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Safeguarding critical customer-care information in medical monitoring equipment.

As insurance companies pressure hospitals to return patients to their homes as quickly as possible, the need for portable and highly-reliable medical monitoring and data logging equipment is becoming increasingly important. The information generated and stored by this equipment can often be of a life-and-death nature, so any possibility of data loss cannot be tolerated. Because this medical equipment frequently enters the homes of patients who will administer simple treatments on their own, the manufacturers have put a premium on compact size, low power consumption and 100 percent fault-tolerant memory storage. To meet these requirements, highly-reliable, non-volatile semiconductor memory must be employed to ensure that patient information is not accidentally lost during a disruption of power caused by a failure of the power grid, unintentional disconnection during transportation to or from the hospital, or due to the patient's own movement.

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While the equipment can span the gamut of medical applications such as portable infusion pumps, vacuum-assisted wound closure devices, portable ventilators and cholesterol measurement systems, each of these devices shares a common need to record and store critical patient information for days, weeks or even months, and ensure that this data is protected from loss the entire time. This article will look at IC memory design solutions to determine the most practical technology for these types of medical application.

Current NV Technologies

Design engineers have a number of memory technologies from which to choose: Flash, battery-backed static random access memory (BBSRAM) and electronically erased programmable read-only memory (EEPROM). However, none of these can adequately ensure that all critical design requirements are met. It is possible to combine two or more of these memory types into a single system. But portable designs also must reduce size, decrease power and lower the maintenance of the medical equipment, and these combinations can easily exceed the allowable board space.

EEPROM is frequently used to store small amounts of non-volatile configuration data. However, compared with other non-volatile technologies, EEPROM memory is much lower in density, and it has much slower read and write speeds, making it unacceptable as a single memory technology for these medical application.

EEPROM can be combined with inexpensive Flash memory where configuration data is stored in the EEPROM, and the Flash is used as primary data storage. However, this hybrid solution more than doubles the physical board space. In addition, this combination has unacceptably slow write times, and the Flash memory has limited read/write cycles that can wear out the memory devices, ultimately causing loss of data.

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While traditional SRAM is a volatile memory technology, there are variants that include an on-chip battery to maintain operation of the device in the event of an unexpected power loss. At first glance, this type of memory, known as BBSRAM, appears to be a good solution for these medical applications. However, it falls short in meeting the board space requirements as these devices require IC packages that are physically quite large due to the battery being included inside the package. (See Figure 1.)

Another SRAM approach uses batteries that are mounted externally to the actual IC that supply power to the memory in the event of a power loss. This approach not only exceeds the limited board space budget, but it also requires periodic maintenance of the batteries to ensure that they have sufficient life to fulfill their duties of providing backup power to the SRAM.

Non-volatile SRAM (nvSRAM) is a memory technology that combines the speed of SRAM and the non-volatility of EEPROM, all manufactured on a single semiconductor die and delivered in a single IC package. The amount of EEPROM memory is equal to that of the SRAM, and it serves as a non-volatile backup.

During normal operation, the nvSRAM constantly monitors the power that is supplied to it. If it detects an event that may cause a loss of power, it immediately writes an exact copy of the SRAM's contents to the on-chip EEPROM. Even if power is instantaneously removed from the nvSRAM, a small external capacitor provides sufficient power to the device to ensure that all of the SRAM's contents are successfully copied to the EEPROM.

When the loss-of-power event has passed and the system is ready to resume normal operation, the nvSRAM automatically copies the contents of the EEPROM back to the SRAM and restores it to the state that existed prior to the power failure. The nvSRAM approach meets all of the requirements of portable medical equipment: high speed, low power, infinite read/write cycles (no wearing out of the memory cells), 20-year data retention, small form factor and zero maintenance.

Medical Applications Benefit From nvSRAM

Intravenous (IV) therapy is a classic example of home-based medical equipment. Here, portable infusion pumps used for intravenous therapy require precise measurements ranging from hours to several days and in some cases, even weeks. The nvSRAM's role in these devices is to store calibration parameters for specific medicines, as well as to store parameters programmed into the device by the nurses administering the treatment. If this data were to be lost due to a power failure, the IV equipment would be unable to continue the prescribed treatment to the patient once power was restored with potentially catastrophic consequences.

Another application area for the nvSRAM involves vacuum-assisted wound closure (VAC) systems that use sub-atmospheric pressure to help uniformly draw large wounds closer together. Doctors prescribe this therapy for a variety of chronic and acute wound types such as pressure ulcers, abdominal wounds, burns, trauma wounds and skin grafts. VAC systems can also be equipped with IV capabilities which allow computerized delivery of prescribed fluids into the wound. These VAC systems are similar to the IV therapy machines in that they are portable and require storage of calibration parameters, data logging and time logging for continuous treatment.

Other medical applications center around the recording and storage of the patient's vital signs such as heart rate, breathing rate, blood pressure, temperature, oxygen levels and insulin levels. An example of this is the portable ventilator that assists the patient with breathing. Here, upper and lower limits of vital signs are programmed into the nvSRAM. In the event that one or more vital signs were to fall outside of the predetermined limits, an alarm, wireless page or other type of communication could be generated to alert medical personnel to a problem. It is essential that these pre-programmed limits are not lost due to a power failure. In addition to home-based ventilators, operating rooms use ventilators as a component of anesthesia machines to administer sedatives to a patient during surgery and to measure and record vital signs.

The LASIK procedure uses a laser to cut and reshape the cornea so that light entering the eye refracts correctly. (See Figure 2.) Such a procedure requires extremely precise control over the path and power of the laser, so all of the parameters programmed into the equipment must not be lost, particularly during an unexpected disruption of power during the LASIK procedure. The nvSRAM's store/recall function assures that during a power event, such as a power spike, the equipment knows exactly where it left off and where to resume once power is restored so that no incorrect cuts to the eye can occur.

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Conclusion

The nvSRAM offers an attractive non-volatile memory alternative to medical equipment applications where loss of data cannot be tolerated. This technology meets the stringent requirements demanded by manufacturers for this type of equipment including fast access times, non-volatility, small form factor, low power, high MTBF and zero maintenance.

Timothy O'Connor is an Associate Applications Engineer at Simtek Corporation. He is currently completing his BSEE at the University of California, San Diego.

by Timothy O'Connor, Simtek Corp.
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Title Annotation:Medical Electronics
Author:O'Connor, Timothy
Publication:ECN-Electronic Component News
Date:Nov 1, 2007
Words:1289
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