With all their regulatory and technical requirements, medical devices don't always lend themselves to aggressive cost reduction. At least, not without some extra design effort. “The medical guys are pushing the envelope right now and coming up with designs that deliver the maximum function for a minimum cost,” says Mike Stoddard, a Phillips Plastics project manager responsible for medical molding. One such design comes from the engineers at Johnson & Johnson's Ethicon division. They managed to take a significant amount of cost out of an applicator gun used for the company's Gynecare Profix surgical fasteners.
The fasteners, which are micro-molded from an implantable polypropylene grade, permanently affix a surgical mesh to ligaments during pelvic floor repairs, a type of gynecological hernia operation. Ethicon adapted the fastener design from an earlier fastener used to repair meniscus tears in the knee. For pelvic floor repairs, though, the fasteners needed some modifications to hold the surgical mesh in place. The shape of the meniscus-repair design resembles a single foot and leg, “which works great for a meniscus tear,” explains Mark Howansky, a staff engineer in Ethicon's r&d group. Surgeons use a special gun to send the fastener into meniscal cartilage adjacent to the tear to tack down the loose edges.
But the original fastener design had no feature capable of locking surgical mesh permanently in place. “There, mesh would have just slipped out from under a single-legged fastener,” Howansky says. So Ethicon engineers designed a new fastener with two parallel feet connected by a continuous, u-shaped beam. “Basically, we doubled up the earlier fastener,” explains Howansky. “The two feet are anchored in the tissue while the beam captures the mesh.” Think of it as a flexible plastic staple.
Lots of effort went into optimizing the fastener geometry and micro-molding operation that produces it. But the new fastener's geometry also forced Ethicon engineers to come up with a new applicator gun to install it. While the gun outwardly seems to take a few cues from the design used for the meniscal fastener, it actually has been completely revamped with an eye toward balancing function and manufacturing costs. “The gun is a single-use item with low production volumes so we had to keep the total cost of goods in check,” says Howansky. The hernia surgeries also have a lower reimbursement than the knee repair, making cost all the more critical.
Ethicon's engineers, with some help from design-for-manufacturing experts at Phillips Plastics, made smart design decisions that ultimately allowed them to deliver the new gun for about one-third the cost of the meniscal gun. Here's how they did it:
Since much of the 11-part gun is made from injection molded engineering plastics, the use of molded-in features to allow parts' consolidation presented an obvious path toward cost reduction. But it wasn't an easy one. Howansky notes that this medical device, like most of them, had materials restrictions and functional requirements that limited his choices. All the plastics, for example, had to withstand ETO sterilization and be part of Ethicon's biocompatibility database since the gun is inserted into the body.
Perhaps the most innovative plastic part is the gun's trigger assembly, which has a feature Howansky calls a “passive lock.” It keeps the surgeons from accidently releasing a fastener as they insert the gun into the body. “We thought about a traditional mechanical trigger lock, but the surgeons think they're more trouble than they're worth,” Howansky says. The trigger lock would also have added several more parts, and the associated cost, to the assembly.
So Howansky instead used a cam to create a resistive force profile for the trigger that makes it difficult to release a fastener accidently during insertion, yet not too hard to fire when the time comes. The spring-loaded cam also returns the trigger to the firing position after releasing a fastener.
While there's nothing special about cams per se, Howansky found a clever way to integrate one into the gun while minimizing parts' count — he designed a cam track into the gun's housing. The cam follower is simply a flex arm molded into the polycarbonate trigger. “The passive lock is implemented in two parts that were already there,” he says. To get the geometry that would deliver the right amount of resistance, Howansky created computer and physical models, the latter of which he tested with surgeons to get their feedback on the force profile.
There were a couple of cost trade-offs associated with the part, however. Howansky initially wanted to go with ABS for the housings and trigger for cost reasons. However, he ultimately had to upgrade to a pricier glass-filled polycarbonate. “ABS has adequate flexural properties, but it has a lower yield point than polycarbonate,” he says. “We needed the higher yield to keep the flex arm from cracking.”
