Growth of USB in Medical Devices



USB is becoming the de-facto standard for wired connectivity in healthcare devices.

Among the important trends in the medical market today is the increase in healthcare spending. In their 2011 World Health Statistics report, the World Health Organization said that, on a global basis, the health economy is growing considerably faster than Gross Domestic Product (GDP). They went on to say that, from 2000 to 2008, health spending rose from 8.3% of total GDP, to 8.5%, and the per-capita total expenditure on health at the average exchange rate rose from (US$) $484 to $854. This is a substantial increase during the eight year period.

As a result of this increasing global investment in healthcare, more medical devices are becoming end-user items. Today, medical devices are not restricted to hospitals and doctors’ offices. Rather, we are seeing a significant trend toward home healthcare, where more and more medical equipment is being designed for home use. Products such as remote-monitoring devices for blood pressure and glucose levels, as well as new drug delivery systems, are becoming commonplace in the homes of patients with chronic conditions.

This market trend is also encouraging innovation in the field of medical electronics. As personal medical devices are becoming more ubiquitous, companies are looking into developing products with lower power consumption and the smallest possible form factors, and they’re adding connectivity to enable the easy exchange of data.

The medical industry has long understood the benefits of standards such as USB to implement the exchange of data in medical facilities, which it is now deploying for remote in-home patient monitoring. USB’s fast, reliable and “plug and play” nature is almost perfect for medical applications where devices need to be portable, robust and easily upgradable. Companies are also coming up with new techniques that directly insert isolation within the USB signal path, which eliminates the need for external isolation components and therefore helps to minimize the size and cost of products. The medical industry is truly leveraging USB technology across a wide variety of healthcare devices.

The Continua Health Alliance was formed to provide design guidelines and product certification programs, and has become a benchmark organization in the medicaldevice industry. In particular, it is working with the USB Personal Healthcare Device Class (PHDC) specification to leverage seamless interoperability between devices. This effort means that USB will likely remain the backbone of wired connectivity in healthcare devices for some time.

USB Standardization for Medical Devices
As USB interface adoption in a variety of medical devices increased, a common standard was essential to maintain interoperability. In particular, interoperability was required to guarantee that different devices produced by different vendors could successfully communicate with each other and exchange meaningful data. The Continua Health Alliance began with this goal in mind. They have united elegant technology and medical devices with healthcare-industry leaders to empower patients to exchange vital medical information, and improve the way they manage health and wellness. It has established an ecosystem of connected personal health products and created design guidelines based on connectivity standards. There are nearly 250 technology, medical-device and healthcare companies that have joined this alliance to conform to the interoperability between devices and provide an opportunity to truly manage personalized health and wellness. The complete list of active Continua Health Alliance participating member companies can be found here.

Seeing the growing need for seamless interoperability between personal healthcare devices and USB hosts, in 2007 the USB Implementer’s Forum (USB-IF) developed the USB Personal Healthcare Device Class (PHDC). The introduction of PHDC allowed personal healthcare devices–such as blood–pressure monitors or glucose meters–to connect via USB to consumer electronics such as PCs or mobile health appliances. In 2008, the Continua Health Alliance approved the USB PHDC and established a product–certification program that formalizes interoperability conformation using this USB standard. Since then, the Continua Alliance has regularly provided updated design guidelines that allow devices to maintain seamless interoperability.

The Personal Healthcare Device Class (PHDC)
PHDC is essentially the USB stack designed specifically for medical devices with USB connectivity, and it was designed to enable seamless interoperability between personal-healthcare devices and USB hosts. It defines the functionality necessary for personal–healthcare devices to send standardized data and messages to hosts over USB, using industry standards such as the IEEE 11073-20601 Optimized Exchange Protocol. A typical PHDC-based healthcare system is shown in Figure 1.

