Photoelastic soft material for sensing application

Abstract

Fatigue-resistant photoelastic soft materials are provided for soft robots and other sensing applications. The fatigue-resistant photoelastic soft materials include hydroscopic salt that allow the stress-optical coefficient of the fatigue-resistant photoelastic soft material to be tuned. The fatigue-resistant photoelastic soft materials include molecular entanglements that help to tune mechanical properties of the fatigue-resistant photoelastic soft materials.

Description

PHOTOELASTIC SOFT MATERIAL FOR SENSING APPLICATIONCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 637,595 filed on April 23, 2024. The entire disclosure of the above application is incorporated herein by reference.FIELD

[0002] The present disclosure relates to fatigue-resistant photoelastic soft materials for soft robots and other sensing applications and to soft robots and other sensing applications including fatigue-resistant photoelastic soft materials.BACKGROUND

[0003] This section provides background information related to the present disclosure which is not necessarily prior art.

[0004] Soft materials that can exhibit color changes in response to mechanical forces or deformation is of significant values in many areas due to its ability to provide real-time and dynamic feedback on stress visualization. For example, these materials can be used in smart textiles and health dressings and also for preceptive soft robots. Strategies for designing color-changing soft materials often depends on mechanophore chemistry and photoelasticity. These methods can exhibit inherent limitations for enduring dynamic stress sensing. For example, mechanophores can face exhaustion and become depleted after undergoing several cycles of loading, while interference color observed in photoelasticity can offer a reliable tool to observe stress distribution, repeatedly applying forces can cause scission of polymer chains, which affects the structure of chain alignment with each occurrence and can lead to varying stress states and interference colors under identical mechanical conditions. Accordingly, it would be desirable to provide soft materials having improved long-term performance and systems using the same.SUMMARY

[0005] This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all of its features.

[0006] At least one example embodiments relates to a fatigue-resistant photoelastic soft material.

[0007] In at least one example embodiment, the fatigue-resistant photoelastic soft material includes a polymeric matrix including a plurality of polymeric chains, molecular entanglements existing where the polymeric chains overlap to define the polymeric matrix, and a hydroscopic salt distributed within the polymeric matrix, where the fatigue-resistant photoelastic soft material exhibits different photoelastic colors when subjected to a uniaxial tension and has a low hysteresis toughening greater than or equal to about 3 % to less than or equal to about 5 %, a fracture toughness of greater than or equal to about 2,000 J / m2to less than or equal to about 3,500 J / m2, and a fatigue threshold of greater than or equal to about 250 J / m2to less than or equal to about 450 J / m2.

[0008] In at least one example embodiment, the polymeric chains may be defined by linked monomers and the fatigue-resistant photoelastic soft material may include greater than or equal to about 10 weight percentage to less than or equal to about 20 weight percentage of monomers and greater than or equal to about 10 weight percent to less than or equal to about 20 weight percent of the hydroscopic salt.

[0009] In at least one example embodiment, the monomers may include acrylamide, and the hydroscopic salt may include lithium chloride, calcium chloride, or a combination of lithium chloride and calcium chloride.

[0010] In at least one example embodiment, a stress-optical coefficient of the fatigue-resistant photoelastic soft material may be inversely proportional to an amount of the hydroscopic salt in the fatigue-resistant photoelastic soft material.

[0011] In at least one example embodiment, the fatigue-resistant photoelastic soft material may further include a crosslinker distributed within the polymeric matrix..

[0012] In at least one example embodiment, an increase in a density of the crosslinker may decrease a degree of the molecular entanglements in the fatigue-resistant photoelastic soft material.

[0013] In at least one example embodiment, the fatigue-resistant photoelastic soft material may include greater than or equal to about 0.02 weight percentage to less than or equal to about 0.04 weight percent of the crosslinker.

[0014] In at least one example embodiment, the crosslinker may include N,N’-methylenebisacrylamide.

[0015] In at least one example embodiment, the fatigue-resistant photoelastic soft material may further include a plasticizer distributed within the polymeric matrix.

[0016] In at least one example embodiment, an amount of the plasticizer may be inversely proportional to a modulus of the fatigue-resistant photoelastic soft material.

[0017] In at least one example embodiment, the fatigue-resistant photoelastic soft material may include greater than or equal to about 60 weight percent to less than or equal to about 80 weight percent of the plasticizer.

[0018] In at least one example embodiment, the plasticizer may include water, glycerol, or a combination of water and glycerol.

[0019] In at least one example embodiment, the fatigue-resistant photoelastic soft material may be a pre-stretched fatigue-resistant photoelastic soft material.

[0020] At least one example embodiments relates to a tactile sensor.

[0021] In at least one example embodiment, the tactile sensor may include a fatigue-resistant photoelastic soft material.

[0022] In at least one example embodiment, the fatigue-resistant photoelastic soft material may include a polymeric matrix including a plurality of polymeric chains, molecular entanglements existing where the polymeric chains overlap to define the polymeric matrix, a hydroscopic salt distributed within the polymeric matrix, a crosslinker distributed within the polymeric matrix, and a plasticizer distributed within the polymeric matrix, where the fatigue-resistant photoelastic soft material exhibits different photoelastic colors when subjected to a uniaxial tension and has a low hysteresis toughening greater than or equal to about 3 % to less than or equal to about 5 %, a fracture toughness of greater than or equal to about 2,000 J / m2to less than or equal to about 3,500 J / m2, and a fatigue threshold of greater than or equal to about 250 J / m2to less than or equal to about 450 J / m2.

[0023] In at least one example embodiment, an increase in a density of the crosslinker may decrease a degree of the molecular entanglements and an amount of the plasticizer may be inversely proportional to a modulus of the fatigue-resistant photoelastic soft material.

[0024] In at least one example embodiment, the tactile sensor may further include a contact materials system that includes the fatigue-resistant photoelastic soft material and an opaque soft reflective film.

[0025] In at least one example embodiment, the tactile sensor may further include a photoelastic imaging system that includes a camera, a linear polarizer, a white lightdisposed between the camera and the linear polarizer, a rigid plate that is free of residual stress, and a quarter waveplate disposed between the linear polarizer and the rigid plate.

[0026] In at least one example embodiment, the fatigue-resistant photoelastic soft material may be disposed between the rigid plate and the opaque soft reflective film.

[0027] In at least one example embodiment, the photoelastic imaging system may further include an opaque black box that houses the camera, the linear polarizer, the white light, the rigid plate, and the quarter waveplate.

[0028] At least one example embodiments relates to a method for preparing a fatigue-resistant photoelastic soft material.

[0029] In at least one example embodiment, the method may include contacting pre-gel solution including monomers and a first solvent to a crosslinker to form a pre-gel mixture, curing the pre-gel mixture to form a hydrogel, contacting the hydrogel to a second solvent until equilibrium swelling is established, and contacting the swollen hydrogel and a hygroscopic solution until salt particles of the hygroscopic solution are evenly absorbed by the hydrogel to form the fatigue-resistant photoelastic soft material.

[0030] In at least one example embodiment, the method may further include, at least one of: contacting the pre-gel solution to a thermal initiator, contacting the pre-gel solution to a crosslinking accelerator, and drying the fatigue-resistant photoelastic soft material.

