A sensor plays a crucial role in a system designed to improve the walk of patients with a neurological condition
By Shana Leonard

As if having a spinal injury or stroke wasn't harrowing enough, such neurological trauma can leave patients struggling with permanent nerve damage or disabilities long after they leave the hospital. Afflicting some of these patients is a condition called foot drop. The disability causes a person to drag the toe of the shoe on the ground or slap the front of the foot on the floor, resulting in an awkward walking motion. Uncomfortable and impeding, the condition can also make victims self-conscious and thus lower quality of life.

Treatment for foot drop varies. However, a recent study published in the journal Neurorehabilitation and Neural Repair indicated that devices such as the WalkAide system by Innovative Neurotronics Inc. (Bethesda, MD) could improve the fluidity of a patient's movement. When strapped onto a patient's leg, the discreet device counters the lack of ankle dorsiflexion through functional electrical stimulation, which uses electrical currents to generate activity in the nervous system. Subsequently, the signals prompt the foot to be raised at the proper time for a less affected walking motion.

To ensure efficacy of the device, doctors must customize it to correspond with the patient's personal gait cycle. During the fitting, the clinician places a heel sensor in the patient's shoe to capture the heel up, heel down pattern. Using the data obtained from the sensor, doctors can calibrate an accelerometer in the system. This yields the optimal stimulation pattern and timing of the device.

Although the heel sensor is used solely during fittings, its function is crucial in the individual success of the system. Because of its importance, Innovative Neurotronics needed a company capable of producing the sensors to high standards and also on a tight deadline.

Initially, Plastics One (Roanoke, VA) was involved in a peripheral phase of the process, supplying the electrode cable for the system. However, the company also provided the molded connector and wire component for the prototype heel sensor. Satisfied with the manufacturer's handiwork, Innovative Neurotronics upgraded Plastics One's role in the project by entrusting the firm with the engineering and production of the heel sensor.

"It was a natural value-added service for Plastics One; they were already an approved vendor for two components of the WalkAide system," says Joseph Moser, director of product development and engineering for Innovative Neurotronics. "They were interested in adding more components and labor to the connector and cable and selling a completed production assembly."

For Plastics One, the challenging project was compounded by having to work under the constraints of an aggressive schedule: The firm needed to ship the first 50 sensors within two months.

"Innovative Neurotronics had prototype heel sensors, which were used during device design and testing. However, we realized that sales of the device would be limited by the number of trained clinicians qualified to fit the devices," Moser says. "Therefore, Innovative Neurotronics accelerated the clinical training schedule. That meant we would need more heel sensors than originally planned. Our first training class was early in February; we needed 50 sensors by the end of January." With completed design plans supplied from the start, the company was at an advantage; however, it had to adapt the plans to suit its in-house production processes. The front-end work for the product involved design reviews, fixture design, production routing creation, and obtaining vendors to provide the materials.

In adapting the assembly procedures from Innovative Neurotronics' documentation, Plastics One had to develop several assembly fixtures. Most of the fixture design was done using Solidworks 3-D design software and Feature Cam, which provides data for use by milling machines to produce the designed fixtures. "Coming up with the fixtures and creating a production process to get the finished product out [was a challenge], especially under such a tight timeline," says Todd Mabry, a manufacturing engineer for Plastics One. "For most of the assembly fixtures, we would take the piece parts for the product, develop those in the 3-D design software, and then we would create the assembly fixtures from those parts within the 3-D system, which is a pretty efficient way to get those fixtures out to the floor as well."

To develop the product, the company began by stamping the top and bottom foam pieces for the sensor using a foam die-cutter. Next, the team established the force-sensing resistor (FSR) soldering fixture to allow the soldering of the FSR and the cable assembly of the heel, and right- and left-foot sensors. An air-expulsion clamp was then employed to force air out of the FSR during the silicon process to protect the sensor from perspiration with a moisture-resistant barrier. Assembly fixtures were used to position the FSR on the adhesive foam and to glue cables to the bottom foam pieces. They were also used to tack-weld the top foam onto the assembly.

The assembled sensors were then placed in a heat press. This process activates the foam adhesive and seals the sensor into the assembly. For testing purposes, the sensor was formed onto a prosthetic foot mold from the heat press and cooled to its final form. Afterwards, it underwent electrical tests prior to compatibility testing with the WalkAide unit.

Despite the daunting timeline, Plastics One was able to manufacture and ship the sensors on time. Recently, the company received an order for the manufacture of several hundred more, according to John Richardson, vice president of sales for Plastics One.

The WalkAide system received FDA approval last spring.

Copyright ©2006 Medical Product Manufacturing News

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