GB2504180A - Skin and tissue simulant - Google Patents

Abstract

A skin and tissue simulant for testing the ability of projectiles 4, in particular small metallic or non-metallic ballistic fragments, to perforate human skin, comprising at least one synthetic chamois layer 1 as the skin simulant, a gelatin backing 2 as the tissue simulant, and a means 3 of affixing the skin simulant adjacent to or in contact with the tissue simulant, and further characterised in the skin simulant outer surface provides a target surface for a projectile, and methods of producing and uses thereof. The skin and tissue stimulant is characterised in that the simulant has been selected using an algorithm V50 = 134S-0.35, wherein V50 is the velocity at which perforation is achieved for 50% of the projectiles and S is the sectional density of a given projectile. A method of producing the gelatin tissue simulant is also disclosed.

Description

SKIN AND TISSUE SIMULANT

The present invention relates to a skin and tissue simulant, methods of manufacture, and use as a physical model to allow assessment of skin perforation by fragments, in particular, but not exclusively, small metallic or non-metallic ballistic fragments, to assess projectile injury risk. The skin and tissue simulant is also useful in supporting the development and assessment of protective equipment, or other mitigation approaches, against projectile impact injury.

The outer surface of the mammalian body is comprised primarily of skin, a flexible, water-resistant covering considered the largest organ of the integumentary system. Skin plays a key role in functions including the protection of the underlying tissue, organs and skeleton from infection and injury, homeostasis, somatosensory perception and storage of metabolites. The skin is made of several layers, each with different properties. The outermost layer is the epidermis, which has been reported to range from 0.05 mm to 1.5 mm thickness. Below this layer is the dermis, which has been reported to range from 0.03 mm to 3 mm thick and contains blood vessels and nerve endings. The dermis also dominates the flexibility and strength of the skin due to the presence of collagen and elastin fibres. Below the dermis is the subcutaneous tissue, also termed hypodermis, which is not considered part of the skin itself but connects the skin to the underlying muscle.

Injury to human skin and underlying tissue regions can occur as a result of one or more projectile(s), originating from a source positioned away from a person, striking an area of the person's covered or uncovered skin. If the one or more projectile(s) is of sufficient mass, size and velocity, impact upon an uncovered or covered area of skin may result in skin perforation, causing injury that ranges from slight pain and discomfort, to serious trauma to underlying soft tissue and organs with potentially fatal consequences. Injury from one or more projectile(s) can occur in a numbers of ways, including: projectile impact onto the skin, causing physical effects that include deformation, stretching, compression, tearing and puncturing; injury as a result of transfer of kinetic energy from the one or more projectile(s) to the surrounding skin and tissue; and infection as a result of contamination to the injury site.

Particular groups of professionals operate with an increased risk of skin and tissue perforation injuries from projectiles. For example, military, security and law enforcement personnel can operate in environments or circumstances where there is a likelihood of injury from munitions fired from a weapon, or fragments generated following detonation of a nearby explosive device. Therefore, to help protect personnel against ballistic injury, there is a need to model the likelihood of a projectile of given mass or velocity to perforate human skin. Furthermore, the availability of a skin and tissue perforation model can assess the effectiveness of protective equipment, such as armour, to potentially prevent or minimise skin and tissue injury inflicted by one or more projectile(s) impacting on skin and underlying tissue.

There have been many separate studies in the reported literature investigating the ability of a projectile to perforate skin. Previous studies assessing projectile perforation of skin and tissue models have reported their findings using a variety of unit measurements. For example, the projectile mass has been coupled to the perforation velocity or, alternatively, the velocity required for small or large projectiles to perforate skin reported irrespective of mass. Furthermore, energy density has also been quoted.

It was previously suggested that the velocity of a projectile required for skin penetration was a function of mass and presented area. The sectional density (S) of a given projectile, as defined as mass over cross-sectional area, has subsequently been shown to be a good predictor for skin perforation. This projectile characteristic is advantageous in that it accounts for a projectile's geometry, size and density.

One particular issue with some of the data reported in the literature is the use of threshold perforation velocity', defined as the onset or lowest velocity for a given projectile at which skin perforation occurred. This threshold velocity is entirely dependent on the velocities achieved in the experiment, with the hope that a non-perforation and perforation are achieved within very close velocities. However, due to constraints on the number of shots possible against a particular target this is not always possible. Furthermore, this measurement does not account for the rest of the velocities achieved during experiments, such as any non-perforations at higher velocities.

