Ballistic Testing of an Aerogel/Starch Composite Designed for Use in Wearable Protective Equipment

Abstract

Concussion is a costly healthcare issue affecting sports, industry, and the defense sector. The financial impacts, however, extend beyond acute medical expenses, affecting an individual’s physical and cognitive abilities, as well as increasing the burden on coworkers, family members, and caregivers. More effective personal protective equipment may greatly reduce the risk of concussion and injury. Notably, aerogels are light, but traditionally fragile, non-Newtonian fluids, such as shear-thickening fluids, which generate more resistance when compressive force is applied. Herein, a composite material was developed by baking a shear-thickening fluid (i.e., starch) and combining it with a commercially available aerogel foam, thus maintaining a low cost. The samples were tested through the use of a ballistic pendulum system, using a spring-powered launcher and a gas-powered cannon, followed by ballistic penetration testing, using two electromagnetic accelerators and two different projectiles. During the cannon tests without a hardhat, the baked composite only registered 31 ± 2% of the deflection height observed for the pristine aerogel. The baked composite successfully protected the hygroelectric devices from coilgun projectiles, whereas the projectiles punctured the pristine aerogel. Leveraging the low-cost design of this new composite, personal protective equipment can be improved for various sporting, industrial, and defense applications.

1. Introduction

1.1. Overview

Concussion represents a persistent and serious challenge across sports, industry, and the defense sector, with consequences that extend far beyond the immediate medical costs. The social and economic burdens include reduced work capacity, extensive rehabilitation requirements, and increased strain on families and caregivers. While personal protective equipment (PPE) remains a frontline strategy for mitigating concussion risk, current materials often fail to provide sufficient protection, highlighting the urgent need for more effective solutions [1]. Recent research has focused on advanced materials, such as aerogels and non-Newtonian fluids, for protective applications. Aerogels, known for their exceptional lightness, are limited by their inherent fragility. In contrast, shear-thickening fluids (STFs), a class of non-Newtonian fluids, exhibit a dramatic increase in viscosity under impact or stress, enabling them to absorb and dissipate energy efficiently [2]. Although aerogels and STFs individually present limitations, integrating STFs into foams and fabrics has produced composite materials with enhanced protective properties, including improved flexibility and impact resistance. In this study, ballistic pendulum testing was performed using a head phantom to gauge the protective ability of an aerogel/starch composite material.

1.2. Background

1.2.1. Prior Work Overview

Concussion is a costly affliction. The sources of concussions in society occur across a broad range of activities, including sports, industry, and the defense sector [3,4,5]. Contact and combat sports, such as football and boxing, have particularly high concussion risks [6,7,8,9,10]. However, other sports, including volleyball and basketball, can also be sources of risk [7,8]. Beyond sports, concussion can occur in industrial workplaces, such as construction sites and mines. Although head impacts are a leading cost of lost productivity in heavy industry, common protective gear is often bulky or provides insufficient shielding. In military and defense applications, explosive shockwaves and ballistic objects can cause traumatic brain injuries [11]. Lighter, more efficient protective headwear would have immediate applications in regard to reducing concussion [3,4,5].

1.2.2. Extracting Protective Materials

The development of protective headgear in various sports has historically relied on materials designed to absorb and dissipate impact forces, thereby preventing head injuries. Traditional materials, such as expanded polystyrene (EPS), have been widely used, owing to their compressive energy-absorbing properties; however, they exhibit significant limitations, particularly concerning multi-impact scenarios. EPS deforms plastically upon impact, thus compromising the protection from subsequent impacts [12]. It also exhibits minimal effectiveness in regard to high-velocity impacts, like those encountered in boxing and taekwondo [13].

