Three-dimensional mechanical property measurement system for human skin

Abstract

The application provides a human skin three-dimensional mechanical property measuring system, comprising an in-plane biaxial loading device which is composed of four motors, four connecting members, four loading members and four sensors, realizes loading and stress collection of the human skin in x and y axial directions through displacement control, and a normal loading device which is composed of one industrial camera, one lens and one external light source; and a controller which realizes in-plane stretching / compression loading of the skin through the biaxial loading device, makes speckles move along with skin deformation, and obtains displacement field and strain field of a test area in a loading process based on a speckle image of the skin surface.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of human skin measurement and characterization, in particular to a human skin three-dimensional mechanical property measurement system. BACKGROUND

[0002] Skin is the largest tissue organ of human body, and is also the first barrier of human body, covering the surface of human body, protecting internal organs and tissues from external microorganisms, physical, mechanical and chemical damage, and realizing multiple functions such as sensing and regulating body temperature, metabolism, etc. In recent years, the quantitative analysis of skin mechanical properties has gradually attracted attention in many fields such as robots, surgery, beauty, medical devices, psychology, etc.

[0003] Due to the very complex structure and function of human skin, it is still very difficult to finely characterize the in vivo skin mechanical properties at present. The skin is composed of epidermis, dermis and subcutaneous tissue, and can be regarded as a multi-layer composite material, and the microstructure and mechanical behavior of each layer are different. In addition, the skin shows extremely complex mechanical properties, such as super-elasticity, viscoelasticity, anisotropy, etc., which makes it show the characteristics of "small load, large response" when subjected to mechanical stimulation. These complex properties are actually the basis for the skin to realize multiple functions. The mechanical response of the skin also takes various forms, providing rich information.

[0004] Although many scholars have carried out related research from the aspects of theory, experiment, numerical value, etc., the reliable identification of the constitutive parameters is not only the focus but also the difficulty. In terms of theory, the existing model still cannot completely describe the multi-layer structure, boundary conditions and loading mode of the skin, and there are still many challenges in establishing the complex constitutive model of the skin. With the increasing complexity of mathematical models, more and more mechanical parameters are involved, and the characterization of these parameters is seriously dependent on the acquisition of a large amount of experimental data. It is crucial to use experimental means to obtain various data, but there are still problems such as less quantity, single type and limited dimension of experimental data for in vivo skin. However, many microstructures inside the ex vivo skin have been damaged, and the difference between the mechanical properties of in vivo skin is large. Due to the strong limitation of in vivo skin, the original measurement method is greatly limited, and the current measurement means is difficult to simultaneously obtain these complex mechanical responses. Therefore, it is urgent to develop a new means which can simultaneously realize in-plane information and triaxial loading along the thickness direction, and can collect related data in real time. SUMMARY

[0005] The application provides a human skin three-dimensional mechanical property measurement system, which comprises an in-plane biaxial loading device, which is composed of four motors, four connecting members, four loading members and four sensors, and realizes loading and force collection of the human skin in x and y axial directions through displacement control, and a normal loading device, which is composed of one industrial camera, one lens and one external light source, and further comprises a controller, which measures the three-dimensional mechanical property of the human skin through the following operations: calibrating the imaging system according to a standard image collected; determining the relationship between the voltage and the loading force of the five force sensors based on a standard weight; obtaining the force data and displacement of the skin in the x and y axial directions of the in-plane biaxial loading device in the stretching / compression loading process; obtaining the indentation force and displacement of the skin in the z axial direction of the normal loading device in the loading process and the contact image in the whole normal loading process; preparing a large number of black mark points with different sizes and shapes as speckle features on the test area on the skin surface, i.e. the rectangular area formed by the four loading members; realizing the stretching / compression loading of the skin in the plane by using the biaxial loading device, so that the speckle moves with the deformation of the skin; adjusting the exposure time and the brightness of the ring light source according to the image observed by the industrial camera in real time before the experiment, so as to obtain the clearest measurement image; collecting the image of the skin with speckle in the whole loading process by using the industrial camera; and obtaining the displacement field and strain field of the test area in the loading process based on the speckle image on the skin surface.

