During the last decade, the field of mechanobiology has become an important area of biomedical research. Scientists increasingly study how physical and mechanical properties influence cellular behavior, especially in cancer development and progression. Advanced experimental technologies and theoretical models are now widely used to analyze cellular mechanics and understand how cells interact with their surrounding microenvironment.

Cancer mechanobiology focuses on the physical characteristics of tumor cells and investigates whether mechanical alterations can distinguish malignant cells from healthy cells. While molecular biology has generated extensive information about tumor genetics and signaling pathways, many fundamental differences between benign and malignant tumors remain unclear. Researchers in physics and biomechanics proposed that changes in cell mechanics may play a critical role in malignant transformation and metastasis.

Cells constantly generate mechanical forces and respond to the stiffness and structure of their extracellular environment. Throughout tumor progression, including proliferation, invasion, and metastasis, major alterations occur in both cellular and extracellular mechanical properties. Many studies suggest that cancer cells, or at least aggressive metastatic subpopulations, tend to become softer and more deformable than normal healthy cells. However, this concept remains controversial because mechanical properties may vary depending on cancer type, tumor stage, and microenvironmental conditions.

This review explores the mechanical differences between cancer cells and normal cells and evaluates whether these differences are sufficient to identify cancer cells using biomechanical properties alone. Research in this field mainly relies on cancer cell lines and primary tumor cells analyzed using 2D and 3D in vitro models. The objective is to understand which cellular structures and molecular pathways are responsible for the mechanical changes observed during cancer progression.

Why Could Cancer Cells Be Softer Than Normal Cells?

Cancer progression involves profound modifications in cellular behavior and physiology. According to the classical hallmarks of cancer, tumor cells acquire several abnormal biological properties that support uncontrolled growth, survival, invasion, and metastasis.

Cancer cells can proliferate indefinitely, evade apoptosis, bypass growth suppressors, and modify metabolic pathways to support high energy demands. In addition, multiple signaling pathways controlling cell adhesion, polarity, migration, membrane trafficking, and cytoskeletal organization become deregulated during tumor development.

One of the most important events in cancer progression is metastasis, where tumor cells spread from the primary tumor to distant organs. During this process, cancer cells detach from the tumor mass, penetrate the basement membrane, enter blood circulation, survive mechanical stress in the bloodstream, exit blood vessels, and colonize secondary tissues.

Highly deformable cells may more easily migrate through confined spaces, dense extracellular matrices, and narrow capillaries. This observation led researchers to hypothesize that softer cancer cells possess greater metastatic potential.

However, the relationship between softness and malignancy is more complex than initially expected.

Complexity of Tumor Mechanics

Although individual cancer cells often appear softer, tumors themselves are frequently stiffer than surrounding healthy tissues. Tumor stiffening is mainly caused by remodeling of the extracellular matrix and stromal environment rather than by cancer cells alone.

The tumor microenvironment strongly influences cell behavior because cells continuously sense and adapt to external mechanical cues. Increased matrix stiffness can stimulate tumor progression, invasion, and activation of oncogenic signaling pathways.

Additional biological processes also influence cancer cell mechanics:

• Epithelial-to-mesenchymal transition (EMT)
• Cytoskeletal reorganization
• Loss of cell polarity
• Changes in cell adhesion
• Collective cell migration
• Mechanical confinement during tumor growth

Moreover, metastatic cells do not always migrate individually. In many tumors, cells migrate collectively, where specialized leader cells guide less deformable follower cells through tissues.

Cancer cell stiffness may therefore dynamically change depending on the physical properties of each microenvironment encountered during metastasis.

Mechanics of Cancer Cells

Mechanical properties are closely linked to cancer cell migration, invasion, and metastatic behavior. Cancer cells continuously interact with their physical environment through mechanotransduction pathways that convert mechanical signals into biochemical responses.

Several studies demonstrated strong correlations between cancer aggressiveness and altered mechanical behavior, including:

• Increased cell motility
• Reduced cell-cell adhesion
• Enhanced contractility
• Increased traction forces
• Higher deformability

Highly metastatic cancer cells often migrate faster and generate stronger traction forces than non-invasive cells. However, some studies reported opposite results depending on cancer type and experimental conditions, showing that cancer mechanics remains highly heterogeneous.

Cell adhesion is also significantly altered during tumor progression. The classical switch from E-cadherin to N-cadherin expression during EMT weakens intercellular adhesion and promotes invasion. Nevertheless, recent evidence indicates that cell adhesion alone does not fully explain metastatic potential.

Overall, cancer cells generally display reduced adhesion to neighboring cells and enhanced migratory capacity, which contribute to tumor dissemination.

Microrheology of Cancer Cells

Microrheology refers to the study of mechanical properties at microscopic scales. Over the past two decades, numerous techniques have been developed to measure cancer cell rigidity, elasticity, viscosity, and deformability.

These techniques investigate either whole-cell mechanics or local intracellular mechanics.

Whole-Cell Mechanical Analysis

Several methods evaluate global cellular deformability:

Micropipette Aspiration

Cells are aspirated into small glass pipettes to measure membrane and cortical deformability.

Optical Stretcher

Laser beams deform suspended cells to quantify cellular elasticity and deformability.

Microfluidic Devices

Cells migrate through narrow microchannels that mimic physiological confinement. Transit time and deformation are measured to evaluate mechanical behavior.

These techniques mainly assess the contribution of:

• Plasma membrane
• Actin cortex
• Cytoskeleton
• Nuclear rigidity

Local Mechanical Measurements

Other techniques probe local regions inside or on the surface of cells.

Atomic Force Microscopy (AFM)

AFM is one of the most widely used tools in cancer biomechanics. A nanoscale probe indents the cell surface to calculate elastic modulus and stiffness.

