DEVICES AND METHODS FOR TESTING INTERFACIAL PROPERTIES

Priority Claims and Related Patent Applications

[0001] This application claims the benefit of priority from U.S. Provisional Application Serial No. 62/959,900, filed on January 11, 2020, the entire content of which is incorporated herein by reference for all purposes.

Technical Field of the Invention

[0002] The invention generally relates to devices and methods for testing material and mechanical properties. More particularly, the invention provides novel devices, and methods of using same, for testing the interfacial ( e.g ., frictional) properties of substrates when in sliding contact with one another, for example, one being a medical product and the other being a biomimetic substrate.

Background of the Invention

[0003] Many medical products have a sliding interaction with bodily tissues during their use. It is desirable to study the tribological properties of such interaction during development and ongoing evaluation of such products, in order to understand the forces, modes of lubrication, and wear properties of the tribological pair, i.e., the product and the bodily tissue.

[0004] Friction testing is commonly used to evaluate the performance and compatibility of variety of materials from lubricants, to biofilms, and medical devices to determine the frictional characteristics of a material surface. It is generally determined to be the ease by which two surfaces (often of different materials) slide against each other with or without the presence of a lubricant. Low friction forces in these testing methods indicate that smoother surfaces and less resistance to a sliding motion. High frictional forces tend to correlate to rougher surfaces and higher resistance between the material interfaces during sliding.

[0005] Several standard American Society for Testing and Materials (ASTM) methods are available for testing sliding and measuring sliding forces of medical products and materials. These methods generally measure the initial and moving friction of one material being dragged across another, otherwise known as the static (initial) and kinetic (moving) coefficients of friction. In these standard set-ups, polymers or films are usually tested dry or in the presence of a lubricant that is laid on top of a hard-smooth surface where a sled is placed on top of a film and pulled by a calibrated load.

[0006] These set-ups have several distinct drawbacks that limit their applicability towards a real- use scenario, especially for medical products in use against human organ or tissue. One major drawback is the use of rigid fixture materials (e.g. , tire hard-smooth surface and sled), compared to non-rigid and compliant tissues in physiology (e.g., organs). Other drawbacks include the use of non- hydrated materials, non-physiologic forces and speeds, non-realistic sliding distances, lubricant use inconsistent to physiological environment, and use of non-physiological materials (e.g., plastic). [0007] In the case of measuring condoms, a key drawback in the available method is the fiat-on- flat surface configuration, an unrealistic geometry for measuring friction to replicate intercourse. In addition, the flexibility and elasticity of the condom material is an important feature to reduce risk of breakage and form to its surrounding surfaces during intercourse. Yet within available ASTM setup (ASTM D1894), the gluing, taping, or clamping of the condom situates the material in place, thus restricting the condoms’ material properties. Furthermore, this set-up is not properly made to measure friction for repeated articulations (i.e., use simulation). Although the slide can be pulled back and forth, this does not correlate to a real-use scenario given that the flat-on-flat rigid geometry exhibits different frictional phenomena than curved and compliant tissues, and non-fluid-en closed set-up confines fluid lubricant in a different manner than an enclosed-fluid configuration such as in sexual intercourse.

[0008] Thus, there is an ongoing need for an unproved friction testing device that addresses these drawbacks, especially for measuring friction forces for sexual intercourse and testing condom performance.

Summary of the Invention

[0009] The invention is based in part on devices that test certain material and mechanical properties of two substrates in lubricated or non-lubricated sliding contact with one another ( e.g ., within an enclosed circular environment). One substrate is moved forward and backward in a linearly reciprocating motion, applying force to the opposing substrate. The invention is further based in part on methods for using the device for a variety of purposes, including but not limited to simulated use testing, wear testing, friction testing, durability testing, and sliding force testing. [0010] The invention provides an automatic, reproducible approach to measure and compare the lubricity and durability of condoms and their coatings. The device allows for reproducible frictional measurements of multiple cyclic insertions/articulations in a linear manner.

[0011] In one aspect, the invention generally relates to a testing device. The testing device comprises: a reciprocating linear actuator having a defined axis along which the actuator is configured to move in forward and reverse motions; a cylindrical mandrel engaged to the reciprocating linear actuator, the mandrel having an outer surface for placing a test substrate thereon thereby exposing a surface of the test substrate; a hollow cylindrical housing fixture with an interior bearing liner for slidingly receiving the mandrel with the test substrate placed thereon in a tight sealing; and a sensor coupled to the housing fixture for measuring sliding force experienced by the housing fixture upon receiving the mandrel with the test substrate placed thereon in a tight sealing. [0012] In another aspect, the invention generally relates to a system for measuring friction. The system comprises: an actuator configured to move in reciprocating motions; a first platform engaged to the actuator for securing a first test substrate thereon thereby exposing a surface of the first test substrate; a second platform for securing a second test substrate thereon thereby exposing a surface of the second test substrate; a sensor coupled to the second platform for measuring the frictional force experienced by the second platform, wherein the first platform and the second platform are configured to allow a tight sealing between the surface of the first test substrate and the surface of the second test substrate when the motor-driven actuator is engaged in reciprocating motions.

