US20170196515A1

US20170196515A1 - Contact sensors and methods for making same

Info

Publication number: US20170196515A1
Application number: US15/411,408
Authority: US
Inventors: Andrew C. Clark; David W. Topham
Original assignee: Sencorables LLC
Current assignee: Sencorables LLC
Priority date: 2009-03-06
Filing date: 2017-01-20
Publication date: 2017-07-13
Also published as: WO2010102309A1; US8820173B2; US20150201886A1; US20120068759A1

Abstract

Disclosed herein are novel contact sensors. The contact sensors disclosed herein include a conductive composite material formed of a polymer and a conductive filler. In one particular aspect, the composite materials can include less than about 10 wt % conductive filler. Thus, the composite material of the contact sensors can have physical characteristics essentially identical to the polymer, while being electrically conductive with the electrical resistance proportional to the load on the sensor. The sensors can provide real time dynamic contact information for joint members under conditions expected during use.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/157,963, fled on Mar. 6, 2009, which application is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of Invention
This invention relates to contact sensors, and more particularly to contact sensors for accurately measuring surface contact data at a junction between two members.
Description of the Related Art
Contact sensors have been used to gather information concerning contact or near-contact between two surfaces in medical applications, such as dentistry, podiatry, and in the development of prostheses, as well as in industrial applications, for example to determine load and uniformity of pressure between mating surfaces, and in the development of bearings and gaskets. In general, these sensors include pressure-sensitive films designed to be placed between mating surfaces. These film sensors, while generally suitable for examining static contact characteristics between two generally flat surfaces, have presented many difficulties in other situations. For example, when examining contact data between more complex surfaces, surfaces including complex curvatures, for example, it can be difficult to conform the films to fit the surfaces without degrading the sensor's performance.
More serious problems exist with these materials as well. For example, film-based contact sensor devices and methods introduce a foreign material having some thickness between the mating surfaces, which can change the contact characteristic of the junction and overestimate the contact areas between the two surfaces. Moreover, the ability to examine real time, dynamic contact characteristics is practically non-existent with these types of sensors.
A better understanding of the contact conditions at joints and junctions could lead to reduced wear in materials, better fit between mating surfaces, and longer life expectancy for machined parts. For example, one of the leading causes of failure in total joint replacement prostheses is due to loosening of the implant induced by wear debris particles worn from the polymeric bearing component. A better understanding of the contact conditions between the joint components would lead to reduced implant wear and longer implant life.
What are needed in the art are contact sensors that can provide more accurate and/or dynamic contact information concerning a junction formed between two surfaces of any surface shape.

SUMMARY OF TIE INVENTION

In one aspect, the present invention is directed to a contact sensor. The sensor includes an electrically conductive composite material comprising a polymer and a conductive filler. Generally, the composite material can include any polymer. In certain aspects, the polymer can be an engineering polymer or a high performance polymer. In one preferred aspect, the composite material can include ultra-high molecular weight polyethylene (UHMWPE). In one aspect, the composite material of the sensors can include between about 0.1% and 20% by weight of a conductive filler. The conductive filler can be any suitable material. For example, in one aspect, the conductive filler can include carbon black.
The contact sensors of the invention can define a contact surface. Optionally, a surface of the contact sensors of the invention can be formed to be substantially inflexible so as to replicate a surface that can be placed in proximity to a surface of a second member, thereby forming a junction. In particular, the contact surface of the sensors of the invention can replicate the shape and optionally also the material characteristics of a junction-forming member found in an industrial, medical, or any other useful setting. For example, in one particular aspect, the essentially inflexible contact surface of the sensor can include curvature such as that described by the contact surface of a polymeric bearing portion of an Implantable artificial replacement joint such as the polymeric bearing portion of a hip, knee, or shoulder replacement joint. Alternatively, the contact sensors can be thermoformed into a desired substantially inflexible three-dimensional shape. For example, the contact sensors can be thermoformed for use as a prosthetic device.
In one aspect, the sensor can be formed entirely of the composite material. In another aspect, the contact sensors of the invention can include one or more discrete regions of the electrically conductive composite material and a non-conductive material. For example, the sensors can include multiple discrete regions of the electrically conductive composite material that can be separated by an intervening nonconductive material, e.g., an intervening polymeric material. In one particular aspect, the intervening polymeric material separating discrete regions of the composite material can include the same polymer as the polymer of the electrically conductive composite material.
In another aspect, the sensor can comprise one or more sensing points. The sensing points can be configured to measure current flow therethrough the sensing point during application of a load. In one aspect, the current flow measured at each sensing point can be transmitted to a data acquisition terminal in an additional aspect, the data acquisition terminal can transmit a digital output signal indicative of the current flow measurements to a computer having a processor. In a further aspect, the processor can be configured to calculate the load experienced at each respective sensing point using the digital output signal. In this aspect, the computer can be configured to graphically display the loads experienced at the sensing points as a pressure distribution graph. It is contemplated that the pressure distribution graph can be a three-dimensional plot or a two-dimensional intensity plot wherein various colors correspond to particular load values. It is further contemplated that the computer can be configured to display the pressure distribution graph substantially in real-time. In still a further aspect, the computer can be configured to store the load calculations for the plurality of sensing points for future analysis and graphical display.
In one aspect, the electrically conductive composite material can be located at the contact surface of the sensor for obtaining surface contact data. If desired, the sensor can include composite material that can be confined within the sensor, at a depth below the contact surface, in order to obtain internal stress data.
The electrically conductive composite material described herein can, in one particular aspect, be formed by mixing a polymer in particulate form with a conductive filler in particulate form. According to this aspect, in order to completely coat the polymer granules with the granules of the conductive filler, the granule size of the polymer can be at least about two orders of magnitude larger than the granule size of the conductive filler. For example, the average granule size of the polymer can, in one aspect, be between about 50 μm and about 500 μm. The average granule size of the conductive filler can be, for example, between about 10 nm and about 500 nm.
Following a mixing step, the composite conductive material can be formed into the sensor shape either with or without area of non-conductive material in the sensor, as desired, by, for example, compression molding, RAM extrusion, or injection molding. If desired, a curvature can be formed into the contact surface of the sensor in the molding step or optionally in a secondary forming step such as a machining or cutting step.
During use, the sensors of the invention can be located in association with a member so as to form a contact junction between a surface of the member and the contact surface of the sensor. The sensor can then be placed in electrical communication with a data acquisition terminal, for example via a fixed or unfixed hard-wired or a wireless communication circuit, and data can be gathered concerning contact between the sensor and the member. In one particular aspect, dynamic contact data can be gathered. For example, any or all of contact stress data, internal stress data, load, impact data, lubrication regime data, and/or information concerning wear such as wear mode information can be gathered.
In another aspect, the disclosed sensors can be integrated with the part that they have been designed to replicate and actually used in the joint in the desired working setting. For example, the contact sensor can gather data while functioning as a bearing of a joint or junction in real time in an industrial, medical, or other working setting.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate certain aspects of the instant invention and together with the description, serve to explain, without limitation, the principles of the invention. Like reference characters used therein indicate like parts throughout the several drawings.

FIG. 1 illustrates one aspect of the sensor disclosed herein for obtaining surface contact data of a junction;

FIG. 2 illustrates another aspect of the sensor disclosed herein for obtaining surface contact data of a junction;

FIG. 3 illustrates another aspect of the sensor disclosed herein for obtaining sub-surface contact data of a junction;

FIG. 4 illustrates another aspect of the sensor disclosed herein for obtaining pressure data of a junction, showing two stacked sensor sheets, each sheet having a plurality of spaced conductive stripes, the stacked sensor sheets being oriented substantially perpendicular to each other such that an array of sensing points is formed by the overlapping portions of the conductive stripes of the stacked sensor sheets;

FIG. 5 illustrates a simplified, non-limiting block diagram showing select components of an exemplary operating environment for performing the disclosed methods;

FIG. 6 graphically illustrates the stress v. strain curve for exemplary composite conductive materials as described herein;

FIG. 7 graphically illustrates the log of resistance vs. log of the load for three different composite conductive materials as described herein;

FIG. 8 illustrates the log of normalized resistance vs. log of the load for three different composite conductive materials as described herein;

FIG. 9 illustrates the voltage values corresponding to load, position, and resistance of an exemplary composite material;

FIGS. 10A-10D illustrate the kinematics and contact area for exemplary artificial knee implant sensors as described herein with different surface geometries;

FIGS. 11A and 11B graphically illustrate the log of normalized resistance vs. log of the compressive force for two different composite conductive materials as described herein;

FIG. 12 is a photograph of one aspect of an exemplary mold and press used to form sensor sheets as disclosed herein;

FIG. 13 is a photograph of a sensor sheet according to one aspect disclosed herein, illustrating a plurality of dots comprising a conductive filler;

FIG. 14 is a schematic of a contact sensor in operative communication with a data acquisition terminal, and showing a battery operatively coupled to the data acquisition terminal and a computer coupled to the data acquisition terminal via a Wi-Fi transmitter;

FIG. 15 is a schematic of an exemplary interface circuitry for the data acquisition terminal; and

FIGS. 16A, 16B, and 16C are schematics of exemplary measurement circuitry for the data acquisition terminal.

