A beginner's guide to atomic force microscopy probing for cell mechanics

Mechanical characterization of Xenopus laevis oocytes using atomic force microscopy

Mechanical properties are essential for the biological activities of cells, and they have been shown to be affected by diseases. Therefore, accurate mechanical characterization is important for studying the cell lifecycle, cell-cell interactions, and disease diagnosis. While the cytoskeleton and actin cortex are typically the primary structural stiffness contributors in most live cells, oocytes possess an additional extracellular layer known as the vitelline membrane (VM), or envelope, which can significantly impact their overall mechanical properties. In this study, we utilized nanoindentation via an atomic force microscope to measure the Young's modulus of Xenopus laevis oocytes at different force setpoints and explored the influence of the VM by conducting measurements on oocytes with the membrane removed. The findings revealed that the removal of VM led to a significant decrease in the apparent Young's modulus of the oocytes, highlighting the pivotal role of the VM as the main structural component responsible for the oocyte's shape and stiffness. Furthermore, the mechanical behavior of VM was investigated through finite element (FE) simulations of the nanoindentation process. FE simulations with the VM Young's modulus in the range 20-60 MPa resulted in force-displacement curves that closely resemble experimental in terms of shape and maximum force for a given indentation depth.

Colloidal Probe Technique Optimization for Determination of Young's Modulus of Soft Adhesive Hydrogels

Atomic force microscopy (AFM) is a valuable tool for determining the Young's modulus of a wide range of materials. However, it faces challenges, particularly when assessing adhesive materials like soft poly(N-isopropylacrylamide) (pNIPAM) hydrogels. This study focuses on enhancing the consistency and reliability of AFM measurements by functionally modifying AFM spherical tip cantilevers to address substrate adhesion issues with these hydrogels. Specifically, hydrophobic functionalization with 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOCTS) emerged as the most effective approach, yielding consistent and reliable Young's modulus data across various pNIPAM hydrogel samples. This work highlights the importance of optimizing data acquisition in AFM, rather than relying on postprocessing, to reduce inconsistencies in Young's modulus assessment.

Unveiling nutrient flow-mediated stress in plant roots using an on-chip phytofluidic device

The initial emergence of the primary root from a germinating seed is a pivotal phase that influences a plant's survival. Abiotic factors such as pH, nutrient availability, and soil composition significantly affect root morphology and architecture. Of particular interest is the impact of nutrient flow on thigmomorphogenesis, a response to mechanical stimulation in early root growth, which remains largely unexplored. This study explores the intricate factors influencing early root system development, with a focus on the cooperative correlation between nutrient uptake and its flow dynamics. Using a physiologically as well as ecologically relevant, portable, and cost-effective microfluidic system for the controlled fluid environments offering hydraulic conductivity comparable to that of the soil, this study analyzes the interplay between nutrient flow and root growth post-germination. Emphasizing the relationship between root growth and nitrogen uptake, the findings reveal that nutrient flow significantly influences early root morphology, leading to increased length and improved nutrient uptake, varying with the flow rate. The experimental findings are supported by mechanical and plant stress-related fluid flow-root interaction simulations and quantitative determination of nitrogen uptake using the total Kjeldahl nitrogen (TKN) method. The microfluidic approach offers novel insights into plant root dynamics under controlled flow conditions, filling a critical research gap. By providing a high-resolution platform, this study contributes to the understanding of how fluid-flow-assisted nutrient uptake and pressure affect root cell behavior, which, in turn, induces mechanical stress leading to thigmomorphogenesis. The findings hold implications for comprehending root responses to changing environmental conditions, paving the way for innovative agricultural and environmental management strategies.

Nanoscale Methods for Longitudinal Extraction of Intracellular Contents

Longitudinal analysis of intracellular contents including gene and protein expression is crucial for deciphering the fundamentally dynamic nature of cells. This offers invaluable insights into complex tissue composition and behavior, and drives progress in disease diagnosis, biomarker discovery, and drug development. Traditional longitudinal analysis workflows, involving the destruction of cells at various timepoints, limit insights to singular moments and fail to account for cellular heterogeneity. Current non-destructive approaches, like temporal modeling with single-cell ribonucleic acid sequencing (RNA-seq) and live-cell fluorescence imaging, either rely on biological assumptions or possess the risk of cellular perturbation. Recent advances in nanoscale technologies for non-destructive intracellular content extraction offer a promising solution to these challenges. These novel methods work at the nanoscale to non-destructively access cellular membranes and can be broadly classified into three mechanisms: tip-facilitated aspiration, membrane-based, and probe-based methods. This perspective focuses on these emerging nanotechnologies for repeated intracellular content extraction. Their potential in longitudinal analysis is discussed, the critical requirements for effective repeated sampling are addressed, and the suitability of each technique for various applications is explored. Furthermore, unresolved challenges in repeated sampling are highlighted to encourage further research in this growing field.

Atomic Force Microscopy for the Study of Cell Mechanics in Pharmaceutics

Cell mechanics is gaining attraction in drug screening, but the applicable methods have not yet become part of the standardized norm. This review presents the current state of the art for atomic force microscopy, which is the most widely available method. The field is first motivated as a new way of tracking pharmaceutical effects, followed by a basic introduction targeted at pharmacists on how to measure cellular stiffness. The review then moves on to the current state of the knowledge in terms of experimental results and supplementary methods such as fluorescence microscopy that can give relevant additional information. Finally, rheological approaches as well as the theoretical interpretations are presented before ending on additional methods and outlooks.

Harnessing atomic force microscopy-based single-cell analysis to advance physical oncology

Single-cell analysis is an emerging and promising frontier in the field of life sciences, which is expected to facilitate the exploration of fundamental laws of physiological and pathological processes. Single-cell analysis allows experimental access to cell-to-cell heterogeneity to reveal the distinctive behaviors of individual cells, offering novel opportunities to dissect the complexity of severe human diseases such as cancers. Among the single-cell analysis tools, atomic force microscopy (AFM) is a powerful and versatile one which is able to nondestructively image the fine topographies and quantitatively measure multiple mechanical properties of single living cancer cells in their native states under aqueous conditions with unprecedented spatiotemporal resolution. Over the past few decades, AFM has been widely utilized to detect the structural and mechanical behaviors of individual cancer cells during the process of tumor formation, invasion, and metastasis, yielding numerous unique insights into tumor pathogenesis from the biomechanical perspective and contributing much to the field of cancer mechanobiology. Here, the achievements of AFM-based analysis of single cancer cells to advance physical oncology are comprehensively summarized, and challenges and future perspectives are also discussed. RESEARCH HIGHLIGHTS: Achievements of AFM in characterizing the structural and mechanical behaviors of single cancer cells are summarized, and future directions are discussed. AFM is not only capable of visualizing cellular fine structures, but can also measure multiple cellular mechanical properties as well as cell-generated mechanical forces. There is still plenty of room for harnessing AFM-based single-cell analysis to advance physical oncology.

