Ultrasonic transducer

Abstract

An ultrasonic transducer is disclosed, comprising an actuator configured to receive an electrical signal to generate ultrasonic vibration, at least one mass component. The mass component is coupled to the actuator at a coupling region of the mass component and configured to transfer vibration energy from the coupling region to a vibration output region of the mass component. The mass component comprises a modified compliance region in which there are provided openings that form voids through the mass component, the voids being arranged in a labyrinthine network. The labyrinthine network may be based on a periodic topology and may for example be defined by a triply periodic minimal surface (TPMS).

Description

[0001] ULTRASONIC TRANSDUCER

[0002] Field of the Invention

[0003] The present invention relates to ultrasonic transducers, and to methods for the manufacture and operation of such transducers. Of particular interest is the incorporation of modified compliance regions in such transducers to alter the vibrational modes and / or the vibrational response of the transducer.

[0004] In applications of power ultrasonics, a common type of transducer is the bolt-clamped Langevin transducer (BLT). The BLT configuration typically consists of a stack of piezoceramic rings located at the nodal plane of the first longitudinal mode, sandwiched between two end masses, clamped by a bolt under pre-stress. The piezoceramic rings are often lead zirconium titanate (PZT), which exhibits a high electromechanical coupling factor, a high Curie point, low dielectric loss, and a stable electromechanical response over time and over ranges of temperature [1]. Various geometric features can be incorporated into the end masses (usually the front mass) to (i) introduce vibrational displacement amplitude amplification along the BLT axis and hence deliver sufficient amplitude for a particular application [2], or (ii) enable excitation of composite modes, usually longitudinal-torsional (L-T) or longitudinal-bending (L-B) modes [3], [4]. Examples of amplifying geometries in a BLT front mass or attached horn include conical- cylindrical, stepped-cylindrical, barrel, spool-shaped and barbell [5], and the selection depends on the requirements of the application. For example, a barbell provides a large acoustic radiating surface area for applications where the cavitation of large volumes of fluid is required [6], whereas a stepped- cylindrical profile can provide high displacement amplification but at a cost of high stress at the step region [7]. Some more recently proposed profiles include a Bezier and nonrational B-spline curve, which benefit from higher displacement and lower stress for applications such as ultrasonic welding [8], [9]. It is more challenging to achieve comparable displacement amplitude in small BLTs, such as for miniaturising ultrasonic surgical devices for minimally invasive surgeries

[0010] . For these types of BLT, with a simple front mass or horn profiles, there are important trade-offs between transducer size, resonance frequency, and achievable vibration displacement amplitude and previously a BLT with a stepped profile front mass has most commonly been required to enable the excitation of stable vibrations at displacements sufficient to cut mineralised tissue.

[0005] Known transducer geometries have been limited to simple shapes by their manufacturability, which has until recently relied on subtractive manufacturing techniques. By adopting metal additive manufacturing (AM)

[0011] , it becomes possible to propose a much wider range of BLT front / back mass or horn designs, that can incorporate intricate and internal features and therefore offer potential for new vibrational responses. Powder bed fusion by laser beam (PBF-LB) is based on incremental layer-by-layer material consolidation, facilitating the fabrication of complex components through the controlled melting of metallic feedstock powder with the aid of a high-energy laser supply

[0012] . There have been a small number of studies reported where ultrasonic transducer components with a complex geometry have been manufactured using PBF-LB. One example was a transducer consisting of two coupled and orthogonally aligned BLTs, that excites a bending mode in an attached cylinder and hence elliptical motion of driving parts attached to the two ends of the cylinder. This configuration was designed for a range of industrial applications including ultrasonic motors and ultrasonic-assisted machining

[0013] . Another study

[0014] evaluated the vibrational properties of two different configurations of an ultrasonic surgical device with Ti- 6AI-4V components manufactured using PBF-LB compared with nominally identical devices with components manufactured using subtractive machining processes. It was shown that AM build quality and surface finish were important and the effects of these on performance were more pronounced in one device than the other, due to the complexity of its features. A further adoption of PBF-LB was in a study proposing a new L-T mode ultrasonic transducer for an AIShoMg alloy front mass

[0015] , to act as a waveguide for an ultrasonic needle aimed at lowering needle insertion force into tissue.

[0006] BLTs are finding increased utility in ultrasonic surgical devices. The designs of modern ultrasonic surgical devices either for hard or soft tissue almost always resemble the configuration invented by Paul Langevin and Chilowsky in 1922 for underwater applications

[0016] . Generally, these devices adopt ultrasonic vibrations to enhance cutting performance, employing a transducer mounted in a hand-held device. Ultrasonic scalpels boast improved performance over traditional surgical devices when it comes to soft and hard tissue dissection.

[0007] Ultrasonic devices used in surgical applications typically consist of three main components: a handpiece, an ultrasonic transducer, and a disposable blade or tip. The handpiece contains the transducer, which converts electrical energy into mechanical vibrations. These vibrations are then transmitted to the blade or tip and into the target tissue. For soft tissue, this mechanical energy generates frictional heat, leading to the simultaneous cutting and coagulation of tissue. Unlike traditional surgical tools, which rely on sharp edges or heat alone for tissue incision and cauterisation, ultrasonic scalpels offer a combination of mechanical cutting and thermal coagulation. This dual-action mechanism can result in cleaner, more precise incisions with minimal bleeding and collateral damage to surrounding structures. For bone cutting, the precise interaction between the cutting blade and tissue depends on the features of the blade cutting edge and the direction of cut.

[0008] In classical BLT profiles in ultrasonic devices, the present inventors have realised that four main problems arise:

[0009] • The vibrational modes of a transducer are not easily tuneable.

[0010] • The heat generated by the transducer vibration under an applied load during cutting can be a challenge for soft and hard tissue cutting in surgical applications because of the risk of tissue burning. Tissue burning from the ultrasonic scalpel could result in a prolonged post-operative recovery for the patient.

[0011] Solid metal front and back transducer mass components are heavy and obtrusive limiting fine movement or prolonged operating time when used hand-held by a surgeon. • The resonance frequency and displacement amplitude are affected by the geometry of the transducer, so increasing cutting power must be balanced with maintaining a reasonable transducer size.

[0012] In WO 2023 / 007013 A1 , researchers from the present inventors’ research group have previously disclosed an ultrasonic transducer for surgical applications in which the front mass, for example, has an arrangement of openings formed through an annular wall of the front mass in order to increase the mechanical compliance of the front mass and thereby reduce the overall length of the ultrasonic transducer required for producing a specific vibration amplitude at a specific operating frequency such as 55 kHz.

[0013] The present invention has been devised in light of the above considerations.

[0014] Summary of the Invention

[0015] The present inventors have realised that further improvements to ultrasonic transducers are possible, and they have further realised that these improvements may have wider applicability than to the area of technical application and technical effects discussed in WO 2023 / 007013 A1.

[0016] Accordingly, in a first aspect, the present invention provides an ultrasonic transducer comprising: an actuator configured to receive an electrical signal to generate ultrasonic vibration; and at least one mass component coupled to the actuator at a coupling region of the mass component and configured to transfer vibration energy from the coupling region to a vibration output region of the mass component; wherein the mass component comprises a modified compliance region in which there are provided openings that form voids through the mass component, the voids being arranged in a labyrinthine network.

[0017] In a second aspect, the present invention provides a surgical tool comprising an ultrasonic transducer according to the first aspect. The transducer may be configured with an operating frequency in the range 10-100 kHz.

[0018] In a third aspect, the present invention provides a welding tool comprising an ultrasonic transducer according to the first aspect. The welding tool may be configured to achieve satisfactory welding for a range of metals, such as aluminium, copper, nickel, magnesium, and alloys thereof, and thermoplastic materials. The transducer may be configured with an operating frequency in the range 10-100 kHz.

