A Tunable Coacervate Matrix Emulates Embryonic Extracellular Matrix (ECM) Biophysical Cues To Regulate Stem Cell Fate

During early embryogenesis, the uterine environment undergoes marked biophysical changes that guide the embryonic cell fate. However, replicating these stage-specific cues in vitro remains challenging. Herein, we introduce a gelatin-based coacervate matrix with phase-transition-mediated tunable mechanics to recapitulate the biophysical cues of pre- and peri-implantation stages. Driven by reversible hydrophobic interactions, liquid-liquid phase separation produces coacervates with ultradynamic structures that enable dramatic volume expansion during cell proliferation in preimplantation stage. Furthermore, the liquid-like coacervate emulates the loosely organized immature extracellular matrix (ECM) of the uterine fluid environment, providing moderate cell-matrix interactions that preserve stemness. Additionally, the coacervate-solution transition allows the efficient harvesting of highly viable embryonic stem cell colonies. Moreover, upon stiffening through the coacervate-hydrogel transition, the matrix promotes peri-implantation-like invasive behaviors, including enhanced cell-matrix adhesion and secretion of ECM-degrading enzymes. These findings establish the biomimetic coacervate matrix as a versatile platform for clonal growth, stemness maintenance, and lineage initiation, offering new opportunities for developmental modeling and therapeutic applications.