CHAPEL HILL, N.C. - Scientists at the University of North Carolina at Chapel Hill have invented a solid-state metal sensor -- roughly the thickness of three human hairs -- that can measure the acidity (pH) of chemical solutions, including body fluids.
A report describing the invention, including successful laboratory tests in animals, is published in the December issue of Analytical Chemistry.
"This particular electrode can be applied to any solution ? from a chemical process in the laboratory to the measurement of tissue pH in physiologic studies," says co-inventor Dr. Wayne E. Cascio, associate professor of medicine at the UNC-CH School of Medicine.
But according to new findings by Cascio and his UNC-CH colleagues, the pH electrode may even prove useful for monitoring the hearts of hospitalized patients, especially those in intensive coronary care. Attached to the tip of a catheter and then threaded via vein to the heart, the pH electrode may offer advance warning when cardiac muscle is becoming more acidic, a condition that occurs during ischemia. Ischemia takes place when the supply of oxygen is cut to portions of the heart and can arise from fat-clogged coronary arteries. This condition could lead to a heart attack.
"If the pH is falling, indicating more acidity, then you know that the tissue is ischemic," Cascio, a cardiologist, explains. "The process of ischemia generates acid, and the degree of acidification offers an index of the severity of ischemia. But a falling pH also seems linked to other electrical and metabolic changes that occur and can predispose to heart rhythm disturbances such as ventricular fibrillation and sudden death," he adds.
In terms of pH in the human body, metabolic actions occur properly when fluids are close to chemical neutrality ? neither too acid nor too alkaline. On a scale of zero to 14, a pH of below 7 signals acidity. In cardiac tissue, ischemia results in the loss of protons from cells, which leads to increased acidity.
Currently, measures of pH typically require glass electrodes. Though stable and reliable, these thin-walled glass bulbs are too large for use in the blood vessels and heart.
The recently invented electrodes have a working lifetime of at least one month and are highly accurate (within about two hundredths of a pH unit) and respond rapidly. They also were shown insensitive to the effects of fluid changes in sodium, potassium, calcium, magnesium, lactate, dissolved oxygen, ascorbate, and urate, which are important for physiological applications in medicine, biotechnology, and drug development.
Cascio and his co-authors explain that the pH electrode is a layer of iridium oxide bonded by electroplating to a platinum surface. This is then covered with a protective polymer. The electrode can be bonded to a variety of materials. According to Cascio, it would be "fairly simple" to attach a pH electrode to the type of catheter commonly used for studying the electrical activity of the heart. "These could be indwelling -- placed in patients where continuous measurements of pH might be an advantage," he says. "The electrode can be generated in any size or configuration."
The electrode?s sensing layer -- called AEIROF (anodic electrodeposited iridium oxide film) -- also may be used for monitoring pH in the esophagus and the gut. "The monitoring of pH in the gut has been shown useful in patients who have septic shock, an overwhelming bacterial infection," Cascio says. "Then there are people with peptic ulcer disease or clinical conditions involving overproduction of acid in the stomach. Gastroenterologists routinely measure this with special tubes, so you can see putting this type of electrode on the end of those tubes as well."
In their application for patent, the researchers note that the AEIROF electrodes will be inexpensive to produce and can be used alone or grouped with other biosensors such as potassium, glucose or lactate electrodes. "In general, the electrodes of the present invention may be used in many situations where standard glass pH electrodes are now used," they say.
Along with Cascio, other AEIROF collaborators in the UNC-CH department of medicine are Dr. Timothy A. Johnson and Larry Dunlap. From the department of chemistry are Dr. Sayed A. Marzouk, Stefan Ufer, and Dr. Richard P. Buck, a pioneer in biosensors. Supporting this research is a grant from the National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland.
Note to media: Dr. Cascio can be reached at 919-966-5201 (email: wcascio@ischemia.card.unc.edu) A photograph of Dr. Cascio for publication is available.
UNC-CH School of Medicine contact: Lynn Wooten, 919-966-6046. Email: lwooten.est1@mail.unch.unc.edu. He may also assist you in obtaining copies of the report in Analytical Chemistry.
Journal
Analytical Chemistry