A Medical Electronics Manufacturing Fall 1997 Feature

MEMORY

Portable Data Carriers Add Function and Convenience to Medical Instruments

Steve Serber

Plug-in memory devices provide increased efficiency and productivity to medical equipment.

Portable memory devices continue to proliferate thanks to the wide variety of storage formats available and the diversity of potential applications. UPC bar codes, magnetic-stripe PIN cards, and cards that store data on integrated circuits are just some of the common ways used to physically carry electronic data from one location to another. The technology is finding its way into more and more medical equipment applications, as manufacturers seek to increase precision and efficiency without sacrificing convenience and ease of use. This article explores the state of the current technology and looks at four instances in which plug-in memory devices have successfully enhanced the capabilities of medical equipment and medical systems.

An integrated circuit die bonded and wired to a printed circuit board.

The Limits of Early Technologies

Early technologies involving bar code strips and readers have enjoyed limited success in uses involving medical instrumentation. Although they are relatively inexpensive, bar codes can be hard to use and are vulnerable to unfavorable end-use conditions. Moreover, they provide read-only memory, and many medical instruments really require read-write capability for maximum benefit. In some uses, bar code strips can be difficult to read, which reduces the efficiency of the whole data-process management process.

The Alternative: Plug-In Devices

Today, effective alternatives are being built around inexpensive, small-capacity nonvolatile semiconductor memory devices that can be simply plugged into medical instruments and equipment. Nonvolatile memory provides full read-write capability and retains data when power is removed. These memory tokens can be specifically packaged to facilitate removal and transportation and to protect the sensitive circuits inside from hazardous conditions. A variety of mating receptacles are available to simplify connection. The circuit inside the device can include read-write or one-time programmable semiconductor memory and even microprocessors. The designer can specify the size and shape of the token itself. Systems can employ bar codes, magnetic-strip cards, low-capacity read-write memory buttons, low-capacity read-write memory keys, and high-capacity read-write memory keys. Examples include Dallas Semiconductor's Touch Memory metal packaging, Datakey's molded plastic packages, various offerings from "smart-card" and PCMCIA-card manufacturers, and a number of proprietary packages.

Each application requires at least some customization. In some cases, the memory-device manufacturers provide off-the-shelf hardware to support their devices. Some good examples of this are the bar code scanners and magnetic card readers from the retail industry that transfer over to the medical environment. In these cases, the application developer must produce software to perform the desired functions. In applications where the portable data device works with a specialized piece of medical equipment, the developer must produce both the electronic interface between the data device and the equipment and the supporting software.

To ensure compatibility and efficiency, designers can choose to adhere to the industry standards­such as the ISO 7816 standards for IC cards and the PCMCIA standards for memory modules­created to identify the physical, electronic, and logical formats for the equipment and data. Inventory management is one area where standardized equipment would be the logical choice. The functional and environmental requirements for this application are similar to those of other industries. In this case, a hospital can mark its equipment with bar code labels, which can be purchased along with the requisite printers and scanners from companies that make these items for many different applications. The software is also similar from one hospital to the next, ensuring a wide available market.

Hazard Analysis and Device Validation

The risk of data corruption is still a very real problem for any portable data application. In most cases, the application designer can incorporate special information sequences in the data, known as error-detecting and error-correcting codes, that allow the instruments to determine whether the data they are reading are valid. In some cases, the instruments can use these codes to reconstruct the correct data. If a product is involved in personal safety, it is up to the application developer to perform the failure mode and effects analysis for the product.

Validation and verification of the data carriers and their programming systems varies with the application. In equipment inventory, the need for reliability is considerably more relaxed as compared to applications involving life-critical equipment, so the verification process is not as strict. Applications involving personal identification and access control have special needs for security and protection from copy and forgery.

Calibration of Medical Instruments and Equipment

One area in which small portable memory devices can have a very large impact is calibration. Many new patient-monitoring techniques use sensitive electronic transducers and sensors. The manufacturing processes for some of these sensors produce wide variations in signal output from one unit to another. By enabling the patient monitors to adjust for these variations, the manufacturer can achieve a higher production yield for the transducers and sensors. In fact, several manufacturers now include a portable memory device with each sensor that contains the sensor's specific calibration information. The instrument operator plugs in the memory device and the sensor simultaneously, so the instrument can make the necessary adjustments. Because the memory devices can be produced at relatively low cost, they are well suited for this one-time or limited-use application.

An example of an alternative physical form for a memory device.