Mechanical properties of human tumour tissues and their implications for cancer development

The mechanical properties of cells and tissues help determine their architecture, composition and function. Alterations to these properties are associated with many diseases, including cancer. Tensional, compressive, adhesive, elastic and viscous properties of individual cells and multicellular tissues are mostly regulated by reorganization of the actomyosin and microtubule cytoskeletons and extracellular glycocalyx, which in turn drive many pathophysiological processes, including cancer progression. This Review provides an in-depth collection of quantitative data on diverse mechanical properties of living human cancer cells and tissues. Additionally, the implications of mechanical property changes for cancer development are discussed. An increased knowledge of the mechanical properties of the tumour microenvironment, as collected using biomechanical approaches capable of multi-timescale and multiparametric analyses, will provide a better understanding of the complex mechanical determinants of cancer organization and progression. This information can lead to a further understanding of resistance mechanisms to chemotherapies and immunotherapies and the metastatic cascade.

Atomic force microscopy in disease-related studies: Exploring tissue and cell mechanics

Despite significant progress in human medicine, certain diseases remain challenging to promptly diagnose and treat. Hence, the imperative lies in the development of more exhaustive criteria and tools. Tissue and cellular mechanics exhibit distinctive traits in both normal and pathological states, suggesting that "force" represents a promising and distinctive target for disease diagnosis and treatment. Atomic force microscopy (AFM) holds great promise as a prospective clinical medical device due to its capability to concurrently assess surface morphology and mechanical characteristics of biological specimens within a physiological setting. This review presents a comprehensive examination of the operational principles of AFM and diverse mechanical models, focusing on its applications in investigating tissue and cellular mechanics associated with prevalent diseases. The findings from these studies lay a solid groundwork for potential clinical implementations of AFM. RESEARCH HIGHLIGHTS: By examining the surface morphology and assessing tissue and cellular mechanics of biological specimens in a physiological setting, AFM shows promise as a clinical device to diagnose and treat challenging diseases.

Pericellular Matrix Formation and Atomic Force Microscopy of Single Primary Human Chondrocytes Cultured in Alginate Microgels

One of the main components of articular cartilage is the chondrocyte's pericellular matrix (PCM), which is critical for regulating mechanotransduction, biochemical cues, and healthy cartilage development. Here, individual primary human chondrocytes (PHC) are encapsulated and cultured in 50 µm diameter alginate microgels using drop-based microfluidics. This unique culturing method enables PCM formation and manipulation of individual cells. Over ten days, matrix formation is observed using autofluorescence imaging, and the elastic moduli of isolated cells are measured using AFM. Matrix production and elastic modulus increase are observed for the chondrons cultured in microgels. Furthermore, the elastic modulus of cells grown in microgels increases ≈ten-fold over ten days, nearly reaching the elastic modulus of in vivo PCM. The AFM data is further analyzed using a Gaussian mixture model and shows that the population of PHCs grown in microgels exhibit two distinct populations with elastic moduli averaging 9.0 and 38.0 kPa. Overall, this work shows that microgels provide an excellent culture platform for the growth and isolation of PHCs, enabling PCM formation that is mechanically similar to native PCM. The microgel culture platform presented here has the potential to revolutionize cartilage regeneration procedures through the inclusion of in vitro developed PCM.

Mechanistic of Vesicular Ethosomes and Elastic Liposomes on Permeation Profiles of Acyclovir across Artificial Membrane, Human Cultured EpiDerm, and Rat Skin: In Vitro-Ex Vivo Study

Acyclovir (ACV) controls cutaneous herpes, genital herpes, herpes keratitis, varicella zoster, and chickenpox. From previously reported ACV formulations, we continued to explore the permeation behavior of the optimized ACV loaded optimized ethosome (ETHO2R) and elastic liposome (ELP3R) and their respective carbopol gels across artificial membrane, cultured human EpiDerm, and rat skin. Transepidermal water loss (TEWL), scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and atomic force microscopy (AFM) were used to investigate the mechanistic perspective of permeation behavior. The size values of reformulated ELP3-R and ETHO2-R were observed as 217 and 128 nm, respectively (close to previous report), whereas their respective gels showed as 231 and 252 nm, respectively. ETHO2R showed high elasticity, %EE, and low vesicle size. These were investigated for the diffusion rate of the drug permeation (3 h) across the artificial membrane, cultured human EpiDerm, and rat skin. ETHO2GR showed the highest permeation flux (78.42 µg/cm2/h), diffusion coefficient (8.24 × 10-5 cm2/h), and permeation coefficient (0.67 × 10-3 cm/h) of ACV across synthetic membrane, whereas diffusion coefficient (2.4 × 10-4 cm2/h) and permeation coefficient (0.8 × 10-3 cm/h) were maximum across EpiDerm for ETHO2GR. ETHO2R suspension showed maximized permeation flux (169.58 µg/cm2/h) and diffusion rate (0.293 mg/cm2/h1/2), suggesting the rapid internalization of vesicles with cultured skin cells at low viscosity. A similar observation was revealed using rat skin, wherein the permeation flux (182.42 µg/cm2/h), permeation coefficient (0.3 × 10-2 cm/h), and diffusion rate (0.315 mg/cm2/h1/2) of ETHO2R were relatively higher than ELP3R and ELP3GR. Relative small size (128 nm), low viscosity, ethanol-mediated ultra-deformability, high drug entrapment (98%), and elasticity (63.2) are associated with ETHO2R to provide remarkable permeation behavior across the three barriers. The value of TEWL for ETHO2R (21.9 g/m2h) was 3.71 times higher than untreated control (5.9 g/m2h), indicating ethanol-mediated maximized surficial skin lipid perturbation at 3 h of application, whereas the respective ETHO2GR-treated rat skin had TEWL value (18.6 g/m2h) slightly lower than ETHO2R due to gel-based hydration into the skin. SEL, CLSM, and AFM provided a mechanistic perspective of ETHO2R and ELP3R-mediated permeation across rat skin and carrier-mediated visualization (skin-vesicle interaction). AFM provided detailed nanoscale surface roughness topographical parameters of treated and untreated rat skin as supportive data to SEM and CLSM. Thus, ethosomes ETHO2R and respective gel assisted maximum permeation of ACV across rat skin and cultured human EpiDerm to control cutaneous herpes infection and herpes keratitis.

