Rheolution Article | April 2021

Why is viscoelasticity so important in the human body?

by Dr. Dimitria Bonizol Camasao
Senior Application Specialist, Rheolution Inc.

If you look at any scar on your skin you will see a difference in viscoelasticity. Scars result from the biological process of wound repair that can happen in any organ or tissue of the body. A scar has a clearly different appearance, texture, and often hardness than the surrounding tissue. On the skin, this difference probably won’t cause major problems beyond esthetics. However, in internal tissues and organs, scars can compromise functioning. A heart attack, for example, can cause scar formation in the heart muscle, which leads to loss of muscular power and possibly heart failure. An important point to extract from this example is that all the components of our body show a viscoelastic behavior to some extent and this behavior is related to their function in the body. In other words, cells, tissues, and organs have a mix of viscous and elastic responses when submitted to mechanical forces and this viscoelastic response is part of their main physiological role. Going into more details about viscoelastic behavior: on one side, a material can behave as a (viscous) fluid capable of flowing when submitted to a force. On the other side of the spectrum, a material can behave as an (elastic) solid capable of deforming under an applied force and returning instantly to its original state when the force is removed. A viscoelastic object will then show both types of responses. It will deform with the force and it will slowly return to its original state after the force is removed.

How can this viscoelastic behaviour correlate with an organ?

The human body is constantly under the influence of a number of forces at different scales. Starting with larger scales, gravity is a downward pull or force that the earth exerts on the whole body. Bones, muscles, and the heart act against gravity to enable the body to stand up, move and send blood to our brain, for example. Muscles apply motive forces to different parts of the human body when walking, running, stretching, or moving in general. On a smaller scale, blood pushes against the walls of blood vessels at each heartbeat. Gases within the lung’s alveoli exert forces against their walls making them expand which in turn expands the lungs and chest during breathing. All these organs and tissues are composed of proteins, water, and cells which respond either passively or actively to these forces. The response to all these forces is very specific to each tissue and this response contributes to the functioning of the body.

What are the practical applications of this information?

The understanding of viscoelasticity in the human body is extremely important, mainly for the two reasons listed below:

1. Differences in the tissue viscoelasticity can be related to the presence of diseases

Human tissues such as skin, muscle, cardiac, and adipose tissues have their own specific viscoelasticity. Medical imaging known as elastography is used to non-invasively map the viscoelastic properties of soft tissues inside the body. This information is useful for detecting the presence and severity of diseases. For example, liver fibrosis is the formation of an abnormally large amount of scar tissue when the liver attempts to repair and replace damaged tissue. The presence of fibrosis should result in harder regions with higher values of stiffness. Therefore, the results of elastography can show the presence and extent of fibrosis which can help in the choice of the best treatment.

2. Treatments and therapies involving the use of any type of biomaterial should resemble the natural viscoelastic response of the site of implantation

Biomaterials are used for the fabrication of medical implants that are placed inside or on the surface of the body. A biomaterial in a very broad definition is a biological or synthetic substance that, when implanted in the body, will cause a controlled host response. An ideal biomaterial should mimic the properties of the implantation site. In our last publication, for example, we saw that researchers are trying to develop alternative vascular grafts to the current stiff synthetic grafts that are on the market. This stiffness mismatch prevents the use of available grafts to replace small-diameter vessels. Chemists, physicists, biologists, and engineers have been combining their expertise to develop biomaterials that reproduce biological and mechanical properties of different human tissues, so they can maintain a similar response under the action of external or internal forces.

To exemplify the importance of matching the properties of native tissues, we can use gears as an analogy for the organs and tissues of the human body, illustrated below:

Let’s say that each one of these metallic gears (here represented in orange) turns at a specific speed to maintain the body working. Now imagine that we replace one of these gears with another one of similar dimensions but made of hard rubber (represented in navy). Do we expect that this system will keep running exactly the same way as before? As an example, the friction between rubber and metal is different from the one between metal and metal, and therefore the force resisting the relative motion of the gears will be different. Maybe the slight differences will pass unnoticed at first, but with time, the accumulation of this difference can impact different parts of the system.

As we can see, viscoelasticity is a physical property strictly related to the function of the tissue. Understanding this relationship and being able to measure this property is important for detecting the presence and severity of diseases, improving health care, and developing novel treatments and therapies for diseases with unmet medical needs.

On the decellularization of organs to produce tissue-specific extracellular matrices for tissue engineering

Decellularization is a process that removes cellular components from organs, leaving behind the extracellular matrix (ECM). This ECM, composed of proteins, proteoglycans, and glycoproteins, serves as a natural scaffold for tissue engineering applications. Decellularization can be achieved through chemical, physical, or biological methods. The resulting decellularized ECM, known as dECM, can be utilized to create hydrogels, bioinks, or coatings for enhancing cell adhesion and tissue regeneration.

Towards a standard method for mechanically characterizing soft organs

In October, our focus was on the mechanical characterization of soft organs. Understanding the mechanical properties of tissues is crucial for advancements in tissue engineering and regenerative medicine. However, measuring the mechanical properties of soft tissues is not straightforward and often involves customized approaches. Standardized and reproducible methods are needed to accurately characterize the mechanical properties of soft organs. By establishing consistent measurement techniques, researchers can gain a deeper understanding of tissue mechanics and develop effective treatments and therapies.

Viscoelastic properties of porcine and sheep soft organs

The emerging field of tissue engineering and regenerative medicine have the noble goal to develop lab-grown human tissues or alternative biomaterials to assist in their self-healing. In order to be functional in the human body, these biomaterials need to meet specific requirements of the intended site of implantation both in terms of biological, biochemical, and physical properties.

Measuring the viscoelastic properties of lungs through complementary techniques

Mechanical testing of soft tissues and organs is crucial for biomaterial development. A study from the University of Massachusetts compared different testing techniques and evaluated the impact of sample freezing on lung tissue. The findings highlighted variations in mechanical properties depending on the technique used and showed a slight difference between fresh and frozen samples. Standardized testing methods are necessary for reliable measurements and advancements in tissue engineering.

Bone regeneration enhanced by using a viscoelastic hydrogel

In a recent study, researchers from the University of California at Davis investigated the impact of the viscoelasticity of the cell's environment on bone formation by mesenchymal stromal cells (MSCs). They prepared alginate hydrogels with different mechanical properties and loaded them with MSC aggregates. The results showed that while both hydrogels supported high cell viability, the viscoelastic hydrogel promoted significantly higher calcium production by the cells compared to the elastic hydrogel. Calcium is an essential component for bone regeneration. These findings emphasize the importance of the cell's external environment, specifically its viscoelastic properties, in influencing cellular behavior and tissue regeneration.

Teens’ Tendency To Take Risks Exposed By Their Brain’s Viscoelasticity

A research study conducted by the University of Delaware explored whether the physiological differences in the maturation of specific brain regions could explain the increased risk-taking tendencies during adolescence. The study, titled "Viscoelasticity of reward and control systems in adolescent risk-taking" and published in the Neuroimage Journal, proposed that risk-taking behaviors are influenced by two opposing brain systems: the reward system (socioemotional system) and the cognitive control system, responsible for regulating impulsive responses. Due to the chronological development of these systems, an imbalance may occur, potentially making adolescents more prone to engaging in risky activities.