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Abstract

The most important piece of safety equipment is developed as a reinforced piece of body armor to resist attacks to the upper parts of the body so as to save the lives of its wearers to offer protection against stabbing with sharp-tipped objects. The majority of commercial stab resistant armors are not comfortable for users to wear during their whole duty shift. The three-dimensional (3D) printing has given great opportunity to develop equipment for a particular and individual application with the incorporation of performance and comfort. Stab protective armor has been developed by 3D printing without compromising the protection performance for a particular energy level to improve the comfort of the armor vest so that users are willing to wear during their whole duty shift. In this study, air permeability is used to measure the comfort tendency of the protective armor as a safety gear without reducing the protection performance. In this study the effect of textile materials and structures, shapes of 3D prints as the segmentation of scales, size of scales, parts of the full vest, attachments, and air exposure sides of the panel are investigated. The result revealed that the air permeability of the 3D printed protective armor vest improved the comfort as compared to the commercially available armor vests of both from a single plate and large sized segmented scales.

Keywords

Air permeability FDM improved comfort stab-protective clothing comfort of armor vest protection performance

Introduction

Stab- protective clothing is the most important piece of safety equipment that helps saving the lives of its wearers. A stab vest is a reinforced piece of body armor designed to resist attacks to the upper part of the body (abdomen, chest, neck, back, and sides) that can be worn underneath or over clothing and offers protection against stabbing with sharp-tipped knives, needles, nails, and other sharp objects.

It is concerning to observe that the significance of personal protective equipment (PPE) as a preventive measure against assaults is steadily increasing in modern society. This trend is no longer limited to police forces, but is also becoming more noticeable in many other professions. According to the Federal Situation Report 2021, published by the Federal Criminal Police Office in Germany, a total of 88,626 incidents (2021) were recorded where police officers became victims of violent acts. This number represents an increase of 4.5% compared to the previous year and is part of a trend that has shown a steady increase in such events since 2013. Physical attacks occupy the second place in the crime statistics with 14,645 recorded offenses, behind resistance against state authority. There has also been an increase in violent acts against fire and emergency service personnel since 2018. In 2021, 2160 violent acts were recorded in which employees of these professional groups were attacked.¹ Furthermore, this trend can also be observed in the assaults against employees of Deutsche Bahn for the year 2022, where an increase in violent acts of 22% was recorded, partly due to the enforcement of mask mandates and the introduction of the 9-euro ticket, resulting in overcrowded trains.²

Research indicates that an officer who is not wearing body armor is 3–4 times more likely to suffer a fatal injury if stabbed in the torso than an officer who is wearing body armor. Thus, police officers, military, transport, and correction administrators should encourage their staffs to wear stab vests during the whole duty shift.^3,4 The level of protection required in soft and sensitive bodily regions is determined by the type of attacks that are likely to be encountered,⁵ the type of weapons those used for puncture, and the stabbing techniques of assailants.⁶

Though protection and comfort are always conflicting, the designers and developers of body armor should consider comfort, flexibility and other ergonomic issues for acceptance along with coverage and service life in addition to stab protection performance for a predefined energy level.^7–9 This can change the well-known fact that a high level of protection from attacks is typically achieved at the expense of physiological comfort¹⁰ which reduces the working period and efficiency of the wearer.⁹ The use of body armor has always been an issue in terms of ease of body movement and cognitive functions^11,12 which is the main cause users to be reluctant to wear an uncomfortable protective vest but should not be drastically compromised by the design of the protective armor.^4,9 Therefore, the interaction between the protective vest and the body is an important factor that needs to be considered when designing body armor. Advanced body armor technologies aim to reduce body armor vest weight in order to enhance the comfort level by using smaller protective elements and plates on the areas of the body where more motion happens or where the radius of the curvature is lower.¹³

The selection of advanced materials (both for performance and comfort) and advanced body armor technologies can reduce the body armor vest weight^14–16 so that appropriate armor design should ideally allow the flow of excessive metabolic heat away from the body (thermo-physiological property), which can be reflected by a combination of air permeability, thermal resistance, and moisture evaporation.^17–19 Researchers described that the increase in the stab resistance was attributed to the coating that bound the reduced micro-porous nature of the cloth and its raw material, increased thickness and bending resistance which resulted in reduced comfort properties.¹⁹ In order to be thermally comfortable when the body is heating up and sweating, the garment should be able to transfer heat and moisture away from the skin to the atmosphere^19–21 and this transfer is managed by the air permeability property of the vest.

