Toughening and functionalization of bioactive ceramic and glass bone scaffolds by biopolymer coatings and infiltration: a review of the last 5 years

Abstract

Inorganic scaffolds with high interconnected porosity based on bioactive glasses and ceramics are prime candidates for applications in bone tissue engineering. These materials however exhibit relatively low fracture strength and high brittleness. A simple and effective approach to improve the toughness is to combine the basic scaffold structure with polymer coatings or through the formation of interpenetrating polymer-bioactive ceramic microstructures. The polymeric phase can additionally serve as a carrier for growth factors and therapeutic drugs, thus adding biological functionalities. The present paper reviews the state-of-the art in the field of polymer coated and infiltrated bioactive inorganic scaffolds. Based on the notable combination of bioactivity, improved mechanical properties and drug or growth factor delivery capability, this scaffold type is a candidate for bone and osteochondral regeneration strategies. Remaining challenges for the improvement of the materials are discussed and opportunities to broaden the application potential of this scaffold type are also highlighted.

Keywords:

• bioactive glasses
• bioceramics
• bone tissue engineering
• calcium phosphates
• composite materials
• drug delivery
• growth factors
• hydroxyapatite
• scaffolds

Acknowledgements

We thank Mr Clemens Randow for experimental support.

Financial & competing interests disclosure

A Philippart received partial support from EU ITN project ‘GlaCERCo’. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

Key issues

• Bone may become damaged or suffer disease over the course of a lifetime.

• For the regeneration of bone in tissue engineering strategies, a convenient approach is to apply an engineered porous structure called ‘scaffold’, usually in combination with bioactive molecules such as growth factors and relevant cells. In particular, also for load bearing bone, the structural competence of the scaffold is of prime importance for its handling prior to implantation and due to the fact that the scaffold will be subjected to mechanical loads in vivo while the tissue regeneration process is taking place. Additionally, the scaffold needs to be handled by the surgeon and therefore scaffolds must exhibit sufficient structural integrity.

• Different biomaterials are being considered for BTE, including bioactive glasses, bioceramics, for example, hydroxyapatite, inorganic filler reinforced biopolymer composites as well as polymer-coated or infiltrated ceramic or BG scaffolds and organic–inorganic hybrids.

• Glass and glass–ceramics scaffolds are brittle and exhibit relatively low mechanical strength, they are not suitable for load-bearing application.

• Brittle inorganic scaffolds can be coated or infiltrated with a bioresorbable polymer offering an attractive alternative to counteract the low mechanical properties drawbacks.

• Polymer coating (and infiltration) can be achieved by simply impregnating porous scaffolds with the polymer of choice dissolved in an organic solvent or water. In most cases, the polymer used is synthetic in origin, including PCL, PDLLA or PLGA; more recently, also polymers of natural origin such as gelatine, silk, alginate, collagen and chitosan have been used to coat BG, glass–ceramic or ceramic scaffolds.

• Polymer-coated inorganic scaffolds can be classified in three groups: CaP-based scaffolds; silicate-based scaffolds and composite scaffolds suggested for osteochondral TE, as a special case in which polymer-coated scaffolds offer notable advantages over single-phase scaffolds.

• In addition to providing a toughening effect, the addition of a soft biodegradable polymer to inorganic scaffolds provides also a vehicle for the controlled in situ delivery of therapeutic drugs or other bioactive molecules, for example, growth factors.

• A range of different drugs has been incorporated to date in such scaffolds including antibiotics, nonsteroidal anti-inflammatory drugs such as ibuprofen and drugs for the treatment of bone diseases. Another application of such scaffolds is for the in situ delivery of growth factors such as bone morphogenetic proteins (e.g., BMP-2) and VEGF.

• The present review will hopefully stimulate further research in the field of polymer coated/infiltrated inorganic scaffolds as an attractive scaffold type with drug delivery capability for bone and osteochondral tissue engineering.

Notes