DEFINITIONS OF TERMS

For purposes of the present disclosure, the following terms are herein defined as follows:
The term “primary particle” is intended to refer to the smallest particle, generally spheroid, of a material such as carbon black.
The term “aggregate” is intended to refer to the smallest unit of a material, and in particular, of carbon black, found in a dispersion. Aggregates of carbon black are generally considered indivisible and are made up of multiple primary particles held together by strong attractive or physical forces.
The term “granule” is also intended to refer to the smallest unit of a material found in a dispersion. However, while a granule can also be an aggregate, such as when considering carbon black, this is not a requirement of the term. For example, a single granule of a polymer, such as UHMWPE or conventional grade polyethylene, for example can be a single unit.
The term “agglomeration” is intended to refer to a configuration of a material including multiple aggregates or granules loosely held together, as with Van der Waals forces. Agglomerations of material in a dispersion can often be broken down into smaller aggregates or granules upon application of sufficient energy so as to overcome the attractive forces.
The term “conventional polymer” is intended to refer to polymers that have a thermal resistance below about 100° C. and relatively low physical properties. Examples include high-density polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), and polypropylene (PP).
The term “engineering polymer” is intended to refer to polymers that have a thermal resistance between about 100° C. and about 150° C. and exhibit higher physical properties, such as strength and wear resistance, as compared to conventional polymers. Examples include polycarbonates (PC), polyamides (PA), polyethylene terephthalate (PET), and ultrahigh molecular weight polyethylene (UHMWPE).
The term “high performance polymer” is intended to refer to polymers that have a thermal resistance greater than about 150° C. and relatively high physical properties. Examples include polyetherether ketone (PEEK), polyether sulfone (PES), polyimides (PI), and liquid crystal polymers (LC).
Contact stress, synonymous with contact pressure, is herein defined as surface stress resulting from the mechanical interaction of two members. It is equivalent to the applied load (total force applied) divided by the area of contact.
Internal stress refers to the forces acting on an infinitely small unit area at any point within a material. Internal stress varies throughout a material and is dependent upon the geometry of the member as well as loading conditions and material properties.
Impact force is herein defined to refer to the time-dependent force one object exerts onto another object during a dynamic collision.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to the following detailed description, examples, and claims, and their previous and following description. Before the present system, devices, and/or methods are disclosed and described, it is to be understood that this invention is not limited to the specific systems, devices, and/or methods disclosed unless otherwise specified, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.
The following description of the invention is provided as an enabling teaching of the invention in its best, currently known aspect. Those skilled in the relevant art will recognize that many changes can be made to the aspects described, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof.
As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “sensor” includes aspects having two or more sensors unless the context clearly indicates otherwise.
Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may or may not occur, and that the description includes examples where said event or circumstance occurs and examples where it does not.
Presented herein are contact sensors, methods of forming contact sensors, and methods of advantageously utilizing the sensors. In general, contact sensors can be utilized to gather dynamic and/or static contact data at the junction of two opposing members such as a junction found in a joint, a bearing a coupling, a connection, or any other junction involving the mechanical interaction of two opposing members, and including junctions with either high or low tolerance values as well as junctions including intervening materials between the members, such as lubricated junctions, for example. Dynamic and/or static data that can be gathered utilizing the disclosed sensors can include, for example, load data, lubrication regimes, wear modes, contact stress data, internal stress data, and/or impact data for a member forming the junction. The contact sensors disclosed herein can provide extremely accurate data for the junction being examined, particularly in those aspects wherein at least one of the members forming the junction in the working setting (as opposed, for example, to a testing setting) is formed of a polymeric material.
Beneficially, the sensors described herein can be configured to replicate either one of the mating surfaces forming the junction. Optionally, the sensors described herein can be essentially inflexible when positioned proximate the junction. As such, in a laboratory-type testing application, the sensor can simulate one member forming the junction, and contact data can be gathered for the junction under conditions closer to those expected during actual use, i.e., without altering the expected contact dynamics experienced at the Junction during actual use. For example, the disclosed sensors can provide contact data for the junction without the necessity of including extraneous testing material, such as dyes, thin films, or the like, within the junction itself.
In one aspect, the sensor can be formed of a material that essentially duplicates the physical characteristics of the junction member that the sensor is replicating. Accordingly, in this aspect, the sensor can exhibit wear characteristics essentially equivalent to those of the member when utilized in the field, thereby improving the accuracy of the testing data. According to one particular aspect of the invention, rather than being limited to merely simulating a junction-forming member, such as in a pure testing situation, the sensor can be incorporated into the member itself that is destined for use in the working application, i.e., in the field, and can provide contact data for the junction during actual use of the part. It is contemplated that the sensors described herein can be used in a variety of working settings, including, for example and without limitation, in industrial working settings, medical working settings, and the like.
In an additional aspect, the contact sensors disclosed herein can be formed to be substantially inflexible. In this aspect, it is contemplated that the contact sensors can be thermoformed as desired into a three-dimensional shape. In one aspect, it is contemplated that the desired shape of the contact sensors can be a substantially sheet-like member. Optionally, the desired shape of the contact sensors can substantially replicate the three-dimensional shape of a selected structure of a subject's body, including, for example and without limitation, a bone, limb, or other body member. Accordingly, it is contemplated that the contact sensors can be thermoformed to function, for example and without limitation, as prosthetic devices for use as a replacement for, or in conjunction with, the selected structure of the subject's body. It is further contemplated that the desired shape of the contact sensors can substantially replicate the three-dimensional shape of a selected structure outside the body of a subject that is configured to bear loads, including, for example and without limitation, textile devices, vehicle parts and components, anthropomorphic test devices such as crash test dummies, building components, and the like.
In various aspects, the contact sensors disclosed herein can comprise an electrically conductive composite material that in turn comprises at least one non-conductive polymer material combined with an electrically conductive filler. In another aspect, the composite material disclosed herein can comprise an electrically conductive filler that can provide pressure sensitive electrical conductivity to the composite material, but can do so while maintaining the physical characteristics, e.g., wear resistance, hardness, etc., of the non-conductive polymeric material of the composite. Thus, in this aspect, the sensors disclosed herein can be developed to include a particular polymer or combination of polymers so as to essentially replicate the physical characteristics of the similar but non-conductive polymeric member forming the junction or three-dimensional structure to be examined.
This combination of beneficial characteristics in the composite materials has been attained through recognition and/or development of processes for forming the composite materials in which only a small amount of the electrically conductive filler need be combined with the polymeric material. As such, the physical characteristics of the composite material can more closely resemble those of the starting polymeric material, and the sensor can closely replicate the physical characteristics of a non-conductive polymeric member forming a junction.
This feature can be particularly beneficial when considering the examination of junctions including at least one member formed of engineering and/or high performance polymers. When considering such materials, the addition of even a relatively small amount of additive or filler can drastically alter the physical characteristics that provide the desired performance of the materials. In the past, when attempts were made to form electrically conductive composites of many engineering and high performance polymers, the high levels of additives (greater than about 20% by weight, in most examples) that were required usually altered the physical characteristics of the polymeric material to the point that the formed conductive composite material no longer exhibited the desired characteristics of the starting, non-conductive material. Thus, the examination of junctions formed with such materials has in the past generally required the addition of an intervening material, such as a pressure sensitive film within the junction, leading to the problems discussed above.
It should be noted, however, that while the presently disclosed sensors can be of great benefit when formed to include engineering and/or high performance polymeric composite materials, this is not a requirement of the invention. In other aspects, the polymer utilized to form the composite material can be a more conventional polymer. No matter what polymer, copolymer, or combination of polymers is used to form the disclosed composite conductive materials, the composite materials of the disclosed sensors can exhibit pressure sensitive electrical conductivity and if desired, can also be formed so as to essentially maintain the physical characteristics of a polymeric material identical to the composite but for the lack of the conductive filler.
In general, any polymeric material that can be combined with an electrically conductive filler to form a pressure sensitive conductive polymeric composite material that can then be formed into an essentially inflexible shape can be utilized in the contact sensors described herein. For example, various polyolefins, polyurethanes, polyester resins, epoxy resins, and the like can be utilized in the contact sensors described herein. In certain aspects, the composite material can include engineering and/or high performance polymeric materials. In one particular aspect, the composite material can include UHMWPE. UHMWPE is generally classified as an engineering polymer, and possesses a unique combination of physical and mechanical properties that allows it to perform extremely well in rigorous wear conditions. In fact, it has the highest known impact strength of any thermoplastic presently made, and is highly resistant to abrasion, with a very low coefficient of friction. The physical characteristics of UHMWPE have made it attractive in a number of industrial and medical applications. For example, it is commonly used in forming polymeric gears, sprockets, impact surfaces bearings, bushings and the like. In the medical industry, UHMWPE is commonly utilized in forming replacement joints including portions of artificial hips, knees, and shoulders. In addition, UHMWPE can be in particulate form at ambient conditions and can be shaped through compression molding or RAM extrusion and can optionally be machined to form an essentially inflexible block (i.e., not easily misshapen or distorted), with any desired surface shape.
