U.S. patent application number 14/475100 was filed with the patent office on 2015-07-23 for contact sensors and methods for making same.
The applicant listed for this patent is SENSORTECH CORPORATION. Invention is credited to Andrew C. Clark, David W. Topham.
Application Number | 20150201886 14/475100 |
Document ID | / |
Family ID | 42710042 |
Filed Date | 2015-07-23 |
United States Patent
Application |
20150201886 |
Kind Code |
A1 |
Clark; Andrew C. ; et
al. |
July 23, 2015 |
CONTACT SENSORS AND METHODS FOR MAKING SAME
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.
Inventors: |
Clark; Andrew C.;
(Greenville, SC) ; Topham; David W.; (Seneca,
SC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SENSORTECH CORPORATION |
Pendleton |
SC |
US |
|
|
Family ID: |
42710042 |
Appl. No.: |
14/475100 |
Filed: |
September 2, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13254988 |
Nov 11, 2011 |
8820173 |
|
|
14475100 |
|
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|
Current U.S.
Class: |
600/594 ;
600/587 |
Current CPC
Class: |
A61B 5/4595 20130101;
A61B 2560/0487 20130101; A61B 2090/064 20160201; A61B 5/0022
20130101; A61B 5/4528 20130101; A61B 2090/065 20160201; A61B
2560/0481 20130101; A61B 2562/046 20130101; G16H 40/67 20180101;
A61B 2562/164 20130101; A61B 5/686 20130101; A61B 2562/0247
20130101; A61B 5/0004 20130101; A61B 5/6885 20130101; A61B 2562/222
20130101; A61B 5/4566 20130101; A61B 2562/12 20130101; A61B 5/002
20130101; A61B 5/6878 20130101; A61B 5/7264 20130101; A61B 5/4585
20130101; A61B 5/4576 20130101; G01L 1/205 20130101; A61B 5/1036
20130101; A61B 5/4851 20130101; G06F 19/00 20130101; A61B 5/4571
20130101 |
International
Class: |
A61B 5/00 20060101
A61B005/00 |
Claims
1-18. (canceled)
19. A contact sensor system, comprising: a data acquisition
terminal; and a surgical insert defining a contact surface
configured to receive a load applied by an electrically conductive
joint element, the contact surface having selected spaced
conductive portions, wherein the selected spaced conductive
portions define an array of sensing points that are in
communication with the data acquisition terminal, wherein the
surgical insert is configured for insertion therein a selected
joint within the body of a subject, and wherein the conductive
portions of the contact surface, during application of the load,
comprises means for producing an output signal indicative of the
change in electrical resistance experienced across the contact
surface at at least one sensing points, wherein the output signal
produced by each sensing point corresponds to variations in the
load between the electrically conductive joint element and the
contact surface.
20. The contact sensor of claim 19, wherein the surgical insert can
be used in one of a knee joint, a hip joint, a shoulder joint, an
ankle joint, and a spinal joint.
21. The contact sensor of claim 19, wherein the surgical insert
comprises one of a tibial insert, a femoral insert, a patellar
insert, an acetabular insert, a scapular insert, a humeral insert,
a talar insert, and a vertebral insert.
22. The contact sensor of claim 21, wherein the contact surface
extends therein the surgical insert to a depth ranging from about
50 nm to about 1000 nm.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/254,988, filed Nov. 11, 2011, now U.S. Pat.
No. 8,820,173, which is a .sctn.371 national phase of International
Application No. PCT/US10/26576, filed Mar. 8, 2010, which claims
the benefit of U.S. Provisional Application No. 61/157,963, filed
on Mar. 6, 2009, which applications are incorporated by reference
herein in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to contact sensors, and more
particularly to contact sensors for accurately measuring surface
contact data at a junction between two members.
[0004] 2. Description of the Related Art
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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 THE INVENTION
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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 .mu.m and about 500 .mu.m. The average granule
size of the conductive filler can be, for example, between about 10
nm and about 500 nm.
[0015] Following a mixing step, the composite conductive material
can be formed into the sensor shape either with or without areas 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.
[0016] 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.
[0017] 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
[0018] The accompanying drawings, which are incorporated in and
constitute a part 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.
[0019] FIG. 1 illustrates one aspect of the sensor disclosed herein
for obtaining surface contact data of a junction;
[0020] FIG. 2 illustrates another aspect of the sensor disclosed
herein for obtaining surface contact data of a junction;
[0021] FIG. 3 illustrates another aspect of the sensor disclosed
herein for obtaining sub-surface contact data of a junction;
[0022] 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;
[0023] FIG. 5 illustrates a simplified, non-limiting block diagram
showing select components of an exemplary operating environment for
performing the disclosed methods;
[0024] FIG. 6 graphically illustrates the stress v. strain curve
for exemplary composite conductive materials as described
herein;
[0025] FIG. 7 graphically illustrates the log of resistance vs. log
of the load for three different composite conductive materials as
described herein;
[0026] FIG. 8 illustrates the log of normalized resistance vs. log
of the load for three different composite conductive materials as
described herein;
[0027] FIG. 9 illustrates the voltage values corresponding to load,
position, and resistance of an exemplary composite material;
[0028] FIGS. 10A-10D illustrate the kinematics and contact area for
exemplary artificial knee implant sensors as described herein with
different surface geometries; and
[0029] 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;
[0030] FIG. 12 is a photograph of one aspect of an exemplary mold
and press used to form sensor sheets as disclosed herein.
