Research was done on muscles, blood vessels, and stem cells.
Skeletal muscles are anchored by tendons to the bone, stimulated by electric impulses transmitted by nerves, and the actin and myosin filaments within muscle cells allow for muscle contraction. Hachi is required to merge with the skeletal muscles in order to benefit the host. The merging of muscles with Hachi's flesh allows for minimum discrepancy in movement when the host is wounded. To merge with the muscles, Hachi should be attached to the tendons and the torn muscles. This assumes that Hachi can be a compatible replacement for muscles. Furthermore, Hachi must be sensitive to the electric impulses sent by the nerves.
Blood vessels are components of the circulatory system. They deliver blood, oxygen, nutrients and proteins, and removes wastes from different parts of the body. Anything of proper size can travel in the circulatory system - provided that do not clog up the blood vessels. Since the circulatory system is the fastest and most extensive transportation medium, Hachi would use this as habitat, entering into the blood stream as a pill, and dissolving into particles within the stomach. To prevent further blood lost, and other circulatory complications, Hachi would construct artificial blood vessel walls and the inner layer of the blood vessel, the tunica intima, which is composed of endothelial cells. This would regulate blood flow.
Stem cells are cells that are common in all multi-cellular organisms that can differentiate into different specialized cells. These can be utilized to construct the endothelial cells of the tunica intima. Hence, Hachi must have stem cells.
The mechanism of Hachi sprouting out a metallic shield is almost scientifically impossible. So a second possibility to make the main seed Hachi very sensitive to temperature. When the blood vessels are damaged and Hachi is exposed, Hachi would replicate but the original Hachi due its sensitive to the temperature would shoot out a tough material. When this gets to a certain height, it would flower out. Visually, it is the same as the metallic shield sprouting out, but just with a different material.
Hachi is going to be an like a cyborg playdoh, massaging into the body as artificial muscle and blood vessels. It would be very sensitive to temperature and electric impulses. Hachi needs a program to be built into it.
Cited: Wikipedia