Standardized microgel beads as elastic cell mechanical probes

The characterization of cell traction force on nonflat surfaces with different curvature by elastic hydrogel microspheres

It is of great importance to study the detachment/attachment behaviors of cells (cancer cell, immune cell, and epithelial cell), as they are closely related with tumor metastasis, immunoreaction, and tissue development at variety scales. To characterize the detachment/attachment during the interaction between cells and substrate, some researchers proposed using cell traction force (CTF) as the indicator. To date, various strategies have been developed to measure the CTF. However, these methods only realize the measurements of cell passive forces on flat cases. To quantify the active CTF on nonflat surfaces, which can better mimic the in vivo case, we employed elastic hydrogel microspheres as a force sensor. The microspheres were fabricated by microfluidic chips with controllable size and mechanical properties to mimic substrate. Cells were cultured on microsphere and the CTF led to the deformation of microsphere. By detecting the morphology information, the CTF exerted by attached cells can be calculated by the in-house numerical code. Using these microspheres, the CTF of various cells (including tumor cell, immunological cell, and epithelium cell) were successfully obtained on nonflat surfaces with different curvature radii. The proposed method provides a versatile platform to measure the CTF with high precision and to understand the detachment/attachment behaviors during physiology processes.

Quantitative characterization of tumor cell traction force on extracellular matrix by hydrogel microsphere stress sensor

Cell traction force (CTF) is a kind of active force that is a cell senses external environment and actively applies to the contact matrix which is currently a representative stress in cell-extracellular matrix (ECM) interaction. Studying the distribution and variation of CTF during cell-ECM interaction help to explain the impact of physical factors on cell behaviors from the perspective of mechanobiology. However, most of the strategies of characterizing CTF are still limited by the measurement needs in three-dimensional (3D), quantitative characteristics and in vivo condition. Microsphere stress sensor (MSS) as a new type of technology is capable of realizing the quantitative characterization of CTF in 3D and in vivo. Herein, we employed microfluidic platform to design and fabricate MSS which possesses adjustable fluorescent performances, physical properties, and size ranges for better applicable to different cells (3T3, A549). Focusing on the common tumor cells behaviors (adhesion, spreading, and migration) in the process of metastasis, we chose SH-SY5Y as the representative research object in this work. We calculated CTF with the profile and distribution to demonstrate that the normal and shear stress can determined different cell behaviors. Additionally, CTF can also regulate cell adhesion, spreading, and migration in different cell states. Based on this method, the quantitative characterization of CFT of health and disease cells can be achieved, which further help to study and explore the potential mechanism of cell-ECM interaction.

High-throughput viscoelastic characterization of cells in hyperbolic microchannels

Extensive research has demonstrated the potential of cell viscoelastic properties as intrinsic indicators of cell state, functionality, and disease. For this, several microfluidic techniques have been developed to measure cell viscoelasticity with high-throughput. However, current microchannel designs introduce complex stress distributions on cells, leading to inaccuracies in determining the stress-strain relationship and, consequently, the viscoelastic properties. Here, we introduce a novel approach using hyperbolic microchannels that enable precise measurements under a constant extensional stress and offer a straightforward stress-strain relationship, while operating at a measurement rate of up to 100 cells per second. We quantified the stresses acting in the channels using mechanical calibration particles made from polyacrylamide (PAAm) and found that the measurement buffer, a solution of methyl cellulose and phosphate buffered saline, shows strain-thickening following a power law up to 200 s-1. By measuring oil droplets with varying viscosities, we successfully detected changes in the relaxation times of the droplets and our approach could be used to get the interfacial tension and viscosity of liquid-liquid droplet systems from the same measurement. We further applied this methodology to PAAm microgel beads, demonstrating the accurate recovery of Young's moduli and the near-ideal elastic behavior of the beads. To explore the influence of altered cell viscoelasticity, we treated HL60 human leukemia cells with latrunculin B and nocodazole, resulting in clear changes in cell stiffness while relaxation times were only minimally affected. In conclusion, our approach offers a streamlined and time-efficient solution for assessing the viscoelastic properties of large cell populations and other microscale soft particles.

Survival-Associated Cellular Response Maintained in Pancreatic Ductal Adenocarcinoma (PDAC) Switched Between Soft and Stiff 3D Microgel Culture

Pancreatic ductal adenocarcinoma (PDAC) accounts for about 90% of all pancreatic cancer cases. Five-year survival rates have remained below 12% since the 1970s, in part due to the difficulty in detection prior to metastasis (migration and invasion into neighboring organs and glands). Mechanical memory is a concept that has emerged over the past decade that may provide a path toward understanding how invading PDAC cells "remember" the mechanical properties of their diseased ("stiff", elastic modulus, E ≈ 10 kPa) microenvironment even while invading a healthy ("soft", E ≈ 1 kPa) microenvironment. Here, we investigated the role of mechanical priming by culturing a dilute suspension of PDAC (FG) cells within a 3D, rheologically tunable microgel platform from hydrogels with tunable mechanical properties. We conducted a suite of acute (short-term) priming studies where we cultured PDAC cells in either a soft (E ≈ 1 kPa) or stiff (E ≈ 10 kPa) environment for 6 h, then removed and placed them into a new soft or stiff 3D environment for another 18 h. Following these steps, we conducted RNA-seq analyses to quantify gene expression. Initial priming in the 3D culture showed persistent gene expression for the duration of the study, regardless of the subsequent environments (stiff or soft). Stiff 3D culture was associated with the downregulation of tumor suppressors (LATS1, BCAR3, CDKN2C), as well as the upregulation of cancer-associated genes (RAC3). Immunofluorescence staining (BCAR3, RAC3) further supported the persistence of this cellular response, with BCAR3 upregulated in soft culture and RAC3 upregulated in stiff-primed culture. Stiff-primed genes were stratified against patient data found in The Cancer Genome Atlas (TCGA). Upregulated genes in stiff-primed 3D culture were associated with decreased survival in patient data, suggesting a link between patient survival and mechanical priming.

