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.