Therefore, a method considered better for calculating skin perforation is a V50 assessment, defined as the velocity at which perforation is achieved for 50% of the projectiles. This measurement is advantageous in that the V50 has the ability to statistically account for all impacts in an experiment, rather than using just the slowest perforating impact. There are a number of methods of calculation of V50, for example the method described in NATO STANAG 2920: Edition 2 or using statistical analysis. It would be understood by those skilled in the art which methods could be applied for calculation of a V50 as used herein. Ideally, a V50 should also be presented with its corresponding standard deviation or confidence interval to give an indication of its range.

Previous human skin and tissue perforation models include the use of intact skin on Post-Mortem Human Subjects (PMHS). Studies have also been conducted using animal models, for example goat, sheep and pig skin have been shown to have very similar performance to PMHS skin. However, there are a number of issues associated with the use of animal and biological materials, including ethics, storage, supply, control and variability. In particular, the question of variability is considered one of the major factors that needs addressing in order to produce a reliable skin perforation physical model.

An ideal skin simulant has specific mechanical properties with well defined tolerances to ensure repeatability in testing. Consideration of materials for a skin simulant in the literature has focused on man-made materials like rubbers, or imposing strict limits and controls on the biological materials to use. Existing skin and tissue simulants include, for example, those disclosed in US 7,222,525, which are skin and tissue simulants including a gelatin composite block and ether based case polyurethane sheet, in which the gelatin block acts as a tissue simulant and the covering polyurethane sheet acts as the skin simulant. This particular model can be used to determine the perforation potential of a munition fired at the skin and tissue simulant. However, an issue with previously tested skin and tissue simulants is the focus on the performance of the physical model against a single form of projectile, with little or no regard to the model's performance over a range of sectional densities. Therefore, previous skin and tissue sirnulants are not necessarily good models for assessing risk of injury from projectiles as they may only be valid for a particular projectile and it would be unknown how accurate such simulants would be for other projectiles. In light of the wide range of potential projectiles, including low density, non-metallic projectiles, that pose a threat to military, security and law enforcement personnel, there is a need for a skin and tissue sirnulant whereby the materials have been ballistically assessed against a range of projectiles and the performance characterised over a range of projectile sectional densities, giving wider applicability and more confidence in the skin and tissue simulant.

The object of the present invention is to offer an improved skin and tissue simulant, is useful as a physical model to allow assessment of skin perforation, in particular, but not exclusively, by small metallic and non-metallic ballistic fragments (herein referred to as projectiles') over a range of sectional densities. The skin and tissue simulant is also useful in supporting the development and assessment of protective equipment, or other mitigation approaches, against projectile impact injury.

The term perforation' as used herein characterises an event wherein one or more projectile(s) ballistically impact the skin and tissue simulant, pass completely through the entire thickness of the skin simulant and exit the other side. This event includes, but not exclusively, the one or more projectile(s) subsequently penetrating through a measurable distance of the tissue simulant.

The term non-perforation' as used herein characterises an event wherein one or more projectile(s) ballistically impact the skin and tissue simulant and bounce off the skin sirnulant or, alternatively, penetrate a partial, but not complete, thickness of the skin sirnulant, without exiting the other side.

The term pencilling' as used herein characterises an event wherein one or more projectile(s) ballistically impact the skin and tissue simulant and the skin simulant material wraps around the one or more fired projectile(s), without causing a perforation to the skin simulant. Pencilling can result in the one or more projectile(s), wrapped in the skin simulant, subsequently penetrating a measurable distance through the tissue simulant.

In the first aspect of the present invention there is provided a skin and tissue simulant characterised in that the skin and tissue simulant has been selected using an algorithm V50 = 134&°, wherein V50 is the velocity at which perforation is achieved for 50% of the projectiles and S is the sectional density (S) of a given projectile, defined as mass over cross-sectional area. This characteristic of a skin and tissue simulant is advantageous in that this skin and tissue simulant algorithm provides an improved empirical equation, compared to previous skin perforation models, to estimate the is velocity required for skin perforation for a wide range of projectiles, in particular non-deforming projectiles. This skin and tissue simulant algorithm has been based on a larger number of studies (55 data points) than previously reported and it is not particularly sensitive to small amounts of changes to the data set e.g. addition of further velocity measurements. The scatter of the PMHS and animal perforation data interrogated means that this skin and tissue simulant algorithm will give the average skin perforation velocity, with all data points falling within ± 35% of the average predicted perforation velocity. This skin and tissue simulant algorithm should give a good estimation for projectiles, including non-deforming projectiles, of sectional densities between 0.5 g.cm2 and 22.5 g.cm2. The skin and tissue simulant algorithm is based on projectiles with densities between 2.5 g.cm3 and 19.25 g.cm3, mass ranging from 0.03 g to 16 g, diameter between 1 mm and 20 mm and projectile geometries including spheres, cylinders, cubes, airgun pellets, bullets and flechettes.

