Challenging Development Environment

The environment for creating medical electronics is an extremely challenging one. Devices need to undergo extensive regulatory approval processes, developers need to create perfect devices but at low cost; there are the matters of obsolescence, EMC and placement of components, restraining wiring and isolation for patient safety. On top of it all, space in the operating theatres and A&E is also at a premium. How can an engineer navigate around all these problems?

As a way of investigating some of these challenges let’s consider a hypothetical device which has the following key elements. Refer to Figure 1.

It has to perform some kind of patient measurement using a disposable probe connected to the patient. It incorporates power actuators to stimulate the patient and a user interface to display to the nurse. It needs a reasonable degree of instrumentation and signal processing; some hardware interlocks and is powered by a mains power supply.

These blocks are very typical of most medical systems, so how might we address the technical solution to some of these blocks as individual problems?

Fig. 1: Block diagram of a hypothetical medical product showing safety isolation

In our example a designer would immediately note that one would have to meet the requirements of ISO 60601. Typical design risk areas in this standard for this application would be an isolation barrier (6mm creepage) for safety between live mains (230V) and the patient side (probe connected to the patient). Also any wiring would have to be physically tied down to prevent a loose wire touching any metal part of the patient-side of the electronics.

EMC impacts on the patient leakage currents which have to be less than several microamps. One could not use large EMC filters, as this would pass a large earth current, greater than the allowed amount, through the earth and subsequently through the patient. It is therefore essential to design a product that has extremely low EMC emissions by design. (see later).

There is a need for intelligent control both on the patient side to analyse the signal generated from the probe and also on the mains side, to ensure that the power is sequenced correctly and there is no inrush etc.

Exploring each of these 3 items would be a useful insight into medical design.

The first two points can be addressed simultaneously. In this design, let us imagine that the greater cause of the EMC is generated by the switched mode power supply that provides the power for the actuators, the instrumentation and control circuits. In this case we will assume for cost and obsolescence reasons that the decision has been made to design our own power supply. This now gives the designer the opportunity to control the EMC and the purchase of multi-source components.

By carefully accessing the characteristics of many power topologies, with the object of keeping the EMC to the required minimum, a semi-resonant phase shifted full bridge circuit would be used. This has the benefit of being easy to drive; there are control chips available to drive the bridge and the EMC generated by the switching is very low. Taking this approach, the designer uses one design to accomplish many different design objectives. The phase shifted full bridge (see figure 2) is ideal in this situation as it uses a full bridge with two square waves applied to the two halves with a phase delay. If the square waves are in phase there is no output, if the two phases are 180degree out, then the maximum output is delivered. The interesting point is that each half bridge sees only a 50% duty cycle, so there is plenty of time to resonate the edges of the switching waveform. This would give rise to very few EMC harmonics being generated and hence a very small EMC filter would be required. With skill it is possible to create the resonant elements using the parasitic leakage inductance of the transformer and the switching devices. The high primary to secondary isolation needed in this application is ideal in creating the parasitic inductance in the transformer.

The 3rd technical challenge benefits from the use of two embedded controllers with an optical interface across the isolation barrier. The patient-side unit can typically be a TMS320 DSP device with lots of efficient processing power and the primary device can be a PIC or MSP430 style device. By carefully choosing the hardware around these devices, with as much of the safety critical monitoring done in hardware it is possible in many cases to reduce the level of risk (level of concern).

The Development Process

In many medical designs the technical design of the electronics is fairly straightforward compared to the regulatory understanding that is needed. In addition, there are quality requirements including verification and validation of test plans and design history files that are required. In most situations all these aspects would be wrapped up in a ‘Quality Plan’, which would detail the test plans and compliance.

Fig. 2: Design process flowcharts

The design process is absolutely key to medical design (and any other design) and a formalised approach actually aids the creation of simple and concise solutions that are part of an elegant design.

Figure 2 shows an outline of the FDA and ISO design processes. Alongside these is an outline of a typical design process that covers these requirements.

The actual steps may differ according to whether the business is technology led or market led, but typically this would consist of the following stages:

Initial project set up to assign the team and kick start the project. The team needs to understand with clarity and focus the objectives in hand to prevent charging forward with typical engineering enthusiasm. These points go beyond the technical requirements and cover commercial, regulatory and quality implications. In addition there will be roles and responsibilities assigned. In a typical successful company, key people would be selected for the team that possess specific character types as shown up by tests.

The first development stage of a project should focus on how feasible the intended final product is. This is mainly about seeking answers to standard questions. For example, is it technically possible? Is it commercially viable? What are the risk areas and what options are available to reduce these risks? Which specific medical regulatory requirements need to be complied with?

During this stage of the development it is necessary to create a draft specification or basic requirements list, which allows good communications throughout all departments of the business. Priority should then be given to these using a weighting approach to define which are ‘must-haves’ and which are ‘wish list’ items. This prevents too much work being conducted on unnecessary items.

From this the engineering team can create conceptual designs and assess what parts of the project are similar to previous projects. This provides two outcomes. One is a clear focus on the re-use of existing design modules and another is a clear understanding of the areas that are new. Listing these formally allows an easier review of design challenges and also creates the basis for the medical, commercial and technical risk assessment.

The output of this stage is theoretically based. It would normally be in the form of a report outlining the points raised for investigation, the options looked at along with the pro’s and con’s of each option. Finally, it should conclude with recommended choices backed up with reasoning that are assessed during an Initial Design Review. This review provides an opportunity for all departments to sign off against decisions made and ensures that key design drivers, quality and regulatory aspects have been considered.

This article was first published in Electronics World magazine, November 2006

Mike Lloyd is the Managing Director of ML Electronics

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Challenging Development Environment