Patent Publication Number: US-2022225720-A1

Title: Helmet Impact Attenuation Liner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/850,199 filed May 20, 2019 entitled “Helmet Impact Attenuation Liner”, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to an impact attenuation liner for a helmet and, more particularly, to helmet liners having an additively manufactured lattice structure for impact attenuation. 
     BACKGROUND OF THE INVENTION 
     Helmet manufacturers have long dealt with the competing requirements of increased impact performance requirements and lower weight targets. Helmets typically have a rigid shell and a compressible liner disposed within the rigid shell. The compressible liner absorbs impact energy and reduces the amount of energy transferred to the user&#39;s head during an impact. Current technologies for helmet liners are typically foam based and have a homogenous impact profile. Due to the temperature dependence of existing liner materials, the impact performance is limited to the lowest common denominator over the expected operating range, i.e. lowest temperature, lowest impact velocity and energy. The tendency of foam padding to retain moisture and lack breathability, also leads to reduced user comfort during extended use. 
     Further, the homogeneity of existing liner technology often leads to tradeoffs in performance in different regions of the liner and helmet, and prevents optimal performance with respect to weight. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, there is an impact attenuation liner for a helmet including an additively manufactured lattice structure configured to be disposed inside the helmet, the lattice structure including a plurality of cells, each having a plurality of struts and nodes, wherein the lattice structure includes a top surface having a convex curvature corresponding to an inner surface of helmet and a bottom surface having a concave curvature configured to receive a user&#39;s head. 
     In some embodiments, the additively manufactured lattice structure is at least partially comprised of a 3D kagome lattice structure. The 3D kagome lattice structure may include a plurality of layers, each layer having the plurality of cells. Each of the plurality of cells of the 3D kagome lattice structure may have a geometry resembling a parallelepiped. Each of the plurality of cells may include vertices and at least one vertex is coupled to a tetrahedron. 
     In some embodiments, the impact attenuation liner further includes a 3D structure disposed at least partially within the lattice structure. The 3D structure may comprise a different material than the lattice structure. The lattice structure may include a plurality of extending portions and the 3D structure includes a plurality of openings each configured to receive one of the plurality of extending portions. The 3D structure may be an aluminum honeycomb sheet. 
     In some embodiments, the additively manufactured lattice structure comprises a plurality of lattice pads, each of the plurality of lattice pads comprised of an additively manufactured lattice. 
     In some embodiments, the additively manufactured lattice structure comprises a macroscopic cross-linked carbon nanotube structure. 
     In some embodiments, the additively manufactured lattice structure comprises a macroscopic cross-linked carbon nanotube structure with re-entrant angles. 
     In some embodiments, the additively manufactured lattice structure comprises an auxetic macroscopic cross-linked carbon nanotube structure. 
     In some embodiments, the additively manufactured lattice structure is comprised of polyurethane. The lattice structure may be at least partially comprised of a polymer where the polymer is comprised of one or more of polyurethane, polyamide, glass reinforced composites, carbon reinforced composites, thermoplastic polymer such as acrylonitrile butadiene styrene (ABS), polycarbonate, polyetherimide (PEI), polyetheretherketone (PEEK), thermoset polymer, acrylic polyurethanes, methacrylic polyurethanes, polyurea, polyacrylates, polymethacrylates and polyepoxides. 
     In some embodiments, in the additively manufactured lattice structure comprised of a material configured to deform non-elastically. 
     In some embodiments, the plurality of cells each have a size between approximately 1 mm and approximately 30 mm. In some embodiments, a ratio between a thickness of one of the plurality of struts and a size of one of the plurality of cells is between 1:4 and 1:120 and a ratio between the thickness of the one of the plurality of struts and a length of one of the plurality of struts is between 1:1 and 1:60. 
     In some embodiments, the lattice structure is configured to attenuate impact in response to an impact event having a velocity greater than approximately 3.0 m/s. In some embodiments, the lattice structure is configured to attenuate impact in response to an impact event having an energy level greater than approximately 35 ft-lb. 
     In some embodiments, the lattice structure includes a first region having a first level of stiffness and a second region having a second level of stiffness different than the first level of stiffness to provide a different level of impact attenuation than the first region. 
     In some embodiments, the lattice structure includes auxetic cell geometries with re-entrant angles ranging from approximately 180 degrees to approximately 270 degrees. 
     In some embodiments, the lattice structure includes a continuous network of channels to enable management of power and data cabling through the lattice structure. 
     In some embodiments, the impact attenuation liner further includes a stiffening layer coupled to an outer surface of the lattice structure, the stiffening layer configured to function as at least a part of a shell of the helmet. The stiffening layer may have a thickness ranging from 0.020 in to 0.100 in and an elastic modulus ranging from 0.5 GPa to 200 GPa. 
     In some embodiments, the impact attenuation liner further includes a stiffening intermediate layer disposed between the lattice structure and one or more of an outer shell of the helmet and a user&#39;s head, wherein the stiffening intermediate layer has an elastic modulus of approximately 0.5 GPa to approximately 200 GPa. 
     In some embodiments, the plurality of cells have a plurality of struts that are hollow and a plurality of nodes that are hollow. 
     Another embodiment of the present invention provides for an impact attenuation liner for a helmet including an additively manufactured lattice structure configured to be disposed between a shell of the helmet and a user&#39;s head, the lattice structure comprising a lattice structure having a plurality of cells, each of the plurality of cells including a plurality of struts, wherein the plurality of cells are shaped to resemble a hexagonal prism and the lattice structure is at least partially comprised of a material having an elastic modulus between 750 MPa and 100 GPa. 
     In some embodiments, the material has a strain at failure between approximately 40% and approximately 500%. 
     In some embodiments, the impact attenuation liner further includes a 3D structure coupled to the lattice structure, the 3D structure comprising an aluminum honeycomb sheet. 
