WO2005002428A1

WO2005002428A1 - A device for evaluating the dynamic biomechanical properties of skin

Info

Publication number: WO2005002428A1
Application number: PCT/AU2004/000907
Authority: WO
Inventors: John Sugden Field; Michael Vincent Swain; Richard Charles Appleyard; Elizabeth Dawes-Higgs
Original assignee: The University Of Sydney
Priority date: 2003-07-07
Filing date: 2004-07-07
Publication date: 2005-01-13
Also published as: AU2003903479A0

Abstract

Device including housing (12), for evaluating dynamic mechanical properties of skin, having resiliently flexible beam (34) displaceably arranged relative to at least par of housing so as to be displaceable in a reciprocatory manner relative to the housing upon the application of an oscillating electric field. Beam is multimorph comprising laminate of pizoelectric materials. Beam carries engaging element (38) for engaging skin to be evaluated, and displacement measuring arrangement (72, 74) for measuring displacement of the beam. Housing carries pressure controller (56) for controlling pressure applied to skin by engaging element.

Description

"A device for evaluating the dynamic biomechanical properties of skin"

Field of the Invention This invention relates to the evaluation of the dynamic, biomechanical properties of skin. More particularly, the invention relates to a device for, and a method of, evaluating the dynamic, biomechanical properties of skin, such as, for example, the viscoelastic properties of the skin, whether living or artificial skin.

Background to the Invention Skin is a body tissue of living organisms. It consists of the dermis and epidermis. The epidermis or outer layer is comprised of stratified squamous epithelium and the dermis or inner layer is comprised of connective tissue. Skin has many functions, but, in particular, plays an important role in body haemostasis, mainly through its barrier function. Skin flexibility is vital for body motion and skin diseases can therefore be very debilitating and may result in permanent disability. An understanding of the biomechanical properties of skin is important for assessing the ability of the skin to respond to environmental stresses. It also enhances understanding of the reduced ability of aged skin to respond effectively to environmental stresses. For example, it is widely known that there is a decrease in the- rate and extent of elastic recovery in the skin of aged persons. However, elastic recovery has been difficult to quantify with accuracy. In a clinical environment, estimation of the biomechanical properties of skin has so far been largely subjective with evaluation being done by dermatologists. Some difficulty is encountered when comparing ihter-observer and intra-observer results, especially when these are obtained at different times. There is a need for a simple and reliable apparatus that can permit the identification of the biomechanical properties of skin in the clinical environment on a repeatable basis. Devices have been developed in an attempt to quantify the biomechanical properties of skin. However, these have mainly utilised static loading, An example of such a device is described in US Patent No. 4,206,769 which makes use of a suction force applied perpendicularly to the skin surface. Recognised limitations of such a device involve insufficient resolution and difficulties in obtaining repeatable data. Data may also be difficult to interpret. Furthermore, this device cannot measure viscoelasticity or be used on more rigid skin areas and cannot be used to evaluate skin anisotropy. In an attempt to address these issues and to incorporate dynamic loading conditions, other devices have been developed. One such device involves the propagation of shear waves in the outermost layers of the skin in a frequency range of 0.5-30kHz. Examples of such devices are described in US Patent Nos. 4,947,851 and 5,115,808. These devices claim to determine the modulus of elasticity of tissue from the velocity of a surface wave using the equation:- E = p x v² where -

E = modulus of elasticity p = density of the tissue v = velocity of an acoustic wave. Viscoelasticity is not measured as a function of low range frequencies that is more appropriate to the stresses normally experienced by skin. An understanding and exact quantification of the viscoelasticity of skin as a function of low range frequencies is useful in the follow-up of disease progression and in evaluating the efficacy, or otherwise, of treatment Thus, there is a need for a simple and reliable dynamic apparatus that can permit the identification of the viscoelasticity of skin in-vivo iri the clinical environment on a repeatable basis,

