The phrase, "May you live in interesting times" certainly applies to today's medical device design engineers, as they face unprecedented opportunities and challenges created by a rapidly evolving market. Perhaps the most pressing challenge is creating sensor elements which can keep pace with the miniaturization occurring in nearly all medical devices, from simple blood pressure monitors to complex heart-lung machines.

Fig.1 – Modern medical devices, such as these infusion pumps, require compact sensors compliant with the stringent standards that govern medical materials safety.
This article focuses primarily on design strategies you can use to develop compact, cost-effective sensing solutions that provide medical devices with critical real-world data about a patient’s condition or the therapeutic agents being administered to them. Several practical scenarios will explore how designers are using recent innovations in sensor technology to create compact, cost-effective medical equipment that delivers excellent performance, profitability, and safety. Each case will be accompanied by some simple tips for selecting components and examples of products which are best suited to the task.

Why Size and Portability Matter

The trend toward smaller, more portable medical equipment is affecting nearly every area of modern medical care. This is especially evident in patient transport, intake, and ambulatory care environments where the smaller the equipment is, the earlier it can reach the patient. For example, compact lightweight patient monitoring and support equipment allows medical teams to provide high quality care immediately, putting capabilities formerly reserved for the operating room and ICU into local clinics, field care units, and even the tight confines of ambulances and medevac aircraft.

Likewise, smaller respirators, infusion pumps, and vital signs monitors help hospitals improve the quality of in-room and ICU care. (See Figure 1) Even operating rooms can benefit from compact monitors, pumps, and suction equipment, which give a scrub team better access to both the patient and the monitors, life support, and treatment equipment they depend on.

Space-saving designs also enhance a product’s portability, allowing care providers to quickly and easily move equipment from one patient to the next. Highly portable equipment can be quickly deployed into critical situations where seconds count. Portability also allows patients greater mobility during their hospital stay, giving them greater autonomy and comfort and, in many cases, a speedier recovery. The same capabilities can also help health care administrators produce a healthier bottom line by enabling more efficient use of a facility’s technical assets.

The benefits of smaller medical equipment are already helping bring reliable, easy-to-use health monitoring capabilities to home-care environments. These advanced products will play a key role in the migration of patient care services from the hospital to the home, and are being promoted by both private medical insurance companies and the US government’s recently-implemented Affordable Care Act. As a result, forward-thinking manufacturers have already begun development of products that allow quality care to be delivered in the home.

New Sensor Options Enable Compact Designs

Fig. 2 – Dialysis machines require highly-accurate pressure measurements of the patient’s blood pressure and the dialysate fluid which clears toxins from their bloodstream.
Until recently however, sensors have been a challenge in the path toward smaller, more portable devices. These mechanical/analog components have not evolved as quickly as the digitally oriented computing, display, and communication technologies found in laptops, tablets, games, and other consumer products. Now, with smaller sensors and new technologies, design engineers can reduce the size and cost of their designs.

This trend toward smaller devices has begun with the implementation of new, space-saving sensors, and an expanded range of low-volume packaging and mounting options. Other new solutions help designers achieve additional space savings by integrating multiple sensor elements in a single package. In some cases, these advanced integration techniques have allowed the sensors to be placed within small surgical devices and other applications which were previously considered impossible.

Sensor platforms that offer several options for mechanical interfaces, mounting, packaging, and I/O options give medical equipment designers new degrees of freedom. Pressure sensors, for example, provide several options for mating connections (port styles), packaging (dual in-line package, single inline package, surface mount), and outputs (analog or digital), which can be used to satisfy their applications’ demanding functional, cost and board space requirements.

The multiple benefits offered by sensors with board-mountable packages have made them one of the most popular space-saving components for medical and industrial designs. In the case of sensors that measure fluid pressure or flow, board-mount packaging enables the sensing element to be firmly attached to the device’s printed circuit board (PCB) as closely as possible to the patient and/or the liquid media (e.g., blood, chemicals, or water) that it is sensing.

This is especially important in certain applications, such as dialysis machines, as shown in Figure 2, which require precise and accurate measurement of dialysate and venous pressure to ensure a patient’s safety and comfort during treatment. Accurate measurement of pressures in the fluid and blood flows helps ensure that an over- or under-pressure situation does not occur, conditions that can result in either burst blood vessels or air bubbles in the dialysis feed line. For these types of applications a board-mounted package can help provide the close proximity the pressure sensor needs to produce accurate, precise measurements and better response time to changes in the system (See Figure 3).

Besides ensuring accurate and precise measurements, board-mounted sensors enable simpler, more reliable designs that can be quickly assembled using automated equipment. Pressure and flow sensors are typically available in several board-mount package styles, many of which offer options for different orientations for their integrated ports. This can be very helpful for a design that needs a clear path for the tubes that connect the sensor with whichever medium it is monitoring.

Fig. 3 – Dialysis Machine Diagram. (Credit: Honeywell Sensing and Control Medical Applications)
Packaging options that combine the sensor port and a pre-integrated manifold can provide additional space savings for some applications. Eliminating the tubing and related connections between the sensor/port assembly and its target medium results in a simpler, more compact design. The direct connection afforded by manifold-mounted packaging also eliminates many of the points in conventional designs where clogs, leaks, and other types of failures can occur. Manufacturers often find that by eliminating the trial-and-error required to optimize a port’s performance, using pre-engineered manifolds can provide significant savings in a design project’s engineering labor costs and development schedule.