Image credit: Eric Schneider, Keytech Inc
Because most medical devices include both a durable instrument and a consumable, product developers often face a pivotal question during the early design process: What stays on the instrument itself, and what belongs on the consumable?
There is no simple answer to that question. Every product development process brings a unique set of design and implementation challenges. Deciding what goes where usually involves a series of tradeoffs including cost, safety, and other considerations. If you’re making millions of cartridges for a diagnostic test, for example, then you want the cartridge design to be as cheap and as simple as possible. But making cheap and simple cartridges may increase the complexity and cost of the instrument. Finding the right balance is tricky, but there are some universal considerations to keep in mind.
The primary concern with medical devices is always patient and user safety. Every design requires an approach that avoids user or environmental contamination and produces safe, reliable results. If contamination and safety risks remain front and center from beginning to end, then an overall architecture for the device often emerges.
Second, it’s vital to keep the intended user in mind. If the device will live in a controlled environment, like a lab, and only be operated by trained technicians, then a completely different architecture might make sense compared to a device that will be used in the field or at home by patients themselves.
Myriad other issues can help determine what should stay on the cartridge, and what should go in the instrument. Here are a few examples from our experience developing diagnostic instrumentation:
- Assay Reagents. One of our recent diagnostic clients initially wanted its reagents stored in the instrument and dispensed with each use. The reagents could be stored in a large container, which could save consumable costs. That made sense from a financial perspective but we ultimately recommended that the reagent be stored in the cartridge. Why? This device was going to be used in the field — which meant higher contamination risks. If you dispense a reagent into a disposable, you have to seal it securely to avoid biohazards. The tradeoff was unacceptable: Saving money on the cartridges would increase the safety risk.
- Pumping. In most cases, you wouldn’t put all the features of a pump on a diagnostic cartridge because it would be prohibitively expensive. Yet there are times when it makes sense to build some pump components on the cartridge — for example, you might install a small rubber diaphragm that is depressed by an actuator in the instrument. The benefit of that design might be to move fluids while keeping them contained exclusively in the cartridge. We’ve often had to invent new pumping methods customized for the application. Pumping design decisions are largely driven by variables like the volume of fluid you’ll need to move, the mixing requirements, and the accuracy requirements of the pump.
- Valving. For diagnostic applications, valve components generally go on the consumable and are controlled by actuators on the instrument. There are ways to reduce the number of actuators — which can reduce cost — by adding passive valves to the cartridge. In other cases, especially when constrained by cartridge cost or size, it’s possible to move certain valving components (e.g. valves for venting air from the consumable) into the instrument.
- Temperature. When your medical device requires heating, cooling, or both — sometimes repeated in succession — it’s critical to think about where to put heaters and how to keep things cool. Putting reagents that have to be kept cool on a cartridge may lead to high storage costs down the road. (The cost of refrigerating millions of cartridges containing only a small amount of reagent on each will likely far exceed the cost of refrigerating a large container of reagent.) If a device has to apply heating at some stage to perform an assay, then where the heater goes will depend on accuracy, speed, and cartridge cost requirements. For instance, assays that require quick, precise temperature changes in the sample may drive the design towards heaters built into the consumable, whereas assays that require steady, uniform temperatures might drive the design towards including the heating elements in the instrument to save consumable cost. The key to fine-tuning your temperature components is to work hand in hand with assay development teams and run quick tests that evaluate temperature requirements throughout the design process.
- Detection. Detector location will depend almost entirely on the detection method. Optical methods and ultrasound transducers typically go in the instrument, whereas wet detection methods, like electrochemical sensing, usually require on-consumable implementation.
Figuring out what goes where means juggling a lot of variables. The key is to build and test prototypes — early and often. We test, retest, consult with the client, make changes, then test again, and repeat. My advice is to break up big challenges into small and manageable pieces, solve each one in turn, and finally assemble those pieces.