Patent Publication Number: US-10779600-B2

Title: Protective helmets having energy absorbing shells

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the 35 U.S.C. § 371 national stage application of PCT Application No. PCT/US2015/060226, filed Nov. 11, 2015, where the PCT claims priority to U.S. Provisional Application Ser. No. 62/078,007, filed Nov. 11, 2014, and U.S. Provisional Application Ser. No. 62/100,751, filed Jan. 7, 2015, and U.S. Provisional Application Ser. No. 62/147,954, filed Apr. 15, 2015, and U.S. Provisional Application Ser. No. 62/210,018, filed Aug. 26, 2015, all of which are herein incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     Sports concussion and traumatic brain injury have become important issues in both the athletic and medical communities. As an example, in recent years there has been much attention focused on the mild traumatic brain injuries (concussions) sustained by professional and amateur football players, as well as the long-term effects of such injuries. It is currently believed that repeated brain injuries such as concussions may lead to diseases later in life, such as depression, chronic traumatic encephalophathy (CTE), and amyotrophic lateral sclerosis (ALS). 
     Protective headgear, such as helmets, is used in many sports to reduce the likelihood of brain injury. Current helmet certification standards are based on testing parameters that were developed in the 1960s, which focus on the attenuation of linear impact and prevention of skull fracture. An example of a linear impact is a football player taking a direct hit to his helmet from a direction normal to the center of his helmet or head. Although the focus of headgear design has always been on attenuating such linear impacts, multiple lines of research in both animal models and biomechanics suggest that both linear impact and rotational acceleration play important roles in the pathophysiology of brain injury. Although nearly every head impact has both a linear component and a rotational component, rotational acceleration is greatest when a tangential blow is sustained. In some cases, the rotational acceleration from such blows can be substantial. For instance, a football player&#39;s facemask can act like a lever arm when impacted from the side, and can therefore apply large torsional forces to the head, which can easily result in brain trauma. 
     Although the conventional wisdom is that the components of modern protective headgear that are designed to attenuate linear impact inherently attenuate rotational acceleration, the reality is that such components are not designed for that purpose and therefore do a relatively poor job of attenuating rotational acceleration. It therefore can be appreciated that it would be desirable to have means for attenuating not only linear impacts to but also rotational accelerations of the head, so as to reduce the likelihood of brain injury. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood with reference to the following figures. Matching reference numerals designate corresponding parts throughout the figures, which are not necessarily drawn to scale. 
         FIG. 1  is a cross-sectional side view of an embodiment of a protective helmet. 
         FIG. 2A  is a front view of an embodiment of an energy absorber that can be used in the helmet of  FIG. 1 . 
         FIG. 2B  is a side view of the energy absorber of  FIG. 2A . 
         FIG. 3  is a partial detail view of an energy absorbing column of the energy absorber of  FIG. 2 . 
         FIG. 4  is a side view of a further embodiment of an energy absorber that can be used in the helmet of  FIG. 1 . 
         FIG. 5  is a side view of a compressed energy absorber illustrating bending and buckling of its energy absorbing columns. 
         FIG. 6A  is a bottom view of a protective helmet of the type shown in  FIG. 1  immediately prior to impact from another helmet. 
         FIG. 6B  is a bottom view of the protective helmet of  FIG. 6A  during an impact from another helmet. 
         FIG. 7  is a side view of a further embodiment of an energy absorber that can be used in the helmet of  FIG. 1 . 
         FIG. 8A  is a cross-sectional side view of a protective helmet incorporating an energy absorbing outer shell immediately prior to an impact. 
         FIG. 8B  is a cross-sectional side view of the protective helmet of  FIG. 8B  during the impact. 
         FIG. 9  is a rear perspective view of a first embodiment of a passive helmet tether system. 
         FIG. 10  is a rear perspective view of a second embodiment of a passive helmet tether system. 
         FIG. 11  is a rear perspective view of a third embodiment of a passive helmet tether system. 
         FIG. 12  is a rear perspective view of a first embodiment of an active helmet tether system. 
         FIG. 13  is a rear perspective view of a second embodiment of an active helmet tether system. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, current protective headgear is primarily designed to attenuate linear impact. However, it has been determined that both linear impact and rotational acceleration from torsional forces contribute to brain injury, such as concussion. Disclosed herein are energy absorbing systems that comprise means for absorbing energy from impacts to a protective helmet that minimize both translational and rotational accelerations experienced by the head of the helmet wearer. In some embodiments, these means comprise an inner liner that includes energy absorbing columns that are designed bend and buckle to attenuate both translational and rotational accelerations. In some embodiments, the means comprise energy absorbing outer shell that locally deforms upon hard impacts to absorb energy. In some embodiments, the means comprise an energy absorbing tether system that limits linear movement and rotation of the helmet upon hard impact. These various means can be used independently of each other or in conjunction with each other to protect the helmet wearer. 
