## 2 Simulants
### Ballistic Gelatine  
Ballistic gelatine is a widely accepted material for survivability assessment and wound ballistic research. Two types are readily available: Type A, which is derived from acid-treated collagen found in pig skin, and Type B, which is derived from alkali-treated beef skin [12]. As the human anatomy is more closely aligned to that of porcine anatomy [10; 13], the most widely used gelatine is Type A. Type A material consists of an acid-treated collagen protein found in animal products [10] and is manufactured in 10 and 20% mass constructs before being conditioned at specific temperatures before use. Both 10 and 20% constructs' ability to replicate the human body is measured by their strength and stiffness properties, which are referred to as the 'bloom number'. Ballistic gelatine is available in 50-300 bloom constructs [10]; however, the Type A bloom number must reside between 250 and 300 to provide accurate results in wound ballistic research [14]. To provide the bloom number, a 112 g sample of 6.67 \(w/w\) gelatine is manufactured to standardised time and temperature systems before undergoing a compressive test resulting in the bloom figure [15].  
It has been reported that the concentration and temperature during preparation can also influence the performance ability [11] but should be controlled using calibration.  
Currently calibrated using the US Fackler method [16], the 10% construct (Figure 1) is reported to be the most beneficial for uses in which penetrating impact analyses are required [17]. Manufactured with 90 parts water and 10 parts gelatine, the material can be affected by water quality, temperature, and post-manufacture conditioning [15], leading to inconsistencies in output data. However, with variables that can be controlled in lab environments, this material has historically shown its viability in wound ballistic research.  
By contrast, the 20% construct is uncalibrated [17] and is often referred to as 'NATO' gelatine [15], which is manufactured with 80 parts water and 20 parts gelatine powder [5]. It has been reported that 20% gelatine is superior to 10% gelatine when examining deflection and force data, with outputs showing reactions closer to those of human responses [18]; however, this evidence is singular, and the claim that performance is only viable for these measurements cannot be found elsewhere. Similar to the 10% construct, this material also exhibits issues with water quality, temperature, and post-manufacture conditioning, but because of the uncalibrated manufacturing technique [14], researchers can vary their manufacturing method, increasing the difficulty in experiment repeatability.  
Regardless of which construct of ballistic gelatine is used, the shelf life remains poor [15; 14]. Polymer-based gels, such as the brain tissue simulants Sylgard and Styrene-Ethylene-Butylene-Styrene (SEBS), which is a thermoplastic elastomer [19], have been developed to provide a suitable alternative with similar performance properties to those of ballistic gelatine to measure behind-armour blunt trauma and back-face deformation. This may eliminate storage requirements and alleviate shelf-life issues; however, the existing issue of manufacturing variable control remains of concern for survivability.