Intravital measurements of solid stresses in tumours reveal length-scale and microenvironmentally dependent force transmission

In cancer, solid stresses impede the delivery of therapeutics to tumours and the trafficking and tumour infiltration of immune cells. Understanding such consequences and the origin of solid stresses requires their probing in vivo at the cellular scale. Here we report a method for performing volumetric and longitudinal measurements of solid stresses in vivo, and findings from its applicability to tumours. We used multimodal intravital microscopy of fluorescently labelled polyacrylamide beads injected in breast tumours in mice as well as mathematical modelling to compare solid stresses at the single-cell and tissue scales, in primary and metastatic tumours, in vitro and in mice, and in live mice and post-mortem tissue. We found that solid-stress transmission is scale dependent, with tumour cells experiencing lower stresses than their embedding tissue, and that tumour cells in lung metastases experience substantially higher solid stresses than those in the primary tumours. The dependence of solid stresses on length scale and the microenvironment may inform the development of therapeutics that sensitize cancer cells to such mechanical forces.

Varying the Stiffness and Diffusivity of Rod-Shaped Microgels Independently through Their Molecular Building Blocks

Microgels are water-swollen, crosslinked polymers that are widely used as colloidal building blocks in scaffold materials for tissue engineering and regenerative medicine. Microgels can be controlled in their stiffness, degree of swelling, and mesh size depending on their polymer architecture, crosslink density, and fabrication method-all of which influence their function and interaction with the environment. Currently, there is a lack of understanding of how the polymer composition influences the internal structure of soft microgels and how this morphology affects specific biomedical applications. In this report, we systematically vary the architecture and molar mass of polyethylene glycol-acrylate (PEG-Ac) precursors, as well as their concentration and combination, to gain insight in the different parameters that affect the internal structure of rod-shaped microgels. We characterize the mechanical properties and diffusivity, as well as the conversion of acrylate groups during photopolymerization, in both bulk hydrogels and microgels produced from the PEG-Ac precursors. Furthermore, we investigate cell-microgel interaction, and we observe improved cell spreading on microgels with more accessible RGD peptide and with a stiffness in a range of 20 kPa to 50 kPa lead to better cell growth.

Microfluidic tapered aspirators for mechanical characterization of microgel beads

In this study, we report a microfluidic approach for the measurement of mechanical properties of spherical microgel beads. This technique is analogous to tapered micropipette aspiration, while harnessing the benefits of microfluidics. We fabricate alginate-based microbeads and determine their mechanical properties using the microfluidic tapered aspirators. Individual microgel beads are aspirated and trapped in tapered channels, the deformed equilibrium shape is measured, and a stress balance is used to determine the Young's modulus. We investigate the effect of surface coating, taper angle, and bead diameter and find that the measured modulus is largely insensitive to these parameters. We show that the bead modulus increases with alginate concentration and follows a trend similar to that of the modulus measured using standard uniaxial compression. The critical pressure to squeeze out the beads from the tapered aspirators was found to depend on both the modulus and the bead diameter. Finally, we demonstrate how temporal changes in bead moduli due to enzymatic degradation of the hydrogel could be quantitatively determined. The results from this study highlight that the microfluidic tapered aspirators are a useful tool to measure hydrogel bead mechanics and have the potential to characterize dynamic changes in mechanical properties.

Measuring cell deformation by microfluidics

Microfluidics is an increasingly popular method for studying cell deformation, with various applications in fields such as cell biology, biophysics, and medical research. Characterizing cell deformation offers insights into fundamental cell processes, such as migration, division, and signaling. This review summarizes recent advances in microfluidic techniques for measuring cellular deformation, including the different types of microfluidic devices and methods used to induce cell deformation. Recent applications of microfluidics-based approaches for studying cell deformation are highlighted. Compared to traditional methods, microfluidic chips can control the direction and velocity of cell flow by establishing microfluidic channels and microcolumn arrays, enabling the measurement of cell shape changes. Overall, microfluidics-based approaches provide a powerful platform for studying cell deformation. It is expected that future developments will lead to more intelligent and diverse microfluidic chips, further promoting the application of microfluidics-based methods in biomedical research, providing more effective tools for disease diagnosis, drug screening, and treatment.

A new hyperelastic lookup table for RT-DC

Real-time deformability cytometry (RT-DC) is an established method that quantifies features like size, shape, and stiffness for whole cell populations on a single-cell level in real-time. A lookup table (LUT) disentangles the experimentally derived steady-state cell deformation and the projected area to extract the cell stiffness in the form of the Young's modulus. So far, two lookup tables exist but are limited to simple linear material models and cylindrical channel geometries. Here, we present two new lookup tables for RT-DC based on a neo-Hookean hyperelastic material numerically derived by simulations based on the finite element method in square and cylindrical channel geometries. At the same time, we quantify the influence of the shear-thinning behavior of the surrounding medium on the stationary deformation of cells in RT-DC and discuss the applicability and impact of the proposed LUTs regarding past and future RT-DC data analysis. Additionally, we provide insights about the cell strain and stresses, as well as the influence resulting from the rotational symmetric assumption on the cell deformation and volume estimation. The new lookup tables and the numerical cell shapes are made freely available.

Ultrasoft edge-labelled hydrogel sensors reveal internal tissue stress patterns in invasive engineered tumors

Measuring internal mechanical stresses within 3D tissues can provide important insights into drivers of morphogenesis and disease progression. Cell-sized hydrogel microspheres have recently emerged as a powerful technique to probe tissue mechanobiology, as they can be sufficiently soft as to deform within remodelling tissues, and optically imaged to measure internal stresses. However, measuring stresses at resolutions of ∼10 Pa requires ultrasoft, low-polymer content hydrogel formulations that are challenging to label with sufficiently fluorescent materials to support repeated measurements, particularly in optically dense tissues over 100 μm thick, as required in cancer tumor models. Here, we leverage thermodynamic partitioning of hydrogel components to create "edge-labelled" ultrasoft hydrogel microdroplets, in a single polymerization step. Bright and stable fluorescent nanoparticles preferentially polymerize at the hydrogel droplet interface, and can be used to repeatedly track sensor surfaces over long-term experiments, even when embedded deep in light-scattering tissues. We utilize these edge-labelled microspherical stress gauges (eMSGs) in inducible breast cancer tumor models of invasion, and demonstrate distinctive internal stress patterns that arise from cell-matrix interactions at different stages of breast cancer progression. Our studies demonstrate a long-term macroscale compaction of the tumor during matrix encapsulation, but only a short-term increase in local stress as non-invasive tumors rapidly make small internal reorganizations that reduce the mechanical stress to baseline levels. In contrast, once invasion programs are initiated, internal stress throughout the tumor is negligible. These findings suggest that internal tumor stresses may initially prime the cells to invade, but are lost once invasion occurs. Together, this work demonstrates that mapping internal mechanical stress in tumors may have utility in advancing cancer prognostic strategies, and that eMSGs can have broad utility in understanding dynamic mechanical processes of disease and development.