Recent advancements have favored polyurea foams and viscoelastic polymers, which have superior impact attenuation properties compared with traditional foams like ethylene–vinyl acetate and thermoplastic polyurethane [14]. These materials are designed to optimize energy absorption, while avoiding the plastic deformation issues associated with EPS, thus enhancing their durability and effectiveness after repeated impacts [15]. Other advances have utilized composites of Kevlar and polyurea and paper honeycombs filled with STF [16,17]. A study on the use of a Kevlar/polyurea composite, with a honeycomb structure, reported that a 2 mm STF structure could replace up to 10 layers of Kevlar [16]. While the material was lighter and customizable, it required specialized equipment to produce and required less commercially available substitutes [16,17,18]. STFs with a honeycomb structure continue to be an area of active research. Improvements have also been made in understanding the biomechanics of head injuries, recognizing several complexities involved in head impacts, including both linear and rotational accelerations [19].

The significance of accurate modeling in regard to material selection is underscored by research that incorporates the hydration effects of brain tissues into finite element models to simulate mechanical responses during collisions [20]. This has crucial implications for the design of headgear, because accurate models are required to drive effective innovations that offer enhanced protection. In the ballistic impact context, effective protection is often rated using the National Institute of Justice (NIJ) standards [21]. The traditional protection levels are listed in

Table 1. Traditional NIJ ballistic protection levels.

NIJ Level	I	IIA	II	IIIA	III	IV
Round	0.22 LR	9 mm P	0.357 Mag	0.44 Mag	0.308 Win	0.30-06
KE (J)	200	600	800	2000	3600	4100

[16,21].

Additionally, the perceptions and attitudes toward headgear can significantly influence their usage and, consequently, their effectiveness in preventing injuries. For instance, some athletes express concerns related to comfort and aesthetics, which may deter them from using protective headgear altogether [22]. Moreover, it is necessary to ensure that new designs do not unintentionally encourage riskier behavior among athletes due to a false sense of security, a problem noted in studies analyzing the efficacy of protective headgear in rugby and other contact sports [23]. These factors highlight the ongoing need for research that not only addresses material limitations, but also considers the sociocultural dynamics surrounding the use of protective sports equipment [3,4,5].

In conclusion, while traditional materials like EPS have paved the way for protective headgear, the shift toward advanced materials, such as polyurea and viscoelastic polymers, represents a notable improvement in addressing their limitations. However, ongoing scrutiny of the integration of these materials with user attitudes, regulatory standards, and biomechanical modeling is essential to enhance the protective capabilities of headgear across various sports, industries, and the defense sector [3,4,5].

1.2.3. Shear-Thickening Fluids

STFs, such as oobleck, exhibit unique rheological properties; specifically, their viscosity significantly increases under shear stress. Additionally, when force is rapidly applied, such as during impact, oobleck thickens, effectively absorbing and dissipating energy. Oobleck is made from cornstarch and water and is widely used in science demonstrations because of its low cost and the ease of manufacturing, demonstrating how simple materials can exhibit complex rheological behaviors. These behaviors are attributed to the microstructural changes that occur within the fluid, allowing it to transition from a liquid to a solid-like state when subjected to high shear rates or rapid impact. This makes oobleck and other STFs beneficial candidates for reinforcing body armor and protective clothing. When STFs are impregnated into fabrics like Kevlar, they enhance the ballistic performance of the armor by improving its energy absorption and impact resistance, without drastically increasing the weight or reducing its flexibility [1,2,24,25,26].

Additionally, Santos et al. (2020) emphasized the adaptability of STFs for different protective clothing configurations, showing that variations in the STF composition and fabric orientation can further influence the protective efficacy [27]. This highlights the importance of optimization between the composite materials, particularly for specific use cases, such as military or personal protective equipment, where lightness and flexibility are critical, alongside high levels of protection. Notably, oobleck transfers its shear-thickening properties to the fabric after baking, owing to starch’s ability to bind with itself [2,24]. Starch has also been used as an aerogel [28].

Gong et al. (2013) addressed the stab and puncture-resistant properties of fabrics enhanced with STFs, suggesting that their performance can be effectively tuned by manipulating the fluid’s viscosity and the type of dispersing agent [29]. This enhances the protective capabilities of materials against a broader range of threats, from ballistic impacts to knife stabs, depending on the intended application.

