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.
There are two common and proven methods of illuminating a membrane switch: LEDs and electroluminescent panels. The LED is a very effective way of communicating — either close or at a distance — a device status, even at oblique angles and differing light conditions. Advances in polymer surface mounted technologies, as well as commercially available low profile LED packages, offer a reliable and effective way to incorporate status indicators into a membrane switch. The combination of chip bonder and conductive epoxies provide the strength and conductivity necessary to attach .010"-tall LEDs to flexible polymer circuits. LEDs are best suited for status indicators, but can be adapted to provide backlighting. This becomes a design challenge as the overall height of the switch is increased when using LED backlighting.
Electroluminescent (EL) lamps are not interchangeable with LEDs for lighting purposes. The EL lamp is most appropriate for backlighting in low light situations. It is not suitable as an indicator light or in daytime or bright light conditions. The advantage for backlighting is an even light source (no point source of light is evident), and can be provided in a very thin layer. Typical EL thickness is approximately 0.010".
6. Tactile Feedback
Tactile feedback enhances error-free data entry and can be achieved with two different materials. Metal dome constructions are used for high reliability applications where a high number of actuations and extreme temperature cycling is anticipated. Metal domes are offered in a variety of sizes, shapes, and actuation forces. A second option is a formed polyester dome. Actuation force is highly customizable with this approach because there are numerous combinations of dome heights, sizes, and shapes. With either material, the user confirms switch closure by “feeling” the switch dome collapse, which acts as a shorting patch to close the switch loop.
7. RoHS Compliance
Medical instrumentation is moving rapidly toward complete RoHS compliance. Typically, there are no banned substances in the basic building blocks of membrane switches if screen printed conductive inks are used. Pressure-sensitive adhesives, the polyester film used for the substrate, and the inks used for graphics are generally free of restricted substances as well. One component that can be a violation is the connector, if it contains lead. RoHS-compliant connectors are commercially available; these connectors are lead-free and will not have any banned substances in their composition. However, if any lead solder is used with these connectors to connect with a printed circuit board or copper circuitry, they will not meet the criteria. Another violating component, though not commonly used, is any flame-retardant screen printed or otherwise included in the product.
Case Study: Handheld Medical Device
A prime example of a handheld medical device is a portable insulin pump. The D/SPC® circuit pictured in Fig. 2 is manufactured to achieve a lightweight, low profile, sealable, tactile and RoHS compliant user interface. The finished membrane switch assembly is approximately 1.5 square inches and has four useable buttons, sized appropriately, to allow for tactile data entry and menu selection. The internal gasket layer seals the circuit and internal case from the potential exposure to harsh environments.
A sealed tactile membrane switch is a reliable, low-cost, proven approach to meet the demanding needs of the medical instrumentation market. It offers advantages with graphics, cleanliness, sealability, shielding, illumination, tactile feedback, and RoHS compliance. It provides the switch function with less weight and space, at a lower cost than most alternatives. As with any user interface technology, it is very important to choose the correct technology for the application.