Viscotaxis: An Emerging Driver for Directed Cell Migration.

Directed cell migration orchestrates embryonic development, wound repair, immune surveillance and malignant tumor invasion. Viscotaxis-directed cell migration along spatial gradients of extracellular fluid viscosity or matrix loss modulus-has emerged as a candidate mechanotaxis modality that may guide cells through heterogeneous viscoelastic tissues; however, its biological functions and molecular underpinnings remain largely uncharacterized, and direct evidence in mammalian systems is still limited to a small number of studies. This mini-review first defines viscotaxis and distinguishes it from the related but physically different mechanical quantities with which it is frequently conflated-substrate stiffness, matrix viscoelasticity, and stress relaxation-and then summarizes its physiological and pathological relevance across development, tissue homeostasis, immunity, and cancer. We outline the putative multi-step mechanosensory cascade governing viscotactic responses, examine how viscotaxis may synergize or compete with durotaxis, haptotaxis, chemotaxis, electrotaxis, and phototaxis under mixed microenvironmental cues, and compare the distinct hydrodynamic and steric-exclusion mechanisms proposed across spiral microbes, flagellated eukaryotes, mammalian cells, and embryonic tissues. Throughout, we explicitly distinguish evidence obtained under viscosity gradients from that obtained under uniformly elevated viscosity and established findings from working hypotheses. Finally, we discuss current methodological bottlenecks and unresolved conceptual debates and propose biomaterial tools and therapeutic strategies to advance viscotaxis from a biophysical curiosity toward a core principle of cellular mechanobiology.