     Another embodiment of the present invention provides for a helmet system including a helmet having a plurality of comfort pads comprised of foam and an additively manufacture impact attenuation lattice structure disposed within the helmet, the additively manufactured impact attenuation lattice structure having a top surface having a convex curvature coupled to an inner surface of the helmet and a bottom surface having a concave curvature configured to receive a user&#39;s head, a plurality of cells having a lattice geometry, the plurality of cells having a plurality of struts, wherein the plurality of cells and the plurality of struts are comprised of generally rigid polyurethane, and a continuous network of channels disposed throughout the additively manufactured lattice structure, the continuous network of channels configured to enable air to flow through the additively manufactured lattice structure, wherein the lattice structure includes a first region having a first level of stiffness and a second region having a second level of stiffness different than the first level of stiffness to provide a different level of impact attenuation than the first region. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The following detailed description of embodiments of the helmet impact attenuation liner will be better understood when read in conjunction with the appended drawings of exemplary embodiments. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       In the drawings: 
         FIG. 1  is a cross-sectional view of the helmet impact attenuation liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  is a front perspective view of a portion of a helmet impact attenuation liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 3  is a bottom view of an impact attenuation liner system in accordance with an exemplary embodiment of the present invention shown inside a helmet; 
         FIGS. 4A-4K  illustrate exemplary lattice cell geometries that may be used in the helmet impact attenuation liner; 
         FIG. 5  is an exemplary kagome lattice structure that may be used in the helmet impact attenuation liner; 
         FIG. 6  is an exemplary kagome lattice unit cell that may be used in the helmet impact attenuation liner; 
         FIG. 7  is an exemplary parallelepiped unit cell volume for a kagome unit cell that may be used in the helmet impact attenuation liner; 
         FIG. 8  is an exemplary additively manufactured lattice composed of macro scale cross-linked (3,3) carbon nanotubes; 
         FIG. 9  is an exemplary unit cell geometry of cross-linked (3,3) carbon nanotubes; 
         FIG. 10  is an exemplary cell geometry of auxetic cross-linked (3,3) carbon nanotubes; 
         FIG. 11A  is a top view of a lattice composed of cross-linked (3,3) carbon nanotubes; 
         FIG. 11B  is an isometric view of the lattice of  FIG. 11A ; 
         FIG. 12A  is a top view of a lattice composed of auxetic cross-linked (3,3) carbon nanotubes; 
         FIG. 12B  is an isometric view of the lattice of  FIG. 12A ; 
         FIG. 13  is an illustration of a re-entrant angle in accordance with an exemplary embodiment of the present invention; 
         FIGS. 14A-14C  illustrate top views of minimal surface lattice structures with varying cell size and wall thickness for use in the helmet impact attenuation liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 15  is a portion of a helmet impact attenuation liner with dual material in accordance with an exemplary embodiment of the present invention; 
         FIG. 16  is a portion of a helmet impact attenuation liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 17  is a portion of a helmet impact attenuation liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 18  is a portion of a helmet impact attenuation liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 19  is a portion of an integrated helmet shell and liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 20  is a portion of a liner integrated with inner and outer helmet shells in accordance with an exemplary embodiment of the present invention; 
         FIG. 21  is a graph of the relationship between relative density and relative impact performance of a helmet impact attenuation liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 22  is a graph of impact testing of various embodiments of helmet impact attenuation liners in accordance with an exemplary embodiment of the present invention; 
         FIG. 23  is a graph of various stress-strain curves for 3D kagome structure and EPS foam in accordance with an exemplary embodiment of the present invention; 
         FIG. 24  is a graph of various stress-strain curves of various embodiments of lattices composed of unit cells of  FIG. 4  in accordance with an exemplary embodiment of the present invention; 
         FIG. 25  is a finite element analysis of a helmet impact attenuation liner in accordance with an exemplary embodiment of the present invention; 
         FIG. 26  is a graph of stress-strain curves of lattices composed of unit cell of  FIG. 4F  with various re-entrant angles in accordance with an exemplary embodiment of the present invention; 
         FIG. 27  is a graph of stress-strain curves for cross-linked (3,3) carbon nanotube lattice and EPS foam in accordance with an exemplary embodiment of the present invention; 
         FIG. 28  is a graph of stress-strain curves for auxetic cross-linked (3,3) carbon nanotube lattice and EPS foam in accordance with an exemplary embodiment of the present invention; 
         FIG. 29  is a graph of stress-strain curves of the minimal surface lattices of  FIG. 14  in accordance with an exemplary embodiment of the present invention; 
         FIG. 30  is graph of impact testing of helmet impact attenuation liners of  FIG. 15  in accordance with an exemplary embodiment of the present invention; 
         FIG. 31  is graph of impact testing of helmet impact attenuation liners of  FIG. 15  in accordance with an exemplary embodiment of the present invention; and 
         FIG. 32  is a graph of stress-strain curves of various embodiments of helmet impact attenuation liners of  FIG. 15  in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE INVENTION 
     Helmets for head protection are worn in a variety of environments and for various purposes including adventure, sporting, police and military purposes. Helmets may provide protection against projectiles and blunt force impacts. Helmets typically include a helmet shell having a peripheral edge and a retention system (e.g., chinstrap) that may be attached to helmet shell. Helmets also typically include a liner system coupled to an inside surface of the helmet shell to provide a compressible material for comfort and impact energy absorption. The liner system may be composed of a single contiguous structure or multiple distinct structures either of which may or may not completely cover the surface of the helmet shell. The need for a comfortable liner with high impact attenuation is particularly important for defense forces, emergency responders, and industrial personnel operating in high performance environments, as well as individuals wearing helmets for extended periods of time under harsh conditions. 
     Referring to  FIGS. 1-3 and 17-20  wherein like reference numerals indicate like elements throughout, there is shown an impact attenuation liner system  100 , generally designated  100 , in accordance with an exemplary embodiment of the present invention. In certain preferred embodiments of the present invention, impact attenuation liner system  100  includes lattice structure  102 . In one embodiment, impact attenuation liner system  100  may be used as a drop-in replacement for the impact liner of an existing helmet. In another embodiment, impact attenuation liner system  100  may be used as a fully integrated system with the helmet. 