A number of researchers have explored STF-impregnated paper honeycomb structures. Chang et al. (2021) reported a 17% reduction in weight relative to conventional Kevlar, using NIJ tests to verify the performance of the composite. New composite STFs have also been combined with paper honeycomb structures, although this is an area of active research [16,17,18].

In summary, STFs, like oobleck, play a transformative role in the field of ballistic protection, personal protective equipment (PPE), and protective sportswear, by enhancing the impact resistance and energy absorption capabilities of conventional materials. Furthermore, the range of rheological properties provided by STFs enables a level of customization in protective gear, paving the way for advanced protective solutions that neutralize various threats, while maintaining comfort and flexibility [30].

1.2.4. Aerogels

Aerogels are a class of highly porous materials, characterized by a low density and a high surface area, making them suitable for numerous applications, including thermal insulation, energy storage, and environmental remediation. Aerogels have even been used to stop orbital micro-debris in space [31]. Among the different types of aerogels, those fabricated using simple and efficient methods have garnered significant attention in the literature.

One of the simplest methods for fabricating aerogels involves the use of poly-(vinyl alcohol) (PVA), a widely available and inexpensive polymer. PVA-based aerogels can be produced through straightforward freeze-drying techniques, wherein a modified PVA solution is frozen and, subsequently, dried under a vacuum. The resulting materials exhibit advantageous mechanical properties and high biocompatibility [32,33,34,35,36,37]. The freeze-drying process also preserves the porous structure, which is crucial for maintaining the aerogel’s low density and high surface area [33,37,38,39].

Another notable approach is the hydrothermal synthesis of graphene oxide (GO) aerogels. This method leverages the abundant oxygen-containing functional groups on GO, which facilitate the formation of a three-dimensional network through simple hydrothermal reactions. The resulting GO aerogels exhibit useful electromagnetic wave absorption properties and can be produced in a relatively short time compared with traditional methods [40,41]. The scalability of this method has also been demonstrated, allowing for the production of large sheets of aerogels [42].

Additionally, ambient pressure drying techniques have emerged as a promising alternative to supercritical drying, which is often complex and costly. Recent studies have shown that silica aerogels can be synthesized at room temperature, using low-cost precursors like sodium silicate, followed by ambient pressure drying. This method is not only simple, but is also energy efficient [43,44]. The resulting silica aerogels maintain desirable properties, such as low thermal conductivity and high porosity, making them suitable for thermal insulation applications.

Furthermore, the incorporation of nanomaterials into aerogel matrices has led to the development of hybrid aerogels with enhanced properties. For instance, the combination of cellulose nanofibrils with PVA yields hybrid aerogels that exhibit improved mechanical strength and thermal stability [45]. These hybrid aerogels can be easily fabricated using sol–gel processes, followed by freeze drying.

In conclusion, PVA-based aerogels produced via freeze drying, GO aerogels synthesized through hydrothermal methods, and silica aerogels created using ambient pressure drying techniques, are among the easiest to fabricate [46]. These methods not only simplify the fabrication process, but also yield aerogels with beneficial properties, suitable for a wide range of applications [28,36].

1.2.5. The Potential for Aerogel-Based STF Composites

The combination of STFs and aerogels presents a novel approach to enhancing the mechanical properties of materials for impact resistance and energy absorption applications. The integration of these two materials can lead to composites that leverage the unique properties of both. For example, the brittleness of aerogels can be compensated for through the addition of STFs, which are not standalone materials and require some form of substrate or scaffolding, ultimately maintaining a low weight and low cost.

One of the primary benefits of combining STFs with aerogels is the enhancement of the impact resistance. When aerogels are impregnated with STFs, the resulting composite can absorb and dissipate energy more effectively upon impact, reducing the likelihood of damage to underlying structures or components [2,47]. This synergy is particularly valuable in body armor, wherein the need for lightweight yet strong materials is critical. Research has shown that STFs can significantly improve the ballistic performance of fabrics when used in conjunction with aerogels, allowing for thinner and more flexible protective gear, without compromising safety [2,24].