Correlation between biological and mechanical properties of extracellular matrix from colorectal peritoneal metastases in human tissues

Peritoneal metastases (PM) are common routes of dissemination for colorectal cancer (CRC) and remain a lethal disease with a poor prognosis. The properties of the extracellular matrix (ECM) are important in cancer development; studying their changes is crucial to understand CRC-PM development. We studied the elastic properties of ECMs derived from human samples of normal and neoplastic PM by atomic force microscopy (AFM); results were correlated with patient clinical data and expression of ECM components related to metastatic spread. We show that PM progression is accompanied by stiffening of the ECM, increased cancer associated fibroblasts (CAF) activity and increased deposition and crosslinking in neoplastic matrices; on the other hand, softer regions are also found in neoplastic ECMs on the same scales. Our results support the hypothesis that local changes in the normal ECM can create the ground for growth and spread from the tumour of invading metastatic cells. We have found correlations between the mechanical properties (relative stiffening between normal and neoplastic ECM) of the ECM and patients' clinical data, like age, sex, presence of protein activating mutations in BRAF and KRAS genes and tumour grade. Our findings suggest that the mechanical phenotyping of PM-ECM has the potential to predict tumour development.

The energetics of rapid cellular mechanotransduction

Cells throughout the human body detect mechanical forces. While it is known that the rapid (millisecond) detection of mechanical forces is mediated by force-gated ion channels, a detailed quantitative understanding of cells as sensors of mechanical energy is still lacking. Here, we combine atomic force microscopy with patch-clamp electrophysiology to determine the physical limits of cells expressing the force-gated ion channels (FGICs) Piezo1, Piezo2, TREK1, and TRAAK. We find that, depending on the ion channel expressed, cells can function either as proportional or nonlinear transducers of mechanical energy and detect mechanical energies as little as ~100 fJ, with a resolution of up to ~1 fJ. These specific energetic values depend on cell size, channel density, and cytoskeletal architecture. We also make the surprising discovery that cells can transduce forces either nearly instantaneously (<1 ms) or with a substantial time delay (~10 ms). Using a chimeric experimental approach and simulations, we show how such delays can emerge from channel-intrinsic properties and the slow diffusion of tension in the membrane. Overall, our experiments reveal the capabilities and limits of cellular mechanosensing and provide insights into molecular mechanisms that different cell types may employ to specialize for their distinct physiological roles.

AFM evaluation of a humanized recombinant antibody affecting C. auris cell wall and stability

Fungal infections are increasingly impacting on the health of the population and particularly on subjects with a compromised immune system. The resistance phenomenon and the rise of new species carrying sometimes intrinsic and multi-drug resistance to the most commonly used antifungal drugs are greatly concerning healthcare organizations. As a result of this situation, there is growing interest in the development of therapeutic agents against pathogenic fungi. In particular, the Candida genus is responsible for severe life-threatening infections and among its species, C. auris is considered an urgent threat by the Center for Disease Control and Prevention, and is one of the three leading causes of morbidity and mortality worldwide. H5K1 is a humanized monoclonal antibody (hmAb) that selectively binds to β-1,3-glucans, vital components of the fungal cell wall. It has been previously demonstrated that it is active against Candida species, especially against C. auris, reaching its greatest potential when combined with commercially available antifungal drugs. Here we used atomic force microscopy (AFM) to assess the effects of H5K1, alone and in combination with fluconazole, caspofungin and amphotericin B, on C. auris cells. Through an extensive exploration we found that H5K1 has a significant role in the perturbation and remodeling of the fungal cell wall that is reflected in the loss of whole cell integrity. Moreover, it contributes substantially to the alterations in terms of chemical composition, stiffness and roughness induced specifically by caspofungin and amphotericin B. In addition to this, we demonstrated that AFM is a valuable technique to evaluate drug-microorganism interaction.

In Situ Measurements of Cell Mechanical Properties Using Force Spectroscopy

Mechanobiology focuses on how physical forces and the mechanical properties of cells and whole tissues affect their function. The mechanical properties of cells are of particular interest to developmental biology and stem cell differentiation, lymphocyte activation and phagocytic action in phagocytes, and development of malignant tumors and metastases. These properties can be measured on whole tissue and cell culture. Advances in instrument sensitivity and design, as well as improved techniques and scientific know-how achieved over the past few decades, allow researchers to study the mechanical properties of single cells and even at the subcellular level. Particularly, nanoindentation measurements using atomic force microscopy (AFM) mechanically probes single cells and even allows mapping of these traits. This chapter discusses these measurements from the experimental design to the analysis.

Quantifying the Mechanical Properties of Yeast Candida albicans Using Atomic Force Microscopy-based Force Spectroscopy

Fungi can adapt to a wide range of environmental stresses in the wild and host milieu by employing their plastic genome and great diversity in morphology. Among different adaptive strategies, mechanical stimuli, such as changes in osmotic pressure, surface remodeling, hyphal formation, and cell divisions, could guide the physical cues into physiological responses through a complex signaling network. While fungal pathogens require a pressure-driven force to expand and penetrate host tissues, quantitatively studying the biophysical properties at the host-fungal interface is critical to understand the development of fungal diseases. Microscopy-based techniques have enabled researchers to monitor the dynamic mechanics on fungal cell surface in responses to the host stress and antifungal drugs. Here, we describe a label-free, high-resolution method based on atomic force microscopy, with a step-by-step protocol to measure the physical properties in human fungal pathogen Candida albicans.

Cellular Biomechanic Impairment in Cardiomyocytes Carrying the Progeria Mutation: An Atomic Force Microscopy Investigation

Given the clinical effect of progeria syndrome, understanding the cell mechanical behavior of this pathology could benefit the patient's treatment. Progeria patients show a point mutation in the lamin A/C gene (LMNA), which could change the cell's biomechanical properties. This paper reports a mechano-dynamic analysis of a progeria mutation (c.1824 C > T, p.Gly608Gly) in neonatal rat ventricular myocytes (NRVMs) using cell indentation by atomic force microscopy to measure alterations in beating force, frequency, and contractile amplitude of selected cells within cell clusters. Furthermore, we examined the beating rate variability using a time-domain method that produces a Poincaré plot because beat-to-beat changes can shed light on the causes of arrhythmias. Our data have been further related to our cell phenotype findings, using immunofluorescence and calcium transient analysis, showing that mutant NRVMs display changes in both beating force and frequency. These changes were associated with a decreased gap junction localization (Connexin 43) in the mutant NRVMs even in the presence of a stable cytoskeletal structure (microtubules and actin filaments) when compared with controls (wild type and non-treated cells). These data emphasize the kindred between nucleoskeleton (LMNA), cytoskeleton, and the sarcolemmal structures in NRVM with the progeria Gly608Gly mutation, prompting future mechanistic and therapeutic investigations.