AFM studies consistently demonstrate that many cancer cells possess lower Young’s modulus values than healthy cells.

Magnetic and Optical Tweezers

These methods apply forces to intracellular particles or membrane-bound beads to analyze intracellular viscoelasticity and force generation.

Intracellular Microrheology

This technique tracks endogenous vesicles or injected particles inside the cytoplasm to study local mechanical properties and cytoplasmic fluidity.

Rheological Properties of Cancer Cells

Cellular rheology describes how cells deform under mechanical stress.

Rheological parameters include:

• Stress
• Strain
• Elastic modulus
• Viscosity
• Shear modulus
• Relaxation time

Cells behave as viscoelastic materials because they exhibit both:

• Elastic solid behavior
• Viscous fluid behavior

Many experimental studies demonstrate that cancer cells exhibit:

• Reduced elasticity
• Lower stiffness
• Increased deformability
• Higher cytoplasmic fluidity

Several theoretical models are used to interpret rheological data, including:

• Maxwell model
• Kelvin-Voigt model
• Standard linear solid model
• Power-law rheology
• Soft glassy material models
• Poroelasticity models

Power-law behavior is now widely accepted as a characteristic feature of cellular rheology because cells contain highly dynamic and heterogeneous internal structures.

Evidence That Cancer Cells Are Softer

Most biomechanical studies conclude that cancer cells are softer than normal cells.

This observation has been reported in:

• Breast cancer
• Bladder cancer
• Glioma
• Prostate cancer
• Colon cancer
• Pancreatic cancer
• Ovarian cancer
• Thyroid cancer

In many cases:

• Young’s modulus decreases with tumor grade
• Deformability increases with metastatic potential
• Cytoplasmic fluidity becomes higher in aggressive cells

Only a few studies reported stiffer behavior in certain cancer cell types, suggesting that tumor mechanics may depend on tissue origin and experimental methodology.

Despite some variability, the overall consensus strongly supports cancer cell softening during tumor progression.

Cellular Mechanisms Responsible for Cancer Cell Softening

Extracellular Matrix and Microenvironment

The extracellular matrix strongly influences tumor mechanics. Increased matrix stiffness promotes:

• Cell invasion
• Integrin signaling
• Cytoskeletal tension
• Tumor growth

Cancer cells continuously adapt to their mechanical environment.

Cytoskeleton Alterations

The cytoskeleton is one of the main regulators of cell mechanics.

Actin Cytoskeleton

Cancer cells often display:

• Reduced actin organization
• Fewer stress fibers
• Altered actin polymerization
• Lower cortical density

Disruption of actin filaments directly decreases cell stiffness and increases deformability.

Microtubules

Microtubule contribution to cell stiffness appears more variable and may depend on cancer type.

Intermediate Filaments

Proteins such as:

• Vimentin
• Keratins

play important roles in cancer biomechanics.

Vimentin expression usually increases during EMT and metastatic progression, while keratin expression often decreases in epithelial cancers.

Reorganization of intermediate filaments strongly affects cellular rigidity and invasiveness.

Nuclear Mechanics

The nucleus is a major determinant of whole-cell stiffness.

Cancer cell nuclei often exhibit:

• Larger size
• Irregular morphology
• Altered lamin expression
• Increased deformability

Softer nuclei facilitate migration through confined spaces during metastasis.

Plasma Membrane and Intracellular Components

Alterations in membrane lipid composition also contribute to cancer cell softening.

Metastatic cells frequently display:

• Lower membrane rigidity
• Enhanced membrane fluctuations
• Increased active mechanical forces

Internal organelles such as:

• Golgi apparatus
• Endoplasmic reticulum
• Mitochondria
• Endosomal networks

may additionally influence intracellular mechanics.

Active Forces and ATP-Dependent Processes

Cellular mechanics are strongly affected by active molecular processes driven by ATP-consuming motors such as myosin.

Recent studies showed that malignant cells exhibit:

• Higher intracellular force fluctuations
• Increased non-equilibrium activity
• Enhanced cytoplasmic dynamics

These active forces may contribute to the softer and more fluid-like behavior of cancer cells.

Clinical Applications of Cancer Cell Mechanics

Mechanical properties of cancer cells could become valuable biomarkers for:

• Cancer diagnosis
• Prognosis
• Metastatic risk assessment
• Drug response prediction

Several high-throughput technologies are currently under development for clinical applications, including:

• Microfluidic deformability assays
• Optical stretcher systems
• Acoustic wave technologies
• Automated AFM platforms

These approaches aim to rapidly analyze thousands of cells and identify mechanical signatures associated with malignancy.

Current Challenges and Future Perspectives

Despite major advances, several challenges remain:

• Tumor heterogeneity
• Variability between patients
• Differences between in vitro and in vivo conditions
• Lack of standardization across techniques
• Dynamic adaptation of cells to their environment

Most studies still rely on long-established cancer cell lines, which may not accurately represent patient tumors. Future research increasingly focuses on:

• Primary tumor cells
• Patient-derived xenografts
• 3D tumor models
• In vivo mechanical analysis

Combining multiple biomechanical techniques with molecular and genetic analyses will likely improve the accuracy of cancer diagnostics.

Conclusion

Accumulating evidence strongly indicates that cancer cells are generally softer and more deformable than normal healthy cells. Reduced stiffness correlates with enhanced metastatic potential in many cancer types and reflects major alterations in cytoskeletal organization, nuclear structure, membrane composition, and intracellular mechanics.

Cancer biomechanics represents a rapidly growing research field with important implications for understanding tumor progression and developing new diagnostic technologies. Although several technical and conceptual challenges remain unresolved, the mechanical characterization of cancer cells may become an essential component of future precision oncology approaches.