Quantitative reconstruction of time-varying 3D cell forces with traction force optical coherence microscopy

Field Guide to Traction Force Microscopy

Introduction

Traction force microscopy (TFM) is a widely used technique to measure cell contractility on compliant substrates that mimic the stiffness of human tissues. For every step in a TFM workflow, users make choices which impact the quantitative results, yet many times the rationales and consequences for making these decisions are unclear. We have found few papers which show the complete experimental and mathematical steps of TFM, thus obfuscating the full effects of these decisions on the final output.

Methods

Therefore, we present this "Field Guide" with the goal to explain the mathematical basis of common TFM methods to practitioners in an accessible way. We specifically focus on how errors propagate in TFM workflows given specific experimental design and analytical choices.

Results

We cover important assumptions and considerations in TFM substrate manufacturing, substrate mechanical properties, imaging techniques, image processing methods, approaches and parameters used in calculating traction stress, and data-reporting strategies.

Conclusions

By presenting a conceptual review and analysis of TFM-focused research articles published over the last two decades, we provide researchers in the field with a better understanding of their options to make more informed choices when creating TFM workflows depending on the type of cell being studied. With this review, we aim to empower experimentalists to quantify cell contractility with confidence.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-024-00801-6.

Recent advances in optical elastography and emerging opportunities in the basic sciences and translational medicine [Invited]

Optical elastography offers a rich body of imaging capabilities that can serve as a bridge between organ-level medical elastography and single-molecule biophysics. We review the methodologies and recent developments in optical coherence elastography, Brillouin microscopy, optical microrheology, and photoacoustic elastography. With an outlook toward maximizing the basic science and translational clinical impact of optical elastography technologies, we discuss potential ways that these techniques can integrate not only with each other, but also with supporting technologies and capabilities in other biomedical fields. By embracing cross-modality and cross-disciplinary interactions with these parallel fields, optical elastography can greatly increase its potential to drive new discoveries in the biomedical sciences as well as the development of novel biomechanics-based clinical diagnostics and therapeutics.

Subcellular mechano-microscopy: high resolution three-dimensional elasticity mapping using optical coherence microscopy

The importance of cellular-scale mechanical properties is well-established, yet it is challenging to map subcellular elasticity in three dimensions. We present subcellular mechano-microscopy, an optical coherence microscopy (OCM)-based variant of three-dimensional (3-D) compression optical coherence elastography (OCE) that provides an elasticity system resolution of 5 × 5 × 5 µm: a 7-fold improvement in system resolution over previous OCE studies of cells. The improved resolution is achieved through a ∼5-fold improvement in optical resolution, refinement of the strain estimation algorithm, and demonstration that mechanical deformation of subcellular features provides feature resolution far greater than that demonstrated previously on larger features with diameter >250 µm. We use mechano-microscopy to image adipose-derived stem cells encapsulated in gelatin methacryloyl. We compare our results with compression OCE and demonstrate that mechano-microscopy can provide contrast from subcellular features not visible using OCE.

Light-sheet photonic force optical coherence elastography for high-throughput quantitative 3D micromechanical imaging

Quantitative characterisation of micro-scale mechanical properties of the extracellular matrix (ECM) and dynamic cell-ECM interactions can significantly enhance fundamental discoveries and their translational potential in the rapidly growing field of mechanobiology. However, quantitative 3D imaging of ECM mechanics with cellular-scale resolution and dynamic monitoring of cell-mediated changes to pericellular viscoelasticity remain a challenge for existing mechanical characterisation methods. Here, we present light-sheet photonic force optical coherence elastography (LS-pfOCE) to address this need by leveraging a light-sheet for parallelised, non-invasive, and localised mechanical loading. We demonstrate the capabilities of LS-pfOCE by imaging the micromechanical heterogeneity of fibrous collagen matrices and perform live-cell imaging of cell-mediated ECM micromechanical dynamics. By providing access to 4D spatiotemporal variations in the micromechanical properties of 3D biopolymer constructs and engineered cellular systems, LS-pfOCE has the potential to drive new discoveries in mechanobiology and contribute to the development of novel biomechanics-based clinical diagnostics and therapies.

