At present, there are no more than two bulletproof mechanisms of bulletproof vests: bouncing off projectiles and fragments; and continuously dissipating the kinetic energy of bullets through bulletproof materials to prevent non-penetrating damage.

Current body armor can be divided into hard material protection such as metal and ceramics, soft material protection such as high-strength high-modulus fiber, and composite protection combining soft and hard according to the different materials used. These three types of bulletproof materials have quite different protection mechanisms against projectiles such as fragments and projectiles. The ballistic protection mechanism of hard bulletproof materials is to absorb kinetic energy and prevent penetration through the destruction of bulletproof materials to a certain extent, as well as the deformation and fragmentation of projectiles themselves; soft bulletproof materials are generally made of high-performance fibers through high-density textiles. It is formed by overlapping 20-40 layers, using the stretching, breaking and friction of multi-layer fine nets to absorb the kinetic energy of bullets and intercept them. At present, most bulletproof vests integrate the two protection mechanisms together, both soft and hard, to show the true qualities of a hero. The most commonly used ceramic bulletproof armor is a kind of composite body armor, which is made of ceramic bulletproof inserts combined with Kevlar fibers. 

                 

2. Problems facing the development of body armor
Body armor mostly uses artificially synthesized ultra-high molecular weight polyethylene fibers or aramid fibers as the main material. During use, these materials will age due to the influence of external environmental changes such as sunlight, temperature and humidity, resulting in a significant decline in ballistic performance. In general, after 2000 hours of sunlight exposure, the tensile strength will drop to 20% of its original value. In addition, the impact of temperature on the life of bulletproof fibers cannot be underestimated. If the temperature is lower than 10°C or higher than 240°C, the strength of bulletproof materials will be greatly reduced, especially after some materials are violently impacted by bullets. higher than 240°C. The impact of body armor is not only reflected in the natural environment, but also in the user's own internal environment. Wearing bulletproof vests is usually accompanied by strenuous exercise. If the moisture dissipation ability of bulletproof vests is relatively poor, sweat cannot be excreted from the body in time, and the strength of bulletproof fibers will be greatly attenuated after being wet. Therefore, how to better adapt the "delicate" body armor to the harsh battlefield environment and make it more durable is one of the problems that need to be solved urgently.

For the human body, most of the important organs are distributed in the torso. If the torso is injured, the probability of death will be very high. At present, the protective parts of bulletproof vests are generally on the vital parts of the human torso from the throat to the waist, which is why bulletproof vests are often called "bulletproof vests" [1] . However, its protective area is still far from meeting the requirements. The limbs of soldiers on the battlefield are still exposed. If they are unfortunately shot, it will also lead to a reduction or loss of combat effectiveness. However, the limbs are the key parts that determine the flexibility of the human body. Once protected, it will undoubtedly affect the mobility and flexibility of soldiers in combat, thereby reducing the overall combat capability. Therefore, how to improve the comfort and flexibility as much as possible while increasing the protective area is the second problem to be solved urgently.

As far as the current bulletproof vests are concerned, in order to meet the most basic bulletproof functions, most of them use materials with low concave depth and cushioning materials to achieve the purpose of reducing the impact on the human body. However, these materials also make the body armor have the disadvantages of high rigidity and excessive weight, which affects the wearing comfort. Poor flexibility or unreasonable structural design will restrict the freedom and acuity of soldiers' actions, which will not only increase discomfort, but also affect the maneuverability and effectiveness of performing work, putting them in a dangerous situation. In addition, wearing too heavy body armor for a long time will overload the individual soldier, increase the consumption of ineffective power, make it easier to feel fatigue and even cause sports injuries. Therefore, how to balance the weight and softness of the body armor with more protection is the third problem that needs to be solved urgently.

Judging from the scope of use of body armor in recent years, it has broken through the military field and made great strides towards police and civilian security. However, after carefully investigating the use of body armor in the three environments of military, police and civilian use, it is not difficult to find that the ammunition power levels, types and damage degrees faced by these three environments are different. In addition, due to the different environments in which the military, police, and civilians wear body armor, they also put forward different requirements for body armor, such as high protection, high degree of freedom, and lightness and thinness. Therefore, how to balance the use needs of multiple parties and create a body armor suitable for various environments is also the fourth problem that needs to be solved urgently.
                                   

3. Outlook for the future development of body armor

Combining the above problems, it is hoped that through the development of new bulletproof materials and optimized structural design, it will help the development of bulletproof vests in the direction of humanization and diversification.

3.1 New material direction

3.1.1 Bionics-based bulletproof materials
The lobster film with high elasticity and good toughness provides a reference for improving the flexibility of body armor. The lower part of the lobster's tail is connected by a translucent membrane that divides into several parts. This soft membrane is exceptionally tough, with a microscopic, layered and plywood-like structure that makes it resistant to scratching and cutting. The membrane is also elastic to some extent, which allows the lobster to whip its tail back and forth, making it difficult for predators to bite through or tear the tail apart. This flexibility is due to the fact that the membrane is a natural hydrogel material that starts out fluffy and stretches easily until it reaches about twice its original length, when the material becomes stiffer and more flexible. If this material can be used to make bulletproof vests, the goal of integrating safety and flexibility can be achieved: on the one hand, when encountering injuries, the soft membrane can harden and play a protective role; Move for greater comfort, especially in areas of high body movement such as elbows and knees.

