Matrix Supports Tissue Engineering

Biological Strategy

Animals

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Optimize Shape/Materials

Resources are limited and the simple act of retaining them requires resources, especially energy. Living systems must constantly balance the value of resources obtained with the costs of resources expended; failure to do so can result in death or prevent reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. An example of such optimization can be seen in the dolphin’s body shape. It’s streamlined to reduce drag in the water due to an optimal ratio of length to diameter, as well as features on its surface that lie flat, reducing turbulence.

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Modify Material Characteristics

The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.

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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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Animals

Kingdom Animalia (“having breath or soul”): Mammals, reptiles, sponges, corals, insects, birds

This kingdom captures a staggering variety of life—from the smallest coral polyp to the largest elephant. Although animal body shapes and life histories vary, there are a few defining characteristics. Animals are heterotrophs, meaning they gain energy from eating other organisms, and have developed from single-celled ancestors to highly diversified and coordinated multicellular forms. Unlike plant and fungi cells that have rigid cell walls, animal cells are bound by more flexible combinations of proteins. Most animals have cells organized into tissues and are able to move within their environment.

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The matrix associated with individual chondrocytes and other stem cells manages varying demands over time during the tissue engineering process by having temporally dynamic mechanical properties.

“Cell-based tissue engineering holds great potential for therapies involving regeneration and/or replacement of damaged cartilage…The ultimate goal and challenge is to produce a material with structural, biochemical and biomechanical properties similar enough to healthy cartilage so that upon maturation in vivo it can restore physiological function. To achieve this objective, it is important to understand the temporal evolution of the properties of the newly synthesized matrix. At early stages, matrix is formed around individual cells within the scaffold and these cell–matrix composites are often isolated from each other. As culture time increases, growth of neighboring cell-associated matrices leads to the formation of a more continuous neo-tissue which undergoes further evolution in structure and properties in vitro or in vivo. Throughout this process, the cell-associated matrix is important in facilitating cell signaling and mechanotransduction (Millward-Sadler et al.,2000; Millward-Sadler and Salter, 2004). Knowledge of the properties of the in vitro-generated matrix provides an assessment of the quality and ultimate success of a given tissue engineering approach and has great potential to be utilized for optimization…the present study investigates the dynamic oscillatory mechanical behavior (Mahaffy et al., 2000, 2004; Alcaraz et al., 2003; Park et al., 2005; Smith et al., 2005) of the cell-associated matrices of individual chondrocytes cultured in alginate scaffolds up to 28 days. Such time-dependent behaviors are important because tissue-engineered constructs implanted in vivo are expected to undergo cyclic and impact loading that includes frequency components as high as 1kHz, just as native cartilage does…

“In summary, our AFM-based approach has enabled the measurement of the poroelastic dynamic mechanical behavior of the newly developing matrix associated with individual chondrocytes, and can be applied generally to study the dynamic behavior of extremely compliant (~kPa) biological, porous, and hydrated systems at nm-length scales over a broad range of frequencies. The high resolution dynamic mechanical approach described here is able to discern fine differences in the development and maturation of the cell-associated matrix temporally and with the addition of growth factors. The methodologies reported here can thus be employed to assess maturation of matrix synthesized by primary chondrocytes and, additionally, by other cells such as stem cells undergoing chondrogenesis in applications for cartilage tissue engineering. It may also be possible to use this methodology to study the mechanobiological response of single chondrocytes (Shieh and Athanasiou, 2006b) and stem cells to applied dynamic loads over a wide frequency range.” (Lee et al. 2010:469-70, 475-6)

Last Updated September 14, 2016

References

Journal article

Dynamic mechanical properties of the tissue-engineered matrix associated with individual chondrocytes

Journal of Biomechanics | 04/11/2009 | BoBae Lee, Lin Han, Eliot H. Frank, Susan Chubinskaya, Christine Ortiz, Alan J. Grodzinsky

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