DESIGN
Triteq Ltd, Stype Hungerford, UK.
Reuse in medical design
The reuse of functional blocks and modules is proving to be essential for electronics businesses seeking to compete successfully in markets ranging from small consumer products to enterprise-level systems. With the increasing importance of medical electronic products, it is natural that designers and manufacturers should consider design reuse for this sector, to accelerate development and gain competitive advantage.
However, when designing medical devices the benefits of reusing a particular module must be assessed within a much more stringent risk analysis and design review structure, as mandated by legislation such as the Medical Device Directive (MDD). Modules performing particularly difficult or complex functions, or those already proven within the medical environment, are the most likely to deliver a worthwhile advantage.
Quality of reusable designs
An established design for a given function or module may already exist within the original equipment manufacturer’s organisation, it having been developed for a previous product. If the previous product is a medical device, this may strengthen the case for reuse. Conversely, many third parties, including component vendors and dedicated intellectual property (IP) developers, also offer shrink-wrapped modular solutions to certain common features and capabilities. Purchasing or licensing these may simplify board design and reduce development time. There is also a vibrant international market for reusable software or firmware IP.
Other sources of reusable designs can include reference designs published by vendors of components or system solutions. These may be available in a variety of possible formats such as a shrink-wrapped design including computer-aided design files, board Gerber data or something as basic as a notional outline or schematic appearing in an application note. Usually, there is no guarantee that this type of module will meet any kind of product specification, nor indeed that it will function as intended or be manufacturable or testable in production.
Risk-based design
Irrespective of the origin of a candidate reusable module or piece of IP, product developers targeting consumer markets can take an objective view on whether those options will offer sufficient value to the product or the project to justify their use. When developing a medical device, however, this type of assessment is subordinate to a more important level of risk analysis. The MDD mandates a risk-based approach to product design from the outset. Annex 1 of the MDD, which details essential requirements, demands that developers should apply a hierarchy of principles, with inherently safe design and construction being the most important.
This part of the Directive states that a medical device “must be designed and manufactured in such a way that, when used under the conditions and for the purposes intended, they will not compromise the clinical condition or the safety of patients, or the safety and health of users or, where applicable, other persons, provided that any risks which may be associated with their use constitute acceptable risks when weighed against the benefits to the patient and are compatible with a high level of protection of health and safety.”
This, then, must be the primary concern when assessing the suitability of a design or module for inclusion in a new or evolutionary medical device. If the module has previously been used in an approved medical device, this may facilitate its reuse in subsequent medical product designs. If there is no history of use in medical applications, a rigorous risk analysis may expose that it has not been designed using inherently safe principles.
The risk analysis procedures applied to medical device designs are also formalised. The MDD requires risk analysis to be performed according to ISO 14971, Requirements for Safety Risk Management, which describes risk analysis, risk evaluation, risk control and the recording of postproduction information. When developing a medical device, therefore, the best design practice includes identifying all possible hazards arising from use of the product, and the potential causes of each hazard to assess the impact of a failure arising in any part of the design. A robust procedure to aid rigorous analysis of the hazards and risks is required to do this. Based on the severity of the hazard and the likelihood of occurrence, it is possible to calculate a set of risk factors for functional blocks within the design.
Based on these risk factors, decisions can then be taken on whether a given module presents an unacceptable risk to the patient or to users. This analysis may show acceptably low levels of risk. However, further steps may be required to reduce the risk factors associated with one or more functional blocks or modules within the design. As far as reusable designs or third-party IP are concerned, these are likely to represent a greater risk if they originate from outside the medical application space.
In either case, the information gained from this formalised risk analysis is able to guide decisions on whether a given module can be reused or whether redesign or replacement is required.
The MDD requires a Technical File (TF) to be maintained for the product. This must document the risk analysis that is performed to demonstrate that the designer has applied inherently safe design, implemented adequate protection where appropriate in relation to risks that cannot be avoided, and informed users of any residual risks. The TF must also demonstrate that the product meets customer requirements as well as statutory requirement such as compliance with the electromagnetic compatibility regulations.
It is interesting to note that, for a consumer product such as a wireless communication device, the reuse of an existing radio frequency design is likely to speed-up type approval and avoid a large part of the required radio conformance testing. In contrast, in the medical context, reusing an existing design is unlikely to offer appreciable assistance with the approvals procedures.
No easy short cut
Even given that all of these stringent analysis and approvals procedures can be satisfied, all the usual technical and commercial caveats surrounding design reuse also need to be observed if the end product is to be successful in the market place. For example, the reusable design must be integrated with the overall circuit design in the same way as any other module, including, paying attention to voltage levels as well as input and output signal properties, among other aspects.
It is also important to bear in mind that many designs presented as reusable modules are usually not created with any real product specification or manufacturability targets in mind. This is particularly true in the case of most reference designs, especially those that are offered free of charge. There is even less likelihood that the reference design will meet the customer designer’s own unique specification.
It makes good technical and commercial sense to view any reference design or reusable module as a solution in principle, rather than an easy short cut. This maxim is even more pertinent in the context of medical device development. Design reuse rarely excuses engineers from the detailed challenges involved in developing a truly market ready product.
When developing a medical device, the circuit design is a much smaller part of the overall engineering effort. Significant technical advantages or substantial savings in development time may convince a project team to reuse an existing design. In general, a reusable design must be of intrinsically high quality and inherently safe to be suitable for medical use.
Ken Hallis, Technical Director, Triteq Ltd, 3 The Courtyard, Stype Hungerford RG17 0RE, UK, tel. +44 1488 684 554,e-mail: ken.hall@triteq.com, www.triteq.com.