Surface characterization of an ultra-soft contact lens material using an atomic force microscopy nanoindentation method

As new ultra-soft materials are being developed for medical devices and biomedical applications, the comprehensive characterization of their physical and mechanical properties is both critical and challenging. To characterize the very low surface modulus of the novel biomimetic lehfilcon A silicone hydrogel contact lens coated with a layer of a branched polymer brush structure, an improved atomic force microscopy (AFM) nanoindentation method has been applied. This technique allows for precise contact-point determination without the effects of viscous squeeze-out upon approaching the branched polymer. Additionally, it allows individual brush elements to be mechanically characterized in the absence of poroelastic effects. This was accomplished by selecting an AFM probe with a design (tip size, geometry, and spring constant) that was especially suited to measuring the properties of soft materials and biological samples. The enhanced sensitivity and accuracy of this method allows for the precise measurement of the very soft lehfilcon A material, which has an extremely low elastic modulus in the surface region (as low as 2 kPa) and extremely high elasticity (nearly 100%) in an aqueous environment. The surface-characterization results not only reveal the ultra-soft nature of the lehfilcon A lens surface but also demonstrate that the elastic modulus exhibits a 30 kPa/200 nm gradient with depth due to the disparity between the modulus of the branched polymer brushes and the SiHy substrate. This surface-characterization methodology may be applied to other ultra-soft materials and medical devices.

Measuring biological materials mechanics with atomic force microscopy - Determination of viscoelastic cell properties from stress relaxation experiments

Cells are complex, viscoelastic bodies. Their mechanical properties are defined by the arrangement of semiflexible cytoskeletal fibers, their crosslinking, and the active remodeling of the cytoskeletal network. Atomic force microscopy (AFM) is an often-used technique for the study of cell mechanics, enabling time- and frequency-dependent measurements with nanometer resolution. Cells exhibit time-dependent deformation when stress is applied. In this work, we have investigated the stress relaxation of HeLa cells when subjected to a constant strain. We have varied the applied force (1, 2, 4, and 8 nN) and pause time (1, 10, and 60 s) to check for common assumptions for the use of models of linear viscoelasticity. Then, we have applied three models (standard linear solid, five element Maxwell, power law rheology) to study their suitability to fit the datasets. We show that the five element Maxwell model captures the stress relaxation response the best while still retaining a low number of free variables. This work serves as an introduction and guide when performing stress relaxation experiments on soft matter using AFM. RESEARCH HIGHLIGHTS: Cells exhibit linear viscoelastic properties when subjected to stress relaxation measurements at the studied different forces and times. The stress relaxation is best described by a five element Maxwell model. All three used models capture a softening and fluidization of cells when disrupting actin filaments.

Supracellular measurement of spatially varying mechanical heterogeneities in live monolayers

The mechanical properties of tissues have profound impacts on a wide range of biological processes such as embryo development (1,2), wound healing (3-6), and disease progression (7). Specifically, the spatially varying moduli of cells largely influence the local tissue deformation and intercellular interaction. Despite the importance of characterizing such a heterogeneous mechanical property, it has remained difficult to measure the supracellular modulus field in live cell layers with a high-throughput and minimal perturbation. In this work, we developed a monolayer effective modulus measurement by integrating a custom cell stretcher, light microscopy, and AI-based inference. Our approach first quantifies the heterogeneous deformation of a slightly stretched cell layer and converts the measured strain fields into an effective modulus field using an AI inference. This method allowed us to directly visualize the effective modulus distribution of thousands of cells virtually instantly. We characterized the mean value, SD, and correlation length of the effective cell modulus for epithelial cells and fibroblasts, which are in agreement with previous results. We also observed a mild correlation between cell area and stiffness in jammed epithelia, suggesting the influence of cell modulus on packing. Overall, our reported experimental platform provides a valuable alternative cell mechanics measurement tool that can be integrated with microscopy-based characterizations.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Narrow-Gap Rheometry: A Novel Method for Measuring Cell Mechanics

The viscoelastic properties of a cell cytoskeleton contain abundant information about the state of a cell. Cells show a response to a specific environment or an administered drug through changes in their viscoelastic properties. Studies of single cells have shown that chemical agents that interact with the cytoskeleton can alter mechanical cell properties and suppress mitosis. This envisions using rheological measurements as a non-specific tool for drug development, the pharmacological screening of new drug agents, and to optimize dosage. Although there exists a number of sophisticated methods for studying mechanical properties of single cells, studying concentration dependencies is difficult and cumbersome with these methods: large cell-to-cell variations demand high repetition rates to obtain statistically significant data. Furthermore, method-induced changes in the cell mechanics cannot be excluded when working in a nonlinear viscoelastic range. To address these issues, we not only compared narrow-gap rheometry with commonly used single cell techniques, such as atomic force microscopy and microfluidic-based approaches, but we also compared existing cell monolayer studies used to estimate cell mechanical properties. This review provides insight for whether and how narrow-gap rheometer could be used as an efficient drug screening tool, which could further improve our current understanding of the mechanical issues present in the treatment of human diseases.

Tooth Wear and Tribological Investigations in Dentistry

Dental or tooth wear is a physiological process in the life cycle of teeth. Loss of the occlusal surface may cause excessive tooth wear. Several factors may contribute to tooth wear with different intensities and duration in the oral cavity. The oral cavity is generally compared to a tribological system to determine the various types of wear between teeth and restorative materials and assess the amount of dental wear. However, it is challenging to investigate in vitro and in vivo wear owing to the complexity of tooth wear; thus, a clear correlation between in vitro and in vivo data could not be established. This review is aimed at providing an insight into the etiology of tooth wear and tribological investigations in dentistry.

Recent developments in DNA-based mechanical nanodevices

Cellular processes and functions can be regulated by mechanical forces. Nanodevices that can measure and manipulate these forces are critical tools in chemical and cellular biology. Synthetic DNA oligonucleotides have been used to develop a wide range of powerful nanodevices due to their programmable nature and precise and predictable self-assembly. In recent years, various types of DNA-based mechanical nanodevices have been engineered for studying molecular-level forces. With the help of these nanodevices, our understanding of cellular responses to physical forces has been significantly advanced. In this article, we have reviewed some recent developments in DNA-based mechanical sensors and regulators for application in the characterization of cellular biomechanics and the manipulation of cellular morphology, motion and other functions. The design principles discussed in this article can be further used to inspire other types of powerful DNA-based mechanical nanodevices.