The concept of air permeability is widely used to interpret the intrinsic characteristics of fabric by measuring the rate of air flow passing perpendicularly through a known area under a prescribed air pressure differential between the two surfaces of a material²² to indicate the thermal loss in windproof garments as a measure of breathability for face coverings and can prevent infection in functional products. Air permeability is a determining factor to the comfort of a fabric because the breathability of a material is very important and it includes the flow of air through the garment and its sensorial impact on the skin. For example, items such as raincoats, hiking trousers and tents require low air and water permeability to provide resistance to wind and moisture (water and water vapors), which would impact not only the wearer’s comfort, but also the safety and performance of the garment.²³

In all cases, researchers found that the comfort of the stab protective armor vest is dependent on different factors mainly of materials and design as discussed above and not yet improved to a satisfying level for the users. The combined effect of design parameters of 3D prints such as shape and size of the 3D printed scales; textiles structures and the gaps of the available armors need to be studied deeply. The study aims the comparative investigation of the comfort determined by the air permeability of the protective vest of various parameters. The textile products must be comfortable with the exchange of air to the human body in addition to their strength and other mechanical performances. This study focuses on investigating the air permeability of the 3D printed stab protective armor vest, its elements and attachments as well as a comparison of the new vest and commercially available vests.

Materials and methods

Materials

In this study, Original Prusa i3 MK3S 3D printer is used for developing 3D printed protective scales of the segment to use on 100% Kevlar aramid twill woven fabric. Fused filament fabrication (Fused layer modeling-FLM) of the material extrusion technique is used to print PLA filament on aramid woven fabrics for the investigation of effect of size of protective armor vest elements on the air permeability property. PLA is a thermoplastic monomer derived from renewable, organic sources such as corn starch or sugar cane.²⁴ Using biomass resources makes PLA production different from most plastics, which are produced using fossil fuels through the distillation and polymerization of petroleum. PLA is one of the most commonly used 3D printing material because of it is easy to use and made of renewable resources and thus biodegradable.²⁵ Stab protective armor developed by 3D printing with different size and shape of the protective elements, fabrics for the formation of armor vest, commercially available armor vest are the main materials used in this investigation.

Table 1 describes the fabric structure and its features which indicates of the type of the structure and appearance, the type of material from which each fabric is made, the mass in gram per square meter (GSM) and the average thickness after measuring according to ASTM the Standard Test Method for Thickness of Textile Materials.²⁶

Table 1.

Fabric structures and their features.

Material/fiber type	Fabric structure and appearance	Fabric features
100% aramid fabric, uncoated, uncolored,	Twill woven	Thickness: 0.629 mm Weight in g/m²: 220
62% Black Dyneema® (PE-UHMWPE), 38% Cotton	Denim/twill woven	Thickness: 1.023 mm Weight in g/m²: 430
Polyethylene-UHMWPE 100%	Knitted	Thickness: 0.751 mm Weight in g/m²: 235

Controlled parameters

The 3D printing machine and the printing process parameters (Table 2) such as temperature, layer thickness, printing speed, infill type, infill percentage, and the raw material type are kept controlled throughout the sample production in all investigation factors.

Table 2.

Process parameters of 3D printing.

