DEVELOPMENT OF AN ELECTROACTIVE POLYMER SENSOR FOR USE IN A NOVEL ARTIFICIAL CELL

Transcription

1 DEVELOPMENT OF AN ELECTROACTIVE POLYMER SENSOR FOR USE IN A NOVEL ARTIFICIAL CELL Lynna Nguyen*, Christina Haden, Donald Jordan, Pamela Norris Mechanical and Aerospace Engineering Department, University of Virginia *Ln7j@virginia.edu Abstract The ultimate goal of this project is to connect many artificial excitable cells together to produce a medium exhibiting contraction similar to the heart. A proposed polymerbased excitable cell design under development at U.Va. mimics the excitability and refractoriness of biological cardiac myocytes. This contracting medium will be used to develop a tube pump capable of propelling fluids along its length. Re-evaluating current pumping systems has the potential to free up precious space, energy, and weight aboard spacecraft. As a part of these artificial cells, an ion-sensitive probe is needed to detect the presence, location, and concentration of potassium ions within the cell. It is a crucial element needed to incorporate excitability within the artificial cell. For this research project, a potassium-selective ion sensor was reproduced following research by Pandey, Singh, and Shrivastava (2002) and tested for functionality within hydrogels based on research by Barthus, Lira, and Cόrdoba de Toressi (2008). A linear relationship between ion concentration and voltage drop of the sensor within the hydrogel was observed, and therefore it is predicted that the sensor will function properly and be easily incorporated into an artificial excitable cell with a hydrogel foundation. Introduction As space exploration attempts to increase in duration and speed, energy consumption of on-board systems requires careful reevaluation. Conventional pumps used for the transport of fuel and other liquids require precious space, energy, and weight. Therefore, reconsidering the transport modes of fluid throughout these vehicles has the potential to reduce energy consumption in space craft. There are two major types of pumps; positive displacement pumps and dynamic pumps. Generally, positive displacement pumps are lighter and arguably more efficient (Fratarcangeli, 1994). Even so, they have yet to achieve their full potential in terms of weight and energy reduction. Displacement pumps require complex heavy components. Therefore a logical improvement would be to let the tube itself propel fluids throughout the system. This is referred to as a standing wave pump. A few pumps today exhibit this attribute, but they fail to produce a purely peristaltic movement where the tube itself propels the fluid. The ultimate objective of this research is therefore to produce a standing wave pump utilizing electro-active polymer materials. The advantageous characteristics of electro-active polymers include light weight, flexibility, and biological levels of energy efficiency as they utilize very low voltages for activation. In this case, electroactive materials will be used to Nguyen 1

2 mimic the excitable and contractile nature of cells that make up the heart (i.e., cardiac myocytes), through the production of an artificial excitable cell, called the gel cell (Fig. 1). The gel cell has earned this name as it is made predominantly of polymer gel material. Ultimately, a large number of cells will be connected together, producing an artificial membrane capable of propagating a contractile wave down its length and efficiently pumping fluids along in a peristaltic motion. Figure 1. Initial conceptual model of the artificial cardiac cell. The artificial gel cell in its initial design is produced by a strategic combination of several types of electroactive polymers. The basic foundation of the initial design is an ionic liquid polymer gel (ILPG), which contains free flowing potassium ions that permeate throughout. Electrodes within the gel cell are used to create a potential gradient directing ion flow against osmotic gradients. Polypyrrole, which is electropolymerized onto a mesh, is the ion gating membrane that controls the flux of ions and mimics the biological cell membrane. The ion permeability of PPy, dictated by its controllable oxidized/reduced state, can be varied by applying a voltage potential. PPy membranes can alternatively also be used to detect the presence of potassium. This research focused on these potassium selective microelectrodes incorporated into the gel cell design to sense the presence and concentration of potassium ions and to convey excitability to the artificial cell by propagating the signal of cellular activation to its neighbors. These materials combine to produce a theoretical cell concept with an activation threshold and a refractory period. This is the foundation that will later become a connected sheet of cells capable of propagating activation. The fundamental traits of this artificial excitable cell which is designed to mimic a cardiac myocyte include 1) an activation threshold, 2) a refractory period, and 3) the ability to propagate signals. An activation threshold prevents the cell from responding aimlessly to any incoming signal. The refractory period is the duration of time over which the cell cannot be reactivated again, even if the threshold has been reached. It is crucial in establishing unidirectional wave propagation. In a standing wave pump, it would be undesirable for the pump to unexpectedly reverse fluid direction. Each step of the conceptual design was outlined to mimic biology. The flux of ions with electrical gradients and gating membranes will be controlled with electroactive polymers. Cell Cycle Phase 1: The gel-cell is in the resting phase and is ready to be fired. The upper electrode is negatively charged and the tracer ions are in the upper chamber of the gel-cell. The PPy membrane is closed and impermeable to the tracer ion. Phase 2: The gel-cell receives a signal to fire (green pulse), which is analogous to gap junctional conduction for myocytes. This signal charges a negative electrode in the ionsensitve switch, increasing the local tracer ion concentration above a pre-determined threshold and activates firing (red pulse). Nguyen 2