[0031] In at least one example embodiment, in the fatigue-resistant photoelastic soft material, the monomers may define a polymeric matrix including a plurality of polymeric chains where molecular entanglements exist where polymeric chains overlap and salts particles from the hygroscopic solution are distributed within the polymeric matrix.

[0032] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.DRAWINGS

[0033] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations and are not intended to limit the scope of the present disclosure.

[0034] FIG. 1 A is an example schematic illustrating an example fatigue-resistant photoelastic soft material including a monomer, crosslinker, and hygroscopic salt in an undeformed state in accordance with at least one example embodiment of the present disclosure.

[0035] FIG. 1 B is an example schematic illustrating the example fatigue-resistant photoelastic soft material of FIG. 1 A in a deformed state in accordance with at least one example embodiment of the present disclosure.

[0036] FIG. 2 is an illustration providing photoelastic images of a fatigue-resistant photoelastic soft material (like the fatigue-resistant photoelastic soft material illustrated in FIGS. 1 A and 1 B) at varying stretch ratios and also experimental interference colors with path differences in accordance with at least one example embodiment of the present disclosure.

[0037] FIG. 3A is an example schematic illustrating example molecular entanglement modulation for a fatigue-resistant photoelastic soft material including a monomer, crosslinker, and hygroscopic salt, like the fatigue-resistant photoelastic soft material illustrated in FIGS. 1A and 1 B, where the number of molecular entanglements decreases as the crosslinker density increases in accordance with at least one example embodiment of the present disclosure.

[0038] FIG. 3B is a graphical demonstration illustrating a relationship between stretch ratio (x-axis) and nominal stress (y-axis) for fatigue-resistant photoelastic soft materials having different crosslinker densities in accordance with at least one example embodiment of the present disclosure.

[0039] FIG. 3C is a graphical demonstration illustrating a relationship between crosslinker density (x-axis) and shear modulus (y-axis) showing that shear modulus increases with crosslinker density increases in accordance with at least one example embodiment of the present disclosure.

[0040] FIG. 4A is an example schematic illustrating example molecular entanglement modulation for fatigue-resistant photoelastic soft material including a monomer, crosslinker, and hygroscopic salt, like the fatigue-resistant photoelastic soft material illustrated in FIGS. 1 A and 1 B, where a water content of the fatigue-resistant photoelastic soft material increases as the hygroscopic salt concentration increases in accordance with at least one example embodiment of the present disclosure.

[0041] FIG. 4B is a graphical demonstration illustrating a relationship between a Cauchy stress (x-axis) and a birefringence (y-axis) for fatigue-resistant photoelastic softmaterial having different hydroscopic salt densities in accordance with at least one example embodiment of the present disclosure.

[0042] FIG. 4C is a graphical demonstration illustrating a relationship between salt concentration (x-axis) and stress-optical coefficient (y-axis) showing that stress-optical coefficient decreases as salt concentration increases in accordance with at least one example embodiment of the present disclosure.

[0043] FIG. 5A is a graphical demonstration illustrating a relationship between crosslinker density (x-axis) and water content (y-axis), where the hydroscopic salt concentration is about 4 M, in accordance with at least one example embodiment of the present disclosure.

[0044] FIG. 5B is a graphical demonstration illustrating a relationship between salt concentration (x-axis) and water content (y-axis), where the crosslinker density is constant, in accordance with at least one example embodiment of the present disclosure.

[0045] FIG. 6 is a graphical demonstration illustrating a relationship between time (x-axis) and normalized mass (y-axis) of example fatigue-resistant photoelastic soft materials having different hydroscopic salt concentrations in accordance with at least one example embodiment of the present disclosure.

[0046] FIG. 7 is an illustration of an example robotic system including a tactile sensor that includes a fatigue-resistant photoelastic soft material, like the fatigue-resistant photoelastic soft material illustrated in FIGS. 1 A and 1 B, in accordance with at least one example embodiment of the present disclosure.

[0047] FIG. 8 is an illustration of an example tactile sensor, like the tactile sensor used by the robotic system illustrated in FIG. 6, in accordance with at least one example embodiment of the present disclosure.

[0048] FIG. 9A is a graphical demonstration illustrating a relationship between wavelength (x-axis) and reflectance (y-axis) of a reflective film, for example, as used in the tactile sensor illustrated in FIG. 8, in accordance with at least one example embodiment of the present disclosure.

[0049] FIG. 9B is a photoelastic color chart for a tactile sensor like the tactile sensor used by the robotic system illustrated in FIG. 6, including a reflective film, in accordance with at least one example embodiment of the present disclosure.

[0050] FIG. 10 illustrates an example method for preparing fatigue-resistant photoelastic soft materials, like the fatigue-resistant photoelastic soft material illustratedin FIGS. 1 A and 1 B, in accordance with at least one example embodiment of the present disclosure.

[0051] FIG. 1 1 A is a graphical demonstration illustrating the cyclic dynamic loading of an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment, where the x-axis represents cycle number and the y- axis represents stretch.

[0052] FIG. 1 1 B is a graphical demonstration illustrating the color change over various cycles of an example fatigue-resistant photoelastic soft material, prepared in accordance with at least one example embodiment of the present disclosure.

[0053] FIG. 11 C is a graphical demonstration illustrating the relationship between cycle number (x-axis) and normalized birefringence (y-axis) for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure.

[0054] FIG. 11 D is a graphical demonstration illustrating the relationship between time (x-axis) and stretch (y-axis) for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure.

[0055] FIG. 1 1 E is a graphical demonstration illustrating the relationship between time (x-axis) and normalized force (y-axis) for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure.

[0056] FIG. 1 1 F is a graphical demonstration illustrating the relationship between loading speed (x-axis) and normalized birefringence (y-axis) for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure.

[0057] FIG. 12A is a graphical demonstration illustrating a monotonic loading of an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment, where the x-axis represents time and the y-axis represents stretch.

[0058] FIG. 12B is a graphical demonstration illustrating a relationship between stretch ratio (x-axis) and force (y-axis) for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure.

[0059] FIG. 12C is a pictorial illustrating a strain field of a fracture test for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure.

[0060] FIG. 12D is a graphical demonstration illustrating cyclic loading for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment, where the x-axis represents cycle number and the y-axis represents stretch.

[0061] FIG. 12E is a graphical demonstration illustrating a relationship between cycle number (x-axis) and crack extension (y-axis) for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure.

[0062] FIG. 12F is a graphical demonstration illustrating a relationship between energy release rate (x-axis) and dc / dN (y-axis) for an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure.

[0063] FIGS. 13A-13F illustrate different objects rearranged by the example robotic system, like the robotic system illustrated in FIG. 7, and their corresponding images captured by a camera in accordance with at least one example embodiment of the present disclosure.