In an embodiment of the first aspect of the present invention there is provided a skin and tissue simulant characterised in that the skin simulant comprises synthetic chamois.

The term synthetic chamois' as used herein characterises an artificial, leather-like material usually used for polishing. This embodiment offers the advantages of a commercially available skin simulant with tight tolerances in terms of their thickness and mechanical properties, as quoted by the manufacturers. In addition, the option to batch-purchase the skin simulant and calibrate a small sample using previously tested projectiles ensures consistency in skin simulant material. In a further embodiment of the first aspect of the present invention the synthetic chamois comprises one or more materials selected from a group consisting of cotton, viscose, polyvinyl acetate, polyester and nylon-polymide. Further, in one embodiment the synthetic chamois comprises cotton, viscose, polyvinyl acetate, polyester and nylon -polymide. A particular embodiment of the first aspect of the present invention is the use of Rbk Synthetic Chamois Leather 1518 supplied by OneClickTools (www.oneclicktools.co.uk) as a skin simulant.

In a further embodiment of the first aspect of the present invention there is provided a is skin and tissue simulant comprising a first synthetic chamois layer with an inner and outer planar surface for use as a skin simulant, and a gelatin backing for use as a tissue simulant, wherein the inner surface of the first synthetic chamois layer covers all or part of a surface of the gelatin backing. Thus, if one or more projectile(s) ballistically impact the skin simulant at a velocity to enable the projectile to perforate the chamois, then the one or more projectile(s) would also be expected to perforate human skin at the same velocity. The characteristic of a skin and tissue simulant comprising both skin and tissue layers is advantageous as this aspect is likely to provide a representation of human skin and tissue to ballistic impact. A skin layer is likely to decrease the distance that perforating projectiles travel through a tissue layer, which is an important consideration for vital structures under the skin. A tissue layer in a skin and tissue simulant provides a significant effect on a projectile's perforation velocity. It has been reported previously that stretched, un-backed skin simulants required a higher velocity to perforate by a projectile compared to skin simulants backed by 10% gelatin. As such, the presence of a tissue layer provides a more realistic skin and tissue simulant. However, skin simulants tested with a particular backing will only be valid when used in combination with that specified backing and may have a significantly different performance if an alternative backing is used. Therefore, it is important to specify and control the tissue simulant used when assessing skin simulant materials. Types of gelatin used in the present invention as a tissue simulant include Type A Ballistic Grade Gelatin as a tissue simulant, for example Type A Ballistic Grade Gelatin of 250 Bloom or greater.

In a further embodiment of the first aspect of the present invention there is provided a skin and tissue simulant characterised in that there is one or more further synthetic chamois layer(s), each layer with an inner and outer planar surface, wherein each inner surface of the further synthetic chamois layer abuts an outer layer of an adjacent synthetic chamois layer. The term abuts' as used herein includes an inner surface of a further synthetic layer which is next to or touching an outer layer of an adjacent synthetic chamois layer. A further embodiment of the first aspect of the present invention includes the use of two layers of Rbk Synthetic Chamois Leather as a skin simulant. Thus, if one or more projectile(s) ballistically impact the skin simulant at a velocity to enable the projectile to perforate the two layers of chamois, then the one or more projectile(s) would also be expected to have perforated human skin at the same velocity.

In a further embodiment of the first aspect of the present invention there is provided a skin and tissue simulant characterised in that the outer surface of the synthetic chamois layer furthest from the first synthetic chamois layer is a skin simulant target surface for a projectile. This characteristic offers the advantage of assessing by simple visual inspection whether or not one or more projectile(s), fired in the direction of the outer surface of the synthetic chamois layer, perforate the skin and tissue simulant.

In a further embodiment of the first aspect of the present invention there is provided a skin and tissue simulant characterised in that perpendicular distance between the inner surface of the first synthetic chamois layer and the skin simulant target surface is 2.5 mm ± 0.5 mm. This characteristic is advantageous as although a skin simulant material does not necessarily have to replicate the different layered structure of the skin, the overall thickness should be comparable to human skin to increase the likelihood of a comparable ballistic performance. Skin (epidermis and dermis) thickness and mechanical properties can vary greatly between people of different genders, ages, ethnicity, Body Mass Index, as well as different places on the same person. An average male skin thickness of 2.16 mm has been reported, determined from skin thickness values averaged across the thigh, waist, deltoid and suprascapula. In the reported data, values varied from an average thickness of 1.72 mm at the thigh to 2.67 mm at the suprascapula. Therefore, the total skin simulant thickness provided by the present invention is an appropriate value to give a general representation of skin thickness independent of body location.