Parameters
Infill type	Infill density	Layer thickness	Number of roof	Number of floor	Number of wall layers	Nozzle temperature for first layer	Nozzle temperature for the main body	Bed temperature
Gyroid	15%	0.2 mm	2	2	2	220°C	210	60°C

Methods

Quadrilateral and triangular shapes of small (173.2 mm²), medium (648.89 mm²), large (1279.05 mm²) and full vest panel dimensions of the 3D printed segments are considered for this study. As shown in Figure 1, the 3D prints are designed as quadrilateral and triangular plates of different dimensions at different portions of the human body parts for fitting without compromising the protection at 4 mm thickness of each scale. The geometries are also selected after the experimental investigation of various geometries with respect to their performance to impact energy, assembly and fit. The air permeability investigation is carried out on the assembled newly developed armor, the commercially available segmented armor and on the armor of a single plate vest.

Figure 1.

Design of the segmented scales to print.

FX 3300 TEXTEST Air Permeability Tester III is used to measure the amount of air pass at 100 Pa pressure on the 3D printed stab protective armor samples through the 38 cm² testing area (Figure 2) and at various knob range for smooth and less noise air passage for the targeted specimen. The unit of measurement used in this test is mm/s. The testing machine setup, the number of specimens, the sample dimension and the testing are done based on the ISO 9237-95 defined as Textiles-Determination of the Permeability of Fabrics to Air.²⁷

Figure 2.

Air permeability test: (a) schematic diagram and (b) sample clamping in the measuring device.

The 3D printed samples were conditioned in the testing room at 20°C ± 1°C and 65% ± 2% relative humidity for 24 h. In this investigation, 10 specimens are tested for most of the target samples but five specimens also tested for samples with small dimensions.

Analysis

The statistical data has been analyzed using Microsoft excel for the development of figures, and one way ANOVA of the Software Packages for Social Sciences (IBM SPSS 26) for analysis of variance, mean, and generation of the necessary results. In this study, analysis of variance (ANOVA) is used to check the significance of each independent variable such as the shape of the protective segment, dimension of the protective element, different parts of the vest based on the shape of the human body on the dependent variable, the air permeability of the stab protective armor vest and its parts and accessories. The statistical tests generate an F value which is used to determine whether the test is statistically significant that is a ratio of two variances and is calculated as variation between sample means divided by variation within the samples (F = variation between sample means/variation within the samples). Variances are a measure of dispersion, or how far the data are scattered from the mean. Larger values represent greater dispersion. The p-value that corresponds to the F-statistic used to determine if the difference between group means is statistically significant. The P stands for probability and measures how likely it is that any observed difference between groups is due to chance. In this study, the p-value is a test of statistical significance and has 95% probability of confidence level. The actual values of F- and P of this study are generated by the IBM SPSS 26 software from the inserted test data.

Result and discussion

The study is conducted to investigate the air permeability as a determining factor of the comfort of the stab protective armor equipment. The study focused on the investigation of the possible influence of the various factors on air permeability of the protective armor vest. Types of fabric structures, materials used for fabrics formation, shapes of the 3D printed and segmented protective scales, the vest sack which covers the scales, panels of the vest without the vest sack and directions of air pressure on the sample are considered as factors for the investigation of this study. In addition, the fully assembled protective vest, panels of a vest and different attachments are also considered for the study.

The types of materials are classified in this study based on the raw materials of which the fabrics are made from and are Kevlar from aramid group, 62% Black Dyneema^®/38% Cotton and Polyethylene-UHMHPE. The fabrics made from these materials are investigated without the protective segmented scales and showed significant variations on the air permeability properties. Kevlar Aramid woven fabrics on which the printed protective element is attached on it has 269.47 mm/s, 62%Black Dyneema^®/38%Cotton fabric used as an outer layer of the newly developed vest has the air permeability of 35.49 mm/s and Polyethylene-UHMWPE fabric which is a knitted fabric and used as an inner layer of the armor vest has the air permeability of 1062.74 mm/s (Table 3). The outer layer, 62%Black Dyneema^®/38%Cotton denim twill woven fabric has the lowest air permeability while the Polyethylene-UHMWPE knitted fabric has the highest permeability. This combined effect is found because the inner fabric should be more permeable for improved comfort while the outer fabrics are less permeable for enhanced protection with reduced porosity of the fabric.

Table 3.

Descriptions of the air permeability in mm/s of 3D printed stab protective armor.