[0064] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.DETAILED DESCRIPTION

[0065] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0066] The present disclosure relates to fatigue-resistant photoelastic soft materials for soft robots and other sensing applications and to soft robots and other sensing applications including fatigue-resistant photoelastic soft materials. The fatigueresistant photoelastic soft materials are distinguished by their controlled molecular entanglements and adjustable hygroscopic salts and improved long-term repetitive stress sensing. Mechanical properties of the fatigue-resistant photoelastic soft materials can be determined by adjusting the molecular entanglement and a stress-optical coefficient of the fatigue-resistant photoelastic soft materials determined by adjusting water content, allowing the fatigue-resistant photoelastic soft materials to have tunable stiffnesses andsensitivities for various applications, including, for example ultrasoft tissues to semi-rigid ligaments. By adjusting the molecular entanglement, the fatigue-resistant photoelastic soft materials can exhibit low hysteresis toughening (e.g., greater than or equal to about 3 % to less than or equal to about 5 % with a fracture energy greater than or equal to about 100 J / m2to less than or equal to about 500 J / m2) with a fracture toughness greater than or equal to about 2,000 J / m2to less than or equal to about 3,500 J / m2(e.g., about 3,000 J / m2) and a fatigue threshold greater than or equal to about 250 J / m2to less than or equal to about 450 J / m2(e.g., greater than or equal to about 400 J / m2to less than or equal to about 450 J / m2. The fatigue-resistant photoelastic soft materials demonstrate superior dynamic stress sensing performance for 10,000 cycles and maintain reliability under various loading rates as compared to other known optomechanical soft materials.

[0067] FIG. 1 A is an example schematic illustrating an example fatigue-resistant photoelastic soft material 100 in an undeformed state. FIG. 1 B is an example schematic illustrating the example fatigue-resistant photoelastic soft material 100 in a deformed state. Although the fatigue-resistant photoelastic soft material 100 is illustrated in FIGS. 1 A and 1 B as having a general rectangular or l-shape, it should be appreciated that, in various other example embodiments, the fatigue-resistant photoelastic soft material 100 may take a variety of shapes and configurations depending on the proposed application of the fatigue-resistant photoelastic soft material 100.

[0068] As illustrated in FIGS. 1 A and 1 B, the fatigue-resistant photoelastic soft material 100 includes a polymeric matrix (or membrane or network) 110 defined by linked monomers 1 12, where a crosslinker 120, a plasticizer 130, and a hydroscopic salt 140 are distributed within the polymeric matrix 110. The hydroscopic salt 140 may help to prevent dehydration of the fatigue-resistant photoelastic soft material 100 when the fatigue-resistant photoelastic soft material 100 is placed in an ambient environment. The plasticizers 130, which may be water molecules within the fatigue-resistant photoelastic soft material 100, may help to ensure low friction for polymer chains to slide against each other when subjected to mechanical forces. The low friction may help to ensure that the polymer chains do not undergo scission during long-term loading and to help maintain a consistent optical response across various loading speeds.

[0069] Each monomer possesses a dipole and a polarizability (a) may be used to characterize the ability to respond to an external electrical field. When the fatigue-resistant photoelastic soft material 100 is in a relax state, as illustrated in FIG. 1 A, the dipoles of the monomers 112 are randomly oriented, resulting in no directionaldifference in polarizability. In contrast, when a mechanical force is applied, as illustrated in FIG. 1 B, the dipoles of the monomers 1 12 may align and orient along specific directions, causing a difference in polarizability along principal directions. The variation in polarizability leads to refractive index differences ( / .e., birefringence An) along the stress principal directions. When the photoelasticity of the fatigue-resistant photoelastic soft material 100 is examined, due to the birefringence, the fatigue-resistant photoelastic soft material 100 exhibits different photoelastic colors when subjected to a uniaxial tension.

[0070] FIG. 2 is an illustration providing photoelastic images of the fatigue-resistant photoelastic soft material 100 at varying stretch ratios and also experimental interference colors with path differences. As illustrated in FIG. 2, the fatigue-resistant photoelastic soft material 100 has a different color at different stretch ratios. For example, in a resting state the fatigue-resistant photoelastic soft material 100 may have a black color. At a stretch ratio of about 1 .5, the fatigue-resistant photoelastic soft material 100 may have a dark gray color. At a stretch ratio of about 2.0, the fatigue-resistant photoelastic soft material 100 may have a lighter gray color. At a stretch ratio of about 3.0, the fatigue-resistant photoelastic soft material 100 may have an even lighter gray color. At a stretch ratio of about 3.5, the fatigue-resistant photoelastic soft material 100 may have a cream color. At a stretch ratio of about 4.0, the fatigue-resistant photoelastic soft material 100 may have a pale-yellow color. At a stretch ratio of about 4.5, the fatigue-resistant photoelastic soft material 100 may have a light orange color. At a stretch ratio of about 5.0, the fatigue-resistant photoelastic soft material 100 may have a medium orange color. At a stretch ratio of about 5.5, the fatigue-resistant photoelastic soft material 100 may have a deep orange color.

[0071] With renewed reference to FIGS. 1A and 1 B, the monomers 1 12 may include, for example, acrylamide. The crosslinker 120 may include, for example, N,N’-methylenebisacrylamide. The plasticizer 130 may include, for example, water, glycerol, or a combination of water and glycerol. The hydroscopic salt may include, for example, lithium chloride (LiCI), calcium chloride (CaCL), or a combination of lithium chloride (LiCI) and calcium chloride (CaCL). It should be recognized that the in various other embodiments, the fatigue-resistant photoelastic soft material 100 may include, additionally or alternatively, various other polymer systems that form substantial entanglements, including, for example, polyacrylic acid, poly(styrenesulfonate), or a combination of polyacrylic acid and poly(styrenesulfonate). These polymer systems mayhave unique mechanical properties, including modulus and toughness, as well as different photoelastic characteristics.

[0072] In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include the monomers 1 12 in an amount greater than or equal to about 10 weight percentage (e.g., greater than or equal to about 1 1 weight percentage, greater than or equal to about 12 weight percentage, greater than or equal to about 13 weight percentage, greater than or equal to about 14 weight percentage, greater than or equal to about 15 weight percentage, greater than or equal to about 16 weight percentage, greater than or equal to about 17 weight percentage, greater than or equal to about 18 weight percentage, or greater than or equal to about 19 weight percentage). In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include the monomers 12 in an amount less than or equal to about 20 weight percentage (e.g., less than or equal to about 19 weight percentage, less than or equal to about 18 weight percentage, less than or equal to about 17 weight percentage, less than or equal to about 16 weight percentage, less than or equal to about 15 weight percentage, less than or equal to about 14 weight percentage, less than or equal to about 13 weight percentage, less than or equal to about 12 weight percentage, or less than or equal to about 11 weight percentage). In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include greater than or equal to about 10 weight percentage to less than or equal to about 20 weight percentage of the monomers 1 12.

[0073] In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include the crosslinker 120 in an amount greater than or equal to about 0.02 weight percentage (e.g., greater than or equal to about 0.025 weight percentage, greater than or equal to about 0.03 weight percentage, or greater than or equal to about 0.035 weight percentage). In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include the crosslinker 120 in an amount less than or equal to about 0.04 weight percent (e.g., less than or equal to about 0.035 weight percent, less than or equal to about 0.03 weight percent, or less than or equal to about 0.02 weight percent). In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include greater than or equal to about 0.02 weight percentage to less than or equal to about 0.04 weight percent of the crosslinker 120.

[0074] In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include the plasticizer 130 in an amount greater than or equal to about 60 weight percent (e.g., greater than or equal to about 65 weight percent, greater than orequal to about 70 weight percent, or greater than or equal to about 75 weight percent). In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include the plasticizer 130 in an amount less than or equal to about 80 weight percent (e.g., less than or equal to about 75 weight percent, less than or equal to about 70 weight percent, or less than or equal to about 65 weight percent). In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include greater than or equal to about 60 weight percent to less than or equal to about 80 weight percent of the plasticizer 130.

