Described here are polymer compositions, methods, and systems for reinforcing a wall of a heart. The polymer compositions may be adapted to form networks, e.g., cross-linked networks, semi-interpenetrating networks, or interpenetrating networks, and placed within the pericardial space or on one or more pericardial tissues. The mechanical properties of the polymer compositions or networks derived therefrom may then be employed to reinforce a heart wall to prevent dilatation of a chamber of the heart and/or expansion of an infarct, e.g., to treat or prevent congestive or chronic heart failure.