BACKGROUND

The disclosure relates to sensors for stress

utilising piezoresistive nanocomposites . The process and sensor are described particularly in relation to use in prostheses such as tibiofemoral joint prostheses. However, it will be clear to a person skilled in the art that the stress sensing characteristics of the nanocomposites can be utilised for alternative prosthetic devices and other applications in which sensing stress is beneficial.

SUMMARY Disclosed in some forms is a sensor for sensing stress, the sensor comprising a composite composed of carbon nanoparticles and polyethylene. In some aspects the nanoparticles are in the form of carbon nanotubes . In some aspects the polyethylene is in the form of ultra-high molecular weight polyethylene.

The sensor is biocompatible, piezoresistive and can be included in a system as a direct reference. In some forms the sensor is polarized and so has greater

sensitivity in one direction.

In some aspects, disclosed is use of a composite of carbon nanoparticles and polyethylene as a stress sensor. In some aspects the nanoparticles are in the form of carbon nanotubes. In some aspects the polyethylene is in the form of ultra-high molecular weight polyethylene.

In some aspects, disclosed is a process for sensing stress comprising positioning a piezoresistive composite of carbon nanoparticles and polyethylene between two surfaces, applying loading to at least one of the

surfaces, measuring the resistance in the piezoresistive composite. In some forms the process senses stress in a prosthetic device and the surfaces are surfaces of the prosthetic device. In some forms the piezoresistive composite is positioned in relation to a tibiofemoral joint. In some forms the piezoresistive composite is positioned sub-surface. In some forms the depth of the piezoresistive composite with respect to the surface is 0.72a and 1.07a where a is the contact radius under equivalent compressive loading of 1 body-weight.

The process allows for a piezoresistive nanocomposite to be positioned between surfaces in a joint as a noninvasive sensor. The material is generally compatible with the material used for the prosthetic knee joint and acts as a direct reference for stress sensing. The

nanocomposite has the benefit of compatibility with the prosthetic, biocompatibility and directional sensitivity.

BRIEF DESCRIPTION OF THE FIGURES

The disclosure will now be described in view of the Figures, in which,

Figs, la - If depict the steps in manufacture of the nanocomposite of one embodiment of the disclosure,

Fig. la shows a MWNT powder prior to processing;

Fig. lb shows a MWNT suspension in ethanol after ultrasonication;

Fig. lc shows MWNT/UHMWPE inside a ball jar with grinding balls for grinding;

Fig. Id shows a resultant powder mixture after grinding has occurred;

Fig. le shows a MWNT/UHMWPE disc after hot pressing;

Fig. If shows the disc of Fig. le when sprayed;

Fig. 2 is a graphical representation of resistance over time in testing the nanocomposite; Fig. 3 is a graphical representation plotting resistance against the compressing stress of the

nanocomposite;

Fig. 4 shows a 3D model of a ball-on-flat

tibiofemoral joint used to test the nanocomposite;

Fig. 5 shows a cross sectional view of the ball-on- flat joint of Fig. 4;

Figs. 6a and 6b show through-thickness Hertz

stresses, normalized to maximum pressure pmaxlBW (nt = 0:46) .

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE Disclosed in some forms is a sensor for sensing stress, the sensor comprising a composite composed of carbon nanoparticles and ultra-high molecular weight

polyethylene . In some forms, the carbon nanoparticles comprise carbon nanotubes . In some forms the carbon nanotubes comprise multi-walled carbon nanotubes .

In some forms, the composite is polarized. In some forms polarisation is effected by aligning the nanoparticles in a specific orientation.

In some forms the nanoparticles comprise carbon

nanospheres .

In some forms the sensor is adapted for use in vitro in a prosthetic device.

In some forms the sensor is adapted for use in sensing stress in a knee implant. In further aspects, disclosed is use of a composite of carbon nanoparticles and polyethylene as a stress sensor. In some aspects the nanoparticles are in the form of carbon nanotubes . In some aspects the polyethylene is in the form of ultra-high molecular weight polyethylene.

In some forms the composite is used to sense stress in a prosthetic device. In some forms the composite is used in a knee

prosthesis .

In some forms the nanocomposite is positioned in relation to a tibiofemoral joint. In some forms the nanocomposite is positioned sub-surface. In some forms the depth of the nanocomposite is 0.72a and 1.07a where a is the contact radius under equivalent compressive loading of 1 body-weight. In further aspects, disclosed is a process for sensing stress comprising: positioning a piezoresistive composite of carbon nanoparticles and polyethylene between two surfaces, applying loading to at least one of the

surfaces, measuring the resistance in the piezoresistive composite.