Body armor that mimics pangolin scales is self-healing. Although the traditional body armor made of Kevlar fiber is strong and safe, its built-in steel plate or ceramic can no longer be used after being hit by a bullet. But body armor made from pangolin scales can be reused after being hit. The wonder of pangolin scales is that its molecular structure is connected to each other in a magical way, and it can remember the original shape. This feature makes the scales highly resistant to impact. Only water can wake up this memory, and it can perform self-control. repair. The body armor made of this material, no matter how severe the deformation is, just throw it into the washing machine, wait for 3~5 minutes, all the dents, creases and twists of the body armor will self-heal and return to the original shape .

3.1.2 Graphene bulletproof material
Graphene is made up of carbon atoms linked together in a honeycomb pattern, in the form of one-atom-thick sheets that are stronger and lighter. The new material called Diamene studied by Professor Elisa Riedo of the City University of New York is composed of two graphene sheets on a silicon carbide substrate. It has been confirmed that the bulletproof material based on double-layer graphene can be thinner than paper and stronger than paper. diamond. It is as light and flexible as aluminum foil in its normal state, and becomes harder than a block diamond for a short time when sudden mechanical pressure is applied at room temperature [2] . In science, the analogy of non-Newtonian liquids can be used to help understand this material.

It is worth noting that the hardening effect of this new material only occurs when two graphene sheets are used, no more and no less, otherwise, the hardening effect does not occur. Therefore, how to optimize its performance through reasonable configuration still needs more in-depth exploration and experiments.
            

3.1.3 Electronic Body Armor Materials
Electronic body armor can not only be bulletproof, but also capture the signal from the incoming projectile, and process and generate interference signals within a few microseconds, so that the fuze of the projectile can be deceived and explode in advance at a distance of several hundred meters from the target [3] . At present, researchers have developed a fiber circuit board made of multiple pre-stretched elastic yarns and polyurethane-coated fibers, which can be used to make smart body armor. When hit by a machine gun, the body armor can send out a message for help, reporting the location of the person who was shot. According to the researchers, the circuit board can be stretched 1 million times without damage and washed 30 times without deformation. When hit by a bullet, the fiber circuit board can respond. This means that if on the battlefield, smart body armor can send a signal to the medic that the soldier is injured.

3.2 Structural Design Direction

3.2.1 Comfort Design
As an important indicator of body armor, comfort can be improved through a more ergonomic design to improve its comfort and flexibility, which is conducive to various tactical actions and movements. For example, a body armor shoulder strap of the U.S. military adopts an obvious inverted trapezoidal design. The front connection point of the shoulder strap is leaned towards the midline of the body, and the neck should be freed from the shoulder socket to facilitate the traditional shoulder. At the same time, the redundant design is retained to ensure that the shoulder strap can Cooperating with upper limb activities, it can better match different shoulder shapes of different people, as shown in Figure 1.

3.2.2 Lightweight design
The development of bulletproof to lightweight is also one of the future trends. The bulletproof layer has changed the traditional practice of stacking bulletproof fabrics layer by layer, but instead has a three-dimensional design based on the trajectory and state of the projectile. For example, a flexible body armor in the United States has a thinner thickness and a 13% reduction in mass, which will undoubtedly reduce the physical consumption of soldiers and be more conducive to the completion of tactical actions.

3.2.3 Modular design
Modular structure design is adopted to strengthen the functional integration of body armor. The method of integrating body armor and carrying equipment reduces the number of varieties, reduces the load on individual soldiers, and avoids the problem of duplication of functions. It truly realizes the integration of body armor and multi-purpose clothing. The modularity of portable equipment is simply a universal attachment system. A wide variety of attachments provides soldiers with more sub-packages and component choices, which facilitates them to flexibly configure and arrange various equipment, weapons and ammunition according to mission needs. It makes individual equipment more systematic and can cope with different mission environments. For example, the AVS modular combat system can replace every part like a building block, and the back plate with protection and the back piece supported by the restraint belt can be freely selected. In addition, you can also choose different equipment according to different tasks. For example, you need a heavy vest for good protection. Long-distance penetration requires lightweight vests that allow for rapid deployment and strategic maneuvering.

4. Epilogue
Body armor is an important modern armor, and all countries in the world attach great importance to its development. Generally speaking, the development of body armor has gone through a long period. Although there are still some technical bottlenecks, with the development and application of high technology, it is believed that new materials, new technologies and new processes will surely provide new body armor. The design and production of the equipment seek the optimal solution for comfort and bulletproof performance, so as to improve the reliability of bulletproof equipment and effectively enhance the performance of injury reduction.