[0019] In a fourth aspect, the present invention provides a method of manufacture of an ultrasonic transducer according to the first aspect, wherein the modified compliance region is formed using an additive manufacturing process. In a fifth aspect, the present invention provides a method of operation of an ultrasonic transducer according to the first aspect, the method including applying an electrical signal to the actuator to generate ultrasonic vibrations, transferring vibration energy from the coupling region to the vibration output region of the mass component. The ultrasonic transducer may be configured so that at an operating frequency is in the range 10-100 kHz, a displacement amplitude at the vibration output region of the mass component is in the range 1-200 microns peak-to-peak.

[0020] Further optional features of the invention are now set out, these being applicable single or in any combination with any aspect of the invention, unless the context demands otherwise.

[0021] The labyrinthine network of the modified compliance region may be based on a periodic topology. A periodic topology is more easily categorised and computationally simulated than random or semi-random structures. Representing the structure by a unit cell enables the facile changing of parameters (such as the cell wall thickness, cell types, cell size) that consistently affect the whole modified compliance region.

[0022] The surface of the labyrinthine network may be defined by a triply periodic minimal surface (TPMS). A TPMS (a) has a mean curvature (the mean of two principal curvatures at any point) of zero on the surface and (b) is non-self-intersecting, periodic in three coordinate directions and can in principle be extended infinitely.

[0023] The TPMS structure may be further selected from: a) Schwarz primitive cos(X) + cos(Y) + cos(Z)' = c b) Schwarz diamond cos (X) cos (Y) cos (Z) — sin X)sin Y)sin Z) = c c) Schoen gyroid sin (X) cos (Y) + sin (Z) cos (X) + sin (Y) cos (Z) = c d) Schwarz neovius

[0024] 3[cos(X) + cos (Y) + cos Z ] + 4 [ cos(X)cos Y')cos(Z')] = c e) Lidinoid

[0025] 0.5 [sin(2X) cos(Y) sin

[0026] — 0.5 where X = 2nx / a, Y = 2ny / a, Z = 2nz / a, and a is the unit cell parameter and c is a constant. These example TPMS structures are known in chemistry and materials science.

[0027] In some embodiments, the ultrasonic transducer may comprise: a back mass; a front mass; the actuator; an ultrasonic horn arrangement forward of the front mass, wherein the back mass, actuator, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer, and the actuator is held between the back mass and the front mass; wherein the mass component comprising the modified compliance region is one or more of the front mass, back mass and ultrasonic horn arrangement.

[0028] Optionally, the mass component comprising the modified compliance region is one or more of the front mass and the back mass. For example, the mass component comprising the modified compliance region may be the front mass.

[0029] Additionally, the length of the transducer, measured from a proximal end of the back mass to a distal end of the ultrasonic horn arrangement, along the longitudinal axis, may be not more than 40mm. The diameter of the transducer, measured in a direction perpendicular to the length may be not more than 6mm. This is of interest for transducer miniaturisation, wherein such compact transducers may be of use in surgical applications.

[0030] The transducer may be a Langevin transducer. An example of this is a bolted Langevin transducer. These are of interest for surgical or industrial applications due to their low ultrasonic frequency and high power operation characteristics.

[0031] The mass component has a diameter in a direction perpendicular to the longitudinal axis of the transducer. The labyrinthine network of the modified compliance region may extend continuously across the diameter of the mass component. Configuring a mass component in this way modifies the compliance in this region (compared with a fully solid mass component of the same overall shape), whilst maintaining good mechanical stability when compared to a hollow structure.

[0032] In some embodiments, the modified compliance region may provide substantially any one of: a) no conversion from longitudinal to torsional or bending mode vibration; b) conversion from longitudinal to longitudinal-torsional mode vibration; c) conversion from longitudinal to longitudinal-bending mode vibration; d) conversion from longitudinal to longitudinal-bending-torsional mode vibration.

[0033] The geometry can be optimised to produce one of the options above such that the transducer fits the intended application. For example, if torsional mode vibrations are not required, their presence could reduce the energy available for longitudinal vibrations.

[0034] The voids provided by the openings on a planar cross-section of the modified compliance region, taken perpendicular to the longitudinal axis at a position along the modified compliance region, may occupy at least 10% of the cross-sectional area of the mass component at that longitudinal axis position. This is one measure of porosity and describes a minimum ratio of the area of empty space to solid space when a cross-section is taken from the labyrinthine region. This may be measured using cross-sections attained from micro-CT scans or ones received from a digital double transducer. Considering the percentage porosity, a higher value may yield a greater vibrational amplitude but this may also reduce the mechanical strength of the structure. Additionally, structures with greater porosity may be more challenging to manufacture and provide more areas for stress concentration and crack initiation. The percentage porosity can therefore be decided based on the requirements of the specific transducer application.

[0035] The ultrasonic transducer may have a configuration wherein the front mass comprises a proximal portion in contact with the actuator and a distal portion connected to the ultrasonic horn arrangement and one or multiple intermediate portions disposed between the proximal portion and the distal portion, and wherein the modified compliance region is provided in the intermediate portions.

[0036] The modified compliance region may have a porosity of at least 10%, the porosity being defined as the volume of the voids expressed as a percentage of the sum of the volume of the voids and the volume of the solid remainder of the modified compliance region. This is a measure of porosity in three dimensions. A minimum porosity may be the lowest porosity required to achieve useful benefits regarding one or more of:

[0037] • modified compliance

[0038] • the mode (or hybrid mode) of vibration

[0039] • greater thermal management

[0040] • a lightweight transducer.

[0041] For the mass component, the length Ltot of the mass component may be defined as the distance between the coupling region of the mass component and the vibration output region of the mass component, and wherein the modified compliance region has a length of Liab which occupies a proportion of Ltot, and wherein the remainder of the mass component other than the modified compliance region has a length of Lsoiid, so that:

[0042] Ltot=Llab + Lsoiid and:

[0043] Rsolid=(Lsoiid) / (Ltot) and wherein Rsoiid may be at least 0.02 or at least 0.03 or at least 0.04 or at least 0.05. This can also be expressed as a percentage.

[0044] The mass component may have a length proportion of the modified compliance region that is defined by the following equation:

[0045] Rlab=(Llab) / (Ltot) and wherein Riab may be at least 0.02 or at least 0.03 or at least 0.04 or at least 0.05. This can also be expressed as a percentage.

[0046] The mass component may include a first solid region of length Lsoiidt at the coupling region of the mass component and a second solid region of length LSoiid2 at the vibration output region of the mass component, wherein : and Rsoiidi may be at least 0.01 and RSoiid2 may be least 0.01 . This is structurally beneficial for the mass component as it ensures the ends of the mass components are solid (i.e. without voids).

[0047] In some embodiments, the mass component may be configured so that Rsoiidi is greater than Rsoiid2 so that the modified compliance region is closer to the vibration output region of the mass component than to the coupling region of the mass component. Placing the labyrinthine network closer to the output face of the transducer may be preferred when trying to elicit a specific vibrational response, as the position of the modified compliance region within the transducer affects the resulting amplitude and vibrational mode of the device.

[0048] In some embodiments, the mass component may be configured so that Rsoiid2 is greater than Rsoiidi so that the modified compliance region is closer to the coupling region of the mass component than to the vibrational output region of the mass component. Placing the labyrinthine network closer to the coupling region of the transducers may be preferred to maximise the gradient of gain profile of the vibrational response.

[0049] The average porosity of one half of the length of the modified compliance region may be different to the average porosity of the other half of the length of the modified compliance region. The percentage porosity may therefore be varied along the length of the mass component. The gradient of the percentage porosity may be changed to optimise the vibrational amplitude or mode of a transducer. Additionally, a spatially varying porosity may be introduced to increase mechanical strength or reduce stress concentration in the mass component.

