Abstract
Optical Coherence Tomography (OCT) combined with Digital Volume Correlation (DVC) is a suitable technique to investigate the biomechanical behavior of biological tissues at the microscale. However, to characterize the whole thickness of large human or porcine arteries, the use of osmotic tissue clearing agents, such as propylene glycol, is unavoidable due to intrinsic tissue scattering of light. The mechanical response of biological tissues immersed in tissue clearing agents has been poorly investigated so far. Nevertheless, understanding the mechanisms of tissue clearing could be helpful for developing safe optical-clearing methods for possible in vivo applications. The goal of the present work is to combine OCT and DVC to measure displacement and strain fields in porcine aortic tissues immersed in polypropylene glycol and subjected to stress-relaxation uniaxial tension. Displacement and strain fields are measured across the whole thickness of the porcine aortic wall for the first time. Measurement uncertainties and optimal OCT-DVC parameters are determined to define useful technical recommendations for future similar analyses. It is known that the main effect of polypropylene glycol is a significant increase of collagen fibril packing density due to important loss of water (dehydratation). It results in a pronounced stiffening of the aortic tensile response when compared to the response of the same tissue immersed in PBS. This effect is reversible when the aortic tissue is removed from polypropylene glycol and immersed again in PBS. Another effect is a dramatic reduction of the Poisson’s effect during tensile loading. But the OCT-DVC measured strain fields also reveal heterogeneities of this effect among the different layers of the aorta. It appears that the reduced Poisson’s effect is concentrated in the media layer, whereas the adventitia and intima layer keep a usual Poisson’s effect of nearly incompressible tissues. It is concluded that further work should be conducted on how the smooth muscle cells highly present in the media layer are affected by polypropylene glycol immersion for a better understanding of these effects.
Highlights
The mechanical response of biological tissues immersed in tissue clearing agents has been poorly investigated so far
The goal of the present work is to combine Optical coherence tomography (OCT) and digital volume correlation (DVC) to measure displacement and strain fields in porcine aortic tissues immersed in a tissue clearing agent (PG) and subjected to stress-relaxation uniaxial tension
Considering the large number of acquisition and processing parameters, the OCT-DVC method must be preliminarily optimized according to the optical properties, and the correlation parameters
Summary
Optical coherence tomography (OCT) is an imaging modality of micrometric scale permitting the visualization of tissue microstructure at different sub-surface levels (high-resolution and crosssectional images acquisition with a near-infrared light) (Yang et al, 2006; Huang et al, 2009; Tucker-Schwartz and Skala, 2012; Real et al, 2013; Ling et al, 2016; Mahdian et al, 2016). OCT was applied to characterize soft and hard human tissues such as vessels (aorta and coronary arteries), respiratory tract, gastrointestinal tissues (stomach and esophagus), cartilage, skin, nervous systems, cornea and retinal tissues, fatty tissue, cancellous and cortical bone, human enamel, etc For medical applications, this technique has been employed to detect and characterize the morphological alterations of microstructure and structure of human tissues undergoing pathologic changes such as: atherosclerotic plaques, thoracic aortic aneurysms, airways dysfunctions (asthma and bronchiectasis), gastrointestinal tumor tissue, articular cartilage degenerative changes, retinal vascular diseases, microfractures and inflammations in oral tissues, etc. It has been used on biomaterials such as dental implants, three-dimensional (3D) porous scaffolds applied in tissue engineering and hydrogels, among others (Brezinski et al, 1996; Wang, 2002; Wang and Elder, 2002; Yabushita et al, 2002; Yang et al, 2006, 2007; Huang et al, 2009; Prati et al, 2010; Williamson et al, 2011; Li et al, 2012; Real et al, 2013; Jia et al, 2015; Liba et al, 2016; Mahdian et al, 2016; Nebelung et al, 2016; Alibhai et al, 2017)
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