Strain gauge sensor

Abstract

The present disclosure is directed at a strain gauge sensor, and in particular to spongy foams and methods for manufacturing said foams that may be usable in strain gauge sensors. In some embodiments, the methods for manufacturing said foams may comprise depositing a layer of un-cured elastomer on a substrate, curing the elastomer in a vessel containing a fluid that has been brough to a boiling temperature of said fluid such that at least a portion of the cured elastomer is perforated with micropores, immersing the cured elastomer in a liquid within which a conductive material is dispersed, and evaporating the liquid such that the conductive material is left deposited within said micropores. Spongy foams manufactured this way exhibit improved electrical sensitivity to stretching, greater stretchability, and decreased electrical and mechanical hysteresis, among other advantages.

Description

STRAIN GAUGE SENSORFIELD OF THE INVENTION

[0001] The present disclosure relates to strain gauge sensors, and in particular to spongy foams and methods for manufacturing such foams that may be used in strain gauge sensors.BACKGROUND OF THE INVENTION

[0002] Strain gauges, particularly for wearable sensing applications, require a high degree of stretchability, softness, sensitivity, selectivity, and linearity, while avoiding mechanical and electrical hysteresis, overshoot behavior, and slow response / recovery time. However, current strain gauges face challenges in meeting all the aforementioned requirements simultaneously due to trade-off relationships.SUMMARY OF THE INVENTION

[0003] In one aspect, the present disclosure is directed at a strain gauge sensor comprising an elastomer subjected to a curing process in a vessel containing a fluid that has been brought to a boiling temperature of said fluid such that at least a portion of the elastomer is perforated with micropores, and a plurality of conductive nanowires deposited within said micropores.

[0004] In some embodiments, the curing process may be conducted at a pressure exceeding atmospheric pressure.

[0005] In some embodiments, the curing process may be conducted at a temperature between 110-220°C and at a pressure between 5.8-15 pounds per square inch (psi).

[0006] In some embodiments, the micropores may have diameters ranging from 5 to 50 pm.

[0007] In some embodiments, the elastomer may comprise polydimethylsiloxane (PDMS).

[0008] In some embodiments, the conductive nanowires may comprise Ag nanowires.

[0009] In some embodiments, the conductive nanowires may adhere to interior surfaces of said micropores.

[0010] In some embodiments, the micropores may be randomly distributed throughout the elastomer.

[0011] In some embodiments, the micropores may have randomly-varying diameters.

[0012] In some embodiments, the fluid may be water.

[0013] In another aspect, the present disclosure is directed at a method of manufacturing a strain gauge sensor, the method comprising: depositing a layer of un-cured elastomer on a substrate, curing the elastomer in a vessel containing a fluid that has been brought to a boiling temperature of said fluid such that at least a portion of the cured elastomer is perforated with micropores; immersing the cured elastomer in a liquid within which a conductive material is dispersed; and evaporating the liquid such that the conductive material is left deposited within said micropores.

[0014] In some embodiments, the method may further comprise plasma-etching the cured elastomer to change at least part of the cured elastomer to a hydrophilic state prior to immersing the cured and etched elastomer in the solution.

[0015] In some embodiments, the method may further comprise placing a shadow mask defining a desired design on the layer of un-cured elastomer prior to curing the elastomer, such that only portions of the un-cured elastomer within the desired design are exposed during the curing process.

[0016] In some embodiments, the method may further comprise spin-coating or dip-coating said layer of un-cured elastomer onto said substrate, such that said layer has a thickness of between 100-200pm.

[0017] In some embodiments, the conductive material may comprise Ag, Au, Ag-Au core / shell structures, or carbon nanotubes (CNTs) and the conductive material deposited within said micropores comprises conductive nanowires or tubes.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Additional embodiments of the disclosure, as well as features and advantages thereof, will become more apparent by reference to the description herein taken in conjunction with the accompanying drawings. The components in the figures are not necessarily to scale. Moreover, in the figures, like-referenced numerals designate corresponding parts throughout the different views.

[0019] FIG. 1 is a flowchart depicting an exemplary method for manufacturing spongy foams having mechanical and electrical characteristics suitable for use in stretchable strain gauges.

[0020] FIG. 2 depicts an exemplary system for manufacturing said spongy foams.

[0021] FIG. 3 depicts an exemplary series of steps for manufacturing said spongy foams.

