Originally Published MPMN May 2001

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Carbon Fiber–Reinforced Polymer Specified for Prosthetic Device

Other materials could not support weight loads.

In developing the Total Concept adjustable-heel-height articulated ankle, Century XXII Innovations Inc. (Jackson, MI), a manufacturer of prosthetic limbs, experienced difficulties in complying with ISO requirements. The ankle-case housings had to endure at least 2 million cycles of released 80-kg weight loads—equivalent to a person walking 2 million steps. "Over a six-month period, we molded the part using a half-dozen different materials including Kevlar and PEEK," recalls director of engineering Grant Wild. "But these materials just couldn't handle the required 80-kg weight loads."The company's prototype housings were giving out after just 50,000 cycles. "The parts cracked at the knit lines due to the molding materials' inability to take the required weight loads," explains Wild. Kevlar and PEEK are used in many medical devices, but they did not fit this particular application. "All the materials that we tested had good tensile strength, but they were too brittle for cycling loads." The materials had only –1% elongation, which is insufficient for repetitive impact absorption. Realizing that its project was in jeopardy, the company asked its molder, Hilco Technologies (Grand Rapids, MI), for a material with a better combination of tensile strength and elongation properties.The search led to Latamid 66 H2 K/40 Natural, a nylon 6,6 reinforced with carbon fiber and developed by LATI USA (Mount Pleasant, SC). Like Kevlar and PEEK, nylon 6,6 has good tensile strength and toughness for handling repetitive impact loads. However, because it is reinforced with 40% glass fibers, it also achieves a high notched Izod strength for added toughness, as well as +2% elongation at break.Before trial testing, LATI engineers performed mold-flow analysis to optimize gate locations and to verify that knit lines were not in a critical, highly stressed area of the part. After molding one of the ankle-case housings using Latamid, Hilco tested it to see if it could pass the ISO weight-load requirements. The Latamid part withstood more than 3 million cycles. In less than 60 days, the part passed all the ISO testing requirements.After successful trial testing, Century XXII engineers standardized the material for all its ankle housings. In addition to providing a solution to the material-durability problem, Latamid reduced material costs by 66%, according to the company.The ankle prosthetic is available in narrow foot and unisex foot models. The device can be easily adjusted to any fixed angle, from neutral up to 10° dorsiflexion (toe up) or 25° plantar flexion (toe down). Adjustments can be done without any tools and without removing the foot: a simple push-button valve mechanism releases the ankle, allowing the amputee to make the desired adjustment.Wire-Harness Software Speeds Design of Vision-Correction SystemProgram simplifies layout of harness and cable paths.EMbassy Works allows users to map out how wires will interact with connectors.

In the process of designing and building its Pulse FS Laser for the vision-correction surgery market, Intralase (Irvine, CA) experienced an unforeseen challenge. The wiring harness design for the system would take months if created conventionally by using tape measures and string. EMbassy Wire Harness software from Linius Technologies (Westborough, MA) enabled the company to finish the project in a week, substantially accelerating ramp up, according to Intralase engineering manager Bob Ozarski.EMbassy Works offers a wire list import function that automatically creates wires by means of a configurable ASCII from/to list or by manual input. The autorouting function automatically routes the wires and cables into a network of predetermined paths. Design Rule Checks enables verification of design integrity and determines whether the design meets both engineer specifications and corporate standards. Changes in components and cable-routing alterations because of bundle size and data line or power line location are easily and quickly implemented. Design Rule Checks can be modified and an upgraded cable assembly manufactured.Linius' wire harness software can plan out and display harnesses inside chassiss.

Intralase purchased a floating seat of EMbassy Works, a version that operates within the SolidWorks CAD software environment. It allows engineers to build prototypes in parallel while producing virtual cable assemblies. "The physical prototype was being built simultaneously and entered testing almost immediately following the EMbassy Works layout," Ozarski says. Automatic calculations provided wire length and bundle diameter data in a format that was immediately accessible and continually updatable whenever the design changed, he added.

Since Intralase expected to produce approximately140 units during the first run, the company used EM~Nailboard and EM~Report, EMbassy Works' manufacturing and backend functions that streamline the creation of manufacturing documentation. EM~Nailboard automatically creates accurate manufacturing drawings in life-size and not-to-scale versions. EM~Report extracts design data to produce material and labor cost estimates, a bill of materials, and wire run lists.

With the help of EMbassy Works, Intralase engineers were able to define 135 cable routes in 7 days.

Copyright ©2001 Medical Product Manufacturing News