A sentinel, also referred to as a "squiddy" by the Human Resistance, is an autonomous killing machine that patrols the ancient sewers and passageways of the dead human cities of old in search of Zion hovercraft or wandering, defenseless humans unlucky to be walking in the tunnels.

The sentinels replaced the B1 series after their exile by the humans. Over time, the machine AI grew more intelligent, and they built newer, better upgrades for their appearance. When the Machine War broke out, the sentinels were used as search-and-destroy units. They were tasked with picking up the dead and incapacitating live humans in order to study them. After the conclusion of the war in the machines' favor, the sentinel units were further upgraded, and now patrol the sewers of the dead human cities of old, where they seach for wandering Zion vessels or humans unlucky enough to be walking through them at the time.

The articulated tentacles can be hard to print with a perfect fit and if too loose by 0,2mm they wont hold the weight or wont clip together properly. After experimenting with different ideas I have found a solution using elastic bungee cord passed through the hollow center of each tentacle from end too end and stretched lightly. This keeps a pressure on all joints and avoids loose and floppy movement. It greatly reduces the precision needed. To hold the bungee I ended up making small collets that clamp onto the bungie cord at each extremity of the tentacles it worked really well and means no glue needed.