Auxetic materials shrink and expand in the transverse direction when subjected to axial compression and tension, respectively. They respond in a way that is counter-intuitive to common elastic materials and, therefore, exhibit a negative Poisson”s ratio. This ratio measures the deformation in a direction perpendicular to the applied force. Unlike the usual elastic materials, auxetic materials, upon compression, become thinner in the direction perpendicular to the applied force.
These materials have become popular over the past couple of decades for their useful properties, such as good fracture toughness, high indentation/impact resistance, shear modulus, good energy absorption, synclastic deformation, and high transverse strain. They also possess high dent resistance properties and superior surface isotropy.
All these properties have enriched auxetic materials, ensuring that these materials have widespread usability. In the following segments, we will look into some such use cases in the fields of medical science and healthcare, vehicle engineering, protection engineering, apparel, etc.
Auxetic Materials in Healthcare and Medical Sciences
High-density auxetic polyethene is used to develop artificial intervertebral discs for spinal surgery. These discs can bend and twist and potentially provide improved biomechanical performance compared with traditional disc replacement solutions. Their auxetic properties prevent bulges that could injure the surrounding nerve endings, allowing the disc to perfectly mimic the behavior of a natural lumbar intervertebral disc.
An auxetic pedicle screw was also proposed for improving the biomechanical interaction between the surrounding bone and the screw, mainly for the spine.
Research also helped scientists develop an auxetic geometry coronary stent based on 316L medical-grade stainless steel. This stent can expand in both radial and longitudinal directions and be optimized for a specific diameter and vessel length to create a certain lumen volume, minimizing the stent’s negative impact on the vessel wall.
Auxetic materials have also proven useful in other medical applications, such as hip implant stems, fixation for long bones, cardiac patches, nasopharyngeal swabs, orthopedic insoles, and more. They are also valuable in tissue engineering and the fabrication of structures and membranes for in-vitro medical devices.
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Auxetic Materials in Vehicle Engineering
Recent research has shown that auxetic materials and structures may provide a superior design method that improves the energy absorption characteristics of the traditional thin-walled tube bumpers and energy-absorbing boxes.
These structures would have greater rigidity, energy absorption advantage under medium-speed collision and longer buffer time for the energy absorption of the collision and the acceleration-deceleration process of the occupants. They also score highly in terms of energy absorption capacity, energy absorption efficiency, and other performance indices.
One of the researchers in this space specifically suggested a bumper that comprised an auxetic sandwich beam structure and an energy-absorbing block combination optimized by a multi-objective algorithm, which improved the vehicle’s frontal crashworthiness.
Auxetic Materials in Protection Engineering
The use of auxetic structures in protection engineering arose from their high energy absorption, improved indentation compressive strength, and enhanced shear strength properties.
The research helped find auxetic honeycomb sandwich panel structures that could protect the reinforced concrete structure from severe impact and proximity blast loads.
Also, cylindrical sandwich panels with auxetic effect had stronger blast resistance properties, and they could be used to efficiently design robust aircraft shells, spacecraft, submarine hulls, fuel tanks, coal mine shelters, and other curved structures.
Auxetic Materials in Apparel Engineering
The transverse expansion property of auxetic materials under load helps form double curvatures during bending. This property allows the apparel made from these materials to adapt to the changing contours of the human body during movement. Research shows that auxetic fabric maternity wear exhibits lower stress concentration compared to traditional counterparts. It is also comfortable to wear.
This broad range of application possibilities has resulted in a surged demand for auxetic materials. To meet this increasing demand, researchers have developed a three-step recipe for designing auxetic materials on demand.
A Three-Step Procedure for Creating Auxetic Materials on Demand
Researchers at the National Institute of Standards and Technology (NIST) and the University of Chicago have made concerted efforts to accelerate the market introduction of auxetic products.
More precisely, these researchers have developed an algorithmic tool that helps with precise three-dimensional designing of auxetics. According to the study’s co-author, Edwin Chan, a materials research engineer from NIST:
“We can actually optimize the material to have whatever particular mechanical properties and behavior that you want.”
The researchers have presented the world with a unified framework for describing bi-dimensional perfect auxetics. This presentation is an outcome of a greater realization that understanding and controlling the perfect auxetic limit could allow us to rationally design realistic auxetics with a tailor-made geometric structure for which Poisson’s ratio remains negative.
While elaborating on the greater significance of their study, the researchers had the following to say:
“Our theory not only unifies a large class of existing auxetics, encompassing previous auxetic rigid unit systems, but also allows to create a large variety of them, with new and diverse geometries, including the somewhat elusive isotropic auxetic material.”
Since the researchers were interested in understanding the behavior of perfect auxetics, they used the minimal bidimensional model for rotating units. The units comprised a series of polygons connected by springs of zero natural length. Each rigid unit had three degrees of freedom: two translational and one rotational. This model helped ascertain the emergence of the auxetic floppy mode and introduced elasticity into materials so that non-ideal scenarios could also be studied.
The outcome of the scientific investigation was in the form of a recipe that would help build perfect auxetics. It would require three ingredients:
- A bipartite network would allow the units to counter-rotate concerning each other.