Image-based cell sorting using focused travelling surface acoustic waves

Sorting cells is an essential primary step in many biological and clinical applications such as high-throughput drug screening, cancer research and cell transplantation. Cell sorting based on their mechanical properties has long been considered as a promising label-free biomarker that could revolutionize the isolation of cells from heterogeneous populations. Recent advances in microfluidic image-based cell analysis combined with subsequent label-free sorting by on-chip actuators demonstrated the possibility of sorting cells based on their physical properties. However, the high purity of sorting is achieved at the expense of a sorting rate that lags behind the analysis throughput. Furthermore, stable and reliable system operation is an important feature in enabling the sorting of small cell fractions from a concentrated heterogeneous population. Here, we present a label-free cell sorting method, based on the use of focused travelling surface acoustic wave (FTSAW) in combination with real-time deformability cytometry (RT-DC). We demonstrate the flexibility and applicability of the method by sorting distinct blood cell types, cell lines and particles based on different physical parameters. Finally, we present a new strategy to sort cells based on their mechanical properties. Our system enables the sorting of up to 400 particles per s. Sorting is therefore possible at high cell concentrations (up to 36 million per ml) while retaining high purity (>92%) for cells with diverse sizes and mechanical properties moving in a highly viscous buffer. Sorting of small cell fraction from a heterogeneous population prepared by processing of small sample volume (10 μl) is also possible and here demonstrated by the 667-fold enrichment of white blood cells (WBCs) from raw diluted whole blood in a continuous 10-hour sorting experiment. The real-time analysis of multiple parameters together with the high sensitivity and high-throughput of our method thus enables new biological and therapeutic applications in the future.

Acousto-holographic reconstruction of whole-cell stiffness maps

Accurate assessment of cell stiffness distribution is essential due to the critical role of cell mechanobiology in regulation of vital cellular processes like proliferation, adhesion, migration, and motility. Stiffness provides critical information in understanding onset and progress of various diseases, including metastasis and differentiation of cancer. Atomic force microscopy and optical trapping set the gold standard in stiffness measurements. However, their widespread use has been hampered with long processing times, unreliable contact point determination, physical damage to cells, and unsuitability for multiple cell analysis. Here, we demonstrate a simple, fast, label-free, and high-resolution technique using acoustic stimulation and holographic imaging to reconstruct stiffness maps of single cells. We used this acousto-holographic method to determine stiffness maps of HCT116 and CTC-mimicking HCT116 cells and differentiate between them. Our system would enable widespread use of whole-cell stiffness measurements in clinical and research settings for cancer studies, disease modeling, drug testing, and diagnostics.

Sensor technologies for quality control in engineered tissue manufacturing

The use of engineered cells, tissues, and organs has the opportunity to change the way injuries and diseases are treated. Commercialization of these groundbreaking technologies has been limited in part by the complex and costly nature of their manufacture. Process-related variability and even small changes in the manufacturing process of a living product will impact its quality. Without real-time integrated detection, the magnitude and mechanism of that impact are largely unknown. Real-time and non-destructive sensor technologies are key for in-process insight and ensuring a consistent product throughout commercial scale-up and/or scale-out. The application of a measurement technology into a manufacturing process requires cell and tissue developers to understand the best way to apply a sensor to their process, and for sensor manufacturers to understand the design requirements and end-user needs. Furthermore, sensors to monitor component cells' health and phenotype need to be compatible with novel integrated and automated manufacturing equipment. This review summarizes commercially relevant sensor technologies that can detect meaningful quality attributes during the manufacturing of regenerative medicine products, the gaps within each technology, and sensor considerations for manufacturing.

PNIPAAm microgels with defined network architecture as temperature sensors in optical stretchers

Stretching individual living cells with light is a standard method to assess their mechanical properties. Yet, heat introduced by the laser light of optical stretchers may unwittingly change the mechanical properties of cells therein. To estimate the temperature induced by an optical trap, we introduce cell-sized, elastic poly(N-isopropylacrylamide) (PNIPAAm) microgels that relate temperature changes to hydrogel swelling. For their usage as a standardized calibration tool, we analyze the effect of free-radical chain-growth gelation (FCG) and polymer-analogous photogelation (PAG) on hydrogel network heterogeneity, micromechanics, and temperature response by Brillouin microscopy and optical diffraction tomography. Using a combination of tailor-made PNIPAAm macromers, PAG, and microfluidic processing, we obtain microgels with homogeneous network architecture. With that, we expand the capability of standardized microgels in calibrating and validating cell mechanics analysis, not only considering cell and microgel elasticity but also providing stimuli-responsiveness to consider dynamic changes that cells may undergo during characterization.

Quantitative phase imaging through an ultra-thin lensless fiber endoscope

Quantitative phase imaging (QPI) is a label-free technique providing both morphology and quantitative biophysical information in biomedicine. However, applying such a powerful technique to in vivo pathological diagnosis remains challenging. Multi-core fiber bundles (MCFs) enable ultra-thin probes for in vivo imaging, but current MCF imaging techniques are limited to amplitude imaging modalities. We demonstrate a computational lensless microendoscope that uses an ultra-thin bare MCF to perform quantitative phase imaging with microscale lateral resolution and nanoscale axial sensitivity of the optical path length. The incident complex light field at the measurement side is precisely reconstructed from the far-field speckle pattern at the detection side, enabling digital refocusing in a multi-layer sample without any mechanical movement. The accuracy of the quantitative phase reconstruction is validated by imaging the phase target and hydrogel beads through the MCF. With the proposed imaging modality, three-dimensional imaging of human cancer cells is achieved through the ultra-thin fiber endoscope, promising widespread clinical applications.

