What does this method look like?
Heise: It is based on the principles of infrared photometry. The substance we want to measure – glucose – exhibits a specific absorption spectrum in the infrared spectral region, which becomes measurable when the substance is penetrated with specific wavelengths beyond the visible light. Each substance has its own absorption pattern that is as distinctive as a fingerprint. You can identify and quantify substances with it.
In the previous CLINICIP project, we extracted small molecules like glucose, urea and lactate from abdominal subcutaneous adipose tissue with a microdialysis catheter. The fluid in the catheter, the so-called dialysate contained information on the concentration of these small molecules in the body fluid. The continuously harvested dialysate was analyzed outside of the body in a measuring cell with a small bedside spectrometer next to the patient. The infrared absorption spectrum is very well suited for this. The advantage is that we are not just able to analyze one fluid compound such as blood glucose, but several components, which are also of interest to intensive care physicians. Aside from the aforementioned, these include dissolved CO2 and bicarbonate, which offer clues about the pH-value of the examined body fluid and deliver insights into the electrolyte status of the body.
You use a quantum cascade laser as a source of radiation. How can we imagine this?
Heise: The smallest conventional IR spectrometers have approximately a footprint of a DIN A4 paper sheet. During the CLINICIP project, we developed a bedside system for critically ill patients using this format. However, further miniaturization is desirable in this case, since physicians are not very pleased of accepting another device by the hospital bed. Yet miniaturization is difficult, since spectrometers on the one hand have a thermal radiation source so that the device also needs to be designed for heat transfer; on the other hand, the measurement technology is also quite intricate. Further miniaturization of these devices is not expected in the near future.
Quantum cascade lasers represent a cold source for infrared radiation where less heat needs to be removed. At this point, we are using a system the size of half a DIN A4 paper sheet, which we would like to reduce to the size of a pack of cigarettes or a matchbox. Aside from the radiation source, we would then have a cuvette for measuring or a fiber optic probe and corresponding photodetector on this chip. Miniaturization is definitely feasible. You can also see this in modern mobile cell phones for example.