In Vivo Mechanical Stimulation of Tissues by Tyler Russell
Overview
Mechanical stimulation can be used to accelerate the proliferation of certain types of cells and is consequently quite useful in several tissue engineering applications. Mechanical stimulation of tissue can occur both in vivo and in vitro, although most in vitro environments used for such stimulation would likely mimic an in vivo environment. Cell types of interest for mechanical stimulation of tissue in vivo primarily include osteoblasts and also include tendon derived stem cells (TDSCs). Multiple studies have shown that in vivo stimulation of bone cells can increase differentiation and development rate of the engineered tissue.
The proliferation of skeletal muscle has also been found to benefit from mechanical stimulation. Intuitively, it seems to make sense that both bone and muscle tissue would behave in such a way, as each tissue is placed under regular stress in everyday life for most if not all animals. However, this article will focus primarily upon the in vivo mechanical stimulation of bone, as the mechanical stimulation of muscle tissue is generally performed in vitro.
History of Mechanical Stimulation
The concept of mechanical stimulation of bone cells can be traced as far back as the 1920's. Prior to this time, it had been established that bone tissue benefited from being placed under a load or strain, and was made stronger as a consequence. This led to the concept of dynamic mechanical loading, which was suggested to be superior to static loading of bone tissue by scientists such as Jores, and Loeschke and Weinnoldt [2]. This dynamic loading, in contrast with static or continuous loading, of force was believed to elicit an osteogenic response. Studies of dynamic mechanic loading compared to static mechanic loading were not performed until the late 1960's and early 1970's by Hert et. al. as well as Chamay and Tshantz [2]. These early studies were performed in vitro and all came to the same conclusion: static load did not promote osteogenesis in animal bone the same way that dynamic loading did. [2]
In the mid 1990's, it became apparent through the work of Turner et. al. [2] that increased osteogenesis was proportional to the rate of strain that the osteoblasts experienced, or in other words the frequency that the load was applied. This rate was a greater factor in bone proliferation than the magnitude of the force itself [3]. This conclusion would lay the ground for future experiments with mechanical stimulation and bone tissue, both in vivo and in vitro.
Motivation for Mechanical Stimulation Research
Although the human body often possesses the ability to heal itself on its own, the healing process can be quite slow, and sometimes incomplete. Bone has the ability to heal fairly completely from most injuries, though over a very long period of time. The combination of in vivo mechanical stimulation with the human body's own ability to heal seems to expedite the normal process of healing. As well as potentially expediting the healing process, mechanical stimulation of bone tissue in vivo has been shown to lead to a more efficient healing process as well [4].