Conductive fillers as are generally known in the art can be combined with the polymeric material of choice to form the composite material of the disclosed sensors. The conductive fillers can be, for example and without limitation, carbon black and other known carbons, gold, silver, aluminum, copper, chromium, nickel, platinum, tungsten, titanium, iron, zinc, lead, molybdenum, selenium, indium, bismuth, tin, magnesium, manganese, cobalt, titanium germanium, mercury, and the like.
According to one aspect, a pressure sensitive conductive composite material can be formed by combining a relatively small amount of a conductive filler with a polymeric material. For example, the composite can comprise from between about 0.1% to about 20% by weight of the conductive filler, more preferably from between about 1% to about 15% by weight of the conductive filler, and most preferably from between about 5% to about 12% by weight of the conductive filler. Of course, in other aspects, such as those in which the physical characteristics of the composite material need not approach those of the non-conductive polymeric material, the composite material can include a higher weight percentage of the conductive filler material.
In general, the polymeric material and the conductive filler can be combined in any suitable fashion, which can generally be determined at least in part according to the characteristics of the polymeric material. For example, and depending upon the polymers involved, the materials can be combined by mixing at a temperature above the melting temperature of the polymer (conventional melt-mixing) and the filler materials can be added to the molten polymer, for example, in a conventional screw extruder, paddle blender, ribbon blender, or any other conventional melt-mixing device. The materials can also be combined by mixing the materials in an appropriate solvent for the polymer (conventional solution-mixing or solvent-mixing) such that the polymer is in the aqueous state and the fillers can be added to the solution. Optionally, an appropriate surfactant can be added to the mixture of materials to permit or encourage evaporation of the solvent, resulting in the solid conductive composite material. In another aspect, the materials can be mixed below the melting point of the polymer and in dry form. In this aspect, the materials can be mixed by a standard vortex mixer, a paddle blender, a ribbon blender, or the like, such that the dry materials are mixed together before further processing.
When mixing the components of the composite material, the mixing can be carried out at any suitable conditions. For example, in one aspect, the components of the composite material can be mixed at ambient conditions. In other aspects, however, the components of the composite material can be mixed at non-ambient conditions. For example, the components of the composite material can be mixed under non-ambient conditions to, for example and without limitation, maintain the materials to be mixed in the desired physical state and/or to improve the mixing process.
When dry mixing the materials to be utilized in the composite, the exact particulate dimensions of the materials are not generally critical to the invention. However, in certain aspects, the relative particulate size of the materials to be combined in the mixture can be important. In particular, the relative particulate size of the materials to be combined can be important in those aspects wherein a relatively low amount of conductive filler is desired and in those aspects wherein the polymer granules do not completely fluidize during processing. For example, the relative particle size can be important in certain aspects wherein engineering or high-performance polymers are utilized. It is contemplated that the relative particle size can be particularly important during utilization of extremely high melt viscosity polymers such as UHMWPE, which can be converted via non-fluidizing conversion processes, including, for example and without limitation, compression molding or RAM extrusion processes.
In such aspects, the particle size of the filler can beneficially be considerably smaller than the particle size of the polymer. According to this aspect, and while not wishing to be bound by any particular theory, it is believed that due to the small size of the conductive filler particles relative to the larger polymer particles, the conductive filler is able to completely coat the polymer during mixing and, upon conversion of the composite polymeric powder in a non-fluidizing conversion process to the final solid form, the inter-particle distance of the conductive filler particles can remain above the percolation threshold such that the composite material can exhibit the desired electrical conductivity. According to this aspect, when forming the composite mixture, the granule or aggregate size of the conductive filler to be mixed with the polymer can be at least about two orders of magnitude smaller than the granule size of the polymer. In some aspects, the granule or aggregate size of the conductive filler can be at least about three orders of magnitude smaller than the granule size of the polymer.
In forming the composite material according to this aspect, a granular polymer can be dry mixed with a conductive filler that is also in particulate form. Readily available UHMWPE in general can have a granule diameter in a range of from about 50 μm to about 200 μm. Typically, the individual granule is made up of multiple sub-micron sized spheroids and nano-mixed fibrils surrounded by varying amounts of free space.
In one aspect, the conductive filler for mixing with the polymer can comprise carbon black. Carbon black is readily available in a wide variety of agglomerate sizes, generally ranging in diameter from about 1 μm to about 100 μm that can be broken down into smaller aggregates of from about 10 nm to about 500 nm upon application of suitable energy.
Upon dry mixing the particulate conductive filler with the larger particulate polymer material with suitable energy, the smaller granules of conductive filler material can completely coat the larger polymer granules. For example, a single powder particle can be obtained following mixing of 8 wt % carbon black with 92 wt % UHMWPE. Therefore, the UHMWPE particles are completely coated with carbon black aggregates. While not wishing to be bound by any particular theory, it is believed that forces of mixing combined with electrostatic attractive forces between the non-conductive polymeric particles and the smaller conductive particles are primarily responsible for breaking the agglomerates of the conductive material down into smaller aggregates and forming and holding the coating layer of the conductive material on the polymer particles during formation of the composite powder as well as during later conversion of the powdered composite material into a solid form.
Following formation of the mixture comprising the conductive filler and the polymeric material, the mixture can be converted as desired to form a solid composite material. In one aspect, the solid composite material can be electrically conductive. The solid composite thus formed can also maintain the physical characteristics of the polymeric material in mixtures comprising a relatively low weight percentage of conductive filler. For example, in the aspect described above, in which the composite material includes a conductive filler mixed with UHMWPE, the powder can be converted via a compression molding process or a RAM extrusion process, as is generally known in the art. Optionally, following conversion of the powder, the resultant solid molded material can be machined to produce a desired curvature on at least one contact surface.
In other aspects however, and primarily depending upon the nature of the polymeric portion of the composite, other conversion methods may preferably be employed. For example, in other aspects, the polymeric portion of the composite material can be a polymer, a co-polymer, or a mixture of polymers that can be suitable for other converting processes. For example and without limitation, the composite polymeric material can be converted via a conventional extrusion or injection molding process.
The composite material of the disclosed sensors can optionally comprise other materials in addition to the primary polymeric component and the conductive filler discussed above. In one aspect, the composite material can comprise additional fillers, including, for example and without limitation, various ceramic fillers, aluminum oxide, zirconia, calcium, silicon, fibrous fillers, including carbon fibers and/or glass fibers, or any other fillers as are generally known in the art. In another aspect, the composite material can include an organic filler, including for example and without limitation, tetrafluoroethylene or a fluororesin. In this aspect, it is contemplated that the organic filler can be added to improve sliding properties of the composite material.
It is believed that during the conversion process, the polymer particles can fuse together and confine the conductive filler particles to a three-dimensional channel network within the composite, forming a segregated network type of composite material. In operation, the distances between many of the individual carbon black primary particles and small aggregates is quite small, believed to be nearing 10 nm. It is contemplated that when two conductive filler particles are within about 10 nm of each other, they can conduct current via electron tunneling, or percolation, with very little resistance. Thus, many conductive paths fulfilling these conditions can be traced across the image. Moreover, as the polymers are deformable, the conductivity, and in particular the resistance, of the composite material of the contact sensors described herein can vary upon application of a compressive force (i.e., load) to the composite material.
Accordingly, following any desired molding, shaping, cutting and/or machining and also following any desired physical combination of the formed composite material with other non-conductive materials (various aspects of which are discussed further below), the composite materials of the contact sensors described herein, which comprise at least one conductive filler, can be formed into the sensor shape and placed in electrical communication with a data acquisition terminal. For example, in one aspect, the composite material of the sensor can be connected to a data acquisition terminal. In this aspect, the composite material can be connected to the data acquisition terminal by, for example and without limitation, conventional alligator clips, conductive epoxy, conductive silver ink, conventional rivet mechanisms, conventional crimping mechanisms, and other conventional mechanisms for maintaining electrical connection. In another aspect, the composite material can be machined to accept a connector of a predetermined geometry within the composite material itself. Other connection regimes as are generally known in the art may optionally be utilized, however, including fixed or unfixed connections to any suitable communication system between the composite material and the data acquisition terminal. In particular, no particular electrical communication system is required of the contact sensors described herein. For example, in other aspects, the electrical communication between the composite material and the data acquisition terminal can be wireless, rather than a hard wired connection.
In one aspect, the data acquisition terminal can comprise data acquisition circuitry. In another aspect, the data acquisition terminal can comprise at least one multiplexer placed in electrical communication with a microcontroller via the data acquisition circuitry. In an additional aspect, the data acquisition circuitry can comprise at least one op-amp for providing a predetermined offset and gain through the circuitry. In this aspect, the at least one op-amp can comprise a converting op-amp configured to convert a current reading into a voltage output. It is contemplated that the converting op-amp can measure current after it has passed through the at least one multiplexer and then convert the measured current into a voltage output. In a further aspect, the data acquisition terminal can comprise an Analog/Digital (A/D) converter. In this aspect, the A/D converter can be configured to receive the voltage output from the converting op-amp. It is contemplated that the A/D converter can convert the voltage output into a digital output signal. In yet another aspect, the data acquisition terminal can be in electrical communication with a computer having a processor. In this aspect, the computer can be configured to receive the digital output signal from the A/D converter. It is contemplated that the A/D converter can have a conventional Wi-Fi transmitter for wirelessly transmitting the digital output signal to the computer. It is further contemplated that the computer can have a conventional Wi-Fi receiver to receive the digital output signal from the A/D converter.