[0075] In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include the hydroscopic salt 140 in an amount greater than or equal to about 10 weight percent (e.g., greater than or equal to about 12 weight percent, greater than or equal to about 14 weight percent, greater than or equal to about 16 weight percent, or greater than or equal to about 18 weight percent). In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include the hydroscopic salt 140 in an amount less than or equal to about 20 weight percent (e.g., less than or equal to about 18 weight percent, less than or equal to about 16 weight percent, less than or equal to about 14 weight percent, or less than or equal to about 12 weight percent). In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may include greater than or equal to about 10 weight percent to less than or equal to about 20 weight percent of the hydroscopic salt 140.

[0076] Areas where the polymeric chains overlap to define the polymeric matrix 1 10 are referred to as molecular entanglement 150. Although only three molecular entanglements 150 are identified in FIGS. 1 A and 1 B by way of example, it should be appreciated that the fatigue-resistant photoelastic soft material 100 includes other molecular entanglements 150 not specifically illustrated or identified or labeled. In at least one example embodiment, the amount of molecular entanglements 150 may be quantified by rate-dependent storage modulus using rheological characterizations. For example, an average number of monomers between neighboring crosslinks may be greater than or equal to about 6,000 to less than or equal to about 10,000 in the swollen state.

[0077] The fatigue-resistant photoelastic soft material 100 may be subject to one or more stresses. For example, in at least one example embodiment, as illustrated in FIG. 1 B, pulling forces 160A, 160B may be applied to opposing sides of the fatigue-resistant photoelastic soft material 100. As the pulling forces 160A, 160B areapplied, the molecular entanglements 150 may serve as slips links and transmit force along polymeric chains 1 10 and the fatigue-resistant photoelastic soft material 100, as illustrated in FIG. 1 B.

[0078] By adjusting the crosslinker density for the fatigue-resistant photoelastic soft material 100, the degree of molecular entanglement 150 may be adjusted. As illustrated in FIGS. 3A-3C, the degree of molecular entanglement in the fatigue-resistant photoelastic soft material may be dependent on crosslinker density, where crosslinker density is a molar weight ratio between the crosslinker and the monomers. For example, the degree of molecular entanglement may increase in the fatigue-resistant photoelastic soft material as water content of precursor decreases during the synthesis.

[0079] FIG. 3A illustrates a first example fatigue-resistant photoelastic soft material 300 including a monomer 312, crosslinker 320, a plasticizer 330 and hygroscopic salt 340, similar to the fatigue-resistant photoelastic soft material illustrated in FIGS. 1 A and 1 B, and a second example fatigue-resistant photoelastic soft material 350 including a monomer 352, crosslinker 370, a plasticizer 380, and hygroscopic salt 380, similar to the fatigue-resistant photoelastic soft material illustrated in FIGS. 1 A and 1 B, where the first fatigue-resistant photoelastic soft material 300 has a crosslinker density that is less than the second fatigue-resistant photoelastic soft material 350. The first fatigue-resistant photoelastic soft material 300 has first molecular entanglements 345. The second fatigue-resistant photoelastic soft material 350 has second molecular entanglements 390. As illustrated, the molecular entanglement 345 of the first fatigue-resistant photoelastic soft material 300 is greater than the molecular entanglement 390 of the second fatigue-resistant photoelastic soft material 350. The presence of the crosslinker 370 inhibited the formation of molecular entanglements.

[0080] FIG. 3B is a graphical demonstration illustrating a relationship between stretch ratio (x-axis) and nominal stress (y-axis) for a first example fatigue-resistant photoelastic soft material 302 having a crosslinker density of about 4.4 x 10-4, a second example fatigue-resistant photoelastic soft material 304 having a crosslinker density of 3.3 x 10-4, a third example fatigue-resistant photoelastic soft material 306 having a crosslinker density of about 1 .6 x 10-4, and a fourth example fatigue-resistant photoelastic soft material 308 having a crosslinker density of about 1.1 x 10-4. As illustrated, the nominal stress increases as the crosslinker density increases.

[0081] FIG. 3C is a graphical illustration demonstrating a relationship between crosslinker density (x-axis) and shear modulus (y-axis) showing that shear modulusincreases as crosslinker density increases. For example, the shear modulus may range from about 10 kPa to about 50 kPa. The crosslinker may restrict the mobility of polymer chains, thereby reducing the ability of the fatigue-resistant photoelastic soft material to deform.

[0082] With renewed reference to FIGS. 1 A and 1 B, the shear modulus of the fatigue-resistant photoelastic soft material 100 may be adjusted between about 10 kPa to about 50 kPa by altering the number of molecular entanglements 150. The molecular entanglement 150 may affect or determine the low hysteresis toughening of the fatigue-resistant photoelastic soft material 100. In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may have a low hysteresis ratio less than or equal to about 3 percentage.

[0083] With renewed reference to FIGS. 1 A and 1 B, by adjusting the hydroscopic salt, a water content of the fatigue-resistant photoelastic soft material 100 may be adjusted. As illustrated in FIGS. 4A-4C and 5A-5D, water content in the fatigue-resistant photoelastic soft material may be dependent on hydroscopic salt concentration, where water content is a ratio of a mass of water to a total mass of both water and polymer. For example, the water content in the fatigue-resistant photoelastic soft material may increase as the hydroscopic salt concentration increases.

[0084] FIG. 4A illustrates a first example fatigue-resistant photoelastic soft material 400 including a monomer 412, crosslinker 420, a plasticizer, 430 and hygroscopic salt 440, similar to the fatigue-resistant photoelastic soft material illustrated in FIGS. 1 A and 1 B, and a second example fatigue-resistant photoelastic soft material 450 including a monomer 452, crosslinker 470, a plasticizer 480, and hygroscopic salt 490, similar to the fatigue-resistant photoelastic soft material illustrated in FIGS. 1 A and 1 B. The first fatigue-resistant photoelastic soft material 400 has a first hydroscopic salt concentration. The second fatigue-resistant photoelastic soft material 450 has a second hydroscopic salt concentration. As illustrated, the water content of the second fatigueresistant photoelastic soft material 450 is greater than the water content of the first fatigue-resistant photoelastic soft material 400. The increased preserved water molecules may help to separate the polymer chains, resulting in a reduced crosslinker density per volume and leading to a lower modulus.

[0085] FIG. 4B is a graphical illustration demonstrating a relationship between a Cauchy stress (x-axis) and a birefringence (y-axis) for a first example fatigue-resistant photoelastic soft material 402 having a first hygroscopic salt concentration of about 4 M,a second example fatigue-resistant photoelastic soft material 404 having a second hygroscopic salt concentration of about 8 M, and a third example fatigue-resistant photoelastic soft material 406 having a third hygroscopic salt concentration of about 12 M. As illustrated, the birefringence decreases as the hygroscopic salt concentration decreases.

[0086] FIG. 4C is a graphical illustration demonstrating a relationship between salt concentration (x-axis) and stress-optical coefficient (y-axis) showing that stress-optical coefficient decreases as salt concentration increases; FIG. 5A is a graphical illustration demonstrating a relationship between crosslinker density (x-axis) and water content (y- axis), where the hydroscopic salt concentration is about 4 M; and FIG. 5B is a graphical illustration demonstrating a relationship between salt concentration (x-axis) and water content (y-axis), where the crosslinker density is constant.