[0013] In yet another aspect, the invention generally relates to a method for measuring an interfacial property. The method comprises: providing a testing device disclosed herein; placing a test substrate on the outer surface of the cylindrical mandrel thereby exposing a surface of the test substrate; placing an interior bearing liner inside the hollow cylindrical housing fixture so that it slidingly receives the cylindrical mandrel with the test substrate placed thereon in a tight sealing; causing the reciprocating linear actuator to engage in forward and reverse motions thereby driving the cylindrical mandrel having the test substrate placed sliding into and away from the hollow cylindrical housing fixture having the interior bearing liner; and measuring a friction or sliding force experienced by the sensor.

Brief Description of the Drawings

[0014] FIG. 1 depicts a schematic illustration (top) of an embodiment of a device according to the invention and a picture (bottom) of a prototype device. [0015] FIG. 2 depicts a schematic illustration (top) of an embodiment of a device according to the invention and a picture (bottom) of a prototype device.

[0016] FIG. 3 depicts a schematic illustration (top) of an embodiment of a device according to the invention and a picture (bottom) of a prototype device.

[0017] FIG. 4 depicts a schematic illustration of an embodiment of a configuration of the mandrel and housing fixtures with a test substrate.

[0018] FIG. 5 depicts an example of a lining for the housing fixture according to an embodiment of the invention.

[0019] FIG. 6 shows exemplary data on the average peak insertion forces for a silicone lubricated condom and a non-lubricated condom over the test’s duration.

[0020] FIG. 7 shows exemplary data on the friction force time profile (the “raw” force vs time data, collected at 20 Hz, by the force sensor) for an example substrate pair, with positive force values representing withdrawal force and negative force values representing insertion force. Time has been multiplied by reciprocation frequency to convert to cycle number, In this example, reciprocation frequency is 1 Hz. 200 reciprocations (cycles) were performed. Sub-charts show zoomed in X axis (cycle number) range: 50 cycles and 5 cycles.

[0021] FIG. 8 shows exemplary data on the isolated peak forces (in this example, the withdrawal force peaks are shown) versus cycle # for an example substrate pair. The peaks can be plotted as-is, or a running average (in this example, a trailing- 10-cycle running average is shown) can be plotted. [0022] FIG. 9 shows exemplary data on isolated peak force time profiles for three example substrate pairs (red, light green, and dark green) in units of force (Newtons) above, and also normalized by the time-varying force values of the red data (unitless - ratio of light green to red, and dark green to red, plotted as percent difference vs red, i.e. normalized by red).

[0023] FIG. 10 shows exemplary data on a control chart for two types of “product” being measured - light green product and dark green product - and the acceptable ranges for each are indicated with grey and blue bars, respectively. In this example, the X axis representing cycle time corresponds to a control chart that assess the friction of test samples over time, and if friction increases too greatly or decreases too greatly over time, then the operator can infer a potential problem in achieving the quality control specification.

[0024] FIG. 11 shows exemplary data on an isolated peak force time profile, and its corresponding average peak force (averaged over the test’s duration).

[0025] FIG. 12 shows exemplary data on the average peak frictional force over a 200-cycle friction test, for four test sample condoms (A-D, green bars), in comparison with a commercial condom (red bar). Values in parentheses indicate percentage less than the commercial condom’s frictional force. The average peak frictional force of the four test sample condoms can be resolved/distinguished from each other and from the commercial condom’s average force.

[0026] FIG. 13 shows an exemplary comparison of device specifications, test method capabilities, and typical testing conditions, between two other commonly used methods in the condom testing industry, and the device disclosed herein.

Detailed Description of the Invention

[0027] The invention provides devices for testing the material and mechanical properties of two substrates in contact with one another, and methods of use thereof. One of the substrates is a medical product ( e.g ., a condom) and the other is a biomimetic substrate. The disclosed device is design to test certain material and mechanical (e.g., frictional or interfacial) properties of two substrates in lubricated or non-lubricated sliding contact with one another (e.g., within an enclosed circular environment). The usage of the friction device design can be done in an automatic, reproducible approach to measure and compare frictional properties of two substrates sliding over multiple cyclic overtime (e.g., insertions/articulations in a linear manner). One substrate is moved forward and backward in a linearly reciprocating motion, applying force to the opposing substrate. The device measures the interfacial or frictional properties and durability of lubricated condoms in an enclosed environment against a tissue mimetic to simulate penile - vaginal penetration for sexual intercourse. [0028] Designing a test figure able to replicate or mimic the dynamic environment of human intercourse to simulate a real use-case scenario is of importance to properly measure the frictional forces and sliding movement of penetration in a reproducible manner. Testing data obtained in real- use simulations offer a significant advantage to improve the test figures of current medical devices such as condoms.

