Measuring biomaterials mechanics with atomic force microscopy. 1. Influence of the loading rate and applied force (pyramidal tips)

Multi-scale characterization and analysis of cellular viscoelastic mechanical phenotypes by atomic force microscopy

The viscoelasticity of cells serves as a biomarker that reveals changes induced by malignant transformation, which aids the cytological examinations. However, differences in the measurement methods and parameters have prevented the consistent and effective characterization of the viscoelastic phenotype of cells. To address this issue, nanomechanical indentation experiments were conducted using an atomic force microscope (AFM). Multiple indentation methods were applied, and the indentation parameters were gradually varied to measure the viscoelasticity of normal liver cells and cancerous liver cells to create a database. This database was employed to train machine-learning algorithms in order to analyze the differences in the viscoelasticity of different types of cells and as well as to identify the optimal measurement methods and parameters. These findings indicated that the measurement speed significantly influenced viscoelasticity and that the classification difference between the two cell types was most evident at 5 μm/s. In addition, the precision and the area under the receiver operating characteristic curve were comparatively analyzed for various widely employed machine-learning algorithms. Unlike previous studies, this research validated the effectiveness of measurement parameters and methods with the assistance of machine-learning algorithms. Furthermore, the results confirmed that the viscoelasticity obtained from the multiparameter indentation measurement could be effectively used for cell classification. RESEARCH HIGHLIGHTS: This study aimed to analyze the viscoelasticity of liver cancer cells and liver cells. Different nano-indentation methods and parameters were used to measure the viscoelasticity of the two kinds of cells. The neural network algorithm was used to reverse analyze the dataset, and the methods and parameters for accurate classification and identification of cells are successfully found.

Elasticity, an often-overseen parameter in the development of nanoscale drug delivery systems

Nanoparticles have shown an enormous potential as drug delivery systems in the lab. However, translation to the clinics or even market approval often fails. So far, the reason for this discrepancy is manifold. Physicochemical properties such as size, surface potential, and surface chemistry are in focus of research for many years. Other equally important parameters, influencing whether a successful drug delivery can be achieved, are mechanical properties of nanoparticles. Even though this is often not even considered during formulation development, and it is not requested for approval, an increasing number of studies show that it is important to have knowledge about these characteristics. In this article, we discuss examples highlighting the influence of elasticity in nanoscale biological interactions focusing on mucosal delivery and on tumor targeting. Besides this, we discuss the influence of different measurement settings using atomic force microscopy for the determination of mechanical properties of drug carriers.

Micromechanical characterizations and viscoelastic modeling reveal elastic and viscoelastic heterogeneities in ovarian tissue and the significant viscoelastic contribution to the apparent elastic modulus determined by AFM indentation

Ovarian follicles develop in a highly regulated mechanical microenvironment and disruptions to the microenvironment may cause infertility. However, the viscoelastic properties of the ovarian tissue are not well studied. Here, we characterize both the elastic and viscoelastic properties of ovarian tissue from both reproductively older and younger domestic cats using atomic force microscopy (AFM) indentation and viscoelastic models of stress relaxation. Importantly, our analyses reveal the apparent elastic modulus obtained from the conventional AFM indentation measurement is significantly higher than the intrinsic elastic modulus and insignificantly different from the equivalent elastic modulus that is the summation of the intrinsic elastic modulus and the viscoelastic contribution to modulus at time 0. Interestingly, the ovarian cortex of both reproductive age groups has a higher apparent/intrinsic modulus than that of the medulla. Furthermore, two different kinetics of stress relaxation are identified with rate constants of ∼1 s and ∼20-40 s, respectively. Moreover, the rate constant of the slow kinetics is significantly different between the cortex and medulla in the reproductively older ovaries. Finally, these mechanical heterogeneities appear to follow the heterogeneous distribution of hyaluronic acid (HA) in the ovary. These findings may be invaluable to the development of biomimetic follicle culture for treating infertility. STATEMENT OF SIGNIFICANCE: This study investigates not only elastic but also the viscoelastic heterogeneity in both reproductively younger and older ovarian tissues for the first time. Further, by combining AFM indentation measurement and viscoelastic modeling, we show the apparent elastic modulus conventionally reported in the literature for AFM indentation measurement is the summation of the intrinsic elastic modulus and a significant viscoelastic contribution to the modulus at time 0. This is an important consideration for others who use this method to quantify biomaterial properties. In addition, the possible connection between the mechanical and compositional heterogeneities is explored. These findings may be invaluable for designing biomaterials to recapitulate the mechanical environment of the ovary and possibly many other organs for biomimetic tissue engineering.

