Miniaturization of medical devices offers tangible advantages to clinicians and patients alike. Smaller pill cams, for example, are more easily ingested. Likewise, smaller hearing aids are less invasive and therefore more comfortable for the wearer. But before either of these devices — and many others like them — can be reduced in size, their components must be made smaller. Magnetic reed switches are increasingly being used to enable manufacturers to reduce their footprint while maintaining tight sensitivities and performance characteristics.

Fig. 1 – Despite its small size, a micro-miniature reed switch is very responsive — a magnetic wand, when passed over the ingested pill cam, turns on the battery and activates the pill cam after it is in place.
Pill-shaped cameras allow clinicians to look inside a patient to determine what is happening within the gastro-intestinal tract — thus enabling more accurate diagnoses before handling a scalpel. Diminutive pill cams are more easily ingested — and thus induce less stress and the inherent physical manifestations associated with stress (higher blood pressure, increased heart rate, etc.). Limiting the variables that clinicians must consider when making their diagnosis allows them to make better decisions and more accurate diagnoses.

A look inside this unique device shows that a vital part of the diagnostic device — and a key element to its reduced size — is a magnetic reed switch that turns the camera on and off. Activating the camera is accomplished by passing a magnetic wand over the part of the patient’s body where the pill cam is located (when it has reached its desired destination). Powering up the camera when it has reached its destination allows the use of smaller batteries — which in turn decreases the overall size of the device.

Magnetic Reed Switches are hermetically sealed and normally backfilled with Nitrogen. These features are what give the switch the ability of millions of switching cycles without degradation of the switching function. The Standex SR4J series is a surface mount version of the GR150 — a 3.7-mm-long magnetic reed switch (the world’s smallest) in a molded SMT configuration.

Fig. 2 – Miniature components, like the world’s smallest reed switch shown here, are helping to reduce the size of medical devices.
Sensitivity of the switch is also an important consideration. The SR4J series switch, which is used in pill-cam applications, boasts a maximum sensitivity range of 3/5 Ampere Turns (AT) (initial), and is available in a production range from 3 to 20 AT. More sensitivity means that less power is required of the wand that activates the switch once the pill cam is in place — thus there is less patient exposure. In addition to a small footprint and tight sensitivity range, the SR4J series is responsive — with a typical operating time of 0.2 milliseconds, and release time of 0.1 milliseconds (typical). They have a power rating of 1 VA maximum. A switching current (max) of 0.05 Amp, DC is standard, as is a carry current rating (max) of 0.5 Amp, DC. Boasting an operating range of -40 °C to +125 °C, the SR4J is well suited for use wherever small size and sensitivity are required for non-contact position sensing applications.

While medical device manufacturers would like smaller components, reducing the size can be a complicated task. To manufacture the GR150 and SR4J for high volume production, a new microelectronics- based manufacturing system was developed by Standex Electronics for both the manufacturing and test processes.

Miniaturization of in-body devices, including in-the-ear (ITE) and behind-the-ear (BTE) hearing aids, is increasing patient comfort because these smaller devices are less intrusive when they are placed within the body. Magnetic reed switches are used within hearings aids to detect the use of telephones (and cell phones) automatically adjusting the hearing aid for optimum sound. Magnetic reed switches are also used for remote manual stepping of hearing aid amplifier gain.

As with pill-cam applications, sensitivity is also vital to the success of magnetic reed switches for hearing-aid applications, allowing the use of less powerful and potentially smaller magnetics on phones and cell phones — reducing their size and cost.

The durability and long service life of magnetic reed switches, given their operation within a hermetically sealed environment, is even more important to hearing aid users. Millions of trouble-free switching cycles provide what is typically a 20-year service life.

An advantage of applying magnetic reed switches in medical devices is that they have controllable magnetic hysteresis, which can become important, for example, in the complex magnetic circuit of a safety switch. This system design usually includes a separate permanent magnet assembly and the important magnetic variables include the magnet itself, distance variables, temperature, nearby magnetic materials, position variables, and the magnetic sensitivity of the switch assembly.

A specification for a magnetic reed switch assembly will normally include a “sensitivity to close” – Pull In with upper and lower limits. This variable is usually stated in AT (Ampere Turns). The “sensitivity to open” – Drop Out, is sometimes not specified, or if it is specified, it is specified as a maximum or minimum value. Within certain limits, it is possible to specify the “sensitivity to open” as a range or as a percentage of “sensitivity to close.” The ability to control reed switch hysteresis can substantially reduce one important magnetic variable in the magnetic system design.

Magnetic reed switches can be used in applications where the medical device is equipped with low voltage electronics. This fact can substantially reduce the cost and/or size of the sensor or switch assembly that uses the reed switch.

The range in operating temperature can be an important variable in magnetic sensor applications. In an application that includes a complicated magnetic circuit as well as a wide range in operating temperature, one must pay attention to this variable as the design progresses. Permanent magnets increase their output at lower temperatures and decrease their output at higher temperatures. Also, the ability of a reed switch to accept magnetic flux increases at lower temperatures and decreases at higher temperatures. DOEs for magnetic circuits in medical applications should always include detail testing at operating temperature extremes.

If the design team addresses the major magnetic circuit and operational system variables early on in the design effort, 6 Sigma system designs are very feasible.

Miniaturization of medical devices provides many benefits to both patients and clinicians, and a review of magnetic reed switches can be an excellent place to start the size reduction process.

This technology was done by Standex Electronics, Cincinnati, OH. For more information, Click Here .

Medical Design Briefs Magazine

This article first appeared in the March, 2012 issue of Medical Design Briefs Magazine.

Read more articles from this issue here.

Read more articles from the archives here.