Sensors for Medical Devices, Part 1: Gas Monitoring, Mixing, and Delivery Systems

By Jon Gabay

Contributed By Hearst Electronic Products

It seems to work both ways. We spend a lot of time and energy monitoring machines and now, increasingly, machines are also monitoring us. The subject here is not surveillance or security, but rather medical devices that we can wear, keep in close proximity to our body, or connect to us in some way. These devices can log data when needed for diagnostics, or be active all the time (“always on”) to let us know if anything in or on our body needs our attention or the attention of medical professionals.

While some sensing and diagnostic machines are too large and thus nontransportable, others have been size-reduced, and downsizing of healthcare equipment has become the predominant design trend. For example, dialysis machines used to be very large and expensive, but are now available as home units or even as travel units. The same is true of oxygen concentrators.

The sensors engineers have at our disposal will make a world of difference to those whom technology can benefit. This article will look at the needs of medical device manufacturers and the types of sensors they commonly use. As modern medical machines tie so many technologies together, it is impossible to encapsulate every type of medical device in one installment. Therefore, this article will focus on gas monitoring, mixing, and delivery systems. Future articles will explore other types of medical technologies, such as infusion pumps, liquid medication monitoring and delivery systems, and more. All components, datasheets, tutorials, and development kits referenced here can be found on the Digi-Key website.

Delivery of anesthesia gases

The metering out of precise amounts of vapor or gas — a.k.a. inhaled therapies — is one of the most important types of equipment medical teams have to rely on. It is very difficult to perform surgery when the patient is conscious; by the same token, too much anesthesia can cause brain damage, coma, or death.

The metering out of anesthesia gases is very dependent on many factors, including real-time conditions inside the body, which must be monitored as quickly and accurately as possible. Blood oxygen level, blood pressure, brainwave activity, and pain reflex response are some of the conditions that the anesthesiologist must be aware of when dispensing gas.

A basic approach before technology provided a better method was to use gravity as a stable and steady rate controller for drip-activated dispensing. Gravity is still used as a reliable means to regulate medicine injection through a simple IV. The drop size (volume) and drip rate multiply to easily calculate the rate of input of a drug or chemical.

While gravity has served us well, modern pharmaceutical systems need to provide higher levels of accuracy, and even coordinate variable delivery rates between several medications. Mixing of different compounds requires special handling and care. Our embedded microcontrollers need to harmoniously calculate the exact rate of flow, sense pressures, temperatures, and use real-time feedback to control pumps and pressure regulators.

A modern anesthesia delivery system relies heavily on both precise electronics and mechanics (Figure 1). Pressure sensors of input air and gases verify gas movement through known apertures to ensure accurate concentrations. These can include the MLH family of pressure sensors from Honeywell Sensing and Control with ranges from 50 to 8,000 psi. A 50 psi part like the MLH050PGB01E could also serve this function. Its 0.5 to 4.5 V sense output scales well with a 5 V microcontroller’s A/D stage.

Image of sensors for modern anesthesia delivery systems

Figure 1: Sensors for modern anesthesia delivery systems work together with electromagnetic control to ensure exacting levels of precise concentrations of several different compounds.

Flow sensors like the HAF series verify movement and mixing, as well as the operation of fixed open/closed or variable-rate electro-valves. Parts like the Honeywell HAFBLF0050C4AX5 are members of the supplier’s Zephyr HAF Series of manifold-style flow-rate sensors that provide analog or digital outputs directly to embedded systems. A Product Training Module on the Zephyr Airflow Sensors can be found on Digi-Key’s website.

Breathing is fundamental

Other benefits come with the elimination of gravity as a needed mechanism for medical equipment. For instance, when conditions are constantly changing such as in an oxygen-dispensing system on a jet fighter or in other applications where gas sensors must detect the presence and concentration of very specific elements or compounds in gaseous form.

Consider Honeywell’s oxygen sensors, such as the KGZ10 zirconia-based sensor (Figure 2). This sensor uses an internal heating element to bring an internal plate to 700°C, which makes the zirconia operate as an O2 gas pump. A dual, hermetically-sealed chamber between the oxygen pump is used to allow the measurement of the ratio of partial pressure difference and generates a corresponding sense voltage. The sense voltage is linear in output and corresponds to the level of oxygen present. No reference gas is required.

Image of Honeywell’s KGZ10 oxygen sensor

Figure 2: Self-contained oxygen sensors can mount on a PCB probe, or be part of an isolated chassis assembly.

Honeywell's MF010-0-LC4 is suitable for oxygen concentration detection and monitoring for aerospace and ruggedized environments such as on a fighter jet. Using the same hermetically-sealed zirconium dioxide discs, these operate at very low temperatures and can detect a gas mixture’s absolute oxygen content in closed systems, or areas that are not easily accessible. Modern-day cryogenic surgeries are proving beneficial at minimizing physical trauma during extensive procedures. Precise control of lowering body temperature coupled with accurate oxygen concentrations could mean a patient does not suffer brain damage.

Sensors for other gasses can be critical as well. Like oxygen, carbon dioxide in blood and breath is an indicator of the level of metabolism. The sensing of oxygen and carbon dioxide levels need to work hand-in-hand and quality precision sensors for carbon dioxide are readily available to do the job. Take the Amphenol Advanced Sensors T6613-F flow-through CO2 module (Figure 3).

Image of Amphenol Advanced Sensors T6613-F CO2 sensor

Figure 3: The single- and dual-channel CO2 sensor modules use software to eliminate the need for constant calibration.

Based on Telaire CO2 detection technology, the T6613 sensors are lifetime factory calibrated to measure concentration levels up to the 2,000 or 5,000 ppm range with accuracies of 3 to 5 percent of reading with updated readings every 4 seconds. A 19,200 baud UART or I²C interface can query data or status at any time, and a 0 to 4 V analog output is also available. A Product Training Module on Telaire CO2 Sensing Technology is available on the Digi-Key website.

Condensation issues

When precise concentrations and mixtures of gases and elements are needed, detecting and eliminating excess moisture may be required. The ability to sense relative humidity may require a symphony of other sensors (such as pressure), as well as the presence of certain gases or even liquids.

Several moisture and humidity sensors are readily available, depending on the environment and specific requirements. A good place to start would be the EnOcean HSM100 unit, which implements the company’s moisture-and-humidity-sensing technology (Figure 4). This modular sensor assembly is internally calibrated to operate reliably from –20° to +60°C.

Image of EnOcean HSM100 unit

Figure 4: Relative humidity sensing provides a range from 0 to 100 percent relative humidity with a resolution of 0.4 percent accuracy.

Honeywell also makes a chip-level solution with its HIH6130-021-001 surface-mounted total-error-band humidity sensor. This corrects for non-linearities, thermal effects, and features hysteresis. SPI or I²C I/O host processor communications are eliminating the need for analog stages on your board. Honeywell claims 1.2 percent relative-humidity long-term stability over 5 years of operations and the internally-calibrated technology allows replacement without the need for recalibration. A Product Training Module on Humidicon Digital Humidity and Temperature Sensing Technology is available to help engineers.

In conclusion

We have reached a point where wearable medical devices can be designed, built, and sold cheaply enough to improve our quality of life. One of the reasons this is possible is sensor technology has enabled a wide range of off-the-shelf sensors that engineers can use in medical equipment design.

The ability to sense body states and health is improving all the time. The sensors we have discussed in this article are here now and ready to be used in current and next-generation devices. For more information on these parts, use the links provided to access product pages on the Digi-Key website.

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