Summary of the Invention According to a first aspect of the invention, there is provided a device for evaluating dynamic, mechanical properties of skin, the device including:- a housing; a resiliently flexible beam displaceably arranged relative to at least a part of the housing so as to be displaceable in a reciprocatory manner relative to said. at least part of the housing upon the application of an oscillating electric field, the beam heing a multimorph comprising a laminate of piezoelectric materials; an engaging element carried by the beam for engaging the skin to be evaluated; a displacement measuring means arranged on the beam for measuring displ acement of the beam; and a pressure controller carried by the housing for controlling pressure applied to the skin by the engaging element. As indicated above, the beam is, preferably, a multimorph being a laminate of at least two piezoelectric materials. Each ^■piezoelectric material may be a piezoelectric ceramic material. The housing may comprise an outer sleeve, closed off at a first end and a tubular member slidably arranged within the sleeve, the tubular member projecting through an opposed, open end of the sleeve. The first end of the sleeve may be closed off by a connector which is used to establish electrical contact between the deflection meowin means, the pressure controller and controlling equipment to which the device is connected for use. The pressure controller may include an urging means interposed between an anchoring formation in the sleeve and an operatively inner end of the tubular member. The urging means may be a spring of a predetermined spring force maintained under compression within the sleeve. The pressure controller may further include a displacement measuring means for monitoring displacement of the tubular member in a longitudinal direction relative to the sleeve and against the action of the urging means. Conveniently, the spring may be arranged about a spindle extending from an operatively inner end of the tubular member into an interior of the sleeve. The displacement measuring means may comprise a magnetic member carried on an inner end of the spindle supporting the spring, to be displaceable together with the spindle, and a magnetically responsive element fixedly arranged in the interior of the sleeve, the magnetically responsive element being responsive to displacement of the magnetic member relative to the magnetically responsive element The magnetically responsive element may be a magnetic field sensor which operates on a Hall effect principle. Thus, by knowing the spring constant and displacement of the magnetic member, a load applied by the device to the skin can be determined. An operatively outer end of the tubular member may be closed off by an end plate, the end plate having an opening defined in it in which an end of the engaging element is received. The end of the engaging element may lie substantially flush with an outer surface of the end plate. The engaging element may be arranged at an end of the beam in alignment with a longitudinal axis of the beam. An opposed end of the beam may be anchored within the tubular member. The deflection measuring means may be in the form of a strain gauge array attached to the beam. The strain gauge array may be mounted at, or adjacent, that end of the beam anchored witiύn the tubular member. The electric field applied to the beam may be in the form of a voltage arising from a voltage generating means of the controlling equipment that generates a harmonic signal of the appropriate frequency or a combination of frequencies.

Typically, the frequency of the electric field is in a range at which the skin operates and to which the skin is subjected via external factors. Thus the frequency at which the beam oscillates may be in the range of about 1Hz to lOOHz, preferably about 15Hz to 25Hz and, optimally, in the region of 20Hz. The controlling equipment may include a signal processor connectable to the deflection measuring means for assessing the dynamic behaviour of the beam, in use, to evaluate the dynamic, mechanical properties of the skin. The processor may be operable to generate a first signal with the engaging element out of contact with the skin and at least one further signal with the engaging element in contact with the skin. A combiner may combine the signals so as to separate the influence of the beam and the engaging element from the material being evaluated. According to a second aspect of the invention, there is provided a method of evaluating dynamic, mechanical properties of skin, the method including the steps of:- urging an engaging element into contact with the skin, the engaging element being carried on a resiliently flexible beam responsive to an electric field, the beam comprising a multimorph being a laminate of piezoelectric materials; controlling pressure applied to the skin by the engaging element; applying an oscillating electric field to the beam to cause it to oscillate at a predetermined frequency so that the engaging element periodically applies a deforming force to the skin; monitoring the dynamic behaviour of the beam as it oscillates; and extracting from the monitored, dynamic behaviour of the beam, data relating to the dynamic, mechanical properties of the skin. The method may include providing a housing comprising an outer sleeve, closed off at a first end and a tubular member slidably arranged within the sleeve and projecting through an opposed, open end of the sleeve with the engaging element being arranged in the tubular member, the method further including controlling the pressure applied to the skin by an urging means contained within the housing and acting on the tubular member. The method may further include monitoring displacement of the tubular member in a longitudinal direction relative to the sleeve and against the action of the urging means. The method may include assessing the dynamic behaviour of the beam, in use, to evaluate the dynamic, mechanical properties of the skin. Thus, the method may include obtaining a first reading with the engaging element out of contact with the skin and at least one further reading with the engaging element in contact wilh the skin. Further, the method may include combining the readings so as to separate the influence of the beam and the engaging element from the material being evaluated. The method may include using signals of the deflection of the beam to form . transfer functions. Hence, the method may include processing the transfer functions 5 simultaneously to give values of the dynamic properties of the skin, in particular, its dynamic complex stiffness at a predeteπnined frequency. The method may include applying the oscillating electric field to the beam at a frequency at which the skin operates and to which the skin is subjected by external factors.