[0087] The example fatigue-resistant photoelastic soft material reviewed in FIGS. 3A-5D are polyacrylamide hydrogels with the noted crosslinker densities and / or hydroscopic salt concentrations. Because of the hydroscopic nature of salt ions, it is often difficult for an as-synthesized hydrogel to fully swell when directly immersed into a high concentration salt solution. For the different examples, the as-synthesized hydrogel were immersed in pure water until fully swollen and then subsequently submerged into salt solutions with varying concentrations. Once the salt particles were evenly diffused into the hydrogels, the hydrogels were removed from solution and placed in a room environment until the water content reached equilibrium in the air. For example, FIG. 6 is a graphical is a graphical illustration demonstrating a relationship between time (x-axis) and normalized mass (y-axis) of example a first example fatigue-resistant photoelastic soft material 600, a second example fatigue-resistant photoelastic soft material 602 having a first hydroscopic salt concentration, a third example fatigue-resistant photoelastic soft material 604 having a second hydroscopic salt concentration, and a fourth example fatigue-resistant photoelastic soft material 606 having a third hydroscopic salt concentration. The first example fatigue-resistant photoelastic soft material 600 is substantially free of a hydroscopic salt. The first hydroscopic salt concentration is less than the second hydroscopic salt concentration and less than the third hydroscopic salt concentration. The second hydroscopic salt concentration is less than the third hydroscopic salt concentration. The first hydroscopic salt concentration may be about 1 M. The second hydroscopic salt concentration may be about 4 M. The third hydroscopic salt concentration may be about 10 M.

[0088] With renewed reference to FIGS. 1 A and 1 B, when the fatigue-resistant photoelastic soft material 100 includes greater than or equal to about 15 weight percent of the hydroscopic salt, the fatigue-resistant photoelastic soft material 100 may have a water content of about 70 percentage and a stress-optical coefficient of about 6e-30, and when the fatigue-resistant photoelastic soft material 100 includes greater than or equal to about 25 weight percent of the hydroscopic salt, the fatigue-resistant photoelastic soft material 100 may have a water content of about 65 percent and a stress-optical coefficient of about 4.5e-30. The fatigue-resistant photoelastic soft material 100 may have a stress-optical coefficient greater than or equal to about 4e-30 with hygroscopic salt less than or equal to 25 weight percentage.

[0089] In various aspects, the fatigue-resistant photoelastic soft material 100 may be incorporated into a device or system that includes one or more sensors (e.g., cameras) configured to read the color change and one or more programs configured to chart or respond to the identified color change. For example, in at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may be incorporated into smart textiles to monitor sport performance. In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may be incorporated into health dressing to visualize pressure in affected areas. In at least one example embodiment, the fatigue-resistant photoelastic soft material 100 may be provided as a tactile sensor that may be used to gather surrounding information such as to help robots adapt movement and avoid obstacles and collisions.

[0090] FIG. 7 is an illustration of an example robotic system 700 including one or more tactile sensors 750 that includes a fatigue-resistant photoelastic soft material 760, like the fatigue-resistant photoelastic soft material 100 illustrated in FIGS. 1 A and 1 B. The tactile sensors 750 may also include one or more imaging systems, one or more polarizers, and one or more reflective films. As illustrated, the robotic system 700 may include a robotic arm 710 having one or more gripping members 720. The one or more tactile sensors 750 may be disposed on, or integrated into, the one or more gripping members 720 such that when the robotic arm 710 engages with an object 790 the one or more tactile sensors 750 capture at least one of geometry, modulus, spatial position, and stress data of the object 790. The object 790 may include, for example, a screw, a cubic, or a cylindrical.

[0091] FIG. 8 is an illustration of an example tactile sensor 800, such as may be used, for example, as the one or more tactile sensors 750 illustrated in FIG. 7. The tactilesensor 800 includes, for example, a first section or portion defined by a photoelastic imaging system (which may also be referred to as a photoelastic box) and a second section or portion defined by contact materials situated away from the photoelastic imaging system for object interaction.

[0092] The photoelastic imaging system may include, for example, a camera 802, a white light (which may also be referred to as a light source) 804, a linear polarizer 806, a quarter waveplate 808, and a rigid plate 810 that is free of residual stress (e.g., an acrylic plate). As illustrated, in at least one example embodiment, the white light 804 may be disposed between the camera 802 and the linear polarizer 806. As illustrated, in at least one example embodiment, the quarter waveplate 808 may be disposed between the linear polarizer 806 and the rigid plate 810.

[0093] The camera 802 may have an adjustable focus length that can be used to visualize stress patterns. The white light 804 may be a natural while light source (e.g., color temperature: 4000K). The linear polarizer 806 may be configured to polarized the random direction while light to a single direction linear polarized light. The quarter waveplate 808 may be configured to polarized the linear light to a circular polarized light. The rigid plate 810 may help to provide structural support to the tactile sensor 800.

[0094] In at least one example embodiment, the camera 802, the white light 804, the linear polarizer 806, the quarter waveplate 808, the rigid plate 810 may be disposed within (for example, encapsulated by) an opaque black box 812. In at least one example embodiment, the opaque black box 812 may be covered with matte black surfaces selected, for example, to reduce light loss and to improve image quality. Notably, in this instance, the camera 802 and the light source 804 are disposed on the same side of the contact materials, such that the object 890 to be contacted does not block the light path.

[0095] The contact materials may be provided as a two-layer structure that includes a first (or inner) layer including a fatigue-resistant photoelastic soft material 820, like the fatigue-resistant photoelastic soft material 100 illustrated in FIGS. 1 A and 1 B, and a second (or outer layer) including a soft reflective film 822 with strain independent and wavelength-independent reflectance spectrum. As illustrated, in at least one example embodiment, the fatigue-resistant photoelastic soft material 820 may be disposed between the rigid plate 810 and the soft reflective film 822. The soft reflective film 822 opacity is critical for blocking external light, which if unblocked, would change the visual outcomes.

[0096] When the tactile sensor 800 interacts with an object 790, the two layers — the fatigue-resistant photoelastic soft material 820 and the soft reflective film 822 — may undergo deformation simultaneously. The two waveplates function as polarizers converting random light into polarized light that passes the light through the deformed fatigue-resistant photoelastic soft material 820 twice as a result of the reflection process by the reflective film 822. The two waveplates may then function as analyzers, converting the polarized light into colorful photoelastic stress patters that are ultimately captured by the camera.

[0097] The function of the tactile sensor 800 depends on photoelasticity. Photoelasticity is a phenomenon where differences in principal stress can lead to changes in optical properties (e.g., refractive index) of a material. In various aspects of the present instance, when the light source 804 passes through both the linear polarizer 806 and the quarter waveplate 808, the light may be circularly polarized. The polarized light may then traverse the fatigue-resistant photoelastic soft material 820. The polarized light becomes subject to the internal stress distribution of the fatigue-resistant photoelastic soft material 820 and is reflected back to the underlying reflective film 822 and again passed through the fatigue-resistant photoelastic soft material 820. The camera 802 captures the resulting interference colors, which contain information (as noted above) about stress distribution withing the fatigue-resistant photoelastic soft material 820. It should be recognized that, in at least one example embodiment, the fatigue-resistant photoelastic soft material 820 may be pre-stretch, for example, to help avoid ranges of the photoelastic chart from black to white and / or to improve the sensitivity of the fatigue-resistant photoelastic soft material 820 and / or change patterns to help distinguish spatial position of the object 890. The first range of photoelastic color is from black to white. After pre-stretch, the fatigue-resistant photoelastic soft material 820 will change color from yellow to purple and / or green, which evidences more obvious color change, helping to enhance sensitivity.