Atomic Force Microscopy (AFM) Applications in Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an inherited heart muscle disorder characterized by progressive replacement of cardiomyocytes by fibrofatty tissue, ventricular dilatation, cardiac dysfunction, arrhythmias, and sudden cardiac death. Interest in molecular biomechanics for these disorders is constantly growing. Atomic force microscopy (AFM) is a well-established technic to study the mechanobiology of biological samples under physiological and pathological conditions at the cellular scale. However, a review which described all the different data that can be obtained using the AFM (cell elasticity, adhesion behavior, viscoelasticity, beating force, and frequency) is still missing. In this review, we will discuss several techniques that highlight the potential of AFM to be used as a tool for assessing the biomechanics involved in ACM. Indeed, analysis of genetically mutated cells with AFM reveal abnormalities of the cytoskeleton, cell membrane structures, and defects of contractility. The higher the Young’s modulus, the stiffer the cell, and it is well known that abnormal tissue stiffness is symptomatic of a range of diseases. The cell beating force and frequency provide information during the depolarization and repolarization phases, complementary to cell electrophysiology (calcium imaging, MEA, patch clamp). In addition, original data is also presented to emphasize the unique potential of AFM as a tool to assess fibrosis in cardiac tissue.

Is It Possible to Directly Determine the Radius of a Spherical Indenter Using Force Indentation Data on Soft Samples?

An important factor affecting the accuracy of Young's modulus calculation in Atomic Force Microscopy (AFM) indentation experiments is the determination of the dimensions of the indenter. This procedure is usually performed using AFM calibration gratings or Scanning Electron Microscopy (SEM) imaging. However, the aforementioned procedure is frequently omitted because it requires additional equipment. In this paper, a new approach is presented that focused on the calibration of spherical indenters without the need of special equipment but instead using force indentation data on soft samples. Firstly, the question whether it is mathematically possible to simultaneously calculate the indenter's radius and the Young's modulus of the tested sample (under the restriction that the sample presents a linear elastic response) using the same force indentation data is discussed. Using a simple mathematical approach, it was proved that the aforementioned procedure is theoretically valid. In addition, to test this method in real indentation experiments agarose gels were used. Multiple measurements on different agarose gels showed that the calibration of a spherical indenter is possible and can be accurately performed. Thus, the indenter's radius and the soft sample's Young's modulus can be determined using the same force indentation data. It is also important to note that the provided accuracy is similar to the accuracy obtained when using AFM calibration gratings. The major advantage of this paper is that it provides a method for the simultaneous determination of the indenter's radius and the sample's Young's modulus without requiring any additional equipment.

AFM Stiffness Mapping in Human Aortic Smooth Muscle Cells

Aortic Smooth Muscle Cells (SMCs) play a vital role in maintaining mechanical homeostasis in the aorta. We recently found that SMCs of aneurysmal aortas apply larger traction forces than SMCs of healthy aortas. This result was explained by the significant increase of hypertrophic SMCs abundance in aneurysms. In the present study, we investigate whether the cytoskeleton stiffness of SMCs may also be altered in aneurysmal aortas. For that, we use Atomic Force Microscopy (AFM) nanoindentation with a specific mode that allows subcellular-resolution mapping of the local stiffness across a specified region of interest of the cell. Aortic SMCs from a commercial human lineage (AoSMCs, Lonza) and primary aneurysmal SMCs (AnevSMCs) are cultured in conditions promoting the development of their contractile apparatus, and seeded on hydrogels with stiffness properties of 12kPa and 25kPa. Results show that all SMC exhibit globally a lognormal stiffness distribution, with medians in the range 10-30 kPa. The mean of stiffness distributions is slightly higher in aneurysmal SMCs than in healthy cells (16 kPa versus 12 kPa) but the differences are not statistically significant due to the large dispersion of AFM nanoindentation stiffness. We conclude that the possible alterations previously found in aneurysmal SMCs do not affect significantly the AFM nanoindentation stiffness of their cytoskeleton.

The protective effect of biomineralized BSA-Mn3O4 nanoparticles on HUVECs investigated by atomic force microscopy

BSA-Mn3O4 nanoparticles were successfully synthesized by one-step biomineralization method, the mechanism of antioxidant activity of BSA-Mn3O4 nanoparticles in HUVECs was investigated from the perspective of biomechanics.

Measuring Mechanical Properties of Breast Cancer Cells with Atomic Force Microscopy

Using physical models to describe the response of cells to external stimuli has grown popular in the last three decades. The mechanical properties of cells are tightly linked to biochemical signaling pathways related to cell structure, proliferation, differentiation, motility, and cell fate. This chapter aims to describe how to perform mechanical experiments on MCF-7 breast cancer cells using the atomic force microscope. We present a stepwise procedure on sample preparation, force spectroscopy measurements and a guide on data evaluation using standard rheological models. We demonstrate how to derive all viscoelastic parameters of the cell by conducting stress relaxation and creep experiments. Additionally, the reader can find a sample dataset and the code required for data evaluation.

Substrate stiffness modulates the viscoelastic properties of MCF-7 cells

Cells sense stiffness of surrounding tissues and adapt their activity, proliferation, motility and mechanical properties based on such interactions. Cells probe the stiffness of the substrate by anchoring and pulling to their surroundings, transmitting force to the extracellular matrix and other cells, and respond to the resistance they sense, mainly through changes in their cytoskeleton. Cancer and other diseases alter stiffness of tissues, and the response of cancer cells to this stiffness can also be affected. In the present study we show that MCF-7 breast cancer cells seeded on polyacrylamide gels have the ability to detect the stiffness of the substrate and alter their mechanical properties in response. MCF-7 cells plated on soft substrates display lower stiffness and viscosity when compared to those seeded on stiffer gels or glass. These differences can be associated with differences in the morphology and cytoskeleton organisation, since cells seeded on soft substrates have a round morphology, while cells seeded on stiffer substrates acquire a flat and spread morphology with formation of actin filaments, similar to that observed when seeded on glass. These findings show that MCF-7 cells can detect the stiffness of the surrounding microenvironment and thus, modify their mechanical properties.

Learning from Nature: Bacterial Spores as a Target for Current Technologies in Medicine (Review)

The capability of some representatives of Clostridium spp. and Bacillus spp. genera to form spores in extreme external conditions long ago became a subject of medico-biological investigations. Bacterial spores represent dormant cellular forms of gram-positive bacteria possessing a high potential of stability and the capability to endure extreme conditions of their habitat. Owing to these properties, bacterial spores are recognized as the most stable systems on the planet, and spore-forming microorganisms became widely spread in various ecosystems.

Spore-forming bacteria have been attracted increased interest for years due to their epidemiological danger. Bacterial spores may be in the quiescent state for dozens or hundreds of years but after they appear in the favorable conditions of a human or animal organism, they turn into vegetative forms causing an infectious process. The greatest threat among the pathogenic spore-forming bacteria is posed by the causative agents of anthrax (B. anthracis), food toxicoinfection (B. cereus), pseudomembranous colitis (C. difficile), botulism (C. botulinum), gas gangrene (C. perfringens).