Moreover, the rheological properties of STFs can be tailored through the selection of the particle size and concentration, which in turn influences the performance of the aerogel composite. The addition of silica nanoparticles to polyethylene glycol has been shown to enhance the shear-thickening behavior, leading to improved mechanical properties in composite materials [30,48]. The ability to manipulate the viscosity and shear-thickening characteristics of STFs enables the design of aerogel composites that apply to different impact conditions [1,30].

Additionally, the incorporation of STFs into aerogels can enhance their structural integrity. The shear-thickening behavior of the fluid provides a damping effect, which helps mitigate vibrations and reduce the risk of structural failure under dynamic loading conditions [21,49]. This aspect is particularly important in aerospace and automotive applications, wherein materials are often subjected to high-stress environments [47].

In conclusion, the combination of STFs and aerogels offers a promising avenue for the development of advanced materials, with enhanced impact resistance and energy absorption capabilities. A recent composite of aerogel and STFs reported an 80% reduction in impact intensity, heat protection, and prevented electromagnetic interference [50]. By leveraging the unique properties of both components, researchers can create composites that meet the demanding requirements of various applications, from protective gear to structural components in high-performance environments, such as ballistic impacts [50].

1.2.6. Ballistic Impact Testing

Potato cannons are a type of pneumatic or combustion launcher, often used to launch projectiles, such as potatoes and tennis balls. These devices are typically constructed from PVC pipe and utilize various propellants, including combustion gases and compressed air. Potato cannons can launch projectiles at high velocities, sometimes exceeding 200 m [51,52]. The potential for injury, particularly concussion, arises from the blunt ballistic impact of the launched potatoes, which can even cause significant damage to structures [53]. The risk of traumatic brain injury is particularly pronounced when these devices are mishandled or used without appropriate safety measures, such as the use of protective gear or safe backstops [53,54].

Educational initiatives that emphasize the physics behind these devices, while also addressing safety protocols, could mitigate the risks associated with their use. Understanding the internal ballistics and the mechanics of projectile motion can help users appreciate the forces at play and the importance of safety [53,54].

In summary, although potato cannons can serve as engaging educational tools, they also pose serious risks of concussion and other injuries if not handled with care [53,54]. The combination of high projectile velocities and the potential for blunt force trauma necessitates a strong emphasis on safety practices to prevent accidents. As such, they represent a practical method to evaluate a genuine cause of concussion.

2. Materials and Methods

2.1. Experimental Summary

To reduce the occurrence of concussion, the protective material should be light and flexible, but strong enough to withstand and distribute the force of an impact. Simulation was considered, but the inconsistent structure of aerogels and manual fabrication rendered it complex and time consuming relative to low-cost, repeated experimentation. A ballistic pendulum experiment was performed to evaluate different material combinations in this study. The three material cases included no aerogel foam, aerogel foam padding, and aerogel foam with cornstarch baked into it. The presence of a hardhat was included as an additional variable, to determine the efficiency of the material sample as a helmet liner. A spring-powered Nerf gun and a combustion-fired potato cannon were used to launch tennis balls at the targets. Subsequently, ballistic penetration testing was carried out using two functional electromagnetic accelerators (i.e., coilguns), launching cylindrical and disk-shaped metallic projectiles. Following the experimental trials, statistical analyses were conducted, using the R software (version 4.4.2, R Core Team).

2.2. Material Preparation

Aerogel foam was purchased from Ningyan Benuo Technology Company (Yongzhou City, Hunan, China). The 1 cm thick silica aerogel insulation was cut into 18 cm squares, each weighing approximately 19.6 ± 2.5 g. Eight samples were prepared, half with cornstarch (Argo, Westchester, IL, USA) baked into them. Using prior recipes for oobleck, a total of 17 g of cornstarch was hand mixed in 12 mL of room-temperature deionized water for 2 min [1,2,24,25,26]. Each square of aerogel foam was thoroughly soaked in 4 L of deionized water. Half of the samples were coated on both sides with the cornstarch–water mixture. To dry them, the samples were baked in an oven at 76 °C for 90 min.

