Paramedics have a rugged job and any apparatus they use must stand up to the fast-paced, but precise task of stabilizing victims at the scene of an accident and in other medical emergencies. One of the first steps a paramedic takes is to use such an instrument to check heart and lung functions of trauma victims, including pulse rate and blood oxygen saturation.
The N-20 Handheld Pulse Oximeter, developed by Nellcor Oximetry, part of Mallinckrodt Inc. (Pleasanton, CA), does such on-the-scene monitoring. It uses a sensor that simply clips to a patient's finger. A detector measures infrared light that penetrates the tissue from an LED source in the oximeter.
| Brightness of reflective displays can be compared to pure black and white, which has 100% contrast. NCAP liquid crystal displays have roughly three times the brightness of LCDs.
A rubber boot encases the oximeter for shock protection. But since the display cannot be covered, Nellcor engineers turned to NCAP, a flexible, encapsulated liquid crystal display material from Xymox Display Products (Sunnyvale, CA). Glass or hard plastic is more susceptible to field breakage.
Mallinckrodt's Manager of Industrial Design Claude Hutcheson says, "We designed the product around the display since its whole purpose is to convey information. This was key to an improved user interface and ease of viewing using large, seven-segment numerics." LEDs were considered too small and conventional LCDs were looked at, he adds, "but these are subject to breakage and the use of color LCDs would increase cost significantly. We wanted color coding to use red for alerts."
In bright sunlight, emissive displays, such as LEDs, can be saturated with incident light and become difficult to read. Hutcheson notes the reflective NCAP displays are bright and, "just like a piece of paper," become more readable under bright lighting.
With a thin, integral electro-luminescent (EL) backlight for illuminating the unpolarized NCAP display in darkness, the life of the EL is extended. In contrast, polarizers in a standard twisted-nematic (TN) LC display will absorb about 60% of the light passing through them from behind, "requiring a very bright backlight to read in low light, reducing the life of the backlight and using substantial power," says William Ostrander, Xymox Display's director of technology.
| NCAP displays can include membrane-switch touch-screen functions, helpful in prompting new or intermittent users—total display and switch thickness is on the order of 0.05 to 0.06 inch.
Hutcheson says polarizers cause the viewing angle on TN LC displays to be poor, on the order of 60º or so. Without polarizers, NCAP has a 160º viewing angle. Being able to see the NCAP display over a wide angle gives the paramedic greater freedom of movement when caring for and monitoring a patient.
Thin is in. Because of NCAP's flexibility, it can be curved and integrated to have membrane switches for a monolithic, rugged control panel. Such an interface, using steady and blinking light cues, can aid in prompting a user simplifying operation and reducing errors, particularly for new or infrequent operators. Medical devices featuring the touch screen include: remote control units for digital x-ray imaging systems, and control panels for arthroscopes, suction units, and sterilizers.
Hutcheson says initial designs for the Pulse Oximeter did consider a touch screen. But the final design, with one-button functionality, used a displacement switch for increased tactile feel and an uncluttered display. The single button also saved cost and size (thickness). He concludes, "I wish we had more products with this display it's one of our best."
Membrane electronic components from Xymox Technologies: Product Code 4787
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