Impact of Doxorubicin on Cell-Substrate Topology

Variable-Angle Total Internal Reflection Fluorescence Microscopy (VA-TIRFM) is applied in view of early detection of cellular responses to the cytostatic drug doxorubicin. Therefore, we determined cell-substrate topology of cultivated CHO cells transfected with a membrane-associated Green Fluorescent Protein (GFP) in the nanometer range prior to and subsequent to the application of doxorubicin. Cell-substrate distances increased up to a factor of 2 after 24 h of application. A reduction of these distances by again a factor 2 was observed upon cell aging, and an influence of the cultivation time is presently discussed. Applicability of VA-TIRFM was supported by measurements of MCF-7 breast cancer cells after membrane staining and incubation with doxorubicin, when cell-substrate distances increased again by a factor ≥ 2. So far, our method needs well-defined cell ages and staining of cell membranes or transfection with GFP or related molecules. Use of intrinsic fluorescence or even light-scattering methods to various cancer cell lines could make this method more universal in the future, e.g., in the context of early detection of apoptosis.

A primer to traction force microscopy

Traction force microscopy (TFM) has emerged as a versatile technique for the measurement of single-cell-generated forces. TFM has gained wide use among mechanobiology laboratories, and several variants of the original methodology have been proposed. However, issues related to the experimental setup and, most importantly, data analysis of cell traction datasets may restrain the adoption of TFM by a wider community. In this review, we summarize the state of the art in TFM-related research, with a focus on the analytical methods underlying data analysis. We aim to provide the reader with a friendly compendium underlying the potential of TFM and emphasizing the methodological framework required for a thorough understanding of experimental data. We also compile a list of data analytics tools freely available to the scientific community for the furtherance of knowledge on this powerful technique.

Nonlinear microscopy using impulsive stimulated Brillouin scattering for high-speed elastography

The impulsive stimulated Brillouin microscopy promises fast, non-contact measurements of the elastic properties of biological samples. The used pump-probe approach employs an ultra-short pulse laser and a cw laser to generate Brillouin signals. Modeling of the microscopy technique has already been carried out partially, but not for biomedical applications. The nonlinear relationship between pulse energy and Brillouin signal amplitude is proven with both simulations and experiments. Tayloring of the excitation parameters on the biologically relevant polyacrylamide hydrogels outline sub-ms temporal resolutions at a relative precision of <1%. Brillouin microscopy using the impulsive stimulated scattering therefore exhibits high potential for the measurements of viscoelastic properties of cells and tissues.

Resolution-enhanced OCT and expanded framework of information capacity and resolution in coherent imaging

Spatial resolution in conventional optical microscopy has traditionally been treated as a fixed parameter of the optical system. Here, we present an approach to enhance transverse resolution in beam-scanned optical coherence tomography (OCT) beyond its aberration-free resolution limit, without any modification to the optical system. Based on the theorem of invariance of information capacity, resolution-enhanced (RE)-OCT navigates the exchange of information between resolution and signal-to-noise ratio (SNR) by exploiting efficient noise suppression via coherent averaging and a simple computational bandwidth expansion procedure. We demonstrate a resolution enhancement of 1.5 × relative to the aberration-free limit while maintaining comparable SNR in silicone phantom. We show that RE-OCT can significantly enhance the visualization of fine microstructural features in collagen gel and ex vivo mouse brain. Beyond RE-OCT, our analysis in the spatial-frequency domain leads to an expanded framework of information capacity and resolution in coherent imaging that contributes new implications to the theory of coherent imaging. RE-OCT can be readily implemented on most OCT systems worldwide, immediately unlocking information that is beyond their current imaging capabilities, and so has the potential for widespread impact in the numerous areas in which OCT is utilized, including the basic sciences and translational medicine.

