Originally Published March 2000

PROFILE

Custom Flow Sensor Facilitates Manufacture of Dialysis Machine

Karim Marouf, Managing Editor

It is terribly frustrating for engineers to have their well-designed medical devices hampered by a single troublesome component. This was the case with Althin Medical (Miami Lakes, FL) and its System 1000 dialysis machine. The machine's innovative design was acknowledged by three international design awards, but its success was initially complicated by one component—the flow switch.

The System 1000's design is not only aesthetically pleasing, but ergonomic as well.

Dialysis machines purify the blood of deadly toxins and treat overhydration by removing excess fluids. When the patient's blood enters the System 1000 dialyzer, it travels through more than 10,000 hollow fibers, which act like a sheet of cellophane, permeable to some substances and not others. Uremic toxins diffuse through the membrane while blood cells and proteins are retained. Outside the membrane, a constant flow of dialysate (an aqueous solution containing physiologic salts) flushes away the toxins. Simultaneously, water is extracted and the purified blood is channeled back into circulation.

The challenge

A thermal dispersion flow switch was located inside the system's balance chamber, where fluid is metered to ensure accurate removal of the excess fluid accrued by the patient. According to Tom Kelly, Althin Medical's director of marketing, "The sensor worked fine, but it was very sensitive to the manufacturing process. It had a little glass bead resistor that was epoxied into a housing, and if it wasn't assembled just right, the glass would crack."

The FS-W flow sensor was reliable and easy to install in the System 1000 dialysis machine.

Althin Medical contacted Gems Sensors (Plainville, CT) for help in replacing the troublesome sensor. A manufacturer of flow, level, and pressure sensors, Gems supplies products used to maintain flow rates in cooling loops in high-powered lasers and for circulating heated water through hypothermia blankets, among other medical applications. Depending on the application, Gems Sensors provides its customers with off-the-shelf sensors, modified versions of existing sensors, or ones that have been designed from scratch.

The solution

For Althin Medical's application, the FS-W flow sensor was chosen. Scott Hurlbut, Gems Sensors's market specialist, explains, "We had to be able to reliably monitor a very low rate of flow with that sensor, and that was our biggest challenge."

Unlike the thermal sensor that Althin had previously used, which indirectly detected flow via the temperature shifts caused by fluid flowing past it, the FS-W is a piston-type flow sensor that reacts only to flow. The sensor's magnetic piston is displaced by fluid flow, causing it to actuate a hermetically sealed reed switch isolated within the unit's body. An opposing magnet deactuates the switch when flow decreases. Pressure drop is also low since the flow-sensing element moves out of the flow path after switch actuation. The FS-W sensor thus provides positive flow/no flow to accurately identify any interruption of flow in the balance chamber. If an imbalance were to occur, the sensor would automatically signal an alarm.

During a six-month design process, the switch was customized to achieve its performance requirements, and the goal of providing a low flow rate of 115 ml/min was met. One modification involved changing the switch's body to a different plastic—Ultem—to provide chemical compatibility. Since Althin Medical was looking for a simple drop-in replacement for its existing switch, Gems also had to replicate the voltage output of the old thermal sensor. That involved making a small PC board with a pair of resistors to act as a voltage divider network.

"The recipe for our success in Althin Medical's application was that we had something in our line of products that, with some slight modifications, met their application requirements," says Hurlbut. "It is an example of Gems's ability to take a sensor made for one application, modify it slightly, and then meet the demands of another application."

Chris Ball, Gems Sensors's flow products manager, explains, "Engineers don't generally come to us with the blueprints for a switch. They come to us with a problem, and we provide application solutions."

A rewarding success

The System 1000 has gone on to win three international design awards. The machine was the first dialyzer to use a self-prompting touch screen interface. "If you look at older dialysis machines, they look like Flash Gordon—lots of little knobs and dials," says Kelly. The System 1000 has greatly improved ergonomics with smooth surfaces and rounded edges that not only make it aesthetically pleasing but also allow easier cleaning, an important advantage considering the hazards of spilled blood. A judge for International Design magazine commented, "One cannot help but be impressed by the way this machine so clearly addresses the user's requirements. The designers have successfully anticipated the needs of both patients undergoing treatment and the physicians providing treatment."

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