[0098] FIG. 9A is a graphical illustration demonstrating a relationship between wavelength (x-axis) and reflectance (y-axis) of a reflective film, for example, as used in the tactile sensor illustrated in FIG. 8. As illustrated, the reflectance remains consistently around about 60 % across the visible light range under various stretch ratios, ranging from about 1 to 4. The consistent reflective coefficient ensures the photoelastic color, a mixture of light of different wavelengths, and adherence to the Michel-Levy color chart.

[0099] FIG. 9B is a photoelastic color chart for a tactile sensor like the tactile sensor used by the robotic system illustrated in FIG. 6, including a reflective film. As illustrated, the path difference of a tactile sensor including a reflective film is nearly double compared to a tactile sensor including a fatigue-resistant photoelastic soft material but without a reflective film.

[0100] In various aspects, the present disclosure provides methods for preparing fatigue-resistant photoelastic soft materials, like the fatigue-resistant photoelastic soft material 100 illustrated in FIGS. 1 A and 1 B. For example, FIG. 10 illustrates an example method 1 100 for preparing a fatigue-resistant photoelastic soft material.

[0101] The method 1 100 may include contacting 1120 a pre-gel solution and a crosslinker. The pre-gel solution may include, for example, acrylamide and an aqueous solvent (e.g., deionized water). In at least one example embodiment, the pre-gel solution may have a 1 :1 weight ratio of the acrylamide to the aqueous solvent. The method 1 100 may include preparing 1 1 10 a pre-gel solution. In at least one example embodiment, the pre-gel solution may be a 10 grams pre-gel solution that includes contacting about 5 grams of acrylamide and about 5 grams of the aqueous solvent.

[0102] The crosslinker may include, for example, N,N’-methylenebisacrylamide. In at least one example embodiment, the contacting 520 may include adding a comparatively small amount of the crosslinker to the pre-gel solution. For example, less than or equal to about 2e-4 gram of the crosslinker may be added to about 5 milliliters of the pre-gel solution. In at least one example embodiment, the contacting 1120 may include applying a mixing force. For example, the pre-gel solution and the crosslinker may be vortexed until a substantially homogeneous mixture is formed.

[0103] The method 1 100 may include contacting 1 130 the pre-gel solution and a thermal initiator. The thermal initiator may include, for example, ammonium persulfate, Irgacure 2959, or a combination of ammonium persulfate and Irgacure 2959. In at least one example embodiment, the contacting 1 130 may include adding a comparatively small amount of the thermal initiator to the pre-gel solution. For example, greater than or equal to about 1 e-3 to less than or equal to about 2e-3 of the thermal initiator may be added to about 5 milliliters of the pre-gel solution. In at least one example embodiment, the contacting 1 130 may include applying a mixing force. For example, the pre-gel solution and the thermal initiator may be vortexed until a substantially homogeneous mixture is formed. Although in FIG. 10, the contacting 1 130 of the pre-gel solution and the thermal initiator is illustrated as following the contacting 1120 of the pre-gel solutionand the crosslinker, it should be appreciated that, in various other example embodiments, the contacting 1 130 may occur before the contacting 1120 of the pre-gel solution and the crosslinker and / or simultaneously with the contacting 1 120 of the pre-gel solution and the crosslinker.

[0104] The method 1 100 may include contacting 1 140 the pre-gel solution and a crosslinking accelerator. The crosslinking accelerator may include, for example, N,N,N’,N’-tetramethylethylenediamine. In at least one example embodiment, the contacting 1 140 may include adding a comparatively small amount of the crosslinking accelerator to the pre-gel solution. For example, greater than or equal to about 5 microliters to less than or equal to about 10 microliters of the crosslinking accelerator may be added to about 5 milliliters of the pre-gel solution. In at least one example embodiment, the contacting 1 140 may include applying a mixing force. For example, the pre-gel solution and the crosslinking accelerator may be vortexed until a substantially homogeneous mixture is formed. Although in FIG. 10 the contacting 1 140 of the pre-gel solution and the crosslinking acceleration is illustrated as following the contacting 1 120 of the pre-gel solution and the crosslinker, it should be appreciated that, in various example embodiments, the contacting 1130 may occur before the contacting 1120 of the pre-gel solution and the crosslinker and / or simultaneously with the contacting 1 120 of the pre-gel solution and the crosslinker.

[0105] The method 1 100 may include curing 1 150 the pre-gel mixture to form a hydrogel, where the pre-gel mixture include the pre-gel solution as well as at least one of the crosslinker, the thermal initiator, and the crosslinking accelerator. In at least one example embodiment, the curing 1 150 of the pre-gel mixture may include exposing the pre-gel mixture to ultraviolet lights for a preselected period of time. The preselected period of time may be greater than or equal to about 1 hour to less than or equal to about 2 hours (e.g., about 2 hours).

[0106] The method 1 100 may include contacting 1160 the hydrogel to another aqueous solvent (e.g., pure water) to form a swollen hydrogel. In at least one example embodiment, the contacting 1 160 of the hydrogel to the aqueous solvent may include immersing the hydrogel in the aqueous solvent. In at least one example embodiment, the contacting 1 160 of the hydrogel to the aqueous solution may continue until equilibrium swelling occurs ( / .e., to a point where weight no longer changes with time).

[0107] The method 1 100 may include contacting 1 170 the swollen hydrogel and a hygroscopic solution. The hygroscopic solution may include, for example, lithium chloride(LiCI) and another aqueous solvent. In at least one example embodiment, the contacting 1 170 of the swollen hydrogel to the hygroscopic solution may include immersing the swollen hydrogel in the hygroscopic solution. In at least one example embodiment, the contacting 1 170 of the swollen hydrogel to the hygroscopic solution may continue until the salt particles of the hygroscopic solution are evenly absorbed by the hydrogel to form a fatigue-resistant photoelastic soft material.

[0108] The method 1100 may include drying 1 180 the fatigue-resistant photoelastic soft material in an ambient environment (for example, at room temperature, where room temperature is greater than or equal to about 20QC to less than or equal to about 22QC. In at least one example embodiment, the dried fatigue-resistant photoelastic soft material may have a moisture level of about 35 %.

[0109] Embodiments of the present technology are further illustrated through the following non-limiting example.EXAMPLE 1

[0110] To evaluate the suitability of an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure (like the fatigue-resistant photoelastic soft material 100 illustrated in FIGS. 1 A and 1 B) for sensing, including long-term durability and short-term responsiveness, longterm cyclic loading, static relaxation loading, and various speed loading tests were preform on an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure and also three common hydrogels.

[0111] As illustrated in FIG. 11 A, long-term cyclic tensile loading is applied to an example fatigue-resistant photoelastic soft material 1200 prepared in accordance with at least one example embodiment of the present disclosure and two common hydrogels 1210, 1220. The long-term cyclic tensile loading is applied to examine the mechanical and optical properties of the example fatigue-resistant photoelastic soft material 1200 to the two common hydrogels 1210, 1220. A first hydrogel 1210 of the two common hydrogel has similar molecular entanglement of the example fatigue-resistant photoelastic soft material 1200 but lacks sufficient hydroscopic salt to retain water.

