Characterization of conjugated polymer actuation under cerebral physiological conditions

Abstract

Conjugated polymer actuators have potential use in implantable neural interface devices for modulating the position of electrode sites within brain tissue or guiding insertion of neural probes along curved trajectories. The actuation of polypyrrole (PPy) doped with dodecylbenzenesulfonate (DBS) is characterized to ascertain whether it can be employed in the cerebral environment. Microfabricated bilayer beams are electrochemically cycled at either 22 or 37 °C in aqueous NaDBS or in artificial cerebrospinal fluid (aCSF). Nearly all the ions in aCSF are exchanged into the PPy-the cations Na(+) , K(+) , Mg(2+) , Ca(2+) , as well as the anion PO4 (3-) ; Cl(-) is not present. Nevertheless, deflections in aCSF are comparable to those in NaDBS and they are monotonic with oxidation level: strain increases upon reduction, with no reversal of motion despite the mixture of ionic charges and valences being exchanged. Actuation depends on temperature. Upon warming, the cyclic voltammograms show additional peaks and an increase of 70% in the consumed charge. Bending is, however, much less affected: strain increases somewhat (6%-13%) but remains monotonic, and deflections shift (up to 20%). These results show how the actuation environment must be taken into account, and demonstrate proof of concept for actuated implantable neural interfaces.

Keywords: bilayers; brain-machine interface; conducting polymers; movable electrodes.

Figure 1. a) An array of 5 probe devices in air having lengths of 1 mm and widths between 50 μm and 250 μm, deflected out of plane with uniform curvature after fabrication, with no applied voltage. b) A similar set of probes with lengths of 800 μm actuated during penetration into a gel brain tissue phantom. The Parylene probe beams included an insulated electrode trace with an exposed Au electrode site at the tip and an overlying PPy/Au actuator. c) A segmented beam with actuators at three joints, electrochemically reduced to be straight (top) and oxidized to be bent (bottom) in NaDBS solution

Figure 2. Three superimposed images of beam deflection upon reduction, oxidation, and transitioning midway (overhead view with the face of the beam perpendicular to the page). The ruler scale is in cm. The center position illustrates the zero curvature reference point that was used to determine tip deflections, and the arrows drawn over the image indicate the magnitude of the tip deflection

Figure 3. Deflection vs. time (lower red curve) during the first 6 cycles (numbered) of a bilayer in NaDBS at room temperature. The voltage was scanned at 10 mV/sec between +0.4 and −1.0 V vs Ag/AgCl, as illustrated at the top (black curve). Positive deflection indicates PPy expansion (bending PPy-side out), and negative deflection indicates PPy contraction (PPy-side in)

Figure 4. Current (black line) and displacement (thicker red line) as a function of potential during the 19^th CV scan at 10 mV/s under four conditions. The direction of the scan for the displacement curves is indicated by arrows; note that this axis has been reversed to facilitate comparison with the current. The deflection axis is shifted in (b) and (d) relative to (a) and (c), but has the same amplitude (16 mm). The dashed lines in (c) show results from a second sample

Figure 5. a) Maximum deflection of representative bilayers during oxidation and reduction during cyclic voltammetry as a function of cycle number. Black symbols: actuation in NaDBS at room temperature (all samples during cycles 1-10). Filled symbols: actuation in aCSF during cycles 11-20; open symbols: actuation in NaDBS; red triangle symbols, actuation at 37 °C during cycles 11-20; blue circle symbols, actuation at room temperature

Figure 6. EDX spectra of samples cycled in NaDBS at 22 °C and in aCSF at 37 °C, normalized to the S peak