A thin-layer model for viscoelastic, stress-relaxation testing of cells using atomic force microscopy: do cell properties reflect metastatic potential?

Discrimination Between Normal and Cancerous Cells Using AFM

Currently, biomechanics of living cells is in the focus of interest due to noticeable capability of such techniques like atomic force microscopy (AFM) to probe cellular properties at the single cell level directly on living cells. The research carried out, so far, delivered data showing, on the one hand, the use of cellular mechanics as a biomarker of various pathological changes, which, on the other hand, reveal relative nature of biomechanics. In the AFM, the elastic properties of living cells are delivered from indentation experiments and described quantitatively by Young's modulus defined here as a measure of cellular deformability. Here, the AFM studies directly comparing the mechanical properties of normal and cancerous cells are summarized and presented together with a few important issues related to the relativeness of Young's modulus.

Indentation Analysis of Biphasic Viscoelastic Hydrogels

Mechanical properties of soft biological materials are dependent on the responses of the two phases of which they are comprised: the solid matrix and interstitial fluid. Indentation techniques are commonly used to measure properties of such materials, but comparisons between different experimental and analytical techniques can be difficult. Most models relating load and time during spherical indentation are based on Hertzian contact theory, but the exact limitation of this theory for soft materials are unclear. Here, we examine the response of gelatin hydrogels to shear and indentation loading to quantify combined effects of the solid and fluid phases. The instantaneous behavior of the hydrogels is different for each test geometry and loading rate, but the relaxed response, measured by the relaxed modulus, is the same for all tests, within 17%. Additionally, indentation depths from 15-25% of the radius of the spherical indenter are found to minimize error in the estimate of relaxed modulus.

Biomechanical profile of cancer stem-like cells derived from MHCC97H cell lines

Biomechanical properties and cytoskeletal organization of cancer cells are known to be closely related with their aggressive phenotype. In this study, based on atomic force microscopy (AFM), we aimed to evaluate the mechanical property of liver cancer stem-like cells (LCSCs) and compare it with human hepatoma cells (HHCs). LCSCs were enriched from human hepatoma cell line MHCC97H through a sphere culture system. AFM nanoindentation was investigated as a method for measuring the cell stiffness, and reflecting by Young׳s modulus. Microfilament bundles of F-actin were observed with immunofluorescence staining by confocal microscopy. We found that LCSCs show lower Young׳s modulus and higher migration ability compared to MHCC97H cells. Moreover, the decrease in Young׳s modulus is accompanied with a dramatic decline in F-actin content. These results demonstrated a close relationship between the cell Young׳s modulus and metastatic potential of HHCs, which suggest that Young׳s modulus detected by AFM can be used to evaluate metastatic potential of cancer cells.

Anti-pulmonary fibrotic activity of salvianolic acid B was screened by a novel method based on the cyto-biophysical properties

Various methods have been used to evaluate anti-fibrotic activity of drugs. However, most of them are complicated, labor-intensive and lack of efficiency. This study was intended to develop a rapid method for anti-fibrotic drugs screening based on biophysical properties. A549 cells in vitro were stimulated with transforming growth factor-β1 (TGF-β1), and fibrogenesis was confirmed by conventional immunological assays. Meanwhile, the alterations of cyto-biophysical properties including morphology, roughness and stiffness were measured utilizing atomic force microscopy (AFM). It was found that fibrogenesis was accompanied with changes of cellular biophysical properties. TGF-β1-stimulated A549 cells became remarkably longer, rougher and stiffer than the control. Then, the effect of N-acetyl-L-cysteine (NAC) as a positive drug on ameliorating fibrogenesis in TGF-β1-stimulated A549 cells was verified respectively by immunological and biophysical markers. The result of Principal Component Analysis showed that stiffness was a leading index among all biophysical markers during fibrogenesis. Salvianolic acid B (SalB), a natural anti-oxidant, was detected by AFM to protect TGF-β1-stimulated A549 cells against stiffening. Then, SalB treatment was provided in preventive mode on a rat model of bleomycin (BLM) -induced pulmonary fibrosis. The results showed that SalB treatment significantly ameliorated BLM-induced histological alterations, blocked collagen accumulations and reduced α-SMA expression in lung tissues. All these results revealed the anti-pulmonary fibrotic activity of SalB. Detection of cyto-biophysical properties were therefore recommended as a rapid method for anti-pulmonary fibrotic drugs screening.

Three-Dimensional Neural Spheroid Culture: An In Vitro Model for Cortical Studies

There is a high demand for in vitro models of the central nervous system (CNS) to study neurological disorders, injuries, toxicity, and drug efficacy. Three-dimensional (3D) in vitro models can bridge the gap between traditional two-dimensional culture and animal models because they present an in vivo-like microenvironment in a tailorable experimental platform. Within the expanding variety of sophisticated 3D cultures, scaffold-free, self-assembled spheroid culture avoids the introduction of foreign materials and preserves the native cell populations and extracellular matrix types. In this study, we generated 3D spheroids with primary postnatal rat cortical cells using an accessible, size-controlled, reproducible, and cost-effective method. Neurons and glia formed laminin-containing 3D networks within the spheroids. The neurons were electrically active and formed circuitry through both excitatory and inhibitory synapses. The mechanical properties of the spheroids were in the range of brain tissue. These in vivo-like features of 3D cortical spheroids provide the potential for relevant and translatable investigations of the CNS in vitro.

Robot-Guided Atomic Force Microscopy for Mechano-Visual Phenotyping of Cancer Specimens

Atomic force microscopy (AFM) and other forms of scanning probe microscopy have been successfully used to assess biomechanical and bioelectrical characteristics of individual cells. When extending such approaches to heterogeneous tissue, there exists the added challenge of traversing the tissue while directing the probe to the exact location of the targeted biological components under study. Such maneuvers are extremely challenging owing to the relatively small field of view, limited availability of reliable visual cues, and lack of context. In this study we designed a system that leverages the visual topology of the serial tissue sections of interest to help guide robotic control of the AFM stage to provide the requisite navigational support. The process begins by mapping the whole-slide image of a stained specimen with a well-matched, consecutive section of unstained section of tissue in a piecewise fashion. The morphological characteristics and localization of any biomarkers in the stained section can be used to position the AFM probe in the unstained tissue at regions of interest where the AFM measurements are acquired. This general approach can be utilized in various forms of microscopy for navigation assistance in tissue specimens.