To assess the protective efficiency of the material, sensitive electronics were sandwiched between two layers, as shown in Figure 1. Hygroelectric devices generate small amounts of power from ambient humidity, but the power output drops if they are physically damaged [55]. For both the baked composite and the pristine aerogel samples, four custom hygroelectric devices (Ossarocco, Moline, IL, USA) were affixed to each corner. In the event of extensive damage, these devices cease functioning, indicating that severe damage has occurred. The samples were affixed to the side of the ballistic pendulum, directly in front of the launcher. This short range was used to minimize the loss of kinetic energy due to atmospheric drag on the tennis ball projectiles.

2.3. Ballistic Pendulum Construction

Based on prior biomedical models, a ballistic pendulum was used to measure the impact force on a mock head [56,57,58]. The average weight of a human head is between 3.5 and 5.5 kg [56,57]. A lighter weight was used for the ballistic pendulum (1.5 kg) for several reasons: (1) safety in the event of a catastrophic failure, (2) considering the effective limit of the string used to suspend it, and (3) greater odds of a visible response (i.e., displacement of the pendulum).

As shown in Figure 2, the ballistic pendulum consisted of a mock head constructed using generic components. The core of the pendulum was a 1 kg boxing reflex bag (Innolife Sports, Singapore). Cloth rags were used to add weight and volume to the bag, which was placed inside a plastic bag. The bag was then fitted into adjustable boxing headgear (Everlast, London, UK) to provide shape and definition. The total weight was 1.5 kg. A high-density polyethylene hardhat (Pyramex, Piperton, TN, USA) was used in additional tests. With the construction hardhat mounted on the pendulum, the total weight was 1.8 kg. As shown in Figure 3, duct tape was used to mount the aerogel samples over the “face” of the boxing headgear.

2.4. Pendulum Experimental Setup

The ballistic pendulum was suspended beneath a chain link bar. A mobile phone camera was positioned perpendicular to the pendulum, at a distance of approximately 1 m. The launcher was then positioned directly against the sample (i.e., point-blank range). A countdown was initiated until the launch was triggered, and video recording started. Standard tennis balls (58 g, diameter of 6.7 cm) were used as the projectiles. Fresh tennis balls were used to ensure that dirt from the ground did not add additional weight, potentially interfering with the results.

During testing, two launchers were used. As shown in Figure 4, the first was a Nerf-brand Dog Tennis Ball Blaster (Hasbro, Pawtucket, RI, USA). The second launcher was a potato cannon adapted to launch tennis balls, namely the Piezo Launching System (Quarter Mile Cannons, Laredo, TX, USA). The primary propellant was generic hair spray, using a 1–2 s spray into the combustion chamber. The launcher was held at point-blank range for each shot. The kinetic energy, KE, of the tennis ball is expressed by Equation (1). KE (in Joules) is related to the mass, $m_{obj}$, and velocity, $v$ [58].

$$KE=\left(0.5\right)\ast m_{obj}{v}^2$$

(1)

The potential energy $U$ of the pendulum at maximum swing is expressed in Equation (2), a function of the mass $m$, gravity $g$, and height $h$ [58].

$$U=mgh$$

(2)

At the moment of maximum extension, Equation (1) can be set equal to Equation (2) to obtain Equation (3).

$$\left(0.5\right)\ast m_{obj}{v}^2=mgh$$

(3)

In reality, some energy is lost during the transition. However, Equation (4) was simplified for the calculation, which is acceptable in ballistic pendulum experiments, as previously described [58].

$$v={\displaystyle \frac{m}{m_{obj} }} \sqrt{gh}$$

(4)

