Patent ID: 12240401

DETAILED DESCRIPTION

We describe here impact protection devices formed from materials that exhibit a negative Poisson's ratio (“NPR materials”). NPR materials are durable and capable of attenuating or absorbing energy, such as energy from impact. NPR materials in impact protection devices, such as airbags or impact vests, attenuate energy of an impact, thereby reducing the force of the impact felt by a human and preventing injury. NPR materials absorb energy better than non-NPR materials (e.g., positive Poisson's ratio (“PPR”) materials), and lower the initial contact stress of the device with a person reducing the occurrence of sudden contact injury. Impact protection devices such as airbags and impact vests can be formed of NPR materials alone or in conjunction with PPR materials. In some implementations, composite materials that include both NPR and PPR materials are used.

NPR materials in airbags and other impact protection devices can provide enhanced strength during impact compared to traditional materials and can absorb energy from impacts to prevent injury to humans.

Referring toFIG.1A, a vehicle interior100includes a first airbag102and a second airbag104. The first airbag102is located in the steering column106and/or front panel108of a steering wheel110. The second airbag is located in a receptacle112of the dashboard114. The first airbag102is designed to deploy from a constricted and inert position to an expanded state in response to an impact to the vehicle to provide protection to a passenger in the front driver's seat116of the vehicle. The second airbag104is similarly designed to deploy to an expanded state in response to an impact to the vehicle to provide protection to a passenger in the passenger's seat118of the vehicle. When the vehicle is impacted in a crash, the passengers may be thrown forward. The first and second airbags102,104prevent injury to the passengers of the vehicle by providing a cushion between the passenger and the hard surfaces of the vehicle like the dashboard114and steering wheel110, e.g., by absorbing some of the impact force of the passenger being thrown against the front structures of the vehicle.

The first airbag102and second airbag104can be deployed based on the sensing of an impact at the first airbag102and second airbag104, respectively, or based on sensing of an impact elsewhere on or in the vehicle, such as at the front or rear bumper (not shown). Data from an impact sensor (e.g., as shown inFIG.1B) can be relayed to a computer that is communicatively coupled to the first airbag102and second airbag104to prompt the airbags to deploy and inflate. For example, the sensor can be communicatively coupled to an inflation mechanism that fills the first airbag102and second airbag104with a fluid or gas when activated. The inflation mechanism or source (also referred to herein as an expulsion source or mechanism) can be a reservoir filled with a gas or liquid, one or more chemicals that expand when combined with air or with each other, a fluid source, a chemical explosive, a pump, or another mechanism that operates to fill the first airbag102and second airbag104with the fluid or gas in response to an activation signal. The filling of the first airbag102and second airbag104causes the first airbag102and second airbag104to expand from a first state out of the compartment in the steering column106and/or front panel108and receptacle112of the dashboard114, respectively, and into a second deployed state.

While only two airbags are shown in the figure, additional airbags can be included in a vehicle at various positions. The term “airbag” is sometimes used to refer to impact protection devices that are deployed to protect the head or upper body portion of a passenger in a vehicle. As used herein, the term airbag is not so limited and includes impact protection devices positioned to provide protection to a passenger where the passenger's body makes contact in the event of a crash, such as the steering wheel, steering column, head set, dashboard, side support structures, head rests, door panels, and similar positions in rear seats of vehicles to provide further protection to passengers of the vehicle. The airbags can protect not only the passenger's head and upper body, but also the torso, side, or other parts of the passenger's body. The use of NPR materials in airbags positioned throughout a vehicle provide support and impact absorption to a passenger in the case of a crash.

Airbags generally operate by quickly inflating and then releasing when an impact is detected. The quick inflation of the airbag protects the person that is seated in front of the airbag from being thrown against structures in the car, such as the steering wheel or dashboard. The first and second airbags102,104are formed using negative Poisson's ratio (“NPR”) materials.

