ARL-TN-0704 ● OCT 2015

US Army Research Laboratory

Fabrication of Helmet Surrogate for the Walter 

Reed Army Institute of Research (WRAIR)

Projectile Concussive Impact Model of Mild 

Traumatic Brain Injury

by John R Brown

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ARL-TN-0704 ● OCT 2015

US Army Research Laboratory

Fabrication of Helmet Surrogate for the Walter 

Reed Army Institute of Research (WRAIR)

Projectile Concussive Impact Model of Mild 

Traumatic Brain Injury

by John R Brown

Weapons and Materials Research Directorate, ARL

Approved for public release; distribution is unlimited.
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1. REPORT DATE (DD-MM-YYYY)

October 2015

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March 2014–March 2015

4. TITLE AND SUBTITLE

Fabrication of Helmet Surrogate for the Walter Reed Army Institute of 

Research (WRAIR) Projectile Concussive Impact Model of Mild Traumatic 

Brain Injury

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

John R Brown

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

US Army Research Laboratory

ATTN: RDRL-WMM-A

Aberdeen Proving Ground, MD 21005-5069

8. PERFORMING ORGANIZATION REPORT NUMBER

ARL-TN-0704

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12. DISTRIBUTION/AVAILABILITY STATEMENT

Approved for public release; distribution is unlimited.

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The Walter Reed Army Institute of Research (WRAIR) projectile concussive impact (PCI) model is designed for preclinical 

analysis of concussion on male adult Sprague-Dawley rats. This model represents a noninvasive, closed-head injury caused by 

blunt impact and is used to study mild traumatic brain injury (TBI). Materials analysis completed within the US Army Research 

Laboratory (ARL) was verified by WRAIR through experimental testing. Based on these results, ARL designed and fabricated 

composite helmets that assured that the rats in the trial were protected against injury greater than mild TBI or concussion. This 

technical note is a description of the rat helmet fabrication process.

15. SUBJECT TERMS

mild traumatic brain injury, helmet, composite processing

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19a. NAME OF RESPONSIBLE PERSON

John R Brown

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410-306-0830

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iii

Contents

List of Figures iv

1. Introduction 1

2. Procedure 5

2.1 Material Preparation 5

2.2 Mold Preparation 5

2.3 Preimpregnated Material Processing 6

2.4 Postprocessing 9

3. Conclusions 9

4. References 11

List of Symbols, Abbreviations, and Acronyms 12

Distribution List 13
iv

List of Figures

Fig. 1 WRAIR PCI device. The red inset shows a side view of the device 

with a helmet protected rat (laying on its back).....................................2

Fig. 2 Projectile analysis: (a) instrumentation including the injury device and 

a surrogate head with 4 accelerometers, and projectiles: 1. steel sphere, 

2. brass sphere, 3. ceramic sphere, 4. steel cylinder, 5. nylon cylinder, 

6. aluminum cylinder, 7. microcentrifuge cap; (b–i) impact energy 

profiles (energy vs. time) of each projectile. Blue, black, red, and 

green lines represent data obtained from each of the accelerometers 

(top left, top right, bottom left, and bottom right corners respectively; 

(j) the average peak energy of each projectile. ......................................3

Fig. 3 Composite material tensile testing, including: (a) Instron 1125 testing 

machine and composite materials before and after testing (red arrow 

indicates fracture); (b) stress-strain curves of tested samples. Inset 

shows stress-strain curve of the fiber glass/carbon samples..................4

Fig. 4 Rat head mold on caul plate with tack tape and tack tape “ears” 

surrounding the mold .............................................................................6

Fig. 5 Release film applied to each rat head mold. Breather cloth and vacuum 

port added...............................................................................................6

Fig. 6 Distance measurement from the base of the mold is taken to assure 

fiber placement is repeatable (left). Release film is applied once all 

layers of preimpregnated materials are placed (right). ..........................7

Fig. 7 A vacuum bag is placed over the mold and caul plate (left). The bag is 

sealed using the tack tape already applied to the caul plate (right). ......8

Fig. 8 Vacuum bagged system before curing of the preimpregnated 

materials.................................................................................................8

Fig. 9 The rough helmet before final machining (left) and the final helmet 

(right) .....................................................................................................9
1