[0029] In a preferred embodiment, the device of the invention includes a medical device material, device, or biomaterial surface to be tested in a repetitive sliding motion against a substrate material, preferably a biomimetic. In a more preferred embodiment, the device comprises a mandrel onto which a condom to be tested is rolled, that moves repeatedly into and out of a cylindrical housing bearing on its interior surface a vaginal tissue mimetic material, to simulate the use of a male condom during intercourse. In such an embodiment, the force of sliding the mandrel into and out of the housing is recorded via a force gauge, and the force may be compared among various tribological conditions and for various condom types, to compare the frictional performance of various condoms. [0030] In certain embodiments, the friction device is used to evaluate frictional properties of two sliding surfaces within an enclosed environment. The forces can be performed in a sliding ‘back-and- forth’ motion, which can be done single or multiple times up to 1, 10, 100, or 1000 repetitive cycles. Frictional forces are to be automatically measured via a force gauge to monitor changes over the cycles. The speed of insertion can be adjusted to measure friction in a steady linear manner at one velocity, a gradually increasing or decreasing manner, or in a pattern of varying speeds.

[0031] In a preferred embodiment, the frictional properties between a medical device interface against a biomimetic substrate is monitored in the presence with or without lubrication, In a more preferred embodiment, the device measure the frictional properties and durability of lubricated condoms in an enclosed environment against a tissue mimetic to simulate penile-vaginal penetration for sexual intercourse.

[0032] In certain embodiments, the device comprises a bearing in the geometry of a journal bearing, or a sleeve bearing, or a thrust bearing, or a piston. The elements of such a device are: a motor that drives motion, a geometry that drives linear motion if the motor is not a linearly reciprocating motor, a moving fixture, a substrate on the moving fixture and on the housing fixture (the two substrates comprising the tribological pair to be evaluated), the housing fixture, and a sensor for measuring sliding force.

[0033] Various motors may be utilized, for example, gear motor (reduction gear system), screw motor, air-driven, or hydraulic fluid.

[0034] The motor must function with working loads relevant to the scenario to be tested. In some instances, the working load is about 10 Newtons to about 60 Newtons. The duty cycle of the motor is preferably near 100% to allow for continuous motion during the test duration. If duty cycle is not approximately 100%, then motion will need to be paused periodically.

[0035] In certain embodiments, the motor is powered by batteries, e.g., 8 AA batteries.

[0036] In certain embodiments, the motor is powered by a standard household electrical outlet

(120V), generators, solar panels, or other devices to provide electricity.

[0037] In certain embodiments, a person may apply a manual force to move to push and pull moving fixture, by moving it forward and backward, into and out of the housing fixture.

[0038] In regard to the geometry of the motor that drives the linear motion, various configurations are possible. For example, the motor movement can be a rotation which is converted to a linear back- and-forth repetitive motion. This conversion may be accomplished through a variety of configurations, including a Scotch yoke, a typical 2-pivot point system. The motor movement may also be a linear reciprocating that drives the coupled actuator. [0039] Linear motion may be in the form of constant speed (triangle wave), smooth deceleration/acceleration to be gentler on the materials (sinusoidal wave), or other time course. Reciprocation frequency can range, for example, from 0.25 Hz up to 5 Hz whereas the length for testing can be within the range, for example, from 1 to 10 inches.

[0040] The linear motion of a journal bearing (unidimensional convex plane sliding against unidimensional concave plane) disclosed herein can be advantageous in certain applications since it mimics physiologic sliding of curved surfaces against one another.

[0041] The motion described is further advantageous because it allows for cyclic testing, forward and backward, whereas other tests more typical in the art are only a single sliding of substrate against substrate. Continued cyclic testing allows for observing changes in mechanical properties and its effects as a function of cycle number, which is relevant to mimic human physiological activities that involve multiple sliding cycles.

[0042] In regard to the geometry of the moving fixture, various configurations can be employed, for example, rounded cylinder, mandrel, condom former, a catheter itself (with no fixture per se), etc. [0043] In certain embodiments, the moving fixture is to be made of a round, cylindrical, rigid shape using a material such as glass, metal, ceramic, or plastic. In certain embodiments, the moving fixture can be round, cylindrical, non-rigid ( e.g ., elastomer, rubber, silicone, polyurethane, and other compliant materials).

[0044] The moving fixture can have enclosed ends or have one or both open ends.

[0045] In one embodiment, rigid fixtures have advantages related to the ability to quantify contact pressures and other relevant forces, as well as the ability to know the deformation of the rigid substrate is zero, and all the observed deformation of the mated tribological pair comes from the other, non-rigid surface (if the other surface is non-rigid). If both surfaces are rigid, then again, the deformation is zero. It simplifies the modeling of contact, rather than having two non-rigid surfaces where it may be challenging to quantify the deformation of each, the contact pressures, etc.

[0046] For certain testing conditions, non-rigid fixtures may be advantages. For example, multidimensional tribological testing requires that the surfaces be well mated together in order to observe true frictional interface phenomena rather than artifacts arising from non-mated contact points. The moving fixture’s axis of motion, when aligned with and moving into and out of the long axis of the housing fixture, is prone to miniscule alignment errors causing incomplete mating of surfaces if both the moving fixture and the housing fixture (and their substrates) were completely rigid. Using compliant fixtures is one strategy to obviate this problem. [0047] Another strategy to obviate the problem of poor mating during multidimensional tribological testing is the use of negligible-friction angles and pivot points, as well as translation points, to allow for the axis of motion of the moving fixture to remain precisely colinear with the housing fixture’s long axis in the case of the housing fixture being a cylinder. If at least one fixture can both translate and pivot with negligible friction (through the use of low-friction bearings, pivot points, and other mechanisms), then alignment issues will be obviated.

