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Many medical conditions can be treated with tablets, but others require injections under the skin in order for therapeutic drugs to reach the bloodstream. In the case of insulin administration for diabetes treatment, patients need to self-inject the drug daily.

Designing injection pens is a snap (fit), teaserMaking those injections easy and safe is of prime importance for Novo Nordisk, the Danish company that has been a world leader in the production of insulin ever since it was discovered by Canadian scientists in the 1920s. The company innovated beyond standard syringe technology to produce the world’s first patient-friendly self-injection system, the NovoPen, some 25 years ago.

With more than 350 million diabetics worldwide—almost 5% of the global population and growing—demand for insulin pens will likely remain strong into the foreseeable future. Since effective control of the disease is dependent on consistent use of the drug, these delivery systems need to be portable, easy-to-use, reliable, and even resistant to minor misuse by patients.

Small medical device, big design task

An insulin pen may be small, but it is a precision instrument with a number of complex parts that must work in perfect tandem (Fig. 1). Some pens are durable, containing a replaceable drug cartridge, while other disposable ones come pre-filled with the drug.

Injection typically involves twisting a short needle onto the pen, turning a dial to the required dose, and pushing a button to deliver the medication under the skin. After a given number of doses are injected, the cartridge is exchanged for a new one (with a durable device) or discarded (with disposable pens).

Designing injection pens is a snap (fit), fig. 1a

Designing injection pens is a snap (fit), fig. 1b

Audible clicks that occur at key stages of this procedure reassure the patient that they are engaging the device correctly at each step. It looks pretty easy (a one-minute video of a woman checking her blood sugar and then using a NovoPen to inject insulin is available here. But every one of those reassuring clicks represents a challenge that has been overcome by the engineers who created the pens. So do the clicks the patient never hears: those that occur as the pen parts are assembled in the factory before use.

“Parts that click into place with ‘snap fit’ instead of screw connectors are very efficient to assemble within mass production,” says Torben Strøm Hansen, principal scientist in the Device R&D division of Novo Nordisk, near Copenhagen, Denmark. “Snap fit is the commonly used way to connect parts in our device mechanisms, and it also signals reliability when the internal components have optimal connections that don’t rattle. It’s very efficient when designed correctly.”

Getting those designs correct from the start is the task that Hansen and his Mechanical Analysis team focus on in close collaboration with Novo Nordisk’s mechanical designers. “Even though an injection pen is not that big, there are a lot of fine details in its design,” he says. Whatever the configuration of device, the plastic-polymer components must withstand the rigors of both manufacturing and patient use, performing as required at different temperatures and loads.

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