Cellular enrichment through microfluidic fractionation based on cell biomechanical properties

The biomechanical properties of populations of diseased cells are shown to have differences from healthy populations of cells, yet the overlap of these biomechanical properties can limit their use in disease cell enrichment and detection. We report a new microfluidic cell enrichment technology that continuously fractionates cells through differences in biomechanical properties, resulting in highly pure cellular subpopulations. Cell fractionation is achieved in a microfluidic channel with an array of diagonal ridges that are designed to segregate biomechanically distinct cells to different locations in the channel. Due to the imposition of elastic and viscous forces during cellular compression, which are a function of cell biomechanical properties including size and viscoelasticity, larger, stiffer and less viscos cells migrate parallel to the diagonal ridges and exhibit positive lateral displacement. On the other hand, smaller, softer and more viscous cells migrate perpendicular to the diagonal ridges due to circulatory flow induced by the ridges and result in negative lateral displacement. Multiple outlets are then utilized to collect cells with finer gradation of differences in cell biomechanical properties. The result is that cell fractionation dramatically improves cell separation efficiency compared to binary outputs and enables the measurement of subtle biomechanical differences within a single cell type. As a proof-of-concept demonstration, we mix two different leukemia cell lines (K562 and HL60) and utilize cell fractionation to achieve over 45-fold enhancement of cell populations, with high purity cellular enrichment (90% to 99%) of each cell line. In addition, we demonstrate cell fractionation of a single cell type (K562 cells) into subpopulations and characterize the variations of biomechanical properties of the separated cells with atomic force microscopy. These results will be beneficial to obtaining label-free separation of cellular mixtures, or to better investigate the origins of biomechanical differences in a single cell type.

Single cell metastatic phenotyping using pulsed nanomechanical indentations

The existing approach to characterize cell biomechanical properties typically utilizes switch-like models of mechanotransduction in which cell responses are analyzed in response to a single nanomechanical indentation or a transient pulsed stress. Although this approach provides effective descriptors at population-level, at a single-cell-level, there are significant overlaps in the biomechanical descriptors of non-metastatic and metastatic cells which precludes the use of biomechanical markers for single cell metastatic phenotyping. This study presents a new promising marker for biosensing metastatic and non-metastatic cells at a single-cell-level using the effects of a dynamic microenvironment on the biomechanical properties of cells. Two non-metastatic and two metastatic epithelial breast cell lines are subjected to a pulsed stresses regimen exerted by atomic force microscopy. The force-time data obtained for the cells revealed that the non-metastatic cells increase their resistance against deformation and become more stiffened when subjected to a series of nanomechanical indentations. On the other hand, metastatic cells become slightly softened when their mechanical microenvironment is subjected to a similar dynamical changes. This distinct behavior of the non-metastatic and metastatic cells to the pulsed stresses paradigm provided a signature for single-cell-level metastatic phenotyping with a high confidence level of ∼95%.

Strain-rate Dependence of Elastic Modulus Reveals Silver Nanoparticle Induced Cytotoxicity

Force-displacement measurements are taken at different rates with an atomic force microscope to assess the correlation between cell health and cell viscoelasticity in THP-1 cells that have been treated with a novel drug carrier. A variable indentation-rate viscoelastic analysis, VIVA, is employed to identify the relaxation time of the cells that are known to exhibit a frequency dependent stiffness. The VIVA agrees with a fluorescent viability assay. This indicates that dextran-lysozyme drug carriers are biocompatible and deliver concentrated toxic material (rhodamine or silver nanoparticles) to the cytoplasm of THP-1 cells. By modelling the frequency dependence of the elastic modulus, the VIVA provides three metrics of cytoplasmic viscoelasticity: a low frequency modulus, a high frequency modulus and viscosity. The signature of cytotoxicity by rhodamine or silver exposure is a frequency independent twofold increase in the elastic modulus and cytoplasmic viscosity, while the cytoskeletal relaxation time remains unchanged. This is consistent with the known toxic mechanism of silver nanoparticles, where metabolic stress causes an increase in the rigidity of the cytoplasm. A variable indentation-rate viscoelastic analysis is presented as a straightforward method to promote the self-consistent comparison between cells. This is paramount to the development of early diagnosis and treatment of disease.

High-Throughput Assessment of Cellular Mechanical Properties

Traditionally, cell analysis has focused on using molecular biomarkers for basic research, cell preparation, and clinical diagnostics; however, new microtechnologies are enabling evaluation of the mechanical properties of cells at throughputs that make them amenable to widespread use. We review the current understanding of how the mechanical characteristics of cells relate to underlying molecular and architectural changes, describe how these changes evolve with cell-state and disease processes, and propose promising biomedical applications that will be facilitated by the increased throughput of mechanical testing: from diagnosing cancer and monitoring immune states to preparing cells for regenerative medicine. We provide background about techniques that laid the groundwork for the quantitative understanding of cell mechanics and discuss current efforts to develop robust techniques for rapid analysis that aim to implement mechanophenotyping as a routine tool in biomedicine. Looking forward, we describe additional milestones that will facilitate broad adoption, as well as new directions not only in mechanically assessing cells but also in perturbing them to passively engineer cell state.

Type VI Collagen Regulates Pericellular Matrix Properties, Chondrocyte Swelling, and Mechanotransduction in Mouse Articular Cartilage

OBJECTIVE

Mechanical factors play a critical role in the physiology and pathology of articular cartilage, although the mechanisms of mechanical signal transduction are not fully understood. We undertook this study to test the hypothesis that type VI collagen is necessary for mechanotransduction in articular cartilage by determining the effects of type VI collagen knockout on the activation of the mechano-osmosensitive, calcium-permeable channel TRPV4 (transient receptor potential vanilloid channel 4) as well as on osmotically induced chondrocyte swelling and pericellular matrix (PCM) mechanical properties.

METHODS

Confocal laser scanning microscopy was used to image TRPV4-mediated calcium signaling and osmotically induced cell swelling in intact femora from 2- and 9-month-old wild-type (WT) and type VI collagen-deficient (Col6a1(-/-)) mice. Immunofluorescence-guided atomic force microscopy was used to map PCM mechanical properties based on the presence of perlecan.

RESULTS

Hypo-osmotic stress-induced TRPV4-mediated calcium signaling was increased in Col6a1(-/-) mice relative to WT controls at 2 months. Col6a1(-/-) mice exhibited significantly increased osmotically induced cell swelling and decreased PCM moduli relative to WT controls at both ages.

CONCLUSION

In contrast to our original hypothesis, type VI collagen was not required for TRPV4-mediated Ca(2+) signaling; however, knockout of type VI collagen altered the mechanical properties of the PCM, which in turn increased the extent of cell swelling and osmotically induced TRPV4 signaling in an age-dependent manner. These findings emphasize the role of the PCM as a transducer of mechanical and physicochemical signals, and they suggest that alterations in PCM properties, as may occur with aging or osteoarthritis, can influence mechanotransduction via TRPV4 or other ion channels.

Sub-cellular force microscopy in single normal and cancer cells

This work investigates the biomechanical properties of sub-cellular structures of breast cells using atomic force microscopy (AFM). The cells are modeled as a triple-layered structure where the Generalized Maxwell model is applied to experimental data from AFM stress-relaxation tests to extract the elastic modulus, the apparent viscosity, and the relaxation time of sub-cellular structures. The triple-layered modeling results allow for determination and comparison of the biomechanical properties of the three major sub-cellular structures between normal and cancerous cells: the up plasma membrane/actin cortex, the mid cytoplasm/nucleus, and the low nuclear/integrin sub-domains. The results reveal that the sub-domains become stiffer and significantly more viscous with depth, regardless of cell type. In addition, there is a decreasing trend in the average elastic modulus and apparent viscosity of the all corresponding sub-cellular structures from normal to cancerous cells, which becomes most remarkable in the deeper sub-domain. The presented modeling in this work constitutes a unique AFM-based experimental framework to study the biomechanics of sub-cellular structures.

