Studies of the outer-most layer of the vascular wall, adventita as a seperate layer

A small Introduction on different vascular layers:

The walls of all blood vessels, except the very smallest, have three distinct layers, or tunics( 'covering') , that surround a central blood-containing space, the vessel lumen. The innermost tunic is the tunica intima which is in direct contact with the blood in the lumen. This tunic contains the endothelium. The middle tuinic, the tunica media is mostly contained of smooth muscle cells, elastin and collagen fibers. The outermost layer of a blood vessel wall, the tunica externa(adventitia) is composed of loosely woven collagen fibers that protect and reinforce the vessel and anchor it to surrounding structure.

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As for the biomechanical properties of the vascular tissue, there has been quite a large number of studies done. Some of these studies report inflation-extension types of experiments done on scaffold of adventitia removed from a whole vessel. Others, have just removed out the adventitia and focused on media. The question is :

Is it really possible for all type of vessels to take out adventitia out of the vessel mechanically?

To my knowledge, this seems quite a local and specie dependent property. It looks that in some arteries , such as human femoral arteries, you can easily separate the adventitia from the rest of the vessel. However, according my experiments, it is almost impossible to take it as a whole intact cylinder out from common carotid, femoral, abdominal arteries and Jagular,facial,femoral,abdominal veins of rabbits. As for common carotid of rats, I may say, it may be possible though I had never really done it.

let's consider that you have done it. Since it is a kind of bulky collagen fibers, it does not seem really impermeable to liquids. Thus, inflating of this layer, even if we can get it from the artery, seems quite a hard job.

Have you ever tried working with adventitia layer separately in inflation-extension tests? I would appreciate as you inform me on the subject.

picture taken from : reference

Mathematical modeling of biomechanical properties of the venous wall

Despite the abundant literature on blood vessel mechanical properties, blood vessel constitutive models are far less common. Blood vessels are nonlinear, anisotropic and viscoelastic, heterogeneous in the unloaded state and compressible when studying macroscopic characteristic and they behave differently in different temperatures. Despite the long list of attributes, constitutive equations generally account for only a subset of these characteristics.

In general, blood vessels can be treated as pseudoelastic, randomly elastic, poroelastic or viscoelastic . Pseudoelasticity assumes that a material can be modeled using separate equations describing the loading and unloading behavior. Random elasticity, however, assumes that the strain response for a given load is rendered around a definite value that lies on a well defined curve, such that data from both the loading and unloading curves can be included simultaneously. Poroelastic formulations treat a material as a fluid-saturated porous medium and are well suited to model wall transport. Viscoelastic formulations include time-dependent responses in the constitutive equation and are useful for modeling creep, stress relaxation, and hysteresis. Useful reviews are available, concerning the biomechanics of soft biological tissues :

Vito, R.P. and S.A. Dixon, Blood vessel constitutive models-1995-2002. Annual Review Of Biomedical Engineering, 2003. 5: p. 413-439.

Humphrey, J.D., Continuum biomechanics of soft biological tissues. Proceedings: Mathematical, Physical and Engineering Sciences (Series A), 2003. 459(2029): p. 3-46.

Soft tissue mechanics

Soft tissues are inhomogeneous, anisotropic materials which show viscoelastic and nonlinear properties. Morover, even under physiological conditions they undergo large deformations. For instance, in arteries, the in vivo longitudinal stretch ratio may pass more than 1.6. ( in vivo length/ unloaded length=1.6)

These properties make the continuum approach suitable for the purpose of analyzing material properties of soft tissue. Therefore, knowledge of nonlinear solid mechanics by a continuum approach seems essential.

Identification of an appropriate strain energy function (SEF) is the preferred method to describe the complex nonlinear elastic properties of vascular tissues. Once the strain energy function is known, the constitutive stress-strain relationships can be directly obtained from the SEF.

Early formulations of SEFs were purely phenomenological, in the sense that parameters involved in the mathematical expression of SEF bared little physiological meaning. Lately, significant effort has been put into developing structure-based or constituent-based SEF, where the parameters of the strain energy function represent some identifiable physical or structural characteristics of the different components of the vessel wall, such as elastic constants of elastin and collagen, fiber structural characteristics of the collagen network, volume fraction of elastin, collagen and vascular smooth muscle cells, etc.

An example of a constituent -based SEF which considers some structural properties, i.e. the orientation of the collagen fibers relative to the arterial wall’s circumferential direction is the model by Holzapfel and colleagues. The Holzapfel et al. model has been subsequently modified and extended by Zulliger et al. take the waviness of collagen fibers into the account and later to include vascular tone.

The structure-based SEFs did provide a significant improvement over the previous phenomenological SEFs. Furthermore, they supplied scientists more powerful tools to relate morpholoy with mechanical properties of soft tissue. Pa