[0022] FIGS. 4 A and 4B depict exemplary applications for using said spongy foams in sensing applications.

[0023] FIG. 5A depicts an illustrative foam with randomly -ordered micropores distributed throughout.

[0024] FIG. 5B depicts an illustrative foam with micropores distributed throughout in an ordered fashion.

[0025] FIG. 6 depicts a conceptual block diagram of an exemplary strain sensor that includes a spongy foam manufactured using the method of FIG. 1.

[0026] FIGs. 7A, 7B, and 7C are pictures of an exemplary apparatus for manufacturing spongy foams, according to one example. FIG. 7D is a picture of an exemplary spongy foam that was manufactured using the exemplary apparatus.

[0027] FIG. 8A are conceptual pictures of an exemplary spongy foam undergoing stretching, while FIG. 8B are scanning electron microscope (SEM) images of the exemplary spongy foam, according to one example.

[0028] FIG. 9 are further SEM images of the exemplary spongy foam at successively higher magnifications, according to one example.

[0029] FIGs. 10 A, 10B, 10C, 10D, 10E, and 10F show mechanical and electrical performance parameters of the exemplary spongy foam compared with a control group of strain sensors comprising a non-porous elastomer substrate, according to one example.

[0030] FIG. 11 A shows a conceptual picture of the exemplary spongy foam undergoing stretching, while FIG. 1 IB shows a conceptual picture of foam from a control group undergoing similar stretching, according to one example.

[0031] FIGs. 12A, 12B, 12C, and 12D show experimental results showing electrical performance parameters of the exemplary spongy foam compared with a control group of strain sensors comprising foam with multi -walled carbon nanotubes (MWCNTs) embedded therein, according to one example.

[0032] FIG. 13 are microscope images showing cracks and delamination marks forming on the surface of control groups (Ag NWs atop Non-porous PDMS substrate) as they are subject to successively higher stress, according to one example.

[0033] FIGs. 14 A, 14B, 14C, and 14D show measurements of electrical sensitivity, stretching, mechanical and electrical hysteresis, and loading and recovery times of exemplary spongy foams compared with sensors from the control group, according to one example.

[0034] FIG. 15 shows measurements of electrical durability to stretching over multiple stretching cycles for the exemplary spongy foams, according to one example.

[0035] FIG. 16 show microscopic images of the exemplary spongy foam compared to sensors from the control group showing the presence of cracks and buckling in the control group, according to one example.

[0036] FIG. 17 shows the electrical response of the exemplary spongy foam from stretching, hammering, and bending, according to one example.

[0037] FIG. 18 show further measurements of the electrical response of the exemplary spongy foam due to compression, bending, and stretching over multiple hours and / or stretching cycles, according to one example.

[0038] FIG. 19 show images of potential applications of the exemplary spongy foam in stretchable strain gauges worn on multiple locations on a human subject’s body, according to one example.DETAILED DESCRIPTION OF THE INVENTION

[0039] For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.

[0040] The present disclosure relates to strain gauge sensors, and methods for manufacturing such sensors. Such sensors find many applications, including in wearable sensors for attachment on, or implantation within, a subject’s body. For example, strain gauges that can be flexibly adhered on, within, or beneath the skin may be utilized to track human motion and human-robot interaction in real-time, providing valuable information on the magnitude, intensity, and consistency of the strains imposed on muscles or joints. This information can have a significant impact on various aspects of healthcare and human performance. Examples of potential applications include (1) injury prevention through real-time alerts on potentially harmful movements or excessive strain, (2) personalized rehabilitation, allowing medical professionals to monitor progress and adjust treatment plans based on the specific needs of each patient, (3) improved pain management by gaining insights into the root causes of chronic pain through ongoing monitoring of joint and muscle forces, and / or (4) accurate gait analysis, which is crucial in the diagnosis and treatment of conditions such as cerebral palsy, multiple sclerosis, and Parkinson's disease.