     Referring to  FIGS. 1-3 , lattice structure  102  may be an additively manufactured lattice structure. In some embodiments, lattice structure  102  is configured to be positioned within an interior region of a head protection device, such as helmet  200 . More particularly, lattice structure  102  may be configured to be positioned inside helmet  200 . Lattice structure  102  may be configured to be positioned between an outer shell of helmet  200  and a user&#39;s head during use and to provide impact protection to the user. In some embodiments, lattice structure  102  is disposed anywhere within helmet  200 , such as between layers of helmet  200 . In one embodiment, lattice structure  102  is shaped such that it retains the same shape whether or not it is coupled to the helmet  200  and/or the user&#39;s head. In some embodiments, lattice structure  102  is dimensioned to fit along the interior of helmet  200  from the front of helmet  200  to the back of helmet  200 . In some embodiments, lattice structure  102  is configured to entirely fit within the interior of helmet  200  and to not extend beyond the periphery of helmet  200  during use. In some embodiments, lattice structure  102  may be removably coupled to helmet  200 . In another embodiment, lattice structure  102  is fixedly coupled to the interior surface of helmet  200 . In yet another embodiment, lattice structure  102  is integrally formed with helmet  200 . 
     Helmet  200  may be any type of head protection helmet known in the art, for example, those used for sporting, industrial safety, police, or military purposes. In certain embodiments, helmet  200  is a standard infantry ballistic helmet. In some embodiments, helmet  200  is an advanced combat helmet (ACH), an enhanced combat helmet (ECH), a modular integrated communications helmet (MICH), a tactical ballistic helmet (TBH), a lightweight marine helmet, police general duty helmet, a personnel armor system for ground troops (PASGT), or an aircrew helmet, such as an HGU-56/P rotary wing helmet or an HGU 55/P fixed wing helmet. In one embodiment, helmet  200  may be manufactured with additive manufacturing such as 3D printing, and may include a 3D printed shell. For example, helmet  200  may be comprised of a 3D printed outer shell with an integrated 3D printed energy absorbing lattice layer, such as lattice structure  102 . Lattice structure  102  may be configured to provide protection to a user&#39;s head, in addition to decreasing the overall weight of helmet  200  compared to traditional liners and helmets. In some embodiments, lattice structure  102  may be manufactured as a single structure or assembled from separate components. 
     Lattice structure  102  may be made by using additive manufacturing, such as 3D printing. Additive manufacturing may allow for specific geometries within lattice structure  102  that may not be manufactured using traditional techniques. Additive manufacturing may allow for lattice structure  102  to be comprised of different materials thereby varying the impact properties of lattice structure  102 . Using a 3D printer, lattice structure  102  may be created with varying layers of different materials based on the impact attenuation performance desired. For example, lattice structure  102  may be a hybridization of different impact attenuating materials such as a sheet of aluminum arranged in a honeycomb geometry with a lattice structure, a lattice with expanded polystyrene (EPS), a lattice with expanded polypropylene (EPP), a lattice with polyurethane foam, or a lattice with other aluminum honeycomb, polymeric cellular, polymeric engineered, composite cellular, or composite engineered structures. In some embodiments, lattice structure  102  may be a 3D printed lattice structure. The 3D printed lattice structure may be comprised of a single use crushable material. In some embodiments, the material may withstand or rebound from minor impacts, but is configured to deformably crush to absorb larger impacts. By deforming without rebounding, the energy may be more effectively absorbed and attenuated without transferring to the user&#39;s head. In one embodiment, the lattice structure  102  is comprised of polyurethane. 
     Lattice structure  102  may be comprised of generally rigid polyurethane. In some embodiments, a generally rigid material refers to a non-elastic material. Lattice structure  102  may be comprised of a generally rigid material, such as polyurethane, such that lattice structure  102  is permanently crushed when deformed. In some embodiments, lattice structure  102  is comprised of a material configured to deform non-elastically. In some embodiments, lattice structure  102  may include both elastic material and non-elastic material. For example, lattice structure  102  may include a layer of elastic material and a layer of non-elastic material. Lattice structure  102  may include one or more layers of polyurethane. In some embodiments, lattice structure  102  is at least partially comprised of polymeric segments. Lattice structure  102  may be comprised of one or more of polyurethane, polyamide, glass reinforced composites, carbon reinforced composites, thermoplastic polymer such as acrylonitrile butadiene styrene (ABS), polycarbonate, polyetherimide (PEI), polyetheretherketone (PEEK), thermoset polymer such as acrylic polyurethanes, methacrylic polyurethanes, polyurea, polyacrylates, polymethacrylates and polyepoxides. In some embodiments, preferred materials have a high specific modulus and exhibit significant toughness. In general, materials fitting these criteria tend to be rigid polymers with elastomers performing poorly due to low specific moduli. In one embodiment, a preferred material has an elastic modulus greater than approximately 750 MPa. For example, the material may have an elastic modulus between approximately 750 MPa and 100 GPa. In one embodiment, the strain at failure is greater than approximately 40%. 
     In one embodiment, lattice structure  102  may be configured to maintain impact performance over a range of varying temperature conditions. For example, lattice structure  102  may be configured to maintain impact performance between approximately −60° F. to approximately 180° F., approximately −40° F. to approximately 160° F., approximately −20° F. to approximately 140° F., approximately 0° F. to approximately 120° F., approximately 20° F. to approximately 100° F., or approximately 40° F. to approximately 80° F. In one embodiment, lattice structure  102  may be configured to maintain impact performance over multiple impact events at high impact velocities. For example, lattice structure  102  may be configured to maintain impact performance at impact velocities greater than approximately 3.0 m/s, approximately 4.25 m/s, approximately 5.2 m/s, approximately 6.0 m/s, approximately 6.5 m/s, approximately 7.0 m/s, approximately 8.5 m/s, approximately 9.5 m/s, or approximately 10.5 m/s. In one embodiment, lattice structure  102  may be configured to maintain impact performance over multiple impact events at high impact energies. For example, lattice structure  102  may be configured to maintain impact performance at impact energies greater than approximately 35 ft-lb, approximately 45 ft-lb, approximately 55 ft-lb, approximately 65 ft-lb, or approximately 75 ft-lb. Lattice structure  102  may be configured to maintain impact performance at impact energies from approximately 25 ft-lb to approximately 150 ft-lb. In one embodiment, lattice structure  102  may be created to match a single user&#39;s cranial profile. This may be done via additive manufacturing, such as 3D printing, and may not require the use of individualized tooling or hard tooling. 
     Referring to  FIGS. 1 and 2 , lattice structure  102  may include a plurality of layers  114 , each layer  114  comprising cells  104 , which may be comprised of struts or walls  106 . In one embodiment, cells  104  may have a geometry resembling a parallelepiped. However, cells  104  may be other shapes such as frustum, cylinder, cone, pyramid, polygonal, spherical, or combinations thereof. In one embodiment, struts  106  are hollow to decrease the overall weight of lattice structure  102  and impact attenuation liner system  100 . Lattice structure  102  may include nodes  111 . Nodes  111  may be joints where struts  106  meet and connect. Cells  104  and struts  106  may be comprised of polyurethane and may be manufactured via additive manufacturing, such as 3D printing. Struts  106  may have a length and thickness (diameter), which may affect the thickness of lattice structure  102 . For example, struts  106  may have an aspect ratio ranging from 1:1 to 1:120. In one embodiment, the length and thickness of struts  106  affect the impact attenuation properties of lattice structure  102 . 