     In the following disclosure, various specific embodiments are described. It is to be understood that those embodiments are example implementations of the disclosed inventions and that alternative embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure. 
     Described below are energy absorbing systems that can be incorporated into protective helmets that not only address linear forces but also tangential forces that cause the highest shear strains on the brain and the brain stem. By optimizing protection from both linear impacts and rotational acceleration, the transmission of shear force to the brain from head impacts can be reduced and so can the incidence of brain injury, such as concussion. 
     In some embodiments, a protective helmet can be provided with an energy absorbing inner liner that utilizes energy absorbing columns having various lengths and/or cross-sectional dimensions that are sandwiched between two elastomeric layers. The use of columns of varying lengths and/or cross-section dimensions enables protection against impacts over a range of energy levels. When columns of different lengths are used, low-energy impacts will activate only the tallest columns, which are connected to both layers, resulting in low translational accelerations. Higher energy impacts, however, will also activate shorter columns, which are connected to only one layer to prevent bottoming out and unacceptably high translational accelerations. The liner can be designed to provide optimal stiffness by tuning the distribution of columns to control the peak accelerations applied to the wearer&#39;s head during impact. 
     As disclosed herein, the inner liner uses controlled buckling and bending of the columns to mitigate both linear and rotational accelerations experienced by the wearer&#39;s head. Traditional column buckling is a velocity-dependent process that produces high initial forces that drop very low as the column deforms. This fundamental behavior must be overcome if columns are to become an efficient energy absorber for use in protective helmets and other protective equipment. One important advantage of precise column buckling that makes it attractive for use as a helmet liner is the directionality of its resistance forces during oblique impacts that apply rotational moments to the helmet. During this type of impact, the top of the column pushes the helmet in the direction of the applied moment while pushing the player&#39;s head in the opposite direction. An advance of the disclosed liners is that linear impact dissipation can be optimized without adversely affecting the rotational behavior of the columns. 
     A column buckles when the eccentricity, or misalignment, over its length produces a bending moment in the center of the column that overcomes its bending stiffness. Hence, dynamic buckling utilizes forces from axial loading to push the middle of the column laterally. Unless the misalignment is very large, these lateral forces are small relative to the mass of the column. As a result, the column produces a large inertial impulse while dynamic buckling is initiated. This type of impulse can produce dangerous acceleration forces on the player&#39;s head. In some embodiments, the disclosed inner liner overcomes this problem using multiple features. First, the columns of the inner liner can be made of an elastomeric material that provides some level of axial compression during the period in which buckling is initiated to compensate for the magnitude of the inertial spike. 
     Second, the columns can be eccentric relative to the layers between which they lie to reduce the load required to initiate column buckling. These eccentricities take the form of a misalignment of the column ends from the normal direction of the layers so that the columns will have a moment applied upon the onset of loading. This misalignment also results in additional stroke because it can cause the column halves to fold beside themselves as they collapse rather than stacking on top of themselves. Furthermore, the curvature of the inner liner due to the curved nature of the helmet results in further eccentricity in the columns because it is likely that only a small portion of the activated columns will be normal to the impact direction, thus any inertial forces coming from these columns would be small in comparison to the overall forces generated by the sum of activated columns. 
     Third, as mentioned above, the column lengths can be varied. Varying column lengths accomplishes two goals. Firstly, it spreads out the inertial impulse to eliminate the high inertial spike during the early stages of impact. Secondly, it enables the liner stiffness to be increased with higher deflections. 
       FIG. 1  illustrates an example embodiment of a protective helmet  10  that is designed to attenuate both linear impact and rotational accelerations. The helmet  10  shown in  FIG. 1  is generally configured as an American football helmet. Although that particular configuration is shown in the figure and other figures of this disclosure, it is to be understood that a football helmet is shown for purposes of example only and is merely representational of an example protective helmet. Therefore, the helmet need not be limited to use in football. Other sports applications include baseball and softball batting helmets, lacrosse helmets, hockey helmets, ski helmets, bicycling and motorcycle helmets, and racecar helmets. Furthermore, the helmet need not even be used in sports. For example, the helmet could be designed as a construction or military helmet. It is also noted that the principles described herein can be extended to protective equipment other than helmets. For example, features described below can be incorporated into protective pads or armor, such as shoulder pads, hip pads, thigh guards, shin guards, cleats, and other protective equipment in which energy absorption could be used to protect the wearer. 
