Active Soft Hydrogels Reveal Cumulative Molecular Force Dosing in Stem Cell Fate Programming.

Decoupling molecular-scale mechanical signaling from bulk substrate stiffness remains a fundamental challenge in biomaterial design. Here, we develop a soft-yet-active hyaluronic acid hydrogel (∼3 kPa) grafted with near-infrared-driven CD-PNIPAM-RGD molecular actuators to apply programmable, piconewton-scale forces directly to integrins without altering the bulk modulus. By isolating force history as an independent variable, we reveal that human mesenchymal stem cells act as cumulative mechanical integrators. A cumulative force dose of just 7 h (1-h daily pulses) drives irreversible osteogenic commitment on this classically non-permissive substrate. Mechanistically, each pulse bypasses canonical tension-based signaling. Despite negligible cytoplasmic traction, localized molecular pulling drives G-actin nuclear import, intranuclear F-actin polymerization, and rapid YAP activation. By recasting mechanotransduction from a continuous analog response into a discrete, cumulative integration of mechanical inputs, this work establishes active force dosing as a quantitative design principle for next-generation soft biomaterials.