In a further embodiment of the first aspect of the present invention there is provided a skin and tissue simulant characterised in that the tissue simulant is 20% ± 5% by weight/weight percent gelatin conditioned to 10°C ± 2°C. As used herein, the term weight/weight percent gelatin' refers to the relative mass percentage of gelatin to water.

is The term conditioned' as used herein includes measurable or controllable conditions during the preparation and/or storage of the gelatin, for example temperature. The term conditioning cabinet' as used herein includes a cabinet which can deliver measurable or controlled conditions during the preparation of gelatin. A further embodiment of the first aspect of the present invention includes a tissue simulant of 20% ± 5% by weight/weight percent gelatin conditioned to 10°C ± 2°C at 65% relative humidity.

In a further embodiment of the first aspect of the present invention there is provided a skin and tissue simulant comprising a means of affixing the skin simulant adjacent to or in contact with the tissue simulant. This aspect is advantageous in that the close arrangement of skin and tissue simulant offers a representation of human skin and tissue. In a particular embodiment of the present invention, the skin and tissue simulant are held in close or direct contact by a single sheet of PVC Cling FilmTM. This embodiment provides a means of presenting the skin and tissue simulant in a way which is not expected to affect the resulting perforation of the skin and tissue simulant by one or more projectile(s).

In a further embodiment of the first aspect of the present invention there is provided a skin tissue simulant characterised in that the skin simulant has a Shore A Durometer hardness of approximately 35.

In a further embodiment of the first aspect of the present invention there is provided a skin and tissue simulant for testing the ability of one or more projectile(s) comprising metallic or non-metallic material to perforate skin. In a particular embodiment of this aspect of the present invention, testing the ability of one or more projectile(s) comprising metallic or non-metallic material to perforate skin is performed at 20°C ± 2°C and approximately 45% relative humidity. In a further particular embodiment of this aspect of the present invention, the tissue simulant comprises 20% by weight/weight percent gelatin if, when testing the ability of one or more projectile(s) comprising metallic or non-metallic material to perforate skin, the depth of projectile penetration through the tissue simulant, following perforation of the skin simulant, is to be measured. In a further particular embodiment of this aspect of the present invention, when measuring the depth of projectile penetration through the tissue simulant, 20% by weight/weight percent gelatin is characterised in that a 4.5 mm steel sphere at 179.7 ± 4.7 m.s1 produces a penetration depth of 38.1 ± 6.4 mm. If the gelatin penetration depth recorded during calibration is outside this range, that particular gelatin backing should not be used and a new gelatin backing produced.

In the second aspect of the present invention there is provided the use of a skin and tissue simulant of the first aspect for testing the ability of one or more projectile(s) comprising metallic or non-metallic material to perforate skin. In a particular embodiment of this aspect of the present invention, use of a skin and tissue simulant for testing the ability of one or more projectile(s) comprising metallic or non-metallic material to perforate skin is performed at 20°C ± 2°C and approximately 45% relative humidity. In a further particular embodiment of this aspect of the present invention, the tissue simulant comprises 20% by weight/weight percent gelatin if, when used for testing the ability of one or more projectile(s) comprising metallic or non-metallic material to perforate skin, the depth of projectile penetration through the tissue simulant, following perforation of the skin simulant, is to be measured. In a further particular embodiment of this aspect of the present invention, when measuring the depth of projectile penetration through the tissue simulant, 20% by weight/weight percent gelatin is characterised in that a 4.5 mm steel sphere at 179.7 ± 4.7 m.s1 produces a penetration depth of 38.1 ± 6.4 mm. If the gelatin penetration depth recorded during calibration is outside this range, that particular gelatin backing should not be used and a new gelatin backing produced.

In a further embodiment of the second aspect of the present invention there is provided the use of a skin and tissue simulant for testing the ability of one or more projectile(s) with sectional density between approximately 0.5 and 10.6 g.cm2 to perforate skin.