As electrical communications methods and electrical data analysis methods and systems are generally known in the art, these particular aspects of the disclosed contact sensor systems are not described in great detail herein. FIG. 5 is a block diagram illustrating an exemplary operating environment for performing the disclosed methods and portions thereof. This exemplary operating environment is only an example of an operating environment and is not intended to suggest any limitation as to the scope of use or functionality of operating environment architecture. Neither should the operating environment be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment.
The present methods and systems can be operational with numerous other general purpose or special purpose computing system environments or configurations. Examples of well known computing systems, environments, and/or configurations that can be suitable for use with the system and method comprise, but are not limited to, personal computers, server computers, laptop devices, hand-held electronic devices, vehicle-embedded electronic devices, and multiprocessor systems. Additional examples comprise set top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers distributed computing environments that comprise any of the above systems or devices, and the like.
The processing of the disclosed methods and systems can be performed by software components. The disclosed system and method can be described in the general context of computer-executable instructions, such as program modules, being executed by one or more computers or other devices. Generally, program modules comprise computer code, routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. In one aspect, the program modules can comprise a system control module. The disclosed method can also be practiced in grid-based and distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules can be located in both local and remote computer storage media including memory storage devices.
Further, one skilled in the art will appreciate that the system and method disclosed herein can be implemented via a general-purpose computing device in the form of a computer 200. As schematically illustrated in FIG. 5, the components of the computer 200 can comprise, but are not limited to, one or more processors or processing units 203, a system memory 212, and a system bus 213 that couples various system components including the processor 203 to the system memory 212.
The system bus 213 represents one or more of several possible types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, and a processor or local bus using any of a variety of bus architectures. By way of example, such architectures can comprise an Industry Standard Architecture (ISA) bus, a Micro Channel Architecture (MCA) bus, an Enhanced ISA (EISA) bus, a Video Electronics Standards Association (VESA) local bus, an Accelerated Graphics Port (AGP) bus, and a Peripheral Component Interconnets (PCI) bus also known as a Mezzanine bus. The bus 213, and all buses specified in this description can also be implemented over a wired or wireless network connection and each of the subsystems, including the processor 203, a mass storage device 204, an operating system 205, contact sensor software 206, contact sensor data 207, a network adapter 208, system memory 212, an Input/Output Interface 210, a display adapter 209, a display device 211, and a human machine interface 202, can be contained within one or more remote computing devices 214 a,b,c at physically separate locations, connected through buses of this form, in effect implementing a fully distributed system.
The computer 200 typically comprises a variety of computer readable media. Exemplary readable media can be any available media that is accessible by the computer 200 and comprises, for example and not meant to be limiting, both volatile and non-volatile media, removable and non-removable media. The system memory 212 can comprise computer readable media in the form of volatile memory, such as random access memory (RAM), and/or non-volatile memory, such as read only memory (ROM). The system memory 212 typically contains data such as pressure and/or hysteresis data 207 and/or program modules such as operating system 208 and contact sensor module software 206 that are immediately accessible to and/or are presently operated on by the processing unit 203.
In another aspect, the computer 200 can also comprise other removable/non-removable, volatile/non-volatile computer storage media. By way of example, FIG. 5 illustrates a mass storage device 204 which can provide non-volatile storage of computer code, computer readable instructions, data structures, program modules, and other data for the computer 200. For example and not meant to be limiting, a mass storage device 204 can be a hard disk, a removable magnetic disk, a removable optical disk, magnetic cassettes or other magnetic storage devices, flash memory cards, CD-ROM, digital versatile disks (DVD) or other optical storage, random access memories (RAM), read only memories (ROM), electrically erasable programmable read-only memory (EEPROM), and the like.
Optionally, any number of program modules can be stored on the mass storage device 204, including by way of example, an operating system 205 and contact sensor module software 206. Each of the operating system 205 and contact sensor module software 206 (or some combination thereof) can comprise elements of the programming and the load cell module software 206. Pressure and/or hysteresis data 207 can also be stored on the mass storage device 204. Pressure and/or hysteresis data 207 can be stored in any of one or more databases known in the art. Examples of such databases comprise, DB2®, Microsoft® Access, Microsoft® SQL Server, Oracle®, mySQL, PostgreSQL, and the like. The databases can be centralized or distributed across multiple systems.
In another aspect, the user can enter commands and information into the computer 200 via an input device (not shown). Examples of such input devices comprise, but are not limited to, a keyboard, pointing device (e.g., a “mouse”), a microphone, a joystick, a scanner, tactile input devices such as gloves, and other body coverings, and the like. These and other input devices can be connected to the processing unit 203 via a human machine interface 202 that is coupled to the system bus 213, but can be connected by other interface and bus structures, such as a parallel port, game port, an IEEE 1394 Port (also known as a Firewire port), a serial port, or a universal serial bus (USB).
In yet another aspect, a display device 211 can also be connected to the system bus 213 via an interface, such as a display adapter 209. It is contemplated that the computer 200 can have more than one display adapter 209 and the computer 200 can have more than one display device 211. For example, a display device can be a monitor, an LCD (Liquid Crystal Display), or a projector. In addition to the display device 211, other output peripheral devices can comprise components such as a printer (not shown) which can be connected to the computer 200 via Input/Output Interface 210.
The computer 200 can operate in a networked environment using logical connections to one or more remote computing devices 214 a,b,c. By way of example, a remote computing device can be a personal computer, portable computer, a server, a router, a network computer, a peer device or other common network node, and so on. Logical connections between the computer 200 and a remote computing device 214 a,b,c can be made via a local area network (LAN) and a general wide area network (WAN). Such network connection can be through a network adapter 208. A network adapter 208 can be implemented in both wired and wireless environments. Such networking environments are conventional and commonplace in offices, enterprise-wide computer networks, intranets, and the Internet 215.
For purposes of illustration, application programs and other executable program components such as the operating system 205 are illustrated herein as discrete blocks, although it is recognized that such programs and components reside at various times in different storage components of the computing device 200, and are executed by the data processor(s) of the computer. An implementation of contact sensor software 206 can be stored on or transmitted across some form of computer readable media. Computer readable media can be any available media that can be accessed by a computer. By way of example and not meant to be limiting, computer readable media can comprise “computer storage media” and “communications media.” “Computer storage media” comprise volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules, or other data. Exemplary computer storage media comprises, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by a computer.
In various aspects, it is contemplated that the methods and systems described herein can employ Artificial Intelligence techniques such as machine learning and iterative learning. Examples of such techniques include, but are not limited to, expert systems, cue based reasoning, Bayesian networks, behavior based AI, neural networks, fuzzy systems, evolutionary computation (e.g. genetic algorithms), swarm intelligence (e.g. ant algorithms), and hybrid intelligent systems (e.g. expert inference rules generated through a neural network or production rules from statistical learning).
It is contemplated that the contact sensors described herein can optionally comprise one or more sensing points. In one aspect, the contact sensor can include only a single sensing point. For example, the entire contact surface of the disclosed sensors can be formed of the conductive composite material. According to this aspect, the contact sensors can be utilized to obtain impact data and/or the total load on the surface at any time. Such an aspect can be preferred, for example, in order to obtain total load or impact data for a member without the necessity of having external load cells or strain gauges in communication with the load-bearing member. This sensor type may be particularly beneficial in those aspects wherein the sensor is intended to be incorporated with or as the member for use in the field. For example, any polymeric load-bearing member utilized in a process could be formed from the physically equivalent but conductive composite material as described herein and incorporated into the working process to provide real time wear and load data of the member with no cost in wear performance to the member due to the acquisition of conductive capability.
In other aspects, the sensors disclosed herein can include a plurality of sensing points and can provide more detailed data about the junction or the members forming the junction. For example, the plurality of sensing points can provide data describing the distribution of contact stresses and/or internal stresses, data concerning types of wear modes, or data concerning a lubrication regime as well as load and impact data for a member forming a junction. According to this aspect, the composite material can be located at predetermined, discrete regions of a sensor to form the plurality of sensing points on or in the sensor, and a non-conductive material can separate the discrete sensing points from one another. Data from the plurality of discrete sensing points can then be correlated and analyzed and can provide information concerning, for example, the distribution of contact characteristics across the entire mating surface, and in particular can provide contact information under dynamic loading conditions involving, for example, sliding, rolling, or grinding motions across the surface of the sensor.
It is contemplated that the plurality of sensing points can be arranged in any desired configuration along a surface of the sensor. For example, and without limitation, the sensing points can be positioned in a series of parallel rows. Alternatively, the sensing points can be positioned in staggered or overlapping configurations. In one aspect, the sensing points can be substantially evenly spaced. In another aspect, the sensing points can be substantially unevenly spaced.
It is contemplated that selected sensing points among the plurality of sensing points can be activated during the application of a load while the remainder of the sensing points remain deactivated.
FIG. 1 is a schematic diagram of one aspect of the sensor as disclosed herein, including a plurality of sensing points at the essentially inflexible contact surface of the Junction member. Surface sensing points such as those in this aspect can be utilized to determine contact surface data, including, for example, contact stress data, lubrication data, impact data, and information concerning wear modes. The polymeric sensor 10 includes a contact surface 8 for contact with a metallic component (not shown) to simulate the dynamic characteristics of the joint formed between the sensor and the metallic component. In this particular aspect, the contact surface 8 describes a curvature to simulate that of the tibial plateau of an artificial knee implant.