     With continued reference to  FIG. 1 , the helmet  10  generally includes an outer shell  12  and an inner liner  14 . In the illustrated embodiment, the shell  12  is shaped and configured to surround the wearer&#39;s head with the exception of the face. Accordingly, the shell  12 , when worn, extends from a point near the base of the wearer&#39;s skull to a point near the wearer&#39;s brow, and extends from a point near the rear of one side of the wearer&#39;s jaw to a point near the rear of the other side of the wearer&#39;s jaw. In some embodiments, the shell  12  is unitarily formed from a generally rigid material, such as a polymer or metal material. Example materials are described below in relation to  FIGS. 9A and 9B . 
     Irrespective of the material used to construct the shell  12 , the shell includes an outer surface  16  and an inner surface  18 . In some embodiments, the shell  12  can further include one or more ear openings  20  that extend through the shell from the outer surface  16  to the inner surface  18 . The ear openings  20  are provided on each side of the shell  12  in a position in which they align with the wearer&#39;s ears when the helmet  10  is donned. Notably, the shell  12  can include other openings that serve one or more purposes, such as providing airflow to the wearer&#39;s head. 
     As is further shown in  FIG. 1 , a facemask  22  can be secured to the front of the helmet  10  to protect the face of the wearer. In some embodiments, the facemask  22  can comprise one or more rod-like segments that together form a protective lattice or screen. When used, the facemask  22  can, for example, be attached to the helmet  10  at points that align with the forehead and jaw of the wearer when the helmet is worn. The facemask  22  can be attached to the helmet  10  using screws (not shown) that thread into the shell  12  or into fastening elements (not shown) that are attached to the helmet. Although a particular facemask configuration is shown in the figures, alternative configurations are possible. Moreover, the facemask  22  can be replaced with a face shield or other protective element, if desired. 
     The inner liner  14  generally comprises one or more pads that sit between the shell  12  and the wearer&#39;s head when the helmet  10  is worn. In some embodiments, each of the pads is removable from the helmet. For instance, the pads can be configured to releasably attach to the inside surface  18  of the helmet shell  16  with snap, T-nut, or hook-and-loop fasteners. In the illustrated embodiment, the pads include a top pad  24 , multiple lateral pads  26 ,  28 , and  30 , a front pad  32 , a rear pad  34 , and jaw pads  36 . The top pad  24  is adapted to protect the top of the wearer&#39;s head. In the illustrated embodiment, the top pad  24  is elongated in a direction that extends along the sagittal plane of the wearer so as to extend from a rear top portion of the head to a front top portion of the head. The top pad  24  is further curved to generally follow the curvature of the wearer&#39;s head. Accordingly, the top pad  24  forms a concave inner surface that is adapted to contact the wearer&#39;s head. 
     The lateral pads  26 - 30  are adapted to protect the sides of the wearer&#39;s head. The lateral pads  26 - 30  extend from the edges of the wearer&#39;s face to points behind (and above) the user&#39;s ears. Like the top pad  24 , the lateral pads  26 - 30  are curved to follow the curvature of the shell  12  and the wearer&#39;s head. Accordingly, the lateral pads  26 - 30  form concave inner surfaces that are adapted to contact the wearer&#39;s head. 
     The front pad  32  is positioned within the outer shell  12  so as to protect the forehead of the wearer. Like the other pads, the front pad  32  is curved to follow the curvature of the wearer&#39;s head. The forward pads  30  therefore form concave inner surfaces that are adapted to contact the wearer. 
     The rear pad  34  is adapted to protect the rear of the wearer&#39;s head. The rear pad  28  is also curved to follow the curvature of the wearer&#39;s head and forms a concave inner surface that is adapted to contact the wearer&#39;s head. 
     The jaw pads  36  are adapted to protect the jaw of the wearer. As with the other pads, the jaw pad  36  can curved to follow the curvature of the wearer&#39;s head and forms a concave inner surface that is adapted to contact the wearer&#39;s head. 
     Several or all of the above-described pads can be of similar construction. In some embodiments, each of the pads comprise an outer energy absorber  40  that is adapted to absorb translational and rotational energy from helmet impacts and an inner cushion  42  that is adapted to provide comfort to the wearer&#39;s head. The energy absorbers  40  releasably attach to the inner surface  18  of the shell  12 . Details about the construction of the energy absorbers  40  are provided below in relation to  FIGS. 2-8 . It suffices to say at this point, however, that the energy absorbers  40  include energy absorbing columns  44  that dissipate translational and rotational accelerations. 
