Plastics advance medical-device designs

If it's medical it must be good! That's the reaction many Design News readers have when dissecting a story about a plastic material that's instrumental in the design of a medical device. After all, if such a material can pass the exhaustive tests required by the FDA and other regulatory authorities, it could very well provide the solution for many other types of design problems.

The following examples reveal some of the innovative uses these polymers must adapt to in diverse medical applications. For instance, in many cases the material must withstand sterilization procedures, survive the impacts experienced by an EMT unit answering a hazardous, life-saving call, or give the user the assurance that a medical instrument will perform flawlessly over many years of service. Here are some examples of innovative plastic designs:

Urethane foam toughens defibrillator

Double-sided, pressure-sensitive adhesive foam tape might not be the first material a design engineer thinks of when it comes to attaching a critical but fragile component in a ruggedized medical device. That's why engineers at Physio-Control (Redmond, WA) initially developed their prototype LIFEPAK(reg) 10/10C defibrillator/monitor/pacemaker with a plastic mounting bracket securing a liquid crystal display (LCD) screen to the portable unit's plastic case cover.

But the multilayered glass LCD screen repeatedly cracked during drop tests. "We were seeing about 1,000 gs," explains Jim Benson design engineer at Physio-Control.

To avoid plastic brackets bolted around the LIFEPAK 10 display screen, Benson decided to adhere it to its aluminum mounting bracket with a urethane foam fabricated with adhesive tape on both sides. He had successfully used the foam (PORON supplied by Rogers Corp., Rogers, CT) on other defibrillator/monitor equipment using "both the low- and medium-modulus types. However, we had never used it as the primary mounting medium, and we thought this application would work well based on the material specs and our previous experience," Benson is quick to add.

A 2- x 2-inch-square mounting pad, made of 1/8-inch-thick foam, adheres on one side to the four-square-inch printed circuit board in back of the display-screen module. The other side attaches directly to the aluminum mounting bracket in the case cover (see diagram next page).

The die-cut mounting pad accommodates an integrated circuit, cable connector, and nuts holding the display unit to its printed circuit board, and allows for use of pins on an assembly fixture to align the parts. Two strips of 1/16-inch PORON urethane with adhesive on only one side also help to add adhesion and provide shock absorption on the front of the display module. Added urethane strips provide shock absorption on the sides, top, and bottom of the assembly.

LIFEPAK 10 LCD module mounting features a foam tape/ adhesive combination that wards off impact shocks and vibration.

The redesign proved immediately successful, since the unit repeatedly survived the drop tests. It also withstood tests at elevated and cold temperatures. The unit operates under conditions ranging from 10 to 55C. Performance also won't degrade after the unit is stored from ­30 to 65C. "Even at these temperatures, the Rogers material continued to adhere and provide good damping characteristics," Benson adds.

Such performance highlights one of the key design reasons that the Physio-Control units have survived many critical escapades on their life-saving journeys. For example, the LIFEPAK 10C system features:

- An ECG monitor that includes a cardioscope display, strip chart recorder, and status display.

- Noninvasive pacemaker with an output rate of 40 to 170 bpm.

- Defibrillator with paddle controls and a synchronizer that delivers energy discharge within 20 msec of the sync marker on the cardioscope.

Several anecdotes relayed to the company by firefighters, police, and other emergency personnel attest to the unit's survival capabilities.

For instance, a unit accidentally fell into a barge canal in Rochester, NY, during a rope rescue up a steep bank. Although visibility was only about four inches in the muddy water, a SCUBA team found the unit because of the glow given off by the illuminated display. After cleaning and testing, the unit went back in service. Two other units powered right up after sustaining only cosmetic damage following a storage-room fire at a Milwaukee ambulance-service facility.

In another recent incident, paramedics pulled over an erratic driver and set the unit on the ground in case it might be needed. When a background check revealed the car was stolen, an officer moved his patrol car to prevent the suspect from making a getaway and ran over the unit. A thorough check revealed the unit continued to execute every function it was designed to perform.