[0041] Strain sensors may utilize porous elastomers (such as poly dimethylsiloxane (PDMS)), with conductive material, such as conductive nanowires or carbon nanotubes, interlaced within and / or through the pores. As such sensors are stretched and / or deformed, theresistance of the sensor may change, thus allowing circuitry coupled to such a sensor to measure the degree of stretching and / or deformation that the sensor is undergoing. Ideally, the elastomers used in such sensors should exhibit a high degree of stretchability and softness, high sensitivity and linearity while being stretched (e.g., exhibit a greater, and relatively linear, change in electrical resistance while being stretched), and relatively low sensitivity towards other types of deformations (e.g., exhibit a comparatively lesser change in electrical resistance while undergoing compression, bending, or torsion). Such sensors should also avoid mechanical and electrical hysteresis and slow response / recovery times.

[0042] Current sensors face challenges in meeting all of these requirements simultaneously, as optimizing for one requirement often requires trading off on another requirement. For example, strain sensors often face a trade-off between "high stretchability", "high sensitivity", and "high linearity". Highly sensitive sensors require significant structural deformation upon stretching, while highly stretchable sensors must remain intact under large stretching. Achieving both features through simple thin film structures is difficult, as highly sensitive sensors often exhibit high nonlinearity and low stretchability, and vice versa. As a result, developing strain sensors that offer both high stretchability (E > 100%) and high sensitivity(GFs > 50) remains a significant challenge.

[0043] There are also various techniques available for fabricating porous elastomers. Such techniques include lithography, templating, and 3D printing. However, methods such as direct templating (sugar templating, colloidal crystals, and Nickel foam) and lithography often require a template removal process that can be challenging and time-consuming, with high associated costs. Additionally, when using direct templating methods with polystyrene (PS) beads or the lithography method with SU-8 templates (SU-8 being a negative photoresist frequently employed in microfabrication and MEMS (Micro-Electro-Mechanical Systems) processes) to create porous structures, the etching process involves the use of harmful solvents such as dimethylformamide (DMF) and SU-8 developer. 3D printing technique for fabricating porous elastomers can be time-consuming and expensive, leading to low fabrication efficiency.

[0044] Therefore, there is a need for a more efficient and eco-friendly method of creating porous elastomers that can be used in strain sensors that meet the above-mentioned requirements. This disclosure presents a steam-etching process that offers significant advantages over other methods for fabricating porous elastomers for use in strain sensors,such as lithography, templating, and use of polystyrene beads. This steam-etching process utilizes pressurized water molecules to induce the formation of highly interconnected inner pores in elastomers. The process is eco-friendly because it does not require hazardous solvents for etching templates. Additionally, in some embodiments, the method does not require a template removal process or precise printing steps, making the method a straightforward and economically viable approach.

[0045] Strain gauges manufactured using the disclosed steam-etching process exhibit a gauge factor of greater than 8 x 103for strain ranges of greater than 100%. They are able to meet many of the necessary requirements discussed herein with few tradeoffs, including providing high stretchability, softness, sensitivity, selectivity, and linearity, while also avoiding mechanical and electrical hysteresis, overshoot behavior, and slow response / recovery times. The strain gauges presented herein remain highly responsive to stretching while being less affected by other deformation modes, such as compression and bending.

[0046] The methods described herein can include one or more of the described steps in any order. An exemplary method 100 for manufacturing such strain gauges is presented in FIG. 1. Method 100 begins at step 102, where a layer of un-cured elastomer is deposited on a substrate. The elastomer can be any suitable elastomer that, when cured, remains flexible and pliable - one example of a suitable elastomer is polydimethylsiloxane (PDMS). The substrate can be a wafer or glass slide. To remove any dust, the surface of the substrate may first be cleaned, such as by using acetone, isopropyl alcohol, water, and / or any combination of the foregoing. In some embodiments, the layer of elastomer can be deposited using spin-coating or dip-coating with a base to cross-linker ratio of 10: 1 v / v. The resulting layer may have a thickness in the range of 100-200 pm. This step is illustrated as step 302 in FIG. 3.

