Tham khảo tài liệu 'advanced biomedical engineering part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Diffusion of Methylene Blue in Phantoms of Agar Using Optical Absorption Techniques 131 The mixture of agar in water was heated up to 80 C and stirred during 4 min in such a way that all the agar powder is completely dissolved. The resulting solutions were deposited in containers cooled to room temperature and the containers were sealed. Optical detection technique In order to evaluate the diffusion processes a simple optical system was developed. The experimental arrangement is shown in Fig. 2. In this case the samples were contained inside glass tubes 10 cm long X 3 mm diameter . As the light source a 635 nm and 4 mW laser diode with a uniformly opened elliptical spot with an approximate area of cm long and 3 mm wide was used to illuminate the glass tube. The light transmitted through the sample is collected on the opposite side of the tube using a Judson PA-7 16C detector with a working range of wavelengths from 500 nm to pm . This detector consists of a linear array of sixteen photodiodes Fig. 3 with a cross section of 1 mm2 with a separation of 2 mm between two consecutive photodiodes. The detector output is connected to homemade electronics and from that to a National Instruments BNC-2090 device allowing the detection of eight simultaneous signals along the tube. The analog signals are captured using a data acquisition Analog-Digital card PCI-6035. This information is sent to a PC for storage and subsequent analysis. The diffusion process was induced by adding 4 mL of methylene blue solution on the upper side of the tube. As a consequence the methylene solution starts to migrate downwards through the sample and the agar slowly changes color and becomes dyed by the methylene blue. The light transmitted through the sample changes when the dye absorbs the light and this is registered by the photodiodes array detector. In this way the transmitted light is a direct measurement of the changes in concentration and provides the parameters .
