In vivo measurement of human brain elasticity using a light aspiration device

The brain deformation that occurs during neurosurgery is a serious issue impacting the patient "safety" as well as the invasiveness of the brain surgery. Model-driven compensation is a realistic and efficient solution to solve this problem. However, a vital issue is the lack of reliable and easily obtainable patient-specific mechanical characteristics of the brain which, according to clinicians' experience, can vary considerably. We designed an aspiration device that is able to meet the very rigorous sterilization and handling process imposed during surgery, and especially neurosurgery. The device, which has no electronic component, is simple, light and can be considered as an ancillary instrument. The deformation of the aspirated tissue is imaged via a mirror using an external camera. This paper describes the experimental setup as well as its use during a specific neurosurgery. The experimental data was used to calibrate a continuous model. We show that we were able to extract an in vivo constitutive law of the brain elasticity: thus for the first time, measurements are carried out per-operatively on the patient, just before the resection of the brain parenchyma. This paper discloses the results of a difficult experiment and provide for the first time in-vivo data on human brain elasticity. The results point out the softness as well as the highly non-linear behavior of the brain tissue.Comment: Medical Image Analysis (2009) accept\'

Cavitation in soft matter

Cavitation is the sudden, unstable expansion of a void or bubble within a liquid or solid subjected to a negative hydrostatic stress. Cavitation rheology is a field emerging from the development of a suite of materials characterization, damage quantification, and therapeutic techniques that exploit the physical principles of cavitation. Cavitation rheology is inherently complex and broad in scope with wide-ranging applications in the biology, chemistry, materials, and mechanics communities. This perspective aims to drive collaboration among these communities and guide discussion by defining a common core of high-priority goals while highlighting emerging opportunities in the field of cavitation rheology. A brief overview of the mechanics and dynamics of cavitation in soft matter is presented. This overview is followed by a discussion of the overarching goals of cavitation rheology and an overview of common experimental techniques. The larger unmet needs and challenges of cavitation in soft matter are then presented alongside specific opportunities for researchers from different disciplines to contribute to the field

Strain-induced alignment in collagen gels

Collagen is the most abundant extracellular-network-forming protein in animal biology and is important in both natural and artificial tissues, where it serves as a material of great mechanical versatility. This versatility arises from its almost unique ability to remodel under applied loads into anisotropic and inhomogeneous structures. To explore the origins of this property, we develop a set of analysis tools and a novel experimental setup that probes the mechanical response of fibrous networks in a geometry that mimics a typical deformation profile imposed by cells in vivo. We observe strong fiber alignment and densification as a function of applied strain for both uncrosslinked and crosslinked collagenous networks. This alignment is found to be irreversibly imprinted in uncrosslinked collagen networks, suggesting a simple mechanism for tissue organization at the microscale. However, crosslinked networks display similar fiber alignment and the same geometrical properties as uncrosslinked gels, but with full reversibility. Plasticity is therefore not required to align fibers. On the contrary, our data show that this effect is part of the fundamental non-linear properties of fibrous biological networks.Comment: 12 pages, 7 figures. 1 supporting material PDF with 2 figure

Quantitative characterization of viscoelastic behavior in tissue-mimicking phantoms and ex vivo animal tissues.

Viscoelasticity of soft tissue is often related to pathology, and therefore, has become an important diagnostic indicator in the clinical assessment of suspect tissue. Surgeons, particularly within head and neck subsites, typically use palpation techniques for intra-operative tumor detection. This detection method, however, is highly subjective and often fails to detect small or deep abnormalities. Vibroacoustography (VA) and similar methods have previously been used to distinguish tissue with high-contrast, but a firm understanding of the main contrast mechanism has yet to be verified. The contributions of tissue mechanical properties in VA images have been difficult to verify given the limited literature on viscoelastic properties of various normal and diseased tissue. This paper aims to investigate viscoelasticity theory and present a detailed description of viscoelastic experimental results obtained in tissue-mimicking phantoms (TMPs) and ex vivo tissues to verify the main contrast mechanism in VA and similar imaging modalities. A spherical-tip micro-indentation technique was employed with the Hertzian model to acquire absolute, quantitative, point measurements of the elastic modulus (E), long term shear modulus (η), and time constant (τ) in homogeneous TMPs and ex vivo tissue in rat liver and porcine liver and gallbladder. Viscoelastic differences observed between porcine liver and gallbladder tissue suggest that imaging modalities which utilize the mechanical properties of tissue as a primary contrast mechanism can potentially be used to quantitatively differentiate between proximate organs in a clinical setting. These results may facilitate more accurate tissue modeling and add information not currently available to the field of systems characterization and biomedical research

