A biocompatible, highly sensitive hydrogel-based T1 thermometer for in vivo MRI applications

Accurate and non-invasive temperature mapping in vivo remains a pivotal challenge for biomedical imaging. Herein, we report the development of a biocompatible hydrogel-based thermometer for high-sensitivity temperature monitoring via T1-weighted magnetic resonance imaging (MRI). The system consists of the inclusion of the tri-phenyl Gd-DTPA derivative (Gd-1) into the self-assembling peptide hydrogel (Fmoc-K2) to yield a nanostructured network capable of amplifying temperature-dependent relaxivity changes. Unlike conventional aqueous Gd-complexes, the Fmoc-K2/Gd-1 hydrogel exhibits a strong positive correlation between relaxivity (r1) and temperature, with a remarkable sensitivity of Δr1/°C = 0.54 s−1/°C at 1T (40 MHz). This enhancement is attributed to temperature-driven modulation of water dynamics within the hydrogel matrix, as confirmed by 17O-NMR and NMRD profiling. The device maintains structural and functional stability under physiological conditions and demonstrates reliable temperature reporting either in vitro and in vivo, with subcutaneously injected hydrogel yielding MRI-based thermal readouts in excellent agreement with invasive thermometry. This work establishes a new class of injectable, responsive soft materials for precise MRI thermometry, offering broad translational potential in thermal therapies and physiological monitoring.