Patent Application
20240350162
This application claims priority from GB2110897.2 filed 29 Jul. 2021, the contents and elements of which are herein incorporated by reference for all purposes.
The present invention relates to an ultrasonic transducer and to a method of operation of an ultrasonic transducer. Such an ultrasonic transducer is of particular, although not necessarily exclusive, interest for surgical applications.
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 [1]. Generally, these devices adopt ultrasonic vibrations to enhance cutting performance, employing a transducer mounted in a hand-held device. In the case of soft tissue cutting some common features can be identified.
The Harmonic® ACE®+Shears together with the SENHANCE™ Ultrasonic (BOWA® Medical, BOWA-electronic Gmbh & Co. KG, Gomaringen, Germany) represent the only two ultrasonic cutting devices compatible with robotic surgery platforms at the time of writing, those platforms, respectively, the Da Vinci® Surgical System (Intuitive® Surgical Inc., Sunnyvale, California, USA) and the SENHANCE™ Surgical System (BOWA® Medical, Gomaringen, Germany).
One of the most recent inventions for Da Vinci® energy instruments is the EndoWrist® (Intuitive® Surgical Inc.). This articulated joint enables a range of motions at the end effector similar to that of the human wrist, thus replicating the experience of open surgery.
Several energy instruments are available at the time of writing with EndoWrist® technology, of 5-10 mm in diameter, which allows them to be inserted through a laparoscopic port and maneuvered inside the human body. To date, an ultrasonic cutting device with EndoWrist® technology does not exist.
In current ultrasonic cutting devices, only axial and rotation movements are permitted, as the transducers are too large to fit through the 5-10 mm trocar. The transducer is axially constrained to the robotic arm, and the waveguide, which transfers the ultrasonic vibration from outside the human body to the end effector inside, cannot be bent. This represents a significant disadvantage for surgical ultrasonic devices. In fact, although ultrasonic cutting has shown to be more precise and effective than other energy instruments, and with excellent coagulation speed, the latter are still preferred due to their dexterity.
U.S. Pat. No. 9,408,622 B2 discloses a bendable waveguide that addresses some of the problems identified above but still provides only limited dexterity.
Ultrasonically-actuated surgical tools are an alternative to electrocautery tools, with the potential to avoid some of the disadvantages of electrocautery tools, whilst achieving similar outcomes.
The Harmonic® ACE+Shears is the only ultrasonic device compatible with the Da Vinci® Surgical System. One of its main drawbacks is the lack of flexibility, as it is not compatible with EndoWrist®. The main reason for this is the size of the ultrasonic transducer which is too long and large to be clamped at the end of an end-effector wristed joint to fit through a burr hole or laparoscopic port.
Long rigid waveguides are preferred to transfer the vibrational energy at ultrasonic frequencies from outside to inside the human body. This, inevitably, limits ultrasonic energy instruments to a few applications not requiring accessibility to difficult sites.
Table 1 outlines the advantages and disadvantages of ultrasonic cutting instruments compared with monopolar and bipolar surgical tools for minimally invasive surgery. Despite the numerous advantages reported in Table 5.1, the lack of flexibility of the device in terms of maneuverability limits its application in several procedures as only axial targets can be reached, due to the long straight waveguide and the lack of articulated joints at the blade tip.
Ultrasonic technology is adopted in robotic abdominal laparoscopic surgery for parenchymal (functional tissue) transections where the parenchyma of an organ is dissected from connective and supporting tissue. Ultrasonic energy is also used in lobectomy procedures for the removal of a part of an organ. Other common laparoscopic procedures where ultrasonic energy is preferred, despite the unarticulated instrument, include gastrectomy, adrenalectomy, splenectomy and hepatectomy [6].
The present invention has been devised in light of the above considerations.
With current state-of the-art technology of Langevin transducer design, miniaturisation of an ultrasonic surgical tool such as a dissector cannot be achieved while still preserving device functionality and performance.
Accordingly, the present inventors have addressed the issues identified above by considering in detail the effect of the mechanical compliance of the components of a transducer along the vibrational energy transfer path of the transducer.
In this disclosure, we present different “developments” of the present invention, each comprising different optional aspects and further optional features. These are presented below as Development A and Development B.
In a first aspect of Development A, the present invention provides an ultrasonic transducer for surgical applications, the ultrasonic transducer comprising:
In a second aspect of Development A, the present invention provides a surgical tool comprising an ultrasonic transducer according to the first aspect.
In a third aspect of Development A, the present invention provides a method of operation of an ultrasonic transducer, the ultrasonic transducer comprising:
In a fourth aspect of Development A, the present invention provides a method of cutting tissue by ultrasonic cutting using an ultrasonic blade, including a method of operating of an ultrasonic transducer as set out in the third aspect and conducting the vibrational energy to the ultrasonic blade.
The transducer may be a Langevin type transducer, such as a bolted Langevin transducer. Such transducers are of interest for surgical applications due to their low frequency and high power operating characteristics.
The ultrasonic actuator arrangement may comprise a piezoelectric material element such as a piezoceramic element. A plurality of such elements may be provided. One or more associated electrodes may be provided in order to conduct a driving signal to the piezoelectric material element(s). Where the front and/or back mass is formed of an electrically conductive material such as metal, the front and/or back mass may provide ground electrical contact(s) for the piezoelectric material element(s).
The vibrational energy transfer path in the back mass, front mass and/or ultrasonic horn may include an annular portion. The annular portion may take the form of a hollow cylinder for example, with the wall of the cylinder extending along the vibrational energy transfer path and, in operation, vibrational energy passing along the wall of the cylinder. The plurality of openings intersecting the vibrational energy transfer path may therefore be provided through the wall of the annular portion. The openings are typically through-holes. Although it is possible in principle to use blind holes, the subsequent operation of the transducer is expected not to be as suitable.
Each opening may have a substantially uniform cross section along its depth. Considering the depth direction of the opening as the direction parallel to the walls of the opening, the depth direction may be substantially perpendicular to the vibrational energy transfer path (e.g. locally in the transducer).
The openings may each have the same size (e.g. the same diameter if the openings have circular cross sectional shape, or other characteristic linear dimension(s) for other shapes). The openings may have the same cross sectional area. Furthermore, the openings may have the same shape as each other, or may be selected from a limited number of shapes (e.g. two, three or four).
The openings may be arranged based on a repeating pattern. For example the openings may be arranged based on a regular lattice such as a square lattice, rectangular lattice, triangular lattice, hexagonal lattice.
Alternatively the openings may be arranged substantially randomly, or offset randomly from a notional regular repeating pattern.
