Cell biomechanics

In the late seventeenth century, English polymath Robert Hooke and Dutch scientist Antonie van Leeuwenhoek looked into ciliate Vorticella with extreme fluid and cellular motion using a simple optical microscope. On 25 December 1907, van Leeuwenhoek described his observations in a letter: "In structure these little animals were fashioned like a bell, and at the round opening they made such a stir, that the particles in the water thereabout were set in motion thereby…which sight I found mightily diverting" Prior to this, Brownian motion of particles and organelles within living cells had been discovered, as well as theories to measure viscosity. However, there were not enough accessible technical tools to perform these accurate experiments at the time. Thus, mechanical properties within cells were only supported qualitatively by observation. With these new discoveries, the role of mechanical forces within biology was not always naturally accepted. In 1850, English physician William Benjamin Carpenter wrote "many of the actions taking place in the living body are conformable to the laws of mechanics, has been hastily assumed as justifying the conclusion that all its actions are mechanical." Similarly, in 1917, Scottish mathematical biologist D'Arcy Wentworth Thompson noted "…though they resemble known physical phenomena, their nature is still the subject of much dubiety and discussion, and neither the forms produced nor the forces at work can yet be satisfactorily and simply explained" in his book On Growth and Form. In the nineteenth century industrialization era, the overall understanding of the cell and tissue mechanics finally developed as it related to the mechanical, structural testing and theory (indentation, beam bending, the Hertz model) of engines, boats, and bridges. At the end of the nineteenth century, the mechanical properties of living cells were able to be experimentally analyzed and examined using techniques provided by large scale engineering mechanics. Since 2008, nanoscale testing and modeling remains fundamentally based on these nineteenth century practices. == Research methods ==

Various studies have been conducted to establish relationships between the structure, mechanical responses, and function of biological tissues (e.g. blood vessels, cardiac muscle, lung tissue). To conduct this research, there have been several developed tools and techniques which are sensitive to detect such small forces. At this time, these techniques are only applicable in a controlled environment (such as a test tube or petri dish). All of these methods ultimately give insight on mechanical properties of cells. Active methods Atomic force microscopy Atomic force microscopy is an interaction between a tip attached to a flexible cantilever and the molecule on a cell surface. The sharp tip can be used to probe single molecular events and image live cells. The relative deformation of the cell and the tip can be used to estimate how much force was applied and how stiff the cell is. Since it is a high force measurement technique, large scale deformations and reorganizations can be observed and mapped. Some drawbacks of this technique include but are not limited to an overestimation of force-versus-indentation curve given no applied force, potential cell damage, variety of tip shapes that determine nature of force-deformation curve. Beads are exposed to magnetizing coils leading to a magnetic dipole moment. A weaker directional magnetic field is then applied to twist the beads through a specific angle or to move the beads lineary. Some disadvantages to this system include the difficulty to control the region of the cell that the beads, no guarantee of complete binding to the cell surface, and loss of magnetization with time. Like AFM, it is also a high force measurement technique, where large scale deformations and reorganizations can be observed and mapped. Cells are incubated on flexible silicone sheet elastic membranes with modifiable surfaces. They are then stretched either in an uniaxial, biaxial, or pressure-controlled manner. The stretching can also occur at different frequencies. The main downside to stretching devices is that they leave behind wrinkling patterns, distorting the actual forces that were applied on the sheets. This method is also referred to as traction force microscopy. Cells are incubated onto a flexible silicone sheet substrate. The cells then apply force onto the sheets causing a wrinkling pattern and is analyzed through the number of wrinkles and patterns. The downside to this method is the difficulty in transforming the patterns into a traction force map leading to potential inaccuracy in identifying forces. Unlike the prior method, the uncertainty of no propagation is not an issue. Rather the cantilever beam can only move in only one direction leading to only one axis being measured. The array of vertical microcantilevers is a technique that overcomes the limitations of the typical micromachined cantilever beam where there are two axes of directions available rather than a single horizontal beam. Although there is an improvement in scale and resolution, it is not suited for rapid- mass production and is quite costly. With delicate properties, minor damage would require reproduction of the device. == Applications and usage ==

In the last 50 years, several studies have been conducted using cell biomechanics leading to greater biological control. A majority of these newly created devices are built to either provide greater insight into the human body's reaction to disease or attempt to eradicate the disease as a whole. Cardiovascular cell mechanics and microcirculation Quantitative passive biomechanical models have been developed to predict cell motion and deformation in the mammalian red blood cell, a cell with a membrane with bending and shearing properties that are dependent upon strain, strain rate, and strain history, and a cytoplasm that in the normal red cell is predominantly a Newtonian viscous fluid, within a living organism. Newly developed (2007) models constitutive to this one show that biomechanical analysis not only is a starting point for prediction of the whole cell and cell suspension behavior, but also provides a reference point for molecular models of cell membranes that originate from the crystal structure of its parts. Deformability-based enrichment devices are an example of this technology. These devices mostly deal with cancer cells from blood. Their main feature is their ability to identify if cancer cells have separated themselves from the tumor and have entered into the bloodstream as CTCs (Circulating Tumor Cells). If they have, these devices have recently also become able to count the number of CTCs in a millimeter of blood. Using this value, medical professionals are able to determine the effectiveness of a chemotherapy treatment. More specific examples include Assistant Professor of Mechanical Engineering at the Whiting School of Engineering Soojung Claire Hur's microfluidic device and Woodruff School of Mechanical Engineering Professor Gonghao Wang's microfluidic device that both deal with breast cancer cells. Hur's device improves metastatic breast cancer cells by balancing deformability-induced and inertial lift forces that pushes larger metastatic cancer cells to move towards the centerline of a microchannel compared to blood cells. Wang's device separates stiffer, less invasive breast cancer cells by having diagonal ridges where only more deformable and highly invasive breast cancer cells can squeeze through. Deformability-based enrichment devices, however, are not only exclusive to cancer cells. An example of this is Nanyang Technological University Researcher Han Wei Hou's microfluidic device that separates and improves red blood cells from normal cells based on their stiffness through margination. Infected red blood cells are generally stiffer, so through his device, stiffer red blood cells would be closer to the vessel wall when normal red blood cells would stay in the center. This allows the deformed red blood cells to be collected via a separate outlet on the sides. == Ongoing research concerns ==

In the 1800s, cells were initially thought to be of homogeneous gels, sols, viscoelastic and plastic fluids. Models currently have been developed into including a viscoelastic continuum, a combination of discrete mechanical elements, or a combination of viscoelastic fluid within a dense meshwork and have been proven to be highly accurate after experimentation. Despite these improved and more refined models, there still remain to be flaws as several experimental proofs (e.g. soft glass rheology phenomenon) refute current existing models. Thus, the time-dependent and predictive theoretical description of cell mechanics remains to be incomplete. It is also not fully understood whether mechanical phenomena are side products of biological processes or they are controlled at the genetic and physiological level through feedback loops, actuation and response pathways given our existing knowledge of cell physiology or neurophysiology. ==References==