[0048] A cylindrical geometry is advantageous compared to flat planar geometries because it is more physiologic. The bodies of animals do not have flat surfaces, and the friction observed in flat interfaces is not directly translatable to that observed with curved substrates. This refers to frictional phenomenon, irrespective of increased insertion causing increased friction force.

[0049] The geometry is further advantageous because increased sliding causes increased friction force. Compared to a sled friction test per ASTM D1894 in which the sliding force is approximately constant over the entire duration of the sliding motion, the sliding force increases when inserting a mandrel into a cylindrical housing, which more closely mimics physiology. The surface contact area increases linearly with further insertion proceeding as a constant speed.

[0050] The substrate to be tested and attached to the moving fixture include, but is not limited to, natural latex, synthetic latex, nitrile, silicones, other rubbers (including polyurethanes, polyamides, polybutadienes), other elastomers, other plastics, metal with a lubricious coating, composite with a lubricious coating, etc. The tested substrate may be molded, draped, or adheres directly to body of the moving fixture. In certain embodiments, the tested substrate is to lay flat and is in contact to the moving fixture. In certain embodiments, clamps, glue, rubber bands, or tape is used to mount the subtracted into the moving fixture.

[0051] As used herein, the term “latex” refers to natural or synthetic latex, which include vulcanized or non-vulcanized. The term “latex polymer” refers to the polymer(s) the latex is formed from. Typically, the latex substrate is hydrophobic. In some embodiments, the latex is non-cytotoxic and/or biocompatible provided that the user does not have an adverse or allergic reaction when in contact with latex. Synthetic latex may include synthetic rubber materials, including nitrile, hydrogenated nitrile, ethylene-propylene, fluorocarbon, chloroprene, silicone, fluorosilicone, polyacrylate, ethylene acrylic, acrylic polymers, styrenebutadiene, acrylonitrile butadiene, polyvinyl acetate, or polyurethane rubbers.

[0052] As used herein, the term “rubber” refers to the elastomeric material which may or may not include vulcanized or non-vulcanized. Typically, the rubber substrate is hydrophobic. The term rubber may compose of the following or a blend of the following items: natural rubber latex (NRL), synthetic latex, neoprene (polychloroprene), nitrile (carboxylated butadiene-acrylonitrile), vinyl(poly vinyl chloride) (PVC), styrene-butadiene rubber (SBR), styrene ethylene butadiene styrene (SEBS), polyurethane, polyisoprene, and other butadiene-based synthetic rubber-based materials. [0053] The substrate to be tested and attached to the housing fixture can include, but is not limited to, biologic materials or tissues (including vaginal tissue, oral and laryngeal and esophageal tissues, urethral tissues, other mucosal tissues, skin, other tissues of interest where medical products slide against said tissues), materials that mimic certain bodily tissues, including biomimetic and bioinspired hydrogels (including gelatin, agar, agarose, collagen, PVA), textiles, elastomers (including silicones), thermoplastic elastomers, and composites (including artificial / synthetic leathers). The material can be hydrated / swollen (partially or completely) with water or other fluid (optionally including dissolved or suspended small molecules or other agents), or the material can be non-hydrated. In certain embodiments, the substrate on the housing fixture is a synthetic or partially synthetic material, intended to mimic sliding properties of skin. The substrate can be attached to the housing fixture by various methods, for example, being glued to the housing fixture or mechanically held in place through interlocking physical elements.

[0054] In certain embodiments, the substrates to be tested on the housing fixture can be the same as the substrates to be tested attached on the moving fixture, and vice versa.

[0055] The substrates and materials to be considered for the moving or housing fixture can be made of, but not limited to, liquid suspensions or solutions ( e.g ., polystyrene, titanium dioxide particles); gelatinous substances (e.g., agar, agarose, collagens, polyvinyl alcohol gels); elastomers (e.g., silicones, latex, rubber, polyuerthanes); epoxy resins; textiles (e.g., cotton, chamois, polytetrafluoroethylene, polyamide, polyester, synthetic or natural leather); metals; other materials (e.g., albumin, cellophane, polycarbonate, other types of skin replica).

[0056] In certain embodiments, the substrates and materials, on the moving or housing fixture, can be made of two or more of a blend or mixture of the above materials. In certain embodiments, the substrates and materials, on the housing or moving fixture, can prepared in multilayers. (See. e.g., http://onlinelibrary.wiley.com/doi/10.llll/srt.12235/pdf (review of synthetic skin materials).) [0057] In certain embodiments, the substrate on the housing fixture is a synthetic or partially synthetic material, intended to mimic vaginal tissue. Such a cylindrical material may be termed “vaginal cuff.” Commercial sources of such tissue include a vaginal cuff.