[0050] The specific volumetric surface area of the modified compliance region may be at least 1200 m2 / m3, expressed as the surface area of the modified compliance region including the surface area of the voids, divided by the sum of the volume of the voids and the volume of the solid remainder of the modified compliance region. A higher specific volumetric surface area would result in better heat dissipation, keeping it cooler in operation. Cooler operation is beneficial for surgical applications as there is a lower risk of soft tissue being damaged by heat during surgery.

[0051] The labyrinthine network may be formed by additive manufacturing. This may more specifically include: selective laser sintering (SLS) or powder bed fusion by laser beam (PBF-LB) or powder bed fusion by electron beam (PBF-EB) or directed energy deposition (DED) or metal filament extrusion.

[0052] The invention includes any combination of the aspects and optional features described except where such a combination is clearly impermissible or expressly avoided. of the Fk

[0053] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0054] Figure 1 shows a generic surgical ultrasonic system with its key components.

[0055] Figure 2A is a schematic diagram of a symmetric Langevin transducer.

[0056] Figure 2B is a schematic diagram of an asymmetric Langevin transducer.

[0057] Figure 3 shows a plurality of mass components (front mass components) A to H. A to E illustrate varying proportions of the modified compliance region and F to H illustrates varying positions and proportions of modified compliance regions.

[0058] Figure 4 provides a view of transducer S2-L20-S78 at rest and an FEA of the transducer when deformed.

[0059] Figure 5 provides a magnified image of the output end of the deformed transducer in Fig.4.

[0060] Figure 6 shows the mode shape predicted from the FEA of transducer S2-L20-S78. The upper curve is the longitudinal component and the lower curve is the bending component of the mode shape. All data is normalised to unit longitudinal displacement amplitude at the end of the back mass.

[0061] Figure 7 provides a view of transducer S2-L30-S68 at rest and an FEA of the transducer when deformed.

[0062] Figure 8 provides a magnified image of the output end of the deformed transducer in Fig.7.

[0063] Figure 9 shows the mode shape predicted from the FEA of transducer S2-L30-S68. The upper curve is the longitudinal component and the lower curve is the bending component of the mode shape. All data is normalised to unit longitudinal displacement amplitude at the end of the back mass.

[0064] Figure 10 provides a view of transducer S2-L50-S48 at rest and an FEA of the transducer when deformed.

[0065] Figure 11 provides a magnified image of the output end of the deformed transducer in Fig.10.

[0066] Figure 12 shows the mode shape predicted from the FEA of transducer S2-L50-S48. The upper curve is the longitudinal component and the lower curve is the bending component of the mode shape. All data is normalised to unit longitudinal displacement amplitude at the end of the back mass.

[0067] Figure 13 provides a view of transducer S2-L75-S23 at rest and an FEA of the transducer when deformed. Figure 14 provides a magnified image of the output end of the deformed transducer in Fig.13.

[0068] Figure 15 shows the mode shape predicted from the FEA of transducer S2-L75-S23. The upper curve is the longitudinal component and the lower curve is the bending component of the mode shape. All data is normalised to unit longitudinal displacement amplitude at the end of the back mass.

[0069] Figure 16 provides a view of transducer S12-L40-S48 at rest and an FEA of the transducer when deformed.

[0070] Figure 17 provides a magnified image of the output end of the deformed transducer in Fig.16.

[0071] Figure 18 shows the mode shape predicted from the FEA of transducer S12-L40-S48. The upper curve is the longitudinal component and the lower curve is the bending component of the mode shape. All data is normalised to unit longitudinal displacement amplitude at the end of the back mass.

[0072] Figure 19 provides a view of transducer S22-L30-S48 at rest and an FEA of the transducer when deformed.

[0073] Figure 20 provides a magnified image of the output end of the deformed transducer in Fig.19.

[0074] Figure 21 shows the mode shape predicted from the FEA of transducer S22-L30-S48. The upper curve is the longitudinal component and the lower curve is the bending component of the mode shape. All data is normalised to unit longitudinal displacement amplitude at the end of the back mass.

[0075] Figure 22 provides a view of transducer S32-L20-S48 at rest and an FEA of the transducer when deformed.

[0076] Figure 23 provides a magnified image of the output end of the deformed transducer in Fig.22.

[0077] Figure 24 shows the mode shape predicted from the FEA of transducer S32-L20-S48. The upper curve is the longitudinal component and the lower curve is the bending component of the mode shape. All data is normalised to unit longitudinal displacement amplitude at the end of the back mass.

[0078] Figure 25 provides a view of transducer S100 at rest and an FEA of the transducer when deformed.

[0079] Figure 26 provides a magnified image of the output end of the deformed transducer in Fig.25.

[0080] Figure 27 shows the mode shape predicted from the FEA of transducer S100. The upper curve is the longitudinal component and the lower curve is the bending component of the mode shape. All data is normalised to unit longitudinal displacement amplitude at the end of the back mass. Figure 28 shows the frequency response functions (FRFs) of the Langevin transducers assembled with front masses (A) - (H) from Fig. 3, extracted from experimental modal analysis (EMA). Each FRF is labelled a-h, corresponding to (A) - (H) respectively.

[0081] Figure 29 shows the measured axial amplitude versus excitation level for (a) transducers A to E in Figure 3 and (b) transducers A, D and F to H in Figure 3 and (c) transducers with the same Riab positioned at different points along the front mass.

[0082] Figure 30 shows embodiments of three front masses with the same modified compliance region proportion, but with each comprising a different TPMS structure; (A) gyroid, (B) diamond, (C) lidinoid, while (D) is for a fully solid mass component.

[0083] Figure 31 provides a view of the diamond embodiment of Fig. 30 at rest and an FEA of the transducer when deformed.

[0084] Figure 32 provides a magnified image of the output end of the deformed transducer in Fig.31 .

[0085] Figure 33 shows the longitudinal mode shape predicted from FEA of the diamond embodiment in Fig. 30.

[0086] Figure 34 provides a view of the gyroid embodiment in Fig. 30 at rest and an FEA of the transducer when deformed.

[0087] Figure 35 provides a magnified image of the output end of the deformed transducer in Fig.34.

[0088] Figure 36 shows the longitudinal mode shape predicted from FEA of the gyroid embodiment in Fig. 30.

[0089] Figure 37 provides a view of the lidinoid embodiment in Fig. 30 at rest and an FEA of the transducer when deformed.

[0090] Figure 38 provides a magnified image of the output end of the deformed transducer in Fig.37.

[0091] Figure 39 shows the longitudinal mode shape predicted from FEA of the lidinoid embodiment in Fig. 30.

[0092] Figure 40 provides a view of the fully solid embodiment in Fig. 30 at rest and an FEA of the transducer when deformed.

[0093] Figure 41 provides a magnified image of the output end of the deformed transducer in Fig.40.

[0094] Figure 42 shows the longitudinal mode shape predicted from FEA of the fully solid embodiment in Fig. 30. Figure 43A illustrates unit cell sections of a labyrinthine network with three different wall thicknesses (1 mm, 2 mm and 3 mm).

[0095] Figure 43B illustrates different variations of the lattice porosity along the length of the mass component.

[0096] Detailed Description of the Invention

[0097] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0098] Reference numbers are included in the drawings. Similar features in different drawings may be provided with similar reference numbers. Some features are described only in respect of one or more drawings and may not be provided with a reference number in all drawings.

[0099] In the current invention, PBF-LB is adopted specifically to enable the design of features into the components of the ultrasonic transducer that are not achievable using subtractive manufacturing techniques. Here, triply periodic minimal surface (TPMS) based structures are incorporated into a BLT to modify the vibrational response, both as a method to deliver composite mode vibrations and to increase the vibrational displacement amplitude amplification compared to solid structures.

