Reliability is a Decisive Factor in Sensor-Based Medical Applications

Driven by rising healthcare costs, an aging population, and the importance of early diagnosis in treating illness and saving lives, sensor development is playing a critical role in the healthcare arena. Sensors are now used in an array of medical applications ranging from disposable and reusable temperature probes to pressure sensors in infusion pumps, airflow sensors in anesthesia delivery systems, force sensors in kidney dialysis machines, and magnetic, humidity, airflow, and pressure sensors in a typical sleep apnea machine.

Sensors are being employed in these and many more medical applications due to their accuracy, intelligence, capability, small size, low-power consumption, and, above all, reliability. This article will examine current medical trends and how it is pushing sensor technology. We will also discuss the reliability standards that must be met, and look at some available parts that exemplify the current sensor state of the art.

Equipment or physiology?

A patient’s heart rate, blood glucose, and body temperature can be monitored to help clinicians diagnose the state of illness based on accurate sensor readings. But therein lies the rub. As in all things, malfunctions exist and it’s often difficult to pinpoint whether the culprit is the device itself or the physiology of the patient.

If a pacemaker fails, for example, it can be that the malfunction occurred because of equipment failure or changes in the patient’s underlying native heart rhythm. Medical body sensors can be implanted or attached to the human body and when inaccurate data is gathered; sometimes it may be attributed to incorrect placement on the body, which, in turn, can seriously influence diagnosis. Fault values combined with abnormal physiological data based on illness make it challenging to determine true faults. Even environmental and logistical factors impact accuracy. Fluorescent lighting can cause data errors in pulse oximeters and devices placed on a patient’s body with adhesive tape may fall off and result in detection error.

All of that notwithstanding, sensor manufacturers working in the rapidly growing healthcare industry must keep safety, accuracy, and reliability moving in the right direction. To ensure that they do regulatory agencies have put in place a variety of standards that must be met.   

Hardware and software standards

Design engineers are, of course, quite familiar with technical standards. However, while technical standards such as IEEE 802 for Wi-Fi define the performance of wireless communications, standards for medical design go much deeper, also often covering design methodology and verification.

In general, the medical hardware industry employs an approach whereby two independent failures are not allowed to harm patients. Standards within the medical industry typically target a specific equipment type such as infusion pumps or dialysis machines. The main medical device electrical safety standard is IEC 60601-1, published by the International Electrotechnical Commission, which establishes general requirements for safety and performance and is the global benchmark for medical electrical equipment.

Apart from IEC 60601 the statutory specifications for medical devices include 21 CFR 820 (the Quality System Regulation for medical devices sold in the United States, which is enforced by the U.S. Food and Drug Administration [FDA]), as well as standards for quality management (ISO 13485, based on the ISO 9001:2000 process model approach and a management systems standard specifically developed for the manufacture of medical devices), risk management (ISO 14971, which specifies a process for a manufacturer to identify the hazards associated with medical devices, including in-vitro diagnostic [IVD] medical devices), usability (IEC 62366, which specifies a process to assess and mitigate risks caused by problems associated with correct use and use errors), and functional safety (IEC 61508).

This last standard, IEC 61508, adds the safe function of a device or system into the development equation. Functional safety ensures that a given apparatus functions correctly in response to inputs. For example, if an infusion pump malfunctions, functional safety protocols will ensure that alarms are activated to signal the malfunction and if relevant the pump is deactivated to protect the patient from harm through overdosing.

Safety considerations go beyond hardware to software and user interfaces. Medical device software is regulated in standard IEC 62304. This standard focuses on the software development process and also defines the typical activities of the system life cycle such as planning, requirements analysis, design, implementation, verification/testing, and release.

The standard describes process and documentation requirements for each phase of the software life cycle. In all five processes are described, with which manufacturers can develop medical device software. While IEC 62304 establishes strict requirements, it is mandatory in Europe; in the U.S. it is recommended only.

