Microsystems for biomechanical measurements

Engineering placental trophoblast fusion: A potential role for biomechanics in syncytialization

The process by which placental trophoblasts fuse to form the syncytiotrophoblast around the chorionic villi is not fully understood. Mechanical features of the in vivo and in vitro culture environments have recently emerged as having the potential to influence fusion efficiency, and considering these mechanical cues may ultimately allow predictive control of trophoblast syncytialization. Here, we review recent studies that suggest that biomechanical factors such as shear stress, tissue stiffness, and dimensionally-related stresses affect villous trophoblast fusion efficiency. We then discuss how these stimuli might arise in vivo and how they can be incorporated in cultures to study and enhance villous trophoblast fusion. We believe that this mechanical paradigm will provide novel insight into manipulating the syncytialization process to better engineer improved models, understand disease progression, and ultimately develop novel therapeutic strategies.

Design, Fabrication, and Validation of a Petri Dish-Compatible PDMS Bioreactor for the Tensile Stimulation and Characterization of Microtissues

In this paper, we report on a novel biocompatible micromechanical bioreactor (actuator and sensor) designed for the in situ manipulation and characterization of live microtissues. The purpose of this study was to develop and validate an application-targeted sterile bioreactor that is accessible, inexpensive, adjustable, and easily fabricated. Our method relies on a simple polydimethylsiloxane (PDMS) molding technique for fabrication and is compatible with commonly-used laboratory equipment and materials. Our unique design includes a flexible thin membrane that allows for the transfer of an external actuation into the PDMS beam-based actuator and sensor placed inside a conventional 35 mm cell culture Petri dish. Through computational analysis followed by experimental testing, we demonstrated its functionality, accuracy, sensitivity, and tunable operating range. Through time-course testing, the actuator delivered strains of over 20% to biodegradable electrospun poly (D, L-lactide-co-glycolide) (PLGA) 85:15 non-aligned nanofibers (~91 µm thick). At the same time, the sensor was able to characterize time-course changes in Young's modulus (down to 10-150 kPa), induced by an application of isopropyl alcohol (IPA). Furthermore, the actuator delivered strains of up to 4% to PDMS monolayers (~30 µm thick), simultaneously characterizing their elastic modulus up to ~2.2 MPa. The platform repeatedly applied dynamic (0.23 Hz) tensile stimuli to live Human Dermal Fibroblast (HDF) cells for 12 hours (h) and recorded the cellular reorientation towards two angle regimes, with averages of -58.85° and +56.02°. The device biocompatibility with live cells was demonstrated for one week, with no signs of cytotoxicity. We can conclude that our PDMS bioreactor is advantageous for low-cost tissue/cell culture micromanipulation studies involving mechanical actuation and characterization. Our device eliminates the need for an expensive experimental setup for cell micromanipulation, increasing the ease of live-cell manipulation studies by providing an affordable way of conducting high-throughput experiments without the need to open the Petri dish, reducing manual handling, cross-contamination, supplies, and costs. The device design, material, and methods allow the user to define the operational range based on their targeted samples/application.

Design and Fabrication by Thermal Imprint Lithography and Mechanical Characterization of a Ring-Based PDMS Soft Probe for Sensing and Actuating Forces in Biological Systems

In this paper, the design, fabrication and mechanical characterization of a novel polydimethylsiloxane (PDMS) soft probe for delivering and sensing forces in biological systems is proposed. On the basis of preliminary finite element (FEM) analysis, the design takes advantage of a suitable core geometry, characterized by a variable spring-like ring. The compliance of probes can be finely set in a wide range to measure forces in the micronewton to nanonewton range. In particular, this is accomplished by properly resizing the ring geometry and/or exploiting the mixing ratio-based elastic properties of PDMS. Fabrication by the thermal imprint lithography method allows fast and accurate tuning of ring sizes and tailoring of the contact section to their targets. By only varying geometrical parameters, the stiffness ranges from 1080 mNm^-1 to 50 mNm^-1, but by changing the base-curing agent proportion of the elastomer from 10:1 to 30:1, the stiffness drops to 37 mNm^-1. With these compliances, the proposed device will provide a new experimental tool for investigating force-dependent biological functions in sensory systems.