1. Introduction

A concussion is defined as a mild form of traumatic brain injury (TBI) generally 

resulting from a blow to the head. Mild TBI makes up about 75% of the brain 

injuries that occur each year. Over 230,000 service members have been diagnosed 

with mild TBI since 2000, which has been designated the “signature injury” on the 

military conflicts in the Middle East.1,2

Most of the preclinical research models designed to study the neurobiology of 

concussion are modified versions of the gold standard models of moderate/severe 

TBI, including the controlled cortical impact (CCI)3 and the fluid percussion injury 

(FPI)4

. To accommodate the study of mild TBI, the force of impact is simply 

reduced, but there is still direct impact to the cortical tissue (exposed via 

craniotomy). There are nonimpact models that can be used instead of these invasive 

techniques; however, since the majority of concussions are caused by impact, 

nonimpact models are not necessarily relevant. The Walter Reed Army Institute of 

Research (WRAIR) projectile concussive impact (PCI) model is designed to 

provide a truly noninvasive, closed-head model for mild TBI.

5 The WRAIR PCI 

testing uses male adult Sprauge-Dawley rats (280–320g; Charles River Labs, 

Raleigh, Virginia, USA) for experimentation. All procedures involving animal use 

were reviewed and approved by the Institutional Animal Care and Use Committee 

(IACUC) of WRAIR. The research was conducted in compliance with the animal 

welfare act, Guide for Care and Use of Laboratory Animals (National Research 

Council) and other federal statutes and regulations. The animals were housed

individually under a 12-h light/dark cycle in a facility accredited by the Association 

for Assessment and Accreditation of Laboratory Animal Care International. Figure 

1 shows the WRAIR PCI device, which consists of an elevated platform and 

restraints to hold the rat in place during testing, a plastic barrel that delivers the 

projectile, and the computer controlled pneumatic train that provides propulsion to 

the projectile. The inset of the figure shows a helmet protected rat before the test 

begins. 
2

Fig. 1 WRAIR PCI device. The red inset shows a side view of the device with a helmet 

protected rat (laying on its back).

5

In initial experimentation, WRAIR used a dry ice driven system where overpressure 

inside a sealed microcentrifuge container would build until the cap of the container 

popped off. The microcentrifuge cap would then become the projectile used to 

impact the rats. The original experimentation also used a steel spoon as the helmet 

to protect the rats. The cap projectile and spoon helmet proved to provide 

inconsistent results, and the US Army Research Laboratory (ARL) set out to create 

a more repeatable system for WRAIR analysis. ARL conducted impact analysis of 

candidate projectiles to determine the most repeatable projectile performance. This 

testing is shown in Fig. 2. 
3

Fig. 2 Projectile analysis: (a) instrumentation including the injury device and a surrogate 

head with 4 accelerometers, and projectiles: 1. steel sphere, 2. brass sphere, 3. ceramic sphere, 

4. steel cylinder, 5. nylon cylinder, 6. aluminum cylinder, 7. microcentrifuge cap; (b–i) impact 

energy profiles (energy vs. time) of each projectile. Blue, black, red, and green lines represent 

data obtained from each of the accelerometers (top left, top right, bottom left, and bottom 

right corners respectively; (j) the average peak energy of each projectile.5

ARL also conducted an analysis of composite material systems to determine if a 

composite solution would be adequate to replace the steel spoon and meet the 

helmet requirements. American Society for Testing and Materials (ASTM)

Standard D3039_D3039M-08, “Standard Test Method for Tensile Properties of 

Polymer Matrix Materials” with an Instron 1125 testing machine and a 10kN load 

cell at a loading rate of 2 mm/min was used for initial characterization. A selection 

of composite samples evaluated is shown in Fig. 3. 
4

Fig. 3 Composite material tensile testing, including: (a) Instron 1125 testing machine and 

composite materials before and after testing (red arrow indicates fracture); (b) stress-strain 

curves of tested samples. Inset shows stress-strain curve of the fiber glass/carbon samples.5

A thorough description of the mechanical and material analysis can be found in 

work done by Chen et al. 2012,

5 which resulted in the selection of the glass fiber 

and carbon fiber composite system to be used for the rat helmets. The fiber glass 

used is an S-2 glass woven textile preimpregnated with an epoxy matrix for bonding 

(7781 fabric style with Cycom 381 resin, Cytec Industries, Inc., Havre de Grace, 

Maryland). The carbon fiber is a unidirectional fiber tape preimpregnated with the 

same epoxy matrix (IM-7 carbon tape with Cycom 381 resin, Cytec Industries, Inc, 
5

Havre de Grace, Maryland). These helmets were fabricated using a 3-dimensional

(3-D) mold of an adult Sprauge-Dawley rat (300 g; Charles River Labs, Raleigh, 

Virginia, USA).

