WO2003026810A1

WO2003026810A1 - Apparatus and method of manufacturing ultrasonic transducers

Info

Publication number: WO2003026810A1
Application number: PCT/GB2002/004379
Authority: WO
Inventors: George Bromfield; Gordon Warnham
Original assignee: The Morgan Crucible Company Plc
Priority date: 2001-09-27
Filing date: 2002-09-27
Publication date: 2003-04-03
Also published as: GB2381691A; GB0222520D0; GB2381691B

Abstract

A transducer has a piezoelectric element (1) lying between opposed parts of a pre-compression system. This system comprises a composite (4) having a matrix and fibres within the matrix aligned in the direction of the pre-compression force, giving high compliance in the system, whereby disturbances to the separation of the opposed parts in use do not significantly disrupt the pre-compression.

Description

APPARATUS AND METHOD OF MANUFACTURING ULTRASONIC TRANSDUCERS

Field of Invention

The present invention relates to surgical instruments and, in general, to apparatus and methods of manufacturing high power sandwich type ultrasonic transducers. One aspect of this invention is a new method of applying pre-load compression stress to the piezoelectric elements. Another aspect of this invention is a method of assembly that ensures that ultrasonic surgical instruments can only be used for one single-use medical procedure. Other aspects are design features and an assembly method that significantly reduces the cost of manufacture.

General Discussion

Hand-held surgical instruments based on piezoelectric transducers are used in various medical and dental fields, such as phacoemulsification and liposuction. Ultrasonic instruments are advantageous because they may be used to cut, coagulate, emulsify, and dissect organic tissue using energy in the form of mechanical vibrations transmitted to a surgical end effector at ultrasonic frequencies at suitable energy levels and using suitable end effectors (needles, probes, blades).

Ultrasonic vibration is induced in the surgical end effector by, for example, electrically exciting a transducer which may be constructed of one or more piezoelectric elements in the instrument handpiece. Nibrations generated by the transducer section are transmitted to the surgical end effector via an ultrasonic waveguide extending from the transducer section to the surgical end-effector.

Sandwich type ultrasonic transducers, also called Langevin transducers, are well known and established for the production of high intensity ultrasonic motion. In United

Kingdom- Patent No. 145,691, issued in 1921, P. Langevin inventor, a sandwich of piezoelectric material positioned between metal plates is described to generate high intensity ultrasound. Sandwich transducers utilizing a bolted stack transducer tuned to a resonant frequency and designed to a half wavelength of the resonant frequency are described in United Kingdom Patent No. 868,784.

High-intensity ultrasonic transducers of the composite or sandwich type typically include front and rear mass members with alternating annular piezoelectric elements and electrodes stacked therebetween. They employ a compression bolt that extends axially through the stack to place a static bias compressive force such that when the transducers operate they always remain in compression, swinging from a value of minimum compression to a maximum peak.

Applying a static bias compressive force to the composite or sandwich transducer using a metal bolt or bolts, has several disadvantages. Metal bolts generally have a high modulus of elasticity and the application of the compressive force results in only minimal extension of the length of the bolt. During high power operation of the transducer there is a compaction of components that results in a small reduction in the overall length of the assembly. Also during high power operation the transducer generates heat both within the piezoelectric ceramic elements and also within the joints between each component. The heat is transmitted to the metal compression bolt that extends axially through the stack and causes the bolt to expand and increase in length. Typically, the value of compressive force applied to a new transducer assembly at build is reduced by a factor of approximately 50% after sustained periods of operation at high power. During subsequent operations at high power the compressive force will be further reduced as the bolt increases in length due to heat generated within the sandwich transducer elements. Other embodiments of the prior art utilize a stud that is threadedly engaged with both the first and second resonator to provide compressive forces to the transducer stack, but with the same problems. Threaded studs are also known in the prior art for attaching and detaching transmission components (needles, blades, anvils) to the transducer assembly. See, for example, U.S. Pat. Nos. 5,324,299 and 5,746,756. Such bolts and studs are utilized to maintain acoustic coupling between elements of the sandwich type transducer or any attached acoustic assembly. Coupling is important to maintain tuning of the assembly, allowing the assembly to be driven in resonance.

