Developing Safe, Reliable Medical Devices

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

Originally Published October 2000

DESIGNER'S NOTEBOOK

Reliability engineering tasks should be performed at each step of the product development process.

Ronald E. Giuntini

As technology improves, product reliability issues become ever more complex. Manufacturers must not only ensure their products are safe for the end-user, but also free of adverse effects on nonusers. Faced with consumer advocate groups and a preponderance of product liability lawsuits, manufacturers have a tremendous challenge—to produce durable, safe, cost-effective, and highly reliable products.

The concept of product reliability itself is often unclear; in many instances it is merely treated as an aspect of product quality. There is an enormous difference between reliability and quality, however. Quality does not ensure that a product is reliable, that it will meet its design life, that it is durable, or that it can survive extraordinary usage; nor does quality address application stress or whether the product is safe. All these issues fall under the reliability category.

To develop a product with a specific characteristic— such as a designated life—that characteristic must be designed into it from the very start. It must be incorporated into the product specifications, then into the conceptual design, the preliminary design, the final design, the prototype test articles, and, finally, the marketed product itself. As simple as this seems, manufacturers often don't work that way. If a manufacturer has selected a design concept and the product has already gone through the preliminary design, the final design, and the prototype phase, it may be too late to insert desirable features and characteristics. Reliability (durability, robustness, safety) must be designed in; it cannot be added later as an afterthought.

In medical device manufacturing, as in any other industry, a reliability program for a product must be tailored to its customers' expectations, which are both market and cost dependent. Customers generally recognize that a product's price is proportional to its features, characteristics, and attributes, such as reliability. Manufacturers must realize that any product is only as good as the reliability methods, processes, and tests employed during its design, development, and manufacture.

The process discussed in this article and shown in Figure 1 is generic, but it indicates the ma-jor tasks to be accomplished in the development of a product. At every step in the development process there are corresponding reliability engineering tasks that should be performed.¹

PRODUCT CONCEPTUAL DESIGN PHASE

The process should begin with the product's design requirements, specifications, features, characteristics, and nature of the end-use environment and applications.² Simultaneously, reliability, maintainability, safety, and human factors requirements and specifications should be entered into the decision process. During the product conceptual design phase, several candidate concepts are usually developed. The selection of the best concept to carry into the preliminary design phase should be based on a systems-effectiveness modeling approach, in which every attribute and characteristic is analyzed and calculated.³ If these reliability-related features are not part of the conceptual design phase, the selection of the best concept will have been based on an incomplete specifications package.

Figure 1. Flowchart of a typical product development process.

In the selection of the most promising concept, a failure mode and effects analysis (FMEA) should be used. Applying the FMEA to candidate concepts provides a means of locating respective weaknesses and helps assure that the selected concept is, in fact, the best. FMEA can be characterized as a systematic method of cataloging failure modes starting at the lower level of assembly. FMEA can be performed using either actual failure modes from field data or hypothesized failure modes derived from design analyses, reliability prediction activities, and other relevant instances when components fail. FMEA offers insight into failure cause-and-effect relationships and provides a disciplined method to proceed part-by-part through the system to assess failure consequences.

In its most complete form, FMEA identifies failure modes at the part level. Each identified part failure mode is analytically induced into the system, and its failure effects are evaluated and noted, including severity and frequency (or probability) of occurrence. Probabilities and severities enable a third factor to be computed, called criticality. Criticality provides a means of ranking the failure modes. In this form, FMEA is upgraded to failure mode, effects, and criticality analysis (FMECA).^4,5

FMECA is used throughout the product development cycle and should also be applied to the manufacturing process to identify failure modes and weak links.⁶ When properly applied, it is a valuable and versatile tool.

A single reliability technique such as FMECA is not an all-purpose tool. For instance, it cannot be used to determine the expected life of the product or its mean time between failures (MTBF). This requires another technique, the reliability prediction. The reliability prediction is only relevant during the preliminary and final design phases.

In the conceptual design phase, prior to any prediction, reliability goals must be allocated to the various subsystems, lower-level functional assemblies, and generic components. The allocation process is vital. The prediction will show how close the design comes to the allocations, so that changes in the design and parts selections can be made with minimal costs—both initially and throughout the product's life cycle.

The allocation process begins with a reliability numerical goal. This is usually expressed as a probability of success— such as 0.95—for some specified time. This number is then allocated to the subsystems. If there were n subsystems, then the average allocated to each would be the nth root of 0.95. Of course, the allocation process is much more complicated than this, but this example shows that the lower one goes into the product or system, the higher the reliability (allocation) must become. If n were 5, then the average allocated probability would be 0.9898. Component-level reliability must be very high.

PRODUCT PRELIMINARY DESIGN PHASE

The best overall design is selected in the conceptual design phase. Next, in the preliminary design phase, that concept is fleshed out. Functions and functional blocks are replaced with generic hardware elements, such as valves, pumps, motors, resistors, and microprocessors. The exact part will not have been selected, but the part type will have been designated. Selection of the exact brand with the exact capabilities is usually accomplished in the final design phase.

In addition to performing FMECA on the preliminary design, a reliability prediction should be made at this stage. The prediction will indicate how close the design is to the allocated probabilities. If necessary, trade-offs can be made to provide higher or lower component reliabilities to achieve the overall subsystem- or system-level goals.

For medical electronics, there are two general methods of prediction: the parts-count reliability prediction method (used in the preliminary design phase), and the parts-stress reliability prediction method (used in the final design phase).⁷

The parts-count reliability prediction method is applicable during the early (preliminary) design phases, when information regarding the product's components is insufficient for the parts-stress analysis model. For parts-count reliability predictions, only the generic part types—including complexity for microcircuits—part quantities, part quality levels, and part usage environment— are needed.

