As technology advances in the medical device space, electronics design engineers are constantly adapting to meet industry requirements for increased functionality, reduced component size, and absolute reliability. For medical implantables, technological innovations are driven by the ability for electronic components manufacturers to superminiaturize electronic circuits and create advancements in the materials and designs available.
Additionally, regulatory guidelines at both the component and device level are becoming increasingly strict. These new guidelines place more weight on reliability grade, size, and durability for life-sustaining applications over other common parameters of importance for consumer devices and electronics.
Looking at the critical components that enable reliable performance of implantable medical devices, capacitors are just one of the many that contribute to the overall longevity of the device (see Figure 1). This article explores requirements, regulations, and testing procedures for medical device capacitors that device designers should be familiar with to ensure that they are selecting the most reliable components, and suppliers, for their medical applications.
Requirements for Medical-Grade Capacitors
In general, between material specifications, leakage resistance, stability, and price, there are numerous factors steering capacitor choice. The complex requirements of medical applications and strict regulatory requirements device manufacturers must adhere to add yet another layer of consideration.
The most widely used source control drawing (SCD) specification for high-reliability capacitors, and by extension, medical implantable devices, is MIL-PRF-55681. This standard defines a mid-K stable dielectric designated as BX. The BX specification has voltage temperature limits in addition to temperature limits of capacitance. The BX dielectric is limited to ±15 percent maximum change in capacitance between 25° and –55 °C or +125 °C and also has a voltage restriction of +15/-25 percent maximum change in capacitance between 25 °C and -55 °C or +125 °C at rated voltage.
Regulatory bodies, such as the International Organization for Standardization (ISO) or the U.S. Food and Drug Administration (FDA), require medical device manufacturers to meet this SCD specification to maintain the highest level of reliability in all phases of development and production of medical applications.
Another widely used SCD specification in the medical device space is MIL-PRF-123. Capacitors covered by this specification may be used in two main applications. The first type is critical frequency determining applications where absolute stability is required (BP and BG), such as cardiac pacing devices that use timing circuits. The second group includes applications where variations in capacitance with respect to temperature, voltage, frequency, and life can be tolerated (BX and BR). Many life-sustaining devices require this kind of variation. These specifications are not limited to implanted devices.
Testing Guidelines and Considerations
When designing medical implantables, minimizing the need for invasive surgery is imperative. This means thorough testing of every component used is critical for ensuring the long-term reliability of the device. Therefore, every high-reliability capacitor should be 100 percent electrically inspected and burned-in at elevated voltage and temperature levels to precondition the parts and comply with established performance criteria.
Each of the tests detailed in Table 1 is necessary to demonstrate that devices meet the specifications discussed. Performing these tests requires specialized equipment, tooling, and significant time investment from a medical device manufacturer’s quality assurance engineering team.
Depending on the SCD requirements, additional testing may be needed on a lot basis after the components have passed a Group A inspection. For example, Group B environmental testing for product group HS consists of the tests specified in table XII of MIL-PRF-123. Additionally, Group C environmental testing consists of the tests specified in table XI of MIL-PRF-55681 for product groups HB and HK. Testing shall consist of the tests specified in table XIII of MIL-PRF-123 for product group HS. For both Group B and C testing, copies of the data shall be forwarded to the purchaser with the parts, which may not be shipped until the conclusion of these tests.
Medical-Grade Capacitors in Practice
Aside from implantables, the demand for portable and wearable medical technology has increased as medicine begins to move away from its conventional setting. Traditionally benchtop devices are becoming miniaturized and increasingly wearable and implantable to improve quality of life for patients. For example, a typical cochlear implant provides a sense of sound to deaf or severely hard-of-hearing patients. The implant is a small device, and in this size range, utilizing a more compact, high-reliability capacitor is required.
Considerations for Choosing a Supplier
Medical devices today incorporate an ever-increasing amount of technology. If a device is to deliver a lifesaving or sustaining function, it must be reliable all the way down to its foundational components.
While choosing the right capacitor for a medical application during design is critical to achieve this functionality, medical device manufacturers also need to choose the right supplier.
But supplier selection is not always an easy task, especially as many companies are simultaneously looking to minimize costs as well. This often results in weaving intricate webs of suppliers from around the world to meet these strict cost goals, presenting logistical challenges for component management, among others. Additionally, medical device engineers and manufacturers are typically facing pressure from consumers and shareholders to beat their competition to market with the most advanced option.
To meet expectations for the functionality and reliability necessary for lifecritical applications, it takes a specialty components manufacturer with a commitment to innovation to drive improvements in components that will support patient safety and quality of life. Working with an experienced supplier will not only help medical OEMs achieve cost and time-to-market goals, but they will act as a trusted partner. Additionally, the right supplier can advise the OEM’s engineering team early in the development process to avoid costly, or potentially life-threatening, mistakes.
When Reliability Is Paramount: Selecting Medical Device Components That Are Built to Last
Since implantable medical devices are embedded into the human body, one of the main end goals is to reduce the physiological burden on patients by minimizing the need for invasive surgeries. By developing devices that use components that are smaller in size, more reliable, and have a longer service life, medical device manufacturers are inherently addressing this requirement.
Finally, when implantable devices fail, everyone pays a price — including patients. By selecting high-reliability capacitors that go beyond typical MLCC designs and requirements, device manufacturers can remove some uncertainty and lessen the possibility of malfunctions, recalls, and replacement surgeries. Ultimately, by minimizing risks and increasing reliability at the component level, medical device manufacturers are developing devices that patients can feel comfortable trusting their lives with.
This article was written by Rich Bestafka, Key Account Manager – Medical at Knowles Precision Devices, Cazenovia, NY. For more information, visit here .