[0031] FIG. 13 is a photograph of a sensor sheet according to one
aspect disclosed herein, illustrating a plurality of dots
comprising a conductive filler.
[0032] 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;
[0033] FIG. 15 is a schematic of an exemplary interface circuitry
for the data acquisition terminal; and
[0034] FIGS. 16A-16C are schematic diagrams of portions of
exemplary measurement circuitry for the data acquisition
terminal.
DEFINITIONS OF TERMS
[0035] For purposes of the present disclosure, the following terms
are herein defined as follows:
[0036] The term "primary particle" is intended to refer to the
smallest particle, generally spheroid, of a material such as carbon
black.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] The term "conventional polymer" is intended to refer to
polymers that have a thermal resistance below about 100.degree. C.
and relatively low physical properties. Examples include
high-density polyethylene (PE), polystyrene (PS), polyvinyl
chloride (PVC), and polypropylene (PP).
[0041] The term "engineering polymer" is intended to refer to
polymers that have a thermal resistance between about 100.degree.
C. and about 150.degree. 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).
[0042] The term "high performance polymer" is intended to refer to
polymers that have a thermal resistance greater than about
150.degree. C. and relatively high physical properties. Examples
include polyetherether ketone (PEEK), polyether sulfone (PES),
polyimides (PI), and liquid crystal polymers (LC).
[0043] 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.
[0044] 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.
[0045] 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
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] In an additional aspect, the contact sensors disclosed
herein can be formed to be substantially inflexible. In this
aspect, the contact sensors can be thermoformed as desired into a
three-dimensional shape. 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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 .mu.m to
about 200 .mu.m. Typically, the individual granule is made up of
multiple sub-micron sized spheroids and nano-sized fibrils
surrounded by varying amounts of free space.
[0067] 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 .mu.m to about 100 .mu.m that can be
broken down into smaller aggregates of from about 10 nm to about
500 nm upon application of suitable energy.
[0068] 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. As can be seen, the UHMWPE particle is 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.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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 connections. 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.
[0074] 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 receiver 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.
[0075] 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.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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 Interconnects (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.
[0080] 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 205 and contact sensor
module software 206 that are immediately accessible to and/or are
presently operated on by the processing unit 203.
[0081] 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.
[0082] 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.RTM., Microsoft.RTM. Access, Microsoft.RTM. SQL
Server, Oracle.RTM., mySQL, PostgreSQL, and the like. The databases
can be centralized or distributed across multiple systems.
[0083] 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).
[0084] 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.
[0085] 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 connections 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.
[0086] 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.
[0087] 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, case 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).
[0088] 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.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.
[0093] 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.
[0094] 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).
[0095] 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.
[0096] 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.
[0097] 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 form 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.
[0098] 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.
[0099] 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.
[0100] 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.
[0101] 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.
[0102] 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 10 can provide real time, accurate,
dynamic contact data for the implantable polymeric bearing under
expected conditions of use.
[0103] 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.
[0104] FIG. 2 illustrates another aspect of the contact sensors as
described herein.
[0105] According to this aspect, the sensor includes multiple
sensing strips 16 across the surface 8 of the 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
necessitate 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.
[0106] 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 8 of the sensor.
[0107] 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.
[0108] 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.
[0109] 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.
[0110] 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.
[0111] 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.
[0112] 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 FIGS. 16A-16C herein.
[0113] 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.
[0114] 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,
the sensor can be formed into the shape of 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.
[0115] 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.
[0116] In this aspect, calibration of 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.
[0117] 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.
[0118] 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.
[0119] 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.
[0120] 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.
[0121] The contact sensors described herein may be better
understood with reference to the Examples, below.
Example 1
[0122] An industrial-grade UHMWPE powder (GUR 1150, available from
Ticona Engineering Polymers) having a molecular weight of
6.times.10.sup.6, density of 0.93 g/mL, T.sub.m of 135.degree. C.,
and an average particle size of 100 .mu.m, was combined with carbon
black (CB) (Printex L-6 available from Degussa Hulls, 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.
[0123] 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.degree. C. and a pressure of 10
MPa. The specimens were then quenched under pressure at a cooling
rate of 50.degree. C./min.
[0124] 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, .alpha.=0.05) between the modulus
of the 8% composite and the modulus of the virgin UHMWPE control
samples that were tested.
TABLE-US-00001 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 (%)
[0125] 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).
[0126] 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.
[0127] 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.
[0128] 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.
[0129] 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 specifics of the measurement
electronics.
Example 2
[0130] 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.times.18
matrix of surface sensing points 12 as shown in FIG. 1. The points
were circular with a 1/16.sup.th inch (1.59 mm) diameter and spaced
every 1/10.sup.th inch (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.degree., 30 .degree., 60.degree., and 80.degree., 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.
[0131] 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.
[0132] 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
[0133] 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
[0134] A 6''.times.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.times.6 inch sensor sheets at
approximately 1/8.sup.th inch thick, and is shown in FIG. 12.
[0135] 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.
[0136] Although HDPE's melt temperature is readily available,
observations were made to confirm how 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.degree. 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.
[0137] 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.
[0138] 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.
[0139] 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.
* * * * *