Collagen is the most abundant protein in mammals and gives mechanical strength, stiffness and toughness to biological tissues like skin, tendon, bone, and vasculature . Collagen fibrils of about 0.1 micrometers in diameters are the basic building blocks of fibrous collagenous tissues and their organization into suprafibrilar structures determines the tissue’s macroscopic mechanical properties. For example, detailed data regarding the organization of strong bundles of collagen might be critical to predict the onset of tissue failure, as it is clinically motivated by a rupture risk assessment of Abdominal Aortic Aneurysm (AAA). Previously proposed structural constitutive models for soft biological tissues [2, 3] integrated information regarding the collagen orientation, and regardless of their popularity, the requested microstructural information is not yet available in the open literature.
Method and Materials
The present study investigated the collagen formation in 12 AAA wall specimens stemming from 9 patients and harvested during elective aneurysm repair at Karolinska University Hospital, Stockholm, Sweden. Specimens of about 1.0 x 1.0 centimeter were squeezed between Plexiglas plates and fixated in formaldehyde for 24 hours. Fixated specimens were dehydrated and embedded in paraffin (Tissue Tek VIP 3000,
Sakura)and sliced at a thickness of 7.0 micrometers (HM 360, Microm). To reinforce the birefringend properties of collagen the slices were stained with Picrus Sirius red before three-dimensional collagen fiber orientations were identified in a polarized light microscope (BX 50, Olympus) equipped with an Universal Rotary Stage (Zeiss). Specifically, the collagen orientations were measured at 36 points at each slice, where three slices across the thickness of the AAA wall were considered. The derived structural information was included in two different structural constitutive models and reported macroscopic mechanical data  was used to estimate mechanical parameters of the constitutive formulations.
Results and Conclusions
Collagen fiber orientation in the AAA wall is considerably spread out and no difference amongst medial and adventitial layers could be identified; a result in line with the layered structure of, e.g., cerebral aneurysms  but in clear contrast to the structural differences amongst the layers of normal arteries . Collagen fibers in the AAA wall are predominantly aligned in circumferential direction, which might explain its higher stiffness along that direction . Naturally, the complex collagen formation cannot be captured by a single (or two) families of collagen fibers and associated constitutive models are not applicable. Collagen turnover is thought to be mediated by the local stress or strain state  and the supra-physiological stresses in the AAA wall might cause the identified pathological collagen orientation.
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6th World Congress of Biomechanics (WCB 2010), Singapore, 1 - 6 August 2010