This cell glows red because it has been injected with nanosensors that fluoresce in the presence of sodium. Implantable and biodegradeable fluorescing nanosensors are under development at Charles Stark Draper Laboratory. The quantum dot sensors could provide the basis for tattoo inks that change color when detecting glucose levels and would alert diabetics to the need for insulin. In vitro biocompatibility studies have produced no indications of cellular injury thus far.
Credit: Heather Clark, Draper Laboratory
At the Charles Stark Draper Laboratory, Inc. (Cambridge, MA) Heather A. Clark (Analytical Chemist, Biomedical Engineering Group Task Leader, Optical Nanosensors) and her team are developing continuous, non-invasive methods for glucose monitoring, that are easy to use, highly accurate and pain-free.
Clark and her group are developing a nanosensor that could be injected into the skin, much like tattoo ink, to monitor an individual's blood-sugar level. As the glucose level increases, the "tattoo" would fluoresce under an infrared light, alerting a diabetic whether or not an insulin shot is needed following a meal. The researchers have already tested a sodium-sensing version of the device in mice, and will soon begin animal tests of the glucose-specific sensor.
In U.S. Patent Application 20090155183, Clark details methods for detecting the presence of a chelatable analyte in a medium using the nanosensor particles. One chelatable analyte is glucose and the medium can be water, blood, plasma or urine. The fluorescent nanosensors can also detect the presence of a chelatable analyte in an animal. The sensor particle can be implanted in the dermis or epidermis where a chelatable analyte, such as glucose, is monitored continuously and nonivasively.
The most reliable way to measure blood sugar is by pricking the finger for a tiny blood sample and using enzyme-laden test strips to detect glucose. In an attempt to free diabetics from this time-consuming and expensive regime, a number of novel glucose-sensing technologies are under development, from implanted devices that continually monitor blood sugar and dispense insulin, to noninvasive sensors that detect glucose through the skin via infrared light.
Heather Clark and her colleagues are developing sensors designed to operate in between these two extremes. The material consists of 120-nanometer polymer beads coated with a biocompatible material. Within each bead is a fluorescent dye and specialized sensor molecules, designed to detect specific chemicals, such as sodium or glucose.
When injected into the skin, the sensor molecule pulls a target chemica, such as sodium into the polymer from the interstitial fluid, which surrounds cells. To compensate for the newly acquired positive charge of a sodium ion, a dye molecule releases a positive ion, making the molecule fluoresce. The level of fluorescence increases with the concentration of the chemical target. Scientists can swap in different recognition molecules to measure different targets, including chloride, calcium, and glucose. The range of concentrations that the sensor can detect can be varied by altering the ratio of the components, depending on whether it is important to measure precise concentrations or more broad variability.
The sodium sensor, which could one day be used to monitor dehydration, has shown early success in animals. When injected into rodents' skin, the beads stay put and fluoresce in response to saline injections. The researchers have developed a glucose sensor that works via a similar mechanism. It has been shown to work in a solution but has not yet been tested in animals.
In the long term, Clark envisions a sensor that would be injected into the surface layers of the skin, shallower than tattoo inks "so that it sloughs off over time," she says. A fluorescence monitor, resembling an optical mouse, would then be used to measure the light emitted by the tattoo, and the sensor would be reinjected periodically.
"It's unique because it doesn't have any components to be used up," says Clark. Glucose strips, for example, use an enzyme to detect glucose, which needs to be continually replaced. "Other monitors, even nanosensors, have a limited lifetime, which makes implanting them difficult," she says.
The systems and methods disclosed in U.S. Patent Application 20090155183 include a sensor particle for detecting the presence of a chelatable analyte, such as glucose. The sensor is comprised of a chromophore and a fluorescent component, such as a quantum dot. The sensor particle further comprises moieties that bind both a clelatable analyte and chromophore reversibly and competitively. In the presence of the chelatable analyte, the moieties bind the analyte, and release the chromophore.
The chromophore absorbs photons of one wavelength in a free state but of a different wavelength in a bound state, and is selected to operate with the fluorescent component such that the chromophore absorbs emissions of the fluorescent substance in only one of the bound and unbound states. The methods can detect the presence of a chelatable analyte in a medium such as water, blood plasma and urine, using the sensor particles developed by Clark and her colleagues.
Diabetes has become a national health-care crisis. According to the 2005 National Diabetes Fact Sheet, an estimated 20.8 million people in the United States suffer from diabetes. The costs associated with diabetic care are also astronomical, with an estimated $132 billion dollars spent in 2002. As a result of a seminal study highlighting the benefits of tight glycemic control, the American Diabetes Association recommends that patients with diabetes should try to control their glucose levels to be as close to normal as possible. With tight glycemic control, the complications associated with diabetes, such as heart disease, blindness and amputation are significantly reduced. Self-monitoring of glucose is essential for regulation, particularly for those with Type 1 diabetes. It is often performed through a finger-stick method three times or more per day. The need to draw blood, even in small quantities, multiple times a day is not desirable.