NPR materials have material characteristics and behaviors that are the opposite of PPR materials under application of mechanical, electromagnetic, light, thermal, and energy forces. An object with a negative Poisson's ratio is an object that has a Poisson's ratio that is less than zero, such that when the object experiences a positive strain along one axis (e.g., when the object is stretched), the strain in the object along the two perpendicular axes is also positive (e.g., the object expands in cross-section). Conversely, when the object experiences a negative strain along one axis (e.g., when the object is compressed), the strain in the object along a perpendicular axis is also negative (e.g., the object compresses along the perpendicular axis). By contrast, an object with a positive Poisson's ratio (PPR) has a Poisson's ratio that is greater than zero. When an object with a PPR experiences a positive strain along one axis (e.g., when the object is stretched), the strain in the object along the two perpendicular axes is negative (e.g., the object compresses in cross-section), and vice versa. NPR materials provide enhanced energy absorption compared to PPR material when used in airbags.

NPR materials can be combined with PPR materials to form “Boolean-Park” materials (“B-P” materials). Because of the NPR material behavior, the use of B-P materials can provide better absorption of forces on the airbag to reduce the forces felt by the passengers of a vehicle.

Referring now toFIG.1B, an airbag system101includes an airbag102, propellant or expansion canister120, computer122, and sensor123. As described above, one or more sensors123on or in the vehicle can detect a crash or impact has occurred and transmit a signal to the computer122. The sensor123can be an electromagnetic sensor, acceleration sensor, load sensor, or other type of sensor. The sensors123can provide a signal to the computer122representing impact conditions. Responsive to receiving the signal, the computer122triggers the canister120to inflate or expand the airbag102. The canister120can include gas or fluid, chemicals that combine to produce an expanding gas or fluid, or other inflating compounds and mechanisms. When the canister120is triggered, the airbag102is expanded from a non-deployed state (e.g., the airbag is folded within a compartment of the dashboard or steering wheel) to a deployed state by quickly filling the airbag102with a fluid, gas, or other compound. The computer can trigger the inflation of the airbag102under pre-programmed conditions.

The airbag102is a shell formed at least in part from an NPR material, which is filled with the fluid or gas when the airbag102is inflated and deployed. Cross section124shows a section of the airbag102shell made from a textile formed from an NPR material128. In an embodiment, the textile is formed from an NPR material128, the NPR material128having a Poisson's Ration of between 0 and −1. In another embodiment, the textile is formed from a combination of an NPR material128and a PPR material130. In some embodiments, the textile is formed from a combination of fibers of the NPR material128and fibers of the PPR material130woven together. The interior129of the airbag102is formed from an NPR material, a PPR material, or a combination of NPR and PPR materials. In some implementations, the interior129is not filled with any material when the airbag102is in the deflated state, and is filled with a fluid or gas to inflate the airbag102responsive to sensing of an impact. In some implementations, the interior129is formed from a cellular matrix incorporating an NPR material, and the cells of the cellular matrix are permeated by the fluid or gas to expand the airbag102. In some implementations, the cellular matrix incorporates both an NPR material and a PPR material. For example, the cellular matrix can have the re-entrant cell structure described below inFIG.3A.

Cross section126shows a second embodiment of the airbag102shell made from a printed material formed from an NPR material132. In an embodiment, the printed material is formed from an NPR material132, the NPR material132having a Poisson's Ratio of between 0 and −1. In another embodiment, the printed material is formed from a combination of an NPR material132and a PPR material134. In some embodiments, the printed material is formed from a combination of the NPR material132and the PPR material134distributed as layers or with the NPR material132embedded within the PPR material134. The printed material forming the airbag102shell can include alternating layers of NPR materials132and PPR materials134. The alternating layers of NPR materials132and PPR materials134can be arranged so that the layers are perpendicular or angled to an outside edge of the airbag102shell, or in some embodiments the alternating layers of NPR materials132and PPR materials134can be arranged so that an NPR material132forms the outer layer at the airbag102shell, with alternating layers of PPR material134and NPR material132beneath the outer layer. The NPR material132outer layer absorbs energy from an impact of a passenger against the airbag102shell, and further energy that is not absorbed can be propagated into the underlying layers which also absorb components of the force of the impact. The interior131of the airbag102is formed from an NPR material, a PPR material, or a combination of NPR and PPR materials. In some implementations, the interior131is not filled with any material, and is filled with a fluid or gas to inflate the airbag102. In some implementations, the interior131is formed from a cellular matrix incorporating an NPR material, and the cells of the cellular matrix are permeated by the fluid or gas to expand the airbag102. In some implementations, the cellular matrix incorporates an NPR material and a PPR material.

