Measuring the elastic properties of biological samples with the AFM

A comprehensive study of AFM stiffness measurements on inclined surfaces: theoretical, numerical, and experimental evaluation using a Hertz approach

Atomic Force Microscopy (AFM) is a leading nanoscale technique known for its significant advantages in the analysis of soft materials and biological samples. Traditional AFM data analysis is often based on the Hertz model, which assumes perpendicular indentation of a planar sample. However, this assumption is not always valid due to the varying geometries of soft materials, whether natural, synthetic or biological. In this study, we present a new theoretical model that incorporates correction coefficients into Hertz's model to account for cone-like and spherical probes, and to consider local tilt at the probe-sample interface. We validate our model using finite element analysis (FEA) simulations and experimental AFM measurements on tilted polyacrylamide gels. Our results highlight the need to include local tilt at the probe-sample contact to ensure accurate AFM measurements. This represents a step forward in our understanding of the elastic properties at the surface of soft materials in the broadest sense.

A Review of Atomic-Force Microscopy in Skin Barrier Function Assessment

Skin barrier function (SBF) disorders are a class of pathologies that affect a significant portion of the world population. These disorders cause skin lesions with intense itch, impacting patients' physical and psychological well-being as well as their social functioning. It is in the interest of patients that their disorder be monitored closely while under treatment to evaluate the effectiveness of the ongoing therapy and any potential adverse reactions. Symptom-based assessment techniques are widely used by clinicians; however, they carry some limitations. Techniques to assess skin barrier impairment are critical for understanding the nature of the disease and for helping personalize treatment. This review recalls the anatomy of the skin barrier and describes an atomic-force microscopy approach to quantitatively monitor its disorders and their response to treatment. We review a panel of studies that show that this technique is highly relevant for SBF disorder research, and we aim to motivate its adoption into clinical settings.

Advantages of integrating Brillouin microscopy in multimodal mechanical mapping of cells and tissues

Recent research has highlighted the growing significance of the mechanical properties of cells and tissues in the proper execution of physiological functions within an organism; alterations to these properties can potentially result in various diseases. These mechanical properties can be assessed using various techniques that vary in spatial and temporal resolutions as well as applications. Due to the wide range of mechanical behaviors exhibited by cells and tissues, a singular mapping technique may be insufficient in capturing their complexity and nuance. Consequently, by utilizing a combination of methods-multimodal mechanical mapping-researchers can achieve a more comprehensive characterization of mechanical properties, encompassing factors such as stiffness, modulus, viscoelasticity, and forces. Furthermore, different mapping techniques can provide complementary information and enable the exploration of spatial and temporal variations to enhance our understanding of cellular dynamics and tissue mechanics. By capitalizing on the unique strengths of each method while mitigating their respective limitations, a more precise and holistic understanding of cellular and tissue mechanics can be obtained. Here, we spotlight Brillouin microscopy (BM) as a noncontact, noninvasive, and label-free mechanical mapping modality to be coutilized alongside established mechanical probing methods. This review summarizes some of the most widely adopted individual mechanical mapping techniques and highlights several recent multimodal approaches demonstrating their utility. We envision that future studies aim to adopt multimodal techniques to drive advancements in the broader realm of mechanobiology.

Profiling native pulmonary basement membrane stiffness using atomic force microscopy

Mammalian cells sense and react to the mechanics of their immediate microenvironment. Therefore, the characterization of the biomechanical properties of tissues with high spatial resolution provides valuable insights into a broad variety of developmental, homeostatic and pathological processes within living organisms. The biomechanical properties of the basement membrane (BM), an extracellular matrix (ECM) substructure measuring only ∼100-400 nm across, are, among other things, pivotal to tumor progression and metastasis formation. Although the precise assignment of the Young's modulus E of such a thin ECM substructure especially in between two cell layers is still challenging, biomechanical data of the BM can provide information of eminent diagnostic potential. Here we present a detailed protocol to quantify the elastic modulus of the BM in murine and human lung tissue, which is one of the major organs prone to metastasis. This protocol describes a streamlined workflow to determine the Young's modulus E of the BM between the endothelial and epithelial cell layers shaping the alveolar wall in lung tissues using atomic force microscopy (AFM). Our step-by-step protocol provides instructions for murine and human lung tissue extraction, inflation of these tissues with cryogenic cutting medium, freezing and cryosectioning of the tissue samples, and AFM force-map recording. In addition, it guides the reader through a semi-automatic data analysis procedure to identify the pulmonary BM and extract its Young's modulus E using an in-house tailored user-friendly AFM data analysis software, the Center for Applied Tissue Engineering and Regenerative Medicine processing toolbox, which enables automatic loading of the recorded force maps, conversion of the force versus piezo-extension curves to force versus indentation curves, calculation of Young's moduli and generation of Young's modulus maps, where the pulmonary BM can be identified using a semi-automatic spatial filtering tool. The entire protocol takes 1-2 d.

Machine learning-based approach for automated classification of cell and extracellular matrix using nanomechanical properties

Fibrosis characterized by excess accumulation of extracellular matrix (ECM) due to complex cell-ECM interactions plays a pivotal role in pathogenesis. Herein, we employ the pancreatic ductal adenocarcinoma (PDAC) model to investigate dynamic alterations in nanomechanical attributes arising from the cell-ECM interactions to study the fibrosis paradigm. Several segregated studies performed on cellular and ECM components fail to recapitulate their complex collaboration. We utilized collagen and fibronectin, the two most abundant PDAC ECM components, and studied their nanomechanical attributes. We demonstrate alteration in morphology and nanomechanical attributes of collagen with varying thicknesses of collagen gel. Furthermore, by mixing collagen and fibronectin in various stoichiometry, their nanomechanical attributes were observed to vary. To demonstrate the dynamicity and complexity of cell-ECM, we utilized Panc-1 and AsPC-1 cells with or without collagen. We observed that Panc-1 and AsPC-1 cells interact differently with collagen and vice versa, evident from their alteration in nanomechanical properties. Further, using nanomechanics data, we demonstrate that ML-based techniques were able to classify between ECM as well as cell, and cell subtypes in the presence/absence of collagen with higher accuracy. This work demonstrates a promising avenue to explore other ECM components facilitating deeper insights into tumor microenvironment and fibrosis paradigm.

Evaluation of endometrial receptivity in women with unexplained infertility by shear wave elastography

OBJECTIVES

The endometrium of most unexplained infertility (UI) patients has been altered histologically. Shear wave elastography (SWE) is utilized to assess the signature of living tissue. This study aimed to explore the value of SWE in evaluating endometrial receptivity (ER) in UI patients.

