How Insulin Pumps Work
An inside look at insulin pump technology
As I walked through the halls of Tandem Diabetes Care, one of the newest pump companies on the scene, a vaguely familiar sight caught my eye. Scrawled on a white board was an equation that transported me back to high school physics: P1V1 = P2V2. Boyle’s law, the gas law that describes how a decrease in pressure corresponds to an increase in volume. Though I never learned exactly what the folks at Tandem were calculating, I did come to understand that insulin pumps are sophisticated devices that must satisfy the laws of math, physics, health, and—to some degree—personal style. Not an easy task.
In people without diabetes, the pancreas releases however much insulin is needed to maintain healthy blood glucose levels—insulin pumps attempt to do the same. These devices are programmed to deliver a small continuous flow of rapid-acting insulin around the clock—basal insulin—as well as on-demand doses selected by the user to cover carbohydrate in meals, snacks, and beverages.
The first insulin pump was designed in the early 1960s and looked like something the Ghostbusters would wear on their backs to blast wicked spirits. Smaller versions started cropping up in the 1970s, but it wasn’t until the mid-1980s that they were designed to deliver precise doses and became user-friendly, according to Nancy B. Dean, director of marketing at Roche, which makes the Accu-Chek Combo pump. “Compared to today’s insulin pump systems, the early insulin pumps were not easy to handle. Some pumps even required the use of a screwdriver in order to adjust the flow rate of insulin, making them difficult to tune and very inaccurate.”
Today, insulin pumps work without hand tools and can fit in the palm of your hand. In most cases, they deliver insulin through an infusion set: tubing that winds from the pump to a needle or cannula—a tiny flexible tube—inserted under the skin. A few tubing-free “patch” pumps, such as the OmniPod, stick to the body and direct insulin through a cannula inserted into the skin beneath the device.
There are eight insulin pumps on the U.S. market, nine if you count the non-electric patch-pump-like V-Go for people with type 2 diabetes (“One a Day,” below). Each pump has its own personality and we won’t cover all their quirks here, but there are some generalities regarding how pumps work. All true pumps require a battery to run—that’s where they get their pumping power. Some pumps have disposable batteries, while others have rechargeable batteries that are powered by plugging them in like a cell phone. They all contain electronics—microchips and such—and a user interface for selecting functions and monitoring insulin delivery. Some pumps double as meters or interact with continuous glucose monitors (CGMs), and so provide a readout of glucose levels as well.
The Pump’s Heart
The most important component of your insulin pump, nestled inside the hard shell, is the part that moves the insulin, according to Michael Hill, MBA, senior director of Medtronic’s Insulin Delivery and Closed Loop Section. “It’s highly important that it be accurate,” he says. When describing the pump mechanism, the Tandem engineers throw around a lot of terms that literally make it sound like they’re talking auto shop: rack and pinions, pistons. At some point, an engineer said that the insulin pump has a transmission.
Most pumps on the market today contain a cylindrical insulin reservoir that looks very much like a syringe. Depending on how much insulin the user selects for delivery, tiny watch-like gears inside the pump rotate a precise amount. This slowly turns a mechanical screw that pushes the plunger ever so slightly to deliver a measured amount of insulin.
Tandem’s t:slim contains a notable exception to the syringe-like design mechanism found in most pumps. The t:slim pump holds its insulin in a clear bag that sits inside a hard-shelled cartridge, and looks nothing like a syringe. Eric Shearin, senior manager of marketing and corporate communications at Tandem, says this change allowed them to shrink down the size of their pump, as the syringe mechanism takes up a fair amount of real estate. The company came up with a different pumping mechanism to go along with the insulin bag approach.
Putting It Together
Tandem can currently make 1 million insulin cartridges per year in a single shift at its San Diego headquarters in an assembly line that includes humans wearing clean gowns and boxy robots. The human-robot teams insert the empty insulin bags into their plastic cases and seal the bag with lasers, leaving only a one-way plug that the user pierces with a needle to fill the bag with insulin. The reservoirs for most other pumps are filled similarly, using insulin from a vial. Asante Snap cartridges come preloaded with insulin.