     Referring to  FIGS. 1-3 , lattice structure  102  may include top surface  107  and bottom surface  109 . In one embodiment, top surface  107  may have a convex curvature and bottom surface  109  may have a concave curvature shaped to receive the user&#39;s head. Lattice structure  102  may include front region  108  and back region  110 . Front region  108  may be proximate to the user&#39;s forehead, and back region  110  may be proximate to the back of the user&#39;s head. In one embodiment, struts  106  of back region  110  may have a thickness greater than struts  106  of front region  108 . In some embodiments, lattice structure  102  may have a first region with struts  106  having a thickness greater than struts  106  of a second region. In some embodiments, lattice structure  102  includes multiple regions having struts  106  of different thicknesses. A transition region may be disposed between front region  108  and back region  110 . The transition region may be an area where of struts  106  transition to struts  106  of increased thickness or decreased thickness. For example, struts  106  of back region  110  may have a ratio of strut length to strut thickness of 1:20 and struts  106  of front region  108  may have a ratio of strut length to strut thickness of 1:10. 
     In one embodiment, struts  106  of back region  110  may have a stiffness greater than struts  106  of front region  108 . In some embodiments, lattice structure  102  may have a first region with struts  106  having a first level of stiffness greater than struts  106  of a second region. In some embodiments, lattice structure  102  includes multiple regions having struts  106  of different stiffness levels. A transition region may be disposed between front region  108  and back region  110 . The transition region may be an area where of struts  106  transition to struts  106  of increased stiffness or decreased stiffness. 
     Referring to  FIGS. 1 and 3 , impact attenuation liner system  100  may be used within helmet system  150 . Helmet system  150  may include additional materials to provide for increased impact attenuation and/or comfort. For example, impact attenuation liner system  100  may include a comfort liner secured to bottom surface  109  of lattice structure  102 . The comfort liner may be configured to provide additional impact attenuation and/or comfort. In some embodiments, impact attenuation liner system  100  includes a plurality of comfort pads  202  secured to bottom surface  109  of lattice structure  102 . Pads  202  may each be configured to provide cushioning between the user&#39;s head and lattice structure  102  during use. Pads  202  may be moveable by the user to position pads  202  based on user preference and head geometry. In some embodiments, a total of two to twelve pads  202  are provided with impact attenuation liner system  100  and are coupled to lattice structure  102 . Impact attenuation liner system  100  may be provided with three, four, five, six, seven, eight, nine, or ten pads  202 . In one embodiment, each of pads  202  has substantially the same shape. In another embodiment, pads  202  may include different shapes. Pads  202  may be square, rectangular, circular, or irregularly shaped. Each pad  202  may have a thickness in a range from about 6 mm to about 20 mm, about 8 mm to about 18 mm, about 10 mm to about 16 mm, or about 12 mm to about 14 mm before compression. In one embodiment, each pad  202  is at least 6 mm thick, at least 8 mm thick, at least 12 mm thick, at least 14 mm thick, at least 16 mm thick, or at least 18 mm thick before compression. In one embodiment, each pad  202  is about 13 mm thick before compression. In other embodiments, each pad  202  has a width of about 40 to about 60 mm and a length of about 80 mm to about 110 mm. In one embodiment, each pad  202  has a width of about 50 mm and a length of about 95 mm. 
     In one embodiment, pads  202  are made from a material that is different than the material used to construct lattice structure  102 . Pads  202  may include a soft or resilient material, such as compressible foam. Pads  202  may include a gel material. In one embodiment, pads  202  include a viscoelastic material or an elastomeric material. In a preferred embodiment, pads  202  are constructed from a breathable material. In some embodiments, pads  202  are manufactured via additive manufacturing, such as 3D printing. In one embodiment, each of pads  202  is made from reticulated foam that is enclosed in fabric. Pads  202  may include a foam that is less dense than the impact-absorbing material of lattice structure  102 . In one embodiment, pads  202  include plastic open cell reticulated foam enclosed in a fleece material. In one embodiment, pads  202  are made from materials that do not substantially absorb or retain water. For example, pads  202  may include foam having open cells that allow for drainage of water. In one embodiments, pads  202  are made from materials that absorb less water than certain polyurethane foams, such as those available under the ZORBIUM® brand. In another embodiment, pads  202  may be made from materials that absorb moisture. 
     In some embodiments, lattice structure  102  may be configured to be non-continuous. For example, lattice structure  102  may be sized and shaped to be individual lattice pads disposed within helmet system  150 . For example, lattice structure  102  may be a plurality of lattice pads, sized similarly to pads  202 . The plurality of lattice pads may be secured to helmet  200 . The plurality of lattice pads may each be configured to provide impact attenuation between the user&#39;s head and helmet  200 . In some embodiments, the plurality of lattice pads may be moveable by the user to position the lattice pads based on user preference and head geometry. In some embodiments, a total of two to twelve lattice pads are provided with impact attenuation liner system  100  and are coupled the interior of helmet  200 . In one embodiment, each of the plurality of lattice pads has substantially the same shape. In another embodiment, the plurality of lattice pads may include different shapes. The plurality of lattice pads may be square, rectangular, circular, or irregularly shaped. In some embodiments, the plurality of lattice pads may include one or more of the different configurations of lattice structure  102  discussed herein. For example, one of the plurality of lattice pads may include cells  104  having a kagome geometry and another one of the plurality of lattice pads may include cells  104  having a gyroid geometry. The plurality of lattice pads may have a thickness ranging from approximately 0.1 mm to approximately 30 mm, approximately 0.5 mm to approximately 25 mm, approximately 1 mm to approximately 20 mm, or approximately 10 mm to approximately 15 mm. 
     In some embodiments, lattice structure  102  is divided into a plurality of islands. Lattice structure  102  may be divided into a plurality of discrete segments to decrease the amount of lattice structure  102  within helmet system  150 . For example, lattice structure  102  may be configured to be a plurality of discrete segments to decrease the overall weight of helmet  200  or to allow space for additional interior components, such as pads  202 . In some embodiments, lattice structure  102  is configured to be a plurality of discrete segments, with pads  202  disposed between the plurality of discrete segments. 