TPE solves adhesion, flexibility problems

While many Americans can rest soundly each night, a condition called obstructive sleep apnea (OSA) continuously plagues the sleep of about 30 million others. OSA relaxes the tongue and airway muscles during sleep, blocking airflow and causing 10- to 40-sec interruptions in breathing as many as 30 times an hour--hundreds of times a night. But OSA patients may now find relief with SleepNet Corp.'s (Manchester, NH) patent-pending "Phantom Mask."

OSA decreases a patient's blood oxygen saturation level, which requires the heart to work harder. Potential long-term health problems, such as coronary heart disease, stroke, psychiatric problems, cognitive dysfunction, and memory loss can occur as a result. "In addition, those with OSA are seven times more likely to have an automobile accident than others in the general population due to fatigue," according to Paul Chisea, CFO for SleepNet.

Since the early 1980s, OSA sufferers have relied on various continuous passage airway pressure (CPAP) devices, that, when connected by a tube to an oxygen source, deliver continuous air flow through the patient's nose during sleep. Until now, however, most CPAP masks consisted of a hard plastic shell that does not always conform to the contour of the wearer's face, causing discomfort and air leaks at the bridge of the nose. In addition, many patients can develop pressure sores from a mask if secured too tightly to compensate for a poor fit around the nose. And the clumsy construction of a hard plastic mask can obstruct vision or cause claustrophobia.

Innovative design of the Phantom Mask allows for an air tube on either side, rather than a cumbersome central tube above the nose.

SleepNet sought to create a flexible mask with a soft shell that would provide an optimum ergonomic, customized fit for a wide spectrum of patients. It also wanted to ensure its new mask could deliver the proper rate of air flow to the wearer, while accomodating appropriate pressure flow of the CPAP device without creating an air "bottleneck."

Design and comfort requirements were interwined when selecting the best thermoplastic materials to fabricate the mask, recalls Luc Boissonneault, marketing director for injection-molder GLP Hi-Tech (St. Jean-sur-Richelieu, Quebec, Canada), the mask's molder. "We needed a material that would offer excellent adhesion to the mask's silicone gel bladder encased in a polyurethane film designed to fit inside the mask to cushion the wearer's face," he explains. The bladder attaches to a pliable metal nose clip that enables the wearer to tighten or loosen the mask to fit.

The CPAP design allows for an air tube on either side of the mask, rather than a central tube above the nose, as in some older masks. "HDPE (high-density polyethylene) was considered early on in the manufacturing design stage, but ruled out because of its poor binding properties," Biossonneault notes.

To withstand the rigors of manufacturing, as well as the mask's end-use environment, the molder needed a material that would provide superior strength, wear, and abrasion resistance. Thermoplastic rubber (TRP) failed to meet these stringent performance requirements.

At the recommendaton of General Polymers, a materials distributor located in Laval, Quebec, Canada, GLP Hi-Tech tested PELLETHANE 2363 80AE thermoplastic polyurethane elastomer (TPE) supplied by Dow Plastics (Midland, MI). "The flexibility of Pellethane elastomers gave the mask the ability to conform to the wearer's face," Boissonneault reveals, "while providing the needed adhesion to the mask's silicone gel bladder."

"PELLETHANE elastomers provide a breath of physical and processing characteristics that set them apart from other TPEs or thermoset rubbers," says Nancy Hermanson, medical market technical leader for Dow Plastics. The materials come in durometers ranging from Shore 70A (the softest) to Shore 70D.

In addition to flexibility, PELLETHANE elastomers reportedly provide excellent tear, tensile, and mechanical strength, resulting in good toughness properties. Their strength was particularly important in order to meet the needs of the mask's eyelets, which must be durable because the headband is threaded through them. "Also, the eyelets need to maintain their shape and resist tearing caused by friction and elongation pressure from patients who move around in their sleep," Boissonneault adds.

Another critical consideration was the ability to unmold the mask's eyelets and ventilation hole without deforming the part. "In this application, PELLETHANE elastomers were the only material able to withstand the process," Biossonneault reports.

"There has been an incredible response to the Phantom Mask since its commercial intro-duction last January," Chiesa enthuses. "We're hearing fromthe field that patients evaluated in sleep labs are actually refusing to give them back."