[0112] A universal testing machine was used to record the stress-stretch data while simultaneously monitoring the color of the same region in each cycle with a camera. The photoelastic color with its cycle number for the example fatigue-resistant photoelastic soft material and the two common hydrogels 1210, 1220 is illustrated in FIG. 1 1 B. Anormalized birefringence value corresponding to the color in FIG. 1 1 B is normalized is illustrated in FIG. 1 1 C. As illustrated in FIG. 1 1 B, the photoelastic color of the example fatigue-resistant photoelastic soft material 1200 remained unchanged up to 10,000 cycles, while the first hydrogel 1210 shows rapid color change around 900 cycles (where dehydration accelerates causing friction between the polymeric chains to increase due to insufficient water content) and the second hydrogel 1220 becomes darker a few cycles thereafter.

[0113] Due to the synergistic effect of the molecular entanglement and the hydroscopic salts, the polymeric chains of the example fatigue-resistant photoelastic soft material 1200 preserve their structure without scission during cycle loading tests as the chains slide against each other and the retained water ensures low friction and sufficient space. The highly entangled hydrogel of the example fatigue-resistant photoelastic soft material 1200 may behave like a spring, responding quickly to mechanical deformation, while the common hydrogels 1210, 1220 behave like a spring combined with a dashpot as a result of their viscoelastic properties, exhibiting a delayed response. For example, as illustrated in FIG. 1 1 D, a constant strain was immediately applied to the example fatigue-resistant photoelastic soft material 1200 and the common hydrogels 1210, 1220 and held for 1 ,000 seconds, during which both the applied force and the photoelastic color were recorded. For the example fatigue-resistant photoelastic soft material 1200, the applied forced reduced about 5 % in the first second then reaching a plateau. In contrast, for the common hydrogels 1210, 1220, the applied force sharply reduced about 40 % in the first 40 seconds and then continuously reduced to about 50 % in the next 1 ,000 seconds. The different is at least in part because glycerol has a larger viscosity than water, resulting in higher friction between polymeric chains and longer time for chains to reach their equilibrium states.

[0114] FIG. 1 1 E shows the normalized force of the example fatigue-resistant photoelastic soft material 1200. FIG. 11 F shows the birefringence of the photoelastic color normalized by the valve at about 0.5 mm / s. As illustrated, the example fatigue-resistant photoelastic soft material 1200 exhibits remarkable stability across different loading speeds, while the birefringence shows an increase trend as loading speed increases.

[0115] In sum, the example fatigue-resistant photoelastic soft material 1200 demonstrated remarkable stability across different speeds and the static relaxation findings affirmed that fatigue-resistant photoelastic soft material can provided real-timeand dynamic feedback on stress sensing, while the common hydrogel 12010, 1230 materials have a larger force relaxation or color change under different loading speeds and may be limited by other time-dependent properties.EXAMPLE 2

[0116] To evaluate the material toughness of an example fatigue-resistant photoelastic soft material prepared in accordance with at least one example embodiment of the present disclosure (like the fatigue-resistant photoelastic soft material 100 illustrated in FIGS. 1 A and 1 B) mechanical tests were performed, including a monotonic fracture test to estimate fracture toughness and a cyclic fatigue test to determine a fatigue threshold. Fracture toughness is a key parameter to describe the ability to resist crack propagation.

[0117] As illustrated in FIG. 12A, a purse shear monotonic loading test was performed for a notched sample to measure critical strength, where the critical strength is defined as the ratio between the sample length before crack propagation and its initial length. In accordance with the equationwhere H is the initial length of sample, S and is nominal stress and stretch ratio measured in pure shear test, the fracture toughness (F) is about 3000 J / m2.

[0118] For long-term dynamic sensing, the fatigue threshold To is a key parameter to describe the resistance to fatigue crack propagation after prolonged cycles of loads. As illustrated in FIG. 12D, a notched sample was subjected to cyclic loading at various stretch levels to measure the crack expansion length every cycle. FIG. 12E displays the curves of crack expansion versus cycle number, where the slope becomes sharper with an increased stretch ratio. The relationship between the crack expansion length per cycle and its corresponding energy release rate (G), for example as calculated using the following equationG = H f^Sd . is illustrated in FIG. 12F.

[0119] A cohesive model and non-linear deformation module may be implemented in finite element software ABAQUS to capture the crack propagation of the notched sample. To implement the cohesive-zone model, the maximum nominal stress (Smax) can be taken as the measured failure stress under the pure shear test, and the maximum nominal separation (8max) can be calculated using the following equationmax ! iTiaxAs illustrated in FIG. 13B, the theoretical curve of force versus stretch ratios, including the critical stretch ratio, shows good agreement with the experimental results. A digital image correlation method on the pure shear tensile test of a notched sample can be used to further validate toughen mechanism. For example, FIG. 12C shows that there is a high degree of agreement between the strain field obtained from digital image correlation experiment and Abaqus simulation.EXAMPLE 3

[0120] To evaluate tactile sensor performance of an example tactile sensor prepared in accordance with at least one example embodiment of the present disclosure (like the tactile sensor 800 illustrated in FIG. 8) an example robotic system (like the robotic system 700 illustrated in FIG. 7) including the example tactile sensor is used to performance various tasks

[0121] FIGS. 13A-13F illustrate different objects rearranged by the example robotic system and their corresponding images captured by a camera (like the camera 802 illustrated in FIG. 8). Because the example fatigue-resistant photoelastic soft material (like the fatigue-resistant photoelastic soft material 100 illustrated in FIGS. 1 A and 1 B) included in the tactile sensor is subjected to a pre-stretch force, the material may display yellow initially.

[0122] FIG. 13A provides images of applying forces on three cylinders of the identical dimensions but with varying modulus, for example, of 3.2 GPa, 3.45 MPa, and 1.34 MPa. When the interacting object is extremely hard, like an acrylic cylinder (for example, as illustrated in FIG. 13A(i)), the tactile sensor contact surface undergoes a large deformation to conform to the shape of the object, resulting in a more pronounced stress pattern (for example, as illustrated in FIG. 13 A(ii)) . In contrast, when the interacting object is softer, like a PU cylinder (for example, as illustrated in FIG. 13A(v)), both the object and the contact surface of the sensor undergo deformation. The surface of the sensor is less deformed compared to interacting with a hard object, leading to a subtler stress pattern (for example, as illustrated in FIG. 13A(vi)).

[0123] FIG. 13B provides images of applying a consistent force to object having varying geometry. The overall pattern aligns with the object geometry while the local color corresponds to the stress level. This is especially evident in the case of pressing an inclined threaded object (for example, as illustrated in FIG. 13B(iii)). The pattern of each thread is very similar, but due to the tilt, each thread experiences a different stress level,leading to variations in color. In contrast, when applying force to an object with a more uniform shape, like a hexagonal wrench (for example, as illustrated in FIG. 13B(v)), the resulting pattern exhibits a uniform color distribution.

[0124] FIG. 13C provides images of applying different forces to the same object. As the applied force intensifies, the contact surface of the sensor experiences increased deformation, resulting in a more distinct pattern. FIG. 14D provides images of the same object at different rotating angles, for example, ranging from 0 degree to 90 degree, when subjected to an identical force. Due to the uniaxial pre-stretch of FPSM, the stress distribution varies even interacting with the same object at different positions, which leads to distinct patterns captured by the sensor’s camera.

