Co-existence of applied compression and perfusion created asymmetry in WSS waveforms.
WSSs induced by applied compression alone (C+P-, C++P-) revealed symmetrical waveforms, in which no WSS was observed at 0, 0.5, and 1 seconds (see Supplemental Video 1 for WSSs of C+P- in whole loading cycles). In compression only configurations, peak WSS values were found at 0.25 sec and 0.75 sec when the moving solid boundary was at maximum velocity, according to our loading sine waveform. The peak values of median WSS were 50 mPa and 100 mPa for C+P- and C++P- configurations, respectively. In contrast, when compression and perfusion were combined, WSS waveforms showed temporal asymmetry in the resulting median WSS. Peak WSSs were higher in the first half of the loading cycle (see Supplemental Video 2&3 for WSSs of C+P+ and C+P++ in whole loading cycles), suggesting that compression-induced fluid flow ‘cancels’ applied perfusion in the second half of the loading cycle. To investigate interstitial flow more deeply, we selected C+P- and C+P+ simulations to generate fluid velocity vectors and WSS heat maps at 0.25, 0.5, and 0.75 sec (Fig. 6). At 0.25 sec, the scaffold walls had maximum downward velocity (~200 um/sec at the top boundary), while at 0.75 sec, the walls had maximum upward velocity. In the compression only (C+P-) simulation, compression pushed the fluid out with the moving scaffold boundary at 0.25 sec, which was then resorbed at 0.75 sec with a similar velocity profile as compression was unloaded (Fig. 6A). Hence the double peak symmetrical WSS waveform. However, in C+P+ simulation, the moving scaffold walls opposed the applied steady fluid flow for the first 0.5 sec. As the fluid was incompressible, the opposing velocity augmented the total upward fluid flow in the scaffold, thus generated much higher WSSs (Fig. 6B). In contrast, at 0.75 sec, the scaffold walls synergized with the direction of applied fluid flow in the upper portion of the scaffold, which reduced the total velocity and WSSs, and even created a downward flow at some local sites.