[0047] At step 104, the elastomer may be cured in a vessel containing a liquid that has been brought to a boiling temperature of said liquid, such that at least a portion of the cured elastomer is perforated with micropores. In some embodiments, the vessel may contain boiling water. This step creates a spongy foam, as the boiling water leaves randomly distributed pores in the curing elastomer. In some embodiments, this steam-etching step may be performed in a pressure vessel using an apparatus similar to that illustrated in conceptual form in FIG. 2. FIG. 2 depicts a pressure vessel 200 comprising a pressure valve 202 that has been partially filled with water 204. Within pressure vessel 200 is disposed a platform 208, which holds a substrate 210 above the water level. On top of the substrate is disposed a layer of elastomer 212. In some embodiments the elastomer 212 may be deposited directly onplatform 208 (i.e., the platform 208 serves as the substrate), or the substrate 212 may comprise an integral part of platform 208. Optionally, a shadow mask 214 defining a cut-out pattern may be disposed on top of the layer of elastomer 212, such that only the portions of the elastomer 212 that are within the cut-out pattern are exposed to steam within the interior of the pressure vessel. This optional mask layer is shown in step 304 of FIG. 3, which shows a shadow mask defining a cut-out design that resembles a letter “P”, and that exposes only the portions of the elastomer layer that are within that design. The pressure vessel 200 may then be brought to a temperature that exceeds the boiling point of water such that the interior of the vessel 200 is filled with steam 206. Optionally, the air within pressure vessel 200 may be brought to an internal pressure that exceeds atmospheric pressure, provided that the temperature of the vessel still exceeds the boiling point of water at said higher internal pressure. In some embodiments, approximately 500g or more of water may be used in a steam-etching process that uses approximately 580 mg or more of polymer. Such a steametching step may be carried out at a temperature of 110-120°C or approximately 120°C for 15-30 minutes, with a pressure of between 5.8-15 pounds per square inch (PSI), or approximately 15 psi, for a batch size of approximately 4x3 inches.

[0048] Returning to FIG. 1, at step 106, the cured porous elastomer may be immersed in a liquid within which a conductive material is dispersed. This step is also illustrated as step 306 at FIG. 3. For example, the liquid may comprise a solution containing a conductive metal (such as Ag (silver) or Au (gold)) dispersed or dissolved throughout. Optionally, the cured elastomer may be first changed into a hydrophilic state by subjecting it to oxygen plasma treatment for 10 minutes - this optional step may facilitate the subsequent absorption and deposition of the conductive material within the porous elastomer.

[0049] At step 108, the cured elastomer is then removed from the liquid and allowed to dry. This step may optionally include spin-casting the cured elastomer to eliminate any residual liquid and then allowing the elastomer to dry in ambient conditions. As the liquid evaporates, the conductive material is left behind and deposited within the micropores of the porous elastomer. For example, if the liquid had included Ag, evaporation of the liquid may leave behind a collection of Ag nanowires (NWs) approximately 25 nm in diameter and 20-25 pm in length deposited throughout the perforated micropores. The resulting spongy Ag foam can then be cut to the desired size for use, creating a highly stretchable and sensitive 3D percolated conductive network structure.

[0050] The spongy Ag foam can be used as a stretchable strain sensor in a plurality of applications. FIG. 4A shows examples of how non-patterned spongy foam can be wrapped around a finger, a can, or a cylinder, while FIG. 4B shows how patterned spongy foam manufactured with a shadow mask can be adhered to a balloon or a plastic container. In each application, the foam may be coupled to circuitry that measures the electrical resistance of the sensor. As the sensor stretches, the electrical resistance can be expected to vary, thus allowing the circuitry to detect and measure the degree of stretching being experienced by the sensor.

[0051] One advantage of spongy foam manufactured using the disclosed steam-etching process is that it results in micropores with randomly distributed size / diameter and placement throughout the elastomer layer. An elastomer with such randomly distributed micropores of varying sizes is depicted in FIG. 5A. This is in contrast with prior methods such as lithography or 3D printing that resulted in an ordered placement of micropores with similar sizes throughout the elastomer layer, as illustrated in FIG. 5B. The random distribution of micropores of differing sizes results in an elastomer layer that is significantly more porous than elastomers having a regular and uniform distribution of micropores.

