Sound Speed Detection of Disease-Relevant Mechanical Changes in Microgel Suspensions: A Proof-of-Concept for Liquid Biopsy

OBJECTIVE: This proof-of-concept study evaluates whether sound speed measurements using Swept-Frequency Acoustic Interferometry can detect disease-relevant stiffness contrasts using gelatin microgels as stand-ins for cells in liquid biopsies. METHODS: Three microgel formulations were tested (175 Bloom 1% w/v; 175 Bloom 2% w/v; 300 Bloom 2% w/v). Sound speed was normalized to 22°C. To determine whether observed differences reflected real separations rather than measurement variability, pairwise differences were benchmarked against a minimum detectable difference (MDD). A physics-based mixing-law framework anchored at measured baseline volume fractions (φ ≈ 3.62%-3.82%) was used to interpret detectability and compare model predictions with experimental results. RESULTS: The 175 Bloom 1% (w/v) microgels showed a higher adjusted mean sound speed than both 175 Bloom 2% (w/v) and 300 Bloom 2% (w/v). Pairwise differences for 175 Bloom 1% (w/v) versus 175 Bloom 2% (w/v) and 175 Bloom 1% (w/v) versus 300 Bloom 2% (w/v) exceeded their MDD, while 175 Bloom 2% (w/v) versus 300 Bloom 2% (w/v) did not. Modeling indicated that equal-magnitude softening of microgel particles produces larger sound speed shifts than stiffening, enabling detectability at lower volume fractions. Model predictions were conservatively biased (∼7%-16% below measured separations) but closely tracked observed contrasts. CONCLUSION: At small volume fractions compatible with liquid-biopsy workflows, sound speed measurements provide a rapid, label-free screening approach that can route samples for validation and support longitudinal tracking under standardized conditions. By integrating physics-based modeling with statistical analysis, this work lays the foundation for clinical acoustic detection of cell-level mechanical changes.