2. Procedure

A 3-D scan of the aforementioned rat was performed and the resultant model was 

fabricated by Fused Deposition Modeling (FDM), a 3-D Printing technology. Two 

molds were fabricated in this manner, one mold consisting of 5 rat head sections 

and one mold consisting of 6 rat head sections. These molds are used to form the 

preimpregnated fabric and tape to the contours of the rat head, thereby creating a 

helmet specifically designed for this type of rat. 

2.1 Material Preparation

The preimpregnated fiber textile and tape are stored in a freezer in order to prevent 

curing of the epoxy matrix. These materials must be removed from the freezer and 

allowed to thaw to room temperature (approximately 2 h depending on 

environmental conditions). The materials remain sealed during thaw to prevent 

moisture uptake. Once thawed these materials are cut into desired 2-dimensional 

(2-D) patterns by a computer controlled cutting table (DCS 2500, Gerber 

Technology Inc, Marblehead, Massachusetts). These patterns are then stored in a 

freezer until approximately 2 h before they are needed. 

2.2 Mold Preparation

The rat head molds are placed on a steel caul plate to prevent warpage during 

processing. The caul plate is cleaned with acetone to make sure no contaminates 

will affect processing. Tack tape (GS 43MR, Coastline International, Amityville, 

New York) is applied to the clean, dry surface of the caul plate and “ears” are made 

with the tack tape to allow for the bagging material to conform over the molds 

without creating unnecessary voids. Figure 4 shows this step.
6

Fig. 4 Rat head mold on caul plate with tack tape and tack tape “ears” surrounding the 

mold

Release film (WL 5200B, Coastline International, Amityville, New York) is fitted 

to each of the rat heads and breather cloth is laid down around each mold. The 

breather cloth (Econoweave 44, Coastline International, Amityville, New York) is 

used to create pathways for the air to escape when the system is placed under 

vacuum. A vacuum port is also added with breather cloth under it (note that the 

breather cloth under the vacuum port must be in contact with the breather cloth 

around the rat heads in order to assure all air can be removed during processing). 

Figure 5 shows this process. 

Fig. 5 Release film applied to each rat head mold. Breather cloth and vacuum port added.

2.3 Preimpregnated Material Processing

Preimpregnated materials (at room temperature) are applied in layers. The first 

layer (bottom most layer, in contact with the release film over the rat head mold) is 

S-2 glass that is placed 0.866 inches from the base of the mold as shown in Fig. 6. 

This measurement is taken during the placement of the initial layer to assure 
7

repeatability. A single drop (0.05 ml) of Freekote (700 NC, ACP composites, 

Livermore, California) mold release is applied to the first layer of S-2 glass. This 

mold release is applied in a location corresponding to the impact location in which 

WRAIR will be testing. A template is used to place this droplet based on the area 

of interest. The mold release prevents the bonding of the layers in a precise location. 

This allows for the helmet to be protective but partially compliant in the desired 

location. The next layer added is carbon fiber tape. Unlike the woven fiber glass, 

the carbon tape only has reinforcing fibers in one direction. This layer is placed so 

that the fiber orientation correlates with the axial direction of the head (in line with 

the spine of the rat). As layers are added, wrinkles should be relaxed by massaging 

them out by hand. Once this layer has been put into place, another drop of Freekote 

is added to the desired location. The third layer is carbon tape, which is placed so 

that the fiber orientation is 90° from the previous layer. Once this layer has been 

placed, another drop of Freekote is added to the desired location. The final layer is 

another layer of the S-2 glass. 

Fig. 6 Distance measurement from the base of the mold is taken to assure fiber placement 

is repeatable (left). Release film is applied once all layers of preimpregnated materials are 

placed (right).

Once all of the preimpregnated material is in place, a layer of release film is placed 

on top, as shown in Fig. 6. 

A vacuum bag (SL 800, Coastline International, Amityville, New York) is placed 

on top of the mold area and is sealed down with the tack tape already on the caul 

plate. When sealing the bag, one should start in between 2 ears and work around 

the caul plate. Take any excess bag created from the ears and seal it along the length 

of the mold. This is shown in Fig. 7. While adhering the bag to the tape, excess 

bagging material is used to make sure the bag can conform to the shape of the mold 

when the system is placed under vacuum. 
8

Fig. 7 A vacuum bag is placed over the mold and caul plate (left). The bag is sealed using 

the tack tape already applied to the caul plate (right).