[0100] Ultrasonic transducers and their design

[0101] A generic surgical ultrasonic system with its key components is shown in Fig. 1 . A high-power signal generator 30 is connected to the power source 32 and controls an ultrasonic transducer, indicated generally by reference number 34. The ultrasonic transducer is a key element of ultrasonic surgical systems, as it converts electrical energy into useful mechanical vibration. A typical operating frequency of the transducer is in the range 10-100 kHz for surgical applications. The ultrasonic transducer is normally located within a casing 36 so it can be handled safely. On one end of the transducer, a probe (waveguide) 38 is attached to guide the wave towards the tissue. As shown schematically in Fig. 1 , the transducer includes a piezoelectric stack 40 sandwiched between a front mass 42 and a back mass 44. A horn 46 is located between the front mass 42 and the waveguide 38.

[0102] An important aspect of the transducer design process is the choice of the materials to be used, this depends on their properties and application requirements. During cyclic loading, a transducer's components will dynamically deform, experiencing high levels of both stress and strain, dependent on both material properties and the shape of the component. Stress concentrations can cause failure, e.g. through cracking, and heating can occur at high strain regions and at interfaces. A typical design guide indicates that the ultimate tensile strength of each material should be 30% higher than the maximum stress experienced by the tool in operational conditions

[0017] .

[0103] Another important parameter in the choice of materials is the acoustic attenuation. When longitudinal sound waves propagate through a medium, their intensity reduces from the source. Energy loss phenomena stem from scattering and absorption, caused by the motion of the ultrasonic wave in other directions than the longitudinal and heat generation due to friction. Materials with low acoustic attenuation are preferred; these include light alloys made with metals such as titanium, aluminium and magnesium; heavier materials such as brass and tungsten should be avoided if possible. A simple way to look at this is via the longitudinal sound velocity: the faster the speed of sound in a material, the less the energy loss

[0018] .

[0104] Ultrasonic energy can be applied to media to generate different vibrational modes, the simplest of which is the longitudinal mode, where consecutive expansions and contractions along the transducer longitudinal axis are observed with lateral motion simultaneously observed, caused by Poisson’s ratio effects

[0019] . The longitudinal mode may also be used to generate other modes by modifying the mass components. Asymmetric longitudinal motion can be achieved by using asymmetric mass component geometries to add lateral vibrations to the longitudinal motion, making the cutting procedure more effective in certain applications, for example in some ultrasonic bone-cutting surgical tools.

[0105] As a starting point for design, the case of a thin rod, where the diameter d is considerably smaller than the length I (c / « I), may be compared to a Langevin transducer to introduce some fundamental concepts. With the assumption of the thin rod, the approximate length of the transducer is half of the wavelength, defined in Equation 1 , where c is the speed of sound in the rod and f the desired operational frequency. The speed of sound in a specific material is defined in Equation 2, frequencies, fn, for the longitudinal vibrational modes n (n = 1 ,2,3...) of a thin rod, can be estimated.

[0106] Fig. 2A is a schematic diagram of a symmetric Langevin transducer. If the transducer can be divided centrally at the nodal plane, each half of the transducer, corresponding to in length can be studied independently.

[0107] Fig. 2B is a schematic diagram of an asymmetric Langevin transducer. The relationship between the resonant frequency (recalling that <u = 2TT ), the acoustic impedance, and the length of both the piezoelectric stack and end masses is expressed by Equation 4.

[0108] The subscripts p and m represent driving (usually piezoelectric) material and end-mass (either the back or the front mass) respectively. lpand lmare the thickness of the piezoelectric ring and the length of the end-mass, respectively. is the acoustic impedance of an end-mass, which can be obtained from Equation 5: where A is the cross-sectional area of the component.

[0109] Equation 4 can be used to approximate the resonant frequency of a transducer with known dimensions or to find an unknown dimension if the frequency and other parameters are known

[0020] .

[0110] The piezoelectric stack 40 is generally made with either cfe-mode rings or a cfej-mode ring. In high power applications, ‘hard’ piezoceramic such as PZT4 and PZT8 is used in even numbers of rings to create the piezoelectric stack, placed between the front and back masses with electrodes in between.

[0111] The total length of the piezoelectric stack is chosen to be approximately one quarter of / . However, the choice is influenced by the available driving electronics, as increasing ceramic thickness needs a higher driving electric field and has higher electrical impedance, higher mechanical losses and higher capacitance, resulting in overall higher costs

[0021] .

[0112] The centre of the stack is best located at the nodal plane, labelled ‘node’ in Fig. 2A, located in the centre of the transducer in a symmetric design, where the strain is highest. The stack position affects both resonant frequency and vibration amplitude, therefore its location is an important aspect of the design process

[0022] .

[0113] Commonly, to allow placement of a supporting flange to attach a casing, the piezoelectric stack is placed away from the nodal position towards the back mass, as shown in Fig. 2B, in an asymmetric configuration. In this configuration, the flange has minimal impact on the vibrational mode. In the asymmetric design, each - 4 section can be analysed independently and if the half of the transducer including the back mass, the piezoceramic stack, and part of the front mass, 11, is considered, Equation 7 can be used to estimate either the frequency or an unknown dimension of the transducer.

[0114] It has been shown by Lierke

[0023] that the maximum efficiency of the transducer is achieved when the piezoceramic stack is centred and its length is half of the total length of the transducer. The greater the offset from the node, the lower the efficiency, as shown in Fig. 6A of WO 2023 / 007013 A1 . When considering the figure of merit, normalised by the maximum of k^ffQm, again the best position for the piezoelectric stack is found to be in the centre of the transducer. The normalised figure of merit is above 90% when the ratio Lpiezo / Ltransducer is between 0.2 and 0.5, as shown in Fig. 6B of WO 2023 / 007013 A1

[0023] ,

[0115] Equation 3 showed the relationship between the natural frequency of the longitudinal mode, the speed of sound and the length of a thin rod. Considering n = 1 , and replacing c with its mathematical definition (Equation 2), Equation 12 is obtained:

[0116] Young’s modulus represents the stiffness of a material, therefore rod-like structures made with materials with low EMwill vibrate longitudinally at lower frequencies than less compliant (more stiff) materials. Thus, in the context of embodiments of the present invention, it is of interest to change (or tune) the stiffness of a rod-like structure to make it resonate at a desired frequency without altering its length.

[0117] Ultrasonic transducers in surgery

[0118] The cutting mechanisms and dynamics of action depend on the specific surgical task. Cavitation is often used in tissue-preserving devices, the direct impact or ‘jackhammer effect’ is one mechanism in bonecutting devices, and the thermal effect is adopted in devices for soft tissue cutting and coagulation.

[0119] In ultrasonic cutting devices used in soft tissue, the thermal effect is desired, and the tissue is heated to the point of denaturation. The interaction between soft tissue and ultrasonic devices is complex, depending on the protein and water content of the tissue undergoing the surgical procedure. In general, tissue with high water content is easier to cut, whereas tissue with high protein content, such as blood vessels, nerves and connective tissue, requires more energy. The temperature can exceed 100 °C which is sufficient to denature proteins and, if the tissue is heated above its critical necrotic temperature, the damage is irreversible and beyond repair.

[0120] The cutting and haemostatic effects are not independent and they happen simultaneously. However, one can be predominant depending on the frequency and vibration amplitude of the blade. Lower frequency and higher vibration result in faster cutting and slower coagulation, while higher frequency and lower vibration cause slower cutting and faster coagulation

[0024] ,

[0025] . Ultrasonic surgical devices with haemostatic dissection capability generally operate at about 55 kHz

[0026] , where the operational frequency of a Langevin ultrasonic transducer is linked to the device length.

