Cascade laser sensor may replace pinprick blood tests

28 Aug 2014

Researchers at Princeton develop mid-IR noninvasive technique; could improve quality of life for diabetics.

Researchers at Princeton University, NJ, USA, have adapted a quantum cascade laser (QCL) source to measure people's blood sugar, and, expect to shrink the laser system to a portable size, when the technique could allow diabetics to check their condition without pricking themselves.

"We are working to turn engineering solutions into useful tools," said Claire Gmachl, the Eugene Higgins Professor of Electrical Engineering and the project's senior researcher. "We hope to improve the lives of many diabetes sufferers who depend on frequent blood glucose monitoring."

Research team member Sabbir Liakat told optics.org, “We are currently in the process of implementing our system in field trials. The system is housed on a mobile cart, and we have already received feedback from two medical centers that are interested in a collaboration. This would allow us to obtain data from their diabetic patients in order to test our system on a larger clinical scale. The next stages of R&D will involve both hardware and software developments.

”On the hardware side, we will miniaturize the system to occupy the smallest area possible, as well as implement a custom laser best suited for our application. The parameters of this laser, namely scanning time, scanning resolution, and tuning range, are based on results from our current data set.

”Other possible applications of this technology include the sensing of any relevant molecules with strong mid-IR absorption features within the upper layers of skin. One specific example is epithelial cancer detection; this has been a recent hot topic regarding possible mid-IR in vivo applications.

“On behalf of Claire and the group, I'd say that this project is a prime example of the importance of fundamental semiconductor device research. While I work on the applications aspect of mid-IR devices, these applications are only possible due to the enormous amount of time spent by my colleagues in perfecting lasers and detectors. The hardware miniaturization aspect of this project goes hand in hand with the push towards improved performance of room temperature mid-IR devices.”

No skin damage

Gmachl’s teams work was recently published in an article in Biomedical Optics Express, which describes how the team measure blood sugar by directing their specialized laser at a person's palm: “The laser passes through the skin cells, without causing damage, and is partially absorbed by the sugar molecules in the patient's body. We use the amount of absorption to measure the level of blood sugar.”

The paper concludes, "Mid-IR spectra obtained from human skin yield clinically accurate predictions for blood glucose levels for concentrations between 75 and 160 mg/dL using both PLSR and derivative spectroscopy techniques. Best-case scenarios with given calibration sets yielded average errors only 2% more than those from a commercial electrochemical meter. Based on these results, we conclude that this application of mid-IR light to noninvasive, in vivo glucose-sensing yields a robust and clinically accurate system that transcends boundaries set in the past which limited the scope of mid-IR in vivo applications."

Lead author Sabbir Liakat, said, “The team was pleasantly surprised at the accuracy of the method. Glucose monitors are required to produce a blood-sugar reading within 20% of the patient's actual level, which was achieved by even an early version of our system. The current version is 84% accurate. It works now but we are still trying to improve it."

Room temperature operation

When the team first started, the laser was an experimental set-up that occupied a moderate-sized workbench. It also needed an elaborate cooling system to work. Gmachl explained that the researchers have solved the cooling problem, so the laser now works at room temperature: “The next step is to shrink it.”

Liakat added, "This summer (2014), we are working to get the system on a mobile platform in order to take it places such as clinics to take more measurements. We are looking for a larger dataset of measurements to work with."

The key to the system is the infrared laser's frequency. Current medical devices often use the near-infrared wavelength range because this frequency is not blocked by water, so it can be used in the body. But these wavelengths do interact with many acids and chemicals in the skin, which makes it impractical for detecting blood sugar.

The mid-infrared range (typically 3–8 µm) however, is not as much affected by the other chemicals, so it works well for blood sugar. But mid-infrared light is difficult to harness with standard lasers. It also requires relatively high power and stability to penetrate the skin and scatter off bodily fluids. The target is actually not blood but a fluid called dermal interstitial fluid, which has a strong correlation with blood sugar.

QCL breakthrough

The Princeton achievement is based on a quantum cascade laser (QCL), which enables the beam to be set to one of a number of different frequencies, notably in the mid-infrared. Recent improvements in QCLs also provided for increased power and stability needed to penetrate the skin.

The new monitor uses the laser, instead of blood sample, to read blood sugar levels. The laser is directed at the person's palm, passes through skin cells and is partially absorbed by sugar molecules, allowing researchers to calculate the level of blood sugar.

Gmachl added, "Because the quantum cascade laser can be designed to emit light across a wide wavelength range, its usability is not just for glucose detection, but could conceivably be used for other medical sensing and monitoring applications."

About the Author

Matthew Peach is a contributing editor to optics.org.