[0125] To correlate the photoelastic patterns with stress distribution, Abaqus software may be used to simulate the contact interaction between a circular thin disc and a uniaxially pre-stretched hydrogel. For example, FIGS. 13E and 13F provide images of the stress field when subjected to different force and various angles. Prior to contact, due to the uniaxially pre-stretch, the hydrogel displayed a uniform stress along x-axis (for example, as illustrated in FIG. 13E(ii)). As the contact force is gradually applied between hydrogel and disc, the difference in principal stress begins to extend outward from the center. The stress distribution forms concentric semicircular patterns along the x-axis, where the stress values exceed the pre-stress level, while simultaneously forming patterns along the y-axis, where the stress values are below the pre-stress level. The simulation results closely align with our experiment photoelastic stress patterns. However, the photoelastic pattern is also related to the material's thickness. The central region experiences a significant reduction in thickness, resulting in a less pronounced color change compared to the stress distribution indicated in the simulation results. To enhance the comparison between the simulation results and the photoelastic patterns, the contact between hydrogel and disc with various rotating angles may be stimulated, for example, as illustrated in FIG. 13F. Due to the presence of uniaxial pre-stress, the difference in principal stress varied with the rotation position, aligning closely with the experiment results.

[0126] Example embodiments are provided so that this disclosure will be thorough and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, thatexample embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well- known processes, well-known device structures, and well-known technologies are not described in detail.

[0127] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0128] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

CLAIMSWhat is claimed is:1 . A fatigue-resistant photoelastic soft material comprising: a polymeric matrix including a plurality of polymeric chains; molecular entanglements existing where the polymeric chains overlap to define the polymeric matrix; and a hydroscopic salt distributed within the polymeric matrix, wherein the fatigue-resistant photoelastic soft material exhibits different photoelastic colors when subjected to a uniaxial tension, and wherein the fatigue-resistant photoelastic soft material has a low hysteresis toughening greater than or equal to about 3 % to less than or equal to about 5 %, a fracture toughness of greater than or equal to about 2,000 J / m2to less than or equal to about 3,500 J / m2, and a fatigue threshold of greater than or equal to about 250 J / m2to less than or equal to about 450 J / m2.

2. The fatigue-resistant photoelastic soft material of claim 1 , wherein the polymeric chains are defined by linked monomers, and the fatigue-resistant photoelastic soft material includes greater than or equal to about 10 weight percentage to less than or equal to about 20 weight percentage of monomers and greater than or equal to about 10 weight percent to less than or equal to about 20 weight percent of the hydroscopic salt.

3. The fatigue-resistant photoelastic soft material of claim 2, wherein the monomers include acrylamide, and the hydroscopic salt includes lithium chloride, calcium chloride, or a combination of lithium chloride and calcium chloride.

4. The fatigue-resistant photoelastic soft material of claim 1 , wherein a stress-optical coefficient of the fatigue-resistant photoelastic soft material is inversely proportional to an amount of the hydroscopic salt in the fatigue-resistant photoelastic soft material.

5. The fatigue-resistant photoelastic soft material of claim 1 , further comprising: a crosslinker distributed within the polymeric matrix..

6. The fatigue-resistant photoelastic soft material of claim 5, wherein an increase in a density of the crosslinker decreases a degree of the molecular entanglements.

7. The fatigue-resistant photoelastic soft material of claim 5, wherein the fatigue-resistant photoelastic soft material includes greater than or equal to about 0.02 weight percentage to less than or equal to about 0.04 weight percent of the crosslinker.

8. The fatigue-resistant photoelastic soft material of claim 5, wherein the crosslinker includes N,N’-methylenebisacrylamide.

9. The fatigue-resistant photoelastic soft material of claim 1 , further comprising: a plasticizer distributed within the polymeric matrix.

10. The fatigue-resistant photoelastic soft material of claim 9, wherein an amount of the plasticizer is inversely proportional to a modulus of the fatigue-resistant photoelastic soft material.11 . The fatigue-resistant photoelastic soft material of claim 9, wherein the fatigue-resistant photoelastic soft material includes greater than or equal to about 60 weight percent to less than or equal to about 80 weight percent of the plasticizer.

12. The fatigue-resistant photoelastic soft material of claim 9, wherein the plasticizer includes water, glycerol, or a combination of water and glycerol.

13. The fatigue-resistant photoelastic soft material of claim 1 , wherein the fatigue-resistant photoelastic soft material is a pre-stretched fatigue-resistant photoelastic soft material.

14. A tactile sensor comprising: a fatigue-resistant photoelastic soft material, the fatigue-resistant photoelastic soft material including: a polymeric matrix including a plurality of polymeric chains; molecular entanglements existing where the polymeric chains overlap to define the polymeric matrix; a hydroscopic salt distributed within the polymeric matrix; a crosslinker distributed within the polymeric matrix; and a plasticizer distributed within the polymeric matrix, wherein the fatigue-resistant photoelastic soft material exhibits different photoelastic colors when subjected to a uniaxial tension, and wherein the fatigue-resistant photoelastic soft material has a low hysteresis toughening greater than or equal to about 3 % to less than or equal to about 5 %, a fracture toughness of greater than or equal to about 2,000 J / m2to less than or equal to about 3,500 J / m2, and a fatigue threshold of greater than or equal to about 250 J / m2to less than or equal to about 450 J / m2.

15. The tactile sensor of claim 14, wherein an increase in a density of the crosslinker decreases a degree of the molecular entanglements; and an amount of the plasticizer is inversely proportional to a modulus of the fatigue-resistant photoelastic soft material.

16. The tactile sensor of claim 14, further comprising: a contact materials system including: the fatigue-resistant photoelastic soft material, and an opaque soft reflective film; and a photoelastic imaging system including: a camera, a linear polarizer, a white light disposed between the camera and the linear polarizer, a rigid plate that is free of residual stress, and a quarter waveplate disposed between the linear polarizer and the rigid plate,the fatigue-resistant photoelastic soft material being disposed between the rigid plate and the opaque soft reflective film.

17. The tactile sensor of claim 16, wherein the photoelastic imaging system further includes: an opaque black box that houses the camera, the linear polarizer, the white light, the rigid plate, and the quarter waveplate.

18. A method for preparing a fatigue-resistant photoelastic soft material, the method comprising: contacting pre-gel solution including monomers and a first solvent to a crosslinker to form a pre-gel mixture; curing the pre-gel mixture to form a hydrogel; contacting the hydrogel to a second solvent until equilibrium swelling is established; and contacting the swollen hydrogel and a hygroscopic solution until salt particles of the hygroscopic solution are evenly absorbed by the hydrogel to form the fatigue-resistant photoelastic soft material.

19. The method of claim 18, wherein the method further comprises, at least one of: contacting the pre-gel solution to a thermal initiator; contacting the pre-gel solution to a crosslinking accelerator; and drying the fatigue-resistant photoelastic soft material.

20. The method of claim 18, wherein, in the fatigue-resistant photoelastic soft material, the monomers define a polymeric matrix including a plurality of polymeric chains where molecular entanglements exist where polymeric chains overlap and salts particles from the hygroscopic solution are distributed within the polymeric matrix.

Images