METHODS

In total, 59 UI patients (UI group) and 52 normal control women (NC group) who received fertility consultation in our hospital were included between January 2022 and June 2023. We divided them into the late-proliferative phase of UI group (LPUI; n = 59), mid-secretory phase of UI group (MPUI; n = 41), late-proliferative phase of NC group (LPNC; n = 52), and mid-secretory phase of NC group (MPNC; n = 45). Transvaginal ultrasonography and SWE were performed during the LP and MP. Endometrial thickness (EMT), uterine artery pulsatility index (UA-PI), endometrial mean elasticity (E-mean), and mean shear wave velocities (SWV-mean) were measured.

RESULTS

There were significant differences in E-mean, SWV-mean, EMT, and UA-PI between the UI group and the NC group during both the LP and MP (p MPNC vs MPUI < 0.05, p LPNC vs LPUI < 0.05). E-mean and SWV-mean decreased with increasing EMT but increased with increasing UA-PI (p < 0.05). The most effective parameter for evaluating ER in UI patients is the E-mean (AUC = 0.89).

CONCLUSIONS

UI patients exhibited thinner endometrium, increased endometrial stiffness, and poor endometrial blood perfusion. E-mean was the most effective parameter to evaluate ER in UI patients. The study preliminarily proved that SWE is a promising non-invasive tool for evaluating the condition of endometrium.

CRITICAL RELEVANCE STATEMENT

This study aimed to explore the significance of endometrial elasticity measured by SWE in evaluating patients with UI. The findings revealed a correlation between EMT, UA-PI, and E-mean. Endometrial elasticity can serve as an effective indicator for predicting ER.

KEY POINTS

1. To explore the significance of endometrial elasticity in assessing patients with UI. 2. The endometrium of UI patient exhibited thinness, stiffness, and poor blood perfusion. 3. Endometrial elasticity serves as a valuable indicator for evaluating endometrial receptivity.

Cell Adhesion, Elasticity, and Rupture Forces Guide Microbial Cell Death on Nanostructured Antimicrobial Titanium Surfaces

Naturally occurring and synthetic nanostructured surfaces have been widely reported to resist microbial colonization. The majority of these studies have shown that both bacterial and fungal cells are killed upon contact and subsequent surface adhesion to such surfaces. This occurs because the presence of high-aspect-ratio structures can initiate a self-driven mechanical rupture of microbial cells during the surface adsorption process. While this technology has received a large amount of scientific and medical interest, one important question still remains: what factors drive microbial death on the surface? In this work, the interplay between microbial-surface adhesion, cell elasticity, cell membrane rupture forces, and cell lysis at the microbial-nanostructure biointerface during adsorptive processes was assessed using a combination of live confocal laser scanning microscopy, scanning electron microscopy, in situ amplitude atomic force microscopy, and single-cell force spectroscopy. Specifically, the adsorptive behavior and nanomechanical properties of live Gram-negative (Pseudomonas aeruginosa) and Gram-positive (methicillin-resistant Staphylococcus aureus) bacterial cells, as well as the fungal species Candida albicans and Cryptococcus neoformans, were assessed on unmodified and nanostructured titanium surfaces. Unmodified titanium and titanium surfaces with nanostructures were used as model substrates for investigation. For all microbial species, cell elasticity, rupture force, maximum cell-surface adhesion force, the work of adhesion, and the cell-surface tether behavior were compared to the relative cell death observed for each surface examined. For cells with a lower elastic modulus, lower force to rupture through the cell, and higher work of adhesion, the surfaces had a higher antimicrobial activity, supporting the proposed biocidal mode of action for nanostructured surfaces. This study provides direct quantification of the differences observed in the efficacy of nanostructured antimicrobial surface as a function of microbial species indicating that a universal, antimicrobial surface architecture may be hard to achieve.

Competition between deformation and free volume quantified by 3D image analysis of red blood cell

Cells in living organisms are subjected to mechanical strains caused by external forces like overcrowding, resulting in strong deformations that affect cell function. We study the interplay between deformation and crowding of red blood cells (RBCs) in dispersions of nonabsorbing rod-like viruses. We identify a sequence of configurational transitions of RBC doublets, including configurations that can only be induced by long-ranged attraction: highly fluctuating T-shaped and face-to-face configurations at low, and doublets approaching a complete spherical configuration at high, rod concentrations. Complementary simulations are used to explore different energy contributions to deformation as well as the stability of RBC doublet configurations. Our advanced analysis of 3D reconstructed confocal images of RBC doublets quantifies the depletion interaction and the resulting deformation energy. Thus, we introduce a noninvasive, high-throughput platform that is generally applicable to investigate the mechanical response of biological cells to external forces and characterize their mechanical properties.

Molecular and cellular level characterization of cytoskeletal mechanics using a quartz crystal microbalance

A quartz crystal microbalance (QCM) is an instrument that has the ability to measure nanogram-level changes in mass on a quartz sensor and is traditionally used to probe surface interactions and assembly kinetics of synthetic systems. The addition of dissipation monitoring (QCM-D) facilitates the study of viscoelastic systems, such as those relevant to molecular and cellular mechanics. Due to real-time recording of frequency and dissipation changes and single protein-level precision, the QCM-D is effective in interrogating the viscoelastic properties of cell surfaces and in vitro cellular components. However, few studies focus on the application of this instrument to cytoskeletal systems, whose dynamic parts create interesting emergent mechanics as ensembles that drive essential tasks, such as division and motility. Here, we review the ability of the QCM-D to characterize key kinetic and mechanical features of the cytoskeleton through in vitro reconstitution and cellular assays and outline how QCM-D studies can yield insightful mechanical data alone and in tandem with other biophysical characterization techniques.

4D Force Detection of Cell Adhesion and Contractility

Mechanical signals establish two-way communication between mammalian cells and their environment. Cells contacting a surface exert forces via contractility and transmit them at the areas of focal adhesions. External stimuli, such as compressive and pulling forces, typically affect the adhesion-free cell surface. Here, we demonstrate the collaborative employment of Fluidic Force Microscopy and confocal Traction Force Microscopy supported by the Cellogram solver to enable a powerful integrated force probing approach, where controlled vertical forces are applied to the free surface of individual cells, while the concomitant deformations are used to map their transmission to the substrate. Force transmission across human cells is measured with unprecedented temporal and spatial resolution, enabling the investigation of the cellular mechanisms involved in the adaptation, or maladaptation, to external mechanical stimuli. Altogether, the system enables facile and precise force interrogation of individual cells, with the capacity to perform population-based analysis.