[0006] Further, the displacement field and the strain field are obtained by: performing correlation calculation on the speckle image on the skin surface to obtain the change of the time-displacement field distribution in the whole loading process; determining the strain field according to the geometric relationship in material mechanics with the known plane displacement; and determining the principal strain size and direction based on the strain field.

[0007] Further, the method for calibrating the resolution of the imaging system obtains the corresponding relationship between the pixel number and the actual size in the contact image, and comprises the following steps: cutting a polyimide film into a square sheet with a size of 30mm*30mm by using a laser marking machine, and cutting holes with radii of 0.5mm, 1.0mm, 1.5mm, 2.0mm, 2.5mm and 3.0mm in the center as calibration test pieces; pasting the calibration test pieces on a fixed frame, loading by using the normal loading device until the hole edges are in close contact with the glass indenter and the calibration test pieces do not deform, and collecting the contact image at the corresponding moment; extracting the contact area based on the image processing method, and fitting the hole boundary by using a circle standard equation to determine the pixel number of the hole radius.

[0008] Further, the expression for determining the strain field according to the geometric relationship in material mechanics is as follows:

[0009]

[0010] where, u x , u y are displacements in x, y directions, ε x , ε y are normal strains in x, y directions, γ xy is shear strain, and the formula is mainly used for small deformation.

[0011] Further, for considering large deformation, the expression of strain field determined by geometric relationship is:

[0012]

[0013] Further, the expression of principal strain size and direction is:

[0014]

[0015] where, ε1, ε2 are maximum and minimum principal strains, respectively, and α is the angle between principal strain direction and x axis. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments described in the present application, and other drawings can also be obtained by those skilled in the art according to these drawings.

[0017] Figure 1 A measurement system of three-dimensional mechanical response of in vivo skin is shown according to an embodiment of the present disclosure;

[0018] Figure 2 A measurement system of in-plane bidirectional loading and deformation of human skin in vivo is shown according to an embodiment of the present disclosure;

[0019] Figure 3 A square sheet calibration specimen cut from a polyimide film by a laser marking machine is shown according to an embodiment of the present disclosure;

[0020] Figure 4 A contact image of the calibration specimen under normal loading until the hole edge is in close contact with the glass indenter and the calibration specimen does not deform is shown according to an embodiment of the present disclosure;

[0021] Figure 5 A pixel number of the contact area is extracted based on an image processing method, and a circular standard equation is used to fit the hole boundary to determine the pixel number of the hole radius according to an embodiment of the present disclosure;

[0022] Figure 6 It is shown that a plurality of black mark points of different sizes and shapes are prepared on the surface of the skin 4 as speckle features.

[0023] Reference signs

[0024] 1-sensor, 2-detachable industrial camera, 3-total connecting member, 4-skin, 5-motor, 6-connecting member, 7-loading member, 8-optical lens assembly, 9-transparent pressure head, 10-lens, 11-external light source. DETAILED DESCRIPTION

[0025] In order to make the personnel in the art better understand the technical solutions in the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the embodiments of the present application shall belong to the scope of protection of the embodiments of the present application.

[0026] In the description of the present application, it should be understood that the orientations or positional relationships indicated by the terms "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0027] In addition, in the description of the present application, unless otherwise specified and limited, it should be noted that the terms "mounting", "connecting", "connecting" should be understood broadly, for example, it can be mechanical connection or electrical connection, it can be the communication between the two elements, it can be direct connection, or indirect connection through intermediate medium, and those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0028] The loading ratio in the present disclosure is defined as: the loading ratio γ refers to the ratio of the stretching ratios in the x-axis and y-axis directions, which can be expressed as:

[0029]

[0030] Where λ x , λ y are the stretching ratios in the x and y directions, respectively.

[0031] As Figure 1As shown, this embodiment provides an in-plane bidirectional loading and deformation measurement system and process for human skin in a living state, including: an in-plane biaxial loading device, consisting of four motors 5, four connecting components 6, four loading components 7, and four sensors 1, which realizes loading in the x and y axis directions and force acquisition of the skin 4 through displacement control; a normal loading device, consisting of an industrial camera 2, an optical lens assembly 8, and a transparent indenter 9, which realizes loading in the z axis direction based on displacement control and acquires the normal force and contact image during the loading process; a general connecting component 3, which connects the in-plane biaxial loading device and the normal loading device to form an integrated loading device; and a control system, which is used to control the movement trajectory of the above multiple loading systems and collect their experimental data.