[0058] In a specific embodiment, the inner diameter of the vaginal cuff is about 32 mm. In such an embodiment, the outer diameter of the vaginal cuff is about 42 mm and has a wall thickness of about 5 mm. When a particular moving fixture (glass mandrel of outer diameter 38 mm) is inserted into such a vaginal cuff, it stretches radially, and the outer diameter increases from about 42 to about 47 mm.

[0059] Substrates that mimic relevant physiologic tissue ( e.g ., synthetic vaginal tissue in the instance of testing a condom for use in intercourse) are advantageous for several reasons. If such substrates are hydrated with water, they allow for water to be involved in the lubrication of the substrate pair (which is the case physiologically). Substrates that are not hydrated with water, e.g. silicones and plastics, create an environment that is not representative of physiology and therefore contradict the recommendation of ASTM G115 that the material selection of the test system should represent the use case. Such substrates are further advantageous if they are flexible and non-rigid, opposed to rigid, because they more closely represent physiology. In the instance of a condom’s articulation during intercourse, it slides against compliant vaginal tissue, therefore a compliant counter-substrate better matches the physiological scenario than, e.g., plastic in the instance of test configurations typical in the art of friction measurement.

[0060] The geometry of the housing fixture may take various configurations. In certain embodiments, the housing fixture is a cylinder. It can be rigid or non-rigid as described herein. The housing fixture may mimic an orifice or a physiological, enclosed environment resembling a biological function.

[0061] In embodiments where the housing fixture is a cylinder, it can be an intact cylinder or it can be a cylinder that has been modified. If modified, it can be cut in portions along the lengthwise dimension parallel with the cylinder’s long axis. In one example, the cylinder can be comprised of two half-pipes; in another example, three third-pipes, and in another example, four quarter-pipes, and so on. Through one or more contracting or expanding hoops around the perimeter of the cylinder, the cylinder fractions can be brought closer together (effectively reducing the diameter of the cylinder) or farther apart in the radial direction (effectively increasing the diameter of the cylinder). This allows a single cylinder (comprised of portions) to be modified to accommodate different diameter moving fixtures, as well as to provide varying contract pressures against the same moving fixture.

The ability to loosen or tighten the moving fixture - housing fixture pair is useful because frictional phenomena vary depending on contact pressure, so the easy of inducing different pressure configurations allows for a robust frictional analysis.

[0062] In certain embodiments, the pressure of the mated pair may be varied by using a compliant housing fixture that can be tighten or loosened in the radial direction through certain means. In one example, a gear hose clamp can be tightened or loosened to provide the adjustment of diameter. [0063] The sensor employed to measure the sliding force may be an axial load cell, can be digital or analog, and logging or non-logging ( i.e ., captures data or not).

[0064] In certain embodiments, an axial load cell or force gauge measures the force in the axial direction when the moving fixture and housing fixture slide against one another. The gauge should measure forces induced by the sliding pair, including initial penetration, insertion, reversal of direction, removal (partial or complete), and re-insertion (partial or complete) of the moving fixture into and out of the housing fixture. Its working range should measure all relevant forces which can be digital or analog.

[0065] In embodiments where the sensor is logging (i.e., captures and records data), its data acquisition frequency should follow typical mechanical engineering principles of mechanical measure, e.g., acquiring and logging data at a frequency of at least about 20 to about 100 times greater than the frequency of cyclic testing. The sensor can record data in a digital manner and allow the data (typically in the force of measured force vs time) to be analyzed via computer software or other means. Compressive forces on the sensor are typically negative (minus sign), whereas tensile forces on the sensor are typically positive. Data can alternatively be captured through other means, e.g., by video capture of the sensor’s force display.

[0066] In embodiments where the sensor is non-logging (i.e., does not record data), a user can observe the measured forces and make manual recordings of the sliding forces.

[0067] In certain embodiments, the sensor is attached to the housing fixture. In other embodiments, the sensor is attached to the moving fixture. As is understood in the art of mechanical testing and force measurement, the sensor can be coupled to either of these elements, and the measured force will be the same.

[0068] In certain embodiments, a lubricant is added to the testing configuration. The lubricant may be a fluid or solid or other phase of matter including materials with properties of both liquids and solids e.g. gels and other viscoelastic materials. Solid lubricants may be, for example, waxes, powders, sacrificial materials layers, and other common solid lubricants. Fluid lubricants and gels may be, for example, gases, aqueous solutions, non-aqueous solutions, suspensions, liquids, and viscoelastic materials.

[0069] In certain embodiments, the lubricant functions over the contact surface area of the sliding pair, and is not confined to any spatial surface or volume other than being compressed between the two contacting materials at the sliding interface. In such embodiments, the lubricant is free and unencumbered from remaining in place, and may flow, run, drip and/or spread. [0070] In specific embodiments, the lubricant is confined to a certain surface or volume. It is not free to flow, run, drip and/or spread; rather, the lubricant is confined through physical or other means to constantly or near-constantly bathe the sliding surfaces. In some such instances, a fixed volume of lubricant is confined in place; in other such instances, an ever-present supply of lubricant is added to the sliding surface area as lubricant flows away from the site, effectively causing the sliding surface area to be constantly or near-constantly bathed in lubricant.