Revisiting tissue tensegrity: Biomaterial-based approaches to measure forces across length scales

Cell-generated forces play a foundational role in tissue dynamics and homeostasis and are critically important in several biological processes, including cell migration, wound healing, morphogenesis, and cancer metastasis. Quantifying such forces in vivo is technically challenging and requires novel strategies that capture mechanical information across molecular, cellular, and tissue length scales, while allowing these studies to be performed in physiologically realistic biological models. Advanced biomaterials can be designed to non-destructively measure these stresses in vitro, and here, we review mechanical characterizations and force-sensing biomaterial-based technologies to provide insight into the mechanical nature of tissue processes. We specifically and uniquely focus on the use of these techniques to identify characteristics of cell and tissue “tensegrity:” the hierarchical and modular interplay between tension and compression that provide biological tissues with remarkable mechanical properties and behaviors. Based on these observed patterns, we highlight and discuss the emerging role of tensegrity at multiple length scales in tissue dynamics from homeostasis, to morphogenesis, to pathological dysfunction.

Traction force microscopy - Measuring the forces exerted by cells

Cells generate mechanical forces (traction forces, TFs) while interacting with the extracellular matrix or neighbouring cells. Forces are generated by both cells and extracellular matrix (ECM) and transmitted within the cell-ECM or cell-cell contacts involving focal adhesions or adherens junctions. Within more than two decades, substantial progress has been achieved in techniques that measure TFs. One of the techniques is traction force microscopy (TFM). This review discusses the TFM and its advances in measuring TFs exerted by cells (single cells and multicellular systems) at cell-ECM and cell-cell junctional intracellular interfaces. The answers to how cells sense, adapt and respond to mechanical forces unravel their role in controlling and regulating cell behaviour in normal and pathological conditions.

Soft Polymer-Based Technique for Cellular Force Sensing

Soft polymers have emerged as a vital type of material adopted in biomedical engineering to perform various biomechanical characterisations such as sensing cellular forces. Distinct advantages of these materials used in cellular force sensing include maintaining normal functions of cells, resembling in vivo mechanical characteristics, and adapting to the customised functionality demanded in individual applications. A wide range of techniques has been developed with various designs and fabrication processes for the desired soft polymeric structures, as well as measurement methodologies in sensing cellular forces. This review highlights the merits and demerits of these soft polymer-based techniques for measuring cellular contraction force with emphasis on their quantitativeness and cell-friendliness. Moreover, how the viscoelastic properties of soft polymers influence the force measurement is addressed. More importantly, the future trends and advancements of soft polymer-based techniques, such as new designs and fabrication processes for cellular force sensing, are also addressed in this review.

A Review of Single-Cell Adhesion Force Kinetics and Applications

Cells exert, sense, and respond to the different physical forces through diverse mechanisms and translating them into biochemical signals. The adhesion of cells is crucial in various developmental functions, such as to maintain tissue morphogenesis and homeostasis and activate critical signaling pathways regulating survival, migration, gene expression, and differentiation. More importantly, any mutations of adhesion receptors can lead to developmental disorders and diseases. Thus, it is essential to understand the regulation of cell adhesion during development and its contribution to various conditions with the help of quantitative methods. The techniques involved in offering different functionalities such as surface imaging to detect forces present at the cell-matrix and deliver quantitative parameters will help characterize the changes for various diseases. Here, we have briefly reviewed single-cell mechanical properties for mechanotransduction studies using standard and recently developed techniques. This is used to functionalize from the measurement of cellular deformability to the quantification of the interaction forces generated by a cell and exerted on its surroundings at single-cell with attachment and detachment events. The adhesive force measurement for single-cell microorganisms and single-molecules is emphasized as well. This focused review should be useful in laying out experiments which would bring the method to a broader range of research in the future.

Computational 4D-OCM for label-free imaging of collective cell invasion and force-mediated deformations in collagen

Traction force microscopy (TFM) is an important family of techniques used to measure and study the role of cellular traction forces (CTFs) associated with many biological processes. However, current standard TFM methods rely on imaging techniques that do not provide the experimental capabilities necessary to study CTFs within 3D collective and dynamic systems embedded within optically scattering media. Traction force optical coherence microscopy (TF-OCM) was developed to address these needs, but has only been demonstrated for the study of isolated cells embedded within optically clear media. Here, we present computational 4D-OCM methods that enable the study of dynamic invasion behavior of large tumor spheroids embedded in collagen. Our multi-day, time-lapse imaging data provided detailed visualizations of evolving spheroid morphology, collagen degradation, and collagen deformation, all using label-free scattering contrast. These capabilities, which provided insights into how stromal cells affect cancer progression, significantly expand access to critical data about biophysical interactions of cells with their environment, and lay the foundation for future efforts toward volumetric, time-lapse reconstructions of collective CTFs with TF-OCM.