CAR T Cell Locomotion in Solid Tumor Microenvironment

The promising outcomes of chimeric antigen receptor (CAR) T cell therapy in hematologic malignancies potentiates its capability in the fight against many cancers. Nevertheless, this immunotherapy modality needs significant improvements for the treatment of solid tumors. Researchers have incrementally identified limitations and constantly pursued better CAR designs. However, even if CAR T cells are armed with optimal killer functions, they must overcome and survive suppressive barriers imposed by the tumor microenvironment (TME). In this review, we will discuss in detail the important role of TME in CAR T cell trafficking and how the intrinsic barriers contribute to an immunosuppressive phenotype and cancer progression. It is of critical importance that preclinical models can closely recapitulate the in vivo TME to better predict CAR T activity. Animal models have contributed immensely to our understanding of human diseases, but the intensive care for the animals and unreliable representation of human biology suggest in vivo models cannot be the sole approach to CAR T cell therapy. On the other hand, in vitro models for CAR T cytotoxic assessment offer valuable insights to mechanistic studies at the single cell level, but they often lack in vivo complexities, inter-individual heterogeneity, or physiologically relevant spatial dimension. Understanding the advantages and limitations of preclinical models and their applications would enable more reliable prediction of better clinical outcomes.

Microsphere sensors for characterizing stress fields within three-dimensional extracellular matrix

Stress in the three-dimensional extracellular matrix is one of the key cues in regulating multiscale biological processes. Thus far, noticeable progress in methods and techniques (e.g., micropipette aspiration, AFM, and molecule probes) has been made to quantify stress in cell microenvironment at different length scales. Among them, the microsphere sensor-based method (MSS-based method) has emerged as an advantageous approach over conventional techniques in quantifying stress in situ and in vivo at cellular and supra-cellular scales. This method is implemented by seven sequential steps, including fabrication, modification, characterization, cell adhesion, imaging, displacement field extraction and stress calculation. Precise control of each step and inter-tunning between steps can provide quantitative characterization of stress field. However, detailed procedural information associated with each step and process has been scattered. This review aims to provide a comprehensive overview of MSS-based method, systematically summarizing the principles and research progresses. Firstly, the basic principles are introduced, and the specific experiment and calculation processes of MSS-based method are presented in detail. Then, recent advances and applications of this method are summarized. Finally, perspectives of the limitations and development trends of MSS-based method are discussed. This specific and comprehensive review would provide a guideline for the widespread application of MSS-based method as an advantageous method for in situ and in vivo stress characterization at cellular and supra-cellular scale within three-dimensional extracellular matrix. STATEMENT OF SIGNIFICANCE: In this review, a method based on a microsphere sensor (MSS-based method) as an advantageous approach over conventional techniques in quantifying stress in situ and in vivo at cellular and supra-cellular scales is introduced and discussed. This technique is implemented by seven sequential steps, including fabrication, modification, characterization, cell junction, imaging, displacement field extraction, and stress calculation. Precise control of each step and inter-tunning between steps can provide quantitative stress field. However, detailed procedural information associated with each step has been scattered. Thus, a comprehensive review collating recent advances and perspective discussions is a necessity to introduce a better option for quantifying the stress field in biological processes at the cellular and supra-cellular scales.

Translating Therapeutic Microgels into Clinical Applications

Microgels are crosslinked, water-swollen networks with a 10 nm to 100 µm diameter and can be modified chemically or biologically to render them biocompatible for advanced clinical applications. Depending on their intended use, microgels require different mechanical and structural properties, which can be engineered on demand by altering the biochemical composition, crosslink density of the polymer network, and the fabrication method. Here, the fundamental aspects of microgel research and development, as well as their specific applications for theranostics and therapy in the clinic, are discussed. A detailed overview of microgel fabrication techniques with regards to their intended clinical application is presented, while focusing on how microgels can be employed as local drug delivery materials, scavengers, and contrast agents. Moreover, microgels can act as scaffolds for tissue engineering and regeneration application. Finally, an overview of microgels is given, which already made it into pre-clinical and clinical trials, while future challenges and chances are discussed. This review presents an instructive guideline for chemists, material scientists, and researchers in the biomedical field to introduce them to the fundamental physicochemical properties of microgels and guide them from fabrication methods via characterization techniques and functionalization of microgels toward specific applications in the clinic.

Modified Red Blood Cells as Multimodal Standards for Benchmarking Single-Cell Cytometry and Separation Based on Electrical Physiology

Biophysical cellular information at single-cell sensitivity is becoming increasingly important within analytical and separation platforms that associate the cell phenotype with markers of disease, infection, and immunity. Frequency-modulated electrically driven microfluidic measurement and separation systems offer the ability to sensitively identify single cells based on biophysical information, such as their size and shape, as well as their subcellular membrane morphology and cytoplasmic organization. However, there is a lack of reliable and reproducible model particles with well-tuned subcellular electrical phenotypes that can be used as standards to benchmark the electrical physiology of unknown cell types or to benchmark dielectrophoretic separation metrics of novel device strategies. Herein, the application of red blood cells (RBCs) as multimodal standard particles with systematically modulated subcellular electrophysiology and associated fluorescence level is presented. Using glutaraldehyde fixation to vary membrane capacitance and by membrane resealing after electrolyte penetration to vary interior cytoplasmic conductivity and fluorescence in a correlated manner, each modified RBC type can be identified at single-cell sensitivity based on phenomenological impedance metrics and fitted to dielectric models to compute biophysical information. In this manner, single-cell impedance data from unknown RBC types can be mapped versus these model RBC types for facile determination of subcellular biophysical information and their dielectrophoretic separation conditions, without the need for time-consuming algorithms that often require unknown fitting parameters. Such internal standards for biophysical cytometry can advance in-line phenotypic recognition strategies.

Correlative all-optical quantification of mass density and mechanics of subcellular compartments with fluorescence specificity

Quantitative measurements of physical parameters become increasingly important for understanding biological processes. Brillouin microscopy (BM) has recently emerged as one technique providing the 3D distribution of viscoelastic properties inside biological samples - so far relying on the implicit assumption that refractive index (RI) and density can be neglected. Here, we present a novel method (FOB microscopy) combining BM with optical diffraction tomography and epifluorescence imaging for explicitly measuring the Brillouin shift, RI, and absolute density with specificity to fluorescently labeled structures. We show that neglecting the RI and density might lead to erroneous conclusions. Investigating the nucleoplasm of wild-type HeLa cells, we find that it has lower density but higher longitudinal modulus than the cytoplasm. Thus, the longitudinal modulus is not merely sensitive to the water content of the sample - a postulate vividly discussed in the field. We demonstrate the further utility of FOB on various biological systems including adipocytes and intracellular membraneless compartments. FOB microscopy can provide unexpected scientific discoveries and shed quantitative light on processes such as phase separation and transition inside living cells.

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.

