Hydro-indentation: a system and method for soft material characterization

Abstract

This system and method removes the complexity introduced by the shape and surface properties of the indenter tip by performing hydro-indentation and by replacing the concurrent force measurement with a local sensing of the resisting pressure at the indented surface. This is achieved by engaging the material surface with a drop of incompressible liquid suspended at the tip of a liquid-filled needle. Being fully covered by the lubricating liquid, the needle tip avoids direct contact with the soft material throughout the indentation. As the liquid drop is depressed against the material surface, pressure builds-up within the liquid. Rather than measuring the applied force, the pressure is measured directly via a pressure sensor.

Description

[0001] Hydro-Indentation: A System and Method for Soft Material Characterization

[0002] This application claims priority of U. S . Provisional Patent Application Serial No . 63 / 762, 327, filed February 24, 2025, the disclosure of which is incorporated herein by reference in its entirety .

[0003] Field

[0004] This disclosure describes systems and methods for characterizing soft material, and specifically using the pressure of an incompressible fluid to determine the characteristics .

[0005] Background

[0006] Traditionally, indentation devices used in industry and academia employ hard-bodied indenters with tips of various shapes, such as conical, spherical, cylindrical and others . These hardbodied indenters are used to characterize materials . As shown in FIG. 1, vertical displacement (5) of the indenter deforms the solid, and the resisting force (F) is concurrently measured. The resulting F - 5 curve is then compared to analytical predictions to evaluate the elastic properties of soft materials, such as the elastic modulus (also known as Young' s Modulus) . However, this process employs simplifying assumptions on the interfacial properties (such as adhesion and friction) between the indenter and the sample . While in certain settings, such as those with stiff materials and at large scales, the interfacial properties may be negligible, in soft and biological materials, these effects determine the contact area of the indenter and can dominate theresponse . This process thus necessitates the decoupling of complicated contact physics, limiting the confidence in recovering material properties and introducing complicated geometric assumptions in post-process . This limitation of current indentation devices for the use in soft materials is well documented in the literature .

[0007] Therefore, it would be beneficial if there were a system and method to characterize soft materials that did not suffer from these drawbacks .

[0008] Summary

[0009] This system and method remove the complexity introduced by the shape and surface properties of the indenter tip by performing hydro-indentation and by replacing the concurrent force measurement with a sensing of the resisting pressure at the indented surface . This is achieved by engaging the material surface with a drop of incompressible liquid suspended at the tip of a liquid-filled blunt needle . Being fully covered by the lubricating liquid, the needle tip avoids direct contact with the soft material throughout the indentation. Instead, as the liquid drop is pushed against the soft material surface, the liquid droplet captured within the geometry of the needle tip becomes pressurized. Rather than measuring the applied force, the pressure is measured directly via a pressure sensor.

[0010] According to one embodiment, a system for characterizing a soft material is disclosed. The system comprises a needle filled with an incompressible liquid; an actuator to move the needle orthe soft material toward the other, such that the incompressible liquid presses against the soft material; a pressure sensor to measure a pressure of the incompressible liquid in the needle as it is pressed against the soft material; and a controller to receive pressure measurements from the pressure sensor and displacement values from the actuator to determine one or more characteristics of the soft material . In some embodiments, a lubricating liquid surrounds a distal end of the needle . In certain embodiments, the lubricating liquid is a same liquid as the incompressible liquid. In some embodiments, the needle comprises a syringe needle . In some embodiments, the controller computes an elastic modulus for the soft material based on the pressure measurements and the displacement values . In some embodiments, the soft material is disposed on a stage, and the actuator moves the stage toward the needle . In some embodiments, the actuator moves the needle toward the soft material .

[0011] According to another embodiment, a method of characterizing a soft material is disclosed. The method comprises pressing an incompressible liquid disposed in a needle against the soft material; and measuring a displacement of the needle and a corresponding pressure of the incompressible liquid at a plurality of points, wherein the displacement and the corresponding pressure are used to characterize the soft material . In some embodiments, force is not used in the characterization. In some embodiments, an insertion rate of a needle is varied in order to capture ratedependence of the soft material . In some embodiments, the soft material is stratified. In certain embodiments, characteristics of each layer of the stratified soft material may be determined. In some embodiments, the soft material is heterogeneous . In some embodiments, the needle is pressed against the soft material in aplurality of locations, wherein the plurality of locations are disposed within one square millimeter . In some embodiments, an elastic modulus of the soft material is calculated from the displacement and the corresponding pressure . In some embodiments, the needle is pressed against the soft material until the soft material is punctured. In certain embodiments, information related to puncture resistance is calculated. In certain embodiments, the needle is continued to be inserted into the soft material after puncture . In certain embodiments, information related to tearing modulus is calculated after the soft material is punctured. In certain embodiments, after the soft material is punctured, the needle is held stationary and information related to viscoelastic relaxation and / or diffusion properties is calculated while the needle is held stationary.

