During locomotion every impact between the foot and ground generates a shock wave that propagates through the human musculoskeletal system and reaches the forehead. Due to the significant difference in the stiffness of bones and soft tissues, it is reasonable to assume that the main carrier of these waves is bone. Thus, the only logical way to measure those waves is the attachment of a wave sensor to the bone. However, such an approach has not found widespread application due to the obvious inconvenience and, more importantly, the possible danger to well being of the subject. An alternative approach, attaching the sensor externally, enables the evaluation of the effects of soft tissue on acquired data and avoids the above-mentioned inconveniences and dangers.
Study of Shock Propagation in Human System
The acquired data may be used for in-situ evaluation of the shock absorbing properties of various footwear, shoe insoles, external prosthesis, etc. The same approach may be used for analysis of the shock protection ability of various car and truck seats. In addition, one can analyze performance of number of exercise machines in order to evaluate the amount of shock waves invading the human musculoskeletal system due to the use of those machines.
A lightweight accelerometer can be attached at the tibial tuberosity, on the sacrum, forehead or any other location where there is a relatively thin layer of skin over the underlying bone. These accelerometers measure the frequency and intensity of shock waves propagated in the longitudinal directions of the tibia, spine and at the forehead. Each accelerometer is attached externally to the point of measurement by a metal holder tightly strapped to the skin by elastic strips. Such attachment is capable of faithfully measuring the amplitude of a shock wave. The subject then walks or runs on a treadmill or over ground at prescribed speed and the acceleration data is acquired. During the test, acceleration data may be recorded continuously or at specified time intervals. The accelerometer data is acquired on-line via A/D converter connected to PC. The data may be sampled at up to 10000 Hz per channel and stored for later processing. The example of the typical acceleration pattern recorded on the tibial tuberosity during walking is shown on the right.
A study of the mechanical compliance of biological cells is critical to the study of the pathophysiology of various diseases and to the search for effective treatments. Biologists hypothesize that the biomechanical properties of osteoblasts change as a function of age. Such change could be a contributing factor to the pathogenesis of osteoporosis. To examine the osteoblasts’ mechanosensitivity a polymer-based MEM device that integrated an electrothermal actuator array, a cell-positioning system, a force sensor, and a thermal sensor on a single chip has been built.
This actuator was used to apply compression to different types of cells and evaluate their time-dependent behavior.
These measurements allows to study mechanotransduction, cell adhesion, and other phenomena of interest in tissue engineering.
The image shown below demonstrates a NIH3T3 fibroblast cell that was compressed 4 µm (25% strain) by the actuator in a cell medium.
Tensegrity based cell modeling
Tensegrity is a tensile/compressive network structures that require prestress in their members before external load is applied to
self-stabilize and resist shape distortion. When a tensegrity is subjected to a mechanical load or deformation it adjusts its configuration and the prestress in its members to reach a new equilibrium state. At each equilibrium state the internal elastic energy of the structure is different depending on the degree of deformation.
Effect of the surface topology on the strain energy of a spread cell was investigated by modelling the spreading cell as a tensegrity structure. Spreading as a way to decrease internal energy toward a minimum energy state is the main hypothesis that is investigated. The cell model was placed at different positions along the wavy surface and the spreading and alignment behavior was observed. The cytoskeleton of a cell was modeled as a tensegrity structure and its strain energy was calculated based on the geometry of surface it is attached to.
The spread configurations of the cell on flat surface (A), trough (B) and peak (C) are shown below.