[0032] like Figure 2 As shown, this embodiment provides a measurement system and process for the three-dimensional mechanical response of in vivo skin, including:

[0033] The in-plane dual-axis loading device consists of four motors 5, four connecting components 6, four loading components 7, and four sensors 1. It achieves loading in the x and y axes and force acquisition on the skin 4 through displacement control. The normal loading device consists of an industrial camera 2, a lens 10, and an external light source 11. It achieves movement in the z-axis direction based on displacement control, changes the focal length to make the image clearer, and acquires the normal force and contact image during the loading process. The main connecting component 3 connects the in-plane dual-axis loading device and the normal loading device to form a whole loading device. The control system is used to control the movement trajectory of the above multiple loading systems and collect their experimental data.

[0034] Based on the above-mentioned loading device, such as Figure 1 and Figure 2 As shown, this embodiment provides a measurement system and process for the three-dimensional mechanical response of in vivo skin. It calibrates five force sensors across three axes, determines the imaging system resolution based on standard image data, writes loading test programs for different axial displacement ratios and rates, controls the in-plane displacement of the biaxial tension-compression loading device, and controls the contact and loading between the transparent indenter and the skin surface. It collects skin force data under biaxial tension-compression conditions and obtains contact images between the indenter and the skin during normal loading and unloading. The following steps, using a polyimide film to simulate human skin, illustrate the specific methods:

[0035] S1. Method for calibrating the resolution of the imaging system: Obtaining the correspondence between the number of pixels and the actual size in the contact image.

[0036] In this embodiment, a laser marking machine is used to cut the polyimide film into 30mm × 30mm square sheets, such as... Figure 3The calibration sample is pasted on the fixing frame, and the normal loading device is used for loading until the hole edge is in close contact with the glass pressure head and the calibration sample does not deform, and the contact image in the corresponding state is collected, as shown in Figure 4 The contact area is extracted based on the image processing method, and the pixel number of the hole radius is determined by fitting the hole boundary using the circle standard equation, as shown in Figure 5 .

[0037] S2, calibration of five force sensors of the loading system

[0038] In this embodiment, 1mg-500g weights are prepared, the force sensor 1 is connected to the small plate on which the weights are placed through a bolt to form a calibration system, the weights with different masses are placed on the plate in turn to determine the force sensor 1 reading, and the relationship between the weight and the force sensor reading is fitted.

[0039] S3, control of the loading device to realize three-axis loading of the skin and data collection

[0040] In this embodiment, the four motors 5 in the in-plane biaxial loading device are controlled to move the loading member 7 to the surface of the skin, reserve the z-direction loading range, and bond the loading member 7 to the skin 4 without force on the skin, so as to ensure that the readings of the corresponding sensors before and after bonding are basically stable without significant change. The loading ratios of the x-axis and y-axis directions in the in-plane biaxial loading device are set, different proportions and different smooth loading paths are set, and the loading displacements of the four motors 5 in the two directions are determined. The industrial camera 2 of the normal loading device is driven by the motor 5, and the z-direction movement displacement of the transparent pressure head is determined according to the distance between the pressure head 9 and the skin 4 and the preset loading depth. The four motors 5 in the in-plane biaxial loading device are controlled to control the in-plane movement distance of the loading member, so as to realize the in-plane tensile / compressive loading in the x-axis and y-axis directions. After the in-plane loading is completed, the pressure head is moved by the motor 5 to realize the normal vertical loading, and the skin is continuously pressed. At this time, the industrial camera 2 transmits the contact image in real time through the optical lens assembly 8. During the loading process, the five force sensors 1 obtain the feedback force data in the process of in-plane tensile / compressive loading and normal vertical loading in real time.

[0041] S4, joint measurement of force and deformation in the biaxial tensile / compressive loading process

[0042] In this embodiment, a large number of black mark points with different sizes and shapes are prepared on the surface test area of the skin 4, i.e. the rectangular area formed by the four loading members 7, as the speckle features, as shown in Figure 6 .

