Contact Force and Biomedical Applications

To demonstrate the feasibility of using the S3F in an artificial heart a preliminary test was conducted by ISSI and PSU. An S3F film was cast onto the wall of an Acrylic Penn State 50cc heart model. This film had a thickness of approximately 1-mm and a shear modulus of about 300 Pa. The S3F data acquisition system included a pulsed LM4X LED array as an illumination source and a PCO.1600 CCD camera for image acquisition. The optically transparent heart model allowed the film to be easily imaged by the data acquisition system. Data was acquired at several phases of the pumping cycle by phase locking the pulsed LED and camera to the pulsatile pump. The average result for the shear stress field at 55 ms (early diastole) is shown in the Figure. This result shows a large region of uniform shear on the heart wall that indicates the flow is moving away from the bottom wall of the heart, then separating along a line. A single frame of data at 300 ms (late diastole) also demonstrates the complex nature of the flow along the back wall of this model. Here the film indicates regions of flow reversal and recirculation indicative of possible flow separation.

S3F can be tuned for a wide range of shear modulus for anywhere from airflow to high-shear contact force.  An example of the range of S3F is shown below on an automotive tire.  The aquisition system including the camera, computers and illumination sources are installed in a flat plate where the tire can roll over top of the sensor while the cameras record the deformation from below through a transparent plate coated with S3F.  The contact patch showing normal forces can be seen on the left below.  Normal force and shear can be calculated from the S3F sensor over a wide range.  Rolling forces of 60,000 pounds have been demonstrated with tires.  

Another application of S3F is measurement of contact forces. An example of this involves investigation of normal and shear forces under a shoe or foot. A preliminary demonstration of the S3F was performed to show the potential of the system. An acrylic plate was prepared with an S3F and the camera and illumination system were positioned below the plate. Data was acquired from below the plate at 15-Hz (limited by the frame rate of the camera) as a subject walked across the S3F surface with a shoe on the foot. The normal and tangential deformation fields at two phases of the walking cycle are shown in the Figure. Note the presence of the shear forces in the region with the maximum load. To demonstrate the spatial resolution of the system, a static picture was acquired with a subject standing on the film. The resulting normal deformation field (upper right portion of the Figure) demonstrates the ability of the system to respond to local forces.