adapter) whose magnification can be varied from 0.7x to 4x. The lens with a working distance of 92mm, has a field of view of 10mm at 0.7x magnification and 2mm at 4x magnification. The depth of field also varies from 1mm at low magnification to 0.1mm at high magnification. The camera can capture at most 17 frames per second. While one of the camera is vertical, the other is inclined at 45 degrees in the vertical plane (see figure - 1). The camera and the lens are mounted on damped mounting rods using a custom designed mounting system. Second subsystem consists of a suit of computer controlled linear translation stages to apply the required multi-axial loads. Two stages are used along each of the two mutually orthogonal directions (see figure 1) to perform displacement controlled bi-axial tests on planar specimens. As seen in figure 1, on top of each of these stages, manually controlled precision stages (25mm travel, 0.01mm resolution) is provided for each of the two directions to allow positioning of the specimen accurately in the field of view of the cameras. The computer controlled linear translation stages (Holmarc Opto-Mechatronics) has a 100mm travel with 0.5 micron precision and its velocity can be set between 2ȝm/s to 6mm/s. The stepper motor used in the translation stage can apply a maximum force of 100 N at its peak velocity. The motor controller communicates with the computer using a RS-232 port. By appropriately actuating these stages uniaxial extension/compression, biaxial extension/compression, rigid body translations can be achieved. The final subsystem is the data acquisition system. It consists of two load cells (Honeywell-Sensotec Model 31E) and a 18 bit analog to digital converter (UEI model PowerDNA with DNA-AI-208 as input layer), which can take 8 voltage input signals and whose sampling rate can be varied between 1 to 8000 samples per second is used for sensing the applied load . The data acquisition system communicates with the computer through the ethernet interface. After, a preliminary study on the expected load that needs to be applied to engender a stretch ratio of 1.2 in the tissue one of the two types of load cells one that can measure loads up to 200N in both tension and compression and the other measures 2.5 N is used. The reported deviation from linearity for the load cells is less than 0.15 percent over the full range and resolution limited by only the analog to digital convertor. All these subsystems are connected to HP Workstation XW4600 with Intel Core 2 Duo processor running at 2.66 GHz and having 4GB RAM. Figure 1 (a) Photograph of the experimental set up (1.Bread board, 2. Stepper Motor Controller (W,X,Y & Z axes) 3. Data acquisition system, 4. Vertical camera and lens, 5. Inclined camera and lens, 6. Z-axis motorized stage, 7. W-axis motorized stage, 8. Y-axis motorized stage, 9. X-translational stage, 10. Y-translational stage, 11. Z-translational stage, 12. Sample, 13. Saline Container, 14. Load cell) (b) Calibration wedge a b 2.2 Details of the Software Modules Software for simultaneous operation (video, motor, DAQ) and synchronous acquisition of data (video, load) from this hardware has been developed in LABVIEW. The experimental protocols are completely automated by separate software modules. Each of these modules has many sub-modules whose details are given below in the order of their execution. 66
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