The present invention relates to improvements in electrical connections and methods of manufacturing them.

It is particularly concerned with small scale electrical connections which could have a variety of applications including the electrical energization of an ultrasound catheter of the kind disclosed in our UK Patent Nos. 2208138, 2212267, 2246632 and US Patent No. 5081993, for example.

In such a catheter electrical signals are transmitted from the proximal end (the end near the surgeon) to the distal end (the end located within the patient) to energize a piezo-electric transducer array to thus generate ultrasonic signals. These signals are radiated from the transducer array and echos caused by contact with body tissue, eg. the inside of an artery, are transmitted back from the transducer array to the proximal end for processing to thus provide a visual representation of the interior of the relevant organ, eg. an artery. This can be done, for example

only, in the way disclosed in our UK Patent No. 2212267. The end result is a picture on a cathode ray tube, for example, of the relevant organ being "viewed" by the transducer array.

For such an application it is necessary for the catheter, and therefore the transducer array at its distal end, to have a relatively very small outside diameter, typically 1.6 mm. Because of this very small size it is difficult to provide the relatively large number of electrical conducting paths required to enable the multi-element transducer array to be energized and the subsequent echos received. This is particularly so in cases where, in order to obtain a high resolution image, the number of transducer elements can typically be thirty-two, and may be as many as sixty-four.

The present invention relates to a miniature coaxial cable which is suitable for use in providing the electrical connection between the proximal and the distal ends of the catheter for the energization of an ultrasound transducer array located at the distal end of that catheter. The present invention also relates

to a process for manufacturing such a miniature coaxial cable.

Although the present invention has particular application in connection with ultrasound catheters it also has general application outside this field.

According to the present invention a coaxial cable has an external diameter no greater than 160 microns (0.16 mm) .

According to the present invention a coaxial cable has an outer conductor comprising a metallized layer carried on an insulating member.

According to the present invention an ultrasonic transducer assembly comprises a transducer array having electrically connected to it a plurality of coaxial cables each one of which has an external diameter no greater than 160 microns (0.16 mm).

According to the present invention a catheter has an ultrasonic transducer array mounted at one end which array is electrically connected to the other end of the catheter by a plurality of electrical conductors

each conductor comprising a coaxial cable, with an external diameter no greater than 1.6 microns (0.16 mm) .

According to the present invention the process for manufacturing a micro-coaxial cable includes the steps of:

(a) Locating an elongated core member or filament within a hollow tubular member or coating surface on the tubular member the latter being made of a material to be coated or deposited on to the said core member or filament;

(b) Evacuating the space between the core member or filament and the hollow tubular member; and

(c) Applying an electrical potential difference between the core member or filament and the hollow member in order to cause the material of the hollow tubular member or coating surface thereon to be sputtered radial inwardly on to the core member or filament.

With this arrangement the material of the tubular member or coating surface, which acts as the cathode in the above process, is selected to be that of the layer to be deposited on the core. It thus forms a sacrificial vacuum chamber itself or a sacrificial lining to the vacuum chamber.

In the case of the manufacture of a coaxial cable according to the present invention, the elongated core member or filament would initially comprise the inner conductor of the cable, eg. a fine copper wire. This wire could be wound from one spool to another spool through the tubular vacuum chamber formed by the sacrificial cathode comprising the tubular member or an inner coating thereon.

The various different conducting and insulating layers required to form the finished coaxial cable could be successively ie. serially, deposited on to the moving core or filament as it passes through the sacrificial vacuum chamber. By this means a continuous process could be provided.

The length of the tubular sacrificial vacuum chamber of cathode would be selected to meet the specific requirements, eg. deposition rates, of the particular application or applications. In some cases it could result in the sacrificial tubular cathode being of considerable length, eg. up to say around 50 metres.

In order to manufacture a catheter incorporating a plurality (typically thirty-two) of micro-coaxial cables running its length the above described method could be applied in a batch process rather than in the continuous process already described earlier in the case where a single length of micro-coaxial cable is being manufactured.