Processing of fast-gelling hydrogel precursors in microfluidics by electrocoalescence of reactive species

Microscopic hydrogels, also referred to as microgels, find broad application in life and materials science. A well-established technique for fabricating uniform microgels is droplet microfluidics. Here, optimal mixing of hydrogel precursor components is crucial to yield homogeneous microgels with respect to their morphology, mechanics, and distribution of functional moieties. However, when processing premixed polymer precursors that are highly reactive, fast or even instantaneous gelation inside fluid reservoirs or the microchannels of the flow cell commonly occurs, leading to an increase of fluid viscosity over time, and thus exacerbating the intrinsic control over fluid flow rates, droplet and microgel uniformity, which are key selling points of microfluidics in material design. To address these challenges, we utilize microflow cells with integrated electrodes, which enable fast addition and mixing of hydrogel precursors on demand by means of emulsion droplet coalescence. Here, two populations of surfactant-stabilized aqueous droplets - the first containing the material basis of the microgel, and the second containing another gel-forming component (e.g., a crosslinker) are formed at two consecutive microchannel junctions and merged via temporary thin-film instability. Our approach provides the ability to process such hydrogel systems that are otherwise challenging to process into uniform droplets and microgels by conventional droplet microfluidics. To demonstrate its versatility, we fabricate microgels with uniform shape and composition using fast hydrogelation via thiol-Michael addition reaction or non-covalent self-assembly. Furthermore, we elucidate the limitations of electrocoalescence of reactive hydrogel precursors by processing sodium alginate, crosslinked by calcium-induced ionic interactions. For this instantaneous type of hydrogelation, electrocoalescence of alginate and calcium ions does not result in the formation of morphologically isotropic microgels. Instead, it enables the creation of anisotropic microgel morphologies with tunable shape, which have previously only been achieved by selective crosslinking of elaborate higher-order emulsions or by aqueous two-phase systems as microgel templates.

Evaluating sources of technical variability in the mechano-node-pore sensing pipeline and their effect on the reproducibility of single-cell mechanical phenotyping

Cellular mechanical properties can reveal physiologically relevant characteristics in many cell types, and several groups have developed microfluidics-based platforms to perform high-throughput single-cell mechanical testing. However, prior work has performed only limited characterization of these platforms' technical variability and reproducibility. Here, we evaluate the repeatability performance of mechano-node-pore sensing, a single-cell mechanical phenotyping platform developed by our research group. We measured the degree to which device-to-device variability and semi-manual data processing affected this platform's measurements of single-cell mechanical properties. We demonstrated high repeatability across the entire technology pipeline even for novice users. We then compared results from identical mechano-node-pore sensing experiments performed by researchers in two different laboratories with different analytical instruments, demonstrating that the mechanical testing results from these two locations are in agreement. Our findings quantify the expectation of technical variability in mechano-node-pore sensing even in minimally experienced hands. Most importantly, we find that the repeatability performance we measured is fully sufficient for interpreting biologically relevant single-cell mechanical measurements with high confidence.

Microfluidic On-Chip Production of Alginate Hydrogels Using Double Coflow Geometry

Microfluidic on-chip production of microgels employing external gelation has numerous biological and pharmaceutical applications, particularly for the encapsulation of delicate cargos; however, the on-chip production of microgels in microfluidic devices can be challenging due to problems such as clogging caused by accelerated progress in precursor solution viscosity. Here, we introduce a novel microfluidic design incorporating two consecutive coflow geometries for microfluidic droplet generation. A shielding oil phase is employed to avoid emulsification and gelation stages from occurring simultaneously, thereby preventing clogging. The results revealed that the microfluidic device could generate highly monodispersed spherical droplets (coefficient of variation < 3%) with an average diameter in the range of 60-200 μm. Additionally, it was demonstrated that the device could appropriately create a shelter of the oil phase around the inner aqueous phase regardless of the droplet formation regime and flow conditions. The ability of the proposed microfluidic device in the generation of microgels was validated by producing alginate microgels utilizing an aqueous solution of calcium chloride as the continuous phase.

On the Determination of Mechanical Properties of Aqueous Microgels-Towards High-Throughput Characterization

Aqueous microgels are distinct entities of soft matter with mechanical signatures that can be different from their macroscopic counterparts due to confinement effects in the preparation, inherently made to consist of more than one domain (Janus particles) or further processing by coating and change in the extent of crosslinking of the core. Motivated by the importance of the mechanical properties of such microgels from a fundamental point, but also related to numerous applications, we provide a perspective on the experimental strategies currently available and emerging tools being explored. Albeit all techniques in principle exploit enforcing stress and observing strain, the realization differs from directly, as, e.g., by atomic force microscope, to less evident in a fluid field combined with imaging by a high-speed camera in high-throughput strategies. Moreover, the accompanying analysis strategies also reflect such differences, and the level of detail that would be preferred for a comprehensive understanding of the microgel mechanical properties are not always implemented. Overall, the perspective is that current technologies have the capacity to provide detailed, nanoscopic mechanical characterization of microgels over an extended size range, to the high-throughput approaches providing distributions over the mechanical signatures, a feature not readily accessible by atomic force microscopy and micropipette aspiration.

Toward deep biophysical cytometry: prospects and challenges

The biophysical properties of cells reflect their identities, underpin their homeostatic state in health, and define the pathogenesis of disease. Recent leapfrogging advances in biophysical cytometry now give access to this information, which is obscured in molecular assays, with a discriminative power that was once inconceivable. However, biophysical cytometry should go 'deeper' in terms of exploiting the information-rich cellular biophysical content, generating a molecular knowledge base of cellular biophysical properties, and standardizing the protocols for wider dissemination. Overcoming these barriers, which requires concurrent innovations in microfluidics, optical imaging, and computer vision, could unleash the enormous potential of biophysical cytometry not only for gaining a new mechanistic understanding of biological systems but also for identifying new cost-effective biomarkers of disease.

