Ferrofluid microrobot driven by an adjustable magnetic tweezer for soft tissue mechanical measurement

Quantitative mapping of tissue mechanical properties is essential for understanding disease progression and guiding therapeutic interventions. However, conventional techniques are constrained by limited spatial coverage and poor mechanical compliance between rigid probes and soft biological tissues. Here, we report an adjustable magnetic tweezer microsystem that actuates a 1 mm vegetable oil-based ferrofluid droplet for in situ mechanical measurement with high spatial coverage and accuracy. The system features a tunable workspace (9-80 mm) that enables two operational modes: large-scale magnetic field gradients for droplet navigation and localized uniform fields for controlled deformation. The ferrofluid's fluid-like compliance facilitates soft-soft interaction with tissue, reducing artifacts from probe stiffness mismatch. Quantitative tests across sucrose solutions (0.8-83 mPa·s) and agar gels (0.4-142 kPa) showed agreement with viscometer and indentation standards (with all errors under 10%, except for the case with 1 mPa·s viscosity, where the Brownian motion at the low viscosity affects the accuracy). The method was further validated in biological tissue, demonstrating 1.6% deviation from reference measurements. This integrated ferrofluid-magnetic tweezer platform provides a minimally invasive, compliance-matched approach for viscoelastic characterization of soft materials and tissues.