Polyimide

CFMs were fabricated as previously described^{33 (link)}. A single carbon fiber (AS4, d = 7 μm; Hexel, Dublin, CA) was used for all experiments. The fiber was inserted into silica tubing (20 µm ID, 90 µm OD, 10 µm coated with polyimide; Polymicro Technologies, Phoenix, AZ) and covered with polyamic acid (Sigma-Aldrich, St. Louis, MO) at one end. The electrode was then heated to 200 °C to polymerize the polyamic acid into a polyimide film for fixation. The silica tubing was then attached to a nitinol wire (Nitinol #1, an alloy of nickel and titanium; Fort Wayne Metals, IN) with a commercial silver conductive paste (Sigma-Aldrich, St. Louis, MO). The carbon fiber-attached nitinol wire was insulated with polyimide tubing (0.0089”ID, 0.0134”OD, 0.00225” WT; Vention Medical, Salem, NH) with the carbon side exposed on the exterior. After the silver paste became solid, the exposed carbon fiber was trimmed under a dissecting microscope to a length of approximately 80–100 μm using a scalpel blade. PEDOT:Nafion coating was applied to the exposed carbon fiber by electrical deposition with a mixture of poly(3,4-ethylenedioxythiophene) and Nafion to increase their sensitivity and selectivity to DA and reduce in vivo biofouling^{36 (link)}. Teflon-coated silver wire (A-M Systems, Inc., Sequim, WA) was prepared as an Ag/AgCl reference electrode.

Carbon fiber microelectrodes were fabricated using an established standardized CFM design at Mayo Clinic (Chang et al., 2013 (link); Oh et al., 2016 (link)). Each microelectrode involved isolating and inserting a single carbon fiber (AS4, diameter = 7 μm; Hexcel, Stamford, CT, USA) into a silica tubing (20 μm ID, 90 μm OD, 10 μm coat with polyimide; Polymicro Technologies, Phoenix, AZ, USA). The connection between the carbon fiber and the silica tubing was covered with epoxy resin. The silica tubing was then attached to a nitinol extension wire (Nitinol #1, an alloy of nickel and titanium; Fort Wayne Metals, IN, USA) by a silver-based conductive paste (Chang et al., 2013 (link)). The carbon fiber attached nitinol wire was insulated with polyimide tubing (0.0089′′ ID, 0.0134′′ OD, 0.00225′′ WT; Vention Medical, Salem, NH, USA) up to the carbon fiber sensing segment. The exposed carbon fiber was trimmed under a dissecting microscope to a length of ∼50 μm. Teflon-coated silver (Ag) wire (A-M systems, Inc., Sequim, WA, USA) was prepared as an Ag/AgCl counter-reference electrode by chlorinating the exposed tip in saline with a 9 V dry cell battery. CFMs were pretested in a flow cell prior to coating deposition with a PEDOT:Nafion deposition solution (Vreeland et al., 2015 (link)), which minimized the effect of biofouling in vivo.