## 2 Active Systems
### Field-Active Fluids in Energy Dissipating Systems
#### 2.2.3 Active Body Armor  
Body armor is any personal equipment worn to protect the body from injury in dangerous situations. The applications of body armors can range from military rests to sport clothes, including personal protective equipments (PPEs) to provide the greatest possible protection for employees in the workplace. Thus, traditionally, field-active fluids have not been used for these sorts of applications due to difficulties in accomplishing the most important constraint, which is lightweight. Nevertheless, during the last decade, new systems, which use active fluids for body armoring applications, have emerged. Here it follows a couple of examples of it:  
* Footwear. Athletic footwear are composed of three parts: the upper, which cosily and comfortably encloses the foot; the sole, which provides traction, protection and a durable wear surface; and a midsole for enhanced protection and shock absorption when the heel strikes the ground. This latter part is particularly important in footwear designed to be used over a long period of time. The midsole should be made of cushioning materials, soft enough to absorb the shock and firm enough to not bottom out before the impact is totally absorbed. Conventional running and walking footwear provide the user with the maximum cushioning, which results in a lower stability, bringing the 'runner's knee' and other athletic injuries as collateral damage. [PERSON] [102] disclosed, among others, an exemplifying embodiment that provides a variable footwear support method that includes one sensor collecting the information about intensity and frequency of foot-falls; and one electric/magnetic field generator adapted to generate an energy field upon a rheological body, which contains the active fluids and is arranged within the sole of the footwear. The energy field controls the viscosity of the fluids and, consequently, adjust the flow rate of ER or MR fluid into and/or out of certain chambers. Thus, this invention allows different degrees of cushioning and support within the sole assembly. Other inventions related to the applications of active fluids to footwear technology can be found in the patents of [PERSON] et al. [103] and [PERSON] et al. [104].  
Figure 2: Sketch of a possible embodiment of an active sandwich structure containing an Electrorheological Fluid, according to the description provided in the works of [PERSON] and co-workers [95; 101].  
* Helmet. In certain sports such as american football, ice hockey, lacrosse, ski, etc. shocks are frequent and many of them result in concussions to the brain. The kinematics that induce a concussion are thought to consist primarily of rotational acceleration-deceleration motions of the head. Thus, any effective device in preventing concussion needs to provide a constraint for head accelerations to below concussive-inducing threshold levels that occur when the wearer is subject to a high impulsive force. In addition, such devices need to be compact and place minimal impediments to the normal motion of the head to allow the user unobtrusive and unhindered operation. [PERSON] et al. (2015) disclosed an invention that connects helmet and shoulders in a way that offers a minimal impediment to the normal motion of the head at typical rates, but provides a nearly rigid connection between the head and torso under impact conditions and, hence, greatly reduces the angular acceleration of the head relative to the body. The primary feature of this invention is a mechanism that effectively creates a telescoping mechanism to allow rotational motions about two orthogonal axes simultaneously (superior-inferior and lateral axis). In addition, the telescoping motion of the plates places the active fluid in a repeatable and consistent shearing motion as the head is moved. Rotation about the third axis (posterior-anterior) is controlled by an active fluid enabled rotational damper, which disallows high rates of angular acceleration that could cause the athlete to suffer a brain concussion.