Portable medical devices are changing the way doctors and patients deal with pain and illness. Developments in battery life let devices run long periods without a recharge. Pumps with microchips control the release of medications that relieve pain, so drugs can be delivered at the point of injury without the sedating effects of a general anesthesia. And insulin pumps automatically inject the right dosage at the right time.

Pumps treat pain on the battlefield

Soldiers injured in battle are being treated with the latest advances to control local pain. Doctors from Walter Reed Army Medical center are using continuous peripheral nerve blocks to reduce acute pain. It works like this: A small ambulatory pump infuses pain medication through a catheter directly to the nerve-bundle responsible for sending pain messages to the brain. This is particularly critical for the first 48 to 72 hours after injury when pain is intense.

At the core of this technique is the ambIT infusion pump from Sorenson Medical Inc., West Jordan, Utah, (sorensonmedical.com). The pumps are lightweight, the size of a TV remote control, and cost much less than other infusion pumps.

In addition to the military, ambIT pumps are used for all types of post-operative local pain management including orthopedic, general, obstetric/gynecological, plastic, and podiatric surgery. Patients often take it home with them. “The pumps deliver a continuous flow of anesthetic to the patient, but can also be programmed to release an additional dose at the push of a button when the patient feels pain,” says Thomas Orsini, President and COO of Sorenson. “This is called a controlled-bolus volume and the pumps have programmable lock-out periods between boluses to ensure patients aren't overdosing.”

“Compared to other infusion pumps, ambIT pumps fill a gap between low-end elastomeric, or mechanical pumps, and high-end electronic ambulatory pumps,” Orsini adds. “Mechanical pumps are often as simple as a balloon filled with drug. They have a fixed volume whereas the ambIT pumps can be hooked up to various IV bags from 50 to 1,000 ml.”

A microchip controls the amount and timing of drug delivered. The pumps run on 2 AA batteries and are about 7-in. long by 2-in. around. The lightweight units, about 6.5 ounces, are carried in a convenient belt pack. Safety features such as occlusion alarms, audible alerts, and anti-free flow protection ensure precise delivery of the drug. The pumps have an expected life of 8 months to two years. They are almost maintenance free when compared to high-end electronic ambulatory pumps that require preventative maintenance every year.

Efficient compressor triples battery life

The Inogen One oxygen concentrator from Inogen Corp., Goleta, Calif. (inogen.net) nearly triples the time respiratory patients can ambulate on battery power. Other concentrators on the market are almost exclusively stationary, designed for in-home use, and sacrifice “quality of life” in areas of noise and vibration. Other portable devices have a dismal battery life of less than 50 minutes, noise level of 55 dBA, and require overhaul at 3,000 hours.

“For the kind of miniaturization and reliability we were targeting, each subsystem had to be optimized, starting with the compressor, which consumes about 80% of the energy,” says Geoff Deane, Inogen vice president of engineering and CTO. The company selected a rotary scroll compressor instead of the industry-standard piston pump. “A lot of development effort went into higher efficiency in the pressure swing adsorption cycle. We developed a sensitive conserver, reduced the size of the electronics, and optimized the software that manages device operation — all to reduce the amount of work the machine has to do.”

The compressor, from Air Squared, Broomfield, Colo., (airsquared.com) delivers more than 80% volumetric efficiency, while drawing less than 40 watts. Critical performance criteria for the compressor included low noise/vibration, long life, oil-free operation, light weight, low energy use, and precise flow and pressure. Deane says his team researched more than a dozen alternatives including helical, rotary vane, and piston pumps before deciding on the scroll compressor. “Until we came upon the rotary scroll compressor, a piston pump was the only device that came close in the pressure and size range we needed, and it would have required work to isolate the noise and vibration caused by valves and balance issues,” Deane adds.

The operating element of a scroll compressor is made of two identical involutes which form right and left-hand components. One scroll is indexed or phased 180 degrees with respect to the other to allow the scrolls to mesh. This indexing creates crescent shaped gas pockets, bounded by the involutes and base plates of both scrolls.

