Multikilohertz optical microelastography for viscoelastic characterization of soft biomaterials demonstrated on...

The stiffness and viscosity of biological tissue determine how mechanical forces are transmitted and dissipated within it, and both are increasingly used to study tumor growth and treatment response. Measuring these properties within small three-dimensional samples is challenging: it requires shear waves at kilohertz frequencies, which attenuate over short distances, while conventional excitation methods introduce additional signal loss that is difficult to separate from the intrinsic dissipation of the tissue.

To enable sub-millimeter viscoelastic characterization of three-dimensional biological constructs in the multikilohertz frequency range using quasi-planar wave excitation and optical detection.

We developed an optical microelastography platform that combines quasi-planar shear-wave excitation using a piezoelectric actuator, high-speed imaging (up to 210,000 fps), and $k$ -space dispersion analysis to estimate frequency-dependent viscoelastic parameters through a Kelvin-Voigt model. The system was validated using polyacrylamide phantoms with tunable viscoelastic properties at five concentrations (15% to 26% v/v).

Validation across polyacrylamide concentrations yielded effective shear moduli of 0.70 to 19.31 kPa, with a minimum resolvable shear wavelength of $\sim 110\mu m$ in the polyacrylamide phantoms. Experimental phase-velocity dispersion showed good agreement with finite-element predictions ( $\mathrm{RMSE}<0.16m/s$ , mean relative error $<3\%$ , $R^2>0.96$ ). Measurements on four melanoma spheroids (237 to 823 µm diameter) produced effective viscoelastic parameters of $\mu =0.45$ to 0.71 kPa and $\eta =0.033$ to $0.097\mathrm{Pa}·s$ ( $R^2\ge 0.96$ ), obtained within specimen-specific analysis bands and consistent with values reported for three-dimensional tumor spheroid models.

This platform demonstrates kilohertz-range viscoelastic characterization of three-dimensional biological constructs at sub-millimeter scales, validated in polyacrylamide phantoms and demonstrated in proof-of-concept measurements on four melanoma tumor spheroids of different sizes. The quasi-planar excitation approach reduces geometric spreading artifacts associated with conventional excitation methods and provides a noninvasive optical tool for studying frequency-dependent mechanical behavior in mechanobiology and tissue engineering.