Microbubble-based indirect measurement of cell loading in hydrogel bioinks during handling of 3D models

We present an inverse-engineering method for estimating physical forces within cell suspensions in 3D scaffolds during fabrication using fluorescent, lipid-coated microbubbles (MBs) filled with an inert fluorocarbon gas. MBs deform and rupture under significant mechanical pressure. In this work, we applied the compressibility characteristics of the MBs to estimate the forces acting on a cell-laden model during fabrication and handling. MBs were encapsulated in hydrogels and subjected to hydrostatic pressure for a specific period, and we demonstrated how calibration curves are generated to estimate the pressure around each MB. We further studied MB response via conventional ultrasound imaging and theoretical modeling. This work demonstrates a simple, scalable approach for estimating physical loads that biological cells experience during extrusion, injection, and other biofabrication processes.