Designing electronic medical devices with use-limiting technology safeguards patients and operators by preventing unauthorized reuse or refurbishment.
By David Smith, Sr. Electrical Engineer, and Tyler Yang, Mechanical Engineer, ATL Technology
It’s no secret that cost is often a critical component of medical care, so it’s no surprise that cost is often the primary factor impacting device misuse. Some patients and providers can be tempted to reuse or refurbish outdated or disposable electronic medical devices. However, when devices are designed for limited lifespans, going outside of the specified use can compromise the safety of patients and care providers.
Making medical devices completely tamperproof can be costly as well, but device designers can help protect patients and providers and keep their costs down by leveraging use-limiting technology, which is designed to prevent the unauthorized reuse and refurbishment of limited-lifespan equipment. Connectors are the heart of any system, and are available with a variety of use-limiting technology to satisfy a broad range of applications and mating cycles.
Use-Limiting Technology Options
Connector Considerations: Some connectors employ a small number of specialized pins that can be used for use-limiting strategies. The number of specialized pins in these connectors will affect their cost, but specifying this type of connector can deliver added value by restricting modifications to the equipment to prevent continued use beyond the rated lifetime.
Sterilization: Sterilization is an essential factor to consider when selecting a use-limiting method as well. Incompatibility with sterilization procedures can help deter unauthorized reuse since the risks of spreading pathogens is well understood in the medical field. However, it can also be a hindrance to more advanced use-limiting technologies. For instance, several sterilization methods, such as electron-beam (e-beam), can destroy certain types of EEPROMs whose smart circuitry can also employ use-limiting measures.
Mechanical Use-Limiting Systems (MULS): Purely mechanical use-limiting systems physically prevent continued use, like a ballpoint pen that, after a prescribed point, employs a mechanism designed to allow users to press the top once to extend the tip and again to retract it before permanently locking to prevent further use. Limited-use connectors can employ this same approach to prevent the mating connector from being reinserted after it has already been mated and unmated once.
Similarly, electromechanical MULS can limit connector mating and unmating cycles to a prescribed number by creating a change in conductivity (i.e., an open or short circuit) to disable the device. In addition, this type of use-limiting mechanism can often be incorporated into existing system architectures with no need for changes to capital equipment hardware.
Advantages of MULS
- Can be used with existing capital equipment
- Fully backward compatible
- Can withstand many environmental factors
- Compatible with all common sterilization methods, including e-beam and gamma ray
- Relatively small
- Fairly low cost
- The electromechanical version is especially difficult to defeat
Disadvantages of MULS
- No protection from water ingress
- Purely mechanical solutions can be defeated by disabling the blocking device
- Provides use-limiting only; no anti-counterfeit provisions
- Occupies space that could otherwise be used by additional contacts
The Simple Electronics Method: Unlike other use-limiting methods that can be made to engage after a prescribed number of mating cycles, the simple electronics method is ideal for single-use, disposable devices. When a simple-electronics cable assembly is first connected, the capital equipment will see a low resistance path (near zero ohms) through the fuse, resulting in full supply voltage at the signal pin. The capital equipment then sends a high current through pin one to blow the fuse. If the cable assembly is removed and later reconnected, the signal pin will see a reduced voltage via the resistance of R1 because there is now no electrical continuity through the fuse. If the capital equipment is properly programmed, it will recognize the assembly as previously used and remain inactive while it’s attached.
Advantages of the Simple Electronics Method
- Fairly simple to design and manufacture
- Fairly low cost
- Can survive most sterilization methods
Disadvantages of the Simple Electronics Method
- Can be counterfeited
- Uses two pins on the disposable device and requires the capital equipment to provide enough voltage to blow the fuse without damaging the resistor
Electrically Erasable Programmable Read-Only Memory (EEPROM): EEPROMs are user-modifiable read-only memory (ROM) that can be erased and reprogrammed repeatedly through the application of higher than normal electrical voltage. Unlike erasable programmable read-only memory (EPROM) chips, EEPROMs do not need to be removed from the device to be modified and can be reprogrammed up to hundreds of thousands of times. EEPROMs also offer tens of kilobytes, but only need a few bytes for use-limiting purposes. The extra memory can be used to store configuration data or capture data from the operation to use for diagnostic purposes. Plus, both standard and custom connector styles allow EEPROMs and ICs to be easily integrated into the enclosed termination area of the connector body with no change to connector form or fit.