For the effective prevention of severe infectious diseases first of all it is necessary to study the molecular structure of bacterial spores and the biochemical mechanisms of sporulation and to develop innovative methods of detection and disinfection of dormant cells. There is another side of the problem: the necessity to investigate exo- and endospores from the standpoint of obtaining similar artificially synthesized models in order to use them in the latest medical technologies for the development of thermostable vaccines, delivery of biologically active substances to the tissues and intracellular structures. In recent years, bacterial spores have become an interesting object for the exploration from the point of view of a new paradigm of unicellular microbiology in order to study microbial heterogeneity by means of the modern analytical tools.

Studying the effect of PDA@CeO2 nanoparticles with antioxidant activity on the mechanical properties of cells

Studying the influence of nanomaterials on the microstructure and mechanical properties of cells is essential to guide the biological applications of nanomaterials. In this article, the effects of the first synthesized PDA@CeO₂ nanoparticles (NPs) with multiple ROS scavenging activities on cell ultra-morphology and mechanical properties were investigated by atomic force microscopy (AFM). After the cells were exposed to PDA@CeO₂ NPs, there was no obvious change in cell morphology, but the Young's modulus of the cells was increased. On the contrary, after the cells were damaged by H₂O₂, the secreted molecules appeared on the cell surface, and the Young's modulus was decreased significantly. However, PDA@CeO₂ NPs could effectively inhibit the reduction of the Young's modulus caused by oxidative stress damage. PDA@CeO₂ NPs could also protect F-actin from oxidative stress damage and maintain the stability of the cytoskeleton. This work investigates the intracellular antioxidant mechanism of nanomaterials from the changes in the microstructure and biomechanics of living cells, providing a new analytical approach to explore the biological effects of nanomaterials.

Burger Model as the Best Option for Modeling of Viscoelastic Behavior of Resists for Nanoimprint Lithography

In this study, Atomic Force Microscopy-based nanoindentation (AFM-NI) with diamond-like carbon (DLC) coated tip was used to analyze the mechanical response of poly(methyl methacrylate) (PMMA) thin films (thicknesses: 235 and 513 nm) on a silicon substrate. Then, Oliver and Pharr (OP) model was used to calculate hardness and Young’s modulus, while three different Static Linear Solid models were used to fit the creep curve and measure creep compliance, Young’s modulus, and viscosity. Values were compared with each other, and the best-suited method was suggested. The impact of four temperatures below the glass transition temperature and varied indentation depth on the mechanical properties has been analyzed. The results show high sensitivity on experiment parameters and there is a clear difference between thin and thick film. According to the requirements in the nanoimprint lithography (NIL), the ratio of hardness at demolding temperature to viscosity at molding temperature was introduced as a simple parameter for prediction of resist suitability for NIL. Finally, thinner PMMA film was tentatively attributed as more suitable for NIL.

Viscoelasticity and Noise Properties Reveal the Formation of Biomemory in Cells

Living cells are neither perfectly elastic nor liquid and return a viscoelastic response to external stimuli. Nanoindentation provides force–distance curves, allowing the investigation of cell mechanical properties, and yet, these curves can differ from point to point on the cell surface, revealing its inhomogeneous character. In the present work, we propose a mathematical method to estimate both viscoelastic and noise properties of cells as these are depicted on the values of the scaling exponents of relaxation function and power spectral density, respectively. The method uses as input the time derivative of the response force in a nanoindentation experiment. Generalized moments method and/or rescaled range analysis is used to study the resulting time series depending on their nonstationary or stationary nature. We conducted experiments in living Ulocladium chartarum spores. We found that spores in the approaching phase present a viscoelastic behavior with the corresponding scaling exponent in the range 0.25–0.52 and in the retracting phase present a liquid-like behavior with exponents in the range 0.67–0.85. This substantial difference of the scaling exponents in the two phases suggests the formation of biomemory as a response of the spores to the indenting AFM mechanical stimulus. The retracting phase may be described as a process driven by bluish noises, while the approaching one is driven by persistent noise.

A designer cell culture insert with a nanofibrous membrane toward engineering an epithelial tissue model validated by cellular nanomechanics

Engineered platforms for culturing cells of the skin and other epithelial tissues are useful for the regeneration and development of in vitro tissue models used in drug screening. Recapitulating the biomechanical behavior of the cells is one of the important hallmarks of successful tissue generation on these platforms. The biomechanical behavior of cells profoundly affects the physiological functions of the generated tissue. In this work, a designer nanofibrous cell culture insert (NCCI) device was developed, consisting of a free-hanging polymeric nanofibrous membrane. The free-hanging nanofibrous membrane has a well-tailored architecture, stiffness, and topography to better mimic the extracellular matrix of any soft tissue than conventional, flat tissue culture polystyrene (TCPS) surfaces. Human keratinocytes (HaCaT cells) cultured on the designer NCCIs exhibited a 3D tissue-like phenotype compared to the cells cultured on TCPS. Furthermore, the biomechanical characterization by bio-atomic force microscopy (Bio-AFM) revealed a markedly altered cellular morphology and stiffness of the cellular cytoplasm, nucleus, and cell-cell junctions. The nuclear and cytoplasmic moduli were reduced, while the stiffness of the cellular junctions was enhanced on the NCCI compared to cells on TCPS, which are indicative of the fluidic state and migratory phenotype on the NCCI. These observations were corroborated by immunostaining, which revealed enhanced cell-cell contact along with a higher expression of junction proteins and enhanced migration in a wound-healing assay. Taken together, these results underscore the role of the novel designer NCCI device as an in vitro platform for epithelial cells with several potential applications, including drug testing, disease modeling, and tissue regeneration.

Research Techniques Made Simple: Analysis of Skin Cell and Tissue Mechanics Using Atomic Force Microscopy

Atomic force microscopy (AFM) is a powerful technique for nanoscale imaging and mechanical analysis of biological specimens. It is based on the highly sensitive detection of forces and displacement of a sharp-tipped cantilever as it scans the surface of an object. Because it requires minimal sample processing and preparation, AFM is particularly advantageous for the analysis of cells and tissues in their near-native state. Moreover, recent advances in Bio-AFM systems and the combination with light microscopy imaging have greatly enhanced the application of AFM in biological research. In the field of dermatology, the method has led to important insights into our understanding of the biomechanics of normal healthy skin and the pathogenesis of a variety of skin diseases. In this Research Techniques Made Simple article, we review the fundamental principles of AFM, how AFM can be applied to the analysis of cell and tissue mechanics, and recent applications of AFM in skin science and dermatology.