     The inner cushions  42  of the pads contact or are at least adjacent to the wearer&#39;s head and/or face when the helmet  10  is donned. The cushions  42  can have any construction that is comfortable for the wearer. In some embodiments, the cushions  42  are foam cushions. In other embodiments, the cushions  42  are air bladder cushions. 
       FIGS. 2A and 2B  illustrate an example energy absorber  50  that can be used in a pad that forms part of a helmet liner, such as the inner liner  14  shown in  FIG. 1 . As shown in  FIGS. 2A and 2B , the energy absorber  50  generally comprises a first or inner layer  52 , an opposed second or outer layer  54 , and a plurality of energy absorbing columns  56  that are provided between the layers, which can bend and buckle to absorb energy. As illustrated in the figures, the inner and outer layers  52 ,  54  comprise thin, generally planar members that are curved to conform to the curvature of the human head and the outer shell  12 . In some embodiments, the layers  52 ,  54  have similar curvatures. The inner layer  52  comprises an inner surface  58  that faces the outer layer  54  and an outer surface  60  that faces the wearer&#39;s head and provides a surface to which an inner cushion  42  can be attached. The outer layer  52  comprises an inner surface  62  that faces the inner layer  52  and an outer layer  64  that can be attached to the inner surface  18  of the outer shell  12 . 
     The energy absorbing columns  56  can comprise elongated cylindrical members that are substantially perpendicular to the inner and outer layers  52 ,  54 . As is apparent in  FIGS. 2A and 2B , the columns  56  can have various lengths or heights. Relatively long columns  66  connect the inner and outer layers  52 ,  54 . Such columns  66  are attached at a proximal end (nearest the wearer&#39;s head) to the inner layer  52  and are attached at a distal end (nearest the shell  12 ) to the outer layer  54 . Shorter columns  68  are only attached to one of the layers  52 ,  54 . In the illustrated embodiment, the proximal ends of the shorter columns  68  are attached to the inner layer  52  while the distal ends of those columns are free ends. In addition to the lengths, the cross-sectional dimensions of the columns  56  can be varied. 
     In some embodiments, the energy absorber  50  can comprise columns of several different lengths. For example, the energy absorber  50  could incorporate columns  56  of 2, 3, 4, 5, or more different lengths, in which case the energy absorber provides multiple stages of energy dissipation. In such cases, relative mild impacts may only affect the longest columns  56  (i.e., the first stage of the energy absorber  50 ) while stronger impacts may affect columns of shorter lengths (i.e., other stages of the energy absorber). This multi-stage approach provides increased stiffness as the deflection of the energy absorber  50  increases, as well as reduction in the inertial spike that comes prior to the onset of buckling in the columns  56 . 
     An important measure of energy absorber efficiency is the achievable absorber deflection divided by its original length. All multi-impact energy absorbers have a maximum useable deflection beyond which the stiffness becomes excessive. This difference is normally referred to as the stack-up distance. In some embodiments, the columns  56  are arranged within the energy absorber  50  in a manner that minimizes interaction between adjacent columns to minimize the possibility of the columns stacking on top of one another as the energy absorber compresses. 
     The thicknesses of the inner and outer layers  52 ,  54 , the lengths and cross-sectional dimensions of the columns  56 , and the ratio of columns attached to both layers versus attached to only one layer can be tailored to achieve a desired load capacity for the energy absorber  50  and the pad in which it will be used. Thicker layers  52 ,  54  will increase the load capacity of the columns  56  because of the stiffened end conditions, thereby enabling the use of thinner columns. However, thicker layers  52 ,  54  will also increase the overall mass of the inner liner  14  because the layers represent the highest volume of material in the system while also reducing the useable stroke. Thus, it is important to optimize the energy absorbers  50  to provide the desired outcome at each location within the helmet  10 , taking into account factors such as available stroke, coverage area in the impact location, frequency of impact in the protected location, and overall liner mass. For instance, the front pad  32  ( FIG. 1 ) may have larger diameter columns and a higher ratio of attached columns than other pads in the liner  14  to increase the pad stiffness due to the inherent weakness in the outer shell at that location and the increased need for protection in the frontal region due to the increased likelihood of impacts in that location. 
     In some embodiments, the outer layer  54  has a thickness of approximately 0.5 to 3 mm and may contain holes for fasteners or ventilation. In some embodiments, the inner layer  52  has a thickness of approximately 0.5 to 2.5 mm. In some embodiments, the energy absorbing columns  56  that are attached to both the inner and outer layers  52 ,  54  have lengths of approximately 18 to 65 mm and cross-sectional dimensions (e.g., diameters) of approximately 3 to 7 mm, while the columns that are attached to only one of the layers have lengths of approximately 8 to 55 mm and cross-sectional dimensions (e.g., diameters) of approximately 2 to 6 mm. In some embodiments, the fraction of columns  56  that are connected to both layers  52 ,  54  is approximately 15 to 40%, but can be increased to as much as 100% if the pad will undergo consistent loading and does not need to provide protection against a variety of impact conditions. While the columns  56  are illustrated in  FIGS. 2A and 2B  as having constant cross-sectional dimensions along their lengths, it is noted that these dimensions can vary along the lengths of the columns. For example, one or more columns  56  can have a larger cross-section at its base than at other points along its length. 