There may be three or more openings. For example there may be 5 or more, 10 or more, 15 or more, 20 or more, 25 or more, 30 or more, 35 or more, 40 or more, 45 or more or 50 or more openings. There are typically fewer than 500 openings, although in some embodiments there may be more openings.
Suitable cross sectional shapes for the openings include circular, oval, elliptical, round, triangular, quadrilateral, rectangular, square, rhombus, pentagonal, hexagonal, pentagonal, octagonal etc. A combination of two or more such shapes may be used for the openings. The openings may have randomly-generated shape. The openings typically have a closed perimeter.
The openings may have a cross sectional area of, for example, at least 0.01 mm2, at least 0.05 mm2, at least 0.1 mm2, at least 0.2 mm2, at least 0.4 mm2, at least 0.6 mm2, at least 0.8 mm2, at least 1 mm2, at least 1.5 mm2, or at least 2 mm2. There is no particular upper limit on the cross sectional area of the openings, except that a suitable number of openings will need to be fitted to the overall size of the transducer in order to have an effect on the compliance of the relevant component.
The openings may be formed by any suitable manufacturing technique, e.g. machining, cutting (e.g. laser cutting) or etching, or by manufacturing the component with net shape or near net shape techniques such as casting, additive manufacture, etc.
The openings may be filled with a filler material. The filler material may have a Young's modulus that is lower (e.g. a factor of at least 5 lower, at least 10 lower, at least 20 lower or at least 50 lower) than the Young's modulus of the material through with the openings are formed. Suitable materials include epoxy materials or other resin-based materials.
Openings may be formed in two or more of the back mass, front mass and ultrasonic horn arrangement.
The operating frequency of the transducer may for example be at least 1 kHz, at least 5 kHz, at least 10 kHz, at least 20 kHz, or at least 30 kHz or at least 40 KHz. The operating frequency of the transducer may for example be at most 200 kHz, at most 150 kHz, at most 100 kHz, or at most 90 kHz, at most 80 KHz or at most 70 KHz. For example, a suitable operating frequency is about 55 KHz.
The displacement amplitude at the distal end of the horn may be in the range 1-200 microns peak-to-peak at the operating frequency.
In the method of operation, the ultrasonic transducer may for example be operated at a power density in the range 10-1000 Wcm−2. The ultrasonic transducer may for example be operated at a power in the range 1-1000 W.
As mentioned, the openings are formed in one or more of the back mass, front mass and ultrasonic horn arrangement. These are referred to here as the vibrational energy transfer path components of the transducer. Each of said components may have a monolithic construction, i.e. formed from a single piece of material without heterointerfaces (grain boundaries are of course permitted). Accordingly, the requirement that the plurality of openings provide an increased mechanical compliance in a direction along the vibrational energy transfer path for an inventive embodiment arrangement is intended to be compared with a reference arrangement in which the ultrasonic transducer is otherwise identical but in which the openings are replaced with the material of the relevant component. Considering the operating frequency of the transducer of an inventive embodiment arrangement compared with that of a reference arrangement, the operating frequency of the inventive embodiment arrangement may be at least 1 kHz (or at least 2 kHz, or at least 3 kHz, or at least 4 kHz, or at least 5 kHz, or at least 6 kHz, or at least 7 kHz, or at least 8 kHz, or at least 9 kHz, or at least 10 KHz) different (e.g. less) than the operating frequency of the reference arrangement. In turn, this allows inventive embodiment arrangements to have a smaller format than other reference arrangements for the same operating frequency. This means that it is more realistic for such transducers to be located for insertion into the body during surgery, rather than locating the transducers outside the body and using elongate waveguides. In turn, this means that the transducer can be located at the distal side of a flexible joint of a surgical device, allowing more convenient positioning of the transducer.
The length of the transducer (measured from the proximal end of the back mass to the distal end of the horn, along the vibrational energy transfer path) may be not more than 40 mm. The maximum diameter of the transducer (measured in a direction perpendicular to the length) may be not more than 15 mm.
Still further, considering an inventive embodiment arrangement compared with a reference arrangement as described above, the inventive embodiment arrangement may demonstrate one or more figures of merit that are the same as or better than the reference arrangement. For example, Kerr for the inventive embodiment arrangement may be the same as or better than keff for the reference arrangement. Additionally or alternatively, Qm for the inventive embodiment arrangement may be the same as or better than Qm for the reference arrangement.
Following on from their work leading to Development A, the inventors have carried out further investigations and consider that their innovations in this technical field can additionally be expressed as a further development, here termed Development B. It is intended that any of the following aspects and/or further optional features can be combined, singly or in any combination, with any of the aspects or optional features referred to with respect to Development A.
In a general aspect of Development B, the present invention provides an ultrasonic transducer for surgical applications, the ultrasonic transducer comprising:
In a first aspect of Development B, the present invention provides an ultrasonic transducer according to the general aspect in conjunction with one or more of the following features.
The transducer may be a Langevin type transducer, such as a bolted Langevin transducer. Such transducers are of interest for surgical applications due to their low frequency and high power operating characteristics.
The ultrasonic actuator arrangement may comprise a piezoelectric material element such as a piezoceramic element. A plurality of such elements may be provided. One or more associated electrodes may be provided in order to conduct a driving signal to the piezoelectric material element(s). Where the front and/or back mass is formed of an electrically conductive material such as metal, the front and/or back mass may provide ground electrical contact(s) for the piezoelectric material element(s).
The vibrational energy transfer path in the back mass, front mass and/or ultrasonic horn may include an annular portion. The annular portion may take the form of a hollow cylinder for example, with the wall of the cylinder circumscribing the longitudinal axis and extending along the vibrational energy transfer path and, in operation, vibrational energy passing along the wall of the cylinder. The plurality of openings intersecting the vibrational energy transfer path may therefore be provided through the wall of the annular portion. The openings are typically through-holes. Although it is possible in principle to use blind holes, the subsequent operation of the transducer is expected not to be as suitable.
The plurality of openings is arranged to increase the mechanical compliance in the direction along the vibrational energy transfer path. Specifically, a longitudinal axial stiffness of the back mass, front mass and/or ultrasonic horn may be modified by the presence of the openings. The axial stiffness affects the resonant frequency at which the longitudinal mode occurs.
Longitudinal-torsional (L-T) mode conversion can occur due to the degeneration of a longitudinal mode into a torsional mode or by the coupling of longitudinal and torsional modes [see, for example, Ultrasonics 52 (2012) 950-988]. In operation, the plurality of openings may provide substantially no longitudinal to torsional mode conversion. For example, the openings may be provided in an achiral array. Additionally, or alternatively, the openings may be provided in a non-helical array. If a helical array is identifiable in the array, then preferably a mirror symmetrical helical array is also identifiable in the array. Avoiding L-T mode conversion preserves the longitudinal mode, which is desirable for soft-tissue cutting.