In the third aspect of the present invention there is provided a method for producing a tissue simulant comprising the following steps in sequence; pie-heating water to 70°C ± 5°C in an appropriate container; producing a gelatin backing of 20% ± 5% weight/weight percent; and storing the gelatin backing at a temperature of 10°C ± 2°C and relative humidity of approximately 65% for up to two days. In a particular embodiment of the third aspect of the present invention, the production of a tissue simulant comprising the following steps in sequence, in which for the second and third steps the water temperature is 70°C ± 5°C: pie-heating water to 70°C ± 5°C in an appropriate container; adding gelatin to the water to a final weight/weight percent of 20% ± 5% whilst stirring the water; stirring the resultant liquid gelatin until all the gelatin is dissolved; removing any foam that has formed on the surface of the liquid gelatin; cooling the liquid gelatin to a temperature of approximately 20°C or below in a mould sufficient to generate a solid gelatin block; and storing the gelatin block at a temperature of 10°C ± 2°C and relative humidity of approximately 65% for up to two days. When measuring the depth of projectile penetration through the tissue simulant, tissue simulant comprising 20% gelatin is calibrated using a 4.5 mm steel sphere at 179.7 ± 4.7 m.s1 to produce a penetration depth of 38.1 ± 6.4 mm. If the gelatin penetration depth recorded during calibration is outside this range, that particular gelatin backing should not be used and a new gelatin backing produced.

In an embodiment of the third aspect of the present invention there is provided a method wherein a skin and tissue simulant is produced by affixing at least one synthetic chamois layer to the gelatin backing.

In the fourth aspect of the present invention there is provided a method for selecting a skin and tissue simulant comprising: selecting a first candidate material as a skin simulant and a second candidate material as a tissue simulant; backing the first candidate material with the second candidate material to produce a potential skin and tissue simulant; ballistically testing the potential skin and tissue simulant with multiple projectiles; calculating V50 for each projectile by using the algorithm V50 = l34S35, wherein V50 is the velocity at which perforation is achieved for 50% of the projectiles is and S is the sectional density (S) of a given projectile, defined as mass over cross-sectional area; and assessing the results to select a skin and tissue simulant. Assessing the results includes using best fit curves to compare the ballistic testing against the predicted V50 for each projectile, wherein a potential skin and tissue simulant is selected on the basis of the level of agreement between the ballistic data and the V50 = 1 34&035 algorithm.

In a further embodiment of the fourth aspect of the present invention there is provided a method for selecting a skin and tissue simulant, wherein the projectiles used for selection have a sectional density between approximately 0.5 g.cm2 and 10.6 g.cm2.

The present invention will now be described by way of examples, firstly with reference to the drawings, in which FIG. 1 is an angled front view of an embodiment of the invention and FIG. 2 is a side view of an embodiment of the invention.

FIG. 1 shows one embodiment from an angled front view wherein at least one synthetic chamois layer(s) (1), providing a total chamois thickness of 2.5mm ± 0.5 mm, acts as a skin simulant and a gelatin block (2) of 20% ± 5% by weight/weight percent gelatin conditioned to 10°C ± 2°C acts as a tissue simulant. The skin simulant is affixed adjacent to or in contact with the tissue simulant by a single sheet of PVC Cling Film'TM (3) wrapped around the skin and tissue simulant.

FIG. 2 shows one embodiment from a side view wherein the skin and tissue simulant, comprising at least one synthetic chamois layer(s) (1), providing a total chamois thickness of 2.5 mm ± 0.5 mm, acts as a skin simulant and a gelatin block (2) of 20% ± 5% by weight/weight percent gelatin conditioned to 10°C ± 2°C acts as a tissue simulant, the skin simulant affixed adjacent to or in contact with the tissue simulant by a single sheet of PVC Cling FilmTM (3) wrapped around the skin and tissue simulant, wherein the skin and tissue simulant is used as a physical model for testing the ability of a projectile (4), of known sectional density and velocity, to perforate the skin simulant.

The present invention will now be described by the particular experimental procedures undertaken to produce the present invention, with reference to FIG. 3, FIG. 4 and FIG. 5.

Generation of an improved skin and tissue simulant algorithm The literature was reviewed to identify previously reported skin perforation threshold and V50 data for PMHS and animal skin perforation models. The PMHS data (number of data points equals 40) was extracted from a number of previous studies using a variety of different projectiles including spheres, cylinders, cubes, airgun pellets, bullets and flechettes. Where possible, all skin perforation data has come from the original authors and, additionally, if the full data set was presented in these original publications (perforation or non-perforation for each shot), the V50 velocities (re)calculated. The limited PMHS testing was supplemented with data from numerous conducted studies looking at skin perforation using animal models (number of data points equals 15), including the use of goat skin, sheep skin and pig skin.

FIG. 3 shows all PMHS and animal data for skin perforation with a wide range of metallic and non-metallic fragments. The animal data agreed well with the PMHS data and confirmed that low density fragments also follow the same relationship for perforation velocities with sectional density.