A computer vision algorithm designed in Python3 (Python Software Foundation, https://www.python.org/ (accessed on 22 January 2025)) was used to measure the deflection. The algorithm detected the center of the pendulum and the number of plotted pixels $p_{path}$ that moved relative to the starting position, depending on the resolution of the cell phone camera. The length of the string $r$ connecting the pendulum was also measured from the image. As shown in Equation (5), a ratio between pixels and centimeters was used to calculate the pendulum height, $h$.

$$h={\displaystyle \frac{r}{p }} p_{path}$$

(5)

Each launcher was used 3 times for each sample configuration: no sample, standard aerogel foam, and baked aerogel foam. After the first set of tests, the tests were repeated with a hardhat placed on the pendulum. The length of the string and the height of the string connecting the pendulum varied with each configuration, so it was measured at the start of each test. Using these values, computer vision was employed to identify the maximum deflection and calculate the kinetic energy of the projectile. Additionally, the power generation of the hygroelectric devices was measured before and after each launch.

2.5. Ballistic Penetration Testing

As previously described, the inertial characteristics of the head phantom and the relatively low mass may result in excessive rotation from a direct impact [3,4,5,11]. Similarly, a potato cannon can produce significantly inconsistent projectile velocities, mainly because of the manual aerosol spray process [53,54]. Hence, ballistic penetration testing is expected to generate more accurate results regarding a material’s protective capability [2,59].

Tennis balls are flexible and compressible upon impact, whereas a rigid projectile reveals the material’s penetration resistance [60,61]. Ultimately, higher velocities and a non-deformable projectile are used to stress the material beyond its capabilities [59]. To ensure the consistency of the launch velocity, coilguns were used instead of firearms, purely mechanical devices, or pneumatic systems [62].

As shown in Figure 5, each sample was fixed on the front side of a box, which was loaded with water bottles and rags to inhibit overpenetration. Two coilguns were used during testing, namely SGP-35 (Arcflash Labs, Los Angeles, CA, USA) and CA-09 (Coil Accelerator, Chicago, IL, USA). The SGP-35 coilgun used small steel dowels as projectiles, each weighing 4 g and carrying 2 J of kinetic energy. The CA-09 coilgun launched metallic disk-shaped projectiles, each weighing 18 g and carrying 18 J of kinetic energy. Each hygroelectric device was shot 3 times with the SGP-35 coilgun, and then 3 times with the CA-09 coilgun, all from point-blank range (at different device locations on the sample). The velocity of each shot was verified using a ballistic chronograph. Two samples of aerogel foam and baked aerogel foam were tested using this method. In the event of device damage or penetration, the next device in the counterclockwise direction was used. The device voltage was measured after each shot, as shown in Figure 6.

2.6. Statistical Analysis

The ballistic pendulum and penetration test results were statistically analyzed. A mixed model analysis of variance (ANOVA), with a post hoc Tukey test, was used to analyze the ballistic pendulum test data. For the penetration test data, a two-tailed t-test was used to determine any statistical differences between both scenarios, as well as shots that penetrated the material. It was hypothesized that the combination of the pristine foam and the hardhat would have lower protective capabilities than the configuration utilizing the baked composite and hardhat, but the efficiency of the baked composite was uncertain. In the penetration tests, it was thought the CA-09 coilgun had a greater chance of penetrating the material, owing to its higher kinetic energy and mass. Furthermore, if the baked composite exhibited superior results in the ballistic pendulum tests, it was expected to withstand penetration testing more easily.

3. Results

3.1. Ballistic Pendulum Experiments

The experimental results from the ballistic pendulum tests are displayed in

Table 2. Experimental results from the ballistic pendulum tests.