[0012] Brief Description of the Drawings

[0013] For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which like elements are referenced with like numerals, and in which:

[0014] FIG. 1 shows an indentation device according to the prior art ;

[0015] FIG. 2 shows the indentation device and soft material according to one embodiment;

[0016] FIGs . 3A-3B show two embodiments of the system used for indentation in more detail;

[0017] FIG. 4 is a schematic graph showing expected pressure and displacement created during one test;FIG. 5 is a schematic graph showing expected pressure and time for the test in FIG. 4;

[0018] FIG. 6 is a schematic graph showing expected pressure and displacement for a stratified material during one test; and FIG. 7 is a schematic graph showing expected pressure and displacement created during one test using a nonlinearly elastic material .

[0019] Detailed Description

[0020] This system and method address the issue of insufficient and inconclusive datasets for soft material mechanical properties traditionally obtained via indentation. The system and method reduce the number of variables, such as friction and geometry, that need to be modeled and removed from data collection. Using hydro-indentation with a small, needle-based approach enables more effective stress-field characterization in soft materials than traditionally larger and more rigid indenters . The reduced contact length scale localizes the stress field beneath the working fluid, allowing analytical solutions and improved assessment of the material' s mechanical response .

[0021] In contrast to the complex boundary conditions introduced by hard-bodied indenters of predefined shapes with varying contact areas, which can be dominated by soft material adhesion and frictional properties, hydro-indentation subjects the surface of the material of interest to an incompressible fluid-based indenter of known diameter circular geometry, and eliminates adhesion and frictional forces .FIG. 2 shows a view of this hydro-indentation. The system includes a hollow needle 10 that is filled with an incompressible liquid 20. The hollow needle 10 may be any suitable size, such as between 14 and 25 gauge, although smaller or larger needles may be used. This incompressible liquid 20 may be any suitable liquid, such as water . In certain embodiments, care is taken to fill the needle 10 to ensure that there are no air bubbles in the liquid. Additionally, the tip of the needle 10 may be coated with a lubricating liquid 30. In some embodiments, this lubricating liquid 30 may be the same as the incompressible liquid 20 or may be a different incompressible liquid, such as mineral oil .

[0022] As the needle 10 is pressed toward the soft material 40, and once the contact is fully established, the contact area 41 remains roughly constant throughout the hydro-indentation procedure . Further, during the displacement (5) of the needle 10, the pressure, P, of the incompressible liquid 20 is measured. Further, the displacement of the needle 10 is also measured. This allows the creation of a pressure-displacement (P-5) curve, as shown in FIG. 2.

[0023] Mathematically, this significantly simplifies the solution. For example, in the linear range of the mechanical response, the pressure becomes linearly dependent on the indentation depth, with

[0024] P =

[0025]

[0026] , wherein a is the radius of the needle 10 and v is the Poisson' s ratio (usually approximately b for soft materials) . This approach dramatically enhances the accuracy of the prediction, since the boundary conditions are well defined.

[0027] FIGs . 3A-3B show the system in more detail . As described above, the system includes a needle 10 filled with anincompressible liquid 20. A mechanical device 50 is used to displace the incompressible liquid 20 into the needle 10 so that a protruding meniscus is created at the distal end of the needle 10. In addition, the system includes a pressure sensor 60 to measure the pressure of the incompressible liquid 20. The pressure sensor 60 is in communication with a controller 70, which receives the pressure measurements from the pressure sensor 60. The controller 70 may include a processing unit, such as a microcontroller, a personal computer, a special purpose controller, or another suitable processing unit . The controller 70 may also include a non-transitory storage element, such as a semiconductor memory, a magnetic memory, or another suitable memory. This non-transitory storage element may contain instructions and other data that allows the controller 70 to perform the functions described herein. Further, the system includes a translation device 80. This translation device 80 is used to move the needle 10 toward and away from the soft material 100. In the embodiment shown in FIG. 3A, the translation device 80 is affixed to the needle 10 to move the needle relative to the soft material 100. In the embodiment shown in FIG. 3B, the soft material 100 may be located on a stage 90, and the translation device 80 may be used to move the stage 90 relative to the needle 10 .

