The spatial presentation of mechanical information is a key parameter for cell behavior. We have developed a method of polymerization control in which the differential diffusion distance of unreacted cross-linker and monomer into a prepolymerized hydrogel sink results in a tunable stiffness gradient at the cell-matrix interface. This simple, low-cost, robust method was used to produce polyacrylamide hydrogels with stiffness gradients of 0.5, 1.7, 2.9, 4.5, 6.8, and 8.2 kPa/mm, spanning the in vivo physiological and pathological mechanical landscape. Importantly, three of these gradients were found to be nondurotactic for human adipose-derived stem cells (hASCs), allowing the presentation of a continuous range of stiffnesses in a single well without the confounding effect of differential cell migration. Using these nondurotactic gradient gels, stiffness-dependent hASC morphology, migration, and differentiation were studied. Finally, the mechanosensitive proteins YAP, Lamin A/C, Lamin B, MRTF-A, and MRTF-B were analyzed on these gradients, providing higher-resolution data on stiffness-dependent expression and localization. mechanobiology | stem cell migration | stem cell differentiation | extracellular matrix | stiffness

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Probing the mechanical properties of tissue on the microscale could aid in the identification of diseased tissues that are inadequately detected using palpation or current clinical imaging modalities, with potential to guide medical procedures such as the excision of breast tumours. Compression optical coherence elastography (OCE) maps tissue strain with microscale spatial resolution and can delineate microstructural features within breast tissues. However, without a measure of the locally applied stress, strain provides only a qualitative indication of mechanical properties. To overcome this limitation, we present quantitative micro-elastography, which combines compression OCE with a compliant stress sensor to image tissue elasticity. The sensor consists of a layer of translucent silicone with well-characterized stress-strain behaviour. The measured strain in the sensor is used to estimate the two-dimensional stress distribution applied to the sample surface. Elasticity is determined by dividing the stress by the strain in the sample. We show that quantification of elasticity can improve the ability of compression OCE to distinguish between tissues, thereby extending the potential for inter-sample comparison and longitudinal studies of tissue elasticity. We validate the technique using tissue-mimicking phantoms and demonstrate the ability to map elasticity of freshly excised malignant and benign human breast tissues.

We present a theoretical framework for strain estimation in optical coherence elastography (OCE), based on a statistical analysis of displacement measurements obtained from a mechanically loaded sample. We define strain sensitivity, signal-to-noise ratio and dynamic range, and derive estimates of strain using three methods: finite difference, ordinary least squares and weighted least squares, the latter implemented for the first time in OCE. We compare theoretical predictions with experimental results and demonstrate a ~12 dB improvement in strain sensitivity using weighted least squares compared to finite difference strain estimation and a ~4 dB improvement over ordinary least squares strain estimation. We present strain images (i.e., elastograms) of tissue-mimicking phantoms and excised porcine airway, demonstrating in each case clear contrast based on the sample’s elasticity.

Optical elastography, the use of optics to characterise and map the mechanical properties of biological tissue, involves measuring the deformation of tissue in response to a load. Such measurements may be used to form an image of a mechanical property, often elastic modulus, with the resulting mechanical contrast complementary to the more familiar optical contrast.Optical elastography is experiencing new impetus in response to developments in the closely related fields of cell mechanics and medical imaging, aided by advances in photonics technology, and through probing the micro-scale between that of cells and whole tissues. Two techniques have shown particular promise recently: optical coherence elastography and Brillouin microscopy; for medical applications, such as in ophthalmology and oncology, and as new techniques in cell mechanics.At every length scale, the mechanical properties of tissue are important. Mechanical and chemical interactions at the molecular and cellular level are fundamentally interwoven in determining biological function, and such interactions and related mechanical properties play an important role in the onset and progression of many diseases, including eye disease, cancer, and atherosclerosis 1,2 . Over the past 25 years, a range of elastography techniques have been developed to image the mechanical properties of tissue 3 . Elastography is now a commercial medical imaging technique, mainly finding application as a diagnostic tool in the assessment of liver fibrosis 4 , and of breast cancer 5 . Primarily based on ultrasonography or magnetic resonance imaging, such elastography provides images, known as elastograms, over centimetre to whole-body depth ranges, at millimetre-scale spatial resolutions far lower than is possible with optics. At higher resolutions, on the cellular scale, the measurement of mechanical properties underpins the field of cell mechanics 2 , which is focussed on understanding cellularscale mechanical properties and how cells respond to physical forces and the mechanical properties of their environment. Cell mechanics is supported by nano-and micro-imaging techniques, such as atomic force microscopy (AFM) and traction force microscopy 6 .Optical elastography is at a much earlier stage of development than the methods employed in medical imaging or in cell mechanics. It is ideally positioned to image mechanical properties on the intermediate scale, between that of cells and organs [7][8] , which presents new opportunities in the understanding, diagnosis and treatment of disease 6 . The use of optics in elastography offers the combination of micro-scale imaging, potential for in vivo deployment and high sensitivity to variations in mechanical properties, and holds promise for a wide range of applications in areas such as oncology, ophthalmology and cell mechanics. Many optical elastography techniques have been proposed, for example, based on optical coherence tomography 7-9 , Brillouin microscopy 10 , laser speckle 11 , ultrasound-modulated optical tomography 12 and dig...

We present optical coherence micro-elastography, an improved form of compression optical coherence elastography. We demonstrate the capacity of this technique to produce en face images, closely corresponding with histology, that reveal micro-scale mechanical contrast in human breast and lymph node tissues. We use phase-sensitive, three-dimensional optical coherence tomography (OCT) to probe the nanometer-to-micrometer-scale axial displacements in tissues induced by compressive loading. Optical coherence micro-elastography incorporates common-path interferometry, weighted averaging of the complex OCT signal and weighted least-squares regression. Using three-dimensional phase unwrapping, we have increased the maximum detectable strain eleven-fold over no unwrapping and the minimum detectable strain is 2.6 με. We demonstrate the potential of mechanical over optical contrast for visualizing micro-scale tissue structures in human breast cancer pathology and lymph node morphology.

We review the development of phantoms for optical coherence tomography (OCT) designed to replicate the optical, mechanical and structural properties of a range of tissues. Such phantoms are a key requirement for the continued development of OCT techniques and applications. We focus on phantoms based on silicone, fibrin and poly(vinyl alcohol) cryogels (PVA-C), as we believe these materials hold the most promise for durable and accurate replication of tissue properties.

We present a novel sample arm arrangement for dynamic optical coherence elastography based on excitation by a ring actuator. The actuator enables coincident excitation and imaging to be performed on a sample, facilitating in vivo operation. Sub-micrometer vibrations in the audio frequency range were coupled to samples that were imaged using optical coherence tomography. The resulting vibration amplitude and microstrain maps are presented for bilayer silicone phantoms and multiple skin sites on a human subject. Contrast based on the differing elastic properties is shown, notably between the epidermis and dermis. The results constitute the first demonstration of a practical means of performing in vivo dynamic optical coherence elastography on a human subject.