Miniaturized optical sensors

People with diabetes rely on finger pricks to withdraw blood or adhesive microneedles to measure and manage glucose levels. In addition to being painful, these methods can cause itching, inflammation, and infection.

Researchers at TMOS, the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems, have taken an important step toward eliminating this discomfort. Their team has discovered new aspects of glucose’s infrared signature and used this information to develop a 5mm optical sensor that could one day be used to provide continuous non-invasive glucose monitoring in diabetes management.

The primary challenge facing affordable, wearable optical glucose testing has been miniaturization and filtering out the glucose signals from water absorption peaks in the near infrared (NIR) spectrum. Essentially, it’s been almost impossible to accurately differentiate between water and glucose in the blood; until now.

In research published in Advanced Sensor Research, the team identified four infrared peaks in glucose allowing selective, sensitive identification in aqueous and biological environments.

The team fabricated a miniaturized glucose sensor established on a 1,600nm to 1,700nm waveband that’s Bluetooth enabled and operates using a coin battery, allowing for continuous glucose monitoring. This compact sensor has demonstrated its viability in detecting glucose levels in the human body ranging from 50mg/dL to 400mg/dL in blood plasma, with a comparable limit of detection and sensitivity to larger, laboratory-based sensors. Its small dimensions could see it integrated into smart watches and other pain-free wearable health trackers.

Lead author, RMIT University Ph.D. scholar Mingjie Yang, says “Until now, there’s no consensus on the unique spectroscopic signature of glucose, largely because the O-H bonds targeted in near-infrared (NIR) spectroscopy for glucose detection are also abundant in water. This similarity makes it challenging to distinguish between glucose and water signals, especially in complex biological fluids and tissues. We optimized spectroscopy setup and analyzed transmittance to identify peaks unique to glucose. Our discovery finally provides the information necessary to move forward with miniaturized optical glucose sensing and we have developed a device prototype to suggest the foundation for a futuristic non-invasive glucose sensor.”

The device prototype uses a surface-mounted device light emitting diode (SMD LED) and circuits made of thin-film copper coated polymide (Cu/PI) only 110µm thick developed with a laser patterning technology. The millimeter-scale and lightweight design of this device makes it considerably more compact than traditional benchtop spectrophotometers. Furthermore, the flexible patch-like design offers the future possibility of direct reading as a wearable device on human skin.

The performance of the device has been evaluated using aqueous glucose solutions as well as in blood plasma. Computational analysis of light-skin interference has been conducted to indicate how the SMD LED will penetrate the skin. Simulation results suggest the promising locations for future exploration of optical glucose sensing in clinical setups.

TMOS Chief Investigator Madhu Bhaskaran says, “The non-invasive nature of optical glucose sensors has the potential to improve patient compliance, reduce discomfort, and lower the risks of infections associated with invasive glucose monitoring. With the right collaborators/partners and the right funding, this can represent an important shift toward continuous and pain-free glucose sensing.”

ARC Centre of Excellence for Transformative Meta-Optical Systems
https://www.tmos.org.au

RMIT University
https://www.rmit.edu.au