[0052] Spongy foam manufactured using method 100 remains effectively responsive to stretching while being less affected by other forms of deformation such as compression and bending (compared to strain sensors manufactured using other methods of creating porous elastomers). As the porous spongy foam manufactured using method 100 is stretched, the internal pore walls rotate and align themselves in the direction of the stretch. This results in lower actual strain on the pore walls and improved stretchability. The microporous structure of the spongy foam manufactured using method 100 exposes a larger portion of its volume to lower levels of strain than non-porous structures, leading to a slower increase in resistance and a significantly increased strain sensing range and sensitivity at large strain ranges. Additionally, as discussed further herein, the values of In (R / Ro) exhibit high linearity (R2 = 0.997) versus tensile strain, enabling prediction of changes in sensitivity. Upon compression, the internal pore walls are crushed, leading to reduction in pore size and a relatively low concentration of strains. Upon bending, the spongy foam shows deformation behaviors similar to stretching and compression. An increase in porosity leads to a larger area experiencing minimal strain due to the concentration of strain in small regions where the pore walls are parallel to the loading axis. The increase in porosity, particularly in compression and bending, enables materials to move into empty spaces, resulting in minimal strain andreduced lateral deformation. Thus, the increase in porosity enables spongy foam manufactured using method 100 to stretch further and exhibit greater sensitivity while being stretched, while being relatively insensitive to other modes of deformation such as compression, bending, and torsion.

[0053] FIG. 6 shows an exemplary strain gauge sensor 600 that uses and / or incorporates the spongy foam manufactured using method 100. Sensor 600 includes the aforementioned spongy foam 602 that may be stretched in the direction indicated by arrow 604. Foam 602 is connected to a processing circuit 608 via electrical leads 606a and 606b, which are in turn electrically connected to foam 602 via terminals 604a and 604b, respectively. Processing circuit 608 may comprise electrical circuitry configured to measure an electrical resistance of foam 604 between terminals 604a and 604b. Such electrical circuitry may include a voltage source configured to establish a voltage differential between terminals 604a and 604b and a current meter configured to measure a current that flows between terminals 604a and 604b as a result of said voltage differential. Processing circuitry may also include a controller, such as a microcontroller or microprocessor, that reads and processes said current measurement and converts it into an estimate of an electrical resistance of foam 602. The electrical resistance of foam 602 varies with the degree of strain imposed on foam 602 in the direction of arrow 604. The controller may therefore optionally convert the estimated electrical resistance of foam 602 into an indication of a degree of strain or stretch being imposed on foam 602. Optionally, said controller may also store the estimate of electrical resistance, and / or the indication of a degree of strain or stretch, in volatile or non-volatile memory, and / or display said output parameters to a user. Although foam 602 is shaped in the form of a rectangular strip, and terminals 604a and 604b are placed on opposing sides of foam 602, it should be understood that foam 602 may be shaped in any arbitrary shape and terminals 604a and 604b may be placed in any suitable location for measuring an electrical resistance of said foam 602.EXAMPLES

[0054] In one example, stretchable strain gauge sensors were manufactured according to method 100 using a pressure rice cooker (Instant Put Duo by Instant Brands, Inc.). For this process, >500g of water and >580 mg of PDMS polymer were used. The uncured PDMS precursor in a mixture of 10: 1 weight ratio of base and curing agents was spun cast onto a glass substrate. The PDMS was cured in using the steam-etching process described herein at a temperature of approximately 120°C for 15-30 minutes with a pressure of 15 psi per batch for a size of 4x3 inches. During this step, a shadow mask with a desired design was placed ontop of the PDMS surface for patterning. FIG. 7A shows a top-down picture of the pressure rice cooker used in this example with a platform 704 placed therein. FIG. 7B shows a picture of the elastomer 706 on top of a substrate (not shown) placed on top of and obscuring platform 704. FIG. 7C shows placement of the optional shadow mask 708 on top of the elastomer 706. FIG. 7D shows a picture of the resulting perforated and cured elastomer defining the pattern “P” from the shadow mask. The fully cured PDMS showed perforated micropores with diameters ranging from 5 to 50 pm. It was then subjected to oxygen plasma for 10 minutes to create a hydrophilic surface, then immersed in a 1 mg ml’1solution of Ag NWs (25 nm in diameter and 20-25 pm in length; Flexoioink, SG Flexio, Inc.), leading to infiltration of the Ag NWs into the perforated micropores. Although optional, the oxygen plasma treatment created a hydrophilic surface on the cured PDMS that facilitated the absorption and deposition of Ag NWs throughout the perforated micropores. The PDMS was then spin-cast to eliminate any residual Ag NWs on its surface, then dried in ambient conditions. The resulting spongy Ag foam was cut to the desired size for use. In this example, the spongy Ag foam was cut to a size of 2 cm x 7.5 cm for mechanical testing using a Mark- 10 mechanical testing system to obtain its stress-strain curves. The resistance of the spongy Ag foam was measured using a Keithley 2400 source meter (Tektronix, Inc.) in a two-wire configuration for electrical testing. The structural details of the spongy Ag foam were analyzed using a high-resolution scanning electron microscope (SEM) (S-4800; Hitachi, Inc.).