3 Phase 3: The switch, connected to external positive and negative DC power leads, reverses the polarity of the upper and lower bucky gel electrodes and positively charges the PPy membrane. The change in polarity of the bucky gel electrodes causes the tracer ions to migrate to the PPy membrane but they will not pass through until the membrane responds to charging and becomes permeable to the ions. The time scale for opening the PPy membrane is expected to be larger than that for switching electrode polarity. Determination of that time scale will be an important sequence of experiments. Phase 4: The PPy membrane becomes permeable to the tracer ions and allows them to pass into the lower chamber. As the potassium tracer ions enter the lower chamber, the potassium selective microelectrode measures an increasing concentration and outputs a voltage analogous to an action potential. Furthermore, as the tracer ions leave the upper region of the gel, the ion concentration in that area drops and the ionsensitive switch reverses to its initial state, causing the PPy membrane to become impermeable to ions again. At this point the gel-cell is in the refractory stage and cannot be fired. Phase 5: With the ion-sensitive switch back in its initial state (because the local concentration of potassium is now lower again), the upper and lower bucky gel electrodes once again reverse polarity and the PPy membrane is again impermeable to ions. The tracer ions are then forced to diffuse through the low conductivity gel in the middle of the cell. (Note that the conductivity of this center gel gives independent control of the refractory period for the gel cell.) Once a threshold number of ions have diffused out, the cell returns to the resting state in Phase 1. Figure 2: A sequence diagram illustrating five phases in a cycle of the artificial excitable gel cell. As a part of this artificial cell, an ionsensitive probe is needed to detect the presence, location, and concentration of potassium ions within the cell for it to function correctly. It will also be a critical component for propagating a signal of cellular activation to its neighbors. This research focuses on reproducing a promising potassium selective sensor following research by Pandey et al. (2002) and testing it in a hydrogel medium following research by Barthus et al. (2008). Materials and Methods The base of the potassium sensor consisted of a polypyrrole (PPy) membrane. Pyrrole was electropolymerized onto a mesh in a conventional three-electrode electrochemical cell. Two types of PPy membranes were deposited onto either gold mesh or stainless steel mesh. The first PPy membrane (ACN PPy) consisted of a solution of acetonitrile (ACN), tetrabutylammonium tetrafluoroborate, deionized water (dih 2 O), and distilled pyrrole. Pyrrole was deposited onto a gold mesh under a potentiostatic deposition at +1.2V versus saturated calomel reference electrode for one hour. The second PPy membrane (dih 2 O PPy) consisted of a Nguyen 3