Brief Description of the Drawings An embodiment of the invention is now described by way of example with 10. reference to the accompanying drawings in which:- Figure 1 shows a three dimensional view of a device_} in accordance with an embodiment of the invention, for evaluating dynamic, mechanical properties of skin; Figure 2 shows a schematic, side view of the device; Figure 3 shows an end view of the device; 15 Figure 4 shows a sectional, side view of the device taken along line IV-IV in Figure 2; Figure 5 shows, on an enlarged scale, an end view of the device; Figure 6 shows a side view of a slidable inner assembly of the device; Figure 7 shows an end view of the assembly; and 20 Figure 8 shows a sectional side view of the assembly taken along line VHI-Vm in Figure 6.

Detailed Description of the Exemplary Embodiment In the drawings, reference numeral 10 generally designates an exemplary 25 embodiment of a device for evaluating dynamic, mechanical properties of skin. The device 10 comprises a housing 1 having a sleeve 14. An inner assembly 16 is slidably arranged in, and relative to, the housing 12. The assembly 16 is shown in greater detail in Figures 6 to 8 of the drawings and will be described with reference to those drawings below. The assembly 16 is slidably arranged in a bore 18 (Figures 2 and 4) of the 30 sleeve 14. The assembly 16 includes a tubular member 20 which projects through an opening 22 in a first, distal end 24 of the sleeve 14. An opposed, proximal end 26 of the sleeve 14 of the housing 12 is closed off by a closure assembly 28. The closure assembly 28 includes a liner 30 which lines a part of the bore 18 of the sleeve 14 and terminates approximately midway along a length of the bore 18 of the sleeve 14, as shown in greater detail in Figure 4 of the drawings. The closure assembly 28 includes an electrical connector plug 31 which is received in an opening 32 of the assembly 28. The connector plug 31 is used to 5 establish an electrical connection between components of the device 10, as will be described in greater detail below, and controlling equipment (not shown) to which the device 10 is connected for use. A resiliently flexible beam 34 is arranged in a passage 36 of the tubular member 20, cantilever-fashion, to be displaceable relative to the tubular member 20. The beam

10 34 is a b moiph comprising a laminate of two piezoelectric materials. Both piezoelectric materials are ceramic materials. If desired the beam 34 can include a central, metallic shim. An engaging element in the form of a tongue 38 is carried at a first, operatively outer end of the beam 34, The tongue 38 is in alignment with the beam 34 with the 15 beam 34 and the tongue 38 being arranged along a longitudinal axis of the housing 12, An opposed end 40 of the beam 34 is anchored in an anchor 42 received in an operatively inner end of the tubular member 20. An operatively outer end 44 of the tubular member 20 is closed off by an end plate 46 (shown most clearly in Figure 5 of the drawings). An opening in the form of a 20 slot 48 is defined in the end plate 46. An end of the tongue 38 is received, in the slot 48 to lie flush with an outer surface of the end plate 46 or to protrude outwardly beyond the end plate 46 to a slight extent. The slot 48 is dimensioned so that limited transverse movement of the tongue 38 is possible. The bore 18 of the sleeve 14 has a constricted distal end region 50 to inhibit 25 transverse movement of the tubular member 20 relative to the sleeve 14. A collar 52 is arranged at the transition between the wider part of the bore 18 and the constricted region 50 to support the inner end of the tubular member 20 and to inhibit withdrawal of the tubular member 20 from the bore 18 of the sleeve 14. A deflection measuring means in the form of a strain gauge array 54 is carried 30 on each side of the beam 34. The strain gauge arrays 54 are arranged on the beam 34 close to the entry of the beam 54 into the anchor 42. The strain gauge arrays 54 measure strain in the beam 34 and, more particularly, outer fibres of the beam 34. The device 10 includes a pressure controller 56 for controlling the pressure applied to the skin, in use, by the device 10. The pressure controller 56 includes a coil

35 spring 58 supported on a spindle 60. The spindle 60 extends from an operatively inner end of the tubular member 20 towards the closure assembly 28 and lies along the longitudinal axis of the sleeve 14 and, hence, the housing 12. An annular insert 62, defining an opening 64, is arranged substantially midway along the bore 18 of the sleeve 14. The insert 62 abuts against an operatively inner end of the liner 30. An end of the spindle 60 passes through an opening 64 in the insert 62.