[0071] In an example with a fixed volume of lubricant: a lubricant reservoir holds excess lubricant. While the moving fixture is removed from the housing fixture when the two are in air-tight contact, lubricant is drawn by suction from the reservoir into the housing fixture where it lubricates the sliding interface. Upon re-insertion of the moving fixture into the housing fixture, lubricant is pushed back into the lubricant reservoir once again.

[0072] In an example with a large surplus volume (or theoretically infinite bath of lubricant): a lubricant reservoir holds excess lubricant. When the two fixtures are not in air-tight contact, lubricant may flow away from the sliding interface due to gravity or due to the reciprocating action of the moving fixture, but as lubricant flows away from the sliding interface, additional lubricant is automatically introduced to the sliding interface via gravity or a pump or any other means.

Effectively lubricant will always be present at the site of articulation.

[0073] In another example with a large surplus volume (or theoretically infinite bath of lubricant): a lubricant reservoir holds excess lubricant, and the reciprocations between mandrel and housing occur while submerged in the lubricant reservoir.

[0074] In certain embodiments, the sliding force may be analyzed during the entire cycle of insertion and removal. In such embodiments, friction force time profiles may be compared among samples, without any work-up of the data. FIG 7. demonstrates the friction force time profile (the “raw” force vs time data, collected at 20 Hz, by the force sensor) for an example substrate pair.

[0075] In certain embodiments, the sliding force is only analyzed during the peak insertion force period (when the majority of the moving fixture has penetrated the housing fixture, just prior to reversal of direction and initiation of removal) and/or peak withdrawal force (when the majority of the moving fixture is inside the housing fixture and removal has just been initiated, i.e. reversal of direction has just occurred). Data may be averaged or smoothed via a variety of methods (i.e. a “running average”), or no smoothing or averaging may be performed. FIG 8. demonstrates the isolated force peaks versus cycle # for an example substrate pair.

[0076] In certain embodiments, the measured sliding force values may be analyzed and interpreted in their current units (units of force, e.g., Newtons or kilograms), or force values may be normalized by other force values, e.g., normalized by the measured values of a control test article, such that all experimental sample values are numerically compared to and are expressed as a ratio of the control sample’s value. FIG. 9 demonstrates isolated peak force time profiles for three example substrate pairs in which two profiles are normalized by the time-varying force profile of the third.

[0077] In certain embodiments, each insertion period’s peak force may be plotted as a function of insertion number (i.e., cycle number). This allows for analysis of each insertion peak force period performed during cyclical testing, and such analysis may be performed regardless of whether the peak insertion force changes substantially versus cycle number. In other embodiments, the peak forces may be averaged together, to allow for the average peak insertion force over a number of cycles. Such analysis can only be meaningfully performed when the peak insertion forces are not changing substantially over the duration of cyclic testing.

[0078] The range of minimum to maximum peak insertion forces can be analyzed, to evaluate the dynamic nature of peak insertion force changing over the duration of cyclic testing.

[0079] The rate of change of peak insertion force can be analyzed to evaluate the change in peak insertion force versus cycle number. This may be useful if some samples lose lubrication more quickly than others.

[0080] In certain embodiments, the isolated peak forces may be averaged over some portion of the cycles performed, or over the entire duration of the cycling. In this way, a single average peak force value may be obtained, corresponding to the average peak friction force during that duration. FIG.

11 demonstrates such an average.

[0081] The disclosed device can be employed to measure various interfacial properties between substrates.

[0082] In certain embodiments, the device can be used to evaluate the lubricity of latex male condoms against a synthetic tissue mimetic. This is accomplished by measuring the frictional force between the condom (e.g., placed over a cylindrical glass mandrel) and a tissue-like countersurface (e.g., synthetic vaginal tissue lining the interior surface of a cylindrical bearing housing) while the mandrel is reciprocated into and out of the housing. A condom specimen is rolled onto the glass mandrel (the moving fixture). The condom may be wetted with a known (measured) amount of applied fluid, or, it may be wetted with excess fluid. The fluid may be water. For example, the mandrel (with condom on it) may be submerged in water for about 10 seconds, nearly to the top of the condom’s beaded retaining ring. It also may be sprayed with surplus water from a spray bottle. The mandrel is positioned at the opening of the housing and the actuator is driven forward (e.g., approximately one inch) to seat the tip of the mandrel within the housing. Reciprocations are performed (e.g., at a frequency ranging about 0.33 Hz to 2 Hz such as about 0.5 Hz). Stroke length may vary, e.g., ranging 4” to 6” such as about 5.5”). Data collection can be programmed via computer to record sliding force at a collection frequency (e.g., about 20 Hz). When the test is complete, the mandrel is removed from the housing, and the housing is cleaned of any potential residual material from the test.

