If indwelling bladder catheters have a bright side, it's that they are usually temporary. Some people, however, aren't so lucky. Spinal cord injuries, multiple sclerosis, and other conditions can all entail a more persistent loss of bladder function that requires extended catheter use. And these long-term catheters can trigger complications ranging from unpleasant to serious. Because they allow urine to flow continuously into a collection bag, they can be inconvenient for patients. More seriously, though, they can up the risk of urinary infections. David Flinchbaugh, a physicist turned engineer, has come up with a way to combat both of these problems.
Called the UroCycler, this patented device from UroSolutions attaches to the outgoing tube of a urinary catheter and functions as a passive shut-off valve. The valve, which relies on carefully selected magnets for actuation, opens automatically when the bladder reaches its normal voiding pressure. Once the urine has drained, the valve then closes without any action on the part of the patient or caregiver.
Flinchbaugh had to cope with several engineering obstacles in coming up with this unique valve. For one, it operates within a tiny pressure window and had to seal to a maximum pressure of 25 cm water. "This translates to about 0.25 psi," Flinchbaugh says. And because the valve has to be so precise and operate reliably through thousands of cycles, Flinchbaugh quickly rejected the idea of using springs. Instead, he based the valves on two ceramic magnets and a silicone elastomer flowbody. The valve works simply enough: Oriented with their opposite poles facing one another, the magnets' natural attraction seats them against the silicone flowbody, where they act much like stoppers to close the valve. Ports in the upstream half of the flowbody allow urine to flow between the magnets. As that urine pressure rises, it overcomes the attractive force of the magnets, forcing them apart and opening the valve.
What wasn't so simple was finding the right magnets for this application. Flinchbaugh reports that he went through a painstaking trial-and-error process that involved matching the separation force of the magnets to the valve's desired opening pressure. He ultimately identified an injection molded ceramic magnet that would work.
He also went through a lengthy process to find the right material for sealing surfaces. It had to be tough enough for thousands of cycles, soft enough to seal under the limited force provided by the magnets, and sticky enough to bond to upstream magnet. It also had to be chemically and thermally resistant to hold up to sterilization processes.
He found a suitable elastomer choice in a silicone rubber from Dow Corning (Midland, MI). With a durometer of roughly 3 Shore A, FDA approval, and chemical resistance, it fit all the requirements other than easy bonding. This last attribute Flinchbaugh imparts by modifying the surface of the elastomer seals. "We've developed a process that allows us to create a tough, non-stick surface on one side of the seal and a tough sticky surface on the side that bonds to the magnets," he explains.
To reduce the possibility of urinary tract fections, Flinchbaugh added a 0.012-inch air intake and filter to the UroCycler valve. Located within the valve's polycarbonate housing, just downstream of the magnets, this filter allows air to enter the tube so the urine can fully drain. "The filter meters the amount of airflow pulled by the venturi effect of the urine flow in the valve's exit," Flinchbaugh says. At the same time, the filter also keeps out microbes. And it has to prevent urine from escaping the catheter in the event that any mishandling-think "squeezing"-of the collection bag creates backpressure in the catheter.
To create the filter, Flinchbaugh once again went through a trial-and-error process, this time with a variety of commercial filter media. He settled on a hydrophobic polyethylene micro-pore respiratory filter that keeps out anything bigger than 0.4 microns. "It's the best choice because it allows air molecules to permeate it, but liquid won't saturate or pass through it."