Hydroxyapatite-reinforced Polyethylene Composite Research Articles

A Multi-component Fiber-reinforced PHEMA-based Hydrogel/HAPEXTM Device for Customized Intervertebral Disc Prosthesis

Spinal disease due to intervertebral disc degeneration represents a serious medical problem which affects many people worldwide. Disc arthroplasty may be considered the future ''gold standard'' of back pain treatment, even if problems related to available disc prostheses are considered. Hence, the aim of the present study was to improve the artificial disc technology by proposing the engineering of a pilot-scale device production process for a total multi-component intervertebral disc prosthesis. The device is made up of a poly(2-hydroxyethyl methacrylate)/poly(methyl methacrylate) (PHEMA/PMMA) (80/20 w/w) semi-interpenetrating polymer network (s-IPN) composite hydrogel reinforced with poly(ethylene terephthalate) (PET) fibers as annulus/nucleus substitute, and two hydroxyapatite-reinforced polyethylene composite (HAPEX™) endplates in order to anchor the multi-component device to the vertebral bodies. Static and dynamic-mechanical characterization show appropriate mechanical behavior. An example of engineering of a suitable pilot-scale device production process is also proposed in order to manufacture custom made implants.

Effect of particle morphology and polyethylene molecular weight on the fracture toughness of hydroxyapatite reinforced polyethylene composite

Fracture toughness testing has been performed on hydroxyapatite-polyethylene composites. Sintered and unsintered grades of hydroxyapatite and two grades of high-density polyethylene were used to make 40 vol % hydroxyapatite composites. Compact tension testing was performed at both room temperature and at 37 degrees C and at three strain rates. The effect of increasing the loading rate from 2 to 200 microm s(-1) was to increase the fracture toughness. Increasing the testing temperature or decreasing the surface area of the reinforcing particles also increased the fracture toughness. However, using a lower molecular weight, injection moulding, grade of polyethylene reduced the fracture toughness. Thus for higher fracture toughness, a low surface area sintered hydroxyapatite in a high-molecular weight polyethylene is required.

Impact behavior of hydroxyapatite reinforced polyethylene composites.

Hydroxyapatite particulate reinforced high density polyethylene composite (HA-HDPE) has been developed as a bone replacement material. The impact behavior of the composites at 37 degrees C has been investigated using an instrumented falling weight impact testing machine. The fracture surfaces were examined using SEM and the fracture mechanisms are discussed. It was found that the fracture toughness of HA-HDPE composites increased with HDPE molecular weight, but decreased with increasing HA volume fraction. Examination of fracture surfaces revealed weak filler/matrix interfaces which can debond easily to enable crack initiation and propagation. Increasing HA volume fraction increases the interface area, and more cracks can form and develop, thus decreasing the impact resistance of the composites. Another important factor for the impact behavior of the composites is the matrix. At higher molecular weight, HDPE is able to sustain more plastic deformation and dissipates more impact energy, hence improving the impact property.

Fatigue characterization of a hydroxyapatite‐reinforced polyethylene composite. I. Uniaxial fatigue

The fatigue behavior of 40 volume % hydroxyapatite-reinforced polyethylene composite (HAPEX™) at 37°C in saline was determined. S-N curves for this material, both in fully reversed axial tension–compression and fully reversed torsion, have been established. In tension, the cycles to failure ranged from 1000 cycles at 13 MPa to more than 1 million cycles at 4.4 MPa while in torsion they ranged from 100 cycles at 14.4 MPa to more than 1 million cycles at 4.8 MPa. Changes in strain range, tangent modulus, and cyclic energy dissipated during fatigue loading also were examined. Torsional fatigue was found to be considerably more damaging than axial fatigue. The fatigue damage accumulation process occurring in the composite was monitored by observing the reduction in modulus and the increase in energy dissipated. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res, 51, 453–460, 2000.

Fatigue characterization of a hydroxyapatite-reinforced polyethylene composite. I. Uniaxial fatigue.

The fatigue behavior of 40 volume % hydroxyapatite-reinforced polyethylene composite (HAPEXtrade mark) at 37 degrees C in saline was determined. S-N curves for this material, both in fully reversed axial tension-compression and fully reversed torsion, have been established. In tension, the cycles to failure ranged from 1000 cycles at 13 MPa to more than 1 million cycles at 4.4 MPa while in torsion they ranged from 100 cycles at 14.4 MPa to more than 1 million cycles at 4.8 MPa. Changes in strain range, tangent modulus, and cyclic energy dissipated during fatigue loading also were examined. Torsional fatigue was found to be considerably more damaging than axial fatigue. The fatigue damage accumulation process occurring in the composite was monitored by observing the reduction in modulus and the increase in energy dissipated.

Fatigue characterization of a hydroxyapatite-reinforced polyethylene composite. II. Biaxial fatigue.

