  Accurate assessment of stiffness distribution is essential due to the
critical role of single cell mechanobiology in the regulation of many vital
cellular processes such as proliferation, adhesion, migration, and motility.
Cell stiffness is one of the fundamental mechanical properties of the cell and
is greatly affected by the intracellular tensional forces, cytoskeletal
prestress, and cytoskeleton structure. Herein, we propose a novel holographic
single-cell stiffness measurement technique that can obtain the stiffness
distribution over a cell membrane at high resolution and in real-time. The
proposed imaging method coupled with acoustic signals allows us to assess the
cell stiffness distribution with a low error margin and label-free manner. We
demonstrate the proposed technique on HCT116 (Human Colorectal Carcinoma) cells
and CTC-mimicked HCT116 cells by induction with transforming growth factor-beta
(TGF-\b{eta}). Validation studies of the proposed approach were carried out on
certified polystyrene microbeads with known stiffness levels. Its performance
was evaluated in comparison with the AFM results obtained for the relevant
cells. When the experimental results were examined, the proposed methodology
shows utmost performance over average cell stiffness values for HCT116, and
CTC-mimicked HCT116 cells were found as 1.08 kPa, and 0.88 kPa, respectively.
The results confirm that CTC-mimicked HCT116 cells lose their adhesion ability
to enter the vascular circulation and metastasize. They also exhibit a softer
stiffness profile compared to adherent forms of the cancer cells. Hence, the
proposed technique is a significant, reliable, and faster alternative for
in-vitro cell stiffness characterization tools. It can be utilized for various
applications where single-cell analysis is required, such as disease modeling,
drug testing, diagnostics, and many more.
