Composite hydrogels for implants.

Hydrophilic composite structures are designed to mimic the transport and mechanical properties of natural soft tissue such as tendons, ligaments and intervertebral discs. Mechanical and viscoelastic behaviour of a soft composite material based on a hydrogel matrix reinforced with bundles of polyethylene therephthalate (PET) fibres is analysed. The typical J-shaped stress-strain behaviour, displayed by natural tendons and ligaments, is reproduced. The mechanical characteristics, such as the extent of the 'toe-in region' and the elastic modulus in the linear region, can be controlled by varying the winding angle of the fibres and the matrix composition. Dynamic mechanical analysis showed the dual behaviour of the composite systems due to the progressive contribution of the PET fibres. Different poly(2-hydroxyethylmethacrylate)/polycaprolactone (PHEMA/PCL) semi-interpenetrating polymer networks (IPNs) hydrogel composite systems reinforced with PET fibres have been investigated for potential use as intervertebral disc prostheses. Compression properties have been evaluated by static and dynamic tests. Uniaxial compression tests on the swollen samples showed an increase of the modulus and maximum stress with increasing content of PCL and PET fibres. Creep behaviour is also dependent on the hydrogel composition. The composite PHEMA/PCL hydrogels showed compression properties similar to those expressed by canine intervertebral discs in different spinal locations.