With such a batch process a plurality (eg. thirty-two) of central filaments would be arranged substantially parallel to one another and in a circular configuration within the sacrificial tubular cathode. Each filament would then be sputtered with the material forming the sacrificial cathode or lining thereof. In such a batch process there would also be a central anode around which the plurality of filaments would be located.

How the invention may be carried out will now be described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a part longitudinal section of a finished transducer arrangement as disclosed in our co-pending patent application No. 911647;.

Figure 2 is a perspective diagrammatical view, partly in section, of the transducer arrangement of Figure i;

Figure 3 is a section taken on the line C-C of Figure 2;

Figures 4 to 6 are cross-sectional views of catheters incorporating coaxial cables constructed according to the present invention;

Figure 7 is a fragmentary perspective view showing a section of a coaxial cable constructed according to the present invention which can be used in the kind of transducer arrangement shown in Figures 1 to 6; and

Figure 8 relates to a process and apparatus for continously manufacturing a micro-coaxial cable according to the present invention; and

Figure 9 relates to a process and apparatus for manufacturing a catheter as shown in Figure 7.

Figures 1 to 3

Reference is made to our co-pending UK Patent Application No. 9116478 and corresponding International Application No. GB 92/01412 for a disclosure of the kind of catheter transducer array to which the present invention can be applied.

In order to energize the transducer array the segments of it require to be electrically connected to the proximal end of the catheter. The present invention provides means for electrically interconnecting the elements of the miniature ultrasound array situated at the catheter tip (distal end) to the corresponding pins on a proximal multi-way connector plug the latter then being connected to electronic signal processing hardware and software in the manner disclosed, for example, in our UK Patent No. 2212267.

These electrical connections must be such as to reduce electrical cross-talk between adjacent interconnections in the confined space available within the catheter body. Another requirement is to provide a well defined electrical transmission line for the transmission of signals between the generation/reception unit (transceiver unit) at the proximal end of the catheter on the one hand and the ultrasound array at the distal end on the other hand and vice versa.

In addition the invention is concerned with a method of providing a low resistance grounded sheath around the interconnections both individual and collectively in order to reduce signal transmission losses and in order to provide adequate radio frequency screening of the whole catheter. The low resistance earth is required because if it is high then the transmission line properties and screening will be degraded.

Referring firstly to Figs . 1 to 3 which show a finished transducer arrangement of the kind to which the present invention may be applied.

The transducer arrangement comprises sixty-four transducer elements 100 made of PZT and arranged in an annular formation and supported on an acoustic backing layer 101 which in turn is carried by a metallised tungsten carbide tubular support or collar 102.

The transducer array 100 is sandwiched between two locating sleeves 103,104 also carried by the collar 102.

Adjacent the sleeve 104 is an annular wire retainer member 105 made by injection moulding from a biocompatible polymer such as ABS so that it has a certain amount of resilience. The member 105 is stepped at its internal diameter to form a first portion 105a which grips the end of the collar 102 and a second portion 105b of smaller internal diameter which grips the outside of the distal end of the inner body 106 of a catheter tube, the catheter having an outer body 107. The inner body 106 is made of a plastic material and has helical or braided metal reinforcement 108 to give it the required tensile strength to enable the catheter as a whole to be extracted from a patient after use.

The transducer array 100 is energised through and resulting echo signals transmitted back by means of four ribbon cables 109 each one of which consists of eight leads to give a total of thirty-two leads .

The distal end 110 of each lead is stripped of insulation and soldered at 111 to bond pad 112 formed on the stepped down (reduced diameter) portion of the rear sleeve 104.

The stripped ends of the wires are located in slots in the wire retainer 105 and gripped by . the sides defining those slots.

The distal end of the outer body 107 of the catheter encloses and grips the insulated distal ends of the ribbon conductors 109. A sealing compound 113 encapsulates the bare ends of the conductors and together with the outside of the outer body 107 forms a continuous smooth external surface.