As can be seen with reference to FIG. 1, the sensor 10 includes a plurality of sensing points 12 at the contact surface 8 of the sensor 10. The sensing points 12 can be formed of the conductive composite material as herein described. Thus, unlike conventional contact sensors, the conductive composite material functions as not only the sensing material, but also as an electrical communication pathway. Each sensing point is configured to produce an output signal in response to the change in resistance experienced by the conductive composite material at the sensing point.
In one aspect, because the conductive composite material provides electrical communication between the sensing points at the contact surface of the sensor, the conductive composite material of each sensor can have a bulk resistance. In this aspect, the bulk resistance can be measured in Ohms per unit length; accordingly, as the length of the sensor increases, the bulk resistance proportionally increases. Therefore, the bulk resistance of the conductive composite material varies from one sensing point to another sensing point. It is contemplated that the farther a particular sensing point is from an electrical connection between the sensor and the data acquisition terminal, the greater the bulk resistance will be at that particular sensing point. Consequently, it is contemplated that the resistance measured at each sensing point will be different even when the change in resistance at some sensing points is identical, in addition, it is contemplated that the sensing points can always have at least some level of electrical communication with adjacent sensing points, even when a load is not being applied. Thus, when a load is applied to one or more sensing points, the sensing points that are subjected to the load can generate current within sensing points that are not subjected to the load (parallel resistance paths).
In order to account for the bulk resistance of the composite material and the parallel resistance paths described herein, the processor of the computer disclosed herein can be programmed to accurately determine the actual change in contact resistance experienced at each sensing point of the sensor based on the digital output signal received from the A/D converter of the data acquisition terminal. In one aspect, the processor can be configured to calculate contact resistance changes at individual sensing points based on the current measurements at each respective sensing point. In this aspect, the processor can calculate the resistance changes as the solution to a series of non-linear equations that describe the load in terms of the current measurements at each respective sensing point. It is contemplated that the processor can be configured to solve the series of simultaneous non-linear equations using one or more conventional algorithms, including, for example and without limitation, the “Newton-Raphson method” and the “node analysis” method. The contact resistance changes calculated by the processor can then be used to determine the actual applied load at each respective sensing point.
It is contemplated that the conductive paths produced by the plurality of sensing points can vary depending on the spatial arrangement of the sensing points. For example, the conductive paths produced by sensing points in a parallel and evenly spaced configuration will be substantially different than the conductive paths produced when the sensing points are positioned in overlapping, staggered, or unevenly spaced configurations.
In use, and with reference to FIG. 1, upon contact of a single sensing point 12 with the metallic component, an electrical signal can be generated and sent via wire 18 to a data acquisition terminal as described herein. In one aspect, this electrical signal can be sent in response to a voltage excitation signal that is processed to the electrical signal by the data acquisition terminal. As one skilled in the art will appreciate, in this example, the metallic component is acting as a first electrode that is mechanically and electrically coupling to the polymeric composite material, which is in turn electrically coupled to a second electrode, i.e., the wire 18. The electrically coupled respective first and second electrodes and the polymeric composite material forms an electrical circuit. Though not expressly shown in the Figure, in this particular aspect, each sensing point 12 of the plurality of sensing points can be wired so as to provide data from that point to the data acquisition terminal. Due to the deformable nature of the polymeric composite material, the characteristics of the generated electrical signal can vary with the variation in load applied at the sensing point 12 and a dynamic contact stress distribution profile for the joint can be developed.
The surface area and geometry of any individual sensing point 12 as well as the overall geometric arrangement of the plurality of sensing points 12 over the surface 8 of the sensor 10, can be predetermined as desired. For example, through the formation and distribution of smaller sensing points 12 with less intervening space between individual sensing points 12, the spatial resolution of the data can be improved. While there may be a theoretical physical limit to the minimum size of a single sensing point determined by the size of a single polymer granule, practically speaking, the minimum size of the individual sensing points will only be limited by modern machining and electrical connection forming techniques. In addition, increased numbers of data points can complicate the correlation and analysis of the data. As such, the preferred geometry and size of the multiple sensing points can generally involve a compromise between the spatial resolution obtained and complication of formation methods.
In this particular aspect as seen in FIG. 1, the composite material forming the surface sensing points 12 can extend to the base 15 of the sensor 10, where electrical communication can be established to a data acquisition and analysis module, such as a computer with suitable software, for example.
In one aspect, the discrete sensing points 12 of the sensor 10 of FIG. 1 can be separated by a non-conductive material 14 that can, in one aspect, be formed of the same polymeric material as that contained in the composite material forming the sensing points 12. In general, the method of combining the two materials to form the sensor can be any suitable formation method. For example, in one aspect the composite material can be combined with a virgin material to produce one or more sensor sheets as described herein. Alternatively, the composite material can be formed into a desired shape, such as multiple individual rods of composite material as shown in the aspect illustrated in FIG. 1, and then these discrete sections can be inserted into a block of the non-conductive polymer that has had properly sized holes cut out of the block. Optionally, the two polymeric components of the sensor can then be fused, such as with heat and/or pressure, and any final shaping of the two-component sensor, such as surface shaping via machining, for example, can be carried out so as to form the sensor 10 including discrete sensing points 12 formed of the conductive composite material at the surface 8.
In many aspects of the invention, the same material can be used but for the presence or absence of the conductive filler for the composite sensing points 12 as for the intervening spaces 14 since, as described above, the physical characteristics of the composite material can be essentially identical to the physical characteristics of the non-conductive material used in forming the composite. According to this aspect, the sensor 10 can have uniform physical characteristics across the entire sensor 10, i.e., both at the sensing points 12 and in the intervening space 14 between the composite materials.
In one particular aspect, the polymer used to form the sensor 10 can be the same polymer as is used to form the member for use in the field. For example, when considering the examination of artificial joints the polymer used to form both the composite material at the sensing points 12 and the material in the intervening space 14 between the sensing points 12 can be formed of the same polymer as that expected to be used to form the polymeric bearing component of the implantable device (e.g., UHMWPE or polyurethane). Thus, the sensor 19 can provide real time, accurate, dynamic contact data for the implantable polymeric bearing under expected conditions of use.
Optionally, the surface 8 of the sensor 10 can be coated with a lubricating fluid, and in particular, a lubricating fluid such as may be utilized for the bearing during actual use and under the expected conditions of use (e.g., pressure, temperature, etc.). In this aspect, in addition to providing direct contact data, the disclosed sensors can also be utilized to examine data concerning contact through an intervening material, i.e., lubrication regimes under expected conditions of use. For example, the sensor can be utilized to determine the type and/or quality of lubrication occurring over the surface of the sensor including variation in fluid film thickness across the surface during use. In one aspect, this can merely be determined by presence or absence of fluid, e.g., presence or absence of direct contact data (i.e., current flow) in those aspects wherein the fluid is a non-conductive lubricating fluid. In other aspects, a more detailed analysis can be obtained, such as determination of variation in fluid film thickness. This information can be obtained, for example, by comparing non-lubricated contact data with the data obtained from the same joint under the same loading conditions but including the intervening lubricant. In another aspect, such information could be obtained through analysis of the signal obtained upon variation of the frequency and amplitude of the applied voltage. In yet another aspect, the sensor can be utilized in a capacitance mode, in order to obtain the exact distance between the two surfaces forming the joint. In one particular aspect, the disclosed sensor can be utilized to determine a lubrication distribution profile of the contact surface over time.
FIG. 2 illustrates another aspect of the contact sensors as described herein. According to this aspect, the sensor includes multiple sensing strips 16 across the surface 8 of de sensor 10. As illustrated, in this aspect, the orientation of the individual sensing strips 16 across the different condoyles formed on the single sensor surface can be varied from one another. Alternatively, and as shown in FIG. 3, strips can be laid in different orientations on separate but identically shaped sensors in a multi-sensor testing apparatus. In any case, by varying the orientation of sensor strips on multiple, but essentially identical surfaces, virtual cross-points can be created when the data from the different surfaces is correlated. In particular, when contacts of the same shape and magnitude at the same location of different surfaces are recognized, a virtual data point at the cross-point can be created. As can be seen in FIG. 3, this aspect can allow the formation of fewer electrical connections and wires 18 in order to provide data to the acquisition and analysis location, which may be preferred in some aspects due to increased system simplicity.
Optionally, it is contemplated that the contact sensors as described herein can be utilized to provide sub-surface stress data. For example, in the aspect illustrated in FIG. 3, multiple sensing strips 16 can be located within a subsurface layer at a predetermined depth of the sensor. According to this aspect, the horizontal and vertical strips 16 can cross each other with a conductive material located between the cross points to form a subsurface sensing point 15 at each cross point. In one aspect, the strips 16 can be formed of the composite material described herein with the intervening material being the same basic composite material but with a lower weight percentage of the conductive filler, and the layer can be laid within the insulating non-conductive polymer material 14. In another aspect, the sensing strips 16 can be any conductive material, such as a metallic wire, for example, laid on either side of a sheet or section of the composite material and the layer can then be located at a depth from the surface of the sensor.
Application of a load at the surface 8 of the sensor can then vary the electronic characteristics at the internal sensing point 15. In particular, the current flow at any point 15 can vary in proportion to the stress at that point. Thus, when data from multiple sensing points 15 are correlated, an internal stress profile for the sensor can be developed at that particular depth.