One end of the coil spring 58 is received in the insert 42 of the tubular member 20 with an opposed end of the spring 58 being received in an annular recess 66 defined in the insert 62. The spring 58 is of a predetermined spring constant and is maintained in slight compression about the spindle 60 when the tubular member 20 is at its maximum extension relative to the sleeve 14 as shown in the drawings. The insert 62, the liner 30 and the connector plug 31 define a chamber 68 in which a further part of the pressure controller 56 is contained. The further part of the pressure controller 56 includes a holder 70 housing a magnetic member or magnet 72.

The holder 70 is formed as an extension of the spindle 60. The pressure controller 56 further includes a magnetically responsive element in the form of a magnetic field sensor 74 fixedly mounted in position in the chamber 68 of the sleeve 14 in spaced relationship relative to the magnet 72. The magnetic sensor 74 is a Hall effect device and monitors displacement of the magnet 72 relative to the sensor 74. Accordingly, by knowing the spring force of the spring 58 and by monitoring the displacement of the tubular member 20 by means of the displacement of the magnet 72 relative to the sensor 74, the pressure applied by the end face 46 of the tubular member 20 and the tongue 38 on the skin being evaluated can be monitored for repeatability purposes. The connector plug 31 defines openings through which leads pass for connecting the magnetic sensor 74 and the strain gauge arrays 54 to the controlling equipment, hi addition, the beam 34 is electrically connected to the controlling equipment. In use, an oscillating electrical field is applied by the controlling equipment to the beam 34. The electrical field has a predetermined frequency, for example 20 Hz, and may, if desired, be a composite field of several frequencies. The beam 34 responds to the applied electric field, the beam 34 being caused to deviate from its rest position in an oscillatory manner as determined by the electric field. When an outer surface of the end plate 46 of the tubular member 20 is in contact with the skin at a predetermined pressure as monitored by the pressure controller 56, the tongue 38 applies a shearing force to skin pinched in the gaps between the sides of the slot 48 and the tongue 38. The force applied to the skin by the device 10 is controlled in a repeatable manner by the spring 60, the magnet 72 and the magnetic sensor 74 in combination. The dynamic characteristics of the mechanical response of the beam 34, with the tongue 38 in contact with the skin, are similar to those of a cantilever and a viscoelastic material in parallel and the characteristics of the viscoelastic properties of the skin can be determined by means of appropriate mathematical functions, more particularly,

Fourier transforms. An initial reading is taken with the tongue 38 out of contact with the skin to determine the free response of the cantilever-arranged beam 34. Further readings are then obtained with the tongue 38 in contact with the skin. The initial and further readings are combined to determine the response of the skin. Thus, signals from the strain gauge arrays 54 are fed to the controlling equipment where the signals are processed to evaluate the dynamic, mechanical properties of the skin and, more particularly, its dynamic, complex stiffness at a predetermined frequency or frequencies. From this, an evaluation of skin characteristics can be made, in particular, the viscoelastic properties of the skin. It is an advantage of the invention that a device 10 is provided which is capable of analysing the viscoelastic properties of skin by characterising the stiffness and loss angle of the skin. Previous devices of which the applicant is aware have not been capable of directly measuring the viscoelastic properties of the skin. Instead, these previous devices quantified the viscoelastic properties in terms of static responses to deformation or values of Young's Modulus obtained from shear wave velocities at relatively high frequencies. This also requires that the density of the skin under investigation be accurately known. A further advantage of the invention is that the device operates within a range of frequencies over which the skin normally functions or to which the skin is normally subjected. Viscoelasticity is frequency dependent and may be significantly different over different frequency ranges. Further, the device is able to quantify the anisotropic

(directional) properties of the skin. Yet a further advantage of the invention is that the device 10 is made of a material which obviates the need for adhesives to be applied to the skin during data collection. In addition, the inclusion of the control means 56 facilitates, control of the downward pressure on the skin by the device 10 to improve repeatability of the measurements made by the device 10. Finally, the device 10 is able to be scaled down in order that subcutaneous tissue does not influence data collection in some body sites. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A device for evaluating dynamic, mechanical properties of skin, the device including:- a housing; a resiliently flexible beam displaceably arranged relative to at least a part of the housing so as to be displaceable in a reciprocatory manner relative to said at least part of the housing upon the application of an oscillating electric field, the beam being a multimorph comprising a laminate of piezoelectric materials; an engaging element carried by the beam for engaging the skin to be evaluated; a displacement measuring means arranged on the beam for measuring displacement of the beam; and a pressure controller carried by the housing for controlling pressure applied to the skin by the engaging element.

