Introduction

Traditionally human cadavers and mammalian tissue have been used as a suitable medium to provide insight into ammunition performance, projectile lethality assessment, and wound ballistics studies [1–6]. However ethical concerns have been raised with the continued use of these materials, and alternative soft tissue analogues have been used in more recent times to ensure research programmes maintain an ethical focus throughout their duration [7–10]. To ensure compliance with ethical requirements, researchers have sought opportunities to utilise different materials that help to reduce their ethical impact without compromising datasets.

Ballistic gelatine has been widely adopted as both the industry and academic standard which has been reported to provide the closest replication of the human torso [5, 11–17]. Its ability to provide a transparent medium in which temporary and permanent cavitation can be analysed via high-speed video which is critical to ensuring improvements in survivability technology and ammunition development. Additionally, the importance of calibration has also been considered with the Fackler method being the industry approved method of ensuring a consistent medium is present within research studies when using 10% gelatine (water/weight (w/w)) [12, 14, 18–20]. The other most used ballistic gelatine variant 20% (w/w) is used within European and NATO countries as an increased density is required to test Full Metal Jacket (FMJ) rounds [11, 12, 19, 21, 22]. However, 20% gelatine is currently lacking a community approved methodology for calibration [11–13, 19, 23].

Whilst the positives of ballistic gelatine are clear, concerns within the academic literature exist, and are most notably raised in relation to temperature dependence, single shot use and manufacturing variability [11, 12, 19, 24–26]. Alternatives are therefore being sought to provide greater economic value to single shot surrogates, simulants that can be used within a broader range of climatic conditions and in more recent times, reducing the amount of material being disposed of enabling research programmes to move toward a more sustainable research practise utilising frameworks such as the 3R principle [12, 27–31].

Perma-Gel has been identified as a suitable alternative that could provide the research community with a viable solution.

Perma-Gel is a synthetic ballistic gelatine substitute mainly used within forensic science, ballistics testing, and medical research [11, 12, 32–34]. Perma-Gel aims to simulate human soft tissue and enable researchers, medical professionals and wider fields to investigate penetration depth, observe temporary and permanent wound cavities, evaluate projectile or fragmentation performance, and to directly compare performance of different bullets or calibres [11, 12, 24, 32, 34, 35]. Manufactured using gellants and mineral oil, this material has been reported to serve the same purpose as traditional 10% ballistic gelatine when evaluating terminal ballistics (i.e. what happens when the bullet or projectile hits a target) but removes the reported limitations [12, 24, 36].

Perma-Gel has been reported to provide additional benefits to traditional Ballistic Gelatine, primarily its ability to be re-used multiple times, removing the need to purchase high volumes of material thereby reducing economic and supply chain burden during testing [37, 38]. This is achieved by melting the material and recasting into the required sample profile. Care and consideration should be given during this process to ensure the material does not degrade - a phenomenon for which the academic literature has reported and has been found to contain numerous conflicting views and findings specifically focussed on the materials ability to continue performing advantageously after repeated melt cycles [12, 24, 33, 39]. Additionally, benefits such as consistency and reproducibility in simulating soft tissue properties are also noted alongside enhanced transparency allowing for visualisation of projectile trajectory inside the media, penetration depth assessment and witnessing cavitation dynamics are also noted [11, 24, 32, 35]. Stability over time compared to ballistic gelatine is also key with no requirement to keep the material refrigerated before testing and greater ability to utilise the material in a wider range of environmental conditions. This attribute is crucial in ensuring a longer lifespan of the material which minimises the probability of suffering from the mould growth and malodorous smell emitted from traditional ballistic gelatine in uncontrolled environments [40–42]. Perma-Gel is delivered in a manufactured state and is ‘ready to use’ having been calibrated to the FBI standard for 10% Ballistic Gelatine at 4 °C, enabling users to link directly to a referenceable source which is both well-known and well understood [43, 44].

Several limitations and critiques of Perma-Gel have been identified and are important to highlight, namely the degradation of mechanical properties caused by thermal cycling (reporting to be a benefit of Perma-Gel) [11, 12, 15, 24, 32]. Repeated thermal cycling of polymeric materials can induce chemical or physical degradation, potentially altering key properties such as elasticity or density (thereby impacting a full assessment of penetration depth and wound profile) [45–47]. For materials like perma-gel, such changes may have important implications for its ability to reliably simulate tissue response in ballistic events, and therefore its applicability in specialisms such as wound ballistics, survivability assessment, lethality and ammunition design, and forensic reconstruction. Additionally, problems occurring during thermal cycling such as air bubble formation (leading to inconsistencies in density and structural integrity), uneven heating (localised degradation or incomplete melting resulting in non-uniformity in material samples), contamination (dust, debris or residue from previous uses) and the potential loss in material (from evaporation, spillage, or residue left within moulds resulting in differences in final sample volume and consistency) can impact the accuracy of test results.

Differences in mechanical properties compared to human tissue (i.e. elasticity, density, or failure modes) are also of concern. Strain rate and mechanical equivalence of Perma-Gel are reported to behave between 10% and 20% Ballistic Gelatine variants, with potential differences in retarding force/exit velocities than human tissue [11, 32, 34]. The literature has reported higher exit velocities in Perma-Gel when compared to other soft tissue surrogate alternatives, which may suggest less retarding force than standard gelatine under specific conditions [15]. This is important to understand to ensure penetration of projectiles are full understood and not overestimated. These differences in mechanical behaviour are important to note as without sufficient control, mapping to tissue behaviour directly may not be exact.

