HKU has developed a breakthrough technology for measuring the rotational motion of cells

Mechanics plays a fundamental role
in cell biology.
Cells use these mechanical forces to explore their surroundings and sense the behavior of
surrounding living cells.
The physical properties of the environment in which cells operate, in turn, affect their function
.
Understanding how cells interact with their environment therefore provides important insights into cell biology and has broad implications in medicine, including disease diagnosis and cancer treatment
.

So far, researchers have developed a number of tools to study the interactions
between cells and their 3D microenvironment.
One of the most popular techniques is traction force microscopy (TFM).

It is the primary method for determining the traction of the cell matrix surface, providing important information
about how cells sense, adapt, and respond to forces.
However, the application of TFM is limited to providing information on
marker translation movements on the cellular matrix.
Information on other degrees of freedom, such as rotational motion, remains speculative due to technical limitations and limited research on the topic
.

Engineering experts at the University of Hong Kong have proposed a new technique for measuring the traction field of cells to address this research gap
.
The interdisciplinary research group is led
by Dr.
Zhiqin Chu from the Department of Electrical and Electronic Engineering and Dr.
Yuan Lin from the Department of Mechanical Engineering.
Using the single nitrogen vacancy (NV) center in nanodiamonds (NDs), they proposed a linear polarization modulation (LPM) method that can measure the rotational and translational motion
of markers on the cell matrix.

The study, which provides a new perspective for measuring multidimensional battery traction fields, has been published in
the journal Nano Letters.
The study, titled "All-optical modulation of a single defect in nanodiamonds: revealing rotational and translational motion in cellular traction fields," was also featured as a complementary cover
of the journal.

The study shows high-precision measurements
of rotational and translational motion of cell matrix surface markers.
The experimental results confirm the theoretical calculations and the results of previous studies
.

NDs with NV centers have ultra-high light stability, good biocompatibility, and convenient surface chemical modification, and are excellent fluorescent markers for
many biological applications.
The researchers found that based on the measurement results of the fluorescence intensity of a single NV center vs.
the direction of laser polarization, high-precision orientation measurement and background-free imaging
can be achieved.

Therefore, the LPM method invented by the team helps solve the technical bottleneck of cell force measurement in mechanobiology, which involves interdisciplinary collaboration
from biology, engineering, chemistry, and physics.

"Most cells in multicellular organisms experience highly coordinated forces
in space and time.
The development of multidimensional cell traction field microscopy has been one of
the biggest challenges in the field.
Compared to traditional TFM, this new technique provides us with a new and convenient tool
for studying true 3D cell-extracellular matrix interactions.
It helps to enable rotational-translational motion measurements in the field of cellular traction and reveals information about cellular traction," he added
.

The main highlight of the study is the ability to represent the translational and rotational movements
of the marks with high accuracy.
This is a big step
forward in analyzing the mechanical interactions of the cell-matrix interface.
It also provides new avenues
of research.

Through special chemicals on the cell surface, cells interact and connect, which are part of
the cell adhesion process.
The way cells create tension during adhesion is mainly described as "in-plane"
.
Processes such as traction stress, actin flow, and adhesion growth are interconnected and exhibit complex directional dynamics
.
LPM methods can help understand the complex moments surrounding the focus adhesion and separate different mechanical loads (e.
g.
, normal traction, shear forces)
at the nanoscale level.
This also helps to understand how cell adhesion responds to different types of stress, and how these mediate mechanical transduction (the mechanism by which cells convert mechanical stimuli into electrochemical activity).

The technique is also expected to be used to study various other biomechanical processes, including immune cell activation, tissue formation, and the replication and invasion
of cancer cells.
For example, T cell receptors, which play a central role in the immune response to cancer, can generate dynamic forces
that are extremely important for tissue growth.
This high-precision LPM technique may help analyze these multidimensional force dynamics and provide insights
into tissue development.

The research team is actively investigating methods to expand optical imaging capabilities and map multiple nanodiamonds
simultaneously.

Lingzhi Wang, Yong Hou, Tongtong Zhang, Xi Wei, Yan Zhou, Dangyuan Lei, Qiang Wei, Yuan Lin, Zhiqin Chu.
All-Optical Modulation of Single Defects in Nanodiamonds: Revealing Rotational and Translational Motions in Cell Traction Force Fields.
Nano Letters, 2022; 22 (18): 7714