Connecting Aortic Stiffness to Vascular Contraction: Does Sex Matter?

This study was designed to connect aortic stiffness to vascular contraction in young male and female Wistar rats. We hypothesized that female animals display reduced intrinsic media-layer stiffness, which associates with improved vascular function. Atomic force microscopy (AFM)-based nanoindentation analysis was used to derive stiffness (Young’s modulus) in biaxially (i.e., longitudinal and circumferential) unloaded aortic rings. Reactivity studies compatible with uniaxial loading (i.e., circumferential) were used to assess vascular responses to a selective α1 adrenergic receptor agonist in the presence or absence of extracellular calcium. Elastin and collagen levels were indirectly evaluated with fluorescence microscopy and a picrosirius red staining kit, respectively. We report that male and female Wistar rats display similar AFM-derived aortic media-layer stiffness, even though female animals withstand higher aortic intima-media thickness-to-diameter ratio than males. Female animals also present reduced phenylephrine-induced aortic force development in concentration-response and time-force curves. Specifically, we observed impaired force displacement in both parts of the contraction curve (Aphasic and Atonic) in experiments conducted with and without extracellular calcium. Additionally, collagen levels were lower in female animals without significant elastin content and fragmentation changes. In summary, sex-related functional differences in isolated aortas appear to be related to dissimilarities in the dynamics of vascular reactivity and extracellular matrix composition rather than a direct response to a shift in intrinsic media-layer stiffness.

Large extracellular vesicles do not mitigate the harmful effect of hyperglycemia on endothelial cell mobility

Extracellular vesicles, especially the larger fraction (LEVs - large extracellular vesicles), are believed to be an important means of intercellular communication. Earlier studies on LEVs have shown their healing properties, especially in the vascular cells of diabetic patients. Uptake of LEVs by endothelial cells and internalization of their cargo have also been demonstrated. Endothelial cells change their properties under hyperglycemic conditions (HGC), which reduces their activity and is the cause of endothelial dysfunction. The aim of our study was to investigate how human umbilical vein endothelial cells (HUVECs) change their biological properties: shape, mobility, cell surface stiffness, as well as describe the activation of metabolic pathways after exposure to the harmful effects of HGC and the administration of LEVs released by endothelial cells. We obtained LEVs from HUVEC cultures in HGC and normoglycemia (NGC) using the filtration and ultracentrifugation methods. We assessed the size of LEVs and the presence of biomarkers such as phosphatidylserine, CD63, beta-actin and HSP70. We analyzed the LEVs uptake efficiency by HUVECs, HUVEC shape, actin cytoskeleton remodeling, surface stiffness and finally gene expression by mRNA analysis. Under HGC conditions, HUVECs were larger and had a stiffened surface and a strengthened actin cortex compared to cells under NGC condition. HGC also altered the activation of metabolic pathways, especially those related to intracellular transport, metabolism, and organization of cellular components. The most interesting observation in our study is that LEVs did not restore cell motility disturbed by HGC. Although, LEVs were not able to reverse this deleterious effect of HGC, they activated transcription of genes involved in protein synthesis and vesicle trafficking in HUVECs.

Revealing anisotropic elasticity of endothelium under fluid shear stress

Endothelium lining interior surface of blood vessels experiences various physical stimulations in vivo. Its physical properties, especially elasticity, play important roles in regulating the physiological functions of vascular systems. In this paper, an integrated approach is developed to characterize the anisotropic elasticity of the endothelium under physiological-level fluid shear stress. A pressure sensor-embedded microfluidic device is developed to provide fluid shear stress on the perfusion-cultured endothelium and to measure transverse in-plane elasticities in the directions parallel and perpendicular to the flow direction. Biological atomic force microscopy (Bio-AFM) is further exploited to measure the vertical elasticity of the endothelium in its out-of-plane direction. The results show that the transverse elasticity of the endothelium in the direction parallel to the perfusion culture flow direction is about 70% higher than that in the direction perpendicular to the flow direction. Moreover, the transverse elasticities of the endothelium are estimated to be approximately 120 times larger than the vertical one. The results indicate the effects of fluid shear stress on the transverse elasticity anisotropy of the endothelium, and the difference between the elasticities in transverse and vertical directions. The quantitative measurement of the endothelium anisotropic elasticity in different directions at the tissue level under the fluid shear stress provides biologists insightful information for the advanced vascular system studies from biophysical and biomaterial viewpoints. STATEMENT OF SIGNIFICANCE: In this paper, we take advantage an integrated approach combining microfluidic devices and biological atomic force microscopy (Bio-AFM) to characterize anisotropic elasticities of endothelia with and without fluidic shear stress application. The microfluidic devices are exploited to conduct perfusion cell culture of the endothelial cells, and to estimate the in-plane elasticities of the endothelium in the direction parallel and perpendicular to the shear stress. In addition, the Bio-AFM is utilized for characterization of the endothelium morphology and vertical elasticity. The measurement results demonstrate the very first anisotropic elasticity quantification of the endothelia. Furthermore, the study provides insightful information bridging the microscopic sing cell and macroscopic organ level studies, which can greatly help to advance vascular system research from material perspective.

The functional cross talk between cancer cells and cancer associated fibroblasts from a cancer mechanics perspective

The function of biological tissues in health and disease is regulated at cellular level and is highly influenced by the physical microenvironment, through the interaction of forces between cells and ECM, which are perceived through mechanosensing pathways. In cancer, both chemical and physical signaling cascades and their interactions are involved during cell-cell and cell-ECM communications to meet requirements of tumor growth. Among stroma cells, cancer associated fibroblasts (CAFs) play key role in tumor growth and pave the way for cancer cells to initiate metastasis and invasion to other tissues, and without recruitment of CAFs, the process of cancer invasion is dysfunctional. This is through an intense chemical and physical cross talks with tumor cells, and interactive remodeling of ECM. During such interaction CAFs apply traction forces and depending on the mechanical properties, deform ECM and in return receive physical signals from the micromechanical environment. Such interaction leads to ECM remodeling by manipulating ECM structure and its mechanical properties. The results are in form of deposition of extra fibers, stiffening, rearrangement and reorganization of fibrous structure, and degradation which are due to a complex secretion and expression of different markers triggered by mechanosensing of tumor cells, specially CAFs. Such events define cancer progress and invasion of cancer cells. A systemic knowledge of chemical and physical factors provides a holistic view of how cancer process and enhances the current treatment methods to provide more diversity among targets that involves tumor cells and ECM structure.