[0055] The resulting spongy Ag foam strain gauge was found to provide a gauge factor of greater than 8 x 103for strain ranges of greater than 100%. As used herein, gauge factor (GF) is defined by the following equation (1):Equation

[0056] where Ro, AR, and & are the initial electrical resistance before stretching, difference of resistance under stretching, and applied strain, respectively.

[0057] The behavior of a strain sensor (e.g., the way in which electrical resistance can be expected to change with applied strain) can be modeled according to Equation 2, presented below:

[0058] Equation

[0059] where A is a constant that depends on the sensor’s geometry and material properties (and may be used to calibrate the sensor), B is related to the previously-defined gauge factor and measures how much resistance changes with strain, and Ro, AR, and a are as previously defined. The coefficient A may be proportional to the number of conducting units within a conducting path but inversely proportional to the number of conducting paths. The coefficient B may be associated with average tunneling distance and the height of potential barrier between adjacent conducting units.

[0060] FIG. 10F shows measured gauge factor relative to percentage strain. By fitting a line to the measured curve (shown as the “Fitting Curve”), the parameters of Equation 2 for the resulting spongy Ag foam strain gauge can be estimated to be A = 1.389, and B = 6.672. The derivative of relative change in resistance (AR / Ro) with respect to the applied strain (a) is equivalent to GF and the value for the aforementioned fitted curve was found to be 9.627-e6 672e. As shown in FIG. 10F, the GF curves from the fitted equation and experimental data showed good agreement.

[0061] The spongy Ag foam was highly porous at the microscale and contained percolation networks of Ag nanowires (NW s) as internal conducting passages. Conceptual pictures of the spongy Ag foam undergoing stretching are shown in FIGs. 8A and 11 A, while FIG. 8B provides scanning electron microscope (SEM) images of the spongy Ag foam undergoing stretching. The spongy Ag foam exhibited elasticity of up to 140% without causing notable damage, due to the three-dimensional (3D) rotation of its internal pore walls to accommodate the deformation. The spongy Ag foam was also bendable, compressible, and able to withstand torsion without compromising its material properties.

[0062] FIG. 9 shows further SEM images of the micropores formed in the spongy Ag foam by the steam-etching process. As shown when zooming in to successively higher magnifications, the steam-etching process not only leaves pores of varying sizes that are randomly distributed throughout the elastomer, it also leaves successively smaller pores formed in the interior walls of larger pores. As shown in image B of FIG. 9, the Ag nanowires from the Ag NW solution are not merely disposed within the micropores, but adhere to the internal surfaces of pores such that, as pores bend and stretch, the adhered nanowires are also caused to bend and stretch.

[0063] FIGs. 10A-F show mechanical and electrical performance parameter of the spongy Ag foam compared to a control group of stretchable strain gauges that featured a network of Ag NWs atop a non-porous PDMS substrate, as illustrated in FIG. 1 IB, for comparison. Thespongy Ag foam exhibited a nearly 2-fold decrease in stiffness compared to the control group, where the corresponding mechanical moduli were measured as 0.9 MPa and 2.0 MPa, respectively (see FIG. 10A). The spongy Ag foam also exhibited a superior tensile breaking strain of -140% compared to the control group (-105%) mainly due to the 3D rotation of the internal pore walls relative to the stretching direction, thus resulting in lower local strains acting on the Ag NWs (see FIG. 10B). The relative resistance change (AR / Ro) of the spongy Ag foam increased exponentially with the applied strain up to a strain of 105%, resulting in a maximum AR / Ro that was almost than 4 times higher than that of the control group (see FIG. 10C). The microporous structure of the spongy Ag foam had a larger portion of its volume exposed to lower levels of strains compared to the non-porous structure, resulting in a slower increase in resistance and a drastically increased strain sensing range. Notably, AR / Ro of the spongy Ag foam was substantially higher, by approximately 140 times, relative to those of a similar spongy foam containing multi walled carbon nanotubes (MWCNTs) at strains between 100-120%, respectively (see FIG. 12B). Also, the R of the spongy Ag foam (-33Q) was substantially lower (by approximately 11,000 times) than the R of the MWCNTs (-360 kQ) (see FIG. 12A). In this example, the average diameter and length of the MWCNTs in the control spongy foam were measured as 10-20 nm and 10-30 pm, respectively. The MWCNTs were blended with isopropanol at a weight ratio of 0.5 wt% and sonicated for an hour to produce a uniform suspension through untangling. The high sensitivity of the spongy Ag foam over the MWCNTs foam is mainly attributed to the fact that cracks are easily induced in the brittle Ag NWs due to their low fracture toughness, leading to a sharp increase in resistance with strain. Cracks and delamination were observed in the control group (i.e., comprising Ag NWs on a non-porous PDMS substrate) when stretched to > 5% and > 22%, respectively (see FIG. 13). As a result, the spongy Ag foam had a high gauge factor (GF > 8* 103) at a large strain of > 100% (see FIG. 10D). In addition, the spongy Ag foam maintained a high linearity(R2= 0.997) in response to strains up to 105% (FIG. 10E). The fitting and measured curves of GF matched well each other for precision calibration (FIG. 10F).