4 solution of dih 2 O, p-toluenesulfonic acid (PTS), and distilled pyrrole. The process of electropolymerizing pyrrole was optimized by graduate student, Christina Haden, in the Soft Materials Laboratory at the University of Virginia. Therefore, for the dih 2 O PPy membrane, pyrrole was deposited on both stainless steel and gold mesh under a galvanostatic deposition at 2 ma versus saturated calomel reference electrode for two hours. The potassium sensor was created following research by Pandey, Singh, and Shrivastava (2002). A sensing solution of poly(vinyl chloride), dibutyl phthalate, tetraphenyl borate, dried tetrahydrofuran, and dibenzo-18-crown-6 as the ionophore was placed onto both sides of a PPy membrane and allowed to evaporate leaving a thin membrane sensing layer, thus creating the potassium sensor. It was then conditioned in 0.01 M KCl aq for 24 hours prior to testing. A negative control sensor was created by depositing the sensing solution onto stainless steel mesh, allowed to evaporate, and conditioned overnight prior to testing. This was used to compare the voltage drop of a sensor with and without a PPy membrane base. To test reproducibility, multiple sensors were created and tested simultaneously (Fig. 3). The potassium sensor was then tested in the experimental setup (Fig. 5) to determine its response time and produce a calibration curve relating voltage drop to potassium concentration. Sensors that were created simultaneously were also tested simultaneously. 5 mm Figure 3. Potassium ion sensors. Hydrogels were formed around both the ACN PPy sensor and the dih 2 O PPy sensor that were formed on gold mesh, creating hydrogel potassium sensors. The hydrogel was created following a recipe by Barthus, Lira, and Cόrdoba de Toressi (2008). A solution of acrylamide, N, N, - methylene bis acrylamide, dih 2 O, N, N, N, N tetramethylethylendiamine, and potassium peroxodisulfate as the catalyst was poured into a mold around the potassium sensor which was left overnight to form a hydrogel (Fig. 4). Figure 4. Hydrogel made following research by Barthus, et al. (2008). The new hydrogel potassium sensor was removed from the mold and allowed to dehydrate to evaporate out impurities that may be within the gel. It then underwent a complete fluid exchange in dih 2 O and was rehydrated overnight. The hydrogel sensor s functionality was then also tested in the experimental setup (Fig. 5) to determine its response time and produce a calibration curve. The experimental setup (Fig. 5) consisted of a buffer solution that was separated from the testing solution by a frit. The setup created an ionic voltage drop between the reference electrode and the sensor, placed in the buffer solution and potassium testing solutions, respectively. Once the sensor was in solution, a Data AcQuisition (DAQ) card recorded the voltage drop produced between the two electrodes using LabVIEW. Nguyen 4

5 Reference Electrode Sensor Table 1. Summary of potassium sensors created and tested. No Potassium Sensor PPy Membrane Base Deposition Mesh Hydrogel 1 Negative Stainless none none Control Steel No 2 ACN PPy ACN Potentiostatic Gold No 3 ACN PPy Hydrogel ACN Potentiostatic Gold Yes 4 dih 2O dih 2O Galvanostatic Gold No 5 dih 2O dih 2O Galvanostatic Stainless Steel No 6 dih 2O PPy Hydrogel dih 2O Galvanostatic Gold Yes Figure 5. Experimental setup used to test the sensor s ability to detect potassium at varying concentrations. A PPy sensor is placed in the testing solution, and a voltage drop is read versus the reference electrode. The simple potassium sensors and the hydrogel-coated potassium sensors were tested in varying concentrations of aqueous potassium chloride (KCl aq ), from dih 2 O to 1 M KCl aq. Testing was performed in three runs: from low KCl aq concentration to high concentration, back down to low concentration, and finally back up to high KCl aq concentration again in order to identify any hysterisis. The hydrogel potassium sensors were hydrated and conditioned in dih 2 O and then tested in the various concentrations of KCl aq. The hydrogel itself was not conditioned in various concentrations of KCl aq. The ACN PPy potassium sensor was tested in ionic liquid, which is the foundation of the ILPG in its initial design. Solutions of ionic liquid saturated with KBF 4 and KCl salt were made to test the interaction between the sensor and the potassium that dissociated into the ionic liquid. Ionic liquid was found to be soluble in water, therefore the sensor was also tested in solutions of ionic liquid and salts diluted with water. A summary of the various potassium sensors created and tested are shown below (Table 1) and are referred to by their respective numbers. Results and Discussion The negative control sensor, which consisted of a stainless steel mesh coated with the potassium sensing solution developed by Pandey et al. (2002), was used as the baseline voltage drop response. It was then compared to the voltage drop response of the PPy potassium sensors and the hydrogel-coated potassium sensors to determine if the PPy membrane or the hydrogel affected the voltage drop response. Two different PPy membrane substrates, ACN PPy membrane and dih 2 O PPy membrane, were tested and compared to see if the different PPy membrane substrates affected the voltage drop response. The two different PPy membranes were then tested in hydrogel to see if hydrogels affected the voltage drop response. Results produced by Pandey et al. (2002) are shown in Figure 6, where a linear relationship is seen with increasing potassium ion concentration, the voltage drop response by the sensor ( E) increases. Similar trends can be observed when testing the reproduced sensors within the range of 100 μm to 100 mm KCl aq. The response of the sensor saturates at concentrations higher than 1 M KCl aq. The sensor demonstrates a directional preference when tested from high concentration to low concentration, and shows Nguyen 5