In further aspects disclosed is a process for sensing stress in a prosthesis comprising: positioning a

piezoresistive composite of carbon nanoparticles and polyethylene between two surfaces of a prosthetic joint, applying movement and/or forces to the joint, measuring the resistance in the piezoresistive composite.

In some forms the prosthesis is a knee prosthesis and the surfaces form part of a femoral part and a tibial part of a tibiofemoral joint. In some forms the piezoresistive composite is positioned sub-surface. In some forms the depth of the piezoresistive composite is 0.72a and 1.07a where a is the contact radius under equivalent compressive loading of 1 body-weight.

In some forms the carbon nanoparticles are aligned. In some forms the alignment in a predetermined orientation may be performed by means of positioning the nanoparticles in an electric field during fabrication, or through mechanical methods during fabrication.

The stress profile of a prosthetic is significant to its efficacy. Accurate and non-invasive testing of the stress profile has benefits for the long term health of the patient and improved prosthetic systems. It can be utilised to test efficacy of existing knee sensors and to test the fidelity of knee simulation machines against stress distribution occurring in use in the body.

Polyethylene and particularly ultra-high molecular weight polyethylene is commonly used as surgical bearing material in prosthetic systems such as knee replacements. The nanocomposite in some forms provides

biocompatible, direct reference load bearing sensing with directional sensitivity. The sensor can be more sensitive in one direction than another. In some forms the nanocomposite combines ultra-high molecular weight polyethylene powder (UHMWPE) and carbon nanoparticles in the form of multi-walled carbon nanotubes (MWNT) to allow the surgical bearing material to act as a sensor. Alternative nanoparticles including nanospheres and single-walled nanotubes can be used in alternative embodiments. In the example the nanoparticles comprised nanotubes which had greater than or equal to 98% carbon basis, 2.1 g/ml bulk density and average dimensions of 10 nm outer diameter, 4.5 nm inner diameter and 3-6 mm length .

The ultra-high molecular weight polyethylene powder utilised in the example had an 180 mm average particle size and 0.94 g/ml density at 25°C. Figure 1 shows the steps in manufacture of the composite. Figure la shows the MWNT powder before the process . The MWNT and UHMWPE raw powders were prepared at 0.1wt%, 0.5 wt%, 1 wt%, 2 wt%, and 5 wt% concentration and the bulk composite was produced from the powder mixture by compression molding. Ultrasonication and ball milling steps were added to de-aggregate the CNT bundles and uniformly disperse them within the UHMWPE matrix before hot pressing into bulk composite. A three-step fabrication procedure is followed to produce the nanocomposite .

The first step comprises ultrasonication. In this example, ethanol is added to MWNT powder in a beaker and ultrasonicated at 20 kHz for 40 minutes. In alternative examples the frequency and timing could be varied. For example ultrasonication could occur for between 20 and 60 minutes. Then UHMWPE powder is added to the CNT-ethanol suspension and ultrasonication is continued for another 20 minutes. Figure lb shows the MWNT suspension in ethanol after ultrasonication. The final suspension is evaporated at room temperature in a fume hood for 24 hours.

The second step comprises ball milling. After evaporation, the powder mixture is ground at 200 RPM for 2 hours, in an in-house-built 400 mL alumina ball jar using 6-mm and 10-mm-diameter alumina grinding balls.

Figure lc shows MWNT/UHMWPE inside a ball jar with grinding balls for grinding. Figure Id shows the resultant powder mixture after grinding has occurred.

The third step comprises hot pressing. In the

example, approximately 1 gram of powder mixture is then compression molded into a solid disc. In the example the disc has a 1 - 25 mm diameter and 0.1-2.0 mm thickness. Variation in disc sizes allows for variation in sensor sizes. In the example, the pressing is achieved through compaction at 10 MPa for 10 minutes, followed by sintering at 170°C and 10 MPa for 10 minutes and finally releasing to cool naturally to room temperature. The timing and temperature can be varied for alternative examples.

In some forms the doped polyethylene disc is fused to virgin polyethylene using resistive heating or induction heating under compression molding.

The resultant disc is shown in Figure le. The

nanocomposite sample disc is sprayed with conductive paint, sputter-coated metal or conductive ink

on top and bottom surfaces to ensure good electrical contact. In Figure If the disc includes silver sprayed layers .