Simultaneous membrane binding of Annexin A4 and A5 suppresses 2D lattice formation while maintaining curvature induction

HYPOTHESIS

Annexin A4 and A5 (ANXA4, ANXA5), both shown to be required for efficient plasma membrane repair (PMR) in living cells, bind as trimers to anionic membranes in the presence of calcium. Both annexins induce membrane curvature and self-assemble into crystal arrays on membranes, observations that have been associated with PMR. However, in-vitro studies of annexins have traditionally been performed using single annexins, despite the recruitment of multiple annexins to the damage site in cells. Hence, we study the potential cooperativity of ANXA4 and ANXA5 during membrane binding.

EXPERIMENTS

Laser injury experiments were performed on MCF7 cells transfected to transiently express labelled ANXA4 and ANXA5 to study the localization of the proteins at the damage site. Using free-edged DOPC/DOPS (9:1) membranes we investigated the annexin-induced membrane rolling by fluorescence microscopy and the lateral arrangement of annexin trimers on the membrane surface by atomic force microscopy (AFM).

FINDING

ANXA4 and ANXA5 colocalise at the damage site of MCF7 cells during repair. A (1:1) mixture of ANXA4 and ANXA5 induces membrane rolling with a time constant intermediate between the value for the pure annexins. While binding of the pure annexins creates crystal lattices, the (1:1) mixture generates a random arrangement of trimers. Thus, curvature induction remains as a functional property of annexin mixtures in PMR rather than crystal formation.

Extensions of the worm-like-chain model to tethered active filaments under tension

Intracellular elastic filaments such as microtubules are subject to thermal Brownian noise and active noise generated by molecular motors that convert chemical energy into mechanical work. Similarly, polymers in living fluids such as bacterial suspensions and swarms suffer bending deformations as they interact with single bacteria or with cell clusters. Often, these filaments perform mechanical functions and interact with their networked environment through cross-links or have other similar constraints placed on them. Here, we examine the mechanical properties-under tension-of such constrained active filaments under canonical boundary conditions motivated by experiments. Fluctuations in the filament shape are a consequence of two types of random forces-thermal Brownian forces and activity derived forces with specified time and space correlation functions. We derive force-extension relationships and expressions for the mean square deflections for tethered filaments under various boundary conditions including hinged and clamped constraints. The expressions for hinged-hinged boundary conditions are reminiscent of the worm-like-chain model and feature effective bending moduli and mode-dependent non-thermodynamic effective temperatures controlled by the imposed force and by the activity. Our results provide methods to estimate the activity by measurements of the force-extension relation of the filaments or their mean square deflections, which can be routinely performed using optical traps, tethered particle experiments, or other single molecule techniques.

Nanomechanics in Monitoring the Effectiveness of Drugs Targeting the Cancer Cell Cytoskeleton

Increasing attention is devoted to the use of nanomechanics as a marker of various pathologies. Atomic force microscopy (AFM) is one of the techniques that could be applied to quantify the nanomechanical properties of living cells with a high spatial resolution. Thus, AFM offers the possibility to trace changes in the reorganization of the cytoskeleton in living cells. Impairments in the structure, organization, and functioning of two main cytoskeletal components, namely, actin filaments and microtubules, cause severe effects, leading to cell death. That is why these cytoskeletal components are targets for antitumor therapy. This review intends to describe the gathered knowledge on the capability of AFM to trace the alterations in the nanomechanical properties of living cells induced by the action of antitumor drugs that could translate into their effectiveness.

Lipid bilayers: Phase behavior and nanomechanics

Lipid membranes are involved in many physiological processes like recognition, signaling, fusion or remodeling of the cell membrane or some of its internal compartments. Within the cell, they are the ultimate barrier, while maintaining the fluidity or flexibility required for a myriad of processes, including membrane protein assembly. The physical properties of in vitro model membranes as model cell membranes have been extensively studied with a variety of techniques, from classical thermodynamics to advanced modern microscopies. Here we review the nanomechanics of solid-supported lipid membranes with a focus in their phase behavior. Relevant information obtained by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) as complementary techniques in the nano/mesoscale interface is presented. Membrane morphological and mechanical characterization will be discussed in the framework of its phase behavior, phase transitions and coexistence, in simple and complex models, and upon the presence of cholesterol.

Visco-hyperelastic characterization of human brain white matter micro-level constituents in different strain rates

In this study, we propose a computational characterization technique for obtaining the material properties of axons and extracellular matrix (ECM) in human brain white matter. To account for the dynamic behavior of the brain tissue, data from time-dependent relaxation tests of human brain white matter in different strain rates are extracted and formulated by a visco-hyperelastic constitutive model consisting of the Ogden hyperelastic model and the Prony series expansion. Through micromechanical finite element simulation, a derivative-free optimization framework designed to minimize the difference between the numerical and experimental data is used to identify the material properties of the axons and ECM. The Prony series expansion parameters of axons and ECM are found to be highly affected by the Prony series expansion coefficients of the brain white matter. The optimal parameters of axons and ECM are verified through micromechanical simulation by comparing the averaged numerical response with that of the experimental data. Moreover, the initial shear modulus and the reduced shear modulus of the axons are found for different strain rates of 0.0001, 0.01, and 1 s^-1. Consequently, first- and second-order regressions are used to find relations for the prediction of the shear modulus at the intermediate strain rates. Graphical Abstract The applied procedure for characterization of brain white matter micro-level constituents. The macro-level experimental data in different strain rates are used in the context of simulation-based optimization to obtain the properties of axons and extracellular matrix material.

Sensing and Modelling Mechanical Response in Large Deformation Indentation of Adherent Cell Using Atomic Force Microscopy

The mechanical behaviour of adherent cells when subjected to the local indentation can be modelled via various approaches. Specifically, the tensegrity structure has been widely used in describing the organization of discrete intracellular cytoskeletal components, including microtubules (MTs) and microfilaments. The establishment of a tensegrity model for adherent cells has generally been done empirically, without a mathematically demonstrated methodology. In this study, a rotationally symmetric prism-shaped tensegrity structure is introduced, and it forms the basis of the proposed multi-level tensegrity model. The modelling approach utilizes the force density method to mathematically assure self-equilibrium. The proposed multi-level tensegrity model was developed by densely distributing the fundamental tensegrity structure in the intracellular space. In order to characterize the mechanical behaviour of the adherent cell during the atomic force microscopy (AFM) indentation with large deformation, an integrated model coupling the multi-level tensegrity model with a hyperelastic model was also established and applied. The coefficient of determination between the computational force-distance (F-D) curve and the experimental F-D curve was found to be at 0.977 in the integrated model on average. In the simulation range, along with the increase in the overall deformation, the local stiffness contributed by the cytoskeletal components decreased from 75% to 45%, while the contribution from the hyperelastic components increased correspondingly.