Recombinant Peptide Production Softens Escherichia coli Cells and Increases Their Size during C-Limited Fed-Batch Cultivation

Stress-associated changes in the mechanical properties at the single-cell level of Escherichia coli (E. coli) cultures in bioreactors are still poorly investigated. In our study, we compared peptide-producing and non-producing BL21(DE3) cells in a fed-batch cultivation with tightly controlled process parameters. The cell growth, peptide content, and cell lysis were analysed, and changes in the mechanical properties were investigated using atomic force microscopy. Recombinant-tagged somatostatin-28 was expressed as soluble up to 197 ± 11 mg g-1. The length of both cultivated strains increased throughout the cultivation by up to 17.6%, with nearly constant diameters. The peptide-producing cells were significantly softer than the non-producers throughout the cultivation, and respective Young's moduli decreased by up to 57% over time. A minimum Young's modulus of 1.6 MPa was observed after 23 h of the fed-batch. Furthermore, an analysis of the viscoelastic properties revealed that peptide-producing BL21(DE3) appeared more fluid-like and softer than the non-producing reference. For the first time, we provide evidence that the physical properties (i.e., the mechanical properties) on the single-cell level are significantly influenced by the metabolic burden imposed by the recombinant peptide expression and C-limitation in bioreactors.

Micro-scale probing of the Rat's oviduct detects its viscoelastic property needed for creating a biologically relevant substrate for In-Vitro- Fertilization

Techniques used in assisted reproductive technology such as In-Vitro- Fertilization (IVF) process, often only replicate the biomechanical environment for embryo. Despite its importance, the biomechanics of the Oviduct tissue that is usually called Fallopian Tube in Human, the natural site of fertilization, has not been replicated nor sufficiently studied. This work studies the time-independent and time-dependent biomechanics of the oviduct tissue by realizing a viscoelastic model that accurately fit on the experimental indentation data collected on the mucosal epithelial lining of the oviduct tissue of rats. Nano-scale experiments with varying indentation rates ranging from 0.3 to 8 μms were conducted using atomic force microscopy (AFM) resulting in instantaneous elastic modulus ranging from 0.86 MPa to 6.46 MPa correspondingly. This result showed strong time dependency of the mechanical properties of the oviduct. An improved viscoelastic equation based on the fractional viscoelastic model was proposed. This modified relation successfully captured all the experimental data found at different rates (R² > 0.8). Using the proposed model, the pure elasticity of the oviduct (i.e., about 317.2 kPa) and the viscoelastic parameters were found.

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.

A-to-I RNA editing of Filamin A regulates cellular adhesion, migration and mechanical properties

A-to-I RNA editing by ADARs is an abundant epitranscriptomic RNA-modification in metazoa. In mammals, Flna pre-mRNA harbours a single conserved A-to-I RNA editing site that introduces a Q-to-R amino acid change in Ig repeat 22 of the encoded protein. Previously, we showed that FLNA editing regulates smooth muscle contraction in the cardiovascular system and affects cardiac health. The present study investigates how ADAR2-mediated A-to-I RNA editing of Flna affects actin crosslinking, cell mechanics, cellular adhesion and cell migration. Cellular assays and AFM measurements demonstrate that the edited version of FLNA increases cellular stiffness and adhesion but impairs cell migration in both, mouse fibroblasts and human tumour cells. In vitro, edited FLNA leads to increased actin crosslinking, forming actin gels of higher stress resistance. Our study shows that Flna RNA editing is a novel regulator of cytoskeletal organisation, affecting the mechanical property and mechanotransduction of cells.