Mechanical trapping of the nucleus on micropillared surfaces inhibits the proliferation of vascular smooth muscle cells but not cervical cancer HeLa cells

The interaction between cells and the extracellular matrix on a topographically patterned surface can result in changes in cell shape and many cellular functions. In the present study, we demonstrated the mechanical deformation and trapping of the intracellular nucleus using polydimethylsiloxane (PDMS)-based microfabricated substrates with an array of micropillars. We investigated the differential effects of nuclear deformation on the proliferation of healthy vascular smooth muscle cells (SMCs) and cervical cancer HeLa cells. Both types of cell spread normally in the space between micropillars and completely invaded the extracellular microstructures, including parts of their cytoplasm and their nuclei. We found that the proliferation of SMCs but not HeLa cells was dramatically inhibited by cultivation on the micropillar substrates, even though remarkable deformation of nuclei was observed in both types of cells. Mechanical testing with an atomic force microscope and a detailed image analysis with confocal microscopy revealed that SMC nuclei had a thicker nuclear lamina and greater expression of lamin A/C than those of HeLa cells, which consequently increased the elastic modulus of the SMC nuclei and their nuclear mechanical resistance against extracellular microstructures. These results indicate that the inhibition of cell proliferation resulted from deformation of the mature lamin structures, which might be exposed to higher internal stress during nuclear deformation. This nuclear stress-induced inhibition of cell proliferation occurred rarely in cancer cells with deformable nuclei.

The fractional viscoelastic response of human breast tissue cells

The mechanical response of a living cell is notoriously complicated. The complex, heterogeneous characteristics of cellular structure introduce difficulties that simple linear models of viscoelasticity cannot overcome, particularly at deep indentation depths. Herein, a nano-scale stress-relaxation analysis performed with an atomic force microscope reveals that isolated human breast cells do not exhibit simple exponential relaxation capable of being modeled by the standard linear solid (SLS) model. Therefore, this work proposes the application of the fractional Zener (FZ) model of viscoelasticity to extract mechanical parameters from the entire relaxation response, improving upon existing physical techniques to probe isolated cells. The FZ model introduces a new parameter that describes the fractional time-derivative dependence of the response. The results show an exceptional increase in conformance to the experimental data compared to that predicted by the SLS model, and the order of the fractional derivative (α) is remarkably homogeneous across the populations, with a median value of 0.48 ± 0.06 for the malignant population and 0.51 ± 0.07 for the benign. The cells' responses exhibit power-law behavior and complexity not associated with simple relaxation (SLS, α = 1) that supports the application of a fractional model. The distributions of some of the FZ parameters also preserve the distinction between the malignant and benign sample populations seen from the linear model and previous results while including the contribution of fast-relaxation behavior. The resulting viscosity, measured by a composite relaxation time, exhibits considerably less dispersion due to residual error than the distribution generated by the linear model and therefore serves as a more powerful marker for cell differentiation.

Microscale consolidation analysis of relaxation behavior of single living chondrocytes subjected to varying strain-rates

Besides the elastic stiffness, the relaxation behavior of single living cells is also of interest of various researchers when studying cell mechanics. It is hypothesized that the relaxation response of the cells is governed by both intrinsic viscoelasticity of the solid phase and fluid-solid interactions mechanisms. There are a number of mechanical models have been developed to investigate the relaxation behavior of single cells. However, there is lack of model enable to accurately capture both of the mechanisms. Therefore, in this study, the porohyperelastic (PHE) model, which is an extension of the consolidation theory, combined with inverse Finite Element Analysis (FEA) technique was used at the first time to investigate the relaxation response of living chondrocytes. This model was also utilized to study the dependence of relaxation behavior of the cells on strain-rates. The stress-relaxation experiments under the various strain-rates were conducted with the Atomic Force Microscopy (AFM). The results have demonstrated that the PHE model could effectively capture the stress-relaxation behavior of the living chondrocytes, especially at intermediate to high strain-rates. Although this model gave some errors at lower strain-rates, its performance was acceptable. Therefore, the PHE model is properly a promising model for single cell mechanics studies. Moreover, it has been found that the hydraulic permeability of living chondrocytes reduced with decreasing of strain-rates. It might be due to the intracellular fluid volume fraction and the fluid pore pressure gradients of chondrocytes were higher when higher strain-rates applied.

Distinct impact of targeted actin cytoskeleton reorganization on mechanical properties of normal and malignant cells

The actin cytoskeleton is substantially modified in cancer cells because of changes in actin-binding protein abundance and functional activity. As a consequence, cancer cells have distinctive motility and mechanical properties, which are important for many processes, including invasion and metastasis. Here, we studied the effects of actin cytoskeleton alterations induced by specific nucleation inhibitors (SMIFH2, CK-666), cytochalasin D, Y-27632 and detachment from the surface by trypsinization on the mechanical properties of normal Vero and prostate cancer cell line DU145. The Young's modulus of Vero cells was 1300±900 Pa, while the prostate cancer cell line DU145 exhibited significantly lower Young's moduli (600±400 Pa). The Young's moduli exhibited a log-normal distribution for both cell lines. Unlike normal cells, cancer cells demonstrated diverse viscoelastic behavior and different responses to actin cytoskeleton reorganization. They were more resistant to specific formin-dependent nucleation inhibition, and reinforced their cortical actin after detachment from the substrate. This article is part of a Special Issue entitled: Mechanobiology.

Mechanical responses of cancer cells on nanoscaffolds for adhesion size control

A mechano-reciprocal interaction plays a critical role for cancer cells searching for favorable metastasis sites. For this study, we utilized nanoscaffolds that can control the maturation of focal adhesions in order to investigate how cancer cells mechanically respond to their nanoenvironments. We found that prostate cancer cells showed linearly decreasing proliferation rate and mechanical stiffness as the size of nanoislands on nanoscaffolds where the cells were grown decreases. This mechanical signature was exacerbated for less metastatic prostate cancer cells. However, there was no dependence of mechanical responses on the geometric properties of nanoscaffolds for breast cancer cells, despite the acute inhibition of adhesion and the abrupt mechanical changes. We believe that our holistic approach that utilizes atomic force microscopy (AFM) and nanoscaffolds can reveal which mechano-reciprocal interactions are crucial for metastasis and, thus, provide useful information for anti-cancer drug development targeting integrin-associated signaling.

