Nano- and ultra-miniature switches provide the reliability and flexibility required in medical devices
BY JEROME SMOLINSKI
C&K Components, Newton MA
http://www.ck-components.com
Highly reliable ultra- and nano-miniature switches are playing a major role in the medical industry today to meet the needs of space-constrained applications, such as hearing aids and drug-dispensing transdermal patches. Whereas previously these applications typically relied on available miniature pushbutton technology, these same applications are now being better served by smaller, more robust, flexible detect and tactile switch designs.
Flexibility in design
The flexibility of switch components is instrumental to their ability to meet the application-specific requirements of medical equipment, particularly as devices such as hearing aids, glucose meters, blood pressure cuffs, and transdermal patches become increasingly smaller by design.
Detect and tactile switches are inherently smaller and more flexible than many other switch technologies, such as pushbuttons. These types of switches afford the capability of multiple mounting and actuation configurations, providing greater flexibility along with the ability to configure the switch for application-specific needs.
One example is by employing side-actuated ultra-miniature detect switches in blood pressure cuffs, making it much easier for the user to operate, given the design of the cuff. And, because custom detect and tactile switches often involve custom-tooled contacts, they can be tailored to meet any application in an extensive number of markets, including the medical device arena.
The HDS Series micro-miniature, side-actuated detect switches are ideal for use in glucose meters and blood pressure cuffs
‘Zero defect’ reliability
Reliability is another major consideration that switch manufacturers must take into account when designing for portable medical equipment, particularly in applications where the switch can be exposed to environmental conditions, such as moisture, humidity, dust and other contaminants, including sweat. In terms of corrosion, sweat is one of the most, if not “the” most, aggressive substances. In hearing aid applications, this is a major concern.
Materials and sealing play a major role in combating sweat and other environmental elements to guarantee a highly reliable product. The combination of standard stainless steel for fixed contacts and domes, and palladium/silver/copper alloy materials, provides a robust solution to withstand corrosive elements.
The other consideration is to design the switch either with or without plating. Plating the stainless steel contacts with gold or silver to resist corrosion requires the deposition of a nickel underlay to chemically prepare the surface to accept the plating; however, this degrades the stainless steel itself and creates molding inserts or “tunnels” which allow for the ingress of fluids. As a result, designing the switch without plating is much more effective.
Unfortunately, without plating, the switch exhibits extremely high and variable contact resistance, which is not compatible with the current levels seen in these types of medical devices and thus compromises the switch design. With all of these considerations in mind, the most effective solution is to design a switch without plating, using the materials mentioned above, and then to seal the switch using Teflon films with either acrylic or silicon adhesives. This provides a robust solution with sealing to IP57 specifications, and doesn’t allow sweat or other liquids to corrode the switch.
The KMT0 Series switches from C&K Components are designed without plating, sealed with Teflon films and adhesives to prevent the ingress of fluids, and provide maximum reliability
Many medical devices require “zero defect” components, given the nature of the application, and in the portable equipment used in the field there is no room for device failure. From glucose meters requiring an extremely high cycle count and high level of reliability to initiate the process of analyzing blood and accurately reading the blood sugar level of a patient; to switches in transdermal patch applications operating properly to ensure the proper dosage of medication is being dispensed, the reliability of tactile and detect switches used in medical devices is imperative.
Ultra- and nano-miniature components
While flexibility and reliability are paramount, there are also significant size constraints associated with some medical devices, forcing switch manufacturers to develop highly reliable ultra- and nano-miniature components. Ultra-miniature switches are often being deployed in transdermal patches to deliver post-surgical drugs by triggering the dispensing of pain killers, antibiotics and various other drugs through the skin.
Along with transdermal patches, ultra- and nano-miniature tactile switches are being designed into hearing aids to help reduce the overall size of the device, while at the same time minimizing the visibility of the hearing aid behind the patient’s ear and providing increased user comfort via a less bulky device. Some nano-miniature tactile switches available on the market today are offered in package sizes as small as 2.6 x 3.0 x 0.65 mm.
As mentioned above, ultra-miniature detect switches are being designed into portable blood pressure cuffs for their flexibility, but also for reduced size and weight, as well as ease of assembly. By reducing the overall weight, the blood pressure cuffs are much easier to transport when used in the field. At the same time, the reduced size of the switch, and subsequently the cuff, contributes to ease of assembly and in turn, efficiency of use in the field.
Ultraminiature switches are often being deployed in transdermal patches to deliver post-surgical drugs by triggering the dispensing of pain killers, antibiotics and various other drugs through the skin.
The micro-miniature tactile switch shown above is designed into transdermal patches for the dispensing of drugs
As glucose meters, blood pressure cuffs, hearing aids and transdermal patches become smaller in size and more advanced in functionality, switches will be able to keep pace due to flexibility in design, advanced materials and material combinations, and micro- miniature configurations. ■
For more on miniature switches, visit http://www2.electronicproducts.com/ElectromechanicalSwitches.aspx
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