Nanoscale geometry determines mechanical biocompatibility of vertically aligned nanofibers

Vertically aligned carbon nanofibers (VACNFs) are promising material candidates for neural biosensors due to their ability to detect neurotransmitters in physiological concentrations. However, the expected high rigidity of CNFs could induce mechanical mismatch with the brain tissue, eliciting formation of a glial scar around the electrode and thus loss of functionality. We have evaluated mechanical biocompatibility of VACNFs by growing nickel-catalyzed carbon nanofibers of different lengths and inter-fiber distances. Long nanofibers with large inter-fiber distance prevented maturation of focal adhesions, thus constraining cells from obtaining a highly spread morphology that is observed when astrocytes are being contacted with stiff materials commonly used in neural implants. A silicon nanopillar array with 500 nm inter-pillar distance was used to reveal that this inhibition of focal adhesion maturation occurs due to the surface nanoscale geometry, more precisely the inter-fiber distance. Live cell atomic force microscopy was used to confirm astrocytes being significantly softer on the long Ni-CNFs compared to other surfaces, including a soft gelatin hydrogel. We also observed hippocampal neurons to mature and form synaptic contacts when being cultured on both long and short carbon nanofibers, without having to use any adhesive proteins or a glial monoculture, indicating high cytocompatibility of the material also with neuronal population. In contrast, neurons cultured on a planar tetrahedral amorphous carbon sample showed immature neurites and indications of early-stage apoptosis. Our results demonstrate that mechanical biocompatibility of biomaterials is greatly affected by their nanoscale surface geometry, which provides means for controlling how the materials and their mechanical properties are perceived by the cells. STATEMENT OF SIGNIFICANCE: Our research article shows, how nanoscale surface geometry determines mechanical biocompatibility of apparently stiff materials. Specifically, astrocytes were prevented from obtaining highly spread morphology when their adhesion site maturation was inhibited, showing similar morphology on nominally stiff vertically aligned carbon fiber (VACNF) substrates as when being cultured on ultrasoft surfaces. Furthermore, hippocampal neurons matured well and formed synapses on these carbon nanofibers, indicating high biocompatibility of the materials. Interestingly, the same VACNF materials that were used in this study have earlier also been proven to be capable for electrophysiological recordings and sensing neurotransmitters at physiological concentrations with ultra-high sensitivity and selectivity, thus providing a platform for future neural probes or smart culturing surfaces with superior sensing performance and biocompatibility.

Reduced platelet forces underlie impaired hemostasis in mouse models of MYH9-related disease

MYH9-related disease patients with mutations in the contractile protein nonmuscle myosin heavy chain IIA display, among others, macrothrombocytopenia and a mild-to-moderate bleeding tendency. In this study, we used three mouse lines, each with one point mutation in the Myh9 gene at positions 702, 1424, or 1841, to investigate mechanisms underlying the increased bleeding risk. Agonist-induced activation of Myh9 mutant platelets was comparable to controls. However, myosin light chain phosphorylation after activation was reduced in mutant platelets, which displayed altered biophysical characteristics and generated lower adhesion, interaction, and traction forces. Treatment with tranexamic acid restored clot retraction in the presence of tPA and reduced bleeding. We verified our findings from the mutant mice with platelets from patients with the respective mutation. These data suggest that reduced platelet forces lead to an increased bleeding tendency in patients with MYH9-related disease, and treatment with tranexamic acid can improve the hemostatic function.

Nanomechanical Characterization of Bacterial Polyhydroxyalkanoates Using Atomic Force Microscopy

Polyhydroxyalkanoates are a promising class of biopolymers that can allow the production of sustainable plastic materials. The mechanical properties of such materials are very important for possible industrial applications, but the amount of polymer required for common mechanical testing can be orders of magnitude more than what is possible to achieve with a lab-scale process. Nanoindentation with the Atomic Force Microscope allows an estimation of the Elastic Modulus that can be used as a preliminary measurement on PHA when only a limited amount of material is available. Poly(hydroxybutyrate-co-hydroxyvalerate) copolymers were analyzed, with moduli ranging from 528 ± 62 MPa to 1623 ± 172 MPa, according to both the composition and the crystallization kinetics.

