Noninvasive holographic sensor system for measuring stiffness of soft micro samples

<b>Significance</b>: Measuring cell stiffness is essential in cellular biomechanics, particularly in understanding disease progression, including cancer metastasis and tissue mechanics. However, conventional techniques such as atomic force microscopy and optical stretching present limitations, including invasiveness, low throughput, and complex sample preparation. These factors restrict their applicability in dynamic and sensitive biological environments. <b>Aim</b>: This study introduces a noninvasive holographic sensor system for evaluating the stiffness of soft microscale samples. <b>Approach</b>: The proposed system integrates holographic imaging with acoustic stimulation using an off-axis Mach-Zehnder interferometer combined with bulk acoustic waves. This setup allows for label-free, high-throughput measurements while preserving sample integrity. The system was validated with polyacrylamide beads engineered to mimic cellular stiffness, ensuring precise and repeatable stiffness assessments. <b>Results</b>: Measurement errors caused by spatial variations were minimized through a structured imaging approach and a calibration strategy, improving uniformity across different regions. These corrections enhanced the consistency and reliability of stiffness assessments. Experimental validation demonstrated stable stiffness measurements regardless of sample size variations. Repeatability tests further confirmed the system's robustness, producing consistent results across multiple trials. <b>Conclusion</b>: The findings highlight the potential of this holographic sensor system in advancing cell biomechanics research, cancer diagnostics, and mechanobiology. By offering a noninvasive, high-throughput alternative for mechanical property assessments in biological samples, this method contributes to improved characterization of cellular stiffness in biomedical applications.