# Implementation of Portable Automatic Tourniquet with High-Elasticity Biocompatible Strap
## 2 Methods
### Biocompatible Strap with High Elasticity  
The material used for the strap in an automatic tourniquet is important because it is in direct contact with the human body. Therefore, the development of a biocompatible strap is important. Additionally, the strap also needs the mechanical properties of high elasticity and tensile strength, which result in superior hemostatic properties when applied to patients. The elongation characteristics of various strap materials were evaluated using a Push-Pull Gauge (Attonic ARF-50), and the optimum strap material, with excellent temperature and tensile strength characteristics, was selected. Generally, the tourniquet is used at room temperature (5-35 \({}^{\circ}\)C), but it may be used in situations of  
Figure 1: Proposed automatic tourniquet system: (**a**) prototype automatic tourniquet system; (**b**) concept image of the final automatic tourniquet system. extreme temperatures [17; 18]. Here, we used a thermo-mechanical analyzer (TMA) (Q400 TMA, TA Instruments, USA) to evaluate the temperature characteristics of straps. Notably, TMA are generally employed to measure the deformation of samples over a wide range of temperatures. Latex tubing made of rubber material is typically used as a conventional hemostatic substitute strap, and it exhibits somewhat superior characteristics in terms of tensile strength. However, the latex causes an allergic reaction in some patients, and has poor wearability because of the high friction coefficient of the latex. Alternatively, buckle-type tourniquets can be used for urgent hemostasis in field situations rather than being used in a hospital. These are inconvenient due to the need for manual operation and the inadequate strap material. In this paper, the temperature characteristics of straps were evaluated to determine the optimal strap for the proposed automatic tourniquet system. The experimental environment for evaluating the temperature characteristics for each material using TMA is shown in Figure 2, and the TMA results are shown in Figure 3.  
The temperature and dimension changes of various strap materials, consisting of Teflon, latex tube, silicone tube, and thermoplastic elastomer, were analyzed by TMA. Although the appropriate temperature range of the practical use of straps is 20-40 \({}^{\circ}\)C, the straps were analyzed to investigate their thermal stability at a wide temperature range. Figure 3: TMA analysis results for various straps: (**a**) Teflon; (**b**) latex tube; (**c**) silicone tube; and (**d**) thermoplastic elastomer. Figure 2: Environment of thermal test for various straps by TMA. On the basis of the thermal stability of the straps, the temperature ranges were determined and analyzed. Teflon has a thermal expansion coefficient (\(\alpha\)) of 1180 \(\upmu\)m in the temperature range from 20 to 60 \({}^{\circ}\)C, which shows that it displays large thermal deformation. The latex tube used widely as a conventional tourniquet has an \(\alpha\) of 495.3 \(\upmu\)m in the temperature range from 27 to 40 \({}^{\circ}\)C. Although latex tube is widely used currently, it displays large thermal deformation at a narrow service temperature range. The silicone tube has an \(\alpha\) of 296.3 \(\upmu\)m in the temperature range from 20 to 200 \({}^{\circ}\)C, indicating that the thermal deformation is relatively small. Finally, TPE as proposed in this paper has the most stable \(\alpha\) compared with the other materials: 185.5 \(\upmu\)m in the temperature range from 20 to 60 \({}^{\circ}\)C. In short, TPE is the most suitable material for use as an automatic tourniquet snap when considering the temperature range of practical use (20-40 \({}^{\circ}\)C). To determine the optimal strap (i.e., wire) for the automatic tourniquet system, the mechanical properties of straps made of six different materials (i.e., Teflon, PVC, latex, TPE, PE, silicone) were analyzed by a universal testing machine (i-UT50, Excelab, Republic of Korea) (results in Figure 4). The elongation was measured by applying a constant force of 1.5 kgf.