New
materials, prototyping and scanning technologies are raising the bar for
running shoe design as seen in the newly developed Brooks DNA cushioning system
that features a custom elastomeric compound that conforms to individual
runners' weight and gait biomechanics.
New
approaches to running shoe design also meet environmental demands, including the
use of recycled content, avoidance of PVC and potential degradability.
"Most people
don't really expect something that's as familiar as running shoes to have as
much 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 biomechanics
for the running shoe company, which was founded in 1914 and competes against
giants like Nike and Reebok.
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One of the
Holy Grails for companies like Brooks is finding a design that provides
excellent energy absorption (cushioning) with a great return (or bounce back).
Until this
year, Brooks relied on a cushioning system called HydroFlow, in which a system
of chambers sitting under the heel area of the midsole contained silicone-oil
compound to dampen impact. When pressure was applied to the central chamber,
the liquid was displaced into the outer ducts. As the pressure on the chamber
decreased, the liquids returned.
The patented
invention was designed to overcome a breakdown
of resiliency, or elasticity, when closed cell viscoelastic foams made of ethyl
vinyl acetate (EVA) and polyurethane are subjected to the punishment of
distance running.
Earlier this
year the company introduced Brooks DNA as its new cushioning system.
"The big
difference in the new technology is that it is soft and conformable at the
lower energies, which makes it more comfortable than in the past," says
Campbell. "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 perform
more poorly. So runners who are faster or larger often end up with a worse
solution."
Typically,
smaller runners also end up with a poor solution because most footwear
technologies 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, or
energy return system that is adaptable to those different energy inputs, you
end up not getting the best solution for everybody.
Some running
shoes advertise an electromechanical system to achieve the right kind of
cushioning. Sensors are tied to tiny electric motors that adjust the tension on
a stainless-steel cord that runs through a flexible heel.
"The problem
with those systems is that even if you have superfast sensor systems, they
don't respond as fast as impact occurs when running," says Campbell. "That's
why we felt we had to go down the pathway of developing a materials technology
that's able to react at that speed so that a runner can get the performance
that is really desired."
Injection Moldable
One of the
suppliers Brooks approached was
PolyOne GLS Thermoplastic Elastomers of McHenry, IL.
"We're not
polymer specific," says Rick Noller, director of global marketing, PolyOne GLS
Thermoplastic Elastomers. "So when Brooks came to us, we had a barrel of
different polymers and suppliers we could work with." Brooks wanted a material
that could be injection molded, a faster and more productive process than the
compression molding historically used to manufacture shoe components.
Brooks also
wanted a material that fit well into assembly and other operations after
molding. "Those requirements had kept styrenic block copolymers out of the
running for these applications," says Noller.
PolyOne GLS
developed a custom alloy described as a non-linearly viscous material, a SEBS
(styrene-ethylene-butadiene-styrene) block copolymer-based material that
includes proprietary additives. The material's resiliency is a function of the
level of impact to the material. Campbell describes the unique formulation as a
cold-flowing liquid elastomer, a very viscous material. Brooks has filed for
patent protection on its cushioning invention.
The Brooks
Future Concepts team also had several environmental mandates for the new
material. For example, the assembly process for the DNA uses a water-based
polyurethane adhesive instead of a solvent-based adhesive. There is also a reduction
in industrial waste in the injection molding process because Brooks' contract
manufacturers use hot runner systems that eliminate waste from sprues. The
regrind that is created is reintroduced into the molding machine at rates of 10
to 15 percent of the total compound.
In addition,
the one-material DNA system replaces the four-material HydroFlow cushioning
system: two types of TPEs (one injection molded and one blow molded), a
polyurethane thermoset gel and a silicon oil fluid.
Sand Imprint
Brooks began
testing the materials and at the same time began rapid prototyping on shoe and
part design to match as closely as possible the pressure pattern that is
generated by a runner going through a gait cycle.
"The shape
of the part looks very much like the imprint of your foot in the sand," says
Campbell. "So if you were running on the beach in wet sand, the imprint is where
you land on the heel of your foot and then roll forward to the outside of your
foot and then onto the ball of your foot and then onto your toes. The design of
DNA mirrors that pattern because those parts of your foot are where the highest
pressures are generated."
Visual
prototypes were generated quickly on an Objet
Eden 500v 3-D printer, which has a build size of 500 x 400 x 200 mm. Functional
testing materials are still not generally available on 3-D printers, but
several soft, even elastomeric, grades have been introduced in recent years.
Impact
testing was conducted on samples of the PolyOne GLS material based on
ASTM F1976, which is widely used to test general cushioning
characteristics of athletic footwear. Testing showed that the average cushioning
improvement from the HydroFlow to the DNA cushioning system was 30 percent.
Brooks
conducts two other types of testing to determine fitness of the new material.
A
3-D
imaging system from Motion Analysis
Corp. captures details of how the body moves in three dimensions in Brooks'
Biomechanics Research Lab. "We can measure energy absorption, as well as more
important things such as how fast different joints are being loaded," says
Campbell. "If you load joints too quickly when running, you don't allow your
natural body's systems for shock absorption to handle those loads."
Up to 32 retroflective markers are
attached to a person wearing shoes made of the new materials. Eight infrared
cameras are fixed on the markers. The data are loaded into a software program
that can create a human image to see how different joints are moving relative
to each other.
In a sign of
the growing importance of biomechanics to athletic footwear, Brooks in April
announced partnerships with Professor Dr. Gert-Peter Brüggemann and Professor
Dr. Joseph Hamill, two leading running biomechanics researchers. They will be
studying large populations of runners.
Brüggemann
directs the Institute of Biomechanics and Orthopedics at the German Sport
University in Cologne, Germany, and Hamill is a professor in the Dept. of
Kinesiology at the University of Massachusetts, Amherst
"Because
Brooks will involve my research team during the entire footwear process, I hope
to influence shoe design based on runners' specific biomechanical needs, rather
than simply providing data for shoes that have already been developed," says
Brüggemann.
Wear testing
shows how well the energy absorption and return characteristics hold up over
the life of the shoe. "We want the technology to be well-suited to the life of the
rest of the shoe," says Campbell. Although wear rates vary widely based on the
runner's size and running style, Brooks targets a shoe lifetime of 400 to 500
running miles.
Brooks DNA
debuted in the new Glycerin 8 neutral running shoe released in January at a
suggested retail price of $130.
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