# In-Vitro Experimental Modeling of Oscillatory Respiratory Flow in a CT-Scanned OSAHS Tract
## 2 In-Vitro Experimental OSAHS Flow Model  
### ETA Model with OSAHS  
Creating an anatomically correct transparent experimental respiratory flow model is essential to measure respiration flow inside a human airway by using the PIV technique. The present ETA model was based an anatomical data, which was obtained from a scanned airway of a 48-year-old Chinese male OSAHS patient with a 128-slice spiral CT, under the rest breathing condition. The apnea-hypopnea index (AHI) and body mass index (BMI) were 52 and 26.8, respectively. In order to obtain detailed data, CT scanning was performed on the patient along the coronal, sagittal, and cross-sectional directions. The thickness between the CT-scanned slice images was 1 mm. The OSAHS condition of the patient was so severe that the passageway through the nasal cavity was totally blocked in the retro uvula region (RUL) as shown in Figure 1. The ETA model composed of the oral cavity, pharynx, larynx and trachea was adopted to mimic the patient's breathing through the oral cavity. The ETA model with OSAHS was then constructed with the image processing software Mimics [15; 16; 22]. The digital ETA model is illustrated in Figure 2, from which the obstruction resulting from the retro uvula hypertrophy can be clearly observed in Figure 2a. Two representative measurement sections were selected, i.e., the global A-A section viewed from the lateral side and the local B-B section near the obstructed region, as shown in Figure 2a,b.  
To facilitate an accurate experimental measurement, the experimental model was finely fabricated as follows. First, a negative ETA model was printed with water-soluble material using a fused deposition  
Figure 1: Sagittal plane CT image of OSAHS patient.  
Figure 2: Reconstructed ETA model with OSAHS: (**a**) dorsal view; (**b**) lateral view; (**c**) front view; (**d**) four cross-sections.  
modeling (FDM) printer [16; 23; 24]. The melting temperature of the water-soluble material was 220 \({}^{\circ}\)C and the wire diameter was 1.75 mm. The maximum printing error of the negative model was less than 0.1 mm. Second, the negative model was coated with a water-soluble glue. The coating glue functioned as a slice separator of the negative model as the glue dried out. Both the water-soluble material and glue were mainly composed of polyvinyl alcohol (PVA), but they had different physical properties. The dried water-soluble glue will not stick to the external filler silicone, but the water-soluble material will. Third, the coated negative model was put into an optical glass box that was then carefully filled with transparent liquid silicone Sylgard 184 [25]. Before it was poured into the box, the silicone was needed to be vacuumed to remove bubbles in it. If not, the fabricated model wall would absorb tiny bubbles, which then seriously polluted measurement results. The liquid silicone was solidified by curing at room temperature over 48 h. Finally, the negative model and the coating glue were washed with cold water. The fabricated experimental model is shown in Figure 3. The compliance of the human airway tract and the mucus attached to the inner wall [26; 27] were not considered in the present experimental model for the sake of accurate PIV measurements.