Elucidating the Biomechanics of Leukocyte Transendothelial Migration by Quantitative Imaging

Leukocyte transendothelial migration is crucial for innate immunity and inflammation. Upon tissue damage or infection, leukocytes exit blood vessels by adhering to and probing vascular endothelial cells (VECs), breaching endothelial cell-cell junctions, and transmigrating across the endothelium. Transendothelial migration is a critical rate-limiting step in this process. Thus, leukocytes must quickly identify the most efficient route through VEC monolayers to facilitate a prompt innate immune response. Biomechanics play a decisive role in transendothelial migration, which involves intimate physical contact and force transmission between the leukocytes and the VECs. While quantifying these forces is still challenging, recent advances in imaging, microfabrication, and computation now make it possible to study how cellular forces regulate VEC monolayer integrity, enable efficient pathfinding, and drive leukocyte transmigration. Here we review these recent advances, paying particular attention to leukocyte adhesion to the VEC monolayer, leukocyte probing of endothelial barrier gaps, and transmigration itself. To offer a practical perspective, we will discuss the current views on how biomechanics govern these processes and the force microscopy technologies that have enabled their quantitative analysis, thus contributing to an improved understanding of leukocyte migration in inflammatory diseases.

Investigating Tissue Mechanics in vitro Using Untethered Soft Robotic Microdevices

This paper presents the design, fabrication, and operation of a soft robotic compression device that is remotely powered by laser illumination. We combined the rapid and wireless response of hybrid nanomaterials with state-of-the-art microengineering techniques to develop machinery that can apply physiologically relevant mechanical loading. The passive hydrogel structures that constitute the compliant skeleton of the machines were fabricated using single-step in situ polymerization process and directly incorporated around the actuators without further assembly steps. Experimentally validated computational models guided the design of the compression mechanism. We incorporated a cantilever beam to the prototype for life-time monitoring of mechanical properties of cell clusters on optical microscopes. The mechanical and biochemical compatibility of the chosen materials with living cells together with the on-site manufacturing process enable seamless interfacing of soft robotic devices with biological specimen.

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.

Mechanical properties of cell- and microgel bead-laden oxidized alginate-gelatin hydrogels

3D-printing technologies, such as biofabrication, capitalize on the homogeneous distribution and growth of cells inside biomaterial hydrogels, ultimately aiming to allow for cell differentiation, matrix remodeling, and functional tissue analogues. However, commonly, only the mechanical properties of the bioinks or matrix materials are assessed, while the detailed influence of cells on the resulting mechanical properties of hydrogels remains insufficiently understood. Here, we investigate the properties of hydrogels containing cells and spherical PAAm microgel beads through multi-modal complex mechanical analyses in the small- and large-strain regimes. We evaluate the individual contributions of different filler concentrations and a non-fibrous oxidized alginate-gelatin hydrogel matrix on the overall mechanical behavior in compression, tension, and shear. Through material modeling, we quantify parameters that describe the highly nonlinear mechanical response of soft composite materials. Our results show that the stiffness significantly drops for cell- and bead concentrations exceeding four million per milliliter hydrogel. In addition, hydrogels with high cell concentrations (≥6 mio ml^-1) show more pronounced material nonlinearity for larger strains and faster stress relaxation. Our findings highlight cell concentration as a crucial parameter influencing the final hydrogel mechanics, with implications for microgel bead drug carrier-laden hydrogels, biofabrication, and tissue engineering.

Label-Free Multivariate Biophysical Phenotyping-Activated Acoustic Sorting at the Single-Cell Level

Biophysical markers of cells such as cellular electrical and mechanical properties have been proven as promising label-free biomarkers for studying, characterizing, and classifying different cell types and even their subpopulations. Further analysis or manipulation of specific cell types or subtypes requires accurate isolation of them from the original heterogeneous samples. However, there is currently a lack of cell sorting ability that could actively separate a large number of individual cells at the single-cell level based on their multivariate biophysical makers or phenotypes. In this work, we, for the first time, demonstrate label-free and high-throughput acoustic single-cell sorting activated by the characterization of multivariate biophysical phenotypes. Electrical phenotyping is implemented by single-cell electrical impedance characterization with two pairs of differential sensing electrodes, while mechanical phenotyping is performed by extracting the transit time for the single cell to pass through microconstriction from the recorded impedance signals. A real-time impedance signal processing and triggering algorithm has been developed to identify the target sample population and activate a pulsed highly focused surface acoustic wave for single-cell level sorting. We have demonstrated acoustic single-particle sorting solely based on electrical or mechanical phenotyping. Furthermore, we have applied the developed microfluidic system to sort live MCF-7 cells from a mixture of fixed and live MCF-7 population activated by a combined electrical and mechanical phenotyping at a high throughput >100 cells/s and purity ∼91.8%. This demonstrated ability to analyze and sort cells based on multivariate biophysical phenotyping provides a solution to the current challenges of cell purification that lack specific molecular biomarkers.

Interaction of a Migrating Cell Monolayer with a Flexible Fiber

Mechanical forces influence the development and behavior of biological tissues. In many situations, these forces are exerted or resisted by elastic compliant structures such as the own-tissue cellular matrix or other surrounding tissues. This kind of tissue-elastic body interactions are also at the core of many state-of-the-art in situ force measurement techniques employed in biophysics. This creates the need to model tissue interaction with the surrounding elastic bodies that exert these forces, raising the question of which are the minimal ingredients needed to describe such interactions. We conduct experiments in which migrating cell monolayers push on carbon fibers as a model problem. Although the migrating tissue is able to bend the fiber for some time, it eventually recoils before coming to a stop. This stop occurs when cells have performed a fixed mechanical work on the fiber, regardless of its stiffness. Based on these observations, we develop a minimal active-fluid model that reproduces the experiments and predicts quantitatively relevant features of the system. This minimal model points out the essential ingredients needed to describe tissue-elastic solid interactions: an effective inertia and viscous stresses.

Effect of elastic modulus on inertial displacement of cell-like particles in microchannels

Label-free microfluidic-based cell sorters leverage innate differences among cells (e.g., size and stiffness), to separate one cell type from another. This sorting step is crucial for many cell-based applications. Polystyrene-based microparticles (MPs) are the current gold standard for calibrating flow-based cell sorters and analyzers; however, the deformation behavior of these rigid materials is drastically different from that of living cells. Given this discrepancy in stiffness, an alternative calibration particle that better reflects cell elasticity is needed for the optimization of new and existing microfluidic devices. Here, we describe the fabrication of cell-like, mechanically tunable MPs and demonstrate their utility in quantifying differences in inertial displacement within a microfluidic constriction device as a function of particle elastic modulus, for the first time. Monodisperse, fluorescent, cell-like microparticles that replicate the size and modulus of living cells were fabricated from polyacrylamide within a microfluidic droplet generator and characterized via optical and atomic force microscopy. Trajectories of our cell-like MPs were mapped within the constriction device to predict where living cells of similar size/modulus would move. Calibration of the device with our MPs showed that inertial displacement depends on both particle size and modulus, with large/soft MPs migrating further toward the channel centerline than small/stiff MPs. The mapped trajectories also indicated that MP modulus contributed proportionally more to particle displacement than size, for the physiologically relevant ranges tested. The large shift in focusing position quantified here emphasizes the need for physiologically relevant, deformable MPs for calibrating and optimizing microfluidic separation platforms.

