LOW RESISTANCE ELECTRICAL PICK-UP

Abstract: A low resistance electrical pick-up, such as a fetal scalp electrode (17), comprises a stainless steel wire (10), with an arcuate needle (18) on the outer end of a radial arm (19), the arcuate needle (18) being plated with nickel (14) and then silver (15), optionally with an intervening layer of copper (16), and then an area (11) of the silver layer (15) at least on the outside of the curve of the needle (18), and possibly spaced from the point of the needle, is chloridized and then coated with bio-compatible plastics (12).

Description: LCW RESISTANCE ELECTRICAL PICK-UP This invention relates to a low resistance electrical pickup - more particularly - but not exclusively - for use in a fetal scalp electrode or other biosensing electrode. It is known that a chloridized silver wire can constitute a low resistance electrical pick-up, but the silver chloride is not bio-compatible. The primary object of the invention is to overcome this difficulty. Another object is to provide a fetal scalp electrode or other biosensing electrode with a low resistance electrical pick-up capable of subcutaneous introduction. According to the present invention, a low resistance electrical pick-up comprises a length of wire having an area of its surface coated with silver chloride and the area of silver chloride coated ith a bio-compatible plastics material. Such a material is that known under the registered trade mark "Tecoflex". The wire may be a silver wire having a length chloridized and then coated with bio-compatible plastics over the chloridized length, and any further non-chlor idized length or lengths of the wire may also be coated with the bio-compatible plastics; alternatively, any further length or lengths of the wire may be coated with a non-conducting varnish. According to an aspect of the invention of independent significance, a low resistance electrical pick-up comprises a length of stainless steel wire having a first coating layer of nickel, and a subsequent coating layer of silver, optionally with an intervening layer of copper, an area of the silver layer being chloridized and then coated with bio- compatible plastics material, e.g., "Tecoflex". A further length or lengths of non-chloridized silver layer may be coated with a non-conducting varnish. In a particular application of the invention to a fetal scalp electrode or other biosensing electrode of the type having an arcuate needle of stainless steel - which may serve as a reference electrode and/or measuring electrode and/or introducer for a separate electrode - the arcuate needle is plated with nickel and then silver, optionally with an intervening layer of copper, and an area of the silver layer at least on the outside of the curve of the needle, and possibly spaced from the point of the needle, is chloridized and then coated ith bio-compatible plastics, and the remainder of the silver layer is coated with a non-conducting varnish. The coating of the chloridized silver area can be given a thin coating of "Tecoflex" by dipping it into a solution of "Tecoflex" in tetrahydrof uran (THF) which is allowed to set (approximately 12 to 24 hours) before the remaining areas of the electrode are painted ith a non¬ conducting varnish. A number of embodiments of the invention will now be described by way of example only, with reference to the accompanying diagrammatic drawings, in which:- Figure 1 is a fragmentary enlarged side elevation of a basic low resistance electrical pick-up in accordance with the invention; Figure 2 is an even greater enlarged cross-section on the line II-II of Figure 1; Figure 3 corresponds to Figure 1 but illustrates an alternative basic embodiment of the invention; Figure 4 also corresponds to Figure 1 but illustrates a preferred embodiment of the invention; Figure 5 is an enlarged cross-section on the line V-V of Figure 4; Figure 6 corresponds to Figure 5 but includes a slight modification; Figure 7 is a greatly enlarged skeletal perspective view of the head member of a "Sur gicraf t-Copeland" fetal scalp electrode (FSE) including a low resistance electrical pick-up in accordance with the invention; Figure 8 is a fragmentary ECG trace obtained using a FSE as in Figure 7; and Figure 9 is a fragmentary ECG trace obtained with an un-modified "Surgi craft- Co pel and" FSE for comparison with the trace of Figure 8, the paper speed in Figure 8 being twice that in Fi gur e 9. The thickness of the coating layers in Figures 1 to 6 have been greatly exaggerated, even compared to the enlarged scale, for the sake of clarity. In Figures 1 and 2, a low resistance electrical pick¬ up comprises a length of wire 10 having an area 11 of its surface coated with silver chloride and the area of silver chloride coated with a bio-compatible plastics material 12, such as that known under the registered trade mark "Tecoflex". In Figure 3 r a length of silver wire 10 has a length 11 chloridized and then coated with bio-compatible plastics 12 over the chloridized length. Any further non-chloridized length (or area) or lengths 13 of the wire 10 in either of the above embodiments may be coated with a non-conducting varnish. However, the preferred embodiment illustrated by Figures 4 and 5 comprises a length of stainless steel wire 10 having a first coating layer 14 of nickel, and a subsequent coating layer 15 of silver, an area (or length) 11 of the silver layer being chloridised (indicated in Figure 4 by the thicker broken line) and then coated ith bio-compatible plastics material, e.g., "Tecoflex". Further lengths 13 of non-chloridized silver layer 15, and preferably also adjacent exposed lengths of the nickel layer 14 and stainless steel wire 10 may be coated with a non-conducting varnish. Figure 6 corresponds closely to Figure 5 but indicates a layer 16 of copper between the nickel layer 14 and the silver layer 15. In Figure 7 f the head member 17 of a "Surgicraft- Copeland" FSE, as described in GB-PS 1316 072 or GB-PS 1 523 263 (or US-PS 4151 835) , has an arcuate needle 18 formed on the outer end of a radial arm 19 from a main stainless steel wire 10, the arcuate needle 18 being plated with nickel 14 and then silver 15 (optionally with an intervening copper layer, not shown, but refer to Figure 6) , an area (not shown) of the silver layer at least on the outside of the curve of the needle 18, and possibly spaced from the point of the needle, is chloridized and then coated with bio-compatible plastics 12, and the remainder of the silver layer is coated with a non-conducting varnish. The fundamental advantage of the low resistance "Surgicraf t-Copeland" FSE of Figure 7 can be appreciated by comparing Figures 8 and 9. The bare stainless steel arcuate needle electrode of the unmodified "Surgicraf t-Copeland" FSE is highly resistant to low frequency electrical activity, so the trace of Figure 9 is erratic, and it also shows enormous variability because of the added effect of f eto-maternal movement. In contrast, the trace of Figure 8 shows that low frequency electrical activity from the fetus can be picked up with great accuracy by the low resistance "surgicraft- Copeland" FSE of Figure 7, and consistent waveforms (P,QRS and T waves) of the cardiac cycle can be more easily identified. This allows more complex fetal ECG waveform analysis and reduces the need for extensive signal processing and averaging, as has been necessary with the unmodified "Surgicraf t-Copeland" FSE. The invention is also applicable to other types of fetal scalp electrodes, such as the spiral electrodes of Corometrics Medical Systems Inc. US Re 28990, the clip-like electrodes of T.C. Neward EP 0 007 702, and the outwardly splayed electrodes of Kontron Inc. EP 0 099077, or to other electrodes or probes, such as the outwardly splayed anchor needles in the cardiac probe of . Mohl EP 0 004 967.