Ageing modulates human dermal fibroblast contractility: Quantification using nano-biomechanical testing

Dermal fibroblasts play a key role in maintaining homoeostasis and functionality of the skin. Their contractility plays a role in changes observed during ageing, especially in processes such as wound healing, inflammation, wrinkling and scar tissue formation as well as structural changes on extracellular matrix. Although alternations in skin physiology and morphology have been previously described, there remains a paucity of information about the influence of chronological ageing on dermal fibroblast contractility. In this study, we applied a novel nano-biomechanical technique on cell-embedded collagen hydrogels in combination with mathematical modelling and numerical simulation to measure contraction forces of normal human dermal fibroblasts (NHDF). We achieved quantitative differentiation of the contractility of cells derived from 'young' (< 30 years old) and 'aged' (> 60 years old) donors. Transforming growth factor β1 (TGF-β1) was used to stimulate the fibroblasts to assess their contractile potential. NHDF from aged donors exhibited a greater basal contractile force, while in contrast, NHDF from young donors have shown a significantly larger contractile force in response to TGF-β1 treatment. These findings validate our nano-biomechanical measurement technique and provide new insights for considering NHDF contractility in regenerative medicine and as a biomarker of dermal ageing processes.

Obesity-associated Adipose Stromal Cells Promote Breast Cancer Invasion Through Direct Cell Contact and ECM Remodeling

Obesity increases the risk and worsens the prognosis for breast cancer due, in part, to altered adipose stromal cell (ASC) behavior. Whether ASCs from obese individuals increase migration of breast cancer cells relative to their lean counterparts, however, remains unclear. To test this connection, multicellular spheroids composed of MCF10A-derived tumor cell lines of varying malignant potential and lean or obese ASCs were embedded into collagen scaffolds mimicking the elastic moduli of interstitial breast adipose tissue. Confocal image analysis suggests that tumor cells alone migrate insignificantly under these conditions. However, direct cell-cell contact with either lean or obese ASCs enables them to migrate collectively, whereby obese ASCs activate tumor cell migration more effectively than their lean counterparts. Time-resolved optical coherence tomography (OCT) imaging suggests that obese ASCs facilitate tumor cell migration by mediating contraction of local collagen fibers. Matrix metalloproteinase (MMP)-dependent proteolytic activity significantly contributes to ASC-mediated tumor cell invasion and collagen deformation. However, ASC contractility is also important, as co-inhibition of both MMPs and contractility is necessary to completely abrogate ASC-mediated tumor cell migration. These findings imply that obesity-mediated changes of ASC phenotype may impact tumor cell migration and invasion with potential implications for breast cancer malignancy in obese patients.

Physicochemical Tools for Visualizing and Quantifying Cell-Generated Forces

To discern how mechanical forces coordinate biological outcomes, methods that map cell-generated forces in a spatiotemporal manner, and at cellular length scales, are critical. In their native environment, whether it be within compact multicellular three-dimensional structures or sparsely populated fibrillar networks of the extracellular matrix, cells are constantly exposed to a slew of physical forces acting on them from all directions. At the same time, cells exert highly localized forces of their own on their surroundings and on neighboring cells. Together, the generation and transmission of these forces can control diverse cellular activities and behavior as well as influence cell fate decisions. To thoroughly understand these processes, we must first be able to characterize and measure such forces. However, our experimental needs and technical capabilities are in discord-while it is apparent that we should study cell-generated forces within more biologically relevant 3D environments, this goal remains challenging because of caveats associated with complex "sensing-transduction-readout" modalities. In this Review, we will discuss the latest techniques for measuring cell-generated forces. We will highlight recent advances in traction force microscopy and examine new alternative approaches for quantifying cell-generated forces, both of individual cells and within 3D tissues. Finally, we will explore the future direction of novel cellular force-sensing tools in the context of mechanobiology and next-generation biomaterials design.