Looking at the software and firmware involved, Scott Nowell, CEO of Validated Software, a company providing supporting software/firmware for embedded devices that must comply with industry-specific development standards, isn’t certain that overall reliability has improved as much as it could.  “I don’t see that the overall reliability of medical devices has improved. The overall requirement is still to get a product to market as fast and as cheap(ly) as possible. Like it or not, corners are cut, particularly at smaller companies.”

Where Nowell does see improvement is on the hardware side where previous systems included processors plus peripherals. Now many of the peripherals are built into the processors and this integration will naturally result in some reliability improvement, he explains.

“We deal in many safety-critical industry segments. Avionics, for example, relies on more stringent requirements and details are specified more clearly than medical in that there are specific things that the industry requires for safety. The FDA is nowhere near as stringent,” Nowell says. “Big medical equipment companies typically do a good job because they are risk adverse. Small companies sometimes just can’t afford it.”

While medical device safety/reliability has room for improvement, the underlying sensors used in these devices have made great strides in providing greater accuracy and reliability to patient care. Following are some examples.

Among medical sensors designed to be disposable, Freescale’s MPX2300DT1CT-ND high-volume pressure sensor is a unique device based on piezoresistive technology combined with integrated thin-film temperature compensation and calibration. The low-cost disposable sensor combines the performance of Freescale’s shear stress pressure-sensor design and the use of biomedically approved materials. Materials with a proven history in medical situations have been chosen to provide a sensor that can be used with confidence in applications, such as invasive blood pressure monitoring. It can be sterilized using ethylene oxide. The portions of the pressure sensor that are required to be biomedically approved are the rigid housing and the gel coating.

When looking at accuracy in sensors, Honeywell Sensing and Control touts its ability to not only give specs regarding accuracy, but they claim they are alone in providing specs that indicate total error band (Figure1). For applications that include anesthesia delivery, heart pump, nebulizers (a device that converts liquid medicine into a fine mist you inhale by breathing through a mouthpiece), sleep apnea, ventilators, and more, Honeywell’s Zephyr digital airflow sensor, the HAFLF0100C4AX5, provides a digital interface for reading airflow over specified full-scale flow and compensated temperature ranges. The thermally isolated heater and temperature-sensing elements ensure that the sensors perform fast in responding to air or gas flow, especially in critical medical applications.

Figure 1: Honeywell’s total-error-band measurement is important in applications where a high level of accuracy is required.

The requirements for sensors that can be inserted through an incision—typically at the tip of a catheter—are less critical than those for implantables but will still need FDA approval. Depending on the surgical procedure, these sensors need to function for a few minutes up to a couple of hours and can be powered via external sources.

Catheter ablation sensors are an example of sensors temporarily inserted through incision. The catheter tip contains a source for RF energy and a force-load cell sensor. It is critical that the force applied by the catheter tip to the target tissue not exceed maximum values to avoid any possibility of perforating the target tissue.

Measurement Specialties’ Microfused sensing technology holds the promise of providing a triaxial force-sensing system able to measure tissue-contact forces in all three dimensions simultaneously. For instance the FX1901, intended for OEM use in laboratory, hospital, or consumer-product applications (Figure 2) is a 1 percent load cell device with full-scale ranges of 10, 25, 50, or 100 and 200 lbf compression. Operating at very-low strains, Microfused technology provides an essentially unlimited cycle life expectancy, high over-range capabilities, and a ratiometric span of 20 mV/V.

Figure 2: The combination of stamped flexures and micro-miniaturized MEMs strain gages in the Measurement Specialties FX1901 permits low cost to be achieved in high-volume OEM applications such as disposable medical devices.

In summary, whether the end product is a disposable blood pressure monitor or a more complex life-saving device such as a heart pump, all elements targeted for use in medical applications must feature high quality and reliability. This article has briefly reviewed the challenges and design safety standards affecting sensors used in medical applications. It has also looked at several specific parts representative of how far suppliers have been raising the bar.

For more information on the sensors discussed in this article, use the links provided to access product information pages on the DigiKey website.