Analyzing Auxetic Cellular Structures for Personal Protective Gear Designs

upper side of our new running shoe design is made of breathable mesh with silicone molded to top surface that adds structure, while being flexible

The primary objective of this study is to create a new athlete shoe design utilizing auxetic cellular structures.The secondary objective is to analyze the properties of various auxetic cellular structures for a wearer's body protection from impacts.Through the exploration of auxetic cellular structures and their application to personal protective equipment (PPE) including sports apparel and accessories, the outcomes of this study will contribute to create designs that prevent or reduce injuries caused by forceful impact.
Auxetics are material structures that have a negative Poisson's Ratio (Hu, Zhang, & Liu, 2019).The Poisson's ratio is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force (Greaves, et al., 2011).When stretched, a cellular structure becomes thicker in the two directions perpendicular to the applied tension.When compressed, it becomes thinner in the two directions perpendicular to the applied compression.Tensile deformation is considered negative.Compared to conventional material structures, auxetic structures have significantly higher energy absorption, fracture resistance, shear resistance, and impact resistance (Kelkar et al., 2020).The auxetic structures are categorized into re-entrant structures, chiral structures, rotational rigid structures, and other miscellaneous structures, such as origami structures (Hu, Zhang, & Liu, 2019).Each type has difference geometric structures and deformation mechanisms.To meet the research objectives, the various cellular auxetic mechanical structures are explored and evaluated using ASTM D 5034 Grab Test method.Three aspects of auxetic materials are explored to optimize both protective properties of athletic shoe designs: Laser cut, woven, or injection molded forms for upper side of shoe; micro level cellular structure foam for insole; and thermoplastic polyurethane (TPU) auxetic forms for the shoe base.
The upper side of our new running shoe design is made of breathable mesh with silicone molded to top surface that adds structure, while being flexible (See Figure 1.).The midsole consists of a lattice auxetic structure (i.e., a type of re-entrant structure), which provides the even bed of support, while giving bounce-back and cushion to impacted areas.The heel counter is made of rigid support of a vertical triangular auxetic structure that flexes when compressed.As a result, it effectively stabilizes heel.The outer sole comprises stiff and durable rubber combined with an inverted hexagon shape of auxetics.This specific re-entrant structure is selected for the outer sole forms as they shift and expand when force is applied.The inverted hexagon auxetic structures effectively accommodate wearers' movement as they can be pushed, squished, and stretched by foot.
Traditional running shoes have an outsole made of carbon rubber and an upper made of knit polyester or nylon mesh.Switching from traditional athletic shoe materials and designs to incorporate auxetic foams and structures poses many benefits.Shock absorbing auxetic structures combined with foam and rubber help reduce repeated stress and prevent injuries related to pressure points.The new shoe design structures are designed to withstand high stress on a molecular level.The result of this study demonstrates that auxetic materials have many advantages over traditional materials in the application of sports related personal protection equipment.Auxetic foams are more shock absorbing, lightweight, flexible, forms to the body, and have more tensile strength as compared to traditional open and closed cell foams.This opens the doors to countless possibilities where auxetic foams, structures, and knits could be applied.Auxetics could be a paramount asset in reducing sports injuries.Examples of potential designs include helmets to prevent traumatic brain trauma, cycling gear, protective elbow and knee pads, and so many more applications in an effort to reduce injuries.

Figure 1 .
Figure 1.New Athletic Shoe Design