Medical Device Link.

DESIGN

Image: Vishay Semiconductors GmbH, Heilbronn, Germany

Infrared (IR) wireless communication is most commonly found in consumer electronics such as mobile phones, personal digital assistants (PDAs), MP3 players and printers. IR communication is based on light, which will not interfere with sensitive medical equipment. Bluetooth and WiFi use radio frequency (RF), which can cause interference. An IR wireless interface offers the medical equipment designer a versatile and economic wireless transceiver.

Figure 1. Typical infrared transceiver printed circuit board base.

The IR transceiver consists of an IR light emitting diode (LED); a p-type, intrinsic, n-type (PIN) diode; and an amplifier application specific integrated circuit assembled to a wire lead frame or printed circuit board. The assembly is then moulded or cast with epoxy lenses (Figure 1). Typically, the epoxy contains filtering material that blocks visible light and allows only the IR light to pass through. Currently, more than five suppliers of IR transceivers offer packages with heights from 4 mm down to 1.6 mm. Data rates from 9.6 kbit/s to 16 Mbit/s are supported. The transceivers are surface-mount devices that are typically lead-free and capable of withstanding lead-free processing temperatures. For special customised requirements in emerging sectors such as e-textiles and wear-on-monitoring equipment, the transceivers can also be assembled on flexible materials. IR transceivers mounted on flexible material were developed in 2005 for the medical equipment market (Figure 2). The graphic of the flexible IR transceiver is expected to have a great impact on these applications in 2006.

Figure 2. Flex-IR transceiver.

The transceiver’s transmit and receive lines are connected to input/output (I/O) ports on a controller. In most cases, controllers support the IrDA encoding scheme and allow direct connection. If not supported, an encoder/decoder (Figure 3) integrated circuit is required to bridge the controller’s universal asynchronous receiver/transmitter (UART) to the transceiver’s I/O ports. This encoding scheme reduces the duty cycle of the IR LED, extending its life and lowering internal heat generation. The widespread availability of IR-compatible controllers makes it possible to feature IR wireless communication with just the addition of a transceiver and a simple RC circuit (an electric filter composed of a simple resistor and capacitor). This is why IR is a lower cost and attractive wireless technology.

Figure 3. Interface circuit sample of IR transceiver with USB microcontroller.

The controller side (encoder/decoder) interfacing with the IR transceiver is well understood. Most manufacturers of embedded processors, Super-UARTS and bridge-chipsets are providing their products with internal IR (encoder/decoder) facilitating a simple and direct interconnection to these IR transceivers. The widespread acceptance of this technology means that it requires just a few passive components when connected to an IrDA compatible controller. Even as a standalone adapter with an RS232 interface or combined with a USB-Bridge, the number of parts is small, typically 2–5 (Figure 4).

Figure 4. Adapter circuit: example of an IR transceiver to a RS232 serial port.

The standards

The Infrared Data Association (www.irda.org), formed in 1994, is a not-for-profit organisation whose goal is to develop globally adopted specifications for IR wireless communication. It is the principal source of information regarding the physical layer as well as protocol layers that provide coexistence across applications and platforms. By adhering to these standards, software and hardware manufacturers are providing ready-made solutions that meet IR-layer specifications and form the building blocks used by consumer electronics and medical equipment manufacturers.

Figure 5. (click to enlarge) Schematic showing typical data encoding and SIP pulse definition.

IR wireless technology is a line-of-sight communication. Adjusted by the power-profile, it can maintain a communication distance of up to one metre. Greater distances are also feasible depending on baud rate and power availability. Figure 5 shows an example of a standard UART pattern and the equivalent pulse shortening as arranged for the optical transmission. The IrDA standard prescribes the link negotiation at 9.6 kbit/s regardless of the eventual baud mode sustained between the two devices involved in a transaction. Thus, a 16-Mbit/s capable device would be able to ramp down to whatever baud rate the second device is capable of maintaining. The serial IR interaction pulse of 1.6 µs is generated every 8.7 µs to sustain an already established communication (Figure 5), which avoids unnecessary search-n-discovery sequences. Communication is only terminated by command or by a lengthy interruption of the light-path, typically of 10–20 s.