Atomic Force Microscopy of Proteins

Atomic force microscopy (AFM) enables imaging of surface-deposited proteins and protein structures under physiological conditions, which is a benefit compared to ultra-high vacuum techniques such as electron microscopy. AFM also has the potential to provide more information from the phase in tapping mode or from functional AFM modes. The sample preparation, probe selection, and imaging conditions are crucial for successful imaging of proteins. Here we give a detailed account of the steps toward imaging of soft samples in both air and liquid along with the basic theory underpinning these details.

Engineering approaches for characterizing soft tissue mechanical properties: A review

From cancer diagnosis to detailed characterization of arterial wall biomechanics, the elastic property of tissues is widely studied as an early sign of disease onset. The fibrous structural features of tissues are a direct measure of its health and functionality. Alterations in the structural features of tissues are often manifested as local stiffening and are early signs for diagnosing a disease. These elastic properties are measured ex vivo in conventional mechanical testing regimes, however, the heterogeneous microstructure of tissues can be accurately resolved over relatively smaller length scales with enhanced spatial resolution using techniques such as micro-indentation, microelectromechanical (MEMS) based cantilever sensors and optical catheters which also facilitate in vivo assessment of mechanical properties. In this review, we describe several probing strategies (qualitative and quantitative) based on the spatial scale of mechanical assessment and also discuss the potential use of machine learning techniques to compute the mechanical properties of soft tissues. This work details state of the art advancement in probing strategies, associated challenges toward quantitative characterization of tissue biomechanics both from an engineering and clinical standpoint.

Bacterial-nanostructure interactions: The role of cell elasticity and adhesion forces

The attachment of single-celled organisms, namely bacteria and fungi, to abiotic surfaces is of great interest to both the scientific and medical communities. This is because the interaction of such cells has important implications in a range of areas, including biofilm formation, biofouling, antimicrobial surface technologies, and bio-nanotechnologies, as well as infection development, control, and mitigation. While central to many biological phenomena, the factors which govern microbial surface attachment are still not fully understood. This lack of understanding is a direct consequence of the complex nature of cell-surface interactions, which can involve both specific and non-specific interactions. For applications involving micro- and nano-structured surfaces, developing an understanding of such phenomenon is further complicated by the diverse nature of surface architectures, surface chemistry, variation in cellular physiology, and the intended technological output. These factors are extremely important to understand in the emerging field of antibacterial nanostructured surfaces. The aim of this perspective is to re-frame the discussion surrounding the mechanism of nanostructured-microbial surface interactions. Broadly, the article reviews our current understanding of these phenomena, while highlighting the knowledge gaps surrounding the adhesive forces which govern bacterial-nanostructure interactions and the role of cell membrane rigidity in modulating surface activity. The roles of surface charge, cell rigidity, and cell-surface adhesion force in bacterial-surface adsorption are discussed in detail. Presently, most studies have overlooked these areas, which has left many questions unanswered. Further, this perspective article highlights the numerous experimental issues and misinterpretations which surround current studies of antibacterial nanostructured surfaces.

Shear-Enhanced Dynamic Adhesion of Lactobacillus rhamnosus GG on Intestinal Epithelia: Correlative Effect of Protein Expression and Interface Mechanics

The oral uptake of probiotic microorganisms as food additives is one widely used strategy to sustain and improve the homeostasis of intestinal microbiota that protect the intestinal epithelia from attack by pathogenic bacteria. Once delivered to the ileum and colon, probiotics must adhere and form colonies on mucus that coats the surface of intestinal epithelial cells. Although an increasing amount of knowledge about the genetic and molecular level mechanisms of probiotics-mucus interactions has been accumulated, little is known about the physicochemical aspects of probiotics-mucus interactions under physiological shear in intestines. In this study, we established well-defined models of intestinal epithelial cell monolayers based on two major constituents of gut epithelia, enterocytes and goblet cells. First, the formation of a polarized cell monolayer sealed by tight junctions was monitored by transepithelial electrical resistance over time. The establishment of tight junctions and secretion of mucus proteins (mucin) was confirmed by immunofluorescence staining. In the next step, we measured the elasticity of cell monolayer surfaces by indentation using particle-assisted atomic force microscopy. The effective elastic modulus of goblet cell-like cells was 30 times smaller compared to that of enterocyte-like cells, which can be attributed to the secretion of a 3 μm thick mucin layer. As probiotics, we used Lactobacillus rhamnosus GG (LGG), which is one of the most widely used strains as food additives. To investigate the dynamic adhesion of LGG to the intestine model surface, we transferred the epithelial cell monolayer into a microfluidic chamber. A distinct difference in dynamic adhesion between two cell types was observed, which could be attributed to the difference in the mucin expression amount. Remarkably, we found that the dynamic LGG adhesion is enhanced by the increase in shear stress, showing a maximum binding efficiency at 0.3 Pa. Finally, we examined the persistence of LGG adhesion by a stepwise increase in the shear stress exerted on adherent LGG, demonstrating that LGG could withstand high shear stress even beyond that of physiological stress. The obtained results present a large potential to quantitatively understand the influence of engineered foods and probiotics on the homeostasis of microbiota on the surface of intestinal epithelia.