An empirical equation was developed, based on data including that shown in FIG. 3, and fitted to the PMHS and animal skin perforation data. This equation is based on more data than was available for the creation of any of the other previously published prediction algorithm and is therefore likely to give a more reliable estimate of skin perforation for a wide range of projectiles. The skin and tissue simulant algorithm for the present invention is given as V50 = 134S°35, wherein V50 is the velocity at which perforation is achieved for 50% of the projectiles and S is the sectional density (S) of a given projectile, defined as mass over cross-sectional area.

The skin and tissue simulant algorithm V50 = 134S°35 has been based on all the PMHS and animal skin perforation data analysed. These data sets were combined due to the similarities of the animal data to the PMHS data. The skin and tissue simulant algorithm should give a good estimation for projectiles, in particular non-deforming projectiles, of sectional density between 0.5 g.cm2 and 22.5 g.cm2. The skin and tissue simulant algorithm is based on projectiles with densities 2.5 g.cm3 to 11.2 g.cm3 (also one datum point was for tungsten, 19.25 g.cm3), mass of 0.03 to 16 grams, diameter 1 mm to 20 mm and a wide range of projectile geometries (spheres, cylinders, cubes, airgun pellets, bullets and flechettes). Due to the number of data points used as the basis for this equation (number equals 55), it is not particularly sensitive to small amounts of new data, or small changes in the original data. The scatter of the PMHS and animal perforation data means that this skin and tissue simulant algorithm will give the average skin perforation velocity for a projectile of given sectional density, with all data points falling within ± 35% of the predicted perforation velocity from the skin and tissue simulant algorithm.

Selection of candidate skin and tissue simulants 12 candidate materials were selected as skin simulants for ballistic testing, including a variety of rubbers, of varying thicknesses and hardness, and real and synthetic chamois leathers. Backing of the skin simulant appears to have a significant effect on the V50.

Skin simulants tested with a particular backing will only be valid when used in combination with that specified backing and may have a significantly different performance if a different backing is used. Therefore it is important to specify and control the tissue simulant used when assessing skin simulant materials. The present authors have found that 20% ± 5% by weight/weight percent gelatin conditioned to 10°C ± 2°C was the most representative tissue simulant for penetrating fragments and bullets and was therefore used as the tissue simulant during the assessment of skin simulants.

Production of skin and tissue simulant models The 12 materials selected as skin simulants were each backed by a 20% gelatin block conditioned to 10°C prepared according to the following method. Type A Ballistic Grade Gelatin 250 (or greater) Bloom was mixed with water in a 1:4 ratio, for example 1kg dry gelatin powder to 4kg water pre-heated to 70 ± 5 °C. During mixing the water was stirred using an appropriate implement and the gelatin flakes slowly added. When all the gelatin was added, the liquid gelatin was stirred for five minutes, covered and allowed to stand for five minutes, stirred once more for five minutes and allowed to stand for a further 45 minutes. Any foam formed on the surface of the gelatin was scraped off. If the gelatin still appeared opaque prior to pouring, it was allowed to stand until clear. The liquid gelatin was decanted into mould(s) sufficient to generate individual gelatin blocks of 30 cm by 15cm by 15cm. This step may be done in a conditioning cabinet. The liquid gelatin mixture was cooled in the mould(s) to a temperature of 20°C. The gelatin block was removed from the mould and stored at a temperature of 10°C and 65% relative humidity for several hours until the temperature in the block has stabilised. The gelatin block may be stored for up to two days. If the depth of projectile penetration through the tissue simulant, following perforation of the skin simulant, is to be measured, the gelatin was calibrated using a 4.5 mm steel sphere at 179.7 ± 4.7 m.s1 to produce a penetration depth of 38.1 ± 6.4 mm. If the gelatin penetration depth recorded during calibration was outside this range, that particular gelatin backing was not used and a new gelatin backing was produced. To prepare each skin and tissue simulant perforation model, a skin simulant was affixed adjacent to or in contact with the long side of a 30cm by 15cm by 15cm gelatin block using a single sheet of PVC Cling FiImTM.

Selected projectiles for skin simulant testing The 12 materials selected as skin simulants were ballistically tested for use in a skin perforation model, backed by 20% gelatin at 10°C. These materials were each assessed with a number of different metallic and non-metallic projectiles to give their performance over a range of projectile sectional densities, giving wider applicability and more confidence in the simulant performance. The projectiles shown in the table below were chosen to allow evaluation of each target material over the applicable sectional density, so that the down-selected skin and tissue simulant can be used for a wide range of projectiles. The projectiles were supplied by Dejay Distribution Ltd. The steel spheres were manufactured to AISI 316. The table below gives the optimal performance in terms of V50 of the projectiles, to aid the assessment of the skin simulants, calculated from the skin and tissue simulant algorithm V50 = 134S°35.

