Med-Tech Innovation Materials
results shown in Figure 6. All ZTA has much higher KIC values than the control. The highest KIC value achieved at CERAM is 7.2 MPa.m1/2
and the potential exists to
achieve greater fracture toughness than that. These and other complete results will be published in a separate article.
Toughening mechanisms The basic model of micro fracture mechanics shown in Figure 1 contains the following Equation [2]:
[2] Where rh is classically called “yielding” zone or plastic
zone. With respect to ceramics, it is better called the “micro deformation/fracture” zone. The size of rh
toughness KIC and the critical triaxial stress σh are
influenced by these three parameters. To maximise fracture toughness KIC, it is necessary to enlarge the micro deformation/fracture zone rh
of this enlargement has turned a sharp crack tip (Figure 1) to an effectively blunted crack front; the bluntness is defined by rh
, that is, extended micro deformation and/
or micro fracture zone. The effect of this is to ease the stress concentration at the sharp crack/defect tip. It also absorbs/consumes a large amount of energy during micro deformation and/ or micro fracture. Both these
effects will make catastrophic failure less likely to occur, hence increased fracture
Figure 6: Fracture toughness KIC of zirconia toughened alumina and alumina control
toughness for that ceramic. This is what is
seen in Figure 5 (the toughened alumina) and (the effect) in Figure 6, which shows that the KIC of the toughened alumina is significantly increased compared with the control alumina.
Figure 7: Microstructure of ceramic and polymer hybrids (porous ceramic is brighter phase and polymer darker phase)
Development of ceramic polymer hybrids Based on the above micro fracture mechanics model, we have been developing a range of toughened ceramics in combination with other
materials such as polymers to form hybrid composites. Figure 7 is one example of
ceramic hybrids. This ceramic
compound is composed of a ceramic foam (brighter phases, highly loaded) and mixture of polymers (darker phases). This kind of ceramic hybrid has the advantage of biocompatibility and bioactivity and the hardness of ceramic with the flexibility of the toughened polymer. It
24 ¦ April 2011 . The effect , fracture
is therefore easier to design and develop a material with good control of its microstructure. It is possible to provide the right set of requirements and toughening of the ceramic and hence there is great potential for orthopaedic applications.
Acrylic polymers are the first polymeric materials ever used for orthopaedic applications and they are still used today. Two well known drawbacks are brittle and large volume contraction at application (during the resin curing). We can make the materials much tougher and stronger. Based on the micro fracture toughness model, we can make rh
, that is, “yielding” or “micro deformation/
fracture” much larger than that of ceramic toughened ceramic discussed above. Therefore, the new material no longer has the problem of brittle fracture as in the case of ceramic hip joints. It will not be sensitive to sharp crack/ defects inherited with the materials, hence catastrophic failure can be prevented. For example, by design, a sub-micron sized particle with a rubber core and acrylic shell was introduced to toughen the matrix of a modified acrylic resin.
Based on that, the material is further strengthened by adding ceramic powders to make a hybrid. The fracture toughness of the hybrid has increased (more than tripled than that of the pure acrylic polymers). This is because rh
, “micro deformation/fracture” zone has increased to
great extent. By the flexibility of changing the formulation and the way to make the hybrid, it is now possible to design the mechanical properties to match the needs of biological bone, rather than simply use one material such as metal or ceramic.
In addition, the new ceramic hybrids can be designed to possess better biocompatibility and, most importantly, bioactivity with tailored properties to meet different application needs. For example, for certain applications, it is desirable to facilitate new bone growth based on artificial bone matrix. This can be achieved by combining HA with bioresorbable calcium phosphate to make polymer hybrids. Changing the ratio of HA and the calcium phosphate will ensure the rate of degradation of calcium phosphate matches the rate of new bone formation.
The company’s work has also shown that ceramic hybrids can be made by employing resorbable polymer such as poly(α-hydroxy acids) to modify bioactivity for different applications. In general, various forms of ceramic hybrid compounds can be made into different microstructures for different applications. These applications potentially include, but are not limited to, spinal fusion, suture anchors, fixation and trauma screws, femoral implants, dental implants, total and partial joint replacement.
Bioactive materials are the future
Bioactive materials will be the new materials technology in the future. The body treats all artificial materials as “enemies.” Metal and polyethylene used in hip and knee joints are classified safe, but are, in fact, not bio-inert and are potential hazards when their debris migrates into other parts of body. This is obviously a major concern. Therefore, biocompatibility is, at the very least, a basic
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