Parameters	Levels	N	Mean	Std. deviation	Std. error	Minimum	Maximum
Type of materials the fabrics are made	Aramid	10	269.4670	2.98	0.94	265.57	274.50
	Dyneema	10	35.4899	1.14	0.36	33.25	36.91
	Polyethylene	10	1062.7390	68.90	21.79	978.70	1188.28
Shape of scales front side	Triangular scales	5	4.50	0.35	0.16	4.10	4.90
Shape of scales front side	Quadrilateral scales	5	12.40	1.05	0.47	10.80	13.50
Shape of scales back side	Triangular scales	5	21.68	2.21	0.99	19.40	23.90
Shape of scales back side	Quadrilateral scales	5	16.22	2.96	1.32	13.30	19.50
Vest sack	Old vest sack	10	2988.20	58.40	18.47	2906.00	3072.00
Vest sack	New vest sack	10	37.18	0.65	0.21	36.09	38.45
Resistant scales of the panel from front	Old vest	10	7.99	10.48	3.31	0.30	35.00
	New vest	10	41.41	9.70	3.07	28.60	55.40
	A plate of vest	10	0.00	0.00	0.00	0.00	0.00
Direction of the testing air on the scales of the new vest	Front	10	41.41	9.70	3.07	28.60	55.40
Direction of the testing air on the scales of the new vest	Back	10	34.33	9.01	2.85	16.70	44.00
A panel of the fully assembled resistant	Old vest	10	3.70	1.95	0.62	0.00	6.00
A panel of the fully assembled resistant	New vest	10	11.09	2.45	0.77	7.10	14.70
Direction of air on the fully assembled Vest panel	Air on the outer side of the panel	10	11.09	2.45	0.77	7.10	14.70
Direction of air on the fully assembled Vest panel	Air on the inner side of the panel	10	9.0520	2.46	0.78	5.10	11.85
Panels of the new vest	Front panel	10	11.96	1.38	0.44	9.40	14.70
Panels of the new vest	Back panel	10	11.09	2.45	0.77	7.10	14.70
Attachment on	Shoulder	5	29.60	16.44	7.35	16.00	56.00
	Side	5	11.60	2.88	1.29	9.00	16.00
	Collar	5	27.76	5.16	2.31	22.00	34.00

Shapes of the 3D printed scales of the newly developed stab protective armor is also investigated for its effect on the comfort of the armor without compromising the protection performance. As shown in Table 3, shapes of the scales of the 3D printed protective elements has two categories as triangular and quadrilateral (Figure 3) which tested by applying air pressure from the face and back side of the sample. The triangular geometries have overlapped arrangements of the chamfered scales during segmentation while the quadrilateral geometries have a topological interlocking segmentation of the scales. The distance between the triangular scales is larger than the distance between the quadrilateral scales during segmentation by considering the flexibility and cover for protection of the skin of the human body.

Figure 3.

Shapes of 3D printed scales and corresponding distance: (a) triangular chamferred and overlapped segmentation with 2.17 mm gap between scales and (b) quadrilateral topologically interlocked segment of scales with 1.82 mm gap between scales.

The permeability resistance arises from the type of segmentation than the distance between scales. The triangular scales with overlapped segmentation have low air permeability in the protector side as compared to the quadrilateral scales with topological interlocking segmentation. The deeper chamfering of the triangular geometries between overlapped scales limits the passage of air than the topologically interlocked segmentation of the quadrilateral geometries. However, the two geometries showed an increment of the air permeability on the reverse side of the protection element relative to its front side for the scales appeared in a slight opening relative to the neighbor scale by the closing round clamp of the top testing ring.

The air permeability of the type of fabric structure used to cover the protective elements which is called vest sack in this research because it is used to insert and hold securely in place the 3D printed protective element also investigated. This research considers two different vest sacks of the commercially available one and the newly developed. As shown in Table 3, the newly developed vest sack has far limited permeability (37.18 mm/s) relative to the commercially available one (2988.20 mm/s). In this research, the commercially available vest sacks are named as old, and have a spacer like structure of honeycomb appearance with more visible porous structures while the new vest sacks are made from the 62%Black Dyneema^®/38%Cotton denim twill woven fabric and Polyethylene-UHMWPE knitted fabrics of an overall reduced porosity of the vest sack.