[0043] Four motors 5 in the in-plane biaxial loading device are controlled to move the loading member 7 to the skin surface, reserve the z-direction loading range, and bond the loading member 7 to the skin 4 without force, so as to ensure that the corresponding sensor readings before and after bonding are basically stable and do not change significantly.

[0044] The loading ratios of the x-axis and y-axis directions in the in-plane biaxial loading device are set respectively, different proportions and different smooth loading paths are set, and the loading displacement of the four motors 5 in the two directions is determined.

[0045] The industrial camera 2 of the normal loading device is driven by the motor 5, and a suitable lens focal length is selected according to the distance between the industrial camera 2 and the skin 4 and the camera parameters;

[0046] According to the image observed by the industrial camera in real time, the exposure time and the Figure 2 brightness of the external light source 11 are adjusted to obtain the clearest measurement image;

[0047] By controlling the four motors 5 in the in-plane biaxial loading device, the in-plane movement distance of the loading member is controlled, the in-plane stretching / compression loading in the x-axis and y-axis directions is realized, and the speckle moves with the skin deformation;

[0048] During the loading process, the four force sensors 1 in the in-plane biaxial loading device obtain the feedback force data in the in-plane stretching / compression loading and the normal vertical loading in real time, at this time, the industrial camera 2 continuously collects the contact image in real time through the Figure 2 lens 10;

[0049] The speckle image on the skin surface is calculated to obtain the change of the time-displacement field distribution in the whole loading process;

[0050] According to the geometric relationship in material mechanics, the expression of the strain field is determined as:

[0051]

[0052] Where, u x , u y are the displacements in the x and y directions, ε x , ε y are the normal strains in the x and y directions, and γ xy is the shear strain. This formula is mainly used for small deformation. For large deformation, the expression of the strain field determined by the geometric relationship is:

[0053]

[0054] Based on the strain field, the principal strain size and direction are determined:

[0055]

[0056] Where ε1 and ε2 are the maximum and minimum principal strains, respectively, and α is the angle between the principal strain direction and the x-axis.

[0057] In summary, compared with the prior art, the present invention has at least the following advantages and beneficial effects:

[0058] (1) The in vivo skin three-dimensional mechanical response measurement system proposed in this invention can be divided into an in-plane biaxial loading device and a normal loading device. It can be used alone to obtain the force-deformation relationship under in-plane and normal loading respectively, or it can be used together to achieve triaxial loading and measurement. This device can accurately measure experimental data related to the complex mechanical behavior of skin hyperelasticity, anisotropy, and multilayer structure in real application scenarios. It is applicable to different parts of the human body (face, forearm, back, abdomen, thigh, etc.), different environments (room temperature, cold, warm, dry, humid, etc.), different contact conditions (dry, lubricated, cracked, etc.), and different loading rates (fast, slow). It has a wide range of applications and strong operability, and solves the problems of insufficient data and single application scenarios in the current mechanical performance characterization.

[0059] (2) The in vivo skin three-dimensional mechanical response measurement system proposed in this invention has flexible application scenarios and can be transformed as shown. For example, by replacing the transparent indenter and optical lens assembly with a camera lens and external light source, an in-plane bidirectional loading and deformation measurement system for human skin in a living state is formed, realizing the force-displacement relationship during biaxial tension / compression. Combined with digital image correlation technology, the displacement field, strain field, and principal strain field of the test area can also be obtained. This device can be used to study skin deformation under different biaxial loading ratios and loading rates. In addition, by designing biaxial compression experiments, the buckling behavior of skin under different loading states can be studied, providing experimental methods and technical means to clarify the mechanical problems in the skin aging process.

[0060] (3) This invention is applicable to a wide range of applications, including characterization of complex mechanical properties, application of prestress, and research on contact behavior of various soft materials such as living skin, ex vivo skin, bionic skin, and silicone, and has certain practical value.

[0061] The above embodiments are only used to illustrate the embodiments of this application, and are not intended to limit the embodiments of this application. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the embodiments of this application. Therefore, all equivalent technical solutions also fall within the scope of the embodiments of this application, and the patent protection scope of the embodiments of this application should be defined by the claims.