Temporal heterogeneity in single-cell gene expression and mechanical properties during adipogenic differentiation

Adipose-derived stem/stromal cells (ASCs) respond heterogeneously when exposed to lineage-specific induction medium. Variable responses at the single-cell level can be observed in the production of lineage-specific metabolites, expression of mRNA transcripts, and adoption of mechanical phenotypes. Understanding the relationship between the biological and mechanical characteristics for individual ASCs is crucial for interpreting how cellular heterogeneity affects the differentiation process. The goal of the current study was to monitor the gene expression of peroxisome proliferator receptor gamma (PPARG) in adipogenically differentiating ASC populations over two weeks, while also characterizing the expression-associated mechanical properties of individual cells using atomic force microscopy (AFM). Results showed that ASC mechanical properties did not change significantly over time in either adipogenic or control medium; however, cells expressing PPARG exhibited significantly greater compliance and fluidity compared to those lacking expression in both adipogenic and control media environments. The percent of PPARG+ cells in adipogenic samples increased over time but stayed relatively constant in controls. Previous reports of a slow, gradual change in cellular mechanical properties are explained by the increase in the number of positively differentiating cells in a sample rather than being reflective of actual, single-cell mechanical property changes. Cytoskeletal remodeling was more prevalent in adipogenic samples than controls, likely driving the adoption of a more compliant mechanical phenotype and upregulation of PPARG. The combined results reinforce the importance of understanding single-cell characteristics, in the context of heterogeneity, to provide more accurate interpretations of biological phenomena such as stem cell differentiation.

Toward the realization of reproducible Atomic Force Microscopy measurements of elastic modulus in biological samples

The validation of the AFM method for elastic modulus E measurement in soft materials (E <5 MPa) is still missing. The interest of measurements in materials with E <5 MPa is mainly biological, including soft tissues and single cells. For the diagnosis of malignant human tumors, a change in cell elasticity, within tissues, has recently been recognized as a marker of metastatic potential. To measure a cell elasticity difference, reproducible E measurements in biological samples are needed. In this work a robust method for a metrological validation of E measurements in the range 500-5000 kPa was developed, based on the realization of thick E standard samples and on the study of the interactions between the measurement process and the sample at micro- and nano-scale. E measurement reproducibility limit of 4% has been reached. This allows designing a very sensitive and reproducible measurement of E in biological samples representing thus a powerful diagnostic tool for cancer detection.

hMSCs bridging across micro-patterned grooves

hMSCs spanned across a groove with 100 μm width.

Nanomechanical characterization of living mammary tissues by atomic force microscopy

The mechanical properties of living cells and tissues are important for a variety of functional processes in vivo, including cell adhesion, migration, proliferation and differentiation. Changes in mechano-cellular phenotype, for instance, are associated with cancer progression. Atomic force microscopy (AFM) is an enabling technique that topographically maps and quantifies the mechanical properties of complex biological matter in physiological aqueous environments at the nanometer length scale. Recently we applied AFM to spatially resolve the distribution of nanomechanical stiffness across human breast cancer biopsies in comparison to healthy tissue and benign tumors. This led to the finding that AFM provides quantitative mechano-markers that may have translational significance for the clinical diagnosis of cancer. Here, we provide a comprehensive description of sample preparation methodology, instrumentation, data acquisition and analysis that allows for the quantitative nanomechanical profiling of unadulterated tissue at submicron spatial resolution and nano-Newton (nN) force sensitivity in physiological conditions.

Effects of an antimitotic drug on mechanical behaviours of the cytoskeleton in distinct grades of colon cancer cells

Biomechanical behaviours of cells change during cancer progression due to alterations in the main cytoskeletal proteins. Microtubules play a vital role in mitosis and in supporting the integrity of the cell due to their ability to withstand high compressive loads. Accordingly, microtubule-targeting agents (MTAs) have become one of the most promising classes of drugs in cancer therapy. This study evaluated changes in visco-elastic parameters induced by an appropriate concentration of an antimitotic drug in two different grades of colon cancer cells. Actin microfilaments and microtubules contents in the cells were evaluated by Western blot analysis and fluorescence intensity calculation. Micropipette aspiration experiments showed that the MTA had distinct mechanical effects on different cell lines. The more aggressive the cells, the greater the reduction in elasticity and viscosity. Invasive cells had a higher initial instantaneous Young's modulus than primary cells, but this reduced to approximately one half of the values for primary cells after 48 h of drug treatment. A considerable association was seen between the changes in mechanical properties and the microtubule to F-actin microfilament content ratio, which decreased with MTA treatment.

Local viscoelastic properties of live cells investigated using dynamic and quasi-static atomic force microscopy methods

The measurement of viscoelasticity of cells in physiological environments with high spatio-temporal resolution is a key goal in cell mechanobiology. Traditionally only the elastic properties have been measured from quasi-static force-distance curves using the atomic force microscope (AFM). Recently, dynamic AFM-based methods have been proposed to map the local in vitro viscoelastic properties of living cells with nanoscale resolution. However, the differences in viscoelastic properties estimated from such dynamic and traditional quasi-static techniques are poorly understood. In this work we quantitatively reconstruct the local force and dissipation gradients (viscoelasticity) on live fibroblast cells in buffer solutions using Lorentz force excited cantilevers and present a careful comparison between mechanical properties (local stiffness and damping) extracted using dynamic and quasi-static force spectroscopy methods. The results highlight the dependence of measured viscoelastic properties on both the frequency at which the chosen technique operates as well as the interactions with subcellular components beyond certain indentation depth, both of which are responsible for differences between the viscoelasticity property maps acquired using the dynamic AFM method against the quasi-static measurements.

3D Viscoelastic traction force microscopy

Native cell-material interactions occur on materials differing in their structural composition, chemistry, and physical compliance. While the last two decades have shown the importance of traction forces during cell-material interactions, they have been almost exclusively presented on purely elastic in vitro materials. Yet, most bodily tissue materials exhibit some level of viscoelasticity, which could play an important role in how cells sense and transduce tractions. To expand the realm of cell traction measurements and to encompass all materials from elastic to viscoelastic, this paper presents a general, and comprehensive approach for quantifying 3D cell tractions in viscoelastic materials. This methodology includes the experimental characterization of the time-dependent material properties for any viscoelastic material with the subsequent mathematical implementation of the determined material model into a 3D traction force microscopy (3D TFM) framework. Utilizing this new 3D viscoelastic TFM (3D VTFM) approach, we quantify the influence of viscosity on the overall material traction calculations and quantify the error associated with omitting time-dependent material effects, as is the case for all other TFM formulations. We anticipate that the 3D VTFM technique will open up new avenues of cell-material investigations on even more physiologically relevant time-dependent materials including collagen and fibrin gels.

