Sensors transcutaneous

Garg S, Zisser H, Schwartz S, Bailey T, Kaplan R, Ellis S, Jovanovic L. Improvement in glycemic excursions with a transcutaneous, real-time continuous glucose sensor. Diabetes Care 2006, 2, 44-50. 

Transcutaneous determination of ethanol with an oxygen electrode covered by AOD has been described by Clark (1979). Stepwise increases of the ethanol concentration in rat blood resulted in a curve reflecting the ethanol injections, returning to the initial value only after several hours. Disturbances were caused by variations in body temperature and blood pressure. In the paper cited, Clark developed the concept of a sensor for volatile enzyme substrates. 

To reduce the invasivity, numerous supplementary methods have been evaluated to determine their usefulness in replacing some invasive methods or obtaining additional data. Some of these tested methods involve transcutaneous sensors for PO2 and PCO2 partial pressures in the tissue (tcp02 and tcpC02) and transcutaneous measurements of oxygen saturation in peripheral vessels or invasive intravasal measurements of oxygen saturation. 

Transcutaneous measurement of partial pressures is based on the gas permeability of human skin. An electrochemical sensor is placed on the skin which is heated to increase arterial blood in superficial blood vessels [1]. 

The electrochemical measurement of PO2 by use of a polarographic Clark cell offers the advantage of designing small and compact sensors which show a linear response to oxygen partial pressure. Figure 23-4 presents a sectional view of a combined transcutaneous sensor for tcp02 and tcpC02. 

Figure 23-4. Sectional view of transcutaneous pOj/pCOj sensor (Drager).

The value of the partial pressure measured at the skin surface depends in a complex way on blood partial pressure, constitution of the skin, local perfusion, metabolism in the associated tissue, cardiac output, and application temperature. An increased temperature of 43 °C raises the gas permeability and expands the capillary vessels of skin which are filled with more artial blood. The local hyperemia has the disadvantage of limiting the application time at a certain site. Assuming stable circulation conditions, transcutaneously measured values correlate with arterial partial pressure by a factor of 1.2 (neonates) to 1.0 (small children) [1]. The measured value for adults proved to be very unreliable. In the case of unstable conditions or shock with a reduction of peripheral blood flow, the transcutaneous value drops very early. Inconvenience in routine use is caused by long preparation times of the sensor, the need for periodic membrane changes, the long run-in time of freshly prepared sensors, the necessity for periodic calibrations and the slow response time to changes in partial pressure. 

HOlscher, U, A Novel Approach for an ECG Electrode Integrated into a Transcutaneous Sensor in Continous Dvnscutaneous Monitoring, Huch, A., Huch, R., Rooth, G., (eds.) New York Plenum Press, 1988, pp. 291-293. 

Most innovations in oxygen measurement have been in the engineering of sensors rather than in the electrochemistry. They nearly all rely on amperometric detection following the application of a suitable reducing potential. Innovations in design include miniaturisation for insertion into blood vessels [12,13], the inclusion of heaters for the transcutaneous measurement of blood gas [14,15] and shaping for mounting on the eye for measurement via the palpebral conjunctiva [16,17]. 

Wireless bidirectional communications and telemetry to aU stimulators and sensors, which eliminates the use of both transcutaneous leads (which are susceptible to infection), and surface applied coils and stimulators. 

Since the original discovery of this phenomenon over 50 years ago, there has been progressive development in instrumentation to measure oxygen saturation along three different paths bench-top oximeters for clinical laboratories, fiber optic catheters for invasive intravascular monitoring, and transcutaneous sensors, which are noninvasive devices placed against the skin. 

In 1954 Leland Clark demonstrated that a platinum cathode would measure the oxygen concentration of blood when it and a reference electrode were covered by an oxygen permeable membrane. Later in that same year Stow and Severinghaus showed that carbon dioxide could be estimated in blood with a glass electrode fitted with a gas permeable membrane. In the seventies the Huchs demonstrated that mechanical adaptations of these devices could be utilized to provide transcutaneous (non-invasive) measurement of arterial blood gas concentration if the skin area surrounding the sensor was heated to 44 - 45°C. 

There has been much effort in recent years to provide continuous chemical monitoring of critically ill patients or patients undergoing heart surgery. It has proved very difficult to fulfill the increased demands in such applications by any sensors. Some of the problems are sensor sterilization and calibration, effect of varying temperatures, sensor deterioration or cellular encapsulation due to continuous direct contact with blood, thrombogenicity of the sensor devices, small size, and patient safety. Noninvasive systems, e.g., transcutaneous oxygen and carbon dioxide sensors, are less affected and have had considerable success, particularly in infant care. O2 and CO2 can diffuse across the mildly heated skin to the sensors and the measured values correlate well with arterial samples. 

Bendjelid K, Schutz N, Stotz M, et al. Transcutaneous PCO2 monitoring in critically ill adults clinical evaluation of a new sensor. Crit Care Med 2005 33 2203-2206. 

The efficacy of end-tidal CO2 and transcutaneous CO2 measurements are extremely close for older children and adults. With currently available sensors, the latter has been shown to correlate extremely well with Pac02. Although both can be accurate in assessing stable Paco2 during mechanical ventilation, both can underestimate increases in hypercapnia and can be an optimal evaluation for therapy (19,43,44). 

In the present study, as a first step toward the evaluation of human brain stiffness using a tactile resonance sensor, we determined the standard values of the transcutaneous viscoelastic properties near cranial defects under stable conditions before cranioplasty. 

This study was approved by the ethics committee of Shimane University Hospital (IRB 475). The background populations were inpatients admitted for unilateral decompressive craniectomy (DC), which prevents brain herniation due to acute intracranial hypertension, at our institution between 2006 and 2010. In this study, seven subjects underwent transcutaneous measurements of viscoelastic properties through cranial defects by a tactile resonance sensor and CT scans within three days before cranioplasty, which was most often done around one month after DC. Their transcutaneous viscoelastic properties via the cranial defects were determined under normal conditions. 

As previously noted, medical devices are developed and marketed under strict regulatory requirements to ensure safety and efficacy. The transcutaneous nature of the minimally invasive CGM systems on the market and in development creates additional requirements on the materials, processes, packaging, and delivery of the devices. In vitro diagnostic devices are considered to be a subclass of medical devices, and a separate EU regulation (98/79EC) applies. In contrast to in vitro diagnostic devices, medical devices need, e.g., a biocompatibility assessment, and for invasive sensors also sterilization, disinfection requirements apply. As such, the materials used in the constmction of the transcutaneous part of the sensor, and the complete sensor itself, must be biocompatible in the sense that the sensor exhibits no toxicity toward the surrounding tissue. 

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