Claims

1. A system for measuring the three-dimensional mechanical properties of human skin, comprising an in-plane biaxial loading device, which is composed of four motors, four connecting members, four loading members, and four force sensors, and a normal loading device, which is composed of one industrial camera, one lens, and one external light source or one industrial camera, one optical lens assembly, and one transparent indenter, wherein the industrial camera of the normal loading device is driven by a motor, and the total connecting members connect the in-plane biaxial loading device and the normal loading device to form an integrated loading device, and a control system for controlling the movement trajectory of the in-plane biaxial loading device and the normal loading device and collecting experimental data; the resolution of the imaging system is calibrated according to the collected standard images; when the normal loading device is composed of one industrial camera, one optical lens assembly, and one transparent indenter, the in-plane biaxial loading device and the normal loading device are controlled to achieve skin triaxial loading and data collection, comprising: Four motors in the in-plane biaxial loading device are controlled to control the in-plane movement distance of the loading member, realize in-plane stretching / compression loading in x-axis and y-axis directions, and after in-plane loading is completed, the transparent indenter is moved by the motor control to realize normal vertical loading, continuously press into the skin, and the industrial camera continuously collects contact images through the optical lens assembly; feedback force data during in-plane stretching / compression loading and normal vertical loading is obtained in real time during the loading process; when the normal loading device is composed of one industrial camera, one lens and one external light source, force and deformation combined measurement is realized through the biaxial stretching / compression loading process, including: a large number of black mark points with different sizes and shapes are prepared on the skin surface test area as speckle features; four motors in the in-plane biaxial loading device are controlled to move the loading member to the skin surface, and a z-direction loading range is reserved, and the loading member is bonded with the skin under the condition that the skin is not stressed; the industrial camera of the normal loading device is driven by the motor, and the appropriate lens focal length is calculated according to the distance between the industrial camera and the skin and the camera parameters; according to the image observed by the industrial camera in real time, the exposure time and the brightness of the external light source are adjusted to obtain the clearest measurement image; four motors in the in-plane biaxial loading device are controlled to control the in-plane movement distance of the loading member, realize in-plane stretching / compression loading in x-axis and y-axis directions, and make the speckle move with the skin deformation; during the loading process, four force sensors in the in-plane biaxial loading device obtain feedback force data in real time during in-plane stretching / compression loading and normal vertical loading, and the industrial camera continuously collects contact images through the lens in real time; the speckle image on the skin surface is calculated to obtain the change of time-displacement field distribution during the whole loading process; the strain field is determined according to the geometric relationship in material mechanics based on the known plane displacement; the principal strain size and direction are determined based on the strain field.

2. The human skin three-dimensional mechanical property measurement system of claim 1, wherein, The imaging system resolution calibration method obtains the corresponding relationship between the pixel number and the actual size in the contact image, and includes the following steps: a laser marking machine is used to cut a polyimide film into a square sheet with a size of 30mm*30mm, and holes with radii of 0.5mm, 1.0mm, 1.5mm, 2.0mm, 2.5mm and 3.0mm are cut in the center as calibration test pieces; the calibration test pieces are pasted on a fixing frame, and the normal loading device is used for loading until the hole edge is in close contact with the transparent indenter and the calibration test piece does not deform, and a contact image at the corresponding time is collected; based on an image processing method, the contact area is extracted, and the hole boundary is fitted by using a circle standard equation to determine the pixel number of the hole radius.

3. The human skin three-dimensional mechanical property measurement system of claim 1, wherein, The expression of the strain field determined according to the geometric relationship in material mechanics is: where u x , u y are displacements in x, y directions, ε x , ε y are normal strains in x, y directions, and γ xy is shear strain. This formula is for small deformation.

4. The human skin three-dimensional mechanical property measuring system of claim 3, wherein, For large deformation, the expression of the strain field determined according to the geometric relationship is: 。 5. The human skin three-dimensional mechanical property measurement system of claim 3, wherein, The expression of the principal strain size and direction is: Wherein, ε1 and ε2 are the maximum principal strain and the minimum principal strain respectively, and α is the angle between the principal strain direction and the x-axis.

Images