tensile strength, or rigidity. Elastin - as its name suggests - is a highly elastic protein with an important role in tissues that need to return to their original positions after deformation, such as skin, blood vessels, and lungs. The relative concentrations of these two main determinants, along with other less influential matrix components, determine the rigidity of the ECM. For example, collagen concentration has been reported to be correlated to matrix stiffness, both in vivo and in vitro (gels). Measuring rigidity In biological research, the rigidity (or stiffness) is commonly measured using Young's modulus of elasticity, the ratio of stress to strain along an axis, in Pascals. Thus, a material with a high Young's modulus is very rigid. The most precise and well-established method to measure Young's modulus of a tissue relies on instruments - such as the Instron load cell device - that directly apply a mechanical load and measure the resulting deformation. Now, the Young's modulus of a tissue can be easily and accurately estimated without excision using a variety of elastography techniques. These methods induce distortion in the tissue and measure the mechanical properties, usually with ultrasound or magnetic resonance imaging (MRI). Young's modulus has been repeatedly used to characterize the mechanical properties of many tissues in the human body. The stiffness of animal tissues varies over several orders of magnitude, for example: Bovine articular cartilage - 950 kPa Mouse skeletal muscle - 12 kPa Guinea pig lung - 5-6 kPa Human fibrotic liver - 1.6 kPa, healthy human liver 640 Pa Swine brain - 260-490 Pa Synthesizing varying rigidity Matrices of varying stiffness are commonly engineered for experimental and therapeutic purposes (e.g. collagen matrices for wound healing). Durotactic gradients are simply made by creating 2-dimensional substrates out of polymer (e.g. acrylamide or polydimethylsiloxane) in which the stiffness is controlled by cross-linking density, which in turn is controlled by cross-linker concentration. The polymer must be coated with a material that the cell can adhere to, such as collagen or fibronectin. The gradients themselves are often synthesized as hydrogels using microfluidic gradient generators followed by photopolymerization. An advancement to this technique is the use of 3D matrices, which are able to guide cell migration in conditions that are more relatable to the natural three dimensional environment of the cell. Molecular mechanisms in durotaxis The site of cellular contact with the extracellular matrix is the focal adhesion, a