Since the physics of failure are different for electronic, nonelectronic, and mechanical parts, the reliability models for these part types are different; however, the basic process is the same.

The reliability prediction provides the quantitative baseline needed to judge a design early in the development process. Potential difficulties can be identified and improvements made long before full-scale production begins. As every manufacturer knows, it is much cheaper to find the flaws during the design phase than to redesign the product to correct flaws after the product hits the market.

The reliability prediction process is a relatively inexpensive method of assessing the quality of the design. It identifies the highest contributors to failure and enables the designer to select other parts or make changes that produce a more reliable and durable product.⁸ Predictions may be used to evaluate the need for environmental controls, to employ redundancies, or to trade off other reliability enhancing techniques against cost, space, or volume.

Reliability prediction enables the designer to examine a number of factors affecting the rate of failure and to test various options for reducing the failure rate by performing sensitivity analysis (i.e., varying the factors and experimenting with various schemes).

PRODUCT FINAL DESIGN PHASE

In the final design phase, all components, parts, and assemblies should be designated and incorporated into breadboards and engineering models for form, function, and fit testing. Prototypes should be built for life stress testing and other reliability tests. Everything possible should be done to identify flaws, weak links, and failure modes and mechanisms.

During this phase, another, more detailed FMECA and another reliability prediction need to be performed to parallel the design. The parts-stress reliability prediction method is much more detailed and requires unique parts data and the use of failure-rate prediction equations. Its accuracy is not where the prediction's value lies, however; the value lies in the prediction's ability to identify the parts' relative reliability so that prudent parts selections can be made commensurate with the budget and the market for the product.

The idea is not necessarily to produce a product with the highest reliability but with the highest affordable reliability. Keeping this in mind ensures that the product can be sold competitively. The claim that reliability is too expensive and will force the product out of the competitive range is deeply flawed. Reliability can and should be tailored to the consumer pocketbook.

The development and testing of prototypes also occurs in the final design phase. The idea is to test the prototypes to find the weak components or design features. Often, it is desirable to perform a cycle test or a duration test to determine the failure rate or the MTBF. In this type of test, a quantity of products is run until failure, and the MTBF is statistically calculated for some level of statistical confidence. The product life can also be estimated by specifically engineered accelerated aging tests that can be correlated to the design of the product.

Depending on the type of product, other tests of a developmental nature that could be performed during this phase are electrostatic discharge, radiated emissions, and radiated susceptibility. For some products, other suitable tests such as high pressure or low pressure, or even drops from various heights and orientations could be valuable.

MANUFACTURING

After prototype testing and design modifications have been implemented, the product is ready to be released to manufacturing (Step 7 in Figure 1). In Step 8, manufacturing begins. A manufacturing-related reliability engineering task that has received some attention in the last few years is the process FMEA. This technique examines the process by which the product itself is built. For some products it is very valuable that the process FMEA be extended to an analysis of how the product is maintained and used.⁶ The maintenance and use of FMEA should be performed during the design phases so that any features that affect the maintenance or the use can be appropriately addressed.

SAFETY

The safety and reliability of a product overlap only when unreliability can result in a risk to the user. Obviously, a product that fails at an unexpected or crucial usage time can be hazardous.⁹ One aspect of reliability is to determine the expected product life so that the product will not be used beyond a designated time without preventive maintenance or replacement of the low-reliability parts.

CONCLUSION

Reliability engineering is a sometimes overlooked discipline in the development of medical devices. It should be an integral part of the product development process from initial concept to design to testing. Many device manufacturers tend to underestimate the value of reliability engineering, however; they fail to see it as a method for producing reliable products in the shortest time and at the lowest cost. Trying to add reliability late in the product development cycle requires costly redesigns and delays. There is no substitute for a thorough product reliability process that parallels the product development process.

REFERENCES

1. Benjamin S. Blanchard, Logistics Engineering and Management, (Englewood Cliffs, NJ: Prentice-Hall) 22–23.

2. Wilton P. Chase, Management of System Engineering (New York City: John Wiley & Sons) 23, 68–69.

3. Ronald E. Giuntini, Design Reliability Seminar, John F. Kennedy Space Center, FL, 1992–1994.

4. Procedures for Performing a Failure Mode, Effects and Criticality Analysis, MIL-STD 1629A, U.S. Dept. of Defense, November 20, 1980.

5. Ronald E. Giuntini and Christopher Scott Martin, "Failure Mode, Effects, and Criticality Analysis (FMECA) of the SPIROS Drug Delivery System, Task A—Part 1," Dura Pharmaceuticals Handbook (December 3, 1996), 5–10.

6. Katsushige Onodera, "Effective Techniques of FMEA at Each Life-Cycle Stage," in Proceedings of the Annual Reliability and Maintainability Symposium (Philadelphia: 1997), 50–56.

7. Military Handbook, Reliability Prediction of Electronic Equipment, MIL-HDBK-217F, U.S. Dept. of Defense, December 2, 1991, 2–3.

8. Ronald E. Giuntini, "Reliability Prediction of the Electronic Systems of the SPIROS(tm) Drug Delivery System Task A—Part 2," Dura Pharmaceuticals Handbook, (Revision January 30, 1997), 12–14.

9. Robert J. Firenze, The Process of Hazard Control (Dubuque, IA: Kendall/Hunt), 184.

Ronald E. Giuntini, PhD, is manager of the reliability engineering staff at Wyle Laboratories (Huntsville, AL). He is a registered professional engineer in industrial and systems engineering, a certified reliability engineer, and a certified professional logistician.

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