The effects of measurement parameters on the cancerous cell nucleus characterization by atomic force microscopy in vitro

Cancer is now responsible for the major leading cause of death worldwide. It is noteworthy that lung cancer has been recognised as the highest incidence (11.6%) and mortality (18.4%) for combined sexes among a variety of cancer diseases. Therefore, it is of great value to investigate the mechanical properties of lung cancerous cells for early diagnosis. This paper focus on the influence of measurement parameters on the measured central Young's moduli of single live A549 cell in vitro based on the force spectroscopy mode of atomic force microscopy (AFM). The effects of the measurement parameters on the measured central Young's moduli were analysed by fitting the force–depth curves utilising the Sneddon model. The results revealed that the Young's moduli of A549 cells increased with the larger indentation force, higher indentation speed, less retraction time, deeper Z length and lower purity percentage of serum. The Young's moduli of cells increased first and then decreased with the increasing dwell time. Hence, this research may have potential significance to provide reference for the standardised detection of a single cancerous cell in vitro using AFM methodologies.

Cancer is now responsible for the majority leading cause of death worldwide and it is noteworthy that lung cancer has been recognised as the highest incidence (11.6%) and mortality (18.4%) for combined sexes among a variety of cancer diseases. Therefore, it is of great value to investigate the mechanical properties of lung cancerous cells for early diagnosis. This paper primarily investigated the morphological properties and the influence of measurement parameters on the measured local elastic moduli of single live A549 cell in vitro using the AFM‐based force spectroscopy mode. In practice, there are many factors for incorrect or inaccurate experimental results using AFM to measure the characteristics of live cells, such as non‐homogeneous nature of cells, probe geometry and size, mechanical analysis model, substrate stiffness and different measurement parameters. The various measurement parameters have become the huge impact factor to influence the measurement result. Hence, this research may have potential significance to provide reference for the standardised detection of a single cancerous cell in vitro using AFM methodologies.

Intranuclear HSV-1 DNA ejection induces major mechanical transformations suggesting mechanoprotection of nucleus integrity

Maintaining nuclear integrity is essential to cell survival when exposed to mechanical stress. Herpesviruses, like most DNA and some RNA viruses, put strain on the nuclear envelope as hundreds of viral DNA genomes replicate and viral capsids assemble. It remained unknown, however, how nuclear mechanics is affected at the initial stage of herpesvirus infection-immediately after viral genomes are ejected into the nuclear space-and how nucleus integrity is maintained despite an increased strain on the nuclear envelope. With an atomic force microscopy force volume mapping approach on cell-free reconstituted nuclei with docked herpes simplex type 1 (HSV-1) capsids, we explored the mechanical response of the nuclear lamina and the chromatin to intranuclear HSV-1 DNA ejection into an intact nucleus. We discovered that chromatin stiffness, measured as Young's modulus, is increased by ∼14 times, while nuclear lamina underwent softening. Those transformations could be associated with a mechanism of mechanoprotection of nucleus integrity facilitating HSV-1 viral genome replication. Indeed, stiffening of chromatin, which is tethered to the lamina meshwork, helps to maintain nuclear morphology. At the same time, increased lamina elasticity, reflected by nucleus softening, acts as a "shock absorber," dissipating the internal mechanical stress on the nuclear membrane (located on top of the lamina wall) and preventing its rupture.

Action of Mechanical Forces on Polymerization and Polymers

In this review, we summarize recent developments in the field of the mechanochemistry of polymers. The aim of the review is to consider the consequences of mechanical forces and actions on polymers and polymer synthesis. First, we review classical works on chemical reactions and polymerization processes under strong shear deformations. Then, we analyze two emerging directions of research in mechanochemistry—the role of mechanophores and, for the first time, new physical phenomena, accompanying external impulse mechanical actions on polymers. Mechanophores have been recently proposed as sensors of fatigue and cracks in polymers and composites. The effects of the high-pressure pulsed loading of polymers and composites include the Dzyaloshinskii–Moriya effect, emission of superradiation and the formation of metal nanoparticles. These effects provide deeper insight into the mechanism of chemical reactions under shear deformations and pave the way for further research in the interests of modern technologies.

