Research done on tendons, anatomy of muscles, and upper layer of epidermis.
Tendon is a type of fibrous connective tissue that connects muscle to bone or muscle to muscle. Tendons are designed to withstand tension, and are required for body movements. Tendons are composed of water, type-I collagen, tenocytes (a type of fibroblasts) cells, and small amounts of minor fibrillar collagens, fibril-associated collagens, and proteoglycans. Tenocytes produce collagen molecules that aggregrate and produce collagen fibrils. These collagen fibrils are then organized into fibers. The tendon organ is formed with these fibers and various types of connective tissues. The vertically-organized arrangement of collagen fibrils is responsible for the tendon's strength. Blood vessels run parallel to the collagen fibers within the tendon. The inner bulk of tendons do not contain nerves but some outter tissues of the tendon contain nerve endings.
Muscles join tendons by having collagin fibers from within the muscle organ connect with those of the tendons. Tendons join bones by inserting collagin fibers into the bone at an enthesis, where the inserted collagen fibers materializes and integrates into bone tissue.
Hachi's flesh must attach itself to the collagin fibers of the torn tendons in order to connect properly with the skeletal muscles, and act as an artificial muscle. Futhermore, it must extend collagin fiber into the bone to join with the bone.
The structure of a skeletal muscle is as follows:
A bundle of muscle fibers create the fascicle, a bundle of fascicle form the endomysium, and the perimysium wrap around the endomysium and blood vessels to create the skeletal muscle while the tendon attaches the muscle to the bone. Now pretend that the muscle disappears because a Hollow slashed it off (we're in the Bleach world), Hachi pops up in place of the muscle and attaches to the torn tendons. To address the problem of what if the Hollow slashes off half the muscle: Hachi's built-in stem cells would create an adjacent layer of similar formation and specialization, developing a compatible boundary, and connecting the torn muscle tissue and Hachi effectively.
Steering away from Hachi's organic parts, and going on to the synthetic brain: Hachi would normally be hibernating, swimming along the blood vessels. While hibernating, Hachi also "subconsciously" memorizes the blood pressure and the balance of chemicals in different parts of the body. (Each Hachi has a designated location where it swims slowly and freely about.) Once an injury is inflicted, the shift in balance alerts and activates Hachi, and Hachi rushes to wound. Hachi then proceeds to execute its healing program.
The outtermost part of the epidermis is composed of dead stratified squamous epithelial cells. The squamous epithelial cells are arranged with one layer on top of another on a basement membrane. Skin is regarded as keratinized stratified squamous epithelium. This just means that the skin is kept hydrated by keratin. The stratified squamous epithelial cells provide abrasion protection for the rest of the epidermis. If rubbed or scratched, a layer of the epithelial cells may come off and be replaced with the one below it, while more of these dead cells layers would produced, or rather, shedded by the body. Hachi may incorporate these qualities for its outtermost layer; the layer that would rest side by side with the human skin.