Cell culture assays have become standardized methods in the toolbox also of medical engineers e.g. to test new materials for cytotoxicity as an alternative for animal testing . Therefore, a broad range of established cell lines are available, which are comfortable to handle. Nevertheless, biological systems like cell cultures are sensitive to temperature and CO2 fluctuation. Thus the employment of a suitable incubator is important , because the probe has to reside for a long time in an optimal thermal and gaseous environment e.g. 37°C, 10 Vol-% CO2, >90% rh. Though it is not required to ensure the absolute niveau of the parameters rather than keeping them as constant as possible within the limits . The inspection of atmosphere constancy of an incubator is regulated in the DIN 12880 .
During access to the incubator due to door opening the climate is exchanged with the surrounding atmosphere. Based on our experience a duration of more than 30 s the distinct gaseous components have emanated to the external environment. Furthermore the inside of the incubator is cooling down. Improvement of the door opening kinematic may hold potential to reduce said exchange and also induced turbulence i.e. air movement during door opening. This may lead to lowering impact on the samples, saving valuable and costly resources and lower the risk to introduce potential harmful particles as carrier for bacteria  into the incubator’s atmosphere. Investigations via experimental light section flow visualization ,  and CFD simulation have been conducted to compare an adjusted door opening concept to the conventional incubator design.
2 Materials and methods
The typical carbon dioxide incubator relies on a two-fold swing door concept consisting of an outer temperature regulated metal door and an inner glass door. In contrast to the standard design a sliding door concept, as shown in Figure 1, is suggested to replace the inner door. It is expected to reduce turbulence and induced suction to the inner gas volume when being operated. The new door concept is compared to the state of the art by CFD-simulation and an experiment of visualization of fluid flow via light section. The simulation is performed with ANSYS Fluent®. The geometry of an incubator prototype is used to define the inner volume with three perforated shelves (modelled as pressure jump boundary conditions) in place. Both door concepts are modelled to comply with the real dimensions. The sliding door model is derived from the working prototype, used for the light section investigation. Its opening area is slightly reduced compared to the swing door. The movement of both doors is realized by using dynamic meshes with user defined functions (UDF) applied to the moving boundaries. The k-ε turbulence model is applied. The setup of the light section experiment is shown in Figure 2. A mock-up of an incubator is used enclosing an inner volume of approx. 180 l which is temperature regulated (37°C) and filled with effect fog for every run of the experiment. The aperture of the mock-up allows mounting of either a swing door or the prototype of the sliding door concept. The inner volume is also equipped with three perforated shelves. During door opening the fog emanates and is visualized by a laser plane projected perpendicular to the aperture plane. Light section images are taken with a camera system from door opening until after 30 s. Characteristic dimensions for simulation and experiment are listed in Table 1.
3.1 Simulation results
As a result of the post-processing of the simulated data Figure 3 shows the magnitude of the induced velocity in the vertical symmetrical plane of the incubator in the relevant area during door opening with the respective opening kinematic. Both opening processes are simulated to be finished after 1 s. Especially at the beginning of the swing door opening the induced velocity in the opening gap between the door and the incubator is very high due to the suction of the opening door. The velocity magnitude remains high throughout the opening process as a result of the fluid displacement, thus there are also eddies disturbing the atmosphere inside the incubator. In contrast, the sliding door concept induces only little air movement during door opening in the area surrounding the moving part of the door, leading to much smaller eddies. Figure 4 visualizes the average and extremal values of carbon dioxide concentration in the incubator over the time span between the closed door and 15 s after the beginning of opening, determined at the coordinates within the incubator defined in DIN 12880 . The rapid decline of the minimum curve of the swinging door during the first second is due to the described high velocities close to the door, resulting in a fast mixing between the inner and surrounding atmosphere, whereas in the sliding concept the suction is much smaller, resulting in much less mixing. Exchange of atmosphere is then achieved only by natural convection and diffusion leading to a much smaller amount of climate loss during door opening. After door opening these mechanisms result in a steady decline of the before established atmosphere. The atmosphere using the sliding approach is declining slower.
3.2 Light section results
The resulting movement of fluid from the start (t = 0 s) until finish (t = 1 s) of the door opening process is compared for the swing and the sliding door in Figure 5 . The inner atmosphere (visualized by fog) seems to adhere to the moving swing door (t = 0.5 s, opening angle approx. 45°). Furthermore two major eddies can be recognized on the upper and the lower edge. After door movement has completed (t = 1 s, opening angle 90°) two air drafts on the upper and the lower level can be seen that reach out by the length of the swing door.
The upward movement of the sliding door generates small delicate eddies. After completion of opening procedure the primary movement direction of the atmosphere is upwards due to natural convection which remains the dominating effect as shown in Figure 6. After 10 s the atmosphere of the swing door has emanated to a higher extent than for the sliding door setup. At t = 20 s the inner atmosphere is nearly depleted whereas emanating atmosphere from the mock-up with the sliding door is still clearly visible. Just before closing the door after 30 s the atmosphere has been exchanged completely with the environment for both concepts.
4 Discussion and conclusion
The comparison of the suggested sliding with a standard swing door concept shows that there is a high influence during the opening process on fluid movement of door kinematic which is obvious in the simulation as well as in the experiment. The suction of the swing movement induces higher fluid velocity and bigger eddies, which may favor the introduction of particles from the outside and thus increase the risk for contamination. Furthermore the atmosphere exchange or depletion occurs faster for the swing than for the sliding door though this outcome might be biased due to the reduced aperture area. Regarding the performance of both concepts over the standardised duration of 30 s according to DIN 12880  one can state that the atmosphere depletes completely for both concepts. An improvement of retainment might be possible by using the option to individually define the aperture position and area by adjusting the sliding door to the position and aperture size required for access to the inside (cf. Figure 1). A clear advantage is the reduced turbulence of air induced by the sliding mechanism, since this is favorable for clean working conditions as well as clean room environments.
Conflict of interest: Authors state no conflict of interest. Material and methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The conducted research is not related to either human or animal use.
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About the article
Published Online: 2016-09-30
Published in Print: 2016-09-01
Citation Information: Current Directions in Biomedical Engineering, Volume 2, Issue 1, Pages 329–332, ISSN (Online) 2364-5504, DOI: https://doi.org/10.1515/cdbme-2016-0073.
©2016 Matthias B. Schuh et al., licensee De Gruyter.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0