Hierarchical micro-/nanotopography for tuning structures and mechanics of cells probed by atomic force microscopy

Extracellular matrix plays an important role in regulating the behaviors of cells, and utilizing matrix physics to control cell fate has been a promising way for cell and tissue engineering. However, the nanoscale situations taking place during the topography-regulated cell-matrix interactions are still not fully understood to the best of our knowledge. The invention of atomic force microscopy (AFM) provides a powerful tool to characterize the structures and properties of living biological systems under aqueous conditions with unprecedented spatial resolution. In this work, with the use of AFM, structural and mechanical dynamics of individual cells grown on micro-/nanotopographical surface were revealed. First, the microgroove patterned silicon substrates were fabricated by photolithography. Next, nanogranular topography was formed on microgroove substrates by cell culture medium protein deposition, which was visualized by in situ AFM imaging. The micro-/nanotopographical substrates were then used to grow two types of cells (3T3 cell or MCF-7 cell). AFM morphological imaging and mechanical measurements were applied to characterize the changes of cells grown on the micro-/nanotopographical substrates. The experimental results showed the significant alterations in cellular structures and cellular mechanics caused by micro-/nanotopography. The study provides a novel way based on AFM to unveil the native nanostructures and mechanical properties of cell-matrix interfaces with high spatial resolution in liquids, which will have potential impacts on the studies of topography-tuned cell behaviors.

Mechanical Control of Cell Migration by the Metastasis Suppressor Tetraspanin CD82/KAI1

The plasma membrane is a key actor of cell migration. For instance, its tension controls persistent cell migration and cell surface caveolae integrity. Then, caveolae constituents such as caveolin-1 can initiate a mechanotransduction loop that involves actin- and focal adhesion-dependent control of the mechanosensor YAP to finely tune cell migration. Tetraspanin CD82 (also named KAI-1) is an integral membrane protein and a metastasis suppressor. Its expression is lost in many cancers including breast cancer. It is a strong inhibitor of cell migration by a little-known mechanism. We demonstrated here that CD82 controls persistent 2D migration of EGF-induced single cells, stress fibers and focal adhesion sizes and dynamics. Mechanistically, we found that CD82 regulates membrane tension, cell surface caveolae abundance and YAP nuclear translocation in a caveolin-1-dependent manner. Altogether, our data show that CD82 controls 2D cell migration using membrane-driven mechanics involving caveolin and the YAP pathway.

Estrogen Modulates Epithelial Breast Cancer Cell Mechanics and Cell-to-Cell Contacts

Excessive estrogen exposure is connected with increased risk of breast cancer and has been shown to promote epithelial-mesenchymal-transition. Malignant cancer cells accumulate changes in cell mechanical and biochemical properties, often leading to cell softening. In this work we have employed atomic force microscopy to probe the influence of estrogen on the viscoelastic properties of MCF-7 breast cancer cells cultured either in normal or hormone free-medium. Estrogen led to a significant softening of the cells in all studied cases, while growing cells in hormone free medium led to an increase in the studied elastic and viscoelastic moduli. In addition, fluorescence microscopy shows that E-cadherin distribution is changed in cells when culturing them under estrogenic conditions. Furthermore, cell-cell contacts seemed to be weakened. These results were supported by AFM imaging showing changes in surfaces roughness, cell-cell contacts and cell height as result of estrogen treatment. This study therefore provides further evidence for the role of estrogen signaling in breast cancer.

Focal adhesion dynamics in cellular function and disease

Acting as a bridge between the cytoskeleton of the cell and the extra cellular matrix (ECM), the cell-ECM adhesions with integrins at their core, play a major role in cell signalling to direct mechanotransduction, cell migration, cell cycle progression, proliferation, differentiation, growth and repair. Biochemically, these adhesions are composed of diverse, yet an organised group of structural proteins, receptors, adaptors, various enzymes including protein kinases, phosphatases, GTPases, proteases, etc. as well as scaffolding molecules. The major integrin adhesion complexes (IACs) characterised are focal adhesions (FAs), invadosomes (podosomes and invadopodia), hemidesmosomes (HDs) and reticular adhesions (RAs). The varied composition and regulation of the IACs and their signalling, apart from being an integral part of normal cell survival, has been shown to be of paramount importance in various developmental and pathological processes. This review per-illustrates the recent advancements in the research of IACs, their crucial roles in normal as well as diseased states. We have also touched on few of the various methods that have been developed over the years to visualise IACs, measure the forces they exert and study their signalling and molecular composition. Having such pertinent roles in the context of various pathologies, these IACs need to be understood and studied to develop therapeutical targets. We have given an update to the studies done in recent years and described various techniques which have been applied to study these structures, thereby, providing context in furthering research with respect to IAC targeted therapeutics.

Peak force tapping atomic force microscopy for advancing cell and molecular biology

The advent of atomic force microscopy (AFM) provides an exciting tool to detect molecular and cellular behaviors under aqueous conditions. AFM is able to not only visualize the surface topography of the specimens, but also can quantify the mechanical properties of the specimens by force spectroscopy assay. Nevertheless, integrating AFM topographic imaging with force spectroscopy assay has long been limited due to the low spatiotemporal resolution. In recent years, the appearance of a new AFM imaging mode called peak force tapping (PFT) has shattered this limit. PFT allows AFM to simultaneously acquire the topography and mechanical properties of biological samples with unprecedented spatiotemporal resolution. The practical applications of PFT in the field of life sciences in the past decade have demonstrated the excellent capabilities of PFT in characterizing the fine structures and mechanics of living biological systems in their native states, offering novel possibilities to reveal the underlying mechanisms guiding physiological/pathological activities. In this paper, the recent progress in cell and molecular biology that has been made with the utilization of PFT is summarized, and future perspectives for further progression and biomedical applications of PFT are provided.

Protective effect of syringic acid via restoring cells biomechanics and organelle structure in human lens epithelial cells

We have previously reported that syringic acid (SA) extracted from D. aurantiacum var. denneanum (kerr) may be used to prevent diabetic cataract (DC). However, the underlying mechanisms through which SA prevents DC in human lens epithelial cells (HLECs) remained unclear. In the present study, we employed single-molecule optics technologies, including transmission electron microscopy (TEM), atomic force microscopy (AFM), laser scanning confocal microscopy (LSCM) and Raman spectroscopy, to monitor the effect of SA on HLECs biomechanics and organelle structure in real-time. TEM suggested that SA improved the ultrastructure of HLECs with regard to nuclear chromatin condensation and reducing mitochondrial swelling and degeneration, which may aid in the maintenance of HLECs integrity in the presence of glucose. AFM revealed a reduced surface roughness and stiffness following SA treatment, suggesting an improved viscoelasticity of HELCs. Raman spectrometry and LSCM further revealed that these changes were related to a modification of cell liquidity and cytoskeletal structure by SA. Taken together, these results provide insights into the effects of SA on the biomechanics of HLECs and further strengthen the evidence for its potential use as a novel therapeutic strategy for DC prevention.