Not far from the cartridge facility is the pump plant, really just a single room. There, people insert the preassembled pump mechanism, battery, and, finally, the electronics into one half of the pump case. They manually connect the wires from the electronic components to the pump and battery. Then, the case is closed, and the device is sealed shut. By the time I arrived, that day’s shift had gone home, and all that remained were a few dozen assembled pumps, lined up in rows, pumping away and chirping occasionally. Shearin says that each pump is tested, churning through as much fluid in less than a day as it typically would in a month of use to make sure the pump is working properly before sending it out into the world.
Building in Safety
Pump people love to talk about safety—for good reason. An overdose of insulin can be fatal. Beyond the uncountable number of safety tests manufacturers perform during production, insulin pumps have built-in safety.
In Medtronic pumps, insulin is kept in check by the combination of a force sensor and an encoder. A force sensor uses conductivity to detect how much force the plunger applies to the insulin. The pump’s electronics can interpret this force in terms of how far the plunger will move under that force. If the software thinks the force is too much, the pump will stop delivering insulin and run an error message. The encoder looks at insulin delivery from a different angle, by checking on the location of the piston. “The encoder is measuring how many times the gears are spinning,” says Hill. The number of gear turns indicates the location of the plunger
With the Tandem pump, the design helps protect the user from delivering too much insulin. The t:slim’s infusion set is not directly connected to the insulin reservoir—instead there is a shuttle that holds only a third of a unit at a time and carries insulin between the reservoir and the infusion set. This setup is a safety feature, according to Shearin: In the case of a malfunction, insulin from the reservoir is blocked from entering the tubing; there’s no direct path.
The atmosphere at Tandem is what you would expect of a California tech company—open work areas, games, and joviality. The engineers were excited to show off a 3D printer they use to make models of pump parts. This helps them get a feel for the parts’ size and shape in the real world, and to see how they might fit together in the cozy confines of a next-generation pump case.
Very soon, Tandem expects to come out with a pump that is integrated with a Dexcom continuous glucose monitor (CGM) so that glucose readings show up on the pump interface. Also in the pipeline is a large-reservoir cartridge that, via a clever design, would work in the company’s existing pump for people who require large amounts of insulin. Another project is related to efforts to develop an artificial pancreas—an experimental device that automatically doses insulin based on CGM glucose readings. This pump includes space for two cartridges, one for insulin and one for glucagon, a rescue hormone used to treat too-low blood glucose levels.
Medtronic is also moving toward an artificial pancreas with its pump, and is currently the closest to that goal with the MiniMed 530G With Enlite. The pump, with an integrated CGM, shuts off insulin for up to two hours should glucose levels fall below a certain level. Medtronic’s next step is to include software that can predict when a low is likely to occur, says Hill. “Instead of an air bag that waits until you crash to deploy, this puts on the brakes early.” Then, once the program determines it’s safe, the pump can turn itself back on. It may not be long before insulin pumps are running themselves, and pumpers can think about something else.
The Tandem t:slim (image courtesy of Tandem Diabetes Care) pumping mechanism is designed to work with its bag-style insulin reservoir. A mobile component inside the device—an insulin shuttle—slides between the reservoir, where it fills with insulin, and the infusion line, where it delivers insulin. If you want to see the shuttle in action, check out this video: http://bit.ly/1oa17dd.
One A Day
The V-Go (image courtesy of Valeritas) is an insulin pump for people with type 2 diabetes that’s replaced every 24 hours. Because it doesn’t use electric power from a battery, some, including its maker Valeritas, call it a disposable insulin delivery device instead of a pump. Like a patch pump, the device sticks to the body and delivers a continuous stream of rapid-acting insulin at a rate of 20, 30, or 40 units of insulin per 24 hours, depending on model. At mealtime, a user can press a button to deliver an additional 2 units per push, for a max bolus of 36 units. How does the V-Go get by without power? It’s spring loaded. A spring slowly presses out the basal insulin. The mealtime insulin is delivered by the physical push of the button.
Insulin Pumps Sold in the U.S.
See more details about insulin pumps at diabetesforecast.org/pumps-jan14.