Can innovative fiberglass composite insoles improve running shoe performance and durability?
Everyone knows that running shoe soles typically have three layers: a durable outer outsole, a midsole that absorbs most of the impact, and an innersole that makes direct contact with the foot.
To create the lightest and most durable athletic shoes possible, numerous carbon fiber composite midsoles and other shoe components have emerged and are now commercially available. These carbon fiber composite-reinforced shoes are often targeted at elite athletes, offering runners enhanced stride and speed.
However, carbon fiber is also very stiff, which can negatively impact a runner's feet, legs, and muscles, explains Gordon Brown, president of sporting goods developer Flexi-StiX LLC (Anderson, South Carolina, USA). He believes that for the average runner, fiberglass is a better, more flexible, and forgiving material choice, enhancing running shoe performance by providing smoother, more consistent energy return to the runner's foot.
Brown and Milliman recently developed and commercialized their latest innovation, a patent-pending running shoe insole made of unidirectional (UD) continuous glass fiber reinforced thermoplastic prepreg sandwiched between two layers of neoprene foam. In the 1990s, Brown and Milliman began prototyping continuous fiberglass/epoxy midsoles. For running shoes, the goal was maximum cushioning and a spring-like effect to return energy to the runner. These midsoles were roughly shaped like two sine waves, one on top of the other, wet-laid in an aluminum mold, pressed, and then baked to cure. "These midsoles worked," Brown says. "They were flexible, lengthened the stride, and created a spring effect, but they were labor-intensive and ultimately not commercially viable." The team decided to switch to a fiberglass composite insole, offering cost-effectiveness for consumers without redesigning the entire shoe. The first insole design involved laying tiny strips of pultruded fiberglass/epoxy resin along the width of the insole. The placement allowed them to flex in the middle, allowing for a smooth and dynamic motion, extending the stride with a gentle, spring-like effect. "This design was successful in practice, but it was also labor-intensive to manufacture," Brown says. He notes that the development and increasing availability of UD thermoplastic prepregs over the past few years "has been a game changer." He explains that the game-changing nature of these materials is twofold. First, the UD and continuous orientation of the fibers, if aligned across the width of the shoe, as in the original pultruded fiberglass rod design, can extend a runner's stride in this way. "For running shoes, it's all about increasing forward motion," Milliman says. When a runner presses down on the insole with their foot, a regular neoprene insole absorbs the applied energy, but a fiberglass composite insole reflects that energy back to the runner, providing a more controlled spring effect and extending their stride. And the use of thermoplastics instead of thermosets allows for a lower modulus, which makes the insole more comfortable for the wearer and also helps it conform to the foot. "The heat from the foot slightly warms the thermoplastic, enough to cause it to mold to the shape of the foot," Brown says.
After hand-crafting several prototypes, Brown and Milliman worked with materials partner Avient (Avon Lake, Ohio, USA) to produce sheets of the sandwich material consisting of three layers: a layer of fiberglass/thermoplastic prepreg between two layers of neoprene foam. These sheets were then die-cut into insoles that match various shoe sizes and can replace or be placed under existing insoles. Two different thicknesses (5 or 8 mm) are available, depending on the customer's desired cushioning.
Marketed as the FlexSpring insole, runner trials over the past year have demonstrated successful results. Users have experienced a longer stride length and an increase in strides per minute (also known as runner's cadence), enabling them to run faster and break personal records. Furthermore, they are extremely durable, potentially extending the life of running shoes by three times by reducing stress on the midsole material. Brown added that insoles could eventually be used in other types of high-performance shoes beyond running shoes, such as basketball shoes and work shoes worn by assembly line workers. "They're comfortable, the wearer feels less fatigued, and they make the shoes more durable. There's a lot of potential."
