A mechanism is a system of parts working together to create motion or transmit force. Mechanisms can be the defining feature of a successful medical device, but deserve careful consideration as the wrong choices can lead to safety and efficacy problems, which means added effort to fix problems in the best case or patient harm in the worst. Though typical engineering practices should always be considered in parallel (parts costs, availability, working envelope, off-the-shelf options, cost, tolerances, etc.), these seven design considerations are specifically tailored to medical devices, and are best reviewed not only during development, but before design even starts.
 
1. Precision
The number one requirement of a mechanism is usually precision and accuracy, regardless of whether it is related to the safety or efficacy of the device. However, when precision is safety or efficacy related, it must be considered extremely carefully. How precise does it need to be and what will happen if it is less precise than the requirements state? How will precision in verification be measured? What happens to precision after a number of uses or if it is dropped, and how can the confirmation be made that it continues to be precise over its lifetime?
 
2. Lifetime
Lifetime is important for both precision and longevity. In a well-designed device, the mechanisms are often the first point of failure, which means they often determine a device’s expected lifetime or maintenance schedule. Make sure to consider the implications of service calls or field replacements when choosing a mechanism with low reliability. Plan who will maintain the device accordingly, when and how quickly they can fix it in case of failure, and what that means to patient health.
 
3. Travel Limits
An upfront understanding of the desired travel range, accuracy of end detection, and the consequences of over-travel will enable a holistic design from day one. Assuming the mechanism isn’t actuated by the user (i.e., the user doesn’t move the mechanism manually), limit switches, light gates, encoders, and a myriad of other detectors are available to feed information back with a certain degree of accuracy.


Example of an extra simple Walker Base mechanism
 
In medical devices, the success or failure of these sensors can have a huge impact on the safety or efficacy of the device. IEC 60601 requires these types of components to be single-failure safe, which means their failure modes should be analyzed thoroughly and contingencies put in place. Some sensors may need to be made redundant, or hard stops added in case of sensor failure (see IEC 60601-1 Section 9.2.3.2). Another option is to use higher reliability sensors or those with built in failure warnings to reduce risk of unexpected failure.
 
4. Mechanical Safety
If a mechanism is exposed to (or used by) an operator or patient, there will almost certainly be pinching or crushing hazards. These hazards should be protected or mitigated to be as safe as possible. Follow a general standard such as IEC 60601-1 (Clause 9) which defines safe distances, gaps, usability, and protective guard design, as well as recommendations regarding the use of emergency stops.
 
5. Debris
Most mechanisms are likely to give off some debris from rubbing surfaces. This can be large chips or fine dust that may interfere with biological samples, optics, other gears or mechanisms, electronics, etc. Oil is often present on mechanisms and may have the same effect. Both oil and debris can create particulate small enough to circulate with even weak air currents and travel internally over the device or externally to other devices and surfaces. This could lead to contamination, loss of biocompatibility, interference with electronics or optics, or a host of other problems. Make sure the debris environment is understood, the mechanism works with the device’s expected lifetime, and it doesn’t pose a risk to other devices in the vicinity.
 
6. Fringe Cases
It is very important to consider the fringe cases of a medical device’s intended use environment. What happens if the device is bumped? What will it do when powered off or interrupted during motion? What happens in 35-degree heat? These are potential causes of safety or efficacy issues in the worst case and must be considered in the risk management procedure. From a business perspective, they can cause costly field failures, lack of user adoption, or the need for more service calls. Although predicting these fringe cases is usually not difficult, it is time consuming. (It will still be less time consuming than fixing a problem in production, however.)
 
7. Usability
Last, but never least, mechanism usability can make or break a device’s success. It is not just market adoption on the line. The device will fail usability testing or validation if it is not well designed. Here are some things to consider: if the mechanism requires input from a person, how much effort is required to use it? Will it feel weak or break easily? Is it going to be loud? Is it obvious to use in the way one thinks it will be used? This is an area worth investing a lot of time in to give users a satisfied, effortless feel and to enable successful usability testing and validation.
 
Now that the seven key considerations when designing mechanisms for a medical device are understood and all requirements are documented, it is time to start thinking about the actual design. Don’t forget to periodically review those requirements and make sure to make the most up-to-date design decisions. Lastly, verify and validate often with good verification and validation plans to ensure expectations and assumptions translate into reality.
 

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Nigel Syrotuck is a mechanical engineer at StarFish Medical. His background includes a diverse project development portfolio including sustainable power solutions, assisted living devices, and nano-satellite design.