The achiral nature of the array may be determined by the relative positions of the openings optionally in combination with the shape of each opening. For example, the relative position of each opening and the shape of each opening may be arranged such that the array of openings is superimposable onto a mirror image of itself. The mirror image may be defined with respect to a plane of reflection parallel to the longitudinal axis.
In some embodiments, the plurality of openings is not provided in the ultrasonic horn arrangement. Accordingly, to the extent that the present disclosure proposes that the openings are provided in one or more of the back mass, front mass and ultrasonic horn arrangement, and the transducer optionally including further features disclosed herein, it is expressly to be understood that a modification of this disclosure is that the openings are formed only in the back mass and/or front mass and not in the ultrasonic horn arrangement.
The arrangement of openings may be defined in relation to the geometric centre or centroid of each opening. The geometric centre is the average position of all points along the edge of the opening. The openings (e.g. geometric centres) may be provided in a reflective symmetrical array, such that, for at least part of the array, there may be at least one plane of reflective symmetry parallel to and coincident with the longitudinal axis. Alternatively or additionally, for at least part or parts of the array, there may be at least one plane of reflection symmetry perpendicular to the longitudinal axis.
The openings may be arranged based on a repeating pattern. For example, the openings may be arranged based on a two-dimensional regular lattice mapped onto the surface of the front mass and/or back mass. The regular lattice may comprise a unit cell which is defined by its lattice parameters a, b and B, where a and b are the lengths of the respective edges of the unit cell and β is the angle between the edges. The regular lattice may comprise a unit cell in which β is 90 degrees and/or a is equal to b. Regular lattices such as a square lattice, rectangular lattice, triangular lattice, hexagonal lattice may be suitable.
Alternatively the openings may be arranged substantially randomly, or offset randomly from a notional regular repeating pattern.
There may be three or more openings. For example, the openings may be formed in the front mass and/or the back mass and there may be 4 or more, 6 or more, 8 or more, 10 or more, 12 or more, 16 or more, 20 or more, 24 or more, 30 or more, 40 or more, or 50 or more openings. There are typically fewer than 500 openings, although in some embodiments there may be more openings.
The openings may be longitudinally offset from each other along the longitudinal axis. This longitudinal arrangement may comprise two or more openings. For example, there may be 3 or more, 4 or more, 5 or more, 8 or more, 9 or more, or 10 or more openings which are longitudinally offset from each other.
The openings may be arranged in an axial direction parallel to the longitudinal axis. The openings may be formed in the front mass and/or the back mass and for a plane parallel to and ending at the longitudinal axis coinciding with a maximum total number of openings intersecting the plane, the maximum total number of openings intersecting the plane may be at least 2 openings. For example, there may be at least 3 openings, at least 5 openings, at least 6 openings, or at least 8 openings intersected by the plane. The plane may coincide with the openings through their geometric centres.
Alternatively, or additionally, the openings may be arranged circumferentially. For example, the openings may be formed in the front and/or back mass and for a planar cross section taken perpendicular to the longitudinal axis at a position along the longitudinal axis coinciding with a maximum total number of openings intersecting the plane, the maximum total number of openings intersecting the planar cross-section may be at least 2 openings. For example, there may be 4 or more, 6 or more, 8 or more, 10 or more or 12 or more openings which are intersected by the planar cross-section. The planar cross-section may coincide with the openings through their geometric centres.
Alternatively, or additionally, the openings may be formed in the front and/or back mass and for a planar cross section taken perpendicular to the longitudinal axis at a position along the longitudinal axis coinciding with a maximum total number of openings intersecting the plane, the openings may occupy at least 10% of the circumference of the front mass or back mass, respectively. For example, the openings may occupy at least 20%, at least 30%, at least 40%, at least 50%, or at least 60% of the circumference of the front mass or back mass, respectively.
Openings may be formed in two or more of the back mass, front mass and ultrasonic horn arrangement.
Each opening may have a substantially uniform cross section along its depth. Considering the depth direction of the opening as the direction parallel to the walls of the opening, the depth direction may be substantially perpendicular to the vibrational energy transfer path (e.g. locally in the transducer).
Suitable cross sectional shapes for the openings include circular, oval, elliptical, round, triangular, quadrilateral, rectangular, square, rhombus, pentagonal, hexagonal, pentagonal, octagonal etc. A combination of two or more such shapes may be used for the openings. The openings may have randomly-generated shape. The openings typically have a closed perimeter.
The openings may each have the same size (e.g. the same diameter if the openings have circular cross sectional shape, or other characteristic linear dimension(s) for other shapes). The openings may have the same cross sectional area. Furthermore, the openings may have the same shape as each other, or may be selected from a limited number of shapes (e.g. two, three or four).
The openings may have a cross sectional area of, for example, at least 0.01 mm2, at least 0.05 mm2, at least 0.1 mm2, at least 0.2 mm2, at least 0.4 mm2, at least 0.6 mm2, at least 0.8 mm2, at least 1 mm2, at least 1.5 mm2, or at least 2 mm2. There is no particular upper limit on the cross sectional area of the openings, except that a suitable number of openings will need to be fitted to the overall size of the transducer in order to have an effect on the compliance of the relevant component.
The openings may be formed in the front mass and/or the back mass and each opening may comprise a plane of reflection symmetry parallel to the longitudinal axis.
Each opening may comprise a longitudinal length in a direction parallel to the longitudinal axis and a circumferential width in a circumferential direction perpendicular to the longitudinal axis. The circumferential width may be greater than the longitudinal length. For example, the value of the longitudinal length may be a percentage of the circumferential width less than 95%, less than 90%, less than 80%, less than 60%, less than 50% or less than 40%.
An opening may have a vertex angle θ which is bisected by a transverse plane perpendicular to the longitudinal axis. Alternatively, or additionally, an opening may have a vertex angle θ which is bisected by a plane parallel to the longitudinal axis. In some embodiments, the lateral (e.g. circumferential) and longitudinal dimensions of the opening are determined for a given cross-sectional area based on the size of the vertex angle θ.
The openings may be formed by any suitable manufacturing technique, e.g. machining, cutting (e.g. laser cutting) or etching, or by manufacturing the component with net shape or near net shape techniques such as casting, additive manufacture, etc.