Sandwich type transducers for surgical instruments and industrial use generally are relatively high Q devices (100 or over), and during operation are driven at resonance, and maintained within a relatively narrow frequency range by feedback control methods known in the art. See, for example, U.S. Pat. Nos. 5,630,420 and 5,026,387 which describe systems incorporating and controlling sandwich type transducers. Sonar transducers in contrast operate over a wide frequency and therefore have a low Q (10 or under). Controlling piezoelectric sandwich transducers for surgical applications is particularly difficult as these instruments are usually steam sterilised prior to use. The high temperature and subsequent cool down period causes significant changes to the resonant frequency and impedance of the transducer such that it is difficult to maintain a precise level of vibration amplitude within the end effector. The frequency range of new transducers as manufactured has to be accordingly reduced to compensate for the large changes due to the operational environment.

It is difficult and expensive to manufacture sandwich type transducers for surgical applications due to the steam sterilisation method and the high Q/narrow resonance range in which these devices operate. It is common to individually tune every transducer at least once during the manufacturing process. Even with the tight tolerances currently available with modern manufacturing processes, tolerance "stack-up" issues present challenges to designers of sandwich type transducers. "Stack-up" issues occur as normal variations due to combining multiple parts, each part having design tolerances, such that variations due to each part sum together to produce a significant variation.

It is also expensive to assemble and test sandwich type transducers for multiple-use surgical procedures that require steam sterilisation prior to each use. The piezoceramic elements have to be hermetically sealed within a pressure vessel that is penetrated by electrical cables, lumens and the waveguide. For certain ultrasonic surgical procedures it is necessary to provide a means of irrigation to, and aspiration of fluid from the surgical site. The irrigation and aspiration lumens are usually an integral part of the transducer assembly. The horn is typically made of titanium, as this does not cause any reaction with operative or bodily fluids. However, titanium is a high cost material for use in a single use device. A transducer horn made from a relatively inexpensive material, such as aluminium, would be more appropriate.

A more compliant and precise mechanism for applying the static bias compressive force to the composite of sandwich transducer that also eliminates torsional stress within the piezoceramic elements is desirable. There is also a need for single use low cost ultrasonic surgical instruments with design features that render the device inoperable should it be negligently or deliberately used for more than one surgical procedure.

The new technology set out herein has been developed in response to a need to both reduce the cost of manufacture and improve performance of high power transducers. The cost reduction measures are generally applicable to medical applications where the assembly of existing transducers is relatively complex and expensive. These applications include cataract fragmentation, scalpels, coagulators, dental sealers, suture welding, liposuction and soft tissue aspirators. The performance improvement technology is applicable to all high power transducers that currently use a metal bolt or stud to provide a pre-load stress to the piezo-ceramic crystals. Applications include industrial (ultrasonic welding and cleaning), navy sonar, commercial sonar, and medical. Performance improvements are derived from an accurate method of controlling and retaining amount of pre-load that is applied to the piezo-ceramic elements and an increase in coupling coefficient. Piezo-ceramic material is inherently weak in tension and therefore requires a mechanical bias force or pre-load to ensure it remains in compression. In the prior art, transducers that vibrate in a longitudinal mode have been pre-loaded using either internal or external torqued metal bolts. The present new method is intended to replace the metal bolt with a more compliant pre-compression system, advantageously in its simplest terms a fibreglass tube that is ideally manufactured using the pultrusion method. For the prior applications above, steel bolts are used to apply between 25MPa to 60MPa of stress to the piezo-ceramic. It is common practice to estimate or calculate the maximum peak to peak dynamic stress that will occur within each of the piezo-ceramics used in the assembly. The bias pre-load applied by the bolt is usually set to this value and therefore includes a safety factor of 2 with respect to the tensile forces with the assembly. This safety margin is needed to allow for seating of the components, thread creep etc. Applying this stress typically results in the steel bolt stretching by only 0.025mm. It is easy to see how only minor changes due to seating of components at high drive level can significantly reduce the level of pre-load stress, hi practice, measurements have confirmed that need for the safety factor of 2 as only approximately 50% of applied stress is retained after use. For some high power applications, applying the extra stress at build causes the piezo-ceramic properties to degrade significantly and adversely affects the performance of the transducer throughout its life.