[0083] In certain embodiments, the device can be used to evaluate the durability of lubrication of lubricated condoms with or without additional personal lubrications. The frictional forces overtimes can be monitored over a period of, for example, 10, 50, 100, or 1000 cyclic articulations. In addition to lubricant diminishing, material wear of condoms can occur and this cannot be studied as effectively by a single-slide test.

[0084] Thus, in one aspect, the invention generally relates to a testing device. The testing device comprises: a reciprocating linear actuator having a defined axis along which the actuator is configured to move in forward and reverse motions; a cylindrical mandrel engaged to the reciprocating linear actuator, the mandrel having an outer surface for placing a test substrate thereon thereby exposing a surface of the test substrate; a hollow cylindrical housing fixture with an interior bearing liner for slidingly receiving the mandrel with the test substrate placed thereon in a tight sealing; and a sensor coupled to the housing fixture for measuring sliding force experienced by the housing fixture upon receiving the mandrel with the test substrate placed thereon in a tight sealing. [0085] In certain embodiments, the reciprocating linear actuator comprises a motor capable of automated driving a reciprocating motion at a controlled speed.

[0086] In certain embodiments, the interior bearing liner of the housing fixture is selected from the group consisting of leather, suede, tissue, a biomimetic material, and rubber (synthetic or natural), etc.

[0087] In certain embodiments, the sensor comprising a force gauge.

[0088] In certain embodiments, the testing device is linked to or includes a computer for control of the reciprocating linear actuator and/or for receiving output from the axial force gauge.

[0089] In certain embodiments, the housing fixture is configured to receive the mandrel with the test substrate placed thereon in a sealing (e.g., tight or substantially air tight).

[0090] In certain embodiments, the mandrel is configured to receive a condom rolled up onto the mandrel.

[0091] In certain embodiments, the mandrel is made of a material selected from the group consisting of glass, metal, porcelain and rubber (synthetic or natural) [0092] In certain embodiments, the mandrel has a circular cross-section with an outer diameter in the range from about 0.5 to about 2.5 inches ( e.g ., about 1 to about 2 inches).

[0093] In certain embodiments, the housing fixture has a circular interior cross-section with a diameter in the range from about 0.5 to about 2.5 inches (e.g., about 1 to about 2 inches).

[0094] In certain embodiments, the interior bearing liner and or the test substrate is lubricated with a lubricant.

[0095] In certain embodiments, the lubricating material comprises a water-based lubricant or a silicone-based lubricant.

[0096] In certain embodiments, the reciprocation frequency ranges between about 0.1 Hz to about 5 Hz (e.g., about 0.2 Hz and 4 Hz, about 0.3 Hz and 3 Hz, about 0.5 Hz to about 2 Hz).

[0097] In certain embodiments, the sliding length traveled by mandrel in the interior of the housing fixture ranges from about 1 inch to about 10 inches (e.g., about 2 to about 9 inches, about 3 to about 8 inches, about 4 to about 7 inches).

[0098] In certain embodiments, the test substrate is a condom.

[0099] In certain embodiments, the test substrate is a biologic tissue or a mimic thereof.

[00100] In certain embodiments, the test substrate is a medical product.

[00101] In another aspect, the invention generally relates to a system for measuring friction. The system comprises: an actuator configured to move in reciprocating motions; a first platform engaged to the actuator for securing a first test substrate thereon thereby exposing a surface of the first test substrate; a second platform for securing a second test substrate thereon thereby exposing a surface of the second test substrate; a sensor coupled to the second platform for measuring the frictional force experienced by the second platform, wherein the first platform and the second platform are configured to allow a tight sealing between the surface of the first test substrate and the surface of the second test substrate when the motor-driven actuator is engaged in reciprocating motions.

[00102] In certain embodiments, the actuator uses a motor-driven linear actuator having a defined axis along which the actuator is configured to move in forward and reverse motions.

[00103] In certain embodiments, the first platform is a cylindrical mandrel engaged to the reciprocating linear actuator, the mandrel having an outer surface for placing the first test substrate thereon thereby exposing a surface of the test substrate.

[00104] In certain embodiments, the second platform is a hollow cylindrical housing fixture with an interior bearing liner as the second test substrate for slidingly receiving the mandrel with the first test substrate placed thereon in a tight sealing. [00105] In certain embodiments, the sensor measures a sliding force experienced by the second platform upon receiving the first platform with the first test substrate placed thereon.

[00106] In yet another aspect, the invention generally relates to a method for measuring an interfacial property. The method comprises: providing a testing device disclosed herein; placing a test substrate on the outer surface of the cylindrical mandrel thereby exposing a surface of the test substrate; placing an interior bearing liner inside the hollow cylindrical housing fixture so that it slidingly receives the cylindrical mandrel with the test substrate placed thereon in a tight sealing; causing the reciprocating linear actuator to engage in forward and reverse motions thereby driving the cylindrical mandrel having the test substrate placed sliding into and away from the hollow cylindrical housing fixture having the interior bearing liner; and measuring a friction or sliding force experienced by the sensor.

[00107] In certain embodiments, the test substrate is a condom.

