Taking a quick departure from cell printing, it may be useful at looking at how far bioengineering has come in all methods. Jokenhovel et al. engineered a heart valve using specialized cell tissue. The result was an autologous fibrin-based heart valve complete with extracellular matrix. The question that had to be answered though is what method of conditioning was best to ensure cell alignment, attachment, and strengthening. It is important in all bioengineering that the end result be true to the actual structure in the body; therefore, a heart valve should not just look like a heart valve, but work like one for as long as the natural one as well. Jockenhovel et al. placed their scaffold and cells in a bioreactor under three conditions: dynamic conditioning (slowly increasing BPM), optimized dynamic conditioning (BPM changes randomly between 5BPM, 10BPM, and 15BPM), and the control is stirred (where bioreactor materials are continually circulated). 
The image above is the bioreactor used, and displays the exact set-up. The ability to change BPM, and partial pressures of Oxygen and Carbon Dioxide is also shown. This an important measure to take because organs must be conditioned under the same environment that they will be working in.
After running 12 day bioreactions under every condition, what was found was that optimized dynamic conditioning allowed for the strongest cell alignment and attachment.
The figure above shows three cell histologies, all including one of the valve flap (leaflet) and one of the valve wall. What is shown is that native valves have blue collagen ECM that allows it to stay attached and strong through pumps. The control valve does not have these markings of collagen, and also has major tears that display no alignment or attachment of cells. The dynamic conditioned valve, however, has an abundant amount of ECM collagen and seems to be even stronger than the native valve. This dense population of cells and ECM seems to support the practice of exercising an organ in a rigorous environment, equal to its native environment.
This study is important because it focuses on the necessity of conditioning a bioengineered organ. This will come in handy for organ printing because after an organ is printed it must be conditioned to work true to nature. In other words, a clump of cells arranged in the shape of a heart is not enough to be a heart, the scientists must also figure out how to best condition the heart to be as strong (or stronger) than the heart it will be replacing.
Jockenhovel: Conditioning a Bioengineered Organ
