Just What the Doctor Ordered

09.01.2010 // Posted by: Murray Slovick // Posted in: Articles, New Technology

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Meeting requirements is essential to medical connector design

The other day I was reading an interesting column by Jon D. Pearson in EET/Embedded.com titled “I Don’t Need No Stinkin’ Requirements.” Here’s the gist of it: Pearson writes about deadline pressures facing embedded developers as a result of ongoing changes in specs and additional feature requests during the design process. He then tries to make the case that design requirements cannot be known up front so you might as well just jump in and start writing code or designing circuits without a rigorous requirements gathering and analysis process.

It may well be true that engineers in the embedded space do of necessity have to start writing code very early in the process, as Pearson suggests, but if you are specifying product for, say, a medical application, well, good golly, Miss Molly, there are certainly plenty of hard and fast upfront requirements—not to mention a big basket full of unbreakable standards and regulations. I think even the most jaded designers among us will agree that not taking medical product requirements seriously from the outset is about as clever as not taking a field full of land mines seriously.

Let’s take connectors as an example. A critical component of any medical device, interconnects must meet multiple challenges and requirements, many of which are well known in advance, including, but not limited to, compact packaging, tight signal integrity tolerances, durability—a high number of mating and actuation cycles-- reduced losses due to resistance and stringent electromagnetic interference (EMI), RF and crosstalk characteristics.

Further driving up the requirements quotient is the fact that the rising cost of healthcare has significantly changed how patients are receiving treatment, with in-home care becoming more common. As a result, smaller, portable medical equipment is being designed to travel to homes with a concomitant need for smaller components and miniature cables/connectors-- connectors with 0.5mm or smaller contacts are frequently used by medical OEMs--that easily can be disconnected for cleaning or disposal.

To facilitate portability miniature connectors also are being designed specifically to be “built-in” to the housing of a medical diagnostic device to provide a docking station for either charging of the battery inside the instrument and/or transmitting data for further analysis.

Even imaging equipment, which previously occupied a separate room in a hospital, is now being developed on a smaller scale, including field-operable CT scanners, laptop-sized mobile ultrasound systems and portable monitoring stations. The miniaturization of connectors for these devices must follow suit.

Defibrillators, too, are becoming more portable, particularly as they meet the need for carriage in helicopters, ambulances, and other first responder vehicles. As a result, top medical device manufacturers are looking for robust equipment that can withstand extreme shock and vibration, and in the case of connectors that also means the ability to withstand high voltage, often 7.5 kV or more.

Reliability and EMI/RFI

Since many healthcare devices are attached to or implanted in patients, high reliability is always at the top of the medical connector requirements list. Functionally that means secure contacts, high mating cycles, long lifetimes and durable materials to survive the rigors of everyday use in environments that can include the presence of a variety of fluids, mating and un-mating practices with little thought about the well being of the connection, and multiple forms of general abuse (e.g., a hospital cart running over a cable).

Second only to reliability on the requirements list is the need for electrical transparency. Connectors and cables must not impair transmitted signals via electromagnetic radiation (EMI/RFI), crosstalk, attenuation or other interference-related factors.

RF emissions can negatively affect diagnostic equipment by reducing image clarity and adding signal noise. Resolution is key since diagnosis and treatment depend on the medical practitioner being able to interpret data or images clearly and accurately; image noise can result in a “false positive” or other mis-diagnosis, making reduction of these emissions a primary concern.

If the connector is inside the medical instrument electrical noise will be further amplified and as the level of chip processing increases, issues of signal speed, crosstalk, and EMI coupling play a greater role in the design process.

Protecting against EMI and RFI signals also is vital in devices such as pacemakers and patient monitors as signal noise can affect a pacemaker’s operation and corrupt data within patient monitoring devices. Shielding techniques can include the use of non-magnetic materials and coatings, gaskets, enclosures and other products or materials that block out EMI and RF interference. Cables will feature jacketing and insulation to limit the undesirable effects of EMI/RFI and interconnects and components such as inductors and capacitors typically will be built with materials that thwart interference.

EMI/RFI prevention requirements in medical applications have been steadily climbing. Not long ago a 0.3 tesla value was acceptable for RF connectors (tesla is the unit of measure for magnetic flux). Today, however, the magnets within MRI and other imaging and diagnostic equipment are more powerful, resulting in most manufacturers of medical “big iron” requesting that products handle exposure to 3-tesla conditions without causing image distortion.

On the materials side, simply reducing the amount of magnetic iron and nickel in use will not by itself achieve the desired result, so, instead, many medical applications are specifying connectors with non-magnetic alloys and brasses, as they offer zero or low-magnetic fields, keeping EMI/RFI from interfering with equipment.

Complex interference issues also have driven manufacturers to develop high-performance and cost-effective suppression filter solutions to lower EMI susceptibility. In particular, the power line cord going into a device can act as an antenna to bring EMI either into the system or out of the system. A filter typically located right where the power cord comes into the piece of equipment will keep potentially disruptive or damaging line transients and EMI out of medical gear.

These filters are designed to provide common and differential mode performance while meeting the stringent UL leakage requirements for medical or dental equipment. In general, power supply designers must consider three kinds of EMI coupling paths: Below 1 MHz, differential-mode conducted energy dominates; between 1 and 30 MHz, common-mode conducted interference takes charge and above 30 MHz radiated interference is the principal villain.

Climbing Data Rates

Medical chipsets are capable of handling multiple input data/signal channels transferring a wide range of sensor and device information at rates of up to 25 Gb/second. For MRI, CT, and other imaging applications higher-density connector arrays allow a greater number of imaging slices, resulting in more accurate diagnosis.

The evolution of information and diagnostic displays is another case in point. Higher data rates are coming to the forefront as high-res digital video IP cores are developed for processor and graphics chips, enabling medical information monitors to migrate from older MPEG-2, MPEG-4 and JPEG2000 technologies to H.264 high-definition video.

To meet increasing data rate requirements connector manufacturers have significantly boosted the density of their products, packing more speed and performance into less space. For example, consider the high-speed board-to-board connectors for applications involving high component count mezzanine cards. Since the pin count is directly related to the resolution of scans generated by MRI machines, up to 1,000-pin, one-piece mezzanine connectors may be in use in this application.

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