Medical Device Link.

Originally Published EMDM November 2003

ENGINEERING INSIGHT

The one-piece housing of disposable nerve stimulators from Medtronic Xomed simplifies assembly and decreases the number of operations by nearly 75%, according to the company.

In order to improve its disposable nerve stimulators and locators, Medtronic Xomed (Jacksonville, FL, USA), teamed with the University of Florida’s educational initiative, Integrated Product and Process Design (IPPD; Gainsville, FL, USA). The redesign project included goals such as a 50% cost reduction and production of the device to six-sigma standards. The team also wanted to design the products to allow the use of manufacturing processes, as well as to improve its ergonomics.

Nerve stimulators are used during surgical procedures. During surgeries, relying on visual recognition of nerve structures can be difficult, and errors can lead to irreparable damage. The systems enable surgeons to find nerves by inducing small electrical currents to excite and identify them.

In redesigning the stimulators, the team first focused on the housing and on the assembly process. It was believed that major cost reductions could result by simplifying the manufacturing process and decreasing the need for rework. 

A seven-step assembly process was devised. The moulded cylindrical housing is loaded into a machine and dimpled for component placement. The internal components are loaded into the housing from the top of the part and then sealed with a rolling operation. The new concept incorporates a clamshell design. The cylindrical housing is split along its length, with an assembly hinge and snap fits.  The one-piece housing provides visibility and increases control during the installation of the internal components. Designed for manufacturability, the new housing reduces the number of assembly operations by nearly 75% and eliminates secondary work.

The Right Resin

During early design concepts, two rapid prototypes of the housing were constructed on a Viper si2 stereolithography (SL) system. While suitable for form and fit analysis, the prototypes could not be used for functional testing of the assembly hinge and snap fits. The rigid SL material did not offer the required flexural properties. To overcome this, small segments of the early design concept were machined in ABS for functional testing of the snap fits.

However, to obtain a functional analysis of the final design, assessing the functionality of the assembly hinge was vital. IPPD used Somos 8120 resin made by DSM Somos (Milan, Italy) to create new prototypes. The resin is a high-speed liquid photopolymer. It produces accurate, flexible, high-impact-strength parts using stereolithography. “Somos 8120 is ideally suited for this project,” says Michelle Wyatt, account manager for DSM Somos, “because the material is especially useful in functional applications, such as medical products, where flexibility and impact strength are critical requirements.” 

Working Prototypes

Upon receiving the prototypes, the team immediately noticed the difference from the previous SL versions. “The Somos 8120 prototypes were great,” says David Rumsey, an IPPD team member. “They were definitely more flexible than the previous prototypes, and the finish and colour were much better.”
At the end of the project, the team made its presentation of the redesign to Medtronic Xomed. They used the prototypes to show design and manufacturing concepts. 

“The team has done an excellent job with the new housing design,” says Mike Maszy, manufacturing engineer at Medtronic Xomed. “I am equally impressed with the job they have done on design for manufacturing. We are confident that this work will deliver, if not exceed, the 50% cost reduction we are targeting.” 

DSM Somos also offers materials for selective laser sintering. Its powder elastomeric polymer can be sintered to produce highly flexible parts with rubberlike characteristics.

To learn more about DSM Somos, circle #8 on the Reader Service Card, or go to RequestLink at www.devicelink.com/emdm. 

Susan Wallace

Copyright ©2003 European Medical Device Manufacturer