Bringing sensors & MCUs to a wirelessly monitored hospital bed -

Bringing sensors & MCUs to a wirelessly monitored hospital bed

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

Sensors play an important role in satisfying our inquisitiveness to know about our surroundings. By far, temperature is the most measured parameter using a sensor.

The most primitive method to measure would have been by touch and then by the use of external metals. The initial measurements were just comparative and did not have a scale. Sensors have come a long way from that point and we now have highly accurate measuring devices.

With advances in technology, various sensors measuring pressure, humidity, proximity, and motion, among others, have been developed. To form a complete application, however, use of these stand-alone sensors is not enough.

Sensors must be interfaced to a microcontroller to form a complete system. Interfacing sensors with microcontrollers has revolutionized different applications and is continuing to do so. Many fields like medical, industrial, and automotive are benefiting from the latest innovations in sensor technology.

Until recently, sensing has been implemented using hardwired circuits, which can be inconvenient to deploy in the areas where they are mounted. To simplify deployment, next-generation sensors are transferring the data they record using wireless technology. A microcontroller that can interface with sensors and also has wireless capabilities makes continuous monitoring of received data possible.

This article explores the technology involved in interfacing of sensors for medical applications, using hospital beds as an example. Hospitals are places where all of us have felt the need for a little more comfort, and hospital beds/mattresses can be made more comfortable through technological improvements possible using sensors and microcontrollers. Making patients more comfortable helps the patients, doctors, and also medical caretakers.

Present-day systems use sensors that are hardwired to a PC next to the bed. The use of sensors detects the conditions of the patient and the data is collected and transferred using a microcontroller.

Doctors and nurses need to visit the patient frequently to examine his/her current condition. Using the proposed system, data can be sent wirelessly to the nurses' station, allowing continuous monitoring of the patient.

Such an approach also reduces the number of wires present in the already confusing patient environment. This system can also be used at places other than hospitals, including home monitoring of patients with special needs.

For the purpose of this article, only temperature, humidity, pressure and capacitive sensors are addressed. Any other sensor that can be interfaced with a microcontroller can be used in a similar method.

These sensors can monitor the condition of the patient and, based on the thresholds chosen, record useful data and/or trigger alarms for attention from doctors or nurses over wireless networks. This will not only help current patients, but the data recorded can be helpful for case studies of certain medical conditions.

Choosing a wireless configuration
There are several microcontrollers available on the market with the requisite wireless capabilities which, when coupled with sensors, could implement a wireless hospital bed.

But in this proof of concept, performed at room temperature, a PSoC FirstTouch Starter Kit with CyFi Low-Power RF was used, with a low power 2.4 GHz RF system built on direct sequence spread spectrum technology.

On the transmission side, the sensors were interfaced with a microcontroller and then connected to a wireless subsystem using a second microcontroller to process and send the data using a wireless transmitter.

At the receiving station, a wireless receiver (PC Dongle) is connected to the computer, which collects and displays the data on the monitoring station through a Graphical User Interface (GUI). The block diagram of the application is shown in the Figure 1 below .

Figure 1. In this proof of concept wireless hospital bed, sensors on the transmission side were interfaced an MCU and connected to a wireless subsystem using a second MCU to process and send the data using a wireless transmitter

Designing the sensor and wireless subsystems as separate boards facilitated testing of different sensor interfaces over the wireless network. For the final system, these two subsystems would likely be integrated into a single unit.

Each transmitter has a different ID associated with it, and a single receiver can bind to different transmitters. Using such an approach, multiple patients' can be observed from the same station simultaneously.

The range of this wireless system is 400 m in line of sight and 200 m within a typical hospital environment. Thus it has the capacity of covering patients in one floor and sending data to the nurse station in charge for that area. As the receiver has to bind to a particular transmitter, the data from the each patient can be uniquely identified.

Sensor Interface
The technology of interfacing the sensor with microcontrollers already exists and there are numerous articles in journals describing the methods. Adding wireless technology to the sensors given below takes the system to the next generation.

Temperature sensing. Temperature sensors in the medical field have been used from time immemorial to measure the body temperature and monitor the medical condition of patients. With a temperature sensor in the bed/mattress, measurement of absolute temperature of the patient will not be accurate, as the sensor is not attached to the body.

However, such a system allows for continuous monitoring of a patient's differential change in temperature. An array of thermocouples can be used to measure temperature from different significant parts of the body.

With the use of thermocouples, only one junction of each thermocouple will be closer to the surface towards the patient while the interface circuit can be hidden within the mattress.

Humidity Sensing. A humidity sensor can be integrated into the hospital mattress to check either the perspiration level or for conditions like incontinence. Though excessive perspiration is not a disease in itself, it might be a revealing symptom.