     Referring to  FIGS. 1-4J , lattice structure  102  may include cells  104 , which may be various sizes and shapes. Cells  104  may be the same shape and size throughout lattice structure  102  or cells  104  may be different shapes and sizes throughout lattice structure  102 . Cells  104  may be arranged within lattice structure  102  in a specific geometry. For example, cells  104  may be arranged in a body centered cubic geometry ( FIG. 4A ), a cubic geometry ( FIG. 4B ), a diamond geometry ( FIG. 4C ), a fluorite geometry ( FIG. 4D ), a hexagonal prism geometry ( FIG. 4E ), an auxetic geometry ( FIG. 4F ), a 3D kagome geometry ( FIG. 4G ), a face centered cubic geometry ( FIG. 4H ), a gyroid geometry ( FIG. 4I ), a tetrahedral geometry ( FIG. 4J ), or a voronoi geometry ( FIG. 4K ). In one embodiment, cells  104  may be arranged in a combination of different geometries. For example, front region  108  of lattice structure  102  may have cells  104  arranged in a one geometry and back region  110  of lattice structure  102  may have cells  104  arranged in a different geometry. 
     Referring to  FIGS. 4G and 5-7 , cells  104  may be arranged in a 3D kagome (tri-hexagonal) geometry. The 3D kagome geometry may be similar to tri-hexagonal tiling, but in 3D geometry. The 3D kagome geometry of cells  104  may resemble a parallelepiped. In some embodiments, when cells  104  are viewed as a layer, the cross-sectional view of the parallelepiped of cells  104  resembles a hexagonal prism. Viewing cells  104  as a layer results in the parallelepiped geometry of cells  104  resembling tetrahedrons and hexagonal prisms arranged such that each side face of the hexagonal prism is shared with a face of an adjacent tetrahedron. For example, the cross-sectional view of cells  104  of the 3D kagome lattice structure may show each hexagonal prism of the including six tetrahedrons disposed around the perimeter of the hexagonal prism. The tetrahedrons may be connected at their vertices such that each tetrahedron has another tetrahedron connected at each of its vertices. 
     The 3D kagome geometry of cells  104  results in lattice structure  102  having a rigid and efficient structure for absorbing energy. The 3D kagome geometry of cells  104  may result in absorption of energy associated with low velocity blunt force impacts. For example, cells  104  may be configured to attenuate impact in response to an impact event having a velocity greater than approximately 4 m/s, approximately 5 m/s, approximately 6 m/s, approximately 7 m/s, approximately 8 m/s, approximately 9 m/s, or approximately 10 m/s. In some embodiments, cells  104  are be configured to attenuate impact in response to an impact event having a velocity greater than approximately 4.25 m/s, greater than approximately 5.2 m/s, greater than approximately 6.50 m/s or greater than approximately 7.0 m/s. Referring to  FIG. 5 , cells  104  may be in the shape of 3D kagome geometry  500 , which forms a series of tetrahedral elements joined at the vertices when tessellated to fill a volume. The microstructure of 3D kagome geometry  500  can be exploited by additively manufacturing a macroscopic analog, such as via 3D printing. Referring to  FIG. 6 , cell  104  may be unit cell  400  having a 3D kagome structure. Unit cell  400  may have nodes  402  and struts  404 . Referring to  FIG. 7 , unit cell  400  may be visualized as parallelepiped  700 . Parallelepiped  700  may illustrate the bounding volume of unit cell  400 . Unit cell  400  may have critical angles α and β. Critical angles α and β may allow the structural response of the unit cell and by connection the lattice as a whole to be tuned to exhibit the desired behavior when subjected to impact. 
     In one embodiment, the density of lattice structure  102  may be altered by changing the size and shape of cells  104  and struts  106  via additive manufacturing. By changing the size and shape of cells  104  and struts  106 , the density and impact properties of lattice structure  102  may be altered in a single additive manufacturing step. In one embodiment, cells  104  may be comprised of different materials throughout lattice structure  102 . For example, cells  104  may be made of varying materials throughout the thickness of lattice structure  102 . Cells  104  may have a size ranging from approximately 0.1 mm to approximately 30 mm, approximately 0.5 mm to approximately 25 mm, approximately 1 mm to approximately 20 mm, or approximately 10 mm to approximately 15 mm. In a preferred embodiment, the size of cells  104  is approximately 5 mm. Struts  106  may have a thickness ranging from approximately 0.1 mm to approximately 5 mm, approximately 0.5 mm to approximately 3 mm, or approximately 1 mm to approximately 2 mm. The ratio of the thickness of struts  106  to the size of cells  104  may vary. For example, the ratio of the thickness of struts  106  to the size of cells  104  may range from approximately 1:1 to approximately 1:300, approximately 1:50 to approximately 1:250, or approximately 1:100 to approximately 1:200. In a preferred embodiment, the ratio of the thickness of struts  106  to the size of cells  104  ranges from approximately 1:4 to approximately 1:120. 
     Further, the ratio of the thickness of struts  106  to the length of struts  106  may vary. For example, the ratio of the thickness of struts  106  to the length of struts  106  may range from approximately 50:1 to approximately 1:300, approximately 25:1 to approximately 1:200, or approximately 1:1 to approximately 1:100. In a preferred embodiment, the ratio of the thickness of struts  106  to the length of struts  106  ranges from approximately 1:4 to approximately 1:60. The density of struts  106  per node  111  may vary. In one embodiment, density of struts  106  per node  11  is the number of struts  106  that meet at each node  111 . This number may differ based on the desired geometries of cells  104 . For example, density of struts  106  per node may range from approximately 1:1 to approximately 1:20, approximately 1:1 to approximately 1:15 or approximately 1:5 to approximately 1:10. 
     In one embodiment, cells  104  within lattice structure  102  may be arranged to create a network of channels within lattice structure  102 . For example, the arrangement of cells  104  within lattice structure may create a continuous network of channels  115  to provide for improved airflow and breathability through lattice structure  102 . In one embodiment, channels  115  of lattice structure  102  may provide airflow and increase breathability compared to standard liners, resulting in a significant increase in a user&#39;s comfort. Lattice structure  102  may also include channels  115  to allow for threading of cables and wires for cable management during use of impact attenuation liner system  100 . Channels  115  disposed within lattice structure  102  may be configured to not affect or sacrifice the impact attenuation performance of impact attenuation liner system  100 . 
     In one embodiment, lattice structure  102  is configured to provide specific impact attenuation performances at specific locations. For example, lattice structure  102  may be configured to match specific performance characteristics in front region  108  and different performance characteristics in back region  110 . In another example, lattice structure  102  may be configured to provide greater or lesser impact attenuation at the crown or front of the head versus the left and right sides. Lattice structure  102  may include specific regions which may be configured to crush upon impact. For example, lattice structure  102  may have regions strategically placed throughout lattice structure  102  which may be configured to initiate crushing in order to control the transfer of impact energy on a first and/or second impact event. In one embodiment, lattice structure  102  may allow for the interchangeability of the strategically placed regions by the user in the field based on situation specific performance characteristics. For example, situation specific uses of impact attenuation liner system  100  may require increase or decrease of the thickness of struts  106  of lattice structure  102  to allow for varying impact attenuation. 