Claims: CLAIMS 1. A low resistance electrical pick-up comprising a length of wire having an area of its surface coated with silver chloride and the area of silver chloride coated with a bio-compatible plastics material. 2. A low resistance electrical pick-up as in Claim 1, wherein the wire is a silver wire having a length chloridized and then coated with bio-compatible plastics over the chloridized length. 3. low resistance electrical pick-upas in Claim 2, wherein, any further non-chloridized length or lengths of the wire is/are coated with the bio-compatible plastics. 4. low resistance electrical pick-u as in Claim 2, wherein any further non-clor idized length or lengths of the wire is/are coated with a non-conducting varnish. 5. A low resistance electrical pick-up comprising a length of stainless steel wire having a first coating layer of nickel, and a subsequent coating layer of silver, an area of the silver layer being chloridized and then coated with bio-compatible plastics material. 6. A low resistance electrical pick-up as in Claim 5, wherein an intervening layer of copper is provided between the nickel and silver layers. 7. A low resistance electrical pick-up as in Claim 5 or Claim 6, wherein a further length or lengths of non- chloridized silver layer is/are coated with a non-conducting varnish. 8. A fetal scalp electrode or other biosensing electrode of the type having an arcuate needle of stainless steel, wherein the arcuate needle is plated with nickel and then silver and an area of the silver layer at least on the outside of the curve of the needle, is chloridized and then coated with bio-compatible plastics, and the remainder of the silver layer is coated with a non-conducting varnish. 9. A fetal scalp electrode or other biosensing electrode as in Claim 8, wherein an intervening layer of copper is provided between the nickel and silver layers. 10. A low resistance electrical pick-up substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings. 11. A low resistance electrical pick-up substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings. 12. A low resistance electrical pick-up substantially as hereinbefore described with reference to Figures 4 and 5 of the accompanying drawings. 13. A low resistance electrical pick-up substantially as hereinbefore described with reference to Figure 6 of the accompanying drawings. 14. A fetal scalp electrode substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.

Inventors: BAND DAVID MARSTON, PENMAN DAVID GERARD, BAND, DAVID, MARSTON, PENMAN, DAVID, GERARD
Applicants: SURGICARFT LTD, SURGICRAFT LIMITED
Application year: 1989
Main classification: A61B5
IPC codes: A61B5