Custom Elastomer Conforms to Runner's GaitCustom Elastomer Conforms to Runner's Gait
August 9, 2010
Newmaterials, prototyping and scanning technologies are raising the bar forrunning shoe design as seen in the newly developed Brooks DNA cushioning systemthat features a custom elastomeric compound that conforms to individualrunners' weight and gait biomechanics.
Newapproaches to running shoe design also meet environmental demands, including theuse of recycled content, avoidance of PVC and potential degradability.
"Most peopledon't really expect something that's as familiar as running shoes to have asmuch technology, engineering and design work go into it," says Derek Campbell,Future Concepts manager at Brooks Sports, Bothell, WA. A materials engineer,Campbell leads a team that explores innovations in materials and biomechanicsfor the running shoe company, which was founded in 1914 and competes againstgiants like Nike and Reebok.
One of theHoly Grails for companies like Brooks is finding a design that providesexcellent energy absorption (cushioning) with a great return (or bounce back).
Until thisyear, Brooks relied on a cushioning system called HydroFlow, in which a systemof chambers sitting under the heel area of the midsole contained silicone-oilcompound to dampen impact. When pressure was applied to the central chamber,the liquid was displaced into the outer ducts. As the pressure on the chamberdecreased, the liquids returned.
The patentedinvention was designed to overcome a breakdownof resiliency, or elasticity, when closed cell viscoelastic foams made of ethylvinyl acetate (EVA) and polyurethane are subjected to the punishment ofdistance running.
Earlier thisyear the company introduced Brooks DNA as its new cushioning system.
"The bigdifference in the new technology is that it is soft and conformable at thelower energies, which makes it more comfortable than in the past," saysCampbell. "When you look at the competing technologies that are out there,often as you put more energy into them either they don't change, or they performmore poorly. So runners who are faster or larger often end up with a worsesolution."
Typically,smaller runners also end up with a poor solution because most footweartechnologies are designed for the middle and scaled out to the other two ends:small women and large men. If you don't have a technology in the cushioning, orenergy return system that is adaptable to those different energy inputs, youend up not getting the best solution for everybody.
Some runningshoes advertise an electromechanical system to achieve the right kind ofcushioning. Sensors are tied to tiny electric motors that adjust the tension ona stainless-steel cord that runs through a flexible heel.
"The problemwith those systems is that even if you have superfast sensor systems, theydon't respond as fast as impact occurs when running," says Campbell. "That'swhy we felt we had to go down the pathway of developing a materials technologythat's able to react at that speed so that a runner can get the performancethat is really desired."
Injection Moldable
One of thesuppliers Brooks approached was PolyOne GLS Thermoplastic Elastomers of McHenry, IL.
"We're notpolymer specific," says Rick Noller, director of global marketing, PolyOne GLSThermoplastic Elastomers. "So when Brooks came to us, we had a barrel ofdifferent polymers and suppliers we could work with." Brooks wanted a materialthat could be injection molded, a faster and more productive process than thecompression molding historically used to manufacture shoe components.
Brooks alsowanted a material that fit well into assembly and other operations aftermolding. "Those requirements had kept styrenic block copolymers out of therunning for these applications," says Noller.
PolyOne GLSdeveloped a custom alloy described as a non-linearly viscous material, a SEBS(styrene-ethylene-butadiene-styrene) block copolymer-based material thatincludes proprietary additives. The material's resiliency is a function of thelevel of impact to the material. Campbell describes the unique formulation as acold-flowing liquid elastomer, a very viscous material. Brooks has filed forpatent protection on its cushioning invention.
The BrooksFuture Concepts team also had several environmental mandates for the newmaterial. For example, the assembly process for the DNA uses a water-basedpolyurethane adhesive instead of a solvent-based adhesive. There is also a reductionin industrial waste in the injection molding process because Brooks' contractmanufacturers use hot runner systems that eliminate waste from sprues. Theregrind that is created is reintroduced into the molding machine at rates of 10to 15 percent of the total compound.
In addition,the one-material DNA system replaces the four-material HydroFlow cushioningsystem: two types of TPEs (one injection molded and one blow molded), apolyurethane thermoset gel and a silicon oil fluid.
Sand Imprint
Brooks begantesting the materials and at the same time began rapid prototyping on shoe andpart design to match as closely as possible the pressure pattern that isgenerated by a runner going through a gait cycle.
"The shapeof the part looks very much like the imprint of your foot in the sand," saysCampbell. "So if you were running on the beach in wet sand, the imprint is whereyou land on the heel of your foot and then roll forward to the outside of yourfoot and then onto the ball of your foot and then onto your toes. The design ofDNA mirrors that pattern because those parts of your foot are where the highestpressures are generated."
Visualprototypes were generated quickly on an Objet Eden 500v 3-D printer, which has a build size of 500 x 400 x 200 mm. Functionaltesting materials are still not generally available on 3-D printers, butseveral soft, even elastomeric, grades have been introduced in recent years.
Impacttesting was conducted on samples of the PolyOne GLS material based on ASTM F1976, which is widely used to test general cushioningcharacteristics of athletic footwear. Testing showed that the average cushioningimprovement from the HydroFlow to the DNA cushioning system was 30 percent.
Brooksconducts two other types of testing to determine fitness of the new material.
A 3-Dimaging system from Motion AnalysisCorp. captures details of how the body moves in three dimensions in Brooks'Biomechanics Research Lab. "We can measure energy absorption, as well as moreimportant things such as how fast different joints are being loaded," saysCampbell. "If you load joints too quickly when running, you don't allow yournatural body's systems for shock absorption to handle those loads."
Up to 32 retroflective markers areattached to a person wearing shoes made of the new materials. Eight infraredcameras are fixed on the markers. The data are loaded into a software programthat can create a human image to see how different joints are moving relativeto each other.
In a sign ofthe growing importance of biomechanics to athletic footwear, Brooks in Aprilannounced partnerships with Professor Dr. Gert-Peter Brüggemann and ProfessorDr. Joseph Hamill, two leading running biomechanics researchers. They will bestudying large populations of runners.
Brüggemanndirects the Institute of Biomechanics and Orthopedics at the German SportUniversity in Cologne, Germany, and Hamill is a professor in the Dept. ofKinesiology at the University of Massachusetts, Amherst
"BecauseBrooks will involve my research team during the entire footwear process, I hopeto influence shoe design based on runners' specific biomechanical needs, ratherthan simply providing data for shoes that have already been developed," saysBrüggemann.
Wear testingshows how well the energy absorption and return characteristics hold up overthe life of the shoe. "We want the technology to be well-suited to the life of therest of the shoe," says Campbell. Although wear rates vary widely based on therunner's size and running style, Brooks targets a shoe lifetime of 400 to 500running miles.
Brooks DNAdebuted in the new Glycerin 8 neutral running shoe released in January at asuggested retail price of $130.
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