     In some embodiments, different levels of impact attenuation can be achieved by having lattice structure  102  with different densities of the impact-absorbing material at the different locations. In some embodiments, lattice structure  102  may include denser material at locations where greater impact attenuation is desired. In other embodiments, lattice structure  102  may have a variable thickness, for example, such that lattice structure  102  is thicker at portions where greater impact attenuation is desired. Lattice structure  102  may be lined with another material. For example, lattice structure  102  may be lined with a soft material to provide comfort to the user. In another example, lattice structure  102  may be lined with a hard material to provide more protection and impact attenuation to the user. 
     In one embodiment, additively manufactured auxetic structures may be created within lattice structure  102  to increase specific energy absorption in localized areas. For example, cells  104  may be arranged, via additive manufacturing, in an auxetic geometry throughout specific regions of lattice structure  102  to increase energy absorption in those specific regions. The term “auxetic” as used herein generally refers to a material or structure that has a negative Poisson&#39;s ratio. As such, when stretched, auxetic materials become thicker (as opposed to thinner) in a direction perpendicular to the applied force. Likewise, when compressed (e.g., by a blunt impact), auxetic materials become thinner in a direction traverse to the applied force. This contraction of the material acts to draw material in from outside of the impact zone to add supplemental energy absorption. This occurs due to the hinge-like structures (sometimes called a “re-entrant” structure) that form within auxetic materials. Conventional materials, including conventional foams (e.g., expanded polypropylene (EPP)), typically have positive Poisson&#39;s ratio, meaning that the materials tend to expand in a direction perpendicular to the direction of compression. Conversely, when a conventional material is stretched, it tends to contract in a direction transverse to the direction of stretching. A rubber band is a good example of an article with a positive Poisson&#39;s ratio, in that when stretched, the rubber band becomes thinner. 
     Referring to  FIG. 8 , auxetic structures may be used to create lattice structure  102 . For example, additively manufactured macro scale cross-linked carbon nanotubes (MSCLCNTs)  800  may be used to create lattice structure  102 . In some embodiments, MSCLCNTs may be modelled after a superposition-based cross-linking of (3,3) carbon nanotubes. In some embodiments, MSCLCNTs may be an auxetic variant of a superposition-based cross-linking of (3,3) carbon nanotubes. MSCLCNTs may be cross-linked to form a continuous orthotropic material and may be modelled after various permutations achieved by rolling a graphene sheet. In some embodiments, the continuous orthotropic material may have different configurations. For example, at least eight distinct configurations may be created based upon graphene sheets rolled to form CNTs in various rotational orientations and the cross-linking strategy used to combine the CNTs. These discreet configurations may also vary based on the bonding behavior of carbon atoms of the CNTs and the macro scale counterparts can additionally be formed in configurations that are not found in these discreet configurations of atomic scale CNTs. In some embodiments, lattice structure  102  may be produced by additively manufacturing a macroscopic analog of atomic structure of the CNTs. The MSCLCNT structures may provide for low velocity impact attenuation. 
     Referring to  FIGS. 9-12B , cell  104  may be created similarly to atomic scale CNTs and may have a cell geometry following that of a superposition-based cross-linking of (3,3) carbon nanotube  1100  ( FIGS. 9, 11A-11B ) or novel auxetic variant of the macro scale superposition-based cross-linking of (3,3) carbon nanotube  1200  ( FIGS. 10, 12A-12B ). MSCLCNTs ( FIG. 9 ) may have angle  602  and novel auxetic MSCLCNTs ( FIG. 10 ) may have angle  604 . Angle  602  may be greater than approximately 90° and angle  604  may be greater approximately 180°. However, angle  602  may be between approximately 90° and approximately 180°, between approximately 120° and approximately 160°, or between approximately 140° and approximately 150°, and angle  604  may be between approximately 180° and approximately 360°, between approximately 210° and approximately 330°, or between approximately 240° and approximately 270°. The auxetic MSCLCNT of  FIG. 10  may be created by changing angle  602  of the MSCLCNT of  FIG. 9 . The modification of angle  602  to angle  604  is significant as auxetic structures have been shown to outperform their standard counterparts in energy absorption due to their inherent structural behavior under loads that cause large deformations. The unit cell structures of both the MSCLCNT ( FIG. 9 ) and the novel auxetic MSCLCNT ( FIG. 10 ) may be contained within a hexagonal prism volume and may be comprised of 18 nodes  606  and 21 struts  608  connecting nodes  606 . The unit cell structures of both the MSCLCNT ( FIG. 9 ) and the novel auxetic MSCLCNT ( FIG. 10 ) may contain redundant struts. In some embodiments, unit cell structures of both the MSCLCNT ( FIG. 9 ) and the novel auxetic MSCLCNT ( FIG. 10 ) are tessellated to fill a volume similar to a honeycomb with the MSCLCNT structure oriented such that energy is attenuated by compressing the MSCLCNT structures along their longitudinal axis. In the preferred embodiment, many of the MSCLCNT structures are packed to form a layer of tubes with the longitudinal axis oriented to be coincident with the loading axis. 
     Referring to  FIG. 13 , cells  104  may have re-entrant angle α and struts  106 . The re-entrant angle may be the angle at which struts  106  come together at node  111 . In some embodiments, as the re-entrant angle decreases, the shape of cells  104  may resemble a rectangular shape. In some embodiments, as the re-entrant angle increases, the shape of cells  104  may resemble a bowtie shape. In one embodiment, cells  104  of lattice structure  102  may have auxetic geometries with re-entrant angles a ranging from approximately 180° to approximately 360°, approximately 210° to approximately 330°, or approximately 240° to approximately 300°. In some embodiments, the re-entrant angle is any angle that results in a lattice structure  102  having a negative Poisson&#39;s ratio 
     Referring to  FIGS. 14A-14C , various configurations of cells  104  arranged in a minimal surface, often referred to as a gyroid geometry, are illustrated. In one embodiment, such as  FIG. 4I , cells  104  may be configured in a gyroid geometry and may have faces  113  instead of struts  106 . Specifically,  FIGS. 14A-14C  show lattice structure  102  with varying sizes of cells  104  and varying thicknesses of faces  113 .  FIG. 14A  shows lattice structure  102 ′ with cells  104 ′ having a larger size than  FIGS. 14B and 14C , and with thinner faces compared to  FIGS. 14B and 14C .  FIG. 14B  shows lattice structure  102 ″ with cells  104 ″ having a smaller size than  FIG. 14A  and with thicker faces compared to  FIG. 14A , but thinner faces than  FIG. 14C .  FIG. 14C  shows lattice structure  102 ′″ with cells  104 ′″ having a similar size to  FIG. 14B , but with larger faces than  FIGS. 14A and 14B . 
