The development of systems for the growth of osteoblasts on bioerodible polymeric matrices was explored. Three classes of bioerodible polymers were studied as possible matrix supports for osteoblast growth: the poly(anhydrides), poly(phosphazenes) and poly(1actic acid/glycolic acid) copolymers. Neonatal calvarial cells from Sprague-Dawley rats were seeded onto polymer disks at a density of 1 x lo4 cells/cm2. Initial attachment and spreading, rate of growth and morphology were determined, and retention of osteoblast-like phenotype was assessed through measurements of alkaline phosphatase activity in the presence and absence of 2,25(OH)2 vitamin 0 3 . All results were considered relative to tissue culture polystyrene.Cells were found to attach to all polymers at 8 hr post-seeding. By 24 hr, cell numbers on all polymers were found to be decreased, except for poly(1actic acid/ glycolic acid). Rat calvarial osteoblasts seeded on poly-(lactic acid/glycolic acid) reached confluency and retained their phenotype.Successful construction of viable osteoblastbioerodible polymer composite materials, as presented in our study, may find their usefulness as grafts for atrophic non-unions of bone, for healing craniofacial and other defects and for use as prosthetic implants or coatings. 'To whom correspondence should be addressed, at M.I.T., Building 56, Room 141, Cambridge, Massachusetts 02139, U.S.A.Composite systems of osteoblast cultures may also find their usefulness in furthering our understanding of bone differentiation, maturation and metabolism in a matrix environment.

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An understanding of bone cell response and extracellular matrix production to a biomaterial is crucial for development of new prosthetic devices. The nature of the cellular-biomaterial surface interface will depend upon a number of factors including substrate properties (surface chemistry, charge, topography) as well as biological cellular concerns (i.e. adsorption of attachment factors to the surface, growth factors). The quality of the matrix and bone-bonding may be influenced by these factors. Recently, a short-term in-vitro cell culture assay has demonstrated the initial attachment and spread of human derived bone cells on metallic (titanium and stainless steel) and polymeric surfaces to be dependent on the adsorption of adhesive attachment factor proteins (fibronectin and vitronectin) to the substratum surface [1]. The morphological appearance of human osteoblasts cultured on titanium and stainless steel with time also demonstrated differences compared to tissue culture plastic [2]. Little data however, is available for the mitogenic and gene expression levels of primary human bone cells cultured on commonly used orthopaedic materials and the response of these cells to growth factors. The present study examined the mitogenic response and steady state mRNA expression levels of primary human bone cells cultured on metallic substrates to provide further insight into the nature of cell-substrate interactions.