An acoustically transparent quarter-wave matching layer 114 encapsulates the transducer elements 100 to not only isolate them from the patient but to

acoustically match the vibrating transducer elements to the body fluid of the patient, e.g blood.

The end of the transducer arrangement/assembly is provided with a rounded annular end cap 115 to give the distal end of the catheter a smooth rounded end in order to facilitate its insertion into the patient's artery.

It will be appreciated that given the very small size of the transducer arrangement and the very limited amount of space between the inner and outer bodies of the catheter there is a potential problem in being able to provide enough electrical leads to energise all the transducer elements and carry signals representative of the ultrasonic echoes .

The desired frequency of vibration of the transducer elements in the radially outer direction is about 20 MHz in this embodiment. To achieve this the transducer annulus requires to be a particular thickness for a given material, in this case about 75μm thick.

With this embodiment there are thirty-two electrical leads from the proximal to the distal end of the catheter to energise the transducer array and to transmit the echo signal. If the annulus 100 were to be divided into thirty-two segments, to match the number of leads, it would result in each segment being a relatively low depth to width ratio (referred to as the aspect-ratio) of roughly 80:120. As a result there would be a significant lateral or tangential ultrasonic signal at ninety degrees to the desired radial signal which lateral signal would interfere with the radial signal.

In order to eliminate this interference or at least reduce it to an acceptable level the depth of the discrete transducer elements needs to be significantly greater than their width.

To achieve this each transducer element is divided into two and by this means provides each element with the required aspect ratio to ensure that any lateral signal is not significant. Depending upon the geometry and size of the transducer arrangement and the manufacturing technology available each element

may be divided into a greater number than two to give the desired performance.

Each of the thirty-two leads energises a pair of transducer elements, typically through a single associated conducting bond pad 112 the elements of a pair being separated by a slot 117 and the pair itself being separated from the two adjacent pairs by slots 116.

Both the slots 116 and the slots 117 extend the length of the cylindrical PZT transducer member 100 but the slots 116 in addition extend into the rear sleeve 104.

The portions of the slots 116 in the member 100 are 0.13mm deep and 0.025mm wide whereas the slots 117 are only 0.09mm deep but the same width.

The description so far and Figures 1 to 3 are substantially as disclosed in our copending Application No. 9116478 and International Application No. GB92/01412.

In the construction so far described the electrical conductors 109 are in the form of four ribbon cables, each ribbon cable including eight separate conductors 109, as shown in Figure 3.

The present invention is concerned with not only providing micro-coaxial cables as such but in replacing the ribbon cables shown in Figure 3 with micro-coaxial cables according to the present invention.

Figures 4 to 6 are similar views to Figure 3 but showing the ribbon cables of Figure 3 replaced by micro-coaxial cables according to the present invention.

Figures 4 to 6 show three possible arrangements of those coaxial cables but the present invention is not limited to any of these arrangements. The same reference numerals have been used in Figures 4 to 6 to designate equivalent elements to those shown in Figure 3.

Figure 7 shows in a greatly enlarged fragmentary perspective view the construction of a coaxial cable according to the present invention which can be used in the catheter/transducer arrangement of Figures 1 to 3 in the place of the electrical conductors 109.

As indicated earlier in connection with Figures 1 to 3, each pair or group of transducer elements 100 which form a single electrical transmission channel is electrically connected to the other proximal end of the catheter by means of a single coaxial cable to form coaxial "transmission lines". The ultrasound catheter may have thirty-two, forty-eight, or sixty-four or more such coaxial interconnections. The number is determined by the number of electrical channels required for the particular application and limited by the overall dimensions of the catheter body. In this case there are sixty-four transducer elements and thirty-two channels .