A vacuum line is attached to the vacuum port via a small incision in the bag directly 

above the vacuum port. Once a vacuum line is attached, approximately 

0.7 atmosphere of vacuum is applied in order to assure the vacuum bag can 

sufficiently secure the preimpregnated material to the rat head molds. During this 

step, spread the bag, by hand, around the vacuum port and rat head molds to assure 

there are no wrinkles in these areas. When wrinkles are removed, 1.0 atmosphere 

of vacuum is applied to the system. Care is taken to make sure that the 

preimpregnated materials are still in the desired location. The fully vacuumed 

system is shown in Fig. 8. 

Fig. 8 Vacuum bagged system before curing of the preimpregnated materials
9

Once the vacuum bagging is complete, the preimpregnated materials need to be 

cured. For these materials, the system is heated to 250 °F and soaks at that 

temperature for 2 h and then allowed to return to room temperature overnight. 

2.4 Postprocessing

Once the system has returned back to room temperature, the bag, release film, and 

breather cloth is removed. The consolidated and cured preimpregnated material is 

now a rigid composite and is the rough estimate of the final part. A template of the 

final helmet shape is used to assist in final machining. A hand grinder with a cutting 

blade can be used to precisely cut out the finished helmet. A rough part and finished 

helmet is shown in Fig. 9.

Fig. 9 The rough helmet before final machining (left) and the final helmet (right)

As described previously, WRAIR requires a section of the helmet to be more 

compliant than the overall structure. This is noted by the location of the drops of 

Freekote that are added to the preimpregnated layers, at desired locations, during 

processing. The final step in the fabrication of these helmets is to delaminate the 

section of the helmet correlating to the area where the Freekote drops have been 

added. A weight drop is used to strike this position to assure that each layer of the 

helmet delaminates from the adjoining layers. 

3. Conclusions

ARL conducted material and mechanical analysis in the support of the WRAIR PCI 

model. This analysis showed that a new helmet system was required to allow 

WRAIR to collect accurate and repeatable results using their PCI device. ARL then 

created a process for fabricating these helmets. ARL fabricated multiple test 
10

helmets in order to establish the validity of the process. This program has since 

transitioned to the Edgewood Chemical Biological Center’s Advanced Design and 

Manufacturing (ADM) Branch for the production of hundreds of rat helmets for 

WRAIR research. 
11

4. References

1. Leung L, Larimore Z, Holmes L, Cartagena C, Mountney A, Deng-Bryant Y, 

Schmid K, Shear D, Tortella F. The WRAIR projectile concussive impact 

model of mild traumatic brain injury: re-design, testing and preclinical 

validation. Annals of Biomedical Engineering. 2014;42(8):1618–1630.

2. DVBIC. DoD worldwide numbers for TBI. 2013 [accessed 2015 July 28]. 

http://dvbic.dcoe.mil/dod-worldwide-numbers-tbi. 

3. Shitaka Y, Tran HT, Bennett RE, Sanchez L, Levy MA, Dikranian K, Brody 

L. Representative closed-skull traumatic brain injury in mice causes persistent 

multifocal axonal injury and microglial reactivity. J Neuropathol. Exp. Neurol. 

2011;70:551–567.

4. Shultz SR, Bao F, Omana V, Chiu C, Brown A, Cain DP. Repeated mild lateral 

fluid percussion brain injury in the rat causes cumulative long-term behavioral 

impairments, neuroinflammation, and cortical loss in an animal model of 

repeated concussion. J Neuropathol. Exp. Neurol. 2012;29:281–294.

5. Chen Z, Leung LY, Mountney A, Liao Z, Yang W, Lu XC, Dave J, DengBryant Y, Wei G, Schmid K, Shear DA, Tortella FC. A novel animal model of 

closed-head concussive-induced mild traumatic brain injury: development, 

implementation, and characterization. J Neurotrauma. 2012;29:268–280.
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List of Symbols, Abbreviations, and Acronyms

2-D 2-dimensional

3-D 3-dimensional

ADM Advanced Design and Manufacturing

ARL US Army Research Laboratory

ASTM American Society for Testing and Materials

CCI controlled cortical impact

FDM Fused Deposition Modeling

FPI fluid percussion injury

IACUC Institutional Animal Care and Use Committee

PCI projectile concussive impact

TBI traumatic brain injury

WRAIR Walter Reed Army Institute of Research
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