Murray Slovick published an overview on TTI MarketEye on capacitor selection considerations for medical application.
Capacitors for Medical Applications: Component Selection Considerations
Within the medical industry, electronics are finding their way into more applications, from large, imaging equipment down to smart tags for surgical packs. On these pages, MarketEYE contributor Dennis Zogbi has forecast that the global medical technology market will reach $515 billion by 2022 to support aging populations and emerging economies.
The medical devices industry is one of the most highly regulated sectors in the world; its regulating bodies include the International Standards Organization (ISO) and the U.S. Food and Drug Administration (FDA). These regulations, as well as the demands placed on electronic medical devices, have ramifications for the component selection process. In this article we will explore those impacts given that tantalum capacitors and multilayer ceramic capacitors (MLCCs) are the most popular types of capacitors for medical applications.
Regulatory Standards
Globally, the primary standard governing medical device design is formally known as IEC 60601-1, “Medical electrical equipment – Part 1: General requirements for basic safety and essential performance.” The European (EN 60601-1) and Canadian (CSA 60601-1) versions of the standard are identical to the IEC standard. IEC 60601-1 is a type test standard, not a standard for process certification. Consequently, it applies to a device design rather than a manufacturer’s processes.
The FDA regulates all medical devices marketed in the US, which are grouped into three broad classes depending on the device’s risk, invasiveness, and impact on the patient’s overall health. These classes are as follows:
- Class I – Lowest Risk: Examples of Class I devices include manual toothbrushes and reusable surgical scalpels. Class I devices are subject to far fewer regulatory requirements than Class II or III devices.
- Class II – Moderate Risk: Class II electronics include test and scan equipment. A non-invasive blood pressure monitor is an example of a Class II device.
- Class III – Highest Risk: Devices that are inserted into the human body, including permanent implants, smart medical devices and systems such as pacemakers and defibrillators.
Capacitor Choices
Passive components have an important role in medical systems and are part of diagnostic, imaging, patient monitoring, and pharmaceutical delivery and dispensing applications. In particular, implantable medical electronic devices are usually powered by batteries or capacitors that have to be removed from the body after completing their function due to their non‐biodegradable properties.
Capacitors are employed for use in implantable medical devices such as defibrillators, insulin pumps and pacemakers, as well as in portable and wearable devices (including electrocardiograms, ultrasonic echo devices and blood gas analyzers). They are required to have high reliability, offer long service life and pass stringent screening checks. Meeting customer demand today often also means miniaturization and advancements in capacitor materials and design.
Tantalum capacitors are used in most of the pacemakers and defibrillators manufactured each year. There are many reasons to choose tantalum, including their inherent reliability, self-healing capabilities (tantalum capacitors have low resistance paths through the dielectric which can self-heal, repairing the potential fault site), and their ability to pack high capacitance values into small case sizes.
MLCCs are attractive for medical devices because they are usually compact in size, offer high reliability and large capacity, and have predictable temperature coefficients. They also offer the most stable capacitance with respect to applied voltage.
Generally speaking, MLCCs are normally chosen for applications with capacitance ranges below 1 μF, and tantalum capacitors are selected for applications with capacitance values above 10 μF. In between (the 1–10 μF range), choices depend on relative size, requirements for capacitance stability over temperature and voltage, and rated voltage capability.
As MLCC technology can go to much smaller dimensions, MLCCs can be manufactured in case sizes that are not practical for tantalum capacitors while solid tantalum capacitors with MnO2 cathodes are attractive because they have no wear out mechanism. For tantalum capacitors, DC leakage current (DCL) is one of the most important electrical parameters. Compared to ceramic capacitors, tantalum capacitors have high leakage currents. The DCL of a tantalum capacitor also increases with an increase in temperature.
Capacitors fail due to various factors, including manufacturing processes and design defects such as cracks and voids that occur during production, materials that wear out, operating temperature, voltage, current, humidity and mechanical stress. These internal flaws can result in leakage instability, increased leakage current or even catastrophic dielectric breakdown. Some of the factors that can accelerate these defects include product assembly, thermomechanical stress and how the device is used. Frequently, failures can be attributed to the degradation of a given material. For example, thin layers of silicon dioxide are used as a dielectric for capacitors or as the gate oxide for a MOS semiconductor device. Time Dependent Dielectric Breakdown (TDDB) failures of capacitors occur due to the degradation of this insulation material.
Reliability assessment is an essential process in the production of components and electronic devices. Life Data Analysis predicts how products will operate throughout their lifetimes by analyzing data from a sample set of failures. In particular, the Weibull reliability assessment method – a mathematical technique frequently used to analyze various types of life data in order to predict failure rates based on studying sample behavior – is commonly used by capacitor manufacturers to assess reliability.
Usage Considerations
Among the sterilization methods available for high-volume medical devices is gamma radiation from Cobalt-60, a radioisotope which continuously emits gamma rays. During sterilization, gamma rays efficiently eliminate microorganisms from the medical device. From a circuit applications standpoint, however, the most important effect of radiation on a capacitor is the induced conductivity in the dielectric material. When exposed to ionizing radiation, capacitor leakage resistance decreases; as such, radiation can degrade the electrical performance of the part.
Dimensional change of the capacitor plate spacing is the principal cause of capacitance changes during irradiation. This change is due to pressure buildup from gas evolution and swelling which results in physical distortion of capacitor elements and thus changes the spacing. This dimensional change is most pronounced when radiation-sensitive materials, generally organics, like polystyrene, polyethylene terephthalate and polyethylene are used in one or more parts of the capacitor’s construction.
Changes in organic materials due to radiation are more pronounced, and so these are less satisfactory in a radiation environment than those capacitors employing inorganic dielectrics. Electrolytic capacitors (aluminum and tantalum) are capable of extended radiation exposure, with tantalum being more radiation-resistant.
One More Choice
While choosing the right capacitor for a medical application is not a trivial task, engineers will find online component selectors and circuit configurators readily available to help locate parts by product family, application or key parameters.
You also need to choose the right supplier. An experienced supplier can advise your design team early in the development process to avoid costly mistakes and find components that meet demanding specifications. The best way to ensure that components are standards compliant is by sourcing directly from suppliers or from authorized distributors.