Replacing the steel or other metal bolts with a pultruded fibreglass tube or rod has significant advantages. Young's Modulus for 'E glass' pultruded tube as described later herein is 38Gpa and is 207 GPa for high tensile steel and 117 GPa for titanium alloy. It follows that Ε glass' pultruded tube is 5.4 times less stiff or 5.4 times more compliant than high tensile steel. As an illustrative example, applying a pre-load of 25 MPa to the stack of a typical ultrasomc transducer with a high tensile steel bolt would result in a bolt extension of 20 microns. Substituting pultruded tube would result in an extension of 110 microns. During assembly and high power use there is some compaction of components. If the overall length of the transducer stack assembly compacts by 5 microns then the bolt extension relaxes to 15 microns and the stack pre-load is reduced to 18.75 MPa, a high proportion and showing the danger of loss of pre-load altogether during the operating stress cycle of the transducer. Using a pultruded tube the 5 microns stack compaction would result in the tube extension being reduced by a small proportion to 105 microns and the pre-load being correspondingly reduced only to 23.86 MPa. The initial level of preload stress can therefore be significantly reduced leading to improved performance. The yield stress for an 'E Glas' pultruded tube is approximately 760 MPa and this is comparable with steel that has a yield stress of approximately 1000 MPa. A more compliant pre-load member will also increase the coupling coefficient and band width of a sonar transducer, as discussed further later herein.

Applying piezo-ceramic pre-load using pultrusion tubes will eliminate or significantly reduce residual torsional stress and thereby reduce the risk of radial cracking during assembly. Maintaining a uniform longitudinal stress distribution within the piezo-ceramic is important for optimum high power performance and will eliminate the symptomatic 'wear rings' at the mating surfaces. When applying torque to a bolt there is also a tendency for components within the assembly to rotate and thereby degrade the mating surfaces, whereas pultrusion tubes are torqued by an in-line pull.

The ability to precisely adjust and maintain the level of pre-load within a piezoelectric transducer assembly effectively imposes a practical limit on the maximum velocity within the component parts of the transducer assembly. In practice it has been observed that the maximum tip displacement or velocity is effectively limited by increasing current distortion as the positive (tensile) dynamic stress cycle approaches and exceeds the preload bias stress. This drive condition is also usually characterized by excessive heating, sudden drop in resonant frequency.

Another way to reduce cost is to use a pultruded tube as the cylindrical part of the housing. This has considerable advantages because it provides insulation from the high potential connection within the transducer. The housing can also be used to apply compressive pre-load to the piezo-ceramic crystals.

For certain high power applications the heat generated within the piezo-ceramic crystals is the principal design limitation. An external tube used in conjunction with doughnut shaped piezo-ceramic crystal rings enables the hole through the centre to be used to both locate the ring and dissipate heat. The placement of a rod or slug of high thermal conductivity ceramic material like beryllia or high grade alumina can be used to both locate the piezo crystals and provide electrical insulation. In the prior art the metal centre bolts were relatively ineffective in dissipating heat because they needed to be electrically insulated from the inner surface of the piezo-ceramic crystals. The bolts are usually covered with a nylon sleeve or wrapped with polyimide tape. The thermal conductivity of a fibreglass pultruded tube is significantly better than nylon and polyimide and the assembly method shown in Figure 3 and Figure 5 therefore provides adequate heat conduction to the end masses.

The new technology has the potential to reduce the manufacture cost of high power medical transducers to a level whereby they can be used only once. This has considerable advantages because, for example, viruses causing diseases such as BSE or CJD are resistant to current sterilization procedures. In France for example the duration of the required steam autoclave procedure has been significantly increased and it is becoming increasing difficult to ensure that existing re-usable surgical transducers will operate reliably over multiple autoclave cycles. In view of other risks, including AIDS, it is believed that there is a premium in medical procedures that reduce the actual or perceived risk of infection.

A significant aspect of this new technology is thus a novel assembly method, that applies pre-load to the piezo-ceramic crystals using bonded components. Should an unscrupulous user attempt to steam autoclave the transducer the heat will cause the bonded joints to fail and the transducer will be rendered completely inoperable. Moreover, the damaged assembly will be beyond economic repair and the individual component parts will have only a nominal scrap value. The same safety factor can be built in even if a mechanically deformed joint is used, suitably chosen resins for the matrix of the composite softening at autoclave temperatures.