Polycarbonate did initially pose some friction problems. “Polycarbonate on polycarbonate is a really bad friction condition,” Howansky says, noting that the problem was solved by switching the trigger to a more expensive internally lubricated grade.
And to mold the trigger without a parting line where the surgeons finger would rest, Howansky asked Phillips Plastics, which both molds and assembles the gun, to create a more complex tool with two-side actions. It added some cost but allowed the parting line to shift to a more ergonomic location on the edge of the trigger.
Another key plastic part on the gun is the long plastic shield that protects the gun's needle-like firing mechanism during insertion into the body. Measuring 10 inches long, the thin, strip-like polyethylene part also contains tiny locating features and snap fits at either end. “Precision features separate by 10 inches of plastic,” is how Howansky puts it. The use of a single long part here did keep the parts' count in check. But it also created molding challenges for Phillips — challenges made all the worse by the high shrink of the plastic chosen for this part.
Phillip's Stoddard, though, helped turn lemons into lemonade with a simple observation about the shrink and tiny nub-features on the front of the shield. Looking at the tooling design, the nubs appear trapped in tool steel. But Stoddard thought the shrink — 0.020 inch/inch — might be able free the nubs. “That was just enough to allow the 0.050-inch nubs to clear the tool steel after a bit of cooling,” he says. Howansky adds that turning the shrink into positive saved thousands in tooling costs. “We were expecting expensive actions in the tool to release the nub features,” he says.
Phillips provided other design for manufacturing advice for the part, saving even more money. To assemble the gun's housing, for example, Phillips engineers recommended a combination of sonic welding and molded-in press-fit pins. “I had gone into the project thinking one joining method or the other,” Howansky recalls. “Phillips suggested we use both on the same assembly.”
According to Stoddard, sonic welding the entire joint would have required an expensive, tricky horn design given the gun's shape. What's more, the gun may have required multiple set-ups on the welding machine, adding to the assembly costs. It would have also required molded-in energy directing features that would have added to tooling complexity.
Relying entirely on press-fits wouldn't work either since there wasn't enough space in all sections of the gun — especially the tip — to accommodate necessary pins and bosses. So Phillips created an assembly set-up in which the tip of the gun is sonic welded while the bulkier handle sections go together with the press-fits. “It's the best of both worlds from an assembly standpoint,” Stoddard says.
Not all of the cost saving design work involved plastics. In fact, some of the biggest cost-out moves involved the gun's metal firing mechanism. Prior to application, the “feet” of the surgical fasteners ride inside parallel hollow needles at the tip of the gun. A metal rod within the tubes then pushes the fasteners out and into the tissue upon actuation by the trigger. Howansky took cost out of these normally expensive custom metal components in a couple of ways.
For the actuation rod, he abandoned a costly manufacturing method used on the gun that installs the previous meniscal fastener. While effective, the earlier rod design consisted of coiled wire laser-welded to a straight steel rod with overmolded plastic piece at the end to provide the connection to the rest of the gun. This assembly costs roughly $15 each, and Howansky would have needed two of them to accommodate the new “doubled-up” fastener.
So he came up with a new actuation rod based on one long length of coiled wire partially encased in a stainless steel hypo tube. Ethicon's metal supplier came up with a process to bend this long length of wire-and-tube in half with each end serving as an individual actuation rod. The bend itself also serves a purpose. It loops though a hole molded into the trigger, creating a linkage without the need for extra parts or a complex assembly operation. “Again, existing components take on extra functions,” says Howansky.
From a performance standpoint, this tube design gives the actuator the stiffness it needs while retaining the flexibility of the coiled wire. And flexibility here is key. The wire has to traverse an arced actuation path — through the needles that form the tip of the instrument and engage the tissue during surgery. Ethicon creates these needles from two sharpened hypo-tubes welded into a stamped backplate.
Total cost for the new firing mechanism is under $10, versus a budget-busting $30 for two of the old-style actuators.
“This design kills three birds with one stone,” says Howansky. It eliminated an expensive end-to-end welding, overmolding and assembly operations. And eliminating manufacturing steps and components is the name of the game when you're trying to cut costs.