Diabetic control typically requires daily monitoring of blood glucose levels. This involves finger pricking 2–10 times a day in order to obtain a sample for assay. Many find the finger prick tedious or painful and fall into noncompliance. Others may also dislike the assorted “paraphernalia” that must be carried around. Research into the “next-tech” for diabetes care has led many researchers to investigate tear glucose sensing as an alternative. As promising as many of these devices are, they are many years from market. Here the development of a near market tear glucose sensor is reported.
More than 23 million people in the U.S. have been diagnosed with diabetes mellitus. Roughly 90 percent are Type II (insulin resistant) diabetics, and these individuals are required to monitor themselves daily. The standard tool for such monitoring is a self-monitoring of blood glucose (SMBG) sensor. In order to SMBG, a diabetic will have to clean an area on or around the finger, use a lancet to prick the finger, and draw blood. This blood is then placed onto an electrochemical sensor that uses an applied voltage to assist in the enzymatic oxidation of glucose into D-glucono-1,5-lactone which hydrolyzes into gluconic acid thereby producing a current. The current is measured by a meter and correlated to blood glucose concentrations. Alcohol wipes, lancets, disposable sensors, and the meter are among the common paraphernalia a diabetic must carry.
Alternative site testing (other parts of the body), noninvasive optical means of sensing glucose through the skin, or soft contact lens sensors have been envisioned or used. Some do not remove the noncompliance issue as they still involve pricking, and many are too far from market. Since the 1930s, tear fluid has been studied and analyzed as an alternative to blood. Until recently, composition and correlation to blood has been somewhat limited and confusing. But understanding rapid capture, minimally abrasive, low-volume sampling with little to no evaporation is key.
The Tear Touch sensor replaces the finger prick (alcohol wipes and lancets) with a thermoset fluidic system that contains a soft, compressible foam material to capture tear fluid from the eye. The capture material is a soft polyurethane foam that wicks tear fluid from the conjunctiva (white part of the eye) in a controlled volume and rapid timeframe. The tear fluid is then delivered to a sensing region (via a fluidic system) where an enzymatic assay of the glucose in tears can be made. The sensor and assay themselves are quite similar to modern SMBG sensors, but due to low concentration of glucose in tears (10–100 times lower than in blood) careful volume and concentration control and prevention of evaporation of reagents and sample are also important. The target range for a sensor to operate is between 0–18.2 mg/dL (0–1000 mM) and have a reproducibility under 20 percent. The linear range of the Tear Touch sensor falls in this region with a lower limit of detection of 0.7 mg/dL (41 mM) and a reproducibility of 15.9 percent in purified solutions.
Currently, interferents to the enzymes, such as other carbohydrates, are being investigated as well as removing or blocking electrochemical interferents: uric and ascorbic acid and acetaminophen. Other improvements include: new thermoplastic materials that are easier to manufacture are being investigated; a more ergonomic design, as the system in current use was made for bench testing; and the most practical means of sterilization. After these improvements have been made, a limited human subject phase would begin using standard fasting glucose testing.
Other applications of this technology would be testing tear fluid for stress sensing purposes, traumatic brain injury, sampling a forensics site for particulate or biological material, or environmental or security purposes.
This technology was done by Jeffrey T. La Belle, Ph.D. of the School of Biological and Health Systems Engineering at Arizona State University, Tempe, AZ. For more information, visit http://labellelab.asu.edu/