Advantages of EEPROMs
- Wide variety of physical and memory sizes
- Use-limiting and anti-counterfeiting can be accomplished with only 64 bytes, leaving plenty of available memory for configuration data or other purposes
- Unique features, including unique serial number and one-time write block capabilities
- Highly configurable
- Easy to communicate between device and capital
Disadvantages of EEPROMs
- Requires four connector pins for power, ground, data, and clock
- Often needs to be mounted to a PCB within the connector or elsewhere in the device
- Susceptible to certain sterilization methods, especially gamma ray and e-beam
1-Wire EEPROM: 1-Wire EEPROMs provide combinations of memory, mixed signal, and secure authentication functions via a single contact serial interface. With both power and communication delivered over the serial protocol, 1-Wire devices provide key functionality for system designs that need to keep additional interconnect pins to a minimum. Spare connector pins between the capital equipment and device are typically limited, so it can be a genuine challenge to add authentication and usage monitoring without increasing connector complexity and cost. The 1-Wire solution not only solves these equipment connectivity challenges, but also enables implementation using a single, dedicated connector contact.
Advantages of 1-Wire EEPROMs
- Smaller package size than EEPROMs
- Fewer pins (two vs. four) offer space and cost benefits
- Can sometimes be soldered directly to connector pins to eliminate the need for a PCB
- Available in a variety of features and sizes
- Capable of sterilization immunity
Disadvantages of 1-Wire EEPROMS
- Communication protocol is less common than I2C for EEPROMs Some devices are susceptible to damage by radiating sterilization methods
- More expensive than EEPROMs
- Shorter communication distance capabilities than EEPROMs
Microprocessors: Microprocessors are also useful tools for use-limiting purposes and are already built into many disposable handheld devices. In these cases, existing microprocessors can be leveraged with software to perform use-limiting functions — or even add new functionality, like switching and sensing capabilities — without adding new components. They are also relatively inexpensive, priced similarly to EEPROMS, and since microprocessors have a four-pin connector for the power, ground, and data, there is no need for extra wires and connector pins for buttons and sensors, or their associated cost. In addition, if a device doesn’t already have a microprocessor, one can be installed at the rear of the connector, where it can be protected and encapsulated by the overmold.
Advantages of Microprocessors
- Very inexpensive to add use-limiting functionality to an existing processor
- Usually doesn’t require additional pins
- Broad customization capabilities
- Cost about the same as EEPROMs and offer many useful peripherals (e.g., temperature sensors, A/D, GPIO for buttons, etc.)
Disadvantages of Microprocessors
- Can be complex and expensive to design, develop, and debug (i.e., high coding, software, and firmware validation costs)
- May be susceptible to certain sterilization methods
- In-circuit programmable
Radio-Frequency Identification (RFID): RFID is gaining traction as a convenient way to offer use-limiting capabilities in disposable devices. Often thought of as a wireless EEPROM, RFID has no pins or connectors. Instead, it uses a small chip and an antenna for power and to communicate from the disposable device to the capital equipment. Devices with an RFID reader can also be reprogrammed through packaging. However, RFID can only be selected when designing a new capital system, as it is not backward compatible. It can be an ideal choice when designing a new system, though, as it can provide both use-limiting and counterfeit prevention capabilities. Some companies even offer proprietary solutions with an RFID chip embedded into connector shells to maintains interconnect compatibility with existing hardware and avoid consuming any of the connector pins.
Advantages of RFID
- Extremely small footprint
- Uses no connector pins for power or communication
- Extremely adaptable and effective for both use-limiting and anti-counterfeiting purposes
Disadvantages of RFID
- High cost for the RFID reader on the capital equipment side
- Complex to design in
- Possible noise issues
- Susceptible to irradiating sterilization methods
Selecting the Right Use-Limiting Technology
Use-limiting technology provides crucial protection for patients and medical providers by preventing unauthorized reuse or refurbishment once a product has come to the end of its rated lifecycle, so it’s important to select the right use-limiting technology for a given design to meet price and performance specifications. The optimal solution will achieve a balance between cost, impact to the capital equipment, simplicity of implementation and operation, backward compatibility, use of space, the number of additional wires and pins, and any additional capabilities that may be required, such as counterfeit prevention features. With the right use-limiting method in place, medical device manufactures can ensure that their products will be effectively protected from misuse to protect patients and providers.
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