Biomechanical Characterization at the Cell Scale: Present and Prospects

The rapidly growing field of mechanobiology demands for robust and reproducible characterization of cell mechanical properties. Recent achievements in understanding the mechanical regulation of cell fate largely rely on technological platforms capable of probing the mechanical response of living cells and their physico–chemical interaction with the microenvironment. Besides the established family of atomic force microscopy (AFM) based methods, other approaches include optical, magnetic, and acoustic tweezers, as well as sensing substrates that take advantage of biomaterials chemistry and microfabrication techniques. In this review, we introduce the available methods with an emphasis on the most recent advances, and we discuss the challenges associated with their implementation.

MEMS measurements of single cell stiffness decay due to cyclic mechanical loading

The goal of this study was to measure the mechanical stiffness of individual cells and to observe changes due to the application of repeated cell mechanical loads. 28 single baker's yeast cells (Saccharomyces cerevisiae) were fatigue tested and had their stiffness measured during repetitive loading cycles performed by a MEMS squeezer in aqueous media. Electrothermal micro-actuators compressed individual cells against a reference back spring; cell and spring motions were measured using a FFT image analysis technique with ~10 nm resolution. Cell stiffness was calculated based on measurements of cell elongation vs. applied force which resulted in stiffness values in the 2-10 N/m range. The effect of increased force was studied for cells mechanically cycled 37 times. Cell stiffness decreased as the force and the cycle number increased. After 37 loading cycles (~4 min), forces of 0.24, 0.29, 0.31, and 0.33 μN caused stiffness drops of 5%, 13%, 31% and 41% respectively. Cells force was then set to 0.29 μN and cells were tested over longer runs of 118 and 268 cycles. After 118 cycles (~12 min) cells experienced an average stiffness drop of 68%. After 268 cycles (~25 min) cells had a stiffness drop of 77%, and appeared to reach a stiffness plateau of 20-25% of the initial stiffness after approximately 200 cycles.

Micro- and nano-force evaluation of bioengineered muscle cells: a non-contact two-dimensional biosensing using surface acoustic wave devices

A high degree of cell-generated force measurement is required to evaluate the biomechanical performance of bioengineered muscle tissues. However, the conventional cantilever types of direct force measurement methods have limitations in developing a non-contact two-dimensional force sensing device for a single muscle cell. In this paper, a method is proposed and discussed by using focused surface acoustic wave and magneto-optic Kerr measurements. To depict the capability of the proposed method, a conceptual design of such a sensory device is demonstrated for non-contact two-dimensional force measurement of a single muscle cell.

Mechano-transduction: from molecules to tissues

Biological mechano-transduction and force-dependent changes scale from protein conformation (â„« to nm) to cell organization and multi-cell function (mm to cm) to affect cell organization, fate, and homeostasis.

External forces play complex roles in cell organization, fate, and homeostasis. Changes in these forces, or how cells respond to them, can result in abnormal embryonic development and diseases in adults. How cells sense and respond to these mechanical stimuli requires an understanding of the biophysical principles that underlie changes in protein conformation and result in alterations in the organization and function of cells and tissues. Here, we discuss mechano-transduction as it applies to protein conformation, cellular organization, and multi-cell (tissue) function.