Statement of Invention

At its broadest the invention provides a transducer having piezoelectric element lying between opposed parts of a pre-compression system, characterised in that the pre- compression system comprises a composite having a matrix and fibres within the matrix aligned in the direction of the pre-compression force, giving a high compliance in the system whereby disturbances to the separation of the opposed parts do not significantly disrupt the pre-compression.

Particularly it provides a surgical instrument for delivering energy to an operative site, having a piezoelectric element lying between an energy delivery member and a preloading member stressed by a compression member secured to the energy delivery member and/or the pre-loading member, characterised by the pre-loading member being a composite comprising a matrix and fibres within the matrix aligned in the direction of the pre-loading force and by the securing being by crimping or other mechanical deformation.

A further particular provision is of a sonar transducer with a high coupling coefficient and wide band width.

Transducers used for sonar and ultrasonic cleaning applications are fundamentally different from those used in medical applications that involve for example tissue fragmentation, coagulation and suture welding. Sonar transducers are designed to transmit and receive energy over a wide frequency range and therefore have a low value of Q factor (typically 4). Similarly, some ultrasonic cleaning transducers are required to operate over a wide frequency because they are required to induce cavitation with a corresponding range of bubble size. To couple energy efficiently their horn geometry is reversed such that the area of the radiating surface is greater than the area of the piezo- ceramic drive stack. The operating bandwidth is related to the transducer's ability to convert electrical energy into mechanical energy for the transmit case and vice versa when operating in receive mode. This energy conversion process is degraded by the centre bolt that effectively throttles the maximum displacement at extreme ends of the stack assembly. Providing a more compliant method of pre-stress application will therefore increase the energy conversion, increase effective coupling coefficient, and increase the operational bandwidth. Subsidiary features of the invention are set out in the claims and discussed at length herein,

The matrix/fibrecomposite is advantageous as a material that, for a given pre-load, gives substantially more extension of the pre-loading member than hitherto and ensures that pre-load can be reliably maintained against manufacturing tolerances and variations from conditions of use.

In a specific composite the matrix is of plastics, particularly an epoxy resin, and the fibre is an inorganic, particularly glass, fibre, the composite suitably being made by the 'pultrusion' process in which alignment of the fibres is secured by drawing through a die rather than by simple extrusion of a matrix material containing the fibres.

Conveniently, the compression member is itself a composite such as forms the pre- loading member.

An advantage of the composites is the inexpensive manufacture that they offer, so that one-use surgical instruments are practical and degradation under post-use sterilisation does not arise. Instruments can be assembled using adhesives, cured with the pre-load applied, and adhesives can moreover be chosen that yield at autoclave sterilisation temperatures making it impossible to use the instrument more than once. Even in a mechanically secured assembly the system can be designed to fail by softening of the matrix of the composite.

Instruments designed using a single pultruded tube or rod that replaces a number of discrete components eliminate many of the potentially expensive machining operations and assembly steps used in prior art sandwich transducers, i such embodiments the pultruded rod or tube is used as the central assembly mandrel for the component parts. In these embodiments the pultruded rod or tube is used to apply a compressive bias force across the piezoceramic elements. Since the pultruded tube or rod is fabricated from materials that a high value of electrical insulation resistance, the piezoceramic elements and their shim interspacing electrodes can be assembled directly over the rod or tube. In prior art a metal bolt would require a sleeve or tape to align components and provide electrical insulation. Pultruded tubes and rods are fabricated from uniaxial glass fibres within an epoxy matrix and they are conceptually similar in construction to multi-stranded cable conductors. As such the well proven crimping or swaging methods used for anchoring components to electrical cables can also be used to attach components to the pultruded rod or tube. Crimping or swaging components together provides a low cost means of permanent attachment that prevents subsequent disassembly of components. The crimped or swaged joint method is superior to bonding with epoxy adhesive. These joints can withstand higher values of shear stress, are quicker to make and are more reliable than bonding methods that are inherently process dependent.

Particular Embodiments

The embodiments described below use pultruded glass fibre tubes available from the company known as EXEL who manufacture the tube at a plant located in Finland (Kivara Factory, Muovilaaksontie 2, FIN - 82110 Heinevaraa). EXEL have perfected the manufacture of high quality tubes used primarily in the manufacture of optical telescopes, ski poles and windsurfer masts. In the present developments we use an 'E glas' composite that has a nominal Young's Modulus =38GPa, Density = 1800 Kg/m³, Maximum stress = 760 MPa.