Projectile Sectional Predicted V50 using skin and density (5) tissue simulant algorithm (g.cm2) (m.s1) 9 mm steel sphere 4.79 78.0 9mm glass sphere 1.53 116.4 6mm glass sphere 1.03 133.6 3 mm glass sphere 0.51 170.6 6 mm steel sphere 3.19 90.0 9 mm ceramic sphere 2.32 100.6 mm steel sphere 10.63 59.0 Skin and tissue simulant testing methodology Due to the number of shots required for this type of V50 assessment, and the numerous target materials, the skin simulants were evaluated in stages. All simulants were tested S with a 9 mm steel sphere, to give a V50 for a central projectile sectional density.

Promising materials were then additionally tested with a 6 mm glass sphere and 9 mm glass sphere. This evaluated the material at the lower range of sectional densities, most appropriate for the majority of small fragments. When the material was shown to be in close agreement with the published data, further testing was done with a selection of 3 mm glass sphere, 6 mm steel sphere, 9 mm ceramic sphere and 20 mm steel sphere.

This gave the performance at the low and high extremes of sectional density, as well as additional low to middle sectional density points. This was conducted against a limited number of the selected materials.

All firings were performed at 20°C ± 2°C and approximately 45% relative humidity, with the skin simulants stored under these conditions for a minimum of 24 hours. Gelatin blocks at 10°C were used within a 20 minute window from removal from the conditioning cabinet, before returning for conditioning.

Projectiles were fired using a Pressure Housing weapon system, with a separate smooth bore barrel for each different diameter projectile. The projectiles were propelled using rechargeable 37 mm compressed Airmunition cartridges, using pressures of 3 to MPa. For extremely low velocities required for the 6 mm glass sphere against some materials, 0.32" blank cartridges with 0.05 to 0.08 g of WIMMIS 0704 propellant were used instead of the Airmunition.

Projectile velocities were measured using Oebler Model 57 Infrared Ballistic Screens with 0.8 m separation, connected to a Nicolet Sigma 10 oscilloscope and additionally by MSI solid state velocity equipment with a 1 m separation.

V50 perforation velocity for each fragment was calculated using statistical probit analysis and in most cases was additionally able to give a 95% confidence interval on the V53 velocity. It also was used during the trial to show when the 95% confidence interval was sufficiently small to give an accurate and reliable result, at which point the testing for that simulant was stopped.

The STANAG 2920 method for calculating V50 was also used to give an additional comparison of material performance. This method takes an average of the velocities of non-perforations and the same number of perforations, all within 40 m.s1. It is a more widely recognised method for calculation of V50 for armour material performance.

All assessments were based on skin simulant perforation. A failure of the material in the same way as a perforation was recorded if the gelatin backing was penetrated as a result of pencilling.

Results Averages of 14 shots were performed for each material and projectile size to get a value for the skin perforation V50. The velocities recorded by the Qehler and MSI velocity equipment were very similar (a Pearson's product-moment correlation coefficient of s 0.9983 with a p value of 2.2x1016), with an average velocity difference of only 0.6 m.s1 (0.4%) between the two systems. This comparison was performed for 388 shots of velocities between 40 and 250 m.s1 for the 6 mm and 9 mm projectiles. When the velocities from the Qehler and MSI equipment were plotted against each other, this resulted in a straight line best fit with a gradient of 1.002 and R2 value of 0.998. The velocities recorded from the Oehler equipment were used to calculate the V50 data, using velocities from the MSI equipment if the Oeheler equipment failed to record a shot. The Oehler velocity equipment was able to consistently record velocities down to m.s1 for the 9 mm spheres and 40 m.s1 for the 6 mm spheres.

FIG. 4 shows the V50 performance of all the skin simulant materials under constant conditions with each of the different projectile sectional densities. The error bars show the ± 95% confidence interval. The solid line shows the skin and tissue simulant algorithm and the dashed lines show the upper and lower limit of all the PMHS data, used for assessing the performance of the simulants. All the materials follow the trend of lower perforation velocity with increasing sectional density of the projectile.

