Elastic Measurements in an Operating Shoe Sole BME 301 Steven Pauls, Timothy Rand Brian Schwartz, Brant Kochsiek Advisor: Professor Naomi Chesler Client: Professor David Beebe March 5, 2004

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Problem Statement Device should fit into the sole of a shoe Inform runner when shoe is worn Decrease incidence of injury due to shoe wear Measure the elasticity of the shoe sole Direct measurement Other methods

Problem Motivation Running shoes last 300-500 miles Worn shoes not always “worn” Incidence of injury increases with worn shoes Diagnostic tool in relation to running style

Client Requirements Lightweight Ergonomic with sole Operable for life of the running shoe Should not hinder performance Have clear indicator

Elastic Effect Elastic recoil cushions impact Age/Wear decreases elastic effect Similar stresses create more strain on material

Shoe Materials Ethylene and Vinyl Acetate (EVA) Polyurethane (PU) Upper Footbridge Midsole Outsole http://www.dummies.com/WileyCDA/DummiesArticle/id-450,subcat-SPORTS.html

Pronation Styles Wet sand imprints indicating foot shape and pronation style Most common strides (note: underpronation is also termed supination) http://www.dummies.com/WileyCDA/DummiesArticle/id-450,subcat-SPORTS.html

Shoe Wear Patterns Forefoot Striking Neutral Overpronation Supination http://www.fleetfeethouston.com/tips/basics.asp

Principles Design Matrix Electrical Mechanical Chemical Complexity 3 1 Feasibility 2 Cost Performance Total 11 8 7

Strain Gauge Directly affected by elasticity (ε = σ ∕ E) Inexpensive Small Requires calibration Incorrect alignment http://www.dur.ac.uk/richard.scott/gauges.html

Conductive Polymer Force Sensor Resistive elements Medium to high pressure range Compact Indirect Measurement Requires calibration http://interlinkstore.com

Force Differential Impact creates force (F), smaller dispersal force (f) F α f as a function of shoe sole force absorbance Force absorbance during impact is a function of elasticity Set ratio F:f will trigger sensor, indicate elastic wear

Retroreflectors Advantages Disadvantages Small, lightweight, & cheap Only taking one measurement (distance) instead of two Virtually no wear on device Disadvantages Need for clear polymer in some portions of the shoe sole Distance measurement may not correlate to shoe sole elasticity

Future Work Polymer which degrades faster than EVA or polyurethane Integrate retro-reflector, strain gauge, & a conductive polymer force sensor into polymer mold Perform tests to determine which device is able to detect wear (elasticity) the best Determine correlation between shoe wear and output of each device Select ultimate wear value Integrate chosen device into a shoe sole for field testing Possible patent application

Prototype Design Matrix Retro-Reflectors Strain Gauge Conductive Polymer Performance Cost Reliability Size/Weight Totals

Acknowledgments Professor Naomi Chesler Professor David Beebe Professor John Webster UW Biomedical-Engineering Dept.