Floor Materials to Prevent Hip Fractures
Floor Materials to Prevent Hip Fractures
Designers: Saeed M. Huda, Timothy K. O’Brien, Tujuana M. Shaw
Client Coordinator: Alan W. Eberhardt, Department of Biomedical Engineering
Fall-related hip fractures are a major source of injury in elderly persons in the United States. The idea for the current design was to develop flooring materials that remain stiff under normal walking conditions (low strain rates) and soften to provide cushion during a fall (high strain rate). The use of shear-thinning fluids in sandwich composites offers a promising method for such floor systems. Shear-thinning fluid is a strain rate sensitive material, which caters to a flooring system that will absorb impact. Based on understanding this fact, the present design study was performed to investigate new materials’ abilities to minimize the peak force seen during impact and to maximize the amount of energy attenuated. The target force values utilized were based on research conducted by Hayes and coworkers who showed hip fractures occurred within 0.78 – 4.00 kN range at energies of 5-50J.
The floor would consist of a shear-thinning fluid component encased in a silicone rubber liner. The silicone rubber selected for this project was uncured Smooth-SilTM 930, (Smooth-On Inc). The ratio to mix parts rubber and catalyst was 10:1. The rubber cured in 16-24 hours. The shear-thinning components tested were Laponite® RD (Rockwell Additives) and Carbopol® (Noveon). Laponite® RD is a synthetic silicate clay that mimics the naturally occurring smectite mineral called hectorite. The open network structure of Laponite® RD allows the chains to align under high shear rates, which causes a thinning response in the material. Carbopol® acts in a similar manner. Previous analyses of the rheology of these products with water alone, suggested that Laponite® has a greater capacity to shear thin. By mixing both the silicone rubber and the shear- thinning component, it was thought that a composite material could be developed that exhibited the strength and resiliency (of the rubber) and shear-thinning ability (Laponite® RD or Carbopol®). In essence, the flooring system was designed to be a shear-thinning rubber. The silicone rubber/powder mix was poured into seven cm aluminum containers. The samples were degassed in a vacuum chamber for five minutes and allowed to cure for 16 – 24 hours. The samples were then post-cured in a recirculating air furnace for two hours at 80 ºC then one hour at 100 ºC. All samples were made at a thickness of 1.27 cm (0.5 in.).
The testing was performed using guidance from ASTM F 1931-98, using a drop tower (Dynatup Instron Model 830-1). The initial setup incorporated the use of a striker head affixed to a load cell to determine energy, force, and velocity values during an impact. A high speed motion detector, operating at 300 Hertz, was also used to record rebound information during the testing. Both the load cell and motion detection data were recorded electronically and converted to spreadsheet form for further review. Testing for comparative values of impact force and time to peak load was performed using a 3.5 kg mass dropped from a height of five cm. A rectangular (7 cm x 10 cm) striking surface was attached to the load cell. Each sample was impacted three times with a minimum of one minute between each successive strike.
The TUP software generated several key values for the samples tested. Among these values are peak force (kN) and energy produced at peak force (J). Peak force values from the six trials performed on each sample were plotted as function of the amount of shear-thinning agent present. Figure 6 shows the peak force values of all the silicone rubber/shear thinning agent samples tested. The 12.5 weight percent silicone rubber/Laponite® RD samples showed the lowest force out of all the samples tested.
The results achieved during this project indicate the mechanical properties of the silicon rubber / Laponite® system are a potentially viable solution for this type of a flooring system. Further testing and characterization of the compound is needed to fully quantify the role Laponite® contributes to energy absorption. Advantages of the silicone rubber/ Laponite® compound over the close cell foam include sanitation and ease of fabrication.
The primary disadvantage is cost. The projected production cost (Table 1) is in excess of sixteen times the cost of commercially available closed cell rubber foam mats.
Table 1. Projected costs of flooring system (8’ x 8’ room).
Material Cost/Square Foot: $38.73
Estimated Fabrication Cost/Square Foot: $3.49
Estimated Installation Cost/Square Foot: $2.00
Material Cost for 8’ X 8’ Floor: $2,478.78
Installation Cost for 8’ X 8’ Floor: $128.00
Total Cost for 8’ X 8’ Floor: $3,180.97
Cost/Square Foot: $44.22
Figure 15. Laponite containing material exhibited the lowest impact force (left) and second highest energy dissipation (right) among the materials tested (from left to right – open cell, closed cell, Laponite, Carbopol)