Towards nanoscale electrical measurements in liquid by advanced KPFM techniques: a review

Fundamental mechanisms of energy storage, corrosion, sensing, and multiple biological functionalities are directly coupled to electrical processes and ionic dynamics at solid-liquid interfaces. In many cases, these processes are spatially inhomogeneous taking place at grain boundaries, step edges, point defects, ion channels, etc and possess complex time and voltage dependent dynamics. This necessitates time-resolved and real-space probing of these phenomena. In this review, we discuss the applications of force-sensitive voltage modulated scanning probe microscopy (SPM) for probing electrical phenomena at solid-liquid interfaces. We first describe the working principles behind electrostatic and Kelvin probe force microscopies (EFM & KPFM) at the gas-solid interface, review the state of the art in advanced KPFM methods and developments to (i) overcome limitations of classical KPFM, (ii) expand the information accessible from KPFM, and (iii) extend KPFM operation to liquid environments. We briefly discuss the theoretical framework of electrical double layer (EDL) forces and dynamics, the implications and breakdown of classical EDL models for highly charged interfaces or under high ion concentrations, and describe recent modifications of the classical EDL theory relevant for understanding nanoscale electrical measurements at the solid-liquid interface. We further review the latest achievements in mapping surface charge, dielectric constants, and electrodynamic and electrochemical processes in liquids. Finally, we outline the key challenges and opportunities that exist in the field of nanoscale electrical measurements in liquid as well as providing a roadmap for the future development of liquid KPFM.

FC_analysis: a tool for investigating atomic force microscopy maps of force curves

Background

The collection and analysis of Atomic Force Microscopy force curves is a well-established procedure to obtain high-resolution information of non-topographic data from any kind of sample, including biological specimens. In particular, these analyses are commonly employed to study elasticity, stiffness or adhesion properties of the samples. Furthermore, the collection of several force curves over an extended area of the specimens allows reconstructing maps, called force volume maps, of the spatial distribution of the mechanical properties. Coupling these maps with the conventional high-resolution topographic reconstruction of the sample’s surface, provides a deeper insight on the sample composition from the structural and nanomechanical point of view.

Results

In this paper we present the open source software package FC_analysis that automatically analyses single force curves or entire force volume maps to yield the corresponding elasticity and deformability images. The principal characteristic of the FC_analysis is a large adaptability to the various experimental setups and to different analysis methodologies. For instance, the user can provide custom values for the detector sensitivity, scanner-z sensitivity, cantilever’s elastic constant and map’s acquisition modality and can choose between different analysis methodologies. Furthermore, the software allows the optimization of the fitting parameters and gives direct control on each step of the analysis procedure. Notably, to overcome a limitation common to many other analysis programs, FC_analysis can be applied to a rectangular portion of the image, allowing the analysis of inhomogeneous samples. Finally, the software allows reconstructing a Young’s modulus map at different penetration depths, enabling the use of modern investigation tools such as the force tomography.

Conclusions

The FC_analysis software aims to become a useful tool for the analysis of force curves maps collected using custom or commercial Atomic Force Microscopes, and is especially useful in those cases for which the producer doesn’t release a dedicated software.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2265-4) contains supplementary material, which is available to authorized users.

Mechanical and migratory properties of normal, scar, and Dupuytren's fibroblasts

Mechanical properties of myofibroblasts play a key role in Dupuytren's disease. Here, we used atomic force microscopy to measure the viscoelastic properties of 3 different types of human primary fibroblasts derived from a same patient: normal and scar dermal fibroblasts and palmar fascial fibroblasts from Dupuytren's nodules. Different stiffness hydrogels (soft ~1 kPa and stiff ~ 50 kPa) were used as cell culture matrix to mimic the mechanical properties of the natural tissues, and atomic force microscopy step response force curves were used to discriminate between elastic and viscous properties of cells. Since transforming growth factor-β1 (TGF-β1) is known to induce expression of α-smooth muscle actin positive stress fibers in myofibroblasts, we investigated the behavior of these fibroblasts before and after applying TGF-β1. Finally, we performed an in vitro cell motility test, the wound healing or scratch assay, to evaluate the migratory properties of these fibroblasts. We found that (1) Dupuytren's fibroblasts are stiffer than normal and scar fibroblasts, the elastic modulus E ranging from 4.4, 2.1, to 1.8 kPa, for Dupuytren's, normal and scar fibroblasts, respectively; (2) TGF-β1 enhances the level of α-smooth muscle actin expression and thus cell stiffness in Dupuytren's fibroblasts (E, ~6.2 kPa); (3) matrix stiffness influences cell mechanical properties most prominently in Dupuytren's fibroblasts; and (4) Dupuytren's fibroblasts migrate slower than the other fibroblasts by a factor of 3. Taking together, our results showed that mechanical and migratory properties of fibroblasts might help to discriminate between different pathological conditions, helping to identify and recognize specific cell phenotypes.

Mask-free fabrication of a versatile microwell chip for multidimensional cellular analysis and drug screening

Cells are frequently studied because they are basic structural, functional, and biological units of living organisms. Extracting features from cellular behaviors can facilitate decision making in medical diagnoses and represents an important aspect in the development of biomedical engineering. Previous studies have just focused on either the individual cell or cell clusters separately, which leads to a great lack of information. Microwell technologies could address the challenges of in vitro cellular studies, from individual cell studies to 3D functional assays, by providing more information from smaller sample volumes and enabling the incorporation of low-cost high-throughput assays in the drug discovery process. To this end, the present study describes an easy-to-use method for fabricating a versatile microwell chip that utilizes a digital micro-mirror device printing system, and the chip can be employed in multidimensional cellular analysis, ranging from the single cell to the 3D spheroid level. The microwell manufacturing process, using a digital mask in place of a conventional physical mask, is based on shadowed light and is full of flexibility. Three different dimensions (single cell (1D), cell monolayer (2D) and cell spheroid (3D)) of one cell type can be formed using a microwell array and the analyses of biological characteristics are achieved separately. Single cells and cell clusters can be controlled via customized geometries of microfabricated selectively adhesive poly(ethylene glycol) diacrylate (PEGDA) wells. The effects of shape on cellular growth and hybrid tissue layers were investigated by peeling off the microwells. Furthermore, 3D multicellular spheroids were successfully established in a controllable and high-throughput format. Preclinical drug screening was investigated and distinct differences were observed in the tolerance response to drug testing between the 2D and 3D conditions. The study results further demonstrate that the high-density microwell chip is an easy-to-use multidimensional cellular analysis and rapid drug screening technique, and it could be easily adapted for a wide range of biological research applications.