Previously, the uniaxial fatigue behavior of 40 volume % hydroxyapatite-reinforced polyethylene composite (HAPEXtrade mark) at 37 degrees C in saline was studied. Fatigue limits between 37 and 25% of the ultimate strengths of the material were established. This study investigates the biaxial fatigue behavior of the same material using various combinations of axial and torsional stresses. The addition of torsional to axial loading significantly reduced the fatigue life. When torsion at 25% of the ultimate strength was applied in addition to axial loading at 25% of the ultimate tensile strength, the fatigue life remained more than 1 million cycles. Out-of-phase loading was less detrimental to the fatigue life of the composite than in-phase. Fatigue damage was monitored by hysteresis loops, the increase in dissipated energy, the reduction in tangent modulus, and the increase in strain values.

In vitro response of osteoblasts to hydroxyapatite-reinforced polyethylene composites.

A primary human cell culture model was used to investigate a range of hydroxyapaptite (HA)-reinforced high-density polyethylene (HDPE) composites (HAPEX). These materials are being developed as potential bone-substitute materials. When designing and optimizing a second-generation biomaterial, it is important to achieve a balance between mechanical and biological properties without compromising either. Biochemical and histological parameters have been used to compare the biological response of 20% and 40% volume HA in HDPE. Cellular DNA and incorporation of tritiated thymidine was measured to assess cell proliferation. Alkaline phosphatase (ALP) production was used as a marker of osteoblast phenotype expression. In this preliminary study, osteoblasts cultured on the 20% HAPEX showed a greater increase in the rate of proliferation and osteoblast expression as indicated by an increase in ALP activity compared to the 40% HAPEX over the time period studied. Osteoblast-like cells showed a flattened morphology on both composites and in some cases a greater covering was observed on the 20% HAPEX. These results indicate that the composites may not be identical in terms of bioactivity and that further research on surface topography and physico-chemical properties is required to assess fully the biological response of these composites.

Influence of sterilization by gamma irradiation and of thermal annealing on creep of hydroxyapatite-reinforced polyethylene composites.

Sterilization of medical devices by gamma (gamma)-irradiation is common. The effect of irradiation on a bone replacement material, HAPEX (hydroxyapatite-reinforced polyethylene composite), was investigated. Unfilled and hydroxyapatite-filled polyethylene at 0.20 and 0.40 filler volume fractions were gamma-irradiated at 2.5 Mrad, and the modified properties were studied by differential scanning calorimetery, isochronous experiments, and creep tests. The effect of thermal annealing of the samples from 140 degrees C also was examined. The results suggest that both irradiation and annealing increase creep resistance of the materials. These are associated with the formation of crosslinks and an increase in crystallinity, respectively.

Influence of Ringer's solution on creep resistance of hydroxyapatite reinforced polyethylene composites.

In vitro creep studies of polyethylene, both unfilled and filled with hydroxyapatite at 0.20 and 0.40 volume fraction, have been performed. The samples were immersed in Ringer's solution at 37 degrees C for 1, 7, 30, 90 and 150 days prior to isochronous and creep tests in the same condition. The creep properties of unfilled polyethylene is unaffected by the immersion, but the isochronous modulus and the creep resistance of filled polyethylene were reduced. The effect increased with increasing volume fraction and time of immersion. This reduction is related to the penetration of the solution into the material, softening the interface.

In vitro mechanical and biological assessment of hydroxyapatite-reinforced polyethylene composite.

In vitro performance of hydroxyapatite (HA)-reinforced polyethylene (PE) composite (HAPEX) has been characterized from both mechanical and biological aspects. The mechanical properties of HAPEX, such as tensile strength and Young's modulus, showed little change after immersion in a physiological solution at 37 and 70 degrees C for various periods. In addition, the biological response of primary human osteoblast-like (HOB) cells in vitro on HAPEX was assessed by measuring DNA synthesis and osteoblast phenotype expression. Cell proliferation rate on HAPEX was demonstrated by an increase in DNA content with time. A high tritiated thymidine ([3H]-TdR) incorporation/DNA rate was observed on day 1 for HAPEX, indicating a stimulatory effect on cell proliferation. The alkaline phosphatase (ALP) activity was expressed earlier on HAPEX than on unfilled PE and increased with time, indicating that HOB cells had commenced differentiation. Furthermore, it was found that the HA particles in the composite provided favourable sites for cell attachment. It appears that the presence of HA particles in HAPEX may have the advantage of acting as microanchors for bone bonding in vivo.

Creep in polyethylene and hydroxyapatite reinforced polyethylene composites

Isochronous stress-strain relationships and long term creep performance for unfilled and hydroxyapatite filled polyethylene composites have been studied. The tests were carried out in a buffered (pH=7.5) Ringer's solution at 37°C. It was observed that the inclusion of hydroxyapatite does not remove the non-linear viscoelasticity of polyethylene. The creep resistance is found to increase with increase in volume fraction of hydroxyapatite. The creep failure of composites at long times can occur due to debonding of the interface.

Predictive modelling of hydroxyapatite-polyethylene composite

A predictive model for hydroxyapatite-reinforced polyethylene composite has been developed using the finite element analysis method. The simulation is based on the analysis of a representative cell. Results for the complete material can be derived using a spatial statistical material model. Predicted values of Young's modulus are found to be in reasonable agreement with experimentally measured values over a wide range of hydroxyapatite volume fraction. The predictive model can be used to investigate the micromechanical behaviour of the material. This investigation leads to significant elucidation of the failure processes for this material.