[0121] In robotic surgery, ultrasonic dissectors are used predominantly in parenchymal transections to separate the functional tissue of an organ from the connective and supporting tissue, and in lobectomies to remove a lobe or a portion of an organ

[0027] ,

[0028] ,

[0029] . Popular robotic procedures involve the use of ultrasonic dissectors to perform specific intra-operative tasks, these include: hepatectomies, splenectomies, enterectomies, adrenalectomies, and thyroidectomies. A study on robot-assisted thyroid surgery

[0030] compared a wristed bipolar electrocautery instrument (Vessel Sealer Extend, Intuitive® Surgical, Inc.) with an ultrasonic dissector (Harmonic® ACE + Shears, Ethicon® Endo-Surgery).

[0122] The direct comparison showed that the use of the ultrasonic device reduced the intra-operative blood loss and improved the dissection margins and speed of the seal when compared with the standard electrocautery instrument. However, a higher risk of patient injuries such as burns and involuntary tissue perforations was found with the ultrasonic device, due to the combination of the high temperature of the waveguide and the lack of instrument manoeuvrability. Therefore, an ultrasonic dissector that is lighter, better cooled and more compact would address these issues.

[0123] Transducers incorporating a mass component comprising a labyrinthine network

[0124] The present invention is based on the realisation that if a portion of the mass components comprises a modified compliance region, the resulting transducer would address many of the aforementioned issues. Specifically, the present inventors have realised that further advantageous effects can be provided over and above the disclosure of WO 2023 / 007013 A1 and that the resultant transducer can have wider utility.

[0125] Furthermore, the present invention is based on the realisation that if a portion of the mass components comprises a modified compliance region, the resulting transducer could be made more compact, lighter and operationally cooler with specific operational modes. The modified compliance region is characterised by the voids provided by the openings formed through the mass component, the openings being arranged in a labyrinthine network. In the context of the present disclosure, a labyrinthine network can be understood as a plurality of intersecting paths or voids in three dimensions, the voids opening at the front mass surface. The voids are arranged such that a straight line cannot pass through one opening of a labyrinthine network, through an axial centre line of the mass component and exit another opening at the surface of the mass component without intersecting solid material of the mass component.

[0126] Figure 3 shows different mass component configurations of a transducer. A to G show mass components with varying proportions of labyrinthine networks, whilst F to H varies both the proportion of the labyrinthine networks and their distance from the output face of the mass component. These properties are noted in Table 1 . In the various figures of this application, the surface of the labyrinthine network can be mathematically defined as a lattice with specific topological properties: a triply periodic minimal surface (TPMS). It is important to note that the labyrinthine network is not limited to a TPMS structure, and the figures serve to illustrate embodiments of the invention.

[0127] Table 1 : Nomenclature of front masses constituted by sections with varying percentage of length.

[0128] Percentage of Length Percentage of Length in Percentage of Length in

[0129] Front Mass Type in Upper End Solid Middle Lattice Section Lower End Solid Section

[0130] Section (%) (%) (%)

[0131] S100 100 0 0

[0132] S2-L20-S78 2 20 78

[0133] S2-L30-S68 2 30 68

[0134] S2-L50-S48 2 50 48

[0135] S2-L75-S23 2 75 23

[0136] S32-L20-S48 32 20 48

[0137] S22-L30-S48 22 30 48

[0138] S12-L40-S48 12 40 48

[0139] In the context of the present disclosure, lattices are a category of porous structure that possess a defined and controlled topology

[0031] . Unlike periodic topologies based on cylindrical beams / struts, and the cubic Bravais lattices, TPMS are of special interest in this research because of two key features: (a) the mean curvature (the mean of two principal curvatures at any point) is zero on the surface and (b) they are non- self-intersecting, periodic in three coordinate directions and can be extended infinitely

[0032] . Some TPMS- based lattices (e.g., Schwarz primitive, Schwarz diamond, and Schoen gyroid) are well known in chemistry and materials science. Their corresponding mathematical equations of implicit surfaces are as follows

[0031] ,

[0032] : a) Schwarz primitive cos(X) + cos(Y) + cos(Z)' = c b) Schwarz diamond cos (X) cos (Y) cos (Z) — sin X)sin Y)sin Z) = c c) Schoen gyroid sin (X) cos (Y) + sin (Z) cos (X) + sin (Y) cos (Z) = c d) Schwarz neovius

[0140] 3[cos(X) + cos (Y) + cos Z)] + 4 [ cos X)cos Y)cos Z)] = c e) Lidinoid

[0141] 0.5 [sin(2X) cos(7) sin(Z) + sin (27)cos (Z)sm(X) + sin ( Z^cos (X)sin (7)]

[0142] — 0.5 [cos(2X)cos(27) + cos (27) cos(ZZ') + cos (2Z) cos(2X)] + 0.15 = c where X = 2nx / a, Y = 2ny / a, Z = 2nz / a, and a is the unit cell parameter and c is a constant.

[0143] If the constant c in the above equations is changed, a family of surfaces are obtained possessing the symmetry and topology of the same TPMS

[0033] , but have a different level-set structural porosity and nonzero mean curvature values.

[0144] Topology generating software (nTopology Inc, USA) has been used to model the hierarchical mass component structures. In the software, mass component cylinders are designed with varying proportions of labyrinthine networks, comprising TPMS unit cells (e.g. gyroid). A Boolean union is conducted with solid parts to create hierarchical structures with changeable longitudinal symmetries. Geometryindependent structures can be flexibly designed, as many parameters like cell type, cell size, wall thickness, physical length and position of the labyrinthine can be altered. With the above software, variable-shape longitudinal cross-section ultrasonic mass components with porous structures can be achieved easily. Additionally, gradient structures can be implemented - these are defined by their changing percentage porosity along the length of the modified compliance region as illustrated in Figure 42B. In Figure 42B the wall thickness of the labyrinthine network is varied along the length of the mass component, resulting in a corresponding change of percentage porosity. In the example shown the mass component can be generated from fully dense to fully labyrinthine.

[0145] The selection of different unit cell types and changeable cell size and wall thickness enable the design of structures tailored to desired ultrasonic vibrational responses and hence the design of novel ultrasonic devices. Figure 42A shows the unit cell with varying wall thicknesses; it’s clear that as the thickness increases from 1 mm to 3mm the percentage porosity is reduced, this would alter the compliance and therefore change the vibrational response of the resulting mass component when part of or coupled to a transducer. A larger percentage porosity may yield a greater vibrational amplitude, but it will also reduce the mechanical strength of the structure. Additionally, structures with greater porosity are more difficult to manufacture and provide more areas for stress concentration and crack initiation. The percentage porosity is therefore be decided based on the requirements of the specific transducer application.

[0146] Figure 3 incorporates a gyroid lattice structure, as defined above, with different sizes and locations of the lattice. The nomenclature indicates the length percentage of solid (S) and labyrinth (L) in the mass component. The size of the solid region in each mass component can also be described by the following ratio:

[0147] Rsolid — (Lsoiid) / (Ltot) where Ltot = Liab + Lsoiid and the size of the modified compliance region can be described by the following equation:

[0148] Rlab — (Llab) / (Ltot)

[0149] In each embodiment of Figure 3 (B)-(E), there is a proportion of solid region, Riab of 0.02 or 2% positioned at the end of the mass component, where the end of the mass component acts as the output face of the transducer. This is to provide structural reinforcement to the modified compliance region and to facilitate easy coupling of further mass components.