     In some embodiments, the columns  56  can be slightly eccentric to reduce the magnitude of the inertial spike that occurs upon impact. This eccentricity can come in the form of an angling of the columns  56  from the direction normal to the inner surface of the inner and/or outer layers  52 ,  54 .  FIG. 3  illustrates an example of this form of eccentricity. As shown in this figure, a column  56  is offset from the normal direction of the inner surface  58  of the inner liner  52  by an angle θ, which, for example, can be an acute angle up to approximately 15 degrees. Other possible forms of eccentricities include a predefined curve or kink manufactured into the columns.  FIG. 4  illustrates an example of this. In this figure, an energy absorber  70  having an inner layer  72 , and outer layer  74 , and a plurality energy absorbing columns  76 . Some of the columns  78  comprise a medial kink  80  that facilitates buckling. 
     Although the energy absorbing columns  56  have been described as comprising cylindrical members, which typically comprise circular cross-sections, it is noted that other cross-sectional geometries are possible. For example, the columns  56  can have an elliptical, polygonal, or other non-uniform cross-section. In addition, the columns  56  can have a twisted configuration in which the cross-section changes along the length of the columns. For example, if the column  56  had an elliptical cross-section, the orientation of the ellipse can rotate as the length of the column is traversed to form a twisted shape. Such a shape can force the columns  56  to twist while buckling, which both increases the energy dissipation rate in the later stages of collapse and forces the top half of the column to land beside the bottom half, which reduces the stack-up distance and maximizes available compression in the energy absorber. 
     Each of the inner liner  52 , outer liner  54 , and the energy absorbing columns  56  can be made of an elastomeric material. In some embodiments, these components are made of a thermoplastic elastomer (TPE), such as thermoplastic polyurethane (TPU). BASF Elastollan 1260D U is one commercial example of a TPU. Other suitable TPEs include copolyamides (TPAs), copolyesters (TPCs), polyolefin elastomers (TPOs), and polystyrene thermoplastic elastomers (TPSs). 
     TPU may be preferable for construction of the energy absorbers for a variety of reasons. This material can be made to be soft enough to provide consistent initiation of the buckling process, has a rapid relaxation time to assure high rates of energy dissipation, and has proven to be both durable and tolerant of large temperature variations. Both the viscoelastic nature of TPU and the sensitivity of column buckling to impact speed enable the energy absorbing columns to absorb greater amounts of energy as impact speed increases. This is important for helmets that must attenuate high speed impacts and simultaneously provide optimum protection of helmet wearers who experience large numbers of low speed impacts. Furthermore, TPU is a low cost, versatile, and commercially available material. It offers a long list of performance characteristics that are desirable in an environment involving energy management, such as athletic equipment and military applications. For instance, all grades of unreinforced TPU have high elasticity with elongation to break values of 300 to 1000%, tensile strength to yield of 10 to 45 MPa, hardness values of 52 to 98 on the Shore A scale and 22 to 95 on the Shore D scale, and material densities in the range of 1.05 to 1.53 g/cc. 
     TPU also has a low glass transition temperature of −69 to −17° C., meaning that it will retain its elastic properties over the a broad range of temperatures, such as that in which sports are played. In addition, TPU provides excellent abrasion resistance, impact strength, weather resistance, and antimicrobial properties. Additionally, TPU can be modified to suit a particular application by adding fillers, colorants, or stabilizers. One desirable performance characteristic is that TPU can be optimized to achieve effective damping with optimal rebound speed (e.g. short relaxation time). Finally, TPU provides fabrication flexibility, can be injection molded, and can be bonded or welded though a variety of processes. 
     One potential problem associated with varying column length is the possibility that shorter columns will slide, resulting in a bending mode of failure rather than buckling. The bending failure mode greatly reduces energy dissipation rates. To combat this issue, a texture can be added to the inner surfaces of the inner and outer layers as well as the outer surfaces of the columns. Such a texture is schematically illustrated in  FIG. 3  as texture  82  and can cause the columns to lock in place once contact is made with other columns and/or the inner and/or outer layers. In some embodiments, the texture  82  can comprise a rough surface that is formed on the layers/columns during energy absorber fabrication (e.g., injection molding). In other embodiments, this texture  82  can comprise a geometric (e.g., metal) mesh that is integrated into the surfaces during fabrication (e.g., injection molding). 