The NPR materials can be created by arranging nano- or micro-spheres or sponges of positive PPR material in a particular configuration so that the spheres or sponges behave as NPR materials. The combination of the NPR materials with PPR materials forms B-P materials, which can be advantageously applied in airbags and other impact protection devices, as shown in the cross-sections124and126ofFIG.1B. B-P materials have a lower density, better strength-to-weight ratio, greater porosity, larger surface area, and better dimensional stability than conventional materials. Further, the use of B-P materials in airbags and other impact protection devices is more efficient than use of conventional materials because a smaller amount of material can be used in the manufacturing process.

In some implementations, the airbag102shell or interior material is formed as a cell-structure or lattice including the NPR material132, or the NPR material132in combination with the PPR material134. In some implementations, components of the airbag102shell are formed of an NPR composite material that includes both an NPR material (e.g., an NPR foam material) and a PPR material, e.g., alternating layers of NPR material and PPR material (as illustrated inFIG.1Bcross section126), or a matrix of PPR material with NPR material embedded therein, or a matrix of NPR material with PPR material embedded therein.

In some implementations, the airbags102and104are formed as an inflatable shell formed of an NPR material, PPR material, or combination of NPR and PPR materials. The airbags102and104include a cellular matrix formed at least in part from an NPR materials positioned within the inflatable shell. In some implementations, the cellular matrix expands from a constricted state to an expanded state in response to a stimuli or in response to the inflatable shell being inflated.

NPR materials have a lower density than PPR materials, e.g., than PPR materials of a similar composition or than PPR materials having similar mechanical properties, and airbags including NPR materials can thus be lighter in weight and use less material than similar objects formed of PPR materials.

In some implementations, at least a portion of the airbag102shell is formed as a textile including an NPR material128as described above, and at least a portion of the airbag102shell is formed as a printed material (e.g., a 3-D printed material) including an NPR material132as also described above. In some implementations, the airbag102shell contains only one of a textile or a printed material formed from an NPR material.

In an embodiment, the first and second airbags102,104are formed as an inflatable shell of the NPR material which is inflated upon impact, as illustrated inFIG.1B. The NPR material in the inflatable shell is impacted by a human passenger of the vehicle during or immediately after the impact, and the force of the human passenger's impact to the inflatable shell is absorbed by the NPR material so as to lessen the force felt by the human passenger and prevent injury. The inflatable shell can then slowly deflate after the impact to provide space for the human passenger.

In another embodiment, the first and second airbags102,104are formed as a cushion of the NPR material which is exposed to the human passenger upon impact. The NPR material in the cushion is impacted by a human passenger of the vehicle during or immediately after the impact, and the force of the human passenger's impact to the cushion is absorbed by the NPR material so as to lessen the force felt by the human passenger and prevent injury. In such embodiments, the cushion can be further expanded by inflation or can be non-inflated.

The first and second airbags102,104absorb the impact energy of the passenger without causing injury to the passenger. While conventional airbags can sometime expand too quickly or can expand when no force is detected and can cause harm to passengers in the vehicle, the first and second airbags102,104formed from NPR materials can better absorb the energy of the passenger being moved through the vehicle to protect the passenger. The NPR material airbags102,104can lower the initial contact stress when the passenger first contacts the airbag, thereby reducing the incidence of sudden contact injury. Because the NPR material deforms more easily than conventional non-NPR materials, the airbags are less likely to cause harm to the passenger. Additionally, the NPR material airbags are resistant to rupture. When conventional airbags rupture, they can expose the passenger to chemicals and materials that are within the airbag for inflation purposes and can cause injury or illness to the passenger.

In some embodiments, airbags as described above can be used on the exterior of a vehicle to protect pedestrians, structures, animals, or the car itself from impact in the case of a crash.

The greater absorption of stresses by the NPR material used in the airbags described above can be useful in various other impact protection devices, such as helmets, and impact vests or flotation devices.

