Frequent glucose monitoring is a fundamental part of diabetes management, and good glucose control is important for long-term health outcomes. New types of electrochemical sensors that allow for continuous glucose monitoring (CGM) have become an important tool for diabetes management, although they still have drawbacks such as short lifetime and a need for frequent calibration. Other technologies are still being researched for CGM, in an attempt to replace the electrochemical sensors. Optical methods have several advantages for CGM, including potentially long sensor lifetimes and short measurement times, and many developments have been made over the last decades. This paper will review optical measurement methods for CGM, their challenges, and the current research status. The different methods will be compared, and the future prospects for optical methods will be discussed.

Optical properties of the human brain in the wave-length region from blue to near infrared are presented. There are significant variations in the optical penetration depth from the neonatal and to the adult brain. Typical values for the penetration depth in the adult brain are: 0.5 mm for blueigreen. 1 .S mm for red and 3.5 mm for near infrared. The values for the neonatal brain are typically 2-3 times larger.

IntroductionPatients with diabetes type 1 (DM1) struggle daily to achieve good glucose control. The last decade has seen a rush of research groups working towards an artificial pancreas (AP) through the application of a double subcutaneous approach, i.e., subcutaneous (SC) continuous glucose monitoring (CGM) and continuous subcutaneous insulin infusion. Few have focused on the fundamental limitations of this approach, especially regarding outcome measures beyond time in range.MethodsBased on insulin physiology, the limitations of CGM, SC insulin absorption, meal challenge, and physical activity in DM1 patients, we discuss the limitations of the double SC approach. Finally, we discuss safety measures and the achievements reported in some recent AP studies that have utilized the double SC approach.ResultsMost studies show that a double SC AP increases the time in range compared to a sensor-augmented insulin pump and shortens the time in hypoglycemia. Despite these achievements, the proportion of time spent in hyperglycemia is still roughly 20–40%, and hypoglycemia is still present 1–4% of the time. The main factors limiting further progress are the latency of SC CGM (at least 5–10 min) and the slow pharmacokinetics of SC-delivered fast-acting insulin. The maximum blood insulin level is reached after 45 min and the maximum glucose-lowering effect is observed after 1.5–2 h, while the glucose-lowering effect lasts for at least 5 h.ConclusionsAlthough using a double SC AP leads to significant improvements in glucose control, the SC approach has severe limitations that hamper further progress towards a robust AP.

Rapid, accurate and robust glucose measurements are needed to make a safe artificial pancreas for the treatment of diabetes mellitus type 1 and 2. The present gold standard of continuous glucose sensing, subcutaneous (SC) glucose sensing, has been claimed to have slow response and poor robustness towards local tissue changes such as mechanical pressure, temperature changes, etc. The present study aimed at quantifying glucose dynamics from central circulation to intraperitoneal (IP) sensor sites, as an alternative to the SC location.Intraarterial (IA) and IP sensors were tested in three anaesthetized non-diabetic pigs during experiments with intravenous infusion of glucose boluses, enforcing rapid glucose level excursions in the range 70-360 mg/dL (approximately 3.8-20 mmol/L). Optical interferometric sensors were used for IA and IP measurements. A first-order dynamic model with time delay was fitted to the data after compensating for sensor dynamics. Additionally, off-the-shelf Medtronic Enlite sensors were used for illustration of SC glucose sensing.The time delay in glucose excursions from central circulation (IA) to IP sensor location was found to be in the range 0-26 s (median: 8.5 s, mean: 9.7 s, SD 9.5 s), and the time constant was found to be 0.5-10.2 min (median: 4.8 min, mean: 4.7 min, SD 2.9 min).IP glucose sensing sites have a substantially faster and more distinctive response than SC sites when sensor dynamics is ignored, and the peritoneal fluid reacts even faster to changes in intravascular glucose levels than reported in previous animal studies.This study may provide a benchmark for future, rapid IP glucose sensors.

Currently used temperature sensor systems do not provide sufficient spatial resolution and can not be used as an integrated part of minimally invasive treatment. Few magnetic resonance (MR) compatible sensor systems are available. A distributed fibre Bragg-grating sensor system for use in biological tissue was constructed. Ten Bragg gratings were inscribed in the core of an optical fibre. The fibre was mounted into tubes made of MR-compatible materials. An opto-electronic unit connected to the fibre was used for signal generation and detection. Communication with a PC allowed presentation and logging of temperature data. The system was calibrated to the temperature range –195.8°C to 100°C. Experiments were conducted during freezing (cryoablation) of porcine liver in vivo . The system yielded a temperature profile with 6.5 mm spatial resolution and 5 s temporal resolution. Both mechanical stability and MR compatibility were acceptable and will allow routine use.