The piezoelectric ceramic material used in the element typically conforms to the published US Navy type I or US Navy Type HJ specification.

The invention is discussed further below and is also illustrated by, but not limited to, the embodiments in the following drawings where in part sectional elevation:-

Figure 1 shows the operative parts of a prior-art surgical instrument; Figure 2 shows the operative part of a surgical or industrial instrument in accordance with a first embodiment of the invention: Figure 3 shows a surgical or industrial instrument in accordance with a second embodiment of the invention;

Figure 4 shows a surgical instrument in accordance with a third embodiment of the invention; Figure 5 shows a part-sectional elevation of a transducer used in a surgical instrument in accordance with a fourth embodiment of the invention, where the instrument is provided with a means of irrigation or aspiration through a tube or lumen that passes through its centre.

Figure 6 shows a corresponding part-sectional elevation in accordance with a fifth embodiment of the invention, with a solid pultruded rod though the centre, suitable for surgical and industrial applications;

Figure 7 shows a part-sectional elevation of a surgical instrument in accordance with a sixth embodiment of the invention.

Figure 8 shows a part-sectional elevation of a sonar or ultrasonic cleaning transducer in accordance with a seventh embodiment of the invention; and

Figure 9 shows a part-sectional elevation of such a transducer in accordance with an eighth embodiment of the invention.

Figure 2 shows an instrument in accordance with a first embodiment of the invention which may be compared with the prior art instrument of figure 1. A piezoelectric element

1 comprising two piezoelectric ceramic rings and thin electrodes is held in compression by a pre-load between a steel rear mass 2 and the horn 3. The pre-load is provided directly in Figure 1 but in Figure 2 by a pultruded glass fibre tube 4 under compression in a sliding fit with a high-tensile cap-head bolt 5. A washer 6 is inserted between the head of the bolt and the rear face of the pultruded tube. The cap head bolt is screwed into the transducer horn adjacent to the piezoelectric ceramic at 7, securing the arrangement.

Tightening the cap-head bolt 5 forces the pultruded tube 4 to remain under compression.

The washer 6 ensures that the torsional force on the tube and consequently on the piezoelectric ceramic is kept to a minimum. Figure 3 shows an instrument in accordance with a second embodiment of the invention. The construction of this instrument is broadly similar to that described in the first embodiment, but the pre-load on the piezoelectric element 11 is provided by an external pultruded glass fibre tube 14 under compression. The pultruded tube is compressed by tightening the cap head bolt 15 which is screw threaded into the transducer horn at 17, and forcing the washer 16 against the end of the pultruded tube remote from the piezoelectric element. The compressed tube provides the pre-load evenly across the piezoelectric element, reducing the amount of torsional force applied. The length of the outer tube, 14, is such as to produce a length equal to λ/4, from the anti-nodal distal face of the rear mass, where λ is the wavelength of the resonant longitudinal vibration within the pultruded tube, to allow decoupling of the vibrational energy.

Figure 4 shows a third embodiment in accordance with the invention with parts corresponding to the above including a rear mass 32 and an inner pultruded tube 34 applying the pre-load under compression, forming a sliding fit with an outer pultruded tube 39 under tension. The outer tube is bonded at 38 to the transducer horn 33 and to a housing 43 surrounding the transducer hom. The pre-load is provided by a sliding bung 41 which is forced into place by a removable screw tensioner 42. The removable tensioner 42 is clamped on to the end of the outer pultruded tube remote from the piezoelectric elements 31. The sliding bung 41 is then forced against the end of the inner pultruded tube 34 remote from the piezoelectric element by tightening the tensioner screw. This results in forcing the inner tube 34 to remain in compression and the outer tube 39 in tension. The sliding bung 41 is then held in place either by bonding or by a mechanical joint, and the tensioner 42 removed. The sliding bung 41 may also be positioned by other means, a hydraulic press, for example, acting on one or both of the tubes 34, 39 to provide the correct amount of compression / tension, such that when the bung is secured to the tube 39 and the press is released, the desired pre-stressing results. This embodiment is particularly advantageous as the piezoelectric ceramic elements may be in the form of discs, rather than a ring. There is also no torsional compressive force on the piezoelectric ceramic. This reduces the likelihood of radial cracking of the piezoelectric material whilst applying the pre-load and during use. The invention is not limited to the use of pultruded fibreglass tubes, being suited also to tubing or rods of other fibre composites or materials with appropriate elastic moduli and yield strength suitable for use with any of the embodiments described above. Increased compliance of the pre-compression system may also be achieved by reducing the load carrying cross-section of that system, but for comparable moduli this requires the use of materials having a higher yield strength, in order to achieve a given pre-compression.