Atomic force microscopy for revealing micro/nanoscale mechanics in tumor metastasis: from single cells to microenvironmental cues

Mechanics are intrinsic properties which appears throughout the formation, development, and aging processes of biological systems. Mechanics have been shown to play important roles in regulating the development and metastasis of tumors, and understanding tumor mechanics has emerged as a promising way to reveal the underlying mechanisms guiding tumor behaviors. In particular, tumors are highly complex diseases associated with multifaceted factors, including alterations in cancerous cells, tissues, and organs as well as microenvironmental cues, indicating that investigating tumor mechanics on multiple levels is significantly helpful for comprehensively understanding the effects of mechanics on tumor progression. Recently, diverse techniques have been developed for probing the mechanics of tumors, among which atomic force microscopy (AFM) has appeared as an excellent platform enabling simultaneously characterizing the structures and mechanical properties of living biological systems ranging from individual molecules and cells to tissue samples with unprecedented spatiotemporal resolution, offering novel possibilities for understanding tumor physics and contributing much to the studies of cancer. In this review, we survey the recent progress that has been achieved with the use of AFM for revealing micro/nanoscale mechanics in tumor development and metastasis. Challenges and future progress are also discussed.

Nuclear Deformation Causes DNA Damage by Increasing Replication Stress

Cancer metastasis, i.e., the spreading of tumor cells from the primary tumor to distant organs, is responsible for the vast majority of cancer deaths. In the process, cancer cells migrate through narrow interstitial spaces substantially smaller in cross-section than the cell. During such confined migration, cancer cells experience extensive nuclear deformation, nuclear envelope rupture, and DNA damage. The molecular mechanisms responsible for the confined migration-induced DNA damage remain incompletely understood. Although in some cell lines, DNA damage is closely associated with nuclear envelope rupture, we show that, in others, mechanical deformation of the nucleus is sufficient to cause DNA damage, even in the absence of nuclear envelope rupture. This deformation-induced DNA damage, unlike nuclear-envelope-rupture-induced DNA damage, occurs primarily in S/G2 phase of the cell cycle and is associated with replication forks. Nuclear deformation, resulting from either confined migration or external cell compression, increases replication stress, possibly by increasing replication fork stalling, providing a molecular mechanism for the deformation-induced DNA damage. Thus, we have uncovered a new mechanism for mechanically induced DNA damage, linking mechanical deformation of the nucleus to DNA replication stress. This mechanically induced DNA damage could not only increase genomic instability in metastasizing cancer cells but could also cause DNA damage in non-migrating cells and tissues that experience mechanical compression during development, thereby contributing to tumorigenesis and DNA damage response activation.

Evaluation of quasi-static and dynamic nanomechanical properties of bone-metastatic breast cancer cells using a nanoclay cancer testbed

In recent years, there has been increasing interest in investigating the mechanical properties of individual cells to delineate disease mechanisms. Reorganization of cytoskeleton facilitates the colonization of metastatic breast cancer at bone marrow space, leading to bone metastasis. Here, we report evaluation of mechanical properties of two breast cancer cells with different metastatic ability at the site of bone metastases, using quasi-static and dynamic nanoindentation methods. Our results showed that the significant reduction in elastic modulus along with increased liquid-like behavior of bone metastasized MCF-7 cells was induced by depolymerization and reorganization of F-actin to the adherens junctions, whereas bone metastasized MDA-MB-231 cells showed insignificant changes in elastic modulus and F-actin reorganization over time, compared to their respective as-received counterparts. Taken together, our data demonstrate evolution of breast cancer cell mechanics at bone metastases.

Nucleotides-Induced Changes in the Mechanical Properties of Living Endothelial Cells and Astrocytes, Analyzed by Atomic Force Microscopy

Endothelial cells and astrocytes preferentially express metabotropic P2Y nucleotide receptors, which are involved in the maintenance of vascular and neural function. Among these, P2Y₁ and P2Y₂ receptors appear as main actors, since their stimulation induces intracellular calcium mobilization and activates signaling cascades linked to cytoskeletal reorganization. In the present work, we have analyzed, by means of atomic force microscopy (AFM) in force spectroscopy mode, the mechanical response of human umbilical vein endothelial cells (HUVEC) and astrocytes upon 2MeSADP and UTP stimulation. This approach allows for simultaneous measurement of variations in factors such as Young’s modulus, maximum adhesion force and rupture event formation, which reflect the potential changes in both the stiffness and adhesiveness of the plasma membrane. The largest effect was observed in both endothelial cells and astrocytes after P2Y₂ receptor stimulation with UTP. Such exposure to UTP doubled the Young’s modulus and reduced both the adhesion force and the number of rupture events. In astrocytes, 2MeSADP stimulation also had a remarkable effect on AFM parameters. Additional studies performed with the selective P2Y₁ and P2Y₁₃ receptor antagonists revealed that the 2MeSADP-induced mechanical changes were mediated by the P2Y₁₃ receptor, although they were negatively modulated by P2Y₁ receptor stimulation. Hence, our results demonstrate that AFM can be a very useful tool to evaluate functional native nucleotide receptors in living cells.

Mechanobiological evaluation of prostate cancer metastasis to bone using an in vitro prostate cancer testbed

Prostate cancer exhibits a propensity to metastasize to the bone, which often leads to fatality. Bone metastasis is characterized by complex biochemical, morphological, pathophysiological, and genetic changes to cancer cells as they colonize at bone sites. In this study, we report the evaluation of MDA PCa2b prostate cancer cells' nanomechanical properties during the mesenchymal-to-epithelial transition (MET) and during disease progression at the metastatic site. Bone-mimetic tissue-engineered 3D nanoclay scaffolds have been used to create in vitro metastatic site for prostate cancer. A significant softening of the prostate cancer cells during MET and further softening as disease progression occurs at metastasis is also reported. The significant reduction in elastic modulus of prostate cancer cells during MET was attributed to actin reorganization and depolymerization. This study provides input towards direct nanomechanical measurements to evaluate the time evolution of cells' mechanical behavior in tumors at bone metastasis site.

Measuring (biological) materials mechanics with atomic force microscopy. 2. Influence of the loading rate and applied force (colloidal particles)

Atomic force microscopy (AFM) is the most often used tool to study the mechanical properties of eukaryotic cells. Due to their complex assembly, cells show viscoelastic properties. When performing experiments, one has to consider the influence of both loading rate and maximum load on the measured mechanical properties. Here, we employed colloidal particles of various sizes (from 2 to 20 μm diameter) to perform force spectroscopy measurements on endothelial cells at loading rates varying from 0.1 to 50 μm/s, and maximum loads ranging from 1 to 25 nN. We were able to determine the non-linear dependence of cell viscoelastic properties on the loading rate which followed a weak power law. In addition, we show that previous loading at high forces leads to a stiffening of cells. Based on these results we discuss a road map for determining cell mechanical properties using AFM. Finally, this work provides an experimental framework for cell mechanical measurements using force-cycle experiments.