     In some embodiments, the energy absorbers can be manufactured in two parts. The first part can comprise the outer layer and all the necessary features for attaching the energy absorber to the outer shell  12 , while the second part can comprise the inner layer and the columns that are connected thereto. The two parts can be produced through injection molding or another commercial manufacturing process. Once formed, the two parts can be bonded together through use of welding or an adhesive. Alternatively, the layers and columns could each be manufactured as separate parts. In such a case, the columns can comprise notches at their ends that enable them to be snapped into place into pre-formed holes in the inner and outer layers. The columns could then be bonded to the layers through welding or adhesion. 
     As discussed above, the energy absorbers are designed to deform upon impact to dissipate energy.  FIG. 5  illustrates such deformation. As shown in this figure, an energy absorber  90  comprises an inner layer  92 , an outer layer  94 , and a plurality of energy absorbing columns  96 . The outer layer  94  has been pushed in toward the inner layer  92  because of an external (downward) force and, as a result, the columns  96  of the energy absorber  90  have bent and/or buckled under this force, thereby dissipating energy. 
       FIGS. 6A and 6B  illustrate operation of the energy absorbers when incorporated into a protective helmet  100 . As is apparent in this figures, the helmet  100  includes an inner liner  102  comprising multiple pads  104  having energy absorbing columns  106 .  FIG. 6A  shows the helmet  100  prior to impact. In this state, the helmet  100  is centered on the wearer&#39;s head.  FIG. 6B  shows the helmet  100  upon receiving a tangential impact from another helmet  110 . As can be appreciated from this figure, the energy absorbing columns  106  near the point of impact have deformed to absorb the energy of the impact. In addition, the helmet  100  has rotated relative to the wearer&#39;s head to dissipate the rotational force imparted by the helmet  110  instead of delivering it directly to the wearer&#39;s head. In such a case, the wearer&#39;s head can remain relatively stationary, at least in terms of rotation, while the helmet  100  rotates. Once the force is removed, however, the energy absorbing columns  106  can return the helmet  100  to its original orientation on the head. 
     It is noted that the energy absorbers can comprise other components besides columns between their inner and outer layers. For example,  FIG. 7  illustrates an energy absorber  120  comprising an inner layer  122 , an outer layer  124 , and a plurality of energy absorbing columns  126 . Provided between the inner and outer layers  122 ,  124  in a free space between the columns  126  is a supplemental energy absorbing member  128 . The member  128  can comprise a foam element or an air bladder element that provides increased energy dissipation where needed, such as near the front of a helmet. In the illustrated embodiment, the member  128  is generally elliptical with its long axis extending along the normal directions of the inner and outer layers  122 ,  124 . 
     The basic premise of impact energy management is to optimize energy absorption in each component of a system. So, while the inner liners described above can be used to absorb energy, other components of the helmet can likewise be designed to absorb energy. Once such component is the outer shell of the helmet. 
     The shell material in most commercial football helmets is made of polycarbonate (PC) alloys or acrylonitrile butadiene styrene (ABS) plastic in thicknesses ranging from 3 to 4 mm. While these materials have high impact resistance, they exhibit a highly elastic response to impacts. Therefore, the energy absorbed by the shell material is minimal. Greater energy could be absorbed, however, if the shell was made of a deformable, energy absorbing material. 
     With reference back to  FIG. 1 , the outer shell  12  of the protective helmet  10  can be made of such a material. In some embodiments, the shell  12  is made of a polyethylene (PE) composition, such as high density polyethylene (HDPE). HDPE is a class of thermoplastic polymers that incorporate long chains of polyethylene mers with molecular weights in the range of approximately 100,000 to 3,000,000. HDPE is a suitable replacement for the elastic PC or ABS materials used in current football helmets, whether or not the helmets include an inner liner of the nature described above. 
     A protective helmet must meet the requirements under its working conditions. Football helmets are required to absorb energy, resist gouging, fatigue, and creep, operate in extreme ambient temperatures (−12° C. and 52° C.), accept paint and dyes, and be readily manufacturable. HDPE is a low-cost, versatile, and commercially available material. It offers a long list of performance characteristics that are desirable in an environment involving energy management, such as athletic equipment and military applications. Specific parameters of a suitable HDPE composition include the following: 
     Tensile Strength to Yield: ˜25-31 MPa 
     Rockwell Hardness (Shore D): ˜55-75 
     Elongation to Break: ˜900-1300% 
     Flexural Modulus: ˜1000-1500 MPa 
     Melt Flow Index: ˜5 to 8 g/10 minutes 
     Additionally, HDPE offers a lower density (0.95 g/cm 3 ) when compared to conventional PC (1.2 g/cm 3 ) or ABS (1.05 g/cm 3 ) formulations. A lower density can be advantageous by providing lower weight to the wearer or a thicker geometry for the same weight. In some embodiments, the shell has a thickness of approximately 2.4 to 4 mm. HDPE also offers a low glass transition temperature of −70° C. to −80° C. 