[0150] Design of front masses with gyroid lattice structures

[0151] To simplify the overall geometry of front masses for Langevin transducers and avoid the displacement amplitude being further augmented by an ultrasonic horn, a uniform cylindrical shape was chosen to be modified with gyroid lattice structures. Cylindrical components (10 mm in diameter, 45-55 mm in length) with selectively applied gyroid structures were generated by a specialised implicit modelling software (nTop, nTopology). A cube-shaped gyroid unit cell with edge length of 4 mm, and wall thickness of 0.9 mm was selected to infill the lattice section, then Boolean united with solid sections to model the hierarchical front masses for Langevin transducers. Two design strategies, namely fixing the same percentage of length from the upper and lower end were exploited to prototype these structures. The nomenclature in Error! Reference source not found, indicates the exact percentage of length from each section of the components. Fig. 3 shows the geometries of these implicit body prototypes (with the same diameter but varying length) before quadrangular meshing and exporting CAD file in *.stl format.

[0152] The inclusion of labyrinthine networks results in a large surface area when compared with the conventional ultrasonic transducer. The modulation of surface area is used to efficiently control the working temperature of the transducer system without exceeding some critical temperature limits (e.g., 55-60 °C is the temperature of bone necrosis). This is due to the better heat dissipation that comes with increased mass component surface area and forgoes the risk of soft tissue burning when in contact with parts of the transducer system.

[0153] The inclusion of labyrinth networks also produces lightweight ultrasonic surgical devices. Hollow structures involved in mass components reduce the total mass, which provides greater convenience for surgeons if the transducer is incorporated into handheld devices. Lighter surgical devices mean prolonged use is possible without surgeon fatigue and greater control of precise movements.

[0154] Furthermore, the labyrinthine networks in mass components enhance the vibrational response when compared with conventional transducers. The resonance frequency of a BLT is easily tuneable by changing the total length of mass components. At the same time, amplitude gain and desired vibration mode (e.g. first longitudinal or hybrid mode) can be adjusted by changing the cross-section, physical length and position of the labyrinthine network. The modified compliance of the labyrinthine region increases the vibration displacement of the transducer. Also, the introduction of a longer compliance region can enable mode coupling (e.g. longitudinal and torsional mode). Therefore, a new generation of ultrasonic devices for, for example, cutting or drilling can be defined. It is also possible to incorporate a labyrinthine network that enables miniaturisation of the transducer, for example for integration of ultrasonic surgical devices with surgical robots for minimally invasive surgeries.

[0155] Manufacture

[0156] Titanium alloys have been selected for manufacturing front masses because of their high specific strength, superior biocompatibility and enhanced corrosion resistance. Ti-6AI-4V (wt. %) is regarded one of the most commonly used titanium alloys in biomedical applications, including ultrasonic surgical scalpels. The feedstock material is accessible, affordable and available as a pre-alloyed powder for additive manufacture. PBF-LB can be used to produce the complex geometries required for the mass components of this invention. It was chosen over conventional techniques when considering: supply chain simplification, component design flexibility, life-cycle energy consumption of tailored parts, and the minimisation of environmental impact.

[0157] Powder bed fusion by laser beam:

[0158] Grade 23 plasma atomized Ti-6AI-4V (Carpenter Additive, Carpenter Technology Corporation) in the particle size range 15-53 pm was used to manufacture front masses for Langevin transducers. Eight specimens were manufactured using a laser powder bed fusion system (RenAM500M, Renishaw PLC) equipped with a 1080 nm ytterbium laser focused to a spot size range of 70-75 pm at the powder bed. All samples were produced under argon atmosphere using the inhouse processing parameters: layer thickness of 30 pm, laser power of 200 W, point distance of 90 pm, exposure time of 60 ps, and hatch distance of 90 pm. Standard pin structures were used to support these specimens and thereafter removed.

[0159] Transducer materials:

[0160] Four hard PZT rings (PIC-181 , PI Ceramic) with high mechanical quality factor were used for each BLT. The dimension and piezoceramic material properties are listed in Error! Reference source not found.. The properties of back mass, front mass, electrode, and prestress bolt are shown in Error! Reference source not found..

[0161] Table 2: PIC-181 PZT material properties.

[0162] Outer Diameter (mm) 10

[0163] Inner Diameter (mm) 5

[0164] Thickness (mm) 2

[0165] Density p (kg / m3) 7850

[0166] Relative Permittivity S33T / £0 1200

[0167] Relative Permittivity EHT / SO 1250

[0168] Piezoelectric Charge Coefficient dsi (C / N) -1.2 x IO'10 Piezoelectric Charge Coefficient djj (C / N) 2.65 x ICT10Piezoelectric Charge Coefficient dis (C / N) 4.75 x ICT10Elastic Compliance Coefficient SnE(m2 / N) 1.18 x ICT11Elastic Compliance Coefficient SJJ1' (m2 / N) 1.33 x ICT11

[0169] Mechanical Quality Factor Q 2000

[0170] Table 3: Transducer metallic material properties obtained in AZO material website.

[0171] Young’s

[0172] Component Material Density (kg / m3) Poisson’s Ratio

[0173] Modulus (GPa)

[0174] Front Mass Ti-6A1-4V 4430 113 0.31

[0175] Back Mass, Stainless

[0176] 8000 193 0.26

[0177] Prestress Bolt Steel 316L

[0178] Electrode Copper 8930 127 0.34

[0179] Characterisation methods

[0180] Finite element analysis:

[0181] To miniaturize ultrasonic devices, transducers with half-wavelength of the L1 mode were modelled using FEA software (Abaqus-Simulia, Dassault Systemes). Transducers were tuned to resonance frequencies around 20 kHz and piezoceramic rings were centred at nodal planes in modal analysis after importing CAD files of transducer components. Models were successfully run after applying with a series of interactions, constraints located in the interaction surface of metallic components, and electrical boundary conditions from the PZT rings. The predicted resonance frequency results were used to finalise the dimension of back and front mass. In addition, gain value, mode shapes and waveforms of these transducers were collected to compare with the relevant experimental results afterwards.

[0182] Transducer performance characterisation:

[0183] The vibration characteristics of each transducer were measured using a 3D scanning laser Doppler vibrometer (MSA-100-3D, Polytec GmbH) under an 8 V peak-to-peak excitation. Regarding the experimental modal analysis (EMA), each transducer was placed and levelled on a soft sponge-like material mounted on a motorized stage. A grid of vibration measurement points was defined on the surface of the transducer. For the solid sections, three lines of grid points were defined and for the lattice section a single line of grid points was defined. To characterise the transducers, a periodic chirp excitation signal was used, exciting a frequency range of 1-100 kHz. Frequency response functions (FRFs) were then calculated from the input excitation signal and the output vibration response measurement at each grid point, enabling modal frequencies and mode shapes to be identified.

[0184] To study the vibration responses of the BLTs excited in resonance at different excitation levels, the input and output signals were controlled and recorded using a data acquisition hardware (USB-6345, National Instruments) controlled by the LabVIEW program. Transducers were excited via upward frequency sweeps with a resolution of 5 Hz through a narrow bandwidth across the resonance frequency of L1 mode, using a burst sine signal generated from a signal generator (Agilent 33210A, Agilent Technologies) and amplified by a power amplifier (HFVA-62, ETS Solutions). Their vibration amplitude was measured using a one-dimensional (1 D) laser Doppler vibrometer (OFV 303, Polytec GmbH) from the BLT front output face.

[0185] Finite element analysis results

[0186] Figures 4 to 27 illustrate the undeformed and deformed mode shapes of the transducers depicted in Figure 3, modelled in finite element analysis (FEA). In each transducer, A to H, the normalised axial vibration displacement amplitude distribution along the length of the transducer is the upper graph (longitudinal component of the mode) and the transverse amplitude distribution along the length of the transducer is the lower graph (bending component of the mode). It can be seen how the hierarchical structure of the front mass can control both the longitudinal amplitude gain and the excitation of a hybrid longitudinal-bending mode. For example, S2-L20-S78 (B) and S2-L75-S23 (E) show a dominant longitudinal mode and a negligible bending mode contribution; whereas S2-L30-S68 (C) and S2-L50-S48 (D) clearly depict a hybrid longitudinal-bending mode. In general, longitudinal amplitude gain increases with a combination of the increased length of the compliance region and the lattice extending towards the coupling region of the transducer. It can be seen here that, a transducer with a completely solid mass component (A) has a gain of 1 .2 whereas transducer (E) has a gain of 5.1 .