In yet another aspect, in lieu of strips, the conductive filler may be arranged on the sensor sheet as a plurality of dots, as shown in FIG. 13. In this aspect, there would be reduced opportunity for cross-talk when the sensor sheets were thermoformed into shape. In this aspect, it can be appreciated that the electrical connections necessary to perform the load analysis can be challenging due to the number of connections required. As such, application of current to one of the sheets may be achieved using a sheet of flexible conductive material such as, for example, mesh or foil. In use, a sensor sheet having a plurality of conductive dots can be configured for coupling with electrodes proximate each respective conductive dots. Following application of a load with a metallic or other conductive element, it is contemplated that current can flow through the conductive filler therein the sensor sheet, thereby permitting calculation of the applied loads.
FIG. 14 is a schematic of a contact sensor in operative communication with a data acquisition terminal, and showing a battery operatively coupled to the data acquisition terminal and a computer coupled to the data acquisition terminal via a Wi-Fi transmitter.
In use, it is contemplated that a plurality of sensor sheets can be thermoformed in substantially identical three-dimensional sizes and orientations. In one aspect, the sensor sheets can be placed in a stacked relationship with adjacent sensor sheets. In this aspect, it is contemplated that no fusing between adjacent sensor sheets will occur. In another aspect, the configurations of the portions of conductive filler therein the sensor sheets can be selected to create overlap between the conductive portions of adjacent sensor sheets. For example, and without limitation, the conductive portions of one sensor sheet can be oriented substantially perpendicularly to the conductive portions of an adjacent sensor sheet prior to stacking of the sensor sheets. It is contemplated that upon application of a load to the sensor sheets, each respective sensor sheet can function as an electrode such that no additional contact with a conductive element is required to produce current therethrough the sensors sheets. It is further contemplated the overlap between the conductive portions of the sensor sheets can create cross points for measuring loads applied to the sensor sheets.
In another aspect, as shown in FIG. 4, the sensors can include multiple stacked polymer sensor sheets. In this aspect, each polymer sensor sheet has a plurality of conductive stripes of conductive material that are separated by non-conductive polymeric stripes. In the illustrated example, the vertical conductive stripes on one sheet, “columns,” and the horizontal conductive stripes on the underlying sheet, “rows,” are positioned relative to each other so that, at the places where these columns and rows spatially intersect, the conductive areas of the two sheets are in physical and electrical contact with each other. In one aspect, the exemplary interface electronics illustrated in FIG. 15 can be used with appropriate control software within the data acquisition terminal to connect one column to a voltage source and one row to a current-to-voltage circuit, in order to measure the current through the conductive polymer materials. In one aspect, it is contemplated that each column/row pair, i.e., the internal junction points 15, can be measured, one at a time, to provide a complete set of current measurements. As illustrated, the stacked sensor sheets being oriented substantially perpendicular to each other allow for the formation of an array of sensing points by the overlapping portions of the conductive stripes of the stacked sensor sheets.
In one aspect, these current measurements do not represent the currents at the pressure-sensitive points in the stacked polymer sheets where the stripes overlap. Rather, the current measurements are actually external measurements at external points (also called “nodes”, which are generally near the outer edges of the material. The measurement data are processed in software within the data acquisition terminal in order to calculate the individual currents that are present at each measurement point where the columns and rows overlap, and then this information is used to determine the pressure that is applied at each measurement point. An exemplary, non-limiting, schematic of the measurement circuitry is provided in FIG. 16 herein.
In yet another aspect, both subsurface contact data and surface contact data can be gathered from a single sensor through combination of the above-described aspects.
In one aspect, the sensor may comprise a thermoformable polymer, such as, for example and without limitation, ultra high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), polyphenolyne sulfide (PPS), low density polyethylene (LDPE), or polyoxymethylene copolymer (POM). In this aspect, the sensor can be formed into the shape of at least a portion of an artificial joint bearing. For example and not meant to be limiting, a portion of a prosthetic limb. Pressure mapping of portions of a joint bearing can provide data necessary to fit the prosthetic limb to the user with lower wear. In this aspect, a polymer capable of stretching is advantageous due to the non-uniformity of the shape of the prosthesis.
In another aspect, the sensors can be manufactured in a two stage process. First, the non-conductive sheets of thermoformable polymer can be molded from raw material. Second, the conductive strips can be added to the sensor sheet and placed back into the same mold. In this manner, flow of the non-conductive polymer into the conductive region of the sheet, and flow of the polymer with conductive filler into the non-conductive region of the sheet can be minimized to ensure that, when thermoformed, there is no cross-talk between adjacent conductive strips.
In this aspect, calibration of the each sensor can be performed prior to the thermoforming step, as calibration after thermoforming can prove to be more difficult. It is believed that the characteristics of the sensors do not substantially change during the thermoforming process.
In one aspect, such calibration may be desired as each individual sensor can have individually unique electrical properties that must be calibrated to a standard in order to achieve a desired degree of load measurement accuracy. Further, it is believed that the individual sensors can experience hysteresis when the sensors are unloaded. Thus, it is contemplated that conventional signal processing components configured to correlate the voltage or current to the load of the respective sensor can be implemented using software configured to correlate the load during loading and load during unloading. To compensate for the observed hysteresis effect, it is also contemplated that the software can be configured to calculate the load during a static position—when the load is substantially constant—by using a mean point between a calculated load value during loading and a calculated load value during unloading.
When used in making a prosthetic limb, for example, the sensor sheets can be thermoformed into the shape of a cup for receiving the anatomical limb. Once the sheets are used to map out the force distribution in the cup, the sensor sheets can be adjusted accordingly. This process can be repeated until the forces are substantially uniformly distributed as desired. Once the desired level of force distribution is achieved, a mold, such as for example, a plaster mold, can be made of the interior portion of the cup. Then the mold can be used to form the cup out of materials that are suitable for the prosthesis.
Optionally, the composite materials produced as described herein can be incorporated into one or more sensor sheets. In one aspect, a method for producing the sensor sheets can comprise providing a plurality of substantially circular virgin sheets comprising at least one virgin material. In this aspect, the virgin material can comprise, for example and without limitation, virgin UHMWPE. In another aspect, the method for producing the sensor sheets can comprise providing a plurality of substantially circular composite sheets comprising at least one composite material as disclosed herein. In this aspect, the composite material can comprise, for example and without limitation, a mixture of carbon black and UHMWPE. In an additional aspect, the virgin sheets can have an outer diameter substantially equal to an outer diameter of the composite sheets. In yet an another aspect, the virgin sheets can have an inner diameter substantially equal to an inner diameter of the composite sheets. In a further aspect, the method for producing the sensor sheets can comprise positioning the virgin sheets and the composite sheets can be stacked in a desired configuration. In this aspect, the desired configuration can comprise a single stack of alternating virgin and composite sheets such that virgin sheets are intermediate and in contact with composite sheets and composite sheets are intermediate and in contact with virgin sheets.
In an additional aspect, while the virgin and composite sheets are stacked in the desired configuration, the virgin and composite sheets can be subjected to a conventional compression molding process for heating and then fusing the virgin and composite sheets together. In another aspect, the compression molding of the virgin and composite sheets can produce a substantially cylindrical billet. In this aspect, the substantially cylindrical billet can be substantially hollow. In a further aspect, the billet can be placed on a conventional mandrel. In this aspect, the mandrel can be configured to spin at a desired rate. In still a further aspect, the method for producing the sensor sheets can comprise spinning the mandrel, thereby turning the billet as the mandrel spins. In another aspect, the method can comprise subjecting the billet to a conventional skiving machine. It is contemplated that the skiving machine can comprise a blade for slicing or shaving off a thin layer of the billet. In operation, the blade of the skiving machine advances toward the billet at a constant rate as the billet rotates on the mandrel, thereby producing the sensor sheets. In one aspect, the sensor sheets can be of substantially uniform thickness. In this aspect, it is contemplated that the sensor sheets can have a thickness ranging from about 0.001 inches to about 0.050 inches, more preferably from about 0.002 inches to about 0.030 inches, and most preferably from about 0.003 inches to about 0.020 inches.
The contact sensors described herein may be better understood with reference to the Examples below.
An industrial-grade UHMWPE powder (GUR 1150, available from Ticona Engineering Polymers) having a molecular weight of 6×10⁶, density of 0.93 g/mL, T_mof 135° C., and an average particle size of 100 μm, was combined with carbon black (CB) (Printex L-6 available from Degussa Huls, Dusseldorf, Germany) having a primary particle size of 18 nm and dibutyl phthalate absorption of 120 mL/100 g. Amounts of each powder were placed in a 120 mL plastic sample container and initially manually shaken for 5 minutes to obtain four different samples having CB weight percentages of 0.25%, 0.5%, 1%, and 8%. The samples were then mixed for 10 minutes on a common laboratory vortex at the maximum speed setting.
Virgin UHMWPE powder and the four UHMWPE/CB powder mixtures were then compression-molded into rectangular sheets 12 cm long, 8.5 cm wide, and 2 mm thick using a mold consisting of a 2 mm thick Teflon frame sandwiched between 2 stainless steel plates that were coated with Teflon mold release spray. The powders were processed in a laboratory press (Carver Laboratory Press, Model C, Fred S. Carver Inc., Wabash, Ind.) equipped with electric heaters for 20 minutes at a temperature of 205° C. and a pressure of 10 MPa. The specimens were then quenched under pressure at a cooling rate of 50° C./min.
Tensile tests were performed to obtain stress-strain curves for each composite and for the control. Results can be seen in FIG. 6 for the control (20), 0.25% CB (22), 0.50% CB (24), 1.0% CB (26), and 8.0% CB (28). From these stress-strain curves, the modulus of elasticity was determined for each composite, and these values were compared to those obtained for the control specimen. Both the control specimens and the composite specimens were formed from the same stock of virgin UHMWPE powder (GUR 1150) by using the same processing parameters of temperature, pressure, time, and cooling rate. The results from the tensile tests can be seen in Table 3, below. It was determined that there was no statistically significant difference (p=0.32, α=0.05) between the modulus of the 8% composite and the modulus of the virgin UHMWPE control samples that were tested.