     Referring to  FIGS. 15-18 , lattice structure  102  may include second 3D structure  300 . Second 3D structure  300  may be used to provide additional impact attenuation properties to impact attenuation liner system  100 . For example, lattice structure  102  and second 3D structure  300 , in combination, may attenuate a force from an impact event more than just lattice structure  102  alone. In another example, second 3D structure  300  may be configured to attenuate high energy impacts, while lattice structure  102  may be configured to attenuate low energy impacts. This results in the combination of lattice structure  102  and second 3D structure  300  attenuating a wider range of impact events compared to lattice structure  102  alone. In one embodiment, second 3D structure  300  may be comprised of a different material than lattice structure  102 . Second 3D structure  300  may be comprised of polymeric foams such as EPS, EPP, or polyurethane foam, or other cellular materials such as a sheet of aluminum honeycomb. In a preferred embodiment, second 3D structure  300  is a sheet of aluminum honeycomb. In one embodiment, second 3D structure  300  is a sheet of pre-crushed aluminum honeycomb such as that disclosed in U.S. Patent Application Publication No. 2018/0140037, which is hereby incorporated by reference in its entirety. Second 3D structure  300  may be pre-crushed to allow for impact attenuation during an impact event. In some embodiments, second 3D structure  300  is a cellular sheet composed of a metallic, composite, or polymeric material. 
     Referring to  FIG. 15 , second 3D structure  300  may be disposed at least partially within lattice structure  102  such that second 3D structure  300  and lattice structure  102  are overlapping with one another. In one embodiment, the combination of second 3D structure  300  and lattice structure  102  may include a liner to provide comfort to the user. The combination of second 3D structure  300  and lattice structure  102  may include pads  202  discussed above. In one embodiment, lattice structure  102  includes extending portions or projections  120 , which may extend from lattice structure  102 . For example, projections  120  may extend from top surface  107  or bottom surface  109  of lattice structure  102 . In a preferred embodiment, projections  120  may extend from top surface  107  of lattice structure  102 , away from a user&#39;s head. Second 3D structure  300  may include openings  302  which may be configured to receive projections  120 . In one embodiment, second 3D structure  300  is disposed within lattice structure  102  where projections  120  are disposed within openings  302 . 
     Referring to  FIG. 16 , second 3D structure  300  may be configured to cover the interior of helmet  200 . In one embodiment, an adhesive may be used to secure second 3D structure  300  to lattice structure  102  and/or helmet  200 . In some embodiments, hooks may be incorporated into lattice structure  102  to couple to and retain second 3D structure  300 . 
     In another embodiment, second 3D structure  300  is disposed on top of lattice structure  102 . Second 3D structure  300  may be removably attached to lattice structure  102  during use. For example, second 3D structure  300  may be disposed between lattice structure  102  and a shell of helmet  200 . Referring to  FIGS. 17-18 , second 3D structure  300  may be disposed on top of lattice structure  102 , such that lattice structure  102  is disposed between the user&#39;s head and second 3D structure  300 . In yet another embodiment, second 3D structure  300  may be sandwiched between two structures. For example, second 3D structure  300  may be sandwiched between two 3D structures. Second 3D structure  300  being sandwiched between two lattice structures may allow impact attenuation liner system  100  to attenuate higher impact energies. In some embodiments, both lattice structure  102  and second 3D structure  300  may include openings  112 . Openings  112  may be configured to provide breathability and airflow to a user&#39;s head. For example, both lattice structure  102  and second 3D structure  300  may include openings  112  in the same location such that aligning lattice structure  102  and second 3D structure  300  together results in alignment of openings  112 . 
     Referring to  FIGS. 19 and 20 , impact attenuation liner system  100  may be used as a fully integrated system with a helmet. For example, as shown in  FIG. 19  lattice structure  102  may be fully integrated with exterior helmet shell  203 . By way of another example, as shown in  FIG. 20 , lattice structure  102  may be fully integrated with exterior helmet shell  203  and may include interior helmet shell  205 . Interior helmet shell  205  may be an intermediate stiffening layer that may be disposed between lattice structure  102  and a user&#39;s head. In some embodiments, the intermediate stiffening layer may function as one or more of exterior helmet shell  203  and interior helmet shell  205 . In one embodiment, interior helmet shell  205  may have an elastic modulus ranging from approximately 1 GPa to 200 GPa, approximately 25 GPa to 175 GPa, approximately 50 GPa to 150 GPa, or approximately 75 GPa to 125 GPa. In one embodiment, exterior helmet shell  203  may have an elastic modulus ranging from approximately 1 GPa to 200 GPa, approximately 25 GPa to 175 GPa, approximately 50 GPa to 150 GPa, or approximately 75 GPa to 125 GPa. 
     Referring to  FIG. 21 , a graph of the relative impact performance based on relative density of a material is illustrated. Specifically,  FIG. 21  illustrates the relationship between lattice density relative to bulk material from which the lattice is made, and impact performance. The area bounded between 0 to 0.4 relative density and 0.1 to 1 relative impact performance indicates the operating envelope where lattices, such as lattice structure  102 , perform optimally for impact attenuation. Current foam technology will follow the normalized performance plot shown but absolute performance will significantly underperform lattice structures as shown in  FIG. 22 . 
     Referring to  FIG. 22 , a graph illustrating acceleration over time of various impacts of lattice structure  102  compared to EPS foam is shown. As illustrated in the graph of  FIG. 22 , the impact performance after one, two, and three impacts of lattice structure  102  outperform that of the EPS foam liners as the EPS foam liners fracture after the first impact and are thus unusable for subsequent impacts. 
     Referring to  FIG. 23 , a stress-strain graph illustrating compression test results of lattice structure  102  being comprised of a 3D kagome lattice compared to other materials is shown. As shown in the stress-strain graph of  FIG. 23 , 3D kagome lattice material has a higher energy absorption capability than EPS. Further, one embodiment of the 3D kagome lattice may absorb approximately 13% more energy per unit volume than EPS foam. However, in some embodiments, the 3D kagome geometry of cells  104  of lattice structure  102  may absorb between approximately 5% to approximately 75% more, approximately 10% to approximately 50% more, approximately 10% to approximately 45% more, approximately 20% to approximately 35% more, or approximately 25% to approximately 75% more. Experimental testing of helmets with lattice structures  102  being both EPS and 3D Kagome reflect the static compression analysis of the different materials. 
     As shown in Table 1, experimental testing of one embodiment indicates an 11% decrease in linear acceleration of lattice structure  102  being comprised of a 3D kagome structure compared to EPS. Stated another way, one embodiment of the 3D kagome lattice had an 11% increase in energy absorption compared to the EPS. This increase in energy absorption translates to an increase in impact performance and allows a smaller volume of the 3D kagome lattice material to do the same energy absorbing work as a much larger volume of traditional polymeric foams, such as EPS. In use, the increase in energy absorption of the 3D kagome lattice material translates to smaller helmets that provide greater impact protection to the user while also facilitating increased airflow and comfort. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Impact performance of 3D Kagome lattice vs EPS 
               