The openings may be filled with a filler material. The filler material may have a Young's modulus that is lower (e.g. a factor of at least 5 lower, at least 10 lower, at least 20 lower or at least 50 lower) than the Young's modulus of the material through with the openings are formed. Suitable materials include epoxy materials or other resin-based materials.
The front mass may comprise a proximal portion in contact with the ultrasonic actuator arrangement and a distal portion connected to the ultrasonic horn arrangement and an intermediate portion disposed between the proximal portion and the distal portion. The openings may be provided in the intermediate portion.
The proximal portion and the intermediate portion may have substantially the same outer diameter. For example, the proximal portion and the intermediate portion may be formed integrally with each other. The intermediate portion and the proximal portion may have substantially the same outer diameter. For example, the front mass may comprise a substantially uniform cross-section. For example, the front and/or back mass may be a cylinder having a substantially constant diameter. The uniform cross-section and constant diameter are regarded as such without consideration for variations resulting merely from the presence or formation of openings.
An apparent elastic modulus of the intermediate portion may be not more than 80% of the apparent elastic modulus of a notional reference intermediate portion, the notional reference intermediate portion being identical to the intermediate portion except for the openings not being present.
The length of the transducer (measured from the proximal end of the back mass to the distal end of the horn, along the longitudinal axis) may be not more than 60 mm. For example the length of the transducer may be not more than 40 mm. The maximum diameter of the transducer (measured in a direction perpendicular to the length) may be not more than 15 mm.
The operating frequency of the transducer may for example be at least 1 kHz, at least 5 kHz, at least 10 kHz, at least 20 kHz, or at least 30 kHz or at least 40 KHz. The operating frequency of the transducer may for example be at most 200 kHz, at most 150 kHz, at most 100 kHz, or at most 90 kHz, at most 80 KHz or at most 70 KHz. For example, a suitable operating frequency is about 55 KHz.
The displacement amplitude at the distal end of the horn may be in the range 1-200 microns peak-to-peak at the operating frequency.
In the method of operation, the ultrasonic transducer may for example be operated at a power in the range 10-1000 Wcm−2.
As mentioned, the openings are formed in one or more of the back mass, front mass and ultrasonic horn arrangement. These are referred to here as the vibrational energy transfer path components of the transducer. Each of said components may have a monolithic construction, i.e. formed from a single piece of material without heterointerfaces (grain boundaries are of course permitted). Accordingly, the requirement that the plurality of openings provide an increased mechanical compliance in a direction along the vibrational energy transfer path for an inventive embodiment arrangement is intended to be compared with a reference arrangement in which the ultrasonic transducer is otherwise identical but in which the openings are replaced with the material of the relevant component. Considering the operating frequency of the transducer of an inventive embodiment arrangement compared with that of a reference arrangement, the operating frequency of the inventive embodiment arrangement may be at least 1 kHz (or at least 2 kHz, or at least 3 kHz, or at least 4 kHz, or at least 5 kHz, or at least 6 kHz, or at least 7 kHz, or at least 8 kHz, or at least 9 kHz, or at least 10 kHz) different (e.g. less) than the operating frequency of the reference arrangement. In turn, this allows inventive embodiment arrangements to have a smaller format than other reference arrangements for the same operating frequency. This means that it is more realistic for such transducers to be located for insertion into the body during surgery, rather than locating the transducers outside the body and using elongate waveguides. In turn, this means that the transducer can be located at the distal side of a flexible joint of a surgical device, allowing more convenient positioning of the transducer.
Still further, considering an inventive embodiment arrangement compared with a reference arrangement as described above, the inventive embodiment arrangement may demonstrate one or more figures of merit that are the same as or better than the reference arrangement. For example, keff for the inventive embodiment arrangement may be the same as or better than keff for the reference arrangement.
Additionally or alternatively, Qm for the inventive embodiment arrangement may be the same as or better than Qm for the reference arrangement.
In a second aspect of Development B, the present invention provides a surgical tool comprising an ultrasonic transducer according to the first aspect of Development B.
In a third aspect of Development B, the present invention provides a method of operation of an ultrasonic transducer according to the first aspect of Development B, the method including applying an electrical signal to the ultrasonic actuator arrangement to generate vibrations to be conducted into the front mass and into the ultrasonic horn arrangement along a vibrational energy transfer path and amplitude amplified by the ultrasonic horn arrangement.
In a fourth aspect of Development B, the present invention provides a method of cutting tissue by ultrasonic cutting using an ultrasonic blade, including a method of operating of an ultrasonic transducer as set out in the third aspect and conducting the vibrational energy to the ultrasonic blade.
The invention includes any combination of the aspects and optional features described except where such a combination is clearly impermissible or expressly avoided.
Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:
Further background to the present invention, and 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.
A generic surgical ultrasonic system with its key components is shown in
Each of the system's components has its own role and importance, and the design of each part follows certain rules to achieve the desired frequency, performance, and effect in tissue.
The design guidelines for Langevin transducers are the result of many years of successful applications. Numerous books, papers and theses have reported extensively on them. See for example Refs 8, 9 and 10. A brief overview of significant design specifications is given below, as a guide for design considerations.
The schematic perspective cutaway view of a classic d33-mode Langevin-type ultrasonic transducer shown in
The ultrasonic transducer is, in many known devices, inspired by the sandwich configuration introduced by Langevin and Chilowsky, firstly applied in underwater sound projectors in 1918 [1]. The Langevin design, also called ‘Tonpilz’, from the German for ‘singing mushroom’, simply consists of a stack of piezoelectric rings (normally piezoceramic), with intervening electrodes 52, the stack prestressed between a back mass 44 and a front mass 46 with a prestressing bolt 50 [8]. These types of transducers are also known as bolted Langevin transducers (BLTs). In
As a starting point for design, the case of a thin rod, where the diameter d is considerably smaller than the length l(d<<l), 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, A, 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,
where EM is the Young's modulus, and ρ the material's density. Equation 3 shows how the natural frequencies, fn, for the longitudinal vibrational modes n (n=1,2,3 . . . ) of a thin rod, can be estimated.
The relationship between the resonant frequency (recalling that ω=2πf), the acoustic impedance, ζ, and the length of both the piezoelectric stack and end masses is expressed by Equation 4.
The subscripts p and m represent piezoelectric and end-mass (either the back or the front mass) respectively. lp and lm are the thickness of the piezoelectric ring and the length of the end-mass, respectively. ζm is the acoustic impedance of an end-mass, which can be obtained from Equation 5:
and ζp is the acoustic impedance of the piezoelectric element obtained from Equation 6:
where A is the cross-sectional area of the component.
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 [12].