From the description it will be seen how in order to overcome the main disadvantages of the prior art, namely the variation in pre-load at high drive frequencies and detrimental effect of torsional forces on the performance of the piezoelectric element, the invention provides a method of applying and maintaining the pre-load in a reliable manner, advantageously with the force directed longitudinally. The amount of pre-load applied is dependent on the stress-strain cycle of the piezoelectric element as it vibrates longitudinally.

With typical prior compression systems, the variation in pre-load at high drive frequencies is due to the inherent 'stiffness' of the cap head bolt used to keep the piezoelectric element under compression. Such bolts are typically made from high tensile steel with a Young's modulus of around 207 GPa. With a pre-load of 50MPa, this leads to the bolt stretching by around 0.025mm, i.e. a pre-compression system compliance of 5 x 10^"7 .Mpa^"1. At high drive frequencies, even minor variations in the seating of components will lead to a variation in pre-load.

When a pre-load of 25MPa to 50MPa is applied to the fibreglass tube of the preferred embodiments, the material stretches by about 0.125mm, i.e. a pre-compression system compliance of 2.5 x 10^"6 to 5 x 10^"6 m.Mpa^"1. Thus the system using a pultruded fibreglass tube is five to ten times less sensitive to changes in the spacing of the pre-compression system parts adjacent to the piezo ceramic rings or discs. The increased compliance leads to a more stable pre-load at resonant frequencies, even allowing for seating of components. Pultruded fibreglass tubes are particularly suitable, for both materials cost and properties for applying pre-loads. The pultrusion process ensures that the glass fibres are aligned in the plastic matrix along the length of the tube, giving strength in compression as well as increased compliance.

Variable pre-load during use leads to variation in both the resonant frequency and impedance of the transducer. At a stress of 25MPa, a change in pre-load of 25% can lead to a variation of resonant frequency of approximately +/- 5%, and in impedance of approximately +/- 12%. This is made worse by the thermal expansion of the prior art bolts at high frequencies. This is undesirable for delicate medical applications. Since the variation of both resonant frequency and impedance is asymptotic with applied stress, the use of higher pre-loads might be thought advantageous to try and reduce variability in performance. However, the use of such higher pre-loads and relatively stiff materials encourages failure of the piezoelectric material by cracking during manufacture and use of the instrument. The stability of the pre-load available from the present invention allows lower pre-loads to be used, resulting in less risk of damage to the piezoelectric components.

Figure 5 shows a transducer formed using crimping/swaging and used in a surgical instrument in accordance with a fourth embodiment of the invention. A piezoelectric element 801 comprising two piezoelectric ceramic rings and thin electrodes is held in compression by a pre-load between a steel rear mass 802 and a transducer horn 803. The pre-load is provided by a pultruded tube 804 within the barrel of the snout section of the horn 803 and crimped to it at join 805. After this crimping (stepl), the two piezoelectric ceramic rings, the thin electrodes and rear mass 802 are assembled over the pultruded tube. The tube outside diameter and the inside diameter of the components are dimensioned to maintain a sliding fit and thereby provide radial alignment of the external surfaces. As step 2, the components are compressed against the horn 803 mating surface by providing a means of anchoring the rear mass 802, gripping the portion of pultruded tube that extends beyond the distal end of the rear mass, and stretching the tube by a suitable jacking mechanism that is attached to the tube and bears against the rear mass anchor. The level of preload within the piezoelectric elements can be measured by monitoring the voltage generated within the element as the tube is extended. Once the required level is achieved, as step 3, the rear mass is anchored to the pultruded tube by a crimping or swaging process at join 806. The pultruded tube that extends beyond the rear mass can then be cut off.

Figure 6 shows a part-sectional elevation of a transducer in accordance with a fifth embodiment of the invention and where the pultruded tube 804 of Figure 8 is replaced with a solid pultruded rod 807 though the centre of the transducer element 801. Transducers of this type can be used for surgical and industrial applications. The construction and assembly of this instrument is similar to that described in the previous embodiment.