In some forms the nanocomposite is polarized. The material can be polarized by melting under the presence of a strong AC electric field or by mechanically stretching the composite. This polarization can make the

nanocomposite more sensitive in one direction than another. It also has the benefit of strengthening the composite. The resulting nanocomposite is also biocompatible and can be load bearing. The bulk of the material is UHMWPE, meaning the material should behave in a similar manner to a standard UHMWPE implant. As a result, the nanocomposite can be used for measurement of direct loads.

In order to test the piezoresistivity of the

nanocomposite, the nanocomposite sample was sprayed with silver paint on top and bottom surfaces then sandwiched between two copper electrodes for cyclic loading. In the example cyclic loading occurred in a Shimadzu AG-X 50 kN universal testing machine. The force was set to rise up t 5 kN and then go down to 10 N in each cycle, at a strain rate of 0.5 mm/min. For a disc sample of 25 mm diameter, this force range was equivalent to a stress range of 20 kPa-10 MPa. The resistance was measured through an NI SCXI-1503 module and LabView GUI, using a two-wire connection and a 50 uA current source. Figure 2 illustrates ten loading cycles and the measured compression force and resistance under cyclic compression for a 1 wt% CNT/UHMWPE sample. Notably, there was a downward trend of the peak resistance at 10N compression force over ten cycles, indicating that a settling time is needed for the measurement to be stable.

Plotting the resistance against the compressing stress, as in Figure 3, we can see a non-linear decay of the resistance with increasing stress, repeated through ten loading-unloading cycles.

The variation between cycles got smaller at higher stress. There was also a noticeable difference in the slopes of the compressing and releasing curves.

As illustrated in Fig. 4, a ball-on-flat model is used to mimic the tibiofemoral knee joint. In the Figure, the femoral component is a CrCo sphere 10. In the example the sphere has a 28 mm radius. The UHMWPE tibial insert is a flat disc 12. In the example it has a 30 mm surface diameter and 10 mm thickness. The CrCo Alloy has a

Materials Elastic Modulus of 200 E (GPa) and a Poisson

Ratio of 0.3. The UHMWPE has a Materials Elastic Modulus of 0.683 E (GPa ) and a Poisson Ratio of 0.46.

The Hertz contact stress is used under the assumption that the two contacting bodies are isotropic materials, the contact is frictionless and only linear elastic deformation occurs. The initial contact point between the spherical object and the flat disc gradually expands to an elliptical contact area due to elastic deformation from static loading. The 2D trajectory of the body with a deformed contact area is shown in Fig. 5.

When the joint is compressed with an uniaxial force F, the maximum pressure under the contact area can be calculated.

In this analytical estimation, we assume that the stress value obtained indirectly from the sensor

resistance reading, is the principal normal stress, D _sz . It was also reported that the average knee joint loading can rise up to 3.9 times bodyweight (BW) in level walking or 8 BW in downhill motion due to change in flexion angle.

Therefore, we compute the change in principal stress, D _sz , when compressive force F increases from 1 BW to 4 BW and from 1 BW to 8 BW, as a quantitative indicator of the local stress resolution through the tibial thickness.

Fig 6a shows changes in principal stress Dsz when F increases from 1 BW to 4 BW and from 1 BW to 8 BW. Fig. 6b shows Von Mises stress when F equals 1 BW, 4 BW and 8 BW. Fig. 6b illustrates the above functions and shows that the highest stress resolution can be achieved at a depth of 0:72a - 0:98a. On the other hand, Von Mises stress, O _? which is an indicator of the distortion energy and is widely used was examined and used to identify the plastic yielding point of the material. As can be seen from Fig. 6b which plots the O M curves for three different loading conditions of 1 BW, 4 BW, and 8 BW, the peak OV occurs at a depth of 1.07a beneath the contact surface, corresponding to the case of F = 8BW. Therefore, in this example an embedded sensor is preferably located away from the peak to minimize

distortion energy and avoid potential mechanical failures.

Combining two conditions on Von Mises stress and principal normal stress, we can estimate the best

subsurface depth to minimize the local distortion energy and maximize the stress resolution is 0:72a <= depth < 1.07a with a as the contact radius under equivalent compressive loading of 1 body-weight.

While the disclosure includes numerical information about resistance and stress, it will be clear that alternative figures will still fall under the scope of the application. While the disclosure speaks of use of the nanocomposite and method as a sensor in knee prostheses, the nanocomposite can be utilised for alternative stress testing and piezoresistive applications .

In some forms, the sensors are positioned in a spatially distributed array. Multiple devices can b spaced apart over the surface in a regular or an ir array of sensors to provide the best possible senso stress. - li lt is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the disclosure, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.