The piezoelectric stack 40 is generally made with either d33-mode rings or a d31-mode ring. In high power applications, ‘hard’ piezoceramic such as PZT 4 and PZT 8 is used in even numbers of rings to create the piezoelectric stack, placed between the front and back masses with electrodes in between.
The total length of the piezoelectric stack is chosen to be approximately one quarter of l. 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 [13].
The centre of the stack is best located at the nodal plane, labelled ‘node’ in
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
It has been shown by Lierke [15] 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, keff2, as shown in
The electrodes are normally chosen with material properties (density, elastic modulus, and acoustic impedance) similar to the piezoelectric materials to avoid unwanted stress concentrations. Note that here we refer to “electrodes” as separate entities from any metallisation formed on the piezoelectric materials.
In hand-held transducers, serpentine electrodes 52a, 52b, shown in
Metallization on the piezoelectric elements (rings) is normally prepared with 3-10 μm layers obtained through sputtered depositions, electroplating, or special coatings of Cr, Ni, or Au, or other combinations depending on the material suppliers, and these form the interface between the piezoelectric elements and the electrodes.
A prestressing bolt is used to apply and distribute the prestress within the stack. Titanium (or Ti-based alloys) is the preferred material due to its high strength and capability to withstand cyclic loads. The primary reason for prestress is to prevent the piezoceramic from experiencing excessive tensile stress during vibration as piezoceramics are typically almost seven times weaker in tension than in compression. Moreover, the prestress helps to stabilise both the resonance frequency and impedance magnitude and, in addition, it ensures continuity of electrical contact during high level of vibrations [16].
The fundamental desired characteristics of a prestressing bolt are low stiffness, achieved with long bolts, small shank diameter, and low Young's modulus, and high resistance to cyclic loads. The threads of the bolt stretch when the front mass stretches, and the generated friction between the bolt and the front mass can cause undesired heating and subsequent failure (e.g. via fatigue failure) of the thread. The stiffness of a prestressing bolt may be equal to or lower than the front mass stiffness, and may be lower than the piezoceramic stack stiffness. However, long transmission bolts suffer more from mechanical failure [17].
Generally, the required and optimal prestress to apply is not specified and it changes with each design and with the piezoelectric material used. The typical prestresses applied to hard piezoceramics are in the range 25-50 MPa for PZT 4 and 30-79 MPa for PZT 8 [16]. It is important to note that piezoceramic performance degrades with increasing prestress, which causes reductions in piezoelectric coefficients and maximum operating temperature; it may also cause piezoelectric material depolarisation and increase mechanical losses. All these parameters are also known to deteriorate more over time when prestress is applied.
Regarding piezocrystal behaviour under prestress conditions, it has been reported (see references to [20]) that uniaxial pressure, in the range of 0-60 MPa, negatively affects electromechanical properties, with the same consequences as highlighted for piezoceramics. Additionally, piezocrystals may also experience phase transitions under the simultaneous combination of high-power electric field and uniaxial pressure [21].
The back mass has a principal role as an inertial mass, and it also distributes the prestress laterally across the piezoceramic rings. It is normally a solid cylinder, but it has been reported that conic shapes can be used, increasing the bandwidth of the transducer [22]. A rule of thumb for a back mass made of steel indicates that its length should be at least 45% of the diameter of the piezoceramic stack, and its diameter should be at least equal to that of the piezoceramic stack. If other materials are used instead of steel, then the Young's modulus may be used as a comparison, and dimensions adjusted accordingly, in inverse proportion. For example, if a material with half the Young's modulus of steel is used, then the length of the back mass should be doubled (see references [8] to and [23]).
The front mass transfers the vibrational energy to the horn and the probe/blade and often includes a flange which connects to the case. The flange sometimes includes holes for air cooling (see
The horn is a mechanical amplifier which increases the vibration amplitude from the front mass: the vibrational energy travelling through a constant cross-sectional area remains constant; but if the cross-sectional area is progressively reduced along the direction in which the vibrational energy is travelling, then the vibrational energy density and the amplitude increase.
Different horn designs can be used such as stepped, linear or tapered, and exponential, as will be understood by the skilled person. Some horns convert the vibration from the longitudinal mode into a torsional or a transversal mode by using incisions on the surface. In some cases, multiple horns are linked together in cascade, with the amplification sections called ‘boosters’ [25].
The case protects the user from the high voltage and current needed for transducer excitation and also from excess heating. It is designed to clamp the device at its nodal plane so that the fundamental vibrational mode of the device will not be affected.
The waveguide or probe is generally a rod like structure which guides the wave to the tip-end. In devices for soft tissue cutting used in laparoscopic surgery, the tip-end is normally a blade at the end of a long wave guide. It is normally made of Ti6Al4V alloy (90% titanium, 6% aluminium, 4% vanadium, 0.25% iron and 0.2% oxygen) in order to handle the cyclic stresses of the ultrasonic vibrations and at the same time to ensure that it can withstand any loading. The length of the transmission rod is an integer number of half wavelengths, at the operational resonance frequency of the device, and it attaches to the horn at an antinode [26].
The blade at the end of the probe transfers the ultrasonic energy to the tissue and it is shaped according to the desired effect/application. Various blade designs are outlines below.
Suitable blade tips:
As already indicated, an important aspect of the transducer design process is the choice of the materials to be used, which depends on their properties and application requirements.
The Young's modulus, EM, quantifies the stiffness of elastic materials, defined in Equation 8:
as the ratio between stress σ and strain ε, where F is the force applied to the material through a cross sectional area A, and di is the change of length and the initial length is l, along an axis.
The elasticity indicates how a material restores to its original shape after distortion, and the restoring force is proportional to the stress applied. The elasticity is described by Hooke's Law, Equation 9:
where k represents the stiffness.
As is widely understood, Hooke's Law is valid only in the elastic region of the stress-strain curve, which is the part of the curve up to the yield strength. A general material may undergo:
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 are due to scattering and absorption, which are caused by motion of the 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 material 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 [10].
During cyclic tensile 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, e.g., with sharp corners and step profiles. These forces can concentrate, causing heating and failure e.g., through cracking. 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 [30].
Transducer components should also be acoustically matched. The speed of sound in a material is dependent on Em and ρ (Equation 2), but when another material is present, some of the energy will be transmitted forward and some will be reflected back from the interface between the materials, with the possibility that some energy will be converted into a different wave mode.
The amount of energy reflected back is expressed with the reflection coefficient, Rc, in Equation 10:
where ζ1 and ζ2 are the acoustic impedance of the first and second media respectively.
