Electrical Impedance Spectroscopy as a Powerful Analytical Tool for Monitoring Microbiological Growth on Medical Implants
Louise M. Oliver (University of Ulster, UK), E. T. McAdams (University of Ulster, UK), P. S.M. Dunlop (University of Ulster, UK), J. A. Byrne (University of Ulster, UK), I. S. Blair (University of Ulster, UK), M. Boyle (Royal College of Surgeons, Ireland) and K. G. McGuigan (Royal College of Surgeons, Ireland)
Copyright: © 2008
Hospital-acquired infections (HAI) are defined as infections that are neither present nor incubating when a patient enters the hospital (Bourn, 2000). Their effects vary from discomfort to prolonged or permanent disability and they may contribute directly or substantially to a patient’s death. HAI’s are estimated to cost the National Health Service (NHS) in England £1 billion annually (Bourn, 2000) with as many as 5,000 patients dying as a result of acquiring such an infection (Anon, 2001). Not all hospital-acquired infections are preventable but Infection Control Teams believe that they could be reduced by at least 15%, with yearly savings of £150 million (Anon, 2001). Central intravascular catheters have been found to be a common source of infection. Catheters can become infected via a number of different routes with the infection proliferating in multiple areas along the catheter surface. It has been reported that over 40% of the identified micro-organisms causing hospital-acquired infection were Staphylococci, an organism that is typically found on the natural skin flora (Bourn, 2000).
Key Terms in this Chapter
Solution Resistance (Rs): The opposition to current flow through the solution/electrolyte by the migration of ions. In general, the solution resistance is considered to decrease with increase in microbial growth.
Impedance: The opposition to flow of an alternating current. Impedance is a complex entity that describes all resistance to current flow. Impedance is composed of resistance (the real component) and capacitive and or inductive reactance (the imaginary components).
Constant Phase Element: In order to facilitate equivalent circuit modelling, the ideal capacitance is often substituted by an empirical constant phase element (CPE). The CPE is not physically realisable with ordinary lumped electric components but it is usually described as a capacitance that is frequency dependent. The use of a CPE reflects the nonhomogeneity of the system under investigation.
Hospital-Acquired Infections (HAI): Infections that are neither present nor incubating when a patient enters the hospital. Effects vary from discomfort to prolonged or permanent disability where HAI’s may contribute directly or substantially to a patient’s death.
Charge Transfer Resistance (RCT): The DC current that manages to cross the electrode-electrolyte interface experiences a charge transfer resistance. The expression for RCT is derived from the Butler-Volmer equation where: R is the gas constant, T is temperature (K), F is the Faraday constant, n is the number of electrons involved in the electrode reactions, and i0 is the exchange current density.
Biofilm: Microbial biofilms are defined as a matrix-enclosed bacterial population adhered to a surface or each other. An organic matrix protects these sessile organisms. Biofilms are characterized in part by the production of a network of highly hydrated extracellular polysaccharides (EPS).