            
           
           
               
               
            
               
                   
                 Peak Accel. [G] 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Expanded Poly Styrene 
                 171.2 
               
               
                   
                 3D Kagome Lattice 
                 152.1 
               
               
                   
                 Difference: 
                 −11% 
               
               
                   
                   
               
            
           
         
       
     
     Referring to  FIG. 24 , a stress-strain graph is shown comparing different geometries of cells  104  via static compression testing. As illustrated in the stress-strain graph of  FIG. 24 , cells  104  configured in a diamond hexagonal geometry are able to maintain a higher amount of stress compared to the geometries of tetrahedral 10×1 mm, tetrahedral 15×2 mm, cubic, or hexagonal/truncated hexagonal. Further, the area under the curve representing the diamond hexagonal geometry of cells  104  is the greatest compared to the other geometries, and thus is the toughest compared to the geometries of tetrahedral 10×1 mm, tetrahedral 15×2 mm, cubic, or hexagonal/truncated hexagonal. Therefore, the diamond hexagonal geometry of cells  104  provides better impact attenuation performance compared to the geometries of tetrahedral 10×1 mm, tetrahedral 15×2 mm, cubic, or hexagonal/truncated hexagonal. 
     Referring to  FIG. 25 , a finite element analysis of lattice structure  102  undergoing axial compression is illustrated, where cells  104  of lattice structure  102  are arranged in a tetrahedral geometry. As illustrated in  FIG. 25 , when a force is applied to the surface of lattice structure  102  in  FIG. 25 , the force dissipates through the layers of cells  104  of lattice structure  102 , where cells  104  are arranged in a tetrahedral geometry. This ensure that the force is attenuate throughout lattice structure  102 . In practice, this results in the user of impact attenuation liner system  100  feeling a force significantly less than the force of the impact event. For example, the user may hardly feel the impact event or may not sustain a head injury from the impact event due to impact attenuation liner system  100 . Further, a decrease in the force felt by the user from an impact event may also translate to lower cranial acceleration experienced by the user, which may reduce head injuries. 
     Referring to  FIG. 26 , a stress-strain graph illustrating compression test results of cells  104  arranged in an auxetic bowtie geometry with varying re-entrant angles. As illustrated in the stress-strain graph of  FIG. 26 , as the re-entrant angle increases within the range of 180 degrees to 270 degrees, there is greater energy absorption, which translates to improved impact performance. 
     Referring to  FIGS. 27 and 28 , a stress-strain graph illustrating compression test results of lattice structure  102  being comprised of MSCLCNTs and auxetic MSCLCNT, respectively, compared to other materials is shown. As shown in the stress-strain graphs of  FIGS. 27 and 28 , one embodiment of MSCLCNTs and auxetic MSCLCNTs have a higher energy absorption capability than EPS. The static compression of one embodiment of MSCLCNT and auxetic MSCLCNT structures show an improvement in energy absorption compared to EPS. Referring to  FIG. 27 , one embodiment of the MSCLCNT lattice absorbs 21% more energy per unit volume than the EPS foam. Referring to  FIG. 28 , the stress-strain graph shows a 35% increase in specific energy absorption of the MSCLCNT lattice over the EPS foam. This increase in specific energy absorption translates to an increase in impact performance and allows for a smaller volume of the MSCLCNT lattice material to do the same energy absorbing work as a significantly larger volume of traditional foams, such as EPS. In use, this translates to smaller helmets that provide greater impact protection to the user while also facilitating increased airflow and comfort. 
     Referring to  FIG. 29 , a stress-strain graph is illustrated displaying test results from static compression of lattice structures in  FIGS. 14A-14C . As illustrated in the stress-strain graph of  FIG. 29 , changing the size of cells  104  and the thickness of struts  106  allows for the tailoring of impact performances of lattice structure  102 . 
     Referring to  FIG. 30 , a graph is illustrated where second 3D structure  300  is a sheet of aluminum honeycomb (ALHC). As illustrated in  FIG. 30 , first impact performances of ALHC, and hybrids ALHC with EPP foam and ALHC with lattice structure  102  are illustrated. As illustrated by the graph of  FIG. 30 , the ALHC with EPP foam hybrid improve first impact performance. As illustrated by  FIGS. 30 and 31 , the ALHC with lattice hybrid maintains first impact performance but significantly improves second impact performance relative to both traditional foam and ALHC with EPP foam hybrid. ALHC without second 3D structure  300  was completely crushed by the first impact therefore a second impact was not practical for testing purposes. 
     Referring to  FIG. 32 , a stress-strain graph is illustrated displaying test results from static compression of various embodiments of impact attenuation liner system  100 . As illustrated in the stress-strain graph of  FIG. 32 , a hybrid embodiment comprising lattice structure  102  and second 3D structure  300  being an aluminum honeycomb sheet is able to maintain a higher amount of stress compared to lattice structure  102  alone or the aluminum honeycomb sheet alone. Further, the area under the curve representing the hybrid embodiment is the greatest, and thus is the toughest compared to lattice structure  102  alone or the aluminum honeycomb sheet alone. Therefore, the hybrid embodiment of impact attenuation liner system  100  provides better impact attenuation performance compared to lattice structure  102  alone or the aluminum honeycomb sheet alone. 
     It will be appreciated by those skilled in the art that changes could be made to the exemplary embodiments shown and described above without departing from the broad inventive concepts thereof. It is understood, therefore, that this invention is not limited to the exemplary embodiments shown and described, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the claims. For example, specific features of the exemplary embodiments may or may not be part of the claimed invention and various features of the disclosed embodiments may be combined. The words “front”, “back”, “lower” and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the impact attenuation system. Unless specifically set forth herein, the terms “a”, “an” and “the” are not limited to one element but instead should be read as meaning “at least one”. 
     It is to be understood that at least some of the figures and descriptions of the invention have been simplified to focus on elements that are relevant for a clear understanding of the invention, while eliminating, for purposes of clarity, other elements that those of ordinary skill in the art will appreciate may also comprise a portion of the invention. However, because such elements are well known in the art, and because they do not necessarily facilitate a better understanding of the invention, a description of such elements is not provided herein. 
     Further, to the extent that the methods of the present invention do not rely on the particular order of steps set forth herein, the particular order of the steps should not be construed as limitation on the claims. Any claims directed to the methods of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the steps may be varied and still remain within the spirit and scope of the present invention.