Oxygen-Mapping Sensor Could Improve Organ Transplants, Skin Grafts

A new, flexible oxygen mapping device could help to prevent rejection of organ transplants and skin grafts.

by Menaka Wilhelm
December 10, 2018

In a breakthrough that could improve the success rate of organ transplants and skin grafts, a  University of California, Berkeley, lab measured blood oxygen levels across several different body parts using a checkerboard of organic electronics that sit atop the skin. The blood oxygenation metric is particularly important for organ transplants and skin grafts. If the body rejects one of these procedures, the oxygen in the affected organ can drop. Monitoring oxygenation could help detect rejections earlier.

Mapping blood oxygenation is tricky. Commercial oximeters measure single-point blood oxygen levels at the fingertip, however they can’t assess larger areas like the ribcage or upper arm. Specialized medical imaging can track oxygenation anywhere in the body, but it requires expensive, lengthy procedures.

 “With this sensor array, you can just place it on top of the organ and get an idea of if the whole organ is getting oxygen or not,” said Yasser Khan, the research leader and a Ph.D. candidate in the university’s department of electrical engineering and computer sciences.  

Kahn created the flexible, 2-D oximeter almost from scratch. His team first revamped the materials and manufacturing of an oximeter. Ultimately, they harnessed those developments to create a new sensory approach.

Commercial surface-level oximeters use semiconductor components to measure blood oxygenation. Those work well on a small, flat, well-defined surface But the device’s rigid components can’t conform well to larger parts of the body that has a curved or uneven geometry.

“From a measurement perspective, this mismatch produces noisy signals,” Khan said. Pockets of air between a flat sensor and rounded body parts introduce error.”

Khan and his team partnered with a UK-based materials science company, Cambridge Display Technology Limited, to replace semiconductor electronics with plastic, flexible equivalents.

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The replacement semiconductors made the sensors printable. “If you can print the sensors as we print newspapers, you can print large volumes and that will reduce the costs,” Khan said.

Khan and his colleagues created their first flexible oximeter, a small oxygen sensing array that wrapped around the fingertip, in 2014.

A predecessor for the new flexible oximeter, the wraparound fingertip sensor leveraged the same basic principle as commercial devices: blood carrying oxygen absorbs different wavelengths of light than deoxygenates blood.

Using organic LEDs shining red and green light, then measuring the light transmitted across the fingertip, the sensor calculated a ratio of deoxygenated to oxygenated blood.

The fingertip sensor flexes to match the body’s contours, but monitoring an area like the forearm or the rib cage was still out of the question. The device relied on shining light through whatever it measured.

Khan and his team opted to modify their measurements. Rather than quantifying the light that moved through blood, they quantified the light that bounced back, so that all of the sensor’s machinery could operate from a single surface.

Khan and his colleagues aimed to compare red light and near infrared light in these reflection measurements. At these wavelengths, oxygenated and deoxygenated blood still behave differently. Near infrared light scatters less and reflects stronger.

“If you go farther to the near infrared, basically the contrast is higher between the red and near infrared light,” Khan said.

While semiconductor LEDs have long provided many types of light, organic LEDs are still a relatively new development.

“It’s a pretty challenging problem,” said Cunjiang Yu, a professor of mechanical engineering at The University of Houston, who also works on flexible electronics. “You need to have different wavelengths of LED, which is not trivial.”

It took a few years for technology company Cambridge Display Technology to produce proprietary new light emitters in the near infrared range. Khan then worked out the mathematical models to translate light measurements into oxygenation values.

In control tests, the team used an altitude simulator to change a volunteer’s oxygen levels. The sensor read blood oxygenation from the forehead and also tracked commercial oximeter readings.

They’ve now tested the sensor all over the body. When a blood pressure cuff changed a volunteer’s forearm circulation, the sensor picked up oxygenation differences.

In other tests, the oximeter delivered blood oxygen values to the stomach and leg.

Khan and his team have filed a patent on the device. One day, these kinds of sensors will be inexpensive to manufacture.  For now, large-scale commercialization is limited because factory infrastructure for producing organic electronics is still developing.

Meanwhile, Khan and his team are hoping to incorporate these blood oxygen monitors into multimodal sensors that track more than one metric of healing.

Yu sees another frontier to explore. Beyond flexibility, there’s stretchability, which would allow these kinds of sensors to measure an even wider range of injuries, he said.

Menaka Wilhelm is an independent writer.

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