Development of a 3-D Physical Dynamics Monitoring System Using OCM with DVC for Quantification of Sprouting Endothelial Cells Interacting with a Collagen Matrix

The extracellular matrix (ECM) plays a key role during cell migration, proliferation, and differentiation by providing adhesion sites and serving as a physical scaffold. Elucidating the interaction between the cell and ECM can reveal the underlying mechanisms of cellular behavior that are currently unclear. Analysis of the deformation of the ECM due to cell-matrix interactions requires microscopic, three-dimensional (3-D) imaging methods, such as confocal microscopy and second-harmonic generation microscopy, which are currently limited by phototoxicity and bleaching as a result of the point-scanning approach. In this study, we suggest the use of optical coherence microscopy (OCM) as a live-cell, volumetric, fast imaging tool for analyzing the deformation of fibrous ECM. We optimized such OCM parameters as the sampling rate to obtain images of the best quality that meet the requirements for robust digital volume correlation (DVC) analysis. Visualization and analysis of the mechanical interaction between collagen ECM and human umbilical vein endothelial cells (HUVECs) show that cellular adhesion during protrusion can be analyzed and quantified. The advantages of OCM, such as fine isotropic spatial resolution, fast time resolution, and low phototoxicity, make it the ideal optic tool for 3-D traction force microscopy.

Three-dimensional imaging of cell and extracellular matrix elasticity using quantitative micro-elastography

Recent studies in mechanobiology have revealed the importance of cellular and extracellular mechanical properties in regulating cellular function in normal and disease states. Although it is established that cells should be investigated in a three-dimensional (3-D) environment, most techniques available to study mechanical properties on the microscopic scale are unable to do so. In this study, for the first time, we present volumetric images of cellular and extracellular elasticity in 3-D biomaterials using quantitative micro-elastography (QME). We achieve this by developing a novel strain estimation algorithm based on 3-D linear regression to improve QME system resolution. We show that QME can reveal elevated elasticity surrounding human adipose-derived stem cells (ASCs) embedded in soft hydrogels. We observe, for the first time in 3-D, further elevation of extracellular elasticity around ASCs with overexpressed TAZ; a mechanosensitive transcription factor which regulates cell volume. Our results demonstrate that QME has the potential to study the effects of extracellular mechanical properties on cellular functions in a 3-D micro-environment.

Optical elastography and tissue biomechanics

Mechanical forces play an important role in the behavior and development of biological systems and disease at all spatial scales, from cells and their constituents to tissues and organs. Such forces have a profound influence on the health, structural integrity, and normal function of cells and organs. Accurate knowledge of cell and tissue biomechanical properties is essential to map the distribution of forces and mechanical cues in biological systems. Cell and tissue biomechanical properties are also known to be important on their own as indicators of health or diseases state. Hence, optical elastography and biomechanics methods can aid in the understanding and clinical diagnosis of a wide variety of diseases. We provide a brief overview and highlight of the Optical Elastography and Tissue Biomechanics VI conference, which took place in San Francisco, February 2 and 3, 2019, as a part of Photonics West symposium.

Computed optical coherence microscopy of mouse brain ex vivo

The compromise between lateral resolution and usable imaging depth range is a bottleneck for optical coherence tomography (OCT). Existing solutions for optical coherence microscopy (OCM) suffer from either large data size and long acquisition time or a nonideal point spread function. We present volumetric OCM of mouse brain ex vivo with a large depth coverage by leveraging computational adaptive optics (CAO) to significantly reduce the number of OCM volumes that need to be acquired with a Gaussian beam focused at different depths. We demonstrate volumetric reconstruction of ex-vivo mouse brain with lateral resolution of 2.2  μm, axial resolution of 4.7  μm, and depth range of ∼1.2  mm optical path length, using only 11 OCT data volumes acquired on a spectral-domain OCM system. Compared to focus scanning with step size equal to the Rayleigh length of the beam, this is a factor of 4 fewer datasets required for volumetric imaging. Coregistered two-photon microscopy confirmed that CAO-OCM reconstructions can visualize various tissue microstructures in the brain. Our results also highlight the limitations of CAO in highly scattering media, particularly when attempting to reconstruct far from the focal plane or when imaging deep within the sample.

