Electrical medical device design: robust, ingress-protected products
Published: July 2024
Will Davies
Design Consultant
James McLusky
Director
From highly controlled environments to the rigours of emergency use or simply uncontrolled home use, all electrical medical equipment must be safe and reliable. The health of a user and patient is dependent on this, and mandatory regulatory standards such as ISO 60601 for ingress protection, impact and electromagnetic interference, help ensure this.
Throughout the 20-year history of Shore, we have repeatably designed products for a variety of industries to function in the most demanding environments. This has built a wealth of expertise that we can draw upon in developing highly robust medical devices that not only exceed reliability and robustness requirements but are cost-effective and delivered on reduced development timescales.
In part 1 of this series, we looked at five key areas of the successful integration of electronics into medical products. This article focuses on the design of robust enclosure elements that are key to any effective electro-mechanical product from the outset.
1. Designing for ingress protection
The IP (Ingress Protection) code is a two-digit scale, with the first digit describing a product’s level of protection from solids, and the second from liquids. This rating has a dual purpose: firstly, to prevent unwanted foreign objects entering the enclosure which may damage the device, and secondly to protect the user from harm through coming into contact with dangerous internal components.
Depending on the level of IP required, the outer edges of the enclosure can range from simple overlapping walls to a more complex labyrinth design, a separate gasket component, or an overmoulded seal profile. Several factors may influence selection; for example, a gasket seal may satisfy a waterproofing requirement, or a lip seal can be designed to ‘self-energise’ – increase sealing performance – under a pressure differential.
The minimum IP rating for ISO 60601 equipment is often cited as IP22. However, the actual requirement is contextual; for example, a device subject to a strict cleaning regime with liquids may demand a higher rating. To achieve this, elastomeric keymats can incorporate a perimeter seal detail; breathable membranes can be used to seal speaker grilles and permit pressure normalisation; adhesive closed-cell foam can be used around displays; and TPE plugs can be used to seal screws and provide exterior surface continuity.
QardioCore ECG – the world’s first wearable, continuously monitoring electrocardiogram. Intended to be worn 24/7, competing requirements of comfort, flexibility, robustness and shower-proofing were all achieved using a combination of structural engineering, overmoulding, ultrasonic welding, and Flexible Printed Circuit (FPC) sections to link rigid PCB segments.
2. Impact and environmental protection
The most obvious thing to consider when designing a product to survive drops and impacts is the enclosure material – toughness is the most desirable property here. However, to meet the needs of ISO 60601 or ISO 11608, material selection alone is often not enough. Impact protection is all about absorbing or redirecting the impact energy, which will be a complex variable based on the mass of the object, the velocity at impact, the surface area affected, and the duration of the impact.
Intelligent design goes a long way to enhancing a product’s impact resistance. By including these concepts from the beginning, a design can be achieved that is not only elegant but also cost-effective.
- Extend the impact: Shock absorption and flexibility can be built into the enclosure itself or by adding overmoulds or elastomeric elements to provide external bumpers, elongating the duration of the impact and thus directly reducing its force.
- Soften the edges: Design out sharp corners to increase the impact area and avoid concentrated stress. Similarly, by engineering in simple stress relief and managing the weld lines in plastic components you can remove areas of weakness and prevent the propagation of cracks.
- Material selection: This is also influenced by environmental protection considerations, such as corrosive environments or cleaning agents, UV exposure, and high or low temperatures. Does the device contain a drug product that requires refrigeration? Low temperatures can reduce material toughness. Conversely, high temperatures can accelerate creep, affecting function.
Before specialising in medical and pharma products, Shore designed and engineered the Argus MiTic thermal imaging camera. Designed for 1st responder fire-fighters, the camera is extremely robust, utilising every trick in the book (and developing new ones) to withstand very demanding environmental and physical conditions such as a 250°C flame, IP67 ingress protection, EMI Shielding, repeated 2m drop onto concrete and being run over by a fire truck.
3. Fire containment in ingress protected products
The enclosure may also be required to provide a fire enclosure function, a critical safety feature under ISO 60601. This purpose is to contain and isolate electrical components and wiring that could pose a fire risk, ensuring that if a fault occurs then any fire remains confined within the enclosure and does not spread to other parts of the device or its environment, thus protecting patients, users, and surroundings from potential harm.