AFM-based dual nano-mechanical phenotypes for cancer metastasis

An enhanced mechanical compliance is considered to be a mechanical indicator for metastatic cancer cells. Our study using atomic force microscopy (AFM) revealed that breast cancer cells agreed well with this hypothesis. However, prostate cancer cells displayed a reverse correlation; less metastatic prostate cancer cells were more mechanically compliant. Two-dimensional AFM force spectroscopy was performed to characterize dual mechanical properties-the cell-substrate adhesion as well as the mechanical compliance. Interestingly, prostate cancer cells displayed a strong positive correlation between the cell-substrate adhesion and metastatic potential. However, there was no clearly observable correlation between the cell-substrate adhesion and the metastatic potential despite variations in mechanical compliance of breast cancer cells. These results suggest that the correlation between the dual mechanical signatures and metastatic potential be uniquely identified for cancer cells originating from different organs. We postulate that this correlation could reveal which step of cancer progression is favorable in terms of physical interaction between cancer cells and micro-environments. We expect that based on the "seed and soil hypothesis", the identification of the dual mechanical phenotypes, could provide a new insight for understanding how a dominant metastatic site is determined for cancer cells originating from specific organs.

Cell mechanics: principles, practices, and prospects

Cells generate and sustain mechanical forces within their environment as part of their normal physiology. They are active materials that can detect mechanical stimulation by the activation of mechanosensitive signaling pathways, and respond to physical cues through cytoskeletal re-organization and force generation. Genetic mutations and pathogens that disrupt the cytoskeletal architecture can result in changes to cell mechanical properties such as elasticity, adhesiveness, and viscosity. On the other hand, perturbations to the mechanical environment can affect cell behavior. These transformations are often a hallmark and symptom of a variety of pathologies. Consequently, there are now a myriad of experimental techniques and theoretical models adapted from soft matter physics and mechanical engineering to characterize cell mechanical properties. Interdisciplinary research combining modern molecular biology with advanced cell mechanical characterization techniques now paves the way for furthering our fundamental understanding of cell mechanics and its role in development, physiology, and disease. We describe a generalized outline for measuring cell mechanical properties including loading protocols, tools, and data interpretation.We summarize recent advances in the field and explain how cell biomechanics research can be adopted by physicists, engineers, biologists, and clinicians alike.

Determination of Strain-Rate-Dependent Mechanical Behavior of Living and Fixed Osteocytes and Chondrocytes Using Atomic Force Microscopy and Inverse Finite Element Analysis

The aim of this paper is to determine the strain-rate-dependent mechanical behavior of living and fixed osteocytes and chondrocytes, in vitro. First, atomic force microscopy (AFM) was used to obtain the force–indentation curves of these single cells at four different strain-rates. These results were then employed in inverse finite element analysis (FEA) using modified standard neo-Hookean solid (MSnHS) idealization of these cells to determine their mechanical properties. In addition, a FEA model with a newly developed spring element was employed to accurately simulate AFM evaluation in this study. We report that both cytoskeleton (CSK) and intracellular fluid govern the strain-rate-dependent mechanical property of living cells whereas intracellular fluid plays a predominant role on fixed cells' behavior. In addition, through the comparisons, it can be concluded that osteocytes are stiffer than chondrocytes at all strain-rates tested indicating that the cells could be the biomarker of their tissue origin. Finally, we report that MSnHS is able to capture the strain-rate-dependent mechanical behavior of osteocyte and chondrocyte for both living and fixed cells. Therefore, we concluded that the MSnHS is a good model for exploration of mechanical deformation responses of single osteocytes and chondrocytes. This study could open a new avenue for analysis of mechanical behavior of osteocytes and chondrocytes as well as other similar types of cells.

Characterization of mechanical and regenerative properties of human, adipose stromal cells

The stromal vascular fraction (SVF) of human adipose tissue is a heterogeneous population, with component cell types that may or may not contribute to its regenerative potential. Recent findings indicate that single-cell mechanical biomarkers are characteristic of cell type and can be used comparably to gene and protein expressions to identify cell populations. In this study, we characterized mechanical properties and differentiation potential of cell types present in the SVF. Fluorescence-activated cell sorting was used to isolate four distinct populations based on surface markers: endothelial cells (EC), adipose-derived stem cells (ASCs), pre-adipocytes, and smooth muscle cells (SMC). Atomic force microscopy was used to mechanically characterize sorted cell populations and unsorted SVF. Differentiation capabilities of sorted and unsorted populations were evaluated by quantifying lipid production and calcified matrix deposition. Cells populating the SVF exhibited a range of mechanical properties, with ECs, ASCs, pre-adipocytes, and unsorted SVF cells being significantly more compliant than SMCs. Lineage-specific metabolite production was most robust in SVF cells, followed by ASCs, with the other cell types showing little or no potential, suggesting the unsorted populations may benefit from a paracrine response that is lacking once the cells are sorted into more uniform cell populations.

Nanobiomechanics of living cells: a review

Nanobiomechanics of living cells is very important to understand cell-materials interactions. This would potentially help to optimize the surface design of the implanted materials and scaffold materials for tissue engineering. The nanoindentation techniques enable quantifying nanobiomechanics of living cells, with flexibility of using indenters of different geometries. However, the data interpretation for nanoindentation of living cells is often difficult. Despite abundant experimental data reported on nanobiomechanics of living cells, there is a lack of comprehensive discussion on testing with different tip geometries, and the associated mechanical models that enable extracting the mechanical properties of living cells. Therefore, this paper discusses the strategy of selecting the right type of indenter tips and the corresponding mechanical models at given test conditions.

Validation of the effects of TGF-β1 on tumor recurrence and prognosis through tumor retrieval and cell mechanical properties

Background

In vivo, the transforming growth factor-beta1 (TGF-β1)-induced epithelial to mesenchymal transition (EMT) occurs in seconds during cancer cells intravasation and extravasation. Although it has been established that cellular stiffness can change as a cancer cell transformed, the precise relationship between TGF-β1-induced mesenchymal stem cell mechanics and cancer prognosis remains unclear. Accordingly, it is hard to define the effects of EMT on cell mechanical properties (CMs), tumor recurrence and metastasis risks. This study bridges physical and pathological disciplines to reconcile single-cell mechanical measurements of tumor cells.

Methods and results

We developed a microplate measurement system (MMS) and revealed the intrinsic divergent tumor composition of retrieval cells by cell stiffness and adhesion force and flow cytometry analysis. After flow cytometry sorting, we could measure the differences in CMs of the Sca-1⁺-CD44⁺ (mesenchymal-stem-cell-type) and the other subgroups. As well as the stiffer and heterogeneous compositions among tumor tissues with higher recurrence risk were depicted by MMS and atomic force microscopy (AFM). An in vitro experiment validated that Lewis lung carcinoma (LLC) cells acquired higher CMs and motility after EMT, but abrogated by SB-505124 inhibition. Concomitantly, the CD31, MMP13 and TGF-β1 enriched micro-environment in the tumor was associated with higher recurrence and distal lung metastasis risks. Furthermore, we report a comprehensive effort to correlate CMs to tumor-prognosis indicators, in which a decreased body weight gain ratio (BWG) and increased tumor weight (TW) were correlated with increased CMs.

Conclusions

Together, we determined that TGF-β1 was significantly associated with malignant tumor progressing. In terms of clinical applications, local tumor excision followed by MMS analysis offers an opportunity to predict tumor recurrence and metastasis risks.

