Technical pitfalls in middle ear gas studies: Errors introduced by the gas permeability of tubing and additional dead space
Conclusions We demonstrated errors introduced by the gas permeability of the tubing and additional dead space. Materials with practically no permeability, such as glass, must be used to overcome the loss or gain of gas through the walls of tubes used for studying ME gas variations. Experiments conducted at a constant volume (variable pressure) require the smallest possible tubing volume in order to obtain good sensitivity and improve the accuracy of the results. Objectives Experimental studies that investigate middle ear (ME) gas exchanges, using either pressure measurements or volume changes, are conducted using various tubing connections between the ME and a measuring device. The aims of this study were to highlight experimental problems due to the errors introduced by (i) the gas permeability of the tubing used and (ii) additional dead space in experiments conducted at constant volume. Material and methods The problem of the gas permeability of the tubing was addressed by comparing three types of tube (silicone, polyethylene, PVC) with a glass tube. Horizontally placed tubes were connected to a syringe filled with pure CO2 via a gas-tight valve. The end of each tube tested was plunged into colored water (5% Coomassie Brilliant Blue R-250). The tube was washed out with CO2 until gas bubbles were seen leaving it. The valve was then closed and the movement of water in the tube was observed. The same experiments were repeated for pure O2. The problem of the error introduced by the additional dead space volume was addressed at a theoretical level using the well-known gas laws. Two conditions were studied: condition A, in which the experiment was conducted at constant pressure and the volume was measured; and condition B, in which the experiment was conducted at constant volume and the pressure was measured. The main outcome measure of each condition was the calculated variation in the final number of moles of gas involved. Results No water movement was observed in glass tubes. In contrast, plastic tubes exhibited significant gas permeability effects for both CO2 and O2. The colored solution advanced at a faster rate with CO2 than O2 but differently for each type of tubing. For condition A, gas transfer was independent of the volume of the external tubing and was accurately measured by the displacement of the droplet in the lumen. In contrast, for condition B, the pressure variations were influenced by the volume of the tubing.
Transmucosal gas-loss rates in middle ears initially filled with O2 or CO2

This study investigates the role of different gases in clearance of gas in the middle ear cavity (ME) by its mucosal blood flow. A rat model was used to measure gas volume changes in the ME cavity at constant pressure without ventilation. We disturbed the normal gas composition of the ME by filling it with O2 or CO2, measured the consequent changes in gas volume over time and compared these results with previously obtained ones for air and N2. The first 5 min of the primary transient phase (phase I) for O2 or CO2 was characterized by a volume loss decrease of -0.49 ± 0.34 μL and -46.28 ± 8.49 μL, respectively, with volume loss increase for air and N2 differing greatly, at +0.17 ± 0.17 and +2.31 ± 0.81, respectively. The CO2 value of -46.28 μL showed that a volume of gas equivalent to that of the ME cleft volume was eliminated within the first 5 min. In the second phase (phase II), all gases showed a linear decrease in volume, which presumably represents a steady-state gas loss rate. However, the gas loss rate of -0.307 ± 0.170 μL min-1 for O2-filled MEs was significantly higher than the mean of -0.124 μL min-1 for all other gases. We used a previously established mathematical model to calculate the effective ME mucosal blood flow rate under steady-state (phase II) conditions. The blood flow results for O2-filled MEs differed greatly from those of the other gases (89.0 ± 49.28 vs. 26.5 μL min-1, on average), which suggest that the model used to calculate blood flow should be modified if used with O2-filled MEs. Further work should involve a comparison of our method with different methods to verify ME blood flow rate.
Influence of preparation conditions on microdomain formation in poly(urethane urea) block copolymers
Selected poly(urethane urea) block copolymers were prepared under different conditions and their microphase-separated morphologies analyzed primarily with small-angle X-ray scattering (SAXS). Preparation conditions were varied by adjusting the temperature and vacuum pressure during solution casting. Copolymers with relatively high hard segment contents prepared under “low” vacuum conditions, where solvent is removed comparatively slowly and the copolymers spend a longer period of time in the presence of a plasticizer (i.e., solvent), produced films with higher degrees of phase separation. The longer casting times associated with the low vacuum pressure conditions also resulted in larger mean interdomain spacings. We speculate that this may be due to secondary hard domain coalescence. Attenuated total reflectance FT-IR and wide-angle X-ray diffraction experiments were performed, but these were not sensitive to the microdomain organization of these copolymers.