Motion mastered

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Developments in organic LED technology could soon revolutionise aspects of patient care, especially for monitoring babies. Andrew West finds out more

The pulse oximeter represents one of the greatest advances in patient monitoring. It is a simple, non-invasive technique using relatively cheap technology, yet its introduction into medicine in the 1980s has led to thousands of lives being saved and has brought about its ubiquitous presence in every well-equipped operating theatre.

Despite their simple design and ease of use there are problems associated with pulse oximeters that restrict their application and accuracy. Now, Ian Sage and his team at QinetiQ in Farnborough, UK, are developing new, fully flexible pulse oximeter probes, which they hope will lead to even easier monitoring, more accurate results and better patient care.

Pulse oximeters are used to monitor continuously the oxygen saturation of haemoglobin in arterial blood. Because feedback from the machine is instantaneous, it is used extensively to monitor patients who may become hypoxic: when the oxygen saturation of haemoglobin falls dangerously low. Patients who are heavily sedated or anaesthetised might suffer from hypoxia, which explains why pulse oximeters are essential tools in operating theatres. Pulse oximeters are also widely used to monitor premature babies’ blood.

However, the technique isn’t without its flaws. ’Pulse oximetry has many drawbacks,’ says Katie Rochester, a research scientist working at QinetiQ. ’Although it is a safe and non-invasive technique, it is highly affected by movement; false alarms are common. It is however a technique used in ever more situations.’

A standard pulse oximeter operates in a very simple fashion. Light at two specific wavelengths is shone through tissue that contains an arterial pulse; this is usually a finger in adults or a hand or foot in babies. By using pulsing tissue, the variable absorption of light by bone, tissue, skin and pigmentation between patients is ignored by the sensor, because only changes in light absorption over time are measured. Oxygen-saturated haemoglobin absorbs strongly in the near IR wavelength region and is monitored using light at 805 or 940 nm depending on the machine. Desaturated haemoglobin absorbs strongly in the red region of the visible spectrum at 660 nm. A sensor opposite the light emitting diodes (LEDs) in the pulse oximeter detects the difference in absorption between red and IR light during systole and diastole and, using a calibration algorithm based on human volunteer data, estimates the haemoglobin oxygen saturation.

Various factors can cause a pulse oximeter to give a false reading or no reading at all; too much background light can decrease the signal-to-noise ratio, giving poor and false readings as the variable absorption of light is too small; flickering light can be misread by the sensor as a pulse; motion at the pulse oximeter will mean the light reaching the detector continually changes, making readings inaccurate and some motion may even be interpreted as a pulse.

’False alarms might seem like just an inconvenience, but they waste the valuable time of hospital staff and increase the anxiety of parents with premature babies if they occur regularly,’ says Andrew Kirke, a senior staff nurse at the Leicester Royal Infirmary. ’If pulse oximeters could be made more reliable, there would be real patient benefit, along with benefit for families and nurses.’

Now, the group at QinetiQ have designed a way to remove the motion problem without decreasing device sensitivity. The team turned its back on traditional LEDs and silicon photoconductor sensors housed in bulky, rigid plastic. Instead, they coated thin plastic under high vacuum with sequential layers of an organic LED (OLED) and a phthalocyanine dye to act as an organic photoconductor. Using a thin film instead of a rigid body allows the emitter and sensor to fit close to the patient’s skin, permitting movement without disturbing haemoglobin oxygen saturation readings. Further, when the outside of the film is coated with an opaque layer, background light can be excluded from reaching the sensor, which increases sensitivity.

Rochester came up with the idea while doing other OLED research. ’I became aware of the problems with pulse oximeters during a meeting with a visiting anaesthetist,’ she says. ’Later, I was working on novel OLEDs for another project and I realised flexible OLEDs may provide a solution.’

Standard small molecule OLEDs have been used along with light emitting polymer devices, both working equally well. Because of their simple design, the new pulse oximeters can be made to virtually any size. This is important when premature babies are being monitored and is another major advantage over current pulse oximeter designs. The team is also investigating inkjet printing of polymer OLEDs, which would make device manufacture even more simple and cheap. The pulse oximeters could also be produced in huge numbers using this method.

Rapid, cheap and simple production would be an important factor for such pulse oximeter devices, as they are not designed for long-term use. Flexible OLEDs traditionally suffer from degradation over relatively short time periods. This is because they are sensitive to air and moisture that enter the OLED if the plastic coating becomes worn or damaged. ’This is not so much of a problem for us as the pulse oximeter sensors will be disposable and have a short working lifetime,’ says Rochester. ’We are confident that current encapsulation technologies will be sufficient to give the shelf life and working lifetime necessary.’

The team hope to get a fully functional and reliable device to market as soon as possible and they are currently optimising their pulse oximeter designs to maximise their efficiency and ease of use. The final challenge will be to get the medical community, which, says Rochester, is notoriously conservative, to accept the new technology. However, she is optimistic that doctors and nurses will see the benefits. Rochester adds: ’After speaking to anaesthetists and other end-users, we are confident that the flexible sensor would give real improvements to patient monitoring.’

With more accurate results and fewer alarms, patients and worried parents of sick children should also be better off.