Measurement of bulk mechanical properties of tissue using laser speckle rheology

Tutorial on laser speckle rheology: technology, applications, and opportunities

SIGNIFICANCE

The onset of several diseases is frequently marked with anomalous mechanical alteration of the affected tissue at the intersection of cells and their microenvironment. Therefore, mapping the micromechanical attributes of the tissues could enhance our understanding of the etiology of human disease, improve the diagnosis, and help stratify therapies that target these mechanical aberrations.

AIM

We review the tremendous opportunities offered through using optics for imaging the micromechanical properties, at length scales inaccessible to other modalities, in both basic research and clinical medicine. We specifically focus on laser speckle rheology (LSR), a technology that quantifies the mechanical properties of tissues in a rapid, noncontact manner.

APPROACH

In LSR, the shear viscoelastic modulus is measured from the time-variant speckle intensity fluctuations reflected off the tissue. The LSR technology is engineered and configured into several embodiments, including bench-top optical systems, endoscopes for minimally invasive procedures, portable point-of-care devices, and microscopes.

RESULTS

These technological nuances have primed the LSR for widespread applications in diagnosis and therapeutic monitoring, as demonstrated here, in cardiovascular disease, coagulation disorders, and tumor malignancies.

CONCLUSION

The fast-paced technological advancements, elaborated here, position the LSR as a competent candidate for many more exciting opportunities in basic research and medicine.

Evaluation of cell and matrix mechanics using fluorescence excitation spectroscopy: Feasibility study in collagen gels containing fibroblasts

BACKGROUND AND OBJECTIVE

Collagen gels containing cells are commonly used in tissue engineering, wound healing, and cancer research to investigate the interplay between cells and the extracellular matrix (ECM), as changes in the density and stiffness of the microenvironment are known to play a role in many diseases or pathological conditions. In these gels, the stiffness is regularly determined using destructive methods, such as indentation and tensile tests. Certain molecules native to cells and the ECM display fluorescence upon irradiation with ultraviolet light. The objective of the present study was to investigate the feasibility of using the endogenous, or innate, fluorescence of collagen gels containing fibroblasts as an optical marker to evaluate changes in the mechanical properties of the ECM.

MATERIALS AND METHODS

Human foreskin fibroblasts cells at concentrations of 50,000 and 100,000 cells/ml were cultured in three-dimensional gels of type I collagen for 16 days. Fibroblast cells remodeled the ECM, contracting and increasing the stiffness of the gel. During this remodeling process, changes in mechanical properties and fluorescence were measured with an indentation test and a spectrofluorometer, respectively. Force and displacement measurements from the indentation test were used to calculate the elastic modulus of the gel. Maps of fluorescence intensity, at excitation/emission of 240-520/290-530 nm, were used to identify the wavelengths of interest.

RESULTS

Fluorescence excitation/emission maps exhibited two distinct excitation/emission bands whose intensities increased as the fibroblasts remodeled and increased the stiffness of the ECM: The 290/340 nm band ascribed to tryptophan and the 330/390 nm band ascribed to cross-links of collagen (pepsin-digestible cross-links). A Spearman correlation analysis, between the elastic modulus of the gel containing fibroblasts and the fluorescence of cross-links of collagen, resulted in R = 0.95 (P < 0.05) and R = 0.77 (P = 0.12) for 50,000 and 100,000 cells/ml, respectively.

CONCLUSIONS

The endogenous fluorescence intensity ascribed to pepsin-digestible cross-links of collagen may serve as an optical marker to evaluate changes in the mechanical properties of the ECM; this is relevant to collagenous tissues for which pathological states are related to mechanical alterations, such as keratoconus in cornea and osteoarthritis in articular cartilage.