[0028] Having described the system and method, an example of this application is provided. FIG. 4 shows a graph of pressuredisplacement and FIG. 5 shows a graph of pressure-time according to one test wherein the soft material is gelatin. In these figures, the solid line ( from point 100 to point 130) represents the insertion of the water-filled blunt needle into the gelatin. Thedashed line ( from point 130 to point 140) shows the needle remaining stationary, allowing observation of material relaxation.

[0029] In this test, the needle travels into the gelatin wherein the pressure built up in the fluid material properties is directly measured by the pressure sensor. This insertion distance may be tracked up-to and past penetration, after which fracture, and the relaxation of the material can be observed and quantified. Specifically, as shown in FIG. 4, the needle 10 is controllably pressed against the surface of the gelatin until point 110, at which point, the top surface of the gelatin is punctured. After this point, a transition from puncture to tearing happens until point 120 and the peak value at 120 may represent the resistance to tearing at the specific rate of needle insertion. Then, the insertion continues and pressure is seen to decrease, which is represented by the slope between point 120 and point 130.

[0030] Further, from FIG. 4, for a brittle material like gelatin, note that a linear relationship between pressure (P) and displacement (5) exists between point 100 and point 110. The slope of this interval may be represented by the theoretical relationship £

[0031] P = -8. Note that the slope of this curve correlates with the known elastic modulus E of the soft material .

[0032] Further, at point 130, the travel of the needle 10 is stopped. Note in FIG. 4, that the displacement of the needle 10 remains constant until point 140. Also note that the pressure decreases during the time in which the needle is stationary. This is due to the relaxation of the gelatin. Note, FIG. 5 shows this relaxation as a function of time where pressure decays over time scales much longer than the hydro-indentation process .Thus, this system and method may determine nonlinear elastic properties, and rate dependent response such as viscoelasticity and poroelasticity, with high repeatability across various soft materials . Additionally, one skilled in the art may infer other characteristics of the soft material from this graph. For example, point 110 may be used to determine information related to the puncture resistance of the soft material 100. Tearing of the soft material occurs between point 120 and point 130; and the slope and length of this portion of the solid line may be used to determine information related to parameters associated with tearing, such as tearing modulus . Finally, information related to viscoelastic relaxation and / or diffusion properties may be determined based on the slope, time duration and length of the dashed line from point 130 to point 140.

[0033] Note that the graph of pressure-displacement may differ for a nonlinearly elastic material, such as polydimethylsiloxane (PDMS) (e . g. , ratios of approximately 45 : 1) , rubbers, and even biological tissues . FIG. 7 shows such a graph, where PDMS is used as the soft material . In this figure, the solid line (from point 100 to point 130) represents the insertion of the water-filled blunt needle into the PDMS . The dashed line ( from point 130 to point 140) shows the needle remaining stationary, allowing observation of material relaxation. In this test, the needle travels into the PDMS wherein the pressure built up in the fluid material properties is directly measured by the pressure sensor . This insertion distance may be tracked up-to and past penetration, after which fracture, and the relaxation of the material can be observed and quantified. For this material, the interval between point 100 and point 110 may be nonlinear due to materialdeformation beyond the linear regime prior to puncture . Additionally, puncture, and specifically, the engagement pressure between the needle 10 and the bulk of these nonlinearly elastic materials, may cause a pressure-drop between point 110 and point 120 .

[0034] This minimally invasive method, free from complicated geometries, controls, stress states, and friction, which are all common issues in current indentation methods, enables rapid and reliable soft material testing.

[0035] This system and method allows a large array of applications . First, due to the size of the needle, it is possible to characterize the soft material at a plurality of locations whose proximity to one another is determined by the diameter of the needle chosen for testing. For example, selecting a 30G needle enables the characterization of a soft material multiple times within a region of one square millimeter .

[0036] Further, the size of the needle also allows the characterization of material that is heterogeneous in the length and width directions .

[0037] Additionally, the use of pressure of incompressible liquid also allows characterization of stratified materials in the thickness or depth direction. Specifically, FIG. 6 shows a stratified material, wherein the top layer has a lower ratio of gelatin weight to water than the lower layer . FIG. 6 shows a difference in the elastic deformation before puncture (slope between points 200 to 210 and 230 to 240) and resistance of the two materials to tearing (at points 220 and 250) by recording twodifferent peak values after transition from puncture to tearing for each material . At point 210, the top surface is punctured, and there is a transition from puncture to tear up to point 220. The insertion continues until the lower layer is reached at point 230, which elastically deforms until the puncture at point 240. As the needle punctures the lower layer, the pressure increases to transition from puncture to tearing up to point 250. From point 250 to point 260 there is a steady tearing of the material and relaxation. The dashed line ( from point 260 to point 270) shows the wait period of the needle from the stratified material . Thus, as shown in FIG. 6, if there is a stratified material, where there are different materials at different depths, the system and method described herein may characterize each of these materials .