The skin simulant which performed closest to the skin and tissue simulant algorithm, during ballistic assessment at 20°C ± 2°C and approximately 45% relative humidity using a range of projectiles, was the two layers (2.5 mm in total) synthetic chamois backed by 20% gelatin conditioned at 10°C. It was characterised to give V50 values for seven different projectiles, suggesting it is a suitable skin simulant for projectile sectional densities between 0.5 g.cm2 and 10.6 g.cm2. It is likely to still be suitable for deforming projectiles, within the stated sectional density range. The performance of this skin simulant may change significantly if backed by a different material. If used with a different backing, it would then require revalidation with the PMHS and animal skin perforation data to ensure its validity.

FIG. 5 shows the performance of the two layer synthetic chamois compared to all the s PMHS and animal skin perforation data. There is good agreement between the PMHS and animal data with the results for the 2.5 mm synthetic chamois over the tested projectile sectional density range (0.5 g.cm2 to 10.6 g.cm2), with all points for the synthetic chamois falling well within the scatter of the PMHS and animal data. If the specification of the synthetic chamois is changed by the manufacturer over time without warning, a suitable replacement synthetic chamois could be used after re-validation.

Claims (16)

1. CLAIMS1. A skin and tissue simulant characterised in that the skin and tissue simulant has been selected using an algorithm V50 = 134S°35, wherein V50 is the velocity at which perforation is achieved for 50% of the projectiles and S is the sectional density (8) of s a given projectile, defined as mass over cross-sectional area.
2. 2. A skin and tissue simulant according to claim 1 characterised in that the skin simulant comprises synthetic chamois.
3. 3. A skin and tissue simulant according to the preceding claims comprising a first synthetic chamois layer with an inner and outer planar surface for use as a skin simulant, and a gelatin backing for use as a tissue simulant, wherein the inner surface of the first synthetic chamois layer covers all or part of a surface of the gelatin backing.
4. 4. A skin and tissue simulant according to any of the preceding claims characterised in that there is one or more further synthetic chamois layer(s), each layer with an inner and outer planar surface, wherein each inner surface of the further synthetic chamois layer abuts an outer layer of an adjacent synthetic chamois layer.
5. 5. A skin and tissue simulant according to claims 3-4 characterised in that the outer surface of the synthetic chamois layer furthest from the first synthetic chamois layer is a skin simulant target surface for a projectile.
6. 6. A skin and tissue simulant according to any of the preceding claims characterised in that perpendicular distance between the inner surface of the first synthetic chamois layer and the skin simulant target surface is 2.5 mm ± 0.5 mm.
7. 7. A skin and tissue simulant according to any of the preceding claims characterised in that the tissue simulant is 20% ± 5% by weight/weight percent gelatin conditioned to 10°C±2°C.
8. 8. A skin and tissue simulant according to any of the preceding claims comprising a means of affixing the skin simulant adjacent to or in contact with the tissue simulant.
9. 9. A skin tissue simulant according any of the preceding claims characterised in that the skin simulant has a Shore A Durometer hardness of approximately 35.
10. 10.A skin and tissue simulant according to any of the preceding claims for testing the ability of one or more projectile(s) comprising metallic or non-metallic material to perforate skin.
11. 11. Use of a skin and tissue simulant according to any of the preceding claims for testing the ability of one or more projectile(s) comprising metallic or non-metallic material to perforate skin.
12. 12. Use of a skin and tissue simulant according to any of the preceding claims for testing the ability of one or more projectile(s) with sectional density between approximately 0.5 and 10.6 g.cm2 to perforate skin.
13. 13.A method for producing a tissue simulant comprising the following steps in sequence: a) pre-heating water to 70°C ± 5°C in an appropriate container; b) producing a gelatin backing of 20% ± 5% weight/weight percent; c) storing the gelatin backing at a temperature of 10°C ± 2°C and relative humidity of approximately 65% and storing for up to two days.
14. 14.A method according to claim 13 wherein a skin and tissue simulant is produced by affixing at least one synthetic chamois layer to the gelatin backing.
15. 15.A method for selecting a skin and tissue simulant comprising: a. selecting a first candidate material as a skin simulant and a second candidate material as a tissue simulant; b. backing the first candidate material with the second candidate material to produce a potential skin and tissue simulant; c. ballistically testing the potential skin and tissue simulant with multiple projectiles; d. calculating V50 for each projectile by using the algorithm V50 = wherein V50 is the velocity at which perforation is achieved for 50% of the projectiles and S is the sectional density (S) of a given projectile, defined as mass over cross-sectional area; e. and assessing the results to select a skin and tissue simulant.
16. 16.A method for selecting a skin and tissue simulant according to Claim 15, wherein the projectiles used for selection have a sectional density between approximately 0.5 g.cm2 and 10.6 g.cm2.