Launcher	Padding	Hardhat	H (cm)	H (std)	V (m/s)	V (std)	KE (J)	KE (std)
Spring	None	No	5.43	3.52	10.73	8.64	3.42	2.22
Spring	Baked	No	3.01	1.03	7.99	4.67	1.90	0.65
Spring	Unbaked	No	4.64	1.75	9.92	6.09	2.92	1.10
Spring	None	Yes	2.72	1.31	7.53	5.23	1.68	0.81
Spring	Baked	Yes	5.11	0.06	10.32	1.09	3.17	0.04
Spring	Unbaked	Yes	5.30	0.51	10.51	3.27	3.28	0.32
Gas	None	No	101.20	3.10	46.32	8.11	63.73	1.95
Gas	Baked	No	29.41	49.86	24.97	32.52	18.52	31.40
Gas	Unbaked	No	90.64	18.28	43.84	19.69	57.08	11.51
Gas	None	Yes	52.30	2.20	33.03	6.78	32.41	1.36
Gas	Baked	Yes	22.03	12.53	21.44	16.17	13.65	7.77
Gas	Unbaked	Yes	148.75	1.45	55.71	5.50	92.18	0.90

, and the results for the cases with no hardhat are shown in Figure 7.

The lowest deflection (2.72 ± 1.31 cm) was observed using the spring-powered launcher with no padding and a hardhat, which also exhibited the lowest kinetic energy (1.68 ± 0.81 J). The highest deflection height (148.75 ± 1.45 cm) and kinetic energy (92.18 ± 0.90 J) were observed using the potato cannon with the aerogel foam and a hardhat. For every sample, all the hygroelectric devices continued functioning.

3.2. Ballistic Pendulum Statistical Analysis

The ANOVA results are displayed in

Table 3. ANOVA analysis of ballistic pendulum testing results.

Condition	Df	SumSq	MeanSq	Fvalue	Pr (>F)
Launcher	1	49,502	49,502	467.565	<2 × 103
Padding	2	21,593	10,797	101.978	0.567
Hardhat	1	35	35	0.329	<2 × 103
Launcher/Padding	2	21,956	10,978	103.689	0.722
Launcher/Hardhat	1	13	13	0.127	<2 × 103
Padding/Hardhat	2	6460	3230	30.508	<2 × 103
Launcher/Padding/Hardhat	2	5677	2839	26.812
Residuals	108	11,434	106

.

The Tukey test results for the three different padding configurations are shown in

Table 4. Tukey test results for the padding configurations used in the ballistic pendulum testing.

Condition	Diff	Lower	Upper	p_Adjust
Baked vs. Unbaked	32.63253	27.16481	38.10025	<0.0001
None vs. Baked	19.64638	14.17866	25.1141	<0.0001
None vs. Unbaked	−12.9862	−18.4539	−7.51843	<0.0001

.

The padding material and launcher were significant (p < 0.001), but the hardhat was not (p > 0.05).

3.3. Ballistic Penetration Tests

The ballistic penetration tests results are shown in

Table 5. Ballistic penetration test results for aerogel foam samples.

Sample	Launcher	Average Voltage (V)	Av. V (StD)	Post-Testing Voltage	Test V (StD)	V Change
Unbaked	SGP-35	0.58	0.01	0.57	0.04	0.01
	CA-09	0.60	0.04	0.37	0.10	0.23
Baked	SGP-35	0.54	0.01	0.57	0.01	0.04
	CA-09	0.60	0.01	0.59	0.01	0.01

, and the results of an impact shown in Figure 8.

No significant difference or visual damage was observed using the SGP-35 coilgun for either sample type (p > 0.05). Using the CA-09 coilgun, a significant difference in the voltage drop (0.23 ± 0.1 V) was observed between the pristine aerogel foam and the baked composite (p < 0.01).

Additionally, two CA-09 shots penetrated both layers of the pristine aerogel foam, but no shots penetrated the baked composite. The top layer of one baked sample was damaged, but the second layer stopped the projectile.