Impulsive stimulated Brillouin microscopy for non-contact, fast mechanical investigations of hydrogels

The mechanical properties of tissues and cells are increasingly recognized as an important feature for the understanding of pathological processes and as a diagnostic tool in biomedicine. Impulsive stimulated Brillouin scattering (ISBS) is promising to overcome shortcomings of other measurement methods such as invasiveness, low spatial resolution and long acquisition time. In this paper, we present for the first time ISBS measurements of hydrogels, which are model materials for biological samples. We demonstrate ISBS measurements discriminating hydrogels of different stiffness. ISBS measurements with lateral resolution close to cellular level are presented. These results underline that ISBS microscopy has a high potential for biomedical applications.

Microrheological quantification of viscoelastic properties with photonic force optical coherence elastography

Photonic force optical coherence elastography (PF-OCE) is a new approach for volumetric characterization of microscopic mechanical properties of three-dimensional viscoelastic medium. It is based on measurements of the complex mechanical response of embedded micro-beads to harmonically modulated radiation-pressure force from a weakly-focused beam. Here, we utilize the Generalized Stokes-Einstein relation to reconstruct local complex shear modulus in polyacrylamide gels by combining PF-OCE measurements of bead mechanical responses and experimentally measured depth-resolved radiation-pressure force profile of our forcing beam. Data exclusion criteria for quantitative PF-OCE based on three noise-related parameters were identified from the analysis of measurement noise at key processing steps. Shear storage modulus measured by quantitative PF-OCE was found to be in good agreement with standard shear rheometry, whereas shear loss modulus was in agreement with previously published atomic force microscopy results. The analysis and results presented here may serve to inform practical, application-specific implementations of PF-OCE, and establish the technique as a viable tool for quantitative mechanical microscopy.

Quantitative characterization of 3D bioprinted structural elements under cell generated forces

With improving biofabrication technology, 3D bioprinted constructs increasingly resemble real tissues. However, the fundamental principles describing how cell-generated forces within these constructs drive deformations, mechanical instabilities, and structural failures have not been established, even for basic biofabricated building blocks. Here we investigate mechanical behaviours of 3D printed microbeams made from living cells and extracellular matrix, bioprinting these simple structural elements into a 3D culture medium made from packed microgels, creating a mechanically controlled environment that allows the beams to evolve under cell-generated forces. By varying the properties of the beams and the surrounding microgel medium, we explore the mechanical behaviours exhibited by these structures. We observe buckling, axial contraction, failure, and total static stability, and we develop mechanical models of cell-ECM microbeam mechanics. We envision these models and their generalizations to other fundamental 3D shapes to facilitate the predictable design of biofabricated structures using simple building blocks in the future.

Automated sensorless single-shot closed-loop adaptive optics microscopy with feedback from computational adaptive optics

Traditional wavefront-sensor-based adaptive optics (AO) techniques face numerous challenges that cause poor performance in scattering samples. Sensorless closed-loop AO techniques overcome these challenges by optimizing an image metric at different states of a deformable mirror (DM). This requires acquisition of a series of images continuously for optimization - an arduous task in dynamic in vivo samples. We present a technique where the different states of the DM are instead simulated using computational adaptive optics (CAO). The optimal wavefront is estimated by performing CAO on an initial volume to minimize an image metric, and then the pattern is translated to the DM. In this paper, we have demonstrated this technique on a spectral-domain optical coherence microscope for three applications: real-time depth-wise aberration correction, single-shot volumetric aberration correction, and extension of depth-of-focus. Our technique overcomes the disadvantages of sensor-based AO, reduces the number of image acquisitions compared to traditional sensorless AO, and retains the advantages of both computational and hardware-based AO.