There are four criteria that must be considered when choosing material for an implanted electrode: (1) tissue response, (2) allergic response, (3) electrode-tissue impedance, and (4) radiographic visibility. This paper discusses these four criteria and identifies the materials that are the best candidates for such electrodes. For electrodes that make ohmic contact with tissues: gold, platinum, platinum-iridium, tungsten, and tantalum are good candidates. The preferred insulating materials are polyimide and glass. The characteristics of stimulator output circuits and the importance of the bidirectional waveform in relation to electrode decomposition are discussed. The paper concludes with an analysis, the design criteria, and the special properties and materials for capacitive recording and stimulating electrodes.

Small-intestine submucosa (SIS) is cell-free collagen, 100 mu thick, derived from the small intestine. It has been used as a vascular graft and has the highly desirable property of remodeling itself to become host tissue. To date there has been limited reporting on its preimplantation mechanical properties as a vascular graft. In this study, compliance, elastic modulus, and burst pressure have been measured on 5- and 8-mm SIS grafts. The compliance (percent of diameter increase for a pressure rise from 80 to 120 mmHg) was 4.6% av (range 2.9 to 8.6%) for the 5-mm grafts. For the 8-mm graft, the increase in diameter for the same pressure rise was 8.7% av (range 7.2 to 9.5%). The modulus of elasticity (E) increased exponentially with increasing pressure according to E = E(o)e(alphaP), where Eo is the zero-pressure modulus and alpha is the exponent that describes the rate of increase in E with pressure; the units for E, Eo, and P are g/cm2. The mean value for Eo was 4106 (g/cm2 range 1348-5601). The mean value for alpha was 0.0059 (range 0.0028-0.0125). At 100 mmHg, the mean value for E was 8.91 x 10(3) g/cm2 (range 1.02-8.80 x 10(3)). The mean burst pressure for 5.5-mm grafts was 3517 mm Hg (range 2069-4654). In terms of preimplant compliance, the small-diameter SIS graft is about (1/2) as compliant as the dog carotid artery, about four times more compliant than a typical vein graft, and more than an order of magnitude more compliant than synthetic vascular grafts.

The direct-current (Faradic) resistance is important because it is the highest impedance that an electrode-electrolyte interface can attain. In this study, the Faradic resistance (Rf) of identical pairs of 0.5 cm2 electrodes of bare and chlorided silver, tin and chlorided tin, nickel-silver, copper, and carbon was measured in contact with 0.9% saline at room temperature. It was found that for positive and negative current flow, the data fit the expression Rf=Rf0 e(-alpha i) (with a high coefficient of determination), where Rf0 is the zero-current Faradic resistance and alpha is a constant that describes the manner in which Rf decreases with increasing current (i). It was found that chlorided silver exhibited the lowest Rf0; removing the chloride deposit increased Rf0 by more than sixfold. Likewise, chloriding tin reduced Rf0 by a factor of about 2. Electrolytically cleaning an electrode reduced Rf0. The highest value for Rf0 was for carbon. This paper concludes with a summary of the data for Rf0 scaled to 1 cm2 electrode area for the electrode materials measured in the present study and data from the published literature.

A three-part analysis was undertaken to increase understanding of the occurrence of pressure ulcers in lithotomy positions. An innovative measuring device was used to determine capillary pressure. Ankle blood pressure was measured compared to ankle height in 11 participants. Ankle systolic and diastolic pressure decreased approximately 20 mmHg per foot of elevation. Calf and heel capillary-support pressures were measured in 15 participants in the standard lithotomy position. Capillary-support pressure for the calf was substantially less than for the heel. Heel capillary-support pressures were measured in 16 participants in the high lithotomy position. As heel height increased, capillary-support pressure also increased.

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