Chemical Functionalization of a Hydroxyapatite Based Bone Replacement Material for the Immobilization of Proteins

Abstract

Hydroxyapatite (HAP) has a great potential as a bone replacement material because of its similarity to the crystal structure of inorganic matrix of bone. Several years ago we showed that porous hydroxyapatite ceramics (Endobon, Merck) can be covalently modified for the immobilization of proteins [Jennissen, H.P. (1999) PCT Patent WO9926674A2] opening the possibility for the immobilization of growth factors and morphogens. Application of this technique to the immobilization of recombinant human bone morphogenetic protein 2 (rhBMP-2) on hydroxyapatite would be of high medical interest, because of the decisive role of bone morphogenetic proteins in bone development and osteogenesis. In this paper it will be shown that the model protein ubiquitin and rhBMP-2 can be immobilized non-covalently and covalently after chemical modification of a hydroxyapatite ceramic surface (Bio-Oss® Cancellous Block, Geistlich, = HAP-wafers). It could be shown that only small amounts of protein are adsorbed to non-functionalized HAP surfaces (control). In contrast ubiquitin and rhBMP-2 can be very efficiently immobilized non-covalently (up to 2.4 mg protein/g HAP) and covalently (up to 9.7 mg protein/g HAP) on porous HAP-wafers. In desorption experiments it is shown that the bound rhBMP-2 is slowly released making such surfaces applicable as long term drug delivery devices for enhancing bone growth and osteointegration of implant materials.