FIGS.2A-Billustrate an example impact vest formed with NPR materials. The impact vest200has a front side (FIG.2B) and back side (FIG.2A). The impact vest200includes a vest body240, first shoulder strap242and second shoulder strap244. The impact vest200defines an aperture245sized to accommodate a wearer's head, and apertures246and249to accommodate the wearer's arms when the wearer is wearing the impact vest200. A bottom portion247of the vest body240is open to allow the wearer's legs and lower body to extend through. The vest body240includes multiple cushions positioned on the vest body240, including first and second upper chest cushions254A-B and first and second lower chest cushions256A-B on a front side of the vest body240(illustrated inFIG.2B), and first and second upper back cushions248A-B, central back cushion250, and first and second lower back cushions252A-B on a back side of the vest body240(illustrated inFIG.2A). The vest body240is meant to be worn closely to a body of a human user, so as to at least partially absorb forces impacting the vest body240to lessen the forces felt by the human user as a result of an impact.

The cushions254A-B,256A-B,250,248A-B, and252A-B are formed at least in part from an NPR material. The NPR material of the cushions absorbs the energy of an impact at the vest body240so that the full impact force is not felt on the body of the wearer of the impact vest200. The NPR material can be formed from one or more types of fibers, such as polyamide fibers or PET fibers. The NPR materials forming the cushions can be 3-D printed as a matrix of cells, an open-celled foam, or a closed-cell foam. When the impact vest is subject to an impact, for example when a user falls or jumps into a body of water or an object or projectile hits the impact vest, the NPR materials in the cushions deform and, as a result, absorb some of the energy associated with the impact. The absorption of the impact energy at the cushions prevents the full force of the impact from being felt at the skin of the user, and the presence of the multiple cushions positioned on the impact vest200also serve to displace the impact force over the surface area of the vest to further protect the wearer.

In some implementations, the cushions can be formed from a combination of the NPR material and a PPR material. In some implementations, the cushions are formed from fibers of the NPR material and PPR material. In some implementations, the fibers include polyamide fibers including nylon 6,6, or polyester fibers, including PET, polyethylene terephthalate. In some implementations, the cushions are formed from a matrix of NPR and PPR materials. In some implementations, the NPR materials are embedded in a matrix of PPR materials. In some implementations, the NPR materials form a matrix in which the PPR materials are embedded. In some implementations, the cushions are formed from a material having cells of the NPR materials and cells of the PPR materials, with the NPR material cells interspersed among the PPR material cells. In some implementations, the PPR material cells and NPR material cells are formed as a 3-D printed foam material. In some implementations, the cushions are formed from alternating layers of PPR materials and NPR materials. The outermost layer of a cushion can be formed from the NPR material, with a layer of the PPR material formed beneath. In some implementations, the layers of PPR and NPR materials are formed within the cushion so that the boundaries of the layers are orthogonal to the outermost surface of the cushion.

The combination of the NPR material and PPR material forms a B-P material. B-P materials have a lower density, better strength-to-weight ratio, greater porosity, larger surface area, and better dimensional stability than conventional materials. Further, the use of B-P materials in airbags and other impact protection devices is more efficient than use of conventional materials because a smaller amount of material can be used in the manufacturing process.

Impact vests200can be used by a wearer engaging in various water-based activities and sports, such as kite surfing, wind surfing, water skiing, jet skiing, surfing, and cliff diving. Impact vests200used for such activities can incorporate additional cushions that serve to add flotation to the impact vest200so that the vest functions to protect against impacts and to provide flotation in the water. Impact vests200can also be used in non-water based activities, such as playing paintball or airsoft games, grappling, martial arts, baseball, military operations, or other activities. Impact vests200can also be designed for non-human users, including for dogs, horses, or other animals.

The impact vest200includes a belt strap253, with a buckle255. The belt strap253wraps around the impact vest200from the back to the front and buckles at the buckle255at the front of the impact vest200. In some implementations, additional belts or buckles are also included on the impact vest200to allow the wearer to fit the impact vest200more closely to their body, and to prevent the impact vest200from falling off. For example, in some implementations, the first and second shoulder straps242and244are adjustable. In some implementations, the impact vest200also includes a strap from the front of the vest to the back of the vest that is meant to pass between the wearer's legs, to keep the impact vest attached when the wearer is in the water.