Exchangeable colloidal AFM probes for the quantification of irreversible and long-term interactions

An original method is presented to study single-colloid interaction with a substrate in liquid environment. Colloids, either in solution or adsorbed on a surface, are fixed by suction against the aperture of a microchanneled atomic force microscopy cantilever. Their adhesion to the substrate is measured, followed by their release via a short overpressure surge. Such colloid exchange procedure allows for 1), the quick variation of differently functionalized colloids within the same experiment; 2), the investigation of long-term interactions by leaving the colloids on a surface for a defined time before detaching them; and 3), the inspection of irreversible interactions. After validation of the method by reproducing literature results obtained with traditional colloidal atomic force microscopy, the serial use of colloids with different surface functionalization was shown on a micropatterned surface. Finally, concanavalin A-coated colloids were allowed to adsorb on human embryonic kidney cells and then detached one by one. The adhesion between cells and colloids was up to 60 nN, whereas individual cells adhered with 20 nN to the glass substrate. A cellular elastic modulus of 0.8 kPa was determined using the attached colloid as indenter.

Physical biology in cancer. 4. Physical cues guide tumor cell adhesion and migration

As tumor cells metastasize from the primary tumor location to a distant secondary site, they encounter an array of biologically and physically heterogeneous microenvironments. While it is well established that biochemical signals guide all stages of the metastatic cascade, mounting evidence indicates that physical cues also direct tumor cell behavior, including adhesion and migration phenotypes. Physical cues acting on tumor cells in vivo include extracellular matrix mechanical properties, dimensionality, and topography, as well as interstitial flow, hydrodynamic shear stresses, and local forces due to neighboring cells. State-of-the-art technologies have recently enabled us and other researchers to engineer cell microenvironments that mimic specific physical properties of the cellular milieu. Through integration of these engineering strategies, along with physics, molecular biology, and imaging techniques, we have acquired new insights into tumor cell adhesion and migration mechanisms. In this review, we focus on the extravasation and invasion stages of the metastatic cascade. We first discuss the physical role of the endothelium during tumor cell extravasation and invasion and how contractility of endothelial and tumor cells contributes to the ability of tumor cells to exit the vasculature. Next, we examine how matrix dimensionality and stiffness coregulate tumor cell adhesion and migration beyond the vasculature. Finally, we summarize how tumor cells translate and respond to physical cues through mechanotransduction. Because of the critical role of tumor cell mechanotransduction at various stages of the metastatic cascade, targeting signaling pathways involved in tumor cell mechanosensing of physical stimuli may prove to be an effective therapeutic strategy for cancer patients.

Biomechanical profile of cancer stem-like/tumor-initiating cells derived from a progressive ovarian cancer model

We herein report, for the first time, the mechanical properties of ovarian cancer stem-like/tumor-initiating cells (CSC/TICs). The represented model is a spontaneously transformed murine ovarian surface epithelial (MOSE) cell line that mimics the progression of ovarian cancer from early/non-tumorigenic to late/highly aggressive cancer stages. Elastic modulus measurements via atomic force microscopy (AFM) illustrate that the enriched CSC/TICs population (0.32±0.12kPa) are 46%, 61%, and 72% softer (P<0.0001) than their aggressive late-stage, intermediate, and non-malignant early-stage cancer cells, respectively. Exposure to sphingosine, an anti-cancer agent, induced an increase in the elastic moduli of CSC/TICs by more than 46% (0.47±0.14kPa, P<0.0001). Altogether, our data demonstrate that the elastic modulus profile of CSC/TICs is unique and responsive to anti-cancer treatment strategies that impact the cytoskeleton architecture of cells. These findings increase the chance for obtaining distinctive cell biomechanical profiles with the intent of providing a means for effective cancer detection and treatment control.

FROM THE CLINICAL EDITOR

This novel study utilized atomic force microscopy to demonstrate that the elastic modulus profile of cancer stem cell-like tumor initiating cells is unique and responsive to anti-cancer treatment strategies that impact the cytoskeleton of these cells. These findings pave the way to the development of unique means for effective cancer detection and treatment control.

A novel pneumatic micropipette aspiration method using a balance pressure model

This paper presents a novel micropipette aspiration (MA) method based on a common pneumatic micro-injection system. This method is the first to quantify the influence of capillary effect on aspiration pressure using a balance pressure model, and in return, uses the capillary effect to quantify the aspiration pressure. Subsequently, the seal between the cell and the micropipette is detected to judge and exclude the ineffective MA attempts. The rationality of the balance pressure model is validated by the designed micropipette-filling experiments. Through applied to elasticity-determination of the cells with different sizes, the feasibility and versatility of this MA method are proved. With abilities to quantify aspiration pressures and detect the seam between the cell and the micropipette, our method is expected to advance the application of the commercial pneumatic injector in the MA of cells. Moreover, with the quantified volume of the liquid entering into the micropipette during MA process, our method also has a potential applicability to the study of the permeability of the cell membrane in the future.

Mechanical properties of cancer cytoskeleton depend on actin filaments to microtubules content: investigating different grades of colon cancer cell lines

Biomechanical properties of cancer cells have been proposed as promising biomarkers to investigate cancer progression. Cytoskeletal reorganization alters these characteristics in different grades of cancer cells. In the present study based on the micropipette aspiration method, whole body evaluation for two different colon cancer cells was performed to determine viscoelastic parameters of the cells. A finite element model was developed for verification of experiments and predicting some behaviors of cells. Western blot analysis and fluorescence intensity for actin microfilaments and microtubules were performed to measure cell content of the proteins. It was illustrated that the proportion of microtubules and actin microfilaments is different in grade I and grade IV colon cancer cells in a manner that microtubules attain an effectual role in progressive reorganization of cytoskeleton in transition from nonaggressive to malignant phenotypes in cancer cells. Furthermore, it was concluded that larger instantaneous Young's modulus value for high grade cells is related to the existence of extensively build-up actin networks at the cell cortex. Based on the cell mechanics results, a simple parameter is suggested for sorting different grades of colon cancer cells.

Mechanical stiffness as an improved single-cell indicator of osteoblastic human mesenchymal stem cell differentiation

Although it has been established that cellular stiffness can change as a stem cell differentiates, the precise relationship between cell mechanics and other phenotypic properties remains unclear. Inherent cell heterogeneity and asynchronous differentiation complicate population analysis; therefore, single-cell analysis was employed to determine how changes in cell stiffness correlate with changes in molecular biomarkers during differentiation. Design of a custom gridded tissue culture dish facilitated single-cell comparisons between cell mechanics and other differentiation biomarkers by enabling sequential measurement of cell mechanics and protein biomarker expression at the single cell level. The Young's modulus of mesenchymal stem cells was shown not only to decrease during chemically-induced osteoblast differentiation, but also to correlate more closely with the day of differentiation than did the relative expression of the traditional osteoblast differentiation markers, bone sialoprotein and osteocalcin. Therefore, cell stiffness, a measurable property of individual cells, may serve as an improved indicator of single-cell osteoblast differentiation compared to traditional biological markers. Revelation of additional osteoblast differentiation indicators, such as cell stiffness, can improve identification and collection of starting cell populations, with applications to mesenchymal stem cell therapies and stem cell-based tissue engineering.