Stress driven fractionalization of vacancies in regular packings of elastic particles

Elucidating the interplay of defects and stress at the microscopic level is a fundamental physical problem that has a strong connection with materials science. Here, based on the two-dimensional crystal model, we show that the instability mode of vacancies with varying size and morphology conforms to a common scenario. A vacancy under compression is fissioned into a pair of dislocations that glide and vanish at the boundary. This neat process is triggered by the local shear stress around the vacancy. The remarkable fractionalization of vacancies creates rich modes of interaction between vacancies and other topological defects, and provides a new dimension for mechanical engineering of defects in extensive crystalline structures.

DryMass: handling and analyzing quantitative phase microscopy images of spherical, cell-sized objects

BACKGROUND

Quantitative phase imaging (QPI) is an established tool for the marker-free classification and quantitative characterization of biological samples. For spherical objects, such as cells in suspension, microgel beads, or liquid droplets, a single QPI image is sufficient to extract the radius and the average refractive index. This technique is invaluable, as it allows the characterization of large sample populations at high measurement rates. However, until now, no universal software existed that could perform this type of analysis. Besides the choice of imaging modality and the variety in imaging software, the main difficulty has been to automate the entire analysis pipeline from raw data to ensemble statistics.

RESULTS

We present DryMass, a powerful tool for QPI that covers all relevant steps from loading experimental data (multiple file formats supported), computing the phase data (built-in, automated hologram analysis), performing phase background corrections (offset, tilt, second order polynomial) to fitting scattering models (light projection, Rytov approximation, Mie simulations) to spherical phase objects for the extraction of dry mass, radius, and average refractive index. The major contribution of DryMass is a user-convenient, reliable, reproducible, and automated analysis pipeline for an arbitrary number of QPI datasets of arbitrary sizes.

CONCLUSION

DryMass is a leap forward for data analysis in QPI, as it not only makes it easier to visualize raw QPI data and reproduce previous results in the field, but it also opens up QPI analysis to users without a background in programming or phase imaging.

Rapid analysis of cell-generated forces within a multicellular aggregate using microsphere-based traction force microscopy

We present a new approach to measuring cell-generated forces from the deformations of elastic microspheres embedded within multicellular aggregates. By directly fitting the measured sensor deformation to an analytical model based on experimental observations and invoking linear elasticity, we dramatically reduce the computational complexity of the problem, and directly obtain the full 3D mapping of surface stresses. Our approach imparts extraordinary computational efficiency, allowing tractions to be estimated within minutes and enabling rapid analysis of microsphere-based traction force microscopy data.

High-throughput cell and spheroid mechanics in virtual fluidic channels

Microfluidics by soft lithography has proven to be of key importance for biophysics and life science research. While being based on replicating structures of a master mold using benchtop devices, design modifications are time consuming and require sophisticated cleanroom equipment. Here, we introduce virtual fluidic channels as a flexible and robust alternative to microfluidic devices made by soft lithography. Virtual channels are liquid-bound fluidic systems that can be created in glass cuvettes and tailored in three dimensions within seconds for rheological studies on a wide size range of biological samples. We demonstrate that the liquid-liquid interface imposes a hydrodynamic stress on confined samples, and the resulting strain can be used to calculate rheological parameters from simple linear models. In proof-of-principle experiments, we perform high-throughput rheology inside a flow cytometer cuvette and show the Young's modulus of isolated cells exceeds the one of the corresponding tissue by one order of magnitude.

Dancing with the Cells: Acoustic Microflows Generated by Oscillating Cells

The interaction of a sound or ultrasound wave with an elastic object, such as a microbubble, can give rise to a steady-state microstreaming flow in its surrounding liquid. Many microfluidic strategies for cell and particle manipulation, and analyte mixing, are based on this type of flow. In addition, there are reports that acoustic streaming can be generated in biological systems, for instance, in a mammalian inner ear. Here, new observations are reported that individual cells are able to induce microstreaming flow, when they are excited by controlled acoustic waves in vitro. Single adherent cells are exposed to an acoustic field inside a microfluidic device. The cell-induced microstreaming is then investigated by monitoring flow tracers around the cell, while the structure and extracellular environment of the cell are altered using different chemicals. The observations suggest that the maximum streaming flow induced by an MDA-MB-231 breast cancer cell can reach velocities on the order of mm s^-1 , and this maximum velocity is primarily governed by the overall cell stiffness. Therefore, such cell-induced microstreaming measurements, including flow pattern and velocity magnitude, may be used as label-free proxies of cellular mechanical properties, such as stiffness.

A new framework for characterization of poroelastic materials using indentation

To characterize a poroelastic material, typically an indenter is pressed onto the surface of the material with a ramp of a finite approach velocity followed by a hold where the indenter displacement is kept constant. This leads to deformation of the porous matrix, pressurization of the interstitial fluid and relaxation due to redistribution of fluid through the pores. In most studies the poroelastic properties, including elastic modulus, Poisson ratio and poroelastic diffusion coefficient, are extracted by assuming an instantaneous step indentation. However, exerting step like indentation is not experimentally possible and usually a ramp indentation with a finite approach velocity is applied. Moreover, the poroelastic relaxation time highly depends on the approach velocity in addition to the poroelastic diffusion coefficient and the contact area. Here, we extensively studied the effect of indentation velocity using finite element simulations which has enabled the formulation of a new framework based on a master curve that incorporates the finite rise time. To verify our novel framework, the poroelastic properties of two types of hydrogels were extracted experimentally using indentation tests at both macro and micro scales. Our new framework that is based on consideration of finite approach velocity is experimentally easy to implement and provides a more accurate estimation of poroelastic properties. STATEMENT OF SIGNIFICANCE: Hydrogels, tissues and living cells are constituted of a sponge-like porous elastic matrix bathed in an interstitial fluid. It has been shown that these materials behave according to the theory of 'poroelasticity' when mechanically stimulated in a way similar to that experienced in organs within the body. In this theory, the rate at which the fluid-filled sponge can be deformed is limited by how fast interstitial fluid can redistribute within the sponge in response to deformation. Here, we simulated indentation experiments at different rates and formulated a new framework that inherently captures the effects of stimulation speed on the mechanical response of poroelastic materials. We validated our framework by conducting experiments at different length-scales on agarose and polyacrylamide hydrogels.