In operation, one scroll remains fixed and the other attaches to an eccentric, driven by an electric motor. As the moving scroll orbits around the fixed scroll, the pockets formed by the meshed scrolls follow the spiral toward the center and diminish in size. The compressor inlet is at the periphery of the scrolls. The entering gas is trapped in two diametrically opposed gas pockets and compressed as the pockets move toward the center and the discharge port in the fixed scroll. No valves are needed because the discharge port is isolated from the inlet. This reduces noise and improves unit durability.

Because scroll compressors use true rotary motion, they can be dynamically balanced for nearly vibration-free operation. Air delivery is continuous, which almost eliminates inlet or discharge pulsation and associated noise. Reliability comes from only two primary moving parts and no inlet or discharge valves to break or make noise. No valves mean there are no associated valve losses.

“Air Squared conducted additional development and customization to reduce energy consumption of the compressor by 40% through a tweaking of the manufacturing process and a proprietary technology that greatly improves sealing of the oil-free machine,” says Deane. The pump manufacturer also developed a way of fine-tuning the discharge flow of the compressor so each unit delivers the required rate ±0.2 l/min.

The scroll compressor inside the Inogen One weighs just two pounds, including its motor, and measures only 4.5 × 3 × 3.3-in. The entire Inogen One weighs less than 10 pounds, including battery, and measures about 12 × 6 × 12-in. It can deliver 1 to 5 l/min in nine flow settings, varying the speed of the compressor from 1,500 to 2,700 rpm to meet demand. The lithium ion battery provides two to three hours of independent use on a three-hour recharge, with ac and dc capability for stationary and mobile operation.

Small and stylish

An insulin infusion pump attaches directly to the body and automatically injects insulin at scheduled, programmable intervals. The Deltec Cozmo Pump was developed by Smiths Medical MD Inc., formerly Deltec Inc., St. Paul., Minn., (www.cozmore.com).

“When we surveyed patients and endocrinologists, we found that size was a major issue with insulin infusion pumps,” says Clint Vilks, Senior Program Manager at Deltec. “People with diabetes wear their pumps 365 days a year. If the pump is even the slightest bit too big, it can present problems — from clothes not fitting over the pump properly to simple comfort issues. Users wanted something effective, but also small and stylish — almost like it wasn't even there.”

The Cozmo Pump is cell-phone size and delivers insulin through a short, flexible plastic tube inserted just under the skin. Operation is similar to a cell-phone keypad. The cartridge holds 300 units of insulin and uses one AAA battery.

Producing the pump presented challenges, notably the need to design and manufacture parts small enough to fit into the pump's sleek profile, yet capable of delivering big performance. Deltec enlisted the help of Kerk Motion Products, Hollis, N.H., (kerkmotion.com) to produce a lead screw and nut that would compress the syringe to inject the insulin. Deltec replaced the human element — the finger that would push down on the syringe's plunger — with a lead screw and nut. The lead screw pushes on the drive rod, which is actually a complex machined plastic plunger. The threaded nut works with the screw to set in motion the pump's inner lock mechanism, which uses a quarter-turn thread to engage the disposable insulin syringes. A small motor rotates the lead screw.

The insulin dose accuracy could be affected if there is any unwanted or free flow, or if the overall fit of the plunger in the mechanism is loose. What's more, the precision fit of the lead screw and nut ensures proper pump performance and creates sealing features that help maintain a strict separation between the medication and the pump parts — an absolute mandate in any medical device.

“This case could not use injection molding, the typical production method for a plastic nut, because it would create a ‘parting line’ that would interrupt the seal in the plunger,” says Tom Solon, Application Engineer at Kerk Motion. “Instead, we used a precision CNC screw machining process. The nut is made of a self lubricating acetal plastic and the screw has a proprietary TFE coating that increases lubricity and typically extends the normal nut life by more than 300%.”