Resiliency of the plasma membrane and actin cortex to large-scale deformation

The tight coupling between the plasma membrane and actin cortex allows cells to rapidly change shape in response to mechanical cues and during physiological processes. Mechanical properties of the membrane are critical for organizing the actin cortex, which ultimately governs the conversion of mechanical information into signaling. The cortex has been shown to rapidly remodel on timescales of seconds to minutes, facilitating localized deformations and bundling dynamics that arise during the exertion of mechanical forces and cellular deformations. Here, we directly visualized and quantified the time-dependent deformation and recovery of the membrane and actin cortex of HeLa cells in response to externally applied loads both on- and off-nucleus using simultaneous confocal and atomic force microscopy. The local creep-like deformation of the membrane and actin cortex depends on both load magnitude and duration and does not appear to depend on cell confluency. The membrane and actin cortex rapidly recover their initial shape after prolonged loading (up to 10 min) with large forces (up to 20 nN) and high aspect ratio deformations. Cytoplasmic regions surrounding the nucleus are shown to be more resistant to long-term creep than nuclear regions. These dynamics are highly regulated by actomyosin contractility and an intact actin cytoskeleton. Results suggest that in response to local deformations, the nucleus does not appear to provide significant resistance or play a major role in cell shape recovery. The membrane and actin cortex clearly possess remarkable mechanical stability, critical for the transduction of mechanical deformation into long term biochemical signals and cellular remodeling.

A Brief Review of the Biophysical Hallmarks of Metastatic Cancer Cells

A hallmark of metastatic cancer cells is their invasion through the basal membrane and endothelial layer, which requires a highly elastic cytoskeleton and nucleus. Therefore, cellular deformability can serve as a universal biophysical marker for detecting a tumor's propensity for invasion, migration, and metastasis. In this review, we define the importance of the biophysical features of cancer cells in tumor metastasis and summarize the state-of-the-art technology for the study of cell biomechanics. This review will serve as a brief introduction to the interdisciplinary character of cancer cell biophysics for cancer biologists, physicists, and engineers.

Mechanical properties of human amniotic fluid stem cells using nanoindentation

The aim of this study was to obtain nanomechanical properties of living cells focusing on human amniotic fluid stem (hAFS) cell using nanoindentation techniques. We modified the conventional method of atomic force microscopy (AFM) in aqueous environment for cell imaging and indentation to avoid inherent difficulties. Moreover, we determined the elastic modulus of murine osteoblast (OB6) cells and hAFS cells at the nucleus and cytoskeleton using force-displacement curves and Hertz theory. Since OB6 cell line has been widely used, it was selected to validate and compare the obtained results with the previous research studies. As a result, we were able to capture high resolution images through utilization of the tapping mode without adding protein or using fixation methods. The maximum depth of indentation was kept below 15% of the cell thickness to minimize the effect of substrate hardness. Nanostructural details on the surface of cells were visualized by AFM and fluorescence microscopy. The cytoskeletal fibers presented remarkable increase in elastic modulus as compared with the nucleus. Furthermore, our results showed that the elastic modulus of hAFS cell edge (31.6 kPa) was lower than that of OB6 cell edge (42.2 kPa). In addition, the elastic modulus of nucleus was 13.9 kPa for hAFS cell and 26.9 kPa for OB6 cells. Differences in cell elastic modulus possibly resulted from the type and number of actin cytoskeleton organization in these two cell types.

Adipose-derived stem cell fate is predicted by cellular mechanical properties

Adipose-derived stem cells (ASCs) show great promise for tissue engineering applications and cell-based therapies because of their multipotency, relative abundance and immunosuppressive properties. However, ASCs must be isolated from heterogeneous cell populations present in adipose tissue. In this brief report, we provide a concise summary of the history and use of cellular mechanical properties as novel, label-free biomarkers to predict the differentiation potential of ASCs toward adipogenic, osteogenic and chondrogenic lineages. Additionally, we have found that passage number influences the mechanical properties of ASCs along with a discussion of potential environmental factors that could affect these properties. Altogether, this report provides evidence for the reliability of cellular mechanical properties as biomarkers for ASC differentiation potential and outlines how they can be used to sort ASCs with lineage-specific preferences for particular applications.

Free-radical polymer science structural cancer model: a review

Polymer free-radical lipid alkene chain-growth biological models particularly for hypoxic cellular mitochondrial metabolic waste can be used to better understand abnormal cancer cell morphology and invasive metastasis. Without oxygen as the final electron acceptor for mitochondrial energy synthesis, protons cannot combine to form water and instead mitochondria produce free radicals and acid during hypoxia. Nonuniform bond-length shrinkage of membranes related to erratic free-radical covalent crosslinking can explain cancer-cell pleomorphism with epithelial-mesenchymal transition for irregular membrane borders that "ruffle" and warp over stiff underlying actin fibers. Further, mitochondrial hypoxic conditions produce acid that can cause molecular degradation. Subsequent low pH-activated enzymes then provide paths for invasive cell movement through tissue and eventually blood-born metastasis. Although free-radical crosslinking creates irregularly shaped membranes with structural actin-polymerized fiber extensions as filopodia and lamellipodia, due to rapid cell division the overall cell modulus (approximately stiffness) is lower than normal cells. When combined with low pH-activated enzymes and lower modulus cells, smaller cancer stem cells subsequently have a large advantage to follow molecular destructive pathways and leave the central tumor. In addition, forward structural spike-like lamellipodia protrusions can leverage to force lower-modulus cancer cells through narrow openings. By squeezing and deforming even smaller to allow for easier movement through difficult passageways, cancer cells can travel into adjacent tissues or possibly metastasize through the blood to new tissue.

Cell mechanics, structure, and function are regulated by the stiffness of the three-dimensional microenvironment

This study adopts a combined computational and experimental approach to determine the mechanical, structural, and metabolic properties of isolated chondrocytes cultured within three-dimensional hydrogels. A series of linear elastic and hyperelastic finite-element models demonstrated that chondrocytes cultured for 24 h in gels for which the relaxation modulus is <5 kPa exhibit a cellular Young's modulus of ∼5 kPa. This is notably greater than that reported for isolated chondrocytes in suspension. The increase in cell modulus occurs over a 24-h period and is associated with an increase in the organization of the cortical actin cytoskeleton, which is known to regulate cell mechanics. However, there was a reduction in chromatin condensation, suggesting that changes in the nucleus mechanics may not be involved. Comparison of cells in 1% and 3% agarose showed that cells in the stiffer gels rapidly develop a higher Young's modulus of ∼20 kPa, sixfold greater than that observed in the softer gels. This was associated with higher levels of actin organization and chromatin condensation, but only after 24 h in culture. Further studies revealed that cells in stiffer gels synthesize less extracellular matrix over a 28-day culture period. Hence, this study demonstrates that the properties of the three-dimensional microenvironment regulate the mechanical, structural, and metabolic properties of living cells.