This calibration technology has been effectively applied to blood-glucose monitoring. Diabetics seeking to manage their own therapy must regularly monitor their blood-glucose levels, typically using inexpensive and simple monitoring systems. Patients place a drop of their blood on a disposable test strip and insert it into the analyzer, which generates a blood-glucose reading in a matter of seconds. Most of the test strips come with a calibration device for the monitoring instrument. Before using each new batch of strips, the patient performs a calibration procedure that tells the instrument how to adjust for the sensitivity of that batch. Some of the older instruments had complex calibration procedures. Many of the newer machines, however, include a small plug-in device that simplifies the calibration operation significantly.

Blood-gas monitoring is another area in which calibration data can enhance instrumentation. Current technology makes it possible to insert a sensor-tipped catheter into a patient's bloodstream to measure the levels of oxygen, carbon dioxide, and nitrogen in real time­a great improvement over the previous process, which required a laboratory analysis that could take several hours to produce results. As with the blood-glucose monitor, there is significant variation in sensitivity from one sensor to another. Moreover, sensitivity is measured early in the manufacturing process, well before sterilization and packaging.

To deal with these issues, one manufacturer attached a portable, inexpensive, nonvolatile read-write memory device to the sensor with a tether strap. During manufacture, the calibration data are written into the memory. When the sensor is used, the operator simply inserts it and the calibration device into the reader instrument.

This technique poses a new set of problems for the portable data device. In the case of the manufacturer mentioned above, the token used in the blood-gas monitor was specifically developed as an alternative to a memory key. Not only did the device need to withstand static electricity, blood, and chemicals, it also had to survive the autoclave sterilization process. Moreover, the price tag had to be low enough to make the token a disposable item. The environmental needs were met using the same high-temperature antistatic plastic as were used in the keys and by plating the contacts with gold. Cost was contained by changing the shape to one that did not require the same tight manufacturing tolerances.

Equipment Setup and Data Recording

A manufacturer of physical therapy equipment sought to design an exercise bike for patients wanting to perform their therapy exercise at home. The company wanted a product that would provide some measure of assurance that the exercise regimen was being followed as prescribed, even without the direct supervision of a physical therapist.

The final design incorporated 4-kbit serial-I/O memory keys, which allow the therapist to set up the exercise routine on the bike electronically and collect patient performance data. A computer in the bicycle controls the exercise program, measures the patient's heart rate, work, and rate of perceived exertion, and records the data on the keys. Each key can hold personalized data from 5­10 physical therapy sessions, letting the physiologist in charge track each patient's progress and perform trend analysis on a personal computer. In addition to producing increased efficiency, more reliable tracking, and higher-quality care, the rugged keys provide an easy way to program instructions that a patient can use at home. When it's time to check on the patient's progress, the patient need only bring the key into the doctor's office­not the treadmill or bike.

Restricted Computer Access

One manufacturer used portable data carriers as part of a system to communicate information from bedside terminals directly to a hospital mainframe. One important goal was to maintain security without requiring the medical staff to log on and off the computer by typing passwords at bedside terminals all day long.

A serial key and reader was developed for the job; each staff member automatically logs on by inserting a key and logs off by pulling the key out. A computer-systems administrator assigns the keys­programmed with a unique code­to individual workers. The computer network keeps track of the valid codes, the key holder for each key, and the types of information that the key holder may access. Each key holder is responsible for keeping his or her key secure. The key is small enough to be carried conveniently and durable enough to withstand the rigors of being shoved into pockets repeatedly. Moreover, the keys prevent unauthorized operation because the terminals simply cannot transmit information unless a key is inserted.

This log-in system provides a good example of effective use of nonstandard equipment. In this case, bar code readers would be too expensive to install in each patient's room, and magnetic-strip cards would fail in the high magnetic fields surrounding an MRI machine. The high number of insertion cycles that the token receives makes the memory key a practical alternative.

Per-Use Billing

The budget crunches facing the medical industry have had a significant impact on the amount of money available for capital equipment. To get around this situation, some companies have made their equipment available to medical service providers on a special basis. In these cases, the institutions pay for the equipment on a per-use basis rather than as an upfront capital item.

To control the use and provide a convenient billing method, the supplier provides a plug-in key or card that limits the number of times the equipment can be used. This is similar to the prepayment schemes used with postage meters and prepaid telephone cards. Once the credit on the device is used up, the service provider replaces it with a new device and either throws the old one out or returns it for recharging. The result is a more attractive arrangement for the manufacturer, who might find an easier sale, and the hospital or clinic, which can negotiate around capital investment restrictions.

Conclusion

Portable data carriers are a valuable resource for designers of medical instruments and equipment. They are among the most practical and versatile technologies in use today, and are especially appropriate for medical instrument and equipment applications. When the right device is paired with the right application, the combination can provide significant benefits, including increased efficiency and productivity, more reliable tracking, more convenient use, and, ultimately, better patient care.

Steve Serber is applications engineer for Datakey, Inc. (Burnsville, MN).