hiPSC Modeling of Inherited Cardiomyopathies

OPINION STATEMENT

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) represent a powerful new model system to study the basic mechanisms of inherited cardiomyopathies. hiPSC-CMs have been utilized to model several cardiovascular diseases, achieving the most success in the inherited arrhythmias, including long QT and Timothy syndromes (Moretti et al. N Engl J Med. 363:1397-409, 2010; Yazawa et al. Nature. 471:230-4, 2011) and arrhythmogenic right ventricular dysplasia (ARVD) (Ma et al. Eur Heart J. 34:1122-33, 2013). Recently, studies have applied hiPSC-CMs to the study of both dilated (DCM) (Sun et al. Sci Transl Med. 4:130ra47, 2012) and hypertrophic (HCM) cardiomyopathies (Lan et al. Cell Stem Cell. 12:101-13, 2013; Carvajal-Vergara et al. Nature. 465:808-12, 2010), providing new insights into basic mechanisms of disease. However, hiPSC-CMs do not recapitulate many of the structural and functional aspects of mature human cardiomyocytes, instead mirroring an immature - embryonic or fetal - phenotype. Much work remains in order to better understand these differences, as well as to develop methods to induce hiPSC-CMs into a fully mature phenotype. Despite these limitations, hiPSC-CMs represent the best current in vitro correlate of the human heart and an invaluable tool in the search for mechanisms underlying cardiomyopathy and for screening new pharmacologic therapies.

LV twisting and untwisting in HCM: ejection begets filling. Diastolic functional aspects of HCM

Conventional and emerging concepts on mechanisms by which hypertrophic cardiomyopathy (HCM) engenders diastolic dysfunction are surveyed. A shift from familiar left ventricular (LV) diastolic function approaches to large-scale (twist-untwist) and small-scale (titin unfolding-refolding, etc.) wall rebound models, incorporating interaction and dynamic distortions and rearrangements of myofiber sheets and ultrastructural constituents, is suggested. Such an emerging new paradigm of diastolic dynamics, emphasizing the relationship of myofiber sheet and ultraconstituent distortion to LV mechanics and end-systolic shape, might clarify intricate patterns of early diastolic rebound and suction, needed for LV filling in many of the polymorphic phenotypes of HCM.

Actuation means for the mechanical stimulation of living cells via microelectromechanical systems: A critical review

Within a living body, cells are constantly exposed to various mechanical constraints. As a matter of fact, these mechanical factors play a vital role in the regulation of the cell state. It is widely recognized that cells can sense, react and adapt themselves to mechanical stimulation. However, investigations aimed at studying cell mechanics directly in vivo remain elusive. An alternative solution is to study cell mechanics via in vitro experiments. Nevertheless, this requires implementing means to mimic the stresses that cells naturally undergo in their physiological environment. In this paper, we survey various microelectromechanical systems (MEMS) dedicated to the mechanical stimulation of living cells. In particular, we focus on their actuation means as well as their inherent capabilities to stimulate a given amount of cells. Thereby, we report actuation means dependent upon the fact they can provide stimulation to a single cell, target a maximum of a hundred cells, or deal with thousands of cells. Intrinsic performances, strengths and limitations are summarized for each type of actuator. We also discuss recent achievements as well as future challenges of cell mechanostimulation.

Mechanical interactions of mouse mammary gland cells with collagen in a three-dimensional construct

An effort to understand the development of breast cancer motivates the study of mammary gland cells and their interactions with the extracellular matrix. A mixture of mammary gland epithelial cells (normal murine mammary gland), collagen, and fluorescent beads was loaded into microchannels and observed via four-dimensional imaging. Collagen concentrations of 1.3, 2, and 3 mg/mL were used. The displacements of the beads were used to calculate strains in the 3D matrix. To ensure physiologically relevant materials properties for analysis, the collagen was characterized using independent tensile testing with strain rates in the range of those measured in the cell-gel constructs. 3D elastic theory for an isotropic material was employed to calculate the stress. The technique presented adds to the field of measuring cell-generated stresses by providing the capability of measuring 3D stresses locally around a single cell and using physiologically relevant materials properties for analysis. The highest strains were observed in the most compliant matrix. Additionally, the stresses fluctuated over time due to the cells' interaction with the collagen matrix.