Current state of knowledge
Three aspects limit today’s biocomposites for bone substitutes :
Materials candidates for biocomposites set up: Calcium phosphate ceramics (hydroxyapatite HA, tricalcium phosphate TCP and more recent biphasic calcium phosphate powders BCP) and some types of biocompatible glasses (called bioglasses) are biocompatible and osteoconductive, and can bond to bone thus avoiding the risk of a fibrous connective tissue interface. Unfortunately, the main limitation to the use of calcium phosphates and bioglasses are their poor mechanical properties. High-tech ceramics such as alumina and zirconia answer the need for strong, fatigue resistant ceramics for implants. These ceramics are however considered as ‘bioinert’ since no direct bone–material interface is created. A soft tissue interlayer always shields the bone from the implant. The metallic implants have the advantage of high mechanical strength and inert behaviour but poorer biological properties compared to bioglasses and ceramics. Besides inert polymers like PE, biodegradable synthetic polymers have been used in orthopaedics since 1960’s: polyglycolic acid (PGA) and polylactic acid (PLA) and their colpolymers PLGA, Polycaprolactone) (PCL), collagen, hydrogels, etc. Their degradation rates can be adjusted from several weeks to several years.
The proposed innovative bone substitute will be composed of composites combining inorganic and organic materials.
Manufacturing of implants: Natural bone is a biologically formed composite with variable density ranging from very dense and stiff (cortical bone) to a soft and foamed structure (trabecular bone). Calcium phosphates with graded and architectured porosity and functionality have been developed in attempts to mimic the structure of natural bone. This hierarchical architecture can be obtained by impregnation of an organic scaffold by inorganic slurry followed by thermal treatment or by using rapid manufacturing techniques.
The development of new processes for biomaterials feedstocks of various types (ceramic, polymer, bioglass, metals) is a keypoint to be discussed in this COST Action. The design of the biomaterials devices is obviously a critical parameter to be studied.
Functionalization of implants : Over the last two decades, research efforts intensified on developing doping materials in order to increase the biological activity (silicon and yttrium), to avoid the proliferation of bacteria at the implant surface (Ag, Cu, F and Zn). One of the main problems related to these doping approaches is the deleterious effect on the mechanical properties. One innovative solution is the phagotherapy which uses bacteriophages virus infecting bacteria according to a specific bacteria–bacteriovirus receptor. Another route is to use the family of antimicrobial peptides to fight against the pathogen agents. However, it is necessary to control the release of the antimicrobial peptide based on the cellular activity or the presence of infectious agent.
The development of a new in vitro bone model would reduce the need for animal testing when screening biomaterials on the basis of bone-implant interaction, and provide greater opportunity to develop a more standardised model than is possible with animals, and ultimately it could be rendered more complex or tailored to represent a specific medical application.