The study also considers the permeability performance of the protective part of the vest separately from the vest sack with the parameter name resistant scales of the panel from front (Figure 4) because the front panels of the two armor vests are investigated in this research.

Figure 4.

The face (left) and reverse (right) side of the front panel with the protective scales of a vest.

The new vest which is formed from the quadrilateral 3D printed scales is far more permeable (41.41 mm/s) than the commercially available protective part (7.99 mm/s) of the vest. In this investigation, the protective part from a single plate is also considered and the result revealed that it is not allowing air to pass through it (0.00 mm/s). The newly developed as segmented protective scales of an armor is comfortable for the security officers to wear during their whole duty shift with improved heat regulation from the air passing to the body and releasing to the surrounding. Similarly, the air permeability of the full vests also studied to check the comfort of the new vest with the comparison to the commercially available vest (Figure 5) and as the result in Table 3 revealed that the new armor vest allow for more air to pass (11.09 mm/s) while the commercial one limited permeability (3.70 mm/s).

Figure 5.

High-strength aramid fabric to which a special plastic in lamella form is inseparably applied by Armadillo Tex GmbH, Germany.²⁸

The air passage in to the newly developed vest is also studied by classifying in to two parts as front panel and back panel of the whole vest. The front panel has more permeability (41.41 mm/s) than the back panel of the vest (34.33 mm/s) when air is applied from the protection side of the armor vest (Table 3).

The direction of air passage on the fully assembled new vest panel has been investigated in this research as air applied on the outer side of a panel and on the inner side of a panel. The result revealed that though not significant the air on the outer side of the panel showed slightly higher permeability than the air on the inner side of the panel (Table 3). The front and back panels of the new vest showed similar permeability performance, 11.96 and 11.09 mm/s, respectively.

In this study attachments are also investigated for they are 3D printed stab protective segments to provide cover for the sensitive organs with improved protection performance. These attachments are named in this research as the shoulder, collar and attachments on the right and left sides of the human abdomen which are designed ensure comfort for the cognitive functions of the human body. For this reason, all attachments are developed from a triangular shape segmented scales and according to the air permeability test results the shoulder attachment has higher permeability (29.60 mm/s) followed by collar and side attachments, 27.76 and 11.60 mm/s, respectively. The geometries used in these attachments are all triangular but have different distance between geometries. The distance between the scales on the shoulder attachment is slightly wider than the consecutive scales on the collar and side attachments based on the location of sensitive organs to fatal injury.

According to Table 4, the result from the one way analysis of variance (ANOVA), the majority of the levels of factors showed significant influence while some of the factors showed not significant effect on the air permeability of the stab protective armor.

Table 4.

Analysis of variance of the air permeability property of stab protective armor vest.

Parameters		Sum of squares	df	Mean square	F	P
Type of materials the fabrics are made	Between groups	5,797,554.88	2	2,898,777.44	1828.02	0.000
Type of materials the fabrics are made	Within groups	42,815.23	27	1585.75
Shape of scales front	Between groups	156.03	1	156.03	255.78	0.000
Shape of scales front	Within groups	4.88	8	0.61
Shape of scales back	Between groups	74.53	1	74.53	10.94	0.011
Shape of scales back	Within groups	54.52	8	6.81
Vest sack	Between groups	43,542,506.67	1	43,542,506.67	25,527.01	0.000
Vest sack	Within groups	30,703.37	18	1705.74
Resistant scales of the panel of the front panel	Between groups	9651.75	2	4825.87	71.01	0.000
Resistant scales of the panel of the front panel	Within groups	1835.00	27	67.96
Direction of the testing air on the new vest	Between groups	250.63	1	250.63	2.86	0.108
Direction of the testing air on the new vest	Within groups	1577.53	18	87.64
A panel of the fully assembled resistant	Between groups	273.06	1	273.06	55.90	0.000
A panel of the fully assembled resistant	Within groups	87.93	18	4.89
Direction of air on the fully assembled vest panel	Between groups	20.77	1	20.77	3.45	0.080
Direction of air on the fully assembled vest panel	Within groups	108.25	18	6.01
Panels of the new vest	Between groups	3.79	1	3.79	0.96	0.340
Panels of the new vest	Within groups	70.85	18	3.94
Attachments	Between groups	980.89	2	490.44	4.80	0.029
Attachments	Within groups	1220.75	12	101.73