Figure 6. (click to enlarge) Block representation of IEEE 1073.3.2 standard in its adaptation to the IrDA standard.

In support of usability and the advantages of IR wireless technology in medical information bus systems, the following standards provide guidelines and specifications relating to design and application:

• ISO 11073-30300, Health informatics, point-of-care (POC) medical device communication, transport profile of infrared wireless.
• IEEE 1073.3.2/3, Standard for Medical Device Communications, IrDA Based Transport Profile, IR wireless system (http://www.ieee.org) (Figure 6).

Figure 6. (click to enlarge) Logic block diagram offers an example of integrating IrDA and IEEE standards in a clinical environment.

IR wireless technology has also been adopted by other relevant standards bodies such as CEN, the European standards body, by its e-Health standardisation focus group (www.cenorm.be/cenorm/index.htm); and by the Health Level Seven (www.hl7.org), a not-for-profit health standard organisation focussed primarily on developing clinical and administrative data (Figure 7).

Medical telemetry has been around for more than 40 years and in recent times employing a wider use of RF-based equipment and the IEEE 802.11 protocol as well as Home RF. The option of RF (ISM 2.4 GHz) wireless technology presents some advantages in specific areas. However, it is currently undergoing greater scrutiny in medical environments, not for its higher cost of ownership or complexity of operation, but for its susceptibility to interferences affecting factors such as latency, which reduces throughput performance in RF-based equipment.

Expanding applications

IR wireless technology with its short-distance, line-of-sight characteristics is immune to eavesdropping or other techniques used to intercept data.
Home diagnostics. The home diagnostics market is estimated to have a compound annual growth rate of 11% by 2010.1 In blood-sugar testing applications, IR wireless communication means that users do not need to worry about connection setup and configurations every time they need to transfer data, and transfer takes place in a few seconds.

Figure 8. Standard defibrillator that also uses IrDA to inter-exchange data.

Critical equipment. Portable defibrillators (Figure 8) are deployed under critical conditions. Data relating to patients’ electrocardiograms, incident events and deployment records can be transmitted via IR wireless communication to PDAs and laptops for data analysis, record keeping and documentation.

Database automation. IR wireless technology has triggered a database-automation trend of portable and autonomous data collection relating to administration, prescriptions, patient records, POC and bookkeeping in general, coexisting with actual hospital information systems and popular operating systems. All these software packages have expanded their functionality by making use of the advantages of personal and secure wireless data interexchange via IR. Physicians and health providers can use these systems to collect (transmitting to/from central system) on their PDAs large numbers of patient IDs with patient data, analysis results, diagnosis, prescriptions and pathologic-related information. This information is available at home or in the office at all times, which enhances clinical decisions, efficiency and recovery rates.

Future success

IR wireless technology’s benefits of reliability, security and simplicity go beyond simple peer-to-peer utilisation. The IR local area network (LAN) is revolutionary approach to shedding cumbersome cabling, connectors and even reducing the large amount of paper generated in conventional administration. Implementation of portable applications, improving POC facilities and diagnostics requires the total integration of portable IR wireless communication with LAN. Some examples of success stories are:

• Long Island Jewish Medical Center (New Hyde Park, New York, USA) which has more than 1400 affiliated physicians, and the University of Kansas Hospital (Kansas City, Kansas, USA) have successfully implemented IR wireless LAN from Clarinet Systems (www.clarinetsys.com) complimented with PatientKeeper solutions (www.patientkeeper.com).

• Charleston Medical Area Center Inc. (Charleston, West Virginia, USA, www.camc.org) has installed a core hospital information system from
Siemens (www.siemens.com) with multilocation IR wireless communi-cation from Clarinet Systems to download and synchronise physicians’ rounds report with their PDAs.

The integration of IR wireless technology with medical equipment will boost the functionality and appeal of medical devices and make them easier to operate for the user, while maintaining security and reliability.