- Resistance: The enclosure components providing protection must be manufactured from materials that are flame-retardant and resistant to ignition or heat deformation. Depending on the risk these materials must meet specific flammability standards. For plastics, a UL flammability rating is typically used.
- Containment: The design of the enclosure must contain any flames, sparks, or molten material that might be generated. This typically involves a robust construction and the consideration of any ventilation openings. Close consideration of the need for flame barriers or other protective measures ensures that ventilation does not compromise fire containment.
4. Vibration protection
Often overlooked, resistance to vibration remains critical. Often the harshest experience will be during shipping, but depending on the product this could also be when in use or storage. Ensuring products can withstand these vibrations is crucial for their longevity and performance.
- Cable management: Key design strategies to cope with vibration include implementing cable strain relief, which provides flexible support for a cable where it exits an enclosure (or internally at a mounting point) to mitigate against local fatigue and wear. In parallel, internal cable mounting is also essential, reducing excessive free movement and further reducing the risk of fatigue.
- Shock isolation: In extreme cases, another strategy is the use of shock or damping mounts for sensitive components. This can be achieved with simple solutions like an o-ring, an overmoulded detail, or a dedicated isolator product. Simply adding elastomeric feet or bumpers on the exterior of the product can further absorb and reduce the impact of vibrations.
- Shake proof: Employing vibration-resistant fasteners, such as specialist screws for plastics, star washers, or threadlock-tipped machine screws, can significantly enhance the product’s resilience.
From Shore’s archives: the Trig Avionics TX56 Nav/Com unit – compact, easy to use, and engineered to withstand extreme vibrational forces such as those experienced in a helicopter. This uses a die-cast aluminium chassis to provide greater accuracy, lower part count, integrated EMI shielding and cable support, also eliminating issues with harmonics seen in typical sheet metal assemblies.
5. Design for repair
Accidents happen, parts wear out. Not every eventuality can be predicted. Any robustness claim suggests a long service life; to truly prolong this, serviceability should be designed in from the outset. The easier it is to service and repair a product, the lower the cost of doing so and hence the burden of ownership is reduced. Furthermore, extending a product’s lifetime through repair is a huge contributor to improving its sustainability.
The following aspects should be considered for service design, whether by the user or a dedicated service centre:
- Ease of access: Use common fasteners or, if safety allows tool-free methods to enable straightforward access to key service elements such as seals.
- Clear labelling: Both internally and externally to guide servicing, and/or clear instructions within a service manual, avoiding technical jargon as much as possible.
- Self-diagnostic system integration: Enable the rapid identification of faults and appropriate service actions. This could be detailed in on-screen information, or more simply communicated through status LEDs.
- Design for modularity: More complex sub-assemblies such as drive units can be removed and replaced as a single piece. Such modules could be returned, repaired, updated and reintegrated by the manufacturer.
- Ergonomic service design: Ensure that the product is easy to manipulate and handle during the repair process.
The Smith+Nephew RENASYS EDGE negative pressure wound therapy device is a great example of a robust ingress-protected medical product capable of withstanding knocks, drops, and hospital-grade cleaning. By design the structure naturally manages liquids and is engineered for rapid servicing, reducing time between patients, prolonging its service life, and lowering its carbon footprint.
Although medical electrical equipment and integrated healthcare devices have some unique design challenges, products from other industries often provide valuable insights for overcoming issues like ingress protection and device robustness. Many techniques are transferable across categories, though some may need adjustments, such as ensuring materials are biocompatible and tough enough for drop testing.
Shore benefits from extensive experience across various sectors, including medical, pharmaceutical, consumer, and industrial. This diverse knowledge allows us to address demanding robustness requirements in medical products, leading to true innovation and market leadership. With this expertise, we consistently meet and exceed expectations and test requirements.
If you’re looking for a design partner with expertise in electronic integration and device robustness, contact our Business Development Manager, Holly Milston, to learn more about working with us.
This was part 2 of our electrical medical device design series.