Each of these miniature coaxial cables comprises a central inner copper conductor 200 surrounded by an inner insulating layer of material 210 which in turn carries the outer coaxial conductor 220. There may also be an outer insulating layer 230 for the cable

itself when used individually. However, when used in a catheter this outer layer 230 is not required.

The inner coaxial conductor 200, in this example is a copper tin alloy and has a diameter of 0.04 mm. The inner insulating layer 210 has an outer diameter of

0.105 and is made from PFA. This is a co-polymer of tetrafluroethylene and perfluroalchyvinylether which is one of a group of fluorinated polymers of low permitivity. An alternative material is FEP. The outer coaxial conductor 220 has an external diameter of 0.110 to 0.115 and is made of electroless copper.

The outer insulating layer 230 has an outside diameter of 0.125 mm and is made of Parylene (Registered Trade Mark) which can be deposited by a chemical vapour deposition (CVD) process.

To reduce transmission line losses , and to give scope for adjusting the characteristic impedance of the line, the inner conductor 200 will be of low

resistivity, eg. less than 2.2 x 10 —8 Ω m, and the insulation will be of low dielectric constant and low dielectric loss for high frequency applications, eg.

relative permittivity of less than 2.1 and a dielectric loss tangent of less than 0.001.

The outer coaxial conductor 220 is formed as a metal coating on the exterior of the inner insulating layer 210. There are a number of ways in which this outer coaxial conductor can be formed one of which is by electroless plating as indicated earlier. Alternatives could be by means of electroplating, after deposition of a seed layer, or by vacuum sputtering or by other physical vapour deposition processes.

In order to provide good adhesion of the metal layer 220, or layers, of the conductor to the insulator 210 the surface of that conductor is activated either chemically or physically. For example, in the electroless deposition of copper on to the PFA insulated mono-filament 210 chemical etching of the PFA grain boundaries enhances the physical adhesion of the copper to the insulating material.

As an alternative corona-etching may also be used in order to activate the surface of inert polymers (PTFE or PFA) .

In the case of vacuum sputtering, adhesion of a conducting material such as copper, silver or gold to a polymeric insulation is enhanced by the deposition of a seed-layer of chromium, nickel-chromium or stainless steel.

Protective sputtered layers may be used to reduce the oxidation of the copper.

Electroless deposition of a seed layer of copper is followed by electroplating. This is the preferred metallization route on the grounds of cost. However, vacuum sputtered multi-layers may be more physically flexible.

Electroplating can be used to thicken an existing metallic seed-layer. Electroplated layers tend to be more brittle than electroless or vacuum deposited layers .

In this example the metallization outer coaxial conductor 220 is in the range 0.001 to 0.004 mm thick in the case where the ground return resistance is determined by thirty-two such metallized layers in parallel. Thus, for example, if each interconnection

has a resistance of thirty-two ohms per metre in the outer coaxial conductor 220 then the resistance of the assembly of thiry-two such interconnections in parallel is reduced to one Ohm per metre if the coaxial cables have no individual outer insulation and are in contact with one another (See the later description in relation to Figures 4 and 5).

In order to promote electrical contact between the outer conductors 220 of the individual coaxial connections an additional helical winding of bare copper foil, or strip (not shown), may be used around the total assembly of thirty-two interconnections.

This winding (not shown) binds the interconnects to the inner catheter body and increases the number of physical contact points between individual interconnects and provides an additional electrical ground return. The interconnections themselves can be arranged to run parallel to the catheter axis between the inner and outer bodies or be wound in a loose helical arrangement to give optimum flexibility to the whole catheter assembly (see Figures 4 and 5).

In the arrangement just described (but not shown in Fig 4) all the thirty-two outer coaxial conductors 120 are electrically interconnected and because of this the electrical resistance of each outer conductor is in effect reduced by a factor of thirty-two as indicated earlier.

However, if the arrangement is such that the miniature coaxial cables are to be used individually then a lower resistance metallization may be desirable in order to reduce resistive losses. This may be required in the arrangements shown in Figure 6 or in non-catheter applications of the invention.