[00108] In certain embodiments, the test substrate is a biologic tissue or a mimic thereof.

[00109] In certain embodiments, the test substrate is a medical product.

[00110] In certain embodiments, the interior bearing liner of the housing fixture is selected from the group consisting of leather, suede, tissue, a biomimetic material, and rubber (synthetic or natural). [00111] In certain embodiments, the method comprises causing the reciprocating linear actuator to engage in forward and reverse motions at least 10 times. In certain embodiments, the method comprises causing the reciprocating linear actuator to engage in forward and reverse motions at least 100 times. In certain embodiments, the method comprises causing the reciprocating linear actuator to engage in forward and reverse motions at least 500 times.

[00112] In certain embodiments, the interior bearing liner and or the test substrate is lubricated with a lubricant.

[00113] In certain embodiments, the lubricating material comprises a water-based lubricant or a silicone-based lubricant.

[00114] In certain embodiments, the interfacial property is the frictional force between the substrates in sliding contact with each other.

[00115] In certain embodiments, the interfacial property is durability testing.

[00116] The method is useful for multiple reasons. One use for the method relates to manufacturing and/or quality control, in its use to establish control charts, which are charts that show an expected range of values to be observed, and performance occurring outside the specified range indicates a potential problem in manufacturing operations or a problem in achieving the quality specification. FIG. 10 demonstrates a control chart for two types of sample, and the acceptable / anticipated ranges for the samples, based on prior history of assessing specimens of the same sample type.

[00117] Another use for the methods relates to its differences and advantages over other methods commonly used in the industry. FIG. 13 conveys a comparison of device specifications, test method capabilities, and typical testing conditions, between two other commonly used methods in the condom testing industry, and the device disclosed herein. In particular, some advantages of the present device and test method relate to other devices and test methods not operating at physiologic speeds or frequencies, or not reciprocating (i.e., only executing one stroke), not using physicochemically physiologic bearing materials, and using flat-on-flat bearing geometries opposed to ajoumal bearing (i.e., mated cylinder-within-cylinder) geometry.

Examples

Example 1 - Evaluation of a commercially available silicone lubricated condom verses a non- lubricated condom

[00118] The frictional properties of a commercial leading silicone lubricated condom were compared to a standard non-lubricating condom to evaluate their lubricating durability after 200 cycles. Condom samples were unrolled onto a glass mandrel, exposed to water for 10 seconds, and immediately mounted onto the testing device. The housing interior was a synthetic leather material, mimetic of skin, with an elastic foam sublayer. Throughout the 200 cycles, the mandrel was inserted into and removed from the interior housing fixture lined with a tissue biomimetic at a rate of 0.5 Hz with a stroke length of 4.5”. The force of insertion and removal was recorded and measured in Newtons using Neulog software. FIG. 6 demonstrates the average peak insertion forces for a silicone lubricated condom and a non-lubricated condom over the test’s duration.

Example 2 Evaluation of four test sample condoms made with four levels of slipperiness, versus a commercially available lubricated condom

[00119] Frictional properties were assessed as described in Example 1, for four test sample condoms (A-D, green bars), in comparison with a commercial condom (red bar). Each successive test sample, green bars from left to right in the chart, was produced with increasing amount of known slipperiness, and the difference in slipperiness among the four samples can be resolved/distinguished by the test method described herein (where increasingly slipperiness corresponds with the decreasing frictional force) (FIG. 12). Values in parentheses indicate percentage less than the commercial condom’s frictional force.

[00120] Applicant’s disclosure is described herein in preferred embodiments with reference to the Figures, in which like numbers represent the same or similar elements. Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

[00121] The described features, structures, or characteristics of Applicant’s disclosure may be combined in any suitable manner in one or more embodiments. In the description, herein, numerous specific details are recited to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that Applicant’s composition and/or method may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the disclosure.

[00122] The term “about” when used in connection with a value can mean 5% of the value being referred to. For example, “about 100” refers to a value in the range of 95 to 105.

[00123] In this specification and the appended claims, the singular forms "a," "an," and "the" include plural reference, unless the context clearly dictates otherwise.

[00124] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described. Methods recited herein may be carried out in any order that is logically possible, in addition to a particular order disclosed.

Incorporation by Reference

[00125] References and citations to other documents, such as patents, patent applications, patent publications, journals, books, papers, web contents, have been made in this disclosure. All such documents are hereby incorporated herein by reference in their entirety for all purposes. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material explicitly set forth herein is only incorporated to the extent that no conflict arises between that incorporated material and the present disclosure material. In the event of a conflict, the conflict is to be resolved in favor of the present disclosure as the preferred disclosure.

Equivalents

[00126] The representative examples are intended to help illustrate the invention, and are not intended to, nor should they be construed to, limit the scope of the invention. Indeed, various modifications of the invention and many further embodiments thereof, in addition to those shown and described herein, will become apparent to those skilled in the art from the full contents of this document, including the examples and the references to the scientific and patent literature included herein. The examples contain important additional information, exemplification and guidance that can be adapted to the practice of this invention in its various embodiments and equivalents thereof.