- Initially and at rest, every pair of neighboring polygon’s position vectors and the vertex between them have to be collinear.
- The ratio between the distance of a polygon to one of its vertices and the distance of its neighbor to the same vertex must be a constant in the network.
The researchers highlight that their theory’s uniqueness lies in the fact that it is not restricted to lattices and may apply straightforwardly to disordered networks. This allows the building of the first isotropic perfect auxetic materials. The researchers are also helpful in that their theory could be expanded into 3D polyhedral materials.
While this research is a breakthrough, work on auxetic materials has been ongoing for quite some time now. In the following segments, we will look at companies that have been conducting groundbreaking work on them.
#1. Nike
One of the leading brands to have done sophisticated work in this space of auxetic structures is Nike. Nike’s patent no US9402439B2 deals with auxetic structures that facilitate building soles with auxetic structures.
Usually, footwear comprises at least two major components:
- An upper that provides the enclosure for receiving the wearer’s foot
- A sole secured to the upper that works as the primary contact to the ground or playing surface
The sole may include three subsegments: an inner sole, a midsole, and an outer sole. In the Nike footwear patented, at least one layer of the sole is made of an auxetic structure. It is referred to as the auxetic layer.
Running, turning, leaping, or accelerating with these shoes puts the auxetic layer under increased longitudinal or lateral tension, increasing its length and width. Thus, it provides improved traction and absorbs some of the impacts of the playing surface.
Nike believes that these shoes may prove beneficial for a range of sports and recreational activities, including tennis and other racquet sports, walking, jogging, running, hiking, handball, training, running or walking on a treadmill, and team sports such as basketball, volleyball, lacrosse, field hockey, and soccer.
For the full fiscal year 2023, Nike reported revenues of US$51.2 billion, up 10 percent compared to the prior year and up 16 percent on a currency-neutral basis.
#2. Airbus
Airbus Defence and Space received a patent on a deformable auxetic structure and manufacturing process. The invention aims to provide lightweight protection against high-energy impacts in aircraft airframes and systems by a single integrated sandwich panel that leverages the bidimensional auxetic behavior in the two directions forming the panel’s surface.
Airbus suggests that such protection would be highly beneficial in highly integrated rear-end engine-driven aircraft configurations, including Open-Rotor or Boundary-Layer Ingestion architectures. In these configurations, protection provisions (shieldings) are required due to safety concerns against high-energy impacts on the fuselage from Propeller Blade Release (PBR) and Engine Debris (Uncontained Engine Rotation Failure small fragment and third disc).
The structure features an auxetic arrangement consisting of a plurality of interconnected adjoining tridimensional auxetic cells. This design ensures that regardless of whether the surface element has a planar surface, a curved surface, or a composition of different planar surfaces, the resulting structure will achieve auxetic behavior on each of these planes. Additionally, it will exhibit auxetic behavior in more than one dimension perpendicular to the main direction of impact.
In 2023, Airbus registered a revenue of more than 65 billion Euros, a significant increase from 58.8 billion Euros earned in 2022.
Creating Auxetic Materials: What the Future Looks Like
The research that opened our discussion could achieve many breakthroughs. It could provide a simple-to-follow model for creating, designing, and characterizing rotating unit auxetics. Models that validate the theory were potent for straightforward simulation for testing material properties while ignoring bending forces. However, the models were conducive to accommodating bending if needed. The work led to establishing the ground rules for creating never-seen-before isotropic perfect auxetics. The only thing it required was more research on dynamic and vibrational analysis.
While research and exploration is a never-ending process in science and technology, There has to be more investigation on the viability of auxetic materials as reinforcement, especially for load-bearing applications. Excessive deformations are required to initiate the advantages derived from auxeticity. These deformations could exceed the serviceability limits specified for structural elements in static loading. In this regard, researchers have pointed towards the requirement of bespoke designs that satisfy both strength and stiffness requirements.
In the field of biomedical application, the need for 3D auxetics arises, which can only be achieved by additive manufacturing procedures. These procedures should be enhanced to ensure the long-term performance of the medical devices. The advanced algorithm-based process to create auxetics could be useful in these cases, as most additive technologies lead to intrinsically anisotropic results due to the layer-by-layer process, which may affect the mechanical performance of the solutions.
While these challenges remain, researchers are hopeful that some of the future trends, if scaled up successfully, could lead to more enthusiastic adoption of auxetics. Such trends include creating synergies between 4D printing and auxetic geometries, manufacturing of shape memory materials and mechanisms, etc.
Auxetics may also find increased use in biohybrid materials, where an auxetic scaffold colonized by cells can lead to more autonomous actuators. According to researchers, these materials are capable of outperforming currently available “smart,” active, or multifunctional materials and structures.
One of the prime examples of the use of Biohybrids is in the form of microbots that come with auxetic chassis and mechanisms operated by living cells. These solutions, especially the electric stimuli-activated cardiomyocytes or musculoskeletal cells, can be used as surgical actuators in the future.
Altogether, auxetics presents us with a minefield of opportunities. All it needs is more research and resources to scale up and provide feasible solutions that last a very long time.
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