     It is important to note that energy absorbing outer shells can be too soft. If the local deformation of the shell is too high, impacting helmets can become entangled or interlocked such that high forces can be generated parallel to the surface of the shell. This type of loading produces high rotational accelerations in the helmet. High rotational accelerations are also produced when hard accessories, such as fasteners, gouge into the surface of the shell material. Typical HDPE&#39;s may not offer sufficient slip (low friction) or abrasion resistance to counter mechanical interlock as described above. The polyethylene of the HPDE can be compounded with one or more additives to combat these issues. Such additives can include a processing stabilizer that protects the polymer at high temperatures, a heat stabilizer that inhibits degradation of the end product, a slip agent that reduces friction between surfaces (i.e., increases slip), and an ultraviolet stabilizer that inhibits environmental degradation. ADDCOMP ADD-VANCE 148 and 796 are two example commercial multi-functional additives that could be used. A range of approximately 1 to 8% by weight of the additives can be compounded with the PE base in the composition. 
     Football helmet shells are often dyed in a similar color in which they will be painted or coated after manufacturing. The HDPE composition described herein can readily accept up to 3.5% by weight, which is suitable for the range of colors on the market. Once a HDPE helmet has been manufactured, its surface energy can be increased by 2-5 dynes/cm through corona treatment or other processes to impart wettability, which enables paint particles to adhere to the helmet. 
       FIGS. 8A and 8B  illustrate the effect of constructing the outer shell  12  of the protective helmet  10  out of an energy absorbing material, such as HDPE.  FIG. 8A  shows the helmet  10  prior to impact.  FIG. 8B  shows the helmet  10  upon receiving an impact to the top of the shell  12 . As can be appreciated from this figure, the shell  12  has locally deformed at the point of impact and thereby dissipates some of the impact force. As before, the energy absorbing columns  56  have also deformed near the point of impact. 
     Another way in which energy imparted to a protective helmet can be absorbed is tethering of the helmet. Tethering a helmet involves attaching one or more flexible tethers between the wearer&#39;s helmet and an object securely anchored to the wearer&#39;s body. Such tethers greatly increase the helmet&#39;s resistance to motion by firmly securing the helmet to the wearer&#39;s upper body. If properly designed, tethers can reduce peak accelerations by as much as 80 percent by raising the effective mass of the head and helmet from approximately 13 lbs. to over 70 lbs. 
     An effective helmet tether system can incorporate the following features: A) enables the head/neck complex to freely rotate and posterior flex when not being struck; B) provides resistance to acceleration when helmet is struck; C) cannot apply excessive force to helmet; D) cannot obstruct players vision; and E) easily attaches and detaches from the helmet. 
     A helmet tether system can be designed as a passive or an active system. Passive tether systems are designed to resist extreme motions, such as excessive deflection or velocity. Active tether systems, however, incorporate sensors that sense when an impact has either begun or is about to occur and includes actuation mechanisms that actively respond to such sensed conditions. 
       FIG. 9  illustrates a first embodiment of a passive helmet tether system  140  that links a protective helmet  142  to an article  144  (shoulder pads in this example) worn by the helmet wearer. The system  140  includes multiple tethers  146  that extend between the helmet  142  and the shoulder pads  144 . More particularly, a first or upper end of each tether is attached to the interior or exterior of the outer shell  148  of the helmet  142 , and a second or lower end of each tether is attached to the outer shells  150  of the shoulder pads  144 . In the illustrated embodiment, the lower ends of the tethers  146  are attached to and wrapped around spools  152  that are fixedly mounted to the shoulder pad outer shells  150 . The spools  152  are free to rotate to enable lengthening of the tether  146  to enable turning of the head until the maximum length has been reached, at which point the tether limits further helmet movement. By limiting the degree to which the helmet  142  can be move relative to the body, the tether system  140  limits the forces that can be transmitted to the wearer&#39;s head. In some embodiments, the tethers  146  comprise high-strength, flexible, inelastic cords. Example inelastic cord materials include steel, nylon, polypropylene, and polyethylene. 