		0.25 wt %	0.50 wt %	1 wt %	8 wt %
n = 4	Control	CB	CB	CB	CB

Young’s Modulus	214.8 ± 21.1	208.48 ± 7.68	211.9 ± 7.74	212.6 ± 6.82	208.9 ± 11.1
(Mpa)
Tensile Strength	30.8 ± 3.98	29.1 ± 2.23	32.6 ± 3.49	31.9 ± 2.43	31.7 ± 1.03
(Mpa)
Yield Strength	17.8 ± 0.75	18.0 ± 0.87	17.8 ± 0.93	15.2 ± 0.96	22.2 ± 1.07
(Mpa)
Elongation at	390 ± 77.0	360 ± 18.0	390 ± 18.0	340 ± 23.0	290 ± 41.0
Break (%)

The elastic modulus values obtained were comparable to those obtained by Parasnis and colleagues for thin-film UHMWPE specimens (see Parasnis C, Ramani K. Analysis of the effect of pressure on compression molding of UHMWPE. Journal of Materials Science: Materials in Medicine, Vol. 9, p 165-172, 1998, which is incorporated herein by reference). The values obtained for tensile strength, yield strength, and elongation at break compared closely to the values cited in the literature (for example, see is Li S. Burstein A. H., Current Concepts Review: Ultra-high molecular weight polyethylene. The Journal of Bone and Joint Surgery, Vol. 76-A, No. 7, p 1080-1090, 1994, which is incorporated herein by reference).
FIG. 7 shows a plot of the log of the resistance as a function of the log of the compressive load applied to the UHMWPE/CB composites of 0.5% (24), 1% (26), and 8% (28). The plot shows that the composites have the same slope, but that the intercepts are different, with the 0.5% composite having the highest intercept, and the 8% composite having the lowest intercept. The value of resistance changed by about two orders of magnitude for each composite. The correlation coefficients of each regression line indicated a good fit. When the values of resistance were normalized (shown in FIG. 8), the curves for the three composites were very similar, suggesting that the amount of CB only affected the magnitude of the resistance. Thus, the relative response to applied load appeared to be independent of the amount of CB. It should be noted that the control sample and the 0.25% CB sample had high resistance for all loads tested and thus were not included on FIGS. 7 and 8.
FIG. 9 shows the voltage values corresponding to the compressive load, the compressive displacement, and the resistance of the 8 wt % CB composite while the composite was loaded cyclically with a haversine wave at 1 Hz. The top curve corresponds to the compressive stress, the middle curve corresponds to the compressive strain, and the bottom curve corresponds to the resistance of the sensor material. This data represents the cyclic response of the material, indicating that it does not experience stress-relaxation at a loading frequency of 1 Hz. The results of this cyclic testing show that the peak voltage values corresponding to resistance remain nearly constant over many cycles. Therefore, the data seem to indicate that the sensor material should be well suited for cyclic measurements since the readings do not degrade over time.
Monitoring the electrical resistance of the composite material while applying a compressive load revealed the force-dependent nature of the electrical properties of the material. Because of the nano-scale dispersion of the conductive filler, the material's electrical response to applied load was nearly ideal for all of the percentages tested. That is, the log of the material's resistance varied linearly with respect to the log of the applied load. This linear relationship makes the material well suited for use as a sensor.
The data show that the linear relationship holds true for 0.5%, 1%, and 8%, with the difference between the three being the value of the resistance. As all three percentages showed good sensor properties, specific formulations could be developed based on other criteria, such as the specifies of the measurement electronics.

