Bovine liver tissue was used as a tissue model for in vitro tests with the designed diffusing fibers, in that the chromophores, such as dead endothelial cells and blood vessels, would still be able to absorb the visible laser light () significantly.23,24 The liver specimens were acquired from a local slaughter house, and they were cut into segments in size (approximately 1 cm thick) and stored at 4°C prior to the experiments. Figure 2(a) illustrates an experimental set-up of photocoagulation tests with the fibers. A circular tissue holder (7 cm in diameter and 1 cm in thickness) was prepared, and a 1 cm thick tissue sample was placed at the bottom of the holder along the curvature. The curved surface of the tissue sample partially reflected the transverse anatomic features of human uterus [Fig. 2(a)]. The diffusing fiber was located 1 cm above the tissue surface, so the incident light was uniformly irradiated on the tissue due to the curved geometry of the sample. A conventional clinical laser (, quasi-cw mode, High Performance System, CA, USA) was employed for laser coagulation, and the input power was maintained at 120 W in order to entail tissue coagulation through the diffusing tip. The average irradiance on the tissue surface was calculated to be approximately . Three fiber conditions were tested: bare diffusing fiber, capped diffusing fiber, and capped diffusing fiber with a 1 mm thick layer of polyurethane (PUR). As PUR is a raw material for balloon catheters for endoscopic applications,25 so the last condition represented the final device design to incorporate the capped diffusing fiber into a balloon catheter. However, for the sake of experimental simplicity, a thin layer of PUR was placed on top of the target tissue [Fig. 2(a)] instead of using a balloon catheter. During the tests, a PUR layer was superimposed upon the tissue sample as shown in Fig. 2(a) (left image). Prior to coagulation tests, the optical transmission of each fiber (bare diffusing, capped diffusing, and capped diffusing with a PUR layer) was roughly measured with a photodetector to be 99%, 97% and 94.5%, respectively. Figure 2(b) presents the intensity profile along the capped fiber tip that was measured every 5 mm. In addition, the transmission loss of PUR at 532 nm was measured with the detector to be less than 2.5%, which experimentally verified negligible light absorption (i.e., optically transparent with no light scattering) at 532 nm by the PUR layer. The coagulation threshold was preliminarily determined for the three fiber conditions by varying irradiation times from 2 to 8 s (1 s increments; a sample per each condition). The onset of visible discoloration on tissue surface was considered the physical evidence of coagulation. A number of irradiation times (4, 7, 15, 30, 60, 90, 120, 150, and 180 s) were then evaluated for the three conditions to identify the temporal evolution of photocoagulation on the tissue surface. Due to difficulty of preparing fresh tissue specimens with a large uniform surface area, each condition was tested only with a single liver sample for evaluating surface coagulation. All the specimens were placed under saline environment (room temperature) during the laser irradiation, and the temperature of the liver tissue was maintained at approximately 20°C. Postexperimentally, the radial depth (i.e., into the tissue) and 2-D area of coagulation on the tissue surface were quantified and compared with an image processing software tool (Image J, National Institute of Health, MD, USA). The image in Fig. 2(a) (bottom of the right image) showed a cross-sectional part of the photocoagulated tissue. In order to measure coagulation thickness, each irradiated specimen was cross-sectioned into three pieces [Fig. 2(a)], and the coagulation thickness of each piece was measured five times (). The discolored zone (tan color) represented coagulation necrosis and the red color was the preserved native tissue. A two-tailed Student’s -test was used for statistical analysis and meant statistically significant.