Rheology of Cystic Fibrosis Sputum

People with cystic fibrosis (CF) suffer from a sticky and thick sputum, which obstructs airways and impedes clearance and thus can cause chronic lung infections. Gel-forming mucins are a main component in the sputum and they are critical to the onset of the disease as well as in determining sputum flow. Rheology is used to study viscoelastic properties of CF sputum and can help to gain insight in flow properties during respiratory clearance processes such as ciliary beating and coughing. Despite the increasing use of shear rheology in medical CF research, little is known about inertia limitations at high frequency oscillations (100 rad/s), which are commonly used to mimic coughing. We therefore investigated the flow behavior of CF sputum in oscillatory shear. We found that the low sample modulus (~10 Pa) and the onset of instrument inertia limits measuring frequencies to 10 rad/s to mimic coughing events, which is one order of magnitude lower than the commonly used frequency. Furthermore, using a smooth and a roughened plate geometry, we find that the roughness of the plate geometry has little effect on small amplitude oscillations but shows substantially different results during large amplitude oscillations and yield measurements. Both protocols are potentially important to study clearance performance. Our findings will support future medical studies in choosing the best possible measurement protocol and geometry for CF sputum rheology and the raised awareness for limitations will assist the interpretation of obtained data