6 erratic behavior when tested from low concentration to high concentration. It should be noted that each sensor was tested in dih 2 O which is plotted as 1 μm ( M) on all graphs. ACN PPy sensor. The inclusion of a negative voltage drop seen by sensor 2 at 10 μm KCl aq indicates a potential reading error and low potassium ion detection at this concentration. Figure 6. Results found by Pandey et al. (2002). Figure 7 shows the relationship of increasing voltage drop with increasing KCl aq concentration for the ACN PPy potassium sensor (sensor 2) and the dih 2 O PPy potassium sensor (sensor 4). For each data set, testing was performed on two sensors simultaneously. Sensor 2 compared to sensor 4 shows a fairly consistent lower voltage drop response, especially at lower concentrations. There is a larger variability demonstrated by sensor 4, therefore, the difference may not be statistically significant because sensor 2 readings are within the error bars of the sensor 4 readings. The standard deviation for sensor 2 is affected by the fact that some data points for the various concentration levels were missing. The similar results by these two PPy only sensors show that both ACN PPy and dih 2 O PPy membranes respond similarly to KCl aq and are therefore interchangeable. The dih 2 O PPy potassium sensor may be preferred over the ACN PPy sensor, however, due to the harmful chemicals needed in creating the Figure 7. Potassium sensor (on gold mesh) and negative control sensor responses to varying KCl aq concentrations. Figure 8 compares the voltage drop response of dih 2 O grown PPy membranes deposited onto either gold or stainless steel (s.s.) mesh. For each data set shown, testing was performed on two sensors simultaneously. Sensor 5 (PPy on s.s. mesh) shows a consistently lower voltage drop response, but is within the standard deviation of sensor 4 s (PPy on gold mesh) voltage drop response; therefore the difference may not be statistically significant. The similar results by sensor 4 and sensor 5 imply that either mesh can be used when creating the potassium sensor. The difference in the response could be due to the sensor s difference in polypyrrole coverage when depositing the PPy membrane. The difference in PPy coverage could be due to the deposition conditions, since the gold mesh sensor underwent a galvanostatic deposition at 3 ma and the stainless steel mesh deposition was performed at 2 ma. Another factor that affects deposition is the total deposition time. Nguyen 6

7 Increased deposition time increases overall mesh coverage, however when performing simultaneous PPy membrane depositions, increased deposition time is necessary to reach a similar PPy membrane coverage as that found for a single deposition. shown to amplify the voltage response with an increased sensitivity as compared to the negative control and PPy only sensors (Fig. 11 and Fig. 12). Figure 8. Negative control sensor and dih 2 O PPy potassium sensor response, on gold and stainless steel mesh. Figure 9 shows the overall comparison of sensors 1, 2, 4 and 5, which were not covered in hydrogel. A negative control sensor, where only a stainless steel mesh and the sensor solution developed by Pandey et al. (2002) were used, demonstrated the effect PPy has on voltage response of the sensor. The negative control sensor follows the trend with increasing KCl aq concentration and increased voltage response, although attenuated compared to the PPy membrane with sensor solution (sensors 2, 4 & 5). When the negative control sensor is compared to sensors 2, 4, and 5, it is less sensitive and shows a lower detection limit than the sensors with a PPy membrane base. Therefore, the PPy membrane base is shown to have an amplifying affect on the sensor s voltage response, which implies that differences in PPy membrane coverage could explain small differences in voltage response. When the potassium sensor is incorporated into the hydrogel material, it is Figure 9. Comparing all PPy potassium sensor responses. Figure 10 shows the voltage drop response of the ACN PPy potassium sensor (sensor 2) tested in ionic liquids. With the introduction of water, the solubility of the potassium chloride salt in ionic liquid was expected to increase; thus, a higher voltage drop was expected due to the increased availability of potassium. However, the ion specificity of the sensor was shown to be poor in ionic liquid, making it unable to detect potassium levels. The data shows no significant difference between the voltage drop response from the ionic liquid (which has no potassium) and the ionic liquid saturated with various potassium containing salts (KBF 4 and KCl). Nguyen 7

