Nanoindentation of histological specimens: Mapping the elastic properties of soft tissues

Localised micro-mechanical stiffening in the ageing aorta

Age-related loss of tissue elasticity is a common cause of human morbidity and arteriosclerosis (vascular stiffening) is associated with the development of both fatal strokes and heart failure. However, in the absence of appropriate micro-mechanical testing methodologies, multiple structural remodelling events have been proposed as the cause of arteriosclerosis. Therefore, using a model of ageing in female sheep aorta (young: <18 months, old: >8 years) we: (i) quantified age-related macro-mechanical stiffness, (ii) localised in situ micro-metre scale changes in acoustic wave speed (a measure of tissue stiffness) and (iii) characterised collagen and elastic fibre remodelling. With age, there was an increase in both macro-mechanical stiffness and mean microscopic wave speed (and hence stiffness; young wave speed: 1701 ± 1 m s⁻¹, old wave speed: 1710 ± 1 m s⁻¹, p < 0.001) which was localized to collagen fibril-rich regions located between large elastic lamellae. These micro-mechanical changes were associated with increases in both collagen and elastic fibre content (collagen tissue area, young: 31 ± 2%, old: 40 ± 4%, p < 0.05; elastic fibre tissue area, young: 55 ± 3%, old: 69 ± 4%, p < 0.001). Localised collagen fibrosis may therefore play a key role in mediating age-related arteriosclerosis. Furthermore, high frequency scanning acoustic microscopy is capable of co-localising micro-mechanical and micro-structural changes in ageing tissues.

Indentation versus tensile measurements of Young's modulus for soft biological tissues

In this review, we compare the reported values of Young's modulus (YM) obtained from indentation and tensile deformations of soft biological tissues. When the method of deformation is ignored, YM values for any given tissue typically span several orders of magnitude. If the method of deformation is considered, then a consistent and less ambiguous result emerges. On average, YM values for soft tissues are consistently lower when obtained by indentation deformations. We discuss the implications and potential impact of this finding.

The role of substratum compliance of hydrogels on vascular endothelial cell behavior

Cardiovascular disease (CVD) remains a leading cause of death both within the United States (US) as well as globally. In 2006 alone, over one-third of all deaths in the US were attributable to CVD. The high prevalence, mortality, morbidity, and socioeconomic impact of CVD has motivated a significant research effort; however, there remain significant knowledge gaps regarding disease onset and progression as well as pressing needs for improved therapeutic approaches. One critical area of research that has received limited attention is the role of biophysical cues on the modulation of endothelial cell behaviors; specifically, the impact of local compliance, or the stiffness, of the surrounding vascular endothelial extracellular matrix. In this study, the impact of substratum compliance on the modulation of cell behaviors of several human primary endothelial cell types, representing different anatomic sites and differentiation states in vivo, were investigated. Substrates used within our studies span the range of compliance that has been reported for the vascular endothelial basement membrane. Differences in substratum compliance had a profound impact on cell attachment, spreading, elongation, proliferation, and migration. In addition, each cell population responded differentially to changes in substratum compliance, documenting endothelial heterogeneity in the response to biophysical cues. These results demonstrate the importance of incorporating substratum compliance in the design of in vitro experiments as well as future prosthetic design. Alterations in vascular substratum compliance directly influence endothelial cell behavior and may participate in the onset and/or progression of CVDs.

Characterizing the elastic properties of tissues

The quality of life of ageing populations is increasingly determined by age-related changes to the mechanical properties of numerous biological tissues. Degradation and mechanical failure of these tissues has a profound effect on human morbidity and mortality. Soft tissues have complex and intricate structures and, similar to engineering materials, their mechanical properties are controlled by their microstructure. Thus age-relate changes in mechanical behavior are determined by changes in the properties and relative quantities of microstructural tissue components. This review focuses on the cardiovascular system; it discusses the techniques used both in vivo and ex vivo to determine the age-related changes in the mechanical properties of arteries.

Remodeling and homeostasis of the extracellular matrix: implications for fibrotic diseases and cancer

Dynamic remodeling of the extracellular matrix (ECM) is essential for development, wound healing and normal organ homeostasis. Life-threatening pathological conditions arise when ECM remodeling becomes excessive or uncontrolled. In this Perspective, we focus on how ECM remodeling contributes to fibrotic diseases and cancer, which both present challenging obstacles with respect to clinical treatment, to illustrate the importance and complexity of cell-ECM interactions in the pathogenesis of these conditions. Fibrotic diseases, which include pulmonary fibrosis, systemic sclerosis, liver cirrhosis and cardiovascular disease, account for over 45% of deaths in the developed world. ECM remodeling is also crucial for tumor malignancy and metastatic progression, which ultimately cause over 90% of deaths from cancer. Here, we discuss current methodologies and models for understanding and quantifying the impact of environmental cues provided by the ECM on disease progression, and how improving our understanding of ECM remodeling in these pathological conditions is crucial for uncovering novel therapeutic targets and treatment strategies. This can only be achieved through the use of appropriate in vitro and in vivo models to mimic disease, and with technologies that enable accurate monitoring, imaging and quantification of the ECM.

