Agarose (Thermo Fisher Scientific, Waltham, MA) was dissolved in ultrapure water to 2.5 wt% and mixed with PEDOT:PSS (PH1000, Ossila, Sheffield, UK) and water in various ratios to make hydrogels with final concentrations of 0.5 or 1.0 wt% Agarose and 0.0, 0.2 or 0.6 wt% PEDOT:PSS (Table 1). PEDOT:PSS was sonicated for 15 min prior to hydrogel fabrication, and water was warmed to extend gelation time, allowing for adequate mixing. Hydrogel pre-polymer was deposited into glass-mounted PDMS molds (1.5 mm thick x 8 mm I.D.) and allowed to set at room temperature for at least 10 min to make 80 μL gels. After complete gelation, hydrogels were transferred to 24 well plates containing 1 mL of either αMEM (Invitrogen, Waltham, MA) supplemented with 10% FBS (GenClone, San Diego, CA) and 1% penicillin-streptomycin (Gemini Bio Products) or ultrapure water to remove any unreacted reagents. Gels stored in αMEM were maintained in standard cell culture conditions (i.e., 37°C, 5% CO2), whereas gels stored in water were kept at room temperature. Pure PEDOT:PSS gels were fabricated by mixing PEDOT:PSS and 4-dodecylbenzenesulfonic acid (Sigma, St. Louis, MO) at 3 v/v% according to Zhang et al.[27 (link)] and used as controls for some biochemical assays (Fig. S1A).