly identical membranes produced under different deposition conditions (PPy(a) and PPy(b)) were compared in solution. The slightly denser and thicker membrane (PPy(b)) produced nearly an order of magnitude higher flux rates than PPy(a), suggesting that an active component of ion transport occurs predominantly through these membranes via binding and release mechanisms in PPy. Next, experiments performed on PPy membranes in hydrogel demonstrated that flux rates through these assemblies were proportional to hydrogel pore size. In addition, ion transport was reduced by two orders of magnitude through PPy/hydrogel as compared to PPy in solution. Finally, the potentiometric response of potassium sensing electrodes was quantified in novel configurations of hydrogel versus hydrogel, demonstrating their ability to observe relative KCl(aq) concentrations in this material. These results lay the foundation for the development of the gel-cell by providing a cardiac myocyte membrane analog made from electroactive polymer materials, and identify key variables which can be varied to optimize ion transport in these assemblies.