Metal-injection molding cuts parts 70%

Also on the subject of breathing, studies show that more than 70% of patients with press-and-breathe inhalers frequently use them incorrectly. That's why Aradigm Corp. (Hayward, CA) created the SmartMist(TM) respiratory management system. The technology determines when the patient is breathing correctly and releases medication only when conditions are optimal.

With the use of a microprocessor, the SmartMist monitors the patient's breathing and medication usage. It even stores over one month's data for physicians to access at a later time via an integrated serial port connection. In short, the system combines drug delivery, peak flow monitoring, and compliance monitoring in one unit.

"When choosing a primary vendor for the system, we looked for a minimum-risk solution," notes Carl Ritson, principal mechanical engineer with Aradigm. The company found just such a partner in Phillips Plastics Corp. (Hudson, WI) when it came to many of the system's critical prototypes and the production schedule.

Four of Phillips' business units worked with Aradigm through the development and production phases of the program. Prototype Technologies built two prototype tools for the chassis and MDI latch. Aradigm needed the two key components in order to test and ensure plastic could be used in a design that involved high stresses.

Phillips' Customer Center-Central built stereolithography parts for three components that make up the case and switch array to see that they fit correctly and were functional. Then Phillips' Short Run Solutions, a business unit specializing in low-volume production runs (100 to 100,000 parts annually), molded 20 of the unit's plastic parts. A secondary pad printing operation also takes place at Short Run Solutions. Finally, three of the system's internal components--escapement lever, escapement drum with gear, and cam shaft--are produced at Phillips' Powder Metal Molding(TM) (PMM) operation, a custom metal-injection molder of complex metal parts.

Phillips engineers became involved with the Aradigm program at the design phase. "Phillips doesn't just make a tool and mold parts; their personnel actually participate in determining that the design is correct and moldable, making a lot of suggestions along the way while we were still designing the housing and various parts," notes Diane Jachinowski, Aradigm's director of product development, respiratory products. "It was that interactive participation throughout our entire project that set the standard."

Nowhere did this involvement become more obvious than in the design of the system's venturi. Phillips engineers suggested that it would be less costly to mold the component in two pieces, venturi left and right, and heat stake them together. "That's what we ended up doing," remarks Ritson. "The savings were realized primarly from tooling, since the tool would have been so complicated that it would cost far more than two separate tools to produce the parts."

The design also involved complex metal parts. Two alternatives proved too expensive--machining and fabricating. The third alternative: metal-injection molding.

Using metal-injection molding, metal parts for the Aradigm project went from as many as 10 parts down to three. Phillips' Powder Metal Molding (PMM) operations performed the work, including design modification and materials selection. Aradigm's primary material requirements were wear resistance and hardness; PMM's team of metallurgists found the right alloy in iron-nickel-steel.

PMM uses commercial feedstock, which diminishes the uncertainties involved when metal injection molders choose to formulate their own feedstock. "Since this is our first product, we wanted to maintain supply and quality," explains Martin Fogle, senior engineer at Aradigm.

The technology employed at PMM (see diagram) also played a role in Aradigm's decision. "PMM has a continuous feed process--parts move through a gradual progression of temperatures and atmospheres," Fogle explains. "Virtually everyone else we visited used a batch process. We found the continuous process much more economical and productive."

The end result: the SmartMist system, which just hit the market, was designed, produced, and shipped on time and ahead of the competition.

What this means to you

- Plastics used in medical-device designs can also prove problem solvers in other troublesome design applications.

- Employing plastics in a design can often reduce the number of components needed, cut assembly costs, and get a product to market faster.

- Plastic resin producers, compounders, and molders can provide a wealth of technical design services to help make the right material selection for your next design project.

The ABCs of metal-injection molding

In spite of all the publicity about how plastics are replacing metals, the two materials can be very compatible. Nowhere is this synergy more apparent than in metal injection molding as performed at Phillips Plastics Corp., Hudson, WI. Here's a quick lesson on how plastics and powder metals work together to make a complex metal part.