     In some embodiments, the spools  152  can comprise internal torsion springs (not shown) that take up any slack that forms in the tethers  146 . In other embodiments, the spools  152  can further comprise internal locking mechanisms (not shown), such as centrifugal brakes, that automatically lock the angular orientations of the spools, and therefore halt lengthening of the tethers  146 , upon a threshold angular acceleration being reached. The threshold angular acceleration can be one that is associated with movements of the helmet  142  that exceed the speed with which the wearer can move his or own head, which are indicative of a helmet impact. 
       FIG. 10  illustrates a second embodiment of a passive helmet tether system  160  that links a protective helmet  162  to an article  164  (shoulder pads) worn by the helmet wearer. The system  160  is similar to the system  140  in that a first or upper end of each tether  166  is attached to the outer shell  168  of the helmet  162 , and a second or lower end of each tether is attached to the outer shells  170  of the shoulder pads  164 . However, this embodiment comprises no spools. Instead, the tethers  146  comprise flexible, elastic cords that resist movement as they are stretched. Example elastic cord materials include elastomers such as synthetic rubber, and TPU, and fiber-reinforced elastomers. 
       FIG. 11  illustrates a third embodiment of a passive helmet tether system  180  that links a protective helmet  182  to an article  184  (shoulder pads) worn by the helmet wearer. This system  180  is also similar to the system  140  shown in  FIG. 9 . Accordingly, the system  180  comprises multiple tethers  186  having a first or upper end attached to the outer shell  188  of the helmet  182 , and a second or lower end attached to the shoulder pad outer shells  190 . In this embodiment, however, an extension mechanism  192  is provided along each tether  186 . Lengths of the tethers  186  are wound around an internal spool (not shown) within the extension mechanism  192 , which also includes an internal torsion spring (not shown) that takes up slack. The extension mechanism  192  can further include a locking mechanism (not shown) that automatically locks the angular orientation of the internal spool, and therefore halts lengthening of the tether  186 , upon a threshold angular acceleration being reached. 
       FIG. 12  illustrates a first embodiment of an active helmet tether system  200  that links a protective helmet  202  to an article  204  (shoulder pads) worn by the helmet wearer. The system  200  includes multiple tethers  206  that extend between the helmet  202  and the shoulder pads  204 . More particularly, a first or upper end of each tether is attached to the interior or exterior of the outer shell  208  of the helmet  202 , and a second or lower end of each tether is attached to and wrapped around spools  210  that are releasably mounted to the shoulder pad outer shells  212 . The system  200  further comprises pre-tensioned springs  214  that are attached at one end to a spool  210  and attached at the other end to the shoulder pad outer shell  212 . In addition, the system  200  includes an impact sensor  216 , such as an accelerometer, that is mounted to the helmet  202  or the wearer&#39;s head. The impact sensor  216  is in communication with a central controller  218  that is adapted to actuate the spools  210 . 
     During use of the system  200 , the spools  210  are free to rotate to enable lengthening of the tether  206  to enable turning of the head until an impact that exceeds a force threshold is sensed by the sensor  216 . When such an impact occurs, the central controller  218  activates actuation mechanisms (not shown) associated with each spool  210  that halt further rotation of the spools and decouple the spools from the shoulder pads  204 . When this occurs, the tethers  206  will no longer lengthen and the springs  214  will pull down on the spools  210  to remove slack from the tethers. 
       FIG. 13  illustrates a second embodiment of an active helmet tether system  220  that links a protective helmet  222  to an article  224  (shoulder pads) worn by the helmet wearer. The system  220  includes multiple inelastic tethers  226  having a first or upper end attached to the outer shell  228  of the helmet  222 , and a second or lower end attached to and wrapped around spools  230  that are fixedly mounted to the shoulder pad outer shells  232 . 
     The system  220  further comprises multiple sensors  234 , such as accelerometers, that are mounted at multiple points on the helmet wearer&#39;s body (multiple locations of the shoulder pads  224  in the example of  FIG. 13 ). The data collected by the sensors  234  can be provided to a central controller  236  that executes a control algorithm that determines from wearer&#39;s body posture and motion that a helmet impact is likely to occur. In such a case, the central controller  236  can activate pre-tensioning mechanisms (not shown) associated with each spool  232  that wind the tethers  226  onto the spools  230  to prepare the head for an impending impact. In some embodiments, the control algorithm comprises a heuristic algorithm that adapts to the individual helmet wearer over time. In the case of a sports helmet, data from both practice and live game play can be used to refine the heuristic algorithm. In some embodiments, the pre-tensioning mechanisms can comprise electro-active materials used to form the tethers  226 , such as dielectric elastomers. Signaling of such electro-active tethers could be used to induce changes in stiffness and strain.