In this case, for example, up to 0.01 mm thick metallization would be preferable. In addition the external insulating covering 230 will be necessary to protect the outer conductor 220 from oxidation and physical damage. As indicated earlier this protective coating could be formed by the deposition of Parylene (Registered Trade Mark) in layers 0.001 to 0.05 mm thick, preferably 0.001 to 0.005 mm thick.

A typically coaxial interconnection cable for an intravascular catheter could have the following dimensions and properties:-

Inner conductor:

Material: copper-tin alloy

Tensile Strength > 600 MPa

Diameter: 0.04 mm 8

Resistivity < 2.2 x 10 Ωm

Insulation:

Material: PFA of FEP Outer Diameter: 0.105 mm Relative permittivity: 2.01 Dielectric Loss Tangent: 0.001

Outer Conductor: A sputtered seed layer followed by and electroplated bulk layer

Material: Electroless copper Thickness: 0.001 mm to 0.004 mm

Outer Insulator:

None

Several methods for providing a low resistance ground return are possible. These methods include filling the space between the inner and outer bodies of the catheter with conducting ink, graphite, silver-loaded silicon rubber or silver epoxy resin or the combination of any such conducting fillers with a bare copper counter winding.

As indicated earlier the basic method of manufacturing the miniature coaxial cable according to the present invention involves forming the outer coaxial conductor as a metallic layer on the inner insulating member 210. In order to achieve this the following factors have to be borne in mind, or steps taken during the manufacture of the coaxial cable.

As indicated earlier the inner insulating member 210 comprises a copper alloy conductor coated with PFA insulation. The outer surface of this insulating member has to be prepared prior to the metallization step in order to give it the required surface properties . The properties which are of particular importance are its physical integrity, chemical composition and macro-molecular structure (eg. crystallinity) . Heat annealing can be used to modify the physical integrity and macro-molecular structure in order to give a uniform substrate without deleterious locked-in stresses. Heat annealing may be applied to both the seed layer as well as to the bulk layer.

Prior to the metallization step the substrate surface is treated to improve the binding of the copper of the outer coaxial conductor 220 to the PFA inner insulating member 210. In order to do this air or water abrasion and/or chemical etching is used to alter the substrate surface mechanically and chemically in order to facilitate the attachment of the metallisation outer coaxial conductor 220 to it. Any appropriate physical or chemical surface

activation, eg. sputter etching, chemical etching abrasion, can be used.

The metallization step itself is achieved by using a "seed-layer" which is put down on the outer surface of the insulating material 210 by sputtering. This initial step is followed by electrolytic deposition of the copper. The "seed-layer" may include metals other than copper.

The final copper surface can then be burnished to increase its conductivity and to stress relieve it.

A coating of tin-lead alloy may then be applied to the copper surface to ease the assembly of the coaxial cables into the catheter body.This coating forms a low-friction coating as well as a surface protection coating.

Figures 4 and 5

These Figures show, in cross-section, a number of different embodiments/configurations of micro-coaxial cables constructed according to the present invention and located in the space 300 which exists between the

inner body 106 and the outer body 107 of a catheter. As indicated earlier equivalent elements to those of Figure 3 have the same reference numerals.

Each of the coaxial cables 109 is of the construction shown in Figure 7 but without the outer insulating layer 230.

In order to ensure that the earth return resistance is below a required level it is arranged that the plurality of coaxial cables shown in each of the embodiments of Figures 4 and 5 either do or will contact one another. This contact may be achieved as a result of the micro-coaxial cables following a twisting configuration within the space 300. Thus with such an arrangement it is not necessary to have the external insulation layer 230.

In contrast in the embodiment of Figure 6, because the coaxial cables are actually embedded in the material of the catheter body it is necessary to employ a somewhat different arrangement for ensuring that the earth return has a value below the aforementioned predetermined value. This could be achieved by interconnecting them in a manner other than relying on

contact between adjacent coaxial cables. As an alternative a thicker earth coating would be required on each cable in order to get the assembly down to the desired resistance as described earlier.