Microparticle traction force microscopy reveals subcellular force exertion patterns in immune cell-target interactions

Force exertion is an integral part of cellular behavior. Traction force microscopy (TFM) has been instrumental for studying such forces, providing spatial force measurements at subcellular resolution. However, the applications of classical TFM are restricted by the typical planar geometry. Here, we develop a particle-based force sensing strategy for studying cellular interactions. We establish a straightforward batch approach for synthesizing uniform, deformable and tuneable hydrogel particles, which can also be easily derivatized. The 3D shape of such particles can be resolved with superresolution (<50 nm) accuracy using conventional confocal microscopy. We introduce a reference-free computational method allowing inference of traction forces with high sensitivity directly from the particle shape. We illustrate the potential of this approach by revealing subcellular force patterns throughout phagocytic engulfment and force dynamics in the cytotoxic T-cell immunological synapse. This strategy can readily be adapted for studying cellular forces in a wide range of applications.

Microscale Interrogation of 3D Tissue Mechanics

Cells in vivo live in a complex microenvironment composed of the extracellular matrix (ECM) and other cells. Growing evidence suggests that the mechanical interaction between the cells and their microenvironment is of critical importance to their behaviors under both normal and diseased conditions, such as migration, differentiation, and proliferation. The study of tissue mechanics in the past two decades, including the assessment of both mechanical properties and mechanical stresses of the extracellular microenvironment, has greatly enriched our knowledge about how cells interact with their mechanical environment. Tissue mechanical properties are often heterogeneous and sometimes anisotropic, which makes them difficult to obtain from macroscale bulk measurements. Mechanical stresses were first measured for cells cultured on two-dimensional (2D) surfaces with well-defined mechanical properties. While 2D measurements are relatively straightforward and efficient, and they have provided us with valuable knowledge on cell-ECM interactions, that knowledge may not be directly applicable to in vivo systems. Hence, the measurement of tissue stresses in a more physiologically relevant three-dimensional (3D) environment is required. In this mini review, we will summarize and discuss recent developments in using optical, magnetic, genetic, and mechanical approaches to interrogate 3D tissue stresses and mechanical properties at the microscale.

Colloidal crystals of compliant microgel beads to study cell migration and mechanosensitivity in 3D

Tissues are defined not only by their biochemical composition, but also by their distinct mechanical properties. It is now widely accepted that cells sense their mechanical environment and respond to it. However, studying the effects of mechanics in in vitro 3D environments is challenging since current 3D hydrogel assays convolve mechanics with gel porosity and adhesion. Here, we present novel colloidal crystals as modular 3D scaffolds where these parameters are principally decoupled by using monodisperse, protein-coated PAAm microgel beads as building blocks, so that variable stiffness regions can be achieved within one 3D colloidal crystal. Characterization of the colloidal crystal and oxygen diffusion simulations suggested the suitability of the scaffold to support cell survival and growth. This was confirmed by live-cell imaging and fibroblast culture over a period of four days. Moreover, we demonstrate unambiguous durotactic fibroblast migration and mechanosensitive neurite outgrowth of dorsal root ganglion neurons in 3D. This modular approach of assembling 3D scaffolds from mechanically and biochemically well-defined building blocks allows the spatial patterning of stiffness decoupled from porosity and adhesion sites in principle and provides a platform to investigate mechanosensitivity in 3D environments approximating tissues in vitro.

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.

Mechanoresponsive Hydrogel Particles as a Platform for Three-Dimensional Force Sensing

We introduce a novel concept for mechanosensitive hydrogel microparticles, which translate deformation into changes in fluorescence and can thus function as mechanical probes. The hydrogel particles with controlled polymer network are produced via droplet microfluidics from poly(ethylene glycol) (PEG) precursors. Förster resonance energy transfer donors and acceptors are coupled to the PEG hydrogel network for reporting local deformations as fluorescence shifts. We show that global network deformations, which occur upon drying/rehydration, can be detected via a characteristic fluorescence shift. Combined characterization with confocal laser scanning microscopy and atomic force microscopy (AFM) shows that also local deformation of the particles can be detected. Using AFM, the mechanical properties of the particles can be quantified, which allows linking strain with stress and thus force sensing in a three-dimensional environment. Microfluidic material design allows for precisely varying the size of our hydrogel microparticles as well as their mechanical properties and polymer network structure with regard to the choice of the macromolecular precursors and their functionalization with fluorophores. Thus, concomitant changes in mechanical properties and mechanosensitivity qualify these hydrogel microparticles as an adjustable material platform for force sensing in structural mechanics or cell culturing.

3D Microenvironment Stiffness Regulates Tumor Spheroid Growth and Mechanics via p21 and ROCK

The mechanical properties of cancer cells and their microenvironment contribute to breast cancer progression. While mechanosensing has been extensively studied using 2D substrates, much less is known about it in a physiologically more relevant 3D context. Here it is demonstrated that breast cancer tumor spheroids, growing in 3D polyethylene glycol-heparin hydrogels, are sensitive to their environment stiffness. During tumor spheroid growth, compressive stresses of up to 2 kPa build up, as quantitated using elastic polymer beads as stress sensors. Atomic force microscopy reveals that tumor spheroid stiffness increases with hydrogel stiffness. Also, constituent cell stiffness increases in a Rho associated kinase (ROCK)- and F-actin-dependent manner. Increased hydrogel stiffness correlated with attenuated tumor spheroid growth, a higher proportion of cells in G0/G1 phase, and elevated levels of the cyclin-dependent kinase inhibitor p21. Drug-mediated ROCK inhibition not only reverses cell stiffening upon culture in stiff hydrogels but also increases tumor spheroid growth. Taken together, a mechanism by which the growth of a tumor spheroid can be regulated via cytoskeleton rearrangements in response to its mechanoenvironment is revealed here. Thus, the findings contribute to a better understanding of how cancer cells react to compressive stress when growing under confinement in stiff environments.