In some implementations, the impact vest200includes one or more cushions254A-B,256A-B,250,248A-B, and252A-B which can be deployed from a non-expanded state to an inflated state upon detection of water to provide additional flotation to a wearer. Such an impact vest200can include sensors to detect water coupled to inflation mechanisms to inflate the cushions. As described above, the inflation mechanism can be can be a property of one or more cushion materials in response to contacting water, a reservoir filled with a gas or liquid, one or more chemicals that expand when combined with air, water or one another, a fluid source, a chemical explosive, a pump, or another mechanism that operates to inflate the cushions of the vest200. The impact vest200can be used to provide safe and non-bulky flotation for infants, children, or other individuals who may need additional floatation in water environments.

The NPR material airbags102,104and NPR material impact vest200use NPR materials to provide protection to a human being against injuries resulting from impact forces. The NPR materials absorb energy upon impact, adding to the energy absorption provided by inflation of a cushion or shell, or simply lessening the impact felt by the wearer. The NPR material used in the first and second airbags102,104or the impact vest200is capable of absorbing stress without lateral deformation.FIG.3Aillustrates the deformation of the NPR material compared to a non-NPR material when a force is applied.

Materials with negative and positive Poisson's ratios are illustrated inFIGS.3A and3B.FIG.3Adepicts a hypothetical two-dimensional block360of NPR material with length L1and W1in a non-stressed state. TheFIG.3Astructure is called a “re-entrant cell.” When the block360is compressed along its length by the application of force F, the material deforms into the shape shown as block362. The length L2of block362is less than the length L1of block360, but the width W2of block362is the same as the width W1of block360. The NPR material block360compresses along the length, and remains constant in width.

FIG.3Bdepicts a hypothetical two-dimensional block364of PPR material with length L3and W3in a non-stressed state. In contrast to the NPR block360ofFIG.3A, when the PPR block364is compressed along its length by the application of force F, the material deforms into the shape shown as block366. The length L4of block366is less than the length L3of block364, but the width W4of block366is the greater than the width W3of block364: the material compresses along its width and expands along its length.

The dimensional changes to the NPR material, such as the lateral deformation depicted inFIG.3A, take place due to the absorption of mechanical, thermal, electromagnetic, or ultraviolet energy, which is absorbed at a higher rate than in non-NPR or PPR materials. The NPR material used to form the airbags (such as airbags102and104inFIG.1A), impact vests (such as impact vest240inFIGS.2A-B), and other impact protection devices described above can be produced by a variety of mechanisms, including molding and 3-D manufacturing.

The NPR material used for airbags (such as airbags102and104inFIG.1A) or impact vests (such as impact vest240inFIGS.2A-B) can be NPR fibers, such as polyamide fibers including nylon 6,6, or polyester fibers, including PET, polyethylene terephthalate. NPR fibers can be used in combination with PPR fibers to produce materials for use in the impact protection devices. The NPR fibers and materials can have a Poisson's ratio of between −1 and 0, e.g., between −0.8 and 0, e.g., −0.8, −0.7, −0.6, −0.5, −0.4, −0.3, −0.2, or −0.1. NPR materials can have an isotropic Poisson's ratio (e.g., Poisson's ratio is the same in all directions) or an anisotropic Poisson's ratio (e.g., Poisson's ratio when the material is strained in one direction differs from Poisson's ratio when the material is strained in a different direction).

NPR materials for insulators can be foams, such as polymeric foams, ceramic foams, metal foams, or combinations thereof. A foam is a multi-phase composite material in which one phase is gaseous and the one or more other phases are solid (e.g., polymeric, ceramic, or metal). Foams can be closed-cell foams, in which each gaseous cell is sealed by solid material; open-cell foams, in which the each cell communicates with the outside atmosphere; or mixed, in which some cells are closed and some cells are open.

An NPR foam can be polydisperse (e.g., the cells of the foam are not all of the same size) and disordered (e.g., the cells of the foam are randomly arranged, as opposed to being arranged in a regular lattice). An NPR foam can be a cellular structure having a characteristic dimension (e.g., the size of a representative cell, such as the width of the cell from one wall to the opposing wall) ranging from 0.1 μm to about 3 mm, e.g., about 0.1 μm, about 0.5 μm, about 1 μm, about 10 μm, about 50 μm, about 100 μm, about 500 μm, about 1 mm, about 2 mm, or about 3 mm.

