Answer:
Complex systems are confined in a small space with multi directional functionality which is difficult for humans to achieve and hence it is difficult for scientists to build a heart.
Step-by-step explanation:
The special complex organ such as heart and kidney have cells that are specialized to perform specific functions.
Creating cells that perform special functions is a difficult task. Along with this, the internal structure of such organs is very very complex and also the entire system is limited to only a small volume or area.
Thus, complexity further rises with space limitation.
Another major issue is that these special organs are connected to multiple systems with in the body in a way that all systems work in co-ordination.
Establishing such co-ordinations is a next to impossible task.
Hence, it is difficult for scientists to build a heart.