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.

Reconstituted virus–nucleus system reveals mechanics of herpesvirus genome uncoating

The viral replication cycle is controlled by information transduced through both molecular and mechanical interactions. Viral infection mechanics remains largely unexplored, however, due to the complexity of cellular mechanical responses over the course of infection as well as a limited ability to isolate and probe these responses. Here, we develop an experimental system consisting of herpes simplex virus type 1 (HSV-1) capsids bound to isolated and reconstituted cell nuclei, which allows direct probing of capsid–nucleus mechanics with atomic force microscopy (AFM). Major mechanical transformations occur in the host nucleus when pressurised viral DNA ejects from HSV-1 capsids docked at the nuclear pore complexes (NPCs) on the nuclear membrane. This leads to structural rearrangement of the host chromosome, affecting its compaction. This in turn regulates viral genome replication and transcription dynamics as well as the decision between a lytic or latent course of infection. AFM probing of our reconstituted capsid–nucleus system provides high-resolution topographical imaging of viral capsid docking at the NPCs as well as force volume mapping of the infected nucleus surface, reflecting mechanical transformations associated with chromatin compaction and stiffness of nuclear lamina (to which chromatin is tethered). This experimental system provides a novel platform for investigation of virus–host interaction mechanics during viral genome penetration into the nucleus.

Mechanical test and friction-mapping on recycled polypropylene beads using atomic force microscopy

Mechanical tests at sub-micron scales using force microscopy are often used for the characterization of materials. Here we report the mechanical, tribologic, and morphological characterization of recycled polypropylene beads using force spectroscopy and lateral-force microscopy. The compression-elastic moduli calculated using the Hertzian model for polypropylene beads was between 0.448 ± 0.010 and 1.044 ± 0.057 GPa. The grain size analysis revealed a significant correlation between the grain size and measured compression-elastic moduli. Friction-maps of recycled polypropylene beads obtained using lateral-force microscopy were also reported for 25 μm² scanning areas.

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.

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.

Time- and Zinc-Related Changes in Biomechanical Properties of Human Colorectal Cancer Cells Examined by Atomic Force Microscopy

Simple Summary

We aimed to study how cellular zinc status (adequate vs. deficiency), closely related to colorectal cancer, does affect the nanomechanical properties of cell lines HT-29 and HT-29-MTX during their early proliferation (24–96 h). These properties and their variations can be characterized by means of Atomic Force Microscopy (AFM), a technique that allows perpendicular indentation of cells with a sharp nanometric tip, under controlled speed and load, while recording the real time variation of tip-to-cell interacting forces on approach, contact, and retraction segments. From each of these sections, complete information about the respective elastic modulus, relaxation behavior, and adhesion is extracted, thus identifying cell line- and zinc-related nanomechanical fingerprints. Our results show how the impact of zinc deficiency on the mechanical response of the cells underlines the relevance of monitoring the nutritional zinc status of tumor samples when analyzing cancerous tissues or single cells with AFM, particularly regarding the development and validation of biomechanical fingerprints as diagnostic markers for cancer.

Abstract

Monitoring biomechanics of cells or tissue biopsies employing atomic force microscopy (AFM) offers great potential to identify diagnostic biomarkers for diseases, such as colorectal cancer (CRC). Data on the mechanical properties of CRC cells, however, are still scarce. There is strong evidence that the individual zinc status is related to CRC risk. Thus, this study investigates the impact of differing zinc supply on the mechanical response of the in vitro CRC cell lines HT-29 and HT-29-MTX during their early proliferation (24–96 h) by measuring elastic modulus, relaxation behavior, and adhesion factors using AFM. The differing zinc supply severely altered the proliferation of these cells and markedly affected their mechanical properties. Accordingly, zinc deficiency led to softer cells, quantitatively described by 20–30% lower Young’s modulus, which was also reflected by relevant changes in adhesion and rupture event distribution compared to those measured for the respective zinc-adequate cultured cells. These results demonstrate that the nutritional zinc supply severely affects the nanomechanical response of CRC cell lines and highlights the relevance of monitoring the zinc content of cancerous cells or biopsies when studying their biomechanics with AFM in the future.

