Inertial sensing of water content in tumor spheroids

Cellular water content governs the concentration of all biomolecules inside a cell, thereby influencing the physical and functional properties of the cell. However, measurements of water content in physiologically relevant cell culture models remain largely unavailable, particularly in three-dimensional (3D) models such as tumor spheroids and organoids. Here, we achieve such measurements using an industrial-grade capillary steel tube. The steel tube functions as a mechanical resonator that inertially senses the buoyant mass of particles. For microgram-scale particles ≥ 400 micrometers in diameter, we achieve <1% precision error in buoyant mass with a 5-minute acquisition interval. By sequentially measuring the buoyant mass of individual, patient-derived glioblastoma tumor spheroids derived from patients with glioblastoma in media of different densities and cell permeabilities, we determine the absolute and fractional (volume/volume) water content of the spheroids, along with their dry mass, volume, and density properties. We achieve ~0.5% precision error in fractional water content with a throughput of three spheroids per hour. This enables us to detect both interspheroid heterogeneity in fractional water content and acute responses to kinase inhibition. Overall, we establish a simple and accessible technique for quantifying water content in living 3D cell culture models, opening previously unexplored avenues for studying biophysical regulation in multicellular systems.