Differences in material properties are often controlled via use of Calibration. Calibration to the FBI standard at 4 °C has previously been reported as being of benefit to Perma-Gel, however some concern exists regarding is appropriateness given the differences in material density. The literature is widely populated when using this procedure for traditional ballistic gelatines, yet paucity of literature exists on this procedure’s applicability to Perma-Gel and its findings on how well this material remains calibrated throughout its life when considering thermal cycling and prolonged exposure to environmental conditions without sufficient refrigeration.

Economic and sustainability should also be considered. Perma-Gel has been found to have a greater upfront economic impact to a research programme, however by re-melting and recasting the material can provide savings throughout the research programmes duration [15]. The authors have also noted potential concern regarding the disposal of the material. Unlike traditional ballistic gelatine, Perma-Gel contains mineral oil, which is reported to not be bio-degradable [48, 49]. Should incorrect post processing and disposal of the material take place, this could have a negative impact on the environment and each research establishments sustainable goals [12, 15, 50–52].

Noting the above limitations, it would suggest that Perma-Gel is best suited to being used comparatively rather than assuming absolute fidelity to human tissue and must include calibration or validation steps if possible. Researchers should track and limit re-use of blocks, documenting any changes in appearance (colour, clarity, bubbles, voids) and mechanical responses where possible to ensure any differences do not negatively impact the output data. Additionally, consideration should be given to maintaining consistent environmental conditions such as temperature, block size, orientation, and backing support to minimise variability and acknowledge that the results (temporary cavity size, material displacement, and fragmentation behaviour) may not directly translate into human wound outcomes for which appropriate caveats should be included in the research discussion.

Consideration of the materials limitations is important, but so is the comparability with existing Soft Tissue Surrogates. PermaGel has most commonly been used within the wider literature to replicate the aims of Ballistic Gelatine [11, 12, 15, 21, 24, 32, 33, 53, 54]. To articulate the benefits of PermaGel compared to the industry standard, Table 1 highlights they key features deemed important within wound ballistics and testing and the differences between both materials.

Feature/Attribute | Ballistic Gelatine | Perma-Gel
Material | - Gelatine made from animal collagen diluted to 10% or 20% by mass [11, 12] | - Polymer/synthetic gel formulated to mimic soft tissue [34, 36]
Primary Advantage | - Industry standard (historic use) [12, 43]- Well understood baseline | - Reusable, [24, 39]- Shelf-stable, [24, 39]- Consistent across batches. [24, 39]
Clarity/Visibility | - Clear-ish but can be cloudy [12, 15]- Visibility decreases with re-use (hence single shot preference) | - Very Clear – excellent for observing wound tracks and formation [12, 24, 39].
Temperature/Storage | - Must be refrigerated and used fresh [12, 18, 19]. | - Stable at room temperature- Long Shelf Life [24, 39].
Reusability | - Traditionally used a single shot use material due to issue regarding clarity. | - Designed to melted and reused many times [24, 39].
Batch Consistency | - Medium – Standardised procedures are available, however risk of incremental differences during manufacture [19]. | - High (factory formulated for uniformity)
Calibration | - Calibration procedures available for 10% variant (Fackler) [12, 43].- No industry accepted calibration procedure for 20% variant [12] | - Manufacturer supplies procedure to match 10% gelatine reference [44].
Behavioural Differences | - Good replication of average soft-tissue response (Industry Standard) [11, 12] | - Good replication of average soft-tissue response [24, 34, 39]- Projectiles may behave differently vs. organic gelatine [34]
Preparation | - Mix and cool to set [14, 15, 18, 19, 55]- Susceptible to spoilage and mould if stored incorrectly [18, 19] | - Melt/recast and pour as directed [24]- Minimal spoilage
Cost | - Lower per block cost but reoccurring expense and logistics [15]. | - Higher initial cost, but lower long term due to reuse [12, 15]
Practical Lab Needs | - Refrigeration/freezer and more lab preparation and disposal required [12, 15, 19] | - Oven or heat source to melt/recast [24]- Less cold storage
Health/Sanitation | - Biological material requires hygienic handling and disposal | - Low bio-hazard risk- No rotting/smell
Limitations | - Homogenous substitute – no skin, bone, layered tissue or vascular perfusion1- Variability from organic material. | - Homogenous substitute – no skin, bone, layered tissue or vascular perfusion

Although Table 1 has highlighted that PermaGel is akin to Ballistic Gelatine, differences do exist and therefore use of this material should be undertaken with caution. Perma-Gel is marketed as being equivalent to 10% gelatine, but the literature indicates that its mechanical response may differ in high strain rate environments [21, 34, 54]. Additionally, previous studies have conflicting views on how many cycles of melting and re-casting are available to the user before material degradation begins [12, 24, 33]. Boackle et al. have reported a that Perma-Gel can be reused up to 12 times, however this is contradicted by discussions between Tischler and Mabbott who have cited 10–15 times [24, 33]. Such inconsistencies within open literature make the decision on reusability challenging and pose additional risk of inaccuracies within datasets.

This study aims to provide the academic literature with a single point of source data that provides both academia and industry with a clear understanding of the number of melting cycles required before degradation becomes an issue, and an assessment of the degradation severity within a ballistics setting. By assessing changes in penetration depth material consistency, and elasticity across multiple re use cycles, the authors aim to determine the practical limitations of Perma-gel as a reusable wound ballistics simulant.