Culturing substrates influence the morphological, mechanical and biochemical features of lung adenocarcinoma cells cultured in 2D or 3D

Alternative models such as three-dimensional (3D) cell cultures represent a distinct milestone towards capturing the realities of cancer biology in vitro and reduce animal experimentation in the preclinical stage of drug discovery. Significant work remains to be done to understand how substrates used in in vitro alternatives influence cancer cells phenotype and drug efficacy responses, so that to accurately link such models to specific in vivo disease scenarios. Our study describes how the morphological, mechanical and biochemical properties of adenocarcinoma (A549) cells change in response to a 3D environment and varying substrates. Confocal Laser Scanning (LSCM), He-Ion (HIM) and Atomic Force (AFM) microscopies, supported by ELISA and Western blotting, were used. These techniques enabled us to evaluate the shape, cytoskeletal organization, roughness, stiffness and biochemical signatures of cells grown within soft 3D matrices (PuraMatrix™ and Matrigel™), and to compare them to those of cells cultured on two-dimensional glass substrates. Cell cultures are also characterized for their biological response to docetaxel, a taxane-type drug used in Non-Small-Cell Lung Cancer (NSCLC) treatment. Our results offer an advanced biophysical insight into the properties and potential application of 3D cultures of A549 cells as in vitro alternatives in lung cancer research.

AFM-based detection of glycocalyx degradation and endothelial stiffening in the db/db mouse model of diabetes

Degradation of the glycocalyx and stiffening of endothelium are important pathophysiological components of endothelial dysfunction. However, to our knowledge, these events have not been investigated in tandem in experimental diabetes. Here, the mechanical properties of the glycocalyx and endothelium in ex vivo mouse aorta were determined simultaneously in indentation experiments with an atomic force microscope (AFM) for diabetic db/db and control db/+ mice at ages of 11–19 weeks. To analyze highly heterogeneous aorta samples, we developed a tailored classification procedure of indentation data based on a bi-layer brush model supplemented with Hertz model for quantification of nanomechanics of endothelial regions with and without the glycocalyx surface. In db/db mice, marked endothelial stiffening and reduced glycocalyx coverage were present already in 11-week-old mice and persisted in older animals. In contrast, reduction of the effective glycocalyx length was progressive and was most pronounced in 19-week-old db/db mice. The reduction of the glycocalyx length correlated with an increasing level of glycated haemoglobin and decreased endothelial NO production. In conclusion, AFM nanoindentation analysis revealed that stiffening of endothelial cells and diminished glycocalyx coverage occurred in early diabetes and were followed by the reduction of the glycocalyx length that correlated with diabetes progression.

Decorating fiber nanotip with single perovskite quantum dot and other luminescent nanocrystals synthesized in oil-phase

We report a simple yet robust method for fabricating single perovskite quantum dot (QD) decorated fiber nanotips. In this method, a single QD is directly picked up and subsequently glued on the apex of a specially fabricated cantilever fiber tip with a high success rate (approx. 70%) without using expensive close-loop feedback systems. Thanks to the flexibility and robustness of the fiber tips, no damage of the tips was observed in the process. Moreover, nanocrystal (NC) dispersing technique was developed to avoid undesired aggregations of QDs, and it guarantees that only one QD is glued each time. Finally, we demonstrate that this technique can also be applied to other oil-phase synthesized NCs, including CdSe QDs and upconversion luminescent NCs. It leads to many important applications on probing the local environment using high performance luminescent nanoprobes.

Nonlinear compliance of elastic layers to indentation

Thin samples adherent to a rigid substrate are considerably less compliant to indentation when compared to specimens that are not geometrically confined. Analytical corrections to this so-called substrate effect exist for various types of indenters but are not applicable when large deformations are possible, as is the case in biological materials. To overcome this limitation, we construct a nonlinear scaling model characterized by one single exponent, which we explore employing a parametric finite element analysis. The model is based on asymptotes of two length scales in relation to the sample thickness, i.e., indentation depth and radius of the contact area. For small indentation depth, we require agreement with analytical, linear models, whereas for large indentation depth and extensive contact area, we recognize similarity to uniaxial deformation, indicating a divergent force required to indent nonlinear materials. In contrast, we find linear materials not to be influenced by the substrate effect beyond first order, implying that nonlinear effects originating from either the material or geometric confinement are clearly separated only in thin samples. Furthermore, in this regime the scaling model can be derived by following a heuristic argument extending a linear model to large indentation depths. Lastly, in a large indentation setting where the contact is small in comparison with sample thickness, we observe nonlinear effects independent of material type that we attribute to a higher-order influence of geometrical confinement. In this regime, we define a scalar as the ratio of strains along principal axes as obtained by comparison with the case of a point force on a half-space. We find this scalar to be in quantitative agreement with the scaling exponent, indicating an approach to distinguish between nonlinear effects in the scaling model. While we conjecture our findings to be applicable to other flat-ended indenters, we focus on the case of a flat-ended cylinder in normal contact with a thin layer. The analytical solution for small indentation associated with this problem has been given by Hayes et al. (J Biomech 5:541-551, 1972), for which we provide a convenient numerical implementation.

Dynamic modeling of trolling-mode AFM: Considering effects of cantilever torsion, nanoneedle flexibility and liquid-nanoneedle interactions

Trolling mode atomic force microscope (TR-Mode AFM) significantly reduces the hydrodynamic drag generated during operation in liquid environments. This is achieved by utilizing a long nanoneedle and keeping the cantilever out of liquid. In this research, a continuous mathematical model is developed to study TR-Mode AFM dynamics near a sample submerged in the liquid. Effects of cantilever torsion, nanoneedle flexibility, and liquid-nanoneedle interactions are considered in the model. In order to derive the equations of motion, Hamilton's principle and assumed mode method are used. System operation in dynamic mode is numerically simulated and the accuracy of the results is verified by comparison with the results of the finite element method. Displacements of different components of the system are also compared with each other and the dominant displacements are determined.

A Validation Study of the Repeatability and Accuracy of Atomic Force Microscopy Indentation Using Polyacrylamide Gels and Colloidal Probes

The elasticity of soft biological materials is a critical property to understand their biomechanical behaviors. Atomic force microscopy (AFM) indentation method has been widely employed to measure the Young's modulus (E) of such materials. Although the accuracy of the method has been recently evaluated based on comparisons with macroscale E measurements, the repeatability of the method has yet to be validated for rigorous biomechanical studies of soft elastic materials. We tested the AFM indentation method using colloidal probes and polyacrylamide (PAAM) gels of E < 20 kPa as a model soft elastic material after having identified optimal trigger force and probe speed. AFM indentations repeated with time intervals show that the method is well repeatable when performed carefully. Compared with the rheometric method and the confocal microscopy indentation method, the AFM indentation method is evaluated to have comparable accuracy and better precision, although these elasticity measurements appear to rely on the compositions of PAAM gels and the length scale of measurement. Therefore, we have confirmed that the AFM indentation method can reliably measure the elasticity of soft elastic materials.