[0064] For reliable sensing, the spongy Ag foam should operate stably under cyclic loads without causing any damage or deterioration. The spongy Ag foam showed no notable alteration in AR / Ro even after > 1,000 cycles of stretching at an applied strain of up to 5%, in contrast to the control group, which experienced a more than 4-fold increase in AR / Ro (FIG. 14A). The results were consistent at the applied strain of up to 20% (FIG. 15), while thecontrol group could not be examined at these levels, due to cracks and buckling of the Ag NWs (FIG. 16). FIG. 14B shows that the mechanical hysteresis (HM) of the spongy Ag foam remained lower than that of the control group when stretched more than 1,000 cycles due to the reduced viscoelasticity of the elastomer (i.e., PDMS) caused by the presence of the perforated micropores. The HM decreased in the first cycle due to an initial hysteresis effect (i.e., irreversible plastic deformation) and then remained stable during subsequent cycles (i.e., reversible elastic deformation). Similarly, the electrical hysteresis (HE) of the spongy Ag foam remained lower than that of the control group in response to the stretching cycles due to the reversible elastic deformation of the micropores (FIG. 14C).

[0065] The mechanical and electrical hysteresis of the spongy Ag foam was evaluated by plotting the AR / Ro versus strain curves, with the resistance of each cycle (100th, 500th, 1,000th) set as the initial value. The extent of hysteresis was determined from the width of the hysteresis loop. The variation in area beneath the AR / Ro versus strain graph between loading and unloading states was quantitatively defined as electrical hysteresis and by the following equation 3 :Equation 3: HE= 'Al~Au' X 100 % Au

[0066] where AL and Au are the areas of relative electrical signal change versus strain in the loading and unloading states, respectively. Mechanical hysteresis (HM) was defined as the difference in area under stress-strain curves during loading-unloading cycles and computed by the following equation 4:Equation 100 %

[0067] where AL and Au are the area under the stress-strain graph in loading and unloading states, respectively.

[0068] FIG. 14D presents the response (left column) and recovery (right column) time of the spongy Ag foam (top row) compared to the control group (bottom row) at an applied strain of 5%, with a loading and unloading rate of 500 mm min'1. The response and recovery time of the spongy Ag foam were measured as -266 ms and -292 ms with the corresponding latency of 26 ms and 52 ms, respectively. These response and recovery time were at least 1.6-fold faster than the control group. In addition, no notable overshoot and relaxation behavior was observed for the spongy Ag foam, in contrast to the control group.

[0069] Due to its highly porous structure, the spongy Ag foam exhibits selective electrical responsiveness to stretching, while being relatively electrically unresponsive to other types ofdeformations such as compressing and bending. For example, the micropores of the spongy Ag foam can be closed when compressed or bent without substantially altering the percolation pathways of Ag NWs, resulting in minimal resistance changes. FIG. 17 presents that the AR / Ro of the spongy Ag foam remained barely changed upon hammering and bending, while exhibiting a high response during stretching. These results were maintained after 4 hours of compression with a weight (1 kg) and > 1,000 cycles of bending (FIG. 18).