Example 2

Compression molding was used to form 2 rectangular blocks of 1150 UHMWPE doped with 8 wt % carbon black filler as described above for Example 1. The blocks formed included a 28×18 matrix of surface sensing points 12 as shown in FIG. 1. The points were circular with a 1/16^thinch (1.59 mm) diameter and spaced every 1/10^thinch (2.54 mm). The blocks were then machined to form both a highly-conforming, PCL-sacrificing tibial insert (Natural Knee II, Ultra-congruent size 3, Centerpulse Orthopedics, Austin, Tex.) and a less conforming PCL-retaining tibial insert (Natural Knee II, Standard-congruent, size 3, Centerpulse Orthopedics, Austin, Tex.) as illustrated in FIG. 1. The implants were then aligned and potted directly in PMMA in the tibial fixture of a multi-axis, force-controlled knee joint simulator (Stanmore/Instron, Model KC Knee Simulator). Static testing was performed with an axial load of 2.9 kN (4.times.B.W.) at flexion angles of 0°, 30°, 60°, and 80°, to eliminate the effects of lubricant and to compare the sensor reading to the literature. The dynamic contact area was then measured during a standard walking cycle using the proposed 1999 ISO force-control testing standard, #14243. Data was collected and averaged over 8 cycles. A pure hydrocarbon, light olive oil was used as the lubricant due to its inert electrical properties.
Static loading of the sensors showed that the contact area of the ultra congruent insert was significantly higher than that of the standard congruent insert at all angles of flexion tested. The data closely agreed with results found in the literature from FEA analysis.
The results from dynamic testing with a standard walking protocol, shown in FIG. 12, show the effects that the lubricant had on the dynamic contact area. Contact area was registered by the sensor when physical contact occurred between the femoral component and any sensing point, allowing electrical current to flow. Because the lubricant was electrically insulating, fluid-film lubrication over a sensing point caused no contact to be registered at that point. The lower contact area measured for the ultra-congruent insert during the stance phase of gait was due to the fluid-film lubrication that occurred with the more conforming insert. The rapid changes in contact area measured for the standard-congruent insert during the mid-stance phase suggests that the mode of lubrication is quite sensitive to the dynamic loading patterns.

Example 3

Tecoflex SG-80A, a medical grade soft polyurethane available from Thermedics Inc. (Woburn, Mass.), was solution processed and molded including 4 wt % and 48 wt % CB to form two solid sample materials. FIGS. 11A and 11B graphically illustrate the resistance vs. compressive force applied to the samples for the 4% and 48% non-surfactant mixed samples, respectively. As can be seen, both samples showed pressure sensitive conductive characteristics suitable for forming the sensors as described herein where the value of resistance can be controlled with the amount of conductive filler added.

Example 4

A 6″×6″ mold was constructed from normalized, pre-hardened 4140 steel with a Rockwell hardness of HRC 32-35. The mold was designed and built to mold 6×6 inch sensor sheets at approximately ⅛^thinch thick, and is shown in FIG. 12.
The mold was used to form “virgin” non-conductive sheets from raw high density polyethylene (HDPE) in powder form, similar to the fashion to form the sheets of UHMWPE in Example 1. HDPE works well in applications in which the sensor sheets need to be thermoformed. However, HDPE's low gel viscosity makes it a challenge to keep adjacent regions of the sensor sheet separated from one another when forming the sensor sheet.
Although HDPE's melt temperature is readily available, observations were made to confirm bow the sensor sheets would behave in the mold. A thermocouple was used to measure the temperature in the oven. It was determined that the transition temperature of the HDPE was 255° Fahrenheit. The mold was heated by upper and lower platens, which also apply compressive force it was noted that, even with the correct temperature being applied, some smearing could occur in the sensor sheet if compressive forces were not applied evenly. Any flow of the gel caused smearing.
Once the virgin sheets were constructed, the conductive regions were added and the sheets were placed back into the same mold. Since neither the mold nor the press were perfectly square, the sheets that were produced varied by 10-20 thousands of an inch. To minimize these variances, the mold sections and the press sections were labeled on each corner. Once the mold was labeled, different mold and press alignments were tested to determine the alignments the produced the sheets with the least variance.
Raw material in powder form was melted in the mold under the optimal alignment determined previously to produce the virgin sensor sheet. The conductive filler portions were placed on the virgin sensor sheet. Then, the sheet with the conductive filler was placed back into the mold with the same alignment to ensure that any dimensional variance that were present during the initial molding will also be present for the second molding. It was found that this procedure reduced material flow inside the mold during gel state which reduced smearing.
These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various aspects may be interchanged either in whole or in part. Furthermore those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims.

Claims

We claim:

1. A contact sensor, comprising:

a plurality of polymer sensor sheets, each polymer sensor sheet having selected spaced conductive portions integrally formed therein, wherein adjacent polymer sensor sheets are stacked such that portions of the conductive portions therein the sensor sheets overlap and create an array of sensing points at the overlapping portions of the stacked sensor sheets, and

means for determining the pressure applied at least one sensing point of the array of sensing points,

wherein the conductive portion of each polymer sensor sheet is formed from a pressure sensitive conductive composite material that comprises an electrically conductive filler and a polymeric material.

2. The contact sensor of claim 1, further comprising a data acquisition terminal, wherein each spaced conductive portion is in communication with the data acquisition terminal.

3. The contact sensor of claim 1, wherein the conductive portions of each polymer sheet form conductive stripes extending the substantial length of the polymer sheet.

4. The contact sensor of claim 3, wherein the conductive stripes in one sensor sheet are oriented substantially perpendicular to the conductive portions of an adjacent sensor sheet.

5. The contact sensor of claim 2, wherein the data acquisition terminal is programmed to selectively connect one conductive stripe of an underlying polymer sheet to a voltage source and one conductive stripe of the overlying polymer sheet to a current-to-voltage circuit to measure the current through the sensing point formed at the overlapping portions of the stacked sensor sheets.

6. The contact sensor of claim 2, wherein the data acquisition terminal is programmed to measure the current at each sensing point of the array of sensing points.

7. The contact sensor of claim 3, wherein the data acquisition terminal is programmed to process the current measurements at at least one sensing point to determine the pressure that is applied at each sensing point.

8. The contact sensor of claim 1, wherein the polymer sensor sheet is formed from a substantially inflexible composite material.

9. The contact sensor of claim 1, wherein the non-conductive potion of each polymer sheet is formed from a polymeric material.

10. The contact sensor of claim 9, wherein the polymeric material used in the conductive and non-conductive portions are the same polymeric material.

11. The contact sensor of claim 10, wherein the polymeric material is a thermoformable polymer.

12. The contact sensor of claim 9, wherein the polymeric material is selected from a group consisting of: ultra-high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), polyphenolyne sulfide (PPS), low density polyethylene (LDPE), or polyoxymethylene copolymer (POM).

13. The contact sensor of claim 9, wherein a desired amount of conductive filler can range from about 0.1% to about 20% by weight of the pressure sensitive composite material.

14. The contact sensor of claim 9, wherein the conductive filler comprises carbon black.

15. The contact sensor of claim 9, wherein the pressure sensitive composite material further comprises ceramic fillers, aluminum oxide, zirconia, calcium, silicon, fibrous fillers, carbon fibers, glass fibers, and/or organic fillers.

16. A contact sensor, comprising:

a data acquisition terminal;

a plurality of polymer sensor sheets, each polymer sensor sheet having selected spaced conductive portions integrally formed therein, wherein each spaced conductive portion is in communication with the data acquisition terminal, wherein adjacent polymer sensor sheets are stacked such that portions of the conductive portions therein the sensor sheets overlap and create an array of sensing points at the overlapping portions of the stacked sensor sheets, wherein the conductive portion of each polymer sensor sheet is formed from a pressure sensitive conductive composite material that comprises an electrically conductive filler and a polymeric material,

wherein the data acquisition terminal is programmed to selectively connect one conductive stripe of an underlying polymer sheet to a voltage source and one conductive stripe of the overlying polymer sheet to a current-to-voltage circuit to measure the current through the sensing point formed at the overlapping portions of the stacked sensor sheets, and wherein the data acquisition terminal is programmed to determine the pressure applied to at least one sensing point of the array of sensing points based on the measured current.

17. The contact sensor of claim 16, wherein the conductive portions of each polymer sheet form conductive stripes extending the substantial length of the polymer sheet.

18. The contact sensor of claim 17, wherein the conductive stripes in one sensor sheet are oriented substantially perpendicular to the conductive portions of an adjacent sensor sheet.

19. The contact sensor of claim 16, wherein the data acquisition terminal is programmed to measure the current at each sensing point of the array of sensing points.

20. The contact sensor of claim 19, wherein the data acquisition terminal is programmed to process the current measurements at at least one sensing point to determine the pressure that is applied at each sensing point.