The closer the acoustic impedance of the materials, the more energy will be transmitted through the interface, which is desirable for efficient devices. Back and front masses should be made with different materials, with the ‘lower density’ one used for the front mass. This contributes to a larger vibrational amplitude at the front of the transducer. Hence, Equation 11:
should be respected to achieve maximum energy transmission between the piezoelectric stack and the front mass [10], [31].
Ultrasonic energy can be applied to media to generate different vibrational modes, the simplest of which is the longitudinal mode shown in
The longitudinal mode may also be used to generate other modes by modifying the waveguide and/or the horn as noted previously. Asymmetric longitudinal motion can be achieved by using asymmetric blades to add lateral vibrations to the longitudinal motion, making the cutting procedure more effective in certain applications, as previously mentioned for ultrasonic bone-cutting surgical tools.
The cutting mechanisms and dynamics of action depend on the specific surgical task. Cavitation is used in tissue-preserving devices, the direct impact or ‘jackhammer effect’ in bone cutting devices, and the thermal effect is adopted in devices for soft tissue cutting and coagulation.
In ultrasonic cutting devices used in soft tissue, the thermal effect is desired, and the tissue is heated to the point of denaturation. The tip of a device to generate this effect has a decoupled bifurcation, as shown in the inset of
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.
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 [25], [26].
The Da Vinci® Surgical System (Intuitive Surgical, Inc.) was introduced along with the EndoWrist® above. The Harmonic® ACE+Shears (Ethicon® Endo-Surgery) is to date the only Da Vinci®-compatible ultrasonic surgical tool. One of the drawbacks of this tool is its lack of maneuverability, i.e. incompatibility with the EndoWrist® technology. This incompatibility is due to several interrelated reasons which are discussed below.
First we consider the operational frequency and device configuration. Ultrasonic surgical devices with haemostatic dissection capability generally operate at about 55 kHz [34]. The operational frequency of a BLT is linked to the device length. Generally, BLTs are designed to operate primarily at their first longitudinal vibrational mode, L1, corresponding to half-wavelength. This puts a constraint on the device length. Additionally, the piezoelectric stack volume and position affect the device's efficiency and functionality. This determines the number of piezoelectric ring elements and their diameter within the stack to achieve the required vibrational amplitude performance. Under these conditions, the device cannot be miniaturised without compromising its operational frequency and without degrading performance, i.e. blade longitudinal vibration amplitude (about 80 μm) [35].
Next we consider the waveguide. From the previous conditions, it emerges that the resulting ultrasonic transducer is effectively too long and large to fit into a laparoscopic port; therefore, a waveguide is used to transfer the vibration from the transducer outside the human body to the end effector (blade) inside the human body. Consequently, this design limits the maneuverability of the blade in view of the need for the waveguide to be long enough to allow the transducer to be outside the body. In order to improve the maneuverability of the blade, it would be useful to include a wristed joint integrated at the end effector of the device. However, this would present an interruption in the waveguide and therefore an interface including a discontinuity. In that case, the ultrasonic wave would not be transmitted towards the blade but would be reflected, resulting in a non-moving blade.
The present inventors have considered whether any previous developments in this field would be useful to assist in the miniaturisation of an ultrasonic transducer that would be useful for surgery.
For example, in [36], BLT optimisation was carried out to miniaturise an ultrasonic scalpel for vessel cutting and sealing and integrate the device with a multi-degree of freedom end-effector for the Micro Hand® S robotic system (Tianjin University, China). A 55 kHz resonating device approximately 50 mm in length, and 10 mm in diameter was developed. The device was successfully integrated and mounted beyond a wrist-like joint and blade displacement of more than 100 μm was reported. Ex vivo experiments were carried out on chicken tissue, demonstrating the functionality and the potential of the device. Effectively, however, the work reported does not present any innovative miniaturisation strategy; since the length of the fabricated device is approximately what is expected for a 55 kHz resonator and is too large for practical laparoscopic surgery.
[37] and [38] report a folded horn transducer, in which the design of the horn allowed a reduction of the horn length by a factor of two whilst maintaining the same operational frequency. While this design reduces the overall length of the device, it does not decouple from the actual horn length.
Flextensional transducers are assembled with a piezoelectric disc sandwiched between two cymbal-shaped metal end caps. Several improvements have been reported for this design, such as the introduction of bolts to prevent failure due to the bonding epoxy layers because of high-power driving. Flextensional transducers can have a relatively small format. However a disadvantage of this design is the possible asymmetry arising from the epoxy bonding layers, which may alter the vibrational mode of the device. Moreover, typical designs exploit the radial mode of a piezoelectric disc, which is not suitable for anisotropic piezocrystals.
A planar ultrasonic silicon scalpel was reported in and [43]. This design used PZT piezoceramics and it was able to achieve a blade vibrational amplitude of about 50 μm at 68 kHz. The device dimensions were length=80 mm, thickness=20 mm and width=22.5 mm, which makes it unsuitable for miniaturisation with the present configuration and material properties of Si. This design was also shown and demonstrated with piezocrystals [23].
Another use of the d31 mode demonstrated the use of piezocrystals for the actuation of a standard needle for anaesthesia [44]. The design incorporated back and front masses and, for this reason, it can be termed a pseudo-Langevin device for its similarity with the piezoelectric ring stack configuration. This design achieves a needle tip displacement of less than 10 μm at 70 VPP. Moreover, the operating frequency of the device is approximately 80 kHz, and the total length without the needle is more than 40 mm. This means that to achieve 55 kHz the device total length must be increased, making this design even less suitable for miniaturisation purposes. A further drawback of these designs is the presence of a bonding layer, in this case made of conductive epoxy, which could fail under high vibrational stresses. Another bonding-related issue with this design can be attributed to the corrupted vibrational mode symmetry which causes stress concentration points which may cause device failure at high vibrational stresses, i.e. detachment from the substrate and cracking.
Next we consider the needs of robotic surgery, relevant to the present disclosure. The superior dissection quality and the speed of sealing of ultrasonic devices, over standard electrocautery, is reported in many studies, e.g. [45]. It is interesting to note that, only a few studies mention the generation of surgical smoke during the procedure [46], [47]. This may reduce laparoscopic visibility and cause delay in the overall procedure time due to endoscope cleaning.
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 [5], [6], [48]. Popular robotic procedures, involving the use of ultrasonic dissectors to perform specific intra-operative tasks, include: hepatectomies, splenectomies, enterectomies, adrenalectomies, and thyroidectomies. A study on robot-assisted thyroid surgery compared a wristed bipolar electrocautery instrument (Vessel Sealer Extend, Intuitive® Surgical, Inc.) with an ultrasonic dissector (Harmonic® ACE+Shears, Ethicon® Endo-Surgery).