[0070] To showcase an example of the spongy Ag foam for motion tracking, multiple specimens were designed to measure strain changes ranging from minor (0-5%) to substantial (5-50%) in various body parts, including the facial muscles, neck during speaking and coughing, wrists for pulse measurementjoints during bending, upper arms during muscle contractions, abdomen for measuring respiration rate, and eyelids during blinking, from a volunteer (see FIG. 19, inset A). The spongy Ag foam was fitted with Cu tapes (CST5, Digi- key, Inc.) on both sides to act as electrical connectors, then mounted on various body parts. During measurement, the Cu tapes were linked to a Keithley 2400 source meter (Tektronix, Inc.) in a two-wire setup. The spongy Ag foam showed a detectable change in resistance when responding to the skin deformations or vibrations caused by the various body motions (FIG. 19, inset B). For a further demonstration of human-robot interaction, the bending of finger joints was tracked and processed using a single-board microcontroller to coordinate robot fingers in expressing sign language words, such as “peace”, “love”, and “rock” (FIG. 19, inset C).

[0071] Various aspects of the present disclosure may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments. Thus, while the present teachings have been described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments or examples. On the contrary, the present teachings encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art. Accordingly, the foregoing description and drawings are by way of example only.

Claims

CLAIMSWe claim:

1. A strain gauge sensor, comprising: an elastomer subjected to a curing process in a vessel containing a fluid that has been brought to a boiling temperature of said fluid such that at least a portion of the elastomer is perforated with micropores; and a plurality of conductive nanowires deposited within said micropores.

2. The strain gauge sensor of claim 1, wherein the curing process was conducted at a pressure exceeding atmospheric pressure.

3. The strain gauge sensor of any of claims 1-2, wherein the curing process was conducted at a temperature between 110-120°C and at a pressure between 5.8-15 pounds per square inch (PSI).

4. The strain gauge sensor of any of claims 1-3, wherein the micropores have diameters ranging from 5 to 50 pm.

5. The strain gauge sensor of any of claims 1- 4, wherein the elastomer comprises polydimethylsiloxane (PDMS).

6. The strain gauge sensor of any of claims 1-5, wherein the conductive nanowires comprise Ag nanowires.

7. The strain gauge sensor of any of claims 1-6, wherein the conductive nanowires adhere to interior surfaces of said micropores.

8. The strain gauge sensor of any of claims 1-7, wherein the micropores are randomly distributed throughout the elastomer.

9. The strain gauge sensor of any of claims 1-8, wherein the micropores have randomly- varying diameters.

10. The strain gauge sensor of any of claims 1-9, wherein the fluid is water.

11. A method of manufacturing a strain gauge sensor, comprising: depositing a layer of un-cured elastomer on a substrate; curing the elastomer in a vessel containing a fluid that has been brought to a boiling temperature of said fluid such that at least a portion of the cured elastomer is perforated with micropores; immersing the cured elastomer in a liquid within which a conductive material is dispersed; and evaporating the liquid such that the conductive material is left deposited within said micropores.

12. The method of claim 11, further comprising plasma-etching the cured elastomer to change at least part of the cured elastomer to a hydrophilic state prior to immersing the cured and etched elastomer in the solution.

13. The method of any of claims 11-12, further comprising placing a shadow mask defining a desired design on the layer of un-cured elastomer prior to curing the elastomer, such that only portions of the un-cured elastomer within the desired design are exposed during the curing process.

14. The method of any of claims 11-13, wherein said curing is conducted at a pressure exceeding atmospheric pressure.

15. The method of any of claims 11-14, wherein said curing is conducted at a temperature between 110-120°C and at a pressure between 5.8-15 pounds per square inch (PSI).

16. The method of any of claims 11-15, further comprising spin-coating or dip-coating said layer of un-cured elastomer onto said substrate, such that said layer has a thickness between 100-200 pm.

17. The method of any of claims 11-16, wherein the micropores have diameters ranging from 5 to 50 pm.

18. The method of any of claims 11-17, wherein the elastomer comprises polydimethylsiloxane (PDMS).

19. The method of any of claims 11-18 wherein said conductive material comprises Ag, Au, Ag-Au core / shell structure, or CNTs and the conductive material deposited within said micropores comprise conductive nanowires or tubes.

20. The method of any of claims 11-19, wherein said conductive material comprises conductive nanowires or tubes that adhere to interior surfaces of said micropores.

21. The method of any of claims 11-20, wherein the fluid is water.

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