[0187] Figures 31 to 42 compare the mode shapes of transducers assembled by three triply periodic minimal surface (TPMS) mass components and a fully solid front mass via FEA modal analysis. The different TPMS geometries are visualised in Figure 30. The transducers exhibit the same longitudinal mode but with different amplitude gain values, in the following order: (a) gyroid, (b) diamond, (c) lidinold and (d) fully solid front mass. This exemplifies the effect of labyrinth geometry on vibrational output characteristics, and so the alteration of said geometry can be used to optimise the vibrational response of the transducer. All three TPMS geometries produced a larger amplitude gain than the fully solid front mass, but their individual responses were different. Alternative labyrinthine geometries can therefore be used to produce different vibrational responses.

[0188] FRFs extracted from EMA:

[0189] Figure 28 presents a set of FRFs with the corresponding vibration modes of Langevin transducers, extracted from EMA. In Fig. 28(a-b), a dominant longitudinal mode is obtained in S100 and S2-L20-S78. It can be seen there exists sufficient frequency separation (more than 13 kHz) between L1 and the neighbouring bending mode to avoid modal coupling. Longitudinal (L1) and bending (B4) hybrid motion are likely coupled in S2-L30-S68 and S2-L50-S48 due to very small frequency gaps (less than 0.8 kHz) between the two vibration modes, as shown in Fig. 28(c-d). S32-L20-S48 (see Fig. 28f) exhibits a similar FRF to S100 and S2-L20-S78 with no mode coupling. With a further increase of percentage of lattice structure in the front mass, longitudinal motion is dominant in S22-L30-S48 and S12-L40-S48 (see Fig. 28(g-h)). It is found that the higher order bending modes (e.g., B8, B9, B10) are not excited in both these transducers.

[0190] Experimental vibration responses:

[0191] The measured amplitude of axial displacement of the transducer output face at increasing excitation levels is shown in Figure 29. Fig. 29a shows that increasing the proportion of lattice in the front mass generally increases the achievable ultrasonic vibration amplitude but this can additionally be controlled by the positioning of that portion of lattice (see in Fig. 29b). Figure 29c provides data for transducers that have the same proportion of the lattice in the front mass but are located at different positions along the length. Here it can be clearly seen that lattice regions close to the coupling region produce a greater amplitude than those further away.

[0192] Discussion

[0193] Vibration mode control enabled by hierarchical lattice front masses:

[0194] As a benchmark transducer, the fully solid S100 vibrates in L1 mode with a resonance frequency around 20 kHz. For transducers assembled with front masses with the same percentage of solid bar (2%), the S2-L20-S78 transducer exhibits a pure L1 mode because the lattice volume is not sufficiently high to excite L-B coupling (see Fig. 28b). With a further increase of percentage of lattice structure in the front mass, an L-B hybrid mode is achieved in S2-L30-S68 and S2-L50-S48. Transducer S2-L75-S23 vibrates in the L1 mode because the lattice occupies most of the front mass. Again, a large frequency gap between the relevant longitudinal (L1) and bending (B6) mode guarantees this. For transducers (S32- L20-S48, S22-L30-S48, and S12-L40-S48) assembled with front masses with the same percentage (48%) of solid bar, they all vibrate in L1 around 20.0 kHz without mode coupling. The front masses possess the same features; the lattice is sandwiched between a large volume of solid bar. A conclusion is that the vibration behaviour of the transducers is dominated by the volume percentage of lattice structure. Also, a comparison of vibration characteristics between S2-L30-S68 and S22-L30-S48 illustrates that different mode shapes can be achieved in transducers with the same volume but different spatial positions of the lattice structure.

[0195] Vibration amplitude enhancement and device miniaturisation achieved by lattice structures: By comparing the vibration amplitude for each transducer, gyroid lattice structures have been found to dramatically enhance the achievable displacement amplitude (see Fig. 29(a, b)). In the lattice section of the front masses, a gyroid unit cell with pores results in a volume of the transducer that is more compliant than solid sections. According to the classic Hooke’s law, front masses with different compliance generate different strain for the same stress. A higher displacement amplitude can be obtained in the transducers assembled with a front mass with higher compliance. Promisingly, the achievable amplitude was shown to be doubled by incorporating the highest lattice volume front mass (S2-L75-S23) compared with using the fully solid front mass (S100). As illustrated in four transducers comprising two transducer pairs (S2- L20-S78 and S32-L20-S48, S2-L30-S68 and S22-L30-S48) having front masses with the same volume but different spatial positions of lattice structure. Interestingly, higher vibration displacement amplitude is achieved in the transducers with lattice structure located closer to the PZT stack. It can be found that the displacement amplitude gradient of the fully solid transducer is lower at the end but higher in the middle of the front mass (see Fig. 27). Displacement amplitude is higher in S32-L20-S48 and S22-L30-S48 because the high gradient is closer to the piezoelectric stack. It is noteworthy to mention that these vibration-enhanced transducers can be driven at a lower level of voltage. For example, the vibration amplitude of S2-L75-S23 driven at a very low voltage is the same as S100 driven at a six times higher voltage (see Fig. 29a). Therefore, less active power will be converted into heat in the PZT stack under electrical excitation. These devices are likely to perform better at the lower excitation level because piezoceramic properties degrade less under driving.

[0196] In addition to the desired amplitude enhancement, Langevin transducers can be reduced in size by incorporating lattice structures. Shorter front masses were used to assemble the four amplitude-enhanced transducers (S32-L20-S48, S22-L30-S48, S12-L40-S48, and S2-L75-S23) compared with S100. The four transducers all vibrate in a pure L1 mode but the wavelength of longitudinal-mode motion (used for the transducer length calculation) in these lattice transducers can be decreased by about 13 mm. These compliant structures can also be integrated into other transducer configurations, such as flextensional transducers, to achieve device miniaturisation

[0034] . Miniature devices bring benefits of, for example, integration of ultrasonic surgical devices with surgical robots to enable more minimally invasive surgeries (MIS) to be carried out with ultrasonic surgical tips

[0024] .

[0197] Further improvements for structural integrity:

[0198] Given the nature of the powder bed fusion by laser beam (PBF-LB) and TPMS structures, fatigue damage and micro-cracks may occur because of stress concentrations in regions of high vibration displacement amplitude. In some cases, a PBF-LB processed surface will have adhered powder that can provide crack initiation sites. Under cyclic loading at the ultrasonic frequency (20 kHz), small cracks can propagate rapidly through the lattice structures, especially if they have thin wall thickness (e.g. less than 1 mm).

[0199] Structural integrity of lattice front masses can be improved via different approaches. Considering geometrical optimisation, a systematic FEA prediction that involves the modal analysis and stress distribution of the proposed prototypes can be used to evaluate their structural integrity. The lifespan of a lattice front mass under cyclic vibration can be increased by using greater wall thickness unit cells. However, the trade-off between structural safety and amplitude gain should be considered. Importantly, an ultrasonic horn can be integrated to achieve a high displacement amplitude. In addition, AM build parameters can be optimised for different regions of the component to reduce sites of defects. For example, the porosity can be controlled; contour scan parameters can be set using the lowest energy density to yield a better as-built surface finish

[0035] ; post-processing treatments can improve structural integrity (e.g. heat treatment with a slow cooling rate has demonstrated to improve the ductility of PBF-LB processed Ti-6AI-4V due to the microstructural evolution (main acicular martensite a' is transformed to lamellar a+p)

[0036] ,

[0037] ; chemical etching has been adopted to reduce surface roughness and significantly improve the fatigue life of PBF-LB processed Ti-alloy components

[0038] ,

[0039] (e.g. the titanium dissolution process in nitric-hydrofluoric acid solution has been widely recorded

[0040] ,

[0041] and due to different chemical reactions, Ti3+adsorbed cations can be oxidized either as soluble titanium fluorides or insoluble titanium oxide).

