These single-use accessories, supplied in sterile, airtight packaging, are only opened before a procedure begins. Being disposable, they can be more economical, lighter, and more flexible, as they don't need to withstand the sterilization process or be robust enough for repeated use. Manufacturers are concerned that third parties might refurbish and re-sterilize the unit for reuse, thus exceeding its design life and compromising hygiene. Along with these primary concerns, other hazards exist, such as using the wrong accessory or accidentally using an expired unit.
If digital control or measurement is already part of the system, then an electronic communications link can be added to the disposable section. Figure 1 shows the simple configuration required for the system to identify each accessory, obtain other data, and store usage data on an integrated memory device embedded in the disposable accessory.
A disposable accessory can contain different types of data that a medical device manufacturer may be interested in obtaining.
Usage statistics:
Disposable accessories can also record that the unit has been used and the serial number of the equipment used. Recording this information on the accessory itself protects against inappropriate reuse with another system, even if this data can also be stored on the equipment. These devices can also measure the total usage time or the number of uses for accessories with a defined lifespan.
Calibration Information:
All manufactured fittings should be identical, but due to manufacturing tolerances, processes, and chemistry, significant variation may occur between units or batches. The system can compensate for these variations if they are detected during production. Fittings can be tested during manufacturing to determine their performance, and their compensation values can be stored in memory. Storing this data within the fitting ensures perfect compensation for users and guarantees that the fitting and its calibration data remain separate and unmixed.
Expiration Date:
To program the medical accessory with an expiration date, the system console only needs to display the current date and time to compare them with the accessory's expiration date. Once this is done, it's easy to verify if its storage period has expired.
Model Number
The accessory model number can be used to ensure that it is the correct accessory for the system, as well as to indicate the accessory type for systems that incorporate various categories of disposable items.
Serial Numbers:
Healthcare management may require the system to store the serial numbers of each item used to collect warranty information and perform statistical failure analysis. If the manufacturer has a communication link with the equipment in the application environment, recall management could be automated with an instruction for the equipment to reject certain serial numbers.
Implementation:
The only requirement is an interface to the accessory if the system already uses a microprocessor or microcontroller. A serial protocol is well-suited for the limited amount of data that needs to be transmitted, requiring only a single interface to the accessory, provided electrical contact can be established between the accessory and the system. This also meets the requirements for low cost and small size of disposable medical accessories. If the system has an internal serial bus, this can be extended to the accessory.
A serial memory device can be used to minimize complexity and components, as the accessory's electronics only need to communicate and store data. Serial memories with various communication buses are available; Table 1 outlines the advantages and disadvantages of each option.
SPI provides fast communications, but the 6 lines and power lines require larger and more expensive connectors between the system and the accessory.
The I2C™ protocol and single-wire buses, such as Microchip's UNI/O® protocol, are much better suited for these applications. Slower communication speeds are no longer an issue given the limited data volumes involved in disposable medical devices. The reduced number of power lines required by these protocols allows for smaller, lower-cost connectors and potentially more reliable communication. Implementing the two-wire I2C communication protocol is straightforward and also has the advantage of being synchronous, thus not having strict timing requirements.
Many microprocessors and microcontrollers are I2C-compatible thanks to the dedicated hardware they incorporate to manage the bus, unlike single-line communication protocols that often require software to manage the bus—although this software is readily available—or the use of a conversion bridge device. Several proprietary single-line protocols compete with each other. The major single-line protocols offer self-synchronization or clock signal recovery, which eliminates the need for high-precision oscillators at both ends. The low data transmission rates allow the use of long, highly capacitive cables and simple signal noise filtering. Power can be supplied through the data line with many single-wire communication protocols, thus eliminating the need for a connection to the mains supply voltage, but some external components may still be required.
Memory Device Selection:
Many vendors offer I2C or single-wire interfaces for low-cost serial EEPROM (Electrically Erasable Programmable Read-Only Memory) devices. Their storage capacity ranges from 128 to over one million bits, allowing the memory to be tailored to the needs of each application. Small packages, such as SOT-23, SC-70, and CSP (Chip Scale Package), allow for easy integration into disposable accessories without significantly affecting their size and form. EEPROM technology also offers highly durable storage for over 200 years at elevated temperatures.
The system can rewrite the data stored in the accessory while it is in operation, allowing usage information to be updated and the system to indicate whether the device should be discarded. Alternatively, the accessory's data can be written to save its usage history, as shown in Figure 2. Writes to the EEPROM are performed quickly, but it is advisable to read the memory afterward to verify that the write operation is complete, as the accessory can be disconnected at any time. The EEPROM is characterized by its long operating life, allowing each bit of memory to be rewritten more than a million times and usage data to be updated without "wearing out" the memory.
Software write protection with EEPROM SPD (Serial Presence Detect) provides enhanced security because an instruction is required to lock and unlock a memory section for writing. This protects against erroneous writes caused by deviations from normal operating conditions. Other EEPROM devices incorporate permanent write protection systems that, once activated, cannot be unlocked. This can be used to indicate that an accessory has already been used by writing usage information to memory before activating the protection. The protection system prevents other users from manipulating the information but still allows the system to read the usage data. SPD devices are I2C-based and are produced by major EEPROM manufacturers. These devices are also standardized, which avoids problems arising from a single source of supply. SPI EEPROM devices with added options are available to protect specific memory sections from writing in sophisticated systems that can tolerate a high number of connections.
Programming embedded memory during the manufacture of disposable accessories gives manufacturers the ability to program model numbers/types, serial numbers, batch calibration, expiration dates, or the maximum number of uses allowed on each device, so that it can be tracked and used appropriately throughout its operational life.
Note: UNI/O is a registered trademark of Microchip Technology Inc. in the U.S. and other countries. All other trademarks mentioned belong to their respective companies
Author:
Jonathan Dillon - Microchip Technology Inc.