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 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 maneuverability. Therefore, an ultrasonic dissector with a flexible joint, mounted at the end of the robotic shaft beyond the wrist, would address these issues.
The present invention is based on the realisation that it is possible to engineer the stiffness of components of an ultrasonic transducer in order to address some of the design constraints identified above and to decouple fr for the longitudinal mode from the device length. The word “metastructure” is used in the present disclosure (similarly to the concept of a “metamaterial”), indicates a structure engineered to exhibit mechanical properties which differ from those of the bulk material from which the structure is made.
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:
Young's modulus represents the stiffness of a material, therefore rod-like structures made with materials with low EM will 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.
A review of mechanical metamaterials shows some strategies to change the mechanical properties of a structure by engineering the unit cells forming the overall lattice. In particular, it is possible to alter the apparent Young's modulus of a structure in one or more directions, typically leading to an increase in anisotropy of the Young's modulus for the structure.
To implement this concept for ultrasonic transducers, a study was performed and results successfully indicated that a transducer cylindrical component can be engineered to modify its apparent Young's modulus and therefore modify the frequency of the longitudinal mode without altering the transducer length. In this study, three different formats for a hollow cylindrical component were considered, each being suitable for use as a front mass of a BLT transducer.
As shown in
The SpringNet sample shown in
The Holes sample shown in
The shape of holes for each sample was changed based on
For a non-hollow cylinder, E=121 GPa (i.e. the Young's modulus of the bulk material) and the frequency response is shown by the upper dashed line. For a hollow cylinder, the apparent Young's modulus is 42 GPa and the frequency response is shown by the lower dashed line. Each sample incorporating holes arranged as described has a lower resonant frequency for a particular length of cylinder compared with the hollow cylinder. Or, put another way, adding holes reduces the length of the cylinder needed in order to achieve a particular resonant frequency. Furthermore, decreasing angle θ for each type of hole leads to a reduction in resonant frequency. Note that the annular wall thickness of the hollow cylinder and the samples incorporating holes is the same.
Based on the study reported above, further investigations were carried out based on the honeycomb (HC) shaped holes. This choice was made because the HC showed great capability in lowering the resonant frequency without introducing other vibrational modes in proximity to L1. The abbreviation TSM used herein refers to Tuneable Stiffness Metastructure.
HC-TSM angle θ and the number of HC-TSM features per plane (f.p.p.), were investigated under the hypothesis that these variables should have an impact on the apparent Young's modulus, hence on fr for the L1 mode. All the models investigated in this study are shown in
Similarly,
Table 2 shows the effect of the introduction of HC-TSM on the relevant transducer parameters of the L1 mode. The key point that emerges from this design study is that the L1 mode was altered without modifying the length of the transducer. In addition, minor but positive changes were observed in other transducer properties, such as reduced electrical impedance magnitude at fr and improved operational bandwidth.
It is therefore possible to make a comparison between a “standard” 55 kHz transducer and an exemplary HC-TSM transducer, where the transducers differ in the construction of the front mass based on the discussion set out above. The most important initial comparison that can be made between the two transducer models is the capability of the HC-TSM device to resonate at the L1 mode with a lower frequency than the standard device despite being of equal length and made from the same materials, as shown in
Table 3 compares the device parameters for the L1 mode of the devices. The introduction of HC-TSM enabled a transducer design which was 20.5% shorter in length than the standard design with approximately the same fr. Moreover, the HC-TSM transducer presents 30% lower electrical impedance at resonance and 28% more bandwidth than the standard design.
In each of
Referring in particular to
In operation, the arrangement of openings provides substantially no longitudinal to torsional mode conversion. This preserves the longitudinal mode. This is achieved by the holes being provided in an achiral array, which is superimposable onto a mirror image of itself.
In this embodiment, there are 24 holes in total, which are all formed in the front mass. The holes each have the same depth and cross sectional area. The holes are provided in a reflective symmetrical array (with respect to the geometric centre of each hole), such that there are multiple planes of reflective symmetry parallel to and coincident with the longitudinal axis. One such plane of reflective symmetry is shown in
For a plane parallel to and ending at the longitudinal axis coinciding with a maximum total number of holes intersecting the plane (for example, plane R shown schematically in
The achievable vibrational amplitudes of standard and HC-TSM 55 kHz transducers were compared. The HC-TSM transducer showed a 33% higher displacement at the blade tip than the standard design for the same driving signal.
In a further study carried out in order to further explore the insights presented above, a number of front masses having differing arrays of openings were proposed to assess the resulting resonant frequencies. The front mass structures were modelled on arrangements of holes formed in a hollow cylindrical front mass, the front mass having an outer diameter of 10.00 mm.
The general shape of each hole used in this further is that of a rhombus (with rounded corners of internal radius of curvature 0.10 mm) with two lines of symmetry. Each hole is aligned to the longitudinal axis of the front mass (and therefore to the longitudinal axis of the transducer) such that one line of symmetry is parallel to the longitudinal axis while the other line of symmetry is perpendicular to the longitudinal axis. In other words, a first pair of opposite interior angles of the rhombus are bisected by a plane parallel to and coincident with the longitudinal axis, while a second pair of opposite interior angles are bisected by a plane perpendicular to the longitudinal axis. Each front mass structure comprises one of five types of rhombus-shaped hole having different dimensions. As illustrated in
The value of the longitudinal length as a percentage of the circumferential width is 57.73% for the holes with a 120° axial angle and 26.79% for the holes with a 150° axial angle, given that each side length of the rhombus-shaped holes is 1.00 mm.
Three parameters were altered between each front mass: the axial angle of the holes, the number of holes in the axial direction (i.e. arranged parallel to the longitudinal axis), and the number of holes in the circumferential direction (i.e. arranged perpendicular to the longitudinal axis).
For simplicity in this further study, the arrangement of the holes was based only on rectangular lattices superimposed onto the cylindrical geometry of the front mass.
The number of holes in the axial direction was varied between 1, 3 and 5. The number of holes in the circumferential direction was varied between 2, 4 and 8. Therefore, 45 new front mass structures were investigated in total.
The following nomenclature was developed for the purposes of identifying each front mass structure: XA_YP_Z, where X, Y and Z represent the number of holes in the axial direction, the number of holes in the circumferential direction, and the axial angle, respectively.