Atomic Force Microscopy Studies of the Interaction of Antimicrobial Peptides with Bacterial Cells

Antimicrobial peptides (AMPs) are promising therapeutic alternatives to conventional antibiotics. Many AMPs are membrane-active but their mode of action in killing bacteria or in inhibiting their growth remains elusive. Recent studies indicate the mechanism of action depends on peptide structure and lipid components of the bacterial cell membrane. Owing to the complexity of working with living cells, most of these studies have been conducted with synthetic membrane systems, which neglect the possible role of bacterial surface structures in these interactions. In recent years, atomic force microscopy has been utilized to study a diverse range of biological systems under non-destructive, physiologically relevant conditions that yield in situ biophysical measurements of living cells. This approach has been applied to the study of AMP interaction with bacterial cells, generating data that describe how the peptides modulate various biophysical behaviours of individual bacteria, including the turgor pressure, cell wall elasticity, bacterial capsule thickness, and organization of bacterial adhesins.

Microrheology of growing Escherichia coli biofilms investigated by using magnetic force modulation atomic force microscopy

Microrheology of growing biofilms provides insightful information about its structural evolution and properties. In this study, the authors have investigated the microrheology of Escherichia coli (strain HCB1) biofilms at different indentation depth (δ) by using magnetic force modulation atomic force microscopy as a function of disturbing frequency (f). As δ increases, the dynamic stiffness (k_s) for the biofilms in the early stage significantly increases. However, it levels off when the biofilms are matured. The facts indicate that the biofilms change from inhomogeneous to homogeneous in structure. Moreover, k_s is scaled to f, which coincides with the rheology of soft glasses. The exponent increases with the incubation time, indicating the fluidization of biofilms. In contrast, the upper layer of the matured biofilms is solidlike in that the storage modulus is always larger than the loss modulus, and its viscoelasticity is slightly influenced by the shear stress.

Elastic properties of hydrogels and decellularized tissue sections used in mechanobiology studies probed by atomic force microscopy

The increasing recognition that tissue elasticity is an important regulator of cell behavior in normal and pathologic conditions such as fibrosis and cancer has driven the development of cell culture substrata with tunable elasticity. Such development has urged the need to quantify the elastic properties of these cell culture substrata particularly at the nanometer scale, since this is the relevant length scale involved in cell-extracellular matrix (ECM) mechanical interactions. To address this need, we have exploited the versatility of atomic force microscopy to quantify the elastic properties of a variety of cell culture substrata used in mechanobiology studies, including floating collagen gels, ECM-coated polyacrylamide gels, and decellularized tissue sections. In this review we summarize major findings in this field from our group within the context of the state-of-the-art in the field, and provide a critical discussion on the applicability and complementarity of currently available cell culture assays with tunable elasticity. In addition, we briefly describe how the limitations of these assays provide opportunities for future research, which is expected to continue expanding our understanding of the mechanobiological aspects that support both normal and diseased conditions. Microsc. Res. Tech. 80:85-96, 2017. © 2016 Wiley Periodicals, Inc.

Introduction to Atomic Force Microscopy (AFM) in Biology

The atomic force microscope (AFM) has the unique capability of imaging biological samples with molecular resolution in buffer solution over a wide range of time scales from milliseconds to hours. In addition to providing topographical images of surfaces with nanometer- to angstrom-scale resolution, forces between single molecules and mechanical properties of biological samples can be investigated from the nano-scale to the micro-scale. Importantly, the measurements are made in buffer solutions, allowing biological samples to "stay alive" within a physiological-like environment while temporal changes in structure are measured-e.g., before and after addition of chemical reagents. These qualities distinguish AFM from conventional imaging techniques of comparable resolution, e.g., electron microscopy (EM). This unit provides an introduction to AFM on biological systems and describes specific examples of AFM on proteins, cells, and tissues. The physical principles of the technique and methodological aspects of its practical use and applications are also described. © 2016 by John Wiley & Sons, Inc.

Load Rate and Temperature Dependent Mechanical Properties of the Cortical Neuron and Its Pericellular Layer Measured by Atomic Force Microscopy

When studying the mechanical properties of cells by an indentation technique, it is important to take into account the nontrivial pericellular interface (or pericellular "brush") which includes a pericellular coating and corrugation of the pericellular membrane (microvilli and microridges). Here we use atomic force microscopy (AFM) to study the mechanics of cortical neurons taking into account the presence of the above pericellular brush surrounding cell soma. We perform a systematic study of the mechanical properties of both the brush layer and the underlying neuron soma and demonstrate that the brush layer is likely responsible for the low elastic modulus (<1 kPa) typically reported for cortical neurons. When the contribution of the pericellular brush is excluded, the average elastic modulus of the cortical neuron soma is found to be 3-4 times larger than previously reported values measured under similar physiological conditions. We also demonstrate that the underlying soma behaves as a nonviscous elastic material over the indentation rates studied (1-10 μm/s). As a result, it seems that the brush layer is responsible for the previously reported viscoelastic response measured for the neuronal cell body as a whole, within these indentation rates. Due to of the similarities between the macroscopic brain mechanics and the effective modulus of the pericellular brush, we speculate that the pericellular brush layer might play an important role in defining the macroscopic mechanical properties of the brain.

AFM-based force spectroscopy for bioimaging and biosensing

AFM-based force spectroscopy shows wide bio-related applications especially for bioimaging and biosensing.

Biomechanical assessment in models of glaucomatous optic neuropathy

The biomechanical environment within the eye is of interest in both the regulation of intraocular pressure and the loss of retinal ganglion cell axons in glaucomatous optic neuropathy. Unfortunately, this environment is complex and difficult to determine. Here we provide a brief introduction to basic concepts of mechanics (stress, strain, constitutive relationships) as applied to the eye, and then describe a variety of experimental and computational approaches used to study ocular biomechanics. These include finite element modeling, direct experimental measurements of tissue displacements using optical and other techniques, direct experimental measurement of tissue microstructure, and combinations thereof. Thanks to notable technical and conceptual advances in all of these areas, we are slowly gaining a better understanding of how tissue biomechanical properties in both the anterior and posterior segments may influence the development of, and risk for, glaucomatous optic neuropathy. Although many challenging research questions remain unanswered, the potential of this body of work is exciting; projects underway include the coupling of clinical imaging with biomechanical modeling to create new diagnostic tools, development of IOP control strategies based on improved understanding the mechanobiology of the outflow tract, and attempts to develop novel biomechanically-based therapeutic strategies for preservation of vision in glaucoma.