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.

Indenting soft samples (hydrogels and cells) with cantilevers possessing various shapes of probing tip

The identification of cancer-related changes in cells and tissues based on the measurements of elastic properties using atomic force microscopy (AFM) seems to be approaching clinical application. Several limiting aspects have already been discussed; however, still, no data have shown how specific AFM probe geometries are related to the biomechanical evaluation of cancer cells. Here, we analyze and compare the nanomechanical results of mechanically homogenous polyacrylamide gels and heterogeneous bladder cancer cells measured using AFM probes of various tip geometry, including symmetric and non-symmetric pyramids and a sphere. Our observations show large modulus variability aligned with both types of AFM probes used and with the internal structure of the cells. Altogether, these results demonstrate that it is possible to differentiate between compliant and rigid samples of kPa elasticity; however, simultaneously, they highlight the strong need for standardized protocols for AFM-based elasticity measurements if applied in clinical practice including the use of a single type of AFM cantilever.

Elastic hysteresis loop acts as cell deformability in erythrocyte aging

The decrease in cellular deformability shows strong correlation with erythrocyte aging. Cell deformation can be divided into passive deformation and active deformation; however, the active deformation has been ignored in previous studies. In this work, Young's moduli of age-related erythrocytes were tested by atomic force microscopy. Furthermore, the deformation and passive and active deformation values were calculated by respective areas. Our results showed that erythrocytes in the densest fraction had the highest values of the Young's modulus, deformation, and active deformation, but the lowest values of passive deformation. Moreover, values of the deformation and active deformation both increased gradually with erythrocyte aging. The present data indicate that the elastic hysteresis loop between the approach and the retract curve could be regarded as erythrocyte deformability, and cellular deformability could be characterized by energy states. In addition, active deformation might be a crucial mechanical factor for clearing aged erythrocytes. This could provide an important information on erythrocyte biomechanics in the removal of aged cell.

Microtubule disruption changes endothelial cell mechanics and adhesion

The interest in studying the mechanical and adhesive properties of cells has increased in recent years. The cytoskeleton is known to play a key role in cell mechanics. However, the role of the microtubules in shaping cell mechanics is not yet well understood. We have employed Atomic Force Microscopy (AFM) together with confocal fluorescence microscopy to determine the role of microtubules in cytomechanics of Human Umbilical Vein Endothelial Cells (HUVECs). Additionally, the time variation of the adhesion between tip and cell surface was studied. The disruption of microtubules by exposing the cells to two colchicine concentrations was monitored as a function of time. Already, after 30 min of incubation the cells stiffened, their relaxation times increased (lower fluidity) and the adhesion between tip and cell decreased. This was accompanied by cytoskeletal rearrangements, a reduction in cell area and changes in cell shape. Over the whole experimental time, different behavior for the two used concentrations was found while for the control the values remained stable. This study underlines the role of microtubules in shaping endothelial cell mechanics.

Measuring biomaterials mechanics with atomic force microscopy. 1. Influence of the loading rate and applied force (pyramidal tips)

Atomic force microscopy (AFM) is today an established tool in imaging and determination of mechanical properties of biomaterials. Due to their complex organization, those materials show intricate properties such as viscoelasticity. Therefore, one has to consider that the loading rate at which the sample is probed will lead to different mechanical response (properties). In this work, we studied the dependence of the mechanical properties of endothelial cells on the loading rate using AFM in force spectroscopy mode. We employed a sharp, four‐sided pyramidal indenter and loading rates ranging from 0.5 to 20 μm/s. In addition, by variation of the load (applied forces from 100 to 10,000 pN), the dependence of the cell properties on indentation depth could be determined. We then showed that the mechanical response of endothelial cells depends nonlinearly on the loading rate and follows a weak power‐law. In addition, regions of different viscous response at varying indentation depth could be determined. Based on the results we obtained, a general route map for AFM users for design of cell mechanics experiments was described.