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Figure 1: Typical PHDC-based healthcare system

Similar to the USB 2.0 specifications that define various device classes such as human interface device (HID), communications device class (CDC) and Mass Storage, the PHDC specification has been further sub-categorized into the following three themes, each focusing on a specific area for the typical usage of personal medical device:

  1. Physical Fitness: For devices that focus on enabling people to stay healthy and fit, such as exercise watches, heart-rate monitors and exercise bikes.
  2. Disease Management: For devices that focus on detecting, monitoring and treating diseases, such as drug-delivery systems.
  3. Independent Aging: For devices that help seniors live unassisted, such as activity monitors and medication reminders.

The above categorization system helps define the data and message structure for each device, depending on its usage and focus area. The PHDC specification does not use a new data or messaging format to exchange data. Instead, it uses the existing IEEE 11073-20601 Optimized Exchange Protocol, which allows for vendor-defined data and messaging standards (Figure 2).

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Figure 2: PHDC’s data-exchange mechanism

The software architecture of the PHDC stack ensures code robustness, portability and reliability in embedded systems development. A USB PHDC-based solution can be seen as a tiered system consisting of the core USB PHDC stack and an IEEE Optimized Exchange Protocol that communicates with the stack and translates the data to external API applications that the devices can use. Several layers of software abstraction isolate the end application from low-level communication drivers, allowing code portability and ease of use. A high-level view of a typical PHDC-based system is shown in Figure 3.

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Figure 3: High-level view of a USB PHDC-based solution

The PHDC specification has simplified USB communication for personal healthcare devices, and has expanded the use of USB to a vast array of personal-healthcare devices. Additionally, the PHDC stack is helping companies achieve faster time to market for their products, while leveraging cross-business expertise and enabling the market expansion of personal healthcare devices.

The complete PHDC specification can be viewed at http:// www.usb.org/developers/devclass_docs/Personal_ Healthcare_1.zip

A Look Ahead
With PHDC in place, another challenge that remains for employing USB in medical devices is to overcome electrical isolation issues. To accomplish this goal, companies are focusing more on EMC and ESD tests that will offer consumers safer and more robust USB devices.

The Continua Health Alliance recently made its most recent design guidelines available to anyone as a free download. These design guidelines were previously available only to Continua Alliance members during interoperability testing. Why this is important? The guidelines can now help all medical developers build end-to-end, plug-andplay systems more efficiently, by facilitating seamless connectivity between personal connected healthcare devices and services, such as smart phones and remote monitoring devices. It will also allow vendors to create devices that make the collection and sharing of personal health data convenient and secure for consumers and healthcare providers, liberating system integrators to develop innovative solutions.

To request a design guideline document from Continua, please visit http://www.continuaalliance.org/products/design-guidelines.html.

Conclusion
As home-based medical care becomes more pervasive, the personal healthcare products market continues to grow. To better enable this growing trend, companies are adopting interoperability standards that allow consumers to use a wider variety of devices and connect them together for remote-monitoring purposes. The Continua Health Alliance continues to work with the PHDC specification to leverage seamless interoperability between devices. As a result, USB is becoming the de-facto standard for wired connectivity in healthcare devices.

Additional Resources
Microchip Technology offers a free PHDC stack that is compatible with all of our 8-bit, 16-bit and 32-bit USB PIC® microcontrollers. You can download the stack today, as a part of the Microchip Libraries for Applications, at www.microchip.com/mla. Inside the library, PHDC code examples are labeled as USB\Device – PHDC, such as Device – PHDC – Blood Pressure Monitor.

If you have any questions related to the subjects covered in this article, please contact us at http://support.microchip.com.

 


khare_manas

Manasi Khare is senior corporate applications engineer at Microchip Technology, focusing on USB applications based upon Microchip’s analog devices and PIC microcontrollers. She joined the company in 2006. Before joining Microchip, she received an MS degree in Electrical Engineering from West Virginia University in 2005. As a part of her Master’s thesis research, she worked on the statistical analysis portion of an iris-recognition system. Her work was published in the journal IEEE Transaction on Information Forensics and Security, in 2006. After graduation, Manasi briefly worked for the International Biometric Group in New York, NY as a Research Associate Engineer.

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