Many people unknowingly suffer from type II diabetes, and it is rapidly becoming a disease of epidemic proportions for all ages, genders, and ethnicities. In the US alone, this disease costs patients, employers, and insurance companies 174 billion dollars collectively each year. In addition, the number of people worldwide suffering from diabetes rose from 30 million to 230 million over the past two decades. For example, China has the largest number of diabetics in the world over age 20; a total of 39 million people are afflicted with this disease. Likewise, India has the second largest number of diabetics in the world, with about 6 percent of the adult population or an estimated 30 million diabetics.
In some Caribbean and Middle Eastern countries, the percentage of diabetic people ranged from 12 to 20 percent. In the world's poorest nations, the disease is a quick death sentence. For example, an insulin dependent person in Mozambique, who requires daily injections of insulin, may not live more than a year, or a person in Mali may not live more than 30 months. However, Americans who receive timely and proper treatment live many years with the disease. As alluded to above, weight gain, excessive carbohydrate intake, and lack of exercise leads to a greater risk of developing Type 2 diabetes.
While Type 1 diabetes is responsible for only 5% to 10% of the total reported cases, approximately 90% to 95% of the reported cases are Type 2 diabetes. In either form, diabetes is characterized by high blood sugar levels that result from the body's inability to make insulin or to properly utilized insulin (i.e. insulin resistance). These deficiencies lead to a host of complications including heart disease, cancers, non healing wounds, kidney failure, blindness, stroke, peripheral nerve pain, congenital birth defects, less resistance to infection, and numerous types of heart and blood vessel diseases.
Currently, most diabetics fall between the ages of 40 and 59, and millions of diabetics die worldwide every year due to complications or improper treatment. However, these statistics are likely to change in the future, and some estimates indicate that the number of diabetics could grow to 350 million worldwide during the next two decades.
A continuous monitoring system would be highly advantageous for patients and healthcare providers alike. It has become the goal of glucose sensor research, and continuous monitoring systems of many varieties are pursued by countless researchers in the field. The benefits of continuous monitoring over the finger-stick method are numerous. First, the finger-stick method is both painful and inconvenient for the patient, which can lead to noncompliance. Second, a single-point measurement gives static information on the concentration of blood glucose, with no knowledge of the trend, or in other words, whether the level is going up or down.
A continuous monitoring system would be highly advantageous for patients and healthcare providers alike. It has become the goal of glucose sensor research, and continuous monitoring systems of many varieties are pursued by countless researchers in the field. The benefits of continuous monitoring over the finger-stick method are numerous. First, the finger-stick method is both painful and inconvenient for the patient, which can lead to noncompliance. Second, a single-point measurement gives static information on the concentration of blood glucose, with no knowledge of the trend, or in other words, whether the level is going up or down.
Third, monitoring at night, a time when levels could dip dangerously low, is either not performed or especially inconvenient. Continuous monitoring systems have been pursued in many different forms, and some are commercially available, such as the Guardian® from Medtronic MiniMed (Northridge, Calif.), and the Gluco Watch Biographer from Animas (West Chester, Pa.). Both of these systems work by sampling glucose from the interstitial space, the extracellular space in the dermis, rather than the blood. Currently, they are approved as monitors to track trends in glucose but highs and lows are verified by a finger-stick test. Some reports have shed doubt on the accuracy of nighttime monitoring in patients whose glucose is tightly controlled.
Commercially available systems for continuous or finger-stick measurements rely on electrochemical biosensors. Glucose oxidase is the most well-known of the biological recognition units, and the enzyme provides a highly selective sensor platform. Enzyme-based sensors are difficult to implement as implantable glucose sensors, since the enzyme limits itself in a confined environment. Oxygen, required for function, regionally depletes, and hydrogen peroxide, a by-product of the reaction, can lead to enzyme degradation. Most often the read-out is electrode-based, which is an added challenge for miniaturization and biological implantation.
Commercially available systems for continuous or finger-stick measurements rely on electrochemical biosensors. Glucose oxidase is the most well-known of the biological recognition units, and the enzyme provides a highly selective sensor platform. Enzyme-based sensors are difficult to implement as implantable glucose sensors, since the enzyme limits itself in a confined environment. Oxygen, required for function, regionally depletes, and hydrogen peroxide, a by-product of the reaction, can lead to enzyme degradation. Most often the read-out is electrode-based, which is an added challenge for miniaturization and biological implantation.
FIG. 6. Wide field fluorescence microscopic image of a suspension of sensor particles developed by Clark. Nanometer-sized glucose-sensitive quantum dots (GSQDs) in solution are shown in FIG. 7.