2. The device of claim 1 in which each piezoelectric material is a piezoelectric ceramic material,

3. The device of claim 1 or claim 2 in which the housing comprises an outer sleeve, closed off at a first end and a tubular member slidably arranged within the sleeve, the tubular member projecting through an opposed, open end of the sleeve.

4. The device of claim 3 in which the pressure controller includes an urging means interposed between an anchoring formation in the sleeve and an operatively inner end of the tubular member.

5. The device of claim 4 in which the urging means is a spring of a predetermined spring force maintained under compression within the sleeve.

6. The device of claim 4 or claim 5 in which the pressure controller further includes a displacement measuring means for monitoring displacement of the tubular member in a longitudinal direction relative to the sleeve and against the action of the urging means.

7. The device of claim 6 in which the displacement measuring means comprises a magnetic member carried on an inner end of a spindle supporting the spring, to be displaceable together with the spindle, and a magnetically responsive element fixedly arranged in the interior of the sleeve, the magnetically responsive element being responsive to displacement of the magnetic member relative to the magnetically responsive element.

5 8, The device of claim 7 in which the magnetically responsive element is a magnetic field sensor.

9. The device of any one of the preceding claims in which an operatively outer end of the tubular member is closed off by an end plate, the end plate having an opening

1.0 ^" defined in it in which an end of the engaging element is received.

10. The device of any one of the preceding claims in which the engaging element is arranged at an end ofthe beam in alignment with a longitudinal axis ofthe beam.

15 11. The device ofany one ofthe preceding claims in which the deflection measuring means is in the form of a strain gauge array attached to the beam.

12. The device of any one of the preceding claims in which a frequency of the electric field is in a range at which the skin operates and to which the skin is subjected

20 via external factors.

13. A method of evaluating dynamic, mechanical properties of skin, the method including the steps of:- urging an engaging element into contact with the skin, the engaging element 25 being_. carried on a resiliently flexible beam responsive to an electric field, the beam comprising a multimorph being a laminate of piezoelectric materials; controlling pressure applied to the skin by the engaging element; applying an oscillating electric field to the beam to cause it to oscillate at a predetermined frequency so that the engaging element periodically applies a deforrriing

30 force to the skin; . _' monitoring the dynamic behaviour ofthe beam as it oscillates; and extracting from the monitored, dynamic behaviour ofthe beam, data relating to the dynamic, mechanical properties ofthe skin.

35 14. The method of claim 13 which includes providing a housing comprising an outer sleeve, closed off at a first end and a tubular member slidably arranged within the sleeve and projecting throngh an opposed, open end of the sleeve with fhe engaging element being arranged in the tubular member, the method further including controlling the pressure applied to the skin by an urging means contained within the housing and acting on the tubular member. ^'5 15. The method of claim 14 which further includes monitoring displacement of the tubular member in a longitudinal direction relative to the sleeve and against the action ofthe urging means.

10 16. The method ofany one of claims 13 to 15 which includes assessing the dynamic behaviour of the beam, in use, to evaluate the dynamic, mechanical properties of the skin.

17. The method of claim 16 which includes obtaining a first reading with the 15 engaging element out of contact with the skin and at least one further reading with the engaging element in contact with the skin.

18. The method of claim 17 which includes combining the readings so as to separate the influence of the beam and the engaging element from the material being evaluated.

20 19. The method of claim 16 or claim 17 which includes using signals of the deflection ofthe beam to form transfer functions.

20. The method of claim 19 which includes processing the transfer functions 25 simultaneously to give values of the dynamic properties ofthe skin.

21. The method of any one of claims 13 to 20 which includes applying the oscillating electric field to the beam at a frequency at which the skin operates and to which the skin is subjected by external factors.