Comparison of the viscoelastic properties of cells from different kidney cancer phenotypes measured with atomic force microscopy

The viscoelastic properties of human kidney cell lines from different tumor types (carcinoma (A-498) and adenocarcinoma (ACHN)) are compared to a non-tumorigenic cell line (RC-124). Our methodology is based on the mapping of viscoelastic properties (elasticity modulus E and apparent viscosity η) over the surface of tens of individual cells with atomic force microscopy (AFM). The viscoelastic properties are averaged over datasets as large as 15000 data points per cell line. We also propose a model to estimate the apparent viscosity of soft materials using the hysteresis observed in conventional AFM deflection-displacement curves, without any modification to the standard AFM apparatus. The comparison of the three cell lines show that the non-tumorigenic cells are less deformable and more viscous than cancerous cells, and that cancer cell lines have distinctive viscoelastic properties. In particular, we obtained that E(RC-124) > E(A-498) > E(ACHN) and η(RC-124) > η(A-498) > η(ACHN).

Microfluidics separation reveals the stem-cell-like deformability of tumor-initiating cells

Here we report a microfluidics method to enrich physically deformable cells by mechanical manipulation through artificial microbarriers. Driven by hydrodynamic forces, flexible cells or cells with high metastatic propensity change shape to pass through the microbarriers and exit the separation device, whereas stiff cells remain trapped. We demonstrate the separation of (i) a mixture of two breast cancer cell types (MDA-MB-436 and MCF-7) with distinct deformabilities and metastatic potentials, and (ii) a heterogeneous breast cancer cell line (SUM149), into enriched flexible and stiff subpopulations. We show that the flexible phenotype is associated with overexpression of multiple genes involved in cancer cell motility and metastasis, and greater mammosphere formation efficiency. Our observations support the relationship between tumor-initiating capacity and cell deformability, and demonstrate that tumor-initiating cells are less differentiated in terms of cell biomechanics.

Label-free enrichment of adrenal cortical progenitor cells using inertial microfluidics

Passive and label-free isolation of viable target cells based on intrinsic biophysical cellular properties would allow for cost savings in applications where molecular biomarkers are known as well as potentially enable the separation of cells with little-to-no known molecular biomarkers. We have demonstrated the purification of adrenal cortical progenitor cells from digestions of murine adrenal glands utilizing hydrodynamic inertial lift forces that single cells and multicellular clusters differentially experience as they flow through a microchannel. Fluorescence staining, along with gene expression measurements, confirmed that populations of cells collected in different outlets were distinct from one another. Furthermore, primary murine cells processed through the device remained highly viable and could be cultured for 10 days in vitro. The proposed target cell isolation technique can provide a practical means to collect significant quantities of viable intact cells required to translate stem cell biology to regenerative medicine in a simple label-free manner.

Cytoskeletal stiffness, friction, and fluidity of cancer cell lines with different metastatic potential

We quantified mechanical properties of cancer cells differing in metastatic potential. These cells included normal and H-ras-transformed NIH3T3 fibroblast cells, normal and oncoprotein-overexpressing MCF10A breast cancer cells, and weakly and strongly metastatic cancer cell line pairs originating from human cancers of the skin (A375P and A375SM cells), kidney (SN12C and SN12PM6 cells), prostate (PC3M and PC3MLN4 cells), and bladder (253J and 253JB5 cells). Using magnetic twisting cytometry, cytoskeletal stiffness (g') and internal friction (g″) were measured over a wide frequency range. The dependencies of g' and g″ upon frequency were used to determine the power law exponent x which is a direct measure of cytoskeletal fluidity and quantifies where the cytoskeleton resides along the spectrum of solid-like (x = 1) to fluid-like (x = 2) states. Cytoskeletal fluidity x increased following transformation by H-ras oncogene expression in NIH3T3 cells, overexpression of ErbB2 and 14-3-3-ζ in MCF10A cells, and implantation and growth of PC3M and 253J cells in the prostate and bladder, respectively. Each of these perturbations that had previously been shown to enhance cancer cell motility and invasion are shown here to shift the cytoskeleton towards a more fluid-like state. In contrast, strongly metastatic A375SM and SN12PM6 cells that disseminate by lodging in the microcirculation of peripheral organs had smaller x than did their weakly metastatic cell line pairs A375P and SN12C, respectively. Thus, enhanced hematological dissemination was associated with decreased x and a shift towards a more solid-like cytoskeleton. Taken together, these results are consistent with the notion that adaptations known to enhance metastatic ability in cancer cell lines define a spectrum of fluid-like versus solid-like states, and the position of the cancer cell within this spectrum may be a determinant of cancer progression.

Effect of matrix on cardiomyocyte viscoelastic properties in 2D culture

Cardiomyocyte phenotype changes significantly in 2D culture systems depending on the substrate composition and organization. Given the variety of substrates that are used both for basic cardiac cell culture studies and for regenerative medicine applications, there is a critical need to understand how the different matrices influence cardiac cell mechanics. In the current study, the mechanical properties of neonatal rat cardiomyocytes cultured in a subconfluent layer upon aligned and unaligned collagen and fibronectin matrices were assessed over a two week period using atomic force microscopy. The elastic modulus was estimated by fitting the Hertz model to force curve data and the percent relaxation was determined from stress relaxation curves. The Quasilinear Viscoelastic (QLV) and Standard Linear Solid (SLS) models were fit to the stress relaxation data. Cardiomyocyte cellular mechanical properties were found to be highly dependent on matrix composition and organization as well as time in culture. It was observed that the cells stiffened and relaxed less over the first 3 to 5 days in culture before reaching a plateau in their mechanical properties. After day 5, cells on aligned matrices were stiffer than cells on unaligned matrices and cells on fibronectin matrices were stiffer than cells on collagen matrices. No such significant trends in percent relaxation measurements were observed but the QLV model fit the data very well. These results were correlated with observed changes in cellular structure associated with culture on the different substrates and analyzed for cell-to-cell variability.

A general approach for the microrheology of cancer cells by atomic force microscopy

The determination of the viscoelastic properties of cells by atomic force microscopy (AFM) is mainly realized by looking at the relaxation of the force when a constant position of the AFM head is maintained or at the evolution of the indentation when a constant force is maintained. In both cases the analysis rests on the hypothesis that the motion of the probe before the relaxation step is realized in a time which is much smaller than the characteristic relaxation time of the material. In this paper we carry out a more general analysis of the probe motion which contains both the indentation and relaxation steps, allowing a better determination of the rheological parameters. This analysis contains a correction of the Hertz model for large indentation and also the correction due to the finite thickness of the biological material; it can be applied to determine the parameters representing any kind of linear viscoelastic model. This approach is then used to model the rheological behavior of one kind of cancer cell called Hep-G2. For this kind of cell, a power law model does not well describe the low and high frequency modulus contrary to a generalized Maxwell model.