A heparin-mimicking sulfonated reverse thermal gel (SRTG) as a heparin-binding therapeutic protein delivery system. This system is designed to turn from low viscous liquid to a physical gel by exposure to body temperature alone. This allows direct deployment through a small gauge needle or catheter at target area with minimal surgical intervention. A unique aspect of this system is that it possesses a net negative charge due to the presence of sulfonate groups. This allows the SRTG to mimic heparin function, binding and preserving the bioactivity of positively charged therapeutic proteins, providing localized and sustained release of such proteins.