The Materials Science of Skin: Experimental Characterization, Constitutive Modeling, and Tear Resistance

Skin is the outermost layer of the body and acts as a primary protective barrier against external agents such as heat, light, infection, and injury. Additionally, skin regulates a broad range of physiological parameters and hosts several vital components. In order to fulfill these functions throughout life, skin must be able to withstand and recover from significant deformation as well as mitigate tear propagation that can occur during growth, movement, and injuries affecting its integrity. Hence, characterizing the mechanical behavior of skin and understanding the underlying mechanisms of deformation at different spatial scales is essential in a large spectrum of applications such as surgery, cosmetics, forensics, biomimetics and engineering of protective gear or artificial grafts. In this dissertation, a comprehensive list of experimental techniques that have been developed over the years to test skin’s nonlinear elastic, viscoelastic, and dissipative properties are reviewed. To identify parameters affecting its behavior, a significant number of models have been developed, some of which are detailed here. The principal structural elements within the dermis, and especially the arrangement and orientation of the collagen fibrils and fibers, are presented; their incorporation into the constitutive models is discussed. We conduct a detailed investigation of the evolution of the collagen architecture of the dermis as a function of deformation, which reveals new aspects that extend our understanding of the mechanical response of porcine skin. The dermis is found to have a tridimensional woven structure of collagen fibers, which evolves with deformation. After failure, we observe that the fibers have straightened and aligned in the direction of tension. Time-dependent and dissipative effects are quantitatively established. Digital image correlation techniques were implemented to quantify skin’s anisotropy; measurements of the Poisson ratio and their evolution are reported for the first time. Based on new observations, we propose that fiber braiding is at least partially responsible for the monotonic increase of the tangent modulus of skin with strain, as well as its dissipative response to cyclical loads. We incorporate these findings in a constitutive framework incorporating fibril stiffness, interfibrillar frictional sliding, and the effect of lateral fibers on the extension of a primary fiber, using analytical and coarse-grained modeling approaches. The representation of these important physical processes that occur during deformation of the dermis represents an advance in our understanding of these phenomena. Finally, we estimate the toughness of porcine skin by conduct two types of experiment on pre-notched specimens, placing the tissue under shear Mode III and opening Mode I. We obtain two distinct toughness values of J_IIIc≈20.4 kJ/m^2 and J_Ic=30.4 kJ/m^2, indicating notable differences between these two modes of crack propagation. Digital image correlation is used to plot strain profiles around the tip of the crack, from which a strain-based criterion for crack propagation is established. The evaluation of the structure at the crack tip and regions undergoing more uniform states of deformation is conducted by ex situ transmission electron microscopy and in situ environmental scanning electron microscopy

Stress relaxation in epithelial monolayers is controlled by the actomyosin cortex

Epithelial monolayers are one-cell thick tissue sheets that separate internal and external environments. As part of their function, they have to withstand extrinsic mechanical stresses applied at high strain rates. However, little is known about how monolayers respond to mechanical deformations. Here, by subjecting suspended epithelial monolayers to stretch, we find that they dissipate stresses on a minute time-scale in a process that involves an increase in monolayer length, pointing to active remodelling of cell architecture during relaxation. Strikingly, monolayers consisting of tens of thousands of cells relax stress with similar dynamics to single rounded cells and both respond similarly to perturbations of actomyosin. By contrast, cell-cell junctional complexes and intermediate filaments do not relax tissue stress, but form stable connections between cells, allowing monolayers to behave rheologically as single cells. Taken together our data show that actomyosin dynamics governs the rheological properties of epithelial monolayers, dissipating applied stresses, and enabling changes in monolayer length.Peer ReviewedPostprint (published version

Characterization and Assessment of Mechanical Properties of Adipose Derived Breast Tissue Scaffolds as a Means for Breast Reconstructive Purposes

Decellularized adipose tissue (DAT) has shown great potential for use as a regenerative scaffold in breast reconstruction following mastectomies or lumpectomies. Mechanical properties of such scaffolds are of great importance in order to mimic natural adipose tissue. This study focuses on the characterization of mechanical properties and assessment of DAT scaffolds for implantation into a human breast. DAT samples sourced from multiple adipose tissue depots within the body were tested and their elastic and hyperelastic parameters were obtained. Subsequently simulations were conducted where the calculated hyperelastic parameters were tested as a real human breast model under two different gravity loading situations (prone-to-supine, and prone-to-upright positions). DAT samples were also modelled for post-mastectomy, and post-lumpectomy reconstruction purposes. Results show that DAT shows similar deformability to that of native tissue, and varying DAT depots exhibited little intrinsic nonlinearity. Finally, contour defects were not observed for the samples under either loading conditions