Figure 7 shows a further surgical instrument in accordance with a sixth embodiment of the invention and comprising a piezoelectric element 801 and end effector 809 with a tube or lumen passing through the centre. A pultruded tube 804 is used to form the horn snout at 811, and as assembly step 1 the truncated horn is crimped or bonded into place over the pultruded tube 804 at join 805. Steps 2 and 3 are as for the embodiment of Figure 5. Finally as step 4, the end effector 809 is pennanently attached to the distal end of the snout of the horn using a crimped ferrule at 812.

Such instruments are particularly useful as phacoemulsifiers, coagulators, dental sealers, suture welders, soft tissue aspirators and as other instruments in fields such as key-hole surgery and liposuction. However, as noted above, transducers embodying the present invention may find broader applications.

Thus the last two embodiments in Figs. 8 and 9 are sonar and ultrasonic cleaning transducers with the transducer horn of earlier embodiments replaced by a transducer piston 903. Other parts are similar to those used before, with piezoelectric elements 901, a rear mass 902 and a bolt 908 applying pre-compression load via an internal pultruded tube 904 or an external tube 905. The rear mass is secured to a plate 906 bearing on the piezoelectric elements, but otherwise has sliding fit with adjacent members. A washer 907 or bearer plate 910 applies the load of the bolt evenly to the pultruded tube. It will be noted that since the radiating pistons or horns used in sonar and ultrasonic cleaning transducers are relatively thin (10mm to 20mm) it is not practical to bond or crimp the pultruded rods or tubes. The shear stress would exceed the bond strength of adhesives and there is not space to use the crimping method. The pultruded tubes are therefore used in compression in conjunction with a metal centre bolt. In the embodiment of Figure 9, as with that at Figure 3, the length of the outer tube 905, outwards from the anti-nodal distal face of the rear mass 902, is λ I A where λ is the vibration wavelength.

Claims

1. A transducer having a piezoelectric element lying between opposed parts of a pre- compression system characterised in that the pre-compression system comprises a composite having a matrix and fibres within the matrix aligned in the direction of the pre- compression force, giving high compliance in the system whereby disturbances to the separation of the opposed parts in use do not significantly disrupt the pre-compression.

2. A transducer according to claim 1 in which the force is applied in manufacture by compressing the composite.

3. A transducer according to claim 1 in which the force is applied in manufacture by extending the composite and securing it in its extended form.

4. A transducer according to claim 2 comprising two members both of a composite as set out in claim 1 but one of larger physical dimension than the other, the force being applied in manufacture by extending that one member and securing it in its extended form to compress the other.

5. A transducer according to any preceding claim, wherein the matrix is plastics, particularly an epoxy resin, and the fibre is an inorganic fibre, particularly glass fibre.

6. A transducer according to any preceding claim, where in the composite is made by pultrusion.

7. A transducer according to any preceding claim, assembled using adhesive cured with the pre-compression force applied, particularly an adhesive that yields at autoclave sterilisation temperatures so that the transducer cannot be sterilised for re-use.

8. A transducer according to any one of claims 1 to 6, assembled using swaging, crimping or other mechanical deformation with the pre-compression force applied.

9. A transducer according to any one of claims 1 to 6, wherein the matrix of the composite yields at autoclave sterilisation temperatures so that the transducer cannot be sterilised for re-use.

5 10. A transducer according to any preceding claim, having a high Q value and incorporated in a surgical instrument or an ultrasonic welding or other industrial instrument suited to such a Q value.

11. A transducer according to any preceding claim, having a low Q value and 10 incorporated in a sonar instrument or an ultrasonic cleaning or other industrial instrument suited to such a Q value.

12. Manufacture of a transducer by assembly of a piezoelectric element and pre- compression system to form the transducer of any preceding claim, particularly by the

15 steps of compressing, or extending, and securing set out in claim 2, 3 or 4.

13. Manufacture of a transducer according to claim 12, using the step set out in claim 7 or claim 8.

20 14. A surgical instrument for delivering energy to an operative site having a piezoelectric element lying between an energy-delivery member and a pre-loading member stressed by a compression member secured to the energy delivery member and/or the pre-loading member, characterised by the pre-loading member being a composite comprising a matrix and fibres within the matrix aligned in the direction of the pre-

25 loading force and by the securing being by crimping or other mechanical deformation.