With the large number of user interface options available today, it is easy to forget why the membrane switch is an excellent choice for medical instrumentation. The basic advantages are still the same as they were two decades ago, when membrane switches became the interface of choice for a multitude of products, including toys and games, high-end appliances, industrial controllers, and automotive controls. These advantages have been enhanced by developments in materials, processes, and design expertise.
Membrane switch applications are well suited for medical electronics, which are limited to a specific or small number of functions, are portable, and require a high degree of visibility. Membrane switches are particularly suitable for handheld medical applications and portable devices because they are lightweight, low profile, durable, easy to clean, and RoHS-compliant.
The remainder of this article discusses seven key features and characteristics of membrane switches that contribute to the optimal design of a medical user interface.
The biggest advantage of a graphic overlay on a medical instrument is its ability to be seen in a 180° radius. Warning lights or other functional indicator lights can be viewed from a distance at various angles. Touchscreens do not provide the same ease of signal detection and identification. As the top layer of the membrane switch, the graphic overlay is the direct interface between the product and the end user. Besides the obvious function of defining the switch locations and functions, it can also serve purposes of product enhancement and differentiation. With an essentially unlimited color palette, the graphic overlay can provide aesthetically pleasing yet highly functional characteristics to the product. Very simple color and artwork adjustments provide low-cost model or functional differentiations even though the lower switch layers remain constant, which can help to keep total program costs down.
Hard coated overlay materials have excellent resistance to adverse environments. Selective applications of texture coatings provide durable and pleasing background and interface areas, while allowing lower layer displays to be highly visible and undistorted. In addition, lower layer displays and LEDs can be hidden using appropriate dead fronts and transparent colors. Embossing may be used to give the panel a threedimensional look and feel.
An antimicrobial film was recently developed for use in typical membrane switch applications. It is commercially available, can be screen printed with common inks or embossed, and can be used for clear window applications. The manufacturer, MacDermid Autotype (Meadows, IL), provides the following description: “The Microban® technology is incorporated into the Autotex AM textured hard coat during the manufacturing process. This process ensures even distribution of the antimicrobial agent throughout the textured hard coat and the film surface. When bacteria come into contact with Autotex AM with Microban®, the antimicrobial function disrupts the bacterial cell wall killing or inhibiting bacterial growth. The result is that the film surface of Autotex AM provides dependable and constant protection against bacterial contamination.” This should merit consideration for any medical application.
Membrane switches can easily provide protection against electrostatic discharge (ESD). An inner layer of the switch assembly can take such discharges to ground. Innovative uses of double-sided polymer circuitry (D/SPC®) can provide the path to ground as part of the switch circuitry, thus eliminating the need for extra shield tails or tabs. The use of D/SPC can also help to reduce the footprint of the switch circuit, eliminating potential problems with discharges to the edges of the membrane panel. This shield layer can simultaneously provide EMI/RFI protection. Printed shield layers can be tailored to each unique application to provide optimum shielding effects and prevent unwanted interference from incoming or outgoing signals. Shielding can generally be accomplished without compromising the performance or aesthetics of the switch.
One of the biggest threats to any type of user interface is the risk of water or chemical ingress into the switch cavity. A membrane switch can be sealed against various types of environments typically encountered in the portable medical device industry. This can be achieved with a full perimeter seal or gasket within which the circuit tail exit is routed. With the proper design and material choices, not only can the switch array be sealed against the environment, but it can also seal the enclosure to which it is mounted. The sealed membrane switch can also provide highly visible status indication through the use of low profile surface mounted LEDs.