The use of polyacrylamide gels for mechanical calibration of cartilage--a combined nanoindentation and unconfined compression study

This study investigates polyacrylamide (PA) gel as a calibration material to measure the nanomechanical compressive modulus of cartilage using nanoindentation. Both nanoindentation and unconfined compression testing were performed on PA gel and porcine rib cartilage. The equilibrium moduli measured by the two methods were discernable. Nanoindentation has the advantage of distinguishing between spatially dependent constituent properties that affect tissue mechanical function in heterogeneous and hierarchically structured tissues such as cartilage. Both sets of measurements exhibited similar positive correlation with increasing gel crosslinker concentration. The compressive modulus measurements from compression in the PA gels ranged from 300 kPa-1.4 MPa, whereas those from nanoindentation ranged from 100 kPa-1.1 MPa. Using this data, a method for relating nanoindentation measurements to conventional mechanical property measurements is presented for porcine rib cartilage. It is shown that based on this relationship, the local tissue modulus as measured from nanoindentation (1.1-1.4 MPa) was able to predict the overall global modulus of the same sample of rib cartilage (2.2 MPa), as confirmed by experimental measurements from unconfined compression. This study supports the use of nanoindentation for the local characterization of cartilage tissues and may be applied to other soft tissues and constructs.

Noninvasive multimodal evaluation of bioengineered cartilage constructs combining time-resolved fluorescence and ultrasound imaging

A multimodal diagnostic system that integrates time-resolved fluorescence spectroscopy, fluorescence lifetime imaging microscopy, and ultrasound backscatter microscopy is evaluated here as a potential tool for assessing changes in engineered tissue composition and microstructure nondestructively and noninvasively. The development of techniques capable of monitoring the quality of engineered tissue, determined by extracellular matrix (ECM) content, before implantation would alleviate the need for destructive assays over multiple time points and advance the widespread development and clinical application of engineered tissues. Using a prototype system combining time-resolved fluorescence spectroscopy, FLIM, and UBM, we measured changes in ECM content occurring during chondrogenic differentiation of equine adipose stem cells on 3D biodegradable matrices. The optical and ultrasound results were validated against those acquired via conventional techniques, including collagen II immunohistochemistry, picrosirius red staining, and measurement of construct stiffness. Current results confirm the ability of this multimodal approach to follow the progression of tissue maturation along the chondrogenic lineage by monitoring ECM production (namely, collagen type II) and by detecting resulting changes in mechanical properties of tissue constructs. Although this study was directed toward monitoring chondrogenic tissue maturation, these data demonstrate the feasibility of this approach for multiple applications toward engineering other tissues, including bone and vascular grafts.

Quantifying Micro-mechanical Properties of Soft Biological Tissues with Scanning Acoustic Microscopy

In this study we have established a new approach to more accurately map acoustic wave speed (which is a measure of stiffness) within soft biological tissues at micrometer length scales using scanning acoustic microscopy. By using thin (5 μm thick) histological sections of human skin and porcine cartilage, this method exploits the phase information preserved in the interference between acoustic waves reflected from the substrate surface as well as internal reflections from the acoustic lens. A stack of images were taken with the focus point of acoustic lens positioned at or above the substrate surface, and processed pixel by pixel using custom software developed with LABVIEW and IMAQ (National Instruments) to extract phase information. Scanning parameters, such as acoustic wave frequency and gate position were optimized to get reasonable phase and lateral resolution. The contribution from substrate inclination or uneven scanning surface was removed prior to further processing. The wave attenuation was also obtained from these images.

Tissue elasticity and the ageing elastic fibre

The ability of elastic tissues to deform under physiological forces and to subsequently release stored energy to drive passive recoil is vital to the function of many dynamic tissues. Within vertebrates, elastic fibres allow arteries and lungs to expand and contract, thus controlling variations in blood pressure and returning the pulmonary system to a resting state. Elastic fibres are composite structures composed of a cross-linked elastin core and an outer layer of fibrillin microfibrils. These two components perform distinct roles; elastin stores energy and drives passive recoil, whilst fibrillin microfibrils direct elastogenesis, mediate cell signalling, maintain tissue homeostasis via TGFβ sequestration and potentially act to reinforce the elastic fibre. In many tissues reduced elasticity, as a result of compromised elastic fibre function, becomes increasingly prevalent with age and contributes significantly to the burden of human morbidity and mortality. This review considers how the unique molecular structure, tissue distribution and longevity of elastic fibres pre-disposes these abundant extracellular matrix structures to the accumulation of damage in ageing dermal, pulmonary and vascular tissues. As compromised elasticity is a common feature of ageing dynamic tissues, the development of strategies to prevent, limit or reverse this loss of function will play a key role in reducing age-related morbidity and mortality.