In some examples, NPR foams are produced by transformation of PPR foams to change the structure of the foam into a structure that exhibits a negative Poisson's ratio. In some examples, NPR foams are produced by transformation of nanostructured or microstructured PPR materials, such as nanospheres, microspheres, nanotubes, microtubes, or other nano- or micro-structured materials, into a foam structure that exhibits a negative Poisson's ratio. The transformation of a PPR foam or a nanostructured or microstructured material into an NPR foam can involve thermal treatment (e.g., heating, cooling, or both), application of pressure, or a combination thereof. In some examples, PPR materials, such as PPR foams or nanostructured or microstructured PPR materials, are transformed into NPR materials by chemical processes, e.g., by using glue. In some examples, NPR materials are fabricated using micromachining or lithographic techniques, e.g., by laser micromachining or lithographic patterning of thin layers of material. In some examples, NPR materials are fabricated by additive manufacturing (e.g., three-dimensional (3D) printing) techniques, such as stereolithography, selective laser sintering, or other appropriate additive manufacturing technique.

In an example, a PPR thermoplastic foam, such as an elastomeric silicone film, can be transformed into an NPR foam by compressing the PPR foam, heating the compressed foam to a temperature above its softening point, and cooling the compressed foam. In an example, a PPR foam composed of a ductile metal can be transformed into an NPR foam by uniaxially compressing the PPR foam until the foam yields, followed by uniaxially compression in other directions.

In some examples, NPR foams are produced by transformation of PPR foams to change the structure of the foam into a structure that exhibits a negative Poisson's ratio. In some examples, NPR foams are produced by transformation of nanostructured or microstructured PPR materials, such as nanospheres, microspheres, nanotubes, microtubes, or other nano- or micro-structured materials, into a foam structure that exhibits a negative Poisson's ratio. The transformation of a PPR foam or a nanostructured or microstructured material into an NPR foam can involve thermal treatment (e.g., heating, cooling, or both), application of pressure, or a combination thereof. In some examples, PPR materials, such as PPR foams or nanostructured or microstructured PPR materials, are transformed into NPR materials by chemical processes, e.g., by using glue. In some examples, NPR materials are fabricated using micromachining or lithographic techniques, e.g., by laser micromachining or lithographic patterning of thin layers of material. In some examples, NPR materials are fabricated by additive manufacturing (e.g., three-dimensional (3D) printing) techniques, such as stereolithography, selective laser sintering, or other appropriate additive manufacturing technique.

In an example, a PPR thermoplastic foam, such as an elastomeric silicone film, can be transformed into an NPR foam by compressing the PPR foam, heating the compressed foam to a temperature above its softening point, and cooling the compressed foam. In an example, a PPR foam composed of a ductile metal can be transformed into an NPR foam by uniaxially compressing the PPR foam until the foam yields, followed by uniaxially compression in other directions.

NPR-PPR composite materials are composites that include both regions of NPR material and regions of PPR material. NPR-PPR composite materials can be laminar composites, matrix composites (e.g., metal matrix composites, polymer matrix composites, or ceramic matrix composites), particulate reinforced composites, fiber reinforced composites, or other types of composite materials. In some examples, the NPR material is the matrix phase of the composite and the PPR material is the reinforcement phase, e.g., the particulate phase or fiber phase. In some examples, the PPR material is the matrix phase of the composite and the NPR material is the reinforcement phase.

NPR materials can exhibit various desirable properties, including high shear modulus, effective energy absorption, and high toughness (e.g., high resistance to indentation, high fracture toughness), among others. The properties of NPR materials are such that an item that includes an NPR material undergoes a different (e.g., smaller) change in dimension when absorbing energy than a comparable item formed of only PPR material.