4. Discussion

4.1. Summary

Aerogel foam can effectively reduce the impact force of projectiles. Repeated measures showed significant differences in the pendulum height for the different materials (p < 0.001). Due to its irregular inertial characteristics and unconstrained dimensions of movement, the ballistic pendulum results had high variability, because much of the kinetic energy resulted in spinning the pendulum [58]. A consistent reduction in the total upward deflection was noted for the baked samples, and rotation of the head during blunt trauma has previously been observed [3,4,5]. However, the results demonstrated the clear resilience of the baked composite. During the potato cannon tests without a hardhat, the baked composite only registered 31 ± 2% of the deflection height observed in regard to the other cases. While a prior work involving an aerogel/STF composite reported an impact absorption of 80% relative to the 69 ± 2% observed with the baked composite, but costlier materials were used [50]. The hygroelectric devices were well-protected and continued to function at peak efficiency, withstanding fire from the two coilguns, a potato cannon, and a spring launcher. Hardening of the aerogel foam to improve impact resistance with baked starch may be beneficial in several applications, providing lighter and improved protection for athletes, workers, and soldiers.

4.2. Limitations

The exploratory approach adopted in this study had some limitations. First, high variability was observed in regard to the ballistic pendulum, in terms of both the deflection height and rotation. The rotation of the pendulum has harmful consequences in terms of concussion biomechanics, but in terms of the ballistic impact, it simply reduces the kinetic energy and hinders the measurement [3,4,5]. Fixing the launcher in place and restraining the lateral movement, instead of manual operation, may improve this situation in future work. Similarly, potato cannons have inherently high variability [53]. To improve this, an automatic gas-feeding system could be implemented. However, the complexity and design of such a system were beyond the scope of this study.

As shown in Table 1, the NIJ standards are often used to measure armor’s effective ballistic protection, and existing protective foams are evaluated based on these standards, but such an evaluation would have required extensive testing with conventional firearms [24,29]. Given the time constraints, firearm ballistics and robust determination of the NIJ level were not included herein, and, therefore, the performance of the aerogel and baked composite were not compared with that of existing protective foams. Another constraint was the lack of material simulation and finite element analysis, which would have been complex due to the manual fabrication of the samples and the variable structure of the aerogel. Finally, only one STF sample type was considered herein, even though silica nanoparticles are frequently used in STF-based protective materials [24,29]. Starch was used because of its low cost and the relative ease of handling, but further optimization and alternative materials must be examined in future work.

4.3. Future Work

Aerogel/STF composites have the potential to provide high-performance ballistic protection, especially considering that each material has been independently validated [24,29,31,50]. Optimizing a composite comprising commercial aerogel foam and starch has the advantage of utilizing available materials and scalable processes. The baking process can be adjusted to ensure the most effective cross-linking among the molecules. The moisture-trapping properties of aerogel may also be exploited to improve the efficacy of hygroelectric devices, enabling energy harvesting or sensing capabilities in wearable protective equipment and devices [40,41,55]. Regarding testing methods, a more consistent impact delivery system and conventional pendulum could also be utilized to ensure consistent force delivery during testing. Additionally, conventional ballistic testing is necessary to determine the effective NIJ protective level [24,29].

5. Conclusions

Concussion is a costly healthcare issue affecting sports, industry, and the defense sector [3,4,5]. Close-contact activities have long been associated with high concussion risks [6,7,8,9,10]. Existing PPE and protective gear can be bulky and impractical in many circumstances, but composite materials offer unique potential [30]. In particular, aerogel and STF materials have been independently validated for both ballistic and blunt force trauma protection [24,29,31]. Commercially available aerogel foam was impregnated with starch and baked to develop and evaluate their combined impact resistance. Composite samples were subjected to ballistic pendulum testing, followed by ballistic penetration testing. During the potato cannon tests without a hardhat, the baked composite only registered 31 ± 2% of the deflection height observed for the other cases, using nothing or just the aerogel foam. While prior work reported a higher rate of impact absorption, more expensive materials were used [50]. The baked composite protected the hygroelectric devices from coilgun projectiles, whereas the projectiles punctured the pristine aerogel foam. Using this new composite, wearable PPE can be further improved for various sporting, industrial, and defense applications.