Article of the month | April 2021
Challenges in the mechanical characterization of engineered tissues
by Dr. Dimitria Bonizol Camasao
Senior Application Specialist, Rheolution Inc.
Have you ever thought about how blood flows to every single cell in our body in a continuous cycle? Yes, the heart is the major organ responsible for making this happen but less mentioned, the blood vessels have also a key active role in that.
The vascular system is composed of an impressive 19,000 km of interconnecting vessels. The heart provides energy to the system by beating from 60 to 100 times per minute. Blood vessels expand when receiving the pumped blood. They absorb part of this energy and their contraction restores the remaining part to the system. With their decreasing diameter and slightly different mechanical properties, the network of vessels perfectly orchestrates the delivery of blood from the heart into all other tissues and vice-versa. The mechanical behavior of blood vessels is characteristic of viscoelastic materials and the lack of this important feature has been shown to be a major cause of failure of vascular grafts. In a recently published review article, a team of researchers from Laval University (QC, Canada) led by Prof. Diego Mantovani (Laboratory of Biomaterials and Bioengineering) discussed the importance of a proper mechanical characterization of potential vascular grafts and critically summarized the methods reported in the literature.Composition and structure of blood vessels helps the circulation of blood
The authors mention that the organization of the blood vessel wall into layers and the presence of different types of cells and proteins result in a very unique mechanical behavior. And it is this specific mechanical response to the cyclic pulsation that allows the blood to circulate through the whole body.
This is very important to have in mind when developing vascular grafts for the treatment of cardiovascular diseases. For the moment, vascular grafts composed of synthetic biomaterials are used in clinics. However, as we can expect, their different structure and composition result in very different mechanical properties when compared with blood vessels. They are usually much stiffer and less compliant. This prevents their use in a number of cases, especially for small-diameter vessels, in which case the patient’s own blood vessels are needed to be used as a graft.
Several research groups are working on alternative vascular grafts using tissue engineering techniques. By using a scaffold (3D tubular support composed of biomaterial) to provide the shape of blood vessels, and cells to provide the biological activity, researchers have been able to develop great constructs that closely mimic the properties of blood vessels.
Blood vessels and viscoelasticity
The authors highlight the importance of analyzing conscientiously the mechanical properties of these vascular grafts in development. One challenge mentioned there is that conventional mechanical testing (based on compression and tensile deformations) was initially designed for the characterization of solid elastic materials such as metals. Blood vessels and bioengineered tissues have a mixed behavior between viscosity and elasticity.
Since compression and tensile testing instruments are the only testing technologies currently available, researchers have been adapting their methodology for characterizing soft biomaterials and soft tissues like blood vessels. For example, baths have been introduced to maintain the samples hydrated and the instruments have been adapted to hold the soft sample in place without damaging it.
To evaluate their viscoelasticity, the review mentions that creep and stress relaxation protocols have been adapted and applied using tensile or compression testers. In this way, the time-dependent response of the specimen can be evaluated. The obtained data is often fit with mathematical models for viscoelastic materials to extract the initial (viscous phase) and equilibrium (elastic phase) elastic modulus.
However, the lack of a standard procedure is a major reason for the great variety of methods, protocols, specimen shape and size, testing parameters, and data analysis present in the literature. This large variability prevents comparison among studies and it can hinder the advancement of the field. In other words, it is difficult to say how far or how close these potential vascular grafts are from blood vessels in terms of mechanical properties.
Beyond the large variation of adapted technologies
In addition to the large variability, the authors pointed out a second shortcoming in the mechanical characterization of these potential vascular grafts. In a number of cases, the mechanical analysis does not take into consideration the real application of the sample. For example, the strength at break and elastic modulus are often reported but how are these characteristics related to the functioning of the vascular graft in the hemodynamic environment? Is it enough to seek higher values of these properties?
The authors think that these properties can be used in the first steps of the vascular graft development. However, other techniques that better mimic the environment of blood vessels, such as pressure-based tests performed with a fluid applied in the tubular graft, can better predict the mechanical behavior of these constructs in the body. For example, values of pressure versus diameter can be plotted and compared to the response of blood vessels in the human body.
Overall, we can see the great importance of mechanical properties in designing proper grafts for maintaining the natural continuous cycle of blood. As the authors mentioned, “The continuous quest for a vascular substitute with superior mechanical performance may just be hindered by the lack of a consistent and physiological-relevant analysis of their mechanical properties”.
Bioengineered blood vessels have the potential to better maintain the natural continuous cycle of blood in our body, and the evaluation of this ability is key for their successful clinical translation. If such a clinical application is achieved at a large scale, it could represent a main treatment for one of the leading causes of death worldwide.
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