The guiding principle behind the "S" crutch was finding a way to leverage geometrical and material properties to make the ultimate crutch. A truly optimal crutch springs back slightly to return energy back to the user, absorbs shock to minimize force on sensitive limbs, provides stability throughout the gait cycle, and, of course - is lightweight and slim. The S shape, combined with the incredible specific strength of glass fiber-polymer composites, provided all these attributes in an elegant and simple way.
A POGO STICK FOR INSPIRATION?
We explored a variety of crutch types - from canadian style to rocker styles. We also took inspiration from other products which communicate sturdiness - like machine tools and wheel chairs. In the end we decided to prototype something that combined a spring and a rocker mechanism together; which would provide elasticity as well as a longer gait; thereby reducing the peak shock force felt by the user.
We made modular pieces that could be inserted into the bottom of a traditional crutch for testing. Energetic and force studies confirmed that this approach was indeed less energy intensive than traditional underarm crutches, so we decided to move forward with this prototype as inspiration for future models.
GOOEY, MESSY LAYUPS
Pursuing material choice as a second axis of innovation beyond the geometry, we chose polymer-composites as an optimal material family given their optimal strength-to-weight ratios. But laying up composite materials on a curved 3-D surface typically requires multiple molds, which turns out to be quite expensive and complicated. To overcome this, we cut up uniform strips of glass fiber and helically laid them up around the mold. We then impregnated the glass-fiber with epoxy resin.
FORCE MODELING: REDUCING RADIUS OF CURVATURE
One thing that we wanted to do was design our product for safety and misuse. So, we assumed the worst possible situation: a 95th percentile male falling off a step. Using this scenario, we did a back of the envelope force calculation and input that into our FEA software. We noticed that our first set of designs failed at the bend so we iterated through our CAD model until we arrived at a safety factor of 2 compared to the ultimate yield strength of our composite.
Welding and bending the finishing touches.
TESTING, TESTING 1...2...3...
Here we performed a series of instron tests to determine the energy lost (and conversely, the energy "returned") to the user during each gait cycle. Above are the individual instron tests of the rocker spring prototype, the S crutch, and the traditional underarm crutches. We then tabulated the fraction energy recovered (i.e. percent hysteretic area vs. total potential energy) for different versions of our prototype: green being 3 layers of glass fiber, blue being 5, and red being 7. As shown, there is a bell curve for the number of fiber layers needed; too many and it is not compliant enough (and breaks), while too few and it is too compliant (and breaks).
THE FINAL RESULTS
We tested the final prototype vs the rocker-spring and traditional underarm crutches - via instron, user testing, and force plate testing. The final prototype achieved a 29% reduction in peak shock force and 33% reduction in energy intensity!