Measuring the variation in perspiration will help log the information for certain medical conditions. It can also give an indication to the medical caregiver to attend to the patient when required.

Multiple humidity sensors can be arrayed, like in the case of temperature sensors, depending upon the patient and caregiver's requirements.

Capacitive and Proximity sensing. A capacitive sensor can be used to detect the presence of the patient on the bed. If the patient has a case of unconscious movement, the capacitance measured will fluctuate with each change in position.

If the patient is moving restlessly or if the patient is out of bed when he/she is not supposed to, the system can trigger an alarm for the medical caregiver.

Utilizing the same technology as a capacitive sensor, a proximity sensor can be placed at specific locations in the bed or mattress, such as close to the head post.

By moving his or her hand close to it, the sensor can be simply activated by patients with special needs. This will help avoid the situation where the patient has to call for help by the press of a button, which requires more motor skills.

Pressure sensing. Pressure sensors are important in medical conditions where patients have moving disabilities. These sensors will detect and prevent any unrelieved pressure on the patients body ” for example, monitoring the formation of a pressure ulcer. The mattress developed for automated body position is discussed in [2]. Adding wireless transmission to capability will take it to the next level of comfort and sophistication.

Linking to the Nurse Station GUI
Once a transmitter binds to a receiving station, data can be sent to and collected by an application at the receiving station that uses a GUI to monitor the data from each individual sensor.

GUI-based monitoring tools can simply display data or can be designed to provide extensive display options and features. The GUI could also store setups, a particular useful feature for enabling monitoring different parameters in patients with different medical conditions. The setup can be stored and loaded when a particular transmitter (patient) is connected.

Figure 2 below shows the graphs obtained by interfacing different sensors and is an example of the patient monitoring system GUI. With centralized control, caregivers also have the option of setting alarms that can be triggered when a high or low limit is exceeded.

Figure 2. In this example of a patient system GUI, graphs obtained by interfacing different sensors are linked to centralized control where caregivers also have the option of setting alarms that can be triggered when a high or low limit is exceeded.

Experimental system setup
In the experimental setup, the high limit of the temperature was set at 26 and the low limit was set at 14. For the humidity, the high limit was set at 60. The alarm window also shows whether the high limit (H) or the low limit (L) was exceeded.

Similar limits can be set for the pressure or capacitance or any other sensor that is interfaced to the system. The medical caregiver can monitor these graphs and can switch between different patient charts and attend to the patient as per the alarms.

Safety Considerations
Given the needs of the hospital environment, safety is a primary concern. When a system is powered through wired lines, not only does it require the manual installation of wires, it also poses the threat of electrical shock in case of short circuit.

A battery-powered wireless sensor system, on the other hand, avoids such safety problems. The wireless system can be made to run in full power mode, low power mode, or switched off based on the requirement to ensure longer battery life. The bed/mattress system will need an access panel to this battery to replace it when required.

There had been concerns in past, about electromagnetic interference (EMI) issues in hospitals due to use of wireless technology. After a detailed study [4], [5], it was shown that if recommended separation distances from medical equipment are observed, then wireless technologies are suitable for use in hospitals including intensive care units and operating rooms.

Extra care should be taken in maintaining these distances when the patient has a pace maker or other life supporting medical equipment.

Although sensors have been around for a long time, the wires attached to them have limited their applications range. The wireless sensor monitoring system in hospital mattress proposed in this article improves patient comfort while freeing up caregivers by providing continuous monitoring of patients on many different levels.

Monitoring is superior compared to wired systems which must be manually checked. Patients can be monitored without being disturbed and caregivers have full access to sensor logs using an intuitive, convenient interface.

And, because of the convenience of wireless monitoring, this application can also be used in other places other than hospitals like homecare and ambulances.

Archana Yarlagadda is an Applications Engineer at Cypress Semiconductor specializing in Programmable System on Chip (PSoC) application designs. She has her Masters in Analog Circuits from University of Tennessee, Knoxville. Her areas of interest are analog, digital circuits, and microcontrollers.

1. Chiou-Fan Chen, Jer-Junn Luh, Yao-Ming Cheng , “Design and clinical evaluation for patient status monitoring system of air-mattress” Biomedical Engineering: Applications, Basis and Communications (BME), 2008
2. Martin Bates, “A refresher tutorial on sensor design using microcontrollers”, Embedded Systems Design, 2007
3. PSoC FirstTouch Starter Kit with CyFi Low-Power RF (CY3271)
4. Tan K-S, Hunberg I, Wadhwani J, “Electromagnetic interference in medical devices”, Medical Electronics Manufacturing. 2001
5. Boyle J, “Wireless technologies and patient safety in hospitals”, Telemed J E Health. 2006

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