[0200] Conclusion:

[0201] As disclosed herein, TPMS lattice structures can provide enhancement of the displacement amplitude of ultrasonic transducers. It is also possible to achieve hybrid mode vibration in such transducers.

[0202] ***

[0203] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0204] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0205] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0206] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0207] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0208] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%. A number of publications are cited above using numbers in square brackets such as [1]. These are cited in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. The entirety of each of these references is incorporated herein. Full citations for these references are set out below.

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Claims

Claims:1 . An ultrasonic transducer, the ultrasonic transducer comprising: an actuator configured to receive an electrical signal to generate ultrasonic vibration; and at least one mass component coupled to the actuator at a coupling region of the mass component and configured to transfer vibration energy from the coupling region to a vibration output region of the mass component; wherein the mass component comprises a modified compliance region in which there are provided openings that form voids through the mass component, the voids being arranged in a labyrinthine network.

2. An ultrasonic transducer according to claim 1 wherein the labyrinthine network is based on a periodic topology.

3. An ultrasonic transducer according to claim 1 or claim 2 wherein the labyrinthine network is defined by a triply periodic minimal surface (TPMS).

4. An ultrasonic transducer according to claim 3 wherein the TPMS is selected from: a) Schwarz primitive cos(X) + cos(Y) + cos(Z)' = c b) Schwarz diamond cos (X) cos (Y) cos (Z) — sin X)sin Y)sin Z) = c c) Schoen gyroid sin (X) cos (Y) + sin (Z) cos (X) + sin (Y) cos (Z) = c d) Schwarz neovius3[cos(X) + cos (Y) + cos Z ] + 4 [ cos(X)cos Y')cos(Z')] = c e) Lidinoid0.5 [sin(2X) cos(Y) sin— 0.5where X = 2nx / a, Y = 2ny / a, Z = 2nz / a, and a is the unit cell parameter and c is a constant.

5. An ultrasonic transducer according to any one of claims 1 to 4, the ultrasonic transducer comprising: a back mass; a front mass; the actuator; an ultrasonic horn arrangement forward of the front mass, wherein the back mass, actuator, front mass and ultrasonic horn arrangement are arranged along a longitudinal axis of the transducer, and the actuator is held between the back mass and the front mass; wherein the mass component comprising the modified compliance region is one or more of the front mass, back mass and ultrasonic horn arrangement.

6. An ultrasonic transducer according to claim 5 wherein the length of the transducer, measured from a proximal end of the back mass to a distal end of the ultrasonic horn arrangement, along the longitudinal axis, is not more than 40mm.

7. An ultrasonic transducer according to claim 5 or claim 6 wherein the transducer is a Langevin transducer.

8. An ultrasonic transducer according to any one of claims 1 to 7 wherein the mass component has a diameter in a direction perpendicular to the longitudinal axis of the transducer and the labyrinthine network of the modified compliance region extends continuously across the diameter of the mass component.

9. An ultrasonic transducer according to any one of claims 1 to 8 wherein the maximum diameter of the transducer, measured in a direction perpendicular to the length, is not more than 15 mm.

10. An ultrasonic transducer according to any one of claims 1 to 9 wherein in operation, the modified compliance region provides substantially any one of: a) no conversion from longitudinal to torsional or bending mode vibration; b) conversion from longitudinal to longitudinal-torsional mode vibration; c) conversion from longitudinal to longitudinal-bending mode vibration; d) conversion from longitudinal to longitudinal-bending-torsional mode vibration.

11. An ultrasonic transducer according to any one of claims 1 to 10 wherein, for a planar cross section taken perpendicular to the longitudinal axis at a position along the modified compliance region, the voids occupy at least 10% of the cross-sectional area of the mass component at that longitudinal axis position.

12. An ultrasonic transducer according to any one of claims 5 to 7 wherein the front mass comprises a proximal portion in contact with the actuator and a distal portion connected to the ultrasonic horn arrangement and an intermediate portion disposed between the proximal portion and the distal portion, and wherein the modified compliance region is provided in the intermediate portion.

13. An ultrasonic transducer according to any one of claims 1 to 12 wherein the modified compliance region has a porosity of at least 10%, the porosity being defined as the volume of the voids expressed as a percentage of the sum of the volume of the voids and the volume of the solid remainder of the modified compliance region.

14. An ultrasonic transducer according to any one of claims 1 to 13 wherein, for the mass component, the length Ltot of the mass component is defined as the distance between the coupling region of the mass component and the vibration output region of the mass component, and wherein the modifiedcompliance region has a length of Liab which occupies a proportion of Ltot, and wherein the remainder of the mass component other than the modified compliance region has a length of Lsoiid, so that:Ltot=Llab + Lsoiid and:Rsolid=(Lsoiid) / (Ltot) and wherein Rsoiid is at least 0.02.

15. An ultrasonic transducer according to claim 14 wherein:Rlab=(Llab) / (Ltot) and wherein Riab is at least 0.02.

16. An ultrasonic transducer according to claim 14 or claim 15 wherein the mass component includes a first solid region of length Lsoiidt at the coupling region of the mass component and a second solid region of length Lsoiid2 at the vibration output region of the mass component, wherein:Rsolidt=(Lsolidl) / (Ltot)Rsolid2=(Lsolid2) / (Ltot) and Rsoiidt is at least 0.01 and Rsoiid2 is at least 0.01 .

17. An ultrasonic transducer according to claim 16 wherein the mass component is configured so that Rsoiidi is greater than Rsoiid2 so that the modified compliance region is closer to the vibration output region of the mass component than to the coupling region of the mass component.

18. An ultrasonic transducer according to claim 16 wherein the mass component is configured so that Rsoiid2 is greater than Rsoiid 1 so that the modified compliance region is closer to the coupling region of the mass component than to the vibration output region of the mass component.

19. An ultrasonic transducer according to any one of claims 1 to 18 wherein the average porosity of one half of the length of the modified compliance region is different to the average porosity of the other half of the length of the modified compliance region.

20. An ultrasonic transducer according to any claims 1 to 19 wherein the specific volumetric surface area of the modified compliance region is at least 1200 m2 / m3, expressed as the surface area of the modified compliance region including the surface area of the voids, divided by the sum of the volume of the voids and the volume of the solid remainder of the modified compliance region.21 . An ultrasonic transducer according to any one of claims 1 to 20 wherein the labyrinthine network is formed by additive manufacturing.

22. A surgical tool comprising an ultrasonic transducer according to any one of claims 1 to 21 .

23. A welding tool comprising an ultrasonic transducer according to any one of claims 1 to 21 .

24. A method of manufacture of an ultrasonic transducer according to any one of claims 1 to 21 , wherein the modified compliance region, in which there are provided openings that form voids through the mass component, the voids being arranged in a labyrinthine network, is formed using an additive manufacturing process.

25. A method of operation of an ultrasonic transducer according to any one of claims 1 to 21 , the method including applying an electrical signal to the actuator to generate ultrasonic vibrations, transferring vibration energy from the coupling region to the vibration output region of the mass component, the ultrasonic transducer being configured so that at an operating frequency in the range 10- 100 kHz, a displacement amplitude at the vibration output region of the mass component is in the range 1-200 microns peak-to-peak.

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