The types of materials used for the production of fabrics (F = 1828.018, p = 0.000), shapes of the 3D printed protective scales from front side (F = 255.779, p = 0.000), shapes of the 3D printed protective scales from the back side (F = 10.937, p = 0.011), type of the vest sack (F = 25527.008, p = 0.000), resistant scales of the front panel (F = 71.007, p = 0.000), panels of the fully assembled resistant (F = 55.898, p = 0.000) and attachments showed significant influence on the air permeability of the protective armor vest. However, directions of the air flow during testing of the protective scales (F = 2.860, p = 0.108) and the fully assembled armor panel (F = 3.453, p = 0.080), and panels (front and back) of the new vest (F = 0.961, p = 0.340) showed not significant influence on the air permeability of the armor vest (Table 4).

In summary, the air permeability of the stab protective armor as a measure of the comfort found to be dependent of various factors which are material, structure, process, design, size and shape related parameters. As presented in Figure 6, the type of materials used in the construction of the fabrics to use as a vest sack, design of the segmented scales as shape of scales, the size of resistant scales of a vest, panels and attachments on different parts of the human body has significant impact on the air permeability of the armor vest. The permeability of the fabrics used in the construction of the vest sack is directly related to its construction density. The denser the fabric will have higher the cover factor and results in limited porosity for reduced permeability but enhanced protection performance to sharp objects such as knife because the materials used in the development of the new stab resistant armor vest are high performance materials. Though the new vest sack results in low permeability, the final assembled new vest shows significantly improved air permeability as compared to commercially available segmented scales of the same equipment. The air permeability in the new vest is improved because of the multiple segmentation lines of the scales in different directions of the protective armor vest as compared to the limited number of segmentation lines between group of scales and within two consecutive scales. Therefore, material, shape and dimension of scales, segmentation and attachments have significant influence on the air permeability whereas directions of air flow to the full vest and its scales and panels of a vest have low effect.

Figure 6.

Air permeability of the stab resistant armor.

Conclusion

The air permeability as a measure of comfort of the stab protective armor has been investigated deeply. The permeability of the 3D prints on textiles significantly influenced by the type of fabrics used to cover and hold secure the protective elements, shapes of the protective elements and sides of the protective panel placement on the human body. The permeability of 3D prints on aramid woven fabrics significantly influenced by the type of the fabric structures used to cover the protective scales and attachments to give better protection of the user sensitive body parts such as neck.

As investigated in this research, the directions of the air flow during testing of the protective scales with and without vest sack and the two panels of the armor those called front panel and back panel of the new vest has not showed significant influence on the air permeability of the armor vest. Protective gear involving segmented scales will provide better comfort to a human torso than a vest made of a single piece of stab-resistant material, for example, ceramics, polycarbonate sheet, or other metals.

The research and development of protective equipment, decorative elements and fashion elements from 3D printed scales based on natural and personal inspiration is on-going by researchers and designers. The protective textiles those developed by 3D printing of the protection elements on textiles such as stab resistant armor vest are evaluated for the comfort properties in addition to its protection performance. This helps to improve the willingness of the wearer to use the protective armor vest frequently and during the whole duty shift without assuming it is not only obliged to wear but also happily needed to wear in the work place.

Footnotes

Acknowledgements

The IGF research project 21622 BR of the Forschungsvereinigung Forschungskuratorium Textil e. V. is funded through the AiF within the program for supporting the “Industriellen Gemeinschaftsforschung (IGF)” from funds of the Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Dereje Berihun Sitotaw

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