FIG.4shows a schematic depiction of the change in diameter of a material400upon impact. Although the material400inFIG.6is shown as a rounded ball, a similar deformation occurs in materials of other shapes. Prior to impact, the material400has a diameter d1in the direction of the impact and a diameter d2in the direction perpendicular to the impact. If the material400is a PPR material, the material undergoes significant deformation (e.g., elastic deformation) into a shape402, such that the diameter in the direction of the impact decreases to d1PPR and the diameter in the direction perpendicular to the impact increases to d2PPR. By contrast, if the material400is an NPR material, the material undergoes less extensive deformation into a shape404. The diameter of the shape404in the direction of the impact decreases to d1NPR, which is approximately the same as d1PPR. However, the diameter of the shape404in the direction perpendicular to the impact also decrease, to d2NPR. The magnitude of the difference between d2and d2NPR is less than the magnitude of the difference between d2and d2PPR, meaning that the NPR material undergoes less deformation than the PPR ball.

FIGS.5A and5Bshow plots of diameter versus time for a substantially spherical PPR material with a Poisson's ratio of 0.45 and an NPR material with a Poisson's ratio of −0.45, respectively, responsive to being struck with an equivalent force. In this example, the NPR material undergoes a smaller initial change in diameter than does the PPR material, and the oscillations in diameter are smaller in magnitude and dampen more quickly. Again, althoughFIGS.5A and5Bare specific to substantially spherical materials, a similar behavior occurs in NPR and PPR materials of other shapes. The material of an insulator can be selected to balance rigidity and elasticity.

FIG.6illustrates examples of NPR-PPR composite materials. An NPR-PPR composite material602is a laminar composite including alternating layers604of NPR material and layers605of PPR material. The layers604,606are arranged in parallel to a force to be exerted on the composite material602. Although the layers604,606are shown as having equal width, in some examples, a laminar composite can have layers of different widths.

An NPR-PPR composite material608is a laminar composite including alternating layers of NPR material and PPR material, with the layers arranged perpendicular to a force to be exerted on the material608. In some examples, the layers of a laminar composite are arranged at an angle to the expected force that is neither perpendicular nor parallel.

An NPR-PPR composite material612is a matrix composite including a matrix phase611of NPR material with a reinforcement phase612of PPR material. In the material612, the reinforcement phase612includes fibers of the PPR material; in some examples, the reinforcement phase612can include particles or other configuration. In some examples, NPR-PPR composite materials can have a matrix phase of a PPR material with a reinforcement phase of an NPR material.

FIG.7illustrates the mechanical behavior of PPR and NPR/PPR composite materials. A hypothetical block700of PPR material, when compressed along its width w, deforms into a shape702. The width w1of the compressed block702is less than the width w of the uncompressed block700, and the length11of the compressed block702is greater than the length1of the uncompressed block: the material compresses along the axis to which the compressive force is applied and expands along a perpendicular axis.

A block704of NPR/PPR composite material includes a region708of NPR material sandwiched between two regions706of PPR material. When the block704of composite material is compressed along its width, the material deforms into a shape710. The PPR regions706compress along the axis of compression and expand along a perpendicular axis, e.g., as described above for the block700of PPR material, such that, e.g., the width w2of a region706of uncompressed PPR material compresses to a smaller width w4and the length12of the region706expands to a greater length14. In contrast, the NPR region708compresses along both the axis of compression and along the perpendicular axis, such that, e.g., both the width w3and length13of the uncompressed NPR region708are greater than the width w5and length15of the compressed NPR region708.

FIG.8illustrates an example method of making an object, such as casing, window, or other object, formed of an NPR material. A granular or powdered material, such as a polymer material (e.g., a rubber) is mixed with a foaming agent to form a porous material50. The porous material50is placed into a mold52. Pressure is applied to compress the material50and the compressed material is heated to a temperature above its softening point. The material is then allowed to cool, resulting in an NPR foam54. The NPR foam54is covered with an outer layer56, such as a polymer layer, and heat and pressure is applied again to cure the final material into an object58.

In some examples, a material can be formed into an NPR material by forming nanoscale or microscale structures, such as spheres or tubules, with the material.

Other methods can also be used to fabricate an object formed of an NPR material or an NPR-PPR composite material. For example, various additive manufacturing (e.g., 3D printing) techniques, such as stereolithography, selective laser sintering, or other appropriate additive manufacturing technique, can be implemented to fabricate an object formed of an NPR material or an NPR-PPR composite. In some examples, different components of the object are made by different techniques. For example, an inner layer may be 3D printed while the out layer is not, or vice versa. Additive manufacturing techniques can enable seams to be eliminated.

Other embodiments are within the scope of the following claims.