[0038] The system described above has many advantages . By incorporating the proposed indentation system and method, the reliance on complicated geometry and modeling assumptions of contact physics (such as adhesion, indentation geometry, and friction) is eliminated. Thus, there is no need for the decoupling of force measurements with relevant contact physics . This system and method reduces the reliance on complicated geometry indenters made of materials such as titanium or steel, typically shaped as cones, cylinders, or spheres . Instead, a readily available syringe and syringe needle cap system may be used. By filling the syringe with an incompressible fluid, coating the tip with a lubricating fluid, and performing the test with readily available control and data acquisition systems, the process becomes more broadly accessible .

[0039] Further, this system and method is versatile and applicable to a wide range of soft materials, including biological tissues .It is minimally invasive, with the size of the indenter being as small as the host needle, which is only limited by the manufacturing methods used in producing the host needles .

[0040] Moreover, this system and method enables unique capabilities in the measurement of additional material properties . In traditional systems, once the indentation limit of the material is exceeded and the indenter penetrates the sample, the commonly used force measurement is complicated by the depth dependent frictional resistance along the indenter . Hence, the recorded force cannot serve as a direct measure to determine the tearing and fracturing behavior of the material . In contrast, in the present system, by recording only the pressure that drives the penetration at the fracture tip, frictional effects are removed and a direct measure of the material resistance to penetration is obtained. As stated above, this allows the observation of information related to other parameters, such as puncture resistance, tearing modulus, viscoelastic relaxation and / or diffusion properties .

[0041] Hydro-indentation has all the advantages of existing indentation methods . Hydro-indentation can measure both timedependent and quasi-static material responses while being highly repeatable and minimally invasive .

[0042] The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings . Thus, such other embodiments and modifications are intended to fall within the scope of the presentdisclosure . Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes . Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.

Claims

What is claimed is :

1. A system for characterizing a soft material, comprising:a needle filled with an incompressible liquid;an actuator to move the needle or the soft material toward the other, such that the incompressible liquid presses against the soft material;a pressure sensor to measure a pressure of the incompressible liquid in the needle as it is pressed against the soft material; anda controller to receive pressure measurements from the pressure sensor and displacement values from the actuator to determine one or more characteristics of the soft material .

2. The system of claim 1, further comprising a lubricating liquid surrounding a distal end of the needle .

3. The system of claim 2, wherein the lubricating liquid is a same liquid as the incompressible liquid.

4. The system of claim 1, wherein the needle comprises a syringe needle .

5. The system of claim 1, wherein the controller computes an elastic modulus for the soft material based on the pressure measurements and the displacement values .

6. The system of claim 1, further comprising a stage on which the soft material is disposed, and wherein the actuator moves the stage toward the needle .

7. The system of claim 1, wherein the actuator moves the needle toward the soft material .

8. A method of characterizing a soft material, comprising:pressing an incompressible liquid disposed in a needle against the soft material; andmeasuring a displacement of the needle and a corresponding pressure of the incompressible liquid at a plurality of points, wherein the displacement and the corresponding pressure are used to characterize the soft material .

9. The method of claim 8, wherein force is not used in the characterization .

10. The method of claim 8, wherein an insertion rate of a needle is varied in order to capture rate-dependence of the soft material .

11. The method of claim 8, wherein the soft material is stratified .

12. The method of claim 11, wherein characteristics of each layer of the stratified soft material may be determined .

13. The method of claim 8, wherein the soft material is heterogeneous .

14. The method of claim 8, wherein the needle is pressed against the soft material in a plurality of locations, wherein the plurality of locations are disposed within one square millimeter .

15. The method of claim 8, wherein an elastic modulus of the soft material is calculated from the displacement and the corresponding pressure .

16. The method of claim 8, wherein the needle is pressed against the soft material until the soft material is punctured .

17. The method of claim 16, wherein information related to puncture resistance is calculated.

18. The method of claim 16, wherein the needle is continued to be inserted into the soft material after puncture .

19. The method of claim 18, wherein information related to tearing modulus is calculated after the soft material is punctured .

20. The method of claim 18, wherein after the soft material is punctured, the needle is held stationary and information related to viscoelastic relaxation and / or diffusion properties is calculated while the needle is held stationary.

Images