8 larger linear range from 10 μm to 1 M KCl aq. However, there is also a significant increase in variability of readings for this sensor. Figure 10. ACN PPy sensor response to various salts dissolved in IL compared to pure IL. The artificial gel cell in its initial design utilized ionic liquid polymer gel as the foundation of the cell. However, because the sensor developed by Pandey et al. (2002) cannot detect potassium in the ionic liquid, it will not function properly in the polymer gel cell. Because of other problems encountered with ILPG as a foundation for the artificial cell, hydrogels were investigated as a potential new foundation for the gel cell due to its hydrophilic nature and its ability to absorb aqueous KCl. Therefore, the sensing membrane developed by Pandey et al. (2002) was hypothesized to function appropriately in the hydrogel reproduced following research by Barthus et al. (2008). To test functionality of the PPy sensor in this new material, a hydrogel was formed around sensors 2 and 4 to create the hydrogel-coated potassium sensors, now referred to as sensors 3 and 6. Figure 11. Voltage drop response of ACN PPy potassium sensor with and without hydrogel. This was not the case for sensor 6, however. Figure 12 compares the voltage drop response of dih 2 O potassium sensor with and without hydrogel (sensors 6 and 4, respectively). The addition of hydrogel to sensor 4 did not amplify the voltage drop response; the voltage drop response shows significantly less variability when compared to sensor 3. Because the voltage response by sensor 4 (without hydrogel) and sensor 6 (with hydrogel) are very similar, using hydrogel as the foundation of the gel cell would result in similar functionality. Figure 11 compares the voltage drop response of an ACN potassium sensor with and without hydrogel (sensors 3 and 2, respectively). The voltage drop response by sensor 3 is significantly higher than the response from sensor 2. Therefore, the addition of hydrogel to sensor 2 to make sensor 3 had an amplification effect on the voltage drop. The amplification effect created an improved detection limit down to the lowest concentration (10 μm KCl aq ) and a Nguyen 8

9 Figure 12. Voltage drop response of dih 2 O PPy potassium sensor with and without hydrogel. Figure 13 shows the linear relationship of increasing concentration and increasing voltage drop response of the ACN PPy hydrogel potassium sensor (sensor 3) and the dih 2 O PPy hydrogel potassium sensor (sensor 6). It also shows the relative amplification effect of the hydrogel on the voltage drop response. It can be seen that the response for the hydrogel around the ACN PPy sensor is significantly greater than the hydrogel around the dih 2 O PPy sensor. When the negative control sensor is compared to sensors 3 and 6, it is less sensitive and shows a lower detection limit than the sensors with a PPy membrane base and a hydrogel. Therefore, the PPy membrane base with a hydrogel is shown to have an amplifying affect on the sensor s voltage response. Figure 13. Comparison of ACN PPy and dih 2 O PPy hydrogel sensor responses. Conclusion An artificial excitable cell (gel cell) is being developed that will ultimately be combined into a contracting medium similar to the heart. This contracting medium will be used to develop a tube pump capable of propelling fluids along its length. A vital component of this excitable cell is an ion sensitive probe to detect the presence and location of potassium ions within the cell. The ion sensor developed by Pandey et al. (2002) was found to function very well in KCl aq solutions, however, it was found not to work properly in the ionic liquid foundation of the gel cell. Hydrogels were tested as a potential replacement for ILPG as the foundation of the gel cell. The two PPy membrane substrates, ACN PPy membrane and dih 2 O PPy membrane, demonstrated a higher response than the negative control sensor. Each PPy sensor functioned in hydrogel, displaying a linear relationship of increasing potassium ion concentration and increasing voltage drop response. The ACN PPy hydrogel sensor displayed an amplification in response in hydrogel versus no hydrogel (sensors 3 and 2, respectively); whereas, the dih 2 O PPy Nguyen 9

10 hydrogel sensor displayed identical response with or without hydrogel (sensors 6 and 4, respectively). The response of the potassium sensor within the hydrogel shows great potential for using this new material as the base of the artificial cell. The hydrogel s ability for complete fluid exchange with aqueous concentrations of KCl aq allows for free flowing potassium ions to permeate throughout; therefore allowing proper function of the gel cell. Acknowledgements Thanks to graduate student Christina Haden, research scientist Donald Jordan, and faculty advisor Pam Norris for their continued support and guidance throughout this entire project. References Barthus, R. C., Lira, L. M., & Cόrdoba de Toressi, S. I. (2008). Conducting polymerhydrogel blends for electrochemically controlled drug release devices. Journal of the Brazilian Chemical Society 19(4), Fratarcangeli, C. (1994). Study of fuel pump performance testing and its implications on product accessibility Pandey, P. C., Singh, G., & Srivastava, P. (2002). Electrochemical synthesis of tetraphenylborate doped polypyrrole and its applications in designing a novel zinc and potassium ion sensor. Electroanalysis, 14(6), A special thanks to the NanoStar Institute, the National Science Foundation, the Center for Diversity in Engineering, the Virginia Space Grant Consortium, and the Ligon-Lamsam Opportunity Fund for their generosity in funding this research project. Nguyen 10