Physiologically relevant microparticles with mechanically tunable properties for acoustophoretic and microfluidic device calibration

Mechanophenotype of biological cells has demonstrated correlation with biomolecular states and cell function. Hence, new methods to measure mechanophenotype at high throughput are of growing interest. Acoustophoretic microdevices can characterize cell mechanical features; however, calibration particles with physiologically relevant properties are needed to quantify and optimize device performance. Currently, conventional polymer microspheres are rigid and do not replicate cell deformation and compressibility. To address this, we developed monodisperse, tunable, cell-like microparticles (MPs) from polyacrylamide hydrogel, fabricated with a microfluidic droplet generator. Size and compressibility are adjusted by fabrication parameters, and density is adjusted by incorporation of nanoparticles (NPs). Here, we present for the first time microparticles of reduced density and acoustic contrast (lower than unloaded MPs) achieved by loading MPs with nanoparticles of low molecular weight alkanes. We produced the NPs by sonication and photopolymerization before addition to the MP precursor. NP-loaded MPs were less dense than unloaded MPs at 1005.9 and 1013.6 kg/m³, respectively, and they exhibited negative acoustic contrast by acoustophoresis in aqueous medium while that of unloaded MPs was positive. These particles extend the tunable range of acoustic contrast, mimicking and exceeding that of most biological cells and could also aid cell separation when conjugated to cells.