Sylgard 184 elastomer base and curing agent

EHTs were cast on polydimethylsiloxane (PDMS) microposts as previously described [26 (link),27 (link)] Briefly, Sylgard 184 Elastomer Base and Curing Agent (Dow, 1317318, Midland, MI, USA) were mixed at a 10:1 ratio and cured in a custom 3D-printed mold for 18 h at 65 °C, with one flexible post and one glass rod filled stiff post per set of posts, 6 posts per array. Cured post arrays were removed from the mold and trimmed of excess PDMS. A total of 500k Day 25 hiPSC-CMs and 100k human Hs27a stromal cells were mixed with 3 U/mL thrombin from bovine plasma (Sigma, T4648, St. Louis, MO, USA) and 5 mg/mL bovine fibrinogen (Sigma, E8630, St. Louis, MO, USA) in 100 μL EHT media (sterile filtered RPMI, B27 supplement, 5 g/L aminocaproic acid (Sigma, A2 504-256-100G, St. Louis, MO, USA), penicillin/streptomycin). The cell slurry was added into 2% agarose wells between posts in a 24-well plate and incubated for 80 min at 37 °C, 5% CO₂. Then, 350 μL EHT media was added to the wells, and tissues were incubated for 10 min at 37 °C, 5% CO₂. Posts were carefully moved to a fresh 24-well plate in 2 mL EHT media, and tissues were cultured on posts for 3 weeks with media change every other day.