Figures 8 and 9

These figures show diagrammatically a continuous process (Figure 8) and a batch process (Figure 9) for manufacturing a micro-coaxial cable according to the present invention (Figure 8) and a micro-coaxial cable array for incorporation in a catheter (Figure 9).

Referring to Figure 8, which is a purely diagrammatic representation of the process according to the present invention for continuously manufacturing a micro-coaxial cable, also according to the present invention.

An elongated core member or filament, in this case in the form of a very fine copper wire 400 is wound from a first spool 401 on to a second spool 402 in the direction indicated by the arrow.

In moving from the spool 401 to the spool 402 the wire or filament 400 passes through a hollow tubular member 403 which is either itself formed from material to be deposited on to the filament 400 or alternatively supports an inner lining 404 made of such a material.

The tubular hollow member 403 is part of a vacuum chamber which includes two sub-chambers 405 and 406 which house the spools 401 and 402 respectively. In an alternative construction the spools could be located outside the vacuum chamber as a whole with appropriate seals being provided for where the filament 400 enters and leaves the vacuum chamber.

The required vacuum is maintained within the vacuum chamber by means of pumps 407 and 408.

Means are provided (not shown) for applying an electrical potential difference between the tubular member 403 and the filament 400 so that the tubular member 403 or its lining 404 will act as a sacrificial anode in a sputtering process by which the material of the member 403 or its lining 404 is transferred

radially inwardly on to the surface of the filament 400, in known manner.

With the kind of process and apparatus disclosed in Figure 8 it is possible to provide a continuous manufacturing processes for a micro-coaxial cable. This is achieved by constructing the tubular cathode 403 of a sufficient length, and with appropriate different linings 404 positioned successively one after the other within the tubular member 403. With this arrangement there is thus formed a series of successive zones each zone being designed to carry out a specific operation in relation to the travelling filament 400.

For example the first zone could be a cleaning zone by which the filament 400 is cleaned immediately after it leaves the spool 401. There could then follow any number of sputtering and/or cleaning stages by which one or more layers are deposited on to the central filament 400. These possibilities are indicated diagrammatically in Figure 8 by "layer 1", "layer 2" and "layer n" .

It must be emphasized that Figure 8 is merely a diagrammatic representation of one possible way in which a process according to the present invention could be realized.

Referring to Figure 9, this show diagrammatically and in cross-section the application of the process according to the present invention to the formation of a plurality of micro-coaxial cables, arranged in a circular configuration, and suitable for use in the manufacture of a catheter having the construction already described earlier, for example in relation to a construction similar to that shown in Figure 5A.

Whereas the process previously described with reference to Figure 8 is a continuous process the process illustrated diagrammatically in Figure 9 is a batch process.

In this batch process the desired array of mono-filaments 500 are arranged in a circular configuration around a central stainless steel anode 501. This arrangement is located coaxially within an outer sacrificial hollow tubular cathode 502, in a similar manner to that described with reference to

Figure 8. As with the arrangement of Figure 8, instead of the outer tubular member 502 itself being the sacrificial cathode it could instead be provided with an inner lining 503 which is made of the material which it is desired to deposit on to the mono-filaments 500.

In Figure 9 there is also provided a cylindrical stainless steel mesh 504 located between the circular array of mono-filaments 500 and the outer cylindrical cathode 502.

The array of mono-filaments 500 would be held in a jig (not shown) and could be subjected to a succession of sputtering operations in order to deposit different materials successively. This would be done by having different outer cylindrical sacrificial cathodes 502 (or associated sacrificial linings 503).

The function of the stainless steel mesh 500 is to permit sputter etching prior to the actual sputtering of the material which is desired to deposit on the mono-filaments 500.

Figure 9 is merely a very diagrammatic representation of a batch process incorporating the present invention and other arrangements could be employed.