Each front mass has an outer circumference of 31.42 mm. Therefore, for a planar cross section taken perpendicular to the longitudinal axis at a position along the longitudinal axis coinciding with a maximum total number of holes intersecting the plane, the proportion of the circumference occupied by the maximum number of holes (or circumferential fill-factor) for each front mass structure is presented in Table 5 below.
Inspecting the changes in resonant frequency between the 150 1A_8P_150, 3A_8P_150, and 5A_8P_150 devices having the greatest axial angle, an initial 5.5% decrease in resonant frequency is observed, followed by a 4.5% decrease. This trend is exhibited in each series of device with a common angle; thus, increasing the number of axial holes from three to five prompts a smaller decrease in resonant frequency when compared to the change in resonant frequency after increasing the number of holes from one to three, indicating that an inversely proportional relationship exists between the resonant frequency and the number of axially arranged holes.
Hence, further increasing the number of holes in the axial direction will have a diminishing effect on the resonant frequency. The number of axial holes is limited by the dimensions of the transducer, with further increases in the number of holes risking impairment of the structural integrity.
Taking the 5A_X_150 class of devices (where X is either 2P, 4P, or 8P), for example, there is a 2.2% decrease followed by an 8.8% decrease in resonant frequency when increasing the number of holes from two to four and from four to eight, respectively. This trend is exhibited by each of the classes of devices; thus, increasing the number of holes in the circumferential direction from two to four generates a small change in resonant frequency when compared to increasing the number of holes from four to eight. Such a non-linear relationship suggests that further increases to the number of perpendicular holes continue to effectively reduce the resonant frequency. The number of holes, however, is limited by the dimensions of the holes relative to the circumference of the transducer. Similar to the number of holes in the axial direction, increasing the number of holes with impair the structural integrity of the front mass, with stress analyses similarly aiding in understanding the maximum number of holes possible in this direction without adversely affecting the stability of the front mass.
The resonant frequency decreases as the angle of the hole increases. Devices that incorporate holes with angles less than 90° exhibit an expected increase in frequency when compared to the standard solid model, as clarified by the dividing, dotted black line. A decrease in resonant frequency was anticipated for devices featuring holes with an axial angle greater than 90°, with eleven of these eighteen devices exhibiting such a decrease. Hence, we may conclude that although increasing the axial angle of the holes is an effective strategy to reduce the resonant frequency of a given device employing an array of holes, it is through the combination of modifying the three parameters related to the array of holes that one would see the greatest decrease in resonant frequency.
The results of this study demonstrate that increasing the number of circumferential holes has the greatest effect on the resonant frequency, followed by the angle of the holes, and the number of axial holes. However, modification to all three of these parameters is advantageous.
The greatest decrease in resonant frequency was observed for the 5A_8P_150 device; a 10.5% decrease in frequency was observed from the solid model resonant frequency to attain a resonant frequency of ˜35 kHz. Comparing this device to a commercially available BLT with a resonant frequency of 40 KHz (manufacturer part number: SMBLTD45F40H) demonstrates that the designed device exhibits a lower frequency despite possessing a similar length (5A_8P_150=53 mm, SMBLTD45F40H=53.75 mm).
To ensure that modification of the front mass structure is a viable strategy to miniaturise ultrasonic transducers, the extent of longitudinal or axial displacement for the modified devices should be similar or greater than that of the standard solid or hollow models. In particular, the transducer should have a suitable gain which is greater than that of the solid and hollow standard models. The gain is determined by the ratio of the maximum axial displacement of each end of the transducer, i.e. the front end (distal end) of the front mass and the back end (proximal end) of the back mass. The axial displacement is the positional change of a part of the ultrasonic transducer in a direction parallel to the longitudinal axis, relative to its equilibrium position. Therefore, the position where the displacement is equal to zero corresponds to the node of the device.
The greatest increase in gain relative to the standard, solid model occurred for the 5A_8P_150 front mass type, which provided an 82.4% increase in gain. This device also exhibited the greatest decrease in resonant frequency.
It is important to recognise that differences in resonant frequency are caused by changes in mass as well as the influence of the altered front mass structure on mechanical compliance and wave propagation. Therefore, the effect of mass loss on transducer operation was investigated by considering the volume of mass removed for each front mass type for comparison with the resonant frequency of each front mass type.
Comparing the masses and resonant frequencies of the 5A_8P_30 (40.2 kHz) and 5A_8P_150 (35.3 kHz), it is observed that, despite possessing the same mass (6.54 g), there is a difference in resonant frequencies of 4.9 kHz, corresponding to a 12.2% decrease in resonant frequency for a 0.0% decrease in mass. It is therefore abundantly evident that the decrease in resonant frequency is driven primarily by changes to the structure of the front mass rather than by variations in mass.
In a further study, three transducer models having different arrangements of openings were proposed for direct comparison with the ‘standard’, solid model and a ‘folded’ front mass model. The standard solid model uses solid front and rear masses except that there is a passage as required through the rear and front mass for the bolt. All five models include a front mass, back mass, two piezoelectric rings, two electrodes and a bolt extended through the back mass and coupled to the front mass. Each transducer has the same total length equal to 67 mm and the same total diameter equal to 15 mm. The material metal making up each device, including the front mass, the back mass and the bolt is Titanium (Ti) and the electrode material for each device is Copper (Cu). Each device model uses the same piezoelectric type PZ26 (MEGGITT) [52].
Table 6 shows the resonant frequency of the L1 mode and the gain from the centre of the piezoelectric stack to the distal end of the front mass for each transducer device model. As shown by the previous design studies, the frequency of the L1 mode was altered without modifying the length of the transducer. Compared to the standard model, all three modified devices provide a lower resonant frequency and a higher gain. The resonant frequency decreased for the folded front mass model relative to the standard model. Notwithstanding this, the FM-mod and FM&BM-mod2 models have lower resonant frequencies than the folded front mass model. This proves that the mechanical compliance of a transducer device may be reduced to a greater extent by having a plurality of openings instead of a folded front mass arrangement.
All three transducer models having arrays of holes formed in the front mass and/or back mass have a larger gain than either the folded front mass model or the standard model. The FM&BM-mod2 transducer device model has the lowest resonant frequency out of the five transducer models presented in Table 6.
As an observation, it is noted that the FM&BM-mod device model has a higher resonant frequency than the Folded device model. Without wishing to be bound by theory, it is considered that this is likely to be due to a combination of both higher modal density and the lattice itself dominating the vibrational response. The results are of interest in particular in view of the considerable increase in gain compared to the folded horn structure, given that there is only <1 kHz difference in resonant frequency.
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.
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.
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.
Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.
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.
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 in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2110897.2 | Jul 2021 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/071469 | 7/29/2022 | WO |
